Reaction of Monoaluminum Phosphate with Alumina and Magnesia

Reaction of monoaluminum phosphate Al(H2PO4)3 with alumina and magnesia and thermal change of the phosphate as well as of the reaction products were studied for fundamental research of the phosphate as a combining material of refractories. On heating, the phosphate changed to an amorphous material and then to Al(PO3)3 (type B) above 450°C and type A above 550°C (Figs. 1 to 3). By treating an aqueous solution of the phosphate with aluminum hydroxide at a ratio of P/Al=2, AlH3(PO4)2⋅3H2O crystallized whose thermal change is shown in Figs. 4 and 5. Thermal change of amorphous AlPO4⋅nH2O is shown in Fig. 6. Much AlPO4 (Tridymite form) appeared at 140°C while Berlinite appeared predominantly when P/Al ratio was larger than 1 (Fig. 7).Magnesia reacted rapidly with the solution of the phosphate; the setting of the mixtures of the solution, aluminum hydroxide and magnesia occurred more rapidly with increasing amount of magnesia (Table 3 and Fig. 8). The reaction products consisted of MgHPO4⋅3H2O and amorphous AlPO4⋅nH2O when P/(Al+Mg) ratio was 1. Magnesia interrupted the crystallization of AlPO4 (Tridymite form), which appeared mainly at 650°C as Mg2P2O7 crystallized (Fig. 9). When the ratio was smaller than 1, Mg3(PO4)2 formed above 900°C while Mg(PO3)2 formed above 400°C when the ratio was larger than 1 (Figs. 10 and 11).