Maximising water recoveries
The first gold from the Pogo mine was poured on 12 February 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. Paste thickeners have continued to gain wide acceptance in a number of applications where process constraints demand either the optimum settled density or maximum water recovery. In many cases, the use of paste technology can lead to improved process performance, project economics, or both. Pogo, operated by Teck Cominco, is a world-class gold deposit located some 140 km southeast of Fairbanks, Alaska. For the Pogo mine, achieving process targets using paste technology meant the difference between a flow sheet that was workable and met strict environmental criteria for zero discharge and one that was not.
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.
The critical stages
Outotec (formerly Outokumpu Technology), working closely with Teck Cominco, developed a test campaign to determine the best 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 best thickening technology to achieve and effectively handle these densities.
Accurate 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.
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.
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 we are talking about installation at a remote site.
...Equals optimal design
Testwork also resulted in optimal engineering through the addition of mechanical design features including rotating and static pickets, 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.
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. Whilst 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 excellent 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.
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. •
Contact:
Outotec 1/25 French’s Forest Road, French’s Forest, NSW 2086 Australia,
Tel: +61 2 9984 2500
Email:
karl.deitz@outotec.com