Hybrid nanofiltration–membrane crystallization system for the treatment of sulfate wastes

An integrated nanofiltration–membrane crystallization (NF–MC) system has been tested for the removal of sodium sulfate from aqueous wastes originated by the production process of base raw materials (Ni–H) for special rechargeable batteries. Nanofiltration experiments on NF90-2540 FilmTec™ modules showed high rejection values to sodium sulfate (>99%) when operated at pressure of 3.45 × 106 Pa; it was possible to achieve a maximum permeate recovery factor of 50% when starting from 60 g l−1 Na2SO4 feed solution. The membrane crystallizer, operated downstream to the pre-concentration NF unit, was able to produce thenardite crystals (the anhydrous form of sodium sulfate) grown from slurry with density up to 21 kg m−3; the solid product exhibited narrow size distribution (coefficient of variation ∼30%). Polypropylene hollow-fibre membranes showed a hydrophobic character stable over 2 days of continuous operation.