From rock to powder
December 10, 2009
After miners extracted ore from the bowels of the mountain, the silver had to be separated from the worthless rock. For a few years, Aspen’s miners accomplished that task by hand sorting. They chose the purest pieces and chipped away useless rock with hammers and chisels. The constraints of that time-consuming process limited prospectors to only the purest of ore. Resulting treasure then was transported by mules over the mountains to mills and smelters in Leadville. Only high-grade silver left the valley profitably.
When less costly transportation became available, greater quantities of ore and lower grades became economically viable. Miners turned to three mechanized methods of separating silver from rock: stamp mills, ball crushers and rod mills.
Like many Aspen youths, I played with baseball-sized steel balls that I scavenged from the town’s surrounding mines. They had once crushed ore in ball crushers. If you have seen the smaller version, a rock polishing tumbler, you understand the concept. Ore is dumped into thick-walled cylinders that contain six- to 15-pound steel spheres. The cylinders then rotate at a speed sufficient to make the balls fly back and forth, smashing the ore with each bounce. If rock tumblers are not familiar to you, then think of what happens when you throw your tennis shoes into your clothes dryer. Heavy steel balls can do much more damage. If you think your tennis shoes make a racket, imagine what steel on steel sounded like.
The stamp mill was more commonly used at gold mines. The iconic remnant at most of Colorado’s east slope mines, as well as those of California, the stamp mill replaced miners who smashed rock with their sledgehammers. Ore was placed under steel columns that weighed between 500 and 750 pounds, and stood eight to ten feet high. A revolving cam at the top raised the columns about a foot before dropping them onto the ore beneath. The Colorado Iron Works in Denver manufactured many of these crushers. They could be arranged into batteries of eight to 60 in mill buildings. Steam and, later, electricity, powered them.
Although ingeniously invented devices fed the ore under the cylinders automatically, mill workers were still needed to keep the material moving freely, a dangerous and unpleasant job. The decibel level of a ball crusher was a whisper in comparison to a battery of stamp mills. If the material passing through a stamp mill was too coarse, then workers would have to collect it and run it through again for finer grains. Ball crushers replaced stamp mills primarily because a ball crusher could run as long as necessary before workers retrieved the ground ore.
Rod mills, similar to ball crushers, were less common. Ore was placed in large revolving cylinders. Instead of balls, steel rods pinched the material between the rods until a fine powder was produced. The ear-shattering noise was more acceptable than that of a stamp mill or ball crusher. Both ball and rod mills worked with either dry material or slurry of rock in water, which made it easier to fill and to empty the cylinders.
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Aspen’s mines, at different times, employed all of these methods. Later, some mines added jaw crushers for breaking larger rocks into sizes small enough to fit into the other crushers. Once ore was pulverized, silver could be separated through mechanical and chemical processes. In the end, the metal was smelted into gleaming bars of silver.