hydrargillite → boehmite transformation

ISSN 0020�1685, Inorganic Materials, 2010, Vol. 46, No. 7, pp. 747–753. © Pleiades Publishing, Ltd., 2010.Original Russian Text © G.P. Panasyuk, V.N. Belan, I.L. Voroshilov, I.V. Kozerozhets, 2010, published in Neorganicheskie Materialy, 2010, Vol. 46, No. 7, pp. 831–837.

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INTRODUCTION

The hydrargillite → boehmite transformation is atechnologically important process which allows thepreparation of partially dehydrated aluminum hydrox�ide (boehmite) from aluminum trihydroxide(hydrargillite), a widespread material that is com�monly produced from bauxite by the Bayer process orits modifications. Calcination of the resultanthydrargillite yields alumina, a raw material for alumi�num production. The yearly world production ofhydrargillite is millions of tons [1].

At the same time, hydrargillite calcination can beused to produce partially dehydrated aluminumhydroxide (boehmite) and various aluminas (γ�, δ�, α�and other forma of alumina). Each of these materialshas its own application area. In particular, boehmite iswidely used as a filler for plastics and fire retardant, inwater purification systems, and in other advancedtechnological applications. Boehmite is also an inter�mediate in the preparation of corundum by an auto�clave process. Autoclaving boehmite with controlledparticle size and shape, one can produce corundumwith tailored properties [2].

Boehmite formation has been the subject of exten�sive studies (see, e.g., Refs. [3, 4]), in particularbecause there is considerable interest in the prepara�tion of nanocrystalline boehmite and, accordingly,various nanoparticulate aluminas [5]. Alumina nanoc�rystals can be produced by a variety of processes,including hydrothermal treatment of hydrargillite.

Therefore, hydrothermal treatment is of interest inconnection with the possibility of using hydrargillite, aninexpensive, readily available raw material. It can be con�verted to boehmite with tailored properties and then, bysubsequent heat treatment, to various aluminas.

The mechanism of the hydrargillite → boehmitetransformation is still open to question. Data available

in the literature give no unique answer to this question.The mechanisms proposed to date include redissolu�tion [6] and a solid�state transformation [7].

This paper examines the mechanism of boehmiteformation during hydrothermal treatment ofhydrargillite with the aim of gaining further insightinto the nature of the hydrargillite → boehmite trans�formation.

EXPERIMENTAL

The raw material used was MDGA hydrargillite(fine�particle aluminum hydroxide), manufactured byAO Pikalevskii Glinozem. Figures 1a–1c show scan�ning electron microscope (SEM) images and the par�ticle size distribution of unprocessed MDGA.

Hydrothermal treatment was performed in 18�cm3

autoclaves at 200 and 250°С. The material was placedin a steel insert, which was then mounted in an auto�clave and filled with water with allowance for a con�stant meniscus level. In the case of treatment in watervapor, an appropriate amount of water was pouredbetween the autoclave wall and the insert.

After sealing, the autoclave was introduced into apreheated electric furnace and held at constant tem�perature. The process parameters are presented inTable 1. After cooling, the autoclave was opened, andthe samples were withdrawn from the inserts, washedwith distilled water, and dried at 100°С. The sampleswere characterized by a variety of techniques: SEM(Cam SCAN S2 instrument), transmission electronmicroscopy (TEM) (JEOL JEM�1001), thermo�gravimetry (TG) (SDT Q600 system), differentialthermal analysis (DTA), X�ray diffraction (XRD)(DRON�3 diffractometer, СuK

α radiation), and IR

spectroscopy (Nicolett Nexus Fourier�transform IRspectrometer, 400–4000 cm–1).

