Sand stockpiling and handing, Sand and gravel conveyor, Sand and ...

Sand Stockpiling and Handing Sand Making Plant Segregation is probably the greatest problem that occurs because of stockpiling and handling, but certainly other problems such as degradation and contamination may adversely affect product quality. Every possible precaution is required to be taken to protect the product quality from the point of manufacture to the point where the aggregate leaves the Producer's control. Sand Cone Stockpiles Although the cone stockpile is very common in the aggregate industry, two stockpile procedures may easily reduce product integrity. Roll-down segregation obviously occurs in full circle around the pile, and very high piles are difficult to adequately remix before shipping. These piles usually are being replenished with fresh material as old and new material is being removed, which keeps the product size in a state of continual change . In some cases the "front-to-back" segregation adds extra coarse material thrown forward and extra fines carried back for even greater variability. In addition, some piles are not fully retrieved for several years and the new product that is added to the old pile may even have different production targets. Situations like these add up to serious problems for predicting gradation uniformity in the retrieved product. The final element of a cone pile that adds to the effects of both the roll-down and front-to-back phenomena is an excessively high drop from the end of a fixed conveyor to the top of the pile . This procedure should be avoided. Use of cone stockpiles should be kept to a minimum and used with extreme caution. Radial Stockpiles A radial stacker is a compromise solution for conveyor-built stockpiles, especially if kept less than 20 ft. The proper technique is to keep the end of the movable conveyor less than a meter from the top of the pile and raise the conveyor with the pile to the full height. Then the conveyor is moved horizontally with the pile in small increments. In this manner the pile is constructed at one end while the products are retrieved at the other end. Although roll-down segregation does occur from the sides of the pile, a continual remixing of coarse and fine material occurs longitudinally as the pile advances. Proper retrieval may take care of the edges. Truck Built Stockpiles If piles from the end of the product belts are thoroughly remixed then truck- built stockpiles are capable of greatly minimizing segregation, if the trucks are loaded properly. The best truck-built stockpiles are those that are constructed one dump high with each dump placed against previously dumped material. This procedure, because of the low profile, reduces roll- down segregation and allows remixing during load out. However, these stockpiles require more space than the others mentioned. A technique that may help reduce the required area is to restock some dumps on top of other dumps with a large end loader operating from ground level. In this case, care is required to be taken to place the upper lift back from the edge far enough that a long sloped face is not created that would cause the same kind of roll- down problem that this type of pile is meant to eliminate. Layered Stockpiles A layered stockpile, if built correctly, may also greatly minimize segregation. Unfortunately these types of stockpiles are very difficult to build properly. Each layer is placed uniformly across the top of the pile in thin horizontal lifts. Care is required to be taken to keep the edge of each new lift set back from the edge of each previous lift so as not to create long sloped edges. This is best done with a large clam shell crane, which is slow and tedious, or with specially made equipment that may place the layers without being on the pile. A compromise is to allow hauling equipment on top of the pile; however, this procedure causes degradation of the product, and the pushing equipment may move the material over the edges causing severe segregation . Generally, these activities are poorly managed, and the stockpile takes on the shape of a ramp and spills over. These situations are very detrimental to product quality. Sand Stockpiling Sand Cone Stockpiles The Producer is required to write standard operating procedures on building stockpiles for each product and to educate all those involved in their responsibilities in the procedure. Most stockpiling problems are created because of inconsistent management. The procedures are required to become part of the Quality Control Plan. Illustrations at the end of this chapter indicate the different techniques that may be used for stockpiling and retrieving. The Segregation Index (S.I.) indicated with each example is a numerical index where the numbers are associated only with the other techniques and indicate greater segregation severity as they become higher. Degradation Degradation or breakdown of the product is often caused by equipment running on top of the aggregate when the aggregate is being stockpiled . When this occurs, the degraded portion of the pile is required to be discarded before shipping. The difficulty lies in knowing where the "bad" material begins and ends. Extensive sampling and testing in these cases may be needed prior to shipping to determine what product is not good enough to ship. Degradation may also occur during retrieval where some of the lower portion of the pile is carelessly run over with equipment while loading out. A Producer is required to know which products tend to degrade with handling and make appropriate allowances. For example, many stone sands increase in minus No. 200 content each time they are loaded and moved. In some cases old stockpiles may degrade through weathering. Piles two years and older are required to be rechecked for gradation before shipping and possibly even for mineral quality. Sand Contamination Contamination  is usually the result of carelessness and poor housekeeping. In order to save space, stockpiles of different products are placed close together and as they grow in size they grow together. Equipment also may track dirt or other foreign matter into the product pile area. Old piles are subject to wind-blown fines over time and are required to be checked for this before shipping. 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