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FASTMET and FASTMELT Processes of Ironmaking

• satyendra
• November 15, 2013
• 1 Comment
• blast furnace, DRI, EIF, FASTMELT, FASTMET, HBI, Heat Fast process, iron making, pellets, RHF,

FASTMET and FASTMELT Processes of Ironmaking

FASTMET process is a coal based process of ironmaking. It enables the conversion of metallic oxides from either iron ore fines or steel plant metallurgical waste, into metalized iron. FASTMELT process is the FASTMET process with the addition of an ‘electric iron melting furnace’ (EIF) to produce liquid iron or hot metal. Kobe Steel in collaboration with Midrex Technologies, Inc., a subsidiary of Kobe Steel in the USA, has developed this process. FASTMET is a unique process uses a rotary hearth furnace (RHF) to reduce agglomerates containing coal with a high reduction ratio and high productivity. Fig 1 shows simplified cross section and plan view of a RHF.

Fig 1 Simplified cross section and plan view of a RHF

The FASTMET process converts iron ore pellet feed, iron ore fines and / or steel plant metallurgical waste into direct reduced iron (DRI) using pulverized non-coking coal as a reductant. The end product DRI can be either hot briquetted to produce hot briquetted iron (HBI), or discharged as hot DRI into transfer containers, or cooled if cold DRI is needed. Hot DRI is directly discharged from the RHF into EIF and is melted to produce hot metal. The hot metal can be cast into pig iron. The reduced iron / hot metal can be fed into a primary steelmaking furnace.

Besides facilitating the recycling of the metallurgical waste to the basic oxygen furnace / electric arc furnace feed, there are two major targets of the process. One is the higher metallization of iron oxides, which reduces the burden on the melting process. Another is high removal ratio of zinc, which reduces concentration of zinc within the recycling loop. For blast furnace feed, high compression strength is needed, which is also achieved by adjusting binder and mixing ratio of raw materials with the FASTMET process.

Historical development

Reducing iron ore in a RHF was first attempted by Midland Ross Co., a forerunner of Midrex Technologies, Inc. The process, called ‘Heat Fast’, was unique in that it involved composite pellets, consisting of iron ore and carbonaceous material, which are pre-heated in a grate, pre-reduced in an RHF and cooled in a shaft cooler. The ‘Heat Fast’ process was tested successfully at 2 tons per hour (tph) pilot plant in Cooley, Minnesota, from 1965 to 1966.

Simultaneous to the development of ‘Heat Fast’, the natural gas-based Midrex DRI process, which offered a higher quality product than ‘Heat Fast’ and lower operating costs (natural gas was priced very low at that time) was also being developed. Due to the then low operating costs of the natural gas process, the ‘Heat Fast’ process work was halted and commercial development was never undertaken.

Midrex revived its interest in using the RHF for direct reduction in the early 1980s. Several studies were conducted which indicated that an RHF-based process could be developed to produce direct reduced iron at an attractive price. In the 1990s, the price of natural gas rose and then a FASTMET pilot plant was built and commissioned at Midrex technical centre with a 2.75 m diameter RHF having a production capacity of 160 kilograms per hour. More than 100 campaigns were run from 1992 to 1994.

Kobe Steel and Midrex Technologies, Inc. collaborated to restart the development of a RHF coal based process with the intent of commercializing the technology. Building upon the ‘Heat Fast’ pilot plant work dating back to the 1960s, improvements were made to the technology which resulted in higher productivity, improved product quality, greater process flexibility, and increased process efficiency. The end result was the development of the FASTMET process. Kobe Steel and Midrex have taken the FASTMET process one step further in developing the FASTMELT process which needs the hot discharge of DRI from the RHF and charging by gravity into an EIF, a melting furnace specifically designed for melting FASTMET hot DRI to produce hot metal.

A demonstration pilot plant was built in 1995 at the Kakogawa works of Kobe Steel limited (KSL). The plant had 8.5 m diameter RHF and a production capacity of 2.5 tph. The plant was in continuous operation from 1995 to 1998. Through various demonstration operations conducted there, Kobe Steel and Midrex Technologies, Inc. had established the FASTMET process for commercialization. In 2000, a first commercial FASTMET plant was supplied to the Hirohata works of Nippon Steel Company (NSC) for reducing 190,000 tons per annum (tpa) of steel mill waste. The FASTMET process was recognized for its ability to efficiently recycle ironwork dust. The details of the first five plants which have been commissioned are given in Tab 1.

