Recovery of rare earth elements from coal fly ash by integrated physical separation and acid leaching

Journal Pre-proof Recovery of rare earth elements from coal fly ash by integrated physical separation and acid leaching Jinhe Pan, Tiancheng Nie, Behzad Vaziri Hassas, Mohammad Rezaee, Zhiping Wen, Changchun Zhou PII:

S0045-6535(20)30305-2

DOI:

https://doi.org/10.1016/j.chemosphere.2020.126112

Reference:

CHEM 126112

To appear in:

ECSN

Received Date: 31 July 2019 Revised Date:

27 January 2020

Accepted Date: 3 February 2020

Please cite this article as: Pan, J., Nie, T., Hassas, B.V., Rezaee, M., Wen, Z., Zhou, C., Recovery of rare earth elements from coal fly ash by integrated physical separation and acid leaching, Chemosphere (2020), doi: https://doi.org/10.1016/j.chemosphere.2020.126112. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Credit Author Statement

Jinhe Pan: Conceptualization, Methodology, Formal analysis, Investigation, Writing - Original Draft Tiancheng Nie: Validation Formal analysis, Investigation, Behzad Vaziri Hassas: Methodology Formal analysis, Writing - Original Draft Mohammad Rezaee: Writing - Review & Editing, Visualization Zhiping Wen: Validation, Resources Changchun Zhou: Supervision, Funding acquisition

Based on REY occurrence theory and results of physical separation and acid leaching, an integrated procedure is shown in graphical abstract, which contains couple parts (classification, magnetic separation, leaching and solid/liquid separation) done and two ongoing parts: roasting for sample with hard leaching and purification from leachate.

1 2

Recovery of Rare Earth Elements from Coal Fly Ash by Integrated Physical Separation and Acid Leaching

3

Jinhe Pan1,2, Tiancheng Nie1, Behzad Vaziri Hassas2, Mohammad Rezaee2, Zhiping Wen1, Changchun Zhou*1

4 5

1

6 7 8 9

2

10 11

Key Laboratory of Coal Processing & Efficient Utilization, Ministry of Education, School of Chemical Engineering and Technology, China University of Mining & Technology, Xuzhou 221116, China Department of Energy and Mineral Engineering, Earth and Mineral Sciences (EMS) Energy Institute, Center for Critical Minerals, The Pennsylvania State University, University Park, 16802 PA, USA

*Corresponding author. E-mail address: [email protected] (C. Zhou)

Phone number: +86 0516 83591066

12 13

Abstract

14

Coal fly ash (CFA) is one of the most promising secondary sources of rare earth elements and

15

yttrium (REY). This research first studied the modes of occurrence of REY in CFA collected

16

from a China’s power generation plant which utilizes a coal feedstock with an elevated REY

17

content. The fact that rare earth minerals remain in CFA and REY associate with metal

18

oxides was proved by emission-scanning electron microscope with an energy-dispersive

19

X-ray spectrometer. The technical feasibility of recovery of REY from CFA was then studied

20

through conducting various physical separation methods followed by acid leaching. It was

21

found that REY are concentrated in fine particle size, non-magnetic and middle density

22

fractions. Using combined physical separation processes, the REY of CFA was enriched from

23

782 µg·g-1to 1025 µg·g-1. The acid leaching process was optimized for various parameters via

24

the Taguchi three-level experimental design. Upon optimization, the physical separation

25

product was leached at the optimum condition and 79.85% leaching efficiency was obtained.

26

Based on the obtained results, a conceptual process flowsheet was developed for recovery of

27

REY from CFA. Such recovery maximizes REY resources utilization and enhances

28

sustainability of CFA disposal.

29 30

Keywords:

31

Coal fly ash; Rare earth elements; Modes of occurrence; Physical separation; Acid leaching

32

1. Introduction

33

Rare earth elements and yttrium (REY) are vital to the modern society as they are used in

34

high-tech industry and a variety of consumer goods such as computers, cell phones, catalysis,

35

fluorescent lighting, permanent magnets, medical devices and advanced defense technology

36

(Hower et al., 2016a; Dai et al., 2018; Salem et al., 2019). However, there is a sharp

37

discrepancy between the high demand for and low production of REY with the limited raw

38

materials and feasible resources (Seredin and Dai, 2012; Dai et al., 2016; Zhang et al., 2018).

