emew Blog

Recovering precious metals and base metals from old electronic waste

Posted by Alex Barshai on Nov 4, 2022 10:43:20 AM

What if I told you that we send hundreds of millions of dollars to the landfill every year? If you look around the room you are in right now, you will likely see a multitude of electronic devices. Our reliance on electronic devices in everyday life is growing and rapid technological advances continuously bring updated models onto the market. This means that more devices are being discarded, with more being sent to the landfill than ever.

Several base and precious metals form an essential component of electronic devices. The properties of these metals make them ideal for recycling and reuse. Yet, studies have shown that only 15-20% of electronic devices are being recycled. Leaving a huge untapped volume of valuable metals that could be sold or reused sitting in landfills.  Effectively recovering metals from these devices could have multiple ecological benefits from reducing the economic burden of recycling to reducing the demand for mining ore. There are multiple methods for extracting metals, mainly pyrometallurgy and hydrometallurgy.

The scale of electronic waste 

Electronic waste (referred to as E-waste) is the broad term given to waste containing electrical components, ranging from computers to commercial machinery. E-waste is categorized as hazardous waste due to the presence of toxic chemicals such as mercury, lead and brominated flame retardants. E-waste is a complex mix of plastics and metals and coupled with hazardous chemical components make proper waste management both laborious and costly. Due to these issues, a high proportion of E-waste is sent to the landfill with very little being recycled.

The United Nations estimates that only 15-20% of E-waste generated is recycled. The STEP initiative (Solving the E-waste problem) from the United Nations University estimates that in 2014, 41.8 million tonnes of E-waste was produced, and this trend has continued to increase. To put that into perspective, that equates to 14lb of E-waste per person globally or over 4000 Eiffel towers. Canada alone has produced 750,000 metric tonnes of E-waste to date. E-waste poses a large environmental issue; however, it is also a largely untapped unconventional resource of base and precious metals.

Recovering precious metals

E-waste contains several base and precious metals such as gold, silver, copper, nickel and palladium. Having an effective method to extract the metals means they can be reused or if the metals are extracted at a high purity, can be sold to offset the cost of processing and recycling E-waste. These metals are typically found in printed circuit boards, making them the most metal abundant components. This makes printed circuit boards an attractive resource for metal recovery, however, they are a diverse and varied mix of metals, soldering and polymers. This composition complicates the recycling process and necessitates the development of technologies to improve metal recovery.

 

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The value in metal recovery 

As an example of the potential worth of recovering metals from electronic devices. A UN report estimated that by recycling 1 million mobile phones we can recover approximately 24 kg of gold, 250kg of silver, 9kg of palladium and over 9,000kg of Copper. If we assume that 100% of the metal is extracted and current metal prices according to the London Metal Exchange (22/1/20), this is a potential value of 2,099,387 US dollars. Here is the breakdown of each metal.

The value in metal recovery This example highlights the potential value found in recycling E-waste. In the last quarter of 2019 alone, mobile phone sales topped 380 million. The average lifespan of a mobile phone is now 2-3 years. That means that in just a couple of years, hundreds of millions of dollars worth of metal will go to landfill. Now expand that to other electronic devices across the globe and it’s easy to see why the market for ‘Urban mining’ is gaining traction. It is well known that the metal content of electronic devices is more concentrated compared to mined ore.

 

 Metal

 LME price ($USD)

 Volume

 Total value ($USD)

 Gold

 50.07/g

 24kg

 1,201,680

 Silver

 0.57/g

 250kg

 143,327

 Copper

 6.103/g

 9000kg

 54,927

 Palladium

 77.7/g

 9kg

 699,453

 

An EPA study calculated that one metric ton of printed circuit boards contained 40 to 800 times as much gold and 30 to 40 times the amount of copper mined from one metric ton in the United States. On top of that, a recent feasibility study from China demonstrated that recycling metals could be up to 13 times more cost effective than mining and processing ore. With all these benefits of recycling metals, the urban mining market is continuously growing, and it is expected to be worth 50 billion dollars in 2020.

