US2892543A

US2892543A - Aerator assembly with pulp elevating discharge

Info

Publication number: US2892543A
Application number: US567798A
Authority: US
Inventors: Arthur C Daman
Original assignee: Mining Process and Patent Co
Current assignee: Mining Process and Patent Co
Priority date: 1956-02-27
Filing date: 1956-02-27
Publication date: 1959-06-30
Anticipated expiration: 1976-06-30

Description

June 30, 1959 A. c. DAMAN ,8

AERATOR ASSEMBLY WITH PULP ELEVATING DISCHARGE Filed Feb. 27, 1956 4 Sheets-Sheet 1 IN VEN TO ARTHUR C. DAM/7N ATTORNEYS A. C. DAMAN June 30, 1959 2,892,543 AERATOR ASSEMBLY WITH PULP ELEVATING DISCHARGE Filed Feb. 27,. 1956 4 Sheets-Sheet 2 INVENTOR. fl/PTHu/P' C. DAM/IN ATTORNEYS ,AERATOR ASSEMBLY WITH PULP ELEVATING DISCHARGE Filed Feb; 27, 1956 A. C. DAMAN June 30, 1959 4 Sheets-Sheet 3 INVENTOR. ARTHUR C, DAM/IN ATTORNEYS June 30, 1959 A. c. DAMAN 2,392,543

AERATOR ASSEMBLY WITH PULP ELEVATING DISCHARGE Filed Feb. 27, 1956 4 Shee ts-Shee't 4 INVEN TOR. ART/1 UP 6. DAM/7N ATTORNEYS United States Patent AERATOR ASSEMBLY WITH PULP ELEVATIN G DISCHARGE Arthur C. Daman, Denver, Colo., assignor to Mining Process and Patent Company, Denver, Colo., a corporation of Delaware Application February 27, 1956, Serial No. 567,798

13 Claims. (Cl. 209-169) This invention relates to aeration of the type employed in froth flotation treatments or the like, and more particularly relates to the aeration unit of such a treatment.

In present flotation practice the tendency is to increase the size and capacity of the cells and associated parts and this has resulted in an increase in the power requirement of the impellers used therein. In order to obtain the desired aeration and circulation, the horsepower per cell cost of operation has become a substantial factor in the overall cost of the processing.

Another tendency is toward coarser grinds with the result that considerable difliculty is encountered in many treatments in floating the coarser fraction. Usually the finer sizes float quite readily but larger sizes may not receive enough elevating component from the aeration to carry into the froth and recirculation of such material is necessary in order to get an adequate amount into the froth. Consequently, the aerator has to allot one part of its capacity to primary circulation and another part to recirculation. This factor has occasioned resort to larger sized impellers and more intense agitation with resulting increase in power demand.

The present invention is based on the discovery that by a relative increase in the working surface of an impeller and a reduction in its peripheral dimension a desired degree of aeration and agitation can'be attained with lowered power consumption. This result can be attained by utilizing an inner pulp-receiving surface disposed at more than 45 degrees to the horizontal with an arrangement of radial blades extending upwardly from said surface, and covering said impeller with a stationary memas compared with the former practice of discharging along a substantially horizontal path. The confining effect of the cover member assists the pumping action of the impeller and also increases the mixing action within the enclosure before the pulp discharges across the periphery of the impeller.

Certain features of'the mixing arrangement shown and described but not claimed herein have been claimed in my copending application Serial No. 488,523, filed February 16, 1955, for FrothFlotation Process and Aeration Apparatus Used Therein.

Accordingly it is an object of this invention to provide a simple, durable and eflicient aerator for pulps which is adapted for installation in existing flotation cells as. a

replacement part as well as in specially designed ..aerating machines.

2,892,543 Patented June 30, 1959 Another object of the invention is to provide an aerator for pulps which provides a high degree of mixing and aeration in a simple, efiicient and economical manner.

A further object of my invention is to provide a simple, durable and efficient aerator assembly permitting use of a novel shape of flotation cell providing a low cost, high volume treatment.

