WO1996023581A1

WO1996023581A1 - Slurry forming process and slurry transfer and densification through vacuum pumping

Info

Publication number: WO1996023581A1
Application number: PCT/US1996/001271
Authority: WO
Inventors: Chih-Chung Wang; Kenneth Robert Taylor
Original assignee: Duracell Inc.
Priority date: 1995-02-01
Filing date: 1996-02-01
Publication date: 1996-08-08
Also published as: JPH10513114A; JP4111999B2

Abstract

Disclosed is a method for the production of slurries, which are uniform in density and consistency, and also an apparatus for the transfer or conveyance of a slurry and the simultaneous densification thereof through the intermediary of vacuum pumping. The method comprises a bag slurry forming process involving mixing dry anode components within a sealable, gas-tight, liquid impervious, flexible, collapsible bag (20); and, then adding a wet component (26) to the bag, collapsing said bag, and tumbling-kneading the components to form a blended slurry. The apparatus (40) is designed for the vacuum pumping of a slurry, particularly a zinc-containing slurry utilized in the preparation of zinc anodes for alkaline batteries, which facilitates the transfer of the slurry and the concurrent densification thereof in conjunction with the elimination of any gas bubbles which are entrapped in the slurry.

Description

SLURRY FORMING PROCESS AMD SLURRY TRANSFER AMD DENSIFICATION

THROUGH VACUUM PUMPING

The present invention relates to a method for the production of slurries, and, in particular is directed to a method for the production of slurries for electrodes which are utilized in electrochemical cells and, more particularly, pertains to a method of preparing zinc anode slurries employed in producing anodes for alkaline batteries.

The present invention further relates to an apparatus for the transfer or conveyance of slurry and the simultaneous densification thereof through the intermediary of vacuum pumping. Specifically, the invention pertains to an apparatus for the vacuum pumping of a slurry, particularly a zinc-containing slurry utilized in the preparation of zinc anodes for alkaline batteries, which facilitates the transfer of the slurry and the concurrent densification thereof in conjunction with the elimination of any gas bubbles which are entrapped in the slurry. Moreover, pursuant to a further aspect, the invention is also directed to the provision of a method of transferring and simultaneously densifying of a slurry while eliminating entrapped gas bubbles therefrom through the intermediary of a vacuum pumping process utilizing the apparatus of the invention.

Zinc anodes which are normally employed in alkaline batteries are generally produced through the formation of a slurry consisting of various active components. Such components include zinc powder, an electrolyte solution such as an aqueous solution of potassium hydroxide, and further anode components, such as surfactants and gellants. The slurry which is constituted from those components is ordinarily mixed in a blending apparatus and subsequently metered into the individual cells of the batteries to form the anodes thereof. A difficulty in the production of electrodes, such as the zinc anodes of the alkaline batteries, that often is encountered, is a lack uniformity from one battery electrode to another, and, consequently, from battery cell to cell. This inconsistency frequently adversely affects the operating characteristics and functioning of the resulting batteries. In the alkaline battery industry, which is a highly competitive and technologically developed field of activity, any significant or noticeable deviations in the physical and operating characteristics of a battery cannot be tolerated by a reputable manufacturer. One particular problem which causes lack of uniformity in the production of battery anodes resides in the undesirable inclusion of entrapped air bubbles in the electrode material, because of the presence of the bubbles in the slurry which is employed to form the anodes. The air bubbles cause variations in the density of the anode slurry, resulting in a lack of uniformity within the electrode structure itself. The foregoing results in inconsistent battery performance, directly proportional to the density of the anode material within each specific cell. Another problem which may cause deviations in the performance of the battery cells is the unwanted inclusion of contaminants in the anode composition, which often are introduced from contact of the anode components with the internal surfaces of the mixing device during blending of the anode slurry materials.

In order to address these problems, a primary effort in industry has been directed to the development of methods of producing electrochemical cells, particularly alkaline batteries utilizing zinc anodes, in an attempt to eliminate any contaminants and any entrapped air bubbles in the slurry which is employed for the production of the zinc battery anodes.