39

The REY supply crisis has aroused concerns and stimulated scientific research and

40

technological developments for the recovery of REY from secondary sources (Zhang et al.,

41

2018a and 2018b; Haberl et al., 2018; Soyol-Erdene et al., 2018, Mihajlovic et al., 2017 and

42

2018, Dai et al. 2017, Wu et al. 2019). These resources have also been previously considered

43

for the recovery of other elements including gallium, germanium, aluminum, and silicon

44

(Arroyo et al., 2014; Yao et al., 2014; Kazemian et al., 2010).

45

Coal fly ash (CFA), is the main coal combustion byproduct of which large amounts

46

worldwide are just disposed in the way of landfill or accumulation and might cause water and

47

soil pollution, disruption of ecological cycles(Woszuk and Bandura, 2019). CFA contains

48

higher REY concentration than that of original coal as the carbon content burns during the

49

combustion processes; thereby the REY concentration increases about 8-10 folds (Seredin et

50

al., 2012; Zhang et al., 2015; Stuckman et al., 2018). As a result, CFA could become a

51

reliable future source for REY, if cost-effective extraction processes are developed (Dai et al.,

52

2014a; Hower et al., 2016b). The development of such extraction methods requires an

53

understanding of the modes of occurrence of REY in CFS, followed by exploring efficient

54

physical and chemical recovery processes.

55

Several studies have investigated the modes of occurrence and distribution of REY in

56

different CFA samples collected from US (Hood et al., 2017; Taggart et al., 2016; Lin et al.,

57

2017a and 2017b, Kolker et al., 2017), England (Blissett et al., 2014), Poland (Blissett et al.,

58

2014; Smolka-Danielowska et al., 2010), and China (Pan et al., 2018 and 2019; Dai et al.,

59

2014b). However, a further thorough characterization of REY occurrence in CFA and their

60

association with the physical properties for a feasible recovery is crucial. Sequential chemical

61

extraction processes have been viewed as the most common quantitative methods to

62

determine the geochemical association and leachability of trace elements in various

63

feedstocks including CFA (Filgueiras et al., 2002; Long et al., 2009; Rao et al., 2008). This

64

procedure provides significant data to identify the modes of occurrence and evaluate scalable

65

REY extraction methods and processes (Taggart et al., 2018).

66

It has been suggested that the REY in CFA can be potentially recovered by acid leaching

67

(Franus et al., 2015). However, REY recovery from CFA through acid leaching is

68

challenging as these elements are entrained in the predominant glassy alumniosilicate phase,

69

hindering their solubilization and requiring strong acidic conditions. Preconcentration of REY

70

and removal of impurities, such as unburn carton, and ferric oxides, using physical separation

71

techniques prior to leaching are of interest (Lin et al., 2017a and 2018; Hower et al., 2013;

72

Kolker et al., 2017). This paper addresses a thorough understanding of the distribution and

73

modes of occurrence of REY in CFA, and potential recovery of REY through physical

74

separation and chemical extraction processes. Based on the obtained results, a conceptual

75

recovery flowsheet was developed.

76 77

2. Materials and Methods 2.1 Material

78

Representative CFA samples were received from a power generation plant, located in

79

Southwest China, and utilizing the late Permian coal which contains elevated content of REY

80

and other critical elements. REY-bearing minerals in the coal sources, identified by Scanning

81

Electron Microscopy / Energy Dispersive X-Ray Spectroscopy (SEM-EDS) analysis,

82

included rhabdophane, silicorhabdophane and florencite (Dai et al., 2014c). Upon arrival to

83

the laboratory, the sample was dried at 105 °C under N2 gas for 2 h, split into representative

84

sample lots, and stored in vacuumed plastic bags for characterization, and experimental

85

procedures.

86

The minerals present in the CFA sample were identified and their content quantified using

87

X-ray Diffraction (XRD) by the Rietveld method (Rietveld, 1969). The dominant phases in

88

the samples were found to be amorphous (70%) followed by mullite (20%), which are mainly

89

generated by pyrolysis of clay minerals in coal (mainly, kaolinite and illite) in different

90

temperatures during the combustion process. Quartz, maghemite, anhysrite, and lime also

91

occurred in minor quantities (as listed in Table 1). These findings were supported by the

92

results obtained from X-ray fluorescence (XRF) analysis. The sample was primarily

93

comprised of silica and aluminum oxides (account for about 70% of the sample). Low

94

maghemite content (3%), despite relatively high iron oxide content (13%), reveals the fact

95

that some of the Fe bearing minerals entrapped in the amorphous phase. Ca content in fly ash

96

is likely combined with various minerals to form Ca-bearing minerals/phases (e.g., anhydrite

97

and lime), which have also been reported as the carrier of REY (Dai et al., 2014d). The

98

majority of sulfur content in the CFA occurred in anhydrite phase.

