Calcium Carbonate

Calcite                    : CaCO3

Refractive Index         : 1.66-1.74
Specific Gravity         : 2.71
Mohs Hardness          : 3

Aragonite              : CaCO3

Refractive Index        : 1.68-1.69
Specific Gravity        : 2.95
Mohs Hardness         : 3.5-4

The mineral calcite is the major or sole constituent of most commercial calcium carbonate products. These include natural limestone, marble, and chalk, plus most precipitated calcium carbonate. Aragonite is a metastable polymorph of calcite that typically has an acicular crystal shape. Natural aragonite products are less common, but precipitated varieties are available. Many calcium carbonate deposits are the remains of the shells and skeletons of ancient sea life. The color, purity, density, and crystal morphology depend upon the influence of waves and water currents before burial, and upon temperature, pressure, and tectonic activity after burial. The most common mineral impurities are quartz and clay. 

The most common substitutes for calcium are other divalent cations, such as magnesium, strontium, and barium, although the amount of substitution is usually no more than a few percent. The exception is magnesium, which can substantially replace calcium to form the mineral dolomite, CaMg(CO3)2. Calcium carbonate rocks, a common constituent of the Earth's surface, range from high-calcium limestones containing >95% calcite to dolostones containing 90% or more dolomite. Chalk is a fine-grained, white, friable, high-purity limestone. Marble is a dense, hard, low porosity stone composed substantially or solely of calcium carbonate. It is formed by the action of heat and pressure on buried limestone beds. 

Marble used for ground calcium carbonate products is chosen for color and purity. Marble for decorative and dimension stone (blocks, fascia slabs, tile) is selected for its characteristic shadings or veining, caused by minor mineral impurities, and for its ability to take a polish. Travertine is a banded,
dense calcium carbonate also used as decorative and dimension stone. It is formed by rapid chemical precipitation around natural hot springs. A similar material is precipitated from cold water solutions as stalagmites and stalactites. This is known as onyx marble (true onyx is banded quartz), Mexican onyx,
Algerian onyx, and oriental alabaster (true alabaster is dense gypsum).Aragonite sand comprises extensive marine deposits off the south Florida coast. It is recovered by suction dredging and after drying and screening grades about 96% calcium carbonate. Most is used locally in cement manufacture.

 

TYPES

Filler uses for calcium carbonate generally require white color and a high degree of mineralogical purity, plus control of particle size and shape, surface area, and liquid absorptivity. Natural calcium carbonate fillers are generally called ground limestone or ground calcium carbonate but may also be sold as ground chalk, ground marble, or whiting. The synthetic alternatives are known as precipitated calcium carbonate, or PCC.

Ground calcium carbonate 

Natural calcium carbonate ores high in chemical and mineralogical purity are wet or dry ground to a wide range of  products. Dry-ground calcium carbonates, comprising nominal 200 to 325 mesh products, are among the least expensive white fillers available. They are simply ground from ore but may also be beneficiated by air separation. Wetground products are produced in finer particle size ranges and may be beneficiated by washing or flotation. They are informally classified by particle size as fine ground (FG; 3 to 12 micrometers median, 44 micrometers top), and ultrafine ground (UFG; 0.7 to 2 micrometers median, 10 micrometers top). There is some overlap between these classifications from one producer to another. As dry grinding technology advances, dry-ground products in fineness ranges previously associated with wet-ground grades are becoming more common. Wet processed FG and UFG products are of necessity more expensive than dry-ground products due to the cost of drying and in some cases remilling to break up agglomerates of ultrafine particles. Wet-ground fine and ultrafine products are also sold in 75% solids slurry form for highvolume paint and paper applications and in stearic acid- and stearate-treated forms for use in polymers. 

Precipitated calcium carbonate 

Precipitated calcium carbonate (PCC) is produced for applications requiring any combination of higher brightnesss, smaller particle size, greater surface area, lower abrasivity, and higher purity than is generally available from ground natural products. In the US PCC is most commonly made by the carbonation process. Limestone controlled for coloring oxides (e.g. of Mn and Fe) is calcined to calcium oxide and carbon dioxide. The calcium oxide (burnt lime) is then slaked with water to form calcium hydroxide (milk of lime). The carbon dioxide liberated on calcining is then reintroduced to precipitate calcium carbonate. Manipulation of process variables determines particle size and shape, surface area, and whether the product is isomorphous calcite or acicular aragonite. PCC products are also made by the lime-soda process, where milk of lime is reacted with sodium carbonate to form a calcium carbonate precipitate and a sodium hydroxide solution. 

This process is used by commercial alkali manufacturers to make a relatively coarse PCC as a byproduct of sodium hydroxide recovery. A third production route is to react milk of lime with ammonium chloride, forming ammonia gas and a calcium chloride solution. This solution is purified and reacted with sodium carbonate to form a calcium carbonate precipitate and a sodium chloride solution. This process is the simplest of the three, but to be economical it is usually carried out in a satellite facility adjacent to a Solvayprocess soda ash plant. Although still common elsewhere, the Solvay process became obsolete in the US in 1986. PCC products are typically offered as fine (0.7 micrometer median) and ultrafine (0.07 micrometer median) grades, with and without stearate surface treatments.

