Comparison of Silica Leaching Behaviors from the Acid-Leached

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Comparison of Silica Leaching Behaviors from AcidLeached Residue of Catalytic Gasification and Combustion Li Li, Zhiqing Wang, Jiejie Huang, Shaohua Ji, Yangang Mei, Yongwei Wang, and Yitian Fang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01894 • Publication Date (Web): 05 Sep 2017 Downloaded from http://pubs.acs.org on September 6, 2017

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Comparison of Silica Leaching Behaviors from Acid-Leached Residue of Catalytic

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Gasification and Combustion

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Li Li†,‡, Zhiqing Wang†,∗, Jiejie Huang†, Shaohua Ji†, Yangang Mei†,‡, Yongwei Wang§, Yitian Fang†

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†

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Sciences, Taiyuan, Shanxi 030001, People’s Republic of China

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‡

University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China

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§

Key Laboratory of Coal Clean Conversion & Chemical Engineering Process, College of Chemistry

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and Chemical Engineering, Xinjiang University, Urumqi 830046, China

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ABSTRCT:

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of

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Acid-leached waste residue, produced during Al extraction from coal ash, has been destroying

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the environment seriously. To utilize this high silicon residue (70% or more) and accelerate the

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industrialization of Al and Si extraction from coal ash, a high-silicon coal was selected to prepare

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catalytic gasification ash (CGA) and combustion ash (CA). Then, the obtained CGA and CA were

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leached by H2SO4 to remove aluminum and obtain silica-rich residue. After that, acid leached

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catalytic gasification residue (ACGR) and acid leached combustion residue (ACR) were leached by

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NaOH aqueous solution to extract SiO2. In this process, some possible influential factors such as

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NaOH concentration, liquid-solid ratio, reaction temperature and time on the extraction yield of SiO2

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(EY-SiO2) and modulus of sodium silicate (SSM) were investigated respectively. Besides,

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orthogonal experiments L9(34) were conducted to analyze their interrelationship. In addition, to

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acquire the variation of crystal structure, the method of X-ray diffusion analysis was conducted.

∗

Author to whom correspondence should be addressed. Tel.:+86-0351-2021137. E-mail address:

[email protected].

ACS Paragon Plus Environment

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Moreover, with the help of UV-Vis spectrophotometer, the content of SiO2 in residues were

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accurately determined. The results showed that the reactivity of CGA in acid leaching process was

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much higher than that of CA, resulting in the content of SiO2 in ACGR can reach 89.65% and the

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dominant mineral was amorphous SiO2. It is the amorphous structure that ACGR has better reactivity

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compared with ACR. Simultaneously, the orthogonal experiments indicate that NaOH concentration

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and liquid-solid ratio are the significant influencing factors for SSM, while temperature is the

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significant influencing factor for EY-SiO2. More significantly, relatively high SSM can be produced

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using ACGR and this can help to break through the bottleneck of sodium silicate production by wet

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process.

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Keywords: Catalytic gasification ash; Combustion ash; Acid leached catalytic gasification residue;

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Acid leached combustion residue; Sodium silicate

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1. INTRODUCTION

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Coal ash is a solid waste formed by coal combustion and gasification. In recent years, more and

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more ash has been discharging owing to the extensive consumption of coal in power plants and coal

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chemical industries. Most of them are stored in open-air landfill, which not only occupy amounts of

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cultivated land,1, 2 but also cause serious damages to the soil and water system. What is more, these

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damages are very difficult to be repaired. Thus, many methods have been developing to recycle,

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dispose and utilize coal ash environmentally. For example, a portion of coal ash was utilized as

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building and subgrade materials. However, this utilization has a potential risk of secondary pollution

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to the environment. In addition, many scholars pay their attention on the extraction of Al3-7 from coal

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ash. Nevertheless, acid-leached residue produced during the process of Al extraction usually contains


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a large amount of silicon (70% or higher).8 Therefore, an environment-friendly and economical

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effective method, which can convert the residue into other useful products, is essential.

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Some high-value silicon materials, such as sodium silicate, silica gel, white carbon black and

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aerogel, are ideal products can be derived from coal ash. Among them, sodium silicate solution can

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act as a widely used chemical raw material9-11 to produce silica gel, silica aerogel,12, 13 white carbon

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black and zeolite molecular sieve14, 15 in the chemical industry. In addition, it is widely used in the

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glass, ceramics, and cement as a major component and in pharmaceuticals, cosmetics and detergents

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industries as bonding and adhesive agents. Several techniques have been used to produce sodium

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silicate solution. In which, dry and wet processes are widely used. For dry process, quartz sand reacts

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with sodium carbonate (Na2CO3) at melt temperatures ranges from 1400 °C to 1500 °C, and then the

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product is dissolved in water under pressure. For wet process,16, 17 quartz sand reacts with NaOH

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aqueous solution in a autoclave reactor, at temperature of 180 °C to 250 °C and under saturated

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steam pressure.

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Meanwhile, Na2CO3 and NaOH are efficient and cheap catalysts in the process of gasification.

