Isolation of Elements: General Principles and Processes
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Introduction to Metallurgy
Our planet earth is a vast source of elements which are distributed in its crust, water bodies and atmosphere. Out of these elements nearly 80 per cent are metals which occur either in the combined state or in free state (called motive state). Metals occurring in free state are copper, silver, gold and platinum group metals. Not only metals some non-metals also occur in the free state, such as, carbon, sulphur, nitrogen, oxygen and group 18 elements (the noble gases). Apart from metals and non-metals some elements occur as metalloids which show both the properties of metals and non-metals. Metalloid silicon is the backbone of electronic industry and solar cells.
Distribution of elements in the above three categories in shown in the periodic Table (Fig. 6.1, Ref www.wikipedia.org)
Some most abundant elements in the combined form as solutes are:
In earth crust In sea water
O, Si, Al, Fe, Cl-, Na+, SO42-
Ca, Na, K and Mg MG2+, Ca2+ and K+
Some life supporting metals are iron, calcium and magnesium. Chlorophyll, a compound of magnesium, is responsible for the photosynthesis process in releasing oxygen.
General principles of metallurgy
For any application of a metal it has to be produced in a pure state. Here lies the importance or metallurgy. Metallurgy involves the initial purification and concentration of the ore and its subsequent reduction to metal.
Minerals and ores
Naturally occurring sources of metals are called minerals which are generally contaminated with impurities such as days and siliceous matter.
A mineral which is rich in the metal compound and which can be used to extract metal economically is termed as an ore. Thus, all ores are minerals but all minerals are not ores. The impurities which are generally present in ores are called gangue.
Following is the list of some important ores of a few metals:
Photograph of metals
Photograph of metals
Al2O3 . 2H2O
Na3 Al F6
Photograph of metals
Cu Fe S2
CuCO3 . Cu(OH)2
Photograph of metals
From the above list of ores and also from literature (www.wikipedia.org)
You will find that metals generally occur as:
Steps in the extraction of metals
Concentration of ore
Reduction of ore
(Chemical reduction or electrochemical reduction)
Refining of metal
Concentration of ore
Ores are usually contaminated with sand and clay minerals called gangue. Therefore, the first step to obtain the metal from the ore is to remove as much gangue as possible. To do so the ore is crushed to fine particles and subjected to the following methods of concentration:
- Hydraulic washing
- Magnetic Separation
- Froth flotation method
Hydraulic washing is done with an upward flow of water. In this process lighter gangue particles are washed away leaving behind the desired heavy are particles.
This method is based on the different magnetic behavior of gangue particles and the ore. The conclutration of ore is done by putting the dried crushed ore on a conveyor belt moving around a powerful magnetic roller. In this way the ore is separated from the gangue particles. As an example, magnetite is ferromagnetic and on (Fe3O4).
Passing over a magnetic roller it gets carried away and made free from non-magnetic gangue.
This method is designed for the concentration of sulphide ores. The method is based on the relative density of gangue particles and ore particles. Either of two can be made to float on the aqueous surface with air bubbles and be collected. This is achieved by adding some chemical compounds in water. The arrangement is shown is Fig. 6. Air is blown with pressure to create froth which engulphes either the gangue or ore particles. Following compounds:
Frothers: Synthetic detergents, pine, oil, eucalyptus oil or coal tar.
Collectors: X anthates .
These impart water repellent properties to the surface of the ore particles to be floated.
Froth Stabilisers: Cresols and aniline.
Depressants: Sodium cyanide.
The purpose of a depressant is to make ineffective one component of the mixed ore. For example, from a mixture of ZnS (sphalerite) and PbS (galena) ZnS is NaCNwhile heavier PbS particles float on the surface.
