Study On Process And Mechanism Of Vanadium Precipitation By High Pressure Hydrogen Reduction From Vanadium-rich Solution

In China,vanadium-bearing shale is an important raw material for vanadium extraction.Vanadium precipitation is an indispensable procedure in the vanadium extraction process.However,the amino-nitrogen problem caused by the widely used ammonium salt vanadium extraction process hinders the green development of the vanadium extraction industry from vanadium-bearing shale.In this paper,the eco-friendly high pressure hydrogen reduction technology was introduced into the vanadium precipitation procedure of vanadium extraction process from shale in order to solve the amino-nitrogen problem caused by the traditional ammonium salt vanadium extraction process.The feasibility of vanadium precipitation was confirmed by thermodynamic analysis.The optimal process conditions of vanadium precipitation under the addition of catalyst or not were determined the experimental design.The mechanisms of vanadium precipitation were deeply studied with the help of XRD,SEM,and TEM analysis.A novel green vanadium precipitation process was finally developed.The main contents and conclusions are as follows.?1?The thermodynamic mechanisms of vanadium precipitation by high pressure hydrogen reduction were studied via Eh-pH diagram under the different reaction conditions.Combined the thermodynamic results with the characteristics of actual vanadium-rich solution from shale,the suitable raw material solution for high pressure hydrogen reduction was ascertained to be the alcaline pentavalent vanadium-rich solution.Another finding is that keeping a lower solution pH value is more conducive to the proceeding of the high pressure hydrogen reduction.?2?Through the research of vanadium precipitation controlling factors in high pressure hydrogen reduction without catalyst,it was proved that the unactivated hydrogen could only reduce V?V?to V?IV?in this system.The pure phase NaV₂O₅single crystal could be prepared from V?V?-rich solution with a vanadium precipitation rate of more than 99%by controlling the reaction conditions.The optimum conditions for NaV2O5 formation were determined as follows:initial pH value of vanadium-rich solution was 4.0,reaction temperature was 200°C,partial pressure of hydrogen was 4 MPa,and reaction time was 2 h.According to the phase composition analysis and electron microscopy analysis of precipitates obtained at different time,the mechanism of the preparation of the single crystal of NaV2O5through high pressure hydrogen reduction was revealed as follows:hydrolytic precipitation of pentavalent vanadate ions,reductive dehydration of pentavalent vanadates,intercalation of sodium ions,and oriented growth of NaV2O5 crystal nucleus.?3?According to the investigations to the vanadium precipitation process and mechanism by high pressure hydrogen reduction with the introduction of palladium chloride catalyst,the activity of hydrogen was significantly enhanced after palladium chloride catalystwas introduced and the single phase of rhombohedral V2O3 was successfully precipitated from the pentavalent vanadium-rich solution.The optimum conditions for vanadium precipitation were determined as follows:initial pH value was 6.0,reaction temperature was 250°C,partial pressure of hydrogen was 4 MPa,reaction time was 2 h,and the addition of palladium chlorine was 10 mg/L.Under this condition,the vanadium precipitation efficiency reached 99.23%and the purity of V2O3 products reached 99.83%.According to the phase composition analysis and the electron microscopy analysis of precipitates obtained at different time,the mechanism of V2O3 formation was revealed as follows.Hydrogen was firstly activated by palladium chloride,and then pentavalent vanadium was reduced by activated hydrogen and rod-like NaV2O5 was crystallized out from the vanandium-bearing solution,which was further transformed to flake-like VO₂?H₂O?_0.5 after dissolution and recrystallization.Through reduction,dehydration and denatrium flake-like VO₂?H₂O?_0.5 was transformed to flake-like VOOH,which was then transformed to flake-like V₂O₃ through dehydration and denatrium.Finally flake-like V₂O₃aggereated with each other and transformed to rhombohedral V2O3.?4?In order to make sure the influence and mechanism of impurities on vanadium precipitation by high pressure hydrogen reduction,the effect of the major impurities of Na,P and Si on vanadium precipitation was investigated.The upper limit of the permitted imputies in vanadium-rich solution was confirmed,and the influence mechanism of different impurities on the high pressure hydrogen reduction process was revealed.It was determined that Na has no harmul effect on the vanadium precipitation process and results.P can complexes with vanadate ions and reduce its activity,then the electromotive force of hydrogen reduction reaction is depressed,which makes the reaction incomplete,resulting in the decrease of vanadium precipitation efficiency and product purity,and the aimed product of V2O3 is also difficult to be obtained;Si will convert to H₃Si₃O₇ in the reaction process and adsorb on the surface of vanadium precipitation intermediates,hindering the contaction between hydrogen and vanadium-containing intermediates.Then the high pressure hydrogen reduction reaction is suppressed,which makes the reaction unable to complete,resulting in the decrease of vanadium precipitation efficiency and product purity,so the aimed product of V₂O₃ is difficult to generate.The concentration of P and Si in vanadium-rich solution should be controlled below 0.5 g/L and 0.1 g/L respectively to guarantee that the purity of V₂O₃ product is higher than 99%.?5?Based on the characteristics of high alkalinity of the vanadium precipitation mother liquor by high pressure hydrogen reduction,the mother liquor was recycled as a stripping agent back to the stripping process.Through the study of the influences of the recycling of vanadium precipitation mother liquor on the technical indexes and the accumulation behavior of impurity elements,it was determined that the recycling of vanadium precipitation mother liquor has no significant effect on the original process.?6?It was proposed that the preparation process route of the high purity V₂O₃product in vanadium precipitation by high pressure hydrogen reduction with the introduction of catalyst.Compared with the traditional vanadium precipitation technology by ammonium salt,this technology has both economical benefit and environmental benefit.