Title:
MIXER
United States Patent 3578876


Abstract:
A mixer is provided with a plurality of impellers which are mounted to hang vertically in a tank. The upper impeller is disposed at a fixed vertical depth and the lower impeller is movable vertically. The shaft of the lower impeller extends through the shaft of the upper impeller and is normally coupled thereto to rotate therewith. The shaft of the lower impeller can be uncoupled from that of the upper impeller and axially supported when both shafts are not rotating. Once uncoupled, the shaft of the upper impeller can again be rotated while the shaft of the lower impeller remains stationary. In alternate embodiments, the shaft of the lower impeller can be fixed at a plurality of vertical positions and the power function of the upper impeller can be changed.



Inventors:
STRATIENKO ANDREW
Application Number:
04/831117
Publication Date:
05/18/1971
Filing Date:
06/06/1969
Assignee:
KING OF PRUSSIA RESEARCH AND DEVELOPMENT CORP.
Primary Class:
Other Classes:
366/293, 366/330.1
International Classes:
B01F7/16; (IPC1-7): B01F7/24
Field of Search:
416/120,121,124,130,169,170,125,126,127,130 259
View Patent Images:
US Patent References:
2898094Paint mixer1959-08-04O'Neill
2668694Apparatus for agitating materials1954-02-09Kline et al.
2609185Method and apparatus for increasing fluidity of material1952-09-02Eisner
2516917Agitator1950-08-01Roubal
2209287Apparatus for mixing1940-07-23Simpson
1467537Air propeller1923-09-11Dornier



Primary Examiner:
Powell Jr., Everette A.
Claims:
I claim

1. In a mixing apparatus having a plurality of impellers mounted to hang vertically on a plurality of drive shafts from a support means, the improvement which comprises:

2. The invention of claim 1 wherein the shaft of the lower impeller is provided with means to raise or lower said lower impeller shaft to a plurality of fixed predetermined vertical positions.

3. The invention of claim 2 wherein said coupling means provides a means whereby said shafts remain coupled while said lower impeller shaft is raised or lowered to a predetermined fixed position.

4. The invention of claim 1 wherein means are provided for varying the speed of said impellers.

5. The invention of claim 1 wherein means are provided coacting with said upper impeller to vary the power output thereof.

6. The invention of claim 1 wherein said coupling means comprises a member slidably connected to said shaft of the lower impeller for rotation therewith and having means extending therefrom for engaging complimentary means inwardly disposed on said hollow shaft.

7. The invention of claim 6 wherein said member includes a plurality of hollow tubular members bearing against said lower impeller shaft and being keyed for rotation therewith by means of a key, said shaft having a longitudinally extending slot forming a keyway for said key, said member having splines about the outer surface thereof, said hollow shaft having mating splines for engaging the splines of said member.

8. The invention of claim 1 wherein said coupling means comprises a longitudinally extending keyway in said lower impeller shaft, a key disposed in sliding engagement in said keyway, a collar disposed about said lower impeller shaft having a plurality of holes therein, said collar engaging said key for rotation therewith, a coupling member attached to said hollow shaft for rotation therewith having a plurality of holes therein, and a plurality of pins disposed to extend through the holes of said collar and into the holes of said coupling member, said pins being in sliding engagement with said collar and said coupling member.

9. The invention of claim 8 wherein removable support means are provided for engaging said shaft of said lower impeller and supporting it on said collar and means are provided for raising said collar when said support means has been removed, and bearing means are provided for supporting said collar when said pins have been removed.

10. The invention of claim 1 wherein said means for uncoupling the shafts comprises disengageable coupling means and means attached to said lower impeller shaft for raising or lowering said lower impeller shaft.

11. The invention of claim 6 wherein means are provided attached to said member to slide said member axially along the shaft of said lower impeller to disconnect said member from the hollow shaft so that said upper impeller shaft can be rotated while said lower impeller shaft remains stationary.

