Stabilization of Chromium in Synthetic Slags with FeSO₄ and FeS₂

Abstract

In this work, the effects of the slag basicity (mass ratio CaO to SiO₂) and the addition of FeSO₄ or FeS₂ into the slag on the stability of the mineralogical species in the slag containing chromium compounds were studied. The chemical stability was evaluated by leaching the slags with an acid solution. The main Cr-compounds in slags with low basicity (CaO/SiO₂ = 1) and FeSO₄ were FeCr₂O₄ and Ca₃Cr₂Si₃O₁₂, while Cr₃S₄ and Ca₃Cr₂Si₃O₁₂ were formed in slags with FeS₂. The slags with high basicity (CaO/SiO₂ = 2) and FeSO₄ contained CaCr₂O₄, FeCr₂O₄ and Ca₅(SiO₄)₂SO₄. The slags with FeS₂ and high basicity produced FeS·Cr₂S₃, FeCr₂O₄ and Ca₃Cr₂Si₃O₁₂. The results showed that the lowest chromium concentration levels in the leaching liquors corresponded to slags with CaO/SiO₂ = 1 and high FeS₂ contents, owing to the stable binding of chromium in the compounds FeCr₂O₄, Cr₃S₄ and Ca₃Cr₂Si₃O₁₂.

1 Introduction

There is an increasing interest in finding ecological solutions for the safe disposal of industrial by-products that can cause environmental risks due to the mobility of the toxic elements therein. In particular, environmental concerns are on the rise due to the heavy metal content in stainless steel and ferrochrome slags, especially Cr [1, 2].

The stone-like properties of metallurgical oxide slags make them attractive in the civil engineering field as construction material; therefore, to prevent the leaching of heavy metals such as chromium, it is desirable to treat them prior to its application or even landfilling. Chemical 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. It has been proposed to fix the chromium into stable mineral phases by adding some materials to the liquid slag in the transfer ladle [1]. For instance, blast furnace slags can be used to stabilize Cr(III) and Cr(VI) in contaminated soils to decrease their leachability to safe levels [3, 4]; blast furnace slags have also been used for both physical and chemical immobilization of chromium in Portland and blended cements [3].

The influence of several slag forming agents over the leaching behavior of Cr has been analyzed. For instance, adding MgO reduces the refractory wear down and modifies the slag composition, lowering the Cr-contents in the leachate [5] which can also be attained by adding spinel forming materials such as bauxite [6, 7]. In either case, the environment-friendly behavior of the slags is improved.

In a previous work [8] the authors studied the effect of slag basicity (CaO/SiO₂) and MgO and Al₂O₃ contents on the stability of the mineralogical species and the leachability of slags. It was found that CaCr₂O₄ and CaCrO₄ were 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 MgCr₂O₄ as main mineralogical species.

In this work, synthetic slags containing chromium have been prepared and the effects of slag basicity and FeSO₄ and FeS₂ contents on the stability of the mineralogical species formed have been evaluated. The morphology and composition of the slags have been analyzed by X-ray powder diffraction (XRD) and scanning electron microscopy coupled with energy dispersive spectroscopy (SEM–EDS). The chemical stability of Cr has been evaluated by analyzing the leaching levels of chromium according to the Mexican Waste Norms [9].

2 Materials and Experimental Procedure

The slags were prepared from laboratory reagent-grade compounds (CaO, FeSO₄·7H₂O, FeS₂, CaF₂, SiO₂ and Cr₂O₃) which were previously ground into fine powder whose particle size was between 45 and 74 μm. Two synthetic slag systems were prepared:

$${\text{Slags}}\;{\text{A}}:{\text{CaO}},{\text{CaF}}_{ 2} ,{\text{SiO}}_{ 2} ,{\text{Cr}}_{ 2} {\text{O}}_{ 3} + {\text{FeSO}}_{ 4}$$

$${\text{Slags}}\;{\text{B}}:{\text{CaO}},{\text{CaF}}_{ 2} ,{\text{SiO}}_{ 2} ,{\text{Cr}}_{ 2} {\text{O}}_{ 3} + {\text{FeS}}_{ 2}$$

The FeSO₄ and FeS₂ content in each type of slag ranged from 0 to 9 mass %, Cr₂O₃ and CaF₂ were 10 % and the slag basicity, defined as the ratio of CaO content to the SiO₂ content (mass %), was 1 or 2. The Cr₂O₃ content tested in these slags was higher than those actually found in ferrochrome or stainless steel production. Still this high Cr₂O₃ concentration was used in order to observe clearly the effect of chromium on the leaching behavior of the slags. Table 1 shows the mixture compositions for slags A. The experimental compositions for slags B were similar to those shown in Table 1, but FeS₂ was used instead of FeSO₄.

