Classifying – separating – and assorting solids – Precedent preparation of items or materials to facilitate... – Sorting special items or sorting by methods and apparatus...
Patent
1995-01-06
1996-12-24
Bollinger, David H.
Classifying, separating, and assorting solids
Precedent preparation of items or materials to facilitate...
Sorting special items or sorting by methods and apparatus...
209312, 209318, 209321, B03B 100
Patent
active
055866603
DESCRIPTION:
BRIEF SUMMARY
It is customary, in the screening of a stream of bulk material, to administer the stream of bulk material from above to the screen surface, on which it is distributed in layer form. By vibration and/or a descending gradient, the layer can be kept in motion and the screen overflow conveyed away. Even when the layer is kept in motion by vibration, it is in a comparatively dense settled state.
U.S. Pat. No. 2,200,472 discloses a screening apparatus for removing dust and coarse foreign bodies from a rubber granulate, in which the material is guided by way of a screen surface arranged at an inclination, through which passes from the bottom upwards a gas stream which lifts the dust and foreign bodies of low specific gravity to a greater extent than the granulate which is lifted to a lesser extent and which, after the stream of bulk material has passed through, falls back onto the screen. In the region of the air jet itself, the granulate cannot fall through the screen counter to the effect of the air jet. Whether, after the air-jet region, it is in a state conducive to the screening effect is not disclosed in the publication.
It is also known from DE-A-2,219,179 to separate light-weight fractions from a stream of bulk material to be screened, by means of a blowing stream passing through upwards from below. The screenability of the remaining particles is not varied thereby, since, when the air stream passes through, they do not lose contact with the conveying surface.
The invention relates to a screening apparatus, in which a stream of bulk material is to flow for the greatest part through a screen surface, whilst only foreign bodies, such as pieces of metal or lumps of material, are retained. The object on which it is based is to reduce the screen surface required for this purpose.
In the solution according to the invention, the screen surface is preceded, with approximately the same conveying direction, by a fluidized-bed conveying surface in the conveying path of the stream of bulk material. As a result, the stream of bulk material passes as a fluidized bed onto the screen surface. In the fluidized-bed state, the bulk material (at least its fraction of finer grain) is flowable. This makes the movement actions associated with the screening operation easier, in particular, on the one hand, the passage of the screen underflow through the screen apertures and, on the other hand, the separation within the material bed. The screening capacity per unit area of the screen surface is increased thereby; the necessary extent of the screen surface is reduced. The height requirement consequently decreases simultaneously. The fluidized-bed conveying surface can be a conventional pneumatic conveying trough, through the porous bottom of which gas is introduced into the material bed lying on the conveying surface, with the result that this material bed is converted into the flowable state. The conveying movement can be induced by an appropriate descending gradient of the conveying surface.
The fluidized-bed state cannot be maintained for long above the screen surface, because the gas establishing the fluidized state and separating the particles from one another can easily flow off upwards and downwards. It is therefore expedient if the screen surface is so short in the conveying direction in relation to the conveying speed that the fluidized-bed state of the screen overflow is maintained. More precisely, at the end of the screen-conveyor stage, the bulk-material bed should be in a state in which it is still so flowable that it can flow off from the screen surface. At the same time, the remaining operating conditions, in particular the descending gradient and a possibly provided vibration of the screen surface, are also to be taken into account. These influences have a beneficial effect on the conveyance and flowability of the material bed. If a descending gradient or vibration are added, the fluidized state therefore need not be so pronounced at the end of the screening stage that the material could still flow off solely as a
REFERENCES:
patent: 2200472 (1940-05-01), Erdmann
Bollinger David H.
Claudius Peters Aktiengesellschaft
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