OCCURRENCE OF METALS 1) Elemental Form e.g. Ag, Au, Pt – noble metals. 2) Aluminosilicates and Silicates Metal + Al, Si, O e.g. Beryl = Be3Al2Si6O18 Hard to extract metals. 3) Nonsilicate Minerals Oxides – Al2O3, TiO2, Fe2O3 Sulfides – PbS, ZnS, CuFeS2 Carbonates – CaCO3

Metallurgy the process of obtaining a metal from its ores Preliminary treatment to concentrate ore: Floatation. Hindered settling Magnetic separation 2) Further purification and reduction to obtain the metal in its elementary state: Hydrometallurgy – leaching. Pyrometallurgy – roasting, smelting. Electrometallurgy. 3) Final purification and refining of the metal.

Hydrometallurgy Metal is refined from ore using aqueous reactions Example: Dissolve Au by forming complex ion with CN 4Au(s) + 8CN(aq) + O2(g) + 2H2O(l)  4[Au(CN)4](aq) + 4OH(aq) Kf[Au(CN)4] = 2x1038 Pure gold is then obtained by reduction: 2Au(CN)4(aq) + 3Zn(s)  3Zn2+(aq) + 8CN-(aq) + 2Au(s) Similar process for silver (dissolves as [Ag(CN)2])

SILVER Found as pure metal (Ag) or sulfide (Ag2S) [Ag(CN)2] Kf = 1 x 1021 4Ag + 8CN(aq) + O2 + 2H2O  4[Ag(CN)2](aq) + 4OH(aq) Ag2S + 4CN(aq)  2[Ag(CN)2](aq) + S2(aq) Practice problem: Use Kf, with E0 and Ksp values (from tables) to calculate Keq for these reactions.

COPPER Electrolyzed to Cu Copper containing ore (CuFeS2) is stirred with aqueous H2SO4 + O2 2CuFeS2(s)+2H+(aq)+SO42(aq) + 4O2(g)  2Cu2+(aq) + 2SO42-(aq) + Fe2O3(s) + 3S(s) + H2O \ / 2CuSO4(aq)  Electrolyzed to Cu

Electrometallurgy Electrorefining of Copper Slabs of impure Cu are used as anodes, thin sheets of pure Cu are the cathodes. Acidic copper sulfate is used as the electrolyte. The voltage across the electrodes is designed to produce copper at the cathode. The metallic impurities do not plate out on the cathode. Metal ions are collected in the sludge at the bottom of the cell.

Electrometallurgy

Hydrometallurgy of Aluminum Aluminum is the second most useful metal. Bauxite: Al2O3.xH2O. primary ore for Al impurities: SiO2 Fe2O3 Bayer Process Bayer process: bauxite (~ 50 % Al2O3) is concentrated to produce aluminum oxide. Dissolve bauxite in strong base (NaOH) at high T, P Al2O3 dissolves [Al(H2O)2(OH)4] hydrated metal complex Filter out solids Fe2O3, SiO2 do not dissolve Lower the pH so that Al(OH)3(s) precipitates Takes advantage of the amphoteric nature of Al oxide.

Electrometallurgy of Aluminum Hall process is used to obtain aluminum metal. Problem: Al2O3 melts at 2000C and it is impractical to perform electrolysis on the molten salt. Hall: use purified Al2O3 in molten cryolite (Na3AlF6, melting point 1012C). Anode: C(s) + 2O2(l)  CO2(g) + 4e Cathode: 3e + Al3+(l)  Al(l) The graphite rods are consumed in the reaction.

Electrometallurgy of Al The Hall Process Anode: C(s) + 2O2-(l)  CO2(g) + 4e- Cathode: Al3+(l) + 3e-  Al(l)

Electrometallurgy of Sodium Sodium is produced by electrolysis of molten NaCl. CaCl2 is used to lower the melting point of NaCl from 804C to 600C. At the cathode (iron): 2Na+(aq) + 2e  2Na(l) At the anode (carbon): 2Cl-(aq)  Cl2(g) + 2e All metals in Groups I and II are obtained by molten salt electrolysis

Pyrometallurgy Pyrometallurgy: using high temperatures to obtain the free metal. Calcination is heating of ore to eliminate a volatile product: PbCO3(s)  PbO(s) + CO2(g) Roasting is oxidation of the ore: Burns off organic matter. Converts carbonates and sulfides to oxides: 2 ZnS(s)+ 3O2(g) 2ZnO(s) + SO2(g) 3. Less active metals are often reduced HgS(s) + O2(g)  Hg(l) + SO2(g)

The Pyrometallurgy of Iron sources of iron: hematite Fe2O3 and magnetite Fe3O4. Iron Ore: Iron oxides and SiO2 Add limestone and coke Coke is coal that has been heated to drive off the volatile components.

Blast Furnace

Pyrometallurgy of Fe Reactions 2C(s) + O2(g)  2CO(g) + heat heat + C(s) + H2O(g)  CO(g) + H2(g) Fe3O4(s) + 4CO(g)  3Fe(l) + 4CO2(g) Fe3O4(s) + 4H2(g)  3Fe(l) + 4H2O(g) Coke: 1) heats furnace 2) reduces iron Why is limestone (CaCO3) added?

