Carbonate precipitation for water control in mining operations

Abstract

A method includes delivering, to a rock formation in a mine, a carbonate deposition mixture, the carbonate deposition mixture comprising one or more reagents to precipitate a carbonate in the rock formation in an amount sufficient to reduce a porosity in the rock formation.

Description

[0001] CARBONATE PRECIPITATION FOR WATER CONTROL IN MINING OPERATIONS

[0002] RELATED APPLICATIONS

[0003] This application claims priority to U.S. Provisional Application Serial No. 63 / 570,507, filed on March 27, 2024, the entire contents of which are hereby incorporated by reference in its entirety.

[0004] BACKGROUND

[0005] Water ingress into mines is a major operational challenge. Dewatering requires extensive infrastructure and energy. Reducing the groundwater flow into operating mines would reduce the expense and difficulty of dewatering. Additionally, during underground backfill, cement is often added to tailings and pumped into abandoned workings. Reducing the cement concentration required during backfill and the permeability of the backfilled tailings is also beneficial to mine operations.

[0006] SUMMARY

[0007] Water ingress into mines can be controlled by precipitating calcium carbonate on and / or within rock formations in a mine. An example of a process by which calcium carbonate can be precipitated in a mine is microbially induced calcium carbonate precipitation (MICP). This is a bio-geochemical process that induces calcium carbonate precipitation within a matrix. This process uses a biological process to create a calcium carbonate deposit in the matrix by introducing a suitable bacterium or bacteria into the matrix and creating an environment in which the bacteria precipitate the calcium carbonate deposit. MICP can be used to fill porosity and strengthen materials. Accordingly, in certain aspects, this disclosure features the use of MICP to control water flow in a mining operation. More generally, other pathways for precipitating calcium carbonate can also be used.

[0008] In general, in a first aspect, the invention features a method that includes delivering, to a rock formation in a mine, a carbonate deposition mixture, the carbonate deposition mixture comprising one or more reagents to precipitate a carbonate in the rock formation in an amount sufficient to reduce a porosity in the rock formation. Among other advantages, the MICP processes described here can provide an inexpensive and environmentally-friendly way to control water flow in mining operations. For example, the components used in the process, as well as the calcium carbonate, has low toxicity and low cost. The processes can also reduce the amount of cement otherwise necessary to effectively backfill mine shafts with tailings from mining operations. In particular, including MICP mixtures with the tailings can facilitate cementation of the backfilling without the need for additional cement. Furthermore, the backfill process can be performed more efficiently than comparable processes in which cement is included in the backfill before pumping the backfill into the mine shaft.

[0009] Other advantages will be apparent from the drawings, the description, and the claims.

[0010] BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1 is a flow chart showing an example process for controlling water flow in a mining operation using MICP.

[0012] FIG. 2. is a flow chart showing another example process for controlling water flow in a mining operation using MICP.

[0013] Like elements in different drawings are identified by like reference numbers.

[0014] DETAILED DESCRIPTION

[0015] Microbially induced calcium carbonate precipitation (MICP) is a biochemical process that can occur in soils, caves, freshwater, marine sediments, and hypersaline habitats. Generally, calcium carbonate can be precipitated in three polymorphic forms, which in the order of their usual stabilities are calcium carbonate, aragonite and vaterite. The main groups of microorganisms that can naturally induce the carbonate precipitation are photosynthetic microorganisms such as cyanobacteria and microalgae; sulfatereducing bacteria; and some species of microorganisms involved in the nitrogen cycle.

[0016] Several mechanisms have been identified by which bacteria can induce the calcium carbonate precipitation, including urea hydrolysis, denitrification, sulfate production, and iron reduction. Two different pathways, or autotrophic and heterotrophic pathways, through which calcium carbonate is produced are known. There are three autotrophic pathways, which all result in depletion of carbon dioxide and favor calcium carbonate precipitation. In heterotrophic pathways, two metabolic cycles can be involved: the nitrogen cycle and the sulfur cycle. Many different species and consortia of both engineered and natural bacteria are capable of carbonate precipitation. Various species and consortia may be needed to successfully accomplish MICP given the water and rock chemistry at a particular mine site.

