Thermally Activated Al(OH)3 Part II—Effect of Different Thermal Treatments

Abstract

In this paper, the thermal decomposition of crystalline Al(OH)₃ was studied over the temperature range of 260–400 °C for particles with a size between 10 and 150 µm. The weight losses and thermal effects occurring in each of the dehydration process were assessed using thermogravimetry (TG) and differential scanning calorimetry (DSC) thermal analysis. X-ray diffraction (XRD) patterns, refined by the Rietveld method, were used for mineral phase identification, phase composition analysis, and crystallinity degree determination. Moreover, the particle size distributions and their corresponding D10, D50, and D90 numeric values were determined with a laser analyzer. We observed a strong relationship between the calcination temperature, the initial gibbsite grade particle size, and the crystallinity of the resulting powders. Hence, for all endothermic effects identified by DSC, the associated temperature values significantly decreased insofar as the particle dimensions decreased. When the gibbsite was calcined at a low temperature, we identified small amounts of boehmite phase along with amorphous new phases and unconverted gibbsite, while the powders calcined at 400 °C gradually yielded a mixture of boehmite and crystalized γ-Al₂O₃. The crystallinity % of all phase transition products declined with the increase in particle size or temperature for all the samples.

1. Introduction

Alumina (Al₂O₃) and its partial hydrates prove to be extremely valuable materials in many practical applications, usually as desiccants [1,2,3], fillers [4,5,6], adsorbents [7,8,9,10] for the removal of oil and grease from industrial waters [11], organic powder coatings [12,13,14], porous ceramics [15,16,17], high-temperature ceramic oxides [18,19,20], ceramic filtration membranes [21,22,23,24], and catalysts [25,26,27,28,29], only if the precursors are properly prepared and thermally treated [30,31,32]. Among the desired properties for these applications are a specific pore size and pore size distribution [33,34], a high specific surface area [35,36,37], and the degree of crystallinity, which also plays an essential role [38,39,40,41]. When discussing the synthesis routes for α-Al₂O₃, the thermal transformation of aluminum oxy-hydroxides remains the most studied [42].

Gibbsite (Al(OH)₃), a natural aluminum trihydrate, is a component of bauxites, beside boehmite (AlOOH), and diaspore (AlO₂H), and without any doubt, as an industrial product, is the only one manufactured at an industrial scale among all the hydrated aluminas. Diaspore is a remarkable product because it is the only aluminum oxy-hydroxide that can be easily and directly dehydrated to α-alumina due to its structural resemblances to α-Al₂O₃ (a hexagonal close-packed structure). The transformation of boehmite to diaspore is possible only if very high pressures are applied [43]. Boehmite is an aluminum oxy-hydroxide with important applications in its natural state or as a precursor for different materials used in catalysis and adsorption processes. The most known research target in the low-temperature alumina domain is growth promotion of the γ-Al₂O₃ phase [44,45]. Gibbsite Al(OH)₃ is industrially produced by Bayer’s process and its final particle size and morphology are strongly influenced by the process parameters [46,47]. Normally, gibbsite is fully transformed into α-alumina at 1100–1300 °C. This high calcination temperature leads to large crystallites with peculiar grain growth and to a decrease in density associated with poor mechanical properties for the sintering of ceramic bodies [48]. It is therefore necessary to reduce the calcination temperature and several studies suggest doing this by reducing the initial gibbsite particles’ size [49,50]. Jang et al. [50] concluded that milling the gibbsite for 8 h leads to a transformation temperature of 1000 °C, while Tsuchida and Ichikawa [51] achieved this transformation at a temperature of 1100 °C after 30 min of mechano-chemical activation. For instance, Kano et al. [52] reported a temperature reduction from 1350 to 1020 °C by grinding the gibbsite powder for 60 min, with no additional improvements if the grinding time is extended. This effect is expected to happen also when the gibbsite is calcined at a lower temperature to obtain the transition aluminas as precursors. The successive decomposition of gibbsite, after the thermal treatment, did yield combinations of different compounds, the most common being boehmite and/or transition alumina phases (including χ, κ, γ, δ, θ, and η), that were more or less amorphous [38,41]. From a structural point of view, these intermediary products have the aluminum atoms surrounded by oxygen atoms in a tetrahedral and octahedral structure. The ratio between the AlO₆ units (characteristic of octahedral coordinated α-Al₂O₃) and the AlO₄ units (characteristic of tetrahedral coordinated transitional alumina) changes for different polymorph phases, which ultimately modifies their properties, including the crystal packing, electronic structure, and dielectric constant [32,35]. Important aspects of the gibbsite dehydration process and the corresponding transition phases have been studied for years at various heating rates. After extensive research, the literature reports unanimously indicate the order in which the compounds are formed during the thermal transformation, but not the kinetics of the dehydration processes or their associated mechanisms [53].

