Note: Descriptions are shown in the official language in which they were submitted.
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For many years the possibility of making an iron or steel
elongate workpiece from purified iron oxide ore by a route
involving powder metallurgy has occupied the attention of
those developing new metallurgical processes. Most proposals
involve the manufacture of iron powder which is then proces-
sed at ambient temperatures by conventional powder procedures
to form iron or steel strip or bar. While such processes are
technically successful, they are difficult to operate econom-
ically in competition with traditional methoas of manufactur-
10 ing steel in large tonnages.
It is an object of the present invention to provide amethod of, and apparatus for, producing an elongate work-
piece by an economical route which does not involve the
formation of iron powder.
According to the first aspect of the present invention,
there is a process in which pellets of purified iron oxide
containing not less than 98% of iron in the form of elemental
iron or iron oxides bonded with an organic binder and which
may have been partially chemically reduced are heated to at
20 least 900C in a reducing atmosphere to form fully reduced
sponge pellets and while at hot rolling temperature the
sponge pellets are introduced into the gap between the rolls
of a rolling mill and are rolled into a substantially dense
elongate workpiece.
The purified iron oxide ore is formed into pellets having
a size between 1.5 and 15 mm using an organic binder and the
sponge pellets formed therefrom are fed into the rolling mill
along with any required alloying material where they are hot
rolled to form iron or steel strip or bar or
B -2-
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or other small section.
Hot rolling of iron powder as opposed to pellet
rolling has been proposed for iron but because of difficulties
of heating, sticking tendencies and difficulties of feeding
it has largely been abandoned as a practical proposition.
The benefit and difference of rolling of sponge
pellets is two-fold. Firstly sticking and feeding problems
are greatly diminished. Secondly, and a very important
consideration, is that the undeformed sponge pellet in the
roll nip has an extremely low overall density. There is
porosity within the pellets themselves and additional
porosity between the pellets. They are therefore super-
compressible compared with iron powder and a much greater
diminution in thickness is used to obtain a solid strip or -
bar than with iron powder, and far lower rolling loads are
used. Consequently it is possible to get a much more
uniform distribution across the width of a rolled strip
leading to less edge and surface cracking than with the
rolling of iron powder.
In the description of the process of the invention
the term steel is frequently used even when the material may
in fact be free from carbon. Strictly, the product, when
free from carbon, is an iron matrix containing very large
numbers of fine irreducible oxide inclusions. me structure
is therefore entirely different from either wrought iron or
conventional low carbon steel. It is in fact a new material
for which no short name has yet been found. In this
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context it will frequently be referred to either as sponge
iron when in the un-consolidated state or as steel when
consolidated even though in some cases it is free from carbon.
When the product is carburised there is, of course, no
ambiguity in naming the product.
It is necessary to feed into the rolling mill
pellets which have an average size sufficiently large to
avoid the undesirable effect of particle starvation at the
roll nip caused by the efflux of gas when the rolls are
operating at reasonably high speeds. It is also desirable
to ensure good flow of the pellets into the nip of the
rolling mill when rolling light sections. The preferred
shape is therefore rounded or near spherical and the preferred
size range lies between 1.5 and 15 mm diameter. Most of
the successful experimental work has been carried out with
pellets in the range of 3 mm to 7 mm.
~ uring the pelletising process it is essential to
avoid contaminating the high purity iron oxides with any
deleterious material which would remain in the steel strip
after processing. At the same time it is found necessary
to use a binder when making the green pellets to ensure that
they are sufficiently hard to withstand subsequent processing,
prior to chemical reduction, with the minimum of fracture.
The term green pellets is used to distinguish pellets that
have not been heated to a high temperature from those that
have. Such binders as Bentonite which are commonly used
in industry for assisting pelletising and which contains
silica, are excluded. me most satisfactory binders
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have been found to be organic materials which on heating
decompose to give only carbon or gaseous products which
can escape from the pellets and therefore cause no
contamination of the final product. Sulphur and
phosphorous containing binders are to be avoided.
