Study on the Transformation of Arsenic and Lead in Pyrite During

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Study on the Transformation of Arsenic and Lead in Pyrite During Thermal Conversion Guo-chang Song, Wen-ting Xu, Pan Ji, and Qiang Song Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b02028 • Publication Date (Web): 16 Aug 2019 Downloaded from pubs.acs.org on August 27, 2019

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Energy & Fuels

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Study on the Transformation of Arsenic and Lead in Pyrite During Thermal Conversion

3

Guo-chang Song, Wen-ting Xu, Pan Ji, Qiang Song*

4 5 6

Key Laboratory of Thermal Science and Power Engineering of Ministry of Education, Department of Energy and Power Engineering, Tsinghua University, Beijing 100084, China

7

Abstract The forms of arsenic (As) and lead (Pb) in coal have an important

8

influence on their transformation during combustion. Considering that As and Pb in

9

coal are partially associated with pyrite, pyrolysis and oxidation experiments were

10

conducted on pyrite in a fixed-bed reactor between 600 and 1000 °C to study the

11

transformation of As and Pb in pyrite during thermal conversion. During pyrolysis, As

12

remained essentially unreleased below 700 °C, and the release ratio increased with

13

increasing temperature above 700 °C. At 1000 °C, As was almost completely released

14

into the gas phase. During oxidation, As remained essentially unreleased below 800 °C,

15

and the release ratio increased with increasing temperature above 800 °C, reaching

16

61.67% at 1000 °C. The gas-phase release ratio of As showed a dynamic change that

17

rapidly increased with time, and the change was significantly correlated with the

18

dynamic transformation of the pyrite. During pyrolysis, As was released into the gas

19

phase by a two-step decomposition of FeAs2. In an oxidizing atmosphere, FeAs2 was

20

oxidized to form As2O3 and Fe2O3, which could further react to form FeAsO4. As was

21

retained in the solid phase, and the release ratio of As was therefore lower. During

22

pyrolysis, Pb was released between 600 and 1000 °C, and the release rate and release

23

ratio increased with increasing temperature. During oxidation, Pb remained unreleased

24

at 600 °C, and the release ratio increased with increasing temperature above 700 °C,

25

reaching 87.69% at 1000 °C. No significant correlation was found between the release

1

ACS Paragon Plus Environment

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of Pb and the conversion of the pyrite, as Pb exists in the form of PbS in pyrite. During

27

pyrolysis, Pb was released into the gas phase by the decomposition of PbS. Part of the

28

PbS in an oxidizing atmosphere was oxidized into PbO or PbSO4, which remained in

29

the solid phase, resulting in a relatively low release ratio.

30

1

INTRODUCTION

31

Coal combustion is a significant source of heavy metals (such as As, Se, Pb, Cr,

32

Cd and Hg) in the atmosphere.1 Arsenic (As) and lead (Pb) in the coal cause widespread

33

concern as they are volatile during coal combustion and extremely harmful to

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ecosystems and human health after being emitted into the environment.2-3 Some

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countries and regions in the world have set limits for As and Pb emissions from coal-

36

fired units.4-5 The study of As and Pb transformation in the coal combustion process

37

and the development of control technology for As and Pb emissions are therefore

38

essential.

39

Field studies of As and Pb emissions from coal-fired power plants show that, after

40

undergoing coal combustion and flue gas cooling, As and Pb exist mainly in a

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particulate state with only small quantities in the gas phase. Most of the particulate As

42

and Pb can be removed by particulate control devices,6 whereas the As and Pb in

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escaping fine particles and in the gas phase are emitted into the atmosphere and cause

44

pollution.1 The combustion of coals with high contents of As and Pb may result in As

45

and Pb emissions exceeding standards. The emission characteristics of As and Pb differ

46

among power plants,7-9 which are mainly due to the difference in combustion equipment,

47

combustion conditions and fuel properties.

