UNIT 8: Metals PDF

# UNIT 8: Metals - State how metals occur in nature. - Name two ores of the following metals: Na, Mg, Ca, Al, Fe, Cu, Zn - Describe general metallurgical processes. - Concentration of the ore: Gravity separation, magnetic separation, froth floatation process, leaching. - Roasting or calcina...

# UNIT 8: Metals - State how metals occur in nature. - Name two ores of the following metals: Na, Mg, Ca, Al, Fe, Cu, Zn - Describe general metallurgical processes. - Concentration of the ore: Gravity separation, magnetic separation, froth floatation process, leaching. - Roasting or calcination of the ore. - Reduction of the ore: Electrolytic reduction for reactive metals, reduction with the help of reducing agents like Na, Al, K, C, CO, etc. - Refining of the crude metal: Electrolytic refining, liquation. - Describe the methods of extraction of Fe, Al, Zn from the respective ores like Haematite, Zinc blende and bauxite. Types of iron. Differences between pig iron and wrought iron. - Differentiate between metals and non-metals (both physical and chemical properties). - State the simple chemical properties of Na, Ca, Mg, Zn, Fe, and Cu with respect to: - Oxygen and atmosphere. - Water. - Acids (Dilute and concentrated). - Preparation and uses of some common compounds of metals. ## OCCURRENCE OF METALS Earth's crust is mainly made up of sand and silicates. At many places, the earth's crust also contains metallic compounds and even free metals. The most abundant element in the earth's crust is oxygen (48%) and the second most abundant is silicon (26.3%). The most abundant metal is aluminum (7.73%) and the second most abundant metal is iron (4.75%). Metals that have low chemical reactivity are found in free state. Gold, silver, and platinum are partly found in the free state. Most of the metals occur in the combined state. The naturally occurring compounds of metals which are generally mixed with the other matter such as soil, sand, limestone and rocks are known as minerals. The rocky impurities like soil, sand, etc. present in an ore is called gangue or matrix. Those minerals from which metals are commercially extracted at a comparatively low cost and minimum effort are called ores. All ores are minerals but all minerals are not ores. ### Table 1: Some metals and ores Metal | Ore | Chemical name | Formula ------- | -------- | -------- | -------- Sodium | Rock salt | Sodium Chloride |NaCl | Albite | Soda feldespar | NaAlSi3O8 | Salt Petre | Sodium Nitrate | NaNO3 Magnesium | Magnesite | Magnesium Carbonate | MgCO3 | Dolomite | Magnesium Calcium Carbonate | MgCO3.CaCO3 | Epsom salt | Magnesium sulphate heptahydrate | MgSO4.7H2O Calcium | Limestone | Calcium carbonate | CaCO3 | Dolomite | Magnesium Calcium Carbonate | MgCO3.CaCO3 | Gypsum | Calcium sulphate dihydrate | CaSO4.2H2O Aluminum | Bauxite | Aluminium Oxide dihydrate | Al2O3.2H2O | Cryolite | Sodium Aluminium fluoride | Na3AlF6 Iron | Haematite | Ferric Oxide | Fe2O3 | Magnetite | Ferroferric Oxide | Fe3O4 | Iron Pyrites | Ferric Sulphide | FeS2 Copper | Copper Pyrite | Cuprous Ferric Sulphide | CuFeS2 | Ruby Copper | Cuprous Oxide | Cu2O | Copper glance | Cuprous Sulphide | Cu2S Zinc | Zinc Blende | Zinc Sulphide | ZnS | Calamine | Zinc Carbonate | ZnCO3 # Metallurgy Metallurgy is an art and science of extracting pure metals from their ores. Metallurgy involves three stages: 1. **Preliminary treatment of the ores:** In this stage, ore is concentrated and made ready for reduction. 2. **Reduction:** In this stage metallic compound is reduced to metal. 