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Method for agglomerating wet-collected fume for use in metallurgical furnaces and agglomerates produced thereby |
| RE30060 |
Method for agglomerating wet-collected fume for use in metallurgical furnaces and agglomerates produced thereby
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| Patent Drawings: | |
| Inventor: |
Kreiger |
| Date Issued: |
July 31, 1979 |
| Application: |
05/903,297 |
| Filed: |
May 5, 1978 |
| Inventors: |
Kreiger; John W. (Bethlehem, PA)
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| Assignee: |
Bethlehem Steel Corporation (Bethlehem, PA) |
| Primary Examiner: |
Rutledge; L. Dewayne |
| Assistant Examiner: |
Lewis; Michael L. |
| Attorney Or Agent: |
O'Keefe; Joseph J.Wilkinson; Charles A.Simitz; John S. |
| U.S. Class: |
75/746; 75/770 |
| Field Of Search: |
75/1; 75/3; 75/4; 75/5; 75/25; 75/41; 159/4CC; 159/48R; 23/313R; 23/313P; 264/111; 264/117 |
| International Class: |
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| U.S Patent Documents: |
2205043; 3403018; 3652260; 3770415; 3770416; 3836353; 3851827 |
| Foreign Patent Documents: |
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| Other References: |
Cavaghan, N. J., et al.; "Utilizaton of In-Plant Fines", Journal of the Iron and Steel Institute pp. 538-542. (6/70)..
Cahn, D. S., et al.; "Compression Testing of Green and Dry Iron Ore Pellets"; Society of Mining, AIME Transactions vol. 241, pp. 475-481 (12/68).. |
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| Abstract: |
A method for preparing moist iron-bearing fume for use in steelmaking furnaces to recover the iron contained in the fume. The fume is thickened, substantially instantaneously dried, impact-fractured and balled. Moisture is added to the fume on the balling device to bind the fume to produce "green" balls which resist degradation during handling and transport. The "green " balls have sufficient strength to resist degradation during handling prior to and during charging to steelmaking furnaces. |
| Claim: |
I claim:
1. A method for utilizing moisture-bearing iron-containing fume from steelmaking furnaces to produce "green" balls suitable for charging into steelmaking furnaces to recover said iron,comprising:
(a) substantially instantaneously drying said fume,
(b) impact-fracturing a portion of said fume whereby the Blaine surface area of said fume is not less than about 8,000 square centimeters per gram, and
(c) balling said fume on a balling device while adding sufficient moisture to produce effective agglomeration.
2. The method of claim 1 wherein the Blaine surface area of the fume after impacting in step (b) is not less than 12,000 square centimeters per gram.
3. The method of claim 1 wherein the "green" balls formed in step (c) have a size within the range of about 3/8 of an inch to about 11/2 inches.
4. The method of claim 1 wherein the "green" balls formed in step (c) have a moisture content within the range of about 7% to about 12%.
5. The method of claim 1 wherein the fume being balled is basic oxygen furnace fume.
6. The method of claim 1 wherein the fume being balled is open hearth furnace fume.
7. The method of claim 1 wherein the fume being balled is a mixture of basic oxygen furnace fume and open hearth furnace fume and water leached dry-collected open hearth furnace fume.
8. The method of claim 5 in which the steelmaking furnace from which the fume is derived is a basic oxygen furnace which has an exhaust hood positioned a short distance above the top of the basic oxygen furnace.
9. A method for utilizing fume formed in steelmaking furnaces wherein said fume is comprised of at least one fume taken from the group consisting of wet-collected open hearth fume, wet-collected basic oxygen furnace fume, water leacheddry-collected open hearth fume, dry-collected basic oxygen furnace fume and moist admixtures of iron-containing fume from steelmaking furnaces, said fume being treated and formed into "green" balls characterized by having sufficient strength to surviveat least ten drops from a height of 18 inches onto a flat steel plate and a compression strength of not less than 15 pounds .[.per square inch.]., said method comprising;
(a) substantially instantaneously, drying said fume,
(b) passing said fume through a one-point contact impact-fracturing step to fracture a portion of said fume to form dry, fresh, clean angular fracture surfaces thereon, and to obtain a Blaine surface area of not less than about 8,000 squarecentimeters per gram in said fume, and
(c) balling said fume on a balling device while adding sufficient moisture to form green balls having a moisture content of between about 7% to about 12%, and a size within the range of 3/8 of an inch to 11/4 inches in diameter.
