• Magnesium: recycling
  • As concern for the environment has grown in recent years, the importance of recycling has become more evident. The more materials that are recycled, the fewer natural resources will be consumed and the fewer waste products will end up in landfills, in the water and air.
  • A large amount of waste in the form of chips and discards is produced in the machining process of casting and sheets. Nearly 1/3 of magnesium used to fabricate structural products ends as new scrap, so it is really essential to find ways for its efficient recycling in order to manage the use of primary Mg.
  • It’s possible to remove oxides and impurities during the recycling scrap process, but not elements like copper and nickel (which have a major impact on the corrosion resistance). Scrap is often sold and used in other products like steel (alloying) or anodes. With die-casting the scrap percentage is about 40-60% and because of that, most die casters recycle the material (internal or external) themselves.
  • For extrusion processes the scrap percentage is much less, at about 25% percent.
  • Recycled magnesium is also an important component of the supply within the magnesium industry, particularly in the USA, where the magnesium market is presently protected from Chinese imports by high antidumping duties. In 2012, about 24,000 tonnes of secondary production was recovered from old scrap. The largest magnesium recycling facility in the world is owned by Advanced Magnesium Alloys Corporation (Amacor) and is located in Indiana, USA, in close proximity to the North American die casting industry.
  • There are various methods for recycling magnesium both as process scrap and also as ELV scrap. Several of these processes have been proven effective and efficient in recycling through recent studies. Re-melting of magnesium chips is a common recycling process-however, due to magnesium's susceptibility to oxidation, this process can be costly. A proposed solution to this issue is the use of hot extrusion as a solid state recycling method. Since the metal is not melted, a special protective environment or additional caution is not required. A study conducted at the Harbin University of Science and Technology in China showed that solid-state recycling of magnesium alloy chips is an efficient method of recycling.
  • The Brunel Centre for Advanced Solidification Technologies (BCAST) developed the Melt Conditioned High Pressure Die Casting (MC-HPDC) process for recycling high quality magnesium cast components. This process consists of imposing intensive shearing directly to the alloy melt before it is poured into the die. Attached to a standard high pressure die casting machine is a twin screw that is used to inflict the high shear mixing to the melted magnesium. The mixing improves the overall uniformity in chemistry and temperature of the melt. The MC-HPDC process was shown to be an excellent candidate for physical recycling of high grade magnesium alloy scrap through research and testing. The castings produced had consistent ultimate tensile strength and elongation properties that were comparable to those of the primary material tested.
  • The same quality criteria in regards to chemical composition and oxide content must be met for both recycled alloy ingots and primary metal. Processing ELVs is most commonly done through shredding for economic reasons. This method results in the mixing of different magnesium alloys as well as their integrated elements. Magnesium can be contaminated with iron, nickel and copper all of which are detrimental to the corrosion resistance of the metal. Through the addition of manganese, the levels of iron can be reduced. Nickel and copper on the other hand may only be controlled through distillation or dilution. In large commercial applications, dilution is impractical as it will not aid in greatly reducing the amount of new material used. Distillation on the other hand consumes approximately 5-7.5 kiloWatt•hr/kg (7,738.61-11,607.91BTU/lb).
  • Re-melting alloys consumes at the most, 50% of the energy that is required for distillation. Secondary alloys are developed for creation of new components from re-melted scrap metal with a minimum amount of primary metal while still achieving the desired and needed composition of the alloy. A recent study completed in Germany shows that it is possible to address the problems associated with mixed alloying elements and impurities in recycling within a single alloy system. These results promote recycling of ELVs through re-melting which uses minimal primary metal and energy.
  • Despite the existence of various current methods of recycling magnesium, there is still room for improvement. The further development of recycling methods is becoming a challenge in the technical, economic and environmental fields.