Abstract
In this research, the effect of addition of 1, 2 and 3wt% chromium oxide on rheological, physical, and mechanical properties of a low-cement high alumina castable has been investigated. For this purpose, tabular alumina, calcium aluminate cement 80PX, reactive alumina, silica fume, and industrial grade Cr(III) oxide (Cr2O3) were used as raw materials. Then different compounds were prepared and fired at 815°C, 1200°C and 1500°C for measurement of density, percentage of permanent linear changes, cold crushing strength, and cold modulus of rupture. The results showed that the optimum rheological properties, density, cold crushing strength, and cold modulus of rupture were obtained for the samples containing 2wt% of chromium oxide in thier composition. It was mainly due to formation of a solid solution phase in the grain boundaries and the filling of the pores.
1.Introduction
In the last decades there has been an increasing trend among refractories towards replacement of bricks by castable refractories[1]. Quality requirement of these products is also increasing. Especially low-cement castables have been widely used in steel industry due to their superior rheological and physical propertiesh[2]. Initially, castable refractories were composed of only cement and aggregates. Then, the addition of deflocculants and fine fillers have followed with the aim to optimize the control of properties such as workability as a function of time[3]. According to the obtained results, develop the properties of self-flowing castables are under effect of several item such as: the Andreasen coefficient, the percentage of silica fume, the percentage of calcined alumina, the use of tabular alumina, white corundum and brown corundum, the use of high firing temperature[4].
Most castables are based on calcium aluminate cement (CAC) bonding although there is a trend away from lime-containing systems due to its fluxing action at high temperatures in aluminosilicates. Lime/silica (C/S) ratio is an important factor in determining the amount and viscosity of the liquid as well as hot strength and creep resistance of the refractory[5]. LCC‘s have increased refractoriness and improved high temperature properties due to their lower CAC and thus lime contents[6]. Hydration starts immediately on mixing the castable with water forming different calcium aluminate hydrates with various morphologies depending on the water/ cement ratio, curing condition (especially temperature and relative humidity) and impurities. All hydrated compounds dehydrate by 550°C forming anhydrous, extremely fine active lime and alumina which react to produce calcium aluminates[7]. The fine fractions of the matrix phases, including products of hydration, initially react among themselves and at higher temperature they, or their reaction products, may react with the coarser aggregate phases. In conventional CAC-bonded castables C12A7, CA and CA2 form in this sequence with increasing temperature until >1300°C when CA2 reacts with alumina to form hexagonal platelets of CA6[8].
The lifetimes of linings made from these castables are limited by their wear rate arising from slag penetration and structural spalling. So that a thorough understanding of the corrosion mechanisms is desirable[9]. Since the slag is the most corrosive component in the melt, its composition has a critical effect on the corrosion mechanism. Unfortunately, slag composition varies greatly between steelworks and even between different batches in the same steelworks, so that a complete model of slag corrosion of a refractory is difficult to achieve[10].
Al2O3-Cr2O3 refractories are widely used as lining materials for various high temperature furnaces due to excellent mechanical properties and corrosion resistance to molten slag. The high stability and extremely low solubility of Cr2O3 in molten slag are responsible for the highly corrosion resistance[11]. Recently, calcium aluminate cement (CAC) bonded Al2O3-Cr2O3 refractories as castables are applied successfully due to more convenient for installation and repairing of the furnace linings. Cr2O3 (trivalent chromium) can be oxidized into highly toxic, mutagenic, potentially carcinogenic and water-soluble Cr(VI) compounds in presence of alkali and alkaline earth elements, their salts and oxides under oxidizing atmosphere at high temperature[12]. The oxidation states of chromium in solid state change with an increase in temperatures according to the following order: Cr(III)→Cr(VI)→Cr(III). Thus, spent Cr2O3-containing refractory castables especially those used in the 700–1100 °C temperature range must be treated carefully before land filling. And the conversion of Cr(III) into Cr(VI) compounds (CaCrO4 and Ca4Al6CrO16) can also be facilitated by CAC due to the presence of CaO component. Therefore, it is of great significance for environmental protection and popularization to achieve detoxification of Al2O3-Cr2O3-CaO castables when using CAC as binders[13].
