Bulk Material Storage and Storage Yard Machines
Bulk Material Storage and Storage Yard Machines
The term ‘bulk handling’ is used in an iron and steel plant for the handling and storage of very large quantities of solid raw materials such as ores (iron ore, limestone, and dolomite etc.) and coal (coking coals, and non-coking coal). These materials are used in huge quantities in the plant. There exists normally a mismatch between the consumption of the bulk materials in the plant and their arrival in the plant and hence, the bulk materials are required to be stored. The storage of the bulk materials is also necessary to take care of any disruptions in the transport system or in the mines. Handling and storage of large quantities involve careful study of the choice of materials handling equipment as well as storage method.
Different methods are used for the storing of the bulk materials. The elements which determine the selection of storage method for the bulk materials include (i) nature of the material, (ii) quantity of materials to be stored, (iii) charging and discharging rate, and (iv) frequency of arrival and supply to the consuming units. These factors determine the storage methods for the bulk materials, selection of suitable charging /stacking, and discharging / reclaiming equipment. Some of the common storage methods used for the bulk materials is described below.
Bins – Bins are large containers of cylindrical or rectangular shape made from steel sheet / plate supported on legs. Filling of bins is from the open or covered top and discharge is from the bottom opening. Inside agitators or outside vibrators are frequently used for facilitating the flow of materials from the discharge opening. Conveyors, bucket elevators, and pneumatic conveyors are normally used for delivery of materials into the bins. Discharge of materials from the bins at the desired rate is carried out by using different types of feeders.
Bunkers – Bunkers are essentially large bins used for storage of materials. They are normally made of concrete. These can be a few hundred meters in length. Belt conveyors with automatic trippers and dumpers etc. are used for filling of the bunkers. Discharge of the materials from the bunkers at the desired rate is carried out by using different types of feeders.
Stock-house bins – Stock-house bins actually consists of a number of conventional bins grouped in an elevated structure for batch delivery of the materials to the process. A blast furnace has such stock-house bins its stock house. The bins are normally filled by belt conveyors. The discharge is through mechanical bin gates either into the skip or onto the conveyor belt.
Silos – Silos are tall cylindrical structures normally made of reinforced concrete. These are used for storage of materials like coal and coke etc.
Sheds – Several materials which need to be stored under a cover for one reason or other, but are difficult to be stored in a bin or silo, are generally stored in a shed. Sheds can be upto 100 meters in width and several hundred meters in length. Sheds are covered at top by corrugated GI (galvanized iron) sheets or corrugated asbestos sheets supported on full width strusses. Materials are normally brought in by belt conveyor and distributed in the shed by a distributing conveyor with self-propelled tripper or by a shuttle conveyor. Discharge is normally through reclaiming conveyors working below floor levels. Equipments like bulldozers, power hoe, or a drag scraper are used for moving materials towards feeding pockets for the reclaiming conveyors. Sheds are used for short to long duration storage of different materials.
Open pile storage – Bulk ores and coals are frequently stored in open storage piles (Fig 1). A number of different methods and types of equipments are used for handling these plied materials. The main types of equipments used in the open pile storage are (i) stacking equipment, (ii) reclaiming equipment, and (iii) belt conveyors. Materials which are not weathered easily and need to be stored and reclaimed at substantial high hourly rate are kept in the open pile storage. The piles can contain materials from 5,000 tons to some million tons, while handling rate can vary from 100 tons per hour to 2,000 tons per hour. The capacity of the open pile storage normally ranges from 7 days to 45 days requirement of the consuming unit of the plant. Such wide variation of pile size and handling rate has influence on the selection of type and size of materials handling plant and equipments needed.
Fig 1 Open pile storage methods and layouts
The two basic open pile storage layouts (Fig 1) are (i) longitudinal pile arrangement, and (ii) circular pile arrangement. The longitudinal arrangement allows easy future expansion but needs more space. The circular arrangement prevents any future expansion but has a more compact storage area. Longitudinal pile arrangement is used for high storage capacity.
