JME

Journal of Mining & Environment,

Vol. 9, No. 3, 2018, 641-655.
DOI: 10.22044/jme.2018.6855.1518

Distribution of trace elements in coal and coal fly ash and their recovery
with mineral processing practices: A review
I. Kursun Unver* and M. Terzi
Department of Mining Engineeering, Faculty of Engineering, Istanbul University, Istanbul, Turkey

Received 5 March 2018; received in revised form 26 March 2018; accepted 29 March 2018
*Corresponding author: ilginkur@istanbul.edu.tr (I. Kursun Unver).

Abstract
Today coal is among the most important energy sources. In order to meet the world's energy demands,
low-calorie lignite with a high ash content is generally used in the large capacity coal-fired thermal power
plants. As a result of coal firing, wastes such as fly ash, slag, and flue gas are also produced. Subsequently,
toxic trace elements within coal are transferred to wastes such as slag, fly ash, and flue gases. Large amounts
of these, which are usually stored in collection ponds or stockpiles, are problematic in terms of environment.
Although coal fly ash (CFA) has been utilized in construction and several other industries for decades, its
current ratio of utilization is still quite limited. As an important fact, CFA also contains many valuable
metals including germanium (Ge), gallium (Ga), vanadium (V), titanium (Ti), and aluminum (Al). In
addtion, coal and CFA can be regarded as alternative sources of radioactive elements. Therefore, they also
have a great potential in terms of the precious metals and trace elements they contain. In this study, the
present literature on the distribution of trace elements in coal and coal ash during firing and ore preparation
processes and their recovery possibilities with mineral processing practices are reviewed. While many
research works on the subject clearly indicate that the large amounts of the ashes produced from firing of
coal could be problematic in terms of environment, many studies and practices also show that coal
combustion products also have a great potential in terms of the precious metals and trace elements.

Keywords: Coal, Trace Elements, Fly Ash, Utilization.

1. Introduction to trace elements in coal

In primary energy consumption, coal takes the produced in Turkey annually [6]. Inorganic
second place with approximately 29% share after matters in coal are the main source of fly ash [7].
oil, which takes the first place with approximately Ninety-five percent of the mineral matters in coal
33% share as of 2015 [1]. As a result of coal are clay minerals, carbonate minerals,
firing, wastes such as fly ash, slag, and flue gas sulfides/disulfides, silica, and sulfates [8]. Silica
are also produced. Coal fly ash (CFA) is a finely minerals such as quartz, feldspar, and clay
divided residue, which is mainly composed of minerals detritically enter the environment due to
spherical glassy particles and has resulted from decrease in the water flow rate in the marsh
the combustion of pulverized coal. After its environment. Minerals such as kaolinite and
occurrence in the boiler, CFA is then transported pyrite are mostly syngenetic with diagenetic
by flue gases from the combustion zone to the processes during the burial and charring
particle removal system [2-3]. Thermal power processes. The third type of minerals that can be
plants generate large amounts of fly ashes [4]. observed in coals are the minerals such as calcite
Globally, China is the largest producer of fly ash, and pyrite that are epigenetically formed in
followed by Russia and USA [5]. On the other fractures and cracks after carbonization [9-11].
hand, an average of about 45 million tons of coal The inorganic matters in coal include trace
are burned and 20 million tons of fly ash are elements, and they are not uniformly distributed.
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The coal–mineral and mineral–mineral especially with As and Se. Group IIA includes B,
associations play an important role in the Mn, and Mo, which should be taken into
formation of fine particles and in the subsequent consideration in leachates from wastes and for
condensation of trace elements in various phases reclamation after mining. Relatively high
[12]. concentrations of U and Th should be avoided in
Elements such as Fe, Mg, As, Zn, Cu, F, Th, and order to minimize radioactivity from fly ash [16].
V whose contents in coal are generally less than Problems with the other group IIB elements are
1000 ppm are generally considered as the trace unlikely, although high-Cl coals may cause some
elements in coal [13]. As expected for a natural corrosion and add to acidity in the atmosphere.
substance with a long and diverse history, coal The concentrations of trace elements in group III
contains most of the elements in the periodic coals are not expected to give troublesome effects.
classification, and there is data for 73 trace Toxicity of elements per se is a trap to be avoided,
elements [14-15]. The major trace elements especially because many elements may be
present in coals are given in Figure 1. essential or hazardous [16]. The associations of
The group I elements are known to be hazardous some trace elements with inorganic mineral
in some circumstances but their concentrations in phases and organic phases found in coal are given
most coals are low. However, care should be in Table 1.
taken to check for possible untoward effects,

1 2
H He
3 4 5 6 7 8 9 10
Li Be B C N O F Ne
11 12 13 14 15 16 17 18
Na Mg Al Si P S Cl Ar
19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54
Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe
55 56 57 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86
Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn
87 88 89 104 105 106 107 108 109 110 111 112 113 114
Fr Ra Ac Rf Db Sg Bh Hs Mt Uun Uuu Uub Uut Uuq
58 59 60 61 62 63 64 65 66 67 68 69 70 71
Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
90 91 92 93 94 95 96 97 98 99 100 101 102 103
Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lw
Figure 1. Major trace elements present in coals (Adapted from [15]).

