Washing of scraps coming from municipal collection

Washing of scraps coming from municipal collection

 

For mixed plastics we mean the light fraction of municipal plastics separation.

The results are normally not very exiting in terms of purity because what can be found into this mixture is what follows:

LD, LLDPE and HDPE film coming from shopping bags, packaging etc. and few bottles.

Heavy printed PP film that's the eye catching wrap of most food products.

PVC film and blister pack that's still around.

Multi-layers films used in the food industry to preserve perishable food.

PS trays both vacuum molded and foamed.

Same trays out of PET

Cellophane, leather, cleaning pads, cotton fabrics, wood, some old shoes, you name it.

So, with a "raw material" like this you shouldn't expect much and here is why:

First, to make a system delivering a good quality product at the end will cost a fortune, and nobody will make any money out of it.

Second because, even thinking to the most sophisticated unit for the separation of whatever you expect as normal contamination, there will always be something you didn't expect and quality will be spoiled anyway.

The matter has many variables and a lot depends by the starting point.

If you get the "all plastics" to be separated by yourself, you will have some valuable PET bottles out of the stream, some HDPE bottles, drums etc, that's a good material to be recycled and going deeper with some manual sorting, a stream of LDPE by its own that after washing and separation will be good for film applications.

The rest is the material we were talking about before so, making an average value, this material is the one you have been paid for. (when you got it)

If you have only this material, meaning somebody else took the good part of the mixture and left the garbage to you, you better ask for a fair amount of money before messing with it.

And now, how to handle it and what you should expect as final product.

Even with the very best, sophisticated set up that will remove all PVC, PET, PS and other sinkable plastics, with a set of washers and dryers to remove all paper, foamed materials, some wood etc, what remains is still contaminated by some multi-layers film, cleaning pads, foamed Polyurethane that will not break apart and surely something else.

All this will be a pelletizing and filtration job.

And this is only what's related to separation while, like any other film washing line, the other two difficult points are cutting and drying.

Cutting because heavy contamination will make your cutting system to be maintained pretty often and drying because films are getting thinner day by day and, as you may understand, the thinner, the more difficult to dry.

Cutting first: the only approach in this case will be the single shaft shredder that's described in the machinery section of this site, because it is, specially for high production rates, the less expensive machine in terms of "currency" per Kg.

One of the things we didn't mention yet is this kind of system should be sized for production rates not less than 1 ton/hour and, if more, much better.

Drying is the other issue; mechanical centrifugal dryer are performing well if thickness of film is over 40/45 microns, while if thickness is less, like it normally is in this case, together with the high throughput required, moisture content you should expect at the end is well over 10%, meaning no extruder will handle it.

You will say hot air drying system will fix the problem and you're surely right but how much it cost ? (we don't like it)

In our opinion, the best ratio energy/drying effect is achieved by the continuous agglomerator that's described in the machinery section of this site.

Doesn't matter how much (till 1800 Kg/hour - 4000 lb/hour) and which level moisture content, up to 20% this machine will deliver to the extruder a melted material with a residual mixture content of less than one % all extruder can easily handle.

Little space required, continuous, no pipes jamming, no possibility to go on fire, no operator required.   What you think ?

Let's go now to the final stage of the line that's filtering and pelletizing.

Which kind of extruder should be used for this purpose ?

A single extruder will do it, no question, considering the fact that single screw extruders don't mix materials much.

A twin screw extruder, much better for compounding but delivering less pressure and making filtration a little more difficult.

To state what's best it is compulsory to know which quality material is getting out of washing line and see if quality is to the level to try to get a good pellet, at least for injection molding applications.

So, difficult to say and to be evaluated time to time according with what you're starting with.

Back to extrusion, the best way to go, is a combination of the two extruder; first twin screw extruder will provide for compounding and first filtration ( backflushing filter with a screen 0,5 mm) and a second single screw, a short L/D one, for further filtration, down to 0,2 mm and pelletizing.

This, of course, if your customer will pay for the higher quality.

As far as filtration, once again, you can get more details into "polymer filtration", in the machinery section and, as we say there, the "very best" device is yet to be invented but everyday somebody finds something new about it so let's keep the window open and see what happens next.

If you need any more deeper information about one of the different steps of the process