A study on the preparation of fine and low soda alumina

Int. J. Miner. Process. 80 (2006) 126 – 132 www.elsevier.com/locate/ijminpro

A study on the preparation of fine and low soda alumina Yi Yong Park a , Suong Oh Lee b , Tam Tran c , Seong Jun Kim a , Myong Jun Kim a,⁎ a

Department of Civil, Geo and Environmental Engineering, Chonnam National University, Republic of Korea b KC Corporation, Chonnam, Republic of Korea c School of Chemical Engineering and Industrial Chemistry, UNSW, Sydney, 2052, Australia Received 4 November 2005; received in revised form 9 February 2006; accepted 8 March 2006 Available online 27 April 2006

Abstract A simple process to produce fine and low soda α-alumina (α-Al2O3) from a commercial grade aluminium trihydroxide (gibbsite, Al(OH)3) produced by KC Corporation Ltd was developed. There are two options for this process with the first one producing low soda α-alumina (b 0.05% Na2O) having a mean particle size of 50 μm. The second option yields a fine product with a mean size of less than 10 μm. In the first option, a plant aluminium trihydroxide containing 0.20% Na2O was first fluidized with nitrogen at 400–600 °C to yield an amorphous activated alumina. This intermediate product was then treated with acetic or oxalic acid, washed with water and heated to 1200 °C to form calcined α-alumina, having a Na2O content of less than 0.05%. A 20 min leaching using 0.2 M acetic or oxalic acid could yield an alumina product containing 0.04% Na2O. In the second option, a new technique for the preparation of fine and low soda α-alumina was evaluated using an attrition mill working also as a leaching vessel at 80 °C. Fine (b 10 μm in mean particle size) and low soda (b0.04% Na2O) alumina was produced by a 20 min leaching step with 0.2 M acetic acid and concurrent attrition milling. © 2006 Elsevier B.V. All rights reserved. Keywords: bayer process; low soda alumina; fine alumina; activated alumina; aluminium trihydroxide

1. Introduction Fine and low soda alumina has been used in many industrial applications. This material is the feed for the production of alumina ceramic and other refractory products, which have been used in a wide range of engineering fields due to their excellent chemical stability, electrical and mechanical properties. Commercial grade “fine and low soda α-alumina” has a mean particle size (D50) of b 10 μm and contains 0.03% to 0.10% Na2O (Watson et al., 1963).

⁎ Corresponding author. Tel.: +82 62 530 1727. E-mail address: [email protected] (M.J. Kim). 0301-7516/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.minpro.2006.03.006

Alumina (Al2O3) is produced mainly by the Bayer process from the calcination of aluminium trihydroxide (Al(OH)3, gibbsite) produced by precipitation from a sodium aluminate liquor (Hind et al., 1999). During this precipitation, an appreciable amount of caustic NaOH referred to as soda (and expressed as %Na2O in the product chemical analysis) is occluded within the alumina structure which cannot be easily removed by simple washing in water or acids. Such an impurity reduces the electrical insulating quality of alumina porcelains for example when used in making spark plugs or other ceramic insulators (Watson et al., 1963). Fine alumina also has extensive application in the manufacturing of dense ceramic materials or catalytic supports. The presence of ultrafine (0.5–5 μm) or nano-

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sized alumina will improve the sintering process during alumina ceramic production (Ting and Lin, 1995; Yeh and Sacks, 1988). Fine-grained alumina can be produced during calcination by seeding a Bayer aluminium trihydroxide with fine α-alumina (α-Al2O3) produced from worn-out balls used during wet milling earlier (Yoshizawa et al., 2004). The addition of fine seeds not only lowers the calcination temperature required to produce α-alumina but also produces a fine material having better powder characteristics and properties than the unseeded materials. Sarikaya et al. (2002) evaluated a technique of making fine alumina by first evaporating an emulsion of aluminium nitrate in oil. The precursor material is then calcined at 1000 °C to produce a fine α-alumina. Another technique for making ultrafine alumina (100 nm particle size range) was also suggested by Morinaga et al. (2000) based on the thermal decomposition of nano-sized ammonium aluminium carbonate hydroxide produced from the reaction of ammonium aluminium sulphate with ammonium bicarbonate. Various methods have been proposed to produce fine and low soda α-alumina from Bayer aluminium trioxide. Isupov et al. (2001) recently proposed a laboratory technique for making fine and low soda materials by first treating technical-grade aluminium trihydroxide (of 85–90 μm in mean particle size) containing 0.14% Na2O from an alumina plant with Li salts (Li chloride, nitrate and sulphate) at 90 °C. This allows the intercalation of Li ions into the aluminium hydroxide structure. The subsequent calcination at 1200 °C formed the final product which is plate-shaped having mean diameters in the range 5–10 μm. Further grinding in a mortar produced a finer product of 3 μm in mean particle size. Plant practice to produce low soda alumina was based on the use of aluminium fluoride during calcination, from which fluoride would react with the soda and was removed in a gaseous form (Watson et al., 1963). Concerns over fluoride emission to the atmosphere have negated the use of this technique in plant practice in recent time (Weinstein, 1977; MacLean et al., 1982). By heating an aluminium trihydroxide containing 0.26% Na2O together with a siliceous material to 1280 °C a product containing 0.02% Na2O was produced. In this process, the soda reacts with the silicate forming quartzite and is separated from alumina by screening (Watson et al., 1963). This technique however poses a potential contamination of the final alumina product by unreacted silica. Calcination with boric oxide was also suggested as a method to remove soda (Gitzen and Ill, 1963). In this technique, the aluminium trihydroxide is mixed with boric oxide

