BACKGROUND

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; be partly or fully covered or open to the environment; be formed by mobile equipment; 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.

THE PROBLEM

When ore stockpiles are formed by overhead conveyor, sifting segregation 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.

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 SOLUTION

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 for successful pile design.

Jenike & Johanson has 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.

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.

Jenike & Johanson is prepared to assist you with determining the most economical and effective method of reclaiming your bulk solids from a stockpile. Without knowing the properties of the bulk solid, a gravity stockpile reclaim system may be incorrectly selected over a more feasible technique and as a result flow problems and poor efficiencies can occur.

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