Recycling composite materials can be more complicated than recycling waste made up of a single material. A single item may contain different components, joined in different ways, which behave differently when shredded or separated; it is important to be aware of this before processing the waste.
Here at Silmisa, as manufacturers of industrial shredders and other recycling machinery, we’ll explain why this happens.
When waste is made up of several materials
Some waste materials appear to be a single piece, but inside they consist of different components. Composite materials combine two or more materials to achieve specific properties. A piece of fibreglass-reinforced resin is an easy example to understand.
That does not mean that any waste consisting of several parts is automatically a composite material. A multilayer material has layers bonded together, whereas a mixture may contain plastic, copper or aluminium without forming a single structure. The way in which they are bonded determines what needs to be done next to separate them.
The problem arises when trying to separate some fractions from others
Crushing a piece does not automatically separate its components. For the materials to be separated afterwards, those parts must first be made sufficiently loose.
Here, they describe the type of joint, the hardness, the behaviour under shear and the size of each component. Two pieces of waste that look similar on the outside may react very differently when crushed. That is why it is important to know how the part is constructed before proceeding.
Joints, layers and reinforcements that do not separate at the first cut
Several layers may be bonded together with adhesives or form a very thin structure. In multilayer materials, an initial cut may leave some of the layers still joined together. Something similar happens with reinforced materials: breaking the piece does not necessarily guarantee that the fibre will separate from the resin.
Densities, hardnesses and behaviours that react differently
Composite waste may contain a combination of flexible rubber, hard metals, lightweight plastics or fibres. Each component reacts differently to cutting, impact and grinding. This difference affects the resulting particle size and how each component can subsequently be sorted.
Three examples that show why not all compounds are treated in the same way
This difficulty is best understood by looking at the types of waste commonly found in a plant. When recycling composite materials, there is no single solution that works for everything, as the structure, strength and the specific material to be recovered all vary.
A tyre, a cable and a technical component each present different challenges. Sometimes steel needs to be extracted, at other times copper needs to be recovered, and on other occasions layers or reinforcements need to be separated. This difference affects the size reduction process and the subsequent stages.
Tyres: rubber reinforced with steel and fibres
This waste consists of a combination of rubber, steel and textile reinforcements. When tyres are recycled, these components must be gradually separated so that they can be sorted. The elasticity of the rubber and the hardness of the metal mean that the process requires several stages of reduction and sorting.
Cables and WEEE: metals bonded to plastics and coatings
Cables and WEEE (waste electrical and electronic equipment) contain a mixture of metals, plastics and coatings. Recycling electronic waste requires these components to be separated before they can be sorted. If copper and plastic remain bonded together in the same particle, it will be more difficult to obtain clean materials.
Technical components and multilayer materials: separating them without losing any usable material
In reinforced parts and multi-layered materials, grinding too little may leave components joined together, but grinding too much can also produce fine particles or damage usable fibres. The correct size depends on how the part is made and which part you wish to recover for subsequent use.
Downsizing serves to free up components, not just to make the waste smaller
Crushing also serves to break up the structure of the waste and expose parts that were previously covered or enclosed. In many composite materials, this step prepares the material for screening, magnetic separation or density sorting.
A cable illustrates this well. If, after being cut, the copper remains attached to the plastic, it is still a composite particle. When both parts are separated, the subsequent equipment can process them separately.
Grinding the material further does not necessarily mean better separation. The particle size distribution – that is, the size of the particles – must be suited to the material and the end objective, as excessive grinding can produce dust and fine particles that complicate recovery.
The final purity depends on how thoroughly the fractions have actually been separated
Once the material has been reduced and loosened, it is time for classification. A density classifier can utilise differences in the density and behaviour of the particles to separate them, but it works best when the fractions are sufficiently loose.
If a particle binds copper and plastic together, it no longer behaves like either of them separately. This can reduce the purity of the fractions obtained.
Furthermore, the level of purity required depends on the intended use of the recovered material. A raw material that is to be reused in production may require a more thorough separation process than one with less stringent requirements.
What you need to know about the material before deciding on a treatment method
Before deciding how to treat waste, it is important to know what it contains and what you wish to recover. In the recycling of composite materials, this information helps to determine the size of the reduction and the separation stages.
There are several points worth looking at:
- Composition: which materials make up the waste and which ones are worth recovering.
- Type of bond: whether they are bonded, laminated, coated, mixed or integrated.
- Hardness and elasticity: how each component responds to cutting or grinding.
- Input size: what reduction is required before the next stages.
- Target purity: the level of purity that each fraction must achieve.
- Subsequent process: whether this will involve screening, magnetic separation, density grading or another method.
First, we need to understand what goes in and what we want to achieve; then we decide how to reduce, release and separate the material. It is this very approach that guides us at Silmisa when we analyse the material, the production process and the recovery objective before designing a machine or a recycling line.