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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-haĪf 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 ; in 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
of the white lead, both chemically and physically, materially affects its value as a pigment. This character will depend upon the character of the solution of lead which is used, the temperature at which the reaction between the carbonic acid gas and the lead salt takes place, and the strength of the solutions used; on these points information is scanty, and very few of the inventors of white-lead processes have mentioned the influence of any of them. The character and basicity of the lead salt will have some influence on the result; the basicity should be due to the presence of lead hydroxide, and not to lead oxide, or, at all events, the latter should be present in only small quantities. To ensure the production of lead hydroxide, water seems to be necessary,
and therefore should be used in sufficient quantity. The quantity of carbonic acid should be so regulated that not more than two-thirds of the base present is converted into carbonate ; if too much gas is used, then all the base will be liable to be converted into carbonate, and the white lead has a tendency to become crystalline; the difficulty is to ascertain when sufficient gas has been used. The strength of the solution of lead will also have some influence, but the diversity of opinion among whitelead makers as to the proper strength is great; some prefer strong solutions, others weak ones. As a rule, weak solutions give the finest precipitates, and strong solutions give the coarsest. The temperature at which the operation is conducted will have some little influence; cold solutions will cause the formation of fine precipitates, while hot solutions tend to give rise to crystalline precipitates, due to the fact that the reaction between the carbonic acid and the lead salt takes place too readily; still it is not desirable to work with solutions that are too cold; the best temperature is from 100° to 120° F.
5. Fourmentin Process.—This was proposed many years ago, and somewhat resembles Milner's process.
Litharge is taken and treated with salt in such proportions as to convert it into oxychloride of lead; this body is placed along with water in a number of cylindrical vessels fitted with radial beaters. Carbonic acid is sent in, while the temperature is maintained at the boiling point. When the reaction between the acid and the lead has finished, the current of gas is stopped and the product run into a boiler, in which it is boiled with a quantity of finely-powdered carbonate of lime, equivalent to the amount of salt used in the preparation of the oxychloride, this boiling being continued until, on taking out a sample, and filtering off and testing the clear liquor with ammonia and
ammonium sulphide, no precipitate forms; the period of boiling varies from two to four hours. When the boiling with the carbonate of lime has been continued long enough, the operation is stopped, and the white lead allowed to settle out, collected and finished in the usual way.
6. Spence Process.—The principle of this process consists in boiling a salt of lead (the oxide or carbonate gives the best results, but the sulphate or other salt which can be dissolved by caustic soda may be used) with a solution of caustic soda until the alkali is saturated with lead; then a current of carbonic acid is passed through the liquor, and white lead is precipitated, while carbonate of soda is formed. The latter can be causticised by means of lime, and used over again. The white lead which is precipitated is collected, washed, and finished in the ordinary way. This process has not been used on a commercial scale.
7. MacIvor Process. The principle of this process depends upon the fact that when litharge is acted upon by acetate of ammonia under the combined influence of heat and pressure, it is converted into basic acetate of ammonia and lead, while ammonia is liberated in the free condition, and dissolves in the water which is present to form the liquor ammonia of commerce. Then, when a current of carbonic acid gas is passed through the mixture of basic acetate of lead and ammonia, the lead is precipitated as basic carbonate or white lead, of good colour and covering power ; while acetate of ammonia is re-formed and can be used again for dissolving a fresh batch of litharge. The process is carried out somewhat in the following manner :-Into a digestor made of strong iron plate lined with lead, is placed a solution of acetate of ammonia of not less than 5 per cent. strength and a quantity of litharge which has been previously very finely ground. The proportions of the two will depend upon the strength of the solution of ammonia acetate which is used; for that given, 1 ton of litharge is used for 1,200 gallons of liquor. The digestor is closed. The acetate solution is sent through a heater so that it may have a temperature of from 60° to 100° C., and then into the digestor, passing into it from a pipe fitted with a conical spreader at its lower end; the acetate solution flows upwards through the litharge, effectually agitating the mass and so assisting its solution ; from the digestor the liquor is drawn off from the upper portion by means of a pump and passed through the heater and again into the digestor, this cycle of flow being continued until all, or nearly all, the litharge is dissolved. The solution of basic acetate of lead and ammonia is now passed
through a filter-press into a cooler, from which it flows into a carbonator. The cooling of the liquor causes the separation of much of the basic acetate of lead in the form of fine crystals, so that in the carbonator a fine magma is presented to the action of the carbonic acid gas, which is sent into it from any convenient Bource. A circulation of the mass in the carbonator is kept up by drawing off from the upper portion of the carbonator and forcing it by means of a pump through a pipe, with a conical spreader at its end, to the bottom of the carbonator; in this way every part of the mass of liquor and crystals is made to come in contact with carbonic acid gas. The white lead is rapidly formed as a fine white precipitate. When it is considered that the carbonation is finished the whole mass is passed through a filterpress, so as to separate the white lead formed, while the liquor, which consists of a solution of acetate of ammonia, together with unchanged basic acetate of lead and free ammonia, is sent to the digestor to be used again. The process is a rapid one; the solution of the litharge in the acetate of ammonia does not take long, while the conversion of the basic acetate of lead into white lead in the carbonator is almost instantaneous. The process is being worked by a limited company.
Defects of Wet Precipitation Processes.—The main fault of the wet precipitation processes appears to be the variable nature of the product unless great care be always taken to work under the same conditions. It may be presumed that the white lead precipitated will vary in composition as a consequence of variations in the basicity of the lead salt, the strength of the solution, and the temperature. Some experiments made by the author show that these have an influence on the composition of the white lead thrown down.
Basicity.—Thus, when the ratio of basicity of the lead salt is Pb 2 C, H, 0,: Pb O, the white lead had the composition81.44 per cent,
H20; while, when the ratio of basicity was Pb 2 C, H,02 : 2 Pb O, the white lead contained
83.65 per cent.
C02, which shows that the greater the basicity the more nearly does the precipitated white lead approach to the normal carbonate, Pb CO2, in composition.
Bischof Process.-(See p. 45.)
Temperature also has some influence. Raising the temperature at which the precipitation takes place appears to lead to less lead carbonate being formed and to the white lead containing more lead hydroxide or even lead oxide. Thus, white leads precipitated from solutions of basic lead acetate (Pb2C,H,O,:PbO) at varying temperatures contained varying amounts of lead oxide (Pb O) and carbonic acid (C 02) as follows:Temperature.
PbO. 212° F.,
85.27 150° F.,
13.07 82:57 100° F.,
13-63 78.87 70° F.,
136 789 Thus, at temperatures varying from 70° to 100° F., the results were practically identical; at 150° F. less carbonic acid was absorbed, and still less at 212° F.
The degree of dilution of the solutions of basic lead acetate employed will have some influence; experiments made by the author showing that the lead is more easily precipitated from weak solutions than from strong ones.
4th GROUP. PRECIPITATION PROCESSES WITH
ALKALINE CARBONATES. When a solution of sodium carbonate, or other alkaline carbonate, is added to a solution of lead, a white precipitate of a more or less basic carbonate of lead is obtained; insoluble basic or neutral salts of lead, such as the oxychloride or the sulphate, are also acted upon by alkaline carbonates, and basic lead carbonate is formed; these reactions are formulated in the following equations :Pb 2 C, H, 02, 2 Pb H, 0, + 2 Na, C03 2 PbC03, Pb H, 02 Basic acetate of lead.
Sodium carbonate. + 2 Na C, H, 0,
+ 2 NaOH Sodium acetate.
The chief difficulties met with in carrying out the processes