Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02361933 2001-08-O1
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Process for Producina Marl Slaa
The invention relates to a process for producing marl slaps
and marl slag cements or mixed components for mixed cements
from marl having a basicity Ca0/Si02 of < 2Ø
When producing cement, calcareous and argillaceous stones, in
particular limestone and lime marl, are used as starting
materials and burned to cement clinker. The lime contained in
the starting materials is completely bound to silicic alumina
and optionally iron, thus forming the cement minerals usual to
portland cement clinker, such as alite, belite, brown
millerite and glass. In order to ensure the desired sintering
to cement clinker at comparatively low temperatures of about
1350°C, relatively high-quality starting materials and, in
particular, highly calcareous lime marl are required. Lime
marl, which usually has a basicity of between 3 and 4 and
besides lime also contains Si02, A1203 and iron oxides,
however, occurs in nature not only in the form of relatively
high-quality and largerly pure lime marl, but rather as a
common or low-quality marl in substantially higher amounts.
The marl that is substantially more wide-spread is
characterized by basicities of between 0.8 and 2 and
frequently is found also in the form of argillaceous marl.
Those comparatively low-grade, yet substantially more wide-
spread starting products in conventional cement production
processes cannot be used without expensive purification and
lime enrichment procedures and are available in large amounts
as cheap raw materials.
Slag cements and, in particular, blast furnace slag cements
likewise exhibit hydraulic properties and it has already been
demonstrated that, by optimizing the slag chemistry and, in
particular, by adjusting basicities and aluminate contents as
well as applying special activation procedures, metallurgical
slaps will be improved to the extent that they correspond to a
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strength development in concrete and are at least equivalent
to clinker cement.
The present invention aims to render wide-spread and cheap raw
material marls having low basicities apt for economic
utilization in the production of slag cements or mixed
components for mixed cements while, at the same time, allowing
for the adaptation of the desired product qualities to the
respective requirements to a high degree. To solve this
object, the process according to the invention essentially
consists in that in a first process step argillaceous marl or
a mixture of marl and clay having a basicity of < 2.0 is
dried, preheated and calcined and that, after this, the
obtained product in a second process step is melted in a
separate melting furnace at higher temperatures than applied
in the first process step and is granulated from the melt.
Calcination is an endothermic reaction, whereas melting
constitutes an exothermic reaction. Due to the fact that
argillaceous marl, i.e. relatively low-grade marl, having an
elevated A1203 content or a mixture of low-quality marl and
clay having a basicity of below 2 is dried, preheated and
calcined in a first process step, it is initially safeguarded
that the high amounts of COZ released during calcining need
not be heated to the temperatures usually required in cement
production for sintering. The calcination of hydrate, sulfur
and carbonate compounds is, thus, carried out at comparatively
low temperatures such that the large gas amounts released
occur at accordingly low temperature levels, thus enhancing
the thermal efficiency and hence the economy of the process.
On account of the relatively low purities of the starting
substances used, calcination takes place already at lower
temperatures than would be the case with highly pure calcium
carbonates. Due to the fact that the obtained product in a
second process step is subsequently melted in a separate
melting furnace at higher temperatures than in the first
process step, there is the possibility to subsequently correct
the composition of the melt by any means whatsoever and it is
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feasible, by appropriately granulating the melt, to provide
the desired glass portion and ensure that any undesired
crystallization will be largely avoided during cooling.
Drying, preheating and calcining in a first process step at
accordingly low temperatures, moreover, allows for the use of
substantially more coarse-grained charging substances as
compared to known cement clinker sintering processes such that
raw material preparation and, in particular, grinding will not
be required, as a rule, but the coarse charging stock merely
will have to be finely broken. The process according to the
invention also calls for a substantially lower heat
consumption than the clinker process.
Advantageously, the process according to the invention is
carried out in a manner that the first process step is
realized in a suspension type heat exchanger, a rotary tubular
kiln, a multiple-hearth furnace or a shaft furnace, or in a
fluidized bed or cyclone preheating unit. In a particularly
advantageous manner, a suspension type heat exchanger may be
employed, whereby it is feasible, in particular if a rotary
tubular kiln or shaft furnace is employed in the first step,
to choose an even coarser particle size of the charging stock,
drying, preheating and calcining of a charging material having
particle sizes of, for instance, up to 40mm being readily
feasible.
