Production of micro-crystalline boehmite from hydrothermal processing of Bayer plant alumina tri-hydrate

Powder Technology 235 (2013) 556–562

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Production of micro-crystalline boehmite from hydrothermal processing of Bayer plant alumina tri-hydrate Chi Jung Oh a, Youn Kyu Yi b, Seong Jun Kim c, Tam Tran b, Myong Jun Kim b,⁎ a b c

Korea Alumina Ltd., Mokpo, Republic of Korea Department of Energy & Resources Engineering, Chonnam National University, Kwangju, Republic of Korea Department of Environmental Engineering, Chonnam National University, Kwangju, Republic of Korea

a r t i c l e

i n f o

Article history: Received 20 February 2012 Received in revised form 26 September 2012 Accepted 19 October 2012 Available online 25 October 2012 Keywords: Alumina trihydrate Hydrothermal leaching Microcrystalline Boehmite

a b s t r a c t Plant alumina trihydrate (ATH, Al(OH)3) from Korea Alumina (KC) Ltd's Bayer alumina plant was used to synthesize boehmite, a new product used as fire retarding fillers or laminating materials. The hydrothermal treatment conditions were optimized from tests conducted with ATH-water slurries (30, 40 and 50 w/v%) at different temperatures (160–220 °C), reaction time (10–80 min) and mean feed sizes D50 (1.5–50 μm). Well crystalline boehmite was formed at a temperature > 200 °C after 60-minute treatment. The reaction pressure (1.0–2.0 MPa) has no effect on the process. The hexagonal crystal of the starting gibbsite changes to plate-like shape as boehmite is formed. After the treatment, the boehmite mean particle size (4–25 μm) is generally smaller than the original gibbsite feed (8–50 μm). For finer ATH feed (1.5–2.5 μm) there is no change in particle size after the hydrothermal treatment. The highly crystalline boehmite product containing b 0.05% Na2O and b 0.01% Fe2O3 has higher purity and whiteness than the ATH feed. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Several studies have been conducted to identify conditions to produce boehmite, (γ-AlOOH) as precursors for making alumina (Al2O3) which possesses certain specific surface areas, shapes, crystallite, pore and particle sizes for different applications. Raw materials used for processing boehmite are mainly freshly precipitated aluminum hydroxide (alumina trihydrate, ATH). Sol gels containing micro-crystalline and nano-sized boehmite of different crystal shapes (plate-like, ellipsoid, rhombic) are used to produce γ-, δ- or θ- alumina substrates for catalysts, silicon chips and as additives in glass and high temperature ceramic applications [1–3]. Boehmite is also used for the lamination of printed circuit boards (PCBs). Although having inferior fire retarding properties with 1400 J/g boehmite energy adsorbed when temperature rises, compared to 2200 J/g for ATH, boehmite is more stable at higher temperatures. Hollingbery and his co-workers stated that boehmite decomposes in air to liberate water at 340–350 °C compared to 180– 200 °C for ATH [4,5]. It is therefore more suitable for use as laminating materials for the fabrication of halogen-free, lead-solder free and thermally stable PCBs. Reviews of the properties and specific requirements for boehmite and other oxides as additives in these applications have been presented in the literature [4–6]. The most common techniques to produce boehmite are via hydrothermal digestion of different types of ATH or seeded precipitation

⁎ Corresponding author. E-mail address: [email protected] (M.J. Kim). 0032-5910/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.powtec.2012.10.041

