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An excursion into manufacturing processes with a focus on CO2 generation

CO2 balance sheet: aluminum versus alumina ceramics

Dr. Meinhard Kuntz, Manager Oxide Development at CeramTec GmbH

There are a number of advantages that result from the production of LED lamps using alumina ceramics as a material for heat sinks, reflectors or mountings. These range from excellent thermal management of the CeramCool® system to electrical insulation, aesthetics and haptics. Ceramic can replace established technical solutions made from aluminum. The LCA or “ecobalance” with regard to the production of the product contributes to an objective evaluation of a possible large-scale substitution. This article offers an estimate of the CO2 equivalents between aluminum and alumina ceramics of energy-relevant processing methods. Substituting aluminum with alumina ceramics reduces CO2 emissions into our atmosphere by roughly a factor of two using a volume comparison. When you consider the weight of the end product, aluminum contributes about three times the amount of CO2.

Bauxite is a rock mainly comprising minerals that are various hydrous aluminum oxides (e.g. boehmite, gibbsite, diaspore) and typical impurities mainly of iron, silicon and titanium. In particular, the typical dark reddish brown color is a consequence of a significant iron content. This raw material is strip mined and serves as the basis for the production of aluminum metals and alumina ceramics. Using the Bayer process, a common method invented by Karl Josef Bayer in 1888, bauxite is mixed with a sodium hydroxide solution, finely ground and heated with steam. The aluminum hydroxide dissolves, the impurities precipitate and are separated from the solution with filters (red mud). As the solution cools down, the purified aluminum hydroxide finally precipitates.

The purification of the hydroxide requires around 0.5kWh of electricity per kg of ceramic alumina powder, which develops through calcination. This is achieved by heating aluminum hydroxide to 1200°C, which removes the water, converts the hydroxide into oxide and results in Al2O3. The calcination kilns are fired with natural gas, using roughly 0.5m³ per kg of Al2O3. The manufacturing process for metallic aluminum and alumina ceramics is identical up to this point. Using stoichiometry, 530g of aluminum is obtained from 1000g of Al2O3. For the CO2 balance sheet this means that the energy consumption from the Bayer process is about twice as high for 1 kg of aluminum than for 1 kg of alumina ceramics. This fact is considered in the tabular overview in the calculation by a factor of two.

The processes go different ways after calcination. By far, the most amount of energy is required for fused-salt electrolysis, during which metallic aluminum is obtained from the alumina powder. The melting temperature of Al2O3 is very high (apprx. 2200°C). It is thus necessary to mix the alumina powder with cryolite (Na3AlF6), which reduces the melting temperature down to approximately 950°C. At fused-salt electrolysis the aluminum metal is deposited at the cathode in the electrolytic cell. The anode consists of carbon blocks that decompose to CO2 during the reaction. This process consumes 15 kWh of electrical energy and additionally directly releases 1.2 kg of CO2 per kilogram of aluminum. Another important factor to consider in the ecobalance is the formation of harmful gases such as carbon monoxide and hydrogen fluoride. Aluminum takes shape when it is cast; the metal is melted at temperatures of around 700°C, which consumes comparatively little energy.

After calcination the ceramic powder is ground, dried and, depending on the production process, combined with 2-12% organic binding agents. Die casting is the preferred method for forming powder bodies in mass applications with complex geometries. Dry pressing is suitable for the large-scale production of less complex components. Electricity consumption of 0.4kWh / kg was assumed together with grinding and drying in evaluating the production process. Sintering requires the most amount of energy in the production of ceramic components, consuming 1 m³ / kg of natural gas. Moreover, approx. 0.4 kg of CO2 is released for each kilogram of ceramic sintered as the organic binding agents are burned.

This concludes the process evaluation and energy estimates regarding the production of aluminum and alumina ceramic components. A tabular summary of all CO2 relevant process steps provides a good overview here. Figures from the Bayrisches Landesamt für Umwelt, Infozentrum UmweltWirtschaft [Bavarian State Office for Environment, InfoCenter Environmental Economics] were used in calculating the CO2 equivalents of various energy sources. According to these figures, the production of 1kg of metallic aluminum creates roughly 13.5kg of CO2 as opposed to 4.8kg for 1kg ceramic alumina. When you consider the weight of the end product, aluminum contributes about three times the amount of CO2.

The density of aluminum is 2.7 g / cm³; Al2O3 has a density of 3.9 g /cm³ (factor 1.44). Replaced by the same volume (Al2O3: 4.8 kg CO2 × 1.44 = 6.9 kg CO2), the CO2 balance is still significantly better for alumina ceramics than for aluminum. Using a rough estimate, substituting aluminum with alumina ceramics for a component of the same size reduces CO2 emissions into our atmosphere by approximately a factor of two.

Whenever considering CO2 we should ask the question at which point a comparison no longer makes sense. Right here, with the heat sink as the end product? One step further, after it has been populated with electronic components? Another step further, in its use, e.g. as a LED lamp which saves more energy with ceramic thanks to more efficient cooling for its service life of 15,000 or even 40,000 hours? There are too many imponderables to be able to provide a single simplification. What appears to be more interesting and calculable, however, is the final stage – disposal or recycling. Metallic aluminum in part can be partly reused through recycling; in contrast, a sintered ceramic component cannot be reused in the production of ceramics. Yet ceramic is absolutely chemically stable and entirely neutral for the environment. Disposal is not critical; worn-out ceramic components are used, among other things, as a filling material in road construction. With metallic aluminum, on the other hand, metal ions can be released into the groundwater under unfavorable conditions. Also considering the harmful products from fused-salt electrolysis, there are many convincing ecological arguments in favor of using modern advanced ceramics.