Oversize fraction

Screening (continuous, commercial) and sieving (batch, laboratory test generally confined for size determination) are essentially mechanical separations of particles based on size, accomplished by using a perforated surface that serves as a go-no-go gauge. Both processes, like all other separation processes, have the drawback that a complete separation is seldom obtained, and some potential oversize/undersize particles are always left in the undersize/oversize fraction. 

The Snrbec-ART Environmental, L.L.C. (Snrbec), soil washing technology is a process based on mining and mineral processing principles that incorporates physical and chemical separation techniqnes (D12463A, p. 3). The technology separates and treats oversized fractions and sand fractions so that they can be placed back on the site as clean backfill. Contaminants are concentrated in the fines, and this fraction can be managed separately for further treatment or disposal. 

Fig. 1. Grinding mill with product classification and recycled oversize fraction.

The recycle line is described by plug flow so that it represents some delay in recycling the oversize fraction of particles. 

BANCO CENTRIFUGAL CLASSIFIER. This is a proprietary instrument that is essentially a centrifugal air elutriator. Air and dispersed powder samples are drawn thru the cavity of a rotating hollow disc in a radially inward direction against centrifugal forces. The powder particles are thus divided into under- and oversize fractions, collected, and weighed. Separation into different size-fractions is made by altering the air velocity. About 20g of sample are required for analysis, and 8 size determinations can be made in 2 hours (Ref. 10)... 

Fig. 9 Elutriators can use gravity or centrifugal force to separate fine particles into undersize and oversize fractions. (A) Gravity elutriator (B) cyclone (centrifugal elutriator) and (C) impactor. (From Ref. l)...

Screening involves passage of particles over apertures in a screen deck (or mat ), which allow particles smaller than the aperture size to pass through the deck, thereby dividing the product into undersize and oversize fractions [5.4-5.6]. The apertures may be square, rectangular, or circular. 

A second attempt (run NITAC-S) was based on more aggressive process conditions. In order to reduce the buffer capacity of sediment, the 500 pm oversize fraction, which had high calcium carbonate content due to the presence of numerous shells fragments, was preliminarily removed through sieving. This fraction accounted... 

Figure 7.17 shows the cumulative mass of Ca, Mg, Pb, Zn, Ni, and Cd over time measured in the cathodic and anodic chambers. As mentioned, Ca and Mg mostly accumulated at the cathode, and their migration toward the electrodes clearly started readily at the beginning of the test conversely, the presence of trace metals in the chambers could only be detected after 5-10 days of treatment. The onset of heavy metal mobilization was found to correspond to -10% electro-removal of Ca from sediment. It should be emphasized that this adds to -30% of Ca removal accomplished through the preliminary mechanical separation of the 500-pm oversize fraction. 

Particulate material fed to a screen can be considered to be composed of two fractions, an oversize fraction consisting of particles that are too large to pass through the screen apertures, and an undersize fraction consisting of particles... 

When one extends the liquid addition period, the bar continues to abrade the granules. Favorably, any particles that have been fractured can be rewetted and the fracture sruface can easily bind to another particle. This mechanism tends to hold down the oversized fraction without the expected increase in the undersized fraction. 

The solid output of the mash separation unit has a particle size >10 mm and joins the oversized fraction of the first 80 mm screening step for composting. They are mixed with the solid digestate and with the oversized lignocellulosic fraction originating from a parallel green waste composting process. 

After the active phase, the feedstock is screened again at 40 mm and the oversized fraction is sent for further active composting. The finer fraction is moved into a second tunnel for a 45 day maturation phase. After maturation, the compost is screened for... 

Oversize fraction from refining stage of green... 

In practical terms the cut size fractionation of an elutriator is never a clean cut fractionation and the oversize fraction often contains some of the particles smaller than the cut size and vice-versa. In industrial situations the efficiency of an elutriative fractionation is measured by what is known as a coarse grade-efficiency curve. This curve is often referred to as Tromp s curve after its originator [3]. To be able to calcu-... 

Figure 5.3. The basic steps for generating a Tromp s curve to evaluate the efficiency of an elutriator. a) Size distribution of the (xjwder sample, b) Size distributions of the undersize and oversize from the elutriator. c) Tromps curve for the elutriator spans the size range between the lower limit of the oversize fraction and the upper limit of the undersize fraction. The Index of Fractionation Efficiency is the ratio of the values x25nt75.

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