The Design of Ore Pass Chutes
John Rozentals Pr.E B.E. MSAIME MSAICE
Acknowledgements : Bionic Research Institute - Chute Design Conference 1992
ALTHOUGH the basic principles of bulk solids flow have been understood for many years, the design of ore pass chutes poses special problems.
This paper reviews the problems and motivates the need for further basic research on this topic.
John Rozentals is a mechanical and structural engineer. He is a Partner of BRI Tec Consulting and an associate with Hamilton Associates.
DESIGN OF ORE PASS CHUTES
Ore passes are used to drop mined ores in to underground bins and loading points. From there the ores can be hoisted to the surface.
The ore in ore passes can be unloaded through side discharge chutes (box fronts), or underflow chutes.
A spate of box front failures in recent years resulted in the serious injury or death of operators and caused production losses. Safe and reliable standards need to be established for the design of ore pass chutes and associated structures.
ESSENTIAL FEATURES OF ORE PASSES
Although the basic principles of bulk solids flow are now well established, flow in ore passes is subject to special problems. The essential features of ore passes are as follows:
- small diameter (typically 1.5 to 2.8 m)
- very tall shafts (60 - 100 m)
- large lump size (typically 1/5 of shaft diameter)
- hopper - minimal
- chute width - about of shaft diameter
- sides of shaft - rigid and rough
- water in ore pass - a constant problem
- properties of ore vary
- loading of the ore pass is generally not central
- ore passes may be vertical or inclined
The width of the chute opening should exceed three times the maximum lump size.
In assessing chute width the particle size as well as the shape of slabs should be considered. If the ore is generally well broken, it may be adequate to consider the largest fragments normally encountered -rather than the absolute maximum size. The use of a grizzly at the dump point will provide a measure of control over the largest size fragments that enter the ore pass.
If a grizzly is used the spacing between the grizzly bars becomes the nominal lump size. Ore fragments too large to pass through the grizzly bars are broken at the grizzly.
If a grizzly is not used the largest ore fragments need to be estimated by inspecting samples of broken ore.
In some cases slab shaped ore fragments may occur. The chute width should be three times the maximum length of the slab.
The chute height should be about 80% of the width.
The slope of the chute bottom is usually 40 to 45 deg, or greater.
A number of different types of gates may be used. Choosing a gate for a given situation basically depends on the size distribution of the ore - particularly the amount of fines.
To design safe and reliable chutes it is necessary to establish design pressures in the chutes.
To simplify the conceptual reasoning divide the problem into two parts:
- What is the magnitude of the downwards force acting at the bottom of the ore pass?
- What are the forces acting within the chute?
FORCES IN AN ORE PASS
The vertical ore load in the shaft of the ore pass is resisted by friction along the sides of the shaft, and by pressure at the bottom.
A limiting horizontal pressure is reached when the weight of ore in any section is exactly balanced by side friction.
The bulk density gives a measure of the weight of ore to be supported. Estimates of bulk density of ore can vary arbitrarily from 1600 to 2000 kg/m3. To make a fair assessment of the pressures in an ore pass the bulk density of the ore should be measured.
ANGLE OF FRICTION
The second significant variable in estimating side pressure is the coefficient of friction between the ore and the shaft walls.
The coefficient of friction should not be assumed. If the sides of the shaft are rough the limiting value of the friction coefficient will be that of ore against ore. This value can be easily determined. It is better to measure than to guess.
The limiting value of side pressure assumes a simple case of non-cohesive ores.
Adhesion of the fines to the shaft wall can change the value of the limiting horizontal pressure.
To date this aspect has not been effectively investigated.
Ore passes do not have specially shaped hoppers at the base. They generally rely on dead material to form a natural hopper. Flow in an ore pass will therefore be some form of funnel flow.
Flow pressures have been studied for a variety of silos. Flow pressures observed during discharge are often higher than those calculated by the Janssen formula.
However, ore passes have length to diameter ratios well in .excess of any silo experiments. Experiments on long silos indicate that overpressures may have little effect once a considerable depth of ore has been filled.
This subject warrants further investigation.
SIDE FORCES AT A BOX FRONT
Box front chutes draw off ore from the side of the ore pass. Very little quantitative information is available on eccentric discharge pressures.
DOWN-LOADS ON CHUTES
The downwards pressure on bottom discharge chutes is generally taken as K times the side pressure.
To arrive at reliable values K should be obtained by experiment.
Water can enter ore passes inducing very large hydraulic pressures. The hydraulic pressures can be so large that, by comparison, the pressures based on dry ore properties become meaningless
For example the limiting side pressure for dry ore may be 40 kPa - while hydrostatic pressure from containing a head of 50 m of water would be 500 kPa.
PROPERTIES OF SLURRIES
The chute designer cannot assess how much water- will enter a particular ore pass. Water from underground storage dams, for example, can seep into ore passes turning clayey ores into slurries which become difficult to handle and control.
Similarities have been noted between mud-rush slurry and the properties of wet concrete. But so far very little research has been conducted on the properties of slurries in ore passes.
FORCES IN CHUTES
Under flow conditions forces in a discharge chute are low. However arching does occur in many ore passes. Large impact forces can result from the collapse of arched material. The magnitude of such impact forces has not been adequately studied.
The magnitude of forces in ore pass chutes are poorly defined for reasons discussed Current design approaches to the selection of operating cylinders for radial gates at ore passes tend to over-estimate the frictional forces under operating conditions.
High hydraulic pressures do not necessarily produce high surface friction forces on the face of the gates. Water has a low coefficient of friction.
ORE PASSES PRESSURE
LIMITING HORIZONTAL PRESSURE
SIDE PRESSURE : Non Cohesive Ore
Assume balanced condition at which added weight of segment is exactly balanced by wall friction.
Weight of Ore in Segment :
W = (π / 4) D H ρ
Wall Friction :
F = π D H h
F = W :
(π / 4) D H ρ = π D H h
h = ρ D / 4
LIMITING HORIZONTAL PRESSURE WITH COHESIVE ORE
SIDE PRESSURE : Cohesive Ore
Cohesive Shear stress at Wall :
Ý = C + h
Friction Force :
F = π D H (C + h)
F = W :
= (π/4) D H ρ
C + h = ρ (D/4)
h = ρ (D/4) - C
h = (ρ (D/4) - C) /
THE SOLUTION TO THE DESIGN PROBLEM
Bulk solids handling is an extremely difficult science requiring basic research, practical interaction with the "real world" of mining and mineral processing, as well as skills in techniques of problem solving.
The design of ore pass chutes is met with far too many unknowns.
A design procedure where the value of basic design forces can vary by 300% to 1000% depending on which values are used in which formulae - can hardly be called adequate. It should be acknowledged as mathematical guesswork.
It is recommended that a program of basic experiments be initiated to define more clearly the relevant design parameters.
- D .F .Hambley "Design of ore pass sys terns for underground mines" CIM Bulletin, Jan 1987.
- D. Griesl, T. Jurgen ' " Flow behaviour of run-of-mill ores and transfer raise design" Berakademie Freiberg, Nov 1990.
- "Guidelines for the assessment of loads on bulk solids containers" Inst. Engineers, Australia 1986.
- D. Griesl, J. Tomas "Fiesverhalten von rohhaufwerk und auslegung van rollochern" Bergakademie Freiberg, 1990.