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3. Receipt. Calcium is obtained by electrolysis of molten chloride.
4. Physical properties. Calcium is a silver-white metal, very light (ρ = 1.55 g/cm3), like the alkali metals, but incomparably harder than them 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 brick-red. Like alkali metals, calcium metal is usually stored under a layer of kerosene.
6. Application. Due to its high chemical activity, calcium metal is used for the reduction of 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 produce certain alloys, in particular lead-calcium, necessary for the manufacture of bearings.
7. The most important calcium compounds obtained in industry.
Calcium oxide is produced industrially 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 releases a large amount of heat:
CaO + H 2 O ═ Ca (OH) 2 + Q
Calcium oxide is the main integral part quicklime, and calcium hydroxide - slaked lime.
The reaction of calcium oxide with water is called lime slaking.
Calcium oxide is mainly used to produce slaked lime.
Calcium hydroxide Ca(OH) 2 is of great practical importance. It is used in the form of slaked lime, lime milk and lime water.
Slaked lime is a fine, loose powder, usually gray in color ( component calcium hydroxide), slightly soluble in water (1.56 g dissolves in 1 liter of water at 20 0 C). A dough-like mixture of slaked lime with cement, water and sand is used in construction. Gradually the mixture hardens:
Ca (OH) 2 + CO 2 → CaCO 3 ↓ + H 2 O
Lime milk is a suspension (suspension) similar to milk. It is formed when excess slaked lime is mixed with water. Lime milk is used to produce bleach, in the production of sugar, to prepare mixtures necessary in the fight against plant diseases, and for whitewashing tree trunks.
Lime water is a clear solution of calcium hydroxide obtained by filtering lime milk. It is used in the laboratory to detect carbon monoxide (IV):
Ca(OH) 2 + CO 2 → CaCO 3 ↓ + H 2 O
With prolonged passage 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 calcium bicarbonate is heated, clouding occurs again:
Similar processes also occur in nature. If water contains dissolved carbon monoxide (IV) and acts on limestone, some of the calcium carbonate is converted to soluble calcium bicarbonate. At the surface, the solution warms up and calcium carbonate precipitates out of it again.
* 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 releases hypochlorous acid. Even carbonic acid can displace 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. The following types of gypsum are distinguished: 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 C:
2 → (CaSO 4) 2 ∙ H 2 O + 3H 2 O
If you mix alabaster powder with water, 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 plaster. Pure alabaster is used to make artistic items, and in medicine it is used to apply plaster casts.
If natural gypsum CaSO 4 ∙ 2H 2 O is heated at more high temperature, then all the water is released:
CaSO 4 ∙ 2H 2 O → CaSO 4 + 2H 2 O
The resulting anhydrous gypsum CaSO 4 is no longer able to add water, and therefore it was called dead gypsum.
Water hardness and ways to eliminate it.
Everyone knows that soap foams well in rainwater (soft water), but in spring water it usually foams poorly (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 unsuitable for cooling internal combustion engines and powering steam boilers, since when hard water is heated, scale forms on the walls of cooling systems. Scale does not conduct heat well; therefore, overheating of motors and steam boilers is possible, in addition, their wear is accelerated.
What types of hardness are there?
Carbonate, or temporary, hardness is caused by the presence of calcium and magnesium bicarbonates. It can be eliminated in the following ways:
1) 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 lime milk 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 sulfates and chlorides of calcium and magnesium.
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 ↓
Carbonate and non-carbonate hardness add up to the total water hardness.
IV. Consolidation of knowledge (5 min.)
1. Based on periodic table and theories of atomic structure, explain what properties of magnesium and calcium are common. Write down equations for the corresponding reactions.
2. What minerals contain calcium and how are they used?
3. Explain how to distinguish one natural mineral from another.
V. Homework(3 min.)
Answer the questions and complete exercises 1–15, § 48,49, solve exercises 1–4, pp. 132–133.
This is exactly what a lesson plan looks like at school on the topic “Calcium and its compounds.”
Based on the above, 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 using the example of carbon and its compounds............ ........................................................ .........5 1.1 Using interdisciplinary connections to develop students...
Activity. Search for methods and forms of teaching that contribute to education creative personality, led to the emergence of some specific teaching methods, one of which is game methods. The implementation of game teaching methods in the study of chemistry in compliance with didactic and psychological-pedagogical features increases the level of students' training. The word "game" in Russian...
And hygienic requirements); compliance of educational and physical activity with the age capabilities of the child; necessary, sufficient and rationally organized motor mode. Under health-saving educational technology(Petrov) understands a system that creates the maximum possible conditions for the preservation, strengthening and development of spiritual, emotional, intellectual, ...
ELECTROLYSIS
One of the methods for producing metals is electrolysis. Active metals occur in nature only in the form of chemical compounds. How to isolate these compounds in the free state?
Solutions and melts of electrolytes conduct electric current. However, when current is passed through an electrolyte solution, chemical reactions. Let's consider what will happen if two metal plates are placed in a solution or melt of an 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 electrons in the circuit move from one electrode to another, an excess of electrons appears at one of the electrodes. Electrons have a negative charge, so this electrode is negatively charged. It is called the cathode. A deficiency of electrons is created at the other electrode and it becomes positively charged. This electrode is called the anode. An electrolyte in a solution or melt dissociates into positively charged ions - cations and negatively charged ions - anions. Cations are attracted to the negatively charged electrode - the cathode. Anions are attracted to a positively charged electrode - the anode. At the surface of the electrodes, interactions between ions and electrons can occur.
Electrolysis refers to processes that occur when electric current is passed through solutions or melts of electrolytes.
The processes occurring during the electrolysis of solutions and melts of electrolytes are quite different. Let's consider both of these cases in detail.
Electrolysis of melts
As an example, consider the electrolysis of a sodium chloride melt. In the melt, sodium chloride dissociates into ions Na+
and Cl - : NaCl = Na + + Cl -
Sodium cations move to the surface of a negatively charged electrode - the cathode. There is an excess of electrons on the cathode surface. Therefore, electrons are transferred from the electrode surface to sodium ions. In this case, the ions Na+ transform into sodium atoms, that is, reduction of cations occurs Na+ . Process equation:
Na + + e - = Na
Chloride ions Cl - move to the surface of a positively charged electrode - the anode. A lack of electrons is created on the anode surface and electrons are transferred from anions Cl- to the electrode surface. At the same time, negatively charged ions Cl- are converted into chlorine atoms, which immediately combine to form chlorine molecules C l 2 :
2С l - -2е - = Cl 2
Chloride ions lose electrons, that is, they oxidize.
Let us write down together the equations of the processes occurring at the cathode and anode
Na + + e - = Na
2 C l - -2 e - = 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 involved in the reduction of sodium cations must be equal to the number of electrons involved in the oxidation of chloride ions. Therefore, we multiply the first equation by 2:
Na + + e - = Na 2
2С l - -2е - = Cl 2 1
Let's add both equations together and get the general reaction equation.
2 Na + + 2С l - = 2 Na + Cl 2 (ionic reaction equation), or
2 NaCl = 2 Na + Cl 2 (molecular reaction equation)
So, in the example considered, we see that electrolysis is a redox reaction. At the cathode, the reduction of positively charged ions - cations - occurs, and at the anode, the oxidation of negatively charged ions - anions. You can remember which process occurs where using the “T rule”:
cathode - cation - reduction.
Example 2.Electrolysis of molten sodium hydroxide.
Sodium hydroxide in solution dissociates into cations and hydroxide ions.
Cathode (-)<-- Na + + OH - à Анод (+)
On the surface of the cathode, sodium cations are reduced, and sodium atoms are formed:
cathode (-) Na + +e à Na
On the surface of the anode, hydroxide ions are oxidized, 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 reaction of sodium cations and in the oxidation reaction of hydroxide ions must be the same. Therefore, let's 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
Let's add both equations together and get the electrolysis reaction equation:
4 NaOH à 4 Na + 2 H 2 O + O 2
Example 3.Consider the electrolysis of the melt Al2O3
Using 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 2000º C), so special additives are added to it to lower the melting point to 800-900º C. In the melt, aluminum oxide dissociates into ions Al 3+ and O 2- . H and cations are reduced at the cathode Al 3+ , turning into aluminum atoms:
Al +3 e à Al
Anions are oxidized at the anode O2- , turning into oxygen atoms. Oxygen atoms immediately combine into O2 molecules:
2 O 2- – 4 e à O 2
The number of electrons involved in the processes of reduction of aluminum cations and oxidation of oxygen ions must be equal, so let’s 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
Let's add both equations and get
4 Al 3+ + 6 O 2- à 4 Al 0 +3 O 2 0 (ionic reaction equation)
2 Al 2 O 3 à 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 the solution contains water molecules, which can also interact with electrons. Recall that in a water molecule, the hydrogen and oxygen atoms are connected by a polar covalent bond. The electronegativity of oxygen is greater than that of hydrogen, so the shared electron pairs are biased toward the oxygen atom. A partial negative charge arises on the oxygen atom, denoted δ-, and a partial positive charge arises on the hydrogen atoms, denoted δ+.
δ+
N-O δ-
│
H δ+
Due to this shift 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 by the negative pole - to the positively charged electrode - the anode. At the cathode, reduction of water molecules can occur, and hydrogen is released:
At the anode, oxidation of water molecules can occur, releasing oxygen:
2 H 2 O - 4e - = 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 occurs at the cathode depends on the nature of the metal. Whether metal cations or water molecules will be reduced at the cathode depends on the position of the metal in range 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 voltage series to the right of hydrogen, metal cations are reduced at the cathode and free metal is released. If the metal is in the voltage series 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 evolution or hydrogen evolution can occur, and sometimes both hydrogen and metal evolution can occur simultaneously. In general, this is a rather complicated case; a lot depends on the reaction conditions: solution concentration, electric current, and others.
One of two processes can also occur at the anode - either the oxidation of electrolyte anions or the oxidation of water molecules. Which process actually occurs depends on the nature of the anion. During the electrolysis of salts of oxygen-free acids or the acids themselves, anions are oxidized at the anode. The only exception is fluoride ion F- . In the case of oxygen-containing acids, water molecules are oxidized at the anode and oxygen is released.
Example 1.Let's look at the electrolysis of an aqueous solution of sodium chloride.
An aqueous solution of sodium chloride will contain sodium cations Na +, chlorine anions Cl - and water molecules.
