The effect of additives on Al 2 O 3 SiO 2 SiC C ramming monolithic refractories

MAHT 090263 The effect of additives on Al 2 O 3 SiO 2 SiC C ramming monolithic refractories E. Karamian, A. Monshi and S.M. Siadati* Department of Materials Engineering, Isfahan University of Technology (IUT), Isfahan, 84156-83111, Iran *E-mail: s_m_siadati@yahoo.com ABSTRACT The aim of this study is to describe the effect of containing additives on increasing cold crushing strength (CCS) and bulk density (BD) of Al 2 O 3 SiO 2 SiC C monolithic refractories. Two series of carbon containing monolithics were prepared from Iranian chamotte (Samples A) and Chinese bauxite (Samples B), as 65 wt.% in each case together with, 15 wt.% SiC-containing material regenerates (crushed sagger) 10 wt.% fine coke (a total of 90% aggregate) and 10 wt.% resole (phenol formaldehyde resin) as a binder. Different types of additives (such as silicon and ferrosilicon metal) are added to a batch of 100 g of mixture and the physical and mechanical properties (such as BD, apparent porosity and CCS) are measured after tempering at 200 C for 2 h and firing at 1100 C and 1400 C for 2 h. After low temperature tempering at 200 C, silicon and ferrosilicon contribute to the formation of stronger cross linking in the resulting structure and provides CCS values as high as 65 MPa. After high temperature sintering, at 1400 C, SiC whiskers of nano sized diameter are formed due to the presence of Si and FeSi 2 and increases the CCS values of the refractories as high as 3 4 times in sample containing 6 wt.% ferrosilicon metal as additive, compared to the material without additive. The temperature of 1100 C is a transient temperature, used in high temperature sintering. Keywords: silicon additives, ferrosilicon additives, ramming, monolithic refractories, carbon 1. INTRODUCTION The development of Al 2 O 3 SiO 2 SiC C resin bonded ramming monolithic refractory for iron making applications (such as iron and slag runners in blast furnaces, furnace bottom and electric-furnace spouts) [1] is a very important recent refractory development which can be considered to have evolved from the high performance chamotte carbon and bauxite carbon refractories. This class of refractory not only shows superior slag corrosion and erosion (wear) resistance, but also excellent thermal shock resistance and mechanical properties. Monolithic refractories containing bauxite aggregate has shown higher densification and mechanical properties due to the presence of some impurities such as iron which help liquid phase sintering, and consequently, improved mechanical properties [2]. Probably, the high glassy content of the matrix of chamotte results in high surface tension and higher amounts of needles of mullite in chamotte, and has caused more nucleation of SiC whiskers at high temperature, consequently, resulting in improved strength and higher density [3]. These refractories consist mainly of aluminosilicate and carbon, which are bonded by carbon bonds formed by carbonisation of phenol formaldehyde-resin (resole) during firing of the refractories [1,3]. Carbon does not wet molten metal and will not melt, that is an advantage excellent for refractory use. Its great weakness is that it will oxidise at low temperatures, above 600 C, which creates high porosity, leading to lower density and reduced strength [1,4]. In order to improve the oxidation resistance of carbon-containing refractories, antioxidants (such as aluminum and silicon metal) have been often added [5]. However, by examining the behaviour of Al in such refractories, the following problems have been observed: (1) Low melting point Al particles, gives decreasing refractoriness; (2) the reaction of Al with water, therefore Al cannot be used in carboncontaining monolithic refractories in which watercontaining binders are used. Based on the above consideration, it is necessary to develop new antioxidants with excellent hydration resistance and high melting point, such as Si-containing antioxidants. Mostly, metallic silicon and aluminum are added to the material as a sintering agent to improve the mechanical strength and abrasion resistance under oxidation. The effect of metallic silicon grain size on the properties of alumina carbon brick has been studied, and it is reported that the added metallic silicon reacts under the reducing atmosphere with carbon in the brick and produces b-sic bond which contributes to improvements of the mechanical strength and the abrasion # 2009 Science Reviews 2000 Ltd doi: 10.3184/096034009X12570016241884 MATERIALS AT HIGH TEMPERATURES 26(4) 000 000 1

