A partnership between the Karara development team and Outotec has helped the Karara Iron Ore Project move into production phase with considerable savings in cost and construction time.
The partnership resulted in an innovative approach to design options for tailings thickening at the Karara Iron Ore Project in Western Australia with benefits such as lower overall cost, reduced thickener sizes and minimised site construction time.
Located 300km north of Perth in Western Australia, the Karara Iron Ore Project is a 50-50 joint venture between Gindalbie Metals Ltd and Chinese steel producer, AnSteel.
The project involves an integrated development of the world-class magnetite deposit to produce both high grade magnetite concentrate on-site and blast furnace quality pellets in north-eastern China. The DSO hematite phase is the first stage of the Karara project and is scheduled for 2011.
Karara’s initial request for the tailings thickeners included thickener sizing using a flux rate of 0.18 t/m²hr at a feed rate per stream of 474 tph, which resulted in the initial selection of 2 x 58m high rate thickeners for preliminary plant layouts.
Design considerations also included overflow clarity of maximum 500ppm, underflow density of 65% w/w solids and potentially challenging civil costs due to thickener construction into the side of a rock hill.
Outotec carried out a series of testwork programmes that examined many scenarios for an optimised design and delivery of the project targets.
After carrying out an initial thickening testwork on a Karara tailings sample in February 2007, Outotec conducted further testwork in August 2009, which included sample preparation to mimic expected site water conditions, comprehensive testwork in flocculant and coagulant optimisation, along with dynamic bench-scale testwork at Outotec’s West Perth laboratory.
Outotec Vane Feedwell
One important innovation in the Karara tailings thickener emerged from the August 2009 testwork wherein the flocculated aggregates were observed to be quite sensitive to shear, causing the overflow water to lose clarity.
This problem was addressed by utilising a new feedwell design called the Outotec Vane Feedwell, which featured vanes and a radially sloped shelf and interconnected upper and lower zones.
Designed to dissipate energy and prevent short circuiting, the Outotec Vane Feedwell was an important inclusion in the Karara thickener design to improve the clarity of the overflow.
Increased solids flux and rise rates in the testing process proved very successful, resulting in significantly reduced thickener sizes and lower capital expense for Karara.
Flocculant dilution and distribution
Given the significance of flocculant dosing on optimum thickener performance, the Karara and Outotec teams discussed reliable methods for dilution and distribution of the flocculant solution at full scale level.
Outotec proposed using a bridge-mounted open ‘flocculant mixing box’, which allows the operator to set the dilution rate accurately.
Bolted tank design
After a detailed analysis of all thickener construction options including considering an on-ground caisson design, the bolted tank design was approved as the most suitable option.
The bolted tank design offered a better value proposition in spite of higher capital fabrication cost. The fully bolted thickener design also significantly reduced site risk since no hot work is required at site, all weights are known and all rigging is performed from permanent lifting lugs or shackles through flange holes.
In addition to a major reduction in the amount of confined space work, the design also lowered the amount of time spent working at heights during fabrication, welding and surface treatment.
Key outcomes from the Karara-Outotec partnership:
Extending from early process design stage to fabrication, the partnership between Karara and Outotec has delivered benefits including:
- Reduction in the size of two 58m high rate thickeners to a pair of more economical and efficient 42m Outotec high rate thickeners
- Innovative thickener design of Outotec’s Vane Feedwell ensures optimised flocculant use and overflow clarity
- Effective reuse of plant water with a limit of 500 ppm suspended solids in thickener overflow
- Minimised on-site construction time with a bolted tank design that significantly reduces site work