Ore-Dressing - Encyclopedia

GEOGRAPHICAL NAMES

ORE-DRESSING, one of the principal processes in the work of mining. When the miner hoists his ore 1 to the surface, the contained metal may be either in the native uncombined state, as, for example, native gold, native silver, native copper, or combined with other substances forming minerals of more or less complex composition, as, for example, telluride of gold, sulphide of silver, sulphide of copper. In both cases the valuable mineral is always associated with minerals of no value. The province of the ore-dresser is to separate the " values " from the waste - for example, quartz, felspar, calcite - by mechanical means, obtaining thereby " concentrates " and " tailings." The province of the metallurgist is to extract the pure metal from the concentrates by chemical means, with or without the aid of heat. There are also a number of non-metallic minerals which do not have any value, or at best do not reach their highest value until they have been subjected to some form of mechanical preparation; among them are diamonds, graphite, corundum, garnet, asbestos and coal. Ore-dressing, for the purposes of this article, may be divided into three parts: (1) properties of minerals which render aid in their separation; (2) simple opera tions; (3) operations combined to form processes or mills.

1. The specific gravity of minerals varies greatly, some being heavy, others light. The rate of settling in water is affected by the specific gravity in this way: of two particles of the same properties. size but different specific gravity, the heavier settles more rapidly than the lighter, while of two particles of different specific gravity which settle at the same rate in water, that of higher specific gravity is of smaller diameter than the other. The same statements are true in regard to settling in air, and in regard to momentum in air when the particles are thrown out in a horizontal direction. Colour, lustre and fracture are of especial value in hand-picking, to aid the eye in selecting the mineral sought. Instances are, of colours, the white of quartz, the pale straw colour of felspar, the dull yellow of limonite, the brass yellow of chalcopyrite, the pale metallic yellow of pyrite; of lustres, the vitreous of quartz, the adamantine of diamond and cerussite, the resinous of blende, the earthy of limonite, and the metallic of pyrite; and of fractures, the cleavage planes of felspar and galena, the conchoidal fracture of quartz and pyrite, the granular of some forms of magnetite and blende. Magnetism is a most direct and simple method of separating minerals where it is available. The discovery that by the use of electro-magnets of great 1 The O. Eng. word was Ora, corresponding with Du. oer, the origin of which is unknown. The form " ore " represents the O. Eng. ur, brass; cf. Lat. aes, Skt. ayas. 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.

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. I) or of 1. - Blake Breaker. the Gates Comet type (fig.

a Movable jaw. 2). All of these machines b Fixed jaw. break by direct pressure, caused by a movable jaw, ,a (figs. I, 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 4 in. diameter. The machine is generally supplied with ore in lumps not larger than 9 in. in diameter, .and crushes them to about I. 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, ,a, Movable jaw. by ball mills, by Chile edgestone mills, by b, Fixed jaw. tube mills and by arrastras. The gravity c, Gear with eccen- stamp (fig. 4) is a pestle of goo-lb weight tric hub and with more or less, which is lifted by a revolving loose fit on the cam and falls by the force of gravity to spindle. 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 syin. 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 Rolls. 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 oar 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.

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.

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 FIG. 4. - Gravity Stamp. 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, I. 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.

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 w FIG. 6. - Hydraulic Classifier. varies from grains of 5 millimetres to grains of I 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 FIG.

FIG. 2. - Gates Breaker.

FIG. 3. - Crushing FIG. 5. - Trommel or Revolving Screen.

a, a?

box classifiers are suited to treating finer sizes than the hydraulic classifiers, and therefore follow them in the mill treatment. Picking Floors, the first of the final operations of separation, are areas on which men, boys or girls pick out the valuable mineral which is rich enough to ship at once to the smelter. The picking is often accowpanied and aided by breaking with a hammer. Picking tables are generally so constructed that the pickers can sit still and have the ore pass before them on a moving surface, such as a revolving circular table or travelling belt. Stationary picking tables require the ore to be wheeled to and dumped in front of the pickers. Picking out the values by hand has the double advantage that it saves the power and time of crushing, and prevents the formation of a good deal of fine slimes which are difficult to save.

Jigs treat ores ranging from 1 in. in diameter down to h in. If an intermittently pulsating current of water is passed up through a horizontal sieve on which is a bed of ore, the heavy mineral and the quartz quickly form layers, the former beneath the latter. The machine by which this work is done is called a jig, and the operation is called jigging. In the hand jig the sieve is moved up and down in a tank of water to get the desired separation.

In the power jig (fig. 7) the sieve, a, is stationary and the pulsating current is obtained by placing a vertical longitudinal par tition, c, extending part of the way down to the bottom of the jig box.

The sieve, a, is firmly fastened on one side of the partition, and on the other a piston or plunger, d, is moved rapidly up and down by an eccentric, causing an up-and-down current of water through the sieve, a. The sieve is fed at one end, e, with a constant supply of water and ore, and the quartz overflows at the other. Clear water (" hydraulic water") is brought by the pipe, i, into the space, g, below called the hutch, to regulate the condition of the bed of ore on a. The constantly accumulating bed of concentrates is either discharged through the sieve into the hutch, g, or by some special device at the side. On jigs where the concentrates pass through the sieve, a bed of heavy mineral grains too large to pass holds back the lighter quartz. The quartz overflow from one sieve, a, generally carries too much value to be thrown away, and it is therefore jigged again upon a second sieve, b. In jigging difficult ores, three, four, five and even six sieves are used. A succession of sieves gives a set of products graded both in kind and in richness, the heavier mineral, as galena, coming first, the lighter, as pyrites and blende, coming later. The best jigging is done upon closely sized products using a large amount of clear water added beneath the sieve. Very good jigging may, however, be done upon the products of hydraulic classifiers, where the heavy mineral is in small grains and the quartz is large, by using a bed on the sieve and diminished hydraulic water, which increases the suction or downward pull by the returning plunger.

