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4th Group. Precipitation methods based on the action of alkaline carbonates on various lead salts.
5th Group.—Miscellaneous methods. Most of these are now obsolete and are only of historical interest.
1st GROUP.-STACK METHOD. Only one process is included in this group, the old Dutch or stack process. No process which is now in use or which has been proposed can claim the antiquity that this process can, and, notwithstanding all the labours of chemists and white lead
makers to supersede it, for reasons which will be pointed out presently, it still remains the best process for the manufacture of white lead. Tradition assigns its discovery to the Dutch and to a person named Stratingh in particular. It must be at least 300 years old. Since 1787 this process has been carried on without much alteration in its details. The Dutch method is used in all
parts of the world for the manufacture of white lead, and there is but little variation in the details of the
process and in the construction of the plant used in different countries.
The plant used in the stack process is shown in Figs. 2 to 5. A shed of brickwork, Fig. 2, is built, the size of which varies a little, but averages 16 feet long by 13 feet wide and 20 feet high; this may have either a lean-to roof, as shown in the figure; or, as in some works, two of these sheds are
built back to back, with a single-ridge Fig. 3.
roof between them,
In some places parts of the structure are built below the level of the ground, but there is no advantage to be gained by so doing. A large white-lead works will have a number of these sheds, so as to keep the workmen fully occupied with filling and emptying them.
A number of earthenware pots are provided. These pots vary in size at different works, but an average size is 8 inches high by
Fig. 5. 4 inches in diameter. In shape they resemble crucibles (see Fig. 3), but have a shelf inside, as shown. In the bottom of these pots is placed some weak acetic acid or vinegar; this diluted acid contains about 2 to 3 per cent. of actual acetic acid. On the shelf inside the pot is placed a roll of thin sheet lead (Fig. 4),
made from a strip of lead 2 feet long by 4} inches broad. In a stack of ordinary dimensions some 11,000 to 12,000 of these pots will be used, and they will contain about 800 to 900 gallons of weak acid.
The stack is built up as follows:- First, a layer of ashes, upon which is placed a layer of spent tan of about 3 feet in thickness. In the older Dutch method horse-dung was used, but this is open to some disadvantages which will be pointed out presently; the use of tan was introduced in England so that this modification of the Dutch process is sometimes spoken of as the English method. This layer of tan is pressed down very firmly and is evenly spread, on it is placed a layer of the pots, which layer is kept at a distance of about 6 inches from the sides of the shed. In some works the outside rows of pots are made of larger size than the others, so as to act as supports for a layer of flooring boards. In other places the pots are all of one size and wooden supports for the boards are provided.
On the top of the pots is placed a layer of lead buckles or gratings (Fig. 5). These are placed face to face in a layer of about 3 to 5 inches thick; above these comes the layer of flooring boards, a space of about 6 inches being left between them. On the top of the boards another layer of tan, then a layer of pots, then a layer of gratings, then another layer of boards, and so on until the stack is completely built up. The number of sets of layers varies from seven to eleven. The doorway through which the filling is done is closed as the work progresses by boarding, but a small space is left at the top through which the progress of the operation can be observed, and fresh additions of material made as required to allow for sinking of the tan, &c.
The quantity of lead used varies considerably, or from about 3 tons to 7 tons in a layer of materials, so that in a large stack there may be something like 85 tons of lead.
In stacks of very large area it is usual to construct chimneys throughout the mass, whereby the steam which is produced during the operation is carried off; in stacks of small area, these chimneys are not required, as the space around the side walls of the shed affords a sufficient outlet.
When the stack is built up it is left for a period of about three months. During this period the stack gets quite hot (140° F.) through the fermentation of the tan which sets in ; large quantities of carbonic acid gas are given off, and the acetic acid is converted into vapour.
The “blue lead” is gradually converted into " white lead.” At the end of three months the stack is pulled to pieces. As the boards are removed it is found
that the lead which has been corroded still retains the form of the blue lead, but is more bulky in volume, is white or greyish in tint, and opaque.