12. The invention of claim 7 wherein means are provided retaining said key in said member for movement therewith and means are provided connected to said member for moving said member axially to engage or disengage the splines and thereby couple or uncouple the shaft of the lower impeller to or from the shaft of the upper impeller.

13. The invention of claim 8 wherein said pins are disposed axially with respect to said shafts and can be removed from said collar in order to disconnect said shafts from one another.

Description:
BACKGROUND OF THE INVENTION

The present invention relates to mixers, and more particularly to mixers for heavy industrial uses in processes involving the suspension of solids in liquids.

In industrial uses such as the mining of coal, it is common to remove the coal dust by mixing it with water to form slurry. The slurry is often transported to receiving tanks where it is mixed with water or other solvents to form a homogeneous mix and then is again transported for further processing. Where tremendous quantities of slurry are moved and mixed, the quantities themselves become an important factor in determining what methods and apparatus should be used. Prior art devices such as that shown in U.S. Pat. No. 3,142,478 issued July 28, 1964 illustrate one method of handling large quantities of slurry. In said patent it is suggested to raise and lower the mechanism progressive while it is rotating and agitating in a tank. Such an apparatus in use could become jammed as where for example, the shaft and impeller are in their lowest position when the machine is shut down. Almost immediately the solids would begin to settle out of the liquid and build up on the bottom of the tank. If the machine was not started promptly, the impeller would become embedded in the ever increasing mass of solid particles settling out of the liquid. In such a situation it would be difficult if not impossible to get the machine functioning again without resorting to draining the tank and digging the impeller out of the settled mass of particles.

SUMMARY OF THE INVENTION

The present invention overcomes the deficiencies of the prior art by providing a plurality of shafts and impellers which can be commonly driven and which can be uncoupled so as to be separately driven. In the preferred embodiment of my invention I have provided a mixer of a type which is most preferably mounted vertically, as for instance on the top of a tank, so that its shaft and impellers extend downwardly into the tank. Its main shaft has an impeller which is preferably fixed at predetermine depth. A second shaft extends through the main shaft and supports a second impeller which is positioned below the first impeller. The second shaft is normally coupled to the first shaft to rotate therewith. When the shafts are not rotating the second can be uncoupled from the first shaft and axially supported. Once uncoupled the main shaft can again be rotated while the shaft of the lower impeller remains stationary.

In alternate embodiments of my invention, I provide means for fixing the lower impeller in a plurality of vertical positions in which it may be either coupled or uncoupled from the main shaft.

In still further embodiments of my invention, I provide means for varying the power input and output of the main shaft and impeller.

Accordingly, it is an object of my invention to provide a new and novel mixer principally for industrial uses, having a plurality of impellers and means for rotating one independently of the others.

It is another object of this invention to provide a mixer for use in applications of the type described in which means are provided for reagitating a slurry in which the suspended particles have settled out and rendered immobile at least one impeller of the mixer, whereby the impeller may be again freed.

It is another object of this invention to provide a mixer for use in applications of the type described which has the capacity for varying its power in accordance with various functions it must perform, so that the power input can be selected for the best efficiency in the particular operation.

These and other objects of my invention will become apparent with reference to the following disclosure and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevation of an embodiment of my invention shown in operative position in its environment in a tank (partially broken away), with alternate positions shown in phantom;

FIG. 2 is a section taken as indicated by the lines and arrows 2-2 in FIG. 1;

FIG. 3 is a section taken as indicated by the lines and arrows 3-3 in FIG. 1;

FIG. 4 is a section taken as indicated by the lines and arrows 4-4 in FIG. 2, a portion being broken away for clarity;

FIG. 5 is a section taken as indicated by the lines and arrows 5-5 in FIG. 2;

FIG. 6 is a view taken as indicated by the lines and arrows 6-6 in FIG. 2 showing only a partial section for the sake of clarity;

