**3.6. Genetic features relations of coal and coal ash based on chemical composition**

Minerals in coal are both detrital and authigenic in nature and their distribution in the inorganic matter are variable. Authigenic minerals in coal are mainly sulfides, carbonates and sulfates of Fe, Mg and Ca [58]. The chemical composition in this detrital authigenic index (DAI) also symbolizes the different index mineral (IM) in coal. For example, the 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 ash could be deduced [51].

60 Analytical Chemistry

(*LLD = Low level detection*)

**composition** 

*3.5.3. Rare Earth Elements (REE)* 

Pb 0.02 8.49 38.50 1.41

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

Element LLD Coal Coal ash EF Element LLD Coal Coal ash EF Si 79200.0 238700.0 0.94 As 231.08 47.67 0.064 Al 60400.0 126400.0 0.65 V 0.06 54.77 117.41 0.67 Fe 2600.0 37000.0 4.43 Cr 1.26 53.77 187.64 1.09 Ca 1300.0 41700.0 9.98 Co 0.02 24.25 17.53 0.22 Mg 600.0 5900.0 3.06 Ni 0.22 18.94 57.92 0.95 Mn 100.0 400.0 1.24 Cu 0.68 39.96 46.30 0.36 K 5100.0 6700.0 0.41 Zn 0.31 38.80 53.99 0.43 Na 500.0 600.0 1.06E-05 Rb 0.04 30.28 37.21 0.38 P 1200.00 2000.0 0.52 Sr 0.00 614.01 1270.70 0.644 Ti 4200.00 13500.0 1.00 Zr 0.01 99.09 392.21 1.23 U 0.002 3.82 10.25 0.84 Nb 0.01 10.35 35.45 1.07 Hf 0.01 2.84 10.77 1.18 Mo 0.00 3.24 6.31 0.12 Ta 0.004 0.82 2.65 1.01 Sn 0.07 4.07 8.95 0.68 W 0.00 119.17 6.99 0.02 Cs 0.01 6.16 5.92 0.30 Th 0.003 14.40 36.39 0.79 Ba 0.06 1200.18 1062.15 0.28

**Concentration (ppm)**

the combustion process in the Tutuka power station.

\* 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*)

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.

Minerals in coal are both detrital and authigenic in nature and their distribution in the inorganic matter are variable. Authigenic minerals in coal are mainly sulfides, carbonates and sulfates of Fe, Mg and Ca [58]. The chemical composition in this detrital authigenic index (DAI) also symbolizes the different index mineral (IM) in coal. For example, the

**3.6. Genetic features relations of coal and coal ash based on chemical** 


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

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

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

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 always include data for LOI.


Mineralogy and Geochemistry of Sub-Bituminous Coal and Its Combustion Products from Mpumalanga Province, South Africa 63

0.00 0.25 0.50 0.75 1.00

O+K<sup>2</sup> O

**Ferrocalcic Ferric Calcic**

0.2

0.4

SiO2

+Al

2

O3

+TiO2

0.6

0.8

**Calsialic**

1.00 0.0

CaO+MgO+Na2

0.2

0.4

+TiO**2**

O3

0.6

SiO2

+Al2

0.8

**Calsialic**

O+K**<sup>2</sup>** O 0.00

**Ferrosialic Ferrocalsialic Sialic**

1.0

0.25

0.50

0.75

Fe

2

O3

+MnO+SO

3

+P

2

O5

The volatile matter on dry and wet basis of pulverised coal sample is (5.6; 6.3) respectively (Table 5). The volatile matter of the pulverised coal sample used in this study is considerable lower than the American coal (31.36 %), Polish coal (32.61 %), Lafia-Obi (29.37 %) and Chikila (44.27 %) [66, 62]. Volatile matter, apart from its use in coal ranking, is one of the most important parameters used in determining their suitable applications [67]. Volatile matter does not form part of the coal; it is usually evolved as tar during carbonization. High-volatile bituminous coal due to its high volatile matter content generates high pressure during carbonization which is detrimental to the coke oven walls ([68, 69]. The above-mentioned data indicated that the coal used in the present study can be classified as medium volatile

