Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to a process for the
production of sintered magnesite, sintered dolomite or the
like, the raw material first being size-reduced to the fineness
of meal, subsequently pre-calcined in loosened form with hot
gases in a first heat-treatment stage and compacted while still
hot, after which the compacted material is sintered in a
second heat-treatment stage. The invention also relates to an
installation for carrying out this process.
Refractory bricks, monolithic mixtures etc. of
sintered magnesite, sintered dolomite and the like are required
in the steel industry, in the cement industry and in other
branches of industry where heat treatments are carried out
at relatively high temperatures (for example above 1500 C).
On account of the need for refractoriness and resistance
to slag, sintered materlals of increasingly higher quality
are required, the trend being towards increasingly purer
materials which, unfortunately, can only be sintered at
relatively high temperatures (above 1800C) and under
difficult conditions.
In known processes, the raw material obtained from
quarries is first size-reduced, subsequently pre-calcined
in a first heat-treatment stage (so-called kauster calcination
with elimination of CO~) and then at least partly cosled,
ground and compacted, after which the compacted material is
sintered in a second heat-treatment stage. In this two-stage
process, ~haft furnaces, travelling grates or even revolving
tubular kilns are normally used for the first heat-treatment
stage whereas revolving tubular kilns are predominately used
for the second heat-treatment stage.
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The main disadvantages of these known p~ocesses
lie in the fact that only a very limited particle size range
of the ra~ material can be processed in the heat exchangers
used for the first heat-treatment stage, in the fact that pre-
calcination and compaction have to be followed by cooling
to enable grinding to be carried out and in the fact that the
grinding and compaction of the cooled material involve the
danger of partial hydration which adversely affects the
sin~ering operation.
By contrast, in a process developed by Applicants,
the raw material is size-reduced to the fineness of meal before
the first heat-treatment stage, subse~uently pre-calcined in
suspended or fluidised form (i.e. in loosened form with hot
gases) in the first heat-treatment stage and then compacted
while still hot. In this way, therefore, the raw materlal
is size-reduced sufficiently finely before any heat treatment,
after which the entire slze-reduced raw materlal is pre-
calcined in loosened form with the hot gases which itself
leads to an extremely uniform and, by comparison with the
above-described processes, much more economical pre-calcination
of the raw material.
Another advantage of this process lies in the
fact that, before compaction, the adequately siæe-reduced
and pre-calcined ra~ material is then pressed~or compacted
/ while still hot, i,e. without any intermediate cooling and
size-reduction, which gives better pressings and higher
press outputs by comparison wlth the known processes
described above.
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The object of the present invention is to further
develop the earlier process to the extent that, in particular,
the compaction level and also the density and strength of
the pressings obtained are further increased.
According to the invention, this object is achieved
in that, before compaction, the pre-calcined material is
intermediately stored, de-aerated and pre-consolidated.
In the extensive tests on which the present
invention is based, it was found that, in the first heat-
treatment stage, the raw material size-reduced to the fineness
of meal is generally "aerated" to a considerable extent by
the hot gases in which it is treated in loosened form, in
other words it is heavily permeated by the hot treatment
gases (for example the waste kiln gases from the second
heat-treatment stagej. As a result, the pre-calcined
material is loosened up to a very considerable extent, giving
a weight per litre of from about 0.4 to 0~7 kg/l depending
on its fineness. However, this highly loosened pre-calcined
material is not conducive either to a relatively high
compaction level or to relatively intense consolidation
and hardening of the pressings to be formed. In these tests,
it ~as found that both the compaction level and also the
consolidation and hardening of the pressings bear a certain
proportionality to the weight per litre of the pre-calcined
material, in otherwords they decrease with decreasing welght
per litre, i.e. to highly loosened ~aterial.
No~, in the process according to one aspect of
the invention, the precalcined material is quite deliberately
subjected to intermediate storage and, at the same time,
de-aerated and pre-consolidated, de-aeration generally being
understood to mean the removal of the treatment gases still
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present in the ma-terial from the pre-calcination step.
Greatly increased weigh-ts per litre of the pre-calcined mater-
ial are obtained by this de-aeration and pre-consolidation,
thereby establishing the essential pre-requisites for
increasing the compaction level and improving the density
and strength of the pressings.
It has proven -to be particularly favourable to
intermediately store the pre-calcined material for a period
of from about 30 to 90 minutes and preferably for a period of
from ~5 to 80 minutes. The intermediate storage period is
essentially determined by the fineness of the material to
be treated.
A particularly favourable pre-condition for the
required incrqase in the compaction level is established by
pre-consolidating the pre-calcined material to a powder density
of from about 0.85 to 1.~5 kg/l during its intermediate
storage~
In general, the pre-consolidation of the
pre-calcined material during its intermediate storage~may
be achieved simply by appropriate and adequate de-aeration.
