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Zinc
Bearing Wastewater Treatment
System
Process Description
Zinc is most often found in plating and galvanizing operations.
In plating shops, the zinc is often complexed with cyanide and the
cyanide must be treated to free the zinc before precipitation can
occur. Traditional cyanide destruct systems use sodium hypochlorite
to oxidize the cyanide. Like
copper, zinc can be precipitated as the hydroxide
salt. Finally, it can be removed by ion exchange in methods similar
to copper.
Precipitation of the insoluble hydroxide salt is the most common
form of treatment. This salt is formed by adjusting the pH of the
water to about 10-10.5 to form the precipitate. If cyanide is
present, it must be pre-treated before entering the Hydro-Flo
Technologies
treatment system. When other complexing agents are present,
Hydro-Flo engineers
can design a treatment system using metal trapping chemistry.
Ion exchange can be used to remove zinc from wastewater.
Hydro-Flo ion exchange systems are designed to
treat plating rinse water with trace amounts of metals. The water is
sent through cation and anion resin beds, along with activated
carbon and/or media filtration to produce deionized water that can
be returned to the process. The advantage offered by Hydro-Flo ion exchange
systems is that the resin is regenerated on-site, eliminating the
need for bottle haul off. Since the regenerate waste will contain
any copper and other metals removed during treatment, a vacuum
distillation system can be used to concentrate the regenerate even
further to reduce the amount of liquid waste hauled away. The
purified water from the vacuum distillation system can also be
returned to the process.
The typical method to remove zinc
from wastewater is as follows:
Stage 1 Precipitation:
pH is adjusted upward to a pH of 8-9.5 to the optimum chrome
hydroxide precipitation point. Often, a coagulant such as ferric
sulfate is added to enhance metal co-precipitation and the formation
of “pin floc”.
Stage 2 Flash mix:
The wastewater with it’s precipitated pin
floc is introduced to the flash mix zone where a polymer flocculent
is added. This stage maximizes flocculent dispersion throughout the
coagulated wastewater.
Stage 3 Flocculation:
The wastewater is now introduced to the slow mix zone to agglomerate
the pin floc into larger rapid settling particles.
Clarifier, Inclined Plate:
The flocculated wastewater is introduced into the clarifier where
the settling particles accumulate in the sludge chamber. The
clarified water then exits the clarifier and flows downstream to
sewer or further treatment if necessary.
Clarifier Sludge Handling:
The accumulated sludge is periodically removed from the clarifier
and sent to a sludge holding tank where it further thickens for
disposal or dewatering.
Sludge Dewatering:
Sludge dewatering is typically handled by a Filter Press. After
processing a batch of "sludge" the filter press is emptied of
“chrome cake” which is a semi solid of approximately 20-35 %
solids. Chrome cake is high in chrome and sulfite and should be
disposed of according to environmental regulations.
APPLICATIONS:
Plating shops are found in typically two
categories, captive and independent shops. Some industries operate
their own captive, in house plating operation while others outsource
to an independent plating operation.
Typical industries include:
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