99

Table 1. Compositions of CFA sample Phase (wt.%) Major Elements (wt.%) b Amorphous 70 Anhydrite 1.1 SiO2 43.66 MgO 0.78 S Mullite 20.4 Lime 1.1 Al2O3 25.61 TiO2 1.78 P Quartz 4.5 Fe2O3 12.89 K2O 2.08 LOI c 0.78 Maghemite 3 CaO 6.69 Na2O a

0.93 0.27 3.91

a: quantified by XRD analysis b: quantified by XRF analysis c: loss of ignition based on ASTM standard D3174 (ASTM, 2008)

Table 2. Minor and trace elements in coal fly ash

100 Elements Content (µg·g-1) Elements Content (µg·g-1) Elements Content (µg·g-1) Elements Content (µg·g-1)

Mn

V

Ba

Sr

Zn

Cr

Cu

As

Ni

Li

Co

851.4

391.5

1338

2624

280.4

272.3

206.6

132.6

116.4

69.91

51.51

Ga

Pb

Se

Rb

U

Be

Cs

Cd

Tl

Ag

Hg

48.85

31.70

26.50

9.89

4.95

4.27

1.78

0.52

0.51

0.41

0.08

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Y

Ho

138.7

290.3

30.28

113.6

21.59

3.51

20.88

3.23

20.04

109.9

3.98

Er

Tm

Yb

Lu

CREY1

UREY2

EREY3

Coutl

‫ܻܧܴܥ‬ ܴܶ‫ܻܧ‬

REY

12.02

1.66

10.88

1.56

262.3

211.4

308.4

0.85

34%

782.1

101

1. CREY: critical REY;

2. UREY: uncritical REY;

3. EREY: excessive REY;

102

The elemental content of CFA was analyzed using Inductivity Coupled Plasma – Mass

103

Spectroscopy (ICP-MS) (Table 2) (Pan et al., 2019). The analysis also showed the samples

104

contain elevated content (782 µg·g-1) of REY. To evaluate the market value and assessment

105

of REY-bearing ores, outlook coefficient (Coutl), which is the ratio of critical (Nd, Eu, Tb, Dy,

106

Y, and Er) to excessive (Ce, Ho, Tm, Yb and Lu) REY, is utilized. The larger the Coutl, the

107

higher profitability of the resources(Seredin and Dai, 2012). The Cout of the CFA was

108

calculated to be 0.85. This large Coutl, along with the fact CFA carry no mining cost, the

109

recovery of REY from this source could be potentially economically viable. The elevated

110

strontium content of CFA was related to the high Sr content of the coal sources which is

111

adjacent to a strontium mine in Sichuan Province. The other minor elements in the sample

112

were found to be in the elemental range of most CFA samples.

113

2.2 Modes of Occurrence of REY in CFA

114

The modes of occurrence of-of REY in the CFA sample were studied using a sequential

115

chemical extraction procedure as outlined in Table 3 (Usero et al., 1998; Mittermüller et al.,

116

2016; Pan et al., 2019). The procedure identifies the modes of occurrence/associations of

117

REY as one or more of the following categories: 1) ion-exchangeable, 2) acid soluble, 3)

118

metal oxides, 4) organic or sulfide, and 5) aluminate silicate. For this procedure, a 2 g

119

representative dry sample was stirred with the chemical reagent(s) at a pre-determined

120

volume, time, and temperature. The solution at each stage was then centrifuged and filtered,

121

and the solid was subjected to the next step of the procedure. The leachate of each step and

122

digested solid of the last step were submitted for elemental analysis using ICP-MS. The data

123

obtained from the elemental analysis were then utilized to identify the modes of occurrence

124

of REY in the CFA.

