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ELECTROLYTIC CADMIUM

by

I-
l‘

Harold V? Fairbanks

Submitted in partial fulfillment of the requirements
for the degree of Master of Science in the
Graduate behool,Michigan State College,
Department of Physical Chemistry
June,1959

 

 

Appreciation

The writer wishes to express his appreciation to
Dr. D.T. Ewing for his helpful suggestions
and guidance during the course of

this investigation.

121516

Electrolytic Cadmium

The primary objective of this investigation was to
study the'anode corrosion of cadmium in an acid sulfate bath,
and the factors effecting the purity of the cadmium deposited

at the cathode.

Previous Work

F.C. Mathers and H.M. Marblel give a good historical
review of the literature in regard to cadmium deposited from
solutions of cadmium salts of inorganic acids, ammoniacal sol-
utions, and solutions of cadmium salts of organic acids. The
cadmium cyanide baths at this time were the only type which

proved very satisfactory.

In experimental work done by Mathers and Marble, they
state that "the sulphate baths were very unsatisfactory. They
seemed to give Spongy deposits more easily than the other baths".
They also noted that there was a gradual loss in the free acid
content in the baths due to the chemical solution of the anode
being greater than the removal of the cadmium at the cathode.

They also used addition agents; such as, clove oil, glue, and
peptone, to reduce the tendency which cadmium has toward "tracing"

at the cathode.

There are two distinct methods of producing cadmium
electrolytically. The distinguishing feature is whether the
cathode is stationary or of the rotating type. The Mammoth
Electrolytic Zinc plant of the U.5. Smelting, Refining and
Mining Co. at Kenneth, Cal.2 produces electrolytic cadmium by
first purifying the cadmium and then electrolyzing it onto
rotating cathodes. The cadmium is first separated from COpper,
thallium, arsenic and iron which constitute the more trouble-
some impurities. Then the cadmium is leached with sulfuric acid
and run into the electrolytic cells which contain lead anodes
and rotating aluminum cathodes. The cathodes rotate at 1 1/2
R.P.M. and are operated at 15 amps./s.ft. giving a cell voltage
of 4 volts, and a cathode current efficiency of 85;. The cadmium
is stripped off the cathode by means of a Special tool. It is

then melted in a furnace and cast into bars and anodes.

In 1925 the Anaconda Cepper Mining company at Great
Falls, Mont.5 started production of electrolytic cadmium. Their
process is to separate cadmium from its impurities and to elect-
rolyze the cadmium from a sulfate solution onto stationary
cathodes. The cadmium deposited is said to be better than
99.95» pure. The main impurities are lead, cepper, zinc, and

iron which together are less than .05; of the cathode deposit.

The electrolytic cells are heated to 55°C and the
cathodes Operated at 4.25 amps./s.ft. giving a bath voltage of
2.6 volts. Lead anodes are used in the cells. Glue is added

to the electrolyte to decrease the cadmium tendency to "tree"

at the cathode. 80-90” cathode efficiency is obtained from

these cells.

(

932 s Wern'ch4 studied the effect of pH,

FJ

In
current density, and temperature upon the deposition of cadmium
from sulfate baths. He states that as the pH is increased up
to 6, the grain size of the cadmium at the cathode becomes
smaller. An Optimum being reached at a pH of 5.7. pH above 6
gives spungy deposits. He found that "trees" occur when the
C.u. (current density) exceeds 4.5 amps./s.dm. At higher temper-
atures he finds that the anode efficiency increases while the

cathode efficiency decreases. His anode current efficiency for

cadmium in the sulfate baths were approx. 102.5%

Mernick brings out in his discussion the fact that

"the'structureless' type of deposit obtainable from cadmium
cyanide solution is unobtainable from cadmium sulfate solutions

in the absence of an 'addition agent'".

Experimental

The apparatus used consisted of a battery of recte
angular glass jars, ammeter, and resistance connected in series
with a u.C. source. (See Fig.) The source was a U.C. generator

in the daytime and an Edison battery at night.

