Because manganese is a high negative potential of the metal, since the US Bureau of Mines in the 1920s raised the production of electrolytic manganese metal diaphragm, the world has been using a neutral MnSO 4 - (NH 4) 2SO 4 -H 2 O system was subjected to cathode Diaphragm electrolysis. It is necessary to adjust ammonia and add antioxidants in electrolysis. 4. The difference between the metal manganese potential and the hydrogen potential φ value φ=-1.1795+0.0295lg[Mn 2+ ]+0.0591pH (5) (1) Adding ammonia to the MnSO 4 -(NH 4 ) 2 SO 4 -H 2 O system solution can increase the stability of the solution, that is, the hydrolysis pH of Mn 2+ increases, and the potential of the metal manganese becomes negative; (1) The effect of adding ammonia to an aqueous solution of MnSO 4 -(NH 4 ) 2 SO 4 -H 2 O system is: at the beginning, the free ammonia [NH 3 ] increases, the equilibrium pH value in the solution increases remarkably, and the metal manganese acts on the hydrogen potential. The difference φ is significantly increased, indicating that it is advantageous for the preferential precipitation of manganese metal; Figure 3 is a plot of φ, pH (=pH A ), [NH 3 ], and φ in various solutions. From Figure 3, the effect of solution composition can be seen: The above calculation results are important for selecting the electrolyte composition and determining the electrolysis conditions. It clarifies the quantitative buffering effect of the (NH 4 ) 2 SO 4 content on the solution and the necessity of [NH 3 ] addition and its most appropriate addition. the amount. At present, the high ammonia consumption of some domestic electrolytic manganese plants is one of the important factors affecting economic benefits. Choosing the most appropriate ammonia addition should attract people's attention.
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As for the principle of the electrolysis process, many articles about the country in the article, but more emphasis on qualitative discussion. The relevant staff performed an electrochemical balance calculation on the metal manganese electrolysis to illustrate the important role of the solution composition and ammonia adjustment.
Under negative polarization conditions, two competing electrochemical reactions will occur on the stainless steel cathode:
Mn 2+ +2e - ===Mn
φ Mn2+/Mn =-1.1795+0.02951lg[Mn 2+ ]
2H 2+ +2e-===H 2 (g)
φ H+/H2 =-0.0591pH
Based on the MnSO 4 -(NH 4 ) 2 SO 4 -H 2 O electrolyte, the technicians proposed charge balance, total ammonia balance and Mn 2+ hydrolysis balance, and performed thermodynamic calculations.
1. The charge balance is provided in solution [MnSO] 4 = A, [ (NH 4) 2SO 4] = B, after adding 3 NH, NH 3 generated with Mn 2+ Mn (NH 3) 2+ and Mn (NH 3) 2 2+ two complexes, the formation constants are 
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Based on the above equations (1), (2), (3), (4), and (5), we can make Figures 1, 2, and 3.
Fig. 1 is a φ-pH diagram of a Mn-NH 3 -SO 4 2- -H 2 O system. See Figure 1:
(2) The pH of the added ammonia solution increases, and the hydrogen electrode potential φ H +/H 2 is much larger than the metal manganese φ Mn 2+ / Mn , that is, the potential difference φ increases, which is favorable for the preferential electric reduction precipitation of Mn:
(3) The amount of ammonia added is limited. For a solution of [MnSO 4 ]=[(NH 4 )2SO 4 ]=1oml/L, [NH 3 ] addition is equal to 0.4089 mol/L. [next]
Figure 2 is a graph showing the effect of ammonia addition on φ, pH, and pH A in A = 0.5 mol/L, B = 1 mol/L solution.
As can be seen from Figure 2:
(2) The amount of ammonia added ([NH 3 ] addition ) is limited. For the solution of A=0.5mol/ and LB=1mol/L, the amount of ammonia added is 0.3488mol/L (ie 0.3488×17). =5.930g / L). Adding more than this limit of Mn 2+ will hydrolyze and precipitate Mn(OH) 2 ;
(3) [NH 3 ] increases from 0 to 0.15 mol/L, and the pH and potential difference φ in the solution increase greatly, but when [NH 3 ] is added up to 0.2 mol/L, although φ increases, the solution pH has The pH A value close to the hydrolysis of Mn 2+ indicates that 0.2 mol/L [NH 3 ] addition is more appropriate. [next]
(1) When fixed A, ie [Mn 2+ ] T = 0.5 mol/L, and the solution equilibrium pH is equal to Mn 2+ hydrolysis equilibrium pH A (pH=pH A ), increase B (ie (NH 4 ) 2 When the SO 4 concentration is increased from 0.75 mol/L to 1.2 mol/L, the increase in φ and the increase in pH A (from 7.885 to 7.9548) indicate that the electrolyte must maintain a high (NH 4 ) 2 SO 4 content, generally maintained. 140g / L, that is, B = 1mol / L;
(2) When B is fixed, ie (NH 4 ) 2 SO 4 =1 mol/L, the decrease of A, ie [Mn] T , decreases from 1 mol/L to 0.5 mol/L, resulting in an increase in pH A , which is increased by 7.7313. To 7.9225, it indicates that the electrolyte should be low in manganese content;
(3) As the amount of B in the solution increases, on the one hand, the amount of ammonia added increases, on the other hand, the free ammonia content [NH 3 ] in the solution also increases, and the increase in pH inevitably leads to more loss of ammonia volatilization, as shown in Table 1.
In summary, in the practice of metal manganese electrolysis production, it is always tried to use high (NH 4 ) 2 SO 4 (B) and low Mn content (A) catholyte, and adding ammonia can increase the pH. Because the pH increase not only facilitates the preferential precipitation of Mn (compared to H 2 ) as explained by the electrochemical balance, but also contributes to increasing the overvoltage of hydrogen and suppressing the precipitation of H 2 in terms of kinetics, thereby improving the cathode current efficiency.
The electric equilibrium of the electrolysis process of manganese is clarified from the principle of charge balance. The quantitative calculation shows the buffering effect of (NH 4 ) 2 SO 4 on the catholyte and the important effect of ammonia adjustment on pH adjustment.
The above-mentioned theoretical calculation of the potential difference of metal manganese to hydrogen, that is, the chemical potential φ preferentially precipitated by metal manganese, will contribute to further kinetic analysis of the electrode process.
Table 1 Relationship between ammonia loss and cathode pH
pH value
6
7
8
9
Tons of Mn liquid ammonia loss / (kg·t -1 )
8
10
12
28
November 15, 2019