Oversize/overflow

The products are an oversize (underflow, heavies, sands) and an undersize (overflow, lights, slimes). An intermediate size can also be produced by varying the effective separating force. Separation size maybe defined either as a specific size in the overflow screen analysis, eg, 5% retained on 65 mesh screen or 45% passing 200 mesh screen, or as a d Q, defined as a cut-off or separation size at which 50% of the particles report to the oversize or undersize. The efficiency of a classifier is represented by a performance or partition curve (2,6), similar to that used for screens, which relates the particle size to the percentage of each size in the feed that reports to the underflow. 

Actual screens do not or provide an incisive separation. Rather, the screen analyses of the overflow and the underflow are like those shown in Figure 2.13 (C). The overflow contains a good amount of particles smaller than the desired cut diameter, and the underflow contains particles bigger than the cut diameter. The two curves overlap, as shown in Figure 2.13 (C). It may also be added that the mass of the two outgoing streams will not equal the individual masses of A and B unless it occurs that the undersize material in the overflow is equal to the oversize material in the underflow. 

The equations derived from consideration of material balances over a screen are found to be useful in calculating the ratios of feed, oversize, and underflow from the screen analysis of the three streams and knowledge of the desired cut diameter. Let F stand for the mass of the feed flow, D for the mass of the overflow flow, B for the mass of the underflow flow, mF for the mass fraction of material, A in feed, mD for the mass fraction of material A in the overflow and mB for the mass fraction of material A in the underflow. The mass fractions of material A are shown in Figure 2.13 (C). The mass fractions of material B in the feed, the overflow, and the underflow are 1 - mF, 1 - mD, and 1 - mn respectively. Since the total material fed to the screen must exit either as underflow or as overflow,... 

The screen efficiency of a screen (known also as effectiveness of a screen) is a measure of the success of a screen in closely separating materials A and B. Provided the screen works perfectly, all of material A would be in the overflow, and all of material B would be in the underflow. An usual measure of screen effectiveness is the ratio of oversize material A that is actually in the overflow to the amount of A entering with the feed. These amounts are D mD and F mF, respectively. Thus... 

Kettle reboilers consist of a bundle of tubes in an oversize shell. Submergence of the tubes is assured by an overflow weir, typically 5-15 cm higher than the topmost tubes. An open tube bundle is preferred, with pitch to diameter ratios in the range of 1.5-2. Temperature in the kettle is substantially uniform. Residence time is high so that kettles are not favored for thermally sensitive materials. The large shell diameters make kettles uneconomic for high pressure operation. Deentraining mesh pads often are incorporated. Tube bundles installed directly in the tower bottom are inexpensive but the amount of surface that can be installed is limited. 

Centrifugal force deposits the oversize particles against the bowl wall, from which they are conveyed by the helix. The overflow fractions flow around the helix to the liquid-discharge ports. Size of separation is controlled by feed rate and degree of centrifugal force. 

Hence, the weight fraction oversize is calculable by measurement of P for different values of S at constant W and Q. The quickest analytical procedure is to ealculate P from gravimetric or chemical analysis of feed and overflow suspensions. Choice of flowrate and speed can be made in accordance with prior knowledge of approximate size and use of derived theoretical expressions, or by trial and error to establish the rate at which P approaches unity with maximum S. 

Example 2.—Oversize in the overflow greater than desired. 

With a higher allowable amount of oversize in the overflow, the ratio of tube-mill feed to original feed increases, under the stated conditions. 

Example 3.—Oversize in the overflow less than desired. 

Wet classifiers are mostly used to yield two product-size ranges, oversize and undersize, with a break commonly in the range between 28 and 200 mesh. A rake classifier operates at about 9 strokes/min when making separations at 200 mesh, and 32 strokes/min at 28 mesh. Solids content is not critical, and content of the overflow may be 2-20% or more. 

Classification or size separation is the unit operation that separates the particles according to their size. In general, it can be carried out dry or wet. The fraction of the feed having size less than the size of the screen aperture is the undersize, while that of greater size is the oversize. The fraction of the undersize that flows through the apertures is called underflow (or passing), while the particles that do not pass are called overflow (or remaining). In solid-waste treatment, classification is carried out almost always on screens and in dry, because the particles to be classified are relatively coarse and wet processes are undesirable. 

Disc screens are composed of a series of parallel, horizontal, and equally spaced bars or shafts, which are installed perpendicular to the flow of the material. Each shaft is equipped with interlocking serrated or star-shaped discs, installed at equal distances across the width of each bar. Undersized material falls in the space between the discs, while oversized material overflows the discs. The adjustment of the distance between the discs determines the materials size to be separated as underflow. After suitable adjustment, the same screen can be used to separate materials into different size. 

If Xq and represent the mass fractions of oversize material, i.e. particles larger than a, in the feed, overflow and underflow, respectively, and Xp, x and x j represent the corresponding undersize mass fractions, then a balance on the oversize material gives... 

Equation 9.138 gives a measure of the success of recovering oversize particles in the overflow stream, equation 9.139 of undersize material in the underflow stream. For perfect screening both Eq and Eq will be unity. Equations 9.138 and 9.139 require values of the mass flow rates F, O and U, but substitution from equations 9.132 and 9.133 can eliminate these quantities ... 

Aperture, pm Cumulative Feedstoek mass fraetion Overflow oversize Underflow... 

The separation or screening unit includes a series of sieves that separate hot aggregates (inclusive of reclaimed asphalt if used and fed before screening) into fractions (usually between four and six) of specified size and reject the oversized aggregates. The fractions of aggregates are stored temporarily in hot bins. Each bin is equipped with an overflow tube of appropriate diameter to prevent the material from overflowing into other bins. 

Diagram of mass balance in a separator M mass flow rate of solids in the feed, M mass flow rate of separated solids, M, mass flow rate of nonseparated solids, F(x) cumulative percentage oversize of feed solids, F (x) cumulative percentage oversize of separated solids, F,(a ) cumulative percentage oversize of nonseparated solids, Q. volumetric flow rate of feed suspension, U volumetric flow rate of underflow suspension, O volumetric flow rate of overflow suspension. 

The primary screen oversize consists of grid metal, separator plastics and case plastics. It is fed to a water fiUed tank where the polypropylene case material floats and is collected and washed. The remainder of the material is extracted from the bottom of the tanks by drag or screw conveyor and is passed to a hydrodynamic separator or elutriating column where an upward flow of water separates out plastic material as overflow from metallic lead, which sinks to the base of the column and is extracted by drag or screw conveyor. 

For classification of particles above size r and below size r respectively, into the underflow and the overflow of a solid-liquid classifier, define component 1 to be the undersize particles and component 2 to be the oversize particles. Show that the extent of separation based on mass fractions in such a case is given by... 

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