Fig. 1. A schematic representation of various sub-processes in coal flotation.
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Fig. 2. Process variables in flotation.
2.1. Coal
Coal is defined as a heterogeneous combustible sedimentary rock formed from plant remains in various stages of preservation by processes, which involved the compaction of the material buried in basins, initially of moderate depth [IHCP, 1963] with an ash content of less than 50% [ECE-UN Document, 1991]. Some other classifications of coals are also given in the literatu毕业论文http://www.751com.cn re [Lemos de Sousa et al., 1992]. Three main parameters are considered in classifying coals, namely type, which refers to the petrographic composition, rank, which refers to the level of coalification, and grade, which refers to the amount of inorganic matter content.
Microscopically, coal has a cross-linked network structure of polymeric macromolecules as indicated by insolubility and swelling of co本文来自辣文论文网原文请找QQ752018766al in an organic solvent [Iino, 2000 and Marzec, 2002]. Macroscopically, it is made up of finely mixed discrete organic entities known as macerals, which fall into three main groups with different physical and chemical properties: vitrinite, exinite (liptinite) and inertinite [Jimenez et al., 1998]. The bands of these macerals, which can be distinguished by naked eye, are called lithotypes. The main lithotypes are vitrain (vitrinite rich), fusain (inertinite rich), clarain (vitrinite and exinite rich) and durain (inertinite and exinite-rich). Vitrinite is the major maceral group in humic coals and contributes significantly to their behavior in industrial processes ranging from flotation to combustion to coking.
Although differences in wetting behavior of various macerals is well recognized, the quantification of wetting behavior of a given coal sample remains a formidable task. For example, vitrain and fusain differ in elemental composition, oxygen-containing functional groups, hydrophobicity and electrokinetic behavior [Shu et al., 2002], therefore, display different degrees of floatability [Burdon, 1962, Sun and Cohen, 1969, Sarkar et al., 1984, Arnold and Aplan, 1989, Holuszko and Laskowski, 1996, Agus, 1997 and Zheng, 1997]. [Aplan and Arnold, 1986] who studied various US coals using contact angle to quantify the hydrophobicity of coal macerals found that the order of hydrophobicity from the highest to the lowest was as follows: liptinite>vitrinite>inertinite with typical contact angles ranging from 90° to 130°, 60° to 70° and 25° to 40°, respectively. Nearly the same ordering of lithotypes and macerals for floatability was observed in conventional and column flotation tests [Sun and Cohen, 1969, Brown, 1979, Arnold and Aplan, 1988, Kizgut, 1996, Attia, 1999, Barnwall, 2000 and Hower et al., 2000]. Hydrophobicity of coal depends strongly on its rank as was shown by the contact angle measurements [Gutierrez-Rodriguez et al., 1984]. The captive bubble contact angle varied from 0° for the lignites to 55° for the bituminous coals, decreasing down to around 30° with further increase in rank to anthracite.
It should be noted however that a given coal would display a distribution of contact angles owing to its heterogeneous structure. In a recent study, [Polat and Chander, 1999] showed, using a modified contact angle measurement method, that the surface of a hvA bituminous coal displayed a distribution of captive bubble contact angles ranging from 40° to 58° (Fig. 3a–c). The same figure also contains the case where the contact angles are measured in the presence of a PEO/PPO block copolymer. It can be observed that adsorption of a promoter not only changes the hydrophobicity of the surface, but it also seems to make the surface more uniform with respect to its wetting character (Fig. 3d).
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Fig. 3. The captive bubble contact angles on a hvA bituminous coal by the modified contact angle method [Polat and Chander, 1999]. The coal sample was from Pittsburgh seam. Data in each figure correspond to a different set of contact angle measurements from 40 bubbles. Graphs a, b and c are repeat test to demonstrate the reproducibility of the method. Graph d is under identical conditions except for the presence of the block copolymer L-64.
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