THE MANUFACTURE OF WHITE LEAD.
The processes by which white lead is or was manufactured may be
divided, according to the principal operations, in the following
manner:—
1. Processes in which metallic lead is subjected to the action of
acetic acid vapour, whilst the vessel in which this operation is
conducted is exposed to a higher temperature. In the oldest so-called
Dutch method this increase of temperature is effected by the
decomposition of manure, by which the vessels containing the lead and
acetic acid are surrounded. In consequence of the heat produced by this
fermentation, acetic acid and water are volatilised, and oxygen also
being present, lead acetate is formed.
Also, as a consequence of the oxidation of the lead, heat is produced,
which accelerates the process, and lead oxide is formed in large
quantity, which unites with the neutral acetate already formed to
produce a basic compound. The vessels in which this process is taking
place are in an atmosphere containing much carbonic acid produced by
the fermentation of the surrounding organic matter; this carbonic acid
converts the basic lead acetate into white lead.
The German or Austrian method is to be regarded as an improvement on
this rough process. The heat necessary for the normal course of the
chemical reactions is produced from fuel; the carbonic acid formed by
the combustion of the fuel is used to convert the basic lead acetate
into white lead.
2. In the above methods the manufacture of white lead commences with
the production of white lead from metallic lead and acetic acid. In the
so-called French method a solution of basic lead acetate is decomposed
by carbonic acid into white lead and neutral lead acetate, which latter
is again converted into basic acetate.
3. The English method. The principle of this process consists in
moistening litharge with a solution of lead acetate and exposing it to
the action of carbonic acid, whereby white lead is formed.
Methods for manufacturing white lead, which are often advanced as
entirely new processes, may be always traced to one of the above, from
which they deviate but little in principle, and the deviations cannot
always be regarded as improvements. In the following detailed account
of the manufacture of white lead we shall adhere to the classification
just given, according to which there are three principal methods: (1)
Manufacture of white lead from metallic lead, acetic acid and carbonic
acid (Dutch and German processes); (2) manufacture of white lead from
basic lead acetate (French process); and (3) from litharge moistened
with lead acetate solution (English process).
We should remark that every year “new” processes for the manufacture of
white lead are patented. The majority of these will not be mentioned;
to those acquainted with the principles of chemistry they at once
appear impracticable.
MANUFACTURE OF WHITE LEAD FROM METALLIC LEAD.
(a) DUTCH PROCESS.
This essentially primitive process, when properly conducted, produces
white lead of good colour and covering power, which are the properties
for which this pigment is valued. It is now seldom used,[2] because
other methods give a product of equal colour in a shorter time. It is,
however, of economic interest, as showing how a branch of industry
may rise from crude beginnings to a high state of perfection. The
operations comprised in the Dutch process are as follows:—
[2] In England the Dutch process is in general use.—TRANSLATOR.
(1) Casting the lead into sheets; (2) placing these sheets in pots and
arranging the pots in the stacks (placing the pots containing lead
and acetic acid in the bed of manure); (3) removing the pots from the
stack; (4) separating the white lead formed in the pots; (5) further
purification of the impure white lead by grinding, washing and drying.
1. =Casting the Lead into Sheets.=—At first sight it would appear
unsuitable to cast lead in sheets, since this metal can be readily
rolled into sheets of any thickness. Experience has, however, shown
that rolled lead is only slowly attacked by acetic acid vapours, whilst
cast sheets are rapidly attacked.
An iron pan about one metre in diameter, with an iron cover furnished
with a pipe opening into a flue, is used for melting the lead. This
arrangement is designed to protect the workman from the dangerous
vapours evolved from the molten metal. At the workman’s side of this
cover is a counterpoised slide, which only remains open when the
counterpoise is held in check by a lever. In front of the kettle
there is an iron plate movable about a horizontal axis. The lead
being heated to just above its melting point, the workman takes 7 to
8 kilogrammes of metal in a ladle, and pours it on the plate, which
is horizontal. The lead solidifies in a very short time, but before
it is completely solid the plate is inclined towards the pan so that
the still liquid lead runs back into it, leaving a very thin sheet on
the plate. The hard sheet is removed from the plate, and the latter
cooled by cold water to be ready for a new casting. The sheets made in
this way are not more than 1 to 2 millimetres thick. They are then cut
into strips of a width to suit the size of the pots in which they are
converted into white lead; the width of the sheets is generally 5 to 6
centimetres. Since the rate at which the white lead is formed depends
on the surface of the metallic lead, instead of continuous sheets the
lead is generally cast into gratings. For this purpose an iron plate,
upon which are intersecting strips, is used instead of the flat plate.
Plates for the casting are also used containing grooves intersecting
at right angles. In the first case, plates are obtained in which are
openings meeting at right angles, and in the second case, according to
the distance of the grooves apart, a more or less wide-meshed lattice
work.
[Illustration: FIG. 4.]
2. =Building up the Stacks.=—The rolled-up lead plates are placed in
the pots. These (Fig. 4) are somewhat conical in shape; they have at
some distance from the bottom a projecting ring, or sometimes three
projections only, upon which the lead spiral rests. Before the spirals
are put in position, a quantity of ordinary vinegar, about a quarter of
a litre, is poured in. There must be sufficient room below the spiral
so that it shall not be in contact with the vinegar. The insides of
the pots are glazed at least half-way up, so that the liquid does not
penetrate the porous earthenware.
The pots have a capacity of about 1 litre, and a diameter at the top
of 10 centimetres. If lead plates are used, the pots are about 20
centimetres high; if gratings are used the pots may be lower, by which
there is economy in room and a larger number can be placed in one stack.
The stacks, built up of pots and manure, are of different sizes; it
is not advisable to make them too small, or the loss of heat would
be considerable. A stack 4 to 5 metres long, 3½ metres wide and 6 to
7 metres high will contain 6,000 to 8,000 pots and 9,000 to 11,000
kilogrammes of lead.
The stack consists of a rectangular pit walled on three sides; the
fourth side is open, with the earth dug out in the form of an inclined
plane, in order to permit the introduction of the pots and the manure.
