SINTERING PROCESS

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: Allied Colloids Limited Sintering Process The charge for metallurgical processes such as blast furnace or direct reduction furnace processes iε nowadays, to an ever increasing extent, subjected to various pretreatments either to improve the efficiency of the process or to permit the use of lower grade materials or both. One pretreatment involves the comminution of metallurgical ore and the conversion of the comminuted ore to relatively strong pellets. The comminuted ore is agglomerated in the preεsence of a small amount of water, and also a binder, to form agglomerates and these are then fired to form hard pellets. The components of the 5 agglomerates and the conditions of firing are selected such that some degree of sintering occurs within each agglomerate (to produce the fired pellet) whilst avoiding any substantial aggregation and sintering of the agglomerates to one another. The binder that is f) traditionally used for the production of these sintered pellets iε bentonite but more recently there have been proposals to use water soluble organic polymer and in Canadian patent 890,342 it iε proposed to use a highly water swellable, cross linked, acrylic polymer. It is C - stated that the water tolerance and green strength of the initial agglomerates iε improved and that the dried and sintered product can have improved strength propertieε. This is, of course, very valuable in view of the possible high loading presεureε that may exiεt within, for 0 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 duεt (BOS duεt) , Stock Yard du≤t and so forth. A common SUBSTITUTE SHEET way of converting 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 10 a firing 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. _. - In order that the process operates efficiently, it is essential that the burden is porous throughout the process, aε otherwise air cannot be drawn through it sufficient to fire the entire burden. For this reason the dusty or other starting material ' iε converted into the form of agglomerateε in order that theεe agglomerateε 20 form a porouε burden through which the air can be drawn to cause firing. Since the agglomerates are formed solely for the purpose of giving porosity to the material that iε to be fired, it iε 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 of water in the wet agglomerateε has to be selected very carefully since if 30 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 formation. In particular, the agglomerates that have been fired are hotter than those that have not, and , t - so the process tends to drive off water from the S UBSTITUTESHEET initially fired agglomerates and to provide a condensation front at which the water condenses on to unfired agglomerates. Accordingly the water content of the unfired agglomerates 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 the burden is greatly reduced. Thus, if the water content is too low the agglomerates will be too dusty and if it is too high they 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 sensitive to the amount of water in the burden. A sintering process according to the invention is one in which a porous burden is formed of carbonaceous material and metallurgical material and comprises wet unfired agglomerates, 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 cover 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 iε applied to a moving strand by which it is carried ^ as a layer from a firing position at which the upper surface of the layer iε fired, over wind boxes or other means by which air is drawn through the layer, and to a sinter-breaking position at which the sintered masε is broken into lumpε 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 the burden so as to spread the firing of the burden from the surface where it iε initially fired. The burden must also be sufficiently flowable that it is possible to apply it to, for inεtance, the moving strand or other surface on which it is to be supported during firing. Usually substantially the entire burden is in the form of the wet ' agglomerates but sometimes it iε convenient for part of the burden (e.g., at least 25%, usually at least 50% and preferably at leaεt 75% by weight) to be in the form of wet agglomerates and the remainder to be other particulate material having size and εhape εuch that the resultant burden is porous and flowable. Some at least of the metallurgical material (usually at leaεt 25% uεually at least 50% and preferably 0 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 agglomerates. The carbonaceous material may be incorporated in wet agglomerates or may be included in other particulate 5 form. The particulate material of which the burden iε formed generally haε particles (before agglomeration) mainly in the range 50 to βOOOμm, often in the range 50 to 500μm, and so is particularly liable to become Q non-porous during sintering. If coarεer particles are included (for instance coke) the proportion iε uεually insufficient to impart permeability to the burden. The amount of coke or other carbonaceous material is generally in the range 2 to 6% by weight of the burden. 