Enrichment implication of froth flotation kinetics in the separation and recovery of metal values from printed circuit boards

Latest trends and challenges in PVC and copper recovery technologies for End-of-Life thin cables

This review investigates the latest trends in separation technologies regarding hard-to-recycle thin cables, specifically in the form of end-of-life wire harnesses (WHs). The cables in WHs mainly contain copper (Cu) and poly(vinyl chloride) (PVC), which is commonly used to insulate and sheath cables. This review reveals that most separation technologies prioritize the recovery of Cu and overlook that of PVC. The recovery of high-purity PVC is very important because of its incompatibility with other plastics or Cu during recycling treatments. Through this investigation, we confirm that physical treatments, such as stripping and chopping, are insufficient to recover high-purity PVC from thin cables. Instead, a combination of chemical (e.g., swelling of PVC insulation or sheathing of cables under a suitable solvent) and physical (e.g., ball or rod milling and mechanical agitation of swollen cables) treatments can be used to achieve the recovery of high-purity PVC and Cu both for recycling. We believe that recovering metals and plastics from end-of-life cables is vital for sustainable waste management, offering several environmental and economic benefits.

Review on the gentle hydrometallurgical treatment of WPCBs: Sustainable and selective gradient process for multiple valuable metals recovery

The metal resource crisis and the inherent need for a low-carbon circular economy have driven the rapid development of e-waste recycling technology. High-value waste printed circuit boards (WPCBs) are an essential component of e-waste. However, WPCBs are considered hazardous to the ecosystem due to the presence of heavy metals and brominated organic polymers. Therefore, achieving the recycling of metals in WPCBs is not only a strategic requirement for building a green ecological civilization but also an essential guarantee for achieving a safe supply of mineral resources. This review systematically analyzes the hydrometallurgical technology of metals in WPCBs in recent years. Firstly, the different unit operations of pretreatment in the hydrometallurgical process, which contain disassembly, crushing, and pre-enrichment, were analyzed. Secondly, environmentally friendly hydrometallurgical leaching systems and high-value product regeneration technologies used in recent years to recover metals from WPCBs were evaluated. The leaching techniques, including cyanidation, halide, thiourea, and thiosulfate for precious metals, and inorganic acid, organic acid, and other leaching methods for base metals such as copper and nickel in WPCBs, were outlined, and the leaching performance and greenness of each leaching system were summarized and analyzed. Eventually, based on the advantages of each leaching system and the differences in chemical properties of metals in WPCBs, an integrated and multi-gradient green process for the recovery of WPCBs was proposed, which provides a sustainable pathway for the recovery of metals in WPCBs. This paper provides a reference for realizing the gradient hydrometallurgical recovery of metals from WPCBs to promote the recycling metal resources.

Eco-friendly approach for enhancing the floatability of non-metallic components in waste printed circuit boards: Adding gutter oil during dry grinding

Waste printed circuit boards (WPCBs) are an attractive secondary resource that is challenging to dispose of due to its complexity. Reverse flotation is an effective method to remove non-metallic particles (NMPs) to obtain metals from WPCBs. Nevertheless, the removal of NMPs is usually inadequate in the present flotation practice. Thus, to provide a clean approach to improve the removal efficiency of NMPs, the method of adding gutter oil during dry grinding process was adopted to enhance the hydrophobic sites on the surface of NMPs to improve the floatability. The surface morphology of NMPs was analyzed by SEM, the results show that the rough morphology inhibited the adhesion of bubbles, while water occupied the cracks and pores, making it challenging for collector adsorption, which result in unstable particle-bubble adhesion. The results of FTIR indicate that both NMPs and gutter oil have -CH3, -CH2, -C = O, -C-O functional groups, which promotes the adsorption of gutter oil on the surface of NMPs. The contact angle (CA) results show that the adsorption of gutter oil on the particle surface is conducive to the formation of enhanced CA. Furthermore, the flotation enhancement effect was verified by flotation kinetic experiments. The accumulated floats yield of NMPs conditioned by gutter oil during grinding is increased from 67.05% (NMPs without conditioning) to 95.02%, and the resin recovery is increased by 31.10%. It is demonstrated that dry grinding with gutter oil can strengthen the floatability of NMPs, which provides a potential approach to increase the flotation efficiency of WPCBs.

Enhanced flotation efficiency of metal from waste printed circuit boards modified by alkaline immersion

Efficient recycling of waste printed circuit boards by flotation has become a research focus. In this study, waste printed circuit boards were treated by alkaline immersion to enhance the flotation efficiency. Firstly, the SEM-EDS analysis of the crushed products shown that metal and nonmetal were completely liberated in the -0.25 mm fraction. When the printed circuit boards were modified by alkaline immersion, the recovery of metal increased from 64.34% to 72.35%. Further, the mixture of metal and nonmetal at the edge of nonmetal was discovered by EPMA. This was the cause of metal loss during the flotation process. Secondly, by adjusting the alkaline immersion time and pH value, a good flotation effect was achieved at 40 min alkaline immersion time and the pH = 11. Meanwhile, the XPS analysis of nonmetal found that the intensity of the OH peak was significantly enhanced, while the intensity of the O peak was evidently decreased. The change of the resin molecular structure indicated that the O linked to the benzene ring was broken under the action of alkaline immersion, resulting a free bond was generated on the benzene ring. This made the free OH adsorb to the free bond. This conduct promoted the dispersion of nonmetal in the slurry due to the increased nonmetal surface energy and metal hydrophilicity. Thus, this study provides a new route to improve the flotation efficiency of waste printed circuit boards.

