Considerations For The Feeding And Use Of Alternative Iron Materials In The EAF
Jeremy A.T. Jones
Over the past 10 years, the amount of steel made in the EAF has grown by almost 50 % and the EAF route now accounts for approximately 35 % of steelmaking capacity worldwide. In North America, the EAF now accounts for 40 % of steelmaking capacity. The reasons for this phenomenal growth are well documented and include, amongst other factors, the lower capital cost as compared to the coke oven-blast furnace-BOF route and the ability of EAFs to produce steel qualities that are acceptable for the lower end of the flat products market. In fact, several integrated mills are now considering the implementation of EAF technology within their operations in order to add incremental capacity or to cope with forecast hot metal shortages in the future. As a result, it can be expected that the demand for steel scrap and scrap substitutes will continue to grow.
It is surprising to note however, that very little effort has been placed on scrap management over the past 10 years. The greatest emphasis has been on residuals in the scrap and their resulting effect on the steel quality. Prior to 1970, EAF operations produced mostly medium to high-alloyed steels. Over the period of 1970 to 1985, this changed and these operations took the market for carbon steels away from the integrated producers especially in the area of long products. Material input costs for an EAF operation typically lie in the range of 55 to 75 % depending on the amount of local scrap that is available (1,2). With such a large portion of the liquid steel cost coming from the cost of scrap, it is surprising that more attention has not been placed on optimizing this aspect of steelmaking! By comparison, energy cost makes up approximately 12 % of the cost of liquid steel and over the past 15 years, the major focus of EAF technology has been placed on making the EAF more productive and more energy efficient (1). It should be noted thats crap affects much more than the cost of liquid steel in EAF steelmaking. Scrap can also have an enormous affect on many of the EAF operating parameters such as:
• Power input efficiency
• Chemical energy efficiency
• Scrap layering
• Heel practice
• Flux addition practices
• Effectiveness of foamy slag practices
• Slag chemistry profile
• Off-gas system operations and capacity
• Damage to furnace refractory
• EAF cycle times
• Damage to w/c furnace panels
To name but a few. All of these factors can be influenced in one way or another by the physical and chemical properties of the scrap. Some of the key scrap parameters include:
• Chemistry and residual levels
• Bulk density
• FeO, SiO2, Al2O3 content
• Oil, grease and coatings content
Many of these parameters have been studied to determine their effect on EAF operations but in very few cases has there been a concerted effort to develop an optimized strategy to provide the best possible control of the EAF.
The key to high productivity in the EAF is to achieve uniform, consistent operation from one heat to the next. Many high productivity EAF shops have adopted the practice of tapping a uniform heat of steel with respect to chemistry and then making the desired steel grade in the ladle. This has resulting in significant improvements in EAF productivity but it is easy to see that the fluctuations that occur in the scrap mix with respect to density, scrap sizing, carbon content and silicon content cause considerable disturbances in EAF operations leading to:
• Variations of +/- 10 % in tap-to-tap time
• Variations of +/- 5 % in power consumption
• Variations of +/- 5 % in oxygen consumption
• Variations in tap temperature of +/- 30 F
• Wide variations in slag chemistry leading to poor foaming efficiency
• Damage to the furnace sidewalls and roof resulting from arc flare
• Over-loading of the EAF offgas system
Several comprehensive scrap management models have been developed and are in use in some EAF meltshops. These models tend to focus on residual requirements for the steel grade being made, the properties of the scrap materials, the actual inventory of materials on hand at the plant and plant logistics (2). Approximately 75 % of prompt scrap is available as identified, unmixed material that is low in residuals (2). The amount of variation in the chemistry and physical properties of such scrap is minimal. Obsolete scrap however, originates from a wide variety of sources – automobiles, appliances, structural steel, miscellaneous sheet products etc. (2). The level of variation in these materials is considerable. Not only does obsolete scrap tend to contain high levels of residuals but also these levels will fluctuate considerably based on the source of the scrap. Scrap is supplied to the EAF facility based on sizing and density – thus the EAF operator is faced with considerable fluctuation in scrap chemistry, sometimes from one heat to the next. Typically, most operations use the mean chemical analysis of the scrap to determine scrap blending requirements. In order to achieve a high success rate for acceptable chemistry, it is necessary to take into account the inherent variability in scrap chemistry (2). This approach tends to result in an increase in cost for the scrap charge but at the same time also achieves a much higher level of consistency in EAF operations and much fewer “off-chem” heats. The question that we must ask ourselves is “What is the value of consistency in EAF operations?”. Unfortunately this is a very difficult question to answer and the answer will be specific for a specific EAF operation.
THE NEED FOR AI MATERIALS – In the next ten years raw materials will become a major concern in the iron and steel market for the industrialized world. In North America, many consultants are already forecasting a shortage of prime #1 grade scrap. Prime # 1 scrap is currently used in operations producing flat products and high-grade long products due to the low level of residual elements (Cu, Sn, Ni) that this scrap contains.
Demands for lower and lower residual levels continue to grow as operators seek to further increase casting speeds and improve product physical properties. Over the past 10 years several million tons of flat rolled, EAF based, steelmaking capacity came on line in North America. Unlike previous EAF minimills that displaced North American integrated steelmaking capacity, these facilities displaced a considerable amount of the flat products currently imported from overseas. A large increase in demand for prime scrap is expected to result to meet the needs of the new North American EAF shops.
Historically, the United States has been a major exporter of steel scrap. Recently, the amount of scrap exported has dropped considerably. However, only a small fraction of domestic scrap is suitable for the production of flat products. It is possible though, touse lower grade scrap that contains residual elements, if this scrap is blended with “clean iron units” so that the resulting residual levels in the steel following melting meet the requirements for flat rolled products. Clean iron units are typically in the form of:
• Direct reduced iron (DRI)
• Hot briquetted iron (HBI)
• Pig iron
• Molten pig iron (hot metal)
More and more EAF operations are turning to the use of alternative iron materials as feedstocks for the EAF. On average, pig iron now makes up 5 – 10 % of EAF feed worldwide. In 1998, worldwide DRI production was 37.1 million tons. DRI/HBI use is growing and is projected to reach 60 – 65 million tons per year by the year 2005. Alternative iron materials are used to provide better quality, lower residual steel as well as to provide greater consistency to EAF operations. The remainder of this paper shall focus on the types of alternative iron materials available to EAF operations aswell as the methods used to charge these materials and the effect that these materials and charging methods have on EAF operations.
Classification of AI Materials
Many attempts have been made to establish the value in use of various AI materials. This has been discussed previously (3). Alternative iron sources can take on many different forms. Even within a single product type such as DRI, variations in chemistry and metallic content can greatly affect the way in which the material can be used in the EAF. One of the biggest concerns facing EAF operations is to meet product quality requirements at minimum cost. Without a clear understanding of the effect of alternative iron characteristics on EAF operations, it will be difficult for the EAF facility to determine the least cost mix of scrap to melt in the furnace. Several parameters are used to specify the quality and characteristics of DRI/HBI. As other technologies have been developed, these parameters have also been used to grade the products. Thus the following parameters can be applied to any alternative iron product.
The key parameters used to specify DRI quality are % metallization, % carbon and % gangue. Percent metallization refers to the % of iron in the DRI that is present as metallic Fe:
% met = [(% Fe metallic)/(% Fe Total)] * 100
Typically steelmakers prefer a metallization of about 92 % or higher. Some operations now classify the alternative iron source by its’ “useable iron content”. Useable iron content refers to the amount of metallic iron present in the feedstock (% Fe metallic) and is an indication of the amount of the feed, which will directly contribute to the steel bath. Any iron that is not in this form will be present as iron oxide and must be reduced in the bath to recover the iron units. This will require both energy input and a source of reductant, usually carbon. In the case of pig iron or hot metal, the iron is 100 % metallized.
