One of the ways to objectively determine the readiness of bituminous coke and to obtain coke with specified properties is the development of scientific and technological foundations for assessing the molecular and supramolecular structure of coke based on its specific electrical resistivity. A necessary condition for ensuring high-quality coke is its proper readiness, which is ultimately determined by the degree of order in the macromolecular structure in the form of graphite-like blocks. It has been proven that in this state, coke acquires semiconductor properties and its specific electrical resistivity decreases.
Coke for modern blast furnaces using pulverized coal fuel must have minimal specific electrical resistivity – no higher than 0.1 Ohm∙cm. For other applications, the specific electrical resistivity of coke should be higher. The main factor influencing the specific electrical resistivity of the produced coke is the final coking temperature, which indeed serves as an objective indicator characterizing the coke's readiness. As the final coking temperature increases, the supramolecular structure of the coke becomes more ordered, approaching the structure of graphite to some extent, which leads to a reduction in the coke’s specific electrical resistivity.
At the same time, there is an urgent need for a theoretical analysis of the processes that lead to changes in the electrical conductivity of coke. Such an analysis was carried out to justify the determination of the nature of the relationship between the coke's specific electrical resistivity and the final coking temperature. In turn, determining the numerical parameters of this relationship made it possible to establish a rational level of final coking temperature for producing coke for various uses, which has significant practical importance both for coke production and its application.
To objectively assess the specific electrical resistivity of coke, a two-probe method for measuring the resistance of coke powder was proposed. This method allows for obtaining a representative coke sample for resistance determination with minimal time and labor. The device for measuring the specific electrical resistivity of coke in accordance with DSTU 8831:2019 was improved, which significantly reduced the error of the obtained results and allowed the device to be used in studies of the dependence of coke's specific electrical resistivity on the final coking temperature.
As a result of theoretical studies, a well-founded hypothesis was proposed that coal is a dielectric due to the presence of a large number of σ-bonds (carbon in sp3- hybridization state) in the side chains of its macromolecule. That is, the width of the forbidden energy band is up to 6 eV. The electrons of these bonds are practically unable to reach the conduction band and become carriers of electric current. During coking, due to deep cracking, the macromolecules are almost completely stripped of their side chains, and the majority of carbon is present in the form of condensed polyaromatic structures (sp-hybridization of the carbon atoms' electron shells). In this state, half of the valence electrons participate in forming π-bonds, whose electron clouds are oriented perpendicular to the layers of the condensed carbon structures. These electrons are associated with a significantly smaller band gap (2 eV), are less tightly bound to atomic nuclei, and can relatively easily become carriers of electric current.
It was determined that the factors significantly affecting the specific electrical resistivity and conductivity of coke are governed by the general band theory of the physical structure of solids. According to the theoretical analysis, both intrinsic and impurity (extrinsic) conductivity of semiconductors increase rapidly with temperature, following an exponential law. This hypothesis was experimentally confirmed through laboratory coking of the industrial coal blend from PJSC “ZAPORIZHCOKS” in a laboratory furnace developed by SE “UКHIN” with electric heating.
The processing of the obtained experimental data allowed for the determination of the numerical characteristics of this dependence. A rational range of final coking temperatures was established for producing coke for different application purposes. For example, to obtain blast furnace coke with a specific electrical resistivity not exceeding 0.1 Ohm∙cm, the final coking temperature must be at least 957°C, which generally corresponds to standard practices in coke chemical production. To obtain blast furnace coke with even lower specific resistivity, a higher temperature is required, whereas for ferroalloy coke, which requires higher specific electrical resistivity, a lower coking temperature is appropriate.
