Spontaneous combustion of coals and coal-shales

Spontaneous combustion of coals and coal-shales

International Journal of Mining Science and Technology xxx (2018) xxx–xxx Contents lists available at ScienceDirect International Journal of Mining ...

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International Journal of Mining Science and Technology xxx (2018) xxx–xxx

Contents lists available at ScienceDirect

International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst

Spontaneous combustion of coals and coal-shales M. Onifade, B. Genc ⇑ The School of Mining Engineering, University of the Witwatersrand, South Africa

a r t i c l e

i n f o

Article history: Received 8 November 2017 Received in revised form 24 January 2018 Accepted 15 May 2018 Available online xxxx Keywords: Spontaneous combustion Coal-shales Proximate and ultimate analysis Wits-Ehac index Wits-CT index

a b s t r a c t Spontaneous combustion of coal is a well-known phenomena around the globe. Apart from the coal itself, burning coal-shales is becoming a problem in the South African coal mines. Serious incidents of spontaneous combustion have been reported as a result of self-heating of reactive coal-shales. The intrinsic properties and spontaneous combustion tests of 28 selected coal and coal-shale samples were conducted and a relationship between the two has been established. Intrinsic properties were obtained by using the proximate and ultimate analysis, and spontaneous combustion liability tests results were obtained by using the Wits-Ehac and Wits-CT indices. The experimental results show that intrinsic properties of these materials complement to the spontaneous combustion liability tests results. Comparative analyses of intrinsic properties and spontaneous combustion characteristics indicate similarities between the mechanism of coal oxidation and that of the oxidative processes undergone by coal-shales. For the tested samples, coal samples have a higher intrinsic spontaneous combustion reactivity rating than the coal-shales. Furthermore, an increase in carbon, moisture, hydrogen, volatile matter, nitrogen and a decrease in ash content indicate an increased proneness to self-heating. The concentration of pyrite found in the coal-shales accelerates self-heating. The event of spontaneous combustion can occur if coal-shales absorb sufficient oxygen when subjected to atmospheric conditions. Ó 2018 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction The sedimentary rock which comprises a lot of organic matters is a reactive porous media. This includes coal, shale and oil sand [1–7]. Reactive porous medium is sedimentary materials with pore spaces embedded in the solid, together with a certain amount of a carbon-rich element [8–14]. This enables the rock to be permeable to different fluids like water, air and considerably increases its surface area. Therefore, making the organic particles reactive as it permits oxidation to occur once oxygen is supplied [15]. Such reactive material might experience spontaneous heating. Many investigations have been carried out on self-heating characteristics of coal both experimentally and computationally but with limited studies on coal-shales self-heating [16–19]. There have been no studies conducted to understand the self-heating characteristics of coalshales when subjected to atmospheric conditions. Coal is a readily combustible rock consisting more than 70% by volume and 50% by weight of carbonaceous material [20]. The inorganic non-combustible compounds which produce sulphur

⇑ Corresponding author at: Wits University, School of Mining Engineering, P.O. WITS, 2050 Johannesburg, South Africa. E-mail addresses: [email protected] (M. Onifade), Bekir.Genc@wits. ac.za (B. Genc).

compounds and ash is the non-carbonaceous matter and mineral matter in coal. Carbonaceous shales are sedimentary rocks which possibly originated from peats comprising less than 50% of organic material [21]. Coal seam, spoil heaps and waste dumps comprise of weathered coal, clays, pyritic shales, coal-shales and other strata associated with them. The chemical reactions between oxygen and external active structures of coal particles which releases heat are the required constituents of self-heating [1,2,13]. Spontaneous combustion is a process in which oxidation reaction takes place without the interference of an external heat source. The increase in temperature is caused by the heat liberated by coal through chemical reactions [22]. The spontaneous heating of coal with a potential transition into endogenous fires constitutes a direct risk to the safety of the working conditions and unfavourably influence the mine environments. There are two points of concern when investigating the self-heating rate of carbonaceous materials due to the presence of oxygen. Firstly, the accumulation of overburden materials (such as shale and sandstone etc.) on the coal surface when a new coal seam is exposed to atmospheric conditions. This is common in coal mining operations both in the past and present. The second is geology of the material, particularly during the formation of the deposits. The self-heating of coal-shale has been reported to be the likely source of spontaneous combustion in selected bands of a coal

