Clean Metallurgy Of Wolframite In Hydrochloric Acid System And Comprehensive Utilization Of Various Components

Tungsten is a strategic rare metal and is known as the teeth of industry.Wolframite is an important tungsten extraction resource.The traditional tungsten extraction process based on high-pressure alkali digestion of wolframite produces a large amount of waste residue and wastewater,and the alkali leaching residue has been listed as hazardous waste.It is an urgent demand of the industry to develop efficient tungsten extraction process from wolframite to realize clean extractive metallurgy.In view of the environmental problems faced by alkali digestion process,a process of hydrochloric acid digestion of wolframite-Na₂CO₃ leaching of crude tungstic acid-solvent extraction of Na₂WO₄-recycling of raffinate and acid digestion solution for clean tungsten extraction was developed.The method of atomization-pyrolysis of acid solution was proposed to realize the recycling of HCl and comprehensive utilization of Fe and Mn.The raffinate containing Na₂SO₄ was converted into Na₂CO₃ solution by adding Ba CO₃ for recycling.The tungsten and silicon were further recovered from tungsten extraction residue,and the tin,tantalum and niobium in residue were highly enriched.Moreover,resource efficiency analysis and life cycle assessment（LCA）study were carried out on the tungsten extraction process.The main conclusions obtained in this paper were as follows.（1）The digestion behavior and mechanism of wolframite concentrate by hydrochloric acid were systematically studied.The digestion of wolframite by hydrochloric acid was with good thermodynamic trend.The wolframite digestion efficiency reached 99.3%under the optimized conditions of concentrate fineness D（95）=20μm,HCl concentration of 7 mol/L,liquid-solid ratio of 3:1,reaction temperature of 90℃and reaction time of 4 h.The digestion of wolframite in hydrochloric acid was a process of tungsten compounds dissolution-H₂WO₄ deposition and continuous formation.The kinetics of wolframite digestion process conformed to the unreacted core shrinking model,which was divided into two stages.The first stage was a surface reaction controlling process while the second stage was transformed into a hybrid controlling process by surface reaction and internal diffusion due to the formation of solid H₂WO₄ film.The kinetic equations of the two reaction stages were as follows.1-（1-x）^1/3=0.19×C^1.60·D^-0.81·exp（-40570/RT）·t4×[1-（1-x）^1/3]+[1-2x/3-（1-x）^2/3]=1.41×C^0.75·D^-0.31·exp（-38160/RT）·t（2）To verify the applicability of hydrochloric acid digestion method,the digestion behavior difference and mechanism between wolframite and scheelite in the hydrochloric acid digestion of mixed wolframite-scheelite concentrate were studied.In the hydrochloric acid treatment,scheelite digestion efficiency was much higher than that of wolframite,and the preferential digestion order of tungsten minerals was Ca WO₄>Mn WO₄>Fe WO₄.The crystal structure analysis on tungsten minerals showed that the Fe,Mn and Ca atoms in wolframite and scheelite crystals were connected to the O atoms.The interaction between Ca,Mn,Fe and O increased in order,resulting in the increased dissolving difficulty of Ca,Mn and Fe from mineral crystals.And the crystal structure of wolframite was tighter than that of scheelite,which leaded to the worse decomposability of wolframite than that of scheelite.Part of wolframite can be converted into scheelite by mechanical activation with adding Ca（OH）₂,thus improving the digestion efficiency of wolframite.（3）A clean tungsten extraction route of Na₂CO₃ leaching of crude tungstic acid-tertiary amine extraction-raffinate conversion and circulation was proposed to realize near zero discharge of sodium salt wastewater in the extraction process.At the optimized conditions of Na₂CO₃stoichiometric ratio of 1.2,reaction temperature of 40℃,liquid-solid ratio of 4:1,and reaction time of 60 min,the tungsten leaching efficiency reached 99.6%.For Na₂WO₄ solution containing 121.26 g/L WO₃,the complete extraction of tungsten could be obtained by one-step with an organic phase of 15%N235 or 15%Alamine 308 and under the optimized conditions of extraction p H<3.0,O/A=1,and extraction time of 4 min.The extraction mechanism of N235 and Alamine 308 for tungsten was similar,while the extraction performance of Alamine 308 was better than N235.The saturated WO₃ loading capacities of 15%Alamine and 15%N235 were179.0 g/L and 156.4 g/L.The stripping efficiency of tungsten from N235and Alamine 308 reached 99.98%with NH₃·H₂O as stripping agent and at the conditions of 4 mol/L NH₃ and stripping time of 10 min.The（NH₄）₂WO₄ solution with more than 260 g/L WO₃ and low K content was obtained,which could be used to produce high-quality ammonium paratungstate（APT）.The raffinate containing high Na₂SO₄ concentration was converted into Na₂CO₃ solution by adding Ba CO₃ with a conversion efficiency of more than 89%.The obtained Na₂CO₃ solution could be recycled for crude tungstic acid leaching process.