Engineering Chemistry S Datta

420 ENGINEERING CHEMISTRY
may be employed as the collector. A concentration of 0.04 lb per ton of rock suspended in 4 tons
of water has given good results. Frothing aids in separation of desired materials from the
undesired ones.
Manufacturing Procedure
Two methods of manufacture are used:
• Wet process
• Dry process
The wet process was the original one. For a time it was gradually displaced by the dry
process. But the wet process is now being adopted largely for new plants for more accurate
control and mixing of the raw mixture.
In the wet process, the solid material after dry crushing is reduced to a fine state of
division in wet tube or ball mills and passes as a slurry through the bowl classifiers or screens.
The slurry is pumped to correcting tanks where rotating arms make the mixture homogeneous
and allow the final adjustment in composition of raw materials to be made. This slurry is
filtered in a continuous rotary filter and fed into the kiln. The moisture content of the feed is 40%.
The dry process is especially applicable to natural cement rock and to the mixtures of
limestone and shale or slate. In this process, the materials are roughly crushed in jaw crushers
followed by gyratory mills, then dried, sized and more finely ground in tube mills. This dry
powdered material (moisture content 2%) is fed directly to the kiln. Heat is provided by burning
oil, gas or pulverised coal, using preheated air from cooling the clinker. 80-160 lbs of fuel are
necessary per barrel (376 lbs) of cement.
The tendency in recent years has been to lengthen the rotary kiln in order to increase its
thermal efficiency. Dry process kilns may be as short as 150 ft, but in the wet process,
300-500 ft kilns are not uncommon. The internal diameter is usually 8 to 15 ft. The kilns are
rotated at from 1/2 to 2 r.p.m., depending on size. The kilns are slightly inclined so that materials
fed in at the upper end and travel slowly to the lower end, taking from 2-3 hrs (Fig. 19.6).
In order to obtain greater heat economy, unit operations are used for removing the part
of the water from slurry by slurry filters. Waste heat boilers are used to conserve heat.
Because the lining of kiln has to withstand severe abrasion and chemical attack at the
high temperatures in the clinkering zone, the choice of refractory linning is difficult. For this
reason, high alumina bricks and high magnesia bricks are widely used for the lining of the
kiln, although Portland cement clinker itself is satisfactory for the purpose.
Reactions at various temperatures in the kiln
Temperature Reactions Heat change
100°C Evaporation of water (H 2
O↑) Endothermic
> 500°C Al 2
O 3
. SiO 2
. xH 2
O ⎯⎯→ Al 2
O 3
.SiO 2
+ xH 2
O Endothermic
> 900°C CaCO 3
⎯⎯→ CaO + CO 2
↑ Endothermic
900°C *5CaO + 3 Al 2
O 3
⎯⎯→ 5 CaO . 3Al 2
O 3
to 2CaO + SiO 2
⎯⎯→ 2CaO . SiO 2
1200°C 3CaO + SiO 2
⎯⎯→ 2CaO . SiO 2
2CaO + Al 2
O 3
⎯⎯→ 2CaO . Al 2
O 3
U
V
|
3CaO + Al 2
O 3
⎯⎯→ 3CaO . Al 2
O 3 W
|
Exothermic
(Contd...)