374.C 2.6Mta炼焦煤选煤厂初步设计(CAD图纸联系本人) 翻译.doc

科技大学英文翻译 题 目 英文翻译 院（系、部) _化学与化工学院_专业及班级 _矿物加工 _姓 名 _ _指 导 教 师 _ _日 期 _2010-3-20_Physical and chemical interactions in coal flotation AbstractCoal flotation is a complex process involving several phases (particles, oil droplets and air bubbles). These phases simultaneously interact with each other and with other species such as the molecules of a promoting reagent and dissolved ions in water. The physical and chemical interactions determine the outcome of the flotation process. Physical and chemical interactions between fine coal particles could lead to aggregation, especially for high rank coals. Non-selective particle aggregation could be said to be the main reason for the selectivity problems in coal flotation. It should be addressed by physical (conditioning) or chemical (promoters) pretreatment before or during flotation. Although the interactions between the oil droplets and coal particles are actually favored, stabilization of the oil droplets by small amounts of fine hydrophobic particles may lead to a decrease in selectivity and an increase in oil consumption. These problems could be remedied by use of promoters that modify the coal surface for suitable particleparticle, dropletparticle and particlebubble contact while emulsifying the oil droplets. The role of promoters may be different for different types of coals, however. They could be employed as modifiers to increase the hydrophobicity of low rank coals whereas their main role might be emulsification and aggregation control for high rank coals. In this paper, a detailed description of the various phases in coal flotation, their physical and chemical interactions with each other in the flotation pulp, the major parameters that affect these interactions and how these interactions, in turn, influence the flotation process are discussed.Author Keywords: coal; flotation; aggregation1. IntroductionConventional froth flotation for fine coal cleaning suffers mainly from two problems: (i) a lack of selectivity for fast floating high rank coals due to the flotation of middlings and entrainment of mineral fines in the froth, and (ii) low recoveries for heavily oxidized or low rank coals due to poor adhesion between bubbles and particles. These shortcomings can be addressed appreciably by selection of better process control and by use of multi-stage flotation circuits Olson and Aplan, 1984 and Arnold, 2000, which, in turn, requires a good understanding of the roles and behavior of various components in the flotation pulp and the mechanisms involved.In this paper, a detailed description of the various phases in the coal flotation pulp, their interactions with each other and how these interactions affect the flotation process are discussed.2. Disperse phases in coal flotationThere are three dispersed phases that constitute flotation pulp: coal particles, oil droplets and air bubbles. These phases interact in water as the medium through various sub-processes during flotation which are identified in Fig. 1. Several parameters affect these sub-processes and hence the outcome of the flotation process. These parameters are divided into four groups as illustrated in Fig. 2. These are: material, chemical, operational and equipment parameters. The parameters that might fluctuate and need adjustment on a regular basis (e.g. daily) are referred to as Level I parameters. Those that are set during the design stage or after a major renovation are referred to as Level II parameters. Some parameters are not controlled due to inherent material characteristics and practical limitations, and they are referred to as the Level III parameters. Some examples of these parameters are listed in Fig. 2. A detailed discussion of various phases and the effect of their interactions on the flotation process are presented below.Full-size image (21K)Fig. 1. A schematic representation of various sub-processes in coal flotation. View Within ArticleFull-size image (34K)Fig. 2. Process variables in flotation. View Within Article2.1. CoalCoal 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 literature 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 coal 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. 3ac). 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).Full-size image (14K)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. View Within Article2.1.1. Effect of the size and locking of coal particles on flotationMany studies have been conducted to determine the effect of particle size, shape and degree of particle-locking (liberation) on coal flotation. For example, Varbanov, 1984 concluded that the flotation rate depends strongly on particle size but not as much on particle shape. The particle size, where a maximum in the flotation rate and the final recovery is obtained, varies widely depending on the conditions of operation Robinson, 1960, Rastogi and Aplan, 1985, Polat et al., 1993, Polat et al., 1994a and Polat et al., 1994b. The flotation rate increases initially, reaches a maximum and decreases afterwards with increasing particle size. This is due to the combined effect of the collision, and attachment/detachment sub-processes, dominant in small and large sizes, respectively Al Taweel et al., 1986. Nevertheless, the exact relationship between the particle size and flotation rate is complex and not well understood, most probably due to the aggregation of fine particles in flotation