Phosphorus speciation during the production of phosphate fertilizers using a metallurgical acid residue

Full text: Soluble phosphate fertilizers are produced in the fertilizer industry by the acidification of rock phosphates (RP) using pure acids. Therefore, the use of acid residues (AR) to solubilize RP is a potential way of reusing any sort of AR. This way, the objective of this work was to evaluate the use of a metallurgical AR to produce phosphate fertilizer from three contrasting RP commonly found in Brazil. This AR comes from the leaching of pegmatite by a mixture of three acids (H2SO4/HF/HCl) in order to extract Ta and Nb. The work was carried out in a 3 x 5 factorial design: three RP (Araxá, Patos, and Bayóvar) and five RA concentrations (0,0; 12,5; 25,0; 50,0; and 75,0 % v/v in water). The changes occurring in the original RP and the P solubility of the products following the addition of AR were evaluated by X-ray diffraction (XRD), P K-edge XANES, and water soluble P (Pwater) and neutral ammonium citrate soluble P (PNAC). In terms of P solubility, the reactivity of the RP increased with increasing AR concentration. It means that the concentrations of Pwater and PNAC increased the higher the AR concentration. This increase in RP reactivity was also accompanied by the disappearing of apatite (P-Ca) peaks in the XRD, which is the main P species in the RP and not readily bioavailable. Phosphorus K-edge XANES spectrum of P-Ca species is characterized by the absence of pre-edge structures and the presence of strong post-edge shoulders. As AR concentration increased, these features disappeared in the products, agreeing with the XRD. What XRD could not show was the formation of non-crystalline P-Ca and P-Fe species. As AR concentration increased, an important feature arose, which was the pre-edge structure, characteristic of amorphous P-Fe species. At the end, linear combination fitting showed that as the AR concentration increased, P-Ca species present in the RP changed to more soluble species of P-Ca, with the formation of great amounts of amorphous P-Fe. (author)