High-Temperature Chemical Stability of Cr(III) Oxide Refractories in the Presence of Calcium Aluminate Cement

Abstract

Al₂O₃-CaO-Cr₂O₃ castables are used in various furnaces due to excellent corrosion resistance and sufficient early strength, but toxic Cr(VI) generation during service remains a concern. Here, we investigated the relative reactivity of analogous Cr(III) phases such as Cr₂O₃, (Al_1−xCr_x)₂O₃ and in situ Cr(III) solid solution with the calcium aluminate cement under an oxidizing atmosphere at various temperatures. The aim is to comprehend the relative Cr(VI) generation in the low-cement castables (Al₂O₃-CaO-Cr₂O₃-O₂ system) and achieve an environment-friendly application. The solid-state reactions and Cr(VI) formation were investigated using powder XRD, SEM, and leaching tests. Compared to Cr₂O₃, the stability of (Al_1−xCr_x)₂O₃ against CAC was much higher, which improved gradually with the concentration of Al₂O₃ in (Al_1−xCr_x)₂O₃. The substitution of Cr₂O₃ with (Al_1−xCr_x)₂O₃ in the Al₂O₃-CaO-Cr₂O₃ castables could completely inhibit the formation of Cr(VI) compound CaCrO₄ at 500–1100 °C and could drastically suppress Ca₄Al₆CrO₁₆ generation at 900 to 1300 °C. The Cr(VI) reduction amounting up to 98.1% could be achieved by replacing Cr₂O₃ with (Al_1−xCr_x)₂O₃ solid solution. However, in situ stabilized Cr(III) phases as a mixture of (Al_1−xCr_x)₂O₃ and Ca(Al_12−xCr_x)O₁₉ solid solution hardly reveal any reoxidation. Moreover, the CA₆ was much more stable than CA and CA₂, and it did not participate in any chemical reaction with (Al_1−xCr_x)₂O₃ solid solution.

1. Introduction

Cr₂O₃-containing refractories possess remarkable corrosion resistance due to their extremely low solubility and high chemical stability against molten slag. Therefore, they are widely used as lining materials in incinerators, gasifiers, glass furnaces, non-ferrous smelting, etc. [1,2,3,4,5,6]. In addition, refractories as castables have become a popular choice in recent decades because of the energy-saving manufacturing process, convenient for installation and repair works, where binders’ chemistry plays a crucial role [7,8,9]. Calcium alumina cement (CAC) binders are the most widely used since they exhibit fast setting and strength development, stable thermo-mechanical behaviour, and resistance to slag attack [10]. However, Cr₂O₃ can oxidize into toxic Cr(VI) products at high temperatures upon reaction with alkali or alkaline earth metals/oxides/compounds under an oxidizing atmosphere [11,12,13]. The Cr(VI) compounds pose a severe threat to humans and the environment since they are toxic, carcinogenic, highly soluble in water, and quickly enter the food cycle [14]. Therefore, it is of significant environmental and practical significance to inhibit the generation of Cr(VI) when applying Al₂O₃-CaO-Cr₂O₃ castables as lining materials.

The Al₂O₃-CaO-Cr₂O₃ system was not investigated in detail earlier though numerous Cr(VI) reduction techniques were described for other applications [15,16,17]. Generally, Cr(VI) formation was closely related to the atmosphere and basicity of other components in the Cr₂O₃-containing materials [18,19]. For the Cr₂O₃-containing refractory linings, since the operating conditions and service atmosphere in a given furnace can hardly be changed in practical production, most related work has focused on Cr(VI) minimization using some additives at high temperatures. The acidic components such as SiO₂, TiO₂, Fe₂O₃, and P₂O₅ can effectively suppress the Cr(III) oxidation during thermal treatment of Cr₂O₃-containing refractories [19,20,21,22]. However, these oxide additives usually result in low melting point phases in the matrix, deteriorating either the thermo-mechanical properties or the slag corrosion resistance [23,24]. Previous research indicated that incorporating chromium into solid solution phases can reduce the risk of Cr(VI) formation in the Cr₂O₃-containing materials [25,26,27]. For example, the investigation of the Al₂O₃-Cr₂O₃-CaO-MgO pure system confirmed that composite spinel Mg(Al,Cr)₂O₄ could co-exist with CA₂, where chromium existed in +3 state [25,27]. Again, Wu et al. [28] studied the effect of temperature on Cr(VI) formation in a pure (Al,Cr)₂O₃ system with CAC in air atmosphere, where (Al_1−xCr_x)₂O₃ was found to be chemically stable against CAC up to 1100 °C, beyond which Ca₄Al₆CrO₁₆ (hauyne) phase predominantly start from.

