US3378141A

US3378141A - Froth flotation apparatus

Info

Publication number: US3378141A
Application number: US354026A
Authority: US
Inventors: Warman Charles Harold
Original assignee: Research and Development Pty Ltd
Current assignee: Research and Development Pty Ltd
Priority date: 1963-03-28
Filing date: 1964-03-23
Publication date: 1968-04-16
Anticipated expiration: 1985-04-16
Also published as: GB988674A; NL6403298A; SE300097B; BE645740A; ES298091A1; OA00009A

Abstract

988,674. Froth flotation ; mixing appliances. RESEARCH & DEVELOPMENT PTY. Ltd. March 26, 1964 [March 28, 1963], No. 13036/64. Headings B1C and B2H. A froth flotation cell for concentrating ores &c. has, for generating an annular mixing zone 74, a rotor provided with inclined surfaces which, when rotated, cause the pulp or solution surrounding the rotor to be mixed with air or other gas from an axial zone 71 within the rotor, the axial zone being circumferentially delimited, as by the line 72, and maintained free of liquid by the movement of the inclined surfaces which impel the mixture downwardly through and from the mixing zone. As shown for two such cells in series, each rotor is supported in a bearing 8, driven by a motor 7, and supplied with air down its central portion from either atmosphere or a common pressurized manifold 6 depending on the position of a selector-valve. The mixture is directed along the bottom of each cell by guide vanes 42 disposed on a removable plate 41, Figs. 1 and 2, and the froth discharged from one or each cell over an adjustable weir 69 by a paddle 52 which is oscillated in the direction of the arrows by an eccentric arrangement 55; the paddle may be adjusted in height by moving the fulcrum point 62 by a handwheel. The pulp flows from one cell to the next and then over an adjustable weir in a discharge box. Each rotor is formed of straight or curved bars which may have a cross-section as shown in Fig. 4 or a cross-section which is circular, elliptical or blade-shaped and increases in size in a downward direction. The rotor may be made of cast metal or rubber-covered metal. Fig. 7 shows how replaceable metal bars can be fitted by split callets 30 with tapered sockets in the hub portion of the rotor ; if desired, the bars may be provided with a rubber slip-on cover. Further cells in series may be provided.

Description

April 16, 1968 c. H. WARMAN 3,373,141

mom FLOTATION APPARATUS Filed March 25, 1964 y 4 Sheets-Sheet 1 4 Sheets-Sheet 2 C- H. WARMAN FROT-H FLOTATION APPARATUS M fi i w H w $1 H April 16, 1968 Filed March 25, 1964 FIG 3 73 7 wwk xw April 16, 1968 c. H. WARMAN 3, 7

FROTH FLOTATION APPARATUS Filed March 25, 1964 4 sheetwhee" 5 FIG 6 April 16, 1968 c. H. WARMAN 3,378,14ll

FROTH FLOTATION APPARATUS Filed March 25, 1964 4 Sheets-Sheet 4 37 "M' w. 40 \IILHMHHM i n uumnn 29 United States Patent 3,378,141 FROTH FLOTATION APPARATUS Charles Harold Warman, Castlecrag, New South Wales, Australia, assignor to Research and Development Pty. Ltd., Perth, Western Australia, Australia, a company of Western Australia Filed Mar. 23, 1964, Ser. No. 354,026 Claims priority, application Australia, Mar. 28, 1963, 28,944/63 6 Claims. (Cl. 209-169) This invention relates to flotation concentration processes and aeration and distribution means for performing the like.

Flotation processes are widely used for the concentration of ores and operate by mixing air as bubbles with a pulp in which the ore in a subdivided solid form is maintained in suspension, the pulp having been conditioned in such a manner that a certain component or certain components of the ore adhere and are concentrated in the air-water bubble interfaces and are removed in the froth formed above the pulp by the upward settling of the bubbles, the froth overflowing a weir with or without the assistance of a paddle or scraper.

Other flotation processes are known for concentrating solutions which operate by mixing air as bubbles with the solution which is such that a certain component or certain components of the solution concentrate in the air-water bubble interfaces and can be removed in relatively higher concentration in a froth formed above the solution.

Other processes are in use in which air is required to be mixed with a pulp or solution to dissolve therein and/or perform certain reactions with constituents thereof, the residual gas escaping from the upper surface of the solution or pulp with substantially no formation of froth.

