Stockpiles provide an economical means of storing large quantities of both fine and coarse ore in mining and quarrying operations. They may vary in size up to one-half million tons; they can be partly or fully covered or open to the environment; they may be formed by mobile equipment such as scrapers or front end loaders or by overhead conveyors such as radial stackers, shuttles, or trippers; and reclaim may be from the pile surface by bucket wheel, scraper reclaimer, and front end loader, or by gravity via various types of feeders or gates housed in tunnels beneath the pile.

One of the most popular methods of tunnel reclaim from large ore piles is to use a combination of several belt feeders discharging onto a common reclaim belt conveyor. The belt feeders may be in line above the reclaim belt or may feed at right angles to it.

When ore stockpiles are formed by overhead conveyor, sifting segregation [220K QuickTime video] occurs, with the fines located mainly at the center of the pile and the coarse particles at the periphery. The precise boundary between the fines, which may have cohesive strength, and the coarse ore, which is generally free-flowing, is difficult to define and can vary significantly with operating conditions. For example, the pile may be partly drawn down and then refilled; or the percentage of fines in the ore coming from the mine may change with time as the crusher settings change due to wear or as the type of ore being mined changes.

As a result of segregation, even the presence of as little as 10% fines (-5 mesh) in an ore can lead to serious flow problems in stockpile reclaim via tunnel systems. If the fines are cohesive and gain strength as a function of consolidation pressure, then ratholes may form above discharge hoppers, severely reducing the live capacity of the stockpile, or arches may form across hopper outlets, leading to flow stoppages. The flowability of most ores is primarily affected by:

• Percentage and size distribution of fines
• Moisture content
• Storage time at rest
• Presence of clays, talcs, etc.
• Freezing conditions

The key to reliable reclaim is to design the pile reclaim system taking into account the likely variation in flow properties of the ore during the life of the mine or quarry. Flow properties tests should be run on -5 mesh samples screened from the various ore and mineral types likely to be encountered. These tests should be performed over a range of moisture, temperature, and storage times at rest to provide the basic flow properties test data essential for successful pile design.

We have recently performed a series of tests on scale models of a variety of large storage piles. The purpose of these tests was to compare and measure the live capacity for a pile comprised of coarse, free-flowing material with the capacity of a pile comprised of the same material mixed with different percentages of fines. We found that when no more than 5% fines was added to the coarse product, complete drawdown occurred, and the live capacity could be accurately calculated using a 3D CAD program based on knowing only the measured "drawn down" angle of repose. However, 15% fines in the mixture led to severe ratholing. In this case the critical rathole diameter must be known as a function of effective head in order to accurately calculate live capacity.

Using a 3D CAD program, we have developed a way of estimating the live capacities of piles for both free-flowing and non-free-flowing materials based on flow properties test data measured in our laboratories. The photos show a typical six feeder discharge arrangement from a tent shaped pile for both free-flowing and cohesive materials. In the example shown, an increase in fines content from 0% to 15% led to a dramatic 75% reduction in stockpile live capacity.