Table 1. Distribution and association of some trace elements in coal [17, 13].
Inorganic
Minerals Associated elements
Clay Minerals, Kaolinite, Al, Ba, Bi, Cr, Cs, Cu, Ga, K, Li, Mg, Na, Ni, P, Pb, Rb, Sn, Sr, Ta, Th, Ti,
Montmorillonite, Feldspars U, V, Y, and rare earths.
Iron Sulphides, Pyrite, Sphalerite As, Cd, Co, Cu, Fe, Hg, Mo, Ni, Pb, S, Sb, Se, Ti, W, Zn.
Calcite, Dolomite, Ankerite C, Ca, Co, and Mn.
Sulphates Ba, Ca, Fe, and S.
Heavy Minerals, Turmaline B.
Co and W (carbonates and sulfides), Ni, Cu, Pb (clay minerals and sulfides),
Others
S (sulfides, sulfates and organic matters), C (carbonates and organic matters).
Organic
Minerals Associated elements
- C, N, S, Be, B, Ge, V, W, V, Zr

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1.1. Toxic trace elements and heavy metals arsenic, bismuth, cadmium, cerium, chromium,
In general terms, metals may be found in the cobalt, copper, gallium, gold, iron, lead,
lower-left side of the diagonal of semi-metals or manganese, mercury, nickel, platinum, silver,
metalloids (from boron to polonium) in the tellurium, thallium, tin, uranium, vanadium, and
periodic table, which may act as both metals and zinc are defined as heavy metals, which among 35
non-metals. Metals are distinguished from metals are considered dangerous for the human
non-metals by their capacity to lose electrons, health. However, most important heavy metals
forming positively charged ions or Lewis acids, whose exposures possess danger to the human
and by their speciation-dependent affinity for health are lead, cadmium, mercury, and arsenic,
abiotic and biological interactions. Several metals which are metalloids and are usually defined as
such as iron, cobalt, copper, manganese, heavy metals [23-24].
molybdenum, and zinc are required at certain As stated earlier, coal is a material that contains
concentration levels by the human body, while relatively high concentrations of trace elements
others are toxic above the not-observed adverse when compared with other geological materials
effect concentration level such as lead, cadmium, [25]. Significant amounts of heavy metals are also
mercury, and arsenic, which have no beneficial present in coal [26-27]. Furthermore, coal
influences on the human health [18]. Heavy combustion products are enriched 4 to 10 times in
metals are usually defined as metals with a density terms of some trace elements. If this fact is
greater than 4-5 g/cm3. Heavy metals such as considered together with the high fly ash
arsenic (As), chromium (Cr), cadmium (Cd), production rate of the power stations (a 1000 MW
copper (Cu), lead (Pb), mercury (Hg), nickel (Ni), power plant produces about 1,000,000 tons of
and zinc (Zn) are of great concern due to their wastes per year), some ideas of the environmental
potentially adverse health effects to humans. importance of heavy metals in fly ash may be
While certain amounts of exposure to Cu and Zn obtained. Therefore, an important variety and
are not harmful, exposure to elevated quantity of some potentially toxic trace elements
concentrations or excessive intake of these including Cd, Co, Cu, Ni, Pb, Sb, and Zn are
elements are also damaging [19-20]. Similar to Cu mobilized during the energy production in
and Zn, while Fe is an essential element for the coal-fired power plants. In this respect, the fly ash
growth, high contents of tissue Fe has been found constitutes an important source of pollutants for
to be associated with cell injury and several groundwater contamination and biological uptake
pathological conditions including liver and heart [25].
disease and cancer [21-22]. Owing to their
chemical characteristics, metals remain in the 1.2. Toxicity of trace elements
environment, in many cases only changing from The effects of some toxic elements in coal on
one chemical state to another and eventually human health is given in Table 2.
accumulating in the food chain. Antimony,

Table 2. Effects of some toxic elements in coal on human health [28].

Element Possible adverse effects on human health
As Anemia, nausea, renal signs, ulcer, skin and lung cancer, defective births.
Be Respiratory and lymph, lung, spleen and kidney disorders, carcinogen effects.
Cd Pulmonary emphysema and fibrosis, kidney disease, cardiovascular effects, carcinogenic effects
Hg Neural and kidney damage, cardiovascular effects, birth problems.
Mn Respiratory effects.
Ni Skin and intestinal disorders, carcinogen effects.
Anemia, neural and cardiovascular problems, growth retardation, stomach and intestinal problems,
Pb
carcinogenic effects, birth problems.
Se Stomach and intestinal disorders, lung and spleen destruction, anemia, cancer, teratogenic effects.
V Acute and chronic pulmonary dysfunctions.

1.3. Radioactive elements 50 Bq kg-1 for 40K. In general, the content of

Like many other materials found in nature, coal natural radionuclide activity in coal is smaller
contains traces of 238U, 232Th, and 40K with decay than the average concentrations in the Earth’s
products. The average natural radionuclide crust. However, low-quality coal such as lignite
activity concentrations in coal are given as 20 Bq (especially young lignite) has been observed to
kg-1 for 238U, 232Th and decay products, and have a high uranium content [29]. Uranium in

