Mineralogical Characteristics and Arsenic Release of High Arsenic Coals from Southwestern Guizhou, China during Pyrolysis Process

Abstract

Coal is the primary energy source in China, and coal pyrolysis is considered an essential and efficient method for clean coal utilization. Three high arsenic coals collected from the southwestern Guizhou province of China were chosen in this study. Low-temperature ashing plus X-ray diffraction analysis (XRD) was used to identify the minerals in coals. The three coals were pyrolyzed in a tube furnace in an N₂ atmosphere at 950 °C, 1200 °C, and 1400 °C, respectively. Environment scanning electron microscope (ESEM), XRD, X-ray fluorescence analysis (XRF), and inductively coupled plasma-mass spectrometry (ICP-MS) were adopted to determine the morphology, mineral compositions, and element compositions and arsenic contents of the coal pyrolysis ashes, respectively. It can be found that minerals in coal are mainly composed of quartz, pyrite, muscovite, and rutile. The minerals in the ashes generated from coal pyrolysis mainly contain quartz, dehydroxylated muscovite, iron oxide minerals, mullite, and silicon nitride. Oldhamite and gupeite exist at 950 °C and 1400 °C, respectively. The morphologies of oldhamite and gupeite at these temperatures are irregular block-shaped particles and irregular spherical particles, respectively. The mineralogical transformations in the process of coal pyrolysis affect coal utilization. The arsenic release rate is higher than 87% during pyrolysis at 1400 °C. The arsenic in organic matter is more able to be volatilized than mineral components. The retention time can slightly influence the arsenic release rate, and the influence of temperature is much more significant than the influence of retention time. The understanding of mineral evolution and arsenic environmental emission is helpful for the safety of high-arsenic coal pyrolysis.

1. Introduction

Coal is the leading fossil fuel for Chinese energy consumption; its consumption in China provides more than 65% of the primary energy [1,2]. The primary pattern of Chinese coal utilization is combustion for electricity generation, while coal pyrolysis plays a vital role in coal’s efficient and clean utilization [3,4]. However, coal utilization will cause all sorts of pollution, e.g., noxious gas, suspended particles, and trace elements [5,6,7,8], which may affect human health and result in severe disease [9,10]. Many inorganic matrices can be identified in coal and may significantly affect coal utilization [11,12,13]. The releasing characteristics of pollution have relations with not only their own chemical and physical properties but also their occurrence forms or affinities with minerals [14,15,16]. For example, mineral transformations in coal combustion may strongly affect ash deposition and slagging characteristics [17,18]. The emissions of particles have a growing impact on the environment, especially those submicron particles enriched in toxic trace elements more easily [19,20].

Ruch and Finkelman defined that mineral materials in coals included both inorganic amorphous substances and mineral materials [21,22]. Mineral transformations significantly impact the formation of ashes [23,24]. Minerals, e.g., quartz, montmorillonite, and kaolinite, did not crush during combustion, while pyrite, calcite, siderite, and white mica will break at different speeds [25]. Benfell et al. found that hollow cenospheres with high expansion ratios and porosity will formulate smaller ash particles, while compact char with low porosity produced large ash particles [26]. The minerals in coal mainly include clay minerals, carbonate minerals, sulfide, oxides, and hydroxides [27,28], possibly the leading carriers for trace elements in coal [20,29]. Many researchers believe some trace elements strongly correlate with syngenetic or epigenetic minerals in coal [30,31,32]. Tian et al. used low-temperature ashing technology combined with a float-sink density separation method to investigate trace elements’ affinities with coal minerals [33,34]. It was found that the inorganic associated arsenic mainly occurs in heavy-density fractions in which the main mineral was pyrite [34,35]. The transformation of mineral phases during coal combustion is strongly dependent on ashing temperature and residence time. Sulfates and simple oxides are stable at temperatures lower than 815 °C [36].

