Industrial Carbon and Graphite Materials. Группа авторов
Figure 6.1.1.4 Development of the needle coke CTE [Method, DIN 51909].
This extremely low CTE of needle coke and an excellent graphitizability give outstanding thermal shock resistance and electrical conductivity of graphite material produced from this coke. Graphite electrodes in electric arc furnaces for steel scrap melting are exposed to enormous electric currents of up to about 150 kA and require these properties to perform in their applications.
Another important parameter is the sulfur content, which limits the speed of graphitization cycles during the electrode manufacture and is of environmental concern. Figures of the real density give information about the coke quality but are also a measure of the quality of calcination.
The more isotropic anode or sponge coke is mainly used in anodes for electrolytic aluminum production. Requirements on coke quality for this application are a low metal content (vanadium, alkali and alkaline‐earth metals) to minimize catalytic oxidation and contamination of the aluminum bath [12]. The quality of anode‐grade coke has decreased in the last years. This trend is expected to continue due to the increasing usage of heavy crudes with high sulfur and ash content. Some low‐sulfur anode cokes and calcined low‐sulfur fuel coke are also used in cathode production for the aluminum electrolysis and for the furnace lining production for the steel blast furnace.
An overview on the various coke types by their CTE and purity as used in the carbon and graphite industry is given in Figure 6.1.1.5.
Horizontal drilling and the fracking technique to produce natural gas provide also a light crude oil, so‐called tight oil. Due to this oil production in the United States, their crude oil imports declined dramatically, and the world noticed falling crude oil prices down to around 30 $/b. Any impact of tight oil on the petroleum coke quantity or quality is not clear yet.
Figure 6.1.1.5 Various coke types and their usages.
6.1.1.1.2 Coal‐Tar Pitch Coke
CTP coke is made from coal tar after its distillation to a soft pitch. If subsequent coking is performed in a delayed coker, a CTP needle coke can be obtained. Coking in a chamber furnace gives more isotropic coke types, particularly if the feed is air‐blown in advance. These coke types are of greater hardness and strength than the petroleum cokes.
Currently produced CTP needle cokes are of competitive quality in CTE and graphitizability compared with petroleum‐derived coke. Their main disadvantage is the irreversible expansion phenomenon driven by the release of nitrogen during the graphitization of the electrode during its manufacture. The difference in this so‐called puffing between a petroleum‐based coke and a CTP‐based coke is shown in Figure 6.1.1.6. Most critical is the expansion velocity in the temperature regime between 2000 and 2300 K. This problem (so‐called nitrogen puffing) has not been solved on an industrial scale for the fast lengthwise graphitization technology with the consequence that this type of coke is mostly used in old Acheson graphitization furnaces, which require long temperature cycles.
Modern Western world lengthwise graphitization Castner furnaces are running at short temperature heating cycles (<16 hours), which have been adjusted to petroleum needle coke. The prevention of nitrogen puffing would make CTP needle coke technically fully competitive to petroleum needle coke. The estimated annual global capacity of CTP needle coke is at about 500 000 t with the tendency to exceed the petroleum needle coke production in the future (Figure 6.1.1.7). The puffing phenomenon had attracted research from academia and industry from the past until today [13–19]. A comprehensive review on the puffing phenomenon had been given recently [20, 21].
Depending on the coker feed (ash content) and coking conditions, semi‐isotropic and isotropic CTP cokes are produced. Isotropic coke grades are used in special fine‐grained graphite products. They are also of interest for their application in graphite‐moderated nuclear power plants. For this application low boron and vanadium contents are the main criteria. A certain amount of this coke goes into anode and blast furnace lining manufacturing.
Figure 6.1.1.6 Expansion of petroleum and coal‐tar pitch coke‐derived electrodes during graphitization [22].
Figure 6.1.1.7 Estimated petroleum and coal‐tar pitch needle coke capacities [22].
6.1.1.1.3 Anthracite
Anthracite is a fossil carbon material with the highest level of coalification. Compared with normal coal, it is low in ash and volatiles. The main usage of anthracite is in power plant and house heating. In the field of industrial carbon, it is used in cathodes for aluminum electrolysis and blast furnace linings and in Söderberg pastes for aluminum production. For the usage as raw material for industrial carbon production, anthracite has to be calcined in order to achieve the desired high electrical and thermal conductivity. Calcining can take place in gas‐fired shaft furnaces (1500–1800 K) or by electrical heating (1500–3300 K). The calcined anthracites are known as gas‐calcined anthracite (GCA) and electrically calcined anthracite (ECA). Depending on the source, anthracites behave differently during calcining. Some grades disintegrate during high‐temperature heat treatment. Petrographic analyses on the content of different macerals are applied to predict the quality of anthracite in this respect. Under heat treatment above 2300 K, anthracite partially graphitizes. The electric calcining furnaces have a broad temperature distribution, in particular at high temperatures. This gives an inhomogeneously calcined anthracite with varying physical properties. Efforts to improve the homogeneity of these furnaces have been made [23, 24].
6.1.1.1.4 Binder Materials
Binder materials are needed not only to glue the unmeltable solid carbon particles together but also to give the mixture of solids and binder plasticity under forming temperatures. Their most important task however is the formation of the unmeltable carbon material, CTP coke, or petroleum coke. Thus the binder has to be of carbonaceous nature as well and should provide reasonable carbon yield after carbonization. When the final product is a synthetic graphite, a polyaromatic substance like CTP [Table 6.1.1.2 pitch properties is the only substance to provide the necessary material properties]. Precondition is the formation of the so‐called mesophase, which enables the later graphitizability. The apparent density of the baked material can be increased by impregnation with a carbonaceous material and subsequent rebaking. The efficiency of this process