? Figure 2 Effect of additives on chamotte based cured at 200 C for 2 h. (Samples A). linear polymer cross-linked polymer Figure 1 Linear polymer and cross-linked polymer. resistance under oxidation. The influence of b-sic bond on the mechanical strength has yet to be clarified [6]. Generally, the modulus of rupture vs. firing temperature is increased at a given temperature (1100, 1200, 1300, 1500 C) by increasing the grain size of silicon up to more than 100 microns, but the elastic modulus is decreased. The amount of SiC formed decreases and the remaining Si increases for coarse particles. It is concluded that the strength and durability of the slide gate plates can be improved, not by increasing the amount of silicon metal, but by decreasing the grain size to enhance silicon reaction. The results of other workers show that SiC additions in the range from 4 to 16% are beneficial to the improvement of mechanical properties as well as the thermal shock resistance [7]. From the investigation of the thermodynamics and from observation of the results after use, it is evident that the SiC materials have a large influence on oxidation prevention. SiC behaviour in Al 2 O 3 SiC C bricks is illustrated in reactions below [8]. SiCðsÞþCOðgÞ ¼SiOðgÞþ2CðsÞ SiOðgÞþCOðgÞ ¼SiO 2 ðsþþcðsþ ð1þ ð2þ The most successful and commonly used ramming mixes were based on phenol-formaldehyde resins. The cured strength of these products was high because of polymerisation of the resin, which is called a resit structure, at about 200 C. At elevated temperatures, however, the products become weak because of the oxidation in the carbonaceous bond phase, which creates high porosity, lower density and reduced strength [1]. Advantages of a resole binder is that it is environmental friendly, easy to mix, with high residual carbon. A disadvantage of a resole binder is the pyrolization of the resist structure, that is called polymer carbon, at temperatures about 300 500 C, and the products became weak (Figure 1). The usage of additives such as silicon and ferrosilicon is supposed to reinforce the resit structure by increasing the cross linking. The cross linking phenomenon was discovered by Charles Goodyear in 1839 and involved heating the raw rubber with sulfur (the Vulcanisation process). The mechanism appears to involve addition of sulfur units to the double bonds in the rubber with the production of cross links between the polymer chains [9]. Cross-linking reactions in general are those that lead to the formation of insoluble and infusible polymers in which chains are joined together to form a three dimensional network structure. A sample cross linking reaction is exemplified by polymer chains with several functional groups, designated A, that are capable of reacting among themselves to form chemical bonds, A22A, (Figure 2). If these polymer chains are exposed to conditions such that the functional groups do react, then all the chains in the reaction vessel will be tied to each other through A 2A bonds [10]. In principle, the polymer molecules in the reaction vessel will have formed giant molecules. Crosslinking changes and generally improves physical properties. A chemical reaction that leads to formation of a chemical bond can be used for cross linking of polymers if the requirements of effective average functionally per molecule are met. Phenol-formaldehyde resins including resoles and novolacs are technologically useful cross-linking systems. Cross linking agents must carry two or more functional groups per molecule [10]. 2. MATERIALS AND METHODS Two series of compositions were prepared. The first composition (samples A) containing as aggregates a combination of 2 MATERIALS AT HIGH TEMPERATURES www.scilet.com