It is significant to understand mineral compositions and transformations in the process of coal pyrolysis; it has much insight into the safe operation and environmental protection. Three high arsenic coal samples, which were collected from southwestern Guizhou, China, were selected to conduct the coal pyrolysis experiments. Further, the characteristics of minerals transformations and the arsenic release in the process of coal pyrolysis are also presented in this study.

2. Geological Setting

Guizhou is a crucial coal-producing region located in southwestern China, and the area has some ore deposits of gold, arsenic, mercury, etc. [30,37]. The coal in this area has attracted a lot of attention due to some environmental concerns, since arsenic, mercury, fluorine, and chromium levels are comparatively high in the coals of this area [38,39,40,41]. Three high arsenic coal samples in this study were collected in Xinren County of Guizhou province. The coal-bearing strata of this area mainly are Late Permian anthracite and Triassic bituminous coal [37,42]. The collected position of coal samples is shown in Figure 1; the LT-K1 and LT-D2 were collected in Lantana, and JL-CQ was collected inJiaole.

3. Experimental Section

3.1. Low Temperature Ashing

Three high arsenic coals studied were air dried, crushed, and sieved to obtain a representative fraction with a particle size of less than 75 μm. Low Temperature ashing technology is an effective method for the non-destructive study of mineral in coals [33], since organic substance can react with oxygen plasma at a very low temperature (120–150 °C). In order to investigate the original minerals in coals, the pulverized coals were ashed at a low temperature by using a K1050X low temperature oxygen plasma asher (Quorum, UK). The low temperature ashes (LTAs) were collected to conduct further analysis.

3.2. High Temperature Pyrolysis of High Arsenic Coals

The pyrolysis experiment was carried out in a tube furnace at N₂ atmosphere with pyrolysis temperatures set at 950 °C, 1200 °C, and 1400 °C, and the retention times were 0 min, 30 min, and 60 min, respectively. The pyrolysis set-up is shown in Figure 2. In order to calculate the ash yields, the set-up must accurately record the weights of the coal samples and ashes. The coal pyrolysis ashes (CPAs) were collected and placed in a drying vessel to keep dry. The yields of CPAs are calculated by Equation (1) showing as following:

Y_CPA = (M_CPA/M_C) × 100%

(1)

where: Y_CPA is the yield of CPAs; M_CPA is the mass of CPAs after combustion; M_C is the mass of coal before combustion.

3.3. Analysis Techniques

The morphology and elementary composition of the ash particles were studied using a Quanta 200 (FEI, Eindhoven, The Netherlands) microscope equipped with an energy dispersive X-ray spectroscopy (EDX). Minerals investigations were conducted using a X’Pert PRO diffractometer (PANalytical B.V., Almelo, The Netherlands). For testing, 40 kV of accelerating voltage and 40 mA of current were used. The 2θ interval of the XRD patterns were recorded between 10–85° or 90° and the scanning step was 0.026°. The reference intensity method was adopted to conduct a semi-quantitative analysis of samples [43,44,45]. XRF analysis adopted the XRF analyzer of EAGLE Ⅲ (EDAX Inc., San Diego, CA, USA), which can determine the content of main elements in samples [46]. The arsenic content was determined via ELANDRC-e inductively coupled plasma-mass spectrometry (PerkinElmer, Waltham, MA, USA).The proximate analysis and ultimate analysis of coals were confirmed via TGA2000 industrial analyzer (Las Navas, Castilla, Spain) and EL-2 elemental analyzer (Elementar Vario, Frankfurt, Germany), respectively.

4. Results and Discussion

4.1. Minerals in High Arsenic Coals

4.1.1. Coal Property

The proximate and ultimate analyses of the three high arsenic coals are listed in

Table 1. Proximate analysis and ultimate analysis.

Coals	Proximate Analysis (wt.%)				Ultimate Analysis (wt.%)
	Mad	Aad	Vad	FCad	Cad	Had	Sad	Nad	Oad
LT-K1	1.33	45.33	9.64	43.59	42.35	0.90	9.72	0.43	1.49
LT-D2	2.31	37.00	4.72	49.72	54.89	0.41	6.66	0.58	1.71
JL-CQ	1.45	14.33	8.21	76.72	57.91	0.75	6.67	0.47	19.66

Note: ad: air-dried basis; M: Moisture; V: Volatiles; A: Ash; FC: Fix carbon.

