Ciiap. II.

COMPOSITION AND USE OF MINERALS.

6Co

ami also for washing poorer lead. Its principle is that of separating the different kinds ofore, hy passing them through plates full of holes of various sizes.

The ore when brought from the mine is grated, that is to say, those portions are thrownaside which are considered not to contain lead, and the large pieces of ore are broken upwith hammers, and made to pass through the crushing mill ; after which it comes on to aseparator, or broad plate with holes in it, of ^ inch in diameter ; that which remains on theplate and cannot get through the ^ inch holes is sent back to the crushing mill to be re-ground ; that which passes through the holes is carried by the water down to plate No. 2.,the holes of which are -j inch in diameter; what remains on this plate, or is too large topass through the holes, is taken to the sieve or the shaking apparatus, where the smallportions called smiddum go through, the cuttings are thrown out, and the small ore at thebottom is taken to the bingstead.

The chats which come out of the sieves or shaking apparatus are ground, and come to aplate No. 1., and the rough, which cannot pass through the holes, is taken off and sent backto the chat mill again; what falls through is carried by the water to the surface of plateNo. 2., which is laid in an inclined position; the smiddum passes over and the sludge goesthrough ; the former is then put into the shaking apparatus, to be separated in the samemanner as in other washing-places; the cuttings are taken out, and the lead is removedto the bingstead.

The Sludge Sejyarator is carried by water to the surface of plate No. 1.; the holes are 1 ofan inch in diameter; the rough sludge is carried off by the water and the small fallsthrough the holes, and runs into trunks or buddies, the rough passing through anotherhuddle.

The slime is made to pass over plate No. 1., when the lead drops through, and is carriedto the inclined plate No. 2.; the rough passes over, and the small, which falls through, isafterwards treated as we have already described. The lead which remains at the upper endof the huddle or trunk is put into the dolly-tub, and the matter at the lower end is put intoanother separator of the same kind.

The cuttings which come from the sieves are carried hy water to a small grate; thestones remain above the grate, and are then removed and thrown away; what falls throughgoes on to an inclined plate with holes in it; the rough is put back again to undergo thesame operation.

The Smelting Mills are buildings for reducing the crude ore to lead, and separating thesilver contained in it.

The operations consist of roasting the ore; smelting the roasted ore at the smelting hearth ;roasting and smelting the ore in one operation at the smelting furnace ; refining the metal, byexposing the lead to the flames of a reverberatory furnace, by which it is converted intolitharge, and the silver left behind ; separating the silver and lead, removing the greaterpart of the latter, and sending to the refining furnace the portion containing the silver.

Roasting the ore in a reverberatory furnace is precisely the same in principle as that adoptedfor puddling iron, and at the balling furnace used for heating the iron before passing throughthe rolls. A bing of lead ore is introduced at one time, and heated to ignition, but not tomelting; too little or too much heat is considered equally had. The flame of the firestrikes against the ore, and when it shows a yellow flame, it is stirred with a paddle oriron rod with a broad end. The stirring must be repeated five, six, or seven times in aheat. This depends on the nature of the ore, varying from one hour and a half to threehours. A barrow, containing a bing of ore, is wheeled from the bingstead, and placedover the furnace ready to be let in when required. When the ore is sufficiently roastedit is raked forward by degrees and let fall into a cistern, and the heated water which fliesup is prevented by an iron plate from reaching the workmen. A reverberatory furnace isone in which the flame and heat are carried forward by the draught of air, and dashedagainst the bodies to be smelted.

When one heat is over, another bing is let into the furnace, and the same work is re-peated. Two men are engaged eight hours at a time; they are succeeded by two others,who work eight hours ; the first set return and work another eight hours: in this mannerthey proceed day and night for four days in the week, it being considered a great saving offuel not to let the furnace cool.

Roasting the ore drives off the sulphur from the galena or sulphuret of lead, of which itis composed as well as the antimony and other matter more volatile than the lead. Thesmall dust ore is made to adhere together, whereas, if it were to be put into the smeltinghearth, and exposed to the blast, a great portion would be lost.

The roasted ore is let fall into water to prevent its forming unwieldy lumps, as it woulddo if left to cool in a heap. The ore is then taken from the water and carried to thesmelting-house.

Of the Smelting Hearth. — Its usual dimensions are 22 inches long and 22 broad, andabout the same in depth ; but these vary at different places. It is made of cast-iron, andcharged with half-melted matter of former operations, with peat and coal, and the roasted