The invention is directed to ammonium paratungstate decahydrate containing at least 75% of crystals having a length of at least 200 μm and having a ratio of length to width of less than 4.5:1.

 

The present invention relates to a novel process for preparing highly pure ammonium paratungstate hydrates and ammonium paratungstate decahydrate having a selected crystal structure.

 

Ammonium paratungstate hydrates (hereinafter referred to as APTs) are known intermediates for producing tungsten metal, tungsten-containing catalysts or hard materials based on tungsten, for example tungsten carbides.

 

The preparation of highly pure APT is carried out essentially via acid or alkali digestion or fusion of tungsten-containing concentrates or tungsten scrap with subsequent purification stages involving precipitation processes and liquid-liquid extraction. The purified solution is generally concentrated by evaporation, resulting in APT finally crystallizing out. The publication “Kristallographische Untersuchung der Ammoniumparawolframate (Zeitschrift für Kristallographie, vol. 120, pp. 216-228 (1964))” by M. Hähnert states that APT×10H2O is formed on slow evaporation of an ammonium tungstate solution prepared from WO3 and NH3 solution. The crystals have an acicular morphology. The bulk density of the product is 0.7 g/cm3. This is not an industrial process but a preparative laboratory process.

 

Further routes to the preparation of APT×10H2O are described in the article “Characterisation of various commercial forms of ammonium paratungstate powder, Journal of Material Science, No. 10 (1975), pages 571-577”.

 

In one process variant, an ammonium tungstate solution prepared from tungstic acid and an excess of NH3 solution is subject to freeze drying. The product formed has a crumbly poorly defined crystal morphology and the bulk density is 1.03 g/cm3. In a second process variant, an ammonium tungstate solution prepared as described above is slowly neutralized with HCl solution at room temperature. The acicular crystals formed have a length of 16-70 μm and a width of 2-14 μm and the bulk density is 1.46 g/cm3. Both processes form APT×10H2O having a low bulk density and the necessary product purity can be achieved only when using clean tungstic acid. In addition, a freeze drying procedure in one case and the use of clean HCl solution in the other case leads to very high process costs. When HCl is used, the product is contaminated with chloride as a result of the process.

 

In the specialist book “Metallurgie der seltenen Metalle, Seligman, Krejn and Samsonov (1978), Metallurgia publishers (USSR), pp. 62-63”, an industrial process for preparing APT×10H2O is described as follows. Scheelite concentrate is digested with HCl solution at 90-100° C. so as to form tungstic acid. The tungstic acid is subsequently dissolved in NH3 solution and the solution is cooled. The resulting ammonium tungstate solution is slowly neutralized with HCl solution to a pH of 7.3-7.4 while stirring. After allowing to stand for 24 hours, the APT×10H2O product is separated off. The acicular crystals have a length of 15-25 μm and a width of 1-3 μm and the bulk density is 0.98 g/cm3. The crystallization yield here is 85-90%. This product still contains considerable amounts of impurities. A further disadvantage of this process is the high consumption of clean HCl solution and the large amount of W-containing NH4Cl solution (mother liquor) which has to be worked up.