Table 1 Mixture compositions of slags A used for experiments in mass pct

100 g of each slag system were prepared as follows. The powders were homogeneously mixed and placed in an Al₂O₃ crucible which was placed in a graphite protecting crucible inside an open induction furnace. Each mixture was heated for 30 min at a temperature 50 °C above its melting point, to ensure complete homogenization. It was worth to note that it was difficult to estimate theoretically the melting temperature of these slags; then, the slags were heated between 1450 and 1550 °C, depending on the composition, and the melting temperature was checked mechanically using a stainless steel bar which was introduced in the melt. The furnace temperature was controlled within ± 5 K with an R-type thermocouple (Pt–Pt, 13 %Rh).

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. The morphology, shape and compositions of the solid crystallized phases were investigated: the presence of crystals with homogeneous composition and well-developed facets, was taken as an 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 these materials was evaluated by the following leaching technique, according to the Mexican environmental regulations [9]: 25 g of each slag was 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 (Whatman 542) and the chromium present in the leachate was determined by atomic absorption spectrophotometry. It was worth noting that this chemical analysis gave the total chromium in the leachate, including Cr³⁺, Cr⁵⁺ and Cr⁶⁺.

3 Results and Discussion

3.1 Result of Slags A (FeSO₄)

Figure 1 shows the XRD patterns of the slag A without FeSO₄ with different basicity ratios. Cuspidine (Ca₄Si₂O₇F₂), calcium chromate (Ca₃(CrO₄)₂), calcium chromite (CaCr₂O₄) and free Cr₂O₃ are present in both cases, despite the difference in basicity. The valence of chromium in the compounds CaCr₂O₄ and Ca₃(CrO₄)₂ are 3+ and 5+, respectively. Figure 2 shows the SEM micrographs and the EDS semi-quantitative analysis for some chemical species found in this slag (CaSiO₃, Cr₂O₃ and CaCr₂O₄).

Fig. 1

XRD patterns of slags A with 0 % FeSO₄ and a CaO/SiO₂ = 1; b CaO/SiO₂ = 2

Fig. 2

SEM micrographs of the crystalline phases of the slags A with 0 % FeSO₄ and CaO/SiO₂ = 1. a CaSiO₃; b Cr₂O₃; c CaCr₂O₄

The XRD patterns for slags A with 9 % FeSO₄ are shown in Fig. 3. FeCr₂O₄, CaCr₂O₄ and the complex compound Ca₅(SiO₄)₂SO₄ are obtained for both the slag basicities. However, calcium silicates of the garnet group, andradite (Ca₃Fe₂Si₃O₁₂) and uvarovite (Ca₃Cr₂Si₃O₁₂), are present in slags with CaO/SiO₂ = 1. The Ca₅Cr₂SiO₁₂ with valence Cr⁵⁺, is observed in slags with CaO/SiO₂ = 2. The garnet group comprise of several minerals with related chemical formulas. The generic formula for the common garnets is: X₃ ²⁺Y₂ ³⁺Si₃O₁₂, where X represents Ca, Fe²⁺, Mn or Mg, and Y represents Al, Cr or Fe³⁺. The species Ca₅Cr₂SiO₁₂, a pentavalent Cr⁵⁺ compound, is formed in the slags with high basicity. Cr⁵⁺ and Cr⁶⁺ form stable anions in aqueous solution [10] and it is expected that the leachability of chromium by acid solutions may be high in these slags.

