Re-Oxidation of DRI is related to the theory of corrosion caused by atmospheric oxidation in presence of oxidizing gas and moisture. The reasons for the high tendency of DRI to oxidize in the presence of air, and atmospheric moisture, rain are as follows..
HIGH POROSITY
Due to reduction of iron oxide of the Iron ore in the solid state, the porosity of the mass increases 50-70%. Thus, the surface to volume ratio of the iron ore becomes very high, as a result of which re-oxidation of DRI takes place at a faster rate.
ROUGH SURFACE
The ruptured surface of the DRI permits dust to accumulate and become solidly lodged. This cause rapid condensation of moisture, and thus accelerates corrosion
POOR THERMAL CONDUCTIVITY
Sponge iron produced either by coal or gas based route posses poor thermal conductivity as a result, heat generated by exothermic re-oxidation reaction within the pile cannot be dissipated away and lead to temperature build-up in DRI, and thus, leads to accelerates the corrosion.
HIGH POROSITY
Due to reduction of iron oxide of the Iron ore in the solid state, the porosity of the mass increases 50-70%. Thus, the surface to volume ratio of the iron ore becomes very high, as a result of which re-oxidation of DRI takes place at a faster rate.
ROUGH SURFACE
The ruptured surface of the DRI permits dust to accumulate and become solidly lodged. This cause rapid condensation of moisture, and thus accelerates corrosion
POOR THERMAL CONDUCTIVITY
Sponge iron produced either by coal or gas based route posses poor thermal conductivity as a result, heat generated by exothermic re-oxidation reaction within the pile cannot be dissipated away and lead to temperature build-up in DRI, and thus, leads to accelerates the corrosion.
RE-OXIDATION REACTION
In Presence of Air
Hot sponge iron will react with oxygen present in air to form ferric oxide (Fe2O3) and Ferrosoferric Oxide (Fe3O4).
4Fe + 3O2 = 2Fe2O3 -1760 kcal/kg Fe
The reaction is quite sluggish in dry air, at ambient temperature.
3Fe + 2O2 = Fe3O4 -1590 kcal/kg Fe
In presence of Moisture
The reaction continues as long as sponge iron remains hot and sufficient oxygen is available. Moisture pressure enhance the reaction rate.
2Fe + 3H2O = Fe2O3 + 3H2
This is a corrosion reaction.
In the presence of dissolved oxygen
This reaction is accelerated by dissolved oxygen, which depolarized the cathodic area and produces ferrous hydroxide.
2Fe + 3H2O + O2 = 2Fe(OH)2
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