Membrane distillation and dispersive solvent extraction in a closed-loop process for water, sulfuric acid and copper recycling from gold mining wastewater

Abstract

Membrane distillation (MD) and dispersive liquid–liquid extraction (d-LLE) were integrated for gold mining wastewater beneficiation and better use of natural resources. MD presented an average permeate flux of 5.51 ± 0.14 L/m2·h, without a significant flux decay (p < 0.001), and rejection efficiencies for acid and metallic species>98.7%. Five different extractants were studied for sulfuric acid recovery from MD concentrate. Considering the extraction and stripping efficiency, tris-2-ethylhexylamine (TEHA) was the most appropriate alternative for sulfuric acid extraction, requiring 2 stages of extraction and 1 of striping to achieve a recovery>95%. The endothermic nature of acid extraction contributed to the integration of d-LLE and MD, designating a proper destination for MD concentrate, a stream often forgotten while discussing membrane separation processes for wastewater treatment. Copper was then selectively recovered from the raffinate from acid extraction (βCu/Fe = 4,274, βCu/Al = 176, and βCu/Mg = 1,262) by Lix 84-I. A single contact stage ensured that 99.3% of copper was recovered. Fourier-transform infrared spectra demonstrated that the acid extraction mechanisms involve amine salts formation, whereas copper is extracted by anion exchange processes. The economic assessment was favorable for water (US$/m3 0.23) and copper (US$/kgCu 4.25) recovery, with prices lower than currently practiced for both commodities. For sulfuric acid, the operational expenditures corresponded to US$/kgH2SO4 0.423, slightly higher than its marketable price. Overall, the integrated system could be inserted in a perspective that departs from the linear focus of raw material utilization, prioritizing recycling and reuse processes.