Analysis of current efficiency by metal manganese electrolysis

Electrolytic manganese metal is a high-energy process, to increase the current efficiency (η _c) is a practical problem in the production of most concern. The current efficiency is directly related to the production and economic benefits of metal manganese and the quality of the product.
Through the simultaneous electrolysis of Mn and H ₂ , the staff clarified the quantitative relationship between the ammonia adjustment and the (NH ₄ ) ₂ SO ₄ content in the catholyte to the cathode current efficiency η _c , and discussed the important technical conditions affecting the current efficiency η _c . .
I. Simultaneous Electrolysis of Manganese Electrolytic Cathode Mn and H ₂
It has been mentioned that two phase-to-competitive reactions are carried out on the electrolytic manganese cathode, and the reaction of precipitation of manganese metal belongs to diffusion control reaction, and the reaction of hydrogen evolution (H ₂ ) belongs to electrochemical control reaction. Then according to the principle of equilibrium reaction, the necessary condition for simultaneous discharge of Mn ²⁺ and H ⁺ should be the precipitation of Mn, and the precipitation potentials of the two reactions of H ₂ are equal, ie

(1) where i _d and i _d∞ are the manganese precipitation diffusion current density and the ultimate diffusion current density (A/m ² ).
Cathode current efficiency That is, reducing the current density i _H2 of the precipitated H ₂ is the main way to increase η _c .
It can be seen from the formula (1) that i _H2 increases as the value of φ increases, but decreases as the pH value and a _H2 increase, and vice versa.
Regarding the i _{d ∞} value in (1), we take the diffusion coefficient of Mn ²⁺ D=10 ^-5 cm/S, [Mn ²⁺ ]=1mol/L10 ^-3 mol/cm ³ , and take the thickness of the diffusion layer=0.025 Cm, then i° _d∞ =0.0772A/cm ² =772A/m ² .
i _d∞ =i° _d∞ , [Mn ^{2 +} ] takes electrolytic catholyte [MnSO ₄ ]=[Mn ²⁺ ]=0.5mol/L, then i _d∞ =772×0.5=386A/m ² .
(1) where a _H2 is the Tafel coefficient, and the hydrogen exchange current density on the metal Mn in the 0.05 mol/L _{H 2} SO ₄ solution is i ₀ = 10 ^-7 A/cm ² .
The overvoltage of H ₂ on the metal is related to pH. At 25 ° C, η _H2 = a _H2 + 0.0591 pH + 0.1182lgi
η _H2 =-0.1182lgi° ₀ +0.0591pH+0.1182lgi
I° ₀ is the standard exchange current density at pH=0.
In 0.05 mol/L H ₂ SO ₄ , i ₀ = 10 ^-7 A/cm ² , which is calculated by data. The pH of the 0.05 mol/L H ₂ SO ₄ solution is 1.231, then i ₀ =i° ₀ •[H ⁺ ] ^0.5 , ie 10 ^-7 =i° ₀ •10 ^{-1.231×0.5} ,i° ₀ =10 ^-6.325
Then a _H2 =-0.1182lg10 ^-6.325 =0.755V
Therefore, the overvoltage of H ₂ on Mn is η _H2 =0.755+0.0591pH+0.1182lgi _H2[next]
Using the values ​​of i _d∞ and a _H2 derived above, and using the φ value calculated by the electrochemical balance, substituting into equation (1), the data of Table 1 and Table 2 can be calculated, and the amount of ammonia added can be quantitatively determined. The effect of NH ₃ ] _addition and [NH ₄ ] ₂ SO ₄ (B) concentration on cathode current efficiency η _c is shown in Figures 1 and 2 .

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As seen from Table 1 and Figure 2, the addition of ammonia thermodynamically increases the value of φ, kinetically increasing the pH of the solution to increase the overvoltage of hydrogen. The combined results of thermodynamic and kinetic effects are all improved by the current efficiency η _c .

It can be seen from Table 2 and Fig. 2 that increasing (NH ₄ ) ₂ SO ₄ also increases the potential difference (Δφ) and pH, which is advantageous for the improvement of the current efficiency η _c , but the effect is not as significant as the ammonia adjustment.
Second, the main factors affecting the efficiency of metal manganese deposition current
The main factors affecting the current efficiency of metal manganese are:
1. Electrolyte composition The manganese electrolyte includes a new liquid (purification liquid), a catholyte, and an anolyte.
(1) New liquid, the new liquid sent to manganese electrolysis is pure solution after purification, Mn ²⁺ 34~38g/L, (NH ₄ ) ₂ SO ₄ 100~120g/L, pH 6.5~7. In order to maintain The high cathode current efficiency, the impurity content of the foreign electrolytic manganese new liquid is shown in Table 3.

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As a result of the domestic iron effective measures, in addition to iron 1mg / L or less, using the SDD (ziram sodium) in addition to heavy metals, nickel and cobalt can be purified to 0.5mg / L or less.
(2) Catholyte 1Mn ²⁺ content, industrial manganese content of catholyte fluctuates between 15~18g/L. Too high will lead to the precipitation of manganese ammonium double salt and destroy the electrolysis process.
2 (NH4) ₂ SO ₄ content, increasing the concentration of (NH ₄ ) ₂ SO ₄ can increase the stability of Mn ²⁺ in solution, preventing its hydrolysis at high pH, ​​and it can also increase the conductivity of the solution.
In the foreign manganese industry, the (NH ₄ ) ₂ SO ₄ content fluctuates between 120 and 140 g/L (without selenium ), and the domestic is 100 to 120 g/L (with selenium).
3 pH adjustment, the test confirmed that with the increase of the pH of the catholyte, the current efficiency increased significantly, but decreased after pH>9. The relationship between catholyte pH and current efficiency is shown in Table 4.

In order to increase the pH of the catholyte, 1:1 ammonia water is added in practice. The amount of ammonia in the production of manganese is generally 0.08t, and the ammonia concentration in the catholyte is generally 3.2g/L.
It should be noted that the amount of ammonia adjustment should be appropriate. Excess ammonia leads to the hydrolysis of Mn ²⁺ on the one hand, and the increase in free NH ₃ due to an increase in pH, which leads to loss of ammonia. See Table 5 for the relationship between the pH of the catholyte for ammonia loss.

Generally, the pH of the catholyte is preferably controlled at 7-8.
(3) Anolyte The anolyte contains about Mn ²⁺ (14~15) g/L, and contains H ₂ SO ₄ 40g/L.
When SeO ₂ is added as an antioxidant, the pH is preferably controlled at 6.8 to 7.2, and when SO _{2 is} used as an antioxidant, the pH is preferably controlled at 7.8 to 8.2. [next]