3.3 Ceramic processing of alumina

Introduction

While aluminum is the most abundant metal in the earth's crust ( ca. 8%) and aluminum compounds such as alum, K[Al(SO ₄ ) ₂ ].12(H ₂ O), were known throughout the world in ancient times, it was not until the isolation of aluminum in the late eighteenth century by the Danish scientist H. C. Öersted that research into the chemistry of the Group 13 elements began in earnest. Initially, metallic aluminum was isolated by the reduction of aluminum trichloride with potassium or sodium; however, with the advent of inexpensive electric power in the late 1800's, it became economically feasible to extract the metal via the electrolyis of alumina (Al ₂ O ₃ ) dissolved in cryolite, Na ₃ AlF ₆ , (the Hall-Heroult process). Today, alumina is prepared by the Bayer process, in which the mineral bauxite (named for Les Baux, France, where it was first discovered) is dissolved with aqueous hydroxides, and the solution is filtered and treated with CO ₂ to precipitate alumina. With availability of both the mineral and cheap electric power being the major considerations in the economical production of aluminum, it is not surprising that the leading producers of aluminum are the United States, Japan, Australia, Canada, and the former Soviet Union.

Aluminum oxides and hydroxides

The many forms of aluminum oxides and hydroxides are linked by complex structural relationships. Bauxite has the formula Al _x (OH) _3-2x (0<x<1) and is thus a mixture of Al ₂ O ₃ (α-alumina), Al(OH) ₃ (gibbsite), and AlO(OH) (boehmite). The latter is an industrially important compound which is used in the form of a gel as a pre-ceramic in the production of fibers and coatings, and as a fire retarding agent in plastics.

Heating boehmite and diaspore to 450 °C causes dehydration to yield forms of alumina which have structures related to their oxide-hydroxide precursors. Thus, boehmite produces the low-temperature form γ-alumina, while heating diaspore will give α-alumina (corundum). γ-alumina converts to the hcp structure at 1100 °C. A third form of Al ₂ O ₃ forms on the surface of the clean aluminum metal. The thin, tough, transparent oxide layer is the reason for much of the usefulness of aluminum. This oxide skin is rapidly self-repairing because its heat of formation is so large (ΔH = -3351 kJ/mol).

Ternary and mixed-metal oxides

A further consequence of the stability of alumina is that most if not all of the naturally occurring aluminum compounds are oxides. Indeed, many precious gemstones are actually corundum doped with impurities. Replacement of aluminum ions with trace amounts of transition-metal ions transforms the formerly colorless mineral into ruby (red, Cr ³⁺ ), sapphire (blue, Fe ^2+/3+ , Ti ⁴⁺ ), or topaz (yellow, Fe ³⁺ ). The addition of stoichiometric amounts of metal ions causes a shift from the α-Al ₂ O ₃ hcp structure to the other common oxide structures found in nature. Examples include the perovskite structure for ABO ₃ type minerals (e.g., CeTiO ₇ or LaAlO ₃ ) and the spinel structure for AB ₂ O ₄ minerals (e.g., beryl, BeAl ₂ O ₄ ).