Tuning the diffusion dialysis performance by surface cross-linking of PPO anion exchange membranes--simultaneous recovery of sulfuric acid and nickel from electrolysis spent liquor of relatively low acid concentration

The results of the development of the industrial diffusion dialysis technology and the unit based on it for sulfate acid recovery from nickel electrolysis waste have been considered. Unlike most acid recovery systems, this system has a relatively low acid concentration and the main aim is to recover both nickel and acid sulfate by recycling the waste and the recovered acid to the respective steps of electrolysis process. So the waste volume control seems to be the most important thing. To satisfy with this new request, the membrane is surface-cross-linked with aqueous ammonium to decrease waste volume expansion caused by the water osmosis. The results showed that the best membrane for such operation is the one that cross-linked at least 8 h with a volumetric expansion factor (volumetric ratio of waste to feed) less than 1.1. Pilot diffusional runs were conducted with this membrane at various feed flow rate and flow ratio of stripping water and feed. After comprehensively considering all factors, the range of feed flow and the flow ratio has been recommended to be 1.2-1.8 l/h and 1.05-1.1, respectively. Under these conditions, nickel leakage can be controlled within 4% and the acid recover ratio can attain as high as 66-72%. The recovered acid can be recycled to the back-extraction step by mixing it with high concentration acid and the waste recycled to the initial leaching stage by adjusting the acid concentration to recover valuable metal nickel and the residual acid. Therefore, the new technology discards nothing and shows many advantages whether in environmental aspect or economical aspect and thus should be deserved attention.