PROCESS FOR ELECTRODIALYTICALLY CONTROLLING THE ALKALI METAL IONS IN A TIN-PLATING PROCESS

METHODE DE CONTROLE PAR ELECTRODYALISE DES IONS DE METAUX ALCALINS EN ETAMAGE

English Abstract

ABSTRACT OF THE DISCLOSURE
A tin-plating bath containing alkali metal
hydroxide is circulated through the anode compartment of
an electrodialytic cell. The cathode compartment contains
an electrolyte and when a current is impressed across the
electrodialytic cell, the alkali metal ions in the plating
bath migrate through the cation permselective membrane
separating the compartments and into the cathode compart-
ment. The stannate ions remain in the tin-plating bath in
the anode compartment. The removal of the alkali metal
ions controls the alkali metal hydroxide content of the
plating bath. This electrodialytic treatment of the bath
eliminates the conventional chemical treatment to remove
the excess alkali metal hydroxide. The electrodialytic
cell may be positioned in the tin-plating bath and where
feasible utilize the same power source as the plating
bath. Alkali metal stannate may also be recovered from
the rinse water by passing the rinse water through a
center compartment of a three compartment electrodialytic
cell bounded by a cation and an anion or neutral perm-
selective membrane.

Excess alkali metal ions may be removed from either the
plating bath or the rinse water of an acid bath by passing
the bath or rinse water through the center compartment of
a three compartment cell that is bounded on both sides by
a cationic permselective membrane. The alkali metal ions
are transferred from the center compartment, thus removing
the excess alkali metal ions from the bath and rinse
water.
- 2 -

Claims

The embodiments of the invention in which an ex-
clusive property or privilege is claimed are described as
follows:
1. A process for controlling the alkali metal ions in an
electrochemical tin-plating process comprising, withdrawing a
solution from said tin-plating process, said solution con-
taining an excess of alkali metal ions and a tin compound,
introducing said solution into the anode compartment of an
electrodialytic cell, removing from said solution by electro-
dialysis a portion of the alkali metal ions that is in excess
of the alkali metal ions necessary to maintain the tin compound
in solution, and returning said solution with said portion of
said alkali metal ions removed therefrom to said tin-plating
process.
2. A process for controlling the alkali metal ions in an
electrochemical tin-plating process as set forth in claim 1 in
which, said solution is withdrawn from the plating bath tank,
and said solution with said portion of said alkali metal ions
removed therefrom is returned to said plating bath tank.
3. A process for controlling the alkali metal ions in an
electrochemical tin-plating process as set forth in claim 1 in
which, said solution is withdrawn from the rinse tank, and said
tin compound in said solution is returned to the plating bath
tank.
- 27 -

4. A process for controlling alkali metal hydroxide in a
tin-plating bath comprising, electrodepositing tin from an
aqueous tin-plating bath containing an alkali metal stannate in
a tank, generating an alkali metal hydroxide in said bath,
removing a portion of said bath containing said alkali metal
hydroxide and said alkali metal stannate from said tank and
introducing said portion as an anolyte into the anode com-
partment of an electrodialytic cell, removing from said
anolyte by electrodialysis a portion of the alkali metal ions
that is in excess of that which is necessary to maintain the
concentration of alkali metal ions in said anolyte at a level
so that the concentration of the alkali metal stannate remains
substantially the same in said portion, and returning said
portion of said bath from said anode compartment having said
portion of said alkali metal hydroxide removed therefrom and
said alkali metal stannate retained therein to said tank.
5. A process for controlling alkali metal hydroxide in a
tin-plating bath as set forth in claim 4 which includes, intro-
ducing an electrolyte solution into the cathode compartment of
said electrodialytic cell, and separating said anode com-
partment and said cathode compartment with a cation perm-
selective membrane.
- 28 -

6. A process for controlling alkali metal hydroxide in a
tin-plating bath as set forth in claim 5 which includes,
impressing a current across said electrodialytic cell, and
transferring a portion of the alkali metal ions from the anode
compartment through said cation permselective membrane into
said cathode compartment.
7. A process for controlling alkali metal hydroxide in a
tin-plating bath as set forth in claim 4 which includes, posi-
tioning an insoluble anode and an insoluble cathode in said
electrodialytic cell, said anode and cathode being insoluble in
said plating bath.
8. A process for controlling alkali metal hydroxide in a
tin-plating bath as set forth in claim 5 in which, said elec-
trolyte solution in said electrodialytic cell cathode compart-
ment includes an acid, forming an alkali metal hydroxide in
said cathode compartment, and reacting said alkali metal
hydroxide with said acid in said electrolyte.
9. A process for controlling the alkali metal hydroxide
in a tin-plating bath as set forth in claim 4 in which said
bath includes potassium stannate, generating potassium hy-
droxide in said bath, and removing a portion of said potassium
hydroxide from said bath in the anode compartment of said
electrodialytic cell.
-29-

10. A process for controlling alkali metal hydroxide
in a tin-plating bath as set forth in claim 4 in which said
bath includes sodium stannate, generating sodium hydroxide
in said bath, and removing a portion of said sodium hydrox-
ide from said bath in the anode compartment of said electro-
dialytic cell.
11. A process for controlling alkali metal hydroxide
in a tin-plating bath comprising, electrodepositing tin from
an aqueous tin-plating bath containing an alkali metal stan-
nate and insoluble anodes, introducing an alkali metal stan-
nate into said bath to replenish the tin content of said
bath, generating an alkali metal hydroxide in said bath,
circulating said bath through an anode compartment of an
electrodialytic cell having a cation permselective membrane
separating said anode compartment from the cathode compart-
ment, circulating an electrolyte through said cathode com-
partment, impressing a current across said electrodialytic
cell, and migrating alkali metal ions from said tin-plating
bath in said anode compartment through said cation perm-
selective membrane into said cathode compartment to remove
a portion of said alkali metal hydroxide from said tin-plating
bath, said portion being that which is in excess of that which
is necessary to maintain the tin in solution as alkali metal
stannate while maintaining the concentration of alkali metal
ions in the anolyte at a level so that the concentration of
alkali metal stannate remains substantially the same in said
anolyte.
- 30 -

