Study On The New Process And Mechanism Of Harmless Treatment And Source Reduction Of Hazardous Wastes From Tungsten Hydrometallurgy

China has the largest tungsten reserves in the world,and currently hydrometallurgy is the main method of tungsten metallurgy both domestically and internationally.Currently,most tungsten concentrates in China are wolframite and scheelite mixed concentrates,mainly produced by the process of alkaline pressure leaching-ion exchange（alkaline extraction）-evaporation crystallization to produce APT.The residue waste from tungsten conctntrates by alkaline pressure leaching is called tungsten residue wastes（TRW）,mainly composed of iron（Fe）,manganese（Mn）,and calcium（Ca）oxides.In addition,it also contains high value metal elements such as W,Sn,Ta,Nb,Sc,as well as harmful elements such as arsenic（As）,lead（Pb）,mercury（Hg）,copper（Cu）,zinc（Zn）.At home and abroad,end treatment is widely adopted for TRW,mainly containning tungsten recovery and As removal.Tungsten recovery has problems such as low recovery efficiency and secondary residues still being hazardous waste,while roasting or reduction smelting for As removal has high energy consumption and is prone to secondary pollution.The ammonium tungstate solution containing molybdenum is obtained from the leach solution by the main process of ion exchange or alkaline extraction.Currently,tungsten enterprises mainly use copper salt precipitation method to remove molybdenum,and the generated slag is molybdenum removal slag（MRS）,with a Mo content of 20%～30%,WO₃ content of 2～10%,and Cu and S content of about 30%.Currently,most of the MRS in industry is sold and processed as molybdenum concentrate,resulting in waste of resources such as W and Cu.The reduction of MRS at the source is in line with current environmental protection concepts.The ion exchange method using polyamine based weakly alkaline resin and the precipitation method using molybdenum trisulfide have industrial applications for molybdenum removal.However,Mo is recovered as Mo S₃,with average economic value and easy generation of H₂S.The new process of quaternary ammonium salt extraction for molybdenum removal has the characteristics of high molybdenum removal efficiency,almost no tungsten loss,no hazardous waste,no introduction of impurity ions,molybdenum recovery in the form of high-value ammonium molybdate,and wide adaptability of raw materials.Laboratory bench experiments have been completed,but continuous operation tests on a pilot scale have not been conducted.The extraction and stripping performance and parameter optimization after organic phase circulation still need to be investigated.This article focuses on the study of end treatment and source reduction of TRW and MRS generated in the tungsten hydrometallurgy.The main content and conclusions of the study are as follows:（1）The element occurrence state and As migration behavior of TRW were studied.As in TRW mainly exists in the form of ionically bound As,strongly adsorbed As,iron/manganese（II）oxide bound As,amorphous iron arsenate（AFA）,and arsenopyrite（Fe As S）,accounting for 5.91%,6.75%,18.16%,5.66%,and 55.31%of total arsenic,respectively.The iron/manganese（II）oxide bound As and AFA are uniformly distributed and encapsulated in Fe and Mn oxides.The main existing forms of W are amorphous Fe WO₄ and Mn WO₄,which are uniformly distributed in compounds composed of Fe Mn Si O;S mainly occurs in the TRW in the form of arsenopyrite and pyrite.During the long-term open-air storage of TRW,arsenopyrite and hydrated iron oxide（HFO）undergo weathering.The weathering products mainly include goethite,sulfate,and ionically bound As（+5）.Due to their strong bioavailability and migration ability,the leaching toxicity of tungsten alkali slag is higher than 5 mg/L.In addition,the high loading of As on hydrated iron oxide（HFO）inhibits its conversion to goethite,resulting in almost unchanged leaching toxicity of Pb,Zn,Cu,and Hg.