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Modes of occurrence of rare earth elements in coal fly ash—a case study Jin-he Pan, Changchun Zhou, Cheng Liu, Mengcheng Tang, Shanshan Cao, Tingting Hu, Wanshun Ji, Yulin Luo, Mingzhong Wen, and Ningning Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02052 • Publication Date (Web): 14 Aug 2018 Downloaded from http://pubs.acs.org on August 14, 2018
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Modes of occurrence of rare earth elements in coal fly ash—a case study a
Jinhe Pan , Changchun Zhou a
a,
*, Cheng Liua, Mengcheng Tanga, Shanshan Caoa, Tingting Hua, a
Wanshun Ji , Yulin Luob, Mingzhong Wenc, Ningning Zhang ,* a. Key Laboratory of Coal Processing & Efficient Utilization, Ministry of Education, School of Chemical Engineering and Technology, China University of Mining & Technology, Xuzhou 221116, China b. Advanced Analysis & Computation Center, China University of Mining & Technology, Xuzhou, Jiangsu 221116, China c. Jiangsu Design and Research Institute of Geology for Mineral Resources, Xuzhou, Jiangsu, 221006, China *Corresponding author E-mail address:
[email protected] (Changchun Zhou).
[email protected] (Jinhe Pan)
ABSTRACT Coal fly ash (CFA), the main byproduct of coal combustion, has been regarded as an attractive source of rare earth elements plus yttrium (REY). Understanding how REY occurs in CFA is essential for designing an effective method for recovery of REY from CFA. To clarify the modes of occurrence of REY, we used particle size analysis, a sequential chemical extraction procedure (SCEP), and a scanning electron microscope with an energy dispersive spectrometer (SEM-EDS) to study REY in CFA from Guizhou Province, China. The relationship between the content of REY and particle size was consistent with previous results and implies that particle separation can be used to enrich the chemical extraction of REY from CFA. The results of the SCEP revealed the proportions of REY in different modes (silicate-aluminate > organic or sulfide > acid soluble > metal oxides > ion-exchangeable form) and can also be used as a guide for reagent choice in REY leaching and extraction. The results of the SEM-EDS analysis showed that REY in the CFA (La, Ce, and Nd as proxies) is significantly associated with Al and P. They also illustrated that Al has a more important
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role in REY occurrence than Si in aluminosilicates. Key words: coal fly ash; rare earth elements; modes of occurrence; sequential chemical extraction procedure; scanning electron microscope
1. Introduction Rare earth elements (REEs) and REEs plus yttrium (REY), called “the vitamin of industry” [1], are extremely important strategic materials. Some of the major end uses for REEs include use in automotive catalytic converters, permanent magnets and rechargeable batteries for hybrid and electric vehicles, numerous medical devices, defense applications, and applications in other fields[2]. The rapid increase of the world’s demand for rare earths also raises the crucial issue of supply vulnerability[3]. Seeking alternatives to rare earth mines and recycling rare earth elements economically has great significance to improve the current situation of rare earth resources and increase national strategic security. For this purpose, the US Department of Energy (DOE) initiated a Rare Earth Element Program to recover rare earth elements from coal and coal byproducts via cost-effective and environmentally benign approaches. The DOE suggests a threshold of 300 µg/g REY as a minimum interest level for potential REY feedstock and requires the goal of a minimum of two weight percent in concentrate production[4]. Coal fly ash (CFA), the main byproduct of coal combustion, has been regarded as a processed resource that has potential to be an attractive source of REY[5-10]. The various REY are not equally abundant or equally valuable, so Seredin[11] and Dai[12] classified REYs into three economic clusters based on their relative demand in industry: critical (Nd, Eu, Tb, Dy, Y, and Er), uncritical (La, Pr, Sm, and Gd), and excessive (Ce, Ho, Tm, Yb, and Lu). They also summarized modes of REY occurrence in coal as syngenetic clastic and pyroclastic minerals, diagenetic and epigenetic minerals of authigenic
