Synthesis of Magnesium Aluminate Spinel Powder from the Purified Sodium Hydroxide Leaching Solution of Black Dross

Abstract

Synthesis of magnesium aluminate spinel (MgAl₂O₄) was investigated by employing ball milling and co-precipitation methods. The starting materials (aluminum hydroxides) were obtained from the purified sodium hydroxide leaching solution of black dross. The characteristics of the synthesized spinel was analyzed through X-ray diffraction (XRD), scanning electron microscopy (SEM) images. In this work, the effect of calcination temperature and time on the formation of spinel by the two methods was compared. Calcination temperature showed a great effect on the formation of spinel in both methods. The results showed that the co-precipitation method has many advantages over the ball milling method. In ball milling method, complete conversion of the starting materials to spinel was impossible even at 1500 °C, while complete conversion to spinel was accomplished at 1000 °C for 5 h by the co-precipitation method. The average size of the spinel synthesized at these optimum conditions of the co-precipitation method was about 17 nm. A process can be developed to synthesize spinel from the black dross which is regarded as hazardous materials.

1. Introduction

Aluminum dross contains alumina, metal oxides and nitrides and some kind of salts [1]. Black dross results from the re-melting of used aluminum cans and is a valuable resource for alumina. Black dross is categorized as a hazardous material owing to the evolution of toxic gases when in contact with moisture. In hydrometallurgical processes for the recovery of alumina from black dross, alkaline leaching leads to purer aluminate (III) solution than acid leaching [2,3].

In our previous work, aluminate solution with purity higher than 99.9% was obtained by removing a small amount of silicate(IV) from the NaOH leaching solution of mechanically activated black dross [4]. In order to recover alumina or other compounds from this purified sodium aluminate solution, crystallization of aluminum hydroxide is necessary and precipitation with hydrogen peroxide was investigated to obtain aluminum hydroxide [5,6]. Magnesium aluminate spinel which is the only stable compound in the MgO-Al₂O₃ system can also be formed from this aluminum hydroxide [7]. Magnesium aluminate spinel has some excellent properties, such as high melting point (2135 °C), chemical inertness, excellent mechanical strength at high temperatures, high thermal shock resistance, and low density [8,9]. These properties make the MgAl₂O₄ an excellent refractory material. The properties of alumina and spinel depend on those of raw aluminum hydroxides. Due to the aforementioned issues, the employment of appropriate starting materials is of particular importance in synthesizing spinel.

Several methods have been employed to synthesize spinel powder, including solid-state reaction [10], wet chemical methods like the precipitation [11], the aerosol method [12], sol-gel of double or semi-alkoxides [13], organic gel-assisted citrate method [14]. The preparation of spinel powders by solid-state reaction requires higher calcination temperature and prolonged reaction time compared to the wet chemical method. However, more experimental steps should be employed in wet chemical methods.

The purity and chemical homogeneity of starting alumina powder also affect the properties of the synthesized materials. We have reported the recovery of alumina from black dross [5]. Until now, few works have been reported on the synthesis of spinel from alumina which has been recovered from black dross [15,16]. This motivated the idea of investigating the synthesis of spinel by employing alumina/aluminum hydroxide which has been recovered from the purified leaching solution of mechanically activated black dross. In this study, solid-state and wet methods were employed to synthesize spinel precursor by employing the recovered alumina. Ball milling was selected for solid-state method, while co-precipitation was selected as wet method. The effect of synthetic conditions (reaction temperature and time) on the thermal evolution and structural features of the spinel was compared between these two methods. The co-precipitation method showed several advantages over ball milling method in terms of reaction temperature and the physical properties of thus synthesized spinel powders. A whole process can be developed to synthesize spinel from the black dross, which will add much value to the economics of the process.

2. Materials and Methods

2.1. Materials

The aluminum hydroxide powders were obtained from the precipitation of sodium aluminate solution by using hydrogen peroxide (Daejung Chemicals and Metals Co., Ltd., Shiheung, Korea, 30%) at the following conditions: 20% H₂O₂, the volume ratio of H₂O₂ to leaching liquor: 1.5, 20 °C, 3 h, 400 rpm. The precipitated Al(OH)₃ was calcined at 1200 °C for 5 h to obtain alumina (Al₂O₃) [5]. Magnesium oxide powder (MgO) (Daejung Chemicals and Metals Co., Ltd., Shiheung, Korea, 96%), and hydromagnesite (Mg₅(CO₃)₄(OH)₂·4H₂O) (98% purity, Aldrich) were used as starting materials for magnesium source.

2.2. Procedure

2.2.1. Formation of Magnesium Aluminate Spinel by Ball Milling

An equimolar amount of hydromagnesite and alumina powders was mixed to synthesize single-phase spinel. The powder mixture was mechanically activated in a planetary ball mill (Fritsch Pulverisette 7 Bead Mill, Fritsch, Idar-Oberstein, Germany) under ambient conditions. The powder mixture was prepared as follows: the mixture of 2 g alumina and 2 g hydromagnesite was added into a vessel with 40 g agate balls (a ball of 6 mm in diameter). The weight ratio of ball to powder mixture was controlled to 10 and the mixture was milled for 1 h at a rotation speed of 500 rpm. Heat treatment of the ball-milled powders was carried out under air atmosphere at different temperature, time.

