SE463079B - SETTING AND DEVICE FOR SIGNING CORN-MATERIAL MATERIAL - Google Patents

Description

15 20 25 30 35 CQW LN 'CI) \ C2 \ O 2 paration limit 12 mm. It is not uncommon for a fraction prepared in this way to contain 30-35% by weight of sub-grains.

Traditionally, the above-mentioned problems have been solved by making the visibility holes of the lower visor deck 10-20% larger than the desired separation limits, in the example above 14 mm large holes. Since screening for obtaining different fractions usually takes place in several stages, this procedure causes inconvenience to the extent that the next smaller fraction simultaneously has a relatively high proportion of upper grains. Especially in the production of so-called short fractions, eg 8-1 mm, a change in the neck size has a very large effect on the proportion of defective grains.

When producing the fraction exceeding 22 mm, a hole size of 24 mm is traditionally chosen in the corresponding manner in the upper sight deck.

When sieving according to the above-mentioned method, the different fractions will thus still contain a relatively high proportion of error grains, often in the order of 10% by weight of upper grains and 20% by weight of lower grains.

However, the market places ever higher demands on clean fractions for use as ballast in concrete or asphalt, for road construction and for many other purposes. The requirements are particularly high when the material is to undergo transhipments, for example to and from ship's hold, whereby the larger grains in the material are easily crushed so that the proportion of sub-grains increases. Recently, fractions have been in demand on the market in which the total proportion of defective grains is less than 15% by weight.

There is thus a need to be able to produce very pure fractions, which contain grains within precisely defined separation limits and which have a significantly lower proportion of defective grains than has hitherto been possible to achieve by conventional sieving methods.

A main object of the invention is thus to provide a method of sieving granular material for obtaining pure fractions with a proportion of defective grains which are significantly less compared to fractions prepared by previously known sieving methods. .

Another object is to provide a device for sieving granular material, by means of which said pure fractions can be produced and which consists of a few simple and inexpensive components.

These and other objects, which will appear from the following description, have now been achieved according to the invention by providing the method and the device defined in the following claims 1 and 2.

In short, the solution proposed according to the invention to the problems discussed above can be said to consist in that the upper separation limit of the current fraction is determined in a first sieve while the lower separation limit is determined in a second sieve. It 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 described in more detail in the following with reference to the accompanying drawings, in which Fig. 1 shows sieving according to prior art, Fig. 2 shows the grain distribution in the obtained finished fraction, Fig. 3 shows sieving according to the invention, Fig. 4 shows the grain distribution in an intermediate product and in finished fraction and Fig. 5 shows screening according to an alternative embodiment.

Fig. 1 schematically shows an example of conventional sieving of crushed stone material to obtain a finished fraction with a lower grain size or separation limit of 12 mm and an upper separation limit of 22 mm. The granular material is fed to a screen 1, comprising an upper screen deck 2 with 24 mm screen holes and a lower screen deck 3 with 14 mm screen holes. The size of the sight holes thus exceeds the two separation limits by about 9 and 17%, respectively, as is usual. The grains which are 465 10 15 20 25 30 35 079 4 larger than 22 mm must be removed from the upper screen deck 2 while grains which are smaller than 12 mm must pass both screen tires 2, 3. Between the screen tires 2, 3 the finished fraction 12-22 mm is obtained, which is removed from the lower screen deck 3.

As can be seen from Fig. 2, the finished fraction '| contain about 20% by weight of lower grain and about 10% by weight of upper grain, while the rest consists of grain of the desired size.

Note that the percentages given are only examples of the order of magnitude that is usually in question.

Fig. 3 shows an example of two-stage screening according to the invention. The screening device 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.

Unlike before, all grains exceeding 22 mm are removed from the upper sieve deck 5. Grains less than 12 mm must pass through both sieve decks 5, 6. Between the sieve decks 5, 6 an intermediate product is obtained, which for the most part contains grains in the size range 12- 22 mm but which also has a relatively high proportion of lower grains.

The intermediate product is fed to a second screen 7 with a single screen deck 8, the screen holes of which are 15 mm in size, ie slightly exceeding the lower separation limit of 12 mm. A large part of the grains which are smaller than 12 mm, ie lower grains, thus pass through the holes in the screen deck 8, whereby the finished fraction obtained at the end of the screen deck 8 will contain a very low proportion of bottom grains. The lower grains that pass through the holes in the screen deck 8 can be screened again, possibly after a crushing. Alternatively, these sub-grains can be mixed with the grains less than 12 mm obtained in the first sieve 4.

According to the invention, the sieving thus takes place in two steps, where the upper grains are separated in the first step while the lower grains are separated in both the first and the second step. For best results, the second sieve 7 sieve deck 8 should have a slightly larger hole than the first sieve 4 lower sieve deck 6.

As can be seen from Fig. 4, the resulting finished fraction 12-22 mm will thereby contain very low proportions of grain outside the desired range.