Previously, efforts have been exerted to avoid and/or eliminate the presence of entrapped or entrained air bubbles from an anode slurry through the employment of various blending or mixing devices. For example, one such attempt is the utilization of a mixing device such as a Ross blender, which mixes under vacuum so as to avoid entrapment of bubbles in the slurry during mixing. This attempt to evacuate the mixing chamber to effect the removal of entrained air or gas bubbles from the slurry is essentially rather complicated due to the need for the utilization of an evacuated system in conjunction with the operation of the blender mechanism. Although somewhat effective in eliminating air entrainment, this technique is frequently subject to undesirable ancillary effects, such as an uneven mixing of the constituents of the slurry. Consequently, anode slurries produced in this manner still do not possess the required uniform density and consistency and fail to completely eliminate, or even to reduce to a desirable extent, the presence of entrained air bubbles in the anode slurries. Moreover, in addition to the above-cited drawbacks of insufficient mixing action of the constituents of the slurry and low slurry density obtained due to entrapped air bubbles, there has also been encountered a problem of contamination of the anode material particularly from contact with the internal metallic surfaces of the mixer implements and walls.

Now, a slurry forming process has been developed, whereby improved density and/or lack of contamination can be accomplished. Pursuant to a preferred aspect of the invented slurry forming process, the components of a zinc anode composition, such as active zinc particles, aqueous potassium hydroxide electrolyte solution, and other components, such as gellants, surfactants, and other ingredients, are filled into a flexible container or bag, with the filled bag then being placed into a mixing device, preferably in the form of a driven rotatable rigid cylindrical container, through the intermediary of which the slurry mixing is to be effectuated. Preferably, the bag is made of a collapsible material, and, most preferably, a material that is gas-tight and liquid-impervious. Since all of the components are contained within the bag, there is no opportunity for direct contact between the components and the inside surfaces of the mixer device. Hence, the chance for contamination of the anode composition from the mixer is eliminated. The bag may be constructed of any suitable flexible, collapsible material; the use of plastic bags, such as polyethylene bags, has produced particularly favorable results.

It is preferred to mix the dry slurry components in a separate step prior to the addition of any wet slurry components. To accomplish the most effective mixing of dry ingredients, it is preferred to inflate the flexible bag containing the dry anode components, so that the bag will generally conform to the shape of the interior of the drum surface and allow the confined dry components freely to mix when the drum is rotated.

Pursuant to a particularly preferred embodiment of the invention, the flexible bag, within the rotatable drum, may then, optionally, be flushed with oxygen before the blending step with wet components added to the bag. In this manner, the bag will contain an atmosphere consisting essentially of oxygen rather than air,- and, consequently, any gas which is entrapped in the slurry during subsequent wet mixing will be bubbles of oxygen rather than air. Entrapped oxygen, advantageously, will react with the zinc component contained in the slurry,- the resultant zinc oxide formed by the zinc and oxygen is considered to be harmless to the performance of the alkaline battery cell. Zinc oxide is soluble in the potassium hydroxide which ordinarily is employed as an electrolyte slurry component, and also is an additive which heretofore has been purposefully added to the electrolyte. To accomplish the most effective slurry blending of the anode composition after the addition of wet components, such as the aqueous electrolyte solution, it is preferred to deflate and collapse the flexible bag before re-initiating mixing after the addition and sealing of a wet component within the bag. The wet and dry components within the collapsed bag undergo a kneading-type action, as the bag is tumbled within the rotating drum, to effect a uniformly blended slurry composition. Subsequent to the mixing of the slurry within the sealed bag, the mixed slurry then is transferred to an individual battery cell producing assembly.

In the embodiment wherein the atmosphere within the sealed bag is air, after completion of the mixing of the slurry, there will be, in all likelihood, some air bubbles remaining undissolved in the slurry. As cited above, this entrapped air can adversely affect the quality and consistency of the anode slurry. Consequently, through the intermediary of vacuum pumping arrangement, there can be carried out the transfer of the slurry in a manner intended to eliminate the presence of any residual air or gas bubbles encountered in the anode slurry pumped therethrough, so as to formulate a substantially deaerated slurry product of uniform density and consistency.