10. The method of claim 9 wherein the fume is wet-collected open hearth fume.
11. The method of claim 9 wherein the fume is wet-collected basic oxygen furnace fume.
12. The method of claim 9 wherein the fume is dry-collected water-leached open hearth fume.
13. The method of claim 9 wherein the fume is a mixture of wet-collected open hearth fume and dry-collected water-leached open hearth fume.
14. The method of claim 9 wherein the fume is a mixture of wet-collected open hearth fume and wet-collected basic oxygen furnace fume.
15. The method of claim 9 wherein the fume is water-leached dry-collected open hearth fume and wet-collected basic oxygen furnace fume.
16. The method of claim 9 wherein the fume is wet-collected open hearth fume and water-collected dry-collected open hearth fume and wet-collected basic oxygen furnace fume.
17. The method of claim 11 wherein the fume is collected from a basic oxygen furnace in which the hood is positioned a short distance above the top of the furnace.
18. A method for preparing wet-collected fume containing iron oxides formed in steelmaking furnaces and water-leached dry-collected fume containing iron oxides from steelmaking furnaces for reuse in said steelmaking furnaces to recover the ironcontained in said fume, said method comprising:
(a) mixing said wet-collected fume and water-leached dry-collected,
(b) substantially instantaneously drying said mixture,
(c) impact-fracturing said dried mixture,
(d) charging said impacted fume onto a balling device,
(e) adding water in an amount sufficient to ball the dried fume on the balling device while controlling the operation thereof to produce degradation resistant "green" balls having a moisture content of between 7% and 12%.
19. The method of claim 18 in which the steelmaking furnace is a basic oxygen furnace which has an exhaust hood placed a short distance above the top of the basic oxygen furnace.
20. A method of forming degradation-resistant "green" balls from moist iron-containing fume derived from steelmaking furnaces for subsequent charging to steelmaking furnaces comprising:
(a) substantially instantaneously drying said fume,
(b) fracturing at least a portion of said dried fume to form clean, dry, angular fracture surfaces upon at least some of the fume particles, and upon agglomerations of fume which may be present, the dried fume, including said fractured portion,at the completion of fracturing having a Blaine surface area of not less than 8,000 square centimeters per gram, and
(c) balling said fume on a balling device while adding sufficient moisture to produce degradation-resistant "green" balls.
21. The method of claim 20 in which fracturing is such that a Blaine surface area of not less than 12,000 square centimeters per cubic gram is attained. |
| Description: |
BACKGROUND OF THE INVENTION
This invention is directed to a method for preparing wet-collected iron-bearing fume and water-leached dry-collected iron-bearing fume and mixtures thereof for use in steelmaking furnaces to recover said iron. More specifically, the invention isdirected to a method for treating wet or moist fume, which method includes drying, impacting-fracturing and forming the fume into "green" balls on a balling device. The "green" balls have sufficient strength to resist degradation during the handlingrequired to transport and charge them into a steelmaking furnace. "Green balls" may be defined as agglomerates which have been formed into balls or pellets and which have not been further subjected to a heat hardening. The "green balls" may be eithermoist or may have been dried, but will not have been hardened at elevated temperatures such as would cause bonding of the component particles due to sintering together of the particles.
Fume formed in steelmaking furnaces during the refining of molten iron consists of fine particles of various elements and metallic and non-metallic compounds, such as iron oxides, zinc oxides, lead oxides, sulfur and sulfur compounds, carbon,silica, alumina, lime, magnesia and the like. These particles are volatilized at the high temperatures within the steelmaking furnaces near the surface of the molten metal, condensed at lower temperatures and carried out of the furnaces by the furnaceexhaust gases. The particles range in size from about one tenth of a micron to 25 microns or more in diameter. Usually more than about 90% of the particles will be less than 25 microns in diameter. The nature of fume from steelmaking furnaces isdiscussed in some detail in the following articles: (a) "Progress Review No. 63: The Formation and Suppression of Oxide Fume in Steelmaking"; Munro et al, Journal of the Institute of Fuel; March 1971, pp. 156-163, (b) "A New Look at the Cause ofFuming"; Morris et al, Journal of Metals; July 1966, pp. 803-810.
