[Footnote 1: A paper recently read before the British
Association.]
By FREDERICK RANSOMS, A.I.C.E.
So much has been said and written on and in relation to Portland
cement that further communications upon the subject may appear to many
of the present company to be superfluous. But is this really so? The
author thinks not, and he hopes by the following communication, to
place before this meeting and the community at large some facts which
have up to the present time, or until within a very recent date, been
practically disregarded or overlooked in the production of this very
important and valuable material, so essential in carrying out the
great and important works of the present day, whether of docks and
harbors, our coast defenses, or our more numerous operations on land,
including the construction of our railways, tunnels, and bridges,
aqueducts, viaducts, foundations, etc. The author does not propose to
occupy the time of this meeting by referring to the origin or the
circumstances attendant upon the early history of this material, the
manufacture of which has now assumed such gigantic proportions--these
matters have already been fully dealt with by other more competent
authorities; but rather to direct the attention of those interested
therein to certain modifications, which he considers improvements, by
means of which a large proportion of capital unnecessarily involved in
its manufacture may be set free in the future, the method of
manufacture simplified, the cost of manipulation reduced, and stronger
and more uniformly reliable cement be placed within the reach of those
upon whom devolves the duty and responsibility of constructing works
of a substantial and permanent character; but in order to do this it
will be necessary to allude to certain palpable errors and defects
which, in the author's opinion, are perpetuated, and are in general
practice at the present day.
Portland cement is, as is well known, composed of a mixture of chalk,
or other carbonate of lime, and clay--such as is obtained on the banks
of the Thames or the Medway--intimately mixed and then subjected to
heat in a kiln, producing incipient fusion, and thereby forming a
chemical combination of lime with silica and alumina, or practically
of lime with dehydrated clay. In order to effect this, the usual
method is to place the mechanically mixed chalk and clay (technically
called slurry), in lumps varying in size, say, from 4 to 10 lb., in
kilns with alternate layers of coke, and raise the mass to a glowing
heat sufficient to effect the required combination, in the form of
very hard clinker. These kilns differ in capacity, but perhaps a fair
average size would be capable of producing about 30 tons of clinker,
requiring for the operation, say, from 60 to 70 tons of dried slurry,
with from 12 to 15 tons of coke or other fuel. The kiln, after being
thus loaded, is lighted by means of wood and shavings at the base,
and, as a matter of course, the lumps of slurry at the lower part of
the kiln are burned first, but the moisture and sulphurous gases
liberated by the heat are condensed by the cooler layers above, and
remain until the heat from combustion, gradually ascending, raises the
temperature to a sufficient degree to drive them further upward, until
at length they escape at the top of the kiln. The time occupied in
loading, burning, and drawing a kiln of 30 tons of clinker averages
about seven days. It will be readily understood that the outside of
the clinker so produced must have been subjected to a much greater
amount of heat then was necessary, before the center of such clinker
could have received sufficient to have produced the incipient fusion
necessary to effect the chemical combination of its ingredients; and
the result is not only a considerable waste of heat, but, as always
occurs, the clinker is not uniformly burnt, a portion of the outer
part has to be discarded as overburnt and useless, while the inner
part is not sufficiently burnt, and has to be reburned afterward.
Moreover, the clinker, which is of excessively hard character, has to
be reduced by means of a crusher to particles sufficiently small to be
admitted by the millstones, where it is ground into a fine powder, and
becomes the Portland cement of commerce.
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:
Per cent.
Carbonic acid, anhydrous 0.4
Sulphuric acid, anhydrous 0.26
Silica soluble 24.68
Silica insoluble 0.6
Alumina and oxide of iron 10.56
Lime 61.48
Magnesia, water, and alkalies 2.02
------
100
Again, fineness of the particles results in their being speedily
heated to a uniform temperature, so that they do not serve as nuclei
for the condensation of the moisture existing in the furnace gas. The
calcined material, on reaching the lower end of the furnace, is
discharged on to the floor or on to a suitable "conveyer," and removed
to a convenient locality for cooling and subsequent grinding or
finishing. It, however, is not in the condition of hard, heavy
clinkers, such as are produced in the ordinary cement kiln, which
require special machinery for breaking up into smaller pieces before
being admitted between the millstones for the final process of
grinding; nor does it consist of an overburnt exterior and an
underburnt core or center portion; but it issues from the cylindrical
furnace in a condition resembling in appearance coarse gunpowder, with
occasional agglutinations of small friable particles readily reduced
to fine powder in an ordinary mill, requiring but small power to work,
and producing but little wear and tear upon the millstones. The
operation is continuous. The revolver or furnace, once started, works
on night and day, receiving the adjusted quantity of powdered material
at the upper or feed end, and delivering its equivalent in properly
burnt cement at the opposite end, thus effecting a great saving of
time, and preventing the enormous waste of heat and serious injury to
the brickwork, etc., incidental to the cooling down, withdrawing the
charge, and reloading the ordinary kiln.