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They can not only accelerate the gasification reaction18, 19 at relatively low temperature, but also

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react with minerals in coal to inactivate the catalyst, and this inactivation cannot be avoided unless

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ash content in the coal is 0%. Fortunately, Ding et al.20 found that the added Na and K can react with

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minerals in coal to deactivate as aluminosilicate and nepheline. While these deactivated catalysts, i.e.

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aluminosilicate and nepheline, are high-Al substances, which can dissolve in acid and remove Al in

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them. In fact, aluminosilicate and nepheline indeed are the target production during the process of

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calcine, which is an essential step to increase the reactivity of raw material in the process of sodium


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silicate production used rice husk ash21, 22 or coal gangue. Thus, it is feasible that extraction of Al

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and Si from CGA.

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In addition, SSM (mole ratio of SiO2:Na2O), as an important evaluation parameter for the

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quality of sodium silicate, is a bottleneck in the wet sodium silicate production for a long time,

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because the SSM is relatively low and hard to be applied in industry when atmospheric pressure is

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used. Therefore, elevating temperature and pressure are obliged to solve this problem. CGS, as a

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kind of high reactive activity feedstock, can partially solve this problem in the gasification process.

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This can contribute to cut down the sodium silicate production costs by saving energy during the

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calcine in the pretreatment process and avoiding the high temperature and pressure condition in the

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reaction process. Thus, CGA may be more suitable to produce sodium silicate solution in

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comparison.

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In this study, a special high-silicon coal was chosen to prepare CGA and CA. Then, acid

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leaching process was conducted to move Al and other impurities in them, and compared their

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reactive activity simultaneously. To turn this high-silicon acid residue into valuable sodium silicate

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solution, they were made to react with NaOH aqueous solution. In this process, influence factors,

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such as NaOH concentration, liquid-solid ratio, reaction temperature and time were investigated

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separately. Moreover, in order to have a better understanding of their interaction, the orthogonal

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experiments L9 (34) were carried out to analyze significant influencing factors of EY-SiO2 and SSM.

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Surprisingly, the parameter of SSM can reach 2.0 or more at water bath and atmospheric pressure.

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This result can compare with the product using traditional process at high temperature and high

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pressure.


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2. EXPERIMTNTAL SECTION

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2.1. Coal Sample.

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A special high-silicon coal was selected as the raw material of this experiment. The raw coal

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was initially dried at room temperature for 48 h, and then was ground and sieved to less than 125 µm.

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After that, the sieved coal particles were dried for 6 h at 110 °C, and stored in a desiccator as coal

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sample. The proximate and ultimate analyses of the coal are listed in Table 1, and the ash

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composition is listed in Table 2. It could be seen that the contents of ash in the coal, Si and Al in the

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ash are high. Thus, this coal sample is preferable for this study.

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Table 1. Proximate and Ultimate Analyses of Coal Sample proximate analysis (wt %, ada) M

A

0.80

27.77

a

ultimate analysis (wt %, dafb)

V

FC

20.70

50.73

b

C

H

Oc

N

S

86.42

5.29

4.05

1.48

2.76

c

ad = air-dried basis. daf = dry and ash-free basis. by difference

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Table 2. Ash Compositions (wt %) of Coal Sample SiO2

Al2O3

Fe2O3

CaO

MgO

TiO2

SO3

K2O

Na2O

P2O5

50.11

38.33

6.30

0.61

0.42

0.84

0.36

0.85

0.16

0.07

2.2. Preparation of CGA and CA.

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Two processes were used to prepare CGA, including the preparation of Na2CO3-loaded coal

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sample and gasification of this sample. Na2CO3-loaded coal sample was prepared by aqueous

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solution impregnation method. The steps were as follows: 23 g of Na2CO3 was added into 100 mL of

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deionized water that had been heated to 60 °C and stirred it until Na2CO3 was fully dissolved. Then,

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100 g of coal sample was added slowly to the stirring Na2CO3 aqueous solution. The resulting

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solid-liquid mixture was stirred at room temperature and then changed into viscous slurry.

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Subsequently, the slurry was evaporated in an oven at 110 °C for 8 h. The obtained dried slurry was


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grounded, sieved to less than 125 µm and stored in a desiccator as Na2CO3-loaded coal sample.

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The gasification process was conducted by gasifying Na2CO3-loaded coal sample at 815 °C in

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CO2 atmosphere. The detail processes were as follows: first, 10 g of Na2CO3-loaded coal sample was

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evenly spread in an alumina boat. The boat was fixed on the one end of the pull-push rod and located

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in the cool zone of the tubular furnace. Then, the tubular furnace was heated under 300 mL/min of

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CO2. When the temperature of the isothermal zone reached 815 °C and stabilized for a while, the

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boat located in the cool zone was pushed to the isothermal zone of the tubular furnace by moving the

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pull-push rod and the gasification began. After 2.5 h, the boat was re-pulled to the cool zone until the

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sample temperature fell to nearly room level. The ash produced in the boat was CGA.

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The preparation process of CA was similar to that of CGA except that the Na2CO3-loaded coal

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sample was replaced by coal sample and CO2 flow was replaced by O2 flow.

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2.3. Preparation of ACGR and ACR.

