The two most important sources of metallic lead are its natural sulphide (galena) and its carbonate (white lead ore). The ore, when mined, is hand - sorted free from refuse, broken small, and washed clean. It is then treated by one of the following methods for the extraction of the metal.
The ore - hearth or Scotch furnace is a form of blast - furnace well suited for the treatment of ores containing little silica; here it is used on poor ores, but on the Continent (Przibram in Bohemia and Pesey in Savoy) it is employed on the richer Kinds. It is one of the simplest forms of furnace, as may be seen from the illustration of it in Fig. 135. It consists merely of a small forge, measuring about 2 ft. high and 1 1/2 by 1 ft. in area, arched above to allow the "fume" or vapours to be carried through long flues (sometimes upwards of 3 miles), wherein much oxide and sulphate of lead are condensed and recovered. The operation involved is simply oxidation of the sulphur com bined with the lead, thus: PbS+ 20= Pb + SO2. The temperature necessary is therefore but small, and must never rise to such a point as to endanger fusion of the sulphide. The ore is previously spread on the bed of a reverberatory furnace in charges of about 1/2 ton, and gently roasted for some 8 hours with constant stirring. By this step much of the antimony and part of the sulphur are expelled, and the hot roasted ore is raked out into a tank of water, where it breaks up.
Thus reduced in size, it is charged into the ore - hearth, whose sides and floor are lined with cast - iron plates, leaving an opening about 1 ft. high in front; this is occupied by the " work stone," a sloping iron plate, upon which the materials are drawn for inspection as the operation proceeds. Quantities of 25 to 30 lb. of ore are introduced at a time, and the draught is stopped every 10 minutes to rake out halt - fused and exhausted matters, and to replenish the fuel and mineral. The draught enters at a, about midway down the back of the furnace b; c is the work stone; d is a cast - iron pot which catches the molten lead running from the floor of the hearth, and heated by a separate fire e. This plan is apt to entail considerable loss of metal by vaporization and to occasion unpleasant fumes; but it is eminently simple, the low temperature renders the metal free from hard impurities, and requires but little fuel, which may be of the poorest description. In Scotland, peat blocks are used, the furnace being filled with them at starting. The first charge of ore is " browse," the mixture of cinders and half - fused ore from a previous operation; this is immediately preceded by a little coal, to momentarily increase the heat.
At short intervals the charge is withdrawn on to the work stone for manipulation. Shining glassy lumps of "grey slag" (a mixture of silicates of lead and lime) are picked out for treatment at a higher temperature in the "slag hearth" (p. 342); and the cleansed browse is returned into the furnace. A little lime is added to regulate the readiness with which the browse fuses, according to circumstances. Each time a new charge is introduced, the entrance to the blast - pipe is closed to exclude dust. According to Dr. Ballard, the ore hearth appears to be the only plan adopted for lead - smelting in Scotland, Northumberland, Durham, and Cumberland. The form used by Cookson and Co., of Howdon, near Newcastle, is shown in Fig. 136. It consists of an oblong cast - iron tank or well a, about 2 ft. 6 in. wide, 2 ft. from front to back, and 6 in. to 1 ft. deep, capable of containing 12 cwt. to 2 tons of lead, with which it is filled to the brim, the surface of the lead forming, in fact, the floor of the hearth. The floor thus formed is enclosed at the sides with blocks of cast iron, and another block of cast iron is placed behind, and is perforated for the passage of the twyer 6, which conducts the blast into the furnace about 2 in. above the surface of the lead in the well.
A brickwork shaft c proceeds upwards from the hearth to the flue, and behind it is blind flue or pit d, into which the "hearth ends" (dusty matter which comes off with the "fume") may fall, and from which it is removed as convenient by the door e. The front opening to the hearth is sometimes provided with a sliding shutter /, which, by means of a counterpoise, can be raised or lowered in its grooves so as nearly to close in the front of the hearth. Extending forwards from the front of the hearth, and inclining downwards at an angle from it, is an iron plate called the "fore - stone" or " work - stone," in which is a groove h leading towards an iron pot i, kept hot by a little fire j beneath. The ore is fed in either from the front or through a hopper k at the side. The chimney of the fire which keeps the lead pot hot is shown at l. The fuel used may be either peat or coal, the latter being general. A fire being made upon the hearth, and piled up chiefly behind, a moderate blast is put on, the ore (sometimes previously calcined) is thrown in, and the shutter (if there be one) is let down.
After the lapse of a few minutes, the workman introduces a poker and stirs up the fuel and ore, and from time to time repeats the process with fresh small quantities of ore, adding fuel as it appears requisite. At intervals of a few minutes, he raises the shutter, draws forward a portion of the charge on to the fore - stone, and picks out from it portions of "grey slag," which he pushes aside and ultimately throws off on the floor of the workshop at the side of the hearth. As the lead forms, it runs into the well, and overflows along the channel of the fore - stone into the pot set to receive it, from which it is ladled into moulds. It is a process which requires constant manipulation of the charge, 2 workmen being continually occupied in adding ore 3 or fuel, poking up the charge, etc, at intervals of a few minutes. Lime is used, as in the Flintshire process, to thicken the slags. (Dr. Ballard's Report.)