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Influence of the milling conditions on the thermal decomposition of Bayer gibbsite
Journal Pre-proof Influence of the milling conditions on the thermal decomposition of Bayer gibbsite
J.M. Rivas-Mercury, J.R.M. Sucupira, M.A. Rodríguez, A.A. Cabral, A.H. De Aza, P. Pena PII:
S0032-5910(19)31017-4
DOI:
https://doi.org/10.1016/j.powtec.2019.11.057
Reference:
PTEC 14939
To appear in:
Powder Technology
Received date:
26 March 2019
Revised date:
1 November 2019
Accepted date:
18 November 2019
Please cite this article as: J.M. Rivas-Mercury, J.R.M. Sucupira, M.A. Rodríguez, et al., Influence of the milling conditions on the thermal decomposition of Bayer gibbsite, Powder Technology(2019), https://doi.org/10.1016/j.powtec.2019.11.057
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© 2019 Published by Elsevier.
Journal Pre-proof
Influence of the Milling Conditions on the Thermal Decomposition of Bayer Gibbsite ‡
‡
§
J M. Rivas-Mercury , J. R. M. Sucupira‡; M. A. Rodríguez , A. A. Cabral , A. H. §
§
De Aza , P. Pena ‡
Instituto Federal de Educação, Ciência e Tecnológica do Maranhão – IFMA Av. Getúlio Vargas, 04 - Monte Castelo - CEP 65025-001- São Luís – Brasil
§
Instituto de Cerámica y Vidrio, ICV-CSIC; Kelsen, 5, 28049 - Cantoblanco, Madrid,
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Spain
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ABSTRACT A synthetic gibbsite (Al(OH)3) produced by the Bayer process was mechanically ratios
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activated by attrition milling for 24 hours with various grinding ball-to-powder
(BPR=5, 10 and 20). Changes in its structure were studied by thermal analysis and X-
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ray diffraction. As a result of increasing ball-to-powder ratio, the grain size of gibbsite decreased while its specific surface area increased. Only for these materials, the formation
of
nanocrystalline
boehmite
(AlO·OH)
and
amorphous
aluminium
al
hydroxides was also observed. Upon having been heat treated between 200 – 1200ºC ( 2
rn
hours), boehmite was detected at the temperature range of 200 and 350ºC for the BPR10 sample, while the boehmite of BPR20 was transformed to γ- Al2O3 at 500ºC. Further,
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only α-Al2O3 was detected at 1200ºC for all samples. Finally, combining the TG-DSC, XRD and SEM results, it was proposed a mechanism for the thermal decomposition of the non-milled and milled samples.
Key words: Gibbsite; milling; mechanical activation; amorphization; thermal behaviour.
Corresponding author: Miguel A.
Rodríguez.
917355843)
1
(email: [email protected]; fax: +34
Journal Pre-proof
I. INTRODUCTION Gibbsite (Al(OH)3) is an aluminium trihydroxide produced on the industrial scale by the Bayer process [1]. This material is a precursor of boehmite (AlOOH) or pseudoboehmite (poor crystalline boehmite), and can be used as a catalyst carrier, a reinforcement of ceramic composites and to obtain transition aluminas (-Al2O3) and
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corundum (-Al2O3) materials, which are widely used in many structural and functional applications [2-7].
Crystalline and amorphous aluminium hydroxides with a low alkali content can be
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prepared by several methods like seed precipitation [8], inorganic aluminium salt deposition [9], sol-gel [10,11] microemulsion [12], and long- and short-term grinding
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[13, 14]. All these methods lead to the formation of high-purity aluminas, but involve
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high annealing temperatures for long times to convert hydroxides into oxide, which is often undesirable because it leads to powders with poor characteristics from the point of view of shape forming (agglomerates, coarse crystal sizes, porous particles).
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Recently, interest in powders processing has increasingly focused on the application of
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mechano-chemical activation to develop new materials and processes [15]. This is because highly reactive surface areas form that change the physical behaviour of solids,
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and can be attributed to both the permanent rearrangement of the crystal lattice and the structural changing of grains during grinding [16,17]. It has been reported that mechanical activation offers significant advantages over other
powder synthesis
methods, such as the polymeric precursor method [18,19], sol-gel [20], and the use of solvents, which lead to large amounts of liquid or gas waste formation and need to be utilised in other processes to prevent environmental impacts. Gibbsite is often the predominant mineral in bauxite ores and a fundamental understanding of its activation is required [21-23]. Several research works have studied the effect of grinding on aluminium hydroxides and have reported that mechanochemical treatment leads to the formation of aluminium atoms that are pentacoordinated on the surface of gibbsite particles, such as the “gel-like” phase (amorphous to X-rays) stabilised by protons and water molecules [24-28]. The rupture of a significant proportion of Al-OH bonds releases molecular water, which is immediately
2
Journal Pre-proof adsorbed onto activated surfaces from where it can be endothermically desorbed at 125ºC [29,30]. The aim of this work was to investigate the effect of changes in the ball-to-powder ratio (BPR) on the physico-chemical properties of coarse mechanically activated synthetic gibbsite (Al(OH)3) in order to provide information on how the grinding can modify the structure of the Gibbsite and its thermal behaviour. II. EXPERIMENTAL
II. 1. Description of the original sample A coarse-synthetic gibbsite of a nearly uniform size (mean grain size, d50 ≈ 93 m),
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prepared by the Bayer process and supplied by ALCOA (San Ciprián, Lugo, Spain), was used. The chemical analyses of gibbsite by X-ray fluorescence spectroscopy (XRF, Magi X; Phillips, The Netherlands) and the alkaline analyses (Na and K) by flame
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emission spectrometry (Perkin Elmer FES 2100) indicated that gibbsite was composed
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of 65.0 wt% Al2O3, 0.2 wt% Na2O, 0.1 wt% TiO 2, 0.04 wt% SiO 2, 0.03 wt% CaO,
and was of 99.7 wt% purity grade.
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II. 2. Attrition Milling of Gibbsite
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0.006 wt% MgO and 0.001 wt% K 2O. The sample underwent 34.6 wt % ignition loss
Milling was carried out in a vertical laboratory batch attritor (home made at the Instituto
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de Cerámica y Vidrio, ICV-CSIC. Spain) at a rotational speed of 1,000 rpm, with a
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milling vessel of stainless steel coated internally with teflon and with a volumetric capacity of 0.8 L. Gibbsite powder was milled at room temperature (25C) using 3
partially stabilised zirconia (3 mol% yttrium, PSZ, density, 6,065 kg/m ) as milling balls (3 mm in diameter). Isopropyl alcohol was used as the process controlling agent to prevent agglomeration of powders. The liquid/solid was maintained at 0.50 and the ballto-powder weight ratios (BPR) were 0 (original sample, without milling) , 5:1, 10:1 and 20:1 (by weight), hereinafter referred to as BPR0, BPR5, BPR10 and BPR20. To avoid system overheating and
its consequences (alcohol evaporation), a mantle with
circulating cooling water (20C) was attached to the bowl. The milling was carried out for 24 hours, stopping after each 1 hour interval for cooling during 15 minutes. After completing the milling process, suspensions were sieved, poured into beakers, and dried in a stove with circulating forced air for 12 h at 110C.
3
In order to study the thermalJournal evolutionPre-proof of gibbsite BRP10, thermal treatments at constant heating rate of 10ºC/min up to 200, 350, 500, 800, 1000 and 1200ºC holding maximum temperature during 2 hours were carried out. Heat treatments were done in an electric furnace electronically controlled (±1°C). II.3. Physical Characterisation of Powders The volumetric N 2 adsorption method was used to determine the BET (BrunauerEmmett-Teller) specific surface area (SSA) of powders. The tested powder was placed inside the sample container with a known volume. The adsorbed volume of gas was determined from changing the pressure associated with gas adsorption on the sample powder surface. A surface area analyser (ASAP 2020, Micromeritics Corporation,
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USA) was used and operated with N 2 (99.999 %) as the adsorbate gas. After degassing samples at 70C, the temperature was raised to 150C (10C/min) and this temperature
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remained for 5 h in vacuum. The monolayer capacity and the SSA were calculated by the BET model.
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Particle size distribution was measured using a laser scattering-based particle size analyser (Master Sizer S; Malvern Instruments, UK). All the particle size measurements
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were taken on slurry samples. Sodium hexametaphosphate was used as dispersant. Particle dispersion and deagglomeration were also ensured by means of ultrasonic
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treatment prior to taking measurements. The true density of the gibbsite powders was
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II. 4. Thermal Analysis
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measured by He-picnometry (Multipicnometer, Quantachrome,USA).
Thermal differential and thermogravimetric (DSC –TGA) analysis were conducted with an STA 409 (Netzsch, Germany) thermo-balance at 20ºC/min up to 1,100ºC (maximum temperature) in flowing air (50 ml/min), and using Pt crucibles and -Al2O 3 powder as references. II. 5. X-Ray Diffraction The diffractograms of samples BPR0, BPR5, BRP10 and BPR20 were recorded in the continuous mode in a X’PERT-PRO X-ray diffractometer (UK) equipped with an X’Celerator detector (RTMS), a PW3050/60 goniometer with a theta-theta design, and a step size of 0.017° (2) within an angular range of 5 - 70 (2) using Cu-K radiation ( = 1.540598 Å) at 40 kV and 35 mA, a Soller slit of 0.04 (rad) in the incident and diffracted beam, and a beta nickel filter. The XRD patterns were analyzed by the High
4
JournalThe Pre-proof Score Plus 3.0e software (Panalytical). crystalline phases were indexed using the following files from the Inorganic Crystal Structure Database (ICDS), collection code: 006162 (Al(OH)3-Gibbsite) and 100391 (AlOOH-Boehmite). Rietveld
refinement were performed
on sample BPR0 following the strategies
recommended by McCusker et al. [31] using the FullProf Suite software. Firstly, the global variables were refined (sample displacement, background and scale factor). Afterwards, the following parameters were refined: lattice parameters, profile variables (W, V and U, besides Peak Shape or Asymmetry), the preferred orientation for specific phases, atomic coordinates and individual thermal factors. Finally, the Pseudo-Voigt profile adjustment function was changed to a Pseudo-Voigt 3 profile function (FJC
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Asymmetry). The refinement quality indicators RWP (R-weight standards or patterndependent disagreement factor), Rp (residual of least-squares refinement) and goodness-
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of-fit (GoF) obtained minimum values, which indicates that fits to any given phase, are very satisfactory.