FIG. 7 is a partial section similar to FIG. 2 showing portions of the apparatus enlarged and moved to an alternate position;

FIG. 8 is a view, similar to FIG. 1, of the preferred embodiment of my invention;

FIG. 9 is a view taken as indicated by the lines and arrows 9-9 in FIG. 8;

FIG. 10 is a view similar to FIG. 9 showing a portion of the apparatus with certain parts removed and moved to an alternate position; and

FIG. 11 is a view taken as indicated by the lines and arrows 11-11 in FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Although specific forms of the invention have been selected for illustration in the drawings, and the following description is drawn in specific terms for the purpose of describing these forms of the invention, this description is not intended to limit the scope of the invention which is defined in the appended claims.

Referring to the embodiments shown in FIGS. 1--7 I shall describe one apparatus in accordance with my invention which has particular applicability to installations wherein an unusually long shaft is necessary, particularly of the type which could not be readily transported. This embodiment is somewhat more complicated than the preferred embodiment as shown in FIGS. 8--11. However, many of the features shown are similar or are identical to those shown in the preferred embodiment. Referring then to FIG. 1, the mixer is shown in elevation in its normal operative relation to the environment wherein it will probably be most useful, that is, a vessel or tank designated generally 20. The tank is shown partially broken away for the sake of disclosing the mixer in full view, and partially in phantom to indicate that its size may vary greatly. A slurry 22 is shown within the tank and consists of finely divided solid particles suspended in a liquid. As long as the liquid is properly agitated, these particles will remain suspended and they most preferably will be suspended as to form a homogeneous mixture. However, should agitation cease, the particles will settle out and filter to the bottom of the tank where they will pile up like silt. Depending on the constituents of the slurry, this silt may amount to a third, more or less, of the volume of the tank. Whatever the mixture and size of the tank may be, it is within the contemplation of my invention to provide a mixer which has an impeller permanently fixed at a predetermined vertical depth where it will always be above the silt level and will always be suspended in the liquid. This height is determined by the solid content of the liquid and the dimensions of the tank.

In FIG. 1 the impeller 30 is supported on the main shaft 32 at the desired predetermined level. The shaft 32 is supported and driven by the apparatus disposed on the frame on top of the tank 20. The frame 34 consists of a plurality of upright structural steel beams. As shown particularly in FIG. 2 the frame 34 has welded thereto a plurality of structural steel plates, 36, 38, 40 and 42. Suitable reinforcing plates 44 and 48 are welded or otherwise interconnected to the plate 42 to form a rigid support assembly together with web members 50 and 52. Additional web members and supporting plates may be provided as desired to support the load of the motors and superstructure and provide a rigid frame for mounting the shafts in driving relation with the motors.

A plurality of drive motors, as at 55 and 56, having integral speed-reducing boxes 57 and 58 are mounted on plate 40 in radial relation about a common center. The output shafts 41 and 43 of the boxes 57 and 58 respectively are connected with pinion shafts 59 and 60 through flexible couplings 45 and 46. The pinion shafts 59 and 60 extend through the plate 40 and are journaled in bearings in the plates 40 and 42. Each pinion shaft supports a helical pinion 61 and 62 respectively that mates with a common ring gear 63 which is keyed by means of a key 64 to the cylindrical midsection 65 of the main shaft. The midsection 65 is journaled in thrust bearings 66 supported within pillow blocks which are welded to the frame 34. Cylindrical sleeve 68 is disposed about the midportion 65 and retains the bearings and the hub 70 of the ring gear 63 in spaced relation. The bearings are retained on shoulders on the midmember 65 and the frame as shown. An annular locking member 72 is threaded onto the upper end of the midportion 65 and locks the hub 70 in axial position on the midportion 65. In this manner rotation of the ring gear 63 will cause rotation of the midportion 65 in the bearing 66.

