Separative recovery with lime of phosphate and fluoride from an acidic effluent containing H 3PO 4, HF and/or H 2SiF 6

Abstract

Fluoride content and flow-rate of fertilizer plant wastewater from phosphoric acid and/or triple superphosphate (TSP) production lead to the discharge of several thousand tons of fluoride (F −) per year and even more for phosphate (PO 4 3−). Since sustainability is an important environmental concern, the removal methods should allow phosphorus and fluoride to be recycled as a sustainable products for use as raw materials either in agricultural or industrial applications. In the present work, separative recovery with lime of these two target species was investigated. A preliminary speciation study, carried out on the crude effluent, showed that two forms of fluoride: HF and H 2SiF 6 are present in a highly acidic medium (pH ∼ 2). Evidence that fluoride is present under both free (HF) and combined (H 2SiF 6) forms, in the phosphate-containing effluent, was provided by comparing potentiometric titration curves of a crude wastewater sample and synthetic acid mixtures containing H 3PO 4, HF and H 2SiF 6. In a second step synthetic effluent containing mixtures of the following acids: HF, H 2SiF 6 and H 3PO 4, were treated with lime. The behaviour of these compounds under lime treatment was analysed. The data showed that fluoride has a beneficial effect on phosphate removal. Moreover, by acting on the precipitation pH, a “selective” recovery of fluoride and phosphate ions was possible either from phosphoric acid/hydrofluoric acid or phosphoric acid/hexafluorosilicic acid mixtures. Indeed, the first stage of the separative recovery, led to a fluoride removal efficiency of 97–98% from phosphoric acid/hydrofluoric acid mixture. It was of 93–95% from phosphoric acid/hexafluorosilicic acid mixture. During the second stage, the phosphate precipitation reached 99.8% from both acidic mixtures whereas it did not exceed 82% from a solution containing H 3PO 4 alone. The XRD and IR analyses showed that during lime treatment, a H 2SiF 6 hydrolysis occurred, instead of CaSiF 6 solid formation, leading to CaF 2 precipitate. Calcium fluoride and calcium phosphate based-by-products resulting from the two-step treatment process can be used as raw materials in several industrial sectors, such as ceramic and phosphate fertilizer industries.