a few of
these may be described. In all cases, however, the main operations are
the same, viz. intimately mixing the raw materials, drying the
mixture, if necessary, and burning it at a clinkering temperature
(about 1500 Deg. C. =2732 Deg. F.). Thus when hard limestone is the
form of calcium carbonate locally available, it is ground dry and
mixed with the correct proportion of clay also dried and ground. The
mixture is slightly damped, moulded into rough bricks, dried and
burned. A possible alternative is to burn the limestone first and mix
the resulting lime with clay, the mixture being burned as before. By
this method grinding the hard limestone is avoided, but there is an
extra expenditure of fuel in the double burning.

Other kilns.

Many different forms of kiln are used for burning Portland cement.
Besides the chamber kilns which have been described, there are the
old-fashioned bottle kilns, which are similar to the chamber kilns,
but are bottle-shaped and open at the top; they do not dry the slurry
for their next charge. Their use is becoming obsolete. There are also
stage kilns of the Dietzsch type, which consist of two vertical
shafts, one above the other, but not in the same vertical line,
connected by a horizontal channel. At this middle portion and in the
upper part of the lower shaft the burning proper proceeds; the upper
shaft is full of unburnt raw material which is heated by the hot gases
coming from the burning zone, and the lower shaft contains clinker
already burned and hot enough to heat the incoming air which supplies
that necessary for combustion at the clinkering zone. A pair of
Dietzsch kilns, built back to back, are shown in fig. 2. There are
other forms of shaft kiln, such as the Schneider, in which there is a
burning zone, a heating and cooling zone as in the Dietzsch, but no
horizontal stage, the whole shaft being in the same vertical plane.
Another form is the Hoffmann or ring kiln, made up of a number of
compartments arranged in a ring and connected with a central chimney;
in these compartments rough brick-shaped masses of the raw materials
are stacked, and between these bricks fuel is sprinkled. At a given
moment one of these compartments is burning and at its full
temperature; the air for combustion is drawn in through one or more
compartments behind it which have just finished burning, and is
thereby strongly