Effect of Temperature on Hexavalent Chromium Formation in (Al,Cr)₂O₃ with Calcium Aluminate Cement in Air

Abstract

Cr₂O₃ containing refractories castables are widely used as linings of various furnaces because of its remarkable corrosion resistance. However, Cr₂O₃ can be oxidized to toxic Cr(VI) by calcium aluminate cement phases (CA and CA₂) which are used as hydraulic binder in castables. In the present research, one of the most stable Cr(III) phase viz., (Al,Cr)₂O₃ solid solution was synthesized and then reacted with cement phases in order to check the formation of Cr(VI). The (Al, Cr)₂O₃ solid solution and calcium aluminate cement (70% Al₂O₃) were mixed in the ratio 1:1 (mass%) and heated in the temperature range of 500–1500°C in air. The phase compositions and formation/leachability of Cr(VI) were investigated using XRD, XPS and the leaching tests. The (Al,Cr)₂O₃ solid solution didn’t react with calcium aluminate cement at 500–900°C, while it partially converted to a Cr(VI)-containing phase at 1100–1300°C. However, (Al,Cr)₂O₃ and cement phases react to form a ternary Cr(III) phase (Ca(Al,Cr)₁₂O₁₉) at 1500°C.

1. Introduction

Cr₂O₃ has excellent corrosion resistance and thermal shock resistance, its use can reduce the materials consumption of any refractory system,^1,2) and it was often used as linings of the glass tank, gasification, waste melting furnaces and pyrometallurgical processes.^3,4,5,6) But the Cr₂O₃ (III)-containing refractories can be oxidized to Cr(VI) in the presence of alkali metal and alkaline earth metal-oxides like K₂O, Na₂O and CaO.^{7,8,9,10,11,12,13)} Cr(VI) is known to be highly soluble in water and toxic and thus easily enter into ground water and food chain,^{14,15,16,17,18)} which poses a serious threat to the human and environment.^{19,20,21,22,23)} Therefore, how to safely use Cr-containing refractories has become an issue of great concern.

In recent years the most significant trend in refractories has been the increased use of castables. Calcium alumina cements (CAC) are used as hydraulic binders in refractory castables because they provide fast setting time and high green mechanical strength,²⁴⁾ which has a rich history of over 80 years.²⁵⁾ In monolithic refractories, calcium aluminate cement, one of the most widely used binders, promotes initial hardening and mechanical strength before firing.²⁶⁾ In the early of hardening process, the CAC dissolves in water and releases calcium, aluminum and hydroxyl ions. After suturing the water, these ions form calcium aluminates hydrates that harden the structure.^27,28) After firing, it creates suitable strengthening phases such as CA₂ and CA₆ due to chemical reaction which can improve the properties of refractory castables.^29,30,31,32) So, calcium aluminate cements exhibit most stable mechanical behavior over the wide range of applicable conditions in all of refractory binding systems.³³⁾ Unfortunately, the main phase of CAC (CA and CA₂), can drive the conversion of Cr(III) to Cr(VI) in castables during the serving process.^34,35,36,37) Song and Garbers-Craig investigated the reaction between the cement phases (C₃A, C₁₂A₇, CA and CA₂) and Cr₂O₃ in air and CO₂. It was found that the Cr(VI)-containing compound Ca₄Al₆CrO₁₆ can form in the air at 1300°C, while a Cr(III)-containing phase (Ca₆Al₄Cr₂O₁₅) formed when Cr₂O₃ reacts with cement phases in CO₂ atmosphere.³⁸⁾

