Contributed by Mark Evans, Founder and Chief Executive Officer of Camston Wrather
The prior installment of this series (Part 1) touched on the consumption and disposal volumes of electronic waste (e-waste) and its impacts. This article will cover a technical overview of current resource recovery methods, both formal and informal.
Metal Fractions (MF) and Non-Metal Fractions (NMF)
Most electronic devices contain printed circuit boards (PCBs) that are generally composed of a non-conducting substrate laminate, printed conducting tracks, and smaller electronic components mounted on the PCB. The substrate is primarily composed of glass fiber reinforced with epoxy resin, or paper reinforced with phenolic resin. Both substrates are covered with a brominated flame retardant, which protects the PCBs when the components are assembled to the boards in the manufacturing theater. Taken as a whole, there are basically two types of recyclable materials in PCBs: The first is the metal fraction (MF), such as Cu, Fe, Sn, Ni, and Zn; and the second is the non-metal fraction (NMF), such as polymers, glass and ceramic materials. However, it is the presence of the precious metals Au, Ag, Pt, and Pd that makes PCB recycling attractive and economical.
In general, the formal recycling of PCBs follows three stages: comminution (dismantling, milling, grinding, and/or magnetic separation), followed by either thermal (heat) processing (pyrometallurgical) or non-thermal processing (hydrometallurgical). The ideal goal of the first stage is to liberate the MFs from the NMFs, which is critical to further downstream treatment.
The two main types of PCBs are made of glass fiber reinforced with epoxy resin, commercially referred to as (FR-4), and cellulose paper reinforced phenolic resin (FR-2). Both types of PCBs can contain the following thermoplastic resins: Polyethylene terephthalate (PET), high-density polyethylene (HDPE), polyvinyl chloride (PVC), low-density polyethylene (LDPE), polypropylene (PP), acrylonitrile butadiene styrene (ABS), polycarbonate (PC), and polystyrene (PS).
Whether mining metals from ore or PCBs, the beneficiation process is similar. Raw materials are identified, secured, transported, classified, and milled, resulting in a concentrated feedstock. Milling requirements vary for different types of complex or refractory ores, and the same is true of PCBs. However, whereas ores are readily reduced in size through traditional milling technology, the same is not true for PCBs.
All polymer thermoplastics have a different “glass temperature,” i.e., the point at which the polymer becomes brittle, amorphous or crystalline. In the current recycling of PCBs, separating the metal fractions (MFs) from the various types of polymers (NMFs) becomes problematic because the mechanical engineering and milling threshold of the equipment cannot completely account for the glass temperatures associated with the various polymer substrates. (Fig. A) In short, the target metals are not fully liberated from the polymer substrate and are discarded with the NMFs, resulting in a 10 to30 percent loss in metal revenue.
Moreover, an additional five to 20 percent of target metal loss can occur through magnetic separation. Magnetic separation is a means to liberate the ferrous metal fractions from the non-ferrous metals after grinding and is universally practiced in PCB recycling. However, very little research exists addressing the unique phenomenon of gold particles being magnetized during separation. Figure B depicts the phenomenon of gold being magnetically attracted to a neodymium magnet. Although gold is not magnetic, Au-Cu-Fe plating and/or Au-Ni alloys are often used in electronics, which would account for some of the ferromagnetic attraction. Still, pure gold and other non-magnetic metals can be electrostatically charged during the grinding and milling stages producing MF loss. Comminution, as a stand-alone mechanical-physical recycling technology, is not an effective or reliable process for the production of single stream recycled metals.
Pyrometallurgical processing is the traditional approach for metal recovery from PCBs. Pyrometallurgical techniques include incineration, sintering,plasma arc smelting, blast furnace or copper smelting, and high temperature gas roasting. Currently, more than 70 percent of PCBs that are formally recycled are treated in smelters rather than through mechanical-physical processing. This is an important distinction and the most salient point of this installment:
Of the 15 percent of e-waste that is recycled (6.3 million metric tons), approximately 70 percent (4.41 million metric tons) of it is processed using incineration or pyrometallurgical processing, leaving approximately 1.89 million metric tons to be recycled through hydrometallurgical or other technologies.