Because of their small size and weight, particles of fume are easily carried out of the furnaces through the furnace stacks to the atmosphere. Therefore, in order to prevent pollution of the atmosphere, the exhaust gases from steelmakingfurnaces are customarily passed through air pollution control devices, for example wet scrubbers, dry electrostatic precipitators, bag-houses and the like which remove a substantial portion if not all of the fume from the exhaust gases. Since fume fromsteelmaking furnaces usually contains a relatively high percentage, for example over 50%, of iron in the form of iron oxides, it is economically desirable to recover the iron. It is possible to reuse the fume as charge material for steelmaking furnaces. However, because of the fine particle size of the collected fume, it is impractical to charge the fume, without some further treatment, into steelmaking furnaces, because the fume would merely be blown or carried out of the furnaces by the exhaust gases.
Wet-collected fume has in the past been agglomerated in order to increase the effective size of the particles. However, the moist agglomerates so formed do not possess sufficient strength to support a load and hence cannot be successfullytransported, stored or charged into steelmaking furnaces without degradation of the agglomerated fume. Attempts to increase the strength of the moist agglomerates by low temperature drying have failed and in addition the dried agglomerates have spalledand cracked.
Moist fume can be balled and the "green" balls dried and then treated at elevated temperatures within the usual sintering or indurating ranges to produce heat-hardened pellets which do have sufficient strength to support a load and which can betransported and charged into steelmaking furnaces. However, the production of heat-hardened pellets is expensive because of the temperatures involved and has not generally been commercially attractive; nor has it been accepted by the Steel industry.
Dry-collected fume from steelmaking furnaces, such as is frequently collected from open hearth furnace exhaust gases, can contain relatively high contents of sulfur, for example 0.30% by weight. Because of the relatively high sulfur content, thefume cannot be used as a charge material in steelmaking furnaces without preliminary treatment to lower the sulfur content. The dry-collected fume is usually treated by water leaching to reduce the sulfur content to acceptable levels, for example, under0.20% by weight. The water leached dry-collected fume is, however, difficult to agglomerate. The agglomerates formed from water leached fume to not possess sufficient strength to support a load and cannot be transported, stored or charged intosteelmaking furnaces without degradation of the agglomerates unless the agglomerates are also first treated at elevated temperatures in order to heat harden them.
There has been, therefore, a need for a simple, relatively inexpensive method for preparing moist, iron-bearing fume for use in steelmaking furnaces to recover the iron.
It is an object of this invention to provide a method for preparing moist iron-bearing fume for use in steelmaking furnaces, comprising substantially instantaneously drying the fume, impact-fracturing the dried fume and balling the impacted fumeon a balling device to form "green" balls.
It is another object of this invention to provide a method for preparing high sulfur iron-bearing dry-collected fume for use in steelmaking furnaces, which method includes water leaching the dry-collected fume, substantially instantaneouslydrying the leached fume, impact-fracturing the dried fume and balling the fume on a balling device to form "green" balls.
It is another object of this invention to provide a method for preparing a mixture of high sulfur iron-bearing dry-collected fume and iron-bearing wet-collected fume for use in steelmaking furnaces. The method includes leaching the dry-collectedfume, mixing the leached fume with the wet-collected fume, substantially instantaneously drying the mixed fumes, impact-fracturing the dried mixed fumes, and balling the dried mixed fumes on a balling device to form "green" balls.
It is still another object of this invention to produce "green" balls suitable for charging into steelmaking furnaces from iron-bearing wet-collected fume and mixtures of the wet-collected fume and high sulfur, iron-bearing leached dry-collectedfume, said "green" balls being characterized by having the strength to survive at least ten drops from a height of 18 inches in a drop test and having a compression strength of at least 15 pounds .[.per square inch.]. and which resist degradation duringhandling and transport.
SUMMARY OF THE INVENTION
In accordance with the present invention wet-collected iron-bearing fume, dry-collected and leached iron-bearing fume and/or mixtures of wet-collected iron-bearing fume and water leached iron-bearing dry-collected fume are substantiallyinstantaneously dried in a spray dryer. The dried fume is impact-fractured in a one point impact step to increase the surface area and to form dry, fresh, clean, angular fracture surfaces on a portion of the fume. The impacted fume is then balled on aballing device into "green" balls. Moisture required for balling is added to the fume on the balling device. The "green" balls produced in the process are characterized by having sufficient strength to survive ten drops from a height of 18 inches in adrop test, have a compression strength of at least 15 pounds .[.per square inch.]. and are sufficiently strong to support a load and resist degradation during transportation and handling and can be charged into steelmaking furnaces.
