OPERATIONAL PROCESS OF CUPOLA FURNACE

 The different steps involved in cupola furnace operation process are : 

 1. Preparation Of Cupola . 

 The First Operation in preparing a cupola is to clean out the slag and refuse on the lining and around the tuyeres from the previous run. Any bad spots or broken bricks are repaired with a daubing mixture of fire clay and silica sand or ganister. The preparation of the sand bottom in cupola is begun as soon as the patching of the lining has been completed. The bottom doors are raised and held in this position by metal props. The bottom sand is introduced through the charging door and is rammed well around the lining and across the intersection of the bottom doors. This layer of sand is built up to a height of 100 to 200 mm above the cast iron door. The surface of the sand bottom is sloped from all direction towards the tapping hole so that the molten metal can be drained completely from the cupola at any time. An opening about 35 mm diameter is provided for the removal of the slag, and a tap hole is formed around a wooden pattern about 20mm in diameter. The cupola should be thoroughly dried before firing. 

2. Firing the Cupola 

In firing a cupola, a fire of  kindling wood is ignited on the sand bottom. This should be done 2.5 to 3 hours before the molten metal is required. On the top of the kindled wood, a bed of coke is built. When the wood is burning well coke is dumped into the well from above in several portions making sure that the coke begins to burn too. The coke is added to a level slightly above the tuyeres and the air blast is turned on at a lower than normal blowing rate to ignite the coke. As soon as red spots begin to show over top of the fuel bed, additional coke is introduced into the cupola to reach a height of 700 to 800 mm above the upper row of  tuyeres. The coke bed must be thoroughly hot before it is finished off to its final height. The height of the coke bed is determined by using a measuring rod which has been prepared to indicate the distance from the sill of the charging door to the top of the coke bed. The layer of coke resting on the sand bottom before beginning the heat is called bed charge. The amount of  coke in the bed is dependent upon the pressure of the air supplied to the cupola. The height of the bed charge or coke - bed is very important to the cupola operation ; it affects the temperature, melting rate, and chemical composition. Other things being equal, low equal, a low bed will yield cooler metal than one which than one which is high. 

3. Charging the Cupola 

As soon as the coke bed is built up to the correct height and ignited uniformly throughout, alternate layers of pig iron, coke and flux (limestone) are charged from the charging door until the cupola is full. Suitable scrap is also added along with the pig iron , to control the chemical com-position of the iron produced. The proportion of this scrap is ordinarily from 25 to 50 percent of the total weight of the metal poured. When considerable steel scrap is used along with pig iron., a small amount, say from 2 to 4 percent of ferro-manganese is used as a deoxidizer. The weight of the metal charge should be from 10 to 15 percent of hourly out-put of the cupola. The Object of adding flux is to remove impurities in the iron, and to protect the iron from oxidation, to reduce the melting point of the reduce the melting point of the slag, and to increase its fluidity for easy disposal. Besides limestone, fluorspar and soda ash are also sometimes used as fluxing material. The quantity of limestone required may be 30 to 40 kg per tonne of iron melted or 25 percent by weight of coke charged. The ratio between the metal melted and coke charged depends on a great number of factors. So it is not possible to give definite recommendations for this ratio which can be achieved on different classes on work. 

RATIO OF METAL MELTED TO COKE CHARGED 

75% steel scrap                                                                                          - 12 :1 to 15:1 

High Phosphoric Iron (Heavy Castings)                                                    - 11:1 to 14 :1 

High Phosphoric Iron (Light Castings)                                                      - 10:1 to 12 :1

Medium Phosphoric Iron (engineering castings)                                       - 8:1 to 10:1

Low- Phosphoric Iron for High duty and automobile castings                  - 6:1 to 7:1

Charges containing 50-75 percent steel scrap                                            - 6:1 - 8:1 

4. Soaking Of Iron : 

After the cupola is fully charged up to the charging door. The charge should soak in the heat for about 45 minutes. The charge gets slowly heated since the air blast is kept at a lower than normal blowing rate (practically kept shut) during this time. This causes the iron to get soaked. 

5. Air Blast :

At the end of  the soaking period, full blast is turned on. Before turning on the blast. the tuyere openings and the tapping hole are kept closed. After the blast has been on for a few minutes, say about to minutes. molten metal starts accumulating in the hearth. When the metal in the cupola starts melting, the rate of charging should be equal to the rate of melting. so that the furnace is kept full throughout the heat. At the end of melt the charging is stopped but the blast is kept on until all the metal has melted.

6. Tapping and Slagging : 

The first tapping can be made 40 to 50 minutes after the full air blast is turned on. During this period, sufficient metal is collected in hearth above the sand bed. When slag  accumulates in the well, the slag hole is opened and the slag is run off, preferably into a bogie for easy removal. Molten metal is collected in ladles and is carried to the molds for pouring. The same procedure is repeated until all the metal is melted and operation is over. 

7. Closing the cupola :

When the operation is over, the blast is shut off and the prop under the bottom door is knocked down so that the bottom plates swing open. This enables the cupola remains to drop on to the floor or into a bucket. They are then quenched and removed from underneath the cupola. 

Generally, cupolas are run continuously as are blast furnaces, but are worked only for such periods as may be required. At many foundries the melting period does not exceed 4 hours, but cupolas may be operated continuously for 10 hours or more. 

EFFICIENCY OF CUPOLA :

Thermal or Melting Efficiency of a cupola in percent is expressed as :

( Heat Utilised in Preheating, melting and superheating / Potential Heat in coke + Heat from oxidation of Fe,Si,Mn + Heat in air blast)  * 100 

The efficiency of a cupola varies from 30 to 50 percent depending on 

1. Coke rate or Coke ratio expressed as the inverse of metal-fuel ratio in percentage.

2. Blast rate, and

3. Mean Coke Size.

AIR REQUIREMENTS OF CUPOLA :

For Complete Combustion of the fuel in the furnace, about 8.4 cu m of air is required per kg of coke charged 10 :1, which is considered a satisfactory figure, the coke required per ton of Iron will be 1000 / 10 Kg, ie. 100 Kg. Thus, the volume of air required per ton of melted is 

8.4 * 100 = 840 cu. m 

To allow for leakage, etc., the air supplied is generally a little in excess i.e. about 900 cu. m per ton of iron.