CA2035901A1

CA2035901A1 - Sintering process

Info

Publication number: CA2035901A1
Application number: CA 2035901
Authority: CA
Inventors: John R. Field
Original assignee: Individual
Current assignee: Ciba Specialty Chemicals Water Treatments Ltd
Priority date: 1989-07-07
Filing date: 1990-07-09
Publication date: 1991-01-08
Also published as: JPH04500701A; WO1991000930A1; GB8915639D0; EP0433423A1

Abstract

Abstract A porous burden is formed of carbonaceous material and powdered or other metallurgical material and comprises wet unfired agglomerates of the metallurgical material and water absorbent particulate polymeric material is included in the burden. The burden is fired from one surface and air is drawn through the burden from the fired surface so as to fire substantially the complete burden and to convert it to a sintered mass, and the sintered mass is broken into lumps of sinter.

Description

W09t/00930 PCT~GB90/0lOS4 ... ~ .

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Allied Colloids Limited Sir.tering Process The charge for metallurgical processes such aS ~last furnace or direct reduction furnace processes is nowadays, to an ever increasing extent, subjected to ~arious pretreatments either to improve the efficiency o~
the process or to permit the use o~ lower grade materials or both.
One pretreatment invo~ves the com~inution of metallurgical ore and the conversion of the comminuted ore to relatively strong pellets. The comminuted ore is agglomerated in the pressence of a small amount of water, and also a binder, to form agglomerates a~d these are then fired to form hard pellets. The components of the agglomerates and the cor.ditions of firing are selected such that some degree of sintering occurs within each agglomerate (to produce the fired pellet) whilst avoiding any substantial aqgregation and sintering of the agglomerates to one another. The binder that is traditionally used for the production of these sintered pellets ic bentonite but more recently there have been prcposals to use water soluble organic polymer and in Canadia~ patent 890,342 it is proposed to use a highly water swellable, cross linked, acrylic polymer. It is stated that the water tolerance and green strength of the . initial agglomerates is improved and that the dried and sintered product can have improved strength properties.
This is, of course, very valuable in view of the possible high loading pressures that may exist within, for instance, a blast furnace.
Another pretreatment is the conversion of metallurgically useful dust to a sinter that can be charged to the furnace. This dust might otherwise be wasted. Examples are, for instance, Basic Oxygen Steel dust (BOS dust), Stoc~ YaFd dust and co forth. A common .

WO9l/00930 PCT/GB90/0l054 ~3..'~

way of c~nverting this dusty or other metallurgical material to a sinter that can be charged to a metallurgical furnace is to fire one surface of a burden ~i.e., a mass) of the material and to draw air through the burden so as to cause firing to occur throughout substantially the complete burden and to convert it to a sintered mass, which is then broken into lumps of sinter.
This process conveniently is carried out on apparatus known as a sinter strand~ In this, the burden is carried as a layer on a moving strand that leads from a firins position at which the upper surface of the layer is fired, over means by which air is drawn through the layer, and to a sinter breaking position at which the sinter mass is broken into lumps.
~n order that the process operates efficiently, it is essential that the burden is porous throughout the process, as otherwise air cannot be drawn through it sufficient to fire the entire burden. For this reaEon the dusty or other starting material is converted into the form of agglomerates in order that these agglomeratec form a porous burden throush which the air can be drawn to cause firing. Since the agglo~erates are formed ~ solely for the purpose of giving porosity to the material ~~ that is to be fired, it is desirable to bond them as easily as possible and it is desirable that they should remain unfired. Conveniently therefore they are bonded by water.
Unfortunately the amount o. water in the wet agglomerates has to be selected very carefully since if it is too low the agglomerates do not bond satisfactorily and if it is too high they tend to collapse and prevent the passage of air, leading to inadequate firing and sinter ~ormation. In particular, the agqlomerates that have been fired are hotter than those that have not, and so the process tends to drive off water from the "",,, ~, "~

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initially fired agglomerates and to provide 2 condensation front at which the water condenses on to unfired agglomerates. Accordingly the water content of the unfired agslomerates may increase as the condensation front descends through the burden, leading to collapse of the agglomerates.
The optimum amount of water in the agglomerates is generally around 10~ and it is necessary to control the water content of the burden very accurately since if the water content deviates from the optimum by more than, for instance, about 1% (based on the weight of agglomerates -or about 10~ based on the weight of water) the porosity of ~he burden is greatly reduced. Thus, if the water content is too low the agglomerates will be too dusty and i~ it is too high the~ will collapse, especially as the condensation front advances.
It would therefore be desirable to be able to modify the sintering process so as to make it much less sen itive to the amount of water in the burden.
A sintering proces~ according to the invention is one in which a porous burden is formed of carbonaceous material and me~allurgical material and comprises wet unfired aqglomerates, the burden is fired from one surface, air is drawn through the burden from the fired surface to fire substantially the complete burden and to covert it to a sintered mass, and the sintered mass is broken into lumps of sinter, and in this process water-absorbent particulate polymer is included in the burden. -. .
The process can be conducted on a static burden, but is preferably conducted continuously on a sinter strand.
Thus the burden is applied to a moving strand by which it is carried as a layer from a firing position at which the upper surface Gf the layer is fired, over wind boxes or J-.
~ther means by which air is drawn through the layer, and , W09l/00930 PCT~G~90~tO54 2 ~3 ~

