Refractory, Definition, Types, Materials and Applications, a Guide
African Pegmatite is a leading processor, miller and supplier of an increasingly wide range of refractory materials suitable for a wealth of applications where quality matters.
Introduction To Refractory Materials
Refractory materials are used in some high-intensity and highly industrial applications, requiring robust protective materials. These applications range from furnaces and kilns to incinerators, and even reactors. Refractories are used to make crucibles and similar uses like moulds for casting glass, metals and for flame retardant and heat deflecting surfaces for even as high temperature as rocket launching structures.
These materials are classified by multiple factors, with designations led primarily by the material’s chemical or physical properties, with physical characteristics further broken down into form, method of manufacture and fusion temperature.
Chemical Classification
Chemical classification of a refractory material refers to whether it is a basic, acidic or neutral material. Broadly speaking, any material type is best suited to a refractory environment that is akin to it. For example, basic refractories are best used when the refractory environment is also basic or produces a basic byproduct.
Acidic
Acidic refractories are those which react with bases (alkaline materials) at high temperatures. Some of the most common examples of acidic refractories include fire clay, silica and aluminosilicates. Acidic refractories are best suited to where the atmosphere around the refractory, or byproducts of the process contained within the refractory, is acidic itself. As acids do not react with acids, this means there will be no attack and thus no material degradation, ensuring a long service life for the refractory material(1).
Basic
Basic refractories will often react with acids at high temperatures. Basic refractories, therefore, are most suited to when the atmosphere in the local environment and/or the slag produced will be basic as well. Magnesite, zirconia and dolomite are widely used refractories. Basic refractories are often the highest temperature refractories, however this means they come at a greater cost(2). Non-ferrous metal production is the most common use for basic refractories.
Neutral
Owing to their broad tolerance of both acidic and basic atmospheres and slags, neutral-type refractories are used across many applications. oxides and carbon are the two most common subcategories of neutral refractories, with carbon refractories being particularly valued in reducing environments(3), and owing to their lack of reactivity with acids and bases, neutral oxide refractories are often considered superior refractories, prized for their expansive utility and performance over a broad array of settings.
Physical: Form
The first of the physical classification subcategories is form, this is where refractories are grouped by final appearance. The three broad categories are shaped, unshaped (often referred to as ‘monolithic’, and also contains the major subcategory, castables) and fibrous.
| Refractory type | Examples |
| Shaped | Refractory bricks, fire bricks |
| Unshaped (monolithic) | Castables, gunning and ramming mixes, mortars, spraying and coating mixes, monoliths |
| Fibrous | Ceramic fibres |
Shaped Refractory
Some examples based on the shaped form classification include fireclay, refractory bricks and silica brick. Various base substances within these example categories might be coal dust, refractory chrome, refractory chrome sand, and glass powder.
Within these substances, there are variations within each type. Taking the example of fire bricks, these are broken down into five subgroups of classification based on their levels of refractoriness, and thus this defines the environments in which they can be used, as follows.
- Super Duty (PCE 33)
- High Duty (PCE 31)
- Medium Duty (PCE 29)
- Low Duty (PCE 15)
- Semi Silica (S10, > 72%)
PCE stands for pyrometric cone equivalent, and is a broad measure of the heat tolerance of a material, with higher PCE numbers corresponding to greater stability at higher temperature, thus it can be stated that these classifications are related to the softening temperature of the material. A PCE number of 33 roughly corresponds to stability of heating to 1,710 °C at a heating rate of 150 °C per hour, whereas a PCE of 15 is stable only as far as 1,425 °C at the same heating rate.
(Pyrometric cones themselves are measures of the heat produced inside an oven, kiln or other hot environment. Developed originally for the firing of clays for ceramic uses, pyrometric cones are small clay cones of known deformation points used inside the kiln, these deform when they reach a certain temperature, providing insight into the kiln temperature and the evenness of heating.)
Based on these criteria, lower PCE materials such as many fire bricks, their use is recommended in lower temperature environments, for example nlet zones, coolers in cement kilns, pizza ovens and backup lining in steel and glass applications.
