WET-PROCESS OF MANUFACTURE
A simplified flow-chart for the production for the cement by the wet-process is given in below figure. The raw materials ( chalk and clay) are mixed with water to a creamy consistency in wash-mills. These are circular tanks in which a central vertical spindle carries a rotating steel framework from which heavy harrows are suspended. Grating are provided above the perimeter walls of the wash-mill tanks, and when the harrows have broken the raw materials down to the size of the grating openings, the slurry is splashed through the gratings to an external annual trough.The was-mills are generally of three grades known as Primary , secondary and screening with the roughest harrows and coarsest screens at the primary mills. The different grade s of mills are arranged in series so that the slurry which has passed the screens of the primary mills is fed by gravity to the secondary mills and so on, until on leaving the screening mills only 5 per cent of the slurry is retained on a B.S. 170 mesh test sieve.
The slurry is then pumped to tall mixing tank where it is kept agitated by compressed air introduced at the bottom of the tanks. Here the proportion of raw materials are checked and adjusted as required. The slurry is then passed to large slurry storage tanks where it is kept stirred by vertical paddles suspended from rotating arms. Here it waits it's turn until the kilns are ready to revive it .
The kilns are frequently about 300 ft.long, 12ft. diameter, made up of steel plate and lined with firebrick. They are set at an angle of about 2.5 degrees to the horizontal and mounted on rollers at intervals of 60ft. Or so and geared so as rotate slowly about their axis . The slurry is fed into the upper end of the kilns and, owing to the combined effect of the slope and rotation, gradually works it's way down to the lower end where the kiln is fired. This the slurry meets the hot gases and flames which pass up
Through the kiln and thence to dust collectors and the chimney.The slurry is first dried out at the upper(cooler) part of the kiln, and as it descends and reaches the zone of maximum temperature (about 2800°F) near the lower end, the constituents combine chemically .At this stage the slurry has been burned into extremely hard nodules about the size of walnuts known as clinker, which falls out of the lower end of the kiln into a cooler where a current of air cools it down from its white heat. The cooling air is thus heated to about 800°F and becomes the secondary combustion air at the lower end of the kiln, so economising in fuel.
The clinker is then ground in steel tube grinding mills with about 3 per cent of gypsum to retard the setting process of the finished cement. The mills are horizontal rotating cylinders often about 30 ft. Long and 6.5 ft. diameter divided into three or four compartments :each compartment contains hard-steel and cast iron balls, which are lifted by the rotation of the mill so as to fall and break the clinker by crushing and attrition. Where there are four compartments in the mill, the first generally contains ball about 4 in . Diameter, the next has balls about 3 in., next 2., and the last 1 in. The cement passes from one compartments to another, and grinding continues until for ordinary Portland cement only about 5 per cent is retained on a B.S .170 mesh test sieve ; and for rapid-hardening Portland cement only about 1.0 per cent residue is retained. The cement is then conveyed to large silos where it is stored until required for packing or Bulk loading and dispatch.
Source from :-
OSCAR FABER'S
REINFORCED CONCRETE
DRY-PROCESS OF MANUFACTURE :-https://26stream.blogspot.com/2017/12/portland-cement-02.html
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