Copper fittings were used throughout for electrical

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connections. However, all cathodes and anodes were suspended

in solution by means of iron wire.

The baths were made by dissolving a weighed amount
of cadmium oxide into a solution made by adding a measured
amount of concentrated sulfuric acid into approximately 1 L
of distilled water. After the resulting solution has cooled,
it is diluted to 1 1/2 L by the addition of distilled water.
‘he solution was then placed in a 2 L rectangular glass vessel.
The cathedes were stationary steel plates having as area of
exactly .1 sq.ft. The aneues were cadmium alloys having an
area approximately equal to .1 sq. ft. They were made by melting
cadmium with a weighed amount of the element which was added
as impurity. A quantitative analysis was then made of the
resulting alloy. Standard wet methods and Spectrographic methods

of analysis were employed throughout the inv’stigation.

In a separate part of the research dealing with the
recovery of cadmium from contaminated baths, the cathodes used
were made of aluminum and had an area of 5 sq.ft. The anOdes
were made of lead, having an area of 5 sq.ft. As to the elect-

rical system used, it was the same as shown above.

In the separation of cadmium sulfate as crystals from
a contaminated bath, the solution was placed in 16" evaporating
dishes which were heated on a steam bath. The crystals obtained

were washed sparingly with distilled water. Then redissolved in

distilled water, the solution made approx. 1.5N with cone.
sulfuric acid. Again the cadmium sulfate was allowed to
crystallize out. This process was repeated until the cadmium

sulfate was of the desired purity.

The anodes and cathodes were weighed dry before elect-
rolysis. After electrolysis the uncles and cathodes were washed
free from cadmium solution (the anodes being also washed free
from adhering sludge) with distilled water, dried, and then

weighed. Analytical balances were used for the weighing".

hr

All of the baths were operated at room temperature
during this investigation. The chemicals used were of the

"Chemical Pure" grade.

Experimental Results and Discussion

The investigation may be divided into three parts:
anode corrosion, deposition at the cathode, and the removal of

cadmium from contaminated cadmium baths.

Anode corrosion.— This includes the anoue current eff—
iciencies of several cadmium alloys used as anodes in baths
of different compositions, the study of which metals alloyed
with cadmium will go into solution and which ones are held
back as sludge, and the analysis of the sludge formed at the

anode.

6

Table No. 1 shows the agparent anode current efficiency
with different impurities alloyed in the cadmium anode. It may
be readily seen that all the apparent anode efficiencies for
cadmium alloys are approximately 100“ or better. Having nickel

present as impurity greatly increases the apparent anode eff—

A
D.
C D
:3
F4
(D
CL!
{.4
F“
U)

iciency. a present in the cadmium the apparent anode
efficiency is decreased. A word about the type of sludge formed.
Nickel and silver in cadmium produced a black sludge. Copper,tin,

mercury, and lead in cadmium produces a spongy metallic colored

sludge.

Table No. 11 shows the apparent anode efficiency when
cadmium anodes containing the same amount of impurities were
electrolyzeu in acid sulfate baths which differ in respect to
free sulfuric acid content and cadmium sulfate content. It may
be noticed that the more the free acid present the greater is the
aytarent anode efficiencie . The more the cadmium sulfate presert

the less the aybarent anode efficiency.

The anodes were weighed dry before electrolysis. After
electrolysis, the anodes were washed free from loose sludge and
solution from the bath with distilled water, 'hen dried and
weinhed, The difference between the weight of the anode before
and after electrolysis divided by the amount of current passed,
expressed in grams of cadmium, gives the aiparent current eff-
iciency of the anOde. It is the apparent current efficiency

due to the fact that the difference in weight includes sludge

Table

No. 1

Apparent Anode Current Efficiency with Different

Impurities Alloyed in the Cadmium Anode
( C.D. = 5 Amps./s.ft.)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

