examples of good and bad chute design

Examples of Good and Bad Chute Design

Jonathan Page B.Sc. (Mech Eng) GCC.

Acknowledgements : The Bionic Research Institute, Chute Design Conference 1991

Anglo American Corporation have reviewed the design of chutes handling hard materials such as gold and diamond ores.

This paper provides user feedback. Current chute design philosophy is explained, and examples of good and bad chute designdiscussed.

Jonathan Page is Divisional Mechanical Engineer at Anglo American Corp. heading up the mechanical design section. Thesection concentrates on materials handling, shaft conveyances, and high pressure piping and pumping.

SYNOPSIS

This paper gives feedback from the Anglo American Corporation on chute design for hard materials (diamond and gold ores).The current design philosophy is explained and examples of good and bad chute designs are given.

Great variability in the material characteristics can compromise the final chute design. Before design it is essential to test thematerial characteristics. Where possible, final designs should be tested and optimised in pilot plants.

Current practices to combat wear problems are given.

1. INTRODUCTION

The Anglo American Corporation of South Africa Limited administers various coal, diamond and gold mines.

The bulk materials handling equipment that is common to all the mines includes belt conveyors, hoppers, silos, feeders andchutes. Chutes are the linking pieces in the materials handling chain. They are the essential means of guiding and directingmaterials between the various storage, transport and process items of equipment. Thus chutes have a common purpose.

The Mechanical Engineering Department has developed optimum chute designs over the years. Such successful designs haveevolved on a trial-and-error basis, where site modifications and the feedback of operating experience have directed the work.

This paper will give practical examples of good and bad chute designs for hard materials (diamond and gold ores).

2. PRINCIPLES OF CHUTE DESIGN

There are five basic design objectives:-

to guide material on to a conveyor belt, at the speed of the belt, in the direction of belt travel.

to eliminate material spillage.

to enclose material dribbles.

to enclose material from operating personnel.

to eliminate dust liberation.

3. CURRENT DESIGN PHILOSOPHY

Examples of Good and Bad Chute Design http://www.ckit.co.za/secure/conveyor/papers/bionic-research-1/f-bri1-p...

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Chutes are transfer points in a materials handling plant. They often demand more attention and can be the source of moredowntime than the conveyors or equipment that precede or follow them. Ideally the chutes are designed first, and then theplant equipment and structures are placed around them. This is sometimes not possible due to other constraints. Forexample, such constraints are the presence of critical supporting steelwork, space restrictions underground and existing plantwhere modifications are required. As a result, chutes may suffer and the final design can be a compromise (Fig 1 shows hownot to design a chute).

The nature and characteristics of the equipment before and after the chute are clearly defined. If a conveyor belt, what is thebelt width, belt speed, loaded I material profile, material trajectory? What is the height through which the material must fall?Does the material have a single flow path, or are there alternative flow paths?

Fig 1: How not to

3.1 Material Characteristics

Although chutes have a common purpose, they have to accommodate a wide variety of material characteristics. Minerals arefound distributed in many different geological areas. In addition, each geological deposit often contains materials with differentproperties.

There are two types of diamond deposits: a volcanic pipe and an alluvial deposit (along river paths and coastlines). In volcanicpipes, material near the surface is weathered and produces a high proportion of fines. Deeper underground, the materialbecomes harder and less weathered. However, this material weathers quickly and produces fines due to crushing andprocessing stages towards the smaller size ranges.

Gold mines are usually at considerable depth. The rock is dense, hard and abrasive. However, there are a few open cast goldmines where the material characteristics are significantly different.

Therefore, there is a variation in material characteristics:

between diamond and gold ores

between ores in different deposits

between ores during the life of a mine.

Material characteristics can change from season to season, or even day to day, on the same mine.

Chutes are designed for the worst possible conditions and material characteristics. It is essential to derive by testing thematerial characteristics such as size distribution, maximum lump size, moisture content, angle of repose etc. For changingconditions, the spectrum of characteristics is required.

