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about by the heat of the fermenting tan, or dung, causing the evolution of acetic acid from the liquid in the pots, which, attacking the lead, causes the production of the normal acetate of lead, thus
2 H C2 H2 02
Pb2 C2 H2O2
The normal lead acetate, under the influence of water and heat, parts with some of its acetic acid and passes into the basic acetate,
2. 2 Pb 2 C, H, O2 + 2H2O = Pb2C, H, 02, Pb H, 02 + 2HC2H30, Normal lead
Acetic acid. acetate.
The acetic acid thus set free is ready to attack a further quantity of blue lead. The basic acetate is now attacked by the carbonic acid evolved by the fermenting tan, the acetic acid it contains is liberated and its place taken by the carbonic acid, and white lead is formed, thus
3. 3 Pb 2 C2 H3 , Pb H,02 + 4CO2
2 (2 Pb C03, Pb H, 02)
6 H C, H302
Although the reactions shown in the above equations are those usually accepted as representing the formation of white lead from blue lead in the stack process, yet they are probably not quite correct; the evolution of hydrogen in the first step in the process is rather improbable; a better explanation would be the following :-(1) By the action of moisture and oxygen on the lead there is formed lead hydroxide, thus
Then this being acted on by the acetic acid forms the normal or neutral acetate and water, thus
2. Pb H, 02
+ 2 H C, H, 02 =
Pb Cg H30, + H2O
The normal acetate now combines with lead hydroxide to form basic lead acetate, thus
3. Pb 2 C2 H30, + 2 Pb H, O,
Pb 2 C, H, 02, 2 Pb H, 0,
Basic lead acetate.
This is now acted upon by the carbonic acid with the formation of white lead and normal acetate, thus
4. 3[Pb 2 C, H3 02, 2 Pb H, 02] + 4CO2
3 Pb 2 C, H, O,
+ 2 [2Pb C 03, Pb H, 02] + 4H2O
The normal acetate thus reproduced then forms more tribasic acetate by the reaction shown in equation 3. It is again decomposed by the carbonic acid, as shown in equation 4, so that a continuous cycle of changes is set up; the lead being oxidised to lead hydroxide, and this converted into white lead, the basic carbonate, pari passu with its formation.
As a rule, nearly all the blue lead is converted into white lead, one ton of lead producing one and a quarter ton of white lead, the amount varying from time to time according to the degree of perfection with which the corrosion has proceeded.
The great fault of the Dutch process is the great length of time required (8 to 12 weeks), the great amount of capital it takes to construct a stack of lead, and the loss of interest which takes place on the capital while the lead is in process of making. Then there is always a risk, owing to some defect, of producing a useless and imperfectly corroded lead, which has to be sent to the smelting furnace and again reduced to blue lead. Hence it is that inventors have turned their attention to devising other methods of producing white lead which shall be free from the defects of the Dutch process; so far, however, no such method has been discovered.
White lead, as made by the process described above, is a basic carbonate of lead having the composition Lead carbonate, Pb C 03,
68.95 per cent. Lead hydroxide, Pb H, 02,
Lead monoxide, Pb O,
100.00 therefore having the formula 2 Pb C O3, PbH,02
As will be seen hereafter, when the properties of white lead as a pigment come to be more fully considered, the carbonate is the substance to which white lead owes its colour and body; while the hydroxide with which it is associated, by chemically combining with the oil used to convert the white lead into a paint, imparts to the white lead great covering properties.
Numerous samples of white lead, of both high and low qualities, have been analysed by many chemists, some of which will be given later on. These analyses show that when the composition of any sample varies greatly from the figures above given, it is more or less defective. Generally, an increased proportion of carbonate, while causing the colour to be better, reduces the covering power; on the other hand, an increase in the amount of the hydroxide causes a loss of body and opacity. If the sample contain any monoxide then the tint becomes more yellow or greyish. Some of these points will be touched upon when considering the other processes for the production of white lead. In the meantime the success or non-success of any process depends upon the approximation of the white lead produced to the composition above given.
