Process for removing solubilized metals with dissolved carbon dioxide

Abstract

The invention relates to a process for removing solubilised metals from a solution by carbonate precipitation of the metal. Carbonate is formed in situ by dissolving carbon dioxide in the solution using an enhanced gas-dissolution unit such as a membrane contactor, and raising the pH of the solution. Metals to be removed include scaling metals, e.g. calcium, strontium, barium or combinations thereof.

Description

PROCESS FOR REMOVING SOLUBILIZED METALS WITH DISSOLVED CARBON DIOXIDE Technical Field

[0001] The invention relates to the removal of metals from solution. Background

[0002] Scaling, or precipitation fouling, involves the deposition of solids from a liquid onto surfaces.

[0003] Metals that form such solids are referred to herein as “scaling metals” and the solids formed thereby as “scaling metal solids”.

[0004] In many situations, removal of scaling metals from solution is desirable. For example, only, scaling metal solids can have negative effects on industrial equipment.

[0005] One method for removing scaling metals from solution (which is also referred to herein as solution remediation) is carbonate precipitation. In this method, a carbonate anion (CO32-) in solution reacts with the metal to form a solid metal carbonate species that precipitates. The precipitate is then removed.

[0006] There are several conventional approaches for removing scaling metals from solution using carbonate precipitation.

[0007] In a first approach, the solution to be treated contains the carbonate anion [for example, groundwater from limestone (CaCO3) deposits is often rich in carbonate ion]. In this situation, a pH adjustment with a strong base such as NaOH or CaO leads to the formation of the desired metal carbonate precipitates.

[0008] In a second approach, a carbonate solution derived from soda ash (Na2CO3) is added to the solution to be treated and the metals precipitate as carbonates from a basic solution.

[0009] In a third approach, carbon dioxide gas is added to the solution e.g. by blowing the gas into the solution. If the carbon dioxide dissolves into solution, it can produce carbonate. However, carbon dioxide has low solubility in water under normal conditions. As noted in Cheremisinoff (1994), Handbook of Water and Wastewater Treatment Technology, CRC Press, pg.423, such a treatment “requires a slow-moving influent being fed into a reactor tank of sufficient depth to ensure adequate absorption of the CO2 into solution”. The third conventional approach is used for recarbonation and pH adjustment when treating metal-bearing wastewater after water softening (see, for instance, Crittenden et al., MWH’s Water Treatment: Principles and Design, 3rdEdition, Wiley: 2012). Brief Summary of the Invention

[0010] In one aspect, the invention relates to a process for precipitating a metal solubilised in an aqueous solution, the process comprising: adding carbon dioxide into the solution to form a carbonate enriched solution; and raising the pH of the carbonate enriched solution.

[0011] In another aspect, the carbon dioxide can be added into the solution in a substantially bubble-less manner.

[0012] In another aspect, a gas-dissolution unit can be used to add the carbon dioxide into the solution.

[0013] In another aspect, the metal can be beryllium, radium, calcium, magnesium, strontium, barium, manganese, iron, cobalt, nickel, copper, zinc, cadmium or lead.

[0014] In yet another aspect, the gas-dissolution unit may comprise a membrane.

[0015] In yet another aspect, the gas-dissolution unit may be a hollow fiber membrane contactor. Brief Description of Drawings

[0016] Embodiments of the invention will be discussed with reference to the following figures:

[0017] Figure 1 is a graph of the equilibrium between H2CO3, HCO3- and CO32-in solution at different pH;

[0018] Figure 2 is a cross-sectional schematic representing a packed tower for facilitating gas-dissolution;.

[0019] Figure 3 is a flow diagram of the metal carbonate precipitation process using dissolved carbon dioxide with a membrane contactor;

[0020] Figure 4 shows a comparison of residual calcium concentration in produced water sample after sodium carbonate treatment (a) and carbon dioxide treatment (b);

[0021] Figure 5 shows a comparison of residual barium concentration in produced water sample after sodium carbonate treatment (a) and carbon dioxide treatment (b);

[0022] Figure 6 shows a comparison of residual strontium concentration in produced water sample after sodium carbonate treatment (a) and carbon dioxide treatment (b);

[0023] Figure 7 shows a comparison of residual sodium concentration in produced water sample after sodium carbonate treatment (a) and carbon dioxide treatment (b);

[0024] Figure 8 is a schematic of an example oil and gas use of the invention;

[0025] Figure 9 is a schematic of another example use of the process as a reverse osmosis pre-treatment; and

[0026] Figure 10 is a schematic of another example use of the process in the field of mining. Detailed Description of Embodiments

[0027] The following provides a description of exemplary embodiments of the invention only.

[0028] One embodiment is a process for precipitating metal solubilised in an aqueous solution, the process comprising (a) dissolving carbon dioxide into the solution in a substantially bubbleless manner using an enhanced gas-dissolution unit, and (b) raising the pH of the solution. A suitable gas-dissolution unit is sold as THE TOOB ™ by BioTherm. Another potentially suitable gas-dissolution unit is the Liqui-CelTMEXF Series Membrane Contactor Cartridge sold by Solventum.

