Retrofit to Ok Tedi tailings thickener

The retrofitting of existing thickeners with high performance Outotec thickening technology achieved the technical and environmental objectives the project sought to fulfill.

Background

The Ok Tedi mine, operated by Ok Tedi Mining Limited (OTML), is a major producer of copper concentrate for the world’s smelting market, yielding some 600,000 tonnes of high-grade copper concentrate a year and 500,000oz of gold. The mine is located on Mount Fubilan, Western Province of PNG, with an altitude of approx 1500m. Ok Tedi is an open cut mine, with approximately 160,000 tonnes mined each day. Its processing plant treats an average of 78,000 tonnes per day.

The greatest challenge facing OTML is managing the effect of mining operations on the local environment. The mine has had a major impact on the riverine ecosystems downstream, so environmental monitoring of and research into the Ok Tedi and Fly River systems is a significant focus at OTML. This research is set to continue beyond mine closure, currently forecast to occur in 2013.

Summary

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Customer
Ok Tedi Mining Limited (OTML)
Location
Papua New Guinea
Delivery year
2008
Technology
Outotec tailings thickening
Commodity
Copper

Challenge

  • Cost-efficient design and retrofit of older technology
  • Remote, mountainous location
  • Short delivery leads times
  • Climatic restrictions

Solution

  • 2 x 50m conventional tailings thickeners retrofitted with Outotec’s high rate thickening technology

Benefits

  • Project delivered on time
  • Project delivered on budget
  • Far superior tailings performance since retrofit

Project overview

As part of OTML’s drive to reduce the sulphur content in the tails from the copper processing plant, a new flotation plant was constructed to recover sulphur and reduce it to 0.8% in the final tails. The pyrite concentrate containing sulphur report to a 40m concentrate thickener, namely thickener 1. The tailings from the Pyrite recovery plant reported to two 50m Dorr Oliver conventional thickeners, both of which were to be retrofitted and upgraded. The retrofit and upgrade was to improve the settling rates of solids in the thickener.

The thickener performance is far superior after the upgrade. The pyrite plant is made up of two streams, namely Module 1 and 2. Tailings from each module are pumped to thickeners 2 and 3 respectively.

Testwork

Outotec were initially sent two site samples for bench scale thickening testwork in May 2006. Following this, OTML requested that Outotec carry out further comprehensive studies, including onsite thickener testwork, an engineering and design study, and a retrofitting feasibility study. Outotec was awarded the contract in September 2006.

Outotec’s proposed solution was to completely gut the old conventional tanks of the existing bridges and mechanisms, leaving only the centre caissons, and retrofit each with high performance drives, new feed and dilution systems, new rake mechanisms and bridges. The new full tonnage of 2538tph dry solids running through one of the 50m thickeners would equate to a solids flux loading of 1.29t/m2.h. Results of Outotec laboratory scale test work on OTML tailings samples indicated the performance of the retrofitted tailings thickener would comfortably meet the required 62% underflow density.

Design considerations

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.

Challenging location

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.

Servicing installation and beyond

Outotec’s extensive experience in on-site construction, installation and commissioning around the globe was put into practice at Ok Tedi also. Outotec provided an Installation Supervisory Team, advising on any issues encountered, and generally assisting with the day-to-day assembly and installation of the retrofitted equipment to both units. This Team remained on site until the project was fully completed. Outotec also delivered an onsite tailored training package to operators prior to wet commissioning in December 2007.

Result

The retrofitting of existing thickeners with high performance Outotec thickening technology achieved the technical and environmental objectives the project sought to fulfill.

According to a spokesman at OTML, “the thickener performance is far superior since the Outotec upgrade”.

Retrofitting with new Outotec high rate bridge and mechanism