7.4.2 Non-ferrous metals
The commercially relevant non-ferrous metals and specific and total CO2 emissions from electrode and reductant use are shown in Table 7.6. Annual production of these metals ranges from approximately 30 Mt for aluminium to a few hundred kilotonnes for metals and alloys of less commercial importance. Production volumes are fairly low compared to some of the world’s key industrial materials like cement, steel, or paper. However, primary production of some of these metals from ore can be far more energy intensive. In addition, the production of these metals can result in the emission of high-GWP GHGs, for example PFCs in aluminium or SF6 in magnesium, which can add significantly to CO2-eq emissions.
Generally, the following production steps need to be considered: mining, ore refining and enrichment, primary smelting, secondary smelting, metal refining, rolling and casting. For most non-ferrous metals, primary smelting is the most energy-intensive step, but significant levels of emissions of fluorinated GHGs have been reported from the refining and casting steps.
Table 7.6: Emission factors and estimated global emissions from electrode use and reductant use for various non-ferrous metals
| ||CO2 emissions (tCO2/t product) ||Global CO2 emissions (ktCO2) |
|Primary aluminium ||1.55 ||44,700 |
|Ferrosilicon ||2.92 ||10,500 |
|Ferrochromium ||1.63 ||9,500 |
|Silicomanganese ||1.66 ||5,800 |
|Calcium carbide ||1.10 ||4,475 |
|Magnesium ||0.05 ||4,000 |
|Silicon metal ||4.85 ||3,500 |
|Lead ||0.64 ||3,270 |
|Zinc ||0.43 ||3,175 |
|Others || ||6,000 |
|Total || ||91,000 |
| Note: Indirect emissions and non-CO2 greenhouse-gas emissions are not included. Source: Sjardin, 2003. |
Global primary aluminium production was 29.9 Mt in 2004 (IAI, 2006b) and has grown an average of 5% per year over the last ten years. Production is expected to grow by 3% per year for the next ten years. Recycled aluminium production was approximately 14 Mt in 2004 and is also expected to double by 2020 (Marchek, 2006).
Primary aluminium metal (Al) is produced by the electrolytic reduction of alumina (Al2O3) in a highly energy-intensive process. In addition to the CO2 emissions associated with electricity generation, the process itself is GHG-intensive. It involves a reaction between Al2O3 and a carbon anode: 2 Al2O3 + 3 C = 4 Al + 3 CO2. In the electrolysis cell, Al2O3 is dissolved in molten cryolite (Na3AlF6). If the flow of Al2O3 to the anode is lower than required, cryolite will react with the anode to form PFCs, CF4 and C2F6 (IAI, 2001). CF4 has a GWP of 6500 and C2F6, which accounts for about 10% of the mix, has a GWP of 9200 (IPCC, 1995). These emissions can be significantly reduced by careful attention to operating procedures and more use of computer-control. Even larger reductions in emissions can be achieved by upgrading older cell technology (for example., Vertical Stud Södeberg or Side Worked Prebake) by addition of point feeders to better control alumina feeding. The cost of such a retrofit can be recovered through the improved productivity. Use of the newer technologies, which require a major retrofit, can cost up to 27 US$/tCO2-eq (99 US$/tC-eq) (US EPA, 2006a).
Members of the International Aluminium Institute (IAI), responsible for more than 70% of the world’s primary aluminium production, have committed to an 80% reduction in PFC emissions intensity for the industry as a whole, and to a 10% reduction in smelting energy intensity by 2010 compared to 1990 for IAI member companies. IAI data (IAI, 2006a) shows a reduction in CF4 emissions intensity from 0.60 to 0.16 kg/t Al, and a reduction in C2F6 emissions intensity from 0.058 to 0.016 kg/t Al between 1990 and 2004, with best available technology having a median emission rate of only 0.05 kg CF4/t in 2004. Overall, PFC emissions from the electrolysis process dropped from 4.4 to 1.2 tCO2-eq/t (1.2 to 0.3 tC-eq/t) Al metal produced. IAI data (IAI, 2006b) show a 6% reduction in smelting energy use between 1990 and 2004.
Benchmarking has been used to identify opportunities for emission reductions. The steps taken to control these emissions have been mainly low or no-cost, and have commonly been connected to smelter retrofit, conversion, or replacements (Harnisch et al., 1998; IEA GHG 2000). However, much of the 30% of production from non-IAI members still uses older technology (EDGAR, 2005).
SF6 (GWP = 23,900 (IPCC, 1995)) has been used for stirring and degassing of molten aluminium in secondary smelters and foundries (Linde, 2005). The process is not very common because of cost and technical problems (UBA, 2004). Current level of use is unknown, but is believed to be much smaller than SF6 used in magnesium production.
The main potentials for additional CO2-eq emission reductions are a further penetration of state-of-the-art, point feed, prebake smelter technology and process control plus an increase of recycling rates for old-scrap (IEA GHG, 2001). Research is proceeding on development of an inert anode that would eliminate anode-related CO2 and PFC emissions from Al smelting. A commercially viable design is expected by 2020 (The Aluminium Association, 2003). However, IEA (2006a) notes that the ultimate technical feasibility of inert anodes has yet to be proven, despite 25 years of research.