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coal is mainly associated with the organic matters, contents of the bioliths [34]. The connection
and to a lesser extent occurs in minerals [30]. mechanism of uranium to coal is shown in Figure
During the first half of the 20th century, U-bearing 2.
coals were utilized as an alternative source of U.
Currently, U-bearing coals are considered to be a
potential source of radioactive contamination [31].
In contrast to coals, the natural radionuclide
activity concentrations of the coal combustion
products are much higher than the average
concentrations in the Earth’s crust. The average
natural radionuclide activity concentrations for
40
K, 238U, 226Ra, 210Pb, 210Po, 232Th, 228Th, and 228Ra
are 265, 200, 240, 930, 1700, 70, 110, and 130 Bq Figure 2. Connection mechanism of uranium to coal
(Adapted from [34]).
kg-1, respectively. In this direction, coal-based
thermal power plants are considered as the most
Abruzov et al. (2011) have investigated the
important source of technologically enriched
geochemistry of radioactive elements in more than
natural radioactivity [32].
5000 coal and peat samples of Northern Asia by
In a power plant, concentrations of natural
quantitative methods. They have found that the
radionuclide activity discharged to the atmosphere
average U content in the coals of deposits and
per unit electric power generation depending on
basins of the region ranges from 0.6 to 32.8 ppm,
the factors such as concentration of natural
and for Th, from 0.8 to 9.2 ppm. The authors have
radionuclide activity in coal, coal to ash ratio,
stated that the distribution of U and Th in the
combustion temperature, fly ash and bottom ash
investigated coal basins is irregular due to the
ratio, and efficiency of the filter system.
influence of a combination of several factors such
Therefore, the difference between natural
as heterogeneity of rock composition of folded
radionuclide activities that are gererated per unit
boundaries of the basins, difference of the facies
energy in different plants should be expected [29].
conditions of the coal accumulation, influence of
Concentration of 226Ra in the atmosphere has
volcanism, climatic conditions, and degree of coal
increased by 100 factors in the last 80 years, and
metamorphism. They have also pointed out that
this increase is mainly assocaitaed with coal-based
the high concentrations of U and Th in
thermal power plants [28]. In Greece, it was
coal-bearing deposits are related to the blocks of
calculated that the 1.6 × 1012 Bq for 226Ra, which
rocks enriched with U and Th at the peripheries of
constitutes more than 80% of the 226Ra content in
the basins or connected with the occurrence of a
a lignite coal burning plant, is released by gas or
volcanism in the period of the coal accumulation.
fly ash [33].
In a majority of USA coals covering
As a known fact, the origin of lignites is plants.
approximately 2000 coal samples from the West
Although plants do not contain uranium, humic
United States and 300 coals from the Illinois
acid extracted from lignite shows a greater
Basin, the U concentration falls within the range
adsorption capacity to uranium than lignites itself.
of slightly below 1 to 4 ppm. Coals containing U
Several hypotheses have been raised on how and
in excess of 20 ppm are rare in the United States.
where uranium in lignite coal and other
Th concentrations in USA coals fall within a
carbonaceous materials came from. According to
similar 1-4 ppm range compared to an average
the most held hypothesis on the accumulation of
crustal abundance of approximately 10 ppm.
uranium in the lignites today:
Coals containing more than 20 ppm of Th are
a) Granites that are the primary source of
extremely rare in the United States [31, 35-37].
uranium are subject to chemical degradation;
Also the coal from the Kangal basin, Turkey,
b) Uranium is dissolved in sea water through
disposes a high U concentration ranging between
the solution in the form of alkaline uranyl
5.5 and 131 ppm but the coal from the Sorgun
carbonate;
basin disposes a higher Th (1.4–69 ppm)
c) These compounds are adsorbed on the
compared to the U concentration (<0.7–11 ppm).
lagoon basins or in the marshes by the humic
The highest possible concentration of U in
substances resulting from the decay of organic
Turkish coals (140 ppm) was detected in the
substances under water.
deposit within the Aegean Region [37-40].
Along with carbonization, uranium is bound to the
organic body via strong bonds by the humic acid

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2. Transfering behaviors of trace elements water vapor is also produced. While the slag is
2.1. Coal firing processes collected under the combustion boilers, the fly ash
During combustion, the boiler flame temperature is kept in the electro-filters, and some of it is
can exceed 1600 oC in thermal power plants. A carried by the flue gas. Studies have shown that
number of complex physico-chemical changes trace elements are mostly concentrated on fly ash
take place within the particles under the high [42]. A simplified mechanism of CFA formation
temperature conditions of the boiler flame. The from pulverised fuel combustion is given Figure
particles can be transformed into spherical forms 3.
very quickly by the effect of surface tension On combustion, the trace elements in coal are
forces. In addition, the molten material in the gas transferred to slag, fly ash or gases, and are
stream is partially or completely converted into discharged to the environment. Annually, the total
spherical ash particles [41]. quantities of trace elements produced in coal
During the combustion of pulverized coal in combustion are comparable to the quantities
thermal power plants, the carbon, nitrogen, and mobilized by weathering of crustal rocks [44].
sulfur in the coal are oxidized into carbon oxide The distribution of some trace elements after
(COX), nitrogen oxide (NOX), and sulfur oxide combustion is shown in Figure 4.
(SOX), respectively. During this conversion, some

Figure 3. Simplified mechanism of CFA formation from pulverised fuel combustion [43].

Figure 4. Distribution of some trace elements after combustion [15].

The distribution of trace elements during coal removed by ESP. Lopez-Anton et al. (2011) have
combustion is affected by several physical suggested that the performance of the mills may
processes such as the turbulence, pollution control also siginicantly affect the trace element
mechanism, and temperature in the boiler. As composition of CFA [45-46].
stated earlier, most of the TE contents are According to Helble and Sarofim (1993), several
removed by solid residues, especially by fly ash. other factors such as feed coal particle size and
However, some parts of trace elements are coal type also affect trace element behavior during
released as aerosols or as vaporized elements in coal combustion. They have found that the
the gas phase [12]. In terms of the pollution enrichment in fly ashes is typically lowest for the
control mechanisms, Meij (1994) has suggested sub-bituminous coal among the three different
that the gaseous inorganic trace elements are not coal types investigated, namely bituminous coal,

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sub-bituminous coal, and lignite coal. As and V specific surface areas [49]. Tang et al. (2013) have
have been found to be most volatile in the investigated the distribution of trace element coal
bituminous coal. On the other hand, Cr was found combustion products from two different power
to be the most volatile in lignite. They have plants operating at Huainan, Anhui, China. They
suggested that the differences in the predominant have reported that As, B, Sn, and Zn tend to
forms of the element in the parent coals greatly increase toward smaller particle sizes [50].
affects their behavior during combustion [47]. Generally, less volatile elements such as Cd, Co,
Hg, As, Se, Ni, Pb, Ce, and Zn are predominantly Cu, Cr, Fe, Mn, Ni, Pb, Zn, and oxidized Hg can
associated with sulfide minerals and organic be found together with finely-grained materials
matters. The formation of coal minerals or trace such as fly ash and bottom ash. As and Se
elements in organic matters can significantly represent medium volatility, and spread widely
affect the evaporation limit and thus the rate of through the atmosphere. Many research works on
flue gas being discharged. Trace elements heavy metals such as Pb, Cd, Cr, As, and Hg have
detected in flue gases are mainly related to sulfite been carried out in the field of enrichment
minerals [15]. According to a study conducted in according to different grain sizes, and removal
a coal-fired thermal power plant in India, the Ni, and reaction with absorption or adsorption.
Cr, and Pb concentrations have been found to be However, the actual behavior of these elements
high in the analysis of fly ash that cannot be can not be predicted in laboratories because it is
captured by the power plant panel electro filters conditioned by highly complex processes such as
[48]. According to Kaakinen et al. (1994), the Cu, combustion of burning coal, combustion
Zn, As, Mo, Sb, Pb, 210Po, and Se concentrations temperature, halogen sample concentration, redox
are generally the highest in fly ashes among coal conditions, and interaction between different
combustion products due to vaporization of these samples [13]. A diagram showing the behavior of
elements in the furnace. Subsequently, they trace elements in coal during and after combustion
condense or absorb onto fly ash, which have large is shown in Figure 5.

