Note: Descriptions are shown in the official language in which they were submitted.
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Process and Plant for Producing Titania Slag from Ilmenite
Technical Field
The present invention relates to a process for producing titania slag from
ilmenite,
and to a corresponding plant.
Ilmenite, which beside titanium dioxide contains large amounts of iron oxides
(x=Ti02 + y=FeO + z=Fe203), is one of the most important starting materials,
apart
from rutile, for recovering metallic titanium and titanium compounds, such as
tita-
nium dioxide used for pigment production. Separating the iron from the ore
usually
is effected by electric melting of ilmenite in a metallurgical furnace, the
iron oxides
being reduced to metallic iron, which is precipitated from the slag containing
tita-
nium dioxide. However, a disadvantage of this process is the very high demand
for
electric energy, which is about 2,200 kWh per ton of titania slag and
represents
the main part of the production costs.
From U.S. 3,765,868 there is known a process for producing titania slag from
=
ilmenite, in which the crude ore first of all is partially reduced in a rotary
kiln and is
subsequently cooled to a temperature of at least 150 C, before the magnetic
fraction containing titanium dioxide is separated from the non-magnetic ash
and
char by means of a magnetic separator and is finally molten in an electric
furnace.
This process is also characterized by a high demand for energy. Another disad-
vantage of the aforementioned process is the fact that before the reduction
the
ilmenite used must first be pelletized.
Description of the Invention
Therefore, it is the object underlying the present invention to provide a
process for
producing titania slag, which with at least the same quality of the titania
slag pro-
duced has a rather low demand for energy.
CONFIRMATION COPY
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In accordance with the invention, this object is solved by a process for
producing
titania slag from ilmenite, comprising the steps of:
a) partial reduction of granular ilmenite with a reducing agent in a
reduction
reactor comprising a fluidized bed at a temperature of at least 900 C,
b) transfer of the partially reduced hot ilmenite obtained in step a) into
an
electric furnace, wherein the inlet temperature of the ilmenite entering the
furnace is
at least 550 C,
c) melting the ilmenite in the electric furnace in the presence of a
reducing
agent by forming liquid pig iron and titania slag, and
d) withdrawing the titania slag from the electric furnace.
In accordance with the present invention, it has surprisingly be found that
the
demand for energy for producing titania slag from ilmenite can be reduced by
40 to
50% as compared with the processes known so far, when the ilmenite is
prereduced prior to electric melting and is introduced into the electric
furnace in
the hot condition, i.e. without cooling or upon being cooled only little after
the
partial reduction. Another advantage of this procedure consists in the
increase of
the magnetic susceptibility of the ilmenite with respect to the impurities,
such as
chromium, contained in the starting ore, so that in the case of a magnetic
separa-
tion a reliable separation between fractions containing titanium dioxide and
frac-
tions free from titanium can be achieved.
In principle, the partial reduction a) can be effected in any apparatus known
to
those skilled in the art for this purpose, for instance in a rotary kiln.
Particularly
good results are obtained, however, when the partial reduction a) of the
ilmenite is
performed in a fluidized bed, preferably in a circulating fluidized bed,
namely either
in a one-stage or multi-stage operation. Due to the high mass and heat
transfer in
fluidized beds, a uniform reduction of the material used is thereby achieved
with a
minimum expenditure of energy.
Preferably, the grain size of the granular ilmenite used is less than 1 mm and
particularly preferably less than 400 pm.
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As reducing agent for the partial reduction a) of the ilmenite, all substances
known
to those skilled in the art for this purpose can be used in principle, and in
particular
coal, char, molecular hydrogen, gas mixtures containing molecular hydrogen,
carbon monoxide and gas mixtures containing carbon monoxide, for instance
reformed gas, turned out to be useful. As reducing agent, there is preferably
used
a gas mixture containing carbon monoxide and molecular hydrogen, particularly
preferably a gas mixture of 60 to 80 vol-iY0 carbon monoxide and 40 to 20 vol-
%
molecular hydrogen, and quite particularly preferably a gas mixture of 70 vol-
cY0
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carbon monoxide and 30 vol-% hydrogen in combination with char. If the partial
reduction is performed in a circulating fluidized bed, this can for instance
easily be
realized in that ilmenite to be partially reduced and char are continuously
supplied
to the fluidized-bed reactor via a solids supply conduit, and the solids in
the reactor
are fluidized by a gas mixture containing carbon monoxide and molecular hydro-
gen.
