US20070266824A1

US20070266824A1 - Using a slag conditioner to beneficiate bag house dust from a steel making furnace

Info

Publication number: US20070266824A1
Application number: US11/437,972
Authority: US
Inventors: Joseph L. Stein; John Beatty
Original assignee: ISM Inc
Current assignee: ISM Inc
Priority date: 2006-05-19
Filing date: 2006-05-19
Publication date: 2007-11-22

Abstract

A slag conditioner containing MgO, up to 50% slag-making carbon, 10 to 60% particulates of bag house dust and dropout box particles and 2% to 25% binder is mixed and formed under pressure to produce aggregates which can have the form of a briquette. The MgO content of the mixture comprising: 20% to 90% burned aggregates comprised of particles less than 8 mm of which at least 30% is 0.2 mm or greater and containing between 35% and 94% MgO; and up to 50% light burned magnesite.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

U.S. patent application Ser. No. 10\990,678 filed Nov. 17, 2004

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to using particulate emissions essentially including oxides of iron, calcium, silicon, magnesium, zinc, cadmium and lead collected by an emission containment system for a steel making furnace as an essential component in a slag conditioner in the steel making furnace and, more particularly, to using such particulate emissions from an earlier steelmaking operation in an electric arc furnace as one of the major components of a slag conditioner for the electric arc furnace in on going steel making operations to increase the yield of the steel by the furnace and enrich the particulate emissions with greater quantities of oxides of zinc, lead and cadmium in bag house dust to allow disposal with enhanced commercial value.
2. Description of the Prior Art
The particulate emissions occurring during the operation of electric arc furnaces are contained by a system of ducts that yield particulate byproducts collected in one or more dropout boxes and a bag house. When the furnace charge is comprised of scrap, the particulate emissions are identified as a hazardous material classified by the EPA as KO61 dust. Typically, KO61 dust is generated at a rate of between 20 and 45 lbs of dust per ton produced by the electric furnace. The variance to the dust generation rates depends on the particle size of the lime and the method used to introduce lime products into the furnace during steel making process. When the lime is added to the furnace charge, typically in the form of iron bearing scrap, the lime is of sufficient size in excess of ½ inch aggregates to avoid air borne loss by a negative pressure applied by the exhaust of the bag house collection system. As a typical example, the dust generation rate falls to between 20 to 28 lbs. per ton of steel. When the lime is injected by pneumatic transportation, the lime is very small finer than ½ inch with some fines and has much more dust is easily pulled into the evacuation system and ends in the bag house which accounts for the large difference in bag house dust generation, as high as 45 lbs per ton of steel produced. The chemistry of bag house dust classified by the EPA as KO61 is site dependent and the following two typical examples demonstrate the existence of potential useful commodities. One example of bag house dust generated at a rate of 28-30 pounds per ton of steel from steel scrap and charged lime contained principal chemical components of: 16% CaO; 35% Fe₂O₃; 6% SiO₂; 03% MgO; 2% Al₂O₃; 3% carbon; 28% ZnO; and 7% other. A second example of bag house dust generated at a rate of 38-40 pounds per ton of steel from a steel scrap charge and injected lime contained principal chemical components of: 25% CaO; 29% Fe₂O₃; 5% SiO₂; 2% MgO; 2% Al₂O₃; 6% carbon; 25% ZnO; and 6% other.
It is known in the art to introduce bag house dust with as part of the scrap furnace charge into a steel making electric furnace. However, the particulate size distribution as typically illustrated by the graph of FIG. 1 is unfavorable to the steel making process since generally the 90% of the particles are less than 100 microns and about 50% are less than 10 microns. As a result it is believed a major portion of the dust particulate is drawn off by the emission containment system without participating as a beneficial part of the furnace charge. The operation of the electric steel making furnace alters the chemistry of the resulting bag house dust and the slag. The chemistry of the scrap making up the furnace charges will usually vary with each particular furnace charge. However, the typically chemistry for both the KO61 dust and slag made during the operation of an electric steel making furnace using the dust as part of the furnace charge are for the KO61 dust: CaO 22.1%; Fe₂O₃36.3%; SiO₂6.1%; MgO 3.3%; Al₂O₃2.0%; Carbon 1.3%; ZnO 24.5%; and Other 2.0%. The chemistry for the slag is: FeO 41.0%; SiO₂11.7%; CaO 26.2%; Al₂O₃5.32%; MgO 7.9%; MnO 5.88%; Cr₂O₃2.28%; TiO₂0.37%; P₂O₅0.10%; and SO₃0.16%. Comparing these to chemicals reveals that the absence of zinc content of the slag is believed to occur because the zinc vaporizes and recovered as part of the KO61 dust. The present invention is based on the realization that the KO61 dust and slag contain an abundance of oxides that can be altered by the use of slag conditioner of the present invention to benefit the entire operation of the electric steel making furnace. For example, it can be seen that the iron oxide content of the dust and slag is remarkably high and when successfully chemically reduced will increase the yield of steel by the electric furnace. The MgO content of the dust can be beneficially reduced in the dust to increase the MgO content in the slag whereby the resulting slag composition is affected for chemically preventing erosion of the furnace refractory and the usual foaming of the slag. The calcium oxide content of the dust can be reduced to exercise control of the lime-silica ratio.
