**3. Results and discussion**

#### **3.1. Mineralogy of coal**

The XRD analytical results show that the pulverized coal used in the combustion process in the power station mainly composed of siliceous mineral such as quartz (SiO2), kaolinite [Al2(SiO2O5)(OH)4)] and the non-siliceous mineral for instance potassium selenium chloride (K2SeCl6) and little quantities of pyrite (FeS2) (Fig. 1). The mineral suites in the coal samples used in the present study are consistent with the previous studies [4, 13, 14, 15, 16, 17). Kaolinite is uniformly distributed in the coal samples. This mineral is commonly present in coal as two species with different crystallinity, namely a low crystallinity detrital kaolinite and a high crystallinity neomorphic kaolinite. These genetic types have been described earlier [18, 19]. Kaolinite contains water bound within their lattices.

**Figure 1.** XRD spectra for the pulverised coal sample used in coal fired power station

All of the water is lost during the high-temperature ashing. Pyrite is the main species of sulfur oxidation in the coal samples studied. Pyrite occurs as typical syngenetic framboidal, euhedral and massive cell filling forms [20]. This mineral shows a highly inhomogeneous distribution in the coal samples. Pyrite is probably the most environmentally interesting mineral in the run of mine and beneficiation of coals and their generated wastes because of its propensity to oxidize during weathering and production of sulphuric acid.

**Figure 2.** Infrared (FTIR) spectra of coal sample

50 Analytical Chemistry

reproducibility.

**2.5. Transmission Electron Microscopy (TEM)** 

**2.6. Proximate and ultimate analyses** 

**3. Results and discussion** 

**3.1. Mineralogy of coal** 

Pulverized sample (~1-2) g of the coal and coal ash samples was poured into a small conical container while little amount of ethanol was added to the sample to serve as medium for solution. This solution was then placed inside the centrifuge for few minutes (5 mins), drops of the stirred solution was then placed on a labelled 200 μm and 400 μm of copper grid underlain by a filter paper with a hot lamp light focused directly on the samples to dry up the earlier added ethanol. The resultant mixture was placed inside the air gun channel so as to project the beam on it for image analysis at a nanometric scale. The TEM analysis of study was carried out on TECHNAI G2 F20 X-TWIN MAT 200Kv field emission transmission electron microscopy.

Proximate and ultimate analyses were performed on coal samples based on ASTM Standards [12]. All runs were repeated to check the instrument's results repeatability and

The XRD analytical results show that the pulverized coal used in the combustion process in the power station mainly composed of siliceous mineral such as quartz (SiO2), kaolinite [Al2(SiO2O5)(OH)4)] and the non-siliceous mineral for instance potassium selenium chloride (K2SeCl6) and little quantities of pyrite (FeS2) (Fig. 1). The mineral suites in the coal samples used in the present study are consistent with the previous studies [4, 13, 14, 15, 16, 17). Kaolinite is uniformly distributed in the coal samples. This mineral is commonly present in coal as two species with different crystallinity, namely a low crystallinity detrital kaolinite and a high crystallinity neomorphic kaolinite. These genetic types have been described

earlier [18, 19]. Kaolinite contains water bound within their lattices.

**Figure 1.** XRD spectra for the pulverised coal sample used in coal fired power station

Pyrite is transformed to hematite and sulfur dioxide during coal incineration at 815°C [20]. Some of the sulfur dioxide may remain combined with calcium in the ash, but much is lost [21]. The weathering of pyrite produces acid conditions that may leach trace elements associated with the pyrite and other constituents in the coal [22]. Quartz is the most common mineral in the coal samples studied. This mineral has mostly detrital genesis [23]. The shape of the quartz grains is rounded to semi-rounded, indicating an intensive transport before their deposition in the basin. The content of quartz is also high in the coal fly ash because this mineral is commonly stable/inert at combustion conditions (Fig. 3).

#### **3.2. Mineralogy of weathered drilled ash cores**

The mineralogical analysis by depth of the core ash samples was carried out with X- ray diffraction technique (XRD). This is to better understand the mineralogical changes (i.e. secondary phases) under the real dry disposal conditions. The experimental protocol for this section is presented in section 2.3. The results of the XRD analysis of samples of the drilled weathered dry disposed fly ash aged 2 week, 1 year and 20-year-old showed quartz (SiO2) and mullite (3Al2O3·2SiO2) as main crystalline mineral phases (Fig. 3). Other minor mineral phases identified are; hematite (Fe2O3), calcite (CaCO3), lime (CaO), anorthite (CaAl2Si2O8), mica (Ca(Mg,Al)3(Al3Si)O10(OH)2) and enstatite (Mg2Si2O6). This is in general agreement with

mineralogy reported for other coal fly ashes [24, 25, 26, 27, 28, 29].The eight phases observed in every sample are considered to be characteristic phase assemblages for Class F fly ash. The XRD results obtained from 2-week-old dry disposed fly ash show similar mineralogical composition to the weathered ash except for the absence of calcite and mica. Lime (CaO) presence in coal fly ash may be due to the heat transformation of dolomite mineral or decarbonation of calcite entrained in feed stock coal [30]. The mullite present in fly ash was formed by the decomposition of kaolinite [31], which is entrapped in the parent coal. The gradual reduction in pore water pH is due to chemical interaction of fly ash with ingressed CO2 and percolating rain water. Calcite formation is attributed to chemical weathering due to over time reduction in pore water pH. Previous study had proved that calcite precipitation in weathered fly ash is as a result of chemical interactions of calcium oxide (lime) rich fly ash with ingressed CO2 [32]..

