Preparation And Drying Kinetics Of Nanosized Magnesium Hydroxide

Magnesium hydroxide is widely used as flame-retardant, precursor of nanosized magnesium oxide ceramic, neutralizer in environment protecting field, desulfurizer in flue gas and so on. With the development of nanotechnology, studies on the preparation and application of nanosized magnesium hydroxide are rapidly expanded. And drying is one of the most important parts of the preparation of high quality nanosized magnesium hydroxide in the liquid phase method.Here, monodisperse magnesium hydroxide nanoparticles were prepared in the liquid phase method with the magnesium chloride and basic solution. Through the single factor experiments, the effects of solvent and basic precipitation, the initial concentration of the reaction and feeding methods were studied. The best condition of the preparation of magnesium hydroxide nanoparticles in the monofactor experiments are as followed: magnesium chloride hexahydrate and sodium hydroxide as raw materials, water/glycol (20/80, v/v) mixture as solvent, initial concentration of Mg²⁺ at o.1mol/L, through the positive charging way which dropping sodium hydroxide into the magnesium chloride solution. Under the conditions, monodisperse magnesium hydroxide nanoparticles were obtained with average nucleus size of 36nm.Magnesium hydroxide nanorods and nanosheets were prepared by precipitation conversion method; the drying curves and drying rate curves of three kinds of nanosized magnesium hydroxide were studied and analyzed by thin layer drying models. The drying kinetics of magnesium hydroxide is investigated using both isothermal and non-isothermal methods of kinetic analysis. The results of the drying experiments indicate that the drying rate curves of three kinds of nanosized magnesium hydroxide are similar to each other, and the drying curves all have a set-up period and a falling rate drying period; The Logarithmic model were chosen to be the thin layer drying model of nanosized magnesium hydroxide to describe the drying curves. The diffusion activation energy of moisture transfer in monodisperse magnesium hydroxide nanoparticles, magnesium hydroxide nanorods and nanosheets are separately 26.5kJ/mol, 25.3kJ/mol and 21.53kJ/mol; G(α) = [(1/(1+α))^1/3-1]² =kt is chosen to be the kinetics mechanism function of isothermal drying kinetic analysis. The apparent activation energy of the drying of monodisperse magnesium hydroxide nanoparticles, magnesium hydroxide nanorods and nanosheets are respectively 25.2kJ/mol, 23.9kJ/mol and 24.0kJ/mol. The In(?)-(?) urves of non-isothermal method have to be analyzed bysections. When T≥108℃,the kinetics mechanism function is G(α) =1-(1-α)^1/2,the apparent activation energy is 6.3kJ/mol.