The Effect of Chromium on the Roasting Process of Vanadium Extraction

Vanadium is an important metal used almost exclusively in ferrous and non-ferrous alloys due to its physicochemical and mechanical properties such as grain refinement, high tensile strength, hardness and fatigue resistance; it has a wide range of uses in the metallurgical and chemical sectors [1–3]. There are mainly three kinds of raw materials, bone coal, vanadium slag and waste vanadium catalyst, used to produce vanadium pentoxide in China. In addition, few foreign countries extract vanadium from petroleum residue, including the United States, Japan and others [4]. Vanadium slag is one of the most important sources of vanadium, which accounts for 58% of the production of vanadium [5]. The vanadium slag mixed with salt roasts in an oxidizing atmosphere, vanadium precipitates from the mineral structure and oxidizes to pentavalent vanadium oxide, and then vanadium pentoxide reacts with sodium oxide decomposed from sodium to form soluble sodium vanadate [6–8]. There is a lot of literature on the extraction of vanadium pentoxide from vanadium-containing material using NaCl, Na2SO4, and Na2CO3 as additives [9–13]. The representative vanadium extraction route mainly consists of the following procedures: sodium salt roasting, water leaching, solution purification and V precipitation [5]. However, vanadium slag usually contains 3%–6% chromium (III) oxide in the form of chromium spinel, which is thermodynamically more stable than vanadium spinel and cannot be recovered at vanadium roasting temperatures [14]. Chromium is an important strategic resource, but hexavalent chromium is a highly toxic and carcinogenic substance, and it can corrode and damage the organism [14,15]. Hexavalent chromium is one of the key pollutants [14] that creates air, soil, surface and groundwater pollution [16], causes geological disasters, and is a long-term environmental pollution [17]. Chromium slag is one of the important sources of chromium pollution. Vanadium slag is a source of chromium slag, so it is a difficult problem to extract chromium from vanadium slag. However, there are only a few studies of elements’ effects on the salt roast vanadium process with the addition of calcium (every 1% increase of CaO in slag would cause a 4.7%–9.0% V2O5 loss [18]). This study will investigate the effect of chromium on salt roasting to provide a theoretical basis for the separation of chromium and vanadium in the future. The change in the mineralogy of vanadium slag