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Page 19
This process of manufacture is almost identical in principle and in
practice with that described and patented by Mr. Joseph Aspden in the
year 1824; and though various methods have been patented for utilizing
the waste heat of the kilns in drying the slurry previous to
calcination, still the main feature of burning the material in mass in
large and expensive kilns remained the same, and is continued in
practice to the present day. The attention of the author was directed
to this subject some time since in consequence of the failure of a
structure in which Portland cement formed an essential element, and he
had not proceeded far in his investigation of the cause of the failure
when he was struck with what appeared to him to be the unscientific
method adopted in its manufacture, and the uncertain results that must
necessarily accrue therefrom. Admitting, in the first place, that the
materials employed were considered the best and most economical for
the purpose readily accessible, viz., chalk and an alluvial deposit
found in abundance on the banks of the Thames and the Medway, and
being intimately mixed together in suitable proportions, was it
necessary, in order to effect the chemical combination of the
ingredients at an intense heat, to employ such massive and expensive
structures of masonry, occupying such an enormous space of valuable
ground, with tall chimney stacks for the purpose of discharging the
objectionable gases, etc., at such a height, in order to reduce the
nuisance to the surrounding neighborhood? Again, was it possible to
effect the perfect calcination of the interior of the lumps alluded to
without bestowing upon the outer portions a greater heat than was
necessary for the purpose, causing a wasteful expenditure of both
time and fuel? And further, as cement is required to be used in the
state of powder, could not the mixture of the raw materials be
calcined in powder, thereby avoiding the production of such a hard
clinker, which has afterward to be broken up and reduced to a fine
powder by grinding in an ordinary mill?
The foregoing are some of the defects which the author applied himself
to remove, and he now desires to draw attention to the way in which
the object has been attained by the substitution of a revolving
furnace for the massive cement kilns now in general use, and by the
application of gaseous products to effect calcination, in the place of
coke or other solid fuel. The revolving furnace consists of a
cylindrical casing of steel or boiler plate supported upon steel
rollers (and rotated by means of a worm and wheel, driven by a pulley
upon the shaft carrying the worm), lined with good refractory fire
brick, so arranged that certain courses are set so as to form three or
more radial projecting fins or ledges. The cylindrical casing is
provided with two circular rails or pathways, turned perfectly true,
to revolve upon the steel rollers, mounted on suitable brickwork, with
regenerative flues, by passing through which the gas and air severally
become heated, before they meet in the combustion chamber, at the
mouth of the revolving furnace. The gas may be supplied from slack
coal or other hydrocarbon burnt in any suitable gas producer (such,
for instance, as those for which patents have been obtained by Messrs.
Brook & Wilson, of Middlesbrough, or by Mr. Thwaite, of Liverpool),
which producer may be placed in any convenient situation.
The cement mixture or slurry, instead of being burnt in lumps, is
passed between rollers or any suitable mill, when, it readily falls
into coarse dry powder, which powder is thence conveyed by an elevator
and fed into the revolving furnace by means of a hopper and pipe,
which, being set at an angle with the horizon, as it turns gradually
conveys the cement material in a tortuous path toward the lower and
hotter end, where it is discharged properly calcined. The material
having been fed into the upper end of the cylinder falls through the
flame to the lower side of it; the cylinder being in motion lifts it
on its advancing side, where it rests against one of its projecting
fins or ledges until it has reached such an angle that it shoots off
in a shower through the flame and falls once more on the lower side.
This again causes it to travel in a similar path, and every rotation
of the cylinder produces a like effect, so that by the time it arrives
at the lower and hotter end it has pursued a roughly helical path,
during which it has been constantly lifted and shot through the flame,
occupying about half an hour in its transit.
To some who have been accustomed to the more tedious process of kiln
burning, the time thus occupied may appear insufficient to effect the
combinations necessary to produce the required result; but it will be
seen that the conditions here attained are, in fact, those best suited
to carry out effectively the chemical changes necessary for the
production of cement. The raw material being in powder offers every
facility for the speedy liberation of water and carbonic acid, the
operation being greatly hastened by the velocity of the furnace gases
through which the particles pass. That such is practically the case is
shown by the following analysis of cement so burnt in the revolving
furnace or cylinder:
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