ent practice the raw materials
are mixed in a wash mill with so much water that the resulting slurry
contains 40 to 50% of water. The slurry, which is wet enough to flow,
is ground between millstones so as to complete the process of
comminution begun in the wash mill. Thorough grinding and mixing are
of the utmost importance, as otherwise the cement ultimately produced
will be unsound and of inferior quality. The drying of the slurry is
generally effected by the waste heat of the kilns, so that while one
charge is burning another is drying ready for the next loading of the
kilns. The kilns commonly employed are "chamber kilns," circular
structures not unlike an ordinary running lime kiln, but having the
top closed and connected at the side with a wide flue in which the
slurry is exposed to the hot products of combustion from the kiln. The
farther ends of the flues of several such kilns are connected with a
chimney shaft. The slurry, in drying on the floor of the flue, forms a
fairly tough cake which cracks spontaneously in the process of drying
into rough blocks suitable for loading into the kiln. At the bottom of
the kiln is a grate of iron bars, and on this wood and coke are piled
to start the fire. A layer of dried slurry is loaded on this, then a
layer of coke, then a layer of slurry, and so on until the kiln is
filled with coke and slurry evenly distributed. Fresh slurry is run on
to the drying floors, and the kiln is started. The construction of an
ordinary chamber kiln may be gathered from the accompanying diagram
(fig. l). The operation of burning is a slow one. An ordinary kiln,
which will contain about 50 tons of slurry and 12 tons of coke, will
take two days to get fairly alight, and will be another two or three
days in burning out. Therefore, allowing adequate time for loading and
unloading, each kiln will require about one week for a complete run.
The output will be about 30 tons of "clinker" ready to be ground into
cement. The grinding of the hard rock-like masses of clinker is
effected between millstones, or in modern plants in ball-mills,
tube-mills and edge-runners. It is an important part of the
manufacture, because the finished cement should be as fine and
"floury" as possible. The foregoing description represents the
procedure in use in many English factories. There are various
modifications in practice according to local conditions: