Tungsten Extraction from Scheelite by NaHSO4 Roasting-HCl Leaching

The alkaline decomposition and ion exchange process was widely used to treat scheelite in China,but there were some problems such as a large amount of hazardous waste,a large amount of waste water and high production cost,which seriously restrict the sustainable development of tungsten metallurgy industry in China. Especially after the National Catalogue of Hazardous Waste issued by the Ministry of Environmental Protection of the People′s Republic of China in 2016,China′s tungsten metallurgy industry was facing the dual pressure of economy and environmental protection. In this background,domestic researchers have developed Na2CO3 decomposition-alkaline extraction,phosphorus salt-fluorine salt decomposition of scheelite and sulfur-phosphoric mixed acid decomposition of scheelite and other new processes,have made outstanding contributions to alleviating the production pressure of enterprises in the economic and environmental aspects. However,with the continuous consumption of high-quality tungsten ore,tungsten raw ore grade declined significantly,the amount of flotation reagent input in the beneficiation process increased,resulting in the increase of residual flotation reagent in concentrates,and the traditional alkali pressure decomposition process and the new process are all wet metallurgy process,there are risks to tank spillage,bring inconvenience to production. This paper continued the characteristics of roasting to solve the flotation reagent problem on the basis of a lot of researches on the roasting and decomposition process of scheelite,and put forward a new metallurgy idea of sodium bisulfate scheelite by one-step roasting,which was combined with the advantages of metallurgy slag decomposition of scheelite by sulfur-phosphoric mixed acid,convert CaWO4 into WO3 and CaSO4,and then deal with the roasting product by acid leaching-dissolve in ammonia-purification-evaporative crystallization to produce ammonium paratungstate(APT). In terms of experiments,muffle furnace was used as the roasting experimental equipment to heating the sample which is evenly mixed by scheelite concentrate and NaHSO4. After roasting,hydrochloric acid leaching was used to obtain WO3 enrichment detection. In terms of detection,X-ray diffraction(XRD)and Mineral Liberation Analyser(MLA),Process Mineralogical Parameter Testing System were used to characterize the roasting product.In addition,hydrogen peroxide-hydrochloric acid mixed solution was used to dissolve the residual CaWO4 in the roasting product,and then WO3 content in the solid phase was determined by ammonia tungstate burning gravimetric method which was compared with the MLA test results in order to accurately calculate the conversion rate of calcium tungstate in the roasting experiment. The results showed that the roasting products are mainly WO3,Na2SO4 and CaSO4 after roasting and decomposition of scheelite by sodium bisulfate,and WO3 was mainly divided into two kinds of occurrence states of granular WO3 and micro-granular WO3,which were dispersed in the roasted products,and the other phases in the roasted products were mainly CaSO4 and Na2SO4. A small amount of columnar Na2SO4 was mixed in CaSO4 aggregate of plate-like crystal,and the gap was filled with Na2SO4 and granular WO3,Na2SO4 and CaSO4. The roasting process research showed that the roasting temperature,time and material proportion all had an impact on the decomposition of scheelite by sodium bisulfate roasting. WO3 conversion rate increased with the roasting temperature and time,and eventually tended to be stable. However,the amount of sodium bisulfate added should not be too much,because the water of crystal released by excessive NaHSO4·H2O would lead to scheelite agglomeration during mixing,so that scheelite aggregates would form WO3 shells during roasting,which hindered the contact between inner CaWO4 and outer NaHSO4· H2O,resulting in the decrease of CaWO4 conversion rate. Under the condition that the dosage of 2.5 times of NaHSO4·H2O theoretical dosage,roasting time of 2.5 h,and roasting temperature of 650 ℃,99.5% CaWO4 in scheelite have been converted to WO3. The roasting products were treated with hydrochloric acid leaching. The hydrochloric acid leaching process showed that the leaching concentration,temperature,time,liquid-solid ratio and rotating speed were all favorable for CaSO4 leaching. However,the concentration of hydrochloric acid should not be too high,otherwise,calcium sulfateoccur ion association,tungsten and calcium separation effect would decline. Under the conditions of hydrochloric acid concentration of 3 mol·L-1,leaching time of 60 min,liquid-solid ratio of 15∶1,rota-tion speed of 300 r·min-1 and acid leaching temperature of 90 ℃,the removal rate of CaSO4 in the roasting products reached 98.04%. WO3 in the product was enriched from 28.95% to 95.23%,and the impurities in the acid leaching product were small amount of CaSO4 and SiO2. Acid leaching products were obtained qualified APT products after dissolving in ammonia-purification-evaporation crystallization. Compared with the traditional alkali decomposition process and other new wet processes,NaHSO4 conversion roasting process of scheelite got rid of the troubles of flotation reagents in the metallurgy process,CaSO4 slag was nontoxic and harmless,and could provide a green economy new technological prototype for efficient green utilization of scheelite with no ion-exchange. © 2023 Editorial Office of Chinese Journal of Rare Metals. All rights reserved.