DRAWING OFTHE INVENTION
The drawing of the invention is a diagrammatic flow sheet of the steps necessary to prepare the fume for reuse in steelmaking furnaces.
PREFERRED EMBODIMENT OF THE INVENTION
Wet-collected iron-bearing fume and mixtures of wet-collected iron-bearing fume and/or water leached high sulfur iron-bearing dry-collected fume which has been formed in steelmaking furnaces can, by the present invention, be processed into"green" balls suitable for charging into steelmaking furnaces. (By "green" balls we mean agglomerates generally spherical in shape which have been formed on a balling device, usually without the addition of a binder material other than water, and whichhave in particular not been subjected to high heat to increase their strength and hardness.)
The moist fume particles are initially dried substantially instantaneously. Small agglomerates comprised of adhering fume particles tend to be formed during this drying. The dried fume is then impacted, or impact-fractured, to form dry, fresh,clean, angular fracture surfaces on a portion of the particles and agglomerates. The impacted dried fume is charged onto a balling device. Sufficient moisture, which is required for balling, is added to the impacted fume on the balling device. The"green" balls produced have a moisture content of between about 7% to about 12% when discharged from the balling device.
Basic oxygen furnaces are equipped with two types of exhaust hoods. In the first or "open" arrangement, the exhaust hood is positioned a short distance above the open end of the basic oxygen furnace. As the exhaust gases pass upwardly from thefurnace into the exhaust hood, air is drawn into the exhaust hood through a space between the hood and the furnace. A portion of the oxygen in the air reacts with carbon monoxide in the exhaust gases to form carbon dioxide thereby increasing the amountof carbon dioxide in the exhaust gases. The remaining oxygen reacts with metallic constituents in the gases. As a result, substantially all the iron found in the exhaust gases in the form of fume is usually in a high state of oxidation.
In the second or "closed" arrangement, the bottom of the exhaust hood is contiguous with and rests atop the basic oxygen furnace. Air is thus excluded from mixing with the exhaust gases passing from the furnace. As a result, some of the iron inthe exhaust gases remains in the metallic state and substantially all of the iron is in a lower state of oxidation than in the above described open arrangement. The amount of fume carried from the basic oxygen furnace is also generally less in the"closed" or second arrangement than in the "open" or first arrangement. In any event, fume also varies in chemical composition from plant to plant and from furnace to furnace. A specific analysis which is typical of the general analysis of open hearthfurnace fume and basic oxygen furnace fume is as follows:
______________________________________ Percent in Open Hearth Basic Oxygen Component Furnace Furnace.sup.(b) ______________________________________ *Fe.sup.T 65.2 54.0 Silica 0.64 2.17 Alumina .15 .12 Zinc.sup.+ 1.70 3.16 Lime 1.02 6.09 Magnesia .75 2.73 Sulfur.sup.+ 0.43 .07 Manganese 0.26 .90 Carbon 0.14 1.52 Lead.sup.+ 0.07 .24 ______________________________________ *Total Iron present in the fume as iron oxides. .sup.+ Zinc oxides, lead oxides, sulfur oxides or sulfidesreported as the elements zinc, lead and sulfur, respectively. .sup.(b) Fume from furnaces equipped with air-tight hoods contain some metallic iron and iron in a lower state of oxidation than furnaces not so equipped. The amount of fume generated insuch furnaces is less than in furnaces not so equipped.
A typical size consist of fume from steelmaking furnaces follows:
______________________________________ Microns Mesh Size % ______________________________________ 149-74 -100, +200 0.3 to 4.0 74-37 -200, +400 2.0 to 10.0 37-25 -400, +500 4.0 to 10.0 less than 25 -500 75 to 95 ______________________________________
As noted above, in the table of typical size consist of fume, about 75% to 95% of the particles in fume are about 25 microns or less in diameter. Due to the fineness of the particles, fume, per se, is not charged into basic oxygen steelmakingfurnaces because it would be immediately carried away by the exhaust gases issuing from the furnace. It is thus necessary to agglomerate the fume prior to charging it into a steelmaking furnace.
