Solid-liquid phase equilibrium relationship of high salt wastewater of coal chemical industry and mechanism of crystallization and scaling process

Abstract

Crystallization and scaling problem in the process of falling film evaporation was one of the key problems restricting the evaporation process in near-zero discharge of coal chemical wastewater. We studied the phase equilibrium data of the ternary system Na+, Mg2+//Cl--H2O at 303.15–373.15 K. And, the influencing factors of crystallization and scaling were investigated, the flow characteristics of high-salt wastewater descending film were analyzed and the scaling sites were predicted. Results showed the system was a simple ternary system. Both w(NaCl) and w(MgCl2) were linearly correlated with the temperature. The higher MgCl2 contents, the faster rates of crystallization and scaling; temperature and scaling rate showed a positive correlation; the fitted curve of scaling under different roughness was in accordance with y=ax+b. The flow characteristics in evaporation tubes of four high-salt wastewaters (10% NaCl+9, 11% CaCl2, 10% NaCl+9, 11% MgCl2) were simulated. The risk of precipitation scaling was obtained at 591 mm, 667 mm, 772 mm and 938 mm, which was also verified by falling film evaporation experiments. The phase equilibrium relationship was creatively combined with numerical simulation in this paper. Meanwhile, phase equilibrium data was introduced in the study of crystallization and scaling process. This study provided a theoretical basis for the reasonable control of salt scaling in evaporation process of coal chemical industry. It is benefit to ensure the efficient operation of the evaporator, achieve the recycling of inorganic salt products and maximize economic benefits. The goal of "near-zero discharge" would be achieved eventually.