Abstract: The lg c—pH diagrams at 25 ℃ for containing ions when the total carbon concentration being 1 mol/L are established according to thermodynamic calculation. The lg c—pH diagrams for and at different total carbon concentrations are also established. Using these diagrams, thermodynamic analysis is carried out to show the effect of technical conditions on soda decomposition of scheelite. It appears that the concentration of has direct effect on soda decomposition of scheelite. When the alkalinity is low, will hydrolyze to , accordingly the decomposition rate of scheelite becomes lower because of the decrease of . So in order to get higher leaching rate in industry practice, it is necessary to use enough amount of soda to serve as digestion reagent and, certain amount of NaOH to maintain high enough alkalinity. A re-explanation of experimental phenomenon reported in literature is derived, which shows that the reason causing the decrease of tungsten extraction when using overhigh degree of substitution of NaOH for Na2CO3 lies in the sharp decrease of the concentration, not the increasing attack of gangue constituents, which would consume Na2CO3 that otherwise would have been available to digest scheelite.

Key words: scheelite; soda decomposition; thermodynamics; concentration logarithm diagram