The % carbon in the DRI can be manipulated based on the flow of reducing gas, the residence time of the DRI in the shaft and most easily by adjusting the amount of cooling gas used in the lower zone oft he shaft furnace. Usually, steelmakers prefer a carbon content in the DRI above 1 %. Carbon content in pig iron or hot metal, the carbon content various considerably depending on the process source. Typically pig iron will contain between 3 and 4.5 %. Some DRI is produced with up to 4 % C but various opinions exists as to whether this is economical in terms of the DR plant throughput and the equivalent cost of the carbon to the product.
If excess carbon is present, it can be used as an energy source in conjunction with oxygen injection in order to reduce electrical power requirements. As has been mentioned previously, approximately 1 % carbon is required in the DRI to offset every 6 % FeO in the DRI. There has been much controversy regarding the benefits of high carbon levels in the DRI. Typical charge carbon rates for medium carbon steel production lie in the range of 5 – 25 pounds per ton liquid steel. Coal can have fixed carbon levels ranging from60 to 80 % and recoveries in the EAF also vary from 50 to 80 % depending on the coal size and method of addition to the furnace. Thus most EAF operations are aiming for a melt-in carbon level of approximately 0.2 to 0.3 %.
MIDREX has carried out an extensive study with several of its’ clients and has concluded that high carbon levels in the DRI are not beneficial to the EAF steelmaker (4). MIDREX found that increasing carbon levels up to 1.8 % aided in creating and sustaining a good foamy slag. However, above 1.8 % carbon, there is no economic benefit, as the carbon does not supply any technical need. Indeed at a price of $US120 per ton of DRI, the cost of an increase of 1 % in carbon content would be the value of the carbon minus the reduction in iron in the DRI. If we assume a 90 % Fe content and ratio the composition to accommodate a 1 % increase in carbon content, the Fe content drops by 0.9 % or 9 kg/tonne DRI. If we apply a value of $160/tonne of Fe, this equates to a reduction in value of $1.44 . If we apply a value of $0.15/kg carbon (anthracite and coke), the value of the carbon is $1.50 resulting in a neutral effect on the cost of materials fed to the EAF. However, increased charge requirements to achieve the same tap tonnage will likely extend tap-to-tap times and will negatively impact plant productivity. MIDREX claims that additional carbon requirements in the furnace are most economically achieved through the injection of carbon into the bath (at 80 % recovery). Production of high carbon DRI will negatively impact the productivity of the MIDREX reduction furnace and will displace iron units. Carburization potential is related to iron oxide type. If carburization is increased, the percent metallization will decrease unless the production rate is reduced.
HYL has presented a different view of carbon levels in DRI (4). HYL claims that high carbon levels can be very effective in supplying energy to the EAF. In the case of HYL DRI, it is claimed that 95 % of the carbon is present as iron carbide. HYL feels that their process is capable of achieving carbon levels ranging from 1.5 to 4.5 % in the DRI. At 1.5 % carbon, the iron carbide content would be 25 %. At 4.5 % carbon, the iron carbide content in the DRI would be 55 %. HYL have coupled high carbon DRI production with hot charging of the DRI to the EAF. This may be related to the fact that at carbon levels above 2 % in the DRI, the material becomes very difficult to briquette. Thus for the merchant DRI market, high carbon DRI might not be as readily available as conventional DRI. HYL indicates a cost saving of approximately $US 5 per tonne liquid steel for the case where DRI makes up 50 % of the charge weight and is hot charged at 650 C with a carbon content of 4 %.
In electric furnace steelmaking, some carbon will be contained in the scrap feed, in DRI, HBI or other alternative iron furnace feeds. The amount of carbon contained in these EAF feeds will generally be considerably lower than that contained in hot metal and typically, some additional carbon is charged to the EAF. In the past carbon was charged to the furnace to ensure that the melt-in carbon level was above that desired in the final product.As higher oxygen utilization has developed as standard EAF practice, more carbon is required in EAF operations as a fuel. The reaction of carbon with oxygen within the bath to produce carbon monoxide results in a significant energy input to the process and has lead to substantial reductions in electrical power consumption in EAF operations. The generation of CO within the bath is also key to achieving low concentrations of dissolved gases (nitrogen and hydrogen) in the steel as these are flushed out with the carbon monoxide.
The amount of charge carbon used will be dependent on several factors including:
• Carbon content of scrap feed
• Projected oxygen consumption
• Desired tap carbon
• The economics of iron yield versus carbon cost
• Offgas system capacity
In general, the amount of charge carbon used will correspond to a carbon/oxygen balance,as the steelmaker will try to maximise his iron yield. Typical charge carbon rates for medium carbon steel production lie in the range of 5 – 25 pounds per ton liquid steel. Injection of foamy slag material will typically lie in the range of 10 to 30 pounds per ton of steel produced. It is important to note that the fixed carbon in charge materials can vary significantly and ash content can be as high as 12 %. The method of placement in the charge bucket can have a major impact on carbon recovery in the furnace. In addition, several operators have found that the size of the material also has a big effect on carbon recovery.
When large quantities of pig iron are used, it may not be necessary to add charge carbon. Every 1 % of pig iron in the charge supplies 0.87 lbs per ton of charge carbon (assumes 4 % C in pig iron, and 92 % scrap yield). Thus 20 % pig iron in the charge would supply the equivalent of nearly18 pounds per ton of charge carbon. When improved carbon recovery is taken into account, this amount of pig iron might replace from 20 to 60 pounds per ton of charge carbon. It is also easy to see that use of large amounts of high carbon materials can also lead to extended decarburization requirements.
A major concern with operations using high levels of carbon in the furnace feed, is the generation of CO gas in the furnace. Burning CO through to carbon dioxide releases a very large quantity of heat and if this can be recovered as an energy source in the EAF, a considerable energy savings can result. Buffenoir has analyzed several different types of commercial grade DRI compositions and has developed a model to predict value in use of the DRI in the EAF, with and without post-combustion (5). This work indicates that with post combustion, properly applied, value in use for DRI can be increased by $ US 4 – 12 per tonne of DRI. If using high levels of carbon in the EAF, certain modifications must be made to the operating practices. Notably, oxygen injection must be spread out over the entire heat. Otherwise, the extremely high CO generation rates over a short period of time make it difficult to use post-combustion to recover a significant quantity of the energy generated. In addition, large peak flows of CO are detrimental to offgas system performance and much larger gas cooling requirements will be required.
The maximum practical decarburization rate in the EAF is much less than in the BOF due to the shallow metal bath. Exceeding a rate of 0.1 % C per minute will result in excessive metal splashing and increased fume losses to the offgas system. Typical decarburization rates in the EAF range from 0.06 % C to 0.1 % C. The maximum achievable decarburization rate will place a practical limit on the level of carbon desired in the DRI. Too high a carbon level may actually extend the tap-to-tap time due to oxygen blowing limitations.
Gangue in the DRI is made up of silica and alumina, which is associated with the iron ore pellets. Alkali oxides such as Na2O and K2O and titanium oxide may also be present in the iron ore though usually in smaller quantities (<0.1 %). Sulfur and phosphorus contents in the iron ore are also a concern because they are not removed in the direct reduction process. Usually, about 1.5 tons of iron ore is required to produce a ton of DRI. Thus any undesirable materials in the ore actually become more concentrated in the DRI product. Steelmakers want the gangue content to be as low as possible because this material reports to the slag in the EAF and lime must be added to help maintain the desired slag basicity. Phosphorus is difficult to remove in a high productivity operation and must be kept to a minimum. Pig iron and hot metal can also have the same concerns regarding phosphorus and sulfur content. Typically, pig iron does not contain gangue materials, it does contain silicon and manganese, which will be oxidized, and will eventually report to the slag. Though the oxidation of these materials will provide energy to the process, the resulting materials will affect the slag chemistry and will likely require additional flux additions that will increase energy requirements and operating cost. It should also be noted that some forms of pig iron (notably, some beach irons) can contain a certain amount of sand and dirt. This will be a source of both silica and alumina that willreport to the slag in a similar manner to the gangue materials contained in DRI. These parameters are discussed in more detail in a previous paper (3).
