Prebaked Consumable Carbon Anodes are a specific type of anode designed for aluminium smelting using the process.
The properties of the anode are largely set during the baking process and must be carefully controlled to ensure an acceptable output efficiency and reduce the amount of undesirable byproduct produced. To that end, the aluminium smelting industry has settled on a range of acceptable values for commercial mass-produced anodes for the purpose of consistent, optimal performance.
| Property | Standard | Range |
|---|---|---|
| Baked Apparent Density | ISO 12985-1 | 1.53-1.64 gcm-3 |
| Electrical Resistance | 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 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% |
Higher baking temperatures result in higher density anodes, which exhibit reduced permeability and therefore extend the operational life of the anode. However, excessive density will result in thermal shock and fracturing of the anode upon first use in an electrolysis cell.
Efficient aluminium smelting requires low resistance on the part of the anode. Low resistance results in greater control over the electrolysis cell’s voltage and reduces the energy loss associated with resistive heating.However, anodes with low electrical resistance also exhibit increased thermal conductivity. Anodes that conduct too much heat will oxidize rapidly, reducing or eliminating their smelting efficiency, called “air burn” in industry parlance.
Anodes are subject to a variety of mechanical stresses during creation, transportation and use. Anodes must be resistant to compressive force, resistant to elastic stress, and resistant to impact without becoming brittle. The relationship between compressive strength and Young’s modulus in prebaked anodes usually results in a compromise in the anode’s resistance to compressive force and elastic stresses.
Low anode thermal conductivity results in “air burn”, as noted in Electrical Resistance, above.
Low thermal expansion coefficients are desirable to avoid thermal shock.
Anodes should be relatively impermeable to both carbon dioxide and air generally in order to reduce the opportunity for “carbon dioxide burn” and “air burn”, both of which will reduce the anode’s smelting efficiency.
High grain stability indicates high anode structural integrity, increasing the smelting efficiency of the anode. High Grain stability also minimizes particle degradation during anode fabrication.
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