| Vanadium is an important strategic resource in China,which is widely used in metallurgy,chemical industry,aerospace and other fields.Vanadium titanomagnetite ores are abundant in China.The vanadium slag,which is obtained from smelting of vanadium and titanomagnetite,is the most important intermediate materials for vanadium extract.Great efforts have been taken to extract vanadium from the vanadium slag in vanadium hydrometallurgy.At present,the main industrial process for vanadium extraction from vanadium slag is sodium roasting-water extraction technique and calcification roasting-acid leaching technique.It has been most widely used to extract vanadium by sodium roasting-water extraction.The evolution law of the V-containing phase in V-slag during the roasting process directly determines the extraction ratio of vanadium.However,the current researches about this evolution law only stay at the macro level,which only focuses on the phase changes.This is not beneficial to realizing essential control of the change of vanadium-containing phases..Therefore,the key to make the breakthrough is to research the evolution law at the atomic level,which is the evolution law in the atmosphere change of vanadium atoms during the sodium roasting of vanadium slag.The atomic atmosphere refers to the chemical environment,the space environment and the chemical state of a kind of atom.The evolution law at the atomic level could greatly help to optimize the production conditions and thus to improve the vanadium recovery rate.In this paper,the evolution law of atomic atmosphere of vanadium atoms was studied by combined characterization of SEM-EDS,XRD,XRF and XPS.Results were as below:(1)The chemical atmosphere evolution law of vanadium atom during sodium roasting of vanadium slag: during the initial stag of roasting,V3+,Fe2+/Fe3+ and O2-constitute FexV3-xO4,which stays unreacted.(Fe,Mg)2SiO4 is heated to react to form Ca3Al2Si3O12 and Na4SiO4.When roasting for 10 min,FexV3-xO4 begins to be oxidized,in which V3+ is oxidized to be V4+/V5+ to form FeVO4 and FeV3O8.When roasting for 15 min,V5+ begins to react with Na2CO3 to form Na3VO4,NaV3O8 and NaVO3.When roasting for 50 min,(Fe,Mg)Cr2O4 begins to be oxidized to form Na2CrO4.When roasting for 90 min,FexV3-xO4 has totally reacted to form Na3VO4、NaV3O8 and NaVO3.(2)The space atmosphere evolution law of vanadium atom during sodium roasting of vanadium slag: during the initial stag of roasting,vanadium iron spinel crystal structure stays unchanged,while the olivine reacts with Na2CO3 to form dendritic crystal.When roasting for 15 min,vanadium iron spinel crystal structure begins to broke,the valence bonds between V3+,Fe2+/Fe3+ and O2-are heated to be broken.V5+,Na+ and O2-form sodium vanadate nanowires.When roasting for 30 min,sodium vanadate nanowires grow into bunches.When roasting for 70 min,sodium vanadate nanowires grow into bundles.When roasting for 90 min,sodium vanadate nanowire bundles get bigger,and sodium vanadate nanowire bunches cover the whole surface.(3)The chemical state evolution law of vanadium atom during sodium roasting of vanadium slag: during the initial stag of roasting,the vanadium atom stays unreacted to be V3+.When roasting for 5 min,some of the V3+ is oxidized to be V4+.When roasting for 10 min,the vanadium atom begins to be oxidized to be V5+.When roasting for 50 min,the valence bonds between V3+,Fe2+/Fe3+ and O2-are heated to be broken.V3+ has been oxidized to be V4+/V5+.When roasting for 90 min,the vanadium atom has been oxidized to be V5+.Vanadium iron spinel crystal structure react completely.Sodium roasting-water leaching technique suffers from the emission of corrosive gases sucn as Cl2 andSO2,which causes problems of environment pollution.Calcification roasting-acid leaching technique was thus developed recently.Although it could reduce the environment pollution,it suffers from the low and unstable vanadium recovery due to the formation of V – P heteropoly acid during the acid leaching process.Therefore,in this thesis,the ammonium carbonate leaching method is proposed to selectively leach vanadium and inhibit phosphorous leaching so as to improve the vanadium recovery ratio.The optimal ammonium carbonate leaching conditions were obtained: the solidliquid ratio was 1:20;the leaching temperature was 80 ℃;the residue size was 45-74 μm;the concentration of(NH4)2CO3 was 600 g/L and the leaching time was 70 min.In this condition,the leaching rate of vanadium was 96.0% and the leaching rate of phosphorus was only 9.2%.Results of dynamic analysis showed that the ammonium carbonate leaching of the calcification roasted –vanadium slag followed the unreacted core model.During the initial stage of the leaching process(0-15 min),the leaching rate was controlled by the diffusion of the solid product layer.During the long time stage of leaching(15-70 min),the leaching rate was controlled by dual factors: the diffusion through solid product layer and the surface chemical reaction;when the concentration of(NH4)2CO3 was higher,the effect of the diffusion through solid product layer was increased. |
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