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stopped, and the white lead is collected and finished in the usual way;
In Richardson's method the lead is soaked in a solution of acetate of lead, the action, both of the acetate and the gaseous bodies, to which the soaked lead is subjected being facilitated by the lead being cast into a granular form. After being soaked, the lead is placed on shelves in the chamber, and then subjected to the combined action of steam and carbonic acid gas, the chamber being maintained at a temperature of about 100° F.; the process is continued until all the lead is converted into white lead.
2. Hatfield Process.—This process resembles those just described to some extent, but differs in a few minor particulars.
The chamber is built with a double wall, and the bottom is hopper-shaped. The lead is cast into gratings of the same shape as used in the Dutch process, and placed in trays on shelves in the chamber. Into the chamber is sent water and acetic acid in the form of spray, at the same time the chamber is maintained at a suitable temperature by means of steam pipes. The action of the water and acid is to convert the lead into basic acetate of lead ; when this has been properly formed the water and acetic acid spray is stopped, and a current of carbonic acid gas sent in to form white lead as described above.
3. Thompson's method, patented in 1873 and 1877, was worked by the Innocuous White Lead Co., of London. In this process the chamber is built of brick, with a large door and one or two windows, so that the progress of the operation can be observed. The bottom of the chamber is made like a trough and acid proof, glass being recommended as a material for its construction. The roof is built double, so that any liquid which is condensed will flow down to the sides and not drop on to the corroded lead below. The lead used is cast into gratings which are placed on open trucks fitted with shelves; the lead is soaked in a solution of acetate of lead and then wheeled into the chamber. A quantity of acetic acid is placed in the bottom of the chamber and vapourised by means of steam pipes passing through it, by which means the lead is converted into the basic acetate; when this action is complete, carbonic acid gas is sent in to change it into white lead. This was said to be of good quality. In the practical application of this process, much depends upon the temperature at which the chamber is maintained during the operation. If too high, then there is a tendency to form oxide of lead which does not readily change into white lead ; if too low, then the action of the acid on the
lead is not energetic enough į if too much acetic acid is used, then the tendency is to form normal acetate of lead which reduces the yield of white lead, and at the same time tends to cause this to have too much carbonate in its composition.
A very similar process to this was patented by Morris. In this the lead was used in the form of sponge or wire so as to expose as much surface as possible to the action of the various gases. The acetic acid was placed in vessels on the floor of the chamber and steam and carbonic acid gas passed in, a constant current of these gases being maintained. The white lead was gradually formed, and when complete was collected and finished in the usual way.
4. Gardner's Electric Process.-In 1882, Prof. E. V. Gardner' patented a process for making white lead which, as he considers that electricity plays a part, he calls an “electric process” (English Patents, 731 of 1882, 12,414 of 1890).
The specification of this patent (No. 731, of 1882) is very full, and is well worth reading by white-lead makers. In the specification the conditions most favourable to a successful production of white lead are fully stated, and from it the following is abstracted :
As has been previously pointed out, in making white lead by the chamber methods there are several factors which require attention, if the product is to be a good one. Prof. Gardner states these to be as follows :—The proper formation of what he calls the sub-acetate or sub-nitrate of lead ; these basic salts are the compounds of the normal salts with the hydroxide of lead, and, therefore, have the formulæ Pb 2 C, H, 02, Pb H, O, for the sub-acetate, and Pb 2 N 03, Pb H, O, for the sub-nitrate. It is usual to consider the so-called subsalts of lead as compounds of the normal salts with the monoxide; probably both kinds of salts exist—that is, there are compounds both of the monoxide and of the hydroxide of lead with the normal salts of lead. In white-lead making it is reasonable to suppose that better results would be obtained if the hydroxide compounds were formed than if the monoxide compounds were obtained in the process of making. Hence the conditions most favourable for the formation of the hydroxide should be carefully ascertained.
Temperature is an important factor ; this should be from 120° to 130° F. A lower temperature increases the length of time required for the formation of the subsalts, and so increases the cost of the process, while the quality of the white is deteriorated, owing to its deficiency in hydroxide. Too high a temperature
must be avoided, for although a high temperature increases the rapidity with which the subsalts are formed, yet it is liable to cause them to lose their water of hydration and to pass into the monoxide subsalts; the presence of these in the white lead makes it of bad colour and hence deteriorates the quality. Too little air, acetic acid, and aqueous vapour also tends to prevent the proper formation of the subsalts and, consequently, of the white lead of the best quality, too much acetic acid converts the subsalts into the normal salts and, as is well known, these do not produce white lead of good quality ; besides which, being soluble in water, they are washed off the surface of the lead by the aqueous vapour which condenses on the lead, and are thus lost for the purpose of making white lead. These are a few of the principal conditions which Prof. Gardner points out as being necessary for the proper production of white lead of good quality, and, although given in connection with his own process, yet there is no doubt but that they are applicable to all chamberprocesses and also to some other methods of making white lead.
