| Vanadium-bearing shale is an important vanadium resource in China.With the increasing demand for vanadium in the world,how to extract vanadium cleanly and efficiently from vanadium shale has become an important topic.In view of the environmental problems existing in vanadium shale extraction,oxalic acid,a clean and environmental organic acid,has been successfully used in the extraction of vanadium shale,which has the advantages of good selectivity,high efficiency,safety and easy operation.However,the components of oxalic acid leach solution are complex,and there are many kinds of impurity ions with high concentration.Vanadium,iron,aluminum and other metal ions form a variety of anionic complexes with oxalate,which makes the purification and enrichment of oxalic acid leach solution difficult.In addition,oxalic acid and valuable metals are not recovered in the process of extracting vanadium from vanadium shale in oxalic acid system,and there are few studies on Purification and enrichment process and mechanism of oxalic acid system at home and abroad.Therefore,this paper takes the oxalic acid leaching solution of vanadium shale as the research object,aiming at the technical problems existing in the purification and enrichment of oxalic acid leaching solution,studies the purification and enrichment process and mechanism of vanadium under the oxalic acid system,puts forward a multi-component asynchronous extraction process,and reveals the recovery and separation mechanism of vanadium and valuable components.The main research contents and results are as follows:?1?Study on the solution chemistry of oxalic acid leaching solution of vanadium-bearing shaleThe occurrence and distribution of vanadium,iron,aluminum,magnesium,phosphorus,fluorine and oxalate in the oxalic acid leaching solution were studied,and the coordination process and coordination mechanism of vanadium,iron and aluminum were determined.The main occurrence states of vanadium,iron and aluminum are VO?C2O4?22-,Fe?C2O4?2-and Al?C2O4?2-in the acid leaching solution with pH of0.5-1.0.The order of coordination binding ability of vanadium,iron and aluminum with oxalate is that:iron>vanadium>aluminum.?2?Extraction behavior and separation mechanism of vanadium,iron and aluminum in the oxalic acid systemThe oxalic acid leaching solution does not need to adjust the initial pH and any pretreatment of oxidation or reduction,and the mixed load organic phase fo V=3.10 g/L,Fe=7.81 g/L,Al=2.42 g/L can be obtained directly by six-stage countercurrent extraction under the optimal conditions.The extraction percentagess of vanadium,iron,aluminum and oxalate are 98.60%,99.64%,8.50%and 70.61%,respectively.The separation factor of vanadium and aluminum was 757.14,vanadium and aluminum achieve selective extraction separation.The extraction mechanisms of vanadium,iron,aluminum,oxalate and N263 all accord with the anion exchange mechanism.The molecular structure of extactable complexes of vanadium,iron,aluminum and oxalate with N263 was determined by quantum chemical calculation.The reaction order of?35?G for vanadium,iron,aluminum and oxalate with[R4N·Cl]was as follow:?35?Gox<?35?GFe<?35?GV<?35?GAl.The extraction reaction?35?G of VO?C2O4?22-?-327.75 kJ/mol?is less than that of Al?C2O4?2-?-186.68 kJ/mol?.The extractive reactions?35?G of Fe?C2O4?2-and C2O42--were-311.00 kJ/mol and-327.75 kJ/mol,respectively,which was slightly less than the extractive reactions?35?G of VO?C2O4?22-resulted in severe co-extraction of iron and oxalate.?3?Stripping behavior and separation mechanism of vanadium,iron and aluminum in the oxalic acid systemThe separation and recovery of vanadium and iron in the mixed loaded organic phase were realized by stepwise stripping.Stripping of vanadium:under the optimal conditions,the sripped vanadium solution with V of 12.11 g/L can be obtained by four-stage countercurrent stripping.The stripping percentages of vanadium,iron,aluminum and oxalate are 99.59%,0.12%,99.62%and 98.72%,respectively.The separation factor of iron and vanadium is as high as 2.05×105.Stripping of iron:Iron organic phase?barren organic phase of vanadium?:under the optimal conditions of stripping,the stripped iron solution with an iron recovery of 99.34%can be obtained through three-stage countercurrent stripping,and the Fe?OH?3 with a purity of 98.7%can be recovered.The stripping mechanisms of vanadium,aluminum and oxalate all conform to the anion exchange mechanism.The separation mechanisms of vanadium and iron is that:?1?[?R4N?2·VO?C2O4?2]and[R4N·Fe?C2O4?2]were completely stripped by HCl.Vanadium and iron were transferred from organic phase to liquid phase.?2?Fe?C2O4?2-was ligated to a new complex FeCl4-in high concentration HCl solution.?3?Extraction of FeCl4-:Fe Cl4-was extracted into the organic phase by[R4N·Cl],while VO?C2O4?22-was retained in the liquid phase without coordination reaction with HCl,thus,the separation of vanadium and iron was realized.?4?A process flowsheet for asynchronous extraction of multiple valuable components from vanadium shale oxalic acid leaching solution was proposed.The oxalic acid leaching solution of vanadium-bearing shale realized the asynchronous comprehensive recovery of vanadium,iron and aluminum by adopting the principle process of"solvent extraction-stripping-hydrothermal precipitation-comprehensive recovery of valuable metal".VO2 products with purity of 98.5%and total recovery of 94.87%of vanadium were prepared by hydrothermal precipitation after pretreatment of the vanadium rich solution obtained by acid leaching solution extraction and back extraction.Iron was recovered in the form of Fe?OH?3 in the second stripping,and the total recovery of iron was 96.60%.Aluminum was recovered with K3Al?C2O4?3·3H2O and KAl?SO4?2·12H2O crystals from raffinate and stripped vanadium solution,respectively,and the total recovery of aluminum was 77.42%.In addition,68.18%of oxalic acid was recovered.This process has high comprehensive utilization rate,green environmental protection,and provides a new way for vanadium-braring shale to clean and extract vanadium efficiently. |
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