08108057-Open Hearth Process

8/3/2019 08108057-Open Hearth Process

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Open Hearth process

Ishan Sethi

08108057

Metallurgy , 3rd

year


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Open hearth furnaces are one of a number of kinds offurnace where

excess carbon and other impurities are burnt out of the pig iron to

produce steel. Since steel is difficult to manufacture due to its highmelting point, normal fuels and furnaces were insufficient and the

open hearth furnace was developed to overcome this difficulty.

The charge for open-hearth furnaces is divided into a metal fraction,

including pig iron, steel scrap, deoxidizers, and alloying additives, anda nonmetallic fraction, consisting of iron ore, open-hearth sinter,

limestone, lime, bauxite, and fluorite. Pig iron, which is used in the

molten state or in the form of ingots, is the major carbon source and

provides normal running of the process. The amount of pig iron and

steel scrap in the charge may vary in any proportion, depending on thetype of process, economic considerations, and the grades of the steel

produced


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Ferroalloys and some pure metals, such as aluminum and nickel,

are used as deoxidizers and alloying addiitives in open-hearth

production. Iron ore and open-hearth sinter are used as oxidizingagents, and also as a flux, providing accelerated formation of the

active slag. Scale may also be used as the oxidizing agent.

Limestone, lime, bauxite, and fluorite are used to form slag with

the required composition and consistency, which supports the

oxidizing reactions, removal of harmful impurities, and heating of

the metal.


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In the open-hearth process, as distinct from the converter

processes, the heat evolved as a result of oxidation of

impurities in the metal bath is insufficient for smelting. Thus,

additional heat from the combustion of fuel in the melting

chamber is supplied to the furnace. Natural gas, fuel oil, coke,

and blastfurnace gases are used as such fuels. To providecomplete combustion of the fuel, the quantity of air supplied

for combustion is slightly in excess of the theoretically

required quantity. This produces an excess of oxygen in the

products of combustion, in which the gaseous oxides CO2 and

H2O, which partially dissociate at high temperatures, are also

present. As a result, oxidation of iron and other elements in

the charge takes place


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To improve combustion, some of the air introduced into the

furnace may be replaced by oxygen; gaseous oxygen is also

supplied to the bath to intensify the processes of oxidation. Theslag coating the metal in all subsequent stages of smelting is

composed ofFeO, Fe2O3, CaO, SiO2, MnO, P2O5, and other oxides,

along with the gradually decomposing refractory materials of the

lining, the fluxes, and impurities carried by the charge. The slag

plays an important role: it binds all the impurities that must be

removed from the charge, transfers oxygen from the furnace

atmosphere to the molten metal, transfers heat from the cone of

flame to the metal, and protects the metal from saturation by

gases in the furnace atmosphere and from overoxidation of theiron. In the various stages of smelting, the slag must have the

chemical composition required for fluidity and must be present in

definite amounts in the furnace.


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Types of processes. Open-hearth processes are divided into

acid and basic processes, depending on the composition of

the refractory materials used in preparing the furnace hearth(in the basic process, mainly the basic oxides CaO and MgO;

in the acid process, SiO2). The slag in the basic process

consists primarily of basic oxides; that in the acid process

consists of acid oxides


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More than 95 percent of open-hearth steel is smelted by basic scrap

and scrap-ore processes. The acid open-hearth process is usedmuch less than the basic process because of the difficulty of

removing sulfur and phosphorus from the metal in the acid process;

therefore, the acid process requires charge materials of higher

purity (which are more expensive). Smelting in the acid process is oflonger duration than in the basic process.

However, the nature of the interaction of the metal with an acid

hearth lining and acid slag, which has reduced gas permeability

relative to the basic process, and also the use of high-purity charge

materials, makes possible the production of high-quality steel in theacid process, free of harmful impurities and with low anisotropy of

properties along and across the direction of subsequent pressure

working.. Thus, acid open-hearth steel is widely used in the

production of turbine rotors, large crankshafts, and artillery barrels,

which require high mechanical strength along and across the grain


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Thus, acid open-hearth steel is widely used in the production of

turbine rotors, large crankshafts, and artillery barrels, which

require high mechanical strength along and across the grain


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Carbon Reaction

Carbon forms the single largest impurity in pig iron to be

eliminated during refining. It is characteristically different from

the rest of the impurities in that the oxide product is a gas at

steelmaking temperatures. The reaction of oxidation of carbon

practically does not take place at the slag-metal interface because

of the difficulty in nucleating gas bubbles there. The reaction takes

place at the gas metal interface since it eliminates the necessity of

nucleation of gas bubbles.

