OVERLAND BAUXITE CONVEYING - CABLE HAULED CONVEYORS vs CONVENTIONAL CONVEYORS

George Thomas and Patrick Conley

JLV Industries Pty Ltd

ABSTRACT

In conventional troughed conveyors, the belt performs the dual role of supporting the material load and transmitting the drive tension. In cable hauled conveyors, these functions are separated: the belt solely supports the material and sits on the drive cables, which transmit the drive tension.

This configuration has some advantages, a main one being that much longer flights are possible. A design study is carried out which investigates the fundamental differences between the two systems for various lengths and highlights the advantages of each system.

Three large alumina producers that rely on a cable hauled conveyor to transport bauxite ore from the mine site to the refinery are examined. Key performance indicators such as usage, maintenance down time and polyrim change out rate are compared.

The 51 km overland conveyor at Worsley Alumina, comprising of 31 km and 20 km flights, is considered the ‘benchmark’ in cable hauled conveying. It is demonstrated how the Worsley overland conveyor has benefited from ‘collaborative partnering’ with suppliers to develop quality parts and services meeting various stringent criteria relating to cost, longevity, reliability, environmental concerns and reliability of supply.

KEY WORDS:

overland, bauxite, conveying, cable hauled, troughed, partnering, maintenance

 

OVERLAND BAUXITE CONVEYING - CABLE HAULED CONVEYORS vs CONVENTIONAL CONVEYORS

George Thomas and Patrick Conley

1.0 INTRODUCTION

Cable hauled conveyors have been used worldwide in a variety of bulk solids transportation applications. These types of conveyors are particularly suited to long distances, high lift and horizontal and vertical curves.

Some major alumina producers have utilised cable hauled conveyors for conveying bauxite from the mine site to the refinery. In these instances, the conveyor has formed the ‘lifeline’ of the system. The reliability and efficiency of the conveyor is critical to the overall performance of the company.

To demonstrate the advantages and capabilities of cable hauled conveyor systems, the following topics are discussed:

2.0 COMPARISON BETWEEN CONVENTIONAL TROUGHED CONVEYING AND CABLE HAULED CONVEYING

Cable hauled conveyors were first developed in the late 1940’s in the United Kingdom and have since been constructed worldwide. They have conveyed a wide variety of materials over various lengths up to 30 kilometres for one flight, and in different environments (overland, underground, extreme climatic conditions).

The longest conveyor in the world is a cable hauled conveyor that is located near Collie in Western Australia. This conveyor, owned by Worsley Alumina, is a 51 kilometre bauxite conveyor comprising a 30 km flight and a 21 km flight that was constructed in the early 1980’s.

Other notable bauxite conveyors are Alpart in Jamaica (14.4km) and NALCO in India (14.6km).

The essential differences between cable hauled and conventional conveyors are:

  • the driving tensions are transmitted via two endless steel cables rather than the belt,
  • the belt is supported by the drive cables and is solely required to support the material laterally, not transmit the driving tension longitudinally.

The two systems are shown diagrammatically in the following two figures.

Figure 1

Cable Hauled Conveyor Linestand Configuration

 

 

Figure 2

Conventional Troughed Belt Conveyor Configuration

The key advantages of cable hauled conveying compared with conventional troughed conveying are summarised in the following table:

 