Those skilled in the art of steel making were aware of the implication of the slag composition related to refractory life. It is known from phase diagrams that magnesium oxide is soluble in calcium silicate based liquid slag and that the solubility level depended primarily on the CaO to SiO2 ratio (“C/S”), commonly referred to as lime-silica ratio. When the lime-silica ratio in the slag composition at the end of a heat was greater than 2/1, the slag was found to have a chemical imbalance requiring about 7% MgO to be satisfied. A relationship exists between lining material wear and MgO content of the slag. The maintenance materials of the furnace lining have a high MgO content and became sacrificial donors of the deficient amounts of dissolving MgO to the slag, the damage to the lining limited the vessel campaign to between 400 to 1200 heats. Steelmakers, who added burned limestone for the CaO component in the basic slag, began to add burned dolomite or a blend of the burned limestone and burned dolomite to supply not only CaO but also MgO as a slag addition to satisfy the demand for MgO in the slag. Refractory lining life was improved but wear remained a continuing problem requiring frequent refractory relining which interrupted steel production.
In the 1980's the lining life of Basic Oxygen steel Furnaces (BOF) vessels was improved by changing the composition of slag for the steel making operations by increasing MgO content which made the slag more viscous. The presents of viscous slag combined with the gas blowing capabilities of the BOF, resulted in a practice called slag splashing. A coating of the viscous refractory slag blown onto the furnace walls protected the vessel lining from excessive wear and was practiced after almost every heat. The steelmaking process could be carried out using the renewed slag coating on the refractory lining after each heat. This slag coating process extended the lining life in some instances to more than 10,000 heats in BOF furnaces and attempts were made to apply a similar concept of a slag coating process to the electric arc furnace. U.S. Pat. No. 6,514,312, issued Feb. 4, 2003 contains a disclosure of the slag splashing practice to a BOF vessel.
Accordingly, it is an object of the present invention to provide a useful slag conditioner composition having an MgO component of at least 15% and a size particulate thereof above 0.2 mm for a steel making furnace using particulate emissions of such furnace particularly bag house dust as recycled major component from a furnace facility, particularly the same on site steel making furnace facility.
It is another object of the present invention to provide a useful slag conditioner composition having an MgO component comprised of dolomite alone or without added carbon bearing material at least 15% particulate thereof above 0.2 mm and a water free binder for a steel making furnace using particulate emissions including bag house dust as recycled major component from a furnace facility, particularly the same on site steel making furnace facility.
It is a further object of the present invention to provide an improved briquette first mention of agglomeration, briquetting preferred composition useful to extract oxides, particularly Ca, Si, Fe, Fe₂, and Mg for dispersion in a slag volume as an enhancement to the chemistry thereof and saturating the slag with an MgO content to impart a creamy and easily foamed properties to the slag and a carbon content to chemically reduce iron oxides in the furnace burden and the developing slag including such oxides in the introduced slag conditioner to increase the productivity of the steel making furnace.
It is a further object of the present invention to provide an improved briquette composition or other formed agglomerate including KO61 dust and/or drop out box material and capable of accommodating changes in KO61 chemistry and changes with the KO61 product or blend of materials with one of a plurality of different quantities of such recycled dust and/or material selected according to a varying chemistry and concentrations, disposal value, availability to neutralize the oxides of the slag and reduce the iron as a slag conditioner for slag in an electric steel making furnace.
It is a further object of the present invention to provide an improved briquette composition including KO61 dust and/or drop out box material useful to increase zinc, cadmium, and lead contaminants in particulates collected as particulates classified as bag house dust for enhancing the commercial value by a periodic routing a part of the dust in the recycle circuit for sale or disposed of at a reduced cost to the steel making operation.
A further object of the present invention provides that the recycling of particulate emission including drop out box particulates and/or bag house dust occurs in such quantities so as to match or reduce the dust generation rates of the steel making furnace and reduce the amount of KO61 requiring disposal.
A further object of the present invention to chemically reduce an oxide of iron which are chemically bound in the KO61 material by combining carbon with the KO61 material in slag bath of a steel making furnace and ultimately increase the yield of steel produced by the furnace and lower the dust generation rates, typically by 5 to 10 lbs per ton of steel produced
A further object of the present invention is to provide an improved slag conditioner composition using refuse particulates of MgO and carbon based slag conditioner selected from used crushed brick from either a steel making furnace or ladle therefore, lime, carbon; KO61 bag house material and or drop out box material; dead burned MgO; and binding agents
Another object of the present invention is to alter a slag composition in a steel making furnace by the addition of a select amount of magnesium oxide, a part of which is supplied by recycled quantities of KO61 particulate material to more economically create useful slag properties including an increased viscosity, creamy texture, and an increased ease for foaming of the slag to provide a protective coating on the furnace walls to extend the useful lining life.