oz./gal oz./gal. Amount of Impurity in Anode
CdSO4 H2804 1.2ﬁNi .87ﬁCu .9zAg 4.0st 2.8ﬁSn 1.8%Hg
8 1/2 111% 101% 100% 99% 99.9% 99%
e 1 1/2 111% 101% 99% 99.5% 99.1% 999
s 4 117% 103.5; 100% 101% 100.9s 99.9%
s s 121% 105s 103s 101.1% 101.1% 101.1%
40 1/2 110% 100.9% 99.1% 96.7% 99.0% 99.0;
40 1 1/2 110% 101.2% 99.1% 96.6% 99.0% 98.2%
40 4 112i 102.6» 99.3% 97.4% 99.1% 97.8%
40 8 118.5p‘105p 109s 97.9w 100.9% 98.0%
No. hours baths
electrolyzed 46 52 47 52 62 42
Table No. 11

Apparent Anode Current Efficiency with Cadmium Anode

containing: lﬁNi, 1%Ag, .SﬁPb,

.5%Sn

as impurity

(Baths ran 76.3 Ampere hours)

 

 

 

 

 

 

 

 

 

 

 

 

 

,oz./gal. oz./gal. i oz./gal. oz./gali oz./gal. oz./ga1.y
CdSO4 H2804 hff. CdSO4 H2504 eff. CdSO4 H2804 Lff.
e 20 119s 20 20 114.5% 52 20 111»
8 12 113% 20 12 111.0% 32 12 108%
e e 106s 20 6 106.0; 32 e 107%
e 1 104% 20 1 104.5% 32 1 104;

 

 

 

loss and the chemical action of the free acid contained in the
electrolyte upon the anode. Thus, it is suggestive that the
true anode current efficiency should be found. Table No. 111
shows six test runs in which the sludge w.s analyzed for the
amount of oxygen it contained, and the bath solution analyzed
for the decrease in free acid (thereby, finding the amount of
cadmium which went into solution due to chemical action). From'
this data, it was found that the true anode current efficiency
was equal, within experimental errors, to the cathode current

efficiency. (Under the specified conditions given in Table 30.111)

It has been mentioned above that there is a decrease
in the free acid content of a bath upon electrolysis; therefore,
there must be a correSponding increase in the cadmium content of
the bath. This increase in the cadmium content of the bath upon
electrolysis, having impure cadmium anodes, takes place in two
ways. The first is by the direct action of the free acid upon
the calmium. This action, however, is very slight in comparison
to the second type. It was found that in one hour only .COlg/
so. cm. cadmium dissolved in a cadmium bath containing a solution
~of 1.6N free sulfuric acid (Bath not electrolyzed). The second

type is the chemical action of the free acid upon the cadmium

(D

ontained in the sludge. This action is greatly catalyzed by
the presence of impurities in the anoae. hspecially if nickel

is one of the impurities present in the anode. In all the anoae

Table No.

111

Distrubution of Cadmium Anode upon Corrosion
(C.D. = 4 Amps./s.ft.)

(O

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

oz./ga1. 00.504 20 20 27 27 27 27
oz./gal, H2504 12 12 9 9 9 9
Wt. of sludge 5.56g 3.23g 2.2135g 5.086g .0768g .0707g
Wt. 02 in sludge .0513 0.303 0.195 0.216g 0.0g 0.0g
Wt. metal
in sludge 4.85g 2.953 2.0255g 2.870g .768g .0707g
Increase of Cd
in bath 3.4g 13.5g 10.97g 15.18g *.4732g *.3795g
ht. of cathoue '
deposit 48.33g 48.65g 61.0g 60.95g .6l.lg .61.5g
4
Total of above ‘
three weights 56.58g 65.08g 73.99g 79.00g 61.65g 61.65g
Loss in wt.
of anoue 53.38g 64.90g 74.00g 79.05g 61.65g 61.65g
Anode current _
efficiency 110% 127; 120% 128% 99.9% 99.9%
Cathode current } _ f
efficiency 94.1% 95% 98.6% 98.6% 99% 99.1%
Anode .2ﬁNi,.9% lﬁ_Ni 1% Ni 1%_Ni Pure Pure
composition Ag,.5ﬁSn 5%Ag .SﬁAg .SﬁAg Cd Cd
NO. hours
electrolyzed 61 61 61 61 74 74

 

 

 

 

 

 

 

 

* Difference between weight of anode corrosion minus
weight of cathode deposit plus the weight of metal in the sludge.

