Transfer Station Design for Iron Ore

Belt Conveying

Transfer Station Design Developments in the Iron Ore Industry A.E. Maton, Australia

T

he article discusses the design of transfer chutes in the iron ore industry of Western Australia. It reviews the changes in design over the last 40 years and gives some insight as to why the changes have been necessary. A computational method is used to compare the designs considered, in particular, the impact of the designs on belt wear. The article discusses some of the problems experienced with the new designs as well as looking at modifying existing chutes to improve their performance.

1

Introduction

The iron ore industry in Western Australia has been operating now for some forty years and from those exciting times which established the industry we are experiencing further exciting times expanding the industry to meet increasing world demand. The materials handling industry has contributed to this expansion with advances in conveyor belt design to achieve higher annual capacity being wider, faster and longer. The advances in conveyor design have focused attention on transfer chute design to handle the higher tonnages, without blockages and with less spillage. To achieve higher productivity the materials handling systems needs to be more reliable, less prone to productive outages and less time to repair and return to service, in other words higher utilization and higher availability. This article reviews some transfer designs used in the early days, which prove adequate for the first 20 to 30 years and then reviews current designs in green field projects and highlights some of problems to modify existing designs to increase productivity in brown field projects. Transfer chute design today requires that the operating and maintenance requirements are addressed including eliminating blockages, minimizing spillage, increasing

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The materials handling industry has contributed to the expansion of the iron ore industry with faster and longer high capacity belt conveyor systems. In the course of this development, transfer chute design has become quite a demanding task. Even with today’s new designs, operating problems can occur and improvements are still possible.

conveyor belt life, improved wear life of liners and ease of replacement. All the elements of chute design should be investigated and considered including rock boxes, flat impact plates, curved receiving and discharge plates and lined chutes in order that the transfer station is optimized for the particular application and operating conditions.

2

General Review

The typical transfer chutes reviewed in this article are based on engineering standards current at the time both yesteryear and the present. In the early days there were two concepts, the rock box and the impact plate. The difference between the two concepts were based on subjective preferences with regards to the issues of minimizing operational problems (chute blockages) and minimizing maintenance problems (chute wear) The rock box type minimized wear, where as, the impact plate increases maintenance but gave perceived advantages for centrally loading the burden on the belt and ease of replacement. In practice both types gave good service while the material remained dry and free flowing. Dust generation and degradation of product lump were not considered in detail and both concepts could have been improved in this respect. The chute design approach used in this article has been the application of a continuum method of material flow through the chute and has been applied to analyze the transfer chutes considered. The design of the chutes could have been undertaken using a Discrete Element Method (DEM). The DEM methodology mod-

bulk solids handling · Vol. 27 · 2007 · No. 2

Belt Conveying

els a large system of particles in motion. It is particularly useful where flow stream behavior cannot be solved by a continuum method.

Description

Type I

Moisture [%] Bulk Density [t/m ] 3

Type II

Type III

<5

>10

<5

<4

>12

2.4

2.3

2.5

2.0

2.1

Eff. Internal Friction [°] 55 65 50 45 60 In both methods for the design of transfer chutes, an experienced judgment is reStatic Internal Friction [-] 45 55 40 40 50 quired to account for the coefficient of Wall Friction [-] 38 38 34 30 25 restitution. The estimate of the C of R is Hopper Half Angle [°] 15 12 13 22 17 based on experience of the typical iron ore transfer chutes illustrated in this article. Bmin 0.3 0.3 0.2 0.1 0.25 The solution by DEM requires the application of extensive computational complex- Table 1: Typical iron ore properties ity. The continuum method by compariEffective Internal Angle of Friction - in general, fine and dry solids son requires relatively more manageable numerical techniques. have low values, while coarse and wet solids have large values.

