Comparison of the Hazardous Waste Slag Corrosion Resistance of SioxX-Zero and Cement-Bonded Al2O3–Cr2O3 Castables

Al2O3–Cr2O3 castables are one of the important refractories for hazardous waste incinerators. In this work, the hazardous waste slag corrosion resistance of Calcium Aluminate Cement (CAC) and SioxXZero bonded Al2O3–Cr2O3 castables were comparatively investigated. The results showed that the slag corrosion and penetration resistance were related to the adopted slags (SiO2-rich, CaO-rich slag, and Fe2O3-rich) and bonding systems. Generally, SioxX-Zero-bonded castable has the best corrosion and penetration resistance against SiO2-rich slag, followed by CaO-rich slag and Fe2O3-rich slag. In comparison, the corrosion and penetration resistance of CAC-bonded castables to the three slags are weaker than that of SioxX-Zero- bonded castables. As for SiO2-rich slag, mullite in SioxX-Zero-bonded Al2O3–Cr2O3 castable was stable as more (Al, Cr)2O3 solid solution was formed, resulting in higher viscosity and slower penetration of slag. Because of their lower viscosity, CaO-rich and Fe2O3-rich slags penetrated castables completely. As for CAC bonded castables, CaO reacted further with SiO2-rich slag to generate more liquid slag and increased the CaO content in the slag liquid, leading to the decrease of viscosity of liquid slag and promoting penetration and corrosion. In CaO-rich and Fe2O3-rich slags, more liquid slags with lower viscosity enhanced the penetration behaviour.

1 Introduction High-temperature incineration technology is currently the most effective way to deal with hazardous wastes as it greatly reduces the volume and weight of wastes and recovers waste heat to generate electricity [1, 2]. However, hazardous wastes come from a wide range of sources, including steelmaking, mining, chemistry, medicine and other industries, leading to a variety of wastes, which includes heavy metal-containing waste, organics, medical waste, etc., making chemical compositions of hazardous waste slag different [3, 4]. Therefore, the selection of refractory materials for the lining of hazardous wastes is significantly important [5]. Refractory materials used for the hazardous waste incinerator lining are mainly SiC, Al2O3–SiO2, and Al2O3–Cr2O3. Although having excellent slag resistance, the application range of SiC refractory is limited because it can be easily oxidized [6, 7]. Again, Al2O3–SiO2 refractories are cost-effective and have good thermal shock resistance but easily react with Na2O, K2O, and CaO in the slag to form low-melting phases like nepheline (NaAlSiO4), anorthite (CaAl2Si2O8) and potassium feldspar (KAlSiO4) aggravating the corrosion of the material [8, 9]. On the other hand, because Cr2O3 has good physical and chemical stability [10–12], ultra-low solubility in corrosive media [13, 14], and ultra-high thermochemical stability [15, 16], Al2O3–Cr2O3 refractory shows excellent slag corrosion resistance during service [17–19]. Because of its more convenient construction and repair, calcium aluminate cement (CAC) bonded Al2O3-Cr2O3 castable has been successfully applied as the lining material [20]. However, there is a risk that Cr(III) would oxidize to Cr(VI) in CAC bonded Al2O3–Cr2O3 castable because of CaO and air as a large amount of CaCrO4 and Ca4Al6CrO16 phases form during the heating, leading to the high concentration of Cr(VI) in the castables [21, 22]. Furthermore, alkali metals or alkaline earth metals in slag were also found to lead to excessive Cr(VI)-concentrations in several studies. Lee et al. [23] found uncombined CaO in the slag penetrated the material and reacted with Cr2O3 and Al2O3 to form Ca4Al6CrO16. Additionally, Xu et al. [24] conducted corrosion experiments with copper slag and magnesia-chrome refractories at 1450 ÅãC, finding the obtained concentration of Cr(VI) was much higher than the European Union standard. Therefore, Cr2O3 in Al2O3–Cr2O3 castable may react with slag to generate