1) Elemental Form e.g. Ag, Au, Pt – noble metals. 2) Aluminosilicates and Silicates Metal + Al,...
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Transcript of 1) Elemental Form e.g. Ag, Au, Pt – noble metals. 2) Aluminosilicates and Silicates Metal + Al,...
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
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.
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])
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.
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
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
ElectrometallurgyElectrometallurgy
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.
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.
Electrometallurgy of AlElectrometallurgy 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 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
• 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
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
Blast Furnace
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?
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
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
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
Homogeneous (solution) alloys
substitutional interstitial
Cr in Fe C in low-carbon steel
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
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!
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
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
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)
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 StructuresMetal Crystal Structures
Body-centered cubic (bcc)8 nearest neighborsNot 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)
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.
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
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.