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and heated to effect the solubilisation of soda, forming sodium borate. Water can be used to dissolve the sodium borate, yielding an alumina product containing generally 0.05% Na2O. The soda removal process using boric oxide has posed numerous disadvantages. The borate is easily volatilized in the hot zone of the kiln and then condensed at the cold end. It can also react with the brickwork refractory causing damage to the kiln. Also, boric oxide is a mineralizing agent which promotes crystal growth in alumina during calcination, thus preventing the formation of fine products. At the ever increasing demand for fine and low soda α-alumina it is therefore necessary to install a simple yet effective process in existing Bayer plants to treat aluminium trihydroxide (gibbsite) produced within these plants. Soda can easily be removed by acid washing as long as it is exposed and not trapped within the aluminium hydroxide structure during precipitation. Organic acids have advantages over other inorganic counterparts as they can be completely destroyed to form carbon dioxide during alumina calcination, thus leaving no trace of contamination in the final product. Fine grinding could be applied if the gibbsite agglomerates (aluminium trihydroxide from precipitation) can be transformed into a less rigid structure. This study was conducted to evaluate a technique which can be easily scaled up in Bayer alumina plants to produce low soda α-alumina. The technique is based on the treatment of activated alumina (formed from calcination of the aluminium hydroxide feed) by acetic and oxalic acid to remove the soda. An attrition mill working as a leaching vessel was also used to grind the activated alumina in acetic acid to produce the fine and low soda product for the study. 2. Experimental All chemicals and reagents used in this study were of analytical grade. All solutions were prepared using distilled water. The aluminium trihydroxide used in the experiment was from Korean Chemicals Corporation's alumina refinery which contains 99.7% Al(OH)3, 0.20% Na2O, 0.01% SiO2, 0.01% Fe2O3 and 9.0% moisture. The material has a mean particle size of 50 μm, and a surface area of 0.20 m2/g. Calcination to produce activated alumina was carried out using a vertical tube furnace incorporating a quartz column in which the aluminium trihydroxide feed was fluidized by nitrogen gas. To make activated alumina, nitrogen gas was also flowed into the furnace column at 2–3 cm/s to fluidize 50 g of aluminium trihydroxide at the set test temperature of 300 to 600 °C.

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The preparation of fine low soda alumina was carried out using an attrition mill (KMD-1B, Korea). The product particle size distribution was checked with a particle size analyzer (Fritsch, Analysette 22). An X-ray fluorescence spectrometer (PW 2400, Netherlands) was used to determine the soda content in alumina. X-ray diffraction analysis (XRD) was performed using a diffractometer employing CuKα radiation. The specific surface area and shape of activated alumina were analyzed with a specific surface analyzer (Nova 1000, USA) and a scanning electron microscope (Sterescan 420, Germany). The soda leaching experiments were carried out using a three-necked, round-bottom 1 L flask equipped with a mechanical stirrer set at 250 rpm and a condenser. The reactor was immersed in a thermostatically controlled water bath to maintain a constant test temperature in the range from 25 to 100 °C. In each experiment, a weighed amount of 250 g of activated alumina was suspended in 250 mL of an acid solution. The acid-treated material was then washed with distilled water (250 mL) and then centrifuged using a centrifugal separator to remove the excess organic acid. In a second series of tests, soda leaching and attrition milling were simultaneously conducted using a special attrition mill (1 L capacity, Model: KMD-1B, Korea) working also as a leaching vessel. The attrition mill was first charged with 250 g of activated alumina, 250 mL of 0.1–0.7 M organic acid and zirconia balls (3 mm diameter) added at 30% mill volume. The attrition mill was also jacketed with hot water to heat the mill compartment to 80 °C. The leached product was then calcined at 1200 °C to make calcined alumina (αalumina). 3. Results and discussion

