PTQ Q4 2022 Issue

Coal tar needle coke route Needle coke produced from coal tar pitch originates as a by-product of destructive distillation (during the reduction of iron ore in a blast furnace). The severe thermal environment of the blast furnace (temperatures more than 1000°C) produces stable poly aromatic hydrocarbons (PAH) and aromatic heterocycles (mainly nitro- gen based). The concentration of pure aromatics in HT-CTP is enhanced as a function of the associated thermal kinetics, which destroy any alkyl or heteroatom (hydroxy, thiol or amide) side chains to the aromatic. Apart from needle coke production, HT-CTP is used in the aluminium sector (as a binder pitch for the production of car- bon anodes and cathodes) and in steel smelting (as a binder and impregna- tion pitch for the production of graph- ite electrodes). The use of HT-CTP as a needle pre- cursor in a DCU is determined by a multitude of factors: • The high aromaticity (especially in the two-ring [naphthalene] to seven- ring [coronene] range) are ideal molec- ular (needle coke) precursors as their thermal stability allows for optimal mesophase development, resulting in a highly ordered coke microstructure • In contrast, the high aromaticity has a distinctive disadvantage given its eco-toxicological carcinogenic poten- tial. Apart from quality factors, this attribute may in future deter substan- tial expansion of this route, especially when associated with production sys- tems which are not closed • The high temperature associated with HT-CTP production forms semi- carbonaceous solids (called primary MIQ), which (if not removed using filtra - tion) present an inert physical barrier, introducing crystalline imperfections in the graphene layers. These crystal imperfections disrupt the microstruc- tural order. Any microstructural disor- der limits the electronic properties and increases the CTE of the needle coke • The prevalence of stable nitrogen heterocycles form three-dimensional structures, providing steric hindrance (also known as ONS obstruction) dur- ing mesophase development and dis- rupting the formation of an ordered microstructure

• The thermal stability of nitro- gen-based aromatic heterocycles (including carbazole, pyridine, and quinoline-based derivatives) in HT-CTP results in their incorporation within the coke matrix, even surviv- ing calcination (1350°C). However, within the electrode graphitisation cycle (between 1500°C and 1800°C), nitrogen dissociates, causing irre- versible volumetric expansion ‘puff- ing’, potentially triggering electrode cracks. While the incorporation of inhibitors (such as borates) can reduce nitrogen puffing, this science is comparatively less developed (than sulphur puffing) as the majority of needle cokes are petroleum based. Graphitisation of coal tar (based needle coke) electrodes is energy intensive given the long residence times required to achieve a slow tem- perature increase without electrode cracking • The comparatively higher density of HT-CTP is beneficial in terms of coke yield • The geographical production of HT-CTP-based needle coke may fur- ther be associated with large coal resources (such as China). While theoretically (at least based on aromaticity) HT-CTP should pro- duce the highest quality needle coke, in practice its quality is inconsistent and electrode graphitisation is expen- sive. The high carcinogenic potential of HT-CTP is a further deterrent. Based on published reports (Mackenzie, W 2019), the expansion of Chinese capacity is based on coal tar needle coke alone. Future quality-based initiatives relate to purity (semi-carbonaceous or metals contaminants) to ensure consistency. The graphitisation of electrodes with high nitrogen con- tent is problematic and expensive, although based on the previously mentioned expansion initiatives, processes to limit graphitisation fir - ing times and reduce puffing would be realistic. In an ironic twist, the desirability of HT-CTP (especially the concentration of pure aromat- ics creating well-established micro- structural order) may, in contrast, act as a deterrent when considering the potential carcinogenic impact.

The future is now with our technologies for tomorrow.

Think. Create. Change. Discover our innovative process technology and other solution concepts that make you fit for the future. We look forward to welcoming you at ADIPEC 2022, stand 8718. Let’s discover together where our ideas will take you. www.borsig.de 31.10. – 03.11.2022 in Abu Dhabi, United Arab Emirates. ADIPEC 2022

BORSIG Q4-2022.indd 1

08.09.2022 13:51:55 45

PTQ Q4 2022