Study On Separation And Extraction Of Vanadium From Alkaline Leaching Solution Of Spent SCR Denitration Catalyst

The increasingly stringent NOx emission standards in China have resulted in the widespread use of denitrification technology in coal-fired power plants,leading to the production of a lot of spent SCR catalysts every year.Reasonable recycling and disposal measures can reduce environmental pollution and recover resources,bringing good social and economic benefits.In this study,the alkaline vanadium-containing solution（p H=13～13.5）obtained in the actual engineering disposal of the spent SCR catalyst is used for resource recovery and utilization.The solution also contains impurities of silicon and aluminum ion.The solvent extraction method is used to selectively extract and separate vanadium from the alkaline leaching solution.Aiming at the problem of vanadium extraction and separation in alkaline systems,this paper proposes a process for achieving efficient resource recovery of spent SCR catalysts.The main research conclusions of this study are as follows:Firstly,the separation and extraction of vanadium-containing solutions with different p H values of spent SCR catalyst were studied.The change of p H value led to the change of vanadium existing form and concentration of OH^-ions,which would affect the extraction performance of different extractants.In alkaline conditions,vanadium exists as a vanadate ion,so that neutral extractants cannot combine with vanadium through coordination bonds,and can only achieve a small amount of extraction through weak hydrogen bond associations.Amine extractants can’t achieve solvent extraction due to high concentration of OH^-ions.The extraction effect will be significantly improved if the primary amine N1923 and tertiary amine TOA are modified by protonation to hydrochloride.However,the hydrochloride is unstable and will dissociate under alkaline conditions,making the raffinate become acidic.The quaternary ammonium salt extractant can increase the molecular weight by extending the carbon chain,thus improving the extraction efficiency.However,the introduction of benzene ring substituents will decrease the extraction efficiency.Through the comparison of different extractants,the quaternary ammonium salt ALIQUAT 336 was determined as the extractant.Secondly,the influence of ions impurity was studied and the extraction process conditions were optimized.The effects of extractant concentration,modifier concentration,p H value of the original solution,extraction temperature,extraction time and extraction ratio were studied.Results show that the concentration increasing can enhance the extraction effect with the extractant concentration less than 25%,and the extraction rate tends to be stable when the extractant concentration is larger than 25%.When p H value is 13,Si and Al ions exist in the form of Si₂O₅^2-and Al O₂^-respectively,which have little competitive extraction ability with vanadium.Under the optimal extraction process conditions,the organic extraction system of ALIQUAT 336 showed a higher single-stage extraction rate of 76.34%for vanadium.At this time,the separation coefficient of vanadium-silicon(β_V/Si)was 21.63 and that of vanadium-aluminum(β_V/Al)was 49.84.Therefore,the separation effect is relatively satisfactory,and the selective separation and extraction of vanadium can be achieved.The saturation capacity method,equimolar series method and slope method were used to determine the extraction ratio,combined with FT-IR spectrogram to realize the analysis of the extraction mechanism of vanadium.From the analysis results of the vanadium extraction mechanism vanadium was extracted in the form of VO₄^3-at p H value of 13,with the ALIQUAT 336/VO₄^3-combination ratio of 3 and the saturated vanadium extraction capacity of 8.0109g/L.Finally,the influence of vanadium on stripping process conditions and the preparation of V₂O₅ products were studied,and the process route of solvent extraction of spent SCR catalyst was designed.Combined with the difference of binding ability between different anions and extractant,the 1mol/L Na OH+1mol/L Na Cl solution was selected as the stripping system to realize the enrichment of vanadium.The concentration of the stripping residue was 19.28 g/L,and the p H value was 13.72,under optimal stripping process conditions.Also,a weakly basic ammonium salt vanadium precipitation experiment was carried out.The V₂O₅ products were produced through the process of“p H adjustment→vanadium precipitation→roasting”.The molar ratio of the NH₄Cl added in the vanadium precipitation process to the vanadium in the stripping raffinate was 5,the vanadium precipitation rate was 99.46%,achieving vanadium substantially precipitated.The XRD and XDF results of the V₂O₅ product show that the product purity is 96.7%,containing little Si impurities,whose content is 2.65%.Therefore,the recycling and utilization of vanadium resources is realized.