- Email: [email protected]
Research and development activities on refractory ceramics
571
Research and development activities on refractory ceramics Robert E Moore A relatively small fraction of the total number of articles
published annually (1,500-2,000)
on refractory themes
represent basic studies and the group sampled here does not well reflect the overall commercial activity in refractories. For in the engineering of new methods of conveying and emplacing concretes
example, a very large effort is expended
and in repairing all types of refractories. However, the larger refractory companies are restructuring ‘Research and Development’ staffs and several are greatly expanding investment in research capabilities in the areas of mix design, chemical additives, rheology of solid-liquid systems, modelling of processing and structures, test method development and studies of chemical mechanisms involved in corrosion, bonding, degradation by the atmosphere, and so on.
Addresses University of Missouri-Rolls, 276 McNutt Hall, Rolla, MO 65401, USA; e-mail: [email protected] Current Opinion in Solid State 6 Materials Sdenca 2:571-576
1997,
Electronic identifier: 1359-0266-002-00571 Q Current Chemistry Ltd ISSN 1359-0266
Introduction Refractories scientists currently appear to be looking for ways to work with complex behavior, rather than to attempt to apply idealized concepts to this class of widely ranging materials. Research and development reports strongly reflect the desire to apply monolithic versions of products wherever possible and to understand how to make their installation as predictable and manageable as possible. There are new studies to develop materials which will not simply refract thermal energy and resist chemical attack but will also interact with thermal energy and combine with corrodants to form enhanced materials. Reports on refractory and chemically resistant materials formed in situ via solid-liquid and solid-gas reactions are also beginning to appear. Another trend is to do more modelling and less application of simple concepts to the various behaviors of refractory materials.
Monolithic refractories Emphasis is placed on characterizing the grain size distribution and understanding the role of additives to concretes on their behavior and properties. A method for calculating the screened weight fractions over specified size ranges for each starting material of
a monolithic permitted expression in the form of an Andreassen distribution formula has been determined [I]. The original particle size data and density for each material must be reliably known for input as respective volume fractions for each size range. Several ultra-low and low cement castables were combined in this way and found to have an Andreassen number of 0.25. Content and characteristics of the ultra-fines require manipulation of the process to achieve self-flow or other flow characteristics. The methods of Andreassen and Furnas for designing particle size distributions for refractory concretes are frequently modified to affect specific properties and behaviors, especially workability. Scian and Agletti [2] have systemized such alterations by using a new variable, called the C.C.-M variable, which shifts the overall distribution, appreciably, compared to an n -0.35 Andreassen formulation (higher fines [higher weight percent of fines] and less coarse). Using the same top most size (the same largest diameter of a particle or grain), the porosity and density were much improved. There was less water demand and the MOR (modulus of rupture) was much higher. Simulations of the dewatering processes, typically carried out, are being conducted followed by extensive measurements of the key properties as they change along the thermal gradient imposed by one-sided heating [3]. The measurements included, air permeability, tensile strength (diametrical compression test) Hz0 content, density, and porosity and subsequently, thermal conductivity. These properties and other variables, selected independently, are being used for input into various models of the dewatering process. Real time measurements include weight change and temperature at ten physical locations. A study of the rheology of cement pastes, used in refractory concretes and of self-flow castables incorporating the cement, underscores the complexities of these new monolithics [4]. The coarse grain and fine powder selection and sizing results in initial thixotropic behavior for periods up to six minutes, then dilatant behavior thereafter. All of the following factors play a role in determining the preferred rheological behavior, percent of fine Al203 powder, grain size distribution and grain shape, type and quantity of calcined alumina,. cement type, chemical additive package make-up and the mixing process. The hot wire method for measurement of thermal conductivity was used to define the effects of alumina source and content in conductivities of alumina-spine] concretes [S]. The coarseness of a given aggregate was
572
Ceramics,
composites
and intergrowths
seen to be important; conductivity increased substantially with substitution of a coarse for a medium sized aggregate. However, the corundum content purity contributes even more to the thermal conductivity. Micronized aluminas from two separate sources were used in a study of bonding action of these aluminas in zero-cement castables [6]. They had mean diameters of 0.6-0.8 pm and 1.1 pm, respectively, and, in combination with other coarse aluminas and fumed silica (to constitute the matrix component) they appeared to function as well as ‘hydratable’ aluminas in cold and high temperature bond development.
Data for the development of hydration products, resulting from the reaction between a mono calcium aluminate, cement and a specific alumina are shown in Figure 2. The NaeO content, specific surface and reactivity of a series of aluminas showed the chemical reactivity characteristic to be the most important at 20°C [9].
