A microfocus study on basic-oxygen-furnace slag thin sections to understand the principles of vanadium incorporation

Vanadium in basic-oxygen-furnace (BOF) slags is particularly interesting regarding the recovery of this element, which has been further strengthened by its inclusion in the list of critical raw materials by the European Union in 2017. As BOF slags can have a significant vanadium content, they continue to be a promising secondary vanadium source. Therefore, the binding mechanisms of vanadium in the respective slag minerals must be fundamentally understood to adapt a recovery process, which enables a sufficient yield and is environmentally friendly and resource-saving. This study presents a synchrotron chemical-imaging investigation based on a combined micro X-ray fluorescence, micro X-ray diffraction and micro X-ray absorption near-edge structure analysis, enabling new insights into vanadium incorporation in BOF slags. In contrast to the previously assumed incorporation of vanadium directly into the calcium silicates, it was shown that vanadium accumulates in their boundary regions as tetrahedral V⁵⁺ in a structure deviating from calcium silicates. This structure is most likely a poorly crystalline residuum consisting mainly of calcium, silicon and vanadium (and oxygen) but shows no evidence of a glass phase. Additionally, vanadium occurs as octahedral V⁴⁺ in calcium ferrites and as tetrahedral V⁵⁺ in very small quantities directly in the calcium silicates. The significant enrichment of vanadium in the boundary regions of calcium silicates together with the high oxidations states, resulting in high mobility, can be regarded as advantageous for future recycling strategies.