Chemical formula : Ca(OH)2
(Ca = 54,1%, O = 43,2 %, H = 2,7 %)
Molar mass = 74.093 +/- 0.005 g·mol-1
Lime is mainly obtained by thermal decomposition of limestone (made basic salt CaCO3):
Quicklime is lime out of the oven. Its main component is
calcium oxide (CaO)
After addition of water (hydration), a chemical reaction occurs. It becomes slaked lime Ca (OH)2 used in the building for masonry, construction coatings: lime, natural hydraulic lime.
It contains more or less of magnesium oxide (MgO), but the common name of lime may include different chemical states of this product. They are distinguished in the current language with respect to their uses.
Hydration (or extinction):
Contacting with water, there is a very exothermic reaction.
Note: the increase in the volume is about 30%.
This step is necessary to obtaining slaked lime.
Slaked lime (hydrated lime) is obtained by hydration of quicklime with very pure limestone. It is this which is used in water treatment (for purification).
The solid (slaked lime) is usually in the form of white, corrosive and hygroscopic powders.
Is mainly used per injection in water with saturated solution of Ca[OH]2 : Limewater is the common name of this mixture. Lime is sparingly soluble in water; maximum solubility in fresh water at 15°C is 1.7 gCa (OH)2 /L.
Limewater turns milky in the presence of carbon dioxide due to formation of calcium carbonate, a process called carbonatation:
Main Features (20°C) :
The calcium hydroxide diluted in water dissolves only very
slightly, giving a true solution <Ca2 +,-OH 2> (maximum
dissolution of 2gCa [OH] 2 / L), and therefore past, an
excess of undissolved hydroxide remaining in suspension in water.
This suspension of fine calcium hydroxide particles in water is
called milk of lime.
In general, lime slurries are 50 to 150 g / L (optimum: 100). But on the milk of lime market,we find new generation of milk with high concentration (45%) and high reactivity.
To remove undissolved suspended fraction it must be decant (or
filter) and it finally arrived in clear limewater: what is carried
out in a lime saturator device.
Today lime is packed in 15 kg bag and 25 kg. It must be stored in a dry place.
A formulation for water treatment.
Reaction mechanism of lime with aggressive CO2 (simplified equations):
2CO2 + Ca[OH]2 >>> Ca[HCO3]2
..88 ..........74 ...............162 or 100 mg/L as CaCO3 (10°F <Alk / TAC)
To neutralize 1 mg of CO2, it is necessary to use (74/88) = 0.84mg Ca [OH] 2, which form (162/88) = 1.84 mg calcium bicarbonate [HCO3], and therefore also the neutralization of 1 mg /L to form CO2 (10/88) = 0.1136 ° F / L of TAC (1.136 mg CaCO3/L of Alkalinity)
Standard products used for the production of drinking water: Official French Bulletin - Hydrated Lime: NF EN 12518.
In France, its use is as follows:
Effects on the environment.
Lime increases the pH of the stream, thus representing a potential threat to aquatic fauna and flora (stream, river, groundwater).