Thursday, January 16, 2020

Microwave-ultrasonic Irradiation - the Most Effective Pretreatment Method to Produce Ultra Clean Coal


Coal samples from concentrate of Shahroud coal preparation plant with an ash content of 13.8% and a total sulfur content of 1.89%, without pretreatment and also with pretreatment by microwave irradiation, ultrasonic waves, and microwave followed by ultrasonic (microwave- ultrasonic), was leached with HF and then HNO3 to produce ultraclean coal (UCC) in a batch reactor. Irradiation pretreatments, in the order of microwave-ultrasonic > ultrasonic > microwave had positive effects on acid leaching with HF, especially for coarse size fractions and early in the leaching process. After HF leaching, the HNO3 complementary leaching was more effective on the microwave-ultrasonic pretreated sample, where, after processing, 95.5% ash content of the sample was removed. A comparison between sulfur content of samples before (1.89%) and after leaching (HF followed by HNO3) shows that the sulfur content of nonpretreated sample decreased to 1.26%, whereas this reduction for microwave-ultrasonic pretreated sample was more noticeable by 0.8%. The results indicated that microwave-ultrasonic irradiation can be considered as the most effective pretreatment method in the chemical leaching of coal to produce UCC.


Coal, as one of the world’s most abundant fossil fuels, has the potential to become more important as the source of both energy and chemical feedstock in the next century [1, 2]. However, coal is not a clean fuel since ash and sulfur are its two main constituents.

In recent decades, the two main disadvantages associated with the use of coal in combustion systems include the environmental problems (emission of sulfur oxides and hydrogen sulfide) and the detrimental effects of coal mineral matter on turbine  blades (in gas turbines) [3–5].

The growing concern about these problems and the increasing demand for a highly efficient means of electricity generation and also the possibility to replace the current uses of oil and natural gas with coal have dramatically increased attention to produce ultraclean coal (UCC) using different techniques.

To produce electricity, UCC (coal with less than 1% by weight mineral matter) can be fired in turbines with high-efficiency combustion apparatus and also reduced CO2 emission. UCC also has numerous other applications: a substitute for  petroleum coke to manufacture carbon electrodes that are utilized in the process of aluminum production; a raw material for the production of carbon-based fuels, chemicals, and materials; and in the production of engineering plastics, high- temperature, heat-resistant polymers and carbon fibers. Coal mostly contains  aromatic rich materials (benzene, naphthalene, antracene)  that,  in  comparison to oil, make it an economical source [4, 6–13].

In order to develop a process for UCC production, a thorough understanding of the chemical reactions, which take place when the mineral matter is treated with leaching reagents, is essential. It is generally accepted that the only way to produce UCC, without losing a significant amount of coal, is through a process of chemical leaching [10]. Since the mineral matter contained in bituminous coal is mostly aluminosilicate compounds and pyrite, the main demineralization reagents are hydrofluoric acid (HF) and nitric acid (HNO3) [14–22]. HF is considered to be the most effective reagent for dissolving aluminosilicates from coal [11]. After HF leaching, HNO3 leaching dissolves the pyrite and removes the insoluble fluoride compounds that are formed in the HF-leaching process. It is reported that HNO3 only reacts with pyrite when its concentration exceeds a particular level, suggesting  that  it reacts preferentially with the organic coal structure [12].

A variety of physical and chemical pretreatment methods have been used to increase the efficiency of coal demineralization. In recent years, microwave technology has been investigated as a physical pretreatment method [23–27]. Coal consists of different heterogeneous phases, each with a different dielectric permittivity and, hence, with a different ability to absorb the microwaves. These differences in various phases of coal (coal structure and its impurities) during microwave  irradiation at different degrees (temperatures) led to thermal fractures in the body  and surface of coal sample. These cracks increase the accessible area in the coal body for leach liquors and also decrease its strength [23–27].

To date, few researchers have studied the impact of ultrasonic pretreatment on coal processing [28–31]. The greatest advantage of ultrasonic method is simultaneous removal of ash and sulfur. When ultrasonic is coupled to the coal slurry, two possible outcomes may happen [30]: (a) physical breakage, which breaks the physical bonds between impurities and coal and (b) leaching, which removes some of the impurities by a mass-transfer mechanism.