Table I

Benefits of Cable Hauled Conveying vs Conventional Troughed Belt Conveying

Area

Conventional Conveyors

Cable Hauled Conveyors

Belt and Cables
  • Extra width required for edge wander
  • Difficult to detect damage and corrosion to steel cord reinforcement
  • Belt width determined by material capacity and tensions to be transmitted
  • Tracking of belt in horizontal curves very sensitive to a variety of factors limiting their use and minimum radii
  • Restricted tensile capacity of belt reduces the length of flights
  • More belt splices for given length compared with cables
  • Belt splicing more time consuming to perform
  • Drive cables easily inspected
  • Width of belt solely determined by material throughput
  • Positive tracking of belt in cables and longitudinally flexible belt enable very tight horizontal curves to be easily negotiated (minimum radius below 500m)
  • Belt mechanically spliced which is maintenance free and very easy and quick to change out
  • Positive capture of cables in belt shoeforms makes belt dislodgment highly unlikely
  • Off-centre material loading will not cause belt dislodgment
  • Much larger tensile capacity of driving cables enables much longer flights
Idlers / Pulleys
  • 6 bearings and seals at each idler set - high number of moving parts and friction losses
  • Idler sets spaced more closely together - high number of idlers
  • Belt in contact with idlers over full width of belt - higher indention loss and belt wear and damage
  • Greater spacing between roller (pulley) sets means less bearings and seals over a system reducing friction
  • Belt does not come into contact with rollers preserving belt and reducing friction
Power Requirements
  • Indention loss of rubber cover on idlers
  • Resistance of belt due to troughing force imparted by idlers
  • Agitation of material passing over idlers
  • More rotating mass in system
  • all contribute to higher power requirements
  • No rubber indention loss over width of belt
  • Belt simply supported between ropes
  • Material is not exposed to constant vertical agitation which absorbs power and causes dust
  • High pulley spacing and smaller pulleys
  • all contribute to low power requirements

 

Area

Conventional Conveyors

Cable Hauled Conveyors

Capital Costs
  • Large number of parts per metre
  • More steel work per linestand
  • Restricted in length
  • Restricted in horizontal profile
  • High linestand spacing
  • Simple and economical construction of linestand steelwork
  • Longer flights
  • Able to negotiate tight horizontal curves
  • Able to negotiate high lifts and vertical curves
  • Able to use angle stations to redirect conveyor at 90
     
Running Costs
  • Many moving parts requiring constant monitoring
  • Idlers less easy to change out
  • Many belt splices which require monitoring and maintenance
  • Belt splicing require much longer time to make up
  • More moving parts and friction losses requires more power to run
  • Fewer moving parts require less time to inspect
  • Simple and accessible design of pulleys easy to change out
  • Pulleys are refurbishable
  • Belt is refurbishable
  • Lower power consumption
  • Less cable splices to be maintained
  • Cable splicing is much less time consuming

While the above table shows the benefits of cable hauled conveying, conventional conveying does hold the advantage in some areas because:

  • The high number of parts and service suppliers makes for easy availability and competitive costs.
  • It is highly utilised and accepted in industry.
  • The physical size of drive and gearbox is smaller than the equivalent for a cable hauled conveyor with a mechanical differential (not as significant in recent times with the advent of electronic differential control).
  • The steel cord tension member is protected in belt construction.
  • With recent technological advances, conventional conveying, particularly long distance overland conveyors, have become more economical and competitive with cable hauled conveyors.
    1. Design Study Comparison

To investigate the fundamental differences between conventional belt conveyors and cable hauled belt conveyors, preliminary designs for a number of conveyor lengths were conducted for each conveyor type. The equations used for the calculations are based on standard practices and design codes, which are in part empirically derived.

The general assumptions made for conventional conveying are: 3 idler set carry, 2 idler set return, typical summer temperature (15 C +), friction factor typical for conveyors about 5 km long and series 20 idlers used.

The general assumptions made for cable hauled conveying are: typical summer friction factor of 0.015, maximum pulley load of 1.8kN and minimum carry linestand spacing of 4m.

A summary of the results is presented in the following table.

