Environ. Sci. Technol. 2008, 42, 5272–5276
Electrostatic Separation for Recovering Metals and Nonmetals from Waste Printed Circuit Board: Problems and Improvements JIANG WU, JIA LI, AND ZHENMING XU* School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai, People’s Republic of China
Received March 28, 2008. Revised manuscript received May 7, 2008. Accepted May 8, 2008.
Electrostatic separation is an effective and environmentally friendly method for recycling comminuted waste printed circuit boards (PCB). As a classical separator, the roll-type coronaelectrostatic separator (RTS) has some advantages in this field. However, there are still some notable problems, such as the middling products and their further treatment, impurity of nonconductive products because of the aggregation of fine particles, and stability of the separation process and balance between the production capacity and the separation quality. To overcome these problems, a conception of two-step separation is presented, and a new two-roll type corona-electrostatic separator (T-RTS) was built. As compared to RTS, the conductive products increase by 8.9%, the middling products decrease by 45%, and the production capacity increases by 50% in treating comminuted PCB wastes by T-RTS. In addition, the separation process in T-RTS is more stable. Therefore, T-RTS is a promising separator for recycling comminuted PCB.
Introduction With the sharp increase in waste electric and electronic equipment (WEEE), the treatment of WEEE has become a very important subject. On one hand, WEEE is a rich resource because of its high content of noble metals and rare metals. On the other hand, the hazardous materials in WEEE may cause some environmental problems if there are no appropriate methods for treatment (1). Therefore, the treatment of WEEE is significant not only for resource recycling but also for environmental protection. However, some inexpensive and primordial methods, such as acid washing and burning, often are used to recover valuable metals and nonmetals (2). These environmentally unfriendly and hazardous methods result in serious environmental pollution, such as the release of waste acid liquids, dioxins, and furans, in WEEE treatment areas (3). Electrostatic separation, defined as the selective sorting of charged or polarized bodies in an electric field (4–6), presents an effective way for recycling metals and nonmetals from WEEE without a negative impact to the environment, especially for comminuted waste printed circuit boards (PCB) (7). The roll-type corona-electrostatic separator (RTS) is a classical separator and is shown in Figure 1a. It consists of a grounded roll electrode and other active electrodes (corona* Corresponding author phone: +86 21 54747495; fax: +86 21 54747495; e-mail:
[email protected]. 5272
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electrostatic) connected to a dc high-voltage supply. The granular mixture to be separated is fed on the surface of the rotating roll with a certain speed and passes through an electric field that is generated between the roll electrodes and the active electrodes. After an intense ion bombardment, insulating particles are charged and pinned to the surface of the roll electrode by an electric image force while the conducting particles lose their charge to the grounded rotor and, thrown by centrifugal forces, are then attracted toward the electrostatic electrode (8).
Problems of the RTS The RTS has some advantages in recycling metals and nonmetals from comminuted PCB wastes. However, there are some notable problems to be solved. Middling Products of Separation Process and Further Treatment. Figure 1b shows the middling products of the separation process. There always exist considerable middling products even if the separation processes run under optimal conditions. This phenomenon is due to the faulty charging of particles and other accidental factors. Some reasons, such as the feeding process, nonuniform electric field, and distribution of space charge, lead considerable particles (especially the nonconductive ones) to not acquire enough charge and fall into the middling products. In addition, some accidental factors, such as the variation of ambient conditions, also bring about an increase of middling products. For the high content of metals (generally >50%), middling products need further treatment. Impurity of Nonconductive Products and Further Treatment. Figure 1c shows the impurity of nonconductive products. For comminuted PCB wastes, a perfect separation between metals and nonmetals can be obtained when the particle size is