SUMMARY OF THE INVENTION

More particularly according to the present invention there is provided a slag conditioner comprising by weight a mixture and 2% to 25% binder for bonded agglomerates or larger particles of the mixture, the mixture comprising: 10% to 60% particulates comprised of bag house dust essentially containing oxides of iron, calcium, silicon, magnesium, zinc, lead and cadmium; 20% to 90% burned aggregates comprised of particles less than 8 mm of which at least 30% is 0.2 mm or greater and containing between 35% and 94% MgO; up to 50% slag-making carbonaceous additive; and up to 50% light burned magnesite.
According to a further aspect of the present invention there is provided a method of making steel including the steps of recovering particulate emissions containing oxides of iron, calcium, silicon, magnesium, zinc, lead and cadmium from a particulate containment system including bag house of a electric steel making furnace, forming briquettes comprised of a mixture of aggregates and 15% to 60% of the particulate emissions obtained by the step of recovering and 2% to 30% binder to agglomerate the mixture, the mixture comprising: by weight 15% to 80% dead burned magnesite comprised of particles less than 8 mm of which at least 30% is 0.2 mm or greater and containing between 35% and 94% MgO; up to 40% light burned magnesite; and 5% to 50% carbon selected from the group consisting of: coal; coke; graphite and petroleum coke and iron oxide, introducing an iron bearing charge into the electric steel making furnace, melting and refining an iron bearing charge in the electric furnace while forming an overlying layer of slag, introducing the slag conditioner to the electric steelmaking furnace in an amount needed to raise the MgO level in the overlying layer of slag to between 5% to 14% and thereby impart a creamy slag texture, non leaching for soluble MgO, foam producing to increasing slag volume, and protectively coat refractory sidewalls of the electric steelmaking furnace.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The present invention will be more fully understood when the following description is read in light of the accompanying drawings in which:

FIG. 1 is a graph to schematic illustrates a particle size spectrum in a random sampling of KO61 particulate material;

FIG. 2 is a schematic illustration of a steel making operation incorporating the present invention; and

