Lead Part 14 300151

Sheet - Lead

As lead is made into sheets by rolling, the first process to which the " softened " and desilverized pigs are subjected is melting and casting into cakes of suitable size. About 10 tons are commonly melted at one time, and the liquid metal is, on raising ■ valve in the melting - pot, allowed to escape directly into the iron mould, which receives about 4 1/2 tons. Lifting - hooks are previously adjusted trader the cake, so that, when solidified, it can be lifted out of the mould and carried by a crane on to the table of the rolling - mill. The size of the cake is usually 7 ft. 10 in. by 5 ft. After remaining in the mould some days to cool, it is lifted out, and the rough edges are trimmed round with an adze. It is then placed upon the rolling - mill table, in the direction of its least width - that is, so that the breadth of 7 ft. 10 in. shall be invariable, while the 5 - ft. length is increased by the rolling. The mil} is a single cylinder of cast iron, 30 in. in diameter and nearly 9 ft. long. It is geared directly to the main driving - engine, and has the adjustments common to all rolls. The table is com-posed of small rollers arranged in 2 parallel lines.

Between them is a rack, the teeth:of which serve as fulcra for the insertion of levers, whereby' the cake is pushed under the cylinder. The rolling once begun, the metal passes to and fro under the roll about 150 times, so that it is finally reduced to a sheet 30 ft. long and of the width above stated. It then weighs 30 lb. to the sq. ft. This sheet is cut into smaller pieces by vertical knives, which are actuated by a screw, so that they move across the table and between the cylinders on which the metal rests. The smaller sheets are now rolled into 18 - ft. lengths, and vary in weight from 2 1/2 to 10 lb. per sq, ft. These constitute the usual sheet - lead, stock - the variation being 1/2 lb. to the ft. up to 5 lb., and 1 lb. per ft. thereafter. Their principal use is in chemical works for lining acid - proof receptacles, notably sulphuric - acid chambers and concen - trating - pans, and also for the interior of tanks, cisterns, etc.

Cookson & Co. have introduced an improvement in sheet lead rolling, as follows: - When, lead is required for sheet - making, instead of running out the "market "lead into the usual pigs of about 1 cwt. each, it is run into large blocks of 3 1/2 tons, and by so doing a very great saving both of time and labour is obtained. These 3 1/2 - 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 fi11 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 10 1/2 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 rolls, is much the same as those of other firms; but instead of an engine with - a heavy flywheel, 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.

In 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, 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 Cookson's system this labour 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 - labour, 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 is one of the greatest improvements in this mill, is its being, furnished with circular knives, which can be. set to any desired width, be 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 is far superior to the usual practice of marking; off the sheet with the chalk - line, and than dressing off with hand - knives. The last length of the mill - table forms a weigh - bridge, and a hydraulic crane lifts the sheets from it either on to the warehouse floor, or to the tramway communicating with the shipping - quay. Sheet - lead is also used in the manufacture of the to - called "tinfoil." A sheet of lead is placed between 2 of tin, and the whole is rolled and rerolled until the thin material with which all are familiar is produced. The tin nerves in this case simply as a covering for the lead, and prevents the latter metal communicating its deleterious properties to substances enclosed in it.

The making of sheet - lend for lining tea - chests, etc, is an important industry of Hong - kong. It is made principally in the western districts. On entering one, workmen will be seen with shears cutting out the sheets of lead to the required sizes and shapes. The shears are simply large scissors firmly fixed to a solid block of wood some 2 ft. in height; the lower blade of the shears terminates in a square piece of iron instead of being pointed, as is the upper blade. The sheets of lend are of small size, and somewhat irregular in shape, and this arisen from the method of manufacture. Beneath an iron pan, raised 12 in. or so above the ground, and carefully finished off, is a furnace, and at the side of the pan next the wall is the flue communicating with it. In this pan the lend is melted; when sufficiently hot, the workman takes 2 large square paving - tiles, and smoothly and carefully cavers them with several layers of unsized paper. Having placed these tiles before him, one above the other, the workman raises the upper one with his left hand, and, taking a ladle of the proper size in his right, dips it in the melted lead and then pours its contents on to the lower tile. He then drops the upper tile and quickly presses the lead out into the form of a sheet.