Figure 5. Behavior of trace elements in coal during and after combustion (Adapted from [13]).

In an examplary study, the behavior of heavy  X sample /  40K

 sample
metals has been investigated during a mixture of EF  (1)
straw and paper mud fired in fluidized bed fuels.  X coal / K 
40
coal
It was found that most of the elements escaped 40
from the fluidized bed but were obtained as fly Since K activity is stable in almost all ash types,
40
ash fractions. It was observed that the emission of K is used as the reference radionuclide to
Hg, Pb, Cd, Cr, Cu, Ni, Mn, and Zn was generally determine the enrichment factor. When the
below the emission limits during combined enrichment rate of radionuclides from the coal is
burning of sewage sludge and coal in a fluidized examined, it is seen that the enrichment at 210Po
bed combustor [13]. and 210Pb is the highest, followed by 238U, 226Ra,
228
Coal-to-ash enrichment rates of the radionuclides Ra, and 232Th. Enrichment of natural
are characterized by the enrichment factor (EF) radionuclides in ashes from various types of
(Equation 1) [29]. thermal power plants has been found to vary

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greatly with the particle size. High enrichment with the combustion temperature of the coal and
rates were observed in particles with a diameter of decreases with the particle size (if greater than 1
2.4 μm and the EF values for 226Ra, 238U, and μm) [29]. EFs in terms of the particle size
210
Pb were determined to be 1.9, 2.8, and 5.0, variation in fly ashes are given in Table 3.
respectively. As a rule of thumb, EF increases

Table 3. EFs in terms of particle size variation in fly ashes [29].

Particle size EF
238 228 226 210
(µm) U Th Ra Pb
0-2.5 2.8 1.9 1.2 4.9
2.5-5 2.2 1.6 1.1 4.1
5-10 1.6 1.4 1.1 2.9
>10 1.3 1.0 1.0 1.5

During burning, the 40K and 232Th isotopes to the original coal. 210Pb shows a definite
dissolve with aluminum silicate minerals and depletion in the bottom ash and the fly ash. They
settle as bottom ash, while the uranium isotopes have also stated that uranium also shows a
act according to their chemical or mineralogical tendency for depletion in both ash types. An
forms in the coal. If uranium is mineralized as explanation for the behaviors of U and nuclides
coffinite in coal, it remains in the bottom ash but derived from the 235U decay chain includes a
if it is dispersed as uraninite, it can be volatile in bimodal residence for this element in the coal
the UO3 form and moves by concentrating on fly [51].
ash particles. Radium, a decaying product of In summary, the coal-to-ash natural radionuclide
uranium, behaves in a similar way. However, EF varies depending on the mineralogy of the
228
Ra, a decaying product of 232Th, remains in the coal, type of burning, combustion temperature,
matrix of fly ash particles. The enrichment of and particle size. From the radiological viewpoint,
226
Ra is higher than that of 228Ra in the volatile 40
K, 232Th, and the decay products remaining in
form due to the main elements of both isotopes the bottom as well as 238U, 226Ra, 210Pb, and 210Po
found in different froms in the coal matrix. The that concentrate on fly ash particles are very
210
Po and 210Pb radionuclides are highly volatile, important. 238U, 226Ra, 210Pb, and 210Po, which
and therefore, they evaporate during combustion have different physico-chemical properties,
and concentrate on fine fly ash particles with large exhibit different behaviors and are enriched at
surface area ratios [29]. different ratios during the burning of the carcass,
Two radiation-based techniques have been used leading to radioactive imbalance in the ash, rather
for determining the distribution and relative than coal. In addition to the uranium series, this
abundance of radionuclides in fly ash and bottom radioactive imbalance is also favored by the
ash samples from a Kentucky power plant. The products in the thorium series, and causes
results obtained indicate that radium isotopes are different radiological characteristics of the bottom
not significantly (within l0-15%) fractionated ash and fly ash [28].
from parent 238U and 232Th during coal
combustion. In contrast, 210Pb appears to be 2.2. Coal preperation processes
preferentially enriched in some samples of fly ash, There are several studies conducted in the
and deplete in the bottom ash relative to 238U and literature regarding the behavior of trace elements
226
Ra [36]. in coal preparation processes. Lutrell et al. (2000)
Coles et al. (1978) have investigated the behavior have evaluated the capabilities of coal preparation
of the naturally occurring radionuclides 40K, 210Pb, technologies for the pre-combustion removal of
226
Ra, 228Th, 228Ra, 235U, and 238U in coal-fired hazardous air pollutant precursors (HAPPs) from
power plants. They have stated that the varying coals. They have carried out the washability
degrees of enrichments for all the nuclides studied characterization of three different coal samples
are observed in the fly ash particles. These values and pilot-scale evaluations of a variety of
range from 5.0 for 210Pb to 1.2 for 228Th. The 235U coal-cleaning processes. The results of this study
and 238U EF values are both 2.8. The 226Ra and show that HAPPs tend to be concentrated in the
228
Ra EF values are 1.9 and 1.6, respectively. The mineral matters of coal. On the other hand, it has
thorium nuclides and 40K show little fractionation also been indicated that the substantial reductions
in the fly ash and the bottom ash when compared in trace element content can be achieved via coal