125

1 2 3

4

Table 3. The sequential chemical extraction procedure Temperature Time Reagent Form (°C) (h) 20 mL 1 M MgCl2 25 1 Ion-exchangeable 20 mL 1 M 25 5 Acid soluble NaAC/HOAC pH: 5 50 mL 0.04 M NH2OH▪HCl 95 3 Metal oxides (25% CH3COOH) 7.5 mL 3.5 M HNO3 + 85 5 Organic or sulfide 20 mL 10 M H2O2

12.5 mL 1 M NH4OAC in 3.5 M HNO3 5

Concentrated H2SO4 and HF

25 Digestion

5 Aluminosilicate

131

A scanning electron microscope (FE-SEM; ZEISS ΣIGMA), in conjunction with an energy-dispersive X-ray spectrometer (Oxford X-MaxN 20) (collectively, SEM-EDS), was applied to show morphology and microstructure of REY carriers and also to assume its phase by HSC Geo software. Images were captured via a retractable solid-state backscatter electron detector, which was used to more easily find REY-containing minerals as well as heavy element-containing minerals.

132

2.3 Physical Separation

133

The distribution of REY in the CFA as a function of particle size, magnetic susceptibility,

134

and density was studied to identify the potential processing methods for preconcentration of

135

REY prior to the leaching process. Screening tests according to Chinese standard

136

GB/T477-2008 were conducted to reveal the distribution of REY in each size fraction. A

137

combination of sieves of desired size (125, 100, 74, 55, 45, 38, 25 µm) was selected to wet

138

sieve the fly ash (typically 50 g) in sequence.

139

Magnetic tube tester (i.e., Davis tube) was used to investigate the REY fractionation as a

140

function of magnetic susceptibility. For this test, a 20 g representative sample of CFA was

141

mixed with 2 ml of alcohol and 500 mL of water, stirred uniformly, and fed to the tube. The

142

magnetic field strength was successively adjusted by changing the electric current to 1, 2, 3, 4,

143

and 5 A values, generating 5 fractions with corresponding magnetic field strengths, viz., S1 to

144

S5 and one non-magnetic fraction (i.e., S6).

145

The density fractionation process was conducted using heavy organic liquids, namely

146

tribromomethane, dibromomethane, and trichloromethane, which were mixed to obtain

147

different specific gravity (SG) values as 2.0, 2.2, 2.4, 2.6, and 2.8 g·cm-3. For the density

148

fractionation, a CFA representative sample was placed in a plastic centrifuge tube, and then

149

the heavy media liquid (starting with SG of 2.0) was added. After a thorough dispersion, the

150

mixtures were centrifuged, and then filtered to separate heavy and light solid fractions from

151

the bottom and top of the liquid, respectively. The process was repeated (usually three times)

152

at the same SG until all the materials sank. The sink fraction was then subjected to the next

126 127 128 129 130

153

step using the next higher SG, and the same process was repeated. All the density fractions

154

were then rinsed using ethanol and methanol to wash any remaining organic liquid. The

155

fractions were then dried, weighed, and analyzed for elemental content using ICP-MS to

156

identify REY distribution in various density fractions.

157

Based on the results of the physical separations, a representative CFA sample was subjected

158

to a sequence of effective physical processes to maximize the grade of the feed reported to

159

the downstream leaching process.

160

2.4 Acid Leaching

161

To maximize the REY recovery through acid leaching of the physically processed CFA, the

162

process was optimized for the process parameters, i.e., acid concentration, solid to liquid ratio

163

and stirring rate. For optimization, a three-level statistically designed program was conducted.

164

The three-level based test program was based on an orthogonal array of L9(33) to explore the

165

effect of the parameters on the response variable (i.e., REY recovery). The ranges of

166

parameter values evaluated are reported in Table 4.

167

Table 4. Taguchi orthogonal array parameters for acid leaching experiments Value of Factor A: Stirring Speed B: Acid C: Solid-Liquid Level (rpm) Concentration Ratio (M) (g·mL-1) 200 1 1:5 1 300 2 1:10 2 400 3 1:15 3

168

In each experiment, a 5 g representative CFA sample was mixed with HCl solution with a

169

specified acid concentration, amount of liquid, and stirring rate. The experiments were

170

conducted in capped flasks to prevent liquid loss due to evaporation, and the solutions were

171

mixed using magnetic stirrers. Each experiment was conducted at a constant temperature of

172

60 °C for 2 h, as they were found to be optimal conditions in our previous study (Cao et al.,

173

2018). After leaching, the remaining solid was filtered, the residue materials were rinsed with

174

DI water five times, and the leachates were analyzed for elemental content using ICP-MS.

175

The leaching efficiency (α) and the fly ash equivalent concentration of REY in leachate (β)

176

were calculated by the following equations:

α=

VC2 ×100% MC1

1

ܸ‫ܥ‬ଶ ‫ܯ‬

2

ߚ= 177

Where, V is the volume of leachate (mL), M is the mass of CFA sample (g), C1 is the element

178

content in the feed sample (µg·g-1), and C2 is the element concentration in leachate (µg·mL-1).