The corrosions are not of a uniform character throughout the whole of the stack ; in some places they are porcellaneous and flaky, are firm to handle, do not break up, and give the best quality of white lead ; in other parts of the stack the corrosions are soft, easily crumble to a fine powder or dust when handled, and do not give a good quality of white lead. In some places the lead may be discoloured owing to a variety of causes, such as the presence of tarry matter in the acid (especially when crude pyroligneous acid is used), by droppings of coloured water from the layer of tan on to the lead, &c. In chemical composition the corrosions will vary; in some places they will approximate closely to the normal composition of white lead, 2 Pb CO2, Pb H, 02, in others more nearly to that of 3 Pb C O3, Pb H,0 22 while in others they consist of the normal carbonate, Pb CO.
As the stack is being pulled to pieces the corrosions are carried to the grinding rooms. The method of treating the white lead varies in different works, but the following may be taken as a good example of the usual manner of working The corrosions are first passed through a pair of rolls; these break up the masses, the white lead crumbles to powder, while the unchanged blue lead is flattened out into thin sheets. The crushed materials are then sieved, which separates the white from the blue lead ; the latter is sent to the melting pot where it is melted and re-cast for use in building another stack. The white lead is sent into tanks full of water, where it is thoroughly agitated, and the small traces of acetate of lead which the corroded lead contains washed out of it. While still wet the white lead is ground as fine as possible under edge runners or between rollers, and then dried, when it is ready for sale. As the grinding must be thorough, the lead is passed through several sets of grinding mills.
Grinding white lead is a source of danger to the workpeople, for the fine dust flies about the room in which it is done and is breathed by the workpeople, ho, sooner or later, suffer from lead poisoning ; much of this danger is avoided by grinding the lead in a wet condition only, when the particles of lead are practically too heavy to fly about.
The greatest risk now arises in the packing of the ground lead, as the workmen frequently get some on their hands and eat their meals without previously washing their hands. Lead poisoning may be prevented by drinking water acidulated with
sulphuric acid, whereby the lead absorbed into the system is converted into the harmless sulphate of lead. The great trouble is that the workmen will not take sufficient care to make use of these precautions.
It was stated above that in the early or Dutch modification of this process, horse-dung was used as the source of the heat and carbonic acid necessary to carry on the process; while with dung the process is quicker (only taking from 8 to 9 weeks), yet it is not so good as the English method with tan, the product is not quite so regular in composition, and it is more liable to discolouration owing to the evolution of sulphuretted hydrogen from the decomposing dung, and to its combination with lead to form the objectionable black sulphide.
For a long period of time little or no changes of any moment have been made in the Dutch process of white-lead making. In working on the practical scale, it is found that loss often occurs, due to the sinking of the piles of pots, &c., in places; this may be brought about in many ways, the pressure upon the bottom layers may be so great as to lead to breakages of the pots, supports, &c. When these accidents happen, the lead is wasted, owing to its becoming so mixed with the tan as to be rendered useless.
Mr. J. V. Walton has devised and patented (English Patents, Nos. 3,116 and 16,703 of 1893) some new methods of constructing the stack, the object of which is to prevent losses from sinking or falling in of the stack in any way. The walls of the shed carry beams of 1-section, made of teak, the flanges of which serve as supports for the frames or crates carrying the pots containing the acid and the blue lead. Boards are placed on the beams, and the upper surfaces of the boards are packed with tan. Ventilating flues are built in the walls, and in these thermometers are placed for the purpose of ascertaining the temperature of the shed from time to time; glazed sight-holes are provided to the flues to allow of the thermometer being inspected from the outside. To facilitate the operations, arrangements are made for sending hot air into the shed in the event of the thermometer showing a fall of temperature below that which experience shows is the best.
The theory of the process of white-lead making by the Dutch process, which at present is most favoured by chemists, and was substantiated by some experiments carried out by Hochstetter, is due to Liebig The first action which goes on in the stack is to convert the blue lead into basic acetate of lead; this is brought