Screens mesh size, effect

Compeau et al. (1990) reported a full-scale slurry-phase PCP remediation. The system consisted of soil washing and screening and resulted in clean soil and wash solution. The wash solution was a slurry containing PCP and < 60-mesh-size soil particles at approximately 20% solids concentration. Slurry was treated subsequently in on-site slurry-phase bioreactors. A 50 m3 slurry reactor was operated in batch mode and inoculated by an uncharacterized PCP-mineralizing culture (107 cells/ml of slurry). After 14 days, 370mg PCP/kg slurry had been degraded to below 0.5 mg/kg. For effective biogradation to occur, inoculation was required. 

Ideally, a particle would have the greatest chance of passing through the screen if it struck the surface perpendicularly, if it were so oriented that its minimum dimensions were parallel with the screen surface, if it were unimpeded by any other particles, and if it did not stick to, or wedge into, the screen surface. None of these conditions applies to actual screening, but this ideal situation can be used as a basis for estimating the effect of mesh size and wire dimensions on the performance of screens. 

Fig. 6 Schematic view of the chain structure in the semidilute concentration range. The mesh size is about equal to the effective polymer persistence length feff aod to the screening length (if no salt is added to the system).

A local correlation length f(x) was defined (the length over which excluded volume effects are screened), the polymer layer at the interface then resembling a network of variable mesh size (x). When the hydrodynamic equation is integrated up to a distance correspjonding to the longest mesh size Rp (the Flory radius of polymers in solution), it is found that the hydrodynamic thickness scales... 

As speculated in Chapters 4 and 5 it is believed that the shape of the actual pore structure is a secondary influence (the primary influence being the fineness of the screen) on the effective pore diameter. For example, for Dutch Twill meshes, these openings are complex 3D structures. The equilibrium L/V interface shape is complex and it depends on the size and packing of the wires. If it were possible to observe microscopically a L/V interface within a Dutch Twill weave, it might be possible to imderstand why the 450 X 2750 outperforms the 510 x 3600. Meanwhile, it is much more efficacious to look at deeper correlations between the pore diameter and geometrical properties of the mesh like the warp and shute wire diameters. Since the warp and shute diameters are a maj or factor in determining the pore shape it is likely that they have a relationship with the pore diameter. 

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. 

Sieve analysis using standard mesh screens is commonly used to determine particle size and size distribution of pellets and the reader is referred to standard texts for further information (61). Several types of densities have been defined for pellets based on interparticulate (void fraction) and intraparticulate pore volumes and include true, apparent, effective, bulk and tapped. The bulk and tapped densities may be obtained using simple devices, such as that used to evaluate granulations in tableting, while the true and apparent densities need more complex techniques based on mercury intrusion, gas flow, powder displacement, imaging or minimum fluidization velocity (62). 

Figure 27. Effect of bleaching clay particle size on bleaching efficiency and press rate particle size measured as percentage particles passing through a Tyler200 mesh screen. (This figure is available in full color at http //www.mrw.interscience.wHey.com/biofp.)...

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