Fig. 3

XRD patterns of slags A with 9 % FeSO₄ and a CaO/SiO₂ = 1; b CaO/SiO₂ = 2

The SEM–EDS semi-quantitative analysis confirm the XRD results, as can be observed in the micrographs shown in Fig. 4, corresponding to the slag with 9 % FeSO₄ and CaO/SiO₂ = 1. The trapezohedral crystal (labeled A) contains calcium, chromium, silicon and oxygen. So it is believed that it corresponds to uvarovite (Ca₃Cr₂Si₃O₁₂). High contents of chromium, iron and oxygen are found in the octahedron crystals (labeled B). It has been reported [11] that chromite (FeCr₂O₄) crystallizes in cubes or octahedrons, such as those observed in this sample; thus, it is believed that chromite (FeCr₂O₄) is formed in this slag. The crystal labeled C, contains mainly calcium, silicon, sulfur and oxygen, and they probably correspond to Ca₅(SiO₄)₂SO₄, identified by XRD in Fig. 3.

Fig. 4

SEM micrographs of the crystalline phases of the slags A with 9 % FeSO₄ and CaO/SiO₂ = 1. a Ca₃Cr₂Si₃O₁₂; b FeCr₂O₄; c Ca₅(SiO₄)₂SO₄

Spinel-type compounds, such as FeCr₂O₄, are very resistant to oxidation and dissolution in acid environment; therefore low chromium levels of leaching are expected. It can be seen that the presence of FeSO₄ leads to the formation of chromite FeCr₂O₄ instead of Ca₃(CrO₄)₂ in these slags, diminishing the formation of leachable chromium compounds.

3.2 Result of Slags B (FeS₂)

Figure 5 shows the X-ray diffraction patterns for slags B with 3 % FeS₂ and CaO/SiO₂ = 1 and 2. As has been observed in slags A with FeSO₄, the slags with FeS₂ contain cuspidine (Ca₄Si₂O₇F₂) and calcium silicates. The Cr-based compounds for the slags with CaO/SiO₂ = 1 are Ca₃Cr₂Si₃O₁₂ and Cr₃S₄. The last compound (Cr₃S₄), named brezinaite, is a complex compound formed by CrS and Cr₂S₃, which has been reported to form at very reducing conditions. Figure 5b shows that increasing the slag basicity, the Cr-based compounds are Ca₃Cr₂Si₃O₁₂, CaCrSi₄O₁₀ and CaCr₂O₄. Brezinaite (Cr₃S₄) is also observed in the FeS₂-based slags with high basicity.

Fig. 5

XRD patterns of slags B with 3 % FeS₂ and a CaO/SiO₂ = 1; b CaO/SiO₂ = 2

Figure 6 shows the SEM micrographs and the EDS results for the crystalline phases of the slag with 3 % FeS₂ and CaO/SiO₂ = 1. Crystal labeled A shows a trapezohedral structure with calcium, chromium, silicon and oxygen, probably corresponding to uvarovite (Ca₃Cr₂Si₃O₁₂). Crystal labeled B contains mainly chromium and sulfur and it is believed that it corresponds to brezinaite (Cr₃S₄), identified by XRD in Fig. 5. The triclinic crystal observed in this slag corresponds to wollastonite (CaSiO₃), according to the EDS results.

Fig. 6

SEM micrographs of the crystalline phases of slag B with 3 % FeS₂ and CaO/SiO₂ = 1. a FeCr₂O₄; b FeS·Cr₂S₃; c CaCr₂O₄

Figure 7 shows the X-ray diffraction patterns for slag B with 9 % FeS₂ and CaO/SiO₂ = 1 and 2. The X-ray results show that the mineralogical species obtained in this slag are basically the same as those obtained with 3 % FeS₂. It is worth noting that the main difference regarding the Cr-based compounds is that, with low basicity, brezinaite is obtained, whilst at high slag basicity, calcium chromite (CaCr₂O₄) and FeS·Cr₂S₃ are formed. Magnetite (Fe₃O₄) is also detected in the slags with FeS₂.

