Improvements in the Production of "White Lead" or Basic Carbonate of Lead, and Apparatus therefor.
Year: 1890

page_text: Date of Application, 10th Oct., 1890
Complete Specification Left, 19th July, 1891-Accepted, 10th Nov., 1891
PROVISIONAL SPECIFICATION.
Improvements in the Production of "White Lead" or Basic Carbonate
of Lead, and Apparatus therefor.
We, RALPH WALDO EMERSON MACIVOR, F.1.C. of 47 Clapham Road,
London, and WATSON SMITH, F.I.C. of University College London, Lecturer in
Applied Chemistry, do hereby declare the nature of this invention to be as
follows:-
Our present invention relates to improvements in the production of "white lead"
or basic carbonate of lead, by the treatment of oxide of lead, that is to say, litharge
or "massicot," with a heated solution of acetate of ammonium in a closed vessel
(called the digestor) with agitation and under pressure, so that the said oxide of
lead is transformed with exceeding rapidity into basic acetate of lead, chiefly
consisting of tri-basic acetate, with, it may be, a little hydrate, ammonia being set
free, and the subsequent treatment of the basic acetate thus obtained and after
cooling, with carbonic acid in a separate vessel (called the carbonator). The
acetate of ammonium is by preference first charged into the digestor, and then the
oxide of lead (litharge or massicot) in fine powder, and in the proportion calculated
to be at least somewhat in excess of the quantity necessary to form with the acetic
acid of the acetate of ammonium the tribasic acetate of lead, Pb (C,H,O,), +
2 PbO, or Pb,O₂ (C₂H₂O₂), is added, the acetate of ammonium liquor of a strength
preferably not below 5 per cent., being first before the addition of the oxide of lead,
set in vigorous agitation or circulation by the pump, and heat having been applied
by means of a steam jacket enclosing the digestor. The digestor is closed, and the
temperature rises. As the temperature rises and approaches 100° C. (212° Fah.)
and as the pressure (due more especially to the tension of vapour of ammonia set
free during the reaction with the litharge) increases, so is the rapidity of the
conversion of oxide into basic acetate increased and more and more of this basic
acetate becomes dissolved, whilst at or about 100° C. (212° Fah.) it is entirely, or
almost entirely, in a state of solution.
The degree of heat, or the prolongation of the heat, will depend of course upon
the state of dilution of the acetate of ammonium used, the weaker solutions
requiring less heat than the stronger, to effect complete solution of the basic acetate
formed.
After a brief period of settling, the extraneous colored particles, dirt, and
undissolved oxide of lead &c., fall to the bottom, and the clear liquor now
containing free ammonia, is pumped out through a suitable filter to one of the
carbonators, where it is allowed to cool, the cooling being hastened by cooling
pipes. After cooling thoroughly, the cooled basic acetate, which will now have
crystallised out more or less according to the strength of the solution, to form a
thin or bulky pure white magma of microscopic needle shaped crystals, chiefly
consisting of the tri-basic acetate of lead, is treated with carbonic acid gas whereby
the basic acetate in the presence of the ammonia set free in the reaction, is
converted into basic carbonate of lead of exceptionally high basicity and covering
power.
This basic carbonate or "white-lead," so obtained will be all the purer and finer
in quality because the filtering process of the basic acetate liquor ensuing en route
between the digestor and the carbonator as described, will have removed all those
insoluble sedimentary or other impurities, that would have otherwise coloured the
product and detracted from the purity of its whiteness.
The advantages of the employment of heat and pressure for the formation of the
basic acetate of lead from oxide of lead, and the advantage of thus carrying out the

page_text: conversion of oxide into basic acetate of lead separately from the conversion of that
basic acetate of lead into basic carbonate or white lead, are firstly: That
separation by settlement and, if necessary, filtration from impurities etc., left by the
oxide of lead used and insoluble in the acetate of ammonium is made possible, and
thereby a pigment of exceeding whiteness and purity is obtained. Secondly.-That
the rapidity of the conversion of oxide of lead into basic acetate is immensely
increased it becomes in fact almost instantaneous, much time being saved; and
Thirdly That, since the conversion is effected quickly and in a perfectly closed
vessel, no chance for the escape of ammonia occurs, and in the carbonating stage of
the process, the free ammonia present in the liquid which secures the formation of
a basic carbonate of lead, and the maintenance of this basicity throughout the
conversion, is converted by the carbonic acid into the more permanent but still
alkaline carbonate, and later on, into the still more permanent acetate of ammonium
ready for use over again.
