Technical Paper THE ADVANTAGE OF CALCIUM ALUMINATE CEMENT CONTAINING CASTABLES FOR STEEL LADLE APPLICATIONS

Transcription

1 Page : 1/12 THE ADVANTAGE OF CALCIUM ALUMINATE CEMENT CONTAINING CASTABLES FOR STEEL LADLE APPLICATIONS by Christopher Parr, Li Bin, Benoit Valdelièvre, Christoph Wöhrmeyer, Bruno Touzo Presented at the XXXII ALAFAR meeting, November 2004, Antigua, Guatemala

2 Page : 2/12 ABSTRACT The substitution of bricks by monolithics in steel ladles in Europe as well as Asia has been well documented 1,2. The main drivers behind this substitution are generally considered to be: Reduction of overall specific refractory costs ( material, installation and maintenance) per tonne of steel cast Improved lives and decreased wear rates to enhance steel cleanliness and improve plant flexibility via better ladle availability Reduced environmental impact with endless repair techniques, there is a lower volume of spent refractories for disposal. The weight of these individual drivers varies significantly according to geographic area, country and even steel plant. It is clear that the high rate of penetration of monolithics into steel ladle applications has occurred through the application of a complete solution rather than an individual product offer. It is only by considering the whole refractory usage chain that the full value potential of the monolithic substitution can be realised. Recent ladle lining trends 3,4 are based on alumina castables that contain magnesia and rely on a Calcium Aluminate Cement (CAC) bonding system. This has only been possible due the widespread adoption of a solution approach. The relatively high raw material costs of these castables compared to bricks would have prohibited their use if the focus had been purely material cost. In addition, the full potential of a monolithic solution can only be realised if continuous repair techniques are utilised. The original lining could either be large precast blocks or cast in situ but subsequent repairs by casting or shotcreting must be in situ. This requires the ability to install castable/concrete under field casting conditions. Magnesia containing castables can be conceived either by adding a pre-formed alumina-magnesia spinel aggregate or by the reaction of MgO with Al 2 O 3 to form in situ spinels within the castable matrix. Both types of system are being used widely in steel ladles in many countries around the world 5,6. These two systems present unique challenges in terms of formulation control particularly with reference to placing properties, sufficient to allow field installation, and thermo mechanical behaviour. Calcium aluminate cement (CAC) is the only bonding system which is able to meet all these challenges. CAC assures two key functions; a binder to guarantee a defined working, setting, and de-moulding time as well as an important role in the development of corrosion resistance at high temperature. This paper compares two such magnesia castables (spinel containing and in situ) based on calcium aluminate cement bonding systems with alternative bonding systems. Each aspect of the usage chain is investigated from placing through dry out to in service aspects such as corrosion resistance. The relative advantages of each type of Alumina-Magnesia or Spinel system will be discussed. It will also be shown that in comparison to CAC based systems, no other binder system is at present able to offer the same ability to harden under field conditions as well as provide a high degree of corrosion resistance in ladle lining applications.

3 Page : 3/12 1 Experimental methods Magnesia Containing Castables 1.1 Raw materials and basic formulations "preformed" "in situ" The following raw materials were used for the study and were sourced largely from Chinese suppliers. Raw Material White fused corundum Sintered M-A spinel Sintered magnesia Reactive alumina Fume silica SM1/ Trimeric cyanamide sulfonic acid formaldehyde condensation product Source San Mexia Corundum Cy (China) Liaoning Hai Cheng Hua Yin (China) Hai Cheng, Liaoning (China) Kaifeng Special (China)/B3M12-D Elkem/971U Suzhou Xinlong Corp. (China) The fundamental differences between the two systems are shown in figure 1. It is seen that the two types of magnesia containing castables follow two different formulation logics. The alumina spinel castables are similar in conception to an LC castable whilst the spinel forming castables tend to follow an ULCC formulation logic. A deflocculated bond system is however at the heart of both castable types. The base model compositions for Alumina-Spinel castables are found in Table 1. The function of the Alumina-Magnesia spinel aggregates, in limiting slag ingress due to trapping of Fe and Mn in the spinel lattice, has been well documented 6. The quantity and type of spinel chosen in this example is typical of those values found in the literature 3. Type : Alumina - Sp Al 2 O 3 + Al 2 O 3. MgO Alumina grain and filler CAC Reactive and calcined alumina Additives Type : Alumina - MgO Al 2 O 3 + MgO Alumina grain and filler Magnesia CAC Reactive and calcined alumina (Silica fume) Additives Fig. 1 Formulation logics of magnesia containing castables. These compositions are silica free to ensure that no liquid phase products are formed at high temperature. The base model composition for Alumina-Magnesia castables can be found in table 1. The formulation basis is a ULCC system with smaller or trace amounts of fume silica. The spinel phase is formed in situ and since it is produced by reaction in the matrix it has been found to offer a high degree of corrosion resistance in certain ladle applications. This type of formulation approach presents three inter linked challenges which need to be addressed for reliable application. Impact of magnesia upon castable rheology which leads to short working times and rapid flow decay 7. Risk of magnesia hydration and subsequent destruction of the castable 8. +8% volume expansion when spinel forms in the castable matrix 9. These three issues are somewhat related. For example fine reactive silica (fume or precipitated) is sometimes incorporated into these castables to fulfil several functions. It is primarily added to control thermal expansion 4, through the idea of auto stress relaxation with liquid phase formation