2 NaCl à 2 Na + + 2 Cl -
2H 2 O à 2 H + + 2 OH -
cathode (-) 2 Na + ; 2H+; 2Н + + 2е à Н 0 2
anode (+) 2 Cl - ; 2 OH - ; 2 Cl - – 2е à 2 Cl 0
2NaCl + 2H 2 O à H 2 + Cl 2 + 2NaOH
Chemical activity anions are unlikely decreases.
Example 2.And if the salt contains SO 4 2- ? Let us consider the electrolysis of a nickel sulfate solution ( II ). Nickel sulfate ( II ) dissociates into ions Ni 2+ and SO 4 2-:
NiSO 4 à Ni 2+ + SO 4 2-
H 2 O à H + + OH -
Nickel cations are located between metal ions Al 3+ and Pb 2+ , occupying a middle position in the voltage series, the recovery process at the cathode occurs according to both schemes:
2 H 2 O + 2e - = H 2 + 2OH -
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 down together the equations of the processes occurring at the cathode and anode:
cathode (-) Ni 2+ ; H+; Ni 2+ + 2е à Ni 0
2 H 2 O + 2e - = H 2 + 2OH -
anode (+) SO 4 2- ; OH - ;2H 2 O – 4 e à O 2 + 4H +
4 electrons are involved in reduction processes and 4 electrons are also involved in oxidation processes. Let's add these equations together and get the general reaction equation:
Ni 2+ +2 H 2 O + 2 H 2 O à Ni 0 + H 2 + 2OH - + O 2 + 4 H +
On the right side of the equation there are both H + and OH- , which 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 ions OH- Let's write 2 water molecules and 2 H + ions:
Ni 2+ +2 H 2 O + 2 H 2 O à Ni 0 + H 2 +2 H 2 O + O 2 + 2 H +
Let's reduce two water molecules on both sides of the equation:
Ni 2+ +2 H 2 O à Ni 0 + H 2 + O 2 + 2 H +
This is a short ionic equation. To get the complete ionic equation, you need to add a sulfate ion to both sides SO 4 2- , formed during the dissociation of nickel sulfate ( II ) and not participating in the reaction:
Ni 2+ + SO 4 2- +2H 2 O à Ni 0 + H 2 + O 2 + 2H + + SO 4 2-
Thus, during the electrolysis of a solution of nickel sulfate ( II ) hydrogen and nickel are released 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 equations for the processes occurring during the electrolysis of an aqueous solution of sodium sulfate with an inert anode.
Standard electrode system potential Na + + e = Na 0 is significantly more negative than the potential of the aqueous electrode in a neutral aqueous medium (-0.41 V). Therefore, electrochemical reduction of water will occur at the cathode, accompanied by the release of hydrogen
2H 2 O à 2 H + + 2 OH -
and Na ions + coming to the cathode will accumulate in the part of the solution adjacent to it (cathode space).
Electrochemical oxidation of water will occur at the anode, leading to the release of oxygen
2 H 2 O – 4e à O 2 + 4 H +
since corresponding to this system standard electrode potential (1.23 V) is significantly lower than the standard electrode potential (2.01 V) characterizing the system
2 SO 4 2- + 2 e = S 2 O 8 2- .
SO 4 2- ions moving towards the anode during electrolysis will accumulate in the anode space.
Multiplying the equation of the cathodic process by two and adding it with the equation of the anodic process, we obtain the total equation of the electrolysis process:
6 H 2 O = 2 H 2 + 4 OH - + O 2 + 4 H +
Taking into account that simultaneous accumulation of ions in the cathode space and ions in the anode space occurs, the overall equation of the process can be written in the following form:
6H 2 O + 2Na 2 SO 4 = 2H 2 + 4Na + + 4OH - + O 2 + 4H + + 2SO 4 2-
Thus, simultaneously with the release 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 à Cu 0 2
anode (+) 2H 2 O – 4 e à O 2 + 4H + 1
H+ ions remain in the solution SO 4 2- , because 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 à Cu 0
anode (+) 2Cl - – 2e à 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 - à Cu 0 + Cl 2 (ionic equation)
CuCl 2 à Cu + Cl 2 (molecular equation)
Example 6. Electrolysis of silver nitrate solution AgNO3.
Cathode (-)<-- Ag + + NO 3 - à Анод (+)
cathode (-) Ag + + e à Ag 0
anode (+) 2H 2 O – 4 e à 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 - (full 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. Electrolysis of sulfuric acid solutionH 2 SO 4 .
Cathode (-) <-- 2H + + SO 4 2- à anode (+)
cathode (-)2H+ + 2eà H 2
anode(+) 2H 2 O – 4eà O2 + 4H+
2H+ + 2eà H 2 2
2H 2 O – 4eà O2 + 4H+1
4H+ + 2H2Oà 2H 2 + 4H+ +O 2
2H2Oà 2H2 + O2
Example 9. Electrolysis of potassium hydroxide solutionKOH.
Cathode (-)<-- K + + OH - à anode (+)
Potassium cations will not be reduced at the cathode, since potassium is in the voltage series of metals to the left of aluminum; instead, reduction of water molecules will occur:
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. Electrolysis of potassium nitrate solutionKNO 3 .
Cathode (-) <-- K + + NO 3 - à anode (+)
2H 2 O + 2eà H 2 +2OH - 2H 2 O – 4eà O2+4H+
cathode(-) 2H 2 O + 2eà H2+2OH-2
anode(+) 2H 2 O – 4eà O2 + 4H+1
4H 2 O + 2H 2 Oà 2H 2 + 4OH - + 4H ++ O2
2H2Oà 2H2 + O2
When an electric current is passed through solutions of oxygen-containing acids, alkalis and salts of oxygen-containing acids with metals located in the voltage series of metals 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-add electrons, being inferior in this regard to ionsH + (cm. Cation activity series). During the electrolysis of aqueous solutions of compounds containing these cations, ions perform the function of an oxidizing agent at the cathodeH + , restoring according to the scheme:
2 H 2 O+ 2 eà H 2 + 2OH -
2. Metal cations with positive values of standard potentials (Cu 2+ , Ag + , Hg 2+ etc.) have a greater tendency to add electrons compared to ions. During the electrolysis of aqueous solutions of their salts, the function of the oxidizing agent at the cathode is released by these cations, while being reduced to metal according to the scheme, for example:
Cu 2+ +2 eà Cu 0
3. During electrolysis of aqueous solutions of metal saltsZn, Fe, Cd, Nietc., occupying a middle position in the voltage series between the listed groups, the reduction process at the cathode occurs according to both schemes. The mass of the released metal in these cases does not correspond to the amount of electric current flowing, part of which is spent on the formation of hydrogen.
4. In aqueous solutions of electrolytes, monoatomic anions (Cl - , Br - , J - ), oxygen-containing anions (NO 3 - , SO 4 2- , P.O. 4 3- and others), as well as hydroxyl ions of water. Of these, halide ions have stronger reducing properties, with the exception ofF. IonsOHoccupy an intermediate position between them and polyatomic anions. Therefore, during the electrolysis of aqueous solutionsHCl, HBr, H.J.or their salts at the anode, oxidation of halide ions occurs according to the following 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, oxidizing according to the following scheme:
4 HOH – 4 eà 2 H 2 O + O 2 + 4 H +
.
Tasks.
Z A cottage 1. During the electrolysis of a copper sulfate solution, 48 g of copper was released at the cathode. 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 ionsC 2+ andS0 4 2 ".
CuS0 4 = 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, and water electrolysis occurs at the anode:
Cu 2+ +2e- = Cu12
2H 2 0-4e- = 4H + + 0 2 |1
The general equation for electrolysis is:
2Cu2+ + 2H2O = 2Cu + 4H+ + O2 (short ionic equation)
Let's add 2 sulfate ions to both sides of the equation, which are formed during the dissociation of copper sulfate, and we get the complete ionic equation:
2Cu2+ + 2S042" + 2H20 = 2Cu + 4H+ + 2SO4 2" + O2
2CuSO4 + 2H2O = 2Cu + 2H2SO4 + O2
The gas released at the anode is oxygen. Sulfuric acid is formed in the solution.
The molar mass of copper is 64 g/mol, let’s calculate the amount of copper substance:
According to the reaction equation, when 2 moles of copper are released at the cathode, 1 mole of oxygen is released at the anode. 0.75 moles of copper are released at the cathode, let x moles of oxygen be released at the anode. Let's make a proportion:
2/1=0.75/x, x=0.75*1/2=0.375mol
0.375 mol of oxygen was released at the anode,
v(O2) = 0.375 mol.
Let's calculate the volume of oxygen released:
V(O2) = v(O2) «VM = 0.375 mol «22.4 l/mol = 8.4 l
According to the reaction equation, when 2 moles of copper are released at the cathode, 2 moles of sulfuric acid are formed in the solution, which means that if 0.75 moles of copper are released at the cathode, then 0.75 moles of sulfuric acid are formed in the solution, v(H2SO4) = 0.75 moles . Let's calculate the molar mass of sulfuric acid:
M(H2SO4) = 2-1+32+16-4 = 98 g/mol.
Let's calculate the mass of sulfuric acid:
m(H2S04) = v(H2S04>M(H2S04) = = 0.75 mol «98 g/mol = 73.5 g.
Answer: 8.4 liters of oxygen were released at the anode; 73.5 g of sulfuric acid was formed in the solution
Problem 2. Find the volume of gases released 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:
2КС1 =К+ + Сl
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" = H2 + 20H-|1
2SG-2e" = C12|1
The general equation for electrolysis is:
2СГl+ 2Н2О = Н2 + 2ОН" + С12 (short ionic equation) The solution also contains K+ ions formed during the dissociation of potassium chloride and not participating in the reaction:
2K+ + 2Cl + 2H20 = H2 + 2K+ + 2OH" + C12
Let's rewrite the equation in molecular form:
2KS1 + 2H2O = 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(KS1) = 39+35.5 = 74.5 g/mol
Let's calculate the amount of potassium chloride:
According to the reaction equation, during the electrolysis of 2 moles of potassium chloride, 1 mole of chlorine is released. Let the electrolysis of 1.5 mol of potassium chloride produce x mol of chlorine. Let's make a proportion:
2/1=1.5/x, x=1.5 /2=0.75 mol
0.75 mol of chlorine will be released, v(C!2) = 0.75 mol. According to the reaction equation, when 1 mole of chlorine is released at the anode, 1 mole 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 is released at the cathode, v(H2) = 0.75 mol.
