Formation of ammonium paratungstate tetra- and hexa-hydrate. I: stability

The crystallization of ammonium paratungstate tetrahydrate, ((NH₄)₁₀OH₂W₁₂O₄₂·4H₂O, 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 information it was postulated that APT·6H₂O is metastable. Experiments were carried out to determine whether and under what conditions the hexahydrate 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° to 96°C. APT·6H₂O is not formed when APT·4H₂O seeds are added prior to crystallization. The experimental data are presented in a three-dimensional ternary phase diagram. It was found that the recrystallization of APT·6H₂O into APT·4H₂O is prompted by a decreasing water activity in solution during crystallization. The increase in solution temperature during isobaric crystallization also stimulates the recrystallization process. In industrial practice some residual crystals are always present in the crystallizer. Therefore, APT·6H₂O will not form during industrial crystallization.