Interfacial Reactions between Alumina and Carbon Refractories and Molten Iron at 1,823 K

Interactions of Al₂O₃–C system

Figure 2 shows the microscopic examination of 100% alumina and AS1 substrate heat treated at 1,823 K for 30 min. Figure 2(a) represents a well-sintered body of alumina particles. Figure 2(b) represents alumina–carbon interaction where several microcracks/pores could be seen; these have been marked in red. The change in morphology during alumina–carbon interactions can be attributed to high temperature interaction between alumina and carbon in the presence of Argon gas as well as to the curing of phenolic resin into active carbon. SEM image in Figure 2(b) clearly indicates that the alumina matrix was well sintered in the regions where carbon was not in close contact with gray particles of alumina. However, microcracks were observed in regions where carbon was in direct contact with alumina. This change could be attributed to the generation of suboxide gases through the partial decomposition of alumina with carbon and their passage through the refractory matrix [16, 22].

Figure 2:

(a) Alumina (100%) and (b) alumina–graphite (AS1) pellet after heat treatment at 1,823 K for 30 min indicating microcracks/porous regions (marked as red).

The EDS analysis of the AS1 refractory pellet (Figure 3(a)) shows the interaction between alumina and carbon. Position “A” shows high alumina content with very little carbon representing unreacted alumina. The analysis of position “B” shows the simultaneous presence of aluminum and carbon; a significant reduction in the oxygen peak indicates the disintegration of alumina and the subsequent removal of oxygen through the formation of CO/CO₂ gases [23]. The presence of aluminum and carbon was observed only in those regions, where alumina was in close proximity with significant amounts of carbon. A further confirmation of EDS results was provided by XRD analysis. The XRD pattern of heat-treated AS1 substrate is shown in Figure 3(b). A number of diffraction peaks were observed in addition to the diffraction peaks for synthetic graphite and alumina; the secondary product formed was identified as Al₄C₃. These Al₄C₃ peaks were observed at peak locations of 31.783° and 40.214° as stand-alone peaks, while at 43.483°, 57.444° and 59.712°, Al₄C₃ peaks were also observed in conjunction with alumina peaks. Generally, low intensities for Al₄C₃ peaks indicate that these compounds were formed in very small quantities.

Figure 3:

(a) SEM image and EDS analysis of AS1 sample and (b) XRD pattern of AS1 substrate showing various diffraction peaks. These peaks have been labeled as corundum, C; graphite, G; aluminum carbide, Al₄C₃.

Although the complete carbothermic reduction of alumina is expected to occur at temperatures above 2,073 K; the thermodynamic feasibility of these reactions has been investigated in the temperature range of 1,803–1,923 K. Ramachandran et al. have reported thermodynamic predictions on the addition of 20 wt% of carbon into the (Al, O) system in the presence of 0.1 mol of Ar (g) in the temperature range 1,773–1,923 K. They have reported on the generation of two predominant Al-containing vapor species: Al₂O (g) and Al (g) along with a high concentration of CO (g) [14]. Balmenos et al. performed thermochemical calculations on alumina–carbon interactions under vacuum to predict that almost all Al₂O₃ was converted into Al (g) and CO (g) around 1,773 K [24]. The dissociation of alumina with carbon has also been reported to occur at 1,803 K with the free energy ∆G calculated as ∆G°J = –1,515,000 + 377 T (K) [25, 26]. Given the thermodynamic feasibility of these interactions at 1,823 K, pellets under investigation would have experienced a partial decomposition of alumina into Al₂O (g) or Al (g) leading to the formation of small amounts of Al₄C₃. The presence of phenol binder provided an additional carbon source in the system. Kanno et al. have reported that during the heat treatment in oxide refractory system, the phenol binder devolatilizes to form a reinforcing carbon framework and the active carbon thus formed reacts to form secondary compounds (carbides and/or oxycarbides) in a nonoxidizing medium [27].