48

Laboratory studies show that, during the thermal conversion of coal,


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temperature,10-11 atmosphere,12-13 and mineral and non-mineral components in coal14-16

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affect the transformation of As and Pb. The release ratios of As and Pb increased with

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increasing temperature.10-11 Compared to the oxidizing atmosphere, the inert

52

atmosphere was found to be more conducive to the release of Pb but inhibit the release

53

of As in coal.12-13 Alkaline mineral components, such as CaO and Fe2O3, can react with

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As compounds to form arsenates and thus cause a lower release ratio of As.14 Non-

55

mineral components, such as SO2, can promote the release of As by the competitive

56

reactions with minerals,15 and prohibit the release of Pb by forming PbSO4.16

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Seames and Wendt17 summarized the transformation pathways of As and Pb. As

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and Pb act as semi-volatile trace elements. During the coal combustion at high-

59

temperature, some of As and Pb are directly released into the hot flue gas, and the rest

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are remained in the solid products such as bottom slag and fly ash. During the cooling

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process of the flue gas, part of the As and Pb in the gas phase migrates to the fly ash by

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condensation, physical adsorption or chemical absorption. The transformation of As

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and Pb can be divided into two stages. In the coal combustion stage, the final release of

64

As and Pb is determined by both the thermal stabilities of their original forms and the

65

chemical reactions between the released gaseous As and Pb and minerals in porous

66

combusting coal particles. Thus, it is important to study how the original forms of As

67

and Pb transform during thermal process.

68

As and Pb exist in various forms in coal. According to the sequential chemical

69

extraction method established by Finkelman et al.18, these can be divided into water-

70

soluble form, ion-exchangeable form, oxide-bound form, sulfide-bound form, organic-

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bound form and residual form. The distribution of forms of As and Pb differed among

72

different coals. Sulfide-bound, organic-bound and residual forms are found to be the


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commonly main forms of As in coal.18-19 Oxide-bound, sulfide-bound, organic- bound

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and residual forms are found to be the commonly main forms of Pb in coal.20 The forms

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of As and Pb in coal changed after combustion,19, 21 which means that transformation

76

occurs to each form during combustion.

77

Sulfide-bound form is the main form of As and Pb in some coals. Liu et al.10 found

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that the sulfide-bound As accounted for 40–80% of the total As in six Chinese coal

79

samples using sequential chemical extraction. Shah et al.22 found that 65–70% of Pb

80

was in sulfide-bound form in the bituminous coals used in five Australian power plants.

81

Sulfide-bound As and Pb are often associated with pyrite (FeS2). Kolker et al.23 found

82

that As associated with pyrite accounted for 61–88% of total As in coal samples from

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several places including Pittsburgh, USA. When studying coals from North China, Luo

84

et al.24 found that the average concentration of Pb in pyrite (271.28 mg/kg) was

85

significantly higher than that in raw coals (23.95 mg/kg) indicating that Pb was mainly

86

associated with pyrite in that area.

87

Considering that the sulfide-bound form is one of the main forms of As and Pb in

88

coal and is often associated with pyrite, in the present study pyrite was used as the

89

sample to study the transformation of sulfide-bound As and Pb during thermal process.

90

The pyrolysis and oxidation experiments were conducted in a fixed-bed reactor between

91

600 and 1000 °C. The dynamic release characteristics of As and Pb under different

92

thermal conversion conditions were determined by measuring the contents of As and

93

Pb in solid samples before and after thermal conversion. Based on a comparison with

94

the dynamic conversion characteristics of pyrite and the crystalline structure

95

characterization of products, the transformation pathways of As and Pb during pyrolysis

96

and oxidation were analyzed.


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2

EXPERIMENTAL

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2.1 Sample

99

The raw material used in this experiment was pyrite produced from the Anhui

100

Province in China. The pyrite was pulverized and particles in the range of 37.4–75 μm

101

were selected. The selected pyrite powder was immersed in hydrochloric acid (pH=1)

102

and subjected to ultrasonic vibration for 30 min to remove carbonate impurities, then

103

washed with deionized water, filtered, and dried at 105 °C for 10 h before use. The

104

crystalline structure of the sample was analyzed by an X-ray diffractometer (XRD; D8

105

Discover, Bruker, Germany). Figure 1 shows that the FeS2, the effective component of

106

the sample, has good crystallinity and that the sample contained a small quantity of

107

quartz impurity. The concentrations of Fe, Na, Mg, Al, Si, K, Ca and Pb in the sample

108

were measured by an inductively coupled plasma optical emission spectrometer (ICP-

109

OES; Prodigy 7, Teledyne Leeman Labs, USA) whereas the concentration of As was

110

measured by atomic fluorescence spectrometer (AFS; AFS-9320, Ji Tian, China). The

111

results are listed in Table 1, which shows that the concentration of As was 51.2 μg/g

112

and the concentration of Pb was 197 μg/g.


