Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a method of preventing sticking
when reducing fine grained material containing iron oxides in a fluid bed.
It is a well known fact that one of the reasons that the fluid bed
technique has been used to a very limited extent for producing entirely or
partially metallized iron ore concentrates, is the difficulty of avoiding
sticking at the high reduction temperatures which are required for reasons
of reaction kinetics. Sticking is when the small particles of material in
the bed agglomerate into larger particles and aggregates which in the end
make fluidization impossible.
It has already been proposed to spray oil into a hot fluid bed in order
to effect cracking of the oil to form cokelike products which are deposited
on the iron ore particles and prevents sticking. It has also been proposed
that when reducing in several stages, a suitably low temperature should be
maintained in part of the system for a deposit of carbon to be formed by the
reducing gas containing carbon monoxide, this carbon being deposited as soot
on the particles. Similar proposals have been made to use solid fine grained
carbonaceous ma~erial, in such quantities that the carbonaceous particles
mechanically prevent sticking between the more or less reduced ore particles.
There are drawbacks to all these methods.
The present invention relates to a method of avoiding sticking when
reducing material containing iron oxides in fluid beds supplied with fine
grained iron ore concentrates and fine grained solid carbonaceous material
by effecting the formation in the hot fluid bed of micro-aggregates between
particles of concentrate and the porous, popcorn-like coke particles formed
when the carbonaceous material is
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introduced into thc bed. Micro-aggregates means small aggregates, generally
comprising one co~e particle and one or a few concentrate particles adhered
thereto. Since the majority of the concentrate particles are in this way
aggregated to coke particles, sticking can be completely avoided when reduc-
ing in fluid beds at high temperature with reducing gases, even if the flo~
of carbonaceous material is small in relation to the flow of iron ore concen-
trates.
According to the invention there is provided a method of reduc-
ing fine grained material containing iron oxides in a fluid bed, cha:racterized
in that fine grained solid carbonaceous material consisting of fossil fuels
having a content of at least 15% volatiles i5 introduced at one or more
points in the fluid bed and that the material containing iron oxides is intro-
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duced~in the limited zones formed around said inlets, where the volatiles in
the carbonaceous material are driven off and partially gasified and coked.
In a prepared form of the invention a preferably continuousflow of solid fine grained carbonaceous material having a high content of
volatiles is introduced into a fluid bed at one or several points. At the
same time fine grained material containing iron oxides is introduced in a pro-
portionally adjusted flow within the limited zones around said inlets, where
the volatiles in the carbonaceous material are driven off and partially gasi-
fied and coked. ~hen fine grained carbonaceous material is introduced into
a hot fluid bed consisting, for example, of micro-aggregates of coke and con-
centrate, the particles are heated extremely quickl~ causing tar-like sub-
stances to "sweat out" and accumulate on the surface of the particles. The
more rapid the heating, ~he more tar will have time to accumulate on the sub-
face before it is cracked to gas and coke. Such tar covered particles seem
to have a preferance for adhering to particles of concentrate, thus forming
micro-aggregates of coke and concentrate. Surprisingly enough the aggregates
do not grow over the micro stage. The reason for this is probably that the
3~ period during which the tar layer is sticky before it is gasified and coked
is so short that the particle does not have time to come into contact with
more than a few par~icles of concentrate.
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In order to achieve a high rate of micro aggregate format-
ion~ meaning that a considerable proportion of the concentrate
particles are aggregated with coke particles, the carbonaceous
material should have >15 % volatiles. Better results are
obtained with higher contents of volatile~ such as in bituminous
coal and lignite. The particle size of the carbonaceous material
is dependent on which type o~ fluid bed is desired. For convent-
ional fluid beds the particle size should be <5 % and for fluid
beds o~ the circulating type <0.5mm. For technical reasons of
fluidization the material containing iron oxide should have a
smaller particle size than khe carbonaceous material because
of its higher specific weight.
The dimensions of the zone within which the formation of
micro-aggregateS occurs is dependent on the particle size of
the carbonaceous material and its content of volatiles, as well
as temperature and gas velocity in the bed. A ~ethod which can
be used in certain cases to ascertain that the material contain-
ing iron oxides arrives in the intended zone is to introduce it
mixed to~ether with the carbonaceous material. However, if the
material containing iron oxides is preheated this method cannot
be used due to the ri~k of clogging the inlet nozzles.
A reducing gas ~uitable for reduction of material contaîn-
ing iron oxid~ preferably consists of a mixture of C0 and H2
obtained by partial combustion of carbonaceous fuel with oxygen
2~ or gases containing oxygen, such as air. The ~as may either be
supplied to the ~luid bed from an external gas producer or be
produced internally in the bed by partial combustion of a part
of the carbonaceous material, which then must be supplied in
greater quantities.
