Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND PLANT FOR THE THERMAL TREATMENT
OF GRANULAR SOLIDS IN A FLUIDIZED BED
Technical Field
This invention relates to a method for the thermal treatment of granular
solids in a
fluidized bed which is located in a fluidized-bed reactor, wherein microwave
radiation is
fed into the fluidized-bed reactor through at least one wave guide, and to a
corresponding plant.
There are several possibilities for coupling a microwave source to fluidized-
bed
reactors. These include for instance an open wave guide, a slot antenna, a
coupling
loop, a diaphragm, a coaxial antenna filled with gas or another dielectric, or
a wave
guide occluded with a microwave-transparent substance (window). The type of
decoupling the microwaves from the feed conduit can be effected in different
ways.
Theoretically, microwave energy can be transported in wave guides free of
loss. The
wave guide cross-section is obtained as a logical development of an electric
oscillating
circuit comprising coil and capacitor towards very high frequencies.
Theoretically, such
oscillating circuit can likewise be operated free of loss. In the case of a
substantial
increase of the resonance frequency, the coil of an electric oscillating
circuit becomes
half a winding, which corresponds to the one side of the wave guide cross-
section. The
capacitor becomes a plate capacitor, which likewise corresponds to two sides
of the
wave guide cross-section. In reality, an oscillating circuit loses energy due
to the ohmic
resistance in coil and capacitor. The wave guide loses energy due to the ohmic
resistance in the wave guide wall.
Energy can be branched off from an electric oscillating circuit by coupling a
second
oscillating circuit thereto, which withdraws energy from the first one.
Similarly, by
flanging a second wave guide to a first wave guide energy can be decoupled
from the
same (wave guide transition). When the first wave guide is shut off behind the
coupling
point by a shorting plunger, the entire energy can even be diverted to the
second wave
guide.
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The microwave energy in a wave guide is enclosed by the electrically
conductive walls.
In the walls, wall currents are flowing, and in the wave guide cross-section
an
electromagnetic field exists, whose field strength can be several 10 KV per
meter.
When an electrically conductive antenna rod is put into the wave guide, the
same can
directly dissipate the potential difference of the electromagnetic field and
with a suitable
shape also emit the same again at its end (antenna or probe decoupling). An
antenna
rod which enters the wave guide through an opening and contacts the wave guide
wall
at another point can still directly receive wall currents and likewise emit
the same at its
end. When the wave guide is shut off by a shorting plunger behind the antenna
coupling, the entire energy can be diverted from the wave guide into the
antenna in this
case as well.
When the field lines of the wall currents in wave guides are interrupted by
slots,
microwave energy emerges from the wave guide through these slots (slot
decoupling),
as the energy cannot flow on in the wall. The wall currents in a rectangular
wave guide
flow parallel to the center line on the middle of the broad side of the wave
guide, and
transverse to the center line on the middle of the narrow side of the wave
guide.
Transverse slots in the broad side and longitudinal slots in the narrow side
therefore
decouple microwave radiation from wave guides.
Microwave radiation can be conducted in electrically conductive hollow
sections of all
kinds of geometries, as long as their dimensions do not fall below certain
minimum
values. The exact calculation of the resonance conditions involves rather
complex
mathematics, as the Maxwell equations (unsteady, nonlinear differential
equations)
must ultimately be solved with the corresponding marginal conditions. In the
case of a
rectangular or round wave guide cross-section, however, the equations can be
simplified to such an extent that they can be solved analytically and problems
as
regards the design of wave guides become clearer and are easier to solve.
