Application of corona discharge and electrostatic force to separate metals and nonmetals from crushed particles of waste printed circuit boards

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Abstract

Printed circuit board (PCB) scrap has a metal content of nearly 28%, including an abundance of nonferrous metals such as copper, lead, and tin. The purity of precious metals in PCBs is more than 10 times that of content-rich minerals. Therefore, the recycling of PCBs is an important subject, not only from the viewpoint of waste treatment, but also with respect to the recovery of valuable materials. A new process was investigated which involved mechanical crushing, screening, drying and electrostatic separation via corona discharge. The results show that (1) a two-step crushing process could completely strip metals from base plates; (2) the effect of aggregation opposed the production on fine powders; (3) particle sizes between 0.6 and 1.2 mm are most feasible for separation in industrial application; (4) Corona electrostatic separation is an efficient and environmental means for recovering metals from PCBs.

Introduction

The production of printed circuit boards (PCBs) is increasing worldwide. New technological innovation continues to accelerate the replacement of equipment leading to a significant increase of waste PCBs. The PCBs have nearly 28% metallic content [1] including an abundance of nonferrous metals such as Cu, Pb, and Sn [2], as well as a purity of precious metals that is more than 10 times that of rich-content mineral. Therefore, the recycling of PCB waste is an important subject, not only from the point of view of waste treatment, but also with respect to the recovery of valuable materials.

Chemical and mechanical methods have been the traditional methods of recycling PCB waste. Chemical methods consist mainly of pyrolysis, combustion, hydration, and electrolysis. Pyrolysis and combustion generate atmospheric pollution through the release of dioxins and furans. Hydration and electrolysis produce large quantities of waste acid liquid during the recycling process that must be carefully disposed.

Many researchers have used various mechanical methods to separate metals from PCBs, such as shape separation [3], jigging [4], and density-based separation [5]. However, these methods cannot meet the future requirements of industry because of low efficiency, high cost and environmental contamination.

Corona electrostatic separation has been investigated extensively in the minerals-processing industry. Both the fundamental and practical aspects of electrode system design have been investigated and developed by Iuga et al. [6], [7], [8], [9]. The extreme difference in density and electrical conductivity of metallic and nonmetallic materials provides an excellent condition for the application of corona electrostatic separation in PCB recycling. Some industrial applications of this technology were reported by Botsh et al. [10]. At the time of publication of this paper, however, the utilization of corona electrostatic separation for recovering material from waste PCBs was still in its infancy. In this study, we investigated a new process consisting of mechanical crushing, screening, drying and corona electrostatic separation.

Section snippets

Materials

This study involved 40 kg of PCBs obtained from discarded telephones, printers, computers, and other electric equipment. As shown in Fig. 1, the largest board size used was 250×200×20 mm and the smallest size was 80×35×10 mm. The toxic components and reusable parts were disassembled before crushing.

Pre-processing

The entire pre-process of crushing, screening and drying shown in Fig. 2 turned the PCB boards into particles of homogeneous dimension.

The PCB material consisted of reinforced resin and metal components

Size distribution of PCB scrap

Fig. 5, Fig. 6 are photographs of crude particles and fine particles crushed by the shearing machine and hammer grinder, respectively. Fig. 7 shows size distributions of PCB scrap obtained with the electric shaker. About 96% of the particles were of size smaller than 1.2 mm. The experiments have shown that by using a two-step crushing process, we could completely strip metals from base plates. Material of sample #5 (<0.3 mm) was found to take up nearly 50% of the total particle weight. The

Discussion

Materials were feed by an electric shaking feeder in this study. The effect of aggregation was observed during the process of feeding sample #5. It was difficult to avoid electrostatic charge in the experiment, and the particle size of sample #5 was too small, hence electrostatic forces caused some powders to aggregate. Because nonmetal comprised a large proportion in the materials, two kinds of aggregate were produced, as shown in Fig. 11. In Fig. 11(a), the nonmetal powders (a1, a2 and a3)

Conclusions

It has been shown in the present study that,

  • 1.

    By using a two-step crushing process, metals can be completely stripped from PCB base plates.

  • 2.

    The effect of aggregate opposed regular production on fine powders.

  • 3.

    Particle sizes between 0.6 and 1.2 mm are suitable for separation in industrial applications.

  • 4.

    Corona electrostatic separation is an efficient and environmentally friendly way for recovering metals from PCB scrap.

Acknowledgements

The project is supported by Program for New Century Excellent Talents in University.

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