The most commonly used stacking methods for making longitudinal stockpiles are cone Shell, chevron and windrow. Basically these methods consist of stacking a large number of layers on top of each other in the longitudinal direction of the pile. In open pile storage different methods of stacking (Fig 1) can be used. These methods are described below.
Cone shell method – It is the simplest of the stacking methods. In the cone shell method, the pile is formed by depositing material in a single cone from a fixed position. When this conical pile is full, the stacker moves to a new position and a new cone is formed against the shell of the first one. This process continues in the longitudinal direction of the storage pile until the stockpile is complete.
Chevron method – In the chevron method, the material is deposited by the stacker moving to and fro over the centre line of the pile. However, the chevron stacking method causes segregation of the material with fine particles in the central part of the pile and coarse particles on the surface and at the bottom of the pile. For ensuring proper blending, a chevron pile is hence to be reclaimed from the face of the pile, working across the entire cross section.
Window method – In the windrow method, material is deposited from a number of positions across the full width of the pile. This method needs a luffing and slewing stacker. The windrow method prevents segregation and ensures more even distribution of fine and coarse particles across the pile. The windrow method is preferred in cases where the reclaimer is only operating in one part of the pile cross section at a time or in cases where segregation makes an open pile base unacceptable.
Strata method – The strata method needs a luffing and slewing stacker. In the strata stacking method, the stockpile is built up in inclined layers. The stacker builds the first layer travelling along the length of the storage area with its boom at a low elevation. The stacker then moves towards the stockpile centre in pre-set steps while gradually lifting the boom. The result is superimposed inclined layers, matching the angle of repose of the bulk material and providing a good blending as the reclaimer cuts into each of the many layers.
The chemical and / or physical characteristics of some of the incoming raw materials can differ widely and these materials need homogenization to have uniform characteristics for meeting the requirement of the processes where these materials are to be used. This homogenization operation is to be carried out during the stacking and reclaiming of the materials. This mixing operation when applied to more than one material is called blending. The blending operation, when applied to only one material to make it uniform in characteristics, is termed as homogenizing operation. Since the mixing operation i.e. blending of more type of materials or homogenization of more grade of single material is functionally same, only word blending is normally used for both the blending as well as homogenizing operations. The blending operation needs separate stacking and reclaiming machines. Also, at least two stockpiles are necessary in the storage yard, so that when one stockpile is under formation, the other formed stockpile is used for reclaiming of the material.
If no blending is needed, then the cone shell method is normally chosen. It needs no slewing mechanism / gear in the stacker and simplifies the movement of the stacker during stacking. If blending is necessary, the chevron method or windrow method is used. Chevron method is the preferred choice since it does not need slewing mechanism in the stacker.
There are three main types of storage yard machines which are used for stacking and reclaiming of materials. These are (i) stacker, (ii) reclaimer, and (iii) stacker cum reclaimer. These yard machines are described below.
Stacker is frequently being referred as stacker machine. The stacker is used for preparing the stockpile with the incoming material. The machine travels on rails with the material fed to it through the yard conveyor between the rails. It is provided with a boom with a belt conveyor which is moved up and down with the help of wire ropes or a hydraulic system. Depending on the application, the entire boom system can also be slewed on the sub-structure which travels on wheels running on the rails. The material coming on the yard conveyor is raised by a tripper attached to the stacker and discharged into the boom conveyor which carries the material up to its end and allows it to fall to form a stockpile. The stacker travels continuously to and fro so that the pile is built up layer by layer.
Stacking in a covered stockyard (A-frame building) can be done by a simple tripper car. The tripper car is installed in a structure suspended from the building roof. The travel of the tripper car is PLC controlled and programmed to build either a cone shell or a chevron stockpile.
The type of stacker is selected based on the stockpile layout, the material properties, and the needed stacking method. As an example, a stacker is to reach the whole cross section of the pile, if windrow stacking is needed. For chevron or cone shell stacking, it is sufficient to reach the centre of the pile only. The stacking process can be done by a simple tripper car (overhead) in a covered yard or it can be by a stacker equipped with a travelling and luffing or luffing and slewing mechanism in an open yard. Feeding of the stacker itself is carried out by a belt conveyor with a tripper car. The stacker has the following three basic movements.