The present invention also is directed to the provision of a novel and highly utilitarian apparatus and method for the continuous transfer and simultaneous densification of zinc anode slurries with the concurrent removal of gas bubbles entrained therein; especially during conveyance of the slurry to a battery cell assembly employed in forming alkaline battery anodes. In order to attain the foregoing, the invention sets forth the provision of a slurry vacuum pump arrangement in which a vertically depending tube or riser is adapted to pick-up and aspirate a continuous quantity of anode slurry which has been previously mixed and which has been transported to the locale of the vertical tube through the intermediary of a suitable transport vehicle or dolly, and whereby the upper end of the vertical conduit is connected to a downwardly sloping conduit which is subjected to a subatmospheric pressure or vacuum through the intermediary of a vacuum pump. A continuous stream of slowly downwardly flowing slurry in the sloping conduit received from the upper end of the vertical tube or rise has the bubbles contained therein expanded under the action of the subatmospheric pressure or vacuum to which the slurry is subjected, causing the bubbles entrained therein to rise and burst and to be aspirated away through the vacuum pump. The resultingly degassed and densified slurry flows downwardly from the lower end of the sloping conduit into a vertically depending conduit so as to form a slurry column therein, the lower end of which conduit is connected to the inlet of a vacuum-assisted peristaltic pump. The peristaltic pump then pumps the densified slurry material which is essentially devoid of any entrained air bubbles and which consequently possesses a uniform desired density or consistency through a pump discharge into a discharge conduit leading to a cell assembly for forming the alkaline battery cell anodes. Although the vertical tube which forms the rise for receiving the slurry from the batch slurry mixing unit is basically intermittently submerged in slurry, upon the slurry being upwardly aspirated therethrough, a continuous flow of slowly advancing slurry flow is produced in the downwardly sloping conduit so as to enable the expansion and elimination through a bursting thereof in the vacuum atmosphere by the bubbles which are entrained in the slurry. This elimination of the bubbles of gaseous medium ensures an appropriate densification of the slurry and consistency thereof when continuously flowing downwardly from the sloping conduit towards the peristaltic pump for conveyance by the letter to the battery cell forming assembly.

An important aspect of the invention also resides in the low cost and simplicity of the bag slurry forming process in comparison with the use of a conventional, evacuated blender. Such blenders necessitate a vacuum atmosphere be present during the mixing operation, and consequently are expensive pieces of equipment, rendering the slurry mixing process considerably more cost intensive. In accordance with the present invention, the need for specialized, vacuum blender equipment is avoided, thereby enabling the employment of less expensive and more efficient mixers, of widely varying types, for formulation of the slurry. According to the invention, a sealed liquid-impervious and gas-tight, flexible, collapsible bag containing the anode materials simply is placed in a rotatable vessel so as to be rotatable in conjunction therewith. In order to minimize transfer steps during the formulation and production of the anode slurry, it is possible to contemplate utilizing containers in which the raw materials for producing the slurry are directly supplied to the mixing bags.

Reference may now be had to the following detailed description of preferred embodiments of the slurry forming process of the invention, taken in conjunction with the accompanying drawings; in which: Figure 1 illustrates a diagrammatic cross-sectional view of a rotatable drum, containing a collapsed bag filled with slurry components, representing the kneading-mixing action between the slurry components;

Figure 2 illustrates a perspective side view of a typical cylindrical batch mill utilized for the mixing of slurries; and

Figure 3 illustrates a diagrammatic sectional view of a conical batch mill drum unit employable for the mixing of slurries.

Figure 4 illustrates, generally diagrammatically, a vacuum pumping arrangement for the continuous transfer and densification through the degassification of a slurry;

Figure 4a illustrates a schematic representation of zinc particles between gas bubbles entrained in a zinc anode slurry prior to processing thereof in the vacuum pumping arrangement of the invention; and

Figure 4b illustrates the zinc particles and entrained bubbles in the zinc anode slurry during the vacuum pumping process.