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In order to enrich silicon and remove aluminum and other impurities from ashes, CGA and CA

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were demineralized by acid leaching. ACGR was prepared following steps: CGA was soaked in a

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beaker with aqueous solution of H2SO4 (6 mol/L) for 3 h with stirring at 60 °C in a thermostatic

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water bath. Where the ratio of H2SO4 to CGA was 20 mL:1 g. Then, centrifuge was used to process

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the solid-liquid separation. The solid separated by centrifugation was rinsed thoroughly with

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deionized water until the pH of washed water reached 6-7 to ensure that no residual acid was

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presented. Afterwards, the rinsed solid was dried in an oven at 110 °C for 6 h and then stored in a

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desiccator as ACGR.

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The process of ACR preparation was similar to that of ACGR preparation except for changing


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reaction material from CGA to CA.

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2.4. The Extraction of SiO2.

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Extraction of SiO2 was realized by the reaction between acid leached residue and NaOH

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aqueous solution. The so-called acid leached residue includes ACGR and ACR. The processes were

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as follows: acid leached residue was added into a beaker with aqueous solution of NaOH and stirred

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by a magnetic mixer in a thermostatic water bath. After reaction finished, the resulting solution was

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filtered by suction filtration. Then, the filtered residue was rinsed with deionized water and the

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filtrated solutions were collected as the sodium silicate solution.

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2.5 The Design of Orthogonal Experiments.

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The design of L9 (34) matrix was adopted to investigate the effect of EY-SiO2 and SSM. Influencing

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factors such as NaOH concentration (factor A), liquid-solid ratio (factor B), reaction temperature

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(factor C), reaction time (factor D) were optimized in three leaching processes, respectively. The

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target parameters are SSM (target 1) and EY-SiO2 (target 2).

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Table 3. Factors and Levels in the Orthogonal Experiments Factor

A

B

Concentration

Liquid-solid ratio

(mol/L)

(mL/g)

1

0.45

2 3

Level

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C

D

Time (min)

Temperature (oC)

18.0

90

85

0.55

20.0

120

90

0.65

22.5

150

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2.6 Range Analysis.

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There are two significant parameters in the range analysis: kij and Rj. kij is defined as the average

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value of target j in the same level of each factor. The value of kij can be used to determine the

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optimal level of target j. Rj is defined as the range of the maximum and minimum value of kij. The


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order of Rj can be used to evaluate the influence extent of each factor on target j. A larger Rj means

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the greater impact. For orthogonal experiments L9(34), the relevant calculations are as follows

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(Factor C and Target 1, for example):

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k11 =

Y11 + Y61 + Y81 3

(1)

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k21 =

Y21 + Y41 + Y91 3

(2)

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k31 =

Y31 + Y51 + Y71 3

(3)

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Rc1 = max ( k11 + k21 + k31 ) − min ( k11 + k21 + k31 )

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where Yij is the value of target j in each trial of orthogonal experiments.

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2.7 Characterization Methods.

(4)

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The proximate and ultimate analyses were performed according to the GB/T 212-2008 and

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GB/T 476-2001, respectively. The ash chemical compositions and the content of SiO2 in acid leached

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residues were conducted following the GB/T 1574-2007. The content of Na2O, SiO2 in sodium

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silicate solution and SSM were conducted following the GB/T 4209-2008. All of the “GB/T” refer to

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Chinese National Standards. The crystal structure of CGA, CA, ACGR and ACR were characterized

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by an X-ray diffractometer (D8 Advance, Bruker, Germany) using Cu/Kα radiation (λ = 1.54056 Å),

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a 30 KV tube voltage, a 15 mA tube current, and a scan rate of 2 o/min.

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The EY-SiO2 (η) was calculated using the following equation:

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η=

m2 × w2 ×100% m1 × w1

(5)

where m1 represents the mass of acid leached ash (g), w1 is the content of SiO2 in acid leached


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residue (%), m2 represents the mass of sodium silicate solution (g) and w2 is the content of SiO2 in

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sodium silicate solution (%).

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3. RESULTS AND DISCUSSION

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3.1. XRD Spectrums of CGA and CA.

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Figure 1. The XRD spectra of CGS and CA.

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1. quartz (SiO2) 2. hematite (Fe2O3) 3. andalusite (Al2(SiO4)O) 4. millosevichite (Al2(SO4)3) 5. zeolite

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(Na6(AlSiO4)6) 6. ultramarine (Na7Al6Si6O24S3) 7. sodium aluminum silicate (Na1.55Al1.55Si0.45O4) 8. sodium

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silicate (Na2Si4O9)

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Figure 1 depicts the crystalline structures of CGA and CA samples. It shows that zeolite,

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ultramarine, and sodium aluminum silicate are the major crystalline compounds in CGA. While

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quartz, hematite and andalusite are the major crystalline compounds in CA. By comparing the

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difference of CGA and CA, it can be found that the existing forms of Si in CGA are zeolite,

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ultramarine and sodium aluminum silicate, while that of CA are quartz and andalusite. This

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difference mainly derived from Na2CO3 addition. During the gasification process, Na2CO3 can react


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with SiO2 and produce sodium aluminosilicate. It is thought that the major reaction routes are as

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follows:23

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Na2CO3 +2C → 2Na+3CO

(1)

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2Na+CO2 → Na2O+CO

(2)

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3Na2O+6SiO2 +3Al2O3 → Na6 (AlSiO4 )6

(3)

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3.2. XRD Spectrums of ACGR and ACR.