For the partial reduction a), the process conditions preferably are adjusted
such
that the degree of metallization of the product obtained by this process step
is 50
to 95% and particularly preferably 70 to 80%, based on its iron content.
To further reduce the energy demand of the process, it is proposed in
accordance
with a development of the invention to first of all preheat the ilmenite
before the
partial reduction a) in one or more heat exchangers to a temperature of 500 to
900 C, particularly preferably 600 to 850 C, and quite particularly preferably
about
800 C, and subsequently heat the preheated material in a calcining reactor up-
stream of the reduction reactor, preferably a reactor with stationary
fluidized bed,
to a temperature of more than 900 C and particularly preferably more than
1,000 C.
In accordance with a particular embodiment of the present invention, the
produc-
tion of the char used as reducing agent is effected in one process step with
the
heating of the ilmenite in a stationary fluidized-bed reactor. For this
purpose, the
preheated ilmenite together with coal, preferably coal with a grain size of
less than
5 mm, and molecular oxygen or a gas mixture containing molecular oxygen, is
introduced into a fluidized-bed reactor and heated there to a temperature of
pref-
erably more than 900 C and particularly preferably more than 1,000 C. By means
of this comparatively high carbonization temperature, the formation of
hydrocar-
bons, e.g. tar, which will disturb in the succeeding process steps, can
reliably be
prevented. The fluidization of the solids preferably is effected by means of
the gas
mixture used as reducing agent in the succeeding step of partial reduction,
the
degree of carbonization being adjustable by adjusting the retention time to a
suit-
able value.
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To achieve a particularly efficient procedure, it is proposed in accordance
with a
development of the invention to circulate the fluidizing gas. This can for
instance
be effected such that the off gas from the reduction reactor is passed through
the
heat exchanger(s) used for preheating the ilmenite, subsequently the off gas
possibly is passed through a waste heat boiler by generating steam, in which
steam is generated, before dust is removed from the cooled waste gas and the
same possibly is further cooled, in a CO2 absorber possibly is separated from
the
carbon dioxide obtained during the partial reduction of ilmenite, is heated in
a
succeeding gas heater and finally again supplied to the reduction reactor and
possibly the carbonization reactor as fluidizing gas.
If the crude ilmenite used has a comparatively high content of FeO, it was
found to
be expedient to subject the same to an oxidative pretreatment prior to the
partial
reduction a), in order to rather completely oxidize the FeO to obtain Fe203.
This is
advantageous because FeO is present in a crystal lattice structure, which
largely
resists the attack of reducing gases, whereas the lattice structure of Fe203
result-
ing from the oxidation of FeO allows an efficient diffusion of gas into the
pores of
the material. Preferably, the oxidation is performed such that the FeO content
of
the treated material after the oxidation is less than 5 wt-%, and particularly
pref-
erably less than 3 wt-%.
In accordance with a development of the invention it is proposed to perform
the
oxidation of the crude ilmenite as well as the partial reduction in a
circulating fluid-
ized bed, preferably at a temperature between 600 and 1000 C.
In particular when ilmenite containing chromite is used as starting material
or coal
and/or char is used as reducing agent, it turned out to be advantageous to
subject
the partially reduced ilmenite to a magnetic separation before charging the
same
into the electric furnace, in order to separate the magnetic fraction rich in
titanium
dioxide from a non-magnetic fraction, which substantially contains chromite,
ash
and, if used as reducing agent, char, and to only transfer the magnetic
fraction
obtained thereby into the electric furnace. In this case, the temperature of
the
partially reduced material used during the magnetic separation preferably is
at
least 600 C, particularly preferably at least 675 C, and quite particularly
preferably
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about 700 C. Particularly preferably, the magnetic fraction subsequently is
trans-
ferred into the electric furnace without cooling or heating. The energy
required for
cooling the partially reduced material after the partial reduction on the one
hand
and the energy required for heating the material supplied to the electric
furnace to
the operating temperatures in the furnace on the other hand thus is minimized
without a substantial reoxidation of the partially reduced material taking
place
before entrance into the electric furnace. The non magnetic fraction can be
further
processed and the char in this non magnetic fraction can be reused in the
process,
e.g. as a feed material.