However, in order to obtain particularly high throughputs,
it has proved to be of particular advantage to intensify
pre-consolidation of the pre-calcined material during its
intermediate storage by applying vibration.
The 1ntermediate storage of the pre-calcined
material affords the possibility of another significant process
advantage which lies in the fact that the decomposition
temperature in the first heat-treatment stage can be kept at a
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lower level than would be possible without intermediate
storage or with excessively brief or unsuitable intermediate
storage. Thus, whereas in known processes, calcination is
carried out either a-t extremely high temperatures (for
example above 1100C), which in many cases adversely
affects the pre-calcined material, or at relatively low
temperatures (for example 850 to 900C), at which the
specific throughput is reduced and residual ignition losses
have to be accepted, the raw material in the process according
to the invention is pre-calcined at a temperature of from
about 850 to 1000C and preferably at a temperature in the
range from about 900 to 950C in the first heat-treatment
stage. As already mentioned, however, pre-calcination at
these temperatures is onIy possible if the pre-calcined
material is intermediately stored in accordance with the
invention. During this pre-calcination, the temperature
is kept above the theoretical decomposition temperature of
the particular material being treated, but distinctly below
the temperature level at whch the pre-calcined material
(so-called kauster) would undergo negative property changes
such as, for example, impaired sinterability, delayed
~uenching etc. Although this pre~calcination at the rela
tively low temperatures in question is accompanied by certain
residual ignition losses, such residual ignition losses as
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occur can be recouped by the intermediate storage according ~
.
to the invention. During this intermediate storage, the
gaseous decomposition products are quickly removed (b~ the
de-aeration).
According to the present invention, then, there is
provided a process for the production of re~ractory
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materials from feedstock including the steps of size-
reducing the feedstock into a fine grained material, calcina-
ting the fine grained materials with hot gases, storing the
calcinated materials to effect the de-aeration and settling
thereof, subsequently compacting the material while still
hot, and sintering the compacted materials.
An installation for carrying out the process
according to the invention contains at least one size-
reducing unit, a hot-gas heat-exchanger unit as the first
heat-treatment stage for pre-calcin~ing the size-reduced raw
material, a compacting unit, an insulated intermediate
vessel arranged between the hot-gas heat-exchanger unit and
the compacting unit and a revolving tubular kiln following
the compacting unit as the second heat-treatment stage.
According to ~he invention, the intermediate vessel is
designed for intermediately storing, de-aerating and pre-
consolidating t~e pre-calcined material.
According to a preferred embodiment of the
invention, an intermediate vessel of the type in question may
have a straight, preferably cyIindrical upper part and a
funnel-shaped lower part.
In the tests on which the invention is based, it was
also found that the intermediate vessel provides for
particularly effective de-aeration and pre-consolidation of the
pre-calcined material during its intermediate storage
providing its straight upper part has a height of less than
about 2.5 metres, preferably less than about 1~5 metres, the
height of the straight part of the intermediate vessel being
essentially determined by the fineness of the pre-calcined
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material. This height of the straight upper part is gauged in
such a way that, in the intermediate vessel the gas trapped
in the hot material is ab]e to escape upwards, even from the
lower parts of the material, without being impeded in any way
by the pressure of the overlying materal.
In order to accelerate de-aeration and pre-
consolidation during intermediate storage, particularly in the
case of high-throughput installations, it has also proved to
be of advantage to associate a vibration unit with the intermed-
iate vessel. Providing the vibration unit is suitably
designed and arranged, the height of the straight upper part
of the intermediate vessel discussed in the preceding paragraph
may even considerably exceed 2.5 metres.
According to a further aspect of the present
invention, then, there is also provided an apparatus for
producing refractory materials from feedstock comprising size
reducing means to grind the feedstock into a fine grained
material, heating means for calcinating the fine grained material
with hot gases, insulating receptacle means to temporarily ~
store the calcined material, compacting means for subsequently ;;
compacting the stored material, and means to sinter the
compacted material to form the refractory materials, wherein
the calcined materials are allowed to settle and de-aerate
during the temporary storage thereof.
According to yet another aspect of the present
invention, there is also provided an insulating receptacle ~or
use with apparatus for producing refractory materials from
feedstock including size reducing means to grind the feedstock
into a fine-grained material, heating means for calcinating the
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fine-grained material, compacting means and sintering means,
the receptacle comprising a substantially cylindrical
upper portion adjoining a lower, funnel-shaped portion to
define a hopper-shaped vessel for receiving and temporarily
storing the calcined material to allow for the de-aeration and
settling thereof prior to compaction.