Fig. 5 shows a variant of the screening shown in Fig. 3, all sieve tires being assembled in one unit. The material is fed to an upper screen deck 9, the screen hole size of which corresponds to the upper separation limit of 22 mm. Directly below the upper screen deck 9 there is a screen deck 10 with 12 mm screen holes and a screen deck 11 with 15 mm screen holes placed directly thereafter with respect to the material flow. The screen tires 10, tires 6 and 8 in Fig. 3 and have the same function. The resulting finished fraction is essentially of the same quality corresponding to visibility as in Fig. 3.

In order to return to Fig. 3, the screen deck 5, 6 of the first screen 4 according to the invention must have screen holes which are equal to or slightly larger than the upper and lower grain size limit, respectively. In both cases, the sight holes should be a maximum of about 25% larger than the separation limits. In the second screen 7, the screen holes should be larger than the lower separation limit, preferably about 5-35% larger. Note that these percentages are only guideline values, which are given here for exemplary purposes.

When thick screening elements are used, for example rubber screens where the screening holes can be regarded as channels of a certain length, the holes must be relatively large in relation to the separation limit in order for the grains to be able to pass.

Thin screen elements, such as metal wire screens, can conversely have relatively small screen holes. How much larger than the separation limits the sight holes should be depends, among other things, on the fraction length and on what requirements are placed on the finished fraction, ie how high a proportion of error grains can be accepted. It should be pointed out that the invention is by no means limited to the types of screens described here, but can be used with different types of screens and screen elements.

Furthermore, the size of the sight holes means the limiting transverse dimension of the hole, for example the diameter of circular heels, as is customary in the screening technique.

One skilled in the art will readily appreciate that the screening devices according to the invention shown in Figures 3 and 5 are usually part of an entire screening plant, in which several different fractions are produced. The fraction less than 12 mm can, for example, be further sieved to obtain other fractions within this fraction length. The screening method according to the invention is then applied once more.

Finally, it should also be pointed out that the invention is equally applicable to reverse sieving, ie when the sieving decks are placed one after the other, whereby the first sieving deck has the least sight holes while the last sight deck has the largest sight holes. In this sieving technique, the two sieving steps are reversed so that the lower grains are separated first and the upper grains thereafter. However, the inventive concept defined in the appended claims has complete equivalence with the above-mentioned reversal of the screening steps. fl »

Claims (2)

10 15 20 25 30 J * CK (j l CÖ w QD PATENT REQUIREMENTS A method of sieving granular material to provide a fraction having predetermined lower and upper grain size limits by means of a screening device in which said fraction is obtained while lower grains and upper grains lying outside said grain size limits are separated and removed, the proportion of lower grains and upper grains in the finished fraction shall be less than predetermined values, the method comprising a first sieve step, during which upper grains are separated by means of a first sieve deck (5), the sieve holes, depending on the permissible proportion of upper grains in the fraction, being equal to or slightly larger, preferably a maximum of about 25% larger than the upper grain size limit, while the lower grain is separated by a subsequently placed second sieve deck (6), the sieve holes, depending on the permitted proportion of upper grains in the following fraction, are equal to or slightly larger, preferably a maximum of about 25% larger than the lower grain size limit, whereby an intermediate product containing a certain proportion of sub-grains and e In relation to this, a smaller proportion of upper grains is obtained between the two sieve tires, characterized by a subsequent second sieving step, during which the lower grains of the intermediate product are separated once more by at least one additional sieve deck (8), the sieve holes, depending on the permissible proportion of lower grains in the fraction , is larger, preferably about 5-35% larger, than the lower grain size limit, whereby the material passing the additional screen deck (8) is removed from the continued screening process, whereby the finished fraction is obtained after the additional screen deck (8). ) without the lower grain size limit automatically constituting the upper grain size limit for the subsequent fraction. Device for sieving granular material to produce a fraction with predetermined lower and upper grain size limits during separation and removal. .Ps Ö '\ GJ C? \; | (C) 8 feedingstuffs and upper grains outside the said grain size limits, the proportion of lower grains and upper grains in the finished fraction being less than predetermined values, the device comprising a first sieve having a first sieve deck intended for separating upper grains ( 5), whose sieve hole, depending on the permissible proportion of upper grains in the fraction, is equal to or slightly larger, preferably a maximum of about 25% larger, than the upper grain size limit, and a second, subsequently placed and for separation of sub-grains intended sieve deck (6), the sieve of which, depending on the permissible proportion of upper grains in the subsequent fraction, is equal to or slightly larger, preferably a maximum of about 25% larger than the lower grain boundary, the first sieve giving an intermediate product between the two sieve tires (5 , 6) containing a certain proportion of lower grains and a correspondingly smaller proportion of upper grains, characterized by a second sieve, to which the intermediate product is fed and which has at least one sieve deck (8) intended for further separation of sub-grains, the sieve hole, depending on the permissible proportion of sub-grains in the fraction, is larger, preferably about 5-35% larger, than the lower grain size limit, the material passing the additional sieve deck (8 ) is removed from the continued sieving process, whereby the finished fraction is obtained after the additional sieving deck (8) without the lower grain size limit automatically constituting the upper grain size limit for the subsequent fraction.