Mineralogical phase separation and leaching characteristics of typical toxic elements in Chinese lignite fly ash

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Mineralogical phase separation and leaching characteristics of typical toxic elements in Chinese lignite fly ash Fuli Liu a,b, Shuhua Ma b,⇑, Kun Ren b,c, Xiaohui Wang b a

Liaoning Technical University, College of Mining, Liaoning Fuxin 123000, China CAS Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China c China University of Mining and Technology, College of Chemical and Environmental Engineering, Beijing 100083, China b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Cr, Ni, Mo and Cd had a relatively high

proportion in the iron microbeads.
V, Cr, Mn, Co, Cu, Hg and Pb easily

migrated in the environment.
Cr, Mo, Cd and W were highly

enriched in the quartz-mullite mixture.
Ni, Cu, Zn and As had potential environmental risks in the amorphous component.

a r t i c l e

i n f o

Article history: Received 31 August 2019 Received in revised form 19 October 2019 Accepted 19 October 2019 Available online xxxx Keywords: Lignite fly ash Toxic element Mineralogical phase separation Leaching characteristic

a b s t r a c t To investigate the distribution characteristics of typical toxic elements in different mineralogical phases of fly ash is of significance when fly ash is comprehensively utilized. In this study, lignite fly ash can be preliminarily separated into three mineralogical phases: unburned lignite, iron microbeads and aluminate-silicate microbeads by two methods namely screening and dry magnetic separation. Then, the aluminate-silicate microbeads were subjected to two-step leaching. The first step was to investigate whether toxic elements migrated easily in the environment by column leaching test. In the second step, the aluminate-silicate microbeads were stripped from the surface of the particles to the internal by the acid-base combined leaching method, then the structural characteristics of the product and the trend of toxic elements content were explored. The results showed that there were few toxic elements in unburned lignite and the toxic elements Cr, Ni, Mo and Cd had a relatively high proportion in the iron microbeads. Column leaching results showed that the toxic elements V, Cr, Mn, Co, Cu, Hg and Pb had higher leaching rates, which proved that these elements were significantly enriched on the surface of the particles and easily migrated in the environment. Cr, Mo, Cd and W were highly enriched in the quartz-mullite mixture. Mn, Co, Ni, Cu, Zn and As were highly enriched in the amorphous component. The toxic elements exhibited different leaching rules during the acid-base combined leaching process revealing the complex embedded relationship with constant elements. Ó 2019 Elsevier B.V. All rights reserved.

⇑ Corresponding author at: National Engineering Laboratory for Hydrometallurgy Cleaner Production Technology, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China.

1. Introduction Coal fly ash is an industrial solid waste product collected by a dust removal device in a coal-fired power plant (Shemi et al.,

https://doi.org/10.1016/j.scitotenv.2019.135095 0048-9697/Ó 2019 Elsevier B.V. All rights reserved.

Please cite this article as: F. Liu, S. Ma, K. Ren et al., Mineralogical phase separation and leaching characteristics of typical toxic elements in Chinese lignite fly ash, Science of the Total Environment, https://doi.org/10.1016/j.scitotenv.2019.135095