In these processes and the like the efliciency of their operation depends on the effectiveness of the mixing of the air with the pulp or solution and in the case of the flotation process also with the contact achieved between suspended solid particles and air bubbles. Further in the processes in which a froth is removed from above the surface of the pulp or solution, relatively quiescent conditions are required adjacent the surface to allow the bubbles to separate and to minimise entrainment of pulp or solution in the froth and re-entrainment of the froth in the pulp or solution.

Because of the great difference in density between air and the pulp or solution there is considerable difficulty in mixing them and obtaining the necessary contact by dispersion of the mixture throughout the body of pulp or solution since the lower density mixture tends to rise quickly from the mixing zone to the surface of the pulp or solution with liberation of bubbles thereat, the residual mixture of higher density thereafter moving downwards before returning to the mixing zone. One of the chief objects of this invention is to reverse this normal flow pattern by impelling the air-liquid mixture downwardly from the mixing zone thereby increasing the length of the path and the contact of the air bubbles with the pulp or solution before they are liberated at the surface thereof. Further, by impinging the downwardly moving airliquid mixture from the mixing means against the bottom of the containing vessel or cell in the form of a continuous highly turbulent layer the mixture can be constrained to flow on the bottom of the vessel or cell to the sidewalls thereof before turning upwards towards the surface of the liquid thereby greatly extending the path of the air bubbles and keeping them in extended turbulent contact with the pulp or solution. Another object is to provide relatively quiescent conditions at the surface of 3,373,141 Patented Apr. 16, 1968 the pulp or solution for bubble separation whilst yet employing high energy intensity in the mixing and contacting regions and this objective is attained by impelling the mixture downwardly against the bottom of the vessel or cell as described. A further objective is to enable pulps containing relatively coarse solid particles to be treated, especially in the flotation process. This objective is achieved by the flow characteristics above described, the intense turbulent layer on the bottom of the cell serving to maintain the coarse particles in suspension and facilitate their contact with the air bubbles. Yet another object of the invention is to distribute the air-liquid mixture from the mixing means in as uniform a manner as possible towards the outer limits of the vessel or cell and so enable maximum intensity of mixing and turbulent contact to be employed with the maintenance of a quiescent bubble separating zone above. For this purpose this invention provides a system of curved bafiles arranged in a lobed arcuate pattern and attached to the bottom of the vessel or cell.

For mixing the air with the pulp or solution this invention provides a rotor mounted for rotation about a substantially vertical axis and provided with inclined surfaces whereby pulp or solution surrounding the rotor is mixed with the air from an axial zone circumferentially delimited and maintained substantially free of liquid by the movement of the inclined surfaces which also serve to impel the resulting air-liquid mixture downwardly. Air is conducted to the axial zone by a passage or passages in or a conduit or conduits attached to the shaft which rotatably supports the rotor.

In one embodiment of the invention. the inclined surfaces are provided by inclined rotor bars attached at their upper ends to a disc or boss attached to the vertical shaft supporting the rotor. The axes of the rotor bars may be straight or curved and downwardly the rotor bars may diverge away from the axis of rotation of the rotor. Downwardly the rotor bars may increase in cross-sectional area. Typically the rotor bars are of circular crosssection downwardly increasing in diameter or are any of various blade, vane or elliptical cross-sections of downwardly increasing dimensions and the axes of the bars are straight and downwardly slope in a direction opposite to the direction of rotation of the rotor, the bars being symmetrically disposed with respect to the axis of rotation of the rotor and at a minimum radial distance from the axis adjacent their upper ends where they are attached to the rotor disc or boss. So constructed the rotor is of downwardly increasing outside diameter and its peripheral velocity when rotating likewise increases downwardly causing a downward pumping action on the fluid supplementing the impelling action of the individual inclined bars. The inclination of the bars may vary to suit differing applications but a typical inclination is 45 By varying the inclination of the rotor bars to the vertical the characteristics of the rotor performance may be modified. Thus there is for a given rotor construction or geometry an inclination from the vertical at which the downward impelling effect on the liquid is a maximum. A reduction from this inclination decreases the downward impelling effect in absolute terms and also relatively to the rate at which air is mixed, assuming the tip speed of the rotor bars and the pressure of the air relative to that of the liquid surrounding the rotor remain unchanged, and hence the ratio of air to liquid in the mixture leaving the rotor increases. If the inclination is successively reduced from that giving maximum downward impelling effect a point is reached at which the buoyancy of the air-liquid mixture overcomes the downward impelling effect and the mean vertical component of the momentum of the mixture leaving the rotor then changes from downwards to upwards. At this point the rotor would cease to perform in accordance with this invention. If the inclination from the vertical is successively increased from that giving maximum downward impelling effect this effect again diminishes and again a point is reached where the buoyancy of the air-liquid mixture overcomes the downward impelling effect and the means vertical component of the momentum of the mixture leaving the rotor changes from downwards to upwards. The scope and operation of a rotor according to this invention therefore lie between these limiting points.