Figure 2
Spine1 additions to monolithics, especially concretes, have been very effective and in a study of the role of stoichiometry in spinels, in a series of 4 spinels with Al203 =67, 70, 75 and 90%, compared to a pure Al203 additive, slag test results varied with test method (a rotary method compared to the common cup test) [7]. Figure 1 schematizes the general form of the corrosion front. Excellent characterization of the reaction zones has led to detailed explanations of effects and suggested various corrosion reaction scenarios, including the role of gaseous phases (whisker morphologies were noted.) The development of the composition of the hydration products for CA+A(d). CAHl 0,C2AH8 and C3AH6 are hydrates of alumina commonly occurring in hydrated pure Ca-aluminate cements. AH3 is an alumina hydrate produced from reactive alumina powder. CA+A(d) refers to a mixture of Ca-aluminate and reactive alumina with mean particle size, d. Reproduced with permission from [g].
Fiaure 1
Unaffected
Schematic diagram (not to scale) of the general form of the penetration front. Note Regions I and II in the dense layer. CAs, Ca-hexaluminate;
CA2, Ca-dialuminate;
are the reaction products
and CAS, Ca-aluminosilicate
of sintering. Reproduced
with permission
from [7].
The addition of alumina, spine1 and silicon carbide to a trough castable (a refractory concrete used to form a channel or trough for containment of molten metals and slags) was studied using a response surface design plotting maximum corrosion depth and corroded section area in an induction furnace test with slag or metal [S]. Spine1 additives were very effective; less liquid formation was noted. High concentrations of fine Sic were effective due to extensive oxidation but the presence of spine1 reduced this effect.
1H NMR and TPD (temperature programmed desorption) measurements were used to study the effects of two aluminas on the hydration behavior of mono calcium aluminate at SYC, the aluminas differing only in reactivity as evidenced by reaction with Hz0 (NazO content and specific surface areas were equal [lo]). The microstructures of hardened pastes were different but not so markedly as those observed in hydration at ZO’C (in an earlier study [ll]). The parameters affected include the length of the induction period, degree of hydration, and the composition of hydration products and of the liquid phase. The concentration of the weak acid sites on the aluminas may be determining. The synergistic effects of chemical additives to calcium aluminate cement bonded refractory concretes were observed in the form of the effects of retarder-accelerator combinations [12]. The schematic diagram below (Figure 3) illustrates the systems employed to produce a rapidly expanding range of products. The cement as well as the other high surface area components, oxides, silica fume, clays, and so on, react with the additives used, often as packages to influence the workability, water demand and physical and mechanical properties. The effect of
Research and development
chemical additives on the dewatering of concretes study at the University of Missouri-Rolla.
is under
Figure 3
activities on refractory ceramics Moore
573
must be taken into account as spine]-like phases have been observed to form in the lower temperature regions and they contain high concentrations of contaminants within the pre-spine1 regions. MgO with ten weight percent flake graphite mixes were densified and exposed to a ferritic-base slag at 1,600X [16]. The oxidation resistance of the mixes was studied over the range l,OOO-1,600X. The least corrosion occurred with the purest flake graphite having the largest flake dimension. The corrosion test exposures revealed a dense MgO zone which is consistently absent in field post-mortems.
CAC
Addltwes Reactive phase
Components influencing the properties in a refractory concrete. [CAC, calcium aluminate cement.] Reproduced with permission from [121.
Steel contact refractories The role of so-called metal antioxidants continues receive attention from researchers. A second challenge in the clogging of pouring nozzles.
to is
A mechanism which orders the effectiveness of additives, including metals used singly and in the form of the carbides of Zr, Mg and Ca as well as B4C is based on the reduction of Pco and an increase in P& which results in the deposition of higher amounts of secondary MgO [13]. This result was varied over the range of l-760Torr and showed a very strong effect of the reduced pressure, especially below 1OOTorr at 1,600”C for 1 hour. The effect of the thermal gradient on the reactions which occur in MgO-C-Metal refractories used in convertors, ladles and electric furnace slag lines, has been studied using cathodoluminescence microscopy combined with SEM and classical light microscopy methods [14]. The results from laboratory simulations, to induce phase development along a thermal gradient, closely parallel those derived from post-mortems (after use or retirement from science). The development of high concentrations of the MgOeAlz03 spine1 is ascribed to gradients in gas concentrations in the hot face as well as in the cold face regions. The nonstoichiometry of the spine1 formed in field specimens has been studied to determine the mechanisms. Sainz &al. [15] conclude that the spine1 reduction reaction mechanism, MgZ(Al+3)z04 Mg+Z(Al+3)(Al+3Mg+2Va)z04
+C = + Mg? + CO ?