4 Page : 4/12 to compensate for the expansive reaction during spinel formation. Secondly, fume silica facilitates rheological control and thirdly it has been suggested as a means to control magnesia hydration. In addition to the base Alumina-MgO and Alumina -Spinel systems three mixed systems containing both Sintered Alumina Spinel as well as Magnesia have been formulated. The ratio of Spinel to Magnesia (Nb. 3) has been varied as well as the relative fineness of the Spinel and Magnesia components (Nb. 1 and 2). Table 1: Basic model formulations The additive systems were held constant for each system. White fused corundum AlSp Sp_1 Sp_2 In order to provide a comparison with the CAC containing systems detailed in table 1. Comparative tests have been run with a cement free system based on a Magnesia Silica Hydrate bonding system. The intrinsic disadvantage of these systems due to their relatively high levels of silica and the negative Sp_3 3-5mm mm mm mesh 6 2 2, mesh 8, ,5 8,5 M-A Spinel( MA-76) 0-1mm mesh Fume Silica 0,5 0,5 0,5 0,5 Chinese sintered MgO 0-1mm mesh Reactive Alumina B3M-12D Calcined Alumina CA Secar Additive +0,20 +0,20 +0,20 +0,20 +0,20 impact this has on steel cleanliness is not considered. The main purpose of the comparison is to compare basic placing properties as well as usage characteristics linked to the durability of each system. These CAC free systems were elaborated according to literature references 10,11 with the same Aluminous aggregate being used as for the CAC based castables. The additive systems were based on mixtures of Boric acid and sodium tripolyphosphate 1.2 Experimental methods Tests were performed according to national Chinese standards. - The flow value was measured with a vibration table at different time intervals; the cone size is Φ70/100mm x 60 mm high. The working time is the time when the vibration flow value of the specimen is lower than 20% - Cold modulus of rupture, cold compressive strengths were measured on 40x40x160 mm specimens according to Chinese standard GB/T and GB/T ; - Bulk density and apparent porosity were measured on 40x40x80mm specimens according to Chinese standard GB/T Hot modulus of rupture was measured on 40x40x160 mm specimens according to Chinese standard GB/T Slag resistance test by static crucible method. Crucible dimensions are mm with a Φ25 25mm hole. After drying at 110 C 24h, 18g slag was put in the hole. The crucible with slag was put in the electro-furnace and heated at the rate of 300 C/h to the test temperature of 1580 C

