Discharge Options for Direct Reduced Iron and its Hot Transport
Discharge Options for Direct Reduced Iron and its Hot Transport
DRI is produced by the direct reduction of iron ore by using non coking coal / natural gas. The two main methods of producing direct reduced iron (DRI) are (i) gas based process in a vertical shaft furnace and (ii) coal based process in a rotary furnace. In both the processes the reduction reactions take place in solid state and the maximum furnace temperatures are in the range of 850 deg C to 1,050 deg C.
In the coal based process, the produced DRI is mixed with char, which is needed to be separated from DRI. Hence DRI-char mixture is cooled in a rotary cooler and then char is separated from DRI by the magnetic separation process. In case of the vertical shaft furnace process, char is not present along with DRI. Vertical shaft direct reduction furnace has traditionally been built for the production of cold direct reduced iron (CDRI). In this furnace, the DRI produced after reduction is cooled in the lower part of the furnace to around 50 deg C. CDRI is temporarily stored in silos for passivation before it is transported to a nearby steel melting shop for its later use.
The major part of the CDRI production is used as a substitute for scrap in steelmaking. DRI is consumed in three primary product forms, namely, lumps, pellets, or hot briquettes. The other secondary product form is cold briquettes made from DRI fines. CDRI is most suited material for the continuous charging in the electric arc furnace (EAF). CDRI has got the property of auto ignition and need special precautions during transport and storages as required by the International Maritime Organization (IMO).
DRI has several pores left after the oxygen is removed by reduction reaction and can easily be re-oxidized. Hence, DRI has a risk of igniting and hence generating heat, making its maritime transport difficult. For this reason, DRI was originally consumed solely within the plant where it was produced. It was against this background that production technology was developed for hot briquetted iron (HBI). HBI is a combined solid form of DRI lump and pellets, hot pressed at around 700 deg C, immediately after its production.
Technically, there are three discharge options available in case of vertical shaft direct reduction furnace. These are cold DRI (CDRI), hot briquetted iron (HBI), and hot DRI (HDRI). The discharge of HDRI improves the specific energy consumption and productivity of the plant, including the downstream steelmaking process. A combination of two discharge methods, cold and hot is being used in several plants to allow flexibility in production planning, which improves the productivity of the furnace.
For the production of HBI, hot DRI is discharged from the vertical shaft furnace at a temperature of around 700 deg C. The hot DRI is sent to briquetting machines to compress it into pillow shape briquettes (HBI) with typical dimension of 30 mm x 50 mm x 110 mm. The produced HBI is cooled on a conveyor. The features and benefits of conveyor cooling of HBI are (i) mist cooling with minimum water, (ii) no sludge, (iii) total automated operation, (iv) proven equipment with high quality standards, (v) improved product quality through soft cooling, (vi) no cracks, less fines, and no reoxidation, and (vi) considerable improvement of HBI quality. Fig 1 shows transport and cooling of HBI.
Fig 1 Transport and cooling of HBI
The most important characteristics of HBI are its high density and low specific area, which improves the resistance to re-oxidation and makes it easier to handle. HBI can be transported and handled using the scrap handling equipment and can be easily batch charged in the EAF. HBI can also be continuously charged in an EAF with specially designed systems.
HBI is 50 % denser than the CDRI and because of this, the tendency of re-oxidation of HBI is reduced greatly. This enables HBI can be stored and handled without any special precautions as recognized by the IMO. Due to high density, the charging of HBI in the EAF is much easier and melting is faster. HBI has facilitated marine transport and enabled the reduced iron to be supplied to the distant places. HBI is now being produced since more than 30 years. It is the desirable method of preparing DRI for storage and transporting it by sea going vessels.
HDRI and the benefits of hot charging
HDRI is discharged from the vertical shaft furnace at a temperature of around 700 deg C and transported in hot condition to the steel melting shop for charging of DRI directly in EAF in hot condition. The charging of hot DRI in EAF directly from a vertical shaft DRI furnace is known as hot charging.
HDRI feed to EAF provides additional sensible heat to the EAF. This results in the reduction of power consumption and tap-to-tap time which is reflected in the increase in the productivity. Besides saving in the electric energy, there is also some savings in other consumables as a result of the electrical energy savings like electrodes, refractories, etc. For HDRI, the temperature at the EAF depends on (i) the discharge temperature at the shaft furnace, (ii) the distance travelled by the HDRI and, and to a lesser extent (iii) the time in the storage bin before being fed into the EAF.
Majority of new steel plants with vertical shaft furnaces for the direct reduction of iron ore feature hot charging options or are planned for future addition of hot charging equipment into the steel melting shop. Latest projects provide for even more flexibility by installing feeding systems in which it is possible to choose between CDRI and HDRI. This allows to manage market changes and to meet the future needs of the steel industry.
