The distribution of arsenic in coal (C), high temperature ash (HA), fly ash (FA) and bottom ash (BA) of Shentou Power Plant in Shanxi Province was studied. The mineral components in the samples were analyzed and observed by X-ray diffraction and optical microscope, and the occurrence of arsenic in the samples was studied by step-by-step chemical extraction. The results show that the arsenic content in FA is the highest and that in BA is the lowest. More than 50% arsenic in coal-fired products enters glassy silicate phase or mullite mineral lattice, 20% ~ 30% is combined with carbonate and iron-manganese oxide, and 15% ~ 20% arsenic is in water-soluble and exchangeable state. Because water-soluble and exchangeable arsenic is easy to enter the water environment, attention should be paid to the potential harm of arsenic in coal-fired products in power plants to the water environment.

Selected Papers on Coal Petrology and Coal Geochemistry in Ren Deyi

Arsenic is a common harmful trace element in coal, and the arsenic released by coal combustion has caused serious harm to human health [1]. The occurrence of arsenic in coal-fired products determines its ability to release to the environment, so it is of great significance to study the occurrence of arsenic in coal-fired products to evaluate the environmental impact of arsenic in coal-fired products. However, there is little research on the occurrence of arsenic as a product of coal combustion.

In this paper, the occurrence of arsenic in coal-fired products of Shentou Power Plant in Shanxi Province was studied by step-by-step chemical extraction.

I. Sample characteristics

Element geochemistry and mineralogical characteristics of 1. samples

The samples used in the experiment are front coal (C), laboratory high-temperature ash (HA), fly ash (FA) and bottom ash (BA) of front coal in Shentou Power Plant, Shanxi Province, in which the laboratory high-temperature ash is obtained after the front coal is ashed at 850℃ for 24h.

The mass fractions of As, Al, Ca, Mg, K, Na, Fe and Mn in the samples were determined by neutron activation analysis (INAA) and atomic absorption spectrometry (AAS) (table 1). After low temperature ashing, the ash content of coal in front of the furnace was determined, and the contents of FA, BA, C and HA were observed by powder X-ray diffraction (XRD) and optical microscope analysis.

2. Elemental abundance of samples

The mass fractions of constant elements Al, Ca and Na in FA and BA are similar. Fe content in BA is the highest, followed by FA and C; The mass fraction of Mn in HA, FA and BA is similar. The mass fraction of As in FA is higher, followed by HA and BA. Obviously, the mass fraction of these elements in C is the lowest, and the Fe/Ca ratio in FA and BA is 0.58 and 0.82, respectively. Obviously, FA and BA are both alkaline ashes, and FA is more alkaline than bottom ash [2].

3. Mineralogical and microcrystalline characteristics of the samples

The coal in front of the furnace is No.4 coal of Shanxi Formation in Pingshuo mining area, and the maximum reflectivity of vitrinite oil immersion is 0.66%. Its organic microscopic components are mainly vitrinite and inertinite. The main minerals are kaolinite and boehmite (γ-AlOOH), followed by calcite and Yingshi, with a small amount of illite. The main components of FA and BA are mullite (Al6Si2O 13), and no other minerals are found. The high background of spectral lines indicates that there are more glassy aluminosilicate phases. The main components of HA are timely and sepiolite. The high background of XRD spectrum, especially the peak of 002, shows that HA contains a lot of carbon residue, which is the product of incomplete combustion caused by insufficient oxygen supply during high temperature ashing. Microscopically, FA and BA are mainly glassy aluminosilicate, followed by glassy microspheres, with the particle size of 10 ~ 200μ m, and most of them are100μ m: the spherical wall is thick. Occasionally, the big ball cavity contains small ball aggregates to form capsules in FA, and there are less carbon residues in FA and BA, but more in HA.

Table 1 sample characteristics

Second, the experimental method

The water-soluble and exchangeable arsenic, iron-manganese oxide arsenic, humic acid arsenic, fulvic acid arsenic, arsenic entering coal molecular structure and arsenic entering mineral crystal lattice were extracted into the solution in six steps, and then the arsenic content was determined by flow injection-hydride generation-atomic absorption spectrometry, and finally the percentage content of arsenic in various forms was calculated. The preparation and experimental methods of experimental reagents are as follows.

