Purification of Metals, Chemistry tutorial

Purification of Metals:

The metals obtained via metallurgical methods still contain certain impurities that persist from the ore or are derived from the flux or the fuel used. In order to obtain pure metal, further purification or refining is essential. There are various methods available for purification, based on the nature of the metal and the kind of impurities present. Some of the refining processes are designed to recover precious metal impurities as well, like, gold, silver and platinum. These methods of refining are as illustrated below:

Liquation:

Crude tin, lead and bismuth are purified via liquation. In this process, the impure metal is positioned at the top of a sloping hearth maintained at a temperature slightly above the melting point of the metal. The metal melts and flows down the inclined hearth to a well leaving behind the solid impurities.

Distillation:

Metals having low boiling points, like zinc, cadmium and mercury can be purified through distillation. The distillation is generally carried out under reduced pressure to allow boiling of the metal at lower temperature.

Electrolysis:

In electro refining, the impure metal is considered as the anode and a strip of pure metal coated by a thin layer of graphite is made the cathode in an electrolytic cell. The electrolyte is an aqueous solution of a salt of the metal. On electrolysis, the impure metal from the anode goes to solution and metal ions are reduced and get deposited on the cathode. Just weakly electropositive metals such as copper, tin and lead that are readily oxidized at the anode and reduced at cathode can be purified in this way. A general reaction can be represented as follows:

M(impure) → Mn+ (aq) + e, at anode

Mn+ (aq) + ne → M (pure), at anode

The other impurities in the metal settle down as anode mud or remain dissolved in the solution. In case of electrolytic refining of copper, an impure copper rod formed the anode, pure copper strip the cathode and copper sulphate solution the electrolyte (figure shown below). The following electrode reactions occur:

663_Purification of copper by electrolysis.jpg

Fig: Purification of copper by electrolysis

Cu(s) → Cu2+ (aq) + 2e-, at anode

Cu2+ (aq) + 2e → Cu (s), at cathode

Therefore, 99.95% pure copper is obtained in this procedure. The more reactive metals like iron that are present in the crude copper, are as well oxidized at anode and pass to the solution. The voltage is so adjusted that they are not reduced at cathode and therefore remain in solution. The less reactive metals like silver, gold and platinum if present, are not oxidized. As the copper anode dissolves, they fall to the bottom of the cell from where they are recovered as a precious anode mud.

Zone Refining:

This process is employed to get metals of very high purity. The fundamental principle involved in this process is alike to the fractional crystallization. A small heater is utilized to heat a bar of the impure metal. The heater melts a small band of metal as it is slowly moved all along the rod. As small bands of metal are therefore melted sequentially, the pure metal crystallizes out of the melt, whereas impurities pass to the adjacent molten zone. The impurities therefore collect at the end of the bar. This end can be cut off and eliminated. High grade germanium and silicon are obtained via purifying them by zone refining (figure shown below).

2391_zone refining.jpg

Fig: zone refining

Parke Process:

Parke method for refining the lead, which is as well a concentration method for silver, relies on the selective dissolution of silver in molten zinc. A small amount of zinc, 1-2% is added to molten lead that comprise silver as an impurity. Silver is much more soluble in zinc than in lead; lead and zinc are insoluble in one other. Therefore, most of the silver concentrates in zinc that comes to the top of molten lead. The zinc layer solidifies first on cooling; it is eliminated and silver is obtained via distilling off zinc, which is collected and utilized over and over again.

Van Arkel de Boer Process:

This process is mainly based on the thermal decomposition of a volatile metal compound such as an iodide. In this process, first a metal iodide is made by direct reaction of iodine and the metal to be purified at a temperature of around 475 to 675 K in an evacuated vessel. The vapors of metal iodide, therefore formed are heated strongly on a tungsten or tantalum filament at around 1300 to 1000 K. The metal iodide decomposes to yield the pure metal, as in the case of zirconium.

Zr (s) + 2I2 (g) → (at 473 - 673K) → ZrI4 (g) → (at 1300 - 1800K) → Zr (s) + 2I2 (g)

Titanium is as well purified by this process. The impure metal is heated with iodine and TiLt therefore formed is decomposed via heating at 1700 K over tungsten filament:

Ti (s) + 2I2 (g) → TiI4 (g) → (1700K) → Ti(s) + 2I2 (g)

The regenerated iodine is utilized over and over again. This method is extremely expensive and is used for the preparation of limited amounts of extremely pure metals for special uses.

