Anthracite In Castings, Refractory Linings And Other Foundry Applications
African Pegmatite is a leading supplier of anthracite, calcined anthracite and electrically calcined anthracite for a variety of foundry applications. Many modern foundries rely on anthracite (and calcined variants) due to its relatively high resistance to thermal shock, strength and chemical inertness.
Anthracite is a type of coal which, in addition to the obvious use as a combustible fuel, has other applications in the refractory/high temperature manufacturing field. As it is combustible, anthracite’s use in high temperature applications require it to be used in a sacrificial manner, in a heat-treated form (calcined) or as a component in a wider refractory. Anthracite is one of the harder forms of coal, with a high carbon content, it burns relatively cleanly compared to its peers. Anthracite’s abundance and thus low cost makes it an attractive material for a variety of applications.
For brevity and clarity, anthracite that has been calcined will be referred to as ‘CA’ throughout this text. A high quality calcination process, for example electro-calcination, is required to attain high quality CA - but this overall requires a good source of anthracite as a starting material(1)
In casting environments, the pore size of the casting mold is important as if pore size is too large, small amounts of molten metal can collect in them and cool, producing ‘whiskers’, which must be milled off the final product, adding time and expense to a process. CA has small and consistently sized pores, and depending on the casting, the CA may behave in a sacrificial method and burn off. In particular utility to metal production, highly calcined anthracite can be used in a monolithic castable graphitic refractory(2). This particular example is formed as a slurry and the monolith is generated in situ in the furnace it is due to line, but the pouring and curing of a slurried refractory around a pattern in the traditional sense is also a popular method. When anthracite is added to refractory castables, castings have been reported as being smoother(3) and with lesser amounts of pollutants due to CA’s relative purity compared to other carbon sources.
In the design of blast furnaces from the 1960’s onwards, anthracite-based refractories are used to line the hearth (i.e. the bottom) of the furnace(4). Such linings are typically found on the order of 3 m thick, and some are water or air cooled, these refractories provided an endurance of around seven years - significantly improved over the previous silica/alumina refractory hearths whose campaigns lasted no more than two years(5). Modern designs of furnace utilise an approach where a ceramic cup is used in contact with the molten metal, with an anthracite-graphite refractory in contact with the furnace’s walls and hearth. Furnace life for the combination ceramic and anthracite-graphite is estimated at 15 years.
Wear of the furnace is particularly concentrated in the hearth, where temperatures often reach their highest and liquid metal flow rate is high. Monoliths based on anthracite have been used in hearths, providing resilience over multiple heating-cooling cycles.
In parts of the blast furnace, a monolith comprising 80% CA has been demonstrated for use in the hearth(6). Here, the liquid metal flow rate is high, meaning turbulence and an uneven level of wear across the lining. CA, in experimental testing, is resistant to thermal shock, oxidation and chemical attack at temperatures exceeding 1,000 °C over the lifetime of the furnace run. Monolith CA is bulk volume stable under furnace conditions(7).
When used as conventional insulation away from the hearth, carbon-type linings are typically on the less than a metre thick(8) and are in refractory brick form as opposed to monoliths. In general terms, thermal stress/shock towards carbon-based linings has historically been a problem, but with the use of CA(9) and suitable fillers in joints(10), combined with temperature dispersion across a large thermal mass, thermal shock is not seen as a significant concern during normal operations in a contemporary furnace environment. Fillers themselves can have significant proportions of anthracite in their composition(11). Alkali attack does remain a concern, especially at the highest of temperatures, but some studies have shown that with the use of microporous CA, the effects of alkali attack are minimised. This paves the way for longer term CA use in linings.
Smelting And Molten Metal Holding Tank Applications
Smelting and holding tank applications require long term resistance to temperature and thermal shock - CA is valued for its resistance to these and to chemical corrosion. Aluminium smelting, in particular, has afforded the use of carbon-based smelter linings and electrodes for many years(12), with electrode examples from the early 1980’s employing up to 75% CA by mass and having performance equal to those with purely graphite electrodes(13). Even accounting for high quality calcining processes, CA is more cost effective than pure graphite whilst retaining high levels of performance. As a lining, the CA is chiefly present for the insulation of the pot and/or holding tank, i.e. to prevent the molten metal setting in situ.Electrodes and linings can be formed in the conventional way as bricks, as monoliths or even from pastes that are largely CA (and small amounts of other carbon types) based(14). As electrodes, a further brief discussion on why CA is suitable for this application can be found below.
As CA is one of the most popular choices of lining of aluminium smelters, with smelter intervals lasting up to six years, shutting down only to replace the cathodes(15), CA is used in some scenarios as both a cathode component and as a lining. The shut down is required due to the significant build up of aluminium carbide and whilst carbide formation is favoured on disordered carbon structures (such as CA)(16), CA is more resistant to electrochemical wear(17), as adhesion of carbides may be stronger to more ordered structures. Therefore, CA can be used alongside other carbon-type materials such as graphite to provide a lining and a cathode that fulfils idealised properties for conductivity and chemical resistance, whilst balancing issues of cost. Uncalcined anthracite would not routinely be used.
Electrically Calcined Anthracite As Electrodes
In addition to lining smelting pots, electrically calcined anthracite (ECA) may be used as the electrodes themselves as well as ramming pastes. ECA is also known for its superior temperature performance and enhanced electrical conductivity. Electrodes can be made from monolithic calcined (or electrically calcined) anthracite, semi-monoliths held together with ramming pastes or by compressing CA or ECA and resin into a form with heating(18).
