Manufacture of steel

The Bessemer process inexpensive process for the mass-production of steel from molten cast iron. The key principle of this process is the removal of impurities by the oxidation with air being blown through the molten iron.
Construction:

Open hearth process:

Open hearth process is the modern process for the manufacture of iron.
This process is based on the following working principles:

a. Oxidation of impurities is done by haematite ore instead of air.

b. Percentage of carbon and silicon are decreased by adding scrap and low grade wrought iron.

c. Fuel is saved by regenerating system of heat.

In this process, open hearth furnace is used. The furnace is lined either with silica (SiO2) or calcinated dolomite (CaO.MgO) depending upon the nature of impurities present in the cast iron. Silica lining is used if the impurities are Mn, Si etc and calcinated dolomite lining is used if the impurities are P, S etc.
A mixture of cast iron, scrap iron, hematite and lime is taken in open hearth process. The mixture is heated by producer gas at 1500ºC. Impurities present in cast iron are oxidized by hematite ore and removed as gas and slag.
The reactions involved in this process are given below:
$$\ce{2Fe2O3 + 3S->4Fe + 3SO2 ^ }$$
$$\ce{Fe2O3 + 3C->2Fe + 3CO}$$
$$\ce{2Fe2O3 + 6P->10Fe + 3P2O5}$$
$$\ce{P2O5 + 3CaO->$\underset{\text{Slag}}{\ce{ Ca3(PO4)2}}$}$$
$$\ce{P2O5 + 3MgO->$\underset{\text{Slag}}{\ce{ Mg3(PO4)2}}$}$$
$$\ce{2Fe2O3 + 3Si->4Fe + 3SiO2}$$
$$\ce{SiO2 + CaO->$\underset{\text{Slag}}{\ce{CaSiO3}}$}$$
$$\ce{2Fe2O3 + 3Mn->2Fe + 3MnO}$$
$$\ce{MnO + SiO2->$\underset{\text{Slag}}{\ce{MnSiO3}}$}$$

Samples of steel are drawn from the open hearth furnace time to time and tested for carbon content. Finally, suitable amount of spiegeleisen is added to the molten mass and stirred to obtain steel. This process takes about8-10 hours for the completion.

Manufacture of steel by Bessemer process:

The Bessemer process is inexpensive process for the mass-production of steel from molten cast iron. The key principle of this process is the removal of impurities by the oxidation with air being blown through the molten iron.

In this process, molten pig or cast iron is charged into a large pear shaped vessel called Bessemer converter. The inner side of Bessemer converter is lined with silica or calcinated dolomite.

Depending upon the nature of impurities present in the cast or pig iron, Bessemer process can be divided into two types:

1. Acidic Bessemer: This process is used if the cast iron contains Mn, Si as impurities. The cast iron is charged into acidic Bessemer converter (lined with SiO2) and heated with blast of hot air as shown in figure. The impurities get oxidized and slag is formed.
$$\ce{Si + O2 ->SiO2}$$
$$\ce{2Mn + O2->2MnO}$$
$$\ce{MnO + SiO2 ->$\underset{\text{Slag}}{\ce{MnSiO3}}$}$$
$$\ce{2C + O2->2CO}$$

Slag is removed and carbon content is increased by the addition of suitable or calculated amount of spiegeleisen. The molten iron is mixed with spiegeleisen to get steel. It is taken out by tilting the converter.

2. Basic Bessemer Converter: This process is used if the cast iron contains P, S, C etc as impurities. The cast iron is charged into basic Bessemer convertor lined with CaO, MgO and heated with blast of hot air. The impurities get oxidized and slags are formed.
$$\ce{2C + O2->2CO ^ }$$
$$\ce{S + O2->SO2 ^ }$$
$$\ce{P4 + 5O2->2P2O5}$$
$$\ce{3CaO + P2O5->$\underset{\text{Thomas slag}}{\ce{Ca3(PO4)2}}$}$$
$$\ce{MgO + P2O5->$\underset{\text{GIlchrist slag}}{\ce{Mg3(PO4)2}}$}$$

Slags are removed and calculated amount of spiegeleisen is assed into the molten iron to obtain steel.

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