The Coal Handbook: Towards Cleaner Production || Coal conveying

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  • Woodhead Publishing Limited, 2013

    628

    19 Coal conveying

    G. L. JAMES, Calibre Minerva, Australia

    DOI: 10.1533/9780857097309.3.628

    Abstract : This chapter mainly discusses the design and operation of conventional trough belt conveyors in mining applications. There are also references to some of the emerging alternatives to troughed belts. The chapter looks at some of the important considerations in the design of large conveyor systems. Finally, integrated crushing and handling systems that receive material from haul trucks and feed the conveyor systems are also reviewed.

    Key terms: coal, transport, troughed, belt, conveyor.

    19.1 Introduction to belt conveyor technology

    This chapter deals with conveying systems commonly employed for bulk material transportation, and covers aspects of their design and some of the key operational features.

    The chapter is not intended to be a comprehensive text on the design details of troughed belt conveyors, or a review of the alternative conveying technologies, but covers details that are important to large conveyor designs that are not commonly found in the standard texts. For more comprehen-sive details on the design of conveyors, the reader is directed to Conveyor Equipment Manufacturers Association (CEMA) (2005), DIN22101 (2000), published papers and design manuals produced by the equipment supply companies, and also to the specifi c mining company standards, specifi cations and detail drawings.

    A troughed belt conveyor consists of a wide belt typically running on three idler rolls. The outer wing rollers are sloped upwards to form the trough shape. The troughed belt then travels over the idler sets to trans-port the load. A conventional troughed belt conveyor has the following components:

    Idlers. These are rollers with bearings that form a trough shape for the belting. The idler sets are typically spaced out between one and three metres.

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    Belting. The belting, which carries the load, rests on the idler rollers. Figure 19.1 shows the typical arrangement of the idlers supporting the belt. The belt is pulled around in a loop with tension/power supplied by drive pulley(s). Drive. Figures 19.2 and 19.3 show the pulleys and drives used to move the belt. Pulleys. The conveyor belt forms a loop. The carry side transports the load and the return side allows continuous cycling of the belt. The pul-leys allow the belt to change direction at the loading and discharge ends, as well as direction changes on the return side. Transfer chute. This is where the material is loaded onto or discharged from the belt. See Fig. 19.4. Conveyor take-up system. The take-up applies tension to the belt to limit the sag between the idlers and prevent slip at the drive pulley. The take-up pulley moves to tension the belt. Take-up systems are typically gravity, but can be winch, screw or hydraulic jack. The typical arrange-ment has a pulley mounted on a trolley. The trolley is connected to a gravity mass in a tower via a cable. See Fig. 19.5.

    19.1 Belt supported by idlers.

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    19.3 Conveyor drives.

    19.2 Conveyor Drive Pulleys.

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    19.5 Conveyor takeup.

    19.4 Conveyor discharge chute.

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    19.1.1 Trends in conveyor design

    Conventional troughed belts

    The long-term trend for belt conveyors continues to be longer, faster, and higher capacity. The highest capacity conveyors include the 40 000 tph 3200 mm wide belts on the Rheinbraun bucket-wheel excavators. The lon-gest single fl ight conventional troughed conveyors include the 17 000 m long 1000 tph conveyor for transporting limestone that crosses the bor-der between India and Bangladesh. More recently, an Australian company, Wesfarmers, commissioned a 20.3 kM 2500 tph 1200 mm wide 7.5 m/s coal conveyor at Curragh North Queensland mine. This was designed by Conveyor Dynamics Incorporated. The installed power is 4250 kW (4 1000 kW and 1 250 kW). There are 2 1000 kW tripper drives located at the mid position.

    The use of the tripper drive concept is an extension of the underground mine intermediate drive arrangements that have been used for many years. These intermediate power injection designs have included piggyback booster belts, powered rollers, and tripper drives. Torsten (1982) described the mathematics of intermediate drive systems and how they could be used to minimise the tensions around horizontal curves. Weigel (1982) described a similar system used in a limestone mine. A more unusual idea, called the Ozomin drive, used the return belt to drive a section of the carry belt. This allowed the drive station to remain at the end of the conveyor. Some of the power from the drive would be shifted to a point along the carry belt, via the return belt.