Let's calculate the volume of chlorine released at the anode:
V(C12) = v(Cl2)-VM = 0.75 mol «22.4 l/mol = 16.8 l.
The volume of hydrogen is equal to the volume of chlorine:
Y(H2) = Y(C12) = 16.8l.
According to the reaction equation, the electrolysis of 2 mol of potassium chloride produces 2 mol of potassium hydroxide, which means that the electrolysis of 0.75 mol of potassium chloride produces 0.75 mol of potassium hydroxide. Let's calculate the molar mass of potassium hydroxide:
M(KOH) = 39+16+1 - 56 g/mol.
Let's calculate the mass of potassium hydroxide:
m(KOH) = v(KOH>M(KOH) = 0.75 mol-56 g/mol = 42 g.
Answer: 16.8 liters of hydrogen were released at the cathode, 16.8 liters of chlorine were released at the anode, and 42 g of potassium hydroxide were formed in the solution.
Problem 3. During the electrolysis of a solution of 19 g of divalent metal chloride, 8.96 liters of chlorine were released at the anode. Determine which metal chloride was subjected to electrolysis. Calculate the volume of hydrogen released 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:
2Н20 + 2е- = Н2 + 2ОH|1
2Cl -2e" = C12! 1
The general equation for electrolysis is:
2Cl + 2H2O = H2 + 2OH" + C12 (short ionic equation)
The solution also contains M2+ ions, which do not change during the reaction. Let us write the complete ionic equation of the reaction:
2SG + M2+ + 2H2O = H2 + M2+ + 2OH- + C12
Let's rewrite the reaction equation in molecular form:
MC12 + 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 mole of chloride of an unknown metal, 1 mole 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 the metal chloride:
The molar mass of the unknown metal chloride is 95 g/mol. There are 35.5"2 = 71 g/mol per two chlorine atoms. Therefore, the molar mass of the metal is 95-71 = 24 g/mol. Magnesium corresponds to this molar mass.
According to the reaction equation, for 1 mole of chlorine released at the anode, there is 1 mole of hydrogen released at the cathode. In our case, 0.4 mol of chlorine was released at the anode, which means 0.4 mol of hydrogen was released at the cathode. Let's calculate the volume of hydrogen:
V(H2) = v(H2>VM = 0.4 mol «22.4 l/mol = 8.96 l.
Answer: a solution of magnesium chloride was subjected to electrolysis; 8.96 liters of hydrogen were released at the cathode.
*Problem 4. During the 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 at both the cathode and anode:
2Н20 + 2е" = Н2 + 20Н-|2
2H2O - 4e" = 4H+ + O2! 1
Let's add both equations together:
6H2O = 2H2 + 4OH" + 4H+ + O2, or
6H2O = 2H2 + 4H2O + O2, or
2H2O = 2H2 + 02
In fact, when electrolysis of a solution of potassium sulfate occurs, the electrolysis of water occurs.
The concentration of a solute in a solution is determined by the formula:
С=m(solute) 100% / m(solution)
To find the concentration of the potassium sulfate solution at the end of electrolysis, you need 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 as C
m(K2S04) = C2 (K2S04) m(solution) = 0.15 200 g = 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 oxygen released:
(O 2)=V(O2) / Vm =14.56l / 22.4l/mol=0.65mol
According to the reaction equation, 2 moles of water produce 1 mole of oxygen. Let 0.65 mol of oxygen be released during the decomposition of x mol of water. Let's make a proportion:
1.3 mol of water decomposed, v(H2O) = 1.3 mol.
Let's calculate the molar mass of water:
M(H2O) = 1-2 + 16 = 18 g/mol.
Let's calculate the mass of decomposed water:
m(H2O) = v(H2O>M(H2O) = 1.3 mol* 18 g/mol = 23.4 g.
The mass of the potassium sulfate solution decreased by 23.4 g and became equal to 200-23.4 = 176.6 g. Let us now calculate the concentration of the potassium sulfate solution at the end of electrolysis:
C2 (K2 SO4)=m(K2 SO4) 100% / m(solution)=30g 100% / 176.6g=17%
Answer: the concentration of the solution at the end of electrolysis is 17%.
*Task 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 + 2N20 = N2 + C12 + 2KON
2NaCl + 2H2O = H2 + C12 + 2NaOH
Let us denote the amount of potassium chloride contained in the mixture by x mol, and the amount of sodium chloride by mol. According to the reaction equation, during the electrolysis of 2 moles of sodium or potassium chloride, 1 mole of hydrogen is released. Therefore, during the electrolysis of x mole of potassium chloride, x/2 or 0.5x mole of hydrogen is formed, and during the electrolysis of x mole of sodium chloride, 0.5y mole of hydrogen is formed. Let's find the amount of hydrogen released during electrolysis of the mixture:
Let's make the equation: 0.5x + 0.5y = 1.5
Let's calculate molar masses potassium and sodium chlorides:
M(KS1) = 39+35.5 = 74.5 g/mol
M(NaCl) = 23+35.5 = 58.5 g/mol
Mass x mole of potassium chloride is equal to:
m(KCl) = v(KCl)-M(KCl) = x mol-74.5 g/mol = 74.5x g.
The mass of a mole of sodium chloride is:
m(KCl) = v(KCl)-M(KCl) = y mol-74.5 g/mol = 58.5y g.
The mass of the mixture is 188.3 g, let’s create the second equation:
74.5x + 58.5y= 188.3
So, we solve a system of two equations with two unknowns:
0.5(x + y)= 1.5
74.5x + 58.5y=188.3g
From the first equation we express x:
x + y = 1.5/0.5 = 3,
x = 3-y
Substituting this x value into the second equation, we get:
74.5-(3-y) + 58.5y= 188.3
223.5-74.5y + 58.5y= 188.3
-16у = -35.2
y = 2.2 100% / 188.3g = 31.65%
Let's calculate the mass fraction of sodium chloride:
w(NaCl) = 100% - w(KCl) = 68.35%
Answer: the mixture contains 31.65% potassium chloride and 68.35% sodium chloride.
Physicochemical properties of the 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 fusibility diagram of the CaCl2-KCl system was studied by O. Menge. The 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 studied by K. Scholich. In it, at 508°, a eutectic with the composition 52% CaCl2, 41% NaCl, 7% KCL is formed
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 = 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 densities of the melt and metallic calcium is still sufficient for the metal to easily float to the surface of the electrolyte
The value of viscosity and surface tension of calcium chloride at the boundary with the gas phase, according to V.P. Borzakovsky are given below
Additions of potassium chloride and sodium chloride to calcium chloride reduce the viscosity of the melt and increase the surface tension at the boundary 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. Additions of up to 25% (mol.) potassium chloride, or up to 55% (mol.) sodium chloride reduce electrical conductivity; 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 studied by V.A. Ilyichev and K.D. Muzhzhalev.
Calcium chloride decomposition voltage (v), measured by Combi and Devato from e.m.f. polarization in the temperature range 700-1000°, expressed by the formula
E = 3.38 - 1.4*10v-3 (t°-700°)
Below is a comparison of the decomposition voltages of several chloride salts at a temperature of 800°.
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 e.g. observed during electrolysis. d.s. responds e.m.f. cells
Ca/CaCl/CaCl2/Cl2.
The decomposition voltage of salts decreases with increasing temperature. Ho, since the temperature coefficients of change in the decomposition voltage for different salts are different, the sequence of separation of a particular metal from a mixture of salts can change with temperature. At the temperatures of electrolysis 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
Basic theory
During the electrolysis of molten calcium chloride, the 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 in the same way as magnesium. Magnesium dissolves slightly in the electrolyte and is protected by a film of electrolyte held on the surface of the metal. Magnesium floating on the surface of the electrolyte is periodically scooped out. Calcium is much more active than magnesium and is not protected by an electrolyte film. Its solubility in electrolyte is high; according to Lorenz's research, 13% of the metal is dissolved in calcium chloride. When it dissolves, subchloride CaCl is formed, which, reacting with chlorine, turns into CaCl2. Under the influence of oxygen and atmospheric moisture, subchlorides form a suspension of calcium oxide in the melt. If the molten calcium is allowed to remain in contact with the electrolyte, then, due to the circulation of the latter, the calcium will be carried away into the region of the anode chlorine and will eventually all turn 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 is formed, permeated with salt, with a large oxidation surface. Melting such metal is very difficult. Therefore, calcium metal with an acceptable current output can only be obtained using 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 drop of metal is formed that well wets the cathode, which, when the cathode is slowly and evenly raised, is removed from the melt along with it and solidifies. In this case, the solidified drop is covered with a solid film of electrolyte, which protects the metal from oxidation and nitriding. By continuously and carefully lifting the cathode, the calcium is drawn into rods.
The conditions for 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, Frery, who paid attention to practical techniques in electrolysis, Brace, who built a 200 A bath, and others.
In Russia, this method was studied and improved in baths with a current from 100 to 600 A (Z.V. Vasiliev, V.P. Mashovets, B.V. Popov and A.Yu. Taits, V.M. Guskov and M.T. Kovalenko , A.Yu. Taits and M.I. Pavlov, Yu.V. Baymakov).
One of the conditions for achieving satisfactory current efficiency is the use of high current density at the cathode. This is necessary so that the amount of metal released per unit time significantly exceeds its dissolution. Depending on the working surface of the cathode, the power of the electrolyzer and other factors, the cathode current density is selected within the range of 50-250 A/cm2. For the normal course of the process, it is important to ensure precise control of the cathode rise. Too rapid rise of the cathode causes a liquid drop of metal to separate and dissolve in the electrolyte. With a slow rise, 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 calcium subchloride and calcium oxide causes thickening of the electrolyte and the formation of foam, which disrupts the normal operation of the bath. When the bath runs cold, the metal on the cathode grows in the form of dendrites.
The current density at the anode is selected as low as possible (about 0.7-1.5 A/cm2) in order to avoid the anode effect. The anode effect occurs when the current density on graphite reaches 8 A/cm2, and on the carbon anode 5.6 A/cm2. The temperature of the calcium chloride electrolyte without additives is maintained at 800-810°, but with the addition of other salts it decreases. Around the cathode, due to the high current concentration, a rim of superheated electrolyte is observed, having a temperature of 820-850 °. Due to the need to maintain the temperature of the electrolyte close to the melting point of calcium (851°), additives to lower the melting point of the electrolyte are not significant, but their role is positive in reducing the solubility of calcium in the electrolyte.
