Characterisation of an industrial Søderberg electrode

Abstract

Søderberg electrode management (together with the initial design of the electrode system) is commonly considered as one of the most important aspects of profitable and safe submerged arc furnace operation, with some even considering it as the most important aspect. Notwithstanding the importance of electrode management, very little research on Søderberg electrodes has been published in the journal peer-reviewed public domain. The overall main aim of this study was to obtain, sample and characterise an industrial Søderberg electrode. The case study electrode (that was obtained from a South African ferrochrome smelter) was cut off approximately 0.5 m below the contact shoes of a submerged arc furnace. The operational history (slipping rate, current, paste levels, etc.) proved that the case study electrode was a representative specimen from a stable ferrochrome submerged arc furnace. Fit-for-purpose sampling procedures and apparatus were developed to obtain samples that were manageable in size and representative of the full-size industrial Søderberg electrode, which was used for the characterisation experiments. These experiments included electrical resistivity (measured with a specially designed two-probe system), degree of graphitisation (calculated from X-ray diffraction), bulk density determinations (measured with a standard American Society for Testing and Materials method and gas pycnometry), porosity determination (with computerised tomography and associated image processing, as well as scanning electron microscopy), compressive breaking strength (with a breaking strength instrument), and thermal shock (with a method developed during this study). The characterisation results indicated no significant electrical resistivity, degree of graphitisation, and bulk density changes from 0.7 to 2.7 m on the non-delta side (outward-facing). In contrast, these characteristics changed relatively significantly on the delta side (inward-facing) of the electrode. The delta-facing tip of the electrode had the lowest resistivity, as well as the highest DOG and bulk density. No significant difference in porosity as a function of length below the contact shoes was observed; however, slight increases occurred near the perimeters. It was postulated that oxidation of carbon resulted in increased pore volumes near the electrode perimeter. This was confirmed by the results obtained from the scanning electron microscopy data, which showed that the pore sizes were significantly larger on the electrode perimeter than on the inner part of the electrode. No significant difference in compressive breaking strength was observed over the electrode area investigated. The thermal shock results showed a very slight decrease in strength at the non-delta side of the electrode compared to the delta side. This characterisation data can be used to verify whether Søderberg electrode models accurately predict important electrode characteristics.