Noble metals (NMs) represent relevant high-value components of WEEE (Waste Electric and Electronic Equipments) and their recovery makes these scraps an intriguing source of valuable raw materials. Among the valuable materials contained in WEEE, Printed Circuit boards (PCBs) represent a real "urban mine" of key metals, containing amount of precious and "critical" metals often larger than in their ores. Sustainable NMs recovery from complex matrix as WEEE is a very challenging issue not satisfactorily solved yet. The most diffused industrial practices for NM recovery from WEEE are based on pyrolysis and electrolysis often involving very harmful and/or energy-demanding processes. Hydrometallurgical methods, deriving from the mineral industry are also insustrially applied, in particular for NMs recycling, but their application is limited by the use of harmuful reagents and by the etherogeneous and composite composition of the scraps which makes the process less effective and selective. Hydrometallurgical methods clearly benefit from prior comminution and separation, able to reduce bulk volume, to expose a greater surface area of contained metals to the leaching and to limit interfering reactions. This important pre-treatment becomes more helpful for improving effectiveness when selective non-aggressive reactants are used to meet sustainability requirements. This is the case, for example, of a sustainable hydrometallurgical NMs recovery method recently set-up by us on a mixed metallic fraction of WEEE, based on the use of non-harmful reagents. We found high effectiveness and selectivity in high purity NMs recovery from this very thin (average particle size: 0.4mm) and concentrated metal sample obtained by means of shredding and electrostatic and magnetic sepration techniques. Nevertheless the very inhomogeneous and almost unknown key metals composition of WEEE and the inefficiency of commercial mechanical pre-treatments plants, in terms of lost of valuable materials during the shredding and separation phases and their cost, still limit the industrial application of sustainable NMs recovery methods. It is worthnoting that the number of companies able to produce in their own a raw shredded material is greatly increasing, while just few of them are currently able to produce a good quality semi-finished metallic material obtained as the output of a selective mechanical separative process. The research activity on high performance comminution and separation technologies is improving their effectiveness but a further effort in combining the pre-treatment set up with the subsequent leaching processes would help in making NMs recovery from these kind of materials more effective and economically sustainable. On these basis, in order to promote the NMs recovery from PCBs and the technology transfer of sustainable hydrometallurgical processes, here we report: i) the accurate NMs characterization by means of quantitative chemical analysis, by a selection of comminuted PCB samples underwent combined mechanical (shredding) and chemical (leaching) pretreatments; and, as a case study, ii) the preliminary results on the application of the sustainable NMs recovery process previously published by us, on a shredded RAM sample, highly eterogeneous in terms of size distribution and composition, provided by companies in order to test the selectivity and effectiveness of the method on a very coars material and when the plastic-vitreous support and the ferrous materials are still present in the mixture.
Noble-metals recovery from printed circuit boards: a multidisciplinary approach towards sustainability
2017
Abstract
Noble metals (NMs) represent relevant high-value components of WEEE (Waste Electric and Electronic Equipments) and their recovery makes these scraps an intriguing source of valuable raw materials. Among the valuable materials contained in WEEE, Printed Circuit boards (PCBs) represent a real "urban mine" of key metals, containing amount of precious and "critical" metals often larger than in their ores. Sustainable NMs recovery from complex matrix as WEEE is a very challenging issue not satisfactorily solved yet. The most diffused industrial practices for NM recovery from WEEE are based on pyrolysis and electrolysis often involving very harmful and/or energy-demanding processes. Hydrometallurgical methods, deriving from the mineral industry are also insustrially applied, in particular for NMs recycling, but their application is limited by the use of harmuful reagents and by the etherogeneous and composite composition of the scraps which makes the process less effective and selective. Hydrometallurgical methods clearly benefit from prior comminution and separation, able to reduce bulk volume, to expose a greater surface area of contained metals to the leaching and to limit interfering reactions. This important pre-treatment becomes more helpful for improving effectiveness when selective non-aggressive reactants are used to meet sustainability requirements. This is the case, for example, of a sustainable hydrometallurgical NMs recovery method recently set-up by us on a mixed metallic fraction of WEEE, based on the use of non-harmful reagents. We found high effectiveness and selectivity in high purity NMs recovery from this very thin (average particle size: 0.4mm) and concentrated metal sample obtained by means of shredding and electrostatic and magnetic sepration techniques. Nevertheless the very inhomogeneous and almost unknown key metals composition of WEEE and the inefficiency of commercial mechanical pre-treatments plants, in terms of lost of valuable materials during the shredding and separation phases and their cost, still limit the industrial application of sustainable NMs recovery methods. It is worthnoting that the number of companies able to produce in their own a raw shredded material is greatly increasing, while just few of them are currently able to produce a good quality semi-finished metallic material obtained as the output of a selective mechanical separative process. The research activity on high performance comminution and separation technologies is improving their effectiveness but a further effort in combining the pre-treatment set up with the subsequent leaching processes would help in making NMs recovery from these kind of materials more effective and economically sustainable. On these basis, in order to promote the NMs recovery from PCBs and the technology transfer of sustainable hydrometallurgical processes, here we report: i) the accurate NMs characterization by means of quantitative chemical analysis, by a selection of comminuted PCB samples underwent combined mechanical (shredding) and chemical (leaching) pretreatments; and, as a case study, ii) the preliminary results on the application of the sustainable NMs recovery process previously published by us, on a shredded RAM sample, highly eterogeneous in terms of size distribution and composition, provided by companies in order to test the selectivity and effectiveness of the method on a very coars material and when the plastic-vitreous support and the ferrous materials are still present in the mixture.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