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origin, and organic compounds[13-17]. The sites of REY in CFA have been reported. Hower et al.[18] chose Ce as a proxy for other REY to study the location of REY in CFA. It was shown that the apparent distribution of Ce occurs throughout fly ash glass. Smolka-Danielowska[19] found monazite in a CFA in Poland, and the concentrations of Ce, Sm, La, and Gd from the back rows of the ash-collection device were higher than those in the front of the device. Kolker et al.[20] used the Stanford-USGS SHRIMP-RG ion microprobe to determine the grain-scale REE portioning of 19 fly ash samples. The results confirmed the occurrence of REEs in aluminosilicate glass and showed a close correlation between REE content and Al2O3. Hood et al.[21] indicated that REY in CFA persists in trace phases because of the size reduction due to thermal shock and occurs in nanoparticles, or a REY carrier at the nanometer scale. To understand how REY occur in CFA in more detail, we investigated an industrial CFA from Guizhou Province, China, to research the relationship between REY content and particle size. Through a sequential chemical extraction procedure and the use of a scanning electron microscope with an energy dispersive spectrometer, modes of occurrence of REY in CFA were identified, and the relevant modes of REY occurrence in coal were inferred from previous research models. The implications for REY recovery from CFA were also evaluated. The conclusion of this study also provides a theoretical foundation for the enrichment and extraction of REEs from CFA.
2. Analytical procedures Samples of raw CFA were collected in May 2017 from the economizer of the Nayong power plant with a special sampling device based on the DL/T 926-2005 method developed by the China Electricity Council[22]. The raw CFA was screened at 120, 140, 200, 270, 325, 400, and 500 meshes (maximum particle sizes of 125, 100, 74, 55, 45, 38, and 25 µm), respectively. The loss on ignition (LOI) of the fly
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ash was determined according to ASTM standard D3174[23]. X-ray fluorescence spectrometric analysis (XRF) was chosen to determine the major element compositions of the raw CFA. Inductively coupled plasma mass spectrometry (ICP-MS) was used to determine REY in the ash samples. To do this, 50 mg of each sample was digested with 4 mL of concentrated HNO3 and 4 mL of concentrated HF for about 3 h in a microwave digestion instrument. After digestion, the aqueous solution was diluted to 100 mL with ultrapure water for ICP-MS analysis. The sequential chemical extraction procedure has been used widely to study the chemical activity and speciation of various elements in different materials[24-34]. The results have been useful for forming scientific inferences pertaining to the modes of occurrence of these elements. Thus, a four-step modified procedure (shown in Fig. 1) was applied to divide chemical species into five types: ion-exchangeable form, acid-soluble form, metal oxides form, organic or sulfide form, and silicate & aluminosilicate form (or silicate-aluminate form). The leaching solution of every procedure was disposed of before ICP-MS analysis.
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2g dry sample +20ml 1mol MgCl2 25℃ 1h Centrifugation
Ion-exchangeable form
Residue +20ml 1mol/L NaAC 25℃ 5h Centrifugation
Acid soluble form
Residue +50ml 0.04mol/L NH2OH▪HCl (25%CH3COOH) 95℃ 3h Centrifugation
Metal oxide form
Residue +7.5ml 20%HNO3+20ml 30%H2O2 85℃ 5h 12.5ml 1 mol/L NH4OAC in 20% HNO3 25℃ 5h
Residue
Centrifugation
Organic or sulfide form
Digestion
Silico-aluminate form
Fig. 1 Diagram of the sequential chemical extraction procedure. A field emission-scanning electron microscope (FE-SEM; ZEISS ΣIGMA) in conjunction with an EDAX energy-dispersive X-ray spectrometer (Oxford X-MaxN 20) (collectively, FE-SEM-EDS) was applied to study morphology and microstructure and also to determine the distribution of some elements in the CFA. Images were captured via a retractable solid-state backscatter electron detector, which was used to facilitate the location of REY-containing minerals as well as other heavy element-containing minerals.
