How to Manufacture Pig Iron: How Pig Iron is made?

Guide for manufacturing and making Pig Iron.

Following three distinct operations are involved in the manufacturing process of pig-iron:

(1) Dressing

(2) Calcination and roasting

(3) Smelting.

(1) Dressing:

The iron ores as obtained from mines are crushed into pieces of size of 25 mm diameter. This is achieved in the rock crushers of ordinary type.

The crushing of ores helps in two ways:

(i) The ore particles of uniform size are obtained.

(ii) The reducing gases penetrate the ores in a better way.

If ores contain clay, loam and other earthy matter, they are washed in a stream to remove such impurities. The perforated trays may be kept in water to remove pebbles and sand. To work in dry condition, the magnetic separators are used to remove the impurities contained in the iron ores.

(2) Calcination and Roasting:

After the iron ores are dressed, they are calcined and roasted. The calcination consists in heating ores in presence of air so that they are oxidized. The water and carbon dioxide are removed from ores by calcination. The roasting consists of making the ores hot and very dry. It is adopted to dissipate the volatile parts, especially sulphur, by heat. Hence the roasting will not be necessary, if ore is an oxide.

(3) Smelting:

The melting so as to separate metal from ore is known as the smelting. It is carried oi t in a special type of furnace, known as the blast furnace. It is in the form of a vertical steel cylinder. The outer shell of furnace is of steel plate about 30 mm to 40 mm thick and its inside surface is covered with a lining of fire-bricks. Its diameter is about 6 m to 8 m and its height is about 30 m to 36 m.

The blast furnaces continue throughout the world to provide the predominant, if not the sole means of producing pig-iron. Their design has practically remained unchanged. They have however grown to an incredible extent in size and efficiency. The capacity of old blast furnaces was around 15000 kN of pig-iron per day. The capacity of present day modern blast furnaces has gone upto 100000 or more kN of pig-iron per day.

Other Methods Of Pig-Iron Manufacture:

In modern age, the attempts are made to modify it or to adopt new methods of manufacturing pig-iron mainly for two reasons:

(i) The height of modern blast furnace has increased. It requires more capital and labour for its working.

(ii) To most of the nations which are producing pig-iron, the coke, which is used as fuel, is becoming either short or inaccessible.

Following are the other alternative methods of pig-iron manufacture:

(1) Electric reduction furnace

(2) Low shaft blast furnace

(3) Sponge iron process.

Some important features of each method will now be briefly described.

(1) Electric Reduction Furnace:

This type of furnace can be adopted at places where electric power can be economically and cheaply generated. There are various forms of this furnace. In one form, the hearth has a diameter about three to four times of stack diameter. The furnace is heated by electrodes passing through the roof.

Following are the advantages of this furnace:

(i) As electric power is used, the coke is required as the reducing agent only. It therefore results in considerable reduction, about 60% or so, in its consumption.

(ii) The external supply of air is not necessary in this furnace.

(iii) It is flexible in operation and can be economically operated for different outputs.

(iv) It is possible to employ raw materials of low grade in this furnace.

(v) It is possible to manufacture iron with low sulphur content by this furnace.

(vi) The quantity of fluxing material required is less.

(vii) The quantity of slag formed is less.

(viii) The gas produced in this furnace possesses higher calorific value. Its quantity is only about 15% or so than that produced in an ordinary blast furnace.

The only drawback of this furnace is that its initial and maintenance costs are somewhat more. Its adoption however largely depends on the relative costs of coke and electric power.

(2) Low Shaft Blast Furnace:

In this furnace, the blast is made rich in oxygen. It therefore depends on the availability of commercial oxygen at cheap rate. In blast of ordinary furnace, nearly 60% is nitrogen which does not take any active part in chemical reactions. It only acts as a carrier of heat. Hence, if blast is enriched with oxygen, the reduction of iron ores can be carried out in a shorter time and it will also result in reduction of stack height.

Following are the advantages of this furnace:

(i) The gas produced possesses high calorific value.

(ii) This furnace consumes fine ores.

(iii) This furnace consumes or depends on oxygen which can be made available from natural air.

(iv) This furnace permits the use of inferior fuels such as lignite, brown coals, etc.

The output from this furnace is comparatively low and it will require an assured supply of large quantity of bulk oxygen.

(3) Sponge Iron Process:

As the name implies, the spongy iron is obtained by the reduction of lumpy iron ore or iron ore pellets from the oxide form into the metallic state. The iron content of the product so obtained varies from 90% to 94% and the balance represents the traces of gangue and residual iron oxide.

Following are the advantages of this process especially with reference to the prevailing conditions in our country:

(i) Conversion into Steel:

The spongy iron can be conveniently converted into steel by melting in an electric arc furnace or used in cupolas to produce heavy duty irons. It is also the direct feed stock for many processes of iron and steel making. It is also increasingly used for improving the productivity of the existing blast furnaces.

(ii) New Technology:

There has been considerable improvement in the method of producing sponge iron and full advantage of such advanced technology can be taken to enhance the production of sponge iron. For instance, the necessity of making iron ore pellets for subsequent conversion into sponge iron may be eliminated in the new projects.

The process is simple and the control factors of process temperature exhaust gas composition and product quality can be easily managed by the latest techniques.

(iii) Production:

There are large reserves of iron ore and non-coking coal in our country and it appears that the annual production of sponge iron will boost to 1000 million kN by the turn of the 2020.

(iv) Raw Material for Mini Steel Plants:

The shortage of steel scrap has been felt by the mini steel plants spread out in the whole country. The sponge iron is very economical and superior to the ordinary steel scrap used by such plants and hence the growth of spongy iron industry will solve the problem of supplying raw material for mini steel plants. The spongy iron will be an excellent substitute for steel scrap used by electric steel melting furnaces.

(v) Replacement of Pig-Iron:

The spongy iron can also replace pig-iron for large iron foundries where there is facility for hot blast cupolas. The pig-iron thus saved could be supplied to the small scale units.

(vi) Shortage of Coking Coal:

There is an acute shortage of coking coal in our country. The spongy iron process requires non-coking coal which is abundantly available in Andhra Pradesh, Orissa, Bengal and Bihar. The spongy iron industry will thus make the steel making less dependent on coking coal which is scarce and is becoming costly day by day.

The small and medium sized spongy iron plants are working economically even in the most advanced countries. Several processes have been developed in Japan and Germany for the economic exploitation of iron ore into spongy iron.