Cr(VI) compounds. Aiming at reducing the excessive concentration of Cr(VI), Nath et al. [25] replaced CAC with silica gel partially, which reduced the concentration of Cr(VI) of Al2O3–Cr2O3 castable. Similarly, Song et al. [26] introduced silica into CAC bonded Al2O3–Cr2O3 castable, reducing the concentration of Cr (VI) by inhibiting the reaction between CaO and Cr2O3. Recently, Zhang et al. [27] studied the phase evolution of the Al2O3-CaO–Cr2O3–SiO2 system, concluding that SiO2 encapsulates Cr2O3 as Cr(III) form within gehlenite phase (Ca2(Al, Cr)2SiO7) and anorthite (Ca(Al, Cr)2Si2O8), resulting the decrease of concentration of Cr (VI). The above results show that the Cr (VI) of Al2O3–Cr2O3 castable can be effectively inhibited by introducing SiO2 into the composition. However, silica gel is unsuitable as a binder for Al2O3–Cr2O3 castable because of high overall water and low mechanical strength [28]. Conversely, SioxX-Zero is a silica-alumina gel binder, requires lower water demand, and provides higher mechanical and physical properties than silica gel under the same experiment conditions conducted by Luz et al. [29]. Therefore, it is a potential method to use SioxX-Zero instead of CAC as the binder for Al2O3–Cr2O3 castable to inhibit the formation of Cr(VI) in serving. In the present work, SioxX-Zero and CAC cement were selected as binders, and three typical hazardous waste slags with different CaO, SiO2, and Fe2O3 contents were used. The corrosion and penetration resistance of SioxX-Zero bonded Al2O3–Cr2O3 castable to hazardous waste slag were evaluated. In addition, the effect of the binder system on the concentration of Cr(VI) generated in slag corrosion were also investigated. With the help of thermodynamic software, specific mechanisms have been proposed. 2 Experimental 2.1 Fabrication of Al2O3–Cr2O3 castables The raw material includes tabular alumina (5–3 mm, 3–1 mm, 1–0 mm, ≤0,074 mm and ≤0,045 mm, w(Al2O3) ≥99,0 mass-%, Zhejiang Zili Alumina Material Technology Co. Ltd./CN); industrial Cr2O3 (≤0,045 mm, w(Cr2O3) ≥98,0 mass-%, Luoyang Yuda Refractory Co. Ltd./CN)); reactive alumina (≤5 μm, w(Al2O3) ≥99,0 mass-%, Kaifeng Special Refractory Co. Ltd./CN); The compositions were bonded with calcium aluminate cement (CAC, Secar71, 70 % Al2O3, 29 % CaO, Yiruishi Aluminate Co. Ltd./CN) or SioxX- Zero (Elkem ASA Silicon Materials/NO). Organic flocculant FS65 (Wuhan Shanda Chemical Co. Ltd./CN) was used as dispersant. After the raw materials are mixed and poured, a cubic crucible specimen of 70 mm Å~ 70 mm Å~ 70 mm and the crucible hole  After curing at 25 ÅãC and humidity of 75 % for 24 h, it is demoulded and dried at 110 ÅãC for 24 h, and the slag corrosion experiment is carried out at 1600 ÅãC for 3 h. 2.2 Characterisation The static crucible method was adopted in this experiment according to the slag resistance test of refractories (GB/T 8931-2007). The chemical compositions of three kinds of hazardous waste furnace slags are shown in Tab. 1, which were not pretreated. The slags with relatively high content of CaO, SiO2, and Fe2O3 are respectively named CaO-rich slags, SiO2-rich slags, and Fe2O3- rich slags, and their basic-acid ratios are 2,7; 0,5; and 1,0; respectively. The hemispherical point of slags was measured by melting point and melting rate tester and the result showed that the hemispherical point of CaO-rich slag is close to 1500 ÅãC, SiO2-rich slag is 1152,3 ÅãC, and Fe2O3-rich slag is 1203,5 ÅãC. 24 g slag was added into crucible specimens, and the static slag corrosion experiment was carried out at 1600 ÅãC for 3 h. According to different slag and crucible specimens, the experimental specimens were named S-SZ, F-SZ, C-SZ, S-CA, F-CA, and C-CA, where S, F, C, SZ, and CA stand for SiO2-rich slag, Fe2O3- rich slag, CaO-rich slag, SioxX-Zero-bonded castables, and CAC-bonded castables, respectively. After the crucible specimen was cooled, it was cut in half along the central axis of the crucible to check