Bumping Tables. - Rittinger's table is a rectangular gently sloping plane surface which by a bumping motion throws the heavy particles to one side while the current of water washes down the quartz to another, a wedge-shaped divider separating and guiding the concentrates and tailings into their reE-spective hoppers. The capacity on pulp of o to 1 in. size is some 4 tons in twenty - four FIG. 8. - Wilfley Table. hours. In the Wilfley table (fig. 8) and those derived from it a gentler vanning motion is substituted for the harsh bump; they have a greatly increased width and a set of riffle blocks, b, at right angles to the direction of flow, c, tapering in height towards the side where the concentrates are discharged, d. This combination has produced a table of great efficiency and capacity for treating grains from in. in diameter down to ain. or even finer. The capacity on in. pulp is from 15 to 25 tons in twenty-four hours.

Vanners are machines which treat ores on endless belts, generally of rubber with flanges on the two sides. The belt (fig. 9) travels up a gentle slope, a, on horizontal transverse rollers, and is shaken about 200 times a minute, either sidewise or endwise, to the extent of about I in. The lower s ft. is called the concentrating plane, b, and slopes 2.78% more or less from the horizontal; the upper 2 ft. of length is called the cleaning plane, c, and slopes 4.45% more or less. The fine ore is fed on with water (technically called pulp) at the intersection of the two planes, d. The vibration separates the ore into layers, the heavy minerals beneath and the light above. The downward flow of the water carries the light waste off and discharges it over the tail roller e into the waste launder, while the upward travel of the belt carries up the heavy mineral. On the cleaning plane the latter passes under a row of jets, f, of clean water, which remove the last of the waste rock; it clings to the belt while it passes over the head roller, and only leaves it when the belt is forced by the dipping roller to dip in the water of the concentrates tank, g. The cleaned belt then continues its return journey over the guide roller h to the tail roller e, which it passes round, and again does concentration duty. Experience proves that for exceedingly fine ores the end shake with steep slope and rapid travel does better work than the side-shake vanner. For ordinary gold stamp-mill pulp, where cleanness of tailings is the most important end, and where to gain it the engineer is willing to throw a little quartz into the concentrates, the end-shake vanner is again probably a little better than the sideshake, but where cleanness of concentrates is sought the side-shake vanner is the most satisfactory. The latter is much the most usual form.

Slime-Tables are circular revolving tables (fig. t o) with flattened conical surfaces, and a slope of 1 k in. more or less per foot from centre to circumference; a common size is 17 ft. in diameter, and a common speed one revolution per minute. These tables treat material of in. and less in diameter coming from box classifiers. The principle on which the table works is that the film of water upon the smooth surface rolls the larger grains (quartz) towards the margin of the table faster than the smaller grains (heavy mineral) which are in the slow-moving bottom current. The revolution of the table then discharges the quartz earlier at a, a, a, a, an intermediate middling product next at b, and the heavy mineral last at c. Suitable launders or troughs and catch-boxes are supplied for the three products. The capacity of such a table is 12 tons or more of pulp, dry weight, in twenty-four hours. Frames, used in concentrating tin ore in Cornwall, are rectangular slime-tables which separate the waste from the concentrates on the same principle as the circular tables, though they a FIG. so. - Convex Revolving Slime-table.

run intermittently. They treat very fine pulp, and after being fed for a short period (about fifteen minutes) the pulp is shut off, the concentrates are flushed off with a douche of water and caught in a box, and the feed pulp is again turned on. Canvas tables are rectangular tables with plane surfaces covered with cotton duck (canvas) free from seams; they slope about s in. to the foot. They are fed with stamp-mill pulp, with the tailings of vanners, or, best of all, with very fine pulp overflowing from a fine classifier. The rough surface of the duck is such an efficient catching surface that they can run for an hour before the concentrates are removed - an operation which is effected by shutting off the feed pulp, rinsing the surface with a little clean water, and hosing or brooming off the concentrates into a catch-box. The feed-pulp is then again turned on and the work resumed. They have been more successful than any other machine in treating the finest pulp, especially when their concentrates are finally cleaned on a steep slope end-shake vanner (the G. G. Gates canvas table system of California).

Buddies act in principle like slime-tables, but they are stationary, and they allow the sand to build itself up upon the conical surface, which is surrounded by a retaining wall. When charged, the tailings are shovelled from the outer part of the circle, the middlings from the intervening annular part, and the concentrates from the inner part. They treat somewhat coarser sizes than the slime-table. The term buddle is sometimes applied to the slime-tables, but the majority confine the phrase to the machine on which the sand builds up in a deep layer.

When wooden blocks or cobble-stones of uniform size are placed in the bottom of a sluice, the spaces between them are called riffles; and when gold-bearing gravel is carried through the sluice by a current of water, a great many eddies are produced, in which the gold and other heavy minerals settle.