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SECTION V.-SEPARATION AND CONCENTRATION.

CHAPTER LXXXI.

THE CONCENTRATION OF AURIFEROUS ALLUVIUM.

As generally used in mining "concentration" refers to the enrichment by mechanical means of the ores that have been raised from veins. So restricted, it may be said that in the mining regions of the west it is confined chiefly to the separation of gold and sulphurets from quartz. But the grand washing operations of the placer gold miner are properly classed with those of concentration. With the aid of water he sweeps away the earth and gravel, and collects the grains of gold, which, by reason of their greater weight, remain behind.

The simplest and most common implements used for concentration are the miner's pan and the horn-spoon. The pan, so much used in California, not only for prospecting but in cleaning up sluices and mills, is at present stamped out of one piece of the best quality of Russia iron, and is a far better article than was formerly in use. It resembles an ordinary tin milk-pan in form, but its sides are more sloping and it is strengthened by a stout wire in the rim. In the gold region of the Carolinas and Georgia, the pan formerly employed was either the ordinary iron frying pan or a light steel pan, a little deeper, and elliptical in form.

The horn-spoon is a very convenient instrument for washing out samples of crushed vein stuff, or any soft material supposed to contain gold. It has one great advantage over a metallic surface, that it does not become enfilmed with air or grease, so as to prevent the perfect contact of the water on its surface. It is made from the large end of the horn of an ox, cut obliquely, and then scraped down to a suitable thickness. A horn that is black at one end makes the best spoon. Its lightness and durability, as well as many other good qualities, make it a favorite implement with gold prospectors.

The batea is another form of washing implement for prospecting and testing. It is a shallow circular plate, made from a single piece of wood, usually by turning in a lathe, and is about twenty inches in diameter and two and a half inches deep at the centre, from which the slope is regular and unbroken to the outer edge. It is much used at the gold mines and washings in Brazil; but in California it is not much known, its use being confined, I believe, to one or two experts who have attained the peculiar skilled manipulation it requires.

The cradle, the tom, and various rockers, are forms of concentrating apparatus familiar to most miners, which need not be here described.

SLUICING.

Sluicing is the simplest form of concentration upon a large scale. It is simply the employment of a current of water upon an inclined plane, which sweeps onward the finer and lighter substances more rapidly than the heavier, and thus effects a separation.

The ordinary board sluice is made of rough pine boards, in sections twelve feet in length, so that they can be fitted one into the other and

thus form a continuous trough, from twelve to twenty inches wide and from ten to twelve inches deep. In order that one section may fit into the next, they are made a few inches wider at one end than at the other. The usual grade or inclination is about twelve inches or from ten to eighteen inches, according to the nature of the materials to be washed. Cleats or riffles are placed across the bottom to arrest the flow of the heavier particles, and thus make a favorable point for the lodgment of the gold and quicksilver. In order to protect the bottom from the action of the larger stones and the violence of the current, a set of false riffles is put in. These are usually placed lengthwise of the box, and consist of slats, nailed to cross-bars of wood, so that the whole may be lifted out when the sluice is cleaned up or the bars need repairs. Both bars and sluice-boards wear out rapidly during active washing by the constant attrition of the stones; and when the boxes are no longer fit to be used, or if for any cause they are no longer to be used, they are dried and burned, and the careful washing of the ashes gives a very remunerative return of gold, often enough to buy a new set of sluice boxes.

Some details regarding the construction and working of the larger sluices have already been given in the chapter upon breaking down rock; and it is only necessary here to revert to the fact that the operations of breaking up and crushing, and of washing and concentrating are inseparably connected in placer mining, as well as in the extraction of gold from quartz taken out of veins. There is one form of sluicing which, however, has not yet been considered. It is the under-current sluice, an improvement introduced in California, and the outgrowth of the gigantic sluicing operations in that State.

UNDER-CURRENT SLUICES.

These are designed to separate the current in the main sluices into two portions, permitting the great bulk of the muddy current and coarse materials to pass on, while the heavier and lower portions are allowed to drop through a grating on the bottom of the sluice into shallower and broader sluice boxes, having a lower grade, and receiving a fresh supply of clean water. The design is to distribute the materials over a greater surface than could be given in the main sluice, and thus allow the gold to settle.

These boxes are made of various widths, from three to nine feet, and from twelve to fourteen inches deep. The grade is usually one in twelve. The grating, through which the stuff is admitted, is made of hard cast iron, with openings an inch wide and eight inches long. The stuff flowing in the under-currents is sometimes divided, a part being dropped into a second system of low-grade boxes, or secondaries, with a width of about thirty inches, and a grade of fourteen or fifteen inches to the box. They receive about one-fifteenth of the water in the under-current. The grating is much finer than that in the main sluice, the spaces being only three-eighths of an inch wide and five inches long. They are very useful for catching quicksilver.

CHAPTER LXXXII.

THE CONCENTRATION OF VEIN-STUFF.

As has already been remarked, concentration, in ordinary mining parlance, is confined in California chiefly to the separation of gold

and sulphurets from quartz. Quicksilver ores and copper ores have, to a small extent at one or two localities, been subjected to concentration, but with these exceptions very little attention has yet been given there to a subject of great importance to the mining interest. The quantity of sulphurets contained in the quartz veins of California rarely exceeds two per cent., and its separation is not attended with any great difficulty, inasmuch as the difference between the specific gravity of the sulphurets and quartz is so great, that, when agitated in water, the particles of sulphuret first find their way to the bottom and form a layer nearly free from the quartz, which settles in an upper stratum. It is upon this difference in gravity of substances, and their consequent different degrees of velocity in passing through water or air, that the operations of concentration are based.

