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Implementation of membrane technology in a base metal refinery
Mocke, Franco Johan
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In this study, the implementation of membrane technology at Anglo Platinum’s base metals refinery to separate acid from metal containing solutions was investigated. The refinery includes a circuit known as the “sulphur removal section”, where the acid in the spent nickel electrolyte is neutralized with caustic soda to remove the excess sulphur from the overall process. Reagent costs associated with acid neutralisation, result in high operating expenditures. An alternative process route is required to improve efficiencies and stay competitive. Nanofiltration was investigated to separate acid from nickel, with the aim of recovering the acid and thereby reducing the need for expensive neutralisation. The objectives of this study were twofold: (1) investigate and simulate the current base metals refinery, and (2) use the understanding and process know-how to investigate the use of nanofiltration by modifying the simulation to include for this technology. The modified process simulation was then used to evaluate the type of membrane required for technical viability. The process investigation of the refinery proceeded with literature studies done on base metals recovery process, chemical reactions and design criteria applicable to the process. A simulation of the base metals refinery was undertaken in Aspen Plus using the information established in the process investigation. The simulation provided insight into the operational issues across the flowsheet, and identified key areas of the process which were sensitive to parameter changes in the sulphur removal section. Areas which were impacted were the electrowinning and copper removal section. The simulation therefore provided a useful tool to predict process variabilities as a result of plant modifications. The investigation into nanofiltration found that it can successfully be used to separate metal ions from acid, subject to the constraints of metal ion concentrations. Pre-treatment of the nickel spent electrolyte was required to remove most of the sodium sulphate in solution, since this can cause fouling and thereby degrade membrane performance. For this reason, a cold crystallization process was introduced for the removal of sodium sulphate. However the sodium removal process caused the sodium sulphate levels in the electrowinning feed to drop below 100 g/l. Therefore minor modifications had to be made to the electrowinning pre-treatment process. The nanofiltration process itself consisted of a series of six nanofiltration stages with dilution of the interstage feed to allow the system to operate below osmotic pressure and wash out all the acid from the system. The modified simulation including the new sulphur removal circuit (nanofiltration process) was completed by integrating the current base metals refinery simulation with the new sulphur removal process, thereby providing a tool where different membrane characteristics could be varied to enable the performance of the overall process to be evaluated. The membrane parameters varied were the nickel rejection, the sodium rejection and the acid rejection. The simulation predicted that each of the cases which varied the mentioned parameters would be technically feasible, although not necessarily economically feasible. The process was most sensitive to acid rejection. The key variables were the amount of water used for dilution, and the membrane size. An exponential distribution was present for the sensitivity of membrane size versus acid rejection; thus realistic membrane sizes can only be achieved if the acid rejection is -100% or less. Furthermore, the addition of dilution water results in the nickel being washed out with the acid, despite nickel rejection being in the region of 99.5%. This demonstrates the importance of the membrane nickel rejection to be as high as possible.
- Engineering