Thermal Decomposition Of Scheelite By Soda To Produce Sodium Tungstate

This paper investigated a novel process to separate tungsten from gangue material to produce sodium tungstate with high purity, in which the sheelite was used as feed and the flowsheet was dry ball milling activation-roasting-high temperature smelting with slag former adding. The main research work included directly high temperature smelting and ball milling activation-roasting-high temperature smelting. The conclusion can be listed as follows:1. The experiment to decompose scheelite by soda through directly high temperature sintering process. The result showed that the decomposition ratio of scheelite was extermely limited(all<60%), even under the conditions when the temperature was higher than900℃, the addition amount of alkali was1.0-2.0times of theory stiochemistry consumption, the reaction time was extend to180minutes and the weight percent of additives to the raw material was more than10%. This result was in accord with theoretical analysis. It proved the process was infeasible. TG-DSC analysis demonstrated that the optimum temperature range for scheelite decomposition was590℃-850℃. Given comprehensive consideration, the reaction temperature of decomposition should be under865℃. And the temperature of smelting should be greater than1000℃. Thus, it is necessary to make decompose reaction process and sintering operation separately.2. The research of soda roasting of sheelite. The results showed that under the conditions:roasting temperature was595℃, reaction time was2hour, alkali additive amount was2.3times of theory stiochemistry consumption, the sheelite decomposition ratio reached93.79%. Adding quartz had a negative effect on decomposition.According to the unsatisfied roasting result obtained from above, the process of mechanical activiation to improve scheelite decomposition ratio was investigated. The technology of wet ball milling activation was used to enhance the scheelite decomposition. The desired parameters were listed as:ball milling time was60min, the rotate speed was100r/min, the number ratio of big ball to small ball was1:8, and the weight ratio of ball to material was12:1. After wet ball milling activation followed by roasting, the sheelite decomposition ratio was98.53%and the mass of WO3in slag was2.1%.After wet ball milling activation, the materials have to be still dried for roasting and high temperature smelting. So the method of dry ball milling activation was also chosed to enhance the sheelite decomposition. The optimum conditions were:ball milling time was20minutes, the rotate speed was250r/min, the number ratio of big balls to small balls was1:10, and the weight ratio of ball to material was16:1. After dry ball milling activation followed by roasting, the sheelite decomposition ratio was98.4%and the mass amount of WO3in slag was2.1%.3. The high temperature smelting. Without adding of slag formers, the smelting results showed that the separation of sodium tungstate and slag was very difficult only by increasing smelting temperature(from1100℃to1500℃). With adding of slag formers, the results showed that the decrease of CaO ratio in slag former and the increase of temperature could improve the property of slag and effectively separate slag and sodium tungstate. The addition of sodium floride can not obviously improve the smelting property.The phase analysis revealed that the main components of slag were:Na1.55(Al1.55Si0.45O4)、Ca(Fe1.4Al0.6)SiO6. And the main phase of sublayer solid was Na2WO4. The analysis result of solid melting point indicated that with the decrease of CaO ratio in slag, the melting point of slag lowered. The obtained slag tapping temperature was1100℃-1200℃with the slag formers of C and D patterns. The melting flowing temperature of sublayer solid(sodium tungstate) was about650℃. Thus, the layered discharge operation of sodium tungstate and slag in industry can be carried out in the temperature range of1100℃-1200℃.4. Based on the above segmentation experiment results, the comprehensive experiment was carried out. The results obtained can be listed as follows:the total amount of WO3in slag was about3.91%in which the undecomposed of WO3was only0.699%and the rest were entrained sodium tungstate. The composition of the sublayer included Na2WO4:85.96%, Na2MoO4:8.05%, P:0.038%(the dephosphorization rate was77.08%), SiO2:0.26%(the desolication rate was94.77%). And the amount of (Ca, Ba) WO4residue obtained from the water dissolved experiment of sodium tungstate solid was about4%.In conclusion, the process of dry ball milling activation-roasting-high temperature smelting with slag former adding, not only effectively decompose scheelite (the decomposition ratio was above98%), but also produce sodium tungstate with high purity through effective separation of tungsten from gangue. The purity of sodium tungstate (include sodium molybdate) was above94%. The dephosphorization rate and the desolication rate were up to77.08%and94.77%, respectively. Molybdenum cannot be separated from tungsten in decomposition and smelting. Sodium tungstate solution with high purity can be obtained by dissolving sodium tungstate solid in water (when CWO3was100g/L, the concentrations of phosphorus and silicon dioxide were0.05g/L and0.38g/L). The residue can be reused for roasting and smelting.