to a sinter-breaking position at which the sintered mass is broken into lumps of sinter.
The burden that is initially formed must be porous.
If it is not porous, it will not be possible to draw air through ~he burden so as to spread the firing of the burden from the surface where it is initially fired.
The burden must also be sufficiently flowable that it is possible to apply it to, or instance, the moving strand or o~her surface on which it is to be supported during 10 firing. - -Usually substantially the entire burden is in the form of the wet agslomerates but sometimes it is convenient for part of the burden (e.g., at least 25~, usually at least 50% and preferably at least 7~ by weight) to be in the form of wet agglomerates and the remainder to be other particulate material havir.g size and shape such that the res~ltant burden is porous and flowable. So~e at least of the metallurgical material lusually at least 25~ usually at least 50% and preferably at least 75~ by weight) will normally have been provided initially in the form of dust, and this material at least should be provided in the form of wet 2gglomerates. The ; ~ carbonaceous material may be incorporated in wet ; agglomerates or may be included in other partic~late ~orm.
The particulate material of which the burden is formed generally has particles (before agglomeration) mainly in the range 50 to 6000~m, often in the range S0 to ~SOO~m, and so is particularly liable to become non-porous during sintering. If coarser particles are included (for instance coke) the proportion is usually insufficier.t to impart permea~ility to the burden. The amount of coke or other carbonaceous material is generally in the range 2 to 6~ by weight of the burden.

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The components of the burden may, apart from the inclusion of the absorbent polymer, be conventional.
Thus the carbor.aceous material is typically coke, usually breeze coke. The metallurgical material may include some fresh metallurgical ore or ore concentrate but usually at least 30~, often at least 50~ and frequently at lea~t 70%, of the burden is recycled metallurgical material such as ~low Dowr dust, Stock Yard dust, BOS -dust, Arc dust or Plasma Arc Furnace dust or bro~en fired pellets or other recycled fines. Although the metallurqical residues are generally iron, they can be of other metals.
The burden usually contains inorganic flux materials which aid sintering and give strength to the final product, for instance limestone and/or dolomite, and may be included as separate materials or as part of the agglomerates.
Some of the agglomerates in the burden may be formed from one component or component mixture whilst others may -~
be ormed from a different component or component mixture. Usually, however, the agglomerates all have a -~
similar composition. Thus if the agglomerates are to ~ ~;
contain more than one material, usually all the materials that are to be agglomerated are pre-blended and then agglomerated.
Agglomeration can be conducted ~y conventional wet agglomeration mixing apparatus by which particulate material is mixed into a homogeneous wet mix and agglomerated, for instance using a cor.ditioner drum, pug mill, ball mixer or trommel mixer. Agglomeration can ~e 3 completed if desired in a secondary agglomeration apparatus. The agglomerated material i5 generally led substantially direct (i.e., without any intermediate treatment) from the agglomeration mixing apparatus (or secondary agglomeration mixing apparatus) to the moving W091/00930 PCT/G~90/0l0~
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strand or other surface that is to support the burden during the firing.
The particle size of the agglomerates may be conventional, for instance 3 to 20mm, often 5 to 15mm.
Although a polymeric binding agent can be included in the agglomerates for the purpose of promoting binding of them, this is generally unnecessary and bonding is usually provided solely by the combination of water (as the sole bonding agent) and the materials that are to be included in the burden in any event.
The water content of the agglomerates will usually be at least 8~ and often at least 10~. Whereas, pxior to the invention, it usually could not be above about 15%
and often had to be within 1 or 2~ of the optimum, lS typically arour.d 10 or 11~, the water content in the invention can be very much higher, for instance up to around 30~ or more without sufferin~ serious loss of permeability. Preferably however the water content is in the ranqe 10 to 20~. Thus good porosity can be 20 maintained despite variability in the water content of the burden.
The absorbent polymer can be included with any part of the burden but is preferably included in some or all of the agglomerates. It can be added deliberately at the agqlomeration stage solely for the purpose of modifying the sintering process. ~lternatively it can be introduced to the agglomeration stage as a mixture with some or all of the metallurgical material, having been mixed with that material to facilitate the recoveFy and handling of that material from an aqueous slurry containing it. For instance we described in EP 195550 that a stic~y wet particulaté mass could be rendered crumbly b~ mixing water-absorbent polymer particles with it and a preferred process according to the invention comprises converting a sticky wet particulate mass of ', ''. ' . , ~ ' .
~ WO91/00930 PCT/GB90/0l054 7 ~3~i~Q~l metallurgical material to a crumbly state by blending it with the polymer and then using the resultant blend as part or all of the material that is to be agglomerated.
By this means it is possible to obtain both the handling advantages that follow from the proposal in EP 195550 and the sintering advantages that are obtained in the invention.
The absorbent polymers that are used in the invention are preferably cross linked synthetic polymers 10 of ethylenically unsaturated monomers. Preferably they -~
are anionic, most preferably being formed from 5 to lOO
mole percent (preferably 30 to lOO mole percent) acrylic acid as free acid or sodium or other alkali metal salt with O to 95 mole percent acrylamide. Particularly i5 preferred polymers are sodium polyacrylate homopolymer ,~
and cGpolymers o~ 30 to 70 weight percent sodium acrylate -with 70 to 30 weight percent acrylamide, together with sufficient cross linking agent to give the desired gel ~apacity. This gel capacity ~gram deionised water absorbed per gram polymer) is preferably at least 25, generally at least lOO and preferably at least 200 typically up to SOO or even 700 or higher. Some of this gel capacity may have been utilised b~fore incorporation into the burden if, for instance, the polymer is premixed with a slurry of particulate material, but the particles should still have a gel capacity of at least 25g/g at the time of firing.
For further information on suitable anionic polymers of the same general type, reference should be made to the EP195550. For information on the general c ass of absorbent polymers that can be used, and that can be natural, modified natural or synthetic and can be linear or cross linked, reference should be made to EP 27701~A, especially page 3 line 29 to page 4 line 21.