（4）In view of the problem that the Fe²⁺and Mn²⁺accumulation reduced the digestion efficiency of wolframite during the recycling of acid mother solution,a process of solution atomization and pyrolysis-medium recycling was proposed.Fe²⁺,Mn²⁺and other metal ions were effectively removed from acid mother solution,and the recycling of HCl and comprehensive utilization of Fe and Mn were synchronously realized.Fe Cl₂ in solution was easy to be decomposed,while the theoretical decomposition temperatures of Mn Cl₂ and Ca Cl₂ were 600℃and 770℃.The removal efficiency of Fe Cl₂,Mn Cl₂ and Ca Cl₂ from acid solution were 90.98%,92.84%and 93.26%respectively at the conditions of solution flow of 30m L/min and pyrolysis temperature of 600℃.When the reaction temperature was 700℃,the chlorides decomposition was almost stable,and micron sized spherical powder of Fe₂O₃ and Mn₂O₃ was obtained.For the solution after atomization and oxidation pyrolysis treatment,the concentrations of Fe²⁺and Mn²⁺were lower than the concentration that affected the wolframite digestion,and the treated solution could be reused to the acid digestion process.The oxidation pyrolysis process included evaporation of aqueous medium and HCl,precipitation of solid chlorides particles containing crystal water,dehydration reaction of chlorides containing crystal water,and oxidation decomposition of chlorides.（5）After the hydrochloric acid digestion of wolframite concentrate and Na₂CO₃ leaching of crude tungstic acid,the yield of tungsten extraction residue was only 4.5%.A variety of valuable metals were highly enriched in the tungsten extraction residue,which contained 8.22%WO₃,1.37%Sn,1.56%Ta+Nb and 67.33%Si O₂.The mineralogical characteristics of tungsten extraction residue were investigated.Alkali digestion-neutralization precipitation separation process was developed to comprehensively recover tungsten and silicon from the residue.The Si O₂and tungsten leaching efficiencies of 94%and 80.18%were obtained at the conditions of Na OH 300 g/L,reaction temperature of 190℃,liquid-solid ratio of 6:1,and reaction time of 2 h.The contents of Sn,Ta and Nb in alkali leaching residue were increased to 4.12%,1.28%and 3.46%respectively with the enrichment ratio more than 3.Based on the different solubility of tungsten and silicon components at different solution p H,the p H of alkali leaching solution was adjusted to 9.8 at the reaction temperature of 50℃with concentrated sulfuric acid as neutralizing agent,obtaining silicon precipitation efficiency of 96.21%.Silica precipitate was further prepared into silica product,and the crude Na₂WO₄ solution after silicon precipitation could be used for further tungsten extraction.（6）Resource efficiency of the clean tungsten extraction process was analyzed,LCA method was used to quantitatively compare the environmental impact of the clean tungsten extraction process with that of traditional process,and the key factors of the new process producing environmental impact were analyzed.The resource efficiency of the new process was 99.22%,which significantly higher than that of traditional process.The overall environmental impact of the new process was also significantly lower than that of traditional process.The main environmental impact categories caused by the new process were resource and fossil energy consumption,acidification,human toxicity（non carcinogenic）,greenhouse gases,smog,and eutrophication,accounting for22.24%,18.81%,18.66%,15.22%,8.87%,and 6.84%of the total environmental impact,respectively.The key stages of the new process producing environmental impact were extraction,hydrochloric acid digestion,atomization pyrolysis,and leaching,accounting for 43.58%,17.61%,13.88%,and 13.13%of the total environmental impact,respectively.The figure,table and reference numbers are 89,34 and 200,respectively.