A review of plastics waste recycling and the flotation of plastics

doi:10.1016/S0921-3449(98)00017-2

Resources, Conservation and Recycling

Volume 25, Issue 2, February 1999, Pages 85-109

https://doi.org/10.1016/S0921-3449(98)00017-2 Get rights and content

Abstract

This paper summarizes the importance of plastic waste recycling and plastic waste separation. Based on an analysis of the physical and chemical characteristics of plastics and plastic waste, the potentials and limitations of several technological processes are discussed. In addition, a review of the surface chemical aspects of plastic flotation is presented. It can be concluded that the flotation of plastics is a fairly flexible technique and could prove to be a useful process for the separation of mixtures of several different types of plastics. However, more research and development effort is required before this technology can be introduced to industry.

Introduction

More and more plastics wastes are generated by industry and householders. Statistics shows that plastics waste in municipal solid waste (MSW) in the USA reached 13.2 million metric tons in 1988 and it comprised 8% of MSW by weight (21% by volume) 1, 2. In fact, plastics waste has become one of the larger categories in MSW, particularly in industrialized countries, as shown in Table 1 [3].

The growth of plastics waste has a great impact on the management of MSW by landfilling and incineration, because available capacity for landfill of MSW is declining and plastics incineration may cause emission and toxic fly and bottom ash which contain lead and cadmium [4].

One alternative way to plastics waste disposal is recycling. Plastics waste recycling is a method of reducing the quantity of net discards of MSW. Although the benefits have not been quantified, plastics recycling also offers the potential to generate demonstrable savings in fossil fuel consumption, both because the recycled plastics can supplement and even compete with ‘virgin’ resins produced from refined fossil fuel and because the energy required to yield recycled plastics may be less than that consumed in the production of the same resins from virgin feedstocks [4]. Therefore plastics waste recycling conserves both material and energy and provides a comparatively simple way to make a substantial reduction in the overall volume of MSW.

Although plastics recycling is important, the estimates made in the late 90's, as shown in Table 2 [3], indicate that only a small percentage of postconsumer plastics waste is recycled around the world. For this reason, attention is being focused on the development of recycling technologies, for example, in the USA, Germany and Japan.

Recycling plastics encompasses four phases of activity, namely collection, separation, processing/manufacturing and marketing. Because only using clean, homogeneous resins can produce the highest-quality recycled plastic products in the existing secondary process (material recycling) and high-value chemical products in the existing tertiary process (feedstock recycling) [4], an effective separation of mixed plastics waste is necessary. Although several separation technologies, including automatic sorting, gravity separation and electrostatic separation, can be applied to separate mixed plastics waste, they have their limitations (see 2 Plastics waste, 3 The physical and chemical properties of plastics). Froth flotation is a major and efficient separation method in mineral processing engineering and could prove useful for mixed plastics separation [5].

Section snippets

Plastics waste

In order to evaluate the potential of plastics flotation in mixed plastics separation and develop more practical technologies, it is necessary to examine the characteristics of mixed plastics waste and the physical and chemical properties of plastics. These include the consumption of plastics, the origin and composition of plastics waste stream, the physical and chemical properties of plastics and plastic composition and polymer composition.

Physical properties

The physical separation of mixed plastics depends closely on the physical properties of the plastics being separated. There are two categories of physical properties, namely fixed physical properties, such as bulk density and physical properties which can be modified, such as surface energy, particle size, shape, surface roughness etc. Fig. 2 [9]lists the density of some plastics. It is shown in the list that some plastics, e.g. PVC and PP, can be separated by applying gravity separation

Plastics flotation

Flotation is undoubtedly the most important and versatile mineral-processing technique and both use and application are being expanded to treat greater tonages and to cover new areas [15]. Compared with mineral flotation originally patented in 1877 [31], plastics flotation is just in its infancy. Plastic flotation research began in the 1970s but there are few full scale applications of this technique in industry at present. However, more and more attention is now being focused on this subject

Conclusions

1.
Statistics shows that plastics waste has become one of the larger categories in MSW. Plastics recycling is important to the plastic industry, energy savings and the environment.
2.
The analysis on the characteristics of plastics waste and the physical properties of plastics demonstrates that although several separation technologies (including automatic sorting, gravity separation and electrostatic separation) can be applied to separate mixed plastics, they generally have their limitations.
3.
The

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ReviewA review of plastics waste recycling and the flotation of plastics

Abstract

Introduction

Section snippets

Plastics waste

Physical properties

Plastics flotation

Conclusions

Framework for assessment of recycle potential applied to plastics

J Environ Eng

Application of mineral-processing technology to plastic recycling

Mining Eng

Plastics Waste Management (Disposal, Recycling and Reuse)

Plastic Wastes (Management, Control, Recycling and Disposal)

Automatic sorting for mixed plastics

BioCycle

Organic Polymer Chemistry

Handbook of Plastics, Elastomers and Composites

Mineral Processing Technology

Review
A review of plastics waste recycling and the flotation of plastics