[0029] Upon dissolution in water, the carbon dioxide (CO2) reacts with water to form carbonic acid, bicarbonate (HCO3-) and carbonate (CO32-).

[0030] The carbonic acid reaction is: CO2 + H2O ↔ H2CO3

[0031] The equilibrium equation, which is pH dependent, is: H2CO3 ↔ HCO3- ↔ CO32-

[0032] The relationship between the equilibrium of the various species and the pH of the solution is depicted in Figure 1. Notably, at higher pH, the concentration of carbonate (CO32-) increases. These carbonate anions can react with metal cations in solution to form solid precipitates.

[0033] Metals that can be removed from solution this way include beryllium, radium, calcium, strontium, barium, magnesium, manganese, iron, cobalt, nickel, copper, zinc, cadmium, and lead. The carbonate species that correspond to these metals are BeCO3 RaCO3,CaCO3, SrCO3, BaCO3, MgCO3, MnCO3, FeCO3, CoCO3, NiCO3, CuCO3, ZnCO3, CdCO3, and PbCO3.

[0034] More than one scaling metal may be present in the solution to be treated.

[0035] Since different metal carbonates have different solubility constants and are formed at different rates, there may be an order as to which metal carbonates precipitate from solution upon a pH increase. For instance, in a solution containing barium, calcium and strontium, barium may precipitate preferentially. This preference may be exploited. For example, the process may be used to preferentially remove barium and strontium from a solution also containing calcium since strontium and barium can be radioactive and can collect in dangerous concentrations as a result of scaling.

[0036] In other embodiments of the invention, the carbon dioxide can be dissolved into solution using laminar jet contactors, wetted wall contactors, stirred cell contactors, spray towers, packed towers or tray / plate towers.

[0037] In a packed tower, the solution can be pumped from the top of the tower, while gas is moved upwards through the packing material. The packing material will break up the flow of solution as it moves down the tower by gravity. This will create increased surface area of contact between the carbon dioxide gas and the solution, which leads to increased and / or faster carbon dioxide dissolution. Examples of packed towers include those supplied by SulzerTM.

[0038] An exemplary packed tower is shown in Figure 2. The liquid is introduced at the inlet 101 at the top of a packed tower containing trays or packing material 102. The liquid flows through the packing material after passing through the liquid distributor 103. The trays or packing material 105 break up the flow of the liquid. By breaking up theliquid flow, droplets of liquid are generated to increase the surface area of diffusion of a gas that is pumped in from the bottom of the tower at inlet 104 and exiting through outlet 105. The carbonated liquid is pumped out of outlet 106.

[0039] There are a variety of packing materials and designs that can be used, depending on the application. The material is generally corrosion-resistant and the design of the packing material allows for calculable mass-transfer surface area and contact time.

[0040] Exemplary packing materials are shown in Table 10.1 of Theodore L. and F. Ricci, Mass Transfer Operations for the Practicing Engineer, Wiley: 2010.

[0041] In a plate / tray tower, steel plates or trays are used. Liquid flows down and is held in trays, which provides time for gas to transfer to solution. This divides the mass- transfer into stages instead of being one continuous tower, and may be selected depending on the liquid properties (i.e. if it foams), or if a longer contact and residence time for the liquid is required.

[0042] As discussed in Luis P. et al. (2013) “The role of membranes in post- combustion CO2 capture”, Greenhouse Gas Sci. Technol.3:318-337, there are at least two approaches to membrane technology. In the first, membranes are used as a selective barrier. Gas-permeation and supported liquid membranes are examples that use this approach. In the second approach, membranes are used as non-selective barriers, and are mainly used to increase the surface for mass transfer between the gas and the liquid. Membrane contactors can be used in the second approach.

[0043] In a membrane contactor, there can be pores filled with gas. The carbon dioxide can be transferred from the gas phase to the liquid phase based on, for instance, a concentration gradient.

[0044] As discussed in Mansourizadeh, A. and A. Ismail “Hollow fiber gas-liquid membrane contactors for acid gas capture: A review” (2009), Journal of Hazardous Materials, 171: 38-53, membrane contactors may provide higher surface area per unit contactor volume, independent control of gas and liquid flow rates without flooding, loading, weeping, foaming or entrainment, small size, known gas-liquid interfacial area, modularity, and scalability, in comparison to other methods of gas dissolution.

[0045] The solubility of a gas in a liquid is proportional to the partial pressure of the gas in equilibrium with that liquid (Henry’s Law). Accordingly, in order to increase the concentration of carbon dioxide dissolved in solution the pressure of carbon dioxide injected into the liquid can be increased. Further, carbon dioxide solubility generally decreases with increasing temperature. Decreasing the temperature of the solution can therefore increase the carbon dioxide solubility.