5 Page : 5/12 2 Experimental results Table 2: Basic physical properties NCC AlSp Sp_1 Sp_2 Sp_3 Water % 5 5 5,2 5 5,3 5,3 Vibration flow % t=0 min t=30 min t=60 min As cast : 24 hours ,5 71,3-107, , ,5 87,5 67,5 CCS : MPa <4, ,5 6, ,25 MOR : MPa <0,8 3,3 4,3 2,7 3,3 3,4 Dried Properties 110 C CCS : MPa ,5 57,8 26,56 40,63 37,5 MOR : MPa 6,8 11,6 >14,0 11,1 >14,0 12,3 BD 110 C g/cm3 AP 110 C % Hot Properties, HMOR 400 C MPa 1100 C MPa 1400 C MPa <3,3 14 5,5 5-7 <1,6 3,22 12,2 9,2 11,7 3,13 14,0 13,2 16,1 13,0 3,16 1 8,9 9,9 2,0 3,21 11,6 8,3 8,8 2,7 3,3 12,0 8,3 11,7 2.1 Placing Properties Placing properties of a castable can be assessed with the measurement of its flow at 30 minutes which gives a good image of its capacity to be cast in situ, and by its cold compressive strength at 24 hour which shows if field demoulding is feasible. A minimum flow value of 60% at 30 minutes is essential to achieve a good placing by vibration. Minimum modulus of rupture of 1.5MPa and compressive strength of 5 MPa are necessary for demoulding. It is clear from the data shown in Fig 2. that the NCC system, although presenting a decent ability for vibration placing, does not develop enough strength in order to achieve an acceptable field demoulding. CCS 24 hr (MPa) CAC containing systems Flow at t30 (%) Fig 2: Placing and hardening properties for different ladle castable systems This is all the more true in that this data has been generated at average ambient temperature (20 C). At low temperatures, this problem would be more acute and any field demoulding would be impossible. By contrast, all the calcium aluminate cement systems exhibit good flowability and excellent strength development, which allow field casting and demolding, even at low ambient temperatures. In comparison with no cement castables, they are the best compromise between placing and hardening properties. 2.2 Dry out of Castables The dry out of CAC containing castables is one area where careful attention needs to be made to the choice of raw materials. At the start of the drying process, the free water (i.e. that which is not combined via hydrates) needs to be removed from the pores. Thereafter there are a series of dehydration reactions. During the heating, the microstructure of the bond phase undergoes various transformations : 1. On heating from ambient temperature to 100 C, the cement hydrates formed initially are converted to the stables hydrates AH 3, and C 3 AH 6, with a release of free water. 2. Between 100 and C, AH 3 and C 3 AH 6 decompose to give amorphous anhydrous relics and water vapour. This water vapour has to escape from the concrete. 3. Above C, the relics of the cement phases and elements of the bonding phase react together to form at high temperature a dense, sintered ceramic. NCC

6 Page : 6/12 These transformations give rise to changes in the macroscopic properties of the castables. It is noteworthy that despite the structural modifications at low temperature the monolith keeps its integrity with sufficient strength, this can be seen in figure 3. The CAC bonded systems all main a significantly higher mechanical strength at intermediate temperatures. This will help to reduce stress within a ladle lining which is normally subject to a large thermal gradient. There is an apparent risk with the NCC systems in that fractures induced by stress at the weakest point (i.e. the point in the lining which attains 800 C) can occur. This leads to cracking and spalling behind the hot face to or in the most extreme cases slabbing and catastrophic failures in service. At 800 C, all hydrated phases have disappeared but sintering has not started. The hot modulus of rupture (HMOR) stays largely unchanged compared to value at 40 C. Above this temperature, sintering begins with Alumina reacting with the decomposed cement hydrates. CCS : MPa NCC NCC Sp_2 Weak point Temperature C Figure 3: Mechanical resistance of ladle castables as a function of temperature Pre-reacted spinel containing castables (AlSp) can be handled as other high alumina castables during this drying and heating period. In case of magnesia-containing mixes it is important to remove both the combined and hydrated water as quickly as possible. Otherwise MgO-hydration can damage the structure of the castable through a destructive expansion mechanism. It is believed that the CAC plays a part in this mechanism and therefore the quantity of CAC should be minimised. The choice of the right type and level of MgO can minimize that risk. After installation, the LCC can contain up to 10% water, most of it being combined in calcium aluminate hydrates. During the dry-out, these hydrates decompose above 100 C. There is restricted space available due to the low porosity, for steam that is generated and its escape is hindered by the low permeability of the LCC. A vapour pressure builds with increasing temperature, which facilitates the hydration of MgO. The maximum vapour pressure that the MgO can be submitted to is limited because an excessively high pressure would lead to spalling. This can happen if the heating rate is too large. Because of the complexity of this system, the parameters controlling the temperature/pressure generated in the castables and the maximum internal pressure that it can stand are not known accurately. There are some standardised tests to define de spalling resistance (JIS R ), however they are very empirical and do not give any idea of the maximum vapour pressure. Some values reported in the literature range between 0.7 to 1.35 MPa. These values have been estimated by simultaneous modelling and experimental measurements. The stability domain at atmospheric pressure of brucite Mg (OH) 2 is limited to 400 C, beyond which, it decomposes to MgO + H 2 O. At higher pressure, the temperature of decomposition increases and can move above 500 C. During the dry-out, the temperature (between 100 and 350 C) and pressure conditions within the castable, falls in the stability domain of brucite Mg(OH) 2, the conditions are then favourable for its formation. The kinetics of the hydration depends strongly on the type and origin of the magnesia. Caustic magnesia hydrates rapidly, even at room temperature. Dead burnt magnesia hydrates very slowly. Various types of commercial products with surface treatment that reduces hydration are also available.