FIG. 3 is a flow diagram of the process for making slag conditioning agglomerates according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The chemical symbol MgO as used herein refers to products recovered from roasting natural magnesite ore in an oven, one product called light burned magnesite takes its name from the common process roasting the ore at a temperature of about 2000° F. for a desired period of time, and second product called dead burned magnesite takes its name from the common process roasting the ore at a temperature of about 3000° F. for a desired period of time. The compound MgO after dead burning develops an observable crystal, periclase, which is chemically resistant to high lime (CaO) containing furnace slag. Also useful in compositions with MgO based refractory brick was chrome ore, which was added for thermal shock resistance for linings in electric arc and open hearth furnaces. The practice of using chemically basic linings caused a chemical change to the slag floating on the steel. The chemical change was a higher lime (CaO) content to the slag for chemical compatible with the refractory lining materials. In addition, the high CaO content of the basic slag improved a necessary metallurgical function of collecting undesirable impurities from the liquid steel bath. Among these impurities better removed by basic slag are sulfur, phosphorus, and silicon depending on the grade of steel being produced.
As used herein dead burned magnesite commonly referred to as DB MgO is an economical source of MgO in a crystalline form made up of aggregates of periclase crystals, predominantly large crystals. These crystals are chemically known as magnesium oxide, MgO. Light burned magnesite commonly referred to as LB MgO is also source of MgO, however the magnesium oxide crystals derived from light burned magnesite or light burned dolomite are smaller and dissolve with greater ease in liquid slag than the magnesium oxide crystals of dead burned magnesite. The MgO constituent in the slag must be sufficient to saturate the slag with MgO thereby prevent absorption of MgO from other sources in the furnace. However, quantities of MgO in excess of the stoichiometric amount are to be present as a solid crystalline suspension to serve as a thickening agent to increase the bulk viscosity of the slag and impart the desired creamy texture to the slag. The MgO held in suspension is most effectively supplied by the relatively larger crystals of MgO derived from dead burned magnesite or dead burned dolomite.
As shown in FIG. 2, an electric steel making furnace 10 receives a charge of scrap material from a furnace charger 12 and during the operations of the furnace for melting and refining the furnace charge there also supplied a slag conditioner from a charger 14. Preferably the composition of the slag conditioner comprising by weight a mixture and 2% to 25% binder for bonded agglomerates or larger particles of said mixture, said mixture comprising: 20% to 90% burned aggregates comprised of particles less than 8 mm of which at least 30% is 0.2 mm or greater and containing between 35% and 94% MgO; up to 50% slag-making carbonaceous additive; and up to 50% light burned magnesite.
During the operation of the furnace, particulate emissions are collected in an exhaust system maintained under negative atmospheric pressure for collection of the particulates in a facility commonly referred to in the art as a bag house 16. At the conclusion of each such the steel making operation, slag is drained from the furnace to a slag pot 18 and refined steel burden is separately discharged from the furnace into a transport vessel 20. According to the present invention, particulates collected in the bag house 16 are routinely removed to a first storage bin 22. Such particulates essentially include FeO, ZnO₂, CaO, SiO₂, MgO, Cd and Pb as emissions during the melting and refining of the scrap charge together with the usual charges of carbonaceous and lime bearing materials. An accumulation of the particulates in the first storage bin occurring under these circumstances will define a generally consistent chemical content but will vary insignificantly with the site specific charge of scrap to the furnace. After the particulate volume in the storage bin 22 is used as a component in an ongoing formulation of slag conditioner briquettes. The briquettes can be introduced as part of the initial furnace charge and/or crushed and introduced as a first recycling formulation of crushed slag conditioning briquettes by charger 14 to an overlying layer of slag in a sufficient amount of to raise the MgO level in slag to between 5% to 14% during melting and refining of an iron bearing charge in an electric steel making furnace.
The operation of the steel producing furnace using the first recycling formulation of crushed slag conditioning briquettes also generates particulates collected in the bag house 16 are routinely removed to a second storage bin 24 or added as replacement quantities to the storage bin 22. The second storage bin 24 is optional. The collection of such particulates will essentially contain ZnO₂, Cd and Pb as emissions during the ongoing operations of the steel making furnace An accumulation of the particulates in the second storage bin 24 occurring under these circumstances will define a generally consistent chemical content but will vary insignificantly with the site specific charge of scrap to the furnace. The storage bin 22, and when used, storage bin 24 will periodically receive quantities of dropout box particulates that are collected from boxes placed at convenient locations about the duct for the fume containment system of the electric steel making furnace. The particulates supplied by the drop out box notably include scale like formations from the duct form generally of higher concentrations of relatively low melting point compounds. Essentially, however, the oxides supplied by dropout particulates are the same as the oxides recovered from the bag house. The briquettes are crushed and introduced as a second recycling formulation of crushed slag conditioning briquettes by charger 14 are altered by variations to the recycled particles and change from a start up quantity used in the slag re-conditioner of between 10 and 60% which is reduced as the recycle program continues to between 10 and 30% and to a minimum of 10 to 25%. The variance being dependent on lower quantities of iron, calcium, silicon and magnesium oxides and increased quantities of zinc, lead and cadmium to an overlying layer of slag in a sufficient amount of to raise the MgO level in slag to between 5% to 14% during melting and refining of an iron bearing charge in an electric steel making furnace.