For example, a sphere of gold eight lines in diameter will fall 100 Prussian inches through water in one second of time, while a sphere of quartz of the same size will fall only about 30 inches in the same time. Thus, a mixture of particles of gold and of particles of quartz could be very easily separated one from the other; and also many other substances could be separated where the difference in the velocity of falling is not so great. But the bulk of particles is also an important element, as will be seen from the inspection of the annexed table, in which the relative velocities with which particles of gold, galena, blende, and quartz of different sizes will fall through water is shown:

Table showing the distance in Prussian inches that spheres of various sizes of different substances will fall through water in one second of time.

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Thus, while a sphere of gold eight lines in diameter is falling 100 inches, galena of the same size will fall 60 inches; blende, 40.8 inches; and quartz, 29.8 inches. But while the sphere of gold eight lines in diameter is falling 100 inches, one of two lines in diameter will fall only 50 inches, or half as fast as the sphere of the same substance with four times the diameter. Further, a sphere of gold 0.707 lines in diameter will fall about as fast as one of quartz with a diameter of eight lines, or one of galena two lines in diameter, and so on. It thus becomes evident that the velocity of fall of substances in water depends not only upon their specific gravity, but upon their bulk and gravity combined, and that for a perfect separation of substances according to their gravity, it is essential that the particles should either be of the same size or that the variation must be confined within certain well-defined limits.

SIZING OF FRAGMENTS-TROMMELS.

From what has already been remarked, it will be seen that a proper sizing of the fragments and particles of crushed ores is an essential prerequisite to successful concentration.

For separating the coarser fragments, such as are suitable for jigging, for example, screens or riddles are used, and for the finer, sieves or perforated plates; while for the separation of the very finest portions resort is had to the action of flowing water.

Screens or riddles are made in a great variety of ways, but are usually flat surfaces of coarse wire or of parallel iron rods, and caused to swing or to jar by rising and falling at one end, so that the stuff may move over the surface by gravity, while the smaller fragments drop through. The product of one riddle may be received upon a second of finer mesh, and the product of the second upon a third, and so on.

For still smaller fragments, sieves in a cylindrical form or trommels are used. They are made to revolve, and are set at an inclination, so that stuff fed into the upper end will gradually descend to the lower, while a portion drops through the mesh and is received either in a suitable box or into an outer concentric cylinder of gauze. This is a form of trommel which was recently exhibited in Paris by Messrs. Huet and Geyler, and which has some novel features. It is not supported upon a shaft passing through from end to end, but is sustained by, and revolves on, trunnions cast upon each of the cast-iron heads or ends, as indicated in the annexed longitudinal section, which represents one of this style of trommels, constructed so as to supply a system of four twin sieves. The crushed stuff is introduced at the hollow trunnion A, and

Section of a Distributing Trommel.

falls upon a grate or perforated iron plate, B, in which the holes are large. The stuff which falls through the plate B, drops upon a second plate, C, perforated with smaller holes, where it is again divided into two sizes, the finer particles dropping through to the outermost plate of all. Each space between the plates has suitable openings at intervals in the cast-iron heads for the discharge of fragments too coarse to fall through the plate below them. This trommel is very compact and will give four sizes of product.

In Europe perforated iron or steel plates are now generally used instead of wire-cloth

screens, which wear out faster. The Exposition of 1867 was rich in samples of perforated plates of all descriptions and very accurately punched. It is essential to the best working effect that the thickness of the plates should always be less than the diameter of the holes punched in them. The space also between the holes in the finer plates should not be greater than the diameter of the holes-in the medium plates half a diameter, and in the coarser plates one-third of the diameter of the holes. In France perforations less than 0.002 in diameter are considered as fine; those between 0.002 and 0.005 are me

dium. The fine numbers begin at 0.0005. The finely-perforated plates for trommels are generally made of copper, and the other sizes of steel, iron, or zinc.

Rittinger adopts one millimetre in diameter as the unit of holes for sizing ores for concentration, and the progression beyond this is geometric, as 1, 2, 4, 8, 16 millimetres, giving for the volumes of the grains that will pass the holes respectively 1, 8, 64, 512, 4,096 cubic millimetres. He divides each of these sizes into four classes, each with four grades, thus:

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Blanket concentration is only a modification of sluicing; a rough surface being substituted for the smooth flat bottom, with riffles and other obstructions, of the sluice. Blankets are in very common use, being at once the simplest and most effective means of arresting the fine particles of gold that escape amalgamation in battery. The blanketing used for this purpose is made specially for it at the woolen mills of the coast, and is very strong, thick, and hairy. It is woven about thirty inches wide, just wide enough to cover the bottom of the strakes or shallow inclined troughs, and to hang over their edges. The troughs are from twelve to sixteen inches wide, with sides from one and a half to three inches high, and are inclined, according to the desired velocity of the current, from five to fifteen degrees. There are usually two or three blanket troughs abreast, receiving the sands as they flow from the battery; but four would be better. While the blankets of one are being washed, the current is turned upon the others, and the greater the surface provided for the flow the less disturbance is caused by the addition of the flow from the first while the blankets are washing.

The upper blankets, where the heaviest of the sands, with the included sulphurets, are deposited, are washed most frequently, sometimes as often as once in every fifteen minutes, but generally once every hour. The second row is taken off only half as often, and the third once in three or four hours; but the time they should be allowed to remain depends upon the amount of stuff which lodges upon them. rough surface must not be permitted to become completely filled with heavy sands.

The

There is a very extraordinary example of blanket concentration in

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