Fig. 18.
The method pursued by Ozouf, in France, is a considerable improvement on Thénard’s process. Pure carbonic acid is used for the precipitation, and white lead of similar composition to that produced by the Dutch process is obtained, since the introduction of the carbonic acid can be regulated according to the volume and strength of the lead solution, and thus white lead of any desired composition can be produced. The most elaborate precautions for the health of the work-people are taken.
The preparation of pure carbonic acid gas is based upon the absorption of this gas from a mixture of gases by a solution of sodium carbonate, and its evolution on heating the solution. The apparatus is shown in Figs. 18 and 19. The products of combustion obtained from the stove, A, are drawn by the air pump, E, through the pipe, C, into the cooler, B, which is regularly fed with cold water by D. The gases compressed in the receiver, E´, deposit moisture there, and then proceed through 3 horizontal cylinders, F, of sheet-iron, provided with agitators, in which the carbonic acid is absorbed by a cold solution of sodium carbonate of 9° B. The unabsorbed gases escape into the atmosphere through G (Fig. 19). The sodium bicarbonate solution is received in the wooden tank, H, after passing through the 3 cylinders, F. The pump, I, of the alternating pump, I I´, lifts the sodium bicarbonate solution out of H and sends it through the pipe, K, into the tubular cylinder, J, which stands upon a cylinder of larger diameter, J´, communicating with it only by the vertical tubes. The bicarbonate solution rises between the tubes in J, passes through the pipe, L, drops in a fine spray through the rose forming its mouth, and by means of the vertical tubes passes into J´ and thence into M, where it is heated by means of a steam coil to 100° C. Carbonic acid is then evolved, and the residual solution of neutral sodium carbonate, after cooling in the vessel, R, by means of the cold coil, is drawn off by the pump, I´, again to enter the cylinders, F, by means of the pipe, K. The carbonic acid evolved in M, together with steam, enters J´ through N, and in rising in the tubes of the cylinder, J, is cooled by the falling spray of bicarbonate solution. The cooling is completed in the coil, O, surrounded by water; the vessel, P, separates the condensed water and passes the gas on into the holder, Q. The pipe, S, connecting P with the suction pipe of the pump, J´, serves to restore to the solution of sodium carbonate the water it has lost, thus maintaining the proper concentration. The cost of 1 cubic metre of carbonic acid is 10 centimes, of 1 kilogramme 5 centimes.
Fig. 19.
For the production of white lead, the carbonic acid, by means of the pipe, U (Figs. 19 and 20), enters the cylinder, T, provided with an agitator and containing a solution of basic lead acetate. By means of the pump, V, the lead solution is fed into the cylinder, T, through W. The absorption of the gas proceeds rapidly; the progress of the operation is followed by the observation of a pointer moving over a scale; as the gas holder sinks the pointer moves upwards. After the precipitation of white lead, the contents of T are emptied into the tub, b, in which rotate rakes attached to a vertical axis of iron plated with copper. When the white lead has settled, the supernatant solution of neutral lead acetate is drawn off through the pipe, c, by means of the pump, d, and conducted into the water-tight vessel, X, containing a stirrer on the vertical axis, W, made of coppered steel. Here litharge is added, and the resulting solution of basic lead acetate is conveyed to the cylinders, T, by means of the pump, V, as already described. The white lead in the tub, b, by putting the stirrer in motion, is washed once with water which has been previously purified by a little lead acetate. It then goes into another tub provided with stirrers, where it is several times washed, sodium carbonate being added to the last wash water until a sample of the white lead is not coloured by a drop of potassium iodide solution. In this way the wash water is obtained free from lead, and the product is said to be of better quality. This, however, is not in accordance with the fact that good Dutch white lead generally contains some lead acetate. The two-cylinder pump, h, which is in connection with the gas holder, forces gas over the surface of the liquid in T in order to drive it into tubs which are not in the position shown for b in the illustration, and into which there is no direct flow. The washed white lead is brought into bags which are pressed in a hydraulic press, dried, ground, sieved and packed in casks. These troublesome and often dangerous operations have been modified by Ozouf in the following manner. The pulp white lead runs from the tub, b, into the hopper, g, where it is kept mixed by a small stirrer, and from which it passes on to the cylinder, f, heated by gas from the inside. In its rotation the cylinder carries along the white lead and dries it, it is then removed by a knife below the hopper, and falls on to an inclined plane. The hopper and cylinder are in a room provided with a good draught.