    The use of high speeds (for example, above 6 m/s), low energy rubbers and intermediate drives will allow longer and higher capacity conveyors. There are barriers emerging that will challenge this trend. These include noise, dust emissions, belt pressures on idler rolls, and lubricant loss. Conveyor systems, mines and populated areas are moving closer together, and environmental standards are changing and becoming more rigorous. There are direct relation-ships between belt speed, belt and idler roll surface geometry, noise, and dust levels. Technology may allow, say, a 9 m/s belt, but night-time noise restrictions at nearby homes may limit the speed to approximately half this value.

    Non-conventional conveyors

    In this context, non-conventional means non-troughed. An emerging tech-nology in belt conveying is the Doppelmayr Ropecon system. This system has applications in rough terrain regions and environmentally sensitive zones. The conveyor can pass over these regions at a high level, as the conveyor is supported by cables between pylons like a suspended cable bridge. The span

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    between the pylons can be up to 1500 m. The conveyor is a box-type fl at belt with fl exible vertical concertina side walls. The belt is supported by axles and wheels that travel on the suspended cables. That means the items that require maintenance, i.e. the wheels and bearings, travel around the system and back to a convenient point where the work can be safely done. The longest length to date is approx. 3500 m, and highest capacity approx. 3500 tph.

    Other non-conventional conveyors include High Angle Conveyor (HAC), Cable Belt, Pipe-types, Rail conveyor, Aerial ropeways, Sicon and Aerobelt.

    The HAC system uses an additional conveyor belt to create a sandwich. The upper belt applies pressure to hold the material in place as it moves up the steep incline.

    Cable Belt conveyors use a cross-reinforced belt supported by cables on each side. The belt is almost fl at, with a slight curve, so the supporting belt is separated from the tension member. (A conventional trough conveyor has the tension member within the belt.) The cables are supported by wheels spaced out several metres apart. The longest length is 31 kM + 20 kM and capacities around 4000 tph are located at a bauxite mine, Worsley Alumina Pty Ltd Western Australia.

    Pipe conveyors are similar to a conventional trough conveyor, except the belt is wider so that it can wrap up into a circular or pipe shape. The idler rollers, say six in a hexagonal pattern, hold the belt in the pipe shape. Pipe conveyors can bend around smaller vertical and horizontal curves than con-ventional troughed belt conveyors. Pipe conveyors are good for dusty mate-rials and conveying in and around process plants. The longest lengths are > 8 kM with capacities near 4000 tph.

    The Rail conveyor is being developed at the University of Newcastle, Australia. The system has a troughed belt, support carriages with wheels that run on a railway track.

    Aerial ropeways come in a couple of forms. There is the classic bucket hanging from a rope (like the chairs on a ski lift). The bucket would hold several cubic metres of material. Doppelmayr and others supply this type of machine for low volumes over diffi cult or complex routes. Aerobelt con-veyors are like conventional troughed belt conveyors, except the belt is sup-ported by a cushion of air instead of idlers. There is an air plenum in the shape of a trough with many holes to allow the air to leak out and form the cushion. The longest length is near 800 m, and the speeds range up to 7 m/s. The maximum capacity is near 400 tph.

    The Sicon conveyor has the belt hanging like a teardrop-shaped pouch, with the tension cables on the edge. These conveyors are good for dusty materials and conveying in and around process plants. The highest capacity is around 1000 tph and longest length 2.5 km.

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    19.2 Belt selection

    Belt selection is critical to the reliability of the conveyor system.

    19.2.1 Strength for tension

    The technology of belting has been improving over time. Premature failures, manufacturing, operating costs, and the need for stronger belts has forced the change. A change to self-extinguishing fi re resistant belting in Germany in the 1970s led to a series of early splice failures. The University of Hannover developed a belt splice fatigue testing machine and ran a crash programme to investigate and solve the problem. Flebbe (1988) describes the machine fi rst being used at the University in 1975. Contitech had a similar machine see Alles (1982) for details of the machine and investigations of time strength behaviour of belting. The University machine has been in operation since 1975. The research at the University with the support of the local belting suppliers, such as Contitech and others, led to changes to the DIN standards (Deutsches Institut Fur Normung E.V. the German national standard).