3. **Refining:** In this stage the crude metal obtained after reduction is purified to the required purity. ## Concentration of the ore It is the process of removal of gangue to make the ore rich in the metal. Concentration of the ore is done mainly by the following means: ### Gravity separation This method is used when the ore is heavy and the gangue is light. The ore is washed down over a vibrating sloped table with grooves. The lighter gangue particles are washed away by the water while the heavy mineral settles in the grooves. This is also called levigation or leaching with water. This method is especially employed to concentrate oxide ores like haematite, tin stone etc. ### Froth Flotation The process depends on the preferential wettability of the ore and the gangue particles. The crushed ore is taken in a large tank containing pine oil and water and agitated with a current of compressed air. Gangue is wetted by water and settles at the bottom of the tank. The sulphide mineral is wetted by the oil, froth produced due to the agitation with air. Froth is removed and the mineral separated. This method is especially employed to concentrate sulphide ores. ### Electromagnetic separation This method is used to separate ores which are magnetic in nature. Crushed ore is placed over the conveyor belt, which rotates around the two metal wheels one of which is magnetic. The magnetic particles are attracted by the magnetic wheels and falls separately closer to the magnetic wheel, while the non-magnetic material forms a heap slightly way from the roller. For example, tin stone or cassiterite (SnO2) is concentrated by this method to remove magnetic impurity, Wolframite (FeWO4). ### Chemical separation for Leaching This is a chemical method for concentrating ores. The powdered ore is treated with a suitable reagent that dissolve the ore but not the impurities. Eg. Bauxite ore containing iron oxide is leached with 45% NaOH when water soluble sodium meta-aluminate is formed while the impurities remain undissolved and hence can be separated by filtration. ## Converting ore to suitable form ### Calcination It is a process of heating an ore in a limited supply of air or in the absence of air at a temperature below its melting point. During calcination the following changes takes place: - Moisture is removed. Fe2O3.2H2O → Fe2O3 + 2H2O↑ - Volatile impurities are expelled. - Carbonate and hydroxide ores get converted into oxides. ### Roasting It is the process of heating the concentrated ore strongly in the presence of air at a temperature below its melting point. This process is generally used to convert sulphide ores into metallic oxides. The changes taking place during roasting are: - Moisture is removed. - Volatile impurities are expelled. Impurities like sulphur, arsenic, phosphorous etc are removed in the form of their gaseous oxides. - eg. S+O2→ SO2↑, 4As + 3O2 → 2AS2O3, P4 +502 → 2P2O5 - Sulphide ores are converted into metallic oxides. - eg. 2ZnS + 3O2 → 2ZnO + 2SO2↑, 2PbS + 3O2 → 2PbO + 2SO2↑ - 2Cu2S +302 → 2Cu2O + 2SO2↑ ## Reduction of the Ores ### Thermal reduction Very less reactive metals can be obtained by just heating. - eg. HgS + O2 → Hg + SO2 ### Using reducing agents Ores of the metals present in the middle of the reactivity series like Zn, Fe, Cu and Pb are reduced using some reducing agents like carbon monoxide, carbon, hydrogen etc. ### Electrolytic reduction Electrolytic reduction is used for metals present in the beginning of the reactivity series like Na, Ca, Mg, Al etc which are highly reactive in nature. ## Refining of Crude metal ### Electrolytic refining This is the most important method for the refining of the metals and is based on the phenomenon of electrolysis. The blocks of impure metals are made the anode whereas a thin sheet of pure metal acts as a cathode. An aqueous solution of some salt of the metal is used as an electrolyte. On passing the current, the metal ions from the electrolyte are deposited at the cathode in the form of a pure metal; on the other hand, an equivalent amount of the metal dissolves from the anode and passes into the electrolyte in the form of metal ions. The impurities either go into the solution or settle near the anode as anode mud. +n - Anode reaction- M → Mn + ne - Cathode reaction- M⁺ + ne → M ### Liquation The method is used for metals with low melting points like tin and lead. The impure metal is placed on the slopping hearth of the furnace and is heated gently in anaerobic conditions. The metal melts and flows down leaving behind the impurities on the