into an amorphous intermediate phase (activated alumina), which had a high surface area and was therefore more reactive than the starting materials. Fig. 1 shows the variation of the specific surface area with respect to calcination temperature and time. For the samples used, the surface area reached a maximum of 312 m2/g after 20 min of calcination at 500 °C. Calcination for 5 min at 550 and 600 °C yielded activated alumina intermediates of lower specific surface areas (250 and 200 mg/m2 range, respectively). The high specific surface area of the activated alumina intermediate could be due to the more cracks formed in the micro-structure of the activated material (Fig. 2(b)), compared to raw aluminium trihydroxide (Fig. 2(a)), although both materials seemed to have the same overall particle morphology. The crystal structure of gibbsite (Al(OH)3) and boehmite (AlOOH) however could be completely destroyed at 600 °C as shown in Fig. 3, at which temperature the peaks for gibbsite and boehmite completely disappeared. The transformation starts at 400 °C from which temperature gibbsite peaks slowly disappeared and boemite peaks started appearing as shown on the XRD diffraction pattern. The increase in the surface area could be due to the increasing porosity caused by dehydration. The transformation from gibbsite to boehmite is also due to the loss of water in the structure. The dehydration completes at 600 °C forming activated alumina. The calcination temperature of 500 °C was chosen in this study to produce the mostly activated alumina for the subsequent acid leaching. As shown in the XRD pattern, although some boehmite phase still existed at this temperature and the dehydration of the alumina trihydroxide feed was not completed, the surface area measurements (Fig. 1) show that a too high activation temperature above 500 °C would decrease the surface area. This would not help the leaching stage as the porosity of the activated

3.1. Preparation of activated alumina Specific Surface Area(m2/g)

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The soda which is captured within the crystal structure of aluminium trihydrate is very difficult to be removed by washing. Therefore, the key to make low soda alumina in this study was to convert the aluminium trihydroxide feed material to activated alumina to open its grain structure and thus exposing the soda to leaching solutions. Numerous methods were employed by researchers to produce activated alumina in a laboratory including mechano-chemical activation (MacKenzie et al., 1999, 2000) or flash calcination (Jovanovic et al., 1992) of gibbsite. Using a vertical tube furnace in which the aluminium trihydroxide was fluidized by nitrogen gas at 500–600 °C, the original gibbsite was converted

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Temperature(˚˚C)

Fig. 1. The effect of temperature on the specific surface area of calcined products.

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(a): SEM micrographs of aluminium trihydroxide.

(b): SEM micrographs of activated alumina (500oC, 20 minutes).

Fig. 2. (a): SEM micrographs of aluminium trihydroxide. (b): SEM micrographs of activated alumina (500 °C, 20 min).

material might be lower at too high activation temperature. The left over boehmite after calcination at 500 °C did not seem to greatly affect the effectiveness of the leaching process to remove soda and all will be destroyed at subsequent calcinations to 1200 °C to produce the calcined α-alumina product.

G

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Untreated Al(OH)3

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Acetic acid Oxalic acid

0.04

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Raw (0.2% Na2O)

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2-Theta

Fig. 3. X-ray diffraction patterns of heat-treated or untreated aluminium trihydroxide (G: gibbsite, Al(OH)3, B: boehmite, AlOOH).

Fig. 4. Variation of soda level at various reaction times after leaching plant aluminium trihydroxide (initially containing 0.2% Na2O) with 1 M organic acid at 80 °C.

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Y.Y. Park et al. / Int. J. Miner. Process. 80 (2006) 126–132 0.05

0.05

Acetic acid Oxalic acid

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Fig. 5. Variation of soda level in the alumina product with acid treatment time (leaching conditions: 0.2 M organic acids, 80 °C, activated alumina at 500 °C).

Fig. 7. Variation of soda level of the final product at various concentrations of organic acid (leaching conditions: 20 min, using activated alumina (activated at 500 °C), 80 °C).