When a higher heating rate is applied, the fine gibbsite particles are partially transformed into an amorphous product (ρ-alumina) with increased reactivity and thermal stability until 800 °C [32]. In particular, when the heating rate is in accordance with the equilibrium conditions, the dehydration and the crystal lattice transformation happen to take place at the same time, resulting in crystalline oxidic compounds (boehmite at temperatures higher than T > 180 °C and χ-Al₂O₃ at T ≥ 250 °C) [37].

The purpose of this paper is to investigate the physical, chemical, and mechanical properties of a specific aluminum hydroxide material produced by a specific technology (a Bayer-modified technology), finished off through drying, milling, and classification, and ultimately calcined at low temperatures in order to produce low-temperature aluminas. The selection of the properties to be evaluated was made because of the disputable quality of the raw material in terms of granulometry and purity. Thus, the thermally treated products from this raw material might find applications in domains such as fillers, adsorbents in heavily polluted industrial water, oil and grease removal from industrial waters, organic powder coatings, extenders in other types of coating materials, and fire retardants. Additionally, to improve the quality of the thermally treated materials, the temperature and time of treatment were selected in such a way to promote the emergence of amorphous phases.

According to the literature, each industrial aluminum hydroxide grade has similar properties to the other grades, but also certain specific properties. Thus, during grinding, heating, and other thermal or non-thermal treatments, the final products differ mainly in terms of phase composition and properties. Hence, the thermal decomposition of crystalline Al(OH)₃ was studied for particle dimension classes between 10 and 150 µm at the following temperature values: 260 °C, 300 °C, and 400 °C. Actually, these temperature values were placed in the phase transition interval as follows: 260 °C at the beginning of the transition gibbsite–boehmite; 300 °C close to the end of the transition gibbsite–boehmite; and 400 °C at the beginning of the transition boehmite–γ alumina.

When the principal properties of these low-temperature aluminas are well known, the accumulated data will help us to determine the destinations of each product: a commercial product, a precursor, or a raw material for another product with a particular use.

2. Materials and Methods

2.1. Sample Preparation

Various samples of aluminum hydroxide powders were received from SC ALUM SA. These samples were produced by Bayer’s process and further collected from the newly built line for the production of special grades of dried, milled, and classified aluminum hydroxide that was recently presented by Dobra et al. [47]. In order to study the thermal transformations, the received powders were firstly dried at 60 °C for 24 h, and then heated in an electric furnace (in an air atmosphere) at 260 °C, 300 °C, and 400 °C for 2 h with a heating rate of 5 °C/min. The samples were afterwards slowly cooled in the oven until they reached room temperature. The choice of heating temperature in the thermal treatment was based on literature data concerning the aluminum hydroxide’s activation by advanced grinding, and eventually on expected changes in the phase transition mechanism and amorphization rate. More information regarding their classification and calcination can be found in Table 1.

2.2. Characterization Methods

The kinetics parameters of the thermal transformations studied in this paper were measured by recording the weight losses and thermal effects occurring during the various dehydration processes on a STA 449 F3 Jupiter device (NETZSCH-Gerätebau GmbH, Selb, Germany). The samples were heated at a constant rate of 10 °C/min from room temperature to 1000 °C in a dynamic atmosphere of air with a flow rate of 50 mL/min.

The phase analysis studies by X-ray diffraction (XRD) were carried out using a PANalytical Empyrean diffractometer (Almelo, Netherlands) at room temperature with a characteristic Cu X-ray tube (λ Cu K_α1 = 1.541874 Ǻ), in-line focusing, a programmable divergent slit on the incident side, and a programmable anti-scatter slit mounted on the PIXcel3D detector on the diffracted side. The samples were scanned in a Bragg–Brentano geometry with a scan step increment of 0.02° and a counting time of 255 s/step. The XRD patterns were recorded in the 2θ angle range of 5–90°. The crystallinity and phase composition were refined by the Rietveld method using the HighScore Plus 3.0e software.

