Experimental study of extracting alumina from coal fly ash using fluidized beds at high temperature

Fuel 199 (2017) 22–27

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Experimental study of extracting alumina from coal fly ash using fluidized beds at high temperature Liyan Sun a, Kun Luo a, Jianren Fan a, Huilin Lu b,⇑ a b

State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China

a r t i c l e

i n f o

Article history: Received 7 December 2016 Received in revised form 8 February 2017 Accepted 21 February 2017

Keywords: Fluidized bed Alumina extraction from coal ash Ammonium aluminum sulfate High aluminum fly ash

a b s t r a c t Experiments are conducted to investigate the process of alumina extraction from coal fly ash generated by coal-fired power plants located in the northern parts of China. Coal ash and ammonium sulfate are mixed and granulated under the effect of binder material. The high temperature fluidized bed reactor is used for the first time to recover the alumina in solid state. The whole process is simplified and operated continuously to make sure its feasibility for the industry. The effect of operating parameters, temperature, reaction time and reactants ratio on efficiency is discussed. The experimental results show that this method has the potential to recover alumina efficiently. And results also show that 60% reaction time is saved when compared with the acid process. The maximum of extracting efficiency reaches nearly 90% at high temperature under laboratory conditions. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction High-alumina coal ash is an industreferenrial byproduct which is generated in coal-fired power plants in most of the western and central parts of China and also in the smelting and chemical industries [1]. With the increasing consumption of coal, the output of fly ash from coal-fired power plants has become the largest industrial solid waste in China [2]. Annual generation is continuously increasing from 155 million tones in 2002 to 620 million tones in 2015 [3]. Common coal ash disposal ultimately lead to the accretion of coal fly ash on wide open ground. Inappropriate management and accumulation of coal ash will be danger to both environment and human health [4]. To utilize coal ash and decrease its damage, we need to change coal ash into high valueadded products. Usually, these materials are rich in Al2O3 such that the range is nearly 50% and are equivalent to mid-grade bauxite ores [5]. Considering its high alumina content, high-alumina coal fly ash can be utilized as substitute for bauxite, which otherwise needs to be imported into China in large quantities. Recovering alumina from coal fly ash provides a significant opportunity for converting waste materials to a new aluminum source [6]. And it will be a good alternative and could achieve significant economic and environmental benefits as well. In recent years, there have

⇑ Corresponding author. E-mail address: [email protected] (H. Lu). http://dx.doi.org/10.1016/j.fuel.2017.02.073 0016-2361/Ó 2017 Elsevier Ltd. All rights reserved.

been widespread concerns about the threat of coal fly ash and the continual increase of the bauxite trade price around the world. The increasing awareness of environmental protection and ecological balance has prompted recovering alumina from coal ash to be a research hotpot [7]. Hence, the study of alumina extraction from coal fly ash has attracted extensive attention in recent times. A number of water or alkaline leaching processes for recovering alumina have been reported and can be broadly classified into sintering processes, hydro-chemical process, acid processes and some other special processes. The sintering processes usually couple a reaction of coal ash with sintering agent powder to form soluble alumina compounds under high temperature [8]. The sinter is then treated with water or Na2CO3 solution to draw the alumina compounds from reactants, and the pregnant solution is then subsequently reacted to precipitate out [9]. One of the greatest advantages of this technology is solid industrial foundations. Fundamental knowledge and experiences of sintering process could be drawn on to improve the process. It also has the advantage of being a simple process and mature equipment system [10]. However, the disadvantages include a high energy consumption, an instability of sinter process resulting from narrow sinter temperature and low extraction rate. Sulfur trioxide or ammonia was produced in the sintering process with the addition of auxiliary materials and poses a risk to the surrounding area [11]. Serious dust pollution can easily lead to a very bad working condition. And the dissolution of small amount of silica during leaching is unavoidable which will results in the decrease of the crystallization rate.

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Fig. 1. Schematic diagram of experimental apparatus.