https://doi.org/10.1016/j.ijmst.2018.05.013 2095-2686/Ó 2018 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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seam, highwalls and spoil heaps in Witbank, South Africa (as shown in Figs. 1–3). Spontaneous combustion of coal has been extensively studied in underground and opencast mines of South African mines using small-scale test [23–27]. There is no sufficient information to predict the spontaneous heating of coal-shales. However, these are materials that contain the fuel for spontaneous combustion in coal mines. This paper presents the results of intrinsic properties and spontaneous combustion tests conducted on highly reactive coal and coal-shales in South African coal mines. This will be useful to establish significant relationships between coal and coal-shale in terms of spontaneous combustion.

Fig. 2. Self-heating of highwalls and coal-shales in Witbank, South Africa.

2. Sample collection and preparation 2.1. Sample collection The coal and coal-shale samples used for this work were taken from four coal mines in Witbank area of South Africa and kept in airtight bags to avoid oxidation. Representative in situ samples were obtained from the affected areas in respect of their high propensity to oxidation. Twenty-eight coal and coal-shale samples were collected at selected bands of a coal seam, highwalls and coal-shales for testing. Eight coal and coal-shale samples were taken from the same coal seam at different bands and the remaining 20 from another three different coal seams. 2.2. Sample preparation In order to reproduce the characteristics of freshly crushed coal, the samples lumps were reduced to suitable sizes to obtain representative samples as required for each experimental test. This was done with the use of a crusher and ball mill. The coal particle sizes were limited by the volume required for each test (coalcharacterization tests and spontaneous combustion tests). The determinations of the volatile matter, ash content and moisture content of the coal and coal-shale samples were also carried out according to the American Society for Testing and Material Standards (ASTM) standards [28–30]. The carbon, hydrogen, nitrogen and sulphur were determined using a LECO TruSpec CHNS analyser after calibration with sulfamethazine based on the ISO standards [31]. The results are given in percentage of carbon, hydrogen and sulphur in the sample. The analyses were repeated three times and the average values were recorded. The results for proximate, elemental analysis (percent air-dried) and spontaneous combustion tests (Wits-Ehac and Wits-CT index) carried out on each sample are presented in Tables 1 and 2. Similar sample preparations were previously reported [32–35]. 2.3. Wits-Ehac tests The Wits-Ehac test at the School of Mining Engineering, University of Witwatersrand is a small-scale test in which freshly

Fig. 1. Self-heating of run-of-mine and spoil heaps at Tweefontein Mine, Witbank, South Africa.

Fig. 3. Symptoms of self-heating in mine face and inseam shale in Witbank, South Africa.

pulverized (<212 mm) and dried coal samples of 20–25 g in weight are used. The testing apparatus consists of an oil bath, six coal and inert material cell assemblies, an oil circular, a heater, a flowmeter used for airflow monitoring, an air supply compressor and a computer. The temperatures are recorded every 20 s by the microcomputer during an average four hours of testing. The test apparatus is used to test coals under predefined conditions and combustibility index is obtained by Wade et al., as shown in Fig. 4 [36]. Twenty-eight tests were conducted by using this apparatus. 2.4. Wits-CT tests (new apparatus) A device for predicting the self-heating characteristics of coal, coal-shale and other carbonaceous materials under the influence of airflow without any heating system was recently developed in the School of Mining Engineering, University of Witwatersrand. This apparatus has been used to test coals and coal-shales and an index has been obtained. This index is called the Wits-CT index. The experimental process represents a well-insulated system relative to actual conditions in situ. The results obtained from this apparatus are in-line with work done by Sensogut et al. in coal stockpiles [37,38]. The experimental investigations were carried out immediately after the samples were collected from the mine sites to avoid oxidation. To emulate the characteristics of freshly crushed coal samples, the lumps were reduced in size (<6.39 mm) on the same day that spontaneous combustion tests for each sample were carried out. The particle size used is based on the study reported by Kunni and Levenspiel to determine the surface-volume average particle size from the size distribution of coal [39]. A representative sample was weighed and loaded into the autoclave. The system is sealed at the base and loaded with a sample from the top. The lid of the system was detached during the loading of the sample. The process was repeated till the column was filled to the marked point. The lid was fixed and fastened as soon as the system was filled with the sample. This technique of preparation gives a closely sized sample that would be preferable to measure the rate of oxidation and it ensures the samples are representatives of the coal beds. Each temperature probe was