3.2. Preparation of low soda alumina

acetic acid seems to be the determining factor explaining the more efficient removal of soda during leaching at the same concentration basis as shown later (Figs. 4–7). The leaching of the plant aluminium trihydroxide by 1 M organic acids at 80 °C over 60 min showed a 50% decrease in the soda level as shown in Fig. 4. The original soda level was decreased from 0.20% to 0.10% at best, using acetic or oxalic acid. On the other hand, the leaching of activated alumina (activated to 500 °C) using 0.2 M acetic or oxalic acid could reduce the soda content of the calcined product (calcined to 1200 °C, α-alumina) to below 0.04% as shown in Fig. 5. At 80 °C, it only took 5 min of leaching to reduce the soda level to less than 0.04% Na2O using 0.2 M acetic acid. A longer leaching time further lowers the soda content to 0.036% Na2O after 60 min. Oxalic acid performed slightly worse, although it also reduced the soda to acceptable levels (0.040–0.042% Na2O). The mean particle size of the final product is unchanged at 50 μm.

Most commercial low soda α-alumina products contain N 99.8% Al2O3, b0.05% Na2O, b 0.01% SiO2, and b0.01% Fe2O3. The removal of soda from activated alumina produced in this study was carried out by reacting the activated alumina (mean particle size of 50 μm) produced with acetic or oxalic acid. Soda in the activated alumina reacts with organic acids according to Eqs. (1) and (2) as follows: NaOH þ CH3 COOH→CH3 COONa þ H2 O

ð1Þ

2NaOH þ H2 C2 O4 →Na2 C2 O4 þ H2 O

ð2Þ

It is noted that the stoichiometry of the reaction is different for oxalic acid (2 mol oxalic acid is required to remove 1 mol of caustic NaOH) compared to the leaching using acetic acid (1:1 molar stoichiometry). However the stronger ionization (higher values of ionization Ka) of

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Acetic acid Oxalic acid Raw (0.2% Na2O)

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Fig. 6. Variation of soda level of the final product at various temperatures (leaching conditions: 0.2 M organic acid, using activated alumina (activated at 500 °C), reaction time: 20 min).

60 40 Raw Sample(50µm) 5min(26.3µm) 10min(13.µm) 20min(7.6µm)

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Fig. 8. PSD of raw aluminium trihydroxide and final α-alumina products at different grinding time (mean particle sizes are also quoted). Conditions: 80 °C, 0.2 M acetic acid attrition milling.

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the calcined alumina product with respect to acid concentration is shown in Fig. 7.

Na2O (%)

0.045

3.3. Preparation of fine particle low soda α-alumina 0.04

Acetic acid Oxalic acid

0.035

0.03 Raw (0.2% Na2O)

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10

20

Time (min)

Fig. 9. Variation of soda level of the final alumina product at various grinding times using an attrition mill with simultaneous acid leaching (0.2 M organic acid, 80 °C).

The effect of temperature is shown in Fig. 6. Leaching the activated alumina at 25 °C for 20 min in 0.2 M acetic and oxalic acid would reduce the soda content to less than 0.042% and 0.046% Na2O, respectively. The soda level could be further reduced to b 0.038% by acetic acid as the temperature was raised to 80–100 °C. The acid concentration also affects the removal of soda from activated alumina. The variation of soda in

A jacketed attrition mill was used to produce fine and low soda alumina product in this study. Activated alumina produced at 500 °C (50% pulp density), organic acid (0.2 M) and zirconia balls (3 mm diameter) were fed into this attrition mill heated to 80 °C. The milling at this test temperature in acid was conducted over 20 min and the acid-treated activated alumina was filtered, dried and calcined at 1200 °C to produce the final fine and low soda α-alumina product. Fig. 8 shows the change in particle size distribution (PSD) with respect to grinding time. The results show that after 5-min grinding the mean particle size achieved was 26 μm (D50), 13 μm (D50) after 10 min, and 7 μm (D50) after 20 min grinding with simultaneous acid leaching at 80 °C. Leaching soda and grinding activated alumina simultaneously was carried out at 80 °C over 20 min for both acids. Fig. 9 confirms a better removal of soda for acetic acid compared with oxalic acid over time at 80 °C, with all of samples recovered having soda content of less than 0.045% Na2O.

Al(OH)3 (0.2% Na2O) To make activated alumina Calcination with vertical furnace at 500˚C

Soda removal from activated alumina using organic acids

Fine grinding To make fine and low soda alumina

Grinding activated alumina in organic acids using attrition mill at 80˚C Filtering Separation between acids and solid

Filtering Separation between acids and solid

Dehydration Calcination at 1200˚C

Dehydration Calcination at 1200˚C

Low soda Al2O3 (0.035% Na2O)

Fine low soda Al2O3 (0.04% Na2O) under 10 µm

Fig. 10. Flowsheet for the preparation of low soda and fine α-alumina.