The slag conditioner composition according to the present invention may incorporate different size fractions of aggregate materials and formulations to supply MgO in an effectively sized crystalline form. An underlying discovery of the present invention is intermediate and fine sized crystals of magnesium oxide can be added to a furnace in an agglomerate form and enter efficiently into chemical reactions with liquid slag phase without detrimental loss of smaller sizes of magnesium oxide crystals to exhaust gases during the steel making process. Supplying MgO in the slag by the slag conditioner will save erosion of MgO from the high cost refractory brick linings, gunning repair mixes, and prepared granular bottom repair mixes of the furnace linings. An adequate MgO content in the slag also facilitates the use of relatively small amounts of carbon bearing materials to produce foaming of the slag for protecting the refractory furnace lining and enhancing the operation of the steel making furnace. As used herein dead burned magnesite commonly referred to as DB MgO is an economical source of MgO in a crystalline form made up of aggregates of periclase crystals, predominantly large crystals. These crystals are chemically known as magnesium oxide, MgO. Light burned magnesite commonly referred to as LB MgO is also source of MgO, however the magnesium oxide crystals derived from light burned magnesite are smaller and dissolve with greater ease in liquid slag than the magnesium oxide crystals of dead burned magnesite. The MgO constituent in the slag must be sufficient to saturate the slag with MgO thereby prevent absorption of MgO from other sources in the furnace. However, quantities of MgO in excess of the stoichiometric amount are to be present as a solid crystalline suspension to serve as a thickening agent to increase the viscosity of the slag and impart the desired creamy texture to the slag. The MgO held in suspension is most effectively supplied by the relatively larger crystals of MgO derived from dead burned magnesite or dead burned dolomite.
As shown in FIG. 3, there is a preferred arrangement of apparatus to formulate the slag conditioner of the present invention. As described previously, there is loaded a 1^stMgO Source into a hopper 100 by weight 20% to 90% burned aggregates comprised of particles less than 8 mm of which at least 30% is 0.2 mm or greater and containing between 35% and 94% MgO; up to 50% slag-making additive; and as a 2^ndMgO Source up to 50% light burned magnesite. The slag conditioner also essentially includes between 10 and 60% which can be reduced to 10 to 30% but at least 10 to 25% particulates including bag house dust a common waste byproduct of a steel making furnace operation as a recycled major component from a furnace facility, particularly the same on site steel making furnace facility to effectively supply beneficial constituents to the layer of slag forming during the ongoing steel making process and increase the yield of steel by the furnace. Other components forming the grope of suitable products for inclusion in the slag conditioner of the present invention are refuse particulates of MgO and carbon based slag conditioner selected from used crushed brick from either a steel making furnace or ladle therefore, lime, carbon; KO61 bag house material and or drop out box material, fines of a bonded mixture containing MgO and carbon such as maybe found in other slag conditioning agglomerates and fines of the agglomerates forming a slag conditioner of the present invention mixture.
The slag conditioner is formed as a briquette or other similar agglomerate or as an agglomerate crushed form such as briquettes comprised of an agglomeration of different size fractions of aggregate materials including bag house dust and formulated to supply intermediate and fine sized crystals of magnesium oxide and enter efficiently into chemical reactions with liquid slag phase without detrimental loss of smaller sizes of magnesium oxide crystals and iron and calcium bearing compounds to exhaust gases during the steel making process. During the refining process of the scrap charge in the electric steel making furnace the conditions in the slag and the steel burden allow a chemical reaction to reduce oxide of iron which are chemically bound in the KO61 material by combining carbon with the KO61 material in slag bath of a steel making furnace and ultimately increase the yield of steel produced by the furnace and lower the dust generation rates, typically by 5 to 10 lbs per ton of steel produced. Supplying MgO in the slag by the slag conditioner will save erosion of MgO from the high cost refractory brick linings, gunning repair mixes, and prepared granular bottom repair mixes of the furnace linings. An adequate MgO content in the slag also facilitates the use of relatively small amounts of carbon bearing materials to produce foaming of the slag for protecting the refractory furnace lining and enhancing the operation of the steel making furnace. The slag conditioner of the present invention will alter a slag composition in a steel making furnace by the addition of a select amount of magnesium oxide, a part of which is supplied by recycled quantities of KO61 particulate material to more economically create useful slag properties including an increased viscosity, creamy texture, and an increased ease for foaming of the slag to provide a protective coating on the furnace walls to extend the useful lining life.