The energy saving occurs in case of charging HDRI because of less requirement of energy in the EAF for heating the DRI to its melting temperature. The rule of the thumb is that the electricity consumption is reduced by around 20 kWh/tLS (kilo-watt hours per ton of liquid steel) for each 100 deg C increase in the charging temperature of DRI. Hence, the minimum saving when charging HDRI at over 600 deg C in the EAF is 120 kWh/tLS. An additional benefit of the electrical energy savings is reduction in electrode consumption, since there is a linear relationship. The saving in the electrode consumption of the order of 0.5 kg/tLS to 0.6 kg/tLS is expected. Fig 2 shows relationship of HDRI with power and electrode saving.
Fig 2 Relationship of HDRI with power and electrode saving
The increase in productivity of EAF due to HDRI charging is considerable since use of HDRI reduces tap to tap time and hence the heat duration. As compared with charging of CDRI, an increase of productivity up to 20 % is achieved with HDRI charging. Use of HDRI also results in the reduction in the specific refractory consumption. The saving in the refractory consumption is of the order of 1.8 kg/tLS to 2 kg/tLS. Fig 3 shows performance of EAF with 100 % of HDRI charge.
Fig 3 Performance of EAF with 100 % of hot DRI charge
There are also environmental benefits of HDRI charging. Retaining the sensible heat in the DRI rather than dissipating it to the environment lowers overall emissions in two ways. First, the lower electricity demand reduces power plant emissions per ton of liquid steel produced. Second, for those EAFs employing carbon injection, reduced energy requirements in the EAF result into less CO2 emissions.
Transport of HDRI
The transport of HDRI is critical in many ways. The difficulty with the transport of HDRI is not just that the material is hot, but also that it is to be kept in a non-oxidizing environment. It is a critical requirement, and hence, the transport method of HDRI from the DRI shaft furnace to the EAF is to be capable of delivering HDRI without adversely affecting its quality. The transport method is also to provide maximum operational flexibility. In addition the system is to be reliable, maintenance friendly, and easy to operate.
There are four alternatives which are being commercially available for the transport of HDRI. These are (i) transport in a hot transport vessel, (ii) gravity transport of HDRI, (iii) pneumatic transport of HDRI, and (iv) hot transport conveyor system. Each of these alternatives has its best application, depending on such factors as transport distance, component arrangement, and conveying capacities. Fig 4 shows the discharge options for direct reduced iron.
Fig 4 Discharge options for direct reduced iron
Transport in a hot transport vessel – When the distance between the DRI shaft furnace and the EAF is more than 300 metres or one DRI shaft furnace is to feed two steel melting shops or more, then the transport of HDRI can be done with the use of insulated vessels, normally having a capacitiy ranging from 60 tons to 90 tons. Hot transport vessel system can maintain HDRI temperature if the steel melting shop is delayed in accepting the HDRI. From the direct reduction vertical furnace, the vessel is filled through a pipe with an air tight seal. After one vessel is filled, the pipe is closed and another vessel begins to fill, the filled vessel is transported to steel melting shop either on flat bed rail cars or on flat bed trucks.
At EAF end, the hot transport vessel discharges HDRI on a hot material transport conveyor or a pneumatic transport system for feeding it to the EAF. Pneumatic transport system uses carrier gas to blow the HDRI at high velocity through pipes to the EAF. The high velocity and resultant turbulence can cause considerable breakage and erosion of the HDRI, especially at bends in the pneumatic line. This method can result in fines generation of as much as 8 % to 10 %. Hot conveyor system using enclosed conveyor buckets are less jarring and greatly reduce HDRI fines generation. Essar steel (now ArcelorMittlal / Nippon Steel) has pioneered the use of hot transport vessel for the transport of HDRI in the 1990s. Schematics of transport of HDRI by hot transport vessels alongwith its charging in the EAF through a hot material transport conveyor is shown in Fig 5.
Fig 5 Charging of HDRI in EAF
Gravity transport of HDRI – Midrex has developed this process and named it ‘HOTLINK’ system. This process uses primarily gravity transport. This process use the same technology as used for gravity feed of HDRI for HBI production. HOTLINK system is the direct link combination between a hot discharge gas-based direct reduction vertical shaft furnace and a conventional EAF. It is a simple system which directly couples the hot discharge of the shaft furnace into the EAF by gravity.
The HDRI from the direct reduction shaft furnace is discharged into a surge bin outside and above the steel melting shop. From this surge bin HDRI is gravity fed directly to the EAF. HOTLINK modules are equipped to handle any upset conditions through the surge bin. This system supply HDRI to the EAF as per the demand of the EAF. HOTLINK system is used when the distance between the direct reduction shaft furnace and the EAF is less than 40 metres.