will explain the development of Al-rich spinels observed in bricks taken from service. The role of impurities, however,
In a study of the effects of temperature and atmosphere (inert) on the hot modulus of rupture and hot modulus of elasticity of MgO-C-Metal specimens, no decreases were noted in strength above or below 1,450%, the threshold temperature for the formation of MgO.Alz03 spine1 via oxidation of Al&3 [17]. The modulus decreases substantially, however, due ostensibly to the oxidation of carbon, strength is maintained by formation of reaction products in the exterior of the specimens. Carbon sources in combination with additives and their role in oxidation resistance of graphites in MgO-graphite refractories were studied [18]. Al203 was found to accelerate graphite oxidation; TiOz, ZrOz and MgO functioned as retarders. Pitch to resin ratios are critical to promote carbon cystallinity. B4C addition appears to promote the degree of crystallization. Secondary carbon formed through reactions in situ have poor oxidation resistance. A detailed characterization of a Canadian graphite included some oxidation resistance measurements [19]. The reactivity with oxygen of individual flakes determined by a camera technique suggested that the loss of mass could be expressed by, dm = K2nrh dt
= 2nrpz
where K is the kinetic constant, p the density of graphite, r the flake radius and h the flake thickness. At high temperatures when gas transport may be controlling, effects of flake size and graphite purity may be less influential. The effect on lessened clogging of nozzles (201 of OtSialon-Zr-02 was positive compared to AfzO3, apparently due to formation of a highly fluid phase which washes out. The reaction layer thicknesses, d, of the O*-Sialon-ZrO2 reacting with Al203 from the steel followed, Ad - KAt at 1, 500°C,
and
574
Ceramics, composites and intergrowths
Ad = KAt-Krjt”2
at 1,600”C
A key reaction is that between the rSialon-202 and Al203 to form a glassy phase, the viscosity and amount of which is crucial for anti-clogging behavior of pouring nozzles.
Chemical processes Corrosion mechanisms, bond systems and reactions with atmospheric components have been classical themes in almost all areas of refractory usage. Si3N4 refractory ceramics are increasingly used in slag contact applications and Taira et a/. [Zl] have attempted to define the corrosion mechanism of a Si3N4 material in contact with CaO-Al203-Si02 slags. The CaO:SiOz ratio was held constant at 1.0 while the SiOz content was adjusted. Dissolution rates were measured as a function of specimen rotation speed, CaO:SiOz ratio and temperature. Apparently N3- are incorporated into the silicate anionic structures. When SiOz was absent the N entered the slag by forming AI-N bonds. Evaluations of C-containing refractories based on Al203 and MgO for resistance to copper convertor slags showed them to be ineffective [ZZ]. Optimized sintered, magnesiachrome and fused cast magnesia chrome were developed and shown to perform well as suggested in earlier use on other metal particles. A study of the reaction between Al-killed steel and a tundish (a refractory-lined molten metal container which serves to feed the metal to mold entrances) refractory (based on magnesia and olivine) revealed the attack to be tied to reduction of the Fe0 and SiOz contained in the olivine component [23]. Layered products of reactions reveal a first layer of spine] + liquid, and a second composed of calcium aluminates. Reoxidation is considered to be deleterious to the production of potential inclusions. Brazilian refractory products, based on magnesia-chromites, magnesia-spinels and magnesia, were exposed to Hz0 in two scenarios: partial immersion in water at 70°C in open air for 3 months; and in another exposure specimens were vacuum impregnated with Hz0 and sealed for aging in plastic bags and subsequently dried at 70°C [24]. Infrared spectroscopy, XRD and the weight change on heating served to identify the reaction products in the surface regions and in the interior. The study produced a protocol for the storage of MgO containing refractories. A revisiting of the direct bond in a magnesia-chromite refractory, of the type used in cement kilns, employing scanning and transmission electron techniques revealed, by the latter, that the ‘direct bond’ defined as phase to phase contact exists for MgO-MgO and MgO-chromite at a 0.3nm level of resolution (251. It was not possible to
produce specimens for such a high definition study of a magnesia-spine] for a parallel study of direct bonding. The mechanism of bottom drilling is well known to be associated with metal droplets in glass contact refractories. A recent microscopic and chemical characterization of the refractory around the drilled region suggested the formation of two liquid phases, one of them high in nepheline after crystallization [26]. The surface tension gradient at the interface of the metal droplet and a thin low viscosity glass film produces a shear stress and the resulting drilling phenomenon. Three different aluminas having average grain diameters of 6, 4 and 0.61.tm, respectively were reacted with magnesium hydroxide powder to form stoichiometric formulations of spine], MgO.Al203, and were ultimately densified by isostatic pressing and firing at 1,000, 1,200, 1,400 and 1,600’C [27]. Aggregates were extensively characterized and evaluated in alumina-magnesia castables: only the 1,600’C firing produced a dense aggregate from the fine alumina. A comparative study of the hydration of a single crystal of MgO and of a polycrystalline clinker determined that the hydration of the former consists of the cumulative growth of layers of the hydroxide without disintegration, while the hydration of the latter moves much more rapidly with early disintegration at the boundaries until single crystals remain [28]. There are textural changes occurring in the single crystal hydration and the activation energy was determined to be 65.5 KJ/mol. A B’ sialon bond formed in situ for the production of Al203 slide gate products and pouring components, originates from a mixture of metallic powders and fine alumina heat treated in N2 [29]. The absence of SiO2 in these products and the very fine pore structure makes them more resistant to Ca additions to steels, which attack SiO2 in mullite bonded products. The absence of C reportedly precludes the presence of CO, essential for oxidation of Al in the steel. ALON (aluminum oxide aluminum nitride complex) bonded corundum refractories require the stabilization of the ALON derived from reactions between Al203 and AIN [30]. MgO functions well as a stabilizer and the AlN:Al203 ratio should be close to 11:89. A comprehensive study of reactions between ZrO;! and constituents of cement clinker and/or magnesia and a sulphate mixture occurring at 1,ZOo’C (to represent the cement kiln transition zone) or 1,SOo’C (to represent the sintering zone) showed that more Zr02 reacts at the lower temperature forming CaO.ZrOe. Also ZrOa when used as an additive in cement kiln brick, especially when compared to spine], should yield better performance due to the refractoriness of CaO-ZrOe [31].
