5. characteristics of surface-modified aluminum hydroxide (1)

Malaysian Journal of Chemistry, 2015, Vol. 17(1), 33–44

Red mud is an insoluble residue produced when refining of bauxite to finally obtain pure aluminum. In Vietnam, red mud is a waste from the production of alumina by Bayer process, causing an alarming threat to the environment due to its large quantity, high alkalinity and toxicity. Many research and development activities are going on throughout the world to study the dissolution mechanism of red mud [1,2]. The major composition of Tan Binh bauxite’s residue is: Fe2O3, 47.74% and Al2O3, 18.19%. Therefore, as an effective utilization of bauxite residue, red mud is widely used to recover metallic components and make adsorbents [1, 2].

Adsorbent can be made by mixing red mud in acid to remove residual sodium and increase the

activity of the surface [1, 2]. However the surface change during the acid treatment has not been clarified. Dissolution mechanism of aluminum and iron in red mud has not been studied enough. Our present research deals with the modification of solid phase surface in the dissolution process of iron oxide and aluminum hydroxide in a separate system and in the red mud system.

EXPERIMENTAL

MaterialsPure iron oxide was provided by SLGH (China). Pure aluminum hydroxide and red mud were provided by the Tan Binh Chemical Plant (Vietnam). Red mud samples were supplied by Tan Binh and Tan Rai Chemical Plants.

Characteristics of Surface-modified Aluminum Hydroxide, Iron Oxide and Red Mud in Dissolution Process

T. M. H. Le1*, V. T. Nguyen1, S. T. Dong2 and T. D. Nguyen3

1Institute of Natural Products Chemistry, Vietnam Academy of Science and Technology2Saint Anselm College, 100 Saint Anselm Drive, Manchester, New Hampshire, 03102, USA

3Institute for Tropical Technology, Vietnam Academy of Science and Technology*Corresponding author (e-mail: [email protected])

Aluminum hydroxide (gibbsite) and iron oxide (hematite) are the main components of natural ores and industrial products. They are the basic raw materials for the production of aluminum and steel. Thermodynamic methods were used to study the modification of iron(III) oxide and aluminum hydroxide surfaces at different rates and conditions of dissolution. Instrumentation utilized for this study were X-ray diffraction, field emission scanning electron microscope and Brunauer, Emmett and Teller (BET) surface area analysis. Data from modified samples were evaluated using these methods for source variation with encouraging results. Experimental results indicated that after dissolving in acid, iron oxide had cracks and grooves on its surface but aluminum hydroxide’s surface had no traces. The surface-modified phenomenon was recorded in the process of making adsorbent from red mud generated from alumina production by Bayer process at Tan Binh and Tan Rai Chemical Plants. BET analysis showed that the specific surface area of red mud at Tan Binh and Tan Rai Chemical Plants dissolved in acid at a dissolution rate of 10% which increased by 50% and 90%, respectively.

Key words: Aluminum hydroxide; iron oxide; dissolution; separation; solid phase surface; adsorbent; red mud

Received: November 2014; Accepted: May 2015

Song Cai

Note

34 Le, T.M.H. et al. Characteristics of Surface-modified Aluminum Hydroxide, Iron Oxide and Red Mud in Dissolution Process

InstrumentationThe structure of untreated sample was analyzed by Siemens D5000 X-ray Powder Diffraction. SEM images were captured with an Hitachi S-4800 field emission scanning electron microscope and Brunauer, Emett and Teller (BET) results were measured with a Micromeritics TriStar 3000.

ProcedureParticle size of all samples ranged from 0.041 to 0.22 mm. Approximately 20 g of each sample was added to the reaction system. Red mud samples were dried at 70°C for 5 h and sieved to break down the adhesion between particles.

Dissolution process was carried out in a 250 ml spherical flask in a bain-marie heated at 90°C and stirred by an agitator at a stirring rate of 480 rpm. To examine the effect of dissolution rate on surface change, approximately 20 g of iron(III) oxide and aluminum hydroxide samples were dissolved in 100 ml sulphuric acid at different concentrations from 0.2 M to 2.2 M to dissolve

samples at dissolution rates of 10%, 20%, 30%, 40% and 50%. (This is done by taking 10%, 20%, 30% .40%, 50% of the acid that is necessary to disolution the whole of the aluminum and iron in red mud). The system was heated to boil at 98°C. Samples were diluted in purified water to 100 ml to fix the ratio of liquid to solid at 5:1 (100 ml solution: 20 g sample).

The mixture was allowed to react for 2 h; at which point the solution was filtered. Experimental dissolution rates were re-examined. Iron and aluminum contents in the leach solutions were determined complexometrically by EDTA titration. The solid residue was dried at 70°C for 5 h and SEM and BET measurements were performed on it.

