General Principles & Processes of Extraction of Elements

• The compound of a metal found in nature is called a mineral. A mineral may be a single compound or a complex mixture. Those minerals from which metal can be economically extracted are called ores. Thus all ores are minerals but all minerals are not ores. Ores may be divided into four groups
  
  
  i) Native Ores: These ores contain the metal in free state eg. Silver gold etc. These are usually formed in the company of rock or alluvial impurities like clay, sand etc.
  
  ii) Oxidised Ores: These ores consist of oxides or oxysalts (eg. carbonates, phosphate) and silicate of metal. Important oxide ore includes, Fe₂O₃, Al₂O₃.2H₂O etc. and important cabonate ores are limestone (CaCO₃), Calamine (ZnCO₃) etc.
  
  iii) Sulphurised Ores: These ores consist of sulfides of metals like iron, lead, mercury etc. Examples are iron pyrites (FeS₂). galena (PbS), Cinnabar (HgS)
  
  iv) Halide ores: Metallic halides are very few in nature. Chlorides are most common examples include horn silver (AgCl) carnallite KCl. MgCl₂.6H₂O and fluorspar (CaF₂) etc.
  
  
  Major ores of our interest are:
  
  ORES
  1. Iron
  i) Haematite     Fe₂O₃
  
  ii) Magnetite     Fe₃O₄
  
  iii) Limonite or hydrated ferric oxide      Fe₂O₃. 3H₂O
  
  iv) Siderite     FeCO₃
  
  v) Iron pyrites     FeS₂
  
  
  2. Copper
  i) Copper pyrites or chalcopyrites     CuFeS₂ or Cu₂S. Fe₂S₃
  
  ii) Copper glance or Chalcocite     Cu₂S
  
  iii) Basic copper carbonate or malachite     CuCO₃. Cu(OH)₂
  
  iv) Cuprite     Cu₂O
  
  v) Bornite or peacock ore*     Cu₅FeS₄
  
  vi) Azurite 2 CuCO₃. Cu(OH)₂
  (* The name peacock ore is given because it has a mixture of irridescent colours like peacock's feather.)
  
  
  3. Lead
  i) Galena     PbS
  
  ii) Cerussite     PbCO₃
  
  iii) Anglesite     PbSO₄
  
  
  4. Magnesium
  i) Magnesite     MgCO₃
  
  ii) Dolomite     MgCO₃.CaCO₃
  
  iii) Epsomite or Epsom salt     MgSO₄.7H₂O
  
  iv) Caranallite     MgCl₂.KCl.6H₂O
  
  v) Kieserite     MgSO₄.H₂O
  
  vi) Kainite     K₂SO₄.MgSO₄.MgCl₂.6H₂O
  
  
  5. Aluminium
  i) Bauxite     Al₂O₃.2H₂O
  
  ii) Kaolin     Al₂O₃.2SiO₂.2H₂O
  
  iii) Cryolite      Na₃AlF₆
  
  iv) Felspar     K₂O. Al₂O₃. 6SiO₂
  
  
  6. Silver
  i) Argentite or Silver glance     Ag₂S
  
  ii) Horn silver or Chlorargyrite     AgCl
  
  iii) Ruby silver (pyrargyrite)     Ag₃SbS₃
  
  
  EXTRACTIVE METALLURGY
  It is the process of extracting a metal from its ores. The following operations are carried out for obtaining the metal in the pure form.
  i) Crushing of the ore
  ii) Dressing or concentration of the ore.
  iii) Reduction of metal.
  iv) Purification or refining of the metal
  
• Ores usually contain soil, sand, stones and other useless silicates. These undesired impurities present in ores are called Gangue or Matrix. The removal of these impurities from the ores is known as concentration. The concentration process involve physical as well as chemical method
  
  
  Physical Method
  
  a) Gravity separation: The separation is based on the difference in the specific gravities of the gangue particles and ore particles.
  
  b) Froth Floatation Process: The method is based upon preferential wetting of surfaces by liquids.
  
  c) Electro Magnetic Separator:Whenone componenteither ore or the impurity is magnetic in nature this method can be used for separation.
  
  
  Chemical Methods
  
  a) Calcination: Calcination is a process in which the ore are usually carbonate or hydrated oxide is subjected to the action of heat in order of expel water from hydrated oxide and carbon dioxide from a carbonate. Calcination is generally done in a reverberatory furnace.
  
  b) Roasting: Roasting is a process in which ores (usually sulphide ores) either alone or along with some other materials are subjected to the action of heat and air at temperatures below their melting points in order to bring about chemical changes in them. Calcination is also roasting but in this case we are concerned mainly with the changes due to the expulsion of some ingredients such as water, carbon dioxide and no other chemical change occurs. But during roasting chemical changes like oxidation, chlorination etc. takes places. Roasting is generally carried out in a reverberatory furnace or in a blast furnace.
  
  c) Leaching: It involves the treatment of the ore with a suitable reagent as to make it soluble while impurities remain insoluble. The ore is recovered from the solution by suitable chemical method.
  
