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MEI Online: Plant Operation News: Australasia: July 8th 2009


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:: Outotec Tankcell Train helps Macraes Achieve World Efficiency

New Zealand’s largest gold producing operation, OceanaGold’s Macraes mine, is providing a world-class example of how investment in efficient technology can achieve rapid payback and increased returns even in challenging economic times.

Since the installation there of the world’s largest operational mechanical flotation cells - three Outotec TankCell 300 flotation cells in a single train - gold bearing sulphide recoveries have been boosted to take advantage of strong prices while maintenance and operating commitments have diminished.

Total Macraes production increased 42% to 259,812 oz in 2008, and the company said the outlook for gold is considerably brighter in 2009. Highlights achieved to date by the new installation at Macraes include:

  • Gold sulphide recovery improved by 3%
  • Throughput of 6mtpa achieved
  • Better mechanical and metallurgical stability
  • Lower maintenance, compared to two-train circuit
  • Easier operability, aided by Outotec ACT Control System

The Macraes mineral ore body is low grade and refractory in nature requiring pressure oxidation for processing the sulphide concentrate. Gold grades from the combined open pit / underground operation average 1.4 - 1.8 grams per ton so increased recovery efficiencies are critical to the bottom line.

The old flotation circuit at Macraes - consisting of two single trains - had four key challenges: 1. Restricted throughput due to a lack of flotation capacity2. Difficult to operate, feed split through two flotation trains3. Recovery not optimal, linked to poor performance of column cells and cleaner circuits4. Expensive to operate and maintain

The solution proposed by Outotec and commissioned in November 2007 included three TankCell 300 flotation cells, each with over 300m3 active flotation capacity, plus Outotec’s FrothMaster 2 froth imaging system and ACT Control System to optimize process control and ease of operation.

“The introduction of the new TankCell 300 scavenger cells - the first such commissioning globally - achieved all of the project aims. The circuit is now operating on a single train basis, with operators confirming significant improvement in ease of operation. Throughput at the targeted 6mtpa has been demonstrated," explains Peter Bourke, Principal Metallurgist, Flotation, for Outotec globally.

“This new circuit at Macraes was projected to improve gold bearing sulphide recovery by 2.5%, and economical evaluation based on that yielded a payback time of about 16 months. According to average metallurgical results, the bulk flotation recovery has improved by 3%, hence contributing to an even shorter payback time. This result will be further evaluated through a complete plant survey program. The cost of operating the flotation circuit has decreased with the decommissioning of the old rougher flotation columns.

The TankCell-300 cells are demonstrating excellent mechanical and metallurgical performance, says Mr Bourke. The ACT Control System provides for a consistent and stable froth surface (despite the very low sulphide feed grades) that produces excellent metallurgical results. Mass pull in these cells is automatically controlled by adjusting the air feed rate or the froth depth.

“The strategy that yields highest recovery uses air feed rate as the primary variable and froth depth as secondary. Concentrate grades have increased in the rougher circuit, whilst simultaneously achieving higher recovery through the careful application of froth control."

Independent hydrodynamic and gas dispersion measurements showed similar superficial gas velocity measurements at various locations within each cell. The superficial gas velocities in the TankCell 300s are within the range of previously measured Outotec flotation cells greater than 100m3.

The bubble sizes also ranged between 1.0 and 1.2mm in the TankCell 300s and are within the range of previously measured large Outotec flotation cells. These results further illustrate that the new TankCell flotation cell is now a well-proven design at site.

The successful commissioning of the additional auxiliary reversible agitator (FlowBooster) has shown that there is significant potential to reduce power consumption without any loss in metallurgical performance, with potential to increase coarse particle flotation.

The three TankCell 300 units were installed on an exceptionally small existing plant footprint. In this case, the need for transfer pumps was eliminated, with the TankCell able to gravitate to the scavenger circuit. This makes the operation particularly efficient, says Mr Bourke.

The successful commissioning by Outotec of three TankCell 300 flotation cells at OceanaGold’s Macraes operation in New Zealand represents a groundbreaking worldwide development for flotation technology, says Mr Bourke.

Further analysis on the performance of the new TankCell 300 cells is planned, including full size-by-size metallurgical surveys and ongoing analysis of operating costs.

All of this information will be used for further development of both new TankCell designs and optimisation on existing cells.




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