Chemical effects in the assessment of phosphate rock residues extracted from multicomponent fertilisers

Abstract

Methods of obtaining “reactive” phosphate rock residues from multicomponent phosphate fertilisers which contain soluble phosphorus components have been examined. Extraction using cold neutral ammonium citrate has been found to be more suitable than water, as chemical reactions appear to occur in water extraction solutions to modify the residue, and more impurity species such as calcium sulphate and iron and aluminium phosphates remain in the residues to complicate the process of obtaining reliable solubility figures.

“Deactivation” of the phosphate rock residues (as measured by decreased 2% formic acid solubilities) was found to be due to reactions occurring during the maturing process, and was dependent on the phosphate rock used, with North Carolina phosphate rock-based residues significantly affected, but Arad-based residues almost unaffected. However, for neutral ammonium citrate extractions, only North Carolina-based residues produced solubility figures lower than the original phosphate rock. Some selectivity of reaction in single superphosphate-“reactive” phosphate rock mixtures was indicated which further complicated assessment of the “deactivation” effect; however such an effect appeared to be much less significant than previously suggested.