T C Alex

Influence of reactivity of fly ash on geopolymerisation

Abstract The reactivity of fly ash has been altered through an increase in glass content by air classification and mechanical activation using vibratory and attrition mills. The effect of the reactivity on fly ash geopolymerisation has been investigated with specific reference to edging of fly ash with alkali at 27°C and geopolymerisation schemes involving edging and thermal curing or direct thermal curing at 60°C. The effect of improved reactivity of fly ash on the resulting geopolymers was studied through determination of compressive strength, phase formation by X-ray diffraction and microstructural evaluation by scanning electron microscopy. The improvement in compressive strength is found to be related to the improved reactivity and resulting formation of compact microstructure. Selection of geopolymerisation scheme is found to be a key factor to realise beneficial effect of improved reactivity. Isothermal conduction calorimetry studies along with differential thermal analysis (TG/DTA) were carried out to elucidate the influence of improved reactivity during geopolymerisation. Finer particle size resulted in greater dissolution of fly ash during edging. However, the overall process of geopolymerisation and strength development was found to depend not only on dissolution but also on subsequent stages of geopolymerisation. Mechanically induced reactivity is found to have far greater influence on geopolymerisation and strength development vis-à-vis reactivity induced by finer particle size and higher glass content obtained through air classification.

Mechanical Activation of Al-oxyhydroxide Minerals–A Review

Abstract Advances in the field of mechanical activation of Al-oxyhydroxide minerals (synthetic as well as in natural bauxites) over the years around the globe are described in this review. An overview of fundamental aspects of mechanical activation with select examples of applications/examples in extractive metallurgy preludes the theme, mechanical activation of Al-oxyhydroxides. The focus is primarily on mechanically induced reactivity of largely used (and most commonly available too) Al-oxyhydroxides viz. gibbsite and boehmite. Reactivity, considered in the context of dissolution in alkali solution and transformations during thermal treatment, is reported to increase with mechanical activation. Available information is collaged to answer the question why/how does the observed effects happen. Gaps in the existing knowledge and prospects in Bayer process are also discussed.

Mechanical Activation of Gibbsite and Boehmite: New Findings and their Implications

Abstract Almost the entire metallurgical grade alumina is produced from bauxite using the Bayer process. The leaching conditions in Bayer process depend on various Al-oxyhydroxide minerals/phases present in the bauxite. Among the various bauxite types, gibbsitic (trihydrate) bauxite is easiest to digest. Monohydrate (boehmitic and diasporic) bauxites need more stringent digestion conditions in terms of temperature, pressure and alkali concentration. Mechanical activation during milling, especially high energy milling, results in enhanced reactivity of solids due to physicochemical changes induced by milling. This paper is an overview of our recent research on mechanical activation of bauxite and its constituent phases, gibbsite and boehmite. This paper focuses on the leaching of mechanically activated bauxite ores. Complementary results on mechanical activation of gibbsite and boehmite phases are included for a deeper understanding of mechanical activation of bauxite. Implications of these results are highlighted in the context of the Bayer process.

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

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-a-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.

Mechanical Activation of Al-Oxyhydroxide Minerals — Physicochemical Changes, Reactivity and Relevance to Bayer Process

Overview of our research on ‘structure and reactivity’ of gibbsite and boehmite under varied conditions of mechanical activation, e.g. milling energy and presence of a second phase is presented. Bulk and surface changes induced in the solids by milling are characterized in terms of morphology, particle size distribution, specific surface area and nature of porosity, crystallite size and zeta potential. Results on enhanced amorphisation of gibbsite in presence of a second phase (quartz, hematite etc), changes in zeta potential of gibbsite due to loss of texture during milling and anomalous decrease in surface area of boehmite during milling are reported. Reactivity of the activated solids in sodium hydroxide and variation in thermal transformation temperatures is correlated with physicochemical characteristics of the samples and plausible explanation for the observed correlations presented. Significance of the results with specific reference to bauxite and alumina processing in Bayer process is highlighted.

Effect of the selected inorganic carriers on the properties of mechanocomposites with drugs

Hybrid core–shell type composites, including organicinorganic composites, possess unique proper ties and, hence, find use in many areas of science and technology, in particular, in pharmacy [1]. Mecha nochemical methods are efficient for the preparation of such materials because they shorten the duration of synthesis and avoid using large amounts of organic sol vents [2]. Rather popular inorganic drug carriers are highly porous alumina, silica, and magnesia [3, 4]. Hydroxides, which are mechanically softer and, due to layered structure, can be more easily mechanically activated are even more promising for the use in pharmacy.