Page:EB1911 - Volume 20.djvu/271

 power minerals formerly regarded as non-magnetic are attracted, has made it possible to separate several classes of minerals present in an ore; for example, the strongly magnetic mineral may first be taken out, then the mildly magnetic, and last the weakly magnetic, the non-magnetic being left behind. Adhesion acts when brightly burnished particles of gold issuing with the sand from the stamp mill come in contact with an amalgamated copper plate, for they are instantly plated with mercury and adhere to the copper, while the sand is carried forward by the water. In this way a very perfect separation of the gold from the sand is effected. In the South African diamond fields it has been found that if the diamond-bearing sand is taken in a stream of water over a smooth surface covered with a suitable coating of grease, the diamonds will adhere to the grease while the sand does not.

2. The concentration of ores always proceeds by steps or stages. Thus the ore must be crushed before the minerals can be separated, and certain preliminary steps, such as sizing and classifying, must precede the final operations which produce the finished concentrates. The more important of these simple operations will now be described.

. 1.—Blake Breaker. 𝑎, Movable jaw. 𝑏, Fixed jaw. The ore as mined contains the valuable minerals attached to and enclosed in lumps of waste rock. The province of crushing or disintegrating is to sever or unlock the values from the waste, so that the methods of separation are then able to part the one from the other. In crushing ores it is found wise to progress by stages, coarse crushing being best done by one class of machine, medium by another, and fine by a third. Coarse crushing is accomplished by breakers of the Blake type (fig. 1) or of the Gates Comet type (fig. 2). All of these machines break by direct pressure, caused by a movable jaw, 𝑎 (figs. 1, 2), approaching towards and receding from a fixed jaw, 𝑏. The largest size ever fed to a breaker is 24 in. in diameter, and the smallest size to which the finest crushing commonly done by these machines brings the ore is about in. diameter. The machine is generally supplied with ore in lumps not larger than 9 in. in diameter, and crushes them to about 1 in. in diameter. Medium-size crushing is done mostly by rolls or steam stamps. Rolls (fig. 3) crush by direct pressure caused by the ore being drawn between two revolving rolls held closely together. They make the least fine slimes or fines to be lost in the subsequent treatment, and are therefore preferred for all brittle minerals. The steam stamp works upon the same principle as a steam hammer, the pestle being forced down by steam pressure acting through piston and cylinder with great crushing force in the mortar. Steam stamps have been very successful with the native copper rock, because they break up the little leaves, flakes and filaments of copper, and render them susceptible of concentration, which rolls do not. Fine crushing is done by gravity stamps, pneumatic stamps, by centrifugal roller mills, by amalgamating pans, by ball mills, by Chile edgestone mills, by tube mills and by arrastras. The gravity stamp (fig. 4) is a pestle of 900-℔ weight more or less, which is lifted by a revolving cam and falls by the force of gravity to strike a heavy blow on the ore resting on the die in the mortar and do the work of crushing; the frequent revolution of the cam gives a more or less rapid succession of blows.

Gravity stamps are especially adapted to the fine crushing of gold ores, which they reduce to -in. and sometimes even to -in. grains. The blow of the stamp upon the fragments of quartz not only liberates the fine particles of gold, but brightens them so that they are quickly caught upon the amalgamated plates. The centrifugal roller mills are suited to fine crushing of middle products, namely by-products composed of grains containing both values and waste, since they avoid making much fine slimes. They crush by the action of a roller, rolling on the inside of a steel ring, both having vertical axes. The amalgamating pan is suitable for grinding silver ores for amalgamation where the finest grinding is sought, together with the chemical action from the contact with iron. It crushes by a true grinding action of one surface sliding upon another. The Chile edgestone mill is employed for the finest grinding ever used preparatory to concentration. The arrastra or drag-stone mill grinds still finer for amalgamating. The ball mill is a horizontal revolving cylinder with iron balls in it which do the grinding; the pulverized ore passes out through screens in the cylinder wall. It is a fine grinder, making a small amount of impalpable slimes. It is used for preparation for concentrating. The tube mill is of similar construction, but it is fed through the hollow shaft at one end and discharged through the hollow shaft at the other; the finely ground ore is floated out by water and contains a large proportion of impalpable slimes. It is used for preparation for cyaniding of gold.

A considerable class of workable minerals, among which are surface ores of iron and surface phosphates, contain worthless clay mixed with the valuable material, the removal of which is accomplished by the log washer. This is a disintegrator consisting of a long narrow cylinder revolving in a trough which is nearly horizontal. Upon the cylinder are knives or paddles set at an angle, which serve the double purpose of bruising and disintegrating the clay and of conveying the cleaned lump ore to be discharged at the upper end of the trough, the water meanwhile washing away the clay at the lower end.

Roasting for Friability.—When two minerals—for example, pyrite and cassiterite (tin ore)—one of which is decomposed and rendered porous and friable by heat and oxygen—are roasted in a furnace, the pyrite becomes porous oxide of iron, while the cassiterite is not changed. A gentle crushing and washing operation will then break and float away the lighter iron oxide, leaving the cleaned cassiterite behind.

. 4. Gravity Stamp. Sizing.—This is the first of the preliminary operations of separation. It is found useful in concentration, for dividing an ore into a number of portions graded from coarser sizes down to finer sizes. Each portion is made suitable for treatment on its respective machine. If crushed ore be sifted upon a screen with holes of definite size, two products will result—the oversize, which is unable to pass through the screen, and the undersize, which does pass. If the latter size be sifted upon another screen with smaller holes, it will again make oversize and undersize. The operation can be repeated with more sieves until the desired number of portions is obtained. P. von Rittinger adopted for close sizing the following diameters in millimetres for the holes in a set of screens: 64, 45·2, 32, 22·6, 16, 11·3, 8, 5·6, 4, 2·8, 2, 1·4, 1. Each of these holes has an area double that of the one next below it; this may be called the screen ratio. A process which does not need such close sizing might use every other screen of the above set, and in extreme cases even every fourth screen. In mills the screen ratio for coarse sizes often differs from that for fine. Sizing is done by cylindrical screens revolving upon their inclined axes (fig. 5), by flat shaking screens, and by fixed screens with a comparatively steel slope. Either wire cloth with square holes or steel plate punched with round holes is used. To remove the largest lumps in the preliminary sizing, fixed-bar screens (grizzlies) are preferred, on account of their strength and durability. . 5.—Trommel or Revolving Screen.

. 6.—Hydraulic Classifier.

Sizes smaller than can be satisfactorily graded by screens are treated by means of hydraulic classifiers and box classifiers. The lower limit of screening and therefore the beginning of this work varies from grains of 5 millimetres to grains of 1 millimetre in diameter. A hydraulic classifier (fig. 6) is a trough-like washer through which the water and sand flow from one end to the other. In the bottom, at regular intervals, are pockets or pits with hydraulic devices which hinder the outflowing discharge of sand, 𝑏, by an inflowing stream of clear water, 𝑎. By regulating the speed of these water currents, the size of the grains in the several discharges can be regulated, the first being the coarsest and the overflow at the end the finest. Box classifiers (spitzkasten) are similar, except that the pockets are much larger and no inflowing clear water is used; they therefore do their work much less perfectly. Classifiers do not truly size the ore, but merely class together grains which have equal settling power. In any given product, except the first, the grain of high specific gravity will always be smaller than that of low. The