7 Page : 7/12 Standard tests have been designed to evaluate the risk of hydration of magnesia (JIS R 2212). In these tests, the sample is placed in an autoclave under hydrothermal conditions. The conditions are set so that the liquid/vapour equilibrium is reached. The pressure of the test is then related to the temperature. The standard pressures used are 0.29 or 0.49 MPa. These pressures are lower than the maximum estimated internal pressure in castables. The quantity of brucite formed increases with time until the stresses generated by the expansion are greater than the tensile strength of the castable. At this point the matrix containing the MgO grain disintegrates due to large scale cracking and the integrity of the castable is lost. Hydration of magnesia progresses successively through the grain boundaries leading to the separations of the crystals and the rupture of the castable matrix. In special tests 12 it was shown within an system that magnesia properties like lime/silica ratio, surface morphology, and crystal size e.g. play a decisive role in hydration resistance 12. The results are summarised in figure 4. Different magnesia are tested in a pressure chamber at 150 C and 5 bars. The amount of Magnesia hydrate (brucite Mg(OH) 2 ) that is formed can be measured by XRD. Mg(OH)2/Al2O3 2,00 1,60 1,20 0,80 0,40 0, CaO + SiO2 % of Magnesia Fig 4: Hydration resistance of selected magnesia types The difference in hydration resistance is shown with the higher purity magnesia types (lower impurities) showing generally lower hydration resistance. The choice of magnesia would also be made after due consideration of the impact on spinel formation, microstructure and thermal expansions. Thus, the situation becomes one of managing an inevitable compromise with conflicting demands rather than optimising a single parameter. Some of the more resistant magnesia types are shown in table 3 below. Low purity magnesia types, produced in a rotary kiln seemed to be particularly effective at resisting hydration. Amongst others a high purity, caustic magnesia (CCM) with a high surface area (~20m 2 /g) also showed excellent hydration resistance. Table 3: Hydration resistance of castable with selected types of magnesia MgO % C/S-ratio Sample after 5 h, 150 C, 5 bar DBM Intact HPS 99.4 >5 Intact CCM 97 3 Intact NCM Intact DBM Intact DBM = Dead burned magnesia, HPS = High Purity Surface coated Magnesia, CCM = Caustic magnesia, NCM = Chinese 90% Magnesia, DBM2 = Dead Burned Magnesia all (<0.045 mm) Despite these investigations, the quantity of MgO that can hydrate without damaging the LCC is not known. Therefore, a series of experiments were performed where magnesia was incorporated in increasing amount in an alumina-spinel LCC containing 6% cement (Secar 71). To simulate the vapour pressure conditions, a 3 cm cube sample were placed in large autoclaves at pressures up to 2MPa (T ~ 210 C) for 5 hours, which is greater than the maximum pressure suspected to be generated in LCC. In order to achieve the expected effect on the LCC, a true castable formula with all the aggregates must be used to ensure that the