Mineralogy and Geochemistry of Sub-Bituminous Coal and Its Combustion Products from Mpumalanga Province, South Africa 53

assemblages for Class F fly ash. This assemblage presented in Figure 4 consisted of quartz,

In the 1-year-old ash cores, the most prominent mineral phase is quartz having a peak of almost double that of the other mineral phases in all the core samples from all depths (0- 31m). The peak height of quartz mineral showed a decreasing trend with increasing depth of 1-year-old ash dump (Figure 3). This observed trend showed strong correlation with flushing/leaching of SiO2 and Al2O3 in the ash dump [34]. Thus, the quartz mineral peak height is indicative of rapid dissolution/weathering of aluminosilicate mineral within 1 year of ash dumping. Other mineral phases such ash mullite; calcite, hematite, enstatite, lime, anorthite and mica showed similar trend in the 1 year and 20-year-old ash dumps (Figures 4 and 6). The prominent presence of the quartz peak in the upper depths is due to flushing of other soluble matrix in the fly ash and the mineral phase quartz is easily detectable by XRD

**Figure 4.** Bulk XRD mineral mean peak heights in 1-year-old (AMB 83) Tutuka ash cores (An=anorthite,

The mineral peak height in the 8-year-old section of the ash dump showed anomalous trend which may be ascribed to in-homogenous irrigation with high saline effluent (brine). This implied that the 8-year-old section has received much of high saline effluents than 1 year

The statistical result is shown in Table 1. The relative standard deviation (RSD) in the peak heights of 1-year-old drilled core (Figure 4 and Table 1) showed highly significant variations

The relative standard deviation (RSD) of quartz phase (being < 40 %) indicates less inhomogeneity among the coal fly ash mineral phases. There are however more variation (57-

anorthite (residual coal minerals), mullite, calcite, hematite, mica, enstatite and lime.

indicating there is relative increase in concentration in the upper depth

En=enstatite, Ca=calcite, H=hematite, L=lime, M=mullite, Mi=mica, Q=quartz).

57-65 % Anorthite, Enstatite, Mica, Mullite, Hematite, Calcite and Lime

and 20-year-old sections of the ash dump.

which could be classified into 2 groups:

< 40 % Quartz

**Figure 3.** XRD spectra for the dry disposed ash dumps: (a) 2-week-old (T 87) not irrigated dry ash dump (b) 1-year-old (AMB 83) irrigated and quenched with high salient effluents (n = 2) (c) 20-year-old irrigated and quenched with fresh water.

Previous study has shown that the statistical variations in peak height on the same phase of ash samples could be used to assess the homogeneity and stability of different mineralogical phases [33]. A statistical consideration of the variations in the ashes peak heights for the different phases was used here to appraise the mineralogical distribution and chemical heterogeneity among the coal fly ash samples. Figures 4-6 presents the summary of the mean peak heights of mineral phases determined by XRD analysis of the 1 year, 8 year and 20-year-old ash core samples drawn from different depths of the dumps respectively. The eight phases observed in every sample are considered to be characteristic phase assemblages for Class F fly ash. This assemblage presented in Figure 4 consisted of quartz, anorthite (residual coal minerals), mullite, calcite, hematite, mica, enstatite and lime.

In the 1-year-old ash cores, the most prominent mineral phase is quartz having a peak of almost double that of the other mineral phases in all the core samples from all depths (0- 31m). The peak height of quartz mineral showed a decreasing trend with increasing depth of 1-year-old ash dump (Figure 3). This observed trend showed strong correlation with flushing/leaching of SiO2 and Al2O3 in the ash dump [34]. Thus, the quartz mineral peak height is indicative of rapid dissolution/weathering of aluminosilicate mineral within 1 year of ash dumping. Other mineral phases such ash mullite; calcite, hematite, enstatite, lime, anorthite and mica showed similar trend in the 1 year and 20-year-old ash dumps (Figures 4 and 6). The prominent presence of the quartz peak in the upper depths is due to flushing of other soluble matrix in the fly ash and the mineral phase quartz is easily detectable by XRD indicating there is relative increase in concentration in the upper depth

**Figure 4.** Bulk XRD mineral mean peak heights in 1-year-old (AMB 83) Tutuka ash cores (An=anorthite, En=enstatite, Ca=calcite, H=hematite, L=lime, M=mullite, Mi=mica, Q=quartz).

The mineral peak height in the 8-year-old section of the ash dump showed anomalous trend which may be ascribed to in-homogenous irrigation with high saline effluent (brine). This implied that the 8-year-old section has received much of high saline effluents than 1 year and 20-year-old sections of the ash dump.

The statistical result is shown in Table 1. The relative standard deviation (RSD) in the peak heights of 1-year-old drilled core (Figure 4 and Table 1) showed highly significant variations which could be classified into 2 groups:

< 40 % Quartz

52 Analytical Chemistry

**Lin(Counts)**

**a**

and quenched with fresh water.

