When making a decision regarding a cover for large bulk material stockpiles, it is important to take a long, hard and honest look at several factors. Environmental considerations, operations, erection, project schedule, maintenance, suitability of the cover for the specific application and lifecycle cost analysis are all part of the process.
If we assume that it is not a question of whether to cover our bulk materials, but with what to cover, then we must consider several issues. In industry it is generally accepted that equipment designed or engineered specifically for an application and manufactured in a quality controlled environment will function better and more likely perform as designed for many years without problems. The same approach should be taken with stockpile covers, even though they may be considered a structure. What are the important issues to ponder before selecting a cover for a particular application? A series of questions should be posed which impact not only life-cycle cost, but operational efficiency of the overall storage system. Although the cover should be evaluated like a piece of equipment, the cover has the additional aspects of geographical location and local building codes to evaluate.
There are basic practical considerations to be made when choosing the right cover solution. The location of the project is of primary importance. This has related parameters such as wind, snow, seismic considerations to be accounted for in the design. The material that is to be stored affects the requirements of the structure, for example, coal should be stored in a well-ventilated structure, which then has an impact on the design. The properties of the material must be considered as the bulk density and angle of repose are important in determining the overall storage size when used in conjunction with storage capacity and a particular handling process.
The storage capacity will be determined by the maximum that the owner needs to store at a given facility and this should be relayed to the cover supplier. Based on their experience, the cover supplier should be able to recommend a material handling system for the given application and provide recommendation of size, even before other equipment suppliers are involved.
Storage system configuration
Once the storage capacity is determined, decisions regarding how the material should be stored must be considered. One of the tremendous benefits of circular storage is the ability to store large amounts in very small areas by utilising perimeter walls as an integral component of the storage system; the higher the wall, the greater the storage capacity for a given area. Of course, there are practical limits, but it is not unusual to store coal or iron in 120 metre diameter stockpiles, with material stacked up to 18-19 metres against the wall. This would then limit the choices of suitable stacker reclaimer systems. For example, nearly 200,000 metric tons of coal storage can be achieved with a 120 metre diameter footprint, depending on the design bulk density and angle of repose.
Material handling system
Once the storage system configuration is selected, the material handling system may be a foregone conclusion and the general cover design parameters defined. The material handling system manages the feed and discharge of the material in storage. The selection of the material handling system may be a preference, or it may be driven by the process, or by the required storage capacity. It is also important to recognise the difference between blending and storage, as the reclaim system will typically be different. For the purpose of this discussion we will only consider storage applications. It is highly recommended to seek the opinion of a material handling system supplier or the original equipment manufacturer (OEM), specialising in the material being stored, as to which type of system is suitable and practical for a given application.
How the material will be brought into storage and how it is reclaimed are important questions. The answers will have an impact on shape and size of the cover, not only because of size due to storage volume, but due to the necessary internal clearance envelope for operations within the covered area.
Material delivery is important and we will consider that in all cases the material is brought to the covered storage via conveyor of some type. The material distribution can be as basic as a simple drop from the conveyor onto a simple conical pile, or into a stacking tube to reduce dusting, or via stacking arm of a circular stacker reclaimer. The material reclaim will be manual with front end loader, drawdown hoppers or a circular reclaimer as part of the circular stacker reclaimer.
‘Manual’ system implies front end loader reclaim. ‘Automatic’ implies some type of mechanical reclaim system, whether drawdown hoppers or scraper reclaimers (linear or circular). For automatic reclaim, both linear and circular have their own strengths in particular applications. For large quantity storage applications, the circular systems are by far the most efficient. We will only consider circular systems, but the same line of thinking applies to linear stockpiles and their covers.
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