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

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.