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Based on Rietveld refinement and XRD spectra, diffractograms were also used to calculate the average crystallite size (size of the coherently diffracting domains) using
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the (002) reflexion and Scherrer’s Equation (1):
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DS
k cos
(1)
rn
S
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where DS is the average crystallite size, k is a constant that equals 0.9; is the X-ray wavelength that equals 0.15406 nm, and is the full-width at half-maximum (FWHM) subtracted the instrumental broadening. Instrumental broadening was measured using Cerium 674b as the standard reference material (NIST, SRM 674b). The XRD patterns collected on the CeO 2 standard were treated with Data Viewer 1.4 (Panalytical) to obtain the FWHM of the peak profile, which were used to estimate the crystallite size with Scherrer’s equation for reflexion (002) in all the treated samples. The amorphisation degree (AD) was estimated according to the BPR on the XRD patterns by the method developed by Ohlberg and Strickler, and as described by Alex et al [32], which is given by:
5
( )
(
) Journal Pre-proof
(2)
where AD denotes the percentage degree (%), Io and Bo are the integral intensities of the diffraction peak (002) and the background of the diffraction peak for the non-milled mixture, respectively, while Ix and Bx are the equivalent values for the mechanically activated mixture. The Io, Bo, Ix and Bx integrals were calculated using Data Viewer 1.4 (Panalytical).
II. 6. Field Emission Scanning Electron Microscopy. The powder morphology was studied under several ball-to-powder ratios (BPR)
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conditions by scanning electron microscopy (SEM). After gold coating, powder surfaces were examined under a Hitachi S-4700 (Hitachi, Tokyo, Japan) field emission
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III. RESULTS AND DISCUSSION
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scanning electron microscope (FE-SEM).
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III. 1. Physical and Mineralogical Characterisation of the Original Gibbsite The initial gibbsite powder presented a characteristic morphology, composed of large spherulitic concretions of pseudo-hexagonal platelet-shaped crystals (Fig. 1-a and Fig. 2 -1
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1-b) of Al(OH)3, with a specific surface area of 0.07 m ·g , a mean grain size of 93 -3
(Table 1).
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m and a real density of 2.44 g.cm
Fig. 1 – a) SEM micrograph of the original gibbsite powder (BPR0); b) SEM micrograph detail of the agglomerates of particles of BPR0.
The diffractogram of the non-milled gibbsite powder (Fig. 2) shows only the reflections corresponding to Al(OH)3. Powder diffraction pattern was refined using the Monoclinic Crystal Structure (P21/n Space Group) by the Rietveld analysis with the FullProf software.
6
Journal Pre-proof Fig. 2. – X-ray diffraction patterns of non-milled (BPR0) at RT: experimental and Rietveld refinement. Vertical lines correspond to the Bragg peaks of the identified phase.
The Rietveld refinement of the XRD pattern of the non-milled gibbsite (BPR0) at room temperature yielded the following structure parameters: a = 8.6798±0.0005 Å, b = 5.0757 ± 0.0003 Å, c = 9.7330 ± 0.0005 Å and = 94.5807 ± 0.0005o and V = 427.417 ± 0.004 Å3 . The mean crystallite size was 146nm., with an anisotropy of 0.92 and the maximum strains were 13.46/10000 with an anisotropy of 0.02 (RBragg = 8.60; RWp = 13.20; 2 = 3.80). These results well agree with the data reported in the literature by other researchers [33, 34], and show that BPR0 has a monoclinic crystal structure
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(P21/n,) and exhibits a tabular pseudo-hexagonal habit. The structure can be regarded as stacked hexagonal close-packed (hcp) layers with open planes between successive
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sheets. Each Al cation is octahedrally coordinated to six OH groups and each hydroxyl group is coordinated to two Al cations, which leaves one octahedral site vacant.
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Alternatively, it was envisaged as double layers of hcp hydroxyl groups stacked in an AB BA sequence. The variables of the hydrogen position were not refined during the
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Rietveld analysis because calculations became unstable as the 14 atomic sites were in
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general positions, hence there were too many coordinates to be refined.
III. 2. Milling Studies
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III. 2.1. Grain size and specific surface area. Figure 3 and Table 1 describe the SSA and the mean grain size (d50) of the gibbsite
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powder according to the BPR.
Fig. 3. Average grain size and BET specific surface area of gibbsite according to the activation conditions.
Table 1 – Physical properties of the milled powders 2 -1
SSA (m ·g ) DBET (m) d50 (m)
-3
r (g·cm )
BPR
Reference
0
BPR0
0.07
35
93
5
BPR5
32
0.079
2.1
7
10
BPR10
20
BPR20
Journal Pre-proof 82
0.031
1.5
219
0.012
6.9
DB ET: BET spherical diameter calculated from the values of SSA and measured densities.
2
As seen in Fig. 3 and Table 1, the SSA increased continuously from 0.07 m /g to 219 2
m /g depending on the milling activation conditions. The average grain size decreased from 93 m to ≈ 1.5 m from BPR0 to BPR10. In this study, the energy input changed as BPR varied and took values of 5, 10 and 20. Nevertheless, an increasing d 50 was observed for BPR20 (Table 1), which could be associated with powder agglomeration
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as a consequence of the formation of secondary particles (agglomerates), which arise from the interactions taking place among fine particles through Van der Waals forces.
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As BPR increased, as we can see in Table 1, the diameters of particles (sphereequivalent diameter) determined from the BET SSAs (DBET) were much smaller than
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those ones obtained by the laser diffraction method. This indicates that particle pores and roughness remain accessible in agglomerates and that nitrogen gas can penetrate the
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pore space of particles. As a result, the BET method determined a SSA that provided smaller particles sizes (DBET ) than those measured by the laser diffraction method. This
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also implies that the primary particle agglomerates broke during intensive milling, which led to new different particles, in size and shape factor, as the milling energy
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increased as a result of the high shear introduced by an increase in BPR. The real
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density (r) of the activated powders measured by He pycnometry clearly diminished according to the activation conditions (Table 1), probably due to the surface amorphization of the particles.
III. 2.2. X-Ray Diffraction – XRD III. 2.2.1. Mineralogical and Structural Changes. Figure 4 (a-d) shows the diffractograms of the gibbsite powders that were mechanically activated for 24 hours at various BPRs. As shown in Fig. 4a, BPR0, corresponding to the pure and non-milled gibbsite, the intense (002) Bragg reflection of this mineral is located at 2= 18.319°. From the diffractograms (Fig. 4b), we can observe that the intensity of Bragg's reflection (002) decreased for BPR5. With BPR10, only the reflections of gibbsite were identified (Fig. 4c). As expected, the intensity of the peaks located at 2= 18.335° and 20.6° were further reduced by mechanical activation due to
8
Pre-proof the amorphisation of gibbsite. Journal BPR20 showed four new strongly broadened peaks centred at 14.6°, 28.2°, 38.6°, and 49.3° of 2. These peaks corresponded to the (020), (120), (031; 140), and (051) reflections of boehmite (AlOOH), respectively (Fig. 4d).
Fig. 4 –XRD patterns of the gibbsite powders subjected to the 24 hours mechanical activation in isopropyl alcohol: a) the non-milled sample BPR0, (b) BPR5, (c) BPRB10, (d) BPR20. (Gb) Gibbsite and (Bh) Boehmite).
The presence of boehmite reflections in BPR20 sample (with a 24 hours milling time)
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was explained after considering that more heat was produced given the mechanical energy introduced as a consequence of the increasing number of collisions per time unit
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as the number of balls increased. In parallel, this led to a rising milling temperature, and a faster diffusion process and phase transformation. This transformation takes place at
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temperatures around 250ºC. Another interesting observation made from the XRD data (Fig. 4 (a-d)) was that the diffraction peaks of all the mechanically activated samples
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(BPR5, BPR10 and BPR20) became lower and broader than those of the non-milled one (BPR0). The gibbsite reflections also shifted due to structure distortion. As pointed out
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above, the decrease in the diffraction peak intensities implied the formation of
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III. 2.2.2. X-ray AD
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amorphous material and the nucleation of the nanocrystalline boehmite phase.
The structural disorder due to the increasing amorphous material content was shown by the intensity decreasing of the diffraction peaks as can be observed in the X-ray diffraction patterns (Figure 5-(a)) of the activated samples.
Fig. 5. Variation of the: a) peak broadening and intensity of the Bragg reflections (002) of Gibbsite for increasing BPR; and b) AD and crystallite size with the SSA.
