Titanium white sulfuric acid concentration by direct contact membrane distillation

In this study, synthetic titanium white waste acid was concentrated by direct contact membrane distillation (DCMD) using laboratory prepared PVDF hollow fibers. Tentative efforts were made to decouple the effects of different factors on wetting and DCMD performance. FeSO4 crystals were found homogeneously distributed on the outer surface of the hollow fibers while treating H2SO4/FeSO4 solution. In homogeneously distributed crystals inside the membrane and on the lumen surface were identified. Elemental analysis indicated that the contamination of membrane interior structure occurred with three different aqueous solutions (H2SO4, H2SO4/FeSO4 and NaCl). This indicated that liquid intrusion was a factor non-negligible in causing membrane wetting. Rejection coefficients nevertheless were dependent on the solute type. For H2SO4 and H2SO4/FeSO4, the rejections were maintained at high level throughout the DCMD process before cleaning. In contrast, concentration of NaCl solution witnessed obvious deterioration in rejection when approaching saturation. This difference is referring to the influence of the amount and property of the solute crystallizable in feed solution. Removal of the FeSO4 crystals by HCl acid cleaning restored the flux partially for two types of membranes, but without apparent deterioration in the rejection only for membranes with lower transmembrane flux.