Study On The New Technology Of Synergistic Leaching Of Low Grade Scheelite With Sodium Phosphate And Sodium Hydroxide And Calcium Fluoride

At present,wolframite resources which are easy to beneficiated and smelted are gradually exhausted,and low-grade scheelite has gradually become the main raw material of the tungsten industry.Because low-grade scheelite is associated with minerals such as fluorite and calcite with similar floatability,it is difficult to obtain high-grade scheelite concentrate while ensuring the recovery rate of tungsten during the flotation process.However,the domestic and foreign mainstream sodium hydroxide decomposition and sodium carbonate pressure cooking processes for low-grade scheelite have the disadvantages of large amount of leaching reagents and low tungsten leaching rate.Taking advantage of the characteristics of calcium fluoride associated with low-grade scheelite,this study proposes a new process for synergistic leaching of low-grade scheelite with sodium phosphate-sodium hydroxide-calcium fluoride,which achieves low-consumption and high-efficiency decomposition of scheelite.The research content includes the thermodynamics and kinetics of scheelite leaching process,the dissolution behavior of calcium fluoride,the process of Na₃PO₄-NaOH-CaF₂synergistic leaching of low-grade scheelite and the removal of fluoride from crude sodium tungstate leaching solution.The main findings are as follows:Based on the existing thermodynamic data,the logc-pH diagram of different free total phosphorus concentrations and total fluorine concentrations in the system of Na₃PO₄-NaOH-CaF₂synergistic decomposition of scheelite under 25 ^oC were calculated and drawn.The results of thermodynamic analysis show that scheelite can be effectively decomposed by using sodium phosphate and sodium hydroxide and calcium fluoride.The decomposing product is Ca₅(PO₄)₃F in the range of 4.72?15.10.Increasing the concentration of free total phosphorus,free total fluorine and pH value of solution are beneficial to the decomposition of scheelite.The kinetics and behavior of calcium fluoride decomposition by Na₃PO₄-NaOH were studied.The results show that the decomposition rate and degree of calcium fluoride are greatly affected by temperature,but relatively less affected by the concentration of phosphate.The decomposition process of calcium fluoride is controlled by a chemical reaction,the apparent activation energy of the reaction is 134.57 k J/mol,and the apparent reaction order of phosphate is 0.92.The reaction can increase the leaching rate and leaching rate of calcium fluoride by increasing the temperature,the amount of sodium phosphate and the amount of sodium hydroxide.The research on the kinetics and technology of sodium phosphate-sodium hydroxide-calcium fluoride synergistic leaching of low-grade scheelite was carried out.The results show that the leaching process of low-grade scheelite is controlled by chemical reaction,the apparent activation energy of leaching reaction is 48.70 k J/mol,the apparent reaction order of phosphate is 0.48,and the apparent reaction order of fluoride is 0.13.Increasing the leaching temperature and the amount of sodium phosphate is beneficial to the leaching of scheelite,while the amount of calcium fluoride and the liquid-to-solid ratio are too large or too small are not conducive to the leaching of scheelite.The best process conditions for low-grade scheelite leaching are:temperature 180 ^oC,sodium phosphate dosage 2 times the theoretical amount,calcium fluoride dosage 1.5 times the theoretical amount,sodium hydroxide dosage 14%of the scheelite mass,the liquid-solid ratio is 4:1,and the leaching rate of scheelite reaches 98.35%after 4 hours of reaction..The leaching residue contains a large amount of undissolved associated calcium carbonate,and the selective leaching of low-grade scheelite is realized in the leaching process.The leaching solution of low-grade scheelite contains about 3 g/L of fluorine.In order to reduce the pressure of fluoride removal from the subsequent wastewater,magnesium chloride is used as a fluoride removal reagent to precipitate and remove fluorine in the crude sodium tungstate.The results show that increasing the amount of magnesium chloride and lowering the pH of the solution are beneficial to increase the fluoride removal rate.The best process conditions for fluorine removal are:when the dosage of magnesium chloride is 1.5 times the theoretical amount,the temperature is 80 ^oC,and the time is 90 min,the fluorine removal rate reaches 90.64%.When the initial fluorine concentration in the solution is 1?5 g/L,the fluorine removal rate is stable above 85%.The new technology of sodium phosphate-sodium hydroxide-calcium fluoride synergistic leaching of low-grade scheelite can reduce the requirements of the flotation concentrate scheelite grade during the beneficiation process and increase the recovery rate of scheelite beneficiation;on the other hand,it can achieve High-efficiency and selective leaching of low-grade scheelite,thereby greatly improving the comprehensive utilization of low-grade scheelite resources.