Therefore,
and due to the relatively easy producibility, only rectangular wave guides or
round wave
guides are used industrially, which are also preferably used in accordance
with the
invention. The chiefly used rectangular wave guides are standardized in the
Anglo-
Saxon literature. These standard dimensions were adopted in Germany, which is
why
odd dimensions appear in part. In general, all industrial microwave sources of
the
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frequency 2.45 GHz are equipped with a rectangular wave guide of the typ R26,
which
has a cross-section of 43 x 86 mm. In wave guides, different oscillation
states exist: In
the transversal electric mode (TE mode), the electric field component lies
transverse to
the wave guide direction and the magnetic component lies in wave guide
direction. In
the transversal magnetic mode (TM mode), the magnetic field component lies
transverse to the wave guide direction and the electric component lies in wave
guide
direction. Both oscillation states can appear in all directions in space with
different
mode numbers (e.g. TE-1-1, TM-2-0).
A method for the thermal treatment of granular solids is known from US
5,972,302,
wherein sulfidic ore is subjected to an oxidation supported by microwaves.
This method
is chiefly concerned with the calcination of pyrite in a fluidized bed,
wherein the
microwaves introduced into the fluidized bed promote the formation of hematite
and
elementary sulfur and suppress the formation of SO2. There is employed a
stationary
fluidized bed which is directly irradiated by the microwave source disposed
directly
above the same. The microwave source or the entrance point of the microwaves
necessarily gets in contact with the gases, vapors and dusts ascending from
the
fluidized bed.
EP 0 403 820 B1 describes a method for drying substances in a fluidized bed,
wherein
the microwave source is disposed outside the fluidized bed and the microwaves
are
introduced into the fluidized bed by means of a wave guide. Open wave guides
involve
the risk that the microwave source is soiled by dust and gases and damaged in
the
course of time. This can be avoided by microwave-transparent windows, which
occlude
the wave guide between the reactor and the microwave source. In this case,
however,
deposits on the window lead to an impairment of the microwave radiation.
Description of the Invention
It is therefore the object underlying the invention to make the feeding of
microwaves
into a stationary or circulating fluidized bed more efficient and protect the
microwave
source against resulting gases, vapors and/or dusts.
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In accordance with the invention, this object is substantially solved in a
method as
mentioned above in that a gas stream is fed into the fluidized-bed reactor
through the
wave guide, which is also used for introducing microwaves. Thus, the microwave
source is disposed outside the stationary or circulating fluidized bed, the
microwave
radiation being fed into the fluidized-bed reactor through at least one wave
guide and a
gas stream being passed through the wave guide in addition to the microwave
radiation. By means of the gas stream from the wave guide it is reliably
avoided that
dust or process gases enter the wave guide, spread up to the microwave source
and
damage the same. In accordance with the invention, microwave-transparent
windows in
the wave guide for shielding the microwave source, as they are commonly used
in the
prior art, can therefore be omitted. The same involve the problem that
deposits of dust
or other solids on the window can impair and partly absorb the microwave
radiation.
Therefore, the open wave guides in accordance with the invention are
particularly
advantageous.
An improvement of the method is achieved when the gas stream introduced
through the
wave guide contains gases which react with the fluidized bed and in the case
of a
circulating fluidized-bed reactor can even be utilized for an additional
fluidization of the
fluidized bed. Thus, part of the gas which so far has been introduced into the
fluidized
bed through other supply conduits is used for dedusting the wave guide. As a
result,
providing neutral purge gas can also be omitted.
Another improvement is obtained in accordance with the invention when the gas
stream
introduced through the wave guide has a temperature difference with respect to
the
gases and solids present in the fluidized-bed reactor. In this way, additional
heat can
specifically be introduced into the fluidized bed or the fluidized bed can be
cooled,
depending on the desired effect.
The thermal treatment can not only be effected in a stationary, but also in a
circulating
fluidized bed, wherein the solids circulate continuously between a fluidized-
bed reactor,
a solids separator connected with the upper region of the fluidized-bed
reactor and a
return conduit connecting the solids separator with the lower region of the
fluidized-bed
reactor. Usually, the amount of solids circulating per hour is at least three
times the
amount of solids present in the fluidized-bed reactor.
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The solids can also be passed through at least two succeeding fluidized-bed
reactors,
for instance two fluidization chambers separated from each other by means of
weirs or
partitions, in which the stationary fluidized beds are formed and to which the
electromagnetic waves (microwaves) coming from wave guides are fed. The solids
can
move from one fluidized-bed reactor into the adjacent fluidized-bed reactor.