Leaching is extration of an active ingradient of the low grade ore. This is done by dissolving the desired component in a suitable chemical solution.[
Leaching of low grade carbonate and oxide ores of copper by dilute sulphuric acid:
CuCO3(S) + H2SO4(aq) → CuSO4(aq) + CO2(g) + H2O(l)
CuO(S) + H2SO4(aq) → CuSO4(aq) + H2O(l)
Leaching of amphoteric arebauxide (Al2O3) with hot aqueous sodium hydroxide when impurities such as Fe2O3 and silicates remain
Al2O3(S) + 2NaOH(aq) + 3H2O(l) -
Na [Al (OH)4] is converted to pure Al2O3 by passing CO2 gas and heating the product Al(OH)3:
Na[Al(OH)4](aq)+CO2(g) → Al(OH)3(S) + NaHCO3(aq)
Al(OH)3(S) Al2O3(S) + 3H2O(g)
Leaching of gold and silver with aqueous sodium cyanide solution in the presence of air:
4 Au(S) + 8NaCN(aq) + O2(g) + 2H2O(l) → 4Na[Au(CN)2](aq) + 4NaOH(aq)
Ag(S) + 8NaCH(aq) + O2(g) + 2H2O(l) → 4Na[AgKN)2](aq) + 4NaOH(aq)
The respective metals can be obtained by adding zinc which is a more electropositive metal than either gold or silver:
2Na [Au(CN)2](aq) + Zn(S) → Na2 [Zn(CN)4](aq) + 2 Au (S)
Conversion of ore to oxide
Metals used in huge amounts generally occur as sulphides, oxides or carbonates. For sulphide and carbonate ores it is necessary to convert them into oxide forms prior to their reduction to metals. This conversion is necessary due to the following reason:
- Availability of a less costly reducing agent
- The reducing agent should not interact chemically with the metal produced.
- Availability of a suitable furnace.
- The production of metal should be cost effective.
- Fewer impurities
There is hardly a reducing agent which meets all the above requirements. Electropositive metals such as magnesium, calcium and aluminium can be used for the chemical reduction of oxide ores. These metals can not be used for the large scale production of less electropositive metals because of their high cost. However, carbon as coke fits well as a reducing agent within the above listed parameters. Its oxide, carbon monoxide is also a very good reducing agent. The efficacy of carbon monoxide as a reducing agent increases with the increase in temperature. One serious drawback of coke is that it reacts with many transition metals and some non-transition metals at higher temperatures to form carbides. However, carbon as coke and carbon monoxide remain the two versatile reducing agents for iron ores.
For carbon to be used as a reducing agent the sulphide or carbonate ores have to be converted into their respective oxide forms. Carbon does not reduce sulphide ores to give metals. To find out the reason consider the following two reduction reactions:
2MS (S) + C(S) 2M (l or S) + CS2(g) ……….(i)
MO (S) + C (S) M (l or S) + CO (g) ……… (ii)
For these two reduction reactions by carbon the Gibb's energy of the reaction should be negative. This can happen only when âˆ†G for CS2 will be more negative than âˆ†G for MS (first reaction); and for the second reaction âˆ†G for CO should be more negative than âˆ†G for MO. Thermodynamically the first reaction where CS2 is formed is not feasible, but the record reaction is feasible. It may be noted that CS2 is very much less stable than CO gas. Therefore, the sulphide ores are first converted into the oxide form before reducing them with coke.
This is done by heating the sulphide ores in the presence of roasting the sulphide ors is that a by-product sulphur dioxide (SO2) is obtained which is used to manufacture sulphuric acid.
To get the ores into their respective oxide forms following processes are used:
Calcination is heating the ores in the absence of air. This method is used for the carbonate, hydroxide and hydrated ores
CaCO3(S) CaO(S) + CO2(g)
MgCO3. CaCO3(S) MgO(S) + CaO(S) + 2CO2(g)
CuCO3. Cu(OH)2(S) 2CuO(S) + CO2(g) + H2O(g)
Calcination is generally done is a reverberatory furnace (Fig. 6. Ref www.wikipedia.org). This process makes the ore process and easily workable.
Roasting is heating the ores in the presence of air. This is done mainly for sulphide ores:
2 Fe S2 (S) + 5O2(g) → 2FeO(S) + 2SO2(g)
2Cu2S(S) + 3O2(g) → 2Cu2O(S) + 2SO2(g)
2ZuS(S) + 3O2(g) → 2PbO(S) + 2SO2(g)
Roasting is done in reverberatory furnace (Fig. 6.4 Fef. www.wikipedia.org)
Roasting also removes volatile impurities like sulphur, arsenic and phosphorus as their volatile oxides:
S(S) + O2 (g) → SO2(g)
4AS(S) + 3O2(g) → 2AS2O3(g)
P4(S) + 5O2(g) → P4O10(g)
Student Activity - 1
Metals used in an ordinary filament bulb
Draw the figure or an ordinary bulb
Label various metals used in it
Give reason as to why tungsten metal is used as the filament
Student Worksheet - 1
Which metal is liquid at room temperature
Leaching is generally used for the following ores of metals
In Aluminium-thermite process the reducing agent used is
Heating of ores in the absence of air is known as
Froth flotation process is used to concentrate the following ore
- how do metals occur in nature by virtue of their reactivity
- giving chemical equations describe the process of calcinations and roasting, respectively.
- Why are sulphide ores roasted to their oxide forms before their reduction with coke?
