vvEPA
United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
I
Cincinnati OH 45268
Technology Transfer
Summary Report
Control anjd Treatment
Technology for the
Metal Finishing Industry
Ion Exchange
-------
-------
Technology Transfer
EPA 625/8-81-007
Summary Report
i
i
Control and; Treatment
Technology for the
Metal Finishing Industry
Ion Exchange
June 1981
i.
This report was developed Ipy the
Industrial Environmental Research Laboratory
Cincinnati OH 45268
-------
Environmental research and development in the metal finishing industry is
the responsibility of the Nonferrous Metals and Minerals Branch, Industrial
Environmental Research Laboratory, Cincinnati OH. The U.S. Environmental
Protection Agency hired the Centec Corporation, Fort Lauderdale FL and
Reston VA, to prepare this report. Roger C. Wilmoth is the EPA Project Officer.
Requests for further information can be addressed to:
Nonferrous Metals and Minerals Branch
IERL-USEPA
Cincinnati OH 45268
EPA thanks the following companies and organizations for providing
information and technical review: American Electroplaters' Society; Best
Technology, Inc., Villa Park IL; Dow Chemical Company, Midland Ml; Institute
of Precision Mechanics, Warsaw, Poland; Raytheon Ocean Systems
Company, Portsmouth Rl; and Rohm and Haas Co., Philadelphia PA.
Photographs were supplied by Best Technology, Inc., of Villa Park IL and
Eco-Tec Limited of Toronto ON.
This report has been reviewed by the Industrial Environmental Research
Laboratory, U.S. Environmental Protection Agency, Cincinnati OH, and
approved for publication. Approval does not signify that the contents
necessarily reflect the views and policies of the U.S. Environmental Protection
Agency, nor does mention of trade names or commercial products constitute
endorsement or recommendation for use.
COVER PHOTOGRAPH: Reciprocating Flow Ion Exchanger used for chromic
acid recovery.
-------
Overview
Ion exchange is a versatile
separation "process with potential
for broad application in the metal fin-
ishing industry, both for raw
material recovery and reuse and
for water pollution control. Three
major areas of application have been
demonstrated: j
• Wastewater purification and
recycle
• End-of-pipe pollution control
• Chemical recovery
Although the idn exchange process
has been commercially available
for many years,! widespread interest
in its use for m'etal finishing pol-
lution control h|as developed
only recently. '
The main impetus for the interest in
ion exchange technology is the
broad range of jresins manufactured
today. With pro'per resin selection,
ion exchange can provide an
effective and economical solution to
pollution control requirements.
As a further stimulus to the use of
the process, the metal-bearing
sludge generated by hydroxide treat-
ment systems is considered a
hazardous material and must be
disposed of in pn environmentally
safe manner. The ion exchange
process can concentrate the heavy
metals in a dilgte wastewater into
a concentrated jmetal solution that is
more amenable, to metal recovery
than is a sludge, and this ability
should lead to more widespread use
of the technology.
This summary ijeport is intended
to promote an understanding of the
use of ion exchange in the metal
finishing industry. The sec-
tions that follov/v discuss ion ex-
change process theory in general
and evaluate ea'ch of the three major
areas of application in terms of
performance, state of development,
cost (in 1980 dpllars), and operating
reliability. j
Water Purification and Recycle
In the first area of application,
mixed rinse solutions are deionized
to permit reuse of the treated
water. The contaminants in the
rinses are concentrated in the small
volume purge streams, and are
thereby made more economical to
treat.
Because ion exchange is efficient
in removing dissolved solids
from normally dilute spent rinse
waters, it is well suited for use
in water purification and recycle.
Most of the plating chemicals, acids,
and bases used in metal finishing
are ionized in water solutions
and can be removed by ion ex-
change. Several factors make the ion
exchange process effective for
this application:
• Ion exchange can economically
separate dilute concentrations of
ionic compounds from water
solutions.
• The process can consistently
provide high purity water over a
broad range of loading conditions.
• The resins used for separation
are durable in severe chemical en-
vironments.
Application of the ion exchange
process in a wastewater purification
and recycle system will signifi- -
cantly reduce water consumption
and the volume of wastewater
discharged, thus reducing water use
and sewer fees and the size and
cost of the pollution control system.
Also, for plants that discharge
wastewater directly to waterways
and that are regulated by mass-
based pollutant discharge limits, the
reduction of discharge volume
will allow for higher concentrations
of pollutants in the discharge
and facilitate compliance with these
limits.
-------
Ion exchange acid purification unit used for sulfuric acid anodizing solutions
End-of-Pipe Pollution Control
In the second application, toxic
heavy metals and metal cyanide
complexes are removed selectively
from combined waste streams
before discharge. The key to this ap-
plication is that the ion1 exchange
resins remove only the toxic
compounds and allow the nontoxic
dissolved ionic solids to remain
in solution.
The ion exchange process can be
used in two different forms for
end-of-pipe pollution control: it has
been demonstrated as a means
of polishing the effluent from
conventional hydroxide precipitation
to lower the metal concentration
in the discharge; it has also been ap-
plied as a means of directly treat-
ing wastewaters to remove heavy
metal and metal cyanide pollutants.
Most plating shops can remove
sufficient metal to comply with
wastewater discharge regulations
using the conventional hydroxide
precipitation process. Where
unusually strict limits are placed on
the effluent metal concentration,
however, or where the metals
are complexed with chemical con-
stituents that interfere with their
precipitation as metal hydroxides,
conventional treatment may
not be reliable for compliance with
the discharge limits. Ion exchange
can be used in such cases to
polish the effluent from the conven-
tional treatment and reduce the
metal concentration further. In this
application, the process can provide
a relatively inexpensive means
of upgrading system performance for
compliance with the discharge
regulations.
Ion exchange has been used to a
limited extent to remove toxic pol-
lutants selectively from an untreated
wastewater while allowing most of
the nontoxic ions to pass through.
Approaches employed to facil-
-------
itate this application include
using:
• Weak acid cation resin in an
application of the wastewater-
softening type to remove
heavy metals and other divalent
cations from a- wastewater
solution with a high concentra-
tion of sodium ions
• Heavy-metal-selective weak acid
or chelating cation resin to
remove only the heavy metal
ions while allowing sodium, cal-
cium, and magnesium ions to
pass through
• A stratified bed of resin containing
strong and weak acid cation
and strong base anion resins to
remove heavy metal and metal cy-
anide complex ions from solu-
tion while allowing most of
the wastewater ionic constituents
to pass through
In each of these approaches,
wastewater pretreatment entails pH
adjustment, to ensure that pH is
within the operating range of the
resin, and filtration, to remove
suspended solids that would foul the
resin bed. The pollutants removed
from the wastewater are con-
centrated in the ion exchange regen-
erant solutions. The regenerants
can be treated in a small batch treat-
ment system using conventional
processes. Firms with access
to a centralized treatment system to
I
dispose of the rpgenerant solu-
tions resulting from treatment would
not need tojnstall chemieal de-
struct systems. In neither case would
it be necessary |to invest in sophis-
ticated pH control systems, floc-
culant feed systems, clarifiers,
and other process equipment asso-
ciated with conventional metal
precipitation systems. And, as a
further advantage, ion exchange
units are compact and easy to
automate compared with conven-
tional precipitation systems.
Chemical Recovery
In the chemical recovery application,
segregated plating rinse waters are
treated to concentrate the plating
chemicals for recycle to the plating
bath. The purified rinse water
is also recycled.'
i
Ion exchange, evaporation, reverse
osmosis, and eIectrodialysis
have all been used in the plating
industry to recover chemicals from
rinse solutions. JThese processes
have in common the ability to
separate specific compounds from
a water solution; yielding a con-
centrate of those compounds
and relatively pure water. The con-
centrate is recycled to the plating
bath and the purified water is
reused for rinsing. Determination of
the separation process best suited
for a particular chemical recovery
application usually requires evaluat-
ing both general and site-specific
factors:
• General factors include rinse
water concentration, volume, and
corrosivity, among others.
• Site-specific factors include, for
example, availability of floor
space and utilities (steam,
chemical reagents, electricity, and
so forth) and the degree of
concentration needed to recycle
the chemicals to the bath.
As a rule, ion exchange systems are
suitable for chemical recovery
applications where the rinse
water feed has a relatively dilute con-
centration of plating chemicals
and a relatively low degree of
concentration is required for recycle
of the concentrate. Ion exchange
is well suited for processing cor-
rosive solutions. Ion exchange has
been demonstrated commercially
for recovery of plating chemicals
from acid-copper, acid-zinc, nickel,
tin, cobalt, and chromium plating
baths. The process has also
been used to recover spent acid
solutions and to purify plating solu-
tions for longer service life.
-------
Basic Concepts
Ion Exchange Reactions
Ion exchange is a reversible
chemical reaction wherein an ion
(an atom or molecule that has lost or
gained an electron and thus acquired
an electrical charge) from solution
is exchanged for-a similarly charged
ion attached to an immobile solid
particle. These solid ion exchange
particles are either naturally
occurring inorganic zeolites or
synthetically produced organic
resins. The synthetic organic resins
are the predominant type used
today because their characteristics
can be tailored to specific appli-
cations. ;
An organic ion exchange resin is
composed of high-molecular-weight
polyelectrolytes that can exchange
their mobile ions for ions of similar
charge from the surrounding
medium. Each resin has a distinct
number of mobile ion sites that
set the maximum quantity of ex-
changes per unit of resin.
Most plating process water is used
to cleanse the surface of the parts
after each process bath. To main-
tain quality standards, the level
of dissolved solids in the rinse
water must be regulated. Fresh water
added to the rinse tank accom-
plishes this purpose, and the
overflow water is treated to remove
pollutants and then discharged. As
the metal salts, acids, and bases
used in metal finishing are pri-
marily inorganic compounds, they
are ionized in water and could be
removed by contact with ion ex-
change resins. In a, water deioniza-
tion process, the resins exchange
hydrogen ions (H+) for the posi-
tively charged ions; (such as nickel,
copper, and sodium), and hydroxyl
ions (OH~) for negatively charged
sulfates, chromates, and chlorides.
Because the quantity of H+ and OH~
ions is balanced, the result of the
ion exchange treatment is relatively
pure, neutral water.
Ion exchange reactions are stoichi-
ometric and reversible, and in that
way they are similar to other
solution phase reactions. For
example:
NiSO4 + Ca(OH)2 ;± Ni(OH)2
+ CaS04 (1)
In this reaction, the nickel ions of the
nickel sulfate (NiS04) are ex-
changed for the calcium ions of the
calcium hydroxide [Ca(OH)2] mole-
cule. Similarly, a resin with hydrogen
ions available for exchange will
exchange those ions for nickel ions
from solution. The reaction can be
written as follows:
2(R-S03H) + NiS04 ;±
(R-S03)2Ni + H2S04
(2)
R indicates the organic portion of
the resin and SO3 is the immobile
portion of the ion active group.
Two resin sites are needed for
nickel ions with a plus 2 valence
(Ni+2). Trivalent ferric ions would
require three resin sites.
As shown, the ion exchange reaction
is reversible. The degree the reac-
tion proceeds to the right will
depend on the resin's preference, or
selectivity, for nickel ions com-
pared with its preference for
hydrogen ions. The selectivity of a
resin for a given ion is measured
by the selectivity coefficient K,
which in its simplest form for the
reaction
R-B+ + A+
(3)
is expressed as: K = (concentration
of B+ in resin/concentration of A+
in resin) X (concentration of A+ in
solution/concentration of B"1" in
solution).
The selectivity coefficient expresses
the relative distribution of the ions
when a resin in the A+ form is
placed in a solution containing B+
ions. Table 1 gives the selectivities
of strong acid and strong base ion
exchange resins for various ionic
compounds. It should be pointed
out that the selectivity coefficient is
not constant but varies with changes
in solution conditions. It does
provide a means of determining
what to expect when various ions
-------
are involved. As indicated in Table 1,
strong acid resins have a preference
for nickel over hydrogen. Despite
this preference, the resin can be
converted back to, the hydrogen form
by contact with a concentrated
solution of sulfuric acid (H2SO4):
(R-S03)2Ni + H2SO4 -<•
2(R-S03H) + NiS04 (4)
This step is known as regeneration.
In general terms, the higher the
preference a resin exhibits for
a particular ion, the greater the
exchange efficiency in terms of resin
capacity for removal of that ion
from solution. Greater preference
for a particular ion, however, will
result in increased consumption of
chemicals for regeneration.
Resins currently available exhibit a
range of selectivities and thus
have broad application. As an exam-
ple, for a strong acid resin, the
relative preference for divalent
calcium ions (Ca+2) over divalent
copper ions (Cu+2) is approximately
1.5 to 1. For a heavy-metal-selective
resin, the preference is reversed
and favors copper by a ratio of
2,300 to 1.
Table 1.
Selectivity of Ion Exchange Resins,
in Order of Decreasing Preference
Strong acid cation
exchanger
Barium
Lead
Calcium
Nickel
Cadmium
Copper
Zinc
Magnesium
Potassium
Ammonia
Sodium
Hydrogen
Strong* base anion
exchanger
Iodide
Nitrate
Bisulfite
Chloride
Cyanide
Bicarbonate
Hydroxide
Fluoride
Sulfate
Resin Types !
Ion exchange resins are classified
as cation exchangers, which
have positively charged mobile
ions available for, exchange, and
anion exchangers, whose exchange-
able ions are negatively charged.
Both anion and cation resins are
produced from the same basic
organic polymers. They differ in
the ionizable group attached to the
hydrocarbon netyvork. It is this
functional group |that determines
the chemical behavior of the resin.
Resins can be broadly classified
as strong or weak acid cation ex-
changers or strong or weak base
anion exchangers.
i
Strong Acid Catibn Resins. Strong
acid resins are so named because
their chemical behavior is similar to
that of a strong acid. The resins
are highly ionized in both the acid
(R-S03H) and salt (R-S03Na) form.
They can convert a metal salt to
the corresponding acid by the reac-
tion:
2(R-S03H) + NiCI2
(R-SO3)2Ni + 2HCI
(5)
The hydrogen and sodium forms of
strong acid resins are highly dis-
sociated and the; exchangeable Na+
and H+ are readily available for
exchange over the entire pH range.
Consequently, th|e exchange
capacity of strong acid resins is
independent of sjolution pH. These
resins would be used in the hydrogen
form for complete deionization;
they are used in the sodium form for
water softening (calcium and
magnesium removal). After exhaus-
tion, the resin is :eonverted back to
the hydrogen form (regenerated)
by contact with a strong acid solu-
tion, or the resin can be converted to
the sodium form iwith a sodium
chloride solution, For Equation 5,
hydrochloric acid (HCI) regeneration
would result in a concentrated
nickel chloride (INIiCI2) solution.
Weak Acid Cation Resins. In a weak
acid resin, the ionizable group is a
carboxylic acid (COOH) as opposed
to the sulfonic acid group (SO3M)
used in strong acid resins. These
resins behave similarly to weak
organic acids that are weakly
dissociated.
Weak acid resins exhibit a much
higher affinity for hydrogen ions than
do strong acid resins. This charac-
teristic allows for regeneration to
the hydrogen form with significantly
less acid than is required for strong
acid resins. Almost complete
regeneration can be accomplished
with stoichiometric amounts of acid.
The degree of dissociation of a
weak acid resin is strongly influ-
enced by the solution pH. Conse-
quently, resin capacity depends
in part on solution pH. Figure 1
shows that a typical weak acid resin
has limited capacity below a pH
of 6.0, making it unsuitable for
deionizing acidic metal finishing
wastewater.
Strong Base Anion Resins. Like
strong acid resins, strong base resins
are highly ionized and can be used
over the entire pH range. These
resins are used in the hydroxide (OH)
form for water deionization. They
will react with anions in solution
and can convert an acid solution to
pure water:
R-NH3OH + HCI -»
R-NH3CI + HOH (6)
Regeneration with concentrated
sodium hydroxide (NaOH) converts
the exhausted resin to the hydroxide
form.
