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ILMIY METODIK JURNAL
Isakov Yusuf Khoriddinovich
E-mail:
Doctor of Philosophy (PhD) in Technical Sciences,
Senior Lecturer at the Department of Chemistry,
Faculty of Natural Sciences, Uzbekistan-Finland Pedagogical Institute.
Akramova Yulduz Dostonbek qizi
E-mail:
A student of the Chemistry program at the Faculty of
Natural Sciences, Uzbekistan-Finland Pedagogical Institute.
Pardayev Ulug‘bek Xayrullo o‘g‘li
E-mail: pardayevulugbek125@gmail.com
A student of the Chemistry program at the Faculty of
Natural Sciences, Uzbekistan-Finland Pedagogical Institute.
Bobojonov Jamshid Shermatovich
Doctor of Philosophy (PhD) in Technical Sciences,
associate professor at the Department of Chemistry,
Faculty of Natural Sciences, Uzbekistan-Finland Pedagogical Institute.
UDK 622.785.5
CHEMICAL BENEFICIATION OF LIGNITE COALS FOR REDUCING ASH AND
MINERAL IMPURITIES
Annotation:
In Uzbekistan, the effective utilization of low-grade coal extracted from the Angren
coal deposit—initially considered unsuitable as fuel—is of critical importance. By processing
and upgrading the quality of this coal, it becomes possible to supply high-quality solid fuel for
both the population and various industrial sectors. This approach ensures the efficient use of coal
resources, which are among the country’s valuable mineral assets. To obtain clean and efficient
fuel from lignite—a natural energy source and a key factor in the development of the nation’s
industry and living standards—it is essential to develop waste-free coal beneficiation
technologies.
Key words:
lignite (brown coal), fuel, coal ash, mineral substances, calorific value, chemical
reagent.
Introduction:
Until now, coal has been used directly as fuel without any processing, which
resulted in high ash content and a large amount of waste. To minimize such waste, coal is now
being processed and recognized as one of the most promising sources for the production of solid
fuels. Moreover, it serves as a key raw material for obtaining various chemical compounds and
composite materials [1]. In the national economy, the processing of coal to produce synthetic
fuel has become a strategic priority, especially for countries without their own oil reserves. In
this regard, the development of new technologies for converting domestic coal resources into
alternative fuels has become a necessity [2]. As a result, within a short period, coal has turned
into one of the most vital energy sources required for industrial and transportation development
to meet the energy demands of society.
Literature review:
Lignite coal, also known as brown coal, is an abundant but low-quality
energy resource due to its high ash content, moisture, and non-combustible mineral impurities.
These properties limit its direct application as an efficient fuel and contribute to increased
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environmental pollution during combustion, especially through the release of sulfur oxides (SO₂),
which are responsible for acid rain formation. Therefore, the chemical beneficiation of lignite
has gained increasing attention as a promising method for improving its fuel quality and
environmental performance.
Numerous studies have explored various techniques for the chemical treatment of coal, including
acid leaching using hydrochloric acid (HCl), sulfuric acid (H₂SO₄), nitric acid (HNO₃), and
phosphoric acid (H₃PO₄). Among these, dilute hydrochloric acid (e.g., 0.1 M and 1 M
concentrations) has demonstrated high efficiency in removing acid-soluble mineral compounds
such as calcium (Ca), magnesium (Mg), sodium (Na), potassium (K), and partially iron (Fe) and
aluminum (Al) oxides from coal ash. This selective leaching significantly reduces the ash content
and improves the overall calorific value of lignite.
Earlier research by industrial laboratories and GOST-regulated studies (e.g., GOST 11022-95,
GOST 26148) has confirmed that treating coal with HCl not only reduces ash by 12–13% but
also facilitates partial desulfurization through the dissolution of compounds like FeS₂, CaSO₄,
and FeSO₄. In addition, water washing prior to acid treatment helps eliminate water-soluble salts
such as carbonates, chlorides, and sulfates, thus reducing acid consumption and improving
process efficiency.
Investigations on lignite samples from Angren, Shargun, and Baysun deposits in Uzbekistan
have revealed that mineral content in raw coal ash may reach up to 60%, severely affecting the
combustion performance. Chemical beneficiation of these coals has shown substantial
improvement in reducing both ash and sulfur content, with the optimized use of 0.1 M HCl
yielding the best results. Moreover, the treated coal exhibits a higher heating value (HHV) due to
the removal of non-combustible mineral matter.
