Crystallization of Ammonium Paratungstate from Ammonium Tungstate Solution

The crystallization of ammonium paratungstate (APT) from aqueous ammonium tungstate solutions is a key unit operation in the current production of tungsten powder. The crystal size, crystal size distribution, and degree of agglomeration of ammonium paratungstate tetrahydrate, (NH4)10[H2W12042] ·4H20, (APT·4H20) are all likely to be of importance for its processing to tungsten powder. Both batch and continuous crystallizers are used in industrial practice.

The conventional crystallization process of APT·4H20 is to heat an ammonium tungstate solution to its boiling point, the actual value of which depends on the composition of the solution. The boiling point of the solution increases during the isobaric crystallization of APT.4H20 from approximately 90 ° to 100°C.

The crystallization activity of ammonium paratungstate (APT) can concluded as below:

1) Ammonium hexahydrate (APT. 6H20) was observed at a solution concentration of approximately 300-230 g/kg WO3 and from approximately 90° to 96°C. Ammonium hexahydrate and ammonium tetrahydrate coexist at the period.

2) As temperature increasing above 96 °C, ammonium hexahydrate starts to recrystallize into ammonium tetrahydrate, while -the NH3 and WO3 concentrations decrease and the solution temperature increases with time. APT. 6H20 is never formed in the presence of seeds of APT. 4H20.

3) APT-6H20 is metastable in the dashed trajectory from approximately 88.5 °C to 96 ° C. The solution composition at 96 °C is approximately 220 g/kg WO3 and 40 g/kg NH3.  APT. 6H20 is unstable at temperatures higher than approximately 96 ° C. This implies that only from solutions with initially higher WO3 concentrations will APT start to nucleate at relatively low temperatures, resulting in crystallization of the metastable APT. 6H20.

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.

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