Nath et al. investigated the phase evolution of the Al₂O₃-CaO-Cr₂O₃ refractories castables after treatment at various temperatures, where CaO from cement facilitated the conversion of Cr(III) into Cr(VI) [29]. The main phase of CAC (CA and CA₂) react with Cr₂O₃ in the air to produce CaCrO₄ and Ca₄Al₆CrO₁₆ at mid-temperature (700–1100 °C). At the same time, nearly all the Cr₂O₃ would convert into (Al_1−xCr_x)₂O₃ (0 < x < 1) and Ca(Al,Cr)₁₂O₁₉ solid solution at 1500 °C, which leads to a significant decrease of Cr(VI) compounds amounts [20,29]. Thus, (Al_1−xCr_x)₂O₃ solid solutions having high refractoriness and good chemical stability could be better performing materials with better mechanical properties and slag corrosion resistance [30,31,32,33]. Based on the above research, it can be inferred that substituting Cr₂O₃ with (Al_1−xCr_x)₂O₃ solid solution as a starting component in the Al₂O₃-CaO-Cr₂O₃ castables could be a feasible way to inhibit the formation of Cr(VI) at various temperatures, especially at mid-temperature (700–1100 °C), without compromising other properties. However, systematic work relating to the effect of (Al_1−xCr_x)₂O₃ solid solution on Cr(VI) formation in Al₂O₃-CaO-Cr₂O₃ refractory castables is rare.

The present work aims to inhibit the formation of Cr(VI) compounds in Al₂O₃-CaO-Cr₂O₃ castables by substituting Cr₂O₃ with (Al_1−xCr_x)₂O₃ solid solution as starting chromium-containing constituent. Firstly, (Al_1−xCr_x)₂O₃ solid solutions were pre-synthesized at 1300 to 1650 °C. Secondly, Al₂O₃-CaO-Cr₂O₃ castables with the pre-synthesized (Al_1−xCr_x)₂O₃ solid solution were fabricated and treated at the temperature range of 300–1500 °C in the air since castables would be put to use without firing and a temperature gradient occurs in any furnace linings in actual practice. The phase evolution and Cr(VI) generation of the Al₂O₃-CaO-Cr₂O₃ castables with temperature and the corresponding mechanism were studied using XRD and related software, SEM, and leaching tests. Furthermore, since the (Al_1−xCr_x)₂O₃ and Ca(Al,Cr)₁₂O₁₉ could be formed in the Al₂O₃-CaO-Cr₂O₃ castables at high temperature [20], castables with Cr₂O₃ were pre-heated at 1500 °C to produce the in situ formed (Al_1−xCr_x)₂O₃, whose effect on the Cr(VI) formation for the castables at various temperature was also evaluated.

