Application of membrane distillation and solvent extraction for water and acid recovery from acidic mining waste and process solutions

Direct contact membrane distillation (DCMD) and solvent extraction (SX) were tested in series to recover water and acid from acidic mining waste solutions. In the DCMD step with the synthetic acidic waste solution, the concentration of H₂SO₄ increased from 0.85 M in the feed solution to 4.44 M in the concentrate. Sulphate and metal separation efficiency was >99.99% and the overall water recovery exceeded 80%. After recovery of water with DCMD, the concentrated solution was then subjected to recovery of sulphuric acid using SX with an organic system consisting of 50% TEHA and 10% ShellSol A150 in octanol. Over 80% H₂SO₄ was extracted in a single contact from the waste solution containing 245 g/L H₂SO₄ and metals with various concentrations. After three stages of successive extraction, nearly 99% of acid was extracted, leaving only 2.4 g/L H₂SO₄ in the raffinate. The extracted acid was stripped readily from the loaded organic solution using water at 60°C. After scrubbing the loaded organic solution at an O/A ratio of 10 and 22°C, 98-100% of entrained metals were removed in a single contact with only 4.5% acid lost in the loaded scrub liquor. It was found that the phase disengagement time was in the range of 2-4 min for both extraction and stripping, indicating reasonable fast phase separation.