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New steel from old sheet-metal

Just like cooking in your kitchen at home, producing tinplate requires the right ingredients. These are placed in a gigantic cooking pot in the right quantities and the right sequence, and melted to form a homogenous mixture. Once further ingredients have been added, the material needed for food or beverage cans is obtained.

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An important step towards transforming scrap into new tinplate: a charging box is floated to the converter, and fills it with tinplate scrap compressed into bundles. Pig iron at a temperature of 1,500 degrees is then poured into the cooking pot.
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In around 40 sorting plants nationwide, lightweight packages from the Yellow Sack and the Yellow Bin are separated into fractions. A strong magnet removes tinplate bundles from the conveyor belt.

The heat is stifling at the steel industry’s cooking pots. Flames are shooting out from the converter, in which a mixture of scrap bundles and pig iron at a temperature of 1,500 degrees is being melted to make steel. Tinplate scrap is one of the recipe’s ingredients. And this comes from the Yellow Sack and the Yellow Bin. Every year, consumers throw away 2.5 million tons of lightweight packages (the weight of around 20,000 blue whales) with the Green Dot emblem. This includes approximately eleven percent or 275,000 tons of tinplate: food cans and other packages made of tinplated steel.

Sorting the packages into fractionsboetzel_078
The recycling of tinplate already begins with private consumers, who throw away empty food cans for fruit and vegetables, screw-on lids for glass packages and crowns in the Yellow Sack. This ends up in a collection vehicle, tasked by the dual system with gathering used packages. The journey continues to one of almost 40 sorting plants in Germany equipped for sorting lightweight packages. Special technical features enable the Yellow Sack’s content to be separated into fractions: for example, an air flow pulls plastic films off the conveyor belt, while a magnetic separator extracts tinplate packages from the mass of recyclable materials.
At this juncture, the tinplate is not yet homogenous: residual contents and contaminants like films still adhering to the cans, etc. may account for twelve to twenty percent of the total weight. And that adds up to a lot, since plastic is lighter than tinplate. Which is why another step is required in order to obtain homogenous scrap.

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Specialist companies remove contaminants from the sorted tinplate, after which the tinplate scrap is compressed into bales and delivered to the steelworks.

Contaminants are removed
The mixture of tinplate and contaminants is then processed by specialist companies. One of them is Wilhelm Bötzel in Witten and Herne. In a large hall, the material arrives, either loose or compressed into bales. A digger heaves the bales into a shredder, which crushes them. Another digger pours the recyclable material onto a conveyor belt leading to the interior of the plant. A machine separates the tinplate from the substances adhering to it, which are then used for recovering energy. The material is now so pure that it can be used in steelworks.

This is the next stop for the tinplate, which is compressed into steelwork bundles, each weighing around 200 kilograms, and transported to thyssenkrupp’s Duisburg facility, for example. From the company’s scrap warehouse, the larder of the steel production operation, so to speak, overdimensioned magnets lift the scrap bundles into charging boxes. These are vaguely reminiscent of kitchen scoops, and are moved by a crane to the converter, a cooking pot used in the steel industry that is eleven meters high and 15 meters wide.

The empty converter is first filled with the scrap, then with liquid iron from the blast furnace. This sequence is important, because due to its lesser weight the scrap would otherwise float on the pig iron and wouldn’t mix with it properly. The pig iron is melted in a blast furnace – using ore, coking coal and lime. From a gigantic bucket, guided by another crane, it is tipped into the converter, where flames are shooting out and sparks are flying. Any remaining substances adhering are burned off completely in this part of the process.

Weißblechschrottpakete im Rohstofflager des Stahlwerks (oben). In einer Chargiermulde geht es zum Konverter, in dem der Stahl erzeugt wird (unten).
Bundles of tinplate scrap in the steelworks’ raw material warehouse.