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Figure 2. The XRD spectra of ACGR and ACR.

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1. quartz (SiO2) 2. hematite (Fe2O3) 3. andalusite (Al2(SiO4)O) 4. millosevichite (Al2(SO4)3) 5. magnetite (Fe3O4)

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The crystalline structure of ACGR and ACR were characterized by XRD and the results are

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shown in Figure 2. Quartz, hematite and andalusite are the major crystalline compounds in ACR, this

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result is much same with that of CA (its parental ash). In other words, the minerals maintained their

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original forms during the acid leaching process and theirs forms do not have any obvious change. As

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to the ACGR, a broad peak appears at 2θ=23o in Figure 2. This indicates that the nature of ACGR is


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disordered and this disordered material is amorphous SiO2. Compared with CGA (its parental ash), it

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can be deduced that some reactions between sodium aluminosilicate minerals and acid have

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happened and the form of Si changes from crystalline state to amorphous state due to these reactions.

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The main reaction route generally as follows:

Na6 (AlSiO4 )6 +24H+ →6Na+ +6SiO2 +6Al3+ +12H2O (4)

193 194

The content of SiO2 in ACR is 59.25%, however, this value in ACGR is up to 89.65%.

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Compared with the SiO2 content in coal ash, the content of SiO2 in ACR does not have obvious raise,

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whereas, the content of SiO2 in ACGR increases from 50.11% to 89.65%. In other words, the content

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of SiO2 in ACGR is enriched in this process and most of the impurities were removed by acid

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leaching treatment. Thus, SiO2 has become the major substance in ACGR. This result is identical

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with the analysis of XRD. Hence, CGA has higher react activity in acid leaching process than that of

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CA.

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3.3. Effect of NaOH Concentration on EY-SiO2 and SSM.

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In the hydrothermal condition, SiO2 reacts with NaOH according to the following reaction:

nSiO2 +2NaOH → Na2O⋅ nSiO2 +H2O

(5)

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It can be seen that NaOH aqueous solution plays a key role, and thus, the effect of NaOH

205

concentration on EY-SiO2 and SSM is initially studied. The results of EY-SiO2 and SSM versus

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NaOH concentration are shown in Figure 3. Where these reactions were conducted at 90 oC for 120

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min and the liquid-solid ratio for ACGR was 20 mL/g and that of ACR was 30 mL/g.


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210 211

Figure 3. Effect of NaOH concentration on EY-SiO2 and SSM: (a) ACGR; (b) ACR

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Figure 3a is the results of ACGR. It indicates that when the concentration of NaOH is less than

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0.55 mol/L, the contact between NaOH and ACGR is improved and EY-SiO2 is increased gradually

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with the increasing of NaOH concentration. Nevertheless, the increasing extent of EY-SiO2 is less


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than that of Na2O, in other words, the relative quantity of SiO2 is decreased. Moreover, from the

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chemical reaction equation 5 can be seen that the n in the Na2O·nSiO2 is decreased, resulting in SSM

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is decreased. However, when the NaOH concentration exceeds 0.55 mol/L, there is no obvious

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increases in EY-SiO2, while the SSM still reduces. The reason is that the amount of SiO2 in the

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solution increases and the viscosity of the solution becomes larger with increasing of NaOH

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concentration.24-26 Moreover, the fluidity becomes worse and mass transfer rate was heavily reduced.

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Thus, the above results indicated that 0.55 mol/L is the optimized concentration. In this

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concentration, EY-SiO2 is 96% and SSM is 1.9. Figure 3b is the results of ACR. It indicates that the

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trend of EY-SiO2 is similar to that of ACGR. However, the SSM is always no more than 0.2. More

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precisely, the EY-SiO2 is 76% and the SSM is 0.16 when the concentration of NaOH is 2.94 mol/L.

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In addition, compared with the ACGR, the EY-SiO2 and SSM for ACR are relatively low. The

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reason for this phenomenon is that the amorphous SiO2 is unstable and has a higher reactivity.

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What’s more, the content of SiO2 is a vital factor which decisively affects EY-SiO2 in the

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hydrothermal condition. The content of SiO2 in ACR is relatively lower than that of ACGR. From

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the above analysis, it can be seen that ACGR has better reactivity with NaOH aqueous solution.

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3.4 Effect of Liquid-solid Ratio on EY-SiO2 and SSM.


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232 233

Figure 4. Effect of liquid-solid ratio on EY-SiO2 and SSM: (a) ACGR; (b) ACR

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Figure 4 shows the effect of liquid-solid ratio on EY-SiO2 and SSM, Where the reactions were

235

conducted at 90 oC for 120 min and the total amount of NaOH was fixed. Figure 4a is the results of

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ACGR. It reveals that when the liquid-solid ratio is less than 22.5 mL/g, EY-SiO2 increases rapidly


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with the increasing of liquid-solid ratio. At the same time, SSM initially increases and then become

238

stable. The reason is that an increase in liquid-solid ratio decrease the viscosity of reaction system.