In a particularly advantageous manner, the second process step
is carried out in a melting cyclone, a rotary tubular kiln or
a hearth-type furnace, or in an iron melting oxidation
reactor, whereby even the formation of foamed slag may be
advantageous if a meltdown oxidation reactor is employed.
What is essential in obtaining the desired cement
technological properties, after all, is the adjustment of the
slag basicity of the target slag, it being advantageously
proceeded in a manner that the target slag is adjusted to a
basicity Ca0/SiOz of between 0.9 and 1.85 by mixing marl and
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clay. If, at the same time, an A1203 content of between 6 and
20 wt.-~ is aimed at, a high-quality synthetic blast furnace
slag will be obtained, such A1203 contents being obtainable in
a particularly simple manner by using argil.laceous marls. When
using other marl qualities for the main component, the desired
slag chemistry may be adjusted by the aid of bauxite, clays,
flue ashes or other industrial waste substances such as, for
instance, red muds, sweepings, corundum-containing grinding
dusts or refractory break-offs.
What is also essential for obtaining the desired cement
technological properties is, of course, accordingly rapid
cooling so as to prevent the formation of crystals. Depending
on the basicity of the slag, the slag is characterized by
different viscosities, wherein the slag becomes highly
viscous, in particular, at basicities of above 1.4, and the
formation of crystals can no longer be reliably prevented in
conventional granulation processes such as, for instance,
during granulation in hot water. In the context of the
invention it is, therefore, advantageously proceeded in a
manner that the melt at basicities of > 1.4 is sprayed into a
granulator and, in particular, a vapor granulator. Spray
granulation, in which the cooling of the molten droplets in
most cases is effected by nozzling in water or water vapor,
may be substantially improved even further in that hydrocarbon
is additionally nozzled into the spray granulator. The thermal
decomposition of hydrocarbon withdraws heat from the sprayed
droplets at a cooling gradient of 104 to 105 K/s while
simultaneously forming high-quality synthesis gas, which, as
in correspondence with a preferred further development of the
process according to the invention, can be burned in the first
process step, because the actual heat demand for calcination
arises there.
Since, due to the relatively low purity of the starting
materials, calcination takes place at relatively low
temperatures, the process according to the invention
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advantageously is carried out in a manner that the first
process step is carried out at temperatures of up to 950° to
1000°C, drying being effected at temperatures of from 100 to
210°C, preheating being effected at 210° to 600°C and
calcining being effected at 600° to 1000°C.
In the second process step, in which the slag is melted in
order to subsequently solidify under the formation of a glassy
consistency, it is advantageously proceeded in a manner that
operation takes place at final temperatures of between 1450°
and 1550°C.
As already mentioned, the process according to the invention
stands out for obviating any cumbersome raw material
preparation and, in particular, for not requiring any grinding
of the starting materials. Advantageously, the process
according to the invention is carried out in a manner that the
first process step is realized with finely broken marl having
a mean particle size ranging from 20mm to 30mm.
In order to further enhance the cement technological
properties, it may be proceeded according to the invention in
that by-pass dust from the production of clinker is added to
the charging material. Since a slag melt is produced within
the context of the process according to the invention, it is
thus feasible to introduce into the melt the high alkali
content contained in the clinker production by-pass dust, a
thus formed product being particularly reactive and
characterized by high early strengths. Clinker raw materials
as are used in the conventional cement production process, as
a rule, are characterized by high alkali contents, and the
processing of such alkali-rich raw materials, as a rule,
constitutes a problem of disposal, which can be solved by the
process according to the invention. Also the use of other
industrial alkali-containing waste substances, for instance,
those from paper and cellulose pulp production, which
partially include also high portions of A1203, as well as of
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organic matter such as lignin, whose calorific value may be
utilized, is advantageous.