Hydrargillite → Boehmite TransformationG. P. Panasyuk, V. N. Belan, I. L. Voroshilov, and I. V. Kozerozhets

Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119991 Russia

e�mail: [email protected] October 21, 2009

Abstract—The hydrargillite → boehmite transformation has been studied at 200 and 250°C in water andwater vapor. Samples have been characterized by X�ray diffraction, differential thermal analysis, thermo�gravimetry, IR spectroscopy, and transmission and scanning electron microscopy before and after autoclavingfor various lengths of time. The hydrargillite → boehmite transformation is shown to be a solid�state process,and its steps are identified.

DOI: 10.1134/S0020168510070113

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INORGANIC MATERIALS Vol. 46 No. 7 2010

PANASYUK et al.

RESULTS AND DISCUSSION

According to XRD results, unprocessed hydrargil�lite contained no second�phase inclusions (Fig. 2,scan 1). The DTA heating curve of MDGA showedthree endothermic events: boehmite formation at234.29°С [8], with a weight loss Δm = 1.70%;hydrargillite decomposition and χ�alumina formationat 294.33°С [9], with Δm = 27.60%; and boehmitedecomposition at 503.99°С (weak peak), with Δm =5.11%.

The IR spectrum of MDGA (Fig. 3, spectrum 1)showed bands characteristic of hydrargillite and a band

at 1631 cm–1, arising from the bending mode ofmolecular water, δ(Н2О).

Treatment of MDGA in water at 200°C and a satu�rated water vapor pressure p = 1.6 MPa for 0.5 h hadlittle effect on the particle size and shape (Figs. 1d–1f). Insome particles, we observed stratification and the forma�tion of finer particles in the shape of parallelepipeds andrhombohedra within each layer. The XRD pattern of thatsample (Fig. 2, scan 2) showed only reflections fromhydrargillite, and its DTA and TG curves (Table 2)showed the same features as in the curves of unprocessedhydrargillite. The water loss corresponding to the event at235.65°C increased from 1.55 to 1.92%. The IR spec�

(c) 1

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3 μm 3 μm 300 nm

300 nm1 μm3 μm

3 μm 3 μm

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(g)

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(j) (k)

Fig. 1. (a, b) SEM images and (c) particle size distribution of unprocessed MDGA; (d–k) SEM images of MDGA autoclaved inwater at 200°C and p = 1.6 MPa for (d–f) 0.5, (g–i) 1, and (j, k) 1.5 h.


HYDRARGILLITE → BOEHMITE TRANSFORMATION 749

Table 1. MDGA autoclaving conditions

Sample τau, h Environment t, °C Fill factor, % p, MPa

Hydrargillite 0.5, 1, 1.5, 2, 2.75, 3.5, 4.5

Water 200 50 1.6

3, 4 Water 200 90 56.0

1, 2, 3 Water vapor 200 8 1.6

0.5, 1 Water 250 50 4.0

Hydrargillite heated in air at 600°C for 4 h

0.5, 1, 1.5, 2.5

Water 200 50 1.6

Table 2. DTA and TG results for MDGA before and after hydrothermal treatment in water at 200°C

Sample Δt, °C Δm, % tmax, °C Event

Unprocessed hydrargillite 50–240 1.57 234.29 Boehmite formation

240–350 27.60 294.33 Hydrargillite dehydration

350–700 5.11 503.99 Boehmite decomposition

Hydrargillite autoclaved in water at 200°C

p = 1.6 MPaτau = 0.5 h

50–250 1.92 235.65 Boehmite formation



p = 1.6 MPa τau = 1 h

50–240 1.72 237.07 Boehmite formation

240–360 27.44 280.00 286.84 294.36

Hydrargillite dehydration


p = 1.6 MPa τau = 1.5 h


300–400 1.48 371.18 Boehmite dehydration


p = 1.6 MPa τau =2 h



pwater = 56 MPa τau = 4 h



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PANASYUK et al.

trum of that sample was essentially identical to the spec�trum of unprocessed hydrargillite, with slightly higherintensities of the bands at 2003, 2093, and 2155 cm–1

(Fig. 3, spectrum 2).

