Abstract
The closure of anthropogenic substance and material cycles is a central theme in industrial metabolism and ecology. Its desirability is based on the analogy with biological nutrient cycles that are closed, as a requirement for their long-term sustainability. This thesis sets out to assess the level of closure of the UK iron, steel and aluminium cycles; i.e. three of the main structural 'nutrients' of the global industrial ecology. To investigate this a new time-dependent methodology for material flow analysis (MFA) has been developed. In sectors such as iron, steel and aluminium where the life-span of goods may be long and the life-spans differ between applications, it is vital to include a temporal dimension in the MFA; different products available as scrap entered use at quite different past times. In this analysis, residence time distribution theory, as developed in chemical engineering science, has been successfully adapted to simulate the delay of goods in use. The methodology has been applied to track the flows of iron, steel and aluminium through the UK economy. Historic information on the amounts of these metals going into different groups of goods, together with values for their estimated life-spans, have enabled modelling of the yearly release of iron/steel and aluminium scrap from the use phase in the form of end-of-life scrap. The iron and steel MFA carried out in this work shows that for 2001, the estimated release of end-of-life scrap and prompt scrap significantly exceeds the documented amount of scrap that is consumed within the country or is exported. This indicates a loss of end-of-life scrap of around 30% (corresponding to three and a half million tonnes). For aluminium, the analysis also shows that for 2001, the estimated amount of released prompt and end-of-life scrap is higher than the documented amount of recovered scrap. There is a loss of end-of-life scrap of about 20% (corresponding to 160 thousand tonnes). For both metals, a level of closure was achieved in the MFAs; i.e. modelled amounts of metal emerging from use could be largely balanced with documented amounts of metal being recycled and sent to landfill. The analysis shows that using a distribution of the life-span (as opposed to a fixed life-span) when modelling the delay of goods in the use phase is more important when the input of goods into use shows a significant increase or decrease over time. To achieve and maintain higher recycling rates of these metals it is vital to avoid build-up of alloying and contaminating elements in the scrap cycle. A model for exploring potential contamination build-up in the metal cycle has been developed in this work, which builds on the MFA methodology, incorporating the temporal dimension. It examines consequences for the composition of the metal flows depending on different future scenarios. A case study of exploring potential build-up of tin in the iron and steel cycle between 2000 and 2020 was performed to demonstrate the model. Not surprisingly, both increasing recycling rates and decreasing scrap exports leads to increases in the concentration of tin in metal products. By separating the scrap before remelting and choosing more carefully what type of scrap goes to which production, buildup can be avoided. The methodology presented here should prove useful in further exploring potential contamination in metal products and developing strategies how to avoid it. The MFA studies show there are still improvements to be made in recovering end-of-life iron/steel and aluminium scrap. Small products such as packaging stand out as a major challenge for these metals. Therefore, possible ways of collecting beverage cans were investigated in a case study of used aluminium beverage cans (UBCs). Two main issues explored included the questions: (1) Does transport intensity differ greatly between various types of collection systems, recovery rates and population density. and (2) How significant is the environmental impact of the collection stage compared to the whole life cycle of the can. Overall, the differences in environmental impacts between the collection systems (kerbside, can banks and deposit) are not considerable. Transport per collected unit increases with decreasing population density. However, in the context of the whole life-cycle of aluminium cans, the analysis of the systems shows that over a range of population density, the collection stage makes negligible contribution to en- vironmental burdens. The savings in environmental impact of recovering and recycling the cans after use far outweigh the impacts of collecting them. This very much highlights the need for functional and easily accessible recovery infrastructures for aluminium cans in the UK.