Here Peter Bourke, Outotec’s global technology manager–flotation process and Alan Berzins, project manager–minerals processing, discuss the installation.

Macraes gold was a two train plant. Each train consisted of an Outotec Flash flotation cell on the cyclone underflow which recovered 50%-65% of the sulphide minerals. The cyclone overflow was then fed into rougher column cells with the tail from the columns passing into mechanical scavengers. Concentrate from the rougher and scavengers was reground and passed through a cleaner circuit, with the final concentrate being treated in a pressure oxidation circuit.

This circuit had four major problems:
• Restricted throughput due to a lack of flotation capacity;
• Difficult operation with two flotation trains;
• Less-than-optimal recovery which was linked to the poor performance of the column cells; and
• High operating and maintenance costs.

The circuit capacity was to be expanded while at the same time returning to a single flotation train operation. Previous testwork showed that at least a 2% increase in recovery of sulphides could be expected if the column flotation roughers were replaced with TankCells.

Thus a plan was undertaken in which the two flotation trains would be combined through the installation of a new rougher flotation stage which consisted of three TC- 300 cells, with each of these TankCells having a volume of 300 m3.

The existing rougher columns were to be decommissioned while the train’s two scavenger cells were to become additional cleaner cells. The resulting circuit would have 6 million mt/y capacity through a single flotation train.

The project also sought to incorporate the latest in control technology for the new TankCells. Each TankCell was fitted with the FrothMaster 2 froth imaging system and an overall control platform was supplied for management of air addition, froth speed and level in the new cells.

Installation of new, larger flotation cells at Macraes had to fit within an exceptionally small
existing plant footprint.

Upon arrival at site, the new flotation cells were installed into the circuit and, after mechanical commissioning, were integrated into the circuit in November 2007 during a 48-hour shutdown. Within one hour of the plant restarting after the shutdown, full operation of the flotation circuit was achieved.

The introduction of the new TC-300 rougher cells achieved all of the project aims. The circuit is now operating on a single train basis with operator comments confirming the significant improvement in ease of operation. The TC-300 cells have demonstrated high mechanical and metallurgical stability. The control system provides for a consistent and stable froth surface (despite the very low sulphide feed grades) which provides excellent metallurgical results. Concentrate grades have increased in the rougher circuit while simultaneously achieving higher recovery through the careful application of froth control.

The TC-300s were installed on an exceptionally small existing plant footprint. In this case, gravity flow of tails makes the operation particularly efficient.

OceanaGold said it achieved an increase in bulk flotation recovery of 4%, which exceeded the projected recovery increase of 2.5%. Based on the projected recovery improvement, payback was 16 months, but is now expected to occur sooner. The Macraes processing facility will treat all ore and concentrate produced at the company’s Macraes, Frasers and Reefton mines.

Outotec said further analysis on the performance of the new TC-300 cells is planned, including a full air distribution and bubble size survey, metallurgical surveys and ongoing analysis of operating costs, to be used in further development of both new TankCell designs and improvements on existing TankCells.