Recovery and recycling of metals from waste PCBs using pyrolysis followed by hydrometallurgical processes and characterization

Abstract

Printed circuit boards (PCBs), essential parts of all the electrical and electronic equipment (EEE), are generally composed of ~28% metallic, ~23% plastic and 49% ceramic materials. They are employed to instinctively hold the electronic components using conductive pathways of Cu circuit on its surface. The substantial increase in discarded EEEs containing PCBs has taken place due to accelerating financial growth and high technology innovation. These obsolete PCBs incorporate a noteworthy quantity of metals such as Cu, Fe, Ni, Pb, etc., some of which are lethal to life and all of which are valuable resources. These complex assemblies also embody copious materials like polymers, glass fiber, solder which eliminate harmful materials when disposed to the surroundings. But these accessible metals are being shattered due to improper recycling, causing the evolution of toxic gases and leached hazardous compounds. Thus, there is a need of appropriate execution and efficient methodologies for the recovery of metals present in PCBs, which is both profitable and environmentally worthwhile. Hence, objective of this work aims to design a commercial process to extract metals such as Cu, Ni, Pb etc, highly valued metallic element, from industrial point of view, from the PCBs of computers which appear to be particular among E-wastes available, as far as further utilization is concerned. Recovery of metals from waste PCBs using pyrolysis-physical beneficiation-leaching followed by solvent extraction-electrowinning-cementation process is focused here. Initially, waste PCBs were depopulated by IR (Infrared) heating to remove all the small electronic components followed by their treatment by pyrolysis to recover the valuable. Investigations were carried out to study the influence of the various atmospheres such as argon, air, oxygen and vacuum on the thermal decomposition of waste printed circuit boards (WPCBs). TGA studies showed that the decomposition of the WPCBs started at about 250C and was rapid in the temperature interval of 250-350C. Experiments carried out in the laboratory showed that about 20.43% mass loss was observed during the pyrolysis of WPCBs in a flowing stream of argon at 275C during a period of 4 minutes of holding at the temperature. On the other hand, a maximum of about 2.26% mass loss was recorded when the WPCBs were heated at about 327C for the same time interval in air. Pyrolysis under vacuum atmosphere was also carried out at 300°C for 4 minutes in which 15% mass loss of PCBs were obtained. Mass transfer during the pyrolysis of WPCBs in flowing stream of inert gas was modeled. The predictions of the model were in good agreement with experimental observations. It was found that controlling the flow rate of inert gas and the geometry of the equipment could enhance the rate of pyrolysis significantly. This can lead to significant savings in energy and improvement in the efficiency of pyrolysis. The fumes generated during the pyrolysis process got condensed to generate marketable LD-oil (low density oil). About 10% of the mass of the WPCB could be recovered as oil during experiments which were conducted in argon and vacuum atmosphere. The oil was characterized using FT-IR and found to contain radicals of O-H, C-C, C-H and C-O. The pyrolysed PCBs sample containing metallic components and carbon fractions were used for further set of experiments. The poly-cracked (pyrolysed) PCBs were subjected to comminution to separate the metallic component from non-metallic fractions using double roll crusher/scutter crusher/ball mill. Further, the ground poly-cracked PCBs was scrubbed/washed with water without using any additives to enrich the metallic concentrate up to 98%. Thereafter, this enriched poly-cracked (pyrolysed) PCBs were further screen analyzed to separate high and low metal concentrate (small fraction). These fractions of pyrolysed PCBs were subjected to hydrometallurgical treatment for recovery of metals. The small fraction (-10 mesh size i.e., <2 mm size) of poly-cracked PCBs was leached in 2.4M H2SO4 at 150°C maintaining 20 bar pressure and pulp density 100 g/L. Systematic scientific studies were also carried out to recover base metals (Cu, Fe and Ni) from leach liquor of small fraction of poly-cracked (pyrolysed) PCBs. To recover acid by solvent-extraction process from the leach liquor, the experiments were carried out using 70% TEHA diluted in kerosene, which resulted in the extraction of 99.99% H2SO4 in three stages at O/A ratio 2/1 in 5 min. The recovered acid could be recycled for leaching of materials in the continuous close loop of operation. Subsequently, air sparging was performed to separate and recover Fe as a precipitate. For this, NaOH was added in the leach liquor to increase its pH up to 3.5 to 4.0 and heated at 80°C. Now Fe was removed from the solution by the process of air sparging which results in 80% removal of iron by precipitating it as ferric hydroxide (99.99% pure). After the recovery of acid and iron from the leach liquor, the solution was further processed to recover Cu using 10% LIX 84IC diluted in kerosene. 