Since the industrial revolution, the world has been dominated by electronic devices. They just made our life more comfortable. We bring smartphones, tablets, laptops, and many other electronic devices everywhere.
No matter, portable or stationary ones, they all have electronics inside. Each device’s essential component is printed circuit boards (PCBs) with many circuits placed on the insulating material instead of cables. This paper focuses on precious metals in PCBs and recovery ways.
Metal mine in a box
Electricity is conducted in the circuits thanks to the metals. The main one commonly used to conduct electricity is copper having the chemical symbol Cu. However, PCBs hide much more than just such metal. Let’s take a look at them. The following element commonly found in PCB is tin (Sn), used in soldering paste. The temperature of tin melting is too high to use pure tin in PCBs. Thus, its alloys are applied.
These alloys contain many other metals like zinc (Zn), indium (In), silver (Ag), bismuth (Bi), antimony (Sb), nickel (Ni), gold (Au), and even lead (Pb). Zinc is widely used in devices to protect against corrosion. The same happens in PCBs. Zn is added to prevent conduction tracks from any degradation, oxidation, and corrosion that may occur on-air (humidity and oxygen from air work as corrosion agents) . Likewise zinc, nickel is used to protect soldiers from corrosion.
Additionally, Ni plating improves the plate’s tensile strength . Bismuth takes part in the solder filler. It is usually added in the lead-free solders to maintain proper plasticity while melting. When the PCB heats up, the metals tend to expand while cooling down causes shrinking, so without the addition of Bi, the solder would break away from the PCB . For the same reason, the indium and/or antimony can also be added to solder. It just improves the tensile properties .
Following metal that is widely used in PCBs is silver. It is usually the coat of the copper circuits that enhance solderability. It also protects copper traces from corrosion . Gold is also used for the same reason, while, due to the high price, it is replaced with silver. Sometimes instead of Ag or Au, platinum is used. This noble metal maintains excellent electrical conductivity and protects from copper oxidation,. Due to their high costs, they are used quite rarely .
Each electronic device heats up under operation. To reduce this effect and regulate its temperature, an additional component is used – aluminum. This metal transfers heat away from essential components. Last, while the worst for health is lead. This metal is still added to some soldering fillers to improve tensile properties. Unfortunately, it is well-known for being highly neurotoxic .
How to recover metals?
Waste PCBs processing is a real technological challenge, as it is characterized by a complex structure and diverse material composition. The recycling of WPCBs focuses mainly on the recovery of valuable metals in them. At the same time, plastic components are usually poorly non-recycled.
How to get precious metals from PCBs like gold, nickel, etc.? The recycling process usually includes a few steps like mechanical pretreatment and chemical treatment . The pretreatment purpose is to disassemble electronic waste equipment into individual components. It is possible to separate the hazardous waste from other parts and prepare it for further recycling. When crucial components are separated, then they are processed chemically.
What does it mean in practice? That step usually requires the application of chemical leaching with aggressive compounds like acids. It is called the hydrometallurgical technique. Anyway, the critical stage is the recovery of metals from the separated material. Despite non-environmental friendly methods, they are commonly used due to their high efficiency.
A like acids
Regarding the recovery of base metals like Al, Fe, Sn, Zn, and Cu, the most frequently used chemical compounds are strong acids, also called mineral acids like sulfuric acid (H2SO4), nitric acid (HNO3), and hydrochloric acid (HCl). Additionally, recovery of metals like Cu is commonly performed in strong acids and in the presence of hydrogen peroxide (H2O2).
The mixture H2SO4 + H2O2 is even more aggressive. It helps to recycle metals more efficiently than just with the use of sole acids . Through appropriately selected parameters of the leaching process (apart from the type of chemical reagents, important indicators are also the concentration of the leaching agent (acid used for metal recovery), duration of the process, and temperature), it is possible to obtain high degrees of metal recovery: over 90% of Al, Sn, and Fe , and even 100% of Cu .
Recovery of precious metals like Au, Ag, and Pt, usually requires the application of strong acids and chemical compounds like cyanide, thiourea, thiosulfate, thiocyanate halides [7, 10]. Nevertheless, it is worth paying attention to the fact that it is challenging to selectively separate one given metal. For this purpose, except leaching, additional methods are used in complex metal recovery systems from waste PCBs, such as ion exchange, adsorption, extraction, precipitation, or combinations of these methods [10, 11], but that is the story for the next time.
Did you know that?
- The country which produces the most electro-waste per year in the United States. The number. The mass of their waste is 9.4 mln tons. 
- An estimated 40 percent of heavy metals come from electronic waste. 
- 8 million tons of electronic waste were shipped into developing countries where they aren’t recycled well. 
- By recycling one million laptops, we can save as much energy as 3,500 American homes use. 
- Recycling one million cell phones save about 33 pounds of palladium, 75 pounds of gold, 772 pounds of silver, and 35 thousand copper pounds. 
- There are more active sim cards (over 7.2 billion) than people on Earth (nearly 7.2 billion). 
PCBs are full of precious metals that can be recycled and reused. Unfortunately, to do that, aggressive chemical compounds like mineral acids are required. Luckily, these processes are performed in specialized laboratories. Thus special precautions must be followed. Next time some electronic devices you use stop working, let’s keep in mind that the PCBs hidden inside should be recycled.
This article is a joint work of Jan Dziewulski (Faculty of Chemistry, University of Warsaw), Weronika Urbańska (Department of Environmental Engineering, Wrocław University of Science and Technology), Agnieszka Pregowska (Institute of Fundamental Technological Research, Polish Academy of Sciences), and Magdalena Osial (Faculty of Chemistry, University of Warsaw) as a part of the Science Embassy project. Image Credit – Weronika Urbańska
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