Mond Process:

Some of the metals are purified by obtaining their volatile carbonyl compounds which on heating strongly decompose to provide pure metal. Purification of nickel is completed by this method. Impure nickel is reacted by carbon monoxide at 325 K to provide volatile nickel carbonyl leaving solid impurities behind. Pure nickel is obtained via heating nickel carbonyl at 450 to 475 K:

Ni (s) + 4CO (g) → (at 325K) → Ni (CO)4 (g) → (at 450 - 475K) → Ni (s) + 4CO (g)

Isolation of some important transition metals:

Titanium:

Titanium, which constitutes 0.63% of the earth's crust, is the 9th most plentiful element. Titanium has numerous useful properties. It is as strong as steel, however only around 60% as dense as steel. It is as well highly resistant to corrosion. Main uses of titanium are in aircraft industry for the production of both engines and airframes. It is as well broadly employed in chemical processing and marine equipment.

The two most significant ores of titanium arc rutile, TiO2 and ilmenite, FeO.TiO2, India possesses big reserves of ilmenite in beach sands of south and south-west coasts whereas deposits of rutile are limited. Titanium is extracted from these ores through Kroll process. In this method, rutile or ilmenite ore is first heated by carbon at 1200 K in a current of chlorine gas:

TiO2 + C + 2Cl2 → (at 1200K) → TiCI4 + CO2

2FeO.TiO2 + 6C + 7Cl2 → (at 1200K) → TiCl4 + 2FeCl3 + 6CO

Titanium tetrachloride is separated from FeCl3 and other impurities through fractional distillation. As titanium reacts with nitrogen at high temperature, TiCl4 is reduced with molten magnesium in the atmosphere of argon:

TiCl4 + 2Mg → (at 1225 -1400k) → Ti + 2MgCl2

TiCl4 + 4Na → (at 1225 -1400k) → Ti + 4NaCl

Magnesium chloride and surplus of magnesium are eliminated by leaching by water and dilute hydrochloric acid leaving behind the titanium sponge. Titanium sponge after grinding and cleaning by aqua regia is melted under argon or vacuum and cast to ingots. In place of magnesium, sodium can as well utilize as a reducing agent in this method.

Chromium:

Chromite, FeO.Cr2O3, is the only commercially significant ore of chromium. In order to isolate the chromium, the ore is finely powdered and concentrated via gravity procedure. The concentrated ore is mixed by an excess of sodium carbonate and roasted in the presence of air in such a way that Cr2O3 present in the ore is transformed to sodium chromate:

4FeO.Cr2O3 + 8Na2CO3 + 7O2 → 8Na2CrO4 + 2Fe2O3 + 8CO2

The roasted mass is then extracted with water; Na2CrO4 goes to the solution leaving behind the insoluble Fe2O3. The solution is treated by sulphuric acid to transform the chromate into dichromate:

2Na2CrO4 + H2SO4 → Na2Cr2O7 + Na2SO4 + H2O

The solution is then concentrated whenever the less soluble Na2SO4 crystallizes out leaving more soluble Na2Cr2O7 in solution. The solution is further concentrated to obtain crystals of Na2Cr2O7, which are heated with carbon to get chromium oxide:

Na2Cr2O7 + 2C → Cr2O3 + Na2CO3 + CO

Chromium oxide is then reduced with aluminium via aluminothermic method or by heating with a computed quantity of silicon in the presence of calcium oxide that forms a slag of calcium silicate with silica:

Cr2O3 + 2 Al → 2Cr + Al2O3

2Cr2O3 + 3Si + 3CaO → 4Cr + 3CaSiO3

Iron:

Iron is the second richest metal, aluminium being the first, comprising 5.1% of the earth's crust. Haematite, Fe2O3, having 60-64% of iron is the most significant ore of iron. Other ores of iron are magnetite, Fe3O4, limonite, Fe2O3.3H2O and siderite, FeCO3. Iron pyrites, FeS2 that takes place in abundance is not employed as a source of iron due to the difficulty in removing sulphur.

Iron ores are of high grade. Thus, usually the ores are not concentrated. The ore is crushed into process, around 2 to 10 cm in size and then washed with water to take away clay, sand and many more. The ore is then calcined or roasted in air if moisture is driven out, carbonates are decomposed and organic matter, sulphur and arsenic are burnt off. Ferrous oxide is as well transformed into ferric oxide throughout this method:

Fe2O3.3H2O → Fe2CO3 + 3H2

2FeCO3 → 2FeO + CO2

4FeO3 + O2 → 2Fe2O3

In the iron ore, the main impurities are of silica and alumina. To take away these, lime stone is utilized as a flux. The calcined or roasted ore is then smelted, that is, reduced with carbon, in the presence of lime stone flux. Smelting is completed in a blast furnace as shown in the figure given below. A modern blast furnace is a tall vertical furnace of around 30 metre high and 9-10 meters in diameter at its broadest part. It is designed to take care of volume changes, to allow sufficient time for the chemical reactions to be fulfilled and to facilitate separation of slag from the molten metal. The outer structure of furnace is made up from thick steel plates which are lined with fireclay refractory. The furnace at its base is given with (a) small pipes known as tuyeres via which hot air is blown, (b) a tapping hole via which molten metal can be withdrawn and (c) a slag hole via which slag flows out. At the top, the furnace is given with a cup and cone arrangement for introducing charge that is beginning materials in the furnace.

