Metallurgy

The seven Metals of Antiquity were

1. Gold (ca) 6000 BC
2. Copper,(ca) 4200 BC
3. Silver,(ca) 4000 BC
4. Lead, (ca) 3500 BC
5. Tin, (ca) 1750 BC
6. Iron,smelted, (ca) 1500 BC
7. Mercury, (ca) 750 BC

Out of the Metals of Antiquity, five metals, gold, silver, copper, iron and mercury can be found in their native states, but the occurrence of these metals was not abundant. 

There are three different methods for the conversion of metallic oxides into metal.

• Pyro metallurgy: It is a high temperature process in which ore particles undergo reactions to form intermediate compounds for further processing or to get converted into their elemental or metallic state. It involves the heating of the material obtained from roasting or calcination, in the presence of a suitable reducing agent. During the heating process some chemical substance is added which further reacts with gangue at high temperature and is known as flux.
• Hydro metallurgy: The process of dissolving an ore in a suitable reagent followed by the extraction of the metal either by electrolysis or displacement of the metal by a more electropositive metal is called hydro metallurgy. This process uses aqueous solutions to extract metals from their ores. The most common hydro metallurgical process is the extraction of gold and silver.
• Electro metallurgy: Electro metallurgy is the metallurgical processes in which the extraction of metal takes place in an electrolytic cell. The molten metallic salt is taken as the electrolyte in the cell, with suitable electrodes. The molten metallic salt dissociates in pure metal and is collected at the cathode. For example, Dow process is used for the extraction of magnesium from its salt, magnesium chloride, which is the main component of sea water.

MgCl₂ â†’ Mg²⁺ + 2Cl^-

Iron metallurgy

Iron is usually extracted by using oxide ores like

• Haematite (Fe₂O₃)
• Limonite ( Fe₂O₃ .3H₂O)
• Magnetite (Fe₃O₄)
• Siderite (FeCO₃)
• Iron pyrites (FeS)

Out of these ores, haematite is the best ore of iron. Iron metallurgy involves the following steps.

1. Concentration: Since iron ores are heavier than their impurities, hydraulic washing is used for the concentration of the ore. For iron sulphide, the froth-flotation method is a good method for concentration or ore-dressing.
2. Calcination: The oxide ores of iron are heated in the presence of limited oxygen in a reverberatory furnace. This process is used to remove moisture and non-metallic impurities associated with the ore. The ferrous oxide present in the ore is oxidized to ferric oxide.
3. Smelting: The calcined ore is reduced with carbon in a blast furnace at high temperature. The blast furnace is a tall cylindrical furnace made up of steel and lined with fire bricks. It is slightly narrow at the top and wide at center with a narrow bottom. There is a cup and cone arrangement in the furnace which helps to feed the charge from the top without letting any gas from inside escape. The charge consists of ore, coke and limestone in a 8:4:1 ratio. The temperature of the blast furnace is maintained at 1000 K by blowing hot air in the furnace. Different reactions take place at different temperatures.

History of Metallurgy

Smelting of iron oxide with charcoal demanded a high temperature, and, since the melting temperature of iron at 1,540° C (2,800° F) was not attainable then, the product was merely a spongy mass of pasty globules of metal intermingled with a semiliquid slag. This product, later known as bloom, was hardly usable as it stood, but repeated reheating and hot hammering eliminated much of the slag, creating wrought iron, a much better product.

By 1000 bc iron was beginning to be known in central Europe. Its use spread slowly westward; iron making was fairly widespread in Great Britainat the time of the Roman invasion in 55 bc. In Asia iron was also known in ancient times, in China by about 700 bc.

Lead was removed from the silver by cupellation, a process of great antiquity in which the alloy was melted in a shallow porous clay or bone-ash receptacle called a cupel. A stream of air over the molten mass preferentially oxidized the lead. Its oxide was removed partially by skimming the molten surface; the remainder was absorbed into the porous cupel. Silver metal and any gold were retained on the cupel. The lead from the skimmings and discarded cupels was recovered as metal upon heating with charcoal.