. It can find that the L-K1 and LT-D2 are high ash coals, with the contents reaching to 45.94% and 37.87%, respectively, while the JL-CQ is middle ash coal, around14.54%. All of the three samples are high-sulfur coals and the sulfur contents reach 9.72%, 6.66%, and 6.67%, respectively.

4.1.2. Minerals in Low Temperature Ash of High Arsenic Coals

The XRD patterns of LTAs are shown in Figure 3. The major minerals in the three high arsenic coals are composed of quartz (SiO₂), muscovite (KAl₂Si₃AlO₁₀(OH)₂), pyrite (FeS₂), and a small amount of rutile (TiO₂). Reinmöller et al. studied the minerals in the LTA of Columbian hard coal, which includes gypsum, anhydrite, corundum, quartz, hematite, and kaolinite. The same mineral is only the quartz in the LTAs of the two kinds of coals, and it can be found that the primary minerals in different kinds of coals vary greatly [36].

The chemical compositions and semi-quantitative contents of minerals in LTAs are shown in

Table 2. XRF analysis of LTA (wt.%).

Ashes	LT-K1	LT-D2	JL-CQ
Al2O3	24.9	12.5	24
SiO2	36.8	53.0	47.1
Fe2O3	9.8	9.7	8.2
SO3	21.5	20.0	16.0
TiO2	3.7	2.0	2.1
K2O	2.3	2.1	2.6
CaO	0.6	0.3	0.51
MgO	*	*	1.2

Note: *= Not detected.

and

Table 3. Semi-quantitative analysis of the minerals in coals (LTA) (wt.%).

Ashes	LT-K1	LT-D2	JL-CQ
Quartz	72.7	49.1	67.5
Muscovite	15.9	31.8	25.0
Pyrite	8.0	15.1	4.5
Rutile	3.4	4.0	3.0

, respectively. The inorganic matrix (LTA) of coals mainly contains the elements Al, Si, Fe, and S; it also contains minor amounts of Ti, K, Ca, and Mg. Combing with the XRD patterns of LTAs (Figure 3), it can be found that the elements of Fe and S are presented as pyrite (FeS₂) and K, Si, and Al are presented as quartz and muscovite, while the element of Ti is presented as rutile. Quartz is the most abundant mineral in the coal samples, which may reach as high as 75.2% of the total amount of minerals. Pyrite is a common heavy mineral in coal, and its content in the studied area are in the range of 4.5–15.1%. The muscovite is a kind of the potassic aluminosilicate, and the only clay mineral in the coals studied. Ti-bearing mineral (rutile) can also be identified in coal samples with the contents around 3–4%.

4.2. Mineralogy during Pyrolysis of LT-K1

The XRD patterns of CPAs from LT-K1 are shown in Figure 4. The minerals in CPAs of LT-K1 at 950 °C include quartz, dehydroxylated muscovite (KAl₃Si₃O₁₁), and iron oxide. However, there are different variations of iron oxide at different retention times. At the 0min, the hematite (Fe₂O₃) and maghemite (Fe₂O₃) both exist in the experimental condition of 0 min, while the Fe-bearing mineral is maghemite at 30 min and 60 min. It also includes quartz in 1200 °C, while there are some variations of aluminosilicate minerals and Fe-bearing minerals. The clay minerals disappeared at 30 min and mullite appears at 60 min. The Fe-mineral is maghemite at 0 min, and turns into magnetite at 30 min and 60 min. The minerals in 1400 °C at 0 min are very different than that of 1400 °C at 30 min and 60 min. For 1400 °C, it includes mullite and maghemite at 0 min, while silicon nitride (Si₃N₄) and hematite are present at 30 min and 60 min. The silicon nitride can be formed via carbothermal reduction in the temperature range of 1200–1350 °C [47]. It is also found that the minerals of cristobalite (SiO₂) and gupeite (Fe₃Si) are present due to the high temperature of 1400 °C.