12. A process for recovering alkali metal stannate
from the rinse water in an electrochemical tin-plating
process comprising, withdrawing a portion of the rinse
water from the rinse water tank, said rinse water con-
taining stannate ions and an alkali metal hydroxide,
introducing said portion of rinse water into the neutral
compartment of an electrodialytic cell between an anode
compartment and cathode compartment, said neutral compart-
ment being separated from said anode compartment by a
membrane and separated from said cathode compartment by a
cation permselective membrane, supplying a controlled
amount of alkali metal hydroxide to said anode compart-
ment, impressing a current across said electrodialytic
cell, said stannate ions in said rinse water in said
neutral compartment migrating through said permselective
membrane into said anode compartment and reacting with
said alkali metal hydroxide in said anode compartment to
form a solution in said anode compartment containing an
alkali metal stannate, and withdrawing said solution
containing said alkali metal stannate from said anode
compartment and introducing said solution into the plating
tank.
13. A process for recovering alkali metal stannate
from rinse water in an electrochemical tin-plating process
as set forth in claim 12 in which, said membrane separat-
ing said neutral compartment from said anode compartment
is an anion permselective membrane.
- 31 -

14. A process for recovering alkali metal stannate
from rinse water in an electrochemical tin-plating process
as set forth in claim 12 in which, said membrane separating
said neutral compartment from said anode compartment is a
non-ionic membrane.
15. A process for recovering alkali metal stannate
from the rinse water in an electrochemical tin-plating pro-
cess as set forth in claim 12 which includes, withdrawing a
portion of the plating bath from said plating tank and intro-
ducing said portion into the anode compartment of said elec-
trodialytic cell, said portion of said plating bath contain-
ing an alkali metal stannate and an alkali metal hydroxide,
reacting said alkali metal hydroxide in said portion of said
plating bath with said stannate ions entering said anode
compartment through said anion permselective membrane to
form an alkali metal stannate, and returning said portion
of said plating bath from said anode compartment to said
plating tank, said portion having a reduced amount of alkali
metal hydroxide and an increased amount of alkali metal
stannate.
- 32 -

16. A process for recovering alkali metal stannate
from the rinse water in an electrochemical tin-plating
process as set forth in claim 12 which includes, withdraw-
ing a solution containing an alkali metal hydroxide from
one of the cells of said electrodialytic cell and intro-
ducing said solution into said anode compartment, reacting
said alkali metal hydroxide in said solution with the
stannate ions entering said anode compartment through said
membrane to form an alkali metal stannate in said anode
compartment.
17. A process for controlling the alkali metal ions
in an electrochemical tin-plating process using a tin
halogen complex plating solution comprising, withdrawing
from the tin-plating process a solution containing an
excess of alkali metal ions and a tin halogen complex,
introducing said solution into an electrodialytic cell
neutral compartment between an anode compartment and a
cathode compartment, said neutral compartment being
separated from the anode compartment and the cathode
compartment by cation permselected membranes, supplying an
acid to said anode compartment and an alkali metal hydrox-
ide to said cathode compartment, said alkali metal ions in
said solution introduced into the center compartment
passing through said perselected membrane into said
cathode compartment and hydrogen ions passing from said
anode compartment into said center compartment, and
withdrawing said solution with said excess alkali metal
ions removed therefrom from said neutral compartment and
introducing said solution into the plating tank.
- 33 -

18. A process for controlling the alkali metal ions
in an electrochemical tin-plating process using a tin halogen
complex plating solution as set forth in claim 17 which in-
cludes, withdrawing said solution from the plating tank and
introducing said solution into the neutral compartment of said
electrodialytic cell.
19. A process for controlling the alkali metal ions
in an electrochemical tin-plating process using a tin halogen
complex plating solution as set forth in claim 17 which in-
cludes, withdrawing said solution from the rinse water tank and
introducing said solution into the neutral compartment of said
electrodialytic cell.
20. A process for controlling the alkali metal ions
in an electrochemical metal-plating process comprising, with-
drawing a solution from said metal-plating process, said
solution containing an excess of alkali metal ions and a
plating metal compound, introducing said solution into a
compartment of an electrodialytic cell, removing from said
solution by electrodialysis without changing the composition
of the plating metal compound a portion of the alkali metal
ions that is in excess of the alkali metal ions necessary to
maintain the plating metal compound in solution, and returning
said solution with said portion of said alkali metal ions
removed therefrom to said metal-plating process.
- 34 -

21. A process for controlling alkali. metal hydroxide in a
metal-plating bath comprising, electrodepositing a metal from
an aqueous metal-plating bath containing an alkali metal
compound in a tank, generating an alkali metal hydroxide in
said bath, removing a portion of said bath containing said
alkali metal hydroxide and said alkali metal compound from said
tank and introducing said portion as an anolyte into the anode
compartment of an electrodialytic cell, removing from said
anolyte by electrodialysis a portion of the alkali metal ions
that is in excess of that which is necessary to maintain the
concentration of alkali metal ions in said anolyte at a level
so that the concentration of the alkali metal compound remains
substantially the same in said portion, and returning said
portion of said bath from said anode compartment having said
portion of said alkali metal hydroxide removed therefrom and
said alkali metal compound retained therein to said tank.
22. A process for controlling the alkali metal ions in an
electrochemical metal-plating process using a metal halogen
complex plating solution comprising, withdrawing from the
metal-plating process a solution containing an excess of alkali
- 35 -

