Eutectic Freeze Crystallization process for the treatment of ternary textile wastewater concentrates

Abstract

Textile wastewaters contain high levels of sodium chloride (NaCl) and sulphate (Na₂SO₄) salts. Conventional water treatment uses evaporative crystallization (EC) which is an energy intensive process and is only effective in recovering pure water and a mixed sulphate-chloride salt. The mixed salt is contaminated with hardness and colour and cannot be recycled within the plant. This research investigates the technical applicability of using Eutectic Freeze Crystallization (EFC) in the recovery of water (as ice) and Na₂SO₄ salt. The effect of varying the operating temperature on the recovery and the purity of the ice and the salt was investigated using a synthetic textile wastewater stream containing 1.4 wt% Na₂SO₄ and 4.5 wt% NaCl. Thermodynamic modelling software predicted that the nucleation temperature of ice and Na₂SO₄ to be -3.1 ^oC and for NaCl, -21.6 ^oC. The recovery of ice was predicted to be 49% and 79% for Na₂SO₄.10H₂O at -6.5 ^oC. Experimental validation, limited to operating temperatures above -7^oC, showed a cumulative recovery of 42 % of ice with 94 % purity and 51.3 % recovery with 98 % purity for Na₂SO₄.10H₂O at -6.5 ^oC. Sulphate analysis of the residual mother liquor at this temperature showed that 8.6 g of Na₂SO₄.10H₂O was entrapped within ice crystals, thus reducing the recovery of Na₂SO₄.10H₂O and the purity of ice.  EFC is technically applicable for the recovery of ice and a single salt in a ternary system, however separation of the two phases is problematic due to high ice formation. The stream should first be treated with multistage Reverse Osmosis before EFC, so as to concentrate the stream and reduce ice formation.