Rita Khanna

Concentration of precious metals during their recovery from electronic waste.

The rapid growth of electronic devices, their subsequent obsolescence and disposal has resulted in electronic waste (e-waste) being one of the fastest increasing waste streams worldwide. The main component of e-waste is printed circuit boards (PCBs), which contain substantial quantities of precious metals in concentrations significantly higher than those typically found in corresponding ores. The high value and limited reserves of minerals containing these metals makes urban mining of precious metals very attractive. This article is focused on the concentration and recovery of precious metals during pyro-metallurgical recycling of waste PCBs. High temperature pyrolysis was carried out for ten minutes in a horizontal tube furnace in the temperature range 800-1350°C under Argon gas flowing at 1L/min. These temperatures were chosen to lie below and above the melting point (1084.87°C) of copper, the main metal in PCBs, to study the influence of its physical state on the recovery of precious metals. The heat treatment of waste PCBs resulted in two different types of solid products, namely a carbonaceous non-metallic fraction (NMFs) and metallic products, composed of copper rich foils and/or droplets and tin-lead rich droplets and some wires. Significant proportions of Ag, Au, Pd and Pt were found concentrated within two types of metallic phases, with very limited quantities retained by the NMFs. This process was successful in concentrating several precious metals such as Ag, Au, Pd and Pt in a small volume fraction, and reduced volumes for further processing/refinement by up to 75%. The amounts of secondary wastes produced were also minimised to a great extent. The generation of precious metals rich metallic phases demonstrates high temperature pyrolysis as a viable approach towards the recovery of precious metals from e-waste.

Generation of copper rich metallic phases from waste printed circuit boards

The rapid consumption and obsolescence of electronics have resulted in e-waste being one of the fastest growing waste streams worldwide. Printed circuit boards (PCBs) are among the most complex e-waste, containing significant quantities of hazardous and toxic materials leading to high levels of pollution if landfilled or processed inappropriately. However, PCBs are also an important resource of metals including copper, tin, lead and precious metals; their recycling is appealing especially as the concentration of these metals in PCBs is considerably higher than in their ores. This article is focused on a novel approach to recover copper rich phases from waste PCBs. Crushed PCBs were heat treated at 1150°C under argon gas flowing at 1L/min into a horizontal tube furnace. Samples were placed into an alumina crucible and positioned in the cold zone of the furnace for 5 min to avoid thermal shock, and then pushed into the hot zone, with specimens exposed to high temperatures for 10 and 20 min. After treatment, residues were pulled back to the cold zone and kept there for 5 min to avoid thermal cracking and re-oxidation. This process resulted in the generation of a metallic phase in the form of droplets and a carbonaceous residue. The metallic phase was formed of copper-rich red droplets and tin-rich white droplets along with the presence of several precious metals. The carbonaceous residue was found to consist of slag and ∼30% carbon. The process conditions led to the segregation of hazardous lead and tin clusters in the metallic phase. The heat treatment temperature was chosen to be above the melting point of copper; molten copper helped to concentrate metallic constituents and their separation from the carbonaceous residue and the slag. Inert atmosphere prevented the re-oxidation of metals and the loss of carbon in the gaseous fraction. Recycling e-waste is expected to lead to enhanced metal recovery, conserving natural resources and providing an environmentally sustainable solution to the management of waste products.

Influence of sulfur on the solubility of graphite in Fe–C–S melts: optimization of interaction parameters

Abstract We report a Monte Carlo simulation study of a molten Fe–C–S system at 1600°C with the aim of developing an atomic model with optimum interaction parameters. This model should account for key features of the system, e.g., homogenous nature of Fe–FeS solution, separation of Fe–C–S liquid in two immiscible layers and a decrease in solubility of graphite in iron melts with the addition of sulfur. The atoms in the ternary Fe–C–S system were arranged on a graphitic hexagonal lattice and pair-wise interactions between them were assumed to be short ranged. The simulations were carried out using a combination of canonical and grand canonical ensembles for a range of interaction strengths, carbon and sulfur concentrations. The strength of ionic Fe–S interaction was one of the significant parameters and could lead to electronic distortions around the Fe atom. Local modifications to various interactions due to these distortions were represented by three parameters: e (Fe–Fe), δ (Fe–S) and δ (Fe–C) and their effect on the Fe–C–S system was systematically analyzed. Optimum modulation parameters for this system, which simultaneously simulate key features of a molten Fe–C–S system are: e (Fe−Fe)=1.0, δ (Fe−S)=1.0, δ (Fe−C)=1.0 and 1.5. Even though a slight increase in Fe–C repulsion gives optimum results, our high temperature simulation results clearly show that electronic distortions around Fe due to strong Fe–S bond do not play a significant role in the molten Fe–C–S system.

