13Metallurgy_AlloysMJ

7/30/2019 13Metallurgy_AlloysMJ

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Metals

Metallurgy

How we get metals

Bonding and structure of metals

Alloys, composites

Properties of metals and alloys

Mechanical properties

Electrical properties

Most metals are found in minerals.

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, CuFeS

2 Carbonates CaCO3

OCCURRENCE OFMETALS


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Steps in MetallurgySteps in Metallurgy

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.

HydrometallurgyHydrometallurgyMetal is extracted from ore using aqueous

reactions

Leaching:

a metal is selectively dissolved

Dissolution agent: acid, base, salt.

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)2]! = 2x1038

The gold is then obtained by reduction:

2Au(CN)2!(aq) + Zn(s) " Zn(CN)4

2!(aq) + 2Au(s)


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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) isconcentrated 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 pH, Al(OH)3(s) precipitates

Take advantage of amphoteric nature of Al

Electrometallurgy of Aluminum

Hall process electrolysis cell is used toproduce aluminum.

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.

ElectrometallurgyElectrometallurgy

Electrometallurgy is the process ofobtaining metals through electrolysis.

Two different starting materials:

molten salt or aqueous solution.


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The Hall ProcessTo produce 1000 kg of Al, we need

4000 kg of bauxite,

70 kg of cryolite,

450 kg of C anodes and

56 # 109J of energy.

ElectrometallurgyElectrometallurgy of Alof Al ElectrometallurgyElectrometallurgyElectrorefining of Copper

Because of its good conductivity, Cu is used to

make electrical wiring.

Impurities reduce conductivity, therefore pure

copper is required in the electronics industry.


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Pyrometallurgy: using high temperatures

to obtain the free metal.

Several steps are employed:

Calcination is heating of ore to cause

decomposition and elimination of a

volatile product:

PbCO3(s) " PbO(s) + CO2(g)Roasting is heating which causes

chemical reactions between the ore

and the furnace atmosphere:

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)

PyrometallurgyPyrometallurgysources of iron:

hematite Fe2O3 and magnetite Fe3O4.

Iron Ore: Fe2O3 and SiO2

The Pyrometallurgy of Iron

Add limestone

and coke

Coke is coal

that has been

heated to drive

off the volatile

components.

The blast furnace


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PyrometallurgyPyrometallurgy of Feof 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?

PyrometallurgyPyrometallurgy of Feof 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


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PyrometallurgyPyrometallurgy of Ironof Iron

Product in blast furnace: pig iron

brittle; not strong

Bessemer Converter

O2 (g) bubbled through molteniron to oxidize remaining

impurities

CaO slag still present to remove

impurities

Alloying elements added as

liquid iron is being removed.

Metals (75% of elements)

Lustrous (reflect light)

(almost) all solids

malleable & ductile

good conductors of heat and electricity

oxides are basic ionic solids

aqueous cations (n+)

reactivity increases downwards in family

Properties of metalsProperties of metals


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Free Electron Model

Metals are positive ions in sea of nearly

free electrons

Electrons bond metal ions together but are

free to roam the crystal lattice.

Explains malleability, ductility , and highelectrical and thermal conductivity.

Bonding in metalsBonding in metals

Important physical properties of pure metals:

malleable, ductile, good conductors of heat

and electricity.

Metals are crystals

every atom has 8 or 12 neighbors.

There are not enough electrons for the

metal atoms to make electron pair bonds

to each neighbor.

STRUCTURE andSTRUCTURE and

PROPERTIES of METALSPROPERTIES of METALS


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Body-centered cubic (bcc)

8 nearest neighbors

Not close packed

Close packed(hexagonal or cubic)

hcp ccp

Metal Crystal StructuresMetal Crystal StructuresMalleability 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 - hardclose packed (fcc and hcp)

-12-coordinate - soft

Close-packed planes Non-close packed

slip easily - speed bumps

Cu (fcc) CuZn alloy (brass)

Zn (hcp)


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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 has a yield

stress of ca. 106 psi!

Mechanical Properties of Metals

and Alloys Defects are responsible for importantmechanical properties of metals:

malleability, yield stress, etc.

Non-directional bonding, large number ofnearest neighbor atoms "metallic

structures readily tolerate mistakes

vacancy dislocation(missing atom) (extra plane of atoms)

point defect line defect

Not important Very important

Defects in Metallic Crystals


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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 wehave 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. jetturbine 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)


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Hardening of Alloys (contd.)

Work HardeningandAnnealinghave

opposite effects

Annealing: crystal grains grow, dislocations

move (metal becomes more malleable)

3. Alloying homogeneous or

heterogeneousImpurity atoms or phases pin

dislocations.

ALLOYSAlloys: Mixtures of metals

- often have improved physical properties

1) Homogeneous (Solution) alloy

Mixed at the atomic level

2) Heterogeneous alloy

non-homogeneous dispersions.(e.g. pearlite steel has two phases: almost pure

Fe and cementite, Fe3C).

3) Intermetallic compounds

compounds of two different metals

having definite compositions:

Examples

Cr3Pt razor blades.

Ni3Al jet engines, lightweight and strong. Co5Sm permanent magnets in headsets.

