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Design of Ore Passes

There are four problems which can affect the performance of ore passes:

1. Hangups due to interlocking arches

2. Hangups due to cohesive arches

3.  Piping or Ratholing

4. Wear

1. Hangups due to Interlocking Arches

These hangups occur when the larger fragments form stable arrangements in the ore pass. They more frequently occur at knuckles and constructions such as drawpoints. Particle size and shape relative to the ore pass size affect the probability of forming interlocking arches. Hambley (1987) provides the following guidelines for ore pass design to prevent interlocking arches:

2.   Hangups Due to Cohesive Arches

Cohesive arches form as a result of sticky, fine particles adhering to each other. For material handling purposes, Jenike(1964) suggests particles less than 0.01 inches be considered as fines. These particles exhibit a cohesive component to their shear strength in addition to a frictional component. This resistance is enhanced if moisture is present. If the strength of the fines is sufficient, a stable arch may form across the ore pass. If the ore pass is sufficiently wide, gravity forces will exceed the shear strength within the arch.

The minimum dimension of the ore pass to prevent cohesive arching is determined from:

D>(2c/g)(1+1/r)(1+sin f)

where:

D = ore pass dimension

c = cohesion of fines (psf)

g = density of fines (pcf)

r = length to width ratio of the opening

f = internal angle of friction of the fines

To prevent cohesive arches the mine will need to undertake to do the following:

• when areas high in clay content such as fault zones are being mined, this material should be fed into the ore pass on a gradual basis.
• If there are a lot of fines, the level of the ore pass should be kept low so if there is a hangup, it is near the bottom where it can be removed by high pressure water jetting.
• Material within the ore pass that is high in fines should be constantly moving
• Water inflow into the ore pass should be avoided.

This can be controlled by modifying the size of the chute or hole drawing from the ore pass. Its a subject for detailed engineering stage.

4. Wear

Based on a review of ore pass literature, operators differ in opinion as to whether wear is reduced by operating the ore pass empty or full. Where hangups have been a problem, either from cohesive arches or interlocking arches, ore passes are operated as near to empty as possible (Kiruna Mine).