« VorigeDoorgaan »
would peruse these records and see what has been done in the past. A few of these inventions may be briefly noticed here, and a fuller description given of such as are at present used on a large scale.
Richardson, in 1839, patented the use of the sulphate only as a pigment; in 1853, Carter & Marriott prepared a chlorosulphate of lead made by treating 100 lbs. of litharge with 25 lbs. of salt, and the product so obtained with 5 lbs. of sulphuric acid. Woods, in 1866, took out a patent for the preparation of a white pigment from lead fume, which is a mixture of lead, lead oxide, and lead sulphate; this he treated with hydrochloric acid thus forming a chloro-sulphate; or he calcined the fume in a furnace, whereby it was converted into a white mixture of lead oxide and lead sulphate, which was then treated with hydrochloric acid as before. Groves, in 1826, treated galena with potassium nitrate and sulphuric acid, whereby the lead sulphide was converted into lead sulphate, which was dried and sold as a pigment. In 1866, Messrs. Bell & Fell patented the use and preparation of what they called a sub-sulphate of lead, prepared by precipitating a solution of lead nitrate with sulphuric acid and then boiling with an alkali.
SUBLIMED WHITE LEAD.
This product is the invention of G. T. Lewis, and was first patented in 1879. About 1870 a vein of ore containing both lead and zinc, being a mixture of galena and blende, was discovered in America; this was smelted for lead, which, owing to it being different in properties from ordinary lead, was distinguished as “ Bartlett lead;" the presence of zinc in lead ores is very detrimental, as the zinc cannot wholly be removed from the lead, while it imparts to it properties which, by causing it to become hard and brittle, prevent its application to those uses for which lead is of great service. The process of white-lead making now to be described was to some extent devised to utilise this lead-zinc ore.
The manufacture of sublimed lead depends upon two facts, which also are the principles that underlie some of the ordinary processes of lead smelting-first, when lead ore (galena, lead sulphide) is heated in a furnace with access of air it undergoes oxidation, partly to lead oxide, Pb O, partly to lead sulphate, Pb S 04, the amount of the oxidation depending upon the amount of air which comes in contact with the ore; if this is small then
the sulphate is mostly formed. The temperature also has some influence on the result; in fact, it is probably the chief factor in the process. If low, then the sulphate is chiefly formed ; if high, this is likely to be decomposed into oxide and sulphur dioxide. Secondly, during the operation of lead smelting a large proportion of the lead is carried off in the form of “fume,” which collects in large chambers or flues built for the purpose. This lead-fume is a mixture of metallic lead, lead oxide, lead sulphate; if there be any zinc in the ore it will be mostly found in this fume. It has frequently been noticed that the composition of the fume varies from time to time, according to the conditions under which the furnaces are being worked. Lewis proposes to utilise the two principles laid down and, by carrying out the process in such a manner that the great bulk of the fume produced consists of lead sulphate, to prepare a white pigment from any lead ore. This is effected in the Lewis and other processes based on the same principle by causing the operation to be carried out in a blast furnace.
The preparation of sublimed white lead takes place in two stages; the first may be called the fume stage, in which the lead ore is transformed into fume of more or less complex composition; the second stage may be called the colouring stage, in which the pigment is made of a proper degree of white
In the first stage a furnace is used resembling the Wetherill zinc furnace. Four of these are placed back to back and side by side, and is hence not unlike four American lead-smelting hearths. The furnace is worked from two sides, known as the fronts; a water back, containing tuyeres, is also placed between the bench of furnaces; above the water back is an air chamber divided into two compartments by a partition, so arranged that the air which enters into one of these compartments passes to one front of the bench of furnaces, while the air in the other passes to the other front. This air is heated by passing through the hot walls of the furnaces, and by suitable means is sent as a powerful blast through the tuyeres into the hearth of the furnace, and through any ore which may be on the hearth.
The ore is placed on the hearth of the furnace with the fuel necessary to effect its melting, and air is sent through it; part of the lead is reduced to the metallic state, and is collected by being allowed to flow out of the furnace into a suitable receptacle. Some of the lead is blown by the blast of air into fume, and is more or less oxidised to oxide and sulphate during that operation; this fume
passes up into the collecting hood which is placed above the hearth, and from thence into a large chamber, where it collects on the sides. These chambers are built in two storeys of brick and iron, the openings connecting the two storeys are covered by means of woollen bags; a powerful fan in the second storey draws the gases produced during the process of furnacing through the bags, but the latter retain the lead fume, which therefore collects in the lower storey of the chamber. The woollen bags are shaken from time to time to dislodge the fume which collects on them. At intervals the fume in the lower storey is set fire to, a proceeding which completes the oxidation of the lead fume, and, at the same time, makes the fume more consolidated, and, therefore, better for use as a pigment. The burnt fume has a lead colour, and requires further treatment before it can be used as a white pigment, although it is sold in this state as a lead-grey colour.