In this work, a novel approach has been used to find the most efficient method for UCC production through a comparison of various  techniques. In other words,  the main purpose of this study is to determine the optimized pretreatment conditions in a series of leaching (HF followed by HNO3) of Shahroud coal samples for producing UCC, with the aim of comparing the effects of the leaching procedure on coal samples without any pretreatment  and  on  samples  with  microwave  pretreatment, ultrasonic pretreatment, and finally with microwave followed by ultrasonic (microwave-ultrasonic) pretreatment.

Material and Methods

Coal Samples

A coal sample  was prepared from concentrate at the Shahroud coal preparation  plant. Proximate and ultimate analyses of the sample are presented in Table 1. According to the X-ray diffraction (XRD) analysis, illite, quartz, kaolinite, pyrite, vermiculite, and dolomite are the mineralogical composition of the sample.

Microwave Irradiation

Extensive research has been conducted on the use of microwave heating in coal sciences, including the drying, gasification, desulfurization, pyrolysis, and grindability [32–39]. Microwave is a nonionizing electromagnetic radiation with frequencies in the range of 300 MHz to 300 GHz. Microwave frequencies include three bands: the ultrahigh frequency (UHF: 300 MHz to 3 GHz), the super high frequency (SHF: 3 GHz to 30 GHz), and the extremely high frequency (EHF: 30 GHz to 300 GHz) [40]. Microwaves can be transmitted, absorbed, or reflected depending on the type of material [40]. In coal pretreatment with microwaves, the amount of defects and cracks in the coal structure increases based on the differences in dielectric properties of the mineral matter content [41–42]. The organic component of coal is a relatively poor absorber of microwave energy, whereas some impurities in coal show a different behavior when absorbing electromagnetic irradiation: Water and pyrite can be easily heated with microwave irradiation, whereas the quartz shows resistance [43–50].

In this study, a Samsung microwave oven, equipped with an air circulation system and with variable power at 2.45 GHz, and with the power of 300 to 900 W, was used for microwave irradiation. To determine the optimum time of irradiation, approximately 20 g coal sample with a particle size of -500 mm was spread in a Pyrex container and was subjected to microwave irradiation using a power of 900 W, and with 3-, 5-, 7-, and 9-minute irradiation times. The optimized irradiation time was then used in the pretreatment stage before the HF-leaching process (Table 2).

Ultrasonic Irradiation

In recent years ultrasound technology has been applied for the extraction, desulfurization, and liquefaction of coal [51–54]. Power ultrasound can be used for the removal of sulfur and mineral matter of coal. In addition, ultrasonic  conditioning  can increase the hydrophobicity of slime and pyrite; therefore, it can change the surface of the coal and pyrite particles [9].

Table 1. Proximate and ultimate analysis (wt.%, dry basis) of Shahroud coal sample

Table 2. Ash reduction regarding microwave and ultrasonic pretreatments

Ultrasonic waves are a series of vertical waves alternating unevenly. There are two effects of cavitations formed by ultrasound near the extended liquid-solid inter- faces in the liquid-solid system: microjet impact and shock wave damage [28]. Each cavitated bubble forms a ‘‘hot spot,’’ with its core reaching a temperature of nearly 5000 K and a pressure of more than 50 MPa [55]. The temperature at the interface between the water and the bubble can be as high as 2000 K.  The rate of heating    and cooling can be faster than 109 K=s. As a result, by ultrasonic treatment on the pulp, mechanical, thermal, and chemical effects may happen that can change  the pulp nature [31].

In this investigation, a Q280 multifunction ultrasonic transmitter was used in the tests. The frequency of its ultrasonic head was 43 kHz and the total power of its ultrasonic transmitter was 225 W.  Around  20  grams  of  coal  was  mixed  with  200 mL of water, processed with the ultrasonic transmitter  and was then  filtered  and dried. To determine the optimum time for ultrasonic transmitter, a coal sample with a 500 mm size was subjected to ultrasound irradiation for 5, 10, 20, 30, and 60 minutes. The optimized ultrasonic irradiation time was  used in the pretreatment  stage before the HF-leaching process (Table 2).