Pyrometallurgy of Fe At high T CaCO3  CaO + CO2 CaO + SiO2  CaSiO3(l) Metal + nonmetal  slag oxide oxide basic acidic Limestone (CaCO3) removes SiO2 (and other) impurities slag floats on Fe(l); protects it from oxidation by O2 Slag: cement cinder block building materials

Physical Properties of Metals Metals and Alloys Physical Properties of Metals Important physical properties of pure metals: malleable, ductile, good conductors of heat and electricity. Metals are crystals in which every atom has 8 or 12 neighbors. There are not enough electrons for the metal atoms to make electron pair bonds to each neighbor. Alloys: Mixtures of metals - often have improved physical properties

ALLOYS Homogeneous (solution) alloys: Mixed at the atomic level - one solid phase 2) Heterogeneous alloy: Non-homogeneous solid (e.g. pearlite steel has two phases: almost pure Fe and cementite, Fe3C). 3) Intermetallic alloys – compounds of two different metals having definite proportions: e.g. Cr3Pt – razor blades. Ni3Al – jet engines, lightweight and strong. Co5Sm – permanent magnets in headsets. Au3Bi, Nb3Sn – superconductors

Homogeneous (solution) alloys substitutional interstitial Cr in Fe C in low-carbon steel

SOLUTION ALLOYS Two kinds: Substitutional alloy – when one metal substitutes for another in the structure. metals must have similar atomic radii, metals must have similar bonding characteristics. Interstitial alloy – when a non-metal is present in the “holes” in a metal crystal lattice. Interstitial atoms are smaller The alloy is much stronger than the pure metal (increased bonding between nonmetal and metal). Example steel (contains up to 3 % carbon).

Mechanical Properties of Metals and Alloys Hypothetical situation: Upon graduation, you go to work for Boeing. Your job – select a high-strength Al alloy for jet airplanes. 50 tons cargo Airplane: 500 tons } 150 tons plane structure 300 tons fuel If you can triple the alloy strength, you can triple cargo load (to 150 tons). Material Tensile Yield Stress (psi) pure (99.45%) annealed Al 4 x 103 pure (99.45%) cold drawn Al 24 x 103 Al alloy - precipitated, hardened 50 x 103 big improvement But, “perfect” single crystal Al as a yield stress of ca. 106 psi!

Defects in Metallic Crystals Defects are responsible for important mechanical properties of metals: malleability, yield stress, etc. Non-directional bonding, large number of nearest neighbor atoms  metallic structures readily tolerate “mistakes” vacancy dislocation (missing atom) (extra plane of atoms) point defect line defect Not important Very important

Dislocations Move Under Stress shear force Key point: Moving a dislocation breaks/makes a line of metal-metal bonds (easy) Shearing a perfect crystal means we have to break a plane of bonds (requires much more force)

Hardening of Alloys Structural alloys - e.g., girders, knife blades, airplane wings Need to minimize movement of dislocations. How? Use annealed single crystals (expensive) Some specialty applications – e.g. jet turbine blade Impossible for large items (airplane wings, bridges…) Work hardening - moves dislocations to grain boundaries planar defect (stronger under stress) “Cold working” or “drawing” of a metal increases strength and brittleness (e.g., iron beams, knives, horseshoes)

Hardening of Alloys (contd.) Work Hardening and Annealing have opposite effects Annealing: crystal grains grow, dislocations move (metal becomes more malleable) Alloying – homogeneous or heterogeneous Impurity atoms or phases “pin” dislocations.

Metal Crystal Structures Body-centered cubic (bcc) 8 nearest neighbors Not close packed Close packed (hexagonal or cubic) hcp ccp

Malleability of Metals and Alloys Some metals are soft and ductile (Au, Ag, Cu, Al, etc.) Others are hard (Fe, W, Cr, etc.) Why? Crystal structure is important. Two types: body centered cubic (bcc) - 8-coordinate - hard close packed (fcc and hcp) - 12-coordinate - soft Close-packed planes slip easily Non-close packed - “speed bumps” Cu (fcc) CuZn alloy (brass) Zn (hcp)

Amorphous (Glassy) Alloys Metals are typically polycrystalline Amorphous alloys have superior mechanical properties because dislocations cannot move. http://www.its.caltech.edu/%7Evitreloy/development.htm

Iron and Steels Below 900oC, iron has bcc structure - “hard as nails” Above 900oC, iron is close packed (fcc) - soft Can be worked into various shapes when hot Steelmaking: Carbon steel contains ~ 1% C by weight (dissolves well in fcc iron but not in bcc) Slow cooling (tempering): fcc Fe/1%C  mixture of bcc Fe and Fe3C (pearlite) Fe3C (cementite) grains stop movement of dislocation in high carbon steel - very hard material

STEELS Steel: Fe (pig iron) + small amounts of C Mild Steel: <0.2% C – malleable and ductile used in cables, nails, and chains. Medium Steel: 0.2-0.6% C – tough used in girders and rails. High Carbon Steel: 0.6-1.5% C – very tough used in knives, tools, and springs. Stainless Steel: 73% Fe, 18% Cr, 8% Ni, 1% C.