矿物加工煤的浮选英文文献及参考文献 第3页
2.1.1. Effect of the size and locking of coal particles on flotation
Many 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 reflectan毕业论文
http://www.751com.cn ce, 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 coal–pyrite or coal–ash 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 flotation
The 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 phenolic–OH, 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 solub本文来自辣文论文网原文请找QQ752018766le 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 α-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
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