A Novel Method For The Vanadium Extraction From Stone Coal

As one of the refractory and rare metals, vanadium is of great strategic significance. In China, the carbonaceous shale (stone coal) alone has accounted for most of the domestic reserve of vanadium. Due to the fact that the utilization of conventional technology of extracting vanadium from stone coal is restricted for its incapability in many fields, much attention has been paid to the innovation of the vanadium extracting technology. In the present study, a novel method for extracting vanadium from carbonaceous shale was developed. Fluosilicic acid was employed as leaching agent/leaching assistant in the leaching procedure and peroxide was proposed and tested as oxidant for the conversion of V(IV) to V(V) in the vanadium precipitation procedure. The problems caused by toxic gases were resolved which were inevitable in conventional technology, as there was no roasting involved. Stone coal from different sources including Guizhou Province and Gansu Province were collected for the experiment because it was necessary to determine the scale where fluosilicic acid could be efficient. After the leaching procedure, stone coal from Guizhou Province was selected for the following vanadium extraction procedures including separation and concentration of vanadium from the filtrate by solvent extraction and the vanadium precipitation.Results of the leaching procedure showed that 68% vanadium could be extracted after stone coal from Guizhou was leached at 95°C by 30% H2SO4 for 24h with the liquid-solid ratio of 1:1, while more than 80% vanadium was extracted when it was leached at 95°C by 20% H2SiF6 for 8h with the liquid-solid ratio of 1:1. Stone coal from Gansu was believed to be less leachable, as the vanadium extraction only reach 40% and 60% after leached by 30% H2SO4 for 24h with the liquid-solid ratio of 1:1 and by 30% H2SiF6 for 12h with the liquid-solid ratio of 1:1, respectively. Vanadium leaching efficiency was satisfactory while it was leaching by H2SO4 + H2SiF6, the later being leaching assistant. For stone coal from Guizhou, vanadium extraction yielded to 80% when it was leached at 95°C by 15% H2SO4 + 8% H2SiF6 for 12h with the liquid-solid ratio of 1:1, and for stone coal from Gansu, it was 55% while leached for 16h with other parameters remained the same.In the step of separation and concentration of vanadium from the filtrate, the liquid phase containing vanadium, with pH=3.0, was stirred and contacted for 5 min with organic phase with equal volume comprised of 10% P204 + 5% TBP + 85% sulfonated kerosene, and it was found that more than 60% of vanadium was moved into the organic phase. It could reach up to 99% after a 7th-level counter-current solvent extraction. The vanadium-bearing organic phase was then stripped with 10 wt% H2SO4 for 10min with O/A=10:1, and 70% of vanadium was dissolved into the H2SO4 solution. More than 99% of vanadium could be stripped after a 5th level counter-current stripping.Vanadium in the H2SO4 solution should be oxidized from V(IV) to V(V) by oxidant before it was to be precipitated. In the present study, NaClO3 and H2O2 were selected as the oxidants. For the sample oxidized by NaClO3, 95% of vanadium precipitation yield could be obtained after pH value of the solution was adjusted to 2 and the solution was heated to 95°C, remained for 2 h. A same precipitation efficiency could be reached for the sample oxidized by H2O2 while the pH value of the solution was 4 and the solution was heated to 95°C, remained for 5 h. The vanadium products were analyzed and found to meet the specification of National Standards for vanadium pentoxide in the category of 98%. Based on the theory of mineral dissolution, a new mechanism concerned with the stone coal dissolution in the solution contained H2SiF6 was proposed. H2SiF6 was firstly decomposed to HF, and the stable structure of the mineral was increasingly destroyed by F- generated from HF. The crystal lattice might be collapsed in the following steps: nF-…nH++M-Az+?nF-…nH+?M-Az+ nF-…nH+?M-Az+?nF-…Az+?M–nH+ nF-…Az+?M–nH+?AFn+M–nH+...