The HOTLINK system delivers HDRI to the adjacent EAF at temperatures which are up to 700 deg C, by positioning the shaft furnace just outside and above the exterior wall of the steel melting shop. DRI is discharged hot into the surge bin and then fed directly to the EAF with minimal heat loss. Low velocity gravity transfer keeps physical degradation of HDRI to the minimum, and there is no re-oxidation of the HDRI because of the sealed design of the HOTLINK system. The HOTLINK system is designed with options to produce hot briquetted iron (HBI) or cold DRI (CDRI) without stopping production, when the EAF is not producing. The system is shown schematically in Fig 6.
Fig 6 Gravity transport of HDRI
The HOTLINK system enables a 25 % reduction in the electricity consumption in the EAF while minimizing the consumption of electrode and refractory. The other advantages include (i) no loss of metallization during handling / storage, (ii) increase in the EAF productivity by 20 % because of the reduction in tap-to-tap time, (iii) less expensive DRI handling circuits, (iv) less space requirement for the plant, (v) minimum maintenance and high reliability, and (vi) overall saving in capital cost. The first Midrex direct reduction plant featuring the HOTLINK system for hot charging DRI directly from the reduction furnace into an electric arc furnace (EAF) is part of the Jindal Shadeed steelworks in Sohar, Oman which was commissioned in 2011.
Pneumatic transport of HDRI – Tenova HYL (now Danieli) had developed this transport system and named it as ‘HYTEMP’ process. The HYTEMP system is a pneumatic transport process for the transport of HDRI. It was installed in the Ternium Monterrey Plant in 1998. Fig 7 shows schematic flow diagram of pneumatic transport of HDRI.
Fig 7 Schematic flow diagram of pneumatic transport of HDRI
The HYTEMP system is based in the concept of pneumatic conveying of bulk materials. The characteristic features of the HYTEMP system include (i) totally enclosed system without minimum losses, (ii) carrier gas is coherent with the HDRI, (ii) any non-oxidizing gas can be used as carrier gas (for example, nitrogen, natural gas, or process gas), (iii) no deterioration in the quality of the DRI (i.e. percent metallization of DRI and percent carbon of DRI) during transport and feeding to the EAF, (iv) it is an integrated system and includes continuous and controlled feeding of HDRI to the EAF, (v) gas flow of the carrier gas ranges from 50 Normal cubic meters per ton (N cum/t) to 100 N cum/t of DRI depending on the transport rate, (vi) low power consumption ranging from 4 kWh/t of DRI to 6 kWh/t of DRI, depending on the transport rate, (vii) low nitrogen consumption of around 6 N cum/t of DRI for the complete operation, transport, and continuous feeding to the EAF, (viii) flexible configuration to match the space available in the steel melting shop with an arrangement of bins in series or in parallel, (ix) fully automated system for the transporting of HDRI from the direct reduction shaft furnace to the steel melting shop and subsequent continuous feeding to the EAF, (x) the system does not interfere with the material handling, maintenance, or other activities neither at the direct reduction plant nor at the EAF since the connection between the two units is located at an elevated position, (xi) a system with practically no wearing parts and hence maintenance free (the only wear and tear component is the inflatable seals of the valves), and (xii) a system with very low heat losses.
The system operates by using a carrier gas (either an inert gas or the process gas itself) to carry the HDRI through a pneumatic pipe to a holding bin above the EAF. The carrier gas is removed from the circuit and recycled back to the direct reduction plant and HDRI is charged to the holding bin for continuous feed to the EAF. In this system, there is no mechanical part. The fines particles from the direct reduction shaft furnace are carried with the lumpy material and cushion the transport pipe line. These fines also get charged in the EAF along with DRI lumps in the EAF thus increasing the yield.
The design criteria of the HYTEMP system include (i) the system is designed to transport all the HDRI produced by the shaft furnace and the parameters in the system are automatically adjusted to match the production rate of HDRI at the shaft furnace, (ii) the HYTEMP interlock system is programmed to detect initial jamming in the transport pipe line and for adjusting automatically the conditions to prevent jamming and to restore stable conditions, (iii) the system is designed to operate at an initial dilute phase condition in order to minimize pressure drop, (iv) all the components in contact with HDRI are hard lined to prevent wearing, (v) the gas velocity in the transport pipe line is controlled between 30 metres per second (m/sec) to 60 m/sec, but the solid velocity is much lower than that, (vi) the transport pipe line is designed to operate under tension to prevent bending due to the thermal expansion, and (vii) the system is designed to minimize the carry-over of the fine particles during the disengagement of the carrier gas at the disengagement bin.
