Wellbore oxidation of lixiviants
United States Patent 3894770

Recovery of metal values such as copper values from a wellbore is combined with the pregnant liquor treatment to achieve as part of the recovery operation the pregnant liquor processing; during a gas-lift operation of the pregnant liquor ferrous ions in the pregnant liquor are being oxidized and a suitable lixiviant is obtained for reintroduction in the wellbore; as an embodiment of the invention sulfur dioxide augmented oxygen containing gas is used for treating the pregnant liquor to aid the oxidation and generate sulfuric acid being used up during the copper value recovery.

Huff, Ray Vincent (Acton, MA)
Huska, Paul Anthony (Acton, MA)
Application Number:
Publication Date:
Filing Date:
Primary Class:
Other Classes:
International Classes:
E21B43/28; E21C41/22; (IPC1-7): E21C41/06
Field of Search:
299/4,5 75
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US Patent References:
3728430METHOD FOR PROCESSING COPPER VALUES1973-04-17Clitheroe et al.
3278233In situ leaching of subterranean deposits1966-10-11Hurd et al.

Primary Examiner:
Purser, Ernest R.
What is claimed is

1. In a process for in-situ mining of minerals containing metal values such as copper values whereby a lixiviant is introduced into at least one wellbore and a pregnant liquor removed therefrom, the combination comprising: introducing in a wellbore a gas-lift comprising from 4 to 11 scf of air per gallon of pregnant liquor lifted per 1000 ft.; maintaining in said gas-lift during said lift oxygen; lifting the pregnant liquor at a rate such that the residence time in the wellbore is about 5 minutes to 2 hours whereby the rate of oxidation of ferrous ions in said pregnant liquor is such that nearly 100% conversion can be assured in a wellbore of at least 500 ft.; separating the pregnant liquor and oxygen for oxidizing the pregnant liquor; recovering said metal values in said pregnant liquor; and reintroducing a barren liquor in said wellbore.

2. The process as defined in claim 1 and wherein oxygen is maintained in said gas-lift from air as a source thereof.

3. The process as defined in claim 1, wherein the pregnant liquor is being lifted with an oxygen containing gas including sulfur dioxide.

4. The process as defined in claim 1 wherein the pregnant liquor is being lifted with an oxygen containing gas including sulfur dioxide, said sulfur dioxide being added in an amount sufficient to also generate acid in such pregnant liquor during said gas-lift.

5. The process as defined in claim 4 and wherein sulfur dioxide is introduced as a liquid.

6. The process as defined in claim 1, wherein the pregnant liquor is being lifted by introducing air as gas of a bubble diameter from 100 mm to 1 mm with 0.5% SO2 by volume to 12% SO2 by volume respectively.

7. The process as defined in claim 5 wherein the pregnant liquor is being lifted from a depth of at least 500 ft. from 5 minutes to 2 hours.

8. The process as defined in claim 5, wherein the amount of sulfur dioxide is 0.5-12% based on the total amount of oxygen introduced, and after ferrous ions have been oxidized to ferric ions, the amount, by volume of introduced sulfur dioxide is doubled.

9. The process as defined in claim 1 wherein during the gas-lift ferrous sulfate is oxidized by air augmented with 0.5 to 12% by sulfur dioxide in which ferrous sulfate is in a solution having a pH of 0 to 4, the total iron concentration of which is 10 g/l, by weight, and the temperature of said solution ranges from 30° to 130°C.

10. The process as defined in claim 8 and wherein during the gas-lift ferrous sulfate is oxidized.

This invention relates to the mining of metals, such as copper; more particularly, this invention pertains to the in-situ recovery of metal values associated with sulfidic deposits, such as sulfidic deposits bearing copper values in ore formations lying at great depths. Still further, this method pertains to in-situ wellbore oxidation of lixiviants in a production well in which a metal recovery such as copper recovery is sought to be joined with lixiviant or leaching liquor processing during the transportation of metal values, such as copper values from a wellbore by a gas-lift.