The calcined or roasted ore mixed by coke and lime stone is fed into the furnace. The furnace is lit and a blast of hoi air is passed via the tuyeres. Coke hunts in the bottom of the furnace to form CO2 discharging huge amount of heat that increases the temperature to 2200 K:

C + O2 → CO2; ΔH = 394 kJ

As the hot gases increase, CO2 reacts with extra coke to form CO which is the active reducing agent. As this reaction is endothermic, temperature reduces to 1600 K:

CO2 + C → 2CO; ΔH = 173kJ

1341_Blast furnace.jpg

Fig: Blast furnace

The reduction of iron oxide occurs in a series of steps. At the top of the furnace, where temperature is approximately 800 K, Fe2O3 is reduced to Fe3O4

3Fe2O3 + CO → 2Fe3O4 + CO2

Ongoing downward, where temperature is approximately 1100 K. Fe3O4 is reduced to FeO

Fe3O4 + CO → 3FeO + CO2

Close to the middle of the furnace at a temperature of approximately 1300 K, FeO is reduced to the iron:

FeO + CO → Fe + CO2

In this area, lime stone decomposes to form CaO and CO. CaO then reacts by SiO2, Al2O3 and P4O10 to form the liquid slag:

CaCO3 → CaO + CO2

CaO + SiO2 → CaSiO3

CaO + Al2O3 → Ca(AlO2)2

6CaO + P4O10 → 2Ca3(PO4)2

Iron produced is in the solid state up to this temperature. It is porous and is termed as spongy iron. However as the spongy iron drops down further via the hotter parts of the finance, where temperature is approximately 1600 K, it melts, absorbs some carbon, phosphorus, sulphur, silicon and manganese, and collects at the bottom of the furnace. Slag being lighter floats on top of the molten iron. The molten iron withdrawn from the furnace is known as pig iron. The molten pig iron can be poured into moulds to produce cast iron.

The composition of pig iron or cast iron differs broadly, however on an average it consists of 92-95% Fe, 3-4.5% C, 1-4% Si, 0.1-2% P, 0.2-1.5% Mn and 0.05-0.1% S. Cast iron melts at 1473 K. Because of the presence of impurities, cast iron is hard and brittle. It is so hard that it can't be welded and it is so brittle that it can't be shaped into articles via hammering, pressing or rolling. Cast iron is quite not expensive and is utilized for making drain pipes, fire-grates, railway sleepers, radiators, lamp posts and so on, where economy is more significant than strength.

Wrought iron is the purest form of iron having 0.10-0.25% carbon and impurities of Si, P, S and Mn not more than 0.3%. This is made up by heating pig iron in a reverberatory furnace lined with haematite. Haematite oxidizes C, Si, P, S and Mn to CO, SiO2, P2O5, SO2 and MnO, correspondingly. Therefore MnO combines with SiO2 to form a slag of MnSiO3 and so does Fe2O3 with P2O5 to provide a slag of FePO4. Wrought iron is soft and malleable however very tough. This can be simply welded and forged. Its melting point is around 1773 K and is resistant to corrosion. It is employed to make anchors, wires, bolls, chains and agricultural implements. Owing to its high cost it has been swapped by steel.

Nickel:

Nickel is the twenty-second richest element in the earth's crust. Nickel occurs in the combination with sulphur arsenic and antimony. Significant ores of nickel are as:

a) Pentlandite: A nickel and iron sulphide, (Ni, Fe)9S8, having around1.5% nickel. It is found mostly in Sudbury, Canada. This is as well known as Sudbury ore.

b) Garnierite: A double silicate of nickel and magnesium, (Ni, Mg)6S14O10(OH)8 having upto 8% nickel.

c) Pyrrhotite: An iron mineral, FenSn+1, as well consists of 3-5% nickel.

d) Kupfer nickel, NiAs.

e) Nickel glance, NiAsS.

Pentlandite is the main principal ore of nickel. The metallurgy of nickel comprises several complex steps however the fundamental principle is to change nickel sulphide to nickel oxide and then reduce it by water gas to get the metal.

