Al-Monohydrate (Boehmite) to Al-Trihydrate (Bayerite/Gibbsite) Transformation During High-Energy Milling
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, A.J. KAILATH
, and RAKESH KUMAR
The specific focus of this study is on the boehmite–water interaction during attrition milling of a boehmite prepared by thermal dehydroxylation of gibbsite. Various characterization studies confirm the formation of a new phase, bayerite (an Al-trihydroxide polymorph) during milling of boehmite with water as the dispersant. Bayerite is a more stable (thermodynamically) phase than boehmite. By forming bayerite, the free energy of the interacting system (consisting of disordered metastable boehmite and water) gets reduced. Such a transformation is of relevance in the digestion of Al-monohydrates in the Bayer process of alumina production. Furthermore, the presence of water during attrition milling itself is essential for the transformation. Bayerite does not form during attrition milling of the same boehmite in ethyl alcohol media and subsequent soaking in water up to 36 hours. Therefore, bayerite formation does not depend solely on structural degradation. https://doi.org/10.1007/s11663-020-01771-6 The Minerals, Metals & Materials Society and ASM International 2020
I.
INTRODUCTION
WATER, the universal (and perhaps the most important) chemical on earth, influences all walks of life. Minerals and metal oxides interact with water in contact with it; water from any origin—adsorbed from the atmosphere or released from the solid during its processing—can take part in the interactions. Mineral–water interactions are relevant to various fields; some of them are flotation, leaching reactions, water treatment, catalysis, and many others. Activated mineral surfaces are more prone to such interplays. Solid–water interactions during wet high-energy milling alter the surface chemistry; besides, the solid strength gets lowered as the interfacial tension gets reduced (Rehbinder effect).[1] The milling may induce (1) changes in zeta potential,[2–4] wettability,[5,6] and (2) phase transformations.[7] Comprehensive reviews on the role of water during high-energy milling are available.[8,9]
Boehmite, referred to as aluminium monohydrate (or monohydroxide), is a precursor material for the preparation of various transition aluminas that find a variety of applications.[10] In the Al2O3-H2O system, boehmite is reported to be a thermodynamically metastable phase at room temperature[11]; instead, aluminium trihydrate (or trihydroxide)—notably, gibbsite and its polymorphs—appears as the stable phase. There is no report available on the stability of boehmite during high-energy wet milling. This article describes the physicochemical changes in boehmite during wet attrition milling. The specific focus has been on: (1) how pure water interacts with boehmite under the stresses prevailing in the mill and (2) whether it transforms to its stable phase. Past investigations on high-energy milling of boehmite— limited mostly to dry planetary milling—report amorphization and dehydroxylation.[12–14] Use of attrition milling in this study is significant since the mill holds good promise towards the development of a universal
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