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M. Onifade, B. Genc / International Journal of Mining Science and Technology xxx (2018) xxx–xxx Table 1 Results of intrinsic properties and spontaneous combustion tests for coal samples. Sample

Mad

Vad

Aad

Cad

Had

Nad

Sad

Pad

Oc

WE

WC

CA CB CC CD CE CF CG CH CI CJ CK CL CM CN

2.3 2.3 2.2 2.3 2.3 2.5 2.5 2.4 2.1 1.9 1.6 1.6 1.6 1.6

23.2 21.0 24.1 25.5 24.3 23.6 20.0 26.9 16.7 25.7 22.1 26.1 22.0 23.9

28.0 20.0 33.8 20.5 28.6 46.9 16.8 18.8 48.4 28.1 13.7 22.5 17.0 17.0

54.4 61.4 47.5 61.4 53.6 35.9 66 65.2 36.1 52.4 69.7 58.9 66.7 65.8

3.33 3.36 3.20 3.78 3.41 3.01 3.64 4.21 2.55 3.13 4.02 3.57 3.77 4.2

1.34 1.48 1.35 1.53 1.25 0.89 1.58 1.55 0.85 1.35 1.60 1.45 1.57 1.63

1.91 1.11 3.96 0.86 1.08 3.42 0.64 2.19 1.22 5.30 0.76 3.88 0.59 2.92

0.95 0.59 2.73 0.28 0.43 2.33 0.13 1.13 0.22 4.13 0.36 3.22 0.30 2.30

8.72 10.4 7.99 9.63 9.76 7.38 8.84 5.65 8.78 7.82 8.62 8.10 8.78 6.85

4.64 4.64 4.52 4.6 4.76 4.49 4.91 4.69 3.82 4.46 4.44 4.87 4.76 4.84

6.29 6.96 5.31 6.80 5.42 3.97 7.53 7.51 4.05 6.61 9.10 9.59 7.27 7.91

Note: Mad, Vad, Aad, Cad, Had, Nad, Sad, Pad, Oc, WE and WC are the air-dried moisture (%), air-dried volatile matter (%), air-dried ash (%), air-dried carbon (%), air-dried hydrogen (%), air-dried nitrogen (%), air-dried sulphur (%), air-dried pyrite (%), calculated oxygen (%), Wits-Ehac index and Wits-CT index.

Table 2 Results of intrinsic properties and spontaneous combustion tests for coal-shales samples. Sample

