The recovery of sodium hydroxide from cotton scouring effluents.
Simpson, Alison Elizabeth.
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This dissertation describes the characterisation of, and development of a novel integrated waste management strategy for, hydroxide scouring effluents produced during cotton processing. Such effluents are typical of mineral salt-rich waste waters which are not significantly biodegradable in conventional treatment plants. The proposed strategy focuses on two complementary concepts: process-oriented waste minimisation adopts a systematic approach to identifying potential problems and solutions of waste reduction in the manufacturing process itself; while add-on controls reduce the impact of the waste after it has been generated, by recycling and treatment. The basic procedures for ensuring effective water and chemical management within the scouring process are described. Examples are given of factory surveys, which have resulted in significant chemical and water savings, reduced effluent discharge costs, maximum effluent concentration, and minimum pollutant loading and volume. Pilot-plant investigations demonstrate the technical and economic feasibility of a four stage treatment sequence of neutralisation (using carbon dioxide gas), cross-flow microfiltration, nanofiltration and electrochemical recovery to remove colour and impurities from the scouring effluent and produce directly reusable sodium hydroxide and water. Fouling and scaling of the cross-flow microfiltration, nanofiltration and electrochemical membranes are minimal and reversible if the operation is carried out under carefully selected conditions. A long anode coating life is predicted. Current efficiencies for the recovery of sodium hydroxide (up to 20 % concentration) are 70 to 80 % and the electrical power requirements are 3 500 to 4 000 kWh/tonne of 100 % NaOH. Pilot-plant trials are supplemented by extensive laboratory tests and semi-quantitative modelling to examine specific aspects of the nanofiltration and electrochemical stages in detail. Electromembrane fouling and cleaning techniques, and other anode materials are evaluated. The effects of solution speciation chemistry on the performance of the nanofiltration membrane is evaluated using a combination of speciation and membrane transport modelling and the predicted results are used to explain observed behaviour. Based on the results of pilot-plant trials and supplementary laboratory and theoretical work, a detailed design of an electrochemically-based treatment system and an economic analysis of the electrochemical recovery system are presented. The effects of rinsing variables, processing temperatures, and background rinse water concentrations on the plant size requirements and capital costs are determined. The implementation of the waste management concepts presented in this dissertation will have significant impact on water and sodium hydroxide consumption (decreasing these by up to 95 and 75 % respectively), as well as effluent volumes and pollutant loadings.