Electrolytic production of calcium and its alloys. Give a complete solution to the tasks

3. Receiving. Calcium is produced by electrolysis of its molten chloride.

4. Physical properties. Calcium is a silvery-white metal, very light (ρ \u003d 1.55 g / cm 3), like alkali metals, but incomparably harder and has a much higher melting point, equal to 851 0 C.

5. Chemical properties. Like alkali metals, calcium is a strong reducing agent, which can be schematically represented as follows:

Calcium compounds color the flame a brick-red color. Like alkali metals, calcium metal is usually stored under a layer of kerosene.

6. Application. Due to its high chemical activity, metallic calcium is used to reduce some refractory metals (titanium, zirconium, etc.) from their oxides. Calcium is also used in the production of steel and cast iron, to purify the latter from oxygen, sulfur and phosphorus, to obtain some alloys, in particular, lead-calcium, necessary for the manufacture of bearings.

7. The most important compounds of calcium obtained in industry.

Calcium oxide is produced in the industry by calcining limestone:

CaCO 3 → CaO + CO 2

Calcium oxide is a refractory white substance (melts at a temperature of 2570 0 C), has chemical properties inherent in the main oxides of active metals (I, Table II, p. 88).

The reaction of calcium oxide with water proceeds with the release of a large amount of heat:

CaO + H 2 O ═ Ca (OH) 2 + Q

Calcium oxide is the main component of quicklime, and calcium hydroxide is the main component of hydrated lime.

The reaction of calcium oxide with water is called lime slaking.

Calcium oxide is used mainly for the production of hydrated lime.

Calcium hydroxide Ca (OH) 2 is of great practical importance. It is used in the form of slaked lime, milk of lime and lime water.

Slaked lime is a fine loose powder, usually gray in color (a component of calcium hydroxide), slightly soluble in water (1.56 g dissolves in 1 liter of water at 20 ° C). A dough-like mixture of slaked lime with cement, water and sand is used in construction. The mixture gradually hardens:

Ca (OH) 2 + CO 2 → CaCO 3 ↓ + H 2 O

Lime milk is a slurry (suspension) similar to milk. It is formed by mixing excess hydrated lime with water. Milk of lime is used to obtain bleach, in the production of sugar, for the preparation of mixtures necessary in the fight against plant diseases, for whitewashing tree trunks.

Lime water is a clear calcium hydroxide solution obtained by filtering milk of lime. It is used in the laboratory to detect carbon monoxide (IV):

Ca (OH) 2 + CO 2 → CaCO 3 ↓ + H 2 O

With prolonged transmission of carbon monoxide (IV), the solution becomes transparent:

CaCO 3 + CO 2 + H 2 O → Ca (HCO 3) 2

If the resulting clear solution of bicarbonate calcium is heated, then turbidity occurs again:

Similar processes also take place in nature. If the water contains dissolved carbon monoxide (IV) and acts on limestone, then some of the calcium carbonate is converted to soluble calcium bicarbonate. On the surface, the solution warms up and calcium carbonate precipitates out of it.

* Bleach is of great practical importance. It is obtained by reacting slaked lime with chlorine:

2 Ca (OH) 2 + 2 Cl 2 → Ca (ClO) 2 + CaCl 2 + 2H 2 O

The active component of bleach is calcium hypochlorite. Hypochlorites undergo hydrolysis. This produces hypochlorous acid. Even carbonic acid can replace hypochlorous acid from its salt:

Ca (ClO) 2 + CO 2 + H 2 O → CaCO 3 ↓ + 2 HClO

2 HClO → 2 HCl + O 2

This property of bleach is widely used for bleaching, disinfection and degassing.

8. Plaster. There are the following types of gypsum: natural - CaSO 4 ∙ 2H 2 O, burnt - (CaSO 4) 2 ∙ H 2 O, anhydrous - CaSO 4.

Burnt (semi-aqueous) gypsum, or alabaster, (CaSO 4) 2 ∙ H 2 O is obtained by heating natural gypsum to 150–180 0 С:

2 → (CaSO 4) 2 ∙ H 2 O + 3H 2 O

If you mix alabaster powder with water, then a semi-liquid plastic mass is formed, which quickly hardens. The hardening process is explained by the addition of water:

(CaSO 4) 2 ∙ H 2 O + 3H 2 O → 2

The property of burnt gypsum to harden is used in practice. For example, alabaster mixed with lime, sand and water is used as a plaster. Art products are made from pure alabaster, and in medicine it is used for applying plaster casts.

If natural gypsum CaSO 4 ∙ 2H 2 O is heated at a higher temperature, then all the water is released:

CaSO 4 ∙ 2H 2 O → CaSO 4 + 2H 2 O

The resulting anhydrous CaSO 4 gypsum is no longer able to attach water, and therefore it was called dead gypsum.

Water hardness and how to eliminate it.

Everyone knows that soap foams well in rainwater (soft water), but in spring water it is usually bad (hard water). Analysis of hard water shows that it contains significant amounts of soluble calcium and magnesium salts. These salts form insoluble compounds with soap. Such water is not suitable for cooling internal combustion engines and powering steam boilers, since when hard water is heated, scale forms on the walls of the cooling systems. Scale does not conduct heat well; therefore, overheating of motors, steam boilers is possible, in addition, their wear is accelerated.

What are the types of stiffness?

Carbonate, or temporary, hardness is due to the presence of calcium and magnesium bicarbonates. It can be eliminated in the following ways:

1) by boiling:

Ca (HCO 3) 2 → CaCO 3 ↓ + H 2 O + CO 2

Mg (HCO 3) 2 → MgCO 3 ↓ + H 2 O + CO 2

2) the action of milk of lime or soda:

Ca (OH) 2 + Ca (HCO 3) 2 → 2CaCO 3 ↓ + 2H 2 O

Ca (HCO 3) 2 + Na 2 CO 3 → CaCO 3 ↓ + 2NaHCO 3

Ca 2+ + 2 HCO 3 - + 2 Na + + CO 3 2- → CaCO 3 ↓ + 2 Na + + 2HCO 3 -

Ca 2+ + CO 3 2- → CaCO 3 ↓

Non-carbonate, or permanent, hardness is due to the presence of calcium and magnesium sulfates and chlorides.

It is eliminated by the action of soda:

CaSO 4 + Na 2 CO 3 → CaCO 3 ↓ + Na 2 SO 4

MgSO 4 + Na 2 CO 3 → MgCO 3 ↓ + Na 2 SO 4

Mg 2+ + SO 4 2- + 2Na + + CO 3 2- → MgCO 3 ↓ + 2Na + + SO 4 2-

Mg 2+ + CO 3 2- → MgCO 3 ↓

The carbonate and non-carbonate hardness add up to the total water hardness.

IV. Consolidation of knowledge (5 min.)

1. On the basis of the periodic table and the theory of atomic structure, explain what properties of magnesium and calcium are common. Write down the equations of the corresponding reactions.

2. What minerals do calcium contain and how are they used?

3. Tell us how to distinguish one natural mineral from another.

V. Homework (3 min.)

Answer the questions and do exercises 1-15, § 48.49, solve exercises 1-4, pp. 132-133.

This is how the lesson plan looks at school on the topic "Calcium and its compounds".

Based on the foregoing, the need to fill the school chemistry course with environmental content is obvious. The results of the work done will be presented in the third chapter.

One-time) - 0.01%. 4 Contents Introduction ............................................... .................................................. ..................... 4 Chapter 1. Interdisciplinary connections in the course of the school subject of chemistry on the example of carbon and its compounds ............ .................................................. ......... 5 1.1 The use of interdisciplinary relationships for the formation of students ...

Activity. The search for methods and forms of teaching that contribute to the upbringing of a creative personality has led to the emergence of some specific teaching methods, one of which is play methods. The implementation of game teaching methods in the study of chemistry in the conditions of adherence to didactic and psychological-pedagogical characteristics, increases the level of training of students. The word "game" in Russian ...

And hygiene requirements); correspondence of educational and physical activity to the age-related capabilities of the child; necessary, sufficient and rationally organized motor regime. By health-preserving educational technology (Petrov) means a system that creates the maximum possible conditions for the preservation, strengthening and development of spiritual, emotional, intellectual, ...

ELECTROLYSIS

One of the methods for obtaining metals is electrolysis. Active metals are found in nature only in the form of chemical compounds. How to isolate from these compounds in a free state?

Electrolyte solutions and melts conduct electric current. However, when current is passed through the electrolyte solution, chemical reactions can occur. Let us consider what will happen if two metal plates are placed in a solution or molten electrolyte, each of which is connected to one of the poles of the current source. These plates are called electrodes. Electric current is a moving stream of electrons. As a result of the fact that electrons in the circuit move from one electrode to another, an excess of electrons arises on one of the electrodes. Electrons are negatively charged, so this electrode is charged negatively. It is called the cathode. The other electrode creates a lack of electrons and charges positively. This electrode is called the anode. The electrolyte in solution or melt dissociates into positively charged ions - cations and negatively charged ions - anions. Cations are attracted to a negatively charged electrode - the cathode. Anions are attracted to a positively charged electrode - the anode. Interaction between ions and electrons can occur on the surface of the electrodes.

Electrolysis refers to the processes that occur when electrolytes are passed through solutions or melts of electric current.

The processes occurring during the electrolysis of solutions and electrolyte melts are quite different. Let's consider in detail both of these cases.

Electrolysis of melts

As an example, consider the electrolysis of sodium chloride melt. In the melt, sodium chloride dissociates into ionsNa +
and Cl -: NaCl \u003d Na + + Cl -

Sodium cations move to the surface of a negatively charged electrode - cathode. There is an excess of electrons on the cathode surface. Therefore, there is a transfer of electrons from the electrode surface to sodium ions. In this case, the ionsNa + are converted into sodium atoms, that is, cations are reducedNa + ... Process equation:

Na + + e - \u003d Na

Chloride ions Cl - move to the surface of a positively charged electrode - anode. A lack of electrons is created on the surface of the anode and electrons are transferred from the anionsCl - to the electrode surface. In this case, negatively charged ionsCl - turn into chlorine atoms, which immediately combine into chlorine molecules Cl 2:

2С l - -2е - \u003d Cl 2

Chloride ions lose electrons, that is, they are oxidized.

Let us write together the equations of the processes occurring at the cathode and anode

Na + + e - \u003d Na

2 С l - -2 e - \u003d Cl 2

One electron is involved in the reduction of sodium cations, and 2 electrons are involved in the oxidation of chlorine ions. However, the law of conservation of electric charge must be observed, that is, the total charge of all particles in the solution must be constant.Therefore, the number of electrons participating in the reduction of sodium cations must be equal to the number of electrons participating in the oxidation of chloride ions.Therefore, we multiply the first equation by 2:

Na + + e - \u003d Na 2

2С l - -2е - \u003d Cl 2 1

We add both equations together and we get the general reaction equation.

2 Na + + 2С l - \u003d 2 Na + Cl 2 (ionic reaction equation), or

2 NaCl \u003d 2 Na + Cl 2 (molecular reaction equation)

So, on the considered example, we see that electrolysis is a redox reaction. At the cathode, the reduction of positively charged ions - cations, occurs, at the anode, the oxidation of negatively charged ions - anions. You can remember which process is happening where using the "T rule":

kATOD - KATION - RECOVERY.

Example 2.Sodium hydroxide melt electrolysis.

Sodium hydroxide in solution dissociates into cations and hydroxide ions.

Cathode (-)<-- Na + + OH - à Анод (+)

The reduction of sodium cations occurs on the cathode surface, and sodium atoms are formed:

cathode (-) Na + + e à Na

Hydroxide ions are oxidized on the surface of the anode, while oxygen is released and water molecules are formed:

cathode (-) Na + + e à Na

anode (+) 4 OH - - 4 e à 2 H 2 O + O 2

The number of electrons involved in the reduction of sodium cations and in the oxidation of hydroxide ions must be the same. Therefore, we multiply the first equation by 4:

cathode (-) Na + + e à Na 4

anode (+) 4 OH - - 4 e à 2 H 2 O + O 2 1

We add both equations together and we get the equation for the electrolysis reaction:

4 NaOH а 4 Na + 2 H 2 O + O 2

Example 3. Consider the electrolysis of the meltAl 2 O 3

With this reaction, aluminum is obtained from bauxite, a natural compound that contains a lot of aluminum oxide. The melting point of aluminum oxide is very high (more than 2000 ° C), therefore special additives are added to it, lowering the melting point to 800-900 ° C. In the melt, aluminum oxide dissociates into ionsAl 3+ and O 2-. H cations are reduced on the cathodeAl 3+ , turning into aluminum atoms:

Al +3 e à Al

Anions are oxidized at the anodeO 2- , turning into oxygen atoms. Oxygen atoms immediately combine to form O 2 molecules:

2 O 2- - 4 e à O 2

The number of electrons participating in the processes of reduction of aluminum cations and oxidation of oxygen ions should be equal, so we multiply the first equation by 4, and the second by 3:

Al 3+ +3 e à Al 0 4

2 O 2- - 4 e à O 2 3

We add both equations and get

4 Al 3+ + 6 O 2- а 4 Al 0 +3 O 2 0 (ionic reaction equation)

2 Al 2 O 3 а 4 Al + 3 O 2

Electrolysis of solutions

In the case of passing an electric current through an aqueous electrolyte solution, the matter is complicated by the fact that water molecules are present in the solution, which can also interact with electrons. Recall that in a water molecule, hydrogen and oxygen atoms are linked by a polar covalent bond. The electronegativity of oxygen is greater than the electronegativity of hydrogen, so the common electron pairs are shifted towards the oxygen atom. A partial negative charge arises on the oxygen atom, it is denoted δ-, and on hydrogen atoms, a partial positive charge, it is denoted δ +.

δ+

H-O δ-

│

H δ +

Due to this displacement of charges, the water molecule has positive and negative "poles". Therefore, water molecules can be attracted by the positively charged pole to the negatively charged electrode - the cathode, and the negative pole - to the positively charged electrode - the anode. The reduction of water molecules can occur at the cathode, while hydrogen is released:

At the anode, oxidation of water molecules with the release of oxygen can occur:

2 H 2 O - 4e - \u003d 4H + + O 2

Therefore, either electrolyte cations or water molecules can be reduced at the cathode. These two processes seem to compete with each other. What process actually takes place at the cathode depends on the nature of the metal. Whether metal cations or water molecules are reduced at the cathode depends on the position of the metal in a number of metal stresses .

