CA1074695A

CA1074695A - Method and apparatus for recovering metal values from mineral ores by in-situ mining

Info

Publication number: CA1074695A
Application number: CA286,304A
Authority: CA
Inventors: Robert A. Hard; Donald H. Davidson; Limin Hsueh; Ray V. Huff
Original assignee: Kennecott Copper Corp
Current assignee: Kennecott Corp
Priority date: 1976-09-20
Filing date: 1977-09-08
Publication date: 1980-04-01
Also published as: AU2778977A; US4360234A; AU512749B2; ZA774796B

Abstract

ABSTRACT OF THE DISCLOSURE

A method of and apparatus for use in recovering metal values such as copper from ore formations by in-situ mining, in which a lixiviant is supplied to at least one porous, sintered powdered metal tube of a gas-sparging unit while a gas is supplied under pressure about the tube to cause the gas to penetrate into the interior of the tube in the form of finely divided bub-bles which are wiped from the interior of the tube by the lixiviant passing therethrough, and the lixiviant admixed with the gas bubbles is supplied to the ore formation to leach the metal values therefrom.

Description

~7~6~5 The present invention relates to a method and apparatus for recovering metal values, for example copper, by in-situ mining of ores containing them.
The recovery of copper and other metals by conven-tional procedures, such as open pit mining and under-ground tunneling, has in recent years become costly and time consuming. For these reasons, and also because of the increasing demand for copper, other methods of copper recovery have been proposed. One such method is in-situ mining which makes it feasible to recover copper even at great depths.
In the in-situ mining operation, a well bore is drilled to the level of the ore formation. A liquid lixiviant, such as ammonia-ammonium sulfate or ferric sulphate, is supplied through the well bore to the ore formation wherein it reacts with the metal values in the ore to produce a pregnant liquor containing the metal values. This pregnant liquor is then withdrawn from the ore formation through the same well bore, or through a series of closely spaced bores surrounding a central well bore, by pumping, or through a gas lift operation.
In many instances of in-situ mining, it is advanta~
geous to introduce a gas, such as oxygen, into the lixiviant supplied to the well bore. The combination of gas and lixiviant improves the leaching of the metal r values from the ore formation. Usually an oxidizing gas such as air, oxygen, or air enriched with oxygen is used~
This gas may also be supplemented with a catalyst such as SO2, or an acid formirig gas such as SO3. An example 1C~7~ S

of a system in which a lixiviant containing bubbles is used to leach metals in the so-called "oxygen-water"
system.
It has been found that supplying the lixiviant admixed with finely divided gas bubbles to the ore formation will aid in the removal of metal values contained in the ore.
It is an object of the present invention to provide a method and apparatus for use in the recovery of metal values from ore formations at great depths by in-situ mining.
According to one aspect of the invention, there is provided a process for introducing a finely divided gas into a liquid comprising the following steps: (a) sup-plying the liquid into a porous tube formed of sintered powdered metal and having micropores with a diameter be~
tween the range of 2-1000 microns; (b) supplying the gas to the exterior of the tube under sufficient pressure to cause the gas to penetrate to the interior of the tube;
I and, tc) passing the liquid through the tube to cause the ¦ 20 liquid to shear the gas bubbles from the interior sur~ace of the tube to produce a liquid containing the finely divided gas bubbles.
Preferably, the process is employed in the in-situ mining of minerals and comprises the steps of supplying a lixiviant as said liquid to said tube and impregnating said lixiviant liquid with said gas, and supplying the thus formed lixiviant llquid to mineral ore to leach out the metal values therein and produce a pregnant liquor containing the metal values.
According to another aspect of the invention there is ,~ provided a gas sparging unit for use in introducing finely 1~ ;95 divided gas bubbles into a lixiviant used for in-situ mining of minerals, said device comprising a hollow casing having a first chamber formed therein into which liquid ~
lixiviant is supplied and a second chamber isolated from said first chamber; a plurality of porous tubes formed of sintered powdered metal and having micropores with a diameter between the range of 2-1000 microns, said tubes extending into said second chamber with said tubes having one end in fluid communication with said first chamber;
and, means for introducing a pressurized gas about the portion of said tubes in said second chamber to enable the gas to penetrate into said tubes so that the gas can be wiped from the interior of the tubes by the lixiviant flowing through the tubes to form a lixiviant containing finely divided bubbles.
The gas sparging unit can be connected to a well pipe above the surface of the ore formation; or it can be inserted down the well bore near the ore formation to supply the liquid lixiviant and intermixe~ gas bubbles to the ore.
~ In the drawings, ; Fiy. 1 is a longitudinal sectional view of a gas sparging unit according to one embodiment of the invention, which unit is adapted to be used above the surface of the ore formation;
Fig. 2 is a longitudinal sectional view similar to Fig. 1 of a gas sparging unit according to another embodiment of the invention;
Fig. 3 is a longitudinal sectional view, similar to Yig. 1, of a gas sparging unit according to still another embodiment of the invention, which unit is adapted to be ~t~

