The idea that horizontal storage systems can only be regarded as a cheap but inefficient alternative to vertical silos has been dispelled by the last 30 years of developments in Argentina and Brazil, where it has emerged as a often-used storage solution.
Brazil pioneered the development of the "silos graneleiros," or horizontal silos, in the late 1960s. Argentina began constructing large facilities in the mid-1980s, inspired mainly by what was done in Brazil.
In a storage facility, the main functions are classification, conditioning, mixing and storage. Operational silos used for classification, conditioning and mixing are conveniently vertical, with bin capacities between 500 and 5,000 tons. Storage silos, on the other hand, are for already conditioned material. Horizontal storage silos range from 5,000 to more than 300,000 tons capacity.
The possibility of constructing up to four times the amount of storage for the same capital cost of vertical silos has made horizontal storage particularly attractive in South America. Currently, about 40% of Brazil’s total storage capacity of 90 million tonnes and about 30% of Argentina’s total capacity of 48 million tonnes is in horizontal storage.
Flat storage is widely used by grain dealers and cooperatives; processing industries, mainly oil-seed crushing plants; port terminals; and pre-shipping silos near the ports.
The average capacity of new horizontal silos is 40,000 to 50,000 tons. The largest facility in Argentina holds 170,000 tons of sunflower seeds, equivalent to 300,000 tons of soybeans, and measures 75 by 220 meters.
One of the limitations of the horizontal silo is its separation capacity. For large storage capacity with many separations (for example, less than 2,000 tons per bin), it is more convenient to construct a battery of vertical silos. A horizontal silo can be supplied with division walls, but they are very expensive. Normally, for grain, it is not worth providing for separation under 5,000 tons.
The type of bottom used in a horizontal silo (see illustration) depends on the soil characteristics, the type of product to be stored and the number of yearly rotations.
The width of a horizontal silo determines the height of the stored material pile, so the maximum height of the stored product has to be compatible to the admissible soil resistance and the height storage limitation for the stored products. The length of the silo depends on the storage capacity for a given width and wall height and the limits of the land.
The type of loading, unloading, aeration systems, and roof cover depends on both the design of the silo and the product being stored.
Brazil uses very large receiving hoppers of about 100 tonnes capacity, mainly without truck dumping platforms, with a reception capacity per pit of about 60 to 80 tonnes per hour. Argentina uses much smaller pits of about 40 tonnes, with hydraulic truck dumping platforms with an average reception capacity per pit of 80 to 120 tph, with current upgrades reaching 150 to 200 tph. Reception rates in ports are about 600 to 1,200 tph.
Horizontal storage systems in Argentina and Brazil have shown to be a practical and efficient alternative to vertical silos, challenging some old prejudices against horizontal units.