[0017] Calcium carbonate deposits from MICP can reduce water permeability in soil, rocks, and cement, for example. Water control is a challenge in many mining operations. In porous rock formations, for example, ground water can seep into mine shafts or pits, increasing the difficulty of ore extraction from the mine. Commonly, water is pumped out of the mine to allow mining operations to continue.

[0018] Using the processes described below, water flow in a mine is controlled by forming a calcium carbonate deposit in a rock formation that is part of the mine. The calcium carbonate deposit is formed using MICP. Typically, the MICP delivery process involves injecting a mixture into a rock formation and / or coating a surface of the rock formation with the mixture, which contains at least bacteria, and urea, and may contain salt (e.g., calcium salt, such as calcium chloride), or additional nutrients or pH modifiers. The bacteria catalyze precipitation of calcium carbonate, which can seal pores in the rock formation and reduce water flow through the rock formation.

[0019] The bacteria can permeate the rock formation and result in carbonate formation away from the injection site, e.g., up to meters, tens of meters, or hundreds of meters from the injection site.

[0020] Calcium chloride and urea are both relatively low cost and have relatively low toxicity. Native bacteria may be relied upon or additional ureolytic bacteria may be added to the mix. The mixture can be water-based or can include some other liquid carrier. The mixture can also include nutrients for the bacteria, such as carbohydrates (e.g., a sugar) and / or protein and / or micronutrients. The mixture can include one or more pH modifiers to adjust the pH to provide a more optimal environment for the MICP process. Examples of pH modifiers include hydroxides, such as calcium hydroxide and sodium hydroxide, as well as sodium carbonate. In general, any ureolytic bacteria can be used, including strains that may be native to the injection site. One example of a ureolytic bacteria type is S. pasteurii (strain ATCC 11859). Other examples include Bacillus alkalinitrilicus, Bacillus sphaericus, Bacillus cohnii (BC) ACCC10230, Bacillus Subtilis, Sposarcina ureae, and Sporosarcina pasteurii. The bacterium may be engineered to increase their MICP effectiveness. The mixture can include more than one bacteria type.

[0021] The pH of the mixture can be more than 7, and preferably 8-12. A higher pH can improve the stability of calcium carbonate in the presence of the water in the rock pores. The urea concentration can be 10 to 200g / L, the calcium chloride concentration can be 10 to 400 g / L and additional nutrients such as ammonium chloride or yeast extract can be 10 to 400 g / L.

[0022] The mixture can be applied via multiple coatings or injections, e.g., over a period of hours or days. Each application can use the same mixture, or different mixtures can be applied at different times.

[0023] Calcium carbonate formation can take place over a time frame of days, weeks, or months, over which time the water pressure in the rock formation can be monitored and delivery of the mixture modified based on the monitored pressure. For example, if the water pressure changes more or less than expected (e.g., based on prior measurements, calculation, and / or simulation), additional bacteria and / or nutrients necessary for calcium carbonate formation can be delivered to change the rate of calcium carbonate formation in the rock formation. Increased pressure drop across the hydraulic barrier formed by calcium carbonate precipitation indicates successful blockage of fluid flow.

[0024] While the foregoing discussion is directed to MICPs, more generally, deposition of carbonate minerals to reduce permeability may also be implemented without bacteria. Carbonate deposition can be caused by providing supersaturated aqueous chemistry conditions (high pH, high metal ion concentration, high carbonate ion concentration). Generally, the deposition rate can be controlled by controlling the level of supersaturation, where higher levels of supersaturation result in faster deposition kinetics. Magnesium and calcium carbonates are the most common and robust carbonates for reducing permeability, but other metal ions, such as sodium or potassium, may also be used. Injection of sufficient pH, carbonate ion, and metal ion to achieve supersaturated deposition conditions, without the assistance of bacteria can achieve the aim of permeability reduction. In some cases, water or rock chemistry is sufficient that only one modification may be needed to achieve supersaturation and deposition. For example, only a pH modifier, such as sodium hydroxide, may be needed to change pH conditions such that existing calcium and carbonate ions are then supersaturated and deposited, blocking permeability. In another example, adding magnesium hydroxide to provide an increase in pH along with an increase in magnesium ion concentration is sufficient to supersaturate and deposit magnesium carbonate, blocking permeability. Supersaturation concentrations and deposition kinetics depend on water and rock chemistry and temperature. Favorable pH ranges may be from 8-12, favorable metal ion concentrations may be millimolar to molar, and favorable carbonate ion concentrations may be millimolar to molar.

