Pigments, Paint and Painting: A practical book for practical men

by George Terry

[Fig. 18] is an elevation of their improved cupola furnace; [Fig. 19] is a corresponding vertical section; and [Fig. 20] is a horizontal section as at the level of the principal tuyeres.

This cupola furnace is formed with a deep hearth a (like the American lead smelting cupola) having a siphon outlet b, for withdrawing molten lead when necessary. At a level a little above the siphon outlet b, an outlet c for slag and scoria is provided, and the main blast tuyeres d enter at about the same or a slightly higher level. With this arrangement, a considerable depth of melted lead is constantly maintained, and the choking of materials is thereby avoided at the level of the tuyeres. For a short distance above the main tuyeres d, the interior of the furnace is made with the sides e moderately and gradually widening upwards, and is afterwards continued upwards of uniform diameter or width. The charging door f is between 3 and 4 feet above the main tuyeres d, and the space above the charging door is crossed by two or more arched diaphragms g h, of brickwork, having irregular openings in them, there being doors i k in the side of the furnace above each diaphragm, for inspection and cleaning. The purpose of these diaphragms is to cause the air, gases, and sublimate to become thoroughly intermixed and to ensure the complete combustion of any black smoke and the oxidation of any sulphide of lead that may be present.

An upper series of small tuyeres or jet pipes l is provided for admitting air a little below the lowest diaphragm g, these jet pipes l being supplied by a branch pipe m from the main air blast pipe n, it being found that the introduction of the air and the production of heat can be best regulated by supplying a portion at the upper part of the furnace in this way, in addition to that supplied by the main tuyeres d; and a tap or valve o is fitted on the branch pipe m for adjusting the supplementary supply thus admitted. Above the highest perforated diaphragm h, the interior of the furnace communicates with a lateral flue p, through which the gases and sublimate pass; and at a part of this flue p, sufficiently near the surface for the gases to be still hot enough, an enlargement or chamber r forming a descending part of the flue, is constructed. Into this chamber r a solution of chloride of sodium is introduced as a spray from a number of jet pipes s, for the purpose hereinbefore explained. This chamber r is provided with a door t at its lower part, for periodically removing matters that become deposited in it. The continuation u of the flue communicates with the lower part of the chamber r.

Preferably the spray of chloride of sodium is fine enough to allow of all or nearly all of the water being instantly evaporated, so as to leave the salt in fine particles, and in a favourable condition for being acted upon by the sulphurous acid, steam, and oxygen present in the gaseous currents. The temperature of the chamber r should not be allowed to fall below red heat. The proportion of chloride of sodium used will depend on the amount of chlorination of the lead that may be desired, in addition to what is necessary for converting the zinc and other metals into chlorides. In practice a proportion of salt 2½ to 5 per cent. of the weight of the sublimate formed answers the purpose and yields a good product, but a larger proportion may be used without injury.

It is of great importance to keep the temperature of the upper part of the furnace steadily at a red heat and flaming, as the colour of the sublimate will be inferior if the temperature is either too high or too low. To facilitate the proper regulation of the temperature, a pyrometer (which may be similar to the kind used in ironworks) is employed, which pyrometer is placed in the flue at a distance from the furnace where it cannot be injured; and by a few trials is ascertained what temperature should be indicated by the pyrometer when the temperature in the furnace is what it should be. This point having been ascertained, a glance at the pyrometer will at any time show whether the furnace is working properly or not. The inventors also provide for rapidly cooling the upper part of the furnace without interfering with the lower part, in the event of the heat becoming too great, by arranging a water pipe w, with a set of jets, round the top of the furnace, so that on turning a tap on the supply pipe a spray of water may be applied to the outside of the furnace; and as it is desirable that water applied in this way should not run down to the lower part of the furnace they build gutter plates x into the sides of the furnace just above the charging door f, to lead off any surplus water to a drain pipe.

Sufficient hydrochloric acid may be formed or introduced, as hereinbefore described, not only for forming chloro-sulphite of lead in the condenser, but also for saturating all the free oxide of lead, and for combining with and rendering soluble any iron, zinc, antimony, silver, or other metals. The chlorides thus formed become dissolved in the water of the condenser, and the solution, separated from the insoluble white pigment, may be treated by known processes for recovery of the metals. When the lead ores or other lead-yielding materials contain silver to a greater extent than 5 oz. per ton, a notable quantity of the silver is volatilised, and if it is left in the white pigment it renders the latter sensitive to sunlight; whereas if rendered soluble in the manner hereinbefore described, and separated by any of the known processes, it becomes a source of profit.

The white pigment is washed in the ordinary way; and when chloride of zinc is not completely removed by washing, a small quantity of sulphuric acid may be mixed with the pigment, by adding the same to the last washing water, to convert the chloride of zinc into sulphate, which is not hygroscopic.

The white pigment made as hereinbefore described is a very good and economical material for manufacturing into chrome yellow, this being done by mixing a solution of any suitable chromate or bichromate with the wet pigment; whilst the chrome yellow thus obtained may be converted into chrome orange or red by treating it in the usual way.

Italian Process.—The precise period of the introduction of white-lead manufacture in Italy is unknown, but it was certainly previous to the beginning of the present century. Prior to 1881, the Dutch process was exclusively used in Italy. In 1881 the so-called Brumlen and Dahn process was introduced into Liguria. Somewhat later the Rhenish process was introduced. The Rhenish process is one in use in nearly all the Italian white-lead manufactories. It is employed in a large manufactory at Cogoleto, as follows:—Lead in thin sheets of about 3 feet in length, and 4 inches in width are placed in a clay chamber having the form of a cube, of about the capacity of 5800 cubic feet. In this chamber there is a wooden framework, upon which are hung the sheets of lead. Three of these sheets weigh together about 4½ lb., and as the capacity of the chamber is about 20 tons, it can hold about 30,000 sheets of lead.

On the floor of the chamber are placed twenty-four copper receptacles, each having four circular apertures. These receptacles are all in direct communication with a large pipe of masonry, which, by means of a copper tube, receives the gas coming from a boiler and furnace placed under the chamber. In the boiler, which is also of copper, is placed a mixture of 900 parts of water, and 80 parts of acetic acid concentrated to 40°, and the capacity of the boiler is about equivalent to 25,882 gallons. The furnace serves the purpose of producing carbonic acid.