Particle size distributions in the analyzed samples were determined using a Malvern Panalytical Mastersizer 2000 diffraction analyzer (Almelo, The Netherlands) in the dynamic range of 0.1 to 3000 μm. Samples were dispersed in water using ultrasound and mechanical stirring. The Mie scattering theory specific to the instrument’s software was used for the particle size distribution assessment.

3. Results and Discussion

Differential scanning calorimetry (DSC), thermogravimetry (TG), and X-ray diffraction (XRD) are essential tools in determining the kinetics parameters for all phase transitions of gibbsite. Figure 1 shows the TG and DSC curves of aluminum hydroxide powders heated up to 1000 °C at a heating rate of 10 °C/min. The first decomposition/transition step, between 215 and 260 °C, is best observed for the GDAH-05 sample (minimum at 231 °C) and can be attributed to the partial dehydroxylation of Al(OH)₃ accompanied by a mass loss of up to ~5.63% (which is about one molecule of water for four units of Al(OH)₃). This reaction was hardly observed in the DSC patterns of the fine-grained gibbsite samples. The corresponding endothermic effects are visible at 231 °C (for GDAH-03 and GDAH-04), 231.4 °C (for GDAH-05), and 234.7 °C (for GDAH-02) but they take place with an insignificant loss of water. For sample GDAH-05, this first transition process was quickly followed by the second decomposition/transition step, between 260 and 320 °C, with a 20.51% mass loss at 306.2 °C. For the samples containing fine particles (after milling), this second endothermic effect has a much higher intensity, covers the first one almost entirely, and is associated with a mass loss of 26–27%. This step has a minimum point between 292.9 °C and 306.2 °C in all samples and again can be attributed to gibbsite dehydroxylation, with AlOOH most probably the final product. The results are in good agreement with literature data, which indicate two concurrently occurring processes at this stage, namely the formation of boehmite and the transformation of gibbsite in some amorphous or χ-Al₂O₃ phase [41,42]. The boehmite will undergo transformation to γ-Al₂O₃ above 320 °C. The main endothermic effect for this process was observed at around 500 °C. The final product represents ~64% of the initial mass for all the samples.

Each chemical and phase transformation that generates the overlapping endothermic effects is firmly influenced by the experimental parameters, such as heating rate, water vapor pressure, and particle dimension. It is easy to observe a connection between the temperature at which the decomposition steps occur and the initial gibbsite particle size. Hence, for all identified endothermic effects, the associated phase transition temperatures considerably decreased when the particle sizes were significantly diminished. For instance, the GDAH-04 product, 76.28% of whose particles are <10 µm in size, decomposed to form transition aluminas at 484.4 °C. In comparison, the GDAH-05 product, whose particles are mostly between 45 and 100 µm in size, displayed the same effect at 519.1 °C.

Similar results were first reported by Brown et al. [54], who identified a combination of boehmite and transition aluminas as decomposition/transition products of large-grained Al(OH)₃ particles by observing distinct peaks for each transformation in different thermal analyses (DTA and DSC). These results were further acknowledged by Yamaguchi and Sakamoto [55], who demonstrated that the resulting boehmite was the product of an in situ dissolution process of Al(OH)₃ followed by the recrystallization of AlOOH, a theory later confirmed by several studies [56,57]. The rate of this internal dissolution process proved to be highly dependent on the Al(OH)₃ particle size, since larger particles have an enhanced ability to retain a higher volume of water and thus facilitate boehmite formation [58].

All these transformations of gibbsite and the chemical composition of the calcined samples were confirmed by the XRD analysis as shown in Figure 2 and Table 2. For the dried sample without the thermal treatment, or samples calcined at a low temperature, gibbsite was the main mineralogical phase identified (ICDD PDF4+ [01-080-6432]), while for the samples calcined at 400 °C a mixture of boehmite (ICDD PDF4+ [04-010-5683]) and crystalized γ-Al₂O₃ (ICDD PDF4+ [04-005-4662]) was also formed. There were no X-ray peaks associated with the other polymorphic phases of alumina identified at this stage. However, some amorphous phases with variable concentrations in the phase mixtures were found in all samples. The results are in accordance with literature studies [41,59,60].