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preparation. The trace element removal rate for study, it was found that some trace elements
the total plant varied from 46.7% for mercury to envisaged to be enriched in the sinking coal are
80.0% for lead with a combustible recovery of actually enriched in floating coal at a high rate. It
aprroximately 90% [52]. was suggested that these results were due to the
The pilot-scale test work indicated that elements dependence of such elements on the components
such as arsenic, cadmium, and mercury were in the organic structure. The behaviors of the trace
rejected at approximately the same level as pyritic element contents of the coal studied for the coal
sulfur 50%, while elements such as chromium, preparation processes are as what follow.
cobalt, lead, and manganese were rejected at Th in the Zonguldak-Kozlu coal was found to be
levels approximately equal to that achieved for 4.76 ppm in feed coal and 16.20 ppm in sinking
ash 70%. The rejections of lead and nickel were coal. Furthermore, when the ash after burning of
higher than expected in the light of the these fractions were examined, Th in ash of the
characterization data, showing that these elements sinking fraction and original coal were found to be
tend to associate more intimately with pyrite than 26.20 ppm and 20.00 ppm, respectively. These
other ash-forming minerals. Float–sink data results clearly show that Th is located in the
indicates that trace element rejections can be sinking fraction after the coal preparation, and it is
theoretically improved by reducing the coal also further enriched in the fly ash after the firing.
topsize to liberate mineral matters [52]. In the same way, the U content was found to be
Valissev et al. (2001) have investigated the 1.78 ppm and 4.90 ppm in the original coal the
behavior of elements and minerals during sinking fraction. When viewed from the aspect of
preparation of coal from Pernik, Bulgaria. In this ashes, it was seen that the ash of coal feed had a U
context, they have used several fractions such as content of 7.20 ppm, and U was enriched in the
feed coal (FC); low-grade coal (LGC), which was sinking coal ash with a rate of 8.70 ppm. As, Be,
obtained after coal sieving 30 mm; high-grade Co, Hg, Se, Pb, and Ni, as air pollutant elements,
coal (HGC), which was obtained after coal sieving were also enriched in sinking fractions and their
and dense media separation; and waste products, ashes [15].
namely coal slime (CS) and host rock (HR) of In the Muğla-Yatağan coal, Th was 11.46 ppm in
Pernik CPP. They have found that HGC shows a the original coal and 33.10 ppm in the sinking
relative enrichment of Fe, Ca, S, Mg, Ti, P, Sr, fraction. Additionaly, when the ashes of these
Mn, Co, Ni, Zn, Pb, Cl, Br, illite, chlorite, coals were examined, it was found that the Th
gypsum, calcite, siderite, and pyrite; and reduction contents of the original coal ash and sinking
of Si, Al, K, Li, Rb, Cu, ash yield, quartz, fraction were 50.60 ppm and 70.00 ppm,
kaolinite, and feldspars in comparison with FC. respectively. These results clearly show that Th is
LGC reveals a relative enrichment of Si, Al, K, concentrated in the sinking fraction after the coal
Na, Rb, Mn, Pb, quartz, illite, calcite, and siderite; preparation, and it is further enriched in the ash.
and reduction of Fe, S, Mg, P, Sr, Cr, Ni, Cu, Zn, In the same way, U was found to be 5.88 ppm in
kaolinite, chlorite, feldspars, gypsum, and pyrite the orginal coal and 16.70 ppm in the sinking
in comparison with FC. HGC is abundant in fraction after the sink-float process. On the other
authigenic minerals and the organic matters and hand, it was seen that the Th contents in the ash of
elements associated with them. The authors have the original coal and the sinking fraction were
stated that such constituents are commonly enriched to 20.90 ppm and 36.20 ppm,
disseminated intimately with each other in coal, respectively. As for the air pollutant elements, As,
and their physical isolation-separation by Ni, and Se concentrated in the floating fractions
screening and dense media is limited during coal and in their ashes; in contrast, Be, Cd, Co, Sb, and
cleaning. In contrast, LGC, CS, and, in particular, Pb concentrated in the sinking fractions and their
HR, are normally enriched in physically separable ashes [15].
detrital and authigenic-quartz, illite, mica,
chlorite, calcite, and siderite. Coal minerals, and 3. Leaching properties of trace elements in fly
especially in rock fragments-sandstones, sandy ashes
clays, sandy limestones, and argillites from The formation properties of trace elements in coal
intra-seam layers, roof, and floor strata fallen in have an important role in the formation of trace
the FC during coal mining [53]. elements in fly ash and the distribution of these
Demir (2009) has investigated the behavior of elements in ash particles. Although the elements
trace elements in some major Turkish coals during enriched in the core of fly ash particles are not
coal preparation operations. As a result of this directly exposed to the leaching, the