2. Materials and Methods

Tabular alumina (Al₂O₃) of various size fractions, 5–3 mm, 3–1 mm, 1–0 mm, and ≤0.045 mm, were procured from Zhejiang Zili Alumina Materials Technology Co., Ltd., Shangyu, China. Reactive α-alumina fines of size fraction ≤ 0.005 mm were purchased from Kaifeng Tenai Co., Ltd., Kaifeng, China. Industrial-grade fused chromium oxide (Cr₂O₃) (size ≤ 0.074 mm) was obtained from Luoyang Yuda Refractories Co., Ltd., Luoyang, China. The hydraulic calcium aluminate cement binder, Secar 71 (CA and CA₂ phases), was procured from Imerys Aluminates, Tianjin, China. An organic defloculant, FS 65 (Wuhan Sanndar Chemical Co., Ltd., Wuhan, China), was used as the dispersant. The detailed chemical composition of raw materials is shown in

Table 1. The chemical composition of raw materials (wt%).

Raw Materials	SiO2	Al2O3	CaO	Fe2O3	MgO	Na2O	K2O	Cr2O3
Tabular alumina	0.08	99.30	-	0.02	-	0.28	-	-
Reactive α-alumina	0.28	98.87	0.07	0.13	0.12	0.10	0.005	-
Fused chromium oxide	0.82	0.59	0.38	0.73	0.27	0.14	0.01	94.02
Calcium aluminate cement	0.40	70.6	28.4	0.20	-	-	-	-

.

Cr₂O₃ and Al₂O₃ fine powders with a mass ratio of 8:17 were dry-mixed, pressed into pellets, and then treated at 1300, 1600, and 1650 °C for 3 h in the air to obtain the mixture of Al₂O₃, Cr₂O₃ and (Al_1−xCr_x)₂O₃ solid solution and pre-synthesized (Al_1−xCr_x)₂O₃ solid solution. Thus, obtained pellets were then pulverized to 200-mesh powders before adding them into the castables. The specimen with Cr₂O₃ and Al₂O₃ powders as initial raw materials was labelled as R, while specimens with (Al_1−xCr_x)₂O₃ solid solution pre-synthesized at 1300 °C, 1600 °C, and 1650 °C were designated as S13, S16, and S165, respectively. Specimen R was pre-heated at 1500 °C for 3h (labelled as F15) to produce the in situ formed (Al_1−xCr_x)₂O₃, whose effect on the Cr(VI) formation in the castables at various temperatures was also evaluated then. The castables were formulated based on the Andreasen distribution coefficient (q) value of 0.31, and the specific formulation is shown in

Table 2. The formulation of fine powders undergoing reaction within castables (30wt% of total).

Code	Aggregates Al2O3 (wt%)	Fine Powders (wt%)				Pre-Treatment Temperature (°C)
		Al2O3	Cr2O3	CAC	(Al1−x,Crx)2O3
R	70	17	8	5	-	-
F15	70	17	8	5	-	In situ treated at 1500
S13	70	-	-	5	25	(Al1−x,Crx)2O3 made at 1300
S16	70	-	-	5	25	(Al1−x,Crx)2O3 made at 1600
S165	70	-	-	5	25	(Al1−x,Crx)2O3 made at 1650

Note: 0.1 wt% additional organic dispersant was added to each formulation to make the castables. CAC designates calcium aluminate cement (Here, a commercial Secar 71 cement was used). Each batch contains 8 wt% of Cr₂O₃.

. Each batch was dry-mixed for 3 min in a Hobart mixer followed by wet-mixing (4.0 wt% water, 25 °C) for further 3 min, and then castables were moulded in a vibrating table (1 min) into bars of size 160 mm × 40 mm × 40 mm at room temperature. All specimens were cured at 25 °C and 75% ± 5% relative humidity for 24 h in a standard cement maintainer and dried at 110 °C for 24 h in an electric air oven after demoulding. Dried specimens R, S13, S16, S165, and specimen F15 were finally heated in the temperature range of 300–1500 °C for 3h at peak temperature in air.

To figure out relative oxidation, the mechanisms of the Cr(VI) generation in the castables and the corresponding chemical reactions, fine powders of CAC and CA₆ were mixed with Cr₂O₃ and pre-synthesized (Al_1−xCr_x)₂O₃ (

Table 3. The formulation of cylindrical specimens (wt%).