In order to improve the extraction efficiency of alumina and to prevent boehmitic reversion, Alum SA Tulcea applies a modified technology for the processing of alumino-gibbsitic/goethitic bauxites. The major differences between this method and the standard procedure are: (1) the digestion temperature is raised from 100–110 °C to 140–145 °C in order to increase the reaction rate; and (2) the alkalinity of the Bayer caustic lye is increased by changing the factor a_k = C(Na₂O)/C(Al₂O₃) from 1.35–1.45 to 1.75–1.85 in order to prevent boehmitic reversion [61,62]. The newly created conditions of a higher temperature and alkalinity make possible the homogeneous nucleation and growth of bayerite particles, which will later serve as germs for the precipitation and agglomeration of the gibbsite particles identified by the XRD analysis.

In agreement with the thermal analysis data, the Rietveld refinement results from Table 2 show an increase in boehmite content (%) when the temperature was raised simultaneously with a decrease in the sample’s gibbsite content (%). The thermal treatment at 400 °C proved to be insufficient for the total transformation of the gibbsite to boehmite or any other crystalline phase, mainly in the case of small particle size samples. Furthermore, the highest amount of boehmite was observed in the case of the GDAH-05 sample at all calcination temperatures. This can be attributed to the same increased ability to retain water on the larger particles’ surface, enabling boehmite formation, since GDAH-05 contains particles that are mostly between 45 and 100 µm in size [63]. Additionally, we observed that the crystallinity degree decreased as the thermal transformation temperature increased when the initial gibbsite phase was heated. Since a greater number of defects in the less-crystallized materials occurs very often in similar well-known experiments, a possible explanation might be the production of defects in the crystal at a high temperature, associated with a high degree of mobility at the atomic level when the gibbsite was calcined [38]. Literature studies consider the loss in crystallinity to be a result of the porous surface layer that forms when gibbsite is calcined in air. In these cases, the pore diameters shift with the temperature from 5 (170 °C) to 20 (500 °C) nm [64]. Moreover, from Table 2 it can be seen that up to 400 °C, neither of the two phase transitions mentioned above are fully finished, in accordance with the purpose of the experiment—to promote amorphous phase growth.

The particle size distribution and characteristic diameters (D10, D50, and D10) of the milled samples thermally treated at 260 °C, 300 °C, and 400 °C for 2 h are given in Figure 3. The variation in characteristic diameters with temperature indicates that the fine-grained particles have undergone aggregation and agglomeration processes during the thermal treatment, which leads to an increase in particle size [65].

In contrast, the GDAH-05 sample had a narrow particle size distribution when compared with the other products, with the median diameter D50 sharply reduced from around 70 μm before calcination to around 55 μm after calcination at 400 °C. The same phenomenon was identified also for the other characteristic diameters D10 and D90, in accordance with similar studies [64]. The thermal treatment triggered therefore a decrease in both the coarse and fine fractions of samples containing mostly large particles. The results can be correlated with previous studies on mechanically activated boehmite from the thermal decomposition of gibbsite [63,65].

All our results are in good agreement with best approaches to the subject found in the literature [66,67,68,69,70,71,72].

4. Conclusions

Aluminum hydroxide (gibbsite) samples with particle sizes between 10 and 150 µm were subjected to a thermal treatment in an electric furnace (in an air atmosphere) at 260 °C, 300 °C, and 400 °C for 2 h. At a temperature between ∼200 and 350 °C, the dehydroxylation of the gibbsite crystal lattice into the less-crystalline boehmite took place in two steps, with a considerable decrease in the phase transformation/transition temperature, when the particles sizes were significantly diminished. Additionally, the higher reactivity of the samples containing fine particles was displayed by a decrease in the AlOOH–γ-Al₂O₃ transformation/transition temperature. Calcination of the aluminum hydroxide samples produces, at a low temperature, a highly crystalline gibbsite phase, while at 400 °C a mixture of boehmite and γ-Al₂O₃ with a lower crystallinity degree was formed. Taken together, the experimental data suggest a strong correlation between the initial gibbsite particle size, the calcination temperature, the crystallinity degree, and the particle size distribution in the resulting aluminas calcined at low temperatures.

