Mechanisms Involved in the Roasting of Pellets Composed of Stainless Steel Slag and Sodium Hydroxide to Extract Chromium

Abstract

In order to extract the chromium from stainless steel (SSL) slag, the thermochemical processes involved in the roasting of pellets composed of SSL slag and sodium hydroxide were studied. It was found that Ca₃Mg(SiO₄)₂, Ca₂SiO₄, MgCr₂O₄ and MgO are the main phases from the SSL slag. Most of chromium from the slag was present in spinel phase (MgCr₂O₄). In addition, the chromium was also found in metallic phase (Fe–Cr) and periclase phase (MgO). These three Cr-containing phases were embedded in the silicate phases. During the pellet roasting, the silicate phases were destroyed by liquid NaOH and the low valence of insoluble chromium species such as MgCr₂O₄ were converted to Na₃CrO₄ at 500–700°C and CaCrO₃ at 800°C, respectively. Both were soluble in hot water. The residual chromium was only present in Cr–Fe–O phases. Many cavities were left on the pellet surface during the roasting, which facilitated the diffusion of both air and liquid NaOH. A high chromium extraction was achieved only when liquid NaOH had diffused into the cores of slag particles over 400°C.