Effect of MgO and CaO/SiO₂ on the immobilization of chromium in synthetic slags

Abstract

This work investigated the chemical and mineralogical properties of CaO–SiO₂–Cr₂O₃–CaF₂–MgO slags. Synthetic slags were prepared and the effect of the slag basicity (mass ratio CaO/SiO₂) and MgO contents on the stability of the mineralogical species formed was analyzed. The morphology and composition of the slags were analyzed by X-ray powder diffraction (XRD) and scanning electron microscope-energy dispersive spectroscopy (SEM–EDS), whilst their chemical stability was evaluated by leaching with an aqueous acetic acid solution. It was found that in slags with CaO/SiO₂ = 1, the main Cr-compound was MgCr₂O₄ spinel, which forms octahedron crystals. Small amounts of CaCr₂O₄ and CaCrO₄ were also observed. It was found that with increasing the slag basicity from 1 to 2 the compounds MgCr₂O₄ and CaCr₂O₄ were formed together with the Cr(V)-containing compound complex Ca₅(CrO₄)₃F which forms hexagonal crystals. The results showed that the highest Cr concentration levels in the leaching liquors corresponded to slags with CaO/SiO₂ = 2, probably owing to the formation of CaCrO₄ and Ca₅(CrO₄)₃F, whilst the lowest chromium concentration levels corresponded to MgO-based slags owing to the stable binding of chromium in spinel with MgO. Additionally, potential–pH diagrams for the Ca–Cr–H₂O and Mg–Cr–H₂O systems at 25 °C were calculated.

Introduction

The slags are attractive as construction material due to their excellent technical material properties. However, environmental concerns have been raised about the content and leachability of heavy metals of stainless steel and ferrochrome slags, especially chromium [1, 2]. Kilau and Shah [3] reported that the critical slag composition factors to control the chromium leachability were the CaO/SiO₂ ratio and MgO content. It was reported that adding MgO to slags reduced the refractory weardown and modified the slag composition, lowering the Cr-contents in the leachate [4, 5], whereas Eriksson and Björkman [6] showed that increasing the MgO in the stainless steel slags can change the mineral composition towards formation of merwinite (3CaO∙MgO∙2SiO₂) which prevents disintegration of the slag.

Stabilization and solidification are very effective tools and are becoming standard processes in hazardous waste treatment and disposal. These processes aim to attain mineralogical control of the metals to be stabilized. In previous works we have studied the effect of several slags forming agents, mainly Al₂O₃, on the leaching behavior of Cr [7, 8]. We found that CaCr₂O₄ and CaCrO₄ were present in slags of the system CaO–SiO₂–CaF₂–Cr₂O₃. Gehlenite (Ca₂Al₂SiO₇) and the Cr(VI)-containing oxide complex Ca₄Al₆CrO₁₆ were present when Al₂O₃ was added to the slag. It was observed in the Al₂O₃-based slags that in increasing the slag basicity (mass % CaO/mass % SiO₂) from 1 to 2 the leachability of the slags were notably increased. It was also shown that slags with MgO produced MgCr₂O₄ as the main mineralogical species.

In this work, chromium containing slags were prepared and the effects of slag basicity and MgO content on the stability of the mineralogical species formed were evaluated. The morphology and composition of the slags were analyzed by X-ray powder diffraction (XRD) and scanning electron microscopy coupled with energy dispersive spectroscopy (SEM–EDS). The chemical stability of chromium was evaluated analyzing the leaching levels of chromium according to the Mexican Waste Norms [9]. Additionally, the potential–pH diagrams for the Ca–Cr–H₂O and Mg–Cr–H₂O systems at 25 °C were constructed using the FACTSage thermodynamic software [10].

Materials and experimental procedure

The slag was prepared with laboratory reagent-grade compounds (CaO, MgO, CaF₂, SiO₂ and Cr₂O₃) which were previously ground into fine powder whose particle size was between 45 and 74 μm. The MgO content ranged from 0 to 9 mass %, Cr₂O₃ and CaF₂ were 10 % and the slag basicity B, defined as the ratio of CaO content to the SiO₂ content (all mass %), was held at 1 and 2. In order to observe clearly the effect of chromium on the environmental behavior of the slags, the Cr₂O₃ contents tested were higher than those actually found in ferrochrome or stainless steel production. Table 1 shows the mixtures compositions used for the experiments.