Li K Na Ca Mg Al ¦¦ Zn Fe Ni Sn Pb (H 2) ¦¦ Cu Hg Ag Au

If the metal is in the series of voltages to the right of hydrogen, metal cations are reduced at the cathode and free metal is released. If the metal is in the series of voltages to the left of aluminum, water molecules are reduced at the cathode and hydrogen is released. Finally, in the case of metal cations from zinc to lead, either metal or hydrogen evolution can occur, and sometimes both hydrogen and metal are released simultaneously. In general, this is a rather complicated case, much depends on the reaction conditions: the concentration of the solution, the electric current, and others.

One of two processes can also occur at the anode - either the oxidation of the electrolyte anions, or the oxidation of water molecules. What kind of process will actually take place depends on the nature of the anion. During the electrolysis of salts of anoxic acids or the acids themselves, anions are oxidized at the anode. The only exception is fluoride ionF - ... In the case of oxygen-containing acids, water molecules are oxidized at the anode and oxygen is released.

Example 1. Let's consider the electrolysis of an aqueous solution of sodium chloride.

An aqueous solution of sodium chloride will contain sodium cationsNa +, chlorine anions Cl - and water molecules.

2 NaCl а 2 Na + + 2 Cl -

2Н 2 О а 2 H + + 2 OH -

cathode (-) 2 Na +; 2 H +; 2Н + + 2е а Н 0 2

anode (+) 2 Cl -; 2 OH -; 2 Cl - - 2е а 2 Cl 0

2NaCl + 2H 2 O à H 2 + Cl 2 + 2NaOH

Chemical activity anions in a row decreases.

Example 2. And if the salt containsSO 4 2- ? Consider the electrolysis of a nickel sulfate solution (II ). Nickel sulfate (II ) dissociates into ionsNi 2+ and SO 4 2-:

NiSO 4 a Ni 2+ + SO 4 2-

H 2 O à H + + OH -

Nickel cations are between metal ionsAl 3+ and Pb 2+ occupying the middle position in the series of voltages, the recovery process at the cathode occurs according to both schemes:

2 H 2 О + 2е - \u003d H 2 + 2ОН -

Anions of oxygen-containing acids are not oxidized at the anode ( anion activity series ), oxidation of water molecules occurs:

anode e à O 2 + 4H +

Let us write together the equations of the processes occurring at the cathode and anode:

cathode (-) Ni 2+; H +; Ni 2+ + 2е а Ni 0

2 H 2 О + 2е - \u003d H 2 + 2ОН -

anode (+) SO 4 2-; OH -; 2H 2 O - 4e a O 2 + 4H +

4 electrons are involved in the reduction processes and 4 electrons are also involved in the oxidation process. We add these equations together and we get the general reaction equation:

Ni 2+ +2 H 2 О + 2 H 2 О а Ni 0 + H 2 + 2ОН - + O 2 + 4 H +

On the right side of the equation, there are simultaneously H + andOH - that combine to form water molecules:

H + + OH - à H 2 O

Therefore, on the right side of the equation, instead of 4 H + ions and 2 ionsOH - we write down 2 water molecules and 2 Н + ions:

Ni 2+ +2 H 2 О + 2 H 2 О а Ni 0 + H 2 +2 H 2 О + O 2 + 2 H +

Reduce two molecules of water in both sides of the equation:

Ni 2+ +2 H 2 О а Ni 0 + H 2 + O 2 + 2 H +

This is a short ionic equation. To get the complete ionic equation, you need to add to both sides of the sulfate ionSO 4 2- formed during the dissociation of nickel sulfate (II ) and not participating in the reaction:

Ni 2+ + SO 4 2- + 2H 2 О а Ni 0 + H 2 + O 2 + 2H + + SO 4 2-

Thus, in the electrolysis of a nickel sulfate solution (II ) hydrogen and nickel are evolved at the cathode, and oxygen at the anode.

NiSO 4 + 2H 2 O à Ni + H 2 + H 2 SO 4 + O 2

Example 3. Write the equations of the processes occurring during the electrolysis of an aqueous solution of sodium sulfate with an inert anode.

System standard electrode potentialNa + + e \u003d Na 0 is much more negative than the potential of a water electrode in a neutral aqueous medium (-0.41 V). Therefore, electrochemical reduction of water will occur at the cathode, accompanied by the evolution of hydrogen

2Н 2 О а 2 H + + 2 OH -

and ions Na + coming to the cathode will accumulate in the adjacent part of the solution (cathode space).

Electrochemical oxidation of water will occur at the anode, leading to the evolution of oxygen

2 H 2 O - 4е à O 2 + 4 H +

since the corresponding system standard electrode potential (1.23 V) is significantly lower than the standard electrode potential (2.01 V) that characterizes the system

2 SO 4 2- + 2 e \u003d S 2 O 8 2-.

Ions SO 4 2- moving towards the anode during electrolysis will accumulate in the anode space.

Multiplying the equation of the cathodic process by two, and adding it to the equation of the anodic process, we obtain the total equation of the electrolysis process:

6 H 2 O \u003d 2 H 2 + 4 OH - + O 2 + 4 H +

Taking into account that the accumulation of ions in the cathode space and ions in the anode space occurs simultaneously, the overall equation of the process can be written in the following form:

6H 2 O + 2Na 2 SO 4 \u003d 2H 2 + 4Na + + 4OH - + O 2 + 4H + + 2SO 4 2-

Thus, simultaneously with the evolution of hydrogen and oxygen, sodium hydroxide (in the cathode space) and sulfuric acid (in the anode space) are formed.

Example 4. Electrolysis of copper sulfate solution (II) CuSO 4.

Cathode (-)<-- Cu 2+ + SO 4 2- à анод (+)

cathode (-) Cu 2+ + 2e а Cu 0 2

anode (+) 2H 2 O - 4 е а O 2 + 4H + 1

H + ions remain in the solution andSO 4 2- , since sulfuric acid accumulates.

2CuSO 4 + 2H 2 O à 2Cu + 2H 2 SO 4 + O 2

Example 5. Electrolysis of copper chloride solution (II) CuCl 2.

Cathode (-)<-- Cu 2+ + 2Cl - à анод (+)

cathode (-) Cu 2+ + 2e а Cu 0

anode (+) 2Cl - - 2e а Cl 0 2

Both equations involve two electrons.

Cu 2+ + 2e à Cu 0 1

2Cl - -– 2e à Cl 2 1

Cu 2+ + 2 Cl - а Cu 0 + Cl 2 (ionic equation)

CuCl 2 à Cu + Cl 2 (molecular equation)

Example 6. Electrolysis of silver nitrate solutionAgNO 3.

Cathode (-)<-- Ag + + NO 3 - à Анод (+)

cathode (-) Ag + + e à Ag 0

anode (+) 2H 2 O - 4 е а O 2 + 4H +

Ag + + e à Ag 0 4

2H 2 O - 4 e à O 2 + 4H + 1

4 Ag + + 2 H 2 O à 4 Ag 0 + 4 H + + O 2 (ionic equation)

4 Ag + + 2 H 2 Oà 4 Ag 0 + 4 H + + O 2 + 4 NO 3 - (complete ionic equation)

4 AgNO 3 + 2 H 2 Oà 4 Ag 0 + 4 HNO 3 + O 2 (molecular equation)

Example 7. Electrolysis of hydrochloric acid solutionHCl.

Cathode (-)<-- H + + Cl - à anode (+)

cathode (-) 2H + + 2 eà H 2

anode (+) 2Cl - – 2 eà Cl 2

2 H + + 2 Cl - à H 2 + Cl 2 (ionic equation)

2 HClà H 2 + Cl 2 (molecular equation)

Example 8. Sulfuric acid solution electrolysisH 2 SO 4 .

Cathode (-) <-- 2H + + SO 4 2- à anode (+)

cathode (-)2H + + 2eà H 2

anode (+) 2H 2 O - 4eà O 2 + 4H +

2H + + 2eà H 2 2

2H 2 O - 4eà O 2 + 4H + 1

4H + + 2H 2 Oà 2H 2 + 4H + + O 2

2H 2 Oà 2H 2 + O 2

Example 9. Potassium hydroxide solution electrolysisKOH.

Cathode (-)<-- K + + OH - à anode (+)

Potassium cations will not be reduced at the cathode, since potassium is in the metal voltage series to the left of aluminum; instead, water molecules will be reduced:

2H 2 O + 2eà H 2 + 2OH - 4OH - -4eà 2H 2 O + O 2

cathode (-) 2H 2 O + 2eà H 2 + 2OH - 2

anode (+) 4OH - - 4eà 2H 2 O + O 2 1

4H 2 O + 4OH -à 2H 2 + 4OH - + 2H 2 O + O 2

2 H 2 Oà 2 H 2 + O 2

Example 10. Potassium nitrate solution electrolysisKNO 3 .

Cathode (-) <-- K + + NO 3 - à anode (+)

2H 2 O + 2eà H 2 + 2OH - 2H 2 O - 4eà O 2 + 4H +

cathode (-) 2H 2 O + 2eà H 2 + 2OH-2

anode (+) 2H 2 O - 4eà O 2 + 4H + 1

4H 2 O + 2H 2 Oà 2H 2 + 4OH - + 4H ++ O 2

2H 2 Oà 2H 2 + O 2

When electric current is passed through solutions of oxygen-containing acids, alkalis and salts of oxygen-containing acids with metals in the metal voltage series, to the left of aluminum, electrolysis of water practically occurs. In this case, hydrogen is released at the cathode, and oxygen at the anode.

Conclusions. When determining the products of electrolysis of aqueous solutions of electrolytes, in the simplest cases, one can be guided by the following considerations:

1.Metal ions with a small algebraic value of the standard potential - fromLi + beforeAl 3+ inclusive - have a very weak tendency to re-attachment of electrons, being inferior in this respect to ionsH + (cm. A series of cation activities). In the electrolysis of aqueous solutions of compounds containing these cations, the function of an oxidizing agent at the cathode is performed by ionsH + , while recovering according to the scheme:

2 H 2 O+ 2 eà H 2 + 2OH -

(2) Metal cations with positive values \u200b\u200bof standard potentials (Cu 2+ , Ag + , Hg 2+ and others) have a great tendency to attach electrons as compared to ions. During the electrolysis of aqueous solutions of their salts, the function of an oxidizing agent at the cathode is released by these cations, thus reducing to metal according to the scheme, for example:

Cu 2+ +2 eà Cu 0

3.In the electrolysis of aqueous solutions of metal saltsZn, Fe, Cd, Ni and others, occupying the middle position in the series of voltages between the listed groups, the recovery process at the cathode occurs according to both schemes. The mass of the released metal does not correspond in these cases to the amount of electric current flowing, part of which is spent on the formation of hydrogen.

(4) In aqueous solutions of electrolytes, the function of reducing agents with respect to the oxidizing anode can be monoatomic anions (Cl - , Br - , J - ), oxygen-containing anions (NO 3 - , SO 4 2- , PO 4 3- and others), as well as hydroxyl ions of water. Halide ions have stronger reducing properties of them, with the exception ofF... JonahOH occupy an intermediate position between them and polyatomic anions. Therefore, in the electrolysis of aqueous solutionsHCl, HBr, Hj or their salts at the anode oxidation of halide ions occurs according to the scheme:

2 X - -2 eà X 2 0

During the electrolysis of aqueous solutions of sulfates, nitrates, phosphates, etc. the function of a reducing agent is performed by ions, being oxidized according to the scheme:

4 HOH – 4 eà 2 H 2 O + O 2 + 4 H +

.

Tasks.

Z and dacha 1. During the electrolysis of a copper sulfate solution at the cathode, 48 g of copper were released. Find the volume of gas released at the anode and the mass of sulfuric acid formed in the solution.

Copper sulfate in solution dissociates no ions C 2+ andS0 4 2 ".

CuS0 4 \u003d Cu 2+ + S0 4 2 "

Let us write down the equations of the processes occurring at the cathode and anode. Cu cations are reduced at the cathode, water electrolysis occurs at the anode:

Cu 2+ + 2e- \u003d Cu12

2H 2 0-4e- \u003d 4H + + 0 2 |1

General electrolysis equation:

2Cu2 + + 2H2O \u003d 2Cu + 4H + + O2 (short ionic equation)

Add to both sides of the equation 2 sulfate ions, which are formed during the dissociation of copper sulfate, we obtain the complete ionic equation:

2Cu2 + + 2S042 "+ 2H20 \u003d 2Cu + 4H + + 2SO4 2" + O2

2CuSO4 + 2H2O \u003d 2Cu + 2H2SO4 + О2

The gas evolved at the anode is oxygen. Sulfuric acid is formed in the solution.

The molar mass of copper is 64 g / mol, we calculate the amount of copper substance:

According to the reaction equation, when 2 mol of copper is released at the cathode, 1 mol of oxygen is released in the anode. At the cathode, 0.75 mol of copper was released, even if x mol of oxygen was released at the anode. Let's make the proportion:

2/1 \u003d 0.75 / x, x \u003d 0.75 * 1/2 \u003d 0.375 mol

At the anode, 0.375 mol of oxygen was released,

v (O2) \u003d 0.375 mol.

Let's calculate the volume of released oxygen:

V (O2) \u003d v (O2) "VM \u003d 0.375 mol" 22.4 L / mol \u003d 8.4 L

According to the reaction equation, when 2 mol of copper is released at the cathode, 2 mol of sulfuric acid is formed in the solution, which means that if 0.75 mol of copper is released at the cathode, then 0.75 mol of sulfuric acid is formed in the solution, v (H2SO4) \u003d 0.75 mol ... Let's calculate the molar mass of sulfuric acid:

M (H2SO4) \u003d 2-1 + 32 + 16-4 \u003d 98 g / mol.

Let's calculate the mass of sulfuric acid:

m (H2SO4) \u003d v (H2SO4\u003e M (H2SO4) \u003d \u003d 0.75 mol «98 g / mol \u003d 73.5 g.

Answer:8.4 liters of oxygen were released at the anode; the solution formed 73.5 g of sulfuric acid

Problem 2. Find the volume of gases evolved at the cathode and anode during the electrolysis of an aqueous solution containing 111.75 g of potassium chloride. What substance was formed in the solution? Find its mass.