Oxidation in the converter
Now oxygen at a pressure of 15 bar is blown into the converter through a long pipe. The pig iron has a carbon content of about 4.5 percent. Since that would render the steel brittle, the carbon content has to be reduced to approximately 0.05 percent. When oxygen is blown in, the carbon oxidizes, and the temperature inside the converter rises by several hundred degrees. In around half an hour, about 18 tons of carbon are burned here. Because the melt in the cooking pot must not be allowed to get too hot, it has to be cooled. This is also a task for the scrap, which after a few minutes melts into a glowing mass.

A material sample is taken in order to ascertain whether the melt’s composition is correct. And while cooks rely on their palates when tasting a dish, when it comes to steelmaking chemical and physical parameters are crucial. If the sample is in the target range, things then start to move fast. About 40 tons of slag are poured off, constituents not required for producing steel. The more important step follows immediately: around 380 tons of liquid steel are poured into a ladle, and then cast into slabs.

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A charging box takes them to the converter, in which the steel is created.

Verpackungsstahl_CoilsHot and cold-rolling
Slabs are blocks that are first rolled into sheets 2.5 millimeters thick and then rolled up into what are called coils. Part of the new steel is transported by train to the next stop in packaging steel production, the Rasselstein GmbH company, a thyssenkrupp facility located in Andernach, 150 kilometers away. The steel strip is now passed through a cold-rolling tandem mill: it is pressed flat between two rollers while being subjected to enormous force, and is lengthened like dough under a rolling pin. It is passed through five or six roll-stands at a speed of up to 145 kilometers per hour. To ensure smooth running, the strip is lubricated with rolling oil and water. After this step, the packaging steel is 0.1 to 0.49 millimeters thick, depending on the intended purpose. The steel is then degreased, meaning that the rolling oil and any soiling are removed, and heated up again, so that it remains moldable. Another rolling pass is followed by a finishing process, which finally transforms the material into actual tinplate.

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After the scrap, the steelworkers add the pig iron to the converter from a gigantic ladle. In a violent reaction, paints and coatings are completely burned off the tinplate cans.

Coating with tin
In the tinplating line, the steel coils are welded together to form an endless strip, and then coated with tin. Once it’s been rolled up in coils, thyssenkrupp delivers the material to customers who use it to manufacture food, pet-food and beverage cans, screw-on lids, crowns, and much, much more.

The thyssenkrupp company produces around 1.5 million tons of tinplate every year at its facility in Rhineland-Palatinate for 400 customers from 80 different countries. Most of it, around 70 percent, goes to companies in Europe. “Here in Andernach, we’re not only the world’s biggest facility for packaging steel, but also probably the biggest recycling company for tinplate as well,” says Christian Pürschel, Head of Communications and Market Development at thyssenkrupp’s Tinplate Division. Tinplate recycling enables around 360,000 tons of coal and 800,000 tons of iron ore to be saved in Germany each year.

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After about half an hour, the pig iron and scrap bundles have been totally intermingled. The converter is tilted and around 380 tons of liquid steel flow out of it. The material is first cast into blocks and after numerous downstream worksteps rolled into what are called coils from which then tinplate packages can be formed, for instance.

Moreover, the tinplated material ranks first among recoverable packaging materials in terms of the recycling quota: more than 90 percent of tinplate packages was recovered as feedstock in 2015, according to the figures published by the German Packaging Market Research Institute (GVM). This impressive recovery performance is also attributable to the commitment displayed by companies like thyssenkrupp, who are keen to close the recycling circuit to best possible effect. In 2003, for example, thyssenkrupp set up DWR Deutsche Gesellschaft für Weissblechrecycling, tasked with tinplate recycling and an important partner of Der Grüne Punkt, so as to maintain the currently high level of recycling rates.

For food cans and other tinplate products, the next step involves food and beverage manufacturers, who fill the packages and deliver them to supermarkets where they are purchased by consumers, who empty them, and dispose of them in the Yellow Sack or the Yellow Bin. And here the steel packaging cycle is closed – only to begin again anew.