| Potassium manganate preparation is the key step of potassium permanganate production. There are two methods for potassium manganate industrial preparation: solid-phase method and liquid-phase method. Solid-phase method can not accord with the industrial policies now in China because it has the disadvantages of high energy consumption, low efficiency and environmental pollution, liquid-phase method which is clean and high efficient is the direction of potassium manganate industrial preparation. Pressure hydrometallurgical technology has been applied to potassium manganate preparation to develop the original liquid-phase method. A new process of "continuous preparation of potassium manganate by pressure oxidation in horizontal reactor" was proposed here. The new process is clean and high efficient and it adapts to low grade pyrolusite with MnO2mass fraction of45%.General situation of manganese resource and industrial processes of potassium manganate production were introduced in the paper. Limitations of manganese resources and deficiencies of the existing processes were pointed out. A preliminary process route of "atmospheric burden—atmospheric preheating—pressure oxidation—high temperature digestion—low temperature crystallization—filtration" was proposed by summarizing the existing processes and combining with pressure hydrometallurgical technology. According to the preliminary process route, effects of particle size of manganese powders, temperature, oxygen partial pressure, time, initial mole ratio of KOH-to-MnO2, initial mass fraction of KOH, concentration of oxygen and agitation rate on the convention rate of MnO2-to-K2MnO4were studied by using two kinds of manganese powder—JM-1and JM-2. JM-1with MnO2mass fraction of55.54%represented middle grade manganese powders and JM-2with MnO2mass fraction of44.69%represented low grade manganese powders. Main influence factors were foud out. Two sets of optimized process parameters were obtained:for JM-1manganese powders, the particle size of the sample should be<120μm, the temperature maintains at260±5℃, the reaction time is for2hours, KOH initial mass fraction is about65%, oxygen partial pressure holds at0.3MPa, the KOH-to-MnO2initial mole ratio and the agitation rate is12and600rpm, respevtively; for JM-2 manganese powder, the particle size of the sample also shoule be<120um, the temperature needs to maintain at260±5℃, the reaction time can vary from2hours to2.5hours, KOH initial mass fraction should be65%in the solution, oxygen partial pressure must be holded at0.2MPa, the KOH-to-MnO2initial mole ratio and the agitation rate is15and300rpm, respevtively. Comparing the two sets of process parameters, we can draw the conclusion that the lower grade of manganese powder requires the higher initial mole ratio of KOH-to-Mn02, the lower oxygen partial pressure, and the longer reaction time. Results of the tests on the other kinds of low grade manganese powder also accorded with the law. Crystallization measurements showed that best effect was obtained when the proportion of saturated solution is1.5302. The results of tests on inner flask, cooling coil and agitator of pressure reactor using different kinds of material showed that wear and corrosion mainly occurred in agitator.Under the determined kinetic conditions, the kinetics of pressure oxidation preparing potassium manganate using JM-1manganese ore powder was studied. The reacton undergoed in sub-molten salt at200～300℃, belonged to liquid-solid reaction and followed the kinetic law of "shrinking of unreacted core". When the agitation rate is up to600rpm, external diffusion can be neglected. Under the experimental conditions, kinetic equations of particle size, temperature, oxygen partial pressure and KOH-to-MnO2mole ratio to the convention rate of MnO2were obtained. The reaction activation energy is110.18kJ/mol. The kinetic mathematical model for pressure oxidation process is obtained by SPSS (Statistical Product and Service Solutions): α%=100×{1-[1-1.28x1010×t×p×exp(5.41x10-5/r0)xexp(-13258.35/T)]3}The results of verification tests showed that the calculated values based on the model agreed well with experimental values, indicating that the model had strong reliability, A set of optimum process parameters based on the kinetic mathematical model and practical production was proposed:the particle size of the manganese poeder should be<120μm (the smaller the better under considering of the cost and technique), the temperature should be maintained at260±10℃, oxygen partial pressure should be holded at0.3±0.05MPa, the KOH-to-MnO2initial mole ratio should be12(The value should be more than15while using the low grade pyrolusite.), KOH initial mass fraction should be65%±2%.A new complete process of "continuous preparation of potassium manganate by pressure oxidation in horizontal reactor" was proposed by detailing the preliminary one. The main characteristic of the new process is using a horizontal pressure reactor as the main reaction equipment. The organic combination of burden, pressure oxidation and automatic control systems realized clean, high efficiency, continuous and automatic potassium manganate preparation. Pilot test successfully complited according to the rule of discontinuous first and continous second. The two kinds of manganese powders JM-1and JM-2were used in the pilot test. Mass data was collected and laid a solid foundation for the industrialization. The results of pilot tests showed that the two sets of optimized process parameters obtained in small experiments are accurate and reliable; the kinetic mathematical model has guiding significance to practical production; continuous and automatic potassium manganate preparation can be realized by using the horizontal pressure reactor which is beneficial to improving production efficiency and product quality. The new process adapts to low grade pyrolusite with MnO2mass fraction of45%, effectively improves resource utilization efficiency, and has important significance to potassium permanganate industry. |
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