This invention relates to the sizing or separation of particulate material into relatively smaller, or finer, and relatively larger, or coarser, sizes.
It is frequently desirable during the beneficiation of coals or ores to crush the material received from the mine and then separate the finer material from the coarser material for later treatment. It is also frequently desirable to separate coals, ores, and other particulates according to size so that the different sizes can be used for various different purposes, subjected to varying treatments, or sold as varying grades. Such size separation has most frequently been accomplished by screening the particulate material. Screening is efficient and simple, but is sometimes not satisfactory for particular materials. Some materials, for example, are so hard and abrasive that a screen is soon worn away, thus requiring frequent and costly replacement. In other cases the material being screened may tend to cling to the screen or clump together, blinding or plugging the screen, or else effectively preventing small particulates from being dislodged from larger particulates. Blinding or plugging of the screen orifices is particularly troublesome in many cases when the particulates are moist or hygroscopic. Slightly moist coal is especially difficult to handle because the fine coal particles and especially coal dust particles cling tenaciously to the wires of the screen, effectively plugging the screen in only a short period.
Various methods of cleaning screens such as by rapping or shaking or the like have been developed, but have not been completely effective, particularly on difficult to separate materials such as moist coal particles. Other nonscreen types of separators such as shaking or vibratory tables, various rotating drum type classifiers, spiral classifiers and the like have been developed, but none has been completely satisfactory either for the separation of coal, ores or other particulates. Furthermore, as the quality of ores has declined and as environmental considerations have increased, requiring the removal of sulfur and the like from coals in particular, finer and finer grinding of various ores and coals has become necessary in order to liberate either desirable or undesirable components. Since grinding requires a significant input of energy, it is desirable not to have to regrind material which is already sufficiently fine. It has thus become more and more desirable to quickly size large tonnages of particulates in the cheapest and most efficient manner.