DRI/HBI Use In The EAF
Generally speaking, DRI requires 100 – 200 additional kWh per ton to melt as compared to scrap melting due to the high levels of gangue materials and FeO in the DRI, which must be reduced in the bath. DRI and HBI can be fed to the EAF using several different methods:
• DRI/HBI added to scrap bucket
• Cold DRI fed continuously through the roof
• HBI can be fed to preheating unit such as shaft furnace
• Hot DRI fed continuously through the roof
All of these methods have been utilized in EAF plants around the world.
BATCH CHARGING OF DRI/HBI -If up to 25 % DRI is to be used in the charge make-up, it is usually added in the bucket. If a larger percentage of DRI is to be used it is usually fed through the furnace roof. Typically, the apparent density of DRI is 3.5 g/cc and its bulk density ranges from 1.6 to 1.9 g/cc (6). Steel scrap typically has a bulk density of approximately 1g/cc. Thus DRI is sometimes used to help ”densify” the charge and thus reduce the number of back-charges required to achieve the desired tap weight. HBI is denser than DRI (apparent density 5.0 g/cc, bulk density 2.4 – 2.8 g/cc) and can be charged in the scrap bucket without increasing the number of charges required. DRI tends to float at the slag bath interface while HBI which has a much higher density tends to sink into the bath and melt in a manner similar to pig iron.
Care must be taken with the location of DRI in the scrap bucket as it has a tendency to stick to the furnace shell as the scrap melts down. Gangue materials may tend to become gummy as the DRI/HBI approaches melting temperatures. These HBI accretions on the furnace sidewalls have a tendency to cave in once the scrap is completely melting in and may result in vigorous bath reactions, which may result in mild explosions or slag being expelled from the furnace through the slag door.This can lead to potentially hazardous operating conditions and thus it is recommended that HBI/DRI be charged to the scrap bucket in such a way that it will mix with the other scrap (7). Another problem with bucket charging DRI is that it can be difficult to charge and may extend bucket loading cycles. This in turn can result in delays in supplying scrap to the EAF.
CONTINUOUS CHARGING OF COLD DRI/HBI – One advantage of DRI/HBI is that it can be fed continuously with power on and therefore no thermal losses are incurred by opening the furnace roof. Typically, the apparent density of DRI is 3.5 g/cc as compared to that of steel that has a density of 7.8g/cc. Thus DRI will tend to float at the bath slag interface. Thus the CO generated from the reduction FeO in the DRI does not assist in flushing nitrogen from the steel, as is the case if this reaction takes place in the steel bath.
Thus the lower nitrogen levels achieved with DRI use are attributed to a dilution effect (8), (i.e. the DRI contains very little nitrogen and dilutes the nitrogen contained in the scrap). The DRI does however promote a foamy slag, which is beneficial for preventing additional nitrogen pick-up from air entering the furnace. HBI can also be continuously fed to the EAF though due to its higher apparent density (5g/cc) (6), it tends to sink deeper into the bath and may take longer to melt in.
Typical continuous feed rates range from 27 to 35 Kg/min/MW of power input (7) though federates as high as 40 – 45 kg/min/MW have been reported (6,9). Most continuous feed systems consist of a central day feed bin, which is supplied from the main storage location. Usually a flux bin is also tied into the feed system so that continuous flux additions can also be made via the same feed port on the furnace roof. A feed leg is used to feed the DRI/HBI so that it enters the furnace on the pitch circle between two phases or enters the furnace in the center of the electrode pitch circle. In the case of a single electrode DC furnace, the material is fed into the cavity created as the electrode bores down into the scrap. More recently, dual electrode DC furnaces have been used to melt continuously fed DRI, which is fed between the two electrodes into the “hotspot”. Of course continuous feed of DRI is not without its problems. Great care must be taken to ensure that the power input profile is configured to produce a large enough opening in the scrap so that the DRI can fall directly into the molten pool of steel. If the opening is too small, the DRI can stick to the surrounding scrap, with the result that the continuously fed DRI will back up and will flow down the scrap and stick to the furnace side-walls (10). Thus great care and a certain amount of experimentation is required to achieve optimum performance of the continuous feed system. Care must also be taken to ensure that the DRI federate does not outpace the power input rate. This can result in the formation of “icebergs” on the slag surface and can lead to arc instability and increased energy consumption. In some operations, an oxy-fuel burner is placed such that it will help to melt “icebergs” and will also assist in mixing of the DRI into the bath so that “iceberg” formation will not occur.
Some of the benefits attributedt o an efficient continuous DRI charging operation include (11):
• Greater % power on time improves furnace productivity
• Lower heat losses due to fewer furnace back charges
• Improved slag/bath mixing due to the continuous carbon boil results in faster metallurgical reactions
• Melting of DRI can take place during decarburization of the bath
• Slag basicity can be maintained throughout the heat by matching continuous flux addition to DRI federate
• CO evolution is spread out throughout the furnace cycle resulting in an atmosphere that deters nitrogen pick-up in the steel
• Improved slag foaming results in optimum heat transfer from the arc to the bath
• Reduced electrode breakage when compared to 100 % scrap operation
Though continuous charging requires a modified power input profile and careful attention to balancing the slag chemistry and balancing carbon/oxygen inputs to the furnace, it is the preferred method for adding DRI/HBI to the EAF. It is difficult to achieve the same degree of benefits when batch charging DRI to the EAF because it is very difficult to achieve good mixing of the DRI with the scrap.
CONTINUOUS CHARGING OF HOT DRI/HBI -Hot charging of DRI to the EAF is not a new concept. A variety of systems have been designed to convey hot DRI (HDRI) from a Direct Reduction (DR) furnace to an EAF. These systems include mechanical conveyors (apron type or drag chain), transport vessels (by rail or truck) and pneumatics. These systems, although functional, have inherent maintenance and reliability problems and typically require significant capital investment. (11)
Several of the early DRI processes discharged hot DRI into charge buckets, which were used to transfer the DRI to the EAF. These buckets were open to the atmosphere and considerable re-oxidation occurred especially when there were furnace delays. The amount of re-oxidation resulted in additional energy requirements in the EAF, which frequently offset any savings due to hot charging. As a result, this option was abandoned for many of these early facilities. Some facilities moved instead to the use of sealed transfer containers and succeeded in hot charging material at a temperature of 700 to 800 C. This resulted in a productivity increase of up to 30 %, increased continuous federate by 35 % and a reduction of150 kWh/tonne as compared to charging cold DRI.
More recently, HYL has developed the HYTEMP®system to pneumatically feed the hot DRI from the bottom of the DR reactor to the meltshop. The 4M facility produces DRI with a metallization in excess of 94 % and a carbon content of 4 %. The facility reeds two EAF furnaces, a shaft furnace supplied by Fuchs (EAF # 1) and a dual cathode DC furnace supplied by Danieli (EAF # 2). Best daily operating results are included in the table below (11).
Initial operating results indicate that hot charging of DRI at 600 C can result in power savings of 120 kWh/tonne as compared to a 100% cold DRI charge (11). Power on time was reduced by as much as 13 minutes for these same conditions. The DRI facility can also produce cold DRI.