The construction of the stack is commenced by placing the pots at the
bottom in rows, avoiding interspaces as much as possible. In a stack of
the size mentioned a layer contains 1,000 to 1,200 pots. Between the
pots containing lead and acetic acid are arranged a number of larger
ones containing acid only; the object of these is to furnish acetic
acid vapour. When the pots are in place, 3 or 4 lead plates are placed
on each spiral, the top plate forming the cover; immediately over the
pots strong wooden planks are laid, and upon them a layer of boards,
which must fit so tightly that nothing can fall through. On the boards
is spread out a layer of fresh stable manure, with which the space
between the outside row of pots and the wall is also filled. The layer
of manure is 30 to 40 centimetres thick.
Upon the lowest layer of pots, a second, third and so on are built up
exactly in the same manner, so that the whole stack is filled with
alternating layers of pots and manure. In order to prevent the cooling
of the uppermost layer of pots, it is covered with a thicker layer of
manure 60 to 70 centimetres thick. When lead plates and the taller pots
are used, a stack will generally contain 15 layers; but when gratings
and the smaller pots are employed, 18 layers can be packed into the
same space. In arranging the layers of pots, care should be taken to
leave spaces at tolerably equal distances, so that the air necessary
for the oxidation of the lead may enter. To prevent the cooling of the
stack at the front, where it is not protected by masonry, when full it
is walled up with boards; a board roof also protects the erection from
rain.
In place of manure, spent tanners’ bark can be used, which in the
same manner ferments, producing heat and carbonic acid. In places in
the neighbourhood of large tanneries this spent bark is generally
obtainable at lower prices than stable manure, which is more valuable
for agricultural purposes; the former has also a very considerable
advantage, white lead made by means of spent tanners’ bark being
generally of a purer white than that made with manure. The reason for
this is that in the decomposition of animal excrement small quantities
of sulphuretted hydrogen are produced, a gas which produces black lead
sulphide when it comes into contact with lead compounds.
According to the results of practical experience, pigs’ dung cannot be
used in the manufacture of white lead; so much sulphuretted hydrogen
is evolved from it that the white lead is not white, but has a greyish
tinge.
When bark is used in place of manure, a discolouration of white lead by
sulphuretted hydrogen is not to be feared, but there is the drawback
that a longer time is necessary for the corrosion of the lead, because
the bark decomposes more slowly than the manure, and accordingly gives
out less heat and carbonic acid.
According to the climate of the country, the stacks may be differently
erected. In colder countries it is necessary to sink them in the earth
and surround them with masonry, as directed above; but in warmer
climates such effectual protection against cooling is unnecessary, yet
in all cases it is better to sink the stack in the earth on account of
the regularity of temperature so obtained.
Instead of sinking the stacks in the earth, they may be built in the
open when there is a plentiful supply of manure or bark; but they must
then be surrounded by a very thick layer of manure to prevent cooling.
It is a desirable alteration in the construction of the stacks to
provide the pots with lids, and so avoid the use of the layer of boards
separating each two layers of pots. The object of the cover is simply
to prevent dirt from falling into the pot. It should not fit tightly on
the edge, or the entry of carbonic acid into the interior of the pot
would be made difficult. The lids are therefore rounded and fit loosely
on the pot. When pots with lids are used, the lowest layer is covered
with manure in the ordinary way; upon this again comes a layer of pots,
and so on.
The transformation of the lead may be regarded as complete in four
to six weeks when manure is used, but with bark the time extends to
ten weeks. The quantity of white lead obtained varies in different
cases; for example, from a stack 5 metres long, 4 metres wide, and 6
metres high, into which 12,000 kilogrammes of lead were put, 10,000
kilogrammes of white lead were obtained, and 4,000 kilogrammes of
lead remained unaltered. In another case, for a stack of 8 layers 280
kilogrammes of vinegar and 9,600 to 12,000 kilogrammes of lead were
used, and there was a residue of 10 to 15 per cent. of lead.
3. =Removal and Grinding of the White Lead.=—The stack is pulled down
after the lapse of the necessary time; the lead plates and rolls are
collected in wooden boxes and brought into the room where the white
lead is separated from the metallic lead. Formerly the white lead was
removed from the sheets exclusively by manual labour, an operation
extremely dangerous to the workman. It is quite impossible to prevent
the formation of white lead dust, so that the men were continuously
in an atmosphere charged with the poisonous material, and, as a
consequence, suffered from the various forms of lead poisoning.
In order to diminish as much as possible the injurious effects of white
lead on the health of the workmen, manual labour has been, as far as
possible, replaced by machinery, yet the greatest care is necessary in
the different manipulations of so poisonous a substance as white lead.
When the white lead is removed by manual labour, an operation which
ought to be forbidden, the lead sheets are unrolled and struck
together, whereupon the greater part of the white lead falls off. To
remove the remainder of the white lead the plates are laid one upon the
other and struck with a hammer until the white lead is loosened; or the
plates are cleaned with metal brushes.
The masses of white lead obtained in this way are contaminated by
larger or smaller quantities of metallic lead, from which they must
be freed by a further mechanical operation, namely grinding. The
larger pieces of the white lead, which have a thickness of several
millimetres, were picked out and sold separately under the name of
flake white. This was formerly a highly prized quality of white lead,
its appearance being a guarantee of its purity. The flake white
generally found in the market nowadays is not obtained in this manner,
but by mixing white lead with a solution of dextrin, forming plates
from the paste and drying them slowly in the air.
In order to separate the white lead mechanically from the remains of
the lead sheets, grooved rollers are used, between which the sheets are
passed. In order to guard against dust, the rollers are surrounded by a
closed casing, in which there is also a sieve which serves to separate
the larger pieces of white lead (flake white) from the fine dust. The
arrangement is represented in Fig. 5. The unrolled plates pass through
the opening, B, on to an endless leather band, by means of which they
are carried between the grooved rollers, D and E; after they have
passed through these they go between a second pair of rollers, F and G,
which are placed nearer together; they then fall into the drum-shaped
sieve, H, out of which they leave the apparatus in the direction of K.
The white lead falls through the sieve, is caught in trucks placed at
J, and carried away to the mills.
[Illustration: FIG. 5.]