The chemical compositions and semi-quantitative contents of minerals in CPAs of LT-K1 are shown in

Table 4. XRF analysis of LT-K1 (wt.%).

LT-K1	C950/0	C950/30	C950/60	C1200/0	C1200/30	C1200/60	C1400/0	C1400/30	C1400/60
Al2O3	14.5	13.7	16.9	11.3	20.5	6.6	16.9	18. 5	15.6
SiO2	37.8	31.5	31.8	21.9	40.9	10.2	56.7	38.5	27.0
SO3	20.4	33.2	33.7	47.8	20.2	77.4	11.5	16.3	36. 5
Fe2O3	21.0	13.3	11.9	13.6	10.1	2.6	9.0	19.7	15.5
K2O	3.6	3.4	2.6	2.1	3.8	1.8	2.6	3.5	2.3
CaO	0.06	0.8	0.6	0.3	0.5	0.43	0.5	0.5	0. 6
TiO2	2.5	3.8	2.4	2.6	2.9	1.0	2.7	2.9	2.6
V2O5	0.1	0.2	0.2	0.2	0.2	0.1	0.2	0.3	0.1

Note: Combustion temperature and retention time: C_950/0, 950 °C/0 min; C_950/30, 950 °C/30 min; C_950/60, 950 °C/60 min; C_1200/0, 1200 °C/0 min; C_1200/30, 1200 °C/30 min; C_1200/60, 1200 °C/60 min; C_1400/0, 1400 °C/0 min; C_1400/30, 1400 °C/30 min; C_1400/60, 1400 °C/60 min.

and

Table 5. Semi-quantitative analysis of minerals in LT-K1 (wt.%).

LT-K1	C950/0	C950/30	C950/60	C1200/0	C1200/30	C1200/60	C1400/0	C1400/30	C1400/60
Silicon Nitride	*	*	*	*	*	*	*	50	72
Quartz	72.4	72.8	73.9	75.8	91	78	63	28	9
Cristobalite	*	*	*	*	*	*	1	4	4
De-Muscovite	8.9	16.4	15.1	10.8	*	*	*	*	*
Mullite	*	*	*	*	*	20	21	*	*
Hematite	3.9	*	*	*	*	*	*	14	13
Maghemite	14.8	10.8	11	13.4	*	*	13	*	*
Magnetite	*	*	*	*	9	2	*	*	*
Gupeite	*	*	*	*	*	*	1	3	2

Note: * = Not detected; De-Muscovite = Dehydroxylated Muscovite; Combustion temperature and retention time: C_950/0, 950 °C/0 min; C_950/30, 950 °C/30 min; C_950/60, 950 °C/60 min; C_1200/0, 1200 °C/0 min; C_1200/30, 1200 °C/30 min; C_1200/60, 1200 °C/60 min; C_1400/0, 1400 °C/0 min; C_1400/30, 1400 °C/30 min; C_1400/60, 1400 °C/60 min.

, respectively. The contents of quartz in CPAs of LT-K1 at the temperatures of 950 °C and 1200 °C are higher than 70%, and it turns to cristobalite at 1400 °C. The contents of dehydroxylated muscovite are around 10% at 950 °C and 1200 °C, then it disappears. Mullite is generated from quartz and muscovite at high temperatures and its content is around 20%. Silicon nitride is the dominant mineral in 1400 °C at 30 min and 60 min, up to 72%. Fe mainly presented as the minerals of hematite, maghemite, or magnetite (Fe₃O₄), the contents of which are approximately 10%. The Fe and Si react and present as gupeite at 1400 °C, the contents of gupeite are around 1–3%.

