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

Abstract

The crystallization of ammonium paratungstate tetrahydrate, ((NH 4) 10OH 2W 12O 42·4H 2O, APT·4H 2O) from aqueous ammonium tungstate solutions is a key unit operation in current tungsten powder production. A literature survey showed that ammonium paratungstate hexahydrate (APT·6H 2O) can form under conditions where APT·4H 2O forms as well. Based on this information it was postulated that APT·6H 2O 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 WO 3 and from approximately 90° to 96°C. APT·6H 2O is not formed when APT·4H 2O 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·6H 2O into APT·4H 2O 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·6H 2O will not form during industrial crystallization.