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surface-associated elements are more susceptible recovery of major elements from CFA [57]. The
to leaching in an aqueous environment. Thus the recovery of alumina from CFA was pioneered by
formation properties of elements in coal greatly Grzymek in Poland in the 1950s, and was
control whether an element is mobile and easily developed mainly due to the bauxite embargo
released into the environment or not [54]. during the Cold War. The recovery process was
Be, Cd, Co, Cu, Fe, Mg, Mn, Ni, Pb, Re, Si, Sn, based upon the auto-disintegration of sinter
Th, Ti, U, and Zn show minimum solubilities at containing calcium aluminates and dicalcium
pH 7-10. These elements are characterized as silicate. The sinter was mixed with sodium
insiginificant under alkaline volatile climatic carbonate and underwent a series of complicated
conditions (for example, from moderate acid to chemical treatments including carbonization and
alkaline pH). Their leachability is low and not water scrubbing to produce alumina and Portland
related to their concentration in bulk but depends cement. In 1953, a demonstration plant for
on pH. Thus the high content of any of these recovering 10 thousand tons of alumina and
metals in the fly ash due to their high producing 100 thousand tons of Portland cement
concentration of a coal should not be considered was established in Poland. Recently, a number of
as a limiting factor for various applications. processes have been reported for recovering
However, this definition does not apply to acidic alumina from CFA, which can be grouped into
ashes as the mobility of the above given elements three types: the sinter process, the acid leach
decreases with pH [54]. process, and the HiChlor process [58].
Conversely, oxyanionic constituents such as As, One of the proposed methods for recovering
B, Mo, Sb, Se, V, and W show maximum aluminum from fly ashes is the lime-sinter and
solubilities at pH 7-10. Therefore, a particular lime-soda-sinter processes. CaCO3 is added as a
attention should be paid to the disposal of alkaline limestone in the calcination process and reacts
ashes containing these elements. As, B, Cr, Mo, with the silicon in ash at 1380 °C to form calcium
and Se are especially important in terms of being orthosilicate (2CaOSiO2). In the lime-soda-sinter
potentially harmful to both plants and animals, process, Na2CO3 is added to system in addition to
highly soluble in water, and therefore, highly the limestone. Soluble metals are recovered by
active in surface and ground waters. The crystallization and separation followed by organic
leachability of As and V can be reduced due to the dissolution and ion exchange [13].
formation of solubility limiting phases in the Toraman (1995) has investigated the recovery of
presence of Ca. Ettringite possesses a great metal oxides such as Al, Fe, and Ti by leaching
potential for metaloid removal due to its high methods from Afşin-Elbistan fly ash. In this
affinity for capturing the elements As, B, Cr, Sb, context, direct acid leaching experiments were
Se, and V into the lattice structure [54]. carried out for recovery of Al, Fe, and Ti from fly
ash. The best results are an acid concentration of
4. Recovery of trace elements from coal and fly 300 g/L, a temperature of 90 oC, a mixing rate of
ashes 1500 rpm, a solid/liquid rate of 5% solids at a
Valuable metal recovery is one of the many extraction time of 6 h for Al and Ti, 8 h for Fe.
application areas of CFA. Besides certain heavy Using these conditions, 97% Al, 95% Fe, and
elements, CFA also contains valuable metals 98% Ti extraction recoveries were obtained [59].
including germanium (Ge), gallium (Ga), Matjie et al. (2005) have investigated the
vanadium (V), titanium (Ti), and aluminum (Al), extraction of alumina and titanium from CFA of a
which are extractable if an acceptable process can South African Coal containing 29.2% Al2O3,
be developed [55]. 2.53% Fe2O3, and 1.49% TiO2. Direct leaching
results have indicated that alumina in the mullite
4.1. Aluminum, Titanium, and Iron recovery phase is not readily dissoluble in mineral acids.
from coal fly ash Therefore, mullite-containing fly ash was mixed
CFA contains between 25 and 30% Al2O3, making with calcium oxide and calcined. Subsequently,
it a potential alternative source of alumina for the calcined ash was leached at 80 °C for 4 h with
aluminum production after bauxite, which a 6.12 mol/dm3 sulfuric acid solution. The
contains about 50% Al2O3. This relative precipitation, solvent extraction, and
abundance of Al2O3 in the ash is significant crystallization methods were evaluated to
enough to justify an attempt to exploit it selectively separate Al and Ti. The final products
commercially [56]. There are a number of contained approximately 99.4% alumina and 97%
methods that have been developed for the

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titanium oxide, obtained with the solvent particles. They have also stated that the surface
extraction method [60]. area properties of the particles are important;
however, due to the void spaces in glass
4.2. Recovery of Germanium, Gallium, and cenospheres, large particles can have a greater
Vanadium from coal fly ash effective surface area as long as acids or other
In one of the previous studies on the extraction of leaching agents can make contact with all or most
gallium from CFA by acid leaching, the major surfaces [65].
elements in fly ashes such as aluminum, iron, Kashiwakura et al. (2013) have investigated the
silicon, and calcium were also succesfully dissolution behavior of the REE content of CFA
recovered [61]. CFA is also a potenital source of in a dilute H2SO4 solution. They have stated that
germanium, and many studies have focused on the REEs in CFA dissolve gradually in H2SO4 over
recovery of germanium from CFA by several time, and this finding implies two types of
methods. The first step of most of these processes occurrences of the REEs in the studied CFA
is leaching of fly ash, followed by a process to samples. For the two different CFA samples
separate germanium from other elements investigated, dissolution rates varying in the range
contained in leachates such as arsenic, of 30-50% were obtained under the leaching
molybdenum, nickel, antimony, vanadium or zinc. conditions of 2 h leaching time, 80 oC leaching
Conventional processes are precipitation with temperature, and 10% (w/w) H2SO4 [66].
tannin, distillation of GeCl4, and solvent
extraction (SX). SX appears to be the most 4.4. Recovery of U and Th from coal fly ash
attractive procedure due to the easy scale-up and Various studies have been conducted using
operation control. In an examplary study, a simple several types of acids for the purpose of uranium
hydrometallurgical method for the selective recovery from fly ashes through chemical
recovery of germanium from fly ash (FA) dissolution. Within a previous study, the best
generated in an integrated gasification with results were obtained from the bulk leaching tests
combined cycle (IGCC) process was presented. performed using 10% H2SO4 with concentrations
The method consisted of leaching FA with water of 0.05-0.15 mol/dm3. The results obtained
and the subsequent concentration and selective indicated that 80% of uranium dissolution
separation of germanium by a solvent method. As efficiency was obtained through the use of 180 kg
a result, a high-purity germanium solution was H2SO4 per ton of ash [67]. In another study, where
obtained with germanium extraction yields higher solvent extraction of uranium in the leach solution
than 90% [62]. with trioctilamine was used, the effects of the
There were also several studies on the recovery of diluting agent, phase rate, solvent percentage,
vanadium from coal and CFA. In their exemplary aqueous phase pH, mixing period, and stage
study, Chen et al. (2010) have investigated the numbers out of the parameters affecting uranium
extraction of vanadium from stone coals by the extraction were examined [68].
sulfuric acid leaching method. The extraction rate In another examplary study conducted for the
of vanadium was found to be 95.86% after acquisition of uranium from fly ash, again, the
leaching for 5 h at 90 °C using 6 mol/dm3 H2SO4 H2SO4 leaching method was used. In this context,
[63]. 500 g fly ash was first added to the sample by
adding acid in a concentration of 750 kg
4.3. Recovery of rare earth elements from coal H2SO4/ton fly ash. Water was added slowly to
fly ash provide 40% solid ratio, and then fly ash was
There are a number of studies available on rare dissolved using 16 kg of H2O2/t oxidant at 60 °C
earth elements (REE) extraction and separation for 18 h with stirring. The pulp obtained was
from CFA. The efficiencies obtained in these filtered with the aid of a vacuum pump, and a dark
studies varied in the range of 50–90% due to green-colored solution with a dense structure was
several factors including concentration and mode obtained. The residue with S/L = 2/3 was washed
of occurrence of different elements in the fly ash for 30 min using a 5% H2SO4 solution.After the
[64]. pulp was filtered, a clear green clear washing
For example, Hower et al. (2013) have suggested solution was obtained. The concentration of
that the distribution of Ce throughout the fly ash uranium in the solution was determined
has implications for its recovery in processing in spectroscopically with Arsenazo-III, a specific
this regard. According to the authors, recovery of and sensitive reagent for uranium. After the
Ce requires leaching of the entire cenosphere leaching process, re-washing of the residue was

