WO2024003575A1

WO2024003575A1 - Process for sequestration of carbon dioxide and minerals from industrial waste products

Info

Publication number: WO2024003575A1
Application number: PCT/GB2023/051727
Authority: WO
Inventors: Mia BAISE; Nikolas Anton Amadeus ZWANEVELD; Glenn Robert LEIGHTON
Original assignee: Hydrophis Gas Ltd
Priority date: 2022-06-30
Filing date: 2023-06-30
Publication date: 2024-01-04

Abstract

The present invention provides a process for producing calcium carbonate and optionally magnesium hydroxide, using an aqueous brine solution comprising the steps of: precipitating calcium carbonate and optionally magnesium hydroxide; and removing calcium carbonate and optionally magnesium hydroxide. Optional pre-treatment steps comprise increasing the pH of the aqueous brine solution; concentrating the alkaline aqueous brine solution by nanofiltration; and precipitation and removal of magnesium and/or calcium carbonate.

Description

PROCESS FOR SEQUESTRATION OF CARBON DIOXIDE AND MINERALS FROM INDUSTRIAL WASTE PRODUCTS

FIELD OF THE INVENTION

This invention relates to a process for sequestering industrially produced carbon dioxide and minerals from industrial waste products. In particular, dissolved carbon dioxide (mostly present as bicarbonate) and minerals may be extracted from waste brine produced by desalination plants and converted into industrially useful products. In addition, the process can operate in a way which incorporates carbon dioxide emitted from other industrial sources into industrially useful products.

BACKGROUND OF THE INVENTION

Carbon sequestration refers to the process of capturing and storing atmospheric or hydrospheric carbon dioxide which can be achieved through various chemical and biological processes, but also occurs naturally over long timescales. Industrial waste, such as desalination brine, contains reasonable quantities of metal ions. These ions also form carbonate minerals found in the Earth’s crust which are typically mined and processed at high temperatures and pressures into useful chemicals/compounds for industrial use. The cations within industrial waste sources have the capacity to sequester large quantities of carbon dioxide (as bicarbonates or carbonates) under ambient conditions and over comparatively short timescales, if the thermochemical properties (e.g. alkalinity, pH, temperature, and pressure) are controlled in situ to reduce the solubility of the target mineral.

Some authors have investigated the potential for the application of chemical precipitation techniques to desalination brine to mineralise carbon dioxide variously as magnesium-, sodium-, or calcium-based carbonate minerals. An overview is provided in Sharkh et al.¹ and some specific publications are discussed below.

Ferrini et al. showed that bubbling carbon dioxide through magnesium chloride found in brine, in the presence of ammonia, precipitated magnesium carbonate as nesquehonite, and produced ammonium chloride as a by-product. The suitable range of pH for the formation of nesquehonite in the experimental conditions was obtained by adding ammonia as a weak base.²

¹ https://doi.org/10.1038/s41545-022-00153-6

² https://doi.Org/10.1016/j.jhazmat.2009.02.103 MgCh + C0₂ + H₂0 + 2NH₃ MgCO₃ + 2NH₄C1

Estefan et al. suggested a method which uses an alkali source (such as sodium carbonate, ammonium hydroxide, or sodium hydroxide) to enable the precipitation of Mg- bearing phases from seawater/brine.³ This is used to isolate the magnesium base solvent (lime or brucite depending on the requisite pH swing) which has the potential for long-term sequestration of carbon dioxide in the form of stable carbonates, such as magnesite.

Mg(OH)₂ + CO₂ MgCO₃ + H₂O

W02022030529A1 (Waseda University) discloses a process based on nanofiltration of seawater or brackish water followed by chemical precipitation of alkali earth metal carbonates (mainly but not limited to MgCO₃ ) and evaporative crystallisation to produce NaCl.⁴ The nanofiltration step separates the monovalent and divalent ions to produce a concentrated sodium chloride solution (as permeate) and a concentrated alkali earth metal stream (as concentrate). The permeate can then be crystalised as NaCl and a subsequent electrodialysis step can produce NaOH. This is suggested as a suitable alkaline source for chemical precipitation of carbon dioxide and NF concentrate to form alkali earth metal carbonates.

US20160200587A1 (Industry Foundation of Chonnam National University) discloses a process based on pH assisted precipitation of magnesium from diluted salt liquor/brine using carbonate and hydroxide solutions, more specifically, Na₃CO₃ and NaOH solutions. This produces hydrated magnesium carbonate and suggests roasting at high temperature to manufacture MgO for economic benefit.⁵

Several other authors have refined this process to improve the efficiency of manufacture of Mg-based cement materials (binders) with appropriate properties such as high tensile strength.⁶ During these processes, the alkali source used to enable the precipitation of the precursor, brucite or lime, determines the overall environmental impact of the production process.

Benyahia et al. proposed a process that reacts carbon dioxide with sodium chloride found in brine, also in the presence of ammonia, to produce and sequester carbon dioxide as

³ https://doi.org/10.1016/0032-5910(80)85028-5

⁴ https://patents.google.com/patent/JP2005097072A/en

⁵ https://patents.google.com/patent/US20160200587Al/en

⁶ https://doi.Org/10.1016/j.jcou.2020.101383 sodium bicarbonate. In this case, the by-product of ammonium chloride is regenerated by lime to produce calcium chloride and release ammonia which is reutilised in the process.⁷

Bang et al. investigated a process which uses the alkaline chemical sodium hydroxide (sodium carbonate, ammonium hydroxide, or potassium hydroxide could also be utilised) to create a suspended solution of calcium hydroxide which can be used to mineralise (scrub) carbon dioxide in the form of calcium carbonate.⁸

A variation on this method suggested by Drioli et al. added successive precipitations after the carbon dioxide scrubbing step.⁹ Specifically, this includes precipitation of halite and epsomite via a membrane crystalliser, and the epsomite precipitates in preference to gypsum due to there being minimal calcium left in the retentate post the carbon dioxide scrubbing step.

CN113045060A (China ENFI Engineering Corp) disclose a compressive method of seawater mining, which comprises the following steps: filtering raw seawater to obtain inorganic seawater and organic matters serving as organic fertilizer raw materials; carrying out uranium ion exchange adsorption treatment on the inorganic seawater to obtain uranium elements and uranium-removed seawater; carrying out nanofiltration membrane treatment on the uranium removed seawater to obtain monovalent saline solution and divalent saline solution; respectively carrying out carbonization treatment on a divalent saline solution and carrying out seawater desalination membrane filtration concentration treatment on a monovalent saline solution; the concentrated brine is subjected to electrolytic hydrogen production treatment, and hydrogen generated by an electrolytic cathode, mixed oxidizing gas generated by an anode, high-alkalinity sodium potassium brine converted from an electrolytic solution and the like are separately recovered.¹⁰

⁷ https://patents.google.com/patent/US9475000B2/en

⁸ https://doi.org/10.3390/min7110207

⁹ https://doi.Org/10.1016/j.memsci.2003.09.028

¹⁰ https://patents.google.com/patent/CNl 13045060A/en WO20 18/011567 (University of Aberdeen) discloses a process based on pH assisted precipitation from brine using two sequential carbon dioxide scrubbing steps, using an alkaline reagent, to precipitate calcium carbonate followed by magnesium carbonate.¹¹

WO2019036676A1 (The Regents Of The University Of California) discloses a process using brine to produce alkaline earth metal hydroxides (Ca(OH)2 and Mg(0H)2) and carbonates (CaCO₃ and MgCO₃).¹² For the latter, authors suggest a pre-treatment stage to enrich the divalent ions via membrane filtration or capacitive concentration before mineral carbonation to reduce alkaline reagent concentration. Furthermore, the alkali is suggested to be, but not limited to, an industrial alkaline waste stream.

