Class III., Solids From Solids. - As for grain machines, which are in this class, it may be said that in centrifugal flour bolters, bran cleaners, and middlings purifiers, though theoretically centrifugal force plays an important part in their action, yet practically the real separation is brought about by other agencies: in some by brushes which rub the finer particles through wire netting as they rotate against it.
The principle exhibited in a separator of grains and seeds is very neat. (Pat. 167,297.) See Fig. 32. That part of the machine with which we have to do consists essentially of a horizontal revolving disk. The mixed grains are cast on this disk, pass to the edge, and are hurled off at a tangent. Suppose at A. Each particle is immediately acted on by three forces. For all particles of the same size and having the same velocity the resistance of the air may be taken the same, that is, proportional to the area presented. The acceleration of gravity is the same; but the inertia of the heavier grain is greater. The resultant of the two conspiring forces R and (Mv2)/2 varies, and is greater for a heavier grain. Therefore, the paths described in the air will vary, especially in length; and how this is utilized the drawing illustrates.
In ore machines there is one for pulverizing and separating coal (Pat. 306,544), in which there is a breaker provided with helical blades or paddles, partaking of rapid rotary motion within a stationary cylinder of wire netting. The dust, constituting the valuable part of the product, is hurled out as fast as formed. In this style of machine, beaters are necessary not only for pulverizing, but to get up rotary motion for generating centrifugal force. In the classes preceding, the friction of the basket sufficed for this latter purpose; but here there is no rotating basket and no definite charge. As the material falls through the machine, separation takes place. Various kinds of ore may be treated in the same manner.
An "ore concentrator" (Pat. 254,123), as it is called, consists of a pan having rotary and oscillatory motions. Crushed ore is delivered over the edge in water. The heavy particles of the metal are thrown by centrifugal force against the rim of the pan, overcoming the force of the water, which carries the sand and other impurities in toward the center and away.
The best ore centrifugal or separator is what is called an "amalgamator." The last invention (Pat. 355,958, White) consists essentially of a pan, a meridian section of which would give a curve whose normal at any point is in the direction of the resultant of the centrifugal force at that point and gravity. There is a cover to this pan whose convexity almost fits the concavity of the pan, leaving a space of about an inch between. Crushed ore with water is admitted at the center between the cover and the pan, and is driven by centrifugal force through a mass of mercury (which occupies part of this space between the two) and out over the edge of the pan. The particles of metal coming in contact with the mercury amalgamate, and as the speed is regulated so that it is never great enough to hurl the mercury out, nothing but sand, water, etc., escape. There have been many different constructions devised, but this general principle runs through all. By having annular flanges running down from the cover with openings placed alternately, the mixture is compelled to follow a tortuous course, thus giving time for all the gold or other metal to become amalgamated. There are ridges in the pan, too, against which the amalgam lodges.
It is claimed for this machine that not a particle of the precious metal is lost, and experiments seem to uphold the claim.
A machine for separating fine from coarse clay for porcelain or for separating the finer quality of plumbago from the coarser for lead pencils uses an imperforate basket, against the wall of which the coarser part banks and catches under the rim. The finer part forms an inner cylindrical stratum, but is allowed to spill over the edge of the rim. The mixture is introduced at the bottom of the basket at the center.
Class IV., Gases And Solids. - There is a very simple contrivance illustrating machines of this class used to free air from dust or other heavy solid impurities which may be in suspension. See Fig. 33. The air enters the passage, B (if it has no considerable velocity of itself, it must be forced in), forms a whirlpool in the conically shaped receptable, A, and passes up out of the passage, D. The heavy particles are thrown on the sides and collect there and fall through opening, C, into some closed receiver.
Class V., Gases And Liquids. - The occluded gases in steel and other metal castings, if not separated, render the castings more or less porous. This separation is effected by subjecting the molten metal to the action of centrifugal force under exclusion of air, producing not only the most minute division of the particles, but also a vacuum, both favorable conditions for obtaining a dense metal casting.
Most of the devices for drying steam come under this head. Such are those in which the steam with the water in suspension is forced to take a circular path, by which the water is hurled by centrifugal force against the concave side of the passage and passes back to the water in the boiler.
The centrifugal force of a revolving particle varies, as we have seen, as the square of the angular velocity, so that the effort has been to obtain as high a number of revolutions per minute as was consistent with safety and with the principle of the machine. For example, creamers which are small and light make 4,000 revolutions per minute, though the latest styles run much more slowly. Driers and sugar machines vary from 600 to 2,000, while on the other hand the necessity of keeping the mercury from hurling off in an amalgamator prevents its turning more rapidly than sixty or eighty times a minute.
However, speed in another sense, the speed with which the operation is performed, is what especially characterizes centrifugal extractors. In this particular a contrast between the old methods and the new is impressive. Under the action of gravity, cream rises to the milk's surface, but compare the hours necessary for this to the almost instantaneous separation in a centrifugal creamer. The sugar manufacturer trusted to gravity to drain the sirup from his crystals, but the operation was long and at best imperfect. An average sugar centrifugal will separate 600 pounds of magma perfectly in three minutes. Gold quartz which formerly could not pay for its mining is now making its owners' fortunes. It is boasted by a Southern company that whereas they were by old methods making twenty-five cents per ton of gold quartz, they now by the use of the latest amalgamator make twenty-five dollars. Centrifugal force, as applied in extractors, has opened up new industries and enlarged old ones, has lowered prices and added to our comforts, and centrifugal extractors may well command, as they do, the admiration of all as wonderful examples of the way in which this busy age economizes time.