4.3. Mineralogy duringPyrolysisof LT-D2

The XRD patterns of CPA from LT-D2 were shown in Figure 5. The minerals in CPAs of LT-D2at 950 °C include quartz, dehydroxylated muscovite, hematite, rutile, and oldhamite (CaS). Those at 1200 °C include quartz, mullite, maghemite, and rutile. The minerals includes silicon nitride, mullite, iron oxides, and gupeite at 1400 °C. The mullite continuously increases with the different retention times at 1200 °C. The silicon nitride forms and increases with the retention times at 1400 °C. The diffraction peaks of silicon nitride continue to enhance and newly generate, while those of mullite continuously recede. This demonstrates that the transformations between quartz, mullite, and silicon nitride are carried out at different stages.

The chemical compositions and semi-quantitative contents of minerals in CPAs of LT-D2 are shown in

Table 6. XRF analysis of LT-D2 (wt.%).

LT-D2	C950/0	C950/30	C950/60	C1200/0	C1200/30	C1200/60	C1400/0	C1400/30	C1400/60
Al2O3	13.9	24.3	7.1	21.2	27.9	6.6	19.3	31.9	11.7
SiO2	37.7	36.5	16.4	39.1	40.5	12.5	45.1	37.9	18.2
SO3	28.3	21.3	70.9	25.8	12.0	69.0	20.0	13.9	61.9
Fe2O3	14.5	10.1	3.8	8.9	12.3	6.4	11.5	8.2	3.1
K2O	2.5	3.1	1.4	3.0	2.8	1.2	1.6	2.9	1.9
CaO	*	0.9	0.6	*	0.7	0.6	0.06	1.3	0.3
TiO2	3.0	3.5	0.9	1.6	3.2	1.2	2.2	3.5	2.5
V2O5	0.2	0.2	0.1	0.2	0.2	0.10	0.2	0.2	0.1

Note: * = Not detected; Combustion temperature and retention time: C_950/0, 950 °C/0 min; C_950/30, 950 °C/30 min; C_950/60, 950 °C/60 min; C_1200/0, 1200 °C/0 min; C_1200/30, 1200 °C/30 min; C_1200/60, 1200 °C/60 min; C_1400/0, 1400 °C/0 min; C_1400/30, 1400 °C/30 min; C_1400/60, 1400 °C/60 min.

and

Table 7. Semi-quantitative analysis of minerals in LT-D2 (wt.%).

LT-D2	C950/0	C950/30	C950/60	C1200/0	C1200/30	C1200/60	C1400/0	C1400/30	C1400/60
Silicon Nitride	*	*	*	*	*	*	*	28	40
Quartz	57.5	58.5	59.5	44	35	20	12	*	*
De-muscovite	24.8	18.8	23.7	*	*	*	*	*	*
Mullite	*	*	*	29	44	65	74	54	44
Hematite	7.4	7.3	7.6	*	*	*	12	17	15
Maghemite	*	*	*	27	21	15	*	*	*
Rutile	5.9	12.2	6.9	*	*	*	*	*	*
Gupeite	*	*	*	*	*	*	1	1	1
Oldhamite	4.4	3.2	2.3	*	*	*	*	*	*

Note: * = Not detected; De-Muscovite = Dehydroxylated Muscovite; Combustion temperature and retention time: C_950/0, 950 °C/0 min; C_950/30, 950 °C/30 min; C_950/60, 950 °C/60 min; C_1200/0, 1200 °C/0 min; C_1200/30, 1200 °C/30 min; C_1200/60, 1200 °C/60 min; C_1400/0, 1400 °C/0 min; C_1400/30, 1400 °C/30 min; C_1400/60, 1400 °C/60 min.

, respectively. The contents of quartz in CPAs of LT-D2 are higher than 55% at 950 °C. It decreases with increasing temperature and retention times, and completely disappears at 1400 °C at 30 min. The dehydroxylated muscovite exists at 950 °C with its contents around 20%. Mullite appears in all the experimental conditions of 1200 °C and 1400 °C, up to 74%. Silicon nitride exists in 1400 °C at 30 min, the content of which increases with the retention times. The iron oxide disappears and the gupeite appears at 1400 °C, and the contents of both are only 1%.