Embodiments of the invention are described by way
of the example in the following with reference to the
accompanying drawings, wherein:
Yigure 1 is a diagrammatic general view of an
installation for carrying out the process according to the
invention.
Figure 2 is a simplified vertical section through
an intermediate vessel provided in accordance with the
invention.
Figures 3 and 5 are partial elevations of various
embodiments of vibration bars which may be axranged in the
intermediate vessel.
Figure 6 is a vertical section through another
embodiment of an intermediate vessel (with a vibration cage)
provided in accordance with the invention.
Figure 7 is a partial cross-section on the line
VII-VII through the intermediate vessel shown in Figure 6.
Looking in the direction of movement of the~
material to be treated, the installation for carrying out the
process according to the invention which is illustrated in
Figure 1 includes a preliminary crusher 1, a fine-grinding
unit, for example in the form of a ball mill 2, following the
preliminary crusher 1 and connected thereto by means of a
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valve 6 and conduit 7, a conduit 8 leading to a hot-gas
heat-exchanger unit 3 (as the first heat-treatment stage), a
compacting unit formed for example by a briquetting press 5 of
known type and a revolving tubular kiln 4 following the briquet-
ting press 5 as the second heat-treatment stage. Tubular
kiln 4 is supported by a stationary housing 9 which
includes an inclined chute (not shown) ~or the supply of
material. Between the hot-gas heat-exchanger unit 3 and
the briquetting press 5 there is an insulated intermediate
vessel 20 which is designed for intermediately storing,
de-aerating and pre-consolidating the material pre-calcined
in the heat-exchanger unit 3, as will be explained in detail
hereinafter.
So far as the hot-gas heat-exchanger unit 3 is
concerned, it is pointed out that, according to the invention,
it may with advantage be formed either by a multistage
cyclone heat exchanger or by a fluidised-bed heat exchanger,
in either case of known type. The heat exchanger in question
should in particular be designed in such a way that the raw
material size-reduced to the fineness of meal which is
delivered to it can be pre-calcined in loosened form with the
hot waste gases from the revolving tubular kiln (arrow 13) which
are carried from the revolving tubular kiln 4 to the lower end
of the hot-gas heat exchan~er unit 3 by a pipe 21. The path
followed by the material to be treated is indicated by the
arrow 12.
In this embodiment of the invention, particular
significance is attributed to the design and function of the
intermediate vessel 20 arranged between the hot-gas heat
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exchanger unit 3 and the briquetting press 5, As will be
explained hereinafter with reference to Figures 2 and 6,
this intermediate vessel 20 has a straight upper part and
a funnel-shaped lower part. The straight upper part of
the intermediate vessel 20 is preferably cylindrical.
With relatively low throughputs of material to be
treated, it is sufficient for the straight upper part of
the intermediate vessel to have a height of less than about
2.5 metres and preferably less than 1.5 metres. With this
construction and also with the relatively low throughput
referred to, the gas trapped in the hot material in lower
parts of the intermediate vessel is also able to escape upwards
so that adequate de-aeration and extremely effective
pre-consolidation of the pre-calcined material are achieved
during its intermediate storage. However, this does
presuppose a reasonable residence time in the intermediate
vessel of, for example, up to one hour or even up to 1.5 hours,
depending on the fineness of the material. In this case,
there is no need for other measures (particularly fittings)
to be taken in the intermediate vessel apart from adequate
insulation.
In order to obtain higher throughputs, however,
it is best to associate a vibration unit with the intermediate
vessel.
Fîgure 2 shows a first embodiment of an intermediate
vessel 20 which on its outside has an adequate insulation
23 by which losses of heat are largely avoided. This ;~
intermediate vessel 20 has a straight, cylindrical upper
part 22a, a funnel-shaped lower part 22b (with a taper
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angle ~ at its lower end of preferably - 50 ) and a
cover 22c. The cover of the intermediate vessel is provided
with an inlet 24 for the introduction of pre-calcined raw
material (arrow 12) from the first heat-treatment stage
and, optionally, with a second inlet 25 for return material
from the briquetting press, whereas at the lower end of
the lower part 22b there is an outlet 26 from which the
pre-consolidated material is delivered to the briquetting
press tnot shown in Figure 2).
A vibration unit 27 is associated with the
intermediate vessel 20. In this case, the vibration unit
27 contains a plurality of vibration bars 28 projecting into
the interior 22d of the intermediate vessel. As can clearly
be seen from Figure 2, the vibration bars 28 are preferably
straight and project into the interior of the intermediate
vessel substantially perpendicularly from above (through
the cover 22c), being uniformly distributed over the entire
cross section of the intermediate vessel. Accordingly,
the vibration bars 28 are more or less long corresponding
to their associated vessel section, as also clearly shown
in Figure 2. All the vibration bars 28 may with advantage
be held by a vibration frame 29 arranged over the cover 22c
of the intermediate vessel and may be vibrated by a common
vibration drive 30.
