E&MJ reported in October 2010 (See “SAG Mill Gearless Drives Go to South American Projects,”p. 102) ABB Group’s Minerals Division had won an order from Newmont Mining Corp. to engineer three gearless mill drive (GMD) systems to be installed at its Yanacocha Minas Conga oxide copper-gold project in Peru. Fluor Corp. in Santiago, Chile, was selected as EPCM contractor for the project and Metso Corp. as mill supplier. Metso has since provided additional details on the project, involving what is considered to be the largest SAG ever installed.

Metso said it will design and supply a 42 x 25-ft (12.8 x 7.6 m), 28-MW gearless SAG mill, plus two 26 x 42-ft (7.9 x 12.8 m), 15.6-MW ball mills as well as two VTM-1500-WB Vertimills for the pro-ject. The 42-ft-diameter SAG mill, according to Metso, will be the world’s largest and the first of its kind.

The Conga project is located approximately 560 miles (900 km) north of Lima, at an altitude of 13,400 ft (4,100 m) above sea level. It is situated near the Newmont Yanacocha gold mine, where Metso installed what was then the world’s largest single-stage SAG mill, a 32 x 32-ft (9.75 x 9.75-m) EGL mill, driven by a 16.5-MW ABB gearless motor, in 2008. That mill design was also the first of its kind and was customized for Newmont. According to Metso, it began commercial production within one month of its start-up—another first for a Newmont project.

With the engineering and release for fabrication of these new mills, Newmont is seeking to maintain the option of proceeding with Conga on an accelerated schedule. Full funds approval is expected from the board of directors in the first half of 2011.

Metso said Yanacocha has experienced excellent operating performance from its SAG mill across a wide range of ore types. Kim Hackney, director of the Yanacocha gold mill project, noted at the time of the mill’s installation that “after just the first week of operation our incremental recovery rate was 80%–90%, compared to 60%–75% with leach pads.” Processing higher grade ores from various pits has consequently provided greater flexibility and revenue generation.

An ABB gearless mill drive system will power the SAG mill with a variable speed drive that allows the mill to start smoothly with minimal mechanical stress and with automated “dropped-charge” protection. This, according to Metso, fulfills the customer’s requirements for flexibility and adjustability of the process and enables maximum control over the grinding process.

In announcing the details of its role in the Conga project, Metso noted it has a tradition of developing and producing increasingly larger and more powerful mills. Starting in 1959 with an 18-ft (5.5-m) diameter, 448-KW AG mill, Metso subsequently designed and produced the first 24-ft (7.3-m) AG mill, the first 32-ft (9.75-m) AG mill, and the first 36-ft (11-m) AG mill, while reducing the required energy load to operate such huge machines.

In 1986, Metso unveiled the first gearless SAG mill, which has grown through stages of 38 ft (11.6 m) and 40 ft (12.2 m) up to the size of the giant unit slated for Newmont. According to Randy Will, vice president, capital mining sales–Americas, Metso is looking forward to developing this new mill as the next standard range. “Con-
centrator plants keep getting larger and larger, and this new mill will be a significant development for the future.

“A single line grinding circuit with a 42-ft SAG mill allows for much higher concentrator throughputs, as compared to using a dual line grinding circuit using two SAG mills,” Will said. “The capital cost savings using one line versus two are significant.” He noted the new design does not compromise the traditional aspect ratio.

Will also pointed out Metso has introduced larger cone crushers and Vertimills.  “This will become more important as the copper grades decrease. More ore must be processed to meet the equivalent of current copper production. That’s where customers need the increase in mill size that Metso can now provide.”

New Ownership, Technology Developments in Sample Prep
In an industry that takes considerable pride in its ability to blast and move massive amounts of ore and rock in both surface and underground mining operations, it’s easy to overlook some of the more delicate technologies that play important roles in the value chain from ore to finished product. Sample preparation is one of those areas, and recent announcements in this sector illustrate the current value and sophistication of the technology.

For example, FLSmidth announced on December 5 it had made an offer to acquire ESSA Australia Ltd. for approximately DKK 170 million ($30.9 million). ESSA specializes in the design and manufacture of sampling and sample preparation equipment for the minerals and mining industry. According to FLSmidth, ESSA is active in a number of market segments including mineral sample preparation, laboratory equipment, industrial solids preparation, mineral sizing, metallurgical testing, run of mine sampling, replacement parts and maintenance, laboratory and sampling automation.

At about the same time, Illinois, USA-based Buehler, a manufacturer of scientific equipment and supplies for material preparation and analysis, introduced its EcoMet 250 Pro grinder-polisher with AutoMet 250 power head. New capabilities include the ability to store and recall 32 sample preparation methods via touch screen controls, a Zaxis (Z-axis) function that removes material by depth instead of time and power head speed adjustments from 30 to 60 rpm.

According to the manufacturer, the EcoMet system’s programmable and adjustable functions increase consistency and repeatability, ensure better flatness and enable method optimization compared with manual grinding. An optional PriMet Pro modular dispensing satellite uses peristaltic pumps, not mist technology, to distribute lubricant, diamond and colloidal polishing suspensions. Controlled dispensing is claimed to offer repeatable results regardless of the operator, and to lower consumables costs by preventing over-use.

Buehler’s Rick Wagner, manager of specialty market development, noted mineral preparation is often done by hand. “Minerals are very brittle, and they can have several different hardness variations within a sample,” he said. “Testing and lab personnel believe they must have a ‘hand touch’ on the sample to prevent a pull-out or over-preparing it. Unfortunately, manually prepared samples often have planar issues and exhibit variations between samples.”

While it also allows manual sample preparation, the semi-automatic system provides several tools that help increase quality. Variable speed adjustments provide the flexibility to custom tailor methods to the type and hardness of a mineral. Power head speeds greatly affect abrasive use. Higher speeds impact the abrasive aggressively, while slower speeds are gentler on the sample. The previous system operated at either 30 or 60 rpm, whereas the new AutoMet 250 adjusts between 30 and 60 rpm in 10-rpm increments.

Semi-automatic material removal also helps ensure better flatness—critical when making measurements—because it eliminates hand rocking. And, the Zaxis function enables targeting a specific plane. After determining the average depth of material needed to be removed to reach the damage-free/stress-free section, personnel can program the EcoMet 250 Pro to remove a specific amount of material, rather than programming it by time.

“In mineral preparation, samples are generally prepared to 30-micron thickness,” said Wagner. “The Zaxis function easily allows targeting that thickness. Then, once the sample approaches target thickness, the AutoMet 250 Pro single force sample holder accepts glass slide holders for the final polishing steps.”

Operating parameters, such as speed and force, are adjustable during the preparation sequence. A pause function allows the operator to stop operation during a step, remove a sample for examination, and then complete that step at a later time.

Buehler also offers the EcoMet 250, a non-programmable grinder-polisher with push-button controls and digital readouts.