Process Research On Preparation Of Magnesium Hydroxide Powder Material From Low-grade Magnesite

China’s magnesite resource reserves are very rich,accounting for about 1/4 of the world’s total reserves.In recent years,with the continuous exploitation of magnesite resources,the reserves of high-grade magnesite are getting smaller and smaller,and a large amount of low-grade magnesite is discarded due to the inability to directly prepare high-grade magnesium products,resulting in a great waste of magnesium resources.How to use low grade magnesite resources to convert them into high value-added products and improve the comprehensive utilization rate is an urgent problem in magnesite industry at present.Magnesium hydroxide is an important inorganic weak alkali compound with strong buffering properties,and also has high activity and good adsorption capacity,and non-corrosive,safe and non-toxic,which has a broad application prospect in the fields of environment,medicine,chemical and metallurgy.In this thesis,a new technology of leaching low grade magnesite to extract magnesium element to prepare magnesium hydroxide by ammonium hydrogen sulfate recycling method is proposed with low grade magnesite as raw material.Firstly,the process of leaching low-grade magnesite with ammonium bisulfate was studied in this thesis,and the results showed that increasing the reaction temperature,reaction time,stirring speed,decreasing the mass concentration of ammonium bisulfate and decreasing the particle size of magnesite could improve the leaching rate of magnesium element in magnesite.The response surface method was used to optimize the leaching process conditions,and the best leaching process conditions were:reaction time 7.7 h,reaction temperature 94 ^oC,NH₄HSO₄mass concentration 19.2%,NH₄HSO₄excess rate18.7%,and the magnesium leaching rate was 95.9%under the best conditions.Secondly,the kinetics of magnesite leaching in ammonium bisulfate solution was studied by shrinking unreacted core model and the leaching slag was characterized.The kinetic parameters and rate control steps of the leaching process were calculated according to the Arrhenius equation.The results showed that the kinetic equation of the leaching process of low-grade magnesite by ammonium bisulfate was in accordance with the surface chemical reaction model with an activation energy of 43.40 k J·mol^-1and a kinetic equation of1-（1-x）^1/3=7.843×10⁵e^-43.40/RTt.Finally,the leachate was cooled and crystallized,and the crystallization product was ammonium magnesium sulfate.The magnesium hydroxide powder material was prepared by hydrothermal method using the obtained ammonium magnesium sulfate crystals reacted with ammonia,and the effects of different hydrothermal temperature,hydrothermal time and ammonia concentration on the particle size and morphology of magnesium hydroxide were investigated,the best process conditions were:hydrothermal time 4 h,hydrothermal temperature 160 ^oC,ammonia mass concentration 25%.The purity of the magnesium hydroxide obtained was 98.062%,and the morphology of the sample was spherical particles composed of regular flakes with small particle size,the average particle size was 8.425μm.