The nature of the residual apatites remaining after partial acidulation of phosphate rocks with phosphoric and sulphuric acids

Abstract

The nature of the residual apatites remaining after the partial acidulation of North Carolina, Jordan and Nauru phosphate rocks have been studied using chemical and physical methods of analysis and the agronomic efficiency of residual North Carolina apatite compared to that of unacidulated North Carolina phosphate rock using a glasshouse pot trial technique.

The residual apatites were found to differ from the apatites contained in the original phosphate rocks. Solubilities in 2% citric acid and formic acid were markedly reduced, with the most reactive material (North Carolina) showing the greatest reductions in solubilities and the least reactive (Nauru) exhibiting the lowest reductions.

The unit cell dimensions of residual North Carolina apatite were determined by x-ray diffraction and indicated a lesser amount of carbonate substitution for phosphate in the apatite lattice than was the case in the original phosphate rock. This was confirmed by chemical analysis.

The residual North Carolina apatite initially performed much less well agronomically than the unacidulated phosphate rock, with the increase in yield (rye grass) over control after three harvests for the residual apatite being only 50–60% of that obtained for the unacidulated material. After 11 harvests however, the residual apatite had improved to give yield increases which were 70–80% of those resulting from the original phosphate rock.

The observed differences between the residual apatites contained in PAPRs and unacidulated phosphate rocks appear to be the result of an apatite-monocalcium phosphate interaction which occurs during storage after the main apatite-phosphoric acid reaction has taken place.