Previously disclosed methods and arrangements for the purpose, e. g. DE 195 01 034 C2, operate with a liquid level that remains essentially constant, and in particular those that are intended for washing heavier particles which come from sewage works, primarily sand and gravel from toilets, kitchens and surface water.

The decanting of water and the organic material contained therein facilitates washing of the sand, etc., at a later stage. In order to achieve the best Coanda effect in the previously disclosed methods, it is necessary for the water level to be maintained at a specific level in the washing vessel in relation to a"Coanda tulip"applied to it for the decanting of water and lighter particles contained therein. No great variation in the liquid level is thus permitted. This is a disadvantage, since it is necessary to operate at all times with an essentially completely full washing vessel, which means that the washing is not always as effective.

The principal object of the present invention is thus, in the first instance, to solve the aforementioned problem by simple and effective means.

The aforementioned object is achieved by means of a method in accordance with the present invention as indicated above, which is characterized essentially in that the level

of the process water in the vessel intended for washing the sand is varied, in that a hydraulic pressure difference is achieved so that the received sedimented lighter material that is not intended to be washed is removed by decanting for a predetermined period of time by causing rotation of the lighter material so that it makes its way towards the centre at a level above the upper level of the sand, and in that washing water and/or some other fluid is supplied for the purpose of washing the sand.

The invention also relates to an arrangement for the implementation of a method for the separation of lighter particles from heavier particles in a liquid medium which contains lighter particles and heavier particles that are to be washed, in particular sand, and in particular at sewage works in the incoming waste water, where the heavier particles are removed at the lower end of a vessel and the lighter particles are removed at a level above the outlet for the aforementioned heavier particles.

Previously disclosed arrangements, for example in accordance with the above-mentioned, previously disclosed Patents, are not suitable for use when it is wished to be able to vary the level of the liquid in a washing vessel.

A further principal object is thus also to make available an arrangement which is suitable for use for the implementation of a method in accordance with the present invention.

The aforementioned further object is achieved by means of an arrangement in accordance with the above, and which is characterized essentially in that there is present, in a circular washing vessel which contains a centrally located rotating washing fluid supply device, an outlet part for lighter particles arranged at the centre of a level above the level for heavier material in the vessel and ready for washing, and in that the aforementioned device is so arranged as to supply washing fluid to the inner space of the vessel

during simultaneous stirring of the medium accommodated therein.

The present invention is described below as a preferred illustrative embodiment, in conjunction with which reference is made to the accompanying drawings, in which: Fig. 1 shows a diagrammatic and partially sectioned view of a system for cleaning waste water; Fig. 2 shows a detailed view of a sand washing part and the decanting of lighter material ; Fig. 3 shows a side view of the system ; Fig. 4 shows a variant of the system with a discharge valve; and Fig. 5 shows a top view of the system shown in Fig.

4.

The expression lighter is used to denote particles of a kind which float of their own accord or particles of a kind which do not float fully but which, with the help of a rising force, can be influenced in such a way that they are caused to lift from the heavier particles in the same vessel, i. e. that the density of the aforementioned lighter particles is below or approximately 1 kg/m3. The expression heavier particles is used to denote particles of a kind which do not float at all in water, but which sink rapidly to the bottom, such as stone material which exhibits a density significantly greater than 1 kg/dm3, e. g. 2-3 kg/dm3.

A system 1 in accordance with the present invention includes as a part thereof an arrangement 2, which is intended for washing heavier particles 3, in particular sand and gravel which comes from waste water of various kinds, such as domestic, industrial and surface water, and is intended specifically for treatment plants.

The dumping of contaminated sand which exhibits losses at red heat greater than 5% may not take place after the year 2005, and it is accordingly important to be able to

clean contaminated sand simply and economically so that there is no need to dump it.

An arrangement 2 in accordance with the invention can consist of a circular vessel 4, which exhibits vertical or inclined single walls 5, and a vessel part 8, 9 provided with straight vertical walls 6,7 can be connected to the aforementioned interjacent circular vessel 4 at the top and bottom respectively.

An arrangement 2 for the implementation of a method for the separation of lighter particles 10 from heavier particles 3, which are contained in a liquid medium 11, more specifically water, where the aforementioned sinking particles 3 are to be washed, in particular sand 3 and in particular at treatment plants in the incoming waste water 12, where the aforementioned heavier particles 3 are removed at the lower end 13B of a common vessel 13 in question, and the aforementioned lighter particles 10 are removed at a level 14 above the level 15 of the outlet 16 and 116 for the aforementioned heavier particles, comprises a decanting arrangement 17 and moving washers and stirrers 18,19.

