We often hear the claim that metals can be recycled indefinitely often, without loss of quality.*) Unfortunately, this claim is not true and the myth of perfect and indefinite recycling needs to be busted. The reasons are that a) our waste management industries cannot perfectly separate the different metal fractions from the waste streams, so that there is contamination with other metals in most cases, and b) there are unavoidable losses at all stages of the recycling loop. When a unit of metal, like steel, passes through different product life cycles, it needs to flow through the recycling loop (Fig. 1) several times. Each time it passes through that loop, some more impurities accumulate and some more metal gets lost in obsolete stocks, dissipative losses, shredder and other waste management residues, and slag formation during remelting.
Figure 1: System definition of the metal recycling loop (Pauliuk et al., 2017).
Imagine a situation where 50% of a certain metal gets lost during each recycling loop, because of obsolete stock formation, a low recovery rate from the shredder fractions, and some remelting loss. Then the fraction of the metal present in the technosphere after 0, 1, 2, 3, … life cycles is 1, 1/2, 1/4, 1/8, …, respectively, which is a geometric series. If we assume a product lifetime of 4 years, we can calculate the fraction of an original amount of 100 kg metal present in year 0 after n years: 0-4 years: 100 kg, 4-8 years: 50 kg, 8-12 years: 25 kg, 12-16 year: 12.5 kg, … . From that series we can calculate the average lifetime of the metal (here: 8 years) and the average number of product life cycles it passes through (here: 2).
The same principle, just a bit more sophisticated (with a lifetime distribution, different products, metal qualities, remelting processes, and regions) was applied to the steel cycle in a recent project that I conducted together with my Japanese colleagues Shinichiro Nakamura and Yasushi Kondo (Pauliuk et al., 2017). In this paper, we take 1 ton of steel in different applications, such as passenger vehicles, consumed in 2015 in a certain region, and calculate the whereabouts of that material over the entire 21st century, assuming a certain product lifetime and other process parameters. A typical model result is shown in Figure 2, where the steel remains in cars for about the first 15 years, and then gradually moves to construction, which is the main sink for secondary steel in most cases. Buildings and infrastructures have a much longer average lifetime than vehicles, which means that the usable steel fraction declines slower after the first recycling loop. Still, significant losses occur during each remelting, leading to a gradual decline of the usable steel fraction to almost zero over the entire 21st century when assuming business as usual process parameters. I admit that such a calculation is highly speculative. It is meant as an illustration of the eventual fate of steel under the assumption that we keep on operating the steel cycle processes with the same efficiencies as today.
Figure 2: Distribution of a unit of steel, originally consumed in form of a passenger vehicle, over different product categories and over time (Pauliuk, 2018).
We can then use curves like the one shown in Figure 2 to determine the average lifetime of a metal in the anthroposphere, which in the case of 1 ton is just the area under the curve. The result for different scenarios are shown in Figure 3. The first estimate of the average metal lifetime in the technosphere was published about 12 years ago (Daigo et al., 2005).
Figure 3: Decay curves of steel in the technosphere for different application and recycling scenarios (Pauliuk, 2018).
Two of the most typical steel applications, cars and buildings, show an average residence time of 250-280 years (continuous blue and green line in Figure 3). The values for the two applications are so similar because of the cascading/downcycling of automotive steel into buildings already after one life cycle. With more efficient metal recovery (details in the supplementary material of Pauliuk et al. (2017)), the average lifetime can be expanded to 290-360 years, which is quite long but not exactly infinite.
A closed loop scenario for cars (solid brown line) would be a disaster in light of a ‘circular economy’ for steel, but a significant lifetime extension potential exist for that case (dashed brown line), resulting from more efficient steel recovery and fewer cars going into obsolete stocks. For steel in buildings up to 560 years average residence time can be achieved under a very efficient recycling loop management. Even longer lifetimes are possible, but achieving those would require material efficiency and ‘circular economy’ strategies, in particular, lifetime extension and reuse of construction steel instead of remelting it.
In my opinion the expected average anthropogenic lifetime of a material should be a core resource efficiency and circular economy indicator.
“Glass can be recycled indefinitely […].”, “This [remelting] process does not produce any change in the metal, so aluminium can be recycled indefinitely.” https://en.wikipedia.org/wiki/Recycling_by_material
“The Aluminum Can is 100% recyclable and can be recycled indefinitely.” https://www.wastecare.com/Articles/Aluminum_Cans_Recycling.htm
“Both steel and zinc are 100% recyclable indefinitely without the loss of chemical or physical properties.” https://www.galvanizeit.org/hot-dip-galvanizing/what-is-zinc/zinc-recycling
Obsolete stocks: Materials in products that are no longer used but not available to waste management either. Obsolete stocks include, for example, old cast iron pipes or copper wires that were never recovered, abandoned buildings, abandoned vehicles, or old electronic devices stored in peoples’ houses.
Closed loop recycling: Situation where recycled material enters the same product type or product group as the one where the scrap originally came from.
Daigo, I., Matsuno, Y., Ishihara, K.N., Adachi, Y., 2005. Application of Markov chain model to analyzing the average number of times of use and the average residence time of Fe element in society. Tetsu to Hagane (Journal Iron Steel Inst. Japan) 91, 159–166.
Pauliuk, S., 2018. Critical Appraisal of the Circular Economy Standard BS 8001:2017 and a Dashboard of Quantitative System Indicators for its Implementation in Organizations. Resour. Conserv. Recycl. 129, 81–92.
Pauliuk, S., Kondo, Y., Nakamura, S., Nakajima, K., 2017. Regional distribution and losses of end-of-life steel throughout multiple product life cycles—Insights from the global multiregional MaTrace model. Resour. Conserv. Recycl. 116, 84–93.