[0025] In general, the MICP process described can be used in any suitable mine for water control, including underground mines and open pit mines. In underground mines the MICP mixture can be applied by coating a hanging wall or a footwall, for example. The MICP mixture can be applied by injecting the mixture into a hanging wall or a footwall. In some cases, the MICP mixture can be injected into a crack or a fissure in a rock formation.

[0026] Injection and / or coating can occur before rock bolting the wall and / or installation of a retention system. In some cases, the MICP mixture can be applied before shotcreting a rock formation.

[0027] In some cases, the MICP mixture can be combined with backfill from a mine to reduce groundwater flow and increase strength of backfill. For example, the MICP mixture can be combined with backfill to increase backfill strength and / or reduce cement concentration required to obtain a given compressive strength. For instance, a MICP mixture can be combined with mine tailings and cement that are introduced as backfill into a mine shaft in order to fill the workings and provide support. The MICP can be introduced on the surface before the backfill mixture of tailings and cement is delivered into the mine shaft. Alternatively, or additionally, a MICP mixture can be introduced beneath the surface in the mine shaft, either while the backfill mixture is enroute to their destination in the mine or while or after they’re being deposited at their destination. The unconfined compressive strength (UCS) of the MICP mixture backfill material can be 0.5-5 MPa after 7 (e.g., 28 or more, 365 or more) days of curing time. The UCS of the MICP mixture backfill can be increased relative to a traditional backfill with the same proportion of cement and tailings that lacks the MICP mixture. In open pit mines, the MICP mixture can be applied by injecting the mixture into a pit wall and / or a bench. In some cases, the mixture can be applied by injecting into a high hydraulic conductivity geologic region intersected by the pit.

[0028] Referring to FIG. 1, an example process 100 for controlling water flow in the mine includes drilling (110) one or more holes into the rock formation. The holes can be 10 cm or more deep (e.g., 50 cm or more, 100 cm or more, 200 cm or more, 500 cm or more, such as up to 10 m deep). The holes can have a diameter of 3 cm to 30 cm.

[0029] A MICP mixture is injected (120) into the holes. The mixture is injected at a pressure that exceeds the pore pressure in the rock formation. This can help to ensure that at least some of the mixture permeates into the rock itself.

[0030] Injection is repeated (130) over a period of hours or days to ensure sufficient mixture is introduced into the rock formation.

[0031] Coating or injection volumes to overcome dilution and establish significant porosity reduction are 1 liter per square meter of mine surface or more (e.g., 10 liters per square meter or more, 100 liters per square meter or more, 500 liters per square meter or more)

[0032] Coating or injection pressures to enable the MICP solution to coat or enter the pore spaces in the rock are atmospheric pressure or more (e.g., O.IMPa or more, IMPa or more, lOMPa or more)

[0033] The hydraulic conductance of the rock formation is measured (140) to ensure the conductance is reduced to a target level. Hydraulic conductance can be measured in a variety of ways. For example, conductance can be measured by measuring a pressure in and / or around the rock formation. For instance, pressure can be measured in the ore body being mined, above the ore body, and / or below the ore body. In some examples, hydraulic conductance is measured by measuring the amount of water flowing through various parts of the mine. E.g., how much water is being pumped out of a shaft. In some case, injection (130) can be repeated after the hydraulic conductance is measured to ensure the desired conductance is achieved.

[0034] Finally, upon achieving sufficient reduction in hydraulic conductance, the holes are capped (150). In some cases, the holes can be used for bolting a support structure to the rock formation.