Researchers showed that increase in Cr2O3 amount reflects better corrosion resistance. The major concern of Cr2O3 containing materials is the formation of Cr(VI) compounds which are toxic and carcinogenic. Besides, Cr(VI) compounds are readily water soluble, consequently making its easy entrance to the food chain. however, it is obvious to use minimal amount of Cr2O3 which lead to formation of less amount of Cr(VI)[14].
2.Experimental
2.1. Preparation of sample
Tabular Alumina (Zhejiang Zili Advanced Materials Co. Ltd., China); calcium aluminate cement namely Px80 (Sheng Chuan Advanced Material Technology Co.,Ltd); reactive alumina(CTC20 ALMATIS); silica fume and industrial grade Cr(III) oxide (Cr2O3) were used as the raw materials. The chemical composition and some physical properties of these raw materials are given in Table 1. The castable were formulated (Table 2) with the addition of 0, 1, 2 and 3wt% of Cr2O3 replacing the Al2O3. The formulation is based on the Andreasen distribution with co-efficient (q) of 0.34. Px80 used as binder (10wt% in each). Each batch is dry-mixed in Hobart’s mixer for 2 minutes at room temperature (22°C) followed by wet mixing (5.9% water, 20°C) for further 3 minutes. The flowability of the castables measured by flow-table. Samples were casted at room temperature (22°C) in vibrating table (60 s) into bars of size 160 mm × 40 mm × 40 mm for CMOR test, and into cubes of size 5 mm × 5 mm × 5 mm for CCS test. After casting, specimens were placed at ambient temperature and 75 ± 5% relative humidity for 24 hours and then demoulded and dried at 110°C for 24 hours in an electric air oven. Thus, dried samples were weighed and their dimensions were measured. The specimens were fired at 815°C, 1200°C and 1500°C, followed by natural cooling to room temperature. The heating rates are shown in Table3.
Table 1 Physical and chemical properties of the raw materials.
| Chemical constituent (wt%) | Tabular Alumina | Alumina cement | Reactive alumina | Silica fume | Cr2O3 |
| AL2O3 | 99.3 | 80 | 99.7 | 1.2 | 0.2 |
| SiO2 | 0.18 | 0.35 | 0.03 | 94.5 | 0.09 |
| CaO | – | 18 | 0.03 | 0.3 | 0.06 |
| Na2O | 0.4 | – | 0.12 | 0.5 | – |
| K2O | – | – | – | – | – |
| Fe2O3 | 0.1 | 0.1 | 0.03 | 1.2 | – |
| MgO | – | 0.4 | – | – | – |
| TiO2 | – | 0.1 | – | – | – |
| Cr2O3 | – | – | – | – | 99.25 |
| Bulk Density(g/cm3) | 3.5 | 0.75 | 0.2 | 0.23 | – |
| Phase | Corundum | CA, CA2, αAl2O3 | Corundum | α-quartz |
Table 2 Formulation of the castables specimens.
| Sample code
|
Tabular alumina |
Compositions(wt%)
PX80 |
SF |
Cr2O3 |
|
| Cr0 | 89 | 10 | 1 | 0 | |
| Cr1 | 88 | 10 | 1 | 1 | |
| Cr2 | 87 | 10 | 1 | 2 | |
| Cr3 | 86 | 10 | 1 | 3 |
Table 3 Heating rate of Samples at different temperatures.
