THE ELECTROLYTIC PREPARATION
0:" LEAD BASIC BARBDNATE

THESIS FOR THE DEGREE OF M. 8.
George Stuart Krentei
1933

 

 

 

 

    

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THE ELECTROLYTIC PREPARATION
OF
LEAD BASIC CARBONATE

A Thesis Submitted to the Faculty
of
Michigan State Collge

In Partial Fulfillment of the
Requirements for the Degree
of

Master of Science

Department of Chemistry

By

George Stuart Krentel

June, 1933

I wish to take this means of eXpressing
my appreciation to Dr. D. T. Ewing, whose
supervision and counsel brought about the

attainment of this thesis.

Basic lead carbonate has been known for many years
as a preservative, having been recorded in the writings of
Pliny as a covering for the hulls of ships. However, it
was not used in their paintings, according to analyses made
by Sir Humphrey Davy. The Greek author and scientist, Theo-
phrastus, describes the preparation of White lead as follows
“Lead is placed in earthen vessels over sharp vinegar and
after it has some thickness by a kind of rust, which it
commonly does in about ten days, the vessels are Opened, as
it were, in a kind of fouiess and the rust scraped off;
the lead is then placed over the vinegar again, repeating
over and over the same method of scraping it until the lead
is wholly dissolved. The scrapings are then beaten to pow-
der and boiled for a long time, and what at last subsides to
the bottom of the‘hessel is cerusse.”

This product was used extensively in Greece at
that time as a cosmetic and as an eyesalve. Another method
was in use in Rhodes, that of placing twigs in the jars and
pouring over them acetic acid and then placing lead shavings
on the twigs. The jars were then covered to prevent eva—
poration and after a period of time were Opened and white
lead found therein. These writers did not take into account
the use of carbon dioxide in the air, as well as the mois-
ture, in the formation of the desired compound. Rather they

attributed the result entirely to the action of the acid.

These processes are closely approximated by two of
the more modern methods, namely, the Dutch or stack process,
and the chamber process. In the former, strips of lead are
placed over pots containing acetic acid. There are several
rows of these, piled one on another. The atmosphere is allowed
to circulate throughout the system. The oxygen is necessary
for oxidation and carbone dioxide also present in the air,
forms the carbonate from the acetate. This process requires
three months at normal temperature. The chamber process
employs strips of lead hanging from bars in a large room
into which carbon dioxide, air, water and acetic acid vapors
are sprayed. This method comes to completion in about sixty
days, during which time, a temperature of about 70° is main—
tained by hot gases and also by the heat of reaction;

Another important process is the French, which is
done by treating basic lead acetate with carbon dioxide.
However, this method is not overly popular or successful,
due to the large amount of the normal lead carbonate Which is
formed.

There have been several electrolytic processes
advanced, all of which consist, in the main, in the electro-
lysis of lead anodes or a lead solution, either in the cell
or outside it. One of these, the Luckow process,may he used
for making any insoluble salt. This employs both an anode
and a cathode of lead. The important principle is that a

very dilute solution of the salt will produce precipitation

of the product, the salt referéed to being sodium carbonate,
which is present in a low concentration. There is present
another salt, known as the secondary salt, which will, in
reaction with the lead ion, form a soluble salt. Sodium
acetate or chlorate may be used for this purpose. The object
of the soluble product is to form the insoluble product away
from the electrode, which would otherwise become coated with
the basic lead carbonate, thus decreasing the efficiency
greatly as the migration of the lead ion would be stOpped
almost completely.

The Sperry process, which is used by the Anaconda
Lead Products Company in their factory at East Chicago, In—
diana, was patented by s. A. Sperry (U.s.,1zoes4s,1sis).

This is a continuous process as described hereinafter. The
Principle is very similar to the Luckow process, in that it
uses a salt which forms a soluble product at the anode;

and the cathode solution, a carbonate, forms the basic lead
carbonate, the formula for which is 2PbCOS.Pb(OH)2. The more
correct representation of the product may be shown better by
this formula, Pb(COS.PbOH)2.

