Metallurgy General Principles and processes of Isolation of elements Modes of occurrence of metals Elements occur in nature in two states: Native state and combined state. When the metals are in the elementary form they are said to be in their native state. Eg: Au, Ag, Cu, Pt etc. When the metals are found in the form of their compounds, they are in the combined state. In the combined state the metals are found in the crust of the earth as oxides, carbonates, sulphides, silicates, Phosphates etc. The naturally occurring chemical substances in the earth’s crust which can be obtained by mining are known as minerals. These minerals are associated with large quantity of rocky and silicious impurities. These are collectively known as gangue or matrix. Every mineral cannot be used for its extraction. The mineral from which the metal can be economically and conveniently extracted is called ORE. All ores are minerals but all minerals are not ores. Metal Potassium Calcium Sodium Magnesium 3Aluminium Titanium Zinc Iron Tin
Ore Sylvine Gypsum, Limestone Rock salt Magnesite, Dolomite Bauxite, Diaspore, Kaolinite(a form of clay) Rutile, Ilmenite Zinc blende or sphalerite, Calamine Zincite Haematite Iron Pyrite(Fool’s gold) Magnetite Cassiterite or tin stone
Composition KCl CaSO4.2H2O, CaCO3 NaCl MgCO3, MgCO3.CaCO3 Al2O3.2H2O, Al2O3.H2O Al2(OH)4.Si2O5 TiO2, FeTiO3 ZnS ZnCO3 ZnO Fe2O3.xH2O FeS2 Fe3O4 SnO2
Chromium Manganese Lead Copper
Chromite Pyrolusite Galena Chalcopyrite(copper pyrites) Malachite Cuprite Cinnabar Argentite Nativemetal
Mercury Silver Gold
FeCr2O4 MnO2 PbS CuFeS2 Cu(CO3).Cu(OH)2 Cu2S HgS Ag2S Au
Among metals, Aluminium is the most abundant. It is the third most abundant element in the earth’s crust. It is a major component of many igneous rocks such as mica and clays. ‘Ruby’ and ‘Sapphire’ are impure forms of alumina (Al2O3) Ruby has Cr as impurity while sapphire contains Co as impurity. Iron is the second most abundant metal in the earth’s crust. Most of the metals occur in the oxidized form as their compounds. Therefore metals are generally extracted by subjecting their ores to reduction by chemical methods or by electrolytic methods. The process of extracting pure metal from its ore is known as metallurgy. Common steps involved in the metallurgical operations are: 1. 2. 3. 4. 5.
Crushing and grinding of the ore. Concentration or benefaction of the ore. Preliminary treatment of the concentrated ore. Reduction. Purification or refining of crude metal.
Crushing and grinding of the ore. Most of the ores are obtained in the form of huge lumps. These lumps have to be converted into powered form so that the chemical changes which have to take place at the later stages may become convenient. The huge lumps are broken into small pieces in the jaw crushers. They are further
pulverised in
ball mill.
stamp mill or
Concentration Or Benefation of the ore The ore obtained from the earth is assocoated with rocky and silicious impurities. The removal of unwanted materials such as sand clay etc from the pulverised ore is called concenration, dressing or benefation of the ore. Hydraulic washing Levigation or Gravity separation method
This method is applicable to oxide ores in which the ore particles are heavier than the impurities. The powered ore is washed with running strem of water. The lighter impurities are washed away leaving behind the heavier ore particles.
Froth Flotation process This process is generally used for the concentration of sulphide ores. The finely powered ore and water are taken in a tank. Additional reagents such as pine oil, fatty acids,xanthates etc are Added to the mixture. These reagents increase the nonwettability of the mineral particles are known as collectors. Sometimes froth stabilizers such as aniline and cresols are also added to the mixture. The contents are kept agitated by the blast of air. As a result of agitation the froth is produced. The ore particles are preferentially wetted by the oil and are carried to the surface by the foam. The gangue material which is preferentially wetted by water sinks to the bottom of the tank. The foam is transferred to another tank and washed with water to recover the ore particles. It is possible to separate one sulphide ore from another by adjusting the proportion of oil and water.
Sometimes additional reagents called depressants are used to prevent certain type of particles from forming the froth. For example NaCN is added to a mixture containing PbS,FeS2 and ZnS. NaCN acts as a depressant for ZnS and Iron pyrites and not for PbS. NaCN forms a Zinc complex, Na2[Zn(CN)4], on the surface of ZnS and prevents it from the formation of the froth. Only PbS comes as a froth. To the remaining solution an activator (CuSO4 Solution) is added and the process is repeated. Only ZnS gets into the froth leaving behind the Pyrites. Magnetic Separation Here magnetic and nonmagnetic substances can be separated. Ore can be magnetic or the gangue can be magnetic.