The starting point in the process is purified iron
oxide. This is most economically and conveniently produced
by the treatment of very finely ground iron oxide ores,
either wet or dry, using high intensity magnetic fields
together with dif~erential floation if necessary to yield
a high purity concentrate. Typical concentrates which are
available on the commercial market and are suitable for
the process generally contain 98% or more of iron in the
form of oxides. A typical cpomposition of a concentrate
based on Fe304 is for instance - Fe304 ~7.0%, Fe203 2.4%,
MgO 0.16%, A1203 o.l8%, SiO2 0.06%, TiO2 0.16%. Worldwide,
suilable concentrates based on either Fe304 or Fe203 can
readily be obtained from many ores by standard purification
procedures.
The first stage n the process is the preparation
of green pellets of high purity oxide concentrate by feeding
a thick slurry~of water, concentrate and an organic binder
into pelletising equipment. Pelletising is a standard
metallurgical procedure. To achieve the correct size
distribution the green pellets are screened to avoid either
very large or very small fractions after which they are dried.
Subsequently two slightly di~ferent routes may be followed.
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The green pellets may either be partially reduced with
reducing gases such as H2 or mixtures of H2 and C0 at high
temperature to give a high degree of metallisation - say
9~% - without proceeding to complete reduction. These
partially reduced pellets will be termed pre-reduced pellets.
Alternatively the green pellets may be fed directly into a
reduction reactor where they are fully reduced at high
temperatures as described below.
Both the dried green pellets and the pre-reduced
pellets are hard and are easily handled. The pre-reduced
pellets may, with advantage, be processed at the geographic
site of the ore field. This improves the economy of the
total process and it avoids the shipping of unnecessary
oxygen in the form of combined oxide. It also enables
pre-reduced pellets to be produced which are very hard
and abrasion resistant so diminishing losses caused by
powdering. There is no advantage in taking the reduction
far beyond 95% if transport over large distances is involved,
as some oxidation will occur during subsequent transit and
handling, thus necessitating a subsequent chemical reduction.
Thus the pre-reduced pellets may be transported in this form
to the geographical site where the steel products are made.
The next stage in the process is the full reduction
of either green pellets or the pre-reduced pellets to sponge
iron pellets by means of H2 or mixtures of H2 and C0 at high
temperature. The reduction reactor may take on a variety
of forms including a vertical shaft reactor, but a
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particularly effective form is the horizontal rotating kiln.
It is essential to feed the pellets into the rolling
mill at 900C or above. At temperatures up to 1000C little
agglomeration of the pellets takes place but as the temperature
is raised further, agglomeration increases. However the
amount of agglomeration is not serious and is small compared
with that commonly experienced when powdered oxides are
reduced at similar temperatures.
At the issuing end of the reduction reactor the
hot fully reduced pellets are passed through a distributor
and sizing equipment and fed directly into the nip of a
rolling mill.
It is, of course, necessary to avoid re-oxidation
of the reduced epllets after they pass from the reduction
reactor. A neutral or reducing gas cover is therefore
maintained from the exit of the reduction reactor to the nip
of the rolling mill.
It is possible to make small alloying additions
to the material being processed. In certain cases and
circumstances, metal or allow powders or carbon powder may
be added to the pellets entering the reactor, but in other
cases, notably.with chromium and frequently with manganese
and carbon, it is desirable to make the additions ir, powder
form to the pellets after emergence from the reduction
reactor when the oxygen in the gases and in the particles
or pellets has reached a very low level.
According to a second aspect of the invention,
.. , . . . . .. , .. . .. . .. . . . ~
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apparatus for producing an elongate metal workpiece comprises
a reactor vessel for receiving pellets of purified iron oxide
bonded with an organic binder and including means by which
pellets therein are heated in a reducing atmosphere to form
sponge pellets, a rollng mill for rolling hot sponge pellets
into an elongate workpiece and means providing a path for hot
sponge pellets from the outlet of the reactor vessel to the
gap between the rolls of the rolling mill.
