Page:The American Cyclopædia (1879) Volume XI.djvu/436

 METALLURGY is an inclined plane, near the head of which the ore is deposited in the form of slime, and acted upon by a stream of water distributed uniformly over the board. To prevent the cutting of furrows or channels in the ore bed, the workman continually smooths and con- solidates with a wire brush or piece of plank the layer of concentrated ore deposited near the head of the table. When the table is full of accumulated ore, the water is shut off by means of the riffle, and the layer is divided into four parts, or zones, parallel with the head of the table. The upper zone is concen- trated ore; the second zone is usually about as rich as the original material ; the third is poor, but still rich enough to pay for re- working; the fourth is too poor for further treatment, and is rejected as tailings. Minor subdivisions may sometimes be made to ad- vantage. The manual labor involved in this process has caused it to be more or less su- perseded by mechanical contrivances. One of these is the huddle, which may be consid- ered as consisting of a large number of plane tables placed radially round a central point. They may be arranged with their heads to- gether, the ore being fed in the centre and discharged on the circumference, in which case the huddle forms the frustum of a low cone, and is called a convex huddle ; or they may be grouped with the tails to ward the cen- tre, the feed being on the circumference and the discharge in the centre, constituting a con- cave huddle. In these machines the tedious operation of maintaining an even surface to the ore layer is performed by revolving arms car- rying brushes or scrapers. A concave buddle is usually preferred, by reason of its discharge in the centre, where the working surface is smallest. This secures a maximum force of current for carrying away the worthless mate- rial ; whereas on the convex buddle the cur- rent is strongest in the centre, and is most likely at that point to carry past the proper zone particles of rich ore ; while on the cir- cumference, where the discharge takes place, the current is so much spread out as to have lost the power of carrying away the worth- less portions of the materiaL The percussion table is another improvement on the plane table, in which the smoothing and consolida- ting of the surface of the ore layer is effected by means of a periodical jar communicated to the table itself. This jar is ordinarily given by suspending the table, swinging it from its posi- tion of equilibrium, and allowing its backward swing to be stopped by striking against a sta- tionary block. The consolidation of the ore by brushes on the buddle. already alluded to, is not effective for very small particles. The finest slimes, when treated in buddies, remain too loosely on the surface, and permit the for- mation of furrows or channels ; but on the percussion table the shock imparted to the particles thoroughly shakes them together, and consolidates the mass. The percussion tables until recently employed were stopped at intervals and cleaned up by hand, like the ordinary plane table; and the buddies were treated in the same way, the zones of classified material in the latter case being of course an- nular. But the most recent practice has given rise to continuously working buddies and per- cussion tables. The former are known as rotating tables, and are substantially buddies which revolve slowly under feeding spouts, and from which the dressed ore, instead of being allowed to accumulate on the table, is washed off by clean water as soon as the sep- aration of the grains has been effected. These machines, as well as the continuous percussion tables, were perfected by the late Herr von Rit- tinger of Austria, The rotating tables have been found somewhat complicated and waste- ful of water, and require very careful and skil- ful management ; but the continuous percussion tables are pronounced both cheaper and more convenient than the similar machines of the intermittent type. These continuous tables receive their shock sidewise instead of end- wise, and the result is a distribution of the ore in peculiar curved zones upon the table. The stuff to be washed is delivered upon the tables at an upper corner. The clear water is fur- nished by distributors. The tendency of the pulp is to flow down the slope in a direct line ; but by means of the Literal percussion the path of the heavier particles is changed, and they are gradually thrown toward the side re- ceiving the shock. The combination of this motion, at right angles to the current of water, with the downward motion of the current gives to the particles, according to their size and weight, a more or less curved path, and gradually separates the heavier and richer par- ticles from the poor stuff. By the time they have reached the foot of the table the richest portions have been transferred to the corner diagonally opposite to that upon which they entered, while the middlings and tailings are discharged along the lower edge of the table, in the order of their concentration. By pla- cing compartments below the edge of the table, to receive the different discharges, the products of the classification are conducted away sepa- rately. The best authorities on this subject are the elaborate treatises in German by Rittinger and Gatzschmann. II. EXTRACTION OF MET- ALS FROM THEIR ORES. The chemical processes employed for this purpose depend, 1, on the affinity of carbon for oxygen ; 2. on the mu- tual reaction of an oxide with the sulphide of a metal ; and 3, on the replacement of one metal in combination by another. The reactions in 1 and 2 take place only at high temperatures, while those in 3 may be effected either by fusion or in solution. 1. Metal* reduced from the state of Oxide by Carbon. The affinity of carbon for oxygen at high temperatures is sufficient to decompose most of the metallic oxides. Even the alkaline metals may be thus obtained. A few of the oxides (alkaline