Improvements in the Production of Basic Carbonate of Lead or "White Lead."
Year: 1895

page_text: Date of Application, 25th Nov., 1895
Complete Specification Left, 18th July, 1896-Accepted, 22nd Aug., 1896
PROVISIONAL SPECIFICATION.
Improvements in the Production of Basic Carbonate of Lead or
"White Lead."
I, OSWALD HAMILTON, of the Undershore, Northfleet, in the County of Kent,
Chemical Engineer, do hereby declare the nature of this invention to be as
follows:-
The object of this invention is an improved process for the manufacture of basic
carbonate of lead (white lead) in a more economical manner and of a more regular
composition, while securing at the same time, all the characteristics of the white
lead produced by the Stack process, without many of its disadvantages.
In all the processes hitherto adopted for these purposes, the greatest difficulty
has been found in obtaining a basic carbonate of lead possessing at the same time
the composition and physical properties hitherto claimed to be characteristic of
"Dutch or Stack white lead, while numerous attempts have been made to hasten
the conversion of the blue or metallic lead used in the stacks without much success,
as undue hastening of the Stack process diminishes the value of the product. This
fact has been put forward from time to time as a proof that more rapid processes
must necessarily be failures.
Hitherto, attempts made in the wet way have been failures, since previous
experimentors in their endeavours to secure rapidity of production have overlooked
the fact that the conditions of precipitation exercise the greatest influence on the
physical properties of the precipitate. The wet process described by Mac Ivor in
Specification No. 10426°, and No. 10426** of 1888; by Mac Ivor & others in
No. 6815 of 1889, and by Mac Ivor and another in No. 16093 of 1890, will produce
white lead of good physical properties, but there are so many sources of loss and
such great difficulties in the manipulation of the product that it could never be a
commercial success.
Processes such as that of Mac Ivor, Orr and others, where litharge, or massicot
is mixed with minute quantities of an active agent such as acetic acid etc. fail,
because of the great care required in manipulation and the difficulty of securing a
regular product, while if the process be carried through to secure a proper product,
the length of time required, put these methods out of the question commercially.
In the process I am about to describe, however, none of these difficulties appear,
and all sources of loss can be avoided. In this process, a solution containing
lead acetate and ammonium acetate is employed in proportions of the chemical
equivalents
To this solution is added litharge, which has been powdered in such a manner so
as to secure that all the litharge has been converted into the red variety which
I find a more soluble and of somewhat different physical properties than litharge
which has not undergone this treatment. Great care must be exercised in the
addition of the litharge and the rate of addition should be such that the litharge
goes into solution as it is added, otherwise the litharge will form clots of litharge
surrounded with a coating of basic acetate of lead which prevents the further
action of the liquor or solution upon the inner core of litharge. The solution is

page_text: previously heated to a temperature of 80° C. but the addition of the litharge may
commence at a much lower temperature, provided the above precaution is taken.
The most suitable apparatus to be employed in the process is that described by
me in Specification No. 11,249 of 1892, and the solution should be heated, if the
heating take place in the digesting vessel, by the steam coil described in my
Specification No. 4069 of 1894, as a coil of this shape offers little or no
surface for the formation of the hard cement-like cake of highly basic acetate
of lead which tend to form on hot surfaces during the solution of litharge in acetic
acid or acetate solutions. Moreover, unless the mixing or agitating apparatus
described in the aforesaid specification be adopted, the small quantity of ammonia
given off during solution which it is important should be intimately mixed with the
acetate solution will be lost or not be thoroughly mixed.
During the first stage of solution, the litharge freely dissolves, but towards the
end the litharge must be added cautiously as above described.
When solution is complete, the solution will have an alkaline reaction and will
smell of ammonia being about 16° T. The solution is then filtered if necessary in
any well known manner and the clear solution is pumped into carbonating vessels
where it is treated with carbonic acid gas. The vessel should be of a suitable size
and the strength of the carbonic acid gas should be such that the gas shall
thoroughly mix and carbonate equally the whole mass of the solution. When
the carbonation is properly carried out, the basic carbonate of lead will be of the
right consistency and density, while the mother liquor will be of a specific gravity
of about 4° to 5° T. and will be of approximatly the same composition as the
solution first employed.
The solution containing the basic carbonate in suspension is then passed into filter
presses, the clear solution passing to the digesting vessel as already described for
use over again.
The white lead remaining in the filter press is washed first with a dilute solution
of sodium carbonate to precipitate the lead solution remaining in the cakes of lead
and then the lead is washed with water till the wash waters are quite neutral.
The first wash waters are treated for the recovery of the acetic acid.
The white lead in the filter presses is then removed and placed in suitable pots
or dishes or packed in suitable trays or trolleys which when loaded are run into a
suitable drying chamber heated by hot air and pipes at a temperature of 100° C.
When dry, the pots are emptied and the white lead is placed under edge-runners
which in a few minutes rolls out the air intimately associated with the lead,
reducing it in a compact and dense form for the market.
Dated the 22nd day of November 1895.
COMPLETE SPECIFICATION.
Improvements in the Production of Basic Carbonate of Lead or
"White Lead."
I, OSWALD HAMILTON of the Undershore, Northfleet, Kent, Chemical
Engineer, 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 :-
The object of this invention is an improved process for the manufacture of basic
carbonate of lead known as "white lead" of a more regular composition and in a
more economic manner while securing at the same time all the characteristics of
the white lead produced by the "Stack" process without many of the disadvantages
of the latter.

