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Materials for Civil and Construction Engineers

CHAPTER 4

Aluminum

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Aluminum

Primarily used for containers, packaging, aircrafts,

and automobiles. In civil projects, primarily used for architectural and

finishing elements like doors, windows, and siding

with a small amount used for electrical wiring. Not used extensively for structural members:

expense

strength and ductility

coefficient of thermal expansion

2

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h

2x

x

W

b

In many beam design problems deflection is a limiting criteria. Assume a rectangular simply supported beam, and the height of the beam is fixed by other design considerations, determine the difference in width required for an aluminum beam compared to a steel beam.

The equation for the deflection in a beam is:

EI

xW

48

)2( 3

Using the subscripts a and s for aluminum and steel respectively, and using equal deflection for the aluminum and steel beam:

ssaa IE

xW

IE

xW

48

)2(

48

)2( 33

Which reduces to:

ssaa IEIE For a rectangular section the moment of inertia is:

3

12

1bhI

By substitution and ha = hs:

sa

sa bE

Eb

Since Es = 29E6 and Ea = 10.5E6

Ba = 2.76 Bs

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Aluminum Advantages

Most plentiful metal on earth

One-third the density of steel

High strength-to-weight ratio

Good thermal and electrical conductivity

Anodizing or hard coating for protection

Weldable alloys

Easy to recycle Corrodes slightly but

does not rust High reflectivity Can be die cast Easily machined Nonmagnetic Nontoxic

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Aluminum Production

Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.

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Crushing and Grinding

Four tons of bauxite are required to produce 2 tons of alumina.

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ExtractionAl(OH)3 + Na+ + OH- ---> Al(OH)4- + Na+Böhmite and Diaspore: AlO(OH) + Na+ + OH - + H2O ---> Al(OH)4- + Na+Ores with a high Gibbsite content can be processed at 140 ° C. Böhmite requires 200 - 240°c.  (At 240°c tl the pressure is approximately 35 (atm))Although higher temperatures are often theoretically advantageous there are several disadvantages including corrosion problems and the possibility of oxides other than alumina dissolving into the caustic liquor.After the extraction stage the insoluble bauxite residue must be separated from the Aluminium-containing liquor by a process known as settling. The liquor is purified as much as possible through filters before being transferred to the precipitators. The insoluble mud from the first settling stage is thickened and washed to recover the caustic soda, which is then recycled back into the main process. 

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Calcination"Hydrate", is calcined to form alumina for the aluminium smelting process. In the calcination process water is driven off to form alumina:2Al(OH)3 ---> Al2O3 + 3H2OThe calcination process must be carefully controlled since it dictates the properties of the final product.

PrecipitationCrystalline aluminium trihydroxide (Gibbsite), conveniently named "hydrate", is then precipitated from the digestion liquor:Al(OH)4- + Na+ ---> Al(OH)3 + Na+ + OH-This is basically the reverse of the extraction process, except that the product's nature is carefully controlled by plant conditions, including seeding or selective nucleation, precipitation temperature and cooling rate. The "hydrate" crystals are then classified into size fractions and fed into a rotary or fluidised bed calcination kiln. Undersize particles are fed back into the precipitation stage.

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Filtration-Hydration

Filtration, such as vacuum drum or pressure filters, remove the silica and low solids from the clarified alumina bearing liquor. The liquid containing the dissolved alumina is pumped to tanks called crystalization or precipitation tanks. The liquid is cooled with water from the counter current decantation thickeners, and as it cools, alumina hydrate slowly precipitates from the tank, according to the formula:

The precipitated liquor containing the white alumina is then filtered, to remove the solid alumina from the liquid, using vacuum drum filters or rotary pan filters, where it can be washed as it is filtered. The alumina hydrate (AlOH3) is then dried. It can be further calcined

to Al2O3, alumina, in a rotary kiln at 800 defrees F.

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Cathode Block

Molten Aluminium

Feeder

GasesAnode

Electrolyte

Anode Carbon

1.7 – 2.1 t CO2eq/t AlIAI average = 2.0

Electricity Input15.6 MWh/t Al0 – 20.8 t CO2/t AlIAI average = 5.8

GHG from Primary Aluminium Production

Two PFC (perfluorocarbon compounds - CF4 and C2F6) contribute about 40% of direct primary aluminium GHG emissions

Alumina Production

1.5 – 2.5 t CO2eq/t AlIAI average = 1.9

PFC Generation

0.02 – 24.5 t CO2eq/t AlGlobal average = 1.26

Source: IAI Life Cycle Inventory Data IAI 2003 PFC Survey

GHGs from Primary Aluminium Production

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Designation System for Aluminum Alloys

Alloy Description or Major Alloy Description or MajorSeries Alloying Elements Series Alloying Elements1xxx 99.00% Minimum Aluminum 1xx.x 99.00% Minimum Aluminum2xxx Copper 2xx.x Copper3xxx Manganese 3xx.x Silicon plus copper and/or magnesium4xxx Silicon 4xx.x Silicon5xxx Magnesium 5xx.x Magnesium6xxx Magnesium and silicon 6xx.x Unused series7xxx Zinc 7xx.x Zinc8xxx Other element 8xx.x Tin9xxx Unused series 9xx.x Other element

*worked by being forged or hammered**poured into a mold to give it its shape

Wrought* Aluminum Alloys Cast** Aluminum Alloys

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Stress-Strain Properties of Aluminum

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Tensile Strength of Aluminum

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