Direct-reduced iron (DRI) or Sponge Iron

Direct-reduced iron (DRI), also called sponge iron, is produced from direct reduction of iron ore (in the form of lumps, pellets or fines) by a reducing gas produced from natural gas or coal. The reducing gas is a mixture majority of hydrogen (H₂) and carbon monoxide (CO) which acts as reducing agent. This process of directly reducing the iron ore in solid form by reducing gases is called direct reduction.

Direct reduction, an alternative route of iron making, has been developed to overcome some of these difficulties of conventional blast furnaces. DRI is successfully manufactured in various parts of the world through either natural gas or coal-based technology. Iron ore is reduced in solid state at 800 to 1,050 °C (1,472 to 1,922 °F) either by reducing gas (H₂+CO) or coal. The specific investment and operating costs of direct reduction plants are low compared to integrated steel plants and are more suitable for many developing countries where supplies of coking coal are limited.

The most frequently used reduction gases are CO and H2.

Reduction with CO: When the initial state is hematite, and the temperature is over 570°C, reduction of

iron oxide will occur in three steps:

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Fe2O3 ⇒Fe3O4 ⇒ FeO ⇒ Fe

3Fe2O3 + CO ⇒ 2Fe3O4 + CO2 ΔH = -7.8 kJ /molFe

Fe3O4 + CO ⇒3FeO + CO ΔH = +11.2 kJ/ molFe

FeO+CO ⇒Fe + CO ΔH = -15.7 kJ /molFe

Summing these reactions gives reduction of hematite

to iron:

Fe2O3 + 3CO ⇒ 2Fe + 3CO2 ΔH = -13.3kJ /molFe

Reduction with H2: This reduction is similar to the

one with CO. It occurs in three steps when temperatures are over 570°C and two steps when the temperature is below 570°C:

3Fe2O3 + H2 ⇒ 2Fe3 O4 + H2O ΔH = -1.0 kJ /molFe

Fe3O4 + H2 ⇒ 3FeO + H2O ΔH = +24.9 kJ molFe

FeO + H2 ⇒ Fe + H2O ΔH = +25.4 kJ molFe

Fe2O3 + 3H 2 ⇒ Fe + 3H2O ΔH =+49.4 kJ molFe

From The above Reactions, we can calculate the carbon/Hydrogen required for reduction or can calculate required C/Fe.