A key feature of the smelting process is the enclosed nature of the electrolytic pots.
Emissions from point fed prebake cells, which produce almost 90% of the world’s primary aluminium, are very low – less than 2% of the generated emissions. The balance of the emissions is collected inside the cell itself and carried away to very efficient scrubbing systems, which remove particulates and gases. Computer technology controls the process down to the finest detail, which means that occurrence of the anode effect – the condition that causes small quantities of perfluorocarbons (PFCs) to be produced – can be minimised. All new plants and most plant expansions are based on prebake technology.
For many decades fluoride emissions (as gases and particulates) were considered to be the single most important pollutant from aluminium smelters. Depending on local conditions, fluorides could have a serious environmental impact on the local flora and fauna. Fluorides accumulate in vegetation and can cause damage to coniferous trees. They also accumulate in the teeth and bones of ruminants eating fluoride-contaminated forage.
Plants with modern control systems to remove and recycle the fluorides today do not generate local concerns. Optimum fume collection from the electrolytic cells and specific workplace training on minimisation of fugitive fluoride emissions, as well as improvements in cell technology have driven a 50% reduction in total fluoride emissions per tonne of aluminium production between 1990 and 2010 (around 80% since the 1960s. The industry has committed to a further reduction of at least 35% between 2006 and 2020.
Fluoride “scrubbing systems” use alumina to extract gaseous fluoride from pot gases. This “activated” alumina, which contains the residual fluoride is then used as a feed for the reduction process; in fact, it has a positive impact on cell chemistry and process efficiency over non-activated alumina. Thus the fluoride is recycled through in a closed loop system. Around xxxx% of smelters worldwide employ such scrubbing systems.
Bauxite residue is a by-product of the Bayer Process. It’s also known as ‘Red Mud’ due to the high concentration of iron compounds in the bauxite giving the substance its characteristic red colour. For further information about bauxite residue – its management and utilisation, please click here.
Spent pot lining (SPL) is an unavoidable by-product of the aluminium smelting process. On average, 15-35 kg of SPL is produced per tonne of aluminium. Reporting plants have increased from 51 to 73 plants increasing the reported production from 13 to 16 million tonnes. In 2007, 27% of SPL output was recycled externally out of a total reported output of 406 thousand tonnes of SPL. About 58% of the SPL output was deposited in form of treated deposition or stored pending final deposition or recycling.
The industry has systematically worked to minimize the amount of SPL produced, by extending the lifetime of the lining in the smelter pots. Since the 1970s, SPL has been recognised as a valuable resource for other industries, including as a feedstock in the cement, mineral wool and steel production processes. However, the main barrier to supply of SPL as a feedstock has been economics. Individual smelters do not produce enough SPL to provide a continuous supply of feedstock for a cement plant to justify their conversion to receiving this material.
Through collaboration with potential customers, and between companies to increase regional supply, the recycling of this material has become more viable and widespread.
Managing Director
Emirates Global Aluminium
“Protecting is a core value at Emirates Global Aluminium. Acknowledging the importance of preserving precious natural resources, EGA endeavours to act responsibly to protect the environment wherever the business operates. Optimising raw material and energy consumption, minimising harmful emissions and implementing a ‘reduce, reuse, recycle’ waste philosophy are fundamental to this approach.”