Preferably, the titania slag withdrawn from the electric furnace contains 75
to 90
wt-% and particularly preferably about 85 wt-% titanium dioxide, and the
liquid pig
iron contains more than 94 wt-% metallic iron.
A plant in accordance with the invention, which can be used in particular for
per-
forming the process described above, comprises a carbonization reactor
constitut-
ing a stationary fluidized-bed reactor for carbonizing coal by heating
ilmenite at the
same time, a reduction reactor constituting a circulating fluidized-bed
reactor for
the partial reduction of ilmenite, and an electric furnace.
Preferably, the carbonization reactor is connected with the reduction reactor
via a
connecting passage such that the solids/gas suspension can pass from the upper
part of the carbonization reactor into the lower part of the reduction
reactor, and
downstream of the reduction reactor a cyclone is provided for separating the
solids
from the solids/gas suspension, a solids return conduit extending from the
cyclone
to the carbonization reactor.
In accordance with an embodiment of the invention it is proposed to provide at
least one preheating stage including a solids/gas suspension heat exchanger
and
a downstream cyclone upstream of the carbonization reactor, in which the
ilmenite
is preheated to temperatures of 500 to 900 C, particularly preferably 600 to
850 C
and quite particularly preferably about 800 C, before being charged into the
car-
bonization reactor.
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In accordance with an embodiment of the invention it is proposed to provide a
means for circulating the fluidizing gas in the plant.
In accordance with a particular embodiment of the present invention, the plant
in
addition comprises a magnetic separator.
The invention will subsequently be described in detail with reference to
embodi-
ments and the drawing. All features described and/or illustrated in the
drawing
form the subject-matter of the invention per se or in any combination,
independent
of their inclusion in the claims or their back-reference.
Brief Description of the Drawing
Fig. 1 shows a process diagram of a process and a plant in accordance
with a
first embodiment of the present invention; and
Fig. 2 shows a process diagram of a process and a plant in accordance
with a
= second embodiment of the present invention.
Description of the Preferred Embodiments
In the process for producing titania slag from ilmenite as shown in Fig. 1, a
mixture
of char and ilmenite, which previously were withdrawn from the bins 2, 3 and
were
mixed with each other in the mixing tank 4, is continuously charged via the
solids
supply conduit 1 into the suspension heat exchanger 5 of a first preheating
stage, -
in which the material preferably is suspended and preheated by the off gas
with-
drawn from a second preheating stage. Subsequently, the suspension is con-
ducted by the gas stream into a cyclone 6, in which the solids are separated
from
the gas. The separated solids then are delivered through conduit 7 into a
second
Venturi-type suspension heat exchanger 8, where they are further preheated to
a
temperature of about 800 C, and in a downstream cyclone 9 are again separated
from the gas stream.
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The ore thus preheated is delivered through the solids conduit 7' into the
carboni-
zation reactor 10, to which coal with a grain size of less than 5 mm as well
as
oxygen are supplied via the solids conduit 7". Furthermore, a fluidizing gas
con-
sisting of 70 vol- /0 carbon monoxide and 30 vol-% molecular hydrogen with a
temperature of about 600 C is supplied to the carbonization reactor 10 via the
gas
conduit 11 for fluidizing the solids in the reactor 10 by forming a stationary
fluidized
bed. The oxygen and fluidizing gas supply rate as well as the retention time
of the
solids in the carbonization reactor 10 are adjusted such that a temperature of
about 1,050 C is obtained in the fluidized bed and a sufficient carbonization
of the
coal is achieved. The coal supplied in conduit 7" can be externally predried
and/or
precarbonized before entering the reactor 10.
The gas-solids mixture is continuously passed from the carbonization reactor
10
via the connecting passage 12 into the reduction reactor 13, in which the
solids
are fluidized by the fluidizing gas supplied via the gas conduit 11' by
forming a
circulating fluidized bed, and the ilmenite is reduced by the reducing agents,
in
particular by carbon monoxide, to a degree of metallization of about 70%,
based
on its iron content.