The type of rolling mill into which the hot sponge pellets
are fed depends on whether strip or bar is being produced.
In the case of strip, a rolling mill with large diameter
cylindrical rolls having their axes in a horizontal plane is
needed whereas in the case of rod a similarly arranged mill
with large diameter profiled rolls is necessary.
In order that the invention may be more readily understood
it will be described, by way of example only, with reference
to the attached drawings.
Figure 1 is a schematic sectional side elevation of
apparatus in accordance with the invention for producing
steel strip,
Figure 2 is a plan view of an alternative form of the
rolls of the rolling mill,
Figure 3 is a side elevation of part of the rolls shown
in Figure 2, and
Figure 4 is a vertical longitudinal section of an
alternative apparatus according to the invention.
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Referring to Figure 1, a hopper 1 contains high
purity pre-reduced iron pellets 2 of 3 to 7 mm diameter
which are fed by means of a screw mechanism 3 into a sloping,
rotating refractory lined kiln 4 operating on a reverse flow
principle at a temperature of 950 - 1200C in the hottest
zone. Pre-heated re~ucing gas containing approximately
~1% H2 and ~/3 CO, such as can be produced by the reforming
of naphtha or natural gas is fed into the kiln at 5 and after
use passes out at 6. Part of the gas issuing from 6 is
used as a fuel to preheat fresh ingoing gas and part passes
to heat exchangers and then to cleaning and drying equipment
(not shown) where water and C02 are removed. The clean,
dry gas is then preheated and recirculated together with
fresh reducing gas and re-enters the reactor at 5. The
technique of reducing iron oxides by high temperature gases
together with the necessary recirculation procedure has
been well established both with fluidised bed reactors
and with the well known Midrex process and is widely reported.
In the rotating kiln 4, the pellets are fully
reduced by the high temperature reducing gas to sponge pellets
and this process is` facilitated by the constant movement of
the pellets as~ the kiln revolves. ~ecause of the small
size of the pellets the reduction time in the reactor is
short. In the example given the time is one hour thus
giving a high throughput. A typical rotating kiln would be
10 m in length and 1 m in diameter rotating at a speed of
2 rpm. Because of the rotation of the kiln, agglomeration
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is minimised but some small particles or pellets ~ay grow
to form agglomerated pellets over 10 mm diameter, while
a small amount of fine iron particles or powder is formed.
The particles and pellets 7 travel down the kiln and are
delivered on to a spreading or distributing grid 8 which
has the effect of distributing the pellets uniformly across
the width of a screen 9, i.e. in the direction of the axes
of the rolls onto which they fall. The purpose of the
screen is to remove oversize pellets greater than say,
15 mm diameter. Such oversize pellets are formed in very
small quantities, but any so formed are taken off at 10 to
be cooled, crushed and returned.
The pellets are allowed to fall whilst still hot
and at a temperature of about 950 - 1200C into the nip of
two rollers 11 and 12 which rotate in the same direction.
The spacing of the rollers can be varied at will, but in
this case they are spaced apart by a distance of 7 mm.
The rollers may be of different diameters but one of the
rollers 11 has higher peripheral speed than roller 12. The
high peripheral speed of roller 11 enables particles of less
than 7 mm diameter to pass through while larger particles
or pellets are~subjected to both compression and to very high
shear forces simultaneously as a result of their being
forced against the opposing roller 12. Pellets of sponge
when subjected to this combination of forces are fragmented
even when at high temperatures. The srnaller ~ragments pass
through the rollers whilst the larger fragments are rotated
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1~2~976
and re-enter the rollers to be fragmented once more until all
the larger pellets have been reduced in size to approximately
7 mm. It is an essential feature of the shearing rolls that
they operate in the manner illustrated. If they have
similar peripheral speeds in reverse directions as in the
case of a conventional rolling mill the oversize sponge
pellets or aggregates o~ pellets would simply plastically
deform and densify rather than fracture by shear. The roller
11 is rotated at such a high speed that no substantial
accumulation of pellets occurs at the nip. In the example,
the rollers 11 and 12 are each 50 cms in diameter and have
peripheral speeds of 200 and 20 m per minute respectively.