Fig. 7

XRD patterns of slags B with 9 % FeS₂ and a CaO/SiO₂ = 1; b CaO/SiO₂ = 2

Figure 8 shows micrographs of the crystalline structures obtained in slags B with 9 % FeS₂ and CaO/SiO₂ = 2. The octahedron crystals (labeled A), which contain mainly iron, chromium and oxygen, probably correspond to chromite (FeCr₂O₄), as identified by XRD (see Fig. 7). The crystals labeled B contain mainly chromium, iron and sulfur, and it is possible that they correspond to the complex compound FeS·Cr₂S₃, observed in the XRD results. Elongated crystals (labeled C) correspond to calcium chromite (CaCr₂O₄). Hashimoto et al. [12] also obtained needle-like CaCr₂O₄ crystals by heating a powdered mixture of CaCO₃ and Cr₂O₃, which are very similar to the crystals shown in Fig. 8.

Fig. 8

SEM micrographs of the crystalline phases of slag B with 9 % FeS₂ and CaO/SiO₂ = 2. a FeCr₂O₄; b FeS·Cr₂S₃; c CaCr₂O₄

3.3 Leaching Trials Results

Figure 9 shows the effect of FeSO₄ and the CaO/SiO₂ ratio in the slags on the leaching behavior of chromium. As can be seen, the maximum chromium extractions of 8.8 and 34.3 mg/l Cr are reached when FeSO₄ is not added to the slag and the basicities are 1 and 2, respectively; whilst 4.8 and 27.1 mg/l Cr are leached for slags with 9 % FeSO₄ and CaO/SiO₂ equals 1 and 2, respectively. These results show that the lowest chromium concentration levels in the leaching liquors correspond to FeSO₄-based slags and low slag basicity (CaO/SiO₂ = 1) owing to the stable binding of chromium in the spinel (FeCr₂O₄) and the garnet compound uvarovite (Ca₃Cr₂Si₃O₁₂).

Fig. 9

Effect of the CaO/SiO₂ ratio and FeSO₄ content on the leaching behavior of Cr

Figure 10 shows the leaching results for the slags with FeS₂. It is clear that FeS₂ is better than FeSO₄ in controlling the chromium leachability, which can be due to the stable binding of chromium in brezinaite (Cr₃S₄) and uvarovite (Ca₃Cr₂Si₃O₁₂).

Fig. 10

Effect of the CaO/SiO₂ ratio and FeS₂ content on the leaching behavior of Cr

3.4 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 [13] and its database have been used to determine the potential-pH diagrams at 298.15 K (25 °C) for the Ca–Cr–H₂O, Ca–Cr–Si–H₂O and Fe–Cr–H₂O systems. The E-pH diagrams are shown in Figs. 11 and 12. 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 are constrained to those defined by the region of water stability.

Fig. 11

Calculated Potential-pH diagrams for the a Ca–Cr–H₂O and b Ca–Cr–Si–H₂O systems at 298.15 K

Fig. 12

Calculated Potential-pH diagrams for the Fe–Cr–H₂O systems at 298.15 K

Figure 11 shows the pH values calculated specifically to compare the domains of Ca₃Cr₂Si₃O₁₂ and CaCr₂O₄ and in this way to establish which Ca–Cr compound makes the slag less vulnerable to leaching. The molality of all the aqueous species in these systems has been fixed to m = 1. Figure 11a shows that CaCr₂O₄ becomes unstable when the pH of the leaching solution is slightly acidic, about pH < 6, where Ca²⁺ and Cr(OH)²⁺ ions are produced. The domain of Ca₃Cr₂Si₃O₁₂ is larger than that corresponding to CaCr₂O₄ and it is stable in solutions with pH > 3. This is in agreement with the results of the work by Tae and Morita [14] who reported that the mineralogical species in the immobilization process of hexavalent chromium in wastewater using a granulated BF slag and hydrothermal treatment is the uvarovite (Ca₃Cr₂Si₃O₁₂). It is worth noting that the thermodynamic information of Ca₃(CrO₄)₂, Ca₅Cr₂SiO₁₂ and CaCrSi₄O₁₀ are not available and these species have not been included in the E-pH diagrams.

Figure 12 shows the potential-pH diagrams at 298.15 K (25 °C) for the Fe–Cr–H₂O systems with two ranges of the Fe/(Fe + Cr) molar ratio. The main Cr–Fe compound in this system is the solid iron chromite (FeCr₂O₄). This figure shows that with low Fe/(Fe + Cr) ratio (between 0 and 0.333) FeCr₂O₄ is stable in relatively high acid solution (about pH > 2.2); however, Cr³⁺ and Cr(OH)²⁺ are dissolved in the liquid solution. The stable species for the systems with Fe/(Fe + Cr) > 0.333 are FeCr₂O₄ and Fe₂O₃, avoiding in this way the leaching of any Cr-ion species. In the slags of the present work, the value Fe/(Fe + Cr) > 0.333 is obtained for the systems with 9 mass % of FeSO₄ or FeS₂.

