Study On Species Structure In W-Mo-H₂O₂-H₂O System And Its Application For Separating Molybdenum From Tungsten By Solvent Extraction

Tungsten（W）,as an important strategic metal,is one of the key elements in the preparation of high-technology and sophistication materials,so it has strict requirements on the content of impurities.Molybdenum（Mo）is one of the main impurities in tungsten metallurgy.The properties of W and Mo are similar due to the lanthanide contraction,which makes it difficult to separate them.Therefore,the deep removal of Mo has always been a research focus in the field of tungsten metallurgy.There are some problems in the treatment of tungstate solution containing high molybdenum content by the existing separation methods,such as incomplete Mo removal,large W loss,high cost,and serious environmental pollution,and these problems will be further highlighted with the complexity of tungsten ore resources and the increase of impurity Mo content.The separation method based on the hydrogen peroxide（H₂O₂）solution system is suitable for removing Mo from tungstate solution with high Mo content.It has broad industrial application prospects because of its advantages of high efficiency,low cost and environmentally friendly,which is highly in line with the development concept of clean metallurgy.However,there is a lack of corresponding theoretical research due to the complexity of the aqueous solution chemistry of W and Mo in this system,which makes it difficult to effectively solve the problems exposed in industrial tests such as the poor stability of feed solution,the formation of the third phase and the unsatisfactory removal efficiency of Mo.Therefore,some studies including the formation and conversion behavior of W and Mo species in solution,the extraction equilibrium and mechanism,the formation and elimination mechanism of the third phase and the deep removal mechanism of Mo were carried out in this work by taking the W（VI）-Mo（VI）-H₂O₂ solution and TRPO-TBP extraction system as research object.A new method for preparing W（VI）-Mo（VI）-H₂O₂ solution based on“coordination p H value”regulation was proposed creatively,which successfully solves the above problems from the root and further facilitates the industrial application of satisfactory separation of W and Mo in peroxy system.The main contents and results are as follows:（1）On the basis of the thermodynamic data of species in the Mo（VI）-H₂O、W（VI）-H₂O、W（VI）-Mo（VI）-H₂O and Mo（VI）-H₂O₂-H₂O systems,the thermodynamic model of W and Mo ion equilibrium in these systems were constructed and solved,and then the formation and distribution of W and Mo species were revealed.Furthermore,the possible depolymerization and coordination reactions in the W（VI）-H₂O₂-H₂O and W（VI）-Mo（VI）-H₂O₂-H₂O systems which lack thermodynamic data were analyzed through the characterized peroxy complexes.Compared with the Mo（VI）-H₂O system([Mo]=0.10 mol·L^-1]),tungstate ion has priority over Mo O₄^2-,and its initial and complete polymerization p H values of WO₄^2-in the W（VI）-H₂O system([W]=0.05 mol·L^-1])are about 1.0 higher than that of Mo O₄^2-.In the mixed solution of W and Mo,the two can form W-Mo heteropoly compounds in addition to their own polymerization,and the Mo/W atomic ratio(AR_Mo/W)of heteropoly compounds increase with the decrease of p H value,their formation order is as follows:[Mo W₆O₂₄]^6-,[Mo₂W₅O₂₄]^6-,[Mo₃W₄O₂₄]^6-,[Mo₄W₃O₂₄]^6-,[Mo₅W₂O₂₄]^6-and[Mo₆WO₂₄]^6-.In the H⁺-Mo O₄^2--H₂O₂-H₂O system,the degree of polymerization increases with the decrease of p H value,while the higher order polymolybdate is further depolymerization into mononuclear and binuclear peroxomolybdate with the increase of H₂O₂ dosage.（2）By comparing the characterization results of W and Mo species in solution with four different analytical methods,the research route for analyzing solution composition via Raman spectroscopy was determined.Based on the spectral data of the general aqueous solutions and peroxy complexes of W and Mo,an in-situ characterization method-“Raman spectrum–Gaussian fitting”–was established,which is suitable for species identification and conversion behavior analysis of W and Mo species in peroxy complex solution.The effects of p H value,H₂O₂ dosage and time on the conversion of W and Mo species in peroxy system were investigated by in situ Raman spectroscopy.The results showed that under different p H values and H₂O₂ dosages,tungsten and molybdenum were observed to exist in a variety of peroxy complex forms,including:[HMo₄O₉（O₂）₄]^-(～923 cm^-1),[Mo₇O₂₃（O₂）]^6-(～935 cm^-1),[Mo₇O₂₂（O₂）₂]^6-(～945 cm^-1),[HMo O₂（O₂）₂]^-(～960 cm^-1)and[Mo₂O₃（O₂）₄]^2-(～972 cm^-1);[WO₂（O₂）₂]^2-(～950 cm^-1),[W₂O₃（O₂）₄]^2-(～962 cm^-1)and[W₇O₂₂（O₂）₂]^6-(～975 cm^-1).Reducing p H value and increasing H₂O₂ dosage facilitated the formation of peroxotungstates and peroxomolybdates and slowed down their dissociation.When H₂O₂/（W+Mo）≥2.0（molar ratio）and p H=1.8～3.0,tungsten and molybdenum can basically exist in the form of[W₂O₃（O₂）₄（H₂O）₂]^2-and[Mo₂O₃（O₂）₄（H₂O）₂]^2-.In acidic W（VI）-Mo（VI）-H₂O₂-H₂O solution,the coordination of Mo species is faster and more thorough than that of W species,and peroxomolybdates is more stable than peroxotungstates.In addition,the presence of Mo can slow down the peroxotungstate,thereby improving the stability of the solution.（3）The extraction equilibrium of Mo and W from acid peroxy complex solution by TRPO,TBP and TRPO-TBP extraction systems was studied systematically by slope method,and the extraction distribution models of Mo and W were established.These models indicated that log D_Moand log D_W were positively correlated with p H value and extractant concentration,and negatively correlated with temperature,which can be used predict the extraction behavior and separation effect of W and Mo in Mo（VI）-H₂O₂-H₂O,W（VI）-H₂O₂-H₂O and W（VI）-Mo（VI）-H₂O₂-H₂O solutions with low Mo and W concentrations.Based on the extraction models and the spectral results of extraction compounds,the extraction mechanisms of Mo and W at H₂O₂/Me=6.0 were revealed,that is,the neutral coordination between extractant and peroxy complex,[H₂Me₂O₃（O₂）₄（H₂O）₂]·2R,where Me is Mo or W,and R is TRPO or TBP.In addition,the separation of W and Mo in peroxy complex solution by mixed extraction system was better than that by single extraction system:TRPO-TBP>TRPO>TBP,and increasing the extraction equilibrium p H value was beneficial to improve the separation coefficient(β_Mo/W).Under certain conditions,the TRPO-TBP extraction system has a certain positive synergistic effect on Mo and W extraction,and the synergistic effect on Mo extraction is more obvious and the synergistic extraction compound is TBP·[H₂Mo₂O₃（O₂）₄·2H₂O]·TRPO.Based on the above extraction equilibrium of different systems,the corresponding apparent extraction equilibrium constants were estimated:log K_Mo-TRPO-H=5.8,log K_Mo-TRPO=3.8,log KMo-TRPO-TBP-H=4.3,log KMo-TRPO-TBP=2.3,log KMo-TBP-H=1.5,log KW-_TRPO=3.7。（4）Based on the thermodynamics of the Mo（VI）-H₂O,W（VI）-H₂O and W（VI）-Mo（VI）-H₂O systems and the chemical behavior of W and Mo in peroxy solution,a new method for preparing W-Mo-H₂O₂ solution focused on controlling coordination p H value,the“weakly acidic coordination method”,was proposed.The key feature of this method is that the solution p H value is controlled in a higher range（5.3～6.5）in the first step acidification to perform the peroxy coordination of W and Mo.Subsequently,the effectiveness of this method for realizing the third phase elimination and the deep Mo removal was verified by contrast experiments,and the formation mechanism of the third phase was further revealed.Specifically,the third phase is an insoluble mixed extraction compound formed by a neutral coordination between TRPO and uncomplexed isopolytungstate（mainly metatungstic acid）,i.e.,[H₆W₆O₂₁·4TRPO]and[H₈W₁₂O₄₀·8TRPO].The order of complex conversion of different Mo species is clarified as follow:isopolymolybdate>low AR_Mo/W heteropoly compounds(AR_Mo/W<2)>>high AR_Mo/W heteropoly compounds(AR_Mo/W>2),and it is pointed out,for the first time,that high AR_Mo/Wheteropoly compounds is the key reason to incomplete removal of Mo.Finally,the separation effects of the W（VI）-Mo（VI）-H₂O₂-H₂O solutions with high Mo/WO₃ mass ratio（～12.5%）prepared by different methods were compared through six-stage countercurrent extraction.Compared with the traditional method,the new method can improve the removal efficiency of Mo about 20-fold,and the Mo/WO₃ mass ratio in the raffinate is reduced to 2.71×10^-5,the Mo concentration is less than 3.54 mg·L^-1,while the tungsten loss is less than 2%.More importantly,there is no need for acidity adjustment and solution storage in the extraction process.