179

Upon optimization, the CFA, and product streams with the maximum REE content

180

obtained from each and combined physical separation processes were leached at the optimum

181

condition and the results were compared. The comparison results were utilized in developing

182

the conceptual recovery flowsheet.

183 184 185

3. Results and Discussion 3.1. Modes of occurrence of REY in CFA 3.1.1 Sequential Chemical Extraction Procedure

186

The sequential chemical extraction procedure was employed to study the modes of

187

occurrence of REY in the CFA sample. As the results are depicted in Fig. 1, the majority of

188

REY (~68%) in the sample is associated with the aluminosilicate (i.e., the dominant

189

amorphous) phase. This is consistent with the other studies that found REY in CFA is mainly

190

entrained in the (Al-Si)-Oxide glassy phase, hindering their solubilization in acid leaching.

191

This mode of occurrence was followed by nearly equal acid soluble form (13.56% of REY),

192

as the combustion products of carbonate-containing lime, periclase and other basic oxides

193

due to the existence of calcium, and organic or sulfide form (11.58% of total REY). Miner

194

fractions of REY were also found to be associated with metal oxides (5.25% of REY) and

195

ion-exchangeable (0.99% of REY) phases.

11.58%

0.99%

5.25%

Aluminium Silicate Organic / Sulfide Metal Oxide

13.56%

Carbonate Ion-Exchangable 68.62%

196 197

Figure 1. Modes of occurrence of REY in the CFA sample

198

3.1.2 REY Carrier

199

It was found that the REY occurs in five modes as discussed in section 3.1.1. Further

200

analyses were conducted with the FE-SEM-EDS to reveal the REY carrier in CFA. Fig. 2

201

shows several complex particles with different mineral phases detected in CFA. Point A and

202

B are the carriers, in which the REY was detected. The elemental spectra of both points were

203

obtained by an energy-dispersive X-ray spectrometer (Fig. 3). The element contents of both

204

points are listed in Table 5. The existence of carbon resulted from the carbon tape to fix the

205

CFA particles on the SEM sample holder and the coating process. Obviously, unburn carton

206

in both points is expected as well. Thus, the data for other elements were used to analyze the

207

probable mineral phases using HSC-Geo software. The mineral phase was estimated based on

208

the elemental proportion and expected mineral phases in CFA.

209

It was found that the monazite [(Ce,Nd)PO4], mullite (Al6Si2O13), andradite [Ca3Fe2(SiO4)3],

210

biotite [K2O·MgO·Al2O3·3SiO2] are the most likely mineral phases in point A. While, quartz

211

[SiO2], lime [CaO], merwinite [Ca3Mg(SiO4)2] as well as other silicates probably coexist.

212

The appearance of monazite sub-micron particles proved the fact that the rare earth minerals

213

remain in the CFA with just reduction in the particle size during the coal combustion (Hood

214

et al., 2017). The elemental content at point B is simply lower than that of point A as some

215

major elements (i.e. Ca, P and Mg) are missing. The metal oxides (Fe, Ce and Ti) expected to

216

be dominant phase at point B, followed by small amount of aluminate silicate. These results

217

demonstrated that the REY at point B associate with metal oxide phase, especially the iron

218

oxide. Considering the fact that REY associate with humic acid in low rank coal (Laudal et

219

al., 2018) and aluminosilicate, the theory of REY occurrence in CFA, which was stablished

220

elsewhere (Pan et al., 2019), can be confirmed.

221

Table 5 Element composition of A and B point Point A Point B Element Weight (%) Atomic (%) Element Weight (%) Atomic (%) C 13.00 23.56 C 41.72 59.45 O 37.54 50.66 O 26.73 28.59 Si 10.77 8.28 Fe 11.85 3.63 Ca 9.83 5.29 Ce 6.90 0.84 Ce 8.61 1.33 Si 6.89 4.20 Al 7.33 5.86 Al 3.57 2.26 Fe 5.12 1.98 Ti 1.23 0.44 Nd 4.10 0.61 K 0.76 0.33 p 2.72 1.90 Na 0.35 0.26 Mg 0.5 0.44 K 0.49 0.27

222

223 224

Figure 2. SEM backscattered electron image of REY carrier

225 226

Figure 3. EDS spectra of Point A and B

227

3.2 Physical Separation

228

To maximize the recovery and grade of the REY, and removal of the impurities prior to

229

leaching, thereby minimizing acid consumption and environmental issues of downstream

230

purification processes, REY distribution as a function of material properties were studied.

