When producing aluminum emission of various chemicals is the main concern and procedures to protect the environment aim to prevent fluorine gas and dust from entering the atmosphere. Measures are used to confine these materials within the production cycle and recycle them.

Carbon dioxide
Carbon dioxide (CO2) is the primary greenhouse gas. The amount of CO2 produced from the production of aluminum depends upon the amount of production in the potrooms and has proven to be about 1.5 tons per produced ton of aluminum. Carbon dioxide is also produced when burning fossil fuels such as crude oil and propane gas. ISAL uses crude oil to power the furnaces in the casthouse, and uses propane gas for various heating purposes. In recent years, ISAL has made systematic efforts to reduce the use of fossil fuels, with a move toward electricity instead.

Perfluorocarbons (CF4/C2F6) are strong greenhouse gases. They are produced through anode effects that take place in the pots. The total amount of perfluorocarbons released is dependent on the number of anode effects in the pots and their duration. The occurrence of anode effects today is considerably less frequent than it was 10 years ago, and ISAL is unsurpassed anywhere in the world when it comes to reducing anode effects. It was once considered desirable to produce regular anode effects in the pots, but through better pot technology and increased knowledge of their polluting impact, that requirement has now been abandoned. The development of software to manage the pots makes it possible to predict with considerable precision when an anode effect will occur, thus making it easier to prevent its taking place.

What Is an Anode Effect?
The anodes send an electric current through the electrolyte in the pot. The optimum alumina content within the electrolyte is 2%, allowing good contact with the anodes. If the alumina content within the electrolyte falls under 1%, the electrolyte avoids the anodes. This avoidance results in the formation of a thin layer of gas under the anodes, separating them from the electrolyte and thus disrupting the flow of electricity from anode to cathode. Where electrolyte should be, under the anodes, pockets of gas have formed instead. Gases are poor conductors of electricity and serve only to increase resistivity, requiring more energy to drive the current through the pot. This energy is in the form of voltage, which consequently has to be increased from 4.6 volts up to approximately 30 volts as gas forms.

Concentrated amounts of fluoride can have negative effects upon plants and animals, and the aluminum industry is the largest anthropogenic source of fluoride emissions. Since 1982 ISAL has cleaned fluoride from the pot fumes in the dry adsorption plants and can today boast over 99% removal. If the concentration of fluoride in the surrounding vegetation is under 30 ppm, no damage is done. Regular measurements within the vicinity of ISAL show concentrations far below that level. It is thought that during the eruption of Mt Hekla in 1970, fluoride emissions were in the region of 30,000 tons - the equivalent of 200 years of ISAL emissions at current levels of production and the present state of cleaning equipment efficiency.

Sulfur dioxide
Sulfur dioxide (SO2) is thought to contribute to the formation of acid rain. The anodes contain approximately 1.5% sulfur, and through the electrolysis of alumina the anodes burn. The sulfur bonds with oxygen and produces SO2, which is not removed. SO2 is also produced through the combustion of fossil fuels.

What Are Greenhouse Effects?
Greenhouse effects form the basis for life on Earth. They are natural - and without them Earth's average surface temperature would be -18°C as opposed to +15°C. Solar radiation from the sun penetrates Earth's atmosphere. The planet's surface absorbs most of the radiation, and surface temperature increases. Heat is then released back into the atmosphere from the surface in the form of infrared radiation. The atmosphere intercepts some of the radiation and reflects it back to Earth's surface. This results in further heating of the surface and of the lower parts of the atmosphere. This is why our atmosphere can be likened to a greenhouse that lets the sun's rays in - and keeps the resultant surface heat inside.

What Are Greenhouse Gases?
It is thought that some gases can increase the greenhouse effect in the atmosphere. They are called "greenhouse gases". The increase in greenhouse gas emissions can result in ecosystem disruption through an increase in temperature, a change in weather systems, and a rise in sea level. Examples of greenhouse gases are carbon dioxide (CO2), methane (CH4), ozone (03), water vapor (H2O), nitrous oxide (N2O), sulfur hexafluoride (SF6) and various halocarbons.

Rio Tinto Iceland Ltd. Straumsvik - P.O. BOX 244 - 222 Hafnarfjordur - T: +354 560 7000 - F: +354 560 7070 - isal@riotinto.com