2.2.2. Formation of Magnesium Aluminate Spinel by Co-Precipitation in Acid

Aluminum hydroxide employed in this work was obtained by precipitation from the purified leaching solution of black dross by H₂O₂. The precipitated aluminum hydroxides were separated by vacuum filtration, washed several times with warm deionized water, till the filtrate became neutral to litmus paper, and then dried at 60 °C for 48 h.

First, aluminum hydroxide powder was dissolved in aqua regia which was diluted 4 times for 6 h at 90 °C. MgO powder was added in a 2: 1 molar ratio of Al to Mg. Further refluxing was carried out for 2 h to make the mixed solution homogeneous. The co-precipitated spinel precursors were stirred at 25 °C and 300 rpm. Then, the pH of the solution was kept at 8.5 by controlled addition of 2 M NH₄OH solution. The precipitates were filtered off and washed with de-ionized water and dried at 70 °C for 48 h under air atmosphere.

2.2.3. Apparatus

For the production of magnesium aluminate spinel, powders were calcined in an SX-GD7123 muffle furnace (MF-32GH, Jeio tech, Korea). The characteristics of the magnesium aluminate spinel was investigated by X-ray diffraction (XRD, D8 Advance (Bruker AXS, Karlsruhe, Germany), analytical Field Emission scanning electron microscopy (FE-SEM, S-4800, Hitachi, Tokyo, Japan). The XRD peaks of precursor and calcinated samples were obtained from the database of XRD program high plus PANalytical. The average crystal size was estimated by the full width at half maximum (FWHM) of peaks in XRD data. The FWHM was determined by employing Origin Pro 9.0 software (Originlab, MA, USA, 2012).

3. Results

3.1. Formation of Magnesium Aluminate Spinel by Ball Milling

3.1.1. Effect of Temperature

The solid-state method is a thermally activated process and thus enough energy needs to be supplied to overcome the activation energy of the process. Therefore, the effect of calcination temperature on the synthesis of spinel from the mixture of Al₂O₃ and Mg₅(CO₃)₄(OH)₂⋅4H₂O by ball milling method was investigated. For this purpose, the ball milled powders were calcined at various temperature (600–1500 °C) for 5 h. Figure 1 shows that the XRD pattern of the combustion product calcined at 600 and 800 °C corresponded to those of alumina and hydromagnesite. Peaks corresponding to MgAl₂O₄ spinel started to appear in the XRD when calcination temperature was 1000 °C. However, well-defined peaks for the spinel phase was observed in the XRD when calcination temperature was higher than 1200 °C. This is in good agreement with the reported data [17,18]. The average crystallite size (d) of the samples prepared at different calcination temperatures was estimated from XRD peak broadening using Equation (1) [19].

$$d=\frac{0.9 \lambda }{\beta cos\theta }$$

(1)

where λ is the radiation wavelength, β and θ represent the full width at half maximum value and Bragg’s angle in radian, respectively. The fill widths at half maximum of the different patterns at 2θ = 36.88 were from 0.5404 to 0.2204 when calcination temperature was from 1000 to 1500 °C. This data indicates that calcination temperature affects the reactivity of the magnesium aluminate spinel.

The influence of the calcination temperature on the average crystallite sizes of spinel is described in

Table 1. Variation in the average crystallite size of magnesium aluminate spinel with calcination temperature.

Calcination Temperature (°C)	Average Crystallite Size (nm)
1000	15
1200	24
1500	38

. The average crystallite size increased from 15 to 38 nm as the temperature increased from 1000 to 1500 °C. It might be attributed to the fact that sintering at higher temperature increases the crystallite size, which is in good agreement with the reported data [20,21]. The SEM of the photograph of the combustion product shows the presence of agglomerates of fine primary particles (see Figure 2).

In this case, magnesium aluminate spinel is formed by mass transfer between MgO and Al₂O₃. After the spinel was initially formed, subsequent growth of the spinel becomes more difficult because the two reactants (MgO and Al₂O₃) should move by diffusion through the spinel layer [22], as shown in Figure 3. Besides that, at lower temperatures, surface diffusion of alumina is predominant but bulk diffusion becomes predominant at higher temperatures. Therefore, it is difficult to synthesize spinel at a lower calcination temperature at which bulk diffusion rate is slow [23].

3.1.2. Effect of Reaction Time during Calcination

The effect of calcination time was investigated at 1500 °C for various time (1, 3, 5, 7, 10 h). In our experimental range, the calcination time did not show any effect on the formation of the combustion product calcined (magnesium aluminate spinel) as shown in Figure 4. The results show that the crystalline phase of spinel (MgAl₂O₄) is dominant. However, there still are other phases like MgO and Al₂O₃, indicating that ball milling followed by calcination would not lead to complete conversion to spinel in our experimental conditions.

