Mechanism Research On Oxidation Roasting And Leaching Process Of High Calcium Low-Grade Vanadium Slag

Vanadium is defined as an important strategic material which is widely used in many fields such as steel industry, aerospace industry and chemical industry, etc. In recent years, with the rapid development of iron and steel industry, the ordinary iron ore resources have been decreased. In order to deal with the increasingly serious situation and make full use of vanadium titano-magnetite ore in Panxi region in China, the small blast furnace for smelting of vanadium titano-magnetite ore has successfully developed, more than 30% of vanadium titano-magnetite ore is added in normal iron ore during blast furnace process. In such case, the vanadium content in hot metal reaches above 0.15%. In order to recover the vanadium from the hot metal, the duplex process was used, and then the vanadium in hot metal was selectively oxidized into slag. The V₂O₅ content in vanadium slag otbained is less than 10% while CaO content is more than 5%. Compared with the normal metaurgical slag (12-25% V₂O₅, 0.7-2.5%CaO), the vanadium slag in this study contains lower V and higher Ca content. At present, there is no a better way to extract vanadium from the special vandium slag. In this paper, the feasibility of sodium salt roasting-water leaching and calcium salt roasting-soda leaching for extraction of vanadium from high calcium low-grade vanadium slag were investigated, and the oxidation process and leaching process were studied in detail. The comprehensive evaluation results could provide a theoretical direction for vanadium extraction in industrial production.At first, mineralogical composition and the spot, surface and line analysis of element in vanadium slag were investigated. The results show that high calcium low-grade vanadium slag is composed of spinel phases, olivine phases and pyroxene phases. The spinel presents connection and dispersion state and its particle size is usually 20-30μm. The descending order of element content in spinel phase is iron, vanadium, titianium, oxygen, chromium, manganese, aluminium. The content of vanadium and chromium in center part of spinel phase is higher while the content of titanium, manganese, iron and aluminium in outer part of spinel is higher. The descending order of element content in olivine phase is oxygen, silicon, iron, manganese. The descending order of element content in pyroxene phase is silicon, oxygen, calcium, iron, aluminium, titianium.Oxidation process and water leaching process were investigated using Na₂CO₃ as additive. The effect of oxidation process and leaching process on leaching efficiency of vanadium, phase transition of vanadium slag in oxidation process and roasted samples in water leaching, and formation of sodium vanadates were studied, etc. The results show thatâ‘ The optimum process parameters for vanadium extraction are follows: addition of 18% Na₂CO₃, roasting temperature of 700℃, roasting time of 150min; leaching temperature of 90℃, leaching time of 30min and liquid to solid ratio of 5:1mL/g. Under the optimum conditions, the leaching efficiency reaches above 89.5%. The main impuritie are Si, P and Cr in the leach liquor.â‘¡Phase transition of vanadium slag in oxidation process and roasted samples in water leaching. 1)Slag sample with 18% Na₂CO₃ is oxidized in the temperature range from 273℃to 700℃. Olivine phases and spinel phases are completely decomposed at 500℃and 600℃, respectively. Most of water-soluble sodium vanadates are formed between 500℃and 600℃. When roasting temperature reaches above 700℃, the vanadium-rich phases of sodium vanadates can be obviously observed. However, at temperatures above 800℃, the samples are sintered. Most of vanadium is enwrapped by glassy phase compounds. 2) The major mineral matters of leach residues are hardly changed except that sodium salts dissolve in water; the calcium vanadates in roasted samples are transformed into sodium vanadates by reacting with Na₃PO₄ or Na₂SiO₃ during water leaching, at the same time, the Si and P in the liquor can be removed.â‘¢Formation mechanism of sodium vanadates. When V2O3 reacts with Na₂CO₃, vanadium oxides VOm (1.5<m<2.5) and sodium oxides (e.g. Na₂O, Na₂O₂ ) are formed between 200℃and 400℃. The stable intermediates species (i.e. vanadium bronze) such as NaV₆O₁₅, Na₅V₁₂O₃₂ and NaV₃O₈ are mainly formed between 300℃and 500℃. Then the vanadium bronzes are oxidized to water-soluble sodium vanadates in the temperature range of 500℃-600℃.Oxidation process and soda leaching process were investigated using CaO as additive. The effect of oxidation process and leaching process on leaching rate of vanadium, phase transition and kinetics of vanadium slag in oxidation roasting process, effect of CaO content on oxidation of vanadium slag in roasting process and roasted sapmles in soda leaching, formation mechanism of calcium vanadates and phase transition and kinetics of roasted samples in soda leaching were studied. The results show thatâ‘ The optimum process parameters for vanadium extraction are follows: roasting temperature of 850℃, roasting time of 60min, Na₂CO₃ concentration of 160g/L, leaching temperature of 95℃, leaching time of 150min, liquid to solid ratio of 10:1 mL/g and stirring speed of 400r/min. Under the optimum conditions, the leaching rate of vanadium reaches above 90%. The main impurities are Si and P in the leach liquor.â‘¡Phase transition and kinetics of vanadium slag in oxidation roasting process. 1)The vanadium slag is oxidized from 300℃to 900℃. The olivine phases and spinel phases in vanadium slag are completely decomposed at 500℃and 800℃, respectively. The vanadium-rich phases are formed above 850℃. When roasting temperature is 1000℃, the samples are sintered. 2) Oxidation process of vanadium slag can be described by the unreacted shrinking core model and is controlled by internal diffusion. The apparent activation energy is 126.6kJ/mol in the initial stage (0-10min).â‘¢Effect of CaO content on oxidation of vanadium slag in roasting process and roasted sapmles in soda leaching. The vanadium-rich phases are formed when CaO content is varied from 6.39 to 36.39% in slag sample. The intensity of diffraction peaks of major phases Fe₂O₃, Fe₂TiO₅ and SiO2 in roasted samples decreases with the increase of CaO content. The diffraction peaks of CaFe₂O₄ and CaTiO₃ appear when CaO content in salg sample is 21.39%. 2) During soda leaching, the calcium vanadates in roasted samples is transformed to soluble sodium salt, at the same time, the CaCO₃ is produced and its relative intensity of diffraction peak increases with the increase of CaO content; the other major phases in samples are hardly changed.â‘£Formation mechanism of calcium vanadates. When V₂O₃ reacts with CaCO₃, formation of calcium vanadate is dependent on the molar ratio of the starting materials and calcination temperature. Partial vanadium is transformed into vanadium bronze Ca0.17V₂O₅ at 400℃. The V₂O₃ is oxidized to V₂O₅ at 500℃. At 600℃, when the molar ratio of V₂O₃ and CaCO₃ is 1:1, the main product Ca(VO₃)₂ is formed; when the molar ratio of V₂O₃ and CaCO₃ is 1:2, the main products Ca(VO₃)₂ and CaV₂O₇ are formed; when the molar ratio of V₂O₃ and CaCO₃ is 1:3, the main product CaV₂O₇ is formed. With increasing calcination temperature, Ca(VO₃)₂ is transformed into Ca₂V₂O₇ or even Ca₃(VO₄)₂.⑤Soda leaching process of roasted samples can be described by the unreacted shrinking core model and is controlled by internal diffusion; the apparent activation energy is 51.75kJ/mol.