3

Properties of Iron Ore

As with all elements in a materials handling system the properties of the material to be handled must be known. Otherwise operational and maintenance expectations may be disappointing. In general the properties required with an indication of the typical values to be expected for iron ore are shown as follows. Bulk density:

2.0 to 2.5 t/m3

Angle of Repose (AOR): Lump size: • Primary crushed • Secondary crushed • Lump ore • Fine ore

30° to 40° -300 mm + 0 mm -100 mm + 0 mm - 30 mm + 6 mm - 6 mm + 0 mm

Cohesion (Not to be confused with surface cohesion): • Free flowing AOR < 30° • Normal 30° < AOR < 45° • Slow flowing 45° < AOR < 60°

Static Internal Angle of Friction - the friction developed within the solid at an exposed surface of a pipe. Wall Friction - low wall friction angles are recommended for practical chute design but importantly allows for chute angles to be determined. Cohesion and Adhesion - A cohesive less material is where failure occurs at the chute surface and hence flow will occur. For a cohesive, material failure occurs internally within the material. The non flowing material adheres to the chute surface and may build up and finally cause the chute to block. Hopper Half Angle - The maximum half hopper angle associated with the minimum opening for mass flow to occur. An important parameter to consider to ensure that a transfer chute containing a surge storage on stopping fully loaded will be self cleaning. Cohesive Bridging - The minimum outlet dimension required to prevent the formation of a cohesive arch across an opening Time Enhancement of Contact Friction - The increased strength of the material when prolonged residence time occurs in a chute, usually when loaded stops occur while in production.

Other Properties: • Abrasive • Very abrasive • Sharp edges • Friable • Dusty when dry • Sticky when wet

Testing for Wall Friction - This is usually undertaken for a range of sliding surfaces which are available. The abrasiveness of the material handled and the wear resistance of the sliding surface.

4

5

Table 1 illustrates the range of typical iron properties handled in the industry. Obviously extremes can occur and selective test work will identify difficult materials.

Flowability of Iron Ore

Chute & Lining Material

Flowability parameters are determined by testwork as described in J [1] and TUNRA [2] on a representative sample. Blocked chutes cause production downtime and a knowledge of the flowability parameters will assist to minimize the loss of production.

A chute is usually lined to facilitate, the replacement of the sliding surfaces, increase time between replacements by increasing the wear resistance of the sliding surface, and to assist flow and avoid build up.

Bulk Density - varies with load and moisture content

5.1

Angle of Repose - varies with moisture and clay inclusions

5.1.1 Impact Wear

Internal Friction - high internal friction angles generally indicate increase handling difficulties.

At the point a particle strikes a surface the impact wear is the damage caused by the perpendicular component of the impact

Wear to Chute Surfaces


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Belt Conveying

pact force. The rate of wear depends on the hardness of the material being handled and the wear resistance of the chute lining.

5.2

Lining Materials

In the iron ore industry the following lining materials have been used; availability, cost and wear life of material should be considered. Mild Steel - for temporary use as a wear indicator. Ni-Cr Tiles - these materials have a low Wear Index and was the liner of first choice for resisting abrasive wear. Hard Facings Plate - the facings exhibit low Wear Index and introduced to offset the higher costs of Ni-Cr tiles. Synthetic Rubber - introduced in areas of direct impact where there are perceived cost advantages over rockboxes.

6

Fig. 1:

Transfer Height - The transfer station is a gravity flow device which must have sufficient height to allow flow at the required capacity but not excessively high which wastes the power required to lift the material.

Early design „Rock Box to Rock Box“, dry material

force. Materials such as rubber are most resistant to direct impact and rockboxes where the resultant wear particles report to the flow stream, and hence no replacement of surface is required.

5.1.2 Abrasive Wear At the point of impact of a particle on a surface the abrasive wear is the damage caused by the parallel component of the im-

Fig. 2:

96

Early design „Rock Box to Rock Box“, wet material

Fig. 3:

Design of Transfer Points

Transfer of Belt Cleanings - Belt cleanings are outside the mainstream flow, they occur at belt cleaning devices arranged around the head pulley. Its classification and flowability is different to the mainstream ore and should be subject to separate testwork on a collected sample.. Variation in Flow - Changes occur in the mainstream flow depending on type of ore from the mined face, mine site and region and additional moisture content e.g. due to dust suppression.