The electrolyte used must be as dehydrated as possible and free of 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 promotes the formation of foam in the electrolyte. All this disrupts the normal course of electrolysis. Another harmful impurity in the electrolyte is silica, which, even in small quantities, 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, released during electrolysis, fuse with calcium, lowering the melting point of the cathode metal and making it difficult to draw out.
Electrolysis practice
The industrial production of calcium by electrolysis with a touch cathode began before the First World War in Germany (Bitterfeld) and France (Jarry). Montel and Hardy indicate that electricity consumption ranged from 30,000-50,000 kWh per 1 g of metal, depending on the size of the electrolyser, its design features and the duration of the electrolysis campaign. Calcium chloride consumption was 4.5 kg per 1 kg of metal.
The working chamber of a German bath (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 the anode. The space between the blocks and the bath casing is lined and filled with thermal insulation. An iron cathode with a diameter of 60 mm is fixed above the working chamber of the bath, which moves in the vertical direction and in the horizontal direction to regulate the voltage on the bath. Air cooling is supplied to the cathode and the air, together with the anode gases, is removed through a channel arranged in the wall of the bath. Bath capacity is 40 liters per 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 given. Electrolyte temperature 800-820°, cathode current density 50-100 A/cm2, anodic 1-1.5 A/cm2, bath current 900-2000 A, voltage 20-25 V. The current output fluctuates greatly at different times of the year and depending on air humidity and averages 35-40%. However, the bath provides from 6 to 15 kg of calcium per day. For 1 kg of calcium, about 70 kWh of electricity and 8 kg of salt are consumed. Analysis of impurities in 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, a pilot installation of three baths with a current strength of 2000 A was built, which was soon doubled (Fig. 3). The cathode control was automated, while the operations of periodically adding electrolyte and removing calcium rods were performed manually. Subsequently, new series of baths were supplied for 4000 a, then for 5000 a and, finally, for 10,000 a.
The resulting calcium rods have a diameter from 175 to 350 mm and a length of up to 600 mm. The outside of the rod is covered with a crust of electrolyte. The internal metal part of the rod is quite compact.
It should still be noted that, despite the existing technical achievements, electrolysis with a touch cathode has serious disadvantages: low current efficiency, high power consumption, low extraction of calcium from raw materials, the need to use an electrolyte completely free of impurities 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 increased greatly, to look for fundamentally different methods of production. The search was not unsuccessful.
Liquid cathode electrolysis and production of calcium alloys
Basic theory
Obtaining calcium from a liquid metal cathode eliminates the main difficulties encountered in the isolation 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 recombining with chlorine and makes it impossible for calcium to be oxidized by surrounding gases. This ensures high current output. The possibility of close proximity of electrodes to each other, the absence of a high cathodic current density necessary for electrolysis with a touch cathode, and depolarization during the release of calcium onto liquid cathode allow you to significantly reduce the voltage on the bath. Achieving high performance depends on the choice of cathode, cathode current density, temperature and other process conditions. The cathode metal must be alloyed with calcium, and the magnitude of the cathode current density must correspond to the rate of diffusion of calcium into the alloy. Therefore, stirring the cathode alloy is useful. The nature of the phase diagram of calcium and cathode metal is of great importance. For example, during the electrolysis of calcium chloride with a liquid lead cathode, it is not possible to obtain rich alloys with good current efficiency due to the fact that during the formation of the alloy, the melting temperature, according to the phase diagram (Fig. 4), increases sharply, reaching 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 to 4.4%; the electrolysis temperature was 800-810°. As the calcium content in the alloy increases and the temperature rises, the current efficiency decreases sharply.
Before the amount of calcium in the alloy reaches 1-2%, the cathode current density can only be increased 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 produced alloys containing 62% Ca at a temperature of 840-880° and a cathodic current density of 1.5 A/cm2. To prevent the calcium-rich alloy from floating, 15% potassium chloride was added to the bath, which reduced the density of the electrolyte from 2.03 to 1.84.
According to the Zn-Ca phase diagram (Fig. 5), the electrolytic separation of calcium on the zinc cathode, bringing the Ca content in the alloy to 90%, 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.
Calcium deposition on the copper cathode works well. According to the Cu-Ca phase diagram (Fig. 6), the melting point of the alloy lies below 750° when it contains from 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 pure calcium to be isolated from the alloy by distillation.
Electrolysis practice
In industry, electrolysis is used with lead, zinc and copper cathodes. The production of lead alloys with calcium and barium is organized in the USA at the United Ltd. Company plant. Each bath is an iron crucible placed in brickwork, in which external heating is installed. Approximately 2 tons of pig lead are loaded into the bath. Lead is covered with a layer of melt of pure calcium and barium chlorides 75-100 mm high. In the center of the bath, a graphite anode is immersed with a device for lowering and raising, the movement of which regulates the temperature of the bath. A scallop is formed at the bottom, as well as along the walls of the bath, which prevents current losses that 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 is reduced due to anodic effects, metal dissolution and the formation of calcium and barium carbides. Electrolysis is carried out until an alloy containing 2% alkaline earth metals is obtained (approximately three days of electrolysis). When the desired concentration is reached, the current is turned off and the alloy is released into a ladle, from which it is poured into a general mixer.
In the GDR, a calcium-zinc alloy was produced at the IGF plant.
The bath (Fig. 7) consists of a cast-iron box measuring 2250x700x540 mm, walled up in brickwork. The anode is six coal blocks with a cross-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 accumulates in the bath, from where, with a content of 60-65% Ca, it is periodically scooped out without stopping the bath. The released chlorine is sucked out from above through the cap. Each bath consumes 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 productivity 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, electrolysis of calcium chloride with a liquid calcium-copper cathode, followed by distillation of calcium from the alloy, has received widespread industrial use.
The electrolyzer for producing calcium-copper alloy (Fig. 8) is a rectangular cast-iron bath. The width of the bath is 0.90 m and the length is 3 m. The outside of the bath is lined with refractory bricks and enclosed in a metal casing for mechanical strength.
The anode is a package of graphite bars, which are attached to a metal crossbeam. Current is supplied to the anode through flexible busbars attached to the traverse. The anode can be raised and lowered using the steering wheel. Chlorine is pumped out through flues located on the side of the bath. A copper-calcium alloy is poured into the bottom of the bath, serving as a cathode. The current strength in such an electrolyzer is 15,000 A. Recently, electrolyzers with high current strength have been created. The voltage on the bath is 7-9 V. The daily productivity of the electrolyzer is 15,000 and approximately 300 kg of calcium in the alloy.
The technological regime is ensured by compliance with 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 to 60-65%. The return alloy after distillation contains approximately 30% Ca. The distance between the electrodes is 3-5 cm. The temperature of the electrolyte is regulated by changing the interpolar distance.
Cathode current density is 0.4-0.5 a/cm2, anodic current density is 1.0-1.2 a/cm2. There are indications of using almost twice as high current densities.
The bath is fed with small portions of solid calcium chloride (20-30 kg each). Unlike electrolysers with a touch cathode, this bath can be fed with partially dehydrated raw materials 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 danger of it floating. Scoop out the alloy using a vacuum ladle to a level in the bath of ~5 cm. After draining the rich alloy, load the lean alloy and calcium chloride into the bath
In the electrolysis of calcium chloride with a liquid calcium-copper cathode, the current efficiency is 70-75%. Specific energy consumption is 15,000 - 18,000 kW/h per 1 ton of calcium in the alloy, consumption of calcium chloride is 3.5 g, and graphite anodes are 60-70 k per 1 g of calcium in the alloy. Cast iron baths last 10-14 months.
Introduction
CHAPTER I. Literature review
1.1. Methods for obtaining and recycling calcium chloride 7
1.1.1 Chemical methods 7
1.1.2. Electrochemical methods 10
1.2. Preparation of calcium saccharates and their use as corrosion inhibitors 12
1.3 Electrochemical synthesis of chlorine gas 13
1.4. Carbon dioxide synthesis 16
1.5. Patterns of electrochemical processes in natural waters containing calcium ions 17
1.5L. Electrolysis of thermal waters 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. Methodology for analysis and identification of products 26
2.4. Mathematical processing of the obtained results 33
Chapter III. Experimental data and discussion
3.1. Patterns of electrode reactions in a solution of calcium chloride on various electrode materials 39
3.1.1. Anodic process - kinetics and mechanism of formation of chlorine gas during electrolysis of calcium chloride solution 39
3.1.2. Cathode process - kinetics of formation of hydrogen gas during electrolysis of a calcium chloride solution 45
3.1.3. Preparative aspects of electrolysis aqueous solution calcium chloride 48
3.2. Features of the occurrence of electrode reactions in aqueous solutions (CAC12 + SUCCAROSE) on various electrode materials 50
3.2.1. Cathode process 50
3.2.2. Preparative aspects of the electrochemical production of calcium sucrose 58
3.2.3. Patterns of electrode reactions in the system: (CaC12 + sucrose + Ca(OH)2) 61
3.2.3.1 Anodic process 61
3.2.3.2 Cathode process 62
3.3. Patterns of electrode reactions in the system [CaCl2+NIII3+Ca(III3)2] 65
3.3.1. Anodic process 65
3.3.2. Cathode 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 the work
Relevance of the topic. Almost all natural waters contain calcium compounds in varying concentrations. Large quantities of calcium chloride are formed as waste during the production of soda, the hydrolysis of chlorinated organic compounds and in other production processes.
Known chemical and electrochemical methods for processing calcium chloride have significant disadvantages: decomposition of chloride
calcium at a temperature of 950-1000 C requires the use of special construction materials and high energy costs; during the electrolysis of calcium chloride solutions, an insoluble precipitate is deposited on the cathode (tCa(OH)2* iCaCI2) 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 syntheses of chemical products without the use of reagents, using electro-oxidative and electro-reductive 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 calcium chloride as a raw material - large-scale industrial waste, the processing of which contributes to the protection environment from harmful industrial emissions.
Purpose and objectives of the study. The purpose of the work was to study the law
dimensions of electrode reactions and the production of calcium-containing
liquid compounds from aqueous solutions of calcium chloride.
