Patent specification FI 97527 describes a method and apparatus for removing impurities from powdery or coarse material, especially from chip or fiber material. In this method, the material to be purified is brought onto an array of adjacent rollers and set into motion by means of the rollers, with the result that heavy impurity particles of greater density drift toward the surface of the rollers and are passed via a wide separating gap off the array of rollers.

A drawback with this prior-art system is that the impurity material flow separated via the wide separating gap contains such a large amount of acceptable material that this material flow needs to be passed through an after-purification process.

Previously known are also systems in which light particles are separated from the material flow by deviating the lighter material by means of an air blast. Such a system is described e. g. in specification SU 1219354. A drawback with these solutions is especially the fact that they need a large amount of air, involving a high energy consumption.

The object of the present invention is to eliminate the disadvantages of prior-art solutions and to achieve a system that makes it possible to substantially reduce the amount of lighter (acceptable) material falling through the separating gap, thus reducing the need for after-purification, by providing a relatively small upward air current in the separating gap.

Thus, the method of the invention is characterized in that the conveyor is a roller conveyor with rotating rollers conveying the material on an array of rollers,

and that an upward air blast is provided in the separating gap, said air blast having an intensity such that it lifts the lighter material in the region of the separating gap so that the lighter material is passed on across the separating gap, preventing lighter material from falling down through the separating gap while permitting heavy impurity material to be screened out through the separating gap. Correspondingly, the conveyor used in the apparatus of the invention is a roller conveyor in which rotating rollers convey the material on an array of rollers and the separating gap is provided with an air blasting unit controlled by a control unit and producing in the separating gap an upward air blast of a suitable intensity.

A preferred embodiment of the invention is characterized in that the air blast is applied in an oblique upward direction toward the conveying direction, thus contributing towards carrying the lighter particles across the separating gap.

The invention makes it possible to achieve a very economical purification system in which the amounts of air used are small and which therefore involves a low energy consumption, and in which a small and simple after-purification apparatus can be used or after-purification can even be omitted in cases where the amount of acceptable material separated together with heavy impurity particles is insignificant. Such an apparatus can readily be mounted in conjunction with e. g. a plan sifter.

In the following, the invention will be described in detail by the aid of examples with reference to the attached drawing, which represents an apparatus according to the invention.

The apparatus presented in the drawing comprises an array of a plurality of transverse rollers 1 arranged in sequence. The coarse or powdery material 2 to be purified, which may consist of e. g. waste material containing fibers and impurity particles amongst them, is fed onto the forward end of the array via feed elements (not shown) placed above it, preferably in a direction transverse to the conveying direction over the entire width of the rollers.

The rollers are rotated by a drive mechanism (not shown) in the clockwise direction. The rollers 1 are spaced by narrow clearances A, whose width can be

adjusted if necessary. The rollers may be placed in the same plane as in Fig. 1, but they may also be arranged in other ways.

A typical width of the narrow clearances A between the rollers is 0.2-1.5 mm.

After these narrow clearances A, a wider separating gap B is provided in the array of rollers to allow heavier impurity particles 3 to be separated from the material flow 2. This wider separating gap B typically has a width of 3-40 mm.

The widths of the clearances A, B between the rollers naturally depend on e. g. the materials used.

The rotating rollers 1 convey the material 2 onward (to the right in Fig. 1) on the array of rollers and produce a favorable agitation in the material bed formed on top of the rollers 1, bringing it e. g. into an almost fluidized state, with the result that material particles of greatest density are moved (concentrated) toward the surface of the array of rollers. As the material reaches the larger gap B, the impurity particles 3 closest to the rollers fall down off the array of rollers.

To prevent acceptable material from being removed along with impurity particles 3, an upward air blast is provided the separating gap, produced by a control unit disposed below the array of rollers in the region of the separating gap B by means of an air blaster 4 and an associated nozzle chamber 5. The intensity of the air blast is adjusted to a level such that it will lift the lighter material in the region of the separating gap 3 so that this lighter material will be carried onto the rollers 1 beyond the separating gap B while the separated impurity material 3 falls down through the separating gap B.

In a preferred embodiment, the nozzle chamber 5 is mounted in a somewhat oblique position relative to the vertical direction, slanting toward the conveying direction, so that it blows air as indicated by arrow 6 obliquely forward through the separating gap B, thus contributing towards carrying the lighter material flow across the separating gap B. If the array of rollers has a large width, it is possible to use a number of nozzle chambers placed side by side.

Moreover, the roller clearances preceding the separating gap B are narrower than the separating gap B, and the separating gap B may be preceded by one

or more separating gaps C with no blasting. In addition, the separating gap/gaps C preceding gap B is/are wider than the roller clearances A.

It is obvious to the person skilled in the art that different embodiments of the invention are not limited to the examples described above, but that they may be varied within the scope of the claims presented below.