Since the (002) Bragg reflection of gibbsite is affected mostly by milling, it was used to calculate the AD. By assuming that the amorphous phase in the initial powder was
9
Pre-proof negligible, and by replacing theJournal data taken from the XRD patterns in Eq. (1), the average X-ray ADs were calculated for the milled samples, as depicted in Fig 5-b. This figure clearly shows that the AD growths up to 99% as the BET surface area increased. Crystallite size (Ds) reduced in agreement with decreasing of diffraction peak intensities [27, 28]. The distortion of the crystal structure is reflected in the line broadening, decreasing peak intensity and shifted reflections (Fig. 5-a and Fig. 5-b). In line with Fig. 4, the diffraction peaks obtained from the mechanically activated samples (BPR5, BPR10, BPR20) were lower and broader than those of BPR0 as the intense energy provided by the balls, resulted in a shear mechanism during attrition milling, which led to the partial mechano-chemical dehydration of gibbsite by breaking OH bonds [29,30]. This caused
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the chemically bound water in the materials to be lost, and the boehmite phase was formed by the following reaction:
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Al(OH)3 AlO(OH) + H2O
(3)
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These results agree with Kumar et al. [28,29] and Alex et al. [32], who pointed out that
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gibbsite reactivity is enhanced by the combined effect of particle breakup and amorphisation.
2.3. Morphological evolution of powders during milling
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III.
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Individual particles in feed were present as aggregates of colourless, pseudo-hexagonal platelet-shaped crystals that were ≈ 93 μm in size. The disappearance of the initial
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morphology during milling was detected by FE-SEM. Figure 6-(a-c) shows the FESEM micrographs of the gibbsite powder after the 24 hours mechanical activation in isopropyl alcohol using different weight gibbsite to milling balls ratios.
Fig. 6 – FE-SEM micrographs of gibbsite powder after the 24 hours mechanical activation in isopropyl alcohol. (a) BPR5; (b) BPR10 and (c) BPR20.
The BPR5 micrographs (Figs. 6 (a)) indicate that most particles have a platelet-like structure sized ≈ 3 μm, which evidences the breaking of particles at the grain boundaries. However, some agglomerates of ≈ 300 nm composed of spherical particles
10
Pre-proof smaller than 50 nm were also Journal visible. These results explain the decreasing intensity of (002) reflection, as previously shown in Fig. 5-b. They also indicate that the starting gibbsite powder underwent intense mechanical shear, which was not enough to induce the transformation of gibbsite into boehmite for BPR5, and reached an AD that equalled 42%.
Figs. 6-(b) shows the BPR10 micrographs and reveals how spherical agglomerates of about 500 nm predominated, composed of spherical particles smaller than 50 nm. Compared to BPR5, a higher AD of gibbsite was reached as the consequence of a higher energy input that would explain the changes in SSA (157%) and AD (95%),
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respectively, as confirmed by XRD (Fig. 5b).
The FE-SEM micrographs of BPR20 are shown in Figs. 6-(c). In this sample, all the
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material consisted of agglomerates of nanosized spherical particles. This result supports the XRD phase analysis, i.e., BPR20 consists of nanocrystalline crystallites. The SSA
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and agglomerate size evolution also confirmed the formation of nanoparticles of amorphous gibbsite and boehmite.
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This finding clearly demonstrates that mechanical activation led to a significantly reduced particle size when the gibbsite-to-boehmite conversion was triggered, and the 2
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powder reached a SSA of 219 m /g and an AD of 99% after the 24 hours mechanical
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activation.
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III. 2.4. Thermal Behaviour of Gibbsite III. 2.4.1. Thermal Analysis
DSC and TG curves of the original/non-milled (BPR0) and milled gibbsite powders (BPR5, BRP10 and BPR20) are shown in Fig. 7 a-d.
Figure 7. – DSC and TG curves of gibbsite powders under several mechanical/grinding activation conditions: (a) without grinding (BPR0); (b) BPR5; (c) BPR10; (d) BPR 20. According to Figure 7a, the DSC curve of the non-milled/coarse gibbsite powder (BPR0; d50=93 μm) displays three endothermic peaks, where the maximum of the first broad nonsymmetrical peak occurred at 254ºC; the second was major and symmetrical, and it centred at
11
323ºC; the third one was located Journal at 538ºC Pre-proof (Fig. 7a; Table 2). From thecorresponding TG traces, three steps of weight loss (m) were recorded, with m values approximately equalling 6.6 wt.%, 22.7 wt.% and 5.3 wt.%, respectively. The total weight loss of the BPR0 powders within the temperature range between 20 and 1.050ºC was ≈ 34.7 wt.% (Fig. 7a and Table 2). This value is consistent with the theoretical value of 34.7 wt.% for the dehydration reaction of gibbsite following the consecutive chemical equations:
Al(OH)3 (s) AlO(OH) (s) + H2O (g)
(4)
AlO(OH)(s) ½ Al2O3 (s)+ ½ H2O(g)
(5)
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Table 2: DSC peaks temperature and weigth losses of gibbsite milled powders. Weight loss (wt. %)
Peak3
Peak4
m
323
538
34.7
323
≈ 515
35.3
283
318
≈ 515
35.7
288
313
≈ 515
35.8
Peak2
BPR0
-
254
BPR5
58
283
BPR10
68
BPR20
68
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Peak1
Pr
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DSC Endotherm peaks, ºC
Specimen
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Compared to BPR0, the shape of the DSC and TG curves of the milled gibbsite powders BPR5, BRP10 and BPR20 significantly changed (Fig. 7 b-d). They are characteristic
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thermograms of gibbsite fine particles. Four endothermic peaks and four mass loss steps were detected, which are described in more detail in the next section. In addition, no endothermic peak between 222-255ºC was observed in the non-milled specimens, which is indicative of a reduced grain size due to the mechanical energy introduced by BPR variation. The increasing impact and shear energy reduced the particle size of powders (Table 1 and Figure 7) and induced the mechano-chemical reaction that led to the formation of pseudoboehmite in sample BPR20, as confirmed by the XRD, FE-SEM and BET results. A clear band at ≈ 530ºC of the milled powders was detected, with a further gradual mass loss of 4.9%, which is indicative of the transformation of pseudoboehmite into a transition alumina [37]. The mass loss of samples BPR5, BRP10 and BPR20 is also presented on Table 3, where we can observe that they were all above 34.7 wt.% (average 35.8 wt %). This difference
12
Journal Pre-proof could be attributed to the moisture adsorbed on the surface of the gibbsite particles due to the large SSA presented by these powders after the high-energy milling process [34].
III. 2.4.2. Phase transformation. Influence of granulometry The effect of the granulometry/specific surface (and/or the influence of BPR) on the phase transformations of the mechanically activated gibbsite powders after 2 h of thermal treatments at 200, 350, 500, 800, 1000 and 1200ºC is shown on Figure 8 (a-b) and Table 3. Only the BPR10 and BPR20 diffractograms are presented.
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Figure 8 – Phases identified by DRX after the heat treatment of samples BRP10 and
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BPR20 at several temperatures.
According to Figure 8(a), after heating at 200ºC/2 h, the crystalline phases of the
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BRP10 powders were gibbsite (JCPDS – 33-0018) and small amounts of boehmite (JCPDS – 21-1307). With an increase in temperature to 350ºC/2 h, and given the
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presence of the -Alumina phase (JCPDS – 10-0425), small amounts of boehmite (Bh) and gibbsite (Gb) were detected. At 500ºC/2 h, the traces of boehmite and gibbsite
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disappeared and only χ-Alumina was identified. After the heat treatment at 800ºC/2 h,
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γ-Alumina was detected (JCPDS – 10- 0425). After treatment at 1000ºC/2 h a mixture of α-Alumina (JCPDS – 10-173) and ϴ-Alumina (JCPDS – 35-0121) was detected,
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which disappeared at higher temperatures to give rise to α-Alumina. Table 3 – Mineralogical composition of BPR0*, BPR5, BRP10 and BPR20 powders heat-treated at several temperatures. Temperature (ºC) 200
350
500 800 1,000
Phases by XRD Gb Bh Gb Bh m Am Am
BRP0* ++++ + + + + + +
13
Specimen BRP5 BRP10 ++++ ++++ + + + + + + ++ ++ + + +++ +++ + + + +++ + + ++ ++
BRP20 + +++ +++ + +++ + +++ + ++
Journal Pre-proof
+++
Major phase = ; Abundant phase = Amorphous * - From reference [34]
++
++ ++
+
; Minor phase = ; Absent =
--
++
; Am =
The BPR20 sample showed boehmite reflections and small amounts of gibbsite that disappeared at 350ºC/2 h and was converted into nanocrystalline boehmite or pseudoboehmite. At 500ºC/2 h, the γ-Alumina phase was detected, which was also observed when heat-treated at 800ºC/2 h. At 1000ºC and 1200ºC/2 h, the same phases observed in samples BPR5 and BRP10 were observed in the diffractograms. These results agree with the literature [35-37], which states that depending on the calcination conditions, gibbsite (Al(OH)3) can be transformed into α-Al2O3 (stable
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phase) via a sequence of transformations that may follow several pathways depending on the reaction conditions and various sample-related factors, such as particle size, calcination temperature, heating rate, water vapour pressure and the atmosphere
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surrounding grains (extra-granular environment).
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From these results, it can be concluded that the more vigorous grinding (greater ball-topowder ratio) helps to a faster transformation of the original material, favouring the
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formation of boehmite at low temperature fundamentally.