Chander and Polat, 1995 and Chander et al., 1995. Hence, it is difficult to determine the effect of primary particle size on the rate of flotation of fine coal particles. The oily collector is introduced into an environment with many fine particles, some of which are strongly hydrophobic even for medium rank coals. Polat and Chander, 1994 observed that oil droplets aggregated strongly in the presence of fine hydrophobic particles, while hydrophilic particles enhanced dispersion by preventing the coalescence of droplets through a retardation of film thinning.The association between the organic and mineral matter in coal, which goes from merely physical association to true chemical bonding, is also important. Pusz et al., 1997 who studied the density fractions of coals using vitrinite reflectance, X-ray diffraction, FTIR and Mossbauer spectroscopy found . The most important impurity in coal is sulfur, which is present in the raw coal as organic, sulfatic or pyritic forms. Of these, pyritic sulfur is often the major form and, if reasonably well liberated, is the most readily removable. For successful removal of mineral matter from coal for better froth quality, these impurities must be liberated. In most cases, this could be achieved only at extremely fine sizes Olson and Aplan, 1984.Though the mineral particles decrease the floatability of the associated coal particles due to an increase in the particle density which leads to poor attachment efficiency and higher detachment rates, locked particles do possess a finite probability for flotation since a small fraction of hydrophobic surface is sufficient for attachment to air bubbles Lynch et al., 1981. Within a given size fraction, the particles of lower specific gravity (relatively pure coal particles) float much faster than the locked coalpyrite or coalash particles or liberated pyrite. The use of oil improves the flotation rate of particles of all sizes and specific gravities though the effect is more for the locked or mineral particles Olson and Aplan, 1987, Polat et al., 1993, Polat et al., 1994a, Polat et al., 1994b and Zhou et al., 1993.2.1.2. Effect of the oxidation of coal particles in flotationThe oxidation of coals starts with the physical adsorption of oxygen on the surface to form an oxy-complex. Then, chemical adsorption of oxygen takes place to form polar phenolicOH, carbonyls, phenols and peroxide type oxygenated moieties by the rupture of cyclic rings Schlyer and Wolf, 1981, Tekely et al., 1987, Ramesh and Somasundaran, 1989 and Somasundaran et al., 2000. These polar species leads to the formation of humic acids, which then degrade into soluble acids Fuerstenau et al., 1987. Adsorption of oxygen is exothermic and, besides the moieties formed on the coal surface, such reaction products as CO, CO2 and H2O may be released from the structure Itay et al., 1989. The most susceptible linkages to oxidation were found to be the -CH2 groups to polyaromatics using a variety of techniques such as FTIR, UV Fluorescence and DRIFT spectroscopy Calemma et al., 1988, Kochi, 1973, Kister et al., 1988 and Xiao et al., 1990. An interesting point on oxidation was revealed by Mitchell et al., 1996 who showed that blue-light irradiation was also a strong agent in oxidizing the vitrinite surfaces.It was shown using contact angle, film flotation and flotation tests that oxidation of coals lowers floatability and that lower rank coals were influenced more by oxidation Fuerstenau et al., 1983, Fuerstenau et al., 1987, Fuerstenau et al., 1994, Gutierrez-Rodriguez and Aplan, 1984 and Bolat et al., 1998. The reason for the decrease in floatability is due to the generation of polar phenolic and carboxylic groups, which are known to increase the wettability and increase the surface charge, both of which are known to be detrimental to flotation Wen, 1977. The effect could be substantial. Sarikaya, 1995 reported that upon oxidation the flotation yield dropped from an initial 95% down to 24% for a bituminous coal using alcohol type frother only.Small amounts of residual oxygen are sufficient to bring about oxidation Korobetskii et al., 1990. Natural oxidation mainly affects the external surfaces of coal, hence, for better flotation results the size reduction must be retarded as long as possible Fuerstenau et al., 1994. Polat et al., 1994a and Polat et al., 1994b demonstrated that upon weathering coal particles developed cracks whose extent was a function of coal rank. Low rank coal particles developed extensive cracks where as high rank coals did not seem to be affected physically. This suggests that for low rank coals oxidation might have its adverse effect at relatively larger particle sizes due to the development of cracks which help the transfer of oxygen into interior of the particles. Formation of cracks during oxidation can also result in the production of finer particles, which may be difficult to float.In determining the effect of oxidation on coal floatability, the behavior of the associated mineral matter, especially pyrite, should also be taken into account. Oxidation of pyrite leads to the generation of various soluble inorganics that can adsorb on the coal surface and modify its wettability while pyrite itself was reported to show improved hydrophobicity upon oxidation Tao et al., 1994.2.1.3. Effect of coal particlepromoter interactions on flotationPromoters act as surface modifiers and may alter hydrophobicity depending on the rank