Aluminum and sustainability – this discussion has long been ideologically charged. The criticism to which aluminum production is subjected generally ignores the fact that the multiple reuse of the material significantly reduces energy requirements. Provided, that is, that the recycling industry is equipped to process the volumes of returns expected in the future.
The ecological footprint of aluminum as a material is significantly influenced by the energy required to extract it. With an energy requirement of around 14,000 kWh/t of primary aluminum, the first question that arises is indeed where the material can be used sensibly.
On the other hand, the aluminum industry is experiencing continuous, strong and sustainable growth worldwide. Since ideological arguments have not been able to stop the trend, the real advantages of the material must inevitably prevail.
In fact, the amount of energy used to produce aluminum mentioned above is put into perspective when two important aspects are taken into account: First, aluminum can be permanently reused, with recycling accounting for about 5 percent of the energy used in the smelter. In addition, lightweight aluminum constructions help to save energy in transportation, where most of the material is used today. In addition, what is usually overlooked, is that the energy required for aluminum processing – melting temperature, deformation strength, specific cutting force – is significantly lower than the comparable values for steel processing – this also contributes to energy savings.
Built-in growth engine
The high material value of aluminum makes it economically worthwhile to recycle all reusable aluminum at the end of its service life. Europe, which is poor in raw materials, has come a long way in this direction. In Germany, recycling is currently the most important source of raw materials for the aluminum industry. For years, about two-thirds of the metal produced here has been obtained through recycling. Worldwide, the recycling industry already covers about one third of the aluminum demand.
Worldwide, the recycling industry already covers about one third of the aluminum demand.
As a result of the growing demand for aluminum and its use for an average of ten years, a gap inevitably arises between current aluminum demand and the available recycled material. The smelters close this gap; however, according to theory, the proportion of secondary aluminum should increase relatively. In fact, this increase has considerable momentum: worldwide, it is expected to rise to around 30 million tons of recycled material per year by 2025.
So much for the theory. However, a comparison of production figures shows that in the past the theoretical proportion of material obtained from scrap has not been achieved. This may be mainly due to the fact that the development of a sufficiently efficient recycling industry takes time. In this respect, much has happened in recent years; today an independent industry with specific melting technology is available.
Today an independent industry with specific melting technology is available.
The time when half of the aluminum required will have been secondarily smelted is already foreseeable. In the time thereafter – as shown by the theoretical relationship – consumption quantity and recycling production will continue to converge.
Aluminum recycling technology
In order to be able to process around 30 percent of the world’s aluminum consumption, highly developed smelting technology are required above all. The task is complex: the scrap produced ranges from thin strips to massive ingots. The composition varies within wide limits. The degree of contamination ranges up to 10 percent and above. The task is to produce high-quality aluminum from this heterogeneous starting material, while minimizing energy consumption and largely eliminating environmental pollution.
These requirements can best be met when aluminum products are processed in a closed loop. In this way, a high degree of grade purity is achieved, allowing direct reuse. Examples include beverage cans, scrap from window and facade construction, aviation scrap and several others.
Differentiated melting technology
Where the scrap is unsorted, a differentiated melting technology is used. Depending on the type and impurities, different scrap classes can be distinguished with regard to the melting technology to be used:
– Lumpy scrap up to 1% impurity
– Lumpy scrap up to 5% impurity
– Lumpy scrap with more than 5% impurities
– Processing chips
– Thin-walled packaging material, e.g. printed foils
– Dross, slag or similar.
Different processes and furnace equipment have been developed for these scrap classes. When melting contaminated scrap, the plants use the energy content of the organic contamination for heating. Among other things, the economic side benefits from the lower energy requirement. This leads to a significant reduction in operating costs, but at the same time it also has the favorable ecological effect of low CO2 emissions. The minimal emissions (NOx, dioxins, VOC, no salt) also contribute to the environmentally friendly furnace operation.
Examples of modern recycling furnaces
In the simplest case, a single-chamber furnace meets the requirements, for example, when production scrap with low contamination is to be melted down. These furnaces can be either tiltable melting and casting furnaces or stationary furnaces.
A frequently used furnace variant is the multi-chamber melting furnace for melting down moderately contaminated scrap. Depending on the design, the material is preheated to about 500°C in a preheating chamber or a preheating shaft, during which the organic buildup is smoldered off. The heated and cleaned material is transferred to the metal bath, where it is completely immersed and melted. The carbonization gases are used for heating. This design, which is offered by Hertwich or LOI thermprocess, among others, operates with an energy requirement of between 250 and 350 kWh/ton, depending on the degree of contamination.
Precautions against oxidation losses
When melting thin-walled packaging material, oxidation losses make special precautions necessary. Also particularly troublesome is the fact that aluminum packaging is usually printed, laminated, or otherwise combined with non-metallic components. Pyral AG in Freiberg, Saxony, for example, is involved in the large-scale recycling of aluminum from packaging materials. The flaking process takes place in a rotary kiln in the absence of oxygen at around 500°C. The flue gases produced in this process are used as a fuel. The resulting flue gases are used to heat the kiln, so that this process can take place almost self-sufficiently in terms of energy. A fluidized bed system is then used to clean the light metal, which can then be returned to the recyclable material cycle.
For heavily contaminated scrap (>10%, high iron content) and for slags, rotary kilns have proven their worth, such as those built by the Herten-based company SUG or by Hertwich. These metal losses should not be underestimated: Worldwide, about 3 million tons of dross with an average metal content of at least 50 percent are currently produced annually. With the available processes, up to 1.5 million tons of aluminum could be recovered from this. The metal yield of this furnace is 85 to 90 percent for contaminated scrap and 45 to 65 percent for dross. Energy consumption is low; 250 to 320 kWh/t is required for contaminated scrap and dross.
Text: Alwin Schmitt, Image: Shutterstock