Calcium carbonate (CaCO3) is one of the most widely used minerals today. Also known as limestone, limestone flour, or powdered limestone, it is used in a variety of industrial and commercial applications such as the manufacture of plastics, paints, and adhesives, as well as in environmental applications. Calcium carbonate is also used as a dietary supplement to support the health of bones, muscles, nervous system, and heart. It can serve as an antacid to relieve heartburn, acid indigestion, and upset stomach.

In the U.S., the Food and Drug Administration has affirmed calcium carbonate to be GRAS (Generally Recognized As Safe).1 As long as it meets certain purity requirements, it can be used as a direct food additive, as a pharmaceutical or as an indirect additive in paper products that come in contact with food. Similar acceptances and approvals exist around the world where calcium carbonate is widely used in these applications.

Characteristics and Challenges:

Calcium carbonate's most common natural forms are chalk, limestone, and marble. Although all three forms are identical in chemical terms, they differ in many other respects, including purity, whiteness, thickness and homogeneity. These differences often make the handling of calcium carbonate a complex proposition.

Calcium Carbonate powders can cause many problems in conveying systems, including sticking in hoppers, flood-feeding into convey lines, building up inside convey lines, and blinding over filter bags and cartridges in receivers.

With a bulk density of 35 to 58 lb/cu ft (443-734 kg/cu m), it is a sluggish adhesive powder that readily packs and cakes. It does, however, respond to aeration and direct contact vibration.

Ideally, the conveying line should be as short as possible and rise vertically from the fill point, such as the base of the storage silo, to the inlet point. There should be a minimum number of bends in the line as well.

If the calcium carbonate arrives at the processing facility in bulk bags, the frames used to discharge the bags should ideally be equipped with additional accessories to reduce the need for operator intervention when emptying the calcium carbonate from the bag. This includes features such as spring-loaded frames that will elongate and stretch the bags as they empty and lighten, making them rigid and help eliminate the bridging of the material. Bag activating devices are usually effective in dislodging collections of calcium carbonate, promoting a better flow. Some of these bag activating devices also serve as an airtight seal between the bulk bag and the receiving hopper.

After the bulk bag has been connected to the receiving hopper and the calcium carbonate begins to flow, the air inside the hopper is forced out. Unless this displaced air passes through a filter, airborne dust particles can escape into the surrounding atmosphere, creating a potentially hazardous situation. A dust collector mounted on the discharger frame will contain the calcium carbonate inside the conveyance system. Calcium carbonate dust trapped in the filter media can be returned to the receiving hopper by a pneumatic pulse through the filter.

If the calcium carbonate is being added into the processing stream from smaller (>50 lb./22.76 kg.) bags, a bag breaking station with a dust hood, filtration devices and a pneumatic pulse cleaner is the recommended solution to support the manual unloading of the material. If empty bag disposal is an issue, a dust-tight bag compacting system may be necessary. A hopper screen above the receiving vessel will help protect the operators if the calcium carbonate is being dumped manually into a hopper that is equipped with agitating devices.

Calcium Carbonate
Calcium carbonate is an extremely tricky material to move. Its density is not always indicative of its flow properties. Some grades of calcium carbonate may pack easily and be hard to convey, while others will flow more easily, but tend to fluidize. If a flexible screw is being used to convey calcium carbonate, a screw with a wider, flatter surface than the standard-sized screw is a safer choice. Due to the changing flow properties of calcium carbonate, a higher-rated horsepower motor may be required.

Flexicon's recommended increases to a screw-conveyor motor's normal horsepower and a larger-than-standard shaft size have proven effective in boosting the throughput of the powder in the system. Flexicon has designed and built automated systems that streamline the handling of this difficult material, increasing productivity while controlling dust and waste. Due to the sometimes unpredictable flow characteristics of calcium carbonate, Flexicon-manufactured valves are recommended. Available in a wide array of options, these valves make the tying and untying of bulk bag spouts easy and reduces unwanted material flow, preventing uncontrolled bursts of calcium carbonate into the hopper and conveying equipment and dust into the plant environment.

Should your calcium carbonate application feature the loading the material into bulk bags, the bag capacity will be maximized by use of a vibratory densification deck to level the carbon black as it fills the bag and by the application of load cells to attain the
desired weight. Seals and other dust containment devices will ensure a dust-tight operation.
Flexicon Applications:

One of Mexico's largest plastics processors was able to reduce dust and streamline its process of blending PVC resin and calcium carbonate after installing a Flexicon bulk bag unloading system with pneumatic and flexible screw conveying.

Consultation with a Flexicon specialist will help you decide if a flexible screw or pneumatic solution best fits your calcium carbonate application.

Flexicon's expert design and engineering staff will weigh each parameter and recommend the best solution for you. Upon request, Flexicon's test lab will simulate your calcium carbonate handling functions before the system is installed in your plant.

Flexicon's product line of advanced flow promotion conveyors, high-flow hoppers, deaeration/densification decks and a host of other components and accessories are proven performers that promote flow while reducing degradation, dusting and/or the separation of blends comprised of disparate particles.

1 United States Food and Drug Administration: Select Committee - Calcium Carbonate

Sources: Where noted. All other information courtesy of Flexicon Corporation.


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