1. Introduction
Mines take advantage of gravity to collect broken ore by means of ore passes. The ore passes are raises driven by drill and blast or raisebored. While ore passes are most commonly used for underground mining, they are occasionally employed at open pit operations where they are often referred to as “glory holes.” Long raises are also employed as waste passes to drop quarried rock for cemented rock fill (CRF) from surface directly to the mining horizons.
Ore passes often lead to an underground storage bin to provide surge capacity in the ore stream. In turn, the ore is normally drawn off the bottom of the bin into a chute. The design of passes, chutes, and bins was once a simple exercise and the operations were relatively trouble-free. Today, many hard rock mines experience acute problems – following are likely the main reasons for the problems.
• The trend from selective to bulk mining methods
• The trend toward greater level intervals
• The trend toward mining sequences that displace ground stress
Bulk mining is accomplished with larger blastholes on a wider spacing. The result is that the shot rock contains more and larger lumps as well as more fines due to the pulverizing effect in the vicinity of the large explosive charge. The lumps increase impact and the fines worsen the flow characteristics of the broken ore. Increased level intervals lengthen the ore pass legs and make clearing a hang-up between levels more difficult.
Today, stoping is often completed in a sequence designed to shunt ground stresses away from the mining fronts to the “far field stress regime,” which, unfortunately, may include the country rock surrounding ore passes and underground bins. This phenomenon is usually only significant when mining at great depth.
Another problem relates to specialization and jurisdiction. The successful design and operation of passes and bins is dependent on two separate sciences. The integrity of the walls and protection pillars is a function of rock mechanics while specialists in materials handling are educated and trained to deal with the flow characteristics of the shot rock. Most mines do not retain materials handling technicians on the payroll; therefore, both responsibilities come under the jurisdiction of the rock mechanics department, which may not be qualified or inclined to properly deal with flow of materials.
2. Rules of Thumb
Ore Passes
• The flow regime in an ore or waste pass is determined on the basis of the largest particle size of muck (not some average size). This is the fundamental reason for a grizzly at the dump. For example, if a raisebored pass has a diameter of 2m, particles with a diameter of 0.5m will flow freely (4:1 ratio), particles greater than 1m will not flow (2:1 ratio), and sizes in between will produce intermittent hang ups. Source: Dr. J. D. Just
• Shot rock containing more than 10% fines passing a 200-mesh screen cannot be sent down an ore pass without incurring blockage from cohesive arching. Source: Rudolf Kvapil
• Ore passes should be spaced at intervals not exceeding 500 feet (and waste passes not more than 750 feet) along the draw point drift, with LHD extraction. Source: Jack de la Vergne
• The best inclination for an ore pass in a hard rock mine is 70 degrees from the horizontal. Source: Bob Steele
• The minimum inclination for a short ore pass is 50 degrees from the horizontal. For a long pass, it is 55 degrees. Source: Harry Pyke
• Ore passes cannot be employed to any advantage where the ore dips shallower than 55 degrees from the horizontal. Source: Doug Morrison
• The size of a glory hole in an open pit should not be greater than the cross-section of the haul trucks that dump into it. Otherwise, you are bound to lose a truck, sooner or later. Source: Sergio Chavez
Bins
• An underground bin larger than 15 feet in diameter should be inclined at the bottom, away from the outlet, at an angle of 65 degrees from the horizontal, to obtain mass flow (as opposed to ratholing) where wet fines are present. Source: Doug Hambly
• To determine the live load capacity of a bin in a hard rock mine, the angle of repose may be assumed at 35 degrees from the horizontal (top of bin) and the angle of drawdown assumed at 60 degrees. Source: Al Fernie
Chutes
• For all but sticky ores, the ideal inclination of a chute bottom is 38 degrees from the horizontal. Source: Bob Steele
3. Tricks of the Trade
• Ore passes should be designed to intersect at a level or sub-level to provide access to clear a hang-up and to repair damage from sloughing. Source: Jim Ashcroft
• A steep ore pass should be pulled almost empty when the mine is to be shutdown over a long weekend or holiday. Source: Sergei Moskalenko
• Unless efforts are directed at its avoidance, decant water from hydraulic fill containing nonhydrated particles of binder may find its way into an ore pass and cause a hang-up due to cementation. Source: Keith Vaananen
• A steep ore pass must never be pulled empty otherwise the next dump of muck will “air mail” down the pass and can wipe out the chute. Source: Bob Brown
• A raisebored ore pass is more susceptible to hang ups than one of the same diameter that has been drilled and blasted. Source: Ken Lowe