Overall, the literature suggests that chemical beneficiation, especially using dilute hydrochloric
acid, is a viable method for enhancing the fuel characteristics of lignite coals and reducing their
environmental impact. However, further work is needed to optimize reagent concentration,
process temperature, and treatment time to achieve industrial-scale applicability.
Methodology:
Lignite coal samples of the brands 2BPK, 2BR-B2, 2BOM, and 2BR were
collected from the Angren, Shargun, and Baysun coal deposits in Uzbekistan. Each sample was
air-dried, homogenized, and ground to a particle size below 0.2 mm for laboratory analysis. Prior
to acid leaching, each coal sample underwent water washing to remove water-soluble mineral
impurities. Approximately 10 g of each coal sample was mixed with 100 mL of distilled water
and stirred for 30 minutes at room temperature. The suspension was filtered, and the solid
residue was dried at 105°C. The pre-washed samples were then subjected to chemical
beneficiation using hydrochloric acid (HCl) at concentrations of 0.1 M and 1 M. For each
experiment, 5 g of coal sample was added to 100 mL of HCl solution and stirred for 1 hour at
ambient temperature. After the reaction, the mixtures were filtered, washed with distilled water
to remove residual acid, and dried. Ash content was measured using the accelerated method
according to GOST 11022-95. About 1 g of each treated and untreated sample was placed in a
muffle furnace and heated to 850–875 ± 25°C. The remaining residue was weighed, and ash
content was calculated as a percentage of the original sample mass. The chemical composition of
coal ash was analyzed to determine the concentration of oxides such as SiO₂, Al₂O₃, MgO, CaO,
Na₂O, K₂O, and Fe₂O₃. Potentiometric titration was used for determining SiO₂, Al₂O₃, and MgO.
Flame photometry (using PFP-7 model photometer) was employed for quantifying Na₂O and
K₂O. Standard aliquots were prepared from 0.1 g of ash as per GOST 26148. Total sulfur content
in the coal samples was determined by combustion in a closed system and measuring the
released SO₂. The amount of sulfur was calculated as a percentage of the dry sample weight.
Particular attention was given to the presence of sulfur-containing compounds such as FeS₂,
CaSO₄, and FeSO₄. The lower and higher heating values of coal were determined using a bomb
calorimeter. Both raw and chemically treated samples were tested to evaluate the impact of
beneficiation on combustion properties.
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Results:
Lignite coals of the brands 2BPK, 2BR-B2, 2BOM, and 2BR were chemically treated
with nitric acid, hydrochloric acid, hydrofluoric acid, and sulfuric acid. The best results were
obtained when treated with 0.1 M HCl. Research was conducted on coal samples from the
Angren, Shargun, and Boysun deposits. The technological characteristics of the coals are
presented in Table 1 [9].
Table 1
TECHNOLOGICAL CHARACTERISTICS OF LOCAL COAL FUELS:
eral composition (SiO₂, Al₂O₃, MgO) of the ash derived from lignite coal brands 2BPK, 2BR-B2,
2BOM, and 2BR, the potentiometric titration method was employed [3]. The content of sodium
and potassium oxides (Na₂O, K₂O) was determined using flame photometry. For this purpose, a
PFP-7 model photometer was used. An aliquot was prepared from 0.1 g of ground ash in
accordance with GOST standards (GOST 26148), and each oxide was analyzed individually
through photometric measurement.
The lower and higher heating values of the coal samples were determined using the calorimetric
method [4].
Experimental results revealed that the content of mineral substances in coal ash ranged between
35–60%. These non-combustible components not only lead to increased economic costs (e.g.,
additional fuel consumption and excess oxygen requirements) but also reduce the heat of
combustion of the coal [5]. The presence of inert minerals results in the consumption of excess
oxygen, leading to a decrease in the efficiency of combustion reactions and a reduction in the
amount of heat released during burning [8].
During the experiments, both the higher calorific value (representing combustion of the organic,
enriched portion of the coal) and the lower calorific value (resulting from combustion without
removing mineral impurities) were evaluated [6] (see Table 2).