As noted previously, fume collected from steelmaking furnaces can be wet-collected or dry-collected. In the case of wet-collected fume the fume and the moisture used to remove the fume from the exhaust gases together form a slurry which usuallycontains about 1% solids by weight. On the other hand, dry-collected fume which has been water leached to reduce the sulfur content usually contains between about 5% to about 10% solids by weight. As is well known to those skilled in the art, slurrieswhich contain 1% to 10% solids by weight cannot be agglomerated. It is necessary to reduce the moisture content of the slurries, that is, thicken the fume before the solids can be agglomerated. Both the wet-collected iron-bearing fume and the waterleached iron-bearing fume are thus preferably initially passed to thickeners to be dewatered. The slurries are allowed to remain in the thickeners for a time to allow the solids to settle. The solids taken from the bottom of the thickeners containabout 45% to 65% water by weight. The thickened slurries are then usually mixed together in a second thickener and the solids allowed to settle. The solids removed from the bottom of this second thickener also contain between 45% to 65% water byweight. The moisture content is, however, too high to permit the agglomeration of the solids. It is thus necessary to reduce the moisture content to below about 12% water by weight.
It has been found that the solids can be substantially instantaneously dried, that is, have substantially all the moisture removed, in a flash or spray dryer. The size of the spray dryer is predicated upon the amount of water which it is desiredto remove in a given time. Slurry containing 45% to 65% water by weight can be dried in a dryer which can, for example, accommodate a feed rate of between 80 gallons per minute to 100 gallons per minute of slurry. Feed rates within this range deliverbetween 10 tons per hour to 14 tons per hour of solids on a dry basis.
The slurry is sprayed into the spray dryer in the form of small droplets. Each droplet may contain one or a plurality of particles of fume surrounded by water. The water is substantially instantaneously vaporized by hot gases which are blownupwardly at a temperature of about 1200.degree. F. (649.degree. C.) in the spray dryer to contact the sprayed fume. The hot gases are cooled to about 280.degree. F. (138.degree. C.) as they contact the slurry and vaporize the water. Thesubstantially instantaneous vaporization of water causes the particles of fume in the droplets to adhere to each other and to shrink in size to produce small dense spheroids comprised of adhering particles of fume. The dried fume discharged from thedryer contains individual particles of fume and small dense spheroids of adhering fume particles.
The formation of the small dense spheroids effectively reduces the surface area of the fume. The spray dried fume has been found to have an aggregate surface area of less than 8,000 square centimeters per gram as measured by the standard BlaineSurface Area Test, ASTM C204-73. A material having a Blaine surface are of less than 8,000 square centimeters per gram is difficult if not impossible to ball into a strong agglomerate. It is, therefore, necessary to increase the surface area of thedried fume.
It has been found that the surface area of the fume can be very effectively increased by impacting the fume in an impact mill wherein a portion of the fume particles are subjected to one-point impact fracturing. The one-point impact fracturingstep increases the Blaine surface area of the fume to more than 8,000 square centimeters per gram, and preferably to at least 12,000 square centimeters per gram. Additionally dry, fresh, clean, angular fracture surfaces are formed on a portion of thefume, as opposed to less clean and angular fracture surfaces which would be obtained in other impacting mills, such as, for example, a ball mill where the fume would be subjected to impact and grinding from several directions at once.
It has been found that it is possible to impact fracture only that portion of the dried fume which has been formed into small dense spheroids in a one-point impact mill and thereby to form fracture surfaces which are particularly effective to aidagglomeration. When the fume particles and fume spheroids are impact-fractured, strong "green" balls which will resist degradation may be produced from the impacted material.
The dried fume is fed in a typical impact-fracturing operation onto a rotor which is rotating at a desired speed such that the small dense spheroids will be hurled against a target at a velocity which will result in the fracture of the spheroids. A portion of the coarser particles of fume will also be fractured, but the smaller particles will not be fractured. Consequently, by the use of a one-point impact fracturing mill, it is possible to fracture that portion of the dried fume which has beenformed into spheroids, while maintaining the integrity of the smaller fume particles, thus increasing the surface area without reducing the size of the smaller particles.
Impact fracturing mills are often referred to as "impact grinding mills". However, since "grinding" has a connotation of comminuting between grinding surfaces, or in other words by multiple point impact and also abrasion the terminologyimpact-fracture mill is more correct.
After impacting, the freshly fractured fume particles are formed into "green" balls suitable for charging into steelmaking furnaces. The impacted fume is fed to a balling device, such as a cone or disc type balling apparatus well known to thoseskilled in the art. Dry impacted fume is fed onto the balling device and the moisture required for balling is added to the fume on the balling device.
It is well known to those skilled in the art that to material which is to be balled there must be added moisture in amounts sufficient to obtain a moisture content which varies between 7% to 12% in the balled material in order to attain effectiveagglomeration. The exact range of moisture necessary to ball any given material composition may vary within these limits and is customarily determined by actual trial and error.
