Like a lindworm, the flightless dragon of Norse legend, the conveyor belt snakes out of the tunnel at the end of the Valle del Choapa. It is a dragon that devours mountains. Never-ending heaps of light-colored crushed rock travel down the 1.80-m-wide belt toward the valley. The copper ore comes out of a mine, 13 km away at an altitude of 3,200 m above sea level—the Los Pelambres open-pit copper mine.[hr]
The gigantic hollow is 2.5 km long, 2 km wide, and by the year 2030 will be 1 km deep. Every hour, the belt moves 8,700 t of ore out of the mine and down to the processing plant, which is located at an altitude of 1,600 m. This massive volume of ore is large enough to fill some 200 heavy-duty trucks. Without the conveyor belt, there would be no mine, says operations director Ricardo Funes Maggi, of mine operator Minera Los Pelambres. “The belt facility is the backbone of our operation.”
Although Los Pelambres is located in a remote region of central Chile, near the tallest peaks of the Andes, where it is subjected to storms, snowfall and rock slides, the pit is among the world’s most profitable mining operations. The conveyor facility, which was conceived and built by Siemens and ThyssenKrupp, makes a decisive contribution to the plant’s economic success. The gigantic conveyor belt not only transports ore to the valley below, but also generates electrical power. In fact, it produced 90 mill. kWh in 2007.
“Just as in a hydroelectric power plant, potential energy is converted to electrical energy,” explains Christian Dirscherl, Mining Marketing Manager for Siemens’ Industry Sector. When the belt is fully loaded, gravity pulls the rocks downward and it runs by itself. The ten electric motors needed to start and maintain the belt’s speed operate as generators to produce electricity, at an efficiency of up to 15%.
Even many years after entering service, the conveyor belt is the longest and most efficient of its type in the world. Mining operations are major consumers of electrical power, and in Chile, electricity is a scarce commodity. Even during the planning stages in the late 1990s, investors and operators paid close attention to efficiency, especially since there was no commodity boom in sight at the time. In fact, in 1999, copper prices reached a record low, with 1 t of the metal going for only $1,320. Since then, the price of copper has risen to more than $8,000 per metric ton.
The reason for the rise in prices is, above all, high demand from China. Half of the worldwide annual production of 15.6 mill. t is used in building construction, for example in plumbing and roofing. Because of its good electrical conductivity, copper is also an essential element in electrical devices; it is found in wires and cables, electromagnets, electric motors, generators and transformers, in addition to everyday items such as coins and pipes.
In Los Pelambres, the ore is blasted out of the sides of the open-pit mine. Giant power shovels load boulders, weighing many tons, into 320-t trucks. These transport the ore to a crusher, which breaks the rock between two giant drums. From there, the fragments, now no larger than 30 cm, fall directly onto the belt.
[hr]“Every hour, the belt moves 8,700 t of ore out of the mine and down to the processing plant, which is located at an altitude of 1,600 m.” [hr]
“Because of the low metal concentration of 0.78 % copper, an enormous amount of material has to be moved to make the process profitable,” explains Dirscherl. In other words, the conveyor belt, which consists of three sections, has to perform heavy labor.
Traveling at a speed of 6 m/s, up to 5,100 t of crushed rock can be rolling downward through a tunnel at any given moment. The trip to the processing plant takes 35 minutes. Because of the steep slope, which averages 10%, there is a risk of uncontrolled belt slippage. But in an emergency, 13 disc brakes, each with a diameter of 2.5 m, can stop the entire belt facility within 70 seconds.
The ten drive motors associated with the belt consume power only if the belt is carrying less than 800 t of ore. But in generator operation, they provide up to 17 MW of power, which is fed into the national power grid. The operation generates about 15% of the mine’s own power needs, thereby reducing carbon dioxide output by more than 50,000 t per year. For this achievement, in 2005 Minera Los Pelambres was awarded the National Prize for Power Efficiency by the Chilean Ministry of Economics.
“The motors are subjected to extreme loads; they have to withstand dust, moisture, dramatic temperature swings, and vibration,” reports Dirscherl.
