Study On Extraction And Separation Process Of Mo And V From Spent HDS Catalysts

Based on the current industrial alkali recovery of Mo and V in spent catalysts,there are bottlenecks such as large alkali consumption,long process,and difficulty in recycling free alkali and water.In this study,the spent HDS catalysts were used as raw materials,and a new process for deoiling-oxidative roasting-mixing alkali leaching at high pressure and CO₃^2--HCO₃^-mixed quaternary ammonium salt extraction was proposed.This process not only achieves highly selective leaching and deep separation of Mo and V,but also helps to reduce alkali consumption and opens up a new path for free alkali and water recycling.The optimal conditions for deoiling were determined to be roasted at350℃for 4 h.The effects of different temperatures and times during the oxidative roasting process on the roasting loss of Mo and V,content of C and S,element occurrence state,phase transformation and leaching behavior of Mo,V and Al in spent HDS catalysts were investigated.The results show that:（1）the optimal roasting condition is direct roasting at700℃for 2 h;（2）the loss of Mo and V will occur when the roasting temperature exceeds 700℃;（3）When the roasting temperature is 500℃,the leaching rate of molybdenum reaches more than 99%;the leaching of vanadium is controlled by its oxidation degree and infiltration effect,and the leaching rate reaches 97%at 700℃;（4）Mo,Ni and Al are uniformly distributed in the spent HDS catalyst,while V and Fe exist in an adsorbed state;（5）During the roasting process,the conversion of low-valent compounds of Mo and V into high-valent oxides occurred,the phase transition fromγ-Al₂O₃ toα-Al₂O₃ occurred,and the infiltration of adsorbed vanadium occurred from the surface to the interior.The influence of three alkali systems,Na₂CO₃,Na OH and Na₂CO₃-Na OH,on the leaching effect of Mo,V and Al in the leaching process were studied.1.5 times Na₂CO₃+1.5 times Na OH（theoretical dosage multiple）mixed alkali system was determined as the optimal leaching.The effects of leaching temperature,leaching time and liquid-solid ratio on the leaching effect of Mo and V were investigated under the mixed alkali system.The leaching rates of Mo and V are 99.12%and 96.86%respectively,and the obtained leaching solution is of good quality and has no burden of impurity removal.The extraction effect of mixed quaternary ammonium salt extractant on Mo and V at different pH values was studied.Combined with the ionic forms of Mo and V in aqueous solution,three kinds of extractions,namely selective extraction of vanadium,co-extraction of Mo and V,and selective extraction of Mo were proposed.Through the relevant experiments and theoretical analysis,the separation scheme of selective extraction of molybdenum was finally determined.With the help of the mixed quaternary ammonium salt extraction system,under the optimized conditions of pH value 13.32,ratio O/A=2/1,mixing time 5 min,and reaction temperature 25℃,the extraction yields of Mo and V were 99.83%and 2.31%,respectively,through 5-stage countercurrent selective extraction of Mo.Before stripping molybdenum,the loaded organic phase needs to be washed to remove vanadium and sodium,and 1.5 mol/L Na OH solution is used as vanadium detergent.Countercurrent washing,the removal rate of vanadium is 98%,and the loss of molybdenum is 14.98%;deionized water is used as detergent,under the condition of O/A=7/1,after three-stage cross-current washing,the third-stage washing water[Na⁺]<0.01 g/L.3.4mol/L NH₄HCO₃+1.2mol/L NH₃·H₂O mixed solution was used as the stripping agent for molybdenum,under the condition of O/A=4/1,after 12-stage countercurrent stripping,the stripping liquid[Mo]was 61.68 g/L,and the Mo/V₂O₅ mass ratio was333,which achieved the purpose of deep separation of molybdenum and vanadium.