Energy & Fuels

1 1—pyrite (FeS2) 2—quartz (SiO2)

Intensity (counts)

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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1

1

2 10

20

1 2

30

1

1 2

40

2

11

50

60

1

11 70

80

2-Theta (°)

113 114

Figure 1 XRD result for pyrite sample

115

Table 1 The concentration of main elements in pyrite sample (in wt%)

116

Fe

S

Na

Mg

Al

Si

K

Ca

As

Pb

42.76

49.33

0.54

0.56

0.64

3.13

0.32

1.16

5.1210-3

1.9710-2

2.2 Experimental setup

117

Pyrite pyrolysis and oxidation experiments were conducted in a fixed-bed reactor25

118

(Figure 2). The reactor consisted of a quartz outer tube (internal diameter of 40 mm)

119

and a quartz inner tube (internal diameter of 30 mm), and was heated by an electric

120

heating furnace. A detachable quartz crucible (internal diameter of 34 mm) was

121

connected to the quartz inner tube by a scrub connection, and a quartz membrane

122

(Staplex, USA) was placed on the bottom of the quartz crucible. A water-cooled

123

chamber was installed at the end of the quartz tube for cooling the sample. The reaction

124

gas was pure N2 for the pyrolysis experiments and a mixture of N2 and O2 (in which the

125

volume fraction of O2 was 5%) for the oxidation experiments. The flow rates of N2 and

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O2 were controlled by mass flow controllers (D08-1F, Beijing Sevenstar Electronics

127

Co., China) with a total gas flow of 0.5 NL/min. In the experiment, the electric heating


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furnace was first heated to a constant temperature (in the 600–1000 °C range). After

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continuous introduction of the reaction gas for 30 min, and after the temperature and

130

atmosphere in the reactor had fully stabilized, the quartz crucible containing the sample

131

(1±0.01 g) was quickly inserted into the constant-temperature zone in the reactor. Once

132

the preset reaction time was reached, the quartz crucible was quickly withdrawn to the

133

water-cooled chamber, and the reaction gas was switched to N2 (at 2 NL/min) to ensure

134

rapid cooling of the sample in an inert atmosphere.

135 136 137

Figure 2 Schematic of the fixed-bed reactor system25

2.3 Method

138

The pyrolysis and oxidation experiments on pyrite were conducted for different

139

reaction times (in the 0–60 min range). For each reaction time, the concentration of As

140

and Pb in the solid samples before and after the reaction were compared to determine

141

the gas-phase release ratio. The dynamic release characteristics of As and Pb in pyrite

142

were obtained in this manner. The solid sample was first digested using microwaves.

143

Approximately 50 mg of the sample was placed in a digestion tank with a mixture of 8

144

mL of HNO3, 2 mL of HCl and 2 mL of HF, and digested in a microwave digestion


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apparatus (ETHOS UP, Milestone, Italy) at 240 °C for 40 min. The concentration of As

146

in the digestion solution was measured by AFS, whereas the concentration of Pb was

147

measured by ICP-OES. The concentration of As and Pb in the pyrite (Cp,M, where M=As,

148

Pb), as well as those in the products after thermal conversion (Cr,M), were calculated by

149

combining the mass of samples and the concentration in the digestion solution.

150 151

The gas-phase release ratios (RM) of As and Pb during the thermal conversion of pyrite were calculated according to Equation (1).

152

 C  mr  RM  1  r,M 100%  C  m  p,M p  

153

where mr and mp represent the mass (g) of solid residue and pyrite sample, respectively,

154

which were weighed before and after the experiment.

155

(1)

The conversion of pyrite (X) was calculated from Equation (2).

X

156

mp  mr mp  mr,total

100%

(2)

157

where mr,

158

pyrolysis of pyrite at the particular temperature. mr, total was obtained by the pyrolysis

159

and oxidation experiments on pyrite at various temperatures for 3 and 4 h time periods

160

when the mass of the solid residue no longer changed, indicating the complete

161

conversion of the pyrite.

162 163

total

represents the mass (g) of solid residue after complete oxidation or

The crystalline structure of solid residues was analyzed using an XRD. All samples were scanned between 10° and 80° (2θ scale) with a resolution of 0.02°.


