Refractory Materials Classified
Refractory materials is a broad term for any material that is broadly tolerant of high temperatures and is resistant to chemical attack at those high temperatures.
There are many classes of refractory materials, and a wide array of materials that fall into those categories. Here, a variety of such materials will be discussed and categorised in terms of their chemical composition. There are many other ways of categorising refractory materials, such as how they are made and what physical form they take, for example monolithic or shaped.
Many - but not all - refractory materials share a common production pathway by a sequential process: raw material processing, forming and then firing. Not all refractory materials are used in the conventional sense, some are incorporated into other materials, be those refractory or otherwise. More than 70% of all refractories produced are used in the metal production industry(1).
The classification here does not include all refractory materials, in the sake of brevity. It should be noted, too, that the use of some refractory materials for applications outside their categories does occur under certain qualifications and reasons. The major reason for classification by chemical composition is that within each category, many refractories will behave largely the same for the most common projects, because of how they do or do not react with the in-process atmosphere or the slag produced. Some examples from each class will be detailed.
The three widest classes of refractory material - by chemical composition - are acidic, basic and neutral.
Acidic refractories are those refractories which readily react with bases at high temperatures. The most common examples of acidic refractories are fire clay and silica. Other examples include aluminosilicates and zirconia. Acidic refractories are most suited to where the slag/atmosphere is acidic itself, as this means there will be no attack or reactivity, ensuring long refractory life(2).
Silica is the most common refractory material. Having a melting point of 2,230 °C, its use is widespread (3). Prized for their longevity and resistance to some of the highest temperatures, fire bricks are an example of a predominantly silica refractory material. Other uses beyond bricks have been demonstrated, such as a tundish lining and in ceramics and cements.
Popular for the formation of crucibles and a whole range of metalworking tools, fire clay is a clay composed primarily of hydrous silicates of aluminium, oftentimes with silica present too.
Ground glass, although not a true refractory in its own right, offers enhanced properties to wider refractory materials. Akin to silica, its primary component, ground glass behaves like an acidic refractory. One particularly useful feature when using ground glass as part of a tundish or furnace lining refractory is that it is able to remove iron oxide from molten iron and steel(4). It has been reported that when used as a component in a refractory lining, ground glass can act as a flux. This assists in purifying the molten metal(5). Downstream benefits such as improved machinability may be observed also.
Ground glass has also been used as a component in other acidic-type refractories such as silica fire bricks, where such bricks are found in steel- and glass-making furnaces(6).
Complimentary to the idea of acidic refractories are basic refractories. These often react with acids at high temperatures. Magnesite, zirconia and dolomite are widely used examples. Basic refractories, therefore, are most suited to when the slag/atmosphere produced will be basic too. Typical use cases for basic refractories are for non-ferrous metallurgical operations. Basic refractories are often the highest temperature refractories - but this means increased cost(7).
Zirconia is a basic refractory that is thermally stable up to 15,000 °C, often used for glass furnaces and other furnaces where the highest temperatures are required. Attractively, it does not react with liquid metals and such is a widely utilised material.
Magnesite, MgCO3, is a refractory primarily used for instances with highly basic or iron rich slags, with which it does not react, however their refractoriness is not the highest. Magnesite-chrome and chrome-magnesite (named accordingly with the content of the predominant material) are mixed refractories. Typically used for high temperature outflows of furnaces and furnace linings. Magnesite and chrome mixed refractories offer excellent resistance to spalling (8).
Many of the members of the ‘neutral’ class of refractories fall into two subcategories, oxides and carbon. Neutral oxides are prized for their lack of reactivity with acids and bases, and are often considered superior refractories for their wide utility and performance over a broad number of applications.
Neutral refractories are used across many applications, as they are tolerant to both acidic and basic atmospheres and slags. Carbon-type refractories are often used in reducing environments(9).
Counterintuitively, anthracite has been long used as a refractory. As an example of a neutral refractory material, its inclusion means that there is no reactivity with acidic or basic atmospheres or slags. Calcined anthracite (CA) is a heat-treated version of anthracite which is stronger and significantly more porous than untreated anthracite.