The main shaft 32 is a composite shaft consisting of a plurality of tubular members of varying diameters and thicknesses. The midmember 65 is welded or otherwise suitably fixed to a downwardly extending tubular member 75 which can be of any convenient length which inturn is welded to a substantially cylindrical flange fitting 76. This fitting is bolted to the mating flange of a similar fitting 78 to which another length of tubular shafting 80 is welded or connected in any suitable manner. The shafting 80 is welded or connected in any suitable manner to the hub of the mixer blade assembly 30 as shown in FIG. 1.

The blade assembly is old in the art and need not be described in greater detail, except to say that it has a plurality of spokes each of which has a paddle at the end thereof, which can be disposed at an angle as shown. It is within the contemplation of an alternate embodiment of my invention that the paddles be adjustable toward and away from the main shaft 32. The mechanical means for adjusting these paddles is well known in the art. However, for purposes of this embodiment and the preferred embodiment, it is unnecessary that the paddle portion of the mixer be radially adjustable.

It will be seen from what has been described thus far that the blade assembly is supported by a main shaft at a fixed vertical depth from the frame 34, and that this depth is predetermined by the length of the shafts involved. As was previously stated, this embodiment while coming within the scope of the basic invention, is particularly useful for installations wherein the center shaft is extremely long so that it would be prohibitive to have the shaft made and shipped in one piece.

In accordance with my invention, a second shaft is supported within the main shaft and can be coupled thereto for rotation therewith or uncoupled so as to remain stationary while the main shaft rotates. In alternate embodiments I provide means for raising and lowering the second shaft to various predetermined fixed positions. For purposes of simplicity, I shall describe the structure for coupling and uncoupling the two shafts along with the structure for raising and lowering the second shaft since many of the parts are identical and can be easily interrelated. However, it will be understood that these two functions are separate and can be performed separately by omitting those portions of the apparatus which are completely unnecessary for particular function desired. Accordingly, referring first to FIG. 1 it will be observed that the second shaft designated generally 90 is again a composite shaft. The shaft members 92 and 94 are coupled together by the coupling 96. The shaft member 94 supports the lower blade assembly 98 which is well known in the art and is similar to the assembly 30.

Once again it is unnecessary within the scope of the basic invention to have the blades of the impeller assembly 98 radially adjustable.

The minimum length of the longest straight piece of shafting, represented by the shaft 92, is determined by the distance between the upper end of the fitting 76 and the highest position at which the lower impeller assembly 98 can be raised before the coupling 96 engages the upper impeller assembly 30. The preferred height to which it is desired to raise the lower impeller assembly will be determined by the flow pattern and materials to be mixed, as well as the configuration of the tank. However, it will be appreciated that the lower impeller will not be too closely spaced to the upper impeller since there is generally no advantage in doing so. Accordingly, I have illustrated in phantom view the approximate position the lower blade assembly will assume when it is raised to its highest point. At this point the coupling 96 will not yet engage the upper impeller assembly 30. Of course, if need be the members 76 and 78 can be more closely spaced to the blade assembly 30 so as to further reduce the length of the shaft 92. For purposes of simplicity of illustration these members have been shown spaced from the upper impeller 30. Note that the main shaft 32 is shown in sections connected by phantom lines, as is the tank 20, to indicate that the length can vary and that the parts are not being drawn to scale nor in proportion to actual use.

The means for retaining and coupling and uncoupling the second shaft 90 are more clearly illustrated in FIGS. 2, 5 6 and 7. Referring to these FIGS., the shaft member 92 terminates just above the coupling member 76. Thereafter it is connected by a coupling 100 to a small diameter solid shaft 102. This shaft is supported in bearing 104 which is seated in flanged member 106 extending from a substantially cylindrical hollow shaft 108, which is welded to the midmember 65. The hollow shaft 108 is retained in bearing 110 which is supported on bearing block 112, attached to a frame made of plate 114 which is suitably affixed to the plate 40.