Mad

Vad

Aad

Cad

Had

Nad

Sad

Pad

Oc

WE

WC

SA SB SC SD SE SF SG SH SI SJ SK SL SM SN

1.4 0.9 1.1 1.6 1.7 0.9 0.8 0.8 1.0 0.9 1.0 1.0 0.8 1.5

11.2 13.9 12.7 13.3 15.9 13.5 10.7 8.5 11.9 11.9 11.7 16.0 11.7 16.6

78.5 77.2 74.6 77.3 68.4 76.9 84.3 88.7 79.6 86.9 79.1 74.0 76.9 51.5

11.5 11.0 13.8 10.8 15.8 11.8 6.02 2.66 9.12 3.42 9.75 10.5 12.5 33.7

1.34 1.27 1.60 1.43 1.78 1.40 1.04 0.96 1.41 0.75 1.73 2.14 1.61 2.87

0.34 0.40 0.42 0.32 0.41 0.43 0.29 0.09 0.26 0.08 0.41 0.39 0.52 0.96

0.54 1.56 0.35 2.53 6.90 0.46 0.73 0.41 0.22 0.75 0.16 0.12 0.24 0.31

0.21 1.01 0.15 1.43 4.26 0.19 0.22 0.15 0.05 0.43 0.10 0.04 0.16 0.12

6.39 7.67 8.13 6.02 5.01 8.11 6.83 6.38 8.39 7.19 7.85 11.85 7.43 9.16

3.09 3.06

1.33 1.30 0.91 0.70 1.60 1.36 0.67 0.27 0.95 0.42 1.18 1.34 1.44 3.99

3.27 3.73 3.10

2.98 2.99 3.77

3. Results and analysis 3.1. Results The results for proximate, elemental analysis and spontaneous combustion tests carried out on coal and coal-shale samples are presented in Tables 1 and 2. 3.2. Analysis

Fig. 4. Wits-Ehac test apparatus setup [36].

inserted into the column as the correct sample level was reached. The position of each temperature probe to the column was place vertically and uniformly spaced to the centre as soon as the coal level was reached. Oxygen was supplied and controlled at a fixed flow rate (20 mL/min) by means of a flowmeter before being fed into a manifold attached below the lid of the autoclave. The logging began and the variation in temperatures distribution of the sensors was stored to a computer file every minute. The test period lasts for 24 h. At the end of each experiment, samples are discarded and the autoclave is cleaned. The conditions in which the experimental tests were carried out closely resembled the situation in the mine environment. Twenty-eight tests were conducted with the use of this new apparatus to bring better definition for the Wits-Ehac index. An illustration of the experimental setup is indicated in Fig. 5a while Fig. 5b shows the apparatus setup.

The determination of moisture, volatile matter and ash content of both coal and coal-shale samples is carried out. Tables 1 and 2 provide the results of the proximate analysis of coal and coalshale samples. The air-dried moisture content varies between 1.6% to 2.5% and 0.8 to 1.7% for coal and coal-shales respectively. The lower moisture content in the tested coal samples could be due to the depth of burial and age of the coal seam. It was observed that most of the coals with lower moisture content show high liability to spontaneous combustion. This is in-line with the study reported by McPherson and Beamish and Hamilton [40,41]. Slight differences are observed in the moisture content of the samples which is in-line with the study described by Gurdal et al. [42,43]. The variations in moisture content (drying or wetting) of different bands within a seam have noticeable influences on spontaneous combustion. The interaction between coal and water in the studied area can be due to two opposing processes. Firstly, the heat of evaporation occurs in the first process when the moisture content of the coal is driven by evaporation at the early heating phase. Secondly, the process involves adsorption of water vapour from

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Fig. 5. Wits-CT experimental setup and apparatus.

the air which caused an increase in the coal temperature. The influence of these two processes is dependent on which process dominates significantly to coal oxidation. The behavior of the moisture content of the investigated samples are in-line with the study reported by Pone et al. [45]. Fig. 6 indicates an increase in moisture occurring on the coal surface layers during spontaneous heating. Previous studies reported a similar coal comportment [45–51]. The coal-shales have low moisture contents similar to the tested coal samples. The air-dried moisture content of seam CG to CJ varies between 1.9% and 2.5%, CA to CC ranges between 2.2% and 2.3%, CD to CF between 2.3% and 2.5% and CK to CN have the same moisture content (1.6%) respectively. Seams CK to CN have the lowest moisture content while seam CF and CG have the highest moisture contents. Seam CF and CG have the highest moisture content than others followed by CH and those from CA, CB, CD and CE respectively. It was observed from the study that the samples have roughly the same moisture contents and are more prone to self-heating except for samples CF and CI, with lower liability indices. Coal-shales SE, SD and SN have roughly the same moisture contents among the coal-shale samples. Coalshale SN is more prone to spontaneous combustion from the result of the liability indices compared to other coal-shales. This may be due to the presence of the mineral matters that promotes the selfheating rate. Literature studies revealed that moisture content increases the liability of coal to spontaneous combustion in addition to parameters like oxygen, organic matter type, surface area exposed, mineral content (particularly pyrite) and rank [44,52]. The study shows that an increase in moisture content of coal and coal-shales is enough to provide a high heat loss from evaporation, as the samples temperature increases during oxidation reaction. The air-dried volatile matter content varies between 16.7% to 26.9% and 6.5% to16.6% for the coal and coal-shale samples respectively. The volatile matter for the coals is not less than 20%, except for sample CI (16.7%). Seams with the high volatile matter have high liability to spontaneous combustion as indicated in Table 1. This supports the study reported by Banerjee [53]. Seam CH has the highest volatile matter content than the other seams. The highest volatile matter among all the coal-shales is found in samples SN and SE as shown in Table 2. The two coal-shales have high spontaneous combustion liability index compared to other coal-shales. It

