China is rich in tungsten resources, and yet the main tungsten ore type is scheelite, which is richer than wolframite-scheelite mixed ore and wolframite. With the progressive exhaustion of wolframite resources, unearthing scheelite with predominant reserve is of great significance for the sustainable development of tungsten industry, the guarantee, and the high-efficient utilization of tungsten resources. Directly decomposing scheelite by sodium hydroxide, as the current industrial method, requires high temperature, high pressure, and high alkali, which usually with high cost and causing serious environmental pollution. In view of the above problems, an environment-friendly process for the extraction of tungsten was proposed by using magnesium chloride roasting followed with sodium hydroxide leaching in our previous work. The process was trying to convert not decomposable scheelite-type CaWO4 in the scheelite into the decomposable wolframite-type MgWO4 through roasting conversion, which was easy to be dissolved by NaOH and the highly efficient extraction of tungsten from scheelite was achieved. In order to further clarify the reaction mechanism of the alkali leaching process of the roasting ore, the alkali leaching process of the MgWO4 calcine which was achieved by roasting scheelite concentrate and MgCl2 was studied. The effects of sodium hydroxide concentration, reaction temperature, particle size, stirring speed, and reaction time on alkali leaching process of MgWO4 calcine were investigated. Furthermore, the kinetics study of the leaching process was also carried out. The results showed that the concentration of sodium hydroxide and the reaction temperature were the main influencing factors in the leaching reaction process. The mineral particle size was the secondary factor, and the stirring speed showed no effect on the leaching process. The required time for reaching reaction equilibrium could be effectively shortened by increasing the concentration of NaOH and raising the reaction temperature. The results of the kinetic study showed that the leaching process fitted well to the corresponding volume reaction model, which meant the leaching agent could be spread inside the mineral, and the reaction interface covered both inside and outside of the mineral particles. Moreover, the apparent activation energy was calculated as 89.9 kJ·mol-1, and the reaction order for the sodium hydroxide concentration was 1.718, the influence index of mineral particle size was -0.391. The reaction process was controlled by chemical reaction and the kinetics equation of the process was established as: x/(1-x)=3.16×108CNaOH1.718D-0.391exp[-89900/(8.314T)]t. XRD pattern of the leaching residue showed that the characteristic peaks of MgWO4 decreased significantly, and the characteristic peaks of Mg(OH)2 were appeared. SEM images showed that Mg(OH)2 generated in the surface of the mineral particles, and formed a new layer, which was loose and porous. This work provided a theoretical basis for the efficient extraction of scheelite roasting conversion. © 2022, Youke Publishing Co., Ltd. All right reserved.