Table 2
CALORIFIC VALUES OF LIGNITE COAL GRADES:
Coal Brands
Higher
Heating
Value, kcal/kg
Lower
Heating
Value, kcal/kg
Type of Regulatory
Document
Type
of
Regulatory
Document
Brand
(Group)
Particle
Size, mm
Quality Indicators
Working
Moisture,
Wr, %
Ash
Content
As,
not
more
than %
Angren Coal Deposit
GОSТ 8202-87
Lignite
(Brown
Coal)
2BPK
50-200
30
16
GОSТ 8202-87
2BR-B2
0-200
40
24
GОSТ 8202-87
2BOM
0-50
40
22
GОSТ 8202-87
2BR
0-200
32,5
22
Shargunsoy Coal Deposit
TSh 12-12:1998
Bituminous Coal
1SSSSh
0-13
10
30
TSh 12-12:1998
1SSKOM
13-100
10
25
Baysun Coal Deposit
TSh 12-12:1998
Bituminous Coal
1ТR
0-200
10
24,5
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2BPK
1620-2430
1670-2500
2BR-B2
1530-2350
1570-2480
2BOM
1680-2480
1690-2400
2BR
1590-2350
1600-2450
According to the table, the presence of mineral impurities in coal significantly contributes to a
lower calorific value. Therefore, in order to increase the heating value and improve the quality of
coal, it is necessary to remove non-combustible mineral admixtures. To determine the amount of
mineral content in the coal, the ash content (A°, %) was measured. This experiment was
conducted using an accelerated method in accordance with GOST 11022-95. Coal samples were
heated in a furnace at a temperature of 850–875 ± 25°C, and the ash content was determined as
the percentage of the remaining ash mass relative to the initial sample mass [10] (see Table 3).
Table 3
ASH CONTENT OF LIGNITE COAL GRADES:
Coal Brands
Ash Content, A°, %
2BPK
16
2BR-B2
24
2BOM
22
2BR
22
To determine the most effective beneficiation method, the composition of the coal ash was
analyzed.
Table 4
Mineral Composition of Ash from Lignite Coal Grades:
№
Chemical
Composition
of
Coal Ash, %
Coal Grades
2BPK
2BR-B2
2BOM
2BR
1
SiO
2
57.0
66.4
61
60.2
2
Fе
2
О
3
1.9
1.5
1.6
1.4
3
Аl
2
О
3
27.6
17.1
18.6
19.0
4
CаО
8.3
7.6
6.4
6.9
5
МgО
1.7
2.1
1.8
1.9
6
Nа
2
О
0.5
0.9
0.4
0.6
7
К
2
О
1.1
0.7
0.9
1.4
Many components in the mineral admixture (Fe₂O₃, CaO, MgO, Na₂O, K₂O) are highly soluble
in acids. Therefore, when selecting a chemical reagent for coal treatment, the effect of various
acids on the mineral admixtures present in coal was investigated. However, since some of these
compounds are also soluble in water, the coal was initially enriched with water to remove water-
soluble compounds and reduce acid consumption. For acid treatment, 0.1 N hydrochloric acid
(HCl) was selected. This is because other acids, such as sulfuric acid (H₂SO₄) and phosphoric
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acid (H₃PO₄), form insoluble sulfate and phosphate salts with certain ions in coal, which leads to
an increase in the total mineral content. Therefore, coal samples were treated with 0.1 M and 1 M
HCl, and the results are presented in Figure 1.
Figure 1:Dependence of Coal Ash Content on Hydrochloric Acid (HCl) Concentration.
When the lignite coal samples of brands 2BPK, 2BR-B2, 2BOM, and 2BR were treated with
0.1 M hydrochloric acid (HCl), a significant reduction in ash content was observed. Therefore,
the optimal concentration of hydrochloric acid for treating the studied coal types was determined
to be 0.1 M (see Table 5).
Table 5
Mineral Content (%) of Lignite Coal Grades After HCl Treatment:
Coal Grades
2BPK
2BR-B2
2BOM
2BR
Mineral
Content
Before
Beneficiation, %
41,26
47,12
43,54
42,16
Mineral
Content
After
Beneficiation, %
28,75
33,87
32,52
31,35
When coal was treated with water, it was found that 6–9% of non-combustible mineral
compounds could be removed due to the dissolution of various water-soluble salts—such as
carbonates, sulfates, and chlorides—as well as potassium, sodium, and partially calcium and
magnesium compounds.
When coal was treated with hydrochloric acid (HCl), the mineral admixture content decreased by
12–13% due to the removal of acid-soluble compounds, mainly calcium and magnesium, and to
a lesser extent, iron and aluminum compounds (see Table 6).
Table 6
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Mineral Content (%) in the Ash of 2BR-B2 Lignite After Water and HCl (0.1 M, 1 M)
Treatment:
Ash Content, %
Mineral Content (%)
SiO
2
Fе
2
О
3
Аl
2
О
3
CаО
MgО
Nа
2
О
K
2
О
2BPK + 0.1 M HCl 58.1
2.1
27.6
8.4
1.8
0,6
1.2
2BPK + 1 M HCl
58.4
1.8
28.9
8.1
1.4
0.41
0.9
One of the key indicators of coal quality is its environmental safety, particularly the extent to
which it pollutes the atmosphere with harmful gases during combustion. These gases can react
with water vapor in the air to form acid rain [11]. An analysis of the composition of various
lignite coal grades revealed the presence of sulfur (S), an element known to produce toxic gases.
