Vacuum Separating And Recovering Heavy Metals (Pb, Zn, Cd, Etc.) From Mixed Metallic Particles Of Crushed-separated Waste Printed Circuit Boards

Waste printed circuit boards (WPCBs) are the main component of waste electrical and electronic equipments. The content of metals contained in WPCBs is about more than ten or hundreds times of natural mineral. Therefore, the recycling of WPCBs is of great value. At present, the resource utilization of WPCBs mainly focuses on the recycling of copper and precious metals. However, the toxic and hazardous metals such as lead and cadmium are released into air, soil and water, which may result in heavy metal contamination and bring potential risk to human health. The mixed metallic particles obtained after crush and separation of WPCBs are the research objects in this study. An exploratory vacuum separation equipment is fabricated. Based on the different vapor pressures of metals at the same temperature, the metals can be separated from each other through vacuum evaporation and condensation. This paper studies the separation mechanism of metals with high vapor pressure and low boiling point and finds out the interaction effects of mixed metallic metals during the vacuum separation process. The mixed metallic particles of WPCBs are separated and recovered, which can avoid the release of toxic and hazardous metals (Pb, Cd et al) during the copper smelting and realize the separation and recover of lead and cadmium.Zn and Cd in the mixed metallic particles sublimate and come into gaseous phase directly. Due to the metal oxide layer covering on the surface of Zn and Cd particles, there exists resistance blocking the free evaporation of Zn and Cd atoms. The smaller the particles, the bigger resistance there is. By contrast, Pb and Bi particles both melt first and then evaporate. The evaporation rates become bigger with smaller particles as a result of bigger vapor pressure of the liquid droplet. The evaporation rates of Pb and Bi particles can be described by the Langmuir-Knudsen equation. The criterion of separating the high-vapor-pressure metals (Pb, Zn, Cd and Bi) from each other is obtained by combining both the differences of vapor pressures and evaporation efficiencies. Under the instruction of separation criterion, different kinds of mixed metallic particles are successfully separated with the separation efficiency of the target metal more than 90 wt %.The vacuum separating process of the mixed metallic particle can be divided into four steps from the dynamics point of view: evaporation/sublimation of metallic particles, diffusion I (diffusing among the particles) of the metal vapor, diffusion II (diffusing in the furnace chamber) of the metal vapor and condensation. During the diffusion I step, the layer height of copper particles can block the separation of metals. During the diffusion II step, higher vacuum degree is helpful for separating the high-vapor-pressure metals.For the multi-metal mixture, the difference of the evaporation efficiency of each high-vapor-pressure metal becomes narrower due to the blocking effect of copper particles, which makes separating metals one by one impossible. When both lead and bismuth exist in the mixed metallic particles, Pb-Bi alloy (a kind of negative-deviation alloy) can form. Due to lower vapor pressure of Pb-Bi alloy, the separation of Pb becomes more difficult. It requires heating for 135-150 min at 1123 K to realize 90 % of Pb evaporation. However, nearly 100 % of Cd can be separated from the mixed metallic particles with priority at a lower temperature (1023 K for 60 min) and then Pb and Bi can be separated subsequently at a higher temperature. When both Zn and Pb exist, hearting for 90 min at 1123 K, Zn and Pb can simultaneously evaporate and be respectively condensed according to the different condensation positions and morphologies of Zn and Pb. Based on the difference of evaporation and condensation of each metal, the high-vapor-pressure metals can be separated and recovered by combining the priority separation and simultaneous separation.In the presence of a large amount of Cu particles, Pb separation efficiency of solder-copper mixed particles becomes higher than that of single solder. It becomes much easier to separate Pb due to the multi-evaporation effect and Cu-Sn inter-metallic effect. When the vacuum degree maintains 0.1 - 1 Pa, Pb separation efficiency can attain to over 95 % at 1123 K for 90 min. After the vacuum separation of solder-copper mixed particles, the obtained aggregates contain a certain content of Sn, which plays an important part of enrichment of dispersed Sn and is in favor of further resource utilization.Based on the research, the human-computer interface of the vacuum separation flowchart is built using Labview. The corresponding parameters for separating various kinds of copper-rich particles containing high-vapor-pressure metals can be visualized, which is greatly convenient for establishing the flowchart and guiding the practical production.The noise level, TSP, PM₁₀ as well as the Pb and Cd concentrations in TSP are monitored inside and outside the vacuum separation workshop. The results show that the noise source is mainly the mechanical pump and water pump. Their noise levels are 71.2 and 69.5 dB (A) respectively, in accordance with the national standard. The working environment will not bring any hearing injury for workers. The contents of TSP and PM₁₀ accord with the second level of air standard regulated by national ambient air quality standard. The use of risk assessment strategies given by US EPA indicates that the concentrations of Pb and Cd in TSP are lower than the risk threshold value, and that they will not do harm to the workers'health.In this study, vacuum metallurgy separation is adopted to separate and recover the copper-rich particles of WPCBs after crushing and separating. During the whole vacuum separation process, the copper particles do not melt. Pb, Cd and other metals are separated from the solid copper particles, which realizes the solid separation. Compared with the traditional vacuum metallurgy separation of the metal melt, the solid separation needs lower separation temperature and consumes less energy. This research provides the theoretical foundation for separating and recovering the metals with high vapor pressure and points out an efficient, environmentally-friendly, economic feasible method for the resource recycling and utilization of WPCBs. Meanwhile, this thesis supplies the technical storage for recovering Pb, Cd and some other metals from other kinds of waste electrical and electronic equipments.