To couple the shaft 92 to the main shaft 32 I have provided a splined structure which is keyed to the shaft and which mates with splines on the inner face of the member 76. This structure comprises a hollow cylindrical member 120 disposed about the shaft 92 and keyed thereto by means of the key 122. The key is disposed in a keyway 124 which as shown in FIG. 7, is quite extensive in length and in fact extends beyond the member 78. The key is in sliding engagement in the keyway and can be moved vertically therein. The member 120 has external splines 128 and a keyway which engages the key 122 and terminates at an upper inwardly depending lip 130 which rests on the key 122. The retaining ring 123 as disposed in threaded engagement with the member 120 in order to retain the key 122 against the shoulder 130, so that the key will move with member 120. The splines 128 are designed to mate with splines 132 which extend inwardly from the fitting 76. A similar key and keyway arrangement is provided diametrically opposite as shown in FIGS. 2 and 6.

Rising above the member 120 is a hollow cylindrical shaft 134 which is welded as shown to the member 120. The length of shaft 134 is determined by the amount that shaft 92 is raised before coupling 100 engages ring 136. Conveniently ring 136 is welded to the shaft 134 to allow it to be joined as by means of welding to the thin-walled long cylindrical tube 138. This tube is disposed about the shaft 102 in spaced relation therefrom and terminates in an annular bearing shoe 140 at its upper end which is in sliding engagement with the shaft 102. A threaded hole 142 in the bearing shoe 140 accepts a bolt 146 FIG. 7 in threaded engagement therewith. The bolt is of such length that it extends through the member 108 which is provided with an elongated vertical slot 150 for this purpose. The length of the slot 150 is such that the bolt 146 can travel vertically for a greater distance than required for engagement of the splines 128 and 132.

In operation the splines 128 and 132 are normally in engagement with one another so that the shafts are coupled together and rotate together. If it is desired to uncouple the shafts the machine is first stopped and then the bolt 146 is inserted into the hole 142 FIG. 2 and force is applied to the bolt 146 to drive it downwardly to the position shown in FIG. 7. The force is transmitted through the thin-walled tube 138 and the hollow cylindrical shaft 134 to the member 120. As the member 120 is forced downwardly the lip 130 forces the key 122 to slide downwardly in the keyway 124. The parts are thus always joined in the correct driving relationship even though the splines 128 and 132 are disengaged as shown in FIG. 7.

To reunite the splines the force on the bolt 146 is reversed so that it is pulled upwardly. Even though there be slight misalignment between the splines this is taken care of by the fact that the engaging ends of the splines are beveled as is well known to the art of splined connections.

The means for engaging the bolt 146 to apply force thereto, comprises the collar 152 which fits about the upper terminal end of the shaft 108. The collar 152 is connected by any suitable means, such as bolts, to the flanged terminal end of the plunger 154. The plunger fits about the upper end of the shaft 102 in sliding engagement therewith, and most preferably spaced slightly therefrom so as not to rub against the shaft while the shaft is rotating. The shaft is supported to hang from the upper terminal end of the plunger by means of a split ring 156 set in a groove near the top of the shaft.

A power-operated gear-driven jackscrew designated generally 160 is used to raise the plunger vertically through a ring gear. This structure is not shown in detail, but is well known in the art. The motor driving the plunger or hollow stem of the jack 160 is reversible so that the collar 152 can be moved vertically in either direction. A clearance hole 162 is provided in the collar 152 through which the bolt 146 can pass to engage the bearing 140.

When both shafts are operating in the coupled position, the shaft 92 of the lower impeller receives its driving force through the splined connection. Thus it is unnecessary to have the shaft continue above the splined connection, at least for power transmission purposes. Accordingly, I have designed the light structure including the support shaft 102 which merely holds the weight of the shaft 92 in a vertical position. This shaft is held in axial alignment by means of the superstructure and the bearings 104. In effect the shaft 102 is merely a linkage mechanism in order to support the shaft 92 and can even be multisectioned, as accuracy and strength are not particularly important.

