Electrolysis of Manganese Sulfate Solution--Effect of Electrolytic Conditions on Production Capacity, Cell Voltage and Product Performance

First, the current density
(1) Current density and production capacity of electrolytic MnO ₂ According to Faraday's law, when manganese dioxide is electrolytically produced, the amount of manganese dioxide deposited on the electrode is proportional to the amount of electricity passed. When the total area of ​​the anode assembled in the electrolytic cell is determined, the amount of electricity passed is proportional to the current density. This relationship can be expressed mathematically as follows:
M=qIt=K · J _a · S · t
Where M is the amount of manganese dioxide deposited on the electrode, g;
q——Electrical equivalent, the amount of manganese dioxide precipitated on the electrode when the unit is charged, and the electrochemical equivalent of manganese dioxide is 1.6216 g/(A · h);
I——current intensity, A;
T——the time of energization and electrolysis, h;
J _a - anode current density, A / m ² ;
S - the total effective area of ​​the anode assembled in a single cell, m ² .
According to the above formula, the amount of manganese dioxide deposited on the anode is proportional to the current density. The current density is as large as possible from the viewpoint of increasing the yield of a single tank per unit time.
(2) Relationship between current density and cell voltage In the case where the effective area of ​​the assembled anode in the current tank is constant, the larger the current density, the larger the total current I flowing into the electrolytic cell, and according to Ohm's law, the electrolytic cell The ohmic voltage drop of the system is greater; at the same time, the greater the current density, the greater the polarization of the electrodes. Therefore, the greater the current density, the higher the cell voltage and the greater the power consumption. In terms of power consumption, the current density is preferably small.
(3) Relationship between current density and product performance
1 Influence of current density on chemical composition of electrolytic manganese dioxide The density of electrolytic manganese dioxide decreases as the anode current density increases. As the current density increases, the number of Mn ²⁺ ions discharged on the anode per unit time increases, the deposition rate of manganese dioxide is greater than the growth rate of the crystal grains arranged in a lattice, and the crystals are not aligned, thereby forming loose porous deposits. , causing its density to decrease.

The effect of anode current density on the BET surface area of ​​the product during electrolysis is shown in Figure 1. As can be seen from Figure 1, the BET surface area of ​​the electrolytic manganese dioxide increases as the current density increases. This is consistent with the effect of the above current density on product density. Since loose porous deposits have a larger surface area than the surface area of ​​dense deposits, this is undoubted. [next]
The relationship between the current density and the chemical composition of manganese dioxide is shown in the table below. The higher the current density, the lower the manganese dioxide content in the product, and the higher the content of low-priced manganese oxide and lead , iron , SiO ₂ and the like.
Relationship between current density and chemical composition of MnO2 (%)

As the current density increases, the anode reaction rate increases, and some Mn ²⁺ ions are introduced into the deposit before they are discharged. At the same time, as the current density increases, the anode potential also increases, creating conditions for the side reactions of other low-cost manganese oxides on the anode. Therefore, as the current density increases, the manganese dioxide product has a low-priced manganese gas oxide content, while the MnO ₂ content decreases.
The lead content of the product increases with increasing current density and is related to the anode potential. The theoretical potentials of the MnO ₂ /Mn ²⁺ pair and the PbO ₂ /Pb ²⁺ pair are 1.236V and 1.456V, respectively. The potential required to deposit lead dioxide is 220mV higher than the potential required for the deposition of manganese dioxide. Under the normal conditions of manganese oxide efficiency of 90% to 95%, it is impossible to deposit lead dioxide together with manganese dioxide because the anode potential is lower than the theoretical precipitation potential of PbO ₂ . However, the electrolytically deposited manganese dioxide is porous, and the Pb ²⁺ ions can slowly diffuse into the pores inside the manganese dioxide deposit. Since the local potential in the internal pores of the manganese dioxide deposit is higher than the potential on the surface of the deposit, it is possible that the lead dioxide is deposited in the internal pores of the manganese dioxide deposit. As the current density increases, the Yanghu potential increases and the possibility of lead dioxide deposition increases. This is why the lead content in manganese dioxide increases with increasing current density.
For similar reasons, the iron content also increases with current density.
2 The effect of current density on the discharge performance of manganese dioxide products Electrolytic manganese dioxide is mainly used to manufacture dry batteries. Therefore, the discharge performance of electrolytic manganese dioxide is the main performance index for measuring the quality of products. A large number of studies have shown that current density is one of the main factors affecting the discharge performance of products. [next]
Hui Luo, et al. systematically studied the effects of various electrolysis conditions such as current density on the discharge performance of electrolytic manganese dioxide. They characterized the manganese dioxide by the electrode potential, discharge overvoltage, discharge capacity and discharge energy of manganese dioxide. The discharge performance, the results are shown in Figure 2.