0 20 40 60 80 100

AEA ME E EQ <sup>E</sup> <sup>E</sup><sup>M</sup> <sup>L</sup> <sup>L</sup>

H H <sup>M</sup> <sup>Q</sup> <sup>Q</sup>

Q

**2 wks old ash**

**2-Theta-Scale**

<sup>M</sup> <sup>H</sup> <sup>A</sup>

A

Q

**Lin (Counts)**

**c**

mineralogy reported for other coal fly ashes [24, 25, 26, 27, 28, 29].The eight phases observed in every sample are considered to be characteristic phase assemblages for Class F fly ash. The XRD results obtained from 2-week-old dry disposed fly ash show similar mineralogical composition to the weathered ash except for the absence of calcite and mica. Lime (CaO) presence in coal fly ash may be due to the heat transformation of dolomite mineral or decarbonation of calcite entrained in feed stock coal [30]. The mullite present in fly ash was formed by the decomposition of kaolinite [31], which is entrapped in the parent coal. The gradual reduction in pore water pH is due to chemical interaction of fly ash with ingressed CO2 and percolating rain water. Calcite formation is attributed to chemical weathering due to over time reduction in pore water pH. Previous study had proved that calcite precipitation in weathered fly ash is as a result of

chemical interactions of calcium oxide (lime) rich fly ash with ingressed CO2 [32]..

A= Anorthite E=Enstatite H=Hematite L=Lime M=Mullite Q=Quartz

28m 24m 20m 16m 12m 8m 4m 1m

> A=Anorthite E=Enstatite C=Calcite H=Hematite L=Lime Mi=Mica M=Mullite

**Lin(Counts)**

**b**

0 20 40 60 80 100 120

<sup>L</sup> <sup>M</sup> <sup>M</sup> <sup>Q</sup> <sup>E</sup> <sup>C</sup>

CAH <sup>C</sup> Mi <sup>Q</sup> Mi

Q

Q

**1yr old ash**

31m 27m 23m 19m 15m 12m 7m 3m 1m

A=Anorthite E=Enstatite C=Calcite H=Hematite L=Lime M=Mullite Mi=Mica Q=Quartz

**2-Theta-Scale**

15m 14m 8m 6m 4m 2m 1m

**Figure 3.** XRD spectra for the dry disposed ash dumps: (a) 2-week-old (T 87) not irrigated dry ash dump (b) 1-year-old (AMB 83) irrigated and quenched with high salient effluents (n = 2) (c) 20-year-old irrigated

Mi MiE A E Mi E C H

0 20 40 60 80 100 120

Q=Quartz <sup>L</sup>

**20 yrs old ash**

**2-Theta-Scale**

Previous study has shown that the statistical variations in peak height on the same phase of ash samples could be used to assess the homogeneity and stability of different mineralogical phases [33]. A statistical consideration of the variations in the ashes peak heights for the different phases was used here to appraise the mineralogical distribution and chemical heterogeneity among the coal fly ash samples. Figures 4-6 presents the summary of the mean peak heights of mineral phases determined by XRD analysis of the 1 year, 8 year and 20-year-old ash core samples drawn from different depths of the dumps respectively. The eight phases observed in every sample are considered to be characteristic phase

57-65 % Anorthite, Enstatite, Mica, Mullite, Hematite, Calcite and Lime

The relative standard deviation (RSD) of quartz phase (being < 40 %) indicates less inhomogeneity among the coal fly ash mineral phases. There are however more variation (57-

65 %) in the peak heights of the following mineral phases, namely anorthite, enstatite, mica, mullite, calcite and lime.

Mineralogy and Geochemistry of Sub-Bituminous Coal and Its Combustion Products from Mpumalanga Province, South Africa 55

[An=Anorthite, En=Enstatite, Q=Quartz, Mi=Mica, M=Mullite, Ca=Calcite, H=Hematite, L=Lime].

inherent minerals together with other minerals with which calcium reacts.

showed low variations which could be classified into 3 groups:

< 3 % Anorthite, Enstatite, Quartz, Mica, Mulite,

< 10 % Calcite and Hematite

< 25 % Lime

ashes (1 year, 8 year and 20-year-old dry disposed cores)

**Table 1.** Summary of the statistical analysis of the peak heights of various mineral phases in Tutuka fly

Mean 14955.4 10933.9 9302.0 10754.1 10291.0 10630.0 10513.0 37438.0 Stdev. 4862.3 5729.2 6515.9 5985.0 6284.1 6487.6 5980.7 9230.3 RSD % 32.5 52.4 70.0 55.7 61.1 61.0 56.9 24.7

20 year old ash core samples

**An En Ca H L M Mi Q**

**An En Ca H L M Mi Q**

**An En Ca H L M Mi Q**

Mean 12776.3 13262.8 13614 13474.9 13128.2 13969 13310.4 33003.8 Stdev. 8198.9 8153.39 7813 7797.4 8057.18 7966.25 7759.5 12837.9 RSD % 64.17 61.48 57.39 57.87 61.37 57.03 58.30 38.90

1 year old core ash samples

Mean 200.00 9261.54 1815.38 4453.85 10923.08 11692.31 3276.92 8600.00 Stdev. 0.00 83.56 170.28 381.53 2646.67 264.46 42.13 161.72 RSD % 0.00 0.90 9.38 8.57 24.23 2.26 1.29 1.88

8 year old ash core samples

Mullite showed variations (57-65 %) in the peak heights which might be due to the conversion of clays that contain < 60 % of Al2O3. The mullite peak heights depend on the amount of SiO2 and Al2O3 in the mineral phase (Figure 4 and Table 1). Anorthite is a rare compositional variety of plagioclase feldspar (calcium rich end-member of plagioclase). Anorthite has been found in other fly ashes obtained from coals in which Ca is present in

