This section is from "Scientific American Supplement Volumes 275, 286, 288, 299, 303, 312, 315, 324, 344 and 358". Also available from Amazon: Scientific American Reference Book.
[Footnote: Lately read before the Institute of Mechanical Engineers.]
By NORMAN C. COOKSON, of Newcastle.
The author began by stating that probably in few trades have a smaller number of changes been made during recent years, in the processes employed, than in that of lead smelting and manufacturing. He then briefly noted what these changes are, and went on to describe the "steam desilverizing process," as used in the works of the writer's firm, and in other works licensed by them, which process is the invention of Messrs. Luce Fils et Rozan, of Marseilles. It is one which should commend itself especially to engineers, as in it mechanical means are employed, instead of the large amount of hand-labor used in the Pattinson process. It consists in using two pots only, of which the lower is placed at such a height that the bottom of it is about 12 in. to 15 in. above the floor level, while the upper is placed at a sufficiently high level to enable the lead to be run out of it into the lower pot. The capacity of the lower pot, in those most recently erected, is thirty-six tons--double that of the upper one. Round each pot is placed a platform, on which the workmen--of which there are two only to each apparatus--stand when skimming, slicing, and charging the pots. The upper pot is open at the top, but the lower one has a cover, with hinged doors; and from the top of the cover a funnel is carried to a set of condensers. At a convenient distance from the two pots is placed a steam or hydraulic crane, so arranged that it can plumb each pot, and also the large moulds which are placed at either side of the lower pot. The mode of working is as follows:
The silver lead is charged into the upper pot by means of the crane. When melted, the dross is removed, and the lead run into the lower, or working pot, among the crystals remaining from a previous operation.
When the whole charge is thoroughly melted, it is again drossed; and in order to keep the lead in a thoroughly uniform condition, and prevent it setting solid on the top and the outside, a jet of steam is introduced.
To enable this steam to rise regularly in the working pot, a disk-plate is placed above the nozzle, which acts as a baffle-plate; and uniform distribution of the steam is the result. To quicken the formation of crystals, and thus hasten the operation, small jets of water are allowed to play on the surface of the lead.
This, it might be thought, would make the lead set hard on the surface; but the violent action of the steam acts in the most effectual manner in causing the regular formation of crystals. Owing to the ebullition caused by this action of the steam, small quantities of lead are forced up, and set on the upper edges and cover of the pot. From time to time the valve controlling the thin stream of water playing on the top of the charge is closed, and the workman, opening the doors of the cover in rotation, breaks off this solidified lead, which falls among the rest of the charge, and instantly becomes uniformly mixed with it.
Very little practice enables an ordinary workman to judge when two-thirds of the contents of the big pot are in crystals, and one-third liquid; and when he sees this to be the case, instead of ladling out the crystals ladleful by ladleful, as in the old Pattinson process, he taps out the liquid lead by means of two pipes, controlled by valves, the crystals being retained in the pot by means of perforated plates.
The liquid lead is run into large cone-shaped moulds on either side of the pot; and a wrought iron ring being cast into the blocks thus formed, they are readily lifted, when set, by the crane. To give some idea of the rapidity of the process, it may be mentioned that from the time the lead is melted and fit to work in the big pot, to the time that it is crystallized and ready for tapping, is, in the case of a 36 ton pot, from thirty-five to forty-five minutes; and the time required for tapping the liquid lead into the large moulds is about eight minutes.
Before the lead begins to crystallize, the upper pot is charged with lead of half the richness of that in the lower pot. Thus, when the liquid lead has been tapped out of the lower pot, it is replaced by a similar amount of lead of the same richness as the remaining crystals, by simply tapping the upper or melting pot, and allowing the contents to run among the crystals.
The same operation is repeated from time to time, until the crystals are so poor in silver that they are fit to be melted, and run into pigs for market.
The large blocks of partially worked lead are placed by the crane in a semicircle round it, and pass successively through the subsequent operations. The advantages of the steam process, as compared to the old six-ton Pattinson pots formerly used by the writer's firm, are: (1) a saving of two-thirds amount of fuel used; (2) the saving of cost of calcination of the lead to the extent of at least four-fifths of all that is used; (3) above all, a saving in labor to the extent of two-thirds. The process has its disadvantages, and these are a larger original outlay for plant, and a constant expense in renewals and repairs. This is principally caused by the breakage of pots; but with increased experience this item has been very much reduced during the last two or three years.