Table 1 The chemical composition of raw materials (wt.%) Material Al 2 O 3 SiO 2 CaO MgO TiO 2 SiC C Si Fe S Grain size (mm) Chinese bauxite 85 6.9 0.15 0.1 3.9 0 1,1 3 Iranian chamotte 42 53 0.5 0.5 0 3 Crushed sagger 45 15 38 0 3 Fine coke 10 5 80 0 0.3 Ferrosilicon metal 2 72 23 0 0.15 Silicon metal 1.0 97 1.0 0 0.15 Silica fume 92 0 0.1 Carbon black 98 0 0.1 Sulfur powder 97 0 0.3 chamotte (0 3 mm, 65 wt.%), crushed sagger which is a SiC-containing material regenerates (15 wt.%) and fine coke (10 wt.%) with different amounts of additives such as Silicon and ferrosilicon metal. The second one (samples B) containing as aggregates a combination of bauxite (20 wt.% coarse bauxite (1 3 mm), 45 wt.% fine bauxite (0 1 mm)), crushed sagger (15 wt.%) and fine coke (10 wt.%) with different amounts of additives such as Silicon and ferrosilicon metal. At low temperature of curing (200 C), three other additives such as sulfur, carbon black and silica fume are also used. Since silica fume and carbon black were not so effective in increasing of CCS values, they were eliminated from higher temperature tests (1100 C and 1400 C). Chamotte, bauxite and sagger are used to increase the strength of the refractory. Coke is used to lower the wettability with the iron or steel melt. Both series consisted of 10 wt.% resole (phenol formaldehyde resin) as the binding agent. The chemical compositions and phase contents of the raw materials are given in Tables 1 and 2 respectively. Specimens were prepared by mixing 90 wt.% aggregate and 10 wt.% resole as a binder with different amounts of additives on top of 100 wt.%. A twin blade mixer was used for dry mixing and kneading. The raw materials were dry mixed for 5 min and then the resole liquid was added gradually. Mixing continued for another 5 min. The mixture was packed into metal moulds with a dimension of 50650650 mm and then formed by using a hydraulic press under a low pressure of 4 MPa, to act similar to ramming. Pressed samples were cured at 200 C for 2 h. Finally, some of the samples were fired for 2 h at either 1100 C or 1400 C in a coke environment to avoid an oxidising atmosphere. The specimens cured or fired were characterised according to ASTM standards. The following tests were carried out: bulk density (BD), apparent porosity (AP), cold crushing strength (CCS), also X-ray diffraction (XRD). Scanning electron microscopy (SEM) were performed in some cases. CCS was measured by using a Hydraulic Testing Machine, type Amsler model D3010y2E. BD and AP were determined by using the following equations: BD ¼ WyW f AP ¼ðW s WÞyW f where W, W s and W f respectively are the weight of dried sample, saturated weight with water and weight of the sample in water suspended by a thin thread without contacting the vessel walls, but fully inside the water and reading by a digital balance under the vessel. In fact W f is the weight reduction of the saturated sample in water, which is equivalent to its volume. An XRD instrument (Philips Xpert-MPD System) with Cu K a1 (l 1 ¼ 1:5405 Å) and Cu K a2 (l 2 ¼ 1:5443 Å) and Ni filter was used for phase analysis. A voltage of 30 kv and current of 25 ma were employed. In order to study precisely the phases, the XRD experiments were carried out with a rate of 0.04 degreesysec between 5 80 degrees (2Y). SEM Philips XL30 was used for microstructure investigations. X-ray fluorescence (XRF) was also used for elemental analysis and oxides content calculations (Table 1). 3. RESULTS 3.1 Cold crushing strength and bulk density results Figures 2 5 show the influence of additives on CCS of resole bond, Al 2 O 3 SiO 2 SiC C refractory. Figures 6 8 illustrate BD variations. ð3þ ð4þ Table 2 Phase analysis of raw materials (XRD results) Material Major phase Medium phase Minor phase(s) Chinese bauxite aal 2 O 3 Mullite(3Al 2 O 3.2SiO 2 ) TiO 2,SiO 2 Iranian chamotte Mullite(3Al 2 O 3.2SiO 2 ) Cristobalite (SiO 2 ) Crushed sagger asic Mullite(3Al 2 O 3.2SiO 2 ) a-al 2 O 3 Fine coke Graphite Al 2 O 3,SiO 2 Ferrosilicon metal FeSi 2 Si Silicon metal Si www.scilet.com MATERIALS AT HIGH TEMPERATURES 3

Figure 3 Effect of additives on bauxite based cured at 200 Cfor 2 h. (Samples B). Figure 7 Effect of additives on chamotte and bauxite based fired at 1100 Cfor2h. Figure 4 Effect of additives on bauxite and chamotte based fired at 1100 C for 2 h. Figure 8 Effect of additives on chamotte and bauxite based fired at 1400 Cfor2h. Figure 5 Effect of additives on bauxite and chamotte based fired at 1400 C for 2 h. Figure 6 Effect of additives on chamotte and bauxite based cured at 200 C for 2 h. Figure 9 Micrograph of chamotte carbon refractory resole bond containing 6 wt.% ferrosilicon metal as additive, fired at 1100 C, where small quantities of SiC whisker are observed. 4 MATERIALS AT HIGH TEMPERATURES www.scilet.com

