Selective elemental concentration during the solidification of stainless steel slags for increased Cr recovery with MnO addition

Abstract

The study investigated the effect of MnO on the viscosity and the equilibrium crystallization behavior of CaO‐SiO2‐Al2O3‐Cr2O3‐MnO stainless steel slags to understand the selective concentration behavior of Cr during slag solidification. The apparent viscosity gradually decreases with higher MnO content, which act as a network modifier, resulting in simpler [SiO4]‐structural units. Due to the formation of MnCr2O4 spinel crystals, the tendency of decreasing viscosity is impeded. The equilibrium crystal phase with higher MnO content changes from the Cr2O3 phase to the MnCr2O4 spinel phase during the crystallization process. Distinct separation between Cr‐enriched phases with the amorphous phase can be observed and analysis of as‐quenched slags after thermochemical equilibrium experiments shows that the overall Cr content in the amorphous phase gradually decreases with higher MnO content and lower temperature, indicating Cr can be selectively enriched in a corresponding Cr‐containing crystalline phase. The sizes of the primary Cr‐enriched phases are < 20 μm. With a higher MnO content, the volume of the Cr2O3 phase decreases, and the corresponding volume of the MnCr2O4 spinel phase increases. Understanding the distribution characteristics of the Cr‐enriched phase and the gradually enriched Cr content can provide a new approach for the increased retrieval of stainless steel slags.

Immobilizing chromium in stainless steel slags with MnO addition

“…Due to the structural similarity, Ca According to Cao et al, 17 the lgC Cr 3+ values for MgCr 2 O 4 and FeCr 2 O 4 spinel phases in acid extractant with pH value of 3.2 approximately equals −7.29 and −10, respectively, suggesting the dissolution of Cr from MgCr 2 O 4 and FeCr 2 O 4 spinel phases are also extremely limited and can be neglected, that is, the formation of Cr-contained spinel phase can decrease the overall content of Cr in the amorphous glass phase and subsequently deplete the potential main source of Cr-related ions in the leachate. Thus, in the present study, MnO additions can selectively stabilize the potentially hazardous Cr into a spinel phase and minimize the content of Cr in the potentially leachable amorphous phase 23 to reduce the potential hazards of Cr-containing stainless steel slags to the environment. In addition, MnO additions into the stainless steel slags can assist in selective enrichment and sustainable reclamation of highly valued Cr-enriched phase and subsequently allow reutilization of Cr-containing stainless steel slags back into the metallurgical smelting process or construction materials.…”

“…Figure 1A-F shows some typical morphology of the crystalline phases of the synthesized stainless steel slags with various MnO contents equilibrated between 1300°C to 1500°C. Combining the results of the previously published article, 23 The details of the XRD analysis, the distribution characteristics of the Cr-enriched phases, the enrichment ratio of Cr, and the size distribution of the Cr-enriched phases have been provided in a past publication. 23 To understand the relationship between Cr leaching behavior dependent on the phase distribution, the leaching stability of Cr in the synthesized stainless steel slags was evaluated through a series of standard leaching tests.…”

“…Combining the results of the previously published article, 23 The details of the XRD analysis, the distribution characteristics of the Cr-enriched phases, the enrichment ratio of Cr, and the size distribution of the Cr-enriched phases have been provided in a past publication. 23 To understand the relationship between Cr leaching behavior dependent on the phase distribution, the leaching stability of Cr in the synthesized stainless steel slags was evaluated through a series of standard leaching tests. Although the leaching standard requirement is only for 18 hours, slags were leached up to 48 hours to evaluate the effect of leaching duration.…”

“…When the MnO content is increased from 3 to 5 mass pct., the Cr-related ions content in the leachate decreases significantly, which can be attributed to the initial generation of appreciable amounts of MnCr 2 O 4 spinel in the equilibrium phases with MnO content of 5 mass pct. 23 Figure 2B shows the leaching amount of Cr-related ions from the slags equilibrated at different temperatures with different MnO contents under standard leaching time of 18 hours. The result indicates that the Cr-related ions content in the leachates gradually decreased with lower equilibrium temperature and higher MnO content.…”

“…Cr-containing stainless steel slags Figure 5 shows the XRD results of the stainless steel slags after leaching. Compared with the phase compositions before leaching at 1500°C and 1400°C, 23 spinel phase is extremely restricted and limits the oxidation and water solubility of Cr in an acidic environment. 26,27 When the temperature was reduced to 1300°C, a complex Ca 3 Cr 1.75 Al 0.25 Si 3 O 12 phase was observed after leaching, which can also behave as a Cr-stabilizer having the cubic garnet structure, 28 but the characteristic peaks of Ca 3 Si 2 O 7 and Ca 3-x Mn x Si 2 O 7 phases disappeared.…”