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 FilmTecTM 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, operateddownstreamto the pre-concentrationNFunit,wasable to produce thenardite crystals (the anhydrous form of sodium sulfate) grown from slurry with density up to 21 kgm-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.