First, current efficiency
Current efficiency is an important technical and economic indicator in current deposition, and its concept must be clarified.
According to Faraday's law, 1.6216 g of MnO 2 should theoretically be deposited on the anode by the amount of 1 A · h, but in practice, the amount of MnO 2 produced by the amount of 1 A · h is always less than theoretically expected. the amount of MnO 2, because in addition to MnO 2 deposition, the anode is also deposited on the O 2 on the anode. When the insulation condition of the equipment is damaged, the anode and cathode contact are caused by the operation, etc., which will cause leakage of current. Industrial production, the amount of MnO 2 used when the actual output power by the same percentage, the amount of MnO 2 is theoretically represented current efficiency was calculated by the following formula:
Where η i - current efficiency, %;
M——the actual yield of MnO 2 precipitated in t time, g;
I——through the current between the anode and cathode, A;
T——electric accumulation time, h;
N——the number of electric accumulators;
q——Electrification equivalent of MnO 2 , 1.6216g (A · h).
In the production practice, the current efficiency of MnO 2 is different due to different specific conditions. Generally, the current efficiency of electrolytic MnO 2 fluctuates between 95% and 98%.
Second, the main factors affecting the current efficiency of electrolytic MnO 2
(1) Current density Many scholars have studied the effect of current density on current efficiency when manganese dioxide is deposited by electrolytic acid manganese sulfate solution. It is generally believed that the smaller the current density, the higher the current efficiency.
The anode current efficiency decreases as the current density increases because the anode potential increases as the current density increases. When the anode potential is increased to a certain value, side reactions such as oxygen evolution and Mn 3+ ions are generated, and a part of the current is consumed in the side reaction, thereby reducing the current efficiency of the precipitated manganese dioxide, which is studied by many researchers. The experimental results are proved. Studies by Gentaro et al. showed that current efficiency began to decrease when the anode potential began to rise significantly higher by 0.550 to 0.650 V (vs. Hg/Hg 2 SO 4 -0.5 mol · L -1 H 2 SO 4 ). This deviation in potential indicates that other electrochemical reactions have taken place. Generally control current density is 50~80A/m 2 .
(2) Electrolyte temperature Takahashi Yuhiko studied in detail the relationship between electrolyte temperature and current efficiency. The conclusion is that when the temperature is higher than 50 °C, the electrolyte temperature is linear with the current efficiency, and the mathematical equation is η i =-61.18+1.77T
That is to say, for every 1 °C increase in temperature, the current efficiency increases by about 1.8%. For other different electrolysis conditions, the relationship between current efficiency and electrolyte temperature does not necessarily conform to the above formula. However, the higher the electrolyte temperature, the higher the current efficiency is recognized.
As the electrolysis proceeds, the Mn 2+ ions at the surface of the anode tend to be depleted and must be replenished by mass transfer to continue electrolysis. In the general electrolysis process, mass transfer is accomplished by ion migration, convection, and diffusion processes. However, when the acidic acidic manganese sulfate solution is electrolyzed, manganese dioxide is deposited on the anode under the action of the electric field force, and the Mn 2+ ions do not migrate toward the anode but migrate toward the cathode. That is to say, ion migration is not only detrimental to the replenishment of Mn 2+ ions on the anode surface, but also counteracts. Thus, both the convection and diffusion mass transfer processes must not only bring a sufficient amount of Mn 2+ ions to the anode region to maintain the deposition process, but also counteract the portion of the Mn 2+ ions that migrate from the anode region to the cathode region. Increasing the temperature of the electrolyte not only facilitates the convection of the solution, but also accelerates the diffusion process, thereby reducing the concentration polarization; at the same time, the increase of the electrolyte temperature is also beneficial to the anode discharge reaction of Mn 2+ ions, reducing the electrochemical polarization. . Therefore, the increase in the temperature of the electrolyte allows the electrolysis process to be carried out at a lower anode potential, reducing or even avoiding the occurrence of side reactions such as oxygen evolution and formation of Mn 3+ ions, thereby improving current efficiency. The general electrolysis temperature is 95~100 °C. [next]
Third, the electrolyte composition
1 Influence of sulfuric acid concentration In industrial production, an aqueous solution of sulfuric acid containing sulfuric acid is generally used as the electrolyte. Among them, manganese sulfate provides the Mn 2+ ions necessary for anodic deposition of manganese dioxide; sulfuric acid is added to improve the conductivity of the electrolyte. Practice has shown that the concentration of each component of the electrolyte has different effects on current efficiency, cell voltage and product performance.
Figure 1 shows the relationship between the electrolyte composition and current efficiency. When, when the electrolytic solution H 2 SO 4 and MnSO 4 molar ratio of 0.2 before, the current efficiency with H 2 SO 4 and the molar ratio of MnSO 4 increases seen from FIG. 1, a molar ratio of 0.2, the maximum current efficiency . Thereafter, the current efficiency decreases as the molar ratio increases, and when the molar ratio reaches 0.4, the current efficiency substantially stabilizes.
Ghana Yoshitaro studied the effect of H 2 SO 4 concentration on the anode potential. The test results show that the anode potential increases as the concentration of H 2 SO 4 in the electrolyte increases. Since the anode potential increases as the concentration of 2 SO H 4 rises, then the current efficiency with the increase of the H 2 SO 4 concentration is reduced natural result. Generally, the concentration of H 2 SO 4 in the electrolyte is controlled at 35~40g/L.
â‘¡ Effect of manganese sulfate concentration of manganese sulfate concentration on current efficiency shown in Figure 2, seen from Figure 2, without H 2 SO 4 when, MnSO 4 concentration has little effect on current efficiency, the addition of H 2 SO 4 The impact is slightly increased.
It can be seen that the effect of MnSO 4 concentration on the electrolysis process does not appear to have a large influence on the H 2 SO 4 concentration. However, it should be noted that MnSO 4 is the main component of the electrolyte, which is the Mn 2+ ion necessary to deposit manganese dioxide on the anode. Therefore, the choice of MnSO 4 concentration should take into account the magnitude of the current density. The current density is high and the MnSO 4 concentration is correspondingly high. Generally, the electrolyte requires MnSO 4 90~110g/L, compared with Mn33~40g/L. The new electrolytic solution contains MnSO 4 140~150g/L, which is equivalent to Mn51~55g/L.
The electrolysis process is strict, and the above technical conditions are stably controlled to ensure the quality of the MnO 2 product and have a high anode current efficiency.
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