 It can be seen that this process is being used presently for the utilization of the metallurgical waste of the steel plant. Besides metallurgical waste, the process can also use iron ore fines having size which is equal to pellet feed size (less than 45 micrometers).  This is because the process has got in-built agglomeration steps. FASTMET/FASTMELT process presents an alternative route to ironmaking in capacities ranging from 100,000 tpa to 500,000 tpa. FASTMELT process offers an alternative for mini blast furnace (BF) technology. The refractory used in RHF and EIF are of standard specification normally used in ironmaking. The design of EIF is based on proven designs of EAF (electric arc furnace) and LF (ladle furnace) technologies.

Features of the FASTMET process

The FASTMET process is clearly different from the gas based reduction process using reformed gas produced from natural gas, in that it heats and reduces composite agglomerates, each consisting of iron ore, or steel plant metallurgical wastes, and coal. This simple and unique process involving rapid heating accomplishes a rapid reduction reaction. The agglomerates are placed in one or two even layers over the hearth and are heated using radiation heat. This prevents the oxidation of agglomerates, despite the in-furnace condition of combustion exhaust gas, which has a considerable oxidation potential.

The process can achieve the heating and the reduction of the agglomerates simultaneously and effectively at an ideal air-gas ratio in the RHF. In addition, the combustible gas generated from the carbon in the agglomerates burns in secondary combustion above them. This suppresses the emission of NOx considerably, despite the fact that the RHF is a furnace which has a high temperature environment. This is another feature of the FASTMET process.

As the hearth of the RHF rotates, the pellets or briquettes pass through three zones and are rapidly heated. Each zone has three gas fired burners, whose air / fuel ration can be adjusted to achieve the desired lean or rich combustion in each zone. At the end of the zone 3, the material passes under a dividing wall which separates the final fired zone from the discharge zone. A helical screw conveys the hot DRI product off the hearth into a diverter chute and then by gravity into a nitrogen purged DRI product container or into an EIF.

A heat exchanger installed on the exhaust gas treatment equipment converts the energy contained in the high temperature exhaust gas into energy for heating the air which is used for either burning or drying of the raw materials. This reduces the overall energy consumption.

Raw materials such as steel plant dust can occasionally generate dioxin as they burn. In the FASTMET process, however, the RHF temperature is 1,300 deg C or higher, which is high enough to suppress the generation of dioxin. The exhaust gas from the RHF is cooled rapidly through the temperature region in which dioxin can recombine, hence preventing it from recomposing. Fig 2 shows a typical flow sheet of the FASTMET / FASTMELT process.

Fig 2 Typical flow sheet of the FASTMET / FASTMELT process

Hot off gas leaving the RHF is cooled by means of an evaporative cooler before it enters a heat exchanger. Sensible heat in the exhaust gas is used to preheat the RHF combustion air and the green ball dryer air to around 350 deg C. The exhaust gas leaving the pre-heater is further cooled by a second evaporative cooler before entering a bag house, where the zinc oxide is recovered and sent to the dust silo for storage. An induced draft fan is positioned on the bag house outlet to achieve the required pressure drop for the exhaust gas system and control the RHF under a slightly negative pressure.

Utilities which are needed are plant air, nitrogen, steam, instrument air, make-up water, and liquefied natural gas. The plant also needs an open recirculating process water circuit as well as closed machinery cooling water circuit.

The hot DRI, discharged out of the RHF, has three discharge options namely (i) cooled to produce cold DRI, (ii) briquetted in a  briquette machine to produce HBI, (iii) melted in an is EIF to produce liquid iron. This liquid iron can be cast in a pig caster to produce pig iron.

The DRI has many pores left after the reduction process. If exposed to air for a long time, the metallic iron reoxidizes into iron oxide, deteriorating its quality. If DRI is not used immediately as raw material for a melting furnace or a blast furnace, compacting and densifying the DRI into hot briquette iron (HBI) prevents reoxidation. This allows the storage of reduced iron for an extended period of time without quality degradation. The stored HBI can be fed to a primary steelmaking furnace or to a blast furnace. Whether the reduced iron produced by the FASTMET process is used as DRI in the form of pellets / briquettes or is formed into HBI using HBI equipment depends on the application of the product (iron source) and its storage period.

The metallurgical waste (dust) generated in the steel plant has conventionally been pelletized or sintered in-house to recycle the dust as a raw material for the blast furnaces. The blast furnace raw material, however, contains volatile components, particularly zinc, which vapourizes in the high temperature zone of the blast furnace. However, not all the vapour escapes from the blast furnace. Some portion of the vapour is cooled and trapped by newly-charged materials, remains in the furnace. The accumulated volatile components decrease the permeability of the blast furnace, and considerably impair its productivity.