It has become increasingly more difficult to recover copper by underground mining techniques. Although the most economic way of mining copper is still an open pit mining technique, greater demands on copper have forced a search for alternative sources for the primary metal and new methods for recovery. As shaft sinking and underground tunneling and other underground methods of mining have become more costly and time consuming, other methods have been sought to replace the conventional underground mining techniques. With increasingly more costly recovery of copper, other methods have become attractive for recovery of copper lying at great depths. For example, in copper ore bodies where the overburden is more than 1000 feet, it has become feasible to mine copper such as by various in-situ mining techniques which provide for recovery of copper. Although in-situ mining has been tried, numerous economical, developmental, and technical problems have prevented large scale use of this technique in the recovery of copper. Although oil field techniques give a general background for placing the present invention in a related framework, the specific chemical reactions, which relate to the present invention are not encountered (because oil is generally lifted with natural gas). A general oil field discussion is found in Brown et al., "Field Operations Handbook for Gas Lift," Revised Edition, Otis Engineering Corporation, Dallas, Texas.

Consequently, it has become necessary to painstakingly develop an optimum recovery system, which would have each element in the recovery process perform a number of functions coactively in the sequence in which the element is employed in order to make the system economically viable notwithstanding the fluctuations of the market price for copper.


In the recovery of copper values from sulfidic ores, the oxidation of ferrous ions to ferric ions must be carried out before a suitable leach solution for reuse can be obtained.

There are several methods of carrying out the ferrous to ferric oxidation ranging from the simple direct air oxidation to elegant, but commercially unattractive processes such as photochemical or ultrasonic oxidation methods.

The following are the most frequently described processes: direct oxidation with air or oxygen; bacterial assisted oxidation; direct oxidation using sulfur dioxide and oxygen; radiation assisted oxidation; electrolytic oxidation; photochemically assisted oxidation; and ultrasonically assisted oxidation.

Direct oxidation of ferrous sulfate solutions with air has been repeatedly attempted in both laboratory and plant. The reaction is exceedingly slow and has never proven to be of practical value. Use of oxygen will accelerate the reaction but there is no major improvement over air. Reportedly, the reaction can also be accelerated by heat, pressure or by pH control.

Certain types of bacteria (Thiobacillus Ferrooxidans) use ferrous ions as an energy source. In the process such bacteria convert the ferrous ions to the ferric state. The bacterial activity is dependent on pH, temperature, degree of exposure to sunlight and oxygen availability. Solid surfaces are required to permit the growth of colonies. Certain operating conditions will be very injurious to bacterial growth, thus the process requires careful control. Bacterial oxidation has been carried out in both the laboratory and in waste copper leaching dumps. Although possible, it is not thought to be a likely candidate in a gas-lift operation in in-situ mining processes.

The concept of bacterial oxidation has been largely derived from dump leach experience. It is believed that bacteria play an important role in the dump leaching process. Detailed investigations have demonstrated that two strains of bacteria, namely the thiobacillus ferrooxidans and the thiobacillius thiooxidans, are present in mine water. The thiobaccilus ferrooxidans are capable of oxidizing dissolved ferrous iron to the ferric state, while the thiobaccillus thiooxidans are capable of oxidizing elemental sulfur to sulfuric acid. Both strains will oxidize thiosulfate to sulfuric acid.

These bacteria (mostly motile rod type, approximately 0.5-1.0 μ wide, and 1.0-2.0 μ long) are autotropic, deriving their energy from the iron and sulfuric oxidizing reactions. In addition, for as yet unknown reasons, some organic compounds help increase their activity; one such compound is Tween-20. The principal constraints on the system include: first, in a system using bacterial oxidation the maximum ferric iron concentration should not exceed 15 g/l optimum. A higher concentration of iron is toxic to bacteria. Secondly, bacteria require solid surfaces for their growth and regeneration. In an open pond which would be the most logical choice for the storage of large volumes of ferric sulfate solutions, there may have to be a provision for rock pile colonization areas. A given surface area would be required, the depth below pond surface and compatibility of such colonization areas with the need for pond dredging is a problem. It may not be feasible to provide colonies in production tubing.

With respect to further investigation for ferrous to ferric oxidation, after concluding that oxidation with air or oxygen was too slow to be of any commercial value, U.S. Bureau of Mines in 1912 undertook work on the oxidation of ferrous iron to the ferric state using SO2 and air. It was observed that oxidation of ferrous to ferric by exposure to SO2 and air was very rapid. Unfortunately, not much work has been done with this process in recent years.