99.0% copper was found to be extracted in two stages at pH 2.5, O/A ratio 1/1 in 5 min, leaving Ni in the raffinate. Cu was stripped from the loaded LIX 84IC using 10% (v/v) sulfuric acid. In each set of experiment organic to strip solution ratio was maintained at 1/1. 99.99% Cu was stripped from loaded organic within 5 contacts at a phase ratio (O/A) of 1/1. Cu was recovered completely in form of copper sulfate salt (99.99% pure) by evaporation of the strip solution. Further, nickel was separated completely from the solution (free from acid, Fe & Cu) in two stages using 1% LIX 84IC at pH 4.58, O/A ratio 2/1 in 15 min. Ni was stripped from the loaded LIX 84IC using 10% (v/v) sulfuric acid. 99.99% Ni was stripped from loaded organic in a single contact at a phase ratio (O/A) of 1/1 maintaining equilibrium time 5 min. Ni was recovered completely in form of nickel sulfate salt (99.99% pure) by evaporation of the strip solution. The high metallic concentrate (+10 mesh size i.e., >2 mm size) of poly-cracked (pyrolysed) PCBs was leached in 4M HNO3 at 90°C for 60 min using 30 g/L pulp density and maintaining stirring speed 250 rpm showing complete dissolution for most of the metals present viz. Cu, Fe, Pb and 57.5% Ni. Systematic scientific studies were also carried out to recover acid and metal from leach liquor of the poly-cracked PCBs. To recover nitric acid from the leach liquor, the experiments were carried out using 60% TBP diluted in kerosene, which resulted in the extraction of 99.99% HNO3 in three stages at O/A ratio 3/1 in 12 min. Subsequently, air sparging was performed to separate and recover Fe as a precipitate. The presence of iron may reduce the efficiency of copper metal deposition during electrowinning. For proper precipitation of iron, NaOH was added in the leach liquor (free from acid) to increase its pH in the range of 3.5 to 4.0 and heated at 80°C. Further, Fe was removed from the acid free leach liquor, Fe get precipitated as ferric hydroxide by air sparging. 80% Fe was recovered in form of ferric hydroxide with 99.99% pure. After the recovery of acid and iron, the leach liquor was further processed to recover Cu using 25% LIX 84IC diluted in kerosene. The copper was found to be extracted 99.99% in two stages at pH 2.5, O/A ratio 1/1 in 20 min, leaving Ni and Pb in the raffinate. 99.99% Cu was stripped from loaded organic within five contacts at a phase ratio (O/A) of 1/1 using 10% H2SO4. Cu was recovered completely in form of copper sulfate salt (99.99% pure) by evaporation of the strip solution. In other side, Cu metal sheet (99.98% pure) was also obtained from the strip solution by electrowinning process in 4 hours at temperature 30°C using cathode current density 200 A/m2. After the recovery of acid, Fe and Cu, the leach liquor was further processed to recover Ni using 1% LIX 84IC diluted in kerosene. The nickel was found to be extracted 95% at pH 5.2, O/A ratio 1/1 in 15 min. Ni was stripped from the loaded LIX 84IC using 10% (v/v) sulfuric acid. 99.99% Ni was stripped from loaded organic in a single contact at a phase ratio (O/A) of 1/1 maintaining equilibrium time 5 min. Nickel sulfate salt (99.99% pure) was obtained by evaporation from the strip solution. Further, 99% lead metal (99.49% pure) was recovered from leached solution (free from acid, Fe, Cu & Ni) by cementation process at pH 1.7, 90°C using Al:Pb ratio 2/1 and stirring speed of 370 rpm for 105 min. Another part of the work was also carried out for recovery of copper from the depopulated small electronic devices using physical beneficiation followed by acid leaching, solvent extraction and electrowinning techniques. In this work, small electronic devices were leached in 3M HNO3 at 90°C for 30 min at 10 g/L pulp density, almost 99% Cu dissolved. Further, TBP was used for complete extraction of HNO3 in three stages at O/A ratio 1:1 maintaining mixing time 15 min. The leached solution (now free from acid) contains 0.016 g/L Fe in which NaOH was added to increase the pH of solution from 3.5-4.0 and was mixed at 80°C. Fe was removed from the aqueous solution as it get precipitated as ferric hydroxide by sparging air and then only minor amount of Fe (0.003 g/L) left in the leach solution. 81% Fe was recovered in form of ferric hydroxide with 99.99% pure. Finally, 99% pure Cu metal sheet and small amount of powder upon the sheet was obtained from the leached solution (free from acid and iron) by electrowinning process in 4 hours at temperature 30°C using cathode current density 200 A/m2. The novel pyro-hydro hybrid metallurgical process consisting of pyrolysis-beneficiation-leaching-solvent extraction-electrowinning-cementation for the recovery of metals from waste PCBs has been reported. The developed process is cleaner and environmental friendly with zero waste generation concept and has potential to be commercialized after validation on pilot scale trials.