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3

RESULTS AND DISCUSSION

165

3.1 Release characteristics of As in pyrite during pyrolysis and oxidation

166

Figure 3(a) shows the release ratio of As and the conversion of pyrite at different

167

reaction times during pyrolysis in the 600–1000 °C range. No significant release of As

168

was observed in the 600–700 °C range. The release ratio of As in the 800-1000 °C range

169

tended to increase with time and then stabilize. At 800, 900 and 1000 °C, the time

170

required for the release of As to stabilize were 20, 40 and 10 min, respectively. The

171

release ratios at steady state were 54.08%, 97.61% and 99.85%, respectively. This

172

indicates that the higher the temperature, the faster the release of As and the higher the

173

release ratio. In general, the conversion of pyrite increased with time and then stabilized.

174

When the reaction time was 60 min, the conversion of pyrite at 600 °C was 45.07%. In

175

the 700–1000 °C range, the conversion of pyrite reached 100%. Supplementary

176

experiments showed that the conversion of pyrite reached 100% after 3 h of pyrolysis

177

at 600 °C. With the temperature increasing from 700 to 1000 °C, the times required for

178

the conversion of pyrite to reach 100% was reduced from 60 to 10 min, indicating that

179

the higher the temperature, the faster the transformation of pyrite. In the temperature

180

range where a significant release of As was observed (800–1000 °C), the dynamic

181

release of As was synchronous with the dynamic conversion of pyrite, indicating that

182

they were correlated.


Energy & Fuels

20

20

0

0

60 40

40

20

20

0

0

80

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60

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0

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700 °C

60

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80 600 °C

60

60

40

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20 0

0 0

10

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30

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60

Release ratio of As (%)

800 °C


80

40

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0

0 100

80

80 900 °C

60

40

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0 100

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800 °C

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700 °C

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80 600 °C

60

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20

20 0

0 0

10

Time (min)

183

60

40

100 Release ratio of As (%)

100 Conversion of pyrite (%)

100 Release ratio of As (%)

60


80 900 °C

Conversion of pyrite (%)

80

60

100



100

60


40

80


40

1000 °C


60

80


60

100


80

100


1000 °C


80


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100


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20

30

40

50

60

Time (min) (b)

(a)

184

Figure 3 Release ratio of As (■) and conversion of pyrite (▲) during (a) pyrolysis and (b)

185

oxidation in the 600–1000 °C range

186

Figure 3(b) shows the release ratio of As and the conversion of pyrite at different

187

times during the oxidation of pyrite in the 600–1000 °C range. No significant release


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of As was observed between 600 and 800 °C. At 900 and 1000 °C, the release ratio of

189

As tended to increase with time and then stabilize. The times required for stabilization

190

were 30 and 20 min, respectively. The release ratios at steady state were 12.95% and

191

61.67%, respectively. This indicates that the higher the temperature, the faster the

192

release of As and the higher the release ratio. The conversion of pyrite tended to

193

increase with time and then stabilize. When the reaction time was 60 min, the

194

conversion of pyrite reached 100%. With the temperature increasing from 600 to

195

1000 °C, the time required for the conversion of pyrite to reach 100% was reduced from

196

40 to 20 min, indicating that the higher the temperature, the faster the transformation

197

of pyrite. In the temperature range of 900–1000 °C, where significant release of As was

198

observed, the dynamic release of As was synchronous with the dynamic conversion of

199

pyrite, indicating that they were correlated. Compared to that required for the release

200

of As during pyrolysis, the temperature required for the release of As during oxidation

201

was higher, and the release rate and final release ratio were lower, indicating a

202

mechanism that inhibited the release of As during oxidation.

203

3.2 Release characteristics of Pb in pyrite during pyrolysis and oxidation

204

Figure 4(a) shows the release ratio of Pb and the conversion of pyrite at different

205

times during pyrolysis in the 600–1000 °C range. No obvious release of Pb was

206

observed at 600 °C within 60 min. in the 700–1000 °C range, the release ratio of Pb

207

tended to increase with time. As the temperature increased from 700 to 1000 °C, the

208

release ratio of Pb at 60 min increased from 22.47% to 95.93%. This indicates that the

209

higher the temperature, the faster the release of Pb. The release process of Pb was not

210

significantly correlated with the transformation of pyrite.