In U.S. Pat. No. 2,563,623, underground in-situ leaching in a mine having a tunnel system has been disclosed. However, the processing of the pregnant liquor by oxidizing ferrous ions to ferric ions is fairly uneconomical and is associated with large above-ground systems rendering the disclosed combination unattractive for a number of reasons, such as waste disposal, leakage, evaporation, slow reaction rates, and unfeasible use of catalysts, etc. Another in-situ system is disclosed in U.S. Pat. No. 3,574,599 with above ground lixiviant treatment. In general, ferrous to ferric oxidation is also described in U.S. Pat. Nos. 3,330,650 and 3,436,177.


When ferric sulfate oxidizes copper sulfide minerals, it reduces to ferrous sulfate. In an in-situ mining operation where ferric sulfate is used as the lixiviant, the pregnant solution coming out of the ground is expected to contain substantial quantities of ferrous sulfate.

In any practical surface plant this ferrous sulfate must be reoxidized to the ferric sulfate state prior to being recycled as copper leaching solvent to the underground deposit. For a production of 150 TPD (tons per day) of copper (nominal 50,000 TPY - tons per year) at a 1 gram per liter (gpl) copper concentration in the solution, the volume of recirculating solution or liquor will amount to approximately 36 million gallons per day and even at 5 gpl copper, the volumes of liquor will be about 7.2 million gallons per day. The regeneration of large amounts of solution is very important and the impact of this step on the project economics can be very critical.

It has not been found that in an in-situ mining process an improvement has been discovered which further enhances the in-situ mining technique by having an element in the combination pertaining to the removing of liquids from wellbores act on the liquid lixiviants or pregnant leach liquor whereby a change in the lixiviant's chemistry is brought about as part of the processing sequence while removing the liquor from the well.

Thus, according to the invention, it has been discovered that the treatment of pregnant liquor from a production well can be carried out at the same time while the liquor is being removed from a production well with a simultaneous generation of acid in the wellbore.

Accordingly, it has been discovered that in a gas-lift process for removing liquid lixiviants from wellbores, the elements of time, temperature, pressure and a chemical reaction(s) can be utilized as a combination for treating the pregnant liquor from the production well. Thus, a chemical processing as part of the overall in-situ mining scheme is carried out in the wellbore. It has been discovered that the chemical reactions undergoing by a pregnant liquor in a wellbore, as the pregnant liquor is being carried up and out of the production well, can be effected in such a manner that considerable reduction of surface located processing and facilities thereof, such as surface ponds, vats and tanks, can be eliminated or dispensed with including problems associated therewith.

Accordingly to the present invention, the process is practiced by removal of a column of frothed liquid lixiviant (which is frothed by a suitable gas such as air or enriched air or sulfur dioxide supplemented air). The lixiviant is expelled from the well by a continuous injection of the gas. An oxidizing gas such as oxygen or air enriched with oxygen and/or supplemented with sulfur dioxide is employed either with or without a suitable catalyst to work on the pregnant liquor. The liquor is thus substantially fully treated by the time it arrives at the surface and the ferrous to ferric conversion is achieved.


In accordance with the outlined process above, the gas-lift for removing liquids from wellbores conventionally practiced has further been enhanced by a processing combination relying on time, temperature, pressure and additionally a catalyst such as the metal ion in solution or SO2 which allows a more facile practicing of the in-situ technique of mining deep lying ore formations. The utilization of time, the increased temperature in the wellbore, and the pressure (for the oxidation of the pregnant liquor whereby ferrous ions of the recovery liquor are being converted to ferric ions), makes the in-situ process thus more attractive as a viable process for the mining of metal values.

With respect to the recovery process whereby chalcopyrite (CuFeS2); chalcocite (Cu2 S); covellite (CuS); bornite (Cu5 FeS4) including pyrites (FeS2) are treated and in which the pregnant liquor plays the following role, a schematic representation of the chemical reactions follow.

