Calcium carbonate, or CaCO3 is commonly found as calcite or aragonite. It is the primary component of naturally formed pearls and the shells of organisms such as snails and oysters. Industrially, pure ground calcium carbonate or ‘GCC’ is obtained through the mining of chalk, marble or limestone reserves. While chalk is a fine, crystalline material, the CaCO3 obtained from chalk is more compact. CaCO3 obtained from marble reserves is coarse and crystalline. Calcium carbonate can also be chemically prepared from calcium oxide or ‘quicklime’ using water and carbon dioxide. The calcium carbonate produced in this manner is known as precipitated calcium carbonate or PCC, and it has a controlled, extremely fine particle size.
China, India, the U.S.A and Europe are leading producers of limestone, with China producing nearly 54% of the global throughput. Leading producers of marble in the current markets include Turkey, Italy, Greece and Iran.
Properties of Calcium Carbonate:
- Can exist as a powder or a crystal
- It is usually white in colour when it is a powder, or colourless when it is a crystal
- It is a stable compound.
Production of Calcium Carbonate:
Manufacture of GCC:
Ground calcium carbonate or ‘GCC’ is obtained through the mining and extraction of calcium carbonate from limestone and marble reserves. Limestone and marble are sedimentary rock which consist primarily of calcite.
The extraction process to obtain aluminum hydroxide from bauxite can be detailed as follows: By specifying the required particle size, the mined rock can then be ground in industrial units to match specifications. The suitability of the GCC for use in a given application is determined by the size, color and purity of the GCC manufactured. Finely ground calcium carbonate has particle diameters as small as 1-3 microns, while coarser GCC produced for less stringent applications may be roughly 2millimeters in size. Industrially produced GCC is usually treated with fatty acids to reduce the corrosive effects of acids on calcium carbonate. GCC is a highly useful chemical due to its low cost, desirable properties and wide use in various industrial applications.
Manufacture of PCC:
PCC or precipitated calcium carbonate, is a synthetically manufactured superior form of GCC, with several advantages including regularity of shape, smaller particle size, narrowness of particle size distribution and higher purity. The manufacture of calcium carbonate from limestone reserves is an extremely economical process and can be detailed as follows:
Step 1: Calcination of limestone
CaCO3 = CaO + CO2
First, the limestone ore is calcined in the absence of oxygen at temperatures in excess of 900°C. Heating for the calcination step is usually provided by the burning of natural gas to avoid contamination from coal vapors.
Step 2: Slaking of quicklime
In this process, water is added to the quicklime formed previously to form calcium hydroxide, which is also known as hydrated lime or slake.
The governing equation for the process is:
CaO + H2O =Ca(OH)2
Step 3: Carbonation of slaked lime
The carbon dioxide produced in Step 1 is captured and reused in this step. By adding carbon dioxide to the slaked lime, an insoluble white precipitate of calcium carbonate is formed. The calcium carbonate formed chemically in this manner is known as PCC and it can easily be separated, resulting in highly pure slaked lime with very fine particles.
The chemical reaction can be illustrated as follows:
Ca(OH)2 + CO2 = CaCO3 + H2O
Step 4: Refining
Grit and other fine particles can be separated from the PCC slurry using a fine mesh and filters to produce purified PCC.
Application of Calcium Carbonate
Calcium Carbonate has various applications in a number of industries. Some of the major applications are discussed below:
Calcium carbonate finds its primary use in the construction industry as a starting material for building materials like cement or builder’s lime. Limestone aggregate is also the primary constituent of concrete and is widely used in road construction. When mixed with soil, calcium carbonate helps to transfer cement-like properties to the soil, enabling the foundations of buildings to be firmer and allowing construction vehicles to easily traverse the construction paths. Owing to its ability to be degraded by acid rain, calcium carbonate is never used as the sole component for brick or building material construction, but it is still highly important as a starting material due to its rigidity and stability.
Approximately 3.6 billion tonnes of cement are produced per year, and since cement consists limestone aggregate and various clays in a 1:1 ratio, roughly 1.8 billion tonnes of limestone would need to be produced per year, attesting to the great importance of calcium carbonate in the construction industry.
Around 60% of the calcium carbonate produce is used in metallurgy industry, mainly in the manufacturing of steel, for the formation of slag and for the purification of iron from iron ore in blast furnace.
It is used as an important reinforcing filler in thermoplastic industry to enhance the mechanical, thermal and structural properties such as impact strength, modulus, toughness, etc. Sealants, adhesives and stained glass windows are some of the applications of uncoated calcium carbonate as a filler in the thermoplastic industry.
It is used in the paint industry as an extender that provides the hiding power and high brightness to
the paint. This is due to its pigmentation property. Also, the addition of Calcium Carbonate lowers
the carbon foot-print for architectural indoor paints, hence being environmentally friendly.
In the paper industry, calcium carbonate is widely used as a filler for paper production. Owing to its low prices and optimal particle size, it is preferred to wood fibres as a non-polluting filler in the production of paper. It is currently being used instead of kaolin for the production of glossy paper. The PCC produced can even be coated with several other materials to ensure porosity and guarantee whiteness to the paper produced.
It is used to soften water and also in the treatment of sewage to remove colloidal particles. It is even
used in desulphurisation of flue gas.
Calcium Carbonate food grade is used as a food additive, for example in snacks or bakery items, it is
used as an anticaking agent, a food preservative and as a colour retainer. Calcium carbonate is also
used as a firming agent in canned food.
In the pharmaceutical industry, it is used in manufacturing of antibiotics through fermentation, and
also as an excipient in pharmaceutical tablets. It is used as a supplement for calcium, in the
treatment of osteoporosis, for growing children and pregnant women. It is also used as an antacid.
Healthcare and Food Industry
In the healthcare industry, calcium carbonate is used medicinally as a calcium supplement in antacid production. It is a major component in various commercial antacids such as ‘Tums’ etc. It is also used as an inert filler for tablets and other pharmaceutical products.
Aside from its use in the manufacture of drugs, calcium carbonate is also used as a food additive under the designated code E170. It can be used commercially as an acidity regulator, anticaking agent and as a stabilizer. It can also be used as a non-toxic source of calcium in vegan products such as soy milk and almond milk. Additionally, calcium carbonate is used in several canned or bottled vegetable products as a firming agent.