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2012). China is the world’s largest coal producer and consumer (Li et al., 2015). It is the largest producer of coal fly ash in the world with about 580 million tons of coal fly ash produced per year and its current stockpiles have reached 2.5 billion tons (Liu et al., 2016; Salameh, 2003). The massive accumulation of coal fly ash causes serious harm to the ecological environment and human health (Schwartz et al., 2016). Coal fly ash has become China’s largest single source of solid pollution (Yao et al., 2015). Therefore, comprehensive utilization of large-scale coal fly ash has become one of the environmental problems that China urgently needs to resolve. In recent years, researchers have developed a variety of fly ash high-value utilization technologies such as preparing soil conditioners, wastewater treatment agents (Bayat, 2002; Kuncoro and Fahmi, 2013), foam glass (Chen et al., 2011), zeolite (Zhou et al., 2014), glass ceramics (Rawlings et al., 2006), mullite ceramics (Blissett and Rowson, 2012; Li et al., 2009), etc. Irrespective of the utilization method adopted, the toxic elements in coal fly ash become the key factor that restrict its comprehensive utilization. The migration and accumulation of toxic elements in coal fly ash is a problem that cannot be ignored during the utilization of coal fly ash. In recent years, researchers have conducted various studies on the toxic elements in coal and coal fly ash. Among these studies, the leaching laws of toxic elements in coal fly ash have gained interest (Teixeira et al., 1992; Kozliak and Paca, 2012) because a direct hazard of the toxic elements in coal fly ash is their migration and transformation in the environment (Jambhulkar et al., 2018). Akar et al. conducted coal fly ash leaching experiments through the TCLP-1311 toxicity test method and found that Cd, Co, Cu, Pb, Ni and Zn have high mobility (Akar et al., 2012). Zhao et al. found that the leaching concentration of Cr in coal fly ash was higher than the allowable range, and the pH of the coal fly ash leaching solution was alkaline (Zhao et al., 2018). Choi et al. also studied the effects of coal fly ash on the surrounding groundwater composition through the leaching methods (Choi et al., 2002). Additionally, many studies focused on the transforming behavior of the toxic elements in fly ash. Lopez-Anton et al. explored the behavior of Hg during coal combustion and the oxidation mechanism along the flue gas path, which can be used to identify and quantify the morphology of Hg in coal fly ash (Lopez-Anton et al., 2010). Yuhan et al. used XANES to explore the transformation process of As in the coal combustion process, revealing that As in fly ash may exist in the form of FeAsO4 and Ca3(AsO4)2 (Yang et al., 2019). Christof used air classification to separate fly ash into medium-sized particles of sizes 2.2, 5.4, 9.7, 19 and 43 lm to investigate the relationship between element concentration and particle size in coal fly ash and found that most of the rare elements were enriched in small particles (Christof, 2018). The above scholars mainly studied the possible existence of toxic elements and proved that some toxic elements in coal fly ash will migrate and accumulate in the environment. For the purpose of removing or solidifying toxic elements (Rodella et al., 2014), it is quite necessary to clarify the distribution of toxic elements in coal fly ash particles and determine whether the toxic elements in the different particles are significantly enriched. However, there are insufficient studies on this subject. In the process of power generation, insufficient combustion of raw coal may lead to high residual carbon content in coal fly ash. Unburned carbon is likely to adsorb toxic elements because of its excellent adsorption; hence, toxic elements may be enriched in unburned coal. Besides, coal fly ash contains crystal phases of quartz and mullite, whose surfaces are generally coated with more amounts of amorphous silicate and less amounts of amorphous aluminate, which do not crystallize during the cooling process of the flue gas, what is more, the quartz-mullite mixture and the amorphous component are easily separated because of their

chemical activity differences (Luo et al., 2017a). Generally, a number of other minerals are also present in fly ash in small amounts, e.g., Ca, Fe, Na, K and Mg. They are melted during coal combustion to produce various microbeads. Among these beads, iron microbeads are easily separated for their magnetic character. It is reported that iron microbeads may adsorb heavy metal elements in the flue and easily adsorb organic and inorganic trace contaminants, which are harmful to the environment (Vassilev et al., 2004). In general, there are many confusions in the distribution of toxic elements in coal fly ash. Toxic elements in contaminated soil are imported from outside sources and there are various methods to explore the distribution rules, but there is almost no research method for the native toxic elements in coal fly ash that has been recognized by the public. Therefore, based on the constant elements occurrences and particle structure characteristics of lignite fly ash, a systemic study for clarifying the distribution of toxic elements in lignite fly ash particles was proposed, including V, Cr, Mn, Co, Ni, Cu, Zn, As, Mo, Cd, W, Hg and Pb. It is necessary for the development of new coal fly ash comprehensive utilization techniques and enhancement of zero waste. Firstly, the enrichment of toxic elements in different particles of fly ash was investigated, including unburned lignite, iron microbeads and aluminate-silicate microbeads. Then, the aluminate-silicate microbeads were leached in two steps. The first step was to simulate the natural environment to conduct a column leaching test on the lignite fly ash to verify whether the toxic elements were easily migrating in the environment and affected the surrounding environment. The second step was to gradually peel off the amorphous component in the aluminate-silicate microbeads from the surface to the inside of the particle. The purpose was to verify the potential risk of toxic elements in the lignite fly ash and to clarify the embedded relationship between the toxic element and the constant element. This work can act as a fundamental support for the comprehensive utilization of huge amounts of solid waste, i.e., coal fly ash, and provide a highlight on the composition and occurrence state of trace elements in coal fly ash.

2. Experimental section 2.1. Experimental materials The lignite fly ash used in this experiment was derived from a lignite combustion product sampled from a power plant in Inner Mongolia, China. The reagents used in this experiment included sodium hydroxide, hydrochloric acid, nitric acid and hydrofluoric acid (Guaranteed Reagent, Sinopharm Group).

