| The dissertation research is one branch of the project named "The Mechanism and Experimental Study on Clean Extracting Vanadium from Vanadium bearing Steel Slag(No.200806740008)", which is supported by the Specialized Research Foundation for the Doctoral Program of Higher Education of China. This paper aims at solving technically difficult problem that vanadium can not be effectively reclaimed from vanadium bearing steel slag at present, and providing strong theoretical base for comprehensive utilization of the vanadium bearing steel slag by analysis of process mechanism of extracting vanadium.As one of the important and valuable metallurgical secondary resources, vanadium bearing steel slag produced in steelmaking process of V-Ti magnetite, can be used as the key raw materials for extracting V2O5. However, it is still very difficult to reclaim vanadium from vanadium bearing steel slag due to its high calciμm, high iron, low content and complex occurrence of vanadium dispersed in a variety of mineral phase. Many technologies exist for vannadium extraction, but they can hardly fit resource characteristics of vanadium steel slag, and there are still many problems with those technologies, including high cost, serious pollution,low recovery and so on; At the same time, some new technologies such as selective precipitation, bacterial leaching and slurry electrolysis are not yet complete and practical while better results can be obtained in experimental study of extracting vanadium with those new technologies. Actually, conventional salt roasting process is the only process used commercially. Thus, according to the characteristics of vanadium-bearing steel slag, a new technology for extracting vanadium from V-bearing steel slag through combined process of beneficiation and metallurgy has been developed in this paper. and systematically and further study on the whole process consisting of pretreatment by beneficiation, selective stage leaching, purification and enrichment of acid-leaching solution was carried out.Firstly, the detailed process mineralogy for a certain vanadium bearing steel slag were studied. The occurrence forms, dissemination granularity, distributed characteristics of V, Ca and Fe in slag and the correlations among them were identified, the influence rules of process mineralogy characteristic on extracting vanadium were also discussed. The results can be used as the scientific reference for the pretreatment by beneficiation and the followed hydrometallurgical vanadium extraction procedure.Based on the studies of the process mineralogy, a novel technique of pretreatment by beneficiation was puted forward for the first time. The mineral components of slag after pretreatment was analyzed and the effect of pretreatment on followed leaching was investigated. Results indicate that an iron concentrate with grade of 72.08% can be obtained by magnetic separation method, which not only helps to recover the iron from slag, but also reduces the accumulation of iron ions in leaching solution and cuts down the difficulty of Fe-V separation; Part of acids consumption impurities can be removed by pretreatment of gravity dressing. In this case, the acids consumption would be saved and the adverse effects of impurity on downstream leaching can be minimized. Hence, V-leaching rate increases greatly. Compared with direct acid leaching without pretreatment, V-leaching rate after pretreatment was high above 11%. After pretreatment, the grade of vandium in the gravity concentrate was increased which was favorable for the followed hydrometallurgical vanadium extraction procedure.Most of vanadium in slag is exist in the form of V(IV) which is soluble in acid solution. In the view of this point, the technique of direct acid leaching after pretreatment by beneficiation without roasting is suitable for extracting vanadium. At present, most of researches about leaching of vanadium still focus on water leaching or acid leaching after roasting rather than direct acid leaching without roasting, and no substantive progress on direct acid leaching has been made now. In view of the problems that exist in current work, technique of direct acid leaching without roasting was emphatically researched in order to made a breakthrough on the basis of previous studies.The chemical reactions that may occur in the system of sulfuric acid leaching of slag were calculated, and the potential-pH diagram of V-H2O system was plotted. Thermodynamic study shows that the leaching reactions of main mineral of slag would be thermodynamically easy and completed. Low△Grθand high equilibrium constant of acid dissolution reactions of CaO,FeO,Al2O3 and MgO indicate that these oxides can react with sulfuric acid easily. So, much of sulfuric acid would be consumed due to the high content of these oxides in steel slag, and only a small amount of sulfuric acid react with vanadium compounds.Based on theoretical analysis, the effects of sulfuric acid dosage, stirring strength, reaction temperature and reaction time on the leaching were systematically investigated. Macrokinetics reaction, leaching behavior of related elements and mineral phase transformation were deeply discussed synthetically by these theories of kinetics, thermodynamics, structure chemistry and so on, and modern microscopic measuring and testing technology such as X-ray diffraction, optical microscope, and scanning electron microscope and so on, which established the theoretical foundation for acid leaching of vanadium from steel slag. Based on above work, a new method of selective stage leaching was developed. With two stages acid leaching, impurity element was leached in the first stage and vanadium in the second stage, "solution of iron" without vanadium in it and "solution of vanadium" with relatively few impurities were obtained respectively. So, a better selective leaching seperation between vanadium and iron was achieved significantly. Theoretical analysis and study about selective stage leaching were also carried out so as to clarify relevant mechanism of selective leaching of vanadium.Purification and enrichment of acid leaching solution containing vanadium is an important research topic. And ion exchange and solvent extraction are the most widely studied topic in all researches. In general, solvent extraction is more suitable for recovery of vanadium from acidic solution. In view of low vanadium content, high impurity content in acid leaching solution, traditional solvent extraction method was improved by adding washing procedure of loaded organic phase besides extraction and stripping procedure, and a new high efficient washing reagent "active Na2SO4 solution" has been developed in this paper.The extraction, washing and stripping were systematically researched and the optimum conditions were obtained. Under these conditions, four stages countercurrent extraction rate reached 98.86%, washing loss rate only 1.16%, singlestage stripping rate reached 99.27%. By extraction-washing-stripping, not only the vanadium enriched from 3.5g-L-1 to 20.37g-L-1, but also most of the impurities were removed. Fe concentration was olny 0.15g·L-1 and concentration ratio of V/Fe in stripping solution reached 135.8, so that effective separation between vanadium and iron was betterly achieved.A greater amount of SO42-, CO32- and Na+ was introduced into stripping solution during acid leaching and extraction processes. And there is still no detailed report whether the high quality product could be obtained from this kind of stripping solution at present. So, the effect of various factors on precipitation of vanadium by acidic ammonium salt was studied and the underlying causes was discussed. After Purification and enrichment via a series of procedures, high quality refine V2O5 product whose purity and quality met national standards was obtained, and the recovery of V2O5 was up to 95.92% from acid leaching solution to refine V2O5.Finally, jarosite process was adopted to treat raffinate and washing waste water according to the characteristics of waste water. Results show that with the pH value of 1.2 at 93℃for 3h, the iron removal rate reached 97%, while the chromium removal rate was 96%. A byproduct of high pure red ferric oxide with 72.01% Fe2O3 can be obtained from ammonium jarosite residue.And recycling of waste water through treatment into leaching process would not bring any bad effect to leaching and could save 5% acid consumption.Overall, a new technology of vanadium extraction was developed by integration of single technology of beneficiation pretreatment, selective step leaching and extraction-washing-stripping and so on in the paper. Compared with total vanadium recovery of less than 70% when extracting vanadium from V-bearing solid waste using traditional process, the extracting vanadium index of new technology with total vanadium recovery of 80% is greatly improved. And the new technology would have broad development potential due to the cancellation of the roasting process, which not only increased leaching index, but also saved a lot of equipment investment and decreased the discharge of pollutants. On the other hand, raw materials used in new technology is vanadium bearing steel slag rather than vanadium slag produced in V-Recovering process with the converter. Thus, new technology is the one of waste utilization technologies, and it has significances to the effective utilization of metallurgical secondary resources and full utilization of vanadium resources. This lays the foundation for the development of new technology.
|
PDF Full Text Request