| Summary: | This thesis concerns the selective recovery of low concentrations of Zn from waste materials using a number of integrated unit operations to produce a high purity material of high value. The emphasis of the processes investigated is to carry out these processes with high efficiency and low energy. A literature survey revealed that a promising hydrometallurgical approach was to concentrate Zn from low concentration using polymer enhanced ultrafiltration (PEUF) using polyethylenimine (PEI) as filterable metal chelator; followed by alkaline precipitation of the chloride salt to form Zn(OH)2; the zincate salt in alkaline solution was then electrowon to give pure Zn metal. Two practical areas of work central to this process were investigated with aims of devising mathematical descriptions of these processes to allow process prediction and optimisation of these operations. The first was to model the process using PEI, as water-soluble polymer that binds Zn efficiently. The process involved: the binding of zinc ions to polymer; concentration of the PEI-Zn complex; followed by decomplexation with acid to regenerate the polymer with alkali and concentrated zinc chloride solution. An MS Excel model based on mass balance, PEI-Zn binding phenomena and the separation of the polymer/zinc complex in the membrane system was developed. Each stage of the process was satisfactorily modelled and it was validated by a series of studies related to the complexation phenomena using Langmuir adsorption equilibria. A series of process variables were investigated to allow an assessment the affects of the zinc concentration and its decomplexation from PEI. The model showed a good agreement with observed data. The complex relationship between polymer concentration, complex concentration processes, washing and polymer regeneration on membrane productivity was then investigated and optimised. Under the conditions employed 6 g/L PEI concentration to give maximum ZnCl2 production based upon membrane productivity. Increasing the washing feed/retentate ratio could decrease the membrane productivity and the final concentration ZnCl2. The electrowinning process of zinc from an alkaline zincate solution at stainless steel electrodes was then investigated. Alkaline electrowinning involved dissolving specific amounts of ZnO in 4 mole/L NaOH and the resultant solution was then subjected to a galvanostatic electrowinning process. The effects of initial zinc concentration, current density and NaOH concentration on the process were examined including electrical energy usage, zinc recovery and current efficiency relationships. The experimental results showed that the highest recovery rate could be obtained when varying current densities at 2000 A/m2 and initial Zn concentration of 20 g/L. The experimental results were then used to test the predictions of concentration-time behaviour of an MS Excel based mathematical model of metal deposition kinetics. Reasonable agreement between experimental data and model predictions using a limited electrode surface area model was achieved. Based on the results obtained in this work, 88% of zinc from alkaline solution (20 g/L) with 95% current efficiency and energy consumption of 0.77 kWh/kg zinc were achieved. Using these studies, an integrated zinc recovery process from low grade zinc tailings was investigated further with primary aim of determining the economics and impacts of the process. The leaching efficiency; metal oxide dissolution and other operation to reduce waste (electrodialysis) were incorporated into an MS Excel model so that the overall process could be evaluated. The process was capable determining the production rate at break-even point based on extraction efficiency and Zn ore content. For example, a profit of about £1.22 M/year was made over 2.2 years with an extraction efficiency of 50% from an ore input of 1000 t/day containing 2% Zn. This model provides a tool for further analysis of the process and will provide good targets for process development and improved process feasibility.
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