Seawater treatment process

Abstract

Described is a seawater treatment process that synergistically combines desalination with the production of industrially useful materials. In particular, seawater is valorized to produce high- purity freshwater and high-purity Mg(OH)2, which may be further processed to MgO for use as an additive in green cement. As part of the process, the pH of seawater is adjusted to pH 9.0- 10.5 (e.g., pH 9.5-10.5) in order to precipitate dissolved inorganic carbon, after which the resulting low carbon seawater is subjected to a reverse osmosis process. The pH of the permeated brine is then adjusted to pH 10.0-13.25 (e.g., pH 11.5-13.25) in order to precipitate Mg2+ ions, notably in the form of Mg(OH)2. Additional steps may be included, for example to use the permeated brine to sequester CO2 and / or to produce NaOH, which can be advantageously used in the aforementioned precipitation steps.

Description

SEAWATER TREATMENT PROCESS INTRODUCTION

[0001] The present invention relates to a seawater treatment process. More particularly, the present invention relates to a seawater treatment process for producing high-purity freshwater and industrially useful materials, notably Mg(OH)2. BACKGROUND OF THE INVENTION

[0002] MgO, known as magnesium oxide or simply magnesia, can be acquired from different routes, mainly the dry route and the wet route. The dry route consists of calcinating (also termed calcining) magnesite (magnesium carbonate MgCO3) at a temperature range from 800 to 1000oC [Ruan et al.2016; Ruan et al.2017]. The wet route requires magnesium to be extracted from seawater or desalination brine or other types of saline water and is mostly in the form of brucite (magnesium hydroxide Mg(OH)2) [Dong et al.2017]. Even though the global production of MgO by magnesium carbonate calcination is a mature processing route, it still involves a number of disadvantages, most notably the fact that carbonate is released into the atmosphere as CO2. Another drawback of this dry route is that magnesite is only abundant in certain countries (e.g., China and North Korea), thus limiting access to global markets [Shand et al.2006].

[0003] On the other hand, the wet route provides a more lucrative alternative due to the accessibility by many nations to seawater and brines. Moreover, unlike for carbonates, the calcination of Mg(OH)2 does not release CO2, thus presenting clear environment advantages. Furthermore, brine resulting from seawater desalination is considered harmful to the environment and many efforts are focused on reaching zero liquid discharge (ZLD) to reduce the impact of desalination. In addition, MgO produced through the wet route hydrates at a similar rate as conventionally used Portland Cement, whereas MgO produced through the dry route is dead burned due to the significantly higher calcination temperature and therefore has a much lower reactivity leading to delayed hydration and cracking [Al-Tabbaa et al. 2013]. Thus, producing MgO through the wet processing route could offer an inherently lower carbon feedstock for various application, including cement manufacture.

[0004] The standard method for extracting Mg(OH)2 from seawater involves the addition of an alkali base (e.g., ammonium hydroxide NH4OH, calcium hydroxide Ca(OH)2, or sodium hydroxide NaOH, of which the latter is preferred), which enables the precipitation of Mg2+. The obtained minerals are then separated, washed, dried, and calcinated to produce MgO at a low temperature(less than 700 ◦C). Nevertheless, inefficiencies within this process can pose disadvantages from the standpoint of cost and high energy consumption.

[0005] Membrane based desalination, which is nowadays the most used method of desalination, is plagued by several inefficiencies, such as (1) organic & inorganic fouling and biofouling, and (2) low boron and silica rejection by reverse osmosis under standard operating conditions. Membrane fouling leads to a reduction of the water flux and inevitably increases the transmembrane pressure loss leading to an overall reduction in permeate production at a higher energy consumption. Due to the low rejection of certain seawater impurities, such as boron and silica, a second membrane pass may be required to adhere to drinking water standards increasing operational complexity and energy and consumables consumption.

[0006] The present invention was devised with the foregoing in mind. SUMMARY OF THE INVENTION

[0007] According to a first aspect of the present invention there is provided a seawater treatment process, the process comprising the following steps: (a) providing seawater; (b) adjusting the pH of the seawater to pH 9.0–10.5 to precipitate dissolved inorganic carbon (DIC); (c) separating the precipitated DIC from the seawater; (d) subjecting the seawater resulting from (c) to a reverse osmosis process to produce freshwater and brine; (e) adjusting the pH of the brine to pH 10.0–13.25 to precipitate Mg2+ions; and (f) separating the precipitated Mg2+ions from the brine.