The elemental composition and elemental ratios of the coal sample used in this study are listed in Table 5. The obtained values for C, N and H contents are within the range observed for various types of coal [66, 62]. The C/N ratios of coal sample used in this study are higher than those reported for two south Brazilian coals [71]. On the other hand, similar values of H/C ratios observed in the present study have been obtained from two south Brazilian coals [71]. The fixed carbon of coal sample used in this study is 45.75 %. This is relatively higher than fixed carbons obtained from Chikila (40.83 %) coal. On the contrary, it is considerably lower than the fixed carbons in the Lafia-Obi (61.93 %), American (62.87 %) and Polish (62.60 %) coals [66, 62]. The carbon content of a coal is essential in coke making because it is the mass that forms the actual coke [72]. Therefore based on the fixed carbon the coal used in

**Figure 9.** Ternary oxide plots for classification of the 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-

CaO+MgO+Na2

0.00 0.25 0.50 0.75 1.00

**Ferrocalcic Ferric Calcic**

1.00 0.0

0.2

0.00

**Ferrosialic Ferrocalsialic Sialic**

1.0

0.25

0.50

0.4

SiO2

+Al

2

O3

+TiO2

0.6

bituminous coal according to ASTM specification [70].

this study may be expected not to have good coking qualities.

0.8

**Calsialic**

O+K<sup>2</sup> O

0.75

3

+P

2

O

**5**

+MnO+SO

0.00 0.25 0.50 0.75 1.00

FeO2 3

**Ferrocalcic Ferric Calcic**

1.00 0.0

CaO+MgO+Na2

0.00

**Ferrosialic Ferrocalsialic Sialic**

1.0

0.25

0.50

3

+P

2

O5

0.75

+MnO+SO

Fe

2

O3

old irrigated and quenched with fresh water.

*DAI: ((SiO2+Al2O3+K2O+Na2O+TiO2) / (Fe2O3+CaO+MgO+SO3+P2O5+MnO)).* 

**Table 4.** Genetic features of coal and coal ash based on chemical composition

[61] classified fly ash based on the intersection of the sum of their major oxides: sialic: SiO2+Al2O3+TiO2; calcic: CaO+MgO+NaO2+K2O; and ferric: Fe2O3+MnO+P2O5+SO3 in a ternary diagram. Based on the chemical composition of coal ash, about seven intermediate fly ash subgroups exists, such as sialic, ferrosialic, calsialic, ferrocalsialic, ferric, calcic and ferrocalcic [51] fly ash. The 1-year-old ash core samples were both sialic and ferrocalsialic in chemical composition (i.e. essentially Fe, Ca, Al and Si). Although, the 2 week and 20-yearold dry disposed ash core samples were sialic in chemical composition (i.e. essentially dominated by Al and Si) (Fig. 8). These trends show that in the 1-year-old drilled cores, there is already a significant change in chemistry of ash core due to rapid weathering or due to irrigation with high saline effluents. The coal fly ash transforms into a more clay-like material due to long-term mineralogical changes occasioned by the weathering process.

#### **3.7. Proximate analysis and coal quality**

The result obtained from proximate and ultimate analyses of pulverised coal sample is given in Table 5. The moisture and ash contents on dry and wet basis of pulverised coal sample (0.8 %; 94.43 %; 93.67 %) respectively. These values are higher than the Polish coals (0.58 %; 4.79 %) but the American coal (1.07 %; 5.77 %) was significantly higher in the moisture content. Some Nigerian coal deposits such as Lafia-Obi (2.91 %; 8.7 %) and Chikila coals (5.82 %; 14.9 %) also have considerably higher moisture content [62]. The relatively low moisture content in the pulverised coal sample represents a significant improvement in coal's quality because moisture affects the calorific value and the concentration of other constituents [63]. Nevertheless, the ash content of American coal, Lafia-Obi and Chikila coals are relatively lower than the ash content on dry basis of the pulverised coal used in this study. Similarly the low ash content is an improvement on the coking quality, low ash content is an essential requirement for coke making coals [63].

