Where the stamp mill cannot be built low enough to enable the rock cars to run from grade into the space above the rock bins, the cars must be hauled up, one by one, by special haulage mechanism. Where the mill is built with the main lower floor but slightly elevated above the level of the dumping ground, the stamp sand may eventually have to be carried long distances in launders and sometimes must also be lifted to the necessary height by immense sand wheels. Where mills have been built on the exposed shores of Lake Superior the waste sands are removed from time to time by storms, but if built on bays or sheltered places, the heaping up of the sand is likely to give trouble unless the waste launders from the mill are sufficiently elevated above lake level.
The function of the stamp mill is to crush the rock fine enough to be able to separate it from the copper and to recover a maximum amount of the latter. The copper rock from the mine is dumped from the cars into the stor age bin at the back of the mill, which is of sufficient capacity to provide against slight delays incident to transportation. From the bottom of the bin the rock runs over a feed pan into the mortar of the steam stamp, into which also a stream of water is constantly flowing. Here the rock is crushed to a size small enough to be splashed by the downward action of the stain shoe through screens around the mortar. The larger lumps of cop per are withdrawn from the bottom of the mor tar through a tube against the force of a small stream of water which prevents the lighter material from discharging with them. The principle of the so-called hydraulic discharge is utilized in some form at several stages of the stamp mill process. The particles of rock and copper that pass through the screen around the mortar then go through a cylindrical revolv ing screen or "trommel,) the oversize from which under a practice that is now going out of use is carried back to the stamp, or, under better practice, passes directly to a set of rolls that crushes it to a smaller size,. thus liberating the included copper and increasing by so much the capacity of the stamp. The entire product from the screens and from the rolls then passes through launders to the classifiers and from these the coarser part of it goes to the jigs. In some cases there is placed under the launders near the stamp-head a box into which by apply ing the principle of hydraulic discharge, the larger particles of copper fall, the smaller par ticles of copper and nearly all of the rock pass ing on to the classifiers. A classifier is a part of the launder system, or a long box through which flows the stream of water from the heads, mixed with finely crushed rock and copper particles; in the bottom of the classifier at stated intervals are openings through which particles of copper and rock drop against a head of water at each opening that can be regulated so as to permit particles of different weight to fall out at the different openings, the heaviest through the first opening, the lightest through the last. On the jigs suc
cessively smaller particles of copper are caught on screens of from 10 to 16 mesh, through which gradually sift the very fine copper par ticles; above the copper caught on the screens accumulate particles of rock which include copper called The larger particles of waste rock are carried off gradually at the surface by the jigging motion and by the flow of the water, to the waste launder. The mid dlings are taken off automatically through an air discharge in front of the jigs and are ground in a Chilian mill, whence they pass to the slime tanks. The fine material passing through the jig sieves goes directly to concen trating tables. The finest particles of rock and of topper that flow through the classifiers from the heads go from these to the settling tanks, where the fine particles of copper with a cer tain amount of rock material settle gradually to the bottom, and the overflow from the top of the tanks goes to the waste launders. The set tlings are then treated on tables provided with elevated strips or "riffles) and by a quick re peatedjerky motion in the direction of their length the heavier particles of the pulp, namely, the copper, are gradually separated from the rock matter and collected at the end of the table in tanks or barrels. Middlings or material containing a good deal of rock and some cop per from along the irregular line of demarca tion between copper and rock are collected and retreated in the same way.
Smelting.— The product of the mine that goes directly to the smelter is in the form of masses, often several tons in weight. This "mass* copper with the product of the stamp mill constitutes what is called "mineral.* Large pieces of rock crushed in the breakers at the mine frequently contain small masses which are collected from the rock as the latter goes into the stamp-heads and shipped to the smelter in barrels. This product is called °barrel work.* Some pieces of copper as large as one's fist unavoidably get into the stamp-heads and these are taken out through the hydraulic discharges as described above' and are shipped as "head ings? The finer copper mixed with more or less rock is collected in different grades, the number of these being sometimes as many as six, but in modern practice being reduced to as few as two. The smallest of these grades are called *fines) and 'slimes,' of which many par ticles are often minute and light enough to float on a moderate current of water, and hence are difficult to save.