For example tungstates of Iron and manganese which are magnetic impurities in tinstone are separated by this method. The powdered ore is dropped over the belt revolving around the rollers, one of which is magnetic. The magnetic roller attracts the magnetic part of the ore that forms a heap near it. The non magnetic part forms a separate heap. Leaching It is a chemical method for the concentration of the ore. In this process the powdered ore is dissolved selectively in certain acids, bases or other suitable reagents. The impurities remain undissolved as sludge. The solution ore is filtered and the ore is recovered by precipitation or cyrstallisation Leaching Alumina from Bauxite The principal ore of aluminium is bauxite. It contains SiO2, iron oxides and titanium(IV) oxide as impurities. The powdered ore is treated with 45% NaOH solution at 473-523 K and 35-36 bar pressure. Al2O3 goes into the solution forming soluble sodium aluminate. SiO2 also dissolves as sodium silicate. The solution is filtered to remove the insoluble impurities. Next, aluminium hydroxide is precipitated from the strongly alkaline solution by passing CO2 which lowers the pH. At this stage the solution may be seeded with freshly prepared aluminium hydroxide precipitate. Na[Al(OH)4](aq) + CO2( g) (--- Al(OH)3 +NaHCO3 The sodium silicate remains in the solution. The aluminium hydroxide precipitate is filtered,dried and heated strongly to get purified Al2O3 1470K 2Al(OH)4 (s) ---------- Al2O3 (s) + 3H2O (g) Preliminary Treatment Of the Concentrated Ore Calcination: It is a process of heating the ore in a limited supply of air below its melting point. The process involves ---- removal of impurities, ---- removal of moisture, ---- decomposition of any carbonate ore into oxide. Fe2O3.xH2O ----------- Fe2O3 + xH2O (g) ZnCO3 ------------
ZnO +CO2
PbCO3 -------------
PbO + CO2
CaCO3.MgCO3 -------
CaO + MgO +2CO2
CuCO3.Cu(OH)2 ------ 2CuO + H2O +CO2 Roasting : It is the process of heating the ore in the excess supply of air below its melting point. This process is employed when oxidation of the ore is required. As a result of roasting, ----- moisture is driven away, ----- volatile impurities are removed, ----- the impurities like sulphur, phosphorous, arsenic are removed as their oxides, ----- the ore undergoes oxidation to form metal oxide or sulphate. 2PbS +3O2 ---------- 2Pbo + 2SO2 2ZnS +3O2 ---------- 2ZnO +SO2 2Cu2S +3O2 -------- 2Cu2O + 2SO2 It is advantageous to roast a sulphide ore to the oxide because metal oxides can be reduced to metal more easily than sulphides.
Both calcinations and roasting processes are carried out in a special type of furnace called reverberatory furnace. The SO2 gas produced during roasting is utilized in manufacturing sulphuric acid (H2SO4) Reduction After the preliminary treatment, the ore may be subjected to reduction process by any one of the following methods depending upon its nature. Smelting: Reduction with carbon In this process, the roasted or calcined ore is mixed with suitable quantity of coke or charcoal (which act as a reducing agent) and is heated to a high temperature above its melting point. During melting an additional reagent is also added to the ore to remove the impurities still present in the ore. This additional reagent is called flux. Flux combines with the impurities to form a fusible product called slag.