The pressure drop in the HYTEMP system depends mainly on the total length to be covered, the transport rate, the gas velocity, and the number of changes in direction in the pipe line etc. The route to the EAF in one of the plant has a total length of 253 metres with 5 changes in the direction, two of them from horizontal to vertical and the total pressure drop in the system is around 0.2 MPa.
In the HETEMP system, because of the concept of the pneumatic transport, the HDRI transported is subject to impact in every change of direction and to abrasion in horizontal / vertical sections. Because of this, some fines are generated in the HYTEMP system. However, this generation of fines is not an issue, since the HYTEMP system is designed in such a way that the fines are not lost in the transport gas after disengagement or in the dust collection system of the EAF.
The gas velocity at the point in which the separation occurs is very low, less than 5 % of the velocity at the outlet, and hence, only a very small portion of the fines is carried by the carrier gas. The carrier gas can carry with it only very small particles because of its very low velocity. The fines carried over by the carrier gas are less than 0.5 % of the total DRI transported and it is the ‘total’ loss of DRI in the HYTEMP system. The losses of DRI in the HYTEMP system are less than one tenth of the losses of any standard DRI handling system for The CDRI.
Hot transport conveyor system – Hot transport conveyor system is an insulated conveyor which is used for the mechanical conveying of HDRI in a non-oxidizing environment from the direct reduction shaft furnace to the EAF. In this case, DRI is discharged from the direct reducttion shaft furnace onto a fully enclosed and insulated conveyor, designed to minimize temperature loss and prevent deoxidation. The conveyor has specially formed pans which have a similar form as the buckets. The closed hood of the conveyor contains an interting gas system. The conveyor provides reliable operation at reasonable costs. The HDRI is fed to one of two HDRI bins located above the EAF. When one of the bin is discharging HDRI to the EAF, the second bin is filled with the HDRI by the conveyor.
The conveyor system represents a reliable solution for modern applications, subject to the plant layout, the conveying distance, and the conveying capacity. In this conveyor system, HDRI charging is carried out under seal gas with bucket apron feeder type conveyor in a gas-tight sealed environment. This system is used in plants where the distance between direct reduction shaft furnace and the EAF is up to 200 meters.
The conveyor capacity is linked to the geometry of the conveying system. The greater is the lift, the smaller is the conveying capacity. With limited space, the conveyor is normally as steep as possible. Along horizontal sections, a capacity of 1,200 tons per hour (tph) can be achieved, but when vertical lift is needed, the capacity drops accordingly. Presently, the hot transport conveyor systems are designed with an inclination up to 60 degrees. Conveying capacities of 210 tph up to an elevation of nearly 100 m, and 400 tph up to an elevation of nearly 80 m have already been designed and implemented. Fig 8 shows hot transport conveyor system.
Fig 8 Hot transport conveyor system
The design of the hot transport conveyor system always targets maximum filling of the conveyor buckets to optimize the ratio of volume to surface area. The lift limit is dictated by the chain strength since the whole installation is essentially attached to the chains. Over the past few years chains have been developed which offer specific breaking strength of a minimum of 3,000 kN (kilo-newton) for each single chain. Special know-how, particularly with material temperatures up to 1,100 deg C, is needed for the design of such a chain along with consideration of a multitude of issues which include materials used, lubricants, drives, mechanical components, and safety devices.
Another important feature is the air seal provided by the inert gas shrouding system. This is constantly been improved to enable dust to be contained inside the system and oxygen kept outside. A completely closed conveying system has been developed, using special covering and sealing combined with the inert gas system. No dust is emitted from the conveyor and no spillage is generated underneath. Dust can be collected at defined areas with a common exhaust air system. The precise quantity of inert gas, such as nitrogen or off-gases, not containing oxygen or carbon mono-oxide, can only be determined during operation. Hence, for safety reasons, an excess of gas is used during start-up. Special sensors monitor the safe operation and the oxygen content inside the system. The off-gas temperature, within certain limits, reflects the material temperature. No specialized tools, manpower, or equipment are needed.
The features and benefits of the hot transport conveyor system compared to a pneumatic system include (i) the conveyor system ensures handling of material temperatures up to 1,100 deg C where as a pneumatic system accepts temperatures up to 700 deg C, (ii) the conveyor system reaches conveying capacities of 1,200 t/h with low slopes, while in case of pneumatic system around 50 t/h is feasible, (iii) the conveyor system needs less power which reduces energy consumption, (iv) the conveyor system allows for operation with variable speed, (v) the conveyor system has simple and easy operation with start / stop by just pushing the button, while the preparation / silent time is unavoidable with a pneumatic system.