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Gravity reclaim stockpiles are widely used in the mining and mineral process-ing industries to store bulk solids in large quantities.

For most cohesive bulk sol-ids the critical rathole dimen-sions at the base of the stock-pile are usually very large, rendering complete draw-down impracticable. To meet live capacity requirements while keeping the reclaim hoppers and feeders within practical size limits it is accepted that ratholes need to form above the reclaim openings.

It then becomes necessary to employ multiple outlets as illustrated by the twin outlet stockpile of Figure 1. The draw-down and live capacity may then be optimised by appropriate spacing of the reclaim hoppers. The photograph in Figure 2 of a twin-outlet stockpile designed using these principles clearly shows the ratholes formed to a height above which the draw-down crater forms.

Lead-up to the projectThe project involved the design of a 42.5-metre-high conical gravity reclaim stockpile within the Rio Tinto Iron Ore (RTIO) Mesa A mine construction, approximately 50km west of Pan-nawonica in the Pilbara.

The mine is owned by Robe River Iron Associates, of which Rio Tinto has a 53 per cent operating interest; as such, Mesa A represents part of Rio Tintos iron ore expansion projects. The mine is expected to produce 20mtpa of Robe Valley pisolite ore in 2010 with 25mtpa production targets from 2011 onwards.

TBS became involved in the Mesa A project through its strong collaborative work on bulk material testing and con-ceptual design with Minerva Engineers. Minerva was tasked with the detailed design of the Casper stockpile and gravity reclaim for a train load-out system for HWE, with TBS provid-ing conceptual design capabilities.

A brief history of stockpile designAs with all design approaches and methods, there is a history of evolution. Stockpile design is centred around the work of three prominent researchers: Janssen, Jenike and Roberts.

The pioneering work of Janssen in 1895 established that

the stresses in a bulk material are not hydrostatic, but relate to the internal strength and lateral to vertical stress ratios in the bulk material, which is critical information needed to de-termine the load-out pressure at the stockpile outlet. It took nearly 70 additional years for the more formative period of bulk material design to commence, with the breakthrough work of Jenike in the late 50s and early 60s.

Jenikes understanding of the relationship between bulk material strength and flowability in silos and hoppers high-lighted the need for material testing and proposed a test pro-cedure using the Jenike biaxial shear cell depicted in Figure 3. It is from this work that all of todays bulk material testing is derived.

The work of Roberts since the early 70s has extended Jeni-kes approach to the design concept of all bulk material handling systems being based on physical bulk material measurements. For stockpile design, Roberts recognised the relationship and design constraints of the feeder interfaces and the influence of stockpile pressures on feeder loads. He also recognised the conservative nature of Jenikes radial stress theory and the impact this has on the prediction of stockpile live capacity. This has led to the development of the hoop stress theory for the prediction of rathole formation and geometry leading to much more accurate live capacity prediction.

What iron ore sample to use? It is vital that a truly representative material sample is used in order to determine the flow properties of the bulk material to be stored. In general, the Jenike Biaxial Shear Tester is used to deter-mine the flow properties of the material, however the strength of the bulk solid will depend on a number of factors, which must be considered prior to the material being tested as follows:

Conceptual design of stockpile at Mesa A mine

Australian Bulk Handling Review: September/October 2010

Figure 1 Twin Outlet Stockpile Cross Section.

TUNRA Bulk Solids (TBS) worked on the conceptual design of a 42.5m conical gravity reclaim stockpile at Rio Tintos Mesa A mine in the Pilbara. This case study describes events while also providing background on stockpile design and theory.

71Australian Bulk Handling Review: September/October 2010

Anticipated process moisture range Particle size and distributions Potential wall lining materials and their surface finish Environmental considerations (wind, dust, water) Live capacity requirements Initial filling conditions Discharge conditions Dynamic load out effects

Once those criteria are established, the likely worst case flow and load determinations can be made and bulk material testing can commence. This testing enables the bulk material characteristics to be determined on a quantifiable level.