  
  Reduction of Free Metal
  
  a) Smelting: The reduction of a metal from its ore by a process involving melting is known as smelting. It is generally done in a reverberatory furnace or a blast furnace in a controlled supply of air. Several reducing agents such as sodium, magnesium and aluminium are used for reduction.
  
  b) Flux:The ores even after concentration contain some earthy matter called gangue which is heated combine with this earthy matter to form an easily fusible material. Such a substance is known as flux and the fusible material formed during reduction process is called slag. Slag is usually lighter and floats on the surface of the molten metal.
  
  
  Concentrated  ore + gangue + reduction agent +  flux
  
  Metal + slag + gases
  
  Fluxes are of the following two types:
  i) Acidic fluxes like silica, borax etc.
  
  ii) Basic fluxes like CaO, lime stone (CaCO₃), magnesite (MgCO₃), haematite (Fe₂O₃) etc.
  
  The other methods used for the reduction of metals are electrolytic reduction, hydro metallurgy and amalgamation method.
  
  
  Refining or purification
  
  The metals obtained by the application of above reduction methods from the concentration ores are usually impure. The impure metal is thus subjected to some purifying process known as refining in order to remove undesired impurities. Various process for this are
  a) Liquation process
  b) Distillation process
  c) Cupellation
  d) Poling
  e) Elecrolytic refining
  f) Bessemerisation
  
  
  1. Iron
  
  Extraction of iron from its ores involves two processes.
  i) Smelting to obtain the curde metal.
  ii) Refining of the crude metal
  The iron so obtained by reduction method contains carbon and other impurities and it is known as pig iron or cast iron. The pig iron is then converted to wrought iron or steel according to the requirement.
  
  Smelting is carried out in a furnace, it is almost cylindrical furnace, lined with fire bricks. Iron ore (haematite - Fe₂O₃) is calcined to remove volatile impurities like sulphur and arsenic. The calcined ore (8 parts) along with limestone (1 part) and coke (4 parts) are lifted to the top of the furnace and released when needed. Preheated air at a temperature of about 873 K is passed into the furnace through a number of nozzles. Near the bottom, preheated air comes in contact with the falling coke and combustion of coke take place and region is known as combustion zone
  
  C + O₂ CO₂
  
  i) CO₂ so produced goes upwards and comes into contact with layers of coke and gets reduced to CO.
  
  ii) CO acts as a reducing agent and reduces iron oxide to iron.
  
  FeO₃ + 3CO 2Fe + 3CO₂
  
  Fe₂O₃ + CO 2FeO + CO₂
  
  Fe₃O₄ + CO 3FeO + CO₂
  
  FeO + C Fe + CO.
  
  
  Molten iron thus formed is collected at the bottom from where it is tapped off.
  Limestone which acts as a flux decomposes at about 1073 K. The quicklime reacts with sandy impurities (SiO₂) to form a slag of calcium silicate.
  
  CaCO₃ CaO + CO₂
  CaO + SiO₂ CaSiO₃(slag)
  
  
  The molten slag is less dense than iron and floats on the top. When both molten iron and slag are drawn off, coke and limestone are added. The gases leaving the furnace contain CO and are used to heat the incoming air - blast. The whole blast furnace process is continuous.
  The molten iron tapped off from the furnace is solidified into blocks known as 'pigs'. This form of iron is called 'pig iron' or cast iron. It contains about 3-5% carbon and varying amounts of Mn, Si, S and P.
  
  
  2. Tin
  The chief ore of tin is cassiterite or tin stone, SnO₂. It contains about 10% of tin. The crushed ore is washed with water to remove lighter impurities. The ore is then roasted to remove arsenic and sulphur as volatile oxides. Tin is obtained by reducing SnO₂ with carbon at 1200- 1300⁰C in an electric furnace.
  
  
  SnO₂ + 2 C Sn + 2 CO
  
  The product often contains traces of Fe, which make the metal hard. Fe is removed by blowing air through the molten mixture to oxidise the iron to FeO, which then floats to the surface.
  
  
  3. Copper
  The ore is crushed and concentrated by froth floatation process, this is then roasted in a limited supply of air in a reverberatory furnace to convert iron into iron(II) oxide. Arsenic and antimony present as impurites are removed as volatile oxides.
  
  2CuFeS₂ + O₂ Cu₂S + 2FeS + SO₂
  
  Sand is added to remove the iron as iron silicate slag FeSiO₃ which floats on the surface. Air is blown through the liquid matte of Cu₂S with wazzu some FeS and silica, causing partial oxidation.
  
  2FeS + 3O₂ 2FeO + 2SO₂
  
  FeO + SiO₂ FeSiO₃ (slag)
  
  Iron (II) silicate
  
  Cu₂S + O₂ Cu₂O + SO₂
  
  After some time the air is turned off and self reduction of the oxide and sulphide occurs, giving impure blister copper which is 98% - 99% pure.
  
  Cu₂S + 2Cu₂O 6Cu + SO₂
  
  Impure copper is further refined electrolytically to obtain 99.95 - 99.99% pure copper by using a solution of Cu (II) sulphate as an electrolyte.
  