Cement, when taken from the furnace, weighed 110 lb. per bushel.
Cement, when ground, leaving 10 percent. on sieve with 2,500 holes to
the inch, weighed 121 lb. per bushel, and when cold 118 lb. per
bushel. When made into briquettes, the tensile breaking strain upon
the square inch:
At 4 days was 410 lb. per square inch.
At 6 days " 610 " " " "
At 14 days " 810 " " " "
At 49 days " 900 " " " "
At 76 days " 1,040 " " " "
A cylindrical furnace, such as the author has described, is capable of
turning out at least 20 tons of good cement per day of twenty-four
hours, with a consumption of about 3 tons of slack coal. It will be
readily understood that these furnaces can be worked more economically
in pairs than singly, as they can be so arranged that one producer may
furnish a sufficient quantity of gas for the supply of two cylinders,
and the same labor will suffice; but in order to provide for possible
contingencies the author advises that a spare gas producer and an
extra furnace should be in readiness, so that by a simple arrangement
of valves, etc., two cylinders may always be in operation, while from
any cause one may be undergoing temporary repairs, and by this means
any diminution in the output may be avoided.
The author considers it unnecessary here to discuss either the
advantages or the economy of fuel effected by the employment of gas
producers for such a purpose. These have been abundantly proved in
steel and glass making industries, where a saving of from 50 to 70 per
cent. of the fuel formerly employed has been made. Their cost is
small, they occupy little room, they can be placed at any reasonable
distance from the place where the gas is to be burnt; any laborer can
shovel the slack into them, and they do not require constant skilled
supervision. It is claimed by the author of this paper that the
following are among the many advantages derivable from the adoption of
this method of manufacturing Portland cement, as compared with the old
system:
(1) Economy of space--the furnaces, with their appurtenances,
requiring only about one-fourth the space of what would be
occupied by the ordinary kilns for producing the same quantity
of finished cement.
(2) Continuous working, and consequent economy of fuel lost by
cooling and subsequent reheating of the kiln walls.
(3) Economy of repairs, which are of a simple and comparatively
inexpensive character, and of much less frequent occurrence, as
the continuous heat avoids the racking occasioned by the
alternate heating and cooling.
(4) Economy in first cost.
(5) Economy in grinding, a friable granular substance being
produced instead of a hard clinker, whereby crushers are quite
abolished, and the wear and tear of millstones greatly
reduced.
(6) Economy of labor, the conveyance to and removal from, the
revolving furnace being conducted automatically by mechanical
elevators and conveyers.
(7) Improved quality of the cement, from non-mixture with
fuel, ash, or other impurities, and no overburning or
underburning of the material.
(8) Thorough control, from the facility of increasing or
diminishing the flow of crushed slurry and of regulating the
heat in the furnace as desirable.
(9) Absence of smoke and deleterious gases.
It is well known that in some localities the materials from which
Portland cement is made are of such a powdery character that they have
to be combined or moulded into balls or bricks previous to calcination
in the ordinary way, thus entailing expense which would be entirely
obviated by the adoption of the patent revolving furnace, as has been
proved by the author in producing excellent cement with a mixture of
slag sand from the blast furnaces of the Cleveland iron district, with
a proper proportion of chalk or limestone, which, in consequence of
the friable nature of the compound, he was unable to burn in the
ordinary cement kiln, but which, when burnt in the revolving furnace,
gave the most satisfactory results. The cement so made possessed
extraordinary strength and hardness, and it has been a matter of
surprise that iron masters and others have not adopted such a means of
converting a waste material--which at the present time entails upon
its producers constant heavy outlay for its removal--into a
remunerative branch of industry by the expenditure of a comparatively
small amount of capital. The demand for Portland cement has increased
and is still increasing at a rapid ratio. It is being manufactured
upon a gigantic scale.
Great interests are involved; large sums of money are being expended
in the erection and maintenance of expensive plant for its production;
and the author submits that the development of any method which will
improve the quality and at the same time reduce the cost of
manufacture of this valuable material will tend to increase the
prosperity of one of our great national industries, and stimulate
commercial enterprise. Works are in progress for manufacturing cement
by this improved process, and the author trusts the time is not far
distant when the unsightly structures which now disfigure the banks of
some of our rivers will be abolished--the present cement kilns, like
the windmills once such a common feature of our country, being
regarded as curiosities of the past, and cement manufacturers cease to
be complained of as causing nuisances to their neighbors.
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