 

Extracting metal from electronic devices 

Electronic waste can come in a variety of shapes, sizes and complexity, from smart fridges to handheld games consoles. This means that the extraction and recovery process cannot be uniform for all devices. making the sorting and recycling process more complex and requiring several processing steps before the metal extraction process can begin. 

The first step in the sorting process involves disassembling the electronic device down into smaller components and removing hazardous components. These components can then be sorted into materials for recycling and other metal containing fractions, such as printed circuit boards. The disassembling process typically involves a large amount of intensive manual sorting of materials before moving onto the next step. More autonomous sorting methods are starting to be developed. For example, Apple has developed “Liam and Daisy”, robots that are capable of disassembling multiple iPhones at once and recycle components for reuse. The development of autonomous sorting may reduce labour costs and make the sorting phase more straight forward.

The next step is physical processing to reduce the metal containing components into smaller fractions, including reusable components and printed circuit boards. First components are heated to remove the soldering and then may undergo multiple stages of reduction, using shredders, grinders or pulverizers to break the components down into manageable pieces.

I will discuss 3 methods used after processing to isolate metals from non-metals; physical sorting, pyrometallurgy and finally, hydrometallurgy.

Physical sorting

There are 3 main methods utilized during physical sorting of metal and non-metal fractions. Shredding or pulverization alters the shape of metals and non-metals. Due to its malleability, metals take on a spherical shape under pressure, while non-metals remain non spherical. This shape effect alongside different specific gravities can be taken advantage of to separate different fractions.

Liquid based sorting 

Uses the different specific gravities of metals and non-metals to sort into fractions. Processed PCBs are placed into a liquid solution, often tetrabromoethane or acetone and non-metals float nearer the surface while metals sink nearer the bottom. Liquid sorting is a straightforward process; however, the efficiency is low and is affected by particle size and shape.

Electrostatic separation 

As given away by its name, this process separates materials based on the ability to conduct electricity. Non-metal fractions that do not conduct are sorted out using a vibration screen. The limitation to this method is that it is limited to only small particle sizes.

Magnetic separation 

Magnetic separation is used to recover ferrous metals, such as copper. Magnetic separation is only effective when done prior to crushing. Magnetic separation is performed first and followed by crushing and then undergoes electrostatic processing.

Physical sorting 

Has a number of major limitations that prevent its large-scale use. The sorting methods produce a high potential for losing precious metals and not recovering metals to a high purity. Additionally, there are high operating and energy costs to using these methods.

Pyrometallurgy 

Pyrolysis is currently one of the most commonly used processes. Up to 70% of PCBs are treated in smelters. Essentially, crushed PCBs are incinerated and smelted in furnaces and used to recover copper, gold and silver. Using this method only partially recovers the metals and further refining methods are necessary to attain high purity metals that can be reused or sold. The recovery of base metals from integrated smelters is limited to copper, as iron and aluminium become concentrated in the slag produced. PCBs also contain ceramics and glass, which contribute to higher slag formation and lead to the greater loss of recoverable precious and base metals. There is also an environmental consideration, there is a high risk of dioxin formation and other hazardous toxins release.

Hydrometallurgy

The final method I will discuss is hydrometallurgy. This method uses a combination of caustic or acid leaching followed by a purification technique. There are several different metal purification techniques including cementation, ion exchange, solvent extraction, activated carbon adsorption and electrowinning. In this article, I will focus on the use of electrowinning to extract metals from e-waste. Electrowinning is an effective step in recycling non-ferrous metals, which can be recycled infinitely without losing any properties. When compared to other methods electrowinning can offer a more efficient, controlled and environmentally friendly options, especially when using emews vortex electrowinning technology.

Electrowinning is a relatively straightforward electrolytic process established in the 1800s by an English chemist. During this method, metal ions are leached into a polar solvent to generate an electrically conductive solution. The application of a direct current to this electrolyte solution causes the metal ions to plate onto a cathode which can then be extracted. Electrowinning is most commonly used to recover metals such as gold, silver, zinc, copper, cobalt and nickel.

