Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Process and Plant for the Heat Treatment of Solids Containing Titanium
The present invention relates to a process for the heat treatment of solids
containing
titanium, in which fine-grained solids are heated to a temperature of 700 to
1000 C in a
reactor with circulating fluidized bed and are in part discharged from the
reactor to-
gether with waste gases into a downstream separator, in which the solids are
sepa-
rated from the waste gases and are recirculated to the reactor at least partly
and/or
phase by phase. Furthermore, the invention relates to a corresponding plant.
Such processes and plants are used for instance for the magnetizing
calcination of
ilmenite (X*TiO2Y*FeOZ*Fe203). In the past, a reactor with stationary
fluidized bed was
used for the magnetizing calcination of ilmenite, which reactor has, however,
only a
small control range and a low reaction density. In addition, when using a
reactor with
stationary fluidized bed, only a comparatively low flow rate is possible with
respect to
the container volume. The temperature and retention time control also
frequently is
unfavorable in the case of such reactors with stationary fluidized bed.
Therefore, it is also known to effect a magnetizing calcination of ilmenite in
reactors
with a circulating fluidized bed. For this purpose, hot air is introduced into
the reactor
through a tuyere bottom (gas distributor) for fluidizing the solids. This hot
air is mostly
generated in an external burner, in which e.g. propane and ambient air are
burnt. The
solids discharged from the reactor together with the waste gases are separated
from
the waste gases in a separator and at least partly recirculated to the
reactor. For con-
trolling the recirculation of solids from the separator into the reactor a so-
called "L-
valve" is used, which can be controlled by supplying gas.
Before the further processing of the ilmenite calcined in the reactor, the
same must be
cooled. To this end it is known, for instance, to use a fluidized-bed cooler,
in which the
CONFIRMATION COPY
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product heat is dissipated. In these known processes and plants it is,
however, possible that
changes in the magnetizingly calcined ilmenite occur during the cooling
period, so that the
positive magnetic properties achieved beforehand are deteriorated again.
Therefore, it is the object of the present invention to provide a process as
mentioned
above, in which the product quality is improved and changes of the product
after the
heat treatment are largely avoided.
More particularly, the invention concerns a process for the heat treatment of
solids
containing titanium, in which fine-grained solids are heated to a temperature
of 700 to
1000 C in a reactor with circulating fluidized bed and are partly discharged
from the
reactor together with waste gases into a downstream separator, in which the
solids are
separated from the waste gases and are recirculated to the reactor at least
partly and/or
phase by phase, characterized in that downstream of the reactor and/or the
separator a
fluidized-bed injection cooler is provided, in which the solids are cooled to
below 250 C
by injecting a coolant evaporating in the injection cooler, and that
fluidizing gas is
introduced into the injection cooler with such a gas velocity that the
Particle-Froude-
Number in the fluidized bed is between 0.01 and 10, wherein the cooling in the
injection
cooler is performed quickly within a few seconds within a cooling time
sufficiently short
to prevent or reduce changes in the solids, wherein at least part of the waste
gas of the
reactor is largely separated from solids in the separator and supplied to a
preheating
stage with a drier and a separator upstream of the reactor for drying and
preheating the
solids to be supplied to the reactor, and wherein the waste gases of the
reactor together
with steam-loaded waste gases coming from the injection cooler are cleaned in
a waste
gas cleaning stage downstream of the preheating stage.
In accordance with the invention, this object substantially is solved in that
downstream of
the reactor and/or the separator an injection cooler is provided, in which the
solids are
cooled to below 250 C by injecting a coolant and possibly are cooled further
in another
cooler, for instance a fluidized-bed cooler, downstream of the injection
cooler, fluidizing
gas being introduced into the injection cooler with such a gas velocity that
the Particle-
Froude-Number in the fluidized bed is between 0.01 and 10, in particular
between 0.1
and 1. Preferred ranges of the Particle-Froude-Number in the fluidized bed
also lie
between 0.01 and 0.1, between 0.05 and 0.7 or between 0.5 and 4. Preferably,
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the Particle-Froude-Number at the bottom of the fluidized-bed cooler is
between 0.1
and 0.25, in particular about 0.17. At the top of the fluidized-bed cooler,
the Particle-
Froude-Number preferably is between 0.35 and 0.55, in particular about 0.47.