The hydro-chemical process was firstly proposed in 1990s and alumina can be separated from impurities in a wet alkaline process [12]. This process has already been successfully implemented for treatment of red mud and low-grade bauxite [13]. The hydrochemical process has made great progress in industrial in recent years. The advantages of this process are that it can achieve a high alumina extraction rate and the dealumination slag can be easily decomposed. However, it also has some limitations. The relatively high alkali concentration results in a high viscosity of the mixed slurry [14]. This process was complicated and required higher energy consumption and total cost [15]. High temperature and pressure required led to difficulties to scale-up and improvement of the technology. The purity of product is hard to be controlled with the exist of sodium aluminate hydrate. And the low cycle efficiency of the reaction medium caused by the reaction discipline is another major limitation [16]. In the acid process, the main method now-a-days is that, coal fly ash first reacts with hydrochloric (or sulfuric acid) to generate aluminum chloride (or aluminum sulfate) [17]. The aluminum salts are then crystallized from the acid medium and are subsequently decomposed. This process can thoroughly separate aluminum from silicon; more over, sulfuric, hydrochloric and nitric acids are generally used as leaching agents [18]. Compared with other process, acid process has the advantages of consuming less energy and producing a lower amount of slag. Shemi et al. [19] employed acid process to extract aluminum from coal ash and their results showed that the extraction efficiency is acceptable. Wu et al. [20] leached aluminum from ash by a pressure acid-leaching method. They found that microwave heating has inherent advantages in that it can be selective, controllable and efficient. Xu et al. [21] proposed a new process to extract alumina from ash using NH4HSO4 and H2SO4 mixed solution. The leaching behavior was investigated and optimized conditions were determined according to their results. Guo et al. [22] performed the research about alumina extraction using hydrochloric acid as the leachant. The addition of NaOH and Na2CO3 improved extraction evidently and the mixed additives made the efficiency reached 95% at 700 °C. And the mechanism of improved extraction in the acid solution was analyzed in their work. However, this process still remains pilotplant scale level and several major drawbacks limit the industrial application of this process, including corrosion, low alumina extraction efficiency, high impurity content and the excessive use of acid and fluoride [23]. The strong acid reaction systems require the need for process equipment to resist corrosion, which will

Fig. 2. Photo of X-ray diffraction.

result in a high cost. Also, high recovery cannot be achieved at low acid concentrations and low temperature [24]. Although there are some theoretical research and large-scale experiments, but the extraction efficiency is hard to be controlled in the industry. An industrialized project with an alumina production capacity of 200,000 tones per year was put into production in 2013, but it did not run well and was subsequently stopped because the recovery rate was too low [25]. So such processes are very tough to be fully understood and put into practice. A method that achieves high efficiency and simple process will be essential to the industrial applications for the extraction of alumina from coal ash. In this paper, we have applied a new method for the recovery of alumina from coal ash by using ammonium sulfate in high temperature fluidized bed to form NH4Al(SO4)2 in solid state. Feasibility of the new method has been theoretically proved by researchers [26]. High purity alum is precipitated by the reaction of NH4Al(SO4)2, dissolved in water and then followed by crystallization. Owing to the advantages of fluidized bed, such as the good heat and mass transfer performances, high efficiency in gas-solid contact, a classical contact reactor, the fluidized beds have been widely used in the field of combustion, catalytic cracking and synthesis [27]. The process is more simple by using fluidized bed and purity of production is higher than the other process. Here, we just discuss the first stage of recovery of aluminum: the production of ammonium aluminum sulfate using fluidized bed at high temperature. The

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L. Sun et al. / Fuel 199 (2017) 22–27 1300

Coal fly ash

1200

(NH4)2SO4 , bentonite

increasing gas velocity decreasing gas velocity

1100

Granulation

1000

NH3, SO3 ,recovery

900

Calcining

Crystallizaton, filtration NH3, SO3 ,recovery

800

Pressure drop (Pa)

Washing, filtration

700 600 500 400

Calcining

300 200

Al2O3

100 0

Fig. 3. Schematic diagram of the new alumina extraction process.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

Fluidization velocity (m/s)

experiments are performed at different temperatures, reactants ratio and reaction time to measure the impact of different parameters. 2. Experimental Raw coal ash was obtained from the thermal power plants located in Inner Mongolia, China. Coal fly ash mainly contains

(A)

(NH4)2SO4 : Ash=3 : 1

Fig. 4. Variation of bed pressure drop with fluidization velocity.

