Mechanism of vanadium slag roasting with calcium oxide
Introduction
Vanadium, as a very important transition metal, is widely used in various fields (Liao and Bo, 1985, Moskalyk and Alfantazi, 2003). At present, the primary vanadium resources are vanadium slag, stone coal, steel slag and spent catalyst. Vanadium slag which is a by-product of vanadium titanomagnetite accounted for more than 38% of the world's overall vanadium production in 2009 (Polyak, 2011). Processes comprising roasting, leaching, purification of aqueous solution, and precipitation steps are commonly employed in vanadium extraction from vanadium slag to produce vanadium pentoxide, of which, roasting plays a more important role in the whole process chain of vanadium.
Since problems like difficulties in sodium salt recovery, utilization of tailings, and environmental pollution, exist in the conventional roasting process with sodium salts (i.e. Na2CO3, NaCl and Na2SO4), this process is not favored; hence, it is essential to develop some new processes for the clean production of vanadium oxide. Sub-molten salt (Liu et al., 2013), carbonate salt (Li et al., 2011) and compound salt roasting for vanadium extraction can decrease the amount of poisonous gas discharge; however, they cannot eliminate the hazard of sodium or potassium in the tailings and waste water. Calcium salt roasting was firstly proposed by the Russia Tula factory in 1970s and it was not applied commercially due to lower recovery of vanadium compared with sodium salt roasting (Liao and Bo, 1985). However, it does have benefits of lower cost of additive, no emission of pollutional gas, and no sodium or potassium contained in tailings and waste water, so this process is of increasing interest.
Vanadium slag roasting with calcium oxide is an oxidation process. Vanadium embedded in the slag presents as V3 +, and with roasting, V3 + is oxidized to V4 + and V5 + and converted to vanadate which can be dissolved in a subsequent leaching step. Many investigations (Cao, 2012, Li et al., 2012, Li, 2011) have focused on the effects of roasting parameters, including the ratio of m(V2O5)/m(CaO), roasting temperature and roasting time, on the vanadium recovery. Van Vuuren and Stander (1995) studied the oxidation kinetics of synthetic FeV2O4 over the temperature range of 200–580 °C and the oxidation of synthetic FeV2O4 in a sodium carbonate mixture (Van Vuuren and Stander, 2001). Voglauer et al. (2004) investigated the reaction kinetics of vanadium roasting process in steel slag. In this paper, the thermodynamics of a converter vanadium slag roasting process with calcium oxide was analyzed. Effects of heating rate, added amount of CaO, holding temperature and holding time on oxidation and recovery of vanadium, and experimentally the relationships between the heating rate and the optimum holding temperature were studied. The roasted samples prepared under different roasting conditions were characterized by X-ray diffraction (XRD). The oxidation kinetic equations and apparent activation energy values for a vanadium slag roasting process in the presence of calcium oxide were obtained using differential scanning calorimetry and thermal gravimetric (DSC–TG) methods.
Section snippets
Materials
The chemicals used in this study (CaO, H2SO4, (NH4)2Fe(SO4)2·6H2O, C13H11NO2, H2NCONH2, NaNO2 and KMnO4) were all of analytical grade (Sinopharm Chemical Reagent Co., Ltd, purity > 98%). CaO was used as the additive for vanadium slag roasting and was dried in an oven at 120 °C for 24 h before use. The other chemicals were used to determine vanadium content in the slag with ammonium ferrous sulfate titration method. Water used in experiments was deionized one time. Vanadium slag was supplied by
Thermodynamic analysis for roasting process
Vanadium spinel surrounded by matrix phases—fayalite and augite, is the main vanadium-bearing phase in converter vanadium slag. In order to obtain a high oxidation efficiency of vanadium, the structure of the binding phases should be destroyed firstly to liberate the vanadium spinel and then it is easier for oxygen to contact with vanadium spinel directly. The trivalent vanadium is oxidized into pentavalent vanadium and then converted to dilute acid soluble vanadate.
Reactions possibly occurring
Effects of heating rate and holding temperature
It was found previously that the cooling rate of roasted slag had little effect on vanadium recovery (Cao, 2012). This paper presents the effect of heating rate on vanadium recovery and the relationships between heating rates and the obtained optimum roasting temperatures under the conditions where 6% CaO was added (assuming the vanadium slag weight as 100%), particle size was within 48–75 μm and holding time was 2 h. The results are shown in Fig. 7.
Reducing the heating rate from 4 to 2 °C per
Evaluation of the results
Using calcium roasting process to extract vanadium from converter slag was proposed in the 1970s, however, an understanding of the roasting mechanism of converter slag with CaO is still not clear and needs to be developed.
In this paper, from the thermodynamic analyses, it is found that metal iron and fayalite should be oxidized thermodynamically more easily than vanadium spinel. Some Mn2 + in the slag can be oxidized to Mn2O3, while other Mn2 + can react with V2O5 to form Mn(VO3)2 at temperatures
Conclusions
- (1)
The roasting process of vanadium slag with lime was investigated. Thermodynamic analysis predicts that oxidation of fayalite is more favorable than that of vanadium spinel; formation of calcium vanadates is energetically easier than that of manganese vanadate (MnV2O6) and magnesium vanadate (MgV2O6); oxidation of Fe2 + to Fe3 + in augite is hindered by the chemically stable diopside. Oxidation of fayalite, spinel and augite and formation of vanadates are all exothermic reactions.
- (2)
Lowering heating
Acknowledgments
This project (FMRU2007K10) was supported by the Open Research Fund of Key Laboratory for Ferrous Metallurgy and Resources Utilization of Ministry of Education, Wuhan University of Science and Technology.
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