A method and plant for the screening of especially stone material

Background of the invention

The invention relates to a method for screening, especially of stone material, where the material is supplied to the uppermost end of an inclined sieve, said sieve being submerged in a water-filled vessel, and where the water has a pulsating movement through the sieve and also flows towards the lowermost end of the sieve so that the material on the sieve is fluidized by the upwardly-directed flow and stratified on the sieve in a subsequent order by the downwardly-directed and the longitudinal flow, and in such a manner that the lightest particles lie uppermost and the heaviest particles lowermost, whereby the material to be separated as well as the heavy fraction are dosed and whereby an overflow plate screens out the light fraction.

The invention also relates to a plant for the execution of the method.

Methods of this kind following the density-screening principle are known among other things for the screening of stone materials which can be used as aggregate material in concrete.

For use in concrete, the stone material must be of very high quality if damage due to so-called concrete deterioration is to be avoided. It has been known for several years that a content of flint, especially the lime chalco and lime chalcedon types of flint, in stone materials for concrete, causes a great deal of concrete deterioration, which arises as a result of the porosity of the relevant flint material. Lime chalco flint and lime chalcedon flint are to be found in varying amounts in more or less all gravel and sand deposits both on land and at sea.

A method for the removal of the relevant undesired components from stone material is screening according to specific gravity, in that the specific gravity is less for flint than for "healthy" stone such as granite and quartz.

Stone materials from 0-50 mm can be screened according to specific gravity. It will always be advantageous, and in many cases necessary out of regard for the classification, to carry out the screening of size-screened stone materials, which can be from 0-4 mm, 4-8 mm, 8-16 mm and 16-32 mm. Many earth stones and most of the sea stones can be improved by screening so that they fulfil the requirements for stone in material class M, which contains a max. of 5% light particles, and others so that they fulfil the requirements for material class A, which contains a max. of 1% light particles.

A method as described in the initial part of the description is known from GB-A-615 321, but the known method is not satisfactory since it cannot separate the lighter particles from the heavier ones with the precision required for stone in material class M.

Therefore it is the object of the invention to show a method by which the required precision can be obtained.

A plant for the screening by specific gravity is known from the description in the German patent publication no. 40 06 680. The plant is provided with a cell-wheel in the outlet for the heavy material fraction, and also has a water outlet at the same end of the screening chamber which is intended for the flushing out of the light material fraction which lies uppermost in the material layer. The removal of the heavy particles is regulated by controlling the cell-wheel's speed of rotation on the basis of a given setting which is compared with the signal from a depth sensor which detects the level between heavy and light particles. It is a prerequisite for this plant that there is a relatively large difference in the specific gravity between the heavy and the light components in the stone mixture, or alternatively that there is a great difference between the grain size of the light and heavy particles. If this requirement is not fulfilled, the depth sensor is unable to function satisfactorily, and the result will be a varying content of light particles in the coarse material fraction.

For stone materials of, among other things, earth stones and sea stones for concreting, where the differences in both specific gravity and grain size between the different components in the stone mixture are minimal, this known plant is unsuitable. However, since this plant is intended primarily for use in connection with coal mining operations and mining operations in general, where the conditions with regard to differences in specific gravity are fulfilled, the plant can screen in a satisfactory manner.

Consequently, this and other known screening plants do not satisfy the requirements for screening, in that their ability to screen the prescribed stone materials is not satisfactory.

Advantages of the invention

By using the method according to the invention, where the heavy fraction is dosed such that this fraction constitutes a predetermined part of the supplied material, and the level of the water in the wessel and the water discharge via the overflow are regulated by a bulkhead above the overflow plate, in a surprisingly simple manner there is achieved a specific-gravity screening, especially of stone materials, with a hitherto unknown high screening precision, even when there are differences in specific gravity and/or grain size which are minimal. The increasing demands which are presently placed on the quality of concrete, and the sharpened requirements which are placed on the quality of the stone, can hereby be fulfilled by the method.

This great screening precision is ensured by the combination of the dosing of the supply and the discharge of the material with an overflow plate and a bulkhead, while at the same time the method provides for a relatively large capacity for reasons of its continuous sequence.

The dosing actually ensures that the amount of removed material in the heavy fraction constitutes a certain preselected part of the supplied material. This percentage-wise screening of the stone material is advantageous, the reason being that in the extraction of material, e.g. from gravel pits, the composition of the material is known beforehand, and continuous control can be carried out by laboratory examinations. Consequently, should the composition change during a sequence of screening, it is possible merely to adjust the dosing, after which the quality of the screening is re-established. It is possible for this adjustment to be effected automatically.

The controlled supply and removal of stone materials, together with the screening by percentage of the two fractions, means that the plant can be made relatively simple in its construction, that the process becomes easy to supervise and control, and that the screening becomes independent of an inaccurate detection of the stratification in the screening chamber. The result is a method which distinguishes itself by being able to screen stone materials with very small differences in specific gravity between the different fractions, and which at the same time has a relatively high screening capacity with great screening precision.

The overflow plate ensures a precise removal of the light fraction which lies uppermost in the layer of material, and which is flushed out together with the rinsing water. The amount of the light fraction can be determined by suitable regulation of the level of the plate.

Finally, the regulation of the water level means that the degree of flushing away with the rinsing water can be determined as that layer of material which is cut off by the overflow plate. To this can be added that the damming ensures a calm course of run-off in the area at the discharge end, and at the same time that the stone layer on the sieve is displaced in a uniform manner without any mixing together of the layers before the separation by the overflow plate. Furthermore, a low consumption of water is ensured.

As disclosed in claim 2, by configuring the plant with dosing means with synchronously-coupled electromotors for the regulation of the screening speed, and herewith the capacity, there is ensured a simple construction and means for the regulation of the dosing of material to and from the plant.

