June/July 2007

Metallurgy

The Chilean Mill or Trapiche

By J.G. Watt

The Chilean mill or trapiche is still used in small-scale mining in Chile, Bolivia, Peru, and Argentina. It is a form of edge-runner mill with from one to four heavy wheels, or mullers, running in a circular U-shaped trough. The ore, which is generally hand-shovelled into the water-filled trough, is crushed by the weight of the revolving wheels. The bottom of the trough is protected by a steel-wearing ring or soleplate. Scrapers fixed behind each roller lift the ground ore off the soleplate and keep it in suspension. The resulting slurry exits the mill through a series of perimeter screens near the top of the trough. The mesh size, height above the soleplate, and length of the overflow screens determine the final grind size and daily throughput. It seems that the Chilean mill is a direct descendent of early Roman olive mills which were rotated by hand or animal traction. The ancient Chinese independently developed an identical machine, called shih nien, for grinding grain.

In the trapiche used today, a short horizontal axle connects the wheel(s) to a central vertical shaft that has mounted on it a bevelled crown gear driven by a pinion. A series of belt-driven pulleys connected to a motor via a jackshaft drive the horizontal main shaft on which the pinion is mounted. While electric motors are most common, diesel and gasoline motors are also used. Motors are generally in the range of 5 to 25 hp. For ease of maintenance, the drive mechanism is normally mounted above the circular trough.

In Chile, rollers and troughs were originally made of carved rock. Near Copiapó, there is an abandoned quarry where semi-completed rollers and bowls can be seen. Nowadays, old railway wagon wheels are the usual base framework for the runners; grinding balls, worn mill liners, or other suitable scrap iron are cemented in to attain the desired weight. The circular troughs are of welded steel plate or poured concrete.

The name 'Chilean mill' probably reflects its widespread use in Chile from where it was introduced into neighbouring countries, rather than any claim to having originated there. There was once a town called Chilean Mill near Prescott, Arizona. Today, this is little more than a geographical spot as the trapiche from there is now in a museum in Jerome, Arizona. Likewise, the hamlet of Trapiche, about 85 kilometres north of La Serena, Chile, no longer depends on mining and no Chilean mills are in use there. The townspeople now cultivate carnations for the local and export market.

While it is not known when the trapiche was first used for crushing and grinding rock, it is probable that this use started in Spanish America. Agricola, in De Re Metallica, published in 1556, makes no mention of trapiches or similar machines for grinding ore. His description of milling devices is limited to several varieties of wind, water, human, or animal-driven mills with either one or two horizontally revolving millstones. The use of the trapiche in Chile is documented to at least 1732, and in 1803, there were 130 being used for grinding gold ore there. They have been used in Mexico and the United States since at least as early as 1882. They were also used in the Nova Scotia gold fields during the early 1860s and in Australia at the Walhalla gold field in the mid-to-late 1860s.

Taggart (1927) gives a description of the Chilean mills that were common in plants in the United States in the early 20th century. The Frasers & Chalmers Company and the Bonnot Company both manufactured Chilean mills. Up until the early 1980s, Chilean mills were being used in two copper smelters in Arizona for grinding clay to make plugs for converters and reverbs. These machines were similar to the pan mill, used in English clay-brick factories, in that the pan or trough was rotated, causing the fixed wheels to turn.

The trapiche was the primary grinding machine in the American southwest copper industry during the late nineteenth and early twentieth centuries, but was displaced by the widespread introduction of tube and ball mills. In Chile, it has been, and still is on a somewhat smaller scale, the main grinding machine at hundreds of small mines. According to the mayor of Andacollo, Chile, at mid-2006 there were nine milling plants operating in the area, with a total of 49 trapiches, down from the 55 plants with 295 trapiches working in 1970.

Increased copper prices spurred the development of the trapiche as a high-capacity (20 to 40 tonnes per day) primary grinding unit in copper milling. Grind fineness is sacrificed, however, with 80 per cent passing the equivalent of 65 mesh Tyler (212 microns) being the normal target. This is achieved with heavier wheels, each weighing between 2,500 and 3,000 kilograms, and by increasing the velocity of rotation to about 18 rpm. The resulting slurry discharges via perimeter screens to the flotation circuit. The mesh and the height of the screens above the soleplate control the grind fineness. Discharge height is normally four to five inches above the soleplate. The approximately 30 per cent solids pulp density required and the higher capacity (20 to 30 kilograms per minute) make these units suitable for a conveyor belt feed system.

To further increase plant throughput, trapiches were put to use as secondary grinding machines. These were either in open circuit, treating the discharge from a primary grinding trapiche, or in closed circuit, with some type of hydraulic classifier. The characteristics of the regrind trapiche are similar to those of the grinding trapiche although with a steeper trough angle and a slower wheel velocity of around 14 rpm.

The Amalgamating Trapiche

In the amalgamating trapiche, copper plates covered with mercury are suspended from the rim of the trough to hang in the slurry and recover the liberated gold particles through amalgamation. As well, free mercury is dispersed around the bottom of the trough to pick up additional gold from the slurry, increasing the total recovery.

Grinds of about 80 per cent passing 150-mesh Tyler (100 microns) are the norm, with roller weights of 1,000 to 1,500 kilograms and velocities of 11 to 13 rpm. Throughputs are around five to six tonnes per day per mill. Heavier wheels would cause disintegration of the free mercury on the bottom of the trough and the resulting extra fine globules would escape via the overflow discharge screen. Higher rotational velocities would cause centrifuging of the slurry, limiting exposure of the suspended gold particles to the copper plates. Trough angles of around 45 degrees or more are the norm for amalgamating trapiches. The overflow discharge screens are normally 10 to 13 inches above the soleplate. Slurry densities of around 16%, equivalent to a feed rate of around 31/2 kilograms per minute, are aimed for. Two shovellers can easily keep a bank of five or six trapiches operating continuously. The trapiche is periodically stopped and amalgam in the bottom of the trough is manually recovered for retorting. Today, the landscape of northern Chile is dotted with hundreds of old tailings piles, evidence of past trapiche operations. These are often contaminated with mercury.

The author would like to thank those friends and colleagues who supplied data, constructively criticized drafts of this paper, and offered helpful suggestions. Special thanks go to Wiston Rocher, Ed Rood, Jorge Ordenes, and Fathi Habashi.

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