- Describe the principle of leaching with suitable examples.
- Describe the principle of froth flotation process. How is PbS ore concentration ewhen it is contaminated with ZnS?
A mineral with high concentration of metal compound which is used to extract metal profitably.
Occurrence of metals in nature
Undesired materials present in ore.
Process of isolation of metals from ores involving the steps:
Concentration of ore
Reduction of ore to metal
Purification of metal
Concentration of ore
Froth floatation (for sulphide ores)
Extraction with a suitable solvent for low grade ores.
Heating of ores (carbonate or hydroxide) in the absence of air.
Heating of ores (sulphide ores) in the presence of air.
Industrial reduction process to obtain metal from ore.
Reducing agents used in smelting
Carbon as coke
Aluminium (In Alumino-thermite process
Refining of crude metal
Besemerization (known as oxidative refining)
Vapour phase (van Arkel and de Boer, and Mond processes).
Zone refining (for silicon)
Curves of Gibb's energy vs temperature. Used to select a suitable reducing agent.
Classification of ores on the basis of the metal compounds
Concentration of ores on the basis of their chemical nature
Reduction of ore to get the metal choosing a suitable reducing agent
Purification of crude metal based on the nature of impurities present
A mineral having high concentration of a metal compound. ORE
Heating or ore in the absence of air. CALCINATIONS
Heating of ore in the presence of air. ROASTING
Valuable by-product during roasting. SO2 gas
Extraction of low grade ores. LEACHING
Concentration of ore by proving air bubbles. FROTH FLOTATION
A furnace used for the smelting of iron ore. BLAST FURNACE
Process of reduction of metal oxides by aluminium. ALUMINO-THERMITE PROCESS
Process used to obtain very high pure silicon. ZONE REFINING
Carbon monoxide is used to purify nickel. MONDS PROCESS
Zirconium tetraiodide (Zrl4) vapours are decomposed on heated tungsten filament. ARKEL-DE BOER PROCSS
Sodium is obtained by passing electric current in molten sodium chloride. ELECTROLYTIC REDUCTION
ADDITIONAL RESOURCE LINKS
Reduction of ore to crude metal
By using the process of reduction, roasted or calcined ores are converted to crude metal. Different reducing agents are used depending upon the reaction between the metal oxide and the reducing agent.
Reduction with carbon : FeZO3, CuO, ZuO, SuO2, PbO etc.
Reduction with Aluminium : FeZO3, Cr2O3, Mn3O4, TiO2 etc.
Reduction with Magnesium : B2O3, TiCl4, etc
Reduction with hydrogen : WO3, MOO3, GeO2, CO3O4 etc
Reduction with CO : Fe2O4, FeZO3, PbO, CuO
Electrolytic reduction : Electrolyzing of oxides, hydroxides or
chlorides in fused state.
Smelting : This is a process in which oxide of a metal is mixed with coke and a suitable flux. The mixture is heated to a high temperature in a blast furnace. Iron, Copper, Zinc and tin can be obtained by this process. Carbon is a good reducing agent below 983K where as above this temperature CO acts as reducing agent.
ZnO(S) + C(S) Zn(S) + CO(g)
+ 2C(S) Sn(S) + 2CO(g)
Fe2 + 3C(S) 2Fe(S) + 3CO(g)
CuO(S) + C(S) Cu(S) + CO(g)
A flux is a substance which is added to roated or calcined ore during smelting to remove the non-fusible impurities of metallic oxides, silica, and silicates etc. During smelting flux combines with the non-fusible impurity to convert it into fusible material called slag. The slag being light float over the molten metal from where it is removed.
Flux is of two types:
Acidic flux - SiO2 :
Basic flux - Lime stone (CaCO3) and Magnetite (MgCO3)
SiO2 + MgCO3 MgSio3 +
SiO2 + CaCO3 CaSiO3 +
Hydrometallurgy : Copper, Silver and gold are extracted by this process. The process is based on the principle that more electropositive metal can displace less electro positive metal from its salt solution. The one is treated with such seagents that the metal forms a soluble compound. On adding more electropositive metal to the solution, the less electropositive metal present in the solution is precipitated.
Example: Extraction of Copper : Malachite ore is roasted and oxide formed is dissolved in sulphuric acid. On adding scrap iron to the solution, copper is precipitated.
Cu(OH)2 . CuC → 2CuO(S) + H2O(P) + C
CuO(S) + H2S → CuS +
CuS + Fe(S) → Cu(S) + FeS
Extraction of silver : ore is dissolved in NaCN solution and air is blown followed by addition of Zinc turnings. Silver is precipitated.