231

The properties used in this study were particles size, density and magnetic susceptibility that

232

can be potentially used for segregation of the elements of interest. The results are discussed in

233

the following sections.

234

3.2.1 Particle Size Separation

235

Wet sieve analysis was conducted to study REY distribution in various size fractions, i.e.,

236

150-100 µm, 100-74 µm, 74-55 µm, 55-35 µm, 38-25 µm and minus 25 µm. The REY

237

concentration, distribution, and mass yield of particle size fractions are shown in Fig. 4. The

238

results showed that the REY content increased from 608 to 896 µg·g-1 with decreasing

239

particle size, and the highest REY content was measured at the minus 25 µm size fraction.

240

This trend coincided well with the trends concluded by the previous studies (Taggart et al.,

241

2016; Lin R., 2017a; Kolker A. et al., 2017; Dai S. et al., 2014a). The Coutl of the finest size

242

fraction was measured to be 86.68%, which was slightly larger than those of the other size

243

fractions. The REY distribution trend, however, was similar to that of the mass yield of the

244

particle size fractions, which can be the controlling factor for the REY distribution. The

245

distribution of individual REY elements showed the same trend with the total REY, despite

246

some variations (as shown in Fig. S.1 in Supplemental Information (SI)). Therefore, Particle

247

size separation can be utilized for REY recovery and segregate the high REY content

248

fractions. Further grinding may be used to reduce particle size and liberate REY in the CFA

249

structure. However, the fine size reduction requires extensive energy consumption, thereby

250

increasing the process cost.

900

Fraction Yield REY Distribution

Percentage(%)

REY Concentration 30

800

20 700

Concentration(ppm)

40

10 600 0 -150+100 -100+74

-74+55

-55+38

-38+25

minus 25

251

Particle-Size-Fraction (µm)

252

Figure 4. Total REY content, distribution and mass yield in particle size fraction

253

3.2.2 Magnetic Separation

254

Fractionation of the CFA sample based on the particles’ magnetic susceptibility was

255

conducted using the Davis tube. The magnetic susceptibility of the particles decreased from

256

S1 to S6 (i.e., the non-magnetic fraction). As illustrated in Fig. 5, the REY content increased

257

with decreasing the magnetic susceptibility of the particles, with the lowest and highest REY

258

contents of 611 and 879 µg·g-1 which occurred in S1 and S6 fractions, correspondingly. The

259

individual REY followed the same trend (Fig. S.2 in SI). The high REY content of the S6

260

fraction can be attributed to the fact that most of REY was associated with the non-magnetic

261

phase (Lin et al., 2017a). Additionally, the REY distribution followed the pattern of the mass

262

yield pattern (i.e., incre0ased with decreasing the magnetic susceptibility). The non-magnetic

263

fraction had the highest mass yield (i.e., 57%) and 64% of REY occurred in this fraction.

264

Another significant observation was a very high concentration of the iron content (nearly

265

60%) in the S1 fraction.

70

50

Percentage(%)

900

Fraction Yield REY Distribution REY Concentration

800 40 30 700 20 10

Concentration(ppm)

60

600

0 S1

S3

S4

S5

S6

Magnetic Fraction

266 267

S2

Figure 5. Total REY content, distribution and mass yield in different magnetic fractions

268

3.2.3 Density Separation

269

Through the destiny separation, REY were concentrated to a maximum value of 855 µg·g-1,

270

in the SG class of 2.4–2.6 g·cm-3 (Fig. 6). By decreasing the density of fly ash particles, the

271

REY content first increased to its maximum value and then decreased after SG of 2.6. These

272

results indicated that the REY embedded in the middle density fraction, especially that with

273

the same density of quartz. However, the REY segregation among various fractions is not as

274

significant as those of size and magnetic separation. This fact agreed well with the previous

275

studies that found REY are dispersed throughout the glassy phase of the fly ash particles

276

(Kolker et al., 2017). Thus, density separation has a low effect on the enrichment of REY.

277

As for the individual REY, more variations were observed (Fig. S.3 in SI) compared to those

278

of size and magnetic separation. This could be due to dissolution of organically associated

279

REY present in the CFA into the organic solutions utilized in the density separation process,

280

as we also found about 10% REY loss after float-sink analysis.