from sodium aluminate (Bayer process) liquors. Mehta and Kalsotra [7] found that the hydrothermal transformation of gibbsite to boehmite starts at 190 °C with slow kinetics and is completed at 250 °C. According to Al'myasheva et al. [8] several polymorphs of alumina and aluminum oxyhydroxide (AlOOH) can be formed under hydrothermal conditions (water pressure of 1.25–10 MPa and 120–380 °C). Metastable and sub-micron boehmite is formed from amorphous ATH when treated at higher temperatures (350–450 °C) and pressures (2–70 MPa). When a crystalline ATH precursor was used, α-alumina (corundum) was also produced at similar conditions. Boehmite seems to be more stable at temperatures b300 °C and pressure b2 MPa [9]. Nano-boehmite is produced first under hydrothermal conditions before their sols are subjected to calcination, yielding high surface area and superfine γ-Al2O3. As an example, nano-boehmite was produced by treating AlCl3–water–dimethyl sulfoxide mixtures at 180 °C for 3 h [10]. Other studies also use hydrothermal conditions to synthesize boehmite as nano-sheets, rods, wires of one or multi-dimension as precursors for different nano aluminas [3,11,12]. Aging temperature and time were found to determine the specific area and crystallinity of superfine boehmite [13]. De Souza Santos et al. [14] used hydrothermal digestion (200 °C, 72 h) to treat different ATHs with additives (acetic acid, NaOH, etc.) and produce well-crystalline boehmite of different shapes at sub micron sizes, as precursors for α-alumina making. Liu et al. [2] treated a commercial boehmite in nitric acid at 80 °C for 6 h to produce a sol before calcination at 1200 °C to produce mesoporous γ-Al2O3 which has good thermal stability, large surface area and porosity required for making catalysts.

C.J. Oh et al. / Powder Technology 235 (2013) 556–562

557

AlOOH Al(OH)3

Weight Loss (%)

0

10

20

30

A (50

)

220

B (25

)

C (17

)

D (8.0

)

180

E (2.5

)

160

F (1.5

)

200

Al(OH)3

10

100

200

300

400

500

20

30

40

600

50

60

70

80

2 Theta

Temperature (oC)

A large number of patents were also granted for different techniques used to synthesize well-crystalline or semi-amorphous boehmite having surface areas 10–200 m 2/g and particles in the nano to micron size range which are used as sols in the making of alumina-based catalysts or adsorbents [15,16]. Kido et al. [17] mixed an alkaline earth metal (calcium, barium or strontium) oxide with ATH and hydrothermally treated the mixture at 150–300 °C to produce plate-shaped boehmite. By roasting at different temperatures, γ-Al2O3 was formed (at 600–1000 °C) which shifted to different types of alumina (δ- or θ-Al2O3) having plate-shaped crystals and BET areas of 10–30 m 2/g, suitable for high temperature catalyst making. A similar approach was patented by Bauer et al. [18], in which ATH and boehmite seeds were hydrothermally treated at 160–180 °C and 0.7–1.2 MPa with KOH, hydrochloric or formic acid to produce boehmite having BET surface areas of 100–200 m 2/g. The use of different additives added would result in boehmite formation of different shapes, varying from plate to needle-like, ellipsoidal or near spherical. Boehmite used as fire retardants or laminating materials during the production of printed circuit boards (PCBs) however needs to be of micron sizes, dense, well crystalline (for easy dispersion) and should have low BET surface areas. Fukuda [19] developed a patented process which subjected ATH (1–25 μm) to hydrothermal conditions (150–350 °C and 1–10 MPa) to form plate-like boehmite. Several other patents claimed different procedures for making “nano-dispersible” pseudo-boehmite or microcrystalline boehmite having the particle size range of 0.02–10 μm and BET surface area of 20–200 m 2/g [20–22]. According to Droval et al. [23] who tested ATH and boehmite as fillers for different fire retarding thermoplastics, the crystal plate shape, micro-crystallinity and small particle sizes (1–3 μm) determine the quality of boehmite with respect to dispersibility and coating. As

Table 1 Characteristics of different ATH's feeds (A–F) used in the hydrothermal processing of boehmite. Typical analysis

A

B

C

D

E

F

Moisture (%) Loss on ignition (%) Chemical composition (%)

