OSMOTIC SOLUTION MINING

Abstract

A process for solution mining of minerals is disclosed. The process comprises injecting an unsaturated stream (150) at an injection pressure into a mineral formation (130) to dissolve the mineral and extracting a high concentration stream (110) containing said dissolved mineral. The process comprising converting latent osmotic energy present in said high concentration stream into an increase in the total pressure of said stream by passage through an osmotic power unit (200) and generating electricity and reducing to the injection pressure the total pressure of a reduced concentration output stream (150) by passage through a power generating device (250) and using the reduced concentration output stream (150) at the injection pressure as the unsaturated stream (150). A process for storing a fuel in an underground formation is also disclosed.

Claims

1-17. (canceled.)

18. A process for solution mining of minerals, the process comprising: injecting an unsaturated stream at an injection pressure into a mineral formation to dissolve a mineral contained therein, and then extracting a high concentration stream containing said dissolved mineral from the mineral formation; converting latent osmotic energy present in said high concentration stream into an increase in a total pressure of said high concentration stream by passage through an osmotic power unit comprising a semi-permeable membrane which permits a passage of solvent but not a passage of the mineral, and in which the high concentration stream is passed over a first side of the semi-permeable membrane, a low concentration stream being passed over a second side of said semi-permeable membrane; generating electricity and reducing a total pressure of a reduced concentration output stream to the injection pressure by passing the reduced concentration output stream through a power generating device, the reduced concentration output stream being derived from the high concentration stream after passage over the semi-permeable membrane; and using the reduced concentration output stream at the injection pressure as the unsaturated stream injected into the mineral formation.

19. A process according to claim 18, wherein a first fraction of said high concentration stream is passed to the osmotic power unit and a second fraction of said high concentration stream is output from the process.

20. A process according to claim 18, wherein the entire high concentration stream extracted from the mineral formation is passed to the osmotic power unit.

21. A process according to claim 18, wherein a first fraction of the reduced concentration output stream is passed to the power generation device and a second fraction of the reduced concentration output stream is passed to a pressure exchanger in which pressure from the second fraction is transferred to the high concentration stream prior to passage of the high concentration stream over the semi-permeable membrane.

22. A process according to claim 21, wherein after passage through the pressure exchanger the second fraction of the reduced concentration output stream is output from the process.

23. A process according to claim 21, wherein after passage through the pressure exchanger the pressure of the second fraction of the reduced concentration output stream is increased to the injection pressure using a pump, before being combined with the first fraction of the reduced concentration output stream after passage through the power generation device to produce the reduced concentration output stream at the injection pressure.

24. A process according to claim 18, wherein the high concentration stream is pressurised in the osmotic power unit using a pump before passage over the semi-permeable membrane.

25. A process according to claim 18, in which the high concentration stream is passed to a second osmotic power unit, a reduced concentration output stream of the second osmotic power unit being passed to the osmotic power unit for use as the high concentration stream, and wherein a total pressure of the reduced concentration output stream of the second osmotic power unit is equal to the total pressure of the high concentration input stream on entry to the semi-permeable membrane.

26. A process according to claim 18, wherein the power generation device comprises a turbine.

27. A process according to claim 18, wherein the mineral formation is a salt formation.

28. A process for storing a fuel in an underground formation, the process comprising creating and/or maintaining a void in a mineral formation using the process of claim 18, and injecting the fuel therein for storage.

29. A process according to claim 28, wherein the fuel comprises hydrogen, biogas, natural gas, methanol and/or ammonia.

30. A process according to claim 28, wherein the fuel is in liquid or gaseous form.

31. A solution mining system comprising a hydraulic system suitable for connection to a mineral formation, said hydraulic system being arranged to inject an unsaturated stream into the mineral formation at an injection pressure and extract a high concentration stream from the mineral formation; an osmotic power unit arranged to generate electricity through Pressure Retarded Osmosis (PRO), using a difference in concentration between the high concentration stream and a low concentration stream, the osmotic power unit being configured to reduce a total pressure of a reduced concentration output stream derived from the high concentration stream after passage through the osmotic power unit to the injection pressure; and wherein the system is arranged such that the reduced concentration output stream is passed to the hydraulic system for use as the unsaturated stream.

32. A solution mining system according to claim 31, wherein the osmotic power unit and/or the hydraulic system are mounted on a mobile platform.

33. A solution mining system according to claim 31, the system being configured such that a first fraction of said high concentration stream is passed to the osmotic power unit and a second fraction of said high concentration stream is output from the system as a high concentration output stream.