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Therefore, the stability of the solution is improved27, and the diffusion rate of OH- and SiO32- is

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ameliorated. When the liquid-solid ratio is more than 22.5 mL/g, EY-SiO2 and SSM reduces owing

241

to the concentration of OH- decreases and the contact probability between OH- and SiO32- decreases.

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Thus, the optimized liquid-solid ratio is about 20 mL/g. In this condition, the EY-SiO2 can reach 96%

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and the SSM is 1.9. Figure 4b is the results of ACR. It shows that the SSM does not have obvious

244

change and always less than 0.2. For example, the EY-SiO2 is 85% and the SSM is 0.17 when the

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liquid-solid is 35 mL/g. This poor reactivity is also determined by the nature of ACR.

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3.5 Effect of Reaction Temperature on EY-SiO2 and SSM.

247 248


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249 250

Figure 5. Effect of reaction temperature on EY-SiO2 and SSM: (a) ACGR; (b) ACR

251

Figure 5 shows the effect of reaction temperature on EY-SiO2 and SSM, Where the reactions

252

were also conducted for 120 min, while the NaOH concentration and liquid-solid ratio were 0.55

253

mol/L and 20 mL/g for ACGR, while 2.52 mol/L and 36 mL/g for ACR. The EY-SiO2 and the SSM

254

increase gradually with the increasing of reaction temperature. The reason is that the viscosity of

255

solution decreases and the molecular motion rate of the solution increases at higher temperature. At

256

the same time, contact probability of OH- and SiO32- is increased. When the reaction temperature is

257

raised to 90 oC, the EY-SiO2 is 96% and the SSM is 1.9. These values are relatively high and 90 oC is

258

regarded as the suitable temperature when economic factors and facility request are considered.

259

Figure 5b is the results of ACR. It shows that the EY-SiO2 is similar to that of Figure 4a. That is, the

260

EY-SiO2 and SSM are increased with increasing temperature. When the temperature is 90 oC, the

261

EY- SiO2 is 85% and the SSM is 0.17.

262

3.6 Effect of Reaction Time on the EY-SiO2 and SSM.


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Energy & Fuels

263 264

265 266

Figure 6. Effect of reaction time on EY-SiO2 and SSM: (a) ACGR; (b) ACR

267

Figure 6 presents the effect of reaction time on the EY-SiO2 and the SSM, where the reactions

268

were conducted at 90 oC, the NaOH concentration and liquid-solid ratio were 0.55 mol/L and 20

269

mL/g for ACGR, while that of ACR were 2.52 mol/L and 35 mL/g, respectively. Figure 6a is the


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 37

270

results of ACG. It indicates that reaction time can rapidly increase EY-SiO2 before 90 min, but its

271

growth trend slows down when the time is further increased. SSM does not have obvious change in

272

this process. When the reaction time is 90 min, the EY-SiO2 is 92% and the SSM is 1.9. It is also

273

obvious that ACGR has preferable reactivity with NaOH. The reason is that the surface area is

274

increased and porous structure is emerged with the dissolution of Al and other metals during acid

275

leaching process. And hence, porous structure of amorphous SiO228, 29 allows OH- to pass through the

276

pores into particles and the rate of reaction is accelerated. Figure 6b is the results of ACR. It shows

277

that the trend of EY-SiO2 is similar to that of Figure 6a. EY-SiO2 gradually increases and SSM does

278

not have obvious change with the increasing of reaction time. The EY-SiO2 is 82% and the SSM is

279

0.17 when the reaction time is 90 min.

280

3.7 Factor Analysis.

281

Many factors that can influence EY-SiO2 and SSM. Moreover, they can influence each other

282

during the process of reaction and these influences are complicated. Above works have investigated

283

the effect of NaOH concentration, liquid-solid ratio, reaction temperature and reaction time on the

284

EY-SiO2 and SSM. On the basis of these, the approximate scope of experimental conditions can be

285

determined. In order to further determine the optimal reaction conditions and the interrelationship of

286

parameters, four-factor three-level orthogonal experiments L9 (34) were conducted. Table 4. Design of Orthogonal Experiments and Results L9(34)

287

Target 1 No.

A (mol/L)

B (mL/g)

C (min)

D (oC)

SSM

Target 2 EY-SiO2 (%)

1

1

1

1

1

2.795

82.110

2

1

2

2

2

2.501

86.431

3

1

3

3

3

2.051

88.592


Page 19 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

4

2

1

2

3

2.501

86.431

5

2

2

3

1

1.901

82.110

6

2

3

1

2

1.919

96.155

7

3

1

3

2

2.068

92.913

8

3

2

1

3

1.876

97.235

9

3

3

2

1

1.542

79.949

k11

2.449

2.455

2.197

2.079

k21

2.107

2.092

2.181

2.163

k31

1.829

1.837

2.006

2.142

R1

0.620

0.617

0.190

0.083

A1

B1

C1

D2

k12

85.711

87.151

91.833

81.389

k22

88.232

88.592

84.270

91.833

k32

90.032

88.232

87.872

90.753

R2

4.321

1.441

7.563

10.444

A3

B2

C1

D2

Excellent level 1

Excellent level 2

288

Orthogonal experiments were conducted according to Table 4. It illustrates that if SSM is

289

regarded as the target, experiment 1 has the best result. R as the range analysis of orthogonal

290

experiments, the value of it follows the order of A > B > C > D, i.e., A and B are the significant

291

influencing factors. That is NaOH concentration and liquid-solid ratio have an enormous influence

292

on SSM. In addition, judging from the value of k, if the maximum SSM is regarded as the target, the

293

condition A1B1C1D2 should be selected. That is, when the concentration of NaOH is 0.45 mol/L, the

294

liquid-solid ratio is 18 mL/g, the reaction time is 90 min and the reaction temperature is 90 oC, the

295

optimize SSM can be obtained.