Leaching Experiments

HF Extraction

HF is one of the most effective reagents for dissolving the mineral matter content (except for pyrite) of coal [10–12]. In other words, HF is able to penetrate further into the coal matrix and to dissolve the mineral matter (preferentially aluminosilicates) [10]. In a single leaching step and in ambient conditions, HF reacts with the aluminosilicates to predominantly form AlFþ2 , AlF3, and SiF4 [11]. Unfortunately, the formation of insoluble fluoride compounds containing the alkali and alkaline earth elements occurs during leaching with HF. Formation of these fluoride com- pounds inhibits the ability of HF to reduce the level of mineral matter in black coal  to less than 1 wt%. As a result, using the second reagent is necessary [11]. Details of the chemical reactions that take place when the mineral matter in coal is treated with HF can be found in the report by Steel et al. [10, 11].

Samples that had been irradiated with microwave and ultrasound, as well as the untreated sample, were demineralized with hydrofluoric acid. The experiments were performed in a 50 ml Teflon reactor equipped with a thermometric tube and stirrer.

Reaction times of 0.5, 1.5, 2.5, and 3.5 hours were used in the process; particle sizes were 1000,  500,  250, and  75 mm; temperatures were 25, 40, 50, and 60°C; and  acid concentrations were 1.5, 2.5, 3.5, and 5.0 M. After the reaction, the pulp was filtered in a polypropylene funnel to recover the demineralized coal. The filtrate    was washed with hot water, dried in an oven and analyzed for ash percentage.

HNO3 Extraction

Pyrite (FeS2) is essentially the only mineral that is not affected by HF. Also, as it was previously discussed, the major concern that exists with the development of a UCC process by HF is the formation of insoluble fluoride compounds, such as calcium fluoride (CaF2) and magnesium fluoride (MgF2) [10]. Nitric acid (HNO3) is an extremely effective reagent that has been used in conjunction with HF for reducing the level of mineral matter in coal below 1 wt.%. HNO3 has a dual action of reacting with pyrite to form soluble products and dissolving low-solubility fluoride com- pounds that form in the HF-leaching process [12].

In this study, the optimum HNO3 concentration was examined with 0.5, 1, 1.5, and 2 M at 65°C temperature for a coal particle size of 250 mm and leaching time of 1 hour. The final filtrated samples were washed with hot water, dried in oven and analyzed to measure the rate of ash reduction.

FTIR Analysis

Fourier Transform Infrared (FTIR) spectroscopy has been used successfully for analysis of organic and mineral matter in bulk coal [56–59]. The transformations that occurred in native and demineralized coal samples were studied by FTIR spectroscopy. The coal samples were milled to form a very fine powder by potassium bro- mide (KBr). This powder was compressed into a thin pellet, and the spectra were recorded with a Bomer MB-100 instrument equipped with Deuterated Triglycine Sulfate (DTGS) detector. A spectrum was obtained for each sample and the analytical software provided by Bruker was used for spectral treatment.

Results and Discussion

Optimization of Irradiation Time

To evaluate the impact of coal pretreatment on ash reduction by leaching, samples without any pretreatment (untreated), and also samples pretreated with microwave irradiation (MW), ultrasonic energy (US), and finally with microwave followed by ultrasonic (MW-US) methods were leached in mild leaching conditions: HF reaction time of 1.5 hr, particle size of 500 mm, leaching temperature at 25°C, HF concentration of 3.5 M, rotation rate 150 rpm, pulp density 20 wt.%. The results are shown in Table 2. According to these results, the optimum time for microwave irradiation was 7 minutes, whereas that of the ultrasonic was 30 minutes.