Other objects reside in novel details of construction and novel combinations and arrangement of parts all of which will be described hereinafter.

The practice of the invention will be explained by reference to the accompanying drawings. In the drawings, in the several views of which like parts bear similar reference numerals,

Fig. 1 is a side elevation of a flotation machine embodying features of my invention and partially broken toshow interior parts in vertical section;

Fig. 2 is a fragmentary plan view of the stationary bottom enclosure of the aerator assembly shown in the :left hand position in Fig. 1 and of the entire aerator assembly exclusive of the hollow column, shown in the center position in Fig. 1;

Fig. 3 is a fragmentary vertical section of another embodiment of aerator assembly taken along a line transverse to the lengthwise axis of the cell;

Fig. 4 is a fragmentary vertical section through a multi-cell flotation machine showing other embodiments of aerator assemblies in two adjoining cells of the machine;

Fig. 5 is a vertical section, similar to Fig. 3, showing another embodiment of aerator assembly utilizing fea- 'tures of this invention;

:Fig; 6 is a fragmentary vertical section of another form of aerating impeller and associated parts utilizing features of my invention;

Fig. 7 is a vertical section of another embodiment of aerating impeller and associated parts;

Fig. 8 is a top plan view of a novel type of top cover member for aerators utilizing features of this invention;

.Fig. 9 is a fragmentary vertical section of still another form of aerator assembly utilizing features of this invention; 1

Fig. 10 is a front elevation of the impeller shown in Fig.9; and

Fig. 1.1 is a fragmentary section through an impeller of the .type shown in Fig. 9 taken approximately along the line 11-11, Fig. 10-.

.As shown in Fig. 1, a single aeration machine may employ different type aeration units in combination to get different aeration and circulation effects along the length of a course of treatment. The machine illustrated is what is known in the art as a hog-trough flotation cell and comprises an elongated tank 20 of substantial depth *having a feed inlet (not shown) at one end and a regulatable discharge outlet (not shown) at its opposite end which controls the liquid level in the cell. S'uperstruc- 'ture .21 supported at the top of tank 20 provides a rigid support on which the aerator units are mounted and headers 22 are supported on and extend along superstructure 21 for supply of aerating gas to said units.

The upper portion of the tank interior adjacent a froth overflow level 23 is divided by a series of vertical baflles or partitions 24 into separated overflow zones in which surface flow lengthwise of the tank is substantially restrained. These baffles have their lower ends substantially below the surface but distant from the tank bottom so that the main body of pulp in the tank issubject to progressive flow from the feed inlet to discharge outlet of the cell.

The aerator assemblies utilized in the cell shown in Fig. l are numbered 25, 25a and 25b respectively in the direction of flow through the machine. Assembly 25 comprises a vertical shaft 26 journaled at its upper end in bearings 27 enclosed in a hollow housing 28. This housing is mounted for vertical adjustment on a plate 29 supported on superstructure 21 so that the top surfaces of the impeller can be adjusted relative to its associated cover member. In this form a bowl liner or enclosure member 31 is seated on the bottom of tank 20 concentric with shaft 26 and has a plurality of lower openings 32, preferably fan shaped, which function as ingress and egress ports for the circulatory move merit of pulp along the machine. The liner bowl preferably has a series of radial ridges or vanes 33 on its inner surface and inclines at a substantial angle, such as 45 degrees for example.

An impeller 34 is mounted for conjoint rotation with shaft 26 at its lower end and has a plurality of depending vanes 35 on its under surface and another plurality of upstanding blades 36 on its upper surface. The bottom surfaces of vanes 35 preferably are substantially parallel to the top surfaces of the ridges 33 and pass in close proximity thereto during rotation of the impeller. In the arrangement shown, a hollow column 37 is fixedly held on the lower end of bearing housing 28 and carries at its lower end a dished cover member 38 which overhangs the impeller 34 and has a series of depending vanes 39 at its periphery. The under surface of cover 38 preferably parallels the top surfaces 'of blades 36 so that a close clearance may be provided through one or a plurality of openings 43 to enter the hollow interior of shaft 26 for discharge at its lower end. In addition, atmospheric air may be drawn through another nipple 44 by suction influence of the impeller. Recirculating pulp is moved through

openings

45 or 46 in column 37 and entrains the descending air for premixing prior to discharge on the working surface of the impeller. Preferably

nipples

42 and 44 are valve controlled so as to permit selective control of gas introduction.

