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. The contaminants emitted into the environment result from the raw materials used in the process.
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 about 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 pre-bake pots and a higher proportion of contaminants is therefore released through the roof vents.
Greenhouse Gas Emissions (GHGs)
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 pitch contained in the anode generates CO2.
- Fuel consumption, such as natural gas (especially in the foundry and carbon areas).
- 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. This is why reduction measures are principally focused on eliminating the anode effect.
Carbon Monoxide (CO)
Carbon monoxide emissions during electrolysis result from the combustion of the carbon anodes. Some of the oxygen that is released reacts with carbon to produce CO instead of CO2. The CO eventually transforms into CO2 in the atmosphere.
When calculating the greenhouse gas inventory, we consider 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 a percentage of emissions also occurs in the anode bake furnaces.
Carbonyl Sulphide (COS)
During the combustion of the anode in the electrolysis process, some of the sulphur that is present reacts to create COS instead of CO2.
Polycyclic Aromatic Hydrocarbons (PAHs)
PAHs result from the coal-tar pitch used for manufacturing anodes. When the anode is pre-baked, as in new technologies, PAH emissions are negligible. The anodes are baked in special dedicated furnaces, equipped with emissions treatment systems. In the Söderberg potrooms, however, the anodes bake during electrolysis in the potrooms. HAP emissions not captured in the scrubbers escape through the vents.
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 various contaminants.
Various residues are generated by a smelter, with the most significant quantities consisting of spent pot linings and foundry dross.
For the current to be conducted in the pot, there must be an anode and a cathode. 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 potlining is generated. This is a hazardous waste that must be managed appropriately.
Before aluminum is cast into products, all impurities on the surface of metal must be removed in the foundry furnace. These impurities, called dross, are recycled internally or shipped to a subcontractor who extracts the aluminum and returns it to us.