Table 1 Mixture compositions used for experiments in mass % (% Cr₂O₃ = 10, % CaF₂ = 10)

100 g of each slag system were prepared as follows. The powders were homogeneously mixed and placed in an MgO crucible which was placed in a graphite protecting crucible inside an open induction furnace. The slags were heated between 1600 and 1650 °C and the temperature was measured with an R-type thermocouple (Pt–Pt, 13 %Rh). It is worth noting that there are not reliable melting temperature values for the slags of the CaO–MgO–CaF₂–SiO₂–Cr₂O₃ system; then, to be sure that the system were completely melted at the furnace temperature, X-ray diffraction analysis were carried out after cooling the slags to verify the absence of free Cr₂O₃. The case where Cr formed complex compounds (CaCr₂O₄, Ca₅(CrO₄)₃F, etc.) instead of free Cr₂O₃, was taken as evidence that the system was completely liquid.

The slags were left inside the furnace to cool down slowly until room temperature, to promote the formation of well defined crystalline structures. The cooling rate from the melting point to complete solidification was about 8 °C/min. Attention was given to the morphology, shape and compositions of the solid crystallized phases: the presence of crystals homogeneous in composition, with well-developed facets was taken as indication of the attainment of equilibrium.

Samples of each slag were crushed into fine powders and characterized by X-ray diffraction (XRD Bruker D8 Focus) and scanning electron microscopy coupled with energy dispersive spectroscopy (SEM–EDS, Jeol 6300). The chemical stability of the Cr-containing species present in the materials was evaluated by the following leaching technique, according to the Mexican environmental regulations [9]. Twenty-five g of each slag were crushed below 74 μm and contacted with 500 cm³ of an aqueous acetic acid solution at pH 2.88 ± 0.05 in a rotary system during 20 h at 30 ± 2 rpm and 23 ± 2 °C. The solid residues were filtered through ashless filter paper (Whatmann 542) and the chromium present in the leachate was determined by atomic absorption spectrophotometry.

Results and discussion

Identification of species

The XRD patterns for slags with 0 % MgO are shown in Fig. 1. Cuspidine (Ca₄Si₂O₇F₂) and wollastonite (CaSiO₃) are the main species for the slags with CaO/SiO₂ = 1 and 2. Calcium chromite (CaCr₂O₄) is also present in both cases. Calcium chromate (CaCrO₄) was observed in slags with CaO/SiO₂ = 1, whereas a Cr(V)-containing compound, Ca₅(CrO₄)₃F, was detected in slags with CaO/SiO₂ = 2.

Fig. 1

XRD patterns of slags with 0 % MgO and a CaO/SiO₂ = 1, b CaO/SiO₂ = 2

Wollastonite (CaSiO₃) and cuspidine (Ca₄Si₂O₇F₂) are again the main species present in the slag with 3 % MgO and CaO/SiO₂ = 1; magnesium chromite (MgCr₂O₄), CaCrO₄ and CaCr₂O₃ are also obtained, as can be observed in the XRD patterns shown in Fig. 2a. Increasing the slag basicity from 1 to 2, new calcium silicate are produced (Ca₂SiO₄ and Ca₃Si₂O₇) and the main chromium compounds are MgCr₂O₄, CaCrO₄ and Ca₅(CrO₄)₃F. Peaks corresponding to free CaO are also observed.

Fig. 2

XRD patterns of slags with 3 % MgO and a CaO/SiO₂ = 1, b CaO/SiO₂ = 2

Figure 3 shows the scanning electron microscope (SEM) micrograph, X-ray spectra and the composition obtained by this semiquantitative method for the slab-shaped crystals observed in the slag with 3 % MgO and CaO/SiO₂ = 1. These crystals, containing almost the same atom % of Ca and Cr, probably correspond to CaCrO₄, identified by XRD (see Fig. 2). Figure 4 shows the micrographs of the crystalline phases of the slag with 3 % MgO and CaO/SiO₂ = 2. Octahedron crystals (labeled S) have high contents of chromium, magnesium and oxygen; then, they correspond to MgCr₂O₄ spinel. It has been reported [11] that magnesium chromite (MgCr₂O₄) crystallizes in cubes or octahedrons, such as those observed in this sample. Monoclinic crystals, labeled C, correspond to cuspidine (Ca₄Si₂O₇F₂), according to the EDS analysis. The hexagonal crystals (CF) contain calcium, chromium, fluorine and oxygen which probably is the complex compound Ca₅(CrO₄)₃F.

Fig. 3

SEM micrographs of the slag with 3 % MgO and CaO/SiO₂ = 1. Elongated crystals are CaCrO₄

Fig. 4

SEM micrographs of the crystalline phases of the slag with 3 % MgO and CaO/SiO₂ = 2. CF, Ca₅(CrO₄)₃F; C, cuspidine (Ca₄Si₂O₇F₂); S, MgCr₂O₄ spinel

Figure 5 shows the X-ray diffraction patterns for slags with 6 % MgO and CaO/SiO₂ = 1, 2. As in the previous case, the slag with CaO/SiO₂ = 1 contains cuspidine (Ca₄Si₂O₇F₂), MgCr₂O₄ and wollastonite (CaSiO₃). Figure 5b shows that the number of chemical species is increased by increasing the slag basicity. In this case calcium and magnesium silicates are formed together with the Cr-based compounds (MgCr₂O₄, CaCr₂O₄, CaCrO₄ and Ca₅(CrO₄)₃F).

