This study was carried out to develop a process for synthesising ultra-fine boehmite powders by precipitation from the Bayer process liquor at a temperature within the range 85 ~ 95 ^oC. The study was conducted to investigate whether boehmite produced by this technique has the same physical properties as those prepared by hydrothermal synthesis at elevated temperature and commercially available for use as fire retardant in plastic making. As with gibbsite, the precipitation of boemite requires seeding in which α-boehmite seeds were first produced by pyrolysis of a plant ultra-fine gibbsite at 420 ^oC for 60 min. The boehmite seeds of mean particle size (D₅₀) of 0.30 μm were obtained through subsequent hydrothermal synthesis.
 This process would yield a high purity well-crystalline boehmite of 1.30-1.47 μm mean particle size of low surface area (2 ~ 4 m²/g) as required for commercial fire retardants.

Keywords: Boehmite, Bayer process, Hydrolysis, Pyrolysis, Precipitation

Boehmite (α-AlOOH or Al₂O₃.H₂O, alumina mono-     hydrate, AMH) has found many applications in industry, including use as a precursor for making special alumina ceramics, as catalysts or fire retardants in plastic production. When boehmite used as a precursor is calcined for making specialty alumina such as α-Al₂O₃ catalysts its crystal morphology (of different shapes such as rhombic, hexagonal, etc.), purity and size determine the transfor-      mation temperature and other properties of the final product [3]. As a fire retardant boehmite decomposes to alumina (Al₂O₃), releasing water and absorbing heat at a higher temperature (> 350 ^oC) compared to alumina trihydrate/ATH (Al(OH)₃, gibbsite) or magnesium hydroxide (Mg(OH)₂). Boehmite fire retardants require specific particle properties (< 2 μm particles, specific crystal shape, low BET specific surface area, and high purity, etc.).
It is well known that under hydrothermal conditions gibbsite is converted to boehmite at a temperature in the range 100 ~ 350 ^oC, depending on pressure in the range 1 ~ 100 MPa [5, 15]. As an example, Panda et al. [9] could produce nano-crystalline boehmite (crystallite size 35 ~ 75 nm) by treating gibbsite (particle size range of P₈₀ of 3 μm) hydrothermally at temperatures < 350 ^oC and 15 ~ 55 MPa pressure.
In the temperature range 350 ~ 450 ^oC, both boemite and α-Al₂O₃ are formed. One of many techniques developed for the synthesis of boemite as nanowire, nanotube or as nanoparticles (1 ~ 20 nm) of various crystallite shapes (hexagonal, rhombic, ellipsoidal, etc.) employs hydrothermal conditions at temperature in the range 150 ~ 250 ^oC under pressure for several days [7, 12-13, 16]. This was done by using different precursors as seeds, ranging from bayerite Al(OH)₃, gibbsite Al(OH)₃, to pseudo-boehmite sol gel, etc. As an example, flaky spherical boemite could be produced from sodium aluminate (Bayer) liquor at 44.7 g/L Na₂O (caustic NaOH) and alumina set at A/C (alumina to caustic mass ratio, with A as g/L Al₂O₃ and C as g/L Na₂O) of 1.41, using 266 g/L gibbsite seed at 180 ^oC [6].
Several synthesis techniques were also proposed to produce boehmite directly from Bayer or sodium aluminate liquors at temperatures lower than 100 ^oC. Although the solubility of boehmite (30 ~ 40 g/L Al₂O₃ in the temperature range 80 ~ 100 ^oC) is generally lower than for gibbsite (60 ~ 120 g/L Al₂O₃) [6] the precipitation of boehmite is slow and controlled via a kinetic step [14] with an activation energy of 89 kJ/mol, compared to easier gibbsite formation over the same temperature range 70 ~ 90 ^oC. To produce pure boemite without gibbsite contamination the precipitation has to be conducted at a very high total caustic concentration (equivalent to 210 g/L caustic as Na₂CO₃) equivalent to an A/C ratio of 0.67 [4]. Attempts to improve the reaction rate, particle size (via promoting applomeration) and strength (increasing via crystal growth) of boemite crystals have been made during the crystallisation step from aluminate liquors. While agglomeration of boemite is dependent on operating parameters and can be enhanced by longer aging time, higher seed ratio, higher liquor supersaturation (ie large difference of liquor Al₂O₃ concentration from equilibrium value), etc. crystal growth to control crystallite size to within a certain range is difficult.
Wang and co-workers [18] found the crystal growth rate of boehmite is independent of seed sizes or seed ratio and is in the range 0.08 ~ 2.4 μm/h at 80 ^oC, 4 ~ 10 times less than that of gibbsite (2 ~ 7.2 μm/h). Panias and Krestou [11] first precipitated nano-crystalline boehmite (3 ~ 8 nm crystallites) by neutralising a supersaturated Bayer liquor to pH 5 ~ 7 at 30, 60 and 90 ^oC using nitric acid. Precipitates with higher crystallinity were formed at pH 7 and 90 ^oC, which after aging for one week would yield well crystalline boehmite of 22 ~ 37 μm mean particle size. At 90 ^oC, nano-crystalline boemite (30 ~ 80 nm) was produced using a concentrated sodium aluminate liquor (120 g/L Na₂O, 132 g/L Al₂O₃) within 24 h using 230 ~ 1200 g/L seeding with boehmite, without which seeds the reaction is extremely slow [14]. Dash et al. [1] found that to produce gibbsite within 8 h at a reasonable yield (~ 10 g/L boehmite) and at atmospheric conditions (85 ~ 95 ^oC, 1 atm). The Bayer liquors have to be supersaturated at high aluminate concentration > 150 g/L Al₂O₃, and the A/C ratio has to be in the range 1.0 ~ 1.2 with at least 100 g/L boehmite seeds of < 10 μm size added.
By adding an organic modifier, boehmite formation is enhanced, minimising the co-precipitation of gibbsite. Dash et al. [1] found that by adding 100 ~ 300 mg/L tartaric acid the nucleation of gibbsite is prohibited and boehmite is produced at a lower temperature of 80 ^oC. Wang et al. [18] also realised that by adding ethanol the activation energy of precipitation is reduced to 13.7 kJ/mol, shifting the reaction kinetics to a diffusion control mechanism, thus promoting boehmite formation.
The techniques studied and reported to date have been difficult in scaling up for plant production due to long residence time required for aging, either during hydrothermal processing or via precipitation at 85 ~ 95 ^oC. These methods could not produce microcrystalline boehmite with high crystallinity in the narrow size range 1 ~ 2 μm nor having a low surface area. Therefore, precipitation under Bayer process conditions was studied to determine optimum conditions of liquor composition, temperature and reaction time for producing micro-     crystalline boehmite suitable to be used as a fire retardant. The materials synthesised from this study are also compared with existing commercial products currently on the market.