Energy & Fuels

80

80

60

900 °C

60

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40

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100

80

80 800 °C

60

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20

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100

100

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80

60

700 °C

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80 600 °C

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0 0

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Time (min)

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0

Release ratio of Pb (%)

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60


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80 1000 °C


80


100






100


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Page 12 of 22

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30

40

50

60

Time (min)

(a)

(b)

212

Figure 4 Release ratio of Pb (■) and conversion of pyrite (▲) during (a) pyrolysis and (b)

213

oxidation in the 600–1000 °C range

214

Figure 4(b) shows the release ratio of Pb and the conversion of pyrite at different

215

times during oxidation in the 600–1000 °C range. No significant release of Pb was


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observed at 600 °C. In the 700–1000 °C range, the release ratio of Pb tended to increase

217

with time and then stabilize. As the temperature rose from 700 to 1000 °C, the time

218

required for the release ratio of Pb to stabilize was reduced from 10 to 5 min, and the

219

release ratio of Pb at steady state increased from 17.85% to 83.69%. This indicates that

220

the higher the temperature, the faster the release of Pb and the higher the release ratio.

221

The release of Pb during oxidation was not significantly correlated with the conversion

222

of pyrite. Compared to the release rate of Pb during pyrolysis, the release rate of Pb

223

during oxidation was higher, but the final release ratio was lower, indicating that

224

oxidation was beneficial to the rapid conversion of Pb but inhibited its release.

225

3.3 Transformation paths of As and Pb in pyrite during pyrolysis and oxidation

226

Figure 5(a) shows XRD results of solid residues after 60 min of pyrolysis of pyrite

227

in the 600–1000 °C range. The results indicate that the crystalline structure of solid

228

residue after 60 min of pyrolysis of pyrite at 600 °C included undecomposed FeS2, FeS

229

produced from partially decomposed FeS2, and a small quantity of quartz impurity. In

230

contrast, the pyrite was completely decomposed above 700 °C, and the crystalline

231

structure of the solid residues was that of only FeS and a small quantity of quartz

232

impurity. This is consistent with previous studies of pyrite decomposition26, which is

233

shown in Equation (3).

234

2FeS2  2FeS  S2


(3)

Energy & Fuels

2 1—pyrite(FeS2) 2—troilite(FeS) 3—quartz(SiO2)

2

3 2 2

2

2

2

3

2

1—pyrite(FeS2) 2—hematite(Fe2O3) 3—quartz(SiO2)

2

1000 °C

2 22

1000 °C

2

3

2

2

900 °C

2 2

900 °C

2

2

3 3

2

2

3

2

22

2

2

800 °C

2 2

2

3 3

2

Intensity (counts)

2

Intensity (counts)

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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2 3

800 °C

2

2

2

2

22

2

2

2 2

700 °C

700 °C

2 2

2

2

3 3

3

2

2

2

22

2

1 600 °C

600 °C

2 2

1 31 2 1

10

20

30

1

1

2 1

40

2

1

2 50

1 1 1 60

70

80

10

3

20

30

2-Theta (°)

235

2

2

40

50

2

22

60

70

80

2-Theta (°) (b)

(a)

236

Figure 5 XRD results of solid residues after 60 min of (a) pyrolysis and (b) oxidation of

237

pyrite in the 600–1000 °C range

238

The structures of As and S in pyrite have similarities. As is usually assigned to the

239

occupation of S sites27, such as in FeAsS and FeAs2. As shown in Figure 3(a), the


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Energy & Fuels

240

release of As between 800 and 1000 °C was associated with the conversion of pyrite,

241

suggesting a similarity between the release of As and that of S. However, at 600 °C,

242

FeS2 could be fully decomposed and release S, whereas the release of As occurred only

243

when the temperature reached 800 °C, indicating differences in the thermal stabilities

244

of As and S in pyrite. Balaz and Balassaová28 found that FeAsS could be decomposed

245

and release As at 552 °C. However, no release of As was observed in this study below

246

700 °C, indicating that As did not occur in the form of FeAsS in the pyrite sample but

247

mainly as FeAs2. Díaz-Somoano and Martínez-Tarazona29 mentioned that, based on the

248

thermodynamic equilibrium calculation of the coal gasification process, the

249

decomposition of FeAs2 could be divided into two stages: decomposition first into As

250

and FeAs, and then into Fe and As. Similar stages were observed in this study. During

251

the decomposition of pyrite at 800 °C, the reaction shown in Equation (4) occurred,

252

forming FeAs as a solid and releasing As as a gas. When the temperature rose above

253

900 °C, FeAs was further decomposed into As and Fe (Equation (5)), resulting in the

254

complete release of As.