furnace. ## Extraction of Iron Ores of the iron are: - Haematite Fe2O3 - Magnetite Fe3O4 Extraction of the metal from Haematite ore involves the following steps: ### Preliminary treatment The ore is crushed and ground finely. It is then washed in a stream of water to remove the lighter impurities like mud and water soluble impurities. ### Roasting It is then mixed with some coke and roasted i.e heated strongly in a current of air. This is done to burn away all sulphur and arsenic, to remove moisture and to make the ore porous. The carbonate ore loses carbon dioxide to give iron (II) oxide. Further any iron (II) oxide (FeO) present is oxidised to iron (III) oxide (Fe2O3). - FeCO3 FeO + CO2 - 4FeO +302 → 2Fe2O3 + heat ### Reduction of the ore (Smelting) The roasted ore is mixed with coke and limestone in the ratio 3:2 and fed at the top of a tall thick chimney like steel furnace called blast furnace. It has inside fire - brick lining. It is almost 30m high and 9m in diameter at the widest part. Near the base of the furnace, there are jetlike pipes called tuyeres through which hot air at about 1073K is blown under pressure. Molten iron and slag is collected in the well having two outlets. The upper outlet is opened first to remove the molten slag and the molten iron is drained out from the lower outlet into large containers where it solidifies to give pig iron or cast iron. ### Reactions in the blast furnace: As hot air enters the furnace in the lower region it combines with red-hot coke at 2273K forming CO2 liberating a lot of heat. This heat is helpful for further reaction. As it rises through layers of red-hot coke, CO2 is reduced to CO and limestone decomposed to lime and CO2 both of which are endothermic reactions. Hence the temperature of the middle furnace falls down to about 873K to 1073K. Further, lime combines with silica to give calcium silicate (CaSiO3) - C+O2→ CO2 + 97 kcal - C+CO2→2CO -39 kcal - CaCO3 → CaO+ CO2 - 43 kcal - CaO + SiO2 → CaSiO3 (slag) - Fe2O3+3CO→ 2Fe +3CO2 The slag floats on the surface due its low density. The slag separates iron from the hot blast of air, thereby, preventing iron from getting oxidized. - Slag is used for: - Metallic roads when mixed with pitch. - Manufacture of cement and fertilizers. - Making rock wool for insulation. ## Wrought Iron Wrought iron is the purest form of the commercial iron and contains less than 0.5% impurities. It is also called malleable iron. ## Conversion of cast iron to wrought iron Wrought iron can be obtained by heating pig iron on the hearth of a reverberatory furnace lined with haematite (Fe2O3). Carbon present in pig iron gets oxidized to carbon monoxide which escapes. - Fe2O3 +3C → 2Fe + 3CO(g) The impurity of MnO present in the pig iron combines with silica to form slag which floats on the top. - MnO + SiO2 → MnSiO3 Wrought iron thus obtained contains about 0.25% of carbon and traces of P and Si in the form of slag. ## Differences between wrought iron and pig iron | S. No. | Wrought Iron | Cast Iron | | -------- | -------- | -------- | | 1 | It contains impurities of range 0.1% - 1.5% | It contains impurities in greater amounts like Carbon | | 2 | It is stronger and malleable | It is weak and brittle i.e., it is not malleable | | 3 | It is more resistant to corrosion | It is less resistant to corrosion | | 4 | It is used for construction purposes. | It is used as raw material for making bolts, magnets, etc. | ## Aluminium - Ores of Aluminium: Bauxite (Al2O3.2H2O), Cryolite (Na3AlF6) ## Extraction of Aluminium ### Preliminary treatment Bauxite contains 60% pure aluminium oxide. The impurities are Fe2O3, SiO2 and TiO2. Pure alumina is obtained from bauxite by Bayer's process: ### Bayer's Process - Bauxite is ground finely and heated below red hot to remove volatile impurities. - It is then heated under pressure with concentrated caustic soda solution for a few hours at 413K when alumina present in the ore dissolves as sodium meta-aluminate leaving behind insoluble impurities like ferric oxide, sand, etc which are removed by filtration. - On