FeS2 + 3 1/2 O2 + H2 O ➝ FeSo4 + H2 SO4

2FeSO4 + H2 SO4 + 1/202 ➝ Fe2 (SO4)3 + H2 O

CuFeS2 + 2Fe2 (SO4)3 ➝ CuSO4 + 5FeSO4 + 2S

2FeSO4 + SO2 + O2 ➝ Fe2 (SO4)3

FeS2 + 7Fe2 (SO4)3 + 8H2 O ➝ 15 FeSO4 + 8H2 SO4

In considering the above reactions for application to the in-situ mining, the solution, flow rate, ferrous iron content, reaction kinetics and the physical properties of the solutions are considered as basic parameters.

The following table illustrates the operation that must be considered based on prior art considerations: Copper production = 150 TPD Copper loading = 1.0 g/liter Pregnant solution = 36 million gallon/day Ferrous content = 10 g/l (assumed) Fe++ to be oxidized = 1,500 TPD Assumed residence = 2 weeks time (for oxidation) Reactor size (or = 500 million gallons pond) Approximate cost = $12 million

Thus, it is seen, quantitatively, in the event of lean copper loading, the kinetics of the oxidation reaction is extremely important, and elimination of surface facilities quite lucrative. Moreover, with respect to obtaining copper from relatively poor ores, the economic penalties cannot be tolerated, especially if relative resource independence is sought for this country.

In the oxidation of ferrous sulfate to ferric sulfate, the following simplified reactions can be assumed to take place:

4FeSO4 + 2H2 SO4 + O2 ➝ 2Fe2 (SO4)3 + 2H2 O

12FeSO4 + 6H2 O + 3O2 ➝ 4 Fe(OH)3 + 4Fe2 (SO4)3

The critical parameters which control the rate of the reactions are:

Total amount of iron in solution

pH of the solution

Initial ferrous iron concentration & ferric/ferrous ratio


Oxygen partial pressure

The interrelationship of these parameters and their resultant effect on ferrous to ferric iron oxidation are described in the three references mentioned above, U.S. Pat. Nos. 2,563,623, 3,330,650 and 3,436,177. However, the penalties, which are incurred when practicing these processes result from the large ponds, slow reaction rates, and other conditions mentioned above which have been overcome herein.

According to these patents, simple air oxidation or oxidation by oxygen is extremely slow. It takes a week or more to effect the oxidation.

The most desirable conditions for air oxidation at normal pressure and temperature are such that the total iron content be less than 25 g/l (preferably less than 15 g/l) and the solution pH be lower than 2. If the total iron content (all of which can be as ferrous ion) is more than 25 g/l, practically no oxidation takes place within a period of two weeks. If the pH is higher than 2, iron begins to precipitate as a basic ferric sulfate, thus reducing the potential of the solution as a lixiviant for copper containing minerals. In U.S. Pat. No. 2,563,623 mentioned above, it is urged that if there is more than 25 g/l of total iron in the solution, not only will it take longer to oxidize from ferrous to ferric, but precipitation of basic ferric salts will cause plugging problems in the leaching operation. To prevent such precipitation, the pH (according to this patent) must be kept below 2 all of the time the solution is in contact with the ore. Maintaining these pH conditions is difficult and costly because of the acid-gangue reactions. It is urged in this patent that there is no advantage in having more than 10 g/l ferric sulfate in the system.

Air oxidation can be greatly accelerated by raising the temperature and the pressure. As above, pH adjustment is important to prevent precipitation of iron. The reaction rate can be accelerated to the point that the required oxidation can be carried out in a few minutes.

Unlike the oxidation of ferrous sulfate by air (oxygen) or bacteria, the oxidation of ferrous sulfate by SO2 and air (or oxygen) is said to be very rapid - in the order of 1/2 hour for 90% conversion under controlled conditions.

Previous study has indicated that the actual chemical reactions are quite complex but the following equations approximate the reactions:

2FeSO4 + SO2 + O2 ➝ Fe2 (SO4)3 (1) H2 O + SO2 + 1/202 ➝ H2 SO4 (2)

the kinetics of these two reactions are not well understood. Experimentally SO2 and air were bubbled through a solution containing ferrous sulfate with the iron being oxidized first. The sulfur was converted to sulfate in the ferric sulfate and this was demonstrated since the acidity of the solution begins to rise only after most of the iron was oxidized. Solutions containing ferrous sulfate up to saturated levels can be oxidized in this way and the acidity can be raised to 10% or higher.

