Removal of phosphates and fluorides from industrial wastewater

Abstract

The objective was the recovery of phosphates from wastewater in the form of compounds which could be utilized in the fertilizer industry. The influence of pH and multivalent cations (Fe 3+, Al 3+, Ca 2+) on the degree of fluoride and phosphates separation from model solutions and wastewater from wet process phosphoric acid production was determined. A strong inhibiting effect of the aluminium ions on the degree of fluoride precipitation in a form of CaF 2 from a solution with the pH of 2–3 was pointed out. It was found that the separation of fluorides and phosphates by two methods is possible. In the first version with calcium, in the presence of iron and aluminium ions were precipited—mainly phosphates of iron and calcium in the first stage (pH 3); in the second stage (pH 8) fluorides were in the form of CaF 2. The degree of phosphate recovery in the first stage and fluorides in the second stage amounted to ca. 85–91% and ca. 82.5%, respectively. In the second version, from a solution containing a small amount of iron and aluminium, using calcium, fluorides were separated mainly (pH 3) in the first stage and phosphates (pH 8) in the second stage. In this case the degree of precipitation of fluorides in the first stage and phosphates in the second stage amounted to ca. 85–88% and ca. 63–73%, respectively. Depending on the method, the content of phosphates and fluorides in the treated wastewater amounted to 0.005–0.010 g/l and 0.008–0.018 g/l, respectively.