Simulation and control of a small scale sodium sulphate salt splitting plant

Abstract

Base metal refineries produce sodium sulphate solutions as a waste product in the precipitation step. The solution that is formed generally has a concentration of around 100 g Na2SO4/L. Discharging solutions with such high concentrations into the environment can upset the biodiversity of nearby water sources due to its high salinity and, thus, the solution should be treated. Currently, the most widely used treatment method is evaporative crystallisation. Evaporative crystallisation produces potable water, but the large capital cost, high energy consumption and the production of a salt with low to zero economic value establish the need for a better alternative. A continuous electrodialysis with bipolar membranes integrated with disc reverse osmosis setup was designed, built and used to treat a sodium sulphate solution to produce potable water, sodium hydroxide and sulphuric acid. The study aimed to develop a process with a robust control system and to minimise the production cost of sodium hydroxide via manipulation of the salt concentration, temperature and current density. The salt concentration, temperature, current density and flow rates were varied to determine their effects on the performance of the setup. The control system was able to keep the parameters at their respective setpoints throughout an experiment. Circulation flowrate tests showed that mass transfer limitations were negligible at the high flow rates used in this study. A maximum salt splitting rate of 1.08 kg/h/m2 at a power density of 1.76 kW/m2 was found, as well as a minimum specific energy consumption of 3.19 kWh/kg at a power density of 1.81 kW/m2. The sodium hydroxide and sulphuric acid produced had concentrations of 66.5 g/L and 53.7 g/L and purities of 90.5% and 74% respectively, with sodium sulphate being the main impurity in both cases. The lowest cost to produce sodium hydroxide, accounting for equipment cost, operational cost and electricity usage was found to be €0.78/kg at a sodium sulphate concentration of 150 g/L, an operating temperature of 35°C and a power density of 1.81 kW/m2. Using these results, together with a fair cost analysis for each individual case, a decision can be made whether it is economically viable to use electrodialysis with bipolar membranes integrated with disc reverse osmosis to treat a sodium sulphate waste stream.