CUPOLA FURNACE: Various Zones of Cupola Furnace, Charging of Cupola Furnace, Working of Cupola Furnace, Applications of Cupola

CUPOLA FURNACE

Cupola furnace is
employed for melting scrap metal or pig iron for production of various cast
irons. It is also used for production of nodular and malleable cast iron. It is
available in good varying sizes. Fig. shows the various parts and zones of
cupola furnace. The main considerations in selection of cupolas are melting capacity, diameter of shell without lining  or with lining, spark arrester.



Production process of market form of steel supply of different shapes or sections
Production process of market form of steel
supply of different shapes or sections


Production process of market form of steel supply of different shapes or sections
Production process of market form of steel
supply of different shapes or sections

Cupola furnace
Cupola furnace




Various Zones of Cupola Furnace

Various numbers of chemical reactions take place in different zones of cupola. The construction and different zones of cupola are discussed as under.


1. Well

The space between the bottom of the tuyeres
and the sand bed inside
the cylindrical shell of the cupola is called as well of
the cupola. As the melting occurs, the
molten metal is get collected in this portion
before tapping out.




2. Combustion zone

The combustion zone of Cupola is
also called as oxidizing zone. It is located between the upper of the tuyeres
and a theoretical level above it. The total height of this zone is normally from 15 cm. to 30 cm. The combustion actually
takes place in this zone by consuming the free oxygen completely from
the air blast and generating tremendous heat. The heat generated in this zone
is sufficient enough to meet the requirements of other zones of cupola. The
heat is further evolved also due to oxidation of silicon and manganese. A temperature of about 1540°C to 1870°C is achieved in
this zone. Few exothermic reactions takes place in this zone these are represented as


C + O——® COHeat


Si + O——® SiOHeat


2Mn + O——® 2MnO Heat



3. Reducing zone

Reducing zone of Cupola is also known as the protective
zone which is located between the upper level of the combustion zone and the
upper level of the coke bed. In this zone, CO
2 is
changed to CO through an endothermic reaction, as a result of which the
temperature falls from combustion zone temperature to about 1200°C at the top
of this zone. The important chemical reaction takes place in this zone which is given as under.


CO+ C (coke)     ——®   2CO      +    Heat



Nitrogen does not participate in
the chemical reaction occurring in his zone as it is also the other main constituent of the upward moving hot gases.
Because of the reducing atmosphere
in this zone, the charge is protected
against oxidation.



4. Melting zone

The lower layer of metal charge
above the lower layer of coke bed is termed as melting zone of Cupola. The
metal charge starts melting in this zone and trickles down through coke bed and
gets collected in the well. Sufficient carbon content picked by the molten
metal in this zone is represented by the chemical reaction
given as under.


3Fe + 2CO      ——®       Fe3C  +    CO2

5. Preheating zone

Preheating zone starts from the
upper end of the melting zone and continues up to the bottom level of the charging door. This zone contains a number of alternate layers of coke bed, flux and metal charge.
The main objective of this zone is to preheat the charges from room temperature
to about 1090°C before entering the metal charge to the melting zone. The preheating takes place in this zone due
to the upward movement of hot gases. During the
preheating process, the metal charge in solid form picks up some sulphur content
in this zone.


6. Stack

The empty portion of cupola
above the preheating zone is called as stack. It provides the passage to hot
gases to go to atmosphere from the cupola furnace.



Charging
of Cupola Furnace

Before the blower is started, the furnace is
uniformly pre-heated and the metal and coke charges, lying in alternate layers,
are sufficiently heated up. The cover plates are positioned suitably and the
blower is started. The molten metal starts trickling down and collecting in the
well. The height of coke charge in the cupola in each layer varies generally
from 10 to 15 cms. The requirement of
flux to the metal charge depends upon the quality of the charged metal and
scarp, the composition of the coke and the amount of ash content present in the coke. Generally about 40 kg to 50 kg of
limestone, in form of flux, per metric ton of the metal is used. The amount of
this flux to be charged should be properly determined. The excess amount of flux affects the acid
lining of cupola. Lesser amount of the flux than required will result in the
loss of molten metal. First charge received of the molten metal is either
allowed to drain out or used for rough castings. For having desired composition
of the casting, it is essential to control the proportions of its various constituents
at the stage of raw material requirement for melting. It is also necessary due
to number of losses and gains of different constituents take place inside the
cupola during the process of melting. These losses and gains in composition are identified for compensating
purposes. The losses or gains of different constituents during melting as identified are given as under:


1. Iron            –      Loss of about 4%


2. Carbon        –      Gain of about 0.1 to 0.15%.


3. Silicon         –       Loss
of about 10%


4. Manganese    –       Loss of about 15 to 20%.


5. Phosphorus   –       Practically
no change.


6. Sulphur          –      Gain of about 0.03 to 0.05%.




Working of Cupola Furnace

Initially the furnace prop is opened to drop
the existing earlier charge residue. The furnace  is then repaired using rich refractory lining. After setting the prop in
position, the fire is ignited using firewood and then small amount of coke is
used to pick fire. The little oxygen    is then supplied for combustion
. Lime, coke, and metal in balanced proportions are charged
through the charging  door upon the coke bed and at proper
time on starting the blower. Air is
forced from wind box through tuyers into furnace. The forced air rise upward
rough the stack furnaces for
combustion of coke. Besides being fuel, the coke supports the charge until melting
occurs. On increase of temperature, the lime stone melts and forms a flux which
protects the metal against from excessive oxidation. Lime also fuses and
agglomerates the coke ash. The
melting occurs and proceeds and molten metal is collected at the bottom. Molten
metal may be tapped at intervals before each skimming, or the tap-hole may be
left open with metal flowing constantly. In
most cupolas slag is drained from the slag hole at the back of furnace. When metal is melted completely the bottom
bar is pulled sharply under the plates
and bottom is dropped. All remaining slag, un-burned
coke or molten metal drops from the furnace. When the melt charge has cooled on
closing furnace, it is patched and made ready
for the next heat.