| Fired at 815°C | Fired at 1200°C | Fired at 1500°C | |||
| Temperature | Hours | Temperature | Hours | Temperature | Hours |
| 0-400
400 |
5 | 0-400 | 5 | 0-400 | 5 |
| 3 | 400 | 3 | 400 | 3 | |
| 400-600 | 2 | 400-600 | 2 | 400-600 | 2 |
| 600 | 2 | 600 | 2 | 600 | 2 |
| 600-815 | 2 | 600-800 | 2 | 600-850 | 2 |
| 815 | 2 | 800 | 2 | 850 | 2 |
| 800-1000 | 2 | 850-1150 | 2 | ||
| 1000 | 2 | 1150 | 2 | ||
| 1000-1200 | 2 | 1150-1300 | 2 | ||
| 1200 | 2 | 1300 | 2 | ||
| 1300-1500 | 2 | ||||
| 1500 | 2 | ||||
2.2. Characterization
The chemical analysis of raw materials were carried out by using XRF (X-Ray fluorescence). The physical properties of each raw material have been measured according to the standard test method for bulk density of granular refractory materials (ASTM C357). The rheological behavior of the castables was evaluated according to the Standard test method for measuring consistency of castable refractory using a flow table (ASTM C1446). Physical properties such as bulk density were measured in accordance with the Archimedes liquid displacement method (ASTM C20). The mechanical properties of castables, such as cold modulus of rupture (CMOR), cold crushing strength (CCS), were tested according to the standard ASTM C133. For all the samples Phase development studies were performed by powder X-ray diffractometer (XRD) using copper radiation (CuKα, λ = 1.5418 Å) at 40 kV/30 mA (X′Pert Pro, PANalytical, Netherlands), and microstructural development by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS).
3.Results and Discussions
3.1. Chemical Analysis
The chemical analysis of each of the compounds has been measured with XRF and is given in Table 4. As shown in table4, the composition without chromium oxide has 95.51% Al2O3. By adding chromium oxide, the percentage of alumina is reduced because chromium oxide has replaced alumina. Due to the constant percentage of cement, the percentage of CaO in all samples is almost the same and between 1.7 and 1.9% and this indicates that the compounds are low cement.
Table4 Chemical analysis of compounds.
| Chemical constituent (wt%) | Cr0 | Cr1 | Cr2 | Cr3 | |
| AL2O3 | 95.51 | 95.22 | 94.31 | 93.78 | |
| SiO2 | 1.49 | 1.04 | 0.96 | 0.94 | |
| CaO | 1.90 | 1.82 | 1.82 | 1.72 | |
| Fe2O3 | 0.56 | 0.43 | 0.40 | 0.42 | |
| Na2O | 0.3 | 0.3 | 0.29 | 0.29 | |
| Cr2O3 | – | 0.92 | 1.91 | 2.67 | |
| L.O.I | 0.04 | 0.09 | 0.07 | 0.12 |
3.2. Rheological properties
Flowability of tested compounds is given in Table 5. According to the table 5, with the increase in the percentage of chromium oxide, the flowability of castables decreases. In fact, the ,precipitation of fine chromium oxide particles on the grain boundary and the surface of tubular alumina particles reduces flowability. Chromium oxide with a particle size of 1-2 µm, has been replaced by tubular powder with a particle size of 45 µm. Therefore, the fine component in the composition has increased, which causes an increase in adhesion and a decrease in flowability.
3.3. Phase evolution
XRD patterns of 4 compounds after fired at different temprature are shown in the Fig 1 to 3. Corundum (Al2O3) is obviously the major phase in all the specimens because it’s the main phase in tubular alumina. Other identified phases are Hibonite and Gehlenite. By investigation ternary diagram of Al2O3-SiO2-CaO it was found that Hibonite phase was existed at 1500°C, Gehlenite and Grossite phases were existed at 1200°C due to reaction between cement Px80 and silica fume and alumina. At 1500 °C, the major hydrated phases C3AH6 and AH3 converted into CA6 phase. The reactions for generating Grossite, gehlenite and hibonite are as Formulas (1), (2) and (3).
CaO + 2Al2O3 = CaAl4O7 (1)
2CaO + SiO2 + Al2O3 = Ca2Al2SiO7 (2)
CaO + 6AI203 = CaAl12019 (3)
Fig. 1. XRD patterns of the specimens after firing at 815°C.
