Hans Christian Oersted succeeded in 1825 in producing metallic aluminum for the first time through reduction. Frederick Wöhler improved the method between 1827 and 1845, and finally Henri Saint-Claire Deville managed to reduce aluminum in somewhat larger quantities by using metallic sodium. However, the aluminum produced was more expensive than silver, so Napoleon III had food served on aluminum plates at state banquets. For very special guests, however, food continued to be served on golden and silver plates.
It was only through the molten salt electrolysis developed in 1876 by Heroult in France and Hall in the USA independently of each other, with only a few days’ difference, that production in large quantities became possible. In this process, pure alumina (Al₂O₃) is dissolved in liquid cryolite. After applying an electrical voltage, the aluminum ions separate out. However, the application of the electrolysis process only became possible through the process developed by Bayer in 1846 for bauxite digestion, which formed the prerequisite for the production of alumina on an industrial scale.
Since the beginning, production has risen unstoppably (Figure 1.1). World production of primary aluminum in 2019 was almost 60 million tons, and a further increase is predicted.
Figure 1.1: Aluminum production since the beginning of industrial manufacturing; the curve shows the upward trend.
This success of aluminum is due to its special properties. It is light but has good strength and can therefore also be used for load-bearing components. It can be cold and hot formed very well, thus allowing the production of complex shapes and components through cold and hot forming and through casting. By alloying with other metals, the positive properties can be significantly improved.
Another property also proves to be very advantageous: Aluminum has good resistance to many chemicals and does not oxidize. Although very rapid oxidation takes place on the material surface, the resulting thin oxide layer effectively prevents any further progression of oxidation. It therefore does not corrode or weather. With liquid metal, the oxide layer also closes the surface, so that effective protection is also created. Through targeted oxidation, the so-called “anodic oxidation,” the surface can obtain almost any desired color shade.
But the most important property is that aluminum does not change when melted down. Metallic structure as well as chemical and physical properties are preserved.
This makes it possible for aluminum to be given a second and third life after a first life. As shown by the Aluminum Association (GDA), the vast majority of products are not consumed during their lifetime, but merely used, with the possibility of being recycled without loss of their essential properties. Accordingly, the life cycle of a product is not the traditional “cradle-to-grave,” but a renewable “cradle-to-cradle” sequence. This property and the associated possibility for recycling have led to the fact that today around 75% of the nearly 1 billion tons of aluminum that have been produced to date are still in use; some of it has undergone numerous cycles of reprocessing.
Due to its excellent properties, aluminum is used in many applications. Starting with transportation technology in vehicle construction, the low specific weight offers possibilities for energy savings in vehicle equipment as well as in load-bearing components such as the vehicle frame. Although the trend toward electric cars eliminates engine blocks and transmission housings, other components require the use of aluminum to save weight. In local trains and also in long-distance trains, it enables rapid acceleration and deceleration to achieve short cycle times for trains (Figure 1.2). Aircraft construction and space travel would hardly be possible without aluminum. It is also gladly used in mechanical engineering because it can be machined excellently and allows the production of thin-walled cast parts.
Figure 1.2: High-speed ICE train; without the use of aluminum, the required accelerations would hardly be conceivable; the reduced weight results in lower rolling resistance and thus also reduced energy consumption.
The low weight with high strength also plays a role when mass forces occur. Aluminum is also indispensable in the construction industry and in packaging. In all applications, it is advantageous that it requires no special corrosion protection and decorative design of the surface is easily possible. The use of aluminum in different industrial sectors is shown in Figure 1.4.
Thus aluminum is present everywhere today and can no longer be imagined away from life. According to GDA, per capita consumption in Western Europe is 28 kg, measured by production figures, since it has already been defined that aluminum is not consumed but used. Of course, a portion used is also lost during production, use, and recycling. For example, burning always occurs during melting, i.e., oxidation to Al₂O₃. Applications in other processes, such as use as a deoxidizing agent in steel production or use in cosmetic products, also lead to losses. The amount used in space travel or the use in particularly thin coated foils, for which there are no recycling processes, is probably lost forever, albeit with a relatively small share. In recycling processes, a certain percentage that cannot be recovered remains in the raw material, i.e., the aluminum scrap. Overall, a share of 2-4% of the return material is lost.
Figure 1.3: Aluminum production worldwide; parallel to the growth in primary aluminum production, the processing of return aluminum also increases; due to the corresponding service life of the components, the return to the cycle occurs with a time delay.
The production of recycled aluminum runs parallel to the production of primary aluminum (Figure 1.3). Worldwide, the demand is covered by about one-third through secondary aluminum. High-quality alloys go mostly to the automotive industry. It would be desirable to increase the share of recycled aluminum in consumption. Unfortunately, however, the supply of scrap is limited. Although consumption is steadily increasing, the return of aluminum occurs only with a time delay.
Figure 1.4: Use of aluminum in various industrial sectors; the graphic shows the high proportion used in transportation; this results in a considerable influence by the production of the automotive industry on secondary aluminum production.
As a result, scrap is always scarce and raw material procurement always presents a new challenge for secondary aluminum smelters.
In recycling, not only are valuable raw materials used multiple times. Reprocessing also significantly reduces the energy expenditure that occurs in the extraction of primary metal. For the production of primary aluminum for alumina factory and electrolysis, about 170,000 MJ/t are required, but for the production of secondary aluminum only about 12,000 MJ/t (3.4 MW/t). Thus the energy expenditure for the production of secondary aluminum is only about 7% of the expenditure required for the production of primary aluminum.
Also, in recycling, waste materials accumulate both in primary aluminum smelting and in recycling, which must be collected in a landfill.
Nevertheless, it is quite justified to speak of a sustainable circular economy in connection with the industry, especially when the high recycling rates are considered.
Thus a time delay between primary production and return must be taken into account, since the aluminum is only fed into the cycle after its period of use.