Fig. 6. Moisture occurrence on the surface of the coal at Khwezela Mine (Bokgoni Pit), Witbank, South Africa.

was found that coal-shales with high volatile matter are more liable to spontaneous combustion. This characteristic of coalshales with high volatile matter corresponding to high liability indices are similar to coals with the high volatile matter contents. It is observed that coal and coal-shales with high volatile matter are liable to spontaneous combustion compared to other samples with lower values of volatile matter content. The air-dried ash content ranges between 13.7% to 48.4% and 51.5% to 88.7% for both coal and coal-shale samples respectively. This confirms with the characteristic of South African coals [54–57]. The ranges of the results are much related to properties of some Indian coals with ash contents greater than 45% [58]. The cause of this high ash content is due to the peat depositional environment where the condition of flooding of the paleomire occurs periodically during deposition. This is in-line with the study reported by Zˇivotic´ et al. [59–61]. It is known that the physical and chemical properties of coal changes during coal oxidation [62]. The variations in ash contents for samples of the same seam may be attributed to changes in combustion. Gurdal et al. observed changes in ash content when compared with un-oxidized coals [42,43]. It was observed that the ash content of samples within a selected band of a seam varies significantly. Seam CK has very low ash content compared to samples CL, CM and CN from the same seam. Seam CG and CH also have ash content lower than samples CI and CJ from the same seam. The ash content of seam CF and CI are considerably high compared to the other seams. Sample CI with the highest ash content has the slowest self-heating rate while coal samples with low ash content sample have the fastest self-heating rate. The slow and fast self-heating of the coal could be due to the influence of heat sink on existing mineral within a coal. This is in-line with the study reported by Humphreys et al. [63,64]. It was observed that the liability of coal to spontaneous combustion decreases with increasing ash content and vice versa. Coal-shales SN and SE have the lowest ash contents compared with other coal-shales and have the highest self-heating rate. It is shown that coal-shales SH, SJ and SG have high ash contents and displayed slow self-heating rate. This characteristic is related to those exhibited by coal samples. Coal-shales with low ash contents indicate a high propensity to spontaneous combustion than coal-shales of higher ash contents. Therefore, coal and coal-shale samples display similar ash behaviour with respect to spontaneous combustion. Proximate analyses of the tested coal samples have a similar range of moisture, ash and volatile matter contents, compared to other values found in a reported study [40,41,56–58,65,66]. This demonstrates that the obtained results are in-line with the previous studies in this regard. No studies have previously been reported on the proximate analysis of coal-shales. The major elemental constituents present in the coal and coal-shale samples are determined using a LECO TruSpec CHNS