If the melt is to be subjected to a conventional water
granulation procedure, its basicity must be lowered to below
1.4, whereby in those cases the A1203 content of the target
slag must be increased accordingly in order to achieve a march
of strength similar to clinker, of the end product.
In the context of the process according to the invention, the
use of charging materials having elevated dolomite contents
has no adverse effects, either, it merely having to be taken
care that the Mg0 portion of the charging material is adjusted
to below 19 wt . -~ . Up to those amounts, magnesium oxide is
able to enhance the granulating capacity due to the reduced
slag viscosity, which will lead to an increased portion of
amorphous or glassy particles.
In the following, the process according to the invention will
be explained in more detail by way of exemplary embodiments.
Example 1
A marl slag is produced of the charging materials marl and
clay. The starting materials had the following directional
analysis:
CA 02361933 2001-08-O1
Com onent (~) Marl Cla
Loss on 32.5 12.5
ignition
Si02 23 46
A1203
4 24
Fe2O3
2 15
Ca0 33 0.72
Mg0 3 0.33
S03 1 -
K20 0.5 -
Na20 0.2 0.1
Ti02 0.4 1
Sum 99.6 99.7
C/S 1.4348 0.0157
The starting materials were supplemented with clinker furnace
by-pass dust, whereby the early strength of the marl slag was
significantly increased.
In order to adjust the slag basicity Ca0/Si02 to below 1.4 so
as to apply conventional water granulation, the portions of
the two components in the mixture were determined as follows
with a target basicity of 1.1 having been sought:
(C/S) ~ Si02 (clay) - Ca0 (clay)
Marl portion (x) -
Ca0 (marl) - (C/S) ~ Si02 (marl)
1.1 ~ 46 - 0.72
Marl portion - - 6.48
33 - 1.1 ~ 23
Thus, one part of clay was mixed with 6.48 parts of marl,
which resulted in a mixture of 86.6 wt.-~ marl and 13.4 wt.-~
clay. After this, the crude mixture for the intended marl slag
had the following composition:
CA 02361933 2001-08-O1
Marl
sla
Com onent Portion
Si02 37.4
A1203 9.6
Fe203 5
Ca0 41.2
Mg0 3.8
S03 1.2
K20 0.6
Na20 0.3
Ti02 0.7
S~ 99.8
C/S 1.1
After drying at 100° to 210°C, preheating at 210° to
600°C and
calcining at 600° to 950°C, the dried and calcined product was
drawn off a suspension type heat exchanger and introduced into
a melting cyclone. Within the melting cyclone, the temperature
was adjusted to between 1450° and 1550°C, whereupon the melt
was granulated in hot water.
In order to reduce the nitrogen oxide portion, it is also
feasible to realize a progressive postcombustion within the
suspension type heat exchanger by blowing in combustion air,
thereby improving the thermal efficiency. The fuel used in the
melting cyclone in an alternative process conduct, in which
the melt was ejected into a spray granulator, was formed by
using hydrocarbons during granulation, the cracked gas or
synthesis gas formed having been used as a fuel together with
air in the melting cyclone. Moreover, this method of
granulation caused the iron oxide of the slag to be
practically completely metallized and separated by a
subsequent magnetic separation of the slag.
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Example 2
Using a rotary tubular kiln in the first process step and a
hearth-type furnace in the second process step, finely broken
marl having a particle size of about 25mm was used. The marl
slag had the following composition:
Marl
sla
Com onent Portion
Si02 34.6
A12o3 6
Fe203 3
Ca0 49.6
Mg0
K20 0.7
Na20 0.3
Ti02 0.6
Sum 96.8
Due to the relatively high basicity of 1.43, the slag was
disintegrated by spraying into a spray granulator, whereby a
high portion of fine granulates was obtained. On account of
the high basicity, the high portion of fine granulates was
characterized by a particularly high hydraulic activity. Such
___-~___~g ~~3n~-_._a ra~~~.~y_.~~~.~_~omLd n~ 1 nnger~ _
successfully granulated with hot water, since the portion of
fines, which is important from a cement technological point of
view, was hydraulically inactivated on account of the belite
crystal formation.