8 Page : 8/12 particle size distribution of the MgO grain within the microstructure is as close to reality as possible. To obtain an accurate value, it is also important that all the MgO hydrates. A caustic MgO was used and lightly burnt at 1450 C (direct incorporation is not possible as it hydrates partly at room temperature). The hydration degree was verified by increasing the treatment time to see that the hydration did not evolve after 5 hours. The particle diameter was less than 40 µm. For the tests at 0.5 MPa, the sample integrity was checked using an ultrasound technique and standard compressive strength measurement. It was found that there is no visible damage to the sample until a given threshold concentration of MgO. Above this value, the sample crumbles. For higher temperature, the sample was then only controlled visually. The results of these experiments are shown in Figure 5. A very small amount (0.15%) of hydration is sufficient to lead to the destruction of the LCC. The threshold value is slightly lower at low pressure: this is probably due to a more intense hydration of the MgO grains. Pressure / MPa 2,5 2 1,5 1 0,5 No Slaking 0 0 0,5 1 1,5 2 2,5 3 Figure 5. Slaking conditions for a MgO containing castable M a x. M g O c o n t e n t MgO in castable wt(g/kg) Slaking The threshold limit will depend on the castable type, but will probably be of the same order of magnitude for all low cement castables as the microstructure is globally similar and mainly defined by achieving the highest compacity of the particles. These results show that when designing a spinel forming LCC castable, the selection of the MgO raw material is important. For the dry-out process in ladles it is important, not only to increase the temperature fairly rapidly but also to create airflow. The water-saturated air must be removed from the surface of the castable to create a concentration gradient, which accelerates the speed of the dry-out. A rapid dry out and forced air will tend to ensure that the Magnesia doesn t hydrate. It is the presence of hydrothermal conditions (high temperature and pressure) inside the castable for prolonged periods that can cause the destructive magnesia hydration reaction to take place. Additionally, the risk of water/steam explosion can occur especially if the temperature increase is too steep. The design of the heating curve is an important quality parameter for the performance of the monolithic ladle lining especially in the case of MgO containing systems where there is a susceptibility of the magnesia to hydrate. Providing these steps are taken no problems should be experienced with the dry out of Magnesia containing castables. 2.3 High temperature properties After the drying process, increasing temperatures lead to a re-crystallization of the calcium aluminates. The main phases, which are formed during heating up to 1100 C, are calcium monoaluminate (CA) and calcium dialuminate (CA 2 ). In magnesia containing castables the spinel formation starts simultaneously. The periclase reacts with corundum - added as reactive alumina and forms MgAl 2 O 4 -spinel (MA). This in-situ created MA-spinel forms the matrix together with the calcium aluminates and alumina phases. The matrix transformation is accompanied by a volume expansion. As long as this is not too high, it has a positive effect. It counteracts the normal small shrinkage of a

9 Page : 9/12 castable. The expansion due to in-situ spinel formation can be adjusted by the addition of a small amount of fume silica. Further reactions of alumina with CA 2 lead to the formation of calcium hexa aluminate (CA 6 ) which forms, together with the micro-crystalline spinel, a very dense structure. In case of castables with pre-reacted spinel, the reaction of calcium aluminates from CA to CA 2 and CA 6 remains the same. The prereacted spinel can only incorporate a certain amount of alumina to form alumina richer solid solution. It appears from the following graph (figure 6), that the castable systems can be classified in 3 different classes according to their HMOR values: HMOR : MPa NCC AlSp Sp_ Temperature C Sp_1 Sp_3 Figure 6: HMOR of Alumina-Magnesia type castable systems The NCC class shows the lowest performance, the castables all exhibit similar intermediate performances and the AlSp systems show very good temperature strengths which make them the appropriate choice for high abrasion resistance in ladle floors, where high mechanical resistance to impact is critical. Another characteristic of castables is the evolution of their dimensional variations with temperature. As can be seen on the following graph (figure 7), AlSp systems are virtually volume stable whereas systems exhibit significantly positive dimensional variations with temperature. Because of this behavior, they are therefore under compression, which makes them quite appropriate for ladle linings applications. PLC : % 4 3 2,5 2 1,5 1 0,5 0 NCC AlSp Sp_1 Sp_2 Sp_3-0, Temperature C Figure 7: PLC of Alumina-Magnesia type castable systems 2.4 Corrosion resistance The refractory design of a secondary steel ladle consists often in a combination of magnesiacarbon bricks (MgO/C) in the slag line and a monolithic body made from a spinel castable. The spinel castable usually is used for the bottom, the impact area and the side wall. As soon as the ladle is in service, temperatures above 1600 C occur. At this temperatures with in-situ spinel forming castables the remaining magnesia now completely reacts with alumina to spinel. The castable is in contact with steel and during tapping also with slag, very often with a high lime/silica ratio of >3.5. At the hot surface several processes change the structure of the monolithic. A certain amount of slag can infiltrate into and react with the matrix due to the high lime-gradient between the slag and the castable. But different from spinel-free high alumina mixes