 

10
reactions investigated where two or more eleznents were alloyed

with cadmium, nickel was one of the elezrents.

it was found u on ar 'alysis of t: .e slud e, that it con-
tained not only the alloyin3 impurity but also a large percentage

of cadmium. it was also found that approximately as of the sludge

I
by weight was oxygen. The sludge itself bein3 black leads to the
conclusion that there is an.abunuance of the oxides of metals

contained in the sludge.

Part of the current must have be en used at the anode
for the production of these oxides. however, it was proven upon
analysis that this sludge carries down with it several ti.e es its
weight of finely divided cadmium. It is this cadmium, vﬂ ich in
the presence of these oxides (perhaps also an oxide of cadmium)
is catalyzed and is dissolved fairly rapidly by the chemical
reaction of the free acid contained in the bath. Some of the
oxides also dissolves, thus adding an impurity into the cadmium
bath. No free oxygen is liberated at the anode although a gas-
ing may be seen around the anode itself, which is black due to

he formation of the sludge on he surface of the anode, and
also from the sludge which dropped off from the anode. The gas
collected at the anoue was analyzed and found to be hydrogen.
This proves that the cadmium in the sludge is being acted upon
chemically along with the other metals by the free sulfuric acid
Contained in the bath. It is this chemical reaction between the

free acid in the bath and the cadr ium in the sludge along with

11

the free acid attacking the cadmium anode which accounts for

the increase in the cadmium content of the bath upon electrolysis.
Thus, in a bath containing a large amount of free sulfuric acid,
it would be expected that the increase of the cadmium content in
the bath upon electrolysis would be large compared to a bath

containing a small amount of free acid.

If the above fact is true, the reverse should also be
true, namely that a decrease in the free acid content will
decrease the gain of the cadmium content of the bath. Now if
the free acid content be decreased sufficiently, there may be
a point reached at which the cadmium content of the bath is
decreased upon electrolysis. Graph No.1 shows that there is
such a condition reached. A bath Operated under these conditions
has the advantage of an almost comylete separation of cadmium
from its impurities at the anodes. ‘However, the bath has a
higher Operating voltage than when more free acid is present.
Also the tyne of cathode deposit obtained is in the form of

coarse, fairly loose crystals of cadmium.

Turning to Table No. 11 again, it was found, as has
already been stated, that the more the free acid present the
higher is the percent anode efficiency. This may be due to
the fact that the oxygen overvoltage is less in a strong acid
solution. Thus, more of the current is used for the production
of the oxides which makes up the sludge, which in turn drags

down with it a large quantity of metallic cadmium when it (sludge)

12

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13
falls off. And,of course, the more the free acid present the
greater is the reaction of the acid upon the sludge. If more
current is used for the production of oxygen (oxides) when the
amount of free acid present is increased, and if there is a
direct relationship between true anode and cathode efficiency,
the cathode current efficiency should decrease upon an increase
‘of free acid in the sulfate bath. Table No. 111 shows that this

fact is true.

As to the explanation of the fact that an increase in
cadmium sulfate content in the bath decreases the apparent anode
efficiency, it may be due to the degree of freedom of movement
which the components in the bath have. This factor of increaseng
the viscosity due to the increase of solute in the bath, coupled
with the fact that immediately surrounding the anode there is a
dense solution of cadmium sulfate formed from the electrolytic
corrosion of the anoce is what prevents the free acid of the
solution from attacking the anode to a certain extent. Cadmium
sulfate may be seen in the bath, under prOper illumination, to
be falling in a heavy "syrupy" stream directly off the anode and
settling on the bottom of the bath. This fact, of course, points
out that en the operation of a cadmium sulfate bath the concent-
ration of the electrolyte is not constant throughout the bath. The
bath is, in general, more concentrated near the bottom than at the
surface. This fact that there is unequal concentration of the
electrolyte in different parts of the bath, while the bath is
being electrolyzed, greatly affects the amount of impurities

which go into the solution from the sludge.