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III.2.4.3 Mechanism of gibbsite thermal evolution By combining the thermal analysis (TG and DSC curves), X-ray diffraction and SEM
rn
results with some other data reported elsewhere by different authors [35,37-42], it is
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possible to propose a mechanism for the thermal decomposition of coarse gibbsite and fine/activated gibbsite up to 1000°C. a) Coarse-grained gibbsite grains (BPR0, with d50 ≈100 μm): At a temperature range between ∼200–350°C, the dehydroxylation of the crystal lattice of gibbsite takes place in two steps: - Inside grains, gibbsite transforms into boehmite under relatively high water overpressure, and the first endothermic peak observed at 254°C is attributed to the partial dehydroxylation of gibbsite to form boehmite, as given by:
Al(OH)3 (Gib) AlO(OH) (Bh) + H2O (g) m = 6.6 wt.% and G1 = -26.93 kJ/mol
14
(6)
Journal Pre-proof - The second endothermic event (323 °C) corresponds to the loss ofapproximately 22.7 wt.% (G2 = -34.72 kJ/mol), which is attributed to the total conversion of gibbsite into boehmite. - At 538°C, the dehydroxylation of the crystal lattice of boehmite [38-40] took place, which can occur by: AlO(OH)(Bh) ½ Al2O3(Am) + ½ H2O (g)
(6)
m = 5.26 wt.% ; G3 = -24.81 kJ/mol b) Fine-grained gibbsite particles (BPR5, BPR10, BPR20 with d50 < 10 μm): Water was released in three pathways. Firstly, at temperatures below 100 ºC, the water
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that was physically attached to gibbsite powders evaporated, as given by.
(7)
pr
Al(OH)3·(m)H2O mH2O + Al(OH)3
Furthermore at temperatures between 230-350ºC, the two endothermic peaks and/or
gibbsite/amorphous
particles
and
e-
mass loss can be attributed to the decomposition (dehydroxylation) of the small-sized into
the
of
their
transformation
amorphous
aluminium
remaining
coarser
gibbsite
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oxides/boehmite/pseudoboehmite [38,42], and also to the dehydroxylation of the with
formation
amorphous
aluminium
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oxides/boehmite/pseuboehmite, as given by Equations (8) and (9), respectively.
Al(OH)3(Gib) AlO(OH)(Bh) + H2O (g)
(8)
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2AlO(OH) (2 − ν)/2H2O + Al2O3−ν/2(OH)ν
(9)
o
Finally at 515 C, pseudoboehmite is converted into some transition aluminas, as given by: Al2O3−ν/2(OH)ν ν/2H2O + Al2O3
(10)
where ν is the number of residual hydroxyl groups that remain in the transition alumina. These results of thermal analysis are also in agreement with those reported by Tsuchida and Icikawa [30], Mercury et al. [35], Gan et al. [38] and Udhayabanu et al. [39] for coarse-grained gibbsite.
IV. CONCLUSIONS
15
Journal Gibbsite powders were subjected to a 24Pre-proof hours activation with four powder-to-milling ball ratio (BPR) and they exhibited broad gibbsite peaks, unlike the sharp peaks exhibited by the non-milled gibbsite. This finding indicates a significant alteration to both crystallite size and morphology, allied to a 99% AD of gibbsite due to mechanical activation. The X-ray amorphous component retained its original water content. Upon heating, this water was progressively removed over a wide temperature range (100ºC-400ºC), conversely to the well- defined thermal dehydroxylation of the non-milled gibbsite.
Grinding caused a significant proportion of the Al-OH bonds to rupture, but the which it was endothermically desorbed at 125ºC.
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remaining molecular water was immediately adsorbed onto the activated surfaces, from
The decomposition temperature of the coarse gibbsite was above 250ºC. The drop in the
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decomposition temperature of the mechanically activated gibbsite powders reflected the
e-
amount of the mechano-chemically dehydrated H2O molecules, which were reabsorbed
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during milling.
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Acknowledgements:
First of all indicate that Prof. J.M. Rivas, our colleague, and co-author has recently
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passed away and has not been able to see this article published. We wish to thank all the good times spent together and his great contributions, both professional and personal.
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The financial support of the Spanish Ministry of Economy and Competitiveness (MINECO) Projects MAT2013-48426-C2-1-R; MAT2017-00000-C2-1-R and of the Brazilian research funding agency FAPEMA (Maranhão Foundation for Scientific Research and Development) through Project Apoio Cooperações Internacionais Edital n 44/2013 – APCInter is gratefully acknowledged.
16
Journal Pre-proof V. REFERENCES 1 – L. D. Hart. Alumina Chemicals Science and Technology handbook. The American Ceramic Society. Ohio USA, 1990. 2 – R. N. Kleiner, “Alumina and Other Refractory Materials”; pp. 297–305 in Practical Handbook of Materials Science. Edited by C. T. Lynch. CRC Press, Boca Raton, FL, 1989. 3 – E. Dorre and H. Huber, “Physical Properties”; pp. 11–16 and 41–49 in Alumina: Processing, Properties and Applications. Springer–Verlag, Berlin, Germany, 1984.
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4 – E. Dorre and H. Huber, “Mechanical Properties”; ibid, pp. 74–173. 5 – G. S. Brady, H. R. Clauser, and J. A. Vaccari, “Materials, Their Properties and
pr
Uses”; pp. 36–40 in Materials Handbook, 14th Ed. McGraw–Hill, New York, 1991. 6 – W. H. Gitzen. Alumina as a ceramic material. The American Ceramic Society. Ohio
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USA, 1970.
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7 – Yoshimura, M. & Bowen, H. K.. Electrical breakdown of alumina at high temperatures. J. Amer. Ceram. Soc.1981, 64: 404 – 410.
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8 – Han Bing and Li Zhi Hong, Technology of preparing alumina nanoparticles by
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precipitation method. J. Chemical Industry and Engineering 2006, 23: 512–515. 9 – Yang Chang Fu, Cheng Bao Ling, and Dong Zeng Fen, Preparation of superfine
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aluminum hydroxide fine powder by two section seed decomposition. J. China Powder Science and Technology 2008, 5: 26–29. 10 – M. Shojaie-Bahaabad and E. Taheri-Nassaj, “Economical synthesis of nano
alumina powder using an aqueous sol-gel method,” Materials Letters,2008, 62: 3364–3366. 11 – F. Mirjalili, M. Hasmaliza, and L. C. Abdullah, “Size-controlled synthesis of nano α-alumina particles through the sol-gel method,” Ceramics International, 2010, 36: 1253–1257. 12 - Y. Xie, D. Kocaefe, Y. Kocaefe, J. Cheng, and W. Liu, “The effect of novel
synthetic methods and parameters control on morphology of nano-alumina particles,” Nanoscale Research Letters, 2016, 259-269.
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Journal Pre-proof 13 – J. Kano, S. Saeki, F. Saito, M. Tanjo and S. Yamazaki. Application of dry grinding to reduction in transformation temperature of aluminum hydroxides. Int. J. Miner. Process., 2000, 60: 91–100. 14 – V.Z. Baranay, F. Kristaly, I. Szücs. Influence of the short time on the thermal decomposition processes of gibbsite produced by the Bayer process. Mat. Sci. Eng., 2013, 38:15-27. 15 – Q. Zhang, F. Saito. A review on mechanochemical syntheses of functional materials. Adv. Powder Technol. 2012, 23: 523–531.
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16 – Mamoru Senna, Materials design through mechanochemical processing, In HighEnergy Ball Milling, edited by Malgorzata Sopicka-Lizer, Woodhead Publishing, 2010, Pages 63-91.
Mechanochemistry: the mechanical
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17 – M.K. Beyer, H. Clausen-Schaumann,
e-
activation of covalent bonds, Chem. Rev. 2005, 105: 2921–2948. 18 – M. P. Pechini, US Patent 3 330697, 11 July 1967.
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19 – R. E. Prestes Salem, K. A.Guilherme, A. S. A. Chinelatto, A.Chinelatto, Sintering of alumina powders obtained by a Pechini-type method with addition of seeds,
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Materials Science Forum, 2012, 727-728: 1034-1039.
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20 – F. Mirjalili, M. Hasmaliza, L. Chuah Abdullah, Size-controlled synthesis of nano -alumina particles through the sol–gel method, Ceram. Int., 2010, 36: 1253–1257.
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21 – T.C. Alex, R. Kumar, S. K. Roy and S. P. Mehrotra. “Mechanically induced reactivity of gibbsite: Part 1. Planetary milling.” Powder Technol. 2014, 264, 105-113. 22 – M. Kitamura, M. Kamitani, M. Senna. Rapid Hardening of Cement by the Addition of a Mechanically Activated Al(OH)3–Ca(OH)2 Mixture. J. Am. Ceram. Soc., 2000, 83: 523–527. 23 – T.C. Alex, R. Kumar, S. K. Roy and S. P. Mehrotra. “Mechanically induced reactivity of gibbsite: Part 2. Attrition milling.” Powder Technol. 2014, 264, 229-235. 24 – X. Du, X. Su, Y Wan and J. Li., Thermal decomposition of grinding activated bayerite. Mat. Research Bull. 2009, 44: 660-665.
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1
25 – C. C. Yong and J. Wang. “Mechanical Activation Triggered Gibbsite-to-Boehmite Transition and Activation-Derived Alumina Powders.” J. Am. Ceram. Soc. 2001, 84: 1225–1230. 26 – S.-W. Jang, H.-Y. Lee, S.-M. Lee, S. W. Lee, and K.-B. Shim, ‘‘Mechanical Activation Effect on the Transition of Gibbsite to a-Alumina,’’ J. Mater. Sci. Lett. 2000, 19: 507–510. 27 – K.J.D. MacKenzie, J. Temuujin, K. Okada. “Thermal decomposition of mechanically activated gibbsite” Thermochimica Acta, 1999, 327: 103-108.
