Recycling of Printed Circuit Boards

Printed circuit boards (PCB) are fundamental components for almost all electric and electronic equipment being used as platform for installation of integrated circuits and of other electronic devices as well as for connections. They usually contain nearly 70% nonmetal materials, chiefly glass or cellulose fibres embedded in a polymeric matrix; due to the risk of ignition during soldering of the components or the impact with electric current, fire retardance is required for materials used in the PCB production, usually attained by using a brominated fire retardant matrix. Copper, solder, iron, nickel, silver, gold and palladium are contained in the metallic part of PCB, adding value to discarded PCBs if properly recycled. The non metalllic part of PCBs (ceramic and polymer) used to be abandoned as an industrial solid-waste byproduct during the recycling of waste, however this fraction has abundant high-value glass fibers and chemicals potentially recoverable. PCBs dismantled form electrical and electronic equipments constitute an important part of waste of electric and electronic equipment (WEEE) which has to be collected separately for recycling. As an example PCBs represent about 8% by weight of WEEE collected from small appliances [2] and 3% of global WEEE. There is an increasing interest in the end-of-life management of polymers present in WEEE mainly due to high quotas of recycling and recovery set by the European WEEE directives [1] and similar legislation of other countries which can only be fulfilled by including the plastic fraction in recycling and recovery approaches. Disposal of PCB in landfill is no longer accepted in developed countries because of environmental impact and loss of resources; however a successful recycling approach should take into consideration the valorisation of the recycled items to compensate for recycling costs. Nearly all of the current recycling technologies available for PCBs include a sorting/disassembly stage, followed by a crushing stage issuing PCB scraps easily manageable for further treatments. Recovery of base metals from PCB scraps is mostly based on leaching with various reactives and electrolytic refinery; bioleaching of copper and centrifugal separation of solder has also been proposed. The recycling of non metallic fraction (plastic and ceramic) from waste PCBs can be performed by physical and chemicals methods. Physical recycling is a promising method but more work should be done to develop comprehensive and industrialized usage of the recycled items which are now intended mainly as fillers for thermoplastic and thermosetting resins or raw material for concrete. On the other hand the challenge in chemical recycling methods is to compensate for the higher cost of process. Pyrolysis appears to be an emerging option allowing contemporary recovery of useful products such as precious metals, coke or glass fibres (in the residue), fuel and chemicals (in pyrolysis oil and gases). However contamination of oil by harmful compounds remains a severe issue and has a strong impact on material recycling and thermal treatment: in effect bromine-containing phenols are classic examples of risky compounds emitted during pyrolytic recycling of polymers flame retarded with brominated fire retardants forming toxic polybromodibenzodioxins and dibenzofurans when oil is used as a fuel. Dehalogenation of pyrolysis oil is therefore an area of continuing scientific interest. Attempts by depolymerisation in supercritical methanol, in situ treatments with CaO or various catalysts such Fluidized Catalytic Cracker (FCC) Catalyst are reported. Hydrodehalogenation with hydrogen-donating media is a promising option for the destruction of halogen-containing aromatics, which allows transformation of them to non-halogenated aromatics and hydrogen halide. Gasification, combustion and co-combustion of WEEE represent other possible forms of PCBs recycling. Searching for new, environmental friendly recycling technologies should take into consideration that PCB composition in the waste stream is expected to evolve as a consequence of technological progress and environmental legislation. For example the Restriction of use of Certain Hazardous Substances (RoHS Directive) [3], such as lead and some brominated fire retardant additives is likely to influence the composition of the PCB wastes in the near future. Several ecofriendly strategies of fire retardancy of the polymer matrix have been exploited, including incorporation of metal oxides, phosphorous and phosphorous-nitrogen compounds, metal oxides/metal hydrates blends and metal hydroxide/metal oxide blends. At the same time new developments are in progress in the field of TBPPA based fire retardance.