3.2 Microstructure Figures 9 and 10 show fired microstructures of 1100 C firing for 2 h. Figures 11 and 12 show fired microstructures of 1400 C for 2 h. 3.3 Phase compositions Phase compositions by XRD results are given in Figures 13 to 16. 4. DISCUSSION Figure 10 Micrograph of bauxite carbon refractory resole bond containing 5 wt.% silicon metal as additive, fired at 1100 C, which shows that no SiC whiskers are formed. Figure 11 Micrograph of chamotte carbon refractory resole bond containing 6 wt.% ferrosilicon metal as additive, fired at 1400 C, showing SiC whiskers of about 75 nanometres in diameter in the microstructure. Figure 12 Micrograph of bauxite carbon refractory resole bond containing 6 wt.% ferrosilicon metal as additive, fired at 1400 C, which shows SiC whiskers are developed from ferrosilicon metal particles in the microstructure. The effect of additives on sample A containing Iranian chamotte as the main aggregate (65 wt.%) is such that silicon metal and ferrosilicon metal generally increase cold crushing strength at all heat treatments of 200 C, 1100 C and 1400 C and also increase bulk density. Sample B based on Chinese bauxite also shows the CCS increases at 200 C and 1400 C, but the CCS is reduced at 1100 C. SEM photomicrographs have shown that SiC whiskers, of nano sized diameter, have developed in sample A, but not in sample B at 1100 C. The reason for the increase of strength in sample A, which has not been clarified in some previous studies [5], is now evident and is related to the development of nano-sized whiskers of SiC within the microstructure. The reason these develop only in sample A, may well depend on the higher amount of mullite and glassy phase in chamotte, when compared bauxite. Needles of mullite in a glassy matrix phase provides good nucleation sites from which to growth SiC whiskers at the low temperature of 1100 C. At 1400 C, the thermodynamic driving force is high enough for the production of SiC whiskers in both samples A and B. But the concentration of whiskers at a comparable level of 6 wt.% ferrosilicon metal (FeSi 2 ) is more for the chamotte matrix (Figure 11) than for the bauxite matrix (Figure 12). It seems that these whiskers are formed on chamotte aggregates as can be seen in Figure 11, but for the bauxite matrix the whiskers are nucleated on the FeSi 2 particles, spherical particles seen in Figure 12, formed on the outside of the bauxite particles. These figures support the hypothesis that SiC whisker formation nucleates on mullite needles, formed in the chamotte aggregates, due to the surface tension of the glassy matrix. The reason for the strength increase at high temperatures is evidently related to the formation of SiC whiskers. However at 200 C the mechanism for CCS increase cannot be related to SiC whisker formation but is related to cross linking formation in the phenol-formaldehyde resole structure or resit. For this temperature, 5 different additions of silicon metal, ferrosilicon metal, sulfur, silica fume and carbon black were used. The first two were capable of increasing the cross linking, but the other three were not so effective (see Figures 2 and 3). The Si has a strong effect, when in a small amount of 1%, due to the large number of covalent bonds related to pure silicon. The effect is reduced for 2% and above, probably due to an interfering effect of the Si atoms. By considering Figure 1, it is probable that formaldehyde (CH 2 O), which acts as cross linking agent of A for the main monomer of phenol (C 6 H 5 OH), is interfered with when a lot of Si atoms are present. Each is associated www.scilet.com MATERIALS AT HIGH TEMPERATURES 5

with covalent bonds, at the expense of, and hence decreasing, the cross links of the phenol-formaldehyde monomers. Formaldehyde is very reactive and readily polymerises with ionic initiation [11]. Either anionic or cationic materials can initiate the polymerisation reaction. R þ CH 2 O?RCH 2 O ; ðr O ¼ CH 2 Þ þ R þ CH 2 O?RCH 2 O þ ðr O þ ¼ CH 2 or R O CH þ 2 Þ ð5þ ð6þ Figure 13 XRD results of fired chamotte carbon refractory, resole bond. (1400 C, 2 h). Figure 14 XRD results of fired bauxite carbon refractory, resole bond. (1400 C, 2 h). Ferrosilicon is less effective than silicon and shows a maximum CCS at 2% and reduces the strength for 3% and above. The mechanism of the effect is the same as for silicon. Micro silica or silica fume (small amorphous spherical particles of