Effect of sodium chloride on the formation of ice and salt during eutectic freeze crystallization of sodium sulfate with a scenario study of real brine

Abstract

Brine streams from industry is a burden to the environment if not disposed properly. “Zero liquid discharge” (ZLD) is an ideal way of converting saline streams from waste to resource through multiple membrane-based or thermal-based treatment technologies. The utilization of the recovered resource minimizes both costs and environmental impacts. Among the brine treatment technologies, the Eutectic Freeze Crystallization (EFC) is a promising thermal-based technology to obtain water and salt in high purity. EFC technology has higher energy benefit and simpler equipment than conventional evaporation technology. This technology is feasible for continuously recovering pure salt and ice in different stages, and separating them spontaneously. In this study, batch EFC experiment was conducted to determine the solubility curve of Na2SO4 under the different concentrations of NaCl. The effect of NaCl on the solubility of Na2SO4 at the low-temperature interval was investigated. It was found that with the increasing concentration of NaCl, the common ion effect between Na2SO4 and NaCl reduced the solubility of Na2SO4 and depressed the eutectic point (EP) of Na2SO4-H2O system. The decreasing EP showed a linear trend. A thermodynamic model in OLI Studio was used to evaluate the solubility of Na2SO4 as well. The modelling result was compared with the experimental data. The experimental result about the EP of Na2SO4-H2O system has a better agreement to the theoretical value than the eutectic concentration of Na2SO4. Besides, the effect of NaCl was compared with KCl in the OLI Studio, the salting- in effect of KCl promoted the solubility of Na2SO4. A scenario study was carried out to explore a proper process to treat reverse osmosis (RO) concentrate from a demineralised-water-producing (DWP) plant that was rich in Na+, SO42- and Cl-. The thermodynamic model was used to simulate the continuous EFC process and investigate how the different pre-treatment processes affect the recovery of Na2SO4. Four scenarios with different pre-treatment technologies were put forward based on the existing processing facilities: Scenario 1: RO – NF – EFC, Scenario 2: RO – TOC – NF – EFC, Scenario 3: RO – TOC – EFC, Scenario 4: RO – TOC – NF – RO – EFC. All the scenarios could reach more than 92% recovery of H2O. Among the four scenarios, Scenario 4 accomplished the highest recovery of Na2SO4 (98.4%) by -3℃. But the scaling tendency and poor permeate quality of the second RO unit made spontaneously. The gap between ice yield and Na2SO4·10H2O (Mirabilite) yield was significant in all the scenarios. It was suggested only to recover H2O rather than both H2O and Na2SO4, which can be achieved by means of Scenario 1 or 3. In Scenario 2, the close nucleation temperature of ice and mirabilite brought problem for separation work. To conclude, EFC is a newly emerging technology that can achieve a sequential removal of minerals from brine and is sustainable. There are barriers to overcome in impurity studies and those problems deserve attention. Further developments should be stimulated on recovering H2O and salt with an advance retrieving method, for example the design of multi-stage separation process, and the synergistic effect of different ion species and the combined effect of the organic compounds.