There are other criteria applied to these materials too, such as classifications by service temperature and bulk density and other variable temperatures such as thermal conductivity, load-bearing, and insulating properties.
Fibrous Refractory Materials
As per the name, these refractories are fibrous, and they take this form through their manufacturing process which is akin to candy floss manufacture; broadly speaking a molten refractory is extruded and then spun, forming fibres. Fibres can then be aligned and pressed into boards, if needed.
In modern usage, these materials usually take the form of boards and in some cases pliable blankets. They typically provide high temperature insulating materials, with fire blankets being the most widely recognised example of a fibrous refractory application.
Unshaped or Monolithic Refractory Materials
Unshaped refractories are some of the most common materials utilised in the high temperature operations space.
These materials are normally classified by their application use. This could be a mortar, casting, gunning, ramming, and others use. Refractory mortar, for example, is made up of fine particles of heat resistant materials which are used as a binding material, used in binding bricks. There are two types of mortar, heat setting mortar which is heated up to 1,000 °C to create a bond in the bricks, and the other is air setting mortar which is chemically bonded and hardens at room temperature. This second type of mortar is used more in applications needing load bearing such as kilns, towers, stacks and roofs.
Castables
Castables are a crucial and widely used subset of unshaped refractory, where a refractory is as part of a non-solid material which is poured and then cures in situ. There are five groups of castable:
- General Castable
- Low Cement Castable
- Ultra-Low Cement Castable
- No Cement Castable (especially used for organic binding)
- Insulating Castable (for low heat loss applications)
Castable material is carefully selected based on its variable characteristics, depending on the application. These criteria would typically depend on characteristics including, but not limited to:
- Refractoriness
- Resistance to Abrasion
- Strength
- Load-Bearing Capabilities
- Thermal Conductivity
- Thermal Shock Resistance
- Chemical Resistance
Physical: by manufacture and by fusion
The major physical grouping of refractories is by far form. However, that is not to say that valid and useful classification by method of manufacture and by fusion cannot be drawn. The modern refractory engineer will often use fusion temperature as a guide of which refractory is best to use in a particular setting, whilst the method of manufacture may produce a refractory more physically suited to the application the engineer has in mind.
Method of Manufacture
A further way of differentiating refractory materials is their method of manufacture, which itself is a proxy for their end use cases, and these refractory materials are often named after their specific applications and are usually selected based on their specific application needs.
Type of Refractory Application
Gunning mix For applications requiring gunning machines for applying the material
Ramming mix Widely used in the base of electric arc furnaces
Spraying mix For spraying applications with special spraying machines
Coating Materials Where coating methods are involved in the application
Patching mix Manual patching work
Plastic Semi-wet and ready-mix applications for reparation uses in steel industries
Fusion Temperatures
The final and one of the most crucial factors to sort between refractories is fusion temperature, and it is often the final arbiter to which refractories are rated. ‘Normal’ refractories perform in the range of around 1,500 to 1,800 °C; ‘high’ refractories from approximately. 1,800 to 2,000 °C; and super refractories are those with a fusion temperature in excess of 2,000 °C. Fire clay/brick, chromite/chromite-magnesite and zirconia are classic examples of normal, high and super refractories respectively. Naturally, the highest grading of refractory material will also be the most expensive.
Examples Of Refractory Materials
African Pegmatite proudly boasts a competitive and comprehensive product range and deals with the type of base materials you'd find in most refractory products. Some of African Pegmatite’s leading products are listed below.
Coal Dust
Among many other applications across many types of industries, coal dust is used in the manufacturing of refractory bricks. Used in the production of bricks to raise conventional clay to refractory status, coal dust doped refractory bricks have many uses around the world, across many industries, uses include open hearth furnaces, electric arc furnaces, metallurgy furnaces, cement rotary kilns, as well as glass kilns. Coal dust provides the thermal insulation necessary for these bricks to perform as needed at high temperatures, where they have been found to be highly effective as thermal insulators(4).