~C17~95 inserted in a well bore below the ground;
Fig. 4 is a schematic representation of laboratory apparatus for measuring the bubble size of two-phase lixiviant formulations; and, Fig. 5 is a schematic diagram showing~laboratory ,~" !;.,~,, i~.~ t - 4a -apparatus for testing the stability, under various con-ditions, of two-phase lixiviants.
Referring now to the drawings in detail, and more r particu]arly to Fig. 1, a gas sparging unit 10, con-structed in accordance with one embodiment of the present invention, consists of a generally cylindrical casing .!'~
formed from a plurality of annular members which are r welded together to form an elongated cylindrical sleeve 12. The sleeve is closed at one end 14 in any convenient manner, as for example by a flanged cap 16 or the like, and has a first partition plate 18 welded therein which -defines a first chamber 20 within the sleeve. The parti-tion plate 18 extends entirely across the internal diameter of sleeve 12. A second partition plate 22 is located near the opposite or outlet end 24 of the sleeve and defines a second chamber 26 within the sleeve. r A plurality of elongated tubes 28 are mounted in the partition plates 18, 22 with one end 30 of each tube 28 communicating with the interior of the chamber 20. The other end 32 of each tube 28 extends through the parti--tion plate 22, near the outlet end 24 of the sleeve.
These tubes are preferably formed of a porous sintered metal powder having micropores of a diameter of, for example, 50 microns, to permit small gas bubbles to be diffused therethrough. A general useful range of pore diameter is from 2 microns to 1000 microns. A preferred range is from 10 to 100 microns. Such tubes may be formed of stainless steel or similar metallic material.
The size of the pores in a tube is controlle~ by 1~79L6~5 ~.
selecting proper particle size distribution of stainless steel powder and by sintering at a temperature slightly r below the melting point of the stainless steel powder~
The number of such tubes used in a particular gas sparging unit may be varied as desired in accordance with ',~
the amount of gas bubbles required to be introduced into '^
the lixiviant solution and the type of ore formation '~
being treated.
The first chamber 20 of the gas sparging unit 10 in-cludes an inlet opening 34 through which a lixiviant under pressure, such as ammonia and ammonium sulfate or ferric sulphate, is supplied from a source shown by arrow 36 by any convenient pumping apparatus.
The second chamber 26 of the gas sparging unit in-cludes an inlet opening 38, through which the gas to be introduced into the lixiviant solution is supplied under '~
pressure from a source indicated by arrow 40, in any convenient manner.
In the typical in-situ mining operation, the gas supplied will be an oxidizing gas such as air, oxygen, oxygen enriched air, or a combination of oxygen and some catalyst, such as SO2 or SO3, as an acid forming gas.
By supplying gas under pressure in this manner to the chamber 26, the gas is forced to penetrate through the porous tubes 28 in the form of small bubbles deposited r on the interior surfaces of the tubes. Since the upper ends 30 of the tubes are in communication wi-th the chamber 20, the liquid lixiviant supplied to that chamber will flow through the tubes into contact with the small 107469~

bubbles formed therein. The lixlviant moving through the tub~s towards the lower ends 32 thereof will wipe the gas bubbles from the interior surfaces of the tubes and become intermixed with them.
It has been found that the greater the velocity at which the barren lixiviant moves through the tubes, the smaller the bubbles introduced into the lixiviant will r be. Generally the proper velocity of lixiviant in a tube can be calculated from the amount and pressure of n~
introduced lixiviant. Fluid velocity in the range of

