Limestone Storage Domes

keywords: blending bed covers, limestone storage, homogenizing, dust control

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Geometrica is a Texas based company dedicated to building large domes for bulk material storage. This paper presents one of the most common applications for these domes: covering limestone blending beds.

The Need

As is generally known, cement manufacturers have to ensure their product maintains consistent properties, but most limestone quarries produce materials of varying chemical composition. To obtain consistent quality in the final product, quarried raw materials must be blended before being fed into the kiln. Blending can be carried out by means of circular action or longitudinally. Figure 1 indicates a circular blending limestone stockpile. The capacity of stockpiles for limestone range from 10,000 t to more than 100,000 t.

Although stockpiles are created with a slewing stacker to minimize fugitive dust, most cement producers enclose their stockpiles to protect the surrounding environment, and to prevent loss of material due to water runoff and wind.

Geometry

Geodesic domes provide the most efficient shape for a stockpile enclosure because they are lightweight and can span large areas without intermediate supports. The most common dome shape is a segment of a sphere, but it is more economical to use varying radii of curvature for the meridian of the dome -- the goal is to have the dome "hug" the clearance line of the stacker-reclaimer. This results in a smaller dome surface area and better clearance for vehicles around the perimeter. Typical diameters and heights for circular-plan domes of varying capacities are as follows:

Ring stockpile dimensions (Bulk density for limestone = 1400 kg/m3)

Capacity	Pile diameter	Dome diameter	Dome height
Tons	m	m	m
13,000	64	72	22
21,000	75	83	25
34,000	89	97	29
55,000	104	113	34
89,000	122	130	39

Minimum clearance lines for the stacker-reclaimer are given by the manufacturer of the equipment. Additional space is usually provided in addition to the manufacturer's clearance line for a traffic path around the stockpile -- this additional space is specified by the owner.

While geodesic domes are an obvious choice to cover stock piles, traditional construction systems may not translate to a low-cost solution because manufacturing and construction complexity offsets material savings from the more efficient form. Geometrica has attacked this problem by creating domes that can be:

• Designed and pre-fabricated by computers;
• Packaged and shipped in small units that can be manually unloaded in remote locations;
• Assembled by local labor without special equipment; and
• Assembled over a stock pile while the pile remains in operation.

Material

Domes may be constructed of concrete sprayed over air-supported fabric, aluminum or steel. Because of span and access requirements, limestone storage domes are generally built of aluminum, painted steel or galvanized steel. The advantage of painted steel is its low initial cost, but it requires expensive periodic maintenance; whereas aluminum and galvanized steel resist corrosion and do not require maintenance. The location of the building and the type of material stored often determines whether aluminum or steel is most economical over the service life of the structure. For limestone storage, galvanized steel has proven to be the preferred material in dozens of installations because it combines the initial cost advantage of painted steel with superior corrosion resistance.

The Geometrica System

This construction system uses either galvanized steel or aluminum tube components. No welding is required because the connection of the tubes consists of a very efficient, moment-resistant mechanical joint. Geometrica domes may be built in single- or double-layer configurations, depending on the load and span for the particular application. Experience suggests that for the spans required in limestone covers, a double-layer vierendeel geometry is the most cost-effective.

Assembly of the structure is simple and does not require specialized labor. Construction starts at the ring support and moves inward and upward, with the structure acting as its own support. Domes are often built over operating stockpiles since construction can proceed without interrupting operations.

Cladding consists of standard corrugated metal R-type sheets, usually galvanized and painted in the case of steel, or millfinished in the case of aluminum. For more information, see the Geometrica System of Construction.

Example

For a typical dome with a capacity of about 25,000 t, the Geometrica solution is a double-layer geodesic dome of 84 m clear span and 25 m height. The two layers of chord members are approximately 300 mm apart, a distance that is maintained by the vierendeel spacers. The weight of the structure and cladding is usually less than 20 kg/m2 over the developed area of the dome, (unless there is a heavy load of snow). In concrete domes, the weight of the structure far exceeds the weight of the live load it can carry -- but in Geometrical galvanized steel domes, the structure's weight is only a small fraction of the total design load. A galvanized steel dome will be about 25% less expensive than a dome using aluminum cladding; the load carrying capacity will be about the same.

The 84m structure described above would consist of approximately 20,000 chord tubes, 7,000 connectors, and 3,500 spacers. All components would be prefabricated, marked and packaged by section. Chord tubes would be approximately 1.6 m long, easy for crews to handle. Assembly could be completed in less than three months by 20 individuals working a single shift.

Dozens of galvanized steel domes cover circular and longitudinal stockpiles around the world. Figures 4 to 7 show a few of these:

2 x 113m South Africa	73m Senegal	100m Dominican Republic
86m Cambodia	110m Argentina	104m Mexico

Conclusion

Complying with new environmental regulations for air quality does not require stock pile owners to make huge investments or shut down operations. The Geometrica dome solutions presented here not only allow affordable compliance, but also enhance the image of any plant.