5 SUBSTITUTE SHEET The components of the burden may, apart from the inclusion of the absorbent polymer, be conventional. Thus the carbonaceous material iε typically coke, uεually breeze coke. The metallurgical material may include εome freεh metallurgical ore or ore concentrate but usually at least 30%, often at least 50% and frequently at leaεt 70%, of the burden is recycled metallurgical material such as Blow Down dust. Stock Yard dust, BOS dust. Arc dust or Plasma Arc Furnace dust or broken fired pellets or other recycled fines. Although the metallurgical 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 formed 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 by conventional wet agglomeration mixing apparatus by which particulate material is mixed into a homogeneous wet mix and agglomerated, for instance using a conditioner drum, pug mill, ball mixer or trommel mixer. Agglomeration can be completed if desired in a secondary agglomeration apparatus. The agglomerated material is generally led substantially direct (i.e., without any intermediate treatment) from the agglomeration mixing apparatuε (or εecondary agglomeration mixing apparatus) to the moving SUBSTITUTE SHEET strand or other surface that iε 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. 10 The water content of the agglomerates will usually be at least 8% and often at least 10%. Whereas, prior to the invention, it usually could not be above about 15% and often had to be within 1 or 2% of the optimum, _._ typically around 10 or 11%, the water content in the invention can be very much higher, for instance up to around 30% or more without suffering serious loss of permeability. Preferably however the water content iε in the range 10 to 20%. Thus good poroεity can be 20 maintained deεpite variability in the water content of the burden. The abεorbent polymer can be included with any part of the burden but iε preferably included in some or all of the agglomerates. It can be added deliberately at 25 the agglomeration stage solely for the purpose of modifying the sintering process. Alternatively 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 recovery 30 and handling of that material from an aqueous slurry containing it. For instance we described in EP 195550 that a εticky wet particulate mass could be rendered crumbly by mixing water-absorbent polymer particles with it and a preferred process according to the invention - c comprises converting a sticky wet particulate masε of SUBSTITUTESHEET 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 thiε 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 polymerε that are uεed in the invention are preferably cross linked synthetic polymers of ethylenically unsaturated monomers. Preferably they 0 are anionic, most preferably being formed from 5 to 100 mole percent (preferably 30 to 100 mole percent) acrylic acid as free acid or εodium or other alkali metal salt with 0 to 95 mole percent acrylamide. Particularly _. _ preferred polymerε are sodium polyacrylate homopolymer and copolymers of 30 to 70 weight percent εodium acrylate with 70 to 30 weight percent acrylamide, together with sufficient cross linking agent to give the desired gel capacity. This gel capacity (gram deionised water 20 absorbed per gram polymer) is preferably at least 25, generally at least 100 and preferably at least 200 typically up to 500 or even 700 or higher. Some of this gel capacity may have been utilised before incorporation into the burden if, for instance, the polymer iε premixed with a slurry of particulate material, but the particles εhould εtill 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 claεε of 30 abεorbent 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 277018A, especially page 3 line 29 to page 4 line 21. 35 SUBSTITUTE SHEET 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 2000μm. If the polymer iε introduced aε a diεperεion in oil, the particle size iε generally below 50μm. Preferably it iε introduced aε powder having a size typically from 100 to lOOOμm. The amount of polymer is preferably εuch that polymer that iε in equilibrium with the moiεture content of the burden can -absorb further water from the burden, aε a result of vapour condensing in the cool part of the burden as ignition proceedε. Preferably therefore the polymer, when in initial equilibrium with the burden, haε abεorbed 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 usually 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) diεcloεe converting to a friable mass a wet cake of, for instance, magnetite, there was no suggestion that the resultant product should be used for duεt that is to be agglomerated and then subjected to the particular sintering process of the invention, and thus there waε no appreciation of the particular benefits that can be obtained in the invention. It iε 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%, baεed on the dry weight of the mix, of 