Optimization of HF-Leaching Process

Effect of Leaching Time

Untreated and pretreated samples (irradiated by microwave, 900 W for 7 min, and=or ultrasonic wave, 43 kHz for 30 min) were leached with hydrofluoric acid (3.5 M) under reaction times 0.5, 1.5, 2.5, and 3.5 hours, with a temperature of 25°C, particle size of 500 mm, and a pulp density of 20 wt.%. According to the results (Figure 1) ash content is reduced as the leaching time increases; however, the reduction is not significant after 2.5 hours. Furthermore, the amount of ash removed from pretreated samples is considerably more than the untreated sample, and the microwave-ultrasonic pretreatment shows the highest percentage of ash removal in  all spots.

As the leaching time is increased, there is sufficient time for the leaching agent to penetrate into the coal structure to dissolve the remaining mineral matter. Therefore, for long leaching times, the weakening effects of the pretreatment methods are no longer a factor.

Effect of Coal Particle Size

To evaluate the effect of particle size on ash reduction, coal samples were grinded to 1000, 500, 250, 75 mm. In the next step, pretreated and untreated samples were leached with 3.5 M HF at 25°C for 2.5 hours. The results (Figure 2) show that due to the increase in the external surface area per unit mass of coal (increase leach liquor accessibility), the percentage of ash removal increased when particle size was decreased from 1000 to 250 mm. Further size reduction to 75 mm did not significantly change the ash reduction; therefore, particles with 250 mm size were prepared for the next tests. In addition, it can be observed from the results (Figure 2) that pretreatments of samples significantly improved the removal of mineral matter compared with the untreated sample, particularly for coarse size fractions. Also Figure 2 shows that the combination of microwave and ultrasonic-irradiation (MW US) is the most effective method especially when it was used with coarser coal particles. As particle sizes were decreased, the influence of irradiation methods became less significant.

Effect of HF Concentration

The effect of HF concentration on ash removal was investigated by acid concentration from 1.5 to 5 M at a leaching temperature of 25°C for 2.5 hours and with particle size of —250 mm. The results show that HF concentration plays a significant role in ash reduction (Figure 3). For the untreated sample, it was observed that by increasing the acidity from 2.5 to 3.5 M, the percentage of ash removal increased from 59.72% to 71.77%; however, ash reduction was not significant when acidity was increased above 3.5 M. Moreover, in all experiments, the results show improvements in ash reduction for pretreated samples. In addition, at 3.5 M acidity, the microwave-ultrasonic pretreated sample shows the highest improvement (approximately 3%) in ash reduction compared with the untreated sample (Figure 3).

Figure 1. Effect of HF-leaching time on ash reduction for untreated, microwave-irradiated (MW), ultrasound-irradiated (US), and microwave þ ultrasound-irradiated (MW þ US) coal samples (—500 mm, 25°C, 3.5 M HF). (Color figure available online.)

Figure 2. Effect of particle size on untreated, microwave-irradiated (MW), ultrasound- irradiated (US), and  microwave þ ultrasound-irradiated  (MW þ US)  coal  samples  (2.5 h, 25 C, 3.5 M HF). (Color figure available online.)

Effect of HF-Leaching Temperature

The impact of HF-leaching temperature was investigated at the following conditions: HF concentration of 3.5 M, reaction time of 2.5 hours, particle size of 250 mm, and at temperatures of 25, 40, 50, and 60°C. Results show that the effect of leaching temperature is significant (Figure 4). Experiments on the untreated sample show that by increasing the temperature from 25°C to 50°C, the percentage of ash removal was increased by approximately 10% (from 71.77% to 81.13%). However, this increase after 50°C ceased, and the percentage of the ash reduction did not improve at 60°C. Similar to the previous experiments, ultrasound and especially microwave- ultrasound pretreatment methods improved the ash reduction for all examined temperatures.

Figure 3. Effect of HF acid concentration for untreated (common), microwave-irradiated (MW), ultrasound-irradiated (US), and microwave þ ultrasound-irradiated (MW þ US) coal samples (2.5 h, 25°C, —250 mm). (Color figure available online.)

Figure 4. Effect of leaching temperature for untreated, microwave-irradiated (MW), ultrasound-irradiated (US), and microwave þ ultrasound-irradiated (MW þ US) coal samples (2.5 h, 3.5 M HF, —250 mm). (Color figure available online.)

