It is not only the type of plastic that plays a large role in plastics recycling, but also the colour. Plastics in electronics scrap involves a particularly high proportion of black plastics, which can amount to between 60 and 75 % (in toner cartridges, it sometimes amounts to more than 90 %). The reason for this is that not only the fact that black is “modern”, but there is also a role played by cost aspects. While for example colouring a plastic red makes it up to 30% more expensive, the extra costs for a black coloured plastic are minimal.
In sorting post-consumer packaging waste from the “yellow bags or bins” used in Germany to collect such waste, opto-electronic sorting equipment has become widely adopted, which is able through use of near infrared recognition (NIR) to automatically recognise e.g. plastic bottles in PET, PE, PP and other materials, and to clearly identify the plastics by their individual categories. It is then possible with the NIR technique by using downstream air-blast units to sort not only complete bottles, but also flakes and other granulated recyclate materials.
It has of course been attempted to also use NIR sorting in the area of plastics from electronics scrap. There was here however for a long time the large disadvantage that only light colour plastics could be recognised, while dark plastics could not be recognised. When considering that only around 20 to 40 % of plastics in electronic scrap has light colour, on the other hand that good material totals only around 55% of the entire amount used, it was established that this NIR technique was not economic for sorting plastics from electrical & electronics scrap according to the EU’s WEEE waste electrical & electronic equipment directive. A large proportion of recyclable ABS and PS was not recognised, so ended up as non-recyclable waste. Despite further development of NIR sorting, experts still view its economic use with scepticism.
If it is wanted to exploit the valuable “plastics” resource to its fullest potential, it is therefore absolutely essential to also recover the proportion of black plastics in as high as possible purity according to the type of plastic.
Electrostatic separation technology can be used with great success for separation of such mixed plastics from electronics scrap.Completion of the separation process results in receipt of ABS or PS fractions with high purity of in some instances more than 99%. Electrostatic separation takes place irrespective of the colour. Completely black coloured material mixtures can therefore also be easily separated.
But electrostatic separation has a catch:
Complex mixtures, as arise in electronics scrap cannot be simply processed as such. There has to be a form of pre-concentration applied prior to the electrostatic process, so that undesired plastics such as those containing for example flame retardants, can be separated.
A combination of dry and wet processing technologies is recommended for the required plastics pre concentration from these complex material mixtures.
Dry and wet separation processes
It is possible with dry separation processes to separate all undesired foreign materials such as for example film, dust, fibres, etc from course pre-shredded WEEE plastics materials, e.g. those in the particle size area of < 50 mm. A combination of air classifiers, settling tables, or other processes are used for this purpose.
It is furthermore recommended to sieve the fine fraction as much as possible and to remove metal parts that may possibly still be present. The material is then ideally prepared for the following process stage, wet separation.
A number of float-or-sink basins containing fluids of different densities, following one after the other, are used in the wet separation process. With a density of approx. 1.08 kg/dm³ used in the first stage, all product groups (PS, ABS, PP etc) float, due to their lower density, while undesired foreign materials, including flame retarded plastics sink, and can be disposed of.
A subsequent further separation stage with water (density 1.0 kg/dm³) not only rinses away the residual separation fluid from the first separation stage, but also enables separation of PP and PE as the floating fraction and PS and ABS as the sinking fraction. It is in this way that sought-after concentrated PS and ABS and concentrated PP and PE fractions can be produced from WEEE plastics.
Undesired foreign materials
But these PS and ABS mixtures are still significantly contaminated by high amounts of foreign materials. Above all here, wood, which originates for example from shredded loudspeakers, radio housings, palettes etc. Corona-roll type electrostatic separators have proven themselves in separation of wood fractions. Different electrical conductivity between moist wood and dry plastic is used here for separation. The plastic is practically free of wood after this separation stage. Conductive rubber is also simultaneously separated in this separation stage.
Separation of PP
The "sought-after plastics ABS and PS" lie in a density range of approx. 1.05 kg/dm³. But it has been shown in recycling plastics from electronic scrap however that mineral filled polypropylene (so called PP20) also lies around this density area, as a material that has been filled for cost reasons with 20% talc. Presence of PP together with PS or ABS is however undesired, as the quality of granulate or compounds suffers considerably from the PP content.