Coal Dust Refractories
Refractory processes require the best performing materials - such as those supplied by African Pegmatite - including the highest quality milled anthracite, offering versatility and performance across a broad range of high temperature, high demand applications.
Coal dust, or powdered anthracite, is a relatively inexpensive and useful material in a variety of refractory settings such as a component in fire bricks and greensand-type casting applications. In some use cases, coal dust is sacrificial, with its combustion products often affording desirable properties. In comparison to other coal types, such as lignite or bituminous coal, anthracite burns relatively cleanly; reducing health risks to plant workers and having a slightly better environmental profile. Carbon-type refractories are often used in reducing environments(1). In terms of refractory classification, anthracite/coal dust can be grouped under the ‘neutral’ category, i.e. it does not possess the ability to react with other materials (such as molten metals) in an oxidative or reductive fashion - this makes it an ideal candidate as a refractory material component in multitudes of settings. Subject to other pages on this website, calcined anthracite also has refractory properties, and is dealt with on those pages.
Coal Dust In Refractory Bricks
A refractory brick - also known as a fire brick - is one that has a maintains high physical and chemical strength and structure at significantly elevated temperatures, without cracking. They are used to line furnaces and ovens, providing insulation and protection to the heating chamber’s exterior structure. Carbon-containing refractory bricks, such as those containing coal dust, have been extensively used in iron and steelmaking owing to their adaptability and resilience(2).
Coal dust as an additive in the production of refractory bricks has been used for almost one hundred years(3). In one often cited work(4), predominantly clay-based fire bricks with varying amounts of coal dust, in various grind sizes, were made. In all cases, high levels of thermal insulation were observed. The authors concluded that thermal conductivity decreased whilst compressive strength and porosity levels increased with an overall increase in the percentage of coal dust used. With regard to grind size, it was found that with a finer grind (smaller particles), compressive strength and levels of porosity increased, while thermal conductivity decreased. Naturally, the balance of thermal conductivity and compressive strength are key to a successful firebrick. Porosity contributes to thermal insulation, as air is a poor temperature conductor(5), though the authors noted that excess levels of porosity can contribute to a lack of mechanical strength characteristics. Percentage composition attributed to coal ranged from 38 to 68%, and grind size ranged from 20 to 500 μm. Another study probed the same issue, albeit with lower percentage additions of coal dust (5 to 20% by mass), the authors stated that with increasing coal content the mechanical strength increased and thermal conductivity decreases(6).
Porosity is directly related to permeability, and it is stated that permeability is the governing factor in the longevity of refractory materials(7). Porosity is created by the combustion of materials added to the main brick-forming material(8). Pores are created after the material burns off, and the shape of the pores is related to the identity of the material(9), in a process known as ‘burnout’. When coal dust burns, it typically leaves a pore shape that is spherical(10). Thermal conductivity - and thus efficacy as a refractory - also depends on the pore shape, in addition to overall levels of porosity(11). As a general rule, the larger particle size of the additive, the larger the pore size will be realised.
Coal dust is used as an additive in the production of highly thermally stable bricks from red clay mud in developing countries(12), with high levels of compressive strength and porosity, and low water retention.
In addition to coal dust, coal ash waste from thermoelectric power stations has been used in the manufacture of both conventional and refractory bricks(13), with performance similar to commercially available bricks. It has been reported that manufacture of bricks from coal dust and coal ash is a more environmentally method of production(14) as it reduces/reuses waste.
Despite not contributing to the pozzolanic activity of the concrete (the strength afforded through the curing process - this can be influenced by other factors and additives, also available from African Pegmatite), coal dust has been used as an additive in refractory concrete since at least 1910. Thermal performance is increased by its addition, rendering it an ideal choice for high temperature environments and those in consistently hot locations.
Refractory Linings And Coal Dust/Anthracite
In addition to in brick form, anthracite/coal dust can be used as part of refractory linings - in instances where they are part of a monolith structure, for example. Carbonaceous linings of pots for aluminium smelting have been popular for some time(15), with combinations of anthracite and coke/calcined anthracite finding use. Anthracite and Coal Dust derived monolithic linings are desirable due to their low ash content and long term stability. Furthermore in the aluminium production space, cathodes used in the electrolysis stage have been produced which contain up to 45% of calcined anthracite; whilst maintaining stability at high temperature and no loss of electrical conductivity. in addition to more conventional pot-lining settings ensuring the molten metal is held at constant temperature(16).
With regards to refractory linings for iron and steel production, in the blast furnace, a cured monolith comprising 80% anthracite has been used(17). It is crucial that a blast furnace is lined with an effective and long-lasting refractory so as to ensure sustained, reliable production(18). In the modern blast furnace, wear to the lining is particularly concentrated at the bottom of the chamber, the hearth. Here, the liquid metal flow rate is high, meaning turbulence and an uneven level of wear across the lining. Refractories based on anthracite proved their value with high levels of resistance to temperature, even up to multiple heating cycles in excess of 1,000 °C, thermal shock testing, chemical resistance testing and oxidation. In this instance, monolithic anthracite is prized for its bulk volume stability(19). Carbon-type refractory linings of blast furnaces are typically on the order of 700 to 750 mm thick, with lengths around 2 metres(20).
Considering long term service of a particular lining, no material is immune from failure. In general, the more heating and cooling cycles there are, the sooner the refractory will fail. It is noted, however, that a more brittle refractory will fail sooner; coal dust/anthracite refractories are known for their relative suppleness(21). Noteworthy is the outward facing pore size in any carbon-based refractory; if pore size is too large, small amounts of molten metal can collect in them and cool, producing so-called ‘whiskers’. These phenomena can impact on the overall efficacy of the refractory.