2 ft/sec. to 50 ft/sec. has been found satisfactory when porous tubes of 1/4" inside diameter are used. The size of the bubbles can also be varied or controlled by using porous tubes of varying diameters at a fixed flow. In L
this connection tubes having inside diameters of between 1/8" and 1/2" have been found satisfactory when the tubes have pores with diameters ranging between 10 and 100 microns and with lixiviant velocities between 2 ft/sec.
and 50 ft/sec.
The lixiviant solution thus mixed with the fine gas bubbles passes through the lower ends 32 of the tubes 28 to the outlet end 24 of the gas sparging unit. É
In the embodiment of the invention shown in Fig. 1, the gas sparging unit is adapted to be used above the ground. ~ccordingly, the end 24 may be connected in any convenient manner, as for example by an elbow joint, to the well pipe which extends down the well bore. In this embodiment, lixiviant mixed with gas bubbles passes down the well pipe to the ore formation to treat the metal 6g5 values in the ore formation and create a pregnant liquor, in accordance with known processes. As mentioned, the present invention is employed in situations where the introduction of fine gas bubbles into the lixiviant im-proves the chemical process which leaches the metal values of the ore formation.
Another embodiment of the invention is illustrated in Fig. 2 of the drawings. This embodiment of the inven-tion is substantially the same as that illustrated in Fig. 1, and also is intended to be used as a surface sparging unit; i.e., it is used above the ground and the combined mixture of lixiviant and fine bubbles is supplied to the well pipe from the outlet end 24 of the sparging unit above the ground level. Typically these units are arranged in a vertical position so that the tubes 28 therein extend vertically.
In the embodiment illustrated in Fig. 2, the elements which correspond to like elements in the embodiment of Fig. 1 have been identified with the same reference nu- ;
merals. In this embodiment of the invention, however, the second chamber 26 is formed between the partition plate 18 and a partition 44 which has a generally conically shaped guide surface 46 downstream of the L
chamber 26. This partition is also generally circular in configuration and extends completely across the entire internal diameter of the sleeve 12 and receives the ends 32 of the sintered metal powder porous tubes 28. In addition, partition 44 receives the end 47 of a hollow vent tube or conduit 48. This conduit extends through the ~L079L1~5 partition pla-te 18 in a gas and liquid tight seal to the exterior of the sparging unit through the cap 16. The r vent tube 48 and the conically shaped partition 44 allow large diameter gas bubbles to escape from the sparging unit. That is, it may happen that during the operation b of the device, bubbles 45 are formed in the lixiviant solution, as it is discharged from the -tubes 28, which have a diameter, and thus a buoyance, which is so large as to prevent the bubbles from moving downstream with the lixiviant solution into the ore formation. Such bubbles 45 then will rise vertically in the sleeve 12 as is shown r by arrows 49 and can escape from the sparging unit through conduit 480 Again, it is noted that the sparging unit is normally used in a vertical position so that the bubbles can rise vertically through conduit 48.
The embodiment of the invention illustrated in FigO 3 is particularly adapted to be used in the well bore it- j self. This embodiment of the invention includes a sleeve 52 which is also formed from a plurality of cylindrical elements welded together. This sleeve is closed in a fluid tight seal at its upper end by a cap 54 and in-cludes an intermediate partition 56 which, with cap 54, defines a first chamber 58 within the sleeve. A second partition 61 is located downstream of the partition 56, and cooperates therewith to define a second chamber 60 in the sparging unit. A plurality oi sintered metal powder porous -tubes 28 are mounted in the second chamber 60 with their upper ends 62 extending in-to the par-tition 56 and into communication with the chamber 58. The lower ends I

