In this work, Microwave roasting (MR) was used in vanadium (V) extraction from shale for its cleanness and high efficiency. Response surface methodology (RSM) was used to optimize conditions for MR-acid leaching, the enhancement mechanism of MR for V oxidization and structural distortion of muscovite were investigated using density functional theory (DFT) and a variety of micro-Characterization methods. A Box-Behnken experimental design has been used to monitor the V extraction characteristics, as affected by MR temperature, MR time, H2SO4 concentration and leaching time. In the optimized conditions, a V leaching efficiency of 93.4% was reached. Compared with conventional roasting (CR)-acid leaching technique, MR technique had absolute advantages, its roasting temperature was reduced by 115
C, roasting time was reduced by 32 min, H2SO4 concentration was reduced by 10%v/v and leaching time was reduced by 4 h. Selective thermal effect of MR enhanced activations of coal and pyrite and results in cracking of particles, promoting the oxidation of low-valent V. MR had an absolute advantage in dehydroxylation of muscovite, resulting the spacing expansion of AleO(OH) octahedral structure layer as about 1.3 times as that of CR and significantly enhancing the release of V.

Introduction

Vanadium (V) is an important rare element widely used in many fields (such as V-bearing alloy steel, V catalyst, V redox flow battery et al.) due to its special alloy and catalytic properties (Hu et al., 2018a,b; Li et al., 2017; In-Hyeok Choi et al., 2018). V-bearing shale is a unique and abundant V resource in China, with an estimated reserve of approximately 61.88 billion tons (Xue et al., 2017; Li et al., 2014; X.Y. Zhang et al., 2011). Therefore, with the increasing demand for V-bearing products, V extraction form shale has been given substantial and continuous attention. However, the average grade of V (in the form of V2O5) in shale is usually less than 1 wt% and most V stably exists as isomorphism state in low valence in octahedral structure of mica-group minerals (muscovite, illite, phlogopite et al.) replacing Al (Wang et al., 2014; Zhang et al., 2013). Therefore, to extract V from shale, roasting process is necessary for breaking structure of mica-group minerals and oxidizing lowvalent V (Hu et al., 2012; Zhu et al., 2012; Liu et al., 2010). Traditional sodium salt roasting-water leaching process is a well-known method to extraction V form shale. However, due to low extraction efficiency and environmental pollution, it is gradually being replaced by blank roasting-acid leaching process (Fu, 2009; Zhu et al., 2010). As an effective and environmentally friendly technique, the use of microwave in the mineral, metallurgy and chemistry fields has shifted into the focus of attention in recent years (Kaewwichit et al., 2017; Wang et al., 2018; Asomaning et al., 2018). Compared to conventional heating (CR), microwave heating (MR) offers several advantages such as selective heating, rapid heating, and volumetric heating.