Turning now to the drawing, iron-bearing wet-collected fume from various steelmaking furnaces is thickened in individual thickeners 13 and 15 and dry-collected fume from an open-hearth precipitator is water leached to reduce the sulfur contentand is thickened in a thickener 11 as shown. Some water leached fume from a basic oxygen furnace is also passed to thickener 15. The underflows from the individual thickeners 11, 13 and 15 are mixed together in a common thickener 17, which can also beconsidered as a mixing tank, to form a single slurry composed of water and fume and containing between 35% to 55% solids. The slurry of fume from the mixing tank, or common thickener 17, is next dried in a spray dryer 19. A very suitable spray dryerfor this purpose is described and shown in a catalogue entitled "Gentle-ized Spray Drying Process", circulated by Nichols Engineering and Research Corporation, 80 Pine Street, New York, 5, N.Y. The slurry is in the example fed into the spray dryer 19 ata rate of between 80 gallons per minute to 100 gallons per minute. The slurry is atomized by feeding it into a rapidly rotating atomizing head 20 located near the top of the dryer and is flung from the head in very fine droplets. The size of thedroplets is controlled by the diameter of small orifices in the rotating head and by the speed of rotation. Heated air at a temperature of about 1200.degree. F. is blown upwardly toward the atomized slurry by a blower 21 through a duct 22 positionedunder the atomizing head 20. The water in the slurry is substantially instantaneously vaporized as it contacts the hot gases and small dense hardened spheroids having a more or less smooth surface and containing a plurality of fume particles are formed. The fume is usually raised to a temperature of about 280.degree. F. (138.degree. C.) and the temperature of the hot air is reduced to about 280.degree. F. (138.degree. C.) by heat exchange between the particles and the hot gases and vaporization ofthe water. The small dense spheroids formed by the vaporization of water retain their shape and identify and are discharged from the bottom of the spray dryer with the remainder of the fume which may not have been included in spheroids at a temperatureof about 280.degree. F. (138.degree. C.) into any suitable cyclone device or the like 23 which serves to separate the fume particles and spheroids from the gas stream which exits from the spray dryer. The formation of the spheroids reduces the surfacearea of the fume to below about 8,000 square centimeters per gram.
The dried fume is next conveyed to and impacted in an impactor 25, such as a "Centrimil" as described and shown in Catalogue CMG-71 entitled "Centrimil/Centridyne-- The Energy Spectrum Through Centrifugal Force" of Entoleter Inc., a Subsidiary ofAmerican Manufacturing Co., P. O. Box 1919, New Haven, Conn. In the impactor 25, the fume is fed to a horizontal rotating member 27. The speed of the rotating member, for example about 3500 revolutions per minute, imparts a velocity to the fume suchthat the small dense spheroids and a portion of the coarsest particles of fume itself are hurled from the rotating member against a liner or target 29, which is positioned a predetermined distance from the periphery of the rotating member, at a velocitysufficient to cause the spheroids and particles to be fractured. New, dry, fresh, clean, angular fracture surfaces are formed on these particles of fume and fractured spheroids. A portion of the intermediate size particles of fume may also befractured. The smallest particles of fume are hurled from the rotating member, but are not fractured.
The formation of the new, dry, fresh, clean, angular fractures on the surface of a portion of the fume increases the Blaine surface area of the fume from less than about 8,000 square centimeters per gram to more than about 8,000 squarecentimeters per gram and preferably to not less than 12,000 square centimeters per gram.
The impacted fume is next passed to storage in silo 31 or other suitable storage facility for use as needed. The impacted fume is removed as needed from storage and fed to a balling device 33, such as a cone or disc type balling device. Moisture which is needed for balling is added to the fume on the balling device. "Green" balls within a size range of about 3/8 of an inch to 11/4 inches in diameter for use in steelmaking furnaces are produced on the balling device. All the "green"balls are substantially spheroidal in shape.
The operation of agglomerating or balling devices such as discs and cones, drums and the like is well known to those skilled in the art and the operation of such devices forms no part of the present invention, except that all the moisturerequired for balling should be added to the fume material during the actual balling operation.
