Improvements in the Manufacture of White Lead and Neutral Lead Acetate from Lead Ores.
Year: 1891

page_text: Improvements in the Manufacture of White Lead and Neutral Lead
Acetate from Lead Ores.
We, ELWYN WALLER, Professor of Chemistry, and CHARLES AUGUSTUS
SNIFFIN, Mining Engineer, both of the School of Mines, Columbia College,
in the City, County and State of New York, United States of America, do
hereby declare the nature of this invention and in what manner the same is to
be performed to be particularly described and ascertained in and by the following
statement :-
"
This invention relates to the production of white lead, for use as a pigment,
directly from the " carbonate ore, an impure cerussite, which is found in the
vicinity of Leadville, Colorado, and various other places in the western part of the
United States.
We have discovered that, when the native carbonate ore is heated to ebullition
with a solution containing either acetic acid or lead acetate, the ore will yield
more or less of the lead it contains to the solution; and, if the proportion of acetic
acid or lead acetate be sufficient and the period of heating be adequately prolonged,
the whole of the lead contained in the ore treated will be taken up and dissolved
in the solution. When neutral lead acetate is used, basic acctates of lead go into
the solution; but, when acetic acid is employed, neutral or basic acetates of lead
are formed according to the proportion of real acetic acid which is used and the
proportion which is allowed to boil away. With neutral lead acetate, or when we
use less acetic acid than will suffice to form a neutral lead acetate with the lead
present in the quantity of ore treated, basic acetates of lead are formed in the
solution; only mere traces of the other constituents of the ore being dissolved.
When carbon dioxide is added to or passed into the solution of basic lead acetates
thus produced, a precipitate of lead carbonate combined with more or less lead
hydrate is produced, affording amorphous lead hydrocarbonate, if suitable conditions
be maintained. This lead hydrocarbonate is the "white lead" of commerce; being
used extensively as a pigment. After the treatment with carbon dioxide, neutral
lead acetate remains in the solution, and this solution may be employed in the
treatment of a fresh portion of the ore, thus effecting an economy in the acetic
acid employed, which may be used over and over again. The carbon dioxide
evolved in the primary treatment of the ore is collected and employed in the
subsequent precipitation of the lead hydrocarbonate; an important economy in the
production of this gas being thus effected.
The most advantageous proportion of acetic acid or of lead acetate to be used
as regards the amount of lead present in the ore treated is such as will afford
approximately a tri-basic lead acetate. Smaller proportions of the acetic acid or
of the neutral acetate are liable to give rise to more highly basic lead acetates,
which are less soluble in the amount of water employed, rendering the process less
advantageous on account of the necessity for handling larger quantities of the
solution. We do not, however, limit ourselves to particular proportions.
Having thus stated, in a general way, the principles upon which our invention is
based, we will proceed to set forth more definitely how it may be carried out in
practice.
The carbonate ore should be first ground or pulverized so finely that it will pass
through a sieve having forty, fifty or sixty meshes to the linear inch. The finer
it is ground, the more rapid and satisfactory will be the operation. An analysis
should then be made of a fair average portion of the ore to determine the percentage
of lead present, as this percentage varies greatly in ores from different localities.
A convenient amount or charge of the ground ore is then placed in a vessel arranged

page_text: over a furnace, or adapted to be otherwise heated. The volume of the charge will
be governed by the percentage of lead contained in the ore. Water to the amount
of about twenty times the weight of lead in the charge is then poured in, and to
this is added, for the first charge, real acetic acid in the proportion of about
one-fifth the weight of the lead in the charge; or, more exactly, 120 parts of acetic
acid to 621 parts of lead. The amount of water contained in the acetic acid, if
ordinary commercial acid be used, should be reckoned in the calculation as a part
of the water added to the charge. [By real acetic acid is here meant the anhydrous
or actual acetic acid (HC,H,O,), as distinguished from commercial acetic acid,
which is a mixture of acetic acid and water.] If neutral lead acetate be used, the
amount by weight of the acetate should be nearly equal to the amount by weight
of the lead present; or, more exactly, 379 parts of crystallized neutral lead acetate
to 414 parts of metallic lead.
The vessel containing the charge may be of iron or other suitable material, and
the charge should not occupy more than about one-third of its volume. It should
have a close-fitting cover, and be provided with a rotating mechanical stirrer,
and an outlet, which leads through a condenser to a gas-holder, for the carbon
dioxide. After the vessel is charged and the stirrer set in operation, heat is
applied and the temperature raised until the contents boil gently or simmer.
This heat should be maintained continuously for a period of from four to
six hours.