[0008] Suitably, step (b) is conducted at pH 9.5–10.5 and / or step (e) is conducted at pH 11.5– 13.25. More suitably, step (b) is conducted at pH 9.5–10.5 and step (e) is conducted at pH 11.5– 13.25.

[0009] Suitably, Mg2+ions are precipitated as Mg(OH)2. More suitably, the Mg(OH)2 is then calcined to produce MgO, which can serve as an alternative cement additive to CaO.

[0010] According to a second aspect of the present invention, there is provided a seawater treatment process substantially as defined, or as defined, in Figure 1A or 1B.DETAILED DESCRIPTION OF THE INVENTION

[0011] Throughout the entirety of the description and claims of this specification, where subject matter is described herein using the term “comprise” (or “comprises” or “comprising”), the same subject matter instead described using the term “consist of” (or “consists of” or “consisting of”) or “consist essentially of” (or “consists essentially of” or “consisting essentially of”) is also contemplated.

[0012] Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0013] Features described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive. The invention is not restricted to the details of any of the specific embodiments recited herein. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0014] Unless otherwise specified, where the quantity or concentration of a particular component of a given product is specified as a weight percentage (wt.% or %w / w), said weight percentage refers to the percentage of said component by weight relative to the total weight of the product as a whole. It will be understood by those skilled in the art that the sum of weight percentages of all components of a product will total 100 wt.%. However, where not all components are listed (e.g. where a product is said to “comprise” one or more particular components), the weight percentage balance may optionally be made up to 100 wt% by unspecified ingredients.

[0015] As discussed hereinbefore, in a first aspect, the invention provides a seawater treatment process, the process comprising the following steps: (a) providing seawater; (b) adjusting the pH of the seawater to pH 9.0–10.5 (e.g., pH 9.5–10.5) to precipitate dissolved inorganic carbon (DIC); (c) separating the precipitated DIC from the seawater;(d) subjecting the seawater resulting from (c) to a reverse osmosis process to produce freshwater and brine; (e) adjusting the pH of the brine to pH 10.0–13.25 (e.g., pH 11.5–13.25) to precipitate Mg2+ions; and (f) separating the precipitated Mg2+ions from the brine.

[0016] Through rigorous investigation, the inventors have developed a process that allows freshwater and industrially useful materials to be produced at scale in a cost-effective and environmentally friendly manner. The process advantageously combines reverse osmosis technology with precipitation steps in such a manner that affords higher purity products, greater efficiencies, and significantly reduced process downtimes, e.g., as a consequence of scaling and / or fouling. As one part of this, the inventors have determined that removing DIC and Mg2+ions in separate precipitation steps allows for an intervening reverse osmosis process to achieve greater boron and / or silica rejection with notably less scaling / fouling.

[0017] As used herein, the term “seawater” refers to any saltwater having a salinity of >0.5%, a pH of 6.0–9.0 and comprising at least chloride, sodium, and magnesium as dissolved minerals. The seawater provided in step (a) may therefore be water obtained from an expanse of water recognised as an ocean (e.g., the Indian Ocean), sea (e.g., the Mediterranean Sea, Red Sea, Dead Sea, Persian Gulf or Gulf of Oman) or salt lake (e.g., the Great Salt Lake). In most instances, the seawater provided in step (a) will have a salinity of >2.5% and a pH of 7.5–9.0.

[0018] Step (b) may be referred to herein as a first precipitation step, and involves precipitating dissolved inorganic carbon (DIC) from the seawater by adjusting its pH to pH 9.0–10.5, suitably pH 9.5–10.5. Typically, DIC is precipitated as CaCO3. It will, however, be appreciated that other solids, including non-carbon-containing solids, may be precipitated during step (b). More suitably, step (b) comprises adjusting the pH of the seawater provided in step (a) to pH 9.5–10.25. Even more suitably, step (b) comprises adjusting the pH of the seawater provided in step (a) to pH 9.5–10.0. Most suitably, step (b) comprises adjusting the pH of the seawater provided in step (a) to pH 9.6–9.9.