The conveyor belt therefore uses robust, maintenance-free machines made by the Siemens Dynamowerk in Berlin, Germany.[hr]
“…the conveyor belt is the longest and most efficient of its type in the world.”[hr]
At the heart of the conveyor facility is its control and communications system. Sensors connected to a fiber-optic network monitor all significant components around the clock. Engineers anywhere in the world can log into the mining operation’s server through a virtual private network and quickly search for the cause of a fault. An on-site service team is responsible for maintenance of the entire conveyor system, from the rollers to the belt and the motors.
After the ore is transported into the valley, the more complex part of copper extraction begins. The ore has to be separated from the rock, and processed. To achieve this, the roughly crushed metallic compounds are first ground in huge mills measuring 12 m in diameter to produce an ore slurry. “It’s like using a giant washing machine,” says Dirscherl.
Falling lumps of ore, along with iron balls, shatter other fragments. Gearless ring motors wrap around crusher drums. Each of these electric drives has a rating of 15 MW, and rotates the drum purely through electromagnetic forces—in other words, without direct contact. Starts and stops are soft and easy on the machinery.
“Because there are no gears and no transmissions, our ore mills exhibit extremely low wear,” explains Dirscherl.
Gearless drives also have higher efficiency and use less energy than other motors. So far, Dirscherl and his colleagues have sold more than 50 of these mills to mining operations all over the world.
Metal from Foam
Once the rock has been ground, the actual separation process takes place. Los Pelambres uses so-called flotation cells—giant vats, each containing an agitator that mixes the powdered and chemically treated ore with water and atmospheric air.
Because the chemically treated metallic compounds are less wettable than other minerals contained in the ore, they cling to the surfaces of rising gas bubbles. At the surface of the flotation cell, a metal-rich foam collects and is skimmed off. To achieve a copper concentration of 30%, the slurry has to pass through multiple flotation stages. The resulting concentrate ultimately flows through a pipeline to the harbor at Los Vilos, where it is dehydrated and shipped to Japan. There, a smelter produces the pure metal by roasting the concentrate, followed by electrolysis.
Along with copper, Los Pelambres also extracts molybdenum, which is recovered using the same flotation method. The mine’s ores contain 0.02% of this metal, which is used primarily in the production of high-strength steel alloys.
Recently, testing has begun on a new, highly-efficient flotation cell at Los Pelambres. The cell enables molybdenum extraction to be increased by more than two percent. What makes the cell unique is that it operates without an agitator, because the ore slurry is sprayed into the cell by a high-pressure nozzle, along with nitrogen, which is often used as a substitute for atmospheric air in molybdenum extraction. Thanks to this process, a larger proportion of tiny molybdenum particles, most of which were once lost, can now be separated from the slurry.
“An increase of 2% may not sound like much, but given the quantities produced here, it pays off,” says Wolfgang Krieglstein, Sales and Product Manager for flotation cells.
Currently, molybdenum is trading at $77,000 per metric ton, and Los Pelambres produces just under 10,000 t per year. In addition, the cell uses 70% less energy and 90% less nitrogen than comparable cells. Following these promising test results, Krieglstein and his colleagues are now working on a similar cell to increase the company’s copper yield as well.[hr]
“The gigantic conveyor belt not only transports ore to the valley below, but also generates electrical power.”[hr]
The new flotation cell follows a trend in the mining industry toward increased resource conservation. “Process improvement is an increasingly hot topic. The goal is to extract more metal from the mined ore, and use less energy in the process,” comments Dirscherl.
Another key topic is water consumption and treatment. “This is an area that will definitely have to be addressed more aggressively in the future,” says Dirscherl.
Like Australia, in rain-starved Chile, mining and agriculture are the greatest consumers of water, and water rights are hotly contested.
“Water is the critical factor when it comes to developing mining operations in northern Chile,” according to the Chilean economic web site Ecoamerica.
Considering the circumstances, Los Pelambres is looking to the oceans. The company sees desalination plants as a potential answer, even though its mines are located up to 170 km away from the coast, and at altitudes of up to three thousand meters above sea level.