Loose bulk materials are most economically stored outdoors, but some conditions can necessitate storage under cover, writes Frank Warmoth, PE, Manager Business Development Bulk Handling and Terminals at Bruks Rockwood Inc. When the materials are sensitive to contamination or natural elements, or where they pose a threat to the surrounding environment, it becomes necessary to store them in an enclosure. The equipment to store and reclaim a given material and the structure in which it is stored are interdependent and technical solutions have evolved along economic lines, coupling together types of enclosures with the equipment inside.
The first delineation is between manual handling and automatic storage and retrieval. Since manual methods like mobile equipment are inherently lower capacity and dry bulk cargo terminals typically require rapid unloading and loading of materials, only automatic high capacity solutions will be discussed here with emphasis on enclosed systems.
Its properties determine the best equipment to convey a bulk product, place it in storage, and reclaim it for processing or shipment. These decisions and the required storage capacity narrow the choices of storage structure. Process and shipment requirements determine the storage capacity. Requirements for FIFO (first in/first out), separation or blending, and the physical space available further clarify the options.
Generally, FIFO enclosures must be discharged from the bottom of the pile (e.g. gravity flow, plow, vibrating pile discharger, screw reclaimer, etc).
Separation into piles requires a large foot print, often a rectangular enclosure with discrete stacking and reclaiming or separate enclosures like silos and domes. It requires stacking and reclaiming equipment that can access each pile like a tripper, shuttle conveyor, or travelling stacker, and a scraper type reclaimer that reclaim from the top of the pile. Blending is best accomplished by a rake- type bridge reclaimer that reclaims across the full pile cross section.
Some classes of materials have fairly well-established handling and storage methods. An example is the use of silos to store free-flowing materials like grain and cement that are also sensitive to contamination and moisture. Silos should be avoided for materials that are compressible, cohesive, or don’t flow freely. The exception to this is when aggressive reclaiming equipment usually acting on the top of the pile is employed. Mechanical or pneumatic conveyors are used to fill the silos and gravity, often aided by vibratory dischargers is most effectively used to discharge onto conveyors. Silos occupy the smallest footprint per ton of storage.
Where high storage capacity is required, the most economical means of storage is a linear system in a structure with a rectangular footprint. Enclosures of this style require a large footprint per tonne relative to others. This is the easiest system to expand by simply increasing length of rails, conveyors, and structure. Different types of rectangular structures are utilized with size and local capabilities often determining the most economical solution. Most storage halls are open between the outer walls to allow the equipment free access to the material.