3.2 Pilot Plants

Pilot plants or sample plants are useful, not only to the extractive metallurgist and plant operator, but also to the chutedesigner. Such plants enable chute designs to be tested and optimised on the actual material to be handled.

Chute design sometimes needs to be finely tuned to the material being transferred. It is often found that only minor sitemodifications are required to turn a problem chute into a successful one.

3.3 Operating Mines

It is essential to get feedback from operating mines on what does not work and what was done to rectify the problems.Otherwise, previously inadequate design drawings are copied for future projects, thus perpetuating the problems.

3.4 Erection and Maintenance

Underground chutes are constructed of bolted panels in manageable body panel sizes.

Surface chutes are fabricated from longer, welded, box sections. However due attention is paid to access to liners anderection between floors of a building.


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Chutes must be easy to assemble and install. They must be effective in their job and require minimal maintenance. Wearresistant liners must be cost effective, strategically placed and be easily changed.

4. PARAMETERS OF CHUTE DESIGN

The following parameters are used to differentiate between good and bad designs. In the future, the aim is to produce designguidelines for use as an office standard.

4.1 Chute Angles

The angle of repose for dry, run-of-mine gold ore is generally taken to be 38. The optimum chute angle in this case has beenfound to be 50-55. Angles less than this are avoided. The same angle is true for dry, run-of-mine diamond ore, e.g.kimberlite. However, the inclusion of large amounts of fines, together with water, renders the prediction of repose somewhatdifficult. In mud-rush situations, the surcharge angle can be 0. This is, however, an exceptional condition and is not used as adesign case.

In diamond plants, a chute angle of 45 is only acceptable in final recovery and sort house areas where the material is crushed,washed, sized and "guaranteed" to be dry .

Chutes with valley angle geometries need careful attention. In a chute with two adjacent 55 plates, the valley angle will be45. Hence there is the danger of material build-up in the valleys. The valley angle is increased accordingly, or designed out,e.g. in a vertical sided chute.

In a chute extended to carry the fines from belt cleaners, angles in excess of 700 are required to prevent build-up of theoften sticky, wet, fine material that is removed from the belt. The fines (or slimes) that stick to the belt have effectively noangle of repose -they hang upside down. Thus if there is any build-up on the chute plates, it will rapidly lead to blockages orspillage (Fig 2) .

Fig 2: Chute angles

4.2 Conveyor Head Chutes

The trajectory of material flowing from the head pulley of a conveyor belt is predictable. Trajectories are sensitive to beltspeed, material load profile, size distribution and moisture content. The trajectory is estimated from the centre of area of thematerial profile. If the belt line is taken, the discharged material will impact the head chute hood higher up than predicted.Once the trajectory is determined, the chute hood is designed around the material flow path (Fig 3).

Conveyor belt capacities are often subject to change in the life of a process plant. Changing throughput requirements willmost likely produce changing belt speeds. Trajectories and the resultant impact zones will change. Thus the effects on headchute design are assessed (Fig 4).

Inspection access covers are located out of the material impact zone or flow path. Cover plates are sized and hinged so thata man can easily open them. Once opened, they must not fall back with the possibility of injuries (Fig 5) .

Fig 3: Incorrect trajectory

Fig 4: One method to cater for varying loading/speed


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Fig 5: Location of inspection plates

Effective belt cleaning is a "must" for any conveyor belt. However. The removed material has to be efficiently transferredaway from the belt cleaner. In some cases, a belt cleaner performs the duty of a secondary discharge device. Snub pulleysbehind head pulleys produce dribbles which should be suitably handled. Thus. not insignificant volumes of material must becombined with the main stream. Steeper chute angles are important here as mentioned earlier. Such belt cleaners and dribblechutes are often in a most inaccessible area and therefore tend to be neglected. Sufficient access is considered at the designstage of a head chute. Alternatively. the removed material can be handled separately and not combined with the mainstream.

On a safety aspect, the sides of ahead chute enclosing ahead and snub pulley should be extended backwards sufficiently tocover all possible nip points on the belt.