[0019] Any suitable alkali may be used to adjust the pH in step (b). For example, the pH may be adjusted using ammonium hydroxide (NH4OH), calcium hydroxide (Ca(OH)2), and / or sodium hydroxide (NaOH). Most suitably, the pH is adjusted in step (b) using NaOH.

[0020] In step (c), the precipitated DIC (e.g., CaCO3), as well as any other precipitated solids, are separated from the seawater. Any suitable solid-liquid separation process may be used, such as filtration, sedimentation, decantation and / or centrifugation. Suitably, the precipitated DIC and any other precipitated solids are separated from the seawater by a filtration process.

[0021] Seawater that has been subjected to steps (b) and (c) may be used directly in reverse osmosis step (d) (i.e., without any intervening processing steps). Alternatively, the seawater provided in step (a) may be subjected to at least one pretreatment step prior to being subjected to reverse osmosis in step (d). Suitably, the at least one pretreatment step occurs before step (b) and / or after step (c).

[0022] The at least one pretreatment step may reduce scaling and / or fouling during reverse osmosis step (d). It will be understood that the at least one pretreatment step is distinct from steps (b) and (c).

[0023] The at least one pretreatment step may be selected from a filtration process, a chlorination process, a coagulation process, a flocculation process, a sedimentation process, an adsorption process, a flotation process and an oxidant scavenger process. Suitably, the at least one pretreatment step is a filtration process. The filtration process is suitably an ultrafiltration process, microfiltration process and / or a multimedia filtration process.

[0024] In particular embodiments, the seawater resulting from step (c) is subjected to a pre- treatment step before reverse osmosis step (d), wherein the pretreatment step is a filtration step, such as an ultrafiltration process, microfiltration process and / or a multimedia filtration process. In such embodiments, step (b) suitably comprises adjusting the pH of the seawater provided in step (a) to pH 9.5–10.25, more suitably to pH 9.6–9.9.

[0025] Seawater that has been subjected to steps (b), (c) and any pretreatment step(s) is then subjected to reverse osmosis in step (d). Suitably, the seawater subjected to reverse osmosis in step (d) has a pH of 9.0–10.0. At this pH, and those mentioned subsequently, greater boron and silica rejection is achieved, and membrane scaling / fouling is reduced. At lower pHs (<9) the boron rejection by reverse osmosis is reduced. More suitably, the seawater subjected to reverse osmosis in step (d) has a pH of 9.0–9.5. Most suitably, the seawater subjected to reverse osmosis in step (d) has a pH of 9.1–9.4. If too high, the pH of the seawater intended to be subjected to reverse osmosis can be lowered by addition of an acid (e.g., HCl).

[0026] Reverse osmosis step (d) is suitably a single-pass reverse osmosis step. It will be understood that a pass refers to a stand alone reverse osmosis process. The difference between a single pass reverse osmosis process and a double pass reverse osmosis process is that with the latter, the permeate from the first pass becomes the feed water for a second pass. A single pass reverse osmosis step requires significantly less energy than a double pass process.

[0027] As used herein in the context of step (d), the term “freshwater” will be understood to refer to the reverse osmosis permeate, whilst the term “brine” will be understood to refer to the reverse osmosis concentrate (also termed retentate).

[0028] The freshwater produced in step (d) suitably has a boron concentration of <0.5 mg / L. More suitably, the freshwater produced in step (d) has a boron concentration of <0.4 mg / L. Most suitably, the freshwater produced in step (d) has a boron concentration of <0.3 mg / L. A boron concentration of <0.5 mg / L is the standard followed by many countries for drinking water [Ophek et al.2015]

[0029] In particular embodiments, step (b) comprises adjusting the pH of the seawater provided in step (a) to pH 9.5–10.25 and the seawater subjected to reverse osmosis in step (d) has a pH of 9.0–10.0. In such embodiments, the seawater resulting from step (c) is suitably subjected to a pre-treatment step before reverse osmosis step (d).

