Recycling Magnesium

Die casting is the most common method of fabricating magnesium alloy parts. Die casting foundries can manage the amount of process scrap in three different ways. Scrap can be sold on the open market however the metal is often times downgraded for use in the desulphurization of steel or other markets. Due to the high dependence on alternate material supply, the prices paid for scrap vary considerably. Foundries can also recycle magnesium scrap either internally or externally. Often times, this is the most cost effective method of controlling scrap quantities. The process scrap that is generated in die casting is kept within a closed loop system reducing demand of primary material by up to 50%.

There are several factors that dictate the amount of processed scrap that is produced and recycled and the amount of primary metal that is utilized. The ratio of scrap to product for a single shot, the amount of material lost in the melting cycle, the quantity of different components that are cast, the percentage of cast parts that must be rejected during production, the end quality of process scrap, and the recycling operation efficiency all affect the amount of process scrap and primary magnesium utilized.

Methods for Recycling

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 consume 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 further improvement. The further development of recycling methods is becoming a challenge in the technical, economic and environmental fields.

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