The exterior walls n o p q may be built up to form two feeding troughs r, one on each side of the arch or roof, and each provided with an aperture or passage s, communicating with the inside or main chamber, and each aperture or passage is provided with a cover to be put on after the furnace has been charged.
In working with this furnace, crushed ore (native sulphuret of lead) and carbon, preferably in the state of pea or dust anthracite coal, are mixed in equal proportions; the mixture is ignited, and the fumes are oxidised by the blast through the mixture, which also promotes the combustion. Dense white vapours or fumes pass off, and are conveyed to a separate chamber, where they are strained by passing through a screen or series of screens of muslin or other textile fabric. Lime may also be employed in the furnace, in the proportion of 200 lb. of lime to 400 lb. of galena, although the addition of the lime is not necessary in all cases.
Instead of the furnace above described, a muffle furnace may be employed, in which the heat is applied indirectly to the ore, with the precaution of constructing the sole or bottom of this furnace of a material not rapidly acted on by the constituents of the ore; or a reverberatory furnace may be used in which the heat is directly applied. In both of these two cases reducing carbon may or may not be mixed with the ore.
Sometimes, generated gas is employed in place of coal, to effect the same result.
In a later specification Lewis remarks that when white lead pigment is manufactured from galena or other lead ores, in the raw state, or even in the roasted state, by subjecting them to the joint action of heat and air, either with or without reducing means, according to the quality of the lead ores used, the fumes are discoloured by particles of carbon or sulphuret of lead, or both, when they are caught in bags of textile fabric, and are unfit when in this state for use as a white pigment.
The fumes which are produced by this action of heat and air on galena or other lead ore are cooled and then collected in bags, and Lewis prefers to expose the so-collected products to the joint action of heat and air, to destroy or to burn out all the particles of carbon or sulphuret of lead, or both, by either throwing the said fumes on a bright clean anthracite or coke fire, with a blast from the sides or from below, or by throwing them over such fires or into a cupola furnace, or by throwing or blowing them into a generator gas flame, or through externally heated retorts. He then in either case collects the escaping fumes from the furnace or retort in bags or screening chambers.
The best process to be adopted depends upon the kind of fuel in the locality where the fumes are refined, and also upon the purity of the lead fumes. If they contain iron, clay, or the like, it is best to throw them into a coal fire; but if they are pure, one process is about as effective as the other, and the degree of purity of the fuel decides the kind of heating apparatus to be used.
There being such great difference in the purity of fuels, and this irregularity not allowing of uniform results, Lewis prefers to use a furnace in which the flame and heat are produced by burning gaseous fuel with air, which is forced into the furnace with the fumes which have been collected in a previous process.
[Fig. 25] represents a furnace which may be advantageously employed for the purpose when gas is used as fuel.
a represents a blower, into which the fumes are fed from the hopper b. The fumes, being thoroughly mixed with air in this blower, are forced into a chamber c, and then through a series of tuyeres d. At the same time, gas from a generator or producer is admitted by the flue e, and is burned by the incoming blast from the tuyeres d; the volatile fumes produced in the furnace f pass through it and out of the flue g, and are collected in bags or screening chambers. By using gas fuel, which is easily and fully burned, and clean to handle, a fine white pigment is produced.