Mechanical Scraping And Corona Electrostatic Separating Of Waste Printed Circuit Boards

New technological innovation continues to accelerate the replacement of equipment leading to a significant increase of waste electrical and electronic equipments (WEEE). At present, the discarding and eliminating peak of electrical appliances and the electronic products has arrived in China. The annual discarding quantity was more than 1 million tons and large quantities of WEEE flows to the nation from foreign countries. Because there is lack of technology and equipments for recycling WEEE in domestic, backward processing methods caused serious environmental pollution and wasted large quantity of resources. The production of printed circuit boards (PCB) is the basis of the electronic industry as it is the essential part of all most electrical and electronic equipments (EEE). PCBs contain nearly 28% metals which have abundant nonferrous metals such as Cu, Pb, Sn etc, and the purity of precious metals which is about more than 10 times of rich-content mineral. Therefore, recycling of PCB is an important subject not only from the treatment of waste but also from the recovery of valuable materials. The PCBs contain glass fiber, reinforced resin and metals. Their special physical and chemical characteristics make it difficult to recycle them. At present, the major processing method in domestic was density-based separation, which was time-consuming and emits waste to the environment. So, it is necessary to have research on recycling resources from waste PCBs without impact to the environment.On the basis of theoretical analysis, the scraping machines and corona electrostatic separator (CES) were originally created to research the harmless and resources processing for the waste PCBs in this study. At first the scraping characteristic of waste PCBs was studied. The software of MATLAB was used to simulate the distribution of electric field strength in the CES and the program was written to compute the trajectory of metal particle from scraped waste PCBs. The critical rotational speed model of CES was established from the analysis of dynamics of nonmetal particle. The method of applying the CES to separate mixture metal particles was explored. Finally, the complete equipment was manufactured. The complete equipment provided theoretical basis and industrial application experiences for recycling resources from waste PCBs without impact to the environment.The scraping machines and CES for researching the methods of processing the waste PCBs were originally created. Two-step scrapping process which was combined with shearing machine and hammer grinder was adopted. The metal parts were completed striped from base boards of waste PCBs during scraping. The parameters of shearing machine were: Rotational speed (n) was 1440 rpm and the radius of rotor was 0.25 m; the parameters of hammer grinder were: Rotational speed (n) was 2000 rpm and the radius of rotor was 0.2 m. The peak value of direct current power of CES was 30 kV. the electrodes include the filar type corona electrode and the columnar electrostatic electrode. The position of electrodes can be adjusted freely. The surface of rotating roll has chromate treatment and the rotational speed can be adjusted from 10 rpm to1000 rpm.When the particle size of scraped Type-A materials (from waste electronic equipments) was below 1.2 mm and the particle size of scraped Type-B materials (from local PCBs factory) was below 0.6 mm, the metal parts were completely striped from the base boards. The shapes of particles from scraped waste PCBs were spherical, flaky and polyhedral.The PDE tool box from software of MATLAB was used to simulate the distribution of electric field strength in the CES. It was found that the position of maximum value of electric field strength closed to the direction of electrostatic electrode. The way of increasing the applied voltage, radius of static electrode, angle of corona electrode and decreasing center distance, angle of electrostatic electrode were good for the separation. Through the correlation of simulated and experimental results, the good separation results were got under the optimized operating parameter: U= 20KV-30KV, L = L₁ = L₂ = 0.21m, R₁ =0.114, R₂ = 0.019m,θ₁ = 20°andθ₂ = 60°.The program wrote by MATLAB was used to establish the trajectory model for metal particle from scraped waste PCBs. The model was suitable for multi-compositions of operating parameters and provided theoretical support to enhance recycling efficiency and expand the applied domain of CES. From the analysis of relation between the particle size and particle stress, it was found that the small particles (r0 < 0.1 mm) were easy to be impacted from outside factors and broke the separation. So the operating parameters of CES should be adjusted to avoid colliding phenomenon.The charging process and stress of nonmetal particle were analyzed. The critical rotational speed model was established. From the model, the judgment equation of charging critical rotational speed was got. The concepts of the charging critical rotational speed (n^*), the detaching critical rotational speed (n') and the critical rotational speed (N) were proposed from the model. The method of increasing n'are: 1) increasing the number of corona electrodes, width of corona field and electric field strength to increase the charging critical rotational speed (n^*); 2) under the premise of avoiding spark discharge, increasing applied voltage and decreasing discharge gap to increase corona electric field strength and charging value of particles; 3) changing the curvature of rotating roll electrode and position of corona electrodes to decease the loss of charge value on the surface of rotating roll electrode. Large n'could simultaneously improve the purity of metal parts and recycle percentage of nonmetal parts. The critical rotational speed model has the important guiding significance to the setting of operating parameters of CES.The computer simulation was used to compute the trajectories of mixture metal particles and the applied domain of CES was expanded. The impacts of electrical, material and mechanical factors to the particle trajectory were analyzed and the optimized operating parameters for separating copper and aluminum particles were got: U = 30 kV, R₂ = 0.025 m,α= 30°, L = 0.07 m, H = 0.28 m, R₁ =0.125 m, n = 60 rpm, r₀ = 0.2 mm.On the basis of study the theory of scraping and corona electrostatic separating waste PCBs, a set of production line for recycling resources from waste PCBs was manufactured. The products of the line were metal particles, glass fiber powders. The capacity of production line was 300 kg/h and the separating efficiency was 90%. The green processing technology and advanced equipments were provided to make our treatment methods and recovery of waste PCBs reaching the international advanced level, which has great signification for China's economic, social and environmental sustainable development strategies.