Native gold itself often contained quite considerable quantities of silver. These silver-gold alloys, known as electrum, may be separated in a number of ways, but presumably the earliest was by heating in a crucible with common salt. In time and with repetitive treatments, the silver was converted into silver chloride, which passed into the molten slag, leaving a purified gold.

 By 100 bc mercury was known and was produced by heating the sulfide mineral cinnabar and condensing the vapours.

During the 16th century, metallurgical knowledge was recorded and made available. Two books were especially influential. One, by the Italian Vannoccio Biringuccio, was entitled De la pirotechnia (Eng. trans., The Pirotechnia of Vannoccio Biringuccio, 1943). The other, by the German Georgius Agricola, was entitled De re metallica. Biringuccio was essentially a metalworker, and his book dealt with smelting, refining, and assay methods (methods for determining the metal content of ores) and covered metal casting, molding, core making, and the production of such commodities as cannons and cast-iron cannonballs. His was the first methodical description of foundry practice.

Agricola, on the other hand, was a miner and an extractive metallurgist; his book considered prospecting and surveying in addition to smelting, refining, and assay methods. He also described the processes used for crushing and concentrating the ore and then, in some detail, the methods of assaying to determine whether ores were worth mining and extracting. Some of the metallurgical practices he described are retained in principle today.

In England, the gradual exhaustion of timber led first to prohibitions on cutting of wood for charcoal and eventually to the introduction of coke, derived from coal, as a more efficient fuel. Thereafter the iron industry expanded rapidly in Great Britain, which became the greatest iron producer in the world. The crucible process for making steel, introduced in England in 1740, by which bar iron and added materials were placed in clay crucibles heated by coke fires, resulted in the first reliable steel made by a melting process.

it was difficult to keep the carbon content low enough so that the metal remained ductile. This difficulty was overcome by melting high-carbon pig iron from the blast furnace in the puddling process, invented in Great Britain in 1784.

The most important development of the 19th century was the large-scale production of cheap steel. Prior to about 1850, the production of wrought iron by puddling and of steel by crucible melting had been conducted in small-scale units without significant mechanization.

 Another major advance was Henry Bessemer’s process, patented in 1855 and first operated in 1856, in which air was blown through molten pig iron from tuyeres set into the bottom of a pear-shaped vessel called a converter. Heat released by the oxidation of dissolved silicon, manganese, and carbon was enough to raise the temperature above the melting point of the refined metal (which rose as the carbon content was lowered) and thereby maintain it in the liquid state. Very soon Bessemer had tilting converters producing 5 tons in a heat of one hour, compared with four to six hours for 50 kilograms (110 pounds) of crucible steel and two hours for 250 kilograms of puddled iron

Neither the open-hearth furnace nor the Bessemer converter could remove phosphorus from the metal, so that low-phosphorus raw materials had to be used. This restricted their use from areas where phosphoric ores, such as those of the Minette range in Lorraine, were a main European source of iron. The problem was solved by Sidney Gilchrist Thomas, who demonstrated in 1876 that a basic furnace lining consisting of calcined dolomite, instead of an acidic lining of siliceous materials, made it possible to use a high-lime slag to dissolve the phosphates formed by the oxidation of phosphorus in the pig iron. This principle was eventually applied to both open-hearth furnaces and Bessemer converters.

The next significant stage was the introduction of cheap oxygen, made possible by the invention of the Linde-Frankel cycle for the liquefaction and fractional distillation of air. The Linz-Donawitz process, invented in Austria shortly after World War II, used oxygen supplied as a gas from a tonnage oxygen plant, blowing it at supersonic velocity into the top of the molten iron in a converter vessel. As the ultimate development of the Bessemer/Thomas process, oxygen blowing became universally employed in bulk steel production.