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Copper can be found in
many wastewater sources including, printed circuit board manufacturing,
electronics plating, plating, wire drawing, copper polishing, paint
manufacturing, wood preservatives and printing operations. Typical
concentrations vary from several thousand mg/l from plating bath waste to less
than 1 ppm from copper cleaning operations.
Copper can be removed from wastewater by precipitation as an insoluble salt,
precipitation as metallic copper and by ion exchange.
Precipitation of the insoluble salt is the most common form of treatment. The
most common salt precipitated is insoluble cupric hydroxide which is formed by
adjusting the pH of the water to about 9 to form the precipitate. The resulting
precipitate leaves about .1 mg/l of dissolved copper in solution if chelates are
absent. If chelates are present, the final copper concentration can be much
higher.
Other insoluble salts of copper include, cupric carbonate, cupric phosphate and
cupric sulfide. The residual copper concentrations of these precipitates are
10ug/l, 1 ng/l and less than 1 fg/l respectively.
When using these salts, it is necessary to maintain the anion concentration at
high enough levels for precipitation to occur and adjusting the pH so the anion
is available to precipitate the copper. Since the anions are also chelate
copper, any residual treatment anion must be removed from solution by an
additional treatment step. This step adds another cation to form a precipitate
of the residual anion. Calcium is suitable for removing the residual carbonate
and phosphate and ferrous ion is suitable for removing residual sulfide. Thus
this process is a two step process which is more complicated than the single
step copper hydroxide process. However, precipitation by this method can reach
very low copper concentrations.
Copper can be also precipitated as insoluble copper metal. By using a reducing
agent to adjust the ORP to less than ఠmv, copper levels of less than 1 ppb
can be acheived. Typical reducing agents used include, ferrous sulfate, sulfide,
sulfite and carbon. The reduction is done best in the presence of a high surface
area such as diatomaceous earth, carbon or sand to form sites on which the
copper can plate out on. If these surfaces arenలesent, the copper plates out
in solution as a fine metallic suspension which is very difficult to settle or
filter.
When using a reducing agent, the metallic suspension often is seen as high
copper residuals. To remove these residuals, it is necessary to oxidize the
copper back to the ionic state and re-precipitate the copper. This method is
especially necessary when trying to achieve copper levels of less than 1 ug/l.
Ion exchange can be used to remove copper from wastewater. Normally a weak base
cation resin in the sodium form is effective for this treatment. But, anion
resin can also be used if the copper is chelated with a soluble anion or the
anion resin itself can be doped or spent with the desired anion to remove the
copper. This method is useful in selectively removing copper in the presence of
other metals.
The equipment normally used for these processes is a batch treatment system for
high copper concentrations (greater than 1000 mg/l), continuous precipitation if
the copper concentrations are less than 1000 mg/l and the volume is greater than
5000 gpd. Ion exchange or copper reduction is feasible if concentrations are
less than 20 mg/l and the desired effluent requires low copper concentrations,
less than 20 ug/l.
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