Electrodeposition of zinc - Electrode reaction of zinc electrowinning

Electrochemical reaction of zinc electrowinning When only zinc sulfate, sulfuric acid and water are present in the electrowinning solution, the following ionization reaction occurs according to ionization theory:

ZnS0 ₄ ==== Zn ²⁺ + S0 ₄ ^2- (1)

H ₂ SO ₄ ====2H ⁺ + S0 ₄ ^2- _{                           Â}  (2)

H ₂ O ==== H ⁺ + OH ^- (3)

When direct current is applied, the cation moves toward the cathode, and the positively charged Zn ²⁺ receives two electrons to discharge on the cathode to become elemental zinc, and precipitates in a crystalline state on the surface of the cathode.

Cathodic reaction: Zn ²⁺ +2e — ^_→ Zn ↓ (4)

While anions toward the anode, the negatively charged OH ^- discharging two electrons at the anode, and oxygen evolution.

Anode reaction: 20H ^{- _-} 2e ==== ^_  H ₂ 0 + 1/2O ₂

Or H ₂ 0 ^_- 2e ====1/2O ₂ +2H ⁺ (5)

The total electrode reaction formula is:
Direct current ZnS0 ₄ +H ₂ 0 —— ^{_{→ Zn}} ↓ + H ₂ S0 ₄ +1/2O ₂ (6)

In order to gain a deeper understanding of the zinc electrowinning process, the electrochemical processes occurring on the anode and anode in the industrial cell are discussed below.
A process of anodic dissolution of the protective lead anodes currently used industrially most silver 0.5% to 1.0% lead alloy plate as an insoluble anode. For the uncoated anode before use, after the energization, the following reactions occur before the normal anode reaction formula (5) (E ^Ó© = 0.615 V) occurs.
The anodic dissolution of lead first occurs and the formation of lead sulfate covers the surface of the anode.

Pb - 2e ==== pb ²⁺ E ^Ó© ₍₇₎ =-0.126V (7)

Pb+SO ₄ ^2- -2e ==== PbSO ₄ E ^Ó© ₍₈₎ =-0.356V (8)

On the surface of the anode not covered by lead sulfate, lead can be directly oxidized to Pb0 ₂ , ie:

Pb + 2H ₂ 0 -4e — ^_→ Pb0 ₂ + 4H ⁺ _  E ^Ó© ₍₉₎ =-0.655V (9)

As the free surface of the metal nearly completely disappears, the following reaction occurs:

Pb ²⁺ + 2H ₂ 0 - 2e — ^_→ Pb0 ₂ +4H ⁺ E ^Ó© ₍₁₀₎ =-1.45V (10)[next]

After the lead anode is substantially covered by Pb0 ₂ , it enters the normal anode reaction, and as a result, oxygen is released on the anode, and the H ⁺ concentration in the electrolyzed liquid is increased. The equilibrium potential of the reaction (10) with the normal reaction of the anode (5) was compared. It seems that the reaction (5) starts earlier than the reaction (10), but in fact the oxygen evolution reaction occurs after the reaction (10) is substantially completed. This is because there is generally a large overvoltage when oxygen is deposited.
B. Overvoltage of oxygen on the anode. The magnitude of the overvoltage when the oxygen is deposited on the anode is related to the anode material, the anode surface state, and other factors. The overvoltage of oxygen on some metals is listed in the table below.

Zinc electrowinning under different conditions, the measured anode potential is listed in the table below. It can be seen from the table that the oxygen evolution potential is higher than the equilibrium potential and varies with the anode material. For example, when an anode of 0.7% Ag and 2% Ca is used, the anode potential can be lowered compared with the lead anode of 1% Ag. 0.12V, and corrosion can be reduced.

The progress of industrial zinc electrowinning is always accompanied by the precipitation of oxygen on the anode. The greater the overpotential of oxygen, the more power is consumed during electrowinning. Therefore, it is necessary to reduce the overvoltage of oxygen. Since the anode potential of the lead silver anode is lowered, the formed Pb0 _{2 is} finer and denser, has better conductivity, and has higher corrosion resistance, so it is commonly used in zinc electrowinning plants.
C Other Anode Reactions Many other reactions occur in the anode during zinc electrowinning, as follows:

Mn ²⁺ +2H ₂ 0-2e — ^_→ Mn0 ₂ ↓ + 4H ⁺ E ^Ó© ₍₁₁₎ = 1.25V (11)

