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It is inclined 7 to 10 degrees below the horizon; its area is to that of the interior copper-plates about as 12:1. To make a tight joint, it is let into the sill (Fig 1, i) of the housing, and pressed against the latter by means of braces underneath. The sheet copper is fastened on with iron wood-screws. At the angles between the raised rim and the bottom of the table, the copper is either turned up for 1 or 2 inches, or allowed to abut upon the rim, the crack being covered with a lath.
The wooden housings, screen-frames, outer plates, apron, &c., are all caulked with strips of blanket in the cracks and joints.
THE OPERATION OF THE MILL.
The quartz is brought from the mine, unless the mill is in or near the shaft-house, in wagons, generally with two mules each, and unloaded upon the charging-floor, where the necessary spalling is performed, as close as possible to the stamps, either by machinery, or, generally, by hand. Each wagon contains 32 cubic feet, or 35 hundred weight of rock.*
The principal object of the crushing is the simultaneous amalgamation of the free gold in the ore, and hence the ruling idea in the management of the process is to extract directly, and during the crushing, as high a percentage as possible; and but subordinate attention is given to any further treatment of the pulp.
Since the gold is in general finely disseminated, a fine crushing is usual. The number of drops per minute varies between 15 and 45, being generally 22 to 28.†
Feeding is always done by hand, and to all the stamps. The battery
*The practice of hauling rock to mill by the wagon-load, and the absence of means or inclination to weigh it, has led to the measurement of ore in Colorado by cords instead of tons. This is theoretically, in one respect, the most accurate way; since the cost of mining depends on the bulk rather than the weight of the material extracted, and what we desire to know of any vein is its value per fathom, not per ton. But practically there is no ascertained relation between the amount of ore sent to mill and the space actually excavated under ground, since no account is kept of the quantity thrown away in sorting. Moreover, the measurement of ore by cords is fatal to exact business transactions and to proper control by assays, &c., of the milling processes. "Four wagons make a cord,” is no basis for a scientific metallurgical practice. The cord, as might be expected, varies according to the character and size of the pieces, and the closeness with which they are packed in the box or wagon. Mr. Hague gives for a cord of quartz, without heavy minerals, stamping size, loose, 6 tons; for the same, more closely packed, 7 tons, while pure iron pyrites, closely packed, would weigh nearly twice as much. A cord of ordinary ore may weigh 7, 8, or 9 tons, the average of 74 tons being not far from the truth. Mr. Reichenecker puts it at 7, unless his "hundred-weight" are meant for German ones, in which case he very closely agrees with Mr. Hague.-R. W. R.
It is now beginning to be admitted in Colorado that a greater speed of stamps would be more economical of power. The important items of wages, interest, insurance, &c., are no greater for a rapid rate than for a slow one, while the actual effect per horse-power, and hence per unit of fuel consumed, is increased. See my last report, passim, and the chapter in this report on the speed of stamps.-R. W. R.
The self-feeding batteries are not generally approved by American mill men, who claim that a faithful and discreet feeder can increase the product of the stamps by far more than the value of his wages; besides, he need not be confined to that work, unless he attends to several batteries. Very small mills, of three or four stamps, might, I should think, be advantageously self-feeding. An amusing instance is a small threestamp mill belonging to a tenant on the southeastern part of the Mariposa estate, in California. It is run by water-power, and is self-feeding. The proprietor works elsewhere for wages, and the mill, being supplied with ore in the morning, feeds itself and crushes and amalgamates all day (rather rudely, it must be confessed) without any attendance whatever. Under these circumstances a profit is obtained from rock which would otherwise scarcely pay expenses.-R. W. R.
box is kept full for about 6 inches above the dies. The feeding is done in 12-hour shifts. A laborer can feed in one shift, allowing for 14 to 1 hours' average enforced idleness of the mill, about 95 cubic feet or 100 to 110 hundred-weight of hard rock, which he also spalls to first-size before charging. The same man can furthermore supply the quicksilver from time to time and regulate the flow of the battery-water. If the quartz is very hard, an assistant for alternate shifts, or a crushing-machine run by the feeder, is required.