page_text: "
In all the processes hitherto adopted for these purposes the greatest difficulty
has been found in obtaining a basic carbonate of lead possessing at the same time
the composition and physical properties (viz.:-density, body, and covering power)
hitherto claimed to be characteristic of "Dutch or "Stack" white lead, while
numerous attempts have been made to hasten the conversion of the blue or
metallic lead used in the stacks without much success, commercially or otherwise,
since undue hastening of the "Stack" process tends to diminish the value of the
final product. This fact has therefore been put forward from time to time as a
proof that more rapid processes must necessarily be failures.
With regard to the so-called "precipitation" processes, experimenters have,
apparently, all overlooked the fact that the conditions of precipitation exercise
the greatest influence on the physical properties of the precipitate and consequently
have always found that while there has been little difficulty in securing a
* white
lead" of the same percentage composition as "Dutch" white lead yet the valued
and desired properties of the latter white lead are absent.
Some experimenters have attributed these properties (which seemed peculiar to
"Stack" white lead) to the dense and compact form of the lead used and they
have tried to secure the desired properties by using dense oxide of lead with a
dilute solution of acetic acid or its homologues, while even nitrate and other acids
have been used, carbonic acid being blown into the mixture while being stirred.
Now such experiments have been unsuccessful, for, to secure the desired properties
and composition the addition of carbonic acid must be carefully adjusted to the
rate of solution which is so slow and imperfect owing to the pasty condition of the
mass and difficulty in proper mixing, which also prevents the proper and regular
combination of the carbonic acid with the hydrate or basic acetate of the lead, that
its commercial production is out of the question. Again where the digestion of
the oxide of lead and the simultaneous carbonation take place, pure oxide of lead
must be used, otherwise the resulting white lead will be spoiled and its durability
jeopardised.
I am well acquainted with the various processes patented and described by
Messrs. Mac Ivor, Smith and Elmore viz. :-those in which massicot or litharge is
treated with hot or cold solutions of ammonium acetate and will endeavour to show
how the process I have invented differs from those of the former, as well as indicate
their failure.
In the above mentioned processes after careful and expensive experiments on a
commercial scale I found :-
First. That massicot cannot be used for solution in acetate of ammonium as it
will dissolve only with great difficulty.
Second. That the litharge employed must be of a definite physical character.
Third. That the proportions described in their specification will not produce the
stated result, namely 1200 galls. of a 5% solution of ammonium acetate will not
dissolve one ton of oxide of lead as the solubility of tribasic acetate of lead in,
water, much less in an alkaline ammoniacal solution, is not sufficiently higb. Ag
soon as about 7 cwt. of oxide of lead is added, the contents of the digesting vessel
suddenly becomes thick owing to a separation of a highly basic acetate of lead
consequent on such a large excess of ammonia.
Fourth. That the cooling described as requisite is not only unnecessary but on
cooling such a mixture it forms a thick mass which it is impracticable, if not
impossible, to treat uniformly and regularly with carbonic acid.
Fifth. That moreover, the product tends to become too basic for the purposes
of pigment, while the presence at the end of the process of a solution consisting
chiefly of carbonate and acetate of ammonia is undesirable for use over again, as
on the addition of a further quantity of litharge a large precipitation of white lead
occurs in the digesting vessel, which of course entails a considerable loss of good
material.
In the process I am about to describe however none of these difficulties appear,
and all sources of loss can be practically avoided.

