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WATER
TREATMENT - HEX CHROME-1
Remco Notes:
IX - APP #3 - Hexavalent Chrome Recovery
Ion Exchange is used to remove hexavalent Chrome from groundwater,
plating bath rinses, chromate
conversion coating rinses and many other areas where it is more
profitable to recover the
Chrome than it is to sludge it. The recovery process is relatively
simple and effective.
Chrome+6 in effluent water is very low (non detectable) when influent
concentration is less than
20 ppm and the flow rate is at 1 gpm/cu.ft of resin/minute.
The Cr+6 is captured on an anion exchange resin. Cr+3 is captured
on a cation resin (see chrome+3 page).
The resin removes the metal ion by exchanging it for a non-hazardous
ion on the resin
bead. Macroporous resin beads have up to 10 sq. meters of surface
area per gram.
Chrome+6 can be exchanged on either a strong base resin or a weak
base resin. The
resin choice is based on the operating stream parameters. If the
salt level is low and the
hex-Chrome is in an acid form, a simple weak base resin can be
used. When
regenerated with a base such as caustic soda, Sodium chromate
is recovered. This can
be recycled or converted to Chromic acid by another pass through
a different resin
column. Weak base resins generally work best at a pH of 0-4. In
certain situations, a
strong cation exchanger is placed in front of the weak base column
to convert salts to
acids.
The actual effluent level of Chrome+6 is dependent on several
factors. These include the
following parameters:
1. The amount of unreacted resin in the column. If the column
is near exhaustion, the
bed will "leak" more Chromium ions. When the leakage
approaches the maximum
allowable, the column is regenerated. A second identical column
behind the first is
sometimes used to catch leakage. The first column can then be
operated longer. When
regenerated, the column order is switched. This is called lead/lag
operation.
2. The total amount of TDS (total dissolved solids) in solution.
If the salt content in the
water is high, it competes with the chromium for resin sites and
the leakage of Chrome is
higher. This is one of the reasons why it's beneficial to break
off individual rinses before
they are combined and treated. The less TDS in the water, the
easier it is to remove a
specific ion.
3. The pH of the solution. This affects the overall efficiency
of the operation. Each resin
has a optimum operating point where the maximum number of sites
are filled. As the pH
is changed, the capacity of the resin varies. The capacity of
the column, relates to
operating time between regenerations. Sometimes systems are run
at a pH which does
not optimize the total capacity but optimizes the capture of a
specific ion or the passing of
an undesirable ion.
Basic configurations -
1. The simplest recovery configuration is a single column or a
lead/lag pair of columns
containing a weak base resin, recirculating water to a rinse tank.
This works well if the pH
stays low in the rinse (after Chrome plating) or the preceding
bath is mainly chromic acid.
2. The next level of recovery is to place a strong acid resin
in front of the weak base to
exchange cations for hydronium ions. This lowers the pH and allows
the weak base to
exchange the chromate ion. The strong acid resin will remove Iron,
Chrome+3, Ni+2, Na
and other cations. The only thing that is not removed would be
weak bases such as
Carbonates, Silica, and free Cyanide.
3. The third level of recovery adds a strong base after the weak
base to remove
Carbonate and Silica and will "polish" any residual
chromate passing through the weak
base. This system will provide DI quality if recirculated.
Regeneration -
Weak base resins can be eluted (ion recovery) with a salt (sodium
or potassium chloride)
solution. The resin then can be regenerated with another anion
if a non chloride form of
the resin is desired.
Strong base resins are usually eluted with a Caustic soda (NaOH)
solution and are run in
the hydroxyl form.
Strong Acid resins are eluted with a mineral acid such as HCl
or H2SO4 and run in a
chloride or sulfate form. If calcium or magnesium is present,
elution with HCl is preferred.
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