the slag corrosion resistance. Subsequently, the corrosion and penetration index calculations were done according to formulas (1) and (2). The phase composition of the specimen matrix before and after the static slag resistance tests were analysed using X-ray diffraction (XRD, X’Pert Pro, Philips/NL). A field scan electron microscope (SEM) equipped with an energy-scattering X-ray spectrometer (EDS) was used to observe that specimen in the backscattering electron imaging (BEC) mode. The leaching experiment determined the concentration of Cr (VI) in the penetrated layer of the specimen after the static slag resistance test [30]. 1,5 g specimen was ground until the particle size was less than 74 μm, and 30 ml distilled water was added. The leaching solution of Cr(VI) was obtained by stirring (300 r/min, 15 min) and suction filtration (0,45 μm filter paper). After 1,5-diphenyl carbazide complexation, the absorbance of leachate was recorded at 540 nm using a 722 Vis spectrophotometer (Jinghua Instruments, China) to determine Cr (VI) concentration. Where, CI is the Corrosion Index, PI is the Penetration Index, SC is the Corrosion Area, SI is the Penetration Area, SO is the Crucible Area. 3 Results and discussion 3.1 Slag corrosion test XRD patterns of Al2O3–Cr2O3 castables with different bonding systems are shown in Fig. 1. After heating, (Al, Cr)2O3 solid solution appears in both specimens. CaAl12O19 appears in specimen CA due to the reaction of CaO and Al2O3, while mullite appears in specimen SZ due to the reaction between SiO2 and Al2O3. The amount of CaAl12O19 in specimen CA is higher because the peak of CaAl12O19 in XRD is more obvious than mullite, and the expansion effect caused by formation is stronger, making the average pore diameter and porosity of specimen CA [31] which are shown in Fig. 2 and Tab. 2 smaller. Additionally, it is worth noting that the peak of (Al, Cr)2O3 solid solution in specimen SZ is relatively higher than in specimen CA, which indicates that more (Al, Cr)2O3 solid solutions are produced. Fig. 3 shows a cross-sectional view of the crucible specimens after the static slag resistance of three kinds of slag at 1600 ÅãC for 3 h, in which there is no obvious residual slag at the bottom of the crucible except S-SZ. As is shown in Fig. 4, specimen SZ has the best resistance ability to SiO2-rich slag, followed by CaO-rich slag, and the worst ability to Fe2O3-rich slag because the lower index means better resistance ability. Comparing the two kinds of specimens, the corrosion index and penetration index of slags to specimen SZ are less than that of specimen CA, especially for SiO2-rich slag, and the penetration index of S-SZ is much smaller than that of S-CA. Therefore, the resistance of specimen SZ to corrosion and penetration of three kinds of slag is stronger than that of specimen CA, and its resistance to penetration of SiO2-rich slag is much greater than that of specimen CA. 3.2 Phase composition To find out the difference between SioxXZero- bonded and CAC-bonded Al2O3–Cr2O3 castables in terms of corrosion and penetration resistance of hazardous waste slag at high temperatures, the phase composition of reacted layer and penetrated layer of crucible specimens were detected by XRD (Fig. 5). The phase of the reacted layer of the specimens is mainly affected by slag. After SiO2-rich slag corroded the specimen, the phase of the reacted layer in Fig. 5 a was mainly mullite, (Al, Cr)2O3 solid solution, anorthite (CaAl2Si2O8), and nepheline (NaAlSiO4). The content of CaO in CaOrich slag was higher than that in SiO2-rich slag, so the phases in its reacted layer in Fig. 5 c were mainly CaAl12O19, gehlenite (Ca2Al2SiO7), dicalcium silicate (Ca2SiO4) and (Al, Cr)2O3 solid solution. The phase formed after the corrosion of Fe2O3-rich slag was similar to that of SiO2-rich slag. Still, the content of Fe2O3 in the former was higher, so the Ca(Al, Fe)12O19 appeared in the reacted layer in Fig. 5 e. Feldspar was