The use of thermomechanical analysis to characterise Söderberg electrode paste raw materials

Continuous self-baking electrodes, i.e. Söderberg electrodes, are commonly employed in the industrial operation of submerged arc furnaces to conduct electrical energy from a transformer(s) to the smelting zone inside the furnace. Söderberg electrodes are formed from Söderberg electrode paste consisting of two components, i.e. a binder that is mostly a coal tar pitch and a solid filler that can be either coke or calcined anthracite. In this paper it is demonstrated how thermomechanical analysis can be used to characterise the thermal dimensional behaviours of Söderberg electrode paste raw materials. Two case study coal tar pitches, three anthracites and a pre-baked graphite electrode sample were characterised. Results indicated that the method applied can be used to determine the baking isotherm temperature more accurately than previously. Enhanced knowledge of the baking isotherm temperature is critical to ensure safe, profitable and continues furnace operation. The results also indicated that the coal tar pitches shrunk approximately 12% if exposed to temperatures above the baking isotherm temperature up to 1300 °C, while the anthracites calcined at 1400 °C expanded 0.6-1.0% in the same temperature range. The magnitude differences in thermal dimensional behaviour and difference in motion (shrinkage vs. expansion) indicates the vulnerability of a Söderberg electrode baked for the first time at temperatures exceeding the baking isotherm temperature. In contrast to the calcined anthracites, the uncalcined anthracite samples shrunk 6-8% if exposed to temperatures up to 1300 °C. This stresses the importance of the efficiency of the calcination process of the anthracite prior to Söderberg electrode paste production to enhance dimensional stability of the Söderberg electrode paste. The results also indicated that the method detailed in this paper can be used by Söderberg electrode paste producers to optimise the selection of raw materials and to do quality control on calcined anthracite.