Au3Bi, Nb3Sn superconductors for low

temperature, high field magnets


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There are two types of solution alloy:

Substitutional alloy when one metal substitutes for

another in the structure.

atoms must have similar atomic radii

elements must have similar bonding

characteristics.

SOLUTION ALLOYS

Interstitial alloy when a non-metal is present in theinterstices of the metal.

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).

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 notin 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 hardmaterial


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STEELSSTEELS

Steel: Fe (pig iron) + small amounts of C

Mild Steel:


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Band Theory Atomic orbitals (AO) mix to form molecular

orbitals (MO).

Start with 2 AO, end with 2 MO

Start with n AOs, end up with n MOs

In metals energy difference between orbitals in

valence band is small.

Orbital form a continuous band of allowed

energy states.

Bonding in MetalsBonding in MetalsMETAL: ConductorValence electrons do not fill available orbitals

(not enough electrons)

Insulator or semiconductor

Valence band is full (or completely empty).

Energy gap separates valence band from

empty orbitals.

Conduction and InsulationConduction and Insulation


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Insulators: The energy gap is very

large in some solids: these solids will

be poor electrical conductors.

Semiconductors: the energy gap may

be smaller in some solids. The

conductivity of semi-conductors can beincreased with T, or applied fields.

Bonding in MetalsBonding in Metals SEMICONDUCTORS

Add impurities(dopants)to semi-conductorIf impurities donate extra electrons,

then the semiconductor is n-type

e.g. P impurities in Si.

If impurities accept electrons,

then the semiconductor is p-type

e.g. B impurities in Si.

n-type: negative charge carriers (electrons).

p-type: apparent positive charge carriers (holes).


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Properties:

shiny, silvery gray

brittle

Poor thermal conductor

SEMI-METAL

Uses:

alloy (with Al, Mg)

Silicone polymers

Electronic applications

for these applications very pure silicon(


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A diode is a semiconductor with a p-typematerial bonded to an n-type material.

Solar cells (photovoltaics) and lightemitting diodes (LEDs) are both diodedevices.

DiodesDiodes

When no current flows

DiodesDiodes

A diode allows current to flow in only onedirection

Electrons can flow from n-type to p-typeunder forward bias

In a solar cell, light excitation makes currentflow in the opposite direction.

Current flows when the diode is forward biased


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Light Emitting DiodeLight Emitting Diode

When electrons combine with holes, light is

emitted.

The energy of light (E = h$) is the same as theband gap energy Eg

The band gap energy depends on the material

used to make the diode.

LEDs are small light bulbs that produce

useful light from the from the flow of

electrons through a semiconductor

diode.

If the semiconductor used is a silicon

based semiconductor the light

produced is infrared.

Used in TV remote controls

LEDs producing visible light are

typically made from doped Aluminum-

Gallium-Arsenide (AlGaAs) conductor.

Changing the dopants changes the

size of the depletion zone and the color

of visible light produced

LEDsLEDs

::

LL

ightight

EE

mittingmitting

DD

iodesiodes


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LED MaterialsLED Materials

GaP/GaPgreen555

Gap: N/GaPgreen565

GaAs0.15PO.85: N/GaPyellow590

GaAs0.25Po.75: N/GaPorange610

GaAs0.35PO.65: N/GaPred630

GaAI0.35 As/GaAsred660

GaP: Zn-O/GaPred700

Material and structure

of LEDsColor

Wavelength

nm

LEDsLEDs:: LLightight EEmittingmitting DDiodesiodes

More energy efficient than incandescent lighting

LEDs producing visible light are typically madefrom doped Aluminum-Gallium-Arsenide(AlGaAs)


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Where areWhere are LEDsLEDs used?used?Ceramics

Ceramics

inorganic, nonmetallic, solids,

can be covalent network and/or ionic

bonded

Properties:

Crystalline or amorphous (e.g. glass),hard, brittle,

stable to high temperatures,

less dense than metals,

more elastic than metals,

very high melting.

Examples: alumina (Al2O3),

carbides (SiC, Ca2C),Oxides (BeO, ZrO2, Al2O3)

Nitrides (BN).


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SuperconductivitySuperconductivity

Superconductors show no resistance to

flow of electricity.

Superconducting behavior starts below the

superconducting transition temperature, Tc.

Meissner effect: permanent magnets

levitate over superconductors. The

superconductor excludes all magnetic field

lines, so the magnet floats in space.

Superconducting

Ceramic Oxides


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The depletion zone can be removed when

the diode is connected to a battery.

The n-type material is connected to the

negative portion of a battery.

The p-type material is connected to thepositive portion of the battery.

DiodesDiodes

COPPER CONTAININGCOPPER CONTAININGORESORES

Copper containing ore (CuFeS2) is stirred

with aqueous H2SO4 + O2

2CuFeS2(s)+2H+(aq)+SO4

2!(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

Metallurgy is the science and

technology of extracting metals from

minerals.

There are five important steps:

Mining

getting the ore out of the ground

Concentrating

preparing it for further treatment

Reduction

to obtain the free metal in the zero

oxidation state

Refining

to obtain the pure metal Mixing with other metals

to form an alloys.

METALLURGYMETALLURGY

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13Metallurgy_AlloysMJ