the difference in the penetrated layer for the two specimens. The phase in the unreacted layer of specimen SZ had more SiO2 content, so anorthite appeared in the penetrated layer. In comparison, specimen CA had more CaO content, so gehlenite appeared in the penetrated layer. Among these phases, the melting point of nepheline is low, which is about 1150 ÅãC, promoting the penetration of slag into the specimen [32, 33]. Anorthite, gehlenite, and Ca2SiO4 have higher melting points than nepheline, but they will still become liquid phase at 1600 ÅãC. The melting points of CaAl12O19, (Al, Cr)2O3 solid solution, Ca(Al, Fe)12O19 phase, and mullite phase are all higher than 1600 ÅãC, which will slow down the penetration of slag. CaAl12O19 is easy to react with acid slag [34], while (Al, Cr)2O3 solid solution is not easy to react with acidic or alkaline slag to form a lowmelting point phase [35]. Ca(Al, Fe)12O19 is the product of a solid solution reaction between Fe2O3 and CaAl12O19, and its properties are similar to the CaAl12O19 [36]. Mullite is the only stable phase in Al2O3-SiO2 binary system with a high melting point. 3.3 Microstructure The microstructures of all specimens corroded by slag were observed (Fig. 6, Fig. 7, and Fig. 8). The low magnification microstructures (Fig. 6) showed that elements such as Si, Ca, Fe and Na in slag penetrated the castable, making reacted layer, penetrated layer, and unreacted layer in the specimens more clearly identifiable, among which most of the components near the corrosion area were altered, showing as reacted layers. The components near the sub-corrosion area were partially altered, showing as the penetrated layer. In addition, there is a slag layer on the left outer side of part of the specimen reacted layer. The high-mag microstructures (Fig. 7 and Fig. 8) are consistent with the phase obtained from the XRD pattern. From the view of the microstructure point, the difference between specimen CA and specimen SZ is not obvious. In the penetrated layers of specimens S-SZ, C-SZ and F-SZ, anorthite and nepheline (EDS at point 1, point 2 and point 3 in Fig. 7) fill exthe gap between (Al, Cr)2O3 solid solution and mullite, while in specimens S-CA, C-CA, and F-CA, anorthite is replaced by gehlenite (EDS at point 4, point 5 and point 6 in Fig. 7). In the reacted layer of S-SZ and S-CA, the mixture of anorthite and nepheline (EDS at point 1 and point 3 in Fig. 8) is filled between (Al, Cr)2O3 solid solution and (Mg, Fe2+)(Al, Cr)2O4 spinel solid solution (EDS at point 2 and point 4 in Fig. 8). The content of CaO in CaO-rich slag is higher, so in the reacted layer of specimens C-SZ and C-CA, gehlenite and Ca2SiO4 (EDS at point 5 and point 7 in Fig. 8) are filled between (Al, Cr)2O3 solid solution and CaAl12O19 (EDS at point 6 and point 8 in Fig. 8). The structure of the reacted layer after the Fe2O3-rich slag corroded the specimens is similar to that of the SiO2-rich slag, but there is a large amount of Ca(Al, Fe)12O19 and (Mg, Fe2+)(Al, Cr)2O4 spinel solid solution (EDS at point 9, 10, 11 and 12 in Fig. 8) in the matrix. (Mg, Fe2+)(Al, Cr)2O4 spinel solid solution was found in the reacted layer of specimens S-SZ and S-CA, and CaAl2O4 was found in the reaction layer of specimens C-SZ and C-CA. These phases are few, so their characteristic peaks can’t be detected in the XRD pattern. The size of Ca(Al,Fe)12O19 in specimen F-SZ, and F-CA reacted layer is larger than that of CaAl12O19 in the unreacted layer. As the liquid in reacted layer promoted the growth of this phase, the CaAl12O19 formation temperature decreased in the presence of Fe2O3, which promotes its sintering [36]. 