Recent researches showed that the temperature plays a important role on Cr(VI) formation. Al₂O₃–CaO–Cr₂O₃ refractory castables upon heat treatment partially produce Cr(VI)-containing phases CaCrO₄ and Ca₄Al₆CrO₁₆ from 500–1300°C in air while higher temperature (1500°C) favors the formation of (Al, Cr)₂O₃ solid solution.³⁷⁾ It is known that (Al,Cr)₂O₃ solid solution is one of the most stable Cr(III) phase. This system is also well known for its high refractoriness, high thermal shock resistance, and good corrosion resistance against molten salts and slags.¹⁾ Therefore, this system finds its applicability in various types of high-temperature furnaces as an addition in refractories castables. Alumina and chromium oxide form substitutional solid solution over the entire composition range without formation of any eutectic at high temperature due to their similar crystal structure and nearer atomic radii of the cations.^39,40) But data on the reaction of (Al,Cr)₂O₃ solid solution with calcium aluminate cement and whether form Cr(VI) during the serve at different temperature is not available in the literature. The objective of this study was therefore to investigate the reaction between the (Al,Cr)₂O₃ solid solution and the calcium aluminate cement which substitute of Cr₂O₃ in castables at different temperature in order to examine the possibility of Cr(VI) formation in (Al,Cr)₂O₃-containing castables. Since there is temperature gradient in the furnace lining, the sample was heat treated at different temperatures (500–1500°C) in the air. The formation of various phases during the heat treatment of sample was studied using XRD. The generation of Cr(VI) was investigated by XPS and leaching tests.

2. Experimental

2.1. Experimental Procedure

High purity chemicals Al₂O₃ (99%), CaO (99%) and Cr₂O₃ (99.95%), supplied by Sinopharm Chemical Reagent Co. Ltd (Shanghai, China), were employed as the starting materials. Before weighing the batches, Al₂O₃ and CaO powders were calcined at 1000°C for 8 h in a muffle furnace in order to decompose any probable hydroxide and/or carbonate phases. Similarly, Cr₂O₃ powder was dried in an oven at 110°C for 8 h to remove any moisture. The samples were synthesized by conventional solid-oxide reaction route. The calcium aluminate cement (based on Secar 71) and (Al,Cr)₂O₃ solid solution compositions were formulated in Table 1. The composition of the cement is based on commercial Secar 71, Kerneos Aluminates Ltd., as it is one of the most widely used cement. The oxides were weighed in appropriate proportions, thoroughly mixed in a tungsten carbide mill and then pressed into pellets of 20 mm diameter.

Table 1. Compositions of synthesized calcium aluminate cement and (Al,Cr)₂O₃, and their sintering temperatures.

Sample	Composition, mass%			Sintering temperature, °C
	Al2O3	CaO	Cr2O3
Calcium aluminate cement	70	30	–	1550
(Al, Cr)2O3	92.44	–	7.56	1550

The calcium aluminate cement was simulated with a mixture of Al₂O₃ and CaO, heated to 1550°C at a rate of 5°C/min and equilibrated for 6 h in a MoSi₂-heated muffle furnace in air. The resulting pellets were then grounded and reheated again. The (Al,Cr)₂O₃ also was sintered at 1550°C, and the detailed procedure was the same with cement.

XRD patterns of the synthesized (Al,Cr)₂O₃ solid solution was subsequently compared with the standard PDF cards and it confirmed that pure (Al,Cr)₂O₃ was successfully synthesized in the present experiments (Fig. 1). The synthesized cement also was confirmed with XRD (Fig. 1) and it exhibits CaAl₂O₄ (CA) and CaAl₄O₇ (CA₂). As per Al₂O₃–CaO phase diagram (Fig. 2),⁴¹⁾ the composition of the synthesized cement would produce the CA and CA₂ phases. The XRD results of synthesized cements reveal a good agreement with the commercial Secar 71²⁵⁾ which has similar composition.

Fig. 1.

XRD pattern of synthesized (Al, Cr)₂O₃ and cement.

Fig. 2.

The phase diagram of the CaO–Al₂O₃ system. (Online version in color.)