Al2O3, SiO2 and Fe2O3 and Al2O3 content nearly by 50 wt%. Therefore, the fly ash should be seen as a valuable reproductive aluminum-containing mineral resource, not only a common solid waste. First step is to make the ash granulation and bentonite is used as binder. Then we put the particles into

(B)

T=350

(NH4)2SO4 :Ash=3.5 : 1

T=350

40min

50min

Intensity

Intensity

50min

40min

30min

30min

20min 10

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40

50

60

70

20min

80

10

20

30

(C)

40

50

60

70

80

2-Theta(deg)

2-Theta(deg)

(D)

(NH4)2SO4 : Ash= 4 : 1 T=350

T=350

(NH4)2SO4 : Ash= 4.5 : 1

40min

10

20

30

40

50

2-Theta(deg)

60

70

50min

Intensity

Intensity

50min

40min

30min

30min

20min

20min

80

10

20

30

40

50

60

2-Theta(deg)

Fig. 5. XRD patterns of production at different time: (A) S/A = 3.0:1.0; (B) S/A = 3.5:1.0; (C) S/A = 4.0:1.0; (D) S/A = 4.5:1.0.

70

80

25

1.00

0.9

0.95

0.8

0.90

Alumina extraction efficiency

Alumina extraction efficiency

L. Sun et al. / Fuel 199 (2017) 22–27

0.85 0.80 0.75

Simulation data Fit curve

0.70

T=400 C

o

0.65 0.60 0.55 3.0

3.5

4.0

4.5

5.0

5.5

6.0

S : A=3.0 : 1.0 S : A=3.5 : 1.0 S : A=4.0 : 1.0 t = 40 min

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

6.5

300

Ratio (S : A) Fig. 6. Effect of ratio between ammonium sulfate and ash (S:A).

fluidized bed for reaction at high temperature. The products are collected and analyzed by X-ray diffraction (XRD) and chemistry respectively. Fig. 1 shows the schematic of the experimental setup which contains granulation machine and fluidized bed. Particles are generated by the pelletizing equipment continuously and the particle diameter ranges from 2 to 10 mm. The bed is 1000 mm in height and 65 mm in diameter. The temperature is measured by thermocouple which is inserted to the bottom of bed. The fluidization air provided by an air compressor flows through a heating pipe and then flows into the bed. The gas velocity is controlled by valve for fluidization. The temperature can be adjusted from 25 to 1200 °C. A steel filter plate with an averaged hole diameter of 20 lm is installed on the bottom of heat-resistant glass cylinder to support the fluidized particles. Fig. 2 shows the X-ray diffraction (D/max-rB 12KW) which is used for analyzing the products. The first step of using XRD is to grind the products into powder. The size of powders must be small enough to ensure the accurate. Then place powders in a specific glass moulds and compact powders. We can begin measure the content now and the data can be got by the computer. Different peak value represent the different materials. Mass fraction can be calculated according to the intensity based on K-value method [28,29]. A new process of alumina extraction from coal fly ash in solid state in high temperature fluidized bed has been proposed in our laboratory. As shown in Fig. 3, the main step of this process includes granulation, calcining, washing, crystallization and calcining. Ammonium aluminum sulfate is the key intermediate product for preparation of high-purity aluminum oxide. In this process, the first step is granulation using pelletizer to prepare the bed material. Ammonium sulfate reacts with alumina contained in the coal ash at high temperature, but not with the quartz and other amorphous phases. This will help to improve the purity of production after filtration. The reaction is carried out at high temperature in fluidized bed. The ammonium aluminum sulfate is generated as a result of the following chemical reaction:

4ðNH4 Þ2 SO4 þ Al2 O3 ! 2NH4 AlðSO4 Þ2 þ 6NH3 þ 3H2 O The fluidized bed is highly efficient equipment in mass transfer, mixing and reaction. It is also suitable for continuous production which makes it to be operated in an efficient way. In this work, the characteristic of calcining and recovery efficiency is discussed.

350 o Reaction temperature ( C)

400

Fig. 7. Effect of reaction temperature on extraction efficiency.

