30-04-2012, 04:33 PM
Lead and Zinc Smelting
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Industry Description and Practices
Lead and zinc can be produced pyrometallurgically
or hydrometallurgically, depending on
the type of ore used as a charge. In the pyrometallurgical
process, ore concentrate containing
lead, zinc, or both is fed, in some cases after sintering,
into a primary smelter. Lead concentrations
can be 50–70%, and the sulfur content of
sulfidic ores is in the range of 15–20%. Zinc concentration
is in the range of 40–60%, with sulfur
content in sulfidic ores in the range of 26–34%.
Ores with a mixture of lead and zinc concentrate
usually have lower respective metal concentrations.
During sintering, a blast of hot air or oxygen
is used to oxidize the sulfur present in the
feed to sulfur dioxide (SO2). Blast furnaces are
used in conventional processes for reduction and
refining of lead compounds to produce lead.
Modern direct smelting processes include QSL,
Kivcet, AUSMELT, and TBRC.
Primary Lead Processing
The conventional pyrometallurgical primary lead
production process consists of four steps: sintering,
smelting, drossing, and refining. A feedstock
made up mainly of lead concentrate is fed into a
sintering machine. Other raw materials may be
added, including iron, silica, limestone flux, coke,
soda, ash, pyrite, zinc, caustic, and particulates
gathered from pollution control devices. The sintering
feed, along with coke, is fed into a blast
furnace for reducing, where the carbon also acts
as a fuel and smelts the lead-containing materials.
The molten lead flows to the bottom of the
furnace, where four layers form:
Waste Characteristics
The principal air pollutants emitted from the processes
are particulate matter and sulfur dioxide
(SO2). Fugitive emissions occur at furnace openings
and from launders, casting molds, and ladles
carrying molten materials, which release sulfur
dioxide and volatile substances into the working
environment. Additional fugitive particulate
emissions occur from materials handling and
transport of ores and concentrates.
Pollution Prevention and Control
The most effective pollution prevention option
is to choose a process that entails lower energy
usage and lower emissions. Modern flash-smelting
processes save energy, compared with the
conventional sintering and blast furnace process.
Process gas streams containing over 5% sulfur
dioxide are usually used to manufacture sulfuric
acid. The smelting furnace will generate gas
streams with SO2 concentrations ranging from
0.5% to 10%, depending on the method used. It
is important, therefore, to select a process that
uses oxygen-enriched air or pure oxygen.
Monitoring and Reporting
Frequent sampling may be required during startup
and upset conditions. Once a record of consistent
performance has been established,
sampling for the parameters listed in this document
should be as described below.
Air emissions should be monitored continuously
for sulfur dioxide and particulate matter.
Other air emissions parameters should be monitored
monthly. Fugitive emissions should be
monitored annually.