Achieving this goal required solving the following tasks:
study the anodic reaction of chlorine release from aqueous solutions of calcium chloride on various electrode materials;
establish the kinetics and mechanism of electrode reactions in aqueous solutions of calcium chloride, calcium nitrate, calcium acetate and a mixture of calcium chloride with sucrose;
Determine the optimal parameters for the electrochemical synthesis of calcium
F ci-containing compounds: current density, electrolyte concentrations,
current outputs of target products.
The objects of study were electrochemical processes, prote
penetrating on various electrode materials in aqueous chloride solutions
calcium with various additives. The choice of the research object was determined with
on the one hand, the lack of knowledge and complexity of electrode processes in races
systems under review, and on the other hand, the possibility of using waste
Sh
large-scale production of calcium chloride to obtain valuable
products.
Scientific novelty:
A scientific basis for the technology and advanced technological solutions for the electrolysis of aqueous solutions containing calcium ions have been created;
The patterns of occurrence of anodic and cathodic reactions according to
radiation of calcium-containing compounds on various electrode materials
Practical significance works:
For the first time, using calcium chloride as a raw material, such valuable chemical compounds, such as calcium acetate, calcium sucrose, calcium nitrate, carbon dioxide, chlorine and hydrogen gases.
Approbation work. 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 low-temperature and food technologies (St. Petersburg, 2001), International conference " Contemporary issues organic chemistry, ecology and biotechnology" (Luga, 2001), at the final All-Russian conferences "Ecology and environmental management" (St. Petersburg, 2001 and 2002).
Scope and structure of the dissertation. The dissertation 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 high school in priority areas of science and technology", subprogram - "Ecology and rational use of natural resources", section - "Problems of man-made formations and the use of industrial and household waste 2001-2002."
Preparation of calcium saccharates and their use as corrosion inhibitors
Chlorine is used in significant quantities to prepare bleaches (calcium hypochlorite and bleach). By burning chlorine in a hydrogen atmosphere, pure hydrogen chloride is obtained. The corresponding chlorides are used in the production of titanium, niobium and silicon. Iron and aluminum phosphorus chlorides 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, and chlorobenzene. Significant amounts of chlorine are consumed in the synthesis of glycerol and ethylene glycol using chlorine methods, as well as in the synthesis of carbon disulfide.
For water disinfection, chlorine dioxide, obtained through the electrolysis of a sodium chloride solution, is more promising.
According to preliminary estimates, chlorine production in 1987 in the United States amounted to 10.4 million tons. The cost of 1 ton of chlorine is $195. Chlorine is obtained by electrolysis of a NaCl solution. Theoretical basis and designs of industrial electrolyzers are described in the monograph.
Mastering the technology of electrolysis of NaCl brines using ion-exchange membranes makes it possible to reduce (compared to diaphragm or mercury electrolysis) the cost of equipment (by 15-25%) and energy costs (by 20-35%). The profitability of membrane electrolysis is associated with the possibility of producing alkali with a concentration of 40% with an electricity consumption of 200 kWh/t of product. Double-layer membranes allow operation at current densities up to 4 kA/m, which provides more efficient use cheap electricity at night. These advantages fully compensate for the relatively high cost of new membranes (500-700 $/m2).
The effectiveness of using activated cathodes to reduce the overvoltage of hydrogen evolution is discussed. A further reduction in the cell voltage can be achieved by increasing the operating pressure to 5 bar while simultaneously increasing the temperature. The use of oxygen (air), which depolarizes the cathode, replacing the process of hydrogen evolution with the process of oxygen reduction, reduces energy costs to 1600 kWh/t of alkali (if the lost energy intensity of hydrogen is not taken into account). An alternative route is the electrooxidation of hydrogen in a fuel cell.
The experiments of the Hoechst company with a chlorine membrane electrolyzer with a membrane area of 0.1 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 falls again. Various mechanisms for the implementation of the membrane process and the selection of membrane properties and the reasons for their aging are considered. It has been shown that only at a low cost of steam the cost of energy costs in membrane electrolysis can approach that of the mercury method.
The work carried out a systematic study of the process of electrolysis of solutions of chlorides of alkali and alkaline earth metals without a diaphragm. It has been 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 different solubilities of the 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. In the cathode and anode chambers, the pressure is preferably maintained at up to 15 kgf/cm2, which makes it possible to reduce the electrolysis voltage. The method can be applied to the electrolysis of water and hydrochloric acid.
The paper discusses a model of the process of producing chlorine gas in a non-flow electrolyzer.
Electrolysis of thermal waters
Recently, sodium or calcium hypochlorite has been used to purify and especially neutralize water. The increased interest in hypochlorite is largely due to the great possibilities of its use. Application of hypochlorite obtained by electrolysis of sea water for treatment Wastewater, environmentally friendly.
The electrochemical method for producing hypochlorite solutions by electrolysis of aqueous solutions of table salt or natural waters makes it possible to organize this production directly at the places where the solutions are consumed, and there is no need for long-term storage of hypochlorite solutions.
Currently, two methods of electrochemical production of a disinfectant have been used: electrolysis of concentrated solutions of sodium chloride followed by mixing with treated water and direct electrolysis of disinfected water. The electrolysis process, in both one and the other case, depends on the current density at the electrodes, the concentration of sodium chloride, pH, temperature and the nature of the movement of the electrolyte, 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 was studied. The influence of current density, concentration of sodium chloride solution, rate of supply of sodium chloride solution, rate of supply of solutions to the electrode chambers was studied. It has been shown that, under optimal conditions, the current efficiency of sodium hypochlorite is 77% with a specific electricity consumption of 2.4 kWh/kg and sodium chloride of 3.1 kg/kg. The corrosion ability of the anode was determined under experimental conditions.
A method and device for monitoring chlorine-containing compounds during water treatment is proposed, intended mainly for disinfecting water in swimming pools. The generation of a disinfecting solution of sodium hypochlorite is carried out using the electrolytic method, and it is assumed that the water in the pool contains a sufficient amount of chlorides. Water circulates in a closed circuit, in the outer part of which there is an electrolyzer, as well as a filter for water purification.
To disinfect drinking water, the authors of the patent propose to build a mini-electrolyzer into the side surface of the pipeline, in which hypochlorite is electrochemically produced from a dilute chloride-containing solution.
The features of electrolysis of a dilute (0.89%) sodium chloride solution under flow conditions were studied. It has been established that increasing the flow rate leads to a sharp decrease in the yield of chlorate and can significantly increase the productivity and stability of the electrolyzer. The best results were obtained in an electrolyzer with titanium electrodes coated with dispersed platinum with a roughness factor of at least 200, with periodic cathodic activation of the anodes.
The electrochemical process of synthesis of sodium hypochlorite under pressure has been studied. Electrolysis is carried out in an autoclave made of titanium alloy, 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 higher current yields of sodium hypochlorite. Titanium-based ruthenium dioxide, graphite and platinum were used as cathode materials, and titanium served as the cathode.
The use of sodium hypochlorite, obtained by electrolysis of natural waters, is reported to purify water from the Makhachkala-Ternair field from phenol.
Sea water has high mineralization. The mineralization of sea water in general is 3.5% or 35,000 ppm. "1. Of these, only two components (chlorides and sodium) are present in quantities of more than 1%, while the concentration of the other two: sulfate and magnesium is about OD%; calcium, potassium, bicarbonate and bromine make up about 0.001%.The remaining elements are present in very low concentrations.
According to the ratio of individual salts to their sum, the salinity of the waters of the Caspian Sea differs from the oceanic and Black Sea. The waters of the Caspian Sea are relatively poor, compared to the oceanic ones, in Na and SG ions and rich in Ca and SO4 ions. The average salinity of the waters of the Caspian Sea is 12.8-12.85%, varying from 3% at 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 the weak influx of Volga waters.
In recent years, there has been an increase in the flow of salts into the sea, which is associated with an increase in the ionic flow of rivers.
The largest amount of suspended particles present in sea waters contain the same minerals as the surrounding rocks (kaolinite, talc, quartz, feldspar, etc.). Table 1.1. The main composition of the water of the Caspian Sea is presented.
Electrochemical syntheses
The analysis of chlorine-containing compounds was carried out using the following methods: Determination of HC by the Pontius method. 10 ml of electrolyte (pH = 8) with the addition of a small amount of starch was titrated with an OD solution of potassium iodide. Definition of SG. Bring 1 ml of electrolyte to 100 ml with distilled water. Titrate 10 ml of sample with a 0.1 N solution of silver nitrate in the presence of a few drops of CH3COOH + K2ClO4.
Determination of C1CV. Add 25 ml of Mohr's salt to 10 ml of sample. Heat until bubbles appear and cool sharply. Add 5 ml of Reinhart's mixture and titrate with 0.1 N potassium permanganate solution until a pink color appears.
Definition of SY/. Add 10 ml of saturated potassium chloride solution to 10 ml of electrolyte. If a precipitate does not form, then there are no CO/s in the system. Determination of the amount of released chlorine The gaseous chlorine 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 the 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.
Progress of determination. Pipette 100 ml of tap water into a 250 ml conical flask, 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 0.1 mg or more of Ca in a sample. This method is based on the use of Trilon B in the presence of the mu-rexide indicator. The essence of the method is that Ca2+ ions in an alkaline environment form 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 purple acid at pH 12 interacts with Ca ions, forming compounds Pink colour.
Murexide does not react with Mg ions, but if the latter in the water under study is more than 30 mg/l, a precipitate of Mg(OH)2 will form, 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 magnesium concentration.
Reagents: Trilon B - 0.05 N solution. Exact normality is established using a standard 0.05 N solution of MgS04 or prepared from fix-sanal; NaOH - 10% solution; murexide - dry mixture (1 part murexide and 99 parts NaCl).
Progress of the analysis. Pipette 100 ml of the water to be tested into a 250 ml conical flask, add 5 ml of a 10% sodium hydroxide solution, and add a little dry indicator mixture. The solution turns red. The sample is titrated with Trilon B with vigorous stirring until a purple color appears, which is stable for 3-5 minutes. With further addition of Trilon B, the color does not change. A titrated sample can be used as a “witness”, but it should be remembered that a titrated sample retains stable color for a relatively short time. Therefore, it is necessary to prepare a new “witness” if a change in color of the previously prepared one is observed.
Cathode process - kinetics of formation of hydrogen gas during electrolysis of a calcium chloride solution
Considering that platinum is an expensive electrode material, the process of chlorine release was studied using a cheaper material - graphite. Fig. Figure 3.3 shows anodic current-voltage 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, the potential for chlorine release shifts to the anodic side by an average of 250 - 300 mV.