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 100. The permeability, and the sintering, iε greatly improved and the total amount of water that can be tolerated before performance deteriorates is in excess of 25%. A dramatic demonstration of this advantage can be seen from considering the normal process for utilising BOS dust. This is normally collected as a slurry which iε 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 to rotary vacuum filtration to produce a sticky masε that is then transported to the agglomerator mixer for the sinter strand. Unfortunately thiε εticky masε can be sufficiently εticky that transportation is difficult and sometimes its transport is facilitated by flushing with water. This therefore increases the water content, even though it is well known 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 reεpects. The absorbent polymer iε introduced, preferably, to the slurry leaving the cyclone and as a result it is possible to improve the handling of the product that is subsequently filtered on the rotary filter (thus eliminating the need for flushing with water) and it is also posεible, in some instances, to omit the rotary filtration stage altogether. Secondly, the invention gives the described advantage of permitting the burden to be wetter without causing porosity problemε. Thus the invention greatly εimplifies handling, can reduce the equipment that is necessary, and can give very reliable results irrespective of variationε in the water content. The following is an example of the invention. A conventional sinter strand process iε operated as follows. A particulate mixture is formed of about 23% iron ore, 23% BOS dust, 31% return fines, 1% flue dust, 6% limestone, 11% dolomite and 5% breeze coke. Dust such aε BOS dust is supplied to the mixture as a cake taken off * a rotary vacuum filter that follows a cyclone εeparator. The product is blended in the preεence of sufficient water to give a total moisture content of about 11% and iε then agglomerated by mixing in a drum conditioner. The resultant agglomerateε are spread aε a layer on a sinter εtrand. Almost 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 sucked down through the layer and spreadε the fire down through the layer. A condenεation 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 εintered mass. This iε broken into lumpε aε it leaveε the εtrand and can then be further comminuted in known manner. When thiε process is conducted with an amount of water below about 10.5% or above about 11.5% a significant deterioration in permeability of the burden, and thus overall sintering, iε apparent. SUBSTITUTESHEET In the invention, about 0.3%, based 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 100 to 700μm and is a copolymer of approximately equal amounts sodium acrylate and acrylamide cross linked with methylene biε acrylamide in an amount of around 200ppm giving a gel capacity above 100. The permeability, and the sintering, is greatly improved and the total amount of water that can be tolerated before sintering performance deteriorates iε in excess of 25%. In another process, the polymer is blended into the mix that is being fed to the drum conditioner in an amount of about 0.1%, and again the total amount of water that can be tolerated before performance deteriorates iε in excess of 25%.

Claims: CLAIMS 1. A sintering process in which a porous burden is formed of carbonaceous material and metallurgical material and compriεeε wet unfired agglomerateε of the metallurgical material, the burden iε fired from one surface, air is drawn through the burden from the fired surface to fire subεtantially the complete burden and to convert it to a sintered mass, and the sintered masε is broken into lumps of sinter, characterised in that water absorbent particulate polymer iε included in the burden. 2. A process according to claim 1 in which the agglomerates are formed from powdered metallurgical material uεing water aε the sole added bonding agent. 3. A proceεε according to claim 1 in which the agglomerates alεo contain carbonaceouε material. 4. A process according to claim 1 conducted continuously on a εinter 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 procesε according to claim 1 in which the metallurgical powder iε 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 abεorbent particleε are blended iε obtained by cyclone separation and is then agglomerated without any intermediate filtration stage. 8. A proceεε according to claim 1 in which the absorbent polymer particles are added as a powder having a size from 100 to lOOOμm. 9. A procesε 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 0 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 aε a powder having a size of from 100 to lOOOμm. 11. A procesε according to claim 1 in which the absorbent polymer particles have a gel capacity, in the burden, of at least 25 grams deionised water per gram polymer.

Inventors: FIELD JOHN RODNEY, FIELD, JOHN, RODNEY
Applicants: ALLIED COLLOIDS LTD, ALLIED COLLOIDS LIMITED
Application year: 1990
Main classification: C22B1
IPC codes: C22B1