Specimens	CAC	CA6	Cr2O3	(Al1−x,Crx)2O3
C-C	50		50
C-S	50			50
CH-C		50	50
CH-S		50		50

Note: CAC (Secar 71) contains mixture of CaAl₂O₄ (63%) and Ca₂Al₄O₇ (35%), CA₆ = CaAl₁₂O₁₉.

). Then, the mixed powders were pressed into Φ20 mm × 20 mm cylindrical specimens under a pressure of 50 MPa. After being treated at 900 °C and 1300 °C for 3 h in the air atmosphere, the phase compositions and microstructures of the specimens were analyzed by XRD and SEM, respectively.

The crystalline phase compositions were identified by X-ray diffraction (XRD) patterns using a X’Pert Pro diffractometer (PANalytical, Almelo, Netherlands) (Copper Kα radiation (λ = 1.5418 Å) at 40 kV/40 mA, step size 0.02 over a 2θ range of 5–90°) and analyzed by the software of X’pert Pro High Score (Philips, Almelo, Netherlands). Lattice parameters were calculated using X’pert Pro High Score (Philips, Almelo, Netherlands) and Celref 2.0 software. Microstructure morphology was analyzed by scanning electron microscopy (SEM, Nova 400 Nano-SEM, FEI Company, Hillsboro, OR, USA) equipped with energy dispersive spectroscopy (EDS, Oxford, High Wycombe, UK).

Cr(VI) leachability was evaluated using the leaching test according to TRGS 613 standard procedure, which is suitable for determining water-soluble Cr(VI) compounds in cement and products containing cement. Leaching specimens were prepared by crushing and grinding thoroughly before passing through a 200-mesh sieve (≤74 μm). Fine samples were stirred with deionized water as a leaching solution using a magnetic stirrer at a speed of 300 rpm for 15 min (at room temperature) with a solid–liquid ratio of 1:20. Then, leachates were obtained through a 0.45-μm membrane filter with a glass fibre by vacuum. The Cr(VI) concentration in the leachates was determined using a colorimetric method. The Cr(VI) can react in acid condition with the 1,5-diphenylcarbazide (DPC) to form 1,5-diphenylcarbazone, a red complex (0.02–0.2 mg/L chrome). Then, the absorbance of the leachates after the DPC method was recorded at 540 nm, using a 722 Vis spectrophotometer (Jinghua Instruments, Shanghai, China).

3. Results and Discussion

3.1. Pre-Synthesis of (Al_1−xCr_x)₂O₃ Powders

The pre-synthesized powders of the (Al_1−xCr_x)₂O₃ solid solution at different temperatures are observed by XRD (Figure 1). It could be found that both corundum and eskolaite existed as separate phases after dry mixing at 25 °C. After treated at 1300 °C, the eskolaite disappeared with a noticeable reduction of the peak intensity of corundum, while a new phase identified as (Al_1−xCr_x)₂O₃ solid solution was generated. With the increase in the heat treatment temperature, the peak intensity of corundum reduced gradually until disappearance at 1650 °C, while the peak intensity of (Al_1−xCr_x)₂O₃ solid solution increased steadily. After treated at temperatures up to 1650 °C, only the (Al_1−xCr_x)₂O₃ solid solution could be detected in the specimens. So, it could be inferred that we have added the (Al_1−xCr_x)₂O₃ solid solution with the remnant of corundum and eskolaite (sample S13 and S16), while that of S165 is a complete (Al_1−xCr_x)₂O₃ solid solution. In addition, the lattice parameters of the (Al_1−xCr_x)₂O₃ solid solution were calculated in comparison with Al₂O₃ (reference code: JCPDS 01-081-2266, a = b = 4.7569 Å and c = 12.9830 Å) and Cr₂O₃ (reference code: JCPDS 00-038-1479, a = b = 4.9540 Å and c = 13.5842 Å). Since Al₂O₃ has smaller lattice parameters than Cr₂O₃, the (Al_1−xCr_x)₂O₃ solid solution reveals smaller lattice parameters than Cr₂O₃. With the increasing temperature, the (Al_1−xCr_x)₂O₃ solid solution showed decreasing lattice parameters as more Al₂O₃ dissolution is expected at higher temperatures. For example, the lattice parameter a = b = 4.8607 Å at 1300 °C (for sample S13) decreased to a = b = 4.8291 Å at 1600 °C (for sample S16).