The FASTMET process vapourizes heavy metals such as zinc and lead, which had inhibited the recycling of scrap iron, and converts them into crude zinc oxide and the like. This allows the discharging of these elements into exhaust lines without circulating them in the process. The exhaust gas treatment facilities are equipped with a cooling and dust collection system, which, combined with air cooling and water cooling, prevents the volatile components from adhering to the equipment walls. This enables stable and continuous operation for an extended period of time and the collection of crude zinc oxide and the like, using a bag filter. The collected crude zinc oxide is a valuable resource and is recycled along with the reduced iron.

The process and the principle reactions

The FASTMET process starts with the mixing of iron ore fines or steel plant metallurgical waste (containing a high percentage of iron oxide) with pulverized coal, agglomerating the mixture into pellets or briquettes using a pelletizer or a briquetter, drying the agglomerates in a dryer, and placing the agglomerates over the hearth of an RHF in one or two even layers.

The pellets or the briquettes are to be isolated from air when they enter the furnace. Their feed rate is controlled precisely at the same time. FASTMET process contains a feed pipe system enabling the adjustment of the number of pipes according to the size of the furnace, thus simultaneously achieving isolation from the air and volume control. A screw-type leveling system is adopted for placing the agglomerates in one or two even layers.

As the hearth rotates, the briquettes are heated by radiation from RHF zone temperatures of more than 1,300 deg C and the iron oxides are reduced to metallic iron. Reduction of the iron oxide is accomplished primarily by fixed elemental carbon reacting with magnetite (Fe3O4) or hematite (Fe2O3) to form metallic iron (Fe) and wustite in the solid form while evolving carbon monoxide (CO) and carbon dioxide (CO2) gas. Some of the carbon goes into solid solution with the metallic iron to form iron carbide (Fe3C).

Zinc oxide, lead oxide, and other volatile metallic oxides contained in the metallurgical waste feed are also reduced to metallic form and vapourized. These metallic vapours are reoxidized in the exhaust gas before leaving the furnace through the off-take.

A rapid heating method, a proprietary technology of Kobe Steel is adopted for heating the pellets or briquettes which are laid over the hearth, rapidly attaining a high temperature of 1,350 deg C. This heating generates the reaction of oxides and carbon. Residing for 8 minutes to 16 minutes, the agglomerates are converted into DRI, which is discharged out of the furnace or supplied to the downstream process, at a temperature of 1,000 deg C to 1,200 deg C. Different reactions which are taking place between oxides and carbon are (i) Fe2O3＋3C = 2Fe＋3CO, (ii) Fe3O4 ＋4C = 3Fe＋4CO, (iii) Fe2O3 ＋3CO = 2Fe＋3CO2, (iv) Fe3O4 ＋4CO = 3Fe ＋4CO2, (v) FeO ＋CO = Fe＋CO2, and (vi) ZnO ＋CO = Zn＋CO2.

The combustion gas (CO gas) emitted from the pellets / briquettes as a result of the reduction reaction can be used as a fuel for the RHF, which considerably decreases the quantity of fuel supplied to the burner.

The atmosphere in EIF of FASTMELT process consists essentially of CO gas and hence highly reducing. This reducing atmosphere promotes silicon reduction and sulphur removal.　

The stable and continuous discharge of DRI out of the RHF at a high temperature is achieved by such proprietary technology of KSL as elevating the hearth of a reduction furnace. Unlike a blast furnace, the FASTMET can start and stop operation with relative ease depending on the amount of production, which enables production in response to demand.

Reduction mechanism

The reduction reaction kinematics in a direct reduction furnace is normally controlled by the diffusion of the reduction gas from the outside. In the FASTMET process, the reduction reaction occurs inside the carbon composite pellets / briquettes made up of iron ore fines and the pulverized coal. Once the composite pellets / briquettes are heated, CO gas inside them, promote the reduction of iron oxide. Hence the reduction reaction proceeds faster in the carbon composite pellets / briquettes than the reduction reaction happening in the conventional direct reduction process. The basic reduction reactions are considered to be occurring during the FASTMET process are (i) FexOy + yC = xFe + y CO (endothermic reaction), (ii) CO2 + C = 2CO (endothermic reaction), and (iii) FexOy + yCO = xFe + yCO2 (exothermic reaction).