Research and development
Modelling
& testing
Modellers are requiring better property data, especially at a defined temperature. Adaptation of a pulse decay technique for high temperature determination of sonic elastic modulus involves using a standard 3-point MOR support system for bar-shaped specimens [32]. A steel ball is dropped through an alumina tube to induce transient resonance and E (E modulus or elastic modulus or modulus of elasticity) is calculated from the resonant frequency using the expression from ASTM C-1259 (American Society for Testing and Materials). Results at room temperature were essentially coincident with Grind-o-Sonic data; measurements on four refractories were performed over a range from room temperature to 95o’c. Acoustic emission upon impact by a falling weighted anvil onto a castable refractory beam specimen and brick specimens, was seen to be very sensitive to damage in one shot and fatigue-type procedures [33]. The resistance to impact of several castables and brick specimens closely parallel their resistance to thermal shock (strength-after method). Mullite and zircon castables were the most resistant. FEM (finite element modelling analysis) demands the input of the appropriate types of data and data which are accurate, reliable and reflect the presence of the gradient and thermal cycling histories. Several studies reflect the dawning of an awareness of the full challenge and data are beginning to appear which show the general effects of composition of MgO-carbon-antioxidant type and level on key thermal and mechanical properties such as failure stress, MOE (modulus of elasticity), poisson’s ratio and thermal expansion and thermal diffusivity. Hoop stress calculations (estimates of the circumferential tensile stresses in the refractory wall) were made based on linear behavior of the lining materials [34]. FEM modelling of the linings of BOF (basic oxygen furnace) convertors incorporate realistic load deflection behavior for MgO-carbon bricks as the bricks are degraded through exposure to actual conditions in service [35]. This study also provided simulations of the results of relevant laboratory test data, oia, bending and creep at loads sufficient to cause damage. This approach, via modelling, has been deferred for a long time because modellers lacked data on the temperature-time-load-atmosphere data, reflective of the conditions of use. A geometric model to represent the very large expansions which occur when MgO and Al203 compacts are heated at 1,SOOC for 12 h to form MgOAi203 has been developed [36]. The model assumes a heterogeneous distribution of constituent particles in mixtures of spine] MgO and Al203. The contact number ratio, R, proved to be the key parameter in the modelling.
activities on refractory ceramics Moore
575
There is considerable interest today in understanding the development of liquid and gas permeability of refractory and construction concretes. A model for the connectivity of pores follows the modelling of the hydration of tricalcium silicate particles, singly and in packed arrays, which then affords an estimate of the fraction of connected pores as a function of capillary porosity for different water cement ratios [37]. This approach does not address the permeability of concretes containing an appreciable gel phase.
Conclusions The many articles of a highly applied nature (not sampled in this survey) as well as presented reports at key conferences reflect, as does this review, the powerful motivation to supplant fired refractory shapes with either monolithic or precast lining components. Major challenges to do this include a versatile bond for chemically basic products, methods to control the rheology of all types of monolithic systems and processing advances that will insure reliability. Serious attempts at modelling of processing, reaction and wear mechanisms, key properties and behaviors are just beginning to be made. In almost all instances the demand for more insight emanates from the customers of refractories producers. Precompetitive cooperative research efforts as practised in other product sectors should be pursued by the refractories producers to accelerate the generation of required data and development of necessary insights.
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37.