- Travelling – It is the movement of the stacker along the length of the stockpile. Bogies on which the stacker is mounted move the stacker on the rail track along the storage yard.
- Luffing – It is the vertical up / down movement of the boom mostly powered by two hydraulic cylinders.
- Slewing – It is rotation of the boom around its central axis to discharge the material on the storage yard where required.
The construction of a typical stacker with luffing mechanism and different types of stackers are shown in Fig 2. A stacker includes a tripper and main body. The tripper and main body are mounted on bogies to move the stacker on the rail track along the storage yard. The main body includes stacking boom conveyor. The stationary yard conveyor passes through tripper and discharges material on to the boom conveyor. The boom conveyor discharges the material on the storage yard to form the stockpile. The luffing mechanism of a stacker is driven either by a hydraulic drive unit or by a winch.
Fig 2 Stacker construction and types of stackers
The three types of the stackers which are shown in Fig 2 are (i) luffing stacker, (ii) luffing and slewing stacker, and (iii) twin boom stacker. The luffing stacker shown in Fig 2 is without slewing mechanism. The luffing stacker can make stockpile in chevron and cone shell type of formation. The luffing and slewing stacker is used to make stockpiles on both sides of the machine. In addition to chevron and cone shell type of formations (due to luffing capability), it can make stockpile in windrow and strata type of formations also (due to the slewing capability). The twin boom stacker has two booms, one on each side of the stacker with a luffing arrangement. Two booms make it possible to form stockpiles on both sides of the stacker without the slewing mechanism.
The tripper car and yard conveyor is frequently equipped with a bypass arrangement, use of a bifurcating chute, and a diverter / flap gate which enables material to bypass the stacker boom and continue along the yard conveyor for meeting the plant requirement. The material conveyed from the tripper is discharged on the bifurcating chute. The chute has two legs, one for directing the material towards the boom conveyor during stacking and the other for discharging the material in to the central chute over the impact table for bypassing the boom conveyor for direct loading to the yard conveyor. A diverter gate, operated by an electric actuator or hydraulic cylinder closes any one of the legs and allows the material to pass through the other, as needed.
There are several types of reclaimers are available for suiting the specific needs such as buffer storage or storage for blending the material properties (material can be free flowing or sticky material), and reliability. The two types of reclaimers which are in common use are (i) scraper reclaimers, and (ii) bucket reclaimers. Each type has varied designs to suit an application.
In scraper reclaimers, scraper blades (toothed rectangular steel plates) are attached to two parallel strands of chain at regular interval. The travel of chains along with scraper blades scrapes / drags the material from the stockpile and discharges it on the yard conveyor. In bucket reclaimers, which can be bridge type bucket wheel reclaimer, boom type bucket wheel reclaimer, or drum type reclaimer, scooping buckets scoop the material from the stockpile. The scooped material falls on the receiving / intermediate conveyor which can be bridge conveyor, boom conveyor, or barrel conveyor. The receiving conveyor in turn discharges the material on the yard conveyor.
If there is a need for blending then the material is scraped / scooped from the face / front (whole cross section of the pile) of a pile. When blending is not needed, the material is scraped / scooped from the side of a pile. When material is scraped / scooped from the face of a pile, there is normally a harrow / rake attached to the reclaimer which brings the material to the bottom, from where it is taken up by scrapers or buckets. The scraper type reclaimers are lighter in weight and are cheaper as compared to boom type bucket wheel reclaimers. However, in scraper type reclaimer, more numbers of components (chain links, pins, guides and scraper blades) are subjected to material abrasion as compared to boom type bucket wheel reclaimer. In boom type bucket wheel reclaimer, only bucket edges are subjected to abrasion during reclaiming operation which can be made from wear resistant materials and can be easily maintained. Hence, boom type bucket wheel reclaimers are used for heavier duty and reliable operation.
