Resumo:
The conditional solubility of scheelite CaWO 4 in molten NaCI--KC1 (1 : 1) has been studied
either in oxobasic or oxoacidic media. In the former case it is increased by formation of
sparingly soluble CaO according to:
Ca 2+ + 02- ~ CaO(s), pK Ca° = 10800/T-- 5.8 (molality scale)
In the later case, WO 2- behaves as an oxobase according to the following equilibria:
WO42- ~- WO 3 (s) + 02_ pK 0 = 10.0 at 1000 K
3 WO2--¢- W30120+ 02_ pK 3 = 12.7 at 1000 K
The latter equilibrium favours an increase in the W VI solubility when pO 2- is increased, for
instance by injection of HC1. The whole set of results has been summarized by a conditional
solubility diagram of scheelite versus pO 2-.
