Hello PM...below is a brief narrative from a paper I researched couple of years ago discussing cathode and anode electrodes for aluminum smelting and how natural flake graphite will not provide any improvement to the electrode, but would actually accelerate decomposition and wear of the electrode. I am not an expert in this application, but very interesting information, especially the complexity and science involved in aluminum manufacturing.
Electrolyte: The electrolyte is a molten bath of cryolite (Na3AlF6) and dissolved alumina. Cryolite is a good solvent for alumina with low melting point, satisfactory viscosity, low vapor pressure. Its density is also lower than that of liquid aluminum (2 vs 2.3 g/cm3), which allows natural separation of the product from the salt at the bottom of the cell. The cryolite ratio (NaF/AlF3) in pure cryolite is 3, with a melting temperature of 1010 °C, and it forms a eutectic with 11% alumina at 960 °C. In industrial cells the cryolite ratio is kept between 2 and 3 to decrease its melting temperature to 940-980 °C.
Cathode: Carbon cathodes are essentially made of anthracite, graphite and petroleum coke, which are calcined at around 1200 °C and crushed and sieved prior to being used in cathode manufacturing. Aggregates are mixed with coal-tar pitch, formed, and baked. Carbon purity is not as stringent as for anode, because metal contamination from cathode is not significant. Carbon cathode must have adequate strength, good electrical conductivity, and high resistance to wear and sodium penetration. Anthracite cathodes have higher wear resistance and slower creep with lower amplitude than graphitic and graphitized petroleum coke cathodes. Instead, dense cathodes with more graphitic order have higher electrical conductivity, lower energy consumption, and lower swelling due to sodium penetration welling results in early and non-uniform deterioration of cathode blocks.
Anode: Carbon anodes have a specific situation in aluminum smelting and depending on the type of anode, aluminum smelting is divided in two different technologies; “Soderberg” and “prebaked” anodes. Anodes are also made of petroleum coke, mixed with coal-tar-pitch, followed by forming and baking at elevated temperatures. The quality of anode affects technological, economic and environmental aspects of aluminum production. Energy efficiency is related to the nature of anode materials, as well as the porosity of baked anodes. Around 10% of cell power is consumed to overcome the electrical resistance of prebaked anode (50-60 μΩm).Carbon is consumed more than theoretical value due to a low current efficiency and non-electrolytic consumption. Inhomogeneous anode quality due to the variation in raw materials and production parameters also affects its performance and the cell stability.
Prebaked anodes are divided into graphitized and coke types. For manufacturing of the graphitized anodes, anthracite and petroleum coke are calcined and classified. They are then mixed with coal-tar pitch and pressed. The pressed green anode is then baked at 1200 °C and graphitized. Coke anodes are made of calcined petroleum coke, recycled anode butts, and coal-tar pitch (binder). The anodes are manufactured by mixing aggregates with coal tar pitch to form a paste with a doughy consistency. This material is most often vibro-compacted but in some plants pressed. The green anode is then sintered at 1100-1200 °C for 300–400 hours, without graphitization, to increase its strength through decomposition and carbonization of the binder. Higher baking temperatures increase the mechanical properties and thermal conductivity, and decrease the air and CO2 reactivity.The specific electrical resistance of the coke-type anodes is higher than that of the graphitized ones, but they have higher compressive strength and lower porosity.
Soderberg electrodes (in-situ baking), used for the first time in 1923 in Norway, are composed of a steel shell and a carbonaceous mass which is baked by the heat being escaped from the electrolysis cell. Soderberg Carbon-based materials such as coke and anthracite are crushed, heat-treated, and classified. These aggregates are mixed with pitch or oil as binder, briquetted and loaded into the shell. Temperature increases bottom to the top of the column and in-situ baking takes place as the anode is lowered into the bath. Significant amount of hydrocarbons are emitted during baking which is a disadvantage of this type of electrodes. Most of the modern smelters use prebaked anodes since the process control is easier and slightly better energy efficiency is achieved, compared to Soderberg anodes.
The prevailing theory is trying to improve the Pre-baked anode performance by adding a limited amount of flake graphite. Since the anode is the more critical of the cathode and anode electrodes, the focus is to improve conductivity and quality. Because natural flake graphite is already crystalline in nature with conductive traits, adding any natural flake graphite to the anode mix would create weak inclusions in the anode thus creating points of arcing and hot spots in the anode during the smelting process.
Since recycled spent anodes (anode butts) are added back into the anode manufacturing process (ground to fine powder), the anode butts (carbonised pitch & coke) are ground to a bulky morphology (fine powder), yet natural flake graphite with a flakey, thin morphology has a higher DBP, requiring more electrolyte, thus changing the chemistry of the green anode manufacturing process. Anode butts make up 15% to 25% of a new green anode and reduce the cost of production of new anodes.
Research documentation is not readily available to support adding natural flake graphite, but there is little evidence to refute or support such an change to the green anode composition.
