Extracting Chromium from Stainless Steel Slags by NaOH‐Added Pellet Roasting Followed by Water Leaching

Preventing toxic Cr6+ from being eluted from the stainless steel (SSL) slag is still an unsolved problem. An effective extraction of chromium from SSL slag might be a solution. The conventional industrial process in chromium extraction from chromite involves adding Na2CO3 to chromite during the sample roasting. The authors found that if a similar process is used to extract chromium from SSL slag, the chromium extraction was 33% at 800 °C. However, if NaOH is used instead of Na2CO3, the chromium extraction is raised to 96% under the same reaction conditions. Thus, a novel process with NaOH‐added roasting is studied here. The effects of roasting temperature, time, the molar ratio of Na/Cr, and particle size for NaOH‐added pellet samples on the chromium extraction are studied and the optimized values are 800 °C, 2 h, 29.0, and 48–58 μm, respectively. The contents of Cr2O3 in the original slag, the leaching residues obtained after the first and second time of extraction are 3.93, 0.12, and 0.02 wt%, respectively. The Cr extraction is effective and the leaching residue might be safely disposed of in environment, which ultimately solved the environmental problem of SSL slag disposal. We found that some bigger cavities are spontaneously formed on the surface of NaOH‐added pellet than on the surface of Na2CO3‐added pellet during the roasting. This perhaps mainly explained the discrepancy in the chromium extractions of NaOH‐added pellet and Na2CO3‐added pellet. The use of NaOH‐added pellet sample rather than NaOH‐added powder sample not only avoided the sample adhesion, but also significantly increased the Cr extraction.