A mechanical arrangement for the separation of white lead from the
unaltered metallic lead, due to Horn, is represented in Figs. 6 and
7. It consists of a drum, in which is the spindle, _b_, provided with
the arms, _c_. The teeth, _e_, on the arms reach nearly to the lower
curve of the drum, but pass at a somewhat greater distance from the
upper portion so that the pieces of lead can fall down again after
they are carried up. The material is fed in through the hopper, _t_,
the rotation of the arms loosens the white lead from the pieces of
metallic lead, and it is carried on by the water which passes through
the sieve, 3, and the stop cock, _h_, into the settling tank. The
lead pieces which are carried forward have their progress checked by
the bridge, _o_, they are collected by the perforated scoop, _n_, are
raised above the hopper, _m_, and fall out of the apparatus at _r_. If
it is required to continue the treatment of the material in the drum
for a longer period it is only necessary to close the hopper, _m_, by
the slide, _n_.
[Illustration: FIG. 6.]
[Illustration: FIG. 7.]
_White Lead Mills._—Before the white lead is subjected to the real
process of grinding, it is generally first ground dry, or, more
properly, pressed or crushed. This crushing is accomplished by means
of vertical or edge-runner mills, which consist of mill-stones running
round upon a stone bed about a vertical axis. The mill is surrounded by
a wooden casing to prevent the escape of dust.
Wet grinding, which is done between mill-stones, may be carried out in
two ways, with the production of hard or soft white lead. The former is
obtained when the lead acetate is not removed, the latter when all the
lead acetate is washed out.
[Illustration: FIG. 8.]
_Hard White Lead_ presents a shining mass, broken with difficulty. This
appearance is a guarantee against adulteration with barytes; white
lead, which contains this adulterant, does not give a smooth, but an
uneven earthy fracture. Hard white lead is rather difficult to grind,
and requires very careful treatment to be brought into that state of
fine division in which it is usable.
The dry white lead, after powdering under the edge-runners, must pass
through a sieve, which retains the particles of metallic lead, before
it is subjected to wet grinding. The mills used for wet grinding differ
little, or not at all, from the ordinary pattern.
[Illustration: FIG. 9.]
A mill designed by Richter of Königsee, in Thuringia, for wet grinding,
is constructed as follows (Figs. 8 and 9). The shaft, _F E_, is driven
by a water-wheel and the necessary gearing at 60 revolutions per
minute; the bevelled cog-wheel _G_, by means of the cog-wheels _H_ and
_J_, drives the shafts _K_ and _L_, which, by means of the bevelled
cog-wheels _a_ and _b_, communicate to the runner stones of the mills,
_M_, _N_ and _O_, a speed of 140 revolutions per minute. _C D_ is the
framework for the support of the storied arrangement of mills, _f_ and
_g_ are bearings with cast-iron cups to receive the escaping oil, _i_
is a clamp fastening the axle _d_ to _k_. A steel cup in _k_ rests upon
the hardened top of the spindle, _l_; the bed stone rests upon the beam
_n_ (80 millimetres thick) and is prevented from lateral movement by
the surrounding _oo_; _p_ is a wooden support in the opening in the bed
stone, carrying the stuffing box, _q_, which prevents the white lead
from running out, and connects the spindle, _l_, with the bed stone.
The runner stone, _r_, is surrounded by the box, _s_, cemented with
white clay inside and out and fastened to the bed stone. The lever,
_t_, turning about _w_, moved by the screw, _u_, and handle, _v_,
regulates the position of the runner. The diameter of the stones is 95
centimetres.
Thirty kilogrammes of white lead, mixed with water, are fed into the
top mill of the series shown on the right; the mixture flows by the
spouts, _e_, into the lower mills, and is received in vessels, from
which it is emptied into the holder, _P_. From _P_ it goes through the
3 mills on the left. After passing through 6 mills, white lead without
barytes is ready for use. If the white lead contains an admixture of
barytes, it is put through the 3 mills on the left a second time. In 24
hours 900 to 1,200 kilogrammes (18 to 24 cwt.) of white lead are ground.
In dry grinding, mills are used arranged so that the formation of dust
is avoided, a matter which is of particular importance. The usual
size of these mills has a diameter of 90 to 95 centimetres for both
runner and bed stone. The grinding surfaces of both stones have radial
grooves.
[Illustration: FIG. 10.]
Fig. 10 represents the construction of Lefèbre’s white lead mill, which
is designed to give the greatest protection possible to the workmen. At
A, the white lead masses are enclosed in a hopper lined with bronze,
with internal angular projections; by means of M, which is grooved in
a similar manner, the larger lumps are broken up and enter the mill
itself by means of the hopper under A. The mill consists of bed stone,
H, and runner, K. The grinding surfaces are grooved to facilitate the
delivery of the ground white lead. As the illustration shows, the
stones are completely covered by M, so that the escape of dust is
almost completely prevented. The ground white lead is conducted by the
tubes, O O, to the drums, O′ O′, from which it is packed.
In order to obtain hard white lead, the material which has been ground
dry under the edge-runners is mixed with water to a soft paste, or if
water has been added whilst the lead was under the edge stones, more
is now added to make the paste thin enough to be ready for the mills,
where it is ground, being fed in regularly by means of a copper spoon.
The paste issuing from the mill is collected in earthenware pans or
plaster of Paris moulds, in which it is dried.
These pans have usually the shape of a truncated cone, which was the
form in which white lead was formerly brought into trade from Holland.
In drying, water is lost, and the mass of white lead shrinks, so that
the lump can be removed from the mould after a few days on turning
it upside down. The drying is accomplished either in the air or in
artificially heated stoves. The heating must at first be gradual, or
the mass of white lead would shrink so rapidly that the cone would be
full of cracks, and then easily fall to pieces. When once the drying
has reached a certain stage, the temperature of the stove may be
raised to 50° C. (122° F.) without danger of breaking the lumps. When
quite dry, the surface of the white lead, which is now rough, must be
smoothed by scraping, when it is ready for the market.
_Soft White Lead._—The hard white lead prepared as just described
consists of very heavy and very hard lumps of the purest white. When
soft white lead is required, the admixed lead acetate must be removed
by washing. This is accomplished by adding a larger quantity of water,
either when grinding under the edge-runners or in the mills, so that
a thin pulp is formed; this is run into a receiver, in which is a
stirrer. The white lead is not completely prevented from sinking by the
stirrer; a soft mud is deposited at the bottom of the vessel, which
can only be stirred up with difficulty on account of its high density.
