Electrolysis of Manganese Sulfate Solution---Processing of Electrolytic MnO2 Product

Product post-treatment includes stripping, pulverizing, washing, neutralization and drying processes to obtain an electrolytic manganese dioxide finished product that meets user requirements. Post-treatment has a significant impact on the performance of electrolytic manganese dioxide.
First, stripping
The manganese dioxide deposit is taken out from the electrolytic bath together with the anode, washed with water or soaked in hot water to remove impurities such as electrolyte on the surface layer of the deposit, and the manganese dioxide deposit is peeled off from the electrode to obtain a block shape. Manganese oxide semi-finished products. After the product is peeled off, the electrode is re-slotted by appropriate treatment. The peeling of the product is generally manual operation, and there are examples of mechanical peeling in foreign countries.
Second, crush
The bulk manganese dioxide semi-finished product is coarsely crushed by a crusher, and then pulverized by a Raymond machine or other pulverizing equipment to prepare manganese dioxide powder that meets the user's particle size requirements.
Third, washing and neutralization
The pulverized manganese dioxide powder also contains a certain amount of electrolyte (aqueous solution of manganese sulfate and sulfuric acid) and impurities such as electrolyte evaporation inhibitor, so washing and neutralization must be performed to remove Mn ²⁺ and SO ₄ ^2- Wait for impurities and adjust the pH to meet the specified requirements.
(1) Washing of manganese dioxide Washing is the use of water to wash out impurities such as sulfate and sulfuric acid in the product from the surface of the manganese dioxide particles and the pores of the bone, thereby reducing the impurity content in the finished manganese dioxide. Therefore, the washing operation should include two basic steps: mixing and contacting the washing water with the manganese dioxide powder; and separating the washing water from the manganese dioxide powder.
In the production of electrolytic manganese dioxide, the washing of manganese dioxide is carried out in a cylindrical washing tank. A mechanical stirring device is installed in the washing tub. The pulverized manganese dioxide powder is directly introduced into a washing tank or slurried with water, and then pumped into a washing tank, and hot water of about 70 ° C is added, and washing is performed by mechanical stirring. At this time, a uniform manganese dioxide powder suspension was formed in the washing tank, and the manganese dioxide particles were sufficiently mixed with water to come into contact with each other. In this process, the first is the microporous water film inside the H ⁺ , SO ₄ ^2- , Mn ²⁺ plasma-soluble particles on the surface of the manganese dioxide particles. Due to the concentration gradient, the H ⁺ , SO ₄ ^2- and Mn ²⁺ ions in the microporous water film diffuse outward, reaching the surface of the particle first, and then, under the dual action of concentration and stirring, quickly reach the depth of the washing water. Diffusion until the concentration gradients are equal everywhere. At this time, if the washing is continued, the effect is minimal. Therefore, stirring should be stopped and precipitation should be carried out. During the precipitation process, the manganese dioxide particles slowly settle down under the action of gravity. The upper part of the washing tank is a clear washing water, and the lower part is the manganese dioxide precipitation, the clarified washing water is put into the sedimentation tank, and the second The manganese oxide is left in the washing tank, and hot water is added to carry out the next washing.
The diffusion rate of H ⁺ , SO ₄ ^2- and Mn ²⁺ ions inside the manganese dioxide particles to the surface of the particles and the size, shape and morphology of the micropores inside the particles, the size and concentration of the ions themselves, and the absorption of ions by manganese dioxide It is related to factors such as force and can only be measured by experiment. For SO ₄ ^2- , the diffusion of H ⁺ plasma from the surface of the particles into the aqueous washing solution can be approximated by the law of dilution. From the calculation, the following conclusions can be drawn:
1 The more the amount of water is used in each washing, that is, the larger the liquid-solid ratio, the better the washing effect;
2 When the total amount of washing water is constant, the more washing times, the better the washing effect;
3 The less wash water remaining in the wet powder each time, the better the washing effect.
The washing process is a physical process and the washing speed depends on the diffusion speed. Therefore, the influence factors of the two-phase mass transfer rate can be qualitatively discussed by the diffusion equation. If the concentration of SO ₄ ^2- , Mn ²⁺ and other solute in the water film or particles around the manganese dioxide particles before washing is C ₀ , the concentration of solute in the water after washing is C, and the mass transfer driving force is (C ₀ -C). The contact area of ​​the two phases is F, then the diffusion velocity equation is

Where D is the diffusion coefficient.
It can be seen from the above formula that the diffusion rate is proportional to the diffusion coefficient D, the two-phase contact area F, and the difference between the solution and the solute concentration of the surface layer of the manganese dioxide particles.
The diffusion coefficient D is mainly related to the temperature of the wash water. The higher the temperature of the washing water, the faster the movement speed of ions and fractions, the larger the diffusion coefficient, and the better the washing effect.
Increasing the stirring strength, the manganese dioxide solid phase particles and the water phase are intimately mixed to contact, which can increase the contact area of ​​the two phases; at the same time, the agitation can reduce the droplet diameter, which also increases the contact area of ​​the two phases. Therefore, when washing, the stirring effect is enhanced, and the washing effect can be improved.
