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Direct Reduced Iron and its Production Processes

• satyendra
• March 16, 2013
• 12 Comments
• DR process, DRI, HBI, Iron making. Metallization, reformed natural gas, residual elements, Rotary kiln,

Direct Reduced Iron and its Production Processes

Direct reduced iron (DRI) is the product which is produced by the direct reduction of iron ore or other iron bearing materials in the solid state by using non-coking coal or natural gas. Processes which produce DRI by reduction of iron ore below the melting point of the iron are normally known as the direct reduction (DR) processes. The reducing agents are carbon monoxide (CO) and hydrogen (H2), coming from reformed natural gas, syngas or coal. Iron ore is used mostly in pellet and/or lumpy form. Oxygen (O2) is removed from the iron ore by chemical reactions based on H2 and CO for the production of highly metalized DRI.

In the direct reduction process, the solid metallic iron (Fe) is obtained directly from solid iron ore without subjecting the ore or the metal to fusion. Direct reduction can be defined as reduction in the solid state at O2 potentials which allow reduction of iron oxides, but not of other oxides (MnO, and SiO2 etc.), to the corresponding elements. Since reduction is in the solid state, there is very little chance of these elements dissolving (at low thermodynamic activity) in the reduced iron, so the oxides which are more stable than iron remain essentially unreduced.

DRI has a porous structure. This is because DRI is produced by removing O2 from iron ore. It is also known as sponge iron since its structure is just like sponge with a network of connecting pores. These pores results in a large internal surface area which is around 10,000 times greater than the internal surface area of solid iron.

DRI is produced in many forms. These are lump, pellets, hot briquetted iron (HBI), fines, and cold briquetted iron (CBI). HBI and CBI are densified forms of DRI to facilitate its handling and transport. HBI is produced by compacting DRI under very high pressure at temperatures in excess of 650 deg C. This closes many of the pores and limits the contact area which is available for reaction with air. It also increases its thermal conductivity.

Iron content in the DRI is in two forms. One is in metallic form which is known as metallic iron, Fe (M), and the second form of iron which is present in residual iron oxides, Fe (O). The total iron, Fe (T), in DRI is the sum of these two iron components. Metallic iron is the aggregate quantity of iron, either free or combined with carbon (as cementite) present in DRI. Metallization of DRI is a measure of the conversion of iron oxides into metallic iron (either free or in combination with carbon as cementite) by removal of O2 due to the action of the reductant used. Degree of metallization of DRI is the extent of conversion of iron oxide into metallic iron during reduction. It is defined in percentage of the mass of metallic iron divided by the mass of total iron.

History of the DR processes

The first patent for the production of DRI was in 1792 in United Kingdom. It presumably utilized a rotary kiln. The development of the modern DR process began in the middle of nineteenth century. Since 1920 more than 100 DR processes have been invented and operated. Most of them have not survived. The modern era of DRI production began on December 5, 1957 when the HYL process plant started production at Hylsa. The first plant of Midrex process came into operation in May 17, 1969 at Oregon Steel mills in Portland, Oregon. The coal based rotary kiln process combines the Republic Steel-National Lead (RN) process developed in 1920s for beneficiating low grade ores, and the Stelco-Lurgi (SL) process conceived in the early 1960s for producing high-grade DRI.

DRI production process

The DRI production process involves the intimate mixing of prepared (sized) iron ore with a reductant, which is also generally used for heating of the ore bed to the temperature needed to achieve adequate reduction rates. The reductant can be a gas or a solid. Major DRI production processes are either natural gas based or coal based. Feed material for the DR process is either sized iron ore of size ranging from 10 mm to 30 mm or iron ore pellets of size ranging from 8 mm to 20 mm produced in an iron ore pellet plant.

The gas based process uses a shaft furnace for the reduction reaction. The coal based process uses any one of the four types of reactors for the reduction reaction. These reactors are (i) rotary kiln, (ii) shaft furnace, (iii) fluidized bed reactor, and (iv) rotary hearth furnace. Rotary kiln is the most popular reactor for the coal based process.

The principle of the direct reduction of iron ore is shown in Fig 1.

Fig 1 Principle of direct reduction of iron ore

The principle of the process of the direct reduction of iron ore is shown in Fig 2.

