« PreviousContinue »
THE TREATMENT OF AURIFEROUS ORES IN COLORADO. *
It is proposed to describe in the present chapter the processes and apparatus generally employed in Colorado for the treatment of gold ores, leaving out of consideration the operations of mining proper.
1. In the mine there is generally no further separation than that which the miner effects in drilling and blasting, by endeavoring to obtain the pay ore distinct from the barren gangue. All the rock thrown down is generally hoisted to grass; but the two classes are, as far as practicable, kept apart. There is no sorting of the ore underground, according to the size, richness, or mineralogical composition of the fragments.
2. Above ground the rock is roughly sorted, with the aid of a limited amount of spalling, into two classes, waste and mill rock. The waste amounts, generally, to one-half or two-thirds of the whole. This sorting is often accompanied with a selection of the larger masses of pure ore, rarely more than 10 per cent. of the mill rock. The ores consist principally of iron and copper pyrites, frequently associated with other ores of copper and galena, and zinc-blende. The gangue is a mixture of quartz and feldspar.
The further preparation or concentration of the ores is, in most cases, intimately connected with the extraction of the free gold. It consists in crushing by means of stamps, and simultaneous amalgamation, after which the tailings, in many cases, are at once discharged into the rapid creek. Most frequently, however, by various methods, the heaviest portions, with a part of the gold which has escaped amalgamation and of the quicksilver lost by the apparatus, are more or less completely recovered.
I. THE CRUSHING IN STAMP-MILLS.
ARRANGEMENT OF THE BATTERIES. (Figs. 1 and 2.)
These have universally a wooden frame and a cast-iron mortar. The stamps, shaft, and cams are of iron.
As a rule, the ground is excavated down to the not very distant bedrock. Upon this are firmly laid, let in, or set in masonry, a number of longitudinal sills, a, and upon them the cross-sills, b, about 1 foot square and 10 to 14 feet long, and corresponding with the number of stamps. At right angles to these is the battery-log, c, 22 to 30 inches square, of the best pine, the upper surface of which is at the level of the mortar-bed.t
Upon the battery-log the posts, d, are erected. pine, 18 to 20 inches wide and 10 to 12 inches
They are likewise of thick, and about high
* A series of articles on this subject, from the pen of Mr. Albert Reichenecker, of Central City, Colorado, recently appeared in the German Berg-und-Hüttenmännische Zeitung. With the permission of the author, who has also furnished me with original drawings to illustrate this chapter, I translate a large portion of his treatise. It has seemed best not to alter or interrupt Mr. Reichenecker's text; and I have, therefore, put my own observations and comments in the form of foot-notes, with my initials.—R. W. R.
The battery-log or mortar-block here described is not so good as the vertical timbers used for the same purpose elsewhere, and to some extent in Colorado also.-R. W. R.
enough to reach to the upper end of the stamp when it rests on the mortar-bed. The posts are maintained in their upright position by the mortised and bolted guides, g, g', above and below, 8 to 10 inches deep by 7 to 9 inches wide, and by the stays and braces, e, f, on the side of the discharge.
The lower part of the mortar, h, is a solid casting, provided with flanges, through which it is bolted to the battery-log. Two longitudinal sills, i, 6 to 8 inches thick, not quite so high as the cast-iron mortar, but reaching down somewhat over the battery-log, prevent a side movement of the mortar. Frequently the above-mentioned bolts are omitted; but there is cast on each side of the mortar-bed, through its whole length, a flange about two inches wide, on which these sills are firmly laid.
The upper guides, g', lie not more than 13 foot below the upper end of the battery-posts, and the lower guides as low as the stamp-head will permit. This brings them about 6 or 7 feet apart. The guide proper consists below of a cast-iron thimble, g2, consisting of two halves with flanges, which is let in between the guide-timbers, to the rear one of which it is bolted through the flanges. Above, the guide consists of two 3-inch planks, cut out to fit the stems of the stamps, and bolted to the rear timber, which is here the only one. Wooden wedges, driven between the planks, keep them at the proper distance apart to give play to the stamps. Frequently the lower guides are arranged in the same manner, but with both front and rear timbers.
As has been remarked, the battery-box is, up to about 3 inches below the discharge, cast in one piece with the mortar-bed.* The latter is horizontal, 3 to 4 inches thick, and provided under each stamp with a recess about an inch deep, in which is set the cast-iron die, k,† 3 to 4 inches high, slightly decreasing in size toward the top. The upper surface of the die is greater by perhaps half an inch in diameter than the shoe of the stamp. The upper part of the die is of chilled iron; the lower part, say one inch of the height, of softer iron, fits into the recess in the mortar-bed, and is generally circular, though sometimes polygonal in section. The die, like the shoe, can be easily changed when worn out, and in this way the destruction of the mortar-bed is prevented. The iron walls of the battery-box are 14 to 2 inches thick at the bottom, and decrease in thickness somewhat, while the box itself · widens a little toward the top.