Side scraper reclaimer – Side scraper reclaimer is suitable for reclaiming sticky materials. However, since the side scraper reclaimer reclaims material from side of the pile, it is suitable for buffer storage (non-blending) application only. The side scraper can reclaim a pile completely. As the machine rides on / over stockpile, it can move to any area of the pile and start reclaiming or can move to any of the stockpiles in series. Side scraper reclaimer is the most economical solution for small stockyards upto the pile width spans of 30 meters. It is used both for open as well as covered stock yards. It is used extensively for low capacity non-blending applications.
Fig 3 shows a typical side scraper reclaimer. It is also called a pylon or cantilever scraper. The side scraper reclaimer travels on a track arranged along one side of the stockpile. Two parallel strands of chain running on chain guides are installed on boom of the reclaimer. The scraper blades are mounted onto the two strands of the chain at regular interval. The boom is provided with a pivot which is located at the discharge end near the base of the stockpile. A winch using a wire rope and sheave system is used to lift the free end of the boom to place it on the side of the stockpile. At tail end, the chains pass around tail sprockets, and at the head / discharge end pass around driven sprockets. The scraper blades reclaim the material from side of the stockpile and convey it to the discharge point above the outgoing conveyor through an inclined drag trough.
Fig 3 Side scraper and portal scraper reclaimers
Portal scraper reclaimer – A portal scraper reclaimer (Fig 3) is named after the shape of the reclaimer body connecting the two end carriages. This structure is normally similar in shape to an inverted ‘V’ or a portal frame. Since the portal scraper reclaimer reclaims the material from side of a pile, it is principally similar in operation to the side scraper reclaimer. It overcomes the restriction of limited boom length (upto 30 m pile widths) of a side scraper reclaimer which is due to its cantilever type arrangement, because of having rail bogie on both sides of the stockpile. Hence, wider pile widths can be handled by a portal scraper reclaimer.
The portal scraper consists of a portal frame with a scraper chain system consisting of a boom with chain and scraper blades. The portal scraper with a two part articulated boom (two arms) has a primary and secondary scraper chain working on either side of the pile while the portal moves to and fro. The primary and secondary scraper chain systems are linked together at a knee joint. The secondary scraper chain lifts the material to the crest of the pile, feeding the primary scraper chain system. The primary scraper chain system conveys the material to the outgoing belt conveyor. The larger capacity machines can have two scraper booms working in parallel.
In case of indoor operation, the best ratio between the building section and the stockpile section is, in most cases, achieved with a semi-portal scraper reclaimer. The height of the retaining wall inside the building is defined to provide full use of the available storage area. Semi-portal scraper reclaimers are the ideal solution for stockyards with limited space or stockyards divided into compartments.
Bridge type scraper reclaimer
A bridge type scraper reclaimer (Fig 4) is necessary, where reclaiming is to be carried out from the face of a stockpile for blending. The bridge reclaimer is named after the reclaimer body connecting the two end bogies (carriages). In this reclaimer, a scraper chain system is mounted on the bridge type structure. The structure of the reclaimer spans from one end bogie to the other and is a bridge like beam which is parallel to the ground.
In the Fig 4, a travelling luffing stacker unit (no. 5) is to explain stacking and reclaiming operation. The material entering the stock yard on a belt conveyor is discharged from the boom of the stacker traveling on rails alongside the stock yard at a preset speed to make a stockpile (by chevron method). The height above the crest of the pile is kept at a minimum to reduce dust emission. Reclaiming takes place from the face of the stockpile (full cross section of the stockpile) at the natural angle of material slide. The bridge of scraper reclaimer runs on rails on either side of the stockpile. On the bridge is mounted a raking harrow system whose sweeping movements cause the material to slide to the pile base. A scraper chain system mounted on the bridge conveys the material to the outgoing belt conveyor.
The raking harrow (no. 11) is a grid frame mounted onto the bridge and is positioned onto the stockpile face. It is moved across the face of the stockpile with an oscillating / reciprocating action of a rake system. The oscillating rake system disturbs the material in the stockpile across its entire width causing it to slide down the slope of the stockpile to stockpile base, into the path of the rotating scrapers. If the reclaimer is needed to reclaim in both directions of the stockpile, then one harrow is needed for each direction. To loosen sticky and non-free flowing materials, it is necessary to use active live-harrows.