The aerator 25a differs from aerator 25 in the gas introduction arrangement, and hence the bottom liner 31,

impeller 34 and cover 38 are the same as previously described and bear the same reference numerals. The bottom liners 31 of aerators 25 and 25a are mounted in abutting relation and collectively define a progressive circulatory course. Column 37a of this form has an open top end 47 for entrainment of large quantities of atmospheric air. A tubular conduit 48 encloses shaft 26a of the aerator 25a and has its upper end at a higher elevation than the recirculation openings 46a and its lower end substantially below the periphery of impeller 34. Shaft 26a is solid except at its lower end which has one or a plurality of entrance openings 49 admitting gas to an interior passage 50 for discharge at the lower end of the shaft. Gas drawn in through opening 47 passes into the interior of conduit 48 under the suction influence of the impeller as pulp entering through openings 46a impedes the flow exteriorly of conduit 48 with the result that substantial quantities pass into and through passage 50 while the remainder discharges onto the top surface of impeller 34.

The aerator 25b per se forms no part of the present invention as its features have been described and claimed in my copending application, Serial No. 504,950, filed April 29, 1955, for Aerating Apparatus for Flotation Pulps. However, its use in the combination shown in Fig. 1 has not been disclosed previously. Gas from header 22 is supplied through an inner conduit 52 which discharges into a plurality of jet nozzles 53. A plurality of tubular conduits 54 are supported from superstructure 21 and each carries a housing or casing 55 at its lower end which forms an enclosure for one of the nozzles 53. Each casing has a restricted opening 56 arranged to form a venturi passage.

The interior of the conduit 54 is divided by a partition 57 adjacent an opening 58 near but below the liquid level 23 and the effective intake level of opening 58 is controlled by a slat weir 59. The partition 57 has its lower end substantially below the intake level so that the descending gas mixes with recirculating pulp during its passage through the conduit and then gets additional aeration by the inspirator efiect of the discharge through nozzle 53 and opening 56.

Having thus described the structural arrangement of the cell shown in Fig. l, the operation will be more readily understood. A suitably conditioned flotation pulp will be introduced with the discharge outlet closed until the pulp level 23 is established, after which the outlet is opened to balance discharge to feed input. Thereafter the operation is continuous except as interrupted by shutdowns.

Rotation of impeller 34 and the hydrostatic head cause streams of pulp to enter

openings

45 and 46 and pass at relatively high speed until reaching the rotating surface of the impeller. At the same time gas is supplied through hollow shaft 26 to discharge under impeller 34 and other gas which may be atmospheric air or gas supplied from header 22 is introduced through nipple 44 and mixes with the pulp streams entering at 45 and 46. This produces a substantial amount of pro-mixing before the matter reaches the impeller.

Thereafter, this partially aerated pulp is given an ascending component of movement by the centrifugal action of the impeller rotation and is subjected to a substantial beating and mixing by the action of blades 36 running in close proximity to the inclined surface of cover 38. The series of depending vanes 39 on cover 38 form a partial enclosure at the periphery of impeller 34 and provide an additional mixing effect.

At the same time, pulp circulating through open ings 32 mixes with the air discharging through the bottom of the shaft. This mixture is pumped upwardly by the action of vanes 35 and is subjected to beating and mixing as the vanes pass over the ridges 33. The stream of pulp moving long the under surface of the impeller mixes adjacent the periphery with the other stream of pulp passing off the upper surface of the impeller and the depending vanes 39 are arranged to retrain the centrifugal effect so that the pulp streams get additional mixing and the air is thoroughly dispersed in fine bubble formation as it rises to the surface assisted by the elevating component derived from the impeller rotation.