Weak Base Anion Resins. Weak
base resins are like weak acid resins,
in that the degree of ionization is
strongly influenced by pH. Conse-
quently, weak base'resins exhibit
minimum exchange capacity above a
pH of 7.0 (Figure 1). These resins
merely sorb strong acids; they
cannot split salts.
In an ion exchange wastewater
deionization unit, the wastewater
would pass first through a bed of
-------
4 r-
=&
•S.
t
ui
O
1
8
Legend:
weak acid cation resin
weak base anion resin
3456789
SOLUTION pH
SOURCE: Adapted from Schweitzer, P. A., Handbook of Separation Techniques for
Chemical Engineers, New York NY, McGraw-Hill, 1979.
Figure 1.
Exchange Capacity of Weak Acid Cation and Weak Base Anion Resins
as a Function of Solution pH
strong acid resin. Replacement of
the metal cations (Ni+2, Cu+2) with
hydrogen ions would lower the solu-
tion pH. The anions (SO-2, Cl~) can
then be removed with a weak
base resin because the entering
wastewater will normally be
acidic and weak base resins sorb
acids. Weak base resins are pre-
ferred over strong base resins
because they require less regenerant
chemical. A reaction between the
resin in the free base form and
HCI would proceed as follows:
R-NH2 + HCI -» R-NH3CI
(7)
The weak base resin does not have
a hydroxide ion form as does the
strong base resin. Consequently,
regeneration needs only to neutral-
ize the absorbed acid; it need not
provide hydroxide ions. Less
expensive weakly basic reagents
such as ammonia (NH3) or sodium
carbonate can be employed.
Heavy-Metal-Selective Chelating
Resins. Chelating resins behave
similarly to weak acid cation resins
but exhibit a high degree of selec-
tivity for heavy metal cations.
Chelating resins are analogous to
chelating compounds found in
metal finishing wastewater; that is,
they tend to form stable complexes
with the heavy metals. In fact,
the functional group used in these
resins is an EDTAa compound. The
resin structure in the sodium form is
expressed as R-EDTA-Na.
aEthylenediaminetetraacetic acid.
The high degree of selectivity for
heavy metals permits separation of
these ionic compounds from
solutions containing high back-
ground levels of calcium, magnesium,
and sodium ions. A chelating resin
exhibits greater selectivity for
heavy metals in its sodium form than
in its hydrogen form. Regeneration
properties are similar to those
of a weak acid resin; the chelating
resin can be converted to the
hydrogen form with slightly greater
than stoichiometric doses of acid
because of the fortunate tendency of
the heavy metal complex to
become less stable under low pH
conditions. Potential applications of
the chelating resin include polish-
ing to lower the heavy metial
concentration in the effluent from
a hydroxide treatment process, or
directly removing toxic heavy
metal cations from wastewaters
containing a high concentration of
nontoxic, multivalent cations.
Table 2 shows the preference of a
commercially available chelating
resin for heavy metal cations
over calcium ions. (The chelating
resins exhibit a similar magnitude of
selectivity for heavy metals over
sodium or magnesium ions.) The
selectivity coefficient defines
the relative preference the resin
exhibits for different ions. The
preference for copper (shown in
Table 2) is 2,300 times that
Table 2.
Chelating Cation Resin Selectivities
for Metal Ions
Metal ion
KM/Caa
Hg+2.
Cu+2.
Pb+2.
Ni+2.
Zn+2.
Cd"1"2.,
Co+2.,
Fe+2 .,
Win*2.
Ca+2.
2,800
2,300
1,200
57
17
15
6.7
4
1.2
1
"Selectivity coefficient for the metal over cal-
cium ions at a pH of 4.
-------
for calcium. Therefore, when a solu-
tion is treated that contains equal
molar concentrations of copper and
calcium ions, at equilibrium,
the molar concentration of copper
ions on the resin will be 2,300 times
the concentration of calcium ions.
Or, when solution is treated that
contains a calcium ion molarconcen-
tration 2,300 times that of the
copper ion concentration, at
equilibrium, the resin would hold
an equal concentration of copper
and calcium.
Their high cost is the disadvantage
of using the heavy-metal-selective
chelating resins. Table 3 com-
pares the cost of these with the
Table 3.
Cost of Commercially Available
Resins
Resin Cost ($/ft3)
Strong acid cation.
Weak acid cation..
Strong base anion.
Weak base anion..
Chelating cation...
50-100
100-150
150-200
15O-200
200-300
Note.—1980 dollars.
costs of the other commercially
available resins!
Batch and Column Exchange
Systems I
Ion exchange processing can be
accomplished b|y either a batch
method or a column method. In the
first method, th|e resin and solution
are mixed in a batch tank, the
exchange is allowed to come to
equilibrium, then the resin is
separated from 'solution. The degree
to which the exchange takes
place is limited| by the preference
the resin exhibits for the ion in
solution. Consequently, the use
of the resin's exchange capacity will
be limited unless the selectivity
for the ion in solution is far greater
than for the exchangeable ion
attached to the! resin. Because
batch regeneration of the resin is
chemically inefficient, batch
processing by ion exchange has lim-
ited potential for application.
Passing a solution through a column
containing a bed of exchange
resin is analogous to treating the
solution in an infinite series of
batch tanks. Cbnsider a series of
tanks each containing 1 equivalent
(eq) of resin in the X ion form
(see Figure 2). A volume of solution
containing 1 eq of Y ions is charged
into the first tank. Assuming the
resin to have an equal preference for
ions X and Y, when equilibrium is
reached the solution phase will
contain 0.5 eq of X and Y. Similarly,
the resin phase will contain 0.5
eq of X and Y. This separation is
the equivalent of that achieved in a
batch process.
If the solution were removed from
Tank 1 and added to Tank 2, which
also contained 1 eq of resin in the
X ion form, the solution and resin
phase would both contain 0.25 eq
of Y ion and 0.75 eq of X ion. Re-
peating the procedure in a third and
fourth tank would reduce the
solution content of Y ions to 0.125
and 0.0625 eq, respectively.
Despite an unfavorable resin prefer-
ence, using a sufficient number
of stages could reduce the concen-
tration of Y ions in solution to any
level desired.
This analysis simplifies the column
technique, but it does provide
insights into the process dynamics.
Separations are possible despite
Solution Y = 1.0
feed (eq): X = 0
Y = 0.5
X = 0.5
Y = 0.25
X = 0.75
Y = 0.125
X = 0.875
Y = 0.0625
X = 0.9375
Resin after
mixing (eq):
Tank 1
Y = 0.5
X = 0.5
Tank 2
Y = 0.25
X = 0.75
Tank3
Y = 0.125
X = 0.875
Tank 4
Y = 0.0625
X = 0.9375
Note.—Resin has equal preference for X and Y ions. Solution feed contains 1 eq of Y ions. Each batch tank initially contains 1 eq
of resin in X ion form.
Figure 2.
Concentration Profile in a Series of Ion Exchange Batch Tanks
-------
poor selectivity for the ion being
removed.
Ion Exchange Process
Equipment and Operation
Most industrial applications of ion
exchange use fixed-bed column
systems, the basic component of
which is the resin column (Figure 3).
The column design must:
• Contain and support the ion
exchange resin
• Uniformly distribute the service
and regeneration flow through the
resin bed
• Provide space to fluidize the
resin during backwash
• Include the piping, valves, and
instruments needed to regulate
flow of feed, regenerant, and
backwash solutions
Regeneration Procedure. After the
feed solution is processed to the
extent that the resin becomes
exhausted and cannot accomplish
any further ion exchange, the
resin must be regenerated. In normal
column operation, for a cation
system being converted first to the
hydrogen then to the sodium
form, regeneration employs the
following basic steps:
1. The column is backwashed to
remove suspended solids
collected by the bed during the
service cycle and to eliminate
channels that may have formed
during this cycle. The back-
wash flow fluidizes the bed,
releases trapped particles, and
reorients the resin particles
according to size. During
backwash the larger, denser
particles will accumulate at the
base and the particle size will
decrease moving up the column.
This distribution yields a good
hydraulic flow pattern and
resistance to fouling by sus-
pended solids.
2. The resin bed is brought in con-
tact with the regenerant solution.
In the case of the cation resin,
acid elutes the collected ions and
converts the bed to the hydro-
Water outlet
Water inlet
Meter
Backwash controller
Sight glass
Upper manifold
Nozzles
Resin
Regenerant
Graded quartz
Lower manifold
Strainer nozzles
Backwash outlet
SOURCE: Kunin, R. "Ion Exchange for the Metal Products Finishers," (3 pts.). Products
Finishing, Apr.-May-Jiine 1 969.
Figure 3.
Typical Ion Exchange Resin Column
gen form. A slow water rinse then
removes any residual acid.
3. The bed is brought in contact
with a sodium hydroxide solution
to convert the resin to the
sodium form. Again, a slow water
rinse is used to remove residual
caustic. The slow rinse pushes
the last of the regenerant through
the column.
4. The resin bed is subjected to a
fast rinse that removes the
last traces of the regenerant
solution and ensures good flow
characteristics.
5. The column is returned to service.
For resins that experience significant
swelling or shrinkage during regen-
eration, a second backwash
shquld be performed after regenera-
tion to eliminate channeling or
resin compression.
Regeneration of a fixed-bed column
usually requires between 1 and
2 h. Frequency depends on the vol-
ume of resin in the exchange;
columns and the quantity of heavy
metals and other ionized corn-
pounds in the wastewater.
Resin capacity is usually expressed
in terms of equivalents per liter (eq/L)
of resin. An equivalent is the
molecular weight in grams of the
compound divided by its electrical
charge, or valence. For example,
a resin with an exchange capacity of
1 eq/L could remove 37.5 g of
divalent zinc (Zn+2, molecular
weight of 65) from solution. Much
of the experience with ion exchange
has been in the field of water
softening; therefore, capacities will
frequently be expressed in terms
of kilograms of calcium carbonate
per cubic foot of resin. This unit
can be converted to equivalents per
8
-------
liter by multiplying by 0.0458.
Typical capacities for commercially
available cation and anion resins
are shown in Figure 4. The
capacities are strongly influenced
by the quantity of acid or base used
to regenerate the resin. Weak
acid and weak base systems are
more efficiently regenerated; their
capacity increases almost linearly
with regenerant dose.
Cocurrent and Countercurrent Re-
generation. Columns are designed to
use either cocurrent or counter-
current regeneration. In cocurrent
units, both feed and regenerant
solutions make contact with
the resin in a downflow mode. These
units are the less expensive of
the two in terms of initial equip-
ment cost. On the other hand, cocur-
rent flow uses regenerant chemicals
less efficiently than countercur-
rent flow; it has higher leakage
concentrations (the concentration
of the feed solution ion being
removed in the column effluent), and
cannot achieve as high a product
concentration in the regenerant.
Efficient use of regenerant chemicals
is primarily a concern with strong
acid or strong base resins. The
weakly ionized resins require only
slightly greater than stoichiometric
chemical doses for complete
regeneration regardless of whether
cocurrent or countercurrent flow
is used.
Regenerant Reuse. With strong acid
or strong base resin systems,
improved chemical efficiency can
be achieved by reusing a part
of the spent regenerants. In strongly
ionized resin systems, the degree
of column regeneration is the
major factor in determining the
chemical efficiency of the regenera-
tion process. (See Figure 5.) To
realize 42 percent of the resin'stheo-
retical exchange capacity requires
1.4 times the stoichiometric amount
of reagent [2 Ib HCI/ft3 (32 g HCI/L)].
To increase the exchange capacity
available to 60 percent of theoretical
3.0
2.5
2.0
I 1-5
u
1.0
0.5
Legend:
weak acid cation resin
weak base anion resin
strong acid cation resin
strong base anion resin
! REGENERATION LEVEL (Ib/ft3)"
alb NaOH/ft3 for'weak and strong base anion; Ib HCI/ft3 for weak and strong acid cation.
SOURCES: Dowlchemical Company. Dower WGR-2 Weakly Basic Anion Exchange Resin,
T.D. Index 330.1, Midland Ml, Dow Chemical Company, undated. Dow Chemical
Company, "Anion Resins: Selection Criteria for Water Treatment Applications," Idea
exchange 5(2), undated. Rohm and Haas Company, Amberlite* 200, Philadelphia PA,
Rohm and Haas Company, Nov. 1976.
Figure 4.
Resin Exchange Capacities
increases consumption to 2.45
times the stoichiometric dose [5 Ib
HCI/ft3 (80 g HCI/L)].
The need for acid doses considerably
higher than stoichiometric means
that there is a significant concentra-
tion of acid in the spent regenerant
Further, as the acid dose is
increased incrementally, the con-
centration of acid in the spent
regenerant increases. By discarding
only the first part of the spent
-------
100
Legend:
i^HBM with acid reuse
mi - i without acid reuse
cc
8.
2
UI
0
X
I
g
CO
cc
o
U
468
REGENERANT CONSUMED (Ib HCI/ft3) :
'Based on strong acid resin in calcium form. :
10
Figure 5.
Effect of Reusing Acid Regenerant on Chemical Efficiency ;
regenerant and saving and reusing
the rest, greater exchange capacity
can be realized with equal levels of
regenerant consumption. For
example, if a regenerant dose of 5 Ib
HCI/ft3 (80 g HCI/L) were used in
the resin system in Figure 5, the
first 50 percent of spent regenerant
would contain only 29 percent of
the original acid concentration. The
rest of the acid regenerant would
contain 78 percent of the original
acid concentration. If this sec-
ond part of the regenerant is
reused in the next regeneration
cycle before the resin bed makes
contact with 5 Ib/ft3 (80 g/L) of fresh
HCI, the exchange capacity would
increase to 67 percent of theoretical
capacity. The available capacity
would then increase from 60 to 67
percent at equal chemical doses.
Figure 5 shows the improved
reagent utilization achieved by this
manner of reuse over a range of
regenerant doses.
Regenerant reuse has disadvantages
in that it is higher in initial cost
for chemical storage and feed
systems and regeneration proce-
dure is more complicated. Still,
where the chemical savings have
provided justification, systems have
been designed to reuse parts
of the spent regenerant as many as
five times before discarding them.
10
-------
I
Wastewater Recycle
Systems
In usual practice, metal finishing
wastewater is treated and then
discharged to a rjver or sewer system;
as an alternative, the wastewater
can be deionized by ion exchange
and reused in the plating process.
Wastewater deionization will
significantly reduce water consump-
tion and the volume of wastewater
requiring treatment, with the
following primary economic
advantages: !
• Water use and sewer fees are
reduced. •
• Although treatment of pollutants
is not eliminated, the size and
cost of the pollution control
system is significantly reduced.
I
The volume reduction resulting from
wastewater recycling can also
make pollution [discharge limits
easier to achieve. For plants dis-
charging wastewater to municipal
treatment systems, the national
pretreatment standards call
for more lenient discharge limits if
a plant discharges less than
10,000 gal/d (37,000 L/d). Plants
discharging directly to surface
waters are typically regulated by
mass-based pollutant discharge
limits. When translated to a
concentration limit based on a
volume of discharge, these limits
may be difficult to achieve by
conventional pollutant removal
systems. The reduction of discharge
volumes resulting from water
recycle will alloVv for higher concen-
trations of pollutants in the dis-
charge. ]
I
Inorganic plating chemicals such as
acids, bases, and metal salts are
ionized in water solutions and can be
removed from process waters by
ion exchange. Some dissolved
organic compounds, oils, and free
chlorine are typically present
in mixed wastewaters and their
presence constitutes a potential
for fouling or deterioration of the ion
exchange resin. Electroplating
facilities using ion exchange on
mixed wastewaters have found the
resins to be operable and stable,
however, when the recycle system
incorporates wastewater pretreat-
ment to remove constituents
that degrade the resins. When ion
exchange is used to remove
chromate and zinc from cooling
tower blowdown there is similar
potential for resin deterioration.