296

However, if EY-SiO2 is regarded as the target, experiments 8 and 6 have the best results. The

297

value of R follows the order of D > C > A > B, i.e., reaction temperature is the significant

298

influencing factor. Thus, reaction temperature has the most dominant effect on EY-SiO2. Besides,

299

judging from the value of k, if the maximum EY-SiO2 is regarded as the target, the condition


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

300

A3B2C1D2 should be chosen. That is, when the concentration of NaOH is 0.65 mol/L, the liquid-solid

301

ratio is 20.0 mL/g, the reaction time is 90 min and the reaction temperature is 90 oC, the optimize

302

EY-SiO2 can be reached.

303

From the analysis above, it is obvious that the two indicators do not have the same optimal

304

conditions. Consequently, SSM and EY-SiO2 should be balanced to satisfy the actual needs. For the

305

factors A and B, if high SSM is regarded as main target, A1B1 should be selected. And if high

306

EY-SiO2 is regarded as main target, A3B2 should be selected. Different goals correspond to different

307

conditions. Moreover, due to A and B have a dominant effect on SSM, they should be given priority

308

in trade-off process. For factors C and D, no matter high SSM or high EY-SiO2 is regarded as the

309

target. There is no confliction, C1D2 should be selected.

310

4. CONCLUSION

311

The method of comparison was used to study the leaching behaviors of silica in CGA and CA,

312

and the influencing factors such as NaOH concentration, liquid-solid ratio, reaction temperature and

313

time on the effect of EY-SiO2 and SSM were analyzed. In addition, orthogonal experiments L9(34)

314

were conducted to analyze the interrelationship between the influencing factors. Specific conclusions

315

are as follows:

316

(1) The main minerals in CA are quartz, hematite, and andalusite, which have poor reactivity

317

with H2SO4. The minerals in ACR and the content of SiO2 do not have obvious change after acid

318

leaching. While the main minerals in CGA are zeolite, ultramarine, and sodium aluminum silicate,

319

which have good acid solubility. After acid leaching, the dominant mineral in ACGR is amorphous

320

SiO2, and its content in ACGR can reach 89.65%, this value is much higher than that of ACR


Page 20 of 37

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321 322 323 324 325 326

Energy & Fuels

(50.11%). (2) Both EY-SiO2 and SSM produced from ACGR have better results compared with that produced from ACR. (3) Sodium silicate solution with relatively high modulus can be produced at atmospheric pressure in water bath. (4) Concentration of NaOH and liquid-solid ratio are the significant influencing factors to the

327

SSM. While temperature is the significant influencing factor to the EY-SiO2.

328

AUTHOR INFORMATION

329

Corresponding Author

330

*Telephone: +86-351-2021137-801. Fax: +86-351-2021137-802. E-mail: [email protected].

331

ORCID

332

Zhiqing Wang: 0000-0001-9009-9785

333

Notes

334

The authors declare no competing financial interest.

335

ACKNOWLEGEMENTS

336

The work is financially supported by the National Science Foundation of China (21676289), the

337

Natural Science Fund of Shanxi Province (2013021007-2), the research supported by the Chinese

338

Academy of Sciences (CAS) / State Administration of Foreign Experts Affairs (SAFEA)

339

International Partnership Program for Creative Research Teams and Youth Innovation Promotion

340

Association (2014156).

341

REFERENCES


Energy & Fuels

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[1] Liu, H.; Liu, Z. Recycling utilization patterns of coal mining waste in China. Resources,

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Conservation and Recycling 2010, 54, (12): 1331-1340.

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[2] Ahmaruzzaman, M. A review on the utilization of fly ash. Prog. Energy Combust. Sci. 2010, 36,

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[3] Guo, Y.; Zhao, Z.; Zhao, Q.; Cheng, F. Novel process of alumina extraction from coal fly ash by

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pre-desilicating-Na2CO3 activation-Acid leaching technique. Hydrometallurgy 2017, 169: 418-425.

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[4] Nayak, N.; Panda, C. R. Aluminium extraction and leaching characteristics of Talcher Thermal

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Power Station fly ash with sulphuric acid. Fuel 2010, 89, (1): 53-58.

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[5] Park, H. C.; Park, Y. J.; Stevens, R. Synthesis of alumina from high purity alum derived from

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coal fly ash. Materials Science and Engineering: A 2004, 367, (1-2): 166-170.

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[6] Sun, L.; Luo, K.; Fan, J.; Lu, H. Experimental study of extracting alumina from coal fly ash using

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fluidized beds at high temperature. Fuel 2017, 199: 22-27.

