Two or more materials are considered ‘compatible’ when they can be recycled together without negatively affecting their properties. Contamination occurs when recycled material streams become contaminated with fragments of other materials. When combining different materials in a single component or part it is important to consider their compatibility.

Metallurgical plants have over time been optimised to deal as efficiently as possible with a certain carrier metal and the elements geologically/thermodynamically associated with it (eg. energy efficiency, minimising losses of those elements in outer ring). E-waste poses a challenge to such installations because it can contain many metals from different slices of the metals wheel. Imperfect separation is what causes contamination of recycled commodity metals.

If you look at the metals wheel below, you see that the recovery of a certain carrier metal (e.g. aluminium) will lead to the recovery of certain co-elements and the loss (to residue or emissions) of others.

Source: im-mining.com

The compatibility of metals depends on how easily a certain combination can be separated during the recycling process. That is, metallurgical technology is able to separate metals that are thermodynamically compatible. Some combinations, however, are difficult to separate, leading to one of the materials polluting the other’s recycled stream (negatively affecting its properties) or ending up as a waste fraction.

Below are a number of design for recycling guidelines related to metal compatibility for commonly-used metals:

• If the main material in a component is Al (cast), do not attach a part of stainless steel or steel onto it.
• If the main material in a component is Al (wrought), do not attach a part of Al (cast), copper, stainless steel or steel onto it.
• If the main material in a component is Stainless steel, do not attach a part of copper onto it.
•  If the main material in a component is Steel, do not attach a part of copper or stainless steel onto it.
•  If the main material is copper, do not permanently fix a part of iron, lead, antimony or bismuth to it

Above, “permanently fixed” refers to connections that cannot be mechanically separated, while “attached” means parts of metal x in a component which main material is metal y.

Source: Design for and from Recycling: Practical Guidelines for Designers (PolyCE, March 2021)

On the next page we’ll look at the compatibility of plastics.

 

For learners interested in additional information on metal compatibility, the following paper provides further reading:

• (PDF) Challenges of Metal Recycling (researchgate.net)