10 Page : 10/12 or pure spinel mixes, the infiltration stops close behind the surface 13. This is illustrated in figure 8 where the penetrated zone is only around 1 2mm deep. slag impregnated zone unaffected zone Cracks which formed on cooling due to densification and shrinkage of impregnated zone 15 1 mm Figure 9: Open porosity and change of mineralogical content during application of a spinel castable in a steel ladle side wall between 0 to 30 mm behind the hot surface 13 Figure 8. Photomicrograph of hot face in alumina spinel castable The unreacted alumina picks up the lime from the slag and forms CA, CA 2 and CA 6 (Fig.9). The spinel phase remains stable. The formation of CA-phases, between spinel grains, creates a small, but highly densified layer with an open porosity below 10%. This reduction in porosity works as a barrier for further penetration of slag into the matrix. The CA 6 content results partly from the calcium aluminate cement reaction with alumina inside the castable and additionally from reaction of penetrated slag with alumina and calcium aluminate. CA 6 reaches a maximum amount in a 3-10 mm depth layer behind the hot surface of the castable. CA 6 is a refractory mineral phase with a resistance of up to 1800 C. In combination with spinel and the excessive alumina, which is the first reaction partner for lime from the slag, this system enables an optimized resistance against the conditions in the steel side wall and bottom. Table 4: Slag corrosion test results (mm) NCC AlSp AlM- Sp_1 Converter slag -Erosion -Penetration Ladle LF slag - Erosion - Penetration CaO MgO Al2O3 SiO2 C/S 2,1-2,5-5,1 2,5-4,5 5,0-12,0 2,5 1,8 5,8 Converter slag 47,86 10,13 16,2 15,23 3,14 2,5 4,9 9,9 2,5 5,9 11,9 AlM Sp_2 4,5 6,3 11,3 Ladle furnace slag 58,55 8,97 12,48 16,76 3,77 AlM Sp_3 2,0 3,0 4,7 8,7 converter (C/S = 3,14) and a secondary refining ladle slag (C/S = 3,77). The results are shown in table 4 As can be seen in Table 4, the Sp_3 system shows the best performance obtained with the converter slag, as seen from the lowest erosion and penetration values. In the case of a

11 Page : 11/12 more aggressive ladle slag, the system shows a significantly better performance than the other systems. The performances achieved with the NCC system are generally lower than that of the other castable systems, particularly in the case of the more aggressive ladle slag. Hence the importance of CAx phases within alumina/magnesia containing castables. The often held perception that the presence of CaO is negative for slag resistance is not always true. These comments have been verified experimentally with slag tests being conducted with two types of slag, a ladle slag from BOS Table 5 (from Ref 10) Al-MgO system Al-Sp system Sp system Bond type MSH CAC CAC Original thickness Reacted layer Penetrated layer Unaltered layer The systems containing CAC are particularly effective in limiting penetration which provokes rapid wear by spalling. Depending upon the slag type then either or the mixed SP_3 system give the lowest erosion results. Further confirmation of the intrinsic performance of CAC containing castables can be seen in a recent publication 10. The corrosion resistance of precast blocks is assessed after actual use in a ladle. A summary of the findings is in the table below (table 5) The Alumina Spinel system showed the best results relative to the three systems tested. 3 Summary and conclusions Monolithics for steel ladle applications have to be examined from a perspective which encompasses the technical requirements as well as the economical constraints which determine the total life of a ladle. Castables cannot be selected on the material cost alone. It has been shown that in order to achieve the best durability of a ladle castable, in-situ field casting is the best solution. This practically prevents the use of precast shapes or small bricks, which are typical of no cement castables which do not permit feasible in-situ casting. Calcium aluminate monolithics give the best thermo-mechanical properties for steel ladle applications: not only they allow the castables to sustain the harsh environment coming from aggressive slags, but they also give the refractory designer the flexibility to select the best solution in function of the location of the castable in the ladle; Alumina/spinel solutions have been shown to be the best choice for ladle floors where resistance to impact is critical whereas alumina/magnesia solutions are the best choice for ladle walls where mechanical stability, abrasion and erosion resistances are essential. Calcium aluminates play a decisive role in the refractory life of a spinel castable. Independent, if a high alumina castable with pre-reacted spinel or with in-situ formed spinel is used, the CAC ensures controlled workability behavior. The right choice of type of magnesia is important to prevent magnesia MgO-hydration and cracking. During the application of these castables in steel ladles, the reaction between alumina and calcium aluminate to calcium hexaaluminate works in combination with the stable spinel phase as infiltration barrier. This barrier is very significant for mixes with in-situ formed spinel, due to the micro-crystalline matrix of CA 6 and MgAl 2 O 4. Infiltration of lime-rich slag stops close behind the surface. Lime from the slag reacts with alumina from the castable and led to pore volume reduction by forming additional calcium aluminate phases like CA, CA 2 and CA 6.

