Concentration and Purification of Fluosilicic Acid by Membrane Distillation

The process was carried out in a plate and frame module with a poly(vinylidene fluoride) membrane. Experiments were performed with the feed and permeate temperature kept respectively at 333 K (or 343 K) and 293 K. During membrane distillation (MD) of fluosilicic acid solutions, the permeate flux declined from 385 to 11.4 dm3/(m2 d) with an increase of acid concentration in the feed from 2 to 35 wt %, at TF,in = 343 K. The retention coefficient was close to 92%. A combination of membrane distillation with osmotic membrane distillation was also studied. In this case a saturated brine was used 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 concentration of 6% H2SiF6 and 30% P2O5, respectively, were also performed. At such conditions the water vapor as well as volatile fluorine compounds are transported through a hydrophobic membrane from the feed solution to the cold distillate. The distillate was then a fluosilicic acid, practically free of phosphates. The investigations with a raw technical fluosilicic acid were also performed.