Midrex Direct Reduction Corporation has also developed a hot charging system, which they have named HOTLINK™. The Midrex system differs from the HYL system in that the DRI transfer is via gravity. This minimizes degradation of the hot DRI during transport and also minimizes temperature loss. The facility is capable of producing both hot and cold DRI. The plant is capable of providing DRI or HDRI up to 95% metallization with carbon from 0.5 to 3%. Some of the main benefits cited by Midrex include (12):
• Minimal temperature loss since the distance conveyed is short
• Minimal HDRI degradation since material velocities are low
• No re-oxidation of material since the system is sealed
• Low maintenance and high reliability since the system uses gravity for transport and is based on existing technology.
Midrex projects power savings of 20 kWh/tonne for every 100 C in DRI pre-heat temperature (assumes 100 % DRI charge). Power savings estimations are shown in the graph below (12).
The material handling system has several provisions for cold DRI usage. These options are very important to insure that EAF availability and productivity are maximized. The integrated plant has the ability to do any one or all of the following:
• Charge cold DRI directly to the EAF
• Mix cold DRI with HDRI during charging the EAF
• Send cold DRI back to the DR furnace to be reheated
• Sell cold DRI.
If the DR plant is shut down while the meltshop is in operation, then cold DRI can be charged directly to the EAF through the cold DRI Surge Bin. If the plant would like to reduce cold DRI storage while the DR plant is on-line, then cold DRI from the surge bin can be blended with HDRI and charged to the EAF. This option will of course lower the composite charge temperature,thus reducing the savings in power and electrode consumption. Alternatively, a significant amount of cold DRI (up to 10% of furnace discharge) can be added back into the DR furnace for re-heating to avoid lowering the charge temperature. Since the cold DRI is already reduced, it will not consume much reductant. This effectively means the discharge rate of the DR furnace can be increased by almost the same amount of cold DRI that is being reheated. Certainly more energy is required to heat the additional throughput, but nearly the same quantity of oxide can be reduced.
EFFECT OF DRI/HBI CHARGING METHOD ON EAF OPERATIONS – There are a wide range of tabulated effects for various alternative iron s in the EAF. This is primarily due to the fact that within any given product such as DRI or HBI, a number of process parameters may vary quite considerably. These include % metallization, % carbon, % gangue etc. All of these parameters will have an affect on the energy requirement to melt the material. Energy consumption in turn affects furnace productivity and electrode consumption. Certain slag chemistry requirements may require additional lime and MgO additions to offset acid gangue components associated with the DRI. This will result in greater specific slag volume per tonne liquid steel. Slag volume in turn affects the iron yield in the EAF. Thus one can appreciate that there are many complex interactions taking place in EAF operations.
Gangue content in DRI is probably the reason cited most often for steelmakers’ reluctance to use DRI in the EAF. Gangue consists of both acidic and basic components. Acid components include silica, alumina and titanium oxide. Basic components include lime and MgO.Gangue does not contribute to steelmaking in the sense that it basically“comes along for the ride” with the iron in the DRI. Silica and alumina are usually present in the iron ore and also in binders used in pelletizing. CaO and MgO may be present in small quantities in the iron ore but usually result from additions to iron ore pellets. Silica and alumina levels in the DRI are in a large part dependent on the selection of iron ore for the process. Selection of appropriate iron ores is essential for acceptable quality DRI. For most gas based DRI processes, silica levels of 1-3 % in the DRI are typical. Alumina levels of 0.5 -1.5 % are also typical. The biggest concern with gangue materials is that they may require additional flux additions and the resulting slag components consume energy within the steelmaking process. The effect of gangue content has been studied extensively in other papers and will not be discussed here (3, 13, 14).
Gangue levels in the DRI also affect iron yield. If the DRI contributes high quantities of gangue to the slag, the slag volume will be increased and iron losses to the slag as FeO will also increase resulting in decreased iron yield. Typically, flux additions (CaO + MgO) will amount to 6 – 8 % of the metallics charge weight. Net slag generation will be in the range of 10 – 12 % of the charge weight. In the case of a 100 % DRI charge at 4 % acid gangue, the necessary lime addition will be 8 – 12 % of the DRI charge weight (V ratio = 2 – 3) and MgO addition at about 1 – 2 % of the DRI charge weight. At 25 % FeO in the slag, the Fe losses will be almost double those for a 100 % scrap operation (total slag = 24 % of charge weight). An additional yield loss of 2.3 % of the feed Fe results. This is costly in terms of lost iron units, reduced productivity, increased melting costs, increased refractory erosion and increased slag handling and disposal costs. It should also be noted here that dirty scrap can contain a significant amount of FeO and also alumina and silica in the form of dirt. This is frequently a problem when “zeroing out” on scrap piles stored on the ground. It is virtually impossible to retrieve the scrap without bringing along some dirt with it.
The charging method for DRI/HBI is not affected by the gangue content. If given the option, continuous charging of the DRI is preferred because flux additions can also be made continuously so that the slag is always in the optimal basicity range to promote slag foaming. It should be noted that great care must be taken to match the federate of DRI to the power input and likewise with flux addition rates.Otherwise, “icebergs may form on the surface of the slag and these can result in large excursions in slag chemistry which will cause poor foaming, increased energy consumption and decreased arc stability. Product yield may also suffer. Some continuous charging operations also have the ability to charge carbon along with the DRI through the furnace roof.
Batch charging of large quantities (> 20 %) of DRI can result in large swings in both bath chemistry and slag chemistry especially if the DRI segregates out in the charge. These fluctuations make it difficult to maintain consistent operations in the EAF and will likely increase power consumption, electrode consumption and tap-to tap time. A more flexible flux addition strategy must be used with a corresponding reduction in effectiveness of slag foaming. If DRI/HBI is not well distributed in the charge bucket, it can stick to the furnace side-walls.
When this material eventually caves in, a violent reaction may result as FeO reacts with carbon. This can be a safety hazard, and slag may violently shootout of the slag door. It is likely that a CO spike in the offgas will occur, temporarily over-loading the offgas system and potentially causing explosions downstream in the system. Slag chemistry may change very rapidly, with the result that slag foaming may be difficult to control and maintain. If the DRI has insufficient carbon content to reduce the FeO contained in the DRI, the slag FeO level may become too high making it aggressive to furnace refractories and making it difficult to regain good slag foaming in the furnace. Thus several potential issues can arise if DRI is improperly batch charged to the EAF.
Two major concerns with DRI use that are frequently over-looked are fines generation during handling and reoxidation of the DRI. Typically DRI pellets can be expected to generate much more fines than briquetted material. Several users of DRI have measured fines generation during shipping due to handling of the pellets and estimate that the –1 mm fraction typically increases by3-5 %. This is an issue for the end user as this material re-oxidizes easily and can potentially be a fire hazard. Even if this material makes it into the furnace, it is likely that a large portion of the fine material will report to the EAF offgas system. Some users have collected the fine material and have injected it into the furnace into the interface between the bath and the slag. When used in this manner, this material can be a very effective for slag foaming.
It should also be noted that some DRI facilities use lump ore as a feed material. This results in lower feed material costs per to of DRI produced but the lump ore portion of the DRI will generate a greater amount of fines during processing and will produce much greater quantities of fine material during handling. Thus yield losses will tend to be much higher when using this material in the EAF.
Great care must be taken to keep DRI dry. Usually, operations using large quantities of DRI will store the DRI in silos. These silos are usually passivated with nitrogen. DRI is extremely porous and re-oxidation will occur very easily if it is not properly stored. DRI can absorb 10 to 15 % moisture if it is stored uncovered in wet weather (15). HBI has a much lower surface area and several re-oxidation tests indicatet hat it re-oxidizes at approximately 30 % of the rate experienced by DRI pellets. HBI tends to absorb only 3 to 5 % moisture when stored uncovered (15). DRI produced from lump ore tends be as reactive or more reactive than DRI pellets.