The decarburisation rate (dC/dT ) is controlled by the rates ofdiffusion of either carbon or oxygen to the gas metal interface.


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The carbon reaction is however very slow in a hearth process

wherein no gaseous refining medium is used.

The pressure inside a bubble of radius r in molten metal ofsurface tension is given by,

pb = po + 2/ r

where po is the static pressure due to the heads of atmosphere ,

slag and metal compressing the bubble and , the 2/ ris the p

ressure term due to surface tension opposing the growth of bubble. If carbon monoxide bubble is to form homogeneously in the melt,

for the average molecular size of the bubble nucleus of r equal to

nearly 5-8 A o , the pressure within the bubble should have a value of

10 4 10 5 atoms.


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Active sites Inactive sites


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(a)Active sites. The bubble attains hemispherical shape beforethe partial pressure of CO attains its equilibrium value and

hence the bubbles are formed and , being mechanically

unstable, are separated

(b)Inactive sites. The CO partial pressure inside the bubble

reaches its equilibrium value before the hemispherical shape

is attained and hence no separation is possible


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CO bubble can nucleate at the slag-metal or refractory-metal

interfaces. The extent of nucleation at the liquid slag-metal

boundary in negligibly smaller than that t the solid refactory-metal interface.

The carbon reaction can proceed here until thepco inside the

bubble is less than the equilibriumpco . With the progess of the

reaction the bubblepco increases and thereby the bubble tends

to grow in hemispherical shape i.e. the bubble radius equalsthe pore radius. Any further rise in bubblepco after the

attainment of hemispherical shape results in increasing the

bubble radius and hence the pressure term 2/r decreases .

This amounts to a decrease in the internal pressure of the

bubble and as a result the carbon reaction moves further to

the right.


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Beyond the hemispherical shape, mechanically the bubble, as it

grows , becomes more and more unstable till finally it detaches

itself from the refractory wall and rises through the melt. Whilethe bubble ascends thepo term decreases and hence the bubble

size increases . This also helps in pushing the carbon reaction to

the right.

If the pore size is smaller than a certain critical value , the

bubble pco may equal equilibrium pco value before the bubble

attains a hemispherical shape .


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Mechanism of Oxygen transport and Kinetics ofCarbon- Oxygen

Reaction

In hearth processes oxygen from the furnace atmosphere has todiffuse across the slag and the metal layers to reach the pore-

metal interface. It must dissolve in slag in ionic form.

At the gas-slag interface oxygen dissolves as:

{O} + 2e ( O 2-)

And the iron in the slag gets oxidised as:

2(Fe 2+ ) 2 (Fe 3+) + 2e

So the overall reaction is

2(Fe 2+) + (O) = 2(Fe 3+ ) + (O 2- )

Due to thermal diffusion these migrate from gas-slag to slag-metalinterface and a reverse reaction as:

2(Fe 3+ ) + (O 2- ) = 2(Fe 2+ ) + [O]


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Ferric oxide supplies oxygen as:

< FeO> 2(FeO) + [O]

Which is an endothermic reaction . It is known as oreing of

slag.

The over all reaction is

< FeO> + 2[C] 4/3 [Fe] + 2{CO}

H = + 65 kcalAnd is endothermic in nature.


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Importance of Decarburisation Reaction

In steelmaking processes suitable slag of the right chemical

and physical characteristics is always aimed at to ensure

smooth and efficient oxidation of impurities like Si, Mn and P.

except Si and Mn , dephosphorisation poses various

problems and requires time for its required and effectivecontrol. Stirring the slag and metal system does wonder but

stirring in hearth processes have to depend on carbon boil

alone and which is not fast enough to cause required stirring.

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08108057-Open Hearth Process