[0046] The rate at which carbon dioxide dissolves in solution will be affected by the pressure of the injected gas, the temperature of the solution, and also by the dimension of the interface between the solution and the gas. A larger interface will permit faster dissolution. If any bubbles are formed, smaller bubbles will have greater surface area per unit volume and stay in solution longer, for any given volume of gas.

[0047] A membrane provides a high surface area to facilitate carbon dioxide dissolution. For instance, membrane contactors may contain many nanometre size pores which provide a high surface area for gas transfer in a very small space. The carbon dioxide gas is transported through these contactors and diffuses through the pores, thereby entering the solution.

[0048] Alternatively, a membrane contactor may be used where the solution is passed through the contactor, and carbon dioxide gas is transported outside the contactor and diffused through the pores into the solution inside the contactor. Liqui-CelTMcontactors function in this manner. Of course, when these contactors are used, care must be taken to monitor the contactor performance and ensure the contactor is not plugged byprecipitating scaling metals. For these contactors, the pH of the solution in the contactor should be kept as low as possible to minimize scaling.

[0049] Membrane contactors can be operated above atmospheric pressure and configured to minimize bubble generation. Bubble generation can be minimized by maintaining the pressure inside the membrane contactor slightly below the water / liquid side pressure. If the pressure inside the fiber is higher, then the gas can escape in a convective manner and generate bubbles. If the pressure inside the fiber is lower, then the gas can escape through diffusion.

[0050] Liqui-CelTMcontactors, which carry the liquid solution inside the contactor, can also be configured to minimize bubble generation. For these contactors, bubble generation can be minimized by maintaining the gas pressure outside the contactor slightly below the water / liquid side pressure to facilitate diffusion.

[0051] Some embodiments result in saturation and super-saturation of the solution with carbon dioxide.

[0052] In some embodiments, the flow of gaseous carbon dioxide or the flow of solution across the membrane can be controlled by an in-line dissolved carbon dioxide monitor so that sufficient carbon dioxide will dissolve to ultimately form carbonate.

[0053] The desired concentration of carbon dioxide in solution can vary. In one embodiment, the metal to be precipitated is a divalent metal, and the amount of carbon dioxide dissolved in solution is in a molar ratio of 1:1 of carbon dioxide to metal. In another embodiment, a greater than 1:1 carbon dioxide to metal ratio is used.

[0054] In membrane applications, the pressure of CO2to be injected into the solution will depend on the flow of solution over the contactor, the desired concentration of carbonate anions, and the nature of the membrane. A CO2 pressure which is too high may cause breakdown of the structural integrity of the contactor. In one embodimentwhere a polymer hollow fiber membrane contactor is used, a CO2pressure of up to about 80 psi can be applied. In another embodiment with a polymer hollow fiber membrane contactor, a pressure of up to about 60 psi can be applied.

[0055] In some embodiments, the temperature of the solution is controlled prior to contacting the solution with the carbon dioxide. The solution temperature may be selected to increase or decrease the amount of carbon dioxide to be dissolved in the solution. Various methods known in the art for controlling a solution temperature can be utilised.

[0056] In one embodiment, the temperature of the solution is from about 5 to about 50 degrees Celsius.

[0057] In membrane applications, the choice of membranes may also influence the temperature of the solution, as sustained high temperatures may degrade polymer physical properties.

[0058] In some embodiments, the membrane comprises polyethylene / polypropylene fibers, and the solution temperature is from about 5 to about 50 degrees Celsius.

[0059] In further embodiments, the maximum temperature is up to 80 degrees Celsius. However, depending on the polymer used, the temperature of the solution may be up to 90 or up to 100 degrees Celsius.

[0060] There are a number of polymers and other materials that can be used to make membranes.

[0061] For contact with aqueous solutions, a hydrophobic material may be used to help prevent water breakthrough (i.e. ingress of the solution into the contactor) and to help maintain a clean interface between the gas and the aqueous solution. By clean interface itis meant that solids are not deposited on the contactor or in the pores of the contactor, such solids would be detrimental to the formation of the gas / solution interface.

[0062] In some embodiments, the membrane contactors include polymer hollow fiber membrane contactors and ceramic membrane contactors. In some embodiments, examples of hydrophobic polymers for making the polymer hollow fiber membrane contactors include: poly(vinylidene fluoride) (PVDF), polysulfone (PSF), poly(ethersulfone) (PES), poly(benzimidiazole) (PBI), polypropylene (PP), polyethylene (PE), and poly(tetrafluorethylene) (PTFE) (also known as TeflonTM). Other hydrophobic polymers, such as polystyrene or polymethacrylates, can also be used. Copolymers of the above with other monomers may also be used. It is also possible to make membranes with high surface area from a ceramic or glass substrate, to which a polymer coating can be applied to impart hydrophobic properties, if desired.

