The bridge design had to be strong enough to support the tails piping along with the feed piping. Whereas most conventional thickeners facilitate tails piping in an underground trench, site conditions at Ok Tedi required that Outotec’s design had to be strong enough for the alternative approach, yet still light enough not to cause undue stress on the thickener.
All equipment needed to be also designed and fabricated with due consideration to sea freight and PNG road restrictions. In this regard, the drive cages were designed to be split into four segments and assembled onsite, such that they could be transported up the mine access road from the nearby town of Tabubil. This access road contains a number of narrow, single lane bridges, limiting the transportable width of components.
Outotec’s design was to accommodate all of OTML’s key project criteria - the upgrade capable of handling the full new design tonnage; drive and rake designed for the increased torque and new high rate duty; new feed dilution system included to reduce the 41% solids w/w feed to 15-20% solids w/w.
New feed and dilution system
The thickener feed is introduced at approximately 41% solids w/w. Experience and testwork results indicated an ideal feed dilution to around 17% solids w/w. To achieve this, Outotec included both of its renowned feed dilution features, the globally patented Autodil™ and Turbodil™ systems. Autodil™ maintains the density in the feedwell within the optimum range for flocculation, irrespective of feed flow rate or density, whilst Turbodil™, a forced dilution system, uses a low head axial pump to drive dilution water into the feedwell.
Optimised rakes and drive unit
Two long and two short rake arms were designed to rake the floor twice per revolution. The rakes, each fitted with replaceable blades, are of box truss construction, supplied in maximum transportable sections and assembled on site by bolting.
Outotec SCD multi-pinion rake drives are designed for column and caisson thickeners and were therefore the optimum solution for the OTML. They are hydraulically driven high efficiency planetary reducers, driving each pinion and, in turn, the ring gear. OTML’s rake drive unit offers maximum operating torque of 3250kNm, which is triple the normal torque rating for thickeners of this size and duty. Fitting a rake lift to the existing thickeners would have substantially increased supply price, installation price and retrofit downtime, so Outotec’s solution was to incorporate a boosted torque rating to over-ride the need for a rake lift.
Road and river transport to the site was governed by limited infrastructure and climatic conditions, so access to Kiunga port was extremely restricted, with larger items being transported to site during the wet season. The rugged jungle environment receives approx 11 metres of rainfall a year. The most cost-effective solution was therefore to pre-fabricate Outotec’s technology, thereby minimising downtime, risk, hazards and complexity during on-site construction. Given this timeframe, managing different sub-suppliers was also a further challenge – the equipment (from different countries) needed to be shipped and guaranteed to arrive at the same time at port, a logistical challenge of its own.