Basic Theory And Novel Process For Sodium Hydroxide Digestion Of Tungsten Ores By Reactive Extrusion

Tungsten ores is one of the important strategic resources in China. Up to now, tungsten extraction from the tungsten ores economically and efficiently is widely researched in the tungsten smelting industry.The basic theory and novel process were investigated in this paper and the main research contents were as follows:(1) The kinetics of sodium hydroxide leaching of scheelite was investigated systematically. The research results indicated that the leaching rate was controlled by the surface chemical reaction from60to100℃. Increasing of alkali concentration, temperature and the decreasing the particle size of the mineral can improve the reaction rate remarkably. Based on the solution activity, the apparent activation energy and the reaction order of the process were respectively determined as82.0kJ·mol-1and0.68. And the kinetics equation was established as1-(1-x)1/3=54.08a0.68r0-1exp(8.314T/82000)t. However, when the activity was replaced by concentration, the apparent activation energy and the reaction order of the process were determined as64.9kJ·mol-1and1.74, respectively. And the corresponding kinetics equation of the process was established as1-(1-x)3/1=20.12×10-3[NaOH]1.74r0-1exp(8.314T/64900)t. The determined kinetics parameters had obvious deviations with that determined based on the solution activity. This indicated that the activity of solution cannot be replaced directly with concentration to research the kinetics when the solution concentration was high, because its activity was affected by the concentration of solution and temperature.Based on the above study, the effects of mechanical activation on the apparent activation energy of sodium hydroxide leaching of scheelite concentrates and wolframite concentrates were investigated. The results indicated that mechanical activation effect can remarkably decrease the apparent activation energy of sodium hydroxide leaching of tungsten ores, which means that the tungsten ore can be digested with sodium hydroxide under a lower temperature. The leaching process was strengthened by mechanical activation. For scheelite concentrates, the apparent activation of the process was decreased from78.1kJ·mol-1to27.9kJ·mol-1by the mechanical activation after10min and for wolframite concentrates, the apparent activation of the process was also decreased from76.2kJ·mol-1to34.3kJ·mol-1by the mechanical activation after20min.(2) Reactive extrusion technology was widely used to treat the materials with high viscosity. By the technology, mass transfer of material with high viscosity can be solved and mechanical activation effect can be provided for the material. So, it is considerable to apply this technology to develop a novel process of sodium hydroxide digestion of highly viscous scheelite slurry. In view of reactive extrusion technology used in hydrometallurgy field for the first time, reaction engineering about the equipment used for developing novel process (twin screw extruder) was investigated in the paper.The research results indicated that the average residence time of flow in the reactor decreased with the rotating speed of screws increased, and increased with the temperature increased; the quantity of virtual slot calculated by tank column model decreased as the rotating speed of screws and temperature increased; the effect of the rotating speed of screws on the average residence time and the quantity of virtual slot was more significant than the temperature; the axial mixing intensity of fluid increased with the rotating speed of screws and temperature increased.Therefore, twin screw extruder is similar with a tubular reactor which is equivalent of numbers of full-mixed tank reactors in series. The equipment is more efficient than single full-mixed tank reactor (such as autoclave reactor).(3) Based on the above study, a novel technology of sodium hydroxide digestion of tungsten ores by reactive extrusion was developed. The effect of leaching temperature, screw rotating speed, reaction time and NaOH dosage on the decomposition of scheelite concentrates and wolframite concentrates were investigated. The experiments results showed that scheelite concentrate and wolframite can be digested efficiently and continuously by the novel process of reactive extrusion method.For scheelite concentrate, on the digestion conditions of the mass ratio of ores/water2.67:1, the dosage of NaOH2.2times to stoichiometry, the screw rotating speed160rpm and the reaction time3.5h and the temperature just120℃(which was30～60℃lower than that of the traditional processes), the WO3extraction can reach99.18%; While for wolframite concentrate, under the conditions of the mass ratio of ores/water2.67:1, the reaction time3.5h, the screw rotating speed160rpm and the dosage of NaOH2.2times to stoichiometry and the temperature just120℃, the WO3extraction can reached99.13%. Both scheelite concentrate and wolframite concentrate can be decomposed completely by the novel process.Besides, the adaptation of novel process to other tungsten ore had been investigated. The results showed that the novel process can deal with other tungsten ore well with all the WO3extraction more than99%, which meant that the novel process was adaptable rifely to tungsten ores.Compared with the traditional NaOH leaching process, tungsten ors can be decomposed completely by this novel process which is conducted on the conditions of atmospheric pressure, less water dosage, lower temperature and considerable dosage of NaOH.