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The soda removal process is summarized as a flowsheet presented in Fig. 10, for both techniques tested. The flowsheet presents two options for making αalumina. Low soda alumina could be made by treating the activated alumina with organic acids followed by calcination (left branch). Fine and low soda alumina on the other hand required attrition milling simultaneously with acid treatment (right branch).

could be produced by simultaneous organic acid leaching and grinding using a heated attrition mill. The effectiveness of using organic acids to remove soda from activated alumina was confirmed. The two processes should be easily scaled up for plant practice.

3.4. Brief economic analysis

Gitzen, W.H., Ill, B., 1963. Production of low soda alumina, US Patent 3,092,452 June 4, 1963. Hind, A., Bhargava, S.K., Grocott, S.C., 1999. The surface chemistry of Bayer process solids: a review. Physicochem. Eng. Aspects 146, 359–374. Isupov, V., Chupakhina, L., Kryukova, G., Sybulya, S., 2001. Fine α-alumina with low alkali: new approach for preparation. Solid State Ionics 141–142, 471–478. Jovanovic´, N., Novacovic´, T., Javac´kovic´, J., Terlecki-barice´vic´, A., 1992. Properties of activated alumina obtained by flash calcination of gibbsite. J. Colloid Interface Sci. 150 (1), 36–41. MacKenzie, K.J.D., Temuujin, J., Okada, K., 1999. Thermal decomposition of mechanically activated gibbsite. Thermochim. Acta 327, 103–108. MacKenzie, K.J.D., Temuujin, J., Smith, M.E., Angerer, P., Kameshima, Y., 2000. Effect of mechanochemical activation on the thermal reactions of boehmite (γ-AlOOH) and γ-Al2O3. Thermochim. Acta 359, 87–94. MacLean, K.C., McCluno, D.C., Laurence, J.A., Weinstein, L.H., 1982. Advances in understanding effects of atmospheric fluoride on vegetation. Light Met. 82, 933. Morinaga, K., Torikai, T., Nakagawa, K., Fujino, S., 2000. Fabrication of fine α-alumina powders by thermal decomposition of ammonium aluminum carbonate hydroxide (AACH). Acta Mater. 48 (18–19), 4735–4741. Sarikaya, Y., Alemdaro, T., Önal, M., 2002. Determination of the shape, size and porosity of fine α-Al2O3 powders prepared by emulsion evaporation. J. Eur. Ceram. Soc. 22 (3), 305–309. Ting, J.M., Lin, R.Y., 1995. Effect of particle size distribution on sintering: Part II sintering of alumina. J. Mater. Sci. 30, 2382–2389. Watson, D.R., Lippman, A, Jr, Royce, D.V., Royce, B., Pulaski County Jr., 1963. Method for reducing the soda content of alumina, US Patent 3,106,452, Oct. 8, 1963. Weinstein, L.H., 1977. Fluoride and plant life. J. Occup. Med. 19, 49–78. Yeh, T.S., Sacks, M.D., 1988. Effect of particle size distribution on the sintering of alumina. J. Am. Ceram. Soc. 71, C484–C487. Yoshizawa, Y., Hirao, K., Kanzaki, S., 2004. Fabrication of low cost fine-grained alumina powders by seeding for high performance sintered bodies. J. Eur. Ceram. Soc. 24, 325–330.

The prices of chemical grade aluminium hydroxide and calcined alumina are approximately $US300/tonne and $US500/tonne, respectively, on the market. Low soda α-alumina demands a much higher price at $US750–900/tonne for D50 of 50 μm product, whereas finer material (D50 of 1.5 μm) is priced at $US900–1500/ tonne. At KC Corp, the cost of calcination adds approximately $US70–80/tonne of product to the cost of producing raw aluminium trihydroxide from Bayer plants, whereas the acid treatment to produce fine and low soda α-alumina would add another $US160/tonne of product. Although information on the cost of producing fine and low soda alumina from Bayer plants is not easily accessed by the general public, it is believed that most plants would spend up to $US120/tonne to produce the desired product. However, conventional processes would cause more environmental issues related to waste disposal as discussed earlier.

4. Conclusions To produce low soda alumina (under 0.04% Na2O), activated alumina was made initially by calcination, which was then treated with acetic or oxalic acid to remove soda at various temperatures and time intervals. In the first option, activated alumina (specific surface area of more than 320 m2/g) could be produced from plant aluminium trihydroxide by calcination at 500 °C for 20 min using a fluidized-bed tube furnace. Practical conditions for leaching soda from activated alumina were found to be at 0.2 M of organic acid and 80 °C in 20 min. In the second option, fine (of mean particle size of around 10 μm) and low soda (b 0.04% Na2O) alumina

References