3.4 Cr(VI) leaching test The Cr(VI) concentrations of the penetrated layer of six static slag-resistant specimens are shown in Tab. 5. Cr(VI) is only detected in specimen C-CA, and the Cr (VI) concentration of other five specimens are lower than the upper limit of the EU standard (2 mg/kg) and China standard of hazardous waste leaching (5 mg/L). Due to the difference in melting point and composition of hazardous waste slag and the content of CaO and SiO2 in the specimen, the concentration of Cr(VI) in the penetrated layer is different after the slag corroded the specimen. For specimens S-SZ, S-CA, F-SZ, and F-CA, the content of SiO2 in the slag were more than that of CaO in the process of corrosion, which could prevent CaO and Cr2O3 from reacting to form Cr(VI) compound [26, 27]. For specimen C-SZ, SiO2 in the specimen could react with CaO in the slag, inhibiting the Cr(VI) formation [37]. The melting point of CaO-rich slag was close to 1500 ÅãC, so a certain amount of Cr(VI) compound was formed at the contact between CaO-rich slag and the specimen during the heating, causing Cr(VI) detected in the penetrated layer of C-CA. The reason why the detected Cr(VI) concentration of the six specimens was very low is that the material composition contains only 4 % mass-% Cr2O3, and the corrosion experiment is carried out at 1600 ÅãC, in which Cr2O3 tends to be in the Cr(III) form with (Al, Cr)2O3 solid solution [22]. Additionally, hydratable phases such as CA3 and C12A7, which would be hydrated to form a gel that wraps part of Cr(VI) compounds, may exist in specimen C-CA, leading to the low Cr(VI) concentration of measured during the leaching test [22]. It is also possible that Cr(VI) compounds are encapsulated in glassy phases of castables-slag, causing the low Cr(VI) concentration. 3.5 Discussion According to the above experimental results, specimen SZ has the best resistance ability to SiO2-rich slag, followed by CaOrich slag, and the worst ability to Fe2O3-rich slag. This is mainly related to the amount of liquid and viscosity change during the corrosion of hazardous waste slag. The viscosity change diagram of slag and material after the reaction is shown in Fig. 9, where m(R) represents the weight of the refractory matrix and m(S) represents the weight of slag. After the interaction with refractory, the viscosity of SiO2-rich slag drops sharply, while that of CaO-rich slag will rise, and that of Fe2O3-rich slag is unchanged. Using FactSage, the change of phase composition produced by the interaction between three kinds of slag and refractory matrix at 1600 ÅãC and 1 atm atmosphere pressure are simulated (Fig. 10). m(R) represents the weight of the refractory matrix, and m(S) represents the weight of the slag. When A = 1, the total weight of Al2O3 and Cr2O3 in specimen SZ is higher than that in specimen CA, indicating that the content of (Al, Cr)2O3 solid solution in the former is higher, which corresponds with XRD patterns. After SiO2-rich slag reacts with specimen SZ, the weight of Al2O3 and Cr2O3 in the specimen decreases, in which Al2O3 reacts with slag to produce anorthite, nepheline, and other substances. The weight of mullite is unchanged, indicating that slag will react with the specimen SZ to form mullite during corrosion. However, the contents of CaAl12O19, Al2O3, and Cr2O3 decreased after SiO2-rich slag corroded the specimen CA. Compared to SiO2-rich slag, the basic-acid ratio of CaO-rich slag and Fe2O3-rich slag is higher, resulting in the weight of mullite in the specimen beginning to decrease and the weight of Ca(Al, Fe)12O19, CaAl12O19 and feldspar increase during the corrosion. Results of C-CA and F-CA are similar to those of C-SZ and F-SZ, but the feldspar phase is mainly anorthite in specimen SZ, while the feldspar phase is mainly gehlenite in specimen CA. According to the phase changes during the corrosion and the results of FactSage, the viscosity of SiO2-rich slag was still high during the corrosion because SiO2- rich slag hardly reacted with mullite and (Al,Cr)2O3 solid solution in specimen SZ. High viscosity and high-melting-point phase slowed down the penetration of slag, resulting in the weakest penetration to specimen SZ. The viscosity of CaO-rich slag and Fe2O3-rich slag were very low before and after the