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28 – R Kumar, T.C. Alex, M.K. Jha, Z.H. Khan, S.P. Mahapatra and C.R. Mishra. Mechanochemistry and the Bayer Process of Alumina Production. Light Metals 2004,
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ed. P. Crepeau (The Minerals, Metals & Materials Society, Warrendale) pp. 31-34. 29 – R. Kumar, T. C. Alex, J. P. Srivastava, B. R. Kumar, Z. H. Khan, S. P. Mahapatra
e-
and C. R. Mishra. Mechanical Activation of Bauxite: On the Nature of Physicochemical Changes and Reactivity of Gibbsite. Metals Materials and Processes, 2004, 16: 171-
Pr
180.
30 – T. Tsuchida and N. Ichikawa. Mechanochemical phenomena of gibbsite, bayerite
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and boehmite by grinding. React. Solids, 1989, 7: 207-217.
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31 – L.B. McCusker, R.B. Von Dreele, D.E. Cox, D. Louer, P. Scardi, Rietveld refinement guidelines, J. Appl. Crystallogr. 1999, 32: 36–50.
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32 - T.C. Alex, R. Kumar, S.K. Roy and S.P. Mehrotra, Anomalous reduction in surface area during mechanical activation of boehmite synthesized by thermal decomposition of gibbsite. Powder Technology, 2011, 208: 128-136. 33 – H. Saalfeld and M. Wedde. Refinement of the crystal structure of gibbsite, AI(OH)3. Zeitschrift fur Kristallographie, Bd. 1974, 139: 129-135. 34 – R. Tettenhorst, D.A. Hofmann. Crystal Chemistry of Boehmite. Clays Clay Miner. 1980, 28: 373–380. 35 – J. M. Rivas, P. Pena, A.H. de Aza, D. Sheptyakov, X. Turrillas, On the decomposition of synthetic gibbsite studied by neutron thermodiffractometry, J. Am. Ceram. Soc. 2006, 89: 3728–3733. 36 – Wefers, K.; Misra, C. Alcoa Technical Paper No. 19. Alcoa Laboratories:
19
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Pittsburgh, PA, 1987.
37 – P. Alphonse, M. Courty. Structure and thermal behavior of nanocrystalline boehmite. Thermochimica Acta, 2005, 425: 75–89. 38 – B. K. Gan, I.C. Madsenb and J. G. Hockridge. In situ X-ray diffraction of the transformation of gibbsite to -alumina through calcination: effect of particle size and heating rate. J. Appl. Cryst. 2009, 42: 697–705. 39 – V. Udhayabanu, B. S. Murty. Synthesis of Nanocrystalline -Al2O3 from Nanocrystalline Boehmite Derived from High Energy Ball Milling of Gibbiste. Trans
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Indian Inst. Met. 2011, 64: 535–540. 40 – P.T. Tanev and L.T. Vlaev. Effect of grain size on the synthesis of active alumina
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from gibbsite by flash calcination and rehydration. Catalysis Letters. 1993, 19: 351-360. 41 – B. Zhu, B. Fang, X. Li. Dehydration reactions and kinetic parameters of gibbsite.
e-
Ceram. Int. 2010, 36: 93–2498.
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42 – X. Bokhimi, J.A. Toledo-Antonio, M.L. Guzmán-Castillo, F. Hernández-Beltrán. Relationship between Crystallite Size and Bond Lengths in Boehmite. J. of Solid State
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Chem. 2001, 159: 32–40.
20
Journal Pre-proof
Highlights
- Grinding modify structure and thermal behavior of Gibbsite - Alteration in crystallite size and morphology with the amorphisation was observed - The amorphous component retained its original water content. - Grinding caused Al -OH bonds to rupture. Activated surfaces adsorbed resulting molecular water
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- A drop in the decomposition temperature was observed.
21
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
J.M. Rivas-Mercury, J.R.M. Sucupira, M.A. Rodríguez, A.A. Cabral, A.H. De Aza, P. Pena PII:
S0032-5910(19)31017-4
DOI:
https://doi.org/10.1016/j.powtec.2019.11.057
Reference:
PTEC 14939
To appear in:
Powder Technology
Received date:
26 March 2019
Revised date:
1 November 2019
Accepted date:
18 November 2019
Please cite this article as: J.M. Rivas-Mercury, J.R.M. Sucupira, M.A. Rodríguez, et al., Influence of the milling conditions on the thermal decomposition of Bayer gibbsite, Powder Technology(2019), https://doi.org/10.1016/j.powtec.2019.11.057
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© 2019 Published by Elsevier.
Journal Pre-proof
Influence of the Milling Conditions on the Thermal Decomposition of Bayer Gibbsite ‡
‡
§
J M. Rivas-Mercury , J. R. M. Sucupira‡; M. A. Rodríguez , A. A. Cabral , A. H. §
§
De Aza , P. Pena ‡
Instituto Federal de Educação, Ciência e Tecnológica do Maranhão – IFMA Av. Getúlio Vargas, 04 - Monte Castelo - CEP 65025-001- São Luís – Brasil
§
Instituto de Cerámica y Vidrio, ICV-CSIC; Kelsen, 5, 28049 - Cantoblanco, Madrid,
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Spain
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ABSTRACT A synthetic gibbsite (Al(OH)3) produced by the Bayer process was mechanically ratios
e-
activated by attrition milling for 24 hours with various grinding ball-to-powder
(BPR=5, 10 and 20). Changes in its structure were studied by thermal analysis and X-
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ray diffraction. As a result of increasing ball-to-powder ratio, the grain size of gibbsite decreased while its specific surface area increased. Only for these materials, the formation
of
nanocrystalline
boehmite
(AlO·OH)
and
amorphous
aluminium
al
hydroxides was also observed. Upon having been heat treated between 200 – 1200ºC ( 2
rn
hours), boehmite was detected at the temperature range of 200 and 350ºC for the BPR10 sample, while the boehmite of BPR20 was transformed to γ- Al2O3 at 500ºC. Further,
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only α-Al2O3 was detected at 1200ºC for all samples. Finally, combining the TG-DSC, XRD and SEM results, it was proposed a mechanism for the thermal decomposition of the non-milled and milled samples.
Key words: Gibbsite; milling; mechanical activation; amorphization; thermal behaviour.
Corresponding author: Miguel A.
Rodríguez.
917355843)
1
(email: [email protected]; fax: +34
Journal Pre-proof
I. INTRODUCTION Gibbsite (Al(OH)3) is an aluminium trihydroxide produced on the industrial scale by the Bayer process [1]. This material is a precursor of boehmite (AlOOH) or pseudoboehmite (poor crystalline boehmite), and can be used as a catalyst carrier, a reinforcement of ceramic composites and to obtain transition aluminas (-Al2O3) and
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corundum (-Al2O3) materials, which are widely used in many structural and functional applications [2-7].
Crystalline and amorphous aluminium hydroxides with a low alkali content can be
pr
prepared by several methods like seed precipitation [8], inorganic aluminium salt deposition [9], sol-gel [10,11] microemulsion [12], and long- and short-term grinding
e-
[13, 14]. All these methods lead to the formation of high-purity aluminas, but involve
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high annealing temperatures for long times to convert hydroxides into oxide, which is often undesirable because it leads to powders with poor characteristics from the point of view of shape forming (agglomerates, coarse crystal sizes, porous particles).
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Recently, interest in powders processing has increasingly focused on the application of
rn
mechano-chemical activation to develop new materials and processes [15]. This is because highly reactive surface areas form that change the physical behaviour of solids,
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and can be attributed to both the permanent rearrangement of the crystal lattice and the structural changing of grains during grinding [16,17]. It has been reported that mechanical activation offers significant advantages over other
powder synthesis
methods, such as the polymeric precursor method [18,19], sol-gel [20], and the use of solvents, which lead to large amounts of liquid or gas waste formation and need to be utilised in other processes to prevent environmental impacts. Gibbsite is often the predominant mineral in bauxite ores and a fundamental understanding of its activation is required [21-23]. Several research works have studied the effect of grinding on aluminium hydroxides and have reported that mechanochemical treatment leads to the formation of aluminium atoms that are pentacoordinated on the surface of gibbsite particles, such as the “gel-like” phase (amorphous to X-rays) stabilised by protons and water molecules [24-28]. The rupture of a significant proportion of Al-OH bonds releases molecular water, which is immediately
2
Journal Pre-proof adsorbed onto activated surfaces from where it can be endothermically desorbed at 125ºC [29,30]. The aim of this work was to investigate the effect of changes in the ball-to-powder ratio (BPR) on the physico-chemical properties of coarse mechanically activated synthetic gibbsite (Al(OH)3) in order to provide information on how the grinding can modify the structure of the Gibbsite and its thermal behaviour. II. EXPERIMENTAL
II. 1. Description of the original sample A coarse-synthetic gibbsite of a nearly uniform size (mean grain size, d50 ≈ 93 m),
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prepared by the Bayer process and supplied by ALCOA (San Ciprián, Lugo, Spain), was used. The chemical analyses of gibbsite by X-ray fluorescence spectroscopy (XRF, Magi X; Phillips, The Netherlands) and the alkaline analyses (Na and K) by flame
pr
emission spectrometry (Perkin Elmer FES 2100) indicated that gibbsite was composed
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of 65.0 wt% Al2O3, 0.2 wt% Na2O, 0.1 wt% TiO 2, 0.04 wt% SiO 2, 0.03 wt% CaO,
and was of 99.7 wt% purity grade.