Potassium chloride in solution dissociates into K + and Cl ions:

2KS1 \u003d K + + Cl

Potassium ions are not reduced at the cathode; instead, water molecules are reduced. At the anode, chloride ions are oxidized and chlorine is released:

2H2O + 2e "\u003d H2 + 20H- | 1

2SG-2e "\u003d C12 | 1

General electrolysis equation:

2СГl + 2Н2О \u003d Н2 + 2ОН "+ С12 (short ionic equation) K + ions are also present in the solution, formed during the dissociation of potassium chloride and not participating in the reaction:

2K + + 2Cl + 2H20 \u003d H2 + 2K + + 2OH "+ C12

Let's rewrite the equation in molecular form:

2KS1 + 2H2O \u003d H2 + C12 + 2KON

Hydrogen is released at the cathode, chlorine at the anode, and potassium hydroxide is formed in the solution.

The solution contained 111.75 g of potassium chloride.

Let's calculate the molar mass of potassium chloride:

M (KC1) \u003d 39 + 35.5 \u003d 74.5 g / mol

Let's calculate the amount of potassium chloride substance:

According to the reaction equation, during the electrolysis of 2 mol of potassium chloride, 1 mol of chlorine is released. Let the electrolysis of 1.5 mol of potassium chloride release x mol of chlorine. Let's make the proportion:

2/1 \u003d 1.5 / x, x \u003d 1.5 / 2 \u003d 0.75 mol

Allocated 0.75 mol of chlorine, v (C! 2) \u003d 0.75 mol. According to the reaction equation, when 1 mol of chlorine is released at the anode, 1 mol of hydrogen is released at the cathode. Therefore, if 0.75 mol of chlorine is released at the anode, then 0.75 mol of hydrogen will be released at the cathode, v (H2) \u003d 0.75 mol.

Let's calculate the volume of chlorine released at the anode:

V (C12) \u003d v (Cl2) -VM \u003d 0.75 mol «22.4 L / mol \u003d 16.8 L.

The volume of hydrogen is equal to the volume of chlorine:

Y (H2) \u003d Y (C12) \u003d 16.8 liters.

According to the reaction equation, during the electrolysis of 2 mol of potassium chloride, 2 mol of potassium hydroxide is formed, which means that during the electrolysis of 0.75 mol of potassium chloride, 0.75 mol of potassium hydroxide is formed. Let's calculate the molar mass of potassium hydroxide:

M (KOH) \u003d 39 + 16 + 1 - 56 g / mol.

Let's calculate the mass of potassium hydroxide:

m (KOH) \u003d v (KOH\u003e M (KOH) \u003d 0.75 mol-56 g / mol \u003d 42 g.

Answer:16.8 liters of hydrogen were released at the cathode, 16.8 liters of chlorine were released at the anode, 42 g of potassium hydroxide were formed in the solution.

Problem 3. During the electrolysis of a solution of 19 g of bivalent metal chloride, 8.96 liters of chlorine were released at the anode. Determine which metal chloride was electrolyzed. Calculate the volume of hydrogen evolved at the cathode.

Let's denote the unknown metal M, the formula of its chloride is MC12. At the anode, chloride ions are oxidized and chlorine is released. The condition says that hydrogen is released at the cathode, therefore, the reduction of water molecules occurs:

2H20 + 2- \u003d H2 + 2OH | 1

2Cl -2e "\u003d C12! 1

General electrolysis equation:

2Сl + 2Н2О \u003d Н2 + 2ОН "+ С12 (short ionic equation)

The solution also contains M2 + ions, which do not change during the reaction. We write down the complete ionic equation of the reaction:

2СГ + М2 + + 2Н2О \u003d Н2 + М2 + + 2ОН- + С12

Let's rewrite the reaction equation in molecular form:

MS12 + 2H2O - H2 + M (OH) 2 + C12

Let's find the amount of chlorine released at the anode:

According to the reaction equation, during the electrolysis of 1 mol of chloride of an unknown metal, 1 mol of chlorine is released. If 0.4 mol of chlorine was released, then 0.4 mol of metal chloride was subjected to electrolysis. Let's calculate the molar mass of metal chloride:

The molar mass of the unknown metal chloride is 95 g / mol. Two chlorine atoms account for 35.5 * 2 \u003d 71 g / mol. Therefore, the molar mass of the metal is 95-71 \u003d 24 g / mol. Magnesium corresponds to this molar mass.

According to the reaction equation, 1 mole of chlorine released at the anode accounts for 1 mole of hydrogen released at the cathode. In our case, 0.4 mol of chlorine was released at the anode, which means that 0.4 mol of hydrogen was released at the cathode. Let's calculate the volume of hydrogen:

V (H2) \u003d v (H2\u003e VM \u003d 0.4 mol «22.4 L / mol \u003d 8.96 L.

Answer:a solution of magnesium chloride was subjected to electrolysis; 8.96 liters of hydrogen were released at the cathode.

* Task 4. During electrolysis of 200 g of potassium sulfate solution with a concentration of 15%, 14.56 liters of oxygen were released at the anode. Calculate the concentration of the solution at the end of electrolysis.

In a solution of potassium sulfate, water molecules react both at the cathode and at the anode:

2H20 + 2e "\u003d H2 + 20H- | 2

2H2O - 4e "\u003d 4H + + O2! 1

Let's add both equations together:

6H2O \u003d 2H2 + 4OH "+ 4H + + O2, or

6H2O \u003d 2H2 + 4H2O + O2, or

2H2O \u003d 2H2 + 02

In fact, during the electrolysis of a potassium sulfate solution, electrolysis of water occurs.

The concentration of the solute in the solution is determined by the formula:

С \u003d m (solute) 100% / m (solution)

To find the concentration of a potassium sulfate solution at the end of electrolysis, it is necessary to know the mass of potassium sulfate and the mass of the solution. The mass of potassium sulfate does not change during the reaction. Let's calculate the mass of potassium sulfate in the original solution. Let us denote the concentration of the initial solution Cb

m (K2S04) \u003d C2 (K2S04) m (solution) \u003d 0.15 200 g \u003d 30 g.

The mass of the solution changes during electrolysis, as part of the water is converted into hydrogen and oxygen. Let's calculate the amount of the substance of the released oxygen:

(O2) \u003d V (O2) / Vm \u003d 14.56 l / 22.4 l / mol \u003d 0.65 mol

According to the reaction equation, from 2 mol of water 1 mol of oxygen is formed. Let 0.65 mole of oxygen evolve during decomposition of x mole of water. Let's make the proportion:

Decomposed 1.3 mol of water, v (H2O) \u003d 1.3 mol.

Let's calculate the molar mass of water:

M (H2O) \u003d 1-2 + 16 \u003d 18 g / mol.

Let's calculate the mass of decomposed water:

m (H2O) \u003d v (H2O\u003e M (H2O) \u003d 1.3 mol * 18 g / mol \u003d 23.4 g.

The mass of the potassium sulfate solution decreased by 23.4 g and became equal to 200-23.4 \u003d 176.6 g. Now we calculate the concentration of the potassium sulfate solution at the end of electrolysis:

C2 (K2 SO4) \u003d m (K2 SO4) 100% / m (solution) \u003d 30g 100% / 176.6g \u003d 17%

Answer:the concentration of the solution at the end of electrolysis is 17%.

* Problem 5. 188.3 g of a mixture of sodium and potassium chlorides was dissolved in water and an electric current was passed through the resulting solution. During electrolysis, 33.6 liters of hydrogen were released at the cathode. Calculate the composition of the mixture as a percentage by weight.

After dissolving a mixture of potassium and sodium chlorides in water, the solution contains K +, Na + and Cl- ions. Neither potassium ions nor sodium ions are reduced at the cathode; water molecules are reduced. At the anode, chloride ions are oxidized and chlorine is released:

Let's rewrite the equations in molecular form:

2KS1 + 2H20 \u003d H2 + C12 + 2KON

2NaCl + 2H2O \u003d H2 + C12 + 2NaOH

Let us designate the amount of potassium chloride substance contained in the mixture, x mol, and the amount of sodium chloride substance in mol. According to the reaction equation, during electrolysis, 2 mol of sodium or potassium chloride releases 1 mol of hydrogen. Therefore, during electrolysis, x mol of potassium chloride is formed x / 2 or 0.5x mol of hydrogen, and during electrolysis, mol of sodium chloride is 0.5 mol of hydrogen. Let's find the amount of the hydrogen substance released during the electrolysis of the mixture:

Let's make the equation: 0.5x + 0.5y \u003d 1.5

Let's calculate the molar masses of potassium and sodium chlorides:

M (KC1) \u003d 39 + 35.5 \u003d 74.5 g / mol

M (NaCl) \u003d 23 + 35.5 \u003d 58.5 g / mol

The mass x mol of potassium chloride is:

m (KCl) \u003d v (KCl) -M (KCl) \u003d x mol-74.5 g / mol \u003d 74.5 x g.

The mass of a mole of sodium chloride is:

m (KCl) \u003d v (KCl) -M (KCl) \u003d y mol-74.5 g / mol \u003d 58.5 y g.

The mass of the mixture is 188.3 g, let's draw up the second equation:

74.5x + 58.5y \u003d 188.3

So, we solve a system of two equations with two unknowns:

0.5 (x + y) \u003d 1.5

74.5x + 58.5y \u003d 188.3g

From the first equation we express x:

x + y \u003d 1.5 / 0.5 \u003d 3,

x \u003d 3-y

Substituting this x value in the second equation, we get:

74.5- (3-y) + 58.5y \u003d 188.3

223.5-74.5y + 58.5y \u003d 188.3

-16y \u003d -35.2

y \u003d 2.2 100% / 188.3g \u003d 31.65%

We calculate the mass fraction of sodium chloride:

w (NaCl) \u003d 100% - w (KCl) \u003d 68.35%

Answer:the mixture contains 31.65% potassium chloride and 68.35% sodium chloride.

Physicochemical properties of electrolyte

The melting point of calcium chloride is 774 °. In some cases, potassium chloride (melting point 768 °) and sometimes sodium chloride (melting point 800 °) are added to the electrolyte.
The melting diagram of the CaCl2-KCl system was investigated by O. Menge. A compound CaCl2 KCl is formed in the system and there are two eutectics, at 75% (mol) CaCl2 with a melting point of 634 ° and at 25% (mol) CaCl2 with a melting point of 587 °.
The CaCl2-NaCl system gives a eutectic at 53% (mol) CaCl2 with a melting point of about 494 °.
The state diagram of the CaCl2-KCl-NaCl system was investigated by K. Sholich. A eutectic of the composition-52% CaCl2, 41% NaCl, 7% KCL is formed in it at 508 °
The electrolyte recommended by Ruff and Plato contains 85.8% CaCl2 and 14.2% CaF2 and melts at 660 ° The density of calcium chloride, according to Arndt, is expressed by the equation: d \u003d 2.03-0.00040 (t ° - 850 °) ...
According to V.P. Borzakovsky, the density of CaCl2 at 800 ° is 2.049; at 900 ° 2.001, at 1000 ° 1.953 Additions of potassium chloride or sodium chloride reduce the density of the melt However, even with significant additions of alkali metal chlorides, the difference in the density of the melt and metallic calcium is still sufficient for the metal to easily float to the electrolyte surface
The value of the viscosity and surface tension of calcium chloride at the interface with the gas phase, according to V.P. Borzakovsky, are given below

The addition of potassium chloride and sodium chloride to calcium chloride reduces the melt viscosity and increases the surface tension at the interface with the gas phase
The electrical conductivity of calcium chloride is, according to Borzakovsky: at 800 ° 2.02 ohm-1 / cm3, at 900 ° 2.33 ohm-1 / cm3; a value close to these data was obtained by Sandonini. Additives up to 25% (mol) of potassium chloride, or up to 55% (mol) of sodium chloride reduce the electrical conductivity; a further increase in additives increases the electrical conductivity of the melt
The vapor pressure of calcium chloride is significantly higher than that of KCl, NaCl, MgCl2. The boiling point of calcium chloride is approximately 1900 °. The total vapor pressure in a mixture of calcium chloride with the indicated chloride salts was investigated by V.A. Ilyichev and K.D. Muzhavlev.
Decomposition voltage of calcium chloride (V) measured by Combi and Devato by emf polarization in the temperature range 700-1000 °, expressed by the formula

E \u003d 3.38 - 1.4 * 10v-3 (t ° -700 °)

Below is a comparison of the decomposition stresses of several chloride salts at 800 ° C.

In practice, with a current output of 60-85%, the reverse emf on the bath is 2.8-3.2 V. Drossbach points out that the reverse emulsion observed during electrolysis. etc. with. the emf answers cell

Ca / CaCl / CaCl2 / Cl2.

The decomposition voltage of salts decreases with increasing temperature Ho, since the temperature coefficients of change in the decomposition voltage are different for different salts, then the sequence of the release of a particular metal from a mixture of salts can change with temperature. At electrolysis temperatures of calcium chloride, a discharge of magnesium and sodium ions is possible. Therefore, the electrolyte of the calcium bath must be free from impurities of these salts.

