HERAMBIENTE – Glass sorting plant
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The act of screening, of passing material through a screen.

Screening or sorting is a mechanical process whereby a material (as in the case of wood screening) or a mix of materials (as in the case of waste sorting) is separated into homogeneous size fractions.


How screening works

Screening generates 2 or more fractions:

- the underscreen fraction with the smaller parts of those produced;

- the overscreen fraction which is the material that remains over the separation gap and that reaches the end of the screening process, it is the bulky or “oversize” fraction.

- intermediate fractions vary in number and depend on the needs of the user and are produced through decreasing settings of the gaps or screen meshes.

Thus, in principle, when screening, the bulkier material remains over the separation system (rollers or sieves), whilst the smaller material passes through; the number of screening decks generates the number of fractions in which the material is subdivided.


“Green” screening and “dry” screening

This is the process entailing the subdivision in homogeneous size fractions applied to recycled or virgin wood (the latter coming from forest cutting). Virgin or prime bare wood, hence damp, releases surface sap and resins that tend to bind the wood mix, soiling the sieves and obstructing the holes, quickly reducing sorting efficiency. Thus, under these conditions, using a roller screen, maybe provided of “V”-profiled rollers, allows screening without losing efficiency and in the worst case obstruction. If the material is particularly polluted and rich in thread-like and filamentary materials, which could twist in the rollers, the use of the STEPPER SCREEN, that is patented for these difficult operating conditions, is highly recommended.


Screen types (rotating, oscillating, disc rollers, etc.)

There are various types of screens depending on the screening needs and the material involved. They can be broken down into 5 main categories as follows:


ROLLER SCREENS: they consist of a bed of rotating rollers that thanks to the rotating action of the rollers move the material forward generating a side-shift oscillating movement that facilitates the stratification of the material by size. The side-shift action on the material occurs thanks to the push operated by the rollers whilst the oscillating movement occurs as the material follows the hilly profile of the rollers given by the alternating diameters of the rollers. The fractions generated by a roller screen range from the smallest to the largest following the flow of the material, as the size of the particles changes in relation to the distance of the rollers and as a result, at the start of the sieve the rollers are close to each other and eventually become distant until the maximum size is reached. The shape of the rollers is of fundamental importance for their correct operation as in the case of the Dynascreen (a Pal’s patent) where the particular “V” profile accentuates the oscillating effect of the particles and thanks to the interlocking of adjacent roller profiles the separating efficiency of this screen is similar to that of an oscillating mesh screen, which is not possible for a roller screen with no interlocking profile, it being unable to sort long particles.

Disc screens for example can fraction flakes of different sizes using “square” discs as in the Quadradyn screen that is used in the OSB (Oriented Strand Board). It is worth noting that in the case of Quadradyn, thanks to the patented profile of the rollers and discs, the screening occurs without damaging the particles.


PADDLE SCREEN: this is a screen where rollers, instead of rotating, perform an alternating tilting motion as in the case of the Stepper Screen (a PAL’s patent). In this screen the bed of paddles pushes the material forward by giving oblique impulses to the material placed upon it; this expedient allows to treat difficult materials by drastically reducing machine stops, overcoming the problems of traditional disc screens, which tremendously suffer from entanglement and twisting of long and filamentary bodies always present in good quantity in waste and recycled wood.


BALLISTIC SCREEN: it is mainly used in waste sorting plants for its ability to sort material according to physical properties such as volume, weight, shape (2D flat and 3D rolling waste) and size; to do this, it uses inclined paddles that perform a rotary motion, this in turn generates an opposite flight movement causing a different performance for each type of material.  Ballistic screens are generally used for coarse and particular selections as they only combine sorting by size (inaccurate) with the features of the material processed.


OSCILLATING SCREENS: Like gold diggers who use a container consisting of a metal mesh to sift the bottoms of rivers, these screens adopt the same concept, but on a larger scale.

In oscillating screens, the opposite occurs compared to the screens described so far, as the larger fraction remains in the screen while the finer one passes gradually into the screens below.

