Waste salt separation by antisolvent crystallization process: Mechanism study of Na2SO4-NaCl-solvent ternary phase diagrams and its life cycle assessment

Abstract

In order to reutilize waste salt resources effectively, antisolvent crystallization (ASC) method was introduced to separate some inorganic salts with high purity from pyrolyzed waste salt of chemical industry. Methanol, chosen as the antisolvent, was added to saturated mixed salt solutions with stirring to gradually crystallize Na₂SO₄, and then NaCl was obtained through the evaporative crystallization of the filtrate. ASC process could effectively separate Na₂SO₄ of a purity above 90 % from the simulated salt mixture when the ratio of NaCl to Na₂SO₄ was lower than 2.5. The recovery and reuse of methanol in ASC process was also investigated. Applying ASC process to the waste salt samples with the Na₂SO₄ contents of 73.6 %, 21.66 %, and 14.39 %, the product purity met the industrial standards (Na₂SO₄ > 98 %, NaCl>98.5 %), the yields remained relatively stable around 90 % for NaCl, and 98.79 %, 81.92 % and 60.62 % for Na₂SO₄, respectively. By constructing the ternary phase diagram of Na₂SO₄-NaCl-solvent systems, the solvent benefited for the preferential crystallization of Na₂SO₄ was evidenced in the order: methanol > recovered methanol > methanol-water(1:1) > recovered methanol-water(2:1) > water. Due to the extremely low solubility of Na₂SO₄ compared to NaCl in methanol-water, Na₂SO₄'s water solvation shell was broken and its methanol-water solvation shell cannot formed, which resulted in the crystallized of Na₂SO_4. While NaCl remained in methanol-water solvation shell, achieving effective separation of the mixed salts. Life cycle assessment showed that the ASC process for waste salt with various Na₂SO₄ content has excellent economic (929–1888 CNY/t) and environmental feasibility (847–1968 kg CO₂ eq./t).