A new green process to produce activated alumina by spray pyrolysis

1 Introduction

High-alumina fly ash [1] is a type of industrial waste discharged from the high-alumina coal burning process employed in many electric power plants. The piling and storing treatment of high-alumina fly ash leads to the occupancy of vast lands [2] and the serious pollution of soil, air, and water. At present, China has over 50 billion tons of high-alumina coal reserves, from which 14 billion tons of high-alumina fly ash (alumina content >40%) are produced. The total bauxite resources in China, which are mostly low-grade (alumina content >45%), are 3.8 billion tons. The reserve of alumina within the high-alumina fly ash is more than thrice of that within the bauxite resources in China. The alumina content of the high-alumina fly ash is over 40 wt% and equivalent to the A1₂O₃ content of low-grade bauxite [3], indicating that the high-alumina fly ash is a promising resource for the production of alumina [4]. The comprehensive utilization of coal fly ash has been studied since the 1970s [5] when the problems of energy crisis, resource exhaustion, and environmental contamination garnered public attention throughout the world [6]. Alkali sintering and acid leaching methods using the high-alumina fly ash as raw material have been proven to be effective for alumina production [7, 8]. The alkali process requires high energy consumption [9] and produces more solid wastes. In the traditional acid process, the AlCl₃ solution has been treated by NaOH or NH₃·H₂O to obtain Al(OH)₃, and then decomposed into Al₂O₃ by using the roasting process. However, the problems of water and acid recycling have limited the development of this method.

Currently, activated alumina is considered one of the most useful oxides [10] and has been used in many engineering applications, such as adsorbents [11], electrical materials [12], plasma spray coatings [13], wear-resistant composites [14], and catalyst support [15]. This is because activated alumina has a large surface area, pore size, and pore volume. The annual demand for activated alumina has grown steadily. In the traditional production process, the activated alumina is obtained by heating and the partial dehydration of Al(OH)₃. However, the presence of the monohydrate results in a low specific surface rating to active the product, and the removal of the monohydrate is impractical [16].

Spray pyrolysis is a key area in the development of science and engineering. Compared with the conventional processes, spray pyrolysis [17] can greatly improve the factory production capacity by making the process shorter and by reducing the waste residue, water, acid, and alkali emissions. Spray pyrolysis has been widely researched in the area of oxide powder production [18], such as γ-Ga₂O₃-Al₂O₃ solids [19], Al₂O₃ nanoparticles [20], carbon nanowalls [21], tungsten oxide thin films [22], and alumina thin films [23]. Through the spray pyrolysis method, the composition and particle size can be easily controlled. However, few studies have been done on the preparation of activated alumina by using the spray pyrolysis technology. In order to solve the water and acid recycling problems during the alumina production process with acid leaching [24, 25], a spray pyrolysis method was presented in this research with AlCl₃ solution as a raw material to prepare the activated alumina.

In this paper, the AlCl₃ solutions were made from pure chemicals in order to verify the feasibility of the experiment. The properties of the activated alumina prepared by the proposed spray pyrolysis were investigated by the phase and apparent morphology analyses. The effects of the pyrolysis parameters on the reaction efficiency of activated alumina preparation from the AlCl₃ solution were also experimentally studied.