Dated 8th October 1890
R. W. EMERSON MACIVOR.
WATSON SMITH.
COMPLETE SPECIFICATION.
Improvements in the Production of "White Lead" or Basic Carbonate
of Lead. and Apparatus therefor.
We, RALPH WALDO EMERSON MACIVOR, Analytical & Consulting Chemist,
85, Gracechurch Street London and WATSON SMITH, Lecturer in Chemical
Technology, University College, London, 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 :-
Our present invention relates to improvements in the production of "white lead
or basic carbonate of lead, by the treatment of oxide of lead, that is to say, litharge
or "massicot," with a heated solution of acetate of ammonium in a closed vessel
(called the digestor) with agitation and under pressure, so that the said oxide of
lead is transformed with exceeding rapidity into basic acetate of lead, chiefly
consisting of the tri-basic acetate, ammonia being set free, and the subsequent
treatment of the basic acetate thus obtained after filtering or otherwise removing
insoluble impurities, and after cooling, with carbonic acid gas in a separate vessel,
(called the carbonator). The acetate of ammonium is by preference first charged
into the digestor, and then the oxide of lead (litharge or massicot) in fine powder,
and in the eqivalent proportions calculated to be at least somewhat slightly in
excess of the quantity necessary to form with the acetic acid of the acetate of
ammonium the tribasic acetate of lead Pb (C,H3O2)2 + 2 PhO or Pb3O₂ (C₂H₂O₂),
is added, (this being in allowance for the certain amount of insoluble and uncon-
vertible matters in the litharge), the acetate of ammonium liquor of a strength
preferably not below 5 per cent., though it may be stronger, being first, before the
addition of the oxide of lead, set in vigorous agitation and circulation by the pump,
and heat having been applied by means of a steam heater as shown in the drawing
and as will be explained later on. The digestor is closed and the temperature rises.
As the temperature rises and approaches 100° Centigrade and as the pressure due
more especially to the tension of vapour of ammonia set free during the reaction
with the litharge, increases, so is the rapidity of the conversion of the oxide into
basic acetate increased and more and more of this basic acetate becomes dissolved,
whilst at or about 100° Centigrade, it is entirely in a state of solution. The degree
of heat, or the prolongation of the heat, depend of course upon the state of dilution
of the acetate of ammonium used. Something more specific will be said later on
regarding this question of degree of heat.
The clear liquor together with extraneous coloured particles, red lead, dirt and

page_text: undissolved matters, is pumped out through a suitable filter to one of the car-
bonators, being carried en route through a cooling-pipe system, and being let cool
further if necessary in the carbonator itself. After cooling thoroughly the cold
basic acetate which may now have crystallized or separated out more or less
according to the strength of the solution, is treated with carbonic acid gas whereby
the basic acetate in the presence of the ammonia set free, is converted into basic
carbonate of lead of exceptionally white colour, and with high basicity and covering
power, proper care being taken as specially indicated later on. There are several
reasons why greater whiteness of the product is secured, the principal one being
that filtration from solid and insoluble coloured impurities has taken place, previous
to the carbonating process.
The advantages of the employment of heat and pressure for the formation of the
basic acetate of lead from oxide of lead, and the advantage of thus carrying out the
conversion of oxide into basic acetate of lead separately from the conversion of that
basic acetate of lead into basic carbonate or white lead, are Firstly;―that
separation by filtration from impurities and so forth left by the oxide of lead used
and insoluble in the acetate of ammonium, is made possible, and thereby a pigment
of exceeding whiteness and purity can be obtained, besides high basicity with
corresponding body and covering power.
Secondly-That the rapidity of the conversion of oxide of lead into basic
acetate of lead is immensely increased-it becomes in fact almost instantaneous,
much time being saved, and Thirdly :-that the conversion is effected quickly and
in a perfectly closed vessel, no chance of the escape of ammonia occurs, and in the
carbonating stage of the process, the free ammonia present in the liquid assists in
securing the formation of basic carbonate of lead, and the maintenance of this
basicity throughout the conversion.
This free ammonia is converted by the carbonic acid into the less volatile but
still alkaline carbonate, and later on, this ammonium carbonate reacts upon the
basic acetate of lead, converting it into basic carbonate of lead, acetate of
ammonium, still less volatile, being simultaneously produced, ready for use over
again.