..... , . ., . , .. _ ~ ~

~7~95 64 of the tubes 28 extend through -the partition 61 and into communication with the discharge end 66 of the sleeve r 52. A11 of the joints between the sleeve 52, the parti~
tions 56, 61 and the tubes 28 are formed to be liquid and gas tight, as for example by welding.
Liquid lixiviant under pressure is supplied to the first chamber 58 of the gas sparging unit in this embodi- r ment of the invention through a conduit 68 from a source of lixiviant, in the same manner as described above with respect to the embodiment of Fig. 1. Thus, the liquid lixiviant can flow through the opened ends 62 of the tubes r 28 through the tubes to the discharge end 66 of the sparging unit.
Gas is supplied from a convenient source to the second chamber 60 through a tube or conduit 70 which ex-tends, in liquid tight relation, through the cap 54 and r the partition 56O
The embodiment of the invention shown in Fig. 3 operates in substantially the same manner as the pre-viously described embodiments in that the pressurized gas supplied to the chamber 60 is caused to penetrate through ~, the porous tubes 28 so as to form small bubbles on the inner surfaces of the tubes from which they are wiped by lixiviant flowing therethrough. In this manner the fine bubbles are introduced into the lixiviant and discharged -therewith thrcugh the discharge end 66 of the sparging unit.
As mentioned, this sparging unit is intended to be used in the well bore itself, and is dropped down the well F
I

~L~7~ :

bore in any convenient manner. Of course, -the conduits 68, 70 are connected to other conduits (not shown) which r extend up the well bore to the surface where the sources of gas and lixiviant are located. The diameter of the generally cylindrical sleeve 52 is, in this embodiment, formed to fit within the predrilled well bore, so that the unit can be readily lowered down the bore hole to the r desired elevation for treatment of the ore formation.
- For example, the sparging unit of Fig. 3 may have an outside diameter of approximately 2" and a length of approximately 40". The porous tubes, which may range for ff example from 3 to 8 tubes, typically will have an outside diameter of .420" and an inside diameter of .250", while their length may be for example 31". b EXAMPLE I

A gas sparging unit was constructed from 8 pieces of sintered stain.less steel tube each with a 0.25" inside diameter, a 0.42" ~utside diameter, and a 15" length.
The tubes had pores with an average diameter of 40 microns.
The casing was made of 4" stainless steel pipe. The unit was pretested and found to produce gas bubbles with diameters in the range of 0.1-0.3 mm. L~
The unit was shipped to a test site where the copper ore body is mainly in the form of chalcopyrite lying at a depth of 2000-4000 ft. below the ground. The average copper grade was estimated to be 0.5%. An ammoniacal solution (3 molar per liter ammoniurn nitrate and 1 molar per liter ammonia) was injected at a rate of 10 gallons ~74695 per minute and gaseous oxygen at a rate of 12 standard cubic feet per minute into a well at the interval of 3200-3300 ft.
The solution was recovered in a separated well 70 ft.
away from the injection well. The solution was produced at 10 gallons per minute. The copper concentration in the produced solution was up to 1.2 grams per literO r The efficiency of the sparger of the present inven-tion can be increased by utilizing a surfactant in the lixiviant.
A further modifica-tion of the process and apparatus of the present invention consists in the inclusion of a twisted stainless steel strap 71 (see Fig. 3) having one spiral per inch within the porous tubes 28. The spiral is designed to create an angular velocity component in addi-tion to the longitudinal velocity component.
In order to evaluate the effect of the surfactant as well as the effect of the spiral 71, a series of tests were conducted.
Referring to Fig. 4, laboratory apparatus for produc-ing two-phase lixiviants and for measuring the size of bubbl-es dispersed therein is shown. The apparatus consists f of a sparger 10' and a bubble viewer 12'. The viewer 12' comprises a clear plastic case, 0.25 inches in depth, -~
2.25 inches wide and 6 inches high. The outlet of the viewer 12' (not shown) is partially submerged in a beaker full of water which keeps the viewer full of fluid during experimenta-tion. The upper portion 14' of the viewer 12' contains a layer 16' of glass beads which reduces vortex r , ,~, .... , ..... , _, .. . ... .