Electrolysis with touch cathode

Foundations of the theory

During the electrolysis of molten calcium chloride, calcium released at the cathode, as in the production of magnesium or sodium, is much lighter than the electrolyte and therefore floats to the surface of the bath. However, it is not possible to obtain calcium in liquid form similarly to obtaining magnesium. Magnesium dissolves slightly in the electrolyte and is protected by an electrolyte film held on the metal surface. Magnesium floating on the surface of the electrolyte is periodically scooped up. Calcium is much more active than magnesium and is not protected by an electrolyte film. Its solubility in the electrolyte is high, according to Lorentz's research, 13% of the metal dissolves in calcium chloride. When it dissolves, CaCl subchloride is formed, which, reacting with chlorine, turns into CaCl2. Under the influence of oxygen and moisture in the air, subchlorides form a suspension of calcium oxide in the melt. If the molten calcium is allowed to remain in contact with the electrolyte, then, thanks to the circulation of the latter, the calcium will be carried away to the region of the anode chlorine and eventually all will be converted into calcium chloride. In addition to dissolving in the electrolyte, calcium, being on the surface of the bath, actively reacts with the gases surrounding it.
When calcium is released below its melting point, a spongy dendritic metal permeated with salt is formed with a large oxidation surface. Smelting such a metal is very difficult. Therefore, metallic calcium with an acceptable current efficiency can be obtained only by the Rathenau and Süter method - electrolysis with a touch cathode / The essence of the method is that the cathode initially touches the molten electrolyte. At the point of contact, a liquid metal droplet that wets the cathode well is formed, which, when the cathode is slowly and evenly raised, is removed from the melt together with it and solidifies. In this case, the solidifying drop is covered with a solid electrolyte film, which protects the metal from oxidation and nitriding. By continuously and carefully lifting the cathode, the calcium is drawn into the rods.
The conditions of electrolysis with a touch cathode on an electrolyte of calcium chloride and fluoride were further studied and improved by Goodwin, who developed an apparatus for laboratory experiments, Freri, who drew attention to practical methods in electrolysis, Brace, who built a bath for 200 A, and others.
In Russia, this method was studied and improved on baths with amperage from 100 to 600 A (Z.V. Vasiliev, V.P. Mashovets, B.V. Popov and A.Yu. Taits, V.M. Guskov and MT Kovalenko , A.Yu. Taits and M.I. Pavlov, Yu.V. Baimakov).
One of the conditions for achieving a satisfactory current efficiency is the use of a high current density at the cathode. This is necessary in order for the amount of metal released per unit time to significantly exceed its dissolution. Depending on the working surface of the cathode, the power of the electrolyzer and other factors, the cathode current density is selected in the range of 50-250 A / cm2. For the normal operation of the process, it is important to ensure accurate control of the cathode rise. Raising the cathode too quickly causes a liquid drop of metal to detach and dissolve in the electrolyte. Ascending slowly, the calcium overheats and melts away from the rod. Metal separation can also be caused by overheating of the electrolyte. The dissolution of calcium in the electrolyte with the formation of subchloride and calcium oxide causes the electrolyte to thicken and foam, which disrupts the normal operation of the bath. When the bath is cold, the metal on the cathode grows in the form of dendrites.
The current density at the anode is chosen as low as possible (about 0.7-1.5 A / cm2) in order to avoid the anode effect. The anode effect begins when the current density on graphite reaches 8 A / cm2, and on the carbon anode 5.6 A / cm2. The temperature of the electrolyte of calcium chloride without additives is maintained at 800-810 °, with the addition of other salts it decreases. Around the cathode, due to the high concentration of the current, a rim of an overheated electrolyte is observed, having a temperature of 820-850 °. In view of the need to maintain the electrolyte temperature close to the melting point of calcium (851 °), additives for lowering the melting point of the electrolyte are not essential, but their role is positive for lowering the solubility of calcium in the electrolyte.
The electrolyte used must be as dehydrated as possible and not contain harmful impurities. The moisture contained in the electrolyte decomposes with the release of hydrogen at the cathode, which, combining with calcium, forms calcium hydride, which is accompanied by an increase in temperature at the cathode. In addition, moisture contributes to the formation of foam in the electrolyte. All this disrupts the normal course of electrolysis. Another harmful electrolyte impurity is silica, which, even in small amounts, causes calcium to dissolve in the electrolyte. As a result, subchloride is formed and the electrolyte thickens, which makes it difficult to separate calcium at the cathode. Impurities of magnesium and sodium are undesirable, since they, being released during electrolysis, are fused with calcium, lowering the melting point of the cathode metal and making it difficult to pull out.

Electrolysis practice

The industrial production of calcium by electrolysis with a touch cathode was started even before the First World War in Germany (Biterfeld) and France (Jarry). Montel and Hardy indicate that the power consumption ranged from 30,000 to 50,000 kW * -h per 1 g of metal, depending on the size of the electrolyzer, its design features and the duration of the electrolysis campaign. The consumption of calcium chloride was 4.5 kg per 1 kg of metal.

The working chamber of the German bathtub (Fig. 2) has an octagonal shape with a diameter of 400 mm and a height of 350 mm. It is lined with carbon blocks that serve as an anode. The space between the blocks and the bath casing is lined and filled with thermal insulation. Above the working chamber of the bath, an iron cathode with a diameter of 60 mm is fixed, which moves in the vertical direction and to regulate the voltage on the bath - in the horizontal direction. Air cooling is supplied to the cathode and air, together with anode gases, is discharged through a channel arranged in the wall of the bath. Bath capacity 40 l for 90 kg of melt. Electrolyte composition,%: 35.46 Ca, 63 Cl, 0.35 CaO, 0.03 SiO2 (max.), 0.04 Fe2O3 + Al2O3 (max.). In addition, 1-1.5 kg of potassium chloride is added to the bath, and sometimes a small addition of fluoride salt is added. Electrolyte temperature 800-820 °, cathode current density 50-100 A / cm2, anode 1-1.5 A / cm2, current on the bath 900-2000 A, voltage 20-25 V. The current efficiency varies greatly at different times of the year and depending on the humidity of the air and averages 35-40%. However, the bath provides 6 to 15 kg of calcium per day. 1 kg of calcium consumes about 70 kWh of electricity and 8 kg of salt. Analysis of impurities in the cathode metal,% (wt.): 0.01-0.08 Mg 0.01-0.05 Si, 0.1-0.3 Fe + Al, 0.05-0.07 Mn, 0.008 -0.03 N, 0.7-1.6 Cl.
According to Bagley's description, in the USA (Michigan) in 1939 an experimental installation of three baths with an amperage of 2000 A was built, which was soon doubled (Fig. 3). The control of the cathode was automated, while the operations of periodic addition of electrolyte and removal of calcium rods were performed manually. Subsequently, new series of baths were supplied for 4000 a, then for 5000 a and, finally, for 10000 a.

The resulting calcium rods have a diameter of 175 to 350 mm and a length of up to 600 mm. The shaft is covered with an electrolyte crust on the outside. The inner metal part of the rod is quite compact.
It should nevertheless be noted that, despite the existing technical advances, electrolysis with a touch cathode has serious drawbacks: low current efficiency, high power consumption, low calcium recovery from raw materials, the need to use an electrolyte completely free of H2O, SiO2, etc. compounds, the difficulty of constructing a bath of greater power, etc. All this forced in the last decade, when the demand for calcium has greatly increased, to look for fundamentally different methods of obtaining. The search was not unsuccessful.

Liquid cathode electrolysis and production of calcium alloys

Foundations of the theory

Obtaining calcium on a liquid metal cathode eliminates the main difficulties encountered in the separation of pure liquid metal. The fusion of calcium with the cathode metal located at the bottom of the bath under the electrolyte prevents it from dissolving in the electrolyte and reuniting with chlorine and makes it impossible for calcium to be oxidized by the surrounding gases. This ensures a high current efficiency. The possibility of close proximity of the electrodes to each other, the absence of a high cathode current density required for electrolysis with a touch cathode, and depolarization during the release of calcium on a liquid cathode can significantly reduce the voltage across the bath. Achieving high performance depends on the choice of the cathode, cathode current density, temperature and other process conditions. The cathode metal must be alloyed with calcium, and the value of the cathode current density must correspond to the rate of calcium diffusion into the alloy. Therefore, it is beneficial to stir the cathode alloy. The nature of the diagram of the state of calcium and cathode metal is of great importance. So, for example, in the electrolysis of calcium chloride with a liquid lead cathode, it is not possible to obtain rich alloys with a good current efficiency due to the fact that during the formation of the alloy the melting temperature, according to the phase diagram (Fig. 4), sharply increases, reaching at 28% Ca 1106 °.

V.M. Guskov and V.F. Fedorov obtained a good current efficiency (89.3%) by stirring the Pb-Ca alloy and saturating it with calcium up to 4.4%; the electrolysis temperature was 800-810 °. With an increase in the calcium content in the alloy and with an increase in temperature, the current efficiency decreases sharply.
Before the amount of calcium in the alloy reaches 1-2%, the cathode current density can be increased only up to 2 A / cm2. With a further increase in the amount of calcium in the alloy, the current density must be reduced. A similar pattern was established by A.F. Alabyshev.
Due to the different nature of the Ca-Al phase diagram, A. Yu. Taits and A.V. Golynskaya electrolysis of calcium chloride with a liquid aluminum cathode obtained alloys containing 62% Ca at a temperature of 840-880 ° and a cathode current density of 1.5 A / cm2. To prevent the calcium-rich alloy from floating up, 15% potassium chloride was added to the bath, which reduced the electrolyte density from 2.03 to 1.84.
According to the phase diagram of Zn-Ca (Fig. 5), the electrolytic release of calcium on a zinc cathode with an increase in Ca content to 90% in the alloy is possible at temperatures not exceeding 720 °. However, it is difficult to obtain very rich alloys on a zinc cathode due to the floating and suspension of alloy particles.

The deposition of calcium on a copper cathode proceeds well. According to the Cu-Ca phase diagram (Fig. 6), the melting point of the alloy lies below 750 ° with a content of 25 to 70% Ca, the alloy of this composition does not float, its density even with a content of 60% Ca is 4.4 at a density electrolyte 2.2. The electrolytic production of calcium-copper alloys is of exceptional interest for the production of pure calcium. The large difference in the vapor pressure of copper (boiling point 2600 °) and calcium (boiling point 1490 °) allows distillation to isolate pure calcium from the alloy.

Electrolysis practice

The industry uses electrolysis with lead, zinc and copper cathodes. The production of lead alloys with calcium and barium is organized in the United States at the United Ltd Company plant. Each bath is an iron crucible, placed in brickwork, in which an external heating is arranged. The bath is loaded with about 2 tons of pig lead. Lead is covered with a layer of pure calcium and barium chlorides with a height of 75-100 mm. In the center of the bath, a graphite anode is immersed with a device for lowering and raising, the movement of which regulates the bath temperature. At the bottom, as well as along the walls of the bath, a garnissage is formed, which prevents current losses, which are possible due to its flow from the anode to the walls of the bath, bypassing the liquid lead cathode. Calcium and barium released during electrolysis are absorbed by molten lead. It is noted that the efficiency of the process decreases due to anodic effects, metal dissolution and the formation of calcium and barium carbides. Electrolysis is carried out to obtain an alloy containing 2% alkaline earth metals (approximately three days of electrolysis). When the desired concentration is reached, the current is turned off and the alloy is released into the ladle, from which it is poured into the general mixer.
In the GDR, at the IGF plant, a calcium-zinc alloy was obtained.
The bathtub (Fig. 7) consists of a cast-iron box with dimensions 2250x700x540 mm, walled in brickwork. The anode is six carbon blocks with a section of 200X200 mm, suspended on a common shaft with a manual drive for lowering and lifting. Zinc is poured into the bottom of the box, and the alloy is accumulated in the bath, from where, with a content of 60-65% Ca, it is periodically scooped out without stopping the bath. The evolved chlorine is sucked from the top through the hood. Each bath draws a current of 10,000 A at a voltage of 25 V. The electrolyte is an alloy of calcium chloride with 18% potassium chloride. Electrolysis temperature 750 °. The capacity of the bath is 4 kg of calcium in the alloy per hour, the plant produced 10 tons of alloy per month.
In recent years, the electrolysis of calcium chloride with a liquid calcium-copper cathode, followed by the removal of calcium from the alloy, has received wide industrial application.
The electrolyzer for producing a calcium-copper alloy (Fig. 8) is a rectangular cast iron bath. The bath is 0.90 m wide and 3 m long. The bath is lined with refractory bricks on the outside and enclosed in a metal casing for mechanical strength.

The anode is a package of graphite bars, which are attached to a metal traverse. The current is supplied to the anode through flexible busbars attached to the traverse. The anode can be raised and lowered using the handwheel. Chlorine is evacuated through gas ducts located on the side of the bath. A copper-calcium alloy is poured onto the bottom of the bath, serving as a cathode. The current strength in such an electrolyzer is 15000 amps. Recently, electrolyzers for high amperage have been created. The voltage across the bath is 7-9 volts. The daily productivity of the electrolyzer is 15,000 and about 300 kg of calcium in the alloy.
The technological mode is ensured by observing the following conditions. Electrolyte temperature 675 ° -715 °. The electrolyte composition is 80-85% calcium chloride and 15-20% potassium chloride. The electrolyte level in the bath is 20-25 cm, the level of the cathode alloy is 5-20 cm. The alloy is saturated with calcium up to 60-65% - The return alloy after distillation contains about 30% Ca. The distance between the electrodes is 3-5 cm. The electrolyte temperature is regulated by changing the pole-to-pole distance.
The cathodic current density is 0.4-0.5 a / cm2, the anode current density is 1.0-1.2 a / cm2. There are indications about the use of almost twice higher current densities.
The bathtub is fed with small portions of solid calcium chloride (20-30 kg each). Unlike cells with a touch cathode, this bath can be fed with partially dehydrated feedstock containing up to 10% moisture. Its final dehydration occurs on the surface of the bath.
The alloy is removed when the calcium content does not exceed 65%. With a richer alloy, there is a risk of floating. The alloy is scooped out using a vacuum ladle to a level in the bath of ~ 5 cm.After draining the rich alloy, the poor alloy and calcium chloride are loaded into the bath
When electrolysis of calcium chloride with a liquid calcium-copper cathode, the current efficiency is 70-75%. The specific energy consumption is 15,000 - 18,000 kW / h per 1 ton of calcium in the alloy, the consumption of calcium chloride is 3.5 g, and the consumption of graphite anodes is 60-70 K per 1 g of calcium in the alloy. Cast iron baths work for 10-14 months.

Introduction

CHAPTER I. Literary Review

1.1. Methods for the production and disposal of calcium chloride 7

1.1.1 Chemical methods 7

1.1.2. Electrochemical Methods 10

1.2. Obtaining calcium saccharates and their use as corrosion inhibitors 12

1.3. Electrochemical synthesis of gaseous chlorine 13

1.4. Synthesis of carbon dioxide 16

1.5. Regularities of the course of electrochemical processes in natural waters containing calcium ions 17

1.5L. Thermal water electrolysis 17

1.5.2. Electrolysis of sea water 20

1.6. Conclusions from the literature review 23

CHAPTER II. Experimental procedure 24

2.1. Polarization measurements 24

2.2- Electrochemical syntheses 25

2.3. Method of analysis and identification of products 26

2.4. Mathematical processing of the results obtained 33

Chapter III. Experimental data and their discussion

3.1. Regularities of the course of electrode reactions in a solution of calcium chloride on various electrode materials 39

3.1.1. Anodic process - kinetics and mechanism of chlorine gas formation during electrolysis of calcium chloride solution 39

3.1.2. Cathodic process - kinetics of hydrogen gas formation during electrolysis of calcium chloride solution 45

3.1.3. Preparative aspects of electrolysis of an aqueous solution of calcium chloride 48

3.2. Features of the course of electrode reactions in aqueous solutions (SAC12 + SUCHAROSE) on various electrode materials 50

3.2.1. Cathodic process 50

3.2.2. Preparative aspects of electrochemical production of calcium saccharate 58

3.2.3. Regularities of the electrode reaction in the system: (CaCl2 + sucrose + Ca (OH) 2) 61

3.2.3.1 Anodic process 61

3.2.3.2 Cathodic process 62

3.3. Regularities of the course of electrode reactions in the system [CaCl2 + NH3 + Ca (III3) 2] 65

3.3.1. Anodic process 65

3.3.2. Cathodic process. 68

3.3.3. Preparative aspects of the electrochemical synthesis of calcium nitrate 74

3.3.4. Preparative aspects of electrochemical production of carbon dioxide 75

3.4 Electrochemical production of calcium acetate 78

3.4.1. Features of the cathodic process in the electrosynthesis of calcium acetate on various electrode materials 79

3.4.2. Preparative aspects of calcium acetate electrosynthesis 87

Literature

Introduction to work

Relevance of the topic. Almost all natural waters contain calcium compounds in various concentrations. Large amounts of calcium chloride are formed as waste in the production of soda, hydrolysis of chlorine-containing organic compounds and in other industrial processes.

