Zinc Diecasting Alloys
Zinc is essential to life, it is clear that there are overwhelmingly more cases of health problems resulting from shortage of zinc in the body than from excess zinc.
Zinc has a vital role in healthy growth; its function is to control the enzymes that operate to renew the cells of our bodies. An adequate level of zinc in the human body can be maintained by an intake of 10-20mg per day. This is normally provided by eating a balanced diet containing meat, fish and vegetables, although the latter contain significantly less zinc and it is in a less readily available form.
Shortage of zinc leads to underdevelopment and blood and skin diseases.
Zinc promotes the healing of wounds and rashes, hence the use of zinc oxide in a multitude of ointments and adhesive plasters.
The other added elements in the zinc alloys, aluminium, copper and magnesium can also be considered safe. Only aluminium is injurious to the human body at low levels, but the widespread occurrence and use of this material without widespread resulting problems demonstrates that it is not readily transported into the body.
Certain of the impurities mentioned in the specifications of zinc alloys such as cadmium and lead are known to be toxic, but it should be noted that they are maintained at very low levels. In fact the levels of these elements in zinc alloys are probably lower than in many other materials where their possible occurrence is not mentioned.
Zinc is a comparatively low/moderate energy consumer to mine and smelt. It is difficult to be more precise because of the conflicting nature of the data available on zinc and potential competitor materials.
The energy required to form the zinc alloy into a casting is theoretically very low indeed but in current practice it varies in the range 10 – 30 GJ/tonne. It is comparable to the energy used in injection moulding and lower than that needed to diecast aluminium. There is clearly room for improvement here if energy costs should rise.
In automotive use the weight of a component is important in terms of energy consumption because it increases the fuel consumption of the vehicle. Zinc is obviously at something of a disadvantage here because of it’s high density but optimisation of casting design will very significantly offset this
The environmental issues surrounding the provision of raw materials of all types are many and various, as are those connected with the transformation of those materials into finished products.
In zinc production some of the most important issues are concerned with other elements found in the naturally occurring ores. Emissions of these materials from mines and smelting plants are monitored carefully to ensure that they remain within acceptable limits. To date zinc production has not proved any more problematic from an environmental angle than most other metals and there is no reason to suppose that such issues will threaten the supply of zinc in the foreseeable future.
The casting of zinc alloys is notable by its cleanliness and lack of noxious emissions, and the vast majority of process scrap is recycled within the diecasting foundry.
Zinc diecasting alloys can be remelted and re-used many times without detriment to their properties provide they are not severely oxidised or contaminated with injurious elements. It is normal to limit the use of remelt material to about 50% in the diecasting foundry, but this enables the complete consumption of process scrap, ie sprues and runners and any reject castings, leading to minimal wastage of material. In fact the only scrap zinc material that the diecaster needs to dispose of is the skimmings from the surface of the melt and perhaps some machining swarf. The amount of material involved is about 5% or less of the weight of castings produced. All of this material has significant scrap value and is eventually processed to turn it into valuable products including zinc dust, and zinc oxide
Scrap assemblies incorporating zinc diecastings and castings with metallic or organic finishes can be “sweated” together with old scrap resulting from the shredding of such things as cars and domestic appliances. This again yields a product suitable as feedstock for the production of zinc dust and oxide or for re-smelting into “primary” zinc.
This recycling system is in place now; it is not just a theoretical possibility. The great majority of zinc diecasting scrap that becomes available is recycled.
In recent years large quantities of shredder scrap have been exported to South East Asia. There, amongst other things, it has been turned into secondary zinc diecasting alloys. The quality of these alloys is highly variable and at worst could lead to very unreliable castings. Castings from this part of the world should be independently checked as to their composition. This is not to say however that all zinc diecasters in that part of the world are disreputable. In fact there are many good ones.
These alloys comply with directive 2000/53/EC of the European Parliament and of the Council of 18 September 2000 on end-of life vehicles. They also comply with the RoHS Directive (European Parliament and Council Directive on the Restrictions of the use of certain Hazardous Substances in electrical and electronic equipment (2002/96/EC)).
In summary zinc alloys have very low toxicity, are not energy intensive to produce or process, and the technology and infrastructure required to economically “recycle” the metal content of zinc diecastings has existed and been used for many years. Making zinc alloys essentially “environmentally friendly” materials.