Screened surfaces

Material that remains on a given screening surface is the oversize or plus material, material passing through the screening surface is the undersize or minus material, and material passing one screening surface and retained on a subsequent surface is the intermediate material. 

The screening surface may consist of woven-wire, silk, or plastic cloth, perforated or punched plate, grizzly bars, or wedge wire sections. 

Aperture Aperture, or screen-size opening, is the minimum clear space between the edges of the opening in the screening surface and is usually given in inches or millimeters. 

Other applications Desliming—Removal of extremely fine particles from a wet material hy passing it over a screening surface. Conveying—In some instances transport of the material may he as important as the operation. Media r ecovery—A comhination washing and dewatering operation. 

Screen Surfaces The selection of the proper screening surface is veiy important, and the opening, wire diameter, and open area should all be carefully considered. The four general types of screening surfaces are woven-wire cloth, silk bolting cloth, punched plate, and bar or rod screens. 

Method of Feed The screening machine must be fed properly in order to obtain maximum capacity and efficiency. The feed should be spread evenly over the full width of the screen cloth and approach the screen surface in a direction parallel to the longitudinal axis of the screen and at as low a practical velocity as is possible. 

Angle of slope The optimum slope of inclined vibrating screens is that which will handle the greatest volume of oversize and still remove the available undersize required by the standards of the particular operation. To separate a material into coarse and fine fractions, the bed thickness must be limited so that vibration can stratify the load and allow fines to work their way to the screen surface and pass through the opening. Increased slope naturally increases the rate of travel, and at a given rate it reduces the bed thickness. 

Vibration Amplitude and Frequency Speed and amplitude of vibration should be designed to convey the material properly and to prevent blinding of the cloth. They are somewhat dependent upon the size and weight of the material being handled and are related to the angle of installation and the type of screen surface. The object, of course, is to see that the feed is properly stratified for the most efficient separation. 

Power Requirements The power required in jigging depends on the screen area, the size of material treated, the percentage of opening in the jig screen, the depth of the bed, the length of stroke, and the choke frequency. The power required for plunger-type jigs treating 12.7-mm 0/ -in) material is about 7 W/m" (0.1 hp/ft") jig screen surface. 

In the polar pores, the diffusion coefficient of all ions is strongly reduced relative to the bulk values. No counterion dependence is observed for the SDC of CP. A more detailed analysis shows that the ion SDC depends on the ion s relative position in the pore [174]. In the case of the K ion, this dependence is particularly strong. K ions forming contact pairs with the surface charges are almost completely immobilized on the time scale of the simulations. The few remaining ions in the center of the pore are almost unaffected by the (screened) surface charges. The fact that most of the K ions form contact pairs substantially reduces the average value of the normalized K SDC to 0.2. The behavior of CP is similar to that of K. The SDC of sodium ions, which... 

Figure 9.51. Shapes of bubbles (u) screen surface—thin oil layer (ftl chromium plated and polished surface...

The probability of passage decreases as the particle size tends to approach the size of the aperture. Thus, to ensure that efficient screening of particles takes place, many opportunities to pass through the screen must be provided to them. This is accomplished by moving the screen. For efficient screening both horizontal and vertical movements are required. The vertical movement is intended to lift the particles out of the blocking positions in the apertures and the horizontal movement ensures that when the particles fall they are presented at different positions on the screen surface. For any given aperture size the optimum conditions of the horizontal movement (vibration frequency) and the vertical movement (stroke) of the screen are related. 

Vibrating screens, horizontal and inclined screening surfaces vibrated at high frequencies (1000 to 7000 Hz). High capacity units, with good separating efficiency, which are used for a wide range of particle sizes. 

Shifting screens, operated with a circular motion in the plane of the screening surface. The actual motion may be circular, gyratory, or circularly vibrated. Used for the wet and... 

The screen area which is required for a given operation cannot be predicted without testing the material under similar conditions on a small plant. In particular, the tendency of the material to clog the screening surface can only be determined experimentally. 

These difficulties are obviated to some extent by arranging the screens in the form of concentric cylinders, with the coarsest in the centre. The disadvantage of all screens of this type is that only a small fraction of the screening area is in use at any one time. The speed of rotation of the trommel should not be so high that the material is carried completely round in contact with the screening surface. The lowest speed at which this occurs is known as the critical speed and is analogous to the critical speed of the ball mill, discussed in Chapter 2. Speeds of between one-third and a half of the critical speed are usually recommended. In a modified form of the trommel, the screen surfaces are in the form of truncated cones. Such screens are mounted with their axes horizontal and the material always flows away from the apex of the cone. 

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