Ammonium Paratungstate Crystallization

The physical properties of the crystallized APT can be influenced by crystallization conditions to some extent. They are of importance for the further processing to metal powder by hydrogen reduction. 

The crystallization is not only a conversion of the dissolved to the solid salt, but the last purification step in the hydrometallurgical process. Most of the impurity elements still present in the process solution have much higher solubility than APT and are consequently enriched in the mother liquor. 

Impurity concentrations of the feed solution to the crystallizer, of the mother liquor, and of the crystallized APT are related. For a given required APT, purity limits in impurity concentrations of the mother liquor are allowed, and these limits in combination with the impurity level of the feed solution determine the degree of evaporation. Upper impurity 

The crystal slurry is separated from the mother liquor by filtration and washing with deionized water. Filtrate and wash water are recycled and, finally, the APT is dried. 

On a technical scale, batch and continuous operation are used. Batch crystallization leads to better consistency of the product, but the continuous process is more economic. 

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Ammonium paratungstate is recovered from the strip solution by evaporative crystallization, dried and calcined to produce blue oxide 208... 

Scheelite, natural or synthetic, is attacked by hydrochloric add in an autoclave. An almost insoluble residue of WO,-5 1120, contaminated by silicates from the gangue, is obtained. On treatment with ammonia this oxide-hydrate is dissolved to form a colorless solution and purified by filtration. After evaporation a white substance crystallizes, ammonium paratungstate APT (or ammonium parawolframate APW) with the formula (NH ) j H j Wj O g. APT has been an important substance in the worldwide trade in tungsten products. When this compound is ignited pure WO, is formed. 

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