~7~695 formation while the viewer is filled with a lixiviant.
The sparger comprises a one quar-ter inch inside r diameter sintered stainless steel porous tube 18', en-closed by pipe 20' which may be filled with pressurized gas through gas inlet 22'. A polyvinyl chloride plug ~
24' sealed to the bottom of pipe 20' by an O-ring 26' -serves as an air tight connection between the sparger 10' r and the viewer 12'.
In use, the apparatus is filled with liquid, and water or ammoniated water is introduced through the top of porous tube 18' at a given flow rate. Gas tnitrogen, air, oxygen, or oxygen enriched air) is introduced through gas inlet 22' under pressure and thereby forced through the porous walls of tube 18'. The gas may also include various gaseous oxidants comprising acid forming gases such as SO2, SO3, or NO2. The gas bubbles produced with-in the tube 18' are then wiped from the interior walls of - tube 18' and carried through plug 24' and glass beads 16' into the viewing area 13' of viewer 12' by the liquid flow. Using this procedure and apparatus, it is possible to study the effects of various parameters on the bubble size and stability of li.xiviants produced, e.g., the effect of the gas flow rate, liquid flow rate, inclusion L
of the spiral, and the inclusion of various additives combined with the liquid phase of the lixiviants. The object of the experiments was to produce a stable, two-phase lixiviant which could be delivered to -the leaching interval of an in-situ mine at a reasonable flow rate without phase separation. In this regard, it has been p .. . .. _ _ $6~5 discovered that the success of such lixiviants in in-situ mining techniques depends on the size of the gas L
bubbles being generally about lO-lO0 microns.
The size of a single gas bubble, in general, can be determined quite easily from its ascending velocity in a L
fluid of known viscosity. However, the apparatus of Fig. ~
4 was developed since there was no established method for L
measuring the size of large numbers of gas bubbles in a fluid. The size of bubbles present in the viewing area 13' of the apparatus of Fig. 4 may be easily determined if a photograph is taken of viewing area 13'. The photo-graphic method was employed because it was both direct and simple.
From a series of experiments on various lixiviants produced using the apparatus of Fig. 4, a lixiviant has been developed which makes the in-situ mining of the type described above more efficient. Specifically, the addi-tion of an effective amount of one or more surfactants to the aqueous phase of the two-phase lixiviants produced enabled the production of bubbles of the desired size range and substantially reduced coalescence of bubbles.
With a surfactant, the size of the bubbles is within the range of O.i to 0.5 mm (lixiviant at atmospheric pressure).
Without a surfackant, two-phase lixiviants produced under identical conditions have a bubble size range of 1.0 to 1.5 mm. In short, the addition of the surfactant yields a 3 to 15 -times improvement in gas bubble size distribu-tion in -the low pressure simulator of Fig. 4.
Furthermore, some attempts at in-situ mining r ..... ., .. -- .. ..... .. .
.~_ .

1~746~S
L

operations which failed when conventional two-phase lixiviants were employed because of the formation of de-bilitating gas pockets, were rendered successful by using the modified process of the instant invention employing the surfactant stabilized lixiviant.
With conventional two-phase lixiviants, the minimum downward velocity of the lixiviant is about one foot per second. At slower velocities, pockets of gas tend to form and the upward rise of the bubbles exceeds the down-ward flow of the liquid. However, the addition of as little as 25 parts per million surfactant in the lixiviant reduces the minimum downward velocity of the liquid phase by a factor between about 3 and 5.
It is preferred that the lixiviant also contain an agent which enhances the stabilization of Ca ions in solution since these are oEten found together with the metal values of interest. A suitable Ca++ ion stabilizer is a sodium polyacrylate sold under the registered trademark "Calnox". In general, other sodium polyacryl-ates can be used as scale inhibitors.
The amount of surfactant added per volume of liquid phase of the lixiviant will vary with the particular surfactant used. Satisfactory lixiviant compositions have been made which include only 25 parts per million D

surfactant and mixtures of 25 parts per million surfactant with 75 parts per million sodium polyacrylate. As an additional bubble coalescent inhibitor, aluminium ions in the form of 1-2 grams per liter A12(SO4)3 has been found to be effective.

1~7~6gX

The advantages and features of the process of the invention will be further understood fro~ the following examples, which in no event should be construed as limiting.

