Device for aerating fluids, in particular during flotation
BACKGROUND OF THE INVENTION 
     The present invention relates to a device for aerating liquids or fluids, especially during flotation, and is embodied as a ring injector, in the housing of which, through which the fluid flows, there is disposed a middle piece that forms an annular channel with the housing, whereby at the level of the air discharge openings in the wall of the housing, the annular channel has its narrowest cross-sectional area; following this narrowest location of the annular channel in the direction of flow of the fluid, the annular channel undergoes a sudden increase in cross-sectional area, adjoining which a mixing and dispersing section leads to the outlet of the housing. 
     Devices of this type are known, and are used, for example, during flotation in order to separate the solid materials contained in a slurry from one another. The difficult-to-moisten solid particles are deposited on air bubbles that are generated in such devices, which are also known as gasifiers, injectors, or reactors. The solid material deposited on the air bubbles is generally discharged over the edge of the separating or flotation vessel. 
     With all aerating devices of this type it is desirable to keep the size of the air bubbles as small as possible, because this facilitates the deposition of solid particles during the flotation. 
     A device for mixing gases and liquids was already disclosed 50 years ago in U.S. Pat. No. 1,810,131. This device shows features of an aerating device such as was later disclosed (DE-OS No. 34 12 431) as a device for aerating dispersions, especially as a flotation device for delivering fibrous material suspensions or dispersions with a ring injector. 
     Air is supplied to the ring injector via air supply passages in the mixing section. As a result, the annular gap formed in the ring injector is aerated on the inside and the outside. The mixing section and the middle piece, which is centrally disposed within the mixing and dispersing section, are stepped, as a result of which, similar to a water-jet pump, the air is drawn into the inner and outer regions of the dispersion stream that is formed. 
     It is an object of the present invention to substantially improve devices of the aforementioned general type so that the specific energy consumption is reduced and the degree of selectively is increased. 
     This device should be especially suitable for the flotation of coal and other preparation material. 
     SUMMARY OF THE INVENTION 
     To realize this object, it is proposed pursuant to the present invention that the aerating device of the aforementioned general type have, as the slurry enters the aeration chamber, a gap width that leads to a minimum slurry speed V 1  of 2.0 m/s. The middle piece of the device extends to the outlet of the housing and the diameter remains constant. The annular channel has the same cross-sectional area from the sudden increase to the outlet of the housing, and the length of the mixing and dispersion section is more than 20 times the annular gap width of the annular channel. 
     Due to the uniform speed in the annular channel resulting from the constant cross-sectional area, the thorough mixing of air bubbles and solid material particles during the course of the mixing section is particularly favorable, since during the course of the entire mixing section the specific energy input is constant. 
     Tests in a coal-flotation plant have shown that at a slurry speed of greater than 2 m/s, especially at 2.0 to 4.0 m/s, the yield is optimum. 
     Further structural details will be described in detail subsequently. 
     The splitting of the core or essential portion of the aerating device into the distributing cone and the middle piece, which can be screwed together or inserted into one another, makes it possible to combine different distributing cones and middle pieces. Depending upon the requirements, it is possible to use distributing cones having different diameters of the base of the straight circular cone, and/or cones having different angles between the base and the side lines of the circular cone, i.e. with increased or decreased slope. By changing the base diameter of the distributing cone, the cross-sectional area of the annular channel is varied at the level of the air discharge openings. The slurry distribution can be influenced by the angle of inclination of the cone. 
     The strips disposed on the middle piece and/or in the upper part of the housing serve to center the middle piece within the mixing and dispersion section. Such strips can also have a helical shape, and it has been shown that in so doing a swirl flow overlaps the axial flow of the slurry-air-mixture; this swirl flow has an advantageous effect upon the deposition of the solid particles on the air bubbles. 
     This is also true for the embodiment of a helically grooved or slotted middle piece and/or a similarly grooved or slotted inner wall of the upper part of the housing. In this case, centering of the middle piece can be effected, for example, by centering pins that hold the middle piece in the upper part of the housing. 