The battery-water is supplied for rich rock in sufficient quantity only to prevent the pulp from catching on the aprons, say about 28 cubic feet of water per cubic foot of rock crushed. For rock poor in gold, (or poor in ore, which is generally the same thing,) the supply of water is increased, to increase the rate of working, so that the quantity amounts to 33 cubic feet and upwards to one cubic foot of rock crushed. The foregoing proportions of 28 and 33 cubic feet to one represent weights of the cubic foot of rock crushed, of about 125 and 108 pounds respectively. Per stamp per minute, the volume of the battery-water averages cubic foot. It has in summer its natural temperature; in winter it is warmed only enough to prevent congelation upon the apron, &c.
Amalgamation.-For the purpose of amalgamating the gold quicksilver is introduced through the charging-slit from time to time, generally once in two hours, and in quantities dependent upon the richness of the ore. Care is taken to distribute it as evenly as possible under the stamps. The quantity of the quicksilver employed is, on the average, three times as much as is afterwards recovered in the amalgam.*
The process of amalgamation is as follows: The quicksilver is finely divided by the stamps, and thus acquires the opportunity to coat or amalgamate the fine particles of gold. This fine division of the mercury is proved by the fact that two-thirds of the quantity charged generally escapes in the battery-slimes. The violent motion ("swash") of the battery-water, produced by the fall of the stamps, the particles of gold, amalgam or quicksilver are carried with the pulp upon the copper plates, to which they have an opportunity to adhere.
Very frequent addition of small quantities of quicksilver gives no better result than charging the same aggregate once in two hours. seems to follow that the quicksilver is but slowly expelled from the battery little by little, after it has caught a quantity of gold about equal to that in the quicksilver which is retained by the inner plates. That which escapes through the screens is thinly fluid, and contains but a small percentage of gold, while the amalgam of the plates is either a pasty mass or a hard layer. Now, since the quicksilver which arrives on the plates is, as has been shown, nearly equal in gold contents with that which escapes, it follows that nearly the whole gold contents of the amalgam of the plates are the result of the enrichment, by floating particles of gold in the pulp, of the quicksilver adhering to the plates.t The same purpose is served by the outer as by the inner copper plates, as the pulp flows over them.
*This loss seems very large, but the fact of its existence is indisputable. Mr. Hague speaks of too of a pound of quicksilver per ton of rock, which is about the same thing. Even at present high prices of quicksilver, however, the value of the loss is not very great, only gold always escapes when mercury does.-R. W. R.
This passage is obscure. Mr. Reichenecker's meaning appears to be, that the quicksilver in the battery takes up a small percentage of gold, and, while still fluid, is thrown by the swash, some upon the interior plates, some through the screen. That which catches on the plates gathers more and more gold, and grows stiffer and harder in consistency. Finally, the same process takes place with the amalgam caught on the outer plates of the apron.-R. W. R.
The movement of the pulp in the battery consists in alternate rising above and falling below the inner plates, combined with wave motions longitudinally along the battery-box. Outside, the pulp streams with uniform velocity over the inclined table, a movement which does not, like that within the battery, present moments of rest, favorable to the deposition of the gold. The only place where this is at all the case on the outer plates is the upper end of the apron, upon which the discharge falls from a height of several inches.*
According to foreging data these outer plates have an inclination of 7 to 10 degrees and a width of 10 to 12 inches per stamp, upon which about one-fourth of a cubic foot of pulp is discharged per minute. The discharge moves over them in a very thin layer, which moves (as in the case of shaking-tables, buddles, &c.) much more swiftly on the surface than on the bottom. Such particles of the gold and amalgam as sink with the larger and heavier pieces of ore to the bottom slide or roll slowly along, and have opportunity to adhere; but smaller particles are swept along by the more rapid surface-current of the watery sheet of pulp, without being able to reach the bottom. To this latter class belongs the greater part of the gold, especially of that which escapes through the seives, and almost all the particles of quicksilver and amalgam. Moreover, the absolute quantity of gold which can come in contact with the outer plates is less than that which may touch the inner plates, by the amount retained upon the latter. For all these reasons, the inner plates must furnish the greatest part of the gold obtained in the mill.