Pentlandite ore is crushed and subjected to froth flotation method. The concentrated ore that comprises of FeS, NiS and CuS, is roasted in the excess of air. The FeS is transformed to FeO, while NiS and CuS remain unchanged. The uncombined sulphur, if present, is as well oxidized to SO2:

2FeS + 3O2 → 2FeO + 2SO2

S + O2 → SO2

The roasted mass; is mixed by silica, lime stone and coke and is smelted in the blast furnace. Therefore FeO combines with SiO2 to provide FeSiO3 and CaO made by the decomposition of lime stone reacts with surplus of SiO2 to form CaSiO3. CaSiO3 and FeSiO3 both form slag that being lighter floats on the molten mass:

CaCO3 → CaO + CO2 

CaO + SiO2 → CaSiO3

FeO + SiO2 → FeSiO3

The slag is constantly removed. Molten mass now consists of impure sulphides of nickel and copper and a few quantity of iron sulphide. It is known as matte. The matte is heated in the Bessemer converter that is fitted by tuyeres for passing hot air in a controlled manner. The remaining iron sulphide is transformed to iron oxide that is slagged off as FeSiO3. The bessemerised matte comprising of NiS and CuS is roasted again to transform sulphides into oxides:

2Nis + 3O2 → 2NiO + 2SO2

2CuS + 3O2 → 2CuO + 2SO2

The mixture of oxides is treated by sulphuric acid at 350 K, whenever CuO dissolves to give CuSO4, whereas NiO remains unaffected. Residue of NiO is dried and reduced by water gas to provide crude nickel:

2NiO + H2 + CO → 2Ni + H2O + CO2

Crude nickel having iron and copper as impurities is purified through Mond process.

Copper:

Copper is found in both the native and also the combined slate. Native copper is found in USA, Mexico, USSR and China. Native copper is around 99.9% pure; however it is merely a minor source of the metal. In the combined state, copper is mainly found as the sulphide, oxide or carbonate ore. Copper takes place as sulphide in chalcopyrites or copper pyrites, CuFeS2 and in chalcocite or copper glance, Cu2S. The oxide ores of copper are cuprite or Ruby copper, Cu2O and malachite, Cu(OH)2.CuCO3. Copper pyrites is the major ore of copper. Workable deposits of copper ore take place in Khetri copper belt in Rajasthan and Mosabani and Rakha mines in Bihar.

For extraction of copper, the sulphide ore is concentrated through froth flotation procedure and is then roasted in air whenever some sulphur is eliminated as SO2:

2CuFeS2 + O2 → Cu2S + 2FeS + SO2

The mixture of Cu2S and FeS therefore obtained is subjected to smelting with coke and silica in the blast furnace. FeS is changed to FeO that reacts by SiO2 and is slagged off as FeSiC3:

2FeS + 3O2 → 2FeO + 2SO2

2FeO + SiO2 → FeSiO3

The molten mixture of Cu2S and remaining FeS is termed as matte. This is transferred to a Bessemer converter (figure shown below) and a blast of hot air mixed by silica is blown via the molten mass. As an outcome, residual FeS is transformed into a slag of FeSiO3 and Cu2S is reduced to copper. The supply of air is so adjusted that around two thirds of Cu2S is transformed into Cu2O. The two then react altogether to give copper metal. The additional step comprising reduction with carbon is therefore avoided:

693_Bessemer converter.jpg

Fig: Bessemer converter

2FeS + 3O2 → 2FeO + 2SO2

FeO + SiO2 → FeSiO3

2Cu2S + 3O2 → 2Cu2O + 2SO2

2Cu2O + Cu2S → 6Cu + SO2

The copper therefore obtained is known as blister copper as bubbles of escaping SO2 throughout cooling give it a blister like appearance. Blister copper is around 99.0% pure and is employed as such for many purposes.

Alloys:

Metals encompass a property of combining by other metals to form alloys. An alloy might be stated as a solid which is made by a combination of two or more metallic elements, however it itself consists of metallic properties. Most of the alloys are solid solutions. For illustration, brass an alloy of copper and zinc is a solid solution of zinc in copper. In brass some of the copper atoms of face-centered cubic lattice are arbitrarily substituted via zinc atoms. Likewise, bronze an alloy of copper and tin is a solid solution of tin in copper. However not all alloys are solid solutions. Some of the alloys like bismuth-cadmium alloys are heterogeneous mixtures having tiny crystals of the constituent metals. Others like MgCu2, are intermetallic compounds which contain metals combined in the definite proportions.

The main aim of making alloys is to impart some desirable properties to a metal. For illustration, gold is too soft for making jewellery. Thus, to make it hard, it is alloyed with copper. Solder, an alloy of tin and lead, comprises of a melting point lower than that of both of its constituents. Pure iron is soft, ductile and it is simply corroded. Stainless steel, an alloy of iron, chromium, nickel and carbon is hard, tough and highly resistant to corrosion.

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