Research On Crystallization Kinetics And In-Situ Modification Of Magnesium Hydroxide

Magnesium hydroxide,as an important chemical raw material,is widely used in the pharmaceutical,chemical,environmental protection,aerospace,food and other industrial fields.It is commonly used as flame retardant in polymers and inorganic materials,acidic waste liquid neutralizer for regional environmental protection and raw materials for the production of magnesium oxide.In many fields of application,magnesium hydroxide is commonly used as a flame retardant for polymeric materials,but in China magnesium hydroxide prepared still has problems of non-uniform particle size distribution,high production cost,high specific surface free energy and severe agglomeration among particles,resulting in a very poor dispersion in the matrix of organic materials.Therefore,it is a hot spot to explore a better way to produce magnesium hydroxide inorganic flame retardant with good performance in current research and development.In this paper,sodium hydroxide and magnesium nitrate hexahydrate were used as raw materials,and polyethylene glycol was used as modifier.In-situ modification of the liquid phase direct precipitation method was used to prepare magnesium hydroxide with smaller particle size and good dispersibility.Using the concentration detection method,the influence of the reaction temperature,the initial reagent concentration of the raw material,the type and amount of modifier on the crystallization reaction process during the crystallization reaction of magnesium hydroxide was investigated by measuring the conductivity in the solution.The results showed that nucleation and growth rate constants in the reaction process gradually increased with the increase of the initial reagent concentration in a special reaction temperature,and when the initial reagent concentration was a certain value,nucleation and growth rate constants also increased with the increase of reaction temperature.In crystallization process,both of nucleation and growth rate increased with the increase of the amount of polyethylene glycol in a special molecular weight of polyethylene glycol.When the amount of polyethylene glycol was a constant,nucleation rate increased and growth rate firstly decreased and then increased with the increase of molecular weight of polyethylene glycol.The nucleation rate constant was much larger than the growth rate constant in crystallization process,so the reaction rate in crystallization process was basically based on the growth rate.The experiment also found that the increase of temperature led to the decrease of the specific surface Gibbs function,the activation energy during the crystallization reaction was independent of the initial reagent concentration,and the average nucleation activation energy of crystals was greater than the average growth activation of crystals.It showed that the activation energy decreased with the reaction rate decreasing.Effects of stirring speed,reaction temperature,reaction time,initial reagent concentration of magnesium ion and the dosage of PEG6000 on the particle size and dispersibility of magnesium hydroxide were investigated by single-factor experiments.The optimized experimental conditions of magnesium hydroxide obtained by orthogonal experiment were as following: stirring speed was 600 r/min,reaction temperature was 343.15 K,reaction time was 45 min,initial reagent concentration was 1.0 mol/L and the dosage of PEG6000 was 4%.The average particle size of magnesium hydroxide prepared is about 0.199 ?m and the dispersity is also good.The unmodified and modified magnesium hydroxide were characterized by FTIR,X-ray diffraction(XRD)and scanning electron microscopy(SEM).The results showed that the modified magnesium hydroxide product particle size was smaller,the crystal shape was more regular and its dispersibility in the organic matrix liquid paraffin was also improved.The study of the crystallization kinetics of magnesium hydroxide under low concentration conditions provides certain control strategies and theoretical guidance for the preparation of modified magnesium hydroxide under high concentration conditions.The research on the modified preparation process of magnesium hydroxide under high concentration conditions provides a basis for the application of magnesium hydroxide in organic materials.