Flux + Impurities ------- Slag. The selection of flux depends upon the nature of impurities. If impurities are acidic in nature, the flux is basic,i.e., lime (CaO). On the other hand, for basic impurities, an acidic flux, silica (SiO 2) is used. For example, P4O10 + 6 CaO ------ 2Ca3(PO4)2 Acidic impurity Basic flux Slag CaO + SiO2 -------- CaSiO3 Basic impurity acidic flux Slag. The metal oxide, during the reduction process gets reduced to metal. 823K Fe2O3 + 3C ------------2Fe + 3CO ZnO + C
1600K -------------- Zn + CO
1473-1573K SnO2 + 2C -------------- Sn + 2CO. The process of reduction with Carbon is carried out in the blast furnace
Reduction with Aluminium Certain metal oxides such as Cr2O3 and Mn3O4 are not easily reduce d with carbon. In such cases Al is used as reducing agent because it is more electropositive than Chromium or manganese. The process of reduction of oxides with aluminium is called aluminothermy. Some examples are: Cr2O3 +2 Al --------- Al2O3 +2Cr 3Mn3O4 + 8Al ------- 4Al2O3 + 9Mn In some cases the reduction is carried out by alkaline earthelements like magnesium. TiCl4
+Mg ------- 2MgCl2 +Ti
Reduction wirh Hydrogen or water gas Hydrogen is very efficient reducing agent. However, its use is limited to those ores where carbon or other reagents are not suitable. The roasted ore is heated in small trays in the current of hydrogen gas. For example, tungsten and indium are obtained by reduction of their oxides with H2. WO3 + H2 -------- W + 3 H2O In2O3 +3 H2 --------- 2In + 3 H2O Similarly, extraction of nickel is carried out from its oxide by the use of water gas. 2NiO +CO + H2 --------- 2 Ni + CO2 + H2O Auto-reduction Certain metals are obtained from their ores by simple roasting without using any reducing agent. For example , mercury is obtained by roasting its ore cinnabar.(HgS) in air. HgS(s) +O2 (g) -- Hg(g) + SO2 2HgS(s) + 3 O2(g) ---- 2HgO(s) + 2SO2 (g) 2HgO (s) + HgS(s) ---------- 3 Hg (l) + SO2(g) Electrolytic Reduction
The highly electro positive elements such as alkali metals and alkaline earth metals and aluminium can not be reduced using other reducing agents like carbon. They are obtained by the electrolytic reduction of their fused salts. This is called electrometallurgy. For example, sodium metal is extracted by electrolysis of molten sodium chloride containing other salts as impurities. NaCl(l) ------- Na+ + Cl— At the cathode: Na+ +e-- -------- Na At the anode : 2 Cl-- --------- Cl2 + 2e— Displacement Method.(hydrometallurgy) Some metals like gold and silver are extracted from their concentrated ore by leaching. They are dissolved in suitable reagents leaving behind insoluble impurities. The metal is obtained by reprecipitation or crystallization. For example silver ore is leached with dilute solution of sodium cyanide. Silver dissolves forming a complex sodium argento cyanide. The solution is treated with scrap Zinc which displaces silver from the complex. Ag2S +4 NaCN < --------------------- > 2Na [Ag(CN)2] + Na2S Sodium argento cyanide 2Na[Ag(CN)2
+ Zn ---------------> Na2[Zn(CN)4] + 2Ag Sod.tetracyano Zincate (II) ppt
Similarly native gold is leached with potassium cyanide solution and it is recovered from the solution by addition of scrap Zinc. 4Au + 8 KCN +O2 +2 H2O ---------- > 4K[Au (CN)2] + 4 KOH 2K [Au (CN)2] + Zn ----------- > K2[Zn(CN)4]
+ 2 Au (ppt)
Purification or refining of crude metal The process of removal of impurities from the crude metal is termed as refining. Liquation This Process is applied to the metals having low melting points such as tin lead & bismuth.
The impure metal is placed on the sloping hearth maintained at the temperature above the melting point of the metal. The metal melts and flows down the sloping hearth into a receiver leaving behind the solid impurities. Zone refining method When metals are required in a ultra pure state, the zone refining method is used. The principleemployed states that the impurities, which lower the melting point of a metal remain preferentially dissolved in the liquid phase and purer metal will emerge in the solid phase. In actual practice, a cylinder of impure metal is kept in a tubular furnace and the heater is made to move in one direction at a very slow speed. The solid material crystallizes as the heater moves along the tube, and the advancing zone contains liquid with higher impurity content. The process is repeated a number of times till the desired level of purity is obtained.
The Zone refining method
Van Arkel method In the Van Arkel method impure metal is first converted into one of its unstable compounds. Heating the unstable compound of the metal at higher temperatures, decomposes it to give pure metal. For example, pure titanium can be obtained from impure titanium by converting it into tetraiodide, and then decomposing this tetraiodide to pure Ti.