255

FeAs 2  FeAs  As

(4)

256

FeAs  Fe  As

(5)

257

Figure 5(b) shows XRD results of solid residues after 60 min of oxidation of pyrite

258

in the 600–1000 °C range. The results indicate that the crystalline structure of the solid

259

residue between 600 and 1000 °C included Fe2O3 and a small quantity of quartz

260

impurity, indicating that the pyrite was completely oxidized. Dunn et al.30 studied the

261

oxidation of pyrite using a thermos-gravimetric reactor, proposing that FeS2 was

262

directly oxidized to Fe2O3 at a lower temperature, as shown in Equation (6), but at

263

higher temperatures FeS2 was first decomposed into FeS and S2 and then oxidized to


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

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Fe2O3, which is shown in Equations (3), (7), and (8).

265

4FeS2  11O 2  2Fe 2 O3  8SO 2

(6)

266

2FeS  5O 2  2Fe 2 O3  2SO 2

(7)

267

S2  2O 2  2SO 2

(8)

268

The transformation paths of S during the oxidation of pyrite suggest two

269

transformation paths of As in FeAs2: direct oxidation and oxidation after decomposition.

270

Figure 3 suggests that As was essentially not released below 800 °C during oxidation,

271

whereas a significant release of As was observed during pyrolysis at 800 °C. This

272

indicates that the path of FeAs2 being decomposed and releasing As into the gas phase

273

in the oxidizing atmosphere was unlikely, and that the direct oxidation of FeAs2

274

occurred instead. The oxidation of FeAs2 should produce As2O3 together with Fe2O3 as

275

shown in Equation (9). According to the thermodynamic equilibrium calculation of the

276

coal combustion process by Wang and Tomita13, As2O3 could react with Fe2O3 to form

277

FeAsO4, as shown in Equation (10). As wasn’t released below 800 °C, which indicates

278

that the generated As2O3 was solidified as FeAsO4 by the excess of Fe2O3. However, a

279

significant release of As occurred at 900 and 1000 °C. this may be because of the

280

increasing rate of As2O3 production with the increasing temperature, and because the

281

As2O3 was not fully absorbed by Fe2O3 within the limited time but released into the gas

282

phase instead. There is also a possibility that the decomposition of FeAs2 was

283

accelerated when the temperature rose, resulting in a partial release of As into the gas

284

phase by the decomposition of FeAs2 when competing with the oxidation reactions.

285

Therefore, the higher the temperature, the higher the gas phase release ratio of As.

286

Compared to that of pyrolysis, the release ratio of As in the oxidizing atmosphere was

287

lower owing to the formation of FeAsO4 in the solid phase by oxidation.


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288

4FeAs 2  11O 2  2Fe 2 O3  8As 2 O3

(9)

289

As 2 O3  O 2  Fe 2 O3  2FeAsO 4

(10)

290

If the temperature was further increased, FeAsO4, the solid product of As, would

291

be decomposed. We placed the solid product of pyrite oxidation at 1000 °C in a muffle

292

furnace and calcined it at 1100 and 1300 °C for 30 min in an air atmosphere. The release

293

ratios of As were 1.72% and 99.61%, respectively. This indicates that the FeAsO4

294

formed at 1000 °C was still stable at 1100 °C, but was completely decomposed at 1300

295

°C. The transformation paths of As are summarized in Figure 6(a).

296

Pb is most likely to be mixed with fine galena (PbS) particles in pyrite31. The

297

release of Pb and the conversion of pyrite were therefore relatively independent, which

298

is confirmed in Figure 4. Díaz-Somoano and Martínez-Tarazona29 pointed out that PbS

299

would be decomposed into Pb and S. The release of Pb during pyrolysis could be

300

considered as the decomposition of PbS, as shown in Equation (11). The increase in

301

temperature promoted the decomposition of PbS and accelerated the release of Pb into

302

the gas phase. When the pyrolysis time was extended from 60 to 120 min, the release

303

ratio of Pb at 600 °C increased from 2.46% to 16.22%, and the release ratio at 1000 °C

304

increased from 95.93% to 99.49%. This suggests that PbS was completely decomposed

305

given a sufficiently long time period for pyrolysis.

