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Novel technology for recovery of water and solid salts from hypersaline brines: eutectic freeze crystallization
Expanded Title:Two major problems currently facing South African water users are the declining availability of sufficient quantities of water and the deterioration of the quality of the available water. However, with the increasing use of water recycling, the result has been an increased generation of inorganic brines and concentrates. Eutectic freeze crystallization (EFC) offers a novel, sustainable method for treating brines and concentrates that were previously regarded as recalcitrant due their complex nature and were consequently discharged to evaporation ponds. With EFC, pure water and pure individual salts can be recovered, thereby making a significant leap towards achieving zero effluent discharge. Eutectic freeze crystallization has been shown to be effective in separating a single salt and water, but has yet to be applied to complex hypersaline brines that are typical of reverse osmosis retentates in South Africa. Thus, the aim of this research was to investigate the applicability of EFC to the hypersaline brines and inorganic effluents produced by industries. The experimental work aimed at investigating the effect of complex aqueous chemistry and impurities on the EFC process. The presence of impurities, even in small concentrations, had a significant depressing impact on the eutectic temperature of the binary system. Maintaining a critical solid mass content i.e. the amount of ice and salt crystals in the reactor was found to be of significant importance as it directly affected the purity and yield of the crystalline products. Thermodynamic modelling of the effects of salts on eutectic temperatures was carried out and demonstrated that, at these relatively low concentrations, the ice always crystallizes first, followed by the higher hydrated salts. No significant shifts in salt freezing points were observed due to the relatively low concentration of salts in the retentate. Experimental studies were carried out on synthetic brines to establish the eutectic temperatures and compositions. A preliminary economic evaluation was conducted to provide an approximation of the expected operating and capital costs associated with using EFC. These were compared to triple-effect evaporative crystallization (EC) using two brines broadly representative of typical South African industrial brines i.e. consisting of Na2SO4 and NaCl. A basis of 100m3/day of brine was used. The operating cost calculated for using EFC to treat Brine 1, without heat integration, with a cooling requirement of 534kW was R28/m3. In contrast, the operating cost for a triple-effect EC process to treat Brine 1 was R132/m3. The operating cost calculated for using EFC to treat Brine 2, without heat integration, with a cooling requirement of 556kW was R29/m3. In contrast, the operating cost for a triple-effect EC process to treat Brine 2 was R126/m3. Hence, the operating cost savings of using EFC over EC are 79% and 76% for Brine 1 and Brine 2 respectively. Eutectic freeze crystallization offers an innovative solution for the treatment of hypersaline brines. It is a technology that can be used either in isolation or in conjunction with other water treatment processes such as reverse osmosis, towards achieving zero effluent discharge sustainably. Future studies will focus on further refining the understanding of the scientific fundamentals together with investigating key operating parameters that will enable the process to be tested at pilot scale.
Date Published:01/08/2010
Document Type:Research Report
Document Subjects:Wastewater Management - Industrial, Sanitation - Hygiene, R & D - Information technology
Document Keywords:Guidelines, Operation and Maintenance, Technology, Water Quality
Document Format:Report
Document File Type:pdf
Research Report Type:Standard
WRC Report No:1727/1/10
ISBN No:978143120006
Authors:Lewis A; Nathoo J; Randall D; Zibi L; Jivanji R
Project Leader:Lewis A
Project No:K5/1727
Organizations:Crystallization and Precipitation Unit Department of Chemical Engineering University of Cape Town
Document Size:1 912 KB
Attachments:Executive Summary 1727.pdf
Table of contents 1727.pdf
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