Early design „Impact Plate to Rock Box“, dry material


Fig. 4:

Early design „Impact Plate to Rock Box“, wet material

Belt Conveying

Transitional Operation - The starting and stopping of the loaded belt varies the discharge trajectory. The range is vertical to near horizontal at high belt speed. The transfer chute contains retained material which on restart of the conveyors must commence flowing. Other Considerations - Valley Angles and Corner Effects and Fillets are details in the chute design to minimize the initiation of build up material which may eventually cause chute blockage. Discharge Trajectories - From the point at which the material leaves the head pulley to the point of impact on the collecting device e.g. rock box or impact plate. Collecting the Flow - At the point of impact this device maybe a rock box, a flat impact plate either vertical or inclined or a curved plate. Transferring the Flow - A series of sliding surfaces or free fall trajectories which direct the mainstream flow to the discharge chute. Chute Capacity and Flow Velocity - The flow rate through the chute under gravity. At changes of direction and cross section and impact points the velocity must be computed before chute capacity can be determined. Discharging the Flow - Collects the flow and directs the mainstream flow onto the belt. It may be a rock box, a flat inclined plate or a curved chute.

7

Discussion

7.1

Early Transfer Chute Design

In the 1960‘s the iron ore was generally free flowing, dusty and abrasive and two concepts of transfer chute were considered a) a rock box to rock box and b) an impact plate to rock box.

Fig. 5:

Transfer chute design with curved impact plate, dry material

Fig. 6:

7.1.1 Rock box to Rock box Figs. 1 and 2 show the typical arrangement. The drop height of material is 4 m belt line to beltline and the belt speed is in the order of 3.0 m/s.

7.1.2 Impact Plate to Rock box Figs. 3 and 4 show the typical arrangement. All other details are as Figs. 1 and 2. These arrangements were generally developed from a formalized standard design and generally taken for granted with manageable levels of maintenance undertaken at scheduled maintenance times. The exception was that the impact plate was subject to excessive wear rates when handling primary and secondary crushed ore and was superseded in some cases by the rock box for these products.

7.2

Deterioration of Flowability Properties

During the late 1970‘s as the iron ore being handled was becoming more moist with increases in dust suppression water, introduction of beneficiation plants and increasingly mining below the water table. The result was the flow properties changed for the worse. The effect of these conditions is shown in Figs. 2 and 4 for the typical transfer chute designs. The obvious unwelcome change was the build up in the lower rock box particularly when handling Fine Ore. In addition for conveyors handling fine ore was the appearance of the phenomenon of the so called rhino horn particularly in the collecting rock box of Figs. 1 and 2.

7.3

Consideration to Improve Design

During the 1990‘s the problems of the typical transfer design were aggravated by the opening up of different iron ore bodies notably

Transfer chute design with curved impact plate, wet material


Fig. 7:

Design with curved impact plate for larger drop heights

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Belt Conveying

Fig. 8:

Feeder/curved plate design, dry material

Fig. 9:

Feeder/curved plate design, wet material

7.4

with markedly different flowability properties particularly in the Fine Ore product. In addition there was the need to increase ship loading capacity with the obvious first step being increased belt conveying speed mainly for brown field projects with modifications to improve the existing transfer chute design.

Fig. 10:

Feeder/flat plate design, dry material

Further Improvements are Required

In general the Fig. 7 design worked acceptably well on Fine Ore minimizing bog outs and shutdowns during shiploading. However the down side is a marked by an increase in wear, in the curved plate, the discharge chute and the conveyor belt. Maintenance was required more often, requiring longer downtime and hence the lowering of the availability of the conveying and handling system.

The collecting and discharging rock boxes in Figs. 1 and 2 were cut back. The design of Figs. 3 and 4 the impact plate was changed to a curved impact plate and the lower discharging rock box cut back. The curved impact plate was introduced to improve the chute capacity for major plant upgrades. The effect of these changes on the typical transfer chute with a curved impact plate is as shown in Figs. 5 and 6. The build up in the transfer chute with rock boxes discharge boots require higher transfer stations but for existing plants this can be at a very expensive cost.

This lower availability was made more critical when handling Lump Ore, on dual product conveyors, and proved disastrous when installed on systems handling Secondary Crushed Ore with associated belt speed increases for increased production requirements. The application of curved impact plates and metal discharge chutes has required increased investigations of chute wear and, in particular, belt wear. To reduce belt wear the curved discharge chute prove advantageous at the expense of chute wear but, is justifiable, on the basis of chute versus belt replacement costs, particularly if the chute is designed for easier replacement in scheduled shut down times.