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II. 2. Attrition Milling of Gibbsite
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0.006 wt% MgO and 0.001 wt% K 2O. The sample underwent 34.6 wt % ignition loss
Milling was carried out in a vertical laboratory batch attritor (home made at the Instituto
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de Cerámica y Vidrio, ICV-CSIC. Spain) at a rotational speed of 1,000 rpm, with a
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milling vessel of stainless steel coated internally with teflon and with a volumetric capacity of 0.8 L. Gibbsite powder was milled at room temperature (25C) using 3
partially stabilised zirconia (3 mol% yttrium, PSZ, density, 6,065 kg/m ) as milling balls (3 mm in diameter). Isopropyl alcohol was used as the process controlling agent to prevent agglomeration of powders. The liquid/solid was maintained at 0.50 and the ballto-powder weight ratios (BPR) were 0 (original sample, without milling) , 5:1, 10:1 and 20:1 (by weight), hereinafter referred to as BPR0, BPR5, BPR10 and BPR20. To avoid system overheating and
its consequences (alcohol evaporation), a mantle with
circulating cooling water (20C) was attached to the bowl. The milling was carried out for 24 hours, stopping after each 1 hour interval for cooling during 15 minutes. After completing the milling process, suspensions were sieved, poured into beakers, and dried in a stove with circulating forced air for 12 h at 110C.
3
In order to study the thermalJournal evolutionPre-proof of gibbsite BRP10, thermal treatments at constant heating rate of 10ºC/min up to 200, 350, 500, 800, 1000 and 1200ºC holding maximum temperature during 2 hours were carried out. Heat treatments were done in an electric furnace electronically controlled (±1°C). II.3. Physical Characterisation of Powders The volumetric N 2 adsorption method was used to determine the BET (BrunauerEmmett-Teller) specific surface area (SSA) of powders. The tested powder was placed inside the sample container with a known volume. The adsorbed volume of gas was determined from changing the pressure associated with gas adsorption on the sample powder surface. A surface area analyser (ASAP 2020, Micromeritics Corporation,
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USA) was used and operated with N 2 (99.999 %) as the adsorbate gas. After degassing samples at 70C, the temperature was raised to 150C (10C/min) and this temperature
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remained for 5 h in vacuum. The monolayer capacity and the SSA were calculated by the BET model.
e-
Particle size distribution was measured using a laser scattering-based particle size analyser (Master Sizer S; Malvern Instruments, UK). All the particle size measurements
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were taken on slurry samples. Sodium hexametaphosphate was used as dispersant. Particle dispersion and deagglomeration were also ensured by means of ultrasonic
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treatment prior to taking measurements. The true density of the gibbsite powders was
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II. 4. Thermal Analysis
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measured by He-picnometry (Multipicnometer, Quantachrome,USA).
Thermal differential and thermogravimetric (DSC –TGA) analysis were conducted with an STA 409 (Netzsch, Germany) thermo-balance at 20ºC/min up to 1,100ºC (maximum temperature) in flowing air (50 ml/min), and using Pt crucibles and -Al2O 3 powder as references. II. 5. X-Ray Diffraction The diffractograms of samples BPR0, BPR5, BRP10 and BPR20 were recorded in the continuous mode in a X’PERT-PRO X-ray diffractometer (UK) equipped with an X’Celerator detector (RTMS), a PW3050/60 goniometer with a theta-theta design, and a step size of 0.017° (2) within an angular range of 5 - 70 (2) using Cu-K radiation ( = 1.540598 Å) at 40 kV and 35 mA, a Soller slit of 0.04 (rad) in the incident and diffracted beam, and a beta nickel filter. The XRD patterns were analyzed by the High
4
JournalThe Pre-proof Score Plus 3.0e software (Panalytical). crystalline phases were indexed using the following files from the Inorganic Crystal Structure Database (ICDS), collection code: 006162 (Al(OH)3-Gibbsite) and 100391 (AlOOH-Boehmite). Rietveld
refinement were performed
on sample BPR0 following the strategies
recommended by McCusker et al. [31] using the FullProf Suite software. Firstly, the global variables were refined (sample displacement, background and scale factor). Afterwards, the following parameters were refined: lattice parameters, profile variables (W, V and U, besides Peak Shape or Asymmetry), the preferred orientation for specific phases, atomic coordinates and individual thermal factors. Finally, the Pseudo-Voigt profile adjustment function was changed to a Pseudo-Voigt 3 profile function (FJC
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Asymmetry). The refinement quality indicators RWP (R-weight standards or patterndependent disagreement factor), Rp (residual of least-squares refinement) and goodness-
pr
of-fit (GoF) obtained minimum values, which indicates that fits to any given phase, are very satisfactory.
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Based on Rietveld refinement and XRD spectra, diffractograms were also used to calculate the average crystallite size (size of the coherently diffracting domains) using
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the (002) reflexion and Scherrer’s Equation (1):
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DS
k cos
(1)
rn
S
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where DS is the average crystallite size, k is a constant that equals 0.9; is the X-ray wavelength that equals 0.15406 nm, and is the full-width at half-maximum (FWHM) subtracted the instrumental broadening. Instrumental broadening was measured using Cerium 674b as the standard reference material (NIST, SRM 674b). The XRD patterns collected on the CeO 2 standard were treated with Data Viewer 1.4 (Panalytical) to obtain the FWHM of the peak profile, which were used to estimate the crystallite size with Scherrer’s equation for reflexion (002) in all the treated samples. The amorphisation degree (AD) was estimated according to the BPR on the XRD patterns by the method developed by Ohlberg and Strickler, and as described by Alex et al [32], which is given by:
5
( )
(
) Journal Pre-proof
(2)
where AD denotes the percentage degree (%), Io and Bo are the integral intensities of the diffraction peak (002) and the background of the diffraction peak for the non-milled mixture, respectively, while Ix and Bx are the equivalent values for the mechanically activated mixture. The Io, Bo, Ix and Bx integrals were calculated using Data Viewer 1.4 (Panalytical).
II. 6. Field Emission Scanning Electron Microscopy. The powder morphology was studied under several ball-to-powder ratios (BPR)
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conditions by scanning electron microscopy (SEM). After gold coating, powder surfaces were examined under a Hitachi S-4700 (Hitachi, Tokyo, Japan) field emission
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III. RESULTS AND DISCUSSION
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scanning electron microscope (FE-SEM).
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III. 1. Physical and Mineralogical Characterisation of the Original Gibbsite The initial gibbsite powder presented a characteristic morphology, composed of large spherulitic concretions of pseudo-hexagonal platelet-shaped crystals (Fig. 1-a and Fig. 2 -1
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1-b) of Al(OH)3, with a specific surface area of 0.07 m ·g , a mean grain size of 93 -3
(Table 1).
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m and a real density of 2.44 g.cm
Fig. 1 – a) SEM micrograph of the original gibbsite powder (BPR0); b) SEM micrograph detail of the agglomerates of particles of BPR0.
The diffractogram of the non-milled gibbsite powder (Fig. 2) shows only the reflections corresponding to Al(OH)3. Powder diffraction pattern was refined using the Monoclinic Crystal Structure (P21/n Space Group) by the Rietveld analysis with the FullProf software.
6
Journal Pre-proof Fig. 2. – X-ray diffraction patterns of non-milled (BPR0) at RT: experimental and Rietveld refinement. Vertical lines correspond to the Bragg peaks of the identified phase.
The Rietveld refinement of the XRD pattern of the non-milled gibbsite (BPR0) at room temperature yielded the following structure parameters: a = 8.6798±0.0005 Å, b = 5.0757 ± 0.0003 Å, c = 9.7330 ± 0.0005 Å and = 94.5807 ± 0.0005o and V = 427.417 ± 0.004 Å3 . The mean crystallite size was 146nm., with an anisotropy of 0.92 and the maximum strains were 13.46/10000 with an anisotropy of 0.02 (RBragg = 8.60; RWp = 13.20; 2 = 3.80). These results well agree with the data reported in the literature by other researchers [33, 34], and show that BPR0 has a monoclinic crystal structure
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(P21/n,) and exhibits a tabular pseudo-hexagonal habit. The structure can be regarded as stacked hexagonal close-packed (hcp) layers with open planes between successive
pr
sheets. Each Al cation is octahedrally coordinated to six OH groups and each hydroxyl group is coordinated to two Al cations, which leaves one octahedral site vacant.
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Alternatively, it was envisaged as double layers of hcp hydroxyl groups stacked in an AB BA sequence. The variables of the hydrogen position were not refined during the
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Rietveld analysis because calculations became unstable as the 14 atomic sites were in
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general positions, hence there were too many coordinates to be refined.
III. 2. Milling Studies
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III. 2.1. Grain size and specific surface area. Figure 3 and Table 1 describe the SSA and the mean grain size (d50) of the gibbsite
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powder according to the BPR.
Fig. 3. Average grain size and BET specific surface area of gibbsite according to the activation conditions.
Table 1 – Physical properties of the milled powders 2 -1
SSA (m ·g ) DBET (m) d50 (m)
-3
r (g·cm )
BPR
Reference
0
BPR0
0.07
35
93
5
BPR5
32
0.079
2.1
7
10
BPR10
20
BPR20
Journal Pre-proof 82
0.031
1.5
219
0.012
6.9
DB ET: BET spherical diameter calculated from the values of SSA and measured densities.
2
As seen in Fig. 3 and Table 1, the SSA increased continuously from 0.07 m /g to 219 2
m /g depending on the milling activation conditions. The average grain size decreased from 93 m to ≈ 1.5 m from BPR0 to BPR10. In this study, the energy input changed as BPR varied and took values of 5, 10 and 20. Nevertheless, an increasing d 50 was observed for BPR20 (Table 1), which could be associated with powder agglomeration
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as a consequence of the formation of secondary particles (agglomerates), which arise from the interactions taking place among fine particles through Van der Waals forces.
pr
As BPR increased, as we can see in Table 1, the diameters of particles (sphereequivalent diameter) determined from the BET SSAs (DBET) were much smaller than
e-
those ones obtained by the laser diffraction method. This indicates that particle pores and roughness remain accessible in agglomerates and that nitrogen gas can penetrate the
Pr
pore space of particles. As a result, the BET method determined a SSA that provided smaller particles sizes (DBET ) than those measured by the laser diffraction method. This
al
also implies that the primary particle agglomerates broke during intensive milling, which led to new different particles, in size and shape factor, as the milling energy
rn
increased as a result of the high shear introduced by an increase in BPR. The real
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density (r) of the activated powders measured by He pycnometry clearly diminished according to the activation conditions (Table 1), probably due to the surface amorphization of the particles.