metal ions and a metal halogen complex, introducing said solu-
tion into an electrodialytic cell neutral compartment between an
anode compartment and a cathode compartment, said neutral
compartment being separated from the anode compartment and the
cathode compartment by cation permselected membranes, supplying
an acid to said anode compartment and an alkali metal hydroxide
to said cathode compartment, said alkali metal ions in said
solution introduced into the center compartment passing through
said permselective membrane into said cathode compartment and
hydrogen ions passing from said anode compartment into said
center compartment, and withdrawing said solution with said
excess alkali metal ions removed therefrom from said neutral
compartment.
23. Apparatus for controlling the concentration of alkali
metal ions in an electrochemical tin-plating bath while plating
tin on a metallic member comprising, a tin-plating tank contain-
ing a first liquid with said alkali metal ions, means for
positioning a metallic member in said tin-plating tank, said
tin-plating tank forming an anode compartment with an anode
positioned in said tin-plating tank in contact with said first
liquid, a cathode compartment being located in said tank, at
least one cation permselective membrane forming a portion of
said cathode compartment and separating said anode compartment
from said cathode compartment, said cation permselective mem-
brane being positioned in said tin-plating tank, a cathode
- 36 -

positioned in said cathode compartment, means to supply a second
liquid to said cathode compartment, means to withdraw said
second liquid from said cathode compartment, and said anode and
cathode being positioned to continuously remove a portion of
said alkali metal ions from said first liquid and transfer said
portion of said alkali metal ions through said cation permselec-
tive membrane into said second liquid in said cathode compart-
ment while said metallic member is being plated with tin in said
tin plating tank.
24. Apparatus for controlling the concentration of alkali
metals ions in an electrochemical tin-plating bath while plating
tin on a metallic member as set forth in claim 23 in which, said
cathode compartment is formed by the side walls and bottom wall
of said tin-plating tank and said cation permselective mem-
brane.
25. Apparatus for controlling the concentration of alkali
metal ions in an electrochemical tin-plating bath while plating
tin on a metallic member as set forth in claim 23 in which, said
cathode compartment has at least one wall formed of a cation
permselective membrane.
- 37 -

26. Apparatus for controlling the concentration of alkali
metal ions in an electrchemical tin-plating bath while plating
tin on a metallic member as set forth in claim 23 in which, said
cathode compartment includes a pair of generally rectangular
cation permselective membranes positioned in spaced relation to
each other, a generally U-shaped member positioned between said
permselective membranes and forming said cathode compartment,
and means to support said cathode compartment in said tin-
plating tank.
27. Apparatus for continuously controlling the concentra-
tion of alkali metal ions in an electrochemical metal plating
bath while plating a metal on a metallic member comprising, a
metal plating tank containing a first liquid with said alkali
metal ions therein for continuously electrochemically plating a
metallic member, means for positioning said metallic member in
said tank, an anode positioned in said metal plating tank in
contact with said first liquid, a cathode compartment within
said metal plating tank, a cation permselective membrane forming
at least a portion of the enclosure forming said cathode com-
partment in said metal plating tank, said cation permselective
membrane being positioned within said metal plating tank, a
cathode positioned in said cathode compartment, means to supply
a second liquid to said cathode compartment, means to withdraw
said second liquid from said cathode compartment, and said anode
and cathode being positioned to continuously remove a portion of
said alkali metal ions from said first liquid and transfer
- 38 -

said alkali metal ions through said cation permselective mem-
brane to said second liquid while said metallic member is being
plated in said metal plating bath.
28. Apparatus for continuously controlling the concentra-
tion of alkali metal ions in an electrochemical tin plating bath
while plating tin on a metallic member comprising, a tin plating
tank containing a first liquid with said alkali metal ions
therein for continuously plating a metallic member with tin,
said tin plating tank forming an anode compartment containing
said first liquid with said alkali metal ions therein, means for
positioning said metallic member in said tank, an anode posi-
tioned in said tin plating tank adapted to be in contact with
said first liquid, a cathode compartment within said tin plating
tank, a cation permselective membrane forming at least a portion
of the enclosure forming said cathode compartment in said tin
plating tank, said cation permselective membrane being posi-
tioned within said tin plating tank, a cathode positioned in
said cathode compartment, means to supply a second liquid to
said cathode compartment, means to withdraw said second liquid
from said cathode compartment, and said anode and cathode being
positioned to continuously remove a portion of said alkali metal
ions from said first liquid and transfer said alkali metal ions
through said cation permselective membrane into said second
liquid while said metallic member is being plated with tin in
said tin plating bath.
- 39 -

Description

~i33~
This invention relates to a process ~or electro-
dialytically controlling the alkali metal ions in a tin-
plating process and more particularly to a process for con-
trolling the alkali metal hydroxide in the tin-plating bath,
the rinse water or both.
Alkali metal stannate tin plating, using potas-
sium or sodium stannate is a commercial method of plating
tin to base metal substrates. Conventionally this process
has been carried out using soluble tin anodes. The use of
tin anodes causes a substantial number of control problems.
If the anode current density is too low, the tin dissolves
in the form of stannite or stannous tin which causes rough,
poorly adhering plate. If the anode current density is too
high, an insoluble film forms on the surface of the anodes
and the tin does not dissolve. These problems require
accurate control of the anode current density which will
vary as the soluble tin anodes are consumed. This in turn
limits the range of current density useable in alkali metal
stannate.
An alternative to using soluble anodes is to use
insoluble anodes. As there is no dissolution of the anode
material, the allowable current densities are much greater
and more flexible~ However, plating from the solution causes
the generation of two moles of alkali metal hydroxide for
each mole of stannate consumed.
K2Sn(OH6) i 2KOH + Sn + 2H2O + 2
-- 3 --