[0035] The process can involve additional steps. For example, the wall of the rock formation can be shotcreted after the holes are capped.

[0036] In some examples, the process can involve fewer steps than those described in FIG. 1. For example, the holes need not be capped. Also, conductance measurements are not always necessary, e.g., where water flow stops entirely a measurement may not be necessary.

[0037] While the example described above involves injecting the MICP mixture into holes drilled in a rock formation, other implementations are possible. For example, MICP mixture can be sprayed onto a wall of a rock formation in a mine. An example process 200 for such an implementation is shown in FIG. 2. Here, an exposed wall of a rock formation is sprayed (210) with a MICP mixture. This is repeated (220) periodically as necessary until sufficient calcium carbonate deposit has formed and / or sufficient bacteria and nutrients have been supplied to ensure sufficient calcium carbonate will be formed. Optionally, after this, the wall can be sprayed (230) with shotcrete to coat and seal the wall.

[0038] While the foregoing examples involve precipitation of calcium carbonate, more generally the techniques described for water containment in mines can be applied using other precipitates too. For example, the techniques described herein can be applied to precipitate magnesium carbonate as an alternative, or in addition, to calcium carbonate.

[0039] The following numbered paragraphs are non-limiting examples of various embodiments of the present disclosure.

[0040] 1. A method, including: delivering, to a rock formation in a mine, a carbonate deposition mixture, the carbonate deposition mixture including one or more reagents to precipitate a carbonate in the rock formation in an amount sufficient to reduce a porosity in the rock formation. 2. The method of paragraph 1, wherein delivering the carbonate deposition mixture to the rock formation includes coating a surface of the rock formation with the carbonate deposition mixture.

[0041] 3. The method of paragraph 1, wherein delivering the carbonate deposition mixture to the rock formation includes injecting the carbonate deposition mixture into the rock formation.

[0042] 4. The method of paragraph 1, further including, prior to delivering the carbonate deposition mixture, determining a pore pressure of the rock formation and the carbonate deposition mixture is delivered at a pressure that exceeds the pore pressure.

[0043] 5. The method of paragraph 1, further including drilling one or more holes in the rock formation and the carbonate deposition mixture is delivered to the rock formation by injecting the carbonate deposition mixture into the holes.

[0044] 6. The method of paragraph 5, further including capping the one or more holes after delivering the carbonate deposition mixture.

[0045] 7. The method of paragraph 1, further including measuring hydraulic conductance of the rock formation after delivering the carbonate deposition mixture.

[0046] 8. The method of paragraph 1, wherein the carbonate deposition mixture is delivered with backfill to the mine in an amount sufficient to increase a compressive strength of the backfill.

[0047] 9. The method of paragraph 1, wherein the carbonate deposition mixture is delivered periodically in a sequence of batches.

[0048] 10. The method of paragraph 1, wherein determining the pore pressure includes measuring the pore pressure.

[0049] 11. The method of paragraph 1 , wherein the solvent is water.

[0050] 12. The method of paragraph 1, wherein the carbon deposition mixture is a

[0051] MICP mixture including a bacteria, a nutrient, urea, and a salt.

[0052] 13. The method of paragraph 12, wherein the bacteria is a ureolytic bacteria.

[0053] 14. The method of paragraph 13, wherein the ureolytic bacteria is S. Pasteurii.

[0054] 15. The method of paragraph 12, wherein the salt is a calcium salt or a magnesium salt.

[0055] 16. The method of paragraph 15, wherein the calcium salt is calcium chloride. 17. The method of paragraph 1, wherein the mine is an open pit mine.

[0056] 18. The method of paragraph 16, wherein carbonate deposition mixture is delivered by coating a pit wall or a bench of the open pit mine.

[0057] 19. The method of paragraph 16, wherein open pit mine includes regions having different hydraulic conductivity and the rock formation is in a region having higher hydraulic conductivity than another region.

[0058] 20. The method of paragraph 1, wherein the mine is an underground mine.

[0059] 21. The method of paragraph 19, wherein carbonate deposition mixture is delivered by coating a hanging wall or a footwall of the underground mine.