3.2. Cr(VI) Leachability

The Cr(VI) concentration in Al₂O₃-CaO-Cr₂O₃ castables treated at various temperatures was evaluated by leaching test according to the TRGS 613 standard procedure (Figure 2). The details of Cr(VI) reduction compared to the reference specimen R is presented in

Table 4. Relative Cr(VI) reduction (%) of specimens compared to R at different temperatures.

Specimens	Temperature (°C)
	110	300	500	700	900	1100	1300	1500
S13	43.7	−19.5	61.7	81.9	16.1	21.2	10.5	12.6
S16	47.4	48.6	93.4	95.0	57.2	24.0	28.0	38.7
S165	38.5	58.0	87.4	98.1	67.6	35.8	−91.4	−202.4
F15	-	-	-	98.9	99.1	99.0	93.5	−30.8

. With the addition of the pre-synthesized (Al_1−xCr_x)₂O₃ solid solution, a noticeable decrease in the Cr(VI) concentration was observed. The specimens with (Al_1−x,Cr_x)₂O₃ pre-synthesized at higher temperature exhibited relatively lower Cr(VI) concentrations at the same heat treatment temperature (exception for specimen S165 at 1300 and 1500 °C). For example, at 700 °C, the total amount of Cr(VI) reduced drastically from 1233.2 mg/kg in specimen R (without (Al_1−xCr_x)₂O₃) to 223.7 mg/kg in specimen S13 (a reduction of 81.9%), and reduced further to 24.0 mg/kg in specimen S165 (a decrease of 98.1%). However, at 1300 °C, specimen S165 exhibited an even higher Cr(VI) concentration than the reference specimen. Moreover, the temperature corresponding to the maximum Cr(VI) concentration shifted from 900 °C for R to 1100 °C for the pre-synthesized (Al_1−xCr_x)₂O₃. The specimen F15, pre-heated at 1500 °C, exhibited extremely low Cr(VI) concentration at all heat treatment temperatures studied. It was concluded that the chromium would present as Cr(III) together within the solid solution of (Al_1−xCr_x)₂O₃ and Ca(Al,Cr)₁₂O₁₉ after the pre-heating treatment at 1500 °C [20]. Therefore, it is plausible that the reoxidation of these stable solid solution phases did not occur. Although the mid-temperature (700–1100 °C) was favourable for Cr(VI) formation, the total amount of Cr(VI) in F15 was still only 13.0-17.3 mg/kg (a decrease of ~98.9–99.1% compared to specimen R). These values are below the allowable Cr(VI) limit of the Environmental Protection Agency (EPA), United States (5 mg/L is equivalent to 100 mg/kg) [34].

