title: Separative recovery with lime of phosphate and fluoride from an acidic effluent containing H3PO4, HF and/or H2SiF6.
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 (PO4(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 H2SiF6 are present in a highly acidic medium (pH approximately 2). Evidence that fluoride is present under both free (HF) and combined (H2SiF6) forms, in the phosphate-containing effluent, was provided by comparing potentiometric titration curves of a crude wastewater sample and synthetic acid mixtures containing H3PO4, HF and H2SiF6. In a second step synthetic effluent containing mixtures of the following acids: HF, H2SiF6 and H3PO4, 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 H3PO4 alone. The XRD and IR analyses showed that during lime treatment, a H2SiF6 hydrolysis occurred, instead of CaSiF6 solid formation, leading to CaF2 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.
authors.affiliation: Unité de Recherche Chimie Industrielle et Matériaux, Ecole Nationale d'Ingénieurs de Sfax, BPW 1173-3038 Sfax, Tunisia.
authors.forename: Mbarka
authors.initials: M
authors.lastname: Gouider
authors.forename: Mongi
authors.initials: M
authors.lastname: Feki
authors.forename: Sami
authors.initials: S
authors.lastname: Sayadi
mesh_terms: D000143:Acids; D017610:Calcium Compounds / Q000737:chemistry*; D004566:Electrodes; D005308:Fertilizers / Q000032:analysis; D005459:Fluorides / Q000737:chemistry*; D006858:Hydrofluoric Acid / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis; D007202:Indicators and Reagents; D007220:Industrial Waste / Q000032:analysis; D010087:Oxides / Q000737:chemistry*; D010710:Phosphates / Q000737:chemistry*; D017680:Potassium Compounds / Q000737:chemistry*; D011199:Potentiometry; D012824:Silicic Acid / Q000737:chemistry*; D012996:Solutions; D013054:Spectrophotometry, Atomic; D014874:Water Pollutants, Chemical / Q000032:analysis / Q000302:isolation & purification; D014961:X-Ray Diffraction
chemical_list: D000143:Acids; D017610:Calcium Compounds; D005308:Fertilizers; D007202:Indicators and Reagents; D007220:Industrial Waste; D010087:Oxides; D010710:Phosphates; D017680:Potassium Compounds; D012996:Solutions; D014874:Water Pollutants, Chemical; D012824:Silicic Acid; C035291:hexafluorosilicic acid; C013216:potassium phosphate; C016538:lime; D005459:Fluorides; D006858:Hydrofluoric Acid