Stawell Gold recovery exceeds expectations through flotation upgrade

A major Australian gold mine at Stawell in Central Victoria has far exceeded its own expectations following an upgraded flotation circuit developed in partnership with Outotec Services.

The Stawell Gold Mine of Northgate Minerals, Toronto, aimed to increase recoveries by 3.5% from the upgraded plant. This upgrade was implemented after Stawell changed its production profile to process lower grade ore at higher throughput rates.

The mine, which has produced more than two million ounces in its 26-year history, previously employed a flotation circuit consisting of a bank of eight mechanical trough cells in the rougher circuit, followed by two banks of two OK3 Outotec cells as cleaners. The feed rate to the cells was between 90-105 tph, at 50-55% solids. The overall flotation circuit was not performing at optimal rate due to entrainment problems in the rougher cells when feed density increased from 45 to 55% solids, typically at 105 tph.

In anticipation of future production levels and as part of Stawell’s focus on operational excellence, it was decided to upgrade the flotation circuit. Following a site audit from Outotec Services, a two TankCell-20 configuration equipped with larger TankCell-30 mechanisms was proposed to help optimise flotation. The larger mechanisms would allow operation at very high percent solids (50% and over).

The TankCell design also allows a much deeper froth depth and better concentrate grade through optimised launder lip length and surface area. Outotec Services was commissioned to handle the complete turnkey solution of the new rougher circuit, including design, supply, installation and commissioning.

The schedule was demanding but achievable, with work to commence in February 2010 and be completed in September 2010, just 30 weeks later. It was decided to adopt the partnering approach between Stawell and Outotec Services, because this collaborative method ensured open communication, with all parties having greater ownership of the project and its aims.

The close teamwork between Stawell and Outotec Services resulted in meticulous planning and the site remaining fully operational at all times. Pipework and electrical easement ducts, for example, were rerouted early in the project. Tie-in points for new cells and rerouting of pipework were also planned up front in the project and all disruptive work was completed during shutdowns.

The project overcame a number of challenges, including an extremely limited footprint, which was adjacent to a gabion wall, close to the run-of-mine pad and also close to a reagents shed, which could not be moved. Additionally, existing process requirements at Stawell required specific elevations for the new TankCells. Structural stability was the main issue when designing the tank support structure due to the height of the tanks and the limited footprint. Sufficient stiffness was required such that the operation frequencies of the TankCells would not interfere with the natural frequency of the tank support structure. Through FE modelling of the structure, section sizes and bracing orientations were optimised to produce the required stiffness.

The civil work was also highly challenging. Following the discovery of an underground slab during the excavation work, test work on core samples from various layers was required. Fortunately, only part of the slab needed to be removed and all parties worked hard to ensure the project stayed within project timelines.

The weather proved to be an additional hurdle as the site experienced the wettest seasonal weather in recorded history during the project. Simple tasks, such as curing of the paintwork, normally taking 24 hours, took days instead. Despite such challenges, the turnkey installation of the new rougher circuit, along with blowers for the complete flotation circuit, was completed within deadlines. Because all tie-in points had been already carefully planned upfront, commissioning was a seamless exercise. On 13 September 2010, just 30 weeks from commencement, the new circuit, complete with TankCell technology, was commissioned and brought on line.

Designed to cope with projected increases in production and considerably more operator friendly than its predecessor, the new TankCell-20 cells have quickly proved their worth at the site. The air demand for the old rougher cells, for example, was estimated at over 3000 Am³/h, whereas the estimated air demand on the Outotec TankCells is a maximum of 992 Am³/h.

The Outotec FloatForce rotor-stator mechanism, with its unique design, delivers enhanced flotation cell hydrodynamics and improved wear life and maintenance.

Following the flotation upgrade at Stawell, the targeted recovery rate improvement was projected at 3.5%. The actual recovery rate, however, improved instantly by 4.5%. Payback was also impressive, occurring within less than four months.

“The projected payback was 5.5 months, so it was a pleasant surprise when it happened so soon” explains Jodie Hendy, Senior Metallurgist at Stawell.

“Maintenance on the Outotec TankCells has also been minimal since the upgrade. Basically, we check the cells during shutdowns, but there has been no maintenance required in the nine months since commissioning.

“The TankCells have really delivered on their reputation. Basically, they do exactly what they are supposed to do.”