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A

alumina. An oxide of aluminum (Al2O

3), and

the compound from which aluminum metalis commercially obtained.

aluminum. A versatile, silvery-white metal.When exposed to the atmosphere, alu-minum rapidly forms an oxide film that pre-vents it from reacting with air and water.This gives it exceptional corrosion-resistantproperties. Aluminum is not found in natureas a free metal, like gold, but is chemicallybound to other elements. Aluminum is themost abundant metal in the Earths crust(8.1%). Atomic number, 13; atomic mass,26.982; melting point, 993.52 K; boilingpoint, 2698 K.

anode. A positively charged mass or surface thatattracts negatively charged ions (anions). Theanode used in the Hall-Heroult process iscomposed of carbon. The oxygen-containinganions react on the anode surface, releasingoxygen that consumes the carbon to formcarbon dioxide.

anode-cathode distance (ACD). The geometriclinear distance between the anode and thecathode is a critical measurement in an elec-trolytic cell. This distance affects the voltageand energy requirement of a cell.

anode effect. An aluminum-industry idiom usedto describe a process upset where the anodereaction shifts from producing oxygen to flu-orine, and the cell voltage increases. Anodeeffects are primarily the result of having in-sufficient alumina dissolved in the bath andavailable at the anode for reduction.

B

bath. An aluminum-industry idiom referring tothe cryolite-based electrolyte pool in the re-duction cell.

bauxite. A prime source of alumina, found as acollection of small, reddish-brown nodulesin a light-brown, earthy matrix. Commercialbauxite ore contains 30 to 60 wt% alumina.

Bayer process. A process developed by KarlBayer in 1888 that refines bauxite ore intoalumina grains. It is the process currently inuse worldwide.

C

calcining. The process of heating a material toa sufficiently high temperature to drive offvolatile components or to oxidize the mate-rial without fusing it. The aluminum indus-try uses calcining in the Bayer process toproduce alumina and to prepare coke foranodes.

carbon dioxide equivalents (CDE). The pre-

ferred unit of measure used to compare theimpact of different greenhouse gases. It is cal-culated by multiplying the quantity of a green-house gas emission by the global-warmingpotential of the gas. The results are commonlyexpressed in terms of a million metric tons ofcarbon dioxide equivalent (106 TCDE).

carbon equivalents (CE). A unit of measureused to compare the impact of different green-house gases. It is calculated by multiplyingthe carbon dioxide equivalent by 12/44, the

mass ratio of carbon to carbon dioxide.

APPENDIX I

Glossary

Aluminum Recycling and Processing for Energy Conservation and Sustainability

John A.S. Green, editor, p 257-260

DOI: 10.1361/arpe2007p257

Copyright 2007 ASM International

All rights reserved.

www.asminternational.org

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258 / Aluminum Recycling and Processing for Energy Conservation and Sustainability

carbothermic reduction. An alternative processto electrolytic reduction. The carbothermicprocess reduces alumina in a high-tempera-ture furnace with carbon.

castings. Metal objects that are cast into ashape by pouring or injecting molten/liquidmetal into a mold. This book divides cast-ings into ingot and shape categories. Ingotcastings are produced in molds of very sim-ple cross section, and shape castings arecomplex structures.

cathode. A negatively charged surface that at-tracts positively charged ions (cations). Thecathode surface in the Hall-Hroult process isthe molten aluminum pad, which rests directlyon the cell carbon lining. The aluminum-

containing cations react on the cathode sur-face, releasing the aluminum as free metal.

chloride reduction. An alternative process toalumina electrolytic reduction in which alu-minum chloride is used as the feed to thereduction cell.

coke. A carbon product of the crude oil refiningindustry. Green or raw coke contains 8 to10% moisture and 5 to 15% volatile organicmaterials. Coke is calcined in thermal kilnsto remove moisture and volatile organic

materials.cryolite. Na3AlF

6, a mineral that, when molten,

dissolves alumina to form aluminum andoxide ions. It is the main component used inthe electrolyte bath for aluminum production.

D

dross. The material that forms on the surface ofmolten aluminum as it is held in a furnace. Itis composed of impurities that have surfacedas a result of gas fluxing, oxidized aluminumthat is the result of molten aluminum exposureto the furnace atmosphere, and aluminum thatbecomes entrapped in the surface material.Dross is periodically skimmed off the surfaceof molten aluminum and processed to recoverits aluminum content.

dusting. An aluminum-industry idiom used todescribe fine carbon anode particles that arelost in the electrolyte bath or atmosphereduring electrolytic reduction. Dusting resultsin a loss of productivity.

E

electrolysis. An electrochemical process inwhich the charged species in an electrolyte

are attracted to electrodes, where they reactwith the electrons of the electrical current.Positively charged ions migrate to the cath-ode, and negatively charged ions migrate tothe anode.

electrolyte. A nonmetallic electrical conductorin which current is carried by the movementof ions.

extrusion. The process of forcing the metalingot (or billet) to flow through a die to createa new cross section.