Navi P, Pignot C: Simulation of cement hydration and the connectivity of the capillary pore space. Adv Cem Based Mater 1996, 4~50-67.
of
Research and development activities on refractory ceramics Robert E Moore A relatively small fraction of the total number of articles
published annually (1,500-2,000)
on refractory themes
represent basic studies and the group sampled here does not well reflect the overall commercial activity in refractories. For in the engineering of new methods of conveying and emplacing concretes
example, a very large effort is expended
and in repairing all types of refractories. However, the larger refractory companies are restructuring ‘Research and Development’ staffs and several are greatly expanding investment in research capabilities in the areas of mix design, chemical additives, rheology of solid-liquid systems, modelling of processing and structures, test method development and studies of chemical mechanisms involved in corrosion, bonding, degradation by the atmosphere, and so on.
Addresses University of Missouri-Rolls, 276 McNutt Hall, Rolla, MO 65401, USA; e-mail: [email protected] Current Opinion in Solid State 6 Materials Sdenca 2:571-576
1997,
Electronic identifier: 1359-0266-002-00571 Q Current Chemistry Ltd ISSN 1359-0266
Introduction Refractories scientists currently appear to be looking for ways to work with complex behavior, rather than to attempt to apply idealized concepts to this class of widely ranging materials. Research and development reports strongly reflect the desire to apply monolithic versions of products wherever possible and to understand how to make their installation as predictable and manageable as possible. There are new studies to develop materials which will not simply refract thermal energy and resist chemical attack but will also interact with thermal energy and combine with corrodants to form enhanced materials. Reports on refractory and chemically resistant materials formed in situ via solid-liquid and solid-gas reactions are also beginning to appear. Another trend is to do more modelling and less application of simple concepts to the various behaviors of refractory materials.
Monolithic refractories Emphasis is placed on characterizing the grain size distribution and understanding the role of additives to concretes on their behavior and properties. A method for calculating the screened weight fractions over specified size ranges for each starting material of
a monolithic permitted expression in the form of an Andreassen distribution formula has been determined [I]. The original particle size data and density for each material must be reliably known for input as respective volume fractions for each size range. Several ultra-low and low cement castables were combined in this way and found to have an Andreassen number of 0.25. Content and characteristics of the ultra-fines require manipulation of the process to achieve self-flow or other flow characteristics. The methods of Andreassen and Furnas for designing particle size distributions for refractory concretes are frequently modified to affect specific properties and behaviors, especially workability. Scian and Agletti [2] have systemized such alterations by using a new variable, called the C.C.-M variable, which shifts the overall distribution, appreciably, compared to an n -0.35 Andreassen formulation (higher fines [higher weight percent of fines] and less coarse). Using the same top most size (the same largest diameter of a particle or grain), the porosity and density were much improved. There was less water demand and the MOR (modulus of rupture) was much higher. Simulations of the dewatering processes, typically carried out, are being conducted followed by extensive measurements of the key properties as they change along the thermal gradient imposed by one-sided heating [3]. The measurements included, air permeability, tensile strength (diametrical compression test) Hz0 content, density, and porosity and subsequently, thermal conductivity. These properties and other variables, selected independently, are being used for input into various models of the dewatering process. Real time measurements include weight change and temperature at ten physical locations. A study of the rheology of cement pastes, used in refractory concretes and of self-flow castables incorporating the cement, underscores the complexities of these new monolithics [4]. The coarse grain and fine powder selection and sizing results in initial thixotropic behavior for periods up to six minutes, then dilatant behavior thereafter. All of the following factors play a role in determining the preferred rheological behavior, percent of fine Al203 powder, grain size distribution and grain shape, type and quantity of calcined alumina,. cement type, chemical additive package make-up and the mixing process. The hot wire method for measurement of thermal conductivity was used to define the effects of alumina source and content in conductivities of alumina-spine] concretes [S]. The coarseness of a given aggregate was
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Ceramics,
composites
and intergrowths
seen to be important; conductivity increased substantially with substitution of a coarse for a medium sized aggregate. However, the corundum content purity contributes even more to the thermal conductivity. Micronized aluminas from two separate sources were used in a study of bonding action of these aluminas in zero-cement castables [6]. They had mean diameters of 0.6-0.8 pm and 1.1 pm, respectively, and, in combination with other coarse aluminas and fumed silica (to constitute the matrix component) they appeared to function as well as ‘hydratable’ aluminas in cold and high temperature bond development.
Data for the development of hydration products, resulting from the reaction between a mono calcium aluminate, cement and a specific alumina are shown in Figure 2. The NaeO content, specific surface and reactivity of a series of aluminas showed the chemical reactivity characteristic to be the most important at 20°C [9].
