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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.