CARMA - Reactivity of Carbon and Refractory Materials used in Metals Production Technology

The CARMA project "Reactivity of Carbon and Refractory Materials used in Metals Production Technology" deals with material issues in modern aluminium production plants. While the carbon anode is consumed in the electrochemical reaction and regularly needs to be replaced, the carbon cathode and the lining materials are expected to be inert and contribute to long, stable and reliable lifetime. Anodes are fabricated from different coke materials mixed with a binder to form a solid anode block. Due to shortage of high quality cokes, anode producers have started to test and include coke materials of lower quality and with higher impurity levels. In some cases, this also includes changes significant differences in the carbon structure. The performance of the anodes may therefore change and parasitic reactions may lower the carbon yield of each anode. In CaRMa, advanced x-ray synchrotron beamlines in Canada and Australia have been used to determine the composition of sulfur and metallic impurity species in the cokes. Sulfur compounds can be grouped in to main configurations and some of the performance parameters can be directly related to this. The results are presented in several scientific journal papers as well as at industrial science conferences. A PhD thesis was completed in the fall of 2019. The anodes are baked to approximately 1200 °C, placed in pits in a baking furnace. The pit walls are severely degraded over time both due to chemical reactions and thermal stress. Through theoretical and on-site excavations and measurements, the phase transformations taking place in the pit walls have been determined. The gas composition in the pits have for the first time been measured on-line over several baking cycles. The measurements have given important information on the gas exposure of refractory materials during the baking cycle, which is also implemented in CFD-modelling of the degradation behaviour. The results are presented in several scientific journal papers as well as at industrial science conferences. A PhD thesis was completed in the fall of 2019. Unwanted wear of the cathode has become increasingly important for the aluminium industry, both due to increased current density to increase the productivity as well as more energy efficient production. Many different mechanisms causing cathode wear have been proposed over time, however, most of them fail to explain the observed wear patterns. New mechanisms, not previously established, have been established in CaRMa, in particular the effects of superheat, wetting and the influence of sodium. This has contributed to a new understanding of the wear mechanism as well as means to reduce the wear. The results are presented in several scientific journal papers as well as at industrial science conferences. A PhD thesis was completed in the fall of 2019. The insulating materials in the electrolysis cell are crucial for the heat balance. If the material properties of the insulating materials change too much during operation, e.g. dimensional or thermal changes, this will affect the thermal balance of the cell and the operation and lifetime of the cell. CaRMa has determined the relationship between phase changes in insulation materials and the stability and effects on the properties of the insulation materials exposed to conditions that may develop during the lifetime of the cell. This has resulted in the production of more resistant insulation materials. The results are presented in several scientific journal papers as well as at industrial science conferences. A PhD thesis was submitted for evaluation late 2019 with defence scheduled for spring 2020.

The CARMA project "Reactivity of Carbon and Refractory Materials used in Metals Production Technology" has focused on basic scientific research related to critical phenomena in modern aluminium production. The four focused areas of research have been on anode raw materials and anode performance, anode baking and degradation of anode furnace pits, mechanisms causing carbon cathode wear, and degradation mechanisms, stability and improvements of insulation recipes. The project has resulted in 4 PhD theses, more than 25 scientific journal papers. The results have also been presented at numerous industrially relevant scientific conferences.

Parasitic chemical and electrochemical reactions are some of the most pronounced material challenges in aluminium production today. In previous efforts to understand these reactions the focus has been on classes of materials, while recent studies indicate s that the materials' microstructure and trace element impurities may have a significant effect on reaction kinetics. The main research methods will be on electrochemical and chemical reactions to understand material properties and reaction kinetics in en vironments simulating the conditions in operations, i.e. the effects of exposure to bath, metal and volatiles on individual materials properties (microstructure, phases and impurities). This competence building project is a collaboration between SINTEF an d NTNU and the industrial partners Hydro, Alcoa, Elkem and Skamol. The precursor materials affect both microstructure and impurity content of carbon electrodes. Especially in anodes the range of inherent properties are extensive, both with respect to str ucture and impurities, and industrial tests do not predict all variations in anode performance. Impurity and structural effects will be studied by electrochemical, chemical and physical methods. Degradation of cathode materials are often limiting the life time of aluminium cells. Both chemical and electrochemical methods will be used to understand initiating reactions, mass transport mechanisms and kinetics related to bath, metal and volatiles. Many of the same mechanisms are evident in anode firing kilns, i.e. exposure to volatiles from impurities in the anode cokes and recycled butts. In addition, the materials in the kilns are subject to temperature cycling and recrystallization. The experimental work will be supported by modelling, using multiphysics f inite element modelling and atomistic simulations based on density functional theory. Besides established methodology, new experimental and characterization methods will be developed.