Partial acidulation of an ‘unground’ phosphate rock: I. Preparation and characteristics

Partially acidulated phosphate rocks were prepared from unground North Carolina phsophate rock and H₃PO₄ by (i) mixing phosphate rock with the requisite amount of H₃PO₄, (ii) mixing with a portion of the acid followed by adding the remaining acid during granulation and (iii) single-step acidulation and granulation. The degrees of acidulation were 20, 30, 40 and 50%. Only 20% and 30% acidulations were done by method (iii). The phosphate rock granulated readily on addition of H₃PO₄ either as in method (ii) or (iii) and the products did not need external drying before storage. The citric and water soluble P showed that from the viewpoint of acid-phosphate rock interaction all three methods of preparation were satisfactory. The granules were equally strong as or stronger than commercially available single or triple superphosphate samples tested. The percent degradation on abrasion was less than 4% compared to about 8% for superphosphate and 0.4% for triple superphosphate. A sand incubation study suggested an interaction in the partially acidulated phosphate rock between the monocalcium phosphate component and unreacted phosphate rock which initially increased the solubility of P.