An investigation of extraction procedures for assessing the phosphate rock residue in multicomponent fertilisers

Five chemical extractants-water, neutral ammonium acetate, neutral ammonium citrate (hot and cold) and alkaline ammonium citrate-have been assessed on their ability to efficiently isolate the residual phosphate rock present in multicomponent fertilisers without affecting the phosphate rock. Phosphoric acid-based partially acidulated phosphate rocks (PAPRs) were manufactured along with several LONGLIFE (LL, single superphosphate to which reactive phosphate rock is added ex-den) materials for the assessment of these extractants.Each extractant was assessed by analysing the resultant residual phosphate rock isolated from each multicomponent fertiliser and comparing its chemical properties with the original phosphate rock used to manufacture the products. An analysis of extractable phosphorus was also undertaken. X-ray diffraction analysis was performed on selected phosphate rock residues as an independent method of examining the extraction techniques.Water and neutral ammonium acetate were found to be unsatisfactory for use as extractants; neither removed calcium sulphate when present, while the latter also did not appear to extract all the monocalcium phosphate component. The remaining three extractants all removed soluble phosphorus components and impurity species from the fertiliser products, leaving the residual phosphate rock relatively unaffected. Residue analyses were only slightly modified to those of the original rocks, although there were some inconsistencies in results obtained using hot neutral ammonium citrate. X-ray diffraction analysis showed no significant differences between residues produced from neutral ammonium citrate or alkaline ammonium citrate extractions, but water and neutral ammonium acetate residues confirmed the presence of calcium sulphate in LL and the presence of monocalcium phosphate in a neutral ammonium acetate extracted PAPR.From this study, cold neutral ammonium citrate and alkaline ammonium citrate appear to be equally suitable for isolating the phosphate rock from a multicomponent fertiliser, leaving the residual rock substantially unmodified from the original phosphate rock. However, there was a slight preference for cold neutral ammonium citrate due to its ease of preparation and use.