The Particle-Froude-Numbers are each defined by the following equation:
Frp= ______________________________________
l(Ps¨ P f) *d *g
Pf
with
effective velocity of the gas flow in m/s
pf = effective density of the fluidizing gas in kg/m3
Ps = density of a solid particle in kg/m3 (apparent density)
dp = mean diameter in m of the particles of the reactor inventory (or
the secon-
dary agglomerates formed) during operation of the reactor
gravitational constant in m/s2.
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When using this equation it should be considered that dp does not designate
the mean
diameter (d_50) of the material used, but the mean diameter of the reactor
inventory
formed during operation of the reactor, which can differ significantly from
the mean
diameter of the material used (primary particles).
In this process in accordance with the invention, the product withdrawn from
the reactor
or the separator first of all is cooled very much in the injection cooler to
e.g. about 100
to 200 C within a very short time. Changes in the magnetizingly calcined
ilmenite
during the cooling period largely can be avoided in this way. Due to the rapid
cooling, a
particularly high product quality of the magnetizingly calcined ilmenite thus
can be
achieved. This high product quality ensures a high degree of separation during
a sub-
sequent magnetic separation. Due to the large temperature range during cooling
it is
necessary to not only take care of changes in the product, but to also
properly adjust
the quantity and velocity of the gas introduced into the injection cooler for
fluidization,
so that the fluidized bed does not expand too much when the injected coolant
is evapo-
rated. In accordance with the invention, the gas velocity of the fluidizing
gas in the
injection cooler therefore is chosen such that a comparatively dense fluidized
bed is
obtained. The fluidized bed is denser at the bottom of the injection cooler
than at the
top of the injection cooler, as the coolant injected is evaporated there. In
the fluidized-
bed cooler downstream of the injection cooler, the product heat no longer
usable in the
process is dissipated.
Preferably, water is injected into the injection cooler as coolant. The gas
content in the
fluidized bed of the injection cooler then can include between 50 and 70%, in
particular
about 60% steam.
With the process in accordance with the invention, all kinds of ores
containing titanium,
in particular those which additionally contain iron oxides, can be heat-
treated effec-
tively. In particular, the process is suited for the magnetizing calcination
of ilmenite. The
mean particle size (d_50) of the solids supplied to the reactor preferably is
between 75
and 250 pm, in particular between about 100 and 150 pm. The maximum grain size
of
the solids supplied to the reactor is about 2 mm, preferably less than 250 pm.
The grain
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size of the ilmenite magnetizingly calcined in the reactor preferably lies in
the same
ranges indicated above.
The generation of the amount of heat necessary for the operation of the
reactor can be
effected in any way known to the expert for this purpose. In accordance with a
pre-
ferred embodiment of the invention it is provided to supply fuel to the
reactor, by whose
combustion with an oxygen-containing gas inside the reactor the amount of heat
re-
quired for the heat treatment is completely or at least partly generated. In
the last-
mentioned alternative, the other part of the required amount of heat then can
be coy-
ered by supplying hot gases or preheated solids. It is preferred when a
gaseous fuel,
preferably natural gas, is introduced into the reactor through e.g. lateral
lances and/or
bottom tuyeres, and air is introduced into the reactor as fluidizing gas. In
this magnetiz-
ing calcination with air, the product quality is influenced by the oxygen
content. There-
fore, the waste gas discharged from the reactor into the separator should
preferably
have an oxygen content between 3 and 10%, in particular about 5%.
In the process in accordance with the invention, a particularly high product
quality can
be achieved when the retention time of the solids in the reactor is between 10
and 30
minutes, in particular about 20 minutes. The Particle-Fro ude-Number in the
reactor can
lie in a range from about 0.3 to 30, in particular between 0.5 and 15.