The Mesa A mine ore body comes from the Channel Iron de-posit, from which core samples were taken and provided by HWE to TUNRA. As such, the worst-case handling scenarios were de-termined by TUNRA in consultation with the stakeholders.

Figure 2 Twin Outlet Stockpile Ratholes and Drawdown Crater.

Figure 3 Jenike Shear Tester.

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For the high compressive loads expected in tall stockpiles tests were performed at the highest pressures possi-ble using TUNRAs direct shear testers. In order to design other bulk material handling and transportation equipment addition-al research and testing was performed to determine: Wall liner wear life Maximum conveyor inclination angle Typical conveyor surcharge angle Dust extinction moisture

Conceptual designBased on the results of the bulk material characterisation, con-ceptual designs of the stockpile were developed using the fol-lowing design considerations.

As the stockpile is built, the material in the hopper com-presses, leading to some movement of material above the hop-per, setting up a flow channel. This then allows transfer of load from the material in the flow channel to the static materi-al forming the flow channel. The pressure, which then exists above the hopper, will be less than the hydrostatic pressure.

Under flow conditions, it can be assumed that the flow channel will act like a tall bin and hence the Janssen pres-sure relationship can be used to estimate the pressure on the hopper. The Janssen pressure relationship can also be used where the stockpile has been emptied with a stable rathole remaining and is then rebuilt. From determining the Janssen stress and determining the bulk properties of the iron ore the following additional parameters were calculated: Wall loads of the hopper Gate loads Feed hopper geometry Hopper liner wear rate

Live capacity Roberts Hoop Stress theoryFor improved stockpile live capacity determination, Roberts Hoop Stress Theory [1] is considered.

Observations of the sloughing action of the bulk material from the free surface during funnel-flow show that the slope an-gle of the free surface is not normally constant. Rather, the flow over the top surface steepens towards the central flow channel.

In order to analyse the condition for drawdown and reduce the excessive conservatism in previous stockpile live reclaim predictions based on Jenikes original theory, a hoop stress analy-sis is applied to this design. This analysis assumes that the hoop strength in the vicinity of the free surface of the bulk solid is the dominating condition for rathole stability, which varies with sur-charge load (i.e. stockpile height). Using the hoop stress based analysis, the live capacity was predicted to be approximately 61,300t. Subsequent analysis of the as-built stockpile confirmed the actual live capacity within 10 per cent of the predicted ca-pacity at 65,000t, as shown in Figure 3.

A successful outcomeTBSs involvement commenced right at the beginning of the Mesa A design phase late in 2007, with the collaborative work culminating in the completion of the conceptual design in August 2008.

TBSs work on the fill and feeder loads, feeder geometries, dust reduction information, live capacity and load out rates provided Minerva and HWE with the underlying information required for the detailed design of the Casper stockpile.

Under the project management of RTIO, the high quality collaborative work between all stakeholders ensured a suc-cessful mine completion with RTIO stating that the project was completed on time and under budget [2]. The first fully loaded train left the Mesa A Casper stockpile and mine site bound for Cape Lambert on February 19, 2010.

TUNRA Bulk Solids more than stockpile designTBSs versatility in determining the fundamental characteris-tics of bulk materials for handling, transportation and storage systems is comprehensive. TBS has a dedicated team which has grown with the mining and minerals sector to provide on-going testing and conceptual design capability for most bulk material handling and transportation systems. TBS delivers over 200 projects for industry each year.

[1] Roberts, Wiche and Krull, Review of Funnel Flow Theory in Relation to Drawdown and Live Capacity of Gravity Reclaim Stockpiles, Bulk Solids and Powder Science and Technology, Vol.3, No.2.[2] Rio Tinto Press Release, Rio Tinto opens new iron ore mine in the Pilbaras Robe Valley, 22 February 2010.

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Figure 4 - 42.5 m High Mesa A Iron Ore Stockpile prediction and actual stockpile.

Contact: Stephen.Wiche@newcastle.edu.au

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Focus on truckless mining adds customers for SKM

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