Fig. 2. XRD patterns of the specimens after firing at 1200°C.
Fig. 3. XRD patterns of the specimens after firing at 1500°C.
3.4. Physical and mechanical properties
The physical and mechanical properties after drying at 110°C are given in Table 5, after firing at 815°C in Table 6, after firing at 1200°C in Table7 and after firing at 1500°C in Table 8.
3.4.1. Bulk density
At 110°C, the bulk density increases with the increase in the percentage of chromium oxide. The reasons for this are the filling of closed pores by fine chromium oxide particles and and reducing water consumption. At 815°C, the bulk density shows decreasing trends. At this temperature, the structural water has disappeared and the hydrated phases of the cement have been dehydrated. These reactions are associated with weight loss, as a result, the bulk density has decreased very little. At 1200°C, the bulk density has not changed. Because at this temperature, the sintering process has not started in the samples. After firing at 1500°C, the bulk density increases. At this temperature, the hibonite phase is formed, which increases the density.
3.4.2. Cold Crushing Strength
At 110°C, the cold crushing strength shows an increasing trend with the addition of chromium oxide up to 2wt%. Increasing the percentage of chromium oxide up to 3wt% does not affect on the strength at this temperature. In this case, by adding chromium oxide, the water consumption is reduced and as a result, the cold crushing strength increases.
After firing at 815°C, the cold crushing strength decreases due to the dehydration of the hydration phases of cement. The decreasing trend in samples with chromium oxide is steeper. The reason for this is that the fine particles of chromium oxide fill all the closed pores and the increase in density at this temperature shows this issue. When all the pores are filled, the exit of structural water will be difficult and may lead to the creation of micro-cracks in the structure. As a result of the loss of porosities, the growth of cracks from within the context happened more easily, and as a result, the strength decreased with a more severe trend.
After firing at 1200°C, the sintering process has not started, and on the other hand, the cement hydration phases have completely disappeared, as a result, the strength of the code samples G0, G1, G2 are reduced. In the code sample G3 with 3wt% of chromium oxide, the formation of a solid solution of chromium oxide and alumina started at a temperature of 1200°C, due to the high amount of chromium, and it prevented the decrease in strength and led to an increase in strength. This solid agent is deposited on the grain boundaries and prevents the growth of cracks.
After firing at 1500°C, the strength of the samples has increased. At this temperature, the sintering process has been carried out and the creation of Hibonite phase has increased the strength. In the samples containing chromium oxide, the amount of this increase was higher. The reason for this result is the presence of fine chromium oxide particles and participation in the sintering process and its improvement. As it is clear in the XRD analysis (Fig. 3), in the samples containing chromium oxide, the solid solution phase (AlCr2) has been detected and it has increased the strength.
3.4.3. Cold Modulus of Rupture
At 110°C, the addition of 1wt% chromium oxide had no effect on the results of CMOR. By adding 2 and 3wt% chromium oxide, the CMOR has increased. The addition of chromium oxide has reduced water consumption and increased CMOR at this temperature.
At 815°C, the CMOR values have decreased. At this temperature, the hydrated phases of cement, which cause strength, are dehydrated, and on the other hand, the mass has not reached the sintering temperature and forming ceramic bonds. As a result, the CMOR is reduced. At 1200°C, the structural water is lost and the hydration phases of the cement are dehydrated. Modulus of rupture is very sensitive to microcracks. At 1200°C, porosity is created as a result of the exit of structural water in the microstructure. Small pores prevent the growth of microcracks and increase strength. In the sample without chromium oxide, as a result of more water consumption, the remaining porosity in the microstructure was higher and as a result, the increase in strength was higher.
At 1500°C, the modulus of rupture has generally increased due to the formation of the hibonite phase (Fig. 3). The increase in strength was higher in the sample without chromium oxide. As it is clear from the microscopic images (Fig. 4), the sample without chromium oxide has more porosity at this temperature. As a result, the crack has hit the porosity during growth and its growth has been prevented. While in the samples containing chromium oxide, the solid solution formed covers the surface of the samples.