One variant
consists in that between the two adjacent fluidized-bed reactors an
intermediate
chamber is disposed, which is in particular connected with both fluidization
chambers
and contains a fluidized bed of granular solids, the intermediate chamber
having no
associated wave guide. Another variant of the method of the invention consists
in that a
partition with the opening in the bottom region is used for separating the two
fluidization
chambers.
To a particular advantage, the operating conditions, in particular
temperature,
composition of the fluidizing gas, energy input and/or fluidization rate can
be specified
differently for each of several fluidized-bed reactors. In the case of one
fluidized bed or
several succeeding fluidized beds, the solids thus can for instance first be
passed
through a preheating chamber upstream of the first fluidized bed. Furthermore,
downstream of the last fluidized bed serving the thermal treatment a cooling
chamber
may be provided for cooling the solid product.
Another advantage is obtained in that solid deposits in the wave guide are
avoided by
the continuous gas stream through the wave guide. These solid deposits change
the
cross-section of the wave guide in an undesired way and absorb part of the
microwave
energy which was designed for the solids in the fluidized bed. Due to the
absorption of
energy in the wave guide, the same is heated very much, whereby the material
is
subject to a strong thermal wear. In addition, solid deposits in the wave
guide effect
undesired feedbacks to the microwave source.
Suitable microwave sources, i.e. sources for the electromagnetic waves,
include e.g. a
magnetron or a klystron. Furthermore, high-frequency generators with
corresponding
coils or power transistors can be used. The frequencies of the electromagnetic
waves
proceeding from the microwave source usually lie in the range from 300 MHz to
30
GHz. Preferably, the ISM frequencies 435 MHz, 915 MHz and 2.45 GHz are used.
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Expediently, the optimum frequencies are determined for each application in a
trial
operation.
In accordance with the invention, the wave guide wholly or largely consists of
electrically conductive material, e.g. copper. The length of the wave guide
lies in the
range from 0.1 to 10 m. The wave guide may be straight or curved. There are
preferably used sections of round or rectangular cross-section, the dimensions
being in
particular adapted to the frequency used.
The temperatures in the fluidized bed lie for instance in the range from 300
to 1200°C,
and it may be recommended to introduce additional heat into the fluidized bed,
e.g.
through indirect heat transfer. For temperature measurement in the fluidized
bed,
insulated sensing elements, radiation pyrometers or fiber-optic sensors can be
used.
In accordance with the invention, the gas velocities in the wave guide are
adjusted such
that the Particle-Froude-Numbers in the wave guide lie in the range between
0.1 and
100. The Particle-Froude-Numbers are defined as follows:
a
Frp =
BPS -Pf) *dp *g
Pf
with
a - effective velocity of the gas flow in m/s
ps - density of the solid particles or process gases penetrating into the wave
guide in kg/m3
pf - effective density of the purge gas in kg/m3
dP - mean diameter in m of the particles of the reactor inventory (or the
particles formed) during operation of the reactor
g - gravitational constant in m/s2.
To prevent solid particles or generated process gases from the reactor from
penetrating
into the wave guide, gas serving as purge gas flows through the wave guide.
Solid
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particles can for instance be dust particles present in the reactor or also
the treated
solids. Process gases are generated in the processes which take place in the
reactor.
By specifying certain Particle-Froude-Numbers, the density ratio of the
penetrating solid
particles or process gases to the purge gas is considered in accordance with
the
invention when adjusting the gas velocities, which ratio, apart from the
velocity of the
purge gas stream, is decisive for the question whether or not the purge gas
stream can
entrain the penetrating particles. Substances can thereby be prevented from
penetrating into the wave guide. For most applications, a Particle-Froude-
Number
between 2 and 30 is preferred.