FIGS. 8 thru 11 show the preferred embodiment of the invention where like members indicate like parts to the embodiment shown in the first seven figures. In this embodiment the drive mechanism for the upper impeller is identical to that shown in the first embodiment. The lower impeller is supported by a shaft which once again is coupled to the shaft of the upper impeller. However, in this embodiment it is unnecessary to go to great lengths in order to segment the shaft, since it will be used in those applications in which a single continuous piece of shafting is practicable. Accordingly, the shafting 192 is shown connected directly to the lower impeller 98. An additional piece of shafting, such as 94 of FIG. 1, could be used together with a coupling similar to coupling 96 FIG. 1, if this was necessary. However, it is with respect to the upper portion of this shaft that the preferred embodiment of the invention differs from that shown in the previous figures.

The shafting 192 passes through bearings in the upper impeller 30, in the coupling 176 (which can be any standard type of coupling for coupling the sections of the shaft together), and the frame 34.

The shaft is supported for rotation within the shaft of the upper impeller by means which allow it to be uncoupled from the surrounding upper impeller shaft, so that the upper impeller can be rotated, while the lower impeller remains stationary. This same means coacts with means for withdrawing the shaft to various predetermined vertical positions. Once again since some of the parts overlap in their functions both of said means will be described simultaneously, it being understood that if only 1 specified function is desired, then those parts performing functions solely for one means or the other can be dispensed with as desired.

Accordingly, referring more particularly to FIGS. 8 and 9 the superstructure rising above the plate 40 comprises web members 194, 196 supporting a tubular structural steel member 208 which is affixed thereto by any suitable means. Pillow block 198 supports bearing 202 which is journaled on a central support hollow shaft 206, which is connected to the midmember 65 in similar fashion to the shaft 108 FIG. 2. Thus the shaft 206 represents the upper terminal portion of the main driving shaft for the upper impeller. A retaining ring 205 retains the bearings in the pillow blocks.

Seated in a recessed portion of the upper terminal end of the shaft 206 is a generally cylindrical support collar 210. The support collar is keyed for rotation by means of the key 211 to the shaft 206. Seated against the internal sides of the support collar 210, spaced from the shaft 192 is an annular bearing 212 which may be of any suitable type and is depicted for purposes of illustration as a ball bearing. The function of this bearing will be more fully described hereinafter.

Mounted above the support collar 210 and in spaced relation from the bearing 212 is a coupling member 214. The member is generally cylindrical and is keyed for rotation to the shaft 192 by means of the key 193. The key 193 is retained in the keyway 195 in the coupling 214 by means of the annular ring 197 which is in threaded engagement with the coupling 214 and which is spaced from the shaft 192. The keyway 199 in the shaft 192 extends down a considerable length of the shaft so that as the shaft is raised (as will be more fully explained hereinafter) the key remains stationary in the collar 214 and guides the shaft 192. Of necessity then, the keyway 199 must be at least long enough to accommodate the anticipated full vertical travel of the shaft.

The coupling member 214 when it is coupled to the collar 210 rests upon a plurality horseshoe washers 216 (see FIG. 11). Passing through a plurality of holes in the flange 218 of the coupling member 214 are a plurality of pins 220 which pass through the washers 216 and into holes 222 in the support collar 210. The pins have a close sliding fit with the holes, but are not press fit, and can be easily withdrawn. The jackscrews 224 shown in FIG. 9 need not normally be in contact with the support collar 210.

At the upper terminal end of the coupling member 214 is an annular split ring 228 which is fastened thereto by a suitable detachable means, such as bolts, so that it may be readily disconnected. The split ring 228 is disposed in an annular groove 269 in the shaft 192. A plurality of such annular grooves are provided along the upper length of the shaft at spaced intervals for a purpose which will be more fully explained hereinafter.

