Metal recovery from scrap printed circuit boards (PCBs) is a critical part of e-waste recycling and urban mining, primarily due to the valuable metals embedded within.
In general, waste circuit boards are usually made of 40% metals and 60% resin fiber (source)
The current processes typically involve the following steps:
1. Mechanical Pre-Treatment
- Description: The initial phase involves dismantling the PCBs, often by mechanical means like shredding or crushing. The goal is to reduce the size of the PCBs and separate different material components, such as plastic, ceramics, and metals.
- Efficiency: Separation processes like eddy current separation and magnetic separation can recover around 70-90% of ferrous and non-ferrous metals.
- Challenges:
- It’s difficult to efficiently separate fine metal particles.
- Losses in metal recovery occur if components like gold or palladium are not properly segregated during pre-treatment.
Note: some systems appear to be mechanical already, such as this PCB Recycling Machine that does Shredding > crushing > sieving > air separation > electrostatic separation. There is a pretreatment step to get electronic components removed from PCB boards, before the boards put into PCB recycling machine for further crushing and separation.
Another example of a mechanical separation system is the one offered by 3E. The metal recovery rate: 93-98%. The average metal content for the non-metal powder: ≤2-5%
2. Pyrometallurgical Processing (Smelting)
- Description: This is a commonly used method, where crushed PCBs are fed into a smelter at high temperatures (around 1,200°C). The organic materials are burned off, and metals are separated based on their melting points.
- Efficiency: Pyrometallurgical processes can recover over 95% of copper, 90% of gold, and a significant portion of other valuable metals like silver and palladium.
- Challenges:
- High energy consumption, resulting in a high carbon footprint.
- Losses of minor metals like tin, antimony, and some rare earth elements (REEs).
- Generation of toxic byproducts, including dioxins and heavy metal emissions.
3. Hydrometallurgical Processing
- Description: This process uses chemical solutions to leach metals from the shredded PCBs. Cyanide or aqua regia is often used for gold recovery, while sulfuric acid can be used for copper.
- Efficiency: Gold recovery can exceed 95%, and copper recovery is often above 80%. However, this process is less effective for REEs and precious metals like palladium or platinum.
- Challenges:
- Toxic chemicals, especially cyanide, pose significant environmental risks if not managed properly.
- The process can be slow, and yields for some metals are lower than pyrometallurgical methods.
- Difficulty in selectively leaching metals without contaminating the solution with unwanted materials.
4. Bioleaching
- Description: This emerging method uses bacteria or fungi to leach metals out of PCBs. Certain microorganisms can metabolize metal ions, creating a more environmentally friendly alternative to chemical leaching.
- Efficiency: Bioleaching is still under development, but current recovery rates for metals like copper and gold range between 60-90%, depending on the metal and the microorganism used.
- Challenges:
- Longer processing times compared to other methods (weeks to months).
- Low recovery rates for certain high-value metals like palladium or platinum.
- Limited scalability and lack of industrial adoption.
5. Electrometallurgy (Electrowinning)
- Description: Following hydrometallurgical processes, metal ions dissolved in solution are plated onto electrodes through electrowinning, recovering metals like copper, gold, and silver.
- Efficiency: Electrowinning can recover more than 90% of some metals, particularly copper and gold.
- Challenges:
- High energy demand.
- Difficulty in separating metals with similar electrochemical properties.
- The process does not handle certain rare earth metals well.
Overall Process Efficiency:
- Copper: 85-95% recovery (combined methods).
- Gold: 90-95% recovery.
- Silver: Around 90%.
- Palladium and Platinum: 50-80%, depending on method.
- REEs (Rare Earth Elements): Less than 50% recovery in most methods.
Primary Challenges Today:
- Energy Intensity: Many methods, especially pyrometallurgy, require high energy inputs, leading to high operational costs and carbon emissions.
- Toxic Byproducts: Smelting and chemical leaching can release hazardous gases or liquids, requiring advanced systems for managing waste.
- Selective Recovery: Some methods struggle to efficiently recover all metals, particularly minor and rare earth elements, leading to lower overall yield.
- Complexity of Waste: The heterogeneity of PCBs, with various metal types and concentrations, complicates efficient processing.
- Environmental Concerns: The use of hazardous chemicals in hydrometallurgical processes and the toxic emissions from pyrometallurgy raise concerns about pollution.
- Economic Viability: Metal recovery from PCBs is often less profitable due to the fluctuating value of metals and the expensive processes required for extraction.
These challenges are driving innovation in more sustainable and efficient processes, like bioleaching and advanced mechanical separation, though they are still in development phases.
Open questions
- Is there truly a massive global problem when it comes to scrap PCBs?
- Mercury from batteries, LCDs, and some switches
- If so, what is the root cause of this problem?
- What do people do with the mixed metal powder that comes out of physical separation machines?
- After milling, the particles are between 1 and 4 mm. Most of the metal is in the smaller particles.
- The mixed metal powder typically contains hazardous substances.
- Many valuable metals are not contained in the mixed metal powder — they are instead in the mixed resin powder.
- It’s hard to recover Nd and Y from e-waste.
- Is gold really the primary value driver, or is it copper? See conflicting answers from BCG vs. Global Waste Monitor p. 14.