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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 C2 H3 O, 2 Pb H2 O2] + 4CO2

Basic acetate of lead.

1=3

Carbonic acid.

2 [Pb C02, Pb H2 O2]

+ 3 Pb 2 C2 H3 O2 + 4 H2O

Normal acetate
of lead.

White 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

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Fig. 6.-French process for making white lead.

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

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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.

It was worked by the Sankey White Lead Co., but has been discontinued for some time.

4. Martin Process.-Martin's process for the preparation of white lead is based on the action of carbonic acid on solutions of basic acetate of lead; whether this process was ever used on the large scale the author has no knowledge. One great fault of all precipitation-processes for the manufacture of white lead is that they are apt to give a product which is more or less crystalline, a condition fatal to its being of good quality; the colour may be good, but the body is always deficient. The patentee states that this depends upon the proportion of acid solvent of the litharge to the water which is used in the process; if the water be in excess, then too much basic salt is formed, and the carbonic acid, acting too energetically upon this, causes the formation of a crystalline product; therefore the acid solvent must be in excess. Martin prepares a solution of the neutral acetate of lead in one and a-half times its weight of water, or litharge may be dissolved in acetic acid in such a way as to produce a similar solution. 3,600 gallons of this solution are placed in a tank fitted with an agitator; there is then added 4 to 6 tons of granulated lead, and half a ton of litharge. After thoroughly mixing the materials together, carbonic acid gas is passed in for an hour, when all the litharge will have been converted into white lead, then half a ton more litharge is added, and more carbonic acid; în about an hour this second lot of litharge will be converted into white lead, then more litharge is added, carbonic acid being meanwhile sent in; these additions of litharge are continued hourly until sufficient white lead has been formed, when it is collected

and finished in the usual way. If thought desirable, instead of adding the litharge in lots every hour it may be run in in a constant stream. During the operation the temperature is maintained at about 100° F.

The distinctive feature of this process is using the litharge in an undissolved form, and strong solutions of lead acetate. In the absence of practical experience of the process it is not easy to speak definitely on the effect of using such strong solutions, but, judging from the known effects of using strong solutions on the character of precipitates obtained in other cases, one would naturally imagine that the white lead formed would have a crystalline character, and not that amorphous condition which is required in good white lead, still the patentee states that such is not the case.

When the principles which underlie these precipitationprocesses are considered, it becomes evident that the character

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