Nevertheless, the effects have not
been found severe enough to
preclude the successful use of ion
exchange for this application.
Hexavalent chromium (Cr*6) can be
removed if the mixed wastewater
is passed through an anion column.
Cyanide and metal cyanide
complexes are ionized and could
also be removed directly from
the wastewater by anion exchange.
Mixing cyanide wastes with the
rest of the plant's wastewater
is potentially hazardous, however;
toxic hydrocyanic gas (HCN) would
result from contact with acidic
wastes. Therefore, cyanide waste-
waters are normally pretreated
before they are blended with the
rest of the wastewater. In many
cases, an integrated chemical waste
system (Figure 6) can provide
cyanide pretreatment that is low
in cost and easy to operate.
The usual ion exchange sequence
is cationic exchange followed by
anionic exchange. The reverse
sequence is avoided because pass-
ing the solution first through an
anion exchange column would
increase pH and could precipitate
heavy metal hydroxides.
System Description
An ion exchange wastewater recycle
system is shown in Figure 7. The
major process components include:
• Wastewater storage
• Prefilters
• Ion exchange columns
• Regeneration system
• Batch treatment for regenerant
solutions
• Deionized water storage
Wastewater Storage. A collection
sump or storage tank is needed
to provide a surge volume in the
1,1
-------
Workplace
Cyanido plating bath
Chemical rinse
with NaOCI
solution
Water
rinse
To wastewater
purification and
recycle
Figure 6. [
Integrated Chemical Rinse to Oxidize Cyanide Compounds
system and allow the exchangers to
be fed at a constant rate. The
unagitated collection tank can also
be used to settle coarse solids
in the wastewater. The collected
solids can be pumped out at
regular intervals and disposed of.
Tank design should allow any free oil
to separate and then collect
on the surface of the wastewater.
Regular skimming can then purge
the oil from the recycle system.
Prefilters. Activated carbon columns
are commonly used as ion exchange
prefilters. The carbon columns
provide a versatile pretreatment
system; they can:
• Filter out suspended solids that
could hydraulically foul the
columns.
• React with free chlorine or
other strong oxidants that could
physically degrade the resin.
• Adsorb organics that would
otherwise build up in the recir-
culated wastewater.
• Adsorb oils that would gradually
foul the resin.
The columns are typically back-
washed daily to remove collected
suspended solids. The backwash
water goes either to the waste-
water storage tank or to the batch
treatment tank. Carbon replace-
-lon Exchange Columns-
Wastewater
Wastewator
discharge
Solids to
disposal
Batch treatment
tank
Deionized water
storage
Legend:
C = conductivity probe
NC = normally closed
Figure 7.
Ion Exchange Wastewater Purification and Recycle System
12
-------
ment frequency depends primarily on
loading of oils or organics. If the
carbon is not replaced, organic
impurities can gradually build up in
the recycle water. Some long
chain organic molecules will foul
strong base resins. Oil not removed
by pretreatment collects on the
resin and reduces its exchange ca-
pacity, resulting in more frequent
regeneration and higher operating
costs. Cleaning solutions are
available from resin manufacturers
to restore the performance of
oil-fouled resin beds.
Ion Exchange Columns. In the most
common column configuration,
wastewater passes in series through
a strong acid cation resin column
and then through either a strong or
weak base anion resin column.
Weak base resins have higher ex-
change capacities and require less
regenerant than do strong base
resins. On the other hand, weak base
resins are not effective in removing
weakly ionized bicarbonates,
borates, and silicates, nor can they
operate effectively at high pH.
These limitations may not be a con-
cern for metal finishing waste-
waters, and weak base resins are
recommended. If these anions
are present in significant amounts,
an anion bed containing both
strong and weak base resins can be
used. A bed of this kind will approach
the higher exchange capacity and
regeneration efficiency of a
weak base system but provide com-
plete deionization.
To provide uninterrupted system
operation when column regen-
eration is required, two sets of col-
umns are frequently installed. When
one set has been exhausted,
flow is switched to the off-stream
set and the spent columns are regen-
erated.
Regeneration System. The cation ex-
change column should be regen-
erated with hydrochloric acid after
exhaustion. Despite its higher
cost, HCI is favored over H2SO4 for
regeneration if the wastewater con-
tains a significant amount of calcium.
In such a case,! regeneration with
sulfuric acid can result in pre-
cipitation of cajcium sulfate and
hydraulic fouling of the resin bed.
Calcium sulfate! precipitation can be
avoided by using dilute sulfuric acid
solutions (2 percent by weight).
Strong base anion columns are
regenerated with sodium hydroxide.
Weak base resins can be regen-
erated with sodium hydroxide or
less expensive
such as sodiurr
basic reagents
carbonate.
Batch Treatment for Regenerant j
Solutions. The pollutants removed
by the ion exchange system will I
be concentrated in the regenerant j
and wash solutions. These solutions
must undergo conventional treat- I
ment before bejng discharged. The!
type of pollutants present (Cr*6 and
heavy metals wjould be most com- j
mon) dictates the treatment j
sequence that would be required. I
Deionized Water Storage. A storage
tank is used to provide an inventory
of water for process needs. The \
effluent from the ion exchange col-i
umn should be jmonitored with a
conductivity probe to provide a j
relative index of [the level of dissolved
solids in the treated water. When !
the water conductivity increases to a'
certain level, the columns are
switched and the spent columns !
are regenerated. Because complete;
water deionization is not needed for
most process applications, the
columns are loaded until the maxi- •
mum allowable llevel of impurity !
is reached before they are re- !
generated; regeneration frequency i
and system operating costs are thus
reduced.
(Co
Ion Exchange Column Specification
Columns are usually sized as a func-
tion of the ratio! of wastewater
volume to resin!volume. Recom-
mended rates vary depending
on the application but as a rule
range from 2 to 4 gal/min/ft3 (0.26 to
0.52 L/min/L) of resin. Higher rates
will usually result in higher
leakage, but wil not affect the
quantity of ionic compounds the
resin bed can exchange.
For rinse water recovery, leak-
age of small concentrations of
ionic compounds would not signal
the end of the cycle. Therefore,
rates should be selected from the
higher end of the recommended
range to minimize the initial cost of
the system. Smaller columns will
increase regeneration frequency and
the associated labor cost. For
columns with automated regenera-
tion packages, increased re-
generation frequency will not sig-
nificantly increase operating costs.
Cost
Conventional end-of-pipe treatment
requires removing pollutants
from large volumes of dilute
wastewater. When pollutants are
concentrated into small volume
regenerant solutions, treatment is
usually more economical. More-
over, recycling the purified waste-
water reduces operating costs
associated with water consumption
and sewer fees.
As a rule, treating the concentrated
regenerant solutions will con-
sume chemicals in quantities
smaller than are needed to treat the
same mass of pollutants in a dilute
waste stream. Capital costs of
wastewater treatment systems
depend primarily on the unit
operations required and the volu-
metric flowrate of the wastewater.
Total investment for an ion ex-
change water recycle system
and a simplified batch chemical
destr'uct unit to treat concentrated
solutions will often be less than
that for a conventional chemical
destruct system designed to treat the
total volume of water consumed by
a plant.
Operating Cost. Operating costs for
an ion exchange purification sys-
tem to treat wastewater containing a
variety of heavy metals will include:
• Chemicals for column regen-
eration
13
-------
• Destruct chemicals for treatment
of concentrated regenerant
solutions and purged wash water
• Disposal of the treatment residue
• Labor for column regeneration
and operation of the batch treat-
ment system (if not automated)
• Maintenance
• Resin and activated carbon
replacement
• Utilities
How these costs compare with the
costs of operating a conventional
hydroxide treatment process can be
determined by evaluating the
costs associated with each system
treating the same waste stream.
To simplify the analysis, equal labor,
maintenance, and utility charges
are assumed for both systems.
A typical waste stream (Table 4)
consisting of rinses after nickel, cop-
per, and chromium plating baths
and acid and alkali process baths
will be used in the cost analysis.
In a water recycle system, only
natural alkalinity brought in with
makeup water must be treated;
recycled water has already had its
initial alkalinity removed. The
wastewater used in conventional
treatment, however, contains
all the natural alkalinity brought in
with the fresh water; as a result
more alkali reagent will be consumed
and more solid waste generated.
In light of the foregoing analysis, the
next step is to determine the
required column configuration
and size of the ion exchange unit.
Either of two column configurations
can be used: strong acid and
strong base or strong acid and weak
base. In either case, ion exchange
column sizing is based on volumetric
loading. At the normally recom-
mended service flowrate of 2 gal/
min/ft3 (0.26 L/min/L) of resin, col-
umns containing 15 ft3 (425 L)
of resin will be needed. The ion ex-
change capacity of these columns
will depend on the quantity of
regenerant (dosage rate).
Using the resin capacities given in
Figure 4, the columns will be
Table 4.
Wastewater Characteristics and Ion Exchange Capacity Requirements
Treatment
Item
Ion exchange Conventional
Wastewater characteristic:
Flowrate (gal/min) 30 30
Constituent (ppm):
Cu+2 40 40
Ni+2 40 40
Cr042 50 50
Na+ 20 20
S042 150 220
Total dissolved solids 310 470
Alkalinity, as Ca(HCO3)2 (ppm) 10 70
Wastewater concentrations to be treated by ion exchange (eq/L):
Cations0 : 0.0036 —
Anions 0.00423 —
Ion exchange resin capacity needed for 15-ft3 bed (eq/L):
Cation resin3 ' °-92 —
Anion resin 1-08 —
"Does not include hydrogen ions.
b16-h operating cycle.
regenerated with sufficient acid and
base to provide 1 day's operating
capacity. The plant is assumed
to operate 16 h/d. Table 4 includes
the resin capacity needed for
columns with 15 ft3 (425 L) of
resin.
For the strong acid/strong base
unit, sufficient capacity would be
obtained in the anion column
with a regenerant level of 6.5 Ib
NaOH/ft3 (104 g NaOH/L) of resin.
The anion column would require
greater capacity than the cation col-
umn because the wastewater is
acidic; the higher anionic loading
rate results from the an ions
associated with the hydrogen ion
acidity. Adequate capacity would be
obtained in the cation column with
a regenerant level of 4 Ib HCI/ft3
(64 g HCI/L) of resin. In this case,
however, the combined anion and
cation column regenerant must
be acidified to reduce Cr*6. There-
fore, excess acid regenerant [6.5
Ib/ft3 (104 g/L)] can be used
to balance the excess NaOH in the
anion regenerant.
Table 5 shows the chemical content
and volumes of the regenerant
solutions after they are mixed with
Table 5.
Regenerant Solution Chemical Content
Item
Strong acid/
strong base
850 gal
20.3 Ib CuCI2
21.1 Ib NiCI2
12.1 Ib NaCI
1.6 lbCaCl2
67.0 Ib HCI
1 6.8 Ib Na2CrO4
53.3 Ib Na2S04
3.0 Ib Na2CO3
56.9 Ib NaOH
Strong acid/
weak base
800 gal
20.3 Ib CuCI2
21.1 Ib NiCI2
12.1 Ib NaCI
1 .6 Ib CaCI2
29.5 Ib HCI
1 6.8 Ib Na2CrO4
53.8 Ib Na2S04
3.0 Ib Na2CO3
7.4 Ib NaOH
14
-------
the volume of wash water [50
gal/ft3 (6.5 L/L) of resin] usually
required for the backwash and rinse
stages of regeneration. Chemical
cost of each regeneration cycle is
$29.83 forthe strong acid and strong
base system (based on Table 6).
Regeneration cost can be reduced if
a weak base resin is used in the
anion column. A weak base resin
downstream of the strong acid
column is suited for this application
because the entering wastewater
would always be acidic. Based on the
capacity shown in Figure 5, suf-
ficient resin capacity could be
achieved with a sodium hydroxide
dose equal to 3.2 Ib/ft3 (51 g/L) of
resin. The amount of acid consumed
Table 6.
Chemical Prices
Hydrochloric acid .
Sodium hydroxide .
Hydrated lime
Sulfuric acid
Reagent Description
Carboys, 32% HCI
• Carboys 50% NaOH
; 1 00-lb bags
i 1 00-lb bags
! Carboys 97% H2SO4
Cost ($/lb)a
005
0075
005
020
005
2 50
a1980 dollars.
for regeneration should be reduced
to the minimum required for
column capacity, 4 Ib HCI/ft3 (64 g
HCI/L) of resinj Table 5 includes
the volume and chemical content of
erant chemicals for this column
configuration would cost $16.56 for
each cycle.
Based on treatment chemical
the regenerantjsolutions. Regen- consumption factors (Figure 8) and
,K
Chromium waste ^
(gal/min, Ib Cr+6) t
F
Heavy metals wastes \
(gal/min, Ib metal8) )
Solids generation factors
Legend:
Reduction (NaHS03, H2S04)
3 Ib NaHS03/lb Cr+6
2 Ib H2SCyib Cr+e
0.3 Ib NaHSOg/1,000 gal
0.2 Ib H2SO4/1 ,000 gal
Neutralization [Ca(OH)2]b
1 .2 Ib Ca(OH)2/1 ,000 gal
Neutralization [Ca(OH)2]b
0.1 Ib dry solids generated
Ib Ca(OH)2 consumed
Process step (treatment reagent)
Consumption factor
!
I
j
i
Mb me't
'AlkalFn
Neutralization [Ca(OH)2]
1.7 lbCa(OH)2/1,OOOgal
0
Precipitation [Ca(OH)2]
2.6 Ib Ca(OH)2/lb Cr
2.2 Ib Ca(OH)2/lb metal
Precipitation
Ib dry solids generated
Ib metal precipitated
Cr 2.24
Ni 1 .80
Cu 1.75
Cd 1 .52
Fe+z 1 .83
Zn 1.74
Al 3.1 1
Flocculation
\ 0.02 lb/1 ,000 gal
als expressed as Ib metal ions.
ity consumes lime and adds to solids generation rate.
Figure 8. I
Conventional Treatment Chemical Consumption Factors
15
-------
Table 7.
Daily Treatment Cost Comparison: Ion Exchange and Conventional Systems
Treatment cost ($/d)
Component
Strong acid/ Strong acid/ Conventiona|
strong base weak base
Chromium reduction:
NaHSO3
H.SO..
Ion exchange regeneration:
HO . .
NaOH
Watar and sewer fee at $1/1,000 gal
3.34
(b)
3.76
3.70d
15.21
14.62
40.63
1 .70
3.34
(">
4.08
3.70d
9.36
7.20
27.68
1.70
3.82a
0.63
5.16
4.97B
(b)
(b)
14.58
28.80
Total treatment cost.
42.33
29.38
43.38
•Assumos 10 gal/min of segregated Cr"1"6 wastewater.
""Not required.
°25% solids by weight at $0.20/gal.
dFor 46.3 Ib dry solids.
•For 62.2 Ib dry solids.
Note.—1980 dollars.
chemical costs (Table 6), Table 7
compares the daily cost to operate
the two ion exchange systems
with the costfora conventional treat-
ment system. Although the chem-
ical costs are higher for the ion
exchange systems, when the
savings in water and sewer fees
(assuming $1/1,000 gal) are
considered, the total cost is less
than that of conventional treatment.
The data also indicate that a strong
acid/weak base column config-
uration is considerably less expen-
sive to operate than the strong
acid/strong base configuration. The
economics of the ion exchange
system could be improved further if
the strong acid column regen-
erant were reused.