EXA~PLE A
Effect of Liquid Velocity in Porous Tube Experiments were carried out with a constant gas flow rate of 700 standard cubic centimetérs per minute (SCCM) for various liquid flow rates. At high flow-rate-low gas volume fraction, the gas is well dispersed in r solution. As the liquid flow rate decreases, the volume fraction of gas increases and the flow gradually changes t from bubbly flow to slug flow, i.e., large amoeba-like Lbubbles are formed. As the liquid flow rate is decreased, -there is a "transition" where large agglomerated gas bubbles are formed having a non-spherical shape in excess of 5 ml in size.
The estimated bubble size, as a function of linear velocity in the porous tube and as measured by photo- ~
graphic methods using the apparatus of Fig. 4, is given ti in Tables 1 through 3. It is abundantly clear that the greater the liquid velocity, the smaller the gas bubble size range.

.... . ... _ _ ~L074695 Estimated Bubble Size! With Surfactant, With Spiral Liq flow linear gas volume Run # rate velocity fraction _ Bubble Sizes (GPM) (ft/sec)% (mm) L
424-1 4.8 31.4 3.7 0.5-1.5 -2 3.15 20.6 5.5 1.0-1.5 r

-3 2.2 14.4 7.7 2.0-5.0 Transition 427-16 3.75 24.5 4.7 0.5-1.5 -17 3.15 20.6 5.5 1.0-2.5 -18 2.48 16.2 6.9 1.0-3.0 -19 2.2 14.4 7.7 2.0-5.0 Transition , ~ .
Estimated Bubble Size, With Surfactant, No Spiral In Porous Tube . _ Liq flow lineargas vo].ume Run # rate velocitYfraction_ Bubble Sizes (GPM) (ft/sec) ~ (mm) 419-1 3.65 23.9 4.8 001-0.5 -2 4.05 26.5 4.4 0.1-0.5 ~
-3 3.15 20.5 5.5 0.1-0.5 .

-4 2.65 17.3 3.8 0.2-0.6

-5 2.15 14.1 7.9 0.25-1.0

-6 1.8 11.8 9.3 0.2-2.0

-7 1.6 10.5 10.4 . 0.2-3.0

-8 1.5 9.8 11.0 0.2-4.0 . Transition ~, .

~ `\
~7~L6~5 Estimated Bubble S.ize, With Surfactant, With Spiral In Porous Tube Liq flow linear gas volume c Run # _ rate velocity fraction Bubble Sizes (GPM) (ft/sec) % (mm) 425-4 4.8 31.4 3.~7 0.1-0.5 -5 4.1 26.0 4.3 0.1-0.5 -6 3.15 20.6 5.5 0.2-0.6 -7 2.2 13.7 7.7 0.2-0.75 -8 1.25 8.2 12.9 1-2.5 Transition

-9 0.60 3.9 23.5 2-5.0 426-11 3.75 24.5 4.7 0.1-0.5 -13 3.15 20.6 5.5 0.2-0.6 . -14 2.2 14.~ 7.7 0.3-0.75 .' -15 1.25 8.17 12.9 0.5-2.0 Transition EXAMPLE B

The Effect of Surfactant Addition ?~
As can be seen from a comparison of Tables 1 and 2 1~
above, the bubble size range is significantly smaller in L
a lixiviant containing a surfactant versus a lixiviant without a surfactant. The bubble size range in lixiviants containing a surfactant, as exemplified by a comparison between the 3.5 gpm flow rate, is from 0.1 to 0.5 mm, whereas, without a surfactant, the range is between 1.0 and 1.5 ~. It is estimated that the addition of an ~074~
!
effective amount of surfactant reduces bubble size by a r factor of 15.

EXAMPLE C
!
The Effect of Inclusion of a Spiral Strap ~,.
A twisted stainless steel strap having one spiral per inch was inserted into the interior of porous tube - ~ 18' to create an angular velocity component in addition to the longitudinal velocity component. By comparing the results disclosed in Table 2 with those of Table 3, it can be seen that the spiral can reduce the transition flow rate from 1.5 gpm to 1.25 gpm. However, no notice- ' able effect on bubble size was observed.
Accordingly, it will be seen that a relatively simple and inexpensive constructed gas sparging unit is provided which will enable gas bubbles of small diameter to be finely dispersed into a liquid lixiviant used in the ore treatment process. The dispersion of this gas in fine bubbles uniformly throughout the lixiviant sub- I
stantially improves the recovery of metal values from an ~!
ore formation.

