The present invention relates to the screening of granular material, especially for providing so-called clean fractions of crushed stone material.

The term "screening" as used herein relates to clas¬ sifying granular material in respect of particle size, which is generally carried out by feeding the material over a surface having openings through which fine par- tides pass while coarser particles are conveyed along the surface and finally removed therefrom.

In screening, it is often desirable to be able to se¬ parate the material into fractions, that is different por¬ tions of material with the particles within upper and lower particle size limits, e.g. in ranges of 0-8 mm,

8-16 mm, 16-24 mm etc., professionally referred to as 8-mm range fractions.

A well-known problem in this connection is that it is very difficult to obtain clean fractions, i.e. fractions exclusively containing particles within the particle size range concerned. One always has to accept that the frac¬ tion contains a certain proportion of outsize particles, i.e. both undersize particles of a size below the desired lower particle size or separation limit, and oversize particles of a size above the desired upper separation limit.

When, for example, it is desirable to provide a 12-22 mm fraction, use is generally made of a screen having an upper and a lower screen deck. Theoretically, the upper screen deck would then have openings of a size of 22 mm while the lower screen deck would have openings of a size of 12 mm. The problem which then arises is that it will be difficult for the particles having a size close to the size of the openings to pass therethrough, which means that the fraction will contain a relatively large proportion of undersize particles thus having a size below the lower separation limit of 12 mm. It is not unusual

that a fraction thus provided contains 30-35% by weight of undersize particles.

Traditionally, the above-mentioned problem has been solved by providing the lower screen deck with openings of a size which is 10-20% above the desired separation li¬ mits, in the above example 14-mm openings. Since screening for obtaining different fractions is normally carried out in several steps, this technique involves inconveniences in so far as the following finer fraction will at the same time contain a relatively large proportion of oversize particles. Especially when providing short-range frac¬ tions, e.g. 8-11 mm fractions, a change of the size of the screen openings has a major influence on the proportion of outsize particles. Similarly, for obtaining a fraction above 22 mm, 24-mm openings are traditionally selected for the upper screen deck.

When screening according to the above-mentioned tech¬ nique, the different fractions will thus still contain a relatively large proportion of outsize particles, often about 10% by weight of oversize particles and 20% by weight of undersize particles.

The market however makes increasingly higher demands for clean fractions to be used as aggregate in concrete or asphalt, for road construction purposes as well as many other purposes. These demands are especially severe on materials to be reloaded, for instance to and from a ship's hold, in which case the coarser particles in the material are easily crushed, whereby to increase the pro- portion of undersize particles. In recent years, there has been a demand on the market for fractions where the total proportion of outsize particles should be less than 15% by weight.

There is thus a need to be able to provide very clean fractions containing particles within accurately determin¬ ed separation limits and containing a considerably smaller proportion of outsize particles than it has hitherto been

possible to achieve with conventional screening tech¬ niques.

A primary object of the invention thus is to provide a method for screening granular material in order to ob- tain clean fractions having a proportion of outsize par¬ ticles which is essentially smaller than in fractions pro¬ vided by earlier known screening methods.

Another object is to provide an apparatus for screen¬ ing granular material, by means of which said clean frac- tions can be obtained and which consists of but a few simple and inexpensive components.

These and other objects, which will appear from the following description, have now been achieved according to the present invention by the method and the apparatus as recited in accompanying claims 1 and 3, respectively, and in preferred embodiments in claims 2 and 4, respectively. The proposed solution according to the invention of the problems discussed in the foregoing may briefly be considered to consist in that the upper separation limit of the fraction concerned is determined in a first screen¬ ing operation while the lower separation limit is deter¬ mined in a second screening operation. From this follows that the lower separation limit for a fraction does not automatically constitute the upper separation limit for the following fraction.

The invention and its many advantages will be de¬ scribed in more detail hereinbelow with reference to the accompanying drawings, in which Fig. 1 illustrates screen¬ ing according to previously known technique, Fig. 2 shows the particle size distribution in the final fraction ob¬ tained, Fig. 3 illustrates screening according to the in¬ vention, Fig. 4 illustrates the particle size distribution in an intermediate product and in a final fraction, re¬ spectively, and Fig. 5 illustrates screening according to an alternative embodiment.