Distillation When metals have low boiling points e.g., mercury, zinc, cadmium etc., most of the impurities can be separated by distillation. Their vapours, on condensation in a suitable condenser give the pure metal. Polling Oxide impurity of impure metals can be purified by this method. Poles of green wood are used to stir the molten crude metal sample. The hydrocarbon organic matter in the poles acts as a reducing agent and reduces the oxide impurities. This method is used for refining copper. Extraction of Nickel (Mond’s process) In 1899 Ludwig Mond developed a process for extracting and purifying nickel. The so-called "Mond Process" involves the conversion of nickel oxides to pure nickel metal. The oxide is obtained from nickel ores by a series of treatments including concentration, roasting and smelting of the minerals. In the first step of the process, nickel oxide is reacted with water gas, a mixture of H2 and CO, at atmospheric pressure and a temperature of 50 °C. The oxide is thus reduced to impure nickel. Reaction of this impure material with residual carbon monoxide gives the toxic and volatile compound, nickel tetracarbonyl, Ni(CO)4. This compound decomposes on heating to about 230 °C to give pure nickel metal and CO, which can then be recycled. The actual temperatures and pressures used in this process may very slightly from one processing plant to the next. However the basic process as outlined is common to all. The process can be summarised as follows: Ni
+ 4CO (impure) (pure) Electrolytic method
50°C →
Ni(CO)4
→
230°C Ni
+
4CO.
A large number of metals e.g., copper, silver, gold, nickel etc. are refined by this method. In an electrolytic cell the impure metal is made anode and a thin strip of pure metal is made cathode. A solution of a suitable salt of the concerned metal is used to fill the electrolytic cell. On passing electricity, the anode undergoes dissolution while the pure metal gets deposited at cathode. Insoluble impurities fall below the anode in the form of anode mud.
Electrolytic purification of a metal Thermodynamic Principles of Metallurgy The reduction of metallic oxide usually involves heating it with some reducing agents such as C, CO, hydrogen or some other metal. The reducing agent combines with oxygen of the metal oxide. MxOy +yC -------- > xM = yCO Some metal oxides get reduced easily while others are reduced with difficulty. Some oxides are reduced at relatively low temperatures while others are reduced at high temperatures. Thermodynamis considerations play an important role in deciding the temperature and the choice of reducing agent in the thermal reduction process during metallurgy (pyrometallurgy). For a spontaneous process, the change in Gibbs energy must be negative. The change in Gibbs energy, ΔG for any process at any specified temperature is described by the equation ΔG = ΔH -- TΔS Where, ΔH is the enthalpy change , T is the absolute temperature and ΔS is the entropy change for the process. Consider a reaction such as formation of an oxide. 2M(s) + O2 (g) ------------ > 2MO(s)
In this reaction, the randomness of the system decreases because gases have more randomness than solids. Hence, ΔS for this reaction is negative. Thus if temperature is increased the TΔ S becomes more negative. Since T S is subtracted in equation, ΔG becomes less negative. On the otherhand, if S is positive, on increasing the temperature the value of and becomes more negative. For example, in the reaction, 2C (s) + O2 (g) ------ > 2CO (g)
ΔG decreases
ΔS is positive and ΔG decreases and becomes more negative as temperature increases. If the reactants and products of two reactions are put together in a system and the net Gibbs free energy change, G of the two possible reactions is negative, the the overall reaction will occur. Thus, the process of interpretation of feasibility of a process involves coupling of the two reactions, calculating the sum of their G and then onserving the sign of G. such coupling is easily understood in terms of Ellingham diagram. Ellingham diagram
An 'Ellingham diagram' is a graph showing the temperature dependence of the stability for compounds. This analysis is usually used to evaluate the ease of reduction of metal oxides and sulfides. These diagrams were first constructed by Harold Ellingham in 1944. In metallurgy, the Ellingham diagram is used to predict the equilibrium temperature between a metal, its oxide and
oxygen, and by extension, reactions of a metal with sulphur, nitrogen, and other non-metals. The diagrams are useful in predicting the conditions under which a metal ore will be reduced to the metal. The analysis is thermodynamic in nature and ignores reaction kinetics. Thus, processes that are predicted to be favourable by the Ellingham diagram can still be slow. Ellingham diagrams follow from the Second Law of Thermodynamics [ΔG = ΔH - TΔS] and are a particular graphical form of it. ΔG is the Gibbs Free Energy Change,ΔH is the Enthalpy Change and ΔS is the Entropy Change] The Ellingham diagram plots the Gibbs free energy change (ΔG) for the oxidation reaction versus the temperature. In the temperature ranges commonly used, the metal and the oxide are in a condensed state (liquid or solid) with the oxygen gaseous, the reactions may be exothermic or endothermic, but the ΔG of the oxidation always becomes more negative with lower temperature, and thus the reaction becomes more probable statistically. At a sufficiently high temperature, the sign of ΔG may invert (becoming negative) and the oxide can spontaneously reduce to the metal. As with any chemical reaction prediction based on purely energetic grounds the reaction may or may not take place spontaneously on kinetic grounds if one or more stages in the reaction pathway have very high Activation Energies EA. If two metals are present, two equilibriums have to be considered, so that the metal with the more negative ΔG reduces, the other oxidizes.] Salient Features 1. Curves in the Ellingham diagrams for the formation of metallic oxides are straight lines with a positive slope. 