Formation mechanism and control method of the silicate minerals-based coating (SMC) in blank roasting process of vanadium-bearing shale

High temperature roasting is very important for vanadium (V) extraction from shale, and blank roasting (roasting without additives) followed by acid-leaching is widely used for its high efficiency and environmental friendliness. However, the silicate minerals-based coating (SMC) is easily generated to enwrap the roasted particles and seriously hinders the leaching of V. In this study, formation mechanism of the SMC was investigated and a control method was proposed. Under normal conditions, the SMC forms above 850 degrees C and there are mainly two stages in its formation process. Stage I is solid phase reaction stage at 850 degrees C to 950 degrees C, where V leaching efficiency slightly decreases without obvious melting. In this stage, feldspar reacts with mica under the promotion of CaO to generate new silicate minerals which are insoluble in acid solution. V coexists with other elements via chemical combination in these new silicate minerals. Stage II is melting stage exceeding 950 degrees C, where V leaching efficiency rapidly decreases with obvious melting in roasted particles. In stage II, the new silicate minerals formed in stage I preferentially melt. Microwave roasting in inert atmosphere is an effective control method to weaken the SMC via inhibiting solid phase reaction. Compared with the normal conditions, microwave roasting in inert atmosphere increases V leaching efficiency by 9.6%-91.1%.