The set up for production as given here is very
similar to the actual plant, except that after the run.in
the plant a Doerr thickener is used to precipitate the solid.
Before thickening, the liquid contains about 4% solids and is
about 30% solids after thickening. It is then filtered,

washed and dried to about .2% moisture in an oven. The

filtrate, which is the catholyte or sodium acetate, is re-
turned to the cell for further use. In the laboratory, no
thickener was used, it being necessary either to filter the
entire amount of catholyte or let it settle and decent the
clear liquid. It was found, however, that slow bubbling of
air through the liquid seemed to produce more rapid precipi-
tation. It may be seen that this process is very inexpensive,
inasmuch as the only raw materials needed are lead, water

and carbon dioxide.

The experimental apparatus used consists of a large
tank.A, of about thirty four liters capacity, containing the
anolyte, sodium acetate. There is a siphon arrangement from
this tank to the cell B. This tube has four nozzles, which
fit in the spaces between the anode and cathode. This supplies
the solution in a steady flow through the cell. Tank A is
kept full of anolyte at all times from a series of large
carboys G, into which anolyte and the product are drawn from
the bottom of tank B. The suspension is allowed to settle
before the solution is used again. The catholmeis supplied
in somewhat the Same manner to the cathodes, which are enclosed
in linen sacks. This is to keep the cathode sludge from
dirtying the product. The catholyte is run into the sacks
at the bottom, and the spent catholyte drawn off by a contin-
uous siphon system D from the top of the sacks, and carried
to the carbonating tower E, so that it may he used over. The
lead anodes are about eighty-five sqare inches in area on

one side. There are three of them so that there are four

sides entering to the reaction. The total area is thus 2.38

square feet. There are two steel plates for anodes, which
are about the same size. The carbonator is a glass tube
packed with rocks so that the liquid will present a large
surface to the gas, which is admitted at the side from a

Kipp generator. The charged catholyte is then placed in Tank

F for use in the cell again.

The anodes and cathodes are so made that they hang
on the sides of the cell, and are connected to the switch

board on Opposite sides.

DATA
Series #I

Anode Plate Area - 2.38 square feet.

Temperature - 20° C

0.13. " 10 Amps.

Concentration Anolyte

Concentration
NaZCOS Yield
2% 207 Grams
4% 192 u
e 154 N
8% 142 “
10% 132 "

These results seem to

Analysis
%co %Pb Color
14.0 % 70.02 White
Light green
u I!
u I
H II

indicate other factors re-

maining constant, that with the increase in concentration of

the catholyte, without an increase in the currect density

seems to produce an inhibition in the amount of product

formed and a‘so a discoloration,

as it may be seen that a

smaller amount was formed at the higher concentrations of

the catholyte and were colored green except the first pro-

duct.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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DATA

Series #II
Anode Plate Area - 2.38 Square feet.
Temperature — 20° C.
Current Density - 13.5 Amps.
Concentration Anolyte - 2% NanH302
Concentration Analysis
NagCQ5 Yield 74 002 % Pb Color
2% 184 Grams 13.05% 71.3% White
4% 184 “ Greenish Whit
6% 144 " Green
8% 242 u u
10% 154 .. "

These results show about the same as series #1, that
an increase in the concentration of the Na3003 produces less
rather than more white lead. Also, while the current density
was increased, the yield decreased. The increase in yield at
the end instead of decrease would seem to indicate the for-

mation of some heavy product.

 

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DATA
Series #III
Anode Plate Area - 2.38 Square Feet.
Temperature — 20° C.
Current Density - 16.8 Amperes

Concentration Anolyte - 3% NanHgog

Concentration Analysis
Na2C03 Yield % cog % Pb Color
2% 200 Grams 12.92% 74.5% White
5% 196 I! Brown Yellow
10% 190 " Light Brown

There is another effect noticed here, that of the
formation of lead Peroxide at the anode. Some of it was re-
tained but some was carried over with the oroduct. Here, also,
the current density was increased again, but the yield was still
lower than the first run.

After this, I decided to regulate the flow. It
took about ten runs before I was satisfied. After regulating,
the anolyte flowed at a rate of a liter every half minute,

andthe catholyte of a liter every minute and a half.