It is essential to use a full facing harrow for reducing harrow wear. The full facing harrow covers the total cross-section of the stockpile and hence needs a short stroke i.e. a short travel distance while the small sized harrows (harrows which only cover a small cross-section of the stockpile) need a long stroke to cover the total cross section. A full facing harrow also has the distinctive advantage of ensuring perfect homogenization and a constant material flow onto the outgoing / reclaiming belt conveyor.
The bridge type scraper reclaimer has the disadvantage of being trapped between stockpiles and hence is only able to reclaim from adjacent stockpiles. To overcome this disadvantage, a portal bridge type reclaimer having a liftable / luffable horizontal scraper boom (bridge) can be used which combines the benefits of both the bridge type and the portal type reclaimers. The horizontal scraper boom carries a roller-mounted sledge moving back and forth driven by a rope winch to move its harrow (instead of a rake system in a bridge type reclaimer). The back and forth movement of the sledge drags the harrow like a wiper across the pile slope.
Alternatively, if needed, in case of parallel stockpile arrangements, a transfer car is provided, running in a pit across the open end of the stockpiles, to enable a bridge type reclaimer to be positioned on any pile. A bridge type reclaimer can also be equipped with slewing bogies to permit transverse travel on rails laid at ground level. Integral hydraulic jacks are provided to raise the reclaiming machine for slewing the bogies. With this design, the need for the transfer car and its pit can be eliminated.
Fig 4 Bridge type scraper and barrel type reclaimers
Barrel / Drum reclaimer – Barrel / drum reclaimer (Fig 4) and bridge type bucket wheel reclaimer are used when robust systems are needed for the high performance blending and reclaiming of bulk materials, especially when the materials are semi-hard to hard. As in the case of the bridge type scraper reclaimer, a barrel type reclaimer is used where reclaiming is to be carried out from the face of a stockpile for blending. The barrel reclaimer comprises of a bridge spanning the stockpile which is supported on ‘A’ shaped frames at each side. These frames are carried on motor-driven bogies which run on rails laid on each side of the pile.
The rotating barrel is supported from the bridge structure. The barrel is fitted with a large number of reclaiming buckets arranged along its length and around it circumference. These buckets pick up the material which has been fed to the base of the pile by means of a harrow which traverses across the pile face. The oscillating rake system disturbs the material in the stockpile across its entire width causing it to slide down the slope of the stockpile into the path of the rotating buckets. Material collected in the buckets is discharged onto a belt conveyor contained within the barrel, and is then delivered to a downstream yard conveyor running alongside the stockpile.
For bi-directional operation, the barrel drive is made reversible and buckets are oriented to suit either direction of rotation. In addition, a harrow is provided on each side of the reclaimer. Like the bridge type scraper reclaimer, barrel reclaimer also has the disadvantage of being trapped between stockpiles and is hence only able to reclaim from adjacent stockpiles. If needed, in the case of parallel stockpile arrangements, like in the bridge type scraper reclaimer, a transfer car or slewing bogies can be provided to barrel reclaimers for positioning them on any stockpile.
Bridge type bucket wheel reclaimer – Bridge type bucket wheel reclaimer has similar function like other barrel reclaimers. It uses rotating bucket wheel/s to extract and feed the material in place of a rotating drum. The digging action is more positive with this reclaimer. However, the homogenizing effect is much lesser. The reclaimer is used for heavy-duty application of blending-cum-reclaiming. The reclaimer has a horizontal bridge supported on rail track spanning the stockpile width. The rotating bucket wheel/s makes to-and-fro linear travel along the bridge. The reclaimer moves with slow speed (by steps) into stockpile face. The to-and-fro motion of the rotating bucket wheel and steady advancement of the machine into the stockpile face results in to blending-cum-reclaiming operation.