Despite most installations for the production of aluminium from its ore using an anthracite cathode, there are some examples where ECA has been used as an anode. Classical installations that may have used petroleum coke at the anode have been replaced in some instances by ECA(19). ECA derived anodes can be effective when as little as 20% by weight of anthracite is used - however greater efficiency is achieved when at least 40% by weight is used. Anthracite is particularly favoured because of its low ash content - high ash content carbon sources are associated with low efficiency electrodes that do not last the test of time(20) - far from ideal in a situation requiring constant high throughput for elongated periods.
The cathode has been the more traditionally associated home for CA and ECA materials in the aluminium and other non-ferrous metals smelting sector. The cathode is the positively charged ‘end’. As electrodes, the superior electrical conductivity of the CA and ECA is valued. As materials that will be subject to high temperatures, the long term thermal stability of CA and ECA is prized. Contemporary cathodes based on CA or ECA are made up of around 70% anthracite by weight with the remainder being graphite and some kind of tarry binder. The use of superior quality anthracite is crucial as even modest levels of impurities such as sulfur can increase the thermal expansion of the cathode; thermal expansion leads to an increase in electrical resistance; thereby making the electrochemical process less efficient and requiring more energy input(21). Further benefits to using CA or ECA in electrodes are high mechanical strength, low heat conductivity and a good resistance to oxidation.
Ramming pastes are complex mixtures used by foundrymen to repair any cracks that may form in electrodes or furnace linings - or even to join together monoliths to form the furnace lining in the first place. Ramming pastes can be thought of as a ‘cement’ that holds refractory linings together as well as a ‘sticking plaster’ for quick repairs, albeit a repair that lasts a long time. The use of ramming pastes is useful as it means that a whole electrode does not need to be replaced, for example, or a highly intricate monolith needs to be produced as smaller, easier to make monoliths can just be fixed together instead. The latter procedure is referred to as the formation of a ‘semi monolith’(22) and although performance is not as good as for a single, continuous monolith, cost and complexity are both significantly reduced.
The highest quality ramming pastes are made from calcined or electrically calcined anthracite. Like the material used for the electrodes, it is strong and valued for long term stability. As per research, ECA-based ramming pastes are stronger than those made from conventionally calcined anthracite or synthetic graphite(23). The binder materials used in the formation of the paste do not play a role in the paste’s overall strength - that is to say that the strength is largely due to the presence of ECA. Around 80% by weight of the paste is typically carbonaceous aggregate such as ECA with around 5 weight% being the binder and the balance being pitch(24).
Ramming pastes based upon any of the variants of anthracite are used as cold ramming pastes. The ‘cold’ refers to the fact that they are applied at ambient temperature and allowed to cool at ambient temperature. Whilst this means that some time is required to allow electrodes or pot linings to cool, it means that extra heating to cure the paste is not required. Cold ramming pastes are valued for their high compressive strength and low electrical resistance characteristics(25). Low levels of expelled gases when cold ramming paste is eventually heated in situ suggest that ECA-based cold ramming pastes are a much more environmentally friendly option than other more bituminous options(26).
The Rapoport effect in smelting and electrode applications refers to the tendency of carbon-type cathodes or cathode blocks to expand at temperature due to the penetration of fluorinated and sodium-type compounds, themselves largely relating from impurities in the metal. The Rappoport effect is a physical, not chemical, phenomenon. Such expansion reduces the efficiency of the cathode(27) and decreases its available surface area for electrolysis processes. Calcination temperature and structure of the original calcined material are the major arbiters of whether the Rappoport effect will be observed(28). Small and consistent pore sizing on CA is viewed as advantageous to mitigating the Rappoport effect. It has been reported that Rappoport expansion is inversely proportional to calcination temperature up to 2,000 °C.
Other Foundry Applications
Calcined anthracite has been used as a component in electrodes in furnaces, in addition to the previously-mentioned lining application, for metals such as titanium and the previously mentioned aluminium. Such electrodes are typically largely carbon or graphite based. The chief reasons for the use of CA are its inexpensive nature and interesting electrical resistance profiles(29). Untreated anthracite is relatively highly electrically resistive (i.e. a poor conductor), however CA treated between 600 and 900 °C shows a resistivity loss on the order of two to three orders of magnitude; reaching only 1,000 µΩ at 1,300 °C(30), with similar values being observed in other studies(31). Calcination is also associated with an increase in structural strength, with levels of porosity fluctuating but substantial changes are not noticed. Notably, anthracite begins undergoing graphitisation at ca. 2,200 °C(32).
- Anthracite is a useful and inexpensive form of coal that has many applications in the refractory sphere
- In metal fabrication, CA can be used in the production of molds for molten metal castings
- As a key component of blast furnace linings, CA is valued for its performance and longevity at high temperatures, over long timeframes, in either monolith or refractory brick form. These are widely used in iron and steel production
- CA is used as one of the key materials in smelter pot lining for electrically-refined metals such as aluminium - a key industrially and economically important process
- In the production of other metals, CA can be used as a component in electrodes owing to its structural strength and good electrical conductivity profile at temperature
- Electrically calcined anthracite finds wide use as electrodes, particularly for aluminium smelting, as well as in ramming pastes
Anthracite, calcined anthracite and electrically calcined anthracite are all widely used in the modern foundry - perhaps counterintuitively. African Pegmatite is a leading supplier, miller and processor of the finest quality anthracite - which can be calcined as required.
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