Ag2S + 4NaCN → 2Na[Ag(CN)2] + Na2S
2Na [Ag(CN)2] + Zn → Na2 [Zn(CN)4] + 2Ag
Acid flux - used to remove basic impurities
Basic flux - used to remove acidic impurities
Reduction with hydrogen :Some of he metal oxides (mostly transition metals) can react with carbon at high temperatures to give metal carbides which resist further oxidation. Oxides of these metal, are better reduced by hydrogen gas. i.e.
WO3 + 3H2 W + 3H2O(g)
MOO3 + 3H2 Mo + 3H2O(g)
GeO2 + 2H2 Ge + 2H2O(g)
CO3O4 + 4H2 3Co + 2H2O(g)
Using H2(G), metals are obtained in small scale as hydrogen is highly explosive.
Aluminium reduction method: This method is also called Alumino-thermite process. Some of the metal oxides cannot be reduced by carbon as affinity of oxygen for the metal is more than for carbon, also, metal may form carbide at high temperature. Such metallic oxides are reduced by using aluminium powder. The reaction is initiated by the using barium per oxide and a small piece of Mg ribbon.
Cr2 + 2Al(S) 2Cr(P) + Al2
Fe2 + 2Al(S) 2Fe(P) + Al2
3Mn3 + 8Al(s) 9Mn(P) +4Al2
Function of BaO2 is to provide oxygen to magnesium when lot of heat is volved which initiates the thermite process.
Air reduction :
Sulphide ores of less electro positive metals such as Hg, Pb and Cu etc are heated in air to partially convert the ore into oxide which then reacts with the remaining sulphide in absence of air to give the metal and SO2 gas.
2HgS(S) +3 2HgO + 2S
2HgO(S) +HgS(S) 3Hg + S
Reaction on p-5
This process may also be called ante reduction process.
2PbS + 3O2 2PbO + 2S
2Pbu + PbS 3Pb + S
2Cu2S + 3O2 2Cu20 + 2S
2Cu20 + Cu2S 6Cu + S
Reduction by Electrolysis : The oxides of highly electropositive metals of group I, II and Al element of group etc cannot temperatures and these can form carbides. These metals are obtained by electrolysis of their oxides, hydroxides or chlorides in fused state. To lower the fusion temperatures or to increase the conductivity or both a small amount of other salt is added. The metal is liberated at cathode.
Sodium metal is obtained by electrolysis of fused mixture of Nacl and Cacl2 (down's process) or by electrolysis of fused sodium hydroxide (Costner's process).
Nacl → Na+ + cl-
At anode cl- → Cl + e-
Cl + cl → c
At Cathode Na+ + e- → Na(l)
Aluminium metal is obtained by electrolysis of fused mixture of alumina and Gyolite (Na3[Al F6])
Na3 Al → 3Na F(P) + Al
Al →Al3+ + 3F-
At anode F- → F + e-
F+F → F2(a)
2A+ 6 → 4Al + 3O2(g)
At cathode Al3+ + 3e- → A(l)
Anode gets cosseted by oxygen liberated during electrolysis, which needs replacement from time to time.
Refining of metals: Metals obtained by any of the reduction method except electrolytic reduction contains impurities. Refining of metals is process where by undesired impurities present in the metals are removed. Different refining processes may be applied depending upon the nature of the metal and nature of impurities.
Name of the Process
Metal to be refined
Low melting metals like Sn, Pb, Bi and Hq etc.
Silver containing lead.
(Impure silver containing lead is heated in cupel made of bone ash or cement and a blast of air is passed over the molten mass. The impurities are oxidized and removed with the blast of air)
Fe and Cu
Vapour phase refining
There are two methods
Impure Ni is heated with CO(g)at 323K when volatile Ni (CO)4is formed. These vapours of Ni(VO)4are passed into another chamber maintained at 306K when Ni (CO)4decomposes to pure Ni which gets deposited on small Ni balls kept in the chamber and carbon-monoxide gas is rejected.
Van Arkel Process
Ti, Zr, Hf, V, Th, B are refined by this method. Impure metal is heated with I2, producing volatile T1I4,, ZrI4or BI3. These vapours are passed over electrically heated filament of Tungsten. The vapours decompose, metal gets deposited over the filament and iodine liberated is -----.