For deionization applications,
commercially available resins cost
between $50/ft3 and $200/ft3. Ion
exchange resins usually need replac-
ing every 2 to 5 years, depending
on the type of resin and the process
application. Resins can be dam-
aged by exposure to strong oxidants,
long chain organic compounds,
or oil. With proper selection of resins
and effective pretreatment of the
wastewater, the potential for
resin deterioration and the cost
for replacement will be reduced.
i
Granular activated carbon must be
replaced when its adsorption
capacity is spent. For small scale
applications, regenerating the
carbon is not economically feasible.
Replacement frequency for activated
carbon will depend on the level
of organic compounds in the
wastewater. Carbon adsorption is an
economical means of removing
trace amounts of organic compounds
from solution. If high levels of
organics are present, however, the
cost becomes excessive and alterna-
tive removal techniques should be
evaluated.
Effect of Pollutant Concentration.
The volume of wastewater that
can be deionized by an ion exchange
column is in direct proportion
to the ionic concentration of the
wastewater and is not influenced by
the volume needing treatment.
Consequently, when dilute solutions
are processed, a large volume can
be treated before column capacity is
exhausted and regeneration is
required. On the other hand, conven-
tional treatment processes—such
as chromium reduction, cyanide
oxidation, and metal precipitation—
must adjust the chemistry of the
water solution to achieve the
desired reaction. The chemical con-
sumption associated with these
processes therefore depends
on both the mass of pollutant and
the volume of solution to be treated.
Because its cost is independent
of solution volume, ion exchange
processing is highly efficient
in terms of chemical consumption
when used to treat dilute con-
centrations of ionic contaminants.
Figure 9a shows the relative costs of
deionization and conventional
treatment techniques as a function
of the concentration of waste-
water contaminants for acid-alkali
waste streams and for hexavalent
chromium wastewater. Only chem-
ical treatment costs are included,
not water and sewer use fees.
The treatment steps and assump-
tions used to derive the conventional
treatment cost are presented in
Table 6 ancl Figure 8. Also assumed
is removal of natural alkalinity
during treatment.
Ion exchange does not compare
favorably with hydroxide precipita-
tion of acid-alkali waste streams
except at very dilute concentrations.
For treating typical metal finishing
wastewater, hydroxide precipi-
tation will usually have lower
16
-------
(a) 2.001-
V 1'50
o>
o
8
st
1.00
£E 0.50
Legend:
hexavalent chromium wastewater (H2CrO4)
acid/alkali wastewater (CuS04)
(b) 3.00
100 200 300
CONCENTRATION (ppm)
400
500
100
200 300
CONCENTRATION (ppm)
400
500
"1980 dollars.
bWater and sewer fees assumed at $1/1,000 gai.
Figure 9. j
Cost Comparisons for Ion Exchange and Conventional Treatment Systems:
(a) Chemical Cost Only and (b) Chemical Cost and Water Use Fees
chemical costs. For hexavalent
chromium wastewater, however, ion
exchange has a treatment cost
advantage up to a concentration of
440 ppm of chromic acid (H2Cr04}.
Ion exchange treatment is effi-
cient for this application partly be-
cause, except for contaminants,
chromic acid wastewater has
no cations other than hydrogen;
consequently, treatment by ion
exchange would not affect cation
column operating costs.
In Figure 9a the cost for con-
ventional treatment of acid-alkali
wastes includes the volume-related
cost for lime to adjust solution pH
and to react with naturally occurring
bicarbonate alkalinity, and for
polyelectrolyte conditioners to aid
in precipitant settling. The curve for
chromium reduction using bi-
sulfite includes these cost compo-
nents plus costs for acid needed
to bring the wastewater to required
reaction pH and base for subse-
quent neutralization.
The ion exchange system costs
are based on 90 percent water
recycle; they include the cost for
column regeneration and treatment
of regenerant solutions by con-
ventional techniques. The regener-
ant chemical consumption is
based on a strong acid/weak base
column configuration.
An ion exchange water recycle
system becomes considerably more
attractive than conventional
treatment techniques if the credit for
savings in water and sewer fees
is included in the analysis. Figure 9b
compares treatment costs of the
same two waste streams but
includes a cost equal to $1/1,000 gal
water consumed.
Waste Reduction. The waste
stream volume reduction achieved
by a wastewater deionization
system relates directly to the con-
17
-------
centration level of the dissolved
ionic solids in the wastewater.
The reduction in volume of the waste
stream and its favorable effect on
both the initial and operating
cost of wastewater treatment are
part of the justification for using ion
exchange.
Each cubic foot of resin in a column
system can remove a specific
quantity of ions; regenerating and
washing that volume of resin
will result in a purge stream of limited
concentration.
Consider an ion exchange system
with a strong acid/weak base
column configuration. Assume the
resin in each column has a capacity
of 1.5 eq/L, and that regenerat-
ing the columns produces purge
(regeneration plus rinsing) in
the amount of 50 gal/ft3 (6.5 L/L)
of resin. The maximum concentration
of the ionic solids in the combined
purge streams from both col-
umns, then, would be 0.11 eq/L.
Expressed in terms of a typical metal
salt, the maximum concentration
of copper sulfate (CuS04) in the
purge solution would be 1.75 per-
cent. Using this relationship. Figure
10 presents the volume reduction
for treating wastewater over a range
of ionic concentrations.
The relationship developed in
Figure 10 is based on normal operat-
ing procedures. Concentration
can be improved by selective recycle
of part of the purge stream; how-
ever, the poor chemical efficiency of
the ion exchange process for
treating concentrated solutions
and the poor degree of concentration
achieved make other methods of
treatment more suitable.
Capital Cost. In the metal finishing
industry, most of the wastewater
requiring treatment results from
rinsing operations. Selective
treatment and reuse of rinse streams
by ion exchange can result in
WASTEWATER CONCENTRATION (eq/L)
0.01 0.02 0.03 0.04
0.05
0.5
1,600 3,200 4,800 6,400
WASTEWATER CONCENTRATION (ppm CuSO4)
8,000
Figure 10.
Relationship of Waste Volume Reduction to Wastewater Ionic
Concentration
considerable savings in the invest-
ment necessary for end-of-pipe
treatment systems. This investment
is usually a function of wastewater
flowrate and the required unit
operations. For flows above 15 to 20
gal/min (57 to 76 L/min), auto-
mated continuous treatment
systems are usually recommended.
A deionization water reuse sys-
tem can result in flow reduction suf-
ficient to make a single batch
treatment tank feasible for treating
regenerant solutions and any
concentrated process dumps.
The cost for ion exchange column
systems is increased significant-
ly when dual cation-anion column
configurations are needed for
continued operation during regen-
eration. Automation adds con-
siderably to the initial cost of the unit
but, in addition to savings in labor,
can permit the use of smaller
columns with more frequent regen-
eration. Figure 11 compares costs
for two- and four-column ion
exchange units, automated and
nonautomated, as a function of resin
volume in each column. The systems
illustrated are skid mounted and
preengineered; costs include
the columns, an initial supply of
resin, reagent storage, and internal
piping and valves necessary for ser-
vice and regeneration flow.
18
-------
dual cation/anion columns (4 columns)
cation/anion columns (2 columns)
12
16
20
RESIN VOLUME PER COLUMN (ft3) ,
a1980 dollars. I
Note.—Skid-mounted, preengineered package unit. Includes acid and base
regenerant storage and all internal pipes and valves.
SOURCE: Equipment vendor.
I
I
The cost of the auxiliary equipment
described earlier for ion exchange
water recycle will add consider-
ably to the total capital cost asso-
ciated with using the technol-
ogy. The total cost, however, may
still compare favorably with that for a
conventional end-of-pipe treat-
ment system.
Figure 11. I
Cost for Deionization Units With and Without Automation
19
-------
End-of-Pipe Systems
Ion exchange can be used in two
different ways for end-of-pipe
pollution control. The process has
been demonstrated as a means
of polishing the effluent from con-
ventional hydroxide precipitation
to lower the heavy metal concentra-
tion further, and it has been used
to process untreated: wastewaters
directly for removal of heavy
metals and other regulated pol-
lutants.
Most plating shops can achieve
sufficient metal removal to comply
with discharge regulations by
employing the conventional hydrox-
ide precipitation process. Conven-
tional treatment may! not be reliable,
however, in achieving compliance
with discharge limits in certain
cases, including where:
• Unusually strict limits are placed
on the effluent metal concen-
tration.
o The metals are complexed with
chemical constituents that
interfere with their precipitation
as metal hydroxides.
In such cases, the use of ion ex-
change to polish the effluent can pro-
vide relatively inexpensive up-
grading of system performance for
compliance with the regulations.
The development of special chelat-
ing resins made ion exchange
feasible for selective removal of
trace heavy metals from a water solu-
tion containing a high concentra-
tion of similarly charged, nontoxic
ions. These resins exhibit a strong
selectivity, or preference, for
heavy metal ions over sodium,
calcium, or magnesium ions. Weak
acid cation resins also display
a significant preference for heavy
metal ions, and in some applications
they are superior to the chelating
resins in performance character-
istics. In a polishing applica-
tion, both resins can remove the
heavy metal ions from the waste-
water while leaving most of the
nontoxic ions in solution. The
preference for heavy metal ions
allows a large volume of water to be
treated per unit of resin volume
before the resin must be regener-
ated. The regenerant solution,
which contains a high concentration
of metal ions, is treated upstream
in the conventional process (Figure
12a).
Ion exchange has received limited
commercial application for selective
removal of heavy metal and rnetal
cyanide pollutants from an un-
treated wastewater while allowing
most of the nontoxic ions to pass
through. Various approaches
have been employed to facilitate this
application:
• A weak acid cation resin has
been used in wastewater soften-
ing to remove heavy metals
and other divalent cations from a
wastewater solution with a
high concentration of sodium
cations.
• Heavy-metal-selective weak
acid or chelating cation resin has
been used to remove the heavy
metal ions while allowing
sodium, calcium, and magnesium
ions to pass through.
• A stratified resin bed, containing
strong and weak acid cation
resins ancl strong base anion
resins, has been employed to re-
move heavy metal cations and
metal cyanide complex anions
while allowing other ions to pass
through.
In each of these approaches,
wastewater pretreatment require-
ments consist of pH adjustment to
ensure that pH is within the operat-
ing range of the resin, and filtration to
remove suspended solids that
would foul the resin bed (Figure 12b).
The pollutants removed from the
wastewater are concentrated
in the ion exchange regenerant
solutions. The regenerants can be
treated in a small batch treatment
system using conventional proces-
ses. Firms with access to a cen-
tralized treatment facility that
accepts industrial wastes can use the
20
-------
"""•""" *•
*
Conventional
hydroxide
precipitation
system
•
Solids
removal
it
Metal
hjydroxide
sludge
1
(b)
Wastewater ^BHMB^.
Collection
and
pH
adjustment
!
Filtration
i
~r~
Sludge
i
Waste
storage
£__
Bptch
treatment
system
1 ~T~
To centralized Metal
treatment hjdroxide
facility slludge
— ,
Ion
polishing
I
Regenerant
chemicals
——^
I .
Ion
exchange mm^m^^^. Discharge
treatment
Regenerant
chemicals
-
Legend:
•••• service
regeneration
Figure 12. |
Ion Exchange Systems: (a) Polishing and (b) End-of-Pip'e Treatment
facility to dispose of the regenerant
solutions and need not install
chemical destruct systems. In either
case, no investment is needed
for sophisticated pH control systems,
flocculant feed systems, clarifiers,
and other process equipment
associated with conventional con-
tinuous treatment systems, and
ion exchange becomes attractive
in terms of cost. And, as a further ad-
vantage, ion exchange units are
compact and easy to automate
compared with conventional treat-
ment systems.
I
i
i
Ion Exchange Polishing Systems
Process Description. Figure 13
shows^a treatment system employ-
ing: j
• Hydroxide neutralization to
control pH and to precipitate
heavy metals as metal hydroxides
• Flocculation to agglomerate
the suspended solids
• Clarification afid deep-bed
filtration to remove the precipi-
tated metals and suspended
solids
• Ion exchange polishing to
reduce residual metal solubility
before the water is discharged
For effective metal removal by
hydroxide precipitation, pH must be
controlled within the narrow
range where the metals are least
soluble. Such narrow control
usually requires sufficient retention
time within the pH adjustment
tank to ensure minimum variation in
neutralizing reagent demand.
Multistage neutralizers and sophisti-
cated control loops are also used
to minimize deviation from the pH
21
-------
NaOH
Wastewater
discharge »
Ion exchange columns
Figure 13.
Conventional Treatment System With Ion Exchange Polishing
control set-point. With effective
pH control, most of the metals
in the wastewater will precipitate as
metal hydroxides.
To provide effective removal of
precipitated metals and other
suspended solids, coagulating-
flocculating compounds are added to
the neutralized wastewater to
agglomerate the solids and facilitate
their removal. Most of the suspended
solids can be removed by clarifica-
tion; however, removal of fine
particles (including precipitated
metals) requires filtration. Deep-bed
filters, which remove solids by
passing the wastewater through a
bed of sand and gravel, are used
most frequently.
For most waste streams, the unit
operation sequence of hydroxide pre-
cipitation, fluocculation, clarifi-
cation, and filtration will produce
an effluent with a minimum
heavy metal content and achieve
compliance with discharge permit
regulations. In cases where the
metal content exceeds the permit
limit, and the excess is in the
form of dissolved metals (as opposed
to metal hydroxide particles not
removed during solids separatipn), a
polishing treatment using ion
exchange resins will reduce the
effluent metal concentrations.
The failure of hydroxide precipitation
to reduce metal solubility to the
required level can be caused by one
of the following:
• Failure to control pH within the
narrow range necessary for
minimal metal solubility
• The presence of chelating
compounds that combine with
metals to form complexes not
effectively removed by hydroxide
precipitation
• Discharge limits requiring metal
concentrations below those
which a hydroxide treatment
system can achieve consistently
and reliably
Resin Selection. Ion exchange
heavy-metal polishing systems will
usually use a chelating heavy-metal-
selective cation resin. A resin of
this kind forms an essentially
non-ionized complex with divalent
metal ions. Consequently, once an
exchanger group is converted to the
heavy metal form, it is relatively
unreactive with other similarly
charged ions in solution. Despite
high concentrations of non-heavy-
22
-------
metal cations competing for the ex-
change sites, the resin has suffi-
cient preference for the heavy
metal ions to exhibit a high metal-
holding capacity'per unit of resin
volume. The chelating resins
will effectively remove heavy metal
cations from solutions with a
pH above 4.0.
Weak acid cation resins also have
potential for use in ion exchange
polishing systems. These resins have
the advantage of being less
expensive than chelating cation
resins, and they require less chemi-
cals for regeneration. On the other
hand, weak acid resins are not
effective in acidic solutions;
moreover, they are less selective for
heavy metal cations over other
divalent calcium and magnesium
ions than are chelating resins.
Polishing System Equipment and
Auxiliaries. The ion exchange polish-
ing system consists of:
• Column or columns containing
the resin
• Acid regenerant storage
• Sodium hydroxide regenerant
storage
• Piping and valving to facilitate
on-stream wastewater treatment,
and regeneration and back-
washing of the resin bed
Three ion exchange column configu-
rations for a polishing system are:
• Single column
• Series column
• Parallel column
Unlike deionization systems, which
require both a cation and an anion
resin column, the polishing system
usually requires only one kind
of resin. Consequently, a single
column design is feasible for small
flows where the wastewater dis-
charge can be interrupted to allow
for column regeneration. Discharge
permits are usually based on a
daily composite sample, and this
factor should be considered in
evaluating use of a single column.
Often the composite effluent
quality of a treatment system that is
j i
off stream 10 percent of the time j
for regeneration in any given '
day will achieve the discharge per-'
mit limits. Mechanical failures
and system maintenance are inevi-
table consequences of using the
process, however, and the reliability
of a single column design is prob-
ably inadequate |n most applications.