The design of the transfer stations was also revised for the newer iron ore mines to handle the flowability properties of these new ore types. In general the height of the transfer station was increased from 4 to 6 m with some installations at 7 m to assist with handling the belt cleanings. The collecting device became more commonly the curved impact plate and the discharging chute became a lined chute. A typical design is shown in Fig. 7 Table 3:

Hard rock discharge chute

Epoch

Description

Figure

1960

Rock box/Rock box

Fig. 1 (Dry)

Flat Plate/Rock box 1990 2000

98

Curved Plate/Rock box Curved Plate/Chute

A typical installation is shown in Figs. 8 to 13. This could be advantageous for loading primary crushed ore from an apron feeder to a belt conveyor [8]. For handling Lump Ore, more work is required to correlate the abrasiveness of the handled material with the abrasive index of the lining material to optimize replacement times Exit Velocity ABP and scheduled shut down periods. 2.78 2.75

Fig. 2 (Wet)

3.48

0.84

Fig. 3 (Dry)

2.85

1.78

Fig. 4 (Wet)

4.24

2.80

Fig. 5 (Dry)

2.18

6.53

Fig. 6 (Wet)

2.94

15.8

Fig. 7

4.55

54.37


8

Future Developments

8.1 Wear With the introduction of transfer stations using curved receiving plates and self cleaning discharge chutes, the loss of production due to chute blockages

Belt Conveying

Fig. 11:

Feeder/flat plate design, wet material

Fig. 12:

Feeder/rock box design, dry material

Fig. 13:

Feeder/rock box design, wet material

has decreased. However there has been an increase in maintenance to repair and/or replace chute linings. This increase requires more time allocated for scheduled maintenance, decreasing available operating hours and also due to increased repair downtimes an overall decrease in plant availability.

main advantage to load the flow stream in the direction of the belt is lost. Also top size rocks in primary and secondary crushed ore do not follow the flow stream hence high impact on curved discharge chutes and receiving belt particularly when segregation has occurred outside the control of the feeder.

It is possible that the current changes in transfer chute design are neutral with regard to actual annual production time. It is therefore very important to study the wear rate in conveyor belts and chute linings as well as the operational design of transfer stations to optimize annual productive operating hours.

The flat plate has similar disadvantages and height available restricts the benefits of a suitable flat plate. The rock box protects the receiving belt most and top size rocks are cushioned before discharge to the belt. However, the rock box increases the risk of cohesive blockage and inability to self clean on restart.

8.2

8.5

Belt Wear

Curved Impact Plate in Hard Rock

Belt replacement is the most expensive activity in terms of cost of new belt, and downtime to replace belt. The Abrasive Wear Parameter (AWP) suggested by R [10] is used to produce the comparative results in Table 2.

There is a preference to design this type of transfer chute shown in Fig. 7 in the iron ore industry. However there has been some consideration for secondary crushed ore but to date there is no known installation that works satisfactorily.

8.3

Observations of existing applications on secondary crushed ore suggest that the curved plate does not control the flow stream. The flow stream after impact is random with variable bounce and resultant direction. The effects are obvious with major wear of components and excessive damage to supporting structure . and enclosures. The DEM method may indicate that this behavior will occur and hence be a warning against installing curved plates for this application.

Chute Wear

A chute wear parameter is also suggested by R [10] but for the examples of curved chutes considered in this article more site observations and weight loss data is required before meaningful comparisons would be useful for discussion.

8.4

Curved Discharge Chute in Hard Rock

There appears, on the face of it, a strong case to retrofit the type of chute shown in Figs. 8 to 13 in the iron ore industry. However there has been some consideration for primary crushed ore but, to date there is no known installation that works satisfactorily. Chute blockages both mechanical and cohesive being prevalent.

Table 3:

Hard rock discharge chute

Description

Figure

Exit Velocity

ABP

Feeder/Curved Boot

Fig. 8 (Dry)

3.57

20.17

Feeder/Flat Boot

Fig. 10 (Dry)

6.15

13.36

A perusal of the possible applications shown in Figs. 8 to 13 suggests some of the following reasons and calculation results are shown in Table 3.

