allows the computer parts to communicate and receive power, and the CPU is the
section that carries out the calculations
needed to run a computer.
Silver is also used as a conductor
in circuit boards. It is the best electrical conductor, and it is used in switches,
circuit breakers, and for wiring.
When electronic devices end up in landfills, the metals they contain can leach into
water or be released into the air. If this
happens, the metals could cause environmental hazards or serious harm to human health.
But when computers are recycled, these metals can be separated and reused without being
released into the environment.
When electronics, such as computer
monitors, motherboards, and keyboards, are
recycled, they are taken apart and separated
into plastic cases and other components.
Then, circuit boards, materials from inside a
monitor or keyboard, and hard drives are usually chopped up, and the metal components
are separated from the plastics.
Iron pieces can easily be separated by
magnets. The other metals can be separated
by hand or by using a device called an eddy
current separator (Fig. 2). The metal pieces
are passed along a conveyor belt, at the end of
which is a rapidly rotating permanent magnet.
Non-metallic substances, such as plastics,
are unaffected by the magnet and just drop
off at the end of the belt. Non-magnetic met-
als experience loops of current due to the
changing magnetic field as the metals move
through the magnet. These loops of current
themselves produce their own magnetic field,
which opposes the external field and repels
the metallic bits from the magnet, so they
actually fly off the conveyor belt and can be
collected in a separate bin.
Precious and heavy metals can be recovered from electronic waste by dissolving the
metals with a strong acid. Then, the metals
are usually recovered from the solutions by
electrolysis: An electric current is passed
through the solution, using an inert anode and
a cathode. Electrons that flow through the circuit to the cathode are delivered to the metal
ions in solution, reducing them to the solid
metal on the cathode. The cathode can then
be removed and the deposit isolated.
For example, silver can be recovered by dissolving it in nitric acid (HNO3):
Ag + 2 H+ + 2 NO3–
➞ Ag+ + NO3– + H2O + NO2
Then, the silver ions are reduced to solid
silver through electrolysis:
Ag+ + e– ➞ Ag
To recover gold, two processes are used. One process
uses cyanide ions (CN–):
4 Au + 8 CN– + O2 + 2 H2O
➞ 4 [Au(CN) 2]– + 4 OH–
Then, the [Au(CN) 2]– ions are
reduced through electrolysis,
enabling the recovery of gold:
[Au(CN) 2]– + e– ➞ Au + 2 CN–
The other process to recover
gold involves a strong acid known
as aqua regia, which is a mixture
of concentrated nitric and hydro-
chloric acids—typically three
parts hydrochloric acid to one part nitric acid.
The name means “royal water,” as it is the
only acid that can dissolve gold—the “metal
The gold is oxidized by nitric acid (HNO3),
while the hydrochloric acid (HCl) provides
chloride ions to form a complex ion. The pro-
cess dissolves the gold off of a circuit board
or connector, as follows:
Au + 3 NO3– + 4 Cl– + 6 H+
➞ AuCl4– + 3 NO2 + 3 H2O
Then, the AuCl4– ions are reduced through
electrolysis, allowing the recovery of gold:
AuCl4– + 3 e– ➞ Au + 4 Cl–
The chemical processes are accomplished
in facilities with covered vats of chemicals,
blower-and-hood systems, as well as dust-
catching equipment. The acids used in the
process are cleaned and reused instead of
being released into the environment. A D
Figure 2. An eddy current separator uses a permanent magnet
that spins rapidly inside a non-metallic drum driven by a
conveyor belt. As non-magnetic metals pass over the drum, the
magnetic field creates eddy currents in these metals, causing
them to be repelled from the conveyor belt and to be propelled
forward inside a bin, where they are separated from the non-metallic materials.