Fig. 1 schematically shows an example of conventional screening of crushed stone material for obtaining a final fraction having a lower particle size or separation limit of 12 mm and an upper separation limit of 22 mm. The granular material is supplied to a screen 1 comprising an upper screen deck 2 having 24-mm openings, and a lower screen deck 3 having 14-mm openings. The size of the open¬ ings thus exceeds the two separation limits by about 9 and 17%, respectively, as is customary. Particles larger than 22 mm should be removed from the upper screen deck 2 while particles smaller than 12 mm should pass through the two screen decks 2, 3. Between the screen decks 2, 3, the final 12-22 mm fraction is obtained and removed from the lower screen deck 3. As appears from Fig. 2, the final fraction may con¬ tain about 20% by weight of undersize particles and about 10% by weight of oversize particles, the remainder being particles of the desired size. It should be noted that the percentages indicated are only examples of the order of size generally involved.

Fig. 3 shows an example of two-step screening accord¬ ing to the invention. The screening apparatus comprises a first screen 4 with an upper screen deck 5 and a lower screen deck 6, corresponding to the screen 1 shown in Fig. 1. The difference is that the screen openings of the screen decks 5, 6 agree with the separation limits, i.e. have a size of 22 and 12 mm, respectively.

As opposed to previous technique, all particles above 22 mm are removed from the upper screen deck 5. Par- ti es below 12 mm should pass through the two screen decks 5, 6. Between the screen decks 5, 6, there is ob¬ tained an intermediate product which primarily contains particles in the particle size range 12-22 mm, but which also has a relative large proportion of undersize par- tides.

The intermediate product is fed on to a second screen 7 with a single screen deck 8 having openings of a size of 15 mm, i.e. slightly above the lower separation limit of 12 mm. A substantial part of the particles below 12 mm, i.e. undersize particles, thus pass through the openings in the screen deck 8, whereby the final fraction obtained at the end of the screen deck 8 will contain but a very small proportion of undersize particles. The undersize particles which pass through the openings in the screen deck 8 can be screened again, optionally after recrushing. Alternatively, these undersize particles can be mixed with the particles below 12 mm which are obtained in the first screen 4.

According to the invention, screening is thus effect- ed in two steps, where the oversize particles are sepa¬ rated in the first step while the undersize particles are separated in both the first and the second step. For an optimum result, the screen deck 8 of the second screen 7 should have slightly larger openings than the lower screen deck 6 of the first screen 4.

As appears from Fig. 4, the final 12-22 mm fraction obtained will thus contain very small proportions of par¬ ticles outside the desired size range.

Fig. 5 shows a variant of the screening shown in Fig. 3, all screen decks being gathered in one unit. The material is supplied to an upper screen deck 9 with openings of a size which agrees with the upper separation limit of 22 mm. Directly below the upper screen deck 9, there are provided a screen deck 10 with 12-mm openings and a screen deck 11 with 15-mm openings immediately suc¬ ceeding the screen deck 10 with respect to the flow of material. The screen decks 10, 11 correspond to the screen decks 6 and 8 in Fig. 3 and have the same function. The final fraction is of substantially the same quality as in Fig. 3.

Reverting now to Fig. 3, the screen decks 5, 6 of the first screen 4 according to the invention should have screen openings of a size equal to or slightly above the upper and the lower particle size limit, respectively. In both cases, the size of the screen openings should be at most about 25% above the separation limits. In the second screen 7, the screen openings should have a size above the lower separation limit, preferably about 5-35% above it. It should be noted that these percentages merely are stan- dard values which are here given by way of example only. When thick screen elements are used, e.g. rubber screens where the openings may be regarded as channels of a certain length, the openings must be relatively large in relation to the separation limit to permit passage of the particles. Conversely, thin screen elements, e.g. metal wire screens, may have relatively small openings. How much above the separation limits the size of the screen open¬ ings must be depends, inter alia, on the fraction range and on what demands are placed on the final fraction, i.e. how large a proportion of outsize particles is acceptable. It should be pointed out that the invention is in no way restricted to the screen types here described, but can be used in different types of screens and screen elements. Further, the "size of the screen openings" means the transverse dimension defining the opening, e.g. the dia¬ meter of circular holes, as is customary in screening technology.

A skilled person readily realises that the screening apparatuses according to the invention shown in Figs. 3 and 5 generally are included in a screening plant in which several different fractions are obtained. For example, the fraction below 12 mm may be further screened to provide other fractions within this fraction range. The screening technique according to the invention is then applied once more.

To conclude, it should also be pointed out that the invention is equally applicable to reversed screening, i.e. when the screen decks are disposed after each other, in which case the first screen deck has the smallest open- ings while the last screen deck has the largest openings. In this screening technique, the two screening steps are reversed, such that the undersize particles are first se¬ parated and then the oversize particles. The inventive concept as defined in the accompanying claims however corn- pletely comprises the above-mentioned reversal of the screening steps.