33~l8
This accumulation of alkali metal hydroxide must be
periodically removed from the bath. One possible method is to
neutralize the alkali metal hyæroxide with acid. However,
neutralization with acid causes other problems. The byproduct
of acid neutralization with, for example, acetic acid, is an
alkali metal acetate which is an extremely soluble compound and
accumulates in the plating bath. When the accumulation becomes
too great, the common ion effect causes a drastic decrease in
the solubility of the stannate and the bath must be discarded.
Another possible alternative is to use alpha oxide sols of tin
as a replacement for the stannate. Sols are disclosed in
United States Patents 3,346,468; 3,723,273 and 3,455,794.
The use of the alpha oxide sols while operable will
only work in potassium stannate solutions and will not work in
sodium stannate solutions. The alpha oxide sols have a limited
shelf life and are very expensive. In addition, with the use
of tin sols it is necessary to maintain a high level of free
alkali, that is from five to ten ounces per gallon. This
causes difficulties in a stannate bath when the substrate is
an active metal as, for example, aluminum.
There is a need for a process to plate tin electro-
lytically utilizing alkali metal stannates and insoluble
annodes while preventing the accumulation of alkali metal
hydroxide in the plating bath by removing the excess alkali
metal hydroxide generated in the bath without expensive chem-
ical neutralization.
1, ~
- 4
.. ~
g

33~18
Where the drag out from the plating bath is ex-
, cessive, there is a substantial loss of the valuable potassium
stannate from the plating bath. There is a need for a process
to economically recover the potassium stannate and return it
to the plating bath.
In a halogen acid plating bath, oxidation caused by
I, ,
air introduced into the bath results in a loss of tin and
fluoride compounds, the formation of sodium hydroxide and an
increase in the pH of the bath. Additives to maintain the pH
at desirable levels increase the dissolved solids and reduce
the plating efficiency. There is need for a process to control
the pH and the alkali metal ions in a tin-plating bath, the
rinse water or both.
This invention relates to a process for controlling
the metal ions in an electrochemical tin-plating process where
a solution containing an excess of alkali metal ions and a tin
compound is withdrawn from the plating bath tank, the rinse
tank or from both tanks and introduced into an electrodialytic
cell. In the cell a portion of the alkali metal ions that are
in excess of the alkali metal ions necessary to maintain the
tin compound in solution are removed from the solution and the
solution with the alkali metal ions removed therefrom is
returned to the plating process.
"
-- 5
, . .

1133~18
In one embodiment directed to a process for
controlling alkali metal hydroxide in a tin-plating bath,
tin is deposited electrolytically from an aqueous tin-
plating bath containing an alkali metal stannate and an
alkali metal hydroxide is generated in the plating bath.
A portion of the bath containing the alkali metal hydrox-
ide and alkali metal stannate is removed from the bath and
introduced as an anolyte into the anode compartment of an
electrodialytic cell. A portion of the alkali metal ions
in excess of that necessary to maintain the tin in solu-
tion as an alkali metal stannate is removed from the
anolyte while the concentration of alkali metal ions in
the anolyte is maintained at a level so that the concen-
tration of the alkali metal stannate remains substantially
the same. The treated portion of the bath from the anode
compartment having the alkali metal hydroxide removed
therefrom is returned to the plating bath in the plating
tank.
Apparatus for practicing the above process may
include an electrodialytic cell within the plating tank.
In one embodiment, two sides of the tank form two sides of
the cell and one or both of the other sides of the cell
may be formed from the cation permselective membrane. In
another embodiment, at least one of the walls of the
cathode compartment of the cell comprises a cation perm-
selective membrane which is spaced from the other wall by
a separator and a cathode is positioned in the cathode
compartment. The cathode

11~3418
compartment is suspended in the tank and has an anode
positioned adjacent the cation permselective membrane.
Water is introduced into the cathode compartment of the
cell and the alkali metal hydroxide is removed therefrom.
The anode for the cell may be either the anode for the
plating bath or a separate anode po~itioned adjacent the
permselective membrane. The cathode is positioned within
the cathode compartment of the cell and where feasible the
power source for the electrochemical deposition may also
be used for the electrodialytic separation of the alkali
metal hydroxide.
The alkali metal stannate may be recovered from
the rinse water containing both alkali metal stannate and
an alkali metal hydroxide by introducing a portion of the
rinse water into the center compartment of a three compart-
ment electrodialytic cell. The cell center compartment is
separated from the anode compartment by an anion permselec-
tive membrane or a neutral membrane and separated from the
cathode compartment by a cation permselective membrane. The
anode compartment contains an alkali metal hydroxide or the
plating solution. The stannate ions in the rinse water
introduced into the center compartment pass through the anion
permselective or neutral membrane and react with the alkali
metal hydroxide or the plating solution containing excess
alkali metal hydroxide in the anode compartment to form an
alkali metal stannate which is returned to the plating bath.
The rinse water after removal of the stannate ions may be
further treated and returned to the rinse tank.
-- 7 --

1133418
A process for controlling the alkali metal ions
in an electrochemical halogen tin-plating process using an
acid plating bath includes withdrawing from the tin-plating
process a solution containing an excess of alkali metal ions
and a tin halogen complex. The solution is introduced into
the center compartment of a three compartment electrodialytic
cell. The center compartment is separated from the anode
compartment and the cathode compartment by cation permselec-
tive membranes. An acid is supplied to the anode compartment
and an alkali metal hydroxide is supplied to the cathode
compartment. The alkali metal ions in the solution intro-
duced into the center compartment pass through the permselec-
tive membrane into the cathode compartment of the cell and
hydrogen ions pass through the permselective membrane from
the anode compartment into the cell center compartment. The
solution with the excess alkali metal ions removed therefrom
is then introduced into the plating tank.
The primary object of this invention is to remove
excess alkali metal ions from a solution obtained during the
electrochemical plating of tin and returning the tin compounds
to the plating process.
Another object of this invention is to recover the
tin compounds from the water used to rinse the tin-plated
material.
A still further object of this invention is to con-
trol the pH of an acid tin-plating bath by controlling the
alkali metal ions in the bath.