The known chemical and electrochemical methods of calcium chloride processing have significant disadvantages: chloride decomposition

calcium at a temperature of 950-1000 C requires the use of special construction materials and high energy costs, while electrolysis of calcium chloride solutions, an insoluble precipitate is deposited on the cathode (tCa (OH) 2* иСаСІ2) and over time the passage of electric current through the system stops.

The processing of calcium chloride into more valuable products, using it as a new type of raw material for the production of hydrochloric acid, chlorine, chlorosulfonic acids and aluminum chloride in organic and pharmaceutical production, is an urgent problem.

Particularly promising for these purposes are electrochemical methods that allow the synthesis of chemical products without the use of reagents, using electrooxidative and electroreductive processes.

The choice of research objects in the dissertation work was determined, on the one hand, by the value of the final products, and on the other hand, by the possibility of using as a raw material - calcium chloride - a large-tonnage waste of industrial production, the processing of which contributes to the protection of the environment from harmful industrial emissions.

The purpose and objectives of the study. The purpose of the work was to study the law
measurements of the course of electrode reactions and obtaining calcium-containing
other compounds from aqueous solutions of calcium chloride.

Achieving this goal required the following tasks:

to study the anodic reaction of chlorine release from aqueous solutions of calcium chloride on various electrode materials;

to establish the kinetics and mechanism of the electrode reactions in aqueous solutions of calcium chloride, calcium nitrate, calcium acetate and a mixture of calcium chloride with sucrose;

Determine the optimal parameters of the electrochemical synthesis of calcium
Fth-containing compounds: current density, electrolyte concentration,

current outputs of target products.

The objects of research were electrochemical processes, prote
popping on various electrode materials in aqueous chloride solutions
calcium with various additives. The choice of the object of research was determined, with
on the one hand, the lack of study and complexity of electrode processes in races
monitored systems, and on the other - the possibility of using waste
Sh large-scale production of calcium chloride to obtain valuable

products.

Scientific novelty:

The scientific basis of the technology and progressive technological solutions for the electrolysis of aqueous solutions containing calcium ions have been created;

The regularities of the course of anodic and cathodic reactions were studied.
irradiation of calcium-containing compounds on various electrode materials

Practical valuework:

For the first time, using calcium chloride as a raw material, such valuable chemical compounds as calcium acetate, calcium saccharate, calcium nitrate, carbon dioxide, gaseous chlorine and hydrogen were synthesized.

Approbationwork. The main results were reported and discussed at the XIV meeting on the electrochemistry of organic compounds "News of the electrochemistry of organic compounds" (Novocherkassk, 1998), at the All-Russian scientific and practical conference "Chemistry in technology and medicine" (Makhachkala, 2002), at the International scientific -technical conference dedicated to the 70th anniversary of the St. Petersburg State University of Low Temperature and Food Technologies (St. Petersburg, 2001), the International Conference "Contemporary Problems of Organic Chemistry, Ecology and Biotechnology" (Luga, 2001), at the final All-Russian conferences " Ecology and rational nature management "(St. Petersburg, 2001 and 2002).

The volume and structure of the thesis.The dissertation work consists of an introduction, three chapters, conclusions and a list of references, including 111 titles. The work is presented on 100 pages of typewritten text, includes 36 figures and 6 tables.

The work was carried out within the framework of a grant from the Ministry of Education of the Russian Federation under the program "Scientific research of higher education in priority areas of science and technology", subprogram - "Ecology and rational use of natural resources", section - "Problems of technogenic formations and the use of industrial and domestic waste in 2001-2002." ...

Obtaining calcium saccharates and their use as corrosion inhibitors

Chlorine is used in significant amounts to prepare bleaches (calcium hypochlorite and bleach). Burning chlorine in a hydrogen atmosphere produces pure hydrogen chloride. The corresponding chlorides are used in the production of titanium, niobium and silicon. Chlorides of iron and aluminum phosphorus are also used industrially.

Over 60% of all chlorine produced is used for the synthesis of organochlorine compounds. Large consumers of chlorine include the production of carbon tetrachloride, chloroform, methylene chloride, dichloroethane, vinyl chloride, chlorobenzene. Significant amounts of chlorine are consumed in the synthesis of glycerin and ethylene glycol by chlorine methods, as well as in the synthesis of carbon disulfide.

Chlorine dioxide obtained in the process of electrolysis of sodium chloride solution is more promising for water disinfection.

According to preliminary estimates, chlorine production in the United States in 1987 was 10.4 million tons. The cost of 1 ton of chlorine is $ 195. Chlorine is obtained by electrolysis of a NaCl solution. The theoretical foundations and designs of industrial electrolysers are described in the monograph.

Mastering the technology of electrolysis of NaCl brines using ion-exchange membranes makes it possible to reduce (in comparison with diaphragm or mercury electrolysis) the cost of equipment (by 15-25%) and energy costs (by 20-35%). The efficiency of membrane electrolysis is associated with the possibility of obtaining alkali with a concentration of 40% with an electric power consumption of 200 kWh / t of the product. Double-layer membranes allow operation at current densities of up to 4 kA / m, which ensures more efficient use of cheap electricity at night. These advantages fully compensate for the relatively high cost of new membranes ($ 500-700 / m2).

The efficiency of using activated cathodes, which makes it possible to reduce the overvoltage of hydrogen evolution, is discussed. A further reduction in cell voltage can be achieved by increasing the operating pressure to 5 bar while increasing the temperature. The use of oxygen (air), which depolarizes the cathode, replacing the process of hydrogen evolution with the oxygen reduction process, reduces the energy consumption to 1600 kWh / t of alkali (if we do not take into account the lost energy content of hydrogen). An alternative route is the electrooxidation of hydrogen in a fuel cell.

Experiments of the Hoechst company with a chlorine membrane electrolyzer with a membrane area of \u200b\u200b0.1 m2 are described. It was found that the current efficiency, which decreases with increasing alkali concentration, reaches a minimum at a concentration of 30% and then increases to a concentration of 34%, after which it drops again. Various mechanisms of the implementation of the membrane process and the choice of membrane properties, the reasons for their aging are considered. It is shown that only at a low cost of steam, the cost of energy costs for membrane electrolysis can approach that for the mercury method.

The work carried out a systematic study of the electrolysis process of solutions of chlorides of alkali and alkaline-earth metals without a diaphragm. It is shown that the differences in the course of the anodic process, depending on the nature of the cation of the initial electrolyte, are due to the different solubility of electrolysis products, mainly the solubility of the hydroxides of the corresponding metals.

In a chloride membrane electrolyzer, at least on one side of the membrane there is a porous gas and liquid permeable layer that has no electrode activity. The pressure in the cathode and anode chambers is maintained, preferably up to 15 kgf / cm2, which makes it possible to reduce the electrolysis voltage. The method can be applied to electrolysis of water and hydrochloric acid.

The paper considers a model of the process of obtaining gaseous chlorine in a non-flowing electrolyzer.

Thermal water electrolysis

Recently, sodium or calcium hypochlorite has been used to purify and especially neutralize water. The increased interest in hypochlorite is largely due to its great potential for use. The use of hypochlorite obtained by electrolysis of seawater for wastewater treatment is ecologically expedient.

The electrochemical method of obtaining hypochlorite solutions by electrolysis of aqueous solutions of sodium chloride or natural waters allows organizing this production directly at the places of consumption of solutions, while there is no need for long-term storage of hypochlorite solutions.

Currently, two methods of electrochemical preparation of a disinfectant have found application: by electrolysis of concentrated sodium chloride solutions followed by mixing with treated water and direct electrolysis of disinfected water. The electrolysis process in both cases depends on the current density on the electrodes, sodium chloride concentration, pH, temperature and nature of the electrolyte movement, the material of the electrodes and their passivation, as well as the method of current supply to the electrodes.

The process of electrochemical synthesis of sodium hypochlorite in a membrane electrolyzer with an ORTA electrode and an inorganic ceramic membrane based on 2x0g has been investigated. The influence of current density, concentration of sodium chloride solution, feed rate of sodium chloride solution, rate of solution supply to electrode chambers was studied. It is shown that under optimal conditions the current efficiency of sodium hypochlorite is 77% with a specific power consumption of 2.4 kWh / kg and sodium chloride 3.1 kg / kg. The corrosion ability of the anode was determined under the conditions of the experiment.

The proposed method and device for monitoring chlorine-containing compounds during water treatment, intended mainly for disinfection of water in swimming pools. The sodium hypochlorite disinfectant solution is generated electrolytically, assuming that the pool water contains a sufficient amount of chlorides. Water circulates in a closed loop, in the outer part of which there is an electrolyzer, as well as a filter for water purification.

The authors of the patent propose for the disinfection of drinking water to build into the lateral surface of the pipeline a mini-electrolyzer, in which hypochlorite is electrochemically produced from a diluted chloride-containing solution.

The features of electrolysis of a dilute (0.89%) sodium chloride solution under flow conditions have been investigated. It was found that an increase in the flow rate leads to a sharp decrease in the output of chlorate and can significantly increase the productivity and stability of the cell. The best results were obtained in an electrolytic cell with titanium electrodes having dispersed platinum-coated electrodes with a roughness factor of at least 200, with periodic cathodic activation of the anodes.

The electrochemical process of sodium hypochlorite synthesis under pressure has been studied. Electrolysis is carried out in a titanium alloy autoclave, reinforced inside with fluoroplastic with stirring. Hydrogen gas formed during the cathodic reaction accumulates in the system, increasing its pressure. The studies were carried out under a pressure of 100-150 atm. Due to the fact that the solution is under high pressure, the solubility of chlorine increases, which leads to a higher current efficiency of sodium hypochlorite. Titanium-based ruthenium dioxide, graphite and platinum were used as cathode materials, and titanium served as the cathode.

It is reported about the use of sodium hypochlorite obtained by electrolysis of natural waters to purify the water of the Makhachkala-Ternair field from phenol.

Sea water has a high mineralization. The salinity of sea water in general is 3.5% or 35,000 million "1. Of these, only two components (chlorides and sodium) are present in an amount of more than 1%, while the concentration of the other two: sulfate and magnesium, is about OD%; calcium, potassium, bicarbonate and bromine are about 0.001%, the rest of the elements are present in very low concentrations.

By the ratio of individual salts to their sum, the salinity of the Caspian Sea differs from that of the oceanic and Black Sea. The waters of the Caspian Sea are relatively poor, in comparison with the oceanic ones, in Na and SG ions and are rich in Ca and SO4 ions. "The average salinity of the Caspian Sea waters is 12.8-12.85%, with fluctuations from 3% in the mouth of the Volga to 20% in the Balkhan Bay In winter, the salinity of the waters of the North Caucasus is high, which is explained by ice formation, and a weak inflow of the Volga waters.

In recent years, there has been an increase in the supply of salts to the sea, which is associated with an increase in the ionic runoff of rivers.

The largest amount of suspended particles present in sea waters contains the same minerals as the surrounding rocks (kaolinite, talc, quartz, feldspar, etc.). Table 1.1. the main composition of the Caspian Sea water is presented.

Electrochemical syntheses

The analysis of chlorine-containing compounds was carried out according to the following methods: Determination of CIO by the Ponttius method. 10 ml of electrolyte (pH \u003d 8) with the addition of a small amount of starch was titrated with OD N solution of potassium iodide. Definition of SG. Bring 1 ml of electrolyte to 100 ml with distilled water. Titrate 10 ml of the sample with 0.1 N silver nitrate solution in the presence of several drops of CH3COOH + K2C104.

Definition of C1CV. Add 25 ml of Mohr's salt to 10 ml of the sample. Warm up until bubbles appear and cool sharply. Add 5 ml of Reingart's mixture and titrate with 0.1 N potassium permanganate until a pink color appears.

Definition of СЮ /. Add 10 ml of a saturated solution of potassium chloride to 10 ml of electrolyte. If no precipitate occurs, then CIO / is absent in the system. Determination of the amount of chlorine released The chlorine gas formed during electrolysis is passed through a solution of potassium iodide and the released iodine is titrated with sodium thiosulfate of a certain concentration. Chlorine is determined by iodometric titrimetric method.

Reagents: sodium thiosulfate - 0.005 N solution; KI - 10% solution; acetate buffer mixture. Prepare by mixing equal volumes of 1 N solutions of CH3COONa and CH3COOH; freshly prepared starch solution - 1% solution.

Determination progress. In a conical flask with a capacity of 250 ml, measure with a pipette 100 ml of tap water, add 5 ml of 10% KI solution, 5 ml of acetate buffer mixture and 1 ml of starch solution. Titrate the sample with 0.005 N sodium thiosulfate solution until the blue color of the solution disappears.

To determine the calcium content in waters, the trilonometric method is used, which makes it possible to determine from 0.1 mg and more Ca in the sample. This method is based on the use of Trilon B in the presence of the indicator mu-rexid. The essence of the method lies in the fact that Ca2 + ions in an alkaline medium form a complex compound with murexide, which is destroyed during titration with Trilon B as a result of the formation of a more stable sodium complexonate. Murexide (ammonium salt of purpuric acid at pH 12 interacts with Ca ions, forming pink compounds.

Murexide does not react with Mg ions, but if the latter in the test water is more than 30 mg / L, Mg (OH) 2 precipitates, adsorbing the indicator on its surface, which makes it difficult to fix the equivalence point. Then the test solution should be diluted 5-6 times to reduce the concentration of magnesium.