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analyser. The air-dried carbon content varies between 36.1% to 69.7% and 2.66% to 33.7% for coal and coal-shales respectively as shown in Tables 1 and 2. Seam CI has the lowest carbon and the slowest self-heating rate while seams with high carbon contents have high self-heating rates. Coal-shales SN and SE contained the higher carbon self-heating rate compared to other coal-shales. Seam CL, CK, CN, CG and coal-shales SN and SE show higher liability indices compared to other coal and coal-shales. Most of the coal seams displayed high self-heating rate due to the high carbon contents compared to the coal-shales with considerably low carbon contents. Seam CI and coal-shale SH indicate the lowest liability indices. The capacity to self-heat appears to be directly related to the amount of carbon and ash content present in the samples. Most of the coals with high carbon and low ash content indicate high liability index while the coal-shales which contain more than 15% carbon and less than 70% ash content shows a high propensity to spontaneous combustion (see Tables 1 and 2). It is shown that coal-shales with high carbon and low carbon contents show similar characteristics to coal samples having high and low carbon content with respect to spontaneous combustion. There is a relationship between the spontaneous combustion characteristics and carbon contents of the coal and coal-shales from the experimental studies. Coal-shales with a low carbon content of 13% can be described as a weakly reactive porous medium with respect to self-heating when compared to coal which contains a higher carbon content. The air-dried hydrogen, nitrogen and calculated oxygen content varies between 2.55–4.21%, 0.85–1.63%, 5.65–10.35% and 0.75– 2.87%, 0.08–0.96% and 5.01–11.85% for the coals and coal-shales respectively. Seam CI has the lowest hydrogen of 2.55% and also the slowest self-heating rate compared to other coal seams. Seam CN and CH have hydrogen contents of 4.20% and show a high liability to spontaneous combustion. The highest nitrogen content of 1.63% is found in seam CN and it shows high liability to spontaneous heating. Seam CI has the lowest nitrogen content of 0.85% and indicates a very slow self-heating rate compared to other coal samples. This indicates that coal and coal-shales with high hydrogen and nitrogen content show a high propensity to spontaneous heating and vice versa. Coal-shale SN has the highest hydrogen and nitrogen content of 2.87% and 0.96% respectively. It is moderately prone to spontaneous heating based on the liability indices. Coal-shale CJ with the lowest hydrogen (0.75%) and nitrogen content (0.08%) shows a low propensity to spontaneous heating. Seam CH has the lowest oxygen of 5.65% and the highest oxygen content is found in seam CB (10.35%). Coal-shale SE has the lowest oxygen content of 5.01% while coal-shale SL has the highest oxygen content of 11.85%. This study indicates that the influence of calculated oxygen from the obtained results of ultimate analyses has no effects on the liability indices used to evaluate spontaneous combustion. The percentage oxygen by itself does not seem to indicate the ability of coal to absorb oxygen. The oxygen content of coal does not seem to show a direct relation to the avidity with which coal and coal-shales absorb oxygen. Coal-shales with a high propensity to spontaneous combustion in this study appears not to contain high oxygen content. Hence, the percentage oxygen contents do not seem to indicate the liability of the tested samples to self-heating. The study shows that coal and coal-shales with high carbon, hydrogen and nitrogen contents are more liable to spontaneous combustion. Therefore, the influence of these elements might play a significant role in evaluating the incidents of spontaneous combustion. It was observed that the carbon, hydrogen and nitrogen contents present in the tested coal samples are in-line with the study reported by Czaplicki and Smolka; Department of Minerals and Energy South Africa (DME) and Roberts [65–67].

