How to Remove Metals From Printed Circuit Boards

What Metals Are Found in PCBs?

A printed circuit board contains a mix of metals, with the composition varying depending on the type of PCB. Some of the key metals found in PCBs include:

Metal	Typical Weight % in PCB
Copper	10-30%
Solder (tin/lead alloy)	1-5%
Nickel	1-5%
Gold	0.1-1%
Silver	0.1-1%
Palladium	0.005-0.1%

While copper makes up the largest portion, the trace amounts of gold, silver, and palladium are often the most valuable to reclaim, as they are precious metals with high market values. A ton of mobile phone PCBs can contain 300-350 grams of gold.

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The PCB Metal Recycling Process

The process of recycling metals from PCBs involves several key steps:

Collection and sorting
Dismantling and shredding
Mechanical separation
Chemical separation and purification
Melting and casting recycled metals

1. Collection and Sorting PCBs

The first step is collecting discarded PCBs from e-waste and sorting them by type. PCBs can be separated into three main categories:

High-grade PCBs (from phones, computers)
Medium-grade PCBs (from TVs, monitors)
Low-grade PCBs (from home appliances, toys)

High-grade boards tend to have the greatest concentrations of precious metals. Careful sorting ensures a more efficient recycling process downstream.

2. Dismantling and Shredding

Next, any batteries, capacitors or hazardous components are removed. The Bare PCBs are then fed into a shredder machine and chopped into small, uniform pieces typically 1-2 cm in size. Shredding the PCBs helps liberate the metals from the boards’ fiberglass or plastic base.

3. Mechanical Separation

The shredded PCB fragments then go through a multi-step mechanical separation process to concentrate the metals. Key separation methods include:

Magnetic separation: Powerful magnets pull out ferrous metals like steel and iron.
Density separation: Metals are much denser than plastics. Using shaker tables or centrifuges, the plastic pieces can be separated out, leaving behind metallic fractions.
Electrostatic separation: Metals are conductive while plastics are not. Passing the shredded mix through an electrostatic field separates the two.

After mechanical separation, a concentrated metal powder consisting of copper, gold, silver, palladium, etc. is produced. This powder then undergoes chemical treatment.

4. Chemical Separation and Purification

To further separate and purify individual metals, chemical processes are used, primarily leaching. In leaching, the metal powder is mixed with chemical reagents that selectively dissolve certain metals into a solution while leaving others as solids.

Common leaching methods used in PCB Recycling include:

Leaching Process	Reagents	Metals Extracted
Acid leaching	Sulfuric acid, nitric acid	Copper, nickel, tin, lead
Cyanide leaching	Sodium cyanide	Gold, silver, palladium
Halide leaching	Iodine, bromine	Gold

Once in solution, individual metals can be precipitated out by adjusting pH, separated using solvent extraction, or collected on a solid substrate through electrowinning. Repeating cycles of leaching and separation allows individual metals to be purified to high levels.

5. Melting and Casting

Finally, the separated metals (now in powder or sponge form) are melted in a high-temperature furnace and cast into ingots or bars. The result is high-purity recycled metals ready to be sold and used in the manufacture of new products.

The Benefits of PCB Recycling

PCB recycling offers several important benefits:

1. Resource conservation

Recycling metals from PCBs reduces the need for environmentally-destructive mining of virgin ores. It takes 90% less energy to recover metals through recycling compared to mining.

2. Waste reduction

E-waste is a rapidly growing waste stream, with 50 million tons generated globally per year. Recycling PCBs keeps them out of landfills where they can leach toxic substances.

3. Economic value

The value of the metals in PCBs is significant. A ton of computer PCBs can contain over $10,000 worth of metals at current prices. Recycling creates a secondary supply for these valuable commodities.

Frequently Asked Questions

What is the metal composition of a typical PCB?

Copper: 10-30%
Solder: 1-5%
Nickel: 1-5%
Gold: 0.1-1%
Silver: 0.1-1%
Palladium: 0.005-0.1%

How much gold is in a ton of mobile phone PCBs?

Approximately 300-350 grams, which is 10-12 troy ounces. At a gold price of $1800 per ounce, that’s $18,000-$21,000 worth of gold per ton of phone PCBs.

What are the main steps in PCB recycling?

Collection and sorting
Dismantling and shredding
Mechanical separation
Chemical separation and purification
Melting and casting recycled metals

What chemicals are used to extract metals from PCBs?

Sulfuric acid, nitric acid, sodium cyanide, iodine, and bromine are some of the key reagents used to selectively leach metals from shredded PCB powder.

How much energy is saved by recycling metals vs. mining?

Recycling metals from PCBs uses 90% less energy compared to extracting those metals through mining of virgin ores. Recycling helps conserve energy and reduce carbon emissions.

Conclusion

Printed circuit boards are a rich secondary source for valuable metals like copper, gold, silver and palladium. Developing efficient and environmentally-sound methods for recovering these metals from PCBs is an important part of tackling the growing global e-waste problem. Through collection, shredding, separation, and purification, these metals can be effectively recycled and re-used, providing a sustainable secondary supply. As the use of electronics continues to proliferate, PCB recycling will play an increasingly vital role in conserving precious metal resources and reducing toxic e-waste.

How to Remove Metals From Printed Circuit Boards

Published by Prototype PCB Assembly on June 1, 2024

What Metals Are Found in PCBs?