 

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DATA

Series #IV
Anode Plate Area - 2.38 Square Feet.
Temperature - 20° C.
Concentration Anolyte - 1% Mac Hgog
Catholyte - 8-4%Nagdos
Analysis
C.D. Yield % co; % Pb Color
13.5 215 Grams 12.04” 78.3% White
18.8 320 H 10.06% 78.16% N
21.8 380 “ 8.8 % 85.2 % n
23.5 438 n ”0.9 % 78.34% "

It was at this point after I got the flow regu-
lated, and the carbonator adjusted that the runs were all
a good clear white. Run #3labove comes closest to the stan-
dard as given by the Anaconda Lead Products 00., about

11.15% COB in the sample.

 

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DATA

Series #V
Anode Plate Area - 2.38 Square Feet.
Temperature 20° C.
Concentration Anolyte - 2 $NanH502
Catholyte - 2—4%Na2003
Analysis
0.0. Yield $5003 %Pb Color
18.8 200 Grams 10.16% 79.33% White
23.5 370 a 10.42% 78.62% “
30.2 400 " 11.34% 81.01% "

This series did not differ much from the prece-
ding one, except that a higher current density was reached,
an amperage that could not be obtained except by the higher
concentration of sodium acetate. Some lead metal was formed

at the cathode in this run.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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DATA

Series #VI
Anode Plate Area - 2.38 Square Feet
Temperature - 20° C.
Concentration Anolyte - 2 % NaCzH302
Catholyte - 2—491, Nazca)3
Analysis
0.1). Yield 270 002 72 Pb Color
15.8 275 Grams 10.9 3’. 80.9% White
86.8 445 " 11.26% 79.9% "

These two runs were made over a longer period of
time to Show that production could be carried out over a
long period of time in the laboratory. As may be seen from

the analysis, the results were good.

 

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This eJQeriment was “irst set up on a s all scale
to determine if it could be vorked in the laboratory. The
set up was, on the «hole, the s.ne as the one described here,
except that the lead plates were about half the size of the
ones used in the regular outfit. About ten runs were made
with this outfit and some eroducts were obtained. These
were discarded without analysing. Some difficulty was ex-
perienced, one of which was due to the fact that reservoir
tanks were too small. thither was there any provision made
for the settling out of the product, which was onite finely
divided and prone to stav in solution for some period of
time. After it was seen that the process could be made to
work, a large plant was made which has been described before.
The Anaconda peonle use diaphragms of Irish linen around.the
cathodes, but since the sole purnose of the diaphragm is to
keep anode sludge, snongv lead and iron rust from polluting
the product, linen of the same texture as sheeting was used.
The size of the particle that could pass through the cloth
was not an important factor.

This plant was deSisned to draw a large amperage,
so that it Vis necessary to construct a switchboard. Cast
iron grids were used as resistors, and hooked up so that the
amperage could be varied. Copper cable, size #G, was used
for the connections. As there was no ammeter available for
the annerases as high as was necessary for the cell as de-
signed, a shunt, having a voltage drop of fifty millivolts

for two hundred amperes, was hooked inthe line. The shunt

was connected to a millivoltmeter and the dron in millivolts
read. The millivolt meter read up to 10“ by two's, so that
it “a necessary to multiply each scale division by eight

to determine the amperage. This was first calibrated by
balancing it with another anmeter on the lower ranges.

At first, a series of runs was made with the con—
centration of the anolyte constant, while the catholyte con-
centration was varied. The first product was white and the
remainder was green. The second series had a higher con-
centration of anolyte, while the catholyte was charged as
before. The results were about the same as the first series.
The third series gave still another increase to the anolyte
concentration, with the results staying about the same. The
conclusion could be drawn that the best results came from
the lower concentrations of the catholyte. It was also no-
ticed that with the higher concentrations of the latter that
there seemed to be a loading effect. In other words, the
anolyte would take up carbon dioxide until the production of
white lead was inhibited entirely. The green color was
also produced at the higher concentrations, which was due to
some factor which I was unable to determine. Another diffi-
culty was the production of lead peroxide at the higher
concentrations of the catholyte. This would seem to follow,
as lead peroxide at the higher concentrations of the catho—
lyte, is made normally in a strong solution of sodium car-

