Every revolution requires a catalyst, and it was the need to pump water from mines that sparked the revolution of steam power. By 1650, coal and tin mining were important industries in England, and water had become a serious problem in many mines that had been worked for centuries, particularly the deep tin mines in Cornwall. Pumps required more power than could be supplied manually if the mines were to be kept in operation.
The idea of using steam as an energy source was not new; the Italian physician Branca had used steam for grinding in 1629, making the steam in a metal boiler shaped like a man’s head. The only place the steam could get out was through a tube held in the man’s mouth, from which a jet of steam issued and blew against a wheel, making it turn. The wheel was geared to a shaft lifting two rods that Branca used for crushing medicine in a mortar (Hartman 1940; see Figure 5.5).
The principle of steam power is that when 1 L of water is boiled, it becomes 1,849 L of steam. If a cylinder containing a piston is attached to a boiler, the conversion of water
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TABLE 5.1 Performance of stamp mills on ores in Colorado and California
Source: Rickard 1897. |
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in the boiler to steam will drive the piston to its limit; it will return to its original position when the steam is condensed and a vacuum is formed. Eventually a machine known as a beam engine, shown in Figure 5.6, was built using this principle.
The beam engine consisted of a beam pivoted in the center with one end attached to a piston that moved up and down as fresh steam was introduced and then condensed, and the other end attached to a chain and bucket that could be used to lift and discharge water every cycle. Thomas Newcomen built the first commercially successful beam engines, and by 1729 more than 100 were in operation at coal mines. The loss of heat by the piston and cylinder during every cycle, however, resulted in high fuel costs. In 1765, James Watt modified the Newcomen engine by including a separate condensing chamber. Eventually, Watt formed a partnership with Matthew Boulton, who built steam engines to Watt’s design that reduced the cost of fuel for pumping by two-thirds. Beam engines using steam engines of Watt’s design were soon installed at many mines. Operators realized that this source of power could be used for other purposes, including size-reduction machinery, and steam engines came to be used to drive stamps and crushing rolls.
Although steam-powered drills were available by 1860, it was difficult to use steam power underground. The Walhalla mine in Victoria, Australia, was a rich gold mine from which 14 tons of gold were extracted starting late in the 19th century. Because the siliceous ore was very hard, the owners chose to use steam-powered drills. They installed
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FIGURE 5.6 Appearance and method of operation of the steam-driven beam engine (de Camp 1961)
six large boilers in a huge cavern underground, which required several tons of wood each day to keep the boilers working. It was only the size and wealth of the deposit that made steam-powered drilling possible.
Steam power gave a new direction to inventors interested in crushers and changed ideas about mechanical crushing. Unlike water and wind, steam was an intense and reliable source of energy that could be used anywhere at a controllable rate.
Crushing is the low-energy phase of the comminution process, and grinding is the high-energy phase. Crushed products for direct use are known as aggregates, which are hard inert materials used for mixing various-sized fragments with a cementing material to form concrete or mortar. Aggregates are used in commercial centers, homes, airports, highways, dams, and streets. When aggregates from natural deposits were not available, they were made from rocks by hammer and chisel, or sometimes even in a mortar and pestle, and later in crushers developed for the purpose.
Before 1855, when the jaw crusher was invented, the hammers, drills, and chisels used to break rocks were powered by hand. The broken rocks were sorted into the sizes needed for various grades of aggregates on stationary screens set up at an angle and moved in wheelbarrows and handcarts (Figure 5.7). Screening surfaces were stretched over wooden or metal frames, and the material to be separated was shoveled onto the screening surface by hand. The original surfaces were made of heavy-duty cloth, but the advent of metal wire woven to a specific opening size and then of steel plates punched with sized openings extended the lifetimes of screening surfaces. In addition, operators discovered that, if the screens were vibrated, the flow through and over the screen improved, making vibrating screens a vital part of aggregate production.
In the 19th century, specifications were developed for the size distribution and shape of aggregates. When the automobile powered by an internal combustion engine was invented, more and better roads were needed. Beginning in about 1815, a construction system developed by John MacAdam was used for building roads. In this system, which became known as “macadam” through usage, the surface of a road was formed by interlocking stones that had passed through 50-mm apertures and then sealing the surface.
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FIGURE 5.7 Manual handling of ore after blasting in a quarry |
The specifications for aggregates became explicit, particularly when portland cement-made by burning a mixture of lime and clay—began to be used to make concrete for main roads. Soon asphalt became the main surfacing material (although, where the climate permits, cities now use concrete as the top surface for streets), and, with the invention of the airplane and the subsequent increases in the number, weight, and size of airplanes, sturdy airport runways became necessary. The first surfaces of airport runways were gravel (small aggregate), and later gravel was surfaced with asphalt. Today, runways are gravel covered by reinforced concrete. The requirement for efficient crushing was a hallmark of all these processes.