1) elemental form e.g. ag, au, pt – noble metals. 2) aluminosilicates and silicates metal + al,...
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1) Elemental Forme.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
OCCURRENCE OF METALS
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MetallurgyMetallurgy the process of obtaining a metal from its ores
1) Preliminary treatment to concentrate ore:Floatation.Hindered settlingMagnetic 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.
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HydrometallurgyHydrometallurgy
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])
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SILVERSILVER
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.
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COPPERCOPPER
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
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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.
ElectrometallurgyElectrometallurgy
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ElectrometallurgyElectrometallurgy
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Hydrometallurgy of AluminumHydrometallurgy 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) precipitatesTakes advantage of the amphoteric nature of Al oxide.
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Electrometallurgy of AluminumHall process is used to obtain aluminum metal.
Problem: Al2O3 melts at 2000C and it is impractical to perform electrolysis on the molten salt.
• Hall: use purified Al2O3 in molten cryolite (Na3AlF6, melting point 1012C).
Anode: C(s) + 2O2(l) CO2(g) + 4e
Cathode: 3e + Al3+(l) Al(l)• The graphite rods are consumed in the reaction.
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Electrometallurgy of AlElectrometallurgy of Al
The Hall Process
Anode: C(s) + 2O2-(l) CO2(g) + 4e-
Cathode: Al3+(l) + 3e- Al(l)
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Electrometallurgy of Sodium
Sodium is produced by electrolysis of molten NaCl.
CaCl2 is used to lower the melting point of NaCl from 804C to 600C.
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
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• 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)
PyrometallurgyPyrometallurgy
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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.
PyrometallurgyPyrometallurgy
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Blast Furnace
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Pyrometallurgy of FePyrometallurgy 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 furnace2) reduces iron
Why is limestone (CaCO3) added?
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Pyrometallurgy of FePyrometallurgy 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: cementcinder blockbuilding materials
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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
Metals and AlloysMetals and Alloys
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ALLOYS1) 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
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Homogeneous (solution) alloys
substitutional interstitial
Cr in Fe C in low-carbon steel
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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).
SOLUTION ALLOYS
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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 cargoAirplane: 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!
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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
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Dislocations Move Under Stress
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)
shear force
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Hardening of Alloys
Structural alloys - e.g., girders, knife blades, airplane wings
Need to minimize movement of dislocations. How?
1. Use annealed single crystals (expensive)Some specialty applications – e.g. jet turbine bladeImpossible 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)
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Hardening of Alloys (contd.)
Work Hardening and Annealing have opposite effects
Annealing: crystal grains grow, dislocations move (metal becomes more malleable)
3. Alloying – homogeneous or heterogeneous Impurity atoms or phases “pin” dislocations.
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Metal Crystal StructuresMetal Crystal Structures
Body-centered cubic (bcc)8 nearest neighborsNot close packed
Close packed(hexagonal or cubic)
hcp ccp
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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)
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http://www.its.caltech.edu/%7Evitreloy/development.htm
Amorphous (Glassy) Alloys
Metals are typically polycrystalline
Amorphous alloys have superior mechanical properties because dislocations cannot move.
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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
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STEELSSTEELS
Steel: Fe (pig iron) + small amounts of C
Mild Steel: <0.2% C – malleable and ductileused in cables, nails, and chains.
Medium Steel: 0.2-0.6% C – toughused in girders and rails.
High Carbon Steel: 0.6-1.5% C – very toughused in knives, tools, and springs.
Stainless Steel:73% Fe, 18% Cr, 8% Ni, 1% C.