The granular solids to be treated by the method in accordance with the
invention can
for instance be ores and in particular sulfidic ores, which are prepared e.g.
for
recovering gold, copper or zinc. Furthermore, recycling substances, e.g. zinc-
containing
processing oxide or waste substances, can be subjected to the thermal
treatment in the
fluidized bed. If sulfidic ores, such as e.g. auriferous arsenopyrite, are
subjected to the
method, the sulfide is converted to oxide, and with a suitable procedure there
is
preferably formed elementary sulfur and only small amounts of S02. The method
of the
invention favorably loosens the structure of the ore, so that the subsequent
gold
leaching leads to improved yields. The arsenic iron sulfide (FeAsS) preferably
formed
by the thermal treatment can easily be disposed of. Expediently, the solids to
be
treated at least partly absorb the electromagnetic radiation used and thus
heat the bed.
It was surprisingly found out that in particular material treated at high
field strengths
can be leached more easily. Frequently, other technical advantages can be
realized as
well, such as reduced retention times or a decrease of the required process
temperatures.
The present invention furthermore relates to a plant in particular for
performing the
above-described method for the thermal treatment of granular solids in a
fluidized bed.
A plant in accordance with the invention includes a fluidized-bed reactor, a
microwave
source disposed outside the fluidized-bed reactor, and a wave guide for
feeding the
microwave radiation into the fluidized-bed reactor, a gas supply conduit for
feeding gas
into the fluidized-bed reactor being connected to the wave guide.
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Furthermore, the reactor can be elongated and have a gas-permeable bottom for
the
entrance of fluidizing gas, for instance a bottom provided with hole or slot
openings,
bell nozzles or similar openings suitable for fluidization technology. This
reactor
designed as fluidized-bed channel can be installed horizontally or with a
small angle of
inclination of a few degrees and have a lengthlwidth ratio of at least 1.5 to
1, for
instance 4 to 1. In such a reactor, the treatment and the transport of the
granular solids
can easily be realized in accordance with the invention. To divide the
fluidization
channel reactor in several zones, partitions or weirs can be arranged inside
the
fluidized bed formed in the channel and/or in the gas space located above the
fluidized
bed, depending on the process, an opening being left for the passage of the
granular
solids. It is particularly advantageous when the partitions or weirs are
adjustable for
separating zones, so that the height of the fluidizing material and the slot
height can be
varied for the transfer from zone to zone. The bed depth in the fluidization
channel is
selected such that in each zone an almost uniform energy state is achieved due
to a
thorough mixing. In the case of a suitable fluidizing material, the siphon
principle can
also be used instead of transfer weirs. Microwave inlet openings with wave
guides
connected thereto can be provided in all zones or in individual zones.
Developments, advantages and possibilities for applying the present invention
can also
be taken from the following description of examples and from the drawing. All
described
and/or illustrated features per se or in any combination belong to the subject-
matter of
the invention, independent of their inclusion in the claims or their back-
reference.
Brief Description of the Drawings
In the drawings
Fig. 1 shows the thermal treatment of granular solids in a stationary
fluidized
bed in a schematic representation;
Fig. 2 shows a method variant with a circulating fluidized bed; and
Figs. 3, 4, 5, 6 show method variants with a plurality of stationary fluidized
beds.
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Detailed Description of the Preferred Embodiments
Fig. 1 shows a plant for performing the method in accordance with the
invention for the
thermal treatment of granular solids in a stationary fluidized layer 3 which
is also
referred to as fluidized bed.
The plant includes a fluidized-bed reactor 1, into which granular solids to be
treated are
introduced through a conduit 2. In a chamber, the solids form a stationary
fluidized bed
3 which is traversed by a fluidizing gas, e.g. air. For this purpose, the
fluidizing gas is
passed from below through a gas distributor 4 into the fluidized bed 3. In the
upper
region of the fluidized-bed reactor 1, an open wave guide 5, which leads to a
microwave source 7, is connected to the chamber with the stationary fluidized
bed 3.
