Circular, sustainable management means manufacturing long-lasting products that are reusable, can be repaired and which are recycled at the end of their life. Hot dip galvanized steel meets these criteria and is particularly sustainable as both the steel and zinc can be recycled at the end of life of a product. Steel and zinc can be recycled as often as required without any loss of quality (downcycling). This means that recycled zinc, for example from zinc roofing sheets, can be reused within the hot dip galvanizing process.
At end of life – and if reuse is not feasible – galvanized steel can be recycled easily with other steel scrap in the electric arc furnace (EAF) steel production process. Any zinc remaining from the coating volatilises early in the steel recycling process and is collected in the EAF dust that is then recycled in specialist facilities and often returns to refined zinc production.
Since the early 1980s, a well-established rotary kiln process has been used to process EAF dusts that contain valuable zinc and other elements. An impressive 98% of EAF dusts produced by Europe’s steelmakers are recycled. This process is the most commonly applied method to recycle these dusts but various other innovative processes have also emerged, including the rotary hearth furnace; multiple hearth furnace and low hearth furnace.
The rotary kiln was originally devised for processing of leach residues during primary zinc production and EAF dusts are quite similar in characteristics to those residues – making the technology relatively easy to adapt for recycling. The first kiln to be used for recycling EAF dusts began in Duisburg, Germany in the early 1980s.
A key driver for the recovery of these dusts is their zinc content. The wider use of zinc for coatings on steel, in particular in the automotive sector, has increased EAF dust zinc contents to levels that make their recovery economically attractive. Generally, zinc contents >15% in the EAF dust, make recovery economically viable and most dusts are at this level.
The main product of recycling EAF dust using the rotary kiln process is zinc oxide. This is sold to a primary zinc refinery where it is substituted for mined zinc concentrates. The zinc refinery then produces the same zinc ingots (or other high purity zinc products) that can be used directly in the galvanizing process. This loop can continue infinitely and there is no loss of quality of the zinc that follows this path.
Recycling begins in the galvanizing plant
Galvanizing is always carried out in an industrial plant, which contains all stages of the process. Steel comes in at one end of the plant and the finished galvanized product goes out at the other. The main raw material, zinc, is used very efficiently in the galvanizing process. The dip operation ensures that any zinc that is not applied to the steel remains in the galvanizing bath. Zinc that oxidizes on the surface of the bath (termed ‘ash’) is removed and is readily recycled (sometimes on the same site). Dross formed at the bottom of the bath is removed periodically and also has a high market value for recycling.
If a material was specially designed for the circular economy, hot dip galvanizing would be an excellent example. But, it is here today and has been following these principles for decades.
1 | Recovery of zinc from galvanized steel, without loss of properties, after many decades of service
2 | Steel and zinc can be recycled as often as required without any loss of quality (downcycling)
3 | Dross formed at the bottom of the galvanizing bath has a high market value for recycling
Image: Institut Feuerverzinken, Public Domain