With a given rotor constructed in acordancec with the invention the power absorbed, air mixing rate and mixture fiow rate are functions of the speed of rotation and the pressure of the air relative to that of the liquid surrounding the rotor and it is a feature of this invention to control the power input, air mixing rate and mixture flow rate by varying the rotor speed and/or the pressure of the air relative to that of the liquid surrounding the rotor.

A feature of this invention is the provision of convenient means for detaching and replacing the elements of the rotor that may be subject to wear in operation or for modifying its performance. Thus the rotor bars may be provided at their upper ends with tapered portions upwardly divergent and adapted to fit and be self holding by friction in sockets, or in inserts therein, of complementary shape in the rotor disc or boss. The rotor bars may also 'be provided with replaceable covers of wear and/or corrosion resisting material. The rotor disc or boss may be provided with a downwardly divergent conical bore adapted to fit and be self holding on a corresponding conical surface on the shaft. So constructed worn parts may be readily removed and replaced and rotor bars of different form, spacing, number and inclination can be fitted to provide various combinations of rotor characteristics to suit a wide range of process requirements.

For removal of the froth from above the surface of the pulp or solution this invention provides a horizontal paddle actuated from above by oscillating arms and hangers to describe a flattened elliptical transverse motion whereby on the lower portion thereof the froth is pushed towards and caused to overflow a horizontal weir by which it is separated from the pulp or solution.

Some specific forms of the invention are illustrated in the accompanying drawings wherein:

FIG. 1 is an axial section of a cell of a flotation machine adapted to perform the process according to the invention;

FIG. 2 is a sectional plan on the line AA of FIG. 1;

FIG. 3 is a sectional elevation on the line BB of FIG. 2;

FIGS. 4 and 5 are plan views of variant forms of rotor;

FIG. 6 is an elevation on the line CC of FIG. 4;

FIG. 7 is a sectional view of a replaceable metal rotor bar; and

FIG. 8 is a sectional view on line EE of FIG. 5.

Like parts are illustrated by like characters throughout the specification and drawings.

As shown in the drawings 1 is a rotor mounted on a hollow vertical shaft 2 for clockwise rotation (as viewed in plan) and submerged in a pulp or solution 3 contained in a cell 4 and having an upper free surface 5 through which the shaft 2 passes. The cell 4 is provided with a longitudinal hollow member 6 which mounts the driving motor 7 and the shaft and bearing assembly 8 which rotatably supports and locates the hollow vertical rotor shaft 2. A belt and pulley drive 9 of any suitable form is arranged to transmit power from the motor shaft assembly to rotate the rotor at any given predetermined speed.

The upper portion of the hollow rotor shaft 2 is provided with a plurality of holes 10 in the wall thereof through which air may pass from the annular space around the shaft enclosed by the housing 11 attached upwardly to the underside of the bearing assembly 8 and having at its lower end a labyrinth seal 12 disposed with close running clearance around the shaft 2 adjacrent below the holes 10. The housing 11 incorporates a twoway air control valve 13 which admits air to the housing at a controlled rate either from the atmosphere through the port 14- or via port 15 and piping 16 from the hollow member or manifold 6 which may be supplied with air or other gas at above atmospheric pressure by a fan or other suitable means.

At its lower end the shaft 2 has externally a downwardly divergent conical surface 17 which is self-holding and self-driving in a corresponding conical bore 18 in the boss 19 of the rotor disc 20. Attached to the disc at their upper ends are the equally spaced rotor bars 21 which, in the form of rotor depicted, are straight and downwardly sloped at 45 in a direction opposite to the direction of rotation of the rotor. As shown in FIGS. 4 and 5 the axes 22. of the bars 21 are at right angles to the radii 23 of minimum length drawn from the axis of rotation of the rotor to the pitch circle 24 of the bar axes within the rotor disc so that downwardly from the rotor disc an envelope of the rotor bars is of increasing diameter.