. . ~

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for introducing a finely divided gas into a liquid comprising the following steps:
(a) supplying the liquid into a porous tube formed of sintered powdered metal and having micropores with a diameter between the range of 2-1000 microns;
(b) supplying the gas to the exterior of the tube under sufficient pressure to cause the gas to penetrate to the interior of the tube; and, (c) passing the liquid through the tube to cause the liquid to shear the gas bubbles from the interior surface of the tube to produce a liquid containing the finely divided gas bubbles.

2. The process as defined in claim 1, wherein the liquid is supplied to the tube from a first chamber of a gas sparging unit, which chamber is in fluid communication with the tube, and wherein the gas is supplied to the exterior of the tube from a second chamber in the sparging unit, which chamber surrounds the tube and is isolated from the first chamber.

3. The process as defined in claim 1, wherein the gas is an oxidizing gas.

4. The process as defined in claim 1, claim 2 or claim 3, in which said process is employed in the in-situ mining of minerals and comprises the steps of supplying a lixiviant as said liquid to said tube and impregnating said lixi-viant liquid with said gas, and supplying the thus formed lixiviant liquid to mineral ore to leach out the metal values therein and produce a pregnant liquor containing the metal values.

5. A gas sparging unit for use in introducing finely divided gas bubbles into a lixiviant used for in-situ mining of minerals, said device comprising a hollow casing having a first chamber formed therein into which liquid lixiviant is supplied and a second chamber isolated from said first chamber; a plurality of porous tubes formed of sintered powdered metal and having micropores with a diameter between the range of 2-1000 microns, said tubes extending into said second chamber with said tubes having one end in fluid communication with said first chamber;
and, means for introducing a pressurized gas about the portion of said tubes in said second chamber to enable the gas to penetrate into said tubes so that the gas can be wiped from the interior of the tubes by the lixiviant flowing through the tubes to form a lixiviant containing finely divided bubbles.

6. A gas sparging unit according to claim 5, wherein said casing has an outlet end that is isolated from said first chamber, and wherein the down stream end of said tube is positioned so that lixiviant containing gas bubbles can pass through said outlet end of the casing.

7. A gas sparging unit according to claim 5, wherein said means for introducing pressurized gas to said second chamber comprises an inlet opening formed through said casing.

8. A gas sparging unit according to claim 7, wherein said inlet opening is formed in a partition between said first and second chambers and wherein a gas supply tube posi-tioned within said first chamber delivers pressurized gas through said inlet opening into said second chamber.

9. A gas sparging unit according to claim 8, wherein said gas supply tube, said first chamber and said second chamber are located in axial alignment to enable the unit to be inserted down a well bore.

10. A gas sparging unit according to claim 6, including means for removing gas bubbles trapped upstream of the outlet end of the casing, said means comprising a conduit providing communication between the interior of the casing adjacent the outlet end and the exterior of the casing.

11. A gas sparging unit according to claim 10, wherein said casing comprises a generally vertically extending cylindrical sleeve, said first and second chambers being located in axial alignment in said sleeve and said conduit extending axially within the sleeve through the first and second chambers.

12. A gas sparging unit according to claim 10, including a generally conically shaped guide surface down stream of the second chamber for guiding bubbles into the conduit.

13. A gas sparging unit according to claim 5, wherein the porous tube contains a twisted spiral strap which provides an angular velocity component of the lixiviant in addition to the longitudinal velocity component.

14. The gas sparging unit as set forth in claim 5 wherein said micropores have a diameter within the range of 10-100 microns.

15. The gas sparging unit as set forth in claim 11 wherein said micropores have a diameter within the range of 10-100 microns.

16. The gas sparaging unit as set forth in claim 12 wherein said micropores have a diameter within the range of 10-100 microns.