2 Materials and methods

In this paper, AlCl₃ (Sinopharm Chemical Reagent Co., Ltd., China) was used as the starting material for basic research, with water as the solvent. The tube furnace was connected to a programmable temperature controller, which was capable of controlling the heating ratio and the target temperature. Each AlCl₃ solution (10, 20, and 30 wt%) was placed in the solution tank, which was kept at room temperature. Nitrogen was used as a carrier gas for the AlCl₃ solutions of 0.1, 0.2, and 0.3 MPa. The AlCl₃ solutions were then fed into the reactor by the pressure effect of flowing air through the flow control valve. The spray time was 10 min, and the sample sintering time was 30 min. On completion, the conversion of alumina, the ratio of the alumina content in the activated alumina to that in the activated alumina and alumina chloride, was calculated by measuring the chloride content using the titrimetric method. For the combinational effect trial, an L₉(3⁴) orthogonal test was designed with the following parameters: spray temperatures (800, 900 and 1000°C), carrier gas pressures (0.1, 0.2 and 0.3 Mpa), and AlCl₃ solution concentrations of (10, 20, and 30 wt%) (Table 1). These factors and their corresponding levels are shown in Table 1. The orthogonal table with four factors and three levels is shown in Table 2. According to the results of the orthogonal test, the temperature single trials were also conducted to investigate the influence of the temperature on the products during the spray pyrolysis process.

In order to study the results from the orthogonal tests, the range analysis was utilized in this study. Range analysis is a statistical method to determine the factors’ sensitivity to the experimental results according to the orthogonal experiment. Here, “range” is defined as the distance between the extreme values of the data. The greater the range is, the more sensitive the factor is. The calculation processes of the range analyses are, respectively shown in Eqs. (1–4)

where I_Xm stands for the sum of the experimental results containing the factor X with the m level, K¯Xm stands for the average value of the experimental results containing the factor X with the m level, and R_X stands for the influence degree of the factor X. The influence degree of X is great when R_X value is high.

The variance analysis was also utilized in this study. Variance analysis is a statistical method to determine the factors’ significance to the experimental results according to the orthogonal experiment. The calculation processes of the variance analyses are, respectively shown in Eqs. (5–8)

where m stands for the number of factor level, k stands for the number of experiment repetitions, T stands for the sum of the experimental results, SS stands for the mean sum of squares, DF stands for the degree of freedom, MS stands for the mean square, and F stands for the ratio of MS_X and MS_D.

Next, the phase composition was determined by using the PW3040/60 diffraction instrument with Cu K_α radiation. The X-ray diffraction (XRD) device is from Philips, Netherlands. The microscopic morphology was observed by a scanning electron microscope (SEM) (SU-8000, Japan). The BET specific surface areas of the products under different conditions were carried out by using a Specific Surface Area Analyzer (Model ASAP2020, US).

3 Results and discussion

3.2 Characterization of the activated alumina products in different temperatures

Results in Figure 1 indicate that the main diffraction peaks of standard γ-Al₂O₃ appear at crystal face (311), (400) and (440) [27]. When the pyrolysis temperature of the AlCl₃ crystal reached 900°C, the characteristic peaks of the α-Al₂O₃ appear at the crystal faces (104), (110), (006) and (202). This is because 900°C is the crystal phase transition temperature of the alumina. Given that the γ-Al₂O₃ is the main phase of the activated alumina, the optimal pyrolysis temperature should be lower than 1000°C.

Figure 1:

XRD patterns of the alumina products from the AlCl₃ solution generated by the spray pyrolysis process.

Table 5 lists the nanoparticles sizes calculated with XRD data. The particle sizes were measured by using the Scherrer equation for XRD. The Scherrer formula is given by

Table 5:

Analysis of XRD patterns of alumina products.

Temperature (°C)	hkl	2θ (°)	H	FWHM (°)	Crystallite size (nm)
700	440	67.1133	232.44	0.7680	8.8304
800	440	67.3963	857.72	0.2880	23.4488
900	440	66.5512	564.44	0.3419	12.3027
1000	440	67.1978	529.35	0.6298	10.7384

(9)D=(0.9λ)/(β cosθ),

where D stands for the apparent particle size, β stands for the full-width half-maximum of the XRD line (additional peak broadening) in radians, λ stands for the wavelength used, and θ stands for half the scattering angle. The constant 0.9 in Eq. (9) depends slightly on the symmetry of the crystal.

The calculations showed that the γ-Al₂O₃ and the α-Al₂O₃ had the average grain sizes of about 13 and 12 nm, respectively. Hence, the crystal phase transition of the γ-Al₂O₃ and α-Al₂O₃ has little effect on the activated alumina granularity.

