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

摘要

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 Ca_3Mg(SiO_4)_2, Ca_2SiO_4, MgCr_2O_4 and MgO are the main phases from the SSL slag. Most of chromium from the slag was present in spinel phase (MgCr_2O_4). 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 MgCr_2O_4 were converted to Na_3CrO_4 at 500-700℃ and CaCrO_3 at 800℃, 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℃.