4.3 Chute Widths

Chute widths are usually designed to suit the piece of mechanical equipment that follows the chute. Nevertheless, chutes aredesigned to a minimum width of three times the maximum expected lump size. For example, for a 300 mm lump size, thechute should be 900 mm -1 m square. Ideally, facilities are used to remove rogue material from a materials handling systemas early as possible. In run-of-mine ore, such rogue material, consisting of outsize rock slabs, tramp iron, timber, etc., wouldeasily choke a properly designed chute.

Another important consideration is the volumetric flow rate of the material. Once the chute is sized for the largest lumps, it isalso sized for the volume to be handled.

4.4 Changes In Flow Direction

Where one conveyor transfers to another conveyor, the easiest configuration for the chute is when the conveyors are in-linewith each other. Conveyors at 900 to each other involve chute work that is more complex but can be achieved without manyproblems. A common rule of thumb is to provide twice the belt width as vertical height for in-line transfers, and 3,5 to 4times belt width for 90 transfers. This generally allows sufficient height to enclose dribbles, etc.

Most problems are encountered when conveyors are at a small acute angle. Such small changes of direction involve awkwardchute geometries where there could be restrictions to the flow of material and problems of build up of material on shallowplate angles. The capture of dribbles also becomes difficult, since the scraping area is generally above open space (Fig 6) .

Fig 6: Conveyors at a small acute angle

As a general conveyor design principle, the material should be in constant uniform motion, not intermittent loading. Chutesperform better under uniform flow conditions.

4.5 Dead Boxes

Dead boxes have proved beneficial in gold ore situations where the material is relatively dry .Dead boxes are used to take thedirect impact of material discharged from a conveyor into ahead chute. Other useful applications are in long chutes or highchutes where the momentum of falling material must be broken before reaching the lower conveyor belt. Changes ofdirection are also accomplished in this manner.. Once dead boxes are filled, they form the ideal deflection plate or impact wearplate, where the "plate" hardness is equal to that of the feed material (Figs 7 and 8).


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Fig 7: Typical dead box

Fig 8: Cascade chute

In diamond plants, dead boxes are avoided for run-of-mine kimberlite conditions because of the high clay and moisturecontents. However, they may be used later in the plant where the material is sized and washed, and then only with carefulconsideration in each design case.

In general, dead boxes are avoided where the material is very fine, wet or sticky. Here the dead boxes are not self cleaning,due to the absence of large particles to give a scouring effect, and the fine material will build up and cause blockages.

4.6 Conveyor Feed Chutes

Fig 9: Typical Langlaagte chute dimensions

The Langlaagte chute is a well-known configuration throughout the South African mining industry .This chute was originallyintended for run-of-mine ore situations to get the fines to flow. Standard dimensions, related to belt width, have beendeveloped as a guide (Fig 9) .Where a number of Langlaagte chutes feed a single conveyor belt, every chute except the firstone is designed either higher or pivoted up out of the way when not in use.

The underside of the feed chute skirts should rise, in the direction of belt travel, to a maximum of 50 mm above the belt.Chutes should be positioned at a minimum distance in front of the tail pulley of a conveyor belt. This is to avoid the transitiondistance where belt troughing is not ideal (Fig 10).

Fig 10: Feed chute after transition distance

Radial doors are used successfully in chutes under ore passes or silos. Apart from primary on/off feed control, they performan effective mud-rush control function. (Langlaagte chutes are ineffective against mud-rushes). Radial doors suffer from the


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possibility of jamming when closed. The normal air cylinder used to open the door can be enhanced by adding a knocker armbetween the cylinder rod and the door. Thus the door can be closed with full force, opened with reduced force, or hammeredopen (Fig 11) .

Figure 11: Radial door with knocker

4.7 Flapper Gates

Flopper gates are used when one conveyor is required to feed either of two discharge points. Thus a bifurcating chutearrangement is required where each flow path must satisfy the general chute design guidelines. A critical area is the hinge ofthe gate which should be placed above the apex of the double chute. Thus the gate is self cleaning and rock traps, whichcould jam the gate, are avoided (Fig 12).

