Study On Preparation Of High-purity Mn₃O₄Derived From Pyrolusite

Due to its unique physical and chemical properties, Mn3O4, possessing a tetragonal spinelstructure at room temperature, has been widely used in the various areas of manganese ferrites,highly efficient catalyst, cathode material of lithium manganese oxide and coatings. In thisdissertation, the technology for preparation of Mn3O4was studied and optimized. Manganeseions in a raw material of pyrolusite were leached out by a reductant in an acid solution. After theremoval of the impurities in the obtained manganese ion solution, it was then directly oxidized,resulting in a high-purity Mn3O4powder with a high specific surface area.The techniques of X-ray diffraction (XRD), inductively coupled plasma atomic emissionspectrometry (ICP), laser particle size analyzer and chemical analysis were employed todetermine the phase/chemical compositions in the raw starting material of pyrolusite, whichoriginated from Jianshui county in Yunnan province. The results show that the total manganesecontent in the pyrolusite reaches32.0wt%, equivalent to50.6wt%of manganese dioxidecontained in the ore. The powder pyrolusite shows an average particle size of0.8μm, consistingof the main phases of MnO2, CaCO3and SiO2with the impurities of Ca, Fe and Mg elements.Sulfur dioxide, pyrrhotite, sucrose and powder carbon were used as reductants for leachingpyrolusite. The influencing factors of manganese leaching rate under different leaching processconditions were studied in order to achieve the optimum leaching process. Firstly, the pyrolusitewas treated by using an acid in order to remove the impurities, which can be dissolved in theacidic solution when using sulfur dioxide as the reductant. The results show that washing theore powder with2mol/L nitric acid is beneficial for the removal of the Ca2+and Mg2+impurities, causing slight loss of manganese ions. The ore powder after acid-washing is leachedin a acid solution with SO2gas pumped, and a leaching rate of manganese over94%is achievedunder the conditions as follow: the weight ratio of liquid to solid of8∶1, keeping flow of SO2to be25mL/min and pH value of leaching solution above4. Besides, the impurities content inleaching solution is low so that it’s no need to remove the Ca2+and Mg2+impurities. Theleaching rate of manganese ions when using pyrrhotite as the reductant is more than95.0%,which is obtained under the following conditions: controlling the weight ratio of pyrolusite,pyrrhotite and1.2mol/L sulfuric acid of10∶3∶68, keeping leaching temperature at60℃,reacting duration of3h. Using sucrose as the reductant, the leaching rate of manganese is up to97.0%, which is achieved under the following conditions: controlling the mass ratio ofpyrolusite, sucrose and2.0mol/L sulfuric acid of25∶3∶50, keeping reaction temperature over90℃, reacting duration of5h. Using power carbon as the reductant, the leaching rate ofmanganese is more than91%, which is obtained under the following conditions: controlling themass ratio of pyrolusite, power carbon and2.0mol/L sulfuric acid of5∶2∶10, keepingreaction temperature at85℃, reacting for7h.In this dissertation, pyrrhotite is used as the reductant to extract manganese ions frompyrolusite. Pyrrhotite is a good reductant for leaching pyrolusite because of its advantages oflow cost, high leaching rate, short reaction time, facile and controllable process. However, theimpurities in the extraction solution obtained by this method are so much that they must beremoved for achieving high-purity Mn3O4. Three steps are involved during the removal ofimpurities from the extraction solution: the first is to remove iron, silicon and aluminum ions;the second is to remove calcium and magnesium ions; the third is to remove heavy metal ions.The iron, aluminum and silicon contents can be reduced to0.4mg/L,3.5mg/L and9.8mg/L,respectively, when pH value of the extracting solution is adjusted to be5.5by ammonia. Thecalcium and magnesium contents can be decreased to203mg/L and445mg/L, respectively,when adding ammonium fluoride whose molar amount is1.2times of the total amount ofcalcium and magnesium. Heavy metal ions are removed by a method of barium sulfideprecipitation and the residues of them are0.31mg/L under the optimal conditions: reactiontemperature of50°C, pH value controlled in the range of4.2~4.5, the molar ratio of bariumsulfide and heavy metal ions of1∶1, reaction duration of2h.Mn3O4is derived from manganese sulfate solution purified. Ammonia is added into themanganese sulfate solution to hydrolyze and precipitate manganese ions, and the obtainedprecipitate manganese hydroxide is oxidized by air, resulting in Mn3O4powder. The resultsshow that the optimal pH value of the manganese ion solution is9.5for hydrolysis andprecipitation. Manganese and ammonia ions will form coordination compounds which canreduce precipitation rate of manganese ions if the pH value exceeds9.5. The hydrolysate ofmanganese ions is mainly manganese hydroxide, frequently containing a small amount ofMn2(OH)2SO4. The main reason of excess content of sulfur in the product is that Mn2(OH)2SO4is hard to be oxidized. Sulfate will be released and the sulphur content is decreased whenammonium chloride is added in this solution. The results show that the optimal air oxidationprocess of preparing Mn3O4is as follows: pH value of9.5for precipitating manganese ion, andoxidation temperature of40℃, ammonium chloride used as the catalyst. The powder product,exhibiting a manganese content of71.26%, a specific surface area of12.52m2/g, a particle sizeof less than200nm and a sulfur content of0.016wt%, reaches the national standard of high-purity Mn3O4.