Recycling Waste Plastics in EAF Steelmaking: Carbon/Slag Interactions of HDPE‐Coke Blends

Due to the inherent limitations of current methods of plastic waste disposal, there have been concerted efforts worldwide towards developing alternative, environment friendly and economic recycling processes. With an aim to recycle waste plastics in EAF steelmaking, carbon/slag interactions for a number of blends made of metallurgical coke and HDPE (high density polyethylene) and an EAF slag (34.8 mass-% Fe2O3) have been investigated at 1550°C using a sessile drop arrangement. The rate of gas generation showed an increase with increasing HDPE concentration, reaching a maximum for blend #3 (with appr. 30 % HDPE) and decreasing thereafter. Among all the blends investigated, blend #3 showed significantly higher levels of slag foaming as compared to metallurgical coke. HDPE-coke blends also showed better wetting compared to metallurgical coke with contact angles in some cases improving from 140° to 70° after 10 minutes of contact. These results have been discussed in terms of ash and sulphur contents of carbonaceous residues and dynamic changes in slag composition. Industrial trials on blend #3 showed a good agreement with laboratory results. This work opens novel avenues for the utilisation of plastics wastes as a valuable carbon resource in EAF steelmaking.

Depletion of Carbon from Al2O3-c Mixtures into Liquid Iron: Rate Controlling Mechanisms

Abstract A sessile drop investigation on the kinetics of carbon dissolution from an alumina-carbon composite (75% C, 25% alumina) and a commercial refractory (28.3% C, 66.67% alumina, 5% binder) into liquid iron at 1600 °C is reported. Carbon dissolution from refractory substrates was very slow reaching 0.84% C and 0.1% C, respectively after 60 minutes. Both substrates also showed poor wettability. Experimental studies were supplemented with atomistic Monte Carlo simulations to investigate the influence of composition, temperature and melt turbulence. High carbon systems (100% C and 75% C, balance alumina) were affected by both temperature and melt turbulence to some extent; increased levels of melt turbulence/higher temperatures had no influence on low carbon (30% C) system. While mass transfer was the dominant rate controlling mechanism for high carbon systems, poor wettability of alumina with liquid iron and its significant influence on inhibiting the penetration of liquid iron in the refractory matrix was found to be the dominant rate controlling factor for low carbon refractories.

High temperature investigations on optimising the recovery of copper from waste printed circuit boards

High temperature pyrolysis investigations were carried out on waste printed circuit boards (PCBs) in the temperature range 800-1000°C under inert conditions, with an aim to determine optimal operating conditions for the recovery of copper. Pyrolysis residues were characterized using ICP-OES analysis, SEM/EDS and XRD investigations. Copper foils were successfully recovered after pyrolysis at 800°C for 10-20 min; the levels of Pb and Sn present were found to be quite low and these were generally present near the foil edges. The relative proportions of Pb and Sn became progressively higher at longer heating times due to enhanced diffusion of these molten metals in solid copper. While a similar behaviour was observed at 900°C, the pyrolysis at 1000°C resulted in copper forming Cu-Sn-Pb alloys; copper foils could no longer be recovered. Optimal conditions were identified for the direct recovery of copper from waste PCBs with minimal processing. This approach is expected to make significant contributions towards enhancing material recovery, process efficiency and the environmental sustainability of recycling e-waste. Pyrolysis at lower temperatures, short heating times, coupled with reductions in process steps are expected to significantly reduce energy consumption and pollution associated with the handling and processing of waste PCBs.

A novel approach for reducing toxic emissions during high temperature processing of electronic waste

A novel approach is presented to capture some of the potentially toxic elements (PTEs), other particulates and emissions during the heat treatment of e-waste using alumina adsorbents. Waste PCBs from mobile phones were mechanically crushed to sizes less than 1mm; their thermal degradation was investigated using thermo-gravimetric analysis. Observed weight loss was attributed to the degradation of polymers and the vaporization of organic constituents and volatile metals. The sample assembly containing PCB powder and adsorbent was heat treated at 600°C for times ranging between 10 and 30min with air, nitrogen and argon as carrier gases. Weight gains up to ∼17% were recorded in the adsorbent thereby indicating the capture of significant amounts of particulates. The highest level of adsorption was observed in N2 atmosphere for small particle sizes of alumina. SEM/EDS results on the adsorbent indicated the presence of Cu, Pb, Si, Mg and C. These studies were supplemented with ICP-OES analysis to determine the extent of various species captured as a function of operating parameters. This innovative, low-cost approach has the potential for utilization in the informal sector and/or developing countries, and could play a significant role in reducing toxic emissions from e-waste processing towards environmentally safe limits.

