Plastics Separation

If recycled plastic is greater than 99.5 per cent pure it can displace the use of virgin material in many applications. However, systems such as mechanical/gravity sorting, cryogrinding and hand sorting, currently only produce a mixed product suitable for low value applications.

A new innovative technology uses naturally occurring charge transfer processes to sort plastic wastes to up to a 99.5 per cent purity in one pass through the dry process. This sorted end product can then be reused and may even replace virgin material. Developed by Plas-Sep Limited, based in London, Ontario (and under exclusive license from The University of Western Ontario), this dry electrostatic process costs little and works for both low value commodity plastics and higher value industrial plastics. It also requires no toxic (or polluting) chemicals.

The technology

When two dissimilar non-conducting particles come into contact, charge is transferred; one of the particles becomes negatively charged and the other positively charged. The charge polarity is determined by the so-called triboelectric series. (See Table 1.) Any polymer higher in the table in contact with one lower in the table will charge negatively. For example, polyethylene (PE) will charge negatively in contact with polyethylene-terephthalate (PET), but will charge positively in contact with polyvinyl chloride (PVC).

In the Plas-Sep process, chopped dry particles (5-10 mm size) of mixed plastics are fed continuously into the upper end of a slightly tilted, slowly rotating drum (at the rear — see photo). As the particles tumble over each other they become charged, due to the many and repeated contacts. The quantity and polarity of the charge on each particle depends on the contacts with other particles. Because of the tilt of the drum, the particles migrate to the exit end of the drum where they fall through a strong horizontal electric field. The negatively charged particles are drawn toward the positive electrode while the positively charged particles are drawn toward the negative electrode.

For example, in a mix of 50 per cent PE and 50 per cent polypropylene (PP), the negatively charged PE product falls onto a conveyor near the positive electrode and is drawn off while the PP product is similarly drawn off on the negative side. (See photo.) The yield for the pure product exceeds 80 per cent in a single pass. The material that falls in the middle of the tower (a mixture of PE and PP) can be recycled through the process to increase the yield.

For mixtures of more than two plastics, more than one pass through the process is required. In the first pass for any mixture, one plastic will be the dominant positive charging species while another will be the dominant negative charging species. These will be drawn toward the electrodes and fall in the side bins. The others will be relatively uncharged and tend to fall in the centre bin. If the material in the centre bin is re-run through the process, the charging of the particles will be quite different since the mixture contains a very different set of plastics. Thus, in the new mixture a different pair may be the dominant positive and negative charging species. As a consequence, very complex mixtures can be separated into pure components by a sequence of passes.

Economics of separation

The electrostatic separation process itself is very low cost — less than $0.04/kg for operating costs, including equipment lease, labour and energy. The major costs of the process are related to materials preparation. Material must be clean, dry and chopped to 5-10 mm size (which may already be very similar to recycling requirements).

As a first example, recycled PE and PP currently have values of $0.54 and $0.25/kg. For a unit that operates two shifts per day, 250 days per year with 80 per cent yield, the value of the purified PE and PP ranges between $0.8 to $1.7-million/year, depending on the mixture composition. (The more valuable mixes are higher in PE content.) This revenue should permit profitable operation of a separation plant, quick recovery of the capital costs and a reasonable return on investment.

Automotive tail light assemblies are another example of more valuable polymers. These consist of approximately 50 per cent polymethyl methacrylate (PMMA) and 50 per cent acrylonitrile butadiene styrene (ABS). The cost of virgin PMMA is $9.50/kg while ABS is valued at $3.50/kg. These materials can be separated electrostatically, but a second purification step is required for the PMMA that must exceed 99.9 per cent purity for recycling. Because of the second step, the throughput for the total process is reduced and 250kg of PMMA and 250kg of ABS are produced per hour. Because of the smaller quantity of such material available for separation, assume a one-shift operation for 200 days per year. Using the values for the virgin materials, over $4-million of products are produced per year. If the recycled materials can command 50 per cent of the value of virgin materials, then again there is a very good margin for return on investment.

Written by James Brown, P. Eng., president of Plas-Sep Ltd., based in London, Ontario.

Table 1. Triboelectric Plastics Series
PTFE, Teflon negative charging
PVC, polyvinyl chloride V
PE, polyethylene V
PP, polypropylene V
PS, polystyrene V
PET, polyethylene terephthalate V
Lexan V
Acrylic positive charging

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