The energy consumption of the process can be reduced in that in the separator
at least
part of the waste gas of the reactor is separated from solids and supplied to
a preheat-
ing stage upstream of the reactor. The preheating stage can, for instance,
comprise a
heat exchanger such as a Venturi drier, and a separator such as a cyclone or
the like.
In this way, the solids supplied to the reactor are dried and preheated,
whereby the
heat treatment in the reactor is facilitated. A multi-stage preheating of
solids is also
possible, the waste gas of the reactor being cooled step by step.
In accordance with a development of this invention it is provided that the
waste gases
of the reactor together with the e.g. steam-loaded waste gases of the
injection cooler
are cleaned in a waste gas cleaning stage downstream of the preheating stage.
The
gases then can possibly be recirculated to the process.
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In accordance with a preferred embodiment of the invention it is provided that
the
recirculation of solids from the separator into the reactor is effected in a
self-regulating
manner. In this way, an intensive internal and external re-mixing of the
solids treated in
the reactor can be effected, so that a uniform temperature and reaction
profile is
achieved in the reactor.
A plant in accordance with the invention, which is in particular suited for
performing the
process described above, includes a reactor with circulating fluidized bed,
downstream
of which a seperator is provided. Downstream of the reactor and/or the
separator there
is furthermore provided an injection cooler and downstream of the same a
separate
fluidized-bed cooler.
More particularly, the invention also concerns a plant for performing a
process for the
heat treatment of solids containing titanium as defined herein, comprising a
reactor with
circulating fluidized bed, downstream of which a separator is provided,
characterized in
that an injection cooler, for using a coolant evaporating in the injection
cooler, is
provided downstream of the reactor and/or the separator, and that a separate
fluidized-
bed cooler is provided downstream of the injection cooler, the fluidized-bed
cooler
having cooling coils through which a coolant is passed countercurrently,
wherein
upstream of the reactor a preheating stage for the solids is provided, the
preheating
stage comprising a drier and a second separator, and the drier is connected
with the
waste gas conduit of the separator downstream of the reactor, and wherein the
second
separator of the preheating stage, the injection cooler and the fluidized bed
cooler are
connected with a waste gas cleaning stage via conduits.
In the injection cooler, the product can be cooled quickly, i.e. within a few
seconds, to
temperatures between e.g. 100 and 200 C by injecting for instance water. This
rapid first
cooling is decisive for the product quality, as for instance during the
magnetizing
calcination of ilmenite changes in the product are possible during a too long
cooling
time. The final cooling of the product then is effected in the separate
fluidized-bed
cooler, which is provided downstream of the injection cooler, the fluidized-
bed cooler
having cooling coils through which a coolant is passed countercurrently.
Preferably, cooling coils are provided in the fluidized-bed cooler, through
which a coolant
is passed countercurrently. These cooling coils can for instance be combined
to form
cooling bundles.
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In the fluidized-bed cooler, the product heat no longer usable in the process
can be
dissipated particularly effectively, when the same has two or more chambers
through
whose bottom fluidizing gas is introduced by means of a blower. The fluidizing
gas on
the one hand is used for cooling the product and at the same time effects an
intensive
intermixing of the solids to be cooled.
For adjusting the temperatures necessary for the heat treatment of the solids,
the
reactor preferably has a, lance assembly and/or bottom tuyeres opening into
the same,
e.g. disposed laterally, which are connected with a supply conduit for
especially gase-
ous fuel. In this way, the fuel is directly burnt inside the reactor in the
presence of the
solids.
=
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In accordance with a preferred embodiment of the invention, a self-regulating
U-shaped
seal is provided between the reactor and the separator, by means of which the
supply
of solids from the separator into the reactor is controlled. An expensive
control system,
for instance by using an L-valve known from the prior art, thus can be
omitted.
To decrease the energy consumption of the plant, a preheating stage can be
provided
upstream of the reactor, in which the solids are dried and preheated. The
preheating
stage includes a drier, which is connected with the waste gas conduit of the
separator
downstream of the reactor, so that the heat generated in the reactor by
internal com-
bustion of the fuel can be utilized for predrying the solids.
