Immobilization of Chromium in Slags using MgO and Al₂O₃

Abstract

Chromium containing slags from stainless steelmaking may be leached by acidic environments, therefore they should be treated before being stockpiled or land filled. In this work, synthetic slags were prepared and the effect of CaO/SiO2, Cr2O3, MgO and Al2O3 contents on the stability of the mineralogical species formed was analysed. The morphology and composition of the slags were determined by XRD and SEM‐EDS, whilst their chemical stability was evaluated by leaching with an aqueous acetic acid solution. It was found that CaCr2O4 and CaCrO4 are present in slags prepared with neither MgO nor Al2O3. The Al2O3‐based slags mainly produced Ca2Al2SiO7 and the Cr(VI)‐containing oxide complex Ca4Al6CrO16, whilst MgO‐based slags produced Mg Cr2O4 as main mineralogical species. Additionally, Eh‐pH diagrams for the Ca‐Cr‐H2O and Mg‐Cr‐H2O systems at 25°C were constructed. The results showed that the lowest chromium concentration levels in the leaching liquors corresponded to MgO‐based slags owing to the stable binding of chromium in spinel with MgO. It was also observed in the Al2O3‐based slags that when increasing the slag basicity from 1 to 2, the leachability of the slags was notably increased.

Effects of MnO and CaO/SiO2Mass Ratio on Phase Formations of CaOAl2O3MgOSiO2CrOxSlag at 1673 K and PO2=10−10 atm

“…The leaching behaviors of various phases are quite different . It was well accepted that spinel phase is very stable in air and leaching of Cr from spinel phase is negligible . Therefore, modifications of stainless steel slag were proposed by many researchers to enrich Cr in spinel phase and stabilized the slag .…”

“…[4,5] It was well accepted that spinel phase is very stable in air and leaching of Cr from spinel phase is negligible. [5][6][7][8][9] Therefore, modifications of stainless steel slag were proposed by many researchers to enrich Cr in spinel phase and stabilized the slag. [5][6][7][8][9][10] Low basicity was reported to favor the formation of spinel.…”

Effect of FeO on the formation of spinel phases and chromium distribution in the CaO-SiO2-MgO-Al2O3-Cr2O3 system

“…Numerous mineralogical species present in the slags are soluble in aqueous media, such as merwinite, periclase, dicalcium silicate, and lime, although there are other phases, such as wustite, spinel, and glass, being considered as resistant to water dissolution [6][7]. The investigation of García-Ramos et al [8] also showed that the Cr leachability has been suppressed by the presence of MgO or Al2O3 in the slag system of CaO-SiO2-Cr2O3, with the efficiency of MgO for the leaching suppression higher than that of Al2O3, due to the stable binding for Cr in MgCr2O4. In addition, the mineralogical and microstructural analyses performed by Tossavainen et al [9] and Engström et al [10] have shown that merwinite (Ca3MgSi2O8), akermanite (Ca2MgSi2O7), gehlenite (Ca2Al2SiO7), and spinels were major phases in the EAF slag.…”

Stabilisation and Microstructural Modification of Stainless Steel Converter Slag by Addition of an Alumina Rich By-Product

“…When the factor sp exceeded 5 wt%, Cr leaching fell below the German norm of 0.05 mg/l Cr in the leachate. In other studies [13,14], the additions of Al 2 O 3 (3-12 wt%) to a slag basicity of 1.6 was shown to significantly increase the spinel formation and the Cr in the spinel phase decreased as a function of Al 2 O 3 addition. Tossavainen and Forssberg [15] showed that the addition of spinel forming compounds, such as MgO, Al 2 O 3 and FeO, to the slag at high temperature does in fact decrease the level of Cr leaching.…”