Ti(S)+ 2TiTi(s) + 2
Zr(S)+ 2ZnZr(s) + 2
2B(S)+ 32B→ 2B(s) + 3
Highly pure silicon or gernanium required for making semi-conductors are refined by this method. The impure rod of silicon or germanium is surrounded by a heating cir-l which can move from one end to another. The heater is allowed to move in one particular direction. As the heater moves away, the metal capitalizes and impurities move along the direction of the movement of the heater. The process is repeated a number of times when a small portion of the rod gets purified. The end portion of the rod having high concentration of impurities can be cut and disconded.
Most of the metals like copper, silver, gold, aluminium, lead etc are refined by this process. The impure metal is made the anode and a thin sheet of pure metal is made a cathode. The electrolytic solution consists generally of an aqueous solution of a salt containing some acid or a complex of the metal.
Purification of Copper
Anode - Impure copper
Cathode - Thin sheets of pure copper
Electrolyte - An aqueous solution of copper sulphate containing some H2SO4.
Purification of Silver
Anode: Impure silver
Cathode: Thin sheet of pure Ag
Electrolyte - An aqueous solution of ASNO3containing HNO3.
Anode: Impure metal
Cathode: Sheet of pure lead
Electrolyte - A solution of PbS1F6containing 8-10 of H2S1F6.
Purification of Sn
Anode: Impure Tin
Cathode: A sheet of pure tin metal
Electrolyte - An aqueous solution of SNSO4containing H2S1F6.
Thermodynamics of Metallurgical process: The metals are extracted when their oxides are heated with carbon or other metal and by thermal decomposition. For any spontaneous reaction, the Gibb's anergy change âˆ†G must be negative at a particular temperature.
âˆ†G = âˆ†H - Tâˆ†S
âˆ†H is enthal by change during the reaction, T is the absolute temperature and change during the reaction, T is the absolute temperature and âˆ†S is the entropy change during the reaction. The reaction will processed only when âˆ†G is negative. For reaction where âˆ†H is negative and âˆ†S is positive. The reaction proceeds even at low temperatures.
Theoretically, it is possible to decompose all metal oxides if sufficiently high temperature is attainable but oxides of Ag, An and Hg are the only oxides which can be decomposed at easily attainable temperatures. Hence these metals are obtained by thermal decomposition of their oxides.
The choice of reducing agent to obtain the metal from its oxide depends upon the change in Gibb's energy âˆ†G. The plot of Gibb's energy change versus temperature is called.
Ellingham disgram: There diagrams can be drawn for different compounds such as oxides, sulphides, halides etc. using these diagrams one can make a choice of reducing agent and the corresponding temperature at which, the reaction becomes feasible. âˆ†G for the reaction is -ve.
Some salient features of Ellingham diagram are:
The slope for metal to metal oxide is upward as Gibbs energy change decreases with increase of temperature.
The all follow a straight line unless they melt or vaporize. When change in entropy is large, the slope of line also changes for example the Hg-HgO line changes slope at 629K when mercury brills and similarly Mg-MgO changes slope at 1393K.
When temperature is increased, the graph crossed the line âˆ†G=0 at a particular temperature. Below this temperature, âˆ†G being negative, oxide is stable where as above this temperature âˆ†G is positive and the oxide become unstable. Thus it should decompose into metal and oxygen.
In a number of reduction processes, one metal is used to reduce the oxide of the other metal. Any metal can reduce the oxide or the another metal which lie above it in Ellingham diagram.
Ellingham diagrams give an indication whether the reaction is possible or not. These graphs do not predict the kinetics of the reaction. This is a major limitation of Ellingham diagrams.
Ellingham diagram of carbon: Carbon reacts with oxygen to give two oxides
C(S) + O2(g) → CO2(g)
2C(S) + O2(g) → 2CO(g)
Carbon monoxide can further react with oxygen to give carbon dioxide.
2CO(g) + O2(g) → 2CO2(g)
When carbon changes to carbon dioxide, change in entropy (âˆ†S) is very small and âˆ†G hardly shows changes with increasing temperature. The graph of âˆ†G against T is almost horizontal.
When carbon changes to carbon monoxide, âˆ†S is positive and âˆ†G becomes more negative with increasing temperature. As a result, the line shows downward slope. The two lines for carbon to carbon-dioxide and carbon to carbon monoxide cross at 983K. below this temperature formation of CO2 is favoured whereas above this temperatures formation of CO is preferred.
Ellingham diagram of metal sulphide : Some metals occur in nature as sulphides, such as ZnS, CuS and PbS. The reaction for the reduction of these sulphides with carbon is highly
2MS(S) + C(S) → 2M(S) + CS2(g)
unfavourable energetically because of the instability of carbon disulphide. It being an endothermic reaction, sulphide ores are roasted to oxides and their reduced into metals.
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