In 1895, Thomas Edison was responsible for a remarkable innovation in crushing. Edison believed that high-grade deposits of iron ores in the eastern United States were running out, and he decided to develop a deposit of low-grade ore containing magnetite at a treatment rate of up to 6,000 tpd. Minimizing mining and crushing costs was essential to success, and the high capital and operating costs of multiple small crushers was a particular area of concern. He reasoned that the recoverable energy in 1 lb of coal and 1 lb of 50% dynamite was approximately the same but the cost of the dynamite was 100 times the cost of the coal (Dyer and Martin 1910). Consequently, it would be much cheaper to break large pieces of ore by machines driven by steam power than by explosive energy, particularly because much of the dynamite was used for rebreaking rocks from the first explosion. His approach was to break the rock by dynamite into large boulders—up to 5 to 6 tons—and then use huge crushers to break these into small particles.
Edison conceived the bold idea of constructing gigantic rolls which, by the force of momentum, would be capable of crushing individual rocks of vastly greater size than ever before attempted. …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. …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. 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. (Dyer and Martin 1910)
Eventually 2.4-m x 2.1-m Edison rolls were built, and they must have been spectacular to watch. But their costs of operation would have been high, and they soon declined in popularity. The giant rolls were only one of many innovations in the complex mining and processing system devised by Edison that resulted in very fine magnetite particles being extracted from low-grade ores. The rolls were a heroic effort by Edison to reduce crushing and grinding costs to a manageable figure, but ultimately the process was not an economic success. Edison rolls created much interest in the mining industry for a while, and several were installed in other plants, including four sets operating in series on quarried limestone in Edison cement plants. But jaw crushers and gyratory crushers soon proved to be superior, and the giant rolls passed into history.
The crushers that were available in the late 19th century could be fully used in mines only if the ore loading and transport systems from the mine or quarry to the processing plant could keep up with the capacity of the crushers. By then open-pit mining and quarrying had become the predominant mining method, but handwork was a serious limitation to efficiency. At the end of the 19th century, steam power was widely used for loading and transporting ore and rocks, although manual handling continued far into the 20th century.
Beginning in the 1920s and 1930s, large internal combustion engines were developed that could drive large trucks. After World War II, ore trucks began to replace railroad transportation of ore from open-pit mines. In the latter part of the 20th century, primary crushers were built on tracks located in open-pit mines and quarries and moved as the face of the mine moved. Ore could either be dumped by the shovel directly into the primary crusher or could be hauled by truck from the shovel to the primary crusher. The primary crusher discharge was conveyed out of the pit and to the processing plant on wide-belt conveyors. Overburden was also conveyed from the mine face to the overburden dumping area.