Concentration and purification of fluosilicic acid by membrane distillation

The process was carried out in a plate and frame module with a poly(vinylid ene fluoride) membrane. Experiments were performed with the feed and permea te temperature kept respectively at 333 K(or 343 K) and 293 K. During membr ane distillation (MD) of fluosilicic acid solutions, the permeate flux decl ined from 385 to 11.4 dm(3)/(m(2) d) with an increase of acid concentration in the feed from 2 to 35 wt %, at T-F,T-in = 343 K. The retention coeffici ent was close to 92%. A combination of membrane distillation with osmotic m embrane distillation was also studied. In this case a saturated brine was u sed as the cold solution. Under these conditions the driving force (partial pressure difference) was higher than that for pure MD. MD experiments of a solution containing both fluosilicic and phosphoric acids, with the acid c oncentration of 6% H2SiF6 and 30% P2O5, respectively, were also performed. At such conditions the water vapor as well as volatile fluorine compounds a re transported through a hydrophobic membrane from the feed solution to the cold distillate. The distillate was then a fluosilicic acid, practically f ree of phosphates. The investigations with a raw technical fluosilicic acid were also performed.