Part 2
"Look at that!" Luis says. "On a hot day 1 don't want to be standing here." I ask him about all those families living in houses built on the relaves in nearby Andacollo.
"Shouldn't they move to safer ground?"
"They'll all die before they move."
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.
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.
As the wheel rotates on its short axle, and spins around the bowl on its long axle, fines are generated beneath it. The center of the wheel rim is limited to crushing ore as it rolls around. But the inner and outer rim surfaces also rub and slide on the base ring. As they go around, they effectively "smear" the rock beneath the rim in addition to crushing it. The wheel sides are designed to agitate the slurry, without splashing, so that fine particles remain suspended and coarse material settles to the track.
Amalgamation in the Trapiche
Half-a-dozen 12 by 20-inch copper amalgamation plates, coated with mercury on the side facing the wheels, hang over the inside lip of the bowl, submerged in the slurry Gold liberated, by grinding and suspended in the slurry by agitation, is captured in the mercury film by amalgamation. Every few hours the operator removes and cleans the plates one at a time. During clean-up the plate is rubbed forcefully with a hard rubber block. This pushes the amalgam off one end into a bowl. The plate is then sprinkled with fresh mercury, smoothed with a soft sponge, and replaced in the trapiche.
Surprisingly, all of these manual operations are almost identical throughout the country.
During the grinding stage, sulfides in the ore such as pyrite or arsenopyrite can poison portions of the amalgam plate. When this occurs, fresh mercury may not readily adhere to the surface in those spots even after cleaning. Grease and oil from around the mine can also coat mercury and interfere with amalgamation. To combat plate poisoning a mild ammonia liquid is often blended into the plate after cleanup by rubbing briskly with the fingers. This reagent, added from a plastic squeeze bottle, is often simply the urine of the operator.
Gold encapsulated in unground particles will not be attracted to the mercury. Gold occurring in sulfide minerals such as pyrite, arsenopyrite, or galena, will also generally remain with the host particle in the slurry and be discharged to waste. This is lost gold. Mercury leaves the trapiche loosely attached to fine rock particles, and as flour mercury This discharge, along with the lost gold, ends up in the relaves (relavez), the stacked tailings.
Years of work with a particular ore has taught the operator how many hours are required for the best amalgamation. When this optimum point is reached, the trapiche slurry is released through a gate. A series of amalgamation plates may line the discharge launder to capture additional values.
Gold Recovery
The amalgam scraped from the plates is squeezed in a wet chamois or fine woven cloth. Only mercury passes through the fabric. A double Kleenex layer may serve the same purpose. Ultimately, a soft, silvery ball remains in the fabric. This is finger kneaded in the palm of one hand or in the bowl and tightly squeezed in the fabric to produce a hard ball. The result is an amalgam ball that contains about 30 percent gold and 70 percent mercury. All the clean up and gold-recovery operations I observed in Chile were done by bare handed men in almost constant contact with liquid mercury.
The amalgam ball may be retorted to recover both metals separately. A common practice in Chile is to burn the mercury off in a two inch pipe fired with butane. Mill hands stand back from the invisible, toxic vapors. The gold is then melted and cast into a small, rough ingot for marketing.
Other and Current Uses
I find it most curious that, to my knowledge, there is not a single trapiche in the entire U.S. Of course, using mercury for amalgamation is now against the law in all fifty states. But Richards(7) describes the use of trapiches here in the early 1900s, 36 of them operating as grinding units in one Utah plant, and others as far east as North Carolina.
Looking at all gold recovery systems in the world, Taggart(
had this to say about the trapiche in the first half of the 1900s: "Upkeep was low and the machine, essentially foolproof. It is adapted to inside amalgamation and is still used and wisely for some small gold mills."
For grinding alone, without amalgamation, Rickard(9) said the Chilean mill produced a product 80 percent finer than 200-mesh in sizable installations. He observed that in the early 1900s dozens of Chilean mills were kept in operation alongside Krupp ball mills and similar grinding units in Mexico.
The Trapiche Heritage
The little city of Andacollo mentioned previously, has been, for centuries, one of Chile's major sources of gold wealth. Trapiche relaves to forty feet in height dominate the city. Many homes are built on massive amalgamation tailings. Old trapiches, both operating and abandoned, can be seen around the city borders. From senior government officials in La Serena, the capital of the region, I learn that there are ten deaths annually in Andacollo alone due to mercury poisoning.
The amalgam ball from a Chilean trapiche clean-up contains one ounce of gold. It is about 30 percent gold and 70 percent mercury. The economic benefits from gold, in the millions of ounces, recovered by trapiche amalgamation in Chile are now being partially, and painfully, offset by the recognition of environmental damage. The U.S. embargo on Chilean swordfish meat is a case in point. These salt water fish are contaminated with mercury from rivers draining the western slope of the Andes. This runoff flows through the trapiche relave regions, and empties into the Pacific along Chile's 3,000 miles of coastline where it enters the aquatic food chain.
These and other serious environmental consequences could seal the fate of the amalgamation trapiche, as it is known today in Chile.
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Topic: LDM MINE SHAFT PROJECT (Read 1997 times)