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83.77 per cent.
8.21 per cent.
which shows that this sample did not approach Dutch white lead in composition, but contained more carbonate.
2. Mullin Process. In this process, which is not now in use, litharge was ground into a paste with water; the paste was then placed in shallow lead-lined boxes, in layers of about an inch and a quarter thick, the boxes were closed by a lid, and then into them was sent currents of carbonic acid and acetic acid gases; the litharge was gradually converted into white lead. The process was, presumably, not a successful one, or it would not have gone out of use.
3b.-WET PRECIPITATION PROCESSES.
In this group of processes for the preparation of white lead, the lead is used in the form of solution, and the precipitation is effected by means of a current of carbonic acid gas. There are a large number of these processes, and many are still in use on the large scale. The differences between the various processes belonging to this group depend upon a variety of circumstances, such as the method of preparing the solution of lead, and the form of apparatus used, on which to a large extent depends the subsidiary, but not unimportant point, the method of applying the carbonic acid to the lead solution.
1. Thenard Process. This process, from having been worked on a large scale at Clichy, in France, is known as the French process; it is also described in the patent granted to E. Noble in 1808.
The principle of the Thenard process, which is also applicable to many others of this group, is that when a solution of normal lead acetate is boiled with litharge, some of the latter is dissolved, and a solution of basic lead acetate, known as “Goulard's Extract," “Extract of Saturn,” &c., is obtained. The reaction is expressed in the following equation :
Pb 2 C, H, 02
+ 2 Pb O
Pb 2 C, H, 02, 2 Pb H, 02
Basic acetate of lead.
If a current of carbonic acid is passed through this solution of basic acetate of lead, the lead hydroxide it contains is precipitated as a more or less basic carbonate, thus
3 [Pb 2 C, H, O, 2 Pb H, 02] + 4CO2 2 [Pb C 0g, Pb H, 02]
Basic acetate of lead.
+ 3 Pb 2 C, H, O2 + 4H, O
of lead. The normal acetate which is thus re-formed can be used again for preparing a fresh solution of basic acetate of lead; of course, while, theoretically, a very little normal acetate is sufficient for the preparation of a large quantity of white lead, and there should be no loss, practically, a small quantity of new acetate has to be added from time to time to make up for the little loss which does occur.
The apparatus used in carrying out the French process at Clichy is shown in Fig. 6. In a vessel, A, of convenient size, litharge is dissolved in a solution of lead acetate, the solution being accelerated by heating the solution by means of the steam pipe, B; from this vessel the liquor in A runs into another vessel, C, in which all insoluble matter settles out. The clear solution is now run into a trough-shaped vessel, D, into which dip a number of pipes connected with the large main pipe, E, through which a stream of carbonic acid gas from the generating system, F G, flows. This system consists of an oven, F, in which is burnt a mixture of chalk and coke, from which a large quantity of carbonic acid gas is evolved ; this gas is washed in the apparatus, G, by passing it through water, after which it passes into the solution of lead in the vessel, E, precipitating white lead from it in so doing; the length of time of treating depends upon the quantity and basicity of the lead solution, but usually it takes from 12 to 14 hours. At the end of this time the current of gas is stopped and the white lead allowed to settle; the clear liquor, which is a solution of the neutral acetate, is run into a vessel, H, from which it is pumped up by the pump, I, into the vessel, A, to dissolve more litharge for a fresh operation. The mass of white lead which settles at the bottom of the vessel, E, is run into another vessel, K, from whence it passes on to filters to be washed, and then it is finished in the usual way.
The product given by this process is fairly good, but liable to vary in composition from time to time, according to the strength of the solution of basic lead acetate, and to the basicity or pro
portion of lead oxide the lead acetate has dissolved. These are points to which reference will be made in describing other processes.
2. Cory Process.—The same materials are used in this process as in the last, viz., basic lead acetate and carbonic acid gas, but it differs in the form of apparatus used. The process has been worked on a large scale for a long period. It was first
Fig. 7.—Cory's process for making white lead. patented in 1838, and the white lead produced by it is favourably spoken of by users. The author believes that the process is still in use.
The plant used is shown in Fig. 7. A chamber is built of brickwork; the bottom is made watertight and sloping towards one end so that any liquor which may fall upon it drains away into a tank; this chamber is divided by a number of vertical
partitions into compartments; the partitions are so constructed that each alternate one does not quite reach the top while the others do not quite reach the bottom, as shown in the figure ; the object of this is to make the carbonic acid gas, which is sent into the chamber at one end, take a circuitous course before it passes out at the other end. Above the chamber is a tank, the bottom of which forms the roof of the chamber, which bottom is perforated with a large number of fine holes, so that any liquor which may be run into the tank flows through into the chamber below, in a fine stream like rain. In another tank a solution of basic acetate of lead is prepared in the usual way, this flows into the chamber tank and from thence into the chamber ; here it comes into contact with carbonic acid gas which is sent into the chamber, the action between the lead solution and the gas being facilitated by the liquor being in such a finely divided form.
The lead solution falls down to the bottom of the chamber, and thence into settling tanks, where the white lead which is formed settles; it is collected, washed, dried, and finished in the usual
way, while the solution of neutral acetate of lead, which is also obtained, is used over again.
3. Milner Process.-Milner does not use the basic acetate of lead in his process, but prepares his lead solution by taking 4 lbs. of finely-ground litharge, and mixing it with 1 lb. of salt dissolved in 16 lbs. of water, the mixture being made in wooden tanks. The patentee states that these should be made of yellow pine ; oak-wood tanks will not do. In the tanks the mixture is well agitated for about 4 hours, at the end of which time it will have been converted into the basic chloride of lead. When the basic chloride has been fully formed, it is run into covered wooden tanks fitted with agitators; through these tanks a current of carbonic acid gas passes, which, acting on the basic chloride, converts the latter into white lead. Instead of this procedure, the basic chloride may be mixed in lead-lined tanks with caustic soda, and gas is passed into the tanks, as before, until the liquor ceases to be alkaline. This point is ascertained by the workmen taking a little of the mixture out of the tanks from time to time; if it appear viscid, forming a homogeneous mass and an even layer on the sides of the glass, then sufficient gas has not been passed in; if, however, it forms a sort of arborescent pattern on the sides of the glass, the operation is finished; the current of gas is then stopped, and the white lead sent to be finished in the usual way. The
process is said to yield a white lead of good colour and body, and very heavy, weighing about 200 lbs. to a cubic foot.