3.2. Formation of Magnesium Aluminate Spinel by Co-Precipitation in Acid

3.2.1. Effect of Temperature

Aluminum hydroxide employed in this work was obtained by precipitation from the purified leaching solution of black dross by H₂O₂. The precipitated aluminum hydroxides were separated by vacuum filtration, washed several times with warm deionized water, till the filtrate became neutral to litmus paper, and then dried at 60 °C for 48 h.

To investigate the influence of calcination temperature on the size and structure of magnesium aluminate spinel, powders were calcined at different temperature. The X-ray diffraction patterns of as-prepared and calcined spinel at different calcination temperatures for 5 h are depicted in Figure 5. Figure 5 shows a typical pattern of the precursor and the combustion product calcined at 600, 800, 1000 °C. The precursor was amorphous in nature. During the calcination, Gibbsite (Al(OH)₃), MgO, and Nitroamine (H₂NNO₂) were crystallized and characteristic peaks were observed in XRD pattern. Further increase in the calcination temperature (600–1000 °C) sharpens the diffraction peaks of spinel owing to an increase in the amount and crystallinity of the spinel phase. At these conditions, Al₂O₃ and MgO completely disappear and a single phase was obtained. Moreover, the increase in temperature only enhances the intensity of the diffraction peaks and no peak characteristic to impurities or other compounds was observed.

Table 2. Variation in the average crystallite size of magnesium aluminate spinel with calcination temperature.

Calcination Temperature (°C)	Average Crystallite Size (nm)
600	3
800	4
1000	16

shows the average crystallite size of the powders calcined at different temperatures. The crystallite size was calculated from the full width at half maximum (FWHM) of the main intense peak of spinel (2θ = 36.86) using Scherer’ formula Equation (1). The average crystallite size of the spinel increased with the increase of calcination temperature.

It is seen that the temperature range from 800 °C to 1000 °C is the transition region where spinel crystallites grow up rapidly. The formation of spinel phase at this relatively low temperature might be ascribed to the uniform distribution of Mg²⁺ and Al³⁺ ions during the formation of co-precipitating complexes [24,25]. In this process, the reactions could be described as follows [26].

Mg²⁺ + Al³⁺ + 5OH^- = Mg(OH)₂ + Al(OH)₃

(2)

1/2Mg(OH)₂ + Al(OH)₃ = 1/2MgO +1/2Al₂O₃ + 2H₂O

(3)

MgO + Al₂O₃ = MgAl₂O₄

(4)

FE-SEM images of surface morphology of as-prepared and calcined spinel at different temperatures are shown in Figure 6. For the as-prepared sample, Figure 6a shows that the structure is uniform in the form of hydroxide. After the calcination process (Figure 6b–d), fine particles were obtained in strong crystalline form. As the calcination temperature increased, the size of crystalline became larger and its shape spherical owing to the availability of sufficient energy for the formation of strong crystallinity.

3.2.2. Effect of Reaction Time

Calcination time has an obvious effect on the composition and particle size. Figure 7 shows XRD patterns for the powders calcined at 1000 °C for 1, 3, 5, 7 or 10 h, respectively. With the increase of the calcination time, the intensity of MgAl₂O₄ peaks gradually increased up to 5 h and there seemed to be no change thereafter. All synthesized products were magnesium aluminate spinel and no impurities such as aluminum or magnesium oxide compounds were detected.

The advantages of co-precipitation method are that a well crystalline spinel phase can be obtained at a low calcination temperature of 1000 °C for 5 h. The spinel synthesized by this method has fine crystallite size and uniform morphology. The results indicate that the formation temperature of the spinel phase by this method is rather low in comparison with ball milling method. At the optimum condition, the average particles size was found to be in the range of 17 nm (standard error 0.74) is shown in Figure 8. The particle size distribution of the spinel powders was analyzed by using an ImageJ [27] and Origin Pro 9.0 software on the basis of the SEM morphology (Figure 6d). This is in good agreement with the calculated values from the XRD data shown in Table 2.

4. Conclusions

NaOH leaching of the mechanically activated black dross and subsequent purification of the leaching solution by removal of silicate (IV) results in pure sodium aluminate solution. Synthesis of spinel was investigated in this work by employing the precipitated aluminum hydroxide from purified sodium aluminate solution. For this purpose, the effect of calcination temperature and time on the development of spinel structure was studied. The calcination temperature showed a remarkable effect on the formation of magnesium aluminate spinel in both ball milling and co-precipitation methods. Compared to the ball milling method, the co-precipitation method showed several advantages. By employing the ball milling method, peaks corresponding to MgAl₂O₄ spinel started to appear at 1000 °C. In addition, the other phases like Al₂O₃ and MgO could not be completely disappeared even at the calcination temperature of 1500 °C. However, Al₂O₃ and MgO completely disappeared and a single phase of magnesium aluminate spinel was obtained at the calcination temperature of 600 °C by co-precipitation method. In this method, complete conversion to pure spinel with high intensity was possible at 1000 °C for 5 h. The average size of the particles was found to be in the range of 17 nm through SEM morphology which was in good agreement with the data obtained from XRD. A hydrometallurgical process can be developed for the recovery of spinel from black dross.