Fig 12: Flopper pivot above apex

4.8 Construction

Underground chutes have to be transported in confined shaft compartments and narrow haulages. Installation sites are oftenconstricted because of the low headroom and the presence of the conveyor structure. Hence construction should be bymeans of bolted body panels of manageable sizes. Body plates are generally 6mm to 10 mm thick with 70 mm x 70 mm x 8mm angle stiffeners. Side plates are bolted to the body by angle construction, with the connections in shear where possible.The top cover is extended over the sides and is bolted down.

Surface chutes are made from longer/taller box sections welded together, with due attention to liner access and erectionneeds.

The construction material is structural steel to SABS 1431 Grade 300WA.

Adequate access is required by the operating and maintenance staff on the mine. Access is an important consideration inchute design and the associated structural steelwork. Lifting beams are designed for the replacement of liners, and theremoval of bolted panels in maintenance operations. Hence inspection access and maintenance access are importantconsiderations.

4.9 Lining Materials

Side liners are generally 12 mm to 16 mm thick and extend up to 3 times the depth of material flow to cater for surges.Bottom liners are up to 25 mm thick. Liner plates are sized for a maximum mass of 30 kg each, so that replacement inconfined spaces is easier. Dead boxes have lip liners.

Quenched and tempered steel plate, with hardnesses of BHN 400 or BHN 500, is increasingly used for liner material. In highwear applications, these materials are most cost effective in terms of price and life, with thinner plate sizes specified.However, project engineers may choose mild steel for lower wear applications.


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Fig 13: Across line rail mat AND Fig 14: In-line rail mat

In certain circumstances, rail mats have been used on the bottom chute plates. Thus, horizontal rows of rails are bolted tothe chute plate at 900 to the direction of flow, forming a bed of dead boxes. Alternatively the rails are set in line with the flowto allow self cleaning (Figs 13 and 14).

Diamond ore fines and dribbles have been found to move more effectively on PTFE liners such as 'ISOLIDUR" or"VESCONITE", This type of liner is not effective in high impact areas.

4.10 De-Dusting

Fig 15: Dust extraction

For dry diamond ores especially, transfer chutes are de-dusted. Here, chutes are enclosed with a dust extraction systemapplied preferably after the transfer point to the belt. Chute skirt plates have rubber seals to stop too much false air enteringthe chute. The rubber seal should not be made from old conveyor belting, as the resultant seal is ineffective. The proportionsof the chute and skirts are generally dictated by the de-dusting requirements, such as air volume and pick-up velocity (Fig15).

In general, feed chute skirt widths are two thirds of projected belt width, not two thirds of actual belt width as was commonpractice.

The skirt seals are seen as seals and not as a means of containing the material on the belt. There are cases where the skirtsare terminated too high above the belt, resulting in long, floppy seals. These very easily flop over the edge of the belt, makingeffective belt training impossible. This is generally the case when the skirts are too wide as well. The long skirts are alsosubject to material loading, which increases the resistance to motion of the conveyor, adding to power and tension, andaccelerating the conveyor belt top cover wear (Fig 16)

Fig 16: Skirt seals

When de-dusting is not required e.g. on gold mines with wet materials, there is no need for rubber skirt seals. Unnecessaryskirt seals wear out the belt top cover and add to frictional resistances. It is better to redesign the skirts themselves in thisinstance.

5. CONCLUSIONS

Optimum chute designs have been developed over the years. Such successful designs have evolved on a trial-and-error basis,where site modifications and the feedback of operating experience have directed the work.

Great variability in the material characteristics can compromise the final chute design. Before design it is essential to test thematerial characteristics. Where possible, final designs should be tested and optimised in pilot plants.

6. ACKNOWLEDGEMENTS

The author would like to thank the Anglo American Corporation of South Africa Limited for permission to publish this paper.Thanks are also due to colleagues in the Mechanical Engineering Department who have contributed to this paper in manyways.


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