Figure 2
Spine1 additions to monolithics, especially concretes, have been very effective and in a study of the role of stoichiometry in spinels, in a series of 4 spinels with Al203 =67, 70, 75 and 90%, compared to a pure Al203 additive, slag test results varied with test method (a rotary method compared to the common cup test) [7]. Figure 1 schematizes the general form of the corrosion front. Excellent characterization of the reaction zones has led to detailed explanations of effects and suggested various corrosion reaction scenarios, including the role of gaseous phases (whisker morphologies were noted.) The development of the composition of the hydration products for CA+A(d). CAHl 0,C2AH8 and C3AH6 are hydrates of alumina commonly occurring in hydrated pure Ca-aluminate cements. AH3 is an alumina hydrate produced from reactive alumina powder. CA+A(d) refers to a mixture of Ca-aluminate and reactive alumina with mean particle size, d. Reproduced with permission from [g].
Fiaure 1
Unaffected
Schematic diagram (not to scale) of the general form of the penetration front. Note Regions I and II in the dense layer. CAs, Ca-hexaluminate;
CA2, Ca-dialuminate;
are the reaction products
and CAS, Ca-aluminosilicate
of sintering. Reproduced
with permission
from [7].
The addition of alumina, spine1 and silicon carbide to a trough castable (a refractory concrete used to form a channel or trough for containment of molten metals and slags) was studied using a response surface design plotting maximum corrosion depth and corroded section area in an induction furnace test with slag or metal [S]. Spine1 additives were very effective; less liquid formation was noted. High concentrations of fine Sic were effective due to extensive oxidation but the presence of spine1 reduced this effect.
1H NMR and TPD (temperature programmed desorption) measurements were used to study the effects of two aluminas on the hydration behavior of mono calcium aluminate at SYC, the aluminas differing only in reactivity as evidenced by reaction with Hz0 (NazO content and specific surface areas were equal [lo]). The microstructures of hardened pastes were different but not so markedly as those observed in hydration at ZO’C (in an earlier study [ll]). The parameters affected include the length of the induction period, degree of hydration, and the composition of hydration products and of the liquid phase. The concentration of the weak acid sites on the aluminas may be determining. The synergistic effects of chemical additives to calcium aluminate cement bonded refractory concretes were observed in the form of the effects of retarder-accelerator combinations [12]. The schematic diagram below (Figure 3) illustrates the systems employed to produce a rapidly expanding range of products. The cement as well as the other high surface area components, oxides, silica fume, clays, and so on, react with the additives used, often as packages to influence the workability, water demand and physical and mechanical properties. The effect of
Research and development
chemical additives on the dewatering of concretes study at the University of Missouri-Rolla.
is under
Figure 3
activities on refractory ceramics Moore
573
must be taken into account as spine]-like phases have been observed to form in the lower temperature regions and they contain high concentrations of contaminants within the pre-spine1 regions. MgO with ten weight percent flake graphite mixes were densified and exposed to a ferritic-base slag at 1,600X [16]. The oxidation resistance of the mixes was studied over the range l,OOO-1,600X. The least corrosion occurred with the purest flake graphite having the largest flake dimension. The corrosion test exposures revealed a dense MgO zone which is consistently absent in field post-mortems.
CAC
Addltwes Reactive phase
Components influencing the properties in a refractory concrete. [CAC, calcium aluminate cement.] Reproduced with permission from [121.
Steel contact refractories The role of so-called metal antioxidants continues receive attention from researchers. A second challenge in the clogging of pouring nozzles.
to is
A mechanism which orders the effectiveness of additives, including metals used singly and in the form of the carbides of Zr, Mg and Ca as well as B4C is based on the reduction of Pco and an increase in P& which results in the deposition of higher amounts of secondary MgO [13]. This result was varied over the range of l-760Torr and showed a very strong effect of the reduced pressure, especially below 1OOTorr at 1,600”C for 1 hour. The effect of the thermal gradient on the reactions which occur in MgO-C-Metal refractories used in convertors, ladles and electric furnace slag lines, has been studied using cathodoluminescence microscopy combined with SEM and classical light microscopy methods [14]. The results from laboratory simulations, to induce phase development along a thermal gradient, closely parallel those derived from post-mortems (after use or retirement from science). The development of high concentrations of the MgOeAlz03 spine1 is ascribed to gradients in gas concentrations in the hot face as well as in the cold face regions. The nonstoichiometry of the spine1 formed in field specimens has been studied to determine the mechanisms. Sainz &al. [15] conclude that the spine1 reduction reaction mechanism, MgZ(Al+3)z04 Mg+Z(Al+3)(Al+3Mg+2Va)z04
+C = + Mg? + CO ?