Referring to Figure 1, pursuant to the inventive concept, there is contemplated the use of a flexible, collapsible container, such as a sealable gas-tight, liquid-impervious bag 20 containing slurry components 26, which when positioned in a rotatable container, such as a metallic cylindrical mixing drum 22, or the like, will eliminate the contact between the anode components and the interior surfaces of the metallic container. The bag preferably may be inflated to conform to the inner shape of the drum during rotational mixing of the dry components. As illustrated in Figure 1, after addition of wet components, the bag may be flushed with oxygen and collapsed to provide for an improved mixing action and so-called "kneading" of the slurry through the action of the flexible or movable walls of the bag 20 which are displaced towards and away from the drum wall inner surface 24 during tumbling and rotation. This enhanced mixing action of the slurry components, with the concurrent elimination of entrapped air through the alternative use of oxygen to replace the atmospheric medium in the rotatable bag 20, accomplishes a uniform slurry while avoiding the formation of unwanted entrapped air or gas bubbles in the slurry.

In practicing the present invention, the flexible bag 20 is filled with slurry components, and the bag, with the contents thereof, is positioned in the rotatable drum 22, as shown in either Figure l or as part of batch mill 28 in Figure 2. The rotatable drum is then tilted into a substantially horizontal or slightly tilted therefrom position, as shown in Figure 2. The drum and contained bag then are rotated in a motor driven mode and tumbled for a specified period of time, in conformance with the requirements of the slurry mixing process, to knead the wet and dry anode components to form a thoroughly blended slurry. Thereafter, the drum 22, including the sealed bag 20 containing the admixed or blended slurry, may be upended through suitable hoisting means and transported on a cut-away dolly or other suitable transport vehicle to a battery cell assembly. Alternatively, if it is necessary, the slurry may be transported first to a vacuum pumping installation for further elimination of any entrained air or gas bubbles, so as to density and deaerate the slurry to a desired consistency prior to being transferred to the battery cell assembly.

As demonstrated by Figure 3, various conventional mixers may be utilized in practicing the present invention. For example, rather than being cylindrically configured, the mixing drum, containing the bag, may be a conical batch mill 32, which is rotatable about a horizontal axis to provide for the slurry mixing action as illustrated by the arrows shown therein. The various cylindrical or conical batch mill drums, and the like, may also be mounted on transport vehicles or dollies so as to enable the displacement and positioning thereof in an operative relationship with a battery cell assembly, or, if necessary, with a vacuum pumping assembly as mentioned hereinabove, thereby obviating the necessity for the provision of further slurry transport devices.

As set forth herein, the slurry for forming the zinc anodes for alkaline battery cells may have the following components, shown in appropriate weight fractions as illustrated in Table 1 hereinbelow.

TABLE 1

Weight

Additive Fraction (%)

Zinc 65.0

Surfactant Solution 0.2

Gel1ant 0.8

Electrolyte 34.0

In essence, by way of example, a bag slurry forming process may constitute the following general steps:

1. Dry Plating - Add zinc powder to a polymer mixing bag. Inflate and seal the bag and tumble for 10 minutes.

2. Fluxing - Add surfactant solution. Inflate and seal the bag and tumble for 10 minutes.

3. Dry Mixing - Add gellant. Inflate and seal the bag and tumble for 10 minutes.

4. Wet Mixing - Add electrolyte to dry mix. Remove excess atmosphere to collapse the bag (and, optionally, flush the bag contents with oxygen), seal, and tumble for 60 minutes.

5. Vacuum Pumping (optional) - A portion of the slurry is sparged into an evacuated chamber, where entrapped air bubbles are removed, thereby producing a more uniform slurry. When the bag is back-filled with oxygen rather than containing air, Step 5 (Vacuum Pumping) may not be necessary.

Results of tests with regard to the slurry formulations and mixing using the present technique rather than with a Ross blender showed a dramatic improvement in uniform characteristics and density consistency. In addition, slurry formulations processed in an air atmosphere, according to the present invention, thereafter followed by vacuum pumping to remove entrained air bubbles, exhibited improved density characteristics and a visible reduction in air entrapment compared to production slurries normally produced in the Ross-type blender. In essence, pursuant to the invention there are obtained improvements in slurry characteristics and processability, which can be readily ascribed to the improved method of formulating the slurry through the bag slurry manufacturing process.

In order to achieve the desired densification and consistency for a commercially viable and uniform anode slurry, recourse may be had to the invention of Figure 4 of the drawings illustrating, generally diagrammatically, a vacuum pumping apparatus 40 for eliminating gas or air bubbles which are entrapped in the anode slurry.

