Mechanism Of Fluoride In Enhancing Vanadium Extraction From Stone Coal

At present, there are generally low leaching efficiency of vanadium and highacid consumption for vanadium extraction from stone coal. However, fluoride caneffciently enhance vanadium leaching process. This thesis focused on the theory offluoride in enhancing vanadium leaching from stone coal and the effects of fluorideon solvent extraction. Through comparing the changes of vanadium leaching andsolvent extraction with fluoride introduced for vanadium extraction from stone coal,the mechanism of fluoride in enhancing vanadium leaching can be concluded: Freacted with Al and Si to generate [SiF226]-and [AlF5]-during vanadium leachingprocess, which facilitated releasing vanadium into leachate and declined the influenceof chemical reaction on vanadium leaching. This made the previous vanadiumleaching controlled by chemical reaction transformed to that controlled by internaldiffusion. These changes accelerated vanadium leaching and increased the efficiencyand rate of vanadium leaching.Through comparison of vanadium leaching behavior of three typical kinds ofstone coal during direct acid leaching process and blank roasting acid leachingprocess, it was found that occurance of vanadium and iron and content ofacid-consuming minerals like calcite in raw stone coal and blank roasting have greateffects on vanadium leaching efficiency of vanadium. On the one hand, blank roastingbenefited the vanadium leaching because of transforming vanadium (bound toorganics) into vanadium (bound to free oxide) and breaking vanadium-bearingsilicoaluminate minerals down. On the other hand, it also made Pyrite transformedinto acid-consuming Hematite, which increased the consumption of hydrofluoric acid.It was the generation of Hematite covering the benefits from blank roasting thatdecreased the leaching efficiency of vanadium. It was reasonable for this stone coalwhere vanadium and iron mainly existed in forms of silicoaluminate and Pyriterespectively to adopt direct acid leaching process with calcium fluoride.By comparing the effects of fluoride introduced in vanadium leaching on solventextraction during direct acid leaching solvent extraction and blank roasting solventextraction process, it was found that the vanadium extracting process for stone coaland the introduction of fluoride have significant influence on solvent extraction ofvanadium. Direct acid leaching may introduce organics from stone coal, such as protein, into aqueous solution. The organics formed interfacial membrane duringsolvent extraction, which is an important factor for producing emulsification.Blank-roasting can remove the influence of organics, but increase the content ofvarious elements other than vanadium, which may also contribute to emulsification.Fluorine can hinder the production of silicon-bearing colloids, fine particles andinterfacial membrane, and facilitate solvent extraction because of [AlF5]2-and [SiF6]2-.However, the beneficial merit may disappear and emulsification may occur again withtoo high content of various elements other than vanadium.