Page 668 - Materials Chemistry, Second Edition

CAT3525_C22.qxd 1/27/2005 1:00 PM Page 639
Electronics Waste 639
22.6.5 METALS
Once electronic devices and components have been collected and segregated, recoverable metals are
extracted through one of three processes (MOEA, 1995):
● Mechanical reclamation involves shredding and grinding electronic appliances and com-
ponents into a fine powder. Metals are then separated from the powder. The powder is
managed by means of recycling or energy reclamation.
● Chemical reclamation (hydrometallurgy) uses a bath to dissolve components and their
packaging. Metals are then extracted chemically.
● Thermal reclamation (pyrometallurgy).
The technology selected depends on several factors. The choice is typically driven by which
technology will provide the best return on investment. Total value extracted from electronics scrap
can range from hundreds to thousands of dollars per ton depending on the kind of scrap collected
and the technology used to reclaim metals (MOEA, 1995).
Precious metals such as gold, platinum, silver and palladium, and copper and steel-bearing mate-
rials are often sent to a smelter for recycling. Smelting is defined as a high-temperature metallurgical
process in which mineral concentrates and fluxes such as silica and limestone are processed in one or
more steps to separate molten metal from impurities. In terms of electronics wastes, Pb (from CRT
tubes), copper (CRT yokes), and gold (circuit board fingers) are all recoverable by smelting. Some
demanufacturing facilities are equipped with the appropriate furnaces for gold and silver recovery.
Lead is recovered in one of two types of smelters. Primary smelters process sulfide concen-
trates, sulfates, oxides, and metallic scrap. Primary smelting requires a sulfur (S) removal process
such as a sinter or acid plant process to capture S and convert it into H SO . Secondary smelters
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process only Pb oxides and metallic scrap. The secondary production of Pb begins with recovery of
scrap from obsolete or damaged CRTs and new scrap composed of product wastes and smelter-
refinery residues (MFF, 2002).
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During smelting, an oven heats the Pb to a temperature of 1260 C (2300 F). Silica is used as a
fluxing agent to separate molten Pb from impurities, which form a slag. CRT glass can be used in
the blast furnace as a fluxing agent. Scrap materials and fluxing agents are placed in a blast furnace
where coke is burned at high temperatures. Carbon contained in the coke reacts with the hot air to
form gases that chemically reduce the feeds to molten Pb and slag. The fluxing agents, limestone,
and iron form a slag that floats to the top of the molten bath. Lead bullion and the slag flow out the
bottom of the furnace in a continuous molten stream where the slag is separated from the Pb bul-
lion. The molten Pb collects in pots where it is allowed to cool. A residue (“dross”) forms on the
surface of molten Pb as it cools. This dross is removed from the surface and the Pb bullion is fur-
ther processed in a drossing furnace. The molten slag is tapped from the furnace and granulated.
Once the impurities have been removed, the refined Pb is ready to cast (MFF, 2002).
The copper yoke on the CRT possesses positive scrap value and is also recovered in the smelt-
ing process. A CRT contains 0.9 to 2.3 kg (2 to 5 lb) of copper on the yoke. During copper smelt-
ing the feedstock is dried and fed into one of several different types of furnaces. There the sulfide
minerals are partially oxidized and melted to yield a layer of “matte” which is a mixed copper-iron
sulfide, and the slag, which is an upper layer of waste. The matte is further processed by a method
known as converting. The slag is tapped from the furnace and stored or discarded. Another product
of the smelting process is SO which is collected, purified, and converted to H SO for sale. Iron
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and S are removed during the converter process and the remaining molten copper is refined to pro-
duce high purity copper (MFF, 2002).

22.6.6 CIRCUIT BOARDS
The PC circuit board contains copper and very small amounts of gold and other precious and semi-
precious metals that give the circuit board much of its scrap value. Printed circuit boards can be