    The approach adopted by DIN and Hannover University has been to focus on the actual tensions across the belt width, and the fatigue strength of the belt carcass or splice, the ratio giving the safety factor. The com-mon, but perhaps less preferred, approach is to compare the highest aver-age across the belt tension with the non-fatigued carcass ultimate strength. This approach, although common, is not preferred because it masks the actual situation. The work by Flebbe (1988) at Hannover University high-lights a problem with using the ultimate belt strength as a reference for belt safety factors. The reference discusses the test results of two belts ST6600 and ST7500 kN/m. At the time, companies were making claims as to which had made the strongest belt in the world. At fi rst glance, the ST7500 belt would appear to have been the stronger. However, the testing at Hannover University established that the fatigue strengths were 50% and 38%, respectively. So the ST6600 had a strength of 3300 kN/m after fatigue and the ST7500 had a strength of 2850 kN/m. Pedro (2004 ) used Goodyear dynamic splice test machine when reviewing a new splice assembly tech-nique for steel cord belts. Pedro tested a ST1250 kN/m belt and an ST4500 kN/m belt on the Goodyear test-rig.

    Hager (2000) gives an overview of the approach taken by DIN 22101.

    19.2.2 Belt strength for impact and chute design

    Conveyor systems are being built to handle larger volumes and at some mines very coarse materials. This trend may be driven by:

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    The move to the use of central processing hubs. The hubs may be old facilities that are no longer adjacent to the material being mined. So the conveyor systems replace long-haul trucking. A change in the types of materials and the associated crushing equipment. A better knowledge and more confi dence in the design of transfers.

    Looking back to the 1970s, some of the early transfers had simple, large open chute arrangements with low impact idler systems. The designs orig-inated out of Germany, where the material was sticky lignite. The situa-tion called for large open chutes that could handle some build-up, but still allow the huge lumps to pass. These lumps originated from the very large bucket-wheel excavators. The designs used suspended or garland idler sets. Colijn (1973) describes how fi ve-roll suspended sets are used successfully in the open mine pits at Rheinbraun in Germany. He provides a graph of the impact force for various support arrangements. Precismeca (1998) has similar curves for the impact loads on various idler support systems. As the large lumps hit the belt, the suspended sets would change shape and reduce the impact force. These arrangements fell out of fashion, due to the mainte-nance of the links between the idler rolls, cost, and mass of the suspended assembly. The large assemblies required lifting equipment to allow replace-ment. Even then, the work was not easy.

    In Australia, in the 1960s, the iron ore and bauxite mines applied the German designs, making changes to the chutes to accommodate the partic-ular nature of the iron ore and bauxite. Iron ore, being abrasive and heavy, forced a move to rock-box design. This minimised the quantity of wear materials and lowered the impact on the belts. It was possible as the ore was dry and free fl owing. At the bauxite mines the raw bauxite tended to be small, less abrasive, and sticky during the wetter months.

    So the chutes for most materials were kept large and open. The large open simple chute designs worked well. When material is dumped crudely onto a belt, the belt tracking is not that bad.

    As the fl ows increased and the knowledge of the designers improved, the designs slowly changed. The technology improvement had the support of the mining companies and organisations that included: University of Hannover Germany; University of Newcastle in NSW, Australia; Jenike and Johanson, at the University of Wollongong, NSW, Australia; publications by CEMA of the USA; the Mechanical Handling Engineers Association in the UK; and others. These organisations have developed tests for fl ow properties, behaviour, and wear, as well as the science of bin, hopper and chute design.

    In the 1970s, soft fl ow chutes were being used to improve the fl ows through the transfers. These chutes were useful for granular non-sticky materials.

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    The benefi ts were reduced dust, degradation, and impact. Bringing the fl ow up to the outgoing belt speed improved belt life. Coal companies in the Hunter Valley of NSW, Australia and other areas used these chutes with great success at this time. Colijn (1972) reviewed the equations that describe the velocity of...

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