The coarser the particles of gold, the longer will they, after being rendered, by their separation from the ore or gangue, accessible to quicksilver, remain in the mortar-bed, where most of the latter is collected, and the better will be their opportunity of contact with it, by which means their subsequent adhesion to the plates is greatly facilitated. Moreover, the coarse gold will, either not at all, or very seldom, be lifted up to the discharge-level, until they have been pounded by the stamps into a more favorable shape, or crushed altogether, or changed in volume or specific gravity by a partial amalgamation. Hence they will remain in the battery at all events longer than the fine particles, and will be more likely to remain on the inner plates; and only a small portion will, after pulverization or amalgamation, reach the outer plates; while the fine gold, on the other hand, is lifted from the bottom immediately after being set free from the ore, and is held in suspension, together with the particles of amalgam, by the battery-water, until it is caught on one of the plates, or is discharged through the sieve.
While the substances are thus floating about, it is very difficult, on account of the thinness of the pulp, to unite the particles of gold and quicksilver; and this explains the fact already mentioned, that the quicksilver which escapes through the screens contains little gold. Moreover, of the gold escaping in the pulp only a small amount is in the two-thirds of the quicksilver originally charged, which likewise passes the screens. On the other hand, the gold escaping must be free gold in the pulp, almost wholly; and a part of it may be still inclosed in the quartz.
*This arrangement of the outer plates is a defect, which could easily be remedied by giving them, as is frequently done, several drops or stairs, instead of a smooth, uniform inclination.-R. W. R.
+ Specific gravity, modified by the resistance of the medium, determines the precipitation of different bodies suspended in water. But the resistance is proportioned by the surface, and small bodies have more surface in proportion than large ones, hence they do not sink as easily.-R. W. R.
Treatment of the plates and mortar;-removal of the amalgam.-Sev. eral times daily the crushing is suspended, the screens are removed, and foreign substances, principally splinters of wood, which may be floating about in the mortar, are taken out; after which the screens are replaced, the outer plates are rinsed with water, and cleansed from grease and oxide by means of cyanide of potassium, alone or with ammonia, poured on and rubbed with a soft brush or woolen cloth. film of grease, of hydrated copper oxide or copper salts, (carbonate, sebate, cyanidé, &c.,) hinders the combination of the quicksilver with the plates and with the gold. Grease finds its way into the battery partly in the battery-water, and in the ore, partly from the lubricating material employed on the cams and journal-bearings. As for oxidation, the copper contained in the film of amalgam coating the plates oxidizes when, for lack of being completely covered by the flowing batterywater, it is exposed to the air.
Accumulation of pulpt on the outer plates, resulting from occasional misproportion in the amount of battery-water, must be at once removed by rinsing.‡
Once a day the gold amalgam adhering to the outer plates is removed, after the latter have been rinsed, by scraping with a sharp-edged piece of India-rubber; and the plates are cleaned and sprinkled with a little fresh quicksilver, which is then spread over them by means of the brush or woolen cloth already described.
The inner plates are taken out generally twice a week, (but in many cases only once,) and the adhering amalgam is scraped off as far as possible without exposing the copper, a knife or chisel-not too sharpbeing employed if necessary. Quicksilver is then sprinkled upon them and rubbed off again with the above described piece of India-rubber.§ In this operation just so much additional amalgam is removed as leaves behind an even, thin layer, which is necessary for the adhesion of fresh gold. Finally the inner plates, immediately before they are replaced, are cleaned from grease and oxide, (a little more quicksilver being sprinkled and spread upon them,) and rinsed with water.
Generally once a week, when the inner plates are removed, the mortar and the stamps are examined for portions of amalgam which collect in thin depressions and joints, and which, when rich ores are under treatment, amount to considerable value. At the same time the rear housing is removed, the mortar emptied, worn-out shoes and dies are replaced with new ones, and other necessary repairs are made.