Table II

Design Comparison between Conventional and Troughed Conveyors, Key Results

Conventional Belt Conveyor

Cable Hauled Conveyor

Conveyor Length [m]

1000

3000

5000

7500

10000

1000

3000

5000

7500

10000

Power [kW]

142

427

712

1176

1627

129

387

646

969

1319

Idler Set spacing [m]

1

1.5

2

2.5

2.5

4

4

4

4

4

Number of idlers

3667

7333

9167

11000

14667

1333

4000

6667

10000

13333

T2 Tension [kN]

19.5

28.5

50.7

78.4

108.5

42.4

37.1

61.8

92.8

126.3

Total Idler Mass [kg]

73882

147644

184556

221467

295289

18667

56000

93333

140000

186667

Rotating Idler mass [kg]

34833

69667

87083

104500

139333

14667

44000

73333

110000

146667

Belt Mass [kg]

16.8

20.6

27.1

32.9

36.9

25

25

25

25

25

2.2 Discussion On Design Comparison Findings

This study, while not exhaustive, does give a good indication of the relative performance differences between troughed belt and cable hauled conveying.

From the above results, the following fundamental differences between conventional and cable hauled conveyors emerge:

  • The power requirement for a conventional conveyor is more than for a cable hauled conveyor for all lengths. At 1000m length, this difference is about 9%, and at 10,000m length, almost 20%.
  • At shorter lengths, sag requirements dictate the idler spacing for both conveyor types. For a 1000m conveyor length, additional counterweight mass is required to provide enough tension for a 1m idler spacing for conventional systems and 4m linestand spacing for cable hauled systems.
  • At longer lengths, allowable idler loading dictates idler spacing. For conventional systems however, a limit is imposed of 2.5m (carry side) due to bulk material properties and dynamic behaviour of the belt (Ref: Grimmer, Kessler, 1997). Beyond a particular length (and minimum tension) the linestand spacing for conventional systems is based purely on static loading limitations.
  • The tensions in a conventional system are significantly lower than for a cable hauled system. However, in terms of the capacity of the tension bearing medium, a conventional conveyor belt operates closer to its recommended capacity. For example, with a 10000m long conveyor, a conventional conveyor belt operates at 96% of its safe capacity, while the cables of a cable hauled system operate at 83% capacity considering a factor of safety for the cables of 3. Also, a cable hauled system has much more capacity for increasing tensions and lengths.
  • The number of idlers in a conventional system is always greater than in a cable hauled system. This is because of the difference in linestand spacing.
  • The total weight of idlers in a conventional system is substantially higher than for a cable hauled system (4 times greater at 1000m, 1.6 times greater at 10000m).
  • The total idler rotating mass of idlers is higher for conventional systems at lengths below 7500m. Above 7500m, the rotating masses of both system types are comparable.
  • The power requirements and tensions increase linearly with increasing slope for both conveyor types. Conventional conveyor power requirements increase at a higher rate compared with cable hauled conveyors while T2 increases at a higher rate for cable hauled systems.

The above findings clearly indicate that cable hauled conveying compares favourably with conventional conveying at longer lengths.

It must be said that the above study compares conventional (ie. traditional) troughed belt and cable hauled conveyor design. In recent years, the following design trends in conventional troughed conveyors have taken place:

  • increased idler spacing
  • increased tension rating and quality of belts
  • higher speeds
  • multiple drives

These factors have helped reduce the capital and running costs of conventional conveying. However, cable hauled conveying holds the natural advantage as flight length increases, evidenced by the difference in longest examples of both types of conveyor. Worsley Alumina (2300tph, 6.1m/s) with 30km and 21km long flights has the longest cable hauled conveyor system with the longest flights in the world, while ZISCO in Zimbabwe (600tph, 4.25m/s) is the longest conventional conveyor flight at 15.6km in length.

3.0 COMPARISON BETWEEN THREE MAJOR BAUXITE PRODUCERS WHICH RELY ON CABLE HAULED CONVEYORS

Three alumina producers are compared which rely heavily on a long distance cable hauled overland conveyor system to convey bauxite from mine site to the refinery.

These producers are:

  • Worsley Alumina, Australia
  • National Aluminium Limited Company, India
  • Alpart, Jamaica

The main performance characteristics and key performance indicators for each conveyor are shown in the following table.