2.2. Experimental equipment Leaching column: PMMA material with outer diameter of 75 mm, wall thickness of 2 mm and height of 50 cm, The bottom end is provided with a microporous plastic sheet and gauze. The plastic balls of 2 mm in diameter and 5 cm in height are filled on the gauze. The leaching solution is dropped into the fly ash layer above the plastic balls by a flow type peristaltic pump and controlling the liquid flow rate of 0.25 mL/min to ensure that the liquid height in the leaching column is 8–10 cm. The solid phase sample was digested by an MDS-6G microwave digestion instrument (Shanghai Xinyi Company, China) and the digestion system consisted of HNO3 + HCl + HF in the volume ratio of 3: 1: 1. The digestion procedure is microwave power of 800 W, heating rate of 10 °C/min, gradient heating mode is 150 °C for 10 min, 180 °C for 5 min, 210 °C for 25 min, then naturally cooled to room temperature.

Please cite this article as: F. Liu, S. Ma, K. Ren et al., Mineralogical phase separation and leaching characteristics of typical toxic elements in Chinese lignite fly ash, Science of the Total Environment, https://doi.org/10.1016/j.scitotenv.2019.135095

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2.3. Experimental procedure

Ai ¼ ð f 3 U i  f 4 M i Þ = f 5

ð9Þ

The lignite fly ash sample was first dried in a 105 °C oven for 24 h. It was found that 100 lm sieve could effectively screen out unburned lignite by screening lignite fly ash with different size standard sieve, thus a certain amount of raw ash was sieved using a 100 lm standard sieve and the material up the sieve was set as unburned lignite, which was taken from the sieve and recorded the quality. The remaining lignite fly ash was sorted out of the iron microbeads in the lignite fly ash using a laboratory magnetic separator, and the quality of the iron microbeads was also recorded. The aluminate-silicate microbeads obtained after magnetic separation were weighed 1000 g and added to the leaching column. Highpurity water was used as the leaching solution, and the time was started for 24 h when the leaching solution appeared below the leaching column, then the leached lignite fly ash was dried and weighed. The lignite fly ash after the column leaching was subjected to second step of leaching by an acid-base combined leaching method to dissolve the amorphous component in the lignite fly ash according to the difference in reactivity of aluminate and silicate in acid-base system, and the product was dried to obtain quartz-mullite mixture. The detailed process flow and parameters are shown in Fig. 1.

Bi ¼ ðf 4 Mi  f 6 Pi Þ = f 7

ð10Þ

2.4. Calculation methods The mass fractions of the unburned lignite, iron microbeads, aluminate-silicate microbeads, column leaching component, amorphous component and quartz-mullite mixture in the lignite fly ash are calculated according to Equation (1) ~ (7), respectively.

where Ui, Mi, Pi, Ai and Bi are the relative contents of element i in the aluminate-silicate microbeads, column leached lignite fly ash, quartz-mullite mixture, column leaching component and amorphous component, respectively. The absolute contents of the elements in the unburned lignite, iron microbeads, column leaching component, amorphous component and quartz-mullite mixture are calculated using Equations fi Di / Wi  100%, f2 Ei / Wi  100%, f5 Ai / Wi  100%, f7 Bi / Wi  100% and f6 Pi / Wi  100%, respectively. Where Wi, Di and Ei are the relative contents of element i in the raw ash, unburned lignite and iron microbeads, respectively. 2.5. Characterizations The phase composition of the solid samples was analyzed using an Empyrean type X-ray diffractometer (XRD) from Netherlands. The concentration of constant elements in the samples was determined by a Thermo Scientific iCAP 7300 V inductively coupled plasma atomic emission spectrometer (ICP-AES). The content of toxic elements in the samples was determined by the American Thermo Scientific iCAP Qc type inductively coupled plasma mass spectrometer (ICP-MS). The surface morphology of the sample was observed by a JSM-7610F thermal field emission scanning electron microscope (SEM) from Japanese electronic company. The particle size distribution of solid samples was analyzed using a Mastersizer 2000 laser particle size analyzer from Malvern Instruments. The specific surface area of the samples was tested using an AUTOSORB-1-C-TCD type physical chemical adsorption instrument manufactured by Quantachrome, USA.