40

900 Fraction Yield REY Distribution

700 20 600

Concentration(ppm)

Percentage(%)

800

REY Concentration

30

10 500 0 minus 2

2.0-2.2

2.2-2.4

2.4-2.6

2.6-2.8

Density Fraction/ g/cm

plus 2.8

-3

281 282

Figure 6. Total REY content, distribution and mass yield in various density fractions

283

Since size classification and magnetic separation processes were found to be most effective

284

physical separation processes for REY recovery, the combination of both processes was

285

utilized to preconcentrate REY prior to the leaching process. Based on the results, 38 µm cut

286

size and 5 A were selected for the size classification and magnetic separation processes,

287

correspondingly, in a sequence. The fine size fraction of the size classification with 873

288

µg·g-1 REY content, called product 1, was subjected to the magnetic separation. After

289

removal of the magnetic fraction (at 5 amps current), the non-magnetic fraction, called

290

product 2, was enriched to 1025 µg·g-1 REY content. The overall yield and REY recovery of

291

the product 2 which then subjected to the subsequent leaching process were 24% and 31.46%,

292

correspondingly.

293

3.3 Acid Leaching

294

After physical pre-concentration, acid leaching experiments were performed on product 2 to

295

optimize process parameters for maximizing the leaching efficiency and develop a conceptual

296

process flowsheetprocess. Experimenets were conducted to explore the effect of process

297

parameters, namely, stirring rate, acid concentration, and solid-liquid ratio. The optimal

298

conditions were selected based on the final product concentration and overall recovery of

299

REY (Cao et al., 2018). A three levels Taguchi orthogonal array L9(33) was utilized in

300

conducting experimental design, of which the results are summarized in Table 6. The range

301

value analysis was employed to analyze the results, where Ki is the sum of the outcomes of

302

the ith level experiments in each column and R is the range (or the difference between the

303

maximum and minimum) of of K values in each column.

304

The results showed that the largest R values were corresponding to the acid concentration

305

(the most influential factor), followed by solid-liquid ratio and then the stirring rate. This is

306

due to the fact that the acid leaching for the recovery of REY from CFA is a chemical reaction

307

for which the efficiency is more effected by the reactant concentration than the other process

308

parameters (Kashiwakura et al., 2013; Pavón et al., 2018; Funari et al., 2017).

309

As shown in Table 6, that the A3B3C2 process parameters which corresponds to 400 rpm

310

stirring rate, 3 M acid concentration and 1:10 solid: liquid ratio results in the highest REY

311

recovery (~80%) with 819 µg·g-1 REY in the leachate.

312

Table 6. L9(33) orthogonal array design and acid leaching results α β Factors Sample (%) (µg·g-1) No. A B C 1 1 1 1 25.72 263.65 2 1 2 2 67.38 690.73 3 1 3 3 76.24 781.60 4 2 1 2 44.44 455.62 5 2 2 3 53.93 552.91 6 2 3 1 74.01 758.74 7 3 1 3 61.34 628.83 8 3 2 1 65.39 670.35 9 3 3 2 79.85 818.61 K1 169.34 131.50 165.12 Leaching K2 172.39 186.70 191.67 Efficiency K3 206.58 230.10 191.52 (%) R 37.24 98.60 26.55 K1 1736.0 1348.1 1692.7 β K2 1927.9 1914.0 1965.0 -1 (µg·g ) K3 1790.1 2358.9 1963.3 R 192.0 1010.8 272.3

313

After optimization, the raw CFA sample, product 1, and product 2 were leached at optimum

314

condition to determine the effect of the physical separation on the enhancement of the

315

leaching efficiency. The results (Fig. 7) showed that the leaching efficiency significantly

316

increases after each step of the physical separation. The improvement through the size

317

classification is due to the finer size and larger surface area of the product 1 compared to that

318

of the raw CFA, as the leaching of REY from CFA can be described using unreacted core

319

shrinkage model (Liu et al., 2014; Lee and Koon, 2009). The CFA particles can be considered

320

as spherical particles containing REY. If H+ ion in the solution is not limited in the solution

321

by its concentration and the solid-liquid ratio, it reacts with the REY at the surface of CFA

322

particles and dissolve them into the solution. The REY are bound to the vitreous body of the

323

CFA, dissolved and released layer by layer during the leaching process. Therefore, the

324

reduction in the particle size and enhancement of the surface area will enhance such process.

