There are three types of process that can be used for recycling mixed plastic waste: mechanical, chemical, and energy recovery (Al-Salem, et al., 2009).
Mechanical recycling generally involves cutting, shredding, or milling and some form of cleaning, before forming it into a new product (Al-Salem, et al., 2009). For most purposes, mechanical recycling requires single polymer plastics, but there are processes which can deal with mixed plastic – for example, forming it into construction materials.
A manufacturer in the UK makes a composite board called EcoSheet from recycled mixed plastic. It is easier to work with and it can be used for the same things as plywood, including flooring, concrete formwork, and advertising billboards. It doesn’t rot and it can be recycled in a closed loop, back into EcoSheet boarding, even when contaminated with other waste materials (Economist, 2009).
There are a number of different board-like construction products made from recycled mixed plastics, but there is some doubt about whether or not they are better for the environment than timber. A life cycle assessment (LCA) comparing alkaline copper quaternary (ACQ) treated wood with wood plastic composite (WPC) concluded that, among other things, WPC used more fossil fuel, produced more greenhouse gas, and used more water than ACQ-treated wood (Bolin & Smith, 2011). This LCA was, however, carried out on behalf of the Treated Wood Council, so should be viewed with some caution.
An interesting use for mechanically recycled mixed plastic is as a gravel replacement in a sports field drainage system. A study carried out by Williams, et al., in 2010 found the substitution of mixed plastic waste for conventional aggregate to be environmentally beneficial in some cases.
Chemical recycling of plastic aims to convert it into a liquid hydrocarbon form, for use either as fuel or as feedstock for a refinery. A number of techniques have been developed to do this, but some of them can only be used with single polymer plastics and so aren’t suitable for mixed plastic waste. One process which can be used with mixed plastics is pyrolysis, which involves heating plastics in the absence of oxygen and can be used with the majority of plastic types.
A system that is widely used for pyrolysis is a fluidized bed reactor, because they have very good heat transfer characteristics, ensuring an even temperature. However, they are only viable on a large scale and so may not be the best approach for chemical recycling of mixed plastics in Australia, where population density is low and recycling materials would have to travel long distances (Low, et al., 2001).
One issue related to pyrolysis is that it is desirable to restrict the amount of polyethylene terephthalate (PET) and polyvinyl chloride (PVC) entering the system, as neither of them produces any liquid product (Low, et al., 1998). It is clearly a waste of energy to include those plastics in the material being pyrolized as it reduces the efficiency and, therefore, the economic viability of the process.
Where mechanical or chemical recycling of plastic is not viable – either because of economics or because the plastic has become too degraded for such a use – energy recovery may be a valuable alternative. Old landfill sites can be mined and their plastic waste recovered for use as refuse derived fuel (RDF), freeing up space for more garbage and therefore saving money on real estate.
A study carried out in Thailand used waste mined from landfill to produce RDF briquettes which could be used as fuel for a gasification system to generate electricity. The waste was cleaned by manual separation and a trommel screen, shredded, and mixed with a binding agent before being compressed into briquettes. The binding agent used was a paste made from cassava root stem, an agricultural waste product, which was cheap and effective for that purpose.
It was found that approximately 55% plastic waste was the optimum mixture, and the resulting briquettes had an energy value of 26.0 MJ/kg. The sulfur and chlorine content of this RDF was within European Union standards. When burnt in a gasification system, energy production cost was US5c/kWh (Chiemchaisri, et al., 2010).
Mixed plastics recycling has its challenges, but there are a number of different processes which can be used – some of which are relatively low cost and all of which can be used to keep waste out of landfill, create jobs, and produce a financial benefit for the community.
Bolin, C.A., Smith, S. (2011) Life cycle assessment of ACQ-treated lumber with comparison to wood plastic composite decking. Journal of Cleaner Production, Volume 19, Issues 6–7, April–May 2011, Pages 620–629.
Chiemchaisri, C., Charnnok, B., Visvanathan, C. (2010) Recovery of plastic wastes from dump site as refuse-derived fuel and its utilization in small gasification system. Bioresource Technology, Volume 101, Issue 5, March 2010, Pages 1522–1527.
Economist, The (2009) The plastic sausage machine. The Economist, London, UK. Retrieved from http://www.economist.com/node/14255246 on 17/11/12.
Low, S.L., Connor, M.A., Covey, G.H. (1998) The feasibility of using pyrolysis to convert contaminated municipal thermoplastic waste to liquid fuel. Environmental Management: Proceedings of the Second International Conference on Environmental Management (ICEM2) Australia, 10-13 February 1998.
Low, S.L., Connor, M.A., Covey, G.H. (2001) Turning mixed plastic wastes into a usable liquid fuel. 6th World Congress of Chemical Engineering Melbourne, Australia 23-27 September 2001. Retrieved from http://www.coveyconsulting.com.au/Documents/paper_gc_plastic_waste_to_liquid_fuel.pdf on 17/11/12.
Williams, T.G.J.L, Heidrich, O., Sallis, P.J. (2010) A case study of the open-loop recycling of mixed plastic waste for use in a sports-field drainage system. Resources, Conservation and Recycling, Volume 55, Issue 2, December 2010, Pages 118–128.