When this vessel is full, the stirrer is stopped and the milky liquid
allowed to settle, which happens in a short time on account of the
high specific gravity of white lead. The clear liquid above the white
lead is drawn off into a tank lined with cement. It is advisable to
arrange the stirrer so that it may be placed at any height in the
vessel; if this is the case, by gradually lowering the stirrer whilst
in motion, the white lead lying at the bottom can be mixed up with
fresh water. These operations are repeated until all the lead acetate
is removed.
Soda solution is added to the wash waters in order to recover the lead
dissolved in them as lead acetate; lead carbonate is precipitated and
settles at the bottom of the tank. This precipitation may also be
effected by putting lumps of limestone in the tank. The paste remaining
in the washing tubs, which now contains only pure white lead and water,
is filled into bags of closely woven material and the water pressed out
by a gradually increasing pressure, until a stiff pulp remains behind.
This is then completely dried, either in the air or in drying stoves.
Soft white lead forms either irregular lumps or a soft, heavy powder.
The lower qualities of white lead contain a smaller or larger quantity
of finely ground barytes; the higher the proportion of barytes the
smaller is the covering power of the mixture. A very simple method for
detecting barytes in white lead will be given later.
(b) GERMAN PROCESS.
The German or Austrian method of making white lead is also known as the
chamber process, since the formation takes place in closed chambers,
constructed of wood or masonry. In the older processes, cast lead
sheets were bent double and hung on cross bars in a wooden box with
a water-tight bottom, with the precaution that the plates did not
touch. A number of these boxes, generally 90, were arranged in a hot
room, each box being about 1·6 metre long, 0·4 metre wide and 0·3
metre high. On the bottom of each box was poured a mixture of vinegar
or dilute acetic acid and wine refuse, and the box was covered with
a well-fitting lid. The temperature of the room was gradually raised
week by week; during the first week remaining at 25° C., during the
second at 38° C., the third at 45° C. At the commencement of the
fourth week the temperature was raised to 50° C., at which it was kept
for a fortnight. At this high temperature a considerable quantity of
acetic acid is evaporated, causing the formation of lead acetate,
which is converted, by the carbonic acid evolved from the wine refuse,
into basic lead carbonate. When the proper temperatures have been
maintained, on opening the boxes almost all the lead is found changed
into white lead, which is knocked off, and the residual lead used in
casting new plates. It is easy to conceive that the boxes may be well
replaced by brick chambers, in which a large number of lead plates
are brought and into which acetic acid vapours and carbonic acid are
introduced after the room has been closed. Chambers are used which can
contain 12,000 to 12,500 kilogrammes of lead.
These chambers, in which the lead plates are hung upon wooden supports,
have an opening immediately above the bottom, which is connected with
a retort in which vinegar, containing 4 to 5 per cent. of acetic acid,
is boiled. After about 12 hours, by the simultaneous action of the
vapours of acetic acid and the oxygen of the air, lead acetate is
formed. Carbonic acid is now led into the chambers. The carbonic acid
is obtained by burning charcoal in a cylindrical furnace; it is cooled
by being passed through a long iron tube before it enters the chamber.
For 12,500 kilogrammes of lead there are required every day about 482
litres of dilute acetic acid, obtained by mixing strong acetic acid
with water until the mixture contains 4·5 per cent. of the acid, and
18 kilogrammes of charcoal, from which the carbonic acid is obtained.
The time required is 5 or 6 weeks, and the residue of unaltered lead
varies from 10 to 35 per cent.
The white lead obtained by this process is quite usable, but the
process has the considerable disadvantage that there is no control
over the quantities of materials used. In order to produce a certain
quantity of white lead of a certain composition, definite quantities
of lead, oxygen, acetic acid and carbonic acid are necessary. If,
then, the apparatus can be so arranged that the quantities of material
employed can be accurately measured, a great advance will have been
made, for the operation will be no longer conducted at random, but
under definite unchangeable conditions. The quantities of carbonic acid
and acetic acid can be calculated beforehand. The volumes of the gases
required may be measured without difficulty by meters of the type used
for measuring coal gas. Upon this principle are based a number of more
modern methods.
[Illustration: FIG. 11.]
According to the process of Major, the vapours of water and acetic
acid are introduced at the same time into the chambers filled with
lead plates, in order to produce basic lead acetate. This part of the
process requires about 12 hours. Then carbonic acid is introduced; it
is produced by burning charcoal in an iron cylinder, through which air
is forced. By means of this carbonic acid, which is at a comparatively
high temperature, about 60° C., the lead acetate is quickly changed
into white lead. A portion of the lead acetate remains undecomposed;
in order to remove this, at the close of the operation ammonia is
injected, which decomposes the lead salt. The ammonia salts now present
are finally driven out by means of superheated steam. Major’s apparatus
is depicted in Fig. 11. _A_ and _B_ are chambers with horizontal
gratings upon which the lead lies; _C_ is the furnace for burning the
charcoal, provided with a fan. The products of combustion from _C_ pass
under the boiler _D_, their passage being regulated by the valve, _a_;
they then heat the boiler _E_, containing acetic acid, and enter the
chambers, _A_ and _B_, through the flue, _b_, or are directed by the
valve in _b_ into the chimney, _d_. The gases, after passing through
the chamber, find an exit at _e_. The steam pipe, _f_, conducts steam
into the boiler _E_, through _g_, and also into the chambers, _A_ and
_B_, thus heating them and providing the necessary moisture; _h_ is the
funnel for filling _E_; the pipe, _k_, carries the acetic acid vapours
into the chambers. After the thin lead plates have been brought into
the chambers they are closed and the temperature raised to 49° to 60°
C., steam and acetic acid vapours are led in for 10 to 20 hours in
order to form the basic acetate, then the furnace gases are introduced
at a temperature of 60° C. The white lead obtained in this manner can
be finished in the ordinary way by washing and grinding, but it is
better to remove the lead acetate by introducing ammonia, and then hot
air or superheated steam, as previously stated.
[Illustration: FIG. 12.]
[Illustration: FIG. 13.]
The process is complete in 2 to 4 weeks. Gartner, working according to
this process with 150 kilogrammes of lead in a chamber 1·26 metre long,
0·78 metre high and 0·78 metre wide, obtained good white lead in 28
days.
