Magnetic Ore Milling Work - 5

Magnetic Ore Milling Work - 5

 The giant rolls were two solid cylinders, six feet in diameter and five feet long, made of

cast iron. To the faces of these rolls were bolted a series of heavy, chilled-iron plates

containing a number of projecting knobs two inches high. Each roll had also two rows of

four-inch knobs, intended to strike a series of hammer-like blows. The rolls were set face

to face fourteen inches apart, in a heavy frame, and the total weight was one hundred and

thirty tons, of which seventy tons were in moving parts. The space between these two

rolls allowed pieces of rock measuring less than fourteen inches to descend to other

smaller rolls placed below. The giant rolls were belt-driven, in opposite directions,

through friction clutches, although the belt was not depended upon for the actual

crushing. Previous to the dumping of a skip, the rolls were speeded up to a

circumferential velocity of nearly a mile a minute, thus imparting to them the terrific

momentum that would break up easily in a few seconds boulders weighing five or six

tons each. It was as though a rock of this size had got in the way of two express trains

travelling in opposite directions at nearly sixty miles an hour. In other words, it was the

kinetic energy of the rolls that crumbled up the rocks with pile-driver effect. This sudden

strain might have tended to stop the engine driving the rolls; but by an ingenious clutch

arrangement the belt was released at the moment of resistance in the rolls by reason of the

rocks falling between them. The act of breaking and crushing would naturally decrease

the tremendous momentum, but after the rock was reduced and the pieces had passed

through, the belt would again come into play, and once more speed up the rolls for a

repetition of their regular prize-fighter duty.

On leaving the giant rolls the rocks, having been reduced to pieces not larger than

fourteen inches, passed into the series of "Intermediate Rolls" of similar construction and

operation, by which they were still further reduced, and again passed on to three other

sets of rolls of smaller dimensions. These latter rolls were also face-lined with chillediron

plates; but, unlike the larger ones, were positively driven, reducing the rock to pieces

of about one-half-inch size, or smaller. The whole crushing operation of reduction from

massive boulders to small pebbly pieces having been done in less time than the telling

has occupied, the product was conveyed to the "Dryer," a tower nine feet square and fifty

feet high, heated from below by great open furnace fires. All down the inside walls of this

tower were placed cast-iron plates, nine feet long and seven inches wide, arranged

alternately in "fish-ladder" fashion. The crushed rock, being delivered at the top, would

fall down from plate to plate, constantly exposing different surfaces to the heat, until it

landed completely dried in the lower portion of the tower, where it fell into conveyors

which took it up to the stock-house.

This method of drying was original with Edison. At the time this adjunct to the plant was

required, the best dryer on the market was of a rotary type, which had a capacity of only

twenty tons per hour, with the expenditure of considerable power. As Edison had

determined upon treating two hundred and fifty tons or more per hour, he decided to

devise an entirely new type of great capacity, requiring a minimum of power (for

elevating the material), and depending upon the force of gravity for handling it during the

drying process. A long series of experiments resulted in the invention of the tower dryer

with a capacity of three hundred tons per hour.

The rock, broken up into pieces about the size of marbles, having been dried and

conveyed to the stock-house, the surplusage was automatically carried out from the other

end of the stock-house by con- veyors, to pass through the next process, by which it was

reduced to a powder. The machinery for accomplishing this result represents another

interesting and radical departure of Edison from accepted usage. He had investigated all

the crushing-machines on the market, and tried all he could get. He found them all greatly

lacking in economy of operation; indeed, the highest results obtainable from the best

were 18 per cent. of actual work, involving a loss of 82 per cent. by friction. His nature

revolted at such an immense loss of power, especially as he proposed the crushing of vast

quantities of ore. Thus, he was obliged to begin again at the foundation, and he devised a

crushing-machine which was subsequently named the "Three-High Rolls," and which

practically reversed the above figures, as it developed 84 per cent. of work done with

only 16 per cent. loss in friction.