Time consideration in mass flow analysis; a contradiction?

Abstract

In metals industry Mass Flow Analysis (MFA) has a long tradition. Combined with scenario calculation this method can for example be used to estimate the future development of scrap availability and metal composition. Despite the unknown inaccuracy of such calculations what regional distribution of material, lifetime or growth rates concerns highly valuable information can be generated to support the industrial business strategy.
When particular products or metal applications in functional units become the scope of such calculations besides mass flow often energy consumption and emissions appear on the agenda. And, unfortunately, metals are recycled (of course only from a methodological and not from a resource point of view). At that point MFA crosses the border from being a static snapshot of an existing system towards a time depending life cycle approach. Besides volume, quality, and location time is introduced as a fourth dimension and the dilemma starts. In the past one approach was to develop depreciation models to allocate the expenditures of mining and primary metal production of Copper and Aluminium according to the recycled content. But latter is even more disputable since metal recycling cannot be treated as an alternative production route achieving the original properties and chemical composition. And, as a result of its electrochemical behavior, each metal cycle looks completely different compared to any other. Consequently the general acceptance of the existing allocation methods for recycling in life cycle models is still low.
In case of Aluminium the historic primary metal production cumulates to an amount of almost 900 million tons worldwide. With the aid of recycling 75% of this material could be conserved in the current inventory in use. As this stock significantly contributes to todays and future's raw material supply the following question arises: Can metallic materials have a defined history and therewith an ecological hereditary debt, which will be reduced by further recycling cycles? If the answer would be yes, on a highly aggregated level, another more practical question would be whether such a credit should be based on the historic expenditures of primary production or better considering the future recycling potential of the inventory in use. In these cases recycling credits are either overestimated or underestimated depending on the lifetime and herewith the deviation from current End of Life recycling practices.