PROCESSING OF METALS

Many metals do not naturally exist in a free state in the ground. They occur in association with other substances collectively referred to as impurities. The combination of these impurities and the metal is referred to as the ore. It is this ore that is mined and processed so as to extract the required metal. Sometimes two or more metals exist together in the ore.

 

A) Processing ferrous metals

Any metal which contains iron in it is a ferrous metal. After iron ore is dug up from the ground, it is transferred to a blast furnace and smelted. Smelting involves heating the ore until it turns to liquid. Unwanted elements such as silicon, manganese and others are extracted from the ore in this process. Thus, the blast furnace simply is the device in which the ore is smelted and undesirable elements burnt off. Before smelting, coke (a form of coal) and limestone are added to the iron ore.

The mixture of iron ore, coke and limestone is called charge. Smelting is achieved by forcing hot air into the blast furnace. The unwanted elements and other impurities separate from the molten iron to form what is known as slag. This slag is removed. The molten iron is then transferred from the blast furnace and cooled, forming pig iron.

 

The amount of carbon present in pig iron can be as high as 5%. This carbon content is quite high for most purposes of fabrication. Because of this relatively high carbon content of pig iron, it is very hard and difficult to be worked with tools. Consequently, pig iron is not very useful in technology. In order to render it useable, pig iron is converted to other forms of iron through the reduction of the percentage of carbon, which it contains. These forms of iron are discussed below:

 

1) Cast Iron

Cast iron is obtained when pig iron is put in a type of furnace called a cupola furnace. In this furnace, the carbon content is reduced to about 3.5%. The molten iron is then poured from the furnace into special containers called moulds, so as to solidify. This technique of pouring molten metal into moulds to solidify into a desired shape is called casting, hence the name of the iron, cast iron.

Cast iron is slightly more workable than pig iron. The Various types are ductile cast iron, malleable cast iron, grey cast iron and white cast iron. The latter two take their names from colours visible on their broken edges. Grey cast iron is more brittle than white cast iron.

 

2) Steel

Steel is what is obtained when the carbon content of pig iron is reduced to 1.5% or less. Steels are named according to their carbon contents. Thus, the different types are as follows:

• Low carbon steel (also called mild steel: This contains about 0.3% or less of carbon.
• Medium carbon steel: This contains from 0.3% to about 0.5% carbon.
• High carbon steel: This has between 0.6% and 1.5% carbon.

The above listed steels are solely made up of carbon and iron, as a result, they are called plain carbon steels. They are produced using the Bessemer converter or the Open-hearth furnace. The Bessemer converter is a big, sturdy specially designed metal tank in which molten pig iron is poured. Hot air is forced into the Bessemer converter and this burns out the excess carbon contained in the pig iron.

The open-hearth furnace makes use of solid pig iron, and this is melted together with steel scraps. Molten pig iron is later added to this mixture. Hot air forced into the furnace burns out unwanted carbon.

 

3) Alloy Steels

All steels are alloys since they contain both iron and carbon. However, in a technical sense, the term alloy steel is used only for those steels, which contain other elements in addition to iron and carbon. Therefore, alloy steel is a combination of plain carbon steel and one or more other elements. The electric furnace is used for the production of alloy steels and other special purpose steels. The advantage of an electric furnace over a Bessemer converter and an Open-hearth furnace is that it allows greater control to be exercised over the quality of steel to be produced.

In order to produce alloy steel, plain carbon steel together with the other elements needed are placed in the electric furnace and subsequently heated. All undesirable substances most notably sulphur are burnt out of the molten mixture. This molten mixture is then poured into special container called moulds and then cooled to solidify. Some examples of alloy steel are given below:

• High-tensile steels or nickel-chrome alloys: These contain iron, carbon, nickel and chromium. They are used for making machine parts, which require high tensile strength.
• High-speed steels: These are made up of iron, carbon, tungsten and chromium. Certain forms contain cobalt and vanadium. High speed steels maintain the same degree of sharpness of their cutting edges even when hot. Hence, they are much used as cutting edges in machines where friction causes excessive heat production due to continuous cutting.
• Stainless Steels: These essentially are made up of iron, carbon, nickel or chromium. Nickel and chromium are principally used as coating for the metal so as to make it resistant to corrosion. Stainless steels are employed in parts where staining is likely to occur if other steels are used. It should be noted that different types of steels may contain the same elements but in different proportions.

 

B) Processing Non-ferrous Metals

The basic processes used in processing ferrous metals are also employed in the processing of non-ferrous metals. Some non-ferrous metals are copper, aluminium, tin, gold, and tungsten. The ore is loaded into a furnace and melted. The desired metal in a molten state then separates from the mixture. The molten metal is then poured into containers so as to solidify. When done on a large scale, a reverberating furnace is used. When done on a much smaller scale as in the case of neighbourhood smiths, an earth furnace is used. Both types of furnaces are built of clay or other suitable earthen material. The reverberating furnace is further reinforced with steel, on its outer walls.