14

As the sludge falls directly beneath the anode, this
viscous column of cadmium sulfate will displace the electrolyte
around the sludge. This will tend, therefore, to retard or
prevent the acid from acting upon the sludge. In other words
the free acid surrounding the sludge is diluted or diaplaced
by the viscous column Of cadmium sulfate coming from the anode.
This fact is greatly emphasized when the current is turned off
after the bath has run awhile, as then the free acid is again
allowed to diffuse freely around the sludge and react with it.
A large amount of gasing takes place around the sludge and the
solution turns green due to the increase of more nickel ions in
the bath. (Nickel must be present in the anode). The bath should,.
therefore, be kept in Operation continously or the sludge re-
moved before stopping .he electrolysis to prevent as much as
possible the impurities from the sludge from going into the

solution.

It was found that nickel and tin when used as impurities
in cadmium go into solution and contaminate the bath. See Table
No. 1V. Lead, silver, copper, and mercury did not contaminate the
bath when they are used as impurities in the cadmium anode. It
was also noted that the larger the amount'of free acid present in
the bath the greater the quantity of impurity found in the sol:
ution from the anoue. This point follows along with what has
been said in regard to the anode corrosion. That is, oxides of
the sludge fall off of the anode and then is reacted upon by the
free acid in the bath. It would be expected,.within limits, that

the more the free acid present the faster would be the action

Table No.

1V

Amount of Impurities Found in Solution due
to the Impurity being Present in Cadmium Anode
(C.D. at cathode = 5 Amps./s.ft.)

15

 

 

 

 

 

 

 

 

 

 

 

 

oz,/gal. oz./gal. Amount of im urity in anode ﬁbn in
Cdsoé H2804 1.2pNi .87pCu .QﬁAg 4.05Pb 1.1ﬁHg 2.8ﬁbn Cathode
8 8 .39g/L None None None None .BCg/L .245
8 4 .Slg/L " " " " .55g/L 585
8 1 1/2 .18gyL " " " " .55g/L 1.43
8 1/2 .lEg/L " " " " .56g/L 98
4O 8 .22g/L None None None None 1.15g/L .091
40 4 .16g/L " " " "" .89g/L .360
40 1 1/2 .O4g/L " " " "" .BOg/L 1.43
40 1/2 Trace " " " " .BOg/L .98
No. hours baths
were run 159 52 47 52 42 62 62

 

 

 

 

 

 

 

 

 

 

 

16

between the sludge and acid. The above effect is certainly true
in respect to nickel, However, tin did not seem to be affected
much by the free acid content of the bath. This leads to the

conclusion that the tin passes into the solution by electrolytic

action at the anOde.

Cathoae.— This includes cathode current efficiencies,

purity of deposits and conditions affecting the type of deposit.
Table No. V shows the cathode current efficiency in different
types of baths and at different current density. The efficiencies
do not change much due to the difference in concentration of
sulfuric acid and cadmium sulfate in the range used. It may be
seen, however, that there is a slight decrease in the current
efficiency when the current density of the cathode is increased.
It is safe to state from the above facts, that within a broad
range of conditions, the cathode current efficiencies in a cadmium

sulfate bath is usually 95% or above.

As to what effect certain impurities in the bath have
upon the cathode deposit (anodes being pure cadmium), it was
found that the impurities would plate out at the cathode along
with the cadmium when present in the bath.- COpper ions in the
bath come over almost at once and codeposits with cadmium to
‘ such an extent that the bath is quickly depleted of c0pper.
Silver, like copper, when it is present as impurity in the bath
also codeposits with cadmium, however, a black spongy deposit
is formed at the cathode. Zinc ions when present in the bath

up to 1 oz./gal, did not contaminate the cathode deposition.