4.4. Mineralogy during Pyrolysisof JL-CQ

The XRD patterns of CPAs from JL-CQ were shown in Figure 6. The minerals in CPAs of JL-CQ at the experimental condition of 950 °C included quartz, dehydroxylated muscovite, and hematite. Those of both the 1200 °C and 1400 °C include quartz and hematite. It is worth noting that the CPAs of JL-CQ include a large amount of amorphous material. The XRD patterns change little at the different retention times of the same temperatures, but to some extent, they change at the temperature of 1400 °C comparing with 950 °C and 1200 °C.

The chemical compositions and semi-quantitative contents of minerals in CPAs of JL-CQ are shown in

Table 8. XRF analysis of JL-CQ (wt.%).

JL-CQ	C950/0	C950/30	C950/60	C1200/0	C1200/30	C1200/60	C1400/0	C1400/30	C1400/60
Al2O3	24.4	36.0	8.6	24.5	34.0	5.5	25.7	32.1	9.1
SiO2	37.5	29.3	14.1	57	44	8.47	58.9	52.6	9.8
SO3	18.4	4.9	69.3	4.6	5.8	73.5	3.0	2.5	68.9
Fe2O3	14.3	7.2	4.7	10.3	9.4	7.0	10.1	8.2	8.7
K2O	1.8	1.7	1.5	0.5	0.4	1.1	0.3	0.07	1.0
CaO	0.2	0.8	0.4	0.2	1.4	2.8	0.2	1.0	0.6
TiO2	3.0	4.2	1.0	2.7	4.7	1.2	1.6	3.4	1.5
V2O5	0.3	0.2	0.1	0.2	0.3	0.07	0.2	0.2	0.1

Note: Combustion temperature and retention time: C_950/0, 950 °C/0 min; C_950/30, 950 °C/30 min; C_950/60, 950 °C/60 min; C_1200/0, 1200 °C/0 min; C_1200/30, 1200 °C/30 min; C_1200/60, 1200 °C/60 min; C_1400/0, 1400 °C/0 min; C_1400/30, 1400 °C/30 min; C_1400/60, 1400 °C/60 min.

and

Table 9. Semi-quantitative analysis of minerals in JL-CQ (wt.%).

JL-CQ	C950/0	C950/30	C950/60	C1200/0	C1200/30	C1200/60	C1400/0	C1400/30	C1400/60
Quartz	78.4	77.8	80.3	94	88	93	83	73	85
De-Muscovite	14.8	11.6	11.8	*	*	*	*	*	*
Hematite	6.8	10.6	7.9	6	12	7	17	27	15

Note: * = Not detected; De-Muscovite = Dehydroxylated Muscovite; Combustion temperature and retention time: C_950/0, 950 °C/0 min; C_950/30, 950 °C/30 min; C_950/60, 950 °C/60 min; C_1200/0, 1200 °C/0 min; C_1200/30, 1200 °C/30 min; C_1200/60, 1200 °C/60 min; C_1400/0, 1400 °C/0 min; C_1400/30, 1400 °C/30 min; C_1400/60, 1400 °C/60 min.

, respectively. There is abundant amorphous substance extant in CPAs of JL-CQ. The contents of quartz are the highest crystalline substance (73–97%). It contains a small amount of dehydroxylated muscovite at 950 °C, which is similar to that of LT-K1 and LT-D2. The Fe-bearing mineral is hematite, the contents of which are around 6–10% and increase to 15–17% at 1400 °C.