2. The lower the position of a metal in the Ellingham diagram more is the stability of its oxide. For example, the Ellingham diagram for Al is found to be below Fe2O3. 3. A metal found in the Ellingham diagram can act as a reducing agent for a metallic oxide found above it. Thus, Al can act as a reducing agent to Cr which is above it in the diagram. 4. The greater the gap between any two lines, the greater the efficiency of the reducing agent. 5. Stability of metallic oxides decrease with increase in temperature. Highly unstable oxides like Ag2O and HgO easily undergo thermal decomposition. 6. The formation enthalpy of carbon dioxide (CO2) is almost a temperature-independent constant, while that of carbon monoxide (CO) has negative slope. According to Boudouard reaction, carbon monoxide is the dominant compound in higher temperatures, and the higher the temperature, the more efficient reductant carbon monoxide also is. 7. A substance whose formation enthalpy is lower (ΔG line lower on diagram) at given temperature, will reduce one whose formation enthalpy is higher on the diagram. Hence metallic aluminum can reduce iron from iron oxide into metallic iron, aluminum itself oxidizing into aluminum oxide. (This reaction is employed in thermite.) 8. The intersection of two lines imply the equilibrium of oxidation and reduction reaction between two substances. Reduction with using a certain reductant is possible at the intersection point and higher temperatures where the ΔG line of the reductant is lower on diagram than the metallic oxide to be reduced. At the point of intersection the Gibbs
energy is 0(zero), below this point the Gibbs energy is <0 and the oxides are stable,while above the point of intersection the Gibbs energy is >0 and so, the oxides are unstable. Reducing agents In industrial processes, the reduction of metal oxides is effected using carbon. Carbon is available cheaply as coal, which can be rendered to coke). Moreover, when carbon reacts with oxygen it forms gaseous oxides carbon monoxide and carbon dioxide, therefore the dynamics of its oxidation is different from that for metals: its oxidation has a more negative ΔG with higher temperatures. Carbon can thus serve as reducing agent. Using this property, reduction of metals may be performed as a double redox reaction at relatively low temperature. Use of Ellingham Diagrams The main application of Ellingham diagrams is in the extractive metallurgy industry, where it helps to select the best reducing agent for various ores in the extraction process. It also helps to guide the purification of metals, especially the removal of tramp elements. The direct reduction process for making iron rests firmly on the guidance of Ellingham diagrams, which show that hydrogen can alone reduce iron oxides to the metal. Reducing agent for hematite In iron ore smelting, hematite reduces at the top of the furnace, where temperature is in the range 600 - 700 °C. The Ellingham diagram indicates that in this range carbon monoxide will reducing agent since the process 2 CO + O2 → 2 CO2 Is subject to a less negative free energy change than the process: 2 C + O2 →--> 2 CO Thus, in the blast furnace haematite is reduced by CO even in the presence of carbon. The overall equation for this carbothermic reaction is: 2 Fe2O3 + 3 C → 4 Fe + 3 CO2 Reducing agent for chromic oxide-carbon cannot be used At high temperature the Ellingham curve for the reaction 2C(s) + O2(g) --> 2CO(g) slopes down and falls below the curves for all the metals. Hence, carbon can normally act as a reducing agent for all metal oxides at very high temperatures. But chromium formed at these temperatures react with carbon to form its carbide which gives undesirable properties to chromium metal obtained. Hence, for high temperature reduction of chromic oxide, carbon cannot be used.
Alumino thermic process
Thermite reaction proceeding for a railway welding. Shortly after this, the liquid iron flows into the mould around the rail gap The Ellingham curve for aluminium lies below the curves of most metals such chromium, iron, etc. This fact indicates that aluminum can be used as the reducing agent for oxides of all these metals. This result is illustrated as below: The free energies of formation of chromium oxide and aluminum oxide per mole of oxygen consumed are -540kJ and -827kJ respectively. The processes are:
(1)
(2) So Aluminum oxide is more stable than chromium oxide. Equation (2) - Equation (1) gives
ΔG0 = − 287kJ Since Gibb's free energy is negative, aluminium acts as the reducing agent. In pyrometallurgy, Al is used as a reducing agent in the alumino-thermic process.