Samples were ground into powders, a mixed powder of composition in cement: (Al,Cr)₂O₃ was 1:1 (mass%) in a tungsten carbide mill, re-pelletized. At the second sintering step, the pellets were reacted in the air for 8 h (optimized from our previous studies³⁷⁾) in a tube furnace at 500, 700, 900, 1100, 1300 and 1500°C, respectively, after that they were quenched by quickly immersing them into liquid nitrogen.

2.2. Analysis

The chemical valence state of chromium in the samples after heat treatment was analyzed by using X-ray photoelectron spectroscopy (XPS) which were carried out using an ESCA spectrometer (PHI 5000 VersaProbe, Ulvac-PHI, Physical Electronics), equipped with a non-monochromatic Al-Kα X-ray source. Samples were finely ground using a mortar and pestle. The results were acquired from the surface layers of powder samples with spot areas of 500 μm in diameter and penetration depth less than 5 nm. All the spectra were obtained under CAE mode with 35 eV of pass energy and 0.05 eV of step size. The values are reported is the average of result of 15 times scanned of each sample.

The phase compositions of the quenched samples which were ground using a mortar and pestle were evaluated using X-ray diffraction (XRD; X’Pert Pro, Bruker, Germany), with copper radiation (Cu Kα, λ=1.5418 Å) working at 40 kV and 40 mA. Scans were performed in continuous mode at a 2° min⁻¹ scan rate and 2θ scanning ranging from 5° to 90°. Phase identification was done by PANalytical X’Pert Highscore plus software. While the quantitative XRD analysis was performed using TOPAS V5 software.

Leach tests were carried out according to the TRGS 613 standard procedure to determine whether Cr(VI) formed during the heat treatment process. This method is used to determine water-soluble Cr(VI) compounds in cement and products that contain cement. Samples of 1.5 g which were grounded and milled (<100 BS mesh) into powder using a tungsten carbide mill were weighed into leaching bottles, suspended in 30 ml of distilled water (DW), vigorously stirred for 15 min with a magnetic stirrer (stirrer bar 40 mm, 300 rpm) and then filtered through 0.45 μm membrane filter with a glass fiber filter by vacuum. The pH of the leach solution was determined using a PHS-3C Precision pH/mV Meter (Aolilong, China). The concentration of Cr(VI) in the leach solution was measured using a colorimetric method. The Cr(VI) can react in acid condition with the 1,5-diohenylcarbazide to form 1,5-diohenylcarbazone, which forms a red complex (0.01–0.5 mg/l chrome). Then the absorbance was recorded, using a 722 Vis spectrophotometer at 540 nm (Jinghua Instruments, China). In order to leach out all the Cr(VI) present in the samples, the solid residues were sequentially leached until the leach solution became colorless. For each leaching cycle, the solid residues were suspended in 150 ml of distilled water, stirred for 15 minutes using a magnetic stirrer, and then filtered through 0.45 μm membrane filters.

3. Results and Discussion

3.1. XPS Analysis

XPS analysis was used to investigate the chemical valence variation in the samples with temperature as shown in Fig. 3. Gaussian curve fitting of the Cr 2p1/2 and Cr 2p3/2 photoelectron peaks were performed using CASA XPS software in order to deconvolute the peak position that associated with binding energies of 588.7, 586.5, 579.4 and 576.8 eV. Relating these values to standard spectra of various chromium species, it can be assumed that the binding energies of 586.5±0.2 eV (Cr 2p1/2) and 576.4±0.3 eV (Cr 2p3/2) are associated with Cr(III), while the binding energies of 588.7±0.2 eV (Cr 2p1/2) and 579.4±0.1 eV (Cr 2p3/2) are associated with Cr(VI).^42,43) It is clear that the samples which were treated from 500°C to 900°C only Cr 2p1/2 and Cr 2p3/2 associated with Cr(III) could be observed. At 1100°C, it has a maximum Cr(VI) content with a low concentration of Cr(III). With the temperature increased to 1300°C, the intensity and the area covered by the peak associated with Cr(VI) decreased while that of Cr(III) increased. When the temperature was up to 1500°C, only Cr 2p1/2 and Cr 2p3/2 associated with Cr(III) could be observed also, the peak associated with Cr(VI) totally disappeared. Thus, it is clear that the temperature plays an important role in the formation of Cr(VI)-containing compound.

