Study On Source Separation Mechanism Of Fe-Al Coordinated Precipitation During Pressure Acid Leaching Of Vanadium Shale

Vanadium shale is an extremely important and advantageous mineral resource in my country.It contains about 118 million tons of V₂O₅ reserves,accounting for about 87%of my country’s total V₂O₅ reserves.It is one of the important sources of high-purity vanadium production in my country.The pressure acid leaching process has received extensive attention in the field of vanadium extraction from shale due to its high leaching efficiency,strong raw material adaptability,and non-polluting gas emissions.However,the large increase in the leaching temperature leads to the unorganized release of mineral components,and a large number of impurity ions such as Fe and Al will be leached simultaneously with V,which affects the subsequent purification and enrichment of V.To solve the above problems,a coordination precipitant was introduced in the pressure leaching process to separate Fe and Al impurities from the source in this study.By studying the leaching behavior of V,Fe and Al under the participation of different complexing agents,comparing the effects of different complexing agents on the precipitation and separation of Fe and Al,and combining with various characterization methods such as XRD,XPS,SEM-EDS and thermodynamic calculations,the mechanism of Fe and Al ions coordination precipitation was analyzed.Finally the leaching process conditions were optimized by response surface methodology.The main conclusions are as follows:（1）During the pressure acid leaching process,the hydrolysis of Fe and Al and their isomorphic substitution can generate stable supergroup minerals of alunite.The intervention of K₂SO₄ promotes the formation of alumite precipitation,which greatly reduces the Fe leaching rate,but has a weak effect on Al precipitation.The study found that the introduction of K₃PO₄ in the leaching process greatly improved the precipitation rate of Fe and Al because the coordination ability of phosphate ions was stronger than that of sulfate ions.Under the coordination effect of SO₄^2-,Fe and Al doping in the precipitate phase formed jarosite and alunite solid solution（K（Fe,Al）₃（SO₄）₂（OH）₆）;Through the coordination of PO₄^3-,the hydroxyl groups produced by Fe hydrolysis and PO₄^3-tend to attract and replace each other to form a complete unit.After the transfer of hydroxyl ions or protons leads to changes in the number of iron hydroxyl groups and redistribution of charges,Fe forms ferric hydroxyphosphate（Fe₄（PO₄）₃（OH）₃）precipitation.The hydroxyl group formed by Al hydrolysis will be broken under the intervention of PO₄^3-,so that Al finally forms aluminum phosphate（Al PO₄）precipitation.（2）The model equation between V,Fe,Al leaching rate and leaching factors was obtained by response surface methodology,and the optimal process parameters were determined:leaching temperature 185℃,sulfuric acid concentration 9 vol.%,K₃PO₄dosage 7 wt.%,oxygen partial pressure 0.7 MPa,leaching time 4 h.Under this condition,the leaching rates of V,Fe,and Al were 85.8%,3.7%,and 24.9%,respectively,which were close to the predicted results.The significant comparison of leaching parameters found that the K₃PO₄ dosage was more significant than the sulfuric acid concentration on leaching rates of Fe and Al.In the acidic solution environment where SO₄^2-and PO₄^3-coexist,Fe and Al ions will preferentially react with PO₄^3-and precipitate,realizing V selective leaching.