F

feedstock energy. These values represent theenergy inherent in a fuel that is used as mate-

rial. For example, aluminum production usescoke as the raw material in carbon anodes.The energy contribution of a feedstock is ex-pressed in terms of calorific or fuel valueplus the tacit/process energy used to producethe feedstock.

G

global-warming potential (GWP). Green-house gases differ in their abilities to trap

heat. Global-warming potential is used to ex-press the greenhouse effect of different gasesin a comparable way. The heat-trapping abilityof one metric ton of CO

2is the common

standard, and emissions are expressed interms of a million metric tons of CO

2equiva-

lent, or 106 TCDE.greenhouse gases (GHG). Atmospheric gases

that contribute to climate change by increas-ing the ability of the atmosphere to trap heat.

H

Hall-Heroult process. An electrolytic processfor reduction of alumina, developed inde-pendently by Charles Martin Hall and PaulLewis Toussaint Hroult in 1886. Thisprocess is commonly referred to using bothnames, the Hall-Heroult process. It is theprocess used worldwide for commercialaluminum production.

I

ingot. Ingot, as used in this book, describes analuminum casting of simple shape. It in-cludes billets, pigs, sows, T-bar, and othersimple cast semifinished shapes.

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Appendix I: Glossary / 259

K

kilowatt-hour (kWh). A unit of energy.

L

life-cycle assessment (LCA). An internationallyrecognized analysis model of the impact of aproduct on energy, environment, economic,and social values. The LCA extends fromcradle-to-grave: from material acquisitionand production; through manufacturing,product use, and maintenance; and finally,through the end of the product life in disposalor recycling. The LCA is particularly useful

in ensuring that benefits derived in one areado not shift the impact burden to other placeswithin a product life cycle.

O

on-site energy. The energy used within a facil-ity. This is sometimes called primary energy.Electrical on-site energy is the kilowatthours used and does not include the secondaryenergy required for generation and transmis-

sion of electricity. Fuel on-site energy use isbased on the calorific heating value of thefuel and does not include the secondaryenergy required to produce and transportthe fuel.

P

pad. An aluminum-industry idiom used to de-scribe the body of molten aluminum thataccumulates within the Hall-Heroult elec-trolytic cell.

polarization. The nonuniform concentrationgradients that form near electrodes duringthe reduction process. The reactions occur-ring at the anode and the cathode createlocalized conditions that are different fromthe bulk of the bath. The reactions depletethe supply of reactants and increase thequantity of products. Additionally, inaluminum electrolysis, gas is generated atthe anode, which lowers the effective bathconductivity. An electric overpotential is

required to overcome the effects of polar-ization.

pot. An aluminum industry idiom used todescribe an electrolytic cell. The term wasderived from the shape of the first cells.

potline. An aluminum-industry idiom that de-scribes the arrangement of a long row of in-terconnected electrolytic cells (pots).

potlining. An aluminum-industry idiom that de-scribes the refractory and carbon materialsused to line the interior of the cell (pot).

primary aluminum. Refers to aluminum metalproduced directly from alumina feedstock bychemical reduction.

Q

quad. A common abbreviation for a quadrillionBtu (1 quad = 1015 Btu).

R

red mud. The residue of insoluble materialsthat results from extracting alumina frombauxite ore. It is also referred to as bauxiteresidue.

reduction cell. A container holding single ormultiple anodes, cathodes, and an electrolyticbath used for reducing a material.

reverberatory furnace. The most commonlyused furnace type in the aluminum industry.The furnace is box-shaped and consists of a

steel shell with refractory lining. Fuel is fireddirectly into the box, either from the roof or,more typically, from the sidewall. Heat istransferred to the molten metal with convec-tion and radiation.

rolling. A process that results in the reductionof the cross-sectional area of a metal shapeas it is passed through rotating rolls.

S

secondary aluminum. Aluminum metal that isproduced from recycled aluminum productsand wastes.

T

tacit energy. A term used to describe an energyvalue that equals the combination of on-siteenergy (primary energy) consumption, theprocess energy required to produce and trans-mit/transport the energy source (secondary

energy), and feedstock energy (energy inher-ent in fuels used as materials). This book usesthe superscript tf to denote any value thatincludes the tacit and feedstock energy contri-butions. The book does not include the energy

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used to make the equipment or buildings thathouse the process steps (tertiary energy).

U

urban mining. A term that describes the largesource of aluminum available through urbanrecycling programs as compared to bauxitemining.

V

value chain analysis. A method that capturesthe energy and material inputs and outputs of

each processing step (link) and builds the cu-mulative value for each product along thechain. A value chain analysis, or cradle-to-shipping dock analysis, is an integral part ofa life-cycle analysis.