HK1007117B - Method of separating poly(ethylene terephthalate) (pet) and poly(vinyl chloride) (pvc) - Google Patents

Description

The invention relates to a method for separating plastic particles of a plastic compound from plastics of chemically different types, but which are in an overlapping density range, such as polyethylene terephthalate (PET) and polyvinyl chloride (PVC), by an electrostatic separation process using a free-fall separator.

In many countries, huge amounts of mixed plastic waste are collected. A well-known example is single-use plastic beverage bottles. These bottles are made of PVC or PET. In addition, the bottles usually have a polyethylene screw cap, and the PET bottles have a polyethylene (PE) bottom. Direct recycling of the mixed bottle plastics is not possible because PET melts only at 260°C, while PVC decomposes under HCl decomposition above the softening temperature of 160°C and PE melts at 105-135°C.

There is therefore no significant recycling potential for such mixed plastics, so that the waste plastics are usually not collected but disposed of via household waste, i.e. ultimately incinerated or landfilled.

The shortage of landfill space and the resistance of the population to the construction of new incineration plants will severely restrict this type of disposal in the future, including through governmental recycling regulations.

In addition, mixed PVC-containing plastics are not usually profitable and often require a credit from the recycler based on the landfill costs saved. On the other hand, there has long been a market for pure recycled plastics, the prices of which are based on the prices of the new products. Depending on the quality, recycled products can reach up to 60% of the value of the new products. There is therefore a great interest in methods for separating mixed plastics containing PVC. Hydrocyclones are already known to be used for the separation of plastics of different densities.

A process has also been developed which exploits the different melting points of the plastics in a compound for separation. The plastic compound is ground and heated, the first PVC particles to melt are attached to a rotating roller and removed from the compound.

The Commission has also developed a method which uses an X-ray detector to identify PVC bottles by spectroscopy and to sort them mechanically.

GB-A-1,107.574 describes a method for electrostatic sorting of a granular non-conductive material by which the mixture of grains to be sorted is dried in a flowbed, by blowing in the air stream at about 60 to 100 °C. The particles are charged differently and separated from each other by a vertical electric field in the same apparatus, by fluidization and contact, so that particles of a certain polarity are drawn out of the flowbed.

A procedure of the type mentioned at the outset is known from DE-PS 30 35 649.

The plastic mixture is reduced to a uniform particle size of 3-7 mm, then tri-electrically charged and inserted into a free fall separator between the electrodes of which an electrostatic field of 3-5 kV/cm is maintained.

In this process, part of the product is separated according to the charge at the respective electrode, while another part of the compound falls through the free fall separator as so-called intermediate material, as it is not attracted by any electrode due to a deficient electrostatic charge.

The disadvantage of the well-known method is that it involves considerable amounts of capital.

It has now been shown that an increase in the rate of separation can be achieved or that effective separation is possible for certain plastics by subjecting the plastic to a temperature treatment at 70-100 °C for at least 5 min before triboelectric charge.

This simultaneously dries the mixture if the plastic mixture has a residual water content, and it is now possible to separate plastics of equal or very similar density.

It is thought that this heat treatment will cause a change in the surface of the plastic, which will allow for a better electrostatic charge.

The subsequent triboelectric charge of the plastic compound is advantageous at a temperature of 15 - 50°C, preferably 20 - 35°C, in a relative humidity of the surrounding air of 10 - 40%, preferably 15 - 20%.

At such low field strengths, the risk of so-called spray discharges is reduced; the spray discharges can cause the plastic particles to ignite or a dust explosion.

The advantage is that the plastic mixture has a particle size of less than 10 mm, preferably less than 6 mm, whereby the plastic mixture, which consists, for example, of cut-off disposable bottles, is cleaned by slurry treatment in water before the heat treatment of materials such as paper or beverage residues. The cleaning is carried out, for example, in a washing mill or by a turbo-liquid, which may require several cleaning operations depending on the degree of contamination. The drainage of this washed and thus wet mixture is carried out by a drainage device, such as a drainage pipe, on a residual water part of about 2%. The heat treatment can be applied to the triboelectric plastic mixture in a heating bed.

In addition, the dry plastic can be sent through a spiral of sufficient length or pneumatically transported over a certain distance to improve the triboelectric charge.

If the amount of capital goods obtained by a single separation is still too high, the capital goods can of course be washed again, but usually without a further washing operation.

For further separation, the precursors may be fed to another downstream free-fall separator, the intermediate being tri-electrically recharged in a second flow-bed dryer. The residual fraction can be fed into the first flow bed dryer, whereby, if mixed with wet material, the mischarged plastic particles are discharged before recharging, which has a positive effect on the high yield in terms of subsequent separation in the free fall separator.

The following example of the process in a process diagram is intended to illustrate the process of the invention.

Example

A quantity of beverage bottles used, excluding the beverage residue, had the following composition: 19,8% of PVC 76,9% of PET Other 1.2 % of paper The bottle was fed continuously to a wet cutting mill via a dosing device and, with the addition of water, was reduced to a particle size of about 6 mm. The dirty solution, which also contained paper, was removed.

The product was then stirred in a washing machine and transferred to a friction separator, which separated the remaining impurities in the plastic, such as paper, sand and other contaminants.

The plastic carving mixture was re-absorbed with water and fed to a hydrocyclone to separate the polyolefins, the resulting PVC-PET mixture was separated from the liquid on a vibrating sieve, thrown and heat treated and dried in a flow bed dryer for 6 min at 70 to 100 °C.

The final paper residue may be removed from the flow bed by the exhaust and separated from the exhaust by means of a cyclone. The product from the flowbed was continuously fed to a separation system consisting of two free fall separators.The PVC concentrate obtained in the first separation step was transported by means of a spiral slug to the second free fall separator, again producing a selective charge of the plastic particles.

The plastic mixture thus charged was separated in the after-separation separator into a high-percentage PVC concentrate, a medium fraction and an enrichment fraction containing approximately 53% PET.

In total, the bottle mixture could be separated into a PVC fraction with a 99,3% PVC content, a PET fraction with a 99,4% PET content and a PE fraction with a 97,6% PE content.

The purity is thus well above 95%, so that we can speak of essentially pure compounds which can be easily recycled.

The yield (in absolute terms) is: 96,2% of PVC 94,6% of PET 89.7% of PE.

Claims (5)

1. A method of separating reduced-size plastics particles of a mixture of plastics of chemically different kinds which however are in an overlapping density range, for example polyethyleneterephthalate (PET) and polyvinylchloride (PVC) by triboelectric charging in a fluid bed and subsequent separation by means of a free-fall separator, characterised in that prior to the triboelectric charging operation the mixture of plastics is subjected to a heat treatment at 70 to 100°C over a period of at least 5 minutes and the subsequent triboelectric charging operation is performed at a temperature of 15 to 50 °C in a relative moisture content in the ambient air of 10 to 40%.
2. A method according to claim 1 characterised in that the triboelectric charging operation is effected at preferably 20 to 35°C.
3. A method according to claim 1 characterised in that the triboelectric charging operation is effected with a relative moisture content in the ambient air of preferably 15 to 20%.
4. A method according to claim 1 characterised in that for the purposes of better triboelectric charging the mixture of plastics is additionally passed through a spiral screw of sufficient length or is pneumatically conveyed over a given distance.
5. A method according to claim 1 characterised in that the free-fall separator operates at field strengths of 2 to 3 kV/cm.