Take 10g sample and grind it to below 200 mesh for later use. CH3COONa solution (CB= 1mol/L), NH2OH HCl solution (CB=0.04mol/L), NaOH solution (ρB= 10g/L) and Na3PO4 solution (CB=0. 1mol/L) were prepared respectively. Nitric acid solution (CB = 0.2 mol/L); In addition, prepare CH3COOH, HF, HClO4 and other reagents. The step-by-step chemical extraction experiment is as follows.

1. water-soluble state and exchangeable state (P 1)

Weigh 1.0000g of sample into a 50mL triangular flask, add 20mL of prepared CH3COONa solution, electromagnetically stir 1h, transfer to a centrifuge tube for high-speed centrifugation for 30min, remove the supernatant from the 50mL volumetric flask, wash the precipitate in the centrifuge tube with 10mL deionized water, centrifuge for 20min, and combine the washed supernatant into the 50mL volumetric flask. Repeat the washing again. Rinse to scale with deionized water and shake well. Used for the determination of arsenic. The residue of the centrifuge tube is used for the next extraction. With the sample operation, there is a blank in the determination, and the following levels of extraction are blank. The mass fraction of As is determined by the above method.

2. Carbonate and Fe-Mn oxide state (P2)

Add 2. 1 residue into 20ml h3cooh(φb = 15%, containing 0.04molNHOH HCl), add a small funnel into a centrifugal nozzle, and extract 1h in a 90℃ water bath. The following operations are the same as 2. 1.

3. Binding state of humic acid and fulvic acid (P3-FZ, P3- Florida)

Add 10mLNaOH solution (containing 0. 1mol sodium pyrophosphate) to the residue obtained in 2.2, and extract 1 hour in a water bath. Transfer the extract to a beaker of 100mL, and repeat the above operations until the color of the extract turns pale. Adjust the extract in the beaker to pH = 1: 1HCl, heat it to precipitate humic acid, and leave it overnight. Filtering the next day, the precipitate on the filter paper is humic acid. The filter paper and sediment were ashed in a porcelain crucible, and then burned to black in a muffle furnace at 550℃. Dissolve it in a 25mL volumetric flask with HCl solution, and the constant volume is humic acid bound state (P3-FZ). The filtrate is fulvic acid, which is evaporated in a water bath and then transferred to a muffle furnace and burned to black at 550℃. Dissolve it in a 25mL volumetric flask with HCl solution, and the constant volume is fulvic acid bound state (P3-FL).

4. Other Organic Binding States (P4)

The residue in the centrifugal tube obtained in 2.3 was evaporated to dryness in a water bath, and then the low-temperature ashing experiment was carried out. Low-temperature ashing is carried out in plasma below 100℃ for 48 hours to release elements bound to organic matter. After low-temperature ashing, put the ash after low-temperature ashing back into the original centrifuge tube, add 5mLHNO3 and 10mL concentrated H2O2, extract 1h in water bath (90℃), repeat once, combine the clear liquid into 50mL volumetric flask, and fix the volume to other organic binding states (P4).

5. Enter the mineral lattice or be in a single mineral state (P5)

Evaporating the residue obtained in 2.4 in a water bath, then transferring it to a 30mL PTFE crucible, adding HF and HClO4, placing the crucible on a low-temperature electric heating plate, heating until the residue is completely dissolved, transferring it to a 50mL volumetric flask filled with HCl solution, and shaking it to the mineral lattice state (P5).

Three. Results and discussion

The experimental results are shown in Table 2. It can be seen from Table 2 that the sum of arsenic extracted step by step is equal to or close to the total amount of arsenic in the sample, which indicates that it is feasible and effective to study the occurrence state of trace or trace arsenic in coal and coal-fired products by step-by-step chemical extraction, and provides an effective experimental method for studying the occurrence state of trace elements in coal and coal-fired products.

Experimental results of stepwise chemical extraction of arsenic occurrence

Note: The upper data of each column in the table is the extracted absolute mass (μg), and the lower data is the percentage of this mass to the total mass.

