CHEMISTRY OF BORATE IN SALT LAKE BRINE .1 7. STUDY ON LIQUID-SOLID PHASE-DIAGRAM OF THERMODYNAMIC NONEQUILIBRIUM OF MGO-B2O3-28-PERCENT-MGCL2-H2O SYSTEM AT 20-DEGREES-C

The MgO-B2O3-MgCl2-H2O supersaturated solutions containing 28 % MgCl2 and different mole ratio MgO:B2O3 were prepared and kept at 20-degrees -C. The crystallization processes of Mg borates from the solutions hav e been studied with the kinetic method. When a solid began to crystall ize out from the solution, the solution samples were taken at differen t time for determing their densities, pH values and compositions until the density did not change basically. Finally, the solid phase was se perated, washed, dryed at room temperature and identified by IR spectr a, powder X-ray diffraction, thermal analysis and chemical analysis. W ith 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:B2O3 in initial solut ions are 1.5:1, 1:2, 1:4, 1:6 and 1:9, the mechanism of crystallizatio n are polynuclear layer controlled growth. When MgO:B2O3 are 3: 1 and 2:1, the mechanism of crystallization are mononuclear layer controlled growth. We also disscussed the crystallization reaction processes. In this paper, we obtain the liquid-solid phase diagram of supersaturate d solutions of MgO-B2O3-28%MgCl2-H2O system at 20-degrees-C for the fi rst time and call it a phase diagram of thermodynamic non-equilibrium state. There are six solid phases in the diagram. They are H3BO3, MgO. 3B2O3.7.5H2O, MgO.3B2O3.7H2O, 2MgO.2B2O3. MgCl2.14H2O, 3Mg(OH)2.MgCl2. 8H2O and 5Mg(OH)2.MgCl2.8H2O respectively.