FIGS. 4 and 6 illustrate rotors in which the rotor bars shaped in cross-section as shown at 25 are integral with the rotor disc 20. The figures illustrate rubber covered metal construction. In the rubber covered construction illustrated in the figures metal rotor core bars 26 are cast integral with the metal disc reinforcing member 27 and a moulded rubber covering 28 is bonded to and entirely covers the metal reinforcement with rubber thickness so disposed that all parts of the rotor have approximately the same wear life cpectancy.

In FIG. 5 are illustrated rotors with detachable rubber covered bars for which the rubber covered metal rotor disc 20 is provided with tapered upwardly divergent sockets 29 adapted to receive tapered inserts or eollets 30 constructed in halves and of any suitable material such as a plastic of suitable rigidity and wear resistance outwardly conforming to the sockets 29 and internally shaped to conform with the upper ends of the detachable rotor bars. FIG. 7 illustrates in a section a construction suitable for the attachment to the rotor disc of a replaceable metal rotor bar. The bar illustrated is circular in cross-section and provided with an upwardly divergent conical surface 31 conforming to the inner surface of the split inserts or collets 30. To attach the bar to the rotor the tapered end 31 of the bar is passed through the socket 29 in the rotor disc from the underside thereof, the half collets 30 placed around the tapered rotor bar end and the assembly so formed then pushed or driven downwardly into the self-holding conical socket 29 in the rotor disc. FIG. 8 illustrates in a section on the line EE of FIG. 5 a construction suitable for the attachment of a rotor bar with a replaceable rubber cover. The bar illustrated is circular in cross-section and comprises a metal core bar 32 downwardly reducing in diameter from a step or shoulder 33 and upwardly increasing in diameter as at 34 from a parallel section 35 adjacent the step 33. At its upper end the core bar has a cylindrical portion 36 which is truncated by a plane end surface 37 sloping with respect to the bar axis at the angle of inclination thereof so as to lie in a level plane when the bar is in position in the rotor disc 20. A protective cap 38 of thin rubber or equivalent extensible material is placed over and conforms to the portion of the core bar above the step 33 and a thick slip-on wear resistant cover 39 is placed over the lower portion of the core bar and is provided with an internal step and parallel section conforming to the step 33 and parallel section 35 on the core bar which serve to retain the cover 39 in place when the split collet 40 is placed in position around it and the assembly so formed has been pushed or driven into the self-holding conical socket 29 in the rotor disc.

To suppress the angular momentum or vorticity about the vertical axis of the mixture downwardly leaving the rotor and to direct and distribute it in a substantially uniform manner towards and against the side walls of the cell the removable bottom liner plates 41 thereof are provided with a combination of vertical baffle plates 42 curved in plan and so arranged that the distances of their leading edges 43 from the centre of the cell vary and a curve 44 joining them presents in plan view a four-lobed arcuate figure symmetrical about the centre of the cell. The space between the battles also varies, being widest as at 45 between those baflles which direct the flow towards the corners of the cell and a minimum as at 46 between those which direct the flow to the mid portions of the cell walls. The plan curvature of the individual baflles is such that at their leading edges the baffles lie substantially in the direction of the flow approaching from the rotor and at their trailing edges their direction is such as to direct the flow leaving them substantially at right angles to the walls of the cell. As shown in the drawings the minimum distance from the centre of the cell of baffle leading edge is substantially greater than the maximum radial dimension of the rotor so that the mixture flow from the rotor impinges and turns outwards on the floor of the cell before entering the bafiles. To secure the removable liner plates 41 which carry the bafiles a circular clamping plate 47 overlapping the inner edges of the liner plates 41 is downwardly tightened by clamping nut 48 and stud 49 attached to the bottom of the cell.

The vertical location of the rotor with respect to the bottom of the cell is not critical but optimum results are obtained when the clearance between the bottom of the rotor and the bottom of the cell are of the order of one-fourth the bottom outside diameter of the rotor. Likewise the height of the curved baflle plates 42 is not critical, but a minimum height approximately one-fourth the bot-tom outside diameter of the rotor is desirable. So arranged the top of the baflle plates is at the same elevation as the bottom of the rotor bars.