34. A process for solution mining of minerals, the process comprising: injecting an unsaturated stream into a mineral formation to dissolve the mineral contained therein, and then extracting a high concentration stream containing said dissolved mineral from the mineral formation; converting latent osmotic energy present in said high concentration stream into an increase in a total pressure of said high concentration stream by passage through an osmosis unit comprising a semi-permeable membrane which permits a passage of solvent but not a passage of the mineral, and in which the high concentration stream is passed over one side of the semi-permeable membrane, a low concentration stream being passed over another side of said semi-permeable membrane; generating electricity and reducing the total pressure of a reduced concentration output stream derived from the high concentration stream after passage over the semi-permeable membrane by passing the reduced concentration output stream through a power generation device; injecting the reduced concentration output stream into the mineral formation for use as the unsaturated stream; and wherein a first fraction of said high concentration stream is passed to the osmosis unit and a second fraction of said high concentration stream is output from the process as a high concentration output stream or other suitable streams.

Description

DESCRIPTION OF THE DRAWINGS

[0077] Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which:

[0078] FIG. 1 shows an example solution mining process (falling outside the scope of the present invention);

[0079] FIG. 2 shows a first example solution mining process in accordance with the present invention;

[0080] FIG. 3 shows a second example solution mining process in accordance with the present invention;

[0081] FIG. 4 shows more detail of an osmotic power unit for use in the processes of FIG. 2 or 3;

[0082] FIG. 5 shows part of a third example solution mining process in accordance with the present invention, being a variation on the processes of FIG. 2 or 3; and

[0083] FIG. 6 shows an example apparatus in accordance with the present invention.

DETAILED DESCRIPTION

[0084] FIG. 1 schematically illustrates a typical solution mining process (one not in accordance with the present invention), where a suitable low concentration solution 100 is injected using a pump 120 to a subsurface mineral formation 130 such as a subsurface mineral ore to create a high concentration solution 110 that is sent to further processing 140. The mineral ore may comprise sugar or salts for example sodium chloride, potassium chloride, calcium chloride or other salts.

[0085] Because the density increases as minerals dissolve in the low concentration solution 100, pumping energy is required to lift solution with a mass equal to the density difference from the formation to the surface. If the density difference is 200 kg/m.sup.3 and the solution is extracted from a depth of 2 km, an injection pressure of about 39 bar is required (not including pressure losses in the system).

[0086] FIG. 2 shows an example solution mining process in accordance with the invention in which a surplus of high concentration solution 110 is extracted from the substance concentration 130 and recirculated between the mineral formation 130 and a suitable PRO system 200, the remaining high concentration solution 110 is sent for further processing 140. A low concentration solution 100 is fed by a feed pump 120 under low pressure to the PRO system 200 where it mixes with high concentration solution 110 to produce a dilute solution 150. The entire dilute solution 150 mixture is injected into the mineral formation 130 to dissolve additional minerals. The extracted volume of high concentration solution 110 and the reinjected dilute solution 150 must be of equal volume to maintain a constant volume in the mineral formation 150. In this way, the mixing of the low concentration solution 100 and the high concentration solution 110, is moved from taking place in the mineral formation 130 to the osmotic power system 200, where the energy can be harvested, allowing the extraction to be driven by the spontaneous mixing of low concentration 100 and high concentration solutions 110.

[0087] FIG. 3 shows a variation of the process shown in FIG. 2. Here the entire volume of high concentration solution 110 extracted from the formation 130 is sent to the PRO system 200, where it mixes with low concentration solution 100. Part of the resulting dilute solution 150 is sent back the formation 130, while the remaining part of the dilute solution 160 is sent for further processing 140. In this setup the dilute stream 160 sent for further processing 140 will be lower in concentration than the extracted high concentration stream 110. This version of the invention is thus useful for scenarios where the further processing does not rely on and prefers lower concentrations. An example could be discharge of dilute formation water as part of excavation of a cavern for gas or other storage, where disposal of water with high concentrations of minerals may be difficult. It is possible to obtain the same end result from the layout given in FIG. 2 but this requires the further processing step to comprise of an additional osmotic power unit. In the layout in FIG. 3, this can be accomplished in one system using fewer components.

