Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/482

Rh 464 MINING [DRESSING. Centri fugal pulver izers. Classifi cation of ores. Concen tration. Jiggers. pieces of ore are thrown with great velocity against bars or arras, or against each other, and so reduced to powder ; in other machines iron balls or iron rollers are whirled by centrifugal force against an iron casing and grind any mineral contained inside it. These pulverizers are much less used than stone-breakers, stamps, and rolls for the dis integration of metallic ores. (3) Classification of a crushed ore into sizes is absolutely necessary in some cases and advisable in others, because the subsequent concentration is dependent upon the fall of the particles in water, as will be presently explained. Classification by size is effected by sieves. Hand sieves and flat sieves placed one above the other have been super seded at most dressing establishments by cylindrical or conical revolving screens known as trommels. These screens are made of wire web or of perforated sheets of metal, and they are often arranged so as to discharge one into the other, so that the ore from a crusher can quickly be separated into classes of various sizes. With sizes of less than 1 millimetre (-^-g- inch) trommels are no longer employed, and recourse is had to the so-called separators or classifiers. These are boxes in the shape of inverted cones or pyramids into which the finely crushed ore is brought by means of a current of water ; a jet of clean water is often made to rise up in the bottom ; the larger and the specifically heavier particles fall and are discharged with a stream of water at or near the bottom, whilst the smaller and specifically lighter particles flow away at the top. The separators do not effect a true classi fication by size ; they merely cause a division by equivalence, a term which will be explained immediately. (4) We now have to deal with the enriching of the ore, or the concentration of the valuable particles into as small a bulk as is economically advantageous. The concentration is generally brought about by the fall of the particles in water. Occasionally the fall in air is utilized; mercury is employed as a collecting agent in the case of gold and silver, and in a few instances magnetism can be applied. The concentration in water depends upon the difference in specific gravity of the valuable ore and the waste vein stone or rock. A piece of galena with a specific gravity of 7 5 sinks to the bottom more quickly than a similar piece of quartz, the density of which is only 2 6. Nevertheless a large piece of quartz may fall to the bottom as quickly as a small piece of galena. Particles which have equal velocities of fall, though differing in size and specific gravity, are said to be equal-falling, or equivalent. P. von Rittinger shows that a sphere of quartz of inch in diameter would sink in water exactly as quickly as a sphere of galena of ^ inch in diameter, and these two particles are therefore equal-falling. Consequently, before we can separate properly by water it is necessary to classify the particles by size, so that equivalence shall not prevent a separation or lessen its sharpness. It is nevertheless true that in the early part of the fall of equivalent grains the influence of the specific gravity preponderates, and the denser particles take the lead ; therefore, by a frequent repetition of very small falls, particles which have not been closely sized may still be separated. The principal machine for concentrating particles of sizes ranging between 1 inch and -^ inch is the jig or jigger. The hand jigger is merely a round sieve which is charged with the crushed ore and then moved up and down in a tub full of water. The particles gradually arrange them selves in layers, the heaviest on the bottom and the lightest at the top. On lifting out the sieve the light waste can be skimmed off with a scraper, leaving the concentrated pro duct below ready for the smelter or for further treatment. Similar sieves worked by machinery were for a long time employed in dressing establishments, but the introduction of the improved continuous jiggers has led to their abandonment in all works of any importance. The con tinuous jigger is one of the most useful dressing ma chines of the present day. It consists of a box or hutch divided by a partial partition into two compartments ; in one is fixed a flat sieve s (fig. 99), which carries the ore, and in the other a piston p is made to work up and down by means of an ex- centric. The hutch being full of water, the movement of the piston causes the water to rise up and fall down through the ore, lifting it and letting it fall repeatedly. The effect of these frequent lifts and falls is to cause a sepa ration of the previously sized Fi S- 99. ore into layers of rich mineral at the bottom, light waste at the top, and particles of ore mixed with waste in the middle. The great value of these jiggers is the continuous discharge of the products without stoppages for their removal. Several methods are in vogue, viz., the end discharge, the central discharge, and the discharge through the meshes of the sieve. With the first, the enriched product lying at the bottom of the sieve passes out through openings at the end of the jigger, and the amount escaping is governed by an adjustable cap or shutter, by which the size of the openings can be increased or diminished at pleasure ; the middle product can be discharged by openings placed a little higher up, whilst the waste is washed over the top of the end of the jigger at each pulsation. Very often a first sieve simply separates a concen trated product and discharges the poorer product into a second sieve, where a similar separation is effected. With the central discharge, a pipe is brought up through the bottom of the sieve, and the size of the opening for the escape of the concentrated ore is regulated by a cylindrical cap which can be raised or lowered by a screw. The discharge through the sieve is especially adapted for the finer products from the crusher, though it is also used in some cases for grains up to inch in diameter. The mesh of the sieve is chosen so that the par ticles under treatment will just pass through, but above the sieve a layer of clean ore is placed which prevents anything but the heavier particles from being dis charged. The pulsations of the water, as before, cause a separation into layers, and the heavy rich particles find their way through the bed and Fig. 100. drop into the hutch, whence they can be drawn off through a hole at pleasure. The poorer part passes over a simple sill at the end of the sieve, or to a second sieve if necessary. Three or four sieves are occasionally arranged in a row in one machine. Fig. 100 is a section through the two sieves of a Harz sand jig. The pistons act in the manner explained by fig. 99. The smaller sizes are concentrated by a variety of machines. The action of many of them is based upon the behaviour of particles carried down an inclined plane by a thin stream of water. If the gradient of the plane and the strength of the thin current are properly arranged, the denser particles will be deposited and the specifically lighter ones washed away, although they may be equal-falling if allowed to settle in deep water.