heated; the products of combustion pass away through
one or more compartments in front of it and heat their contents before
they are subjected to actual combustion. It will be seen that the
principle of the ring kiln is similar to that of the stage kiln. In
each case the clinker which has just been burned and is fully hot
serves to heat the air-supply to the compartment where combustion is
actually proceeding; in like manner the raw materials about to be
burned are well heated by the waste gases from the compartment in full
activity before they themselves are burned. (It may be noted that here
and generally in this article "burn" is used in the technical sense;
it is technically correct to speak of cement clinker being "burned",
although it is not a fuel; in accurate terms it is the fuel which is
burned, and it is the heat it generates which raises the clinker to a
high temperature, i.e. technically "burns" it.) By this device a great
part of the heat is regenerated and a saving of fuel is effected.

[Illustration: FIG. 2.]

Rotatory kilns.

The methods of burning cement described above are obsolescent. They
are being replaced by the rotatory process, so called because the
cement is burned in rotating cylinders instead of in fixed kilns.
These cylinders vary from 60 to 150 ft. in length, an ordinary length
in modern practice being 100 to 120 ft.; their diameter
correspondingly varies from 6 ft. to 7 ft. 6 in. The cylinders are
made of steel plate, lined with refractory bricks, are carried on
rollers at a slight angle with the horizontal, and are rotated by
power. At the upper end the raw material is fed in either as a dry
powder or as a slurry; at the lower end is a powerful burner. In the
early days of rotatory kilns producer gas was used as a fuel, but with
little success; about 1895 petroleum was used in the United States
with complete success, but at a relatively heavy cost. At the present
time, finely powdered coal injected by a blast of air is almost
universally employed, petroleum being used only where it is actually
cheaper than coal. In the working of this type of kiln the rotation
and slight inclination of the cylinder cause the raw material to
descend towards the lower end. At the upper end the raw material is
dried and heated moderately. As it descends it reaches a part of the
kiln where the temperature is higher; here the carboni