Further developments, advantages and possible applications of the invention
can also
be taken from the following description of an embodiment and the drawing. All
features
described and/or illustrated form the subject-matter of the invention per se
or in any
combination, independent of their inclusion in the claims or their back-
reference.
The only Figure shows a process diagram of a process and a plant in accordance
with
an embodiment of the present invention.
In the process as shown in the Figure, which is suited in particular for the
magnetizing
calcination of solids containing titanium, such as ilnnenite, moist solids are
introduced
into a preheating stage via a screw conveyor 1. This preheating stage
comprises a
Venturi drier 2, in which the raw material is suspended, dried and preheated,
and a
separator 3, e.g. a cyclone, downstream of the Venturi drier 2. The solids
separated
from waste gases in the separator 3 are charged into a reactor 4.
The reactor 4 constitutes a fluidized-bed reactor with circulating fluidized
bed. For
fluidizing the solids, bottom tuyeres are provided in the reactor 4, through
which air is
introduced by means of a blower 5. Via lateral lances 6, natural gas is
supplied to the
reactor 4, which is burnt inside the reactor together with the fluidizing air.
Alternatively
or in addition, fuel can be introduced into the reactor 4 via a conduit 7 by
means of
bottom tuyeres.
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In the fluidized-bed reactor 4, the solids are carried upwards by the
fluidizing gas. Part
of the solids separate out in the reactor and are thereby recirculated to the
circulating
fluidized bed, in order to be carried upwards again by the fluidizing gas.
Together with a
waste gas stream from the reactor 4, the other part of the solids is
discharged upwards
through a conduit 8 and in a downstream separator 9, for instance a cyclone,
separated
from the gas stream for the most part. Through a conduit 10, the solids from
the sepa-
rator 9 are recirculated to the fluidized-bed reactor 4. By means of this
intensive inter-
nal and external remixing a particularly uniform temperature and reaction
profile is
achieved inside the fluidized-bed reactor 4.
The control of the amount of solids recirculated from the separator 9 into the
fluidized-
bed reactor 4 is effected via a self-regulating U-shaped seal 11 which is
provided in the
conduit 10. As a result, a control and regulating unit for metering the amount
of solids
recirculated into the fluidized-bed reactor 4 can be omitted.
The gases which leave the fluidized-bed reactor 4 together with solids through
conduit
8 are heated by the internal combustion of fuel in the reactor 4. The gas
stream sepa-
rated from the solids in the separator 9 is supplied to the Venturi drier 2,
so that the
heat content of the gas stream leaving the separator 9 is utilized for drying
and pre-
heating the solids.
From the fluidized-bed reactor 4 and/or the separator 9 hot solids are
withdrawn and
supplied to an injection cooler 13 via conduits 12a and 12b, respectively. In
the injec-
tion cooler 13, the hot solids are fluidized in a stationary fluidized bed.
For this purpose,
air is introduced into the injection cooler 13 as fluidizing air via a blower
14. The gas
velocity of the fluidizing gas is chosen such that the fluidization in the
injection cooler
13 is low, so that the stationary fluidized bed expands only little. At the
same time,
water is injected into the injection cooler 13 as coolant via conduit 15. The
water is
evaporated in the injection cooler 13, so that the stationary fluidized bed in
the upper
region of the injection cooler 13 expands more than in the bottom region of
the injection
cooler. By injecting water, the hot product is quickly cooled to temperatures
of e.g.
below 200 C.
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Downstream of the injection cooler 13 a separate fluidized-bed cooler 16 is
provided, in
which the product heat no longer usable in the process is dissipated. In the
illustrated
em bodiment, the fluidized-bed cooler has two chambers 16a and 16b, into which
e.g.
water is countercurrently introduced as coolant through schematically
illustrated cooling
coils 17, whereby the product is further cooled to the temperature necessary
for the
further processing, such as the magnetic separation. Via a blower 18, air is
introduced
into the two chambers 16a and 16b of the fluidized-bed cooler 16, in order to
fluidize
and cool the product. The cooled product then is supplied to the further
processing via
a conduit 19.
