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.
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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.
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.
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 iron contained 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%.
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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 leached dry-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 survive at 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 square centimeters 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.
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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 is directed 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 handling required 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 either moist 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 furnace exhaust 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 is discussed 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 of Fuming"; 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 steelmaking furnaces 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 from steelmaking 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 successfully transported, 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 spalled and 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 be transported 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, the fume 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, under 0.20% 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 into steelmaking 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 fume on 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 instantaneously drying 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-collected fume, 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-collected fume, 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, 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 quart of 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 of the "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. The impact 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 form fresh, 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 fracture surface 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 fracture between 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 are fractured 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 our invention, 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 and provides 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, degradation resistant "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 surfaces upon 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 disposed circumferentially 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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| Executed on | Assignor | Assignee | Conveyance | Frame | Reel | Doc |
| May 05 1978 | Bethlehem Steel Corporation | (assignment on the face of the patent) | / |
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