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W~91/00930 PCT/Gn90/010~
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The polymer is usually introduced in powder form, for instance having sizes as proposed in EP 195550 but alternatively it may be introduced in the form of aggregates or a dispersion in oil, as described in EP
277018; The dry particle size of the polymer can be from 1 to 2000~m. If the polymer is introduced as a dispersion in oil, the particle size is generally below 50~m. Preferably it is introduced as powder having a size typical~y from 100 to lOOO~m.
~ he amount of polymer is preferably such that polymer that is in equilibrium with the moisture content of the burden can absorb further water from the burden, as a result of vapour condensing in the cool part of the burden as ignition proceeds. Preferably therefore the polymer, when in initial equilibrium with the burden, has a~sorbed not more than 50% of its gel capacity.
The amount of polymer is usually at least 0.05 and generally at least 0.1% dry weight polymer based on the dry weight of the burden but it is ~sually unnecessary for it to be above 0.4 or 0.5~. If the polymer is being added initially to, for instance, a slurry of BOS or other dust then the amount based on that wet product may be typically 0.1 to 2~ based on that wet product.
It should be noted that although EP 195550 (and EP
277018) disclose converting to a friable mass a wet cake of, for instance, magnetite, there was no suggestion that the resultant product should be used for dust that is to be agglomerated and then subjected to the particular sinterins process of the invention, and thus there was no appreciation of the particular benefits that can be obtained in the invention.
It is particularly preferred in the invention for the mix that is being fed to a drum conditioner or other agglomerator should include also about 0.3~, based on the dry weight of the mix, of a copolymer of approximately . .: ~ , ,.... : .

WO91/00930 PCT/GB90/010~

equal amounts s~dium acrylate and acrylamide cross linked with methylene bis acrylamide in an amount of around 200ppm, giving a gel capacity above lOO. The permeability, and the sintering, is greatly improved and the total amourt of water that can be tolerated before performance ~eteriorates is in excess of 25~.
A dramatic demonstration of this advantage can be ~-;
seen from considering the normal process for utilising ~OS dust. This is normally collected as a slurry which is then settled and the settled product is then passed through a cyclone to separate materials such as lead and zinc from a concentrated slurry that contains the iron residues.~
Normally this concentrated slurry is then subjected lS to rot~ry Yacuum filtration to produce a sticky mass thet is then transported to the agglomerator mixer for the - sinter strand. Unfortunately this stic~y mass can be -sufficiently sticky that transportation is difficult and sometimes its .ransport is facilitated by flushing with water. This therefore increases the water content, even though it is well ~nown that the water content in the ~next, agglomeration, stage must be kept to a critical low value. Thus the supply of the material to the agglomerating stage not only requires settlement, cyclone separation and rotary vacuum filtration but is often accompanied by the deliberate addition of water to the filtered product. In the invention, this process is greatly simplified in two respects. The absorbent polymer is i~troduced, preferably, to the slurry leavirg the cyclone and as a result it is possible to improve the han~ling of the product that is subsequently filtered on the rotary filter ~thus eliminating t~e need for flushing with water) and it is also possible, in some instances, to omit the rotary filtration stage altogether.