325

On the other hand, removal of the magnetic materials in CFA minimizes the acid

326

consumption by low-REY bearing materials. In another word, more H+ ions will be available

327

in the solution to dissolve glassy (i.e., the major REY bearing) particles, thereby maximizing

328

recovery and minimizing acid consumption.

Leaching Efficiency/%

100

Raw Fly Ash Product 1 Product 2

80

60

40

329 330

La Ce Pr Nd Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu REY

Figure 7. The leaching efficiency of feed fly ash, product 1, and product 2

331

4. Conceptual process flowsheet

332

Based on the obtained results, a conceptual process flowsheet was developed for recovery of

333

REY from CFA. Through size classification of raw CFA, the fine fraction will be separated

334

from coarse fraction to preconcentrate the REY. The fine fraction will be then subjected to

335

magnetic separation for removal of magnetic fraction and further preconcentration of REY.

336

The preconcentrate obtained from physical separation (i.e., product 2), will be then leached

337

using acid leaching process at the determined optimum process conditions.

338

The leaching efficiency has been found to be further increased through chemical roasting of

339

CFA prior to leaching (Target et al. 2018). The roasting process, if economically viable,

340

might be included in the process. The leachate can be then fed to purification processes such

341

as

342

ion-chromatography for purification and separation of individual REY and other critical

343

elements (Honaker et al., 2019).

solvent

344 345

extraction,

membrane

technology,

or

continuous

ion-exchange

Figure 8. The experimental flowsheet of the combined process

346

5. Conclusion

347

In this study, the modes of occurrence of REY in CFA was thoroughly studied using a

348

sequential chemical extraction procedure. Physical separation processes were then evaluated

349

for preconcentration of REY. Additionally, leaching process parameters were optimized for

350

maximizing the leaching efficiency. Finally, a conceptual process flowsheet was developed

351

for REY recovery from CFA.

352

The sequential chemical extraction results showed that the glassy aluminosilicate phase is the

353

dominant mode of occurrence of REY in CFA, as 69% of REY were found to be associated

354

with this phase. The other major modes were acid soluble, and organic or sulfide forms,

355

containing 11.58% and 13.56% of REY, correspondingly. The fact that rare earth minerals

356

remain in CFA and REY associate with metal oxides was proved by SEM-EDS. Through

357

physical separation, it was found that the REY can be enriched in fine, middle density and

358

non-magnetic fractions using size classification, density and magnetic separations,

359

correspondingly. The combination of size classification and magnetic separation, fund to be

360

the most effective processes, resulted in enrichment of REY content of raw CFA with 782

361

µg·g-1 of REY to 1025 µg·g-1 with overall yield and recovery values of 24% and 31.46%,

362

respectively. Using the Taguchi three-level experimental design, the leaching process

363

parameters were optimized for maximizing the leaching efficiency. The optimum condition

364

found to be 400 rpm stirring rate, 3 M acid concentration and 1:10 solid: liquid ratio at 60 °C

365

leaching temperature, and 2 h leaching time period. In this optimum condition, a 78%

366

leaching efficiency was obtained from the leaching of preconcentrate product obtained from

367

the physical separation processes, compared to 43% as for raw CFA. Therefore, integrated

368

physical separation and acid leaching processes were found to be a promising approach for

369

REY recovery from CFA, and a conceptual process flowsheet was developed accordingly.

370

Acknowledgements

371

J.P. acknowledges the financial supported by Outstanding Innovation Scholarship for

372

Doctoral Candidates of “Double First Class” Construction Disciplines of CUMT.

373

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Figure List

519

520

Figure 1 Modes of occurrence of REY in the CFA sample

521

Figure 2 SEM backscattered electron image of REY carrier

522

Figure 3 EDS spectra of Point A and B

523

Figure 4 Total REY content, distribution and mass yield in particle size fraction

524

Figure 5 Total REY content, distribution and mass yield in different magnetic fractions

525

Figure 6 Total REY content, distribution and mass yield in various density fractions

526

Figure 7 The leaching efficiency of feed fly ash, product 1, and product 2

527

Figure 8 The experimental flowsheet of the combined process

528

Figure S1 Individual REY distribution patterns in particle size fractions

529

Figure S2 Individual REY distribution patterns in different magnetic fractions

530

Figure S3 Individual REY distribution patterns in various density fractions

Highlights REY mode of occurrence in coal fly ash was studied and confirmed.

REY content of 1025 µg/g in final product was achieved via physical processing.

Integrated physical separation and acid leaching process was suggested for REY recovery.

Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: