(Ca(OH)_(2)-)NaOH-Na_(2)WO_(4)-H2O和(Ca(OH)_(2)-)NaOH-Na_(3)PO_(4)-H_(2)O体系的热力学与应用

Thermodynamics of(Ca(OH)_(2)-)NaOH-Na_(2)WO_(4)-H_(2)O and(Ca(OH)_(2)-)NaOHNa_(3)PO_(4)-H_(2)O systems and their application

白钨矿(CaWO_(4))碱压煮工艺是我国主流的钨冶炼工艺。但是,该工艺的机理仍不是很明确。本文利用相平衡原理,开展了白钨矿浸出过程、浸出渣洗涤过程、浸出液蒸发结晶过程中的热力学研究。根据浸出热力学平衡相图的分析,发现白钨矿中的钙对杂质的浸出有抑制作用(以磷为例),合理地解释了浸出液中磷等杂质始终维持在较低水平的主要原因。在浸出渣稀释-溶解钨酸钠晶体的过程中,利用热力学平衡相图解释了添加磷酸钠作为抑制剂,抑制二次白钨生成的作用机理,并优化了磷酸钠用量、反应时间、反应温度等对逆反应有抑制效果的工艺参数。在钨酸钠浸出液蒸发结晶钨碱分离过程中添加Ca(OH)_(2),在回收利用余碱的同时,还可实现溶液中杂质磷的脱除。该过程的热力学平衡相图表明,蒸发结晶结束区域是Ca_(5)(PO_(4))_(3)OH的稳定区,而CaWO_(4)不稳定,说明了Ca(OH)_(2)的加入不会影响钨的损失。实验结果表明,在蒸发结晶开始时加入理论量的Ca(OH)_(2),可以去除90%的磷,钨的损失率控制在0.2%以下。

The caustic soda autoclaving process for digesting scheelite(CaWO_(4))is a unique and widely used tungsten(W)metallurgy technology in China.However,some experimental phenomena still need a proper theoretical explanation.In this work,some operational details and the theoretical basis of the technology are reported.During the digestion,some impurities,such as P,As,and Si are also digested.The P impurity was taken as an example.Based on the analysis of thermodynamic equilibrium phase diagram analysis,low levels of P impurities in the leaching solution were obtained.However,during the next dilution of the residue,P was supplemented as an inhibitor to reduce W loss.The inhibition of the reverse reaction mechanism is explained.The washed Na2WO4 solution containing excess NaOH can be reused in the evaporation−crystallization process;meanwhile,the supplemented P should be removed by adding Ca(OH)_(2).A theoretical analysis of the thermodynamic equilibrium phase diagram shows that there is an area where Ca_(5)(PO_(4))_(3)OH is stable,but CaWO4 is not.The experimental results show that over 90%of P can be removed by adding the theoretical Ca(OH)_(2) amount at the beginning of the evaporation-crystallization step.The W loss ratio can be controlled below 0.2%.