Fig 2 Principle of direct reduction process

Gas based process

In the gas based reduction processes, a vertical shaft kiln is used in which iron ore is fed into the top of the kiln and finished sponge iron is drawn off from the bottom after cooling it so as to prevent its re-oxidation. The reducing gas is passed through the ore bed, and spent gas is recirculated after heating and reforming to a mixture of H2 and CO in a reformer, where it is also heated to a temperature of 950 deg C, which is the temperature needed to achieve adequate reduction reaction rates.  The shaft furnace works on the counter current principle where the iron ore feed material moves downward in the furnace by gravity and gets reduced by the up flowing reducing gases.

In the gas based DR process, gaseous fuels are used. These fuels are to have the ability to reform or crack to produce a mixture of H2 and CO gas. High methane containing natural gas is the most commonly used gas. Natural gas is reformed to enrich with H2 and CO mixture and this enriched and reformed gas mixture is preheated and sent to the shaft DR furnace.

Gas based process is simple to operate and involves three major steps namely (i) iron ore reduction, (ii) gas preheating, and (iii) natural gas reforming. Presently the gas based process is also available in which the reforming of natural gas is not needed.

The heart of the gas based process is the shaft furnace. It is a cylindrical, refractory-lined vessel and is a key component of the direct reduction process.  It is a flexible as well as a versatile reactor. It can use natural gas, a syngas from coal, coke oven gas, or exhaust gas from the Corex process as the reducing gas.

The reduction reactions take place both with H2 and CO in a gas based DRI process. The reactions which take place with H2 are (i) 3Fe2O3 + H2 =2Fe3O4 + H2O, (ii) Fe3O4 + H2 = 3FeO + H2O, and (iii) FeO + H2 = Fe + H2O. The reactions which take place with CO are (i) 3Fe2O3 + CO = 2Fe3O4 + CO2, (ii) Fe3O4 + CO = 3FeO + CO2, and (iii) FeO + CO = Fe + CO2.

Gas based DRI is not subjected for any magnetic separation since no contamination with non magnetic materials is possible. The gas based process is flexible to produce three different product forms, depending on the specific requirements of each user. The three forms of DRI are cold DRI, HBI or hot DRI.

There are three popular gas based processes. These are (i) HYL process, (ii) Midrex process, and (iii) PERED process. The latest version of HYL process is known as the Energiron process. The flowsheet of the Energiron process is at Fig 3.

Fig 3 Flowsheet of Energiron process

The flowsheet of the Midrex process is at Fig 4.

Fig 4 Flowsheet of Midrex process

Coal based process

In a coal based process, the reactor for the reduction reaction is a rotary kiln which is slightly inclined to the horizontal position. The process of direct reduction is carried out with the operating temperatures maintained in a range from 1,000 deg C to 1,100 deg C. In the rotary kiln, both coal and the iron ore feed material is charged from the same end of the kiln. During the movement of feed material forward the oxidation reaction of carbon in coal and reduction reaction of CO gas is carefully balanced. A temperature profile ranging from 800 deg C to 1050 deg C is maintained along the length of the kiln at different zones and as the material flows down due to gravity the ore is reduced. The basic reduction reactions in the process are (i) C + O2 = CO2, (ii) CO2 + C = 2CO, (iii) 3Fe2O3 + CO = 2Fe3O4 + CO2, (iv) Fe3O4 + CO = 3FeO + CO2, and (v) FeO + CO = Fe + CO2.

The product of the kiln (DRI and char mix) is then cooled in a rotary cooler with external water cooling system to a temperature of 100 deg C to 200 deg C. The product after it is discharged from the kiln is screened and magnetically separated. DRI being magnetic gets attracted and gets separated from non- magnetic char.The separated DRI is screened into two size fractions of +3 mm and -3 mm. -3 mm fractions is sometimes briquetted by using hydrated lime and molasses as binders. The flowsheet of the coal based DR process is shown in Fig 5.

Fig 5 Flowsheet of coal based rotary kiln process for DRI production

Properties of DRI 

The comparison of the properties of the coal based DRI and the gas based DRI is given in Tab 1

The comparison of the composition of the coal based DRI and the gas based DRI is in Tab 2

Advantages of DRI

The various advantages of DRI are (i) it allows dilution of metallic residuals in scrap during the steelmaking, (ii) since it is a manufactured product, it has a uniform composition, (ii) it has a uniform size, (iv) it has low sulphur and phosphorus content as compared to the scrap, (v) if it is charged in blast furnace along with other burden materials, it improves the productivity of the blast furnace.