Carbon anodes, made from calcined
petroleum coke and liquid coal tar pitch, are manufactured on site.
They are suspended in the electrolytic solution in the cells and
used to conduct electricity through the cells during the aluminium
reduction process. The reduction cells are connected in series by
an aluminium busbar, which carries the electrical current to the
cells.
The anodes are consumed in the
process, at a rate in excess of 400 kg carbon per metric tonne of
aluminium.
Carbon anode manufacturing at DUBAL
DUBAL's anode production facility (known as the carbon (or
reduction materials) plant) has the capacity to produce 380,000
anode blocks a year. On-site storage facilities are used for the
raw materials which include aluminium rod assemblies and molten
cast iron, in addition to calcined petroleum coke and liquid coal
tar pitch. Unused portions of the anodes (known as 'butts') are
also crushed, recycled and used as feed stock for new anodes. Three
broad processes are involved, each in a dedicated area of the
carbon plant:
- Greenmill (also known as 'paste plant') - This
unit produces pre-baked anode blocks (also known as 'green
anodes'). After crushing, screening and grinding petroleum coke and
recycled anode butts, the mixture is combined with hot liquid coal
tar pitch to form a paste. A vibro-compaction technique forms the
paste into blocks. The green anodes are then cooled by water jets
and sent to the baking kilns.
- Baking kilns (also known as 'baking furnace') -
All green anodes are stored for a minimum 24-hour cooling period
before further handling. The pre-baked anodes are stacked in the
baking kiln, then baked gradually (the final temperature is
approximately 1,200°C) After approximately 18 days, the green
anodes have reduced in weight by 5 per cent due to the loss of
pitch tar volatiles, and are taken out of the kilns.
DUBAL has four baking kilns operating on natural gas. The
off-gases produced are treated by two FTPs.
- Rodding room - The baked anodes are fitted with
aluminium rod assemblies to enable electricity flow in the
reduction cells. The connection between the anode and the rod is
sealed and secured using molten cast iron. To prevent oxidation,
the rodded anodes are coated with molten aluminium (provided by the
casting operations), then sent to the smelter operations for
use.
Anode butts returned to the carbon plant from the smelter are
cleaned to avoid contamination of the calcined petroleum coke in
the greenmill. The butts are then separated from the rod assembly
and the crushed to the desired size for use in the greenmill. The
removed rod is inspected for damage, and repaired if necessary,
prior to re-use.