Explain Siemens-Martin's open hearth process for the manufacture of steel with diagram.

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Solution

Siemen-Martin'process : The mixture of cast iron $(30 - 50 %)$ , scrap iron haematite $(F e_{2} O_{3})$ and lime is melted in the open hearth furnace which has a basic or acidic lining depending Upon impurities. $F e_{2} O_{3}$ acts as oxidizing agent in the formation of steel and carbon gets oxidized to $C O$ and escapes.
$F e_{2} O_{3} + 3 C \to 3 C O + 2 F e$
$2 F e_{2} O_{3} + 2 S \to 4 F e + 3 S O_{2}$
$10 F e_{2} O_{3} + 3 P_{4} \to 20 F e + 6 P_{2} O_{5}$
$2 F e_{2} O_{3} + 3 S i \to 4 F e + 3 S i O_{2}$
$F e_{2} O_{3} + 3 M n \to 2 F e + 3 M n O$
Now steel containing required amount of carbon is obtained by addition of spiegel.
Advantages
(i) As the process proceeds slowly therefore, its control is easy.
(ii) Better quality of steel can be manufactured,
(iii) The composition and temperature can be easily controlIed.
(iv) As no blast of air is passed therefore, very little steel is lost.
(v) The steel can be directly obtained by the use of ore and scrap iron.

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