Titanium white waste acid concentration by DCMD: Wetting, crystallization, and fouling

We have investigated titanium white waste acid (TWWA) concentration by direct contact membrane distillation (DCMD) using self-made (PVDF)/poly(vinylidene-co-hexafluoropropylene) (P(VDF-co-HFP)) blend hollow fiber membranes. The concentration degree, ferrous sulphate (FeSO4) precipitation and membrane fouling/cleaning were investigated at different temperature and flow rate of feed streams. It was found that membrane with PVDF/P(VDF-co-HFP) ratio of 12/6 (w/w) (M1) had slightly larger contact angle, but very low flux (<6 kg/m2-h). Another ratio with more PVDF delivered membranes (M2) with significantly better flux. With the modules M2 batch of hollow fibers, the feed acid was concentrated up to around 58% at 70 °C (feed) and FeSO4 precipitated out during the DCMD. Surface fouling was confirmed by SEM analysis; the situation of surface fouling is more serious with higher concentration. Tests regarding effect of feed flow rate indicated that higher speed could reduce the fouling degree and foulant particle size. The final sulfuric acid (H2SO4) concentration is highest for modules operated at 150 ml/min of feed flow. Membrane cleaning involving HCl solution rinsing and air drying restored the salt rejection to be higher than 99%, but with depreciation in vapor flux.