I
In none of the polishing system
column configurations is there a
simple means of detecting the
breakthrough ofj metal ions that
would indicate a need for column
regeneration. Mptal breakthrough is
avoided by loading the column
only to some fraction of its
exchange capacity. A series col-
umn configuration, where the
total flow of wastewater passes
through each column, is particularly
reliable in ensuring contact of
the wastewater j/vith a large volume
of unreacted resin. After the
up-stream colurrin is exhausted, it
is taken off stream, regenerated,
and returned to Service as the
down-stream column. This configu-
ration minimizes the possibility
that the resin wijl be exhausted and
that metal breakthrough will
occur. On the other hand, pressure
drop over the syjstem will be
high and each column must be sized
to process the total flow.
A parallel column configuration
employing three'or more columns
has advantages, particularly for
larger flows. Both equipment cost
and reliability are intermediate
between the single and series col-
umn configurations. In a parallel
configuration, each column is
sized based on the assumption that
one column is always off stream
for regeneration.! This design
reduces the total resin volume re-
quirements compared with those of
a series column (design. Using a
bank of small columns does increase
regeneration frequency; many of the
units are designed with automated
regeneration capabilities, how-
ever, and more frequent regeneration
does not increasls the need for
operating labor. ;
Operating Procedure. Operation of
an ion exchange polishing system is
complicated by the lack of prac-
tical means for determining when the
column is exhausted and metal
breakthrough occurs. Unlike
deionization systems, where a con-
ductivity probe will signal the
end of a column cycle, polishing
systems have no simple, direct
technique for continuously monitor-
ing the levels of heavy metals
in the effluent. To compensate for
this lack, the columns are operated
on either a time or flow cycle.
This approach requires determining
the column exchange capacity
and the loading per unit volume of
wastewater. Then, based on the
resin volume in the column,
the volume of wastewater that can
be processed before exhausting
the exchange sites can be estimated.
As a rule, to provide a factor
of safety, a capacity equal to three-
quarters of the actual exchange
capacity is used to determine the
volume that can be processed
per cycle.
For a constant flow system, the
volume capacity can be converted
to a cycle time. A flow totalizer can
be used for variable flow systems
to monitor the cumulative volume
and indicate when the column
should be regenerated. Many manu-
facturers provide automatic
regeneration capabilities with
their column systems. For such sys-
tems, the control mechanism
can be directed to begin regenera-
tion by either a timing device
or a flow totalizer.
The regeneration sequence for a
chelating and a weak acid cation
resin is:
1. Water backwash to remove sus-
pended solids from resin bed
2. Acid regeneration
3. Water wash to remove residual
acid
4. Sodium hydroxide regeneration
5. Water backwash to remove
residual caustic and reclassify
the resin particles
6. Cocurrent fast rinse to ensure
that the resin bed's flow charac-
23
-------
teristics are adequate and to
remove any unused reagents
7. Return to service
The resin is used in the sodium form
even though it adds extra steps
to the regeneration process
and increases the chemical con-
sumption. Treating an alkaline
waste stream with a resin in the hy-
drogen form would gradually result
in conversion of the resin to
either the sodium or calcium form;
however, the exchange for hydro-
gen ions would depress the effluent
pH below the control limitations
and result in a period of noncom-
pliance. Also, the resin exhibits
a greater selectivity for heavy
metals in the sodium form.
Hydrochloric acid is normally
used for acid regeneration although
it is more expensive than sulfuric
acid. Sulfuric acid regeneration
could result in the precipitation of
magnesium or calcium sulfate
during regeneration, and the resin
bed could thus be hydraulically
fouled. This effect can be avoided,
however, if a dilute (2-percent)
sulfuric acid regenerant solution is
used.
The final backwash to reclassify the
resin bed is critical. "Classifi-
cation" refers to positioning the
resin particles so that the largest
particles are at the base of the
column and the particle size
gradually decreases as distance
from the base increases. This
arrangement results in maximum
flowrate per unit of pressure drop
ana makes the bed more resistant
to fouling from suspended solids
in the column feed.
With strong acid and base resins,
an initial backwash before regenera-
tion is usually sufficient to ensure
good flow characteristics during
the service cycle. In the case
of weak acid or chelating resins,
however, the resin beads swell
considerably when converted
to the hydrogen Torm and subse-
Table 8.
Ion Exchange Polishing System Performance Characteristics
Item
Characteristic
Value
Ion exchange column . . . ;
Service:
Wastewater to column
Discharge r
Regeneration:
Flow to column
Purge streams *
Chelating resin
Wastewater volume
Resin capacity
pH
Ni+2
Cu+2
PH
Ni+2
Cu+2
Wash water
5% NaOH
5% HCI
Volume
Nickel
Copper
ioft3
1 20,000 gal/cycle
0.87 Ib Ni/ft3
0.03 Ib Cu/ft3
8.4
8.9 ppm
0.3 ppm
8.4
0.1 6 ppm
0.02 ppm
5OO gal/cycle
68 gal/cycle
65 gal/cycle
633 gal/cycle
0.16%
0.006%
quently shrink when converted to the
sodium form, which necessitates
a final backwash before the column
is returned to service.
All regenerant and wash solutions
are sent to the hydroxide treat-
ment system for processing.
Table 8 presents a typical polishing
system performance with volume
of wastewater processed and the
relative volumes of the regenerant
streams.
System Performance. The number of
ion exchange polishing systems
installed to date is limited;
however, abundant pilot test data
verify system effectiveness in
reducing the soluble metal content
of a neutralized waste stream.
The data from these controlled
experiments can lead to a better
understanding of hpw process vari-
ables and design factors influence
performance.
Volumetric loading for ion exchange
systems is usually expressed in
bed volumes (bv) of solution
treated per hour or in gallons per
minute per cubic foot (liters
per minute per liter) of resin. Both
measures describe'loading in
terms of the volume of solution
treated per volume of resin in a unit
of time. In essence, they define
the length of time the solution is in
contact with the resin.
Figure 14 shows the concentration
profile of the effluent from a pilot
test column containing a chelating
resin. The feed solution has an
initial cadmium concentration of 50
ppm, a calcium chloride (CaCI2)
concentration of 1,000 ppm, and a
pH of 4.0. Tests were run at two
different volumetric loadings:
8 bv/h [1 gal/min/ft3 (0.13 L/min/L)]
and 16 bv/h [2 gal/min/ft3 (0.26
L/min/L)]. The higher loading
resulted in earlier breakthrough.
Assuming the column cycle is
terminated at a cadmium concen-
tration of 2.0 ppm in the effluent, the
8-bv/h system could treat 400 bv
before regeneration, compared
with 325 bv for a system operating
at 16 bv/h.
The influence of volumetric loading
on capacity results in a trade-off
between investment and operating
cost. Specifying a larger, more
expensive column will result
in greater capacity per unit volume
of resin and less frequent and
more efficient regeneration.
24
-------
Legend:
O resin capacity (Ib Cd/ft3)
100
200 300
BED VOLUMES TREATED
500
Note.—Feed solution: 50 ppm Cd+2, 1,000 ppm CaCI2, pH =4.0.
SOURCE: Adapted from Rohm and Haas Company, "Ion Exchange jn Heavy Metals
Removal and Recovery," Amber Hilite No. 162, Philadelphia PA, Rohjm and Haas Company,
1Q7B I
1979.
Figure 14. I
Influence of Flowrate on Chelating Resin Capacity !
Pilot evaluations have also been
performed with actual plating waste-
water. Figure 17 shows the feed and
effluent concentrations of copper.
and nickel when the effluent
from a hydroxide precipitation
system was treated by ion exchange
polishing. After adjustment to
a pH of 8.4, the wastewater still
contained a high level of nickel,
although copperwas removed to less
than 1 ppm. Dissolved ammonia
content was approximately 80
ppm. The weak acid cation resin in
sodium form was ineffective in
removing the nickel and the
test was terminated after 700 bv of
solution had been treated. The
chelating resin in sodium form
consistently removed the nickel to
levels below 0.5 ppm and the
copper to below 0.1 ppm until 1,600
bv of solution had been treated.
The equivalent would be processing
12,000 gal/ft3 (1,600 L/L) of resin
before regeneration would be
needed.
Figure 15 shows concentration
profiles of the effluent from two
pilot test columns. One column
contained a chelating resin,
the other a weak acid resin. Each
column treated a solution with
50 ppm cadmium and 1,000 ppm
calcium chloride at pH 2.07,4.0, and
8.0. Neither resin is effective
at a pH of 2.07. The chelating
resin shows approximately equal
capacity at pH 4.0 and 8.0. The weak
acid resin shows a capacity
increase when pH is increased from
4.0 to 8.0. It is significant that
the weak acid resin showed
greater capacity than the chelating
resin at pH 4.0 and 8.0. Where
they are suitable, the less expensive
weak acid resins are the resins
of choice in metal removal applica-
tions.
Ion exchange polishing is often
considered because hydroxide pre-
cipitation cannot effectively
reduce metal solubility in the pres-
ence of compounds that form
stable complexes with the heavy
metals. Ammonija, a common
constituent of rrrany plating waste-
waters, tends to! increase metal
hydroxide solubility. For example, in
a copper solution containing
dissolved ammonia, the ammonia
would compete for copper ions
as follows:
Cu(OH)2+4NH3
+ 20H-
Cu(NH3)+?
(8)
In the presence jof many chelating
compounds, a chelating resin
is more effective! in removing
heavy metals than a weak acid resin
because it forms a less-ionized
complex with the heavy metal ion.
This effect is demonstrated in
Figure 16, which shows the superi-
ority of the chelating resin in
removing copper from solutions
containing ammonia. A similar
situation would be expected for
other complexed metal ions.
I
When the resin is selected for a
polishing application both weak acid
and chelating cation resins should
be tested. The lower initial cost,
greater capacity, and more efficient
use of regeneration chemicals
make weak acid resins the choice for
those applications where they
are effective in metal removal. Many
wastewater applications, how-
ever, wilt require the chelating
resins' greater affinity for heavy
metals to achieve the necessary
effluent quality.
Ion Exchange Wastewater
Treatment Systems
The conventional practice of con-
verting the heavy metal pollutants
in metal finishing wastewater to
a hydroxide sludge was thought to be
a means of eliminating any envi-
ronmental hazard the metals might
pose. In fact, a solid waste stream is
generated that, although its
volume is much smaller than that of
the wastewater, requires further
controls to ensure that disposal of
25
-------
Legend:
chelating resin
•••HI weak acid resin
O resin capacity (Ib Cd/ft3)
100
200
300 400 500
BED VOLUMES TREATED
600
7OO
800
Note.—Feed solution: 50 ppm Cd+2, 1,000 ppm CaCI2, 8 bv/h.
SOURCE: Adapted from Rohm and Haas Company, "Ion Exchange in Heavy Metals Removal and Recovery," Amber Hilite No. 162, Philadelphia
PA, Rohm and Haas Company, 1979.
Figure 15.
Influence of Solution pH on Chelating and Weak Acid Cation Resin Capacity
the metal residue is environmentally
acceptable.
Ion exchange represents an alterna-
tive means of concentrating the
pollutants. The metals are concen-
trated in the regenerant solutions
and are then in a form more
easily handled and more amenable
to further processing. With the
increasing cost of virgin metals
and the significant cost of heavy
metal waste disposal, the devel-
opment of processes that recover
metals from mixed metal wastes
is inevitable. When metal recovery
is commercialized on a wide
scale, the ion exchange regenerant
solutions will represent a byproduct
of metal finishing operations, not
a waste product. ;
Currently, firms using ion exchange
for end-of-pipe pollution control
must also install small batch treat-
ment systems to treat the regenerant
and wash solutions. These systems.
which use conventional destruct
processes, result in a residue with
the same disposal criteria as
the sludge from a conventional
treatment process. The ion exchange
system may still present a less
costly means of complying with
pollution control regulations. The
key to using ion exchange for waste
treatment is to remove only the
toxic pollutants while allowing most
of the nontoxic ions in solution to
pass through the column. Normally,
the toxic compounds represent
26
-------
Legend: j
•nm weak acid cation resin
chjelating resin
O resin capacity (Ib Cu/ft3)
_
O
O
z
Q.
Q_
O
CJ
44g/L
(NH4)2S04
1.31
100
200 300
BED VOLUMES TREATED
I 400
500
Note.—Feed solution: 50 ppm Cu+2, 1,000 ppm CaCI2, pH = 4.0, ^ bv/h.
SOURCE: Adapted from Rohm and Haas Company, "Ion Exchange in Heavy Metals
Removal and Recovery," Amber Hilite No. 162, Philadelphia PA, Rohm and Haas Company,
1979.
Figure 16.
Influence of Ammonia on Chelating and Weak Acid Cation Resin Capacity
only a small percentage of the ionic
solids in the wastewater. If the
ion exchange system is not selective
in the species it removes from the
wastewater, the column capacity
required and the regenerant chem-
icals consumed will result in prohibi-
tive costs.
Ion exchange has proved successful
in selectively removing many of
the pollutants encountered in
metal finishing wastes. Proper appli-
cation of the process requires
selecting the appropriate resin and
regeneration sequence and, usually,
some pretreatment of the waste-
water before ion exchange.
Process Description. Wastewater
treatment systems employing
ion exchange include the following
components:
• Wastewater collection
• Wastewater pretreatment
• Ion exchange columns
• Ion exchange; column regenera-
tion system '
• Batch treatment for regenerants
(or waste stoijage if regenerants
are shipped 6ff site for treat-
ment or recovery)
Wastewater collection most
frequently consists of gravity drain-
age of rinses to a collection sump
below ground. The sump provides a
storage volume to allow the flow
to the treatmentisystem to be
controlled at a cpnstant rate. If the
ion exchange colpmns employ either
weak acid or weak base resins, the
capacity and performance of the
resins will be influenced by pH.
Consequently, the collection
sump should include coarse pH
adjustment capabilities. The pH ad-
justment system |must only ensure
that the solution jpH does not
deviate from the Abroad operating
range of the resin. This pH control
requirement is substantially dif-
ferent from those of hydroxide pre-
cipitation systems, which need
minimum deviation from the control
set-point.
As a rule, filtration to remove
suspended solids is the only other
pretreatment required. Suspended
solids in the feed would hydraulically
foul the resin bed. Different filters
have been employed, including
deep bed, diatomaceous earth pre-
coat, and activated carbon filters.
In one approach, a filter with
fine resin particles is used to trap
suspended solids. Regardless of the
filter type, the resulting purge
stream containing the suspended
solids must be processed and
disposed of.
The specifications of the column
system containing the ion exchange
resin depend on the flowrate
and the pollutants in the wastewater.
Two potential cases emerge with
respect to pollutants:
• Heavy metal cations alone
• Heavy metal cations along
with cyanides, and complex metal
anions
In the case of wastewater containing
only heavy metal cations, a column
with the sodium form of a weak
acid or heavy-metal-selective chelat-
ing cation resin would be employed.
For a weak acid resin, a pH close
to neutral is recommended. If
a chelating resin is used, the pH can
be slightly acidic (>4.0). In both
cases, strongly basic conditions
should be avoided because
such conditions favor formation
of anionic metal complexes.
For waste streams containing both
heavy metals and cyanides, a
stratified bed of resin has proven
effective. This patented approach
uses a bed of resin with successive
layers of strong base anion, weak
acid cation, and strong acid cation
resins. The wastewater first comes in
contact with the strong base resin,
which selectively adsorbs the
complex metal cyanide ions but
27
-------
(a) 20.0 r-
10.0
I
u
S i.o
(b)
0.1
2.0 r"
200
1.0 -
c\
o
S
o
1
te
Ul
a.
a.
