Oxalic acid and UV enhanced arsenic leaching from coal in flue gas desulfurization by coal slurry

doi:10.11949/0438-1157.20230513

Abstract

Abstract:

Currently, coal is still the main energy source in our country. During coal combustion, arsenic will be discharged into the atmosphere with flue gas. Arsenic is highly toxic and harmful to the environment and human health. Therefore, it is of great practical significance to study the arsenic removal technology from coal. Using N₂, O₂ and SO₂ to simulate flue gas, the effects of main factors on arsenic leaching from coal, As(Ⅲ) oxidation and SO₂ removal were studied in a 1 L photoreactor. The process mechanism was analyzed, and the rate control step of the leaching process was determined. The results showed that both ultraviolet(UV) irradiation and oxalic acid could effectively increase the extraction ratio of arsenic. At 180 min, under pH 2.5 and dark conditions, adding 1.0 mmol/L oxalic acid could increase the arsenic extraction ratio by 32.9%. Under UV irradiation, the arsenic extraction ratio could be further increased by 21.3%, and the ratio of As(Ⅲ) in the leaching solution to the total arsenic was less than 13%, and the SO₂ removal efficiency could also be improved. Low pH was beneficial for arsenic leaching from coal, but unfavorable for SO₂ removal. Both arsenic extraction ratio and SO₂ removal efficiency increased significantly with the increase of temperature. The apparent activation energy for process of arsenic leaching from coal was calculated to be 4.9 kJ/mol, indicating that the arsenic leaching process was controlled by the diffusion process in the reacted solid layer. The free radical quenching experiments showed that sulfate radical and hydroxyl radical were the main free radicals in the oxalic acid-enhanced coal arsenic leaching system under UV irradiation. ATR-FTIR and XPS characterization results further confirmed the measured results of the leaching solution.

Key words: ultraviolet light, oxalic acid, arsenic, free radicals, sulfur dioxide, kinetic model, oxidation

摘要：

以N₂、O₂、SO₂三种气体模拟烟气，在1 L的光反应器中，研究了主要因素对煤砷浸出、As(Ⅲ) 氧化以及SO₂脱除的影响规律，分析了过程机理，确定了浸出过程速率控制步骤。结果表明，紫外线照射和草酸均能有效提高砷浸出率，180 min时，在遮光和pH为2.5的条件下，加入1.0 mmol/L草酸能使砷浸出率提高32.9%，在紫外线照射下，可使砷浸出率再提高21.3%，且浸出液中的As(Ⅲ) 占总砷之比小于13%，还能提高SO₂脱除率。低pH有利于煤中砷的浸出，但对SO₂的脱除不利。煤砷浸出率和SO₂脱除率均随温度的提高而显著增大。动力学分析计算得出煤砷浸出过程的表观活化能为4.9 kJ/mol，说明砷浸出过程受已反应固体层内的扩散过程控制。自由基猝灭实验表明，硫酸根自由基和羟基自由基是紫外线照射下草酸强化煤砷浸出体系中的主要自由基。ATR-FTIR与XPS表征结果进一步证实了浸出液的测定结果。

关键词: 紫外线, 草酸, 砷, 自由基, 二氧化硫, 动力学模型, 氧化

CLC Number:

X 592

Jintong LI, Shun QIU, Wenshou SUN. Oxalic acid and UV enhanced arsenic leaching from coal in flue gas desulfurization by coal slurry[J]. CIESC Journal, 2023, 74(8): 3522-3532.

李锦潼, 邱顺, 孙文寿. 煤浆法烟气脱硫中草酸和紫外线强化煤砷浸出过程[J]. 化工学报, 2023, 74(8): 3522-3532.

Figures/Tables 13

Fig.1 Schematic diagram of the experimental apparatus1—gas cylinder; 2—switch valve; 3—pressure reducing valve; 4—pressure maintaining valve; 5—rotor flow meter; 6—gas mixing bottle; 7—soap film flow meter; 8—inclined tube manometer; 9—agitator; 10—ultraviolet lamp; 11—thermometer; 12—reactor; 13—heating jacket; 14—water cooler; 15—electronic soap film flowmeter; 16—absorption vessel

Fig.2 Effect of UV irradiation and oxalic acid concentration on extraction ratio of iron (a) and on extraction ratio of arsenic from coal (b)

Fig.3 Effects of UV irradiation and oxalic acid concentration on As(Ⅲ)/As(total) ratio (a) and on flue gas desulfurization efficiency (b)

Fig.4 Effect of pH on extraction ratio of iron (a) and on extraction ratio of arsenic from coal (b)

Fig.5 Effects of pH on flue gas desulfurization efficiency

Fig.6 Effect of temperature on extraction ratio of iron (a) and on extraction ratio of arsenic from coal (b)

Fig.7 Variation of 1-(1-F2)1/3 with time (a) and variation of 1-3(1-F2)2/3+2(1-F2)with time (b) during 1.0 mmol/L oxalic acid enhanced process under UV irradiation

Fig.8 Plots of lnkversus 1/T for arsenic leaching process

Fig.9 Effect of temperature on flue gas desulfurization efficiency

Fig.10 Effect of radical quenchers on As(Ⅲ)/As(total) ratio

Fig.11 ATR-FTIR spectra of coal residue and raw coal

Fig.12 As 3d5/2, S 2p3/2 and Fe 2p3/2 XPS spectra of raw coal and coal residue

Table 1 XPS parameters of raw coal and coal residue

煤样	组分	结合能/ eV	半峰宽/ eV	积分面积	面积占比/%
原煤样	黄铁矿铁	707.96	2.00	1016.03	34.16
	Fe²⁺	710.70	2.00	1269.06	42.66
	Fe³⁺	712.78	2.00	689.56	23.18
浸出后煤样	黄铁矿铁	707.01	2.00	219.83	20.70
	Fe²⁺	710.82	2.00	446.60	42.06
	Fe³⁺	713.07	2.00	395.30	37.24
原煤样	As(Ⅲ)	44.04	1.00	87.44	70.94
原煤样	As(Ⅴ)	45.63	1.00	35.82	29.06
浸出后煤样	As(Ⅲ)	44.21	1.00	17.57	26.37
浸出后煤样	As(Ⅴ)	45.72	1.00	49.07	73.63
原煤样	黄铁矿硫	162.97	1.70	309.19	30.59
	有机硫	165.79	1.70	149.74	14.82
	硫酸盐硫	168.29	1.70	551.68	54.59
浸出后煤样	黄铁矿硫	162.22	1.70	57.85	7.89
	有机硫	164.80	1.70	62.66	8.54
	硫酸盐硫	168.73	1.70	612.92	83.57

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