• A raisebored ore pass is more susceptible to hang ups than a rectangular raise of similar size. It is easier for material to bridge across a circular shape than a square one. Source: Archibald and Friesen
• Finger raises are a neat way to feed a long leg of an ore pass. They may be drop-raised from successive levels or from a different location on the same level. Unfortunately, finger raises are fragile with respect to ground stress. Fingers cause problems (mainly brow sloughing) in shallow mines and almost always cause problems at depth. In highly stressed ground, fingers should be avoided. In many cases, a finger may be replaced with an ore transfer station on the level equipped with control chains. Source: Jack de la Vergne
• The only practical inclination for a really long glory hole ore pass is vertical. Source: Gord Graham
• In general, the wear resistance of a concrete lining increases as the concrete strength is increased. Lowering of the water/cement ratio through improvement of the aggregate grading and employing the lowest practical slump is more effective in improving wear resistance than the same reduction in water/cement ratio resulting from an increase in cement content. The entrained air content of the concrete should not exceed 4%. Source: Troxell and Davis
• Normally, the best aggregates (abrasion resistance and toughness) for a concrete or shotcrete lining are igneous rocks called "traprocks" (basalt, Andesite, diabase, diorite, etc.). The second best are massive metamorphosed traprocks (greenstone). Unsatisfactory rocks include granite, schistose greenstone, limestone, marble, sandstone, and slate. Source: Krynine and Judd
• Water should never be used in the attempt to clear a hang-up in an ore pass. Source: Menno Friesen
• The traditional way to clear a hang-up is to wrap sticks of powder to build a charge (which may be single or double primed) around the tip of a long-hole loading stick. Then, push this charge up the footwall of the raise by coupling as many loading sticks as necessary. After the charge has reached the hang-up, it is detonated remotely. Source: Bob Dengler
• The “Sputnik®” is an effective means to safely bring down a hang-up in an ore pass. Source: Claude Bouchard
• The procedure of last resort to blast down a hang-up in mid-raise is to long-hole drill into the hang-up and spring the hole with explosives. Source: Doug Hambly
• The risk of self-heating (sintering that causes hang-ups) by ore from zones rich in sulfides can be overcome by adding waste rock into ore passes with ore that is over 25% sulfide. The mine geologist makes the call. Source: BM&S Corp
• Ground support for an underground bin can be provided by over-drilling blastholes from a pilot raise. Bolts are driven into resin or cement cartridges at the toe of the hole with the drill before the remaining open hole is loaded with explosives and fired. Source: Bill Shaver
• Mechanical rock bolts are unsatisfactory to support the walls of underground bins for the long term. The barring effect of vibration can create sloughing faster than if there were no bolts at all. Grouted steel rock bolts are often employed but grouted fiberglass bolts or cable bolts are considered best in the long run. Source: Menno Friesen
• The sides of chutes should not be steeper than 60 degrees from the horizontal for ore containing no fines because if an arch of lumps develops at a steep angle it becomes solidified and this makes it difficult to knock down. Source: Rudolph Kvapil.
• Chutes that feed conveyors should always have parallel sides (not choking) until the material has landed or is just above the feeder. This means that the choking should always take place in a horizontal fashion by means of the feeder skirt. This way, when the material has reached a position above the feeder it will be shaken through this choke point. Source: Ed Cayouette
• Cylinders for chain control press frames should be installed upside down to save the machined surface of the piston rod from wear and tear. Sources: Peter White and Heinz Schober
• Most crusher/ore pass feed arrangements choke the flow up in the ore pass. It is better to have a high chute that lets the muck expand upwards while being constricted sideways as it exits the ore pass. Source: Peter White
4. Ore Pass Inclination
Most mine operators prefer that the ore pass be 70 degrees or steeper, but not vertical. Others prefer that the ore passes are all inclined at near 55 degrees. The steep ore passes are run nearly full while the ore passes at 55 degrees are run empty. Ore will not run on a footwall inclined at less than 50 degrees, which explains the minimum desired inclination.
One reason for the disparity of opinion among operators may lie in the characteristics of hang-ups. Lumpy ores with few fines tend to hang up in an interlocking arch while ore containing a large amount of fines tends to hang up in a cohesive arch. The lumpy arch is most susceptible to a raise inclination exceeding 60 degrees while a cohesive arch is best avoided in a raise that is near vertical.
In mines employing bulk-mining methods, fines are significant explaining why most operators prefer a steep ore pass. Probably, seventy degrees is the preferred inclination since it may be the best compromise to avoid both types of hang-up.