RESULTS AND DISCUSSION

Dissolution ReactionsRed mud was digested with sulphuric acid and the reactions in the medium were as follow:

2Al(OH)3 + 3H2SO4 → Al2(SO4)3 + 6H2O (1)

Fe2O3 + 3H2SO4 → Fe2(SO4)3 +3H2O (2)

Modification of Iron(III) Oxide’s Solid Phase Surface in Dissolution Process

Analysis of structure and dissolution mecha-nism of Fe2O3 in sulphuric acid. By taking the samples’ X-Ray diffraction (Figure 1) and com-paring its result with the standard diffraction, the result showed that the structure of the sample contained 96.21% hermatite. Hematite is by far the most common form of iron oxides, having important magnetic properties. In hematite structure shown in Figure 2, oxygen ions are hexagonally close packed, and iron is present only in octahedral sites. Dissolution rates for hematite in H2SO4 have been reported to decrease for acid

concentrations above 7 M. Hematite structure is dense, having a density of 5.26 g/cm3 and does not contain cation vacancies. The high density along with the occurrence of iron atoms solely in octahedral sites may be responsible for the slow dissolution of hematite in acid [4].

Iron oxide sample was prepared and dissolved in sulphuric acid at different dissolution rates as described in the experimental section. The diluted solution was titrated to determine the excess quantity of acid and iron. The result was used to evaluate the experimental dissolution level. The dissolution level is described in Table 1.


35 Le, T.M.H. et al.

20 50 60 70402-Theta-Scale

30

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Figure 1. X-ray diffraction of iron oxide.

Figure 2. Structure of hematite [4].


In the dissolution process, reaction rate changes if Fe2+ ions exist in the solution. The reaction rate of the system is described in Equation 3 [5].

This paper focuses only on explaining the principle of solid phase surface, not the electrochemical processes that occurrs in the dissolution process.

Wi = K[H]m[Fe(II)](1–β+) z+[Fe(III)](–1–β+) z+ Ʃn = qn = q (Kred)n[A–]n

(3)Ʃi = q

i = q (Kox)i[A–]i

Although the oxide particles in untreated system were not distributed uniformly in terms of size and geometry, they link together (Figure 3A). At a dissolution rate of 10%, the morphological variation of particle structure was insignificant (Figure 3B). The reaction happened mainly at separate surface between particles which destroy- ed the connection between particles to create blocks. The variation happened at small points on the surface of hematite particle. As the dissolution rate increased, the links between particles were fully broken. This phenomenon led to a rapid formation of small-sized particles. On the surface of larger-sized particles, grooves were formed. This phenomenon happened due to the fact that the structure of hematite consisted of hcp arrays of oxygen ions stacked along the [001] direction. Two thirds of the sites were filled with Fe3+ ions; therefore, H+ ions could easily react with iron atoms [5]. However, once the surface links on large-sized oxide particles cracked, many small particles could be formed. When the dissolution rate reached 20% (Figure 3C), a strong surface growth of solid phase happened. When the dissolution rates were increased gradually to 30% (Figure 3D), 40% (Figure 3E) and 50% (Figure 3F), the formation of smaller-sized oxide particles limited the ability to create grooves and cracks on particle’s surface. Therefore, the specific surface area of solid phase decreased as represented in the BET measurement results (Table 2).

SEM and BET analysis of samples at different dissolution rates indicated that the sample which dissolved 20% in sulphuric acid had the largest specific area. The result is shown in Figure 4. At this dissolution rate, cracks were formed on the surface of large oxide particles. Since the reaction on small particles and particles that separate off the link did not take place completely, the quantity of small particles did not decrease considerably. At higher dissolution rates (20%, 30%, 40%, and 50%), the specific surface area of iron oxide’s solid phase started decreasing.

Modification of aluminum hydroxide’s solid phase surface in dissolution process. By taking the samples’ X-Ray diffraction (Figure 5) and comparing its result with the standard diffraction, the result showed that the structure of the sample contained 79.72% gibbsite. The schematic X-ray diffraction of the aluminum hydroxide sample (Figure 5) showed that the main component of aluminum hydroxide was gibbsite. Gibbsite has layered structure. The layers of gibbsite are connected by weak hydrogen bonds (Figure 6). Each layer consists of two planes of approximately close-packed oxygen, enclosing a plane of aluminium. Aluminum ions occupy two-thirds of the octahedral holes between the two layers [6].

Gibbsite dissolution rate is very high in sulphuric acid compared to its dissolution rate in

Table 1. Experimental dissolution level of hematite samples.