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[7] Zhu, P.-w.; Dai, H.; Han, L.; Xu, X.-l.; Cheng, L.-m.; Wang, Q.-h.; Shi, Z.-l. Aluminum

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extraction from coal ash by a two-step acid leaching method. Journal of Zhejiang

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University-SCIENCE A 2015, 16, (2): 161-169.

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[8] Jinguo, Q.; Songqing, G. Process for recovery of silica followed by alumina from coal fly ash.

358

US 7871583 B2, 2011.

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[9] Guo, F.; Wei, N.; Xiu, Z.; Fang, Z. Transesterification mechanism of soybean oil to biodiesel

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catalyzed by calcined sodium silicate. Fuel 2012, 93: 468-472.

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[10] Kouassi, S. S.; Tognonvi, M. T.; Soro, J.; Rossignol, S. Consolidation mechanism of materials

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obtained from sodium silicate solution and silica-based aggregates. J. Non-Cryst. Solids 2011, 357,


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(15): 3013-3021.

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[11] Gaber, M. A. W. Utilization of Sodium Silicate Solution as A Curing Compound of Fresh

365

Concrete. Journal of American Science 2012, 8, (11): 61-66.

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[12] Bhagat, S. D.; Kim, Y.-H.; Ahn, Y.-S.; Yeo, J.-G. Textural properties of ambient pressure dried

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water-glass based silica aerogel beads: One day synthesis. Microporous Mesoporous Mater. 2006, 96,

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(1-3): 237-244.

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[13] Bhagat, S. D.; Kim, Y.-H.; Moon, M.-J.; Ahn, Y.-S.; Yeo, J.-G. A cost-effective and fast

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synthesis of nanoporous SiO2 aerogel powders using water-glass via ambient pressure drying route.

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Solid State Sciences 2007, 9, (7): 628-635.

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[14] Chareonpanich, M.; Namto, T.; Kongkachuichay, P.; Limtrakul, J. Synthesis of ZSM-5 zeolite

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from lignite fly ash and rice husk ash. Fuel Process. Technol. 2004, 85, (15): 1623-1634.

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[15] Sarkar, B.; Thakur, R. M.; Samant, N.; Prabhu, M. K.; Gopal, R.; Patel, M. B.; Ray, S. K.;

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Venkatachalam, K.; Makhija, S.; Ghosh, S. Process for preparing sodium silicate alkali solution

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depleted of sodium salt and enriched in silica. US 6864202 B2, 2005.

377

[16] Kostinko, J. A. Preparation of sodium silicate solutions. US 4539191, 1985.

378

[17] Trabzuni, F. M. S.; El Dekki, H. M. Sodium silicate solutions. US 8734750 B2, 2014.

379

[18] Wang, Y.; Wang, Z.; Huang, J.; Fang, Y. Catalytic Gasification Activity of Na2CO3 and

380

Comparison with K2CO3 for a High-Aluminum Coal Char. Energy Fuels 2015, 29, (11): 6988-6998.

381

[19] Mei, Y.; Wang, Z.; Fang, H.; Wang, Y.; Huang, J.; Fang, Y. Na-Containing Mineral

382

Transformation Behaviors during Na2CO3-Catalyzed CO2 Gasification of High-Alumina Coal.

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Energy Fuels 2017, 31, (2): 1235-1242.


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[20] Ding, L.; Zhang, Y.; Wang, Z.; Huang, J.; Fang, Y. Interaction and its induced inhibiting or

385

synergistic effects during CO-gasification of coal char and biomass char. Bioresour. Technol. 2014,

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173: 11-20.

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[21] Foletto, E. L.; Gratieri, E.; Oliveira, L. H. d.; Jahn, S. L. Conversion of rice hull ash into soluble

388

sodium silicate. Materials Research 2006, 9, (3): 335-338.

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[22] Liu, X.; Li, Z.; Chen, H.; Yang, L.; Tian, Y.; Wang, Z. Rice husk ash as a renewable source for

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synthesis of sodium metasilicate crystal and its characterization. Res. Chem. Intermed. 2015, 42, (4):

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3887-3903.

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[23] Zhang, F.; Xu, D.; Wang, Y.; Wang, Y.; Gao, Y.; Popa, T.; Fan, M. Catalytic CO2 gasification

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of a Powder River Basin coal. Fuel Process. Technol. 2015, 130: 107-116.

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[24] Nordstrom, J.; Nilsson, E.; Jarvol, P.; Nayeri, M.; Palmqvist, A.; Bergenholtz, J.; Matic, A.

395

Concentration- and pH-dependence of highly alkaline sodium silicate solutions. J. Colloid Interface

396

Sci. 2011, 356, (1): 37-45.

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[25] Provis, J. L.; Kilcullen, A.; Duxson, P.; Brice, D. G.; van Deventer, J. S. J. Stabilization of

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Low-Modulus Sodium Silicate Solutions by Alkali Substitution. Ind. Eng. Chem. Res. 2012, 51, (5):

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2483-2486.

400

[26] Yang, X.; Zhu, W.; Yang, Q. The Viscosity Properties of Sodium Silicate Solutions. J. Solution

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Chem. 2007, 37, (1): 73-83.

402

[27] Winston, A. E.; Dunn, S. E.; Cala, F. R.; Vinci, A.; Lajoie, S. M. Stabilization of silicate

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solutions. US 5234505, 1995.