During calcination - which may be via an electrical heating process, producing electrically calcined anthracite, ECA - anthracite starts to undergo graphitisation at approximately 2,200 °C(10). In effect, due to the graphitisation, synthetic graphite is formed through ECA(11). Graphite itself is a refractory material.
For casted refractories, CA has small and consistently sized pores. Of particular utility to ferrous metal production, calcined anthracite finds extensive use in monolithic castable graphitic refractories(12).
In smelting applications, CA/ECA have interesting electrical resistance profiles(13), whereas anthracite is a poor conductor, CA/ECA is a good one. CA/ECA-produced electrodes for smelting applications have a slow rate of oxidation, high mechanical strength and low heat conductivity. Electrodes are formed from monoliths of ECA, semi-monoliths or a compression process involving high performing resins gluing together smaller monolith sections(14).
CA and ECA are popular refractory choices due to their inexpensive nature, good levels of purity arising from high-quality anthracite, and wide applicability.
As an additive in the production of refractory bricks, coal dust has been long used(15). It has been shown that mostly clay-based fire bricks that had been doped with varying amounts of coal dust, across different grind sizes, were found to be highly effective as thermal insulators(16), raising conventional clay to refractory material status.
Chromite is the naturally occurring ore of chromium. It has a melting point of 2,040°C and is “almost chemically inert”(17). In refractory brick form, chromite is thermally stable well in excess of 1,900°C, whilst maintaining mechanical strength(18). One of the many advantages to chromite refractories is their resistance to deformation, i.e. they maintain a constant volume at high temperatures.
Chrome flour is a finely ground powder of iron chromite and is used extensively alongside the basic refractory magnesite to form magnesite-chrome and chrome-magnesite refractory bricks(19,20), used widely in the construction of furnaces and kilns. Further to this, refractory bricks can also be manufactured using predominantly chromite(21) or with the fellow neutral refractory alumina, which can be added in its unrefined bauxite form, improving significantly the mechanical strength(22).
Other Methods Of Refractory Classification(23)
Aside from chemical composition, there are three other major classification methods for refractories. The descriptions within each category lead to how the finished refractory will be used. For example, an unshaped (form) castable (method) refractory may be used in an aluminium smelting application.
As mentioned, the two broad classifications of refractory materials are ‘shaped’ and ‘unshaped’. ‘Shaped’ refractories are better known as refractory bricks, whereas ‘unshaped’ refractories are better known as monoliths.
Method Of Manufacture
Refractories can be made by several methods, most of which tend to agree with the earlier-mentioned “material processing, forming and then firing”. Examples of refractory production methods include dry pressing, fused casting and hand moulding. Ramming masses, gunning, spraying and castable processes tend to be associated with unshaped forms.
Fusion temperature refers to the temperature range to which refractories are rated. ‘Normal’ refractories perform in the range of ca. 1,500 to 1,800 °C; ‘high’ refractories from ca. 1,800 to 2,000 °C; and super refractories in excess of 2,000 °C. Typical examples are fire clay/brick, chromite/chromite-magnesite and zirconia respectively.
- By chemical composition, refractory materials are grouped into acidic, basic and neutral classes
- Acidic refractories are those which are resistant to acidic slags/environments, examples include silica and ground glass
- Conversely, basic refractories are those that are tolerant of basic slags/environments, such as zirconia and magnesite
- Neutral refractories do not react with basic or acidic slags, and as such they have a wider scope of usages, however in some cases their refractoriness may not be as high. Examples include chromite and anthracite/calcined anthracite
- There are other methods to categorise refractories, including by form, manufacturing method and fusion temperature
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16 M. H. Rahman et al., Procedia Eng., 2015, 105, 121
17 J. O. Nriagu and E. Nieboer (eds.), Chromium in the Natural and Human Environments, Wiley-Interscience, New York, 1988
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19 US Patent US4366256A, 1982, expired
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23 C. Schacht, Refractories Handbook, CRC Press, Boca Raton, United States, 2004 (for entire section)