High-temperature volatility characteristics and pyrolysis mechanism of common sodium salts

doi:10.11949/0438-1157.20200061

Abstract

Abstract:

The common sodium salts (sodium chloride, sodium carbonate, sodium sulphate, sodium formate, sodium acetate, sodium oxalate) in organic wastewater were selected, and their high-temperature volatility characteristics were examined by thermogravimetric analyzer at 25—1400℃. Based on the principle of Gibbs free energy minimum, the volatility characteristics of NaCl, Na₂CO₃ and Na₂SO₄ were calculated thermodynamically. The results showed that in the N₂ atmosphere, NaCl began to be released in the form of NaCl(g) and Na₂Cl₂(g) after reaching the melting point. Na₂SO₄ decomposed to form Na₂O at high temperature, and it continued to decompose to form elemental sodium when the temperature raised. The organic carboxylic acid sodium salt was pyrolyzed to Na₂CO₃ before 600℃, then it decomposed to form Na₂O when the temperature went up, and finally released as sodium vapor. In the air atmosphere, due to the presence of O₂, the decomposition reaction of Na₂O is suppressed, resulting in the decomposition rate of Na₂CO₃ is less than that of N₂ atmosphere.

Key words: sodium salt, high temperature volatility characteristics, thermogravimetric analysis, pyrolysis, thermodynamic, corrosion

摘要：

选取有机废液中常见的钠盐(氯化钠、碳酸钠、硫酸钠、甲酸钠、乙酸钠、草酸钠)，通过热重分析仪，在25~1400℃考察其高温挥发特性。基于Gibbs自由能最小原理，对NaCl、Na₂CO₃及Na₂SO₄的挥发特性进行热力学计算。结果表明，在N₂气氛中，NaCl在达到熔点后以气态NaCl及气态Na₂Cl₂的形式释放，Na₂SO₄升温至885℃分解生成Na₂O，同时Na₂O分解，并以Na单质形式释放。而有机羧酸钠盐在600℃之前均热解为Na₂CO₃，继续升高温度则分解为Na₂O，同样最后以气态Na单质的形式释放。在空气气氛中，由于O₂的存在，抑制了Na₂O的分解反应，致使Na₂CO₃分解速率小于N₂气氛。

关键词: 钠盐, 高温挥发特性, 热重分析, 热解, 热力学, 腐蚀

CLC Number:

TQ 131.1

Jian LI, Ge PU, Jiashan CHEN, Qiwen LIU. High-temperature volatility characteristics and pyrolysis mechanism of common sodium salts[J]. CIESC Journal, 2020, 71(8): 3452-3459.

李剑, 蒲舸, 陈家善, 刘啟文. 常见钠盐的高温挥发特性及热解机理[J]. 化工学报, 2020, 71(8): 3452-3459.

Figures/Tables 7

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Compound	Fusion point/℃	Degradation stage
Compound	Fusion point/℃	Temperature interval/℃	Pyrolysis products	TG/%
NaCl	803	808—1153	NaCl(g),Na₂Cl₂(g)	100.0
Na₂CO₃	851	853—1280	Na₂O,Na(g)	69.9
Na₂CO₃	851	1280—1400	Na(g)	4.4
Na₂SO₄	885	1105—1400	Na₂O,Na(g)	21.5
HCOONa	226	372—440	Na₂C₂O₄，Na₂CO₃	12
		553—573	Na₂CO₃	7.5
		855—1260	Na₂O，Na(g)	31.4
		1260—1400	Na(g)	1.7
CH₃COONa	308	416—530	Na₂CO₃	34.2
		858—1232	Na₂O，Na(g)	35.4
		1232—1400	Na(g)	4.4
NaOOC—COONa	—	540—576	Na₂CO₃	17.6
		864—1190	Na₂O，Na(g)	45.1
		1190—1400	Na(g)	5.9

Compound	Fusion point/℃	Degradation stage
Compound	Fusion point/℃	Temperature interval/℃	Pyrolysis products	TG/%
NaCl	803	808—1153	NaCl(g),Na₂Cl₂(g)	100.0
Na₂CO₃	851	853—1280	Na₂O,Na(g)	69.9
Na₂CO₃	851	1280—1400	Na(g)	4.4
Na₂SO₄	885	1105—1400	Na₂O,Na(g)	21.5
HCOONa	226	372—440	Na₂C₂O₄，Na₂CO₃	12
		553—573	Na₂CO₃	7.5
		855—1260	Na₂O，Na(g)	31.4
		1260—1400	Na(g)	1.7
CH₃COONa	308	416—530	Na₂CO₃	34.2
		858—1232	Na₂O，Na(g)	35.4
		1232—1400	Na(g)	4.4
NaOOC—COONa	—	540—576	Na₂CO₃	17.6
		864—1190	Na₂O，Na(g)	45.1
		1190—1400	Na(g)	5.9

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