will explain the development of Al-rich spinels observed in bricks taken from service. The role of impurities, however,
In a study of the effects of temperature and atmosphere (inert) on the hot modulus of rupture and hot modulus of elasticity of MgO-C-Metal specimens, no decreases were noted in strength above or below 1,450%, the threshold temperature for the formation of MgO.Alz03 spine1 via oxidation of Al&3 [17]. The modulus decreases substantially, however, due ostensibly to the oxidation of carbon, strength is maintained by formation of reaction products in the exterior of the specimens. Carbon sources in combination with additives and their role in oxidation resistance of graphites in MgO-graphite refractories were studied [18]. Al203 was found to accelerate graphite oxidation; TiOz, ZrOz and MgO functioned as retarders. Pitch to resin ratios are critical to promote carbon cystallinity. B4C addition appears to promote the degree of crystallization. Secondary carbon formed through reactions in situ have poor oxidation resistance. A detailed characterization of a Canadian graphite included some oxidation resistance measurements [19]. The reactivity with oxygen of individual flakes determined by a camera technique suggested that the loss of mass could be expressed by, dm = K2nrh dt
= 2nrpz
where K is the kinetic constant, p the density of graphite, r the flake radius and h the flake thickness. At high temperatures when gas transport may be controlling, effects of flake size and graphite purity may be less influential. The effect on lessened clogging of nozzles (201 of OtSialon-Zr-02 was positive compared to AfzO3, apparently due to formation of a highly fluid phase which washes out. The reaction layer thicknesses, d, of the O*-Sialon-ZrO2 reacting with Al203 from the steel followed, Ad - KAt at 1, 500°C,
and
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Ceramics, composites and intergrowths
Ad = KAt-Krjt”2
at 1,600”C
A key reaction is that between the rSialon-202 and Al203 to form a glassy phase, the viscosity and amount of which is crucial for anti-clogging behavior of pouring nozzles.
Chemical processes Corrosion mechanisms, bond systems and reactions with atmospheric components have been classical themes in almost all areas of refractory usage. Si3N4 refractory ceramics are increasingly used in slag contact applications and Taira et a/. [Zl] have attempted to define the corrosion mechanism of a Si3N4 material in contact with CaO-Al203-Si02 slags. The CaO:SiOz ratio was held constant at 1.0 while the SiOz content was adjusted. Dissolution rates were measured as a function of specimen rotation speed, CaO:SiOz ratio and temperature. Apparently N3- are incorporated into the silicate anionic structures. When SiOz was absent the N entered the slag by forming AI-N bonds. Evaluations of C-containing refractories based on Al203 and MgO for resistance to copper convertor slags showed them to be ineffective [ZZ]. Optimized sintered, magnesiachrome and fused cast magnesia chrome were developed and shown to perform well as suggested in earlier use on other metal particles. A study of the reaction between Al-killed steel and a tundish (a refractory-lined molten metal container which serves to feed the metal to mold entrances) refractory (based on magnesia and olivine) revealed the attack to be tied to reduction of the Fe0 and SiOz contained in the olivine component [23]. Layered products of reactions reveal a first layer of spine] + liquid, and a second composed of calcium aluminates. Reoxidation is considered to be deleterious to the production of potential inclusions. Brazilian refractory products, based on magnesia-chromites, magnesia-spinels and magnesia, were exposed to Hz0 in two scenarios: partial immersion in water at 70°C in open air for 3 months; and in another exposure specimens were vacuum impregnated with Hz0 and sealed for aging in plastic bags and subsequently dried at 70°C [24]. Infrared spectroscopy, XRD and the weight change on heating served to identify the reaction products in the surface regions and in the interior. The study produced a protocol for the storage of MgO containing refractories. A revisiting of the direct bond in a magnesia-chromite refractory, of the type used in cement kilns, employing scanning and transmission electron techniques revealed, by the latter, that the ‘direct bond’ defined as phase to phase contact exists for MgO-MgO and MgO-chromite at a 0.3nm level of resolution (251. It was not possible to
produce specimens for such a high definition study of a magnesia-spine] for a parallel study of direct bonding. The mechanism of bottom drilling is well known to be associated with metal droplets in glass contact refractories. A recent microscopic and chemical characterization of the refractory around the drilled region suggested the formation of two liquid phases, one of them high in nepheline after crystallization [26]. The surface tension gradient at the interface of the metal droplet and a thin low viscosity glass film produces a shear stress and the resulting drilling phenomenon. Three different aluminas having average grain diameters of 6, 4 and 0.61.tm, respectively were reacted with magnesium hydroxide powder to form stoichiometric formulations of spine], MgO.Al203, and were ultimately densified by isostatic pressing and firing at 1,000, 1,200, 1,400 and 1,600’C [27]. Aggregates were extensively characterized and evaluated in alumina-magnesia castables: only the 1,600’C firing produced a dense aggregate from the fine alumina. A comparative study of the hydration of a single crystal of MgO and of a polycrystalline clinker determined that the hydration of the former consists of the cumulative growth of layers of the hydroxide without