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Elucidation of the Nature of Structural Heterogeneity During Alkali Leaching of Non-activated and Mechanically Activated Boehmite (γ-AlOOH)

An Erratum to this article was published on 20 May 2015

Abstract

Crystal joints and faces in non-activated boehmite and, state of agglomeration of particles, degree of amorphization, microcrystallite dimension and, strain in mechanically activated boehmite are indicators of structural heterogeneity which influences reactivity of the solid phase. The focus of this paper is on understanding the manifestation of the heterogeneity during alkali leaching of a boehmite (specific surface area—263 m2/g), without and with mechanical activation using planetary milling up to 240 minutes. A two-prong strategy is used for this purpose which involved analysis of the kinetics of leaching by a model-free approach using ‘isoconversional method’ and, in parallel, characterization of the reacting solid after different durations of leaching. Unlike model-fitting methods, the kinetic analysis revealed sample-dependent variation of apparent activation energy with fraction leached. Changes observed in the morphology of samples (by SEM), particle size distribution (by laser diffraction), and crystalline nature (by powder X-ray diffraction) are used to explain activation energy changes and propose mechanisms of leaching. The effect of mechanical activation on rate constant is assessed and it has been found that up to ~23-fold increase in rate is possible depending on the activation time, leaching temperature, and fraction leached. Further, based on binary correlations between activation energy at different fractions leached and initial characteristics of the samples, it is found that the leaching is predominantly influenced by structural changes during milling, namely, degree of amorphization, microcrystallite dimension, and strain, vis-à-vis specific surface area. Significantly, the paper highlights limitation of model-fitting methods used by most researchers to analyze the kinetics of leaching, especially for mechanically activated minerals.

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Abbreviations

α :

Fraction leached

a, b, c :

Lattice parameters (nm)

a/c :

Alkali to caustic weight ratio

A m :

Degree of amorphization (pct)

d 10, d 50, d 90 :

Characteristic particle diameters (µm)

ε :

Microstrain

E a :

Activation energy (kJ/mol)

E aα , E *aα :

α dependent E a (kJ/mol)

ΔE *aα :

Stored energy E aα  − E *aα (kJ/mol)

g(α):

Functional form of integrated reaction model

k/k* :

Rate constant ratio

MCD:

Microcrystallite size (nm)

R :

Gas constant

R 2 :

Correlation coefficient

r xy :

Binary correlation coefficient

SSAGeo :

Geometrical specific surface area (m2/g)

SSABET :

BET specific surface area (m2/g)

t MA :

Milling or mechanical activation time (min)

t :

Leaching or reaction time (min)

t α :

Leaching time for fraction leached α (min)

T :

Temperature (K)

Z,Z* :

Pre-exponential factor

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Acknowledgments

The authors acknowledge the constructive criticism of this work and useful suggestions from Prof. S.P Mehrotra (formerly Director CSIR-NML and presently at IIT Gandhinagar, India). This work was carried out as a part of Department of Science and Technology sponsored project (ILTP/A-2.55).

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Correspondence to Rakesh Kumar.

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Manuscript submitted August 13, 2014.

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Kumar, R., Alex, T.C. Elucidation of the Nature of Structural Heterogeneity During Alkali Leaching of Non-activated and Mechanically Activated Boehmite (γ-AlOOH). Metall Mater Trans B 46, 1684–1701 (2015). https://doi.org/10.1007/s11663-015-0343-9

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Keywords

  • Leaching
  • Apparent Activation Energy
  • Boehmite
  • Gibbsite
  • Isoconversional Method