Preparation and crystallization kinetics of micron-sized Mg(OH)₂ in a mixed suspension mixed product removal crystallizer

Abstract

Magnesium hydroxide is an important chemical, and is usually obtained from seawater or brine via precipitation process. The particle size distribution of magnesium hydroxide has great effects on the subsequent filtration and drying processes. In this paper, micron-sized magnesium hydroxide with high purity, large particle size and low water content in filter cake was synthesized via simple wet precipitation in a mixed suspension mixed product removal (MSMPR) crystallizer. The effects of reactant concentration, residence time and impurities on the properties of magnesium hydroxide were investigated by X-Ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Malvern laser particle size analyzer. The results show that NaOH concentration and residence time have great effects on the water content and particle size of Mg(OH)₂. The spherical Mg(OH)₂ with uniform diameter of about 30 μm was obtained with purity higher than 99% and water content less than 31%. Furthermore, the crystallization kinetics based on the population balance theory was studied to provide the theoretical data for industrial enlargement, and the simulation coefficients (R²) based on ASL model and C-R model are 0.9962 and 0.9972, respectively, indicating that the crystal growth rate of magnesium hydroxide can be well simulated by the size-dependent growth models.

Keywords

magnesium hydroxide / precipitation / micron-sized / crystallization kinetics

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Acknowledgements

This work was supported by the Shanghai Natural Science Foundation (No. 09ZR147900) and the Program for New Century Excellent Talents in University (NCET-08-0776).

Nomenclature

a, b	Equation parameter
B0	Nucleation rate, No.·m-3·h-1
CV	Coefficient of variation
dp	Particle diameter in microns corresponding to the pth percentile cumulative volume, μm
d50	Mean diameter of the particles, μm
D[4,3]	Volume-weighted mean particle size, μm
fi	Volume fraction of particle size i interval in the whole sizes, %
G	Crystal growth rate, μm·h-1
G0	Crystal nuclei growth rate, μm·h-1
L	Crystal particle size, μm
ΔL	Difference between particle size Li+1 and Li, μm
Mi	Suspension density of magnesium hydroxide slurry, g·cm-3
n0	Nuclei population density, No.·m-3·μm-1
ni	Crystal population density of ith size fraction, No.·m-3·μm-1
R	Relative coefficient
Vi	Single crystal size volume with the diameter\overline{L_i}={\displaystyle \frac{L_{i+\mathrm{1}}+L_i}{\mathrm{2}}}, μm-3
ρ	Density of magnesium hydroxide, g·mL-1
τ	Residence time, h

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