Treatment of fluoride concentrates from membrane unit using salt solutions

A detailed study has been carried out for the removal of fluoride from concentrated retentate stream by precipitation overcoming the drawback of membranes. Calcium hydroxide (Ca(OH)2), magnesium hydroxide (Mg(OH2)) and calcium chloride (CaCl2) used for fluoride precipitation, have shown effective results in the pH range varying from 4 to 14. Kinetic studies have shown that the reaction between F and Ca ions in aqueous phase is fast and completed within 20 min at stoichiometric ratio. Reactions carried out with method of excess (keeping calcium concentration very high) showed pseudo-first order kinetics for both Ca(OH)2 and CaCl2.Parameters such as effect of reactant loading, reactant ratio and temperature which affect the particle size were studied. The maximum particle size of calcium fluoride (CaF2) was observed to be around 1 μm and 0.5 μm for Ca(OH)2 and CaCl2 respectively. The reactant ratio studies showed that an increase in the calcium dose decreases final fluoride concentration upto USEPA and WHO standards. Temperature showed an accelerating effect on reaction kinetics for both the reactions. The CaF2 particle size essentially remained unaltered with an increase in temperature for both the reactions. The study showed a successful reduction of fluoride concentration in the concentrated stream of the membrane and can be applied as relatively inexpensive assisting treatment to isolate large amount of fluoride precipitate.