Fig. 20.
The lumps coming from the drying room are placed by workmen wearing respirators in buckets on an endless chain, are carried to the mills, ground and sieved; then, by means of an Archimedean screw, the white lead is conveyed to a cask in which it is evenly pressed by means of a special mechanism. A bell announces when a cask is full.
Manufacture of White Lead by means of Natural Carbonic Acid.—In districts where currents of carbonic acid gas issue from the ground, they can be used in the manufacture of white lead, and are actually utilised for this purpose. Natural carbonic acid may, of course, be used for any of the white lead processes.
In this process, now no longer in use, white lead was obtained by mixing litharge to a stiff paste with a weak solution of lead acetate and exposing the paste to the action of carbonic acid. By continually kneading the mass by means of grooved rollers or of rotating cylinders, through the hollow axis of which carbonic acid was led, the paste was thoroughly brought into contact with the carbonic acid.
By this process a good product is only obtained when pure litharge, entirely free from the oxides of iron and copper, is used. The copper oxide may be removed from the litharge by means of ammonia if this can be obtained at a low price; but oxide of iron cannot be removed, and very small quantities of it are sufficient to impart a yellow tinge to the white lead.
In Payen’s process the lead sulphate obtained in considerable quantities as a by-product in calico printing is the raw material employed. By treating this lead sulphate with a solution of ammonium or sodium carbonate, white lead and ammonium or sodium sulphate are produced. The white lead is then freed from the soluble salts by washing, mixed with a small quantity of lead acetate, and pressed into the drying moulds.
By boiling lead sulphate with caustic soda and passing in carbonic acid (Puissant’s process), a white lead is obtained which differs considerably in composition from ordinary white lead.
Many methods have been proposed with the object of converting insoluble lead salts, obtained as by-products or by an inexpensive process, by treatment with alkaline or alkaline earth carbonates, into white lead. The fact that none of these methods has obtained a permanent footing in the industry shows that each must be accompanied by serious defects, or can only be practicable under peculiar conditions.
Magnesium carbonate is used in Pattison’s process to decompose lead chloride. Dolomite (magnesian limestone) is the raw material for the magnesium carbonate. Coarsely powdered, it is heated at a low red heat, when magnesia is formed, the calcium carbonate remaining almost entirely unaltered, since it requires nearly a white heat for its decomposition. The powder ground in water was, when treated with carbonic acid under a high pressure, soluble, magnesium bicarbonate being formed, the saturated solution of which contains 2·3 per cent. of magnesia, and has a specific gravity of 1·028. The solution of lead chloride contains 1 part of the salt in 126 parts of water; it is mixed with a slight excess of the magnesium carbonate solution as quickly as possible. The liquid is drawn off from the mixing vessel into a large receiver in which a precipitate deposits, consisting of white lead and a little oxychloride. After drying, the precipitate is ground with a small quantity of caustic soda to decompose the oxychloride. A few days afterwards the mass is washed to remove sodium chloride and the product dried.
The process of Dale and Milner is similar to the above magnesia process. Litharge, lead hydroxide or insoluble lead salts are mixed with sodium bicarbonate solution, and, with repeated additions of water, ground until the formation of white lead is completed. The lead compound and sodium bicarbonate are used in equivalent proportions.
According to the process of P. Bronner (German patent 52,262), 3 molecules of freshly precipitated lead sulphate are heated with a solution of 2 molecules of caustic soda, when the basic sulphate 2 PbSO₄.Pb(OH)₂ is formed according to the equation—
3 PbSO₄ + 2 NaOH = 2 PbSO₄.Pb(OH)₂ + Na₂SO₄.
Or 4 molecules of lead sulphate are decomposed by 2 molecules of caustic soda, according to the equation—
4 PbSO₄ + 2 NaOH = 3 PbSO₄.Pb(OH)₂ + Na₂SO₄.
This transformation takes place at a temperature of 70° C. The resulting basic sulphate, although pure white, cannot be used as a pigment on account of its lack of covering power; but by heating with a solution of sodium carbonate it is converted into white lead.
