PRECIPITATION SELECTIVITY OF BORON COMPOUNDS FROM SLAGS

摘要

The effects of slag composition, additive agents and heat treatment on the crystal morphology, the precipitation behavior and the efficiency of extraction of boron (EEB) from slags were investigated. The EEB varied with the slag composition; the further the slag composition deviated from the line between the borate 2MgO.B_2O_3 and the silicate 2MgO.SiO_2 in the MgO-B_2O_3-SiO_2 system, the lower the EEB. The EEB varied with the temperature of heat treatment, the highest EEB appeared at l l00℃. During the solidification of molten slags the behavior of the boron compounds involved two steps: phase separation and crystal precipitation. The micrographs of the phase separation were described using the fractal dimen- sion which was 1.87-1.92. The result of computer simulation indicated that the mechanism of the phase separation was Spinodal decomposition. The fractal dimension of the crystalline phase 2MgO.B_2O_3 whose micrographs showed fractal characteristics was 1.62. The result of computer simulation showed that the growth of the borate 2MgO.B_2O_3 was dependent on the DLA model. The additives TiO_2 and MO. decrease the activation energy of crystallization and make the boron compounds precipitate in the form of the crystalline phase more easily, thus the EEB was increased. The structure of slag was modified by ad- dition of TiO_2. A lot of [B_2O_5] and [BO_3] clusters, which were favorable to make crystals of 2MgO.B_2O_3 and 3MgO.B_2O_3 precipitate, were formed in the slags.