The experiment shows that the low-rank coal samples in front of the furnace are slightly oxidized due to stacking in the open air, resulting in secondary humic acid and fulvic acid, so some secondary humic acid or fulvic acid are extracted, while others are not. The experiment shows that 36.7 1% arsenic in C combines with fulvic acid, but not with humic acid, which is related to the fact that fulvic acid molecules contain more functional groups than humic acid molecules [2]. In addition, arsenic in C mainly enters mineral crystal lattice (56.99%) and organic bound state (36.7 1%). Arsenic mainly enters HA, FA and BA in the form of glass silicate phase or mullite mineral lattice (in the form of AsO3-4), accounting for 86.25% in HA, 60.78% in BA and 47.22% in FA. Secondly, it is combined with carbonate and iron-manganese oxide, accounting for 34.72% in FA, 265,438+0.57% in BA and 8.44% in HA. Thirdly, it is water-soluble and exchangeable (soluble salt of AsO3-4), accounting for 65,438+09.44% in FA, 65,438+05.69% in BA and 5.00% in HA.

Obviously, the occurrence state of arsenic in FA, BA and HA is not exactly the same, which may be the result of different combustion modes. Arsenic in FA, BA and HA mainly enters the glass silicate phase or mullite mineral lattice, because their phase composition is mainly glass silicate phase or mullite. FA and BA in power plants contain 15% ~ 20% of water-soluble and exchangeable arsenic, which is easy to enter the water environment, thus causing potential pollution to the water environment.

Four. Concluding remarks

Through the above analysis, we can get the following understanding:

(1) The arsenic content in FA of Shentou Power Plant is the highest, followed by HA and BA.

(2) FA and BA in Shentou Power Plant are mainly composed of mullite crystal and glassy aluminosilicate phase, and HA also contains more anachrosite and sepiolite; The residual carbon content of FA and BA is very small, while that of HA is more.

(3) When the content of arsenic in the coal in front of the furnace is low and the content of clay minerals is high, arsenic mainly enters the lattice of clay minerals; The secondary fulvic acid produced by open-pit stacking of low-rank coal can combine some arsenic.

(4) Arsenic in FA and BA of Shentou Power Plant mainly enters the glass silicate phase or mullite mineral lattice, but there is still 15% ~ 20% arsenic in water-soluble state and exchangeable state, so we should pay attention to the potential harm of arsenic in FA and BA of power plant to water environment.

Take the exam and contribute.

[1] Belkin H E, Zheng B S, Zhou D X, et al. Preliminary research results on geochemistry and mineralogy of arsenic in coal from endemic arsenism areas in Guizhou Province. Proceedings of the 14th International Pittsburgh Coal Conference. Pittsburgh: Pittsburgh Coal Conference (CDROM ISBN1-890977-14-4). 1997.

Zhao Fenghua. Experimental study on distribution and occurrence mechanism of harmful trace elements in coal and leaching of coal-fired products. [doctoral thesis]. Beijing: Beijing Campus of China University of Mining and Technology, 1997.

Phase study of arsenic in coal-fired residue

Zhao Fenghua 1 Ren Deyi 1Xu Dewei 1 Ying Jinshuang 2 Li Yanan 2 Wang Xiuqing 2

(1.CUMT Department of Resource Development Engineering, Beijing100083;

2. Institute of Geology, Ministry of Nuclear Industry, Beijing 100029)

The distribution of arsenic in coal (C), laboratory high temperature ash (HA), fly ash (FA) and bottom ash (BA) was studied. The phase compositions of C, HA, FA and BA were analyzed by optical microscope and X-ray diffraction. The occurrence of arsenic in these samples was studied by continuous chemical extraction. The results show that the arsenic content in FA is the highest and that in BA is the lowest. More than 50% of arsenic in coal-burning residue exists in glassy aluminosilicate or mullite phase, 20% ~ 30% in carbonate and iron-manganese oxide phase, and 15% ~ 20% in water-soluble state and ion-exchange state. Water-soluble and ion-exchange arsenic is easily soluble in water, so coal residue is a potential pollution source.

Keywords coal residue, arsenic, occurrence state, continuous chemical extraction

(This article was co-authored by Zhao Fenghua, Xu Dewei, Yin, and Wang Xiuqin, originally published in Journal of China University of Mining and Technology, Volume 28, No.4, 1999).