Numerical simulation of electro?thermal coupling process for spent cathode carbon block from aluminum electrolysis cell
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摘要: 廢陰極炭塊是鋁電解槽大修時產生的一種危險固體廢棄物，對其進行安全處置和資源化利用的關鍵是深度分離其中的有價組分炭和氟化鹽。采用火法工藝對廢陰極炭塊進行處理，明確了氟化鹽的揮發溫度。基于氟化鹽的揮發析出性質，設計了高溫熱處理電阻爐，并對其傳熱特性、控溫規律以及氟化鹽有效揮發區域進行了三維數值解析。實驗確定氟化物的有效揮發溫度為≥1700 ℃，該溫度段下其揮發率可達93.1%以上。通過模擬不同供電模式下爐內溫度場的演變規律，得到：在12 V升溫24 h，9 V保溫12 h的供電條件下，升溫階段爐內最高溫度可達2250 ℃，氟化鹽理論揮發區域占比可達98%；采用逐級遞減的電壓供給制度可以保證1700 ℃以上溫度區域維持20 h，大幅度延長了有效熱處理時間，有利于廢陰極炭塊中炭與氟化鹽的深度分離。Abstract: Spent cathode carbon block (SCCB) is considered to be a kind of hazardous waste, because it contains a large amount of soluble fluoride salts and toxic cyanides. The life of an aluminum electrolytic cell is generally 5?8 years, and the SCCB would be produced during the overhaul of the cell. Currently, most SCCBs are piled in landfills or stored for disposal in China. The unreasonable disposal of SCCBs will cause serious pollution and damage to the ecological environment, and wastage of valuable carbon material and fluoride salts. The key to the safe disposal and resource utilization of SCCBs is to separate the carbon and fluoride salts deeply. In this study, SCCB was treated by the pyrometallurgical process, and the characteristics of volatilization temperature of fluoride salts were firstly experimentally determined. For a laboratory-scale self-designed high temperature resistance furnace, a three-dimensional model was built and numerical calculation was performed. The heat transfer characteristics, temperature control law and effective volatilization region of fluoride salts were analyzed in detail. The experimental results demonstrate that the effective volatilization temperature of fluoride is higher than 1700 ℃, and the volatilization rate is higher than 93.1%. By simulating the evolution of the temperature field in the furnace under different power supply modes, it is obtained that under the power supply condition of heating at 12 V for 24 h and holding 9 V for 12 h, the maximum temperature in the furnace during the heating phase can reach 2250 ℃, and the theoretical volatilization volume of fluoride salts can reach 98%. After optimization, a step-by-step decreasing mode of power supply can improve the efficiency of treating SCCBs. Moreover, the treating temperature can be maintained for 20 h at 1700 ℃, which is beneficial to the deep separation of carbon material and fluoride salts in SCCB.
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圖 1 高溫電阻爐結構示意圖及爐膛內部細節
Figure 1. Schematic of the high temperature resistance furnace and its interior detail
圖 2 高溫電阻爐沿x軸方向（x=1.605 m）的橫截面
Figure 2. Cross-section of the high temperature resistance furnace in x direction (x=1.605 m)
圖 3 不同處理溫度與廢陰極炭塊燒損及氟化鹽揮發的關系
Figure 3. Relationship between burning loss of SCCB and volatilization of fluoride salts at different temperatures
圖 4 不同加熱時間下中心截面溫度云圖（Z=0.89 m）.（a）4 h；（b）8 h；（c）12 h；（d）16 h；（e）24 h；（f）36 h
Figure 4. Contours of temperature distribution in the cross section (Z=0.89 m) of the furnace at different heating time: (a) 4 h; (b) 8 h; (c) 12 h; (d) 16 h; (e) 24 h; (f) 36 h
圖 5 監測線1溫度隨時間變化曲線
Figure 5. Variation in temperature of monitoring line with changing of heating time
圖 6 沿監測線方向五層耐火保溫層隨時間溫度分布（X=1.605 m）.（a）24 h；（b）28 h；（c）32 h；（d）36 h
Figure 6. Variations in temperature of refractory insulation along monitoring line at different heating time (X=1.605 m): (a) 24 h; (b) 28 h; (c) 32 h; (d) 36 h
圖 8 監測點溫度隨時間變化
Figure 8. Variations in temperature with heating time at monitoring point
表 1 爐型結構尺寸
Table 1. Structure size of the high temperature resistance furnace
mm Parameters Numbers Length 3210 Height 1680 Width 1780 Thickness of carbon brick 230 Thickness of insulating filler 65 Thickness of high alumina brick 230 Thickness of insulating brick 230 Thickness of rock wool 50 Dimensions of electrode 300×300×805 Dimensions of furnace hearth 500×500×1600 Dimensions of furnace core 50×50×1600 
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表 2 熱物性參數
Table 2. Thermophysical properties used in this study
Parameters Density /（kg?m?3） Specific heat /（J?kg?1?K?1） Thermal conductivity /（W?m?1?K?1） Conductivity /（S?m?1） Carbon block 1500 800 10 1×10?10 Insulating filler 450 1465 0.65 1×10?10 High alumina brick 1500 1100 0.786 1×10?10 Insulating brick 1000 1200 0.3 1×10?10 Rock wool 135 900 0.06 1×10?10 Graphite felt 1400 900 0.01 1×10?10 SCCB 850 840 6 2.127×103 Electrode 1600 700 12.4 1.13×10?5
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表 3 廢陰極炭塊的元素組成與含量
Table 3. Composition and the content of various elements in SCCB
% C F Na Al Ca Fe Else 59.2 16.73 13.62 7.87 1.43 0.94 0.21
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