diluting with water and cooling, sodium aluminate is hydrolysed to give aluminium hydroxide as precipitate. - Al2O3 + 2 NaOH → 2 NaAlO2 + 2 H2O - NaAlO2 + 2 H2O → NaOH + Al(OH)3↓↓ - The precipitate is filtered, washed, dried and ignited at 1273K to get alumina. - 2 Al(OH)3 Al2O3 + 3 H2O ### Reduction of alumina by Hall- Heroult's process As aluminium oxide is very stable, it is not reduced easily by carbon, CO or H2. Hence electrolytic reduction was chosen as a method of refining alumina. Electrolytic reduction is also difficult as the melting point of alumina is very high. Therefore, it was dissolved in molten cryolite (which has a much lower melting point) for electrolysis. Electrolysis is carried out at about 1173K in an iron tank lined with electrodes made of gas carbon, which serves as cathode. The anode consists of a number of graphite rods suspended from the top in the fused electrolyte. The electrolyte is covered with coke powder to avoid heat loss by radiation. ### At the anode - 202- → 20+4e, - C+2O→ CO2 ### At the cathode - Al+3e Al ## Zinc The main ores: - Zinc blende (ZnS) - Calamine (ZnCO3) - Zincite (ZnO) ## Extraction of Zinc from zinc blende Zinc is extracted from its ores by the carbon reduction method. ### Preliminary treatment Ore is crushed and ground and then concentrated by froth floatation process. The powdered ore is washed in a tank of water. In this process a little pine oil is added to the tank and compressed air is blown into the mixture when the earthly materials sink to the bottom of the tank. The sulphide ore particles rise up along with the froth which are skimmed off. An acid is added to break up the froth. The concentrated ore is filtered and dried. ### Roasting The concentrated zinc ore is now roasted at 1173K in excess of air. The sulphide is first converted into its oxide. In the case of calamine, the carbonate decomposes into the oxide on calcination. The roasted ore is thus mainly zinc oxide. - 2 ZnS +3 O2 → 2 ZnO + 2 SO2 - ZnCO3 → ZnO + CO2 ### Reduction The roasted ore is reduced using coke. The oxide is mixed with excess of powdered coke and fed into the top of a vertical retort heated by producer gas to about 1673K. The metallic zinc is formed as a vapour which along with carbon monoxide passes through an outlet near the top of the retort. The vapour is condensed to obtain molten zinc, which is; then cast into ingots. - ZnO + C → Zn + CO This commercial zinc is known as zinc spelter, which is about 97% pure. It contains 1 to 3% lead and traces of arsenic and carbon as impurities. ### Refining It is purified by melting spelter in a furnace when two immiscible layers are formed; the lower layer is of lead because of its heaviness. The upper layer is separated and heated. Arsenic and other impurities are volatilized away to obtain pure zinc. Purification of crude zinc can also be done by fractional distillation. The boiling points of cadmium, zinc, lead and iron are 1040K, 1180K, 2024K and 3273K respectively. When molten impure zinc is distilled, molten cadmium distils over first. Pure molten zinc distils next. Pb and Fe with higher boiling points are left behind. ### Electrorefining Zinc is refined by the electrolysis of acidified zinc sulphate solution using impure zinc as anode and pure zinc as cathode. During electrolysis, zinc from anode dissolves in the solution and pure zinc deposits on the cathode ## Differences Between Metals and Non-Metals | | Metals | Non-metals | | -------- | -------- | -------- | | 1 | Physical state: Metals are solids at room temperature except a few like mercury and gallium which are liquids. | Physical state: Non-metals exist in all three states (only bromine is liquid). | | 2 | Density: Metals have a high density (except a few like Na, K) | Density: They have low densities; diamond is the only example that has a high density. | | 3 | Hardness: Metals are hard and strong (except a few like Na, K) | Hardness: Most of them are soft whereas a few are hard. Diamond is the hardest natural substance. | | 4 | Metals have high boiling and melting