12 Page : 12/12 4 Acknowledgements The authors gratefully acknowledge the experimental support and assistance of Anshan Science and Technology University as well as the resources of Kerneos Research Centre. 5 References 1 Hey, S. J. Gregory, G. S. Hutchesson, D. M. Pickard, D. S. Taylor, and S. B. Tomlinson, Applications of Engineered Castable Systems to Refractory Linings in the Steel Industry, 39 th Int. Coll. on Refr. in Stahl & Eisen Spezial, (1996). [2]Y. Shinohara, H. Yaoi, and K. Sugita, Recent Progress in Monolithic Refractories Usage in the Japanese Steel Industry, New Developments in Monolithic Refractories in Advances in Ceramics (1984). [3]. J. Yamada, S. Sakaki, K. Kasai, T. Matsui, and H. Ishimatsu, Application Technology of Monolithic Refractories in NSC, Proc. UNITECR 95 (Kyoto, Japan, 1995) [3] T. Kanatani, Y. Imaiida, "Application of an alumina-spinel castable to the teeming ladle for stainless steel making", Unitecr 93 proceedings, Sao Paulo, Brazil, [4]. M. Nanba, T. Kaneshage, Y. Hamazaki,H. Nishio, I. Ebizawa, Thermal characteristics of castables for teeming ladle, Taikabutsu overseas, Vol.16, No.3, 1997, pp Cousin, J.-F., Parr, C., Revais, C.: Formulation of spinel castables and the impact of additives and alumina on their rheology, IREFCON Calcutta (1998) 6 Th. Bier, C. Parr, C. Revais : Alafar, Argentina, Vol (1996) [7] Th. Bier, C. Parr, C. Revais, "Workability of calcium aluminates cement based castables containing magnesia", Alafar proceedings, Agentina, [8] M. Rigaud, Cheng Xing, Basic castables for ladle s steel making applications : A review, J. Canadian ceramic society, Vol.66, No.3, August 1997, pp [9] Cheng Xing, M. Rigaud, V. Kovav, Volume stability of various MgO-Al 2 O 3 castable mixes, J. Canadian ceramic society, Vol.66, No.3, August 1997, pp Wei Xiahao, Liang Lanfang, Ren Gangwei, Huang Zhenwu, Zhou Ningsheng, Gao Zhenxin: Investigation on Slag Corrosion of Precast Shapes of Steel Ladle Castables in Al 2 O 3 - MgO system. Proceedings of fourth International Symposium on Refractories, Dalian, China, 2003 p [11] Cecile Odegard, Zhiqiang Chen, Bjorn Myrhe, MgO-SiO 2 -H 2 O bonded MgO castables. Proceedings of fourth International Symposium on Refractories, Dalian, China, 2003 p Parr, C., Bier, T.A., Vialle, M., Revais, C.: An approach to formulate spinel forming castables, UNITECR Berlin (1999), p Naaby, H., Abildgaard, O., Stallmann, G., Wöhrmeyer, C., Meidell, J.: Refractory wear mechanisms and influence on metallurgy and steel quality as a result of the conversion to endless lining at Det Danske Stålvalseværk. XXXVIIth International Colloquium on Refractories, Aachen (1994), p [6] H. Sumimura, T. Yamamura,Y. Yukitoshi, T. Kaneshige, Study on slag penetration of Alumina-Spinel castable, Unitecr 91 proceedings, Germany, 1991, pp