Recovering precious metals

The first step of electrowinning is leaching of metals into solution. There are multiple leaching methods and will depend on the type of metal being recovered. Cyanide leaching was a commonly used method for the recovery of precious metals, gold and silver. However, this method is highly toxic and is no longer as frequently used. Thiourea or thiosulfates are more environmentally friendly, although not as economically promising for larger scale use. This is due to the volume of reagent required for effective leaching and there are also stability issues and slower kinetics. Acid leaching is the most popularly used method due to a high leaching rate and fast kinetics.

Now that the metal is in solution, a direct current can be applied, and metals deposited onto the cathode. The electrowinning method can be used for successively isolating different metals and has been used to successively recover copper, gold and silver. The electrolyte formed after leaching may be composed of multiple metals, which can complicate recovery of a specific metal. In some cases, metals can be specifically electrowon. This depends on the reduction potential relative to each metal.

Recovering precious metals and base metals from old electronic waste

In order to recover the highest value of metals, the metal isolated must be of high purity. One of the drawbacks of conventional electrowinning is that as the target metal is plated, the concentration of metal around the cathode is lower than the rest of the bulk solution. This can lead to the formation of depletion zones which can lead to the deposition of impurities onto the cathode. The emew electrowinning system has improvements that overcome many of the issues with electrowinning. The emew system utilizes a cylindrical tank where the electrolyte is rapidly circulated past the electrode, improving mass transfer and allows the metal to be depleted to a much lower concentration. This system also helps to overcome the issues associated with depletion zones and results in higher purity of plated metals. Additionally, emew electrowinning cells are a closed system which overcome hazardous issues such as acid mist and other noxious gases, making it a much safer system to work with.

Recovering precious metals and base metals from old electronic waste

 

Electrowinning case study 

To highlight the effectiveness of the emew electrowinning system for the recovery of metals from E-waste, I will go through a paper published by Pitroda et al (2017). This paper investigated the feasibility of using emew electrowinning technology for successive recovery of gold, silver and copper metals from printed circuit boards from computer RAM and motherboards.

Electrowinning case study 

In the first step, printed circuit boards were passed through multiple physical processing steps. Firstly, the printed circuit boards were passed through shredding and pulverizing steps. Plastic and metal components were then separated by allowing them to settle in solution. Metals and non-metals are then further separated by electrostatic separation. Ferrous and non-ferrous metals were then separated using a magnetic separation. Removing ferrous metals is important as they can interfere with the leaching process. Leaching of metals was achieved in two steps. Metals were treated first with sulfuric acid, followed by aqua regia to produce the final solution that will undergo electrowinning.

The results demonstrated that using the emew electrowinning system resulted in up to 92% of the available metal content recovered.  91% of copper, 90% of gold and 92% of silver, were recovered from motherboard printed circuit boards. In RAM printed circuit boards, 87% of copper, 87% of gold and 90% of silver was extracted. This highlights that the emew system can be highly effective in recovering metals from electronic waste sources. Using the emew technology can result in highly efficient metal recovery, with metal purity as high as 99.999%

 

 

References

  • Solving the E-waste problem initiative http://www.step-initiative.org/
  • Hsu et al (2019). Advancements in the Treatment and Processing of Electronic Waste with Sustainability: A Review of Metal Extraction and Recovery Technologies. Green Chemistry
  • Kaya, M (2016). Recovery of metals and nonmetals from electronic waste by physical
  • and chemical recycling processes. Waste management
  • Debnath et al (2018). Sustainability of metal recovery from E-waste. Frontiers of environmental science and engineering.
  • Memon et a (2017). Design for recovery of precious and base metals from e-waste using electrowinning process. International Journal of Advance Research in Engineering, Science & Technology

Topics: recovery, metals, electronic waste, electronic, Recovering precious metals