Via conduits, the separator 3 of the preheating stage, the injection cooler 13
as well as
the fluidized-bed cooler 16 are connected with a waste gas cleaning stage 20,
which for
instance has a bag filter. In this waste gas cleaning stage 20, the gas
streams partly
containing solids and/or steam are cleaned. Via conduit 21, the solids can be
charged
from the waste gas cleaning stage 20 into the fluidized-bed cooler 16.
Example (magnetizing calcination of ilmenite)
In a plant for the magnetizing calcination of ilmenite as shown in the Figure,
43 t/h of
moist ilmenite from a storage tank are charged via the screw conveyor 1 into
the Ven-
turi drier 2. In the Venturi drier 2, the moist ilmenite was suspended, dried
and pre-
heated by hot waste gases from the separator 9. In the cyclone 3, which is
provided
downstream of the Venturi drier 2, the dried and preheated ilmenite was
separated from
the gas stream and introduced into the reactor 4 with circulating fluidized
bed.
The waste gas of the cyclone 3 was supplied to the waste gas cleaning stage
20,
therein liberated from solids and supplied to a chimney. The dry ilmenite dust
separated
in the waste gas cleaning stage 20 was passed through conduit 21 into the
fluidized-
bed cooler 16.
Via the blower 5, 13,000 Nm3/h of air were introduced into the fluidized-bed
reactor 4
for fluidization. At the same time, about 700 Nm3/h of natural gas were
supplied to the
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reactor 4 via the lateral lances 6 as well as conduit 7 and were burnt in the
fluidized =
bed together with air. The hot gas obtained heated the ilmenite charged into
the fluid-
ized-bed reactor 4 to about 900 C, the Particle-Froude-Number in the reactor
being
about 1.2. By an excess of oxygen in the reactor 4, a partial calcination of
the ilmenite
was achieved with retention times of the solids between 10 and 30 minutes.
After the
calcination, the oxygen content in the top of the reactor 4 was between 3 and
10%.
Together with the waste gases of the reactor 4, the solids were transported
into the
separator 9, separated there and for the most part recirculated to the reactor
4 through
conduit 10. An amount of ilmenite product, which corresponds to the amount
charged
into the reactor 4, was supplied to the injection cooler 13 through conduits
12a and
12b, respectively. The average particle size (d_50) both of the ilmenite
charged into the
reactor 4 and of the calcined ilmenite was about 100 to 150 pm with a maximum
grain
size of about 250 pm.
The injection cooler 13 was operated as stationary fluidized bed by supplying
about
6300 Nm3/h of fluidizing air into the injection cooler 13 via the blower 14.
At the same
time, about 8 m3/h of water were introduced into the injection cooler 13 via
conduit 15,
so that the hot ilmenite was cooled to about 150 C within few seconds. Due to
the
evaporating water, the steam was about 60% of the total amount of gas in the
fluidized
bed of the injection cooler 13. The gas velocity of the fluidizing air
introduced via the
blower 14 was chosen such that the Particle-Froude-Number at the bottom of the
injection cooler 13 was about 0.17 and at the top of the injection cooler
about 0.47.
The final cooling of the product was effected in the two chambers 16a and 16b
of the
fluidized-bed cooler 16. For fluidization, about 6000 Nm3/h of air were
introduced into
the fluidized-bed cooler 16 via the blower 18. At the same time, cooling water
was
countercurrently passed through the chambers 16a and 16b via conduit 17. In
the
chambers 16a and 16b, the conduit 17 had cooling bundles.
In this way, a magnetizing calcination of ilmenite could be effected, and due
to the rapid
cooling no changes were detected during the cooling period, so that the
calcined ilmen-
ite had a high product quality.
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List of Reference Numerals
1 screw conveyor
2 Venturi drier
3 cyclone
4 fluidized-bed reactor
5 blower
6 lance
7 conduit
8 conduit
9 separator
10 conduit
11 U-shaped seal
12a,12b conduit
13 injection cooler
14 blower
15 conduit
16 fluidized-bed cooler
16a,16b chamber
17 conduit
18 blower
19 conduit
20 waste gas cleaning stage
21 conduit
