Magnetic Ore Milling Work - 6

Magnetic Ore Milling Work - 6

 A brief description of this remarkable machine will probably interest the reader. In the

two end pieces of a heavy iron frame were set three rolls, or cylinders --one in the centre,

another below, and the other above--all three being in a vertical line. These rolls were of

cast iron three feet in diameter, having chilled-iron smooth face-plates of considerable

thickness. The lowest roll was set in a fixed bearing at the bottom of the frame, and,

therefore, could only turn around on its axis. The middle and top rolls were free to move

up or down from and toward the lower roll, and the shafts of the middle and upper rolls

were set in a loose bearing which could slip up and down in the iron frame. It will be

apparent, therefore, that any material which passed in between the top and the middle

rolls, and the middle and bottom rolls, could be ground as fine as might be desired,

depending entirely upon the amount of pressure applied to the loose rolls. In operation

the material passed first through the upper and middle rolls, and then between the middle

and lowest rolls.

This pressure was applied in a most ingenious manner. On the ends of the shafts of the

bottom and top rolls there were cylindrical sleeves, or bearings, having seven sheaves, in

which was run a half-inch endless wire rope. This rope was wound seven times over the

sheaves as above, and led upward and over a single- groove sheave which was operated

by the piston of an air cylinder, and in this manner the pressure was applied to the rolls. It

will be seen, therefore, that the system consisted in a single rope passed over sheaves and

so arranged that it could be varied in length, thus providing for elasticity in exerting

pressure and regulating it as desired. The efficiency of this system was incomparably

greater than that of any other known crusher or grinder, for while a pressure of one

hundred and twenty-five thousand pounds could be exerted by these rolls, friction was

almost entirely eliminated because the upper and lower roll bearings turned with the rolls

and revolved in the wire rope, which constituted the bearing proper.

The same cautious foresight exercised by Edison in providing a safety device--the fuse--

to prevent fires in his electric-light system, was again displayed in this concentrating

plant, where, to save possible injury to its expensive operating parts, he devised an

analogous factor, providing all the crush- ing machinery with closely calculated "safety

pins," which, on being overloaded, would shear off and thus stop the machine at once.

The rocks having thus been reduced to fine powder, the mass was ready for screening on

its way to the magnetic separators. Here again Edison reversed prior practice by

discarding rotary screens and devising a form of tower screen, which, besides having a

very large working capacity by gravity, eliminated all power except that required to

elevate the material. The screening process allowed the finest part of the crushed rock to

pass on, by conveyor belts, to the magnetic separators, while the coarser particles were in

like manner automatically returned to the rolls for further reduction.

In a narrative not intended to be strictly technical, it would probably tire the reader to

follow this material in detail through the numerous steps attending the magnetic

separation. These may be seen in a diagram reproduced from the above-named article in

the Iron Age, and supplemented by the following extract from the Electrical Engineer,

New York, October 28, 1897: "At the start the weakest magnet at the top frees the purest

particles, and the second takes care of others; but the third catches those to which rock

adheres, and will extract particles of which only one-eighth is iron. This batch of material

goes back for another crushing, so that everything is subjected to an equality of refining.

We are now in sight of the real `concentrates,' which are conveyed to dryer No. 2 for

drying again, and are then delivered to the fifty-mesh screens. Whatever is fine enough

goes through to the eight-inch magnets, and the remainder goes back for recrushing.

Below the eight- inch magnets the dust is blown out of the particles mechanically, and

they then go to the four-inch magnets for final cleansing and separation.... Obviously, at

each step the percentage of felspar and phosphorus is less and less until in the final

concentrates the percentage of iron oxide is 91 to 93 per cent. As intimated at the outset,

the tailings will be 75 per cent. of the rock taken from the veins of ore, so that every four

tons of crude, raw, low-grade ore will have yielded roughly one ton of high-grade

concentrate and three tons of sand, the latter also having its value in various ways."

This sand was transported automatically by belt conveyors to the rear of the works to be

stored and sold. Being sharp, crystalline, and even in quality, it was a valuable byproduct,

finding a ready sale for building purposes, railway sand-boxes, and various

industrial uses. The concentrate, in fine powdery form, was delivered in similar manner

to a stock- house.