Introduction including Ecological and Economical Considerations

This section deals exclusively with the modem process of tungsten hydrometallurgy. In this introduction the classical and modem concepts are compared, outlining the differences as well as the advantages and disadvantages. In Figs. 5.6 and 5.7, simplified flow sheets of the classical and modem processes are given. 

Scheelite ore concentrates Wolframite ore concentrates Oxidized scrap 

In the classical procedure, scheelite was mainly processed by acid digestion while wolframite was dissolved by an alkaline digestion. The important solid, pure intermediate was, in both cases, tungstic acid, derived either directly or after several precipitation steps. Tungstic acid is formed by a precipitation, which is a more or less instantaneous process. Foreign ions present in solution during the precipitation are partly entrapped or coprecipitated and contaminate the tungstic acid. 

In the modem process, APT is the intermediate which, in contrast to tungstic acid, is gained by crystallization. Crystallization compared to precipitation is a much slower process, consequently less of the impurities present in the mother liquor will be contained in the crystallized product. 

The second important difference between classical and modem technology is the method used to separate the large amount of sodium ions. The old method used a calcium tungstate precipitation and subsequent hydrochloric acid treatment for that purpose, while in the modem process solvent extraction or ion exchange converts sodium into ammonium isopolytungstate solution. 

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