From the current-voltage curves of chlorine release presented above on electrode materials made of platinum, graphite and ORTA, it follows that with increasing calcium chloride concentration, the process of molecular chlorine release is facilitated due to a decrease in the diffusion component of the process.
To compare the kinetic parameters of chlorine release in Fig. Figure 3.4 shows the corresponding Tafel dependences of overvoltage (n) on the logarithm of current density (lg і) on platinum, graphite electrodes and ORTA.
The corresponding straight line equations, after calculating the coefficients a and b, can be presented in the following form: Using the calculated coefficients a and b, the characteristics of the process were found - exchange current i0 and transfer coefficient a
The parameters for the electrochemical separation of chlorine from a 2M calcium chloride solution are given below:
In Fig. 3.5. For comparative analysis, anodic current-voltage curves for platinum, graphite and ORTA in a 2M calcium chloride solution are presented. As can be seen from the figure, chlorine is released from a calcium chloride solution at the lowest potentials at the ORTA anode, and the current-voltage curve on graphite is shifted by 250 - 300 mV relative to the ORTA curve to the anodic side. Therefore, it is obvious that it is preferable to use ORTA as an anode material in the electrolysis of aqueous solutions of calcium chloride. On graphite, energy consumption will be higher, and the latter is inferior in durability to ORTA, especially at high anodic loads.
Considering that the energy costs during electrolysis also depend on the kinetics of the cathodic process, we studied the patterns of hydrogen evolution from aqueous solutions of calcium chloride on various electrode materials.
In Fig. 3.6. Current-voltage curves of cathodic hydrogen evolution from calcium chloride solutions with a concentration of 0.5 - 2.0 M on a platinum electrode are presented. Analysis of the current-voltage curves shows that with increasing concentration of calcium chloride, the overvoltage of hydrogen evolution increases (by 30-40 mV). A probable explanation may be the formation of a sparingly soluble precipitate of calcium salts, shielding 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 during the electrochemical production of calcium hypochlorite.
Cathode current-voltage curves taken on more affordable electrode materials for practical electrolysis - graphite, steel, copper and titanium - are presented in Figures 3.7 and 3.8. Current-voltage 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 the titanium cathode (Fig. 3.8, curve 2) occurs with the highest overvoltage. This behavior is typical for metals coated with phase oxides in the region of hydrogen evolution potentials and having an inhibitory effect on the process. Therefore, the most suitable cathode material for electrolysis of calcium chloride solution is graphite.
^ CHAPTER 9. ELECTROCHEMICAL PRODUCTION9.1 Theoretical basis of industrial electrolysis
The technology of electrochemical production considers processes in which the main reactions take place in the context of a direct transition of electrical energy into chemical energy, without intermediate conversion of energy into heat.
For this purpose, 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, usually with a high degree of selectivity, which makes it possible to obtain a product with relatively small impurities. Degree beneficial use electricity during electrolysis is relatively large.
Technological processes that can be carried out by electrochemical methods can, in most cases, 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 techno-economic advantages of electrochemical methods are determined by the fact that with their help it is possible to obtain fairly pure products in relatively simple technological schemes. The disadvantages are associated with the need to consume an expensive type of energy (DC energy) and incur costs for 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 produce hydroelectrometallurgically such metals as copper, nickel, zinc, cobalt, cadmium, manganese, chromium, iron, silver, gold, etc., as well as metal powders. Using 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, chrome plating and the application of other coatings, in mechanical engineering - for anodic mechanical processing of products (drilling, cutting, electropolishing, precision complex-profile processing, etc.).
Chemical current sources, such as batteries and galvanic cells, are created on the basis of electrochemical processes.
Electrochemical reactions take place in devices called electrolysers. In them, a direct current passes through electrolytes (solutions or melts - conductors of the second type) from the anode to the cathode. Oxidation reactions take place at the anode, and reduction reactions take place 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 at each electrode. The proportion of the total amount of electricity passed spent on this reaction is its current output.
Practically important current output to the main reaction, which characterizes the perfection of the process. The reaction rate in electrochemistry is understood as current density - the amount of electricity passed per unit time through a unit surface of the electrode at its boundary with the electrolyte.
In practice, current density is determined by dividing its strength by the geometric area of the electrode. Distinguish calculated And 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 surface topography (the presence of convexities 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, reactions that require electrical energy are carried out electrochemically. These costs are characterized by a potential jump that occurs at the electrode-electrolyte interface. If the electrode reaction takes place under 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 needed to initiate a reaction.
Important concept "standard potential". This is the equilibrium potential, determined for the case when the activity of each active substance equal to one. Standard potentials are given in reference tables. Taking into account real conditions and using the Nernst formula, 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, with a current close to zero, and in the presence of conditions for the reversibility of electrode reactions, that the electrolysis process begins.
In practice, a current that differs from zero passes through the electrodes, and electrode processes therefore occur under nonequilibrium conditions. The potential jump at the electrode-electrolyte interface in these cases is greater than the equilibrium one and is called electrode potential. It is impossible to experimentally measure the potential difference between the electrode and electrolyte. Instead, the potential difference between this and a standard (eg, standard hydrogen electrode) electrode is measured. This difference is taken as the electrode potential. The sign rule for potentials follows from this method of defining them.
The difference between the electrode potential 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 taking place 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 from the electrolyte to the electrodes and removal of reaction products from them;
2) movement of electrons between electrodes and ions;
3) secondary reactions on the 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.
Equilibrium potentials are calculated based on average activity(concentrations) of reactants in solution. At the electrodes they are either triggered or built up, so their activity there differs from the average.
The equilibrium reaction potential, calculated from the true activity of substances at the electrodes, differs from the potential calculated from the average activity. The difference between them is the concentration polarization. It is proportional to the work of concentrating or diluting solutions from the average activity value to the true value that is created at the electrodes, and the higher the reaction rate at the electrodes, the greater.
The appearance of the second component of the overvoltage depends on the other two stages of the electrode process - chemical overvoltage or polarization. From the energy side, it is explained as follows. From chemical kinetics it is known that only active molecules with energy 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 to lower the energy barrier and thereby increase the proportion of active particles without changing the temperature. In this part there is an analogy between catalytic and electrochemical processes. The additional jump in potential against equilibrium (chemical polarization) is proportional to the work required to activate the required number of ions or molecules so that the reaction can proceed at a given rate. The higher the reaction rate, the higher the chemical polarization.
The physical picture on the electrodes, which explains the occurrence of chemical polarization, is considered in the double layer theory and the related delayed discharge theory. These theories show that the magnitude 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 selecting the composition of the solution and the electrode material, it is possible to control the chemical polarization.
The actual voltage that should be applied to the terminals of the electrolyser is electrolysis voltage - in order to conduct a 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 conductors of the first and second kind.
Electrical energy consumption per unit of product is directly proportional to the product of the electrolysis voltage and the amount of electricity required to produce the product (taking into account the current efficiency). Of the total energy consumption, only a portion is converted into chemical energy. This part is proportional to the voltage, which is called Thompson voltage. It differs from the decomposition voltage for the following reason: the electrical energy required for the process on the electrodes (at a 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 turns into the internal energy of the system.
The difference between the electrolysis voltage and the Thompson voltage is proportional to the excess heat generated during electrolysis. This is the heating voltage, or the thermal component of the voltage.
The portion of the total energy consumption that is converted as a result of the reaction into the internal energy of the target product is called energy output.
The main technological indicators of electrolysis include: current efficiency, energy utilization coefficient (energy efficiency) and energy consumption coefficient.
Current output (V t, %) is calculated using the formula:
V t = (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 t = k ∙ I∙ τ, (9.2)
Where: I - current strength, A; τ – electrolysis time, h; k is the electrochemical equivalent of the released substance.
K = M/(F∙ z), (9.3)
Where: F is the Faraday constant, equal to 96,500 C or 26.8 A∙h; z is the charge of the ion released on the electrode.
The energy utilization factor (V e, %) is calculated using the formula:
V e = (Wt/W f) ∙100%, (9.4)
Where: W T - theoretical energy consumption, kWh; W f - actual energy consumption, kW∙h.
W t/p = V t/p ∙ J ∙ τ / m t/p (9.5)
Where: V t - theoretical decomposition voltage, V; V p - actual voltage on the electrodes, V.
The theoretical energy consumption (kWh/t) can also be calculated using the equation:
W t = 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 amount (∆V), i.e. the condition will be ensured:
V p = V t + ∆V (9.7)
The theoretical decomposition voltage at the bath electrodes is determined by the equation:
V t = E k - E a (9.8)
Where: E k – actual ion discharge potential at the cathode, V; E a - actual ion discharge potential at the anode, V.
The actual discharge potentials of the ions differ from their equilibrium discharge potentials by the amount of overvoltage, respectively, cathodic E k per and anode E a per, which increase the equilibrium potentials:
E k = E k r + E k ln and E a = E a r + E a l (9.9)
Where: E k p and E a p are the equilibrium discharge potentials of the cation and anion.
The equilibrium potentials of the ion discharge are equal in magnitude and opposite in sign to the equilibrium electrode potentials: E k р = - φ k and E а р = - φ а, which can be calculated using the Nernst formula:
φ k/a = 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 – temperature, K; ak/a - ion activity in solution (melt), mol/l; F – Faraday constant, equal to 96500 Cul.; z is the charge of the electrolyte ion.
Explanation of the condition presented in 9.7 gives the “stress balance” equation:
V p = V t + J ∙∑R = E k – E a + J(R e + R d + R tp) (9.11)
Where: J – current strength, A; R – total resistance of the electrolysis process, Ohm; R e – electrolyte resistance, Ohm; R d - resistance of the electrolyzer diaphragm, Ohm; R tp - resistance of current-carrying paths, Ohm.
^ TEST QUESTIONS FOR TOPIC 9.1
9-1 . What processes are called electrochemical and how do they differ from electrothermal processes? Give examples of both.
9-2. What advantages do electrochemical methods for obtaining substances have over chemical ones?
9-3. Name the areas of application of electrochemical methods.
9-4 . What is the condition for the electrolysis process? What is overvoltage and how does it affect the ion discharge sequence during electrolysis?
9-5 . List the quantitative characteristics of industrial electrolysis and give them a definition.
^ PROBLEMS FOR 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 your answer in kilograms and liters.
