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.

Chemistry of Uranium ore processing

Primary uranium minerals, include uraninite and pitchblende (the latter a variety of uraninite). The uranium in these two ores occurs in the form of uranium dioxide, which—owing to oxidation—can vary in exact chemical composition from UO₂ to UO_2.67. Other uranium ores of economic importance are autunite, a hydrated calcium uranyl phosphate; tobernite, a hydrated copper uranyl phosphate; coffinite, a black hydrated uranium silicate; and carnotite, a yellow hydrated potassium uranyl vanadate.

Uranium ores occur in deposits that are both near-surface and very deep (e.g., 300 to 1,200 metres, ). The deep ores sometimes occur in seams as thick as 30 metres. 

Roasted uranium ores are leached of their uranium values by both acidic and alkaline aqueous solutions. For the successful operation of all leaching systems, uranium must either be initially present in the more stable hexavalent state or be oxidized to that state in the leaching process.

Acid leaching is commonly performed by agitating an ore-leach mixture for 4 to as long as 48 hours at ambient temperature. Except in special circumstances, sulfuric acid is the leachant used; it is supplied in amounts sufficient to obtain a final leach liquor at about pH 1.5. Sulfuric acid leaching circuits commonly employ either manganese dioxide or chlorate ion to oxidize the tetravalent uranium ion (U⁴⁺) to the hexavalent uranyl ion (UO₂²⁺). Typically, about 5 kilograms (11 pounds) of manganese dioxide or 1.5 kilograms of sodium chlorate per ton suffice to oxidize tetravalent uranium. In any case, the oxidized uranium reacts with the sulfuric acid to form a uranyl sulfate complex anion, [UO₂(SO₄)₃]^4-.

Uranium ores that contain significant amounts of basic minerals such as calcite or dolomite are leached with 0.5 to 1 molar sodium carbonate solutions. Although a variety of reagents has been studied and tested, oxygen is the uranium oxidant of choice. Typically, candidate ores are leached in air at atmospheric pressure and at 75° to 80° C (167° to 175° F) for periods that vary with the particular ore. The alkaline leachant reacts with uranium to form a readily soluble uranyl carbonate complex ion, [UO₂(CO₃)₃]^4-.

The complex ions [UO₂(CO₃)₃]^4- and [UO₂(SO₄)₃]^4- can be sorbed from their respective leach solutions by ion-exchange resins. These special resins—characterized by their sorption and elution kinetics, particle size, stability, and hydraulic properties—can be used in a variety of processing equipment—e.g., fixed-bed, moving-bed, basket resin-in-pulp, and continuous resin-in-pulp. Conventionally, sodium and ammonium chloride or nitrate solutions are then used to elute the sorbed uranium from the exchange resins.

Uranium can also be removed from acidic ore leach-liquors through solvent extraction. In industrial methods, alkyl phosphoric acids—e.g., di(2-ethylhexyl) phosphoric acid—and secondary and tertiary alkyl amines are the usual solvents. As a general rule, solvent extraction is preferred over ion-exchange methods for acidic leachates containing more than one gram of uranium per litre. Solvent extraction is not useful for recovery of uranium from carbonate leach liquors, however.

Prior to final purification, uranium present in acidic solutions produced by the ion-exchange or solvent-extraction processes described above, as well as uranium dissolved in carbonate ore leach solutions, is typically precipitated as a polyuranate. From acidic solutions, uranium is precipitated by addition of neutralizers such as sodium hydroxide, magnesia, or (most commonly) aqueous ammonia. Uranium is usually precipitated as ammonium diuranate, (NH₄)₂U₂O₇. From alkaline solutions, uranium is most often precipitated by addition of sodium hydroxide, producing an insoluble sodium diuranate, Na₂U₂O₇. It can also be precipitated by acidification (to remove carbon dioxide) and then neutralization (to remove the uranium) or by reduction to less soluble tetravalent uranium. In all cases, the final uranium precipitate, commonly referred to as yellow cake, is dried. In some cases—e.g., with ammonium diuranate—the yellow cake is ignited, driving off the ammonia and oxidizing the uranium to produce uranium trioxide (UO₃) or the more complex triuranium octoxide (U₃O₈). In all cases, the final product is shipped to a central uranium-purification facility.

Uranium meeting nuclear-grade specifications is usually obtained from yellow cake through a tributyl phosphate solvent-extraction process. First, the yellow cake is dissolved in nitric acid to prepare a feed solution. Uranium is then selectively extracted from this acid feed by tributyl phosphate diluted with kerosene or some other suitable hydrocarbon mixture. Finally, uranium is stripped from the tributyl phosphate extract into acidified water to yield a highly purified uranyl nitrate, UO₂(NO₃)₂.

Uranyl nitrate is produced by the ore-processing operations described above as well as by solvent extraction from irradiated nuclear reactor fuel . In either case, it is an excellent starting material for conversion to uranium metal or for eventual enrichment of the uranium-235 content. Both of these routes conventionally begin with calcining the nitrate to UO₃ and then reducing the trioxide with hydrogen to uranium dioxide (UO₂). Subsequent treatment of powdered UO₂ with gaseous hydrogen fluoride (HF) at 550° C (1,025° F) produces uranium tetrafluoride (UF₄) and water vapour, as in the following reaction:

UO2 + 4HF---> UF4 + 2H2O

This hydrofluorination process is usually performed in a fluidized-bed reactor

Conversion to uranium metal is accomplished through the Ames process, in which UF₄ is reduced with magnesium (Mg) at temperatures exceeding 1,300° C (2,375° F). (In an often-used modification of the Ames process, calcium metal is substituted for magnesium.) Because the vapour pressureof magnesium metal is very high at 1,300° C, the reduction reaction is performed in a refractory-lined, sealed container, or “bomb.” Bombs charged with granular UF₄ and finely divided Mg (the latter in excess) are heated to 500° to 700° C (930° to 1,300° F), at which point an exothermic  reaction occurs. The heat of reaction is sufficient to liquefy the conversion contents of the bomb, which are essentially metallic uranium and a slag of magnesium fluoride (MgF₂):

UF4 +2Mg--> U + 2MgF2

When the bomb is cooled to ambient temperature, the massive uranium metal obtained is, despite its hydrogen content, the best-quality uranium metal available commercially and is well suited for rolling into fuel shapes for nuclear reactors.

Uranium tetrafluoride can also be fluorinated at 350° C (660° F) with fluorine gas to volatile uranium hexafluoride (UF₆), which is fractionally distilled to produce high-purity feedstock for isotopic enrichment. Any of several methods—gaseous diffusion, gas centrifugation, liquid thermal diffusion—can be employed to separate and concentrate the fissile uranium-235 isotope into several grades, from low-enrichment (2 to 3 percent uranium-235) to fully enriched (97 to 99 percent uranium-235). Low-enrichment uranium is typically used as fuel for light-water nuclear reactors.

After enrichment, UF₆ is reacted in the gaseous state with water vapour to yield hydrated uranyl fluoride (UO₂F₂ · H₂O). Hydrogen reduction of the uranyl fluoride produces powdered UO₂, which can be used to prepare ceramic nuclear reactor fuel. In addition, UO₂ obtained from enriched UF₆ or from UF₆ that has been depleted of its uranium-235 content can be hydrofluorinated to yield UF₄, and the tetrafluoride can then be converted to uranium metal in the Ames process described above.