Page 192 - Chemical and process design handbook

Speight_Part II_C 11/7/01 3:08 PM Page 2.133

CALCIUM OXIDE 2.133
melted unless it combines with another substance first to convert it to a
more fluid lava called slag. The molten silicate slag is less dense than the
molten iron and collects at the top of the reactor, where it can be drawn off.
CaO + SiO → CaSiO
2 3
Lime is used in water treatment to remove calcium and bicarbonate ions.
–
2+
Ca(OH) + Ca + 2HCO → 2CaCO + 2H O
2 3 2
Lime is also used in pollution control where lime scrubbers in combustion
stacks remove sulfur dioxide present in gases from the combustion of high-
sulfur coal.
SO + H O → H SO
2 2 2 3
Ca(OH) + H SO → CaSO + 2H O
2 2 3 3 2
Lime is also used in the kraft pulping process for the purpose of regen-
erating caustic soda (sodium hydroxide).

Na CO + CaO + H O → CaCO + 2NaOH
2 3 2 3
The caustic soda is then used in the digestion of wood. The lime is
regenerated from the limestone by heating in a lime kiln.
A large part of portland cement is lime-based. Sand, alumina, and iron
o
ore are mixed and heated with limestone to 1500 C. Average percentages
of the final materials in the cement and their structures are:

.
20% 2CaO SiO 2 Dicalcium silicate
.
50 3CaO SiO 2 Tricalcium silicate
.
10 3CaO A1 O 3 Tetracalcium aluminate
2
.
.
10 4CaO Al O Fe O 3 Tetracalcium aluminoferrite
3
2
2
10 MgO Magnesium oxide
The percentage of dicalcium silicate determines the final strength of the
cement, whereas the amount of tricalcium silicate is related to the early
strength (7 to 8 days) required of the cement. Tricalcium aluminate relates
to the set in the cement, and tetracalcium aluminoferrite reduces the heat
necessary in manufacture.
See Cement.