Autoclaving in water at 200°C and p = 1.6 MPa for1 h had an insignificant effect on the MDGA particlesize and shape (Figs. 1f–1h). Some of the particles hadwell�seen regularly shaped structures on their surfaceand in their bulk. The XRD pattern of that sampleshowed only reflections from hydrargillite. Accordingto its DTA and TG curves (Table 2), the water loss cor�responding to the thermal event at 237.07°C wassmaller (1.72%). The broad peak in the temperaturerange of hydrargillite decomposition was split intothree components at 280.00, 286.84, and 294.36°С.The Δm corresponding to boehmite decomposition(502.81°С) was smaller in comparison with the previ�

ous sample (a decrease from 5.43 to 4.76%). In thestretching region of hydroxyl groups, ν(ОН), thebands at 3621 and 3526 cm–1 remained unchanged,whereas the 3455�cm–1 band transformed into ashoulder, the bands at 3389 and 3373 cm–1 becameweaker, and the bands at 2003, 2085, and 2155 cm–1

increased further. The δ(Н2О) band became stronger,its maximum shifted to 1690 cm–1, and it split into fivecomponents at 1715, 1700, 1690, 1680, and 1655 cm–1.

The MDGA sample autoclaved in water at 200°Cand p = 1.6 MPa for 1.5 h consisted largely of isometricparticles 1 to 3 µm in size (Figs. 1j, 1k). There werealso agglomerates of hydrargillite and boehmite parti�cles. The XRD pattern of that sample also indicatedthe presence of two phases (Fig. 2, scan 3): in additionto reflections from hydrargillite, there was the stron�gest diffraction peak of boehmite, with a d020 spacing of6.13 Å. The DTA and TG curves showed two promi�nent endothermic peaks at 272.05 (hydrargillitedecomposition) and 512.62°C (boehmite decomposi�tion) and a weak feature at 371.18°C with Δm = 1.48%(Table 2). There was also a small endotherm in therange 50–220°С. The IR spectrum of that sample(Fig. 3, spectrum 3) showed absorption bands ofhydrargillite at 3621, 3528, 1024, and 969 cm–1 and, inaddition, features characteristic of boehmite: 3270(shoulder), 3087, and 1077 cm–1. In the range 3000–3620 cm–1, a broad absorption band emerged. Like inthe previous sample, there was a strong absorption at2003 cm–1 with a shoulder at 2100 cm–1. The δ(Н2О)band was shifted to 1689 cm–1.

When the autoclaving time was increased to 2 h,the hydrargillite�to�boehmite transformation reachedcompletion (Fig. 2, scan 4). The particle size distribu�tion of the resultant boehmite is displayed in Fig. 4. ItsDTA and TG curves (Table 2) showed a small endot�herm at 368.07°C with Δm = 1.58% and a strong peakat 510.21°C, due to the complete decomposition of theboehmite. Its IR spectrum (Fig. 3, spectrum 4) wascharacteristic of boehmite. The δ(Н2О) band at1690 cm–1 was weak. After autoclaving at 200°С for2.75 h, it was missing. Autoclaving at this temperaturefor 4.5 h markedly reduced the intensity of the bandsat 2094 and 1974 cm–1 (Fig. 3, spectra 5, 6).

We also studied MDGA samples autoclaved inwater vapor at 200°C and p = 1.6 MPa, in water at250°С and p = 4.0 MPa (Fig. 2, scans 5–9), and inwater at 200°С and a water pressure as high as56.0 MPa. The conversion to boehmite reached com�pletion in 3 h in water vapor (Fig. 2, scan 7), in 4 h inwater at the high pressure, and in 1 h in water at 250°C(Fig. 2, scan 9).