To assist the overflow of the froth layer 50 from the cell 4 the invention provides when necessary a horizontal froth paddle 51 supported and actuated by hangers 52, arms 53 and drag links 54 said arms and drag links being so actuated by a rotating eccentric and linkage system 55 that the pinned

connections

56 and 57 between the arms 53 and drag links 54 respectively and hangers 52 execute the traces shown in FIG. 1 and the paddle 51 describes in the direction of the arrows the flattened elliptical shaped path indicated by the dotted line 58 and the froth is pushed by the paddle toward-s and over a slatted weir 59 adjustable as to height by placing slats of varying width in the slots 60 arranged to accommodate them. By means of a handwheel 61 the fulcrum point 62 of the eccentric and linkage system 55 may be changed and the elevation of the paddle path 58 adjusted to suit the height of the froth overflow weir 59.

The use and operation of the invention may conveniently be described by considering a flotation machine comprising two cells operating in series as depicted in FIGS. 1, 2 and 3 although it will be apparent that any convenient number of cells may be operated as a series bank. Pulp or solution to be processed enters the machine from the feed box 63 and passes successively through the cells via

openings

64, 65 and 66 in the transverse walls thereof to the discharge box 67. A slatted tail weir 68 adjustable for height by adding or removing slats is provided in the discharge box 67 for controlling the height of the surface of the pulp or solution in the cells 4 at a predetermined level below the froth overflow lip 69 and above the top of the rotor disc 20. Processed pulp or solution overflowing the tail weir 68 is discharged from the machine through the pipe opening 70 in the bottom of the discharge box 67. With the rotors turning clockwise in plan at a rotational speed equivalent to a linear speed of 30 to 40 feet per second at the maximum diameter of the rotor bars, an axial zone 71 within and beneath the rotor with boundaries approximately as indicated by the chain dotted line 72 is maintained substantially free of liquid by the outward and downward impelling action of the inclined surfaces 73 of the rotor bars 21. Around the boundary of the axial air zone 71 and extending into the region of intense turbulence adjacent the surface of revolution generated by the outer limits of the rotor bars 21 lies a mixing zone 74 in which pulp or solution entering downwa-rdly from the upper regions of the cell is mixed with air from the contiguous axial zone 71 and the resulting mixture impelled downwardly and outwardly from the mixing to impinge on the bottom of the cell 4 in an annular area adjacent the periphery of the clamp plate 47. The mixture flowing outwardly on the bottom of the cell from the area of impingement thereon passes between the baffles 42 and is distributed in substantially a uniform manner at right angles towards and against the sidewalls of the cell thereafter rising towards the upper free surface 5 of the liquid, turning at and adjacent said surface and flowing substantially parallel thereto with separation of bubbles from the mixture to form the froth above, the residual mixture thereafter turning and flowing downwardly to the mixing zone 74. The path of the mixture flow as above described is shown approximately in FIG. 1 by arrows in the body of the liquid in the cell.

The froth layer 50 which forms above the liquid surface 5 may be flowed between adjacent cells through openings such as 65 for removal from a cell other than that in which it was formed or it may be contained within and removed from the cell of its formation by insertion of the isolating froth boards 75 in the slots provided. Usually the froth is sulficiently mobile to overflow the froth weir 59 without assistance. Viscous or copious froths may require for their removal from the cell the use of paddle mechanism such as herein before described.

In this description of the invention the term air is to be understood to mean air or other gas.

What I claim is:

1. A flotation concentration apparatus comprising a vessel or cell for holding suspension, feed means feeding a suspension into said vessel or cell, discharge means for discharging a non-floated product from said vessel or cell, froth discharge means for discharging a froth product from the vessel or cell, a rotor in said vessel at a level to be submerged in said suspension, a hollow axial shaft on the bottom of which said rotor is mounted with the bottom of the shaft opening into said rotor, gas admitting means feeding gas into said hollow shaft for delivery into the interior of said rotor, rotating means coupled to said hollow axial shaft and rotating said rotor about a substantially vertical axis, said rotor comprising a generally horizontal boss or disc spaced from the bottom of the vessel or cell and supporting a plurality of bars attached thereto at their upper ends in uniform angular relation, symmetrically about said vertical axis, the enveloping peripheral diameter of said bars about said vertical axis diverging downwardly toward the bottom of the vessel or cell, the bars being inclined so that downwardly successive cross sections in horizontal planes of said bars have, with respect to said vertical axis, angular displacements in a direction opposite to the direction of rotation of the rotor, the lower end of the bars being spaced from the bottom of the vessel or cell suflicient to establish and maintain an axial zone free of liquid within and beneath said rotor.

