Industrial Carbon and Graphite Materials. Группа авторов

6.1 Introduction to Polygranular Carbon and Graphite Materials

      The basic principles for modern metal production were developed during the industrial revolution in the eighteenth century. Blast furnaces for iron reduction grew in size and efficiency, and carbon blocks were introduced as lining material. As electric current became widely available, electrochemical reduction processes were expanded to industrial scale. Hall and Héroult independently developed the aluminum electrolysis process that is still used today to melt metallic ores and aluminum scrap. As a result, the demand for synthetic graphite significantly increased. The electrical graphitization processes invented by Acheson [1] and Caster [2] were able to satisfy the growing demand in synthetic graphite.

      The growing utilization of carbon and graphite results from the unique physical and chemical properties that can be achieved within these materials. The most important properties are the high electrical and thermal conductivities, which resemble that of metals. Graphite materials also withstand high thermal shock loads due to the favorable combination of thermal conductivity, thermal expansion, and mechanical properties. Graphite does not melt under standard technical pressures and is resistant even to temperatures above 3300 K. It is only oxidation that limits the application of uncoated graphite materials in air to below 800 K. Graphite is highly resistant to chemicals, which opened up applications in corrosive media. Carbon and graphite properties can be altered widely by the types of raw material used, their combinations, and the applied process technology.

      References

6.1.1 Polygranular Carbon and Graphite Materials

       Hubert Jäger1, Wilhelm Frohs2, Ferdinand von Sturm3, Otto Vohler3, †, and Erhard Wege3,†

       1 Institute of Lightweigth Engineering and Polymer Technology (ILK), Hohlbein Straße 3, Dresden, 01307, Germany

       2 SGL Carbon GmbH, Werner‐von‐Siemens Straße 18, Meitingen, 86405, Germany

       3 Sigri GmbH, Werner‐von‐Siemens Straße 18, Meitingen, 86405, Germany

The production processes for polygranular carbon and graphite materials are equivalent to those applied in the ceramic industry. As the granular carbon raw materials do not melt under heat treatment, they have to be glued together by a binder component that is viscous under mixing and forming conditions. Most important however is that this binder is later thermally converted to coke or char. This conversion is the result of manyfold chemical reactions like cracking, polymerization, and polycondensation. To fulfill this task the binder has to be of carbonaceous nature as well. The production scheme of synthetic carbon and graphite is shown in Figure 6.1.1.1.

      Self‐sintering carbons have been developed from carbonaceous mesophase. Their high prices have limited a broader industrial application, although their material properties are very promising.

      6.1.1.1 The Relevance of Raw Materials

      Petroleum coke, coal‐tar pitch (CTP) coke, anthracite, natural graphite, and carbon black are commonly used as raw materials in the production of industrial carbon artifacts. Although CTP is the common binder material for the solid filler material, in some cases, polymer resins and petroleum‐derived pitch are used.

      Metallurgical coke from mineral coal or calcined lignite coal are used as packing material in ring furnaces and graphitization furnaces but are not common as raw materials for industrial carbon artifacts themselves due to their high ash content and poor graphitizability or even non‐graphitizability.

      The final properties of carbon and graphite products and their performance depend to a large extent on the properties of the raw materials. Therefore, the selection of raw materials is very important in the manufacturing of carbon and graphite artifacts.

Figure 6.1.1.1 Production scheme for synthetic carbon and graphite materials. Source: Courtesy SGL Carbon.

The main raw material sources are petroleum coke (see 6.1.2 petroleum coke) and CTP coke (see 6.1.3 coal‐tar pitch coke) for the solid filler component and CTP as binder (see 6.1.5 tar and pitch). Petroleum coke is maintained as a by‐product during crude oil refining. Coal tar and its derivates (CTP and coal‐tar coke) are by‐products from coal coking (Figure 6.1.1.2). Their availability depends on the trends in the oil and steel industry. Their price as by‐products is less dependent on the crude oil or coal price but on the demand in the market they serve. The rapid growth of China’s economy at the beginning of the twenty‐first century let it become the biggest steel and aluminum producer with more than 50% in both areas. This position has an overwhelming impact on the carbon materials like graphite electrodes, furnace linings, cathodes, and anodes in regard to their demand and pricing.

Figure 6.1.1.2 Sources for petroleum and coal‐tar pitch coke.

Economics of oil refineries and reduced availability of easily accessible fields with crude oil of decent quality have already led to a significant drop in calcined petroleum coke quality for the aluminum industry [1]. With respect to the quantity of sources for raw material, the crude oil production is forecasted to increase in the next 15–20 years, followed by a significant decrease by 2050 [2]. On a long‐term basis, this will have consequences not only for the carbon and graphite industry but also for its main customers, the aluminum and steel industries. Under these circumstances, coal tar, the by‐product from coal coking, may gain importance as an alternative raw material to petroleum residues in the production of the solid coke filler material. However, the coking coal process is itself under significant environmental pressure due to the generation of polyaromatic hydrocarbons (PAHs), which are listed as carcinogenic materials, and its future prospects are somewhat uncertain, particularly in the Western world.

      6.1.1.1.1 Petroleum Coke

      Petroleum coke, the most frequently used raw material as the solid filler component of carbon and graphite artifacts, is produced from residues of crude oil refining. As the main interest of crude oil refining is to produce light clean products as transportation fuels, the technological development of crude oil conversion units has concentrated on that end during the last 70 years. The most common conversion processes are fluid catalytic cracking (FCC) and hydrocracking. The most severe and robust conversion technique is coking, which is able to process heavy, highly contaminated feedstocks.

      Residues from the vacuum distillation of crude oils are by far the most common feedstock for coking,

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