f 1 ¼ m1 = m  100 %

ð1Þ

f 2 ¼ m2 = m  100 %

ð2Þ

f 3 ¼ m3 = m  100 %

ð3Þ

3. Results and discussion

f 4 ¼ m4 = m  100 %

ð4Þ

3.1. Analysis of mineralogical phase characteristics of lignite fly ash

f5 ¼ f3  f4

ð5Þ

f 6 ¼ m5 = m  100 %

ð6Þ

f7 ¼ f4  f6

ð7Þ

The mass fraction of unburned lignite, iron microbeads, quartzmullite mixture and amorphous component obtained by the separation method of Fig. 1 were 3.90, 4.21, 31.56 and 60.33%, respectively and the quality of lignite fly ash was almost unchanged during column leaching. Lignite fly ash is a kind of heterogeneous material. It can be seen from the main element content of lignite fly ash mentioned in Table 1 and the SEM result (Fig. 3a) that lignite fly ash is mainly spherical particles condensed by aluminate and silicate. According to the cumulative particle size curve (Fig. 4), it is found that the size distribution of the lignite fly ash particles is not uniform. The unburned lignite has a larger particle size than the raw ash and has the characteristic of loose porosity (Fig. 3b). The median diameter of the unburned lignite sorted by screening is as high as 548 lm. Because iron microbeads have high iron content, they exhibit strong magnetic properties. From the SEM image of the iron microbeads shown in Fig. 3, it is found that the surface of the iron microbeads is rougher than that of the aluminate-silicate microbeads, and the specific surface area of the aluminate-silicate microbeads is 0.46 m2/g, but the iron microbeads is 0.60 m2/g. Therefore, the surface energy of the iron microbeads is larger than that of the aluminate-silicate microbeads, which resulting in the iron microbeads have a stronger enrichment effect on the submicron fly ash particles than aluminate-silicate microbeads during the condensation of the flue gas. This directly leads to the fact that iron microbeads usually have three microscopic structures namely hollow microbeads, mother beads (internal coated secondary bead particles), and solid microbeads (Fig. 3c, d and e) (Fisher et al.,

where f1, f2, f3, f4, f5, f6 and f7 are the mass fractions of the unburned lignite, iron microbeads, aluminate-silicate microbeads, column leached lignite fly ash, column leaching component, quartz-mullite mixture and amorphous component, respectively. m, m1, m2, m3, m4 and m5 represent the quality of raw ash, unburned lignite, iron microbeads, aluminate-silicate microbeads, column leached lignite fly ash and quartz-mullite mixture, respectively. The solid samples are weighed to about 0.1 g (accurate to 0.0001 g) and dissolved according to the microwave digestion program. Subsequently, the solution is transferred to a 100 mL volumetric flask and the toxic element contents are tested by ICP-MS. The relative content of the elements are calculated using Eq. (8).

wi ¼ C i  100 mL = mi

ð8Þ

where wi is the relative amount of an element in the sample (lg/g), Ci is the concentration of an element in solution as is measured by ICP-MS (lg/L), and mi is the weighing quality of the sample (g). The relative content of the elements in the column leaching component and amorphous component are calculated using Equation (9) and (10) according to the conservation law of element mass.

Please cite this article as: F. Liu, S. Ma, K. Ren et al., Mineralogical phase separation and leaching characteristics of typical toxic elements in Chinese lignite fly ash, Science of the Total Environment, https://doi.org/10.1016/j.scitotenv.2019.135095

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Raw ash Screening / 100 μm

+

-

Unburned lignite

Remaining ash Dry magnetic separation

Aluminate-silicate microbeads

Iron microbeads

Column leaching First alkali treatment: C(NaOH) = 50 mol/L L / S = 4 mL/g, T = 95 °C, t = 150 min First acid treatment: C(HCl) = 3 mol/L L / S = 5 mL/g, T = 75 °C, t = 75 min Second alkali treatment: C(NaOH) = 50 mol/L L / S = 4 mL/g, T = 95 °C, t = 150 min

Acid-base combined leaching (L / S means liquid to solid ratio)

Second acid treatment: C(HCl) = 3 mol/L L / S = 5 mL/g, T = 75 °C, t = 75 min

Amorphous component Quartz-mullite mixture Fig. 1. Process and parameters of mineralogical phase separation of lignite fly ash.

Table 1 The constant element contents in different mineralogical phases. Element / %

Raw ash

Unburned lignite

Iron microbeads

Quartz-mullite mixture

Amorphous component

SiO2 Al2O3 TiO2 Fe2O3 Na2O K2O MgO CaO LOI

56.37 21.94 0.67 3.54 0.93 1.03 2.45 10.07 1.90

27.57 9.78 0.72 3.39 0.60 0.84 1.86 9.19 41.60

11.45 4.11 1.61 68.80 0.23 0.53 6.63 6.17 –

76.40 17.43 0.84 0.18 0.89 0.95 0.68 0.29 –

50.89 26.33 0.51 0.75 1.02 1.12 3.12 15.52 –

Note: LOI means loss on ignition.

1978; Ramsden and Shibaoka, 1982). From the XRD (Fig. 2) and elemental composition (Table 1) of the iron microbeads, their iron content was found to be as high as 68.80% and was present in the form of magnesia iron and ferric oxide, and also had melt of uncrystallized calcium iron and aluminum oxide. From the particle size analysis of the iron microbeads shown in Fig. 4, it is found that

the selected iron microbeads have a larger particle size than the raw ash and the median diameter reached about 60 lm. The aluminate-silicate microbeads of lignite fly ash have a complex composition, which can be divided into two categories according to the difference in activity of aluminate and silicate. One is chemically stable quartz and mullite, and the other is chemically

Please cite this article as: F. Liu, S. Ma, K. Ren et al., Mineralogical phase separation and leaching characteristics of typical toxic elements in Chinese lignite fly ash, Science of the Total Environment, https://doi.org/10.1016/j.scitotenv.2019.135095

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1

Intensity(a.u.)