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[28] Schulmeister, K.; Mader, W. TEM investigation on the structure of amorphous silicon


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Energy & Fuels

405

monoxide. J. Non-Cryst. Solids 2003, 320, (1-3): 143-150.

406

[29] Treacy, M. M.; Borisenko, K. B. The local structure of amorphous silicon. Sci 2012, 335, (6071):

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950-3.

408


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

409

Table Captions

410

Table 1. Proximate and Ultimate Analyses of Coal Sample

411

Table 2. Ash Compositions (wt %) of Coal Sample

412

Table 3. Factors and Levels in the Orthogonal Experiments

413

Table 4. Design of Orthogonal Experiments and Results L9(34)


Page 26 of 37

Page 27 of 37

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Energy & Fuels

Figure Captions

414 415


416

1. quartz (SiO2) 2. hematite (Fe2O3) 3. andalusite (Al2(SiO4)O) 4. millosevichite (Al2(SO4)3) 5.

417

zeolite (Na6(AlSiO4)6) 6. ultramarine (Na7Al6Si6O24S3) 7. sodium aluminum silicate

418

(Na1.55Al1.55Si0.45O4) 8. sodium silicate (Na2Si4O9)

419


420


421

magnetite (Fe3O4)

422

Figure 3. Effect of NaOH concentration on EY-SiO2 and SSM.

423

Figure. 3 (a). The reaction between ACGR and NaOH.

424

Figure 3 (b). The reaction between ACR and NaOH.

425

Figure 4. Effect of liquid-solid ratio on EY-SiO2 and SSM.

426

Figure 4 (a). The reaction between ACGR and NaOH.

427


428

Figure 5. Effect of reaction temperature on EY-SiO2 and SSM.

429


430


431

Figure 6. Effect of reaction time on EY-SiO2 and SSM.

432


433

Figure 6 (b). The reaction between ACR and NaOH

434


Energy & Fuels

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Page 28 of 37

435 436

Table 1. Proximate and Ultimate Analyses of Coal Sample proximate analysis (wt %, ada)

a

M

A

V

FC

0.80

27.77

20.70

50.73

b

ultimate analysis (wt %, dafb) C

H

Oc

N

S

86.42

5.29

4.05

1.48

2.76

c

ad = air-dried basis. daf = dry and ash-free basis. by difference


Page 29 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

437

Table 2 Ash Compositions (wt %) of Coal Sample SiO2

Al2O3

Fe2O3

CaO

MgO

TiO2

SO3

K2O

Na2O

P2O5

50.11

38.33

6.30

0.61

0.42

0.84

0.36

0.85

0.16

0.07


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

438

Page 30 of 37

Table 3. Factors and Levels in the Orthogonal Experiments Factor

A

B

Concentration

Liquid-solid ratio

(mol/L)

(mL/g)

1

0.45

2 3

Level

C

D

Time (min)

Temperature (oC)

18.0

90

85

0.55

20.0

120

90

0.65

22.5

150

95


Page 31 of 37

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Energy & Fuels

Table 4. Design of Orthogonal Experiments and Results L9(34)

439

Target 1 No.

A (mol/L)

B (mL/g)

C (min)

D (oC)

SSM

Target 2 EY-SiO2 (%)

1

1

1

1

1

2.795

82.110

2

1

2

2

2

2.501

86.431

3

1

3

3

3

2.051

88.592

4

2

1

2

3

2.501

86.431

5

2

2

3

1

1.901

82.110

6

2

3

1

2

1.919

96.155

7

3

1

3

2

2.068

92.913

8

3

2

1

3

1.876

97.235

9

3

3

2

1

1.542

79.949

k11

2.449

2.455

2.197

2.079

k21

2.107

2.092

2.181

2.163

k31

1.829

1.837

2.006

2.142

R1

0.620

0.617

0.190

0.083

A1

B1

C1

D2

k12

85.711

87.151

91.833

81.389

k22

88.232

88.592

84.270

91.833

k32

90.032

88.232

87.872

90.753

R2

4.321

1.441

7.563

10.444

A3

B2

C1

D2

Excellent level 1

Excellent level 2

440


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

441 442


443


444

zeolite (Na6(AlSiO4)6) 6. ultramarine (Na7Al6Si6O24S3) 7. sodium aluminum silicate

445

(Na1.55Al1.55Si0.45O4) 8. sodium silicate (Na2Si4O9)

446


Page 32 of 37

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Energy & Fuels

447 448


449


450

magnetite (Fe3O4)

451


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

452 453

454 455 456

Figure 3. Effect of NaOH concentration on EY-SiO2 and SSM: (a) ACGR; (b) ACR


Page 34 of 37

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Energy & Fuels

457

458 459

Figure 4. Effect of liquid-solid ratio on EY-SiO2 and SSM: (a) ACGR; (b) ACR

460 461


Energy & Fuels

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462 463

464 465

Figure 5. Effect of reaction temperature on EY-SiO2 and SSM: (a) ACGR; (b) ACR

466 467


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468 469

470 471

Figure 6. Effect of reaction time on EY-SiO2 and SSM: (a) ACGR; (b) ACR

472 473


Comparison of Silica Leaching Behaviors from the Acid-Leached

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