11334~L8
These and other objects and advantages of this
invention will be more completely disclosed and described
in the following specification, the accompanying drawings
and the appended claims.
Figure 1 is a flow diagram illustrating the tin-
plating tank and the electrodialytic cell with the tin-
plating bath circulating from the tin-plating tank through
the anode compartment of an electrodialytic cell and back to
the tin-plating tank.
Figure 2 is a schematic fragmentary top plan view
of a corner of the tin-plating tank with the cathode compart-
ment of an electrodialytic cell positioned therein.
Figure 2a is a view in section taken along the
lines 3-3 illustrating in elevation the tin-plating tank
with the cathode compartment of the electrodialytic cell
positioned therein.
Figure 3 is a schematic fragmentary perspective
view of the cathode compartment of an electrodialytic cell
suspended in a plating tank.
Figure 3a is a view in section of the cathode
compartment of the electrodialytic cell illustrated in
Figure 3.
Figure 4 is a flow diagram illustrating the tin-
plating tank, the rinse water tank and the three compartment
electrodialytic cell. The circulation of the rinse water to
the center compartment and the recovery of the stannate com-
pound therefrom is illustrated diagrammatically.

~33418
Figure 5 is a flow diagram illustrating a tin-plating
tank, a reclaim rinse tank and a three compartment electro-
dialytic cell. The circulation of the solution from either the
plating tank or rinse tank to the center compartment of the
cell and the recycle of the tin compounds to the plating tank
is diagrammatically illustrated.
Referring to Figure 1, there is illustrated a tin-
plating tank generally designated by the numeral 10 that has
an aqueous tin-plating bath or tin-plating solution 12 that
includes either an aqueous solution of potassium or sodium
stannate. Positioned within the tin-plating bath are a plur-
ality of insoluble anodes 14 which are connected by wires 16 to
a common wire 18. Also positioned within the tank are a
plurality of cathodes 20 that are connected by wires 22 to a
common wire 24. The wires 18 and 24 are connected to the
positive and negative terminals of a source of current which
when energized impresses a current between the anodes 14 and
cathodes 20 to plate tin on objects positioned within the
tin-plating bath 12. The insoluble anodes 14 may be fabricated
from stainless steel or the like.
; The tank 10 has an outlet opening 26 and conduit 28
is connected thereto. The conduit 28 has a valve 30 therein to
control the flow of the tin-plating bath therethrough. A pump
32 is also connected to the conduit 28 and is arranged to con-
trollably withdraw plating solution from the bath 12 and
conduct the plating solution to the anode compartment 34 of an
electrodialytic cell generally designated by the numeral
38.
-- 10 --

1133~8
The electrodialytic cell 38 includes a cation
permselective membrane 40 dividing the cell 38 into an
anode compartment 34 and a cathode compartment 42. In-
soluble anode 44 i8 positioned in the anode compartment 34
and is connected to a wire 46. The wire 46 is connected
at the other end of the positive terminal of a source of
current. The cathode compartment 42 has an insoluble cath-
ode 48 positioned therein. The insoluble cathode 48 has a
wire 50 connected thereto with the other end of wire 50
connected to a negative terminal of the same source of
current.
The electrodialytic cell 38 has an outlet open-
ing 52 in the anode compartment 34 to which is connected a
conduit 54. The plating tank 10 has an inlet opening 56
opposite the outlet opening 52 and conduit 54 is connected
to opening 56. A valve 58 is positioned in conduit 54 to
control the flow of plating bath solution therethrough.
With this arrangement the plating bath solution is circulated
from the plating tank 10 through conduit 28 to the anode com-
partment 34 of the electrodialytic cell 38. The platingsolution is further circulated from the anode compartment
34 through the conduit 54 back to the plating tank 10. With
this arrangement the plating bath solution may be continuously
or intermittently circulated from the plating tank 10 through
the anode compartment of the electrodialytic cell 38 and back
to the plating tank 10.

~13341~
The electrodialytic cell 38 has a conduit 60
arranged to supply an electrolyte such as acid or water
to the cathode compartment 42. The cell 38 has an outlet
opening 62 to which a conduit 64 is connected to withdraw
liquid from the cathode compartment 42 at a controlled
rate. A pump 66 is provided in conduit 64 to control the
flow of fluid from the cathode compartment 42. With this
arrangement the electrolyte within the cathode compartment
42 may be replaced either continuously at a controlled rate
or intermittently depending upon the concentration of the
alkali metal hydroxide in the cathode compartment.
Although only a two compartment cell is illus-
trated it should be understood that other cell arrangements
that include cation permselective membranes may also be
utilized. The anode 44 and cathode 48 are insoluble anodes
and cathod~s and are fabricated preferably from stainless
steel. Other metals may be used which are insoluble in the
plating bath.
With this arrangement the plating #olution con-
veyed to the anode compartment 34 contains an alkali metalhydroxide. When a current is impressed across the electro-
dialytic cell the sodium or potassium ions migrate through
the cation permselective membrane into the cathode compart-
ment of the electrodialytic cell 38 thus reducing the amount
of alkali metal hydroxide present in the plating solution.
The solution with the reduced amount of alkali metal hydrox-
ide is then recycled through conduit 54 to the plating tank
10. As previously stated the recirculation may be contin-
uous or intermittent.
- 12 -
v