The apparatus designed by H. Kirberg for the manufacture of white lead
is illustrated in Figs. 12, 13 and 14. The lead plates are hung upon
the laths, _a_, in the chambers; the supports of the laths, _b_, go
through the slits, _c_, in the supports, _d_, and project through the
walls at _e_. The carriers, _b_, hang from bolts by brass wire. By
striking the end of the laths, _e_, they are made to swing so that the
white lead loosely adhering to the lead plates is shaken off and fresh
surfaces of metal are exposed. The openings in the walls through which
the ends of the laths project are closed by indiarubber. In order to
prevent the production of dust when the chambers are emptied, water is
introduced in a fine spray upon the lead plates from the copper tubes,
_v_, placed under the roof of the chamber. The water washes off the
remains of the white lead from the plates.
[Illustration: FIG. 14.]
In a similar manner, the white lead process may be carried out in a
shorter time when the gases enter the chamber under increased pressure;
but this is attended with difficulties, since continuous supervision of
the apparatus and of the tightness of the chamber is necessary.
[Illustration: FIG. 15.]
The apparatus of W. Thompson (Fig. 15) consists of a chamber, _A_,
constructed with a false roof, _B_, to carry off the condensed vapours,
and provided at both sides with doors, through which the waggons,
carrying the lead, are introduced and removed. The pipes, _n_, which
convey air and carbonic acid, are provided with branches, _e_, which
reach to the sides of the chamber, and are joined to the perforated
tubes, _D_. The pipes, _n_, are provided with reservoirs, _b_, which
prevent the too rapid entry of the air or carbonic acid, and at the
same time give a sufficient heating surface. Upon the waggon, _E_, are
the frames, _v_, which support the lead plates. The plates may be solid
or gratings; they are 3 to 12 millimetres thick, and are arranged at
intervals of about 25 millimetres in the frames. The troughs, _F_, for
the reception of the acetic acid are filled from the reservoir, _Z_.
The steam pipe, _C_, effects the evaporation of the acid; the steam
pipe, _d_, heats the chamber at the commencement of the process, and
also when _C_ is out of use. When the chamber is filled with lead, the
troughs, _F_, are filled with acetic acid of 5 per cent. strength. The
chambers are closed, and steam is led through _C_ and _d_ until the
temperature in the chamber reaches 25° to 50° C., and the evaporation
of the acid begins. Then air is forced in under a small pressure
through _n_, _e_, _D_ during 3 to 4 days, after which follows a mixture
of equal parts of carbonic acid and air, continued until the formation
of white lead is complete. When plates 3 millimetres thick are used,
and the temperature is gradually raised from 25° to 50° C., about 12
days are required; but if the plates are 12 millimetres thick, 28
days are necessary. For the proper carrying out of this process it is
important that the temperature of the chamber should be very gradually
raised from 25° to 50° C.
In the process of P. Rey molten lead is poured in a thin stream into
water, and the “granulated” metal then placed in vessels, in a layer
30 centimetres thick, upon a grating 5 centimetres above the bottom.
In the bottom are narrow tubes which, reaching above the lead, serve
to admit air. Acetic acid is allowed to flow over the lead from vessel
to vessel. In order to obtain the proper solution, a layer of lead, 2
metres thick, is necessary, so that 6 to 7 vessels are arranged, one
above the other. If lead acetate solution be used, the layer of lead
need only be 1·2 metre thick. The solution of basic lead acetate is
then treated with carbonic acid according to the ordinary process.
In addition to the modifications of the German white lead process which
have been described, many others have been proposed. The principle of
many of these methods is, that finely divided lead is much more rapidly
converted into white lead than lead in the form of plates; finely
divided lead exposes an enormously greater surface to the action of the
acetic acid and other materials than do lead plates.
In Rostaing’s method the lead is changed into very small pellets, by
allowing the melted metal to flow on to a rapidly rotating iron disc;
the molten lead, in consequence of the centrifugal force due to the
rapid rotation, assumes the form of very small drops, which are thrown
off the disc and cooled in a vessel of cold water.
Torassa recommends that lead, obtained in pellets by pouring into
cold water, should be brought into a rapidly rotating vessel, whereby
the greater part will be converted into fine dust (?). This lead dust
is said to be then converted into white lead by simple exposure to
air, lead oxide being first formed, and from it basic lead carbonate.
This process is not workable on a large scale. The rapid oxidation of
finely divided lead by air has been applied by several inventors to
the manufacture of white lead. The processes of Woods, M’Cannel and
Grüneberg are founded upon this transformation. In essentials they are
as follows: lead is finely divided in iron or earthenware cylinders,
and either carbonic acid and air, or a mixture of these with acetic
acid vapour, are introduced by means of the axle of the cylinders.
If a special apparatus is not provided, as it should be, on the large
scale, to separate the unaltered lead from the white lead, the lumps
coming from the chambers must be subjected to a process of levigation,
in which the lead, being the heaviest body, will be first deposited;
the last portions deposited consist of the purest white lead, the lower
layers of which will be tinged more or less grey by an admixture of
finely divided lead and lead peroxide.
White lead manufactured by the German process occasionally exhibits
perceptible reddish or greyish tinges, the cause of which lies in the
defective execution of the process. The red tinge denotes the presence
of free lead oxide, caused by the use of an insufficient quantity of
acetic acid. A grey tint is due to the presence of metallic lead or an
excess of lead carbonate.
(c) FRENCH PROCESS.
The French process for making white lead is based upon the reaction
which occurs when carbonic acid is passed into a solution of basic lead
acetate; basic lead carbonate is precipitated and neutral lead acetate
remains dissolved; the latter is again converted into basic acetate,
from which carbonic acid again separates white lead and so on. This
process, at present used on an enormous scale, is due to the French
chemist Thénard, who first put it into operation on the large scale at
Clichy, near Paris. The method is also known as the Clichy process.
The operations of this process are divided into the production of the
basic acetate and the treatment of its solution with pure carbonic
acid, whereby the basic carbonate is precipitated.
1. =Preparation of the Solution of Basic Lead Acetate.=—The preparation
of this compound has been already described; the following is
supplementary to what was previously given. If litharge be used, it is
dissolved in wooden tubs heated by steam. The acetic acid is brought
nearly to boiling by open steam, and the finely ground litharge
gradually added. In consequence of its high specific gravity, the
litharge would quickly sink to the bottom, so that it is advisable
to keep the liquid in motion by means of a stirrer, and to allow the
litharge to fall in in a thin stream. The introduction of the latter is
continued until the specific gravity of the solution indicates that the
liquid contains three equivalents of lead oxide to one equivalent of
acetic acid.
