Process for the Production of Cement from Limestone and Blast- furnace Slag.
Year: 1898

page_text: Date of Application, 27th Dec., 1898-Accepted, 25th Feb.,
1899
COMPLETE SPECIFICATION
Process for the Production of Cement from Limestone and Blast-
furnace Slag.
I, CARL VON FORELL, of Giessen, in the Empire of Germany, Director of a
Manufactory, do hereby declare the nature of this invention, and in what manner
the same is to be performed, to be particularly described and ascertained in and
by the following statement..
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Various processes have already been suggested for utilising slag from blast-
furnaces in the manufacture of cement The essential feature of all such processes
is that triturated slag is thoroughly mixed with limestone in the same form and
burned at a temperature which is not excessive In connection with these processes
attempts have also been made to desulphurate the slag by treating it with car-
bonio acid in the presence of moisture under a special process None of these
methods however have hitherto been adopted in practice to any appreciable
extent, on the one hand, because the quality of the cement obtained 18 in many
respects far from satisfactory, and on the other hand, the cost of production is
too great
If a cheap slag cement is to be produced, care must be taken to combine the
desulphuration of the slag with the process of burning in such a way that the
carbonic acid set free from the limestone, simultaneously expels the sulphur, so
that a special operation for this purpose is not necessary. As however, for the
burning of the limestone mixed with the slag, a certain degree of temperature,
as well as a certain period of time, is requisite, and the final burning of the cement
necessitates a different degree of temperature and a different burning period, it
results in practice that the preliminary burning of the limestone and slag cannot
be combined with the final burning, that is to say, the former is not effected in
the upper parts of the furnace and the latter in the lowest parts of the same, it
being necessary to keep the two operations separate from each other and in such
a manner that, in the preliminary burning, all that is effected is that the carbonic
acid is expelled from the limestone and the sulphur out of the slag, but without
the burnt lime acting on the slag to any appreciable extent at the same time, na
otherwise, not only will the material be baked (become stiff) and the furnace
choked when the preliminary burning is effected, but by dividing the chemical
process, which properly belongs to the final burning operation, between two
separate buining operations, difficulties will be created in supervising the working
and in producing a constantly uniform cement
Triturated slag (slag in the form of powder or sand, etc) is mixed with lime-
stone in pieces, placed in a furnace arranged for continuous working and heated
so that the temperature rises gradually fiom less than 100° C at the mouth of
the furnace, until it reaches a degice sufficient for buining the limestone (that is,
a bright red heat) In this way three different burning areas are produced in the
furnace, in each of which the mixture of limestone and slag under roes a chemical
change which is important for the production of the final product
In the first (the coolest) area, the temperature of which is below 100° C, the

page_text: watery vapors coming from the hotter parts of the furnace, are partially condensed,
so that, by means of the condensed water, in combination with any such moisture
as still remains in the slag, after the triturating or granulating thereof, the
material to be burned will be kept relatively moist in the first area
In con-
sequence the material exercises a powerful absorbing effect on the carbonic acid
coming from the hottest area and retains it The latter 18 thus brought into
intimate contact with the slag and sets up a chemical action therem by decom-
posing the sulphur compounds and setting free the sulphide of hydrogen, which
is traceable as an educt in the furnace gases and then passes into the slag, forming
carbonates in doing so The desulphulation is important in connection with the
solidity of the cement In the chemical alterations which occur in the first
heating area, the limestone takes no pait
If the materal to be burned, and which is charged with moisture, be removed
from the first area to the second, it will here fust be dried, that is to say, the
watery vapour will be expelled, which is partially precipitated (condensed) again
in the first area, so as to be employed again as a solvent for the carbonic acid
The further the material advances in the second heating area, the more effective
will be the drying, which now takes the form of calcining, that 18, the (hydrate)
water taken up by the slag, and in chemical combination, is expelled In this
second area also, the limestone takes no part in the chemical changes which are
effected
In the third area, that in which the temperature, as stated, is raised to a bright
red heat, the carbonic acid is partially driven out of the limestone, the former,
as mentioned, serving in the coolest area as the means for effecting desulphuration
However on account of the burnt lime being in piece form, the chemical action
of the former on the slag 18 purely superficial and should not be at all penetrating,
as this operation must be effected by means of a separate burning process, which,
as already stated, on account of the limits of temperature to be observed, as well
as the duration of the burning period, cannot be combined with the three opera-
tions mentioned, that is, carried out with them in a common (mutual) furnace
When therefore the preliminary burning of the triturated slag with the lime-
stone in piece form is completed, the material is removed and after being reduced
(powdered) is finally burnt.
In addition to an effective and inexpensive desulphuration, the present process
has the further advantage that, after undergoing the preliminary burning, the
material can be easily powdered, so that in this respect, there is a considerable
saving in the cost of production.
Having now particularly described and ascertained the nature of my said
invention, and in what manner the same is to be performed, I declare that what
I claim is
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Process for producing cement from limestone and alag, in connection with which
limestone in piece form and triturated slag (slag in the form of powder, sand, etc)
are burned together in a moist condition in such a manner that the carbonic acid
set free from the limestone drives the sulphur out of the slag, whereupon the
material is reduced to powder and finally burned at a high temperature in the
known manner.