The vacuum pumping apparatus 40 basically includes a vertical tube or rise 42 which has an open lower end 44 thereof adapted to be inserted into a quantity or batch of premixed zinc anode slurry which was formed by being produced in a suitable manner. As shown in phantom lines, the batch of slurry, into which the lower end 44 of the vertical tube 42 is inserted, may be contained in suitable vessel 46, which may be possibly, although not necessarily, supported on a suitable transport vehicle (not shown) . The upper end 50 of the vertical rise or tube 42 communicates through the intermediary of a suitable angel or Y-connector 52 with the upper end 54 of a downwardly sloping or inclined conduit 56 which subtends an acute angle θ with the vertical axis of the tube or riser 42. The lower end

58 of the downwardly inclined conduit 56 is connected by means of a suitably angled Y-connector 60 with a vertically oriented conduit structure 62. The conduit structure has an upwardly extending portion 64 leading from connector 60 and which is connected at its upper end 66 to a vacuum pump 68. A second conduit portion 70 of conduit structure 62 extends vertically downwardly from connector 60 so as to connect at the lower end 72 thereof with an inlet 74 of a vacuum-assisted peristaltic pump 76. The peristaltic pump 76 possesses a discharge outlet 78 having a further conduit 80 attached thereto, and which leads to a battery cell line hopper (not shown) for conveying a predetermined continuous feed of the slurry of zinc anode material from the pump 76 to the battery cells.

As indicated in the graphical representation of Figures 4a and 4b of the drawings, during the formation of the zinc anodes slurry, through the aggregation and mixing of the constituent materials, air and/or gas bubbles are generated and dispersed throughout the slurry, whereby the trapped air bubbles or gas bubbles do not rupture easily, and consequently are difficult to separate out of the slurry. Figure 4a shows the presence of gas bubbles interspaced between the zinc constituents of the slurry before the vacuum pumping of the latter by means of vacuum pumping apparatus 40. However, during the imposition of a vacuum by the vacuum pumping apparatus 40 pursuant to the invention, the zinc particles contained in the slurry are assumed to assist in rupturing the film surface of the bubbles as the bubbles expand and the film thins, as illustrated in Figure 4b.

As the vacuum is increased due to the vacuum action of the vacuum pumping apparatus 40, such as up to 29 inches Hg, the bubble size contained in the slurry will expand. This aspect is indicated in Table 2 hereinbelow, illustrating bubble diameter at increasing slurry depth as the slurry is conveyed upwardly through the riser. Table 2 BUBBLE DIAMETER VS "INCREASING" VACUUM

ABSOLUTE GAUGE BUBBLE

PRESSURE PRESSURE* DIAMETER

(MM Hg) (in. Hg) (urn)

760 -O 50 (original size)

340 -16.54 65

100 -25.98 98

20 -29.13 168

10 -29.53 212 (4x original size)

5 -29.72 267

2 -29.84 362

1 -29.88 456

* Based on an atmospheric pressure of 760mmHg absolute.

OPERATION OF THE VACUUM PUMPING APPARATUS

A flow of slurry is picked up and aspirated at the lower end 44 of the vertical tube or riser 42 from the slurry batch contained in vessel 46 towards the upper end 50 of the vertical riser 42. This suctioning effect or action is produced by the vacuum pump 68 which creates a vacuum or subatmospheric condition in the conduits 42, 56 and 62 of the apparatus 40 of up to about 29 or 30 inches Hg. As the flow of slurry from riser 42 reaches the upper end 54 of downwardly inclined conduit 56, a continuous flow of the slurry streams slowly under the effect of gravity downwardly towards the lower end 58 of the conduit 56 where the latter forms the juncture with the vertical conduit section 62 leading to, respectively, the vacuum pump 38 by means of conduit portion 64 and the vacuum-assisted peristaltic pump 46 through conduit portion 70.

The vacuum pump draws a specified vacuum, for example up to 29 inches of mercury, causing the bubbles entrapped within the slurry flowing slowly down the conduit 56 to be subjected to a reduced pressure, as described hereinbefore, and to expand as shown in Figure 46 and burst, thereby releasing the gas from the slurry and enabling the gaseous medium to be aspirated upwardly through the vertically upwardly extending conduit portion 64 towards the vacuum pump 68 for discharge into atmosphere.