Therefore the pulverised coal used in this study may be expected not to have good coking qualities. An ash content of less than 10 % is recommended for a good coking coal (Bustin et al., 1985). Industrial experience indicates that a 1 wt. % increase of ash in the coke reduces metal production by 2 or 3 wt. % [65].

The volatile matter on dry and wet basis of pulverised coal sample is (5.6; 6.3) respectively (Table 5). The volatile matter of the pulverised coal sample used in this study is considerable lower than the American coal (31.36 %), Polish coal (32.61 %), Lafia-Obi (29.37 %) and Chikila (44.27 %) [66, 62]. Volatile matter, apart from its use in coal ranking, is one of the most important parameters used in determining their suitable applications [67]. Volatile matter does not form part of the coal; it is usually evolved as tar during carbonization. High-volatile bituminous coal due to its high volatile matter content generates high pressure during carbonization which is detrimental to the coke oven walls ([68, 69]. The above-mentioned data indicated that the coal used in the present study can be classified as medium volatile bituminous coal according to ASTM specification [70].

62 Analytical Chemistry

*DAI: ((SiO2+Al2O3+K2O+Na2O+TiO2) / (Fe2O3+CaO+MgO+SO3+P2O5+MnO)).* 

**3.7. Proximate analysis and coal quality** 

metal production by 2 or 3 wt. % [65].

content is an essential requirement for coke making coals [63].

**Table 4.** Genetic features of coal and coal ash based on chemical composition

Sample Name Cr2O3 TiO2 K2O Na2O P2O5 LOI SO3 Sum CaO/MgO SAC 0.01 0.44 0.62 0.14 0.28 67.92 0.25 98.67 1.44 FA 0.03 1.40 0.81 0.15 0.45 8.51 0.05 98.76 4.63

[61] classified fly ash based on the intersection of the sum of their major oxides: sialic: SiO2+Al2O3+TiO2; calcic: CaO+MgO+NaO2+K2O; and ferric: Fe2O3+MnO+P2O5+SO3 in a ternary diagram. Based on the chemical composition of coal ash, about seven intermediate fly ash subgroups exists, such as sialic, ferrosialic, calsialic, ferrocalsialic, ferric, calcic and ferrocalcic [51] fly ash. The 1-year-old ash core samples were both sialic and ferrocalsialic in chemical composition (i.e. essentially Fe, Ca, Al and Si). Although, the 2 week and 20-yearold dry disposed ash core samples were sialic in chemical composition (i.e. essentially dominated by Al and Si) (Fig. 8). These trends show that in the 1-year-old drilled cores, there is already a significant change in chemistry of ash core due to rapid weathering or due to irrigation with high saline effluents. The coal fly ash transforms into a more clay-like material due to long-term mineralogical changes occasioned by the weathering process.

Sample Name SiO2 Al2O3 Fe2O3 CaO MgO MnO SiO2/Al2O3 K2O/Na2O (MgO+CaO)/(K2O+Na2O) DAI SAC 16.94 11.41 0.37 0.18 0.12 0.01 1.48 4.46 0.40 24.53 FA 51.05 23.88 5.29 5.84 1.26 0.05 2.14 5.37 7.39 5.97

**Major elements (%)**

The result obtained from proximate and ultimate analyses of pulverised coal sample is given in Table 5. The moisture and ash contents on dry and wet basis of pulverised coal sample (0.8 %; 94.43 %; 93.67 %) respectively. These values are higher than the Polish coals (0.58 %; 4.79 %) but the American coal (1.07 %; 5.77 %) was significantly higher in the moisture content. Some Nigerian coal deposits such as Lafia-Obi (2.91 %; 8.7 %) and Chikila coals (5.82 %; 14.9 %) also have considerably higher moisture content [62]. The relatively low moisture content in the pulverised coal sample represents a significant improvement in coal's quality because moisture affects the calorific value and the concentration of other constituents [63]. Nevertheless, the ash content of American coal, Lafia-Obi and Chikila coals are relatively lower than the ash content on dry basis of the pulverised coal used in this study. Similarly the low ash content is an improvement on the coking quality, low ash

