Investigation on partial acidulation of rock phosphate using acetic acid and acidic calcium phosphate solutions

This study explores an innovative method for the partial conversion of sparingly soluble rock phosphate (RP) into a biphasic, highly soluble calcium phosphate mixed with crystalline apatite. The RP was reacted with acetic acid and monocalcium phosphate (MCP) (a commercial triple super phosphate) solutions. The formation of low-order crystalline phosphate phases and associated chemical environments were analyzed using Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). The Rietveld refinement of XRD data obtained from progressive acidulation of RP at different time intervals showed the formation of anhydrous forms of calcium-deficient hydroxyapatite (CDHA), dicalcium phosphate (DCPA), monocalcium phosphate (MCPA), and octacalcium phosphate (OCP), with these more soluble mineral phases constituting up to 27%. FTIR analysis confirmed the formation of protonated phosphate (HPO 4 2- ) enriched calcium phosphate, the presence of which was indirectly corroborated by the increase in solution pH and the formation of β-tricalcium phosphate upon calcination. SEM images showed that both acetic acid and MCP treatments led to comparable morphological changes, including irregular surface features, fuzzy boundaries, and localized agglomeration likely from new, more soluble mineral phases. Collectively, our results suggest the formation of protonated calcium phosphate and more soluble minerals during acidification. These phases exhibit high solubility, thereby effectively transforming the original RP into biphasic form containing highly soluble and sparingly soluble original minerals. These findings highlight the potential to optimize acidulation of RP to achieve the desired solubility for target applications.