Fig. 3.

XPS spectra of the sample treated at 500–1500°C. (Online version in color.)

3.2. Phase Evolution

XRD analysis was implemented to identify the presence of different crystalline phases in samples, with the results presented in Fig. 4. The XRD patterns of samples fired at 500°C, 700°C and 900°C was the same. (Al,Cr)₂O₃ is the major phase in the samples fired at 500°C, 700°C and 900°C while the secondary phases were CaAl₂O₄ and CaAl₄O₇. It means the cement and (Al,Cr)₂O₃ solid solution didn’t react from 500 to 900°C. In the case of sample fired at 1100°C, (Al,Cr)₂O₃ still exists as the primary phase and CaAl₄O₇ was the secondary phase in the sample and also contains CaAl₂O₄ but the intensity of it decreased, while a minute amount of Cr(VI)-containing phase namely Ca₄Al₆CrO₁₆ formed as well. The Ca₄Al₆CrO₁₆ which has a cubic crystal structure belongs to the haüynite family (3CaO·3Al₂O₃·CaSO₄, 3SrO·3Al₂O₃·SrSO₄, and 3SrO·Al₂O₃·SrCrO₄). In chrome-haüyne, the SO₄²⁻ group is replaced by CrO₄²⁻, and the chromium ion exists as the oxidation state of Cr⁶⁺.⁴⁴⁾ As the temperature increased from 1100 to 1300°C, the amount of CaAl₄O₇ increased while the amount of (Al,Cr)₂O₃ solid solution and CaAl₂O₄ decreased, the Cr(VI)-containing phase Ca₄Al₆CrO₁₆ also decreased with the temperature increased. But in the sample fired at 1500°C, the peaks of CaAl₂O₄, Ca₄Al₆CrO₁₆ and (Al,Cr)₂O₃ solid solution totally disappeared, leaving behind only cement phase CaAl₄O₇ and Ca(Al,Cr)₁₂O₁₉. The concentration of CaAl₄O₇ decreased compared with 1300°C (Table 2). The Al₂O₃–CaO phase diagram (Fig. 2) also shows that CaAl₁₂O₁₉ is the intermediate phase between Al₂O₃ and CaAl₄O₇. Li and Medina reported that the CaAl_12-xCr_xO₁₉ (x = 0.5−3.5) solid solutions formed by solid state reactions through substitution of aluminum with chromium.⁴⁵⁾ The peaks of CaAl₁₂O₁₉ phase in the sample (fired at 1500°C) were compared with standard CaAl₁₂O₁₉, all of them shifted towards lower 2Ɵ angle which implies some Ca(Al, Cr)₁₂O₁₉ solid solution may be formed.

Fig. 4.

X-ray diffraction pattern of different temperature quenched samples. (Online version in color.)

Table 2. Quantitative XRD analysis of samples treated at 500–1500°C.

Sample	Phase composition, mass%
	CaAl2O4	CaAl4O7	CaAl12O19	(Al,Cr)2O3	Ca4Al6CrO16
500	38.49	12.59	–	48.92	–
700	38.64	12.37	–	48.99	–
900	38.84	12.80	–	48.37	–
1100	20.88	35.27	–	38.92	4.92
1300	3.25	73.58	–	20.6	2.57
1500	–	66.87	33.13	–	–

Amount of crystalline phases in all samples were determined using the Rietveld method (TOPAS V5) as shown in Table 2. It confirmed that just the samples treated at 1100°C and 1300°C have Cr(VI)-containing phase Ca₄Al₆CrO₁₆, and the concentrations of Ca₄Al₆CrO₁₆ are 4.92% and 2.57%, respectively. Combined with XPS analysis results, in the Ca₄Al₆CrO₁₆ phase chromium is present as +6 oxidation state, whereas in Ca(Al,Cr)₁₂O₁₉ and (Al,Cr)₂O₃ chromium are present as +3 oxidation state.