In the oxidation process a third reaction also takes place, particularly in the absence of air (or oxygen):

Fe2 (SO4)3 + SO2 + 2H2 O ➝ 2FeSO4 + 2H2 SO4 (3)

this reaction is not of great importance, under conditions that favor reactions (1) and (2); but ferric sulfate is likely to be in equilibrium with iron in the reduced (ferrous) state.

The oxidation kinetics, which have been investigated, are dependent upon the following factors:

Acidity of the solution (pH)

Total iron concentration (ferrous and ferric)

Ratio of SO2 /oxygen (or air)

Mass transfer of gases into solution (SO2 and oxygen bubble size, etc.)




Ferrous sulphate solution containing appreciable amounts of free acid causes the rate of oxidation to be substantially slower while the formation of acid is also hindered. If the starting solution is lightly acidic, a "period of induction" was observed during which it was hard to get the oxidation reaction started. After the period of induction was over, more acid could be formed and more ferrous sulfate oxidized in spite of the accumulation of acid.

With the lower concentration of ferrous iron, the presence of some ferric iron seems to assist the oxidation, while at higher concentrations of ferrous iron, the effect of ferric iron on the rate of oxidation is practically negligible.

The concentration of ferrous iron is practically immaterial as long as the solution contains enough available ferrous iron to react with oxygen and sulfur dioxide as fast as they are effectively supplied. This means that mass transfer of SO2 and oxygen to the solution is the rate limiting parameter. It is with respect to the mass transfer considerations in the gas-lift in in-situ mining that the discovery has been made on how to utilize these complex interactions to achieve the processing of the pregnant liquor for reuse in a functionally "double duty" approach.

For the stochiometric equation (1) and (2), it is apparent that the ratio of sulfur dioxide to oxygen required by equation (1), iron oxidation, is only half that required by equation (2), sulfuric acid oxidation. The stochiometric relationship also indicates that the absolute amounts of SO2 required by either reactions (equation 1 or 2) are dependent on the amount of ferrous iron in the solution.

Thus, it has been discovered and it has been observed that the effect of the ratio of SO2 /O2 is dependent upon: 1) the type of liquid-gas contact units used, 2) the size of the gas bubbles introduced, and 3) the particular depth of solution in the unit. This indicates the dependency of the reaction rate on mass transfer limitations. For a given type of unit (which made bubbless 1 cm in diameter) the maximum desirable ratio of sulfur dioxide to oxygen by volume is about 1:40 (i.e., 0.5% SO2 if air is used). This high air/SO2 ratio is desirable for two reasons:

i. Rapid oxidation times in the order of 1/2 hr.

ii. The sulfur dioxide efficiency will be almost 100% which means that the tail gases would be essentially free of SO2 thereby requiring no further treatment.

The rate of formation of sulfuric acid is not as sensitive as the oxidation reaction, but it is also impaired by too high concentration of sulfur dioxide.

If the design of equipment is changed so that the bubble diameter is reduced from 1 cm to 1 mm, the maximum percentage of sulfur dioxide permissible in a mixture is capable of being increased to 5-7% by volume (SO2 :O2 = 1:2.75). On the other hand, if SO2 can be introduced as solution (i.e. liquid SO2) and air is introduced as 1 mm bubbles, over 10% of SO2 based on volume of air (SO2 :O2 = 1:1.86) can be used, e.g., up to 12%.

When the required iron oxidation is complete, and assuming that it is desirable to make additional acid, the ratio of SO2 to O2 (i.e., air) can be doubled in a second step as part of the process in order to favor the formation of acid.

From the observed data, it appears that heating the solution above 40°C has no advantage and may even be deleterious. At higher temperatures the solubility of oxygen and sulfur dioxide in the solution decreases and this may be directly causing reduced reaction rates.

The addition of air/SO2 will raise the temperature somewhat since the reaction is exothermic.

There is same effect of increased pressure on reaction rates. Experimental results indicated that the greatest increase in rate of oxidation produced by an increase in pressure to 5 to 6 atmospheres is only 1.5 times that at one atmosphere. The effectiveness of pressure changes with different concentrations of SO2 with the maximum being a 6-7% SO2 by volume. A detailed analysis of the data shows that the effect of gas pressure varies approximately as the cube root of the pressure as a maximum.

