Pig and cast iron are usually obtained from hematite ore. The main steps involved in the process are:
The ore is crushed to small pieces. It is washed with water to remove impurities like clay, sand etc. The washed ore is then subjected to magnetic field where non-magnetic impurities are separated from magnetic ore particles.
ROASTING OR CALCINATION
The concentrated are is heated strongly in the presence of excess of air in shallow kilns. As a result, the following changes occur.
i) Most of the moisture is removed.
(ii) Sulphur and arsenic are oxidized to their oxides.
S + O2 → SO2
4AS + 3O2 →2AS2O3
iii) Iron carbonate decomposes as follows
FeCO3 → FeO + CO2
iv) Ferrous oxide is oxidised to ferric oxide. Thus the loss of iron is avoided as FeO forms a slag with sand,
4FeO + O2→2Fe2O3
FeO + SiO2 → FeSiO3
v) The mass becomes porous which is more suitable to reduction to metallic iron.
3. SMELTING IN THE BLAST FURNACE
The purpose of blast furnace is to chemically reduce and physically convert iron oxides into liquid iron called “hot metal”. The roasted ore is reduced in the blast furnace. The blast furnace is a huge, steel stack lined with refractory bricks. The furnace is about 100 feet high and 25 feet in diameter. The charge consisting of roasted ore, coke and lime stone is added into the furnace by means of double cup and cone arrangement.
The raw materials require 6 to 8 hours to descend to the bottom of the furnace where they become the final product of liquid slag and liquid iron. These liquid products are drained from the furnace at regular intervals. The hot air that was blown into the bottom of the furnace ascends to the top in 6 to 8 seconds after going through numerous chemical reactions. Once a blast furnace is started it will continuously run for four to ten years with only short stops to perform planned maintenance. The following reactions take place in the different Danes of furnace.
Table:6.2. Composition of pig iron.
= 93.5- 95.0%
= 0.30 – 0.90%
= 0.025 – 0.050%
= 0.55 -0.75%
= 0.03- 0.09%
= 0.02- 0.06%
= 4.1- 4.4%
When solid pig iron is melted in a vertical furnace called cupola in the presence of coke and lime, it changes to a molten mass. This molten mass is poured into moulds of desired shape and cooled. The iron thus obtained is called cast iron. It has the following composition;
Fe = 92% C = 4.5% Sn = 1% P = 1.5%
S = 0.25%.
Cast iron is very hard and brittle as it contains iron carbide. It is used for making articles. Such as stones railway sleepers, gutter lids, toys etc. A large quantity of cast iron is used in the manufacture of wrought iron and steel.