1133418
The stannate ions contained in the plating solution
will not migrate through the permselective membrane so that
only the excess alkali metal is removed from the plating bath.
By regulating the current across the electrodialytic cell, the
free alkali metal in the plating bath may be accurately con-
trolled. Although the following example reduces potassium
stannate, it should be understood that either potassium stan-
nate or sodium stannate may be used in the tin-plating bath.
EXAMPLE
A typical electroplating bath utilizing potassium
stannate was prepared containing 1600 mls. of solution. The
solution comprised lS0 g/l of potassium stannate and 23 g/l
excess potassium hydroxide. This cell was operated for a
period of nine hours during which time all the tin units
were replenished by the addition of potassium stannate. The
plating bath was continuously conducted through the anode
compartment of an electrodialytic cell containing a stainless
steel anode. The anode compartment was separated from the
cathode compartment by a cation permselective membrane manu-
factured by the Ionac Chemical Company and designated
"MC-3470". The following tables show that the free alkali
metals hydroxide concentration in the plating bath was kept
essentially constant.
- 13 -

~1334i8
~ ~ ~ ~ 0
O ~ _ I a) ~ ~ ~r ~ t~
~ ,~ ~
a) a~ ~ ~ ~ ~ ao
~D ao
~ .
O ~ . O ~ o er ~ ~ a~
~; ~ ~ O ~D 00 In ~ ~ o
Q) ~ ~ ~ ~ ~i
O ~ ~ t~ ~ N N
o
C~
o a~ 1- ~ CO 1`
. ~ I ~r
m dP
.~
~1 ~ ~r ~D ~D ~ ~ ~
a~ ~ ~ ~ In
I O O _I
~ o
.,, .,,
E~
o ~ n o ~ ~ o ~ o~
~ s~ ~I O U~~ O U~ O _I
o
C~
o o
o~ o~
.,~
r~
m
rl ~ rl fd
E~ 5 H E~ E~
-- 14 --

~334i8
~n ~ oo c~ u~ o
u~ u~ o ~ ~ ~r
~H
E~ I 1` 1` 1` 1` 1` 1`
d~
CO 0~
I ~ u~ 1~ 0 ~ In
~1 q Q
o o o
O ~ a~ ~1 ~
d
O
O .,
S~
U
Q~ ~ ~ u~ ~ o ~ u~ o~
,1 ~
~9 ~D ~ ~` ~` I` t`
C~ ~ ~ U~
O O
O _I ~ ~ ~D r~
,~
-- 15 --
.

11334~8
The plating bath was maintained at a temperature
of about 150 F. and .373 faraday of current at 3.4 volts
and 10 amps was impressed across the plating bath. The
electrodialytic cell was subjected to .1865 faraday of
current at 3 volts and 5 amps. The temperature of the
solution in the electrodialytic cell was about 120 F.
The table setting forth the condition of the
plating bath during a nine-hour run clearly shows the tin
solution concentration and the grams of tin plated with the
per cent efficiency in plating. The column in this table
indicating the free potassium hydroxide concentration shows
the 23 g/l excess potassium hydroxide in the bath initially
and the fact that the free potassium hydroxide concentration
remained substantially the same in the plating bath over the
nine-hour run. The next column shows the potassium hydroxide
generated over the nine-hour run which was approximately
88.77 grams.
Referring to the table for the electrodialytic
cell, the concentration of the potassium hydroxide in the
cathode compartment is set forth in the first column. The
concentration increased over the nine-hour run to include
the potassium hydroxide generated in the plating bath over
the same nine-hour run. The second column shows the amount
of potassium ions that passed through the cation permselective
membrane and formed potassium hydroxide in the cathode compart-
ment. The efficiency of the electrodialytic cell is set forth
in the next column.
- 16 -

113~i8
In the above example, the electrolyte solution
in the cathode compartment 42 was an a~ueous solution of
potassium hydroxide that had a potassium hydroxide con-
centration slightly higher than the potassium hydroxide
concentration in the plating bath at the beginning of the
nine-hour run. If an acid solution were employed as the
electrolyte the potassium ions would form a potassium salt
in the cathode compartment 42.
Referring to Figures 2 and 2a, there is diagram-
matically illustrated one embodiment of an electrodialytic
cell similar to the electrodialytic cell 38 discussed with
reference to the process illustrated in Figure 1. In Figure
2, the electrodialytic cell is within the plating tank lO and
the side walls 12 of plating tank 10 form two walls of the
electrodialytic cell generally designated by the numeral 80.
The other two walls 82 and 84 of the cathode compartment of
the electrodialytic cell 80 are formed of interconnected
cation permselective membranes. Thus, by simply positioning
two permselective membranes 82 and 84 within the plating tank
10 and connecting the permselective membranes to the side walls
and floor or bottom wall 96 of the tank 10 a cathode compart-
ment or chamber 86 is formed within the tank 10. The cathode
88 is diagrammatically illustrated in Figure 2 and is illus-
trated in Figure 2a as a metal cathode 88 suitably supported
within the chamber 86 in a manner similar to the cathode 48
illustrated in Figure 1. In Figure 2 the anode 90 is diagram-
matically illustrated and in Figure 2a is illustrated as an
insoluble metallic anode positioned adjacent the permselective