Table 5 Physical and mechanical properties of the castables specimens after drying at 110°C.
| Sample code | Cr0 | Cr1 | Cr2
|
Cr3 | |
| Flow value (mm) | 150 | 130 | 115 | 90 | |
| BD (g/cm3) | 3.06 | 3.08 | 3.10 | 3.12 | |
| CMOR (MPa) | 14.73 | 14.74 | 16.41 | 17.63 | |
| CCS (MPa) | 77.40 | 92 | 111.40 | 111.75 |
Table 6 Physical and mechanical properties of the castables specimens after firing at 815°C.
| Sample code | Cr0 | Cr1 | Cr2
|
Cr3 | |
| PLC (%) | -0.34 | -0.33 | -0.46 | -0.30 | |
| BD (g/cm3) | 3.03 | 3.03 | 3.05 | 3.05 | |
| CMOR (MPa) | 10.47 | 12.56 | 12.58 | 12.83 | |
| CCS (MPa) | 68.25 | 61 | 76 | 67 |
Table 7 Physical and mechanical properties of the castables specimens after firing at 1200°C.
| Sample code | Cr0 | Cr1 | Cr2
|
Cr3 | |
| PLC (%) | -0.33 | -0.23 | -0.18 | -0.25 | |
| BD (g/cm3) | 3.02 | 3.02 | 3.04 | 3.05 | |
| CMOR (MPa) | 22.57 | 18.04 | 19.86 | 22.55 | |
| CCS (MPa) | 61.67 | 53 | 61.25 | 75.33 |
Table 8 Physical and mechanical properties of the castables specimens after firing at 1500°C.
| Sample code | Cr0 | Cr1 | Cr2
|
Cr3 | |
| PLC (%) | -1.21 | -1.03 | -0.87 | -0.42 | |
| BD (g/cm3) | 3.09 | 3.11 | 3.10 | 3.08 | |
| CMOR (MPa) | 42.30 | 39.47 | 35.99 | 39.65 | |
| CCS (MPa) | 110.67 | 111.00 | 118.50 | 119.50 |
| b |
| a |
| d |
Figure. 4. SEM images of samples fired at 1500°C a)Cr0 b)Cr1 c)Cr2 d)Cr3.
4.Conclusions
The addition of chromium oxide due to the fineness of the particles has reduced the water consumption and improved the flowability and rheological behavior of the castable. As a result, the addition of chromium oxide up to 2 wt% has increased the density, cold crushing strength and cold modulus of rupture at temperatures of 110°C, 815°C and 1200°C. Increasing chromium oxide from 2 wt% to 3 wt% did not have any effect on the mentioned properties. At 1500°C, a solid solution is formed in samples with chromium oxide, which increases the bulk density and cold crushing strength. At this temperature, according to the scanning electron microscope images, the sample without chromium oxide has a higher percentage of porosity, which causes a greater increase in cold modulus of rupture.
5.Refrences
[1] F. Simonin, C. Olagnon, S. Maximilien, G. Fantozzi, L. A. Diaz, and R. Torrecillas, “Thermomechanical behavior of high-alumina refractory castables with synthetic spinel additions,” J. Am. Ceram. Soc., vol. 83, no. 10, pp. 2481–2490, 2000, doi: 10.1111/j.1151-2916.2000.tb01579.x.
[2] M. D. M. Innocentini, F. A. Cardoso, M. M. Akyioshi, and V. C. Pandolfelli, “Drying stages during the heating of high-alumina, ultra-low-cement refractory castables,” J. Am. Ceram. Soc., vol. 86, no. 7, pp. 1146–1148, 2003, doi: 10.1111/j.1151-2916.2003.tb03438.x.
[3] T. Xu, M. Nath, Y. Xu, Y. Li, N. Liao, and S. Sang, “Thermal evolution of Al 2 O 3 – CaO – Cr 2 O 3 castables in different atmospheres,” Ceram. Int., vol. 47, no. 8, pp. 11043–11051, 2021, doi: 10.1016/j.ceramint.2020.12.227.