[0063] In one embodiment, absorption agents are used to help increase carbon dioxide dissolution. Examples of absorption liquids include amines, ionic liquids, and other compounds. Amines, such as MEA, may be used. Such agents have also been used with membrane contactors before (see, for instance, Park et al., “Characteristics of CO2fixation by chemical conversion to carbonate salts” (2013), Chemical Engineering Journal, 231: 287-293; and Park et al., “CO2 (carbon dioxide) fixation by applying new chemical absorption-precipitation methods” (2013), Energy, 59: 737-742). Ionic liquids and other compounds (such as amino acids and NaOH) have also been used with membrane contactors and can be used in other embodiments.

[0064] In one embodiment, the solution in which carbon dioxide is to be dissolved is pre-treated to, or maintained at, a neutral or acidic pH.

[0065] In one embodiment, the solution has a pH of 8 or lower, of 7 or lower, or of 6 or lower.

[0066] Depending on the conditions and the concentration of metal in solution, a caustic (basic) solution may promote metal carbonate precipitation in the gas-dissolution unit (e.g. at the membrane contactor surface), which may be detrimental to the functioning of the gas-dissolution unit. Membrane contactors may be more susceptible to fouling by way of such precipitation due to the small size of the pores in the membrane. However, if the concentration of metal is low, a higher pH (>8) may be used.

[0067] In one embodiment, subsequent to the dissolution of the carbon dioxide into the solution, the pH of the solution is increased. Addition of the base is carried out after dissolution of the carbon dioxide as this prevents clogging of membrane contactor pores by precipitated metal carbonates during the gas transfer.

[0068] The pH can be increased e.g. by addition of a base. In one embodiment, the base may be NaOH, LiOH, or Mg(OH)2. In another embodiment, the base is CaO or Ca(OH)2. In yet another embodiment, the base is an amine base, such as ammonia, monoethanolamine (MEA), di-ethanolamine (DEA), or methyl di-ethanolamine (MDEA).

[0069] In some embodiments, the pH of the solution is increased above neutral. In another embodiment, the addition of the base causes the pH to increase to about 8 or above. In yet another embodiment, the addition of the base causes the pH to increase to about 8.3. In yet another embodiment, the addition of the base causes the pH to increase to about 9 or above. In yet another embodiment, the addition of the base causes the pH to increase to about 10 or above.

[0070] In one embodiment, the addition of base is controlled by monitoring pH and adjusting the injection rate accordingly.

[0071] Increasing the pH pushes the equilibrium between carbonic acid, bicarbonate and carbonate towards the carbonate (see Figure 1). The carbonate can then react with the solubilised metal, leading to precipitation of the metal carbonate.

[0072] Increasing the pH can also lead to gaseous carbon dioxide solute reacting directly with hydroxide anions in solution to form bicarbonate: CO2+ OH- ↔ HCO3- which bicarbonate in turn can form carbonate anions, which will themselves form metal carbonate precipitates.

[0073] In some embodiments, the reaction rate for the reaction between carbon dioxide and the hydroxide anion will be more rapid than the reaction rate for the dissolution of carbon dioxide into solution. The equilibrium between dissolved carbon dioxide and carbonic acid heavily favours the gaseous solute, so the conversion of dissolved gas to carbonic acid is thermodynamically and kinetically slow. By adding base, the formation of carbonates can be sped up through this alternate reaction pathway.

[0074] In some embodiments, increasing the pH can also be a co-precipitation strategy, as metal hydroxide (e.g. MeOH, Me(OH)2, Me(OH)3) precipitates can also form. The concentration of metals to be removed by the process, whether using a membrane or a tower, is not particularly limited. Very low concentrations of metals can be removed, as the solubility product constant of metal carbonates tends to be low. Higher concentrations can be removed with correspondingly larger amounts of dissolved carbon dioxide. The amount of dissolved carbon dioxide needed will vary depending on the metal(s) to be removed, the conditions (e.g., temperature), the nature of the solution, and other factors that a skilled person would take into consideration.

[0075] The maximum concentration of metal that can be removed using a membrane may depend on the operating pressure limitation of the membrane, which may in turn depend on the hydrophobicity of the material and the pore size. The maximum concentration may also depend on the conditions for carbon dioxide dissolution. For instance, in general, as pressure increases, solubility increases; as temperature decreases, solubility increases; and as alkalinity increases, solubility increases. The maximum concentration may also depend on the metal carbonate solubility constant and themolecular weight of the metal. As an example, the concentration of calcium that might be removed with a polyolefin fiber membrane might be 7.8 g / L, and for a fluorinated polymer membrane, 13.2 g / L. However, different values may be obtained depending on the conditions and membrane used.

[0076] The embodiments described above may be combined with other steps. An exemplary process comprising multiple steps is explained below. Each step explained below is optional.

[0077] In one embodiment, prior to the process as defined in the embodiments above contaminants can be removed from solution. These may include, for instance, organic pollutants. Various methods are known to the skilled person for removing such organic pollutants.

