Increasing sustainability on the metallurgical industry by integration of membrane nanofiltration processes: Acid recovery

Abstract

The metallurgical industry generates large volumes of toxic effluents characterised, generally, by high acidity and a noticeable content of metals (Fe, Cu and Zn) and non-metals (As, Sb, Bi). The toxicity of these streams makes necessary treatment before its discharge to the environment or reuse. Sustainable management of these effluents must be focused on the recovery of low added valuable by-products (e.g. strong acids) to reduce the wastes generated along with the treatment (e.g. sludge). Nanofiltration offers clear advantages for acid recovery instead of the conventional treatments such as neutralisation and precipitation, due to the high membrane transport ratios of single charged ions and high rejection of multi-charged ions. The performance of a semi-aromatic poly(piperazineamide) membrane (NF270) was evaluated for the treatment of effluents from copper metallurgical process streams of off-gases treatment trains. These streams are characterised by a high acidity (pH < 1) due to a mixture of strong (H2SO4, HCl) and weak (H3AsO4) acids and the presence of metallic species (Fe, Cu, Zn). The membrane performance was evaluated in terms of acid recovery and metal ions rejection taking into account their aqueous speciation in strong acid media. The transport of the species across the membrane was characterised according to the Solution-Electro-Diffusion model. The membrane permeances to aqueous species (both charged and non-charged) in strongly acidic solutions were calculated. NF270 showed good results for strong acid recovery, exhibiting high rejections of the metallic impurities. The implications of the presence of large amounts of As present as H3AsO4 should involve a selective removal stage using H2S or Na2S2O3.