New or Relined Pots can be restarted-recommissioned in a number of ways. One common practice is a “Shunted” Startup – which involves lowering the anodes down, until they are almost in contact with the Cathode surface, and turning the current on. Initially low (shunted) current is passed, but gradually the pot is brought up to full power – during which process the entire Pot is “Baked Out”, and brought to normal operating temperature, around 970oC.
The Anodes are then raised and Molten Bath is poured into the pot, and the Electrolysis process commences, with (liquid) Aluminium forming at the Cathode, etc.
Prior to the current being turned on, Calcined Petroleum Coke, or more lately, pure Graphite, is piled on the cathode, beneath the Anodes. The current flows from the Anodes, through the Graphite Pile/Bed. Direct contact between Cathode and Anode is not desirable, and Graphite in particular provides a highly conductive path for the current. This permits a better distribution of the current between Anode and Cathode eliminating “Hot Spots” on the Cathode Block, which can generate high Thermal Stresses in the Cathode, which can lead to cracking of the new cathode. These cracks can cause premature failure of the Pot – at a cost of between $300-500,000 per pot Reline.
Bisley offer a granulated Graphite material, which exhibits high Electrical Conductivity. Experience at a number of smelters, who have moved from CPC to Graphite, indicate this is an excellent material for the Shunted Startup practice.
The nominal specification is as follows –
|Fixed Carbon||98.5% min||99.222%|
SIZE – Any size range available between 20um & 9.5mm
|Specific Resistance - µΏ.m||7.00 – 9.00|
|Elastic Modulus – GPA||9.2 – 12|
|Bulk Density – G/cm3||1.5 – 1.65|
|Thermal Expansion Coefficient 10-6 1/oC||2-2.9|