Tests conducted on DRI pellets stored in unprotected piles indicated a reduction in metallization of 7 %. For every 1 % increase in FeO in the DRI, an additional 10.4 kWh/tonne DRI, (theoretically), is required to recover the iron. EAFs are usually 58 to 60 % efficient and therefore as much as 17 kWh may be needed. If the DRI iron yield is assumed to be 89 %, a total of 19 kWh per tonne of steel would be required. Thus a decrease of 1 % in DRI metallization could increase the power requirement per tonne of steel by almost 25 kWh/ton. This could represent an increase of 6.3 % in tap-to-tap time for a 100 % DRI charge based on FeO content alone. Several analyses of the effect of DRI characteristics on the cost of steelmaking have been conducted (13, 14). These studies indicate that a decrease of 1% in DRI metallization results in an increase in conversion costs of $5/ tonne of liquid steel.
Of greater importance than the additional cost associated with recovering the iron units is the effect that the reoxidized iron will have on the slag chemistry in the EAF.The DRI will typically only contain sufficient carbon to reduce the un-metallized portion of the pellet (1 % C for every 6 % FeO). Thus unless the pellet contains excess carbon, there will be a shortfall of carbon in the EAF. This can result in wide fluctuation in the slag FeO content and may result in accelerated wear of furnace refractories. If sufficient carbon injection capacity is not available, iron yield will suffer, as will slag foaming, which will in turn reduce energy efficiency in the EAF.
When all factors are taken into account, most high tonnage steel producers will prefer to maintain as high a metallization as possible in the DRI. It is however surprising to see how few facilities utilize closed storage bins for DRI. For facilities using less than 20 % DRI/HBI in their operations, the best course of action is to purchase HBI if it is available. Even though the cost to briquette HBI is approximately $3 to $5 per ton, the potential losses in re-oxidation of DRI pellets easily out-weigh this additional cost. For facilities utilizing greater quantities of DRI, an enclosed storage system coupled with a continuous feed system should be considered a necessity.
Recently, several operations that utilize large quantities of DRI in the EAF have begun to optimize slag practices and in some cases the furnace design itself has been optimized for use of DRI. The twin cathode DC furnace at Hylsa’s Monterrey facility was specifically designed with a fourth hole sized for low gas velocity so that carry-over of DRI fines would be minimized (16). Eventually this it is intended that this furnace will operate on 100 % hot charged DRI.
A considerable amount of work has been carried out at Tuscaloosa Steel to optimize the flux/slag practices in the EAF. Very early on, it was recognized that the FeO/ SiO2rich slag produced from the DRI greatly increased the refractory wear in the furnace (10). Iron yield is also directly related to the slag volume in the furnace and as a result, lower yields are typically achieved when using large quantities of DRI. Initial estimates of slag weight indicated that 60 to70,000 pounds of slag was being generated per heat ( tap weight = 148 tons liquid steel) when DRI made up 33 % of the scrap charge. Tuscaloosa concluded that slag volume had to be decreased in order to allow for better reaction of injected carbon with the FeO in the slag. Lime addition was reduced and MgO addition was increased to maintain saturation of the slag as the V-ratio decreased. Eventually lime addition was reduced to 55 pounds per ton of scrap charged and MgO additions were adjusted to 30 pounds per ton. This resulted in a yield of over 89 %. At 33 % DRI continuously charged through the roof, the EAF operation at Tuscaloosa achieves an energy consumption of approximately 380 kWh/ton of liquid steel (10)
Though it is recognized that DRI melting contributes significantly to effective slag foaming, very few operations actually attempt to place a value on this. Slag foaming is probably one of the least understood phenomena in EAF steelmaking today and yet it is probably the most critical factor for achieving good energy efficiency in the EAF. It is now generally accepted that in conventional EAF operations, slag foaming consists of two steps. In the first stage of slag foaming, oxygen reacts to form FeO in the bath, which almost immediately reacts with carbon in the bath to produce CO gas, which foams the slag. In the second stage, an excess of FeO is formed which tends to move to the slagbath interface. Carbon is injected at the bath slag interface in order to reduce the FeO and the resulting CO promotes slag foaming.
When CO is evolved within the bath, it will also help to strip nitrogen and hydrogen from the steel. Decarburization is also beneficial for the removal of hydrogen. It has been demonstrated that decarburizing at a rate of 1 % per hour can lower hydrogen levels in the steel from 8 ppm down to2 ppm in 10 minutes.
Foamy slag practice originated with DRI melting operations where FeO and carbon from the DRI would react in the bath to produce CO which would “foam'” the slag. At the start of meltdown the radiation from the arc to the sidewalls is negligible because the electrodes are buried in the scrap. As melting proceeds the efficiency of heat transfer to the scrap and bath drops off and more heat is radiated from the arc to the furnace sidewalls. By covering the arc in a layer of slag, the arc is shielded and the energy is transferred to the bath. When foamed, the slag cover increases from 4 inches thick to 12 inches or more. HYL estimates that the slag thickness when using 80 % DRI will be approximately double those for a 100 % scrap charge. HYL also indicates that at high carbon removal rates and 80% DRI, the foamy slag thickness can be as high as40 inches (17). Claims for the increase in energy transfer efficiency range from an efficiency of 60 – 90 % with slag foaming compared to 40 % without. It has been reported that at least 0.3%carbon should be removed from the bath using oxygen in order to achieve a good foamy slag practice. The following table shows results for efficiency of electrical energy transfer for different levels of arc shielding by the slag (18).
It can be seen that totally immersing the arc in the foamy slag has a hugeeffect on electrical energy transfer efficiency. Slag foaming results in greatly improved thermal efficiency and allows the furnace to operate at high arc voltages even after flat bath is reached. Burying the arc helps toprevent nitrogen from being exposed to the arc where it will dissociate and will enter the steel.
In the case of DC furnaces, it is even more critical to ensure that the arc is buried because, typically, these furnaces tend to operate at much greater arc voltages than conventional AC furnaces. Thus the use of DRI in a DC furnace may be an effective way to ensure good electrical efficiency.
It may prove to be a worthwhile study for some EAF steelmakers to evaluatewhether the additional benefits of improved slag foaming from DRI, are sufficient to offsetthe additional power requirements associated with the increased flux requirements.
Cold Pig Iron Use In The EAF
The world average use of cold pig iron (CPI) in the EAF is approximately 5 %.However, in some parts of the world where scrap is scarce, CPI may be used in quantities up to 60 % (19, 20). Typically, when using CPI, furnace operators prefer small “pigs” which will melt in along with the scrap in the charge. If the CPI consists of very large pieces, it will take longer to melt in and may result in large swings in bath chemistry. Some operations have witnessed large un-melted pieces of pig iron when tapping. Pig iron should be placed in the bottom half of the first charge bucket so that it will melt into the bath early in the heat. This allows for adequate decarburization time, (thus avoiding the possibility of an extended tap-to-tap time), and allows for maximum recovery of energy from CO post-combustion to cold scrap in the furnace.
The high apparent density (S.G. = 3.3)of CPI makes it quite suitable for bucket charging. However, granulated pig iron has the added advantage in that it can be charged continuously through the roof in a manner similar to DRI. This is beneficial in reducing power-off times and maximizing furnace productivity.
EFFECT OF CHARGING METHOD OF CPI ON EAF OPERATIONS – Pig iron is a material in which the chemical composition can vary quite considerably (20). The silicon content can vary from as low as 0.3 wt % up to 2.5 wt %. Manganese content is typically around 0.4 wt % but can be as high as 1.0 %. Carbon content varies between 3.5 and 4.5 and is usually greater than 4 % in most operations. This contrasts sharply with scrap, which typically contains 0.1 % silicon, 0.1-0.3 % carbon and 0.3-0.6 % manganese.
In the case of cold pig iron, a power savings should result from using 10 – 15 % pig iron in the charge. This is due to the silicon, manganese and carbon contained in the pig iron. These act as a source of chemical energy in the bath when oxygen is injected. Pig iron typically contains silicon, which reacts with oxygen to produce silica that reports to the slag. Thus some additional flux additions are required in order to maintain the desired slag basicity.