The resultingly deaerated and densified slurry which passes from the lower end 58 of conduit 56 through the Y-connector 60 will then slide downwardly columnarly within conduit portion 70 under the action of gravity so as to form a column of the densified and deaerated slurry at the end 72 above and the inlet connection 74 to the vacuum-assisted peristaltic pump 76. The pump 76 then conveys the densified slurry through outlet 80 to suitable anode cell line hopper or cell assembly arrangement for forming the alkaline battery cell andoes.

From the foregoing it becomes readily apparent that the inventive method and apparatus 40 for the vacuum pumping of a zinc anode slurry enables the continual transfer and densification of the slurry while simultaneously eliminating any gas or air bubbles entrained therein. In this connection, Table 3 illustrates the slurry densities for a series of anode slurries before and after vacuum pumping by the inventive apparatus, as shown:

Table 3 SLURRY DENSITIES BEFORE AND AFTER VACUUM PUMPING

Slurry Theoretical Density Density Batch No. Density Before Pumping After Pumping

(gr/cc) (@ 25 mmHg)

(gr/cc)

1 (2.97) 2.74 2.89

2 (2.97) 2.85

3 (2.97) 2.62 2.85

4 (2.97) 2.49 2.81

5 (2.97) 2.56 3.04

6 (2.97) 2.73 2.90

7 (2.97) 2.48 3.05

8 (2.97) 2.79 2.95

9 (2.97) 2.78 2.97

10 (2.97) 2.59 2.88

11 (2.97) 2.59 2.84

12 (2.97) 2.57 2.87

13 (2.97) 2.60 2.88

14 (2.97) 2.78 2.97

15 (2.97) 2.79 2.95

16 (2.97) 2.85 2.93

While there has been shown and described what are considered to be preferred embodiments of the invention, it will, of course, be understood that various modifications and changes in form or detail could readily be made without departing from the spirit and scope of the invention. It is, therefore, intended that the invention be not limited to the exact form and detail herein shown and described, nor to anything less than the whole of the invention herein disclosed as hereinafter claimed.

Claims

What is claimed is:

1. A method of forming a slurry comprising: introducing selected slurry components into a flexible bag; sealing said bag to confine said components therein and placing the sealed bag within a mixing device; and, activating said mixing device to agitate said bag, so as to blend the confined components into a slurry composition.

2. A method of forming a slurry of select dry and wet components comprising: introducing the dry and wet components into a flexible, collapsible bag; collapsing said bag and sealing the bag to confine the components therein,- placing the collapsed bag within a mixing device; and, activating said mixing device to agitate said bag, so as to knead and blend the confined components into a slurry composition.

3. A method of forming a slurry in a rotating drum mixer comprising: introducing dry slurry components into a flexible, collapsible bag; placing said bag within the drum of the drum mixer; inflating said bag to conform to the shape of drum interior surface and sealing the bag to confine the components therein,- rotating the drum to mix the dry components; introducing a wet slurry component into said bag; collapsing said bag and sealing the bag to confine the wet and dry components therein,- and, rotating the drum mixer to agitate and tumble said bag, so as to knead and blend the confined components into a slurry composition.

4. The method of Claim 3 wherein the bag is inflated with air before it is sealed and the drum is rotated to mix the confined components.

5. The method of Claim 3 wherein the bag is flushed with oxygen before the drum is rotated to blend the confined wet and dry components into a slurry.

6. The method of Claim 3 wherein dry and wet anode components are blended to form a slurry composition.

7. The method of Claim 6 wherein dry components comprising zinc powder, a gellant, and a fluxing agent are mixed; and a wet component comprising an electrolyte solution is introduced and blended to form a battery anode slurry composition.

8. The method of Claim 3 comprising: introducing selected dry slurry components into said bag; inflating said bag,- rotating the drum to mix the selected dry components; opening said bag and introducing at least one additional dry component into the bag; sealing the bag to confine the components therein,- and, rotating the drum to mix the confined dry components.

