Chile A Story of Gold and Quicksilver
These mills, shown as drawings in Agricolas text, had hard-wood toothed-gear drives with wrought iron axles connected to grinding stones.
Perhaps some of this knowledge crossed the Atlantic with the early Spaniards. A letter dated July 25, 1511, from King Ferdinand VII to his colonists in the New World says, "Get gold, humanely if you can, but at all hazards get gold. "(I)
Another early and very simple Chilean ore grinder is the maray. It dates from the first Spanish efforts to liberate gold from its vein rock after about 1570. The maray is a round boulder about two feet in diameter with two holes in the top. Wooden pegs driven into the holes are fastened to a long wooden pole. The boulder is set in a stone depression with ore. Men sit on either end of the pole and rock back and forth see saw fashion, grinding the ore to a powder. Several of these primitive devices can still be seen in Andacollo, a small city north of central Chile, where they are used occasionally to grind high-grade gold ore.
In 1557, Bartolome Medina introduced the practice of mercury amalgamation into Mexico. From there it spread south (4), and the use of this liquid metal to capture Chilean gold came into widespread use.
The Modern Trapiche
The Chilean trapiche evolved, probably sometime in the early 1700's, into a faster-moving machine with iron tires doing the grinding. The metal bands were mounted on hollow wooden cylinders. These were filled with heavy rock to give them weight. Thick metal tires over seven feet in diameter(5) and two feet wide used in the early years were worn down by as much as 15 pounds of iron per ton of ore ground.
The modern two-wheel trapiche swings on an axle powered by a 10 or 15 HP electric motor. The mechanism rotates at 40-60 rpm, while the wheels ride free on their steel track. The thick, work hardened steel tire is six or more inches wide and is held in place by wooden wedges driven between it and the five-foot diameter wheel. Manganese steel may also serve as the stationary working surface in the bowl base. After a few months of non-stop grinding hard silicate ores to as fine as 150-mesh (6), wear on the thick ring and the wheel rim require replacement of these metal parts.
Installations with ten or more trapiches currently operate in Chile. Ore hauled in by truck is usually dropped or hand-fed into a jaw crusher. From there, the minus two inch material goes by wheelbarrow to stockpiles between each two trapiches. This crushed ore is shoveled over the lip of each bowl at the rate of 600-800 Lbs per hour, about ten tons per 24-hour day.
One-man operations are not unusual in single trapiche mills. All operating installations are labor intensive. The wage of a laborer, working 12-hour days, is about $200 per month.
Grinding takes place between the rotating steel rim and the base ring track.
Reprinted with permission of the author. Dr. Ralph Pray,
Mining & Metallurgy
805 S Shamrock Ave
Monrovia, CA 91016
Telephone: 626-357-6511 Fax: 626-358-8386