The stirrer is not absolutely essential, but is decidedly useful, because, unless
the heat be very carefully regulated, the contents of the vessel are apt to boil
over; as low a heat as possible, consistent with the necessity for keeping the
mass gently simmering, is best. At the end of from four to six hours, about
one-half of the water should have boiled away. The actual length of time
required for maintaining the heat in any given case will depend somewhat upon
the purity of the ore, the management of the heat and similar conditions. When
the period of boiling has terminated, the charge is allowed time to settle.
The water driven off as vapour from the charge, and the carbon dioxide which
is driven off with it, are passed through a coil or condenser, which should be
kept cold in order that the water of condensation may absorb as much of
the carbon dioxide as possible, and this water and the free carbon dioxide are
led from the condenser into a closed vessel or gas-holder, where they are stored.
Pipes from the gas-holder lead the carbonated water and the free carbon dioxide
into a precipitating tank or vessel; the gas entering below the level of the liquid.
The charge in the boiling vessel, as soon as the heat is withdrawn, settles rapidly;
leaving only a few finely-divided particles of the residuum in suspension. To
obtain perfectly pure white lead, these particles should be removed by passing the
supernatant solution in the vessel (now containing basic lead acetates) through a
filter. While still hot, it is led to the tank containing the carbonated water, and
carbon dioxide is passed through the mixture, whereupon a reaction takes place
and the lead is precipitated in the form of amorphous lead hydrocarbonate or
"white lead."
We have discovered that when a solution of basic lead acetate is very dilute, or
when precipitation is induced at once by mingling the solution containing basic lead
acetate with water containing carbon dioxide in solution, and more carbon dioxide
is passed into the solution, a precipitate of amorphous lead hydrocarbonate is
obtained. The uncombined carbon dioxide arising from the precipitating vessel may
be conveyed to a gas-holder for further use.
The amount of carbon dioxide set free from the ore treated should be more than
sufficient for the precipitation of the lead hydrocarbonate in the tank, since lead is
present in the ore as a neutral carbonate (PbCO,) containing 44 parts of carbon
dioxide to 207 parts of metallic lead, whereas the "white lead of commerce
contains from 27 to 36 parts of carbon dioxide to 207 parts of lead. The
more common proportion is 294 parts of carbon dioxide to 207 parts of metallic
lead. However, as some carbon dioxide may be lost by leakage, it may be necessary

page_text: to manufacture a portion of that required by one of the well-known methods. If a
large excess of carbon dioxide be employed, the precipitate tends to become
cystalline, and consequently deficient in covering power or "body" when used as a
pigment. Hence it is advisable to stop the introduction of the carbon dioxide
when the solution in the tank (which is milky) shows a tendency to rapidly clear.
After precipitation, time is allowed for the solution to become clear, when it is
drawn off from the tank for use with another charge; the precipitate being removed,
washed and dried in the usual manner.
Our object in using carbonated water, or water charged with carbon dioxide, in
the precipitating tank, and passing carbon dioxide in a gaseous form through the
solution in that tank, is to avoid crystallization and render the precipitate
amorphous.
The residual ore from the treatment may retain some lead in the form of basic
acetate, probably in a more basic form than the tri-basic, and the amount will be
larger if the solution be allowed to cool in contact with it. If desired, practically
all this basic acetate may be recovered by boiling the residue in water.
In the accompanying drawings, we have shown, diagrammatically, an apparatus
adapted for carrying out our method.
1 is the vessel or retort in which the charge is boiled or simmered for a period
of from four to six hours. This vessel is mounted on trunnions 2 and placed over
a furnace 3, or any form of heating apparatus. It can be tilted for decanting into
a filter 4, whence the solution of basic acetate flows by a pipe 4¹ to the precipitating
tank 5. Within the vessel 1 is a mechanical stirrer, shewn in dotted lines, the
shaft 6 of which is provided with a bevel-wheel 7 adapted to gear with a similar
wheel 8 on a driving shaft 9. The removable cover 11 of the vessel 1 is provided
with a gas and vapour outlet 1", which is coupled by a suitable union with a
jointed pipe 10; the latter leading the carbon dioxide and watery vapour from the
vessel 1 to the condenser 11, whence the free gas and the carbonated water pass to
a holder 12 furnished with a counterweighted cover 12'. The water remains in the
holder and seals the cover, while the free gas rises therein. The carbonated water
from the holder 12 flows into the precipitating tank 5 by way of a cock-controlled
pipe 13; the gas from the cover 12' entering the tank, below the level of the liquid
therein, by way of a pipe 14 having that portion within the tank perforated.
