Fundamental Study On Co-extraction And Separation Of Vanadium,titanium And Chromium From Vanadium Slag

Vanadium,titanium and chromium are important strategic metals,which are widely used in steel,chemical industry,aerospace,new energy and other fields because of their excellent physical and chemical properties.Vanadium slag is an important secondary resource containing vanadium,titanium and chromium,but only the vanadium in vanadium slag is extracted and utilized due to technical limitations.The traditional processes of extracting vanadium from the vanadium slag include sodium salt roasting-water leaching process and calcification roasting-acid leaching process.The principle of vanadium extraction is that the structure of vanadium spinel is destroyed by oxidative roasting,in which the trivalent vanadium(V3+)is oxidized and reacts with additives to form the vanadate(V5+),and then V2O5 is obtained by leaching,purification,ammonium precipitation and calcination.However,the above-mentioned processes have many deficiencies,such as toxic kiln gas in the roasting process,high-salt wastewater and toxic tailings in the water leaching process,high-ammonia nitrogen wastewater in the ammonium precipitation process,no co-extraction of titanium and chromium.The new methods of extracting vanadium from vanadium slag mainly include sub-molten salt method and oxidative sulfuric acid leaching method.The principle of vanadium extraction is that the vanadium in vanadium slag is oxidized and leached in high concentration alkaline or acidic medium.In addition,the vanadium and chromium in vanadium slag can be co-extracted by sub-molten salt method,but the titanium in vanadium slag is still not extracted.If the vanadium(V3+)can be extracted directly from vanadium slag without the oxidation,the vanadium extraction process will be greatly simplified and energy conservation and emission reduction will be realized.In addition,if the co-extraction of titanium and chromium in vanadium slag is realized in the process of extracting vanadium,the comprehensive utilization of vanadium slag can be realized fundamentally.Based on this idea,the co-extraction and separation of vanadium,titanium and chromium in vanadium slag was studied in this paper,and the following progress has been made:(1)Based on the thermodynamic analysis of the leaching reactions of pyroxene,olivine and spinel in vanadium slag,the direct leaching of vanadium,titanium and chromium from the vanadium slag by oxalic acid hydrothermal method was studied.The single-factor experimental research showed that under the better leaching conditions,the leaching rates of vanadium,titanium and chromium in vanadium slag could reach 97.9%,98.4%and 93.3%,respectively.The leaching residue was mainly composed of ferrous oxalate,manganese oxalate and amorphous silica.The vanadium,titanium and chromium ions in the leachate should mainly existed in the form of V(C2O4)33-,TiO(C2O4)22-,Cr(C2O4)33-,respectively.(2)In view of the problems existing in the above leaching methods,such as high reaction temperature,high reaction pressure and large consumption of oxalic acid,the selective reduction of pyroxene and olivine in vanadium slag by hydrogen at the low temperature,the leaching and separation of metallic iron by ferric chloride solution,and the hydrothermal leaching of vanadium,titanium and chromium by oxalic acid were investigated.The thermodynamic analysis and mechanism of the reduction process indicated that the stability order of each phase in vanadium slag was spinel>olivine>pyroxene.Firstly,the pyroxene and olivine were selectively reduced by hydrogen to obtain the metallic iron,in which the silicate structure surrounding the spinel was destroyed.Then,the metallic iron in the reduction slag was selectively leached by the ferric chloride solution,and the leaching rate was more than 98%.Then,the vanadium,titanium and chromium in the iron removal slag were leached by oxalic acid hydrothermal leaching,and the leaching rates could reach 96.8%,94.7%and 95.4%,respectively.This method realizes the co-extraction of vanadium,titanium and chromium under the conditions of relatively low temperature,low pressure and low oxalic acid concentration,and recovers the metallic iron.(3)For the separation of titanium in the leachate,the research on the direct separation of titanium in the leachate by hydrothermal decomposition method was carried out.Under optimal hydrothermal conditions,more than 99%of Ti in the leachate was precipitated in the form of anatase TiO2 with a purity of 95.7%.Aiming at the problems of large oxalic acid loss and high reaction pressure in this method,the recovery of oxalic acid by cooling crystallization mthod and the separation of titanium by hydrothermal decomposition method were studied.The research shows that the recovery of oxalic acid from the leachate was conducive to reducing the hydrothermal decomposition temperature and pressure,and the purity of TiO2 was improved to 96.6%.(4)For the separation of vanadium and chromium in the leachate after titanium separation,the research on the direct separation of vanadium and chromium in the leachate by hydrothermal decomposition method was performed.Under the optimum hydrothermal conditions,the precipitation rates of vanadium and chromium in the leachate were more than 99.9%.The possible reactions of vanadium and chromium during hydrothermal decomposition and the changes of phase and micro-morphology of decomposition products were clarified.(5)In view of the high impurity content of the product obtained by the above separation method,the purification and enrichment of vanadium and chromium by solvent extraction method and the separation of vanadium and chromium from stripping solution by hydrothermal decomposition method were studied.Firstly,the study of extraction and stripping process showed that more than 99%V and 78%Cr in the leachate were enriched in the stripping solution by three-stage acidified N235 extraction and four-stage HCl solution stripping.The concentration of vanadium ion in the solution was improved from 2.40 g/L to 7.15 g/L,the concentration of chromium ion was improved from 1.14 g/L to 2.68 g/L,and the vanadium and chromium were separated from a large number of impurities.The mechanism of extraction and stripping was the anion exchange reaction between V(C2O4)33-,Cr(C2O4)33-complex ions and Cl-ions.Then,the vanadium and chromium ions in the raffinate were separated by hydrothermal decomposition method,and the precipitation rates of vanadium and chromium could reach 98.3%and 97.6%.The purity of the product was greatly improved.(6)The leaching scale-up test of vanadium,titanium and chromium in the vanadium slag showed that the leaching rates of vanadium,titanium and chromium could reach 93.4%,92.2%and 95.4%,respectively.The separation scale-up test of titanium in the leachate showed that the TiO2 with the purity of 95.3%could be obtained.The separation scale-up test of vanadium and chromium in the leachate showed that the mixture of vanadium oxide and chromium oxide could be obtained.