The "zinc process" of desilverizing, which is largely used by Messrs. Locke, Blackett & Co., and was patented in the form adopted by them about fourteen years since. The action of this process is dependent on the affinity of zinc for silver. The following is a brief description of it:
A charge of silver lead, usually about fifteen tons, is heated to a point considerably above that which is used in either the Pattinson or the steam process. The quantity of zinc added is regulated by the amount of silver contained in the lead; but for lead containing 50 oz. to the ton, the quantity of zinc used is in most cases about 1½ per cent, of the charge of lead. The lead being melted as described, a portion of this zinc, usually about half of the total quantity required for the charge, is added to the melted lead, and thoroughly mixed with it by continued stirring. The lead is now allowed to cool, when the zinc is seen gradually to rise to the top, having incorporated with it a large proportion of the silver. The setting point of zinc being above that of lead, a zinc crust is gradually formed, and this is broken up and carefully lifted off into a small pot conveniently placed, care being taken to let as much lead drain off as possible. The fire is again applied strongly to the pot, and when the lead is sufficiently heated, a further quantity of zinc, about one-third of the whole quantity used, is added, when the same process of cooling and removing the zinc crust is repeated. This operation is gone through a third time with the remaining portion--¼ per cent.--of zinc; and if each of these operations has been carefully carried out, the lead will be found to be completely desilverized, and will only show a very small trace of zinc. In some works this trace of zinc is allowed to remain in the market lead, but at Messrs. Locke, Blackett & Co.'s works it is invariably removed by subjecting the lead to a high heat in a calcining furnace. The zinc crusts, rich in silver, are freed as far as possible from the lead by allowing this to sweat out in the small pot, after which the crusts are placed in a covered crucible, where the zinc is distilled off, and a portion of it recovered. The lead remaining, which is extremely rich in silver, is then taken to the refinery, and treated in the usual manner. The writer is given to understand that the quantity of zinc recovered is as high as from 50 to 60 per cent. of the total quantity used.
Although it was said that the rolling or milling of lead remains unchanged in its main features since the first mill was established, yet the writer's firm have introduced many important improvements. When lead is required for sheet making, instead of running out the market lead into the usual pigs of about one hundredweight each, it is run into large blocks of 3½ tons. These 3½ ton blocks are taken on a bogie to the mill-house, where the mill melting pot is charged with them by means of a double-powered hydraulic crane, lifting, however, with the single power only.
Three such blocks fill the pot, and when melted are tapped on to a large casting plate, 8 ft. 4 in. by 7 ft. 6 in., and about 7 in. thick. This block, weighing 10½ tons, is lifted on to the mill table by the same crane as fills the pot, but using the double power; and is moved along to the rolls in the usual manner by means of a rope working on a surging head. The mill itself, as regards the roll, is much the same as those of other firms; but instead of an engine with a heavy fly-wheel, always working in one direction, and connected to the rolls by double clutch and gearing, the work is done by a pair of horizontal reversing engines, in connection with which there is a very simple, and at the same time extremely effectual, system of hydraulic reversing. On the usual method there is no necessity for full or delicate control of lead mill engines; but with this system it is essential, and the hydraulic reversing gear contributes largely to such control. This may be explained as follows:
In all other mills with which the writer is acquainted, when the lead sheet, or the original block, has passed through the rolls, and before it can be sent back in the opposite direction, a man on either side of the mill must work it into the grip of the rolls with crowbars.
In the writer's system this labor is avoided, and the sheet or block is fed in automatically by means of subsidiary rolls, which are driven by power. When it is required to cut the block or sheet by the guillotine, or cross-cutting knife, instead of the block being moved to the desired point by hand-labor, the subsidiary driven rolls work it up to the knife; and such perfect control does the engine with its hydraulic reversing gear possess, that should the sheet overshoot the knife 1/8 in., or even less, the engine would bring it back to this extent exactly.
Another point, which the writer looks upon as one of the greatest improvements in this mill, is its being furnished with circular knives, which can be set to any desired width, and put in or out of gear at will; and which are used for dressing up the finished sheet in the longitudinal direction. This is a simple mechanical arrangement, but one which is found to be of immense benefit, and which, in the writer's opinion, is far superior to the usual practice of marking off the sheet with a chalk line, and then dressing off with hand knives. The last length of the mill table forms a weighbridge, and a hydraulic crane lifts the sheet from it either on to the warehouse floor or the tramway communicating with the shipping quay.