In working with metallic lead, this must be used in a finely divided
form; it is cast, as in the Dutch process, into thin sheets or
gratings, or into flat wires or ribbons. These ribbons are easily
made by melting the lead in a pan provided with a delivery pipe with
stop cock; beneath the latter is brought a vessel, which can be moved
backwards and forwards upon a tram line, filled with water. When the
melted lead is allowed to flow into this vessel, which is being moved
backwards and forwards, the metal forms long, thin ribbons, which
possess a large surface. A wooden tub is almost completely filled with
these lead ribbons, which are then covered with acetic acid. After a
short time the acid is run off, when, by the action of the air, so
energetic an oxidation of the lead takes place that the contents of the
tub become heated, and steam and acetic acid vapours begin to rise.
When this is seen, the original acetic acid is pumped back into the
tub, and left there for some hours in contact with the lead in order
that it may dissolve the lead oxide. When the solution has reached
a specific gravity of 1·1326 to 1·1415, it is drawn off from the
undissolved lead, which again, in a short time, in consequence of the
rapid oxidation, becomes warm, and is treated with fresh acetic acid.
The lead ribbons become finally so thin that they fall together by
their own weight, and form tight masses, upon which air and acetic
acid can no longer act. These residues are then removed from the
solution vessel and new ribbons introduced. The residues have a velvety
appearance; when they are mixed up with water they make it dark, and
from the turbid liquid a fine, velvet-black powder soon separates,
which consists of finely divided silver. The liquid still remains
turbid owing to the suspension of fine particles of carbon. The lead
ores, especially galena, often contain notable quantities of silver;
silver lead is generally desilverised before use; usually, however,
small quantities of silver remain in the lead. When the lead is
dissolved in acetic acid, this silver settles as a soft powder at the
bottom of the vessels in which the lead residues from the solution tubs
are washed.
2. =Preparation of the Carbonic Acid and Precipitation of the White
Lead.=—The carbonic acid required for precipitating the basic carbonate
is obtained either by heating limestone in a small furnace, from which
the gas is drawn by a pump, or directly by burning charcoal. In the
former case very pure carbonic acid is obtained, and, as a by-product,
valuable quicklime; in the latter case precautions must be taken to
produce pure carbonic acid.
[Illustration: FIG. 16.]
The furnace designed by Kindler for preparing carbonic acid (Fig. 16)
consists of a conical furnace, burning coal or coke upon the hearth,
_a_. The passage, _c_, is divided by a vertical wall, in order to avoid
obstruction from the piling up of fuel. The space, _K_, is filled with
limestone, through which the carbonic acid passes; the tanks, _e_,
filled with water, cool the limestone, and the gas, which then passes
through the water in the washing vessel, _D_, is drawn off by a pump.
In the old process at Clichy the apparatus depicted in Fig. 17 was
used. The basic lead acetate was made in the wooden tub, A, provided
with the stirrer, B C. The solution was run off from this vessel
by means of the cock, b, into the settling tank, E, in which the
mechanical impurities separated from the solution. The clear liquid
ran into the decomposing vessel, a large shallow covered tank, holding
9,000 to 10,000 litres. In this tank opened 800 copper tubes, given
off from the large pipe, S. The small furnace, D, in which limestone
was burnt with coke, produced the carbonic acid; from the pipe at the
top of the furnace the carbonic acid was brought into the Archimedean
screw, h K, was washed with water, and pumped into the solution of lead
acetate. The introduction of the carbonic acid was continued from 10 to
12 hours, after which the apparatus was left at rest until the liquid
in the decomposing tank had become quite clear, through deposition
of the white lead. The clear solution, now containing neutral lead
acetate, was run off into the receiver, m, from which the pump, P,
carried it back into the dissolving tub, A, where it was treated with
fresh quantities of litharge. The solution of neutral lead acetate
drawn off from the white lead had approximately a specific gravity of
1·0901. The white lead at the bottom of the decomposing tank was a
tolerably thick paste. It was transferred to the tank, O, and washed
several times with water. The first wash waters, which contained small
quantities of lead acetate, were returned to the dissolving tub. The
resulting white lead formed a very soft powder; it was at once placed
in the drying pans. The white lead prepared by this process is a
precipitate containing no coarse lumps, so that grinding is unnecessary.
[Illustration: FIG. 17.]
Theoretically, the quantity of acetic acid with which the process is
commenced is sufficient to form an unlimited quantity of white lead,
since all the acetic acid brought into the decomposing tank in the form
of basic acetate is returned to the dissolving tub as neutral acetate.
In practice, however, matters are somewhat different. Small quantities
of acetic acid are lost in the wash waters; each time the solution of
lead acetate is pumped back into the dissolving tub, a small quantity
of acetic acid must be added to make up the loss.
[Illustration: FIG. 18.]
The method pursued by Ozouf, in France, is a considerable improvement
on Thénard’s process. Pure carbonic acid is used for the precipitation,
and white lead of similar composition to that produced by the Dutch
process is obtained, since the introduction of the carbonic acid can be
regulated according to the volume and strength of the lead solution,
and thus white lead of any desired composition can be produced. The
most elaborate precautions for the health of the work-people are taken.