In our previous research,³⁷⁾ a Cr(VI)-containing compound CaCrO₄ can form from 500 to 1100°C in Cr₂O₃-containing refractory castables where individual Cr₂O₃, Al₂O₃ and calcium aluminate cements (Secar 71) were subjected for reaction. But in the present research, cement phases (CA and CA₂) virtually didn’t react with (Al,Cr)₂O₃ solid solution from 500 to 900°C and almost no traces of any Cr(VI)-containing phases could be observed. It means that using (Al,Cr)₂O₃ solid solution instead of Cr₂O₃ can restrict the formation of Cr(VI)-containing phase CaCrO₄ at low temperature regime (500–900°C). But at 1100 and 1300°C the Cr(VI)-containing phase Ca₄Al₆CrO₁₆ still can be formed.

The activity of Cr₂O₃ in the (Al,Cr)₂O₃ has been investigated by Besman and Kulkarni.⁴⁶⁾ It was reported that the Cr₂O₃ activities increased with mole fraction of Cr₂O₃ in the (Al,Cr)₂O₃ solid solutions. At the treated chemical composition, the Cr₂O₃ is 0.04 mol%, the activity is ranged from 0.08 to 0.25, from 1100°C to 1500°C. The Cr₂O₃ activities in the (Al,Cr)₂O₃ solid solutions is much lower than that of pure Cr₂O₃ was used (where activity of Cr₂O₃ is equal to 1). And the chromium in (Al,Cr)₂O₃ solid solution is more stable and not to form Cr(VI)-containing compound comparing with that in pure Cr₂O₃.

3.3. Cr(VI) Leaching Tests of the Quenched Samples

The extraction of Cr(VI) was evaluated according to the TRGS 613 procedure. The leach solutions obtained from samples and Cr(VI) concentrations of these leach solutions are shown in Fig. 5. Yellow coloration of the leach solution is an indication of Cr(VI) in the leach solution. It indicated the leach solution from samples fired at 1100 and 1300°C contain a large amount of Cr(VI). While the leach solutions from samples fired at 500–900°C and 1500°C was colorless.

Fig. 5.

Cr(VI) leached as a function of temperature, using the TRGS 613 leach test. (Online version in color.)

The concentration of Cr(VI) leached from samples treated at 500–900°C and 1500°C range from 0.29 mg/l to 2.08 mg/l. All of these were below the allowable EPA limit of 5 mg/l. However, the concentrations of Cr(VI) in the leach solutions from samples treated at 1100°C and 1300°C were up to 117.27 mg/l and 59.86 mg/l, respectively. The pH of the leach solution increased from pH 7.3 of the distilled water to range from 10.91 to 9.54. During the leaching process, Ca and Al containing cement phase presumably dissolved in water as Ca²⁺ and Al(OH)₄⁻, which increased the pH of the leach solution.³⁸⁾ And the amount of chromium ions in the leach solutions was also quantified with ICP. The Cr(VI) concentration leached out (measured from UV-vis) is close to that of Cr_total in leach solution (ICP). It means that most of the chromium present in the leach solutions as Cr(VI) only, on the contrary Cr(III) present in the (Al,Cr)₂O₃ and Ca(Al,Cr)₁₂O₁₉ phase didn’t leach out.

3.4. Repeat Leaching of the Solid Residues

The solid residues that remained after the TRGS 613 test procedure were repeatedly re-leached until the leach solution became colorless. In order to leach out all of Cr(VI) in the samples, leaching was done with an excess amount of distilled water (the solid residues was suspend in 150 ml of distilled water). Figure 6 shows the amount of Cr(VI) as a function of the number of times leached and corresponding leach solutions. The leach solution from samples treated at 500–900°C became colorless after just being leached one time. Similar is the case for samples treated at 1500°C, just after one time leach, the leach solution became colorless. The samples fired at 1100°C and 1300°C became colorless after being leached for three times. It means that there were some water soluble Cr(VI) left in the solid residues of samples fired at 1100°C and 1300°C after the TRGS 613 leaching procedure. The amount of Cr(VI) in the leach solution decreases with number of leaching cycles.

