(376d) Crystallization Separation of Na2so4·10H2o and NaCl from Coal Gasification Water Based on Pseudo-Ternary System (Na2SO4+ NaCl +H2O) Phase Equilibria

Due to high contents of Na₂SO₄ and NaCl, coal gasification wastewater (CGW) is of poor biodegradability and great difficulty to treat. At present, CGW is often delivered to evaporation ponds, producing huge amounts of mixed salts with impurities. With high water solubility, mixed salts can lead to secondary pollution and thus are identified as hazardous wastes which need further treatment. Under this background, a fractional crystallization method for obtaining high-purity Na₂SO₄·10H₂O and NaCl from CGW was developed based on phase equilibria.

The phase equilibria of pseudo-ternary system (Na₂SO₄ +NaCl+H₂O) in CGW were investigated at T= (268.15 to 373.15) K. The CGW was provided by a gas-making plant in the northwest of China. The solubility of Na₂SO₄and NaCl together with the density of the equilibrium liquid phase was determined by the isothermal solution saturation method. The equilibrium solids were investigated by the Schreinemaker’s method of wet residues and X-ray powder diffraction. According to the experimental data, the phase diagrams and the diagrams of density versus composition were plotted and turned out to exhibit diverse characteristics in different temperature ranges, including (313.15 to 373.15) K, (293.15 to 303.15) K, (273.15 to 288.15) K and at 268.15K.

On the base of phase equilibria, a fractional crystallization method was developed to separate pure Na₂SO₄·10H₂O and pure NaCl. Purity analysis of the obtained products was carried out. It was found that both Na₂SO₄·10H₂O product and NaCl product were of high purity (≥98.5% by mass fraction) and their heavy metals contents were far below identification standards for hazardous wastes. The obtained products could be used as raw materials by downstream industries. The fractional crystallization method showed great promise for the treatment of CGW and provided an important approach for solving the pollution problems caused by coal industries in China.