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

Abstract

Preventing toxic Cr 6þ 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 Na 2 CO 3 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 8C. However, if NaOH is used instead of Na 2 CO 3 , 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 8C, 2 h, 29.0, and 48-58 mm, respectively. The contents of Cr 2 O 3 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 Na 2 CO 3 -added pellet during the roasting. This perhaps mainly explained the discrepancy in the chromium extractions of NaOH-added pellet and Na 2 CO 3 -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.[ Ã ] Prof.

Detoxification and Recovery of Spent Carbon Cathodes via NaOH–Na2CO3 Binary Molten Salt Roasting–Water Leaching: Toward a Circular Economy for Hazardous Solid Waste from Aluminum Electrolysis

“…Figure a shows that fluoride leaching efficiency and recovered graphitized carbon purity increased from 96.63 and 95.41% to 96.92 and 96.38%, respectively, with NaOH-to-Na 2 CO 3 mass ratio increased to 20%. This result was attributed to the increasing amount of NaOH with a lower melting point and stronger alkalinity, which improved the fluidity and reactivity of the molten salt system . However, the uniformity of the activated mixture gradually decreased with the increase of NaOH-to-Na 2 CO 3 mass ratio because of the hygroscopicity and viscosity of NaOH.…”

“…This result was attributed to the increasing amount of NaOH with a lower melting point and stronger alkalinity, which improved the fluidity and reactivity of the molten salt system. 41 However, the uniformity of the activated mixture gradually decreased with the increase of NaOH-to-Na 2 CO 3 mass ratio because of the hygroscopicity and viscosity of NaOH. This view can be supported by the fact that the median particle size of the mixture increased with the increase of NaOH-to-Na 2 CO 3 mass ratio, as shown in Figure S1.…”

“…25,43 In addition, Na 2 CO 3 melts and decomposes at temperatures above 851.7 °C. 41 This phenomenon is beneficial for improving the mixing effect and penetration ability of the mixed alkali. 31 The further increase of the alkali melting temperature provides formation conditions for complex and poorly soluble fluorine-containing compounds, limiting the dissociation of fluoride and the purification of graphitized carbon.…”

A Review of Circular Economy Prospects for Stainless Steelmaking Slags

“…It might be a separate "reduction furnace" in which valuable metals could be reduced from the slag to very low contents in a properly stirred reactor under highly reducing conditions, e.g., in the presence of a Fe-C or Fe-C-Cr melt. Also, other reductants like Si and Al are possible [15][16][17][18][19][20][21]. Cr bound is most difficult to reduce in spinels like MgCr 2 O 4 .…”

Inclusions Absorbed by Slags in Interstitial‐Free Steel Production

“…Many studies have reported on the second step of inclusion separation across the interface. However, various grades of steel need distinct slag to achieve different metallurgical tasks, such as the IF steelmaking process, slags in the refining process, and continuous casting have significantly different properties from those in pipeline steel or stainless steel manufacturing . Therefore, reconsidering the separation of inclusions across the interface is significant in understanding inclusions removed by slag in IF steel manufacturing.…”