9-2. From a diaphragm-type chlorine electrolyzer with a load of 40 kA, a liquor volume of 10.6 m 3 containing 130 kg/m 3 of sodium hydroxide was obtained per day. Determine the yield of alkali by current.
9-3. How many baths should there be in a copper refining shop with a capacity of 182.5 thousand tons/year of copper cathode, if the baths operate with a load of 12 kA, and the current efficiency for copper is 96%? Bath utilization rate is 0.96.
9-4. Determine the masses of chlorine gas 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 energy consumption to produce 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.s.) 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. When 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 minutes, 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 the salts in the original solution, if it is known that gases have evolved at the anode, and after electrolysis is completed the solution does not contain metal ions.
9-9. During the electrolysis of a solution of chromium(III) nitrate at the cathode, 31.2 g of chromium was released, which was dissolved in hydrochloric acid. The solution was left in air, and then a 25% sodium hydroxide solution (density 1.28 g/ml) was gradually added to it. The precipitate that initially formed then completely dissolved. How many ml of sodium hydroxide solution was required to dissolve the precipitate?
9-10. Two samples of a binary compound of a certain metal were studied. The first 16 g sample was melted and electrolyzed, yielding 26.312 L of hydrogen measured at 720 mm. Hg and 31 o 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 the equations for the processes occurring.
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 was released at the cathode, and carbon oxide (IV) and hydrogen were released at the anode. Determine which salt was electrolyzed.
9-12. By electrolysis of an aqueous solution of sodium chloride with a mercury cathode, an amalgam was obtained, which was treated with water. To titrate the resulting solution, 7.46 ml of 0.5 M sulfuric acid solution was consumed. Determine the strength of the current passed through the solution if the electrolysis time is 1 hour.
9-13. An aqueous solution of nitrate of an unknown metal was subjected to electrolysis. In this case, 3.78 g of metal and 196 ml of oxygen (n.o.) were released on the platinum electrodes. Determine which metal nitrate is subjected to electrolysis.
9-14. An aqueous solution of copper nitrate was subjected to electrolysis using inert (carbon) electrodes. The electrodes were weighed after completion of electrolysis and one hour after its completion. Will these masses be the same? Justify your answer.
9-15. Determine the energy yield during the electrolysis of alumina in cryolite, if the theoretical electrolysis voltage is 1.12 V, the practical voltage is 4.6 V, and the current yield of the metal is 0.8.
9-16. Calculate the degree of conversion of sodium chloride in an electrolyzer, the catholyte of which contains 120 g/l sodium hydroxide and 190 g/l sodium chloride.
9-17 . Calculate the current output for an electrolyzer at a current strength of 14,000 A, if in 24 hours there were 4000 liters of electrolytic liquor containing 120 g/l sodium hydroxide.
^ 9-18. For the problem conditions 10-17 calculate the energy utilization factor if the practical sodium chloride decomposition voltage is 3.6 V and the current output is 96 %.
9-20. At the medical instruments factory, the surface of most products is coated with a layer of nickel 5.0·10 -5 m thick from 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%. Current strength during electrolysis is 1.9 A.
^ 9.2. Electrolysis of aqueous sodium chloride solution
Electrolysis of an aqueous solution of sodium chloride is used in industry to produce chlorine, hydrogen and sodium hydroxide.
Currently, two methods of electrolysis are used in industry - diaphragm and mercury. The main process in both methods is electrolysis of a saturated solution of table salt. In both methods, the anodic processes are similar; their main product is chlorine gas. Cathode processes are different.
At diaphragm method A steel cathode is used, to which a sodium chloride solution is supplied. 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 - a solution of sodium hydroxide with a concentration of 42-50% (wt.) contains 2-4% (wt.) sodium chloride.
IN mercury electrolysis mercury cathode. Sodium ions, discharged on it, form a sodium amalgam. In a separate apparatus - a decomposer - sodium amalgam is decomposed by water, forming hydrogen and a solution of sodium hydroxide. The decomposer can immediately produce a sodium hydroxide solution with a commercial concentration of 42-50% (wt), which does not contain sodium chloride impurities.
The table salt solution (brine) is purified before electrolysis. The brine is cleaned of calcium and magnesium salts. Cleaning is carried out by precipitation of impurities using strictly dosed precipitation reagents: a suspension of soda and milk of lime.
Precipitation of impurities occurs by reactions:
Mg 2+ + Ca(OH) 2 = Ca 2+ + Mg(OH) 2 ↓
Ca 2 + + Na 2 CO 3 = 2 Na + + CaCO 3 ↓
In addition to chemical cleaning, the brine is freed from mechanical impurities by settling and filtering.
Diaphragm production (Fig. 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. The prepared brine is sent for electrolysis;
2) electrolysis;
3) evaporation of electrolytic liquors. At this stage, weak solutions of sodium hydroxide and sodium chloride obtained by electrolysis are evaporated to a commercial concentration of sodium hydroxide. The salt that falls out in this case 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) removal 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 salt as 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 occurring in a diaphragm electrolyzer depend on the materials and designs of the electrolysers, brine concentration, pH of the medium, current density, temperature and content of oxygen-containing ions.
Rice. 9.1. Block diagram of the diaphragm method:
1- preparation and cleaning of brine; 2 - electrolysis; 3 - evaporation of electrolytic liquors; 4 - removal of sulfides: 5 - cooling, drying and compression of chlorine; 6 - cooling, drying and compression of hydrogen.
In industrial electrolysers, the anode is made of graphite, the cathode is made of iron.
On the iron cathode, the main process is the evolution of hydrogen:
2 Н + + 2ē = Н 2
2 H 2 O + 2ē = H 2 + 2 OH -
The discharge of sodium ions is impossible, since the equilibrium potential of the discharge of sodium ion on the iron cathode in a neutral saturated solution of sodium chloride is much higher (-2.71 V) than that of hydrogen (-0.415 V).
Basic reaction on a graphite anode:
2 Сl - + 2ē = С1 2
In addition to this reaction, side reactions occur at the anode:
2OH - - 2ē= 0.5 O 2 + H 2 O H 2 O - 2ē = 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. Consequently, oxygen should be released first at the anode with a low overvoltage.
As is known from theoretical electrochemistry, parallel electrode reactions occur with such partial current densities that give the same electrode potential. Therefore we can write:
φ a = φ (C1 2) + ψ (C1 2) = φ (O 2) + ψ (O 2) (9.12)
Where: φ a - anode potential, V; φ (C1 2), φ (O 2) - equilibrium potentials for the release of chlorine and oxygen 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 magnitude of the overvoltage increases with increasing current density.
The overvoltage for chlorine release decreases with increasing temperature to a greater extent than that of oxygen. As the current density increases, the process at the anode also shifts towards the release of chlorine. As can be seen from Fig. 9.2, with increasing current density, the potential for chlorine release increases to a lesser extent than that of oxygen. Hypochlorite ions can be discharged at the anode. As a result, oxygen is released:
3 ClO - + 3 H 2 O - 6ē = ClO 3 - + 1.5 O 2 + 2 Cl - + 3 H 2
The presence of hypochlorite ions is caused by partial hydrolysis of chlorine.
During diaphragm electrolysis, oxygen is always released 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 for this is the normal acidity of the anolyte (the solution located in the anode space).
Rice. 9.2. Anodic polarization curves on graphite at 250 °C in 22.6% (wt.) sodium chloride solution:
1- release of chlorine; 2 - release of oxygen.
The presence of hypochlorite and hypochlorate ions in the electrolyte can cause adverse reactions at the cathode:
ClO 3 - + 3 H 2 = 3 H 2 O + Cl - ClO - + H 2 = H 2 O + Cl -
Increasing the alkalinity of the anolyte increases the intensity of oxygen evolution at the anode. Therefore, the electrolysis process in diaphragm electrolyzers is designed in such a way as to minimize the electrolytic transfer of hydroxide ions to the anode. This can be achieved by using filter diaphragm.
The filter diaphragm is made in the form of a porous partition separating the cathode and anode spaces. It prevents mixing of electrolysis products. A flow of anolyte continuously passes through it from the anode space to the cathode.
The flow of the diaphragm and the degree of conversion (approximately) are controlled by the concentration of sodium hydroxide in the catholyte (electrolyte located in the cathode space). In practice, in modern industrial electrolyzers the limiting value of the degree of conversion corresponds to a concentration of sodium hydroxide in the catholyte of 140-150 g/l. When the alkali concentration exceeds its value, the progress of electrolysis deviates from the norm.
Data showing the dependence of current efficiency on alkali concentration are shown in Fig. 9.3. A decrease in current efficiency is observed when working with catholyte having an alkali concentration above 150 g/l.
Rice. 9.3. Dependence of current efficiency on the concentration of sodium hydroxide in the 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 occurring and, therefore, increases the yield. In addition, an increase in temperature increases the electrical conductivity of the electrolyte, thereby reducing the voltage across the bath. Thus, energy consumption is reduced, so electrolysis of sodium chloride solutions is carried out at temperatures of 70 - 80 °C.
Industrial electrolysers with a filter partition are widely used in industry.
The 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. The anode and cathode spaces are separated by a diaphragm, the base for which is the cathode. 3 . The diaphragm covers the cathode on the side facing the anode. Brine - a saturated solution of table salt - is supplied to the anode space.
The anolyte level is above the upper limit of the diaphragm. Chlorine formed at the anode collects in the gas space above the anolyte level. From here the chlorine is discharged into the collector. Due to the difference in liquid levels in the anode and cathode spaces, the anolyte flows through the diaphragm.
Hydrogen is reduced at the cathode, and the anolyte flowing to the cathode changes its composition and is enriched in hydroxide ions. 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 regulate the level of solution in the electrolyzer. Hydrogen is collected in the gas space above the catholyte level, which is then sent to the collector.
TO 
The atholyte coming out of the electrolyzer, otherwise called electrolytic liquor, contains sodium hydroxide 110-120 g/l and sodium chloride 170-180 g/l.
Rice. 9.4. Scheme of a diaphragm electrolyzer:
1- anode; 2 - diaphragm; 3 - cathode; 4 - anode space; 5 – cathode space; 6 - dropper; 7- electrolyzer body; 8 - cover; 9 - drain tube 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 into sodium hydroxide to the number of moles of sodium chloride supplied to electrolysis.