This relatively high speed of rotation compared with the
rate of delivery of sponge pellets from the kiln ensures
that they enter the nip of the rollers 11 and 12 sufficiently
separated from one another that they remain as discrete
particles and are not consolidated into a strip at this point.
The hot pellets at a temperature of between 900 and
1150C then fall into the nip of a rolling mill having rolls
13 and 14 which turn in opposite directions. By hot
compression of the pellets hot rolled strip 6 mm in thickness
and 40 cms in width is produced which is coiled a-t 15. The
speed of the rolls 1~ and 14 of the rolling mill is variable
to enable the rolling mill speed to match the delivery of
pellets at the nip. In the example, the peripheral speed
of each of the two rolls 13 and 14 is the same but is is
variable between the limits of 40 and 10 m/min. The speed
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is always maintained sufficiently low to ensure that sound,
coherent hot rolled strip is produced.
Typical speeds of operation are approximately
15 m per min with a roll gap of 6 mm. The precise speed
of operation is adjusted so that a saturated feed of pellets
is maintained giving an effective diminution of approximately
80% in thickness. It is essential that the rolls be of
large diameter, as it is necessary to diminish the thickness
of the material in one large rolling pass. In the example
the rolls are each 90 cms in diameter and 40 cms in face width.
In the example the speed of the rolls was adjusted
in the region of 15 m per min. Fine adjustments were made
to the speed to ensure that a pellet height 16 of not less
than 15 cms above the line of centres of the rolls was
maintained. At a roll gap setting of 6 mm this gave
appr~ximately an 85% diminution in thickness of the pellet
mass as it passed through the rolls. In these circumstances
the maintenance of a constant height of pellets was not
critical provided the pellet feed was always fully saturated.
Generally speaking, the larger the roll diameter
the thicker is the maximum thickness of strip that can be
produced. If strip much thinner than 6 mm is required using
90 cm diameter rolls it is advisable to fit 2 adjustable
plates just above the roll nip having their planes parallel
~5 to the axes of the rolls. The plates act as a funnel for
the pellets as they fall into the nip of the rolls.
Side plates are fitted to the rolls to prevent
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1~24976
pellets escaping sideways from the roll gap and therefore
ensure that a saturated feed of pellets is maintained.
The sizes, speeds and temperatures above are given
for example only and are in no way intended to impose a
specific lim1t on the process. The roller shearing equipment
is not essential to the process, but it is of value in the
invention because the material being treated is sponge.
This densifies and becomes ductile when compressed at high
temperatures but has not been found to fracture when subjected
to high shear forces at high temperatures. In this respect,
pellets of sponge behave differently from fully dense iron
pellets or particles which have been found to remain ductile
and be plastically deformed at high temperatures under
combined compression and shear. Sponge pellets also differ
in behaviour from particles of iron oxide or other brittle
oxides which will fracture under simple compression between
the contra-rotating roller of a conventional rolling mill.
In the example, oxidation of the reduced sponge
pellets after leaving the rotating kiln is prevented by
maintaining a small flow of reducing gas which is introduced
at ports 17 and 18 in a water cooled enclosure 19 which
is sealed on to the rollers 11 and 12 and the rolls 13 and 14.
