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.
The 2% CTA and formic acid (FMA) solubilities of the WIRs from LLSSPs and PAPRs were markedly lower than those of the original PRs. This reduction in solubility of PRs following partial acidulation was probably related to changes in mineralogical and chemical composition of the WIRs as indicated by the increases in apatite ‘a’ dimension of some residual PRs and shifts in peak positions in DXRD, to increases in the concentrations of Fe, Al and F compounds, and to coating effects of PR particles by Fe, Al and F compounds. This, in turn, may reduce the agronomic value of the residual PR component of PAPR and LLSSP fertilizers, particularly over the short-term.
The solubility of residual PRs following pretreatment with 2% or 4% CTA was slightly lower than that of the original PRs. The pretreatment caused no significant change in the apatite ‘a’ dimension of NC PR. The complexing effects of CTA and its lack of Fe and Al impurities may have prevented the formation of Fe, Al and F compounds. The effect of citric acid on PR reactivity is thus quite different from that of the mineral acids used to prepare LLSSPs and PAPRs.
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References
Bolan NS, Hedley MJ, Harrison R and Braithwaite AC (1990) Influence of manufacturing variables on characteristics and agronomic value of partially acidulated phosphate fertilizers. Fert Res 26: 119–138
Braithwaite AC, Eaton AC and Groom PS (1990) Factors affecting the solubility of phosphate rock residues in 2% citric acid and 2% formic acid. Fert Res 23: 37–42
Braithwaite AC, Eaton AC and Groom PS (1992) Chemical effects in commercial and laboratory mixtures of ‘reactive’ phosphate rock and acidulated fertilizers. Fert Res 31: 111–118
Brown G, Wood IG and Nicholls L (1987) Thermal and mechanical instabilities in the alignment of Bragg-Brentano parafocussing powder diffractometers. Powder Diffraction 2: 7–21
Charleston AG, Condron LM and Brown IWM (1989) The nature of the residual apatites remaining after partial acidulation of phosphate rocks with phosphoric and sulphuric acids. Fert Res 18: 267–273
Frazier AW and Lehr JR (1967) Iron and aluminium compounds in commercial superphosphates. J Agric Food Chem 15: 348–349
Frazier AW and Kim YK (1989) Redistribution of impurities in commercial wet-process acid. Fert Res 21: 45–60
Junge A and Werner W (1989) Investigation on interactions of phosphorus compounds in partially acidulated phosphate rock and fertilizer effectiveness. Fert Res 20: 129–134
Khasawneh FE and Doll EC (1978) The use of phosphate rock for direct application to soils. Adv Agron 30: 159–206
Olsen SR and Sommers LE (1982) Phosphorus. In: AL Page (ed) Methods of Soil Analysis, Part 2. Chemical and Microbiological Properties. 2nd edn, pp 403–430. Am Soc Agron, Madison, Wisconsin, USA
Resseler H and Werner W (1989) Properties of unreacted rock residues in partially acidulated phosphate rocks affecting their reactivity. Fert Res 20: 135–142
Schulze DG (1981) The identification of soil iron oxide minerals by differential X-ray diffraction. Soil Sci Soc Am J 45: 437–440
Stephen RC and Condron LM (1986) An assessment of the agronomic efficiency of partially acidulated phosphate rock fertilisers. Fert Res 10: 269–282
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Condron, L.M., Di, H.J., Campbell, A.S. et al. Effects of partial acidulation on chemical and mineralogical characteristics of residual phosphate rocks. Fertilizer Research 39, 179–187 (1994). https://doi.org/10.1007/BF00750245
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DOI: https://doi.org/10.1007/BF00750245