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The air-dried total sulphur content varies from 0.59% to 5.30% and 0.12% to 6.90% for the coals and coal-shales respectively. Six of the coal samples have sulphur content more than 2% as shown in Table 1, CC (3.96%), CF (3.42%), CH (2.19%), CJ (5.30%), CL (3.88%) and CN (2.92%) respectively. The presence of high sulphur content can be related to the peat depositional environment and conditions, regional volcanic activity (tuff deposits are interbedded in the coal-bearing sequences) and alkaline depositional environments with concentrated sulphide mineralization. Another cause is that marine influenced peat when the sulphate ions in seawater provide sufficient amount of sulphur. This is supported in the study reported by Casangrande et al. [68–70]. It is also reported by Teichmüller and Teichmüller [71] that fresh-water coal deposited in the calcium-rich environment is usually sulphur-rich and this may cause the presence of high sulphur content in some coals [70]. Seam CM has the lowest total sulphur of 0.59% while seam CJ has the highest sulphur of 5.30%. The spontaneous combustion tests on the samples show that both samples are liable to spontaneous combustion. Coal-shales SE (6.90%) and SD (2.53%) have high sulphur content compared to other coal-shales. Coal-shale SL has the lowest sulphur concentration of 0.12% while coal-shale SE has the highest sulphur content of 6.9%. The high sulphur content in coal-shale SE increases with increasing self-heating rate while coal-shale SL with low sulphur content shows a slow self-heating rate as shown in Table 2. This may be caused by the high sulphur minerals occurring in the tested samples. Comparing the total sulphur content to other coal samples around the world, the study shows that South African coal can be considered as low sulphur coal. The values of the total sulphur analysed in the tested coal samples are in-line with the study reported in previous studies [65–67,72–76]. The pyrite content in coal is known to be one of the main factors accelerating the self-heating rate. The pyrite present in the coal and coal-shales was investigated using the inductively coupled plasma optical emission spectrometry (ICP-OES). The concentration of the total sulphur and pyritic sulphur varied considerably from one sample to another due to other sulphur forms occurrence. The high amounts of pyrite are observed in seams CJ, CL, CN and CC while the lowest in seams CG and CI respectively. Seam CJ has the highest concentration of total sulphur while the lowest total sulphur is found in seam CM. The sulphur forms analysed show that the pyritic contents vary between 0.13% to 4.13% and 0.038% to 4.26% for the coals and coal-shales respectively. The high liability indices observed in coal-shales SN and SE compared to all other coal-shales can be associated with the existence of pyrite which reacts with oxygen and moisture. The reason for the self-heating rate of coal-shale SE to be higher than other coal-shale samples except for SN may be due to pyrite concentration of 6.9%. This pyrite content might have considerable effects on the spontaneous combustion process. This supports the study reported by Stach et al. [77]. The results certainly illustrate the advance effects of pyrite oxidation on coal and coal-shales spontaneous combustion. Furthermore, the results show that the oxidation of sulphide minerals might favour the self-heating rate. The results from the tested coal and coal-shales are in-line with the study reported on coal by Pone et al. [44,52,78]. The analysed results show that the concentration of pyrite in the coal and coal-shales accelerates the self-heating rate. The burning areas as seen in Figs. 7 and 8 indicate that the areas of selfheating are causes of severe ingress of oxygen into the coal seam. The distribution of different minerals is formed where gas streams may appear on the coal surface. These conditions allow sulphur deposits in the form of crystals and their aggregate, crust, dendrites and other forms of native sulphur and ammonium chloride to be found. The occurrence of fumarolic acid mineral deposits are signs

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Fig. 7. Forms of crystals and ammonium chloride present on the surface of coalshales at iMpunzi Mine, Witbank, South Africa.

Fig. 8. Blooms of sulphur and ammonium chloride on the surface of coal-shale at Tweefontein Mine, Witbank, South Africa.

of the oxidation stage of coal. This study indicates a similar experience in the burning areas as seen in Fig. 7. Fig. 8 shows a light-yellow to straw coloured sulphur deposits and white powdeish ammonium chloride appeared on the coal surface in one of the studied areas. The presence of sulphur deposits on the coal-shale surface indicates that coal fire may be located deep underground. Figs. 7 and 8 show that pyrite reacts exothermally with oxygen to liberate heat and oxidizes to form sulphuric acid which significantly increases the rate of low-temperature oxidation. The crystals of these hydrated sulphates on the exposed coal surfaces are shown in Figs. 9 and 10. The formation of these hydrated iron crystal sulphates is exothermic and should be controlled effectively to avoid consistent self-heating of coal seams. If the self-heating rate of carbonaceous materials is associated with the heat generated and heat dissipated from a single process, it is expected that the rise in temperature would basically increase the reaction rate of that process. The carbon contents found in the coal-shales varied from one sample to the other, hence, this might indicate that different reactions with different activation energies might take place in the spontaneous combustion process. When the carbon contents for the coal-shales were compared with each other, shales SN and SE are more reactive under the two spontaneous combustion tests than other coal-shales. The Wits-Ehac values of coal-shales SC, SG, SH, SI, SJ and SM could not be determined due to their low reactivity. The results of Wits-CT index obtained from tested samples are in-line with the Wits-Ehac index. The Wits-CT index varies between 3.97 to 9.59 and 0.27–3.99 for the coal and coal-shales respectively as shown in Tables 1 and 2. Seam CL has the highest Wits-CT values while seam CF has the lowest Wits-CT value. Coal-shale SH has the lowest Wits-CT and coal-shale SN has the highest Wits-CT index among the coal-shale samples. It is shown from the analysed results that high-risk coal and coal-shales have higher Wits-CT and Wits-Ehac index values. Most of the fires caused by spontaneous combustion in the affected areas of this