The degree of conversion is calculated using the formula:
X = 1.46 C NaOH / (9.13)
The processes occurring in electrolyzers and their technical performance largely depend on the functioning of the diaphragm. In order for the diaphragm to perform its functions, it must meet the following requirements:
Be dense and strong enough to ensure complete separation of gas products and prevent displacement of anolyte and catholyte;
Have low electrical resistance to avoid voltage loss in the diaphragm;
Have a fairly low hydraulic resistance;
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 for the release of oxygen than chlorine.
A material that is absolutely resistant to the processes of joint electrochemical release of chlorine and oxygen has not yet been found. In practice, they strive to ensure that the materials used degrade 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 distribution of current density over it. In practice, platinum, graphite and magnetite can be used. The best in all respects (except cost) is platinum. In industry, anodes are made exclusively from artificial graphite.
^ Mercury method of electrolysis of aqueous sodium chloride solution 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 its own peculiarities and, according to the technological scheme, differs from the corresponding stage of diaphragm production. This is due to special requirements for the anolyte returned to electrolysis.
The anolyte after electrolysis contains 260-270 g/l sodium chloride, about 0.6 g/l dissolved chlorine, about 5 mg/l calcium, magnesium, heavy metals, and graphite dust impurities.
Rice. 9.5. Block diagram of 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 anolyte, acidification, evacuation, air blowing, and destruction of chlorine residues with reducing agents are successively used. The anolyte is acidified with hydrochloric acid. Vacuuming is carried out at a pressure of 400-450 mmHg.
The reactions occurring at the anode in electrolysers with a mercury cathode are similar to the reactions considered for diaphragm electrolysers.
The cathodic process in electrolyzers with a mercury cathode is fundamentally different from that in a diaphragm electrolyzer, in which hydrogen ions are discharged on a steel cathode.
In mercury electrolyzers, the evolution of hydrogen at the cathode is a side and harmful process. Its development is hindered by the fact that hydrogen is released at a high overvoltage at the mercury cathode or at the sodium amalgam cathode.
A characteristic polarization curve of this process is shown in Fig. 9.6. The figure shows that intense hydrogen evolution occurs at cathode potentials 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.
At the moment of release, metallic sodium reacts with mercury, forming the intermetallic compound NaHg n (sodium amalgam dissolved in mercury). In this case, the work required to reduce the sodium ion is reduced by the amount of energy released during the formation of the amalgam. Sodium amalgam formation potential φk = -1.80 V.
A change in the potential for the release of substances in an electrochemical process due to the occurrence of a secondary reaction at the electrode is called depolarization. Due to depolarization, sodium can be released at the mercury cathode in the form of an amalgam according to the reaction:
Na + + n Hg + ē = NaHg n
This process takes place with virtually no overvoltage.
The main side reaction at the cathode is:
2
H + + 2e - = H 2
Rice. 9.6. Polarization curve
Hydrogen evolution on mercury
Other side processes also take place at the cathode. Sodium amalgam reacts with chlorine dissolved in anolyte according to the equations:
NaHg n + Cl 2 = Na + + CI - + nHg Hg + Cl 2 = Hg 2+ + 2 Cl -
Under the influence of water, amalgam decomposes, releasing alkali:
NaHg n + 2 H 2 O = H 2 + Na + + 2 OH - + n Hg
The reaction in the decomposer consists of two coupled reactions:
2 H 2 O + e - = H 2 + 2 OH - NaHg n – e - = Na + + n Hg
The electrolytic process in an electrolyzer with a mercury cathode takes place in two stages. In the first stage, by electrolyzing an aqueous solution of sodium chloride, chlorine and strong sodium amalgam are obtained. The amalgam obtained after electrolysis contains 0.3-0.5% sodium. At 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-supplied for electrolysis using a mercury pump.
Carrying out the process in two stages makes it possible to obtain a solution of sodium hydroxide with very small impurities of sodium chloride in electrolyzers with a mercury cathode.
The diagram of an electrolyzer with a mercury cathode is shown in Fig. 9.7. It consists of three main parts: electrolytic bath 9 , decomposer 12 and mercury pump 10.
Rice. 9.7. Diagram of an electrolyzer with a mercury cathode:
1 - amalgam; 2 - outlet pocket of the electrolyser; 3 - electrolyzer cover; 4 - anode; 5 - anode conductor and its seal; 6 - anolyte; 7 - space for collecting chlorine; 8 - inlet pocket of the electrolyser; 9 - electrolytic bath; 10 - mercury pump; 11 - decomposer nozzle; 12 - decomposer; 13- sodium hydroxide solution.
Into the 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 sodium chloride solution depleted as a result of electrolysis with chlorine dissolved in it is removed.
Into the decomposer 12 strong amalgam and purified water are continuously supplied. Hydrogen with water vapor, a solution of sodium hydroxide in water and weak amalgam are removed.
Mercury electrolyzers are designed to operate with high current density (5000-10,000 A/m2). As density increases, flow yield improves. In addition, the calculated surface of the cathode decreases (at the same current load), therefore, the required amount of mercury decreases.
Currently, horizontal electrolysers are common. They are an inclined trough of rectangular cross-section, along the bottom of which amalgam flows by gravity. The gutter is covered with a lid 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 working surface area of the anodes approaches the surface area of the cathode. Each anode plate has a current lead leading out through the lid of the electrolyser. There is a seal where the current lead passes through the cover 5 , preventing the release of chlorine into the atmosphere.
During the electrolysis process, 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 the interelectrode distance to be adjusted.
For this purpose, devices of two various types. The first type is designed to lower each anode individually, the second type is designed to lower a whole group of anodes simultaneously.
Anolyte moves on top of the amalgam layer in the electrolyzer in the same direction. ^ 6 .
A gas space is formed above the anolyte layer 7 . The released chlorine collects in it. Chlorine and anolyte are removed from the electrolyzer either together or separately.
The second stage of the electrochemical process takes place in the decomposer. Horizontal decomposers have the form of a steel, hermetically sealed trench installed with a slope. Graphite plates are placed at the bottom of the decomposer 12 . The amalgam flow moves by gravity along its bottom. A solution of sodium hydroxide moves in countercurrent to the amalgam and is removed along with hydrogen at the end of the decomposer.
^ TEST QUESTIONS FOR TOPIC 9.2
9-1. What industrial methods exist for the electrolysis of an aqueous solution of sodium chloride?
9-2. Name the main stages of the diaphragm electrolysis method.
9-3 . What reaction occurs at the cathode during the diaphragm electrolysis method? What side reactions can occur at the cathode during the diaphragm electrolysis method?
9-4 . What is the main substance released at the anode during the diaphragm electrolysis method? What by-product is released at the anode during the diaphragm electrolysis method?
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?
^ TASKS FOR 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 efficiency for CI 2 and NaOH is 96%, and for hydrogen 98%. Calculate: a) the daily productivity of the bath for chlorine and hydrogen (by mass and volume) and 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 produce 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 decomposition voltage during electrolysis of an aqueous sodium chloride solution. The concentration of anolyte is 270 kg/m3, catholyte is 120 kg/m3.
9-4. Calculate the energy utilization factor for an electrolyzer equipped with an iron cathode, where the theoretical decomposition voltage is 2.16 V and the practical voltage is 3.55 V for electrolysis of an aqueous solution of sodium chloride. Current efficiency 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 sodium hydroxide was produced during the day at a current of 40 kA.
9-6. In the diaphragm method for producing 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 was subjected to electrolysis if the initial brine concentration was 310 kg/m3 and the density was 1.197 t/m3.
9-7. Determine the degree of conversion for a catholyte containing 120 kg/m 3 of sodium hydroxide if the initial sodium chloride content was 293 kg/m 3 . Neglect production losses.
9-8. Determine the additional electricity consumption for producing hydrogen weighing 1 ton, caused by an overvoltage of gas evolution h = 0.2 V.
9-9. Calculate the energy consumption to produce chlorine weighing 1 ton in the BGK-17-50 electrolyzer, if the current at the terminals is 25 kA, voltage 3.6 V, current efficiency 96%.
9-10. Determine the current output for a Hooker electrolyzer 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 weekly need of the enterprise for railway tanks with a carrying capacity of 50 tons for the transportation of liquid chlorine, if the enterprise operates 3 series of BGK-17-50 electrolysers, 68 pieces in each series. Electrolyzer load 50 kA, current efficiency 96%.
9-12. Calculate the theoretical energy consumption to produce caustic soda weighing 1 ton and chlorine weighing 1 ton in a diaphragm electrolyzer if the theoretical decomposition voltage of the 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 Solvay V-200 mercury cathode, if the voltage at the electrodes is 4.56 V, the current efficiency is 96%, and the current strength is 190 kA.
9-14. The electrolysis shop has 66 baths with mercury cathodes. They are supplied with a voltage of 250 V from a DC source 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 at 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. A 5 mm layer of mercury flows along the bottom of a mercury electrolyzer, which is 10 m long and 1.5 m wide. At the entrance to the electrolyzer, the mass fraction of sodium in mercury is 0.01%, and at the exit it is 0.2%. Current efficiency 95%. Cathode current density 5000 A/m2. Determine the mass of a 40% sodium hydroxide solution that can be obtained from 1 m 2 of mercury cathode, and the linear flow rate of mercury. Neglect the change in mercury density during amalgam formation.
9-16. Determine the energy output for the mercury electrolyzer 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 a chlorine electrolyzer if the mass fraction of sodium in the incoming mercury is 0.015%, and in the mercury leaving the electrolyzer is 0.21%. The sodium current efficiency is 97%, the electrolyzer load is 25 kA.
9-18. In a horizontal decomposer, which receives 23 tons of sodium amalgam per hour, hydrogen was released with a volume of 56 m 3. Determine the mass fraction of sodium in the amalgam (at standard conditions).
9-19 . The designed annual capacity of one of the enterprises for the production of hydrochloric acid is 80 thousand tons of product with a mass fraction of hydrogen chloride of 34%. Will this enterprise provide chlorine and hydrogen to a workshop with 84 R-ZO type baths, operating according to the enterprise schedule? Current efficiency 96%, load of one electrolyzer 30 kA. The acid yield is 95% of theoretical.
9-20. The diaphragm chlorine electrolyzer has the following performance indicators: chlorine current output 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 all the chlorine produced in 30 days of operation of the electrolyzer? What volume of hydrogen in m3 must the electrolyzer produce to produce this mass of acid, if the volume fraction of hydrogen is 5% greater than stoichiometry?