Note: The remaining balance of 85 percent of PCB waste never enters any stage of formal recycling at all.
The downside of the pyrometallurgical approach is the large amount of waste water that needs to be treated, the lack of recycling of any of the plastic polymers, an efficient Fe and Al oxide recovery process, hazardous emissions of dioxins as a result of burning plastic polymers, and the large amount of capital investment and energy required during processing.
Traditional hydrometallurgy processes involve using acids and other lixiviants – such as sulfate, chloride, iodide, ammonia, and thiourea – in order to leach metals from PCBs. Aggressive acids like nitric acid and nitric acid in combination with hydrochloric acid (aqua regia) are often used in the digestion of base and precious metals from PCBs. Hydrometallurgical processes can offer high metal recoveries suitable for small-scale applications, however, there has been no multi-location rollout based solely on any one hydrometallurgical method.
Cyanide leaching of precious metals has been used for more than a century due to the selectivity and stability of the dicyanoaurate complex. It should be pointed out that although the affinity of cyanide for gold is preferential, cyanide will also form complexes with other MFs in PCBs, including Cu, Fe and Zn. The formation of strongly bound complexes such as those with iron and copper will tie up cyanide that would otherwise be available to dissolve gold and require higher cyanide concentrations and costs.
A routine search on YouTube will yield countless videos of backyard hobbyists attempting to extract gold from e- waste using nitric acid and other chemical compounds. Dissolving metals or reacting organics in nitric acid can release toxic gases. Attempting to dissolve e-waste in a closed container to mitigate those toxic gases will form toxic nitrosyl chloride, nitrogen dioxide, and deadly chlorine gases. These gases will pressurize in the container and ultimately lead to an unfavorable outcome.
The fundamental reasons limiting the commercial multi-location roll-out of nitric acid or cyanide processing plants for PCB recycling is: 1) they are difficult to safely scale or even to permit; 2) the disposal and neutralization of NOx gases, waste water, and byproducts is not economically feasible; and 3) both processes have major toxicity concerns that can cause environmental and worker safety concerns.
As previously reported, it is estimated that a total of 700,000 workers are employed in the informal e-waste collection and recycling industry in developing countries. The backyard operations in Asia and Africa – especially in China, India and Ghana – are problematic because of the adoption of primitive recycling techniques that lead to hazardous elements being released into the environment. Lack of access to appropriate technologies and methodologies, and an infrastructure capable of handling the increasing volumes of PCB scrap, directly leads to the release of dioxins and furans formed during the open-air burning of waste PCBs.
In the last few years, a number of articles have been published on the adverse health effects (such as abnormalities in thyroid function, decreased lung function, premature birth, reduced birth weights and lengths, genotoxicity, and adverse neonatal outcomes) caused by exposure to informal PCB recycling. The proper disposal and recycling of PCB waste is currently a concern, not only because of the large volumes generated, but also because of the heavy metals and toxic substances that PCB waste contains. If not properly treated and disposed of, toxic heavy metals like Pb, Cd, Hg, As, and Cr can be released into the environment. Lui, et al., reported that PCB recycling workers had 20-fold higher chromosomal aberrations than those not exposed to informal recycling.
Looking forward, the next installment will address the need for a more systematic, eco-friendly and multi-disciplinary approach to processing PCB waste streams in which the MFs as well as the NMFs can be recycled while imparting minimum impact on workers and the environment.
Mark Evans is the Founder and Chief Executive Officer of Camston Wrather and a University of California at Berkeley, Alumni.
Connect with author on LinkedIn.
“The Nexus Blog” is a sister publication of “The Nexus” newsletter. To sign up for the newsletter, please email firstname.lastname@example.org, or if you have an ACS ID, login to your email preferences and select “The Nexus” to subscribe.
To read other posts, go to Green Chemistry: The Nexus Blog home.
You must be a registered user to add a comment. If you've already registered, sign in. Otherwise, register and sign in.