Accommodated in the aforementioned circular washing vessel 13 is a centrally located rotating washing fluid supply device 18,19 supported by a rotatably 20 driven vertical drive shaft 21. Also present is an aforementioned outlet part 17 for lighter more or less buoyant particles 10 arranged in the middle thereof at a level 14 above the level 22 for heavier, non-floating material 3 intended for washing received in the vessel 13. The aforementioned device 18,19 is so arranged as to supply washing fluid 23 in the form of water, gas or some other liquid medium to the inner accommodating space 24 of the aforementioned vessel 13 during simultaneous stirring of the medium accommodated therein.

The aforementioned centrally located outlet part 17 for lighter particles 10 is formed by a tubular concentric hopper which exhibits a centrally located passage 25 for a

rotating shaft 21 to the aforementioned stirring and washing fluid supply device 18,19, and in turn a pipe 26 for washing fluid 23 extends internally therein.

The illustrative example comprises a lower and an upper washing fluid supply device 18, 19, of which the lower is so arranged as to rotate as part of the actual process of collecting the aforementioned heavier particles, the sand 3, while the upper acts at a higher level 27, albeit below the level 28 where an inlet 29 for the supply of a liquid medium 12 to the upper part 8 of the washing vessel extends tangentially at a level 28 below an upper outlet 30, intended for other lighter buoyant particles, which leads from the washing vessel 13 at its upper end 13A.

A discharge part 31,32 for sand, etc., from the lower part 13B of the washing vessel is formed by a valve 31 or a conveyor 32, preferably in the form of a rotatably driven screw conveyor 32 accommodated in an inclined tube 33.

The function of the aforementioned invention is for the sinking heavier particles 3 to be received at the bottom in the washing vessel 13, while the lighter particles 10 leave the washing vessel 13 together with the liquid medium 34 either through the upper outlet 30 or through the decanting funnel 17.

After pumping the aforementioned liquid medium 12 into the washing vessel 13, which takes place externally, a valve 35 in an outlet pipe 36 is opened for lighter particles and naturally also waste water, and by creating a hydraulic pressure difference the sedimented material is decanted more easily, for example the organic material that it is not intended to wash away, but instead to separate beforehand from the content of the washing vessel 13, which takes place during a predetermined period. The aforementioned lighter particles may, for example, consist of grains of corn, peas, beans, maize, wood and other similar products that are present in the matter removed from sewage.

The aforementioned lighter material 10 is caused to rotate like a whirlpool and makes its way towards the centre, while the sand 3 or some other non-buoyant particular material deposits itself at a level below the level of the aforementioned organic material 10 or other light material.

The sand, etc., 3, which it is intended to wash, moves past a lower rotating stirrer arm 18 or disc, which exhibits a number of outlet holes 37 to permit water 23 and/or air or some other appropriate washing fluid to flow out as it makes its way downwards 38 in the washing vessel 13.

Means are also provided for regulating the sand level 22 in the vessel 13, so that a certain level of sand is constantly present in the actual washing part 39 of the vessel.

The aforementioned discharge valve 31 or screw conveyor 32 is only opened or operating when it is wished to empty sand 3 from the vessel 13 and when it is wished to transport washed sand 3 from the vessel 13.

In addition to using water 23 as a washing medium, it is also possible, if unavoidable, to consider the use of another medium for washing in the event of it being necessary to mix in suitable chemicals, for example in order to release the sand from any contamination, such as oil, etc. The addition of the aforementioned chemicals preferably takes place by means of the above-mentioned liquid/air supply pipe 26.

In order to permit the washing process for heavier materials of the kind in question to continue, i. e. sand 3 or other non-buoyant particles, it is possible, as shown as an example in Fig. 1, to provide the rotating screw conveyor 32 internally with a central tube 40, which exhibits a number of radial vents 41 intended for the supply of water and/or air 42, which is supplied at the bottom of the axial screw 32 in its enclosing casing 33. The aforementioned central tube 40

is rigidly attached to a bottom end wall 50 at the bottom of the screw casing 33.

Once the sand 3 washed in the vessel 13 has been fed out through the bottom outlet 16 to the screw 32, it is subjected to further washing by means of the additionally supplied washing fluid 42 in the interior of the screw.

Return flushing of fluid 43 from the screw 32 and its enclosing casing tube 33 can, if so desired, be achieved by means of a pipe 45 running from the upper part 44 of the screw and to the washing vessel 13, preferably on a common level 46 with the upper part 13A of the vessel.

The present invention thus does not include any requirements for the level of fluid in the vessel 13 to be kept constant. This is advantageous, since the water level can be lowered before decanting of the lighter particles 10, and fresh water can subsequently be added in conjunction with the following washing process via the rotating washing arms 18,19. It is thus advantageous to be able to lower the water level in the vessel 13 before decanting, as it is not then necessary to wash the sand 3, etc., in such a large quantity of dirty water.

The invention is naturally not restricted to the embodiment described above and illustrated in the accompanying drawings. Modifications are possible, especially with regard to the nature of the various components, or by the use of equivalent technology, without departing from the scope of the protection of the invention as defined in the Patent Claims.