Introduction To Calcined Anthracite
Anthracite is one of the higher quality forms of carbon, itself used for a variety of applications. It is commonly located throughout the world and benefits from a highly developed mining and distribution regime. Improvement processes for the already low-ash and low-volatile organic containing anthracite such as calcination have long been used to modify anthracite to make it into a refractory material, with a porous structure.
Calcination is a process by which a material is significantly heated but not allowed to combust. It is often used to enhance the strength properties of a material, to enhance hardness, or to simply provide a longer lasting material better resistant to erosion or decay.
Anthracite is an often calcined material, with calcined anthracite having a wide bouquet of uses across foundry and metal production applications(2,3). For applications such as in electrodes, calcination decreases the electrical resistivity properties of the material and removes any residual volatile organic compounds. Non-calcined anthracite is an electrical insulator (i.e. it conducts poorly). Studies have shown that CA/ECA calcined as low as 900 °C shows a substantial boost to its electrical conductivity; reaching only 1,000 µΩ of resistance at calcination temperatures of 1,300 °C(4,5).
Ahead of any calcination process, it should be stressed that a high quality anthracite must be used as a starting material(6).
Applications
It is perhaps counterintuitive that anthracite has been long used as a refractory - on the grounds that as a form of coal, it would readily combust. It’s an example of neutral refractory material, meaning that there is no reactivity with acidic or basic atmospheres or slags. As heat treated versions of anthracite, CA and ECA are stronger, more porous and better electrical conductors than untreated anthracite.
Throughout the world, major applications for ECA are the manufacture of electrodes, electrode paste and in the production of steel and aluminium. Typically, ECA can be used wherever CA is used. One vital consideration when choosing between CA and ECA is environmental - the use of gas furnaces to produce conventional CA produces significant quantities of greenhouse gases at site. On the other hand, ECA can be produced using ‘clean’ or renewable energy if the supply is from a suitable source.
For the production of aluminium, ECA is used in the lining of the pot smelter as an insulator in addition to electrodes (see below). For steel and other ferrous metals production, it is used as an electrode for electrical smelting processes(11). ECA/CA can also be used in foundry cupolas(12).
Herein, primary applications of ECA will be discussed, which are the production of electrodes and ramming pastes. With regards to electrodes, modern examples are typically carbon or graphite based, with the carbon being CA/ECA. CA/ECA are used due to their relatively inexpensive nature and well suited electrical resistance properties(13).
During calcination anthracite starts to undergo graphitisation at approximately 2,200 °C(14). In effect, due to the graphitisation, synthetic graphite is formed as part of the process of producing ECA(15). Electrical calcination is more energy efficient than achieving calcination by gas heating. Graphite itself is a refractory material. CA has small and consistently sized pores. Calcined anthracite finds extensive use in monolithic castable graphitic refractories which is of great utility to ferrous and non ferrous metal production. Wide applicability, good levels of purity and low cost make ECA a popular choice in refractory settings.
Specifically for smelting applications, ECA has an interesting electrical resistance that is significantly lower than anthracite(16); ECA is a good conductor. This makes it ideal for use as an electrode for aluminium smelting (more later) and in addition, electrodes based on ECA for smelting have a slow rate of oxidation. Combined with a high level of mechanical and compressive strength and low heat conductivity, ECA is an ideal choice. Electrodes may be formed from monoliths of ECA, semi-monoliths or a composite princess where resins are used to glue together small monoliths or powdered ECA.
Electrodes-cathodes
CA and ECA have long been used for the cathodic portion of an electrochemical cell to produce/extract a variety of elements from solutions and/or for the smelting of non-ferrous metals. CA/ECA are most often used as the cathode (the positively charged ‘end’) and rely on CA/ECA’s excellent electrical conduction properties in addition to long-term bulk stability.
In an electrothermal furnace, heat is provided by passing current through carbonaceous electrodes, such electrodes can be composed primarily or solely of ECA. Electrodes for this application must have a high capacity for electrical conductivity, a slow rate of oxidation, high mechanical strength and low heat conductivity. High quality ECA has these properties. Electrodes can be formed from monoliths of ECA, semi-monoliths (see ramming pastes below) or via a process whereby crushed ECA and a resin are formed into an electrode via compression and heating(17).
When used for aluminium smelting, electrodes based around ECA contain around 70% by weight ECA that has been heat treated to in excess of 1,200 °C, with the balance being tar and milled graphite. Interestingly, research has shown that electrical resistance increases with thermal expansion of the cathode, and this expansion increases with a greater sulfur content in the CA/ECA. Therefore, a crucial factor in cathode production is the selection of the highest quality anthracite(18).