3.3. Phase Evolution of the Castables

To study the effect of the pre-synthesized (Al_1−xCr_x)₂O₃ solid solution on the phase evolution of the castables, phase compositions of the specimens treated at 110–1500 °C were analyzed (Figure 3). In all samples, the main phase corundum and the NaAl₁₁O₁₇ impurity could be detected at all temperatures, and hydrate phase C₃AH₆ was generated at 110 °C but then disappeared at 300 °C due to dehydration. For specimen R, the CaCrO₄ phase could be detected at 300 °C, whose peak intensity increased with the increase in temperature from 300 °C to 900 °C but then decreased with further increasing temperature until disappearance at 1300 °C. The Ca₄Al₆CrO₁₆ was generated at 900 °C, whose peak intensity reached a maximum at 1100 °C but dropped down with further increasing temperature until disappearance at 1500 °C. Moreover, eskolaite existing in the range of 110 °C to 1100 °C reduced in peak intensity with temperature and disappeared at 1300 °C, while the (Al_1−xCr_x)₂O₃ solid solution and CaAl₁₂O₁₉ increased in peak intensity after generating at 1100 °C and 1300 °C, respectively. However, for specimens S13, S16, and S165, no CaCrO₄ phase was detected at 300–1100 °C, indicating chromium that in the (Al_1−xCr_x)₂O₃, the CAC in this temperature range would not oxidize the solid solution. At 900–1300 °C, although the Ca₄Al₆CrO₁₆ phase was still formed in these specimens with pre-synthesized (Al_1−xCr_x)₂O₃, the peak intensity of Cr(VI) compound was much lower compared with sample R. The peak intensity of the Ca₄Al₆CrO₁₆ phase reached the maximum at 1100 °C in Al₂O₃-CaO-Cr₂O₃ castables, and therefore, the highest Cr(VI) concentration for the specimens with pre-synthesized (Al_1−xCr_x)₂O₃ were detected at 1100 °C (Figure 3b). In general, the substitution of Cr₂O₃ with (Al_1−xCr_x)₂O₃ in the Al₂O₃-CaO-Cr₂O₃ castables can almost completely restrict the formation of CaCrO₄ compounds at 300–1100 °C and effectively lower the Cr(VI) compound Ca₄Al₆CrO₁₆ formation at 900–1300 °C, which was following the results of Cr(VI) leachability shown in Figure 2. After being treated at 1500 °C, only the corundum (with NaAl₁₁O₁₇ impurity), the (Al_1−xCr_x)₂O₃ solid solution, and the CA₆ phases were found in specimens R, S13, S16, and S165. The enlarged XRD patterns of the castables (Figure 3c) indicated that samples with (Al_1−xCr_x)₂O₃ pre-synthesized at higher temperature exhibited relative lower peak intensity of the CA₆ phase after being treated at 1300 °C. In addition, specimen F15, which had the same phase compositions as the other four specimens treated at 1500 °C, showed hardly any phase changes with the subsequent heat treatment temperature.

3.4. Reaction Mechanism

The above results demonstrated that in the Al₂O₃-CaO-Cr₂O₃ castables, chromium and calcium would exist in the state of Cr₂O₃/(Al_1−xCr_x)₂O_3, and CAC/CA₆, respectively, which affects the formation and concentration of Cr(VI) compounds CaCrO₄ and Ca₄Al₆CrO₁₆ at mid-temperature (700–1100 °C). Fine powders of CAC/CA₆ were mixed with Cr₂O₃/(Al_1−xCr_x)₂O₃ (pre-synthetized at 1650 °C) to figure out the mechanisms of the Cr(VI) generation in the castables and the corresponding chemical reactions. Then, the mixed powders were treated at 900 and 1300 °C for 3 h in the air; the XRD patterns and SEM microstructure are summarized in Figure 4 and Figure 5, respectively. The plausible chemical reaction equations discussed below in various samples heated at 900 °C and 1300 °C are listed in

Table 5. Chemical reaction equations in cylindrical specimens.