Al₂O₃/Fe system

Pure alumina substrate was reacted with iron by using the sessile drop method at 1,823 K for 30 min in the presence of Ar (g). Microscopic and diffraction analysis confirmed that no interaction had occurred between alumina and iron [23]. A detailed study by Kapilashrami had reported that contact angles of Al₂O₃ with molten iron were generally nonwetting and in the range of 100–141° at 1,803–1,873 K in inert gas atmospheres of helium, argon, hydrogen gases and under vacuum [28]. Due to the nonwetting behavior of alumina with iron, there will be minimal contact between two species. It is therefore unlikely that any appreciable reaction will occur between alumina and liquid iron at 1,823 K. Al₂O₃/Fe reactions have been reported to occur only at temperatures >2,273 K and at high pressures [19]. Figure 4(b) shows the diffraction pattern of 100% Al₂O₃/Fe system; there was no evidence for the formation of additional reaction products.

Figure 4:

(a) SEM image and EDS analysis of 100% alumina sample containing 5% phenolic resin and iron at 1,823 K for 30 min, (b) XRD pattern of 100% alumina sample interacted with iron showing only alumina peaks (corundum, C) and (c) XRD pattern of 100% alumina sample interacted with 5% phenolic resin and iron at 1,823 K for 30 min using sessile drop arrangement. This pattern shows a few peaks in addition to alumina peaks.

Another study was conducted where 5 wt% phenolic resin was added into Al₂O₃ (3 g) substrate. This resin-cured sample was reacted with iron at 1,823 K in a horizontal tube furnace for 30 min. Microscopic investigations showed the penetration of a small amount of iron into the alumina substrate (Figure 4(a)). EDS analysis of the sample in positions “A” and “B” indicated unreacted iron and alumina, whereas the position “C” indicates the alumina–carbon region. Position “D” shows the presence of iron, aluminum and carbon. XRD analysis of the sample as shown in Figure 4(c) revealed the presence of three new diffraction peaks at 41.223°, 47.969° and 70.18°; these peaks were identified as Fe₃AlC. This result indicates that even a small amount of carbon from phenolic resin had reacted with alumina and created channels within alumina causing the penetration of iron through pores and the formation of the reaction product Fe₃AlC. Further investigations are underway to understand the role of active carbon from phenolic resin and its impact on the kinetics of alumina–carbon interaction.

Al₂O₃–C/Fe system

Microscopic investigation and EDS analysis of the refractory pellet AS1–5Fe (Figure 5(a)) showed the combined effect of carbon and iron, which is expected to play a significant role for chemical reactions in the system. Chemical reactions were observed in those regions where iron had penetrated deep in the alumina–carbon matrix. At position “A” the elemental analysis showed that whenever Fe was in close proximity with regions containing alumina and carbon, there was clear evidence for alumina decomposition. Similar results were observed at several other locations in the matrix as well in regions where molten iron had completely covered the surface of alumina–carbon regions. Figure 5(b) and 5(c) shows the microstructure and EDS analysis of the samples AS1–10Fe and AS1–15Fe. The concentration of iron in these systems was much higher than in AS1–5Fe. While the typical elemental analyses observed were quite similar to those observed for AS1–5Fe, the ratio of Fe/Al peak heights was found to increase with increasing Fe levels in the initial pellet.

Figure 5:

Microstructure and EDS analysis of (a) AS1–5Fe, (b) AS1–10Fe and (c) AS1–15 Fe pellets after heat treatment. Alumina (gray region), carbon (dark region) and Fe–Al reaction product (white region) can be seen clearly in these micrographs.

Figure 6 shows carbon-rich regions of AS1–5Fe and AS1–10Fe samples in the presence of iron. Microscopic results show that the presence of iron had an impact on morphology and chemical nature of alumina decomposition such that Al and C were observed at position “A” with trace amounts of iron. The combined presence of C and Fe can enhance the kinetics such that Al₂O₃ gets decomposed into Al₂O and AlO gases and its subsequent reduction into Al (g) with iron saturated carbon. Ar (g) can also act as a carrier gas for aluminum vapors [14, 18] to form either a compound with solid carbon or with liquid iron. The formation of Al (g) has previously been reported in other studies in the presence of carbon saturated iron. [29].

Figure 6:

EDS analysis of AS1–5Fe and AS1–10Fe pellets showing the formation of Al₄C₃ in the carbon-rich regions.