It can be seen from Fig. 2 that the electrode potential of the manganese dioxide is decreased as the anode current density increases during electrolysis. The results of this experiment can be explained by the theory of homogeneous solid phase redox system proposed by Ozawa Sawa. According to the theory of Zeze, the reaction of manganese dioxide in a general dry battery is MnO ₂ +H ₂ O+e ^- →MnOOH+OH ^- (homogeneous solid phase reaction)
Manganese dioxide belongs to a homogeneous solid phase redox system, and its electrode potential is

It can be seen from the formula that the electrode potential φ of manganese dioxide decreases as the ratio of [Mn ³⁺ ] _solid /[Mn ⁴⁺ ] _solid increases. As mentioned above, as the current density increases, the low-cost manganese oxide (mainly the oxide of Mn ³⁺ ions) in the manganese dioxide is electrically ^analyzed , that is, [Mn ³⁺ ] _solid / [Mn ⁴⁺ The solid ratio increases as the current density increases. Therefore, the electrode potential current density of manganese dioxide is increased and decreased. [next]
The discharge overvoltage is the difference between the open circuit voltage and the load voltage. Figure 2 (2) shows that the discharge overvoltage of manganese dioxide is related to the increase of current density, and it can be concluded that the smaller surface area of ​​manganese dioxide is overvoltage. . This indicates that the initial reaction rate control step of the dioxane manganese discharge reaction is proton diffusion in the manganese dioxide lattice, rather than the electrochemical reaction at the solid-liquid interface. Therefore, surface area is not a critical factor.
Fig. 2(3) and Fig. 2(4) show that the smaller the anode current density during electrolysis, the larger the discharge capacity of electrolytically deposited manganese dioxide, and the reason is generally similar to the effect of current density on discharge overvoltage.
The determination of the optimum current density requires consideration of the following two aspects.
A. Product quality From the viewpoint of improving product quality, that is, improving product purity and discharge performance, it is preferable to carry out electrolysis using a low current density. However, using too low current density and lowering the yield is economically uneconomical, so the maximum current density that guarantees product quality should be used. This maximum current density value can only be determined experimentally.
B. Economic Benefits The impact of current density on the economic benefits of electrolytic manganese dioxide production is contradictory. From the perspective of improving productivity and utilization of plant and equipment, it is advisable to use large current density. However, the cell voltage increases as the current density increases, resulting in an increase in power consumption. Therefore, determining the optimal current density from the perspective of improving economic efficiency should be a contradiction between the two aspects.
In industrial production, the anode current density is generally in the range of 0.4 to 1.0 A/dm ² .
Second, the electrolysis temperature
(1) Effect of electrolyte temperature on cell voltage In production practice, the cell voltage decreases with increasing electrolyte temperature, especially when titanium anodes are used. Increasing the electrolyte temperature lowers the cell voltage because the polarization of the anode and cathode (including concentration polarization and electrochemical polarization) can be reduced when the electrolyte temperature is high.
(2) Effect of electrolyte temperature on product properties 1 Effect of electrolyte temperature on physical and chemical properties of products

Figure 3 shows the relationship between the surface area of ​​electrolytic manganese dioxide and the temperature of the electrolyte. It can be seen from Fig. 3 that when electrolysis is performed at a current density of 0.2 to 3.0 A/dm ² , the surface of the electrolytic manganese dioxide decreases as the temperature of the electrolyte increases, without exception. [next]
Studies have shown that as the temperature of the electrolyte increases, the content of MnO _{2 in the} electrolytic manganese dioxide and the value of _x in MnO _x increase, and the content of low-valent manganese oxide and SO ₄ ^2- decreases. This is because the temperature of the electrolyte rises and the anode potential decreases when electrolysis proceeds, thereby reducing or avoiding the occurrence of side reactions that generate low-priced manganese oxide. Other studies have shown that when the electrolyte temperature is low, the product contains more water, and when the electrolyte temperature is high, the combined water content in the product is high.
2The effect of electrolyte temperature on the discharge performance of the product

Figure 4 shows the relationship between the electrolyte temperature and the electrode potential, discharge overvoltage, discharge capacity and discharge energy of electrolytically deposited manganese dioxide. The figure shows that as the electrolyte temperature increases, the electrode potential, discharge capacity and discharge energy of the product manganese dioxide increase, and the discharge overvoltage decreases, that is, the discharge performance of the electrolytic manganese dioxide increases with the electrolyte temperature. improve.
In summary, increasing the electrolyte temperature not only improves current efficiency, reduces cell voltage, but also improves product purity and discharge performance. Therefore, it is generally required that the electrolyte temperature be above 95 °C. [next]
Third, sulfuric acid concentration
(1) Effect of sulfuric acid concentration on electrolyte conductivity

