FORMATION OF AMMONIUM PARATUNGSTATE TETRA-HYDRATE AND HEXA-HYDRATE .1. STABILITY

The crystallization of ammonium paratungstate tetrahydrate, ((NH4)10H2 W12O42.4H2O, APT.4H2O) from aqueous ammonium tungstate solutions is a key unit operation in current tungsten powder production. A literature survey showed that ammonium paratungstate hexahydrate (APT.6H2O) can form under conditions where APT.4H2O forms as well. Based on this info rmation it was postulated that APT.6H2O is metastable. Experiments wer e carried out to determine whether and under what conditions the hexah ydrate is metastable. It was found that the hexahydrate is metastable at a solution concentration of approximately 300-230 g/kg WO3 and from approximately 90-degrees to 96-degrees-C. APT.6H2O is not formed when APT.4H2O seeds are added prior to crystallization. The experimental d ata are presented in a three-dimensional ternary phase diagram. It was found that the recrystallization of APT.6H2O into APT.4H2O is prompte d by a decreasing water activity in solution during crystallization. T he increase in solution temperature during isobaric crystallization al so stimulates the recrystallization process. In industrial practice so me residual crystals are always present in the crystallizer. Therefore , APT.6H2O will not form during industrial crystallization.