Preparation of metallic arsenic from calcium arsenate by carbon thermal roasting reduction
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摘要: 致力于碳熱焙燒還原砷酸鈣制備具有商業價值的金屬單質砷，為推進砷危廢物無害化處理向砷資源化回收利用前進展開科學研究。其中熱重分析表明，砷酸鈣與碳粉混合熱解的質量損失分為3個階段，階段1和階段2為失水過程，階段3為碳還原砷酸鈣生成CaO和砷蒸氣過程。且研究發現，可以利用相邊界反應動力學模型解釋階段3反應機制。而單因素條件實驗結果表明：在溫度1000 ℃、碳配入系數1.4、恒溫時長60 min條件下砷揮發率高達99.94%。X射線衍射儀(XRD)、掃描電鏡能譜儀(SEM?EDS)對反應體系中有關產物表征表明，較優條件下產品砷主要為片狀金屬砷和粉末非晶體砷，焙燒殘渣為CaO。Abstract: Given the widespread application of the lime precipitation process for arsenic removal in the smelting of arsenic-containing minerals, the resourcefulness of calcium arsenate use has received increasing attention. In general, more types of arsenate have different high-temperature characteristics, and the slag type is complicated under mixed reduction roasting and difficult to recover. Additionally, arsenate in the form of calcium arsenate is a more common and inexpensive product in the metallurgical process. Because whether it is arsenic-containing wastewater, arsenic slag, arsenate, and so on, the material can be separated from the system by inexpensive lime precipitation or calcification transformation in simple metallurgical equipment to generate calcium arsenate. Therefore, this paper was devoted to preparing commercially valuable metallic monomers of arsenic by carbon thermal roasting reduction of calcium arsenate and to starting scientific research to advance the harmless treatment of arsenic hazardous waste to arsenic resource recovery and use. Among them, thermogravimetric analysis shows that the mass loss of calcium arsenate mixed with carbon powder pyrolysis is divided into 3 stages: stages 1 and 2 are water loss processes, and stage 3 involves the carbon reduction of calcium arsenate to generate CaO and arsenic vapor. It is found that the stage III reaction mechanism could be explained using the phase boundary reaction kinetic model. The experimental results of single-factor conditions show that the arsenic volatilization rate reaches 99.94% at a constant temperature of 1000 °C for 60 min and a carbon allotment factor of 1.4. The characterization of the relevant products in the reaction system by X-ray diffractometer (XRD) and scanning electron microscope energy spectrometer (SEM?EDS) show that the arsenic product is mainly flaked metallic arsenic and amorphous powdered arsenicunder better conditions, and the roasted residue is CaO.
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圖 1 實驗合成砷酸鈣X射線衍射圖譜
Figure 1. XRD pattern of experimentally synthesized calcium arsenate
圖 3 砷酸鈣與碳粉混合熱解特性。（a）砷酸鈣與碳粉混合熱解的熱重?熱重微商曲線；(b)殘渣X射線衍射圖（Ⅰ原物料；Ⅱ第1質量損失階段，320 ℃；Ⅲ第2質量損失階段，700 ℃；Ⅳ第3質量損失階段，1000 ℃)
Figure 3. Pyrolysis characteristics of calcium arsenate mixed with carbon powder: (a) TG?DTG curves of the pyrolysis of calcium arsenate mixed with carbon powder; (b) XRD plots of the residue (Ⅰ raw material; Ⅱfirst mass loss stage, 320 ℃; Ⅲ second mass loss stage, 700 ℃; Ⅳ third mass loss stage, 1000 ℃)
圖 4 第3個質量損失階段動力學模型擬合結果。（a）三維擴散；（b）相邊界反應；（c）相邊界反應模型結果驗算
Figure 4. Results of the third weightless phase kinetic model fitting: (a) 3D diffusion; (b) phase boundary reaction; (c) phase boundary reaction model result verification
圖 5 焙燒溫度對砷揮發影響圖
Figure 5. Diagram showing the effect of roasting temperature on arsenic volatilization
圖 6 碳配入系數對砷揮發率的影響圖
Figure 6. Plot of the effect of the carbon incorporation factor on the volatility of arsenic
圖 8 產品砷掃描電鏡和X射線衍射圖譜。（a~c）不同放大倍數金屬片砷；（d）粗糙反面金屬片砷；（e）粉末不定型砷；（f）光澤正面金屬片砷；(g) 產品砷X射線衍射圖
Figure 8. Arsenic product SEM, XRD patterns: (a?c) metal flake arsenic with different magnification; (d) rough back side of the metal flake arsenic; (e) powdered unshaped arsenic; (f) glossy front side of the metal flake arsenic; (g) XRD of arsenic product
表 1 合成砷酸鈣的主要成分（質量分數）
Table 1. Main components of synthetic calcium arsenate
% Ca As O Other 19.81 31.43 38.41 10.35 
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表 2 動力學機理函數
Table 2. Kinetic mechanism function
Function number Function name Mechanism Points form G(α) 1 Ginstling?Brounshteine
equationThree-dimensional diffusion $ 1-\dfrac{2}{3}\alpha -(1-\alpha {)}^{\frac{2}{3}} $ 2 Shrink globular Phase boundary reaction $ 1-(1-\alpha {)}^{\frac{1}{3}} $
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表 3 金屬砷的掃描電鏡能譜分析結果（質量分數）
Table 3. Results of EDS analysis of arsenic metal
% number O As 1 1.16 98.84 2 0.85 99.15 3 2.44 97.55 4 23.35 76.65 5 0.72 99.28 6 0.92 99.08 7 1.63 98.37 8 1.14 98.86 10 0.48 99.52 11 1.32 98.68 12 0.38 99.62
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