4

6

1 61 4

ash, thus obtaining the quartz-mullite mixture (Zhang et al., 2016). Fig. 3e shows the formation of quartz-mullite mixture, and through the acid-base combined leaching process, the median particle size of the lignite fly ash particles is reduced from 30 to 10 lm. The amorphous component is effectively removed through the elemental composition of the quartz-mullite mixture and the amorphous component shown in Table 1.

1 Quartz 2 Magnesium iron oxide 3 Ferric oxide 4 Titanium dioxide 5 Akermanite 6 Mullite

1 1

Raw ash

1

1

5

1 3.2. Distribution characteristics of typical toxic elements in different mineralogical phases

1 2 12 1 10

20

1

1 5 2 4 33 1 2 3 33 30

40

50

1 1

Unburned coal

2 3 Iron microbeads 60

70

80

90

2θ(°) Fig. 2. XRD analysis of different mineralogical phases of lignite fly ash.

active amorphous (Luo et al., 2017b). The formation of the amorphous component is because flue gas does not crystallize in time during the extremely rapid cooling process and condenses on the surface of the spherical particles. Selective dissolution methods are often used to study the distribution of elements in ores with complex intercalations. For the complex distribution relationship between the lignite fly ash mixture and amorphous phase, it is difficult to achieve efficient separation by conventional physical methods. Dai et al. studied the distribution of rare elements in coal fly ash by using HF to extract the amorphous component from coal fly ash (Dai et al., 2010). Although HF can effectively dissolve the amorphous component in coal fly ash, it is highly corrosive and easily destroys the crystal phase in coal fly ash. Moreover, the reaction product H2SiF6 of HF and coal fly ash is highly corrosive and toxic. According to the difference of aluminum and silicon activity in lignite fly ash, this experiment uses acid-base combined leaching method to separate the amorphous component in lignite fly

Through the separation of the different mineralogical phases of lignite fly ash, unburned lignite, iron microbeads and quartzmullite mixture were obtained from the lignite fly ash, and the content of elements in the raw ash and the three products were tested separately. The element content in the column leaching component and amorphous component were obtained using the calculation method mentioned in section 2.4. The relative content of toxic elements in different mineralogical phases were shown in Fig. 5. We can find from Fig. 5 that toxic elements show different distribution characteristics in mineralogical phases. V exhibits a relatively uniform distribution characteristic in each mineralogical phase and does not have a significantly high relative content in one phase. Cr shows a high relative content in iron microbeads and quartz-mullite mixture, which are 217.81 and 68.87 lg/g, respectively. The relative content of Ni in iron microbeads is as high as 169.57 lg/g. The relative content of Cu in the iron microbeads and the amorphous component is 54.16 and 72.76 lg/g, respectively. Zn is present in the amorphous component at a higher content of 87.25 lg/g. As is higher in the iron microbeads and the amorphous component, which are 11.62 and 14.19 lg/g respectively. Mo exhibits relatively high concentrations in unburned lignite, iron microbeads and quartz-mullite mixture, which are 20.66, 63.00 and 15.91 lg/g, respectively. W has a small content in unburned lignite and leaching component, which is evenly distributed in the other three phases. Pb is mainly in the leaching

Fig. 3. SEM images of lignite fly ash. (a: raw ash, b: unburned lignite, c–e: iron microbeads, f: quartz-mullite mixture).

Please cite this article as: F. Liu, S. Ma, K. Ren et al., Mineralogical phase separation and leaching characteristics of typical toxic elements in Chinese lignite fly ash, Science of the Total Environment, https://doi.org/10.1016/j.scitotenv.2019.135095

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Cumulative distribution (%)