Coal dust has been used in refractories for almost a century, with the resultant refractories known for their resilience and broad applicability(5). Addition of coal dust to a refractory blend has been shown to increase levels of thermal insulation, with smaller grind sizes leading to better compressive strength, increased porosity and lower thermal conductivity(6). As porosity is related to both insulation and durability, greater levels are naturally desired and coal dust is ideal to provide this, and can be used in a refractory mixture prior to firing at up to 70% inclusion(7,8).
Anthracite
Although it seems counterintuitive to use as a refractory, anthracite is another material which can be added to clays or other materials to give them an enhanced degree of refractoriness, particularly when the anthracite itself has been calcined either by conventional or electrically assisted heating. The calined version is more porous than untreated anthracite which also adds to its strength. Calination causes anthracite to undergo a graphitisation process at around 2,200 °C(9), at which point it also becomes more electrically conductive which is useful for smelting applications. Anthracite, in both forms, is a popular refractory choice due to its relatively inexpensive nature and wide applicability.
Anthracite has been used to line the hearth of blast furnaces since at least the 1960s(10), with service lifetimes exceeding 15 years not uncommon, compared to silica hearths lasting only two years in some cases(11). When formed into a monolith, calcined anthracite is especially resistant to thermal shock, chemical attack and oxidation even at extreme temperatures(12). Anthracite is also used in fillers and joints(13,14) but use in linings is restricted only to microporous calcined anthracite, owing to its better resistance to alkali attack(15).
Refractory Chrome Sand
Chromite is the only naturally occurring ore of chromium. Boasting a melting point of 2,040 °C, it has the luxury of being “almost chemically inert”(16). With no loss of strength, in brick form, chromite is thermally stable to temperatures past 1,900 °C(17). Chromite refractories are valued for their high levels of resistance to deformation. It is used in the production of chrome magnesite bricks, these bricks are most commonly used in the construction of certain types of furnaces and kilns. Chromite is used in refractory applications at a concentration not less than 36%(18).
Used in making blends for ladle sand and nozzle sand. Also used in making bricks. Due to its highly corrosion-resistant nature, chrome sand is also used to produce magnesia chrome refractories. magnesia chrome refractories are used in non-ferrous metals like copper, lead, and zinc.
Filler sand made with chrome sand also prevents molten steel from settling inside tap holes and solidifying (like chrome flour). And chrome sand also allows this solution to be effective at higher temperatures with long processing times.
Specific applications of chromite in refractory settings include as binders, where a refractory material is used to bind together aggregate materials, and chromite has largely replaced phenol, urethane and furan binders(19). As part of resin bonded castables, chromite is a highly valued additive which because of its superior refractory properties makes casting for especially high temperature molten metals such as magnesium alloys(20) and titanium(21) much more viable, alongside conventional iron and steel castings. Because of its expensive nature relative to the end casted products, however, it is rarely used at a greater than 10% concentration by weight in the casting application(22).
Refractory Chrome Flour
Chrome flour is a finely ground powder of iron chromite. Used in the production of chrome magnesite bricks by way of a refractory cement mix, these bricks are then used in the construction of certain high temperature furnaces and kilns. Indeed, refractory bricks can also be manufactured using mostly chromite alone(23). When bricks are made using chromite and alumina, their mechanical strength is noticeably increased - an added extra benefit(24). Resistance to shock and spalling is increased by the addition of chrome flour(25), whilst temperature performance is also increased(26).
Used in the making of blends for ladle sand and nozzle sand. Also used in making a variety of bricks. Chrome flour is used in the production of chrome magnesite refractory bricks, these kinds of bricks are mostly used in the construction of certain types of furnaces and kilns. It has a highly corrosion-resistant nature to it which prolongs the life of the kiln. Chrome flour is also used widely with the basic refractory magnesite to form magnesite-chrome and chrome-magnesite refractory bricks(27,28), which are used widely in the construction of furnaces and kilns best suited to the production of non-ferrous metals like copper, lead, and even zinc from their ores. One issue in the kiln/furnace is the presence of slag and ash, which can be a problem for other refractories, but are widely tolerated by chrome flour(29). Chrome based refractories are the leading choice in gasifiers, and throughout the Fischer-Tropsch process(30).
