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Logisitics Chain only as strong as its weakest link

By Barry Woodbine, AUMUND Group

 

The logistic chain is fast becoming an essential part of the raw materials and fuel management in any business. But any chain is only as strong as its weakest link and for the cement industry it is critical the raw material supply chain from quarry to kiln remains strong at every link to ensure continuous kiln performance and productivity. Dependable handling and storing solutions are critical for efficient materials management. Handling heavy and abrasive materials and working generally 24/7 dependable machinery supported by equally dependable spare parts and maintenance service is critical to maintain production and therefore profitability.

Rising fuel costs and expanding sustainability obligations drive cement manufacturers worldwide to consider not only alternative fuels but also substitute raw materials such as Blast Furnace Slag and Synthetic Gypsum. With a broader range of substitute raw materials, additives and fuels, processing, storage and transportation diversity is increasing putting ever more pressure on the associated handling, loading and storage facilities. The logistics trail may include significant volumes of imports by ship, barge or rail demanding additional handling solutions at the port and at the plant to discharge the vessels or rail wagons and provide adequate storage for economic shipping volume. Slag in particular is problematic in that it is highly abrasive, some five times more so than clinker for example, and due consideration must be made in the machinery design to resist wear, accumulation within chutes and casings and blockages in general, to maintain productivity.

At the modern quarry there are novel solutions now available for the handling of as-mined minerals all with the objective of reducing heavy truck haulage and therefore reducing dust and noise pollution and carbon footprint plus eliminating or at least mitigating the operating costs associated with these huge vehicles. The mobile face crusher concept for example places the primary crusher or sizer at the working face and therefore eliminates the need for haul trucks in the quarry since it is no longer necessary to haul the as blasted rock to a central fixed primary crusher, which, as the quarry expands may be several kilometres from the face, demanding a large fleet of trucks. The mobile primary crusher along with its feeder and discharge conveyor may be mounted to a single tracked undergear, generally with diesel drive motor, for rapid in-pit travel and easy manoeuvrability controlled either from a driver cabin or hand held remote control joystick system.

Generally a tracked excavator is used to feed the rock to the mobile primary crusher directly or via an Apron Feeder with lump sizes typically to 1 metre or more. After the initial sizing, typically down to a maximum lump size of minus 250 mm., the mineral may be easily conveyed on traditional troughed belts to a secondary crusher and screening station either within the quarry or close by or at the cement plant; which in some instances may be as much as 25 km away using a system of overland conveyor haulage between.

B&W Mechanical handling Ltd. (part of the AUMUND Group since 2002) along with ARC Quarries (Hanson Heidelberg) in the UK pioneered the development of the rubber tyred self-propelled mobile Link Conveyor as a means of connecting a mobile face crusher to a fixed in-pit overland conveying system. The major benefit of this solution is the ability to move the crusher and link conveyors quickly from the face to a safe parking position during blasting and thereafter to quickly re-position the equipment at the working face as the bench advances.

However, there are occasions where the flexibility offered by short haul truck operation is still very attractive. In this situation an integral twin shaft rotary sizer in combination with a mobile surface feeder unit, such as the SamsonTM concept, provides an effective and efficient solution easily transportable as the quarry expands. Typically the blasted rock is loaded to trucks and hauled a short distance to the combined feeder and sizer, the sized material is then discharged to a moveable overland conveyor connected to the secondary crusher and screening plant.

In mining operations closely linked to cement plant short haul transportation to a central crushing station is the most popular solution. Here a ground hopper with its associated apron feeder will handle as mined rock up to say 1.5 metres in size providing a controlled feed rate to the crusher to avoid choking or over feeding. Often the feeder drive will include an Inverter variable speed device (VFD) controlled by the crusher drive motor current demand and other downstream sensors. AUMUND, for example, have developed a range of heavy plate feeders for various applications around the cement plant but in this primary feed application the most robust design will be required, based on tracked vehicle chains and close supporting track rollers with heavy steel overlapping plates, up to 80 mm thick in total, and central impact bars to resist deflection of the apron plates under extreme impact loading. An apron plate type feeder provides both the impact resistance to accept the material from the haul truck, often falling 10 metres or more in height, and a controllable output to the primary crusher to avoid overfeeding and choking.

Whilst the receiving and feeding unit are important the choice of crusher is critical to the performance of the installation. For hard rock the gyratory or jaw type primary crusher is typical but for soft shale and clay a rotary sizer may be more suitable. By their very nature rotary sizers are susceptible to flushing when the input material contains a large proportion of fines that simply flow directly through the rolls. To regulate the flow to the on-going conveyors and provide fine control of the final feed rate a SamsonTM feeder may be employed between final sizer and conveyor providing both a controllable output rate and a buffer holding capacity to absorb the peaks delivered from the sizer. Using a variable frequency drive (FVD) linked to a downstream belt weigher the final output from the feeder may be closely controlled and similarly using a level detector to signal the material bed depth in the SamsonTM the primary sizer apron feeder may be controlled to match the intake and output rates of the installation and the associated process plant as a whole. A typical installation at Irish Cement where shale is delivered by dump truck to the primary feeder/size combination and the material is further reduced by a secondary rotary size before falling into the SamsonTM which provides the output regulation.

