Chile A Story of Gold and Quicksilver
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.
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