[0078] In one embodiment, prior to the process as defined in the embodiments above the solution can be passed through a preliminary filter or screen to remove suspended solids. The solids can be removed to prevent fouling of the gas-dissolution unit. Filtration may be accomplished by for instance a belt filter, vacuum filter, disc filter, dissolved air flotation with or without a skimmer, or other means. The choice of filtering will vary based on the quantity of solids in solution, along with space and disposal constraints. If the filtered solids entrain water, they can be dewatered, with recovered water returned to the original solution.

[0079] In one embodiment, following the precipitation of the metal carbonates, a flocculant can be added to the solution. The flocculant can be a polymer flocculant. The polymer flocculant may be an anionic polymer flocculant such as P816E from ChemTreat. The P816E flocculant can be added at a concentration of 2mg per litre of solution. In a further embodiment, after flocculant addition, the solution can be passed through a baffled plug flow reactor, tank system or other suitable resting receptacle to allow flocs to form and settle or rise, depending on their relative density. The meanresidence time can be, e.g. several minutes to 2 hours. In further embodiments, the mean residence time is up to 20 minutes, up to 10 minutes, up to 5 minutes or about 5 minutes.

[0080] In one embodiment, following precipitation the solution can be passed through a high-capacity filter system. This can be very suitable for effluent / wastewater streams from oil and gas production. A high-capacity filter system that produces a cake can be used. Dewatering can follow filtration, and recovered water can be returned to the solution.

[0081] In another embodiment, after precipitation the solution can be filtered to separate contaminants. Filtration can be accomplished using, for instance, a zeolite and / or activated carbon. Other steps, such as desalination, may be included where appropriate.

[0082] In one embodiment, after precipitation the solution can be neutralized with an acid. Following addition of a base in the process embodiments described above, the solution will have an increased pH. An acid can be added to obtain a solution having the desired pH. In some embodiments, the acid can be sulfuric acid, and the pH can be decreased to 6.5 or to 7.5. The addition of acid can be controlled by monitoring the pH and adjusting the addition rate. In one embodiment, sulphuric acid is used, e.g. in the form of a 50% solution, which does not have to be kept heated due to its low freezing point of around -50 degrees Celsius.

[0083] In other embodiments, carbon dioxide can be reinjected into the solution to lower its pH. Addition of carbon dioxide in a basic solution results in formation of bicarbonate, which reduces the solution pH: OH- + CO2↔ HCO3-

[0084] Reinjection of carbon dioxide can be accomplished by way of, for instance, a membrane contactor, as described above, by sparging gas carbon dioxide through the solution, or by passing the solution through a packed tower through which carbon dioxide is flowing.

[0085] In some embodiments, the process mentioned above can be used in the field of oil and gas production, such as shale gas extraction. Once the solution has been treated and metals removed, the resulting solution can be transported and re-injected into geological formations to maintain pressure or transported and used as fracturing water in fracking operations. It can also be used to remove scaling metals from liquid being transported in pipelines used in the oil and gas industry.

[0086] The process mentioned above can also be applied for the carbon sequestration of flue gases. In some embodiments, flue gases contain carbon dioxide as a by-product of combustion. Prior to being released into the atmosphere, the flue gas can be passed through a solution containing solubilised metal. This can lead to carbon dioxide from the flue gas being dissolved into solution. A membrane, such as a selective membrane, can be used. Subsequently raising the pH of the solution can lead to precipitation of the metal carbonate. Examples

[0087] The following examples are provided to illustrate the invention. It will be understood, however, that the specific details have been selected for purpose of illustration and are not to be construed as limiting the scope of the invention.

[0088] Reference is made to the flow diagram of Figure 3, where a membrane contactor is used as the gas-dissolution unit.

[0089] Wastewater stream 201 at acidic or neutral pH containing dissolved metals species such as calcium, barium, strontium or other divalent metal cations which can form metal carbonate species (e.g., Mg, Mn, Fe, Co, Ni, Cu, Zn, Cd, Pb) is pumped through a membrane contactor 202. The carbon dioxide from carbon dioxide storage 203 passes through the carbon dioxide feed 204 and the membrane contactor 202. Excess carbon dioxide and other removed gases pass out through the displaced gas vent 205.

[0090] The gas-liquid contactor will bring the wastewater stream into contact with carbon dioxide gas at elevated pressure to facilitate diffusion of the carbon dioxide into solution. Carbon dioxide, once dissolved in water, will gradually form carbonic acid (H2CO3), which depending on the pH is in equilibrium with bicarbonate (HCO3-) and carbonate species (CO32-). This forms carbonated wastewater stream 206.

[0091] The carbonated wastewater stream 206 is then fed into a vessel such as a tank 207, where it is mixed with a strong basic solution (such as NaOH, Ca(OH)2) to raise the pH as suitable for precipitation (e.g. pH > 10). The strong basic solution is stored in 208, and is fed into the tank 207 via the alkaline solution feeder 209.