Boom type bucket wheel reclaimer – Boom type bucket wheel reclaimer (Fig 5) is the best means of handling and moving large amounts of bulk materials in the shortest possible time and can be designed as reclaimers or as combined stacker / reclaimers. As shown in Fig 5, the reclaimer has a slewing boom. The boom is supported at machine centre by a slew bearing. The bucket wheel is at far end of the boom. Reclaiming is done by combining rotary motion of the bucket wheel with slewing of super-structure. The travel motion of the reclaimer is in steps, to create depth of cut into the stockpile face. The reclaiming operation comprises of travel step, followed by slewing pass, followed by travel step in a repetitive manner, for continuous reclaiming of the selected area. As the blending effect is marginal, the reclaimer is mainly used for reclaiming operation. The reclaiming of material primarily occurs during slewing motion of the boom. The bucket wheel rotates at steady speed, whereas the slewing speed varies automatically during slewing pass. The slewing speed is least when boom is nearly parallel to yard conveyor. The slew speed is highest when boom angle with yard conveyor is nearly perpendicular.
Fig 5 Boom type bucket wheel reclaimer and stacker reclaimer
Combined stacker and reclaimer
Combined stacker and reclaimer also known as stacker cum reclaimer can stack the material to form the stockpile or reclaim the stockpiled material and feed onto the main line conveyor. The stacker is mounted on the centre column, which allows rotation in both directions simultaneously with the vertical movement of the boom. Its height above the crest of the pile is kept at a minimum to reduce dust emission. Reclaiming takes place at the natural angle of slide. A raking harrow is mounted on the bridge reclaimer. The sweeping movements of the harrow system cause the material to slide to the base, where the chain system then conveys it to the centrally placed outlet hopper. The homogenized material leaves the stockyard by an underground belt conveyor. To loosen sticky and non-free flowing materials active live-harrows are available.
A boom type bucket wheel stacker cum reclaimer also called stacker cum reclaimer (Fig 5) is used for stacking material (building stockpiles) on either side of the track rails and subsequently reclaiming the material from the stockpiles and feeding them for onward usage. The boom conveyor is reversible, enabling stacking in the forward direction and reclaiming in the reverse direction (Fig 6). A bucket wheel, like that of a bucket wheel reclaimer is used for positive digging and feeding of material onto the boom conveyor. Stacker cum reclaimer with bucket wheel boom is primarily used where medium, large or very large material flows are to be stacked and then later reclaimed by the same machine. For stacking material, tripper of the yard (main line) conveyor (installed on the tripper car) transports the material to the boom conveyor and then to the stockpile. For reclaiming the material, a bucket wheel is used for feeding of the material onto the boom conveyor and then to the yard conveyor.
Stacker cum reclaimer normally consist of seven main assembly groups namely (i) set of travel gear bogies, (ii) sub-structure, super-structure with slew assembly, (iv) bucket wheel boom and bucket wheel body, (v) pylon or mast and counter weight boom , (vi) tripper and intermediate conveyor structure, and (vii) conveyor systems for boom, tripper and intermediate conveyor.
The stacker cum reclaimer is mounted on rails. The weight of the stacker cum reclaimer is shared by travel bogies, in accordance with permissible wheel load. Travel bogies are of robust construction and are provided with forged and case hardened steel travel wheels. The travelling gear (set of travel gear bogies) consists of driving wheel bogies (each comprises of two wheels), follower wheel bogies (each comprises of two wheels) and single wheels. One of the wheels in each driving wheel bogie is driven by a shaft-mounted drive unit. The drive unit is supported by a torque arm from the bogie. The bogies are provided with wheel fracture props. Wheel fracture props provided in each bogie restricts its fall (normally of 25 mm), in an unlikely event of failure of the wheel axle.
The tripper car moves along with the stacker cum reclaimer. It is connected with the stacker cum reclaimer by a tie member with spherical seat hinge or pins. The tripper lifts the yard conveyor belt and, after passing over the tripper pulley and bend pulleys, returns it to the yard level. The power cable reeling drum and the electrical room are mounted on the bottom frame of the tripper. Since three points in space determines the specific plane, the body of the stacker cum reclaimer is supported on wheel assemblies as ‘three points support’ to avoid twisting of structure. The three points support can have four corner wheel assemblies. The design ensures stability of the stacker cum reclaimer in all applicable conditions.