CN113880344A (Institute of Seawater Desalination and Multipurpose Utilization) outlines a process for carbonation of calcium-based brines.¹³ Calcium is separated and purified through brine nanofiltration and reverse osmosis producing a concentrate containing calcium and a permeate containing sodium and chloride. Caustic soda is prepared from the sodium chloride stream before carbon mineralisation of the high calcium brine with carbon dioxide.

KR101672224B1 (Korea Institute of Geoscience and Mineral Resources) discloses a seawater desalination method of producing metal carbonates. It describes a two-step process which separates metal ions from concentrated brine using membrane technology and reacts the separated metal ions with carbon dioxide to produce carbonate salts (preferably Li2CO3, MgCO₃, or CaCO₃).¹⁴

Molinari et al. conducted a theoretical and experimental study focussing on selective calcium precipitation from desal brine whilst minimising magnesium impurities. They test sodium citrate, sodium hydrogen carbonate, and sodium carbonate for calcium removal and focus in detail on the ideal pH, temperature, and reagent dose for optimum purity of target calcium-based compound.¹⁵

Electrochemical methods to precipitate carbonate-based minerals from brine have also been investigated. House et al. identified that the chloralkali process could be used to render sodium chloride solution alkaline for addition to the oceans.¹⁶ The chloralkali process

¹¹ https://patents.google.com/patent/W02018011567Al/en

¹² https://patents.google.com/patent/WO2019036676Al/en

¹³ https://patents.google.com/patent/CNl 13880344A/en

¹⁴ https://patents.google.com/patent/KR101672224Bl/en

¹⁵ https://doi.Org/10.1016/j.jclepro.2021.129645

¹⁶ https://doi.org/10.1021/es0701816 requires electrical energy, some of which can be recovered by reacting the evolved hydrogen and chlorine in a fuel cell. The process is known as electrolysis.

Anode: 2C1' + Ch + 2e~

Cathode: 2H₂O + 2e“ H₂ + 2OH“

Ion-permeable membrane: 2NaCl + 2H₂O — Cl₂ + H₂ + 2NaOH

In principle, the resulting caustic soda can react with the calcium and magnesium in sea water to sequester carbon dioxide while the chlorine, can be reacted with silicate rocks in such a way as to mimic accelerated weathering of rocks by carbonic acid, but these effects have not yet been proven experimentally.

Recently, advanced electrochemical methods have been developed to improve seawater brine electrolysis. Fernandez-Gonzalez et al. used a bipolar membrane to decompose seawater brine into acids and bases, with the emphasis on the economic value of these products.¹⁷ Xie et al. introduced a new way of decomposing magnesium chloride by an electrolysis process incorporating a gas diffusion anode (GDA), yielding magnesium hydroxide and hydrochloric acid.¹⁸ These processes are intended to be used for carbon sequestration from flue gases. These studies mainly focused on the electrochemical methods for isolating the solvents from brine. The electrolysis-GDA method is particularly appealing in terms of its potentially low energy consumption and simpler configuration of the electrolysis cells, key for keeping both capital and operational costs low for large scale processing.

The Heimdal Group is looking to mineralise carbon dioxide at large scale using desalination brine as a solvent.^{19, 20} The process will use electricity from renewable energy (not alkaline reagents) to manufacture carbon-negative industrial materials, including limestone for making concrete. The seawater is alkalinized via electrolysis/electrodialysis/electrodialysis with bipolar membranes which acts to increase the pH which results in isolation of gaseous hydrogen, chlorine, and a hydroxide sorbent. This is mixed with a separate stream of seawater, leading to the precipitation of calcium, magnesium, and sodium-based carbonate minerals and reducing the saturation of carbon

¹⁷ https://doi.org/10.1080/15422119.2015.1128951

¹⁸ https://doi.org/10.1007/sl l434-014-0388-l

¹⁹ https://techcrunch.eom/2021/08/30/heimdal-pulls-co2-and-cement-making-materials-out- of-seawater-using-renewable-energy/

²⁰ https://www.heimdalccu.com/ dioxide in the water — allows for the absorption of more from the atmosphere when it is returned to the sea.

Ion-permeable membrane: 2NaCl + 2H2O — Ch + H2 + 2NaOH

Another company from California, Ebb Carbon, have devised a similar process using electrochemistry to enhance the ocean’s capacity to capture carbon dioxide via mineralisation of dissolved carbon in desalination brine.²¹

The main limitation of these previous approaches is that mineralised carbon dioxide from complex mixtures of ions cannot be produced in high yield because the absorption of carbon dioxide is low, i.e. the mineralisation rate is slower, and more carbon dioxide must be added compared to an ideal solution. This is because they generate a supersaturated solution, which is thermodynamically metastable at best, and any such concentration represents a kinetic solubility rather than an equilibrium/thermodynamic solubility value intrinsic to metal carbonate. The present invention solves this problem by using additional steps prior to the carbon scrubbing step (such as that described by Bang et aL), comprising initial alkalization and nanofiltration steps to increase the concentration of target minerals with low energy requirement to improve chemical precipitation rate, yield and reagent consumption. This improves chemical precipitation rate, yield, and reagent consumption.

SUMMARY OF THE INVENTION

In a first embodiment, the present invention provides a process for producing calcium carbonate, and optionally magnesium carbonate and/or magnesium hydroxide, using an aqueous brine solution comprising the steps of: a. increasing the pH of the aqueous brine solution to a value above 7 but not more than 10, preferably using caustic soda; b. concentrating the alkaline aqueous brine solution in a system that comprises

²¹ https://www.ebbcarbon.com/ (i) a membrane which resists passage of divalent ions but allows passage of monovalent ions, or

(ii) a membrane which resists passage of monovalent ions but allows passage of divalent ions; c. (I) optionally precipitating calcium carbonate and/or magnesium carbonate by adding an alkalinity source, preferably sodium hydroxide, to the aqueous brine solution such that the pH of the solution is between 9 and 10, preferably from 9.5 to 9.9, and calcium carbonate and/or magnesium carbonate precipitates; c. (II) optionally precipitating magnesium hydroxide by adding an alkalinity source, preferably sodium hydroxide, to the aqueous brine solution such that the pH of the solution is between 10 and 12, preferably from 10.5 to 11.5, and magnesium hydroxide precipitates; c. (Ill) precipitating calcium carbonate by

(i) adding a soluble carbonate salt and an alkalinity source, preferably sodium hydroxide, to the retentate of step b(i) or the permeate of step b(ii) such that the pH of the solution is between 9 and 12, preferably between 9 and 10, and calcium carbonate precipitates, or

(ii) adding carbon dioxide gas and an alkalinity source, preferably sodium hydroxide, to the retentate of step b(i) or the permeate of step b(ii) such that the pH of the solution is between 9 and 12, preferably between 9 and 10, and calcium carbonate precipitates, d. (I) optionally removing calcium carbonate and/or magnesium carbonate produced in step c(i); d. (II) optionally removing magnesium hydroxide produced in step c(II); and d. (Ill) removing calcium carbonate produced in step c(III).

In a second embodiment, pre-treatment steps a and b are omitted, and the invention provides a process for producing calcium carbonate, and optionally magnesium carbonate and/or magnesium hydroxide, using an aqueous brine solution comprising the steps of: c. (I) optionally precipitating calcium carbonate and/or magnesium carbonate by adding an alkalinity source, preferably sodium hydroxide, to the aqueous brine solution such that the pH of the solution is between 9 and 10, preferably from 9.5 to 9.9, and calcium carbonate and/or magnesium carbonate precipitates; c. (II) optionally precipitating magnesium hydroxide by adding an alkalinity source to the aqueous brine solution such that the pH of the solution is between 10 and 12, preferably from 10.5 to 11.5, and magnesium hydroxide precipitates; c. (Ill) precipitating calcium carbonate by

(i) adding a soluble carbonate salt and an alkalinity source, preferably sodium hydroxide, to the aqueous brine solution such that the pH of the solution is between 9 and 12, preferably between 9 and 10 and calcium carbonate precipitates, or

(ii) adding carbon dioxide gas and an alkalinity source, preferably sodium hydroxide, to the aqueous brine solution such that the pH of the solution is between 9 and 12, preferably between 9 and 10 and calcium carbonate precipitates, and d. (I) optionally removing calcium carbonate and/or magnesium carbonate produced in step c(i); d. (II) optionally removing magnesium hydroxide produced in step c(II); and d. (Ill) removing calcium carbonate produced in step c(III).

In a third embodiment, pre-treatment steps a and b are omitted, and the invention provides a process for producing calcium carbonate and magnesium hydroxide, using an aqueous brine solution comprising the steps of: c. (II) precipitating magnesium hydroxide by adding an alkalinity source to the aqueous brine solution such that the pH of the solution is between 10 and 12, preferably from 10.5 to 11.5, and magnesium hydroxide precipitates; c. (Ill) precipitating calcium carbonate by adding carbon dioxide gas and an alkalinity source, preferably sodium hydroxide, to the aqueous brine solution such that the pH of the solution is between 9 and 12, preferably between 9 and 10 and calcium carbonate precipitates; d. (II) removing magnesium hydroxide produced in step c(II); and d. (Ill) removing calcium carbonate produced in step c(III).

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the invention will now be described in detail with reference to the accompanying drawings, in which:

Figure 1 shows a schematic representation of a preferred embodiment of the present invention. Figure 2 shows the results of ion rejection from a Dow NF270 membrane for nanofiltration from pH 7.8 - 9.25.

Figure 3 shows the flux performance of three nanofiltration membranes.

Figure 4 shows the ion rejection performance of three nanofiltration membranes.

Figure 5 shows the ion rejection performance of one of the three nanofiltration membranes under different conditions.

Figure 6 shows the ion rejection performance of another of the three nanofiltration membranes under different conditions.

Figure 7 shows photographs of precipitated solutions at different pHs.

Figure 8 shows the % mineral precipitation at different Ca:CO3 molar ratios.

Figure 9 shows the % mineral precipitation at different reagent concentrations.

Figure 10 shows a schematic representation of the apparatus used for the Examples.

Figure 11 shows a schematic representation of the apparatus used for the alkalization and nanofiltration step in the pilot plant.

Figure 12 shows a schematic representation of the apparatus used for the precipitation step in the pilot plant.

Figure 13 shows a schematic representation of the apparatus used for the filtration step in the pilot plant.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Step a: Alkalization

The first step in the process of the first embodiment of the present invention involves increasing the pH of the aqueous brine solution to a value above 7 but not more than 10.

Preferably, the pH increase in step a is achieved by addition of an aqueous alkali reagent, such as sodium hydroxide, potassium hydroxide, magnesium hydroxide, sodium carbonate or ammonia, or alternatively, an alkali waste or waste water from an industrial process such as fly ash or lime sludge. In an alternative method the hydroxide ions can be produced through electrical methods, such as, electrolysis of water. Preferably the aqueous alkali reagent is an alkali metal hydroxide, preferably sodium hydroxide, most preferably 10 % w/w sodium hydroxide.

It was found that care was required when increasing the pH, because if the pH in any localised part of the solution became too high then it caused precipitation. Thus, in a preferred embodiment, the alkalising agent is pre-mixed with a small quantity of the initial aqueous brine solution or other recycled process stream before being introduced into the main process stream. In an alternative preferred embodiment, the alkalising agent is pre-mixed with a small quantity of the solution which remains after step d of the process before being introduced into the main process stream. The alkalising source may also be a high pH waste stream from this process or another compatible industrial process.

It was also found that at pH up to 9.04 the calcium ions in the aqueous brine solution remained fully dissolved, but at 9.25 or above, there was a precipitation of the calcium (most likely as CaCO₃). 9.04

Thus, in a preferred embodiment, the pH of the aqueous brine solution is increased to a value above 7.5 but not more than 10, more preferably to a value from 7.5 to 9.5, still more preferably to a value from 8.0 to 9.0, most preferably to a value from 8.6 to 9.0. It was found that, up to pH 9.04, the calcium rejection in step b stayed relatively stable at 45-48%.

However, at pH 9.25 the rejection of calcium decreased to 28% (see Figure 2) and the mass balance showed that dissolved calcium had decreased by 23% to 77%. This indicates that there was significant precipitation of calcium at higher pH.

On the other hand, testing showed that rejection of magnesium steadily decreased with increasing pH (see Figure 2), with mass balance indicating that it was either precipitating from solution upon alkalization or being rejected more by the membrane. Without wishing to be bound by theory, the inventors believe this problem may be exacerbated by use of higher concentrations of the alkalising agent and could be mitigated by pre-mixing the alkalising agent with a small quantity of the initial aqueous brine solution (or the solution which remains after step d of the process) before being introduced into the main process stream, as previously described.

The monovalent cations sodium and potassium exhibited a high degree of permeation at all pH values tested (see Figure 2).

With respect to the anions, it was found that the membranes tested gave excellent sulphate rejection (99%) across all pH values tested (see Figure 2). Bicarbonate rejection increased from 52% to 64% with increasing pH and the carbonate rejection was highest at 89% at pH 9.04 (see Figure 2). This discovery of differential rejection of carbonate versus bicarbonate also supports the aim of increasing alkalinity so that more of the dissolved/solvated carbon dioxide is present in carbonate form because this will significantly increase its capture. Step b: Nanofiltration

The second step in the process of the first embodiment of the present invention involves filtering the alkaline aqueous brine solution through (i) a membrane which resists passage of divalent ions but allows passage of monovalent ions, or (ii) a membrane which resists passage of monovalent ions but allows passage of divalent ions. Option (i) is preferred. In such case, the membrane which resists passage of divalent ions but allows passage of monovalent ions is preferably a thin film composite (TFC) membrane. From a structural perspective, these membranes typically comprise a thin, permeable layer (normally polyamide) deposited on top of a thicker, porous support layer (normally polyethersulfone or polysulfone) with an optional layer of non-woven fabric which improves mechanical strength. Such membranes can be modified using different types of polymer supporting membranes and surface modification/functionalisation to improve membrane performance. From a functional perspective, such membranes are able to separate ions and small molecules, preferably with high permeation flux and low energy consumption. Such membranes may achieve a rejection of monovalent ions, such as sodium, potassium and chloride of between 5 and 75%; and a rejection of divalent ions, such as calcium and magnesium and sulphate of between 50% and 100%. The purpose of this nanofiltration step is to retain the divalent ions (specifically calcium, magnesium and carbonate) while minimising the operational pressure. The passage of the monovalent ions (specifically sodium and chloride which are the most abundant) ensures that the osmotic pressure across the membrane and thus pumping energy, is as low as possible. Preferably, the concentration of calcium ions (Ca²⁺) in the retentate from step b(i) or the permeate from step b(ii) is 800 mg/L or more.

During development of the present invention, the inventors directly compared high flux membranes with ion rejection parameters appropriate for a wide range of process applications under consistent conditions. Membranes were tested across the following three functional categories: (a) low driving pressure with medium to high salt passage and medium hardness removal, (b) low driving pressure with high salt passage and high hardness removal, and (c) medium driving pressure with a medium salt passage and high hardness removal. Examples of useful membranes falling into these three functional categories are: DOW NF270 (available from Dow Chemicals Co.), SUEZ SWSR-90 and SUEZ Duraslick NF2540 (both available from Suez Water Technologies). Other possibilities are: DOW NF90 and NF200, Hydronautics nano-SW, nano-SW MAX, and HYDRACoRe, SUEZ DK and DL series, NE8040-7-, Microdyne 8040-NF7-400, and TrisepXN45. (The experimental results showed that the Dow NF270 membrane had a higher flux (i.e. 20 litres per m² per hour (LMH) at 14 bar pressure) than the SUEZ membranes (8.8 and 10.9 LMH respectively at 14 bar pressure) (see Figure 3). This would mean that the SUEZ membranes would need to be operated at a higher pressure and/or more membranes would need to be installed to achieve the same level of water and salt production.

The salt rejection was consistent across the membranes (see Figure 4). Sulphate was rejected at approximately 99% for all membranes and conditions. Magnesium rejection was over 80% under most conditions and the calcium rejection was between 50 - 75%. The calcium rejection was impacted by the membrane choice with the Dow NF270 membrane only rejecting approximately 50% of the calcium but with the SUEZ SWSR membrane rejecting 75%. Overall ion rejection (including NaCl) was 19 - 24% which is the driver of the low osmotic pressure for this system.

Experimental work using DOW NF270 membranes showed that rejection of bicarbonate alkalinity was 52 - 64% and the maximum rejection of carbonate alkalinity was 89% (see Table 1 and Figure 2 below). Thus, increasing the alkalization pH increases the rejection of carbonate alkalinity by converting bicarbonate ions to carbonate ions. Specifically, it was found that for this membrane the recovery of carbonate alkalinity at pH 7.8 (when the majority of alkalinity is in bicarbonate form) was 52% and at pH 9.04 (when the majority of alkalinity is in carbonate form) was 79%. Above pH 9, there was precipitation of calcium carbonate that would foul the membranes.

The analytical results showed similar trends for the other membranes tested with high sulphate rejection, and lower calcium and magnesium rejection (see Figures 5 and 6). The rejection of ions decreased as the recovery increased and the concentration increased. The carbonate rejection was approximately 80% whereas the bicarbonate rejection was -50%. The only significant difference here is that both SUEZ membranes had a slightly higher rejection for magnesium and calcium.

Based on these results, water flux using a range of nanofiltration membranes was effective up to a 50% water recovery without fouling or scaling for all three membranes. At pH 7.8 the rejection of alkalinity (i.e. combined bicarbonate and carbonate anions) was low and increasing the pH up to 9 increased recoveries of alkalinity (and thus the effectiveness of the program for carbon capture and storage). The DOW NF270 membrane had approximately double the flux compared to the SUEZ membranes but a slightly lower recovery of calcium and magnesium. Sulphate rejection was high at 99%+ for all membranes. Increasing the pH of the feed solution above 9 resulted in precipitation of calcium carbonate and is not recommended.

Step c(I): Magnesium and/or calcium carbonate precipitation

An optional third step in the process of the first embodiment of the present invention, or optional first step in the process of the second embodiment of the present invention, is the precipitation of magnesium and/or calcium carbonate which involves the reaction of magnesium and calcium ions with carbonate ions which are already present in the aqueous brine solution.

Mg²⁺ + CO₃ ²’ MgCO₃

Ca²⁺ + CO₃ ²’ CaCO₃

This step is achieved by adding an alkalinity source, preferably sodium hydroxide, to the aqueous brine solution such that the pH of the solution is between 9 and 10, preferably from 9.5 to 9.9, and calcium carbonate and/or magnesium carbonate precipitates.

Step c(II): Brucite precipitation

An optional third (or fourth) step in the process of the first embodiment of the present invention, or optional first (or second) step in the process of the second embodiment of the present invention, is the precipitation of magnesium hydroxide (brucite) which involves the reaction of magnesium ions with hydroxide ions.

Precipitation of magnesium hydroxide is achieved by adding an alkalinity source, preferably sodium hydroxide, to the aqueous brine solution such that the pH of the solution is between 10 and 12, preferably from 10.5 to 11.5, and magnesium hydroxide precipitates. Preferably, the amount of magnesium remaining in the aqueous brine solution after step c(II) is less than 200 ppm.

Step c(III): Calcite precipitation

The third (or fourth or fifth) step in the process of the present invention is the precipitation of calcium carbonate (calcite) which involves the reaction of calcium ions with carbonate ions.

Ca²⁺ + CO₃ ²’ CaCO₃ It must be noted that the concentration of carbonate ions is also a function of pH with higher pH conditions converting bicarbonate ions into carbonate ions until a pH 10.25 at which point most of the alkalinity is in carbonate form.

In the experimental work which led to the present invention, the inventors used sodium carbonate (Na2CO₃) as a source of carbonate. However, in an alternative embodiment, the carbonate can be generated by the reaction of CO2 (e.g. exhaust gas from industry) with the alkaline solution and thus form a carbon capture and storage system (CCS). In another alternative embodiment, the carbonate can be sourced from another gaseous, solid, aqueous, or liquid source including potassium carbonate, magnesium carbonate, or industrial waste sources such as flue gas.

Initial experiments indicated that calcite precipitation could be optimised by adjustment of the pH (see Figure 7). Thus, step c(III). (i) or c(III).(ii) also involves adjusting the pH of the aqueous brine solution to a value above 9 but not more than 12. More preferably, the pH is adjusted to a value above 9 but not more than 10, Most preferably, the pH is adjusted to a value of about 10. Preferably, sodium hydroxide is used as the alkalinity source.

Further experiments indicated that the pH for calcite precipitation is preferably in the range from 9.5 to 10.

Initial experiments also indicated that calcite precipitation could be optimised by adjustment of the Ca:CO₃ molar ratio (see Figure 8). Thus, in a preferred embodiment, step c(III). (i) or c(III).(ii) involves adding a soluble carbonate salt, or carbon dioxide, in an amount sufficient to provide a Ca:CO₃ molar ratio in the range from 0.5 to 2.5 (i.e. 1 :2 to 5:2). More preferably, the soluble carbonate salt, or carbon dioxide, is added in amounts sufficient to provide a Ca:CO₃ molar ratio in the range from 0.5 to 2 (i.e. 1 :2 to 2: 1). Even more preferably, the soluble carbonate salt, or carbon dioxide, is added in amounts sufficient to provide a Ca:CO₃ molar ratio in the range from 0.5 to 1.5 (i.e. 1 :2 to 3:2). Most preferably, the soluble carbonate salt, or carbon dioxide, is added in amounts sufficient to provide a Ca:CO₃ molar ratio of about 1 (i.e. about 1 : 1).

Initial experiments also indicated that calcite purity could be improved by adjusting the reagent concentrations when using step c(III).(i). This helped to reduce the amount of brucite (Mg(OH)2) produced. Such conditions were found to increase the calcite purity up to 63% and importantly reduce the amount of sodium chloride in the solids (see Figure 9). Thus, in a preferred embodiment, step c(III).(i) involves adding a soluble carbonate salt solution having a concentration in the range from 0.5 to 10 % w/v. More preferably, step c(III).(i) involves adding a soluble carbonate salt solution having a concentration in the range from 0.5 to 2 % w/v. Most preferably, step c(III).(i) involves adding a soluble carbonate salt solution having a concentration of about 1 % w/v.

Further experiments indicated that when CO2 is used as the source of carbonate (i.e. step c(III).(ii) is used) it may involve bubbling CO2 through an aqueous solution to provide a carbonic acid solution with a pH of from 5.0 to 5.4, preferably about 5.2, which is then added to the aqueous brine solution. Alternatively, it may involve bubbling CO2 directly into the aqueous brine solution, at a flow rate between 0.2 and 1 mol/h, preferably, about 0.4 mol/h.

Steps c(III).(i) and c(III).(ii) involve adjusting the pH of the aqueous brine solution. This is preferably carried out by adding a hydroxide solution (e.g. sodium hydroxide) having a concentration in the range from 0.1 to 2.0 M. More preferably, this is carried out by adding a hydroxide solution (e.g. sodium hydroxide) having a concentration in the range from 0.1 to 1.5 M. Most preferably, this is carried out by adding a hydroxide solution (e.g. sodium hydroxide) having a concentration of about 0.15 M.

Initial experiments also indicated that calcite purity could be improved by seeded precipitation. Thus, in a preferred embodiment, prior to or during step c(III), calcium carbonate is added to the retentate of step b(i) or the permeate of step b(ii) (or to the aqueous brine solution if steps a and b are omitted) in an amount sufficient to promote precipitation of calcium carbonate. The seed can be either a ground calcium carbonate product added separately, or a precipitated calcium carbonate either isolated or recirculated from this process. Preferably, seed calcite is added in an amount from 0.1 to 20 % by mass based on the theoretical yield of calcium carbonate, more preferably from 10 to 20 % and most preferably about 15 %. Use of about 15 % seed calcite resulted in residual calcium levels which were low (around the 30-100 mg/L range) meaning that most of the calcium precipitated out as a solid. The yield, purity, and filterability were all good and improved relative to the unseeded experiments.

Combining the preferred embodiments described above enabled calcite precipitation with 80-90% recovery and 50-80% purity based on calcium percentage of cations (see Figure 10). Step d(I): Magnesium and/or calcium carbonate removal

When step c(I) is carried out, a final step in the process of the present invention is the removal of magnesium and/or calcium carbonate from the system. This step may be carried out by any suitable method for removing solids from liquids.

Step d(II): Brucite removal

When step c(I) is carried out, a final step in the process of the present invention is the removal of magnesium hydroxide from the system. This step may be carried out by any suitable method for removing solids from liquids. To improve filterability, a flocculant may be used. A preferred flocculant is AN913 which is commercially available from the company SNF.

Step d(III): Calcite removal

The final step in the process of the present invention is the removal of calcium carbonate from the system. This step may be carried out by any suitable method for removing solids from liquids. Preferably, the concentration of bicarbonate (HCO3 ) ions in the aqueous solution remaining after step d is 200 mg/L or less and/or the concentration of carbonate (CO3² ) ions in the aqueous solution remaining after step d is 200 mg/L or less. In one embodiment of the process, steps c and d are repeated until the concentrations of bicarbonate (HCO3 ) and carbonate (COv-) ions in the aqueous solution remaining after step d are reduced to the desired levels.

Filterability was found to be an issue due to particle size and wetness of product. To improve this, a flocculant may be used. A preferred flocculant is AN913 which is commercially available from the company SNF.

EXAMPLES

Step a: Alkalization

Method and Materials

Feed water was desalination brine sourced from the Adelaide Desalination Plant (operated and maintained by Adelaide Aqua Pty Ltd which is a subsidiary of Acciona in South Australia). Sodium hydroxide was 99% purity AR grade mini-pearls and sourced from Chem-supply Australia and was made up into a 1.5M solution by weighing the appropriate mass of solid and then adding tap water and agitating until dissolved. Experimental

1. The Feed tank (Tank 1) was filled with 100L desalination brine.

2. The pH was adjusted by adding 1 ,5M NaOH solution with agitation until the target pH was achieved.

Step b: Nanofiltration - impact of pH on processing (Figure 2)

Method and Materials

The membrane used in this study was a NF270-2540 sourced from Dow-Dupont. Feed water was desalination brine sourced from the Adelaide Desalination Plant. Sodium carbonate was 99.2% purity LR grade anhydrous sodium carbonate sourced from Chem- supply Australia, sodium hydroxide was 99% purity AR grade mini-pearls sourced from Chem-supply Australia. Sodium carbonate (1%) and sodium hydroxide (0.15M) solutions were prepared by weighing the appropriate mass of solid and then adding tap water and agitating until dissolved. AS26 antiscalant was sourced from Genesys. The water was processed in a specially produced membrane processing rig comprising a 2540 300 psi pressure vessel (Wave Cyber) and a 23-bar pump (Grundfos CREI 1-27) and two 100L tanks. Online monitoring of flow rates and conductivity was collected using a Labjack data monitoring system. A diagram of the test apparatus can be seen in Figure 10 below: Experimental

1. A Dow NF270-2540 membrane was installed in the pressure vessel.

2. The Feed tank (Tank 1) was filled with 20L desalination brine and 5ppm of AS26 antiscalant was added.

3. The membrane pump was started, and the feed pressure increased to 14 bar in 2 bar increments.

4. The membrane was allowed to stabilize for 60 minutes and a sample of the feed tank and permeate tank were taken for analysis.

5. Initial pH was measured at 7.7.

6. A sample of 500ml of the feed water was removed from Tank 1 and 10 ml of 1.5M NaOH added before the water was returned to the feed tank with stirring and the resulting pH was measured as 8.11. The membrane was allowed to stabilize for 60 minutes and a sample of the feed tank and permeate tank were taken for analysis.

7. A sample of 500ml of the feed water was removed from Tank 1 and 35 ml of 1.5M NaOH added before the water was returned to the feed tank with stirring and the resulting pH was measured as 9.04. The membrane was allowed to stabilize for 60 minutes and a sample of the feed tank and permeate tank were taken for analysis.

8. A sample of 500ml of the feed water was removed from Tank 1 and 82 ml of 1.5M NaOH added before the water was returned to the feed tank with stirring and the resulting pH was measured as 9.25. The membrane was allowed to stabilize for 60 minutes and a sample of the feed tank and permeate tank were taken for analysis.

9. Addition of more NaOH resulted in visual precipitation and the experiment halted. It was found that care was required when adding the NaOH, as if added too concentrated then it caused precipitation. Thus, the NaOH was mixed with an aliquot of brine before being re-introduced to the feed tank.

Results

The results of the experiment can be seen in the tables below as well as Figure 2.

Ion Rejection pH pH 7.8 pH 9.04 pH 9.25

Bicarbonate 52% 64%

Carbonate 89% 44%

Total Alkalinity 52% 79% 47%

Calcium, Ca 48% 45% 28%

Magnesium, Mg 77% 70% 62%

Potassium, K 9% 10% 9%

Sodium, Na 5% 10% 5%

Sulfur, S 99% 99% 99%

Feed Compostion

Feed Total Alkalinity 250 360 83

Feed Ca 880 870 690

Feed Mg 2800 2700 2600

Feed Sulfate 2200 2100 2100

Mass Balance

Total Alkalinity 103% 141% 32%

Ca 98% 97% 77%

Mg 97% 93% 90%

Sulfate 104% 99% 99%

Table 1 : Ion rejection, feed composition, and mass balance using Dow NF270 membrane for NF between pH 7.8 - 9.25. Step b: Nanofiltration - Comparison of different membranes (Figure 3, 4)

Method and Materials

The membranes used in this study were a NF270-2540 sourced from Dow-Dupont and Duraslick NF2540 and SWSR2540 sourced from SUEZ. Feed water was desalination brine sourced from the Adelaide Desalination Plant. Sodium carbonate was 99.2% purity LR grade anhydrous sodium carbonate sourced from Chem-supply Australia, sodium hydroxide was 99% purity AR grade mini-pearls sourced from Chem-supply Australia. Sodium carbonate (1%) and sodium hydroxide (0.15M) solutions were prepared by weighing the appropriate mass of solid and then adding tap water and agitating until dissolved. AS26 antiscalant was sourced from Genesys. The water was processed in a specially produced membrane processing rig comprising a 2540 300 psi pressure vessel (Wave Cyber) and a 23- bar pump (Grundfos CREI 1-27) and two 100L tanks. Online monitoring of flow rates and conductivity was collected using a Labjack data monitoring system. Experimental

1. A SUEZ SWSR2540 membrane was installed in the pressure vessel.

2. The Feed tank (Tank 1) was filled with 25L desalination brine and 5ppm of AS26 antiscalant was added.

4. The membrane was allowed to stabilize for 60 minutes and a sample of the feed tank and permeate tank were taken for analysis. Initial pH was measured at 7.69

5. The system was halted, and the membrane changed to a SUEZ Duraslick NF2540 membrane. The pump was restarted, and the feed pressure increased to 14 bar in 2 bar increments. The membrane was allowed to stabilize for 60 minutes and a sample of the feed tank and permeate tank were taken for analysis.

6. The system was halted, and the membrane changed to a SUEZ SWSR240 membrane. The pump was restarted, and the feed pressure increased to 14 bar in 2 bar increments. The membrane was allowed to stabilize for 60 minutes and a sample of the feed tank and permeate tank were taken for analysis.

Results

The results of the experiment can be seen in the tables below as well as Figures 3 and

4

Table 2: Ion rejection parameters and flux for the different membranes.

Step b: Nanofiltration - Duraslick NF2540 Ion rejection (Figure 5)

Method and Materials

The membrane used in this study was Duraslick NF2540 sourced from SUEZ. Feed water was desalination brine sourced from the Adelaide Desalination Plant. Sodium carbonate was 99.2% purity LR grade anhydrous sodium carbonate sourced from Chem- supply Australia, sodium hydroxide was 99% purity AR grade mini-pearls sourced from Chem-supply Australia. Sodium carbonate (1%) and sodium hydroxide (0.15M) solutions were prepared by weighing the appropriate mass of solid and then adding tap water and agitating until dissolved. AS26 antiscalant was sourced from Genesys. The water was processed in a specially produced membrane processing rig comprising a 2540 300 psi pressure vessel (Wave Cyber) and a 23-bar pump (Grundfos CREI 1-27) and two 100L tanks. Online monitoring of flow rates and conductivity was collected using a Labjack data monitoring system.

Experimental

1. A Duraslick NF2540 membrane was installed in the pressure vessel.

2. The Feed tank (Tank 1) was filled with 50L desalination brine and 5ppm of AS26 antiscalant was added.

4. The membrane was allowed to stabilize for 60 minutes and a sample of the feed tank and permeate tank were taken for analysis. Initial pH was measured at 7.58

5. 1.5M NaOH was added with stirring until the pH was 9.06. The membrane was allowed to stabilize for 60 minutes and a sample of the feed tank and permeate tank were taken for analysis.

6. The feed pressure was increased to 16 bar and the permeate was directed to the permeate tank (Tank 2) and removed until the feed water tank level had reached 25L (50% recovery) and then the permeate was set to return to the feed tank (Tank 1), so the system was operating on full recirculation mode.

7. The membrane was allowed to stabilize for 60 minutes and a sample of the feed tank and permeate tank were taken for analysis.

Results

The results of the experiment can be seen in the table below as well as Figure 5

Step b: Nanofiltration - SWSR 2540 Ion rejection (Figure 6)

Method and Materials

The membrane used in this study was SWSR 2540 sourced from SUEZ. Feed water was desalination brine sourced from the Adelaide Desalination Plant. Sodium carbonate was 99.2% purity LR grade anhydrous sodium carbonate sourced from Chem-supply Australia, sodium hydroxide was 99% purity AR grade mini-pearls sourced from Chem-supply Australia. Sodium carbonate (1%) and sodium hydroxide (0.15M) solutions were prepared by weighing the appropriate mass of solid and then adding tap water and agitating until dissolved. AS26 antiscalant was sourced from Genesys. The water was processed in a specially produced membrane processing rig comprising a 2540 300 psi pressure vessel (Wave Cyber) and a 23-bar pump (Grundfos CREI 1-27) and two 100L tanks. Online monitoring of flow rates and conductivity was collected using a Labjack data monitoring system.

Experimental

1. A SWSR2540 membrane was installed in the pressure vessel. 2. The Feed tank (Tank 1) was filled with 50L desalination brine and 5ppm of antiscalant was added.

4. The membrane was allowed to stabilize for 60 minutes and a sample of the feed tank and permeate tank were taken for analysis. Initial pH was measured at 7.78

5. 1.5M NaOH was added with stirring until the pH was 9.26. The membrane was allowed to stabilize for 60 minutes and a sample of the feed tank and permeate tank were taken for analysis.

6. The permeate tank (Tank 2) and removed until the feed water tank level had reached 25L (50% recovery) and then the permeate was set to return to the feed tank (Tank 1), so the system was operating on full recirculation mode.

Results

The results of the experiment can be seen in the table below as well as Figure 6

Step c(III)(i): Calcite precipitation (Figures 7-9)

Method and Materials

Feed water was desalination brine sourced from the Adelaide Desalination Plant.

Sodium carbonate was 99.2% purity LR grade anhydrous sodium carbonate sourced from

Chem-supply Australia, sodium hydroxide was 99% purity AR grade mini-pearls sourced from Chem-supply Australia. 70% Nitric acid was sourced from RCI-Labscan. Sodium carbonate and sodium hydroxide solutions were prepared by weighing the appropriate mass of solid and then adding tap water and agitating until dissolved.

Experimental

1. 250ml of desalination brine was placed in a 500ml beaker and stirring was commenced using a magnetic stirrer bar. pH was monitored using a handheld pH meter (Milwaukee Instruments)

2. The required amount of 10% or 1% sodium carbonate solution was added dropwise using over a 15 minute period.

3. Sodium hydroxide solution of a 0.15, 0.5 or 1.5M concentration was added dropwise over a 15 minute period until the resulting pH was 10

4. The solution was allowed to stabilize for 60 minutes.

5. The solution was filtered through #5 Whatman filter paper using a Buchner funnel and the filtrate sent for analysis by a commercial lab using alkalinity, ion chromatography and ICP-MS and ICP-OES.

6. The precipitated residue was dried in an oven at 105C overnight and then weighed to determine the mass yield.

7. Approximately lOOmg of the dried precipitate was sampled, dissolved in 5% nitric acid, and analysed using ICP-MS and ICP-OES

Results

The results of the experiment can be seen in the tables below as well as Figures 7-9

Pilot plant:

Apparatus

An exemplary process according to the embodiments outlined above was also conducted in a pilot plant to investigate its suitability for medium to large scale sequestration of carbon dioxide and minerals from industrial waste products. The apparatus utilised is described below.

Apparatus 1- Alkalization and nano filtration steps

Reference is made to Figure 11, which is a schematic representation for the apparatus utilised for alkalization (step a) and nanofiltration (step b).

Step a:

Brine (1) (e.g. 6000-9000 L/day) is fed into day tank (5) (such as a Bushmans Im³ HDPE Feed water tank, fitted with an IBC mixer and pH monitor). The pH of the brine is then adjusted to within the range of 7 to 10 by feeding in an alkalization agent (e.g. 10% NaOH solution) from alkali tank (2) (e.g. an IBC lOOOL HDPE tank). The flow rate of the alkalization agent is controlled by dosing pump (4) (e.g. Blackstone 316SS, model BL7016) in combination with controller (3) (e.g. Hanna instruments, model B79-16).

