Mineral Phase Transition And Distribution Of Vanadium In Roasting Process Of High Calcium Type Vanadium-bearing Stone Coal

Known as ‘vitamin of metal’, Vanadium is widely applied in industries due to itsmany excellent physical and chemical properties. Stone coal, with its huge reserves inChina, is an important vanadium-bearing resource. High calcium type vanadium-bearing stone coal is one kind of stone coal which is difficult to extract vanadium bywater leaching and the technical problem is to be solved. Most of current researchesrelated to stone coal only investigate the effects of external factors such as roastingadditives and roasting parameters, while the effect of internal factors and themechanism of the roasting process lack of in-depth study.Sodium chloride roasting of a high calcium type vanadium-bearing stone coalfrom Hubei province was investigated in this thesis. The characters of water leachingrate of vanadium (WLRV) and the effect of different forms of calcium on the roastingprocess were discussed, and the mineral phase transition in the roasting process wasrevealed. Then the effect of other major chemical components on the mineral phasetransition and the distribution of vanadium in the roasting process was investigated.The occurrence state of vanadium in the roasted sample and the cause of theunrecovered vanadium were analyzed. Finally, the essence of the raosting process wasdiscussed. The main conclusions are as follows:(1) For this stone coal, under the conditions of roasting with10%NaCl at850℃for2h, the WLRV was only14.28%and the total leaching rate of vanadium(TLRV) was69.14%. Calcium removal by5%volume concentration of HCl followedby roasting was investigated, as a result, the WLRV reached47.54%and the TLRVwas66.16%. As for the high calcium type vanadium-bearing stone coal, the processof calium removal–roasting can remarkably improve the WLRV and reduce theoptimum roasting temperature.(2) The high content of CaO in the raw stone coal was the main cause for the lowWLRV by the traditional roasting process. With the increase of the additive amount ofNaCl from0to4%, mica gradually transformed to feldspar, and the content ofanorthite and the WLRV increased. When adding more than4%NaCl, anorthitetransformed to albite, and the content of f-CaO increased, which could react withvanadium to form more calcium vanadates, as a result, the WLRV decreased.(3) The adverse effects of the different forms of calcium on the WLRV in descending order was CaCO3(CaO)> CaSiO3> CaSO4. As for the high calcium typevanadium-bearing stone coal, roasting parameters could be controlled to transformCaCO3(CaO) to CaSO4which has low activity and high melting point, thus calciumwas stabilized and the generation of water-insoluble vanadates was inhibited, as aresult, the WLRV increased obviously.(4) The excess of SiO2in the roasted sample existed in the form of quartz, whichwouldn’t decrease the WLRV and TLRV and cause the sintering phenomenon, andhad no adverse effect on the roasting process. Fe2O3in the roasted sample existed inthe form of hematite and had no adverse effect on the roasting process. InsufficientMgO in the roasted sample existed in the form of enstatite, while excessive MgO inthe roasted sample existed in the form of forsterite, and MgO had no adverse effect onthe roasting process. Al2O3in the roasted sample existed in the form of spinel andcorundum, while excessive Al2O3in the roasted sample existed in the form of sodalite.Al2O3could react with vanadium to form water-insoluble AlVO4, as a result theWLRV decreased. KCl could transformed (K, Na)-feldspar to sanidine, but didn’thave a significant activation.(5) In addition to sodium vanadates and calcium vanadates, vanadium in theroasted sample also existed in apatite, paragenesis of Ca5(PO4)3Cl with Ca5(VO4)3Cl.In the core of mica, as silica was deficient and nepheline was generated. As a result,the majority of unrecovered vanadium in the roasted sample was present in nepheline,and another small part of unrecovered vanadium was embedded in quartz in the formof sodium vanadates or a replacement of aluminum in mica.(6) For the roasting process of vanadium-bearing stone coal, the most importantchemical composition which had effects on the WLRV and TLRV was CaO and Al2O3,respectively.(7) The essence of the roasting process was mineral phase transition and theredistribution of vanadium. The objective of the roasting process was the oxidationand conversion of vanadium. The roasting process of stone coal could be studied fromthe perspective of mineral composition, the occurrence state of vanadium and thevalence of vanadium.