Fig. 5

XRD patterns of slags with 6 % MgO and a CaO/SiO₂ = 1, b CaO/SiO₂ = 2

The SEM–EDS analysis confirm the XRD results, as can be observed in the micrographs shown in Fig. 6, corresponding to slag with 6 % MgO and CaO/SiO₂ = 2. Hexagonal and monoclinic crystals were observed in this system. According to the EDS analysis, the hexagonal crystal (labeled CF) corresponds to Ca₅(CrO₄)₃F, whereas elongated crystals (labeled C) contain fluorine, calcium and silicon, which might correspond to cuspidine. EDS analysis and XRD results indicate that crystals labeled C₃S are calcium silicate (Ca₃SiO₅).

Fig. 6

SEM micrographs and EDS analysis of the crystalline phases of the slag with 6 % MgO and CaO/SiO₂ = 2. C, Ca₄Si₂O₇F; CF, Ca₅(CrO₄)₃F; C₃S, Ca₃SiO₅

Cuspidine (Ca₄Si₂O₇F₂), calcium silicate (Ca₂SiO₄) and the spinel structured compound (MgCr₂O₄) are present in the slags with 9 % MgO and CaO/SiO₂ = 1 and 2, as can be observed in the XRD patterns shown in Fig. 7. Besides, the slag with CaO/SiO₂ = 1 contains magnesium silicates (MgSiO₃ and Mg₂SiO₄) and in the slag with CaO/SiO₂ = 2 three Ca–Cr compounds were formed; CaCr₂O₄, CaCrO₄ and Ca₅(CrO₄)₃F, where chromium has an oxidation state of 3+, 6+, and 5+, respectively.

Fig. 7

XRD patterns of slags with 9 % MgO and a CaO/SiO₂ = 1, b CaO/SiO₂ = 2

Figure 8 shows micrographs of the crystalline structures obtained in the slag with 9 % MgO and CaO/SiO₂ = 2. There were obtained as tetragonal crystals (labeled C₂S), which contained mainly calcium, silicon and oxygen, probably corresponding to Ca₂SiO₄, identified by XRD (see Fig. 7). The octahedron crystals (labeled S) correspond again to magnesium chromite (MgCr₂O₄). The crystal labeled C corresponds to cuspidine, according to the EDS analysis.

Fig. 8

SEM micrographs of the crystalline phases of the slag with 9 % MgO and CaO/SiO₂ = 2. C, Ca₄Si₂O₇F; C₂S, Ca₂SiO₄; S, MgCr₂O₄; CC, CaCr₂O₄

Finally, elongated crystals (labeled CC) correspond to calcium chromite (CaCr₂O₄). Hashimoto et al. [12] also obtained needle-like CaCr₂O₄ crystals by heating a powder mixture of CaCO₃ and Cr₂O₃, which are very similar to the crystals shown in Fig. 8.

Spinel-type compounds, such as MgCr₂O₄, are very resistant to oxidation and dissolution in acid environments; low chromium levels of leaching are expected in the MgO modified slags from stainless steelmaking. Therefore, it can be said that the presence of MgO leads to the formation of MgCr₂O₄ instead of CaCrO₄ or Ca₅(CrO₄)₃F in these slags, diminishing the formation of leachable chromium compounds; however, the CaO/SiO₂ ratio must be maintained in a low level to avoid the formation of the Ca–Cr compounds, which may be leached in acid environments.

Leaching trials

Figure 9 shows the effect of MgO and the CaO/SiO₂ ratio in the slags on the leaching behavior of chromium. As can be seen, the maximum chromium extraction, 40 mg/l Cr and 98 mg/l Cr, are reached when MgO is not added in the slag and the basicity is 1 and 2, respectively; whilst 29 mg/l Cr and 68 mg/l Cr are leached from slags with 9 %MgO and CaO/SiO₂ equals 1 and 2, respectively. These results showed that the lowest chromium concentration levels in the leaching liquors corresponded to MgO-based slags and low slag basicity (CaO/SiO₂ = 1) owing to the stable binding of chromium in spinel with MgO (MgCr₂O₄).