Mn ²⁺ +4H ₂ 0-5e — ^_→ Mn0 ₄ ^- ↓ + 8H ⁺ E ^Ó© ₍₁₂₎ =1.50V (12)

Mn ²⁺ +2H ₂ 0-3e — ^_→ Mn0 ₄ ^- ↓ + 4H ⁺ E ^Ó© ₍₁₃₎ = 1.71V (13)

C1 ^- + 4H ₂ - 8e — ^_→ ClO ₄ ^- +8H ⁺ E ^Ó© ₍₁₄₎ =1.39V (14)

2C1 - 2e — ^_→ C1 ₂ ^{                              Â}  E ^Ó© ₍₁₄₎ =1.39V (15)[next]

Lead anode reaction is related to anode life and cathode zinc quality. Fluorine and chlorine in the effusion are extremely harmful. It not only makes the anode corrosion of lead intensified, but also causes difficulty in stripping zinc in electrowinning operation and increase in lead and anode consumption. It also leads to an increase in lead in cathode zinc and an increase in fluorine and chlorine in the electric storage tank. Deteriorating operating conditions and seriously affecting the health of workers. Therefore, in industrial production, fluorine and chlorine in the effusion are generally required to be as low as possible.
In addition, since lead and its oxidation products have different bulk densities (g/cm ³ ), such as lead of 0.09 g/cm ³ , Pb 0 ₂ of 0.11 g/cm ³ , and PbS0 ₄ of 0.16 g/cm ³ , lead anodes The surface of the Pb0 ₂ layer may have pores and even partially fall off. Under normal production conditions, the reaction formula (8) for forming PbS0 ₄ is still carried out in a small amount. Although Pb0 ₂ is insoluble in water, PbS0 ₄ has a certain amount of dissolution in the effusion (see table below). In the industrial electrowinning solution, the Pb ²⁺ content can be as high as 5 to 10 mg/L, which shortens the life of the anode and lowers the quality of the precipitated zinc.

In industrial production, the concentration of Mn ^{2+ in the} electrowinning solution can be controlled to reduce the lead content of the precipitated zinc and slow down the chemical corrosion of the lead anode. This is because Mn ²⁺ is oxidized on the anode to form MnO ₂ adhered to the surface of the anode to form a protective film, which hinders the dissolution of lead. Therefore, the progress of the reaction formula (11) is always maintained during the zinc electrowinning process. However, MnO ₂ precipitates excessively on the anode, which increases the burden on the leaching step. On the other hand, it causes the Mn ²⁺ depletion in the effusion and directly affects the quality of the precipitated zinc.
Cathodic Process Under industrial production conditions, the zinc electrowinning solution contains Zn ²⁺ 50-60 g/L and H ₂ SO ₄ 120-180 g/L. If impurities in the effusion are not considered, only two processes can occur on the cathode when energized:
(1) Discharge of zinc ions, precipitation of metallic zinc on the cathode:

Zn ²⁺ + 2e — _^→ Zn
E ^Ó© _{Zn ²⁺ /Zn} = -0.763 + 9.92 x 10 ^-5 Tlga _{Zn ²⁺}

(2) Hydrogen ion discharge, releasing hydrogen gas on the cathode:

2H ⁺ + 2e — _^→ H ₂
E ^Ó© _{H ⁺ /H2} = 0.00+19.84 x 10 ^-5 Tlga _{H ⁺}

In these two discharge reactions, which ion is preferentially discharged is critical for wet zinc smelting. From the potential sequence of various metals (see Table 3), hydrogen has a greater positive charge than zinc, and hydrogen will preferentially precipitate out of solution without precipitation of metallic zinc. However, in industrial production, zinc is electroformed from a strongly acidic zinc sulfate solution. [next]
To explain this phenomenon, we first discuss the reversible electrowinning process, that is, the electrowinning process in which there is no polarization at all. Under reversible process conditions, simultaneous discharge of these two ions is only possible when E ^Ó© _{Zn ²⁺ /Zn} = E ^Ó© _{H ⁺ /H2} . For the sake of simplicity of calculation, let α _{H ⁺} =1 = c _{Zn ²⁺} , α _{Zn ²⁺} = c _{Zn ²⁺} at room temperature, when E ^Ó© _{Zn ²⁺ /Zn} = E ^Ó© _{H ⁺ /H2} :

c _{Zn ²⁺} = 10 ²⁶

Therefore, in the reversible process of H ⁺ and Zn ²⁺ , since the concentration of Zn ²⁺ is unlikely to reach such a large size, it is impossible for both to be simultaneously discharged, and since E ^Ó© _{Zn ²⁺ /Zn} > E ^Ó© _{H ⁺ /H2} , Only hydrogen should be released on the cathode.
The standard electrode potentials of various metals are shown in the table below.
However, in the process of irreversible electrowinning, that is, in the actual electrowinning process, the equation of the ion discharge potential is:

E ^Ó© _{Zn ²⁺ /Zn} = E ^Ó© _{Zn ²⁺ /Zn} - η H ₂

E ^Ó© _{H ⁺ /H2} = E ^Ó© _{H ⁺ /H2} - η Zn ²⁺

In the formula, η H ₂ and η Zn ²⁺ are ^{supervoltages} of hydrogen gas and zinc ion, respectively.
When the actual effusion is an aqueous solution of ZnSO ₄ and H ₂ SO ₄ , the η H ₂ value reaches a large value on the cathode, and η Zn ^{2+ is} close to zero, which is about -0.030V. Therefore, certain conditions can be created during electrowinning. Due to the polarization, the discharge potential of hydrogen ions will be greatly changed, so that the precipitation potential of hydrogen ions on the cathode is more negative than zinc, and thus the zinc ions are preferentially on the cathode. The discharge is precipitated. This is the theoretical basis for the success of zinc electrowinning technology.
It can be seen from the above analysis that the overvoltage of hydrogen is of great significance in the actual production of zinc electrowinning. The main factors affecting the overvoltage of hydrogen at the cathode are:
(1) cathode material and its surface state;
(2) current density;
(3) Electrolyte temperature;
(4) adding an additive in the electrowinning liquid;
(5) Impurities and the like in the electrolyzed liquid.
Regarding the overvoltage at the time of hydrogen evolution, it is important in industrial production and theory, and a great deal of research work has been carried out. The following three figures cite some curves of the relationship between the overvoltage and current density of hydrogen sulfate on the zinc, the temperature of the effusion, the concentration of the neutral salt and the content of the gel.

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In 1905, Tafel proposed the Tafel formula according to the experimental results, pointing out that under the condition that the influence of concentration polarization is negligible, the relationship between the overvoltage of hydrogen and the cathode material and current density is as follows:

η H ₂ = a + blgJ

In the formula, a is equal to the super-voltage of hydrogen per unit cathode current density (J=1), and its value is related to electrode material, electrode surface state, temperature and solution composition. The constant b has little to do with the electrode material.

b = 2.3 x 2RT/F

It can be seen from the Tafel formula that the value of a changes and the overvoltage of hydrogen changes, that is, the overvoltage of hydrogen depends on the material of the cathode. In addition, as the cathode current density increases, the overvoltage of hydrogen also increases, as shown in Table 4. [next]

Adding glue to the electric effluent can increase the over-voltage of hydrogen, but the glue can only reach a certain limit. When the amount of glue is excessively increased, the over-voltage of hydrogen begins to decrease again.
In addition, the presence of certain impurities has a great influence on the electrowinning process. When there are impurities (or even trace amounts) which are more likely to be precipitated in the effusion, these impurities are deposited along with the zinc and the overvoltage of hydrogen on these impurities is lower than that on the zinc, so that hydrogen is strongly precipitated at the cathode. And reduce the rate of zinc production.
The state of the surface structure of the cathode has an indirect effect on the magnitude of the overvoltage of hydrogen. The more uneven the surface of the cathode, the larger the real area of ​​the surface, that is, the smaller the true current density, the smaller the overvoltage of hydrogen.
As the temperature of the effluent increases, the overvoltage of hydrogen decreases, as shown in Table 5. This is because the value of a in the Tafel formula is reduced.

Since the standard electrode potential of hydrogen is much more positive than that of zinc, and there are many factors affecting the overvoltage of hydrogen in the actual electrowinning process, hydrogen deposition is inevitably inevitable under industrial production conditions. The precipitation of hydrogen (also referred to as "burning" in industrial production) is a technical problem often encountered in industrial zinc electrowinning. In severe cases, zinc flakes are not even precipitated. Therefore, the successful application of zinc electrowinning technology depends to a large extent on trying to maintain a high hydrogen overvoltage, so that the hydrogen evolution reaction occurs as little as possible, so that the zinc evolution reaction can still have a sufficiently high current efficiency.

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