Huge amount of stainless steel slags are produced annually during stainless steel-making, leading to important economical and ecological issues regarding their afterlife. Since chromium is one of the major constituents of the stainless steel and more easily oxidized to Cr₂O₃ than iron, the final slag contains a certain content of chromium [1] . It is well known that hexavalent chromium is toxic [2] and it is possible that chromium leaches out from stainless steel slag. Thus, utilization of stainless steel slags is highly restricted.

The leachability of chromium mainly depends on the occurrence of chromium in slags. Numerous mineralogical species present in steelmaking slags are soluble in aqueous media, for example, merwinite, periclase, dicalcium silicate and lime, but other phases viz. wustite, spinel and glass are considered as resistant to dissolution [3] [4] [5] [6] . Therefore, the part of chromium enclosed in soluble mineral phases could leach out as long as the slag is in aqueous condition. On the other hand, the leaching of chromium can be suppressed by means of adjusting the mineral composition of slag system. Mineralogical phases are significantly affected by slags composition and heat treatment method.

In order to understand the effect of MnO on the mineral composition of stainless slag, CaO-SiO₂-MgO-Al₂O₃-Cr₂O₃ system, experiments are carried out to prepare synthetic slag samples under a certain cooling procedure. Some analysis methods are employed, including X-ray powder diffraction (XRD), scanning electron microscopy (SEM) equipped an energy dispersive spectrometer (EDS), as well as thermodynamic calculations with FactSage 6.2.

Synthetic slags were prepared with analytical grade reagents (CaO, MgO, Al₂O₃, SiO₂, Cr₂O₃, and MnO). The composition of synthetic slag is on the basic of industrial stainless steel slag produced by EAF, shown in Table 1. The compound powders were homogeneously mixed and placed in an Al₂O₃ crucible, which was placed in a graphite crucible inside an induction furnace. The cooling procedure adopted is as follows. Firstly, the mixtures were heated to 1600˚C slowly and kept for 30 min; secondly, the temperature drop down to 1450˚C and held on for 30 min; then the temperature continued to decline to 1300˚C and kept for 60 min; finally, temperature dropped down to 1250˚C and kept for 120 min. Then, the slags were left inside the furnace to cool down to room temperature naturally. The temperature was measured with an R-type thermocouple (Pt, 30% Rh-Pt, 6% Rh).

The mineralogy of the samples was determined with X-Ray Powder Diffraction analysis (XRD, M21x, MAC). Diffraction patterns were measured in a range of 10˚ - 90˚ in 0.02˚/step. Microstructural characterization of the slags was performed using scanning electron microscopy (SEM, Jeol 6480LV), equipped a Thermo Electron NSS energy dispersive spectrometer (EDS). Thermodynamic calculations were performed by FactSage 6.2 using the model of Scheil cooling target phase.

The solidified microstructures of CaO-MgO-SiO₂-Al₂O₃-Cr₂O₃ systems with dif-

ferent MnO contents are shown in Figures 1-3, and the SEM-eds quantitative analysis results are summarized in Tables 2-4. The sample without MnO consists of four mineral phases: merwinite (Ca₃MgSi₂O₈), Larnite (Ca₂SiO₄), spinel [Mg(Cr,Al)₂O₄], and melilite, where larnite, spinel, and melilite are solid solutions on the basic of SEM-eds analyses. The white particles marked “ 4” are chromium-enriched spinel, and the dark areas around Cr-spinel are also spinel, which are rich in alumina. Melilite is a solid solution, consisting of akermanite (Ca₂MgSi₂O₇) and gehlenite (Ca₂Al₂SiO₇) [7] . As shown in Figure 2, the solidified microstructure is constituted by merwinite, larnite, spinel and melilite. The slag with 6 wt% MnO is composed of spinel and matrix. All spinels are in angular shape and unevenly distributed in matrix, which reveals that the spinels have precipitated from liquid slag at 1600˚C.

Figure 4 shows the XRD patterns for the solidified samples with different MnO content. The peaks corresponding to merwinite, akermanite, gehlenite, chromite (MgCr₂O₄), and larnite appear in all samples. In addition, diopside [Ca(Mg,Al) (Si,Al)₂O₆] is identified in 6 wt% MnO slag, while it is not observed in SEM analysis.

The mass fractions of liquid and mineral phases of the CaO-MgO-SiO₂-Al₂O₃- Cr₂O₃ systems are shown in Figure 5, as functions of temperature and MnO content. The solidification processes of these slags calculated using FactSage 6.2 can be summarized as follows and a new phase underlined is calculated to precipitate at each step.

