Calcium carbonate

Calcium carbonate (limestone).
Chemical formula : CaCO₃
(O = 47,96 %, C = 12 %, Ca = 40,04 %)
Molar mass = 100,087 ± 0,006 g·mol^-1

Natural product :
It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells.
Calcium carbonate is the active ingredient in agricultural lime, and is created when Ca ions in hard water react with carbonate ions creating limescale (it is the hard, off-white, chalky deposit found in kettles, hot-water boilers and the inside of inadequately maintained hot-water central heating systems. In French : tartre).
It is also often found as a similar deposit on the inner surface of old pipes and other surfaces where "hard water" has evaporated. Other than being unsightly and harder to clean, limescale seriously impairs the operation or damages various components.
Fouling (corresponding effects of fouling) :
Precipitation fouling or scaling involves crystallization of solid salts, oxides and hydroxides from solutions. These are most often water solutions, but non-aqueous precipitation fouling is also known. Precipitation fouling is a very common problem in boilers and heat exchangers operating with hard water and often results in limescale.
Piping, flow channels – reduces flow, increases pressure drop, increases upstream pressure, increases energy expenditure, may cause flow oscillations, slugging in two-phase flow, cavitation; may increase flow velocity elsewhere, may induce vibrations, may cause flow blockage

Polymorphs of calcium carbonate:
(polymorphism is the ability of a substance to crystallize in different structures depending on ambient conditions).

• aragonite polymorph primairement biogenic origin (eg aragonite: the skeleton of the coral reef-building)
  Shell (mollusc)
  Marble (metamorphic rock)
• calcite, stable in ambient conditions
  limestone
• Chalk (sedimentary rock), the limestone is sandy or micaceous chalk grained
  travertine, tufa and vaterite: rare because unstable under conditions of pressure and normal temperature.

Limestones represent 20% of sedimentary rocks.

Preparation.
The vast majority of calcium carbonate used in industry is extracted by mining or quarrying. Pure calcium carbonate (e.g. for food or pharmaceutical use), can be produced from a pure quarried source (usually marble).

Alternatively, calcium carbonate is prepared from calcium oxide. Water is added to give calcium hydroxide, and carbon dioxide is passed through this solution to precipitate the desired calcium carbonate, referred to in the industry as precipitated calcium carbonate (PCC):

It is commercially available under different solid forms (depending on the manufacturer):

• crushed, pieces 2 to about 4 mm thick,
• Current grain size of 0.5 to 2 mm.

In water treatment; it is used by the percolation of water through a filter limestone (usually). Direct injection in powder form or in suspension has a low yield of neutralization.

NB - The concentration of calcium in drinking water (hardness) is measured in French degrees (°F, 1 degree equivalent to 4 mg/L Ca), also in mg/L as CaCO3.
The presence of limestone in the water shows no disadvantage to health, on the contrary, and so there is no statutory maximum content.

Properties :
Fine white powder; odorless, chalky taste

	Grain product
% CaCO3	80 to 97%
Volumetric mass density (20°C)	2700 to 2900 kg/m3
Melting point	825 °C (aragonite), 1339 °C (calcite)
Maximum solubility at 20°C (in pure water) solubility product (Ksp, 25°C)	15 mg/L 4.8×10-9
Acidity (pKa)	9.0

A formulation for water treatment.
Calcium carbonate will react with water that is saturated with carbon dioxide to form the soluble calcium bicarbonate.
Reaction mechanism of lime with aggressive CO2 (simplified equations):
CaCO3 + CO2 + H2O >>> Ca[HCO3]2
...100.........44 ............................ 162 or 10°F as TAC (100 mg/L as CaCO3)

To neutralize 1 mg of CO2, it is necessary to use (100/44)= 2.273 mg as CaCO₃, which form (162/44)=3.68 mg calcium bicarbonate [HCO3], and therefore also the neutralization of 1 mg /L to form CO2 (10/44)= 0.2273 °F/L asTAC (2.273 mg CaCO3/L of Alkalinity)

Standardization.
Standard products used for the production of drinking water: Official French Bulletin - Calcium carbonate : NF EN 1018.

Uses.
Rocks made of limestone are used as follows:

• water treatment, sludge and household waste
• sculpting material (technique of direct carving)
• stone material of the road: tarmac, serious limestone ballast, a very frequent use
• sand and gravel in the manufacture of concrete, more rarely in the asphalt, for the most hard limestone
• powdered mineral filler in various industrial products (plastics, paints, glues, recurring ...)
• as a flux in melting glass (sand) and the melting of ferrous metals (limestone flux)
• agricultural liming to fight against soil acidification
• calcium intake in the diet of livestock
• raw material for cement manufacture
• coat paper (one ton of paper contains 250 to 300 kg of chalk).

Sources : personal and Wikipedia, the free encyclopedia.

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