Anorthite showed variations (57-65 %) in peak height which could be due to chemical interactions of CaO, SiO2 and Al2O3 in the mineral phase of coal fly ash samples. Enstatite is a magnesium silicate mineral. Enstatite shows variations (57-65 %) in peak heights due to prevailing chemical interactions of SiO2 and MgO in the mineral phase of coal fly ash samples. Hematite (Fe2O3) is a heat transformation product of pyrite in feed coal and accordingly hematite was revealed by XRD in the ash core samples. Previous studies had shown that pyrite (FeS2) was present in the feed coal as a fine-grained mineral [35]. Hematite (Fe2O3) phase showed variations (≈ 57 - 87 %) in peak heights distribution among the fly ash samples. This trend is an indication of the instability of the Fe containing mineral phases in the ash core samples of 1-year-old ash at the dry disposed fly ash dump site. The relative standard deviation (RSD) in the peak heights of the 8-year-old ash (Figure 5 and Table 1)

The relative standard deviation (RSD) of anorthite, enstatite, quartz, mica and mullite (being < 3 %) indicates homogeneity among these coal fly ash mineral phases in this core. There was however more variation (< 25 %) in the mean peak heights of lime (CaO) in the 8-year-old ash samples. The in-homogeneous distribution of calcite and hematite mineral

**Figure 5.** Bulk XRD mineral mean peak heights in 8-year-old (AMB 81) Tutuka ash cores (An=anorthite, En=enstatite, Ca=calcite, H=hematite, L=lime, M=mullite, Mi=mica, Q=quartz).

**Figure 6.** Bulk XRD mineral mean peak heights in 20-year-old (AMB 79) Tutuka ash cores (An=anorthite, En=enstatite, Ca=calcite, H=hematite, L=lime, M=mullite, Mi=mica, Q=quartz).

The in-homogeneous distribution of the calcite mineral phase could be attributed to continuous weathering occasioned by ingress of CO2. There is a direct relationship between the calcite mineral phase peak heights and CaO (weight %). The depletion or enrichment of CaO (weight %) in coal fly ash agrees with the peak height of calcite which is an indication of the role of lime (CaO) in the formation of calcite (CaCO3). The mica mineral phase also exhibited heterogeneous distribution in the drilled weathered cores which could be attributable to continuous weathering process.


[An=Anorthite, En=Enstatite, Q=Quartz, Mi=Mica, M=Mullite, Ca=Calcite, H=Hematite, L=Lime].

**Table 1.** Summary of the statistical analysis of the peak heights of various mineral phases in Tutuka fly ashes (1 year, 8 year and 20-year-old dry disposed cores)

Mullite showed variations (57-65 %) in the peak heights which might be due to the conversion of clays that contain < 60 % of Al2O3. The mullite peak heights depend on the amount of SiO2 and Al2O3 in the mineral phase (Figure 4 and Table 1). Anorthite is a rare compositional variety of plagioclase feldspar (calcium rich end-member of plagioclase). Anorthite has been found in other fly ashes obtained from coals in which Ca is present in inherent minerals together with other minerals with which calcium reacts.

Anorthite showed variations (57-65 %) in peak height which could be due to chemical interactions of CaO, SiO2 and Al2O3 in the mineral phase of coal fly ash samples. Enstatite is a magnesium silicate mineral. Enstatite shows variations (57-65 %) in peak heights due to prevailing chemical interactions of SiO2 and MgO in the mineral phase of coal fly ash samples. Hematite (Fe2O3) is a heat transformation product of pyrite in feed coal and accordingly hematite was revealed by XRD in the ash core samples. Previous studies had shown that pyrite (FeS2) was present in the feed coal as a fine-grained mineral [35]. Hematite (Fe2O3) phase showed variations (≈ 57 - 87 %) in peak heights distribution among the fly ash samples. This trend is an indication of the instability of the Fe containing mineral phases in the ash core samples of 1-year-old ash at the dry disposed fly ash dump site. The relative standard deviation (RSD) in the peak heights of the 8-year-old ash (Figure 5 and Table 1) showed low variations which could be classified into 3 groups:


54 Analytical Chemistry

mullite, calcite and lime.

65 %) in the peak heights of the following mineral phases, namely anorthite, enstatite, mica,

**Figure 5.** Bulk XRD mineral mean peak heights in 8-year-old (AMB 81) Tutuka ash cores (An=anorthite,

En=enstatite, Ca=calcite, H=hematite, L=lime, M=mullite, Mi=mica, Q=quartz).

**Figure 6.** Bulk XRD mineral mean peak heights in 20-year-old (AMB 79) Tutuka ash cores (An=anorthite, En=enstatite, Ca=calcite, H=hematite, L=lime, M=mullite, Mi=mica, Q=quartz).

attributable to continuous weathering process.

**Peak heights**

The in-homogeneous distribution of the calcite mineral phase could be attributed to continuous weathering occasioned by ingress of CO2. There is a direct relationship between the calcite mineral phase peak heights and CaO (weight %). The depletion or enrichment of CaO (weight %) in coal fly ash agrees with the peak height of calcite which is an indication of the role of lime (CaO) in the formation of calcite (CaCO3). The mica mineral phase also exhibited heterogeneous distribution in the drilled weathered cores which could be

1m 2m 4m 6m 8m 14m 15m

An En Ca H L M Mi Q

**Depth**

The relative standard deviation (RSD) of anorthite, enstatite, quartz, mica and mullite (being < 3 %) indicates homogeneity among these coal fly ash mineral phases in this core. There was however more variation (< 25 %) in the mean peak heights of lime (CaO) in the 8-year-old ash samples. The in-homogeneous distribution of calcite and hematite mineral phase (< 10 %) could be attributed to continuous differential weathering occasioned by chemical interaction of fly ash with atmosphere, ingressed carbon dioxide and percolating rain water.