2. A floation concentration apparatus as claimed in claim 1 in which said gas admitting means comprises a housing around the top of said shaft and at least one opening in said shaft within said housing through which the gas is conducted to the hollow shaft.

3. A flotation concentration apparatus as claimed in claim 2 in which said housing encompassing the shaft is sealed to it on either side of said gas admitting opening so as to form a closed annular space around the shaft, and a controllable setting two-way valve in said housing through which gas is transmitted to said annular space at the desired pressure.

4. A flotation concentration apparatus as claimed in claim 1 in which the rotor bars increase in cross-sectional area downwardly.

5. A flotation concentration apparatus as claimed in claim 1 in which the vessel has a substantially square crosssection, and said apparatus further comprises a removable bottom liner plate and a plurality of vertical bafiie plates curved in the horizontal direction and attached to said removable bottom liner plate, for distributing the flow in the cell, the distances of the inner ends of the bafiie plates from the center of the cell varying so as to produce in plan view a four lobed arcuate pattern, and the space between adjacent baflles also varying, being widest between those baflfies which direct the flow towards the corners of the cell.

6. A floation concentration apparatus as claimed in claim 1 in which the rotor disc is readily detachable from the shaft said rotor disc has an axial downwardly divergent conical bore, and said shaft having a corresponding conical surface thereon on which the rotor disc fits and is self-holding.

FRANK W.

References Cited UNITED STATES PATENTS Daman 209-169 X Woods 29-1568 Saltzer 287-124 Ingalls, et al. 209-169 Fahrenwald 209-169 Potts 209-169 X Payne 209-169 X Daman 209-169 Stadthaus et a1. 29-1568 Nelson et al. 259-107 Benozzo 209-169 Rodoz 261-93 X Great Britain.

LUTTER, Primary Examiner.

HARRY B. THORNTON, Examiner.

L. H. EATHERTON, Assistant Examiner.

Claims

1. A FLOTATION CONCENTRATIOAN APPARATUS COMPRISING A VESSEL OR CELL FOR HOLDING SUSPENSION, FEED MEANS FEEDING A SUSPENSION INTO SAID VESSEL OR CELL, DISCHARGE MEANS FOR DISCHARGING A NON-FLOATED PRODUCT FROM SAID VESSEL OR CELL, FROTH DISCHARGE MEANS FOR DISCHARGING A FROTH PRODUCT FROM THE VESSEL OR CELL, A ROTOR IN SAID VESSEL AT A LEVEL TO BE SUBMERGED IN SAID SUSPENSION, A HOLLOW AXIAL SHAFT ON THE BOTTOM OF WHICH SAID ROTOR IS MOUNTED WITH THE BOTTOM OF THE SHAFT OPENING INTO SAID ROTOR, GAS ADMITTING MEANS FEEDING GAS INTO SAID HOLLOW SHAFT FOR DELIVERY INTO THE INTERIOR OF SAID ROTOR, ROTATING MEANS COUPLED TO SAID HOLLOW AXIAL SHAFT AND ROTATING SAID ROTOR ABOUT A SUBSTANTIALLY VERTICAL AXIS, SAID ROTOR COMPRISING A GENERALLY HORIZONTAL BOSS OR DISC SPACED FROM THE BOTTOM OF THE VESSEL OR CELL AND SUPPORTING A PLURALITY OF BARS ATTACHED THERETO AT THEIR UPPR ENDS IN UNIFORM ANGULAR RELATION, SYMMERTICALLY ABOUT SAID VERTICAL AXIS, THE ENVELOPING PERIPHERAL DIAMETER OF SAID BARS ABOUT SAID VERTICAL AXIS DIVERGING DOWNWARDLY TOWARD THE BOTTOM OF THE VESSEL OR CELL, THE BARS BEING INCLINED SO THAT DOWNWARDLY SUCCESSIVE CROSS SECTIONS IN HORIZONTAL PLANES OF SAID BARS HAVE, WITH RESPECT TO SAID VERTICAL AXIS, ANGULAR DISPLACEMENTS IN A DIRECTION OPPOSITE TO THE DIRECTION OF ROTATION OF THE ROTOR, THE LOWER END OF THE BARS BEING SPACED FROM THE BOTTOM OF THE VESSEL OR CELL SUFFICIENT TO ESTABLISH AND MAINTAIN AN AXIAL ZONE FREE OF LIQUID WITHIN AND BENEATH SAID ROTOR.