Movement along the surface is substantially restrained by partition members 24, and the rising bubbles form a froth on the surface into which the elevated mineral is carried and held until removal by overflow or other means. The openings 45 are located near but below this froth and suspended mineral reaching this elevation and not collecting in the froth passes through said openings and receives further mixing and aeration as previously described.

Other circulating solids requiring additional aeration are recirculated through openings 46, while settled solids are drawn through openings 32 in the bottom liner and receive additional aeration in their ascending movement under the impeller action. Some of this material and some of the discharge from the upper surface of the impeller pass into the range of treatment by aerator 25a to maintain the circulatory action of the cell.

In this way large quantities of aerating gas in line bubble formation are brought into repeated contact with the mineral portion of the pulp and due to the degree of recirculation provided, the mineral which is in condition for flotation receives the required amount of 5. aeration to carry to the surface. Mineral not ffloating in such action is subjected to other aerationtreatments in its progress through the cell so that ample flotation time is provided to collect substantially all the desired mineral as concentrate before reaching the final discharge of the machine.

The aerator 25a functions in a manner similar to aerator 25 except that recirculation in preferred practice is effected only through openings 46a. Large quantities of' atmospheric air are drawn through the open end 47 of the hollow column and then pass interiorly of the conduit 48. Some of this gasenters opening 49 and discharges from the lower end of passage 50' to aerate the pulp moving in contact with the bottom surface of the impeller. The remaininggas discharges from the lower end of conduit 48 and mixes with pulp discharging ontothe impeller from openings 46a.

'In other respects, action of this unit duplicates the action of aerator ZSpreviohs'ly described and the description of same will not be repeated. Due to the omission of the Upper recirculation openings, a more-quiescent .pulp condition is maintained in the upper portion of the pulp body which assists the flotation, particularly when brittle froths are formed. Pulp circulating. toward the-final outlet from aerator 25a passes into the range of influence of aerator 255 which provides intense aeration without appreciable agitation except in the area adjoining the bottom of tank 20.

, The unit 25b recirculates pulp through the openings '58 over weirs 59 into the-plurality of conduits 54 which are supplied with gas from header'2'2. The descending stream of gas is shielded from the pulp entering openings accelerates the mixing action and discharges the aerated pulp at relatively high velocity into the circulating body of pulp in which the casings are submerged. This serves to disperse a large volume of gas through the pulp adjacent the cell bottom from which it rises in fine bubble formation' Asa consequence mineral which does not float readily under conditions of more intense agitation is carried to the surface and collected in the froth. The upper portion of the pulp body provides a substantially quiescent zone in which the mineral may rise without material imjpedance, and flotation efficiency is quite high. Most of the mineral content responding to the flotation action has been removed as concentrate before reaching this portion of the circulatory movement and hence the quantity of solid in the upper quiescent portion is relatively small and provides little contact with other solids which might break the flotation attachment. Pulp passing from the final outlet as tailings has its mineral content substantially depleted.

While a machine of the type hereinbefore described will give highly satisfactory results it may be substantially modified to suit particular treatment requirements. For example, all the aerator units may be identical with aerator 25:: or 25b, and also may be modified as shown in Fig. 3 to utilize an aerating arrangement of the type illustrated. Except for the dilierence in aerator unit and the provision of the inlets at the bottom of the cell, Fig. 3 would be a typical cross sectional view through the tank 20 of Fig. 1.

A unit of the type illustrated in Fig. 3 comprises a tank 20x having inwardly tapering lower portions on a .front wall 61 and rear wall 62 interconnected by a fiatbottom portion 63. Each of said walls terminates .inan upright portion having an overflow lip 64 over which 67 is also shown-as an alternative arrangement for gas introduction when a solid shaft is' used.