Therefore the pulverised coal used in this study may be expected not to have good coking qualities. An ash content of less than 10 % is recommended for a good coking coal (Bustin et al., 1985). Industrial experience indicates that a 1 wt. % increase of ash in the coke reduces The elemental composition and elemental ratios of the coal sample used in this study are listed in Table 5. The obtained values for C, N and H contents are within the range observed for various types of coal [66, 62]. The C/N ratios of coal sample used in this study are higher than those reported for two south Brazilian coals [71]. On the other hand, similar values of H/C ratios observed in the present study have been obtained from two south Brazilian coals [71]. The fixed carbon of coal sample used in this study is 45.75 %. This is relatively higher than fixed carbons obtained from Chikila (40.83 %) coal. On the contrary, it is considerably lower than the fixed carbons in the Lafia-Obi (61.93 %), American (62.87 %) and Polish (62.60 %) coals [66, 62]. The carbon content of a coal is essential in coke making because it is the mass that forms the actual coke [72]. Therefore based on the fixed carbon the coal used in this study may be expected not to have good coking qualities.

**Figure 9.** Ternary oxide plots for classification of the 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-yearold irrigated and quenched with fresh water.

O+K**<sup>2</sup>** O

CaO+MgO+Na2


Mineralogy and Geochemistry of Sub-Bituminous Coal and Its Combustion Products from Mpumalanga Province, South Africa 65

showed significant enrichment in the coal ash. On the contrary, W, As, Cs and Ba are

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 proximate analysis revealed that the moisture content, ash content and volatile organic matter of pulverised coal used in this study is relatively low in values compare to the American coal, Polish coal, Lafia-Obi and Chikila coals. The ultimate analysis showed that the fixed carbon of coal sample used in this study is relatively higher than fixed carbons obtained from Chikila coal. On the contrary, it is comparatively lower than the fixed carbons

In conclusion, factors such as the nature of combustion process, type of coal and chemical interaction of fly ash with the ingressed CO2 and percolating rain water would ultimately

*Fossil Fuel and Environmental Geochemistry Group, Department of Earth Sciences; University of the* 

*Environmental and Nano Sciences Group, Department of Chemistry; University of the Western* 

*Environmental Remediation and Water Pollution Chemistry Group, Department of Ecology and* 

[1] Jeffrey, L. S. Characterization of the coal resources of South Africa. The Journal of the

[2] DME (Department of Minerals and Energy), Digest of South African Energy Statistics.

[4] Snyman, C. P. and W. J. Botha, W. J. Coal in South Africa. Journal of African Earth

[5] Renton, J. J. Mineral matters in coal, In: Meyers, R. A. (Ed.). *Coal Structure*. New York

*Resources Management, School of Environmental Studies, University of Venda. X5050,* 

South African Institute of Mining and Metallurgy 2005; 95-102.

[3] Daniel, M. African coat supply prospects, lEA Coal Research, London, 1991.

Department of Minerals and Energy, Pretoria. 2005.

determine the mineralogy and chemical composition of coal combustion products.

considerably enriched in the coal samples used in the present study.

the coal ash used in the present study.

in the Lafia-Obi, American and Polish coals

**Author details** 

L. F. Petrik

W. M. Gitari

**5. References** 

S. A. Akinyemi, A. Akinlua

*Cape, Bellville, South Africa* 

*Thohoyandou, South Africa* 

Sciences*,* 1993; 16 171-180.

Academy Press. 1982.

*Western Cape, Bellville, South Africa* 

Note: OM % = C %\*1.7.

**Table 5.** Proximate and ultimate analyses of South African coal sample *(n = 3)*

The organic matter content calculated (OM %) was calculated from the carbon content by multiplying with a value of 1.7 (Table 3). The derived organic matter content of the coal sample used in the present study is comparatively higher than Lafia-Obi coal (Jauro et al., 2008; Nasirudeen and Jauro, 2011).
