Migration behavior of alkali metals in an iron ore sintering process with the substitution of biomass for coke breeze
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摘要: 通過揮發–冷凝實驗裝置進行小型燒結實驗，運用X射線熒光光譜（XRF）、掃描電鏡–能譜儀（SEM–EDS）及電感耦合等離子體發射光譜儀（ICP–OES）等分析檢測手段，結合Factsage熱力學模擬，對比研究了以木炭和焦粉為燃料，配加含鐵粉塵的鐵礦石燒結過程中，床層堿金屬隨煙氣揮發遷移的規律、燒結前后的堿金屬脫除率以及工藝措施對堿金屬脫除的影響。結果表明，K相對于Na更容易被脫除，揮發至煙氣中的堿金屬化合物主要是KCl，其次為NaCl。增加燃料配比促進了堿金屬元素的脫除；在燃料配比相同的條件下，木炭燒結的堿金屬脫除效果不及焦粉燒結。燒結過程中，排入廢氣中的堿金屬化合物被下部混合料層大量捕獲、吸附，下部床層內捕集的堿金屬氯化物促進了堿金屬的氯化脫除。添加CaCl2后，以木炭為燃料時K和Na的脫除率高于焦粉工況，且產物中K和Na的含量較低。配合氯化脫除工藝將生物質應用于鐵礦石燒結是燒結生產發展的可行方向。Abstract: Iron ore sintering is a process in which iron ore powder, flux, iron-bearing dust, solid fuel (such as coke powder), and return fines are mixed in a certain proportion, granulated, and then processed into agglomerates by high-temperature generated by solid-fuel combustion, which is an important process prior to blast furnace ironmaking. The iron ore sintering process is an important emitter of atmospheric particles in which alkali metal elements in a sinter bed contribute to the formation of fine particles during combustion, aggravating particulate emissions. Using biomass materials such as charcoal to replace coke in the sintering process can significantly alleviate the emission of both greenhouse gases and pollutants. However, owing to the high content of alkali metals in biomass and their poor combustion characteristics, alkali-metal-related problems inevitably arise. In this study, a small sintering experiment was conducted in a volatilization condensation test facility and analyses were performed based on data obtained by X-ray fluorescence spectroscopy, scanning electron microscopy energy dispersive spectrometer, and inductively coupled plasma-atomic emission spectrometry followed by thermodynamic simulation. The purpose of these analyses was to investigate the laws associated with alkali metal migration and enrichment, removal rate of alkali metal elements, and influence of technological measures on removal process in iron ore sintering using charcoal and coke as fuel with iron-bearing dust added. The results show that K is easier to remove than Na, and the alkali compounds volatilized into a flue gas mainly contain KCl with small amount of NaCl. With the same fuel mass fraction the removal rate of alkali metal in the sintering process using charcoal as fuel is less than that using coke. As the alkali metal compounds in the downstream flue gas migrate, they collide with the raw material particles because of the inertial effect. In addition, owing to the low temperature of the raw materials in the low bed, alkali metal compounds tend to condense and deposit on the particles’ surface. During the sintering process, a large number of alkali metal compounds discharged into the waste gas are trapped and absorbed by the low bed, and the alkali metal chloride accumulated in the low bed promotes the removal of chloride from the alkali metal. With the addition of CaCl2, the removal rate of K and Na when using charcoal as fuel is higher than that using coke. Accordingly, the content of K and Na in sintering products with charcoal as fuel is lower than that using coke. The use of biomass as fuel in iron ore sintering in combination with chlorine removal process is feasible and has good prospects.