5. Ore Pass Size
The required cross section of an ore pass is often determined at an operating mine by a standard based upon past experience. For a proposed mining operation, ore pass size may be determined by one or more of the following methods.
• Rule of thumb
• Standards set at other mines with a similar mining environment
• Empirical methods
Rule of Thumb
In this case, rules of thumb (such as diameter equals 3, 4, or 5 times the maximum lump size) are not adequate.
Standards Set at Other Mines
Standards set at other mines are most often used for a proposed mining operation. This is the most practical method provided that the operating environments are comparable.
Empirical Method
An empirical method has been developed in the former Czechoslovakia in which a set of curves was developed for each principal characteristic of the ore to be handled. These include lump size, percentage of lumps, gradation, percentage of sticky fines, etc. To obtain the size of ore pass required, the resulting value, k is inserted into the following formulas.
Square cross-section of side length, of L: L = 4.6 √(d2k)
Rectangular cross-section of width, W: W = 4.6 √(d2k)
Circular cross-section, of diameter, D: D = 5.2 √(d2k)
In which,
d = the size of the biggest lumps
k is determined from a nomograph. For typical shot rock from hard rock mines, the
following values for ‘k’ have been obtained from the nomograph.
k = 0.6 when the content of sticky fines = 0%
k = 1.0 when the content of sticky fines = 5%
k = 1.4 when the content of sticky fines = 10%
In a hard rock mine, all very fine material (passing 200 mesh) is sticky due to water sprayed on the muck pile for dust suppression.
Example
Determine the required size of a square ore pass in the following case.
Facts:
1. The shot rock is sized with a standard 16 x 18 inch grizzly (Texas gate)
2. The content of sticky fines is 2½%
Solution:
L = 4.6 √(d2k) = 4.6 √(1.52x 0.8) = 8.3 feet
In this case, a 7 by 7 raise will be satisfactory when unavoidable overbreak is taken into account.
6. Ore Pass Linings
In bad ground, ore passes are often lined with concrete. In many cases, the concrete is faced with a high strength steel liner that also provides the formwork that is required to pour the concrete. The steel lined ore pass has proven itself worldwide; however, the high cost and time required often make this design impractical.
Different means have been developed for placing a concrete lining to reduce the cost. One mine in Idaho developed a system for concrete lining that consists of a “sock” filled with crushed rock. The brattice cloth sock is suspended in a vertical ore pass and the annulus between the sock and the rock is filled with ready-mix. When the concrete is set, the sock is slit at the bottom and the waste material falls out leaving behind a lined ore pass. A similar method is to employ a long sausage shaped balloon as formwork. After a pour, it is deflated, elevated, and re-inflated for the next lift.
Another type of lining is shotcrete that may be applied remotely with the use of specialized equipment. Shotcrete containing steel fiber may have a tendency to crack and spall in an ore pass, but shotcrete (or sprayed concrete) containing polypropylene fibers does not have this problem. Shotcrete containing an aggregate of abrasion resistant material, such as carborundum, trap, or chert is also employed. The method has proven effective in South Africa and at some locations in North America; however, the high price of the special aggregate makes it cost-prohibitive for most applications.
It is generally accepted that the resistance to wear of concrete and shotcrete is mainly dependent on the aggregate employed. It has been proposed that the most economical procedure is to select an aggregate material that has a relatively high abrasion resistance and toughness, such as traprock (basalt, Andesite, diabase, diorite, etc.) or massive “greenstone” (metamorphosed trap rock).
7. Ore Pass Stability
A circular ore pass is more stable than a square or rectangular ore pass. The latter invites arching on the sides and stress concentration at the corners. The most stable ore pass is a circular raisebored hole; however, this type of pass has two disadvantages. The first is the problem of placing ground support in the raise and the second is the fact that a smooth circular raise is more prone to hang-ups than a rectangular raise that has been drilled and blasted. In highly stressed, burst prone ground, an ore pass usually has to be raisebored for safety reasons. The same reasons make placing ground support in the raise problematic.
The stress regime in the walls of the ore pass may be exacerbated by ground stress shunted from the mining. In some cases, the ore pass may itself become a source of seismic activity. One authority proposed that ore passes should not be employed in burst prone ground. He suggests that ramp haulage or “throw away” mill holes may be employed, instead. The mill holes would be left filled with waste rock after failure.
8. Glory Hole Ore Passes
For some mining applications at high altitude, very long vertical ore passes are required. These long passes are normally excavated with a raiseborer. They are similar to the long waste rock passes employed for CRF except that they are normally run empty while a waste rock pass is designed to be kept nearly full. The glory hole ore passes have all the problems of regular ore passes, plus some of the following problems.