4.5. Morphology of Pyrolysis Ashes of High Arsenic Coal

The electron images of minerals and char in CPAs of coals are shown in Figure 7. There are various char particles in the products of coals pyrolysis. The oldhamite is in the shape of an irregular particle with a diameter of 10 μm (Figure 7a). The minerals in JL-CQ may be wrapped inside carbonaceous particles due to the high carbon content of JL-CQ (Figure 7b). Hematite and maghemite are flocculent spherical particles with adiameter less than 20 μm (Figure 7c–e). Mullite is irregular blocky-shaped particles with its size around 10 μm ((Figure 7c,e). There are two main shapes of silicon nitride; one is hyperfine needle-shaped particles (Figure 7d) and the other is flocculent aggregate (Figure 7d,e). The minor amount of gupeite is in the shape of spherical particles in the diameter of 10 μm (Figure 7d). Though cristobalite in LT-D2 cannot be identified by XRD pattern at 1400 °C, it can be found via SEM (Figure 7e). Quartz in JL-CQ can be found in tiny spherical particles less than 5 μm (Figure 7f).

4.6. Characteristics of Mineral Transformation during High Arsenic Coals Pyrolysis

The muscovite in coal takes place the decomposition reaction which is accompanied by the releasing of hydroxyl functional groups and converts into dehydroxylated muscovite at a relatively lower temperature (<950 °C). The dehydroxylated muscovite disappears with the pyrolysis time increasing and converts into quartz at a higher temperature (>1200 °C), which demonstrates that the pyrolysis temperature and retention times can have an effect on the transformations of muscovite into mullite. The silicon nitride is generated at 1400 °C and its content increases with increasing pyrolysis time, which shows that the process of quartz and mullite converting into silicon nitride is affected by both the pyrolysis temperature and the time. Some quartz can convert into cristobalite at 1400 °C.

The pyrite in coal firstly converts into hematite, then hematite transforms to maghemite with the pyrolysis time increasing at 950 °C, and the maghemite transforms to magnetite at 1200 °C. The Fe-minerals at 1400 °C include maghemite and hematite, and the maghemite converts into hematite with the pyrolysis time increasing. This indicates that the Fe-bearing minerals have some variations with the pyrolysis temperature and time increasing. A small amount of iron oxides may convert into gupeite when the temperature reaches 1400 °C. There is a large amount of amorphous substance in the coal with medium ash yield, and the amorphous substance gradually transforms toward crystalline phase with the temperature increasing [48].

According to the minerals analysis of the three coals in this studied area, it can be found some characteristics of mineral transformations during coals pyrolysis. Minerals’ transformations in the process of coal pyrolysis is shown as follows:

$$\mathrm{Muscovite}\stackrel{1200°\mathrm{C}}{\to }\mathrm{Mullite}+\mathrm{Quartz}$$

(2)

$$\mathrm{Quartz}+{\mathrm{N}}_2\stackrel{1400°\mathrm{C}}{\to }\mathrm{Silicon}\mathrm{nitride}$$

(3)

$$\mathrm{Quartz}\stackrel{1400°\mathrm{C}}{\to }\mathrm{Cristobalite}$$

(4)

$$\mathrm{Pyrite}\stackrel{950°\mathrm{C}}{\to }\mathrm{Hematite}\stackrel{950°\mathrm{C}}{\to }\mathrm{Maghemite}\stackrel{1200°\mathrm{C}}{\to }\mathrm{Magnetite}\stackrel{1400°\mathrm{C}}{\to }\mathrm{Hematite}$$

(5)

$$\mathrm{Maghemite}+\mathrm{Quartz}\stackrel{1400°\mathrm{C}}{\to }\mathrm{Gupeite}$$

(6)

4.7. Characteristics of Arsenic Release during High Arsenic Coals Pyrolysis

The contents of arsenic in the three high arsenic coals are 194.9 μg/g, 127.2 μg/g, and 230.4 μg/g, respectively, which were determined by our research team in previous work [49]. In this study, the pyrolysis ash yield rates of the three coals at 1400 °C with different retention times are shown in

Table 10. Pyrolysis ash yield rates at 1400 °C with different retention times (wt.%).

Coal	1400 °C/0 min	1400 °C/30 min	1400 °C/60 min
LT-K1	58.97	48.62	47.04
LT-D2	61.84	53.00	51.43
JL-CQ	79.45	74.88	75.90

and the arsenic content in pyrolysis ashes were determined as shown in the

Table 11. Arsenic content in per gram of pyrolysis ash (μg/g).