O
U
0.1
0.01
400 600 800 1,000 1,200 1,400 1,'600 1,8002,000
BED VOLUMES TREATED :
Legend:
BBBB column feed
•••H weak acid cation resin column effluent
E:; . a chelating resin column effluent
J
0 200 400 600 800 1,000 1,200 1,400 1,600 1,800 2,000
BED VOLUMES TREATED
Not«.—Feed conditions (average): 0.1 ppm Cr, 275 ppm Ca, 2,200 ppm Na, 0.05 ppm Zn,
80 ppm NH4, pH = 8.4, 8 bv/h.
Figure 17.
Metals Removal Data: (a) Nickel and (b) Copper
allows the rest of the negatively
charged ions to pass through.
It should be noted that, although the
resin will remove complexed
metal cyanides selectively, the
presence of free cyanide will result
in early cyanide breakthrough.
The strong base resin does not show
significant selectivity for hexavalent
chromium or free cyanide over
the sulfate, chloride, and other non-
toxic anions in a wastewater. For
effective use of this type of resin
system, hexavalent chromium
wastes should be treated to reduce
the chromium to the trivalent form
before they are mixed with the
rest of the wastewater. The trivalent
chromium will be removed selec-
tively by the weak acid resin.
The wastewater then cornes in
contact with the weak acid cation
resin in the sodium form. The resin
employed exhibits a strong pref-
erence for multivalent cations. Con-
sequently, cation resin capacity
is a function of the concentration of
calcium, magnesium, and heavy
metal cations. Finally, the resin
makes contact with; a layer of
strong acid cation resin that is pre-
dominantly in the hydrogen form.
The exchange of the hydrogen ions
tends to balance the pH rise that
normally would occur at the
beginning of the cycle.
The system also employs a novel
regeneration sequence for the
stratified resin bed. In a conventional
mixed bed system, cationic and
anionic resins are separated by being
backwashed into discrete layers.
Each layer is then regenerated
independently; acid is brought
in contact with the cation resin and
sodium hydroxide regenerates
the anion bed. The bed is then mixed
with air and the resin types are
distributed equally throughout the
bed.
With the stratified bed used for heavy
metals and metal cyanide, the
resin bed is first backwashed gently
to remove suspended solids and
the resin bed is fluidized. Because
the three types of resins have
28
-------
HCI and
wash
Air
out
i
Legend:
regeneration
sodium cyanide recovery
Compressed air
!H eater
NaOH
SOURCE: C. Terrian; Best Technology, Inc., personal ^communication to P. Crampton, Aug. 10, 1980.
NaCN for
chemical
makeup
Figure 18.
Sodium Cyanide Recovery
different densities, the resin stratifi-
cation can be maintained with
proper backwashing. The strong
base resin is least dense, the weak
acid resin is intermediate, and
the strong acid resin most dense.
After backwash, the bed makes
cocurrent contact with a 20-percent
HCI solution. The acid elutes
the metal cyanide complexes from
the anion resin and replaces
them with chloride ions. The heavy
metals and divalent cations are
removed from the weak acid
cation resin and replaced with
hydrogen ions. The strong acid resin
is also converted to the hydrogen
form.
After a water wash, the bed is
washed with a 20-percent sodium
hydroxide solution. The sodium
hydroxide converts the anion
resin to the hydroxide form and
elutes any metal chloride complexes
formed during the acid wash. The
weak acid cation resin is converted
to the sodium form. The sodium
hydroxide is essentially depleted
by the time it reaches the strong acid
resin. After another water wash,
the column is returned to service.
I
The stratified bed system also fea-
tures cyanide recovery to avoid
the significant dost of treating the
cyanide contained in the acid
regenerant. Thejacid regenerant and
the subsequent!water wash are
routed to a closed-top vessel (Figure
18) where heatjis supplied to
raise the solution temperature to
140° F (60° C) and air is bubbled
into the solution. The result is a
rapid release of IHCN gas. The
liberated gas is (brought in contact
with a caustic soda solution; the
caustic soda absorbs the cyanide,
yielding a sodiujn cyanide (NaCN)
solution that cai-i be used for chem-
ical makeup in the cyanide plating
baths. |
Regenerant solutions from the ion
exchange column are usually
treated in a small batch treatment
system. Depending on the pollu-
tants present, the system may need
capability for cyanide oxidation,
chromium reduction, arid metal pre-
cipitation. The sludge resulting
from batch treatment can be either
settled and disposed of or mechan-
ically dewatered before disposal.
A system treating a combined
wastewater containing both ferrous
ions and cyanides will have a
significant concentration of ferro-
cyanides in the regenerant solution.
These difficult-to-treat cyanide
complexes result from mixing
of the cyanide wastewater with
acidic streams containing dissolved
iron. An additional treatment step
is needed to oxidize the ferro-
cyanides. In this step hydrogen per-
oxide is added to the wastewater,
which is subjected to irradiation by
ultraviolet light. The strong oxidiz-
ing power of this system is effective
in treating the ferrocyanides.
29
-------
When ion exchange column size is
determined, hydraulic and con-
taminant loadings must be consid-
ered. Resin manufacturers recom-
mend volumetric loading rates
in the range of 1 to 2 gal/min/ft3
(0.13 to 0.26 L/min/L) of resin.
Unless the contaminant loading
results in unmanageable regenera-
tion frequency, the hydraulic
loading should be selected from
the high end of the range.
System Performance. Operating data
from ion exchange wastewater
treatment systems is scarce because
of the small number of facilities
employing the technology; however,
the available performance data indi-
cate the potential for application
in metal finishing wastewater
treatment.
In one case, treatment was of a
slightly acidic heavy metal waste-
water containing a moderate
concentration of calcium, magne-
sium, and sodium cations. A
weak acid cation resin in the sodium
form was evaluated for removing
the heavy metals. The resin was able
to remove both the zinc and cad-
mium selectively while allow-
ing most of the calcium ions to pass
through (Table 9). Initially, the
resin exchanged its sodium ions for
calcium ions in solution; however,
the resin then exchanged these
calcium ions for heavy metals. After
70 bv had been processed, the
effluent contained essentially the
same calcium concentration as
the column feed. The column was
regenerated with 3.6 percent HCI fol-
lowed by conversion to .the sodium
form with NaOH. Table 10 shows
the composition of the acid
regenerant solution. In this case,
ion exchange treatment reduced
the waste volume associated with
the pollutants to less than 5 percent
of the original volume.
In a second application, an ion ex-
change waste treatment unit
was installed to treat the combined
waste flow from a plating shop
performing copper, nickel, and
assorted cyanide plating processes.
Table 9.
Removal of Zinc and Cadmium from Wastewater by Weak Acid Cation Resin3
. . . . Total loading
Bed volume sampled (ga|/ft3 resin)
10
25
35
55 ,
70
1 00
150
1 60
1 65
1 75
190 j
200 •
225 :
75
190
260
410
520
750
1,120
1,200
1 ,230
1 ,300
1 ,420
1 ,500
1,680
pH
10.1
10.1
8.3
7.2
6.0
7.0
6.9
6.8
6.8
6.7
6.8
6.8
6.8
Leakage (ppm)
Zinc
0.01
0.01
0.01
0.01
0.16
0.1
0.13
0.25
0.37
0.56
0.64
1.3
6
Cadmium
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
Calcium
1
3
53
303
338
385
404
405
407
404
395
394
395
aNa form.
Note._Feed characteristics: 391 ppm Ca, 91 ppm Zn, 0.12 ppm Cd, 350 ppm Mg, 57 ppm Na, 3.5
ppm Mn, 0.12 ppm Ni; pH =4.7; 8-bv/h (1 -gal/min/ft3) flowrate.
SOURCE: Rohm and Haas Company, "Ion Exchange in Heavy Metals Removal and Recovery," Amber
Hilite No. 162, Philadelphia PA, Rohm and Haas Company, 1979.
Table 10.
Acid Regenerant Composition
Bed volume3
1
2
3
4
5
6 •
7
8
Average
pH
6.0
2.4
0.4
0.3
0.5
2.2
3.0
3.1
Constituent (ppm)
Zinc
600
1 3,000
3,000
2,600
290
3.3
0.2
0.06
2,440
Cadmium
3.2
14
1.8
1.3
0.12
0.01
0.01
0.01
2.6
Calcium
3,380
6,944
1,003
677
67
1.7
0
0.6
1,500
"3.6% HCI in bv 1 through 4; distilled water in bv 5 through 8.
SOURCE: Rohm and Haas Company, "Ion Exchange in Heavy Metals Removal and Recovery," Amber
Hilite No. 162, Philadelphia PA, Rohm and Haas Company, 1979.
The entire wastewater flow was
collected in a single sump equipped
with a pH control system to ensure
that cyanide wastes were not
subjected to acidic conditions. The
ion exchange columns were strati-
fied bed units containing strong
base, weak acid, and strong acid
resins. Table 11 gives the waste-
water composition at various
points in the treatment system and
the concentration of the purge
streams. Sample points are raw feed,
filtered feed, filter backwash,
regenerant purge, and treated
effluent (see Figure 12b).
The design of the ion exchange
wastewater treatment system is es-
30
-------
Table 11.
Treatment of Metal Cyanide Wastewater by Ion Exchange: Pollutant Analysis
Content (ppm) at safnple point
Constituent
Total cyanide
Cadmium
Calcium
Chromium
Copper
Iron
Nickel
Zinc
Raw feed
0 8
61 4
1 37
211
14 2
3 14
42
Filtered
feed
31 4
ft A
00 0
n R9
ft op
0 0
o QO
00
Filter
backwash
Regenerant
purge
Treated
effluent
0.0001
1.637
0.356
0.41
0.195
0.425
2.62
SOU RCE: C. Terrian, Best Technology, Inc., personal communication to P. Crampton, Aug. 10,1980.
Table 12.
Commercially Demonstrated Resin Systems for Wastewater Treatment
Resin system
Application
Weak acid Selective removal of heavy metals from untreated
wastes |
Chelating cation Selective removal of heavy metals from untreated
wastes •
Removal of trace concentrations of heavy metals
from solutions with higfi background cation con-
centrations [
Selective removal of heavy metals from solutions
containing .metal comp'lexing compounds
Stratified bed (strong base, weak acid, j
strong acid) Selective removal of metal cyanide, anionic metal
complexes, and multivalent cations from solution
sentially uniform over the range of
pollutant removal capabilities
it exhibits. Proper resin selection,
however, is the key to effective
and efficient pollutant removal.
Testing to verify performance of a
resin system is essential before
system selection. Table 12 presents
varieties of resin systems in commer-
cial use for pollution control and
the pollutant removal capabilities
of each.
Equipment Cost
Whether ion exchange is used to
polish the effluent of an existing
treatment system or whether it
is applied to treat the waste
directly, the ion exchange unit is
basically the same. The major
equipment cost differences between
the two systems js in the auxiliaries.
For a polishing application after
conventional treatment, the auxiliary
requirements arejprovided by
the upstream process. In a direct
treatment application, however,
these items add significantly to the
total system cost Determinants
of the total system cost include:
• Feed pretreatrrjient requirements
• Volumetric and contaminant
loadings (resin! volume needed)
• Regeneration mode
• Equipment needed to process or
store regeneration solutions and
purge streams
To pretreat the wastewater before it
passes through the ion exchange
column, suspended solids removal
and coarse pH adjustment are
needed; removal of organic com-
pounds may also be required. (Foul-
ing by organics is primarily a
problem with strong base anion
resins.) Organics can be removed
using activated carbon or synthetic
adsorbent materials. The syn-
thetic materials have the advantage
of being regenerate; spent carbon
must be disposed of and replaced.
As a rule, filters that remove or-
ganics are also effective for remov-
ing suspended solids.
Similarly in a polishing application
the upstream process can treat
the regeneration purge streams.
For direct treatment, a batch
destruct system is needed. Table 13
presents typical costs for the
auxiliaries commonly associated
with ion exchange systems.
»
Exchange column size and the asso-
ciated resin volume specifications
depend on the wastewater flovvrate
and the contaminant loadings.
Resin manufacturers usually recom-
mend loadings for resin systems
at about 2 gal/min/ft3 (0.26 L/min/L)
of resin. Flowrate indicates the
flow of solution related to the aver-
age period of time it is in contact with
the entire resin bed. However,
the active zone of an ion exchange
system can be represented as
an exchange front proceeding clown
the column (Figure 19). The depth of
the active front is a function of
the volumetric loading and the
speed of the ion exchange reaction
kinetics. For ion exchange applica-
tions where the columns are
run to exhaustion, the benefits of
low loadings include greater
capacity per unit of resin and more
efficient use of regenerant chem-
icals. In wastewater treatment,
however, the lack of direct measure-
ment techniques to signal column
breakthrough precludes load-
ing the column to exhaustion, and
higher loading rates are recom-
31
-------
Table 13.
Typical Costs for Ion Exchange Equipment Auxiliaries
Auxiliary
pH adjustment tank, by ftowrate in gal/min:a
25
100 ...
Deep bod sand filters, by flowrate in ga!/min:b
75 . 4
100 ,
Batch treatment system, by volume in gal:c
OCft
1 em t
, Installed cost ($)
25,000
32.000
45,000
51,000
25,000
38,000
45,000
49,000
7,000
8,500
10,750
12,250
13,500
•20-mln retention, pH-controlled addition of NaOH, skid-mounted unit.
bOual filters with backwash system and backwash storage, skid-mounted unit.
cAgitatad reaction tank, pH-controlled addition of H2S04 and NaOH, ORP-controlled addition of
NaHS03, manual operation.
Note.—1980 dollars.
Influent water, B+ ions
Legend: '
+ resin containing A+ ions
resin containing 8"*° ions
©©©©©©©©©©
©©©©©©©©©©
©ffi©©©©©©©© (S
© + ©++©++© +
+ +©++©++© +
+ ©++©++©+ + +
+ +©++•©++© +
Exhausted zone
Ion exchange
active zone
Regenerated zone
4^- Treated water, A ions
mended. Regeneration is based
either on time or on cumulative
volume interval. As the interval will
be based on assumed wastewater
concentration established by
earlier testing, a safety factor must
be used in determining the dura-
tion of the cycle.
The columns would typically be
loaded to 75 percent of their actual
capacity before regeneration. That is
to say, there should usually be a
band of unreacted resin left over at
the end of the column on-stream
cycle. For both wastewater
treatment and polishing, higher
volumetric loading rates, if they
still result in a manageable regenera-
tion frequency, offer the advantage
of reduced equipment size and
cpst. Loading rates as high as 20
gal/min/ft3 (2.6 L/min/L) of cross
sectional area [equal to 5 gal/min/ft3
(0.65 L/min/L) of resin volume,
assuming a bed 4 ft (1.2 m) deep]
have been used in some applications.
High loading rates for polishing
systems are particularly advanta-
geous considering that the contami-
nant loading is usually low.
The regeneration sequence is labor
intensive and automation is cost
effective except where regeneration
is needed infrequently. Regen-
eration of a column normally takes
1 to 2 h. As a rule, columns in
multicolumn parallel flow arrange-
ments are designed to operate
at least 4 h before regeneration.
The costs for various column config-
urations are shown in Figure 20
for skid-mounted units that require
only piping and utility connections
for installation. The regenerants
are metered into the units by
eductors. Regeneration is manual
for the single- and dual-bed units.
The three-bed parallel flow unit is
sized based on two columns in ser-
vice while the third is being
regenerated; costs are with and
without automated regeneration.
Figure 19.
Ion Exchange Column in Service
32
-------
3-bed parallel flow, automated regeneration
3-bed parallel flow, manilal regeneration
2-column parallel flow, manual regeneration
1-column, manual regeneration
RESIN VOLUME PER COLUMN (ft3
1980 dollars. Add $200/ft3/oolurnn for chelating resin.
Note.—Skid-mounted unit with weak acid cation resin, acid and, base regenerant,
storage, and all internal piping and valves.