At temperatures are below the melting point of iron, there is hardly any direct reaction with the solid carbon of pulverized coal and hence the reaction as given in equation (i) dominates the reaction kinematics. At higher temperatures of 1,000 deg C and more, the reaction of the generation of CO gas by carbon solution loss as per equation (ii) and the reaction of iron oxide by CO gas as per equation (iii) take place in series inside the carbon composites pellets / briquettes. In these reactions CO gas generation controls the reaction kinematics with its highly endothermic nature. Hence to promote the reaction, it is necessary to supply the heat needed for the reaction to the inside of carbon composite pellet / briquette at the higher temperature of 1,000 deg or more. This means heat is to be transferred efficiently by radiation from the atmosphere to the surface of the pellet / briquette, and by conduction from the surface of the pellet / briquette to its interior. Fig 3 shows the reduction mechanism of carbon composite pellets / briquettes

Fig 3 Reduction mechanism of carbon composite pellets / briquettes

In RHF, the pellets / briquettes are normally heated with zone temperatures of more than 1,300 deg C and are reduced to metallic iron. Residence time on the hearth is typically 8 minutes to 16 minutes. This varies depending on the material being processed, size of pellets / briquettes, and other factors. The rapid reduction rate achieved in the FASTMET process is due to the high reduction temperature, the high heat transfer rate, and the intimate contact of the carbon contained inside the briquettes with the iron oxide. The heat transfer and the different reduction reactions which are taking place in RHF are schematically shown in Fig 4.

Fig 4 Schematic of heat transfer and reduction reactions in RHF

Product

The end product of FASTMET / FASTMELT process can be HBI, hot DRI discharged directly into transfer containers, cold DRI, or liquid iron (hot metal). The metallization achieved during the process is more than 85 %. The temperature of the hot metal produced by the FASTMELT process ranges from 1,450 deg C to 1,550 deg C and it has the typical composition of carbon – 3 % to 5 %, silicon – 0.3 % to 0.6 %, manganese – 0.6 % to 1.2 %, sulphur – less than 0.05 %, and phosphorus – less than 0.03 %.

Exhaust gas treatment and environmental control

The exhaust gas leaving the RHF is fully combusted, containing around 2 % oxygen. Heat exchangers use the thermal energy in the exhaust gas to preheat combustion air for the RHF burners and raw material preparation dryers. Exhaust gas leaves the rotary hearth furnace through the roof and flows through a refractory lined off-take to the exhaust gas duct. Proper location of the exhaust gas off-take relative to the RHF combustion zones is determined by analysis of the feed materials, reduction kinetics, and verified by ‘Computational Fluid Dynamics’. Dilution air is injected into the exhaust gas duct to provide cooling and burn any remaining combustibles (hydrogen and CO) in the exhaust gas stream.

Spray water is added to the primary cooler to cool the gas from more than 1,400 deg C to 1,000 deg C to minimize NOx formation and to provide an acceptable inlet temperature for the recuperator. From the primary cooler exhaust gas flows through the combustion air and dryer air preheater where the heat from the exhaust gas is used to heat combustion air for the rotary hearth burners and the rotary dryer. The exhaust gas exits the combustion preheater to the secondary cooler. Spray water is added to the secondary cooler to cool the gas from around 800 deg C to 120 deg C to provide an acceptable inlet temperature for the bag filter system. The exhaust gas then flows to a jet fabric filter bag house where the crude zinc oxide is collected and then to an ID fan where it is discharged through a stack to the atmosphere.

SO2 control is normally not needed in FASTMET process since most of the SO2 reacts with and is absorbed by the metallic oxides in the gas flue stream. Lime injection can be used to further control SO2. NOx is controlled by the use of low NOx burners and close operational control of air to fuel ratio and combustion temperatures. Dioxins and Furans are destroyed by the high temperatures and long residence time within the RHF. Flue gas cooling rate is controlled to minimize Dioxin and Furan reformation. Particulates are removed from the flue gas by a bag filter system. Crude zinc oxide is collected by the bag filter system and stored in a silo.

FASTMET process is an environmental friendly process. CO2 emission from FASTMELT process is around 1.6 tons per ton of hot metal (t/tHM) against around 2.1 t/tHM CO2 emission in case of mini blast furnace. The emission of NOx is in the range of 0.3 kilograms per ton of hot metal (kg/tHM) to 1.5 kg/tHM and that of SOx emission is around 2.4 kg/tHM.

Benefits of the FASTMET process

FASTMET process provides another option for the handling of the steel plant metallurgical waste. A FASTMET plant located at a steel plant can process the dust and make two primary products, DRI for feed back to the primary steelmaking furnace, and crude zinc oxide for sale to zinc processors. It makes a liability into an asset. The high cost of disposal is eliminated and an inexpensive supply of iron units becomes available.

The process benefits include (i) very low fines generation in the process results in high zinc content and very low iron content of the secondary dust, (ii) high metallization and high zinc removal make reduced iron product recyclable to the primary steelmaking furnace, (iii) no waste is generated for disposal, (iv) high temperature treatment decomposes dioxins, and (v) zinc dust can be treated economically and becomes a product, not a waste.