Materials and reagents
The Bayer liquor used in this study was prepared from a bauxite ore sample from Weipa, Australia (Table 1), the same material has been used in the production of gibbsite/alumina trihydrate (ATH) at the Daejoo-KC alumina refinery at Mokpo, South Korea. As in plant operation the liquor used for the study was achieved by digesting bauxite to a caustic solution to achieve a caustic concentration of 210 ~ 220 g/L (as Na₂CO₃) and A/C ratio of 0.65 ~ 0.68.
All other reagents used in this study are of analytical grade.

Analytical techniques
Aluminum and caustic contents of the synthetic mother liquors and precipitation filtrate were analysed by acid-base neutralization/titration method. The precipitation (%) of boehmite/AMH based on the alumina analyses of the mother liquor and the filtrate was calculated from the following equation:

Precipitation (%) = [Alumina of mother liquor (g/L)
– Alumina in filtrate (g/L)] / Alumina of mother liquor
(g/L) × 100%

The prepared α-boehmite seeds used in this study were analysed using a Particle Size Analyzer (PSD, S3500, Microtrac), and the crystal phase was determined by an X-Ray Diffractometer (XRD, Ultima IV, Rigaku). A Surface Area Analyser (Quadrasorb-SI, Quantachrome) was used to confirm the specific surface area of the seeds. The thermal decomposition and composition of boemite (AMH) was evaluated using a thermal analyzer (DTG-60H, Shimadzu). A Scanning Electron Microscope (SEM, SSX-550, Shimadzu) was also used to evaluate the shapes of the particles produced. Chemical analysis was performed on digested samples using an Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES, ICPS-7510, Shumadzu).

Experimental procedures
The following stages were conducted for the experiments:
(a) Liquor used: The Bayer mother liquor from which boehmite was precipitated from was prepared by varying the A/C ratio, seed mass ratio (mass of boehmite seeds/mass of Al₂O₃ in the liquor),
(b) Precipitation of seeds: α-boehmite seeds were prepared for the precipitation of boehmite by calcination of a KC superfine (< 2 μm) gibbsite/ATH product (alumina trihydrate, Al(OH)₃) at 420 ^oC. The calcined material was then subjected to a hydrothermal reaction, filtered and ground to produce nano-AMH (boemite) material as shown in Fig. 1.
(c) Boehmite precipitation – The boehmite precipitation experiments were carried out by adding the above freshly prepared seeds into the Bayer sodium aluminate liquor at different temperatures, caustic soda concentrations, and amounts of seeds used.

Thermodynamic Study
A thermodynamic simulation of the precipitation conditions was conducted using HSC program [8]. Data obtained (Fig. 2) confirm that boehmite could be thermodynamically precipitated from 95 ^oC, in preference to gibbsite. The formation of gibbsite peaks at 110 ^oC and decreases slowly as the temperature rises to 200 ^oC. More boehmite is formed in this temperature range (95 ~ 200 ^oC).