As disclosed in claim 3, by configuring the sieve so that its slope in relation to the horizontal can be adjusted, a uniform thickness of the layer is ensured for the whole length of the sieve.

As disclosed in claim 4, by configuring the sieve with a uniform mesh size and hole distribution, there is ensured a uniform through-flow of water without any formation of eddy currents, and herewith a more distinct division of the layers, which ensures the screening precision.

As disclosed in claim 5, by using pulsating compressed air to bring the water into pulsation, there can be achieved a uniform water pulsation throughout the whole extent of the sieve. A correspondingly uniform fluidization is hereby achieved, and herewith a uniform distribution and layer stratification.

As disclosed in claim 6, by being able to control the pulsation, this can be adjusted according to requirements in order to achieve the greatest possible screening precision on the basis of the material available.

As disclosed in claim 7, by changing the distribution characteristic of the pulsation, and a possible throttling of the air exhaustion, the downwardly-directed streams of water through the sieve can be retarded, which ensures a calm and therewith a good precipitation of the material layers.

Finally, as disclosed in claim 8, it is expedient to ventilate above the removal cell wheel, in that any variation which may arise in pressure and flow can be equalized, hereby ensuring the calmest possible course of flow at the discharge end, which is of decisive importance for the screening precision.

The drawing

In the following, an embodiment of a plant for the execution of the method according to the invention will be described in closer detail with reference to the drawing, where

fig. 1

shows a longitudinal section through a plant seen in the direction I-I in fig. 2, and

fig. 2

shows a cross-section through the plant seen in the direction II-II in fig. 1.

Description of the example embodiment

In the drawing is shown an example embodiment of a screening plant according to the invention.

As shown in fig. 1, the plant comprises an inclined sieve 1 which is mounted in a vessel 6 in such a manner that the slope of the sieve 1 can be adjusted in relation to the horizontal.

The vessel 6 is filled with water 15 which is supplied through a valve 16 in order to ensure that the water level lies above the sieve 1.

The sieve 1 is configured as a frame with a fine-meshed net in the bottom, said net extending for the whole length of the sieve.

The material 12 to be screened is led to the plant via a rotating cell-wheel 2, the rotational speed of which determines the dosing to the uppermost end of the sieve 1.

At the opposite end of the vessel 6 there extends a horizontal overflow plate 4. This plate can be adjusted in height for the determination of the part of the water which is led out over the plate 4, and which separates the light fraction 13 from the material 12.

At a distance behind the front edge of the plate 4 there is mounted a vertically-extending bulkhead 5. This bulkhead can likewise be adjusted in height for the determination of the discharge opening for the water.

At the lowermost end of the sieve 1 there is a discharge chamber with ventilation duct 11 which opens out over the surface of the water.

Below this chamber there is mounted a second cell-wheel 3, the rotational speed of which similarly determines the amount of that heavy fraction 14 which is screened from the supplied material 12.

The vessel 6, which is shown in cross-section in fig. 2, is built together with a pressure chamber 7 which extends along the one side and for the full length of the vessel 6. The pressure chamber 7 stands in direct fluid connection with the vessel, so that the pulsations which are generated by the alternate blowing-in and evacuation of compressed air from the pressure chamber 7 are transmitted to the water 15 in the vessel 6.

The vessel 6 and the pressure chamber 7 function as connected vessels, whereby a change in the level in the chamber 7 will give rise to a corresponding but opposite change of the level of the water 15 in the vessel 6. The changes in level, or the pulsations, generate the upwardly and downwardly-directed flows of water through the sieve 1, which are used for the specific-gravity screening of the material 12 lying on the sieve.

To effect the sequential injection and exhaustion of compressed air to and from the chamber 7, the top of the chamber 7 has a built-in slide valve 8 comprising an elongated slide which extends for the full length of the chamber. The slide is moved forwards and backwards by a not-shown mechanical activation arrangement, and in doing so it opens and closes a series of ports which alternately connect the chamber with a source of compressed air and the surrounding atmosphere. In fig. 2, the supply of compressed air is indicated by the reference figure 9, and the air exhaustion by the reference figure 10. The air exhaust system has a built-in, not-shown throttle valve. This valve can be regulated and can thus damp the downwardly-directed water flow.

The following is a description of the method:

The raw material 12 is dosed to the plant via the cell-wheel 2 in such amounts that the removal via the cell-wheel 3 constitutes a predetermined part hereof.

The two cell-wheels 2 and 3 are driven by separate electromotors, the rotational speeds of which are regulated by commonly-known frequency transformers. The two cell-wheels 2 and 3 are synchronized via an electronic surveillance and control system to rotate at a certain mutual ratio which can be regulated during operation.

Besides this dosing, the level of the bulkhead 5 and the overflow plate 4, and the angular position of the sieve 1, are set in accordance with the composition of the material and the desired degree of screening. Furthermore, the valve 8 is set for a suitable pulsation of the water 15 through the sieve 1.

During operation, the material 12 is fed to the sieve 1 where it is fluidized by the upwardly-directed flow of water through the mesh of the sieve, and is thereafter precipitated by the downwardly-directed flow of water and the influence of gravity. The heavy fraction will deposit itself lowermost in the layer, while the light fraction will settle uppermost.

The stratification is concluded at the end of the sieve 1, and the separation of the light fraction is effected with the water which is led out over the overflow plate to the discharge.

The heavy fraction 14, on the other hand, will remain in the water and be led out at the end of the sieve to the cell- wheel 3.

The method and the plant ensure a hitherto-unknown high degree of precision in the screening of raw materials 12 having differences in both specific gravity as well as differences in size.

The screening is effected in a completely continuous manner and uses the least possible amounts of water and energy.