Mineralogy and Geochemistry of Sub-Bituminous Coal and Its Combustion Products from Mpumalanga Province, South Africa 57

anorthite is formed via solid-state reactions and not by recrystallization from a

FTIR was used to identify the mineral matter components removed and to monitor any changes in the functional groups resulting due to microbial treatment in the raw and bioleached coals. Figure 2 shows the FTIR spectra obtained for raw and bioleached coals. All

The high mineral content of the coals necessitated the analysis of the mineral matter more closely. Distinct peaks at the regions of 1000, 529 and 413 cm-1 are ascribed to kaolinite [39, 40]. In kaolinite (1:1 layer silicate), one of the silicate anions is replaced by a sheet of hydroxyl groups, and the layer units are linked by hydrogen bonds between the hydroxyl surface of the layer and the oxygen surface of the next layer. The high frequency OHvibrations occur at the region of 3600 cm-1. In dioctahedral silicates, the degree of substitution of aluminium (A1) for silicon (Si) is lower than in trioctahedral silicates [41]. A perpendicular Si-O vibration causes absorption at 413 to 529 cm-1. The Si-O-Si stretching vibrations give two bands at 1594 and 2174 cm-1 Si-O bending vibrations contribute to the

The presence of quartz in the coal sample possibly gives rise to the IR spectrum with absorption frequency at 746 and 684 cm-1 [42, 43, 44]. The potassium selenium chlorides also absorb at 1625 and 1450 cm-1. The small shoulder at 1450 cm-1 could be attributed to potassium selenium chlorides. The spectra indicated that the coal sample used in the present study had little iron sulphide. The iron sulphide (pyrite) is generally the most important of the iron-bearing minerals in coals (basic absorption frequency 413 crn-1). Previous study established that the presence of quartz in the 2-week-old ash gives rise to IR spectrum with absorption frequency at 433-427, 770-764, and 991-996 cm-1. Although, the presence of

mullite is responsible for the series of bands around 540-532 and 1,413-1,000 cm-1 [45].

SEM can provide size and morphology information of particles at submicron scale [46]. The size and morphological characteristics of coal and coal ash particles examined by SEM are exemplified in Fig. 7. SEM observations show that the coal and coal ash particles The coal ash (Fig. 7c, d) consists mostly of spherical shaped and aggregate that contains varying sizes and amount of particles. Conversely, the coal sample consisted of irregular shaped pyrite crystal coated with kaolinite (Fig 7a, b). TEM is the most powerful and appropriate technique for investigating the characteristics of nanoscale particles [46]. The morphology of coal and coal ash were identified using TEM. Transmission electron microscopy (TEM) is the most powerful and appropriate technique for investigating the characteristics of nanoscale particles [46]. The morphology of coal and coal ash were identified using TEM. The TEM images show that the coal and coal ash are made up of

homogeneous melt [38].

**3.3. FTIR spectral analysis of the coal sample** 

strong absorption at 413 and 529 crn-1.

**3.4. Microscopic study of coal and coal ash** 

peaks between 3600 cm- I to 413 cm- 1 were enlarged separately.

The relative standard deviation (RSD) in the peak heights of the 20-year-old ash (Figure 6 and Table 1) showed moderate variations which could be classified into 3 groups:

< 35 % Anorthite


There is obvious similarity in the variation of mineral peak height of 20 year and 1-yearold drilled cores (Figures 4 & 6) which is attributed to in-homogeneity due to textural differences in 1 year and 20-year-old drilled ash cores [34]. Less variation in the mineral phase peak heights is observed with the age of the ashes due to dissolution/precipitation of secondary phases (stability of mineral phases) with time. The decrease or increase in the mean peak height of some minerals in the 3 drilled ash cores suggest variation in the steady state conditions at the interface of mineral particles due to reduction in the ash pore water pH [34]. The RSD in the mineral peak heights showed correlation with already significant change in chemistry of the 3 drilled ash core samples. This showed that significant flushing/leaching of major components of fly ash had taken place within 1 year of ash dumping due to continuous irrigation with high saline effluents [34].

Bulk chemical composition as determined by XRF analysis of all the coal fly ash samples also revealed major presence of MgO in fly ash which could result in the formation of enstatite (Mg2Si2O6) upon concomitant depletion of quartz. Mullite and quartz were the species identified, quartz being the only original unaltered coal mineral phase present [36]. Mullite and hematite are products of the thermal transformation of some minerals present in the coal during combustion. Mullite (3Al2O32SiO2) is a product of aluminosilicate transformation. It has been reported that mullite in fly ash is formed through the decomposition of kaolinite, Al2Si2O5 (OH)4 [31, 35], which is entrapped in the parent coal. For UK fly ashes mullite is reported to form preferentially from kaolinite, whereas illite contributes towards the glass phase [35]. Calcite (CaCO3) was found in all coal fly ash core samples. Calcite precipitation in weathered fly ash is as a result of chemical interactions of calcium oxide (lime) rich fly ash with ingressed CO2 [32] during weathering. The high concentration of calcium oxide as evidenced by XRF analysis [34] of the core samples suggests possible secondary formation of anorthite (CaAl2Si2O8), calcite, and lime in the fly ash dump [28]. Anorthite is a calcium mineral not present in coals. The formation of anorthite requires the mixing of separate calcium and aluminosilicate mineral domains. Thus, as calcium aluminosilicates are not found in fly ashes, their presence in coal-fired boiler deposits has been used to probe the mechanism of deposit formation. Studies of a range of coal fired boiler deposits, having different temperature histories, together with complementary investigations on mineral mixtures and coal ashes, have demonstrated that anorthite is formed via solid-state reactions and not by recrystallization from a homogeneous melt [38].

