11/15/2012 | Autor / Editor: Holger Lieberwirth * / Marcel Dröttboom
But with small batch sizes of about 20 tonnes even with a small stacker as shown in Fig. 8, having a capacity of 200 tonnes per hour only, would not even complete one single layer. Or the piles would have to be so short that the end cones with their adverse effects on the homogenising efficiency would make up for a major part of the total pile volume. Not to talk about change over times from one pile to the other in case of trucks always coming in from various resources.
A conveyor system, layering various material qualities onto the feeding conveyor to the homogenising stockpile would allow the blending of the various material qualities on the conveyor as per a predefined scheme (Fig. 9). The stockpile would serve merely for homogenising the material flow. Linking all the small individual stockpiles with a conveyor system, each feeder and conveyor having only a rather small throughput, would however generate prohibitive high investments costs for equipment and civil works.
The better alternative is, to dump all incoming material from the local mines onto one of two large “incoming coal” piles (Fig. 10). While one of the two incoming coal piles is built up the coal from the second pile is reclaimed and fed to the homogenising piles. With all incoming batches being sampled, the average value of each characteristic data of the coal stored in the incoming coal stockpile is known. Yet, the fluctuation of the values in time is unknown, irrespective of the method of stacking or reclaiming. In the worst case, a complete cross section of the incoming coal pile may be composed of just one type of material, causing very high fluctuation of the values (Fig. 11).
This fluctuation is widely compensated by the homogenising coal pile. While the average values of the data characterizing the incoming coal stockpile are identical to those of the homogenising stockpile the fluctuation of those data can be reduced dramatically.
The exact determination of the stockyard and stacking parameters, such as:
is an optimisation process following mathematic rules described in the literature [2, 3, 4].
The best homogenising of material is reached by the operation of two longitudinal stockpiles each for the incoming coal and the homogenising task. A slightly cheaper option may be circular stockpiles (Fig. 12). With the virtually endless continuing stacking and reclaiming operation, one stockpile for incoming coal and one for the homogenising task are sufficient, saving investment in equipment, particular the stockpile conveyors, and saving space. The stockpile volume is limited, however, by the maximum possible diameter. A certain impact on the blending efficiency has to be considered as well. In particular, since the stacking process is limited to the Chevcon method with its adverse effects on the segregation of lumpy materials.
The selection of the optimum stacking method for longitudinal piles depends mainly on the material characteristics. Good blending and homogenising results require, however, that any cross section of the pile reclaimed contains at least 400 layers of material. Compared to a proper stacking the reclaiming with its various methods has less influence on the homogenising of a material flow. The best homogenising efficiency is achieved while reclaiming at once from the full face. Respective machines would be:
Both machines scrape the material from the pile using full face harrows. Drum reclaimers are particularly suited for high capacity blending/homogenising of very abrasive materials like iron ore or materials which require a gentle handling like iron ore pellets. Application areas of both machines are shown in Table 2.
Reclaimer, which remove only a fraction of the pile cross section at each point in time such as bucket wheel boom reclaimer, show a significantly lower homogenisation effect. Fig. 15 shows a comparison between bucket wheel bridge reclaimer with one or more bucket wheels and a drum reclaimer.
Economic, technological and environmental considerations lead to increasing requirements for quality raw materials. Blending of raw materials has to be more and more complemented by homogenising. In many cases blending and homogenising cannot be done in one process step.
Additional stockyard facilities are often required. This will at least in certain industries potentially lead to a centralisation of the processing plants, offering new opportunities to large stockyard operators such as ports. The example of a cement plant in South East Asia shows, that the proper design of a stockyard and the selection of appropriate equipment, however, very much depends on the local situation, on size and quality of the incoming batches as well as on the requirements of the downstream production processes. n
* Dr.-Ing. Holger Lieberwirth is CEO Asia at Takraf GmbH, Germany, Tel. +49 (0)341/2423-500, E-Mail: email@example.com
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