The chief diflerence between this aerator unit and those previously described resides in the provision of blades 69 mounted for conjoint rotation with shaft 267; at an elevation slightly below openings 46x in the hollow column 37 which supports at its lower end the cover element 38x overhanging. the impeller; The recirculating pulp and aerating gas des'cendingthrough colimm 37x is subjected to a beating action and downward component of movement as it reaches blades 69 after which 'it reaches the top surface of the impeller and is subjected to the same type of mixing action previously described. Additional gas such as compressed air may be introduced through nipple 65 or inlet 67 while a diluting solution orreagen-t may be fed through nipple 65.

The diluted pulp contacting the under surface of'the impeller is given an elevating component of. movement by vanes 35x and is subjected to the same mixing and beating influences previously described at it passes over ridges 33x. Pulp and aerating gas descending onto the upper surface of the impeller are given an-upward' component of movement as previously described'and the mixture is subjected to the beating influences as it passes by the vanes 39x.

Fig. 4 illustrates a mul'ti-cell flotation machine showing another form of aerator unit embodying features of this invention in a two-cell arrangement. It will be understood thatany of the aerator units described herein may be employed in such arrangement, and Fig. 4 is intended as a typical installation. This machine comprises an elo-ngated tank 71 having its interior divided by a plurality of partition members 72 into a plurality of cells C and C. A weir assembly is supported from each partition 72 and comprises an upright wall 73 having an opening 74 partially covered by a series of slats 75 regulating the eflective overflow level through said opening. A short wall member 76 extends downwardly in the cell to form an open-ended enclosure with the wall 73 forwardly of opening 74 so that pulp from the cell has to enter through the open bottom before it canoverflow the Weir 75. Heavier solids not overflowing the weir escape the cell through lower openings 77 into a feed box 78 which supplies a feed inlet conduit 79' of a following-cell.

The form of aerator shown in Fig. 4 differs-from those previously described in the manner of pulp circulation and pre-mixing and in the shaping of the impeller cover. A bowl liner 81 of thetypepreviously described has openings 82 for circulation of pulp and a series of ridges'83 on its inner surface. The ridges 83' terminate at their upper ends in upstanding blade portions 84. The impeller 85 is of upwardly dished contour and has a set-of blades 86 on its upper surface and another .set of depending vanes 87 adjacent the periphery of its under surface.

The impeller 85 is mounted at the lower end. of a hollow shaft 88 which is enclosed by a hollow column 89 carrying at its lower end the dished cover member 90 which overhangs the impeller. The column. 89 is provided with recirculation openings 91 in the arrangement utilized in cell C, or such openings may be omitted as shown in the aerator of cell C. In any event. pulp is fed from the feed box 78 through inlet conduit 79 to the interior of column 89 in its widened portion and premixes with the descending stream of gas beforedischarge onto the dished surface of impeller 85. The action of this aerator is substantially the same as in the forms previously described except that there is less restriction at the periphery of the impeller and hence a greater upward component of movement is present in-the discharge of aerated pulp. The blade portions 84 on the liner 81 direct the pulp travelling upwardly along the under surface of the impeller into impinging relation to the discharge from the upper surface and hence a high degree of mixing is attained.

Figs. 5, 6 and 7 show other modifications of aerator features. In order to simplify description, only modified parts bear reference numerals. In Fig. 5, the base of the hollow column flares outwardly to provide an enlarged mixing chamber 101 through which pulp descends onto the impeller and recirculating pulp is fed into this chamber through a conduit 102 discharging into the center thereof.

Fig. 6 shows another aerating arrangement for use in solid shaft assemblies. The dished surface of the impeller is separated from its hub by an interconnecting portion 103 which forms a recessed portion 104 at the end of the shaft into which a gas nozzle 105 extends from the bottom of the tank for supply of an aerating gas. This arrangement distributes the gaseous discharge effectively under the impeller and shields the end of the nozzle from sands in the case of shutdowns.