### **3.3. FTIR spectral analysis of the coal sample**

56 Analytical Chemistry

rain water.

< 35 % Anorthite

< 60 % Enstatite, Mica and Hematite

≤ 70 % Lime, Mullite and Calcite

high saline effluents [34].

phase (< 10 %) could be attributed to continuous differential weathering occasioned by chemical interaction of fly ash with atmosphere, ingressed carbon dioxide and percolating

The relative standard deviation (RSD) in the peak heights of the 20-year-old ash (Figure 6

There is obvious similarity in the variation of mineral peak height of 20 year and 1-yearold drilled cores (Figures 4 & 6) which is attributed to in-homogeneity due to textural differences in 1 year and 20-year-old drilled ash cores [34]. Less variation in the mineral phase peak heights is observed with the age of the ashes due to dissolution/precipitation of secondary phases (stability of mineral phases) with time. The decrease or increase in the mean peak height of some minerals in the 3 drilled ash cores suggest variation in the steady state conditions at the interface of mineral particles due to reduction in the ash pore water pH [34]. The RSD in the mineral peak heights showed correlation with already significant change in chemistry of the 3 drilled ash core samples. This showed that significant flushing/leaching of major components of fly ash had taken place within 1 year of ash dumping due to continuous irrigation with

Bulk chemical composition as determined by XRF analysis of all the coal fly ash samples also revealed major presence of MgO in fly ash which could result in the formation of enstatite (Mg2Si2O6) upon concomitant depletion of quartz. Mullite and quartz were the species identified, quartz being the only original unaltered coal mineral phase present [36]. Mullite and hematite are products of the thermal transformation of some minerals present in the coal during combustion. Mullite (3Al2O32SiO2) is a product of aluminosilicate transformation. It has been reported that mullite in fly ash is formed through the decomposition of kaolinite, Al2Si2O5 (OH)4 [31, 35], which is entrapped in the parent coal. For UK fly ashes mullite is reported to form preferentially from kaolinite, whereas illite contributes towards the glass phase [35]. Calcite (CaCO3) was found in all coal fly ash core samples. Calcite precipitation in weathered fly ash is as a result of chemical interactions of calcium oxide (lime) rich fly ash with ingressed CO2 [32] during weathering. The high concentration of calcium oxide as evidenced by XRF analysis [34] of the core samples suggests possible secondary formation of anorthite (CaAl2Si2O8), calcite, and lime in the fly ash dump [28]. Anorthite is a calcium mineral not present in coals. The formation of anorthite requires the mixing of separate calcium and aluminosilicate mineral domains. Thus, as calcium aluminosilicates are not found in fly ashes, their presence in coal-fired boiler deposits has been used to probe the mechanism of deposit formation. Studies of a range of coal fired boiler deposits, having different temperature histories, together with complementary investigations on mineral mixtures and coal ashes, have demonstrated that

and Table 1) showed moderate variations which could be classified into 3 groups:

FTIR was used to identify the mineral matter components removed and to monitor any changes in the functional groups resulting due to microbial treatment in the raw and bioleached coals. Figure 2 shows the FTIR spectra obtained for raw and bioleached coals. All peaks between 3600 cm- I to 413 cm- 1 were enlarged separately.

The high mineral content of the coals necessitated the analysis of the mineral matter more closely. Distinct peaks at the regions of 1000, 529 and 413 cm-1 are ascribed to kaolinite [39, 40]. In kaolinite (1:1 layer silicate), one of the silicate anions is replaced by a sheet of hydroxyl groups, and the layer units are linked by hydrogen bonds between the hydroxyl surface of the layer and the oxygen surface of the next layer. The high frequency OHvibrations occur at the region of 3600 cm-1. In dioctahedral silicates, the degree of substitution of aluminium (A1) for silicon (Si) is lower than in trioctahedral silicates [41]. A perpendicular Si-O vibration causes absorption at 413 to 529 cm-1. The Si-O-Si stretching vibrations give two bands at 1594 and 2174 cm-1 Si-O bending vibrations contribute to the strong absorption at 413 and 529 crn-1.

The presence of quartz in the coal sample possibly gives rise to the IR spectrum with absorption frequency at 746 and 684 cm-1 [42, 43, 44]. The potassium selenium chlorides also absorb at 1625 and 1450 cm-1. The small shoulder at 1450 cm-1 could be attributed to potassium selenium chlorides. The spectra indicated that the coal sample used in the present study had little iron sulphide. The iron sulphide (pyrite) is generally the most important of the iron-bearing minerals in coals (basic absorption frequency 413 crn-1). Previous study established that the presence of quartz in the 2-week-old ash gives rise to IR spectrum with absorption frequency at 433-427, 770-764, and 991-996 cm-1. Although, the presence of mullite is responsible for the series of bands around 540-532 and 1,413-1,000 cm-1 [45].

