Effects of partial acidulation on chemical and mineralogical characteristics of residual phosphate rocks

Abstract

Original phosphate rocks (PR) and water insoluble residues (WIR) from mixtures of reactive PRs and single superphosphate, known commercially as longlife single superphosphate (LLSSP), and from partially acidulated PRs (PAPR), were compared in terms of their elemental content, chemical reactivity as indicated by the apatite unit cell ‘a’ dimension and solubility. Phosphate rock reactivity is known to be inversely related to the ‘a’ dimension. Partial acidulation (20%) with commercial grade phosphoric acid resulted in an increase in aluminium (Al), iron (Fe) and fluoride (F) concentrations in the WIRs. The apatite ‘a’ dimensions of WIRs from LLSSPs were greater than those of the respective original North Carolina (NC), Khouribga (KR), Jordan (JR), Sechura (SE) and Arad (AR) PRs added to single superphosphate (SSP), made from Nauru PR (NR)) to produce the LLSSPs. This was attributed to the presence of the less reactive NR in the WIRs left-over from the SSP. Partial acidulation with phosphoric acid increased the apatite ‘a’ dimensions of NC and ElHassa (EH) PRs. The increase in apatite ‘a’ dimension of NC and EH was probably due to selective dissolution of a more reactive fraction of the PRs during partial acidulation. Changes in the apatite ‘a’ dimension following partial acidulation with phosphoric acid were not significant for the other PRs studied, e.g. Gafsa (GF), KR and AR, although differential X-ray diffractograms (DXRD) indicated that the material dissolved during partial acidulation was more reactive than the WIRs and the original PRs. The apatite ‘a’ dimension of NC PR was not affected by pretreatment with 2% or 4% citric acid (CTA). The contrasting response in ‘a’ of NC PR to acidulation with phosphoric and citric acids may be related to differences in the strength of these acids, and/or to the differing environments under which the reactions took place.