0.12 34.5 0.25 0.01 0.01 50 85 b1 1.1 2.4

0.23 34.5 0.25 0.01 0.01 25 88 b1 1.0 2.4

b0.2 34.5 0.25 0.01 0.01 17 90 1.1 0.9 2.4

b0.2 34.5 0.25 0.01 0.01 8 92 1.4 0.65 2.4

b0.3 34.5 0.35 0.01 0.01 2.5 95 2.42 0.25 2.4

b0.3 34.5 0.38 0.01 0.01 1.5 95 3.5 0.25 2.4

D50 (μm) Whiteness Specific surface area (m2/g) Bulk density (g/cm3) Density (g/cm3)

Na2O Fe2O3 SiO2

Fig. 2. XRD patterns of hydrothermal products after digestion of 40 wt.% ATH for 60 min at different temperatures (160–220 °C).

an example, commercial boehmite from Nabaltec has platelet-shape, mean particle diameter (D50) varying between 0.9 and 7 μm, BET area of 2–6 m2/g, whiteness 90–98% and density of 3 g/cm3 [24]. Previous research has highlighted the fact that the conditions for hydrothermal processing boehmite from ATH are very much dependent on the feed properties. Nano-sized ATH having larger BET areas (>50 m 2/g) required much longer time (> 72 h) or use of additives to be converted to nano or micro-sized boehmite having a high BET area needed as precursors for Al2O3 catalyst making. However, boehmite particles used as fire-retarding plastic fillers are of micron in sizes (1–10 μm) and have a low BET area (b10 m 2/g) and a high density. Therefore, the conditions for synthesizing the two types of material are different. The effects of leaching ATH at different hydrothermal conditions on the properties of the final boehmite product have not yet been determined. This investigation focuses on the hydrothermal processing of boehmite using plant ATHs produced by the Bayer process at Korea Alumina (KC) Corp. Low soda and fine ATH products from the plant having mean particle sizes (D50) of 1–50 μm have been produced from a process developed at KC Corp and reported earlier [25]. The current study is to determine the optimum conditions for ATH hydrothermal treatment to produce boehmite, including digestion temperature, pressure, time and ATH/water weight/volume ratio. The objective of the study was to identify the parameters which determine the low porosity, BET specific area, particle size and high packing density of the boehmite product.

0

Weight Loss (%)

Fig. 1. Weight losses of ATHs produced from KC Corp, showing total dehydration (of 32–34% weight losses) at 550–600 °C (forming Al2O3).

10

20

220 200 180

30

160

100

200

300

400

500

600

Temperature (oC) Fig. 3. TGA results of products formed from different hydrothermal treatment temperatures (160–220 °C).

558

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160 oC

180 oC

200 oC

220 oC

Fig. 4. SEM micrographs of products yielded at different temperatures showing flat plate crystals at temperatures above 200 °C, while hexagonal shapes of gibbsite remain after treatment at a lower temperature.

2. Experimental procedures AlOOH Al(OH)3

2.1. Equipment and materials The ATH materials used in this study were sampled from different products produced at KC Corp alumina plant at Mokpo, Korea. De-ionized water was used for the study and chemicals used were of analytical grade. The composition of the solid samples was measured by XRF (PANalytical, model: AXIOS). Information on crystal sizes and mineralogy was determined using the Phillips PW2200 X-Ray Diffractometer. The SEM micrographs were taken using instrument from Shimadzu (model SSX-550). The TGA/DTA measurements were conducted using a Shimadzu instrument (model DTG-60H) heated at 0.1 °C/min. The solid specific surface area (BET) was determined by the nitrogen adsorption method using Quadrasorb-SI, Quantachrome instrument. Particle size distribution and mean diameter D50 were determined using a Microtrac particle analyzer (Model S-3500). The density of the solid samples was measured according to industry standard method (JIS R9301-2-3). The product whiteness was determined by measuring the reflection rate by a photo diode (Kett Instrument, model C-100-3).

80min 70min 60min 50min 40min 30min 20min 10min Al(OH)3

10

20

30

40

50

60

70

80

2 Theta Fig. 5. XRD patterns for products yielded at different time (10–80 min) showing new peaks of boehmite after 60-minute hydrothermal treatment. Conditions: 40% ATH slurries, D50 of 2.5 μm, pressure of 1.7 MPa and 200 °C.