on the surface of the spherical particles, which is why many toxic elements have a relatively high content in the column leaching component and amorphous component. According to the proportion of the quality of different mineralogical phases, the absolute content of typical toxic elements in lignite fly ash can be obtained by the calculation method presented in section 2.4. The results are shown in Fig. 6. From the overall distribution characteristic of toxic elements in the different mineralogical phases, the content of toxic elements in unburned lignite can be negligible, and the proportion of the elements with high absolute contents of Mo and Cd reached 6.65 and 5.50%, respectively. Although the relative content of toxic elements adsorbed by iron microbeads is obvious, the mass content of iron microbeads in the lignite fly ash is only 4.21%, resulting in a small absolute content of toxic elements in the iron microbeads. Only Cr, Ni, Mo, and Cd have high proportions of iron microbeads, i.e., 16.12, 17.20, 21.89 and 19.66%, respectively. The toxic elements Cr, Mo, Cd and W, whose contents are 38.19, 41.41, 52.62 and 31.72%, respectively, are easily embedded in the quartzmullite mixture. The toxic elements embedded in the quartzmullite mixture undergo more complex migration processes compare with toxic elements in other mineralogical phases, the reason is that the quartz-mullite mixture is converted from amorphous component during the cooling process and it takes more energy and time for toxic elements to embed in the quartz and mullite lattice. The crystal structure of mullite has short-range ordered oxygen vacancies, which can be incorporated into various external cations, especially transition metals and rare earth elements. Therefore, it is highly probable that toxic elements are embedded in the crystal structure of mullite during the cooling of flue gas (Hartmut et al., 2015). Column leaching results showed that the toxic elements V, Cr, Mn, Co, Cu, Hg and Pb had higher leaching rates, which proved that these elements were significantly enriched on the surface of the particles and easily migrated in the environment. Because the particle structure of the lignite fly ash is not destroyed during the leaching process, the leaching toxic elements are adsorbed on the surface of the fly ash particles by physical adsorption. The lignite fly ash used in this experiment contained amorphous component with a mass proportion of 60.33%, which directly leaded to a high absolute content ratio of the toxic elements Mn, Co, Ni, Cu, Zn and As in the amorphous component to be 56.46, 49.93, 44.82, 47.90, 58.51 and 72.11% of the total toxic elements in the lignite fly ash, respectively. The

Raw ash Unburned lignite: 3.90% Iron microbeads: 4.21% Quartz-Mullite mixture: 31.56%

120 100 80 60 40 20 0 0.1

1

10

100

1000

10000

Particle size (μm) Fig. 4. Mineralogical phase particle size curve of lignite fly ash.

component and the amorphous component, and the content is as high as 31.38 and 42.99 lg/g. Co has a relatively high content in iron microbeads and amorphous component, which are 49.17 and 31.76 lg/g, respectively. Cd has a prominent content in the iron microbeads, but is rare in the leaching component and the amorphous component. Hg is rarely found in quartz-mullite mixture, and is evenly distributed in other mineralogical phases. Mn exhibits a relatively high content in iron microbeads and amorphous component, which are 643.89 and 810.20 lg/g, respectively. From the above analysis results, it is known that the relative content of toxic elements in unburned lignite is relatively low. The adsorption of toxic elements in lignite fly ash by iron microbeads is obvious. In different mineralogical phases per unit mass, multiple toxic elements have significantly higher adsorption amounts in iron microbeads. This is because the surface energy of the iron microbeads is larger than that of aluminate-silicate microbeads, thus, the iron microbeads not only have a strong adsorption effect on the submicron fly ash particles, but also have a strong enrichment effect on the toxic elements in the flue gas. In the process of condensation of flue gas, fly ash particles gradually form, due to its strong adsorption capacity, toxic elements will be gradually wrapped in spherical particles. Before entering the dust collector, a certain amount of toxic elements are adsorbed

Unburned lignite

Iron microbeads

Column leaching component

Amorphous component

Quartz-Mullite mixture

2.5

60

800

200

Relative content (μg/g)

700

2.0

50

600 40

1.5

500

30

400

100 1.0

300

20

200

0.5

10

100 0

0

V

Cr

Ni

Cu

Zn

As

Mo

W

Pb

Co

0.0

Cd

Hg

0

Mn

Fig. 5. Relative content of toxic elements in the different mineralogical phases.

Please cite this article as: F. Liu, S. Ma, K. Ren et al., Mineralogical phase separation and leaching characteristics of typical toxic elements in Chinese lignite fly ash, Science of the Total Environment, https://doi.org/10.1016/j.scitotenv.2019.135095

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Fig. 6. Absolute content ratio of toxic elements in the different mineralogical phases.