Step b:

The alkalized brine from day tank (5) is fed through nanofiltration unit (6), comprising a feed pump, which controls the flow rate and pressure (for example, NF membrane total flow rate of 20 L/h, retentate and permeate flow rate of 10 L/h, feed pressure of -680 kPa, and retentate pressure of 650 kPa), and a nanofiltration membrane unit (e.g. NF270-2540). The nanofiltration membrane is selected to either:

(i) resist the passage of divalent ions but allow the passage of monovalent ions, and therefore produce a permeate (8) comprising monovalent ions, and a retentate (7) comprising divalent ions; or

(ii) resist the passage of monovalent ions and allow the passage of divalent ions, and therefore produce a permeate (8) comprising divalent ions, and a retentate (7) comprising monovalent ions.

The retentate comprising divalent ions from (i), or the permeate comprising divalent ions from (ii) continues to step (c) of the process.

Apparatus 2- Precipitation step(s)

Reference is made to Figure 12, which is a schematic representation for the apparatus utilised for the precipitation of calcium carbonate using carbon dioxide (Step c(III)(ii)).

Step c(III)(ii):

Carbon dioxide gas (10) is combined directly with concentrate (11) (either the retentate or permeate comprising divalent ions from the nanofiltration step (b)) in a carbon dioxide adsorption column (12) at a flow rate of about 0.41 mol/h to provide a carbonic acid solution with a pH in the range of 5.2-5.6. This is achieved in combination with CO2 pH monitor (13) and controller (14).

The resulting carbonated solution then flows to precipitation tank (15) (e.g. Rotamould 1500L HDPE tank), wherein it is combined with alkali source (9) (such as 50 M NaOH) to achieve a pH in the range of 9.3 to 10.3. The flow rates of the alkali source and the carbonated solution are maintained in the range of 10 to 11 L/h, which is remotely controlled with Controller (17) (e.g. Walchem WPHPW 120 HA-N), in combination with pH monitor (16) (e.g. CWC M-10-A-05M).

The resulting precipitate then overflowed into settling tank (19) (e.g. Rotamould 2400L HDPE tank, fitted with a Phathom TSS probe S20-SWW-880-PP-10-MB) and is treated with flocculant (e.g. AN913, concentration 0.025 g/L at a dose rate in the range of 2 to 4 L/h to promote phase separation of the calcium carbonate and the supernatant (20). After a settling time in the range of about 1 minute, the layers are separated, and the calcium carbonate precipitate slurry (21) continues to step d(III). The supernatant (20) can be recirculated back to the precipitation tank, or directed to waste.

Apparatus 3- Filtration step Reference is made to Figure 13, which is a schematic representation for the apparatus utilised for the filtration and drying of the hydrated calcium carbonate (Step d(III)).

Step d(III):

The wet calcium carbonate collects in filter feed tank (22) (e.g. Polymaster HDPE 4000 L tank), and subsequently passes to filter press (23) (e.g. MBA470 manufactured by Innovative Filtration Solutions Pty Ltd) fitted with air compressor (24) (e.g. IRONAIR 3HP 100L compressor), whereby the calcium carbonate is filtered and washed with tap water (-100L) at 2bar, before increasing the pressure to 7 bar to remove the excess water. The pressure is then maintained, and the calcium carbonate is rinsed and dried two further times (-200L) to provide a solid with a moisture content of 50-60%.

Example 1.

In a first example, the process according to the pilot plant apparatus outlined above was utilised for producing calcium carbonate, starting with desalination brine sourced from the Adelaide Desalination Plant (9227 L).

Composition of desalination brine

Step a

In accordance with the apparatus for step a. above, brine (9227 L) was treated with 10% NaOH to afford an alkalized brine with a pH of 9.

Composition of alkalized brine in day tank

Step b

In accordance with the apparatus for step b. above, alkalized brine was fed through the nanofiltration unit (fitted with DuPont's FilmTec™ NF270-4040 Wet Nanofiltration Elements for Commercial Systems membrane) at a rate of 20 L/min, with a feed pressure of 650 kPa and a NF recovery of 50% to give a permeate and retentate (concentrate).

Nano filtration permeate composition

Nanofiltration retentate (concentrate) composition

Step c(III)(ii)

In accordance with the apparatus for step c(III)(ii) above, the nanofiltration retentate (concentrate) was fed at a rate of 10 L/min through the carbon dioxide adsorption column with carbon dioxide being bubbled through at a constant rate of 0.41 mol/h. This dropped the pH to 5.2-5.6. The carbonated solution then flowed to the precipitation tank to which 50% NaOH was added until a pH of 9.7 was reached. This caused a precipitate to form which then overflowed into the settling tank with the addition of AN913 (0.25g/L).

The precipitate was left to settle from the supernatant, and then the precipitate slurry was transferred to the filter feed tank.

Step d(III)

In accordance with the apparatus for step d(III), the precipitate slurry was transferred from the filter feed tank into the filter press until a pressure above 3 bar was reached and held for 1 minute.

Accordingly, 30.76 kg of solid was obtained with a moisture content of 65%. ICP-MS analysis revealed that the solid was composed of 38% CaCCh, 53% Mg(0H)2 and 9% NaCl.

Therefore, it could be calculated that the yield of CaCCh obtained from the desalination brine was 32% (4.08 kg of a theoretical total of 12.94 kg).

Example 2.

In a second example, the process according to the pilot plant apparatus outlined above was utilised for sequentially producing magnesium hydroxide and calcium carbonate over a 7 day period, starting with desalination brine sourced from the Adelaide Desalination Plant (53,837 L in total was processed, 7,691 L/day (on average)).

Composition of desalination brine

Step a

In this example, no alkalization step was performed prior to nanofiltration. Step b

In accordance with the apparatus for step (b) above, desalination brine was fed through the nanofiltration unit (fitted with DuPont's FilmTec™ NF270-4040 Wet Nanofiltration Elements for Commercial Systems membrane) at a rate of 20 L/min, with a feed pressure of 680 kPa (days 1 to 5) and 780 kPa (days 6 and 7) and a NF recovery of 50% (all days) to give a permeate and retentate (concentrate). The following table highlights the parameters for this step.

Parameters for the Brine, Permeate, Concentrate and NF rejection.

Step c(I)

Using the apparatus for step c(III)(ii) above, the nanofiltration retentate (concentrate) was fed at a rate of 10 L/min into the precipitation tank (bypassing the carbon dioxide adsorption column) to which 50% NaOH was added until a pH of 9.5 to 9.8 was reached. This caused a precipitate to form which then overflowed into the settling tank with the addition of AN913 (0.25g/L).

The precipitate was left to settle from the supernatant, and then the precipitate slurry was transferred to the filter feed tank. Supernatant was sent to a storage tank prior to use in step c.(II) below. The following table highlights the parameters for this step.

Step (d)(1)

In accordance with the apparatus for step (d)(III), the precipitate slurry was transferred from the filter feed tank into the filter press until a pressure above 3 bar was reached and held for 1 minute. Filtrate was also sent to the storage tank prior to use in step c.(II) below.

Solids composition:

The above shows that the alkalinity decreased slightly with increased pH of precipitation, however, it only significantly decreased at 9.8. This was also accompanied by a significant decrease in Mg concentration. Step c(II)

Using the apparatus for step c(III)(ii) above, the stored supernatant from step c(I) and filtrate from step d(I) was fed at a rate of 10 L/min into the precipitation tank (bypassing the carbon dioxide adsorption column) to which 50% NaOH was added until a pH of 10.25 was reached. This caused a precipitate to form which then overflowed into the settling tank with the addition of AN913 (0.25g/L).