page_text: Pb.(C,H,O₂),
(NH,)C,H,O.
As carried out on the large scale I usually employ about 9000 galls. of a 5%
solution and this is capable of dissolving about 35 cwt. to 2 tons of oxide of lead.
I find moreover that larger quantities can be dissolved on the large scale than on
the laboratory scale. Whatever quantities of materials are employed it is
important that the solubility should be taken into account.
To this solution is added litharge which has been powdered in such a manner so
as to secure that all the litharge (preferably made by oxidation of refined lead on
bone ash tests as in cupillation of silver) has been converted into the red variety
(which I find a more soluble and of somewhat different physical properties than
litharge which has not undergone this treatment).
Great care must be exercised in the addition of the litharge and the rate of
addition should be such that the litharge goes into solution as it is added,
otherwise the litharge will form clots surrounded with a coating of basic acetate
of lead which prevents the further action of the liquor or solution upon the inner
core of litharge.
The solution is heated to a temperature of 80° C. but the addition of litharge
may commence at a much lower temperature provided the above precaution is
taken.
The most suitable apparatus to be employed in the process is that described by
me in the Specification of No. 11249 of 1892 and the solution should be heated, if
the heating take place in the digesting vessel by the steam coil described in my
Specification No. 4069 of 1894 as a coil of this shape offers little or no surface for
the formation of the hard cement like cakes of highly basic acetate of lead which
tend to form on hot surfaces during the solution of litharge in acetate solutions.
Moreover, unless the mixing and agitating apparatus described in the aforesaid
specification be adopted, the small quantity of ammonia given off during solution
will not be intimately mixed with the acetate solution. Even where a partly open
vessel is employed if the above named temperature is not exceeded the loss of
ammonia is exceedingly small and does not seriously affect the carbonation process
or the use of the liquor over again, only greater care is requisite.
During the first stage of solution the litharge freely dissolves, but toward the end
the litharge must be added cautiously as above described.
When solution is complete the solution will have an alkaline reaction and will
smell of ammonia being about 14°-16° T.
The solution is then filtered if necessary, in any well known manner, and the
clear solution pumped into carbonating vessels where it is treated with carbonic
acid gas.
The vessel should be of a suitable size and the strength of the carbonic
acid gas should be such that the gas shall mix thoroughly and carbonate equally
the whole mass of the solution. When the carbonation is properly carried out,
the basic carbonate of lead will be of the right consistency and density, while the
mother liquor will be of a specific gravity of about 4° to 5° T. and will be of
approximately the same composition as the solution first employed, and should give
a faint alkaline reaction.
The solution containing the basic carbonate in suspension is then passed into
filter presses, the clear solution passing to the digesting vessel, already described,
for use over again.
The white lead remaining in the presses is washed first with a dilute solution of
sodium carbonate to precipitate the lead solution remaining in the cakes of white
lead, and then the lead is washed with water till the wash waters are quite
neutral,
The first wash waters are treated for the recovery of the acetic acid. The

page_text: white lead in the filter presses is then removed and placed in pots or dishes packed
on suitable trucks or trolleys which are run into a chamber heated by hot air at a
temperature of 100° C.
When dry the pots are emptied and the white lead is placed under edge runners
which in a few minutes rolls out the air intimately associated with the lead,
reducing it to a compact and dense form for the market.
Having now particularly described and ascertained the nature of my said inven-
tion, and in what manner the same is to be performed, I declare that what I
claim is :-
The manufacture and production of the basic carbonate of lead, known as
"white lead" for the production of paints and pigments, from oxide of lead or
litharge, by the employment of a dilute solution of the acetates of lead and
ammonium in the proportions specified and in the manner as herein described.
Dated this 14th day of July 1896.
OSWALD HAMILTON.