Study On Roasting Mechanism Of Vanadium-Chromium Slag Under Different Sodium Salt Additives

Vanadium and chromium are two important strategic resources in China,which are widely used in metallurgy,chemical industry,aerospace and other fields in the form of compounds,or alloys.Hongge ore is a typical high-chromium vanadium-titanium magnetite,whose contents of vanadium and chromium are considerable.The vanadium-chromium slag obtained from Hongge ore will be an important resource for extracting vanadium and chromium in China.At present,the efficient extraction process of vanadium and chromium from vanadium-chromium slag has not been established.In view of this problem,considering the difficulty in industrial application,this research combined the current extraction process of vanadium from vanadium slag to develop a method,compatible with the current production line,for efficient extraction of vanadium and chromium from vanadium-chromium slag.At the same time,the extraction mechanism and related physical and chemical laws were explored,so as to fundamentally regulate the evolution of V-containing and Cr-containing phases to achieve full utilization of sodium salt additives to improve the recoveries of vanadium and chromium.This paper focused on extraction of vanadium and chromium from vanadium-chromium slag by sodium roasting-water leaching process with sodium carbonate,sodium hydroxide and their mixture as roasting additives respectively.The conversion mechanism of vanadium and chromium and the consumption law of sodium salt additive in the process of roasting with different sodium salt additives were studied.Specifically,the major research contents and results are as follows:?1?Phase composition of vanadium-chromium slag and appearance of vanadium and chromium:vanadium,chromium and titanium in vanadium-chromium slag were present in the spinel phase??Mn,Mg,Fe??Mn,Cr,V,Fe?₂O₄?Mg₂TiO₄?;vanadium and chromium elements existed in the spinel phase as V³⁺and Cr³⁺.The spinel phase was surrounded by olivine phase?Fe₂SiO₄?and pyroxene phase?CaAl₂SiO₆?.In the spinel particles,Cr-containing spinel accumulated in the center of the particle,V-containing spinel accumulated in the outer layer of particle core,while Ti-containing spinel accumulated in the edge region.?2?The conversion mechanism of vanadium and chromium during sodium roasting:the V-containing spinel began to undergo oxidation and sodiumation at 300°C to form vanadates,and the amount of water-soluble vanadates reached the maximum at800°C;the vanadates gradually transformed into the most stable NaVO₃ according to the path of NaV₃O₈?1?Na₃VO₄?1?Na₄V₂O₇?1?NaVO₃.The Cr-containing spinel began to oxidize and decompose at 400°C to form?Fe,Cr?₂O₃,NaCrSi₂O₆,etc.,and then Na₂CrO₄ was formed in a large amount at 600°C.The amount of Na₂CrO₄ produced was largest at 800°C,accompanied by a small amount of Na₂Cr₂O₇.According to the valence state quantitative results,vanadium and chromium had the highest oxidation ratios at 800°C.As the temperature continued to rise,the oxidation ratio gradually decreased.This was due to that the sodium salt gradually melt when the temperature was too high,causing the sample to be sintered and the formation of the glass phase such as NaAlSi₂O₆?Ca₃Al₂?SiO₄?₃,which suppressed the oxidation of vanadium and chromium and the reaction with the sodium salt.Therefore,in order to obtain soluble vanadates and chromates containing high-valence vanadium and chromium,the roasting temperature should be controlled at 800°C.?3?Consumption law of sodium salt during sodium roasting of vanadium-chromium slag:in the sodium roasting process,Na combined with V,Cr,Si,Al,Fe,and Ti to form compounds to consume the sodium salt.At low temperature,some olivine was oxidized and combined with Na to form NaFe?SiO₃?₂.When the temperature raised,Na combined with Si to form Na₆Al₄Si₄O₁₇.When V and Cr were not completely oxidized,Na can also combine with them to form Na_0.56V₂O₅,Na₂V₂O₅,Na₂V₅O₁₃ and Na₃CrO₄.After the oxidation of Fe²⁺in the Cr-containing spinel caused its decomposition,then the unoxidized Cr³⁺could also combine with Na to form NaCrSi₂O₆.When V and Cr were completely oxidized,Na reacted with V⁵⁺and Cr⁶⁺to form Na₃VO₄,NaVO₃,Na₄V₂O₇ and Na₂CrO₄.In addition,after oxidation of the Ti-containing spinel,Na combined with Ti to form NaFeTiO₄.When the roasting additive was 20%Na₂CO₃+30%NaOH and the roasting temperature was 800°C,most Na formed water-soluble vanadates and chromates leachable into the leachate,and the effective utilization ratio of sodium salt reached the highest.?4?The optimal roasting process conditions for extracting vanadium and chromium from vanadium-chromium slag by sodium roasting-water leaching are as follows:roasting at 800°C for 2 h with 20%Na₂CO₃+30%NaOH as additives.At these conditions,the oxidation ratios of vanadium and chromium were 83.74%and 80.45%,respectively.