Pollution Prevention and Control Technologies for Plating
Section 4 - Chemical Solution Maintenance
4.7 MEMBRANE ELECTROLYSIS
4.7.4 Technology/Equipment Description
220.127.116.11 Commercially Available Equipment
Membrane electrolysis is the newest of the chromium bath purification
technologies. In the most basic sense, these units consist of:
(1) a tank containing an anode and cathode compartment separated
by a selective membrane(s), and (2) a power source. The membranes
are ion specific in a manner similar to ion exchange resin. They
allow the passage of only positive or negative ions, depending
on the type of membrane. This design element is used in this text
to distinguish these units from ion transfer technologies that
employ non-ion permeable membranes such as ceramic pots or a polyfluorocarbon
material (Section 4.6). Cation specific membranes are used for
chromic acid purification, the most common application for this
technology. When current is applied to the cell, cations present
in the anolyte flow through the cation permeable membrane. The
anions in the catholyte are restricted by the membrane and remain
in that compartment. Trivalent chromium present in the anolyte
(plating bath) is reoxidized to the hexavalent state. Membrane
electrolysis units can be configured with only cation or anion
membranes or both.
It is important to note that Cr+3 is a cation, while Cr+6 combines
with oxygen in solution to form CrO4 and acts like an anion. Therefore,
in a membrane electrolysis system, the Cr+3 that is not oxidized
to Cr+6 will behave like iron and aluminum ions and be transported
through the cation membrane while the Cr+6 remains in the plating
solution. The loss of Cr+3 in this manner is undesirable since
Cr+3 can be restored to a useful state by simple oxidation at
the anode. Further, the catholyte solution, into which the cations
are transported, becomes more quickly spent when Cr+3 passes through
the membrane. The cathode solution is subsequently discarded and
its chromium content will increase disposal costs. Therefore,
units with a rapid Cr+3 oxidation rate will generate less process
residual and the residual will have a lower chromium content.
The available commercial membrane electrolysis units are discussed
in Section 18.104.22.168. Four different membrane electrolysis manufacturers
were identified during the literature search, Users Survey and
Vendors Survey. A comparison of these technologies shows that
there are several basic variations among the units. The following
discussion highlights these differences.
The choice of catholyte solution is one major difference between
the various commercially available units. Most of the units use
mineral acid (typically sulfuric acid) as the catholyte, which
was used by the Bureau of Mines in their early work. With an acidic
catholyte solution, cations flow through the permeable membrane
and into the acidic catholyte. The electroplatable cations deposit
on the cathode and the non-electroplatable cations stay in that
solution as salts. Periodically, the cathode is cleaned and the
catholyte solution is discarded and replaced (370).
One of the available technologies uses a patented caustic catholyte
(combination of alkali reagents). The caustic catholyte causes
the precipitation of cations as they pass through the membrane.
This overcomes any problem of plating-out the cations on the cathode.
This catholyte converts multi-valent metal cations entering the
catholyte solution into insoluble hydroxides. The hydroxyl ions
needed to react with the metal cations are formed at the cathode
of the cell. Precipitation of the cations prevents a loss of conductivity
and eliminates the buildup of a deposit on the cathode; consequently,
the operational period is extended to two or more weeks and there
is no need to clean the cathode.
The overall appearance of the commercially available units varies
significantly. Some of the units are configured similarly to the
Bureau of Minesí prototype units. The chromic acid bath
is pumped to a tank on either a batch or continuous basis. The
tank contains multiple cathode cells, around which the anolyte
(chromic acid bath) is circulated. Another commercial unit has
a conventional electrodialysis design with ìstacksî
of membranes that are kept together in a clamping unit which looks
similar to a filter press. Anode and cathode electrodes are located
at opposite ends of the stack. The plating solution is pumped
through the stack. The pumps and other support equipment are located
on a skid. A third configuration, places the cathode in a ìmembrane
sock,î which is lowered into the plating tank. The catholyte
is circulated from a side tank into the cathode compartment (ref.
The capacities and capabilities of membrane electrolysis units
should be measured in terms of the tramp metal removal rate (g/hr),
which will vary over a range of tramp metal concentrations in
the anolyte. Unfortunately, there is a limited amount of such
data available. Exhibit 4-29 shows the results of a comparative
test of a caustic catholyte membrane electrolysis system and a
single cell porous pot unit.
22.214.171.124 Commercially Available Equipment
This subsection contains a description of commercially available
ion specific electrochemical membrane technology equipment. This
is intended to provide the reader with information and data on
a cross section of available equipment. Mention of trade names
or commercial products is not intended to constitute endorsement
Ionsep Corporation has manufactured electrochemical processing
equipment since 1985 and has sold more than 150 units to metal
finishing firms. Approximately two-thirds of their units were
sold for chromic acid applications. The basic Ionsep unit consists
of a electrolytic cell, a catholyte tank (located near and plumbed
into the electrolytic cell ), and a power source (typically a
500 amp rectifier). The cell is inserted in a protective 6 to
9 in. diameter PVC pipe approximately 36 in. long that is kept
in the plating bath during operation (alternatively, a side tank
is used, especially with larger applications). The pipe is perforated
to permit plating solution to enter and flow around the cell membrane.