Coal	1400 °C/0 min	1400 °C/30 min	1400 °C/60 min
LT-K1	34.48	38.50	35.53
LT-D2	26.22	24.26	23.56
JL-CQ	6.28	3.41	2.60

. The arsenic release rate can be calculated using Equation (7), and the arsenic release rates of the three coals pyrolysis at 1400 °C with different retention times are shown in Figure 8. The calculation formula of the arsenic release rate is shown as following:

ARR = (ACA × AYR)/ACC

(7)

where: ARR is arsenic release rate; ACA is arsenic content in per gram of pyrolysis ash; AYR is pyrolysis ash yield rate; ACC is arsenic content in per gram of coal.

The results show that the arsenic release rate is higher than 87% during the pyrolysis process of the three high-arsenic coals at 1400 °C with different residence times. It indicates that the arsenic release rate is very high at a high pyrolysis temperature of 1400 °C. The release rate of JL-CQ reaches over 97.8%, and the arsenic was basically completely released. The ash content in JL-CQ coal is only 14.51% (Table 1), which is the lowest among the three types of coal. This indicates that JL-CQ coal has the lower mineral component; therefore, less arsenic is bound to minerals, and more arsenic exists in the organic matter. It is more prone to volatilization, resulting in the highest arsenic release rate in JL-CQ coal. The arsenic release rates of LT-K1 and LT-D2 coals are similar. The arsenic release rate of LT-K1 is slightly higher than that of LT-D2. The content of pyrite in LT-D2 is higher than that in LT-K1 (Table 3), and arsenic in coal is more likely to be combined with pyrite [27,29], which will slightly reduce the arsenic release rate during coal pyrolysis.

Comparing the different high temperature retention times (0 min, 30 min, and 60 min), it can be found that the longer the retention time can slightly increase the arsenic release rate, but the degree of improvement is limited. The retention time of LT-K1, LT-D2, and JL-CQ at 1400 °C increased from 0 min to 60 min, the arsenic release rate increases by 1.86%, 3.22%, and 1.31%, respectively. It can be seen that the influence of temperature on arsenic release rate is much greater than the retention time.

5. Conclusions

The mineral transformations and arsenic release in the pyrolysis progress of high arsenic coals in southwestern Guizhou, China, are studied. The main minerals in the high arsenic coals are quartz, muscovite, pyrite, and rutile. The content of muscovite and pyrite in coals are 15–32% and 5–15% (wt.%), respectively. Minerals in the products of coal pyrolysis are mainly composed of quartz, dehydroxylated muscovite, iron oxide minerals (hematite, maghemite, or magnetite), mullite, and silicon nitride. In coals pyrolysis, the muscovite takes place decomposition reaction and converts into dehydroxylated muscovite at a temperature lower than 950 °C. Quartz and dehydroxylated muscovite convert into mullite at 1200 °C, quartz and mullite transform into silicon nitride at 1400 °C, and the content of silicon nitride reaches up to 72%. The pyrite in coals first converts into hematite, then hematite transforms to maghemite and magnetite at different temperatures, while the Fe-bearing mineral will transform back to hematite at 1400 °C. It contains a large amount of amorphous substance in the pyrolysis ashes of the coal with medium ash yield. The low content of oldhamite and gupeite are found at 950 °C and 1400 °C, respectively.

The arsenic release rate is higher than 87% at 1400 °C for all the three coals during pyrolysis. The release rate of JL-CQ reaches over 97.8%, so the JL-CQ coal has a low content of mineral components, and more arsenic is in the organic matter, which is more prone to volatilization. A longer retention time can slightly increase the arsenic release rate, while the influence of temperature is much more significant than retention time.

This work focuses on the mineral transformation and arsenic release during the pyrolysis process, which is necessary for the safe operation and environmental protection of the high-arsenic utilization.