The electric process is carried out in a chamber made of any convenient form and material ; it is necessary, however, that it should be so constructed that the progress of the operation is readily visible. In this chamber are arranged a number of shelves covered with tin, a metal which is electro-negative to lead. Carbon, or any metål which is electro-negative to lead, may be used, but the inventor prefers tin. These shelves are connected together in succession by means of strips of tin, so that when lead is placed on them they form an electric couple. Instead of having the shelves a fixture in the chamber, they may be constructed on an open framework fitted with wheels ; on this, while outside the chamber, the lead gratings are arranged, and when the shelves are full the frame and its contents are run into the chamber, but before doing so the lead is soaked in a solution of acetate or nitrate of lead. The temperature of the chamber is maintained at 120° F. by means of steam which is sent into it for that purpose. At the same time currents of acetic or nitric acid vapours, made by boiling dilute solutions of those acids, are conveyed into the chamber, or dioxide of nitrogen with acetic acid may be used. The atmosphere of the chamber must be in a misty condition, and this is brought about by regulating the current of acid vapours and steam ; this state of affairs is kept up for 48 hours, when a current of pure carbonic acid gas is sent in for 2 hours, then stopped, and the acid gases sent in by themselves for 4 hours, when the admission of carbonic acid is again resumed for 2 hours; these alternation of currents of acid gases and steam for 4 hours, and acid gases and steam and carbonic acid for
2 hours, is carried on for 14 to 15 days, when the whole of the lead will be found to be converted into white lead. During all this time the temperature must be kept at about 120° F., and the atmosphere of the chamber misty; it is for the purpose of closely watching the progress of the process that the chamber is fitted with windows. The carbonic acid gas may be prepared by any well-known method, but the inventor prefers to use a petroleum lamp as the source of it. After the operation of making the white lead is finished, the material is not immediately removed from the chamber, but the acid gases are stopped, and steam only sent in, which serves to wash the product; then, after a time, the current of steam is stopped, and air only admitted, when the white lead becomes dry. It is now taken out of the chamber and finished in the usual way. The product obtained by Gardner's process is of good colour and body, and is equal to, if not superior than, Dutch white lead in its properties. A sample of white lead made by this process examined by the author contained11:72 per cent.
lead hydroxide. 86.16
lead carbonate. 2:13
water. 100.01 It has rather more lead carbonate and less lead hydroxide than Dutch white lead. In point of colour and covering power it is fully equal to Dutch white lead, but is somewhat denser or heavier One advantage said to be possessed by this process over the stack method, is that while in the latter it is essential to work with the purest lead which can be made, in the new process ordinary commercial lead gives excellent results. The process is worked by the Universal White Lead Syndicate at Millwall.
3rd GROUP. PRECIPITATION PROCESSES. When a current of carbonic acid gas is passed through a solution of a basic salt of lead, such as the basic acetate or the basic nitrate, a white precipitate will be obtained, which is due to the combination of the carbonic acid with the excess of lead oxide contained in the basic salt; this precipitate consists of a more or less basic carbonate of lead. At the same time a solution of the normal salt is obtained, because carbonic acid is too weak, under the conditions, to displace any other acid from its combination with lead. This action of the carbonic acid gas is shown in the following equation 3[Pb 2 C2 H202, 2 Pb H202] + 4CO2 2 [2 Pb C 03, PbH,021
+ Pb 2 C2 H202 + 4H2O
Basic acetate of lead.
Normal acetate of lead,
although this may not accurately represent the action which goes on in the majority of cases.
Various salts of lead are used. The differences between the various processes based on the principle just described, depend upon the kind of salt used, and the method of carrying out the operation.
These processes were introduced in the early part of the century, the first patent being dated 1808, and granted to E. Noble. The process described consisted in passing a current of carbonic acid through a solution of lead acetate. A very similar method is known as Thenard's, or the French, process, and will be found described below; while another precipitationmethod is known as the Kremnitz process, having been largely used there for the preparation of white lead.
The precipitation-processes based on the action of carbonic acid gas upon lead salts may be divided into two sub-groups :3a. Dry methods, in which the lead salt is used in the dry state, or, at the most, simply moistened. 3b. Wet methods, in which, the lead is used in the form of a solution.
3a.—DRY PRECIPITATION PROCESSES. 1. Kremnitz Process.—This process owes its name to having been worked at Kremnitz in Germany. It is carried on in a chamber built of brick or wood, having a number of shelves, on which are placed trays containing a paste made of litharge and either acetic acid or lead acetate, usually in the proportions of 100 lbs. of litharge to 18 pints of acetic acid, or an equivalent quantity of lead acetate solution. When the chamber is filled carbonic acid gas is sent into it, this becomes absorbed by the lead oxide present in the paste, the absorption of the gas being facilitated by raking over the paste from time to time, the mass being kept moist, as this increases the absorption of the gas. The mass originally has a yellowish-grey colour, but as the operation progresses it gradually changes into a white; and when all traces of yellow have disappeared, the operation is stopped, and the white lead which is made is first washed with water, then ground and dried.
Care is taken not to pass the carbonic acid in too long, because this would induce the formation of the normal, instead of the basic, carbonate, which means poor white lead. When carefully worked, good results can be obtained by this process.
The following analysis, presumably of a Kremnitz white lead, is given in Wagner's Technologie :