The formation of micro-aggregates gives efficient
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protection against sticking even at high degrees of metallizat-
ion. Thus at 70-80 % metallization, for example~ and 900C the
flow of carbonaceous material required is not greater than that
required to give a product containing carbon corresponding to
its oxygen content plus the desired excess. Such a product can
give crude iron with ~4% carbon upon melt reduction without
extra addition of coke for the reduction process.
The method according to the invention will be further
described in connectlon with its use in two different types OI
fluid bed reactors. In Fig. 1 its use is illustrated with a
conventional fluid bed and in Fi~. 2 its use with a circulating
fluid bed.
Fig. 1 shows a conventional fluid bed consisting of a
reactor chamber 1 with a gas-distributing bottom 2, supply
pipe 3 ~or gas and outlet 4 for consumed gas. The gas supplied
fluidizes the bed above the distributing bottom. A suitable
flow o~ particles is tapped Ofr through an outlet pipe 5. When
the fluid bed is used for reduction of material containg iron
oxides, a preferably continuous flow thereof is supplied through
an inlet pipe 7 into the bed where it is re~uced by a reducing
gas supplied through the pipe 3, and the reduced bed material
is tapped through the outlet pipe 5. According to the invent-
ion coal powder or other carbonaceous materials with a high
content of volatiles are supp~ed continuously in an adjusted
flow through a supply pipe 6. At khe outlet of the supply pipe
6 a zone 8 is formed where the volatiles are driven off and
partially gasified and coked. The supply pipe 7 for the materlal
containing iron oxides is located so that the material is fed
into said zone, whereupon micro-aggregates of coke and concentrate
are formed.
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~ ig. 2 shows a circulating fluid bed, i.e. a fluid bed
in which the velocity of the gas is kept so hi~h that the bed
fills the entire reactor chamber and the gas leaving therefrom
has a high content of solid particles which are separated in a
cyclone and then returned to the reactor chamber. For the
materials in question here, this type of ~luidization is obtain--
ed at a gas velocity of 2.5 - 5 m/sec. The particle size of
material treated in a circulating fluid bed can be smallerthan
that used in a conventional ~luid bed. For instance, the
material containing iron oxidesshould have an average particle
size <0.5 mm, preferably <0.3 mm. The fluidized bed reactor
shown in the drawing, which is intended for reduction of material
containing iron oxides, conssistsof a reactor chamber 10, 11, 12
the upper part 10 being connected to a cyclone 13 having a
return pipe 14 which returns the separated solid particles to
the central part 11 of the reactor chamber. In the lower part 12
the material containin~ iron oxides is reduced by a reducing gas
introduced from below. When this gas reaches the central part 11
of the reactor, the gas is partially combusted together with
the solid carbonaceous material by air supplied through a number
of small nozzles 15, thus generating suffic~nt heat for reaction.
A continuous flow of fine grained carbonaceous material having
a high content of volatiles is supplied to the central part 11
through a su~y pipe 16. The volatiles are very quickly driven
out of the particles of carbonaceous material in zone 17 around
the inlets. Pre-heated material containing iron oxides is intro-
duced in a proportional flow in these zones through supply
pipe(s) 18. In this way micro-aggregates of coke and concentrate
are formed which are carried by the gas through the upper part
0 of the reactor chamber to the cyclone 13 and returned to the
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central part 11 of the reactor through the return pipe 14.
After one or several circuits in the upper part of the reactor
The particles, which are now heated and more or less reduced;
pass down into the lower part 12 of the reactor where they are
further reduced by the strongly reducing ~as there, after which
they are drawn off through the outlet 19. The flow o~ solid
material fed ~o the reactor and the flow of material drawn o~f
therefrom are controlled so that the quantity of solid material
in the reactor remains constant. The transmission o;~ heat ~rom
the upper part of the reactor to its lower part is high due to
the considerable internal circulation of bed material, which
gives extremely good temperature e~ualizationO Partially com-
busted gas leaves the cyclone 13 to a venturi means 20, to which
untreated material containing iron oxides is supplied at 21.
This material is pre-heated by the gases, separated in two
cyclones 22 and 23 and supplied through the supply pipe 18 to
the central part 11 of the reactor. Some of the gas leaving the
cyclone 23 is freed from C02 and H20 in washing means 24 and
used~ after heat-exchanging and pre-heatingg as the reducing gas
in the lower part 12 of the reactor, while the remainder 25 is
is used for some other purposeg such as for providing electric
energy for melting and final reduction of the completely or
partially reduced product obtained.
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