3. Result and discussion In order to determine the minimum fluidization velocity, we measure the pressure drop of bed under different fluidization gas velocity. As shown in Fig. 4, the pressure drop under both increasing and decreasing gas velocity process are measured, respectively. When increasing the gas velocity, slug and plug will occur in the bed first and then change into the fluidization state. Hence the corresponding line strongly fluctuates violently at the critical state. For the decreasing process, the pressure drop changes smoothly. The pressure drop under steady state for the two processes is in the same value. According to the line shown in Fig. 4, the minimum fluidization velocity is nearly 2.2 m/s. Fig. 5 depicts the XRD patterns of the products at different calcination time and the calcination temperature is 350 °C. From the figure, we can see that ammonium aluminum sulfate (NH4Al (SO4)2) peak intensities (10.5°, 24°, 30.5°) gradually increase with the increase of reaction time and alumina peak intensities decrease simultaneously. This outcome reflects the fact that alumina reacted with ammonium sulfate ((NH4)2SO4) causes the reduction of alumina peak intensities. At the same time, the quartz peak intensities keeps nearly constant or increase slightly since the amorphous quartz is generated with mullite decomposition. Moreover, at reaction temperature of 350 °C and after 50 min ammonium aluminum sulfate (NH4Al(SO4)2) peak intensities and quartz peak intensities do not change any more, thus indicating that the reaction is completely performed. Fig. 6 shows the alumina extraction efficiency under different mass ratio between ammonium sulfate and coal fly ash at 400 °C for 50 min. The components of production are analyzed by XRD patterns and chemical method. For XRD method, each material has a K value (or RIR value) which is calculated by K A ¼ IA =IAl2 O3 . And the mass fraction can be calculated by MA ¼

KA

IA P N

Ii i¼1 K i

. So the

alumina extraction efficiency can be given as:

g¼

M AljNH4 AlðSO4 Þ2 MAljAsh

where g is the alumina extraction efficiency. M AljAsh and MAljNH4 AlðSO4 Þ2 are the converted alumina mass in coal fly ash and ammonium aluminum sulfate production after calcination. From Fig. 6 we can see that the alumina extraction efficiency increases with the mass ratio between ash and ammonium sulfate. When the ratio ranges between 3.0:1.0, 3.5:1.0, 4.0:1.0 and 4.5:1.0,

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(A)

(B)

D= 4 mm

D = 6 mm

S :A =4 : 1

S : A=4 : 1

o

T=400 C

o

50min

10

20

30

40

50

60

70

Intensity

Intensity

T=400 C

50min

40min

40min

30min

30min

80

10

20

30

40

50

60

70

80

2-Theta(deg)

2-Theta (deg)

1.0

1.0

0.9

0.9

0.8

Alumina extraction efficiency

Alumina extraction efficiency

Fig. 8. Effect of initial particle diameter on extraction efficiency.

0.7 0.6 0.5

o

T=350 C o T=400 C o T=440 C

0.4

S:A=4:1

0.3 0.2 0.1 15

20

25

30

35

40

45

50

55

60

65

Time (min) Fig. 9. Alumina extraction efficiency under different reaction temperature (S:A = 4.0:1.0).

alumina extraction efficiency increases sharply. And when the mass ratio is larger than 4.5, the extraction efficiency increases slightly or keep constant. This result is attributed to the fact that the alumina in the coal fly ash has been exhausted and more feed reactants will cause no change for the reaction. For this operating condition, we can derive the liner relation between the alumina extraction efficiency and ratio of reactants for predicting reaction rate. And the reasonable proportion is between 4.5 and 5.0. The effects of reaction temperature on alumina extraction efficiency are depicted in Fig. 7. Efficiency increases almost linearly with the increase in reaction temperature under different reactant ratio. This result is attributed to the fact that the reaction rate and equilibrium will be promoted under high reaction temperature, especially at primary stage. At ratio 4.0:1.0, the extraction efficiency increases most sharply with the increase of temperature. The mechanism of temperature promoting is complex and different for different reactant ratio. The main reason for this trend is that proper increase in reaction temperature can improve the reaction rate and shorten reaction time. At ratio 4.0:1.0, the reactants is enough. For other conditions, increasing temperature will result in loss of (NH4)2SO4 and weaken the increase of reaction rate. The maximum alumina extraction efficiency is obtained at reaction

