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‘Understanding the Importance of Precision’
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DC arc plasma disposal for PCBs?
An experimental new solid waste disposal technology setup with DC arc plasma is described below. Being different from conventional combustion or burning such as incineration, it is based on a process called controlled high-temperature pyrolysis, the thermal destruction and recovery process. The final product (vitrified material) and air emission from the plasma treatment is environmentally acceptable.
DC arc is a plasma. In the plasma state electric energy is converted to heat producing a temperature as high as 1000O°C.
Generally a plasma arc torch is used to generate the plasma energy between two electrodes. A carrier gas, passes between the electrodes and transfers energy to the waste material. A plasma-based system has 4 components: a process chamber, a waste feed subsystem, glass and metal recovery subassemblies and a process gas cleaning subassembly.
Experiments and results
The DC arc furnace was designed in the Institute of Plasma Physics of the Chinese Academy of Sciences and applied to treat the printed circuit board. The furnace is a graphite DC arc system with a maximum output power of 150 kW. The waste material comes from the waste pieces of fibre-glass circuit board, with a copper sheet backing, without any additional wiring, electrical components or solder.
The waste pieces of the circuit board were shredded to a size small enough to be fed into the chamber by a screw feeder.
To change and control the glass chemistry for a high leach resistance, glass-forming additives are added into the feeder and mixed with the waste pieces of circuit board. The waste material is mainly composed of fibreglass-reinforced plastic, and is nearly one-half of the weight of organic matter. Depending on the analysis technique, the remainder of the waste is roughly one-fourth of the elemental copper for metal-sheet backing, plus roughly one-third of metal oxides that comprise the glass-fibre. The temperature of the furnace charge was measured with an infrared pyrometer. The temperature of the molten bath is about 1400OC~1800°C.
To absorb heavy metals such as lead and cadmium et al, the device of spraying active carbon is needed and it is possible to treat the circuit board with components containing those heavy metals, then the noble metals in the components will be recovered.
It is very expensive to separate H2 and CO from the gases generated in the plasma arc furnace, because the process scale is too small. At present, the gases generated in the plasma arc furnace are burned in the second combustion chamber by using a Riello diesel oil burner. The emission of gaseous waste to the air is environmentally acceptable.
Conclusions
It has been demonstrated that the operation of the designed graphite DC arc plasma furnace is efficient in pyrolysis of the circuit board. In all the cases the cooled slag is homogenous and vitreous in nature and is of a basalt-like structure resistant to leaching The Vicker's hardness of the slag varies between 6 and 6.5. The final emission of the furnace is friendly to the environment and the slag obtained after the plasma treatment (vitrified material) is environmentally acceptable; it can be returned to the environment as raw materials in the form of agglomerate that may be used for prefabricated components in the construction industry.
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