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carried out to obtain a part of the remaining mineralized on the glass phase as the monazite
uranium. The total uranium recovered after ((Ce, La, Nd, Th)[PO4]) and thorite (Th(SiO4))
washing was also determined by analyzing the forms in Yatagan fly ashes. Radioactive minerals
uranium remaining in the washing residue by the were distributed on the glass phase in both fly
standard addition method using the uranium ashes.
analyzer. As a result, the total uranium gain was Leaching tests were carried out with sulfuric acid
obtained to be 86.4% [13]. using various solid rates, dissolution periods,
Kursun et al. [69-71] have investigated the amounts of acid, and leaching temperatures. The
dissolution characteristics of uranium and thorium results of the experiments for the effect of the
from the fly ash samples taken from the Soma and amount H2SO4 per ton ash on U and Th
Yatagan thermal power plants in Turkey under dissolution efficiency is given in Figure 6.
different leaching conditions, where H2SO4, HCl, As it could be seen in Figure 6, the U and Th
and CH3COOH were used as the leaching dissolution efficiencies for both Yatagan and
reagents, and the best results were obtained using Soma fly ash samples increased to 120 g/per ton
H2SO4. According to the analyses of thin section ash H2SO4 amount, and reached a plateau at this
samples of the Soma fly ashes, uranium was point. Therefore, it was concluded that the
mineralized on the glass phase as a uraninite optimum amount of H2SO4 was 120 g/per ton ash.
(UO2) form, and thorium was mineralized on the On the other hand, the results of the experiments
glass phase as a thorite (Th(SiO4)) form. On the for the effect of pulp temperature on the U and Th
other hand, uranium was mineralized on the glass dissolution efficiency is given in Table 4.
phase as a uraninite (UO2) form, and thorium was

U (%) U (%)
Th (%) Th (%)

(a) (b)
Figure 6. Effect of amount of H2SO4 per ton ash on U and Th dissolution efficiency of Yatagan (a) and Soma (b)
fly ashes (solid ratio: 30%; leaching time, 240 min, pulp temperature, 20 ºC).

Table 4. Effect of pulp temperature on U and Th dissolution efficiency of Yatagan and Soma fly ashes (solid
ratio, 30%; leaching time, 240 min; amount of H 2SO4, 120 g/per ton ash).
Yatagan CFA Soma CFA
Pulp temperature (ºC) U dissolution (%) Th dissolution (%) U dissolution (%) Th dissolution (%)
20 94.12 83.79 90.22 83.48
40 97.03 89.11 94.18 91.78
60 97.12 91.21 94.71 93.21

As it can be seen in Table 4, the pulp temperature The best results were obtained at 30% solids, 120
has a positive effect on the dissolution efficiencies g/ton H2SO4, 240 min leaching period, and 60 °C
of U and Th in both fly ashes. On the other hand, leaching temperature. Under these conditions,
the results clearly showed that the pulp 94.71% uranium dissolution efficiency and
temperature had a more prominent effect on the 93.21% thorium dissolution efficiency for Soma
Th dissolution as it inceased the Th dissoluiton fly ash and 97.12% uranium dissolution efficiency
efficiencies for the Yatagan and Soma fly ashes and 91.21% thorium dissolution efficiency for
with rates of 7.42% and 9.73%, respectively. Yatagan fly ash were obtained. The results