Ichiro Ichiro measured the conductivity of the MnSO ₄ + H ₂ SO ₄ solution using an alternating current of 1000 Hz. As shown in Fig. 5, when the electrolyte is pure MnSO ₄ liquid, the electrical conductivity increases as the concentration of Mn-SO ₄ increases. After adding H ₂ SO ₄ , the conductivity of the electrolyte increases, and the higher the concentration of sulfuric acid, the higher the conductivity of the electrolyte. However, when the concentration of H ₂ SO ₄ exceeds 38 g/L, the conductivity of the electrolyte increases with the concentration of MnSO ₄ . The increase is lowered. Figure 6 shows that the increase in electrolyte temperature and the greater the increase in conductivity with increasing H ₂ SO ₄ concentration.
(2) Effect of sulfuric acid concentration on cell voltage, anode potential and current efficiency

Fig. 7 shows the relationship between the sulfuric acid concentration and the cell voltage. It is shown that when the H ₂ SO ₄ concentration is below 20 g/L, the cell voltage drops sharply as the H ₂ SO ₄ concentration increases. When the H ₂ SO ₄ concentration exceeds 20 g/L, the cell voltage remains substantially unchanged. [next]
(3) Effect of sulfuric acid concentration on product properties Ghana Yuantaro studied in detail the effect of sulfuric acid concentration on the chemical composition, water content and crystal structure of manganese dioxide. The results show that:
The manganese dioxide content of the product 1 increased slightly as the sulfuric acid concentration increased from 0 to 0.75 mol/L. However, when the concentration of sulfuric acid is 2mol/L, the MnO ₂ content of the obtained product is significantly reduced regardless of the temperature of the electrolyte. The total manganese content of the product decreases with the increase of H ₂ SO ₄ concentration; SO ₄ ^{2- of the} product The content increases as the H ₂ SO ₄ concentration increases. The relationship between the value of _x in MnO _x and the concentration of H ₂ SO ₄ is the same.
Both the water absorption and the combined water content in the product showed an increase with the increase in the concentration of H ₂ SO ₄ .
3 When the concentration of sulfuric acid is low, the product is mainly composed of γ-type structure, which contains a small amount of β phase and Ramsdellite phase. When the concentration of sulfuric acid is increased, the product is pure γ phase; when the concentration of sulfuric acid is as high as 2~2.1 mol/L, the product is a structure containing a large amount of a phase and γ phase. The manganese dioxide obtained under the condition of high sulfuric acid low temperature, that is, under the condition that the anode potential is 0.75 V or 0.75 V or more, is a structure in which a phase [is represented by Mn(OH) ₄ or Mn(OH) ₂ ]. .

Fig. 8 shows the relationship between the discharge overvoltage of manganese dioxide (discharge in KOH solution for 45 min, the difference between the open circuit voltage of 24 h of open circuit and the load voltage at 45 min of discharge) and the concentration of H ₂ SO _{4 in the} electrolyte. The graph shows that the discharge overvoltage of the product manganese dioxide tends to be stable regardless of the electrolyte temperature. Manganese dioxide has a low discharge overvoltage, that is, its electrochemical activity is high. Combining the influence of H ₂ SO ₄ concentration on water, crystal structure and electrochemical activity, the crystal structure and bound water of manganese dioxide can be considered. The content is the main factor determining the electrochemical oxidation of manganese dioxide.
Fourth, the electrolyte components
The concentration of each component of the electrolyte is one of the important electrolysis conditions, and the selection of the optimum electrolyte composition should also consider both product performance and economic benefits. The composition of the electrolyte used in industrial production is roughly within the following range:
1MnSO ₄ : 0.5~0.8mol/L
2H2SO ₄ : 0.3~0.5mol/L
3 Iron ion concentration in the electrolyte: mainly affects the product quality, while the reaction Fe ³⁺ +e ^- ===Fe ²⁺ will reduce the current efficiency. Requires iron ion concentration <0.2mg/L.
â‘£ heavy metal ions in the electrolytic solution: primarily affects product quality and reduce current efficiency, no qualitative requirements.
5 Electrolysis cycle: The anode of the general electrolysis process is discharged from 15 to 30 days after loading, and the anode current efficiency can be improved by appropriately shortening the time.
6 foaming agent (sodium dodecyl sulfonate): covering the surface of the electrolyte, is conducive to heat preservation, reducing solution evaporation and improving working conditions. The dosage requires MnO _{2 of} less than 2kg.

Steam Boiler

Gas Fired Steam Boiler,Oil Fired Steam Boiler,Steam Boiler,Coal Fired Steam Boiler

Changzhou machinery and equipment Imp.& Exp.Co.,Ltd , https://www.czautoclave.com