0.8 0.7 0.6 o

T=420 C

0.5

S : A=4.5 : 1.0 S : A=4.0 : 1.0 S : A=3.5 : 1.0 S : A=3.0 : 1.0

0.4 0.3 0.2 0.1 15

20

25

30

35

40

45

50

55

Time (min) Fig. 10. Alumina extraction efficiency under different reactant ratio.

temperature 400 °C, ratio 4.0:1.0 and the value is nearly 75%. That is because the reactant is very sufficient and the reaction has completely took place. Overall, the alumina extraction efficiency increases significantly with reaction temperature. Furthermore, effect of temperature for high reactant ratio is most obvious. The diameter of particles is an important parameter during fluidization. Two kinds particle with different diameter generated by granulation machine are adopted for experiment. The XRD patterns are plotted in Fig. 8 at 400 °C and the reactant ratio is 4:1. From the figure, we can find that the ammonium aluminum sulfate peak intensities (10.5°, 24°, 30.5°) increase with time. When the results of alumina extraction efficiency are compared, the averaged efficiency is very close and that for smaller particle is a little higher, but the difference is more reaction time is needed for particles with bigger diameter. The experiments are carried out at same temperature and fluidized gas velocity. The fluidization and contact efficiency become worse for particles with bigger diameter; also, heating process is hard for the center area. The effects of reaction temperature and time on alumina extraction efficiency are depicted in Fig. 9. At different reaction temperature, alumina extraction efficiency increases with the increase in time from 20 min to 50 (60) min. At reaction temperatures of 350 and 400 °C, alumina extraction efficiency increases sharply before

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40 min and gradually at 50 min. When the results of various temperatures are compared, the alumina extraction efficiency increases fastest at reaction temperature 350 °C, but the maximum value of efficiency is at reaction temperature 440 °C. When the time needed for reaction compared with hydro-chemical process and acid processes (more than 3 h), the present method is timesaving and simple. Fig. 10 shows the effect of reactant ratio on alumina extraction efficiency. The value of efficiency increases with reaction time from 20 to 40 min and does not change after 40 min. When the results of various reactants ratio are compared, the tendency of curve is similar and the maximum of alumina extraction efficiency is obtained under ratio 4.5:1.0. This result is attributed to the fact that the reactant is sufficient to drive reaction completely. The result under ratio 3.0:1.0 decreases slightly after 40 min. This maybe caused by the mass loss due to the fraction and collision between particles for poor strength. 4. Conclusion This research propose a new method for extracting alumina from coal fly ash in solid state by adding ammonium sulfate using fluidized bed at high temperature. The first step to generate the ammonium aluminum sulfate from coal ash is investigated experimentally. Forty minutes are needed for the reaction and 60% reaction time is saved when compared with the acid process method. Another advantage is that we introduce no impurities artificially. The alumina extraction efficiency increases with the increase of reactant mass and the reasonable ratio is about 5:1. The alumina extraction efficiency increases sharply with the increase of reaction temperature and less time is needed at high temperature. The maximum of extracting efficiency reaches nearly 90% at high temperature under laboratory conditions. The economy need to be discussed considering both the cost of high temperature and the reaction time in next stage. And, the possibility of scale-up and improvement of the technology will investigated. Simulation will also be considered during the process of recovery in future work. Acknowledgment This research is supported by Natural Science Foundation of China through Grant No. 51390493. References [1] Ahmaruzzaman M. A review on the utilization of fly ash. Prog Energy Combust Sci 2010;36:327–63. [2] Yao ZT, Xia MS, Sarker PK, Chen T. A review of the alumina recovery from coal fly ash with a focus in China. Fuel 2014;120:74–85. [3] Li J, Zhuang XG, Carlos L, Ana C, Oriol F, Querol X, et al. Potential utilization of FGD gypsum and fly ash from a Chinese power plant for manufacturing fireresistant panels. Constr Build Mater 2015;95:910–21.

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