Alumina and its numerous varieties the Bayer process

Heat treatments of Bayer hydrargillite initially cause dehydration, then a reciystallization to maity types of transition aluminas (khi, kappa, rho, eta, theta, delta, gamma, etc.) that can be classified nnder the collective name of gamma alnminas. The distinctions within transition alnminas are based on macroscopic criteria (particnlariy the specific snrface of the products) and ciystallographic criteria (particnlariy the presence of numerons defects point defects as well as extended flaws). 

The reactivity of alumina powders that must be used for the sintering of ceramics has not been fully understood, especially since it is covered by the shroud of the industrial secrecy in powder preparation treatments. The challenge is to obtain a densified ceramic very close to the theoretical density and with a well-controlled 

For specific uses, particularly for the preparation of ultrapure powders used for the synthesis of sapphire monocrystals, the Bayer process makes room for other methods, including ammonium alum method (dissolution of Bayer hydrargillite in excess sulphuric acid, neutralization in ammonia in NH4A1(S04)2.12H20 form, calcination at about 1,000°C and then milling) or the alcoholate method (based on an aluminum isopropylate). 

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