Table III

Key performance indicators of 4 cable hauled bauxite conveyors

Physical Characteristics

Alpart

Nalco

Worsley

Conveyor 1

Worsley

Conveyor 2

Length

14192

14550

30429

21015

m

Net Lift

-428

-340

-72

-14

m

Capacity

1560

900

2300

2300

TPH

Speed

4

2.4

6.1

6.1

m/s

Belt Width

36

42

36

36

inch

Rope Size

51

51

57

57

mm

Load Rate

108

106

105

105

kg/m3

Annual Tonnes

6

2.4

6.6

6.6

MTPA

Power Capacity

932

2000

7800

5200

kW

Power During Loaded Running

148

460

5000

3800

kW

Operating Characteristics

Daily Operating Hours

20

13

18

18

hrs

Weekly Operating Hours

115

78

80

80

hrs

Annual Operating Hours

6000

3900

4000

4000

hrs

Scheduled Maintenance

2000

900

n/a

n/a

hrs/yr

Unscheduled Maintenance

1280

300-500

n/a

n/a

hrs/yr

Usage Rate

69%

45%

46%

46%

Polyrim Change-out Rate

10000

15000

13000

7000

rims/yr

Rope Life

8

9

6.75

6

years

Belt Life

10

11

14 to 25

14 to 25

years

Number of Pulleys in System

15360

18578

38240

26770

pulleys

Polyrims Changed per Year

33%

39%

17%

13%

3.1 Observations From Performance Comparison

The major replacement cost items in cable hauled conveyors are belting, rope and poly pulleys.

These major cost items, normally envisaged as standard or generic, have a large bearing on the operating costs and efficiency of a cable hauled system. By developing and using higher performance components specifically suited to the unique demands of the system, and developing techniques to maximise the life of these components, significant reductions in operating costs can be realised.

All three conveyor systems have generally performed very well. However, from Table III, each conveyor system has exhibited varying levels of performance in terms of life of major components.

Belt life is a function of cycle time, loading technique, lump size and usage. Worsley experiences much greater belt life compared with the other systems. While longer cycle times contribute, more robust construction and refurbishment of the top cover are significant factors in extending belt life.

Rope life is lower for the Worsley conveyors despite the longer cycle times. This is largely attributable to the ropes operating at a lower safety factor (higher tensions) and the rope construction being steel core rather than fibre core.

Polyrim change out is a regular maintenance event and a major yearly cost item. The Worsley system experiences virtually half the change out rate of the other two systems, despite operating at a much higher speed.

Worsley Alumina has achieved this superior component performance through extensive design and development work in conjunction with the component suppliers. This development is detailed in the next section.

4.0 WORSLEY ALUMINA’S COLLABORATIVE PARTNERING

In the early years of operation, the Worsley overland conveyor experienced many problems largely due to shortcomings in the original design. Some of these problems included belt/rope lift off and dislodgments, belt failures, power shortfall in cold weather and other problems.

The shortcomings relating to inadequate component design required a collaborative effort with external suppliers to develop new designs able to cope with the unique demands of this conveyor system. Prime examples of this are detailed below:

  • The original strapped belt suffered major problems with the steel rods protruding through the edges of the belt (See Figure 3). This was cause by the rods not being bonded to the surrounding rubber. After much developmental work, a refurbishing technique was developed which fixed this problem in the original belt. 85km of the original belt were successfully refurbished and reinstalled into the system.
  • As a consequence of the refurbishment developmental work, a new type pre-troughed belt, called Combi Belt, was developed (See Figure 4). Worsley have embarked on an ongoing replacement program to replace all existing belt with Combi Belt. So far, about 65km out of 102km have been replaced.
  • As the belting in the system is a major replacement cost item, maximising the life of this belt is very significant in minimising operating costs. On-site belt monitoring of the top cover thickness is used to determine belt wear and predict when belts should be removed for refurbishment or replacement. This assists planning and ensures that belts are removed for refurbishment with a minimum amount of top cover remaining. This is essential for a successful refurbishment.
  • The polyrims used at Worsley have been developed over a number of years in a continuous effort to improve quality and reduce wear. The urethane used has been specifically formulated with improved dynamic physical properties which exhibits better wear characteristics for this application.
  • To reduce noise and vibration in the system, polyurethane vibration isolators were developed which reduced noise and improved the life of mechanical components (See Figure 5). These isolators also have the effect of reducing the polyrim wear rate. This is because of a reduced level of vibration that leads to direct impact and wear. This also causes misalignment in the conveyor, further accelerating wear.
  • To further reduce noise in sensitive areas, polyrims using urethane of various hardnesses were developed. These rims reduced noise emissions and are currently in use in noise sensitive areas.

