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The primary use of ferric chloride is to remove impurities in water and wastewater
treatment. The second-largest application is in the production of printed circuit
boards (PCBs).
Ferric chloride’s popularity stems from its physical as well as its economic
properties. Ferric chloride’s rapid hydrolysis in water makes it an ideal
flocculating and precipitating agent. The ferric hydroxide (Fe[OH]3) produced
forms flocs (small clumps or tufts) that adsorb suspended particles of various
materials (e.g., colloids, clays and bacteria). The clumps, with the adsorbed
matter, are then allowed to settle out for removal. Ferric chloride forms precipitates
with hydrogen sulfide (H2S), phosphate (PO4), arsenic as arsenate (AsO4) and
hydroxide alkalinity (OH). Ferric chloride is highly cost-effective. Though
it is not the cheapest water purification material (ferrous salts are usually
less expensive, and alum [aluminum sulfate] is comparatively priced), on a
per-pound basis ferric chloride delivers better cost performance. Typically,
one can expect to use about 30% less ferric chloride than aluminum sulfate
(on a dry weight basis) to achieve similar results. This favorable cost/performance
has allowed ferric chloride to gain market share. Ferric chloride is one of
the few water treatment chemicals that can sequester odors. Unlike hydrogen
peroxide and potassium permanganate, the ferric chloride (and sulfate) reaction
is irreversible.
The following pie chart shows world consumption of ferric chloride:
Ferric chloride competes with ferric sulfate, aluminum salts such as aluminum
sulfate (alum), sodium aluminate, polyaluminum chlorides, and organic polymers
in the treatment of raw water and wastewater. The decision to use ferric chloride
is highly dependent on location to the source, as transportation costs can
be a large part of the delivered cost.
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