Linkages and stock dynamics of the aluminium-gallium system: Alloying elements, impurities and by-products

Abstract

Impending global environmental- and resource related problems require a transformation of the socio-economic metabolism, the inputs, processing and outputs of materials and energy in society. The environmental impacts and resource requirements of metal cycles can be reduced significantly if they approach a steady state where in-use stocks are maintained through recycling rather than primary metal. In a steady state system, linkages between metal cycles, for example through alloys, impurities and by-products, will become increasingly important due to the high share of recycled post-consumer metal. The aluminium cycle is in this regard particularly important: It is responsible for large greenhouse gas emissions and energy use, highly sensitive to alloying elements and impurities, and has a strong linkage to gallium, which is a by-product of aluminium production. In this thesis, these linkages were studied in the context of in-use stock saturation and closed-loop material cycles, to better understand their importance, and to identify strategies that can facilitate a transition towards a steady-state socio-economic metabolism. It was found that higher recycling rates are increasingly difficult to achieve in closed-loop steady state systems, due to the accumulation of impurities. A model of aluminium beverage can recycling showed that stable metal impurity concentrations are reached after 5-15 recycling loops with recycling rates in the range of 45-75%. It is expected that similar results would apply for systems that are more complex. The increasing availability of automotive aluminium scrap represents a future challenge for recyclers due to the large variety of alloys and limited demand for mixed scrap. A global surplus of scrap may occur in the period 2020-2030 unless measures are taken to restructure the recycling system. Allowing recycled material in safety-relevant components, together with an improved sorting of alloys through dismantling or advanced sorting technologies, may delay the surplus with several decades. The global system of production, manufacturing, use and recycling of gallium, and gallium-containing products was described and quantified for 2011. Gallium use is currently driven by neodymium-iron-boron magnets containing gallium as an alloying element and semiconductor applications: integrated circuits, lightemitting diodes and photovoltaic panels. Large material losses occur in the fabrication of intermediate products and devices. Currently, demand is far below the supply potential. Scenario analysis showed that a low stock saturation in the aluminium cycle might cause the supply potential of gallium to fall below future demand, given an increased market penetration of gallium-containing technologies. A number of measures was identified for improving the systemwide material efficiency, among which the most effective are related to process yield improvements or collection of production scrap. The results showed that linkages between material cycles greatly complicate the transition to a steady-state socio-economic metabolism, and at the same time indicated priorities for measures that can be taken to facilitate this transition.