2nd GROUP.-CHAMBER METHODS. In one sense the Dutch method just described is a chamber method, but it has been classed as a separate group because while being made in a chamber, it differs materially from those now to be described. In these processes the operation of whitelead making is carried on in large chambers in which metallic lead is placed, and into which currents of carbonic acid gas, acetic or other acid vapours, are passed, together with air and steam. The different methods are distinguished one from another by the construction of the chambers, the method of admitting the
acid gases, &c., and in other points. Although many such have been invented and will be found described in the Patent Records, yet very few are in actual operation for the production of white lead; and of those which have become obsolete very little is known beyond the, often very scanty, description which is to be found in the specification of the patent which protected the process.
In all the accounts which have appeared describing these processes, and which are evidently copied from one source, the chamber process is given as “the German method ;* but it is a matter of doubt whether the process was invented in Germany or not. The author is inclined to consider it to be of English origin, partly because it is described in a patent taken out in 1749 by Sir James Creed, who makes no mention of having taken it from a foreign source. This was the first patent to describe a chamber process, and since then many have been patented; but the author must refer readers to the Patent Records for an account of these or to a series of articles which appeared in The Chemical Trade Journal in October and November, 1890.
1. Creed or German Process. This process was the first in which chambers were used in conjunction with lead and acid gases. In detail it is carried out as follows:-A chamber of brickwork is built of any convenient size and with few openings, the usual ones are a door to enter into the chamber for the purpose of filling it, and an opening in the roof for ventilation; sometimes a window or two for the purpose of observation is added. The chamber has a number of shelves, on which are placed sheets or gratings of lead; it is immaterial which are adopted, although the gratings expose more surface to the action of the various gases which are used. When all the lead is placed on the shelves, the doors are closed, and currents of aqueous vapour, air, carbonic acid, and acetic acid at once admitted into the chamber. After a period varying from four to five weeks, the white lead will have been formed; it is collected and treated as in the Dutch process.
The chemical action which proceeds is supposed to be the same as that which takes place in the old stack method. The acetic acid acts upon the lead, forming neutral acetate of lead ; this, under the action of the aqueous vapour, is transformed into basic acetate of lead, and this, in its turn, is changed by the carbonic acid into basic carbonate of lead or white lead.
The quality of the product is usually very good, not, perhaps, quite equal to that produced by the Dutch method, but better than that produced by the precipitation processes. It is, how
ever, inclined to be very variable, and the process requires some experience to carry out in the best possible manner to ensure a good product. It is desirable, as far as possible, to cause the white lead to approximate in composition to the formula 2 Pb C O3, Pb H, 02, and to do this it is necessary that the gases should be sent into the chamber in the proper proportions. If excess of acetic acid is present, too much acetate of lead is formed, which is not decomposed by the aqueous vapour and the carbonic acid ; too much of the latter tends to cause the formation of an excess of lead carbonate, and the white lead loses its covering powers. On the other hand, too much steam will lead to the formation of oxide, especially if the temperature be allowed to get high; the oxide so formed being of a yellow tint spoils the colour of the white lead. The same result is brought about by a deficiency of acetic acid. Experience is the only factor which can guide the white-lead maker in adjusting various
proper proportions. The following are some analyses of white leads made by this process quoted by Weise :
1. Firsts, of the best quality ; good both in colour and body.
2. Seconds, not so good as No. 1, but still very serviceable as a pigment.
3. Thirds is only just usable as a pigment.
5. Not usable at all, it contains too much carbonate, and is sent to the smelting furnace.
Various alterations in the details of this method have been made from time to time by various inventors, some of which may be briefly noted. Burton placed the lead in coils on the shelves of the chamber, and passed the current of steam through perforated pipes, thereby converting the lead into hydroxide ; when a sufficient amount of this has been formed the current of steam is stopped, and a current of acetic acid vapour sent in; this, acting on the hydroxide, converts it into the basic acetate; when this action is finished the acetic acid current is stopped, and carbonic acid gas sent in, which acts on the basic acetate and changes it into basic carbonate or white lead. These currents of steam, acetic acid vapour, and carbonic acid gas are sent in successively until all the lead is converted into white lead, then the currents are