Environmental Aspects of the Process
Aluminum is manufactured by a process of electrolysis that separates the raw material, alumina (Al2O3), into aluminum and oxygen. Alumina is dissolved in a cryolite bath to be electrolysed at a lower temperature. The electric current passes into the cell by means of carbon electrodes. Contaminant emissions into the environment result from the raw materials used in the process.
Fluorides
Fluoride emissions result from using a cryolite solution containing approximately 50% fluoride. Large capacity ventilation systems connected to each pot capture and scrub emissions in a process that is approximately 99.9% efficient. Alumina is also used to capture fluorides, which are recycled when the fluorinated alumina is returned to the pots.
A small percentage of emissions is not captured by the scrubbers and escapes directly through the roof vents of the electrolysis buildings. In the Söderberg potrooms at Baie-Comeau, capture rate efficiency is lower than in the prebake pots and a higher proportion of contaminants is therefore released through the roof vents.
Greenhouse Gas (GHG) Emissions
The five main sources of GHGs from smelters are:
- Anode combustion during electrolysis. Oxygen released by electrolytic reaction then reacts with the carbon in the anode to form CO2 .
- Anode baking. Combustion of the pitch contained in the anode generates CO2 .
- Fuel consumption, such as natural gas (especially in the Casthouse and Carbon sectors).
- Fuel consumption by vehicles.
- Anode effects during electrolysis.
During anode effects, a reaction takes place between the carbon in the anode and fluorides in the electrolytic bath, leading to emissions of CF4 and C2F6, two gases with a much higher climate-warming potential than CO2. For this reason, reduction measures are mainly focused on eliminating the anode effect.
Carbon Monoxide (CO)
Carbon monoxide emissions during electrolysis are associated with the use of carbon anodes. A portion of the released oxygen reacts with the carbon to produce CO rather than CO2. The CO eventually turns into CO2 in the atmosphere. Consequently, in evaluating GHG emission inventories, it is assumed that all of the carbon in the anode will react to form CO2.
Sulphur Dioxide (SO2)
Sulphur results mainly from coke, one of the two raw materials used in the production of anodes that conduct current in the pot. It is emitted in electrolysis mainly during combustion of the anode, although some emissions are also generated in the anode bake furnaces.
Carbonyl Sulphide (COS)
During the combustion of the anode in the electrolysis process, a portion of the sulphur present reacts to form COS rather than SO2.
Polycyclic Aromatic Hydrocarbons (PAHs)
PAHs result from the coal-tar pitch used for manufacturing anodes. When anodes are pre-baked, as in new technologies, PAH emissions are negligible. The anodes are baked in special dedicated furnaces, equipped with emission treatment systems. In Söderberg potrooms, however, the anodes bake in the pots during electrolysis. PAH emissions not captured in the scrubbers escape through the vents.
Water
Water does not enter directly into the process of aluminum electrolysis. It is used for domestic purposes
and various types of cooling. Water discharge from a smelter consists mainly of surface runoff containing some contaminants.
Wastes
Aluminum smelters generate various residues, with spent potliners and casthouse dross topping the list in terms of quantity.
Spent potliners
An anode and a cathode are required to allow the electric current to be conducted in the pot. The bottom of the pot consists of carbon cathode and refractories. After several years of operation, the cathode becomes progressively saturated with contaminants and must be replaced. At this point, a residue known as spent potliner is generated. This is a hazardous waste that must be managed appropriately.
Casthouse dross
Before aluminum is cast into various products, all impurities on the surface of the metal are removed in the casthouse furnace. These impurities, called dross, are shipped to a subcontractor who extracts the aluminum and returns it to the smelter.
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Click on the image to enlarge the Aluminum Manufacturing Process Scheme
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