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Key words:
- iron ore sintering /
- biomass /
- alkali metal /
- removal /
- enrichment characteristics /
- occurrence state
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圖 1 焦粉與木炭的微觀形態。（a）焦粉；（b）木炭
Figure 1. Micro morphology of coke and charcoal: (a) coke; (b) charcoal
圖 2 堿金屬揮發–冷凝試驗臺
Figure 2. Experimental system for evaporation and condensation of alkali metal vapors
1—Thermocouple; 2—Hand lift; 3—Electric heating wire; 4—Corundum porcelain boat; 5—High temperature pad; 6—Corundum crucible; 7—Electric furnace control cabinet; 8—Thermocouple; 9—Collecting substrate; 10—Flowmeter; 11—Air compressor; 12—Temperature displayer; 13—Condensation collection system
圖 3 燒結床火焰鋒面傳播的一維示意圖
Figure 3. One dimensional diagram of flame front propagation in sintering bed
圖 4 木炭質量分數對基片表面凝結顆粒的形態和摩爾組成的影響。（a）4%；（b）4.5%；（c）5%；（d）5.5%
Figure 4. Effect of charcoal mass fraction on micro-morphology and mole composition of deposits on probe surface: (a) 4%; (b) 4.5%; (c) 5%; (d) 5.5%
圖 5 基片表面凝結顆粒的EDS面掃描結果（木炭質量分數為5%，3200倍）
Figure 5. EDS mapping of deposits on probe surface (Mass fraction of charcoal is 5%, 3200 times)
圖 6 焦粉質量分數對基片表面凝結顆粒形態和摩爾組成的影響。（a）4%；（b）4.5%；（c）5%；（d）5.5%
Figure 6. Effect of coke mass fraction on micro-morphology and mole composition of deposits on probe surface: (a) 4%; (b) 4.5%; (c) 5%; (d) 5.5%
圖 8 燃料種類及質量分數對K和Na脫除行為的影響。（a）焦粉；（b）木炭
Figure 8. Effect of fuel type and mass fraction on removal behavior of K and Na: (a) coke; (b) charcoal
圖 9 冷凝溫度對基片表面凝結顆粒形態和摩爾組成的影響。（a）250 ℃；（b）300 ℃；（c）350 ℃；（d） 400 ℃
Figure 9. Effect of surface tempetature on micro-morphology and mole composition of deposits on probe surface: (a) 250 ℃; (b) 300 ℃; (c) 350 ℃; (d) 400 ℃
圖 10 摻入NaCl后基片表面顆粒形態和組成（2000倍）
Figure 10. SEM–EDS mapping of deposits on probe surface with NaCl added (2000 times)
圖 11 CaCl2添加劑對K和Na脫除行為的影響。（a）焦粉；（b）木炭
Figure 11. Effect of CaCl2 additive on removal behavior of K and Na: (a) coke; (b) charcoal
圖 12 堿金屬硅酸鹽和鋁硅酸鹽的氯化反應。（a）硅酸鹽；（b）鋁硅酸鹽
Figure 12. Chlorination reactions of alkali metal silicate and aluminosilicate: (a) silicate; (b) aluminosilicate
表 1 燒結原料的化學組成（質量分數）
Table 1. Chemical composition of raw materials (mass fraction)
% Raw material TFe CaO SiO2 Al2O3 MgO K2O Na2O Cl LOI Yandi 58.07 0.08 5.09 1.26 0.07 0.02 0.005 0.008 10.20 Iron-bearing dust 41.59 10.28 4.90 2.07 2.68 2.80 0.32 1.19 19.94 Limestone 0.36 53.40 2.23 0.98 0.48 0.09 0 0 42.73 Dolomite 0.35 31.45 2.26 0.58 19.45 0.08 0.03 0.02 45.50 Coke 1.07 0.62 6.11 4.32 0.04 0.08 0.01 0 86.85 Charcoal 0.20 3.02 0.14 0.06 0.43 0.19 0.13 0.01 94.50 Note: TFe is total Fe, LOI is loss on ignition. 
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表 2 焦粉和木炭的工業分析與元素分析（質量分數）
Table 2. Proximate analysis and ultimate analysis of coke and charcoal (mass fraction)
% Sample Proximate analysis Ultimate analysis M A V FC C H N S O Coke 0.34 20.42 5.20 74.04 85.48 0.31 1.13 0.62 2.67 Charcoal 8.94 4.36 15.53 71.17 77.00 2.39 0.47 0.21 6.63 Note: M is moisture, A is ash, V is volatile matter, FC is fixed carbon.
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表 3 實驗工況表
Table 3. Experimental conditions
Variable Mass fraction of fuel/% Mass fraction of iron-bearing dust/% Mass fraction of
additive/%Evaporation temperature/℃ Collection temperature/℃ Charcoal 4, 4.5, 5, 5.5, 6 5 — 1300 350 Coke 4, 4.5, 5, 5.5, 6 5 — 1300 350 Collection temperature 5 5 — 1300 250, 300, 350, 400 CaCl2 5 5 0.65, 1.30 1300 350 NaCl 5 5 0.6 1300 350
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