[0092] Dissolved metal will precipitate as metal carbonates once pH is sufficiently high, and after several minutes a high molecular weight, water soluble, polymer flocculant solution is added to flocculate all the fine particulates to help with water clarification and filtering of metal waste. The polymer flocculant solution is stored in 210 and fed through the feeder 211 into vessel 217. The solution in the tank 217 may undergo water clarification by sedimentation and settling and the injection of other water clarifying agents.

[0093] The solution is then filtered or dewatered to separate flocculated metals and wastewater at filter 212. The filtered or dewatered sludge should still be basic, and may be recycled back to the tanks as shown in arrow 213 to aid in pH adjustment of carbonated water and to minimize the use of caustic agents.

[0094] The filtered metal free alkaline water 214 is mixed in-line with an acid 215 to bring the pH back to a more neutral value. This results in neutral water which is mostly free of scaling metal 216.

[0095] In a further example, a solution comprising dissolved calcium, strontium, and barium can be subjected to a process as defined. The process can lead to about 90% or more reduction in calcium (for instance, from about 4050 mg / L to about 260 mg / L) and strontium (for instance, from about 267 mg / L to about 22.6 mg / L), and / or can lead to about 99% or more reduction in barium (from about 106 mg / L to about 0.68 mg / L). Experimental Examples

[0096] The following data is provided as a comparison of the carbon dioxide metal precipitation process and a conventional sodium carbonate metal precipitation method. Comparative Example - Conventional sodium carbonate precipitation

[0097] Produced water samples from the oil and gas industry were treated using sodium hydroxide (NaOH, >97.0% purity, Sigma Aldrich) and varying amounts of sodium carbonate (Na2CO3, 99.95-100.05%, Sigma Aldrich) to test the effect of carbonate concentration for metal removal from wastewater.

[0098] A concentrated 3 M NaOH solution was added dropwise to the wastewater until the alkalinity stabilized at approximately pH 10. Sodium carbonate in a powdered form was then added directly to the alkaline wastewater to trigger precipitation of metal carbonates. The mixture was allowed to stir for one minute before the addition of an anionic polymer flocculant. The flocculated solids were then removed by gravity filtration through filter paper. Carbon dioxide triggered metal precipitation

[0099] Produced water samples from the oil and gas industry were treated using carbon dioxide (CO2, >99% purity, Praxair) and sodium hydroxide (NaOH, > 97.0% purity, Sigma Aldrich).

[0100] Carbon dioxide was absorbed or dissolved into the wastewater using a polymer hollow fiber contactor. The alkalinity of the CO2 saturated wastewater was then raised to either pH 10 or pH 11 via addition of a 3 M sodium hydroxide solution. The mixture was allowed to stir for one minute before the addition of an anionic polymer flocculant. The flocculated solids were then removed by gravity filtration through filter paper. Metal analysis

[0101] The filtered wastewater samples were sent to ALS Laboratories for elemental analysis. The samples were analyzed via inductively coupled plasma mass spectrometry (ICP-MS) according to reference method EPA 200.8. Results

[0102] Two different produced water samples from the oil and gas industry were used in the testing. The elements of interest that were present at an appreciable concentration included calcium, barium, strontium, iron, magnesium, and sodium. Table 1 provides a summary of the initial metal composition of the water samples before treatment with sodium carbonate or carbon dioxide. The metal content of the two samples were very similar and suggested that the metal removal efficiency of tests conducted using one sample would be applicable to the other. Water sample 1 was used for sodium carbonate treatment, while water sample 2 was used for the carbon dioxide process. Table 1. Concentration of metals of interest in oil and gas produced water samples as measured using ICP-MS Metal Water sample 1 Water sample 2 (ppm) (ppm) Calcium 4050 4000 Barium 106 104 Strontium 267 278 Sodium 13100 13300 Iron 25.6 24.2 Magnesium 171 169

[0103] In the sodium carbonate metal removal strategy, the alkalinity of the wastewater samples was first adjusted to pH 10. Different amounts of sodium carbonate were then added to the wastewater to give a final concentration 1, 2, 5 g Na2CO3 / L of wastewater. The precipitated solids were then flocculated, and the filtered wastewater was analysed by ICP-MS. The residual metal concentrations in the different sodium carbonate treated samples and carbon dioxide treated samples are shown in Table 2. As the amount of sodium carbonate added increased, the residual metal concentration of the primary scaling metals (calcium, barium, strontium) decreased. Iron was also removed in the process, as the concentration decreased from 25.6 ppm down to below the detectable limits of 5 ppm. The concentration of magnesium also decreased, although the differential was not as high as the other scaling metals.