Fig. 6.

Cr(VI) concentrations as a function of leaching times and corresponding leaching solutions. (Online version in color.)

Mass balance before and after the leaching for Cr(VI) in samples also has been done. The mass percentage of Cr(VI) in the leach test was calculated as per Eq. (1):   

$$w_{Cr(VI)}[\%]=\frac{C_0{V}_0+(C_{Cr(VI)1}+\mathrm{...}+C_{Cr(VI)n})\times V}{m}\times 100$$(1)

where, V_n is the volume of repeat leaching liquor of solid residue and V₀ is the volume of leaching liquor in TRGS 613, in L

C_n is the concentration of Cr(VI) in repeat leach of solid residue and C₀ is the concentration of Cr(VI) in TRGS 613, in mg/L

m is the weight of the synthesized sample used in leaching test, in mg

As predicted from earlier XRD quantitative using the TOPAS V5, in the sample treated at 1100°C and 1300°C, it has an amount of Cr(VI) while it was almost no Cr(VI) in other samples. During the repeat leaching test, the amount of Cr(VI) was the maximum in the sample treated at 1100°C. The leachability of Cr(VI) in the sample fired at 1100°C and 1300°C during the repeat leaching is 0.398% and 0.203%, respectively. Moreover, XRD quantitative analysis of this two samples before leaching showed the mass percentage of Cr(VI) around 0.406% and 0.212%, which is consistent with the leaching result. Thus, all Cr(VI) in samples can be leached out.

The solid residues after multiple leaching were also analyzed by XRD to observe any changes in phase composition during the leach process (Fig. 7). After repeat leaching tests, the Cr(VI)-containing phase Ca₄Al₆CrO₁₆ disappear completely in the samples treated at 1100°C and 1300°C, leaving behind only CaAl₄O₇, CaAl₂O₄ and (Al,Cr)₂O₃ solid solution. The XRD pattern of the sample treated at 700°C remained unchanged before and after multiple leaching. Similar is the case for the samples treated at 1500°C, with both CaAl₄O₇ and Ca(Al,Cr)₁₂O₁₉. It means that the Cr is in solid solution with CaAl₁₂O₁₉/Al₂O₃ to form Ca(Al,Cr)₁₂O₁₉/(Al,Cr)₂O₃ and exists as Cr(III) which is very stable and insoluble in water. While all the Cr(VI)-containing phase Ca₄Al₆CrO₁₆ can be leach out during the repeat leach tests.

Fig. 7.

X-ray diffraction patterns of leach residues of samples before and after multi-leaching. (Online version in color.)

4. Conclusion

It is clear that the temperature plays a very important role in the formation of Cr(VI) between the (Al,Cr)₂O₃ solid solution and cement phases. The (Al, Cr)₂O₃ solid solution didn’t react with cement (CA and CA₂) at 500–900°C. Thereafter, at 1100°C and 1300°C, the Cr(VI)-containing phase haüyne (Ca₄Al₆CrO₁₆) still can form. On the contrary, the sample treated at 1500°C predominantly forms Ca(Al,Cr)₁₂O₁₉ and CaAl₄O₇, and there isn’t any other Cr(VI)-containing phase form. During the leaching test and quantitative XRD analysis, the sample treated at 1100°C exhibit the highest concentrations of Cr(VI). However, all of the Cr(VI)-containing phase in the samples can be completely leach using distilled water through repeated leaching. So the spent (Al,Cr)₂O₃-containing refractory castables from 1100°C to 1300°C must be disposed very carefully.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (No. 51604203) and the Hubei Chutian Scholar Program.

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