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IN this remarkable invention, which has
brought about a striking innovation in a
long-established business, we see another
characteristic instance of Edison's incisive
reasoning and boldness of conception carried into
practical effect in face of universal opinions to
the contrary.
For the information of those unacquainted with
the process of manufacturing Portland cement,
it may be stated that the material consists
preliminarily of an intimate mixture of cement
rock and limestone, ground to a very fine
powder. This powder is technically known in the
trade as "chalk," and is fed into rotary kilns
and "burned"; that is to say, it is subjected
to a high degree of heat obtained by the
combustion of pulverized coal, which is injected
into the interior of the kiln. This combustion
effects a chemical decomposition of the chalk,
and causes it to assume a plastic consistency and
to collect together in the form of small
spherical balls. which are known as
"clinker." Kilns are usually arranged with a
slight incline, at the upper end of which the
chalk is fed in and gradually works its way down
to the interior flame of burning fuel at the
other end. When it arrives at the lower end,
the material has been "burned," and the
clinker drops out into a receiving chamber
below. The operation is continuous, a constant
supply of chalk passing in at one end of the kiln
and a continuous dribble of clinker-balls
dropping out at the other. After cooling, the
clinker is ground into very fine powder, which
is the Portland cement of commerce.
It is self-evident that an ideal kiln would be
one that produced the maximum quantity of
thoroughly clinkered material with a minimum
amount of fuel, labor, and investment. When
Edison was preparing to go into the cement
business, he looked the ground over thoroughly,
and, after considerable investigation and
experiment, came to the conclusion that
prevailing conditions as to kilns were far from
ideal.
The standard kilns then in use were about sixty
feet in length, with an internal diameter of
about five feet. In all rotary kilns for
burning cement, the true clinkering operation
takes place only within a limited portion of
their total length, where the heat is greatest;
hence the interior of the kiln may be considered
as being divided longitudinally into two parts or
zones--namely, the combustion, or
clinkering, zone, and the zone of oncoming raw
material. In the sixty- foot kiln the length
of the combustion zone was about ten feet,
extending from a point six or eight feet from the
lower, or discharge, end to a point about
eighteen feet from that end. Consequently,
beyond that point there was a zone of only about
forty feet, through which the heated gases
passed and came in contact with the oncoming
material, which was in movement down toward the
clinkering zone. Since the bulk of oncoming
material was small, the gases were not called
upon to part with much of their heat, and
therefore passed on up the stack at very high
temperatures, ranging from 1500 degrees to
1800 degrees Fahr. Obviously, this heat
was entirely lost.
An additional loss of efficiency arose from the
fact that the material moved so rapidly toward
the combustion zone that it had not given up all
its carbon dioxide on reaching there; and by the
giving off of large quantities of that gas within
the combustion zone, perfect and economical
combustion of coal could not be effected.
The comparatively short length of the
sixty-foot kiln not only limited the amount of
material that could be fed into it, but the
limitation in length of the combustion zone
militated against a thorough clinkering of the
material, this operation being one in which the
elements of time and proper heat are prime
considerations. Thus the quantity of good
clinker obtainable was unfavorably affected. By
reason of these and other limitations and
losses, it had been possible, in practice, to
obtain only about two hundred and fifty barrels
of clinker per day of twenty-four hours; and
that with an expenditure for coal proportionately
equal to about 29 to 33 per cent. of the
quantity of clinker produced, even assuming that
all the clinker was of good quality.
Edison realized that the secret of greater
commercial efficiency and improvement of quality
lay in the ability to handle larger quantities of
material within a given time, and to produce a
more perfect product without increasing cost or
investment in proportion. His reasoning led him
to the conclusion that this result could only be
obtained through the use of a kiln of
comparatively great length, and his
investigations and experiments enabled him to
decide upon a length of one hundred and fifty
feet, but with an increase in diameter of only
six inches to a foot over that of the sixty-
foot kiln.
The principal considerations that influenced
Edison in making this radical innovation may be
briefly stated as follows:
First. The ability to maintain in the kiln a
load from five to seven times greater than
ordinarily employed, thereby tending to a more
economical output.
Second. The combustion of a vastly increased
bulk of pulverized coal and a greatly enlarged
combustion zone, extending about forty feet
longitudinally into the kiln--thus providing an
area within which the material might be
maintained in a clinkering temperature for a
sufficiently long period to insure its being
thoroughly clinkered from periphery to centre.
Third. By reason of such a greatly extended
length of the zone of oncoming material (and
consequently much greater bulk), the gases and
other products of combustion would be cooled
sufficiently between the combustion zone and the
stack so as to leave the kiln at a comparatively
low temperature. Besides, the oncoming
material would thus be gradually raised in
temperature instead of being heated abruptly, as
in the shorter kilns.
Fourth. The material having thus been greatly
raised in temperature before reaching the
combustion zone would have parted with
substantially all its carbon dioxide, and
therefore would not introduce into the combustion
zone sufficient of that gas to disturb the
perfect character of the combustion.
Fifth. On account of the great weight of the
heavy load in a long kiln, there would result
the formation of a continuous plastic coating on
that portion of the inner surface of the kiln
where temperatures are highest. This would
effectively protect the fire-brick lining from
the destructive effects of the heat.
Such, in brief, were the essential principles
upon which Edison based his conception and
invention of the long kiln, which has since
become so well known in the cement business.
Many other considerations of a minor and
mechanical nature, but which were important
factors in his solution of this difficult
problem, are worthy of study by those intimately
associated with or interested in the art. Not
the least of the mechanical questions was settled
by Edison's decision to make this tremendously
long kiln in sections of cast-iron, with
flanges, bolted together, and supported on
rollers rotated by electric motors.
Longitudinal expansion and thrust were also
important factors to be provided for, as well as
special devices to prevent the packing of the
mass of material as it passed in and out of the
kiln. Special provision was also made for
injecting streams of pulverized coal in such
manner as to create the largely extended zone of
combustion. As to the details of these and many
other in- genious devices, we must refer the
curious reader to the patents, as it is merely
intended in these pages to indicate in a brief
manner the main principles of Edison's notable
inventions. The principal United States
patent on the long kiln was issued October
24, 1905, No. 802,631.
That his reasonings and deductions were correct
in this case have been indubitably proven by some
years of experience with the long kiln in its
ability to produce from eight hundred to one
thousand barrels of good clinker every
twenty-four hours, with an expenditure for coal
proportionately equal to about only 20 per
cent. of the quantity of clinker produced.
To illustrate the long cement kiln by diagram
would convey but little to the lay mind, and we
therefore present an illustration (Fig. 1)
of actual kilns in perspective, from which sense
of their proportions may be gathered.
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