Usually, a maximum of 20 % cold pig iron is used in the EAF because it takes longer to melt in than scrap, especially if it is supplied as large pigs. Small sized pieces are preferable. The pig iron can contain up to 4 % carbon which results in a very high bath carbon level if large amounts of pig iron are used. Removal of carbon with oxygen generates a large quantity of heat but also requires increased blowing times because practical limits exist on the rate at which oxygen can be blown into the steel. Previous studies have indicated that high silicon levels in the pig iron are not necessarily beneficial (3). This is due primarily to the large volume of slag associated with the silicon once it has been oxidized. In order to maintain a slag V-ratio of 2.5, a total of 11.5 kg of slag results per 0.01 wt % (1 kg) Silicon. High iron losses are associated with the increased slag quantities generated and as a result, the total equivalent cost of the energy supplied from the oxidation of silicon can be quite high. Alternatively, oxidation of the carbon in the CPI results in a much lower total cost of energy. These results will obviously vary somewhat with operating practices (i.e. slag V-ratio, tap temperature etc.).
A large part of the attractiveness of the carbon is the fact that no detrimental slag materials are formed. However, it should be noted that high CO levels in the EAF offgas can greatly increase the required size of the offgas system and with these the overall cost of the gas treatment system. One approach is to apply a post combustion program in the EAF. This will not necessarily solve the entire problem as only a portion of the energy generated is recovered to the furnace charge. A finger shaft furnace can allow for considerable heat recovery and may provide an economic solution. Regardless EAF operators are cautioned against raising carbon levels in the furnace above about 2 % due to the extended decarburization times that result.
If the CPI is loaded too high in the scrap bucket, it may stick to the furnace walls. Eventually this material will cave into the bath but may cause an eruption when it melts into a highly oxidized bath.
If granulated pig iron is continuously fed to the EAF, the scrap must be melted back to expose a molten pool of metal into which the CPI will be fed. This is similar in many ways to continuous charging of DRI/HBI and the same restrictions apply. Continuous addition of fluxes is beneficial to maintaining the desired slag basicity. When CPI is added continuously, oxygen injection can be balanced to CPI federate in order to spread CO generation over the whole heat. This is beneficial for slag foaming operations.
Operations using high levels of CPI (>30 %), have found improved steel quality, better EAF operating consistency, improved iron yield and generally a significant reduction in energy consumption (20). With 35 % CPI in the scrap mix, energy consumption on the order of 390 kWh/tonne liquid steel have been achieved (20).
Utilization of pig iron in the scrap bucket can frequently lead to a reduction in the number of back charges required due to the high density of the CPI. However, care must be taken to ensure that the resulting furnace charge does not leave too much freeboard space in the EAF.This can result in a large amount of arc flare to the furnace shell if the charge melts in quickly. In addition, slag FeO levels are unlikely to contain sufficient FeO to promote good slag foaming (due to the large amount of carbon supplied from the CPI). Thus the arc will tend to open up resulting in arc instability and poor energy transfer efficiency. Alternatively the power input profile can be adjusted to use a shorter arc which will help alleviate this problem.
Results for a Brazilian EAF operation utilizing high levels of CPI have identified the following (20):
• CPI is normally charged in the last 1/3 of the bucket in layers of no more that 7 tonnes
• To avoid arc deflection back to the roof, CPI should never be in the top layer of the bucket
• CPI should be surrounded with light scrap such as bushellings and turnings
• Oxygen blowing must commence shortly after power-on in order to decarburize the heat without incurring a delay
• With 35 % CPI, nitrogen levels in the steel were less than 50 ppm
Typically, though pig iron does not contain gangue materials, it does contain silicon and manganese, which will be oxidized, and will eventually report to the slag. Though the oxidation ofthese materials will provide energy to the process, the resulting materials will affect the slag chemistry and will likely require additional flux additions that will increase energy requirements and operating cost. It should also be noted that some forms of pig iron (notably, some beach irons) may contain a certain amount of sand and dirt. This will be a source of both silica and alumina, which will report to the slag in a similar manner to the gangue materials contained in DRI.
It should be noted that if CPI is not well distributed in the charge bucket, cave-ins may occur that are similar to those experienced with DRI use. This will have the same adverse effects on offgas system performance, slag chemistry and slag foaming. If all of the CPI melts in at the same time,it may be difficult to generate sufficient FeO to sustain slag foaming because the bath carbon level will be high. In addition, the silicon must be removed from the bath before carbon can be removed. Thus CO generation in the bath will also be delayed. The net result may be poor slag foaming and potentially damage to the furnace shell and refractories. It may also be necessary to provide the capability to make flux additions during furnace operation so that slag chemistry can be adjusted dynamically.
Hot Metal Use In The EAF
Hot metal production is a standard part of operations in integrated steelmaking. Hot metal is produced in the blast furnace from iron ore pellets. This hot metal is then refined in basic oxygen furnaces to produce steel. However, several integrated facilities have installed EAFs and are now charging hot metal to the EAF. One such installation is Cockerill Sambre in Belgium where up to50 % of the total charge weight is hot metal. This installation gets its’ hot metal from a blast furnace. In several other operations, hot metal is provided via COREX units, mini blast furnaces, or cupolas. In the case of the Saldahna Steel facility in South Africa, the EAF feed consists of 45 % COREX hot metal and 55 % MIDREX DRI.
Hot metal is extremely beneficial for increasing furnace productivity and achieving short tap-to-tap times. Hot metal provides benefits similar to pig iron with the added benefit that all material is already at a temperature of approximately 1430 C. Thus the major portion of the energy requirement in the EAF,(that required for melting the Fe) is already provided. One tonne of hot metal at1430 C supplies approximately 250 kWh in the form of sensible heat, based on Fe content alone. Obviously there are limitations as to how much hot metal can be used in the EAF. Generally, the maximum oxygen injection rate will limit the amount of hot metal use if optimum tap-to-tap times are to be achieved. A study conducted by Paul Wurth indicates that the optimal proportion of hot metal in the EAF charge lies between 25 and 50 % (21). Typical energy savings lie in the range of 3.1 – 3.6 kWh/%pig iron. Using hot metal increases the savings to4.8 kWh/% hot metal (22). Using large quantities of hot metal can decrease the power consumption to 200 kWh/tonne and below and thus can be very beneficial for locations that have a weak electrical grid. Typically hot metal use is between 10 and 50 % of the charge (23). Extending the use of hot metal beyond this point usually results in increased tap-to-tap time due to decarburization rate limitations (21).
The charging of hot metal to the EAF sounds like a simple proposition though it is in fact quite complex. Care must be taken that the hot metal that is charged does not react with the highly oxidized slag that is still in the EAF. Several different techniques for hot metal charging have been used in the past including:
1. Top charging the hot metal onto the liquid heel in the EAF.
2. Boring a hole into the scrap charge and top charging into this void.
3. Pouring the hot metal into the empty furnace via a launder positioned through the slag door.
4. Adding the hot metal continuously to the furnace via a side launder.
Heard et. al. (21) have reviewed all of these techniques extensively. Their assessment of these various charging methods is as follows:
TOP CHARGING OF HOT METAL -Most operations that have attempted to top charge hot metal onto the heel in the furnace have found that this is not an optimum method. The heel must be thoroughly killed to ensure that violent bath reactions do not result (24). Usually aluminum or silicon fines are added to kill the heel. Calcium carbide is probably a better choice because it does not result in increased flux requirements. Charging on top of the heel can also result in high heat fluxes to the water cooled panels and frequently results in damage to these panels.
Several operations bore into the scrap charge to create a cavity and then top charge the hot metal into this cavity. This method has several advantages (21, 23):
• No additional equipment is required
• Charging is rapid – usually 2 –3 minutes
• The scrap helps to chill the hot metal making it less likely to react with the slag in the furnace.
Several disadvantages have also been noted (21):
• Energy loss (radiation) due to an additional roof swing
• Safety hazard – slag may still react with hot metal
• Damage to water cooled panels due to deflection of hot metal stream
• Heavy fuming
• Crane is tied up for duration of charging operation
CHARGING OF HOT METAL VIA SLAG DOOR LAUNDER -Charging of hot metal can also be accomplished by using a launder to introduce the hot metal into the EAF via the slag door. Some of the advantages attributed to this charging method include (21):
• Furnace roof does not have to be opened
• Reduced damage to furnace panels
• Safer operation
• Less fuming than top charging
Some of the disadvantages include (21):
• Maintenance and pre-heat requirements for the launder
• Get fuming during pour from ladle to launder
• Have to clear out slag door area with oxy-fuel burners and/or ram prior to hot metal charging
• Sometimes get back flow of hot metal out of the furnace
• Crane is tied up in charging position for 5 – 15 minutes
CHARGING HOT METAL USING A SIDE RUNNER – In this mode of operation, a permanent side runner is used to feed the hot metal into the furnace. The hot metal ladle is positioned in a tilting device and hot metal charging is performed at a controlled rate over a significant portion of the duration of the heat. Advantages of this method include (21):
• Power is on during pouring
• Flow rate can be modified to prevent back flow of hot metal
• Safer charging conditions
• Crane is free after dropping off hot metal ladle
• Less fuming due to controlled additions
• Additional cost for tilting mechanism
EFFECT OF HOT METAL CHARGING METHOD ON EAF OPERATIONS – Each of the various charging methods for hot metal use in the EAF results in unique effects on the furnace operation. These are discussed by charging method as follows.
Top Charging -Top charging of all of the hot metal early in the heat results in a bath that is very high in both carbon and silica. The furnace is also full of scrap. Scrap must be cleared so that decarburization operations can begin. This can result in an increased tap-to-tapt ime due to the delay required for scrap melting. In addition, it can take some time to form a well-fluxed slag since FeO is required to help flux the lime. The slag at flat bath may not be very fluid and will be difficult to foam due to low FeO content. Almost all of the silicon must be removed from the bath before foaming can begin. This can cause several problems including reduced electrical heat transfer efficiency. Most of the carbon and silicon removal will take place at flat bath resulting in poor energy recovery to the steel. In fact temperature control may become an issue and scrap may need to be added for to cool the bath. Carbon will be removed over a short period of time and may result in over-loading of the offgas system. Alternatively upgrades to the offgas system to accommodate the use of hot metal in the EAF may be quite costly.
Launder Charge Through Slag Door -This method requires that the scrap be melted back from the slag door area prior to hot metal charging. Paul Wurth estimates that this will result in a wait of 10 – 13 minutes before the hot metal can be charged (21). This again reduces the amount of time available for decarburization and may result in an extended tap-to-tap cycle.Once again the bath is high in carbon and silicon that will result in difficulty foaming the slag until flat bath. Silicon must be removed prior to significant formation of FeO which is essential to slag foaming. Thus it is likely that there will be a period during flat bath when there will be poor foaming and significant heat radiated to the furnace shell and roof. A well-fluxed slag will not be formed until late in the heat and energy efficiency will likely suffer. Carbon removal will be over a short period of time resulting in a CO peak flow, which may over-load the offgas system.
Continuous Hot Metal Charging Through a Side Runner -This method provides several process benefits (21):
1. Bath chemical composition remains essentially constant because oxygen injection rate can be matched to the hot metal feed rate to oxidize carbon and silicon at the rate at which they are added to the furnace.
2. Control of silicon oxidation rate can be matched with dolo-lime addition to ensure that large excursions in slag chemistry do not occur.
3. Decarburization takes place while there is still scrap in the furnace. Thus some of this energy can be recovered to the scrap.
4. CO generation rates are controlled and thus the offgas treatment system sizing can be optimized.
5. Bath and slag chemistries are more consistent and thus a closer approach to equilibrium is achieved.
6. Short tap-to-tap times can be achieved due to the fact that decarburization can begin very early in the heat.
7. Slag chemistry can be controlled to give good slag foaming throughout the heat.
Combining Alternative Iron Feedstocks
Several steelmaking operations have begun to consider mixing alternative iron feedstocks in order to balance out some of the disadvantages associated with some feed materials. It has been recognized that charging hot metal reduces overall power requirements and also significantly reduces tap-to-tap time. On the other hand, DRI increases power requirements and extends tap-to-tap times. By combining these two materials in the charge it is possible to offset the detrimental energy consumption effects of DRI and achieve cycle times similar to or lower than conventional EAF scrap based operations.
The SMS- Demag CONARC furnace is based on an operation which combines converter and EAF technologies namely a CONverter ARC furnace. This technology is based on the growing use of hot metal in the EAF and is aimed at optimizing energy recovery and maximizing productivity in such an operation. The use of hot metal in the EAF is limited by the maximum oxygen blowing rates which are dependent in turn on furnace size. The basic concept of CONARC is to carry out decarburization in one vessel and electric melting in another vessel.
A CONARC is operating at the Saldahna Steel facility in South Africa. This operation is based on 45 % COREX hot metal and 55 % DRI feed to the furnaces. Hot metal is charged to one shell and a top lance is used for decarburization. Oxygen injection rates of 150 – 300 Nm3/min will be used. These injection rates are much higher than those typically used in the EAF (35 – 70 Nm3/min). Simultaneously, 10 tonnes of DRI is added. The primary function of the converter cycle is the removal of silicon, carbon and Manganese. Once the aim carbon level is achieved, the electrodes are moved to this shell and additional DRI is fed to make up the balance of the heat. Large quantities of heat are generated during the oxygen blowing cycle. As a result it is important that charge materials be added during this period so that some of this energy can be recovered. This also helps to protect the furnace shell from overheating.
If using this process with just hot metal and DRI, it will be difficult to capture a large portion of the heat generated during the oxygen blow cycle. However, should scrap be used asa portion of the charge, it could be preheated during the oxygen blowing cycle and then charged into the furnace for the electric portion of the cycle. This would allow for a much higher recovery of post-combustion energy.
The following table shows several possible feed mix combinations for the CONARC operation at Ispat Industries Limited (25). One can see the huge impact that both hot metal use and charging of hot DRI have on the energy consumption and the annual plant capacity. As the electrical energy requirement increases, the transformer size is also increased to adjust the tap-to-tap time. The more pig iron and hot metal that is used, the higher the oxygen requirement. It is easy to extrapolate some of the general effects that blending of AI materials can have on EAF operations. The benefit of blending pig iron with DRI is clearly seen from the table.
Table III SMS-MDH Projections for Mixed Feed Operations Compositions for Ispat
Sampaio et al.(19), have developed a model at CVRD which has been calibrated using results from various Brazilian meltshops. Table 14 shows their projections for a mixed charge where pig iron and DRI are used in equal proportions in the EAF.
Table IV Projected Power and Oxygen Consumption for Mixed CPI/HBI Use in the EAF
It is important to note that though the oxygen requirement increases with the use of the alternative iron mixture in the furnace, the electrical power requirement remains constant indicating that the furnace tap-to-tap should not change as long as the operation is capable of supplying the oxygen. The slag volume increases considerably as the use of CPI/HBI increases. This will increase melting costs and will also result in slightly lower iron yield.
It is apparent from the fore-going discussion that alternative iron sources can be combined effectively to give high tonnage operations with short tap-to-tap times. This is especially important for operations producing flat products because typical casting speeds dictate a demand for short furnace cycle times and large heat sizes.
The same comments discussed previously for various alternative iron feeds, apply for the effect of charging techniques on EAF performance when using combinations of these materials. Again the key factors to keep in mind relate to maintaining the carbon/oxygen balance in the furnace and maintaining the slag chemistry in the optimum range for slag foaming activities. With regards to the offgas system performance, it is best if decarburization is spread out as much as possible throughout the heat. This will also result in optimum energy recovery to the scrap and steel bath.
One interesting development in the DRI field is the production of a direct reduced iron material, which contains no carbon(26). CIRCAL is a hot-briquetted iron product, which is produced using hydrogen as the reductant. As a result, it contains no carbon. This material has a total iron content of 94.6 % at a metallization of 93 %. The alumina + silica content is approximately 2.3 %. This material is similar in density to HBI and has a moisture pick-up of less than 3 %. Though many EAFs prefer to have high carbon AI feed materials, this material is quite suitable for operations where clean iron units are required and additional pig iron use is not appropriate. This may occur for the following situations:
• The offgas system has insufficient capacity to handle additional use of pig iron
• Additional pig iron may result in extended tap-to-tap times due to decarburization considerations
• Additional use of conventional DRI/HBI may result in higher slag volume resulting in decreased iron yield
• Pig iron cost is too high to justify use of additional pig iron
The advantage of using CIRCAL is that it can provide additional flexibility to an EAF operation wishing to tailor the amount of AI material used in the EAF and to simultaneously control carbon levels in the furnace along with slag volumes generated during melting operations. CIRCAL can be batch charged or alternatively, it can be continuously fed to the EAF similar to HBI. One can consider that CIRCAL will essentially act in a similar manner to #1 bundles with the added advantage that it willelt into the bath much more quickly due to its size. The lack of carbon content is not necessarily a negative effect on furnace operations, as CIRCAL can be blended with either pig iron or DRI to give the desired carbon/oxygen balance in the EAF. In the future, it is very likely that EAF operations utilizing large quantities of AI materials will actually use blends of the various materials available, in order to engineer specific cycles in the EAF leading to optimized furnace operations. It is expected that AI blending will become standard practice in the near future.
Summary And Conclusions
Many advantages have been attributed to the use of alternative iron materials in the EAF including:
• Dilute residuals – AI materials contain low levels of residuals
• Consistent chemistry – AI materials are consistent in their chemistry. Thus utilization of AI materials will tend to offset the wide chemistry fluctuations common in obsolete scrap.
• Consistent C recovery – recovery of carbon from AI materials typically exceeds 90 %. Charge carbon recovery can vary widely (30 – 80 %) and some charge carbon is also high in ash content (up to 14 %) and sulfur content.
• Better slag foaming – slag chemistry is more consistent when properly using AI materials. The period of CO evolution is typically extended as compared to conventional EAF operations. These factors lead to more optimum slag foaming conditions.
• Greater slag volume – greater slag volume can lead to higher Fe losses but may be beneficial for slag foaming especially for DC operations and AC operations utilizing long arc practices in the EAF.
• Consistent EAF operations
• Lower N content in steel – AI materials are low in nitrogen content and thus result in low bath melt-in nitrogen content. If the AI material results in elevated bath carbon levels, CO formed in the bath during decarburization will strip nitrogen from the bath.
It is important that the furnace operator be cognizant of the effect of the charging method on the operations within the EAF. It has been shown that several rules apply:
1. Traditionally, EAF operations have only been concerned with the residual content of scrap. In order to better optimize scrap melting operations, it will be necessary that these operations factor in other considerations such as carbon content, silicon content, FeO content, oil and grease content and coating content. All of these variables will also have an effect on furnace operations and on the operation of environmental systems.
2. When less than 20 % DRI is to be used in the EAF, HBI is probably the best material to use due to its lower reactivity and also because it is more easily charged via the scrap bucket.
3. When greater than 25 % DRI is to be used, the material should be stored in inert silos and a continuous feeding system should be used to introduce the material into the EAF. Oxidation of DRI can lead to upset conditions in the EAF and will create a carbon/oxygen imbalance. Moisture pickup can seriously impact energy consumption in the EAF.
4. When operating with large quantities of DRI/HBI in the feed mix, slag volumes will tend to be elevated. Iron yield can be improved by operating with a lower slag V-ratio. This will require greater MgO additions in order to prevent excessive refractory erosion. Silica content in AI materials is not always a negative attribute. In some cases, the silicon content of scrap is so low that sand must be added to the furnace in order to give the desired slag V-ratio and the required amount of slag. Generally speaking, the amount of slag generated each heat should range between 6 and 12 % of the tap weight. When operating at high voltage, the upper end of the range is advisable.
5. Pig iron may be used in very large quantities up to 60 % in the EAF. The best results are obtained by layering the material in the scrap bucket. CPI should never be placed on the top of the bucket.
6. Hot metal is best charged continuously using a side-launder. This allows for early bath decarburization and also allows for energy recovery to the scrap. Other charging systems though used at several facilities throughout the world do not result in optimum energy recovery nor optimum tap-to-tap cycle time.
7. Combining AI materials can be very beneficial for both furnace productivity and product quality. The key to successful operation is to ensure that the carbon/oxygen balance is properly maintained in the EAF and to ensure that slag chemistry is maintained in the optimum range for slag foaming.
8. Consistent slag chemistry is of utmost importance for achieving efficient slag foaming. Optimum slag foaming will significantly improve electrical energy transfer efficiency in the EAF and will lower energy consumption. Recent developments for monitoring slag foaming have been implemented by Nupro Corporation to optimize addition of slag foaming materials to the EAF.
9. There is a need for better tracking of the bath and slag chemistry profiles in the EAF. Through the use of simple process models, it is possible to optimize the addition of feed materials, fluxes and energy inputs to the EAF to achieve optimum EAF performance. Nupro is working with Baker Refractories to develop models that can be used to improve EAF operations.
10. The effect of AI utilization on offgas system performance is frequently ignored. This frequently results in over-loading of offgas systems leading to emission problems. Nupro Corporation has developed a model for evaluating the impact of AI use on offgas system operations. Simple process models can be utilized to evaluate the effects of various feeding scenarios to minimize the impact on the fume system.
11. Blending of AI materials in the charge mix can provide additional flexibilityfor EAF operations. Many facilities currently blend pig iron and DRI. New materials such as CIRCAL provide additional flexibility to the manner in which AI materials are usedin the EAF. In the future, it is expected that very specific blends of AI materials will be used in the EAF to achieve specific goals with respect to operational consistency, maintaining the carbon/oxygen balance in the EAF, maintaining optimum slag chemistry for slag foaming throughout the furnace cycle and optimizing charge material costs for a given product chemistry.
12. As is the case with implementation of EAF technologies, each operation will find that some specific feed blends give the best results for their operation. Many process factors will come into play and thus the specific value of different feed materials must be evaluated on a case-by-case basis. It is imperative however, that a proper evaluation be made and that it is recognized that the value of AI materials lies not only in their low residual content but also on gangue content, carbon content, silicon content etc.
Though in the past, AI materials have been used predominantly to control residual levels in the steel, it is expected that the use of these materials will grow in conventional EAF operations. It has been shown that the use of AI materials can greatly enhance the consistency of EAF operations and though these materials may cost more than conventional scrap, the overall benefit to the EAF operation will frequently out-weigh the cost factor. Use of AI materials greatly enhances the capability of the EAF operator to maintain the carbon/oxygen balance in the furnace. This in turn helps to maintain slag chemistry in the optimum range for slag foaming, which in turn impacts heavily on furnace performance. All of these factors are inter-related and must be coordinated in order to achieve optimum EAF performance. The manner in which AI materials are introduced to the EAF has a huge effect on all of these factors and great care should be taken to evaluate the manner in which these materials are utilized with-in any given furnace operation. The number of AI materials available to EAF operations is growing and through blending of these materials, it will be possible to further optimize EAF operations in the future.
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