(1) Sample without MnO

Liquid + Chromite + Larnite (1600˚C) → Liquid + Chromite + Larnite + Merwinite (~1427˚C) → Liquid + Chromite + Larnite + Merwinite + Melilite (~1406˚C) → Solid mixture (~1204˚C);

(2) Sample with 3 wt% MnO

Liquid + Chromite + Larnite (1600˚C) → Liquid + Chromite + Larnite+ Merwinite (~1420˚C) → Liquid + Chromite + Larnite + Merwinite + Melilite (~1370˚C) → Liquid + Chromite + Larnite + Merwinite + Melilite + Mn₂SiO₄ (~1010˚C) → Solid mixture (~970˚C);

(3) Sample with 6 wt% MnO

Liquid + Chromite + Larnite (1600˚C) → Liquid + Chromite + Larnite + Merwinite (~1400˚C) → Liquid + Chromite + Larnite + Merwinite + spinel (~1380˚C) → Liquid + Chromite + Larnite + Merwinite + spinel + Melilite (~1300˚C) → Liquid + Chromite + Larnite + Merwinite + spinel + Melilite + Mn₂SiO₄ (~1000˚C) → Solid mixture (~950˚C).

According to the thermodynamic calculations, for all slag systems, MgCr₂O₄ phase primarily precipitates. This is in good agreement with the experimental results through SEM. The addition of MnO significantly reduces solidus temperature due to lower melt point of MnO. For example, the solidification temperature decreases from 1204˚C to 950˚C when the content of MnO in oxide systems increases from 0 wt% up to 6 wt%. This can explain the precipitation of amorphous phase in the system with 6 wt% MnO. The lower solidus temperature is expected for a better kinetic condition.

By comparing the SEM micrographs of the slags with different MnO contents, it is apparently found that the amount of spinel markedly increases with the

content of MnO increasing. Moreover, the Mn content contained in spinel also significantly increases up to 2.93 at%. This can be explained by solid solution formed through isomorphous replacement. The investigation by L. Zhao [8] reported that Mn could dissolute into MgCr₂O₄ to form spinel through replacing the site of Mg in lattice. In addition, the size of spinel is bigger when the content of MnO is higher due to better mass transfer conditions.

As mentioned in introduction section, the leaching of chromium from slag limits the utilization in other fields. The leachability of chromium mainly depends on the occurrence of chromium in slags. Magnesium chromite (MgCr₂O₄) is considered as save mineral due to the strong bonding of chromium in the spinel and significant stability towards oxidation and dissolution [9] , while merwinite and larnite are unexpected because of being soluble in aqueous media [3] [10] . In fact, for most basic slags, merwinite is present as main mineral phase and it is possible that a small fraction of chromium is enclosed into merwinite. Therefore, this part of chromium might leach out as long as merwinite dissolves into water. Additionally, the investigation by Samada et al. [11] shows that larnite (Ca₂SiO₄) can form solid solution with magnesium chromite (MgCr₂O₄), and the results found the dissolution of larnite weakens the stability of MgCr₂O₄.

As shown in Tables 2-4, chromium is mainly present as spinel solid solution. On the basic of comparison of SEM-eds analyses, the amount of chromium existing in larnite phases reduces with the addition of MnO, while the increase of MnO content does not show great influence on the amount of chromium contained in merwinite. On the other hand, the amount of calcium dissolved into MgCr₂O₄ reduces slightly with the content of MnO increasing. It is not ignored that the amorphous phase is formed in oxide system with 6 wt% MnO. Amorphous phase is expected to suppress the leaching of chromium [12] . Furthermore, the content of chromium impurity enclosed in matrix is low down to 0.04 at%. Therefore, the addition of MnO fluxes could suppress the leaching of chromium from slag through adjusting the slag mineral composition.

The effect of MnO on the mineral composition of CaO-SiO₂-MgO-Al₂O₃-Cr₂O₃ synthetic system during cooling process from 1600˚C was investigated using X-ray diffraction, SEM-eds, and commercial thermochemical software, FactSage 7.0. The addition of MnO significantly reduces the solidus temperature of oxide systems from 1204˚C to 950˚C and promotes the precipitation of spinel by means of isomorphous replacement. The fraction of chromium contained in non-spinel mineral phases decreases and the amount of larnite dissolved into spinel phase slightly reduces with the content of MnO increasing. In addition, the amorphous phase forms when the content of MnO is up to 6 wt%. Therefore, the addition of MnO is beneficial to suppress chromium leaching from slag.

The research is supported by National Natural Science Foundation of China (No. 51404173), Hubei Provincial Natural Science Foundation (No. 2016CFB579), China Postdoctoral Science Foundation (No. 2014M562073), and State Key Laboratory of Refractories and Metallurgy (No. 2014QN21). We thank Dr. Q. Yang and Pro. A. Xu for sample-making assistance.

Yu, Y., Li, J.L., Li, M.X. and Xue, Z.L. (2017) Effect of MnO on Mineral Composition of CaO-SiO₂- MgO-Al₂O₃-Cr₂O₃ System. Journal of Minerals and Materials Characterization and Engineering, 5, 132-139. https://doi.org/10.4236/jmmce.2017.53011