The vibration bars 28 themselves may be designed
in different ways. For relatlvely gentle vibration of the
material intermediately stored in the intermediate vessel 20,
it may be sufficient to provide completely smooth bars.
For intense vibration, however, it is preferred to provide
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the vibration bars with corresponding projections. For
example, the vibration bars 28 are provided over at least
part of their length and preferably over their entire length
with a number of pin-like pro~ections 28a, as also shown
in an enlarged view in Figure 3. These pin-like projections
28a are preferably arranged and designed as projections
protruding substantially radially from the particular
vibration bar 28.
Figure 4 shows another type of vibration bar 28'
which, over at least part of its length, comprises continuous
vanes 28a' on the lines of a conveying screw vane.
Figure 5 shows a somewhat modified embodiment (i.e.
in relation to Figure 4) of a vibration bar 28" which may
again be provided over at least part of its length with
vanes 28a" on the lines of conveying screw vanes. In this
case, however, the individual vanes 28a" may be spaced apart
from one another.
Both the vanes 28a' shown in Figure 4 and also the
~0 vanes 28a" shown in Figure 5 may be made of solid material or
even of perforated material, depending on the particular
application in question.
Figures 6 and 7 show a somewhat modified embodiment
of an intermediate vessel 20l. In Figures 6 and 7, the
design of the actual intermediate vessel 20' is identical
with that of the intermediate vessel 20 shown in Figure 2,
so t~at the same elements have ~een denoted by the same
reference numerals accompanied by an apostrophe. The main
difference between t~ese two embodiments (Figure 2 on the
one hand and Figures 6 and 7 on the other hand) lies in the
construction of the vibration unit 27'.
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The vibration unit provided in the embodiment shown
in Figures 6 and 7 essentially comprises a cage-like
vibration assembly 41 which projects into the interior ~2d'
of the intermediate vessel from above (through the cover 22c')
and which is supported by a vibration frame 27' provided over
the cover 22c' and carrying a vibration drive 30'.
The cage-like vibration assembly 41 essentially
consists of several rings 45 of different diameter arranged
concentrically to the central axis 44 of the intermediate
vessel and of a number of bars 46 which are arranged sloping
downwards towards the central axis 44 of the vessel, the
bars 46 being arranged in stages in such a way that their
upper ends are supported by a ring 45 of relatively large
diameter and their lower ends by a ring of smaller diameter.
In this way, the vibration assembly 41 tapers conically towards
the outlet end of the vessel.
In order further to illustrate the process according
to the invention, some test results are set out in the following
Table. These tests were carried out with finely ground
spathic magnesite, the raw material being pre-calcined
in a first heat treatment stage in the form of a multistage
cyclone heat-exchanger unit, intermediately stored, de-aerated
and pre-consolidated in an intermediate vessel according to
the invention and then compacted in a briquetting press.
As can be seen from the following TabIe, the tests were
carried out with two different raw meal finenesses which
were subsequently consolidated during intermediate storage
on the one hand with vibration and on the other hand without
vibration, the influence of the vibration bars on the different
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ra~ meal finenesses also being apparent. It can also be seen
that, where vibration bars are used, the residence time in
the intermediate vessel can also be considerably shortened
(greater throughput). In this connection, it is also mentioned
that, where vibration bars are used, the height of the straight
part of the intermediate vessel may even amount to more than
2.50 metres.
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TABLE
Example 1 Example 2
without with without with
vibration bars vibration bars
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Fineness of the 20 %>0.09 mm l %>0.09 mm
raw magnesite
Temperature (C) in the
reaction part of the first
heat-treatment stage 920 910
Residence time in the
intermediate vessel (mins.) 60 45 80 55
Residual ignition loss (%)
before the intermediate vessel3.30 2.90 1.95 2.03
after the intermediate vessel 0.33 0.41 0.72 0.33
Powder density (kg/l)
before the intermediate vessel0.70 0.65 0.58 0.53
after the intermediate vessel 1.03 1.08 0.88 1.02
(measured hot)
Temperature (C) at the :
outlet end of the vessel685 735 665755
Briquetting press
Efficiency *) 185 % 275 % 150 %220 %
Proportion of briquettes
> 10 mm ~~65 %75 % 55 ~78 %
Compressive strength of the
briquettes (kg/briquette) 18 25 16 23 :~
Dropping resistance of the
briquettes ~ 1.25m 1.50m 1.00m1.25m
*) Operation without intermediate storage = 100 %, ~ -
for a briquette-yield~with 45 % > 10 mm, compressive
strength 8 - 12 kg/briquette, dropping resistance 0.40 m.
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