Optimum and consistent electrolytic cell performance requires
prebaked consumable carbon anodes which satisfy a range of acceptable commercial values, as shown in Table 1
[1][2]. A selection of these properties will be briefly outlined below.
[edit] Density
Baked anode
density is directly related to anode baking temperature, with sufficiently baked anodes being associated with higher density, reduced permeability and extended life (termed rota in industry), particularly at higher current densities
[3]. Conversely if anode density is too high
thermal shock may be initiated upon setting the anode into an operational pot
[4].
[edit] Resistivity
The specific electrical resistivity determines the ability of an anode to conduct an electric current, which is highly important in
aluminium smelting. Generally, a low specific resistivity is desired to give greater control over cell voltage and reduce energy losses associated with
resistive heating[5]. However if specific electrical resistivity values become significantly low there is a subsequent increase in
thermal conductivity which causes an increased incidence of anode air burn. Air burn is the phenomena whereby the high temperatures of an operational cell and the high levels of oxygen, cause the anode to be oxidised
[6]. To ensure optimum resistivity the anode must be baked at temperatures ≥1000°C
[7][8].
[edit] Mechanical Strength
Incorporating
compressive strength,
flexural strength and
Young’s modulus, each of these mechanical properties must be considered individually and more importantly they must be considered collectively. Compressive strength defines the ability of the anode to withstand a compressive force; high values are required to prevent anode damage caused by handling. Excessively high compressive strength values will result in an increase in Young’s modulus which is not favourable
[9]. Tensile flexural strength is associated with the capacity of an anode to withstand a three point load. Anode tensile flexural strength values are required to be quite high in order to sustain any damage on impact, however if values are too high brittleness may ensue which depletes structural integrity
[10][11]. Young’s modulus denotes the elasticity of a material, low anode values give rise to the initiation and propagation of cracks associated with handling and other external stressors
[12].
[edit] Thermal Conductivity
Thermal conductivity is the ability of the anode to conduct heat. Low anode thermal conductivity is undesirable as anode top temperature will be significantly increased and disproportionate air burn will occur
[13][14]. The coefficient of thermal expansion is a measure of an anode’s ability to expand dimensionally when heated. It is necessary for anodes to have a low coefficient of
thermal expansion so to avoid thermal shock and thus crack formation during anode fabrication and pot setting
[15][16].
[edit] Air Permeability & Grain Stability
The penetration of the anode by both
carbon dioxide and air must be minimised in order to restrict the transport of gases to internal reactive surfaces and thus reduce selective burning
[17]. Both the carboxy reactivity residue and air reactivity residue values should be high to limit carbon dioxide burn and air burn, respectively
[18]. High grain stability will increase anode structural integrity and therefore reduce selective burning and excess carbon consumption. Grain stability also minimises particle degradation during anode fabrication
[19].
Table 1: Acceptable industrial values for prebaked consumable carbon anodes
[20][21][22].
| Property | Standard | Range |
| Baked Apparent Density | ISO 12985-1 | 1.53-1.64 gcm-3 |
| Specific Electrical Resistivity | ISO 11713 | 55-62 μΩ for pressed anodes |
| Compressive Strength | ISO 18515 | 40-48 MPa |
| Young's Modulus | RDC-144 | 3.5-5.5 GPa |
| Tensile Flexural Strength | ISO 12986-1 | 8-10 MPa for pressed anodes |
| Thermal Conductivity | ISO 12987 | 3.5-4.5W mK-1 |
| Coefficient of Thermal Expansion | RDC-158 | 3.5-4.5 x 10-6 K-1 |
| Air Permeability | ISO 15906 | 0.5-1.5 nPm |
| Carboxy Reactivity Residue | ISO 12988-1 | 84-96% |
| Air Reactivity Residue | ISO 12989-1 | 0.05-0.3% per minute |
| Grain Stability | N/A | 70-90% |
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