Low-Temperature Formation of Aluminum Orthophosphate

Factors influencing the low-temperature formation of AIPO₄ and its precursor phases, AIPO₄· x H₂O (1 x 2), were investigated. AIPO₄ 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°, 123°, and 133°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 AIPO₄, AIPO₄· x H₂O (1 x 2), crystallized directly out of solution. x generally decreased as the curing temperature was increased. Upon dehydration of these hydrated reaction products, anhydrous AIPO₄ 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 AIPO₄ phase ultimately produced on dehydration. Phase-pure and highly crystalline AIPO₄-cristobalite (the high-temperature modification) was formed by the dehydration of AIPO₄·H₂O at a temperature as low as 113°C.