In in-situ mining, considerable amount of impurities such as aluminum, magnesium, sodium, calcium, etc., are expected in the pregnant solution. The effects of many of these impurities have been investigated by O. C. Rolston, Bureau of Mines, Bulletin 260 (1927). Copper sulfate is the most deleterious to the iron oxidation and copper concentrations above 0.1 g/l will significantly retard the oxidation and demand a lower sulfur dioxide to air ratio.

Alminum and zinc salts, even at high concentrations (2 to 25 g/l), do not significantly affect the rates of reaction.

Sodium and calcium sulfates in the solution also seem to be harmless. Chlorides of sodium and calcium in small amounts (2-3 g/l) also do not seem to affect oxidation, but in large quantities, these may become injurious.

Theoretically, 0.5 gm mole of SO2 is required to produce 1 gm-mole of iron as ferric sulfate, or 0.57 gm of SO2 (or 0.28 gm of sulfur) is needed to get 1 gm/liter iron as ferric sulfate. In addition, when generating acid in the liquor by means of injected SO2, the amount of SO2 needed is that which will adjust the pH to O:2.

Assuming that 10 gm/liter iron as ferric sulfate produces a pregnant solution containing 2 gm/liter coper (theoretically, 10 g/l Fe+3 will produce about 5.64 g/l copper), i.e., stochiometric efficiency of 35% for the production of 150 TPD copper; then a need to convert 750 TPD of ferrous iron to ferric iron is evident. Hence, approximately 428 TPD of SO2 is required.

In addition, as an example, 500 TPD of acid is needed for reactions with gangue - the additional amount of sulfur required is then 170 TPD.

Accordingly, in an operating wellbore in an in-situ mining operation in which the well is at least 1,000 ft. deep, the leaching liquor after appropriate residence time is being removed from the wellbore as a pregnant liquor. In the production well, the residence time in the wellbore is generally from 5 to 120 minutes while the temperature is in the range from 30° to 130°C. The pressure may be from 300 to 2200 psi depending on the hydrostatic or overpressure being exerted on the well. Hydrostatic pressure is about 435 psi for each 1,000 ft of water below the water table. In a gas-lift, whereby the pregnant liquor is being removed from the wellbore by appropriate distribution and frothing, an illustrative operation is as follows.

For illustrative purposes it is assumed that pregnant liquor is a dilute sulphuric acid solution having a specific gravity of 1.1 and containing dissolved salts including iron, copper, and other metals. The pH of the liquor ranges from 0 to 3. Each installation because of differences in ore compositions will require an individual design but the following general criteria cover the encountered ranges.

A. Pressure Needed:

100 to 130 psi per 1,000 ft. of depth up to 1050 psi maximum and 300 psi minimum.


1. Pressure needed for 4,000 foot well having liquor of 1.1 specific gravity. ##EQU1## 2. Pressure needed for 500 foot well having liquor of 1.1 specific gravity. Use minimum of 300 psi.

B. Gas Volumes Needed

1. Continuous gas-lift: About 4.0 to 6.5 Stand. cubic foot of gas per gallon of liquor per 1000 ft. of lift.


A well lifting from 4000 ft. requires ##EQU2## 2. Intermittent gas-lift: About 6.5 to 10.5 scf of gas per gallon of liquor per 1000 ft. of lift are needed.

The pressure at the bottom of a production well will range between the hydrostatic pressure created by the static column of fluid in the well to a flowing bottom hole pressure. The flowing bottom hole pressure will depend on the desired rate of production and the productivity index, i.e. the rate of fluid production per unit pressure drop.

The temperature is proportional to depth and in igneous rock invironments, a general approximation is a temperature gradient of 2°F per 100 ft. of depth beneath the surface. Therefore, the temperature will be some average ambient temperature plus the gradient times depth divided by 100. In addition, the temperature may be increased by the oxidation process in the production tubing.

The rate of oxidation of ferrous ion to ferric ion will depend on the temperature, pressure, catalytic effects of either dissolved ions; or any other catalytic agents that are added. At moderate contact time (between oxygen in the lifting gas and the solution), depending on the rate of production and height of lift, the conversion will be expected to approach 100%. Contact times may range from 5 minutes to 2 hours.

Inasmuch as the present method calls for using of an oxidizing gas such as air, oxygen, or air enriched with oxygen, it should be supplemented with a catalyst such as sulfur dioxide. The normal residence time of a unit of pregnant liquor in a 2000 ft. wellbore is 4.4 min. if the production rate is 100 gal/min. or 44 minutes if the production rate is 10 gal/min). In a 5,000 ft. wellbore, residence time at 100 and 10 gal/min productions rates is 11 min. to 110 min. respectively. The residence time of the liquor is generally sufficient to convert up to 100% of the ferrous ion to ferric ion, in the indicated range of bubble sizes i.e. 100 mm and larger up to slug flow, but generally 100 mm to 1 mm. It is also noted that at a pressure of 950 psi, corresponding to a column of water at 2,000 ft. depth, the reaction rate of ferrous ion to ferric ion is a function of pressure and temperature. The relatively higher pressure and temperature encountered in wells in in-situ mining of mineral deposits is thereby utilized as a reaction accelerating means during the actual gas-lift.

It is estimated that a potential savings may range from 3 to 10 cents per pound of produced copper in an in-situ mining and thus considerable savings in an economic sense is evident in the use of the presently disclosed invention.

Still further, inasmuch as surface facilities for oxidizing the ferrous sulfate to ferric sulfate are considerably reduced and/or eliminated, the incorporation of the oxidation step (as well as an acid generating step) in the gas lift of pregnant liquor from the bottom of the well is an operating benefit which contributes to the optimization of the in-situ mining of mineral deposits.

In accordance with the invention, the above embodiment has been illustrated to show the various aspects of the invention without the embodiment being a limiting condition on the broader scope of the generic invention disclosed herein.

In accordance with the invention, a gas-lift utilizing a conventional gas-life valve is installed with tubing in a well. Gas is injected into the annulus between the wellbore and tubing at pressures of 100 to 130 psi per 1000 ft. of depth but not less than 300 psi. The rate of gas injection is about 5 standard dubic ft. per gallon of liquid per 1000 ft. of lift.

As longer residence time can be tolerated and readily contributes to the conversion, it is evident that the rate at which the pregnant liquor can be recovered can be further enhanced or slowed down by the capacity of the gas lift, the oxygen, the sulfur dioxide content in the gas lift, and the function of the gas lift on the basis of the original lixiviant solution, temperature and pressure.

As illustrated above, the process of utilizing the gas lift to perform a multiple function contributes to the economic application of the in-situ process. In this aspect, the processing of the pregnant liquid in which coacting chemical reactions takes place during the removal of the pregnant liquid from a wellbore provides a desired aspect of the present combination of elements.

The recovery of the copper values from the pregnant liquor is well known in the art and need not be recited herein.

As it is evident from the above, the novel combination, i.e. process may be practiced in a multiple well field, such as a five-spot pattern field, i.e. a central injection well and production wells in each corner of a generally rectangular pattern.

From this description, an optimized sequencing of the recovery processes in which oxygen-water lixiviant system is used or ferric sulfate lixiviant is used, pregnant liquor recovery is based on the process sequence in a total in-situ scheme of wellbore recovery of copper. Thus, the amount of lixiviant being introduced and the pregnant liquor being produced may likewise be scheduled in an appropriate sequence such that the total combination of wellbores are optimized in a predetermined pattern such as the mentioned five-spot hole array for the maximum utilization and processing of the recovery components and the end product, i.e., metal values. Similarly, a "huff-puff" single well recovery method may be used and other techniques employed in combination with the process herein.

A deposit, such as found near Safford, Arizona may be worked by the above described methods.

As it is evident, the exact reaction mechanisms of the lixiviant or pregnant liquor at the bottom of the wellbore or during gas-lift are not subject to direct observation and, therefore, the above discussion must be understood as being based on experimental evidence, deductions and observed behavior of the reactants. It is likely that the theory or explanation may be different, but the invention sought herein is defined by the claims and equivalents of the elements recited in the claims.