1133~18
membrane 84. The anode 90 is similar to the anode 44
illustrated in Figure 1. Thus, the plating tank 10 forms
the anode compartment of the cell 80 and the cathode com-
partment 86 is formed by the side walls and bottom wall of
the plating tank and the two cation permselective membranes.
Both the cathode 88 and anode 90 are connected to
a suitable source of power which may be the same power util-
ized in the electrochemical plating process. A conduit 92
is arranged to supply water to the lower portion of chamber
86 and an overflow pipe 94 is arranged to withdraw the alkali
metal hydroxide formed within the chamber 86.
Another emboidment is illustrated in Figures 3
and 3a and similar numerals will indicate ~imilar parts.
The electrodialytic cell 80 includes a cathode compartment
86 having side walls formed by a pair of cation permselective
membranes 82 and 84 positioned in spaced relation to each
other. A U-shaped spacer 81 is positioned between the mem-
brane walls 82 and 84 and has an opening in the base for a
water inlet conduit 92 and an opening in an upwardly extending
portion for the outlet 94. A hook shaped portion 83 is pro-
vided to hang the cathode compartment 86 in the plating tank
j 10. A cathode 88 is suitably suspended in the cathode com-
partment 86 and anodes 90 are suitably suspended adjacent the
cation membrane walls 82 and 84.
It should be understood that other modifications
of an electrodialytic cell including the plating tank may be
made such as positioning a three sided cell along the side of
- 18 -

11334~L8
the tank and oxming a recta~gular opening i~ the ~ide of
the tank. A cation Lermselective mcmbrane positioned in the
opening form~ the other ~1all of the cell and ~hen a current
is impressed across the ano(le and cathode the alkali metal
ions migrate from tlle tank throu~h the permselective mem-
brane into the auxiliary tank-like electrodialytic cell
mounted on the side of the platinc3 tank. With this arrange-
ment the tank functions as the anode comyartment of the cell
and the cathode compartment of the cell is positioned extern-
ally of the tank.
Referring to Figure 4 there is diagral~latically
illustrated a plating tank and a rinse tank. The plating
tank holds the aqueous plating bath and the electrochemical
plating process takes place within the plating tank. The
material plated is then transferred to a rinse tank where
the material is thoroughly rinsed with water. Where elon-
gated strips are plated the metal strips pass through the
plating tank and then into the rinse tank. A substantial
amount of the aqueous tin-plating bath is transferred from
the plating tank to the rinse tank by the rapidly moving
metal strips. ~his loss of the plating bath is commonly
referred to as a "drag out" of the plating bath.
When the drag out is excessive, the alkali metal
hydroxide removed from the plating tank reduces the amount of
treatment required by the proce~s previously described and
illustrated in Figure 1 to control the alkali metal hydroxide
in the plating bath. In an extreme situation, the drag out
may be so large that there is little if any insrease in alkali
metal hydroxide beyond a certain point such as 10 07,/gal.
- 19 -
~ !

1133418
This concentration of alkali metal hydroxide may be accept-
able for certain plating operations. Although the plating
quality with the high drag out may be acceptable, there is
an extremely high loss of valuable potassium stannate in the
drag out and the possibility of corrosion problems and safety
problems from the high caustic levels in the rinse water. The
rinse water could be concentrated by conventional methods to
recover a concentrated solution of tin compounds and the
alkali metal hydroxide. This, however, is expensive and
would then result in excessive alkali metal hydroxide in the
plating bath if the concentrated solution were returned to
the plating tank.
As illustrated in Figure 4, the plating tank is
generally designated by the numeral 100 and the rinse tank by
the numeral 102. The transfer of the plating bath to the
rinse tank is indicated diagrammatically by the line 104 and
desingated drag out. The process for recovering the tin units
from the rinse water includes an electrodialytic cell 106 that
has a center compartment 108, an anode compartment 110 and a
cathode compartment 112. The anode compartment llO hasa~an
anode 114 suspended in a manner similar to the anode illus-
trated in Figure l. The cathode 116 in cathode compartment
112 is also suspended in a manner similar to the cathode 48
illustrated in Figure 1.
A conduit 120 is arranged to convey rinse water
cont2ining potassium stannate and potassium hydroxide from
the tank 102 to the electrodialytic cell center compartment
108. Water is introduced into the cathode compartment 112
- 20 -

~,33~18
and the tin-plating bath which includes both the potassium
stannate and potassium hydroxide in solution i5 withdrawn
from the plating tank 100 and introduced into the cell anode
compartment 110 through conduit 124.
When a current is impressed across the anode and
cathode of the cell 106 the stannate ions in the rinse water
in the center compartment 108 migrate. through the anion
permselective membrane or neutral membrane 126 into the anode
compartment 112. In the anode compartment 112 the stannate
ions react with the alkali metal hydroxide in the plating
bath introduced into the anode compartment 110 through con-
duit 124 and form potassium stannate. The plating bath en-
riched in potassium stannate and reduced in potassium hydrox-
ide is withdrawn from the anode compartment 110 through outlet
conduit 130 and returned to the plating bath in the plating
tank 100. The rinse water is withdrawn from the center com-
partment 108 through conduit 132 ~nd the watex introduced into
the cathode compartment 112 is withdrawn through conduit 134.
The rinse water and the water from cathode compartment 112
contain potassium hydroxide and may be neutralized for reuse
or may be discarded.
The potassium ions that migrate from the cell
center compartment 108 to the cathode compartment form an
alkali hydroxide solution in the cathode compartment 112.
In another embodiment illustrated in Figure 4 by
dotted lines the water introduced into the cathode compart-
ment 112 is withdrawn therefrom through conduit 136 as an
alkali hydroxide solution and introduced into the anode

11;~341&~
compartment 110. This eliminates recycling the plating
bath and the alkali metal hydroxide in the solution intro-
duced through conduit 136 from the cathode compartment 112
reacts with the stannate ion in the anode compartment 110
to form pota~sium stannate. The potassium stannate is with-
drawn through conduit 138 and introduced as a part of the
plating bath.
With the above process it is apparent that it is
now possible to recover the tin units in the rinse wate~r by
the migration of the stannate ions from the rinse water in
the electrodialytic cell center compartment 108 to the anode
compartment 110. The plating bath in this embodiment has
substantially the same composition as the plating bath de-
scribed with reference to the process illustrated in Figure
1. The permselective membranes suitable for use in the
electrodialytic cell 106 may be purchased from the same
source as the permselective membrane illustrated in Figure 1.
Referring to Figure 5, there is illustrated diagram-
matically, a tin-plating process in which the material to be
plated is electrochemically plated in the aqeous plating bath
in a plating tank 150. The material after plating is trans-
ported to a first rinse tank 152 where the plated material is
rinsed with water. Subsequent to the first rinse, the material
is transported to a second rinse tank 154 where the material is
again rinsed with water. The transfer of the material from the
plating tank 150 to the first rinse tank 152 is indicated by
the line 156 and the transfer of the material from the first
rinse tank 152 to the second rinse tank 154 is indicated by the
line 158. Where strip steel is being plated it passes at high

