PLASTICS
The recycling of waste plastics is constantly evolving and is becoming increasingly important in many countries. There are still technical, economic and structural problems to be solved, but the industry has a potentially huge future. Plastic is a very durable, widely used material – hence the difficulty of dealing with plastic waste and the fact that not all plastics can be recycled.
We take:
- Plastic waste from production (e.g. PET preform)
- Scrap plastic production waste
Comment
After 31 July 2023, plastic packaging waste cannot be freely traded in Hungary. This type of material can only be disposed of within the MOHU system. As Loacker is not yet a member of the MOHU system, please contact MOHU directly: www.mohu.hu
RECYCLABLE PLASTICS ARE MAINLY USED THROUGH MECHANICAL OR CHEMICAL PROCESSES, WHICH CONSIST OF THE FOLLOWING STEPS
- sorting
- high pressure cleaning
- stream pre-treatment (only for certain wastes)
- grinding
- granulate production
- gasification
- hydrogenation
The sorting of collected plastic waste is essential in order to obtain good quality raw material after recycling. The reason is that most polymers are incompatible, and the properties of the heterogeneous mixtures they form are largely incompatible with those of the starting components. Sorting could be greatly facilitated by the introduction of selective collection, as mentioned in the introduction. In addition, it is important to note that our most commonly used plastics are marked with recycling numbers, introduced by the Society of Plastics Industry (SPI), to facilitate their identification
METHODS OF SORTING
Although some of the automated waste sorting methods are still in their infancy and others have a higher cost, in the long run they are more economical than manual sorting and can ultimately result in better quality raw material. In the case of automated waste sorting systems, light contaminants (e.g. paper), magnetisable and non-magnetisable metals and glass must first be removed from the mixed waste. Several options are already available to separate the remaining mixed plastic waste into its components.
SEPARATION BY DENSITY
The densities of the most commonly used pure plastics are within relatively narrow limits, and the densities of the most common contaminants are generally outside this range. Consequently, density-based separation methods will be excellent for the removal of contaminants, but may also allow the separation of different plastics using flotation media of different densities. The method is well suited for the separation of polyolefins (polyethylene, polypropylene) with lower densities than other plastics from polystyrene (PS), polyethylene terephthalate (PET) or for example poly(vinyl chloride) (PVC).
Several attempts have also been made to develop separation processes in supercritical carbon dioxide and sulphur hexafluoride, as their density can be adjusted to an accuracy of about 0.01 g/cm3 by varying the pressure. Despite their accuracy, these methods are expensive and intermittent. In summary, density difference based methods are well suited for the separation of pure plastics, but not for the separation of plastics containing fillers and foamed plastics.
SPECTROSCOPIC SEPARATION
Infrared spectroscopy is often used to identify plastics. For the separation of plastic waste, this method is only suitable for the separation of larger waste pieces. The cleaned waste pieces, fed individually or in bulk, pass in front of the sensor, which identifies them on the basis of the spectrum obtained. Depending on the result obtained, the piece is transferred to the appropriate container by the special ejection system.
The method allows fast, efficient sorting at a high cost. However, its main drawback is that it is not suitable for separating black-coloured plastics (containing soot), which absorb infrared waves.
RECYCLING AND RECOVERY OF PLASTICS
Plastics are made from non-renewable raw materials, which makes proper recycling even more important. Plastics are collected, sorted and usually baled or prepared for transport to a recycling plant. At the recycler, the plastics are shredded into so-called flakes and then washed and dried. The flakes are then turned into regranulate. The quality of this granulate is significantly influenced by grading, as different plastics melt at different temperatures and cannot be mixed in this state. Therefore, professional recycling and proper sorting are crucial for a good quality granulate.
USE OF SELECTIVE SOLVENT
As the temperature changes, the solubility of different polymers in a given solvent changes, and for some polymers specific solvents can be found. From fractions obtained at different temperatures, or from a polymer mixture dissolved in the specific solvent, the solute can be recovered by precipitation with high purity. The method is most commonly used for the recycling of cables, fibre reinforced composites and composites containing fillers.
Unfortunately, the selective dissolution process often involves the use of hazardous solvents, which require careful handling. The economics of the method are determined by the polymer/solvent ratio achievable in its application; with a high polymer/solvent ratio, and continuous solvent recovery, the method can be operated economically.
OTHER SEPARATION OPTIONS
In addition to the separation technologies mentioned above, there are many other technologies for plastics recycling. For example, electrostatic separation, which exploits the charge transfer that occurs when two polymers are rubbed together. The charged plastic particles are scattered into a horizontal electric field, moving towards the electrode with the opposite charge, and finally fall into a structured collection tray at the bottom of the chamber. The resulting plastic fractions can have a purity of more than 95% in the side compartments. It should be noted that the polymers containing an antistatic agent cannot be separated by this method.
Cryogenic grinding can be used to produce different particle sizes of plastic waste from multicomponent plastic waste, and then to separate the different particle sizes by sieving (e.g. PET, PVC). In addition, centrifugal-based separation methods in the fluid state and the use of fluorescent tracers are currently being explored.
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