Electrodes-anodes
Although less popular, it is possible to utilise ECA as an anode in aluminium production. Requiring the same properties as for a cathode, ECA has largely replaced petroleum coke as the identity of the anode(19). Research showed that anodes with as little as 20% by weight ECA content are feasible, but idealised at around 40%. Again, high purity coal with low ash content should be used in the first instance, as high ash contents are partly responsible for low-lifetime and low-efficiency electrodes(20).
As already mentioned, one of the superior qualities of ECA is its highly conductive nature and so it is apt that a rapidly growing application for ECA is in the production of anodes for modern battery cells. The clearly defined molecular structure and low density mean that such anodes are lightweight in addition to being highly conductive. Use cases include in high end lithium ion cells(21) and in electric vehicle battery arrays(22). Some of the highest performing examples are found in sodium-vanadium-phosphate cells where the deployment of ECA as an electrode enhances the batteries properties in terms of enhanced energy density levels and ability to be charged rapidly(23). As an electrode, ECA can be used as all or part of the electrode mass, often exceeding 50%, or even as a coating.
Electrode/ramming Pastes
Anodes and cathodes when composed of ECA are known for their long term stability, but sometimes incidents will happen causing cracks or other similar damage to them. In such instances, repair is often preferable to replacement, and so ramming pastes are used for the repair to the electrode slab in a similar way to plaster filler used in domestic walls. The highest quality ramming pastes are made primarily from ECA.
Furthermore, semi-monolithic ECA electrodes can be formed by using ramming pastes to join smaller ECA monoliths together, where the ramming paste behaves akin to glue - monoliths are placed together with ramming paste and the whole system is compacted to form one large semi-monolith(24). Although performance is not as good as for a single monolith electrode, cost is significantly reduced, and the strength of the ECA-containing ramming paste is stronger than those pastes made from CA or synthetic graphite according to the secondary literature(25). The same work suggested that the binder material used in the paste, which was quinoline based, had no impact on the density or compressive strength of the paste.
ECA-based ramming pastes are used preferentially as cold ramming pastes - i.e. they are applied and allowed to cure at ambient temperature. This alone confers an advantage in that heating is not required. Such ramming pastes enjoy low electrical resistance properties and high compressive strength values. Data from studies in China indicates ECA ramming paste using phenol formaldehyde resin as binder has resistance in the 50 µΩ region with circa 30 MPa of strength(26). As per patent literature, ramming pastes contain approximately 80% by weight carbonaceous powdered aggregate and up to 5% by weight binder. The balance is typically pitch(27). In addition, cold ramming paste based on ECA can be regarded as an ‘eco-friendly’ material owing to a low toxicity measurement of escaped gases from when the paste is heated(28). ECA can be produced from chipped/powdered anthracite and is the ‘carbonaceous powdered aggregate’ here. ECA ramming pastes are known for their long term stability, and not just as a ‘quick fix’ for damaged monolithic electrodes.
Considerations When Using ECA
In some earlier generation arc furnaces, there could be a radial temperature gradient, which can result in a lack of homogeneity in the calcination process. Alleviated by modern heating elements, this is no longer considered a problem. The Rappoport effect is a phenomenon whereby the pore structure of CA/ECA is caused to expand due to sodium- and fluorine-containing compounds penetrating them(29), such expansion causes inefficiency in electrical conductivity.
In many applications of ECA (and CA) such as in electrodes and ramming pastes, part of the composition commonly includes some kind of resinous or carbonaceous binder and/or filler material. Research has noted the importance of these materials and their interactions with the calcined materials - with these materials also contributing to the long term stability of the electrode or ramming paste in question. When fillers for electrodes had themselves been made from ECA, their overall mechanical strength was less related to variations in the pore structure compared to when simple coke had been used as filler(30). In the ongoing replacement of coke in these situations, high quality ECA is used. Studies with an aim to optimise for ECA have shown that when ash content is set at 0.95% by weight and a volume density of 1.452 g cm-3, supreme compressive strength values of 37.59 MPa and electrical resistance values of 54.72 μΩ m-1 can be achieved(31).
Other Applications
Owing to the nature of high quality ECA undergoing a graphitisation process under heating, synthetic graphite can eventually be formed from ECA(32); anthracite starts to undergo graphitisation at approximately 2,200 °C(33). Although a minor use, powdered ECA can be used as a high-performance carburiser in grey iron production from steel wastes(34). ECA is perceived as a significantly higher quality product than conventional CA, and as such is not used in common CA applications such as in casting molds and as blast furnace linings.
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