To validate results obtained from the EDS analysis of pellets with increasing amount of iron, an epitaxial mapping was drawn for AS1–5Fe, AS1–10Fe and AS1–15Fe systems [21]. The results are shown in Figure 7(a), 7(b) and 7(c); these reveal that with increasing iron levels in the system, the proportion of secondary product containing Fe and Al had also increased. These images also show that the concentrations of products formed were much higher than in that observed in the sessile drop arrangement due to a much enhanced contact between reacting species [23]. Microdiffraction analysis of these samples showed the presence of additional phases in AS1–5Fe, AS1–10Fe and AS1–15Fe samples (Figure 8(a), 8(b) and 8(c)). This additional phase (identified as cubic aluminum–iron alloys) was present at stand-alone peak positions of 30.840°, 44.187°, 64.258° and 81.289°; these peaks were in addition to alumina and carbon peaks. Aluminum–iron alloy (Fe₃Al) also shared peaks at 26.628° and 54.851° with key graphite peaks. The presence of Fe₃Al phase has provided unambiguous confirmation for the reduction of alumina at 1,823 K.

Figure 7:

Epitaxial mapping of reacted pellets at 44.19° for locating the position of Fe₃Al reaction product [(a) AS1–5Fe, (b) AS1–10Fe and (c) AS1–15Fe].

Figure 8:

XRD pattern of refractory pellets [(a) AS1–5Fe, (b) AS1–10Fe and (c) AS1–15 Fe] after heat treatment at 1,823 K for 30 min showing Fe₃Al as reaction product other than corundum (C) and carbon (G).

The probability of Fe₃Al diffraction peaks belonging to iron peaks was also considered, as the location of some of Fe diffraction peaks was quite close to Fe₃Al peaks in the diffraction pattern. Diffraction peaks of iron (JCPDS No. 085–1410) are observed at peak positions of 44.354°, 64.528° and 81.657°; and JCPDS No. 006–0696 shows diffraction peaks of iron to be at peak positions 44.674°, 65.023°, 82.335° and 98.949°. These Fe peak positions are, however, slightly different from the peak positions observed in our Al₂O₃–C/Fe system, which were located at 26.628°, 30.840°, 44.187°, 54.851°, 64.258° and 81.289°. All these peaks provide an excellent match to the diffraction pattern of JCPDS No. 45–1203 for Fe₃Al. No pure Al or Al₄C₃ was observed in these samples, thereby indicating that both of these were intermediate phases and had later transformed into a more stable Fe–Al phase. These results clearly show that reduced Al had reacted either with molten iron or with solid carbon to form Al₄C₃. The Al₄C₃ phase, however, was not detected in the diffraction pattern probably due to rather its small concentration.

These results show that molten iron has played an important role in the carbothermic reduction of alumina at 1,823 K. The role of iron was further analyzed through off-gas analysis, where interaction with alumina, carbon and iron generated far more CO/CO₂ gases than those produced during a blank run of AS1 substrate [23]. It was observed from Al₂O₃–C interactions that a small part of alumina had changed to aluminum and formed Al₄C₃. In the presence of molten iron, Al (g) formed during the carbothermic reduction of alumina showed a tendency to be absorbed within the molten iron. These findings indicate that gas–liquid reactions were more likely to occur than gas–solid reaction in this system. Reduction reactions are likely to occur via gas transport mechanism between alumina and carbon source. A similar observation has been reported by Heyrmanz et al. and Frank et al. [10, 30]. If AlO and Al₂O gases are formed due to the carbothermic reaction of alumina, subsequent carbon pickup by iron would lead to a further reduction of these suboxide gases into aluminum gas, which is later absorbed by Fe to form Fe–Al alloys.

Both theoretical and experimental results have shown a partial conversion of alumina into aluminum and its subsequent interaction with iron or carbon to form carbides. These studies also show that Al (g) formed had partially reacted with Fe, and molten iron had picked up both Al and carbon during these interactions. Khanna et al. have reported on another aspect, highlighting that C and Fe are among some of most important elements involved in redox reactions [23]. While it is well known that the presence of even small amounts of carbon can affect the kinetics of chemical reactions, it is difficult to predict how the simultaneous presence of two reducing agents will have a significant effect on reduction reactions under conditions wherein carbon is present along with molten iron. Iron also contributes as a solvent metal to lower the temperature for the carbothermic reduction of alumina. This feature is similar to observations on molten tin and copper by Frank et al. [30], which were used as solvent metals to lower the temperature for the carbothermic reduction of alumina.