Within the PVC pipe is a cylindrical shaped anode, which is exposed
to the plating bath, and a selective membrane ìsockî
that creates an inner cathode compartment. A cathode and catholyte
solution (an alkaline solution) are located in the sock. This
configuration is termed a two compartment cell where the anode
compartment is the plating bath and the cathode compartment is
the space inside the sock.
In operation, a direct current is passed to the cell electrodes
and the catholyte is continuously circulated from the catholyte
tank (typically a 55-gal. plastic drum with a small circulation
pump located on top) into the sock formed by the membrane. The
cations (e.g., iron) are electrically driven from the plating
bath through the selective membrane and into the cathode compartment.
The metal cations precipitate as hydroxides. Most of the trivalent
chromium present in the bath is converted to hexavalent chromium
at the cellís anode. Some trivalent chromium, which is
a cation, passes from the bath, through the membrane, and into
the catholyte solution. The tramp metal removal rate for a small
Ionsep unit (130 amp unit) is shown in Exhibit 4-29.
The two compartment Ionsep cell is applicable to hard chromium
and chromic acid anodizing baths and other baths with similar
chemistry. Baths with more complex chemistry often require cells
with three or more compartments. For example, a caustic chromium
stripper can be reformulated into sodium hydroxide and chromic
acid with a three compartment cell. The resultant sodium hydroxide
is reused as stripper and the chromic acid is reused in the plating
Large Ionsep systems are configured differently from that described
above. With the larger systems, the electrolytic cell is not located
in the plating tank, but rather, inside of a skid mounted PVC
piping system. With this configuration, the bath is pumped to
the cell and circulates back to the plating tank. Many of the
large systems also have a catholyte recycle capability, while
in the small systems the catholyte solution is discarded weekly.
The catholyte recycle system causes the separation of precipitated
metals from the catholyte which permits the reuse of the catholyte.
Ionsep units and systems are available with capacities from 150
amps to 15,000 amps. In addition to the chromic acid bath maintenance
(chromium plating, chromic acid anodizing and chomate conversion
coating), their equipment has been applied to nitric acid strip,
nitric-HF pickle, sulfuric acid pickle, hydrochloric acid, and
caustic chem mill etch maintenance. They also manufacture a closed-loop
rinse water recovery system that employs ion exchange and electrochemical
regeneration to recover chemicals and enable the reuse of water.
GOEMA, a German-based company, manufactures a family of membrane
technologies for water, wastewater, and bath maintenance. These
technologies include membrane electrolysis, diffusion dialysis,
ultrafiltration, and microfiltration. Their membrane electrolysis
units are designed with a stacked membrane configuration. A ìmembrane
stackî consists of a number of thin membrane compartments
created by alternating layers of ion-specific membranes and spacers.
The spacers are present to permit solutions (e.g., plating solution
and catholyte) to flow between the membrane layers. Either anion-
or cation-specific membranes or both are used to create a stack.
For hard chromium plating and chromic acid anodizing bath purification,
only cation membranes are used. The membranes and flow spacers
are held together by a cell frame and clamping unit, giving the
membrane stack the appearance of a plate and frame filter press.
In operation, plating solution and catholyte solution are pumped
into the stacked cell. Separate inlets and outlets and flow channels
are present for each solution. Anode (lead alloy) and cathode
(stainless steel) electrodes are located at the ends of the stack
in their associated electrolyte. When an electrical potential
is applied across the stack, Cr+3 present in the anolyte (plating
solution) is oxidized to Cr+6 and dissolved tramp metals migrate
toward the cathode and pass through the cation membrane into the
catholyte. Sulfuric acid (20 percent) is typically used as a catholyte
for these applications.
GOEMA units can be purchased in various capacities. The key sizing
variable is the number of compartments that make up the membrane
stack. GOEMAís basic unit (KRME-100) has a rated metals
removal capacity of 100 grams per hour. The membranes are approximately
4 ft2 each with a total membrane area of 20 ft2. The DC power
source is a 1,000-amp rectifier. The flow rate of anolyte through
the stack is approximately 6 gph (144 gpd). A single solution
pass through the stack provides a high cation removal rate. Approximately
one to two tank volumes of treatment will adequately purify a
moderately contaminated bath. Assuming this rate of bath purification,
a 500-gallon bath could be treated in approximately 1 week (two
tank volumes x 500 gallons/144 gpd = 6.9 days) (Based on the manufacturerís
rating for these units.) (ref. 384).
Graver Water Division of the Graver Company (Graver) is the distributor
of Tokuyama Soda membrane electrolysis equipment. These units
have been utilized for the following applications: nickel and
zinc recovery from galvanizing, caustic recovery from aluminum
etch, acid recovery from metal pickling, and aluminum recovery
from nitric acid solutions.
Kinetic Recovery Corporation (KRC) manufactures the Membrane Electrolysis
System 125 (Exhibit 4-30). This unit is designed for the oxidation
of Cr+3 to Cr+6 and the removal of tramp metals from chromic acid.
The basic unit is skid mounted and consists of: electrolysis container
with a drip pan; anode compartment including anode (Ti/Pt expanded
metal), inflow-assembly and electrical connections; stainless
steel cathode; ion exchange membrane; anolyte and catholyte feed
tanks; anolyte filter; catholyte pH control system; rectifier
(15V, 150A); and control panel. The dimensions of the skid are
48" x 21" x 26" (H).
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