In the balling of dry material such as fume about 7% to 12% moisture will normally be used to cause agglomeration of the dry material into balls. The water serves as a binder for the particles causing them to adhere together. Other binders suchas bentonite or the like may also be used along with the water. As is well known, the exact amount of moisture which can be used to effect agglomeration of any given composition of material will vary widely within the broad range of 7% to 12% and cannotusually be more than generally estimated beforehand. Even fumes from the same steelmaking furnace may vary significantly from time to time with respect to the exact range of moisture required for effective agglomeration. Agglomeration, at the presenttime at least, can best be characterized as an art rather than an exact science. The operator of a balling or agglomerating device will thus normally vary the amount of moisture added to the material on the balling device within the broad stated rangesand also the position at which the water is fed into the material until he recognizes that proper agglomeration is being attained.
Although the mixing of different types of fume from several different sources has been shown for illustrative purposes in the single FIGURE, any individual moist iron-bearing fume or any mixture of moist iron-bearing fume can be processed intosatisfactory "green" balls by the method of the invention. In some cases only a single thickener might be used when only a single source of fume is involved, or even when several sources of fume are available, in which case all the fumes would be mixedtogether in the single thickener.
The drop test is a test well established in Industry and consists of allowing balls or agglomerates, for example, the "green" balls of the invention, to fall freely from a height of 18 inches onto a flat steel surface, such as a steel plate. Thenumber or drops which the test material survives before it fractures in an indication of the strength of the balls. It has been found that the final "green" balls of the present invention should withstand not less than ten drops in order to haveadequate strength to be used as raw material for recharging to steelmaking furnaces. "Green" balls which do not have the desired drop strength and compression strength may sometimes survive degradation and be successfully charged into steelmakingfurnaces. However, a large percentage, and in many cases up to 60% or more, of low strength "green" balls will be destroyed, resulting in the recycling of a large amount of fines, which is very undesirable.
By making "green" balls in accordance with the method of the instant invention, however, substantially all of the green balls produced will withstand ten drops in the drop test and in many instances many more drops, up to twenty, thirty, or evenas many as sixty drops. Many of the balls show no indication of fracture no matter how many times they are dropped. By stating that substantially all the green balls will survive ten drops in the drop test, we do not mean to rule out the production ofisolated balls which will not meet specifications. In an agglomeration process there are always occasional deviant or defective balls which for some reason may not have agglomerated together correctly. However, in our process such defective balls willoccur only occasionally and almost invariably well over 90% of the balls produced will withstand ten drops or more in the drop test. The standard weight of eighteen inches in the drop test is significantly related to the stress to which such balls aresubjected during normal handling and was originally picked because the drop from one conveyor belt to another in industrial equipment is often about eighteen inches. The principal consideration in the production of suitable balls for charging into asteelmaking furnace is that the balls shall not be broken up or degraded sufficiently so that a significant amount of fines is produced, since any such fines will be useless or even detrimental in the operation of the furnace.
Wherever percentages are mentioned in these specifications and claims, such percentages are on a weight basis unless otherwise noted.
In a specific example of the invention, 100 tons of basic oxygen furnace fume collected in a wet scrubber and having a chemical composition and size consist as follows:
______________________________________ Chemical Composition ______________________________________ Fe.sup.T 53% SiO.sub.2 2.2% Al.sub.2 O.sub.3 0.15% Zn 2.1% CaO 7.4% MgO 3.5% S 0.12% Mn 0.9% ______________________________________ SizeConsist Mesh Size Cum. % Microns ______________________________________ -100, +200 0.36 74 -200, +400 2.36 37 -400, +500 6.10 25 -500 93.9 25 ______________________________________
was mixed with 100 tons of open hearth fume which was collected in an electrostatic precipitator, leached with water and after leaching, had a chemical composition and size consist as follows:
______________________________________ Chemical Composition ______________________________________ Fe.sup.T 66.3% SiO.sub.2 0.57% Al.sub.2 O.sub.3 0.15% Zn 0.90% CaO 0.72% MgO 0.65% S 0.08% Mn 0.25% ______________________________________ Size Consist Mesh Size Cum % Microns ______________________________________ -100, +200 3.15 74 -200, +400 9.50 37 -400, +500 13.84 25 -500 86.16 25 ______________________________________
and 80 tons of open hearth fume collected in a scrubber and having a chemical composition and size consist as follows:
______________________________________ Chemical Composition ______________________________________ Fe.sup.T 66.2% SiO.sub.2 0.47% Al.sub.2 O.sub.3 0.15% Zn 1.32% CaO 0.93% MgO 0.38% S 0.06% Mn 0.25% ______________________________________ Size Consist Mesh Size Cum % Microns ______________________________________ -100, +200 3.15 149-74 -200, +400 9.50 74-37 -400, +500 13.84 37-25 -500 86.16 25 ______________________________________
The mixture of the three fumes was thickened in a thickener to produce a slurry containing 42% solids. The thickened fume was fed at a rate of 90 gallons per minute into a spray-dryer. Hot gases having an inlet temperature of 1200.degree. F.(648.degree. C.) were blown upwardly to contact the fume and vaporize the water. The gases had an exit temperature of 280.degree. F. (138.degree. C.). The fume had a temperature of about 280.degree. F. (138.degree. C.) when discharged from thedryer. About 121/2 tons of fume on a dry basis were produced in on hour.