The slag making additive also introduced in the hopper 100, by weight, is carbonaceous; preferably having carbon content between 78% and 99.8%. The carbonaceous material preferably has a particle size of less than 6 mm is useful and can be a size fraction of 5×0 mm, or a smaller size fraction of 3×0 mm, but the smallest size fraction is 1×0 mm. The carbonaceous additive can be selected from the group comprising coal; anthracite coal; metallurgical coke; petroleum coke; graphite and petroleum coke. A hopper 102 is supplied, by weight, with 2% to 25% binder for bonding agglomerates or larger particles of the mixture in hopper 100. The weighed quantity of binder can be a liquid such as water or selected from the group consisting of: sodium silicate; ligosulfonate; lignosulfonate solutions; hydrochloric acid; sulfuric acid; magnesium chloride; magnesium sulphate; molasses; pitch; tar; asphalt; bentonite; clays and resins, with or without water added, each with sufficient liquid to form a moldable mixture. Alternate binders to reduce or essentially eliminate the hydroxide binder formed by as the reaction product of water with caustic MgO component of the conditioner will not play a significant role in slag or steel making except to act as a temporary binder for the agglomerated particles, in one case, of dead burned magnesium oxide and coal. Organic binders using 6% water or less are useful to make briquettes in compositions of this invention. Low ignition loss binders permit a higher weight percent of useful steelmaking materials, i.e. MgO and carbon units. Another advantage of the use of low ignition loss binders is that the energy required to decompose hydroxides and/or carbonates from slag conditioners in the melting process is minimized, if not eliminated. The slag conditioner in briquette form can be designed to have a sufficiently low ignition loss so to be exothermic and thus will not deplete energy from the steel making furnace. Another advantage in the use of organic binders is that the need for light burned magnesite as a source for MgO can be replaced with additional dead burned magnesium oxide fines, which are more resistant to hydration, thereby making the life of slag conditioner briquettes longer in storage. The use of organic binders provides another advantage. Binders can be selected that contain little or no water. In this case, alternate materials sensitive to hydration can be employed in slag conditioners in the same particle size ranges. Those alternate materials include but are not limited to burned dolomite and dead burned dolomite. In compositions based on burned dolomite in place of dead burned magnesium oxide, the intermediate particles contribute reactive sources of MgO and CaO, both oxides being useful for steel making slag to produce similar useful results as those compositions based on dead burned magnesium oxide. Some slag conditioner formulations of the present invention provide that the burned aggregates are present between 40% to 80% and, in such a formulation, the light burned magnesite is up to 40% and the binder is between 2% to 25%.
The size range of particles comprising the dead burned aggregate is further defined by a size fraction of 6×0 mm at least 30% being larger than 0.2 mm, preferably the particles are within the range of about 5×0 mm, most preferably a 3×0 mm size fraction but a size fraction of 1×0 mm and includes fines is also suitable. The MgO constituent of the dead burned magnesite and the light burned magnesite may be replaced with burned dolomite aggregate all or in part. The smaller crystals of MgO occur in light burned magnesite particles and comprise at least 80% and not more than 97% MgO in magnesite particles less than 100 mesh, preferably less than 200 mesh to promote the desired ease of dissolution in the slag bath occurring throughout the refining of a heat of steel. The dead burned aggregate may consist of dead burned dolomite and the slag conditioning mixture further include light burned dolomite, each providing a sources of CaO and MgO components to the chemistry of the slag to reduce the sulfur content of the refined molten steel.
The 20% to 90% by weight of burned aggregates in hopper 100 are comprised of two constitute parts, first part are in a size fraction of less than 8 mm with at least 30% of the aggregates being 0.2 mm or greater and containing 35% to 94% MgO, preferably between 80% and 94% MgO and the aggregates of the second part are in a size fraction of up to 50% light burned magnesite containing more than 85% MgO and having a particle size less than 100 mesh and more particularly about 80% or more particles less than 200 mesh. The two constitute parts are separately measured by weight and then loaded into a hopper 10. Dead burned magnesium oxide fines can be used beneficially to lower ignition losses and replace light burned magnesite as a component in a slag conditioner in the briquette form.
Carbon from the dense agglomerates or briquettes of this invention, or particles derived from such agglomerates, react in a more efficient way in the steel making process in an electric furnace including a very effective reducing of slag components to increase the yield of metals such as iron from iron oxides normally found in steel slag. The slag making additive can be carbonaceous, preferably having carbon content between 78% and 99.8%, and/or the additive can be a slag making compatible filler. The measured quantities of burned aggregate and slag additive in hopper 100 and the binder in hopper 102 are loaded into a suitable mixer 104, such as a muller, ribbon, or auger mixer. The mixer 104 is operated for at least two minutes until the aggregates and binder are uniformly dispersed and tempered to form a moldable mass. The tempered mass is then loaded into an agglomerating machine 106, such as a high-pressure briquette press to produce solid 60 mm square briquettes between 30 to 40 mm thick. The briquettes of slag conditioner of the present invention may be formed in other sizes, such as 30×30×10 mm; 40×40×20 mm; 60×40×20 mm; 70×50×40 mm. Other suitable forms of machines 106 for forming agglomerates are a mechanical press, a hydraulic press, a friction screw press, a rotary press, an inclined pelletizing disc and an extruder, all per se well known in the art. The briquettes develop an adequate strength for handling after curing and partially drying in storage room 108 maintained at temperature suitable to promote bonding by operation of the binder and evaporation of residual water, when the binder is aqueous, for example about three days. The density of the briquettes typically exceeds 1.8 g/cc and attains a crushing strength measured according to ASTM test methods modified for 2 cm cubes, to exceed 2000 pounds per square foot. The resulting agglomerates are suitable for charging into an electric furnace with iron bearing scrap charge and fluxes such as burnt lime to alter the chemistry of the slag occurring during the steel making operation. Additionally or alternative the agglomerates maybe crushed to a size suitable for injection into the developing layer of slag overlying the steel burden in the furnace.