Fig. 7 shows another form of impeller mixing in which a portion of the pre-mixed pulp descending onto the impeller is short circuited through a plurality of openings 106 to mix with the stream of material moving along the bottom surface of the impeller while the remainder passes upwardly along the inner surface of the impeller in the manner previously described.

Another type of cover arrangement is shown in Fig. 8. This arrangement may be utilized with dished impellers of the type previously described. The column 116 of this form terminates in a flaring portion, divided into alternate inwardly extending members 111 having conduits 112 and outwardly flaring hollow members 113. The members 111 and 113 merge into a cover portion 114 having a series of depending blade members 115 at its periphery. The members 111 and 113 are open at the bottom and pulp and gas entering from above impinge on other pulp elevated by the impeller blades so that a high degree of mixing and aeration is obtained. Recirculating pulp gets a high velocity movement through conduits 112 and then is subjected to beating and mixing influences in passing the base of member 113 and by impact with the blades 115 at the periphery.

Figs. 9, l and 11 illustrate another aerator form, particularly suited for use in solid shaft assemblies as illustrated. Gas is supplied through a hollow column 120 in the manner previously described and its downward descent is accelerated by the pumping vanes 121 mounted on the shaft 122. The column flares at its bottom in connecting to the dished cover portion 123 and a series of passages or conduits 124 deliver recirculating pulp within the flaring portion of the column. A bottom liner 125 similar to those previously described encloses the under surface gg yan impeller 126 which has a series of elongated slots Some of the aerated pulp fed onto the upper surface of the impeller is drawn through slots 127 and moves upwardly between the impeller and liner 125 while being subjected to a beating and mixing action of the type previously described. Preferably the slots 127 are narrower to the inside and wider to the outside as shown in Fig. 11.

In all the various embodiments illustrated herein, it is intended that the variable features of the assemblies shall be interchangeable. For example, any of the impeller forms may be used with any of the gas supply arrangements shown and described. Both hollow shaft and solid shaft arrangements have been described and any type of impeller may be used with either. In preferred practice, a bottom liner member will be utilized and it may have narrow blades or ridges such as shown at 33 in Fig. 1, or may have larger blades or vanes as shown at 84 in Fig. 4, or the blade may extend upwardly to contact the depending blade of the cover member as indicated by the dash line representation in Fig. 3.

Similarly, the cover member may have depending blades of varying length (compare Figs. 3 and 6 for example) or may omit the depending blades as shown in Fig. 4. The working surface of the impeller may be imperforate as in Fig. l, or apertured as in Figs. 7 or 9.

The length and shape of the blades on the upper surface of the impeller may vary as will the angle of inclination of the impeller surface.

Therefore, it should be understood that the various forms illustrated in the drawings are intended as typical embodiments for use in the practice of the invention, the scope of which is set forth in the hereunto appended claims.

I claim:

1. In an aerator for pulps of the type having a rotary shaft, an impeller mounted on the shaft for conjoint rotation therewith, and a stationary hood overhanging the impeller, the improvement which comprises an inner pulp-receiving surface of the impeller disposed at an angle of at least 45 to the horizontal, blades projecting upwardly from said surface with their upper surfaces disposed at an inclination to the axis of rotation, and an under surface of said hood disposed in substantially parallel relation to said upper surfaces of the blades.

2. An aerator for pulps comprising an upper stationary cover member, a lower stationary enclosure having its top portion spaced from and in proximity to a surface of said upper member, and a frusto-conical impeller mounted for rotation within the enclosure of said stationary members, said impeller having an interior pulpimpelling surface slanting upwardly at an angle of at least 45 to the horizontal, there being an opening in said upper member for gravitational movement of pulp into the impeller interior, and means for supplying an aerating gas into the impeller interior and into the space between the impeller and said lower member.

