Detoxification of hexavalent chromium compounds in incineration fly ash by carbothermal reduction

doi:10.11949/0438-1157.20240058

Abstract

Abstract:

Carbothermal reduction is an effective means of reducing and detoxifying hexavalent chromium compounds in waste incineration fly ash, but carbothermal reduction conditions and changes in the composition of the fly ash can affect the speciation transformation of chromium. Bamboo charcoal (BC) and powered active carbon (PAC) were used to react with Na₂CrO₄ to explore the process of carbothermal reduction on the morphological transformation of $C r (Ⅵ)$ , and the effects of four oxides on the morphological transformation of $C r (Ⅵ)$ in the carbothermal reaction were analyzed. The experimental results showed that Na₂CrO₄ was reduced to NaCrO₂ below 1000℃ and converted to Cr₃C₂ above 1000℃. In the presence of oxides, when the temperature was higher than 1000℃, Fe₂O₃ reacted with the chromium reduced by BC to form FeCr₂O₄. When the CaO content was high, it was converted into CaCr₂O₄, if an excessive amount of BC was added, it was still dominated by Cr₃C₂. When trace amounts of BC were added to calcium-sprayed fly ash, the reduced chromium reacted with CaO to form CaCr₂O₄; when excessive amounts of BC were added, Cr(Ⅵ) was reduced to form Cr₃C₂.

Key words: Cr(Ⅵ) reduction, carbothermal reduction, oxides, fly ash, thermodynamics

摘要：

碳热还原是垃圾焚烧飞灰中六价铬化合物还原脱毒的有效手段，但碳热还原条件和飞灰的组分变化会影响铬的形态转化。使用竹炭（bamboo charcoal，BC）和粉末活性炭（powered active carbon，PAC）与Na₂CrO₄反应探索碳热还原中Cr(Ⅵ) 形态转化的过程，并分析碳热反应中4种氧化物对Cr(Ⅵ)形态转化的影响。实验结果表明，1000℃以下Na₂CrO₄被还原为NaCrO₂，高于1000℃则转化为Cr₃C₂。氧化物存在时，高于1000℃后Fe₂O₃与被BC还原的铬反应生成FeCr₂O₄，在CaO含量高时转化为CaCr₂O₄；若添加过量BC则仍以Cr₃C₂为主。在喷钙的飞灰中，CaO与被还原的Cr反应生成CaCr₂O₄；若添加过量的BC，Cr(Ⅵ)则被还原为Cr₃C₂。

关键词: Cr(Ⅵ) 还原, 碳热还原, 氧化物, 飞灰, 热力学

CLC Number:

X 705

Guodong ZHAO, Zhuo XIONG, Yongchun ZHAO, Junying ZHANG. Detoxification of hexavalent chromium compounds in incineration fly ash by carbothermal reduction[J]. CIESC Journal, 2024, 75(10): 3730-3741.

赵国栋, 熊卓, 赵永椿, 张军营. 焚烧飞灰中六价铬化合物碳热还原脱毒[J]. 化工学报, 2024, 75(10): 3730-3741.

Figures/Tables 21

Table 1 Percentage of four oxides in fly ash

飞灰	氧化物占比/%（质量分数）
飞灰	Al₂O₃	SiO₂	CaO	Fe₂O₃
NJ	0.77	2.24	54.55	0.3
XY	0.74	0.66	3.1	0.39
JN	0.936	1.33	22.59	0.689

Table 2 Chemical compositions and industrial analysis of BC and PAC

样品	化学成分/%（质量分数）						工业分析/%（质量分数）
样品	SiO₂	MgO	CaO	K₂O	Fe₂O₃	Al₂O₃	M	A	V	FC
BC	12.685	4.032	6.986	47.779	4.461	—	6.28	4.73	14.3	74.69
PAC	23.183	6.722	6.936	2.771	5.993	2.668	4.13	1.23	17.75	76.89

Table 3 Specific surface area of BC and PAC

样品	比表面积/(m²/g)
BC	106.5
PAC	1384.4

Table 4 Thermodynamic calculation parameters

样品	SiO₂	Al₂O₃	Fe₂O₃	CaO	Na₂CrO₄	C
P1	1.5	1.5	1.5	1.5	6	2/8
P2	0.9	0.6	0.6	3.9	6	2/8
P3	0.6	0.18	0.12	5.1	6	2/8

Fig.1 Schematic diagram of the connection of the experimental setup

Fig.2 Gibbs free energy change of Eqs.(1)—(8)

Fig.3 Thermodynamic simulations of the reaction of Na2CrO4 with C

Fig.4 XRD patterns of BC/PAC+Na2CrO4 at different holding times

Fig.5 XRD patterns of BC/PAC+Na2CrO4 at different temperatures

Fig.6 XPS spectra of BC/PAC+Na2CrO4 at holding times of 0.5 h and 3 h

Fig.7 Reduction rate of Cr(Ⅵ) by BC/PAC+Na2CrO4 at 1000℃ with different holding time

Fig.8 SEM-EDS images of BC before and after reacting with Na2CrO4

Fig.9 Speciation transformation of chromium during heat treatment

Fig.10 Gibbs free energy change for Eqs.(10)—(19)

Fig.11 XRD patterns of simulated fly ash adding trace BC

Fig.12 XRD patterns of simulated fly ash with excessive BC addition

Fig.13 XRD patterns of trace BC added to three types of fly ash

Fig.14 Reduction rate of Cr(Ⅵ) in fly ash after adding trace BC

Fig.15 XRD patterns of excess BC added to three types of fly ash

Fig.16 Reduction rate of Cr(Ⅵ) in fly ash after addition of excess BC

Fig.17 Speciation transformation of chromium during carbothermal reduction

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