306

PbS  Pb  S

(11)

307

Two pathways may exist for the conversion of PbS during oxidation: the

308

decomposition and the oxidation of PbS. Abdel-Rehim32 studied the oxidation

309

characteristics of PbS by using a thermos-gravimetric reactor and characterized solid

310

products with XRD, and found that the solid product during oxidation at 600 °C was

311

PbSO4·PbO. The form of Pb in the solid phase changed to PbSO4·4PbO as the


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

Page 18 of 22

312

temperature increased to 900 °C, and to PbO at 1000 °C where PbSO4 was decomposed

313

into PbO. The oxidation reactions in which PbS was involved can be described by

314

Equations (12) and (13). The oxidation products of Pb in PbS below 1000 °C were

315

solids, and therefore the gas-phase release of Pb occurring in the experiment would be

316

caused by the decomposition of PbS. The higher the temperature, the higher the release

317

ratio of Pb, because the promotion of PbS decomposition by the increase in temperature

318

is stronger than the promotion of PbS oxidation. Compared to that in an inert

319

atmosphere, the release ratio of Pb in an oxidizing atmosphere was lower owing to the

320

oxidation reactions of part of the PbS to form PbSO4 or PbO, which are the solid forms

321

of Pb. The solid product of pyrite oxidation at 1000 °C was placed in a muffle furnace

322

and calcined at 1100, 1300 and 1500 °C for 30 min in an air atmosphere. The content

323

of Pb remained essentially unchanged below 1300 °C, but reached 0 at 1500 °C. This

324

indicates that PbO formed at 1000 °C stayed stable below 1300 °C and was completely

325

released at 1500 °C due to its volatility, whose boiling point is reported33 as 1472 °C.

326

The transformation paths of Pb are summarized in Figure 6(b).

327

2PbS  3O 2  2PbO  2SO 2

(12)

328

PbS  2O 2  PbSO 4

(13)

329 330

Figure 6 Transformation paths of (a) As and (b) Pb during pyrolysis and oxidation of pyrite

331


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332

Energy & Fuels

4

CONCLUSIONS

333

In this study, pyrolysis and oxidation experiments on pyrite were conducted in a

334

temperature range of 600–1000 °C using a fixed-bed reactor system to investigate the

335

transformation of As and Pb in pyrite during thermal conversion.

336

As was essentially not released during thermal conversion at lower temperatures.

337

The release of As was obvious for pyrolysis above 800 °C and oxidation above 900 °C.

338

The release ratio of As tended to increase with time and then stabilize, strongly

339

associated with the conversion process of pyrite. As the temperature increased, the

340

release of As was accelerated, the release ratio at the time of stabilization increased and

341

reached 99.85% during pyrolysis and 61.67% during oxidation at 1000 °C, respectively.

342

The pyrolysis atmosphere was more conducive to the release of As.

343

No obvious release of Pb occurred at 600 °C. At the temperature above 700°C, the

344

release of Pb was obvious. The release ratio of Pb increased when the pyrolysis time

345

was extended, but tended to increase with time and then stabilize during oxidation. The

346

Pb release process was not significantly correlated with the conversion process of pyrite.

347

As the temperature increased, the release of Pb was accelerated, the release ratio at the

348

time of stabilization increased and reached 87.69% during oxidation at 1000 °C,

349

respectively. The pyrolysis atmosphere was more conducive to the release of Pb.

350

During pyrolysis, As was released into the gas phase by the two-step

351

decomposition of FeAs2. Under oxidizing conditions, FeAs2 was first oxidized to form

352

As2O3 and Fe2O3, which could further react to form FeAsO4. As was retained in the

353

solid phase, and the release ratio of As under oxidizing conditions was therefore lower.

354

Pb was released into the gas phase by the decomposition of PbS during pyrolysis.


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

355

Decomposition and oxidation of PbS occurred simultaneously under the oxidizing

356

conditions. PbS was partially oxidized to PbO or PbSO4 and remained in the solid phase,

357

and the release ratio of Pb under the oxidizing conditions was therefore lower.

358

The transformation behaviors and mechanisms of As and Pb in pyrite during

359

thermal conversion are helpful for understanding the transformation of sulfide-bound

360

As and Pb in coal.

361

AUTHOR INFORMATION

362

Corresponding author

363

* Fax: +86 10 62781740; E-mail address: [email protected].

364

ACKNOWLEDGEMENTS

365

This work was financially supported by the China National Key Research and

366 367

Development Program (2018YFB0605101).

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368

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Study on the Transformation of Arsenic and Lead in Pyrite During

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