Advanced utilization of copper in waste printed circuit boards: Synthesis of nano-copper assisted by physical enrichment

The copper in the waste printed circuit boards (WPCBs) is cleanly recycled by physical methods and presented in the form of nano copper particles by hydrometallurgical, which provides environmental approach to the advanced utilization of metal copper. Copper in WPCBs was first pre-concentrated by gradient enrichment process including gravity separation, mechanical grinding and flotation. The leaching method was then used to dissolve copper from the flotation concentrate in ammoniacal/ammonium salt solutions. Subsequently, reduction treatment was conducted to synthesize nano-copper from leaching solution. The enrichment results of the clean physical separation process show that the grade of copper increased from 16.22% to -38.05% by gravity separation, and the grade of copper further increased to 72.62 % by flotation after dissociation, which avoids overgrinding of low value components. Copper nanoparticles can be prepared effectively, and the recovery of copper in the leaching process reaches 99 %. The particle size of copper nanoparticles obtained by ascorbic acid reduction is tens of nanometers, and the surface of copper nanoparticles is smooth and nearly spherical. The present study proposes an environmentally friendly process of preparing nano-copper from the copper in WPCBs.

Mechanical activation to enhance the natural floatability of waste printed circuit boards

The metal in the waste printed circuit boards (WPCBs) is an excellent secondary metal resource. WPCBs were ground to dissociate, and impurities in the dissociated product were removed by gradient flotation to recover valuable metals in this study. The effects of crushing methods on size composition and dissociation state of the crushed products were studied. Then the gradient flotation experiment was designed to verify the natural floatability of ground materials. Grinding test shows that impact crushing has greater grinding fineness (-0.074 mm) than shear crushing, which is 42.14% and 26.18% respectively with 5 min grinding. The flotation test results illustrate that the natural floatability of impurities increases with the grinding fineness, that is, the yield of floats increases without flotation reagents. For impact crushing and shear crushing, the floats yields are 38.48% and 31.75% respectively, accompanied by 70.53% and 65.46% impurity removal for ground materials with 5 min grinding. Subsequently, 21.61% and 26.35% of impurities can be further removed with the aid of collector. Finally, the recovery of Cu in concentrate reaches 67.84% and 65.75%, respectively. FT-IR proves that the excellent floatability of particles is caused by the significant hydrophobic group. Mechanical grinding has been proved to have double effects of improving dissociation and natural floatability.

Optimization Studies on Recovery of Metals from Printed Circuit Board Waste

The aim of the study was to recover copper and lead metal from waste printed circuit boards (PCBs). The electrowinning method is found to be an effective recycling process to recover copper and lead metal from printed circuit board wastes. In order to simplify the process with affordable equipment, a simple ammonical leaching operation method was adopted. The selected PCBs were incinerated into fine ash powder at 500°C for 1 hour in the pyrolysis reactor. Then, the fine ash powder was subjected to acid-leaching process to recover the metals with varying conditions like acid-base concentration, electrode combination, and leaching time. The relative electrolysis solution of 0.1 M lead nitrate for lead and 0.1 M copper sulphate for copper was used to extract metals from PCBs at room temperature. The amount of lead and copper extracted from the process was determined by an atomic absorption spectrophotometer, and results found were 73.29% and 82.17%, respectively. Further, the optimum conditions for the recovery of metals were determined by using RSM software. The results showed that the percentage of lead and copper recovery were 78.25% and 89.1% should be 4 hrs 10 A/dm².

The influence of waste printed circuit boards characteristics and nonmetal surface energy regulation on flotation

This paper studied the influence of waste printed circuit boards (PCBs) characteristics and nonmetal surface energy (SE) regulation on flotation. First, SEM-EDS was applied to study the appearance and surface element distribution of the glass fiber and copper. The results showed that the glass fiber was present in a bundle and the surface carbon content was 49.42%, which facilitated glass fiber floating. The copper appearance contained many nodules, with a carbon content of 32.54%, which hindered copper sorting. XPS analysis further discovered that copper was mainly present in the forms of CuO, Cu(Met), and Al₂Cu. A FT-IR analysis revealed that the organic matter in the PCBs was essentially the same as the epoxy resin. It was easy to achieve floating and some polar functional groups promoted the adsorption of the flotation reagents. Based on this, by calculating the nonmetal SE, it was found that the proportion of the non-polar component of the nonmetal SE was 94.60%. The EDLVO theory was applied to research interactions between nonmetal particles. Hydrophobic attraction was found to be the main factor causing nonmetal particles to agglomerate. Further, the tannin was used to improve the dispersion of nonmetal by adjusting the nonmetal SE. Furthermore, the hydroxyl and carbonyl groups in the tannin may form hydrogen bonds with the bromine, epoxy and hydroxyl groups in the nonmetal. Finally, flotation test results indicated that tannin added significantly enhanced PCBs flotation efficiency. When the amount of tannin added increased from 0 to 60 mg/L, the recovery of copper increased from 61.92% to 90.53%. Thus, this study provides an alternative approach to improve the flotation efficiency of waste PCBs.

Study of the physicochemical effects on the separation of the non-metallic fraction from printed circuit boards by inverse flotation

Recycling printed circuit boards using green technology is increasingly important due to the metals these contain and the environmental care that must be taken when separating the different materials. Inverse flotation is a process that can be considered a Green Technology, which separates metallic from non-metallic fractions. The degree of separation depends on how much material is adhered to air bubbles. The contact angle measurement allows to determine, in an easy way, whether the flotation process will occur or not and thus establish a material as hydrophobic or not. With the material directly obtained from the milling process, it was found that the contact angle of the non-metallic fraction-liquid-air system increases as temperature increases. In the same way, the increments in concentration of frother in the liquid increase the contact angle of the non-metallic fraction-liquid-air system. 10ppm of Methyl Isobutyl Carbinol provides the highest contact angle as well as the highest material charging in the bubble.