Fig. 9

Effect of the CaO/SiO₂ ratio and MgO content on the leaching behavior of Cr

The effect of slag basicity on the leaching behavior of Cr was analyzed in the Al₂O₃-based slags in a previous work. It was shown that by using Al₂O₃ instead of MgO to modify the slag, the slag basicity strongly affected the Cr extraction, when the CaO/SiO₂ ratio was increased from 1 to 2. This behavior was mainly due to the formation of the Cr(VI)-containing compounds CaCrO₄ and Ca₄Al₆CrO₁₆. In the MgO-based slags, the systems with high slag basicity (CaO/SiO₂ = 2) produced compounds where chromium had a high oxidation state (CaCrO₄ or Ca₅(CrO₄)₃F), which increased the leachability of the slags.

Potential–pH diagrams

Potential–pH diagrams can be used to identify the conditions required to dissolve a metallic compound in an aqueous solution at specific temperature and pressure. The FACTSage computational thermodynamic package [10] and its database were used to determine the potential–pH diagrams at 25 °C for the Ca–Cr–H₂O and Mg–Cr–H₂O systems. To complete the database of FACTSage we included the following thermodynamic information for the solid calcium chromate (CaCrO₄) reported by Lee and Nassaralla [13].

$$ {\text{H}}_{298}^{ \circ } = - 1396.3{\text{ kJ mol}}^{ - 1} ;\,{\text{S}}_{298} = 130.9{\text{J mol}}^{ - 1} \,{\text{K}}^{ - 1} $$

(1)

$$ {\text{Cp}} = 127.92 + 35.73 \times 10^{ - 3} {\text{T}} - 2.257 \times 10^{6} {\text{ T}}^{ - 2} {\text{J mol}}^{ - 1} {\text{K}}^{ - 1} $$

(2)

Figure 10 shows the potential–pH diagrams at 298.15 K (25 °C) for the Ca–Cr–H₂O and Mg–Cr–H₂O systems calculated with the FACTSage thermodynamic software [10]. Water is only stable in the region bounded by the dashed lines. The equilibrium conditions that can be utilized for the leaching of minerals or any solid compound in aqueous solutions at ambient temperature and pressure are constrained to those defined by the region of water stability.

Fig. 10

Calculated potential–pH diagrams for the Ca–Cr–H₂O and Mg–Cr–H₂O systems at 25 °C

Figure 10 was constructed specifically to compare the domains of CaCr₂O₄, CaCrO₄ and MgCr₂O₄ and in this way to establish which compound makes the slag more vulnerable to leaching. The molality of all the aqueous species in these systems have been fixed at m = 1. CaCrO₄ is not stable in acid solutions. CaCr₂O₄ becomes unstable when the pH of the leaching solution is slightly acid, about pH <6, where Ca²⁺ and Cr(OH)²⁺ ions are produced. The domain of MgCr₂O₄ is larger than that corresponding to CaCr₂O₄ and it is stable in solutions with pH >4. Then, it is important to promote the stabilization of chromium in the slag using MgO. It is worth noting that the thermodynamic information for Ca₅(CrO₄)₃F was not available and this species was not included in the E-pH diagrams.

Conclusions

Slags samples of the system CaO–CaF₂–SiO₂–Cr₂O₃–MgO were prepared in order to analyze the effect of the CaO/SiO₂ ratio and MgO content on the stability of the mineralogical species and the leachability of slags. The following results were obtained:

• a. Slags with CaO/SiO₂ = 1 and low MgO content (3 %) contained magnesium chromite (MgCr₂O₄) together with CaCr₂O₄ and the hexavalent chromium species CaCrO₄. Increasing the MgO content, the only Cr-containing compound formed was MgCr₂O₄, a spinel structured compound.

• b. Slags with CaO/SiO₂ = 2 contained MgCr₂O₄ together with CaCr₂O₄, CaCrO₄ and the Cr(V)-containing compound complex Ca₅(CrO₄)₃F which was formed with hexagonal crystals. However, the presence of MgO in the slag leads to the formation of MgCr₂O₄, diminishing the formation of leachable Ca–Cr compounds.

• c. The leaching trials results showed that the slags with high slag basicity produced the highest chromium extraction in the leachates. The presence of MgO in the slag diminished the formation of leachable chromium compounds; however, the CaO/SiO₂ ratio must be maintained in a low level to avoid the formation of Cr(V) and Cr(VI) compounds, which may be leached by acidic solutions. MgO helped to control the chromium leachability, which is due to the stable binding of chromium in MgCr₂O₄. The E-pH diagrams showed that the stability domain of MgCr₂O₄ is larger than that corresponding to CaCr₂O₄ and CaCrO₄; therefore, the formation of the Mg-spinel minimizes the leachability of the slag.

• d. A topic of future research will be the stabilization and solidification as processes of hazardous waste treatment and disposal.