. Metallurgy: Science and technology of extracting metals from their ores and of compounding alloys. Most metals are found in nature in solid inorganic. - презентация

1 . Metallurgy: Science and technology of extracting metals from their ores and of compounding alloys. Most metals are found in nature in solid inorganic compounds (minerals) except Au and Pt, Pd, Rh, Ru, Ir, Os. N.B source of minerals oxides (O 2- ), sulphides (S 2- ) and carbonates (CO 3 2- ). Silicates are abundant, BUT they are generally difficult to treat (not economically viable)

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3 . What is involved? Mining (then crushing and grinding) Concentrating the ore remove the gangue e.g. magnetic separation, flotation, amalgamation Reducing the concentrate to obtain free metal Refining/purifying the metal Alloying mixing metal with other elements to modify properties

4 PYROMETALLURGY Use high temperatures to alter the mineral chemically and reduce it to the free metal. It requires large amounts of energy and often the source of atmospheric pollution. Used in concentration, reduction and refining processes. Some pyrometallugical processes include; Calcination: Heat ore to decompose and eliminate volatile products. Carbonates are often calcined to drive off CO 2 and form metal oxide e.g. PbCO 3 (s) PbO(s) +CO 2 (g) Δ Require temps ~ o C (1000 o C for CaCO 3 ) Hydrated minerals lose H 2 O at ~ o C

5 . Roasting: Themal treatment causes chemical reaction between ore and furnace atmosphere. Leads to reduction or oxidation (depending on atm) and may be accompanied by calcination.e.g. Roasting of sulphides to produce oxides 2ZnS(s) + 3O 2 (g) 2ZnO(s) + 2SO 2 (g) e.g. Sulphides of less active metals can be roasted to the free metal; HgS(s) + 3O 2 (g) Hg(g) + SO 2 (g) e.g. Free metal also obtained by using reducing atm: PbO(s) + CO(g) Pb(l) + CO 2 (g) Smelting: Melting process in which material separates into 2 or more layers in a chemical reaction. Form molten metal + slag

6 . Slag forms when basic metal oxide reacts at high temps with molten silica (SiO 2 ) e.g. CaO(l) + SiO 2 (l) CaSiO 3 (l) Slag = mainly molten silicate minerals (with aluminates, phospates, fluorides and other inorganic compounds) Refining: Treatment of crude, relatively impure metal product to improve purity. Produce pure metal or mixture of well-defined composition. PYROMETALLURGY OF IRON: Source: iron oxide minerals e.g. hematite = Fe 2 O 3, magnetite = Fe 3 O 4 Reduction of iron oxides in blast furnace Charged at the top with mixture of: iron ore + coke + limestone

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8 . Coke = coal heated in absence of air to drive off volatiles; = 85-90% C; Fuel produces heat as it is burned in lower part of furnace Also, source of reducing gases CO + H 2 Limestone = CaCO 3 ; Source of basic oxide in slag formation; Hot air blown in from bottom of furnace for combustion of coke. Reactions in furnace: Combustion of coke: 2C(s) + O 2 (g) 2CO ΔH = -221 kJ ; Water vapour also reacts with coke:2C(s) + H 2 O(g) CO +2H 2 ΔH = +131 kJ Limestone is calcined in upper part of furnace: CaCO 3 (s) CaO(s) + CO 2 ; Iron oxides reduced by CO and H 2 in upper part of furnace:

9 . e.g. Fe 3 O 4 (s) + 4CO(g) 3Fe(s) + 4CO 2 (g); ΔH = -15 kJ Fe 3 O 4 (s) + 4H 2 (g) 3Fe(s) + 4H 2 O(g); ΔH = +150 kJ. Reduction of other elements also occurs in hottest part of furnace where C is the major reducing agent. Molten iron collects at bottom of furnace with a layer of slag on top. Tap furnace periodically to remove iron(also drain off slag). Tapped iron is NOT PURE contains Si, Mn, P, S, C. Formation of steel: Remove impurities by oxidation in a converter; use O 2 or O 2 + Ar as oxidising agent; C & S expelled as CO and SO 2 gases. Si oxidised to SiO 2 and adds to slag. Presence of basic slag NB for P removal: 3CaO(l) + P 2 O 5 (l) Ca 3 (PO 4 ) 2 (l); Monitor O 2 coming off to see when reaction is complete.

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11 Alloying elements added to produce steel. Different elements and different amount to give different properties. Types of steel

12 Hydrometallurgy Leaching: Desired metal containing compound is selectively dissolved. Leaching agents used: H 2 O or aqueous solns of acid/base/saltOften form a complex ion. e.g. Gold concentrated by the process of cyanidation; Leaching agent = NaCN soln in presence of air 4Au(s) + 8CN - (aq) + O 2 (g) + H 2 O(l) 4Au(CN) 2- (aq) + 4OH - (aq) ;Au(CN) 2- is then reduced by zinc powder; 2Au(CN) 2- + Zn(s) Zn(CN) 2 2- (aq) + 2Au(s)

13 Electrometallurgy Electrolysis of molten salt or aqueous solution. e.g. Na, Mg, Al; H 2 O more easily reduced than metal ion. Electrorefining of Copper to obtain pure Cu Anodes = large slabs of crude Cu; Cathodes = thin sheets of pure Cu; Electrolyte = acid soln of CuSO 4 ; Apply voltageget oxidation of Cu at anode and reduction of Cu 2+ at cathode; Oxid. & reduct. more readily than that for H 2 O. Impurities in anode includes: Zn, Ni, As, Se, Te, precious metals incl. Au & Ag Metallic impurities that are more active than Cu are also readily oxidised at the anode, but do not plate at the cathode because their reduction potentials are more negative than that for Cu 2+. Less active metals are not oxidised at the anode collect as sludge below the anode

14 Question: Ni is one of the main impurities in the crude copper of the anode. What happens to the nickel during the electrolytic process?

15 Alloys Alloy = Material containing more than 1 element and has characteristic properties of metals. Modify properties of metal. e.g. Gold is often too soft generally alloy with copper to harden it; 24 karat = pure gold and Jewelry: generally ~14 karats 58% gold; Yellow/white gold depends on metal added for alloying. Alloys: There are Solution alloys, Heterogeneous alloys and Intermetallic compounds. Solution alloys can either be Substitution alloy or Interstitial alloy.

16 Solution alloys are Homogeneous mixtures in which the components are dispersed randomly and uniformly. For Substitution alloy atoms of the solute take position usually occupied by solvent atom. 2 metallic components have similar radii and chemical-bonding characteristics e.g. Ag and Au. When radii differ by >~15%, solubility becomes limited.

17 Interstitial alloy atoms of the solute occupy interstitial positions. Component in interstitial positions must have much smaller covalent radii than the solvent atoms. Interstitial element typically non-metal and bonds to neighbouring atoms. Extra bonds causes metal lattice to become harder, stronger and less ductile.

18 . Heterogeneous alloys: components are NOT dispersed uniformly. e.g. In pearlite (form of steel). We have 2 distinct phases in alternating layers:1) pure iron and 2) Fe 3 C (cementite) Note: the way in which the solid was formed e.g. rapid cooling vs fast cooling of molten mixture leads to different properties. Intermetallic compounds: Homogeneous alloys that have definite properties and compositions. e.g. Ni 3 Al used in jet engines. It is strong and have low density;e.g. Cr 3 Pt coat razor blades. It is hard. e.g. Co 5 Sm permanent magnets in headsets. It has high magnetic strength per unit mass.