It must be understood then that as the carbonic acid passes into and through the
ammoniacal mixture in the carbonator, it continually precipitates or forms basic
carbonate of lead, in presence of more or less of the volatile alkali which however
continually diminishes in quantity as the conversion proceeds.
Were no ammonia present, but only tribasic acetate of lead as in the case of the
earlier methods of precipitating white lead, unless a very slow current of carbonic
acid gas were passed through, some of the first formed basic carbonate of lead
would be in danger of being over-carbonated and losing its basicity, being converted
into mono-carbonate of lead. This danger is much lessened and consequently a
much greater possibility of rapidity of carbonating is conferred in the case of our
process as above described, and by virtue of the ammonia which is present.
Nevertheless the carbonic acid gas must not be passed even into the ammoniacal
liquid containing the tribasic acetate of lead, with such rapidity that distinct
alkalinity to the usual tests ceases to be maintained, and the process must be
terminated whilst the liquid is still alkaline. If a little lead salt on the one hand
and a little ammonia on the other be left in the mother liquors, ultimately obtained
on filtering from the white lead, they will be returned and circulated.
In order that our invention may be more clearly understood we will now refer to
the accompanying drawing which shows an clevation (partly in section) of the
apparatus we employ.
A, is an iron vessel made of boiler plate and lined internally with lead, and it
may be here added that all the apparatus is thus lined or is constructed of material
invulnerable to the action of lead salts, ammonia or acetate of ammonia.
The vessel A, which we term the digestor, may be furnished with a thermometer,
pressure gauge, safety valve, charging hole a, and suitably arranged sampling pipe
with cock.

page_text: Agitation of the contents of the vessel is effected by means of the circulating
pump B, which drawing off supernatant liquor along with air and ammonia vapour
from the upper part of the digestor A, by the pipe 6, forces it through the pipe c
to the heater C, consisting of a coil suitably heated by steam in a steam vessel T,
or by similar means, and then is forced through a pipe D, leading to the bottom of
the digestor A, where it terminates in a cone-spreader E, provided with a
regulating valve El. By means of this spreader the liquid is forced through the
heavy and dense mixture containing the oxide of lead, which it vigorously agitates
and keeps more or less in circulation as indicated.
The charges of litharge and acetate of ammonium will vary according to the
strength of the latter. For a five per cent, strength of acetate of ammonium
solution however it will be best to calculate the proportions as follows:-. about
1200 gallons of five per cent. acetate of ammonium liquor for 1 ton of litharge which
should be finely ground.
The temperature of the liquor may vary between 60° and 100° Centigrade, but
we prefer within these limits the lowest temperature consistent with the sufficiently
rapid conversion and solution of the litharge. The reason of this is simply that the
less the heat employed, the less is the tension of the ammonia, and the chances of
the loss of ammonia are thus minimised; in addition to this, less delay is involved
and less refrigeration or cooling needed before carbonating.
The object of the violent agitation of the litharge amongst the heated acetate of
ammonium in the digestor A, is that caking of the former may be prevented, and a
most rapid conversion of the oxide of lead into basic acetate of lead with minimum
expenditure of heat, secured.
The effect of the heat in the closed space is greatly aided by that of the pressure
due to the tension of the ammonia and aqueous vapour at the increasing
temperatures.
When the litharge is converted into tribasic acetate of lead and brought into
solution, the liquor and sludge of insoluble matters, is preferably pumped through
pipe e, by means of the filter-press pump F, and forced through the filter-press G,
which removes and retains the insoluble matter, allowing the clear liquor to pass to
the cooler H, and through this to the carbonator I, similar in construction to the
digestor A..
Here the cooled liquid is circulated by means of pump J, (in a similar manner to
that adopted in the digestor process already described)-carbonic acid being simul-
taneously pumped in through the pipe d, which is perforated as shown or introduced
in any other way.
When carbonated to the desired extent, the white magma consisting of basic
carbonate of lead and mother-liquor, is drawn through pipe j, by the pump K, into
the filter-press L.
The clear liquid flows through, into the covered mother-liquor tank M, whilst
the press-cakes of white lead after sufficient washing with water are removed and
dried in a suitable manner.
The washings are run off to a weak liquor tank (not shown) for concentration for
use over again. Any inert gases accompanying the carbonic acid, or the latter
alone in excess, pass from the carbonator through pipe g, into the catch-box other
condensing and absorbing apparatus N, containing either dilute acetic acid or cold
water, in order to retain any ammonia carried over, and furnished with perforated
trays or battle-plates n, n', 2.
