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depending upon the action illustrated in the above equations, are to prevent the formation of a highly crystalline neutral carbonate and to ensure that the precipitate shall have the necessary amount of basicity; for, as will be seen from the above equations, the tendency is to form the normal carbonate of lead instead of the basic carbonate.
These methods of preparing white lead early attracted attention from white-lead makers, and many and various have been the processes which have been patented and tried for the manufacture of the pigment by such methods. While there is no doubt that good white lead can be made by them, yet the results seem to be so variable that from a commercial point of view these processes have always been failures.
The first patented process belonging to this group dates from 1797 when the Earl of Dundonald secured a patent for making white lead from the oxychloride of lead.
1. Dundonald Process.—Litharge is taken and is treated with sufficient salt and water as to convert it into the oxychloride of lead, in the manner which will be found more fully detailed on p. 30. The insoluble oxychloride is collected, washed to free it from alkaline salts, then boiled in a solution of potash (potassium carbonate), when it is converted into white lead, which is collected, and, after washing, dried; it is then ready for
A very similar process was patented some years later by James Kier. No record exists as to whether this
process much, if at all, used on the large scale.
2. Pattinson Process.—Mr. Hugh Lee Pattinson, a large lead-smelter of Newcastle, has prepared white lead by many processes ;
“ Pattinson's white lead” (which see) is the oxychloride of lead.
The process, which comes under the present group, has for its object the preparation of ordinary white lead. Chloride of lead prepared by any convenient process is mixed with carbonate of lime in the proportion of their chemical equivalents, 278 to 100, and the mixture is ground with water for several hours, then allowed to stand all night, the clear liquor (which consists principally of a solution of chloride of calcium) run off, more water added, and the grinding resumed for a few hours ; then it is again allowed to stand all night and the clear liquid again drawn off. These operations are continued until the effluent water is tasteless. The white lead, after being finished in the usual manner, is ready for use. Instead of the process just described a solution of the carbonate of lime or of carbonate of magnesia, made by means of carbonic acid, is used to act on the lead chloride.
In another modification of the process, chloride of lead and carbonate of calcium are placed in a revolving cylinder and a current of carbonic acid gas sent into the mixture, preferably the gas is used at a pressure of four or five atmospheres ; after four days the aqueous liquor, which is, as before, a solution of calcium chloride, is drawn off, more water run in and the gas again passed in for two days longer, when the reaction is completed, and the white lead only requires finishing to be ready for use. This process does not seem to have been much used.
3. Dale and Milner Process.—The inventors take litharge or a basic salt of lead and grind it with water and bicarbonate of soda for some time, when white lead is formed. This process was worked on a large scale for a short time, but it was superseded by Milner's process described above. A process patented by Isham Baggs was almost identical with this. The results were not very satisfactory as the white lead obtained was rather too crystalline in structure.
4. Watt and Tebbutt Process. This consisted in treating sulphate of lead, first with lime, then with potash. Cooper uses 25 lbs. of sulphate of lead to 10 lbs. of potash. The action of alkaline carbonates upon lead sulphate is, at the best, but imperfect, and a complete change into carbonate is never obtained.
5. Delafield Process.-Delafield uses nitrate of lead, which he prepares by dissolving one cwt. of litharge in one cwt. of nitric acid and just enough water to form a saturated solution. This is heated by steam to a temperature of about 200° F. When a hot solution of 70 lbs. of potash is run in, white lead is precipitated, which is collected and, after washing, dried. The product is liable to contain too much carbonate and, therefore, to be deficient in body.
6. Rowan Process.-This resembles the Watt and Tebbutt process, only the action between the lead salts and the alkaline carbonate is effected under a pressure of from 30 to 40 lbs.
7. Lowe Process.-In Patent No. 9,122 of 1887, a process for making white lead is described, which consists of the following operations :-50 lbs. of lead acetate, or 43•6 lbs. of lead nitrate, are dissolved in 25 to 30 gallons of water; to this solution is then added 23 lbs. of solid bicarbonate of soda or 26.4 lbs. of solid bicarbonate of potash, when a precipitate of a more or less basic carbonate of lead will be obtained. In another vessel 25 lbs. of lead acetate and 15 lbs. of litharge are digested with 127 gallons of water for 8 to 10 hours, when the product which is obtained is mixed with the precipitate obtained in the first
instance. White lead is formed and is collected and finished in the common way.
An analysis of a sample of white lead made by this process is given in the patent, as follows :Lead monoxide, Pb 0,
86.185 per cent. Carbonic acid, C02,
11.270 Water, H, 0,
2-545 which differs but little from that of the Dutch process white lead.
