1、中文 1770 字 COAL PREPARATION TABLE 7-14. Effect of Geometry and Concentration of Feed Solids on throughput for a 1/6-in, diam hydro cyclone cleaning 1/4-in Varying the distance between the bottom of the vortex finder and the hydro cyclone cone bottom. For example, the washed coal ash can be reduced by
2、 decreasing the diameter of the vortex finder, decreasing the length of the vortex finder, or increasing the diameter of the underflow orifice. Increasing feed-Solids content increases the specific gravity of separation and, therefore, washed coal yield and ash, which indicates the importance of mai
3、ntaining a constant feed-solids content to preserve washed coal quality. Capacity is influenced by cyclone geometry, i.e., the sizes of the overflow, underflow, and inlet openings, and by feed-solids content. The effects of these parameters is given in Table 7- 14.Increasing inlet pressure is a simp
4、le method of increasing capacity without changing hydro cyclone geometry, and washed yield and ash are not significantly affected. However, the penalty is increased pumping cost, and degradation of the coal. Flow sheets Soon after the hydro cyclone was developed, it became evident that performance w
5、as inferior to nearly all other cleaning devices. Consequently, in an effort to improve performance, three two stage circuits, shown in Fig. 764, were developed. In the earliest two-stage circuit, called two-stage relearn or TSR, the refuse from a primary hydro cyclone is simply relearned in a secon
6、dary hydro cyclone, The overflows from the two hydro cyclones are recombined as the washed coal product, and the underflows from the secondary hydro clone contains the final refuse. In more recent installations, one of the products from the secondary hydro cyclone is recirculated to the feed of the
7、primary hydro cyclone. In the two-stage overflow recirculation circuit, TSOR, the primary or first-stage hydro cyclone is adjusted to produce an acceptable clean coal and the secondary hydro cyclone is adjusted to produce a refuse essentially free of misplaced coal. The overflow from the secondary h
8、ydro cyclone, which contains the misplaced coal in the underflows of the primary hydro cyclone, is returned to the feed of the primary hydro cyclone for reprocessing. In the two-stage underflow recirculation circuit, TSUR, and the overflow is relearned in the secondary hydro cyclone. The underflow f
9、rom the secondary hydro clone is recalculated to the feed of the primary hydro cyclone. The overflow from the secondary hydro cyclone contains the washed coal. Each of these circuits has advantages that depend upon the size and specific gravity compositions of the feed, as well as the required washe
10、d coal quality. The TSOR circuit is more effective in recovering washed coal whereas the TSUR circuit is more effective in rejecting heavy impurity. The TSR circuit is most effective when the specific gravity of separation of the two hydro cyclones is similar. Conversely, the performance of TSOR and
11、 TSUR is improved by diverging the specific gravity of separation of the two cyclones. At the present time, the TSOR is the most common circuit. A variation of the TSR circuit has been proposed whereby underflow from the primary cyclone is relearned on a concentrating table rather than a secondary h
12、ydro cyclone. Some plants using jigs to clean the coarse coal utilize hydro cyclones to improve performance on the finer sizes. One method is to relearn the underflow of the washed coal screen, commonly the 1/4-in.material, with hydro cyclones. Another method is to screen the raw coal at about this
13、size and clean the undersize with hydro cyclones. Hydro cyclones have been used ahead of dense-medium cyclones to remove some of the low specific gravity coal and thereby reduce the amount of material sent to the dense-medium plant. The hydro cyclones are adjusted to separate at a specific gravity o
14、f about 1.35 to 1.40. The advantage is that the capacity of the dense-medium cyclone plant can be smaller, thus reducing capital and operating costs. Hydro cyclone Performance As mentioned previously, the quality of the washed coal and refuse products can be regulated by changing the diameters of th
15、e overflow and underflow orifices. However from a performance standpoint, a ratio of overflow diameter to underflow diameter in a range of about 1.7 to 2 gives the best results. Performance at lower ratios is inferior. Also, the solids content in the feed to primary and secondary hydro cyclones shou
16、ld range from 8 to 15 % (by weight). Outside this range, either above or below, performance is adversely affected. Separations obtained in a single hydro cyclone and two-stage circuits (TSR) are shown by the distribution curves in Fig. 7-65. The sharpness of separation of the two-stage circuit is si
17、gnificantly superior to that of a single hydro cyclone. Also, the sharpness of separation of the two-stage circuit is not nearly as sharp as the separations characteristic of a dense-medium cyclone. It follows then that hydro cyclones are not applicable for difficult-to-clean coal or separations at
18、low specific gravity unless followed by a more effective relearning process. Also, they are not suitable for friable coal or where the refuse particles are platy. Table 7-15 gives detailed performance data for two-stage (TSR) hydro cyclones. These data indicate that in general the specific gravity of separation increases and the sharpness of