In stainless steel production environments, filler sand - made with chrome flour, prevents molten steel from settling inside any tap holes and solidifying. The chrome flour refractory allows this solution to be effective at high temperatures with long processing times.
Glass Powder
Although it isn’t a refractory in its own right, the addition of finely milled ground glass can enhance the refractory behaviours of other refractory or semi-refractory materials. Ground glass powder is widely used in the foundry industry as a formulation in fluxes and in the manufacture of tundish liners. A tundish acts as a reservoir and a distribution channel in the continuous casting of steels and aluminium. One particularly attractive property of ground glass when used in a tundish or a furnace lining is that it is able to remove iron oxide from molten iron and steel(31). Silica is the primary component of glass, and hence ground glass behaves in a similar way to silica in the refractories space, i.e. it acts as an acidic refractory. Furthermore, ground glass can also act as a flux, affording benefits such as lower heating energy requirements to a process. Using ground glass as a flux enables a greater tolerance to rapid temperature changes and a superior resistance to harsh chemicals in the final product(32). As with glass in other refractory types, the combination of ground glass with other materials such as kaolin and clay can afford an excellent result, such as the synthetic Wollastonite ceramics as found in rockwool insulation(33). In the traditional fire brick, ground glass has been added to silica to form fire bricks especially suited to glass-making furnaces.
Overall, strength increases in ceramics and refractories formed with ground glass are posited to be stronger because the ground glass provides a better developed network of interactions between itself and clays across different crystal phases(34). Thermally stable tiles and frits(35) can be produced using ground glass, either virgin or from recycled sources, with or without ceramic sludge. Such broad tolerances are a further string in the bow of ground glass as a refractory additive.
Increased glass content has been shown to lower environmental costs(36), whilst providing an overall increase in bulk density, strength and resistance to shrinkage during firing. In refractory cements, the addition of glass is beneficial for pozzolanic development, whilst reducing alkali-silica expansion and improving durability(37).
Typical Applications
Applications for refractory materials are almost incomprehensibly broad. Anywhere a high temperature needs to be tolerated, protected from or maintained, it is highly likely that a refractory material is present in one form or another.
Refractory Bricks Applications
African Pegmatite provides the materials to create specific shaped and custom bricks, including those containing silica, ground glass, anthracite and many others, for a broad range of shaped refractories.
Refractory Lining Applications
Often used in blast furnaces and cement rotary kilns, using shaped and predominantly unshaped materials, lining applications are some of the most common uses of refractory materials. Bolstering industrial production by insulating blast furnaces, smelters and tundishes, linings have a requirement for a large amount of high-performing, long lasting refractory materials.
Boiler Refractory Applications
High end, typically fibrous, refractories are used in heat retention for boiler applications. The use of such insulation materials ensures more efficient heating of the boiler, less heat loss and therefore a lower energy requirement overall - saving the end user money.
Refractories In Consumer Applications
Far from the sole preserve of heavy industry, refractory materials are often found in domestic settings. Aside from the aforementioned fire blankets, other common forms of low-level refractory material uses include fire bricks surrounding the fireplace, garden fire pits, aspects of the components of the lower part of chimneys and even the mortar holding the bricks together. Finally, refractory materials are used in certain glasses used for packaging, offering the glass a better level of thermal performance.
Summary
- Refractories are a broad class of materials united by their strong resistance to high temperature
- They are subdivided into several categories based on their resistance to heat, chemical attack, preparation type and use cases
- Common refractory materials/additives include coal dust, anthracite, chrome sand, chrome flour and ground glass powder
Refractory materials are an essential part of modern industry, providing for a wealth of manufacturing processes. African Pegmatite is a leading supplier and processor of high quality refractory materials, milled to exacting specifications.
References
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