Treatment of industrial wastewater containing sulphates by freeze desalination.
Zikalala, Nomvuselelo
Zikalala, Nomvuselelo
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Abstract
Disposal of multi-component and hypersaline brines generated from mining, energy generation, mineral processing and the pulp and paper industries, are threatening freshwater resources in South Africa. There is currently no environmentally benign and economically viable technology available for the treatment of complex brines. Large volumes of untreated brines are disposed of in evaporation ponds, posing a real threat to groundwater. The feasibility of treating sulphate rich wastewater, landfill leachate and saline Authentic Brine from the mine in South Africa by chemical-free, freeze desalination was investigated using the HybridICE laboratory scale unit. The HybridICE process is a newly developed suspension freeze crystallization technology which employs scraped surface heat exchangers to generate ice from wastewaters and brines which is then separated from the concentrate by means of a static filter The first part of the study aimed at proving that cooling and/or freezing can be used for the recovery of drinking water from sulphate rich solutions associated with various cations. The brine, containing mainly Na2SO4, produced from RO process was cooled after pre-treatment with sodium alkalis to allow Na2SO4.10H2O crystallization. It was found that: (i) If the solution contains only Na2SO4, after pre-treatment with sodium alkalis, it can be removed from solution through cooling down to 0.33 mol/L (31.7 g/L SO4) through crystallization of Na2SO4.10H2O; (ii) the solubility of Na2SO4 upon cooling is influenced by the Cl- concentration; and (iii) The energy required to cool water, containing 100 g/L Na2SO4, from 25°C down to 0°C, amounts to 10.66 kWh/t water. The second part of the study focused on the treatment of landfill leachate by freeze desalination using the HybridICE laboratory scale unit in a 3-stage process. In the first stage, salt removal of 28.9% was achieved by cooling the leachate from 25°C down to -5.4 °C. In the second stage, freeze crystallization was applied for treatment and a crude ice was produced with the TDS reduced from 56 799 mg/L down to 29 173 mg/L in the melted ice. In Stage 3, crude ice produced in Stage 2 was processed further, where freeze crystallization was applied and purer ice was produced where the TDS was lowered from 29 173 down to 2 637 mg/L. The energy utilized to produce 13.17 kg of pure ice was estimated at 4 399 kJ (m.Hv). The third stage of the study focused on evaluating the feasibility of using the HybridICE freeze desalination technology treating a brine salt plume from contaminated groundwater near a mine. In this investigation two series of experiments were carried out. In the first series synthetic brine was prepared to simulate the authentic brine and in the second series of experiments, Authentic Brine from the mine was treated. The Authentic Brine from the mine contained high sulphate and sodium concentrations. Ice recovered from the treatment of synthetic and authentic brines using the HybridICE freeze desalination unit had high purity. The conductivity of the synthetic brine feed was reduced from 17 mS/cm down to 0.62 mS/cm in the melted ice. The conductivity of the authentic brine was reduced from 56.6 mS/cm down to 5.5 mS/cm. The removal of clean ice from the feed brine resulted in a brine with a higher salt concentration, coupled with a smaller brine volume. The HybridICE process was found to be a viable desalination technology in terms of the quality of water produced.
Description
Submitted in fulfilment of the requirements for the degree, Magister Technologiae: Environmental, Water and Earth Sciences in the Department of Environmental, Water and Earth Sciences
Faculty of Science at the Tshwane University of Technology.
Date
2018-10-01
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Tshwane University of Technology
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Keywords
Freeze desalination, HybridICE technology, Sulphate-rich wastewater, Landfill leachate