Environmental Impact of Processing Electronic Waste – Key Issues and Challenges

Extensive utilization of electric and electronic equipment in a wide range of applications has resulted in the generation of huge volumes of electronic waste (e-waste) globally. Highly complex e-waste can contain metals, polymers and ceramics along with several hazardous and toxic constituents. There are presently no standard approaches for han‐ dling, dismantling, and the processing of e-waste to recover valuable resources. Inappro‐ priate and unsafe practices produce additional hazardous compounds and highly toxic emissions as well. This chapter presents an overview of the environmental impact of proc‐ essing e-waste with specific focus on toxic elements present initially in a variety of e-waste as well as hazardous compounds generated during e-waste processing. Hazardous constit‐ uents/ and contaminants were classified in three categories: primary contaminants, secon‐ dary contaminants, and tertiary contaminants. Primary contaminants represent hazardous substances present initially within various types of e-waste; these include heavy metals such as lead, mercury, nickel and cadmium, flame retardants presents in polymers etc. Sec‐ ondary contaminants such as spent acids, volatile/toxic compounds, PAHs are the byproducts or waste residues produced after inappropriate processing of e-waste and the tertiary contaminants include leftover reagents or compounds used during processing. A detailed report is presented on the environmental impact of processing e-waste and the detrimental impact on soil contamination, vegetation degradation, water and air quality along with implications for human health. Challenges and opportunities associated with appropriate e-waste management are also discussed.

Recycling polymeric waste from electronic and automotive sectors into value added products

The environmentally sustainable disposal and recycling of ever increasing volumes of electronic waste has become a global waste management issue. The addition of up to 25% polymeric waste PCBs (printed circuit boards) as fillers in polypropylene (PP) composites was partially successful: while the tensile modulus, flexural strength and flexural modulus of composites were enhanced, the tensile and impact strengths were found to decrease. As a lowering of impact strength can significantly limit the application of PP based composites, it is necessary to incorporate impact modifying polymers such as rubbery particles in the mix. We report on a novel investigation on the simultaneous utilization of electronic and automotive rubber waste as fillers in PP composites. These composites were prepared by using 25 wt.% polymeric PCB powder, up to 9% of ethylene propylene rubber (EPR), and PP: balance. The influence of EPR on the structural, thermal, mechanical and rheological properties of PP/PCB/ EPR composites was investigated. While the addition of EPR caused the nucleation of the β crystalline phase of PP, the onset temperature for thermal degradation was found to decrease by 8%. The tensile modulus and strength decreased by 16% and 19%, respectively; and the elongation at break increased by ~71%. The impact strength showed a maximum increase of ~18% at 7 wt.%–9 wt.% EPR content. Various rheological properties were found to be well within the range of processing limits. This novel eco-friendly approach could help utilize significant amounts of polymeric electronic and automotive waste for fabricating valuable polymer composites.

Atomistic Simulations of Properties and Phenomena at High Temperatures

Abstract Atomistic computer simulations have emerged in recent years as an important research tool to probe experimentally inaccessible regimes, providing deep insights for understanding the influence of chemical interactions and structure on the properties of the solid/fluid phase. While simulations enable a detailed investigation of material processes and phenomena with atomic resolution, their reliability depends on the quality of interaction potentials, the use of an appropriate simulation approach followed by experimental validation. Interaction potentials are usually constructed to describe the zero-temperature ground state properties of materials such as crystal structure, the elastic constants and the cohesive energy; only a few potentials/interaction parameters have been developed to describe the high temperature behavior and thermodynamic properties. Iron and steel have preferentially been used in this article as a case study to illustrate the use of atomistic modeling in process metallurgy. A variety of atomistic computer simulation techniques including Monte Carlo methods, molecular dynamics, and ab initio approaches are critically reviewed along with special algorithms developed to obtain physical properties such as free energy, surface tension, wettability, phase behavior, diffusion, etc. Interatomic potentials for Fe, its alloys and a number of nonferrous metals are presented and critically assessed for application to high temperature properties and phenomena. Atomistic Monte Carlo simulation results have been presented on the carburization and decarburization of molten steel, role of surface-active elements, refractory degradation, phase behavior, diffusion-based phenomena, the influence of defects and defect clusters and solidification simulations. Concluding remarks briefly highlight the current status and limitations of atomistic research at high temperatures. Rational design and advancement in steelmaking technology is expected to ultimately rely on an atomic-scale understanding of various reaction processes.