Specimens	900 °C	1300 °C
C-C	(1)	(3) (4) (5)
C-S	(2)	(2)
CH-C	-	(5) (6)
CH-S	-	-
${4\mathrm{CaAl}}_2{\mathrm{O}}_4{+2\mathrm{Cr}}_2{\mathrm{O}}_3{+3\mathrm{O}}_2\to {4\mathrm{CaCrO}}_4{+4\mathrm{Al}}_2{\mathrm{O}}_3$		(1)
${16\mathrm{CaAl}}_2{\mathrm{O}}_4{+\mathrm{y}({\mathrm{Al}}_{1-\mathrm{x}},\mathrm{Crx})}_2{\mathrm{O}}_3{+3\mathrm{O}}_2\to {4\mathrm{Ca}}_4{\mathrm{Al}}_6{\mathrm{CrO}}_{16}{+(\mathrm{y}+2)\mathrm{Al}}_2{\mathrm{O}}_3$		(2)
${16\mathrm{CaAl}}_2{\mathrm{O}}_4{+2\mathrm{Cr}}_2{\mathrm{O}}_3{+3\mathrm{O}}_2\to {4\mathrm{Ca}}_4{\mathrm{Al}}_6{\mathrm{CrO}}_{16}{+4\mathrm{Al}}_2{\mathrm{O}}_3$		(3)
${16\mathrm{CaAl}}_4{\mathrm{O}}_7{+2\mathrm{Cr}}_2{\mathrm{O}}_3{+3\mathrm{O}}_2\to {4\mathrm{Ca}}_4{\mathrm{Al}}_6{\mathrm{CrO}}_{16}{+20\mathrm{Al}}_2{\mathrm{O}}_3$		(4)
${(1 - \mathrm{x})\mathrm{Al}}_2{\mathrm{O}}_3{+\mathrm{xCr}}_2{\mathrm{O}}_3\to {({\mathrm{Al}}_{1-\mathrm{x}},\mathrm{Crx})}_2{\mathrm{O}}_3$		(5)
${16\mathrm{CaAl}}_{12}{\mathrm{O}}_{19}{+2\mathrm{Cr}}_2{\mathrm{O}}_3{+3\mathrm{O}}_2\to {4\mathrm{Ca}}_4{\mathrm{Al}}_6{\mathrm{CrO}}_{16}{+84\mathrm{Al}}_2{\mathrm{O}}_3$		(6)

. In addition, the qualitative EDS spot analysis (atomic%) was shown in

Table 6. Examples of qualitative EDS spot analysis of the samples (atomic%) for identifying various phases in Figure 5.

Phase	Al	Ca	Cr	O
CaCrO4	-	28.36	41.27	30.37
Ca6Al4CrO16	34.27	18.58	6.34	40.82
CaAl2O4	40.39	16.94	-	42.68
CaAl4O7	49.87	4.13	-	46.00
(Al,Cr)2O3	48.68	-	15.27	36.05

, corresponding to the fractured surface in Figure 5. Needless to mention that the uneven surface of the specimen would only reveal the non-stoichiometric composition to identify the different phases associated with different morphology.

After being treated at 900 °C, the CA phase disappeared in specimen C-C with the formation of many granular CaCrO₄ grains (Figure 5a) via reaction 1. However, the sample C-S was still composed of the initial main phases (CA, CA₂, and (Al_1−xCr_x)₂O₃) (Figure 5b) in addition to forming minute amounts of Ca₄Al₆CrO₁₆ (reaction 2). As the heat treatment temperature increased to 1300 °C, plenty of chrome-hauyne and (Al_1−xCr_x)₂O₃ solid solution (Figure 5e) were generated in specimen C-C (via reactions 3–5), accompanied by the disappearance of CA and significant reduction in CA₂ phase, while specimen C-S possessed relative lower peak intensity of Ca₄Al₆CrO₁₆ although it had similar phases as C-C. Combining the observations of Cr(VI) in Figure 2, with the phase evolution results (Figure 3 and Figure 4), it can be deduced that compared with Cr₂O₃, the (Al_1−xCr_x)₂O₃ solid solution was more stable that would not form CaCrO₄ and could effectively hinder the Ca₄Al₆CrO₁₆ formation when contacted with CAC. Therefore, the substitution of Cr₂O₃ with (Al_1−xCr_x)₂O₃ can effectively lower the Cr(VI) concentration of the castables after being treated at various temperatures (Figure 2). Furthermore, the castables with (Al_1−xCr_x)₂O₃ pre-synthesized at higher temperature exhibited lower Cr(VI) concentration, implying that the stability of the (Al_1−xCr_x)₂O₃ improved gradually with the Al₂O₃ proportion in the solid solution. In addition, in comparison with the CA₂ phase, CA was more likely to react with Cr₂O₃/(Al_1−xCr_x)₂O₃ resulting in the formation of Cr(VI) compounds.