Subsequently, the suspension is conducted by the gas stream into the cyclone
14
downstream of the reduction reactor 13, in which cyclone the solids are
separated
from the gas. Thereupon, the separated solids are recirculated through the
return
conduit 15 into the carbonization reactor 10, whereas the off gas containing
CO,
H2 and CO2 with a temperature of about 1,000 C is transferred via the gas
conduit
16 first into the suspension heat exchanger 8 of the second preheating stage
and
from there via the cyclone 9 and the gas conduit 16' into the suspension heat
exchanger 5 of the first preheating stage, in which the same is cooled to
about
500 C. Via the gas conduit 16", the off gas separated in the cyclone 6
downstream
of the suspension heat exchanger 5 is first conducted through a waste heat
boiler
(not shown), in which the off gas is cooled to approximately 200 C by
generating
steam (4 bar), before it is separated from dust in a scrubber 17 and cooled
further
to about 30 C. The solid/sludge outlet of the scrubber (fines of ore and
carbon)
can be further used in the process, e.g. after pelletizing as feed material to
mixing
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tank 4 and/or to the reactor 10 and/or 13 and/or furnace 22. Subsequently,
carbon
dioxide is removed from the off gas in the CO2 absorber 18, and the gas
mixture
thus purified can be preheated in a heat exchanger, e.g. with the gas from
conduit
16", and is heated to about 600 C in the gas heater 19, before it is
recirculated as
fluidizing gas into the carbonization reactor 10 and the reduction reactor 13
via the
conduits 11, 11'. Furthermore, the value of hydrogen and/or water and/or water
vapour in the circulating gas flow may be controlled e.g. by a hydrogen
permeable
membrane or a water condenser/absorber or water evaporator.
From the reduction reactor 13, a mixture of partially reduced ilmenite and
char with
a temperature of about 1,000 C is continuously withdrawn via the pneumatic
product discharge conduit 20, is cooled to about 700 C in a heat exchanger
(not
shown), and with this temperature is charged to the magnetic separator 21,
where
a fraction rich in titanium dioxide is separated as magnetic product from a
non-
magnetic fraction, which substantially comprises chromite, ash and char,
before
the magnetic fraction is charged into the electric furnace 22.
In the electric furnace operated at about 1,600 C, titania slag with 75 to 90
wt-%
titanium dioxide and liquid pig iron with more than 94 wt-% metallic iron are
ob-
tamed as products. The off gas from the electric furnace contains more than 90
vol-% carbon monoxide and, after dedusting, is burnt in an afterburning
chamber
(not shown), and the hot flue gas is supplied to the gas heater 19 for heating
the
fluidizing gas. Also a part of the circulation gas flow can be burnt and
supplied to
the gas heater 19.
In contrast to the plant described above, the plant shown in Fig. 2
additionally
includes an oxidation reactor 23 upstream of the carbonization reactor 10. Ore
preheated in the suspension heat exchangers 5, 8 is introduced into the
oxidation
reactor 23 via the solids conduit 7' and is fluidized with fluidizing gas
supplied via
the gas conduit 11", which before was preheated in the heat exchanger 24 with
the waste gas from the cyclone 14 downstream of the reduction reactor 13, by
forming a circulating fluidized bed. Furthermore, fuel is supplied to the
oxidation
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reactor 23 via conduit 16". The suspension is conducted by the gas stream into
the cyclone 25 downstream of the oxidation reactor 23, in which the solids are
separated from the gas. One part of the solids is recirculated to the
oxidation
=
reactor 23, while the other part is introduced into the carbonization reactor
10 via
the solids conduit 7". Off gas withdrawn from the cyclone 25 is transferred
via the
gas conduit 26 into the suspension heat exchanger of the second preheating
stage
8 and from there via the cyclone 9, the suspension heat exchanger of the first
preheating stage 5 and the cyclone 6 to a waste gas cleaning unit (not shown).
The invention will be explained below with reference to an example which demon-
strates the invention, but does not restrict the same.