Dissolution level Hematite sampleSample — expected dissolution rate Fe3+ – 10% Fe3+ – 20% Fe3+ – 30% Fe3+ – 40% Fe3+ – 50%Experimental dissolution rate (%) 9.64 19.42 29.34 39.32 49.40



Table 2. Surface area of hematite samples at different dissolution rates

Dissolution level Hematite sampleSample — expected

dissolution rateUntreated Fe2O3 Fe3+ – 10% Fe3+ – 20% Fe3+ – 30% Fe3+ – 40% Fe3+ – 50%

Surface Area (m2/g) 3.8813 4.3718 4.9394 4.1314 3.8623 3.6892

A B

C D

E F

Figure 3. A: SEM images of untreated hematite sample; (B), (C), (D), (E), (F): SEM images of hematite at dissolution rate of 10%, 20%, 30%, 40% and 50%, respectively.


Figure 4. SEM image of hematite dissolved in 20% sulphuric acid at 10 000x magnification.

100011001200130014001500

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Figure 5. X-ray diffraction of aluminum hydroxide.



other acids such as perchloric acid, hydrochloric acid [9]. The dissolution of gibbsite occurs on the edges of unit layers rather than on basal faces [10]. This is due to the unstable bonding of OH groups on the edges of the unit layer [11].

Aluminum hydroxide sample was prepared and dissolved in sulphuric acid at different dissolution rates as described in the experimental section. Diluted solution was titrated to determine the excess quantity of acid and aluminum. The result was used to evaluate the experimental dissolution level (Table 3).

The following results were inferred from SEM images of solid phase of aluminum hydroxide in dissolution process:

● The structure of gibbsite is hexagonal crystal and stacked. The corner and edge of aluminum hydroxide could be observed in Figure 7A. With this structure, the reaction between Al(OH)3 and sulphuric acid in dissolution process occured mainly at the angular position of this structure, not on the surface like in the reaction between Fe2O3 and sulphuric acid. At dissolution

Al atoms

OH– lower layer

OH– upper layer

b

a

Figure 6. Structure of Gibbsite [4, 5].

Table 3. Experimental dissolution level of aluminum hydroxide.

Dissolution rate AluminiumSample — expected dissolution rate Al3+ – 10% Al3+ – 20% Al3+ – 30% Al3+ – 40% Al3+ – 50%Experimental dissolution rate (%) 9.83 19.70 29.10 39.72 49.70


rate of 10%, reaction between Al(OH)3 and acid occured at angular positions, ma-king the surface of the crystal to change significantly (Figure 7B). The angles and corners were flattened. When the dissolution rate reached 20% (Figure 7C), this process occured more intensely. The separation

between layers could be observed on the surface of the crystal.

● When dissolution rate reached 30% (Figure 7D) and 40% (Figure 7E), layers in the crystal structure were divided at more frequent rates. Along with the edges and

A B

C D

E F

Figure 7. A: SEM image of untreated gibbsite sample; (B), (C), (D), (E), (F): SEM images of gibbsite at dissolution rates of 10%, 20%, 30%, 40% and 50%, respectively.



corners of the crystals, other positions on the surface of the particles began to react. These additional positions resulted in the formation of capillaries in hole form. However, if the level of dissolution kept increasing, this phenomenon stopped taking place. BET results in Table 5 showed that Al(OH)3 samples which dissolved 30% in sulphuric acid had the largest specific surface area. Specific surface area of Al(OH)3 was reported to decrease at dissolution rate above 30%.

In single systems, the modification of solid phase followed a general rule. The specific surface area of solid phase surface increased until the solid phase particles reacted so intensely that the particle size started shrinking. The particle surface was not large enough to form cracks or layers, resulting in a decrease in specific surface area.

Modification of Red Mud’s Solid Phase Surface in Dissolution Process

Dissolution of aluminum hydroxide and iron(III) oxide in red mud system. According to Figure 8 and database, the main components of red mud are gibbsite and hematite. Red mud samples

underwent dissolution process at dissolution levels of 10%, 20% and 30% with the same conditions as aluminum hydroxide and iron(III) oxide. Compositions of the solution and of the solid phase after dissolution are shown in Table 4 and Figure 9. There is competition in terms of speed between components in red mud system. The reaction rates of gibbsite and hematite with acid are different, making the ratio between Al3+ and Fe3+ in the residue varies. Also, due to the difference in reaction rate, the growth of red mud particles surface does not follow the rules applied for separated system [12]. Especially, layered separation of aluminum cannot be observed due to the tiny size of gibbsite particle and its equal dispersion into the solid phase of red mud particles.

