Flowsheet at Pogo:
The first gold from the Pogo mine was poured February 12th, 2006.
The Pogo Mill flowsheet consists of SAG and ball mill grinding, followed by gravity recovery, flotation, ultra-fine regrinding, cyanidation and thickening.
The plant disposes of most tailings underground as paste backfill, while remaining tailings are filtered and dry-stacked on the surface.
Maximising water recoveries:
The need to maximise water recovery from tailings was critical to the successful awarding of the many environmental permits necessary for the project, which is close to the Goodpaster River, upstream from the ecologically sensitive Yukon River and a spawning ground for Chinook salmon.
The permits for tailings and water management were the stimuli to conduct additional thickener testwork in October 2003, to guarantee the water balance of the mill.
Until October 2003, only limited testwork had been performed on the thickener streams and, as a result, the thickeners had been sized using a combination of limited testwork, similar applications and their performance based conservative approximations.
Timely completion of the testwork was critical to provide the technical details to support the water permit application.
Close client collaboration:
Outotec (formerly Outokumpu Technology), working closely with Teck Cominco, developed a test campaign to determine the perfect thickening solution for three critical thickening stages in the Pogo flowsheet and to confirm the water balance.
The three streams tested were the flotation tailings, the flotation concentrate and the post CN-destruction CIP tailings.
The test programme was conducted at SGS Lakefield Research (Canada) in October 2003, using samples that had been stored in a frozen state from the Pogo pilot plant.
The objective of the testwork was to confirm the densities required to complete the water balance were indeed achievable in a continuous, dynamic system and also to ascertain the perfect thickening technology to achieve and effectively handle these densities.
Accurate, professional testwork:
The testwork consisted of initial bench scale testing in Outotec’s 94mm bench scale high rate thickener, with subsequent testing in Outotec’s 190mm mini-pilot paste thickener.
The bench scale thickener has been used successfully to size hundreds of high rate thickeners globally, while the 190mm dynamic mini-pilot thickener enables the effects of bed compression and rake design to be investigated.
As such, the need for scale-up adjustment is minimised and, in many cases, proven to be 1:1 with industrial applications.
Accurate testwork- cost savings:
The continuous steady state operation of the 190mm mini pilot paste thickener allowed for significant rheology data to be collected, ensuring accurate sizing and design, thereby avoiding the potentially costly measure of over-conservatism.
190mm Ø Mini Pilot Test Rig 94mm Ø Bench Scale Test Rig
94mm Bench scale, 190mm Mini-Pilot and Plant Data Summary Table
It is natural that where the true underflow rheology characteristics are not known, a degree of conservatism can creep into the raking mechanism design and the result can be more steel in the mechanism and selection of higher torque drives than might be necessary.
Both of these results have a flow-on effect to the bridge and tank design and, in turn, civil design and the underflow and pumping system.
The effects on capital cost can be significant, particularly when talking about installation at a remote site.
When designing a paste thickener raking mechanism, it is of great benefit to know the rheology of the final underflow material.
Accurate testwork- optimal design:
Testwork also resulted in optimal engineering through the addition of mechanical design features including rotating and static pickets, currently under patent by Outotec, thus achieving targeted underflow densities with no evidence of rotating bed formation.
A Concentric Shear Thinning arrangement, currently under patent by Outotec, has ensured reduction in yield stress for transportation of the thickened slurry, and this system is working in conjunction with the rotary lobe pumping system as designed.
Accurate testwork- reliable results:
Results from the plant after five months of operation indicated the 10m pre-leach SUPAFLO paste thickener was operating at an average density of 52% solids compared to 47- 48% in the mini-pilot.
Over the same period, the 12m SUPAFLO high compression tailings thickener has had an average underflow density of 60% solids compared to 62.0 – 62.4% in the mini pilot.
While testwork achieved a higher result, it came at the cost of higher yield stress. The balance between thickener type and pumping capacity has led to a design of 60% being the operating balance for this duty with the pumping being upgraded so densities of 64% and above could be accommodated.
The CIP tailings and CN- Recovery thickeners, using two 7m SUPAFLO paste thickeners, are both able to achieve the tested 49% solids.
The torque values for all thickeners show good correlation with the design point of 25% of the maximum design output torque for normal operation at the design densities.
The patented Concentric Shear Thinning system has been operating successfully at site and is maintaining underflow pumpability.
In conclusion, the testwork conducted during October 2003 facilitated the accurate specification of the design parameters for the thickeners at Pogo.
This testwork has also ensured complete confidence during scale-up, as the Pogo thickeners have all been field-proven, and are successfully operating at or slightly above target densities.
- Testwork and design enabled development within rigorous environmental requirements involving more than 50 individual permits
- 190mm test rig allowed rake design to be trial tested
- Optimised drive design and torque selection ensured resulting underflow rheology measurements more closely replicated those from actual industrial scale
- Thickener underflow handling and pre-treatment design were optimised from this closer approximation to industrial scale rheology