The sub-structure, a ring girder (also called gantry portal), consists of outer ring and inner ring with top and bottom plates welded and is a circular frame of box construction. The sub-structure ring girder with its three legs transfer forces from the slewing super structure of the stacker cum reclaimer over the supporting points of bogie system. A chute is provided in the centre through which material to be handled is discharged to the yard conveyor. An annular surface is machined on the upper inner edge of the top plate (ring girder deck) for accommodating the slew bearing. Impact table is bolted underneath of the sub-structure and is a structural framework of channels and plates. It is to accommodate yard conveyor passing over impact idlers mounted on fixed channel frame. The extreme two wheel balancers (also called equalizers) of the stacker cum reclaimer, away from the tripper car, and the extreme two bogies (balancers) of the tripper car, away from the stacker cum reclaimer are fitted with buffers and track / rail sweeps.
The slewing super-structure includes the slew platform with vertical projections for the pylon bearings, pylon, bucket wheel boom, counter weight boom and the bucket wheel boom conveyor. The slew platform consists of welded rectangular frame box construction with inner drum shell. The rotating connection between the super-structure and the sub-structure circular frame is formed by a slew bearing designed to safeguarded the super-structure from lifting and also absorb alternating axial and radial loading with high tilting moments. The gear rim (outer ring) of the slew bearing is bolted to the sub-structure ring girder by high tensile bolts and the inner ring of slew bearing is bolted on the slew platform by high tensile bolts. Slew movements of the super-structure are introduced by two (or three) slew drives located on the slew platform (or gantry portal), whose drive pinions mesh with the gear rim of the slew bearing. The gear rim and pinion of the slew assembly have a cover / casing which are bolted under the slew platform to protect the gear and pinion teeth from accumulation of the dirt.
The bucket wheel, normally of cell-less design with buckets fixed around its periphery is installed at far end of the bucket wheel boom. The buckets are fabricated from high tensile steel plates having minimum yield strength of 350 MPa and a minimum ultimate tensile strength of 490 MPa. The digging lips are made of wear resisting steel (cast manganese steel). Many times, buckets are provided with replaceable digging teeth of manganese steel with holders welded onto the lips. Normally the bucket wheel is installed at around 10 degrees inclinations to ensure continuous material discharges on to the side chute. All chutes (bucket wheel chute, central chute etc.) are fabricated of steel plates and provided with wear / abrasion resistant liners. Mostly the liners are bolted to the chutes for easy replacement.
While stacking, the bucket wheel chute is lifted up by a hydraulic cylinder to clear the material on the conveyor. The bucket wheel drive is reversible type to facilitate withdrawal of the bucket wheel in the event of excessive under cutting. The bucket wheel is driven either by electric motor with fluid coupling (for starting and safety), brake and gearbox or by hydraulic motor and power pack. The boom for the bucket wheel is of slewing and luffing type. The boom is slewing since it is held by the super-structure, which is supported at stacker cum reclaimer centre by the slew bearing. Normally, the slewing range is around 100 degrees to 105 degrees. The boom operation angle to yard conveyor is normally limited upto 75 degrees.
Luffing (raising / hoisting or lowering) of the bucket wheel boom is carried out by two double acting hydraulic cylinders (and power pack) connected between the pylon and the slew platform. The cylinders are sized such that, in the event of one cylinder failing, the other is able to sustain the total load of the system (out of operation) alone as an emergency feature. Like slewing range, the stacker cum reclaimer is also designed for certain luffing range (for example, maximum down is minus 6 degrees, maximum up is plus 9 degrees and parking is minus 4 degrees). The upper and lower positions of the bucket wheel boom (extreme luffing positions) are limited by limit switches.
Protection of piston rods and seals (wiper ring and rod seal) is very important for long life of the hydraulic cylinders. The piston rods and seals are protected by leather bellows. If piston rod gets exposed to environment (due to damaged bellow, or the bellow coming out from bellow holding down screws), piston rods and seals get damaged resulting in oil leakage.