Reagents: Trilon B - 0.05 N solution. The exact normality is established according to the standard 0.05 N solution of MgS04 or prepared from fik-sanal; NaOH - 10% solution; murexid - dry mixture (1 part murexid and 99 parts NaCl).

Analysis progress. In a conical flask with a capacity of 250 ml, measure with a pipette 100 ml of the test water, add 5 ml of 10% sodium hydroxide solution, add a little dry indicator mixture. In this case, the solution turns red. The sample is titrated with Trilon B with vigorous stirring until a lilac color appears, which is stable for 3-5 minutes. With further addition of Trilon B, the color does not change. As a "witness", you can use a re-titrated sample, but it should be remembered that the titrated sample retains a stable color for a relatively short time. Therefore, it is necessary to prepare a new "witness" if a change in the color of the previously prepared one is observed.

Cathodic process - kinetics of hydrogen gas formation during electrolysis of calcium chloride solution

Taking into account that platinum is an expensive electrode material, the process of chlorine evolution was investigated on a cheaper material, graphite. Fig. 3.3 shows the anodic volt-ampere curves on graphite in aqueous solutions of calcium chloride at a concentration of 0.1 - 2.0 M. As in the case of a platinum electrode, with an increase in the concentration of calcium chloride leads to a shift in the potential of chlorine evolution towards the anode side by an average of 250 - 300 mV.

From the volt-ampere curves of chlorine evolution presented above on electrode materials made of platinum, graphite, and ORTA, it follows that with an increase in the concentration of calcium chloride, the process of evolution of molecular chlorine becomes easier due to a decrease in the diffusion component of the process.

To compare the kinetic parameters of chlorine evolution, Fig. 3.4 shows the corresponding Tafel dependences of the overvoltage (n) on the logarithm of the current density (log і) on the platinum, graphite electrodes and ORTA.

The corresponding equations of the straight lines, after calculating the coefficients a and b, can be represented in the following form: According to the calculated coefficients a and b, the characteristics of the process were found - the exchange current i0 and the transfer coefficient a

The parameters for the electrochemical evolution of chlorine from a 2M calcium chloride solution are given below:

In fig. 3.5. for comparative analysis, anodic volt-ampere curves for platinum, graphite, and ORTA in a 2M solution of calcium chloride are presented. As can be seen from the figure, chlorine is released from the calcium chloride solution at the lowest potentials at the anode from the ORTA, and the volt-ampere curve on graphite is shifted by 250-300 mV relative to the curve on the ORTA towards the anode side. Therefore, it is obvious that OPTA is preferable to use as an anode material for electrolysis of aqueous solutions of calcium chloride. On graphite, energy consumption will be higher, and the latter is inferior in resistance to ORTA, especially at high anode loads.

Taking into account that the energy consumption during electrolysis also depends on the kinetics of the cathodic process, we studied the regularities of hydrogen evolution from aqueous solutions of calcium chloride on various electrode materials.

In fig. 3.6. the volt-ampere curves of cathodic hydrogen evolution from calcium chloride solutions with a concentration of 0.5 - 2.0 M on a platinum electrode are shown. Analysis of current-voltage curves shows that with an increase in the concentration of calcium chloride, the overvoltage of hydrogen evolution increases (by 30-40 mV). A likely explanation may be the formation of a hardly soluble precipitate of calcium salts, which screens the surface of the platinum electrode and the amount of which increases with increasing concentration of Ca + ions. In this regard, there is a noticeable increase in the voltage on the electrolyzer, noted earlier in the work in the electrochemical production of calcium hypochlorite.

Cathode volt-ampere curves recorded on more affordable electrode materials for practical electrolysis - graphite, steel, copper and titanium - are shown in Figures 3.7 and 3.8. Volt-ampere curves show that a low overvoltage of hydrogen evolution after platinum is observed on the graphite electrode (Fig. 3.7, curve 2)? while the electroreduction of hydrogen ions on a titanium cathode (Fig. 3.8, curve 2) proceeds with the highest overvoltage. This behavior is typical for metals coated with phase oxides in the region of hydrogen evolution potentials and which have an inhibitory effect on the process. Consequently, the most suitable cathode material for electrolysis of calcium chloride solution is graphite.

^ CHAPTER 9. ELECTROCHEMICAL PRODUCTION

9.1 Theoretical foundations of industrial electrolysis

The technology of electrochemical production considers processes in which the main reactions take place in an environment of direct conversion of electrical energy into chemical energy, without intermediate conversion of energy into heat.

For this, special technological methods and equipment have been created, based on theoretical electrochemistry and differing from methods in other areas of chemical technology. In electrolysis, the desired reactions can be carried out, as a rule, with a high degree of selectivity, which makes it possible to obtain a product with relatively small impurities. The usefulness of electricity in electrolysis is relatively high.

Technological processes that can be carried out by electrochemical methods, in most cases, can also be carried out by other chemical methods.

The choice of technology should be decided on the basis of a comparative techno-economic analysis, which takes into account the economics of production, the resources of the necessary raw materials, the complexity of the hardware design and other issues.

The technical and economic advantages of electrochemical methods are determined by the fact that they can be used to obtain fairly pure products in relatively simple technological schemes. The disadvantages are associated with the need to spend an expensive type of energy (direct current energy) and incur the costs of creating sources of its production.

Electrochemical methods have found application for the production of hydrogen and oxygen, chlorine, sodium and potassium hydroxide, oxygen compounds of chlorine, for the electrosynthesis of inorganic substances, as well as for the synthesis of organic substances.

The electrochemical method is used to obtain by hydroelectrometallurgical methods such metals as copper, nickel, zinc, cobalt, cadmium, manganese, chromium, iron, silver, gold, etc., as well as metal powders. Using the electrolysis of molten media, it is possible to obtain aluminum, magnesium, alkali and alkaline earth metals (sodium, calcium), beryllium, rare and rare earth metals, as well as elemental fluorine.

In electroplating, electrochemical methods are used for copper plating, nickel plating, chromium plating and the application of other coatings, in mechanical engineering - for anode-mechanical treatment of products (drilling, cutting, electropolishing, precise complex-shaped processing, etc.).

Chemical current sources, such as batteries and galvanic cells, have been created on the basis of electrochemical processes.

Electrochemical reactions take place in devices called electrolysers.In them, a direct current flows from the anode to the cathode through electrolytes (solutions or melts - conductors of the second kind). Oxidation reactions take place at the anode and reduction reactions at the cathode.

According to Faraday's laws, the amount of substance released on the electrodes is proportional to the amount of electricity passed. Several reactions can occur in parallel on each electrode. The share of the total amount of electricity passed, consumed for this reaction, is its current output.

Practically important current output to the main reaction,which characterizes the excellence of the process. The reaction rate in electrochemistry is understood as current density - the amount of electricity passed per unit of time through the unit of the electrode surface at its border with the electrolyte.

In practice, the current density is determined by dividing its strength by the geometric area of \u200b\u200bthe electrode. Distinguish estimatedand true current density,which is determined not by the geometric, but by the actual surface of the electrode. The latter depends on the porosity and topography of the surface (the presence of bulges and depressions) and practically cannot be determined. Obviously, the more developed the electrolyte surface, the lower the true current density and the more it differs from the calculated one.

In industrial plants, electrochemical reactions are mainly carried out that require the consumption of electrical energy. These costs are characterized by a potential jump that occurs at the electrode-electrolyte interface. If the electrode reaction takes place in equilibrium reversible conditions (with a current approaching zero), then the potential jump between the electrode and the electrolyte is called equilibrium potential.The equilibrium potential gives the value of the potential jump required to start the reaction.

Important concept "Standard potential".This is the equilibrium potential determined for the case when the activity of each active substance is equal to unity. Standard potentials are given in reference tables. Taking into account real conditions and using the Nernst formula, the equilibrium potentials can be calculated from them.

The sum of the equilibrium potentials at the anode and cathode is called decomposition stress.It is at this voltage, at a current close to zero, and in the presence of conditions for the reversibility of electrode reactions, the electrolysis process begins.

In practice, a current that differs from zero passes through the electrodes, and the electrode processes therefore proceed under nonequilibrium conditions. The potential jump at the electrode-electrolyte interface in these cases is greater than the equilibrium one and is called potential of the electrode.It is impossible to experimentally measure the potential difference between the electrode and the electrolyte. Instead, the potential difference is measured between the given electrode and a standard (eg, standard hydrogen electrode) electrodes. This difference is taken as the potential of the electrode. The sign rule for potentials follows from this way of defining them.

The difference between the potential of the electrode and its equilibrium potential is called overvoltage.The higher the true current density, the greater it is. Overvoltage at the electrode also occurs when the reactions occurring on it are irreversible. The overvoltage is proportional to the energy that must be expended in order to carry out the electrode reaction at a certain speed.

The electrode reaction goes through the following stages:

1) supply of reacting substances to the electrodes from the electrolyte and removal of reaction products from them;

2) movement of electrons between electrodes and ions;

3) secondary reactions at electrodes (for example, the formation of hydrogen and oxygen molecules from atoms).

In order for the process to proceed at a given speed on the electrodes, a certain amount of electrical energy is required for each stage. At the first stage, it is proportional to the concentration component of the overvoltage, or concentration polarization.

The equilibrium potentials are calculated based on the average activity (concentration) of the reactants in the solution. At the electrodes, they are either triggered or accumulated, so there their activity differs from the average.

The equilibrium reaction potential calculated from the value of the true activity of substances at the electrodes differs from the potential calculated from the average activity. The difference between them is concentration polarization. It is proportional to the work of concentrating or diluting solutions from the average activity value to the true value, which is created at the electrodes, and the greater, the higher the reaction rate at the electrodes.

The appearance of the second component of the overvoltage depends on the other two stages of the electrode process - chemical overvoltageor polarization.From the energy side, it is explained as follows. It is known from chemical kinetics that only active molecules with energies above a certain level (activation energy) enter into reactions.

By increasing the potential jump at the electrode - electrolyte interface against the equilibrium one, it is possible, as it were, to lower the energy barrier and thereby increase the fraction of active particles without changing the temperature. In this part, there is an analogy between catalytic and electrochemical processes. An additional jump in potential against equilibrium (chemical polarization) is proportional to the work required to activate the required number of ions or molecules in order for the reaction to proceed at a given rate. The higher the reaction rate, the higher the chemical polarization.

The physical picture at the electrodes, which explains the appearance of chemical polarization, is considered in the double layer theory and the related delayed discharge theory. These theories show that the value of chemical polarization depends on the structure of the double layer, which is largely determined by the composition of the solution and the ability of the electrode material to adsorb certain components of the solution. Thus, by choosing the composition of the solution and the material of the electrode, it is possible to control the chemical polarization.

The actual voltage to be applied to the terminals of the electrolyzer is electrolysis voltage - in order to conduct the reaction at a given speed, the decomposition voltage is greater by the amount of overvoltage at the electrodes and by the sum of losses in the conductors of the first and second kind.

The consumption of electrical energy per unit of product is directly proportional to the product of the electrolysis voltage by the amount of electricity required to produce the product (taking into account the current efficiency). Only a part of the total energy consumption is transferred to chemical energy. This part is proportional to the voltage, which is called voltage according to Thompson.It differs from the decomposition voltage for the following reason: the electrical energy required for the process on the electrodes (at constant temperature and in equilibrium reversible conditions), proportional to the decomposition voltage, does not fully correspond to the change in the internal energy of the system. It may happen (depending on the properties of the substances participating in the reaction) that part of it during the reaction turns into heat, which passes into the internal energy of the system.

The difference between electrolysis voltage and Thompson voltage is proportional to the excess heat generated during electrolysis. This is the heating voltage, or thermal component of the voltage.

The share of the total electricity consumption that has passed as a result of the reaction into the internal energy of the target product is called energy efficiency.

The main technological indicators of electrolysis include: current efficiency, energy utilization factor (energy efficiency) and energy consumption factor.

The current output (W t,%) is calculated by the formula:

B t \u003d (m f / m t) ∙ 100%, (9.1)

Where: m f - the amount of substance actually obtained during electrolysis, kg; m t - the amount of substance that should be released according to Faraday's law, kg.

M т \u003d k ∙ I ∙ τ, (9.2)

Where: I - current strength, A; τ — electrolysis time, h; k is the electrochemical equivalent of the emitted substance.

K \u003d M / (F ∙ z), (9.3)

Where: F - Faraday constant equal to 96,500 C or 26.8 A ∙ h; z is the charge of the ion released at the electrode.

The energy utilization factor (V e,%) is calculated by the formula:

V e \u003d (Wt / W f) ∙ 100%, (9.4)

Where: W T - theoretical energy consumption, kW ∙ h; W f - actual energy consumption, kW ∙ h.

W t / p \u003d V t / p ∙ J ∙ τ / m t / p (9.5)

Where: V t - theoretical decomposition voltage, V; V p - the actual voltage at the electrodes, V.

The theoretical energy consumption (kWh / t) can also be calculated using the equation:

W t \u003d 10 3 ∙ V t / k (9.6)

The electrolysis process begins if the voltage applied to the electrolyzer (V p) exceeds the theoretical decomposition voltage (V t) by an infinitesimal value (∆V), i.e. the condition will be provided:

V p \u003d V t + ∆V (9.7)

The theoretical decomposition voltage at the bath electrodes is determined by the equation:

V т \u003d E k - E a (9.8)

Where: E k - the actual potential of the discharge of ions at the cathode, V; E a is the actual potential of the ion discharge at the anode, V.

The actual discharge potentials of ions differ from their equilibrium discharge potentials by the amount of overvoltage, respectively, of the cathodic E k per and anodic E a per, which increase the equilibrium potentials:

E k \u003d E k p + E k per and E a \u003d E a p + E a per (9.9)

Where: E k р and E а р - equilibrium potentials of the cation and anion discharge.

The equilibrium potentials of the ion discharge are equal in magnitude and opposite in sign to the equilibrium electrode potentials: E k p \u003d - φ k and E a p \u003d - φ a, which can be calculated using the Nernst formula:

φ k / a \u003d φ 0 k / a ± R ∙ T ∙ Iga k / a / z ∙ F, (9.10)

Where: φ 0 k / a - standard electrode potential, V; R - universal gas constant, J / mol ∙ K; T is temperature, K; a k / a - ion activity in solution (melt), mol / l; F - Faraday constant equal to 96500 Cul .; z is the charge of the electrolyte ion.