#### **3.4. Microscopic study of coal and coal ash**

SEM can provide size and morphology information of particles at submicron scale [46]. The size and morphological characteristics of coal and coal ash particles examined by SEM are exemplified in Fig. 7. SEM observations show that the coal and coal ash particles The coal ash (Fig. 7c, d) consists mostly of spherical shaped and aggregate that contains varying sizes and amount of particles. Conversely, the coal sample consisted of irregular shaped pyrite crystal coated with kaolinite (Fig 7a, b). TEM is the most powerful and appropriate technique for investigating the characteristics of nanoscale particles [46]. The morphology of coal and coal ash were identified using TEM. Transmission electron microscopy (TEM) is the most powerful and appropriate technique for investigating the characteristics of nanoscale particles [46]. The morphology of coal and coal ash were identified using TEM. The TEM images show that the coal and coal ash are made up of

Mineralogy and Geochemistry of Sub-Bituminous Coal and Its Combustion Products from Mpumalanga Province, South Africa 59

particles greater than 200μm. TEM images of coal ash sample showed nearly spherical shaped haematite structure (Fig. 8d), and cluster texture agglomeration of ultrafine particles (Fig. 8c). On the other hand, the TEM images shows that coal sample consist irregular shaped Fe-rich particles (i.e. pyrite) encrusted with Al, Si-rich particles (i.e. kaolinite) (Fig. 8 a, b). The metallifeorus particles such as Fe-rich particles, Al-rich particles and Si-rich particles are not uniformly distributed in the heterogeneous microstructure of coal

Table 2 reports the chemical composition of coal and ash samples. The elements found in coals are commonly classified as major (> 1 wt. %), minor (1-0.1 wt. %) or trace (< 0.1 wt. %) elements. These elements may occur in both organic and inorganic constituents of coal and each element has dominant associations and affinities with different phases in coal [49]. The most abundant major components in both coal and ash samples are Si followed by Al, Fe and Ca. The least abundant components in the pulverised coal and fresh ash are Ti, Mg, Na, K, P, Cr and Mn. The bulk chemical composition and classification systems of coal fly ash always include data for LOI. The LOI and volatile (H2O) components are relatively enriched in the pulverised coal sample. This combustion process thermally converts kaolinite to mullite as indicated in the Figures 1 & 2. This type of fly ash is principally composed of small (10 mm or less) glassy aluminosilicate spheres. The latter are formed by the rapid cooling of the molten mineral matter in the pulverized coal used in the power station boilers [50]. The ratio of Si/Al in the coal ash is ≥ 2 and thus can also be classified as silico-aluminate

Usually elements in coal occur either associated with the inorganic constituents (minerals) or with organic constituents [52]. The enrichment factor (EF) of the inorganic elements was calculated based on the method previously proposed by [53] (Table 2). Major elements such as Fe, Ca, Mg, Mn and Ti were as expected in coal samples but significantly enriched in the coal ash. Although, P, Na and K are slightly enriched in the coal ash samples used in this study. Enriched values in the ash were observed for environmentally significance trace

The distribution of trace elements in coals used for electrical generation is of increasing importance in the assessment of environmental impacts from coal-fired power plants [54]. Trace elements such as U, Cr, Th, V, Ni, Cu, Zn, Rb, Sr, Mo and Sn are slightly enriched in

The slight enrichment of these trace elements in the coal ash is attributed to the combustion process. Simultaneously, trace elements (such as Hf, Ta, Pb, Cr, Zr and Nb) showed

ash [47, 48].

fly ash [51].

elements.

*3.5.1. Major elements* 

*3.5.2. Trace elements* 

the coal ash (Table 2).

**3.5. Geochemistry** 

**Figure 7.** Scanning electron microscopy (SEM) of coal (a, b) and coal ash (c, d). Change the color of the letters so they are visible against the background of the photos

**Figure 8.** Transmission electron microscopy (TEM) of coal (a, b) and coal ash (c, d)

particles greater than 200μm. TEM images of coal ash sample showed nearly spherical shaped haematite structure (Fig. 8d), and cluster texture agglomeration of ultrafine particles (Fig. 8c). On the other hand, the TEM images shows that coal sample consist irregular shaped Fe-rich particles (i.e. pyrite) encrusted with Al, Si-rich particles (i.e. kaolinite) (Fig. 8 a, b). The metallifeorus particles such as Fe-rich particles, Al-rich particles and Si-rich particles are not uniformly distributed in the heterogeneous microstructure of coal ash [47, 48].

#### **3.5. Geochemistry**

58 Analytical Chemistry

**Figure 7.** Scanning electron microscopy (SEM) of coal (a, b) and coal ash (c, d). Change the color of the

C D

A B

letters so they are visible against the background of the photos

**Figure 8.** Transmission electron microscopy (TEM) of coal (a, b) and coal ash (c, d)

Table 2 reports the chemical composition of coal and ash samples. The elements found in coals are commonly classified as major (> 1 wt. %), minor (1-0.1 wt. %) or trace (< 0.1 wt. %) elements. These elements may occur in both organic and inorganic constituents of coal and each element has dominant associations and affinities with different phases in coal [49]. The most abundant major components in both coal and ash samples are Si followed by Al, Fe and Ca. The least abundant components in the pulverised coal and fresh ash are Ti, Mg, Na, K, P, Cr and Mn. The bulk chemical composition and classification systems of coal fly ash always include data for LOI. The LOI and volatile (H2O) components are relatively enriched in the pulverised coal sample. This combustion process thermally converts kaolinite to mullite as indicated in the Figures 1 & 2. This type of fly ash is principally composed of small (10 mm or less) glassy aluminosilicate spheres. The latter are formed by the rapid cooling of the molten mineral matter in the pulverized coal used in the power station boilers [50]. The ratio of Si/Al in the coal ash is ≥ 2 and thus can also be classified as silico-aluminate fly ash [51].