The knowledge gained from the experiences at Worsley Alumina is being utilised by other cable hauled conveyor systems around the world. The belt, poly pulleys, noise and vibration isolators, belt monitoring and other products and services have found successful application in a diverse range of systems. While the basic concepts remain the same, the components require careful review and re-design for some unique systems in order to maximise the benefits to the operator. This requires a two-way flow of information between the operator and the supplier to ensure success. This is significantly different to most conventional conveyor systems that utilise standardised parts that, due to the large amount of users and suppliers, are readily available.

 

Figure 3

Original strapped belt showing rods protruding through the edges

 

Figure 4

Combi Belt developed specifically to meet Worsley Alumina’s requirements

Figure 5

Poly-pulley rocker assembly showing noise and vibration isolators

5.0 CONCLUSION

Cable hauled conveyors are generally better suited to longer distance conveying than conventional troughed belt conveying, particularly through arduous terrain. This has been demonstrated with a basic design comparison of representative lengths up to 10,000 metres. Power consumption, rotating mass, number of idlers and belt mass are all higher for conventional conveyors, increasingly so for longer lengths. There are also many operational benefits with cable hauled conveying such as positive belt tracking, less material agitation, use of simple mechanical belt splicing and easy pulley change-out. These factors make cable hauled conveying the preferred conveying system for long distance overland conveying.

Three major alumina producers that are dependent on a cable hauled conveyor to transport bauxite from the mine site to the refinery are compared. While all three systems perform well, the Worsley system exhibits markedly better performance in terms of belt life and polyrim change out.

The longer belt life is attributable to a superior belt type that was specifically developed for the Worsley system. This belt can be refurbished which greatly extends the belt life. A belt monitoring program, which looks at the top cover wear rate, is in place at Worsley. This is vital for the successful implementation of a belt refurbishment program.

The polyrim change-out rate at Worsley is virtually half that experienced at Alpart and NALCO. This is attributable to the polyurethane used for the rim liners that has superior physical properties, and the use of noise and vibration isolators that reduce mechanical impact and misalignment.

These components and methods were developed and proven over a number of years in a collaborative partnering relationship between Worsley Alumina and external suppliers. Consequently, these components are now being utilised in other cable hauled systems around the world. While these products have been proven in the Worsley conveyor system, the unique demands of other individual systems require that the design and specifications of these components be carefully reviewed and tailored to each unique system. This requires collaboration between the conveyor operator and the supplier to ensure long term success.

 

ACKNOWLEDGMENTS

G Downham and J Pitts, Worsley Alumina Pty Ltd

Steve Smith, Alpart Jamaica

National Aluminium Company Limited India

REFERENCES

Malton, D. (1997). Maximising the capacity of the cable belt conveyor system at Worsley Alumina. Improving Conveyor Performance in Mining Conference Proceedings.

Nordell, L.K. (1998). ZISCO installs world’s longest troughed belt 15.6km horizontally curved overland conveyor. Optimising Conveyor Performance in Mining Conference Proceedings.

Roberts, A.W. and Harrison, A. (1997). Advances in the design of belt conveyors - An overview. Belt Conveying Module - Master of Engineering Practice (Bulk Solids Handling) Course Notes.

Roberts, A.W, Hayes, J.W. and Harrison. (1997). A. economical considerations in the design of belt conveyors. Belt Conveying Module - Master of Engineering Practice (Bulk Solids Handling) Course Notes.