These results demonstrate that, under hydrother�mal conditions, the hydrargillite → boehmite trans�formation reaches completion in 1–2 h and dependsvery little on the fill factor of the autoclave and,accordingly, on the pressure in the reaction medium.Raising the temperature from 200 to 250°C increasesthe reaction rate. The longer transformation time (4 h)

50403020100

Inte

nsi

ty9

8

7

6

5

4

3

2

1

2θ, deg

Fig. 2. XRD patterns of (1) unprocessed hydrargilliteand (2–9) samples produced by autoclaving in waterat 200°C and p = 1.6 MPa for (2) 0.5 (hydrargillite),(3) 1.5 (hydrargillite + boehmite), and (4) 2h (boehmite);in water vapor at 200°C and p = 1.6 MPa for(5) 1 (hydrargillite), (6) 2 (hydrargillite + boehmite), and(7) 3 h (boehmite); and in water at 250°C and p = 4.0 MPafor (8) 0.5 (hydrargillite) and (9) 1 h (boehmite).



at 200°C and 56.0 MPa is determined by the timeneeded to reach the high pressure. Note a slightincrease in transformation time (to 3 h) when the pro�cess is run in water vapor.

SEM examination of the autoclaved samplesshowed that, after hydrothermal treatment for 0.5 h,the particles were split into platelets 20–30 nm inthickness (Fig. 1f). After 1 h, we observed slightchanges in the shape of the particles and their order�ing. It seems likely that the cleavage was caused by thedisjoining pressure [10]. Since no boehmite was

detected in those samples by XRD, IR spectroscopy,DTA, or TG, the formation of such particles was con�cluded to be the first stage of the hydrargillite → boe�hmite transformation. When hydrargillite was auto�claved at 200°C in water, the transformation occurredmainly in 1.5 h and reached completion in 2 h. Similarresults were obtained when hydrargillite was auto�claved in water at 250°C or in water vapor at 200°C.According to the DTA and TG data, increasing theautoclaving time from 0.5 to 1.5 h increases theamount of water occluded in the sample and evapo�

1000200030004000 Wavenumber, cm–1

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5

4

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2094

1974 16

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9233

72

2007

1631

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968

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666

559 51

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Fig. 3. IR absorption spectra of hydrargillite (1) before and (2–6) after hydrothermal treatment in water at 200°C and p = 1.6 MPafor (2) 0.5, (3) 1.5, (4) 2, (5) 2.75, and (6) 4.5 h.

MDGA (unpr.)

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PANASYUK et al.

rated with a slight heat effect in the range 50–240°C(Δm = 1.56% in unprocessed MDGA, 1.92% afterautoclaving for 0.5 h, and 1.72% after 1 h). Autoclav�ing for 2 h eliminates this effect. At an autoclavingtime of 1 h, the splitting of the hydrargillite decompo�sition peak in the DTA curve into three components(Table 2) was due to the presence of water in differentforms in the hydrargillite structure. It is reasonable toassume that the small endotherm in the range 368–371°C with Δm ~ 1.5% in the DTA and TG curves ofthe samples autoclaved for 1.5 and 2 h was due to the

dehydration of the boehmite formed in the bulk ofsome particles.

The IR spectroscopy results demonstrate thatincreasing the autoclaving time from 0.5 to 1.5 hcauses the bands of hydrargillite at 3455, 3392, and3372 cm–1 to gradually disappear; leads to the forma�tion of a broad band in the range 3000–3620 cm–1;gives rise to the bands of boehmite at 3270 and3870 cm–1, on top of the broad band; and increasesand shifts the δ(Н2О) band, splitting it into five com�ponents, with the strongest one at 1689 cm–1.

The present results lead us to the following conclu�sions: The hydrargillite→ boehmite transformation isa solid�state process and is not determined byhydrargillite dissolution or boehmite crystallization.The key processes are water diffusion into the struc�ture of the hydrargillite particles; particle decomposi�tion along the cleavage plane; interaction of the waterwith hydrargillite microcrystals in the structure of theprimary particles; dehydration of the microcrystals,resulting in a rapid hydrargillite → boehmite transfor�mation; and the formation of boehmite particles fromthe microcrystals.

