Page:EB1911 - Volume 20.djvu/272

 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 accompanied 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 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. EB1911 - Ore-dressing Fig 7.png . 7.—Harz Jig.

In the power jig (fig. 7) the sieve, 𝑎, is stationary and the pulsating current is obtained by placing a vertical longitudinal partition, 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 respective hoppers. The capacity on pulp of to in. size is some 4 tons in twenty-four 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 in. or even finer. The capacity on in. pulp is from 15 to 25 tons in twenty-four hours. EB1911 - Ore-dressing Fig 8.png . 8.—Wilfley Table. 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 1 in. The lower 10 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 side-shake, but where cleanness of concentrates is sought the side-shake vanner is the most satisfactory. The latter is much the most usual form. EB1911 - Ore-dressing Fig 9.png . 9.—Frue Vanner. Slime-Tables are circular revolving tables (fig. 10) with flattened conical surfaces, and a slope of 1 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 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 1 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).

EB1911 - Ore-dressing Fig 10.png . 10.—Convex Revolving Slime-table. Buddles 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.

Riffles.—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.

Kieves.—The kieve or dolly-tub is a tub as large or larger than an ordinary oil-barrel, with sides flaring slightly upwards all the way from the bottom. In the centre is a little vertical shaft, with hand-crank at the top and stirring blades like those of a propeller at the bottom. Fine concentrates from buddies or slime-tables are still further enriched by treatment in the kieve. The kieve is filled perhaps half full of water, and the paddles set in motion; concentrates are now shovelled in until it is nearly full, the rotation is continued a little longer and then the shaft is quickly withdrawn and