The mineralogy of air granulated converter slag

Converter slag, also known as Basic Oxygen Furnace slag, is a by-product of steelmaking that is produced in large quantities worldwide. It currently has few applications, because the presence of free lime often prevents the use as aggregate, while the low reactivity makes it undesirable as a cement replacement. Air granulation is a promising way to increase the reactivity of converter slag and enable recycling as a cement replacement and this is the first in-depth characterization of air granulated steel slag. In this study converter slag was separated into different fractions (0.25-0.5 mm, 0.5-1 mm, 1-2 mm, 2-4 mm) after air granulation to study the influence of size and therefore cooling speed on its mineralogy. The air granulated slag fractions were characterized using X-ray fluorescence, quantitative X-Ray diffraction, large area phase mapping based on scanning electron microscopy and energy-dispersive X-ray spectroscopy as well as leaching behavior. The results show that the main minerals in air granulated converter slag are the same as in industrially cooled slag, but that additional perovskite was formed, which has not been reported before. All fractions contained large phenocrysts of Ca₂SiO₄ and Mg-wuestite surrounded by a dense matrix containing the other minerals. The three largest fractions are very similar to each other in chemical composition and microstructure, while the smallest fraction (0.25-0.5 mm) contains a higher content of Mg-wuestite even though the starting composition was the same. Free lime is only present at the detection limit (0.1 ± 0.1 wt%) in all size fractions. The leaching of chromium and vanadium is greatly increased compared to standard cooled converter slag indicating that air granulation results in the greater dissolution of phases containing these elements, which also indicates a greater hydraulic reactivity of granulated slag despite very little amorphous content.