[0060] 22. The method of paragraph 19, wherein carbonate deposition mixture is delivered by injecting the carbonate deposition mixture into a hanging wall or a footwall of the underground mine.

[0061] 23. The method of paragraph 19, wherein carbonate deposition mixture is delivered by injecting the MICP mixture into a crack or a fissure of the underground mine.

[0062] 24. The method of paragraph 20, further including rock bolting or installing a retention system into the underground mine after delivering the carbonate deposition mixture.

[0063] 25. The method of paragraph 21, further including shotcreting a surface of the rock formation after delivering the carbonate deposition mixture.

[0064] 26. The method of paragraph 1, wherein the carbonate is calcium carbonate.

[0065] 27. The method of paragraph 1, wherein the carbonate is magnesium carbonate.

[0066] A number of embodiments are described. Other embodiments are in the following claims.

Claims

What is claimed is:

1. A method, compri sing : delivering, to a rock formation in a mine, a carbonate deposition mixture, the carbonate deposition mixture comprising one or more reagents to precipitate a carbonate in the rock formation in an amount sufficient to reduce a porosity in the rock formation.

2. The method of claim 1, wherein delivering the carbonate deposition mixture to the rock formation comprises coating a surface of the rock formation with the carbonate deposition mixture.

3. The method of claim 1, wherein delivering the carbonate deposition mixture to the rock formation comprises injecting the carbonate deposition mixture into the rock formation.

4. The method of claim 1, further comprising, prior to delivering the carbonate deposition mixture, determining a pore pressure of the rock formation and the carbonate deposition mixture is delivered at a pressure that exceeds the pore pressure.

5. The method of claim 1, further comprising drilling one or more holes in the rock formation and the carbonate deposition mixture is delivered to the rock formation by injecting the carbonate deposition mixture into the holes.

6. The method of claim 5, further comprising capping the one or more holes after delivering the carbonate deposition mixture.

7. The method of claim 1, further comprising measuring hydraulic conductance of the rock formation after delivering the carbonate deposition mixture.

8. The method of claim 1, wherein the carbonate deposition mixture is delivered with backfill to the mine in an amount sufficient to increase a compressive strength of the backfill.

9. The method of claim 1, wherein the carbonate deposition mixture is delivered periodically in a sequence of batches.

10. The method of claim 1, wherein determining the pore pressure comprises measuring the pore pressure.

11. The method of claim 1, wherein the solvent is water.

12. The method of claim 1, wherein the carbon deposition mixture is a MICP mixture comprising a bacteria, a nutrient, urea, and a salt.

13. The method of claim 12, wherein the bacteria is a ureolytic bacteria.

14. The method of claim 13, wherein the ureolytic bacteria is S. Pasteurii.

15. The method of claim 12, wherein the salt is a calcium salt or a magnesium salt.

16. The method of claim 15, wherein the calcium salt is calcium chloride.

17. The method of claim 1, wherein the mine is an open pit mine.

18. The method of claim 16, wherein carbonate deposition mixture is delivered by coating a pit wall or a bench of the open pit mine.

19. The method of claim 16, wherein open pit mine comprises regions having different hydraulic conductivity and the rock formation is in a region having higher hydraulic conductivity than another region.

20. The method of claim 1, wherein the mine is an underground mine.

21. The method of claim 19, wherein carbonate deposition mixture is delivered by coating a hanging wall or a footwall of the underground mine.

22. The method of claim 19, wherein carbonate deposition mixture is delivered by injecting the carbonate deposition mixture into a hanging wall or a footwall of the underground mine.

23. The method of claim 19, wherein carbonate deposition mixture is delivered by injecting the MICP mixture into a crack or a fissure of the underground mine.

24. The method of claim 20, further comprising rock bolting or installing a retention system into the underground mine after delivering the carbonate deposition mixture.

25. The method of claim 21, further comprising shotcreting a surface of the rock formation after delivering the carbonate deposition mixture.

26. The method of claim 1, wherein the carbonate is calcium carbonate.

27. The method of claim 1, wherein the carbonate is magnesium carbonate.

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