Figure 20.
Ion Exchange Unit Costs
Operating Cost
The chemical cost of operating an
ion exchange system relates
directly to the quantity of toxic
contaminants removed from the
wastewater by the resin bed. The
chemical efficiency of the ion
exchange reaction is a function of
the resin selected and of the per-
centage of the resin's exchange
capacity used. This relationship is
I .
shown in Figure 21 for typical strong
and weak acid cation resins over
a range of acid regenerant doses.
The weak acid resin requires signifi-
cantly less regenerant per unit
of exchange capacity.
Figure 21 also shows that the capa-
city of weak acid| resin increases
almost linearly with the amount
of regenerant. That is to say, increas-
ing the regenerant dose 50
percent increases the exchange
capacity by an almost equal ratio.
The strong acid resin, on the other
hand, achieves much greater
chemical efficiency per unit of
regenerant at lower regenerant
doses. Consequently, weak acid
resin systems can be designed to
use the total resin exchange
capacity; this capability reduces
either required resin volume or
regeneration frequency. Strong
acid systems will realize greater
efficiency if they are designed to
use approximately 40 to 60 percent
of the total resin exchange capacity.
Determining, exchange capacity
requirements requires analysis of the
wastewater feed and column
effluent chemical concentrations.
Consider the weak acid resin system
whose performance for removal
of zinc and cadmium was described
in Table 9. Assuming that a concen-
tration of 1 ppm zinc in the efflu-
ent signaled the end of the cycle, 175
bv of solution could be treated
before regeneration. Table 14 gives
the composite feed and effluent
concentrations in milligrams
per liter and equivalents per liter of
solution. The change in the equiva-
lents per liter represents the number
of resin exchange sites that would
be exhausted if 1 L or solution
were passed through tne exchange
column. The test indicated that
each liter of solution treated would
exhaust 0.0145 eq of resin exchange
capacity. Breakthrough occurred
after 175 bv had been treated,
indicating that the resin had a total
capacity of 2.5 eq/L, which is the
same as the resin manufacturer's
data indicated.
Assuming a three-column parallel
flow unit is selected to treat the
50-gal/min (190-L/min) waste
stream and that the columns
are operated on a 4-h cycle, the
necessary column size can be deter-
mined. It is assumed that the resin
capacity is actually 80 percent
of the theoretical capacity. This
adjustment is similar to applying a
fouling coefficient to a heat transfer
surface and accounts for gradual
33
-------
2.5
2.0
1.5
I
5
1.0
0.5
weak acid cation resin
strong acid cation resin
J.
4 6
REGENERANT LEVEL (Ib HCI/ft3)
Figure 21. i
Exchange Capacity versus Acid Regenerant Load for Cation Resins
Table 14.
Resin Capacity Based on Test Results '.
Feed
Product
Constituent
g/L
eq/L
g/L
eq/L
Change (eq/L)
Zinc
0.39
0.35
0.09
0.06
0.03
0.001
0.01 95
0.0292
0.0028
0.0026
0.001 .
(a)
0.3
0.27
C)
0.32
0.02
(")
0.015
0.0225
.(')
0.01 39
o.pooe
'.(')
0.0045
0.0067
0.0028
-O.01 1 3
0.0004
0.0001
"Negligible.
Not*.—Exchange requirements: per liter of feed, 0.0145 eq/L; per 175 bv of feed, 2.53 eq/L of resin.
deterioration in resin performance.
The adjustment yields a resin
capacity of 2 eq/L
The resin volume requirement
calculation per column is shown in
Table 15. The unit is designed
to have two of the three columns on
stream at any time. Assuming
that the column is run to exhaustion,
5.8 ft3 (164 L) of resin would
be needed per column.
No direct indication of column
breakthrough is available for
end-of-pipe process applications.
To prevent discharging high concen-
trations of regulated pollutants, the
columns can only be operated to
some fraction of theiractual capacity;
75 percent is a reasonable safety
factor. Required resin volume
would then increase to 7.8 ft3 (220 L)
per column.
Two safety factors, then, have
been used in sizing the ion exchange
system; one to compensate for a
gradual deterioration of resin
exchange capacity and one to com-
pensate for lack of direct means
of determining column breakthrough.
Table 15 shows regeneration
chemical consumption and cost,
the purge streams from the unit, and
the waste concentration factor.
The purge stream containing the pol-
lutants is approximately 7 percent
of the original volume of wastewater.
Consumption of HCI and IMaOH for
the system was assumed at 120
percent of the stoichiometric reagent
requirement, based on the theo-
retical resin exchange capacity of
2.5 eq/L. Sodium hydroxide needs
are only slightly above stoichiomet-
ric amounts, despite the resin's
preference for being in the hydrogen
form, because the product of the
caustic regeneration reaction is not
ionized. The caustic regeneration
reaction is:
R-H + NaOH -» R-Na + H2O (9)
Once the resin's hydrogen ion
is exchanged, it combines with a
34
-------
Table 15. j
Column Size Determination for Three-Column Parallel Flow Unit
Item
Factor
Flowrate:
Per column
Total
Column cycle
Exchange capacity per liter of feed
Capacity needed per column
Resin volume needed:
Per column
With safety factor
Regenerant consumption per column per cycle:
HCI (based on 100%)
NaOH (based on 100%)
Wash water
Cost per cycle8
Waste concentration factor
25 gal/min
50 gal/min
4h |
0.0145 eq |
4 X 60 X 25 X 13.79 X 0.0145 =
: 330 eq
[330/(2 eq/L)] XJ [1 /(3.79)(7.48)] = 5.8 ft3
5.8/0.75 = 7.8 f*3
45 Ib
50 Ib
390 gal
$15.09
(6,000 gal waste'waterpercycle)/(400gal
purge per cycle) = 15
a1980 dollars.
Table 16.
Annual Cost of Ion Exchange Treatment System
I
Item
Cost
Investment ($) 23,000
Operating cost ($/yr):
Labor, % h/shift at $8/h
Maintenance, 6% of investment
Regenerant chemicals, 4,000 h at 2 h/cycle
2,000
1,400
30,180
Total operating cost I 33,580
Fixed cost ($/yr):
Depreciation
Taxes and insurance
Total fixed cost...
Total annual cost.
2,300
230
2,530
36,110
Note.—1980 dollars. Operation 4,000 h/yr. Does not include water pret
ment system.
•eatment or batch treat-
hydroxide ion to form a non-ionized
water molecule and no longer
competes for exchange sites.
The installed cost of a three-column
parallel flow ion exchange system
with 7.8 ft3 (220 L) of resin.
per column, skid-mounted with
automated regeneration, is $23,000
(Figure 20). Table 16 shows the
total annual cost for a system
operating 4,000 h/yr. Capital and
operating costs of wastewater pre-
treatment and batch treatment
are not included.
A similar analysis can be performed
for a polishing system using per-
formance data from Figure 17
and a flowrate of |50 gal/min (190
L/min). The majoij difference
is in the large volume of solution
that can be treated per unit volume
of resin. In the direct treatment
case, 175 bv could be treated before
resin exhaustion; for the polishing
system (Figure 17) breakthrough
does not occur until 1,600 bv of
wastewater have been treated.
The longer column cycle associated
with polishing often eliminates
the justification for automated
regeneration. If regeneration
is manual, a two-column unit, oper-
ated in either parallel or series
flow with each column sized to
process the total flow, would
probably be most effective in terms
of regeneration frequency and
reliability. For automated units, a
three-column parallel flow unit,
designed to have one column off
stream for regeneration, would prob-
ably be most effective.
Safety features similar to those
used in the direct treatment analysis
will be applied to the polishing
system. The theoretical capacity will
be reduced to 80 percent of the
capacity indicated in the test
data to compensate for fouling,
and the column will be exhausted
to only 75 percent of its actual
capacity to avoid breakthrough
before regeneration. These features
will yield a volume-processing
capability of 960 bv of wastewater
before regeneration.
Regeneration frequency is a function
of column size. Table 17 gives
regeneration frequencies, costs
per regeneration cycle, and annual
costs for units in three sizes, each
operating 4,000 h/yr. Operating
time for each regeneration cycle was
assumed at 1 h. Operating costs
are approximately the same for
all three units, and would therefore
favor the smaller unit, which requires
the least capital outlay. The chem-
ical cost for a polishing system
is significantly lower than that
for the direct treatment system (Table
16) because most of the metals
are removed during conventional
treatment.
Evaluating resin capacity by running
a test column to exhaustion (Figure
17), is time consuming, particu-
larly for a polishing application
35
-------
Table 17.
Annual Cost for Ion Exchange Polishing Systems
Regeneration frequency
Item
16h
24 h
36 h
Column resin volume (ft3)'.
Ion exchange unit cost ($).
6.7 10.0 15.0
15,700 19,000 24,000
Operating costs (S/yr):
Labor, at $8/hb
Maintenance, at 6% of unit cost.
Regeneration chemicals0
2,000
780
1,660
1,330
900
1,660
890
1,140
1,660
Total operating cost.
4,440
3,890
3,690
Fixed costs (S/yr):
Taxes and insurance "• •
Total annual cost
1,570
1 60
1,730
6,1 70
1,900
190
2,090
' 5,980
2,400
240
2,640
6,330
•For chelating resin column. 50-gal/min flowrate.
b1 h labor per regeneration.
cBasod on 120% theoretical resin capacity = 1 eq/L.
Note.—1980 dollars. Systems operating 4,000 h/yr.
where the resin can process a
large volume of solution before
exhaustion. It is more expedient to
pass only sufficient volume through
the column until the column effluent
reaches equilibrium, then analyze
the feed and product for ionic
constituents. The exchange per unit
of feed solution will thus be deter-
mined and, when compared to
the resin's theoretical exchange
capacity (from manufacturer's
literature), can be used to predict
the solution volume the .resin
can process before exhaustion. The
safety factors described earlier
should be used with this approach.
The foregoing, then, are some of the
alternatives and process variables
to be considered in evaluating
ion exchange systems. Actual test?
ing, decisions regarding system
specification, and type of resin
should be left to experts in use of the
technology. An awareness of the
flexibility and power of the ion
exchange process for waste
treatment applications, however,
can aid the metal finisher in obtaining
the most effective system for the
least total cost.
36
-------
Chemical Recovery
and Recycle Systems
Pollution controlj legislation has
affected industry! by increasing
the economic penalty associated
with inefficient ujse of raw materials.
In the plating industry, for example,
loss of raw material in the waste-
water can result ih costs in three dis-
tinct areas: i
• Replacement of this material
• Removal of thfe material from the
wastewater before discharge
• Disposal of thfe solid waste
residue j
In response to the increased cost
of raw materials, iplating shops
are modifying their processes
to reduce their losses. Recent years
also have seen trie cost-effective
application of various separa-
tion processes thkt reclaim plating
chemicals from rinse waters,
permitting reuse of both the raw
material and the water.
Ion exchange, evaporation, reverse
osmosis, and electrodialysis have all
been used in the |plating industry
to recover chemicjals from rinse
solutions. These processes have in
common the ability to separate
specific compounds from a water
solution, yielding la concentrate
of those compounds and relatively
pure water. The concentrate is
recycled to the plating bath and the
purified water is reused for rinsing.
To determine which separation
process is best suited for a particular
chemical recovery, application, it
is usually necessary to evaluate both
general and site-specific factors,
for example: j
i
• General factors would include
rinse water concentration,
volume, and cojrosivity.
• Among site-specific factors are
floor space available, utilities
(such as steam,; chemical
reagents, electricity) available,
and degree of concentration
needed to recycle the chemicals
to the bath. ;
As a rule, ion exchange systems are
suitable for chemical recovery
when the rinse water feed has a
relatively dilute concentration of
plating chemicals and the degree of
concentration needed for recycle
is not great. Ion exchange is
well suited for processing corrosive
solutions. The process has been
demonstrated commercially
for chemioal recovery from acid
copper, acid zinc, nickel, cobalt, tin,
and chromium plating baths. It
has also been used to recover spent
acid solutions and for purifying
plating solutions to prolong their
service life.
Economic Analysis of Recoveiy
Systems
To evaluate the economic benefit of
installing ion exchange or other
recovery processes, the following
determination must be made:
• Quantity and replacement
cost of the chemicals and water to
be recovered
• Savings in wastewater treatment
cost expected to result from
recovery unit installation
• Reduction in solid waste and cost
of sludge disposal expected to
result from recovery unit installa-
tion
In evaluating a plating cnemical
drag-out recovery system, the rinse
water volume and chemical con-
centration must first be measured.
This step will establish the quantity
of chemicals available for recovery.
When the relationships of waste-
water volume and metal content to
the associated wastewater treatment
and sludge disposal cost have
been determined, the potential
savings can be determined. Table 18
shows the economic penalties for
losses of typical plating chemicals.
The high investment cost for
installing an automated recovery
process limits application of
this process to plating operations
with high drag-out rates, as illus-
trated for chromic acid recovery in
Figure 22. The analysis assumed an
37
-------
Table 18.
Economic Penalty for Losses of Plating Chemicals
Cost ($/(b)
Chemical
Replacement Treatment3 Disposal13 Total
Nickel*
As NiS04
As NiCt2
Zinc cyanide, as Zn(CN)2:
Using CI2 for cyanide oxidation
Using NaOCl for cyanide oxidation. . .
Chromic acid, as H2CrO4:
Using S02 for chromium reduction. . .
Using NaHS03 for chromium reduc-
tion
Copper cyanide, as Cu(CN)2:
Using CI2 for cyanide oxidation
Using NaOCl for cyanide oxidation. . .
Copper sulfate, as CuS04
0.84
1.14
1.55
1.55
0.98
0.98
2.05
2.05
0.62
0.31
0.34
0.80
1.68
0.53
0.76
0.80
1 .68
0.31
0.38
0.52
0.50
0.50
0.64
0.64
0.50
0.50
0.34
1.53
2.00
2.85
3.73
2.15
2.38
3.35
4.23
1.27
*At concentration of 100 mg/L in wastewater.
b4% solids by weight at $0.20/gal.
Note.—1980 dollars.
SO
40
ce
UJ
t 30
i
o
cc
20
10
J
1 2 3 | 4 E
DRAG-OUT RATE (Ib CrO3/h)
.—Operating 4,000 h/yr. $30,000 investment cost. Tax rate at 48% of profit.
Figure 22.
Return on Investment in Chromic Acid Recovery Unit
investment cost for the recovery
system of $30,000, with the
unit depreciated over 10 years.
Typical operating, labor, and mainte-
nance costs for an ion exchange
system were used to determine
operating costs. Chemical savings
were derived from Table 18,
which indicated a total saving of
$2.15/lb of H2CrO4 recovered (equal
to $2.50/lb of Cr03): From the
foregoing, a reasonable rate of return
is achieved for a Cr03 drag-out rate
above 3 Ib/h (1.4 kg/h), for which
payback equals 2.8 years. Plating
operations with rates significantly
lower than 3 Ib/h (1.4 kg/h)
would not be economically justified
in installing this recovery system.
Tax credits associated with invest-
ments in pollution control hard-
ware were not included in the
foregoing analysis. The credits would
improve the economy of otherwise
marginal investments, but not
enough to justify installing an auto-
mated recovery system in an
operation with low drag-out rates.
Drag-Out Recovery
by Ion Exchange
The Reciprocating Flow Ion Ex-
changer (RFIE) is the kind of ion ex-
change system most widely used
for chemical recovery from plating
rinses. This proprietary unit was
especially developed for purifying the
bleed stream of a large volume
solution such as the overflow from a
plating rinse tank. It operates
on the principle that, for the short
period of time the unit goes off
stream for regeneration, the buildup
of contaminants in the rinse sys-
tem is negligible.
