Discharge options for Direct Reduced Iron and its Hot Transport
Discharge options for Direct Reduced Iron and its Hot Transport
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 1050 deg C.
In the coal based process, the produced DRI is mixed with char that 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 the case of vertical shaft furnace processes, since char is not present along with DRI, there are three discharge options available. These are cold DRI (CDRI), hot briquetted iron (HBI), and hot DRI (HDRI).
Most of the vertical shaft DRI furnaces have been built for the production of CDRI. In these furnaces the DRI produced after reduction is cooled in the lower part of the furnace to about 50 deg C. CDRI is temporarily stored in Silos for passivation before it is transported to a nearby steel melting shop for its use later. CDRI has got the property of auto ignition and need special precautions during transport and storages as required by the International Maritime Organization (IMO). CDRI is most suited material for the continuous charging in the EAF.
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. 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 with typical dimension of 30 mm x 50 mm x 110 mm. HBI is 50 % denser than the CDRI and because of this the tendency of re-oxidation of HBI is reduced greatly. This enables HBI to be stored and handled without any special precautions as recognized by the IMO. 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.
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 electric arc furnace (EAF) in hot condition. The charging of hot DRI in EAF directly from a vertical shaft DRI kiln is known as hot charging.
Benefits of hot charging
Hot charging provides two major benefits. They are reduction in energy consumption and improvement in the productivity of EAF. Presently most of the steel melting shops around the world have hot charging systems for the EAF. In fact, today, steel melting shops have feeding systems for EAF which are able to choose between cold or hot materials. 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 about 20 kWh/tCS for each 100 deg C increase in the charging temperature of DRI. Thus, the minimum saving when charging hot DRI at over 600 deg C in the EAF is 120 kWh/tCS. 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 to 0.6 kg/t of liquid steel is expected.
The increase in productivity of EAF due to hot DRI charging is significant since use of hot DRI reduces tap to tap time and hence the heat duration. As compared with charging of cold DRI, an increase of productivity upto 20 % is achieved with hot DRI 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 to 2 kg/t of liquid steel.
There are also environmental benefits of hot DRI charging. Retaining the sensible heat in the DRI rather than dissipating it to the atmosphere lowers overall emissions in two ways. First, the lower electricity demand reduces power plant emissions per ton of steel produced. Second, for those EAFs employing carbon injection, reduced energy requirements in the EAF result into less CO2 given off.
Transport of HDRI
The transport of HDRI is critical in various ways. The difficulty with the transport of HDRI is not just that the material is hot, but also that it must be kept in a non oxidizing atmosphere. It is a critical requirement since the transport method of HDRI from the DRI shaft furnace to the EAF is capable of delivering HDRI without adversely affecting the quality of the DRI. It should also provide maximum operational flexibility. In addition the system must be reliable, maintenance friendly and easy to operate.
There are four alternatives which are being commercially available for transporting of HDRI. Each of these alternatives has its best application, depending on such factors as transport distance, component arrangement and conveying capacities.These four alternatives are described below.
HYTEMP process – This process was developed by Tenova HYL and installed in the Ternium Monterrey Plant in 1998 and is a pneumatic transport process for the transport of HDRI. The system operates by using a transport 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 transport gas is removed from the circuit and recycled back to the DR 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 DR shaft kiln are carried with the lumpy material and cushion the transport line. These fines also get charged in the EAF along with DRI lumps in the EAF thus increasing the yield. The system is schematically shown in Fig 1.
Fig 1 Schematics for Hytemp prcess
HOTLINK process – This process uses primarily gravity transport and was pioneered by Midrex. This process use the same technology as used for gravity feed of HDRI for HBI production. The HDRI from the DRI shaft kiln is discharged into a surge bin outside and above the steel melting shop. From this surge bin HDRI is directlly gravity fed to the EAF. HOTLINK modules are equipped to handle any upset conditions via the surge bin. This system supply HDRI to the EAF as per the demand of the EAF. HOTLINK process is used when the distance between the DRI shaft kiln and the EAF is less than 40 meters. The process is shown schematically in Fig 2
Fig 2 Schematics of HOTLINK process
Hot Transport conveyor system
Where the steel melting shop is not adjacent to the DRI shaft kiln (more than 40 m but less than 100 m), an insulated mechanical conveyor is used for the transport of HDRI to the steel melting shop. In this case, DRI is discharged from the DRI shaft kiln onto a fully enclosed and insulated conveyor, designed to minimize temperature loss and prevent deoxidation. The conveyor has specially formed pans that have a similar form to buckets (Fig 3). The closed hood of the conveyor contains an interting 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 this bin is discharging HDRI to the EAF, the second bin is filled with the HDRI by the conveyor. The schematics of hot transport conveyor system is shown in Fig. 4.
Fig 3 Hot DRI conveyor
Fig 4 Schematics of hot transport conveyor system
Transport by hot transport vessels
When the distance between the DRI shaft kiln and EAF is more than 100 meters or one DRI shaft kiln 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 of 60 tons to 90 tons. From the DRI vertical kiln, 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 rails or on trucks. Essar steel has pioneered the use of hot transport vessel in the 1990s. Schematics of transport of HDRI by hot transport vessels is shown in Fig 5.
Fig 5 Schematics of transport of hot DRI by hot transport vessel