reaction, so they penetrated the specimens completely, resulting in a higher penetration index and corrosion index than SiO2-rich slag. The melting point of Fe2O3- rich slag was lower than that of CaO-rich slag, and it reacted with the specimen SZ to form nephelite, which was a low melting point phase, promoting the penetration of slag. Therefore, the penetration and corrosion of Fe2O3-rich slag on specimen SZ were higher than that of CaO-rich slag. Compared to specimen SZ, the corrosion and penetration of three kinds of slags to specimen CA are more. In specimen CA, the SiO2-rich slag reacted with CaAl12O19 to generate low-melting point phases, which increased the liquid amount and reduced the viscosity, promoting the penetration of slag, resulting in the higher corrosion and penetration index compared to specimen SZ. Compared to specimen CA, the reaction between mullite in specimen SZ and CaO-rich slag or Fe2O3-rich slag would produce more liquid. Still, the content of SiO2 in the liquid was also increased, causing higher viscosity. In terms of the results, the penetration index of C-SZ and F-SZ was lower than that of C–CA and F–CA, indicating that the inhibition effect caused by the increase of viscosity on the slag penetration was greater than the promotion effect caused by the increase of liquid. Therefore, the resistance ability of specimen SZ to CaO-rich slag and Fe2O3-rich slag was stronger than that of specimen CA. In addition, there were more (Al, Cr)2O3 solid solutions in specimen SZ, which was not easy to react with slag, so the CaO-rich slag and Fe2O3-rich slag had lower corrosion index than specimen SZ. 4 Conclusion

• The influencing factors of refractory resistance to slag penetration and corrosion are related to the viscosity of the liquid and phases before and after the reaction. SiO2-rich slag hardly reacted with the phases in the specimen SZ, resulting in the high viscosity of liquid during the corrosion and making the lowest corrosion and penetration of SiO2-rich slag. Due to the lower viscosity of CaO-rich slag and Fe2O3-rich slag, two slags penetrated the specimen completely, making the penetration index higher than that of SiO2-rich slag. The nepheline formation promoted corrosion and penetration during Fe2O3- rich slag corrosion, so its indexes were higher than that of CaO-rich slag. Therefore, SioxX-Zero bonded castable has the best corrosion and penetration resistance ability to SiO2-rich slag, followed by CaOrich slag and worst to Fe2O3-rich slag.
• The bonding systems influence refractory resistance to slag penetration and corrosion. For example, compared to specimen SZ, CaAl12O19 in specimen CA reacted with SiO2-rich slag to generate more liquid and reduced liquid viscosity, resulting in a greater penetration index. On the other hand, the liquid generated by the reaction of specimen CA with CaO-rich slag and Fe2O3-rich slag had lower viscosity, causing larger penetration. Thus, the corrosion resistances of CAC-bonded castables and slag penetrations are weaker than that of SioxX-Zero bonded castables.
• For the slag with high content of SiO2 and Fe2O3 in hazardous waste slag, since SiO2 and Fe2O3 are acidic oxides, the formation of Cr(VI) seemed to be inhibited in the slag corrosion. For hazardous waste slag with high content of CaO, SiO2 in the SioxX-Zero bonded castable reacted with secit to inhibit the formation of Cr(VI) at high temperatures. Therefore, Cr(VI) was detected in specimens CA but not in specimen SZ after CaO-rich slag corrosion. Generally speaking, the concentration of Cr(VI) is below the various standard control limits (European and Chinese) for SioxX-Zero bonded castables.

Acknowledgment
The authors gratefully thank the financial support from Natural Science Foundation of Hubei Province (2021CFB370) and National Natural Science Foundation of China International (Regional) Cooperation and Exchange Program (5191101186).

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