Chile A Story of Gold and Quicksilver
By Dr. Ralph E. Pray
In the end, I have to shift gears. The relaves are simply not rich enough to merit the expense, political or financial, required to extract the gold. But there is something else to do here. The trapiche plants are shutting down or converting to grinding plants ahead of froth flotation. The really important work for me to get involved with around the trapiches is either in cleaning up the old relaves, or in removing mercury trapped in the slurries of working trapiches before it's discharged. Cleaning up means writing grant proposals for millions of dollars and moving a lot of dirt. But working with freshly lost mercury is a different ball game. The slurry contamination is in the form of flour mercury, particles so small their individual surface tension exceeds the energy required to effect coalescing into larger bodies of fluid quicksilver. But I have another means of motivating these microscopic specks as they leave the trapiche, before any mercury arrives at the relave. Success in this, I am assured, will permit the resumption of gold trapiche operation throughout Chile. There is plenty of work ahead. The main task is to bring the mercury content of a trapiche slurry from a customary several parts per million to zero.
A very attractive suggestion is made at the highest Chilean technical level that I do my investigation in Chile's principal research facility with Chilean funding. I am presently doing the preliminary research in my Los Angeles lab with no outside assistance, With luck, I'll go back to Chile and continue the work there.
In Chile, the trapiche (tra-peach-ee) method of milling gold ore dates back more than 400 years (1). The first trapiche, also known as a Chilean mill, is a stone wheel weighing two or three tons(2) rotating on an axle. The stone wheel, powered by mules, rolls in a circle over crushed rock, grinding it to a powder. Gold particles break out of the rock during the process. The 19th century working trapiche shown in Photo 1, is a direct descendant of the 16th century apparatus used in Chile, Peru, and Mexico to grind gold ore.
An early improvement to the trapiche is the addition of a second wheel opposite the center post. Water poured in during the grinding makes a thin mud. The early wheels were probably used in the gold fields shortly after the 1570's, simply to grind ore. The mud is flushed from the mill and agitated in water. The gold settles out, and the pulverized rock that's left is washed away. The massive stone wheels used in these early mills can be seen abandoned in many parts of Chile.
Very Early History
The idea of using a carved rock wheel to grind ore was not born in the Americas. Long before the Spaniards arrived in Chile, the rotating stone to grind ore was widely employed in Europe. According to Agricola (3), writing in 1556, there had long been water powered grinding mills in the Carpathian region of Germany, certainly before the discovery of the New World.
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June, 2001.
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