After the precipitation of the white lead in the tank, the remaining solution of
neutral lead acetate is drawn off by a cock 15 for use with the next charge treated.
The rotary stirrer may be removed from the vessel 1 by lifting it with the
cover 1; cleaning and the removal of the residual ore being thus facilitated. The
free carbon dioxide escapes from the tank 5 by an outlet 16.
The apparatus described above is given by way of example only.
By preference, we use acetic acid only with the first charge treated, and for the
succeeding charge or charges we employ the solution of neutral lead acetate from
the precipitating tank for admixture with the water and pulverized ore.
Of course
we may employ acetic acid with each charge, or a solution of the neutral lead
acetate with each charge, but for economical reasons we prefer to operate as above
set forth. In each case, the acetic acid is the active agent, whether combined in
the form of the neutral acetate or not.
We have given the proportions of the parts which we usually employ and which
will produce good results, but we do not limit ourselves to these proportions, nor do
we limit ourselves to the degree of heat employed in the treatment.
When the proper proportions of acid and ore are employed, the first step in our
process may be used to produce neutral lead acetate from the ore. To obtain this
salt from the solution, the latter may be reduced by evaporation and the salt
crystallized out.
Having now particularly described and ascertained the nature of this invention
and in what manner the same is to be performed, we claim :—
First-The herein-described method of producing a solution of a basic lead

page_text: ་
acetate from lead carbonate, which consists in heating the carbonate with water
containing acetic acid or neutral lead acetate.
Second-The herein-described method of producing lead hydrocarbonate or
"white lead" directly from ores containing lead carbonate, which consists in
heating the ores with water containing acetic acid or neutral lead acetate, whereby
a solution of a basic lead acetate is obtained, and then precipitating the lead hydro-
carbonate from this solution with carbon dioxide.
Third: The herein-described method of producing lead hydrocarbonate or
"white lead" directly from ores containing lead carbonate, which consists in first
heating the ores with water containing acetic acid or neutral lead acetate, whereby
a solution of a basic lead acetate is obtained, then mixing this solution with water
charged with carbon dioxide and passing through the mixture carbon dioxide in its
gaseous form, whereby lead hydrocarbonate is precipitated.
Fourth-The herein-described method of producing lead hydrocarbonate or
"white lead" directly from the native lead carbonates, which consists in first
mixing together the carbonate ore, water and acetic acid, or neutral lead acetate,
then heating the mixture to ebullition and maintaining this heat until the lead in
the ore has been converted into a basic acetate and the latter dissolved, then mixing
the solution of basic acetate with water charged with carbon dioxide and passing
through said mixture carbon dioxide in a gaseous form until the lead has been
precipitated therefrom in the form of lead hydrocarbonate.
Fifth-The herein-described method of producing lead hydrocarbonate from a
solution of a basic lead acetate, which consists in mixing the solution with water
charged with carbon dioxide and passing through the mixture carbon dioxide in its
gaseous form, whereby the lead is precipitated as a hydrocarbonate.
Sixth-The herein-described method of producing amorphous lead hydro-
carbonate from a solution of a basic lead acetate, which consists in mixing the
solution while hot with water charged with carbon dioxide and passing through the
mixture carbon dioxide in its gaseous form until the mixture begins to clear.
Seventh-In an apparatus for the manufacture of "white lead" directly from
the native carbonate, the combination, with a closed vessel containing the charge
and connected at the top to a condenser, of the condenser, means for heating the
vessel, a holder in communication with the condenser for the carbonated water of
condensation and gas, and a precipitating tauk connected to the holder, substantially
as and for the purposes set forth.
-
Eighth The herein-described method of producing lead acetates from ores
containing lead carbonate, which consists in mixing together the ore, water and
acetic acid in substantially the proportions specified, then heating the mixture till
ebullition takes place, and maintaining the heat until the lead in the charge is
converted and taken up in solution.
Ninth The herein-described method of making "white lead" from ores
containing.lead carbonate, the same consisting in heating the ore till ebullition
takes place with a solution of neutral lead acetate from a previous charge until
the lead in the ore is converted into a basic acetate which is taken up in solution
and the carbon dioxide from the ore is driven off and stored, and then mixing the
solution of basic acetate with carbonated water and passing therethrough the free
carbon dioxide driven off from the ore.
Tenth The herein-described method of producing neutral lead acetate, which
consists in heating the carbonate ore with water containing acetic acid in proper
proportion.
300
G. G. M. HARDINGHAM,
191, Fleet Street, London, E.C., Chartered Patent Agent.