     
                                           TABLE 1                                 
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Test conditions and results with different                                
ring injectors                                                            
                             Mater-                                       
        Length      Retention                                             
                             ial  Ash                                     
        L.sub.1 of  time in  yield                                        
                                  content                                 
Gap     the     Slurry                                                    
                    the  Deliv-                                           
                             in the                                       
                                  in the                                  
width   mixing  speed                                                     
                    mixing                                                
                         ery ash                                          
                             concen-                                      
                                  concen-                                 
Q.sub.2 section                                                           
            Ratio                                                         
                v.sub.1                                                   
                    section 6                                             
                         content                                          
                             trate                                        
                                  trate                                   
(mm)    (mm)                                                              
            L.sub.1 /Q.sub.2                                              
                (m/s)                                                     
                    (s)  (a %)                                            
                             (m %)                                        
                                  (a %)                                   
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Type 1                                                                    
    6   125 20.8                                                          
                9.1 7.3 · 10.sup.-2                              
                         21.4                                             
                             55.5 7.4                                     
Type 2                                                                    
    3   125 41.7                                                          
                24.5                                                      
                    5.1 · 10.sup.-3                              
                         22.5                                             
                             64.6 7.5                                     
Type 3                                                                    
    5   600 120.0                                                         
                14.8                                                      
                    4.1 · 10.sup.-2                              
                         20.4                                             
                             70.9 4.9                                     
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     Table 1 shows the test conditions and test results that were achieved with inventive ring injectors of the Types 1 to 3. During the flotation tests, at half of a commercial scale, a flotation slurry from a mineral coal wash was used as the feed material. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     One exemplary embodiment of the inventive aerating device is described in detail with the aid of the schematic drawing. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The cross-sectional view of the illustrated aerating device comprises a multi-part housing. The lower part (3) of the housing is provided with the connection flange (1) to which the (not illustrated) slurry line is connected. The arrow indicates the direction of flow of the slurry. The inlet diameter of the aerating device is designated by (Q 1 ). 
     Connected to the lower part (3) of the housing is a housing section in which is disposed the air chamber (4) with the annular air discharge opening. The aerating medium flows to the air chamber (4) via a (not illustrated) air line. 
     By inserting spacer rings (13), the gap width of the air discharge openings in the annular gap of the air chamber (4) can be varied. 
     Disposed above the lower part (3) of the housing with the air chamber (4) is the upper part (7) of the housing. The flange (5) serves for the securement of the aerating device within a line to the (not illustrated) separating vessel or, where the aerating device is disposed externally of the separating vessel, as a line connection on the separating vessel. 
     Disposed within the housing of the aerating device is the core or essential portion, which comprises a distributing cone (2) and the middle piece (10). The distributing cone and middle piece can be screwed together or inserted into each other. The separating location is at the transition of the reduction of the cross-sectional area of the distributing cone to the middle piece of uniform diameter. The middle piece (10) has a diameter that is uniform over its entire length to the outlet (12) of the housing. 
     Disposed at the level of the air outlet from the air chamber (4) is the narrowest location of the annular channel for the through flow of the slurry. This narrowest location (Q 2 ) is formed by the largest diameter of the distributing cone (2). The abrupt increase (11) in the cross-sectional area of the annular channel between the distributing cone and the inner wall of the upper part of the housing, which increase in the cross-sectional area results from the structural configuration of the upper portion of the distributing cone (2), prevents, despite the fact that air is received from the air chamber, an increase in the speed in the flotation slurry. 
     Adjoining the narrowest location of the annular channel is the mixing and dispersing section (6), which has a length L 1  and a uniform gap width (Q 3 ) until the outlet (12) of the housing. 
     Indicated on the middle piece (10) is a helically disposed strip (8) that is intended for centering the middle piece in the upper part (7) of the housing and for generating a swirl flow of the slurry-air-mixture. 
     A holding mechanism (9) in the form of an adjustable pin is disposed on the upper part (7) of the housing below the outlet (12) of the housing. 
     The present invention is, of course, in no way restricted to the specific disclosure of the specification, Table 1, and drawing, but also encompasses any modifications within the scope of the appended claims.