The present invention retains the benefit of slag conditioning briquettes or aggregates of the present invention to maintain the desired MgO content in the slag throughout the steel making operation. The slag appearance exhibiting a creamy texture is a reliable indicator of a surplus of solid MgO crystals which increase the bulk viscosity of the liquid slag. Foaming of the slag by the injection of surprisingly smaller quantities of carbon is sufficient to produce a reaction with the oxygen blown into the furnace or by reaction with FeO in the slag to release CO and CO₂gases to cause a slag to foam. However, when the carbon particles are supplied by the briquette, the carbon is altered to the form of dense particles that penetrate deep into the slag bath so that the reaction with FeO or oxygen creates gas in a position to better foam the slag. The role of the carbon component of the briquettes of this invention, when used in conjunction with the correct type and size of MgO source materials, is associated with a particle of high density in the briquette. Even when the briquettes are crushed and injected as fines, the carbon is associated with a dense, but a finer particle size. The carbon associated with higher density particles from the briquette compositions is consumed very efficiently in the steel making process. The briquettes of this invention provide between 8.5% to 12% MgO to the slag.
Taking into account the observation that both MgO and carbon was being provided to the steel making process more efficiently through the use of the agglomerate of finer materials in the form of a briquette, it was discovered that the purity of the MgO source, the particle size and the density of the grains are important factors controlling the solubility of the MgO units into the slag. Therefore, the dead burned magnesium oxide used in the slag conditioners of this invention shall be not more than 94% MgO purity, and have a grain bulk density or bulk specific gravity of not more than 2.25 g/cc. This observation explains why crushed used refractory brick especially brick containing fused MgO of any size does not provide the expected benefits of slag conditioners, even compared to dolime, dead burned magnesium oxide 15×3 mm.
The improved slag conditioner of the present invention using bag house inkling dropout box particulate contributes to the needed MgO and lime units in the slag The charge weight of briquette material can be reduced. Therefore, the weight of MgO units added to the furnace is reduced while the same MgO level can be realized in the slag. The briquettes also permitted a reduction to the burnt lime added to the furnace. This in turn lowered the CaO and therefore the CaO to SiO₂ratio. Liquid slag at a lower CaO to SiO₂ratio is known to favor lower FeO in the slag. More iron could therefore be converted to steel therefore increasing the yield of tapped steel. The slag conditioner may be introduced to electric steelmaking furnace in an amount needed to raise the MgO level in high lime calcium-silicate slag to between 5% to 14% , although for low lime calcium silicate slag up to 18% is useful and thereby impart a creamy slag texture, non leaching for soluble MgO, foam producing to increasing slag volume, and protectively coat refractory sidewalls of the electric steelmaking furnace. The slag conditioner is introduced in an electric furnace in sufficient quantities to raise the MgO level in a high lime calcium-silicate slag to up to 22% when the CaO to SiO₂ratio is below 1.5. In the event of the failure to develop a creamy slag texture and the slag has the appearance of thin water like texture or consistency and further the slag does not foam well, the quantity of slag conditioner introduced is selected as an amount sufficient to raise the MgO level in a high lime calcium-silicate slag to up to 14% by adding more burned lime to increase the CaO to SiO₂ratio to between 1.8 and 2.1. The charging of the furnace may includes charging an iron bearing metal at two different intervals of time into and between 20%-80% of the slag conditioner is charged during a furst of the two internals of time and between 20%-80% of the slag conditioner is charged during a second of the two intervals of time. Typically, at least 20% of the calculated slag conditioner is charged during a time of heating the iron bearing metal in the furnace. In addition, an objective is to combine other compatible additives into the briquette slag conditioner with or without carbon, to provide finer more reactive materials efficiently to the steel or to the slag. Additives introduced in this manner may include silicon carbide and ferrosilicon for reducing other valuable oxides to metal such as chromic oxide to chrome metal in the manufacture of stainless steel. Alternately, burned dolomite can be added to provide finer more reactive particles of CaO and MgO. Incorporating the finer materials in a briquette form ensures the materials will reach the slag bath interface and be reacted efficiently.
While the present invention has been described in connection with the preferred embodiments of the various figures, it is to be understood that other similar embodiments may be used or modifications and additions may be made to the described embodiments for performing the same function of the present invention without deviating there from. Therefore, the present invention should not be limited to any single embodiment, but rather construed in breadth and scope in accordance with the recitation of the appended claims.