Fig. 9. Hydrated iron sulphates on exposed coal surfaces, iMpunzi Mine, Witbank, South Africa.

Fig. 10. Crystals of hydrated iron sulphates on exposed coal surfaces, iMpunzi Mine, Witbank, South Africa.

study have been identified as having high Wits-Ehac and Wits-CT values. As can be seen from Table 1, seam CL, CK, CH, CN and CG, (which were taken from Khwezela at Bokgoni Pit and Arthur Taylor Mine at iMpunzi mines) showed the highest risk value in the two liability indices. The records of the mines revealed that an incident of spontaneous combustion happened in the seam. These areas are assigned as being high-risk zone and are the zones from which samples were collected. The event of spontaneous combustion in coal mines may be caused by various organic and inorganic constituents of coal-shale within a coal seam. Coal-shales can initiate self-heat when it absorbs sufficient oxygen and moisture when exposed to atmospheric conditions. The heat generated due to the influence of oxygen may exceed the heat dissipated to the surrounding via conduction, convection and radiation and accumulates causing spontaneous combustion of coal-shales to arise. 4. Conclusions This study has established a detailed experimental methodology to evaluate the self-heating characteristics of coal and coalshales under the field conditions. The study illustrates that the tested coals have a high intrinsic spontaneous combustion reactivity rating than the coal-shales. Both materials show that an increase in carbon, moisture, hydrogen, volatile matter, nitrogen and decrease in ash content can complement the proneness to spontaneous combustion. The proximate and ultimate analysis of the samples tested shows that these properties may be used as a tool to measure the liability of spontaneous combustion. Further research on petrographic analysis and detailed geotechnical study of coal and coal-shales is underway to evaluate their influences in predicting and minimizing the event of spontaneous heating. Acknowledgments The authors would like to thank the staff of Glencore and Anglo American mines for their devoted help during sample collections. The authors wish to express gratitude to Coaltech for their financial support. The work presented here is part of a PhD research study in the School of Mining Engineering at the University of the Witwatersrand. References [1] Oliveira ML, Navarro OG, Crissien TJ, Tutikian BF, da Boit K, Teixeira EC, et al. Coal emissions adverse human health effects associated with ultrafine/nanoparticles role and resultant engineering controls. Environ Res 2017;158:450–5. [2] Oliveira MLS, da Boit K, Pacheco F, Teixeira EC, Schneider IL, Crissien TJ, et al. Multifaceted processes controlling the distribution of hazardous compounds in the spontaneous combustion of coal and the effect of these compounds on human health. Environ Res 2018;160:562–7. [3] Gredilla A, de Vallejuelo SFO, Gomez-Nubla L, Carrero JA, de Leão FB, Madariaga JM, et al. Are children playgrounds safe play areas? Inorganic analysis and lead isotope ratios for contamination assessment in recreational (Brazilian) parks. Environ Sci Pollut Res 2017;24:24333–45. [4] Dutta M, Saikia J, Taffarel SR, Waanders FB, de Medeiros D, Cutruneo CM, et al. Environmental assessment and nano-mineralogical characterization of coal, overburden and sediment from Indian coal mining acid drainage. Geosci Front 2017;8:1285–97.

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