3.5 Effect of the Slag Basicity

Different chromium oxides exist in silicate melts. Turkdogan [15] reported that chromium is present in silicate melts, depending on the temperature and composition, in bi- tri- or hexavalent states. The ratio between the concentrations of Cr²⁺, Cr³⁺ and Cr⁶⁺ is regulated by the reactions

$$2({\text{Cr}}^{2 + } ) \, + \, 1/2{\text{O}}_{2} = 2({\text{Cr}}^{3 + } ) + ({\text{O}}^{2 - } )$$

(1)

$$({\text{Cr}}^{{ 3 { + }}} ) + 5/2({\text{O}}^{2 - } ) + 3/4{\text{O}}_{2} = ({\text{CrO}}_{4}^{2 - } )$$

(2)

In oxidizing conditions, the anions of chromates are stable in basic melts, but decompose to form an ion of trivalent chromium in acid melts. In silicate slags silicon is present in the orthosilicate anion which is formed by the reaction:

$${\text{SiO}}_{2} + 2({\text{O}}^{2 - } ) = {\text{SiO}}_{4}^{4 - }$$

(3)

Consequently, the solution of silica reduces the concentration of free anions of oxygen (O²⁻). The addition of SiO₂ diminishes the slag basicity and the equilibrium of reaction (2) is shifted to the left and the hexavalent chromium is reduced to the trivalent state.

It is worth to note that the experiments in this work have been carried out in an open furnace with the oxygen partial pressure close to PO₂ = 0.21 atm. Even though the alumina crucible is placed in a graphite protecting crucible, the experimental setup is not exposed to a reducing atmosphere above the slag system. If air is used as an atmosphere, hexavalent chromium compounds appear in the high-lime region, whereas at high silica slag content, all the chromium is virtually in the +3 oxidation state as has been reported by Glasser and Osborn [16]. So, if air is used as atmosphere, the stable chromium compounds depend only on the CaO/SiO₂ ratio.

4 Conclusions

Synthetic slag samples of the system CaO–CaF₂–SiO₂–Cr₂O₃–(FeSO₄ and FeS₂) were prepared in order to understand the effect of FeSO₄ and FeS₂ content and the slag basicity on the stability of the mineralogical species and the leachability of slags. The following results were obtained:

• (a) Slags without FeSO₄ and FeS₂ contained Ca₃(CrO₄)₂, a pentavalent chromium species (Cr⁵⁺), and CaCr₂O₄. The FeSO₄–based slags produced the complex compound Ca₅(SiO₄)₂SO₄, together with garnet-type compounds (Ca₃Cr₂Si₃O₁₂ and Ca₃Fe₂Si₃O₁₂). The slags with FeS₂ formed Ca₃Cr₂Si₃O₁₂ and the sulfur-based compounds (Cr₃S₄ and FeS·Cr₂S₃).

• (b) The results showed that the slags without FeSO₄ or FeS₂ produced the highest chromium extraction in the leachates. The presence of FeSO₄ in the slag led to the formation of chromite (FeCr₂O₄) and uvarovite (Ca₃Cr₂Si₃O₁₂), diminishing the formation of leachable chromium compounds; however, the CaO/SiO₂ ratio was to be maintained at a low level in order to avoid the formation of Ca₅Cr₂SiO₁₂, where Cr had a valence of 5+.

• (c) The leaching results showed that FeS₂ was better than FeSO₄ in controlling the chromium leachability, which could be due to the stable binding of chromium in chromite (FeCr₂O₄), brezinaite (Cr₃S₄) and uvarovite (Ca₃Cr₂Si₃O₁₂).

• (d) The potential-pH diagrams showed that the stability domain of Ca₃Cr₂Si₃O₁₂ and FeCr₂O₄ were larger than that corresponding to CaCr₂O₄; therefore, the formation of these compounds minimized the leachability of the slag.