Factors influencing the low-temperature formation of AlPO4 and its precursor phases, AlPO4.xH2O (1 less-than-or-equal-to x less-than-or-equal-to 2), were investigated. AlPO4 formed by reaction between 33.3 wt% H3PO4 solution and alumina. Five aluminas (three anhydrous and two hydrated) were utilized. Each differed in particle size, surface area, and crystallinity. The reaction temperatures investigated were 113-degrees, 123-degrees, and 133-degrees-C. The high-surface-area aluminas were sufficiently reactive in the phosphoric acid solution at these temperatures to produce crystalline reaction products. However, only hydrated forms of AlPO4, AlPO4.xH2O (1 less-than-or-equal-to x less-than-or-equal-to 2), crystallized directly out of solution. x generally decreased as the curing temperature was increased. Upon dehydration of these hydrated reaction products, anhydrous AlPO4 was formed, primarily in the berlinite and/or cristobalite modifications. Both the temperature of reaction and the alumina used influence the hydrates that form. In turn, the hydrates which form, the macroscopic assemblages into which they may crystallize, and the morphologies of the crystallites all affect the polymorphic form and the crystallinity of the anhydrous AlPO4 phase ultimately produced on dehydration. Phase-pure and highly crystalline AlPO4-cristobalite (the high-temperature modification) was formed by the dehydration of AlPO4.H2O at a temperature as low as 113-degrees-C.
