Recycling

Abstract

Environmentally sound and cost effective use of magnesium alloys in automotive applications assumes efficient closed loop recycling of die casting returns and post-consumer scrap.Closed loop recycling (Fig. 9.1), is defined as recycling die casting returns and post-consumer scrap back to the quality needed to cast the same part, as produced originally [1-3]. The success of closed loop recycling is important for the magnesium industry because unlike aluminium, there is no other major market or source for returns/ scrap outside the die casting community. This forces recycling processes to be capable of regaining the original chemical composition and cleanliness of the magnesium alloys. It is also the objective of the industry to minimise life cycle costs, energy consumption and CO2 emissions of magnesium products. It is also important for the industry to improve continuously the recycling friendliness of magnesium,both through developing recycling friendly alloy specifications, and through more efficient technologies for recycling die caster returns and post-consumer scrap [2]. Based on life cycle assessments, estimating the total energy consumption for producing a typical magnesium application, the importance of closed loop recycling of both die caster returns and post-consumer scrap can be visualised.The CO2 emissions will in general follow the trend of the energy consumption [1].The energy requirement for melting and recycling magnesium is only about 5% of the energy to produce the same quantity of primary material.As an example, parts produced from a mixture of primary and recycled alloys, assuming closed loop recycling of all die caster returns, represent only 50-70% energy consumption compared to parts produced from primary magnesium only, assuming a die casting yield of 50%. Parts produced from a mixture of primary, recycled die caster returns and post-consumer scrap, assuming that 50% of the net metal is produced from post-consumer scrap, represent only 30-40% energy consumption compared to parts produced from primary metal only. The growth in magnesium alloy consumption is driven by an increasing demand for automotive applications [4]. The automotive industry accounts for 90% of the casting demand.While this consumption generates considerable amounts of process scrap today, in the long run it will give significant amounts of postconsumer magnesium scrap as well. The need for recycling will more or less follow the same trend, illustrating the need for recycling capacity either by the die casters themselves, by primary producers, or by independent recyclers [5].