[0088] FIG. 4 shows an example PRO type osmotic power unit 200, suitable for use in the systems of FIG. 2 or 3. The high concentration solution 110 is pressurized in a pressure exchanger 210 (e.g. a heat exchanger, a rotary pressure exchanger etc.) at a pressure below the difference in osmotic pressure between the high concentration 110 and the low concentration 100 solutions. The pressurized high concentration solution is then sent to one side of a semi-permeable membrane 220, while the low concentration solution 100 is sent to the other side of the membrane 220. The low concentration solution is pressurized using a feed pump 230 prior to being sent to the membrane 220. Due to the difference in osmotic pressure, solvent will spontaneously move from the low concentration side to the high concentration side to equalize the chemical potential across the membrane 220. This creates a dilute solution 150 the total pressure of which is higher than the total pressure of the high concentration stream 110 on input to the semi-permeable membrane 220. A first fraction of this dilute solution 150 is directed to a power generating device 250 such as a turbine to produce electricity. A second part of the dilute solution 150 is passed to the pressure exchanger 210 where pressure from the dilute solution 150 is transferred to the high concentration solution 110.

[0089] Passage through the power generating device 250 reduces the total pressure of the first fraction of the dilute solution 150 to the injection pressure. The dilute solution 150 can then be passed to the mineral formation 130 without the need for any additional mechanical pumping. This may provide a particularly efficient solution mining process, in particular in comparison to those in which the dilute solution 150 is pressurized using a pump driven using electricity generated in the osmotic power unit 200.

[0090] In some embodiments the second fraction of dilute solution 150 output from the pressure exchanger 210 is not reinjected into the mineral formation 130. In the process of FIG. 2, it may be combined with the high concentration solution 110 sent for further processing 140 or disposed of as appropriate, for example into a nearby watercourse. In the process of FIG. 3, the second fraction of dilute solution 150 may be sent for further processing as stream 160.

[0091] In some embodiments, the entire stream 110 going to the pressure exchanger 210 is reinjected into the formation 130. In this case the second fraction of dilute solution 150 output from the pressure exchanger 210 and first fraction of dilute solution 150 output from the power generation device 250 must be combined and reinjected. A pump (not shown) is used to pressurize the second fraction after passage through the pressure exchanger 210 to the injection pressure before it is recombined with the first fraction (which is at the injection pressure already).

[0092] In some embodiments the pressure exchanger is absent. In the same or yet further embodiments a pump is used to pressurize the high concentration solution 110 prior to passing over the membrane 220. This makes all the pressurized dilute solution 150 available for passing through the power generation device 250 (by which passage the pressure of the dilute solution 150 is reduced to the injection pressure) and thereby allows the entire dilute solution 150 to be sent directly for injection.

[0093] Only the high concentration solution 110 must be pressurized at the high pressure (>30 bar) required for injection, whereas the low concentration solution 100 can be pumped to the membrane using a low pressure (<15 bar). Power is needed to drive the pump or pressure exchanger for the high concentration solution 110 and the low pressure pump 230 for the low concentration solution 100, and by operating the PRO process at a pressure higher than the injection pressure, the power generating device can utilize the pressure gradient for energy generation (to power the high pressure pump and the low pressure pump) while the diluted solution 150 can be sent directly for injection.

[0094] It is also possible to use several osmotic power units 200 in combination to enhance the efficiency of the process. FIG. 5 comprises an example of such a system which comprises two osmotic power units (A, B) (though the system could include any number of stages (A, B) in succession of each other) of the type shown in FIG. 4. The dilute solution 150 coming from the prior stage A is used as the high concentration solution 110 for the subsequent stage B. The dilute solution 150 from the subsequent stage B passes through the power generation system 250 to have a pressure on exit equal to the injection pressure. The dilute solution is then reinjected into the mineral formation 130. The two stages A, B can operate at different pressures, with the pressure in the prior stage A being higher than in the subsequent stage B. To maximize energy generation, it is desirable to operate the PRO process at high pressures, but as the pressure is increased, the degree of dilution of the dilute solution 150 that can be obtained is lowered because the osmotic pressure difference decreases as solvent crosses the membrane 220. Operating with dual stages as illustrated thus allows for a greater energy generation and dilution to lower concentrations. This may mean that less additional brine (high concentration solution 110) needs to be extracted from the substance concentration 130 to run the PRO process.

[0095] In a variation of the process of FIG. 5, the pressure exchanger 210 is omitted from both stages A, B. Instead, the high concentration solution 110 is pressurized using a pump and then passes over the membrane 220 in prior stage A to produce dilute solution 150. After passage through the power generation device 250 of prior stage A the dilute solution 150 is used as the high concentration solution 110 of subsequent stage B. In some embodiments, passage through power generation device 250 of prior stage A reduces the pressure of dilute solution 150 to the operating pressure for the membrane 220 of subsequent stage B. That is to say, the dilute solution from prior stage A can be passed directly to the membrane of subsequent stage B without the need for any pumping and thereby removing the need for any additional pump. In this way, there is no need for an additional pump or pressure exchanger to pressurize the solution before it enters the subsequent stage B membrane 220.