C.J. Oh et al. / Powder Technology 235 (2013) 556–562

particle size distribution (PSD), scanning electron microscopy (SEM), X-Ray diffraction (XRD), thermal gravimetric/differential thermal analysis (TGA/DTA) and chemical composition analysis. Different slurry mixtures of ATH in water at 30, 40 and 50 (solid weight/water volume) % were subjected to thermal treatment over a temperature range of 160–220 °C. As there was no effect of slurry density, all subsequent runs were conducted using 40% slurries. Other parameters studied include time (10–80 min) and ATH mean particle size (1.25–50 μm). As shown in the following sections, as long as the digestion temperature was maintained above 200 °C, boehmite would be formed under the conditions tested after 60 min.

Weight Loss (%)

0

10

80 min 70 min 60 min 50 min 40 min 30 min 20 min 10 min 0 min

20

30

559

3. Results and discussion 100

200

300

400

500

600

3.1. Material characterization

Temperature( ) Fig. 6. TGA graphs showing dehydration of products yielded at different reaction time (10–80 min) confirming boehmite existence after 60 minute hydrothermal treatment. Conditions: 40% ATH slurries, D50 of 2.5 μm, pressure of 1.7 MPa and 200 °C.

2.2. Procedures The hydrothermal treatment of ATH samples was conducted using high pressure digestion reactors operated at a controlled-temperature up to 300 °C. For each experiment, an amount of 2.2 kg ATH was mixed with 5.4 L of water and the slurry was added to the reactor before heating began. The agitation speed was set at 200 rpm. The heating took about 120 min to get to the tested temperature and was then controlled to ± 2 °C. After the test was completed, the reactor was cooled to ambient temperature and the boehmite product was filtered from the slurry, washed with de-ionized water and dried at 105 °C. The boehmite product was then subjected to different product characterization measurements including BET area, density,

The characteristics of the ATH feeds are shown in Fig. 1 (thermal gravimetric properties) and Table 1 (chemical composition, moisture content, mean particle sizes, etc.). Two types of ATH were used in this study. Low-soda (Na2O) ATH is generally coarse (8–50 μm) and has low whiteness (b90%). On the other hand fine ATH (1.5–2.5 μm) has higher whiteness (95%) and specific surface area (>2 m2/g) and low bulk density (0.25 g/cm 3). As shown in Fig. 1, the ATH (gibbsite, Al(OH)3) materials start decomposing at ~ 200–250 °C and complete their thermal decomposition to boemite (AlOOH) at 300–350 °C, at which point ~ 30% of weight loss was observed, approximately corresponding to the change in molecular weight from 78 mg/mol for dry gibbsite to 50 g/mol for boemite (AlOOH). This starting thermal decomposition temperature (200–250 °C) is also in line with what reported by Hollibery et al. [4,5] who stated that gibbsite will thermally decompose at 180–220 °C. A further heating to over 500 °C will fully decompose boemite to Al2O3. It is also noted that the air dehydration temperature increases with the increase in mean particle size.

30min

60min

70min

80min

50 µ m

25 µm

2.5 µm

1.5 µ m

Fig. 7. SEM micrographs (top row) showing hexagonal crystals of gibbsite at earlier time (b60 min) and plate-like boehmite formed after 60 min. (Conditions: 40% ATH slurries, D50 of 2.5 μm, pressure of 1.7 MPa and 200 °C) and (bottom row) showing different boehmite products formed from different ATHs (D50 varying from 1.5 to 50 μm). (Conditions: 200 °C, 60 min, 1.7 MPa, 40% ATH slurries).