17

Table No. V

Cathode Current Efficiency with Different Amps./s.ft.

in Respect to Cathode Area

 

 

 

 

 

 

 

 

 

 

 

 

oz./gal. oz./gal. Current Density
CdSOQ HBSOQ 2 4 6 8 10 12 15
a 20 95.8p 95.5» 97.0% 94.5s 95.6% 94.4% 95.5p
5 12 97.0% 96.0% 99.4% 99.32 94.52 95.9% 95.0%
a 5 96.4ﬁ 96.0; 97.9% 93.3% 91.4% 95.42 95.0%
s 1 95.4% 97.0; 97.5% 91.7% 93.4% 93.4; 95.0%
20 20 950403 9405);; 970050 9600;.7 99067.) 9409f.) 90.5%
20 12 97007:; 9600;? 9709?.) 9505;: 98.01% 950450 9500;?
32 20 93.3% 94.0% 97.0% 95.5% 99.1p 95.4; 94.2”
32 12 95.9a 94.5; 97.5” 96.0% 97.19 95.9% 95.0%
32 s 97.0; 95.5% 97.9” 9444p 99.4; 97.9% 93.0%
32 1 98-0h 96.5% 97.9ﬁ 93.39 96.4; 95.92 91.7%
No. of hours
electrolyzed 46.5 23.5 16 10.75 12 7.75 5

 

 

 

 

 

 

 

 

 

 

18

Mercury in the bath codeposited with the cadmium producing a
very loose deposit at the cathode. Tin codeposits from the
solution in percentages less than the tin—cadmium ratio in the
bath. 1t brightens the cadmium deposits but also aids in the
formation of flat leaf like trees (scales) at the cathode.
Nickel does not come over as an impurity until l-Zﬁ nickel
(cadmium content 2 lCCﬁ) is present in the bath. However, the
amount of nickel a5 impurity present in the bath increases
above the l-Zp content. Thus, it may be seen that only zinc
may be present infthe bath in fairly large amount Without cone
taminating the cathode deposit. A spectrographic analysis of
several samples of the cathode deposit showed that zinc was

absent.

Table No. VI shows the amount of nickel and tin
impurities which appear in the cadmium sulfate baths upon
electrolysis. In the same Table is also shown the amount of
tin which codeposited with the cadmium at the cathode. It
should be noted that the Tables No. II and V together with
Table No. VI represents the complete history of twelve different
cadmium sulfate baths run in series, under the same conditions,
and having the same type of anodes and cathodes. After the
cathodes were run at one current density, they were replaced in
the baths by new steel cathodes which were in turn run at a
different current density. The anodes were not removed during

these changes.

Upon analyzing Table ho. VI, t may be seen that the
bath must be electrolyzed several ampere hours before any im-
purities enter intO the electrolyte. The amount of nickel enter-
ing the bath, it may be seen, is greater in baths containing
a large amount of free acid. Tin, like nickel, goes into the
baths containing a large amount of free acid more readily than
in baths containing a smaller amount of free acid. Of the two
metals which enter the bath, tin is the only one which codepo-
sited with the cadmium at the cadhode. Tin came over almost
immediately after entering into the electrolyte. Its appearance
in the cathode may readily be detected by the flat trees (scales)

formed at the cathode.

Referring to Table NO. 1V, there is a column which
shows the percentage of tin found in the cathode deposit from the
baths in which tin entered from the anOde. It is noted that in
these baths tin is the only impurity present. From the data,
it appears that there is an Optimum condition, controlled by the
amount of free acid in the bath, which determines how much tin
will codeposit with the cadmium. However, referring to Table
No. VI, this Optimum condition cannot be readily discerned. The
baths referred to in Table NO. V1 haveboth nickel and tin present

as impurities which may account for this fact.