SiO 2 ) and carbon black, amorphous carbon, do not contribute to a stronger cross linking, probably due to them having an amorphous and non crystalline nature and the submicron size of these amorphous particles. Instead, they have an interfering effect and reduce CCS. Almost the same effects are observed when these additives are used in bauxite based refractories of sample B (Figure 3). Results for samples A and B provide confirmation to each other. The role of sulfur, in increasing of CCS, was good to a degree (Figures 2 and 3), but was not as effective as Si or ferrosilicon. At high temperature, above 800 C, the resit structure is destroyed and graphite phase, as a crystalline carbon state, is formed. XRD results show higher peaks of the graphite phase at 1400 C firing (Figures 13 and 14) when compared to 1100 C firing (Figures 15 and 16). 5. CONCLUSIONS Figure 15 XRD results of fired chamotte carbon refractory, resole bond. (1100 C, 2 h). Figure 16 XRD results of fired bauxite carbon refractory, resole bond. (1100 C, 2 h). 1. The addition of silicon or ferrosilicon metal to aluminasilicate-carbon ramming monolithics, with resole bond, for Iron making applications, improves mechanical properties and develop a high strength and therefore more wear resistance for applications such as blast furnace runners and troughs. 2. For low temperature heat-treatments, of 200 C, these additions contribute to the formation of the resit structure in resole, by increasing the cross linking, and provides CCS values as high as about 65 MPa, which is remarkable for a carbon containing monolithic. 3. There is a limit of 1 wt.% for silicon, and 2 wt.% for ferrosilicon, to increase the cross linking phenomena and thus raise the strength of the refractory. Beyond this the additions interfere with the formaldehyde hence the effectiveness of cross linking is reduced and the strength is lowered. 4. Three additives, silica fume, carbon black and sulfur, were not so effective as compared with silicon and ferrosilicon metal. 5. SiC whiskers of nano-sized diameter are observed to form at 1400 C in both samples containing chamotte and bauxite. At 1100 C these whiskers are only observed in the chamotte based refractory. It is proposed that probably the higher glassy content of the matrix of chamotte has caused a higher surface tension. This, 6 MATERIALS AT HIGH TEMPERATURES www.scilet.com

together with the higher amounts of needles of mullite formed in chamotte when compared to bauxite, has caused more nucleation of SiC whiskers. The temperature of 1100 C is a transient temperature, where any cross linking is destroyed yet SiC whisker formation is not fully formed. REFERENCES [1] Bosall, S.B. and Henry, D.K. (1985) New developments in monolithic refractories. The American Ceramic Society, Westerville, OH, Vol. 13, pp. 331 340. [2] Zawrah, M.F. (2007) Effect of zircon additions on low and ultra-low cement alumina and bauxite castables. Ceramics International, Vol. 33, no. 5, pp. 751 759. [3] Chan, C.F., Aregent, B.B. and Lee, W.E. (1988) Influence of additives on slag resistance of Al 2 O SiO 2 SiC C refractory bond phase under reducing atmosphere. J. Am. Ceram. Soc., No. 81, pp. 3177 3188. [4] Parsons, J.R., Forest, P. and Rechter, H.L. (1974) Refractory composition comprising coarse particles of clay or bauxite and carbon. U.S. Patent 3 810 768. [5] Yamaguchi, K., Zhang, S. and Hashmato, J.YU. (1991) Behavior of antioxidant added to carbon-containing refractories, J. Ceram. Soc. Japan, pp. 341 348. [6] Akamine, K., Suruga, T., Yoshitomi, J. and Asona, K. (2006) Effect of metallic silicon size and heat temperature on properties of Al 2 O 3 C brick. Internationals Feuerfest-kolloquium, Aachen. [7] Fangbao, Ye, Rigaud, M., Liu, X. and Zhong, X. (2004) High temperature mechanical properties of bauxite-based SiC-containing castable. Ceramics International, Vol. 30, pp. 801 805. [8] Nishio, H. and Matsuo, A. (1995) Al 2 O 3 SiC C Bricks for Torpedo Car, Taikabutsu Overseas, Vol. 15, No. 4, pp. 39 46. [9] Streitwieser, Jr. A. and Heathcock, C.H. (1981) Introduction to organic chemistry, second edition. Macmillan Publishing Co. Inc., USA, pp. 595, Chapter 20 (conjugation). [10] Kroschwitz, J.I. (1990) Concise encyclopedia of polymer science and engineering. John Wiley & Sons, Inc., Cross linking, pp. 213 219. [11] Ibid, page 4. www.scilet.com MATERIALS AT HIGH TEMPERATURES 7