C.J. Oh et al. / Powder Technology 235 (2013) 556–562

5.0

0.40

4.5

0.35

BET area (m2/g)

4.0

0.30

3.5 3.0

0.25

2.5

0.20

2.0

0.15

1.5 BET

1.0

0.10

Na2O

3.3. Effect of reaction time The effect of digestion time at 200 °C was tested within the range of 10–80 min, for ATH materials having the mean particle size of 2.5 μm. As shown by the XRD patterns in Fig. 5, the conversion of gibbsite to boehmite is considered complete only after 60 min of digestion. The ATH-free boehmite was confirmed also by TGA (Fig. 6) which shows no further dehydration at 220–250 °C for products formed with treatment time past 60 min. The predominance of plate-like crystals of boehmite is also obvious after 60 min of treatment as shown in Fig. 7 (top row). As shown later, the plate crystal shape can be formed from different feeds ATH having mean particle diameter from 1.5 to 50 μm (bottom row, Fig. 7). The reaction time also affects the Na2O content and BET specific surface area of boehmite as shown in Fig. 8. The BET specific surface area increases as the reaction proceeds as gibbsite is dehydrate. However as plate-like crystals were formed after 60 min, the BET area was unchanged and remained constant at ~3 m2/g. The hydrothermal leaching of sodium from the solid particles was also confirmed with XRF analysis showing a steady decrease with reaction time, reaching a steady state of 0.050% Na2O from 0.35% Na2O feed material.

0.05

0.5 0.0

hexagonal shape of gibbsite is still predominant whereas the materials collected at 200 °C and 220 °C show mostly flat and plate-like particles.

Na2O Content (%)

560

0

10

20

30

40

50

60

70

80

0.00

Reaction time(min) Fig. 8. Variation of BET area and Na2O content with respect to reaction time. Conditions: 40% ATH slurries, D50 of 2.5 μm, pressure of 1.7 MPa and 200 °C.

3.2. Effect of temperature The effect of digestion temperature (160–220 °C) was tested first to identify the conditions at which 40% ATH slurry of mean particle size 2.5 μm was converted to boehmite. The products after 60-minute digestion at different temperatures have different characteristics, as shown in their XRD patterns (Fig. 2) and TGA measurements (Fig. 3). At a temperature below 180 °C, gibbsite ATH was unaffected and its presence is still noticed on the XRD patterns shown (Fig. 1). As shown in Fig. 3, it is clear that the gibbsite phase still existed after treatment at 160 °C and 180 °C and only started dehydrating at 200 °C. However, the materials produced at 200 °C and 220 °C exhibit only sharp XRD peaks of boehmite. TGA analysis also confirms that the dehydration of boehmite products produced at 200 °C and 220 °C has a much higher dehydration temperature of 350 °C. The SEM micrographs of Fig. 4 show the shapes of various particles formed at different temperatures. At 160 °C and 180 °C, the

3.4. Effect of ATH feed size The effect of different ATH materials having particle sizes from 1.5 to 50 μm treated and produced at 200 °C for 60 min was confirmed by the XRD patterns (Fig. 9). Remnants of gibbsite peaks still exist with coarse materials. Also by TG-DTA measurement (Fig. 10) it is clearly shown that the dehydration of coarse ATH still takes place at 200–220 °C for coarse ATH feeds, corresponding to gibbsite to boehmite conversion. For finer materials (b17 μm) the boehmite products formed are stable up to 300–350 °C when small peaks of boehmite decomposition to Al2O3 appear. This indicates that coarse

AlOOH Al(OH) 3 1.5

2.5

8

17 25 50 Al(OH) 3 15

20

25

30

35

40

45

50

55

60

65

70

75

2 Theta Fig. 9. XRD patterns of products from different ATHs having particle size from 1.5 to 50 μm. (Conditions: 200 °C, 60 min, 1.7 MPa, 40% ATH slurries).

C.J. Oh et al. / Powder Technology 235 (2013) 556–562

Weight Loss(%)

a

Table 2 Properties of boehmite product from hydrothermal treatment of plant ATH (2.5 μm) at 200 °C and 1 h.