This catch-box is if necessary also connected with a further condenser so as to
remove all ammonia from the displaced air, or inert gases (if impure carbonic acid
has been used)-of the carbonator. It is a lead lined vessel preferably. The
ammonia carried from each charge thus is tested by measuring the volume of the
solution from the catch-box or other condensing and absorbing apparatus and
estimating the ammonia present in the solution. This amount of ammonia in a
sufficiently concentrated form is then added to the charge in the carbonator when
cold so as to produce a completely neutral solution. The ammonia in the catch-box

page_text: is either added to ammonia stock in the ammonia department, used for dilution of
strong ammonia in making fresh acetate, or strengthened up to further absorption
in the catch-box or condenser until strong enough to add to a freshly run and
cooled charge in the carbonator to replace any ammonia driven off by heat.
The sludge in the filter-press G is washed with water to remove acetate liquors
and ammonia, the weak liquors being run to a separate closed vessel similar to
carbonator I, but smaller, and not shown in the drawing.
Here white lead is precipitated by carbonic acid and the product passed into the
filter-press L, the weak liquors associated with it serving to give a preliminary
washing to a freshly received charge of white lead already in the press. The weak
filtrate liquors thus obtained are preferably sent to the weak liquor tank already
mentioned for subsequent concentration, instead of to the stronger liquor tank M.
The sludge-cakes from the filter-press G, connected with the digestor are suitably
treated for the recovery of lead therefrom.
The mother-liquor contained in tank M from the white-lead filter press L, are
directly returned to the ammonium acetate tank O, by the pump P, for use over
again.
The carbonators may be operated singly as hereinbefore described, or two or
three may be connected together so as to be worked in rotation, the partially
absorbed carbonic acid from one carbonator being completely or more or less
The last of
completely, absorbed in the one, or two, with which it is connected.
such series of carbonators, would of course be connected with the catch-box
arrangement previously described or other condensing and absorbing apparatus.
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 :-
Firstly. The herein described process for the production of white lead or basic
carbonate of lead consisting in the treatment of oxide of lead, that is, litharge or
massicot, with a heated solution of acetate of ammonium in a closed vessel under
pressure and with agitation, the temperature being raised with the pressure, until
not only is the oxide of lead converted into basic acetate of lead, chicfly consisting
of the tribasic acetate, but that basic acetate of lead is dissolved in the hot liquor,
which hot liquor, at a temperature between 60° and 100° C. (212° Fah.) or there-
abouts, is then passed through a suitable filter, by this means, all sediment and
coloured impurities that would impair the whiteness of the white lead ultimately
obtained, being removed from the basic acetate, which becomes thus a pure raw
material for the manufacture of a pure white lead.
Secondly. In the manufacture of white lead or basic carbonate of lead, the
herein described treatment of tribasic acetate of lead in solution or partly in
solution after removal from the extraneous and insoluble constituents of the litharge
or oxide of lead employed, in presence of ammonia, and in the cold, with carbonic
acid gas in sufficient quantity, of the right degree of purity, and at a proper degree
of speed to produce basic carbonate of lead or white lead, the aminonia and acetic
acid being recovered as acetate of ammonium for further use in the process.
Thirdly. In the manufacture of white lead or basic carbonate of lead, the
system of apparatus herein described and shown by the drawing wherein the process
is carried out as a circular one, the whole combined and operating as and for the
purposes herein set forth.
Fourthly. In the manufacture of white lead, and for similar purposes, the herein
described mode of agitating the litharge with hot acetate of ammonia so as to
prevent caking and to effect conversion and solution, and the similar agitating
process in carbonating the tribasic acetate of lead solution produced, so as to keep
up a continual motion of the particles by means of rapid circulation and injection
by the aid of a pump, the cone spreader E, and regulating valve E', as shown, the
upper layers of liquid in either digestor or carbonator being rapidly drawn off by

page_text: the pump, returned, and forced in again to digestor or carbonator respectively, at
the lower parts thereof: with considerable vigour, as may be desired.
Fifthly. In the manufacture of white lead or basic carbonate of lead, the herein
described method of working the carbonating process so that the carbonic acid gas
whether pure or admixed with inert gases, after partial absorption in one carbonator
may be passed into another carbonator or carbonators, the final inert gas being
passed into a catch-box or other convenient absorbing apparatus for the purpose of
retaining any ammonia carried over and that would otherwise be lost.
Dated this 8th day of July 1891.
RALPH WALDO EMERSON MACIVOR.
WATSON SMITH.