The plank housings, l, are fastened to the longitudinal sills, i, and neatly fitted at sides and bottom. They usually reach to the lower guides, and completely inclose the battery-box, with the exception of the
*The latest California pattern of mortar is much higher than this, having bottom and walls cast in one piece. There are several decided advantages in the high mortars. I know of none possessed by the low ones, except greater cheapness of first cost and freight.-R. W. R.
+In the stamp-mill attached (for dry crushing) to the reduction works of Huefeden & Co., at Georgetown, a single solid block is used, instead of a number of dies. This is the form adopted in Germany and Hungary. It permits the use of both sides, by simply turning over the bed when the upper side is worn by the stamps into depressions, and it is claimed that a considerable gain in economy results. One objection that occurs to me is the necessity of turning or changing the whole block when it happens to be worn away in a single spot, though the remainder may still be serviceable. It is doubtful, however, whether this counterbalances the advantages of simplicity and cheapness and the ease with which the whole can be removed at one operation, to clean out the mortar. A more serious objection, perhaps, is the fact that such a die-block wears in circular depressions, which may diminish the effectiveness of the blow of the stamp; while the projecting circular die presents always a prominent surface, clears itself of pulp, and thus brings the quartz always between the iron faces of shoe and dic.-R. W. R.
feed-slit, r, and the discharge, s. The bolts, m, hold the housings firmly to the posts, and wedges driven between them and the guides press them closely to the sills below.
Each battery contains usually four or five, now and then three, seldom six stamps.
The iron stamp consists of four parts: the head, the stem, the collar, and the shoe. These are all cylindrical in form.
The cast-iron head, A, has the same diameter as the shoe, and is 18. to 20 inches long. Its lower surface is provided with a conical recess 6 to 7 inches deep, to receive the shank of the shoe, and a similar recess is bored in its upper surface for the lower extremity of the stem. The interior end of each of these recesses is intersected by a mortise passing through the stamp-head, so that by driving a wedge through this opening the shoe or stem may be loosened and forced out when necessary. In some cases the lower end of the head is turned down in the lathe to receive a wrought-iron ring, 2 to 1 inch thick and 1 to 2 inches high.
The wrought-iron stem, B, has a length of 9 to 11 feet, and varies in thickness, according to the weight of the stamp, from 2 to 3 inches. The lower end is turned to a somewhat conical form to fit the recess in the top of the head. The upper end is also turned down a little for some distance to permit the collar or tappet to be slid over the stem to its place. In some cases the stem has a screw-thread in the neighborhood of the collar, and the latter has a place for a key, by means of which the collar can be screwed up or down on the stem, and then keyed fast in the desired position.
The collar, C, consists of a hollow cast-iron cylinder, 5 to 8 inches in height, 2 to 3 inches thick below, and somewhat conical above. The interior is turned to fit the stem, or with a screw-thread. The lower working surface is sometimes protected with a steel ring; but ordinarily the lower, cylindrical part of the collar, about 2 inches high, is merely made of chilled iron.
The shoe, D, is always of cast iron, the butt, up to within 3 of an inch of the shank, being chilled and the rest cast in sand. The shank is 4 to 6 inches long, having below half the diameter of the butt, and con tracting conically upwards, as shown in the engraving. The diameter of the shoe varies between 6 and 10 inches, the ordinary limits being 7 and 9 inches.
In setting up the stamp, the stem is driven into the head, the collar either simply slipped over and driven down or screwed on, and then keyed fast. The shoe is set in the mortar under the stamp, the shank is surrounded with thin wooden wedges, pointed upwards and kept in place with a string. Then the stamp is allowed to fall, and the shank wedges itself into the head with sufficient firmness to remain fixed. With every subsequent fall of the whole stamp the several parts tend to wedge more tightly into one another. The only exception is the collar, in case it is screwed on.
The weight of the stamp varies between 300 and 750 pounds.* Most
According to Mr. Hague, the Colorado stamps, as a general rule, are heavier, run more slowly, and with greater fall than is usual in the mills of California and Nevada. Some of them weigh 900 pounds each; and although the mills of most recent construction have generally adopted a 500-pound or 600-pound stamp, the average is probably somewhat higher than that at present. Personally, I incline to the opinion that 600 pounds will be found a practically convenient weight, permitting on one hand a high rate of speed, and securing a sufficiently powerful blow with a moderate fall. As regards speed of running, see remarks in my last report, and a chapter in the present volume, further on.-R. W. R.