0

10

20

561

F (1.5

)

E (2.5

)

D (8.0

)

C (17

)

B (25 A (50

) )

Values

AlOOH content (%) Moisture (%) Loss on ignition (%) Chemical composition (%)

99.9 0.21 17.1 0.037 0.006 0.010 2.46 0.001 3.10 1.02 0.78 97.8 3.0

PSD, D50 PSD, +45 μm (%) Specific surface area (m2/g) Bulk density g/cm3

Na2O Fe2O3 SiO2

Tap Loosed

Whiteness Density (g/cm3)

30 100

Typical analysis

200

300

400

500

600

Temperature( )

b

1.5–50 μm. The mean particle size of boehmite produced from coarse ATH (with D50 > 8 μm) also decreased after the hydrothermal treatment as shown in Fig. 11. For fine ATH feeds (1.5 or 2.5 μm), there is no clear change of particle size of the boehmite products. The hydrothermal treatment also leaches Na2O and Fe2O3 from the ATH feeds, yielding purer boehmite products, containing b 0.05% Na2O and b 0.01% Fe2O3. Typical properties of the boehmite product made from the process are shown in Table 2. DTA of the boehmite product shows its thermal stability up to 320–340 °C, where dehydration first took place. The material should be completely converted to Al2O3 at 520 °C as shown on the DTA pattern (Fig. 12). High quality boehmite products therefore can be synthesized by the hydrothermal treatment of plant ATHs having mean sizes b8 μm. The boemite products generally have lower mean sizes compared to the starting ATH feed as confirmed in Fig. 13.

20

DTA( V)

0

-20

F (1.5

)

E (2.5

)

D (8.0

)

C (17

)

B (25 A (50

) )

-40 100

200

300

400

500

600

Temperature( )

4. Conclusions

Fig. 10. TGA graphs showing weight losses of boehmite products from different ATHs having particle size from 1.5 to 50 μm. (Conditions: 200 °C, 60 min, 1.7 MPa, 40% ATH slurries).

ATH of 25 and 50 μm mean diameter is not suitable for the synthesis of boehmite. As shown in Fig. 7 (bottom row), the typical hexagonal crystals of gibbsite were completely converted to flat plate-like boehmite after the treatment at optimum conditions. The plate crystal shape can be formed from different ATH feeds of mean particle diameter

Mean particle size (

)

60

Optimized conditions for the production of boemite used as fillers from plant gibbsite were determined in this study. The hydrothermal treatment of ATH at 200 °C for 1 h produces well crystalline boehmite particles of 1.5–8 μm diameter which are thermally stable in air up to 320–340 °C. However, larger particles (50 μm) of ATH show incomplete conversion to boehmite. The boehmite product formed also has low Na2O (b0.050%) and Fe2O3 (b 0.010%) and high whiteness as a result of further extraction of these impurities during the hydrothermal treatment of ATHs. The hexagonal crystal of the starting gibbsite changes to plate-like shape as boehmite is formed.

DTG

TG(%)

50

Al(OH)3

100

AlOOH

95

0.0005

TG(%)

0.0000 -0.0005

40 90

-0.0010

30

85

-0.0015

20

80

-0.0020 -0.0025

75

10

DTG

70 0

0 A (1.5

) B (2.5

) C (8.0

) D (17

) E (25

) F (50

)

-0.0030

-0.0035 100 200 300 400 500 600 700 800 900 1000

Temperature(oC)

Samples Fig. 11. Decrease of boehmite product mean particle size after hydrothermal treatment.

Fig. 12. TGA and DTA patterns for boehmite product from hydrothermal treatment of plant ATH (200 °C, 60 min, 1.7 MPa pressure).

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C.J. Oh et al. / Powder Technology 235 (2013) 556–562

100

Accumulated weight(%)

90 80 70 60 50 40 30

Al(OH)3 AlOOH

20 10 0

0.1

1

10

100

1000

Paticle size( ) Fig. 13. PSD measurements of boehmite product and ATH feed showing reduction in particle size after treatment.

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