[0104] The carbon dioxide treated wastewater also showed a significant decrease in all scaling metals of interest. Figures 4 to 7 compare the residual dissolved metal concentration (calcium, barium, strontium, sodium) for sodium carbonate treatment and carbon dioxide treatment for the 5 samples listed in Table 2. Table 2. Concentration of metals of interest in oil and gas produced water samples after sodium carbonate treatment or carbon dioxide treatment as measured using ICP-MS Metal Na2CO31 g / L Na2CO32 g / L Na2CO35 g / L CO2pH 11 CO2pH 10 (ppm) (ppm) (ppm) (ppm) (ppm) Calcium 3390 2500 840 706 1290 Barium 62.2 45.8 6.95 <0.2 9.52 Strontium 243 186 63.2 56.3 107 Sodium 14200 14500 15800 14000 14300 Iron <5 <5 <5 <5 <5 Magnesium 162 152 132 <50 127

[0105] The amount of metal removed can also be expressed as a percentage of metal removed, calculated using the following formula: M ^ M % Metal Removed =i f^ 100 where Mi and Mf are the after sodium carbonate orcarbon dioxide treatment. The percentages of metals removed using both sodium carbonate treatment and carbon dioxide treatment are shown in Table 3. From the sodium carbonate data, it can be seen that the percentage of metal removed increases with the amount of sodium carbonate added. With 5 g / L of Na2CO3, over 75% of both calcium and strontium were removed from the wastewater, along with over 90% of dissolved barium. However, this comes at the expense of an increase in dissolved sodium levels after treatment, which rose 20%.

[0106] In comparison, with CO2at pH 10, over 60-65% of calcium and strontium are removed, as well as over 90% of dissolved barium. As with sodium carbonate precipitation, >20% of magnesium was removed and the iron concentration was reduced to below detectable limits. However, the concentration of dissolved sodium increased by only 9%. This was a similar increase in sodium concentration as with 2 g / L of Na2CO3, however, the amount of metal removed was significantly enhanced.

[0107] Increasing the precipitation pH to 11 during carbon dioxide treatment enhanced the metal removal efficiency. With additional base, the percentage of calcium and strontium removed increased to approximately 80%, while > 99.8% of barium was removed (barium was below detectable limits). Both iron and magnesium concentrations were reduced to below detectable limits of 5 and 50 ppm respectively. In addition, the residual sodium concentration increased by only 6.4%, a significantly lower number than when sodium carbonate is used to remove a similar percentage of metal from the wastewater.Table 3. Percentage of metal removed using sodium carbonate induced metal removal at increasing concentrations of sodium carbonate. Metal Na2CO31 g / L Na2CO32 g / L Na2CO35 g / L CO2pH 11 CO2pH 10 (% removed) (% removed) (% removed) (% removed) (% removed) Calcium 16.3 38.3 79.3 82.4 67.8 Barium 41.3 56.8 93.4 >99.8 90.9 Strontium 9.0 30.3 76.3 79.7 61.5 Sodiuma8.4 10.7 20.6 6.4 9.2 Iron >78 >78 >78 >78 >78 Magnesium 5.3 11.1 22.8 >70.4 24.9aAs the sodium concentration goes up due to the addition of NaOH and Na2CO3, the values denoted in the table indicate % of additional sodium metal after treatment.

[0108] In the carbon dioxide treatment, a polymer hollow fiber contactor was used to adsorb / dissolve CO2 into the wastewater. Although the exact CO2 concentration was not directly measured, analogous absorption experiments using deionized water indicated that the dissolved CO2concentration should be approximately 1600 ppm (1.6 g / L of wastewater). As one molecule of dissolved CO2 directly translates into 1 HCO3- / CO32-anion, the level of dissolved bicarbonate / carbonate can be compared between sodium carbonate addition and adsorbed carbon dioxide by factoring in the molecular weight of sodium and carbon dioxide. The calculations are as follows: MW Na2CO3^105.9885gmolMW Na ^ 22.9898 gmol^,% CO2from Na105.9885 ^ 2*22.98982CO3= ^ 100 ^ 56.61%105.9885 With 2gof Na 2 CO 3 , effective CO 3 = 2*0.5661 =1.1323gL L g L

[0109] The MW of CO2is 44 g / mol, and the MW of a CO32-anion is 60 g / mol, therefore the equivalent dissolved CO2 concentration required to provide the same level of bicarbonate / carbonate as the addition of 5 g / L of Na2CO3 is 2.08 g / L. When comparing the results of CO2treatment at pH 11 with 5 g / L of Na2CO3, it can be seen that although a lower effective concentration of bicarbonate / carbonate is used in the carbon dioxide process, the metal removal efficiency is similar to or greater than with sodium carbonate precipitation. These results indicate that carbon dioxide is a suitable replacement for sodium carbonate for removing dissolved metals (calcium, barium, strontium) from produced water in the oil and gas industry.

[0110] Figure 8 demonstrates the use of the invention for cleaning produced water for reinjection into an oil well or oil field to provide field pressurization. In this process, a multi-stage water treatment process is first undertaken taken to remove large solids and reduce the oil content below 2 ppm. This might involve taking produced water from a Free Water Knockout and passing it through skimmers, cyclonic separation, polyacrylamide flocculation with dissolved air flotation, leading to walnut shell and / or microfiltration. Alternatively, this might involve taking produced water from a Free Water Knockout and applying a high molecular weight insoluble oil-based flocculant followed by secondary filtration to directly reduce oil below 2 ppm. Then once the oil has been removed, the process in Figure 3 is initiated to remove scaling metals from the water with no risk of the oil plugging the CO2infusion machinery. Following this, the resulting clean brine can be reinjected to help maintain the pressure in the field. The clean brine provides three benefits: (1) the brine won’t promote scaling which could partially plug the formation, (2) the brine will assist in cleaning the formation as it naturally takes in oil as it seeks chemical equilibrium with the other liquids and compounds in the oil field, and (3) it helps pressurize the field.

