Applications In Refractory Cements, Refractory Materials
Without refractory materials, many modern industrial processes would not be possible due to no high quality iron, steel and aluminium being available to make the machinery required. A refractory material is a crucial part of a furnace or masonry oven, where it both acts as the major thermal barrier between the core and its housing, and as a material that can join together other refractories, such as silica or alumina. Refractory materials dealt with here include refractory cements and bricks.
Introduction To Refractory Cements
Refractory cements, or refractory concretes, are types of concrete with high thermal stability. Whereas traditional Portland cement-based concrete can be damaged or destroyed by high temperatures(1), refractory cements resist such damage. Portland cement is primarily composed of calcium silicates, compared to refractory cement which utilises monocalcium aluminate (CaAl2O4) - made from the high temperature combination of alumina (Al2O3) and calcium carbonate (CaCO3) - amongst other additive compounds. Production is otherwise similar to that of regular concrete(2). For refractory cements, a high-grade source of aluminium oxide is essential for optimal high-temperature performance. Lower grade aluminium-based cements can employ more readily available sources of aluminium such as bauxite - though their thermal performance properties are inferior. Much like regular cement, refractory cement can be fired into brick form, but is more often molded into a particular shape.
What Are They?
Refractory concretes comprise of coarse or finely ground aggregate material, such as sand or aluminium or magnesium oxide, alongside an often porous binder phase as a cement; monocalcium aluminate in high-performing environments. Monocalcium aluminate, as mentioned, is formed from the firing of calcium carbonate (lime) and alumina, either by fusion, clinker or sintering processes(3). It can then be used in an analogous fashion to Portland cement.
The development of the aluminate type cement derived from a requirement of stability - common calcium silicate-based cements were found to lack sulfate resistance and did not have high performance levels at high temperature(4) in the 1920s. By the 1960s, high-quality refractories based on calcium aluminate cements and alumina-type aggregates were becoming available, performing highly in terms of thermal stability, abrasion resistance and against chemical erosion(5,6). By the 1970s, lower cement containing refractories were becoming available, with a significantly reduced cement content, made up with various metal oxides and deflocculants and other additives(7,8). Contemporary refractories are made up with varying quantities of cement, based on the desired application, with a variety of additives.
How Are They Made?
In industrial settings, broadly speaking, refractories are made by either dry pressing, casting or forming. The latter of which has subclasses including by firing (one method for manufacture of refractory bricks) and chemical bonding. A final method of manufacture is into a monolith, which is a solid without a finished form - its form and shape are given to it upon first application.
A refractory used in a furnace is typically either of the brick (fired) type or of a cast refractory cement (akin to cast concrete). Monoliths are also used and are prized for their lack of joins and lengthy service lives. The fired brick type is analogous to traditional methods of forming fire clay - another refractory material, albeit for lower temperature applications like wood fired furnaces - where the material is fired in a kiln until it is partly vitrified. When using refractory cement to form the refractory structure, it can be advantageous to use a low-cement content mixture, which improves the flow properties of the material prior to curing/firing(2).
Chrome In Refractory Environments
Much like additives in the manufacture of regular concrete, the performance of refractory cement can be modulated by the addition of additives. One major class of refractory cement additives is that of chrome compounds; chrome flour, chrome sand (chromite) and directly chromium(iii) oxide (chromia, Cr2O3). It is stated that 18% of all chromite is used for refractory purposes(9). Chrome-based refractory materials are extensively used in furnace environments for the production of iron, steel and aluminium.
Iron Chromite And Chrome ‘Flour’
Iron Chromite is the naturally occurring ore of chromium. It has a melting point of 2,040°C and is “almost chemically inert”(10). Chromite has found use as a refractory material in its own right, as opposed to being a component in refractory cement, with its high chromia content providing a highly stable material resistant to wetting(11), notwithstanding earlier mentioned failure under high weight load.
The use of chrome compounds in refractory environments dates back over one hundred years. Refractories of just chromite had been used in iron and steel making environments however under a high weight load tended to mechanically collapse and fail(12). Western and Central European foundries had been utilising silicate-bonded magnesia bricks which performed well, but were significantly improved by the addition of chromite, particularly in the areas of resistance to shock and spalling(13). By the 1960s, foundries were introducing oxygen directly into their furnaces, resulting in higher operating temperatures and silicate-type bricks began being replaced with magnesite-spiked chromite bricks for better performance at elevated temperatures(14). When working under oxidising conditions, it is imperative to consider the possible oxidation of all compounds present. A known problem with chromate as a refractory is that under certain highly oxidising conditions, chromium(iii) can oxidise to chromium(vi), which is a known carcinogen. The inclusion of a high quantity of aluminium oxide in the refractory component has been found to alleviate such oxidation(15).
Chrome flour is the incredibly finely ground powder of iron chromite and is used extensively in the production of magnesia chrome refractory bricks, for the construction of furnaces and kilns(16,17).
In refractory brick form, chromite alongside alumina and magnesium oxide has been shown to be stable and strong up to 1,900°C(18). It is important to note that this value is lower than the published melting point of chromite, but in brick form alongside other materials, it is far easier to use, handle and contain. A refractory brick comprising solely chromite did not perform any better than any of the chromite-alumina-magnesium mixed composition bricks, this is not a disadvantage, however, as the other materials come at a lower cost than the chrome flour. Noteworthy is the fact that cast magnesite-chrome is denser than it is in brick form, as it is less porous(19,20).