Example
In a plant corresponding to Fig. 2, the suspension heat exchanger 5 was
charged
via the solids supply conduit 1 with raw ilmenite (12 kg/h) having a grain
size of
less than 1 mm with the following composition:
TiO2 50.04 wt-%
Fe203 13.44 wt-%
FeO 32.79 wt-%
MnO 0.58 wt-%
Si02 0.62 wt-%
A1203 0.53 wt-%
MgO 0.68 wt-%
Ca0 0.05 wt-%
0 wt-%
0 wt-%
Others 0.37 wt-%
Loss on Ignition (L01) 0.90 wt-%
Total 100.00 wt-%
Titotal 30 wt-%
Fetotal 34.90 wt-%
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After passage through the first and second preheating stages, the preheated
ore
was introduced into the oxidation reactor 23 via conduit 7', in order to
almost com-
pletely oxidize the FeO to form Fe203. Furthermore, fuel and fluidizing gas
were
supplied to the oxidation reactor 23 via conduit 11". After separating the
solids
from the gas in the cyclone 25 downstream of the oxidation reactor 23, the
solids
were introduced into the carbonization reactor 10 via the solids conduit Tu.
The
oxygen content of the waste gas from the cyclone 25 was 6 vol-%. Furthermore,
oxygen and 7.5 kg/h coal (Blair Athol, Cfix: 62%) corresponding to a ratio
Fe:Cfix of
1 were supplied to the carbonization reactor 10 via the solids conduit 7", and
in the
reactor 10 the solids were fluidized with a gas mixture of 70 vol-% carbon
monox-
ide and 30 vol-% hydrogen by forming a stationary fluidized bed.
From the carbonization reactor 10, the gas-solids mixture was continuously
intro-
duced into the reduction reactor 13 via the connecting passage 12, and the oxi-
dized ilmenite was partially reduced to a degree of metallization of 70%,
based on
its iron content.
Solids withdrawn from the reduction reactor 13 via conduit 20 were first of
all
separated magnetically in the magnetic separator 21, and the magnetic fraction
obtained thereby was charged into an electric furnace 22. The installed trans-
former capacity of the furnace 22 was 2 MVA. The titania slag was tapped every
2
hours, and the sponge iron was tapped twice per day.
In accordance with a chemical analysis, the titania slag and the sponge iron
thus
obtained had the compositions as shown in Table 1. The calculated electric
energy
consumption for the process was 1.004 kWh per ton of slag.
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Comparative Example
For comparison, crude ilmenite with the composition as stated above, which was
subjected neither to an oxidation nor to a partial reduction, was charged into
the
electric furnace 22 described in the above example instead of prereduced
ilmenite,
and molten.
The compositions of the titania slag and the sponge iron thus obtained are set
forth in Table 1. The calculated electric energy consumption for the process
was
2,050 kWh per ton of slag.
Table 1
Chemical composition of the titania slag and the sponge iron obtained in the
Ex-
ample and in the Comparative Example, respectively.
Example Comparative Example
= prereduced
ilmenite crude ilmenite
Femet in the charge 70 wt-% 0 wt-%
Titania slag
TiO2 88.5 wt-% 87.9 wt-
%
FeO 8.1 wt-% 8.8 wt-
%
Sponge iron
Fe 95.5 wt-% 95.2 wt-
%
Si 0.61 wt-% 0.60 wt-
%
FeS 0.68 wt-% 0.71 wt-
%
3.00 wt-% 2.90 wt-
%
Mn 0.12 wt-% 0.12 wt-
%
Consumption of 1,004 kWh/t139 2,050
kWhit
,.siag
electric energy
(calculated)
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List of Reference Numerals
1 solids supply conduit
2 reservoir for char
3 reservoir for ilmenite
4 mixing tank
5 suspension heat exchanger of the first preheating
stage
6 cyclone of the first preheating stage
7,7',7",7" ' solids conduit
8 suspension heat exchanger of the second preheating
stage
9 cyclone of the second preheating stage
10 (carbonization) reactor
11,11',11" gas conduit for fluidizing gas
12 connecting passage
13 reduction reactor
14 cyclone of the reduction reactor
= 15 solids return conduit
16,16',16",16" gas conduit
17 scrubber
18 CO2 absorber
19 gas heater
20 product discharge conduit
21 magnetic separator
22 electric furnace
23 oxidation reactor
24 heat exchanger
25 cyclone of the oxidation reactor
26 waste gas discharge conduit