points. (except a few like Sn, Pb, Na, K, Hg) | Melting points and boiling points: The melting points and boiling points are low (except a few like carbon). | | 5 | Metals are highly malleable (except a few like Na, K) | Non-metals are generally brittle. | | 6 | Metals are highly ductile (except a few like Zn) | Non-metals are not ductile. | | 7 | Metals are good conductors of heat and electricity | Non-metals are bad conductors of heat and electricity (except graphite, gas-carbon). | | 8 | Metals are hard and not brittle (except a few like Zn) | Non-metals are generally brittle. | | 9 | Metals possess a high lustre (metallic lustre) and can be polished | Non-metals have a dull appearance, except a few | | 10 | Metals form alloys and amalgams. | Non-metals do not form alloys or amalgams. | | 11 | Metals usually do not dissolve in liquid solvents. | Non-metals dissolve in many liquid solvents like CS2, Chlorofom, etc. | ## Chemical Properties | | Metals | Non-metals | | -------- | -------- | -------- | | 1 | Electronic configuration: Atoms of metals usually have 1, 2 or 3 electrons in their outermost shell. | Atoms of non-metals usually have 5, 6 or 7 electrons in their outermost shell. | | 2 | Metals form cations by loss of electrons and electropositive. | Non-metals form anions by gain of electrons and are electronegative. | | 3 | Atomicity: Molecules of metals in vapour state are usually mono-atomic. | Molecul of non-metals are usually poly-atomic. | | 4 | Nature of oxides: Oxides of metals are usually basic (PbO, ZnO, Al2O3, SnO2 are amphoteric and Cr2O3, MnO2 are acidic) | Oxides of non-metals are usually acidic (some are neutral like H₂O, CO) | | 5 | Metals more reactive than hydrogen react with acids and replace the hydrogen in them. | Non-metals do not displace hydrogen from acids. | | 6 | Nature of chloride: Metallic chlorides are electrovalent compounds, are electrolytes. | Non-metallic chlorides are covalent and are non-electrolytes (exception: HCI) | | 7 | Oxidising and reducing property: They ionise by loss of electrons and hence are reducing agents Na→ Na⁺ + e- | They ionise by gain of electrons and hence are oxidizing agents (however carbon is a reducing agent) | | 8 | Nature of hydrides: Metal hydrides are unstable. Eg.: NaH, CaH2 | Non-metals form quite stable hydrides like CH4, NH3 and H2O. | ## General Chemical Properties of Metals ### a) Reaction of metals with air - Sodium burns in air when heated to from Na2O2 (Sodium peroxide) but when not heated forms Na2O. - 2Na(s) + O2 → Na2O2 - Calcium burns in air with a brick red flame to form calcium oxide (quick lime) - 2Ca + O2 → 2CaO - Magnesium burns in air to form a mixture of its oxide and nitride. - 2 Mg + O2 → 2MgO - 3 Mg + N2 → Mg3N2 - Aluminium on heating in air forms a mixture of its oxide and nitride - 4 Al + 3O2 → 2 Al2O3 - 2 Al + N2 → 2 AIN - Zinc on heating with air burns with a greenish blue flame to form ZnO - 2 Zn + O2→2 ZnO - Iron when exposed to moist air reacts very slowly to form a reddish brown solid which mainly consists of hydrated ferric oxide - 4 Fe + 3 O2 + 2H2O → 2Fe2O3. 2H2O - But when iron is heated in air it forms a magnetic oxide of iron. - 3Fe+2O2 → Fe3O4 - Copper is not affected by dry air at ordinary temperature. On exposure to moist air, it gets converted either to basic copper carbonate or sulphate depending upon the presence of CO2 and SO2 in air. - 2Cu + O2 + H2O + CO2 → CuCO3.Cu(OH)2 - 4Cu + 3O2 + 2H2O + 2SO2 → 2CuSO4. 