Constituent Minerals and Solubility of Calcined Phosphate of Na_2O-CaO-MgO-P_2O_5-SiO_2 System Made from High-Silica Phosphate Ore

Abstract

A study was made to produce the calcined phosphate of Na_2O-CaO-MgO-P_2O_5-SiO_2 system, by calcining high-silica phosphate ore together with additional sodium, calcium and magnesium carbonates. On calcination of the ore of BPL 67 with sodium carbonate at 1300℃, the carbonate was reacted with apatite and silica nearly stoichiometrically at the Na_2O/P_2O_5 mole ratio of 1.33 to form rhenanite (β-form) and di-calcium silicate, resulting in increase of citric-solubility of P_2O_5 up to 92%. The use of higher-silica ore needed more amount of sodium carbonate to conduct enough the above reaction, and consequently fused partially or mostly the calcined product due to the formation of sodium silicate. On calcination of the ore of BPL about 55 together with additional sodium and calcium carbonates at 1300℃, rhenanite was easily formed even at the Na_2O/P_2O_5 ratio of 1.33 and its amount increased with increase of the mole ratio of CaO/P_2O_5 ; it was formed with a double salt Na_2Ca_5(PO_4)_4 and tri-calcium di-silicate at higher CaO/P_2O_5 ratio than 4.3 to bring no fusion of the calcined product. The amount of the double salt formed reached maximum at the CaO/P_2O_5 ratio about 5, resulting in increase of the citric- and citrate-solubilities of P_2O_5 to about 100% and 80%, respectively. On calcination of the above ore together with additional sodium, calcium and magnesium carbonates, rhenanite was easily formed with akermanite even at 1050℃ and its amount reached maximum at the mole ratios of Na_2O/P_2O_5 of 1.33, (CaO+MgO)/P_2O_5 of 5.75 and MgO/(CaO+MgO) of 0.22, resulting in increase of the solubilities of P_2O_5 to 96% and 92%, respectively. Higher MgO/(CaO+MgO) ratio than 0.22 formed partially enstatite and silicocarnotite, and increased the amount of apatite unreacted to decrease the solubilities.