The RFIE units are more attractive
than fixed bed systems for plat-
ing chemical recovery because the
columns use smaller resin vol-
umes and, therefore, capital costs
and space requirements are usually
lower. The units incorporate
regenerant chemical reuse tech-
niques to reduce operating costs and
38
-------
Spent rinse
water
PACKAGE
UNIT COMPONENTS
I
Note.—Automated control included with package unit. !
(b)
• ON STREAM (LOADING) •
Rinse
water
|
1 REGENERATION
WASHING
Product
Exhaust
(to waste Water
treatment)
i
Purified
Exhaust water
(to waste
treatment
I Cation I
Water
NaOH
NaOH
Figure 23.
Chromic Acid Recovery RFIE System: (a) Hardware Components and (b) Operating Cycle
yield higher product concentration
for recycle. They are sold as skid-
mounted package units, which
are automated to minimize operating
labor requirements. Two basic
units are available for drag-out re-
covery: one for chromic acid recovery
and one for metal salt recovery.
Another unit is designed to deionize
mixed-metal rinse solutions to
recover only thej water and concen-
trate the metals [before treatment.
i
Chromic Acid. Figure 23 shows
the hardware components of an RFIE
chromic acid recovery system
and necessary auxiliaries and de-
scribes the operating cycle. The
segregated rinse water after a
chromium plating bath (or baths) is
pumped to the ion exchange
unit and passes in series through a
cartridge filter, a strong acid
cation resin bed, and a strong base
anion bed. The demineralized water
is returned to the rinse system.
The RFIE unit regenerates itself auto-
matically based either on a cycle
timer or on the conductivity of
the treated water. With the conduc-
39
-------
I
tivity controller, the conductivity of
the treated water is compared
with that of the unit feed. When the
unit is no longer achieving sufficient
conductivity reduction, regenera-
tion is initiated. Regeneration
frequency is based on the quantity of
chromic acid in the rinse and the
unit's resin volume. The unit is
off stream for regeneration for ap-
proximately 20 min. The chromate
ions removed from the rinse are
concentrated in the anion resin bed.
They are eluted in the form of a
sodium chromate solution when this
bed is regenerated with sodium
hydroxide. The sodium chromate so-
lution is passed through a second
strong acid cation resin bed to
convert the sodium chromate to
chromic acid. The recovered chromic
acid solution is stored and used for
chemical makeup in the chromium
plating bath. The product concentra-
tion is approximately 10 percent
chromic acid. After the resin beds
are washed with water, the unit goes
back on stream.
The RFIE units come in several
sizes; higher chromic acid loading
rates require larger resin bed
volume. Ideally, the unit performs
two cycles per hour. Each cycle
reclaims a certain amount of chromic
acid and consumes a set amount
of regenerant chemicals. Table 19
shows the chemical savings,
reagent cost, and amount of chro-
mium recovered per cycle.
Figure 24a presents the purchase
cost of RFIE units for chromic
acid recovery as a function of the
amount of chromic acid the unit can
recover. Including reagent and
product storage, piping and utility
connections, startup, and shipping
expenses, the total installed cost
fora system should be approximately
120 percent of the unit cost.
Metal Salts. RFIE units are recover-
ing plating drag-out from nickel,
copper, zinc, tin, and cobalt plating
rinses. The major area of application
is for nickel plating baths. Two
basic units are used for metal
Table 19.
Performance of RFIE Chromic Acid Recovery Unit3
Item
Value (per cycle)
Regenerant solutions:
NaOH 3.7 Ib
H2SO4 12.2 Ib
Water .
Spent rinse...
Purified rinse ,
80 gal
1,200 gal/cycle; 200 ppm CrO3
1,200 gal/cycle
Product, Cr03 2- Ib each at 10% CrO3
Purge to waste treatment.
SO gal
Chemical savings ($):b '
CrO3. 2 Ib at $2.50/lb.
NaOH at$0.15/lb .'...
H2SO4 at $0.05/lb
5.00
-0.56
-O.61
Total saving per cycle.
3.83
"0.35 ft3 anion resin.
b1980 dollars.
Three-column parallel-flow ion exchange package unit ready for installation
40
-------
(a) 50
o
o
40
30
20
10
4 6
DRAG-OUT RATE (Ib
(b) 50 r
40
30
fc
8
t 20
10
! 8
10
"Based on 2 cycles per hour.
bDual-bed (cation and anion).
°Based on 7.5 cycles per hour.
10
20
30
40
50
DRAG-OUT RATE (Ib NiS04 • 6H20/h) ]
Figure 24.
I
recovery. One employs a cation bed
to reclaim the metal ions and an
anion bed to remove the counter-
ions; the deionized water is recycled
to the rinse station. For applications
where only recovery of the metal
is desired, the anion bed is elim-
inated and the metal-free water is
discharged.
Figure 25 presents RFIE system
I hardware and the necessary auxil-
[ iaries for metal salt and rinse
' water recovery and describes the
operating cycle. The segregated
rinse water after the plating bath (or
baths) is pumped to the ion ex-
change unit and passed, in series,
through a prefilter, a strong acid
cation resin bed, and a strong
base anion bed. The demineralized
| water is returned to the rinse
system. The metal ions concentrate
on the cation resin and are eluted
with either sulfuric or hydrochloric
acid. The concentrated salt solution
(either the metal sulfate or chlo-
ride) is stored and used for chemical
makeup in the plating bath. The
i regenerantfrom the anion bed is sent
to waste treatment.
Metal salt recovery units also come
in various sizes, with unit size
determined by the amount of metal
salts in the rinse water. Each
cycle will reclaim a set amount of
metal salts and consume a set
amount of regenerant chemical.
Table 20 shows chemical savings,
reagent consumption, and the
amount of metal recovered per cycle
for nickel plating recovery.
The purchase cost for an RFIE metal
salt recovery unit is presented Tn
Figure 24b as a function of the
amount of metal salts the unit can
recover. The price is for a unit
with both a cation and an anion bed;
the price is approximately one-
third less for a unit with a si'nglecat-
ion bed. Including trie basic RFIE
unit, reagent and product storage,
piping and utility connections, start-
up, and shipping, the total installed
Equipment Cost of RFIE Units: (a) Chromic Acid Recovery and
(b) Metal Salt Recovery i
41
-------
(a)
Spent rinse
water
Note.—Automated control included with package unit.
PACKAGE UNIT COMPONENTS
~~ T"
ities •*
Utilities
o Water
o Air
(b)
I ON STREAM (LOADING) •
Rinse water
•
Purified water
Filter
I
, REGENERATION
Recovered j
nickel
electrolyte ' Waste
WASHING ,
Water
Waste
Cation
Anion
i
Anion
T
Caustic
Anion
Water
Acid
Acid
Figure 25.
Metal Salt Recovery RFIE System: (a) Hardware Components and (b) Operating Cycle
cost for a recovery system should
be 120 percent of the unit cost.
Acid Recovery Systems
Ion exchange is used to purify
concentrated acids (such as sulfuric,
hydrochloric, and nitric) that have
been contaminated by metal salts.
The process, called acid retardation,
brings an acid solution in contact
with a strong base anion resin. The
resin will sorb the strong acid
but not the metal salts. The acid can
be desorbed with water. This
technique has been commercialized
using reciprocating flow methods
similar to those described for
chemical recovery.
The two process steps are
shown in Figure 26. In the on-
stream step (upstroke), the metal-
42
-------
Table 20.
Performance of RFIE Metal Salt Recovery Unit3
Item
Value (per cycle)
Regenerant solutions: j
NaOH 0.63 Ib i
H2S04 1.2 Ib j
Water 58 gal '
Spent rinse 250 gal/cyciej 600 ppm NiSO4 • 6H2O,
150 ppm NiCI • 6H2O
Purified rinse 250 gal/cycle'
Product, NiS04 • 6H2O 1.7,|b each a{ 17% NiS04 • 6H2O
Purge to waste treatment 58 gal i
Chemical savings ($):b
Anhydrous NiS04, 1 Ib at $1.53/lb 1.53
NaOH at $0.15/lb -0.09
H2S04 at $0.05/lb -0.06
Total savings per cycle .
1.38
°0.35 ft3 cation resin.
b1980 dollars.
(a)
-Water
Compressed
air
Water
metering
tank
Spent
acid
metering
tank
(b)
Compressed air
Spent acid
I
I
Water
metering
tank
Spent
acid
metering
tank
I salt solution
to waste
1
Resin bed
Resin bed
\
Purified acid (product)
Figure 26. j
Acid Recovery System Operation: (a) Upstroke and (b) Ddwnstroke
salt-contaminated acid is metered
into the bottom of the resin bed.
The free acid is sorbed by the resin
and the metal salt byproduct
solution flows out the top of the bed.
In the regeneration step (down-
stroke), water elutes the acid
from the resin, yielding an acid con-
centration equal to that of the
feed solution and a lower concentra-
tion of metal contaminants.
Two applications are seen for this
system:
• Purification of strongly acidic
process baths
• Recovery of excess acid from cat-
ion exchange regenerant solutions
Demonstrated uses of ion exchange
acid purification include removing
aluminum salts from sulfuric
acid anodizing solutions, removing
metals'from nitric acid rack-stripping
solutions, and removing metals
from sulfuric and hydrochloric acid
pickling solutions. The major
area of application is for aluminum
anodizing solutions.
Investment in an acid purification
system is justified by the savings in
purchases of replacement acid
and of neutralizing reagents for treat-
ing the spent acid. The amount
saved depends on the type of acid
to be recovered, the volume and
concentration of the spent acid dis-
carded yearly, and the cost of
treating the spent acid.
Acid purification systems are
available in a range of sizes. Size is
a function of the volume of acid
that can be purified per unit of time;
size requirement is determined
by the rate at which metal salt
accumulates in the acid bath.
Table 21 shows the feed, product,
and waste stream concentration of a
purification system for sulfuric acid
43
-------
Table 21.
Performance of Acid Recovery Unit
for Purifying Sulfuric Acid Pickling
Solution
Item
Performance
Water
Food, at 1 1 gal/h:
H2S04
Iron ,
Product, at 10.4 gal/h:
H2S04
Iron ,
Purge, at 7.6 gal/h:
H2SO4
Iron
Iron removed
Acid recovered
7.0 gal/h
0.94 Ib/gal
1 .1 5 Ib/gal
0.94 Ib/gal
0.74 Ib/gal
0.07 Ib/gal
0.65 Ib/gal
4.9 Ib/h
94%
Table 22.
Cost3 and Iron Removal Capacity of Sulfuric Acid Purification Unit
Unit size (ft3 anion resin)
Item
0.40
2.79
14.12
Unit cost ($)b ,...
Acid feed rate (gal/h)....
Iron removal rate (lb/h)c..
Savings ($/h):c
H2SO4, at $0.05/lb.,
NaOH, at $0.08/lbd
10,000 20,000 56,000
11 80 400
4.9 35.8 179
0.31
0.41
2.26
3.02
11.32
15.09
•1980 dollars.
bSkid-mounted package unit, including filter and automated control systems.
"Based on Table 21.
dFor neutralization.
pickling solution. Based on the
quantity of iron in the purge stream,
the iron salt removal capacity
can be determined from vol-
ume processing capacity. Once
the rate of iron accumulation in the
acid solution has been determined, a
purification Unit with equal salt
removal capacity can be selected
to control the buildup.
Acid purification systems are
inexpensive and simple to install.
Air supply and water are the only
utilities needed. Piping requires only
feed, product, and waste stream
connections. Table 22 gives
approximate costs for units of differ-
ent sizes, the volume of sulfuric
pickling acid they can process, and
the value of the recovered acid.
In another, well-established
application of ion exchange, metal
buildup in dilute acid solutions
is controlled by passing the solution
through a cation exchanger in the
hydrogen form. This approach
has been used for hydrochloric and
sulfuric acid etching solutions
and to remove trivalent chromium
and ferrous ions from chromic
acid solutions.
44
-------
Bibliography
Abrams, I. M. "Selective Removal of
Heavy Metals frorti Wastewaters
by Ion Exchange and Absorb-
ent Resins." Paper read at South
Central Regional fleeting of National
Association of Corrosion Engineers,
1974. ]
i
i
Anderson, R. E. "Some Examples
of the Concentration of Trace
Heavy Metals with Ion Exchange
Resins." In Proceedings: Trace
Heavy Metals in Water, Removal
Process and Monitoring. Nov. 15-16,
1973. Princeton lilJ, Princeton Uni-
versity Press, 1973.
Brown, C. J., D. Dbvy, and P. J. Sim-
mons. "Nickel Sa|t Recovery by
Reciprocating Flow Ion Exchange."
Paper read at 62rjd Annual Tech-
nical Conference bf American
Electroplaters' Society, 1975.
Brown, C. J., D. Davy, and P. J. Sim-
mons. "Purification of Sulfuric
Acid Anodizing Solutions." Plating
and Surface Finishing, 66(1):54-57,
Jan. 1979. ;
Brown, C. J., and C. R. McCormick.
"Pollution Abatement via Re-
source Recovery for a Plastics Plating
Shop." Paper read at American
Society of Electroplated Plastics
12th Annual Meeting, 1979.
Calmon, C., and l-j. Gold. Ion
Exchange for Pollution Control.
Vol. 1. West Palm Beach FL, CRC
Press, 1979. -!
Crampton, P. "Application of
Separation Processes in the Metal
Finishing Industry!" Paper read
at Third Annual EPA/AES Conference
on Advanced Pollution Control in
the Metal Finishing Industry, 1980.
Dormer, K. Ion Exchangers; Proper-
ties and Applications. Ann Arbor
Ml, Ann Arbor Science, 1977.
Dow Chemical Coinpany. "Anion
Resins: Selection [Criteria for Water
Treatment Applications." Idea
Exchange, 5(2), undated.
I
Dow Chemical Company. "Chemical
Processing by Ion Exchange."
Midland Ml, Dow Chemical
Company, undated.
Dow Chemical Company. "Dower
WGR-2 Weakly Basic Anion
Exchange Resin." T.D. Index 330.1.
Midland Ml. Dow Chemical Com-
pany, undated.
Dow Chemical Company. "Weak
Acid Cation Resins." Idea Exchange,
2(3), undated.
Kunin, R., and R. J. Myers. Ion
Exchange Resins. John Wiley and
Sons, 1950.
Kunin, R. "Ion Exchange for the
Metal Products Finishers." Products
Finishing, Apr.-May-June 1969.
(3 pt. article)
Rohm and Haas Company. "Helpful
Hints in Ion Exchange Tech-
nology." Philadelphia PA, Rohm
and Haas Company, May 1972.
Rohm and Haas Company. "Am-
berlite® 200." Philadelphia PA,
Rohm and Haas Company,
Nov. 1976.
Rohm and Haas Company. "Ion
Exchange in Heavy Metals Removal
and Recovery." Amber Hilite No.
162. Philadelphia PA, Rohm
and Haas Company, 1979.
Rohm and Haas Company. "Porous
Polymers and Absorbents—A
Review of Current Practices." Amber
Hilite No. 163. Philadelphia PA,
Rohm and Haas Company, 1980.
Schweitzer, P. A. Handbook of
Separation Techniques for Chemical
Engineers. New York NY, McGraw-
Hill, 1979.
45
-------
U.S. Environmental Protection Wing, R. E. "Processes for Heavy Yeats, A. R. "Ion Exchange Selec-
Agency. Environmental Pollution Metal Removal from Plating Waste- tively Removes Heavy Metals
Control Alternatives: Economics of water." Paper read at First Annual from Mixed Plating Wastes." Paper
Wastewater Treatment Alternatives EPA/AES Conference on Advanced read at 32nd Purdue Industrial
for the Electroplating Industry. Pollution Control for the Metal Waste Conference, 1977.
EPA 625/5-79-016, 1979. Finishing Industry, 1978.
46
U.S. GOVERNMENT PRINTING OFFICE 1881 -757-064/0321
------- |