[0030] The brine produced in step (d) suitably has a pH of 9.0–11.0. More suitably, the brine produced in step (d) has a pH of 9.25–10.5.

[0031] Prior to step (e), the brine produced in step (d) may be concentrated. Any suitable means of concentrating the brine can be used. For example, the brine may be concentrated thermally, electrochemically and / or with the use of a membrane (e.g., using a low-salt-rejection reverse osmosis, LSRRO process, or an osmotic membrane in osmotically assisted reverse osmosis (OARO) process).

[0032] Step (e) may be referred to herein as a second precipitation step, and involves precipitating Mg2+ions from the brine by adjusting its pH to pH 10.0–13.25, suitably pH 11.5– 13.25. Suitably, the Mg2+ions are precipitated as Mg(OH)2. It will be appreciated that other ions may be precipitated alongside Mg2+. For example, Ca2+may also precipitate as Ca(OH)2. Precipitating Mg2+in a second precipitation step (i.e., after reverse osmosis) presents advantages relative to precipitating Mg2+in a first precipitation step (i.e., before reverse osmosis). In particular, due to the Mg2+concentration being higher in brine (i.e., after reverse osmosis) relative to seawater (i.e., before reverse osmosis), the reaction kinetics are notably faster, thereby engendering significant time and cost savings. More suitably, step (e) comprises adjusting the pH of the brine to pH 12.0–13.25 to precipitate Mg2+ions. Even more suitably, step (e) comprises adjusting the pH of the brine to pH 12.0–13.0 to precipitate Mg2+ions. Most suitably, step (e) comprises adjusting the pH of the brine to pH 12.4–12.8 to precipitate Mg2+ions.

[0033] Any suitable alkali may be used to adjust the pH in step (e). For example, the pH may be adjusted using ammonium hydroxide (NH4OH), calcium hydroxide (Ca(OH)2), and / or sodium hydroxide (NaOH). Most suitably, the pH is adjusted in step (e) using NaOH.

[0034] Alternatively, an electro-chemical process may be used to locally adjust the pH in step (e). For example, the pH may be adjusted using an electrolyser.

[0035] In particular embodiments, step (b) comprises adjusting the pH of the seawater provided in step (a) to pH 9.5–10.25 and step (e) comprises adjusting the pH of the brine to pH 12.0–13.25 to precipitate Mg2+ions. Suitably, step (b) comprises adjusting the pH of the seawater provided in step (a) to pH 9.5–10.0 and step (e) comprises adjusting the pH of the brine to pH 12.0–13.0 to precipitate Mg2+ions. More suitably, step (b) comprises adjusting the pH of the seawater provided in step (a) to pH 9.6–9.9 and step (e) comprises adjusting the pH of the brine to pH 12.4–12.8 to precipitate Mg2+ions. In any such embodiment, the seawater resulting from step (c) is suitably subjected to a pre-treatment step before reverse osmosis step (d), e.g., a filtration step, such as an ultrafiltration process, microfiltration process and / or a multimedia filtration process.

[0036] In step (f), the precipitated Mg2+(e.g., Mg(OH)2), as well as any other precipitated solids, are separated from the brine. Any suitable solid-liquid separation process may be used, including filtration, sedimentation, decantation and / or centrifugation. Suitably, the precipitated Mg2+and any other precipitated solids are separated from the brine by a filtration process.

[0037] At least a portion of the brine resulting from step (f) may be used for sequestering carbon dioxide (i.e., carbon capture). For example, carbon dioxide can be injected into the brine, which has the added benefit of lowering the brine’s pH to adequate levels for discharge (approximately pH 8).

[0038] Owing to the fact that DIC is precipitated in a separate, preceding step, the invention allows for Mg(OH)2 to be produced in comparatively higher yield and purity. Mg(OH)2 has a number of industrial uses, e.g., as a food additive, as a fire retardant, in the medical field, and in the treatment of waste water. Furthermore, Mg(OH)2 can be processed into MgO (i.e., magnesia), which is an attractive alternative to CaO (i.e., generated from mined CaCO3) used in the Portland Cement process. The use of Mg(OH)2-dervied MgO instead of CaCO3-derived CaO as a cement additive presents a number of environmental benefits. In particular, Mg(OH)2 can be calcined to MgO at significantly lower temperatures than that required to convert CaCO3 to CaO. Moreover, the calcination of Mg(OH)2 releases water vapour, whereas calcining CaCO3 produces carbon dioxide. Large quantities of carbon dioxide can be sequestered during the curing of MgO- containing concrete. The fact that concrete is the second most consumed material on earth behind water underlines the need to decarbonize the production of cement. Seawater, abundant with magnesium, is therefore a viable source material for green cement manufacturing.