The next operation, in which the pigment is made white, takes place in the “slag-eye furnace" as it is called. This slag furnace consists of a square brick chimney, fitted with a hearth, tuyeres, and water back. In this furnace the fume from the last operation is heated in a powerful blast of hot air, which causes most of it to become completely oxidised to lead sulphate, which passes into condensing chambers, where it collects. These chambers are constructed in exactly the same way as those described above. As a rule, the pigment is ready for sale, but occasionally it may have a slight grey colour; to get rid of this it is treated with sulphuric acid, which causes it to whiten, and sometimes it is ground in order to render it as fine as possible.
Sublimed white lead is a powder of a fine white colour, although sometimes it has a grey tint; the white has a rather bluish hue. It is heavy, its specific gravity being over 6; a cubic foot weighs about 200 lbs. It is quite insoluble in water, is partially soluble in dilute nitric acid, which dissolves out the zinc oxide, lead oxide, or lead carbonate the pigment may contain; as a rule, it is completely soluble in boiling hydrochloric acid, although some samples contain a little barytes, which remains behind as an insoluble residue on treatment with the acid.
In composition it is very variable. Much depends upon the ore from which it is made, and whether anything has been added to it after the pigment was made in the furnaces described above. The following analyses are partly from published accounts, and partly from the author's own results :
70 82-390 per cent.
23 0.554 Zinc oxide, Zn 0,.
7 6.335 Lead carbonate, Pb C 03,
9:421 Water, hygroscopic,
0.350 Water, combined,
1.050 The proportion of oxide of zinc and sulphate of lead contained in the pigment is, to some extent, a measure of the value of the pigment for painting purposes.
Sublimed white lead when well made is a fairly satisfactory pigment. It possesses good colouring power and body or covering power, although in this respect it is perhaps not quite equal to the best white lead. It is more permanent than white lead, because sulphureous gases and vapours have little or no action upon it; it, therefore, keeps its colour longer when used in paint and exposed to air. Being insoluble in water and acids it is not so poisonous as white lead.
It has been noticed that occasionally when mixed with oil and turps, sublimed white lead has a tendency to become gelatinous ; on what this peculiar property depends is somewhat uncertain.
FREEMAN'S NON-POISONOUS WHITE LEAD.
The base of this white lead is the sulphate made from metallic lead by precipitation, it also contains zinc oxide, barytes, and, in the earlier makes, a little magnesia, all the ingredients being mixed together by a process of grinding under edge runners, which causes them to be thoroughly incorporated together, and, at the same time, by consolidating the materials, increases the body or covering power. The pigment was first patented in 1882, but since then other patents have been taken out for improvements in its composition, which, in consequence, has varied a little from time to time. The non-poisonous white lead is a very good pigment, is more permanent under exposure to atmospheric influences than white lead, and is equal to white lead in body or covering power and in freedom of working. It is rather heavier than white lead, weighing about 180 to 190 lbs. to the cubic foot. Its specific gravity is 5.95 to 6.00. It is one of the best substitutes for white lead which have been made.
HANNAY'S CALEDONIAN WHITE LEAD.
Messrs. Andrew French & J. B. Hannay in 1884 took out a patent for the preparation of a chloro-sulphite of lead to be used
as a substitute for white lead. This was an extension of a former process of French's for the production of a sulphite of lead pigment (see p. 57).
To prepare the chloro-sulphite of lead, lead ore, lead-fume, or any lead compound is mixed with coke, and, if the amount of sulphur in the ore or lead used is deficient, with some pyrites, as it is necessary that a large quantity of sulphurous acid (sulphur dioxide) gas should be produced. This mixture is placed in a kind of cupola furnace provided with an air blast. The heating of the furnace and the amount of air blown through is to be so arranged that the lead is oxidised to sulphite, not to sulphate; the sulphite is carried by the blast into a chamber and along flues where it comes into contact with hydrochloric acid gas, which, by its action on it, produces the chloro-sulphite. The hydrochloric acid gas is formed by sending a solution of salt into the flues in the form of a fine spray; the sulphurous acid in the furnace gases acting on the salt under such conditions decomposes it into hydrochloric acid and sulphite of sodium. From the flues the pigment is carried into large condensing chambers where it collects; from time to time the chambers are emptied of their contents, and the pigment washed with water and dried, when it is ready for use.
In a subsequent patent Mr. Hannay describes the production of the sulphate of lead from lead-fume, &c., by heating in a cupola furnace with a blast of air. The fume which is thus produced is carried through a kind of reverberatory chamber, where it is completely oxidised to sulphate; this is carried forward by the draught into large chambers where it collects. If white enough the pigment so obtained is sent out for use; if it has a grey colour due to the presence of metallic lead it is bleached by treatment with an acid. To improve the quality of the pigment it may be mixed with zinc oxide.
One difficulty which is met with in all these subliming processes is that of collecting the lead-fume or pigment. This necessitates the use of large condensation chambers or long flues to ensure the complete deposition of the material, or otherwise there is great risk of much of the material being carried forward by the draught into the chimney of the works and so out into the atmosphere where it is lost.
To provide these large chambers or long flues is not always possible, both on account of the cost of construction and of the space they require. To remedy this, Mr. Hannay has devised a special form of condenser constructed as follows:-A large closed chanıber made of iron is partially filled with water, the flue from