Oscillating screens are multi-layer and consist of sieves placed one upon another starting from the one with the largest wire mesh to the one with the finest wire mesh up to the closed end of the machine. The material processed in this series of wire mesh sieves is screened until it reaches a wire mesh that is finer than its size, hence in vibrating screens the material is separated by screening the larger particles first and so on until the smaller ones are screened.

Furthermore, oscillating screens unquestionably offer the highest screening efficiency in terms of particles according to an extremely narrow Gaussian distribution, since the particles are exposed to an oscillatory motion with no jolting effects, thus separation only occurs as a result of the passage through an opening if the surface projection of the particle is smaller than the opening.

Sometimes oscillating screens are confused with VIBRATING SCREENS that have a considerably lower separation efficiency as they combine a side-shift movement with an important jolting effect that allows the long particles to arrange themselves vertically and then slip into the openings of the sieve thus polluting the lower fraction and effectively reducing separation efficiency.

The structure that supports oscillating screens can stand on elastic joints as in the case of VMESuperscreenMulticross or hydraulic ones as in the Hydrostatic Screen.


DRUM SCREEN: this is often used in processing waste and consists of a rotating drum fitted with a mesh or perforated plate to facilitate the screening of finer material, whilst the coarse material reaches the end of the screen. The VRO Drum Screen can be tilted to facilitate the exit of material in the most adverse cases.


Products that can be screened

Depending on the material to be screened it is possible to choose between the screens listed above. Some screens are specific to certain applications such as for example the sorting of wood flakes for the production of OSB panels, and this is the case of Quadradyn.

To produce wood panels starting from recycled wood polluted by foreign bodies such as filaments and fabrics or in the case of waste sorting, it is advisable to use a screen with clog prevention system such as the Dynascalper or a screen with phase-opposition discs such as the Stepper Screen.

There are also versatile screens that can be used in many applications, not just in the production of wood panels but also in the sorting of waste, compost, food products, salts, minerals and aggregates such as for the Dynascreen or the Oscillating Screen.


Screening in the production of wood panels

In the production of wood panels, it is fundamental to subdivide the material by homogeneous fractions for two main reasons:

- to eliminate pollutants with higher efficiency, removing the least amount of wood;

- to subdivide the material into micro and macro particles to properly feed the production process.


Wood panels can be broken down into the following types:


PB: (Particle Boards) obtained from virgin or recycled wood particles mixed with resin, are made of a core layer with larger particles and 2 surface layers with finer particles, thus can be directly coated, veneered or laminated; they are the preferred panels in furnishing as they are affordable and versatile, their surface being easily customizable.


MDF: (Medium Density Fibreboards) obtained from primary wood fibres (and recently also from recycled wood, thanks to the super-efficient cleaning systems patented by PAL) combined with a resin binder, then pressed into a single layer panel; they are used in furnishing for their versatility in surface processing by means of milling and adding décor papers.

As in the case of PB, their surface can be directly coated, veneered or laminated; they are the preferred panels for furniture doors (kitchens or furniture) and for the possibility of achieving extremely thin and elastic thicknesses (such as wardrobe backs).


OSB: (Oriented Strand Board) obtained from primary and recycled wood strands and characterized by a core layer of finer strand placed between two surface layers obtained from larger oriented strands; they are used in construction for their high mechanical resistance.


LSB: (Light Strand Boards) these differ from OSB panels as their surface also contains a layer of “thin” particles, and this technical choice allows for laminating hence they can also be used in furnishing.


Screening in waste sorting

In solid urban waste sorting it is fundamental to recover the RDF (Refuse Derived Fuel) fraction that can be used to produce energy by incineration and filtering of fumes as it is necessary to fraction the fine part of bio-waste to obtain compost.

In both applications, screening displays particular difficulties due to the presence of foreign bodies such as rags, small bags, plastic wires and cables that may clog machines.

In these cases, the best screening system is represented by odd rollers that operate with an oscillating motion in phase opposition vis-à-vis even rollers, so as to guarantee that the material moves forward as in the Stepper Screen that avoids clogging by dragging any foreign and bulky items to the end of the machine.

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