3. An aerator for pulps comprising upper and lower stationary members arranged to provide at least one peripheral discharge passage therebetween, and a frustoconical impeller mounted for rotation within the enclosure of said members and having an interior, pulp-impelling surface slanting upwardly at an angle of at least 45 to the horizontal and positioned to discharge through said peripheral passage, there being an opening in said upper member for gravitational movement of pulp into the impeller interior, and means for supplying an aerating gas to the interior of the impeller and into the space between the impeller and said lower member.

4. Aeration apparatus comprising a tank for pulp arranged to receive incoming pulp at one end and to discharge treated pulp at its opposite end and having a top overflow for concentrate and a lower outlet for tailings determining a liquid level therein, and an aerating assembly for the tank including a hollow shaft depending in the tank from a point above the liquid level with its bottom adjacent the bottom of the tank, a hollow frustoconical impeller mounted on the lower end of said shaft for conjoint rotation therewith and having an interior pulp-impelling surface slanting upwardly at an inclination to the horizontal greater than 45 a hollow column depending in the tank having a bottom portion overhanging said impeller so as to prevent direct gravitational flow of pulp into its interior portion and upper submerged intake openings for admission of recirculating pulp from the tank, means for delivering an aerating gas through the hollow column to a point of discharge interiorly of said impeller for mixing with pulp descending through said hollow column, and a stationary hollow enclosure disposed beneath said impeller for restraining and directing the agitation of said impeller and having at least one opening arranged to direct the impeller discharge from said slanting surface lengthwise of said tank.

5. In an aerator for pulps of the type having a rotary shaft, an impeller mounted on the shaft for conjoint rotation therewith, and a stationary hood overhanging the impeller, the improvement which comprises an inner pulp-receiving surface of the impeller disposed at an angle of at least 45 to the horizontal, blades projecting upwardly from said surface with their upper surfaces disposed at an inclination to the axis of rotation, and an under surface of said hood disposed in proximity to but spaced from the upper surfaces of said blades.

6. An aerator for pulps comprising upper and lower stationary members arranged to provide a circumferential enclosure with a series of radially spaced passages, a frustro-conical impeller mounted for rotation within said enclosure and having an interior, pulp-impelling surface slanting upwardly at an angle of at least 45 to the horizontal and positioned to discharge through said passages, there being an opening in said upper member for gravitational movement of pulp into the impeller interior, and means for supplying an aerating gas to the interior of the impeller and into the space between the impeller and said lower member.

7. Apparatus as defined in claim 3, in which the lower member has openings adjacent its bottom for circulation of pulp into said enclosure.

8. Apparatus as defined in claim 3, in which the lower member has diametrically spaced openings adjacent its bottom for circulation of pulp into said enclosure.

9. Apparatus as defined in claim 3, in which the lower member has a series of radial ridges on its inner surface.

10. Apparatus as defined in claim 3, in which reagent feeding means are arranged to deliver reagent into said enclosure.

11. Apparatus as defined in claim 4, in which there are two diametrically spaced openings in the enclosure under the impeller for directing said discharge lengthwise of the tank.

12. An aerator for pulps, comprising an upper stationary cover member, a lower stationary enclosure having its top portion spaced from and in proximity to a surface of said upper member, a deeply-dished impeller mounted for rotation within the enclosure of said stationary members having an interior pulp-receiving surface slanting upwardly at an angle of at least degrees to the horizontal, there being an opening in said upper member for movement of pulp into the impeller interior, and means for supplying an aerating gas into the enclosure of said stationary members.

13. An aerator for pulps, comprising upper and lower stationary members arranged to provide a circumferential enclosure having a plurality of peripheral discharge passages, a deeply-dished impeller mounted for rotation within the enclosure and having an interior, pulp-impel- ]ing surface slanting upwardly at an angle of at least 45 degrees to the horizontal and positioned to discharge through said passages, there being an opening in said upper member for movement of pulp into the impeller interior, and means for supplying an aerating gas into the enclosure of said stationary members.

References Cited in the file of this patent UNITED STATES PATENTS 1,309,219 Ruth July 8, 1919 1,549,492 Kraut Aug. 11, 1925 1,901,123 Ruth Mar. 14, 1933