The electromagnetic waves proceeding from the microwave source 7 are passed
through the wave guide 5 and fed into the chamber of the fluidized-bed reactor
1. They
at least partly contribute to the heating of the fluidized bed 3. Furthermore,
purge gas,
e.g. air or nitrogen, is laterally fed into the wave guide 5 through a conduit
6, which
purge gas flows into the fluidized-bed reactor 1 and prevents the ingress of
dust or
process gases from the chamber with the fluidized bed 3 into the wave guide 5.
In this
way, the microwave source 7 is protected against being damaged, and at the
same
time microwave-absorbing soil deposits in the wave guide 5 are prevented
without the
open wave guide 5 having to be closed by a window transparent for microwaves.
If necessary for the process, the stationary fluidized bed 3 can additionally
be heated
by a heat exchanger 8 disposed in the fluidized bed 3. Gases and vapors formed
leave
the chamber of the fluidized-bed reactor 1 through a conduit 9 and are
supplied to a
non-illustrated cooling and dedusting known per se. The treated granular
solids are
withdrawn from the fluidized-bed reactor 1 through the discharge conduit 10.
In Fig. 2, the fluidized-bed reactor 1 constitutes a reactor with a
circulating fluidized bed
(fluidized layer). The solids to be treated are introduced into the fluidized-
bed reactor 1
via conduit 2 and entrained by fluidizing gas introduced into the fluidized-
bed reactor 1,
whereby the circulating fluidized layer is formed. The solids then are at
least partly
discharged from the fluidized-bed reactor 1 along with the gas through a
conduit 11 and
introduced into a solids separator 12. The solids separated therein are at
least partly
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recirculated through a return conduit 13 into the lower region of the
circulating fluidized
layer of the fluidized-bed reactor 1. Part of the solids can also be
discharged through
the discharge conduit 14. Coarse solids, which are deposited at the bottom of
the
fluidized-bed reactor 1, can be removed from the reactor 1 through a discharge
conduit
15. The fluidizing gas for forming the circulating fluidized bed, e.g. air, is
supplied to the
fluidized-bed reactor 1 through a conduit 4a and then first gets into a
distribution
chamber 4h, before it flows into the fluidized-bed reactor 1 through a grid
4i, entrains
the introduced, in particular fine-grained solids and forms a circulating
fluidized layer as
fluidized bed.
A wave guide 5 connects a microwave source 7 with the chamber of the fluidized-
bed
reactor 1, through which wave guide microwaves are fed into the microwave
reactor 1
for heating the granular solids as in the plant in accordance with Fig. 1. In
addition,
purge gas from conduit 6 flows through the wave guide 5, in order to avoid the
ingress
of dirt as well as deposits in the wave guide 5. In the present case as well,
the inner
region of the chamber can again be provided with one or more heat exchangers
for
additionally heating the granular solids, which for a better clarity was not
represented in
Fig. 2.
Dust-laden gas leaves the solids separator 12 through conduit 9 and is first
cooled in a
waste heat boiler 16, before it is passed through a dedusting 17. Separated
dust can
either be removed from the process or be recirculated to the chamber of the
fluidized-
bed reactor 1 through a non-illustrated conduit.
As shown in Fig. 3, two stationary fluidized-bed reactors 1 and 1a are
arranged in
series, an intermediate chamber 1c being located between the chambers of the
two
reactors 1 and 1a. In all three chambers, the solids form a stationary
ffuidized bed 3,
3a, which is traversed by fluidizing gas. The fluidizing gas for each chamber
is supplied
through a separate conduit 4a, 4b, 4c, respectively. The granular solids to be
treated
enter the first fluidized-bed reactor 1 through conduit 2, and completely
treated solids
leave the second fluidized-bed reactor 1a through the discharge conduit 10.
From the
upper region of the chamber of the first reactor 1 a first wall 19 extends
downwards.