33 ~ ~ 8
velocity from the plating bath 15~ to the rinse tanks, and a
substantial amount of the tin units are lost by drag out and
where a halogen plating bath is used, the halogen tin complex
is oxidized and precipates in the bath and rinse water.
A typical halogen electrotinning bath is disclosed in
United States Patent 3,907,653 assigned to the same Assignee
and entitled "Process For Recovering Tin Salts From A Halogen
Tin Plate Sludge". The bath contains a tin fluoride complex
with a fluostannite ion. The fluoride complex in the bath is
stable and does not precipate the basic tin salts when the pH
of the bath is within the range of 2.5 and 4Ø
During the tin-plating of the strip steel, the strip
very rapidly moves through the bath and air is introduced into
the bath by the rapid movement of the strip steel and agitation
from other sources. The air introduced into the bath oxidizes
a portion of the tin fluoride complex. The oxidized tin is in
the form of a stable anionic complex and has the formulation
Na2SnF6. The oxidized tin compound does not have any
adverse effects on the bath but, because of its low solu-
bility, precipitates and settles to the bottom of the bath in a
crystalline mass. Sodium ferrocyanide is also added to the
bath and reacts with the iron drawn in with the rapidly moving
strip of sheet steel to form an iron ferrocyanide compound.
The oxidation process results in a loss of tin and
fluoride compounds and increases the pH of the bath due to
the formation of sodium hydroxide. The loss of tin due to
.,
., ~
- 23 -

~33418
oxidation does not change the fluoride to tin mol ratio
which is desirable to maintain at about 6 and 7 to 1. The
increase in pH however, re~uires the addition of sodium bi-
fluoride (NaF.HF) or hydrochloric acid to maintain the pH at
the desired level of between about 2.5 and 4Ø
There has been a tendency in plating operations
to install countercurrent rinsing after the plating bath with
the solution from the last stage of the countercurrent rinsing
being returned to the plating bath proper. The net result is
that there is a general increase of total dissolved solids in
the plating bath. Such an increase in dissolved colids creates
a need for a means of reducing the pH in the halogen tin-
plating bath without increasing the total dissolved solids.
If the formation or amount of sodium hydroxide or alkali metal
hydroxide in the bath is controlled the pH will also be con-
trolled and can be maintained at the desired level.
The process illustrated in Figure 5 provides a means
for controlling the pH of a halogen tin-plating bath by with-
drawing the plating solution through conduit 160 or the rinse
water through conduit 162 and introducing the solution from
conduit 160 or 162 into a filter 164. Valves 166 and 168 are
provided to control the flow to the filter 164 from either the
plating bath in tank 150 or the rinse water tank 152. After
the solution has been filtered in filter 164 it may be further
treated to remove organic impurities by suitable means and is
thereafter introduced through conduit 170 to an electrodialytic
cell 172 that has a center compartment 174 and an anode com-
partment 176 and cathode compartment 178. The center compart-
ment 174 is separated from the anode compartment 176 by a
- 2 4 -

11334i8
cation permselective membrane 180 and the center compartment
is separated from the cathode compartment by a similar cation
permselective membrane 182. The cathode compartment contains
an aqueous solution of an alkali metal hydroxide and a con-
ventional cathode 184. The anode compartment contains an acid
and an insoluble anode 186. Conduit 188 is provided to supply
make-up acid to the anode compartment 176.
When a current is impressed across the cell the
alkali metal ions in the solution in the center compartment
174 migrate through the permselective membrane 182 to the
cathode compartment 178 and hydrogen ions are passed from the
anode compartment 176 and the solution withdrawn from the
center compartment 174 through conduit 190 has a reduced con-
centration of alkali metal ions. The solution withdrawn from
the center compartment 174 is fed through conduit 190 into the
plating tank 150.
In this manner, excess alkali metal ions in the
plating bath is removed. The tin in solution which is in the
form of either a fluostannite ion (SnF4)~2 or the fluostan-
nate ion (SnF6)~2 does not pass through either membrane 180
or 182 and is returned to the plating bath. In addition,
the three cell compartment prevents the tin from being oxi-
dized as it is necessary to retain as much tin as possible in
the stannous (fluostannite) condition. The process disclosed
in Figure 5 is similar in certain respects to the process dis-
closed in Figure 1. The process in Figure 1, however, the tin
is in the stannic (stannate) form and can be passed across the
anode whereas in the process disclosed in Figure 5 the tin is
- 25 -

1133418
in the stannous condition and must be kept away from the
anode. The permselective membranes 180 and 182 are conven-
tional cation permselective membranes and are similar to
those illustrated in Figure 1.
The embodiments illustrated in Figures 4 and 5
disclose a three compartment cell. It should be under-
stood that cells having a greater number of compartments
may be employed as long as there is a "neutral" compart-
ment between the anode and cathode compartments.
According to the provisions of the patent
statutes, I have explained the principle, preferred
construction and mode of operation of my invention and
have illustrated and described what I now consider to
represent its best embodiments. However, it should be
understood that, within the scope of the appended claims,
the invention may be practiced otherwise than as specif-
ically illustrated and described.
- 26 -