2 PbSO₄.Pb(OH)₂ + 2 Na₂CO₃ = 2 PbCO₃.Pb(OH)₂ + 2 Na₂SO₄.
3 PbSO₄.Pb(OH)₂ + 3 Na₂CO₃ = 3 PbCO₃.Pb(OH)₂ + 3 Na₂SO₄.
By this process, which is harmless to the workmen, the lead sulphate obtained as a by-product in the preparation of mordants for calico printing, can be converted into good saleable white lead. The lead sulphate may also be obtained from litharge, lead acetate or nitrate.
It occasionally happens that white lead has a rose tint, which is clearly perceptible by comparison with a pure white sample. This colouration occurs in white lead made from argentiferous lead. A very small quantity of silver is sufficient to produce the tinge of colour.
Occasionally white lead which has been ground in oil and used for painting turns perceptibly yellow, the colouration being similar to that observed on a surface painted with white lead from which light is almost excluded. The yellow colouration is due to lead oxide. This has been proved by suspending such a white lead in water and treating it with carbonic acid, after which a surface painted with it remains permanently white.
Under the name of white lead, but differing from it in composition, various products are found which consist of lead oxychloride. This compound is also known as Pattison’s white lead.
Pattison’s white lead can be much more cheaply manufactured than real white lead, the raw material employed being the cheap galena. The finely-powdered mineral is boiled with strong hydrochloric acid in closed lead vessels. Sulphuretted hydrogen is evolved, which may be burnt to sulphur dioxide and so used to make sulphuric acid. A hot saturated solution of lead chloride remains, from which the salt separates in small crystals on cooling. The crystals are drained in a basket and washed with cold water to remove the acid. The pure lead chloride is then dissolved in hot water and mixed with lime water. Pattison obtained lime water from dolomite by burning it, treating with a little water to remove the easily soluble salts, and, after the removal of this wash water, treating the residue repeatedly with water in order to obtain a clear solution of pure hydrate of lime. When pure limestone is used, it may be treated with water immediately after burning without any preliminary preparation.
Two equivalents of lead chloride are used to one equivalent of calcium hydroxide. Practical experience showed that the best product was obtained when the precipitation was very rapidly brought about. With this object, both solutions entered the precipitation tanks through pipes with narrow slits at the side, so that the liquids met in a thin layer, in which the precipitation of the pigment was instantaneous. It is also necessary that lead chloride should be in excess throughout. The liquid is allowed to stand for the precipitate to settle, which it does in a brief time on account of its high specific gravity. The solution now contains the small excess of lead chloride in addition to calcium chloride; lime water is added until the liquid turns red litmus paper blue. From the alkaline solution all the lead soon separates as lead hydroxide, which is dissolved in hydrochloric acid, and thus again comes into the process.
In order to utilise the large quantities of hydrochloric acid obtained in the manufacture of soda, Percy described a process in which galena is ground with hydrochloric acid, whereby in 30 to 40 hours all the lead is converted into lead chloride, whilst the stony admixtures are unattacked. The lead chloride is then separated by levigation from the undissolved minerals and washed until free from iron, when it is dissolved in hot water and converted into oxychloride by means of lime water.
Lead Sulphite, PbSO₃, can be obtained by passing sulphur dioxide into a solution of basic lead acetate; lead sulphite is precipitated and a solution of neutral lead acetate remains. The process is similar to the French white lead process, with the difference that sulphur dioxide is used instead of carbon dioxide. Lead sulphite has no advantages over white lead, and is more expensive; it has thus never found practical application.
Lewis and Bartlett’s White Lead Pigment.—In the lead works at Zoplin, in Missouri, galena is smelted with limestone and coal, lead fume being obtained in addition to metallic lead. The lead fume deposits are ignited, and again worked for lead and lead fume. This last lead fume can at once be used as a white pigment; it consists principally of lead sulphate, lead oxide and zinc oxide.
Lead antimonite and antimonate are both heavy, white powders which can be used as pigments. They are dearer than white lead, to which they are inferior in covering power, and which they do not exceed in permanence.
Lead Antimonite is obtained by heating 5 parts of finely powdered antimony with 20 parts of sulphuric acid until a dry, white mass of antimony sulphate is left. This is fused with soda ash, the melt is extracted with water, and lead antimonite obtained by precipitating with lead acetate.
Lead Antimonate is formed by introducing in small quantities at a time a mixture of 1 part of finely powdered stibnite (antimony trisulphide) with 5 parts of sodium nitrate into a red-hot crucible, boiling the mass with water and precipitating the solution with lead acetate.