[0111] Another advantageous use of the process is for pre-treatment of metal- containing brackish feed water destined for purification by reverse osmosis. The scaling metals in this case can rapidly plug the reverse osmosis membrane. Figure 9 presents the process of removing oil from the reservoir via pumps that deliver the brackish water to a facility that implements the process in Figure 3 prior to the brackish water being desalinated by the reverse osmosis unit.

[0112] The invention also has application as a means to avoid the use of evaporation ponds in the production of metals such as magnesium and calcium. Instead of evaporation followed by prilling and electrolysis, the metal rich brine can have carbon dioxide dissolved thereinto to form a carbonate-enriched solution; the pH of the carbonate enriched solution can be raised to precipitate the metal as metal carbonate precipitate using the process of Figure 3 with or without the optionable process component of using a soluble polymer flocculant such as ChemTreat P816E; the precipitate can be calcined to produce metal oxide; and the oxide can be thermally reducing to metal using one of several silicothermic methods such as the Pidgeon, Magnetherm, or Mintek processes. For those skilled in the art, reduction can also be done using Carbothermic or Aluminothermic process, along with Solid Oxide Membrane reduction. Where the brine is rich in more than one metal, and there is a desire to form purified metal rather than alloys, the precipitates can be cyclonically separated so the separated carbonates can be individually calcined and thermally reduced. This is shown in Figure 10.

[0113] Although the foregoing invention has been described in detail for purposes of clarity of understanding, it is readily apparent to those of ordinary skill in the art in light of the teachings of this invention that certain changes and modifications may be made thereto without departing from the scope of the appended claims.

Claims

CLAIMS:

1. A process for use with a metal solubilised in a liquid, the process comprising: dissolving carbon dioxide into the liquid to form a carbonate enriched solution; and raising the pH of the carbonate enriched solution to precipitate the metal as metal carbonate precipitates.

2. The process of claim 1, wherein the carbon dioxide is added into the solution in a substantially bubble-less manner.

3. The process of claim 1, wherein the metal is at least one of calcium, strontium, and barium.

4. The process of claim 1, wherein a gas-dissolution unit is used to add the carbon dioxide into the solution.

5. The process of claim 1, wherein the gas-dissolution unit comprises a membrane.

6. The process of claim 5, wherein the membrane is a polymer hollow fiber contactor.

7. The process of claim 5, wherein the membrane is a ceramic membrane contactor.

8. The process of claim 4, wherein the gas-dissolution unit comprises a packed or plated tower.

9. The process of claim 1, wherein the carbonate enriched solution is saturated or supersaturated with carbon dioxide.

10. The process of claim 9, wherein the applied partial pressure of the carbon dioxide, which is in gas form, is at least as high as the partial pressure of the carbon dioxide which is dissolved.

11. The process of claim 10, wherein the applied partial pressure of the carbon dioxide gas is above atmospheric pressure.

12. The process of claim 6, wherein the temperature of the liquid is from about 5 to about 50 degrees Celsius.

13. The process of claim 1, wherein the liquid is neutral or acidic.

14. The process of claim 1, wherein the liquid comprises an absorption agent.

15. The process of claim 1, wherein the liquid is a liquid that has been filtered to remove suspended solids and organics.

16. The process of claim 1, wherein a base is added to raise the pH.

17. The process of claim 1, wherein the pH is raised to 10 or higher.

18. The process of claim 16, wherein the base is at least one of NaOH, LiOH, or Mg(OH)2.

19. The process of claim 1, further comprising a step of removing the metal carbonate precipitates.

20. The process of claim 1, further comprising the step of adding acid to lower the pH following precipitation of the metal.

21. The process of claim 1, wherein the liquid is produced water from an oil or gas wellhead and the process is performed prior to re-injection.

22. The process of claim 1, wherein the liquid is feed water to a reverse osmosis plant.

23. A method for producing metal from a metal rich brine, the method comprising: dissolving carbon dioxide into the brine to form a carbonate enriched solution; raising the pH of the carbonate enriched solution to precipitate the metal as metal carbonate precipitate; calcining the precipitate to produce metal oxide; and reducing the oxide via a thermal process, such as the Pidgeon process, or through a Solid Oxide Membrane.

24. The method of claim 23, further comprising the step of separating the precipitates using a hydrocyclone prior to calcining.

25. The method of claim 23, wherein the metal is selected from magnesium and calcium.

26. The process of claim 1, wherein the liquid is aqueous.

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