Basic oxygen furnace (BOF) slag accounts for the majority of all residual materials produced during steelmaking and may typically contain certain transition metals. Vanadium, in particular, came into focus in recent years because of its potential environmental toxicity as well as its economic value. This study addresses the vanadium chemistry in BOF slags to better understand its recovery and save handling of the waste stream. The experimental results from the electron probe microanalysis (EPMA) study show that vanadium is preferably incorporated in calcium orthosilicate-like compounds (COS), with two variations occurring, a low vanadium COS (COS-Si) (approx. 1 wt.%), and a high vanadium COS (COS-V) (up to 18 wt.%). Additionally, vanadium is incorporated in dicalcium ferrite-like compounds (DFS) with an average amount of 3 wt.%. Using powder x-ray diffraction analysis (PXRD), EPMA, and virtual component models, stoichiometric formulas of the main vanadium-bearing phases were postulated. The stoichiometries give an estimate of the oxidation states of vanadium in the respective hosts. According to these results, trivalent vanadium is incorporated on the Fe-position in dicalcium ferrite solid solution (DFS), and V⁴⁺ and V⁵⁺ are incorporated on the Si-position of the COS.

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.

This manuscript presents the results from the synthesis and characterization of a slag analogue with a nominal composition of 17 wt% LiMnO₂ and 83 wt% Ca₂SiO₄ encountering fairly high cooling rates in order to study the evolution of Mn-species. The Mn species was also simulated from 1223 K to 1773 K using a thermodynamic model, assuming a homogeneous melt. The micro-composition including the Mn species of the solidified slag was determined experimentally, and was used as basis for molecular dynamics (MD) simulation. The MD simulation provides information on structure and viscosity at high temperatures, which are otherwise difficult to access. These parameters significantly influence oxidation state of redox-active elements and the solidified product. The micro-composition analyzed by electron probe micro analysis (EPMA) and synchrotron-based micro-X-ray fluorescence (micro-XRF) showed that Mn-rich and Ca-Si-rich phases are separated. While the Mn-O phases did not contain noticeable Ca, the Ca₂SiO₄ phase had incorporated 0.6 wt% of Mn. The slag solidified into round-shaped and droplet-shaped grains of a Li-Mn-oxide, some Mn₃O₄ and Ca₂SiO₄. The powder X-ray diffraction (PXRD) confirmed the formation of larnite; the identity of the Li-Mn-oxide, however, remained inconclusive. The Mn oxidation state (OS) was identified using synchrotron-based micro-X-ray absorption near edge spectroscopy (micro-XANES). The Mn-O grains matched well with Li-Mn-oxides and a Mn OS: +3, e.g., LiMn³⁺O₂. Small areas matching hausmannite (Mn²⁺Mn₂³⁺O₄) were also identified. The OS of Mn in the silicate phase could not be identified. For comparison, a slowly cooled slag analogue with similar composition, but higher Si content, was also subjected to micro-XANES. The slowly cooled slag formed long Mn-rich needles in a matrix of large calcium silicate crystals. The Mn-rich crystals matched well with the XANES spectrum of a Mn³⁺ Li-oxide like LiMn³⁺O₂. At the rim of the needles, the Mn-spectra matched well the hausmannite (Mn²⁺Mn₂³⁺O₄) reference. In the silicate phases, Mn had an OS: +2, unambiguously. The melt structure at different temperatures of two compositions, i.e., LiMn³⁺O₂ and Ca₂SiO₄, was simulated using molecular dynamics (MD). They serve as model compositions, assuming a heterogeneous melt. The results show significant different degrees of polymerization and viscosity. Information from MD simulations can support the identification of potentially different oxygen permeability and with that prediction of oxidation states. The bulk composition was identified by inductively coupled plasma optical emission spectrometry (ICP-OES), bulk structure by PXRD and bulk species by lab-XANES. The synchrotron micro analysis including micro-XRD were performed at the microfocus beamline I18 at the Diamond Light Source. Pure reference compounds were prepared and characterized with the same multi-modal approach.

A better understanding of high temperature processes in slags contributes to facilitate knowledge based design of the solidified product. Here, a slag analogue with a nominal composition of 17 wt% LiMnO2 and 83 wt% Ca2SiO4 was synthesized encountering fairly high cooling rates. The Mn species from 1223 K to 1773 K was simulated using a thermodynamic model assuming a homogeneous melt. The micro-composition including the Mn species of the solidified slag was determined experimentally and was used as basis for molecular dynamics (MD) simulation. The MD simulation provides information on structure and viscosity at high temperatures, otherwise difficult to access. These parameters significantly influence oxidation state of redox-active elements and the solidified product. The micro-composition analyzed by electron probe micro analysis (EPMA) and synchrotron based micro-X-ray fluorescence (micro-XRF) showed that Mn-rich and Ca-Si-rich phases are separated. While the Mn-O phases did not contain noticeable Ca, the Ca2SiO4 phase had incorporated 0.6 wt% of Mn. The slag solidified into round shaped and droplet shaped grains of a Li-Mn-oxide, some Mn3O4 and Ca2SiO4. The powder X-ray diffraction (PXRD) confirmed the formation of larnite, the identity of the Li-Mn-oxide however remained inconclusive. The Mn oxidation state (OS) was identified using synchrotron based micro-X-ray absorption near edge spectroscopy (micro-XANES). The Mn-O grains, matched well with Li-Mn-oxides and a Mn OS: +3 e.g. LiMn3+O2. Small areas matching Hausmannite (Mn2+Mn23+O4) were also identified. The OS of Mn in the silicate phase could not be identified. For comparison a slowly cooled slag analogue with similar composition however higher Si content, was also subjected to micro-XANES. The slowly cooled slag formed long Mn-rich needles in a matrix of large calcium silicate crystals. The Mn-rich crystals matched well with the XANES spectrum of a Mn3+ Li-oxide like LiMn3+O2. At the rim of the needles the Mn-spectra matched well the Hausmannite (Mn2+Mn23+O4) reference. In the silicate phases Mn had a OS: +2, unambiguously. The melt structure at different temperatures of two compositions i.e. LiMn3+O2 and Ca2SiO4 was simulated using molecular dynamics (MD). They serve as model compositions assuming a heterogeneous melt. The results show significant different degrees of polymerization and viscosity. Information from MD simulations can support the identification of potentially different oxygen permeability and with that prediction of oxidation states. The bulk composition was identified by inductively coupled plasma optical emission spectrometry (ICP-OES), bulk structure by PXRD and bulk species by lab-XANES. The synchrotron micro analysis including micro-XRD were performed at the microfocus beamline I18 at the Diamond Light Source. Pure reference compounds were prepared and characterized with the same multi-modal approach.