Deciphering the condition presented in 9.7 gives the "voltage balance" equation:

V p \u003d V t + J ∙ ∑R \u003d E k - E a + J (R e + R d + R tp) (9.11)

Where: J - current strength, A; R is the total resistance of the electrolysis process, Ohm; R e - resistance of the electrolyte, Ohm; R d - resistance of the diaphragm of the electrolyzer, Ohm; R TP is the resistance of the current supply paths, Ohm.

^ TOPIC CONTROL QUESTIONS 9.1

9-1 ... What processes are called electrochemical and how do they differ from electrothermal processes? Give examples of both.

9-2. What are the advantages of electrochemical methods of obtaining substances over chemical ones?

9-3. Name the areas of application of electrochemical methods.

9-4 ... What is the prerequisite for the electrolysis process? What is overvoltage and how does it affect the discharge sequence of ions in electrolysis?

9-5 ... List the quantitative characteristics of industrial electrolysis and give them a definition.

^ PROBLEMS TO TOPIC 9-1

9-1. How much hydrochloric acid can theoretically be obtained from electrolytic chlorine and hydrogen per day if the current supplied to the electrolyzer is 1500 A. The mass fraction of hydrochloric acid in the solution is 37.23% (density 1.19 g / ml). Express the answer in kilograms and liters.

9-2. From a chlorine electrolyzer of diaphragm type with a load of 40 kA, a lye with a volume of 10.6 m 3 was obtained per day, containing 130 kg / m 3 of sodium hydroxide. Determine the alkali current output.

9-3. How many baths should there be in a copper refining workshop with a capacity of 182.5 thousand tons / year of cathode copper, if the baths operate with a load of 12 kA, and the current efficiency for copper is 96%? The utilization rate of baths is 0.96.

9-4. Determine the masses of gaseous chlorine and 50% sodium hydroxide solution produced by electrolysis of an aqueous solution of sodium chloride per day if the current passing through the electrolyzer is 150 kA and the current efficiency is 0.95.

9-5. Determine the theoretical electricity consumption for the production of 3 tons of 85% sodium hydroxide and 3 tons of chlorine gas, if the theoretical electrolysis voltage is 2.2 V.

9-6. During the electrolysis of a melt of 24 g of a certain substance, 33.6 liters of hydrogen (n.u.) were released at the anode. Determine the substance that was taken for electrolysis and the volume of 20% hydrochloric acid solution (density 1.1 g / ml) required for the reaction.

9-7. By passing a current of 1 A through the melt of some binary inorganic compound for 8 hours, 2.068 g of metal was obtained. Which compound has undergone electrolysis if the ratio of its components is 1: 0.145 wt%?

9-8. When a current of 0.8 A was passed through 80 ml of a solution containing a mixture of AgNO 3 and Cu (NO 3) 2 for 117 min, a mixture of metals with a total mass of 3.0 g was released at the cathode. Write the electrolysis equations for each salt and determine the molar concentrations of salts in the initial solution, if it is known that gases have evolved at the anode, and after the end of electrolysis, the solution does not contain metal ions.

9-9. During the electrolysis of a solution of chromium (III) nitrate, 31.2 g of chromium was released at the cathode, which was dissolved in hydrochloric acid. The solution was left in air, and then 25% sodium hydroxide solution (density 1.28 g / ml) was gradually added to it. The precipitate that formed at the beginning then completely dissolved. How many ml of sodium hydroxide solution did it take to dissolve the precipitate?

9-10. Investigated two samples of a binary compound of some metal. A first 16 g sample was melted and electrolyzed to give 26.312 L of hydrogen measured at 720 mm. Hg and 31 about C. The second sample weighing 37.23 g, when exposed to water, gave 9.308 g of hydrogen. Establish the formula for the unknown compound and write down the equations of the ongoing processes.

9-11. A current of 2 A was passed through a solution of an organic acid salt for 5 hours. As a result of electrolysis, 12.195 g of metal were released at the cathode, and carbon monoxide (IV) and hydrogen at the anode. Determine which salt has been electrolyzed.

9-12. The electrolysis of an aqueous solution of sodium chloride with a mercury cathode produced an amalgam, which was treated with water. The titration of the resulting solution consumed 7.46 ml of 0.5 M sulfuric acid solution. Determine the strength of the current passed through the solution if the electrolysis time is 1 hour.

9-13. An aqueous solution of an unknown metal nitrate was subjected to electrolysis. In this case, 3.78 g of metal and 196 ml of oxygen (NU) were released on the platinum electrodes. Determine which metal nitrate is electrolyzed.

9-14. An aqueous solution of copper nitrate was subjected to electrolysis using inert (carbon) electrodes. The electrodes were weighed after the completion of the electrolysis and one hour after its completion. Will these masses be the same? Justify the answer.

9-15. Determine the energy efficiency of the electrolysis of alumina in cryolite if the theoretical electrolysis voltage is 1.12 V, the practical voltage is 4.6 V, and the metal current efficiency is 0.8.

9-16. Calculate the degree of conversion of sodium chloride in an electrolytic cell, the catholyte of which contains sodium hydroxide 120 g / l and sodium chloride 190 g / l.

9-17 ... Calculate the current efficiency for a cell at 14,000 A if in 24 hours there were 4,000 liters of electrolytic liquor containing 120 g / L sodium hydroxide.

^ 9-18. For problem conditions 10-17 calculate the energy utilization rate if the practical decomposition voltage of sodium chloride is 3.6V and the current output is 96 %.

9-20. At a medical instrument factory, the surface of most products is coated with a 5.0 · 10 -5 m thick nickel layer made of an electrolyte based on NiSO 4. Determine the duration of electrolysis to obtain a coating of the required thickness on tweezers, the surface of which is 4.3 · 10 -3 m 2, if the density of metallic nickel is 8.9 t / m 3 and the current efficiency is 9 6%. The current strength during electrolysis is 1.9 A.

^ 9.2. Electrolysis of an aqueous solution of sodium chloride

The electrolysis of an aqueous solution of sodium chloride is used in industry to produce chlorine, hydrogen and sodium hydroxide.

Currently, the industry uses two methods of electrolysis - diaphragm and mercury. The main process in both ways is electrolysis of a saturated solution of sodium chloride. In both methods, the anodic processes are similar; their main product is chlorine gas. Cathodic processes are different.

When diaphragm methoda steel cathode is used, to which a sodium chloride solution is fed. Some of the sodium chloride is converted to sodium hydroxide and hydrogen is released. Sodium chloride is separated from sodium hydroxide by evaporation of the solution. At the same time, due to a decrease in solubility, it precipitates. The commercial product is a sodium hydroxide solution with a concentration of 42-50% (wt.) Contains 2-4% (wt.) Sodium chloride.

AT mercury electrolysismercury cathode. Sodium ions, being discharged on it, form a sodium amalgam. In a separate apparatus - decomposer - sodium amalgam is decomposed by water, forming hydrogen and sodium hydroxide solution. In the decomposer, a solution of sodium hydroxide with a commercial concentration of 42-50% (wt), which does not contain impurities of sodium chloride, can be immediately obtained.

A solution of sodium chloride (brine) is purified before electrolysis. The brine is purified from calcium and magnesium salts. Cleaning is carried out by precipitation of impurities with strictly dosed precipitation reagents: soda suspension and lime milk.

The deposition of impurities occurs according to the reactions:

Mg 2+ + Ca (OH) 2 \u003d Ca 2+ + Mg (OH) 2 ↓

Ca 2 + + Na 2 CO 3 \u003d 2 Na + + CaCO 3 ↓

In addition to chemical cleaning, the brine is freed from mechanical impurities by settling and filtration.

Diaphragm production (Figure 9.1) includes the following stages:

1) preparation and cleaning of brine. At this stage, solid table salt is dissolved and the brine is purified from calcium and magnesium ions. Prepared brine is sent to electrolysis;

2) electrolysis;

3) evaporation of electrolytic liquors. At this stage, weak solutions of sodium hydroxide and sodium chloride obtained during electrolysis are evaporated to a commercial concentration of sodium hydroxide. The salt that falls out is separated from the solution, dissolved in water and transferred to the brine preparation stage, where this brine is added to the brine prepared from fresh salt;

4) withdrawal of sulfates. This stage receives sodium chloride obtained at the last stage of evaporation of electrolytic liquors and containing an increased amount of sulfates. Sodium sulfate is isolated from the salt in the form of a commercial product. The purified salt solution is transferred to the brine preparation stage;

5) cooling and drying of chlorine;

6) cooling and drying of hydrogen.

The reactions taking place in a diaphragm cell depend on the materials and designs of the cells, the concentration of the brine, the pH of the medium, the current density, the temperature and the content of oxygen-containing ions.

Figure: 9.1. Block diagram of the diaphragm method:

1- preparation and cleaning of brine; 2 - electrolysis; 3 - evaporation of electrolytic liquors; 4 - withdrawal of sulphides: 5 - cooling, drying and compression of chlorine; 6 - cooling, drying and compression of hydrogen.

In industrial electrolyzers, the anode is made of graphite, the cathode is made of iron.

On an iron cathode, the main process is hydrogen evolution:

2 H + + 2ē \u003d H 2

2 H 2 O + 2ē \u003d H 2 + 2 OH -

The discharge of sodium ions is impossible, since the equilibrium potential of the discharge of a sodium ion on an iron cathode in a neutral saturated solution of sodium chloride is much higher (-2.71 V) than that of hydrogen (-0.415 V).

The main reaction at the graphite anode:

2 Сl - + 2ē \u003d С1 2

In addition to this reaction, side reactions occur at the anode:

2 OH - - 2ē\u003d 0.5 O 2 + H 2 O H 2 O - 2ē \u003d 0.5 O 2 + 2 H +

The equilibrium electrode potential of the discharge of hydroxide ions in a neutral saturated solution of sodium chloride is +0.82 V, and that of chlorine ions is +1.32 V. Therefore, oxygen should first be released at the anode with a low overvoltage.

As is known from theoretical electrochemistry, parallel electrode reactions take place with such particular current densities that give the same electrode potential. Therefore, we can write:

φ а \u003d φ (С1 2) + ψ (С1 2) \u003d φ (О 2) + ψ (О 2) (9.12)

Where: φ a - anode potential, V; φ (С1 2), φ (О 2) - equilibrium potentials of chlorine and oxygen evolution are determined by the Nernst formula and depend on the concentration (activity) of chlorine or hydroxide ions, as well as on temperature; ψ (C1 2), ψ (O 2) - overvoltage of chlorine and oxygen; the overvoltage value increases with increasing current density.

The overvoltage for chlorine evolution decreases with increasing temperature to a greater extent than for oxygen. With an increase in the current density, the process at the anode also shifts towards the release of chlorine. As seen from Fig. 9.2, with an increase in the current density, the potential for chlorine evolution increases to a lesser extent than that of oxygen. Hypochlorite ions can be discharged at the anode. As a result, oxygen is released:

3 СlO - + 3 Н 2 O - 6ē \u003d ClO 3 - + 1,5 О 2 + 2 Сl - + 3 Н 2

The presence of hypochlorite ions is caused by the partial hydrolysis of chlorine.

With diaphragm electrolysis, oxygen is always evolved along with chlorine. The normal level of oxygen evolution is determined by the established technological regime (anode material, current density, temperature, brine composition, etc.). The most important condition in this case is the normal acidity of the anolyte (a solution in the anode space).

Figure: 9.2. Anodic polarization curves on graphite at 250 ° С in a 22.6% (wt.) Sodium chloride solution:

1- chlorine evolution; 2 - oxygen evolution.

The presence of hypochlorite and hypochlorite ions in the electrolyte can cause side reactions at the cathode:

СlO 3 - + 3 Н 2 \u003d 3 Н 2 O + Сl - СlO - + Н 2 \u003d Н 2 O + Сl -

An increase in the alkalinity of the anolyte increases the rate of oxygen evolution at the anode. Therefore, the electrolysis process in diaphragm electrolyzers is constructed in such a way as to minimize the electrolytic transfer of the hydroxide ion to the anode. This is achieved by using filter diaphragm.

The filtering diaphragm is made in the form of a porous partition separating the cathode and anode spaces. It prevents mixing of electrolysis products. Anolyte flows continuously through it from the anode space to the cathode.

The flow rate of the diaphragm and the degree of conversion (roughly) are controlled by the concentration of sodium hydroxide in the catholyte (electrolyte located in the cathode space). Practically in modern industrial electrolysers, the limiting value of the degree of conversion corresponds to the concentration of sodium hydroxide in the catholyte 140-150 g / l. When the concentration of alkali exceeds its value, the course of electrolysis deviates from the norm.

The data showing the dependence of the current efficiency on the alkali concentration are shown in Fig. 9.3. A decrease in the current efficiency is observed when working with a catholyte with an alkali concentration above 150 g / l.

Figure: 9.3. Current efficiency versus sodium hydroxide concentration in catholyte

Elevated temperatures of electrolysis and condensation of sodium chloride in the electrolyte reduce the solubility of chlorine, which reduces the likelihood of side reactions, and therefore increases the flow yield. In addition, an increase in temperature increases the conductivity of the electrolyte, thereby reducing the voltage across the bath. Thus, the power consumption is reduced, therefore the electrolysis of sodium chloride solutions is carried out at temperatures of 70 - 80 ° C.

Industrial electrolysers with a filter baffle are widely used in industry.

A diagram of a modern diaphragm electrolyzer is shown in Fig. 9.4. Electrolyzer body ^ 7 divided into two cavities: anode 4 and cathode 5 space. Graphite anodes are placed in the anode space. Anode and cathode spaces are separated by a diaphragm, the base of which is the cathode 3 . The diaphragm covers the cathode from the side facing the anode. Brine is fed into the anode space - a saturated solution of sodium chloride.

The anolyte level is above the upper border of the diaphragm. Chlorine formed at the anode is collected in the gas space above the anolyte level. From here, the chlorine is discharged to the collector. Anolyte, due to the difference in liquid levels in the anode and cathode spaces, flows through the diaphragm.

Hydrogen is reduced at the cathode, and the anolyte flowing to the cathode changes its composition and is enriched with hydroxide ions. The catholyte contains table salt, sodium hydroxide and a small admixture of sodium chlorate. It is discharged from the cathode space through a drain tube 9 ; its device allows you to adjust the level of the solution in the electrolyzer. In the gas space above the level of the catholyte, hydrogen is collected, which is then directed to the collector.