However, it does not extend down to the ground, so that in the bottom region
an
opening 20 is left, through which solids from the first fluidized bed 3 can
get into the
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fluidized bed 3a of the intermediate chamber 1 c. The intermediate chamber 1 c
extends
up to a weir-like second wall 21, over which the solids from the fluidized bed
3a of the
intermediate chamber 1 c are moved into the chamber of the second fluidized-
bed
reactor 1 a. Corresponding to the plants as shown in Figs. 1 and 2, wave
guides 5 with
purge air conduits 6 and microwave sources 7 are each connected to the
chambers of
the two reactors 1 and 1a, through which wave guides the microwaves and purge
gas
are fed into the reactors 1 and 1 a. In the chambers of the reactors 1 and 1
a, heat
exchanging elements 3 may be arranged in addition.
The gas space 22 above the fluidized bed 3 of the first fluidized-bed reactor
1 is
separated from the gas space 23, which belongs to the chamber of the second
reactor
1 a and the intermediate chamber 1 c, by the vertical wall 19. For the gas
spaces 22, 23
separate gas discharge conduits 9 and 9a exist. As a result, different
conditions can be
maintained in the chambers of the reactors 1 and 1a, in particular different
temperatures can exist or different fluidizing gases can be supplied through
separate
gas supply conduits 4a, 4b, 4c. Furthermore, the two microwave sources 7 can
be
designed differently and perform different functions. In particular,
microwaves of
different frequency or energy can be generated and be introduced through the
wave
guide 5.
As shown in Fig. 4, two stationary fluidized-bed reactors 1 and 1a without
intermediate
chamber are arranged directly succeeding each other, a partition 19 being
disposed
between the two. In the chambers of the two reactors 1, 1a the solids form a
stationary
fluidized bed 3, 3a, which is fluidized by fluidizing gas from several
conduits 4a, 4b, 4c
disposed one beside the other. The granular solids to be treated are supplied
to the
first fluidized-bed reactor 1 through conduit 2, and the treated solids leave
the fluidized-
bed reactor 1a through the discharge conduit 10. From the upper region of the
chamber
of the first reactor 1, a first wall 19 extends downwards, which does,
however, not
extend down to the ground, so that in the bottom region an opening 20 is left,
through
which solids from the first fluidized bed 3 can get into the fluidized bed 3a
of the second
fluidized-bed reactor 1a. Waveguides 5, which are connected to the microwave
sources
7, each extend to the two chambers of the reactors 1 and 1 a. According to the
principle
described already in the previous embodiments, microwaves are fed into the two
reactors 1, 1a through these open wave guides 5, in order to heat the solids
to be
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treated, which absorb the microwave radiation, and reach the necessary process
temperatures. At the same time, purge gas flows into the wave guides 5 through
purge
air conduits 6, in order to avoid deposits in the same. In the chambers of the
reactors 1
and 1a, heat exchanging elements 8 may be arranged in addition.
The gas space 22 above the fluidized bed 3 of the first fluidized-bed reactor
1 is
separated from the gas space 23, which belongs to the chamber of the second
reactor
1a, by the vertical wall 19. There exist separate gas discharge conduits 9 and
9a. As a
result, different conditions can be maintained in the different reactor
chambers 1 and
1a; in particular, the temperatures or the gas phase composition can be
different.
Different fluidizing gases can also be supplied through the respective
conduits 4a, 4b,
4c. Furthermore, the two microwave sources 7 can be designed differently and
perform
different functions.
In the arrangement as shown in Fig. 5, the solids to be treated, which are
supplied via
conduit 2, first enter an antechamber 31 and flow through a first intermediate
chamber
32 in the first fluidized-bed reactor 1. The solids then are discharged from
the same to
flow through a second intermediate chamber 1 c into the second fluidized-bed
reactor
1a and finally through the third intermediate chamber 33 into a cooling
chamber 34,
before the treated and cooled solids are withdrawn through the discharge
conduit 10.
Wave guides 5 with associated non-illustrated microwave sources each open into
the
chambers of the fluidized-bed reactors 1 and 1a, in order to feed microwaves
into the
reactors 1 and 1 a according to the above-described principle. All chambers
include
stationary fluidized beds, to which fluidizing gas is supplied through
separate gas
supply conduits 4a to 4g for each chamber. The exhaust gases are discharged
through
corresponding conduits 9a to 9d.
