Particle size distribution can have a significant impact on the outcome of a recycling process, even though at first glance it may seem as straightforward as breaking down waste into smaller pieces. The size of the material at the end of the process affects how it moves, how it is separated and what can be done with it afterwards. Therefore, before setting an output size, it is important to consider the entire process and the intended use of that material.
At Silmisa, we manufacture machinery for industrial recycling, including grinders and granulators that enable the size of the processed material to be controlled. We explain how particle size distribution affects recycling and what factors need to be taken into account to achieve a suitable end product.
Particle size: what are we actually measuring?
When we measure particle size, we look at how the material is distributed across different sizes after crushing or grinding. If we are aiming for a 10 mm output, for example, this does not mean that all the particles will be exactly the same size. Some particles will be slightly larger, others will fall within the main range, and others will be finer. We refer to each of these groups as a fraction.
This distribution system helps to determine whether the material is being fed at the rate required by the process. It also helps to detect changes in the cut, blade wear or variations in the feed rate.
In a plant, it is important to know what proportion falls within the expected range and how much material falls outside it into larger or smaller fractions.
The same type of waste may require different output sizes
The same material may require a different output depending on what happens next. A plastic entering a washing stage may need to be processed differently from one that goes on to density separation or extrusion. Something similar applies to rubber: the end use determines how much it needs to be reduced in size.
For this reason, the granulator is not selected solely on the basis of the type of waste. It is also necessary to consider the equipment that follows it, its capacity, the desired separation level and the product we wish to recover. Reducing the material further as a matter of course may increase energy consumption and wear and tear without improving the result.
What problems arise if the fraction is too large?
If the particle size is too coarse, the material may fall outside the range required for the next stage. The problem varies depending on the waste, but it usually affects separation, the consistency of the feed and the utilisation of the various fractions.
The most common faults are as follows:
- Less separation: various materials may remain bonded together in a single piece, making it more difficult to separate them properly in the next stage.
- Worse classification: some teams perform better within a specific size range and may become less effective when given fragments that are too large.
Irregular feeding: long or bulky items can make it difficult for them to pass through hoppers, conveyors, screens and industrial dosing systems.
More recirculation: material that does not meet the required dimensions can be returned to the machine for another cut before proceeding.
Monitoring this stage prevents unnecessary work and ensures that each stage receives the most suitable material for carrying out its function.
And what happens when it’s ground too finely?
Reducing the size any further does not always improve the process either. A particle size that is too small can increase dust levels, cause some of the material to end up in the extraction system or as reject, and complicate certain separation processes.
Furthermore, producing smaller particles may require further reduction of the material, which increases the cutting workload. This can result in higher consumption, greater wear and tear, and lower hourly output. In metals, for example, certain very small fractions are more difficult to recover using some separation systems.
When working with paper, it may also be worth avoiding excessive fibre breakage. The aim is to achieve the required size without further reducing a material that is already prepared for the next stage.
Particle size varies depending on the material and the intended use
Each type of waste may react differently to cutting and may require different preparations.
Hardness, elasticity, shape, moisture content or the mix of components all affect how the material behaves inside the machine. Added to this is what we want to do next: wash, separate, store, transport, extrude, recover or recycle a specific fraction.
For this reason, determining the particle size requires an examination of the entire process. An appropriate output ensures that downstream equipment operates more consistently and prevents the material from being reduced more than necessary. The criteria vary considerably between plastics, wood and metals, as each presents different challenges.
Plastics and rubber: consistency for subsequent stages
In the case of plastic and rubber, a consistent output is important when the material is subsequently sent for washing, separation or extrusion. A stable fraction facilitates feeding into this equipment and helps to keep the process within more consistent parameters, without mixing very large pieces with an excess of unnecessary fines.
Wood, paper and cardboard: size versus handling and recovery
When dealing with wood, paper and cardboard, a balance must be struck between shredding and handling. Pieces that are too large make transport or feeding difficult, whilst excessive shredding can generate dust or damage usable fibre. The appropriate size depends on whether the material is to be recycled, sorted, transported or reused at a later stage.
Non-ferrous metals: separating materials for sorting
In the case of non-ferrous metals, the aim is to separate components that are bonded together: aluminium, copper and plastic. Reducing the material helps to separate them later on based on their different properties, but breaking them down into fractions that are too small can complicate their recovery. The aim is to cut the material until sufficient separation is achieved without generating an unnecessary amount of fines.
How is the output size controlled in mills and granulators?
In this machinery, one of the main control elements is the screen. The material remains inside the chamber until it reaches a size that allows it to pass through the holes. Therefore, changing the aperture of the screen directly alters the output we obtain.
Other factors that have an influence include the type of blade, its condition, the gap between the blades and counter-blades, the rotor speed and the amount of waste fed in at any one time. If any of these factors change, the final distribution may also change, resulting in an increase in the amount of coarse or fine material.
At Silmisa, we design and manufacture granulating mills tailored to the material, the required output and the specifications of each process. Before finalising the equipment, we assess the input, the required output and the next stage in the process. This enables us to tailor the grinding process to the plant’s actual requirements and avoid processing the residue more than is necessary.