The precipitate was left to settle from the supernatant, and then the precipitate slurry was transferred to the filter feed tank. Supernatant was sent to a storage tank prior to use in step c.(III)(ii) below. The following table highlights the parameters for this step.

*The filter press was not operated on this day, hence the value of zero

Step (d)(II)

In accordance with the apparatus for step (d)(III), the precipitate slurry was transferred from the filter feed tank into the filter press. Filtrate was also sent to the storage tank prior to use in step c.(III)(ii) below. The following filtration conditions were used.

Solids composition:

*The filter press was not operated on this day, hence the value of zero

Step c(III)(ii)

In accordance with a modified version of the apparatus for step c(III)(ii) above, the stored supernatant from step c(II) and filtrate from step d(II) was treated with NaOH to pH 11.8 to 12 and then fed to a precipitation tank fitted with a carbon dioxide bubbler with carbon dioxide being bubbled through at a flow rate of 10 to 15 L/h. This dropped the pH to 9.5 causing a precipitate to form which then overflowed into the settling tank with the addition of AN913 (0.25g/L). The precipitate was left to settle from the supernatant, and then the precipitate slurry was transferred to the filter feed tank. The following table shows the average results for the overflow going into the settling tank. Most of the Ca is removed and the alkalinity of the solution is high.

Step d(III)

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS:

1. Process for producing calcium carbonate, and optionally magnesium carbonate and/or magnesium hydroxide, using an aqueous brine solution comprising the steps of: c. (I) optionally precipitating calcium carbonate and/or magnesium carbonate by adding an alkalinity source to the aqueous brine solution such that the pH of the solution is between 9 and 10 and calcium carbonate and/or magnesium carbonate precipitates; c. (II) optionally precipitating magnesium hydroxide by adding an alkalinity source to the aqueous brine solution such that the pH of the solution is between 10 and 12 and magnesium hydroxide precipitates; c. (Ill) precipitating calcium carbonate by

(i) adding a soluble carbonate salt and an alkalinity source to the aqueous brine solution such that the pH of the solution is between 9 and 12 and calcium carbonate precipitates, or

(ii) adding carbon dioxide gas and an alkalinity source, to the aqueous brine solution such that the pH of the solution is between 9 and 12 and calcium carbonate precipitates, d. (I) optionally removing calcium carbonate and/or magnesium carbonate produced in step c(I); d. (II) optionally removing magnesium hydroxide produced in step c(II); and d. (Ill) removing calcium carbonate produced in step c(III).

2. Process for producing calcium carbonate, and optionally magnesium carbonate and/or magnesium hydroxide, using an aqueous brine solution comprising the steps of: a. increasing the pH of the aqueous brine solution to a value above 7 but not more than 10; b. concentrating the alkaline aqueous brine solution in a system that comprises

(i) a membrane which resists passage of divalent ions but allows passage of monovalent ions, or

(ii) a membrane which resists passage of monovalent ions but allows passage of divalent ions; c. (I) optionally precipitating calcium carbonate and/or magnesium carbonate by adding an alkalinity source to the aqueous brine solution such that the pH of the solution is between 9 and 10 and calcium carbonate and/or magnesium carbonate precipitates; c. (II) optionally precipitating magnesium hydroxide by adding an alkalinity source to the aqueous brine solution such that the pH of the solution is between 10 and 12 and magnesium hydroxide precipitates; c. (Ill) precipitating calcium carbonate by

(i) adding a soluble carbonate salt and an alkalinity source to the retentate of step b(i) or the permeate of step b(ii) such that the pH of the solution is between 9 and 12 and calcium carbonate precipitates, or

(ii) adding carbon dioxide gas and an alkalinity source to the retentate of step b(i) or the permeate of step b(ii) such that the pH of the solution is between 9 and 12 and calcium carbonate precipitates, d. (I) optionally removing calcium carbonate and/or magnesium carbonate produced in step c(I); d. (II) optionally removing magnesium hydroxide produced in step c(II); and d. (Ill) removing calcium carbonate produced in step c(III). Process according to claim 1 or claim 2, wherein step c(II) and d(II) are not optional. Process according to any preceding claim, wherein step c(I) and d(I) are not optional. Process according to any preceding claim wherein the aqueous brine solution is waste brine from a desalination plant. Process according to any preceding claim wherein the aqueous brine solution is sea water. Process according to any preceding claim wherein the aqueous brine solution is industrial or municipal wastewater. Process according to any preceding claim wherein the aqueous brine solution is a saline ground water. Process according to any preceding claim wherein the aqueous brine solution is waste brine from a reverse osmosis plant. Process according to any one of claims 2 to 9 wherein the pH increase in step a is achieved by addition of an alkali metal hydroxide, preferably sodium hydroxide. Process according to any one of claims 2 to 10 wherein the pH increase in step a is achieved by addition of an alkali waste water from an industrial process. Process according to any one of claims 2 to 11 wherein the pH increase in step a is to a value in the range from 8.0 to 9.0. Process according to any one of claims 2 to 12 wherein the membrane which resists passage of divalent ions but allows passage of monovalent ions is a thin-film composite nanofiltration or reverse osmosis membrane having: a pore size around 1 nm and a molecular weight cut-off between 70 and 500 Da; a rejection of monovalent ions, such as sodium, potassium and chloride of between 5 and 75%; and a rejection of divalent ions, such as calcium and magnesium and sulphate of between 50% and 100%. Process according to any one of claims 2 to 13 wherein the membrane which resists passage of monovalent ions but allows passage of divalent ions possesses the following functional characteristics: either (a) low driving pressure with medium to high salt passage and medium hardness removal; or (b) low driving pressure with high salt passage and high hardness removal; or (c) medium driving pressure with a medium salt passage and high hardness removal. Process according to any one of claims 2 to 14 wherein the concentration of calcium ions (Ca²⁺) in the retentate from step b(i) or the permeate from step b(ii) is 800 mg/L or more. Process according to claim 10 wherein adjusting the pH of the aqueous brine solution is carried out by adding a hydroxide solution having a concentration in the range from 0.1 to 2.0 M. Process according to any preceding claim wherein step c(III)(i) or c(III)(ii) involves adding a soluble carbonate salt, or carbon dioxide, in an amount sufficient to provide a Ca:CO3 molar ratio in the range from 0.5 to 2.5 (i.e. 1 :2 to 5:2). Process according to any preceding claim wherein step c(III)(i) involves adding a soluble carbonate salt solution having a concentration in the range from 0.5 to 10 % w/v. Process according to any preceding claim wherein the soluble carbonate salt added in step c(III)(i) is sodium carbonate. Process according to any preceding claim wherein the carbon dioxide gas used in step c(III)(ii) is carbon dioxide gas from an industrial facility. Process according to any preceding claim wherein the carbon dioxide gas used in step c(III)(ii) is added at a flow rate between 0.2 and 1 mol/h. Process according to any preceding claim wherein, prior to or during step c, calcium carbonate is added to the retentate of step b(i) or the permeate of step b(ii) in an amount sufficient to promote precipitation of calcium carbonate. Process according to claim 22 wherein calcium carbonate is added in an amount from 0.1 to 20 % by mass based on the theoretical yield of calcium carbonate. Process according to any preceding claim wherein steps c(III) and d(III) are repeated until the concentrations of bicarbonate (HCO3 ) and carbonate (CO3² ) ions in the aqueous solution remaining after step d are reduced to the desired levels. Process according to any preceding claim wherein the concentration of bicarbonate (HCO3 ) ions in the aqueous solution remaining after step d is 200 mg/L or less. Process according to any preceding claim wherein the concentration of carbonate (CO3² ) ions in the aqueous solution remaining after step d is 200 mg/L or less.