TO
the atolite released from the electrolytic cell, otherwise called electrolytic liquor, contains sodium hydroxide 110-120 g / l and sodium chloride 170-180 g / l.

Figure: 9.4. Diaphragm electrolyzer diagram:

1- anode; 2 - diaphragm; 3 - cathode; 4 - anode space; 5 - cathode space; 6 - dropper; 7- cell body; 8 - cover; 9 - drain pipe for catholyte

The ratio of the concentration of sodium hydroxide to the concentration of sodium chloride in the catholyte is determined by an important indicator of the technological regime - degree of conversion (X)sodium chloride during electrolysis. This is the name given to the ratio of the number of moles of sodium chloride converted to sodium hydroxide to the number of moles of sodium chloride supplied to electrolysis.

The conversion is calculated using the formula:

X \u003d 1.46C NaOH / (9.13)

The processes taking place in electrolyzers and their technical indicators largely depend on the functioning of the diaphragm. In order for the diaphragm to perform its functions, it must meet the following requirements:

To be dense and strong enough to ensure complete separation of gas products and eliminate the displacement of anolyte and catholyte;

Have low electrical resistance to avoid voltage loss in the diaphragm;

Have a sufficiently low hydraulic resistance;

To be chemically resistant to acids and alkalis so that the diaphragm works for a long time;

Have similar properties and uniformity in all areas.

The best material for the diaphragm is chrysolite asbestos.

The main requirement for the anode material is the greatest overvoltage in the evolution of oxygen than chlorine.

No material has yet been found that is absolutely resistant to the processes of combined electrochemical evolution of chlorine and oxygen. In practice, the aim is to ensure that the materials used are destroyed at a relatively low rate.

Preference is given to materials with low electrical resistivity, since the lower the resistance, the lower the voltage drop in the anode and the more uniform the current density distribution over it. You can practically use platinum, graphite and magnetite. The best in all respects (except for cost) is platinum. In industry, anodes are made exclusively from artificial graphite.

^ Mercury method of electrolysis of an aqueous solution of sodium chloride contains the same stages as the diaphragm, with the exception of evaporation (Fig. 9.5). The preparation and purification of brine in this production has features and according to the technological scheme they differ from the corresponding stage of diaphragm production. This is due to the special requirements for anolyte returned to electrolysis.

After electrolysis, anolyte contains 260-270 g / l of sodium chloride, about 0.6 g / l of dissolved chlorine, about 5 mg / l of calcium, magnesium, heavy metals, graphite dust impurities.

Figure: 9.5. Block diagram of the electrolysis of an aqueous solution of sodium chloride with a mercury cathode:

1- preparation and cleaning of brine; 2- electrolysis: 3 - dechlorination and purification of anolyte; 4 - cooling, drying and compression of chlorine; 5 - cooling, drying and compression of hydrogen.

To remove chlorine from the anolyte, acidification, evacuation, air blowing, and destruction of chlorine residues with reducing agents are consistently used. The anolyte is acidified with hydrochloric acid. Vacuuming is carried out at a pressure of 400-450 mm Hg.

The reactions occurring at the anode in electrolysers with a mercury cathode are similar to those considered for diaphragm electrolyzers.

The cathodic process in electrolysers with a mercury cathode is fundamentally different from that in a diaphragm electrolyzer, in which hydrogen ions are discharged at the steel cathode.

In mercury electrolyzers, hydrogen evolution at the cathode is a side and harmful process. Its development is hampered by the fact that hydrogen is evolved with a large overvoltage at the mercury cathode or at the cathode from sodium amalgam.

A typical polarization curve of this process is shown in Fig. 9.6. The figure shows that intense hydrogen evolution occurs at cathode potentials that are more negative than - 1.9 V. However, at a lower negative potential, another electrode reaction occurs at the mercury cathode - the formation of sodium mercury amalgam, which consumes the bulk of the current.

Metallic sodium at the time of release reacts with mercury, forming the intermetallic compound NaHg n (sodium amalgam dissolved in mercury). In this case, the work required to restore the sodium ion is reduced by the amount of energy released during the formation of the amalgam. Formation potential of sodium amalgam φ к \u003d -1.80 V.

The change in the potential of the release of substances in the electrochemical process due to the occurrence of a secondary reaction at the electrode is called depolarization.Due to depolarization, sodium can be released on a mercury cathode in the form of an amalgam according to the reaction:

Na + + n Hg + ē = NaHg n

This process takes place practically without overvoltage.

The main side reaction at the cathode:

2
H + + 2e - \u003d H 2

Figure: 9.6. Polarization curve

Evolution of hydrogen on mercury

Other side processes also take place at the cathode. Sodium amalgam reacts with chlorine dissolved in the anolyte according to the equations:

NaHg n + Сl 2 \u003d Na + + CI - + nHg Hg + Cl 2 \u003d Hg 2+ + 2 Cl -

Under the action of water, the amalgam decomposes, releasing alkali:

NaHg n + 2 Н 2 O \u003d Н 2 + Na + + 2 ОН - + nHg

The decomposition reaction consists of two coupled reactions:

2 Н 2 O + е - \u003d Н 2 + 2 ОН - NaHg n - е - \u003d Na + + n Hg

The electrolytic process in an electrolytic cell with a mercury cathode takes place in two stages. In the first stage, by electrolysis of an aqueous solution of sodium chloride, chlorine and strong sodium amalgam are obtained. The amalgam obtained after electrolysis contains 0.3-0.5% sodium. In the second stage, the amalgam is treated with purified water. Part of the amalgam decomposes to form sodium hydroxide and hydrogen. The weak amalgam is re-fed to the electrolysis by a mercury pump.

Carrying out the process in two stages makes it possible to obtain a sodium hydroxide solution with very small impurities of sodium chloride in electrolyzers with a mercury cathode.

A diagram of an electrolyzer with a mercury cathode is shown in Fig. 9.7. It consists of three main parts: an electrolytic bath 9 , decomposer 12 and mercury pump 10.

Figure: 9.7. Electrolysis cell with mercury cathode:

1 - amalgam; 2 - outlet pocket of the electrolyzer; 3 - cell cover; 4 - anode; 5 - anode current lead and its seal; 6 - anolyte; 7 - space for collecting chlorine; 8 - inlet pocket of the electrolyzer; 9 - electrolytic bath; 10 - mercury pump; 11 - decomposition nozzle; 12 - decomposer; 13- sodium hydroxide solution.

Into an electrolytic bath 9 saturated sodium chloride solution and weak amalgam are continuously supplied. Chlorine together with water vapor and strong amalgam are removed from the electrolyzer Separately from the amalgam, a solution of sodium chloride depleted as a result of electrolysis with chlorine dissolved in it is removed.

Into the decomposer 12 strong amalgam and purified water are continuously fed. Hydrogen with water vapor, sodium hydroxide solution in water and weak amalgam are removed.

Mercury electrolyzers are designed to operate at high current density (5000-10,000 A / m 2). As the density increases, the flow yield improves. In addition, the calculated cathode surface decreases (at the same current load), therefore, the required amount of mercury decreases.

Horizontal electrolysers are now common. They represent an inclined rectangular gutter, along the bottom of which amalgam flows by gravity. The gutter is covered with a cover 3 , on which graphite flat anode plates are fixed 4 . The distance between the electrodes is 3-5 mm. The plates are placed so closely that the area of \u200b\u200bthe working surface of the anodes approaches the area of \u200b\u200bthe cathode surface. Each anode plate has a current lead through the cell cover. There is a seal in the place where the current lead passes through the cover 5 preventing the release of chlorine into the atmosphere.

During electrolysis, graphite is destroyed. As a result, the interelectrode distance increases and the electrolysis voltage increases. Therefore, in modern designs, the anodes are equipped with a device that allows you to adjust the interelectrode distance.

For this, devices of two different types are used. The first type is designed to lower each anode separately, the second - to lower a whole group of anodes at the same time.

Anolyte moves over the amalgam layer in the electrolyzer in the same direction ^ 6 .

A gas space forms above the anolyte layer 7 ... The released chlorine is collected in it. Chlorine and anolyte are removed from the cell either jointly or separately.

The second stage of the electrochemical process takes place in a decomposer. Horizontal decomposers have the form of a steel, hermetically sealed trough, installed with a slope. Graphite plates are laid at the bottom of the decomposer 12 . The amalgam stream moves by gravity along its bottom. A solution of caustic soda moves in countercurrent to the amalgam and is removed together with hydrogen at the end of the decomposer.

^ TOPIC CONTROL QUESTIONS 9.2

9-1. What are the industrial methods for electrolysis of an aqueous solution of sodium chloride?

9-2. What are the main stages of the diaphragm electrolysis method?

9-3 ... What is the reaction at the cathode in diaphragm electrolysis? What side reactions can occur at the cathode with diaphragm electrolysis?

9-4 ... What is the main substance released at the anode during diaphragm electrolysis? What side substance is released at the anode during diaphragm electrolysis?

9-5. What are the features of electrolysis of an aqueous solution of sodium chloride with a mercury cathode? What is the role of the diaphragm in a diaphragm electrolyzer?

^ OBJECTIVES TO TOPIC 9.2

9-1. The liquor flowing out of the diaphragm chlorine electrolyzer contains 130 kg / m 3 of alkali. The bath operates with a load of 25 kA, the current output for CI 2 and NaOH is 96%, and for hydrogen is 98%. Calculate: a) the daily performance of the bath for chlorine and hydrogen (by weight and volume) and for alkali; b) the volume of liquor flowing out of the bath in 1 hour. Conditions are normal.

9-2. How many hours should the BGK-17-25 electrolyzer operate to obtain chlorine with a volume of 800 m 3, if the current efficiency is 96%, the current strength is 30 kA? Conditions are normal.

9-3. Calculate the theoretical value (V) of the decomposition voltage during the electrolysis of an aqueous solution of sodium chloride. Concentration of anolyte 270 kg / m 3, catholyte 120 kg / m 3.

9-4. Calculate the energy utilization factor for an electrolytic cell equipped with an iron cathode, where the theoretical decomposition voltage is 2.16 V and the practical one is 3.55 V in the electrolysis of an aqueous solution of sodium chloride. Current output 93%.

9-5. Determine the current output for the BGK-17-50 electrolyzer, where 9821 m 3 of electrolytic liquor containing 140 kg / m 3 of caustic soda were obtained during the day at a current of 40 kA.

9-6. In the diaphragm method of obtaining caustic soda, the electrolysis process is completed when the mass fraction of caustic soda in the solution reaches 10%. Calculate what mass fraction of sodium chloride has undergone electrolysis if the initial brine concentration was 310 kg / m 3, and the density was 1.197 t / m 3.

9-7. Determine the degree of conversion for catholyte containing 120 kg / m 3 sodium hydroxide, if the initial content of sodium chloride was 293 kg / m 3. Disregard production losses.

9-8. Determine the additional power consumption for the production of hydrogen with a mass of 1 ton, caused by the overvoltage of gas evolution h \u003d 0.2 V.

9-9. Calculate the energy consumption for the production of chlorine weighing 1 ton in the BGK-17-50 electrolyzer, if the current at the terminals is 25 kA, the voltage is 3.6 V, and the current output is 96%.

9-10. Determine the current efficiency for a Hooker electrolytic cell that produces 225 m 3 / h of catholyte containing 135 kg / m 3 of sodium hydroxide. The electrolyzer operates with a load of 40 kA.

9-11. Determine the enterprise's weekly need for rail tank cars with a carrying capacity of 50 tons for the transportation of liquid chlorine, if the enterprise has 3 series of BGK-17-50 electrolysers, 68 units in each series. Electrolyzer load 50 kA, current efficiency 96%.

9-12. Calculate the theoretical energy consumption for producing caustic soda of 1 ton and chlorine of 1 ton in a diaphragm electrolyzer if the theoretical decomposition voltage of sodium chloride solution is 2.2 V.

9-13. Calculate the energy consumption for the production of 1 ton of caustic soda in an electrolyzer with a mercury cathode type "Solve" V-200, if the voltage across the electrodes is 4.56 V, the current efficiency is 96%, the current is 190 kA.

9-14. The electrolysis shop has 66 baths with mercury cathodes. From a direct current source, a voltage of 250 V is applied to them at a current of 30 kA. Determine the productivity of such a workshop per day for liquor with a caustic soda concentration of 140 kg / m 3 and chlorine with a current output of 96%; voltage on each bath and energy consumption per 1 ton of chlorine and 1 ton of caustic soda (separately).

9-15. On the bottom of the mercury cell, which is 10 m long and 1.5 m wide, mercury flows in a layer of 5 mm. At the entrance to the electrolyzer, the mass fraction of sodium in the mercury is 0.01%, and at the exit, 0.2%. Current output 95%. The cathode current density is 5000 A / m 2. Determine the mass of a 40% sodium hydroxide solution, which can be obtained from 1 m 2 of a mercury cathode, and the linear velocity of the flow of mercury. Neglect changes in mercury density during amalgam formation.

9-16. Determine the energy efficiency for the mercury cell R-101, if here: anode potential -1.42 V; cathode potential 1.84 V; bath voltage 3.55 V; current efficiency 93.7%.

9-17. Calculate the volumetric circulation rate of mercury in the chlorine electrolyzer, if the mass fraction of sodium in the incoming mercury is 0.015%, and in the outlet from the electrolyzer 0.21%. The sodium current efficiency is 97%, the electrolyzer load is 25 kA.

9-18. In the horizontal decomposer, which receives 23 tons of sodium amalgam per hour, hydrogen with a volume of 56 m 3 was released. Determine the mass fraction of sodium in the amalgam (at n.a.).

9-19 ... The design annual capacity of one of the enterprises for the production of hydrochloric acid is 80 thousand tons of the product with a mass fraction of hydrogen chloride 34%. Will this enterprise provide chlorine and hydrogen for a workshop with 84 baths, type R-3O, which is working according to the enterprise's schedule? Current output 96%, load of one cell 30 kA. The acid yield is 95% of theoretical.

9-20. The diaphragm chlorine electrolyzer has the following performance indicators: current efficiency of chlorine 95%; hydrogen current efficiency 99%; load 20 kA. What mass of hydrochloric acid with a mass fraction of hydrogen chloride of 35% can be obtained from the total produced chlorine in 30 days of electrolyzer operation? What volume of hydrogen in m 3 should the electrolyzer produce to obtain this mass of acid, if the volume fraction of hydrogen is 5% higher against stoichiometry?