SEM images show that the activated alumina at the low-temperature sintering of 700°C has a lamellar structure (Figure 2A), but the products at other temperatures have a granular structure. In the spray pyrolysis process, the solid reactants precipitate first from the liquid phase and then transformed into round particles. With rising temperature, the pyrolysis products become irregular and result in rough surfaces. However, the formation of the lamellar structure is not observed. These results confirm that sintering under increasing temperature changes the product structure from a lamellar to an irregular structure with rough surfaces.

Figure 2:

SEM images of the pyrolysis products (A) 700°C, (B) 800°C, (C) 900°C, (D) 1000°C.

The specific surface areas of the products under different temperatures range to 94–52.4 m²/g, the pore sizes range to 12.12–17.78 nm, and the pore volumes range to 0.2655–0.2128 cm³/g. The specific surface areas of the alumina products decrease as the pyrolysis temperatures increases, which is consistent with the SEM results (Figure 2). In the spray pyrolysis process, due to the special properties of the AlCl₃ and the limited liquid surface tension, the specific surface areas of the products are less than that of the common activated alumina (>100 m²/g). The specific surface areas, pore sizes, andpore volumes of the products meet the requirements of the catalyst carrier, which are high purity (>92%), γ-Al₂O₃, big pore size (>7nm), and large pore volume (>0.1 cm³/g).

Based on the above results, the N₂ adsorption-desorption isotherms of the pyrolysis products at 700°C and 900°C were investigated by using the Specific Surface Area Analyzer (Figure 3). From Figure 3, we can see that the adsorption volume of the pyrolysis products increases rapidly when the relative pressure exceeds 0.5. A significant hysteresis loop exists because the capillary condensation, adsorption isotherm, and desorption isotherm do not coincide. According to the classification of the adsorption-desorption isotherms by BDDT (Brunauer S., Deming L. S., Deming W. E. and Teller E classified the adsorption isotherms into five categories, called BDDT classification), the isotherm line belongs to the 4th category. The results show that the micropores and mesopores are distributed throughout the particles of the pyrolysis products. In addition, according to deBoer’s classification of the hysteresis loops, when the relative pressure is 0.9, the hysteresis speed of the desorption line in Figure 3B changes significantly, and when the relative pressure is 0.5, the desorption curve shown in Figure 3A has an inflection point as compared with that in Figure 3B. Both figures show that two pyrolysis products present different hysteresis loops. The hysteresis loop of the product produced at a temperature of 700°C is, therefore, classified as a hysteresis loop type H2 [28] with inkbottle-shaped and parallel-plate pores in the spray pyrolysis product, which coincides with the SEM results. The hysteresis loop of the product in 900°C is classified as type H3, which is associated with the slit-shaped pores. The results also show that with the increase of temperature, the spray pyrolysis technology can improve the specific surface area and increase the pore size of the activated alumina. These findings demonstrate the viability of the proposed process for the production of large pore-sized activated alumina adsorbents.

Figure 3:

N₂ sorption isotherm curves of the alumina products (A) 700°C, (B) 900°C.

4 Conclusions

The results of this study showed that temperature is the most influential factor on the alumina conversion ratio during the spray pyrolysis process. The conversion ratio, which was much higher than that achieved by other alumina production methods, reached above 99% at the following optimal conditions: of temperature of 1000°C, pressure of 0.3 MPa, and AlCl₃ solution concentration of 10 wt%. According to the mineral phase analysis, the as-sprayed nanoparticle activated alumina consists of the γ-Al₂O₃ and α-Al₂O₃, and the γ-Al₂O₃ is the main phase of the activated alumina. The particles of the pyrolysis powders also show a granular structure. The characterizations of the products meet the requirement of catalyst carrier. Finally, the green utilization of high-alumina fly ash and acid recycling can be achieved by using this process.