 

USES

The major filler uses of calcium carbonate, both natural and PCC, are paper, paint, adhesives and sealants, and polymers. Filler uses account for only about 1% of the 700 to 800 million metric tons of calcium carbonate produced in the United States annually. Production is overwhelmingly dominated by
commodity, low-value crushed stone, mainly for civil engineering uses and as aggregate for concrete and asphalt.

Paper

In alkaline papermaking, calcium carbonate is used as a paper filler and coating. Both uses require high brightness, high purity, small particle size, and lack of abrasion. Precipitated products generally retain a performance edge over the best ultrafine wet-ground grades. Commercial PCC products are at a disadvantage, however, in their high cost as dry products and in their difficulty in forming the high solids (usually 75%) slurries, due to their extremely small particle size, that large paper mills prefer. In Europe, where alkaline papermaking has been more common, an acceptable balance of performance
and price has been met with high-quality ground chalk and marble. In the US PCC is preferentially used because the more recent and ongoing conversions to alkaline papermaking have beeen accompanied by the establishment of satellite PCC production facilities adjacent to paper mills. Byproducts of the pulping process are diverted to economically produce PCC, which is pumped to the paper mill in slurry form. About half these US satellite PCC plants produce enough slurry to also supply smaller mills where the construction of a full-scale PCC plant may not be justified. In Europe the establishment of satellite PCC plants is just now gaining popularity. Whether ground natural or precipitated, calcium carbonate is used as a paper filler and in coatings to provide opacity, high brightness, and improved printability due to its good ink receptivity.

Coatings

Gound natural calcium carbonate is the most widely used white pigment in paints because it is available at relatively low cost and has high brightness for TiO2 extension, high purity, low abrasivity, and resistance to weathering. The coarsest grades are used at loadings up to 50% in textured paints. Fine and ultrafine grades, including PCC, are used in a wide variety of other decorative and protective coatings. PCC products, with generaly higher brightness, provide better TiO2 extension. PCC and ultrafine wet-ground grades contribute to rheology and stability and provide good dry hide and gloss retention. Slurries of 75% ultrafine natural calcium carbonate are sold for convenient use in the large-scale manufacture of water-based coatings. Stearate-treated natural and PCC products are used in nonaqueous coatings.

Adhesives and sealants 

Calcium carbonate is the most widely used filler in adhesives and sealants. The coarsest grades of ground natural products are used at high loadings in drywall joint cements and in ready-mix adhesives for heavy wall tile. Somewhat finer and generally off-color natural products are used at high loadings in oil-based putties. Finely ground white grades are used as a general-purpose filler in most types of adhesives, sealants, and gap fillers for their balance of low binder demand and narrow particle size distribution. This allows economically high loadings without adversely affecting flow. High performance polymer-based adhesives and sealants use stearate-coated PCC and ultrafine natural products to control flow and slump on application and to provide low modulus with good tear and tensile properties in the cured state.

Polymers 

Ground natural calcium carbonate is the most commonly used filler in plastics due to its low cost, low abrasion, low oil absorption, low moisture, high brightness, and easy dispersion with conventional mixing equipment. These attributes account for its widespread use in elastomers as well, where it can be used at very high loadings with little loss of compound softness, elongation, or resilience. PCC products, particularly stearate-coated grades, are used as functional fillers in rubber and plastics. In plastics they are used to improve mar and impact resistance, surface gloss, weatherability, shrinkage control, cold flow properties, low and high temperature properties, and dielectric properties, and to reduce plasticizer migration and crazing of molded parts. PCC is most widely used in rigid and flexible PVC and PVC plastisols. Fine and ultrafine natural products are also commonly used in PVC as well as in polypropylene and in polyester molding compounds. Lesser quantities are used in polyurethane foam, epoxies, and phenolic resins. 

In rubber stearate-coated ultrafine PCC is used for its low moisture absorption, good dispersion, and good elastomer-filler contact. This enables it to perform as a semireinforcing filler, imparting good tensile strength, tear resistance, resilience, abrasion resistance, and flex crack resistance. Dry-ground calcium carbonate is the most common filler in carpet backings. It is used to add weight and body and to extend the binder, usually latex, which secures the loop of tufted carpeting. It is also used as the filler in secondary backing and in separate underlay (the foam pad), both of which are usually urethane foam.

 

Other uses

Calcium carbonate meeting pharmacopeia requirements is used as a therapeutic source in anatacids and calcium supplements and as a tableting excipient. Fine particle size products are used as a fire extinguisher foam filler, as an abrasive in household cleaners, as a flux in welding rod coatings, as a diluent in agricultural pesticide dusts, and as a dusting agent to mitigate the explosion potential of coal dust generated during underground mining. Substantial quantites of crushed and ground limestone are also used in the manufacture of portland cement, lime, glass, and metallurgical fluxes, as well as in flue gas desulfurization processes and as a soil amendment.

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