3. Result and discussion 3.1 Coal fly ash properties The results of the X-ray fluorescence spectrometric analysis are shown in Table 1. Silicon, which occurs in quartz, glass, and mullite, accounts for more than 50% of the fly ash. Iron contents of the fly ash are higher than those of many other previously reported power plant fly ashes because the feed coal
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in Guizhou Province is high-pyrite coal. Calcium and magnesium occur as basic oxides after combustion. Similar to the case of iron, these samples have higher S contents than those of many other power plant fly ashes because the feed coal in Guizhou Province contains abundant sulfide minerals. Geochemically, a threefold classification of REY was used in this study: light REY (LREY: La, Ce, Pr, Nd, Sm), medium REY (MREY: Eu, Gd, Tb, Dy, Y), and heavy REY (HREY: Ho, Er, Tm, Yb, Lu). In comparison with the upper continental crust (UCC) [35], three enrichment types were identified[12]: L-type (light-REY; LaN/LuN > 1), M-type (medium-REY; LaN/SmN < 1, GdN/LuN > 1), and H-type (heavy-REY; LaN/LuN < 1). The REY enrichment patterns in this fly ash are characterized by M-type enrichment (LaN/SmN = 0.75 < 1, GdN/LuN = 1.39 > 1) with clear negative Eu anomalies, as shown in Fig. 2. Based on the outlook coefficient (0.7 ≤ Coutl ≤ 1.9) and the percentage of critical elements in the total REY (30% ≤ REYdef ≤ 51%) in Table 2, the CFA may be considered as a promising raw material for REY recovery. Another important conclusion of this study is the relationship between particle size and REY, whereby individual contents or sums of contents decrease with increase of particle size, which is consistent with previous finding[36-38]. Values of Coutl and REYdef in the small size-fraction fly ash are slightly larger than those in the large size-fraction fly ash. REY recovery by particle separation is an economical and feasible method in thermal power plants. Table 1 Major element composition (%). Element
SiO2
Al2O3
Fe2O3
TiO2
MgO
CaO
K2O
Na2O
LOI
Content (%)
46.26
24.39
13.14
3.24
1.25
2.29
0.93
0.63
4.98
S
P
Mn
Zr
Ba
V
Cu
Cr
Ni
0.65
0.100
0.076
0.066
0.054
0.045
0.024
0.020
0.010
Element Content (%)
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Table 2 Rare earth elements in the raw fly ash and size-fraction fly ash (µg/g). Element
Raw fly ash
Plus 100
100-74
74-55
55-38
38-25
Minus 25
Y
62.11
46.87
49.24
58.62
60.98
63.67
70.87
La
91.44
80.84
85.84
86.28
90.4
97.5
100.79
Ce
195.67
174.37
185.13
190.99
198.21
199.47
215.45
Pr
23.52
19.65
20.29
21.53
24.5
26.74
29.63
Nd
88.64
72.49
74.29
80.61
83.87
88.89
93.47
Sm
18.24
14.95
16.65
18.81
19.81
20.09
24.06
Eu
3.36
2.05
2.51
2.74
2.9
3.42
3.8
Gd
16.41
14.79
15.52
19.6
21.86
23.43
26.51
Tb
2.50
1.79
1.9
2.2
2.75
2.99
3
Dy
12.73
10.79
11.5
11.93
12.52
12.65
15.93
Ho
2.79
1.93
2.06
2.24
2.47
2.71
2.98
Er
6.87
5.73
6.03
6.34
6.66
6.84
6.95
Tm
1.23
0.7
0.85
0.9
0.98
1.04
1.43
Yb
6.46
4.64
4.71
5.39
5.82
6.51
7.42
Lu
0.99
0.67
0.69
0.71
0.8
0.85
0.99
LREY
417.51
362.31
382.2
398.23
416.79
432.68
463.39
MREY
97.11
76.3
80.67
95.1
101.01
106.16
120.12
HREY
18.35
13.8
14.47
15.75
16.83
18.08
19.96
REY
532.96
452.29
477.21
508.92
534.53
556.79
603.27
REO
628.50
532.87
562.22
600.09
630.2
656.42
711.31
Critical
176.22
152.47
158.47
179.31
188.64
198.47
216.73
Uncritical
149.60
130.24
138.3
146.24
156.57
167.75
180.98
Excessive
207.14
182.44
193.57
200.4
208.39
210.72
228.47
Coutl
0.85
0.84
0.82
0.90
0.90
0.94
0.95
REYdef
31.96%
33.71%
33.21%
35.23%
35.29%
35.64%
35.93% [12]
Coutl: outlook coefficient of REY ores. REYdef: percentage of critical elements in total REY
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8 7 6
Sample/UCC
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plus 100 100-74 74-55 55-38 38-25 minus 25
5 4 3 2 1 0 La Ce Pr Nd Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu
Fig. 2 REY distribution patterns for size-fractioned fly ashes.
