Basic Research On Recovery Of Vanadium,Tungsten And Carrier From Spent Denitrification Catalyst

As a typical refractory muti-metal hazardous waste from energy industry,the annual emission of spent denitration catalyst reaches to 500,000 m3 in China.Its recycling,disposal and utilization processes become increasingly attractive in past years.In order to meet the urgent demand for the recovery of metal resources from spent denitration catalyst,this thesis proposes a new recycling process to achieve the removal of impurities in the spent catalyst,recovering of vanadium using oxalic acid,and regulation of TiW carrier by dilute alkali.This thesis focuses on the mechanism of leaching vanadium and iron by oxalic acid,the process of stepwise oxidation hydrolysis precipitation to recovery of vanadium and tungsten from saturated oxalic acid leachate,the regulation process for the pore structure of TiW carrier by dilute alkali.The full flowsheet for recovery vanadium,tungsten,and titanium from spent catalyst,preparation of ammonium metavanadate and TiW carrier is proposed.The material flow and economy of flowsheet are studied.The main research contents and conclusions are as follows:(1)Pretreatment and deep purification process by acid leaching for removing of impurities are studied,and the reaction mechanisms of leaching vanadium and iron using oxalic acid are clarified.After sieving and ultrasonic washing at 45?,the amount of fly ash impurity with quartz and mullite phases in spent catalyst is significantly reduced.The spent catalyst powder is leached by oxalic acid to remove the impurities of V and Fe elements.The leaching efficiencies of V and Fe are respectively 84.22%and 96.66%under the conditions of 90? and 1.0 mol/L H2C2O4.The UV-vis full spectrum scanning and color reaction results show that valences for V and Fe elements in the leachate are VO2+ and Fe2+,respectively.These results suggest that V3+,V5+ and Fe3+in the spent catalyst be transformed into stable VO(C2o4)22-and Fe(C2O4)22-complex via dissolution,coordination and redox reactions,thus promoting the leaching of V and Fe.The leachate is circulated for three times and separated by the process of vacuum evaporation-crystallization to recycle H2C2O4.The recovery rate of H2C2O4 reaches at 88%.The impurity content of H2C2O4 is less than 500 mg/kg.The circulating efficiencies of V and Fe elements are kept at 83.52%and 95.15%respectively,indicating that H2C2O4 shows a good leaching cycle characteristic.(2)In order to solve the problems of diversified of impurity elements and low concentrations of vanadium and tungsten elements in saturated oxalic acid leachate,a stepwise oxidation hydrolysis precipitation process by H2O2 is proposed to recovery of vanadium and tungsten from saturated oxalic acid leachate.Under the optimal conditions including a molar ratio of 0.85 and 90?,the precipitation efficiency of W element is 72.37%.After the addition of Cinc,the precipitation efficiency of W increased to 97.10%.The decomposition rate of H2C2O4 in this process is 69.88%and separation factor of W/V element reaches 2957.After refining process,the tungstic acid product including 98.41%WO3 is obtained.Through the condition optimization of second oxidation hydrolysis precipitation process,the precipitation efficiency of V element reaches to 99.89%with a molar ratio of 2 at 100?.The ammonium metavanadate product including 99.07%V2O5 is obta:ined after refining process.The mechanism of the stepwise oxidation hydrolysis precipitation process is studied.It shows that the TiO(C2O4)22-and Wo2(C2O4)22-ions in saturated oxalic acid leachate are oxidatively hydrolyzed to form anatase TiO2 and H2WO4 with the decomposition of H2C2O4,then H2WO4 is converted to H54N10O48W12 with Cinc additive,which promoting precipitation of W.In second oxidation hydrolysis precipitation process,VO(C2O4)22-and Fe(C2O4)22-ions undergo oxidation reaction to generate VO2+ and Fe3+ions,and eventually hydrolyze and precipitate in V2O5·0.5H2O and Fe0.12V2O5.(3)Aiming at the problems of low specific surface area in spent catalyst and containing aluminum silicon elements from promoter,the leaching of Al,Si and the regulation process for the pore structure of TiW carrier by dilute alkali are studied.Through thermodynamic calculation and alkali leaching experiments,NaOH solution is selected as the leaching system.The relationship between reaction conditions and pore structure in the carrier is investigated.The leaching efficiencies of Al and Si elements under the condition of 160? and 2.5 mol/L NaOH are 59.51%and 71.38%,respectively.The specific surface area,pore volume and average pore diameter of TiW carrier were 92.57 m2/g,0.41 cm3/g and 17.78 nm.The content of K,Na and Fe in the recovered TiW carrier after acid washing arrives the limitation of commercial carrier.Then resynthesized catalyst is prepared using recovered TiW carrier and its catalytic performance is studied.The denitration activity of the catalyst is higher than 90%at 300-500?.The resynthesized catalyst shows a good resistance of sulfur and water at 350?.(4)The full flowsheet for recovery vanadium,tungsten,and titanium from spent catalyst is put forward.The industrial scale experiments of full flowsheet are carried out.18wt.%of fly ash impurities are removed by pretreatment process.The powder catalyst with a particle size of less than 75 ?m is obtained.The leaching efficiencies of V and Fe elements in three cycles of acid leaching process are all above 75%.About 81%oxalic acid crystal is recovered from cycled leachate through the vacuum evaporation-crystallization process.The separation factor of vanadium and tungsten in the stepwise separation process reaches 1501.After recovering of vanadium and tungsten,the concentrations of W and V are respectively 0.007 and 0.004 g/L,indicating that the environmental risk from leachate is effectively suppressed.The anatase TiW carrier is obtained by the dilute alkali leaching process.The total content of TiO2 and WO3 in carrier is up to 95.24%,meanwhile concentration of impurity contents the limitation requirement.In addition,the specific surface area of carrier is 81.11 m2/g.Based on material flow analysis of recovery process for spent denitration catalyst,it can be concluded that there are 0.71 t TiW carrier,4.82 kg V2O5 and 3.28 kg WO3 from per ton spent catalyst.The process shows a good economic benefit and realizes the utilization of hazardous waste.