Gigantic grinding mills excel at cutting copper ore down to size so it can be refined. At the Escondida copper mine in Chile’s Atacama Desert, one such mill is scheduled to go into action this summer. But this is no ordinary mill. With inside dimensions of 36-ft diam. and 19-ft long, this semi-autogenous grinding mill (SAG — see next section) will use a 43-ft diam. ring-gear drive to transmit 18,000 hp and handle up to 3,000 metric tons of copper ore per hour. In short, it will be the largest mill in the world.
Following the SAG mill will be two 9,000-hp ball mills, each one 20-ft in diam. and 331/2-ft long, that do the finish grinding. Built by the Allis Mineral Systems Grinding Div. of Svedala Industries Inc., York, Pa., the three mills will process up to 60,000 metric tons of copper per day, according to Stuart M. Jones, director of sales and marketing for Allis Mineral Systems. The Escondida copper mine, the world’s second largest, is jointly owned by RTZ of England and BHP of Australia.
Grinding mill types
In a typical mine operation, a crusher sends large chunks of copper ore, up to 1-ft diam., to a primary, or semi-autogenous grinding mill (SAG), Figure 1, which reduces the ore to smaller pieces, about the size of coarse gravel. Screens sort these gravel-size pieces, recycling large ones through the SAG mill and passing small ones to a ball mill, which further reduces the ore to the size of fine sand. Then the finely ground ore is delivered to flotation cells that separate the copper from other elements. From here, the copper goes to a smelter for refining into a form suitable for use in manufacturing copper-based products.
Basically, a SAG mill is a rotating cylindrical container that relies primarily on the tumbling action of chunks of ore to grind each other down to the size of coarse gravel. A supplemental grinding media (usually steel balls) assists in the process. This type of mill grinds ore that either comes directly from a mine or from a crusher. Its output is either finished size, ready for processing, or an intermediate size ready for final grinding in a secondary ball mill.
SAG mills can operate at a fixed speed or adjustable speed through the use of ac synchronous motors or dc motors. They typically range up to 16,000-hp.
A secondary, or ball mill, works a lot like a SAG mill except that it uses steel balls as the primary grinding media. It normally accepts material that passes through a 1/4-in. screen for hard ores and a 1-in. screen for soft ores, then grinds the ore to a size of 35 mesh or finer. Ballmill capacities range from 75 to 8,000 hp.
Powerful ring gear
To meet the challenge of providing 18,000 hp to the mill, Allis Mineral Systems selected a ring-gear drive with two pinions. Supplying power to the pinions will be two 9,000-hp, 176.5 rpm adjustable- speed synchronous motors. With a speed-reduction ratio of 17.48:1, the drive will produce 9.35-million lbft of torque at 10.1 rpm.
A ring gear drive offers several benefits, such as simplicity, low cost, and ease of installation. But, it requires manufacturing the largest, most powerful ring gear ever made for a grinding mill application. This task was turned over to The Falk Corp., Milwaukee, which also manufactured the two mating pinions.
Making the new gear was particularly challenging — a 43-ft ring gear had never been made due to a 40-ft limitation in gear tooth cutting machines. So Falk modified a Maag shaper cutter machine, extending its capacity to handle up to 46-ft.
Too large to manufacture in one piece, the gear was cast in four circular-arc segments, and their flanges were machined to ensure an accurate match with each other. Workers then bolted the segments together and installed the 190,000-lb assembled gear in a tooth-cutting machine called a “slasher.” This machine makes rough cuts at high speed to approximate the final tooth shape, thereby reducing total cutting time for the ring-gear teeth by about 40%. Then the gear moved to the Maag shaper cutter, which generated the 71/2-deg angle helical teeth, Figure 2, to their proper involute shape, moving at a slower rate to obtain a smooth finish of about 80 min. and an accuracy of AGMA Quality 10. Pinion tooth surfaces were machined to a 32 min. finish, and accuracy of AGMA Quality 12, which reduces friction and dynamic losses.
After inspection, workers disassembled the gear into its four segments and shipped them, along with the two mating pinions to Chile in December 1994. The gears are scheduled to be installed on the SAG mill at the mine in June 1995. They will be driven through clutches by two 9,000-hp, 176.5 rpm adjustable-speed synchronous motors made by GE Ltd. of Canada.
