Recycling of spent nitric acid solution from electrodialysis by diffusion dialysis

Laboratory study of recovering HNO3 solution from acidic effluents discharged by an electrodialysis plant for extracting lithium from concentrated brine at East Taijinar Salt Lake, China, was conducted using diffusion dialysis (DD) with homogenous anion exchange membrane (DF120). The effects of concentrations of nitric acid and metal ions (including Li, Na, K, Mg, Ca) and water to spent acid flow rate (W/A) ratio on the performance of diffusion dialysis were investigated. Results indicated that acid recovery increased with the concentration of the feed. The membrane showed moderate monovalent permselectivity: divalent cations were more effectively retained as opposed to monovalent cations. The selectivity between metal and hydrogen ions were impaired by increasing W/A ratio. The acid recovery reached its maximum at 3.0 mol/L H+, demonstrating that the efficiency at feeding concentrations above 3.0 mol/L would be low for the DF120 membrane. The increase of salt content in the spent acid to 3.0 mol/L and above conceivably impaired the selectivity of the membrane.As such, acid recovery should be best performed with acid effluents with lower-to-medium salt content and hydrogen concentration. Also, a suitably lower W/A ratio should be adopted for effective acid recovery with low water cost.

Research highlights
► Diffusion Dialysis with the homogenous membrane DF120 was used to treat spent nitric acid solutions from electrodialysis lithium extraction.
► The separating performance of the process was investigated with respect to water-to-acid ratio, and acid and salt concentrations in the feed.
► The membrane demonstrated a cationic rejecting sequence as Mg2+>Ca2+ >Li+>Na+>K+>H+.
► The membrane also showed a concentrating effect on acid in the recovery stream as a result of rising salt concentrations.