The pylon, comprising two plate girders connected by K-bracings, is pivoted / hinged to the slew / revolving platform by means of plain spherical bearings. The bucket wheel boom and its ties are pinned to the front of the pylon structure whereas the counter weight boom and its ties are pinned to the rear of the pylon. The bucket wheel boom tie and counter weight boom tie, connects the pylon top with the bucket wheel boom and counter weight boom respectively.
Bucket wheel boom is of fabricated double plate girder construction adequately braced by rolled sections in a K-lattice construction. The bucket wheel with drive and the mechanical components of the boom conveyor are mounted on this boom. Necessary service walkways are provided along both sides of the bucket wheel boom. The operator’s cabin is positioned on the opposite side of the bucket wheel to give good visibility to operator and is kept levelled by means of hydraulic cylinder regardless of the changing inclination of the bucket wheel boom. The counter-weight boom is of two sturdy joists adequately braced together. It supports the counter weight. The counter-weights serve to balance the boom deadweight as well as around 50 % of the live loads and incrustation. This ensures minimum shift of the centre of gravity and similarly minimum possible loading of the boom hoist.
Several times the pylon, bucket wheel boom, counter weight boom with counter weight and ties is called a bascule (or rocker assembly) which in turn is pivoted to slew platform. Electronic belt weigh scale is provided optionally on the boom conveyor to measure the flow and also to regulate the reclaiming capacity of the stacker cum reclaimer.
The stacker cum reclaimer is provided with an anemometer (wind warning device). The anemometer is normally installed on top of the pylon. When the velocity of wind exceeds the set value, the control system of the stacker cum reclaimer gives audio / visual signal to the operator. The operator thereby applies the rail clamps and stops the travel movement of the stacker cum reclaimer. In case of high wind, a heavy lug below the boom head structure is used for the holding down arrangement (guy ropes) of the boom. Alternatively, adequate locking arrangement is provided between the super-structure and lower structure to make the stacker cum reclaimer withstand storms and cyclonic winds.
The holding force of the clamp is produced by means of the friction (coefficient of friction is around 0.25 which corresponds to normal grease free conditions) occurring between clamp shoes and railhead. There are two types of rail clamps namely manual and automatic (hydraulically operated). Automatic rail clamps are released by hydraulic cylinders which actuate limit switches to permit travel motion. As the hydraulic rail clamp is closed through spring operation and opened electro-hydraulically, opening needs electric current. In case of power failure, the clamp closes automatically. Guy ropes (steel wire ropes) connected to the foundations are used for anchoring the boom for parking the stacker cum reclaimer.
The yard conveyor is frequently equipped with a bypass, which is a bifurcating chute (chute with two legs) with diverter gate after the tripper. One leg directs the material towards the boom conveyor during stacking whereas the other enables material to be discharged in to the central chute (bypassing the boom conveyor) for bypassing the boom conveyor and continue along the yard conveyor. Several times the stacker cum reclaimer is provided with a bifurcating chute and intermediate conveyor for bypassing the boom conveyor, a more efficient method (material is not required to be lifted upto the boom conveyor) of bypassing.
Stacker cum reclaimer can be designed for unidirectional flow or reversible flow depending upon the application. In case of reversible flow stacker cum reclaimer, the machine is designed for working with a reversible yard conveyor with the provision for feeding at one end for stacking and discharging at the same end during reclaiming (Fig 6). For stacking the material, the material on the forward moving yard conveyor is raised by tripper and discharged through a vibrating / belt feeder onto an intermediate conveyor which lifts the material and ultimately discharges on to boom conveyor. For reclaiming the material, boom conveyor discharges the material on the yard conveyor after its direction is reversed. In case the chute is not having adequate slope, a vibrating feeder can be used to overcome chute choking problem.
Fig 6 Material flow and stockpile geometry for stacker cum reclaimer
The stockpile geometry of a boom type bucket wheel stacker cum reclaimer is shown in Fig 6. Travelling the stacker cum reclaimer along the length of stockpile does the stacking while slewing the boom does reclaiming. Since the boom cannot reach the farthest point of the stockpile base, a dead stock is left behind after reclamation of the pile. If needed, the dead stock is required to be bulldozed into the reclaimable area so as to be able to reclaim the same by the bucket wheel.