The additional reducing gas subsequently passes into the main
reduction reactor. An inert gas such as nitrogen may also
be used for the purpose of avoiding re-oxidation but:in
this case the flow must be controlled at a low level to
avoid undue dilution of the reducing gases in the reduction reactor
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The rollers 11 and 12 are fitted with scrapers 20
and 21 which preven-t iron particles accumulating near the
seals with the enclosure.
When bar or rod is required, profiled rolls are
used instead of the flat rolls shown at 11 and 12, otherwise
the equipment is similar.
In Figure 2, the nip of the two rolls 22 and 23
is shown in plan. Each roll is adjusted so that the tips
of the rolls 24,which may be slightly rounded, are almost
in contact, or they may actually be in contact. Hot pellets
25 are fed into the roll nip in the same way as with the
cylindrical rolls. They are prevented from spilling sideways
by side plates 26 in Figures 2 and 3 which fit closely to the
sides of the rolls. The pellets fill the roll nip up to a
level 27 such that the level is at or above the level at
which the pellets beging to be consolidated by the rolls.
The diameter of the rolls must be làrge enough for the height
of pelle-ts up to this level to be sufficient to enable a
diminution of thickness at the line of centres of the rolls
to be approximately 80% or more. Diminutions of thickness
below 75% are likely to leave major porosity in the rolled
bar 28 whereas diminution of thickness much greater than 85~o
is generally unnecessary.
In the example the profiled rolls rotate at a
peripheral speed of approximately 22 m per min. The
rotation of the rolls which are 150 cms in diameter and
16 cms wide, causes the pellet mass 29 to consolidate
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commencing at a height above the line of centres of approximately
20 cms, and to be separated by the tips of the profiled rolls
22 and 23 Figure 2 into four square bars 28 having a 2 cm side.
There is a degree of transverse flow of the consolidated pellet
mass into the body of the bars which often issue from the
profiled rolls with a very thin fin joining them. This
however, is readily broken enabling multiple bars to be
produced from a single pellet feed. Other roll configurativn
can be used for the rolling of bar from pellets, but the one
described has been found to be particularly suitable. It
will usually be found necessary to relieve ~he rolls at the
extreme edge so that none of the pellets are rolled between
flat parts of the roll having too small a roll gap. Equally
it is advisable to have a small flat portion at the edges of
the rolls as shown in Figure 2 in order to ensure that the
outermost bars are completely filled.
It is often required to produce steel containing a
small amount of carbon in order to improve mechanical
properties. It will be appreciated that without the addition
of some carbon, as described above, or the use of a carburising
atmosphere in the reduction furnace the sponge pellets emerging
from it, and therefore the steel strip or bar product, will be
virtually free of carbon. The carbon content can be increased
in two ways. Firstly a small amount of finely divided carbon
can be added to the hot pellets. Secondly, and more
effectively, the maintenance of a carburising atmosphere near
the outgoing end of the reduction furnace will ensure that
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the Garbon content is raised. The techniques for controlling
the carbon c~ntent of steels by alteration of the composition
of the reducing gases is well known. A high proportion of
C0 or the presence of residual hydrocarbons will ensure that
the carbon content is raised. The raising of the carbon
content when rolling bar is a matter of some importance as
it is frequently required to have higher mechanical properties
than are attainable with the carbon free product.
Trials have been undertaken with horizontal or
slightly inclined rotating tube type furnaces. It has been
found that slow rotation of the tube when loaded with the
special high purity organically bonded pellets maintained
the pellets in constant rolling motion and thereby avoided
any sticking. High speeds of rotation were not beneficial
because of centrifugal effects and because of the danger of
abrasion and breakdown of the pellets. Low speeds of
rotation varying from 6 rpm to 0.5 rpm, depending on whether
te furnace tube was small or large, were found to be highly
effective in promoting a rolling action of the pellets and
preventing sticking without appreciable degradation of the
pellets. It also enabled pellets to be transported along the
length of an inclined rotating tube from the upper to the
lower section and finally poured into the nip of the rolling
mill. It was found moreover that such horizontal tubes
could be operated satisfactorily when filled with pellets up
to, or even above, half the total tube volume. Even when
half full of pellets, a~y pellets adhering temporarily to the
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metal tube were detached during subsequent rotation. Any
adhesion was therefore only transient. A further observation
was that as a result of the rolling action induced in the
pellets their density increased slightly. They also
became much smoother and more rounded. This was advantageous
from the point of view of subsequent feeding and compaction
by hot rolling.