III. 2.2. X-Ray Diffraction – XRD III. 2.2.1. Mineralogical and Structural Changes. Figure 4 (a-d) shows the diffractograms of the gibbsite powders that were mechanically activated for 24 hours at various BPRs. As shown in Fig. 4a, BPR0, corresponding to the pure and non-milled gibbsite, the intense (002) Bragg reflection of this mineral is located at 2= 18.319°. From the diffractograms (Fig. 4b), we can observe that the intensity of Bragg's reflection (002) decreased for BPR5. With BPR10, only the reflections of gibbsite were identified (Fig. 4c). As expected, the intensity of the peaks located at 2= 18.335° and 20.6° were further reduced by mechanical activation due to
8
Pre-proof the amorphisation of gibbsite. Journal BPR20 showed four new strongly broadened peaks centred at 14.6°, 28.2°, 38.6°, and 49.3° of 2. These peaks corresponded to the (020), (120), (031; 140), and (051) reflections of boehmite (AlOOH), respectively (Fig. 4d).
Fig. 4 –XRD patterns of the gibbsite powders subjected to the 24 hours mechanical activation in isopropyl alcohol: a) the non-milled sample BPR0, (b) BPR5, (c) BPRB10, (d) BPR20. (Gb) Gibbsite and (Bh) Boehmite).
The presence of boehmite reflections in BPR20 sample (with a 24 hours milling time)
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was explained after considering that more heat was produced given the mechanical energy introduced as a consequence of the increasing number of collisions per time unit
pr
as the number of balls increased. In parallel, this led to a rising milling temperature, and a faster diffusion process and phase transformation. This transformation takes place at
e-
temperatures around 250ºC. Another interesting observation made from the XRD data (Fig. 4 (a-d)) was that the diffraction peaks of all the mechanically activated samples
Pr
(BPR5, BPR10 and BPR20) became lower and broader than those of the non-milled one (BPR0). The gibbsite reflections also shifted due to structure distortion. As pointed out
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above, the decrease in the diffraction peak intensities implied the formation of
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III. 2.2.2. X-ray AD
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amorphous material and the nucleation of the nanocrystalline boehmite phase.
The structural disorder due to the increasing amorphous material content was shown by the intensity decreasing of the diffraction peaks as can be observed in the X-ray diffraction patterns (Figure 5-(a)) of the activated samples.
Fig. 5. Variation of the: a) peak broadening and intensity of the Bragg reflections (002) of Gibbsite for increasing BPR; and b) AD and crystallite size with the SSA.
Since the (002) Bragg reflection of gibbsite is affected mostly by milling, it was used to calculate the AD. By assuming that the amorphous phase in the initial powder was
9
Pre-proof negligible, and by replacing theJournal data taken from the XRD patterns in Eq. (1), the average X-ray ADs were calculated for the milled samples, as depicted in Fig 5-b. This figure clearly shows that the AD growths up to 99% as the BET surface area increased. Crystallite size (Ds) reduced in agreement with decreasing of diffraction peak intensities [27, 28]. The distortion of the crystal structure is reflected in the line broadening, decreasing peak intensity and shifted reflections (Fig. 5-a and Fig. 5-b). In line with Fig. 4, the diffraction peaks obtained from the mechanically activated samples (BPR5, BPR10, BPR20) were lower and broader than those of BPR0 as the intense energy provided by the balls, resulted in a shear mechanism during attrition milling, which led to the partial mechano-chemical dehydration of gibbsite by breaking OH bonds [29,30]. This caused
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the chemically bound water in the materials to be lost, and the boehmite phase was formed by the following reaction:
pr
Al(OH)3 AlO(OH) + H2O
(3)
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These results agree with Kumar et al. [28,29] and Alex et al. [32], who pointed out that
Pr
gibbsite reactivity is enhanced by the combined effect of particle breakup and amorphisation.
2.3. Morphological evolution of powders during milling
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III.
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Individual particles in feed were present as aggregates of colourless, pseudo-hexagonal platelet-shaped crystals that were ≈ 93 μm in size. The disappearance of the initial
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morphology during milling was detected by FE-SEM. Figure 6-(a-c) shows the FESEM micrographs of the gibbsite powder after the 24 hours mechanical activation in isopropyl alcohol using different weight gibbsite to milling balls ratios.
Fig. 6 – FE-SEM micrographs of gibbsite powder after the 24 hours mechanical activation in isopropyl alcohol. (a) BPR5; (b) BPR10 and (c) BPR20.
The BPR5 micrographs (Figs. 6 (a)) indicate that most particles have a platelet-like structure sized ≈ 3 μm, which evidences the breaking of particles at the grain boundaries. However, some agglomerates of ≈ 300 nm composed of spherical particles
10
Pre-proof smaller than 50 nm were also Journal visible. These results explain the decreasing intensity of (002) reflection, as previously shown in Fig. 5-b. They also indicate that the starting gibbsite powder underwent intense mechanical shear, which was not enough to induce the transformation of gibbsite into boehmite for BPR5, and reached an AD that equalled 42%.
Figs. 6-(b) shows the BPR10 micrographs and reveals how spherical agglomerates of about 500 nm predominated, composed of spherical particles smaller than 50 nm. Compared to BPR5, a higher AD of gibbsite was reached as the consequence of a higher energy input that would explain the changes in SSA (157%) and AD (95%),
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respectively, as confirmed by XRD (Fig. 5b).
The FE-SEM micrographs of BPR20 are shown in Figs. 6-(c). In this sample, all the
pr
material consisted of agglomerates of nanosized spherical particles. This result supports the XRD phase analysis, i.e., BPR20 consists of nanocrystalline crystallites. The SSA
e-
and agglomerate size evolution also confirmed the formation of nanoparticles of amorphous gibbsite and boehmite.
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This finding clearly demonstrates that mechanical activation led to a significantly reduced particle size when the gibbsite-to-boehmite conversion was triggered, and the 2
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powder reached a SSA of 219 m /g and an AD of 99% after the 24 hours mechanical
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activation.
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III. 2.4. Thermal Behaviour of Gibbsite III. 2.4.1. Thermal Analysis
DSC and TG curves of the original/non-milled (BPR0) and milled gibbsite powders (BPR5, BRP10 and BPR20) are shown in Fig. 7 a-d.
Figure 7. – DSC and TG curves of gibbsite powders under several mechanical/grinding activation conditions: (a) without grinding (BPR0); (b) BPR5; (c) BPR10; (d) BPR 20. According to Figure 7a, the DSC curve of the non-milled/coarse gibbsite powder (BPR0; d50=93 μm) displays three endothermic peaks, where the maximum of the first broad nonsymmetrical peak occurred at 254ºC; the second was major and symmetrical, and it centred at
11
323ºC; the third one was located Journal at 538ºC Pre-proof (Fig. 7a; Table 2). From thecorresponding TG traces, three steps of weight loss (m) were recorded, with m values approximately equalling 6.6 wt.%, 22.7 wt.% and 5.3 wt.%, respectively. The total weight loss of the BPR0 powders within the temperature range between 20 and 1.050ºC was ≈ 34.7 wt.% (Fig. 7a and Table 2). This value is consistent with the theoretical value of 34.7 wt.% for the dehydration reaction of gibbsite following the consecutive chemical equations:
Al(OH)3 (s) AlO(OH) (s) + H2O (g)
(4)
AlO(OH)(s) ½ Al2O3 (s)+ ½ H2O(g)
(5)
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Table 2: DSC peaks temperature and weigth losses of gibbsite milled powders. Weight loss (wt. %)
Peak3
Peak4
m
323
538
34.7
323
≈ 515
35.3
283
318
≈ 515
35.7
288
313
≈ 515
35.8
Peak2
BPR0
-
254
BPR5
58
283
BPR10
68
BPR20
68
al
e-
Peak1
Pr
pr
DSC Endotherm peaks, ºC
Specimen
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Compared to BPR0, the shape of the DSC and TG curves of the milled gibbsite powders BPR5, BRP10 and BPR20 significantly changed (Fig. 7 b-d). They are characteristic
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thermograms of gibbsite fine particles. Four endothermic peaks and four mass loss steps were detected, which are described in more detail in the next section. In addition, no endothermic peak between 222-255ºC was observed in the non-milled specimens, which is indicative of a reduced grain size due to the mechanical energy introduced by BPR variation. The increasing impact and shear energy reduced the particle size of powders (Table 1 and Figure 7) and induced the mechano-chemical reaction that led to the formation of pseudoboehmite in sample BPR20, as confirmed by the XRD, FE-SEM and BET results. A clear band at ≈ 530ºC of the milled powders was detected, with a further gradual mass loss of 4.9%, which is indicative of the transformation of pseudoboehmite into a transition alumina [37]. The mass loss of samples BPR5, BRP10 and BPR20 is also presented on Table 3, where we can observe that they were all above 34.7 wt.% (average 35.8 wt %). This difference
12
Journal Pre-proof could be attributed to the moisture adsorbed on the surface of the gibbsite particles due to the large SSA presented by these powders after the high-energy milling process [34].