bonate. This would deposit directly on the lead and prevent

the lead ions from leaving the electrode. After a short

length of time the anode would be completely covered. Then,
too, if only a small amount was formed, it was very likely
to drop off and would color the lead carbonate brown. This
happened in the 6% sodium carbonate, 3% sodium acetate run.
Another run which was not listed gave the same results. These
results also made it clear that the flow was a factor which
could not be overlooked. There were about ten runs made in
which the flow was experimented with, for which the product
was not retained. The aralyses determined the most satis-
factory flow to be two liters per minute for the anolyte and
.66 liters per minute for the catholyte.

After the concentrations of the two siutions had
been investigated and the flow regulated, runs were made to
determine the most satisfactory current density. As the cell
was first set up, it was planned to have four anodes and
three cathodes. It was found that these were so close to-
gether that the cloth diaphragms, distended with the load of
solution, would touch the lead. While this did not cause a
short, it had a serious tendency to decrease the efficiency
of the cell, so that it was necessary to remove one of each
at this point. The linen was then more firmly restrained by
a rubber binding. All the parts were of rubber or glass,
as the causticity of the basic solutions was, while quite
dilute, nevertheless strong enough over a period of time to
have a very strong corrosive action on wood, metal, stone or

paint.

By changing the current density, the amount of the
product and the character can be radically changed. Also,
if the current density is too high, there is a large amount
of spongy lead formed which accumulates at the cathode on
both the inside and the outside of the cloth sack, which will
cause some impurities to be present if it is not removed.
This may be easily done by passing the anolyte and the sus-
pended product through a coarse cloth directly on its lea-
ving the cell. This serves very well to strain out the
large particles of lead While the product passes through with
ease.

There is one factor in the process as outlined here,
which tends for further economy, namely, that the impurities
in the metallic lead, while they may form products of some
sort, are retained on the anode and may be washed off at
the end of the run. This obviates the purifying of the lead.
Spongy lead was formed on some of the longer runs at high
current densities, which would seem to indicate that the
flow of catholyte was insufficient to precipitate the lead
ions, thus giving them an Opportunity to get to the cathode
and be reduced again to lead. This could be corrected by
lowering the current density or increasing the flow. The
former was the better solution to the problem.

Some of the product was ground in a mortar to a
very fine powder, which was then mixed with linseed oil,

to form white lead in oil. Some was also mixed into paint,

and as raw oil was used, and it was a very slow drying job.

A sample of electrolytic lead chromate was also ground and
mixed into pigment. Some of the pigment was then mixed half
and half with white lead and diluted with oil and turpentine
to a proper consistency. As paints, both had good covering
and hiding qualities.

The samples were analysed in the following manner.
An aspirator is charged with sulphuric acid and weighed. The
sample of white lead is added and weighed again. This gives
the weight of the sample. The sulphuric acid is then let
down on the sample through a funnel. After the reaction has
come to an end and the carbon dioxide has all bubbled through
the small drying chamber, air is drawn through the bottle
to rinse it out and clear out all the carbon dioxide. The
apparatus is weighed again. The loss in weight is the car—
bon dioxide. The lead sulphate is insoluble and is digested,
filtered, dried and weighed to determine the amount of lead
in the sample. The standard amounts are given by the Ana-
conda Cbmpany as 11.35% 002 and 80.14% Pb.

In conclusion, it is noted that the test yields of
basic white lead are obtained under the following conditions,
namely, 2% solution of sodium acetate, a 2--4% solution of
sodium carbonate, and the current density 6.8 amperes per
square foot. The flow of anolyte is two liters per minute

and .66 liters per minute for the catholyte.

BIBLIOGRAPHY

Bowman - Anaconda Electrolytic White Lead. Transactions
of the American Institute of Mining and Metall—

urgical Engineering. Vol. 73. 1926

Mantell - Industrial Electrochemistry.

Mellor - A Comprehensive Treatise on Inorganic and Theo-

retical Chemistry. Vol. VII.

M2239
Nov27’3 9

M82“

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OCT 2 5 ’fl
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