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WO 91tO0930 PCI'/GS90tO1054 .` "``` .
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Secondly, the invention gives the described advantage of permitting the burden to be wetter without causing porosity problems. Thus the invention greatly simplifies handling, can reduce the equipment that is necessary, and can give very reliable results irrespective of variations in the water content.
The following is an example of the invention.
A conventional sinter strand process is operatee as follows.
0A particulate mixture is formed of about 23~ iron 1 ore, ~3~ ~OS dust, 31% return fines, 1~ flue dust, 6~
limestone, 11% aoiomite and 5S breeze co~e. Dust such as ~OS dust is supplied to the mixture as a cake taken off a rotary vacuum filter that follows a cyclone sepa~ator. The product is blended in the presence of lS sufficient water to give a total moisture content of about 11~ and is then agglomerated by mixing in a drum conditioner. The resultant agglomerates are spread as a layer on a sinter strand. A~most immediately after the formation of the layer, it passes underneath an ignition device that ignites the carbon in the agglomerates at the top of the layer. The sinter strand carries the layer over a series of windboxes by which air is sucXed down through the layer and spreads the fire down through the ~-Iayer. A condensation front precedes the layer due to the evaporation of water from the fired region and its condensation in the cooler region. By the time the layer reaches the end of the strand the entire depth has been fired to form a reasonably coherent sintered ma~s.
This is broken into lumps as it leaves the strand and can then be further comminuted in known manner.
~When this process is conducted with an a~.ount of ;water below about 10.5~ or above about 11.5~ a significant deterioration in permeability of the burden, and thus overall sintering, is apparent.

WO9l/00930 PCT/CB90/0~0~
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In the invention, about 0.3~, baced on the dry weight of the mix, of a particulate polymer is blended into the product coming off the cyclone separator or off the rotary vacuum filtration step can be omitted, or this filtration step can be omitted if the polymer is added in sufficient amount to the product from the cyclone. The polymer is a powder having a size mainly in the range lOO
to 700~m and is a copolymer of approximately equal amounts sodium acrylate and acrylamide cross linked with methylene bis acrylamide in an amount of around 200ppm giving a gel capacity above lOO. The permeability, and the sintering, is greatly improved and the total amount of water that can be tolerated before sintering performance deteriorates is in excess of 25~.
In another process, the polymer is blended into the nlix that is being fed to the drum conditioner in an amount of about O.l~, and again the total amount of water that can be tolerated before performance deteriorates is in excess of 25%

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Claims

1. A sintering process in which a porous burden is formed of carbonaceous material and metallurgical material and comprises wet unfired agglomerates of the metallurgical material, the burden is fired from one surface, air is drawn through the burden from the fired surface to fire substantially the complete burden and to convert it to a sintered mass, and the sintered mass is broken into lumps of sinter, characterised in that water absorbent particulate polymer is included in the burden.

2. A process according to claim 1 in which the agglomerates are formed from powdered metallurgical material using water as the sole added bonding agent.

3. A process according to claim 1 in which the agglomerates also contain carbonaceous material.

4. A process according to claim 1 conducted continuously on a sinter strand wherein the burden is applied to a moving strand and is carried as a layer from a firing position at which the upper surface of the layer is fired, over means by which air is drawn down through the layer, and to a sinter-breaking position at which the sintered mass is broken into lumps of sinter.

5. A process according to claim 4 in which the agglomerates are made by agglomerating powdered metallurgical material in the presence of water as the sole bonding agent and the resultant wet agglomerates are led substantially direct to the moving strand.

6. A process according to claim 1 in which the metallurgical powder is supplied to the process as an aqueous mass and in which the absorbent polymer particles are blended with the aqueous mass to provide a friable mass that is then converted to the agglomerates.

7. A process according to claim 6 in which the aqueous mass into which the absorbent particles are blended is obtained by cyclone separation and is then agglomerated without any intermediate filtration stage.

8. A process according to claim 1 in which the absorbent polymer particles are added as a powder having a size from 100 to 1000µm.

9. A process according to claim 1 in which the absorbent polymer particles are formed of 5 to 100 mole percent acrylic acid (or alkali metal salt thereof) and O
to 95% acrylamide and are cross linked.

10. A process according to claim 1 in which the absorbent polymer particles are cross linked polymers selected from sodium polyacrylate hompolymer and copolymers of 30 to 70 weight percent sodium acrylate and 70 to 30 weight percent acrylamide and are added as a powder having a size of from 100 to 1000µm.

11. A process according to claim 1 in which the absorbent polymer particles have a gel capacity, in the burden, of at least 25 grams deionized water per gram polymer.