2Cu(OH)2 - When copper is heated with oxygen it forms a product which is black from outside due to the formation of cupric oxide and is red from inside due to the formation of cuprous oxide but prolonged heating gives CuO. - 3Cu + O2 → CuO + Cu2O - 2Cu + O2 → 2CuO (prolonged heating) ### b) Reaction with water - Sodium reacts vigorously with cold water forming H2 and alkali - 2Na + 2 H2O → 2NaOH + H2 - Calcium reacts smoothly with cold water to form H2 and alkali - 2Ca + 2 H2O → 2Ca(OH)2 + H2 - Mg reacts with boiling water to form H₂ and metal oxide. - 2Mg + H2O → 2MgO + H2 - Al, Zn and Fe react with steam to form H₂ and metal oxide. Though aluminium is a reactive metal its true reactivity is not displayed because of the thin, sticky and non-corrosive oxide layer. Thus we are able to boil water in aluminium utensils. - 2Al + 3H2O → Al2O3 + 3H2 - Zn + H2O → ZnO + H2 - 3Fe + 4H2O → Fe3O4 + 4H2 - Cu does not react with H₂O ### c) Reactions with Acids - Na reacts explosively with dilute acids liberating H2 - 2Na + 2HCl → 2NaCl + H2 - Ca reacts less vigorously with acids liberating H2 - Ca + 2HCl → CaCl2 + H2 - Mg, Zn, Al and Fe reacts with dilute acids with decreasing vigour liberating H2. Though aluminium is a reactive metal its true reactivity is not displayed because of the thin, sticky and non-corrosive oxide layer. Concentrated nitric acid renders Al passive. - Mg + 2HCl → Mg Cl2+H2 - Zn + 2 HCl → ZnCl2+H2 - 2A1 + 6HCl → 2AlCl3 + 3H2 - Fe + 2HCl → FeCl2 + H2 - Cu being below hydrogen in the reactivity series cannot liberate H₂ from acids but oxyacids oxidize Cu. - 3 Cu + 8HNO3 → 3Cu(NO3)2 + 4H2O + 2NO (with dilute acids) - Cu + 4HNO3 → Cu(NO3)2 + 2NO2 + 2H2O (with concentrated acid) - Cu + 2H2SO4 → CuSO4 +2H2O +SO2 ## Some Compounds of Metals Commonly in Use. ### Compounds of Sodium 1. **Common salt ( NaCl )** The main source of NaCl is sea water, which contains 2.7 - 2.9% of NaCl. It is got by evaporation of sea water. CaCl2 and MgCl2 are undesirable impurities present in it because they are deliquescent and make the salt wet during the rainy season. NaCl is an essential constituent of our food. It is used for manufacturing caustic soda, washing soda, baking soda etc. It is also seed in freezing mixtures. 2. **Washing soda – Na2CO3.10H2O** It is manufactured generally by Solvay's process. In this process CO2 is bubbled through brine (NaCl solution) saturated with ammonia. Ammonium bicarbonates formed reacts with sodium chloride to form sodium bicarbonate, which precipitates due to its lower solubility. - NH3 + H2O + CO2 → NH4HCO3 - NaCl + NH4HCO3 → NaHCO3 + NH4Cl The precipitate is separated and ignited to get sodium carbonate. - 2 NaHCO3 → Na2CO3 + H2O + CO2 When sodium carbonate is crystallised from water it will have 10 molecules of water of crystallisation to form (Na2CO3.10H2O). When kept open it slowly loses its water of crystallization to form Na2CO3 monohydrate. - Na2CO3.10H2O→ Na2CO3. H2O + 9H2O Solution of sodium carbonate is alkaline in nature because it hydrolyses in water to give H2CO3 and NaOH. - Uses: Softening of hard water, as an important lab reagent, washing in laundries, manufacture of glass, soap. 3. **Baking soda - NaHCO3** It is obtained as an intermediate product of Solvay's process of manufacture of Na2CO3. - Uses: As a component of baking powder, used in fire extinguishers, used in medicines. 4. **Caustic sode (NaOH)** It is a white translucent solid, deliquescent in nature. It is readily soluble in water to give strong alkaline solution. It is prepared by electrolysis of NaCl solution in a specifically designed electrolytic cell - Castner-Kellner cell or Nelson's cell. - Uses: It is used as a strong base in the lab, it is used in metallurgy. e.g., concentration of bauxite. ### Compounds of Calcium 1. **Quick lime (CaO)** It is prepared by heating limestone (CaCO3) at a temperature of 1070-1270K. - (CaCO3 CaO + CO2) - Uses: Purification of sugar, tanning softening of hard water. 2. **Slaked lime (Ca(OH)2)** It is prepared by adding water to quick lime. It is a white amorphous powder sparingly soluble in water. - CaO + H2O → Ca(OH)2 - Uses: It is used in white washing manufacture of bleaching powder, as laboratory reagent. 3. **Plaster of Paris (CaSO4. 12 H2O)** It is a hemihydrate of CaSO4. It is prepared by heating gypsum (CaSO4.2H2O) at a temperature of 390K. - CaSO4.2H2O - CaSO4.2 H2O + 1/2 H2O - Uses: Making models, used in setting fractures, making casts for making statues and other metal articles. 4. **Bleaching powder:** It is calcium oxy chloride. When chlorine is passed over slaked lime Ca(OH)2, bleaching powder is formed. - 2 Ca(OH)2 + 2Cl2 → CaCl2 + Ca(OCl)2 + 2H2O