Extraction of Alumina from Nawan Kaolin by Acid Leaching

This paper describes the production of alumina from Nawan kaolin by acid leaching with sulfuric and hydrochloric acids. Kaolin was calcined at 850oC and was leached with 6 M acid at 90oC, 5M NaOH followed by HCl solutions were added to the leaching liquor and the precipitated aluminum hydroxide was converted to alumina by calcination at 900oC. Materials were characterized by FTIR, XRD, and SEM techniques. The alumina extraction percent was determined at different leaching times (30-180 min) and solid/liquid ratios (0.05-0.15 g/ml). The purity of kaolin is about 95%. The percent of extraction of alumina rapidly increases with the solid/liquid ratio up to 0.1g/ml then decreases thereafter. The percent of extraction of alumina is higher for HCl than H2SO4. The size of the chloride and sulfate ions is the key factor that controls the percent of extraction of alumina from calcined kaolin under the studied conditions.


INTRODUCTION
Alumina that exists as a stable form (α-alumina) and in a variety of meta-stable forms including γ-alumina 1 is an important industrial mineral, which can be used as an abrasive material and as adsorbent 2 . Alumina has been widely used in advanced technological applications including its use for the processing of high-quality insulators 3 , semiconductors 4 , microelectronics 5 , high-strength materials 6 , ceramics, refractories 7 , biofuel and cellfuel 8 , fireproof plastics 6 and high-grade polishes 9 . Bauxite is the main raw material used in the production of alumina by the Bayer process 10 . But because bauxite is not discovered in Saudi Arabia in economic quantities 11 , kaolinitic clay has been processed for the production of alumina alternative to bauxite 12 . Kaolin is widely used in the manufacture of paper, functional filler, fiberglass, and ceramics 13 . Kaolin is a common weathering product of many tropical and sub-tropical soils 14 and consists of kaolinite mineral, A 12 Si 2 O 5 (OH) 4 , in addition to many accessory minerals such as carbonates, feldspars, and hematite. Kaolinite is a member of the two-layer planar hydrous phyllosilicate minerals called the kaolin group 15 . The structural unit of kaolinite consists of single tetrahedral silica and octahedral alumina sheets bonded together by hydrogen bonding 16 . Leaching process is commonly used in the extractive metallurgy due to its low-cost implication, eco-friendly procedure, low-energy requirement as well as ability to treat low-grade ores 17 . The three processes commonly used for extraction of alumina from clays are; (1) the acid leaching with sulfuric acid, hydrochloric acid or nitric acid to extract the alumina from calcined clay 18,19 . (2) The sulfatization by sintering clay with ammonium sulfate followed by extracting alumina by leaching with hot water 20 . (3) The alkali roasting by sintering clay with lime or soda followed by extracting alumina and silica by leaching with hot water 21,22 . Acids were proved more effective in aluminum extraction, than bases 23 . From an industrial point of view, the leaching of calcined kaolin clay in hydrochloric acid has several significant advantages including; low solubility of silica, 24 the possibility of selective crystallization of AlCl 3 •6H 2 O 25 and recovery of hydrochloric acid from the waste products for reuse in the process 26 . There have been a lot of recent research articles has focused on the process extracting alumina from kaolin by acid leaching where the extraction rate did not exceed 90% 27,28 . The alumina could not be efficiently extracted from kaolin by acid leaching because of the existence of the stable structures of Al-O-Si and Si-O-Si 29 as well as the formation of hydrated silica during leaching reaction that would hinder the contact between leaching agent and leaching nucleus 30 . The rate of extraction of alumina can be enhanced by calcination of kaolin in the temperature range 500-750°C prior to leaching to increase the reactivity of the particles by rendering the aluminum in the solid more soluble and removing organic material in the pores 31 . Taking into consideration that the kinetics of the dissolution of aluminum from calcined kaolin by hydrochloric acid is a liquid-film diffusion controlled process 32 . Hence, the rate of extraction of alumina was enhanced by the hydrothermal leaching of calcined kaolin by hydrochloric acid reaching about 98% 33,34 . As the hydrothermal leaching reduces the resistances caused by diffusion in the presence of hydrated silica 35 . The leaching of calcined kaolin in hydrochloric acid in the presence of fluoride ions also makes higher extractions possible at lower roasting and leaching temperatures 36,37 . Generally, the extraction of alumina from calcined kaolin by leaching with hydrochloric acid depends on the many variables including; fineness of kaolin, period and temperature of calcination, the concentration of hydrochloric acid, liquid/solid ratio as well as period and temperature of leaching 15 38 . The objective of this paper is to study the suitability of producing alumina from kaolin deposits located nearby Nawan, Al Baha, KSA by acid leaching with sulfuric and hydrochloric acids.