The preparation of pure carbonic acid gas is based upon the absorption
of this gas from a mixture of gases by a solution of sodium carbonate,
and its evolution on heating the solution. The apparatus is shown in
Figs. 18 and 19. The products of combustion obtained from the stove,
_A_, are drawn by the air pump, _E_, through the pipe, _C_, into the
cooler, _B_, which is regularly fed with cold water by _D_. The gases
compressed in the receiver, _E´_, deposit moisture there, and then
proceed through 3 horizontal cylinders, _F_, of sheet-iron, provided
with agitators, in which the carbonic acid is absorbed by a cold
solution of sodium carbonate of 9° B. The unabsorbed gases escape
into the atmosphere through _G_ (Fig. 19). The sodium bicarbonate
solution is received in the wooden tank, _H_, after passing through
the 3 cylinders, _F_. The pump, _I_, of the alternating pump, _I I´_,
lifts the sodium bicarbonate solution out of _H_ and sends it through
the pipe, _K_, into the tubular cylinder, _J_, which stands upon a
cylinder of larger diameter, _J´_, communicating with it only by the
vertical tubes. The bicarbonate solution rises between the tubes in
_J_, passes through the pipe, _L_, drops in a fine spray through the
rose forming its mouth, and by means of the vertical tubes passes into
_J´_ and thence into _M_, where it is heated by means of a steam coil
to 100° C. Carbonic acid is then evolved, and the residual solution of
neutral sodium carbonate, after cooling in the vessel, _R_, by means
of the cold coil, is drawn off by the pump, _I´_, again to enter the
cylinders, _F_, by means of the pipe, _K_. The carbonic acid evolved
in _M_, together with steam, enters _J´_ through _N_, and in rising
in the tubes of the cylinder, _J_, is cooled by the falling spray
of bicarbonate solution. The cooling is completed in the coil, _O_,
surrounded by water; the vessel, _P_, separates the condensed water
and passes the gas on into the holder, _Q_. The pipe, _S_, connecting
_P_ with the suction pipe of the pump, _J´_, serves to restore to the
solution of sodium carbonate the water it has lost, thus maintaining
the proper concentration. The cost of 1 cubic metre of carbonic acid is
10 centimes, of 1 kilogramme 5 centimes.
[Illustration: FIG. 19.]
For the production of white lead, the carbonic acid, by means of the
pipe, _U_ (Figs. 19 and 20), enters the cylinder, _T_, provided with
an agitator and containing a solution of basic lead acetate. By means
of the pump, _V_, the lead solution is fed into the cylinder, _T_,
through _W_. The absorption of the gas proceeds rapidly; the progress
of the operation is followed by the observation of a pointer moving
over a scale; as the gas holder sinks the pointer moves upwards. After
the precipitation of white lead, the contents of _T_ are emptied into
the tub, _b_, in which rotate rakes attached to a vertical axis of iron
plated with copper. When the white lead has settled, the supernatant
solution of neutral lead acetate is drawn off through the pipe,
_c_, by means of the pump, _d_, and conducted into the water-tight
vessel, _X_, containing a stirrer on the vertical axis, _W_, made of
coppered steel. Here litharge is added, and the resulting solution
of basic lead acetate is conveyed to the cylinders, _T_, by means of
the pump, _V_, as already described. The white lead in the tub, _b_,
by putting the stirrer in motion, is washed once with water which has
been previously purified by a little lead acetate. It then goes into
another tub provided with stirrers, where it is several times washed,
sodium carbonate being added to the last wash water until a sample of
the white lead is not coloured by a drop of potassium iodide solution.
In this way the wash water is obtained free from lead, and the product
is said to be of better quality. This, however, is not in accordance
with the fact that good Dutch white lead generally contains some lead
acetate. The two-cylinder pump, _h_, which is in connection with the
gas holder, forces gas over the surface of the liquid in T in order
to drive it into tubs which are not in the position shown for _b_ in
the illustration, and into which there is no direct flow. The washed
white lead is brought into bags which are pressed in a hydraulic press,
dried, ground, sieved and packed in casks. These troublesome and often
dangerous operations have been modified by Ozouf in the following
manner. The pulp white lead runs from the tub, _b_, into the hopper,
_g_, where it is kept mixed by a small stirrer, and from which it
passes on to the cylinder, _f_, heated by gas from the inside. In its
rotation the cylinder carries along the white lead and dries it, it is
then removed by a knife below the hopper, and falls on to an inclined
plane. The hopper and cylinder are in a room provided with a good
draught.
[Illustration: FIG. 20.]
The lumps coming from the drying room are placed by workmen wearing
respirators in buckets on an endless chain, are carried to the mills,
ground and sieved; then, by means of an Archimedean screw, the white
lead is conveyed to a cask in which it is evenly pressed by means of a
special mechanism. A bell announces when a cask is full.
=Manufacture of White Lead by means of Natural Carbonic Acid.=—In
districts where currents of carbonic acid gas issue from the ground,
they can be used in the manufacture of white lead, and are actually
utilised for this purpose. Natural carbonic acid may, of course, be
used for any of the white lead processes.
(d) ENGLISH PROCESS.
In this process, now no longer in use, white lead was obtained by
mixing litharge to a stiff paste with a weak solution of lead acetate
and exposing the paste to the action of carbonic acid. By continually
kneading the mass by means of grooved rollers or of rotating cylinders,
through the hollow axis of which carbonic acid was led, the paste was
thoroughly brought into contact with the carbonic acid.
By this process a good product is only obtained when pure litharge,
entirely free from the oxides of iron and copper, is used. The copper
oxide may be removed from the litharge by means of ammonia if this can
be obtained at a low price; but oxide of iron cannot be removed, and
very small quantities of it are sufficient to impart a yellow tinge to
the white lead.
(e) OTHER METHODS.
In Payen’s process the lead sulphate obtained in considerable
quantities as a by-product in calico printing is the raw material
employed. By treating this lead sulphate with a solution of ammonium
or sodium carbonate, white lead and ammonium or sodium sulphate are
produced. The white lead is then freed from the soluble salts by
washing, mixed with a small quantity of lead acetate, and pressed into
the drying moulds.
By boiling lead sulphate with caustic soda and passing in carbonic
acid (Puissant’s process), a white lead is obtained which differs
considerably in composition from ordinary white lead.
Many methods have been proposed with the object of converting insoluble
lead salts, obtained as by-products or by an inexpensive process, by
treatment with alkaline or alkaline earth carbonates, into white lead.
The fact that none of these methods has obtained a permanent footing in
the industry shows that each must be accompanied by serious defects, or
can only be practicable under peculiar conditions.
Magnesium carbonate is used in Pattison’s process to decompose lead
chloride. Dolomite (magnesian limestone) is the raw material for
the magnesium carbonate. Coarsely powdered, it is heated at a low
red heat, when magnesia is formed, the calcium carbonate remaining
almost entirely unaltered, since it requires nearly a white heat for
its decomposition. The powder ground in water was, when treated with
carbonic acid under a high pressure, soluble, magnesium bicarbonate
being formed, the saturated solution of which contains 2·3 per cent.
of magnesia, and has a specific gravity of 1·028. The solution of
lead chloride contains 1 part of the salt in 126 parts of water; it
is mixed with a slight excess of the magnesium carbonate solution as
quickly as possible. The liquid is drawn off from the mixing vessel
into a large receiver in which a precipitate deposits, consisting of
white lead and a little oxychloride. After drying, the precipitate
is ground with a small quantity of caustic soda to decompose the
oxychloride. A few days afterwards the mass is washed to remove sodium
chloride and the product dried.