9. The method of Claim 8 comprising: introducing zinc powder and fluxing agent into said bag,- inflating said bag; rotating the drum to mix the zinc powder and fluxing agent; opening said bag and introducing a gellant into said bag,- sealing the bag to confine the components therein,- rotating the drum to mix the confined components; opening said bag and introducing an electrolyte solution into said bag; flushing said bag with oxygen,- collapsing said bag,- and, rotating the drum mixer to knead and blend the components into a slurry composition.

10. A vacuum pumping apparatus for the transfer and densifying of a flow of a slurry having a gaseous medium dispersed therein, said apparatus comprises conduit means for conveying said slurry along a predetermined flow path; first pump means generating a subatmospheric pressure in said conduit means for separating said gaseous medium from said slurry; and second pump means communicating with a discharge end of said conduit means for dispensing a flow of said degassified and densified slurry from said apparatus.

11. An apparatus as claimed in Claim 10, wherein said conduit means comprises a first conduit portion having a first end immersible into a supply of said slurry; a second conduit portion connected to a second end of said first conduit portion for receiving a continuous flow of slurry from said first conduit portion, said first pump means aspirating said gaseous medium from said slurry during the flow of said slurry through said second conduit portion.

12. An apparatus as claimed in Claim 11, wherein said first conduit portion comprises a vertical riser, said first end of said first conduit portion being the lower end immersible into said slurry, said second conduit portion being downwardly inclined relative to a horizontal plan and having an upper end connected to the second end of said vertical riser for receiving the flow of slurry therefrom.

13. An apparatus as claimed in Claim 12, wherein said first pump means aspirates entrapped gaseous medium from the flow of slurry continuously streaming downwardly through said second conduit portion towards the juncture thereof with a third conduit portion of said conduit means.

14. An apparatus as claimed in Claim 13, wherein said third conduit portion comprises a vertical conduit section extending upwardly from the juncture with said second conduit portion and having an upper end communicating with said first pump means, and a vertical conduit section extending downwardly from the juncture with said second conduit portion end having a lower end connected to an inlet of said second pump means for directing a continuous flow of densified slurry from said second conduit portion to said second pump means.

15. An apparatus as claimed in Claim 10, wherein said gaseous medium comprises gas bubbles entrapped in said slurry, said subatmospheric pressure in said conduit means generated by said first pump means expanding and bursting said bubbles to facilitate deaerating and densifying of said slurry.

16. A vacuum pumping method for the transfer and densifying of a flow of a slurry having a gaseous medium dispersed therein,- said method comprising conveying said slurry along a predetermined flow path; subjecting said slurry to a subatmospheric pressure along said flow path for separating said gaseous medium from said slurry,- venting said separated gaseous medium,- and discharging said flow of degassified and densified slurry for further processing thereof.

17. A method as claimed in Claim 16, wherein said flow path comprises a first conduit portion having a first end immersible into a supply of said slurry,- a second conduit portion connected to a second end of said first conduit portion for receiving a continuous flow of slurry from said first conduit portion, and said gaseous medium is aspirated from said slurry during the flow of said slurry through said second conduit portion.

18. A method as claimed in Claim 17, wherein said first conduit portion comprises a vertical riser, said first end of said first conduit portion being the lower end immersible into said slurry, said second conduit portion being downwardly inclined relative to a horizontal plane and having an upper end connected to the second end of said vertical riser for receiving the flow of slurry therefrom.

19. A method as claimed in Claim 18, wherein said entrapped gaseous medium is aspirated from the flow of slurry which continuously streams downwardly through said second conduit portion towards the juncture thereof with a third conduit portion of said flow path.

20. A method as claimed in Claim 19, wherein said third conduit portion comprises a vertical conduit section extending upwardly from the juncture with said second conduit portion and having an upper end communicating with a pump for generating said subatmospheric pressure, and a vertical conduit section extending downwardly from the juncture with said second conduit portion end having a lower end connected to an inlet of a second pump for directing a continuous flow of densified slurry from said second conduit portion to said second pump.

21. A method as claimed in Claim 20, wherein said gaseous medium comprises gas bubbles entrapped in said slurry, said subatmospheric pressure in said conduit means generated by said first pump expanding and bursting said bubbles to facilitate deaerating and densifying of said slurry.