Chromium(iii) oxide, chromia, Cr2O3, is also refractory material in its own right, and has been proposed as useful in the manufacture of refractory bricks(21). Research has shown that the addition of aluminium oxide (in the unrefined bauxite form) to chromia improves its mechanical strength(22). Chromia’s main use industrially is as a piment; sources of this oxide for refractory applications tend to come from the chromite form (as above)(23), even though sources directly from the ore contain iron, this is not viewed as a problem. In furnace conditions, any residual iron will oxidise to inert iron oxide.
Use Of Chrome Refractories In Gasifiers
Aside from lining of furnaces for iron and steel production, chrome refractories have a significant and important use in the lining of gasifiers. The gasification process converts carbonaceous materials (such as coal, coke and biomass) to synthesis gas, and the most common gasifier type is the entrained flow type gasifier. The inherent problem with the gasification process is the production of ash, which under the high heat conditions collects on the reactor walls as slag, which can flow down the wall of the refractory and potentially penetrate its porous structure - causing unwelcome reactions to occur. Chrome-based refractories are prized in this setting for their robustness and the low solubility of chromia in slag. In general, however, the chromia performed well in laboratory testing with slight and hard-to-quantify structural changes, but it was noted that larger particle size in the refractory and denser packing are most beneficial(24). Synthesis gas is used in the Fischer-Tropsch process for the synthesis of hydrocarbons. Chrome-alumina is the leading choice for refractory materials for gasifiers(25). Due to the heightened cost of chrome components in refractory manufacture, some research has suggested a layered approach to reactor lining(26) or a sectional-type arrangement; a high weight% chromium oxide layer in the hottest section, and a lower chromium oxide (down to 15 weight%, with the balance being alumina) in the cooler zones(27). It is noted that alumina-chromia refractories are superior to alumina and magnesia-chrome refractories in the presence of acidic slag(28).
- Refractory materials are crucial in the provision of globally significant materials such as iron, steel and aluminium.
- Aluminates have become present in refractory concrete/cement offering greatly improved thermal performance character.
- Chromium compounds are noted as a valued component of refractories, often alongside aluminates and magnesites, due to their high thermal stability.
- Chrome-based refractories have found a particular high-importance application in gasification processes, providing for a reliable and resilient process.
1 Q. Ma, et al., Constr. Build. Mater., 2015, 93, 371
2 W. E. Lee et al., Int. Mat. Rev., 2001, 46, 145
3 J. E. Kopanda and G. Maczura, Alumina Chemicals – Science and Technology Handbook, American Ceramic Society, Westerville, United States, 1990
4 A. V. Briebach, Trans. J. Br. Ceram. Soc., 1972, 71, 153
5 D. R. Lankard, Adv. Ceram., 1984, 13, 46
6 G. MacZura et al., in Proc. UNITECR ’95, Technical Association of Refractories Japan, Kyoto, 1997
7 P Krietz et al., Am. Ceram. Soc. Bull., 1990, 69, 1690
8 J. P. Radal et al., Adv. Ceram., 1984, 13, 274
9 J. Barnhart, Reg. Toxicol. and Pharmacol., 1997, 26, 3
10 J. O. Nriagu and E. Nieboer (eds.), Chromium in the Natural and Human Environments, Wiley-Interscience, New York, 1988
11 N. McEwan et al., Chromite—A Cost-effective Refractory Raw Material for Refractories in various Metallurgical Applications in Southern African Pyrometallurgy 2011, eds. R. T. Jones and P. den Hoed, Johannesburg, 2011
12 J. White, Magnesia-based Refractories, in High Temperature Oxides Part I Magnesia, Lime and Chrome Refractories, A. M. Alper (ed.), Academic Press, New York, 1970
13 W. D. Kingery, H, K. Bowen and D. R. Uhlman, Introduction to Ceramics, 2nd ed., Wiley, New York, 1960
14 A. Muan and E. F. Osborne, Phase Equilibrium among Oxides in Steelmaking, Addison Wesley, Reading, United States, 1965
15 A. Muan and S. Somiya, J. Am. Ceram. Soc., 1959, 42, 603
16 US Patent US3360387A, 1967, expired
17 US Patent US4366256A, 1982, expired
18 E Ruh and J. S. McDowell, J. Am. Ceram. Soc., 1962, 45, 189
19 W. E. Lee and W. M. Rainforth, Ceramic Microstructures - Property Control by Processing, Chapman and Hall, London, 1994
20 J. H. Chesters, Refractories: Production and Properties, Institute of Materials, London, 1973
21 K. Matusmoto, Chem. Abst., 1963, 59, 3626
22 T. R. Lyman and W. J. Rees, Trans. Ceram. Soc., 1937, 36, 110
23 H. G. Emblem and T. J. Davies, Rev. Inorg. Chem., 1993, 13, 103
24 H. B. Kim and M. S. Oh, Ceramics Int., 2008, 34, 2107
25 J. P. Bennett, Refractories Applications and News, 2004, 9, 20
26 W. A. Taber, Refractories Applications and News, 2003, 8, 18
27 Z. Guo, Am. Ceram. Soc. Bull., 2004, 83, 9101
28 J. A. Bonar et al., Am. Ceram. Soc. Bull., 1980, 59, 4