In the cooling chamber 34, the fluidized bed includes a cooling means 35 for
an indirect
heat transfer, whose cooling fluid, e.g. cooling water, is heated in the
cooling means 35
and then supplied through conduit 36 to the heat exchanger 37 in the
preheating
chamber 31. There, the cooling fluid releases part of its heat to the solids
in the
fluidized bed disposed there, whereby a very economic utilization of heat is
achieved.
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As variant of another plant in accordance with the invention, Fig. 6 shows a
fluidization
channel reactor 38, in which the fluidized layer is formed in a channel-type
bottom 39
with through openings for a fluidizing gas. The fluidization channel reactor
38 is divided
into four zones 41 a to 41 d separated by adjustable partitions 40, the first
zone 41 a
constituting a preheating zone, the second zone 41 b an oxidation zone, the
third zone
41 c a reduction zone, and the fourth zone 41 d a cooling zone. Downstream of
each of
the zones 41 a to 41 d a separator 42 or a cyclone is provided, which
separates the
solids discharged with the fluidizing gas from the gas stream and recirculates
the same
to the respective zone 41a to 41d. To achieve a high utilization of energy,
the exhaust
gases from the separators 42 are supplied to other zones 41 a to 41 d by means
of a
suitable gas recirculation.
Via a feed conduit 43, the solids to be treated are supplied to the first zone
41a of the
reactor 38. As fluidizing gas, hot exhaust gas from a first combustion chamber
44 is
supplied to the first zone 41a, in order to dry and preheat the introduced
material. The
correspondingly preheated solids flow through the partition 40 into the
oxidation zone
41 b, to which there is likewise supplied hot exhaust gas from a second
combustion
chamber 45. To both combustion chambers 44, 45, supply conduits are connected
for
fuel and air and possibly preheated exhaust gas from other process zones 41a
to 41d.
From the oxidation zone 41 b, the solids are supplied to the reduction zone 41
c. For
protecting the downstream compressor, the exhaust gas from the oxidation zone
41 b
can likewise be supplied to the reduction zone 41 c via a cooler 47. Possibly,
the
exhaust gas is again heated in a heater 49.
To bring the fluidized material to the desired energy state, microwave rays
are
additionally introduced into the oxidation zone 41a and the reduction zone 41c
through
wave guides 46 traversed by purge gas. Due to the microwave radiation, the
solids are
heated by an internal excitation, so that the energy state can easily be
adjusted. In the
last zone 41 d, the treated material is cooled with injected air and
discharged as product
through the discharge conduit 48.
To make the feeding of microwaves into a stationary or circulating fluidized
bed 3, 3a
more efficient and also protect the microwave source 7 against the resulting
gases,
vapors and dusts, the microwave source 7 in accordance with the invention is
disposed
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outside the stationary or circulating fluidized bed 3, 3a and the fluidized-
bed reactors 1,
1a, 38. The microwave radiation is fed into the fluidized-bed reactor 1, 1a,
38 through
at least one open wave guide 5, 46, wherein in addition to the microwave
radiation a
gas stream flows into the fluidized-bed reactor 1, 1a, 38 through the wave
guide 5, 46.
By means of the gas stream, the wave guide 5, 46 is kept dust-free, which
considerably
increases the efficiency of the introduction of microwaves.
Example 1 (Calcination of ores containing pyrite)
Pyrite with grain sizes in the range from 80 to 160 pm is treated in two
successive
fluidized beds 3, 3a, which are designed corresponding to the plant in
accordance with
Fig. 4. Irradiation is effected in both chambers by microwaves with a
frequency of 2.45
GHz. As radiation source, magnetrons are used.