During the washing process, the solute concentration C ₀ in the solid phase decreases continuously, while the solute concentration C in the aqueous phase increases continuously, and the concentration difference (C ₀ -C) decreases with the prolongation of the washing time, so the washing speed varies with each time. The washing time is prolonged and slowed down. Wash should be stopped when the washing speed drops to a certain value. That is to say, it is preferable to use a washing process for a short time. In industrial production, it is generally washed 10 times with hot water for about 40 minutes each time. [next]
In recent years, electrolytic manganese dioxide manufacturers have basically used granules of MnO ₂ pulverized to 10~20mm, which has the advantages of no need to add NH ₄ CI, good sedimentation, easy drying and high recovery of MnO _{2 .} . In addition, NaOH is used instead of NaHCO _{3 for} neutralization. After treatment, electrolytic MnO _{2 is} mixed in large quantities, which is conducive to the stability and improvement of product quality.
(2) Neutralization of free acid and water washing after the test showed that the SO ₄ ^2- ion in the dioxazitan powder can be reduced from the original 2% to 3% to about 1% by water washing, but the pH value Has not changed much. Moreover, only the water-washed manganese dioxide powder has a rapid electrode potential drop rate. This means that water washing does not "completely" remove free sulfuric acid from the manganese dioxide powder. Neutralization must be carried out with an alkaline solution.
Industrial production is generally neutralized with 5%~10% NaHCO ₃ solution or NaOH solution, the temperature is 60~70 °C, and the neutralization time is about 1h. After neutralization, it is washed in hot water. Since a small amount of colloid is formed in the slurry after neutralization, the sedimentation rate of manganese dioxide is very slow. Therefore, it is necessary to add acidic ammonium chloride as a debonding agent to accelerate the sedimentation rate of manganese dioxide. The concentration of the NH ₄ CI solution is 2% to 5%.
(3) Dehydration Dewatering is required after washing to reduce the water content in the manganese dioxide wet material, thereby accelerating the drying speed and reducing energy consumption. The dehydration of the manganese dioxide wet material generally adopts a plate and frame filter press or a disk vacuum filter.
4. Drying of manganese dioxide wet material
The moisture between the capillary water and the pores of the manganese dioxide wet material is collectively referred to as sucking water. After the sucking water is removed, it can be re-adsorbed in the manganese dioxide powder in a humid environment. There is another water in the manganese dioxide. This water is present in the manganese dioxide lattice and becomes a component of the manganese dioxide lattice, which is called bound water or crystal water. The presence of bound water facilitates the diffusion of the two protons in the manganese dioxide lattice, which is advantageous for the electrochemical activity of manganese dioxide. The combined water content of electrolytic manganese dioxide is generally 3% to 5%, and once the combined water is lost, it cannot be recovered.
Choosing the right drying process conditions makes the manganese dioxide wet material dry faster and the product quality is good.
Drying temperature is a process factor that has the greatest impact on drying speed. The higher the drying temperature, the faster the drying speed, but the drying temperature must be strictly controlled to not exceed 110 °C. If it exceeds 110 ° C, manganese dioxide will partially lose crystal water and reduce its electrochemical activity.
The larger the dry area of ​​the dried material, the faster the drying speed, because the evaporation of moisture during drying is carried out on the surface of the material, so the thinner the material is, the faster the drying is.
When drying, the water evaporated on the surface of the material must diffuse into the space and be carried away by hot air (drying medium) or air, and evaporation can be continuously performed. If the flow rate of the hot air is small or the air in the dry space does not flow, the evaporated water quickly saturates the hot air or air in contact with the material, the evaporation speed is slowed down, and even the energy can continue. Therefore, the hot air is used for heating. When the medium is dry, the hot air should pass through the material at a sufficient flow rate; when drying in the drying room by means of steam by heat conduction, an exhaust fan is placed in the drying room to accelerate the flow of air.
The greater the relative humidity of the dry material space, the greater the vapor concentration in the air (or hot air) above the material, so the slower the drying rate. To speed up the drying process, the relative humidity of the dry material space must be reduced. This can be considered in two ways: appropriately increase the hot air flow (ie flow rate), or strengthen the air circulation, so that the evaporated water vapor is drained as soon as possible; appropriately increase the temperature of the dry material space (or hot air), thereby increasing the air above the material ( Or hot air) The maximum amount of water vapor that can be contained, which is equivalent to a decrease in relative humidity.
Drying equipment used in industrial production includes rotary kiln dryers, rake dryers, airflow dryers, and conventional steam ovens.
Since post-treatment has a great relationship with product performance, improvements in post-treatment processes aimed at improving product quality and reducing consumption have been ongoing. According to reports, the post-treatment operation of the product in the production of electrolytic manganese dioxide in Japan adopts a secondary rinsing secondary drying process, that is, the anodic deposit after stripping is first washed with hot water to wash away paraffin and sucking as an evaporation inhibitor of the electrolyte. The electrolyte is then dried, pulverized to a specified fineness, and then subjected to secondary rinsing, neutralization, filtration, and then dried to form a finished product. This secondary rinsing process is undoubtedly beneficial for improving product purity and saving neutralizers. When the coarse product is rinsed for the first time, the electrode corrosion product ( graphite ) and the low-priced manganese oxide on the surface of the product can be washed away, and it is difficult to obtain this effect by using a single rinsing process.