~ hen using the procedure described above it was
~ound to be advantageous to use a rotating heat resistant
metal tube as the rotating member. A suitable material ~or
constructions was a high temperature Ni-Cr alloy. The
good thermal conductivity and high temperature resistance
of this material allowed such a tube to be heated externally
by electrical means or by gas or oil burners. S~ch an
arrangement enabled the pellet charge to be heated conveniently
by external means rather than internally by means of the
high temperature gases used ~or reduction as described in the
embodiment of Figure 1.
Freedom from sticking under the conditions described
above was generally ~ound to be so pronounced that it was
unnecessary to use additional means oi fragmenting any
agglomerates that might otherwise have formed.
Figure 4 represents a vertical longitudinal section
of alternative apparatus for converting approxi~ately 95%
reduced pellets of sponge to iron or steel strip.
Pellets consisting of sponge containing not more
than 5% of iron oxide are introduced by means of a screw
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conveyor 41 from a container or hopper 42 into a large
diameter inclined tube h3 composed of Ni-Cr heat resisting
alloy. The tube is 8 m long and 0.8 m internal diameter.
The tube is surrounded by a gas fired furnace body 44 fitted
with gas burners 45 arranged to fire tangentially so that the
flame does not impinge directly on the tube 43. me tube
is heated to a temperature of 1050-1100C by the burners.
The waste gases exit from the Mue 46 and are
used to preheat incoming reducing gas (H2/C0) to a
temperature of 800-900C which is injected at 47 and is
exhausted at the off-take 48. The inclined tube 43 is
rotated slowly (2 rpm) such that the charge 49 on the tube
gradually works its way along the tube to discharge at the
lower end. The rotating tube has flanges 50 fixed to it at
each end which are resting on freely rotating grooved rollers
51 two at each end positioned with their axes parallel to the
axis of the rotating tube, but not in the vertical plane
containing the axis of the rotating tube. The tube is driven
by a chain wheel 52, the drive not being shown.
During its passage along the tube the charge 49
consisting of pre-reduced pellets us heated to a temperature
between 1000 and 1050C and is rapidly reduced to fully
reduced sponge pellets which are fed through an oscillating
grid 53 fixed below the pellet exit from the tube and inside
an insulated Ni-Cr fixed delivery enclosure 54. The grid
is intended to separate oversize pellets which are taken off
at 55 cooled, broken and returned to the hopper 42. The
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pellets drop through the oscilla-ting sieve 53 fall into a
feeding chute 56 fitted with side plates situated in the
nip of the large diameter rolls 57 which roll the pellets
to form strip 58 which is coiled at 59. An auxiliary
supply of H2/C0 is fed into the fixed delivery enclosure
54 at 60. The excess H2/C0 flames off at nip of the feeding
chute so preventing oxidation of the fully reduced pellets
as they pass through the nip of the rolls.
The rolls are driven by a variable speed motor,
which speed is varied such that a saturated feed of pellets
is maintained in the nip of the rolls without allowing undue
pile-up of pellets in the feeding chute. This is an important
aspect of the control of the process because if the accumulation
of pellets at the base of the feeding chute is insufficient
the feed to the rolling mill will be unsaturated and
unsatisfactory porous and unsound strip will result. Too
large an accumulation of pellets in the feeding chute will
lead to sticking which in turn will prevent continuous
and steady feeding into the rolls. In this connection
vibration of the feeding chute may be of some benefit.
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