Fine Size Limit

Fine Size Limit (See also Single-Particle Fracture above.)... 

Fine Size Limit (See also Single-Particle Fracture above.) It has long been thought that a limiting size is attainable, and, in fact, it is almost a logical necessity that grinding cannot continue down to the molecular level. Nonetheless, recent results suggest that stirred... 

The fluidized-bed system (Fig. 3) uses finely sized coal particles and the bed exhibits Hquid-like characteristics when a gas flows upward through the bed. Gas flowing through the coal produces turbulent lifting and separation of particles and the result is an expanded bed having greater coal surface area to promote the chemical reaction. These systems, however, have only a limited abiUty to handle caking coals (see Fluidization). 

Owing to the feed pan distance usually maintained on wet dmm cobbers, the wear encountered with coarser particles, and the feed velocities requited to move coarse particles, the recommended upper size limits for cobber separators is 10 mm in diameter. Individual ore characteristics requited to obtain Hberation determine the feed size in rougher and finisher feeds. For finishers, where all the nonmagnetics must be overflowed, a sufficiently fine size to accomplish the overflow must be obtained. Typical feed sizes for cobbers are from —841 fim (—20 mesh) to 10 mm for rougher, —420 fim (—35 mesh) to —297 fim (—48 mesh) and for finishers, —63 fim (—270 mesh) to —44 fim (—325 mesh). The magnetic content of the iton ores to be concentrated varies over fairly wide limits. Ores as low as 10 wt % Fe have been successfully treated, as have ores having up to 50 wt % or more Hon. 

There are size limitations. If large areas are surfaced by automatic welding, only tough alloys can be applied without cracking. The cracks tend to stop at the tougher base, but there is no simple answer to the question about the erosion resistance of a surface containing fine cracks. 

The image produced may be viewed either directly by the eye or projected on to a screen. The latter method is usually the more convenient and involves less eyestrain it is less satisfactory than direct observation for fine sizes near the limit of optical resolution. Sizing is commonly achieved by comparison of the particle images with a scale or graticule. Since the process is tedious, sizing is sometimes performed automatically. The smallest resolvable particle size is a function of the wavelength of the light used and varies between 0.5 and 1 p... 

For flotation to be most effective, reagents such as oil (collector) and surfactant (frother) must be added to the aqueous coal slurry. The collector adsorbs on the coal surface and makes it more hydrophobic, while the surfactant facilitates production of a stable froth. According to Leonard (26), the optimum coal sizes for froth flotation are between 50 and 140 mesh (0.3 and 0.106 mm). While conventional froth flotation will successfully remove minerals from coal, it has limited effect on the pyrite content, as pyrite tends to float almost as well as coal. Because of this, researchers have been looking for better ways to remove pyrite from coal. In addition, as previously mentioned, it is necessary to grind coal to very fine sizes in order to sufficiently liberate the mineral matter and pyritic sulfur. But, at these very fine sizes,... 

The liberation of a valuable constituent does not necessarily translate directly into recovery in downstream processes. For example, flotation tends to be more efficient in intermediate sizes than at coarse or fine sizes [Mclvor and Finch, Minerals Engineering, 4(1), 9-23 (1991)]. For coarser sizes, failure to liberate m be the limitation finer sizes that are liberated may still be carried through by the water flow. A conclusion is that overgrinding should be avoided by judicious use of size classifiers with recycle grinding. 

Thus, the lower size limit depends upon the nature of the sedimenting system, e.g. density, temperature etc. It also increases if the analysis time is reduced or the measurement height is decreased. The measured distributions for monodisperse powders will be broadened, the effect increasing as the measurement time is reduced [9]. A corollary to this is that analyses of fine powders, in which a scanning system is used, will give different results at different scan speeds. 

To complicate further our perceptions of nanoparticle toxicity, some recent evidence suggests that, on a mass basis, not all nanoparticle-types are more toxic than fine-sized particles of similar chemical composition. As mentioned previously, the limited numbers of studies that have been reported suggest that ultrafine Ti02 particles produced greater pulmonary inflammation when compared with fine-sized Ti02 particles. However, in contrast to the conclusions of the earlier findings, the results of recent preliminary studies comparing the effects of nano- versus... 

---