3.3. Phase changes and elements removal rate during leaching test It was found through the above studies that most of the toxic elements in the lignite fly ash were enriched in the amorphous component. The acid-base combined leaching method is a process of gradually separating the amorphous component from the surface of the lignite fly ash particles to the inside and is also a process of gradually removing toxic elements. Therefore, the correlation of removal rate between the constant and toxic elements is helpful to reveal the embedding relationship of toxic elements in the amorphous component. In the acid-base combined leaching method, the lignite fly ash reacts with NaOH solution under normal pressure. Owing to the difference in the activity of aluminate and silicate in the amorphous and crystal components, the highly active aluminate and silicate in the amorphous component are dissolved in the NaOH solution (Eqs. (11) and (12)), while quartz and mullite cannot be dissolved. However, when the aluminate and silicate component in the solution reaches a certain concentration, sodalite is adsorbed on the surface of the lignite fly ash particles, thereby preventing the NaOH solution from further reacting with the amorphous aluminate and silicate inside the particles (Equation (13)) (Sun et al., 2016). The sodalite is dissolved from the surface of the lignite fly ash particles with HCl to expose the active aluminate and silicate inside the particles. The amorphous aluminate and silicate are dissolved again using NaOH solution, and the by-product, i.e., sodalite, is dissolved with HCl to obtain the quartz-mullite mixture. When the product is subjected to alkali treatment again, no sodalite is produced and the mass remain constant until complete separation of the amorphous component. In this study, the acid-base combined leaching was carried out twice to completely strip the amorphous component (Hu et al., 2018).

2NaOH + SiO2 = Na2 SiO3 + H2 O

ð11Þ

2NaOH + A12 O3 = 2NaAlO2 + H2 O

ð12Þ

6NaAlO2 + 6Na2 SiO3 + 8H2 O = Na8 Al6 Si6 O24 (OH)2 (H2 O)2 + 10NaOH ð13Þ Combined with the XRD analysis of the product (Fig. 7), the constant elements cumulative removal rate (Fig. 8) and the product morphology analysis (Fig. 9) during the acid-base combined leaching process, we can find that this is the process of the production

Intensity(a.u.)

structure of the elements in the amorphous component is complex, and it is necessary to explore the relationship between the toxic and constant elements in the amorphous component.

19800 13200 6600 0 12900 8600 4300 0 12900 8600 4300 0 10500 7000 3500 0 3300 2200 1100 0 4200 2800 1400 0 3600 2400 1200 0

1 1 Quartz 2 Mullite 3 Sodalite 4 Titanium dioxide

1

Second acid

2

2 1

3 13

Second alkali 150min

1 Second alkali 90min

1 1

First acid

1 1 3 13

First alkali 90min

3

3

1 1

First alkali 60min

1 1

42

Aluminum silicon microbeads

12

2 1

1

1 2 2

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95

2θ(°) Fig. 7. XRD analysis of the acid-base combined leaching process.

Please cite this article as: F. Liu, S. Ma, K. Ren et al., Mineralogical phase separation and leaching characteristics of typical toxic elements in Chinese lignite fly ash, Science of the Total Environment, https://doi.org/10.1016/j.scitotenv.2019.135095

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100

SiO2

Cumulative removal rate (%)

Al2O3 80

CaO K2O

60

MgO TiO2

40

20

0

Aluminate silicate First alkali microbeads treatment

First acid treatment

Second alkali treatment

Second acid treatment

Fig. 8. Cumulative removal rate of constant elements in the acid-base combined leaching process.

and dissolution of sodalite and deep removal of amorphous component from lignite fly ash. The first alkali treatment of lignite fly ash produced a diffraction peak of sodalite for about 90 min and the dissolution of the amorphous component led to an increase in the intensity of the diffraction peak of the crystalline phases, which proves that the dissolution of amorphous component leads to an increase in the proportion of crystalline phases. From the first alkali treatment of the removal rate of aluminum and silicon oxide, it is found that the dissolved aluminate was almost completely converted into sodalite in the solution and adsorbed on the surface of the fly ash particles (Fig. 9a). After the first acid treatment of the

product, it is found that the sodalite is dissolved and the spherical particles of the lignite fly ash are destroyed (Fig. 9b), which leading to the diffraction peak intensity of the crystalline phases further increased. From the removal rate of Al2O3 and SiO2 in the first acid treatment, not only the aluminum in the sodalite is dissolved, but also the activated aluminate in the amorphous component is partially dissolved, what is more, CaO and MgO in the amorphous component can have more than 90% of removal rate. The second alkali treatment for about 90 min produce sodalite phase and the active silicate in the lignite fly ash is further removed. The intensity of the diffraction peak of the quartz-mullite mixture in the acidbase combined leaching process is gradually increasing, indicating that the proportion of the quartz-mullite mixture in the product is gradually increasing, and the quartz-mullite mixture is obtained after the second acid treatment (Fig. 9d). It is find that TiO2 reached a removal rate of 45% after a slight acid-base treatment and it is not removed during the subsequent acid-base treatment, which proves that Ti is mostly present in the quartz-mullite mixture. Fig. 10 shows the change in particle size and specific surface area of lignite fly ash during the acid-base combined leaching process. The peeling process of the amorphous component during the acid-base combined leaching process is indirectly explained from the viewpoint of particle size and specific surface area. The coating of sodalite increases the lignite fly ash particles and reduces the specific surface area and the quartz-mullite mixture with a particle diameter of 10.81 lm and specific surface area of 1.16 m2/g was obtained after the acid-base combined leaching. Fig. 11 shows the trend of the cumulative removal rate of elements from the surface of the lignite fly ash particles to the internal stripping process. The toxic elements Co, Zn, Cu, Hg and Pb have achieved final removal rates of 90.86, 83.89, 83.59, 88.22 and 88.58%, respectively, after the first acid-base treatment, which indicates that most of these five toxic elements are present on the

Fig. 9. SEM analysis of the acid-base combined leaching process. (a: first alkali treatment for 150 min, b: first acid treatment, c: second alkali treatment for 150 min, d: second acid treatment).