In smelter lining applications, anthracite/calcinite and resinous pitch can be used to form a refractory paste, which can effectively fill the gaps between refractory bricks, panels or monoliths(22). Superior compressive strength is conferred by the anthracite present in these pastes, especially at the higher temperature ranges often experienced by such pastes, especially when compared to previous generation asphalt or bituminous carbon based resins.
Greensand And Foundries: The Use Of Coal Dust In Casting Applications
Coal dust is a common additive in the green sand molding process, as a carbonaceous additive that under both combusts and oxidises. In modern green sand casting, anthracite replaces bituminous coal, which combusts upon heating and releases hazardous pollutants such as benzene, xylene and toluene. It is imperative that bituminous coal is replaced with an equally well performing carbonaceous material, but with a less environmentally damaging profile. Experimentally, anthracite(23) emits significantly less hazardous pollutants than bituminous coal does(24). Coal dust use decreases burned-on defects, improves surface finish, and decreases metal penetration. Additionally, it inhibits wetting, a process whereby the molten metal adheres to the sand in the mold and leaves defects on the surface of the product. The use of coal dust also prevents phenomena such as ‘burn on’, where iron oxide is produced on the surface due to the volatile organic compounds given off by the combustion of coal dust(25).
Preventing Wetting In Castings
Wetting is an issue faced in the casting space by many foundrymen, as materials prone to wetting are often used. Wetting is known to create surface defects, yet is often alleviated by using coal dust or anthracite in the sand mixture.
Pyrolysis of the carbonaceous material at high temperatures deposits a thin film of solid carbon at the molten metal - sand interface. Such a layer prevents metal penetration to the sand and sand penetration to the metal. Prevention of penetration means no surface defects can occur, and therefore no post-casting machining of burrs is required. Coal dust that is mostly anthracite, that has a good coking capacity is preferred for this application. Additionally, it will have no more than 30% by weight of volatile organic compounds, less than 0.8% sulfur and low ash(26).
A mild pressure increase is to be expected and is observed due to the pyrolysis expelling gases as well as evaporation of water or other liquids in the sand. Such a pressure increase is moderate and is easily tolerated by the sand and metal, however if hydrogen is released it may penetrate the metal as it has a sufficiently small atomic radius(27).
Coal Dust In Continuous Casting: Tundish Linings
Modern foundries make extensive use of continuous casting techniques to ensure high plant utilisation, driving down the cost of production. One of the tools required to achieve this is a tundish, which is a chute or box-like structure used for the transport of molten metal from one part of the plant to another.
Tundishes are made of steel and possess several layers of insulation between their outside skin and where the molten metal will be handled. Magnesia carbon is one of the most commonly used insulators for its superior thermal performance - it is made of magnesia and a source of carbon like anthracite or graphite. The carbon modulates and largely prevents the expansion of the magnesia at the highest temperatures, making it more suitable for long term use with many heating and cooling cycles. Spalling is decreased with increasing carbon content(28), whilst the overall material’s Young’s modulus is increased with more carbon present(29). In magnesia carbon refractories for tundish linings, anthracite is used in up to 15% by weight. One disadvantage is that degradation of the refractory can be accelerated by the presence of iron oxide that oxidises the carbon(30).
Calcined anthracite has been used in conjunction with a resinous pitch to produce a highly effective refractory paste, used in aluminium smelter pot linings. Continuing the theme of refractory paste, it has been found that anthracite-rich resins are far superior in terms of compressive strength at high temperatures than commonly used asphalt-type resins(31). Coal dust has found utilisation in refractory cements and concrete, with inclusion as part of the aggregate component in a concrete mix as early as the 1910’s and 1920’s(32,33). Although the addition of coal dust does not contribute to the pozzolanic activity of the resultant concrete slab, the inclusion does increase the overall thermal performance. Such performance enhancements are particularly valued where concrete may be subject to intermittent or sustained high temperatures, such as in power stations, as a further line of defence alongside the steel reinforcements within the concrete itself.
- Coal dust (anthracite) is a useful material that is found as additives in refractory applications, particularly attractive owing to its relatively inexpensive nature and ubiquity
- As an additive to refractory bricks, it provides enhanced properties largely by increasing porosity
- In furnace linings, coal dust as a refractory material provides for long term strength and high thermal performance; in steel production it is used to line the high impact areas of the furnace, providing long service lives
- Within casting applications, it is a common additive to green sand molding providing for a more efficient process by preventing defects such as burn on and inhibiting wetting
- In continuous casting, coal dust/anthracite is a vital component in some of the layered materials used for tundish linings, offering enhanced mechanical properties to a largely otherwise manganese based refractory
- In other areas, it is used as a refractory paste and in cements/concretes providing for higher thermal performance characteristics
- The use of coal dust/powdered anthracite across a range of refractory settings is testament to its high performance, despite the seemingly counterintuitive nature of using combustible material in very high temperature scenarios
- Calcined anthracite (not discussed here) has further refractory properties, substantially expanding the scope for an already ubiquitous and dependable material
African Pegmatite is a leading miner, miller and supplier of the highest quality anthracite for a variety of applications, including in refractories. Providing dedicated service, a wide reach and decades of experience, African Pegmatite is the go-to partner for milled anthracite and a full suite of other refractory materials.
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