In summer the mills are usually stopped from Sunday morning to Monday morning; but in winter they are run during this period.
Further treatment of the amalgam obtained in the stamp-mill.—The amalgam obtained from the plates is diluted with a larger quantity of
*The .use of stearine candles, almost mines, leaves much grease in the ore. R. W. R.
universal in our American gold and silver Lamps would on this account be better.
This term, which I should perhaps have explained bofore, is applied to the crushed product escaping through the screens. In dry crushing the "pulp" is dust; in wet crushing, slime mechanically suspended in the battery-water.-R. W. R.
For this purpose, it is well to keep a small rubber hose in readiness, connected with the water supply, so that a stream of water can be at any time directed upon the part of the apron which is thus encumbered.-R. W. R.
The object of this is to dilute as it were with mercury the hard amalgam which cannot be evenly scraped off by means of the knife. Gold-amalgam catches gold better than pure quicksilver or copper-amalgam; hence the care with which the complete removal of all amalgam from the copper plates is avoided. The gold-amalgam on the inner plates is generally harder (because richer) than that upon the tables.-R. W. R.
quicksilver, and worked together thoroughly with the hand into one mass, the adhering particles of pulp being separated and washed away. The impurities, consisting chiefly of copper and lead, which still float on the surface of the mercury, are removed by means of a wet woolen cloth, until the surface is perfectly bright. Finally the mass is pressed through strong, thick canvas-filters.* The amalgam remaining in these contains about 64 per cent. of quicksilver, which is removed by distillation in a cast-iron retort. In this way is generally obtained a porous argentiferous gold, more or less alloyed with quicksilver, copper, and sometimes even lead. This gold is a marketable commodity.
The particles of ore and the other skimmings removed from the amalgam are collected, and generally rubbed fine, and digested with nitric acid, after which the greater part may be treated like the original amalgam from the plates.
Conditions of a good result from the amalgamation.-These are: 1. Very fine stamping.
2. The copper plates must have a coating of gold amalgam. This is secured for new plates either by rubbing on gold amalgam, or by coating them with quicksilver only, (forming copper amalgam,) and then rubbing over them the gold amalgam which gradually collects upon them from the pulp. No amalgam is removed until a hard layer of it has formed on the plates.
3. Use of the proper quantity of water. When the supply is inadequate, the mortar fills up with pulp, the screens are choked and the plates are covered up. The gold and quicksilver have more opportunity to come in contact with one another, but no chance to reach the plates. Too much water, on the other hand, leads to coarser crushing, less complete mechanical exposure of the fine gold, less contact of gold and quicksilver, and a premature sweeping off of both. The addition of quicksilver in larger quantity does not practically overcome these evils, since, though it may cause the formation of more amalgam, the excessive current of the water will sweep it away from both the inner and the outer plates.
4. Proper temperature of the battery-water. A high temperature, indeed, favors the amalgamation; but the amalgam formed is more liquid in proportion to its contents of gold, and, instead of adhering to the inner plates at first contact, tends to run down upon them and fall back into the mortar. At the proper temperature, on the other hand, particles of amalgam adhere to the inner plates, gradually accumulating to dendritic aggregates and undulating radial forms, the mass of which possesses solid consistency and a rough surface. In this way the plate soon acquires a corrugated surface, which greatly facilitates the further accumulation of quicksilver and gold, whereas, at a high temperature, the surface remains more or less smooth. The richer the amalgam on the plates, the more likely is it-especially when the battery-box is accidentally over-filled-to be washed off by the pulp, pulverized by the stamps, and carried out through the screen. Too high a temperature causes similar evils upon the outside plates.
5. The addition of quicksilver in proper quantity. Too much quicksilver is attended by results similar to those of excessive temperature. Moreover, the increased quantity of quicksilver, the battery-water remaining the same, causes much more to be deposited on the outside plates. This may dissolve the thin coating upon them, and expose and
*For small quantities, a piece of dressed buck-skin, or chamois-leather, is employed, in which the amalgam is placed, and the superfluous quicksilver expressed by wringing.-R. W. R.