Applications of Cupola Furnace

Cupola is most widely used for
melting practices for production of grey cast iron, nodular cast iron,
malleable cast iron and alloy cast iron. It can be used for melting some
copper-base alloys, and in duplexing and triplexing operations for making of
steel, malleable cast iron and ductile cast iron. Steel can be also prepared in
cupola by employing duplexing and triplexing

operations. In duplexing melting operation two
furnaces are used, and triplexing operations, three furnaces are employed



Jamming
of Cupola Furnace

Jamming of cupola furnace is a very common
problem for which sufficient care should be taken to prevent it. It occurs
frequently due to the negligence of the furnace operator. The cupola furnace may be either
jammed temporarily or permanently and the same are discussed as under.



Temporary Jamming of
Cupola Furnace

Temporary jamming of cupola furnace means the temporary
suspension of air supply in the tuyere zone due to choking of the tuyere
mouths. This is caused mainly due to the low
temperatures at the tuyere openings resulting in the solidification of
iron and slag around these openings. The air passage
is hence chocked
and the supply of air is temporarily stopped. It results in incomplete combustion of the coke/fuel
inside the furnace and hence leads to rapid reduction of the temperature in the
furnace. Therefore this temporary jamming must be prevented by frequent poking
of this solidified material by means of a poking bar through the tuyeres. The
operator of cupola should always keep himself alert enough for not allowing
such solidification for continue melting
in furnace for a longer
period.



Permanent Jamming of Cupola Furnace

Permanent jamming of cupola furnace
means a complete cut-off of the air supply to the cupola due
to the permanent choking of the air passage. This occurs on account of the
overflow of  the slag into the wind chamber. As the metal melts and starts
collecting in the well of the furnace, its level rises gradually and the
slag being lighter,  always  floats on the top surface  of the molten metal. If, the molten metal is
not tapped out in time due to the negligence of the operator, the level of molten metal in furnace will start rising
in the well and a stage will come when this level approaches to the tuyere level. Any slightest delay further, in tapping  by the cupola operator will cause the slag to
flow through the tuyere openings into the wind belt. Since the slag comes in
contact with the air at low temperature it quickly solidifies in the wind belt as well as in the tuyers and therefore chock the passages
of air flow permanently in
the furnace. Hence the cupola furnace reaches in such undesirable condition on
account    of the above occurrence.
Therefore, to avoid such undesired situation, the operator of cupola furnace should always be vigilant
enough to tap the molten metal out from well of the cupola furnace before the level of molten metal rises up to the
tuyere level. Hence the cupola  furnace may continue melting
for longer period.


Thermal Efficiency of Cupola Furnace

Thermal efficiency of cupola furnace is the ratio
of heat actually utilized in melting and superheating the metal to the heat
evolved in it through various means. The total heat evolved involves the heat
due burning of coke, heat evolved due to oxidation of iron, Si and Mn and heat
supplied by the air blast. During melting it is observed that approximately 48-
70 % of the evolved heat is going as waste.


Precautions for Safety of Cupola Furnace

For
operating the cupola successfully, the following safety precautions must always
be kept into account.


1. For
safety of Cupola, better quality refractory lining must be used for preparing
or repairing furnace so that it can withstand high temperature as produced
inside the furnace during melting,
otherwise it will fuse and mix with molten metal to form slag.


2. The
furnace operator during firing the furnace should always make an effort to
place the metal charge in the centre. He must ensure that the coke charge is
well distributed all around
and towards the firebrick lining to ensure uniform and thorough melting of the metal.


3. As
the air passes through the tuyeres, the temperature near the tuyere openings
will therefore be comparatively lower and consequently the molten iron and slag
will have a tendency to solidify near these openings and block them. This should  be prevented by frequent poking and
removal of these materials by means of a poking rod through the tuyeres.


4. Amount of air supply should be
properly controlled. An excess amount of air will always result in waste of
fuel and lowering of temperature inside and a lesser
amount, than required, will cause
incomplete combustion
of fuel which is undesirable.


5. Tap hole must be properly
closed by means of a well suitable plugging means. Clay mixed with an equal
amount of coal dust forms a very suitable mixture for plugging up the tap hole.


6. In closing the tap-hole, precaution
must be taken to press the plug downwards in the hole so that the splash of the
molten metal, during plugging does not fall on  
the hands of the furnace
operator.


7. Molten metal should always be tapped
out well in time before its level rises too high in the well of the cupola furnace. Any delay of tapping molten metal, the slag floating on the surface
of the molten metal, will start flowing into the wind belt through the tuyeres
and air passage will be choked and it will result in severe problem of the jamming of cupola furnace.

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