8. Condy Process.—This process was patented in 1881, and has been worked on a large scale ; but whether the process is now in use or not the author is unaware. In this process, acetic acid of 1.045 specific gravity is diluted with about five times its volume of water, and allowed to act on granulated lead until a solution of lead acetate of 1.2 specific gravity is obtained ; this solution is evaporated to dryness, when the bibasic lead acetate is obtained. 275 lbs. of bibasic acetate of lead, 112 lbs. of litharge and 5 gallons of water are ground together into a paste. Instead of preparing the bibasic acetate the neutral acetate may be used ; in this case, 189 lbs. are ground with 229 lbs. of litharge and 21 lbs. of water for a few hours, and then left for 48 hours. In either case there is formed the tribasic acetate of lead. The mass is dissolved in 10 times its weight of water, and, then, for every 112 lbs. of litharge in the mass 84 lbs. of solid bicarbonate of soda is added; this precipitates the white lead, which is finished in the usual way. A modified process was described in a later patent. One part of acetic acid of specific gravity 1.045 is mixed with 121 times its weight of water, and the dilute acid so obtained allowed to act upon granulated lead until a solution of specific gravity 1.040 is obtained ; this is mixed with water, and, then, for every 60 lbs. of acid used in preparing the solution, 30 lbs. of solid bicarbonate of soda are added, and the white lead is precipitated.
The white lead prepared by this process has been favourably spoken of; it has a good colour and covering power. In chemical composition it resembles white lead, but the process appears to be somewhat variable in its results, and, therefore, not commercially practicable.
9. Brown's Process.-Mr. Arthur G. Brown has devised an electric process for the production of white lead, which has been worked on a large scale in Cambridge, Mass., U.S.A. A solution of nitrate of soda of 10° B., containing about 1 lb. of the salt to the gallon, is placed in an electrolytic cell with a porous partition; in each of the compartments an electrode is fixed;
the positive one is formed of lead, the negative one of copper. On passing an electric current of suitable strength through the solution, nitric acid collects about the positive electrode and caustic soda about the negative electrode. The nitric acid, however, attacks the lead and forms nitrate of lead. The reactions which take place may be represented by the following equations :
The liquors in the two compartments are drawn off; when mixed together they yield a precipitate of lead hydroxide and a solution of sodium nitrate, according to the equation :
3. Pb2 NO2 + 2 Na O H
Pb H,09 + 2 Na N 02
The solution of sodium nitrate is used for a fresh electrolytic operation.
The next step consists in treating the lead hydroxide with a solution of bicarbonate of soda, when white lead is formed, according to the equation :
The process can be worked in a continuous manner.
The product is described as being of a highly amorphous character, in which respect it is superior to white lead made by the Dutch process; it is stated to have from 20 to 25 per cent. more covering power than the best Dutch lead.
There are 46 white-lead works in Great Britain, 42 use the Dutch method, 3 prepare the pigment by chamber methods, and 1 by precipitation.
5th GROUP. MISCELLANEOUS PROCESSES.
Besides the processes described above, others have been proposed or patented from time to time which are perhaps just worth mentioning, as showing what has been done by
inventors towards the preparation of white lead by other means than the old Dutch process.
Some of these processes do not come within the groups of processes described above, others fall into one or other of them; but as they are only of small importance, and as, in some cases, it is doubtful whether they were ever worked on a large scale, they have been relegated to this division of white-lead processes for description. The processes are rather numerous, and will only be given in outline; for further details the reader is referred to the records of the Patent Office.
Torassa proposed a curious process, which is of interest on account of its novelty only, not from any practical value it may possess. Lead is granulated, and then placed, with a small quantity of water, in a revolving box, or a box fitted with agitators ; in this it is worked until it forms a very fine mud, which is again agitated with air until it has been converted into white lead. The process must have been a slow one, as the amount of carbonic acid in the air is small, and can only convert in a given time but a small quantity of fine lead into carbonate. Wood, a more recent inventor than Torassa, proposed to use the same process, but to hasten the preparation of the white lead by agitating the fine lead mud with carbonic acid ; but even this was not sufficient to make the process a practical
Mullins proposed to make white lead by an ingenious but, from a practical point of view, unsuccessful method. Sponges, saturated with a solution of basic acetate of lead, were suspended by porous strings in a chamber into which carbonic acid was passed ; this, of course, transformed the basic acetate into basic carbonate of lead. The sponges were kept saturated with a solution of acetate by connecting the porous strings with a tank containing the solution, which, by capillary attraction, passed along the strings to the sponges. The process was not used on a large scale.
Martin prepares carbonate of lead so that it shall contain a slight excess of carbonic acid. Hydroxide of lead is prepared by thoroughly agitating granulated lead with air and water. The two bodies are mixed together in the proportion of 8 lbs. of hydroxide to one ton of carbonate, the mixture being made by grinding with water into a paste.
Lewis prepares what he calls white lead from lead or lead ores, by mixing these with anthracite coal, and heating the mixture in a Wetherill zinc furnace with a powerful blast of air; the white lead sublimes, and is collected (see p. 51).