Feeder/Rock box

Fig. 12 (Dry)

2.09

3.72

Feeder/Curved Boot

Fig. 9 (Wet)

2.37

12.76

In Fig. 8 the curved chute is restricted to an included angle of 68° to 70° to ensure self cleaning hence the

Feeder/Flat Boot

Fig. 11 (Wet)

6.36

13.22

Feeder/Rock box

Fig. 13 (Wet)

3.43

0.62


99

Belt Conveying

9

Concluding Remarks

Clearly there is further work to be undertaken to improve the performance of curved and flat impact plates to improve wear life for both impact and abrasive wear. The computation methods using the continuum theory allows for a rational approach to chute design to reduce belt wear rates. For existing designs accurately determining the exit velocity for various options of the proposed chute modifications will show that there is little benefit to be obtained with modifying the chute. However it may be easier to realize that it could be very effective to adjust belt speed to obtain an increase in belt life. In general transfer chute design is becoming more objective but, still remains subjective. Nothing is better than experience in the exercise of good judgment. For current design practice a good staring point is T [4] and R [15] a wide range of experience from 1986 to 2006 encapsulating 40 years experience.

References [1]

J, A.W.: Storage and Flow of Solids. Bulletin No 123 University of Utah, Salt Lake City, 1964.

[2]

A, P.C., M, A.G. and R, A.W.: Bulk Solids Storage, Flow and Handling. University of Newcastle Research Associates (TUNRA) Ltd. Australia.

[3]

K, Z.: The Dynamics of Bulk Solids Flow on Impact Plates of Belt Conveyor Systems. bulk solids handling Vol. 8 (1988) No. 6, pp. 689 - 697.

[4]

T, H.J.: Guide to the Design of Transfer Chutes & Chute Lining. The Mechanical Handling Engineers Association, 1989.

[5]

M, A.E.: Experimental examination of friction factor influence on power consumption for long overland conveyors. MIE Aust Transactions, Vol. ME 15 (1990) No. 3.

[6]

M, A.E.: The effects of Idler Alignment and Belt Properties on Conveyor Belt Power Consumption. bulk solids handling Vol. 11 (1991) No. 4, pp. 801 - 805

[7]

M, A.E.: Review of the effects of idler alignment and belt properties on conveyor belt power consumption. bulk 94 design seminar, Blackpool, UK.

[8]

N, L.K.: Palabora Installs Curved Transfer Chute in Hard Rock to Minimize Belt Cover Wear. bulk solids handling Vol. 14 (1994) No. 4, pp. 739 - 743.

[9]

M, A.E.: Power and capacity review of tubular pipe and trough conveyors. bulk solids handling Vol. 17 (1997) No. 1, pp. 47 - 50.

[10]

R, A.W. and W, S.J.: Interrelation between Feed Chute Geometry and Conveyor Belt Wear. bulk solids handling Vol. 19 (1999) No. 1, pp. 35 - 39.

[11]

M, A.E.: Tubular pipe conveyor design, a review of cross section and belt selection. bulk solids handling Vol. 21 (2001) No. 2., pp. 179 - 182

[12]

M, A.E.: The effects of idler selection on conveyor belt power consumption. bulk solids handling Vol. 22 (2002) No. 1., pp. 46 - 49.

[13]

M, A.E.: Unit Train Loading Systems - Rail Wagon Loading Times. bulk solids handling Vol. 24 (2004) No 2., pp. 92 - 96.

[14]

M, A.E.: Unit Train Loading Systems - Reclaimer Selection and Wagon Weighing. bulk solids handling Vol. 24 (2004) No. 3., pp. 172 - 177.

[15]

R, A.W. and MB, B.: Chute Design Considerations for Feeding and Transfer. Bulkex 2006 Melbourne, Australia. ■

About the Author

A.E. Maton Mr. Bert Maton has been in the engineering industry for 50 years of which 40 years has been in engineering services to the mining and minerals processing industry in Western Australia. Mr. Maton graduated during 1974 in Mechanical Engineering at the Western Australian Institute of Technology. Services have been provided in project and design engineering for a number of major developments and operating facilities in the iron ore, nickel, bauxite, coal and gold. In recent years Mr. Maton has specialised in mining facilities from the ROM receival, crushing, screening, belt conveying, unit train loading and unloading, shipping terminal stockyards reclaiming and shiploading. Contact: Maton Engineering Pty. Ltd. Mr. Albert E. Maton 201 Reservoir Road; Orange Grove WA 6106, Australia Tel.: ++61 (0) 8 945 977 04 Fax: ++61 (0) 8 945 234 96 E-Mail: [email protected]

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Transfer Station Design for Iron Ore

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