• Air blast
• Ricochet
• Coriolis Effect
• Attrition
• Raisebore pilot deviation
Air Blast
Because this type of ore pass is normally designed to feed directly into an underground bin, it is run empty. This means that the ore stream obtains very high velocities resulting in intermittent air blasts due to piston effect that must be relieved by an underground connection to a relief airway. A second ore pass connected to the same bin underground may provide the required relief.
Ricochet
The high velocity of the ore stream produces tremendous impact at the bottom of the raise; therefore, the geometry is designed to provide an impact bed (rock box) at the bottom of the raise. In some cases, liners are required to take care of the ricochet (bounce) from the rock box. Another ricochet phenomenon occurs when the glory hole raise is fed with a conveyor. The horizontal motion of ore on the conveyor continues when the ore stream falls into the raise. The result is a first impact on the far side of the raise and subsequent ricochet to the near side. If nothing is done to mitigate this action, it produces wear in the upper portion of the raise.
Coriolis Effect
The earth’s rotation tends to direct the ore stream towards the East wall (Coriolis effect). A simple calculation reveals that the deviation is only a few inches and may be ignored.
Attrition
The loss of potential energy due to the drop of the ore stream is divided between friction and comminution of the ore. The total potential energy is simple to calculate. The portion of this energy that results in attrition is difficult to estimate in advance. The amount of attrition is important if the ore is to be treated in an autogenous mill. If the ore has a high work index, and a conservative fraction is assumed for comminution, the amount of reduction in lump size is not normally significant. In some cases, such as a limestone quarry or a rock fill quarry, the generation of fines by attrition may result in an unsatisfactory product.
Raisebore Pilot Deviation
The long pilot hole required to back ream a glory hole is subject to lateral deviation. In some cases, the hole may be drilled with the raiseborer when the deviation trend is known by adjusting the pilot hole alignment (Kentucky windage); however, a directional drilling technique is required for very long holes.
In hard rock, a long glory hole is not normally lined. Instead, two glory holes are provided near the same location for the following three reasons.
• Lining a long glory hole properly is more expensive than drilling the hole.
• A second raise provides the required relief to exhaust the air blast.
• If one raise becomes inoperable, the second one is available.
9. Fill Raises
The same principles that apply to glory holes also apply to long waste rock passes that supply aggregate for CRF. In temperate climates, these raises have problems with frozen material during the winter months since they are maintained nearly full. One remedy for this problem is to add calcium chloride to the waste rock. Another problem is ground water in the raise and snow or ice in the fill material that disrupts the water/cement ratio of the cemented fill. (Refer to Chapter 21 – Backfill.)
10. Bins
Underground bins are required to provide surge capacity to the ore stream. In the past, small mines simply slashed out an ore pass between levels to provide a bin. Today, most underground bins stand vertical and have a circular cross section. Larger bins are built with the bottom coned to inhibit rat holing of the ore. In some cases, liners are provided in the cone section to improve ore flow. Until recently, the maximum practical underground bin diameter was approximately 28 feet.
The completion of a 75-foot diameter excavation designed to collect neutrinos from outer space at a depth of 7,000 feet in the Creighton Mine may have removed this cap on the maximum size. Underground bins are usually provided with ground support consisting of fiberglass bolts or cable bolts. Solid steel bolts vibrate when struck by ore particles. This is believed to abet sloughing of the walls.
11. Chutes
Chutes are inclined steel troughs used for transferring shot rock. Near vertical chutes are covered on four sides and equipped with an access door. Inclined chutes are often left open on top to facilitate clearing blockages. The chute width should never be less than three times the size of the largest lump to be free running. Chute inclination in most operating mines varies between 37 and 40 degrees. Steeper chutes may require control chains to stem the muck flow consideration. If the sides of the chute are not vertical, the angle of the intersection of the slopes (valley angle) is a critical consideration because it is always less than the slope of either of the intersecting sides. In hard rock mines, it may be good practice to weld a fillet at the intersection of the plates to increase the valley angle.
Example
Determine the valley angle, C in the following case.
Facts:
1. The back plate of a chute is inclined at 45 degrees to the horizontal, A = 45 degrees
2. The chute side plates are inclined at 54 degrees to the horizontal, B = 54 degrees
Solution: Cot C = [cot2A + cot2 B]½ = [1 + 0.528] ½ = 1.236
Valley angle, C = 39.0 degrees