（2）Based on the occurrence state of As and W and the thermodynamic and kinetic analysis of arsenopyrite leaching,an alkaline pressure oxidation leaching method was creatively proposed to efficiently and selectively extract As and W from TRW,and process parameters were optimized.Under the conditions of particle size 240～280 mesh,Na OH concentration 70 g/L,Na₂CO₃ concentration 20 g/L,liquid to solid ratio 5m L/g,170℃,0.1 MPa oxygen partial pressure,and leaching for 4 hours,the leaching efficiencies of As,W,and S were 98.05%,98.24%,and84.64%,respectively;The content of As in the secondary leach residue is0.05%wt%,which is lower than the national standard upper limit of As in solid waste;The content of W in the secondary leach residue is less than0.2 wt%,the leaching solution can be recycled for 6-7 times to enrich As and W.More than 95%of arsenic in the secondary leaching residue is in a residual state,with arsenopyrite accounting for approximately 1.76%.The vast majority of residual W is uniformly distributed in iron manganese oxides,but there are W-Si-O compounds enriched in tungsten,which is also the reason why the highest tungsten leaching rate can only reach around 84%.The surface of the secondary leach residue is rough,and after one year of weathering,hydrated iron oxide（HFO）transforms into smooth and dense goethite（Fe OOH）on the surface.The leaching toxicity of As further decreases to 0.007 mg/L,and the leaching toxicity of Pb,Zn,Cu,and Hg also greatly decreases.The leaching rate of As and S in TRW by alkaline pressure oxidation leaching follows a diffusion control mechanism,with apparent activation energies of 16.67 k J/mol and 15.66 k J/mol,respectively.The diffusion of reactants and products in the hydrated iron oxide（HFO）solid film layer is the limiting factor for the leaching rate.（3）As and W were separated and rcovered via solvent extraction followed by sulfurization precipitation methods in the enriched leach solution from TRW.The results showed that the recovery eficiency of W was close to 100%,the removal rfficiency of As was 96.68%,the removal efficiency of Si was 86.93%,and the removal efficiency of S was 99.73%.The reverse extraction solution can contain over 230 g/L of WO₃,with a WO₃/As ratio of 20.2,and can be directly incorporated into the leach solution of the main process of tungsten hydrometallurgy.After washing with H₂O₂,As（+5）mainly exists in the form of[As₂W₁₈O₆₂]^6-in the organic phase,while As（+3）is bound to the organic phase through hydrogen bonds in the form of H₃As O₃.The proportion of As（+3）in the extraction residue is about 45%.Under the conditions of molar ratio S/As=6:1,equilibrium p H 3.0,rotational speed 100 r/min,4 hours,and30℃,the arsenic precipitation mother liquor contains 0.08 g/L of As,with an As precipitation efficiency of 99.6%.As（+5）is first reduced to As（+3）and S,and then precipitated by H₂S.（4）The optimal parameters obtained in the pilot test of quaternary ammonium salt extraction for molybdenum removal are organic phase:extraction liquid:detergent:oxidant:reverse extraction agent at a ratio of1:4.2:0.1:0.14:0.16,which can achieve continuous deep Mo removal with almost no tungsten loss.The reffinate can meet the requirements for preparing grade 0 APT,and the strip solution can prepare high-quality ammonium molybdate（WO₃/Mo<1%）.For the WO₃ 200 g/L and Mo 0.323g/L feed solution,when producing 1 ton of APT,using quaternary ammonium salt extraction to remove molybdenum saves 270.58 yuan compared to copper salt precipitation method.As the concentration of Mo increases,due to the high value of molybdenum products in the quaternary ammonium salt extraction molybdenum removal process,the molybdenum removal process may even generate considerable profits,while the copper salt precipitation method will have higher costs due to the doubling of the amount of copper salt added.In addition,this new process has achieved the reduction of molybdenum slag at the source,and has good industrial application prospects.（5）The new process of treating the end of TRW and reducing the amount of MRS at the source can be connected with the current main process of alkaline extraction in tungsten hydrometallurgy.The entire process is environmentally friendly,and the molybdenum in tungsten concentrate is recovered with high value ammonium molybdate,with an overall recovery eficiency of over 99%.In addition,about 2%of As in tungsten concentrate enters the final secondary leaching residue,and about98%of As enters the sulfur arsenic residue.Besides,the quaternary ammonium salt is of the same type as the alkaline tungsten extraction main process extraction agent,that is,the organic phase in the alkaline tungsten extraction reverse extraction solution can be reused during the molybdenum removal process.This green tungsten hydrometallurgy process has certain advantages in environmental protection and economic benefits,and is expected to be promoted and applied in the hydrometallurgy of wolframite and scheelite mixed concentrates.