CA1124976A - Manufacture of elongate workpiece from pelleted material - Google Patents

Abstract

ABSTRACT OF DISCLOSURE
A process in which purified iron oxide in the form of pellets bonded with an organic binder and which may have been partially reduced, are heated in a reducing atmosphere to form sponge pellets and while at rolling temperature the sponge pellets are hot rolled in a rolling mill to form strip or similar elongate workpiece. The apparatus comprises a reactor in which the iron oxide pellets are reduced in a hot reducing atmosphere, a rolling mill and means providing a path from the reactor to the gap between the rolls of the rolling mill.

Description

llZ~9'76 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 llZ4976 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 llZ4976 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 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.

11'~4976 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 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, .. , . . . . .. , .. . .. . .. . . . ~

l~Z4976 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.

llZ4976 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 llZ4~76 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 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 .. . . . ..... ,, .. ~ .

llZ4976 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 . ~

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 llZ4'~76 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 1~24976 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 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 l~Z4976 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 ~lZ4976 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 l~Z~976 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.

Claims (10)

What we claim is:-

1. A process for the manufacture of an elongate workpiece in which pellets having a size between 1.5 and 15 mm are heated in a gaseous reducing atmosphere to form fully reduced sponge pellets and while at a temperature of at least 900°C and in a non-oxidising atmosphere are introduced into the gap between a pair of rolls of a rolling mill to be rolled into a sound, coherent, elongate workpiece, and wherein said pellets either consist essentially of an organic binder and iron oxides and contain 98% or more of iron in the form of oxides or are partially reduced and before reduction consisted essentially of an organic binder and iron oxides and contained 98% or more of iron in the form of oxides.

2. A process as claimed in claim 1, in which the pellets have a size in the range 3 - 7 mm.

3. A process as claimed in claim 1 or 2, in which the pellets are heated to a temperature of between 950 and 1200°C in the reducing atmosphere.

4. A process as claimed in claim 1, in which finely divided alloying material is added to the hot sponge pellets prior to rolling.

5. A process as claimed in claim l, in which the sponge pellets are subjected to a carburising atmosphere prior to being rolled.

6. Apparatus for producing a sound coherent elongate workpiece from pellets having a size between 1.5 and 15 mm and either consisting essentially of an organic binder and iron oxides and containing 98% or more of iron in the form of oxides or being partially reduced and before reduction consisted essentially of an organic binder and iron oxides and contained 98% or more of iron in the form of oxides, said apparatus comprising a reactor vessel having an inlet for receiving the pellets and an outlet, and including means by which pellets in the vessel are heated to at least 950°C in a reducing gaseous atmosphere to form sponge pellets, a rolling mill having a pair of rolls with a gap therebetween, an enclosure connected at one end to the outlet of the reactor vessel and at the other to the rolling mill and providing a path for the pellets from the vessel to the rolling mill, said enclosure having means for introducing a non-oxidising atmosphere therein and means for preventing oversized pellets from entering the gap between the rolls of the rolling mill.

7. Apparatus as claimed in claim 6, in which the reactor vessel is heated internally by hot ingoing reducing gas.

8. Apparatus as claimed in claim 7, in which the reactor vessel is a rotatable refractory lined kiln.

9. Apparatus as claimed in claim 6, in which the reactor vessel is heated by external heaters.

10. Apparatus as claimed in claim 6, in which said means for preventing oversized pellets from entering the gap comprise a pair of spaced apart rollers between which the pellets are passed and means for rotating the rollers in the same direction of rotation at different speeds.