The sulfur content in each coal sample was determined and calculated as a percentage (see Table
7).
Table 7
Sulfur (S) Content (%) in the Composition of 2BR-B2 and 2BOMSh-B2 Lignite Coal
Grades:
Coal Brands
S (%)
2BPK
0.9-2.8
2BR-B2
0.3-3.5
2BOM
0.4-3.4
2BR
0.2-2.4
It is well known that, upon heating, these substances decompose and release toxic sulfur dioxide
(SO₂) into the atmosphere. Therefore, it is necessary to remove sulfur-containing compounds
from coal as well. Through chemical treatment of lignite coal grades, partial desulfurization was
achieved due to the partial solubility in acid of sulfur-containing compounds such as FeS₂,
CaSO₄, and FeSO₄.
Discussion:
The experimental results confirm the effectiveness of chemical beneficiation,
particularly hydrochloric acid (HCl) treatment, in improving the quality of lignite coals. Initial
ash content in raw lignite samples ranged between 35–60%, which is consistent with earlier
reports on low-rank coals from Uzbekistan’s Angren, Shargun, and Baysun deposits. Such high
ash levels not only reduce the calorific value of coal but also increase handling, transport, and
environmental costs.
Water washing was effective in removing 6–9% of the ash-forming mineral matter, primarily
through the dissolution of water-soluble salts such as chlorides, sulfates, and carbonates, along
with minor amounts of potassium and sodium compounds. This preliminary step also helped
reduce acid consumption during subsequent HCl leaching.
The most significant reductions in mineral content were observed after treatment with 0.1 M and
1 M HCl. The 0.1 M solution yielded the best efficiency-to-cost ratio, reducing mineral matter by
12–13% on average. Acid-leachable compounds such as CaO, MgO, Na₂O, and K₂O showed
substantial decreases in concentration, as confirmed by potentiometric titration and flame
photometry. Notably, Fe₂O₃ and Al₂O₃ levels also dropped slightly, suggesting partial solubility
in hydrochloric acid.
The desulfurization effect of acid treatment is particularly important from an environmental
perspective. Sulfur present in lignite coal—primarily in the form of FeS₂, CaSO₄, and FeSO₄—
was partially removed during acid leaching, leading to a decrease in SO₂ emissions potential
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during combustion. Since sulfur dioxide is a major contributor to acid rain, this outcome
enhances the environmental profile of treated lignite.
A direct correlation was observed between the reduction of ash and an increase in calorific value.
Samples treated with 0.1 M HCl exhibited notably higher heating values compared to untreated
coal, due to the removal of inert, non-combustible mineral components. This supports the
feasibility of chemical beneficiation not only for fuel improvement but also for cost reduction in
energy production.
The comparative analysis across different coal grades showed that 2BR-B2 and 2BOMSh-B2
responded particularly well to acid treatment, suggesting that mineralogical composition plays a
key role in the beneficiation efficiency. This indicates the need for tailored treatment approaches
depending on the specific characteristics of the coal deposit.
The results demonstrate that chemical beneficiation using dilute hydrochloric acid is an effective,
scalable method for reducing ash and mineral impurities in lignite coal. This method enhances
fuel quality and supports cleaner, more efficient energy production.
Conclusion:
This study has demonstrated that chemical beneficiation, particularly hydrochloric
acid (HCl) leaching, is an effective approach for reducing ash and mineral impurities in lignite
coals from Uzbekistan. Preliminary water washing removed a portion of water-soluble salts,
while HCl treatment further reduced mineral content by 12–13%, significantly improving the
coal’s combustion characteristics.
The optimized use of 0.1 M HCl was found to be the most effective in terms of both efficiency
and practicality, resulting in the partial removal of calcium, magnesium, sodium, potassium, and
sulfur-containing compounds. As a result, the treated coal samples exhibited an increase in
calorific value and a reduction in sulfur dioxide (SO₂) emissions potential, thereby improving
both fuel quality and environmental performance.
These findings underscore the potential of acid-based beneficiation techniques as a low-cost,
scalable solution for upgrading low-grade lignite coal in regions lacking high-quality fossil fuel
resources. Future studies should focus on process optimization, recovery of valuable by-products,
and the environmental safety of effluent disposal from acid treatment operations.
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