Bigger mills fuel drive technology race
The new Escondida mill represents the latest in an on-going competition between two major types of drives to meet growing capacity requirements. Main competitors are the ring gear, as used at Escondida, and a ring motor (gearless drive), either of which wraps around the mill. A third, less common method, is a conventional speed reducer.
In a ring-gear (or girth gear) drive, the gear is bolted to the outside diameter of the mill shell, Figure 3. In one configuration, each low-speed motor (usually 200 rpm) supplies power to the ring gear through a pinion, and the ring gear turns the mill. Other applications use a higher-speed motor (usually 1,200 rpm) connected to a speed reducer to drive the ring gear. The higher-speed motors are either squirrel cage or wound rotor. Low-speed versions are normally synchronous motors and are used with air clutches.
The Escondida SAG mill will be, by far, the largest gear-driven mill. The next largest is Asarco’s 34-ft diam., 19-ft long, 14,000-hp mill in Arizona.
In a ring-motor drive, Figure 4, a ringshaped motor encircles the mill and drives it directly without gears. The motor consists of rotating rotor elements (poles) that are bolted to the mill shell and a stationary stator assemble that surrounds the rotor elements. Electronics convert incoming power from 50 or 60 Hz to about 1 Hz to create a low operating speed, usually about 10 rpm. The mill shell essentially becomes the rotating element of a large, low-speed synchronous motor. Mill speed is adjusted by changing the frequency of the voltage to the motor. The motor can also be used to inch and spot the mill for maintenance.
Ring-motor drives were first used in cement industry mills and later in mining industry applications. The largest ringmotor- driven mill in the world is a 36-ft diam., 16,000-hp SAG mill that processes up to 2,000 tons of ore per hour. Located at Kennecott Utah Copper near Salt Lake, this mill uses a ring-motor drive, Figure 5. The SAG mill is followed by two 20-ft diam., 7,500-hp gear-driven ball mills.
Another type of drive, less commonly used, consists of a motor-driven speed reducer that is coupled to one of two millsupport trunnions, and drives the mill from the end. These conventional speed reducers, which are said to be more expensive, are used primarily in the cement industry where mill expansion problems make it less practical to use a wraparound drive.
Shifting popularity. Traditionally, the ring-gear drive has been preferred over ring motor and speed reducer drives because of its simplicity and lower cost, plus ease of installation, operation, and maintenance, says Craig D. Danecki, Falk’s manager of marine and general gearing.
The most significant advantage of a large ring-gear drive is the lower initial cost compared to a ring-motor drive. The savings in capital costs for a ring-gear drive ranges from about 1 million dollars for an adjustable-speed system to 2 million dollars for a fixed-speed system. Installation cost savings for a ring gear drive can also be substantial.
Ring gear-efficiency is estimated to be as high as 99 to 99.5%. This high efficiency is due, in part, to lower losses from seals and oil churning compared to conventional multistage speed reducers that drive from the end. Also, ring-gear and pinion tooth surfaces are machined to a high AGMA Quality level to enhance efficiency.
However, ring -gear capacity hasn’t kept pace with increased power requirements of SAG mills. The main limitation: they couldn’t be made in sizes larger than 40-ft diam., which limited the mill size to 34-ft diam. The only method of driving mills over 34-ft diam. was the ring motor, which is said to be more complicated and expensive.
Technology puts gears back in the game. But manufacturing advances have boosted the ability of ring gears to handle the loads required for today’s large grinding mills. Recent improvements that enhance ring-gear strength and durability ratings include:
• Ladle treatments, vacuum remelt, and vacuum degassing that produces cleaner steels in forged pinions, thus reducing discontinuities, which may cause fatigue failures.
• Full-ring risers that reduce shrinkage and contaminants in gear castings and make the hardness more uniform.
• Tooth quality, which has shown the most improvement. Manufacturers now cut the large gears to AGMA Quality 10, thereby increasing their horsepower rating. Pinion teeth are surface hardened and ground to AGMA Quality 12. This is expected to give a life of 8 years per flank or 16 years total when both tooth faces are loaded. This extended life span is accomplished in SAG mills by operating gears in both directions. In a ball mill, the pinion is reversed after teeth become worn on one side.