To validate this mechanism, it is necessary to ascer�tain whether water molecules can penetrate into thestructure of hydrargillite particles. It is reasonable toassume that a hydrargillite particle is an agglomerate ofaligned fine particles and that the forming boehmitehas a similar microstructure.

To verify this assumption, particles of unprocessedhydrargillite and those of the boehmite formed underhydrothermal conditions were examined by TEM.Prior to examination, the samples were treated in an

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20

15

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5

00.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Particle size, μm

Fig. 4. Particle size distribution of the boehmite preparedby autoclaving MDGA in water at 200°C and p = 1.6 MPafor 2 h.

(a) 100 nm 50 nm

50 nm100 nm

(b)

(d)(c)

Fig. 5. TEM images of (a, b) unprocessed MDGA and (c, d) boehmite particles heated at 850°C for 1 h.



aqueous solution of a surfactant, the mixture wasground in an agate mortar, and the resultant suspen�sion was dispersed by sonication.

The hydrargillite particles were found to be faceted(Figs. 5a, 5b) and to range in size from 100 to 200 nm.They were composed of finer, aligned crystals, 1 to5 nm in size. Similar TEM results were obtained forboehmite particles (200°C, 1.6 MPa, 4.5 h) after heat�ing in air at 850°C for 1 h (Figs. 1c, 1d).

REFERENCES

1. Chemistry of Aluminium, Gallium, Indium and Thallium,Downs, A.J., Ed., Glasgow: Blackie Academic & Pro�fessional, 1993, p. 506.

2. Panasyuk, G.P., Danchevskaya, M.N., Belan, V.N.,et al., The Phenomenology of Corundum Crystals For�mation in Supercritical Water Fluid, J. Phys.: Condens.Matter, 2004, vol. 16, no. 14, pp. S1215–S1221.

3. Panda, P.K., Jaleel, V.A., and Usha Devi, S., Hydro�thermal Synthesis of Boehmite and α�Alumina fromBayer’s Alumina Trihydrate, J. Mater. Sci., 2006,vol. 42, no. 24, pp. 8386–8389.

4. Mishra, D., Anand, S., Panda, R.K., and Das, R.S.,Hydrothermal Preparation and Characterization ofBoehmite, Mater. Lett., 2000, vol. 42, no. 1, pp. 38–45.

5. Al’myasheva, O.V., Korytkova, E.N., Maslov, A.V., andGusarov, V.V., Preparation of Nanocrystalline Aluminaunder Hydrothermal Conditions, Neorg. Mater., 2005,vol. 41, no. 5, pp. 540–547 [Inorg. Mater. (Engl.Transl.), vol. 41, no. 5, pp. 460–467].

6. Tsuchida, T., Hydrothermal Synthesis of Submicrome�ter Crystals of Boehmite, J. Eur. Ceram. Soc., 2000,vol. 20, no. 11, pp. 1759–1764.

7. Ne, J. and Ponton, S.V., Hydrothermal Synthesis andMorphology Control of Boehmite, High Pressure Res.,2001, vol. 20, nos. 1–6, pp. 241–254.

8. Danchevskaya, M.N., Ivakin, Yu.D., Martynova, L.F.,et al., Investigation of Thermal Transformations in Alu�minium Hydroxides Subjected to Mechanical Treat�ment, J. Therm. Anal., 1996, vol. 46, no. 5, pp. 1215–1222.

9. Gan, B.K., Madsen, I.C., and Hockridge, J.G., In situX�Ray Diffraction of the Transformation of Gibbsite toα�Alumina through Calcinations: Effect of ParticleSize and Heating Rate, J. Appl. Crystallogr., 2009,vol. 42, no. 4, pp. 697–705.

10. Deryagin, B.V., Concerning the Concept of DisjoiningPressure: Definition and Role in the Statics and Kinet�ics of Thin Fluid Layers, Kolloidn. Zh., 1955, vol. 17,no. 3, pp. 207–214.

hydrargillite → boehmite transformation

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