Transcript No Slide Title

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
1) 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
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) + 4eCathode: 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:
1. Burns off organic matter.
2. 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)
Pyrometallurgy
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 
oxide
oxide
basic
acidic
slag
Limestone (CaCO3)
removes SiO2 (and other) impurities
slag floats on Fe(l); protects it from oxidation by O2
Slag: cement
cinder block
building materials
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
1) 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
Cr in Fe
interstitial
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.
Airplane: 500 tons }
50 tons cargo
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
(missing atom)
point defect
Not important
dislocation
(extra plane of atoms)
line defect
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?
1. Use annealed single crystals (expensive)
Some specialty applications – e.g. jet turbine blade
Impossible for large items (airplane wings, bridges…)
2. 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)
3. 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
Cu (fcc)
Zn (hcp)
Non-close packed - “speed bumps”
CuZn alloy (brass)
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.