#### *3.5.1. Major elements*

Usually elements in coal occur either associated with the inorganic constituents (minerals) or with organic constituents [52]. The enrichment factor (EF) of the inorganic elements was calculated based on the method previously proposed by [53] (Table 2). Major elements such as Fe, Ca, Mg, Mn and Ti were as expected in coal samples but significantly enriched in the coal ash. Although, P, Na and K are slightly enriched in the coal ash samples used in this study. Enriched values in the ash were observed for environmentally significance trace elements.

#### *3.5.2. Trace elements*

The distribution of trace elements in coals used for electrical generation is of increasing importance in the assessment of environmental impacts from coal-fired power plants [54]. Trace elements such as U, Cr, Th, V, Ni, Cu, Zn, Rb, Sr, Mo and Sn are slightly enriched in the coal ash (Table 2).

The slight enrichment of these trace elements in the coal ash is attributed to the combustion process. Simultaneously, trace elements (such as Hf, Ta, Pb, Cr, Zr and Nb) showed

significant enrichment in the coal ash. On the contrary, W, As, Cs and Ba are considerably enriched in the coal samples used in the present study. In summary, most of the determined trace elements were comparatively enriched in the coal ash when compared with the parent material (coal). Therefore, the trace elements relative enrichment in coal ash is attributed to the combustion process in the Tutuka power station.

Mineralogy and Geochemistry of Sub-Bituminous Coal and Its Combustion Products from Mpumalanga Province, South Africa 61

oxides of Si, Al, K+, Na+, and Ti represent minerals and phases such as quartz, feldspars, clay and mica minerals (excluding some kaolinite and illite), volcanic glass, Al oxyhydroxide, and rutile-anatase-brookite, which commonly have dominant detrital genesis in coal. On the other hand, the oxides of Fe, Ca, Mg, S, P, and Mn represent minerals such as Fe-Mn sulphides; Ca-Fe-Mg sulphates, Ca-Mg-Fe-Mn carbonates and Ca-Fe phosphates, which commonly have dominant origin in coal [59]. Based on the ratio of detrital and authigenic minerals (DAI) some genetic information for the formation of fly

> Element LLD Coal Coal ash EF La 0.002 39.90 91.36 0.71 Ce 0.004 91.61 182.42 0.62 Pr 0.002 9.46 19.72 0.65 Nd 0.016 30.84 71.76 0.72 Sm 0.009 5.30 14.38 0.84 Ho 0.003 0.67 2.35 1.09 Er 0.004 1.89 6.65 1.09 Tm 0.002 0.27 0.95 1.08 Yb 0.000 1.81 6.5 1.11 Eu 0.004 0.89 2.68 0.94 Gd 0.012 4.15 12.62 0.95 Tb 0.002 0.57 1.91 1.04 Dy 0.007 3.31 11.91 1.12 Y 0.01 17.50 64.87 1.15 Sc 0.12 9.66 26.50 0.85 Lu 0.002 0.25 0.93 1.15

**Concentration (ppm)**

\* EF = [(X) / (Ti) Ash / (X) / (Ti) Coal], where X means element (Ogugbuaja and James, 1995)

the supply of clastic material into the peat swamp [23].

**Table 3.** Rare Earth Elements (REE) in the coal sample and coal ash from Tutuka Power Station (*n =3*)

The main trend (Table 4) indicates that the coal used in the present study is a higher-ash coals which is enriched in elements associated with probable detrital minerals. Detrital minerals such as quartz, kaolinite, illite, acid plagioclases, muscovite, rutile, apatite and Fe and A1 oxyhydroxides are commonly stable minerals during coalification. Their proportions in coal may remain almost unchanged, while their total amount depends predominantly on

From Table 4, the proportion of detrital minerals is higher in coal sample used in the present study. It has been pointed out that the proportion of detrital minerals in coal increases [60]. The ratio of SiO2/Al2O3 in the coal ash is ≥ 2 and thus can also be classified as silicoaluminate fly ash [51]. The bulk chemical composition and classification systems of coal fly

ash could be deduced [51].

(*LLD = Low level detection*)

ash always include data for LOI.


\* EF = [(X) / (Ti) Ash / (X) / (Ti) Coal], where X means element (Ogugbuaja and James, 1995)

**Table 2.** Major and trace elements in the coal sample and coal ash from Tutuka Power Station (*n = 3*) (*LLD = Low level detection*)

#### *3.5.3. Rare Earth Elements (REE)*

Rare earth elements (REEs) contents in the coal used in the present study are summarized in Table 3. It shows that the bulk of REEs are found in high levels in coal ash when compared with the typical concentration in coal. Rare earth elements (REEs) such as La, Ce, Pr, Nd, Sm, Eu and Gd are slightly enriched in the coal ash. On the contrary, Lu, Y, Dy, Tb, Yb, Tm, Er and Ho are considerably enriched in the coal ash used in the present study. Enrichment of REEs in the coal ash disagreed with the previously held views [55, 56]. Consequently, the obvious enrichment of REEs in the coal ash used in the present study is attributed to the combustion conditions. Rare earth elements in coal appear to consist of a primary fraction which is associated with syngenetic mineral matter [57]. Another portion of the REE can be externally derived or mobilized when primary mineral matter is destroyed or modified.