Modification of red mud particles in dissolution process. BET results of red mud samples at different dissolution rate are shown in Table 5.

According to SEM images and BET measurement results, Tan Binh red mud samples dissolved at dissolution rates of 10% and 20% with a strong growth on surface properties. For Tan Rai red mud, the samples dissolved at 10% and had a strong growth on surface properties. This

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Figure 8. X-ray diffraction of Tan Binh red mud [11].


Table 4. Dissolution efficiency and solid phase composition of Tan Binh red mud in.

Sample — expected dissolution rate

Dissolution efficiency Composition of solid phaseFe3+ (%) Al3+ (%) Fe2O3 (%) Al2O3 (%)

Untreated red mud – – 47.73 18.19Red mud – 10% 0.35 21.24 42.68 27.2Red mud – 20% 1.9 31.88 39.58 24.93Red mud – 30% 13.07 79.6 19.13 12.08

Table 5. Specific surface area of red mud at different dissolution rates.

Red mud — disolution rate

Specific surface area of Tan Binh red mud (m2/g)

Specific surface area of Tan Rai red mud (m2/g)

Red mud – 0% 105.4 54.68Red mud – 10% 154.7 100.91Red mud – 20% 150.8 95.59Red mud – 30% 142.5 91.56Red mud – 40% 137.1 85.50

A B

C

Figure 9. SEM images of red mud images at dissolution rate of (A) 10%, (B) 20%, and (C) 30%.



observation demonstrated the ability to compete between different components to react with acid. Red mud surface formed capillaries without causing debris or corrosion. The reaction rate with sulphuric acid of [Al3+] was faster than that of [Fe3+] ion. Therefore, in solid phase of red mud after dissolution process, the surface properties developed and the ratio between aluminum and iron varied as the composition of iron in the residue increased. Thanks to these characteristics; red mud particles modified by this method have good adsorption abilities [13, 14].

Applications of adsorbents made from red mud. To test the absorption capacity of the above Tan Binh red mud experiments were conducted with ion Ni(II) and As(III, V) from the substance NiSO4 and As2O3 respectively [13]. The experiments were carried out in room temperature conditions with 1 g volume of adsorbent, pH of 6–7 for Ni(II) and As(III,V). Adsorption time was 2 h and the concentration of adsorbed solutions was checked by standard colorimetric method. The absorption capacity results of red mud are shown in Table 6.

The development of aluminum hydroxide and iron(III) oxide’s specific surface when dissolving in sulphuric acid at different dissolution rates provides valuable information about surface growth within dissolution process. From the law of surface variation, we aim to continue testing to find out the optimum conditions for making adsorbents from red mud and other minerals containing aluminum hydroxide and iron oxide.

CONCLUSIONIn red mud system, dissolution of Al(OH)3 in sulphuric acid happened in the form of layer separation. When iron (III) oxide was dissolved in acid, the solid phase surface had grooves and

cracks in all directions. In single systems of Al3+ and Fe3+, when the surface growth increased along with the increase in dissolution rate to a limiting value, there was a formation of smaller particles. The solid surface could not be separated into layers and grooves, resulting in a decrease in specific areas.

The modification of red mud surface within the dissolution process did not follow the principles applied for single systems due to the differences in the level of competition and reaction rate. There was a development in specific surface of particles for solid phase after the sample was dissolved. The development of this surface characteristics enabled dissolved red mud particles to have typical features of adsorbent. Specific surface area of Tan Binh and Tan Rai red mud dissolved in acid at dissolution rate of 10% increased by 50% and 90%, respectively. However, to confirm adsorption capacity, more research needs to be conducted on the characteristics of electrical properties of solid phase surface.

The results of research on red mud confirmed the potential of applying dissolution process for the manufacture of adsorbent from red mud and the recycling of red mud to reduce environmental pollution. Also, red mud can be integrated with the manufacture of flocculants with the secondary wastes treatment process [13].

ACKNOWLEDGEMENTSThe authors would like to express their sincere appreciation to the project “Processing Techno-logy of Red Mud from Taynguyen Bauxite” code named 05/HD-DT.13/CNMT of the programme on scientific research, application and technology transfer to develop the environmental industry (Ministry of Industry and Trade of the Socialist Republic of Vietnam).

Table 6. Ion adsorption capacity of adsorbents made from red mud [13].

IonAdsorption Capacity (mg/g)

Tan Binh red mud – 10% dissolved Tan Binh red mud – 30% dissolvedNi2+ 19.2 35.8

As (III, V) 0 0.79


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5. characteristics of surface-modified aluminum hydroxide (1)

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