The dried fume was impacted in a "Centrimil" having a 24 inch diameter rotor, rotating at a speed of 3500 revolutions per minute to fracture the spheroids formed during drying. The rate of feed was 121/2 tons per hour. The Blaine surface areaof the fume before impacting was 7,500 square centimeters per gram. The Blaine surface area of the fume after impacting was about 12,000 square centimeters per gram.
The fume was next fed at a rate of 35 tons per hour to a balling disc, which was 18 feet in diameter. The disc was rotated at ten revolutions per minute. Sufficient moisture was added to the fume on the balling disc to produce "green" ballshaving a moisture content of about 10% which was the moisture found necessary to ball this particular material. The "green" balls ranged in size from 3/8 of an inch to 3/4 of an inch in diameter. The "green" balls had a compression strength of 18pounds .[.per square inch.]., a bulk density of 130 pounds per cubic foot and survived an average of fourteen drops when dropped from a height of 18 inches onto a flat steel plate in a test in which samples of balls, each sample containing about a quartof balls, were collected and random series of ten balls each were selected from each sample and tested. None of the balls tested from the particular samples survived less than ten test drops and many survived many more than 10 or even fourteen drops. The "green" balls were charged into a hopper car and were transported to the basic oxygen furnace shop. The "green" balls were emptied into a storage bin and were transported by a conveyor system to a basic oxygen furnace. No significant degradation ofthe "green" balls occurred as determined by visual observation and no difficulty was encountered in refining the "green" balls into steel.
As has been pointed out above, it has been found very important in the present invention to fracture the spheroids of fume produced in the spray dryer, together with some of the larger fume particles, in a one-point impact fracturing step. Theimpact fracturing step serves not only to increase the Blaine surface area of the fume, i.e. the fume particles and agglomerations of fume particles, to more than 8,000 and preferably more than 12,000 square centimeters per gram, but also serves to formfresh, clean, dry, angular fracture surfaces upon the fume and spheroids, which fracture surfaces appear to be very important in attaining strong degradation-resistant green balls. The sharp cleavage of the particles which forms the proper fracturesurface appears to be attainable only in a one-point fracture operation such as is attained in an impact fracturing mill when the material is flung against a target and strikes the target against one side causing fracture by impact rather than fracturebetween two surfaces such as is accomplished in a ball mill, a hammer mill or the like. In an impact mill, furthermore, the particles are struck only once and then leave the mill. In various grinding mills, on the other hand, the particles arefractured between opposing surfaces and are, furthermore, usually fractured on ground several times so that original fracture surfaces are refractured several times, thus losing their freshness and initial angularity. While we have described ourinvention, therefore, in connection with the use of an impact fracturing mill of the centripedal type in which particles to be fractured are flung against a target, it will be evident that any other sort of apparatus which fractures particles andprovides fresh, clean, dry angular fracture surfaces could be used. The inventors advance no particular theory at this time as to the exact mechanism by which the fresh, clean, dry, angular fracture surfaces aid in attaining strong, degradationresistant "green" balls. It is only evident that the provision of such fracture surfaces has a very dramatic effect upon the strength of the final "green" balls which are produced.
As will be evident from the foregoing discussion, it is not necessary in the practice of the present invention to use the type of impact fracturing apparatus illustrated, as any other equivalent apparatus which provides similar fracture surfacesupon the material treated may be used. A number of different impact fracturing devices could thus be used, although basically most such devices operate by flinging the material to be fractured from a rotating disc against a target disposedcircumferentially about the disc.
It will likewise be evident that various suitable types of spray dryers or any other type of dryer in which drying occurs substantially "instantaneously" in a hot gaseous atmosphere or the equivalent could be used.
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