Fascination Technology How Used Coffee Capsules Become Clean Aluminum

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Montanuniversität Leoben

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In our "Fascination Technology" section, we present impressive projects from research and development to engineers every week. Today: how a new recycling process produces clean aluminum from used coffee capsules.

Machine on, coffee capsule in, enjoy coffee—as practical as coffee capsules may be, they can pose a potential environmental burden: while Austria does have a collection system for used coffee capsules, the recycling rate is currently only around 30 percent. A key issue for the subsequent recycling process is the high amount of coffee grounds compared to the packaging material, which leads to economic and process-related limitations during the melting process in aluminum smelters where the coffee capsules are melted down.

Nespresso capsules consist of about 0.02 to 0.04 ounces of packaging material, with aluminum being the main component, and contain approximately 0.21 ounces of coffee.

Ass.-Prof. Dr. Eva Gerold

A project at the Department of Nonferrous Metallurgy at the Montanuniversität Leoben, in collaboration with industry partners, aims to develop a new processing method for producing new coffee capsules from used aluminum coffee capsules. "The aluminum from such capsules is very valuable and should be preserved in the spirit of a circular economy," says Assoc. Prof. Dr. Eva Gerold, a researcher at the Department of Nonferrous Metallurgy at the Montanuniversität Leoben. The professor provides an example: "Nespresso capsules consist of about  0.02 to 0.04 ounces of packaging material, with aluminum being the main component, and contain approximately 0.21 ounces of coffee." The challenge is to separate these components from one another and ensure efficient recycling.

Recycling Options for Aluminum Alloys

Alloys in particular pose a challenge. "There is a saying in metallurgy: What gets into aluminum stays in aluminum. This is especially crucial in the context of recycling, since aluminum exists in numerous alloys—each with specific properties depending on the application, such as for car doors or coffee capsules. The former need to be stable and aesthetically pleasing, which is why alloying elements like silicon, zinc, or iron are used. Coffee capsules, on the other hand, must be impermeable, very thin, and rollable. These requirements must be considered not only in the selection and development of the alloy but also in the subsequent recycling process," explains Gerold.

However, alloys for coffee capsules, for instance, differ from manufacturer to manufacturer. Additionally, if the scrap is not well sorted or mixed, it is not easily possible to create a specific alloy from it again. In such cases, casting alloys are usually produced—typically for engine blocks. With the rise of e-mobility, this demand is decreasing, making new utilization options necessary. This is where the researchers come in, aiming to develop alloys and recycling processes that can tolerate various contents of different alloying elements.

Process Produces Aluminum Foils

To test and optimize the new recycling process, an industry partner provides the researchers with capsule materials prepared in various ways, differing in the proportion of organic components and the degree of shredding. "In a first step, the used coffee capsules are shredded, and the contained coffee is separated. Then the oils and lacquers on the capsules are removed. They contain organic components that would contaminate the melt. This transformation process works thermally under the exclusion of oxygen—using, for example, nitrogen. The resulting gases have a high calorific value and are used, energy-efficiently, to heat the melting furnace," explains the researcher.

To minimize oxidation due to the high surface volume of the capsules, they are compacted before melting. "The raw materials are first pyrolyzed in so-called double-chamber furnaces and then pushed into the melting bath to produce metallic aluminum. A salt treatment further helps to remove impurities and achieve an improved composition of the melt," adds Gerold. In subsequent steps, small aluminum ingots are produced, which are rolled into foil with a final thickness of 0.004 inches on the department's in-house mini rolling mill—the desired end material.

By the way, recycled coffee capsules don't necessarily have to become new capsules again, as depending on demand and alloy requirements, they can also turn into a beverage can or a new laptop.

Ass.-Prof. Dr. Eva Gerold

Recycling Process Soon Ready for Industrial Application

In the further course of the project, the aluminum foils will be industrially processed into coffee capsules again by an industry partner, enabling a closed-loop recycling of coffee capsules. "By the way, recycled coffee capsules don't necessarily have to become new capsules," the researcher points out, "as, depending on the requirements and alloy specifications, they could also be turned into a beverage can or a new laptop."

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