MATERIALS AND ExpERIMENTAL
Kaolin was provided from the kaolin deposits located about 13 km to the southeast of Nawan city, Al Baha, KSA within coordinates (19o29'10.27" N and 41o15'20.27" E) as illustrated in the location map in Fig. 1. The kaolin deposits cover an area of up to 10 km 2 where kaolin is found in sub-surface layers covered with sand and marl deposits. Kaolin is also shown in the form of a large number of prominent divergent tongues that rise several meters above the earth's surface and extend up to 300 meters. The area contains an enormous stock of whitish kaolin. The kaolin mineral nature varies from hard stone to soft mud depending on the accessory minerals. Kaolin sample was crushed, ground and screened to particles size below 90 μm. Kaolin was calcined in a muffle furnace at 850 o C for 2 h and was cooled down in the furnace to the room temperature. Two leaching agents were used, are 6 M sulfuric acid (H 2 SO 4 ) or hydrochloric acid (HCl). 10 g of calcined kaolin was added to 100 ml of the 6 M leaching agent in a 250 mL round flask with condenser previously heated at 90 o C (slurry density is 10 g per 100 ml of the leaching agent) and the slurry was stirred at 500 rpm for 3 hours. The slurry was diluted with 100 ml of distilled water as well as was filtered using a Buchner funnel and the dealuminated residue was washed with distilled water and dried at 80 o C. The filtrate was heated to near boiling followed by the addition of an excess solution of 5M NaOH, to convert alumina into NaAlO 2 to facilitate the separation of the hydroxides of iron and magnesium. After filtration, 6M HCl was added to the NaAlO 2 filtrate with stirring to adjust the pH to 7, and the aluminum hydroxide precipitate was filtered, washed with distilled water and dried at 110 o C overnight. Alumina (Al 2 O 3 ) was obtained by the calcination of aluminum hydroxide precipitate in a muffle furnace at 900 o C for 2 hours. The leaching procedure was summarized in Fig. 2. The aluminum recovery percent was determined from the weight of alumina. Leaching of Al from calcined kaolin with H 2 SO 4 and HCl at different leaching time (30,60,90, 120 and 180 min) and solid/liquid ratios (0.05, 0.10 and 0.15 g/ml) using 6 M acid solution at 90 o C were studied as the leaching system was illustrated in Figure 3.
The Al 2 O 3 recovery was calculated by using Equation (2):    Figure 5 illustrates the alumina extraction percent from calcined kaolin with HCl and H 2 SO 4 as a function of the solid/liquid ratio at different leaching times. The alumina extraction percent rapidly increases with leaching time up to 120 minutes. This is due to that as time passes, the number of acid molecules that interact with the particles of calcined kaolin increases. The alumina extraction percent then increases at a slower rate thereafter. This indicates that the leaching time should not exceed 3 hours. The alumina extraction percent rapidly increases with the solid/liquid ratio up to 0.1 g/ml then decreases thereafter. This might be attributed to that at the lower solid/liquid ratio, the amount of leaching agent is sufficient up to the solid/liquid ratio 0.1 g/ml, thereafter, the amount of leaching agent compensation to every particle decreases with increasing the amount of solid in the leaching liquor 41 . The alumina extraction percent from calcined kaolin is higher for HCl than H 2 SO 4 . It can be concluded that the size of the chloride and sulfate ions is the key factor that controls the alumina extraction percent from calcined kaolin under the same leaching conditions. According to the literature, the radius of the ion, R ion , can be defined as:  Figure 4 illustrates the thermogravimetric analysis of kaolin. The weight loss below 200°C is due to the loss of physically adsorbed water. The significant weight loss stage within the temperature range (350-700°C) and the corresponding endothermic peak and mass signal peak at 520°C are attributed to the removal of structural water in kaolin 22 . Besides, a sharp exothermic peak at 981°C is assigned to the phase transformation from metakaolinite to Al-Si spinel or the mixture of γ-alumina, amorphous silica, and mullite 39 . Clay materials contain two kinds of structural water molecules. The water molecules present in the first coordination sphere of the interlayer ions dehydroxylate in the range 300-500°C, whereas the structural hydroxyl groups dehydrate in the temperature range of 500-800°C 40 .  Where R water is an appropriate distance that characterizes the radius of a water molecule, (the average of the reported value is R water = 0.1420 ± 0.0005 nm), and dion-water is the mean intermolecular distance between the ion and water molecule 42 . Several authors determined the ionic radii in aqueous solutions by using X-ray, electron, and neutron diffraction and Monte Carlo and molecular dynamics computer simulation methods 43,44 .

