Electrically Calcined Anthracite
The global market for calcined anthracite is expected to reach 3.4bn USD annually by 2024(1), it is imperative that processes for the efficient and robust calcination of anthracite are leveraged - to produce the best material possible. For brevity, throughout this work electrically calcined anthracite and ‘regular’ calcined anthracite are referred to with abbreviations ECA and CA respectively.
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).
Electrically Calcined Anthracite (ECA)
ECA is anthracite that has not been calcined by a gas oven but rather an electric furnace, and the process is referred to as ‘electro-calcination’. Electro-calcination is functionally similar to the traditional gas-fired method and produces a product that is largely indistinguishable, yet it is operated as a continuous process offering an inherent efficiency advantage over a regular gas furnace(7). The first example of continuous eletro-calcination on an industrial scale was demonstrated in China in the 1980s. Modern electro-calcination plants utilise direct current over alternating current, as efficiencies are greater despite a power reduction of circa 20%(8). One of the major advantages of electro-calcination is much easier control of heat, and a completely uniform heat distribution. This makes for a completely uniform calcined product, ready for further use.
By way of comparison to CA, ECA is often regarded as having slightly higher electrical resistance but with a better long-term stability profile. In manufacturing, ECA tends to be heated at a higher temperature than CA, but for a much shorter time. It has been found that the advantageous properties conferred by calcination arise from temperature of calcination rather than time(9)
CA/ECA manufacturing at the highest temperatures produces a product that is referred to semi-graphitised anthracite. That is, the application of significant temperature causes a graphitisation process on the anthracite - making the material harder and more resilient. Calcination (of both types) is strongly linked to an increase in both compressive and structural strength, related to porosity levels.
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(10). ECA/CA can also be used in foundry cupolas(11).
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(12).
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(13).
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 comprised of 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(14).
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(15). 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(16).
Anodes and cathodes when comprised 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 comprised of 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(17). 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(18). 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(19).
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(20). 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(21). 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(22), such expansion causes inefficiency in electrical conductivity.
Owing to the nature of high quality ECA undergoing a graphitisation process under heating, synthetic graphite can eventually be formed from ECA(23); anthracite starts to undergo graphitisation at approximately 2,200 °C(24). Although a minor use, powdered ECA can be used as a high-performance carburiser in grey iron production from steel wastes(25). 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.
- ECA is a useful material in the production of monolithic and semi-monolithic electrodes and ramming pastes
- ECA tends to be used in these higher-end applications, with conventional CA reserved to broader bulk uses
- Calcination by electrical means is regarded as superior to other methods due to a more even heating, at a higher temperature
- ECA does not require the burning of fossil fuels at site
- ECA has excellent properties in terms of electrical conductivity, mechanical/compressive strength and long-term stability
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1 MarketWatch (online), 2019, Electrically Calcined Anthracite Market (ECA) Incredible Possibilities, Growth with Industry Study, Detailed Analysis and Forecast to 2025, accessed 28 Feb 2020, https://www.marketwatch.com/press-release/electrically-calcined-anthracite-market-eca-incredible-possibilities-growth-with-industry-study-detailed-analysis-and-forecast-to-2025-2019-09-06
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