Aside from the materials coming from the quarry, most cement plants have to handle additives such as gypsum, clay, puzzolana, trass, iron ore, fly-ash or  granulated blast furnace slag plus solid fuels like coal, pet-coke or alternative fuels coming in by truck, rail or waterway. In many modern plants access to inland waterways such as the Mississippi (USA) or the Yangtze (China) are critical factors influencing the logistics solutions available to the operator. For barge transportation the logistics surrounding both loading and discharging the barge must be addressed to minimise capital and operating cost and, at the discharge point, fugitive dust emissions. In this scenario mobile equipment for stockpiling incoming materials and loading of river barges or coasters with a minimum of fixed infrastructure become very attractive. Using mobile plant, such as the StormajorTM by B&W, mining operators may create a load out station from a simple river berth requiring often no fixed civil works; simply a hard standing area for local stockpiling plus a truck access roadway is all that is required.

At the cement plant the materials may be imported either through a dedicated jetty with Eco-Hopper(s) or through a single mobile Eco-Hopper mounted to rubber tyres and loading direct to trucks. In either solution the bulk cargo may be received with minimum dust pollution and transported to local storage either by belt conveyor or by road trucks working on a short haul merry-go-round system.

However, for intake by rail wagon at the cement plant dedicated under rail track hoppers and feeders are the only solution either for use directly with hopper bottom wagons or for flat bottom, open top, wagons via a rotary wagon tippler such as offered by SCHADE (AUMUND Group) in various design configurations to suit the rail wagons, geography, plant configuration and throughput demands. Beneath the truck hopper there are various feeding solutions, apron feeders are the most robust for high loading and impact. For cohesive materials such as clays the Panzer type armoured chain feeder is ideal or, for shallow pit configurations, handling difficult materials the SamsonTM Under-Rail or SUR has significant advantages with a much reduced excavation depth.

Regardless of the material routing by the time it reaches the cement plant it will require storage and probably, certainly if there are multiple sources and/or varying qualities from the same source, blending to ensure a homogeneous mix before addition of other elements and finally blending and elevating to the preheater tower. The homogenisation of limestone is best carried out using a Circular Storage with integral radial stacker and bridge type reclaimer. Using the radial and luffing boom stacker a stockpile may be generated in small height increments working between two set points such that at any point along the stockpile a section would reveal samples of every quality of material delivered to the storage. Using the bridge type reclaimer with full face reciprocating harrow material is drawn down from the full inclined material face and recovered by the lateral chain scraper to a central outlet such that the final output includes elements of every layer from the stockpile. The circular format allows the bridge reclaimer to follow the stockpile around the central supporting column eliminating any interruption to the output flow.

Closely controlled material flow is crucial at the raw meal mill which can be achieved using an apron weigh feeder This equipment combines the reliability and strength of the steel apron construction with a weigh rail to monitor the load level and translate the applied load along with the conveying speed to compute an accurate mass flow rate. For feeding free flowing materials such as clinker the weigh feeder may be based on a pan conveyor design with supporting rollers to each pan.

However, for metering cohesive materials such as clays in particular the overlapping pan principle is impossible to clean and for such materials the “Arched Plate” design presents a smooth conveying surface allowing any material adhering to the plate carrying side to be easily cleaned off at the discharge to minimise spillage and carry-back.

At the raw material section, before grinding, chain bucket elevators lift coarse materials such as limestone and additives including blast furnace slag. The abrasive properties of slag represent a considerable challenge and for such applications AUMUND have developed special solutions with steel faced digging buckets, non-metallic intermediate buckets and rubber lined casings to resist wear and surface accumulation.

After grinding the prepared raw meal must be raised first to the blending silos where complete homogenisation is achieved and after the blending silos raised up to the pre-heater. For both these operations the Belt-Bucket elevator is the accepted industry solution and now with the latest 5 stage cyclone pre-heater towers the operating envelope is now extended to raise up to 175 metres in a single lift. AUMUND Belt-Bucket and Central-Chain elevators may be found in almost any cement plant worldwide handling usually the raw material, mill recirculation, raw meal, hot clinker and finished cement, in fact in all mission critical locations where dependability is absolutely vital.

Culminating at the pre-heater tower we have illustrated various materials handling solutions from the Quarry to Kiln including both novel concepts and conventional solutions achieving above all else dependable performance at every stage of the logistics chain and process operations. Of course, for AUMUND, the conveying train does not stop at the kiln and it is from the kiln cooler to the cement mill that AUMUND are probably best known for their extensive range of steel pan conveyors, designed specifically for hot materials; but that is another story.