[4] K. Cui, T. Fu, Y. Zhang, J. Wang, H. Mao, and T. Tan, “Journal of the European Ceramic Society Microstructure and mechanical properties of CaAl 12 O 19 reinforced Al 2 O 3 -Cr 2 O 3 composites,” J. Eur. Ceram. Soc., no. August, 2021, doi: 10.1016/j.jeurceramsoc.2021.08.024.
[5] M. D. M. Innocentini, A. R. F. Pardo, and V. C. Pandolfelli, “Influence of air compressibility on the permeability evaluation of refractory castables,” J. Am. Ceram. Soc., vol. 83, no. 6, pp. 1536–1538, 2000, doi: 10.1111/j.1151-2916.2000.tb01426.x.
[6] X. Li, J. Li, and L. Zhang, “Enhancing mechanism of improved slag resistance of Al 2 O 3 -spinel castables added with pre-synthesized ( Al , Cr ) 2 O 3 micro-powder,” Ceram. Int., no. August, 2021, doi: 10.1016/j.ceramint.2021.08.235.
[7] J. Zhang, N. Li, W. Zhou, X. Xiong, S. Gao, and G. Ye, “Microstructure Evolution of Hydration Products and Strength Change of Hydratable Alumina-bonded Castables Below 1250 °C,” pp. 0–3, doi: 10.1111/jace.17545.
[8] E. Karadeniz, C. Gurcan, S. Ozgen, and S. Aydin, “Properties of alumina based low-cement self flowing castable refractories,” vol. 27, no. June 2006, pp. 1849–1853, 2007, doi: 10.1016/j.jeurceramsoc.2006.05.050.
[9] M. Ghassemi Kakroudi, E. Yeugo-Fogaing, M. Huger, C. Gault, and T. Chotard, “Influence of the thermal history on the mechanical properties of two alumina based castables,” J. Eur. Ceram. Soc., vol. 29, no. 15, pp. 3197–3204, 2009, doi: 10.1016/j.jeurceramsoc.2009.05.052.
[10] A. Gungor, O. Celikcioglu, and S. Sahin, “The physical and mechanical properties of alumina-based ultralow cement castable refractories,” Ceram. Int., vol. 38, no. 5, pp. 4189–4194, 2012, doi: 10.1016/j.ceramint.2012.02.001.
[11] T. Xu, N. Liao, Y. Xu, M. Nath, Y. Li, and Y. Chen, “Journal of the European Ceramic Society In situ detoxification and mechanical properties of Al 2 O 3 -Cr 2 O 3 -CaO castables with zeolite,” J. Eur. Ceram. Soc., no. August, 2020, doi: 10.1016/j.jeurceramsoc.2020.08.060.
[12] W. E. Lee, W. Vieira, S. Zhang, K. Ghanbari Ahari, H. Sarpoolaky, and C. Parr, “Castable refractory concretes,” Int. Mater. Rev., vol. 46, no. 3, pp. 145–167, 2001, doi: 10.1179/095066001101528439.
[13] J. Wang, L. Yuan, S. Chen, Q. Zhu, X. Sun, and X. Yan, “Thermal shock resistance of Cr 2 O 3 – Al 2 O 3 – ZrO 2 bricks with the microstructure of Cr 2 O 3 aggregates coated by ZrO 2,” no. August, 2021.
[14] M. Nath, S. Song, A. M. Garbers-craig, and Y. Li, “Phase evolution with temperature in chromium-containing refractory castables used for waste melting furnaces and Cr ( VI ) leachability,” Ceram. Int., vol. 44, no. 16, pp. 20391–20398, 2018, doi: 10.1016/j.ceramint.2018.08.032.
Yasaman.Mohammadi (The head of research and development department of ZICO Refractories CO.)
Behzad.Azimi (The factory manager of ZICO Refractories CO.)
