Macraes gold was a two train plant. Each train consisted of a Flash Flotation Cell from Outotec on the cyclone underflow which recovered 50-65% of the sulphide minerals.
The cyclone overflow is then fed into rougher column cells with the tail from the columns passing into Outotec mechanical scavengers.
Concentrate from the rougher and scavengers is reground and passed through a cleaner circuit, with the final concentrate being treated in a pressure oxidation circuit.
This circuit had four key challenges:
- Restricted throughput due to a lack of flotation capacity
- Difficult to operate with two flotation trains
- Recovery was not optimal which was linked to the poor performance of the column cells
- Expensive to operate and maintain
The circuit capacity was to be expanded while at the same time returning to a single flotation train operation. Previous testwork had shown that 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 3 x TC-300 cells, with each of these TankCells having a volume of 300m3.
The tails from these cells would be fed into the existing 2 x TC-150 scavengers. The existing rougher columns were to be decommissioned while the train’s two scavenger cells, 5 x OK38, were to become additional cleaner cells. The resulting circuit would have a 6mtpa capacity through a single flotation train.
The project also sought to incorporate the advanced 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.
The circuit configuration was finalised in May 2007 and construction of the first TC-300 started immediately. This cell, being the first of its size, was subjected to rigorous testing in Perth WA before another 2 TC-300s were constructed in New Zealand.
In June 2007 Outotec began work on site to prepare for installation of the TC-300 cells. Works included civil and earthworks, significant upgrades to the electrical system on site and a detailed plan for integrating the cells into the existing circuit, including design of all pumping, piping additional flotation air blowers and the rerouting of existing slurry piping.
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. Throughput at the targeted 6mtpa has been demonstrated.
Early plant metallurgical results indicate a 4% increase in sulphide recovery, which is double the contracted objective. This result will be further evaluated through a complete plant survey program in the near future. The cost of operating the flotation circuit has decreased with the decommissioning of the old rougher columns.
The TC-300 cells are demonstrating high mechanical and metallurgical stability. The control system provides for a consistent and stable froth surface (despite the low sulphide feed grades) which provides good metallurgical results. Concentrate grades have increased in the rougher circuit, simultaneously achieving higher recovery through the careful application of froth control.
The 3 x TC-300 were installed on a small existing plant footprint. In this case, gravity flow of tails makes the operation efficient. Outotecs’ recent scale-up of TankCell sizes to 200m3 and now to 300m3 is a highlight with flotation efficiencies being easily maintained in these larger cell sizes.
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. All of this information will be used for further development of both new TankCell designs and improvements on existing TankCells.