Improvements in the Manufacture of White Lead.
Year: 1896

page_text: Date of Application, 23rd June, 1896-Accepted, 29th Aug., 1896
COMPLETE SPECIFICATION..
Improvements in the Manufacture of White Lead.
We, WILLIAM PENN TATHAM and WILLIAM. TATHAM, both of 226 South
Fifth Street, in the City of Philadelphia, State of Pennsylvania, United States
of America, Metallurgists and Manufacturing Chemists, 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 :-
We have found that if finely powdered oxide of lead or litharge is stirred
into a solution of neutral or basic acetate of lead, in proper proportions and at
the proper temperature, the oxide of lead or litharge is converted into a mass
composed substantially of crystals of tri-basic acetate of lead mingled with a
mother liquor containing a small quantity of tri-basic and di-basic acetate of lead.
in solution. If the product thus obtained is then subjected to the action of
carbonic action it is converted into a white lead of commerce of superior whiteness.
and body, and a supernatant solution of neutral acetate of lead.
The whole operation can be performed in one vessel, but we prefer to use two.
vessels and proceed as follows:-
In the first vessel containing the neutral acetate of lead we stir in a quantity
of unground litharge until the specific gravity indicates a solution distinctly short
of saturation. The liquid is then drawn off into a second vessel and the proper
amount of finely ground litharge is gradually stirred in until crystallization sets
in. The product thus obtained is a dense mass of small crystals, generally of
such character as to stop the agitator a comparatively small quantity of mother
liquor being formed, usually sufficient to merely moisten the crystals, and presents.
the appearance of a damp curdy mass.
We obtain the best results when the whole amount of litharge added to a
neutral acetate is about two and one-fourth times as much as the oxide of lead
combined with the acetic acid in the neutral solution.
When the two vessels are used we find that good results are to be obtained
by dissolving about 60% of the litharge in the first vessel and the remaining 40°
in the second.
When one vessel is used, the temperature of the original solution should be
about 80 degrees Fahrenheit.
When two vessels are used the temperature of the solution in the second vessel
should be about 160 degrees Fahrenheit before adding the litharge.
We have found that better results are obtained by having the strength of the
original solution, when the litharge is all put in at once, about 16% acetate of
lead; and in the second case, where the powdered litharge is added to the
partially saturated solution, the strength of the original neutral solution need
only be about 13% of acetate of lead.
The next step in the process consists in exposing the crystallized tri-basic
acetate together with the mother liquor itself, to the action of carbonic acid,
preferably, though not necessarily, in the same vessel in which crystallization is
accomplished.
When this exposure to the action of carbonic acid has continued for a sufficient
time, the mass is converted into white lead and neutral acetate of lead in solution..
The neutral acetate of lead is separated by filtering, or by any other suitable
method, and is used over again with a fresh portion of litharge, while the white
lead is washed and dried.

page_text: We do not say that it is necessary for the production of white lead that the
mother liquor should be exposed to the action of the acid. It is difficult and
expensive, however, to separate it from the crystals, and therefore more economical
to allow the whole product, both crystals and uncrystallized liquor, to be exposed
to the action of the carbonic acid.
We are aware that it is not new to subject a solution of basic acetate of lead
to the action of carbonic acid gas, and we do not claim such process. It differs
primarily from our process in the fact that the tri-basic acetate of lead which
we use is subjected in solid form to the action of the carbonic acid. By this
means we are enabled to greatly reduce what may be termed the idle percentage
of lead when the process is used as a continuous one by the repeated saturation
of the final mother liquor after the conclusion of exposure to the gas.
Thus in our process practically all of the litharge which is added at the first
stage of the process is converted into white lead, whereas in such a liquid process
as is set forth in the United States Letters Patent to A. C. Bradley, No. 434,623,
dated August 18th 1890, the patentee states that only about one-halt of the charge
of basic acetate is converted into white lead. The lack of economy in such
method, when the process is conducted upon a large scale, is obvious.
Having now particularly described and ascertained the nature of our said inven-
tion and in what manner the same is to be performed we declare that what we
claim is :-
1. The hereinbefore described process of manufacturing white lead, which
consists in adding litharge to a solution of acetate of lead until a
crystals of tri-basic acetate of lead has been formed, and then exposing said
mass to the action of carbonic acid gas, substantially as and for the purposes
specified.
2. The herein before described process of manufacturing white lead, which
consists in exposing crystallized tri-basic acetate of lead to the action of carbonic
acid gas, substantially as set forth.
Dated this 2nd day of June 1896.
CARPMAEL & Co.,
Agents for the Applicants.