3.2 Sequential chemical extraction procedure The sequential chemical extraction procedure (SCEP) is one of the most common quantitative methods for ascertaining the modes of occurrence of trace elements in the geological and environmental fields. The individual REY recovery rate of the SCEP was from 88.60% to 106.21%, according to the REY content in the raw fly ash listed in Table 2. The results of the SCEP are plotted in Fig. 3 and Fig. 4 to illustrate the modes of occurrence of the total and the individual REY. The main mode of REY occurrence is within silicate & aluminosilicate forms (65.22%), which is consistent with the reports by Hower[18,39,40]. In addition aluminosilicates have a more important role in REY occurrence than silicates, although they seem to be equal in this division. The ratio of REY in organic or sulfide form is 12.12%, as the result of unburnt carbon from the high-sulfur feed coal. This indirectly indicates the existence of REY-bearing aluminum phosphates and sulfates of the alunite supergroup (APS minerals) and REY-bearing organic compounds (humic matter) in the coal[12,41]. The acid soluble form contains quicklime, periclase, and other basic oxides, which dissolve in acetic acid and are mainly combustion products of carbonate minerals. About 10% of the REY exists in this form and is likely transformed from REY-bearing carbonates in the coal. Metal oxides (especially
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iron and manganese oxides) are excellent scavengers for trace metals and are thermodynamically unstable under anoxic conditions. Although the content of total Fe, Ti, and Mn is more than 15%, only 7.11% of the REY was present in metal forms. From this, it was inferred that iron, titanium, and manganese oxides do not attract rare earth elements during coal combustion. REY in ion-exchangeable form may be liberated ionic rare earth ore, which is scattered throughout south China. Rare earth ions are easily liberated with inorganic salt solution in nature. As shown in many reports, rare earth ore remains unchanged during coal combustion (below 1500 °C)[22,42].
80
65.22
Percentage/%
60
40
20
12.12
10.21
7.11
5.35
or m -a l
Si lic o
ic o
rs
um
in a
ul fid
ef
te f
or m
or m sf O
rg an
et al sO xi de M
ci d A
ex ch
an ge
so lu bl ef
or m
fo rm
0
Io n-
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
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Fig. 3 Modes of occurrence of total rare earth elements.
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100
La Ce Pr Nd Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu
80
Percentage (%)
60
40
20
fo rm al um in at e
ul fa te
co Si li
O
rg an ic
or s
O al s M et
fo rm
xi de sf or m
fo rm bl e so lu ci d A
ex ch an ge f
or m
0
Io n-
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
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Fig. 4 Modes of occurrence of individual rare earth elements.