Claims

1. A slag conditioner comprising by weight a mixture and 2% to 25% binder for bonded agglomerates or larger particles of said mixture, said mixture comprising: 10% to 60% particulates comprised of bag house dust essentially containing oxides of iron, calcium, silicon, magnesium, zinc, lead and cadmium; 20% to 90% burned aggregates comprised of particles less than 8 mm of which at least 30% is 0.2 mm or greater and containing between 35% and 94% MgO; up to 50% slag-making carbonaceous additive; and up to 50% light burned magnesite.

2. The slag conditioner according to claim 1 wherein said particulates comprised comprise between 10% and 30% of bag house.

3. The slag conditioner according to claim 1 wherein said particulates comprised comprise between 10% and 25% of bag house.

4. The slag conditioner according to claim 1 wherein said particulates comprised of bag house dust comprise particles in the range of 0.1 and 1000 microns.

5. The slag conditioner according to claim 1 wherein said particulates comprised of bag house dust include particulates of drop out boxes essentially including oxides of iron, calcium, silicon, magnesium, zinc, lead and cadmium.

6. The slag conditioner according to claim 1 wherein said burned aggregate is selected from the group consisting of: dead burned MgO; light burned MgO; burned dolomite; crushed magnesite carbon brick; fines of a bonded mixture containing MgO and carbon and fines of said mixture.

7. The slag conditioner according to claim 6 wherein said carbonaceous additive is selected from the group consisting of: coal; coke; petroleum coke; crushed magnesite carbon brick; ladle slag line brick, steel making furnace brick; fines of a bonded mixture containing MgO and carbon and fines of said mixture.

8. The slag conditioner according to claim 1 wherein said carbonaceous additive is selected from the group consisting of: coal; coke; petroleum coke; crushed magnesite carbon brick; ladle slag line brick, steel making furnace brick; and fines of said mixture.

9. The slag conditioner according to claim 1 wherein said burned aggregate comprises particle less than 8 mm of dead burned magnesite containing between 80% and 94% MgO.

10. The slag conditioner according to claim 1 wherein said burned aggregate comprise particles in a size range of 6×0 mm.

11. The slag conditioner according to claim 1 wherein said burned aggregate comprise particles 3×0 mm.

12. The slag conditioner according to claim 1 wherein said burned aggregate comprise particles 1×0 mm including fines.

13. A method of making steel including the steps of:

recovering particulate emissions containing oxides of iron, calcium, silicon, magnesium, zinc, lead and cadmium from a particulate containment system including bag house of a electric steel making furnace;

producing a slag conditioning by forming agglomerates comprised of a mixture of aggregates and 15% to 60% of said particulate emissions obtained by said step of recovering and 2% to 30% binder to agglomerate said mixture, said mixture comprising: by weight 15% to 80% dead burned magnesite comprised of particles less than 8 mm of which at least 30% is 0.2 mm or greater and containing between 35% and 94% MgO; up to 40% light burned magnesite; and 5% to 50% carbon selected from the group consisting of: coal; coke; graphite and petroleum coke and iron oxide;

introducing an iron bearing charge into said electric steel making furnace;

melting and refining an iron bearing charge in said electric furnace while forming an overlying layer of slag;

introducing said slag conditioner to said electric steelmaking furnace in an amount needed to raise the MgO level in said overlying layer of slag to between 5% to 14% and thereby impart a creamy slag texture, non leaching for soluble MgO, foam producing to increase slag volume, and protectively coat refractory sidewalls of said electric steelmaking furnace.

14. The method of making steel according to claim 13 wherein said step of introducing a slag conditioner includes introducing briquettes, produced by said step of forming, to said electric steel making furnace during said step of introducing an iron bearing charge.

15. The method of making steel according to claim 13 wherein said step of introducing a slag conditioner includes introducing briquettes produced by said step of forming to said electric steel making furnace during said step of melting and refining an iron bearing charge.

16. The method of making steel according to claim 13 including the step of crushing said agglomerates to form slag conditioner particulates for said step of introducing a slag conditioner, and wherein said step of introducing a slag conditioner includes injecting said slag conditioner particulates into said overlying layer of slag.

17. The method of making steel according to claim 13 wherein said particulates comprise between 10% and 30% .

18. The method of making steel according to claim 13 wherein said particulates comprise between 10% and 25%.

19. The method of making steel according to claim 13 wherein said particulates comprise particles in the range of 0.1 and 1000 microns.

20. The method of making steel according to claim 13 wherein said particulates include particulates of drop out boxes essentially including oxides of iron, calcium, silicon, magnesium, zinc, lead and cadmium.