According to the above results, the optimum conditions for HF leaching were determined: HF concentration of 3.5 M, reaction time of 2.5 hours, particle size of 250 mm, and a temperature of 50°C. In addition, in different samples (with and without pretreatments), the results indicated that ash content was reduced in the following order: microwave-ultrasonic > ultrasonic > microwave > untreated samples.

Effect of HNO3 Concentration

Coal samples (with and without pretreatments) under prepared optimized conditions of HF leaching were cleaned further using HNO3 leaching. In order to find a suitable concentration for HNO3 leaching, Nitric acid was examined under 1 hour leaching time at 65°C and at various concentrations: 0.5, 1, 1.5, and 2 M. The results (Figure 5) showed a considerable improvement in ash reduction by the increase in HNO3 concentration. In the experiments conducted on the untreated sample, by increasing the acidity from 0.5 to 1.5 M ash reduction increased from 85.5% to 95.1%. However, raising the acidity to 2 M did not show any improvement. Also, the pretreatment methods were more effective on lower HNO3 concentrations. At the optimized HNO3 concentration (1.5 M), the effect of irradiation was negligible and can be disregarded.

Figure 5. Effects of HNO3 concentration on ash reduction for  untreated,  microwave- irradiated (MW), ultrasound-irradiated (US), and microwave þ ultrasound-irradiated (MW þ US) coal samples (1 h, 65°C, —250 mm). (Color figure available online.)

Table 3. Sulfur analysis of coal samples before and after leaching

A comparison between sulfur content of samples before (1.89%) and after leach- ing (with and without pretreatment) shows that, after two stages of leaching (HF followed by HNO3), the sulfur content of the  untreated  sample  decreased  to  1.26%. This reduction for the microwave-ultrasonic pretreated sample is more noticeable, since the sulfur content was reduced to 0.8% (Table 3).

FT-IR Spectra of Coal Structure

The structures of coal samples (dried in a vacuum oven at 70°C for 2 hours) before and after demineralization were studied by FT-IR spectra (Figure 6).

Figure 6. FT-IR of coal samples before and after leaching procedure on untreated and pretreated (microwave, ultrasonic, and microwave-ultrasonic) samples. (Color figure available online.)

The intense absorbance bands in the range of 3650 to 3000 cm—1 were inter- preted as O-H bonds that are bound to the mineral and organic matter. The absorbance bands at 3000 to 2800 cm—1 correspond to the stretching vibration of aromatic C-H bonds. Since coal contains polycyclic species, the bands observed at 1436, 1431, 1442, and 1432 cm—1 for the microwave, ultrasound, microwave-ultrasound pre- treated, and native, and untreated samples, respectively, can be assigned to skeletal C-C stretching modes. The broad bands for leached samples between 1100 and 1400 cm—1 are due to C-O stretching.

Signals related to mineral matter contained in coal (at 1200–1000 cm—1 and 600–400 cm—1) were significant for the native sample and nonexistent for demineralized samples. In other words, the spectra indicate that the demineralization process of coal samples using the applied pretreatments significantly changed the peaks corresponding to the mineral matter.


Ultraclean coal (UCC) is a high-purity, carbon-based material that can be further processed to produce a wide range of fuels, chemicals, and materials. The well- known method to produce UCC is via chemical leaching: hydrofluoric acid (HF) followed by nitric acid (HNO3). Physical and chemical pretreatment methods can improve the efficiency of the coal-demineralization process. In this study, microwave irradiation, ultrasonic energy, and finally microwave followed by ultrasonic pretreat- ments were used in order to enhance the process of UCC production. Our results indicated that the UCC production process under optimum conditions (HF concen- tration of 3.5 M, reaction time of 2.5 hours, particle size of 250 mm, temperature 50°C) for samples that were pretreated by microwave (900 W for 7 minutes) and then ultrasound energy (43 kHz for 30 minutes) decreased 95.50% of the ash content in coal. The complementary HNO3 leaching under optimized conditions of 1.5 M, at 65°C, for 1 hour can produce a final product with approximately 0.6% ash content.

Source: M. M. Royaei , E. Jorjani & S. Chehreh Chelgani - Department of Mining Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran


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