It should be noted that the tin-cadmium ratio in

the cathoae deposit was, in several of the cases, greater than

Table NO.V1

Amount of impurities Found in Cadmium Sulfate
Baths and the Percentage Tin found Codeposited with Cadmium

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

oz./gal. oz./gal. Impurities found in baths 2 Tin found in
CdbC4 H2505 g/L cathOde deposit
Cathode current density
8 10 12 12 15 15 12 15
' Ni Ni Ni 5n Ni Sn ﬁSn ﬁSn
8 20 .051 .121 .234 .700...396 396 .063 .120
8 12 0.00 0.00 .902 .410 .375 .325 .039 .156
8 6 0.00 0.00. .168 .900 .214 .162 .067 .106
8 1 0.00 0.00 .224 .097 .151 .097 .150 .242
20 20 0.00 0.00 .141 .620 .487 .720 .066 .157
20 12 0.00 0.00 .092 .110 .081- .425 .050 .315
20 6 0.00 0.00 0.00 .228 .021 .360 .083 .242
32 20 0.00 0.F0 .539 .360 .254 .780 .140 765
32 12 0.00 0.00 .131 .394 .224 .650 .082 .785
52 6 0.00 0.00 .051 .515 .234 .425 .082 .810
32 1 0.00 0.00 .031 078 .151 .227 .072 .740
NO. Ampere hour
bath was electr 44 55 66 66 77 77 66 77
lzed

 

 

 

 

 

 

 

 

 

 

 

 

 

21

the tin—cadmium ratio present in the bath. This seems to
contradict a previous statement. However, in the cases described
in Tables NO. 1V and V1 the anodes contain tin as an impurity,
while in the other cases mentioned pure cadmium anodes were

used.

Nickel was not found in any of the cathode deposits
in the baths given in Tables No. IV and VI. This reiterates the
fact already mentioned that nickel will not codeposit with
cadmium until 1—2p nickel is present in the bath. (Cadmium content

equal to 1002).

In this problem of separations, it has been pointed
out that at the anode OOpper, silver, mercury, and lead did
-not go into solution.; but nickel and tin dOs . As tin is'
rarely found in connection with cadmium while nickel is more
Often present as an impurity, special attention was given to

the further study of nickel as impurity.

It was found that in baths Of low free acid content
only a small amount of nickel from the anode sludge dissolves,
and goes into solution. It must also be remembered that the
bath is being built up with mere cadmium sulfate the longer
the bath is run, until the free acid content in the bath is

almost zero. Thus, the chances of the free acid attacking the

sludge are being decreased the longer the bath is electrolyzed.

22

With these facts in mind and the fact that the solution can
tolerate l-Zﬁ nickel (cadmium content = 100%) before an

appreciable amount of nickel codeposits with the cadmium, it
is safe, therefore, to assume that a bath could be operated
several months before becoming contaminated sufficiently to

have a codeposition of nickel and cadmium at the cathode.

As to the types 0f d813081128 formed, there are several
factors involved. The free acid to cadmium sulfate ratio, amount
and type of impurities present in the electrolyte, and the current
density at the cathode play a very large part. To sumf it up a
bath containing a very high acid to cadmium sulfate ratio or one
containing silver ions results in a black Spongy deposit being
formed at the cathode. The closer the acid-cadmium ratio becomes
to l to l the deposit becomes better, passing through the stages
of spongy deposits, treeing deposits, to denser deposits with a
minimum amount of treeing. As the cadmium content is increased
to a larger and still larger ratio with the free acid,the deposit

becomes coarser and less adherent to the steel plate.

Also as the current density of the cathode is increased
from O to 100 amps./sq.ft., the deposit changes from fairly dense
deposit with few trees (C.D. = O-i2 amps./sq.ft.), to less adherent'
deposits and more treeiC.D. = 12-60 amps./sq.ft.). to more or less
spongy deposits at higher current densities. Rotating the

cathode helped to prevent"treeing" on the face of the plate but

not on the edges. A higher current density may be used, with a
rotating cathode ; however, the current efficiency is lower than

when stationary cathodes are used.

The optimium cadmium sulfate bath was found to contain:
ZUoz. cadmium sulfate per gallon; and 20 oz. free sulfuric acid
per gallon. The addition of .5 oz. glue per gallon produces a
smooth, dense deposit of cadmium at the cathode with Only nodules
forming at the upper edge of the plate, The bath should be
Operated at 4 amps./sq.ft. This gives a bath voltage of less than

.3 volts.