3.3 Mineralogy There are many amorphous phases (glass and carbon) in the fly ash, as indicated by the XRD analyses, the mineral mullite being the most abundant. Quartz, magnetite, and hematite were also found in the samples. Further mineralogical analysis was conducted on the samples with the SEM-EDS. Although detection of REY-enriched particles in fly ash is difficult[20], rare occurrences have been recorded with the help of elemental analysis. Fig. 5 shows several complex particles with different mineral phases. In comparison with the gray glass balls, REY-bearing mineral is luminous. Elemental map images of a submicron region in the red box were obtained by energy-dispersive X-ray spectrometer (shown in Fig. 6). The order of the different element contents (after normalization) is O (45.44%), Al (21.04), Ce (10.66%), P (6.84%), Si (6.33%), La (4.42%), Nd (2.54), and other elements (below 2%). From the elemental map, it can be seen that the bright parts in Fig. 6 a or b occur in all mineral phases. Only a few areas at the top and lower right corner are luminous in both pictures. The bright areas in these pictures represent the enrichment of elements in this region. As such, it is inferred that
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the distribution of Al is exclusive from the distribution of Si in this area. The distributions of Al and P were found to coincide with those of La, Ce, and Nd (proxies of rare earth elements). However, the distribution of Si does not show much overlap with the rare earth elements. This directly indicates that the occurrence of rare earth elements is significantly associated with Al and P, which is consistent with Hower’s conclusion[18]. In addition, we can infer that there is little correlation between Si and rare earth elements, but Al has greater control on the occurrence of rare earth elements. This may be the reason that the dominant mode of occurrence of REY in the SCEP study was within aluminosilicates. Furthermore, silicon in aluminosilicates has less effect on REY occurrences than does aluminum in aluminosilicates. Therefore, we present this inference based on the SEM-EDS results. Certainly, further work should be done to examine this relationship.
9 Fig. 5 SEM backscattered electron image of REY carrier.
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A
B
C
D
E F Fig. 6 Elemental map images of the region in the red box in Fig. 5.
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3.4 Implications for REY recovery China, as one of the largest consumers of coal, produces more than 500 million tons of coal ash annually[43,44]. It will be significant for the industry if a large fraction of the unused ash material produced in thermal power plants becomes available as a potential REY resource[20]. Research on the recovery of REY from CFA is actively ongoing, and the economic and environmental impacts remain to be evaluated. The trend of REY being concentrated onto fine particles confirms the occurrence of REY as nanoparticles, or on a REY-enriched carrier at the nanometer scale. This also suggests that particle separation can be used to enrich REY[17,37]. Based on the SEM imaging in Fig. 5 and Fig. 6, the REY-enriched carriers are mainly constituted by Al, P, and O at the nanometer scale[18,21,22]. This also indicates that reagents used for leaching aluminum and phosphorus are also good for leaching REY from fly ash. The identification of modes of occurrence of REY in fly ash based on SCEP provides a foundation for REY extraction. For example, Taggart et al.
[2]
reported an inability to extract a high fraction of
REY in only nitric acid because only 35% of REY was in modes other than aluminosilicates. Extraction of REY from aluminosilicates is essential for recovering REY from fly ash. As such, the results of the SCEP provide guidance for determining which reagent should be chosen and what amount of reagent is sufficient to economically recover REY in the specific part.
4. Conclusion Coal fly ash with M-type REY distribution from the Nayong power plant, Guizhou Province, China, is regarded as a promising REY raw resource for economic development. Particle separation may be a superior method of recovery of REY from the ash based on the content of REY, Coutl, and REYdef. Using a sequential chemical extraction procedure, we summarized the relative importance of
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five types of REY occurrence: ion-exchangeable form, acid soluble form, metal oxides form, organic or sulfide form, and silicate-aluminate form. The silicate-aluminate form is the dominant mode of REY occurrence in the fly ash. We also identified the REE-enriched particles in the fly ash by employing a scanning electron microscope with an energy-dispersive X-ray spectrometer. It was shown that REY in the CFA (La, Ce, and Nd as proxies) is more closely bound to Al/P than Si. From this, it was inferred that there is little correlation between Si and rare earth elements. Aluminum is more important for rare earth element occurrence than silicon and aluminum combined to constitute aluminosilicates. Further work should target economical and environmentally benign recovery of rare earth elements from CFA.
Acknowledgements The authors wish to acknowledge the financial support by the Fundamental Research Funds for the Central Universities (2017BSCXA12).
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