[0096] FIG. 6 shows a schematic diagram of a mobile production unit 350 for use with a salt formation 130. Injection well 310 and extraction well 315 extend from the surface to a salt cavern 330 located within the salt formation 130. An outflow port 340 of production unit 300 is connected to injection well 310 and an inflow port 345 connected to extraction well 315 (these connections being shown with dashed lines in FIG. 6). The mobile unit 350 comprises an osmotic power unit 200, a control system (not shown) and other elements of a solution mining system not shown here for clarity. The mobile unit 350 further comprises an inflow port 360 and output flow port 365, both connected to a water source (not shown). Within mobile unit 350 a hydraulic system connects the osmotic power unit 200 to the various ports as follows (shown by dashed lines in FIG. 6); inflow port 360 is connected to the low-salinity input of the osmotic power unit, outflow port 365 with the waste (low-salinity) output of osmotic power unit 200, outflow port 340 with the osmotic power unit output for the stream derived from the high-salinity input, and inflow port 345 with the high-salinity input of osmotic power unit 200. The total pressure of the osmotic power unit output for the stream derived from the high-salinity input is substantially equal (barring minor pipe flow losses etc.) to the total pressure of the stream at the outflow port 340 and the head of the injection well 310. Accordingly, there is no pump situated between the output from the osmotic power unit 200 and the head of the injection well 310. A portion (not shown) of the high-salinity stream from extraction well 315 is split off upstream of the mobile production unit 350 and sent for further processing, for example use in an industrial process. Once the cavern 330 has been excavated fuel, for example hydrogen, biogas, natural gas, methanol and/or ammonia, may be pumped into the cavern for storage.

[0097] In a variation of the process shown in FIG. 6, the entire high-salinity stream from extraction well 315 is sent to the osmotic power unit 200 and part of the stream derived from the high-salinity input after passage through the osmotic power unit 200 can be discharged through outflow port 365 with the waste stream. In this way the volumetric balance in the cavern 330 can be maintained.

[0098] It will be appreciated that the apparatus of FIG. 6 can be used with other minerals in place of salt.

[0099] The impact of the present invention on the efficiency of the solution mining process can be seen in the consideration of the following systems (all of which produce 100 m3 saturated brine per hour).

[0100] A traditional solution mining process uses an injection pump to pressurize the fluid for injection into the salt formation. As shown below, such a process requires an energy input of 163 kW/hour to operate.

TABLE-US-00001 Process Injection pump Total Flow m.sup.3/h 103.09 Pressure bar 40 Efficiency 0.7 Energy kW −163 −163

[0101] The energy requirements for a solution mining process that uses electricity from an osmotic power unit including a turbine to power an injection pump that pressurizes the fluid for injection into the salt formation is shown below. The feed and draw pumps are used to pressurize the low and high concentration flows respectively prior to passage over the semi-permeable membrane, use of such pumps increasing the efficiency of the osmotic power unit and balancing the flow either side of the membrane. The ERD is an energy recovery device that transfers pressure from the reduced concentration output stream to the high concentration input stream. It is fed by the draw pump. In the example process below the injection pump that returns all diluted saltwater to the salt formation. Such a process requires an energy input of 43 kW/hour to operate.

TABLE-US-00002 Feed Draw Injection Process pump pump ERD Turbine pump Total Flow m.sup.3/h 129 62 62 102 165 Pressure bar 8.9 2 80 80 24.5 Efficiency 0.7 0.63 0.95 0.84 0.7 Energy kW −47.5 −5.7 −8.3 185.5 −167 −43

[0102] The energy requirements for an example process in accordance with the present invention is shown below. No injection pump is needed as the turbine lowers the pressure to the injection pressure. Again, all the diluted saltwater is returned to the salt formation. Such a process requires an energy input of 4 kW/hour to operate. Further, this efficiency may be achieved without a pressure exchanger, thereby reducing the number of components required in the system.

TABLE-US-00003 Feed Draw Injection Process pump pump Turbine pump Total Flow m.sup.3/h 129 63.5 167 — Pressure bar 9.2 80 24.2 — Efficiency 0.7 0.9 0.84 0.7 Energy kW −49 −163 208 0 −4

[0103] Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein.

[0104] Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.