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obtained from the study showed that H2SO4 Pilot-scale and lab-scale testing of traditional
leaching of the uranium and thorium contents dense medium and froth flotation processes
from the fly ashes could be successfully used for showed that substantial reductions in the trace
dissolution of both elements. element content could be achieved via coal
preparation. When viewed from the aspect of
5. Conclusions fractions obtained from coal processing and ashes
Coal is among the most important energy sources. of these fractions, elements such as U and Th as
In primary energy consumption, coal has taken the radioactive elements and As, Be, Co, Hg, Se, Pb,
second place with approximately 29% share after and Ni, as air pollutant elements, were enriched in
oil, which has taken the first place with certain processing fractions and their ashes.
approximately 33% share in 2015. In order to Therefore, valuable metal recovery is one of the
meet the world's energy demands, low-calorie many application areas of CFA, and a
lignite with a high ash content has been generally considerable atteniton has been given to this in the
used in the large capacity coal-fired thermal recent studies.
power plants. As a result of coal firing, wastes Besides certain heavy elements, CFA also
such as fly ash, slag, and flue gas are also contains valuable metals including germanium
produced due to the fact that the coal contains (Ge), gallium (Ga), vanadium (V), titanium (Ti),
relatively high concentrations of trace elements and aluminum (Al), which are extractable if an
when compared with other geological materials. acceptable process can be developed. In addtion,
Subsequently, toxic trace elements such as As, coal and CFA can be regarded as alternative
Cd, Ga, Ge, Pb, Sb, Se, Sn, Mo, Ti, and Zn, sources of U and Th. The U and Th extraction
among the potential contaminants to environment from coal and CFA have been an active research
within coal, are transferred to wastes such as slag, area from the beginning of nuclear era to the
fly ash, and flue gases. recent studies.
Total quantities of the trace elements involved in In conclusion, large amounts of the ashes
coal combustion are large, being roughly produced from firing of coal, which are usually
comparable with the quantities annually mobilized stored in collection ponds or stockpiles, could be
by the natural process of weathering of crustal problematic in terms of environment if necessary
rocks. Generally, less volatile elements such as precautions were not taken. On the other hand,
Cd, Co, Cu, Cr, Fe, Mn, Ni, Pb, Zn, and oxidized coals have high trace element contents, and their
Hg can be found together with finely grained waste products also have a great potential in terms
materials such as fly ash and bottom ash. As and of precious metals and trace elements they
Se represent medium volatility, and spread widely contain. Therefore, the future studies in this area
through the atmosphere. Many research works on should be supported in order to fully emphasize
heavy metals such as Pb, Cd, Cr, As, and Hg have the true potential of these alternative sources.
been carried out in the field of enrichment
according to different grain sizes, removal, and Acknowledgments
reaction with absorption or adsorption. On the The authors would like to thank Dr. Ismail
other hand, coal-to-ash enrichment rates of the DEMIR and Naranbaatar ENKHTAIVAN for
radionuclides are characterized by the enrichment their valuable contributions.
factor. The enrichment of natural radionuclides in
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 ‫‪ /Kursun Unver & Terzi‬نشریه علمی‪ -‬پژوهشی معدن و محیطزیست‪ ،‬دوره نهم‪ ،‬شماره سوم‪ ،‬سال ‪7931‬‬

‫بررسی توزیع عناصر نادر در زغالسنگ و دوده زغالسنگ و بازیابی آنها با روشهای فرآوری مواد معدنی‬

‫‪I. Kursun Unver* and M. Terzi‬‬

‫‪Department of Mining Engineeering, Faculty of Engineering, Istanbul University, Istanbul, Turkey‬‬

‫ارسال ‪ ،1078/9/5‬پذیرش ‪1078/9/13‬‬

‫* نویسنده مسئول مکاتبات‪ilginkur@istanbul.edu.tr :‬‬

‫چکیده‪:‬‬

‫امروزه زغالسنگ یکی از مهم تریم منابع انرژی است‪ .‬عموماً به منظور رفع نیازهای انرژی در جهان‪ ،‬از لیگنیت با انرژی حرارتی کم که همراه خاکستر بااییی اسات‬
‫در نیروگاههای حرارتی با ظرفیت بای استفاده میشود‪ .‬در نتیجه سوختن زغالسنگ‪ ،‬باطلههایی مانند دوده‪ ،‬سرباره و دود تولیاد مایشاود‪ .‬در نتیجاه عنا ار ناادر‬
‫سمی همراه در زغالسنگ به باطلههای ذکر شده انتقال مییابند‪ .‬مقدار زیادی از این عنا ر نادر سمی که معمویً در مخازن و کپهها ذخیارهساازی مایشاوند‪ ،‬باه‬
‫عنوان یک مشکل محیطزیستی مطرح میشوند‪ .‬اگرچه دوده زغالسنگ در دهههای گذشته در ساختوسازها و چندین نایع دیگر مورد استفاده قارار مایگرفتاه‬
‫است اما میزان استفاده فعلی آن بسیار محدود شده است‪ .‬همچنین دوده زغالسنگ دارای بسیاری از فلزات با ارزش از جمله ژرمانیوم‪ ،‬گاالیم‪ ،‬واناادیوم‪ ،‬تیتاانیوم و‬
‫آلومینیوم است‪ .‬عالوه بر این‪ ،‬زغالسنگ و دوده زغالسنگ را میتوان به عنوان منابع جایگزین عنا ر رادیواکتیو در نظار گرفات بناابراین‪ ،‬آنهاا همچناین دارای‬
‫پتانسیل زیادی از نظر فلزات گرانبها و عنا ر نادر است‪ .‬در این پاژوه‪ ،،‬ساابقهی علمای موواو ‪ ،‬در ماورد بررسای توزیاع عنا ار ناادر در زغاالسانگ و دوده‬
‫زغالسنگ در طی سوختن و بازیابی آنها با روشهای فرآوری مواد معدنی مورد بررسی قرار گرفته است‪ .‬در حالی که تحقیقات در مورد ایان موواو انجاام شاده‬
‫است به وووح نشان میدهند که مقادیر زیاد خاکستر تولیدی از سوختن زغالسنگ میتواند مشکالت زیستمحیطی ایجاد کند‪ ،‬همچناین بسایاری از مطالعاات و‬
‫اقدامات نشان میدهد که محصویت ناشی از احتراق زغالسنگ به طور بالقوه دارای فلزات گرانبها و عنا ر نادر هستند‪.‬‬

‫کلمات کلیدی‪ :‬زغالسنگ‪ ،‬عنا ر نادر‪ ،‬خاکستر بادی‪ ،‬بهرهبرداری‪.‬‬