  
  4. Lead
  
  The principal ore of lead is Galena, PbS. The ore is first concentrated by froth floatation process. The concentrated ore is roasted in air to convert it into lead oxide PbO and lead sulphate. Some galena is also left unchanged. If the air supply is now reduced, the unreacted PbS reacts with PbO and PbSO₄ to produce metal
  
  3PbS + 5O₂ 2PbO + PbSO₄ + SO₂
  2PbO + PbS 3Pb + SO₂
  PbSO₄ + PbS 2Pb + 2SO₂
  
  
  5. Magnesium
  
  Magnesium from sea-water is obtained by the Dow's process. It consists of electrolysis of molten magnesium chloride using an iron cathode and a graphite anode. Following are the steps that involved during the process.
  
  1. Mg⁺² is precipitated as magnesium hydroxide by the addition of slaked lime, Ca(OH)₂ to the sea water.
  
  Mg²⁺ + Ca(OH)₂ Mg(OH)₂ + Ca²⁺
  
  2. Magnesium hydroxide on treatment with hydrochloric acid is converted to chloride which is crystallised as MgCl₂.6H₂O
  
  Mg(OH)₂ + 2HCl MgCl₂ + 2H₂O
  
  3. Now for electrolysis, magnesium chloride is fused as follows:
  
  On passing a current of dry HCl gas it get partially dehydrated and the chloride thus obtained is added to a molten mix. of sodium chloride and calcium chloride (temp. range 973 - 1023 K).Under this condition MgCl₂ melt with the loss of water.
  If magnesium chloride hydrate is heated strongly, it hydrolyses to yield magnesia (magnesium oxide ) which is a refractory.
  
  MgCl₂.6 H₂O MgO + 2HCl + 5H₂O
  
  4. Molten mixture of MgCl₂ , NaCl and CaCl₂ is electrolysed. Magnesium is formed at the cathode and chlorine is evolved at the anode. The chlorine is used to make HCl acid which in turn is required for making magnesium chloride.
  
  Cathode : Mg²⁺ + 2e^- Mg
  Anode : 2Cl^- Cl₂ + 2e^-
  
  
  6. Aluminium
  Aluminium is extracted from Bauxite and it involves two steps:
  
  i) Purification of bauxite
  (ii) Electrolysis of pure alumina
  
  
  i) Purification of bauxite: Bauxite is treated with a hot conc. solution (45%) of sodium hydroxide. Aluminium dissolves to form sodium aluminate NaAlO₂ , leaving behind iron oxide which is present as impurity.
  
  Actually the chemistry of aluminates is complex. Aluminium hydroxide which is formed redissolves in excess of sodium hydroxide to form sodium aluminate, NaAl(OH)₄ which is not stable and on heating gets converted to NaAlO₂ which is called sodium meta-aluminate. Silica SiO₂ also dissolves in sodium hydroxide to form soluble sodium silicate, Na₂SiO₃
  
  Al₂O₃.2H₂O + 2NaOH 2NaAlO₂ + 3H₂O
  
  SiO₂ + 2NaOH Na₂SiO₃ + H₂O
  
  The impurities are filtered out and the solution containing sodium meta- aluminate and sodium silicate is seeded with freshly precipitated aluminium hydroxide. Aluminium hydroxide precipitates leaving behind sodium silicate in solution. This is filtered and heated at 1473 K to yield pure alumina.
  
  NaAlO₂ + 2H₂O NaOH + Al(OH)₃
  2Al(OH)₃ Al₂O₃ + 3H₂O
  
  
  ii) Electrolysis of pure alumina : Alumina is dissolved in a fused mixture of cryolite
  
  (Na₃AlF₆) with a little fluorspar ( CaF₂), which lowers the temp. of the melt and electrolysed at about 1173 K. The oxygen evolved at the anode burns the carbon anode producing carbon dioxide and carbon monoxide, due to this anodes should be replaced periodically.
  
  
  Cathode: Al³⁺ + 3e^- Al
  Anode : C + O^2- CO + 2e^-
  C + 2O^2- CO₂ + 4e^-
  
  
  7. Silver
  
  The powdered ore is treated with a dilute solution of sodium cyanide (0.5%) for several hours and the mixture is continously agitated by a current of air. Silver from the ore goes to the solution in the form of complex ion, [Ag(CN)₂]^-.
  
  2Ag₂S + 8CN^- + O₂ + 2H₂O 4 [Ag(CN)₂]^- + 4OH^- + 2S.
  
  The soluble silver complex is removed and treated with zinc dust silver precipitates out.
  
  2[Ag(CN)₂]^- + Zn 2Ag + [Zn(CN)₂]^2-
  
  Further silver is purified by electrolysis of a solution of silver nitrate and nitric acid having impure silver as anode and pure strip of silver as cathode. On passing electric current pure silver gets deposited at cathode whereas impurities fall down below anode as anode mud
  
  At Cathode : Ag⁺ + e^- Ag
  
  At anode : Ag Ag⁺ + e^-.