When it is desired to uncouple the center shaft from the outside shaft the jackscrews 224 are driven against the support collar 210, thereby raising the shaft 192. The pins 220 and the washers 216 are removed. The jackscrews 224 are then backed off so that the coupling member 214 rests on the bearing 212 as in FIG. 10. In this position the outer shaft 206 is free to rotate while the inner shaft 192 will remain stationary particularly if the lower impeller is embedded in the silt at the bottom of the tank.

If it is desired to again connect the inner shaft to the outer shaft, the jackscrews are again driven against the support collar 210, thereby raising the coupling member 214 and the inner shaft 192. The pins 220 are inserted into the holes and the washers 216 are slipped about the pins between the coupling member 214 and the support housing 210 so that both shafts can rotate together once the jackscrews have been backed off.

This shaft is equipped with means for raising and lowering it to a plurality of predetermined positions, which means include the support rig 230 FIG. 8 which rises as part of the superstructure. The support rig has a motor driven jackscrew similar to the jackscrew described in the previous embodiment and designated 260. Depending from the jackscrew is a shaft 262 which is in threaded engagement therewith and is vertically positionable in a similar fashion to plunger 154 shown in FIG. 2.

Coupled to the bottom end of the shaft 262 in any suitable fashion are a plurality of links shown in more detail in FIG. 9. Each link is formed with an eye at one end and a yoke at the other end, so that they can be connected by means of pins to form a chain.

In this embodiment, the jackscrew is used to raise the shaft 262, once the split ring 228 has been removed, until the lower impeller has been raised to a desired position. The annular grooves (such as 269, 270) on the shaft 192 dictate what positions are available. Thus in FIG. 9 the impeller can be raised to the next annular groove 270 (as shown in phantom) and the split ring 228 placed so that shaft again would be mounted to exert a downward forced on a coupling member 214. If necessary some of the links can be removed by removing the pins and reconnecting them so that the shaft can be raised even further. However, it will be appreciated that there must be at least an additional annular groove for each permanent position in height to which it is desired to raise the lower impeller.

The embodiments of my invention which I have described provide a means whereby the lower impeller can be raised and also supported in a predetermined fixed position, and the upper impeller can be uncoupled from the shaft of the lower impeller so as to be rotatable while the lower impeller remains stationary. A number of desirable results flow from provision of such a means on equipment of the type described. For one thing, when the machine is stopped the lower impeller can be quickly and easily raised from its lowermost position before the silt has had a chance to settle and embed it in an immovable position. However, should this condition occur it is possible to uncouple the shafts and allow the upper impeller to rotate. With the upper impeller turned up to full power the flow pattern in the tank will eventually resuspend all of the solid particles and allow the shaft of the lower impeller to be recoupled to the shaft of the upper impeller for rotation therewith.

In order to provide a better means of resuspending the solids I have provided a means for increasing the power input to and power output from the upper impeller. There are two ways in which this can be done. The position of the upper impeller blades can be adjusted radially by means which are themselves well known in the art (as previously described). Alternatively the drive motors previously described can be two-speed motors which function electrically whereby the speed can be changed while the upper impeller is operating to mix the solution and place the particles in suspension. In both of these ways power consumption can be varied greatly. These features are particularly useful in combination with the structure disclosed in greater detail. It will be remembered that the structure is particularly adaptable to a process involving slurries of various compositions which will feed into tanks of varying dimensions. Thus the power needed and flow curves produced during the mixing operation may vary greatly. It is accordingly desirable to provide a means for varying the power input to the upper impeller so as to efficiently provide a means for freeing the lower impeller.

It will be understood that various changes in the details, materials and arrangement of parts which have been herein described and illustrated in order to explain the nature of this invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the following claims.

It will further be understood that the "Abstract of the Disclosure" set forth above is intended to provide a nonlegal technical statement of the contents of the disclosure in compliance with the Rules of Practice of the U.S. Patent Office, and is not intended to limit the scope of the invention described and claimed herein.