Into the first fluidized-bed reactor 1, 182.5 kg/h pyrite are charged. For
fluidizing the
fluidized bed 3, 360 Nm3/h nitrogen are used, which are supplied through
conduit 4a, so
that a height of 20 cm is obtained for the fluidized bed. After the microwave
treatment,
the mass flow rate of the solid reaction products from the first fluidized-bed
reactor 1 is
153.5 kg/h. The first chamber is operated at 550°C and a magnetron
irradiation of 36
kW.
Deoiled compressed air with a volume flow rate of 120 Nm3/h is supplied to the
second
fluidized bed 3a through conduit 4c. The second chamber is operated at
500°C and a
microwave irradiation of 36 kW. After 90 min, a steady state is obtained;
after the
microwave treatment, the mass flow rate of the solid reaction products is
140.2 kg/h.
As purge gas, there is each utilized the gas used for fluidization, i.e. in
the first
chamber nitrogen and in the second chamber deoiled compressed air, which each
have
a volume flow rate of 50 Nm3/h.
The phase content of the pyrite used and of the products of the first and
second
process stages is analysed by X-ray diffraction. In the pyrite, only FeSz can
be
detected. After the first temperature treatment, the solids consist of
substoichiometric
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FeS and FeS2 for instance in accordance with FeSX with x = 1.4. After the
second
stage, no more sulfur-containing products can be detected, the solids
virtually
exclusively consist of hematite.
Example 2 (Calcination of ore containing gold)
On a laboratory scale, gold ore with grain sizes in the range below 250 pm is
treated in
a circulating fluidized bed which is designed as shown in Fig. 2. Irradiation
is effected
by microwaves with a frequency of 2.45 GHz. As radiation source, a magnetron
is
used. For purging, 24 Nm3/h air are supplied to the reactor 1 through the wave
guide 5.
Feed
Type gold ore, ground, dried and
classified
Grain fraction
Max pm 250
Composition Wt-
Org. C 1.05
CaC03 19.3
Ah03 12.44
FeS2 2.75
Inerts, e.g. Si02 64.46
Input, about kg 100
Apparatus
Type of reactor circulating fluidized bed with microwave irradiation
Reactor diameter mm 200
Magnetron 500 W, 2.45 GHz
Microwave coupling wave guide, R26 (43 x 86 mm) designed as secondary air
conduit
Connected: online gas analysis + exhaust gas washing
Operation: continuous
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Test conditions and results
Inlet Outlet
Mass flow rate, gold ore kg/h 195
feed
Primary air C 250
Nm3/h 30
C 50
Oil consumption kg/h 0.70
Secondary air,
preheated by means of Luvo C 425
to
Secondary air, consumptionNm3/h 24
Drier air C 50 320
Nm3/h 70 70
Calcining residue, ex-WS C 400
luvo
kglh 132
Calcining gas, total
Nm3/h 59
°C 600
Composition, dry
CO~ vol-% 23.3
N2 vol-% 74.3
O~ vo I-% 2.4
SO~ ppV 134.1
The phase content of the material used and of the products is analysed by X-
ray
diffraction. After the treatment, neither residual sulfur nor organic carbon
can be
detected in the calcining residue, the solids have a pale gray color.
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List of Reference Numerals:
1,1a fluidized-bed reactor 20 opening
1c intermediate chamber 21 weir, partition
2 conduit 31 antechamber
3,3a fluidized layer, fluidized32 intermediate chamber
bed
4 gas distributor 33 intermediate chamber
4a to gconduits 34 cooling chamber
4h distribution chamber 35 cooling means
4i grid 36 conduit
5 wave guide 37 heat exchanger
6 conduit 38 fluidizatlon channel
reactor
7 microwave source 39 bottom
8 heat exchanger 40 partitions
9 conduit, gas discharge 41 o d zones
conduit a
t
10 discharge conduit 42 separator
11 conduit 43 feed conduit
12 solids separator 44 combustion chamber
13 return conduit 45 combustion chamber
14 discharge conduit 46 wave guide
15 discharge conduit 47 cooler
16 waste heat boiler 48 discharge conduit
17 dedusting 49 heater
19 weir, partition