Author Contributions

Conceptualization, B.S.V. and L.F.; methodology, G.D., S.I. and L.C.; validation, A.B.; formal analysis, A.I.N.; investigation, V.A.S.; writing—original draft preparation, I.A.N.; writing—review and editing, I.A.N., B.S.V. and L.F.; visualization, A.I.N.; supervision, L.F.; project administration, B.S.V.; funding acquisition, B.S.V. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Acknowledgments

This study was made possible by the implementation of the “Endow the Research and Development Department of SC ALUM SA Tulcea with independent and efficient research facilities to support the economic competitiveness and business development” project, co-funded by the European Regional Development Fund through the Competitiveness Operational Programme 2014–2020. Under this project were purchased and commissioned: “Independent equipment/installation for research and development of the technology of wet aluminum hydroxide classification”, “Independent equipment/installation for research and development of technology to obtain the dried aluminum hydroxide”, and “Independent equipment/installation for research and development of the technology of grinding and screening the dried aluminum hydroxide”.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Tables

Figure 1. TG and DSC curves of aluminum hydroxide powders heated at 10 °C/min.

Figure 1. TG and DSC curves of aluminum hydroxide powders heated at 10 °C/min.

Figure 2. X-ray diffraction patterns for all samples.

Figure 3. Particle size distributions for each powder, overlapped before and after calcination. The inserts show the D10, D50, and D90 numeric values for each sample before and after thermal treatment at 260 °C, 300 °C, and 400 °C.

Figure 3. Particle size distributions for each powder, overlapped before and after calcination. The inserts show the D10, D50, and D90 numeric values for each sample before and after thermal treatment at 260 °C, 300 °C, and 400 °C.

Table 1

Samples of dried, milled, and thermally treated aluminum hydroxide classified by particle size dimension.

Sample Code	Dimensions after Milling	Calcination Temperature (°C)
GDAH-02	<45 µm = 98.29%	-
GDAH-02-260	<45 µm = 98.29%	260
GDAH-02-300	<45 µm = 98.29%	300
GDAH-02-400	<45 µm = 98.29%	400
GDAH-03	<20 µm = 92.13%	-
GDAH-03-260	<20 µm = 92.13%	260
GDAH-03-300	<20 µm = 92.13%	300
GDAH-03-400	<20 µm = 92.13%	400
GDAH-04	<10 µm = 76.28%	-
GDAH-04-260	<10 µm = 76.28%	260
GDAH-04-300	<10 µm = 76.28%	300
GDAH-04-400	<10 µm = 76.28%	400
GDAH-05	<45 µm = 0.001%; >150 µm = 6.54%	-
GDAH-05-260	<45 µm = 0.001%; >150 µm = 6.54%	260
GDAH-05-300	<45 µm = 0.001%; >150 µm = 6.54%	300
GDAH-05-400	<45 µm = 0.001%; >150 µm = 6.54%	400

Table 2

Effect of the calcination temperature on the phase composition and amorphous materials.

Sample	Mineral Name			AmorphousPhases (%)
	Gibbsite (%)	Boehmite (%)	γ-Al2O3 (%)
GDAH-02	62.54	0.00	0.00	37.46
GDAH-02-260	46.42	4.15	0.00	49.43
GDAH-02-300	34.60	5.82	0.57	59.00
GDAH-02-400	3.32	7.45	10.61	78.60
GDAH-03	61.67	0.00	0.00	38.33
GDAH-03-260	45.41	2.80	0.00	51.79
GDAH-03-300	38.33	4.99	0.44	56.25
GDAH-03-400	0.05	5.44	12.81	81.70
GDAH-04	62.59	0.00	0.00	37.41
GDAH-04-260	53.23	2.62	0.00	44.15
GDAH-04-300	43.26	5.48	0.64	50.62
GDAH-04-400	2.28	6.86	12.98	77.88
GDAH	55.56	0.00	0.00	44.44
GDAH-05-260	43.48	8.28	0.00	48.24
GDAH-05-300	33.81	11.94	0.00	54.25
GDAH-05-400	0.05	12.86	13.79	73.32

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