Equivalent oil used per bbl. 10 gals. 12 gals.
A rotary kiln is used almost exclusively in the manufacture of cement and recent tests have proven large, long kilns of this type to be the most efficient. (See Fig. 55.) The clay is treated in a small rotary kiln or dryer. The oil required to dry one ton of clay varies from five to six gallons per ton, depending upon the moisture of the clay. After having been dried, the clay is ready for the powder mill, and from there it is conveyed to the top of the roasting kiln, where it is mixed with the necessary amount of lime rock, which has also been reduced to a powder. The main rotary kilns are steel cylinders varying from 6 to 10 ft. in diameter and from 60 to 100 ft. in length. They are set on rollers, at a slope of about to 4 in. to the foot, and lined with resistant fire-brick, to withstand the great heat to which the inner surface is subjected. When oper ating, the cylinder slowly revolves upon the roller by means of a train of gears ; this motion, in connection with the inclination of the cylinder, causes the cement mixture, which is fed into the kiln at the upper end, to move slowly to the lower end of the cylinder where it discharges into a clinker pit. As the raw material slowly works down into the interior of the kiln, the heat generated from the burners first drives off the water and the carbon dioxide from the mixture and then causes the lime, silicia, alum ina, and iron to combine chemically and form a partially fused mixture known as "cement clinker." The operation during this period is closely watched by the operator. The rate of feeding the raw material is regulated by the speed which the cylinder revolves. The temperature of the kiln is regulated by the ap pearance of the clinker.
The physical and chemical changes which occur within the kiln are divided into two stages. During the first, the water and carbon dioxide is driven off, the temperature of the kiln averaging about 1800 de grees F. During the second stage a temperature of 2800 degrees F. is maintained, and the burnt mass is thor
oughly fused. The greatest care must be exercised by the operator during this period, for if the clinker is burned too much or too little, the value of the cement will be greatly reduced. After fusion is com plete, the clinker is cooled and passed to the ball and tube mills where it is ground into a fine powder.
Different cement manufacturers have widely dif ferent views on the method of firing kilns. Some recom mend an intense, concentrated heat as the clinkering or fusing point, claiming that the calcining is effectively carried out by the lower temperatures of the spent gases. Others require a long, sweeping flame, extend ing back many feet into the kiln, claiming that they get a much more perfectly burnt clinker by this method. It is possible that peculiar characteristics of the raw materials make the latter method desirable in some cases ; but in the opinion of the writer, much more economical results can be obtained by the first method.
Some manufacturers 'claim to be able to make cement with from 8% to 9% gal. of oil per bbl. of ce ment. It is difficult to verify these statements, as the utmost secrecy is maintained in all details of operation. Frequently, however, errors can be found in their method of calculating the exact amount of oil used. Some take measurements from faulty meter readings, while others do not allow for the amount of oil re quired to produce the atomizing agent. A careful reading of the report reproduced below will show that the average amount of oil required per bbl. of cement lies between 10 and 12 gallons.
Various types of burners are used in cement kilns. Some use the steam atomizing type, some the high pressure air type, and some the low pressure air type. The last type has proven so much more economical than the others that many plants throughout the coun try have converted from steam or high pressure air burners to low pressure burners. Fig. 56 shows a complete fuel oil installation at a cement plant.