Please cite this article as: F. Liu, S. Ma, K. Ren et al., Mineralogical phase separation and leaching characteristics of typical toxic elements in Chinese lignite fly ash, Science of the Total Environment, https://doi.org/10.1016/j.scitotenv.2019.135095

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Particle size (μm)

1.0 25

0.8

20

0.6

0.4

15

0.2

Specific surface area (m2/g)

30

10 0.0 Aluminate silicate Alkali Alkali Alkali Alkali Alkali First Second Second microbeads

30min 60min 90min 120min 150min Acid Alkali

Acid

Fig. 10. Particle size and specific surface area change during acid-base combined leaching.

Cumulative removal rate (%)

100

80

60

Ni Cr As Cd Hg Pb

40

20

0

Aluminate silicate Column First alkali First acid microbeads treatment leaching treatment

Cumulative removal rate (%)

100

80

60

Second alkali Second acid treatment treatment

V Mn Co Cu Zn Mo W

9

processes reached 48.65 and 56.12%, respectively; however, the rate did not change much during the acid treatment process. The form of Mo is easily removed by NaOH but is not dissolved by HCl. The SiO2 in the amorphous component reacts only with the NaOH solution and does not react with HCl, indicating that the toxic element Mo may have an embedded relationship with SiO2 in the amorphous component. The toxic element Cd show the opposite phenomenon to Mo. The removal rate of Cd was only 4.64% during the first alkali treatment; however, it reached 44.67% after the first acid treatment, which is close to the final removal rate. This indicates that Cd is easily removed by HCl. V, Cr, Ni, As and Mn exhibit a similar removal rate trend to that of SiO2, which indicates that these five toxic elements may have embedded relationships with the aluminate-silicate in the amorphous component. They exist on the surface and inside the particle and gradually dissolve during the acid-base combined leaching process. 4. Conclusions Lignite fly ash can be separated into unburned lignite, iron microbeads and aluminate-silicate microbeads through screening and dry magnetic separation. Toxic elements in unburned lignite are rare. Cr, Ni, Mo and Cd have higher relative contents in the iron microbeads because of the special structure of the iron microbeads. Cr, Mo, Cd and W have a high degree of enrichment in the quartzmullite mixture, which is closely related to the lattice structure of mullite. Column leaching results show that the toxic elements V, Cr, Mn, Co, Cu, Hg and Pb have higher leaching rates, which proves that these elements are significantly enriched on the surface of the particles and easily migrate in the environment. Toxic elements in amorphous component are potentially hazardous and tend to migrate in the environment under more severe conditions. In the acid-base combined leaching process, the toxic elements Co, Zn, Cu, Hg and Pb approach the final removal rate after the first acid-base treatment. The occurrence state of W in the amorphous phenomenon is single and its removal rate in the first alkali treatment is close to the final removal rate. Mo and Cd exhibit high fluidity in NaOH and HCl diluted solutions, respectively. V, Cr, Ni, As and Mn show similar removal rate trends to SiO2, which indicates that they may have an embedded relationship with the aluminum– silicon in the amorphous component. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

40

20

Acknowledgement 0 Aluminate silicate Column First alkali First acid microbeads treatment leaching treatment

Second alkali Second acid treatment treatment

Fig. 11. Cumulative removal rate of toxic elements in the leaching process.

The authors are very grateful to the financial support of the major science and technology projects in Inner Mongolia Autonomous Region and the project name is Preparation and Application Demonstration of Fly Ash Based Soil Conditioner (2060901). References

surface of the particles and have high fluidity in both acidic and alkaline environment. The removal rate of toxic element W in the first alkali treatment process reached 60.49% and did not increase significantly in the subsequent treatment, which indicates that W is in a single form in the amorphous component and is easily removed by the NaOH solution at once. The removal rate of toxic element Mo in the first and second alkali treatment

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Please cite this article as: F. Liu, S. Ma, K. Ren et al., Mineralogical phase separation and leaching characteristics of typical toxic elements in Chinese lignite fly ash, Science of the Total Environment, https://doi.org/10.1016/j.scitotenv.2019.135095