disintegration, while the hydration of the latter moves much more rapidly with early disintegration at the boundaries until single crystals remain [28]. There are textural changes occurring in the single crystal hydration and the activation energy was determined to be 65.5 KJ/mol. A B’ sialon bond formed in situ for the production of Al203 slide gate products and pouring components, originates from a mixture of metallic powders and fine alumina heat treated in N2 [29]. The absence of SiO2 in these products and the very fine pore structure makes them more resistant to Ca additions to steels, which attack SiO2 in mullite bonded products. The absence of C reportedly precludes the presence of CO, essential for oxidation of Al in the steel. ALON (aluminum oxide aluminum nitride complex) bonded corundum refractories require the stabilization of the ALON derived from reactions between Al203 and AIN [30]. MgO functions well as a stabilizer and the AlN:Al203 ratio should be close to 11:89. A comprehensive study of reactions between ZrO;! and constituents of cement clinker and/or magnesia and a sulphate mixture occurring at 1,ZOo’C (to represent the cement kiln transition zone) or 1,SOo’C (to represent the sintering zone) showed that more Zr02 reacts at the lower temperature forming CaO.ZrOe. Also ZrOa when used as an additive in cement kiln brick, especially when compared to spine], should yield better performance due to the refractoriness of CaO-ZrOe [31].
Research and development
Modelling
& testing
Modellers are requiring better property data, especially at a defined temperature. Adaptation of a pulse decay technique for high temperature determination of sonic elastic modulus involves using a standard 3-point MOR support system for bar-shaped specimens [32]. A steel ball is dropped through an alumina tube to induce transient resonance and E (E modulus or elastic modulus or modulus of elasticity) is calculated from the resonant frequency using the expression from ASTM C-1259 (American Society for Testing and Materials). Results at room temperature were essentially coincident with Grind-o-Sonic data; measurements on four refractories were performed over a range from room temperature to 95o’c. Acoustic emission upon impact by a falling weighted anvil onto a castable refractory beam specimen and brick specimens, was seen to be very sensitive to damage in one shot and fatigue-type procedures [33]. The resistance to impact of several castables and brick specimens closely parallel their resistance to thermal shock (strength-after method). Mullite and zircon castables were the most resistant. FEM (finite element modelling analysis) demands the input of the appropriate types of data and data which are accurate, reliable and reflect the presence of the gradient and thermal cycling histories. Several studies reflect the dawning of an awareness of the full challenge and data are beginning to appear which show the general effects of composition of MgO-carbon-antioxidant type and level on key thermal and mechanical properties such as failure stress, MOE (modulus of elasticity), poisson’s ratio and thermal expansion and thermal diffusivity. Hoop stress calculations (estimates of the circumferential tensile stresses in the refractory wall) were made based on linear behavior of the lining materials [34]. FEM modelling of the linings of BOF (basic oxygen furnace) convertors incorporate realistic load deflection behavior for MgO-carbon bricks as the bricks are degraded through exposure to actual conditions in service [35]. This study also provided simulations of the results of relevant laboratory test data, oia, bending and creep at loads sufficient to cause damage. This approach, via modelling, has been deferred for a long time because modellers lacked data on the temperature-time-load-atmosphere data, reflective of the conditions of use. A geometric model to represent the very large expansions which occur when MgO and Al203 compacts are heated at 1,SOOC for 12 h to form MgOAi203 has been developed [36]. The model assumes a heterogeneous distribution of constituent particles in mixtures of spine] MgO and Al203. The contact number ratio, R, proved to be the key parameter in the modelling.
activities on refractory ceramics Moore
575
There is considerable interest today in understanding the development of liquid and gas permeability of refractory and construction concretes. A model for the connectivity of pores follows the modelling of the hydration of tricalcium silicate particles, singly and in packed arrays, which then affords an estimate of the fraction of connected pores as a function of capillary porosity for different water cement ratios [37]. This approach does not address the permeability of concretes containing an appreciable gel phase.
Conclusions The many articles of a highly applied nature (not sampled in this survey) as well as presented reports at key conferences reflect, as does this review, the powerful motivation to supplant fired refractory shapes with either monolithic or precast lining components. Major challenges to do this include a versatile bond for chemically basic products, methods to control the rheology of all types of monolithic systems and processing advances that will insure reliability. Serious attempts at modelling of processing, reaction and wear mechanisms, key properties and behaviors are just beginning to be made. In almost all instances the demand for more insight emanates from the customers of refractories producers. Precompetitive cooperative research efforts as practised in other product sectors should be pursued by the refractories producers to accelerate the generation of required data and development of necessary insights.
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