Experimental Study On Continuous Preparation Of Magnesium Hydroxide With Sodium Hydroxide Method

There are has abundant salt lake resources in China for potassium fertilizer production.In the process,a large amount of magnesium chloride（Mg Cl₂）is produced simultaneously,and this byproduct has not been fully utilized.To prepare magnesium hydroxide（Mg（OH）₂）and magnesium oxide（Mg O）compounds which can be widely used in various industrial fields,may provide an important way to utilize magnesium chloride.Among the methods for preparing Mg（OH）₂with Mg Cl₂as a raw material,the method of using sodium hydroxide（Na OH）complies with the concept of circular economy and has received widespread attention.However,because the reaction process is rapid and difficult to control,produeced Mg（OH）₂is usually too small,resulting in difficult filtration and high chlorine content in the products,which affects the product quality.In this paper,the process with continuous feeding has been studied and the products of Mg（OH）₂with large particle size and easily to be filtered were produced,based on the optimization of the measurement conditions for the laser particle size analyzer,the kinetics of Mg（OH）₂crystallization,and the research on the relation between Zeta potential and filtration performance.The detailed contents are as follows:A method for Mg（OH）₂particle size measurement using Mastersizer-3000 particle size analyzer was established.The influence of circulating pump speed,ultrasonic time,dispersant type and its dosage on the measurement results of Mg（OH）₂particle size distribution,were studied.The results show that the appropriate conditions are 2000 r·min^-1for circulating pump speed,10 min for ultrasonic time,sodium polyacrylate as dispersant,and a mass concentration of 1%for dispersant in the test solution.The particle sizeof Mg（OH）₂measured in the experiment was the secondary particle size.The kinetics of Mg（OH）₂crystallization in the reaction process with continuous feeding were studied.The particle size distribution of Mg（OH）₂product at different reaction times and sampling positions were measured,and the time required for the reaction system to reach a steady state was determined to be three times the average residence time of the material in the reactor.The kinetics data of Mg（OH）₂crystallization under different experimental conditions were obtained using the continuous steady-state method,and a mathematical model was established based on the particle number balance equation combining with the dynamic crystal nucleation and crystal growth equations.The results show that the nucleation rate equation of Mg（OH）₂is,and the growth rate equation is.The relative supersaturation index of magnesium hydroxide crystal nucleation rate and growth rate are 3.244 and 0.790,respectively,and lower supersaturation was more conducive to obtaine larger particle size Mg（OH）₂products.The influence of Zeta potential changes on the particle size and filtration performance of Mg（OH）₂during the process was experimentally studied.The results show that p H value corresponding to the isoelectric point of magnesium hydroxide is 11.20.When Zeta potential value is closer to the isoelectric point,the particle size is larger.The effects of OH^-and Mg²⁺in the solution on the sedimentation and filtration rate of Mg（OH）₂slurry were separately investigated,and the results show that when Mg²⁺is mainly on the particle surface,a positive potential value close to zero is more favorable to sedimentation and faster filtration.The optimization of operation conditions for Mg（OH）₂preparation with continuous feeding was studied.Based on the results of Mg（OH）₂crystallization kinetics and Zeta potential experiments,the experimental condition range was determined.After the reaction system reaching steady state,the influence of different conditions on the particle size distribution,particle size,and filtration performance of Mg（OH）₂products was investigated.The optimal operating conditions obtained are as follows:reaction temperature of 60°C,the feeding rate ratio of Na OH to Mg Cl₂is 1.9 and stirring speed of400 r·min^-1.Under these conditions,magnesium hydroxide products with larger particle size and better filtration performance can be obtained.