For specimens CH-C, no new phases occurred after heat treatment at 900 °C, and only a minuscule amount of chrome-hauyne was generated at 1300 °C (Figure 5g) via Eqs. 6, which also produced Al₂O₃ that subsequently interacted with Cr₂O₃ to develop the (Al_1−xCr_x)₂O₃ solid solution via Eqs. 5. It is worth mentioning that no changes in the phase compositions were detected in specimen CH-S after heat treatment at both 900 °C and 1300 °C (Figure 4). These observations demonstrated that calcium in CA₆ was much more stable than in CA and CA₂, which only caused slight oxidation of Cr₂O₃ and would not take chemical reaction with (Al_1−xCr_x)₂O₃ solid solution. Therefore, specimen F15, in which chromium and calcium existed in (Al_1−xCr_x)₂O₃ and CA₆, respectively, showed no changes in phase composition and extremely low Cr(VI) concentration at various heat treatment temperatures. In the Al₂O₃-CaO-Cr₂O₃ castables, CA₆ could be generated from the reaction between CAC and Al₂O₃ powders in the matrix at 1300 °C (Figure 4). However, for specimen S165, since no Al₂O₃ existed in the (Al_1−xCr_x)₂O₃ powder pre-synthesized at 1650 °C, the calcium would still exist as CA and CA₂ rather than CA₆ at 1300 °C. As a result, specimen S165 possessed an even higher Cr(VI) concentration than the reference specimen R at 1300 °C, suggesting that CA and CA₂ can more easily react with (Al_1−xCr_x)₂O₃ to produce Ca₄Al₆CrO₁₆ compared with CA₆.

4. Conclusions

In the present work, (Al_1−xCr_x)₂O₃ solid solution was pre-synthesized at a different temperatures for the inhibition of the formation of Cr(VI) in Al₂O₃-CaO-Cr₂O₃ castables was systematically investigated. The summarized conclusions are as follows:

(1)

Compared with Cr₂O₃, the stability of the (Al_1−xCr_x)₂O₃ solid solution in contact with CAC was much higher. Furthermore, the substitution of Cr₂O₃ with (Al_1−xCr_x)₂O₃ in the Al₂O₃-CaO-Cr₂O₃ castables can completely inhibit the mid-temperature (300–1100 °C) formation of Cr(VI) compound CaCrO₄ and relatively higher temperature Cr(VI) phase Ca₄Al₆CrO₁₆ (hauyne) drastically reduced at 900 to 1300 °C. Therefore, replacing Cr₂O₃ with (Al_1−xCr_x)₂O₃ can effectively lower the Cr(VI) concentration of the castables after being treated at various temperatures, and a reduction in Cr(VI) amounts up to 98.1% with (Al_1−xCr_x)₂O₃ addition could be achieved. Most importantly, Cr(III) present within the in situ (Al_1−xCr_x)₂O₃ and Ca(Al,Cr)₁₂O₁₉ solid solution phases showed maximum reoxidation resistance and thus need further investigation.

(2)

In comparison with the CA₂ phase, CA was more likely to react with Cr₂O₃/(Al_1−xCr_x)₂O₃, resulting in Cr(VI) compound formation. Simultaneously, calcium in CA₆ was much more stable than in CA and CA₂, which only caused slight oxidation of Cr₂O₃ and would not undergo a chemical reaction with (Al_1−xCr_x)₂O₃ solid solution. Thus, incorporating some Al₂O₃ powders in the matrix of the Al₂O₃-CaO-Cr₂O₃ castables to form CA₆ at a temperature above 1300 °C was also essential for inhibiting Cr(VI) formation when using (Al_1−xCr_x)₂O₃ solid solution as a substitute for Cr₂O₃.