Table No. VII gives the operating voltage of baths of
various composition at different current densities of the anode
and cathode. As to be expected, the higher the current density
the higher the voltage, the less the amount of free acid present
the higher the voltage, and the less the cadmium sulfate present

the higher the voltage.

Reclaiming cadmium.- cadmium may readily be reclaimed
from impure baths by putting in place of the cadmium anodes,
lead anodes. it was found that very little of the impurities
(nickel and tin) of the bath were removed if the bath is
Operated at the preper current density (4 amps./sq.ft.). The
bath voltage is 2.6 volts due to the high oxygen overvoltage
on the lead peroxide anoae. Approximately so; of the cadmium
in the bath may be removed by this method. Aspnngy deposit

is formed when the cadmium content in the bath was reduced to

less than 1 oz./ gal.

Voltage of Bath with Different Current Densities

Table No. VII

being used in respect to Cathode

24

 

 

 

 

 

 

 

 

 

oz./gal. oz./ga1. Current density
CdSOQ H2504 2 4 6 8 10 12 15
8 20 .05 .10 .15 .20 .20 .25 .40
8 12 .05 .15 .20 .20 .22 .35 .50
8 6 .10 .20 .30 .30 .40 .70 .65
8 1 .20 .45' .60 .80 1.20 1.65 2.15
20 20 .03 ‘.15 .20 .20 .20 .35 .40
.20 12 .07 .15 .20 .22- .30 .45 .60
20 6 .10 .20 .20 .35 .40 .65 .90
20 1 .15 .30 .40 .60 .85 1.20 1.40
32 20 .02 .17 .20 .20 .20 .40 .55
32 12 .05 .17 .20 .22 .30 .40 .65
32 6 .09 .22 .25 .30 .60 .65 .90
32 1 .10 .30 .40 .60 .85 1.20 1.50

 

 

 

 

 

 

 

 

 

 

 

25

Another method for reclaiming cadmium from the con-
taminated baths is by crystallizing out the cadmium sulfate by
allowing the solution to evaporate down until about 50; concent-
rated sulfuric acid solution is present; then decanting off the
solution, and washing the crystals with a small amount of water.
This method usually produces fairly pure cadmium sulfate. How-
ever, they may be further purified by recrystallization. Having
recrystallized cadmium sulfate twice from an impure bath, it
was found that approximately 85% of the cadmium sulfate was

recovered.
bummary

1. The Optimum cadmium sulfate bath contains: 20 oz,
per gal. of both cadmium sulfate and free
sulfuric acid, and .3 oz. per gal. glue. The
C.D. of the cathode and anode should be 4 amps.
per sq.ft. giving a bath voltage of less than
.3 volts.

2. Silver, copper, mercury and lead in the cadmium
anoue will not go into solution along with
the cadmium upon electrolysis.

3. Cadmium anode corrosion is always 100% or Over due
to the formation of sludge.

4. When nickel is present in the anode some current
at the anode is used to produce oxides.

5. Cadmium sulfate content increases due to chemical
,action of the free acid upon the cadmium
anode and the sludge.

6. There is a ratio of free acid to cadmium sulfate
where the cadmium sulfate content is neither
increased or decreased upon electrolysis.

26

7. more than 1-2p nickel (cadmium content = 100p) must
be present in the solution before nickel co-
deposits with the cadmium at the anode.

8. Nickel in cadmium goes into the solution in-
directly; as the oxide is first formed which in
turn is acted upon by the free acid of the bath.

9. Cathode efficiencies are related to the amount of
impurities in the anode.

10. Removal of cadmium from contaminated baths may
easily be done by replacing the cadmium anodes

with lead anodes or by crystallizing out the
cadmium as cadmium sulfate.

References

1 Trans. Am. hlectrochem. 500., 25, 297 (1914).

03

Trans, Am. Inst. Mining Met. Eng., 78, 239 (1930).

4 Trans. Am. Electrochem. 500., 62, 27 (1932).

 

 

 

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