Question

Describe smelting of iron in a blast furnace.

Write all the reactions which take place in different zones of the blast furnace.

Answer 1

Reduction of oxide ore into metal: The oxide form of metal is converted into metal by the reduction process.

Different methods are using based on reactivity of metals: (a) self-reduction (b) smelting (c) thermite (d) amalgamation (e) electrolytic reduction

Smelting: The ore is mixed with coke and limestone approximately in the ratio of 8: 4: 1 This mixture is called charge.

The charge is lowered into the furnace from the top through the cup and cone arrangement, and various reactions take place in the furnace at different levels.

(a) Lower region/combustion zone (1500-2000^oC): Exothermic

Coke combines with the heated air coming from the tuyeres and burns to form carbon dioxide.

The reaction, being highly exothermic, increases the temperature in this zone to 2000⁰C. $$\begin{gathered} \mathrm{C}\left(\mathrm{S}\right)+\mathrm{O}\left(\mathrm{g}\right)\to {\mathrm{CO}}_2\left(\mathrm{g}\right)+97\mathrm{Kcal} \\ \mathrm{Carbon}\mathrm{Oxygen}\mathrm{Carbon}\mathrm{dioxide} \end{gathered}$$

(b) Middle region or fusion zone (1500-1000^oC): Endothermic

The carbon dioxide formed in the lower layer moves to the middle layer where it is reduced to carbon monoxide

$$\begin{gathered} {\mathrm{CO}}_2\left(\mathrm{g}\right)+\mathrm{C}\left(\mathrm{g}\right)\to 2\mathrm{CO}-39\mathrm{Kcal} \\ \mathrm{Carbon}\mathrm{dioxide}\mathrm{Carbon}\mathrm{Carbon} \\ \mathrm{monoxide} \end{gathered}$$

At such a high temperature, the limestone(CaCO₃) added decomposes into calcium oxide (CaO) and carbon dioxide(CO₂):

$$\begin{gathered} {\mathrm{CaCO}}_3\left(\mathrm{S}\right)\stackrel{∆}{\to }\mathrm{CaO}\left(\mathrm{s}\right)+{\mathrm{CO}}_2\left(\mathrm{g}\right) \\ \mathrm{Lime}\mathrm{stone}\mathrm{Quick}\mathrm{lime}\mathrm{Carbondi}\mathrm{oxid} \end{gathered}$$

The calcium oxide(quick lime) formed reacts with impurities like sand in the ore and converts them to slag

$$\begin{gathered} \mathrm{CaO}\left(\mathrm{s}\right)+{\mathrm{SiO}}_2\left(\mathrm{s}\right)\to {\mathrm{CaSiO}}_3\left(\mathrm{s}\right) \\ \mathrm{Quick}\mathrm{Silicon}\mathrm{Calcium} \\ \mathrm{lime}\mathrm{dioxide}\mathrm{silicate} \end{gathered}$$

(c) Upper region or reduction zone (1000—500^oC)

The carbon monoxide formed in the middle region moves to the upper region where it reduces the iron oxide to iron.

$$\begin{gathered} \mathrm{FeO}\left(\mathrm{s}\right)+\mathrm{CO}\left(\mathrm{s}\right))\stackrel{{750}^{\mathrm{o}}\mathrm{C}-{600}^{\mathrm{o}}\mathrm{C}}{\to }\mathrm{Fe}(\mathrm{s})+{\mathrm{CO}}_2(\mathrm{s}) \\ \mathrm{Iron}\mathrm{II}\mathrm{oxide}\mathrm{Carbon}\mathrm{Iron}\mathrm{Carbon} \\ \mathrm{monoxide}\mathrm{dioxide} \end{gathered}$$

$$\begin{gathered} {\mathrm{Fe}}_2{\mathrm{O}}_3\left(\mathrm{s}\right)+3\mathrm{CO}\left(\mathrm{g}\right)\stackrel{{600}^{\mathrm{o}}\mathrm{C}-{400}^{\mathrm{o}}\mathrm{C}}{\to }2\mathrm{Fe}\left(\mathrm{s}\right)+3{\mathrm{CO}}_2\left(\mathrm{g}\right) \\ \mathrm{Iron}\mathrm{III}\mathrm{Carbon}\mathrm{Iron}\mathrm{Carbon} \\ \mathrm{oxide}\mathrm{monoxide}\mathrm{dioxide} \end{gathered}$$