The MgO-B~2O~3-28%MgCl~2-H~2O supersaturated solutions containing 28% MgCl~2 and different mole ratio MgO:B~2O~3 were prepared and kept at 20℃. The crystallization processes of Mg borates from the solutions have been studied with the kinetic method. When a solid began to crystallize out for determing their densities, pH values and compositions until the density did not change basically. Finally, the solid phase was seperated, washed, dryed at room temperature and identified by IR spectra, powder X-ray diffraction, thermal analysis and chemical analysis. With the aid of simplex method and digital integration as well as Runge-Kutta digital solution of differential equation system, we fitted the experimental data and evaluated the kinetic parameters of the chosen models with computer. The crystallization kinetic equations have been given. The calculated results show that when MgO:B~2O~# in initial solutins are 1.5:1, 1:2, 1:4,1:6 and 1:9, the mechanism of crystallization are polynuclear layer controlled growth. When MgO:B~2O~3 are 3:1 and 2:1, the mechanism of crystallization are mononuclear layer controlleld growth. We also disscussed the crystallization reaction processes. In this paper, we obtain the liquid-solid phase diagram of supersaturated solutions of MgO--B~2O~3-28%MgCl~2-H~2O system at 20℃ for the first time and call it a phase diagram of thermodynamic non-equilibrium state. There are six solid phases in the diagram. They are H~3BO~3, MgO.3B~2O~3.7.5H~2O, MgO.3B~2O~3.7H~2O, 2MgO.2B~2O~3.MgCl~2.14H~2O, 3Mg(OH)~2.MgCl~2.8H~2O and 5Mg(OH)~2.MgCl~2.8H~2O respectively.

Key words: WATER, MAGNESIUM OXIDE, MAGNESIUM CHLORIDE, BORON OXIDE, CRYSTALLIZATION KINETICS, PHASE DIAGRAM