III. 2.4.2. Phase transformation. Influence of granulometry The effect of the granulometry/specific surface (and/or the influence of BPR) on the phase transformations of the mechanically activated gibbsite powders after 2 h of thermal treatments at 200, 350, 500, 800, 1000 and 1200ºC is shown on Figure 8 (a-b) and Table 3. Only the BPR10 and BPR20 diffractograms are presented.
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Figure 8 – Phases identified by DRX after the heat treatment of samples BRP10 and
pr
BPR20 at several temperatures.
According to Figure 8(a), after heating at 200ºC/2 h, the crystalline phases of the
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BRP10 powders were gibbsite (JCPDS – 33-0018) and small amounts of boehmite (JCPDS – 21-1307). With an increase in temperature to 350ºC/2 h, and given the
Pr
presence of the -Alumina phase (JCPDS – 10-0425), small amounts of boehmite (Bh) and gibbsite (Gb) were detected. At 500ºC/2 h, the traces of boehmite and gibbsite
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disappeared and only χ-Alumina was identified. After the heat treatment at 800ºC/2 h,
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γ-Alumina was detected (JCPDS – 10- 0425). After treatment at 1000ºC/2 h a mixture of α-Alumina (JCPDS – 10-173) and ϴ-Alumina (JCPDS – 35-0121) was detected,
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which disappeared at higher temperatures to give rise to α-Alumina. Table 3 – Mineralogical composition of BPR0*, BPR5, BRP10 and BPR20 powders heat-treated at several temperatures. Temperature (ºC) 200
350
500 800 1,000
Phases by XRD Gb Bh Gb Bh m Am Am
BRP0* ++++ + + + + + +
13
Specimen BRP5 BRP10 ++++ ++++ + + + + + + ++ ++ + + +++ +++ + + + +++ + + ++ ++
BRP20 + +++ +++ + +++ + +++ + ++
Journal Pre-proof
+++
Major phase = ; Abundant phase = Amorphous * - From reference [34]
++
++ ++
+
; Minor phase = ; Absent =
--
++
; Am =
The BPR20 sample showed boehmite reflections and small amounts of gibbsite that disappeared at 350ºC/2 h and was converted into nanocrystalline boehmite or pseudoboehmite. At 500ºC/2 h, the γ-Alumina phase was detected, which was also observed when heat-treated at 800ºC/2 h. At 1000ºC and 1200ºC/2 h, the same phases observed in samples BPR5 and BRP10 were observed in the diffractograms. These results agree with the literature [35-37], which states that depending on the calcination conditions, gibbsite (Al(OH)3) can be transformed into α-Al2O3 (stable
oo f
phase) via a sequence of transformations that may follow several pathways depending on the reaction conditions and various sample-related factors, such as particle size, calcination temperature, heating rate, water vapour pressure and the atmosphere
pr
surrounding grains (extra-granular environment).
e-
From these results, it can be concluded that the more vigorous grinding (greater ball-topowder ratio) helps to a faster transformation of the original material, favouring the
Pr
formation of boehmite at low temperature fundamentally.
al
III.2.4.3 Mechanism of gibbsite thermal evolution By combining the thermal analysis (TG and DSC curves), X-ray diffraction and SEM
rn
results with some other data reported elsewhere by different authors [35,37-42], it is
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possible to propose a mechanism for the thermal decomposition of coarse gibbsite and fine/activated gibbsite up to 1000°C. a) Coarse-grained gibbsite grains (BPR0, with d50 ≈100 μm): At a temperature range between ∼200–350°C, the dehydroxylation of the crystal lattice of gibbsite takes place in two steps: - Inside grains, gibbsite transforms into boehmite under relatively high water overpressure, and the first endothermic peak observed at 254°C is attributed to the partial dehydroxylation of gibbsite to form boehmite, as given by:
Al(OH)3 (Gib) AlO(OH) (Bh) + H2O (g) m = 6.6 wt.% and G1 = -26.93 kJ/mol
14
(6)
Journal Pre-proof - The second endothermic event (323 °C) corresponds to the loss ofapproximately 22.7 wt.% (G2 = -34.72 kJ/mol), which is attributed to the total conversion of gibbsite into boehmite. - At 538°C, the dehydroxylation of the crystal lattice of boehmite [38-40] took place, which can occur by: AlO(OH)(Bh) ½ Al2O3(Am) + ½ H2O (g)
(6)
m = 5.26 wt.% ; G3 = -24.81 kJ/mol b) Fine-grained gibbsite particles (BPR5, BPR10, BPR20 with d50 < 10 μm): Water was released in three pathways. Firstly, at temperatures below 100 ºC, the water
oo f
that was physically attached to gibbsite powders evaporated, as given by.
(7)
pr
Al(OH)3·(m)H2O mH2O + Al(OH)3
Furthermore at temperatures between 230-350ºC, the two endothermic peaks and/or
gibbsite/amorphous
particles
and
e-
mass loss can be attributed to the decomposition (dehydroxylation) of the small-sized into
the
of
their
transformation
amorphous
aluminium
remaining
coarser
gibbsite
Pr
oxides/boehmite/pseudoboehmite [38,42], and also to the dehydroxylation of the with
formation
amorphous
aluminium
rn
al
oxides/boehmite/pseuboehmite, as given by Equations (8) and (9), respectively.
Al(OH)3(Gib) AlO(OH)(Bh) + H2O (g)
(8)
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2AlO(OH) (2 − ν)/2H2O + Al2O3−ν/2(OH)ν
(9)
o
Finally at 515 C, pseudoboehmite is converted into some transition aluminas, as given by: Al2O3−ν/2(OH)ν ν/2H2O + Al2O3
(10)
where ν is the number of residual hydroxyl groups that remain in the transition alumina. These results of thermal analysis are also in agreement with those reported by Tsuchida and Icikawa [30], Mercury et al. [35], Gan et al. [38] and Udhayabanu et al. [39] for coarse-grained gibbsite.
IV. CONCLUSIONS
15
Journal Gibbsite powders were subjected to a 24Pre-proof hours activation with four powder-to-milling ball ratio (BPR) and they exhibited broad gibbsite peaks, unlike the sharp peaks exhibited by the non-milled gibbsite. This finding indicates a significant alteration to both crystallite size and morphology, allied to a 99% AD of gibbsite due to mechanical activation. The X-ray amorphous component retained its original water content. Upon heating, this water was progressively removed over a wide temperature range (100ºC-400ºC), conversely to the well- defined thermal dehydroxylation of the non-milled gibbsite.
Grinding caused a significant proportion of the Al-OH bonds to rupture, but the which it was endothermically desorbed at 125ºC.
oo f
remaining molecular water was immediately adsorbed onto the activated surfaces, from
The decomposition temperature of the coarse gibbsite was above 250ºC. The drop in the
pr
decomposition temperature of the mechanically activated gibbsite powders reflected the
e-
amount of the mechano-chemically dehydrated H2O molecules, which were reabsorbed
Pr
during milling.
al
Acknowledgements:
First of all indicate that Prof. J.M. Rivas, our colleague, and co-author has recently
rn
passed away and has not been able to see this article published. We wish to thank all the good times spent together and his great contributions, both professional and personal.
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The financial support of the Spanish Ministry of Economy and Competitiveness (MINECO) Projects MAT2013-48426-C2-1-R; MAT2017-00000-C2-1-R and of the Brazilian research funding agency FAPEMA (Maranhão Foundation for Scientific Research and Development) through Project Apoio Cooperações Internacionais Edital n 44/2013 – APCInter is gratefully acknowledged.
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4 – E. Dorre and H. Huber, “Mechanical Properties”; ibid, pp. 74–173. 5 – G. S. Brady, H. R. Clauser, and J. A. Vaccari, “Materials, Their Properties and
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alumina powder using an aqueous sol-gel method,” Materials Letters,2008, 62: 3364–3366. 11 – F. Mirjalili, M. Hasmaliza, and L. C. Abdullah, “Size-controlled synthesis of nano α-alumina particles through the sol-gel method,” Ceramics International, 2010, 36: 1253–1257. 12 - Y. Xie, D. Kocaefe, Y. Kocaefe, J. Cheng, and W. Liu, “The effect of novel
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37 – P. Alphonse, M. Courty. Structure and thermal behavior of nanocrystalline boehmite. Thermochimica Acta, 2005, 425: 75–89. 38 – B. K. Gan, I.C. Madsenb and J. G. Hockridge. In situ X-ray diffraction of the transformation of gibbsite to -alumina through calcination: effect of particle size and heating rate. J. Appl. Cryst. 2009, 42: 697–705. 39 – V. Udhayabanu, B. S. Murty. Synthesis of Nanocrystalline -Al2O3 from Nanocrystalline Boehmite Derived from High Energy Ball Milling of Gibbiste. Trans
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Indian Inst. Met. 2011, 64: 535–540. 40 – P.T. Tanev and L.T. Vlaev. Effect of grain size on the synthesis of active alumina
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from gibbsite by flash calcination and rehydration. Catalysis Letters. 1993, 19: 351-360. 41 – B. Zhu, B. Fang, X. Li. Dehydration reactions and kinetic parameters of gibbsite.
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42 – X. Bokhimi, J.A. Toledo-Antonio, M.L. Guzmán-Castillo, F. Hernández-Beltrán. Relationship between Crystallite Size and Bond Lengths in Boehmite. J. of Solid State
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Chem. 2001, 159: 32–40.
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Highlights
- Grinding modify structure and thermal behavior of Gibbsite - Alteration in crystallite size and morphology with the amorphisation was observed - The amorphous component retained its original water content. - Grinding caused Al -OH bonds to rupture. Activated surfaces adsorbed resulting molecular water
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al
Pr
e-
pr
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- A drop in the decomposition temperature was observed.
21
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8