Naked Science Forum
Life Sciences => The Environment => Topic started by: peppercorn on 28/05/2012 11:06:10
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That is, if I put these plastics in the recycling bin, do they simply end up as a layer of 'slag' once all the plastics are melted down together?
I'm amazed that so many differing items (of all shapes, sizes and weights) in our waste can be reasonably well processed (mostly) automatically. But, assuming there is a notable percentage of rejected plastic fraction, does the recycler have any option but to send it for landfill?
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Whilst speration of different materials can be done automatically (such as eddy current magnets for metals and air flotation methods for plastics) , in the UK much of mixed (non putrescible) waste is sorted by hand using picking belts (and cheap labour) as it is more accurate and cheaper than machinery. The big problem with plastic is that it can only really be recycled if it is only one type - and then it becomes quite valuable - for example some music festivals are now offering a 10p bounty on disposable plastic beer "glasses" as a container/ skip of plastic glasses can go a long way towards meeting the waste management costs of the festival. (and as it happens they can be reprocessed in the UK). This results in gangs of kids swarming round collecting them and pulling them out of bins etc. and a much tidier site!
There are some specialist recyclers in the UK, that for example, shred lemonade bottles to make fleece clothing or melt down plastic to make municple furniture however, these are very much the minority.
Chances are the plastic will be baled up and shipped to China as "Mixed rigids" where it is sorted by hand, reprocessed and then returned to the west... I do not know what they do with any "unrecycleable" materials. Depending on the time of year and the distance from a port, it may cost the recycling/skip company to have this baled material taken away (however it would cost more to landfill)
The other "use" for waste plastic is to provide much of the energy in mass burn incenerators / energy from waste plants.
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I was led to believe that the vast majority of sorting can be done by mechanised means; differing weights of paper and plastics separated by a combination of air blowers and electrostatics.
The remaining fraction left as a brittle layer at the bottom of the distillation (eg. thermosets) might not be reusable as plastics (because of chemical cross-bonding that's irreversible) but there ought to be other ways of reusing these hydrocarbons.
The simplest one that springs to mind is as a chipped filler material, as in tarmac or other 'metalled' surface.
Alternatively, is there a chemical processing route by which it can be gasified to a synthesis gas for further processing?
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Locally the municipal dumpsite removes plastic by type manually from the incoming waste, then it is further processed for recycling. It is only a part of the waste, but at least some does not go to be compacted. They sort out cans, bottles, plastic and ferrous and copper in the line, whatever does not get pulled out goes to the dump bin. They get odd things there............
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Is it better to bury the plastic that can't be recycled, or burn it to produce electricity?
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Peppercorn, you are right that there are several technologies that can be used to mechanise the sorting of waste (and in particular plastic waste), these include air seperation, flotation / hydrocyclones and electrostatic methods - all of which have pros and cons. However, all of these require the input material to be clean (and often dry) so that they can be pelletised/ chipped and then separated. I think the issue is that "recycling" can be a multistage process and "contamination" with other waste is one of the first, most difficult (and potentially most costly) problems to address. A secondary issue is that the recyclate is needed in China where many plastic things originate and that exporting mixed baled plastic is cheap because the containers would otherwise be shipped back to the far east empty. This skews the economics of recycling plastic (at least in the UK) and is a disincentive to investment in costly machinery.
Organisations such as WRAP www.wrap.org.uk/ (http://www.wrap.org.uk/) promote recycling and are currently offering loans of up to £1m for mixed plastic recycling projects. They also look at innovate uses of recycled materials - such as using glass chips as a replacement for quarried aggregate in road surfacing .
The use of plastics as fuel is reasonably well established and there are suggestions that technologies such as pyrolysis may produce useful fuels.
In answer to Geezer, most plastic does not break down particularly well in landfill (it may reduce in size due to the physical/ chemical environment but the methanogenic bugs do not readily digest it. Burning it directly raises questions of emissions of dioxins etc. which can be managed through tight control of processes and are often quite tightly regulated. On balance I would argue that use of unrecycleable plastic as fuel is better than burying it.
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Burning [plastics] directly raises questions of emissions of dioxins etc. which can be managed through tight control of processes and are often quite tightly regulated.
Which is why gasification prior to use as a fuel (or chemical feedstock) ought to have the advantage of cleaner emissions as well, perhaps, giving the possibility of reforming the syn-gas stream to liquid fuels (offering a little saving in imported crude). Of course, the gasification reactor would need to be able to withstand quite high levels of impurities in the feed and have good gas clean-up technology to boot.
It's a shame for our economies (thinking of the rich, consumerist 'West' in particularly here) that it is still so cheap to import throw-away plastic goods from China, Indian and others. I've nothing against any of these nations improving their pops. standard of living but, as things are heading, this approach is unsustainable for everyone.
Edit: Interesting link to 'WRAP' btw. Thanks!
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You might be interested in this - not plastics per se, but a major waste headache...
http://www.agg-net.com/news/a90-gets-a-novel-retread-from-breedon-aggregates (http://www.agg-net.com/news/a90-gets-a-novel-retread-from-breedon-aggregates)
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Thanks for another inspiring link!
There's something almost poetic about old tyres being used to help make road surfaces isn't there? :)
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Nice thought is this. If you think like this then try to implement this.And people should be try to not to use plastic material.
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They recycle the milled old roadbed here and relay it after blending it with rubber crumb. It makes the road quieter and it is said to increase life and grip. Here there are no problems of frost and road salt, so a slightly porous coat is not an issue.