Seed preparation
Seed preparation is a critical step for this process. Seeds were produced by roasting gibbsite (superfine < 2 μm gibbsite/ATH from KC plant) at different temperatures to determine the condition for making boemite. XRD analysis (Fig. 3) shows that gibbsite remains stable at less than 350 ^oC during roasting. Above 400 ^oC γ-boehmite/AMH is formed. The pyrolysis of superfine gibbsite to form g-boehmite seeds was therefore conducted at 420 ^oC subsequently for the study. After hydrolysis to 190 ~ 200 ^oC this material is converted to α-boemite/AMH.

Optimum boehmite precipitation temperature
To determine the optimum precipitation temperature, tests were conducted at the caustic NaOH concentration of 220 g/L (as Na₂CO₃), pregnant liquor A/C ratio 0.68, mass seed ratio (boemite AlOOH seed/Al₂O₃ in the liquor, both in g) of 2.0 with the isothermal (fixed) temperature varying from 80 to 100 ^oC. It was found that gibbsite was formed when the precipitation temperature was 85 ^oC or less and boemite could only be produced when the temperature was set at higher than 90 ^oC. These results were confirmed by XRD analysis of the boehmite products as shown in Fig. 4. It was also observed that the precipitation rate was very slow, up to 48 h at which time the A/C ratio still has not reached a steady state (ie. equilibrium has not been reached) as shown in Fig. 5.
Conditions for the tests to determine the optimum precipitation temperature are tabulated in Table 2.

Precipitation with slow cooling
In the Bayer process, the precipitation rate of gibbsite/ATH increases depending on the difference between the initial precipitation temperature and its final value afterwards after cooling. Based on this, it was also applied to boehmite precipitation and the cooling rate was 0.4 ^oC/hr from the starting temperature. Conditions for this test series are tabulated in Table 3.
The experimental results (Fig. 6) show that the lower the initial temperature is, the higher precipitation rate and yield, as expected with lower final A/C ratio. It was also confirmed by XRD analysis (Fig. 7) that gibbsite will be formed if the final temperature is less than 85 ^oC.
The effect of other parameters such as A/C ratios, seed ratio, etc was also studied and results are as follows.

Precipitation at different A/C ratios
Tests were conducted to evaluate the effect of varying A/C ratios during boehmite precipitation according to conditions shown in Table 4.
As expected at a higher A/C ratio the precipitation rate and yield are higher. At all conditions the steady state would be reached slowly merging to an A/C ratio of ~ 0.45 after 48 h (Fig. 8).
It is best therefore to use a liquor having an initial A/C ratio of 0.68 to achieve maximum product yield. This value therefore was chosen for all tests.

Effect of seed mass ratio (AlOOH mass/Al₂O₃ mass in liquor)
Since the boehmite precipitation process requires a higher precipitation temperature compared top gibbsite, a relatively high seed mass ratio is required to increase the initial precipitation rate. The effect of seed mass ratio is shown in Fig. 9, showing a higher precipitation rate at a higher seed mass ratio in the range 0.5 ~ 2:1. Above this 2:1 ratio, there is not much of a change of A/C ratio with respect to the seed mass ratio. The 2:1 seed mass ratio was therefore chosen as optimum conditions as shown in previous results.
Under SEM typical materials through different stages of the process are shown in Fig. 10. It is clear that the raw material (gibbsite) was transformed to g-boehmite after heating to 420 ^oC, when the calcined crystallites tended to sinter to hexagonal agglomerates > 2 μm in sizes. After hydrothermal treatment at 190 ~ 200 ^oC these agglomerates were broken and the a-boemite seeds became cubic in form with reduced sizes to mostly < 2 μm. This mainly cubic shape is maintained after precipitation into the final boemite product.
Adopting the optimum conditions for precipitating boemite, the products obtained are very compatible to typical commercial products such as Nabaltec APYRAL AOH-30 and SR-100 MES produced by TOR MINERALS – USA (Table 5). The boemite produced from this study meets the specifications for particle size with D₅₀ in between 1 ~ 2 μm, surface area 3 ~ 6 m²/g. The purity of boehmite from this study is better than these commercial products.

This study was conducted to investigate the production of 1 ~ 2 μm particle sizes required commercially as a fire retardant. Boehmite seeds (mean particle size 0.30 μm) were first produced by pyrolysis of a plant superfine gibbsite (< 2 μm) at 420 ^oC, which was then subsequently treated by a hydrothermal process at 190 ~ 200 ^oC. These seeds were then used in a precipitation step at optimum conditions determined from this study.

Optimum conditions for precipitation are : caustic (NaOH) concentration of 220 g/L measured as Na₂CO₃, alumina/caustic (A/C) ratio of 0.68, seed mass ratio (boemite/Al₂O₃ in the liquor) of 2:1 and precipitation temperature of 85 to 95 ^oC and in 48 h. Following these conditions the process would yield a high quality crystalline boehmite of 1 ~ 2 μm in sizes, 2 ~ 4 m²/g surface area. The boemite produced from this process are compatible to other high quality commercial products.

The authors acknowledge the support of Daejoo-KC’s laboratories for providing analyses for this study.