碳酸钠分解白钨矿的热力学分析

Thermodynamics on soda decomposition of scheelite

通过热力学计算,绘制了25℃下白钨矿在总碳浓度为1 mol/L Na2CO3溶液中各主要溶解组分的lg c—pH图,以及白钨矿在不同总碳浓度下Na2CO3溶液中CO 32-和WO42-的lg c—pH图。利用这些热力学平衡图对Na2CO3分解白钨矿的工艺条件进行了分析,认为溶液中CO 32-浓度直接影响白钨矿分解,而当碱度较低时,Na2CO3将水解生成HCO3-,从而降低CO32-浓度,进而降低白钨矿的分解率;故在工业实践中,为使钨最大限度地转入溶液,除了保证足够的Na2CO3用量,还必需添加NaOH以维持一定碱度。利用热力学分析所得结论,重新解释了前人的实验现象,认为以过多的NaOH代替Na2CO3造成钨浸出率回落的原因在于,溶液中CO 32-浓度的较大幅度下降,而不是钨矿中脉石分解所引起的Na2CO3消耗。

The lgc-H diagrams at 25℃ for containing ions when the total carbon concentration being 1mol/L are established according to thermodynamic calculation. The lgc-pH diagrams for CO3^2- and WO4^2- at different total carbon concentrations are also established. Using these diagrams, thermodynamic analysis is carded out to show the effect of technical conditions on soda decomposition of scheelite. It appears that the concentration of CO3^2- has direct effect on soda decomposition of scheelite. When the alkalinity is low, CO3^2- will hydrolyze to HCO3^2-, accordingly the decomposition rate of scheelite becomes lower because of the decrease of CO3^2-. 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 CO3^2- concentration, not the increasing attack of gangue constituents, which would consume Na2CO3 that otherwise would have been available to digest scheelite.