Study On A Strengthened Process And Mechanism Of Vanadium Leaching From Shale With Microwave Intervening

Vanadium-bearing black shale is a dominate vanadium resource in China.The properties of large reserves,wide distribution and various occurrences make it significantly valuable.While the low grade?usually at 0.3%¹.0%?and occurs in muscovite lattice make the extraction and utilization process difficult.As the hydrometallurgical extraction process takes the advantages of high adaptability and efficiency,leaching and precipitation process are the key points to guarantee both the recovery and purity of vanadium.However,the requirement of a long time and large acid consumption for leaching,as well as the uncontrollable vanadium precipitation process seriously restricts the development of the vanadium extraction technology.The acid leaching process of vanadium bearing shale with microwave intervening was firstly studied.Then the essence of microwave heating with vanadium shale acid leaching system was revealed and the vanadium crystallization process with microwave intervening was investigated further.An efficient microwave irradiation-assisted hydrothermal synthesis of ammonium vanadate method was established.The intrinsic reason for the ammonium vanadate purity improvement under microwave irradiation was also confirmed.The main conclusions of this paper are as follows:?1?Microwave intervened direct acid leaching process and response surface optimizationThe optimum conditions of direct acid leaching were leaching temperature of 102°C?microwave power is 350 W?,leaching time of 150 min,H2SO4 concentration of 20 vol%,CaF2 dosage of 5 wt%,grain size of-0.074 mm accounts for 75%,vanadium leaching rate reaches 62.51%in microwave heating.Thus the response surface analysis was used for the leaching conditions optimization for the lower leaching rate.The analysis of the significant order of the influencing factors could be sorted as:CaF2 dosage>H2SO4 concentration>leaching time.The interaction between CaF2 dosage and the H2SO4 concentration existed while the microwave power was attributed to no significant factor.The optimization program obtained proved that a significant synergistic effect existed between microwave intervention and leaching system.?2?Microwave intervened leaching process and characteristics of vanadium-bearing shale with roasting intensificationAfter blank roasting treatment,the structure of vanadium-bearing muscovite was activated and the microwave absorbing property was also improved.Thus the dissolution of muscovite under microwave irradiation was enhanced.Moreover,the reductive substances,such as carbon and pyrite were removed and the hot spot interference caused was eliminated.The leaching tendency of roasted vanadium-bearing shale under microwave irradiation was 25.75%higher than direct acid leaching.It also proved that microwave and calcium fluoride indicated a remarkable synergistic effect on vanadium leaching.The leaching rate of vanadium was 88.26%with the leaching conditions at 5 wt%of CaF2,20 vol%H2SO4,102°C and 90 min.Contrast to conventional heated leaching,the leaching time was shortened by 4.5 h and the leaching rate of vanadium was increased by9.75%.Element content analysis of leaching solution showed that the leaching rate of potassium was increased anomalously.Meanwhile,the infrared analysis of the leaching residue showed that the characteristic peak of the mica potassium loss structure appears only in the patterns of the microwave heating leaching residue.Both of the results above proved that microwave intervention promoted the release of potassium.The fitting results of leaching kinetics showed that the activation energy decreased and the ion diffusion rate accelerated under microwave irradiation,and the leaching process from chemical reaction to mixed control with the addition of CaF2 could change.?3?Essence of microwave intervention with vanadium-bearing shale acid leaching systemFor the vanadium-bearing shale ore which contains strong absorbing minerals,a strong dispersing of the solid product layer was caused from the hot spots result from microwave absorbing substance.On the other hand,microwave radiation appeared weak destructive effect on muscovite for the low absorbing capability.After roasting,the microwave absorbing property of vanadium-bearing muscovite was increased.Then the destruction of mica structure by microwave radiation was also strengthened.The diffusion of H⁺and F^-to the solid-liquid reaction interface was promoted by the directional movement of ions in the microwave field.Microscopically,the polarization of metal oxygen bonds?M-O?in microwave field as well as the lattice thermal vibration intensity is strengthened.Due to the highest ion bonds content of K-O,the induced polarization in microwave field is significantly enhanced.Therefore,the K⁺vibration between the mica structure layers was intensified and the migration rate to the liquid phase is accelerated.Because of the escape of K⁺,the electronegativity of mica increases and attracting more H⁺reaches inside of the lattice.Then the reaction of interlayer active sites and the broken of crystal structure was enhanced,the release of vanadium was also accelerated.?4?Microwave intervened vanadium precipitation process and crystallization propertiesThe crystallization process of ammonium vanadate under the microwave intervention was investigated.The optimum process were at the final pH=2.0,90°C,20 g/L and heating for 15 min.The vanadium precipitation rate and the purity both reached more than 99%.The analysis of phase and micromorphology showed that a diamond ammonium polyvanadate with uniform particle size,high purity and complete crystal form were synthesized.As the temperature gradient of the solution was eliminated by microwave heating,the polar vanadium containing ions in the directional vibration were distributed uniformly in the solution which promoted the homogeneous nucleation and growth of ammonium vanadate.The vanadium precipitation time was shortened from 60 min to 15min.The intrinsic reason for the purity improving of the ammonium polyvanadate under microwave irradiation was revealed by crystal analysis.The microwave intervention increased the vibration amplitude of the inter layer cations?NH₄⁺,Na⁺?of ammonium polyvanadate,made the inter layer spacing of[1?00]expanded by about 10%,which was favorable to the substitution of Na⁺exist in interlayer with NH₄⁺.Thus the formation of hydrolysate sodium vanadate was eliminated and the purity of vanadium precipitation products improved.