RESULTS AND DISCUSSION
The reported values of the Cl-(O water) internuclear distance is of the average 0.318 ± 0.006 nm, whereas that of S-(O water) is 0.381 ± 0.007 nm 42 .
Hence, as the chloride ion is smaller in size compared to the sulfate ion, the attack of HCl molecules is more aggressive than H 2 SO 4 on the calcined kaolin particles under the similar conditions and gives the higher alumina extraction percent. This indicates that the optimum solid/liquid ratio is 0.1 g/ml.  The band at 532 cm -1 is due to Al 4+ -O-Si vibrations, where Al 4+ is in octahedral coordination 46 . The band at 465 cm -1 is responsible for the bending O-Si-O vibration 48 . The bands at 3700 and 3621 cm -1 are due to elongation vibrations of OH-groups sitting at the edges of the kaolin platelets 49 . The band at 3618 cm -1 is connected with the internal OH-groups 49 49 . The bands at 1620 and 3685 cm -1 are responsible for the bending HO-H vibrations of adsorbed water molecules 49 . The band at 1086 cm -1 is attributed to Si-O-Si asymmetric stretching vibrations 54 . This band is shifted to a lower frequency as an indication for lowering the degree of polymerization of the silica network 55 may be due to the influence of acid leaching. The FTIR spectrum of alumina, the large broadband at 400-1000 cm -1 is attributed to Al-O-Al stretching vibration arising from the distribution of alumina among the octahedral and tetrahedral sites 56 . The bands at 808, 722, and 562 cm -1 are attributed to vibration of the Al-O bonds of pseudo-boehmite structure 57 . The band at 1080 cm -1 corresponds to the Al-O-H mode of boehmite 57 . The very large band centered at 3440 and 3540 cm -1 ascribed to stretching vibration of bonded OH-groups, isolated OH-groups, and stretching vibrations of adsorbed water molecules. The band at 1614 cm -1 is due to bending of adsorbed water molecules 58 . Figure 8 illustrates the SEM micrographs of kaolin, calcined kaolin and Al 2 O 3 . The SEM micrograph of the kaolin shows hexagonal platelets of kaolinite mineral in addition to a small proportion of halloysite tubular crystals 59 . After being thermally treated, metakaolin attains its original shape. Therefore, the thermal treatment does not drastically modify the morphology 60 . The SEM micrograph of kaolin illustrates the existence of flat plate particles with sizes of around 1 μm was observed, where the flat particles stacked together. Although kaolin was calcined no changes can be observed by microscopic examination. SEM micrograph of the alumina illustrates the appearance of octahedral alumina crystals 61 .

CONCLUSION
This paper studied the suitability of kaolin deposits located nearby Nawan, Al Baha, KSA for the production of alumina by acid leaching with 6 M sulfuric acid or hydrochloric acid for 30-180 min and solid/liquid ratios 0.05-0.15 g/ml. The optimum conditions for the Al recovery from the studied kaolin include the grinding and screening of kaolin to particles size below 90 μm, calcination of kaolin at 850 o C for 2 h leaching with 6 M HCl with the solid/ liquid ratio of 0.1 g/ml for 3 h at 90 o C and 500 rpm in a reflux system. The percent of aluminum recovery from calcined kaolin under the studied conditions reaches about 75 wt%. study in the laboratories of the chemistry department, faculty of science and arts, Mukwah.