Quantitative Phase Analysis in Carbon Steel EAF Slag for the Determination of Phase-Controlled Leaching Mechanism

The production of Electric Arc Furnace (EAF) slag is 150-180 kg per ton of steel, making its recycling essential for the steelmaking process’s sustainability. Despite favourable mechanical properties as its good abrasion resistance, environmental challenges, such as the leaching of harmful elements (e.g., Cr, V, Ba and Mo), limit its direct reuse in industries like cement production, road construction and water filtration. The mineralogical composition of the slag is the key to address its leaching behaviour, as the hydration of specific phases (e.g., larnite and brownmillerite) can release these elements. Current regulations demand minimal environmental impact and require precise quantification of slag phases to identify and mitigate the leaching mechanisms. The slag is generally and rapidly characterized by means of X-Ray diffraction (XRD) and Scanning Electron Microscopy (SEM), however the quantitative analysis obtained by the Rietveld method is not always reliable because of the presence of preferential orientations common to several phases and the high sensitivity to the imposed background. Therefore, it is necessary to implement a methodology to refine the X-Ray analysis. Selective dissolution of larnite and brownmillerite, dangerous for Ba and Cr leaching, can be used for this purpose. In this work, different quantitative techniques were compared to tune-up the Rietveld analysis. These methods facilitated the accurate determination of critical phases like larnite and brownmillerite. SEM and XRD analyses have intrinsic limitation in quantifying phase contributions. In particular the identification of brownmillerite with XRD is overestimated respect the chemical prediction due to its strong crystallographic orientation al low 2θ angle. Selective chemical dissolution is instead the most reliable technique to quantify the phases, since a previous salicylic acid methanol (SAM) dissolution dissolves all the calcium silicates and a following potash sucrose (KOSH) treatment quantify all the calcium aluminates. The difficulties encountered doing only KOSH were associated to the formation of a viscous gel due to the hydration of silicates. If the dissolution of the brownmillerite was not complete, a second KOSH dissolution is required.