The process of Dale and Milner is similar to the above magnesia
process. Litharge, lead hydroxide or insoluble lead salts are mixed
with sodium bicarbonate solution, and, with repeated additions of
water, ground until the formation of white lead is completed. The lead
compound and sodium bicarbonate are used in equivalent proportions.
According to the process of P. Bronner (German patent 52,262), 3
molecules of freshly precipitated lead sulphate are heated with a
solution of 2 molecules of caustic soda, when the basic sulphate 2
PbSO₄.Pb(OH)₂ is formed according to the equation—
3 PbSO₄ + 2 NaOH = 2 PbSO₄.Pb(OH)₂ + Na₂SO₄.
Or 4 molecules of lead sulphate are decomposed by 2 molecules of
caustic soda, according to the equation—
4 PbSO₄ + 2 NaOH = 3 PbSO₄.Pb(OH)₂ + Na₂SO₄.
This transformation takes place at a temperature of 70° C. The
resulting basic sulphate, although pure white, cannot be used as a
pigment on account of its lack of covering power; but by heating with a
solution of sodium carbonate it is converted into white lead.
2 PbSO₄.Pb(OH)₂ + 2 Na₂CO₃ = 2 PbCO₃.Pb(OH)₂ + 2 Na₂SO₄.
3 PbSO₄.Pb(OH)₂ + 3 Na₂CO₃ = 3 PbCO₃.Pb(OH)₂ + 3 Na₂SO₄.
By this process, which is harmless to the workmen, the lead sulphate
obtained as a by-product in the preparation of mordants for calico
printing, can be converted into good saleable white lead. The lead
sulphate may also be obtained from litharge, lead acetate or nitrate.
It occasionally happens that white lead has a rose tint, which is
clearly perceptible by comparison with a pure white sample. This
colouration occurs in white lead made from argentiferous lead. A very
small quantity of silver is sufficient to produce the tinge of colour.
Occasionally white lead which has been ground in oil and used for
painting turns perceptibly yellow, the colouration being similar to
that observed on a surface painted with white lead from which light
is almost excluded. The yellow colouration is due to lead oxide. This
has been proved by suspending such a white lead in water and treating
it with carbonic acid, after which a surface painted with it remains
permanently white.
(f) OXYCHLORIDE WHITE LEAD.
Under the name of white lead, but differing from it in composition,
various products are found which consist of lead oxychloride. This
compound is also known as Pattison’s white lead.
Pattison’s white lead can be much more cheaply manufactured than
real white lead, the raw material employed being the cheap galena.
The finely-powdered mineral is boiled with strong hydrochloric acid
in closed lead vessels. Sulphuretted hydrogen is evolved, which may
be burnt to sulphur dioxide and so used to make sulphuric acid. A
hot saturated solution of lead chloride remains, from which the salt
separates in small crystals on cooling. The crystals are drained in a
basket and washed with cold water to remove the acid. The pure lead
chloride is then dissolved in hot water and mixed with lime water.
Pattison obtained lime water from dolomite by burning it, treating
with a little water to remove the easily soluble salts, and, after
the removal of this wash water, treating the residue repeatedly with
water in order to obtain a clear solution of pure hydrate of lime. When
pure limestone is used, it may be treated with water immediately after
burning without any preliminary preparation.
Two equivalents of lead chloride are used to one equivalent of calcium
hydroxide. Practical experience showed that the best product was
obtained when the precipitation was very rapidly brought about. With
this object, both solutions entered the precipitation tanks through
pipes with narrow slits at the side, so that the liquids met in a thin
layer, in which the precipitation of the pigment was instantaneous. It
is also necessary that lead chloride should be in excess throughout.
The liquid is allowed to stand for the precipitate to settle, which
it does in a brief time on account of its high specific gravity. The
solution now contains the small excess of lead chloride in addition to
calcium chloride; lime water is added until the liquid turns red litmus
paper blue. From the alkaline solution all the lead soon separates as
lead hydroxide, which is dissolved in hydrochloric acid, and thus again
comes into the process.
In order to utilise the large quantities of hydrochloric acid obtained
in the manufacture of soda, Percy described a process in which galena
is ground with hydrochloric acid, whereby in 30 to 40 hours all the
lead is converted into lead chloride, whilst the stony admixtures are
unattacked. The lead chloride is then separated by levigation from
the undissolved minerals and washed until free from iron, when it is
dissolved in hot water and converted into oxychloride by means of lime
water.
=Lead Sulphite=, PbSO₃, can be obtained by passing sulphur dioxide into
a solution of basic lead acetate; lead sulphite is precipitated and a
solution of neutral lead acetate remains. The process is similar to the
French white lead process, with the difference that sulphur dioxide is
used instead of carbon dioxide. Lead sulphite has no advantages over
white lead, and is more expensive; it has thus never found practical
application.
=Lewis and Bartlett’s White Lead Pigment.=—In the lead works at Zoplin,
in Missouri, galena is smelted with limestone and coal, lead fume being
obtained in addition to metallic lead. The lead fume deposits are
ignited, and again worked for lead and lead fume. This last lead fume
can at once be used as a white pigment; it consists principally of lead
sulphate, lead oxide and zinc oxide.
WHITE LEAD-ANTIMONY PIGMENTS.
Lead antimonite and antimonate are both heavy, white powders which can
be used as pigments. They are dearer than white lead, to which they are
inferior in covering power, and which they do not exceed in permanence.
=Lead Antimonite= is obtained by heating 5 parts of finely powdered
antimony with 20 parts of sulphuric acid until a dry, white mass of
antimony sulphate is left. This is fused with soda ash, the melt is
extracted with water, and lead antimonite obtained by precipitating
with lead acetate.
=Lead Antimonate= is formed by introducing in small quantities at
a time a mixture of 1 part of finely powdered stibnite (antimony
trisulphide) with 5 parts of sodium nitrate into a red-hot crucible,
boiling the mass with water and precipitating the solution with lead
acetate.