[0039] At least a portion of the brine resulting from step (f) may be concentrated. Any suitable means of concentrating the brine can be used. For example, the brine may be concentrated thermally, electrochemically and / or with the use of a membrane (e.g., using a low-salt-rejectionreverse osmosis, LSRRO process). Suitably, a portion of the brine resulting from step (f) is concentrated with the use of a membrane (e.g., by a LSRRO process or an OARO process).

[0040] At least a portion of the brine resulting from step (f), optionally concentrated as described hereinbefore, may then be pH adjusted to pH 11.0–14.0, suitably pH 11.0–12.5, to precipitate one or more divalent metal ions. This step may be referred to herein as a third precipitation step. More suitably, in the third precipitation step, the pH of the brine is adjusted to pH 11.1–12.25 to precipitate one or more divalent metal ions. Even more suitably, in the third precipitation step, the pH of the brine is adjusted to pH 11.2–12.0 to precipitate one or more divalent metal ions. Most suitably, in the third precipitation step, the pH of the brine is adjusted to pH 11.4–11.8 to precipitate one or more divalent metal ions. The one or more divalent metal ions may be selected from the group consisting of Mg2+, Sr2+and Ca2+. The precipitated divalent metal ions may then be separated from the brine by a solid-liquid separation process. Any suitable solid-liquid separation process may be used, including filtration, sedimentation, decantation and / or centrifugation. Suitably, the precipitated divalent metal ions are separated from the brine by a filtration process.

[0041] Any suitable alkali may be used to adjust the pH in the third precipitation step. For example, the pH may be adjusted using ammonium hydroxide (NH4OH), calcium hydroxide (Ca(OH)2), and / or sodium hydroxide (NaOH). Most suitably, the pH is adjusted in the third precipitation step using NaOH.

[0042] In particular embodiments, step (b) comprises adjusting the pH of the seawater provided in step (a) to pH 9.5–10.25; step (e) comprises adjusting the pH of the brine to pH 12.0–13.25 to precipitate Mg2+ions; and the process comprises a third precipitation step, as described hereinbefore. Suitably, step (b) comprises adjusting the pH of the seawater provided in step (a) to pH 9.6–9.9 and / or step (e) comprises adjusting the pH of the brine to pH 12.4–12.8 to precipitate Mg2+ions and / or in the third precipitation step, the pH of the brine is adjusted to pH 11.2–12.0. In any such embodiment, the seawater resulting from step (c) is suitably subjected to a pre-treatment step before reverse osmosis step (d), e.g., a filtration step, such as an ultrafiltration process and / or a multimedia filtration process.

[0043] Brine produced by the seawater treatment process may be advantageously used in the preparation of a pH adjusting agent (e.g., an alkali, such as NaOH). Suitably, the pH adjusting agent is produced by an electrochemical process, such as a bipolar membrane electrodialysis (i.e., BMED) process or a chlor-alkali electrolysis process. Most suitably, the pH adjusting agent is produced by a BMED process and the pH adjusting agent is NaOH.

[0044] Any brine produced by the process may be used to prepare the pH adjusting agent. By way of non-limiting examples, the brine may be that produced by step (b); and / or the brine maybe that produced by step (f); and / or the brine may be that produced by step (f) and subsequently concentrated as described hereinbefore; and / or the brine may be that produced by the third precipitation step. Most suitably, the brine used to prepare the pH adjusting agent is that produced by the third precipitation step. Such brine contains extremely low levels of scalants, which could otherwise compromise electrochemical processes such as BMED.

[0045] The pH adjusting agent (e.g., NaOH) prepared from the brine may be used to adjust the pH in step (b) and / or step (e) and / or the third precipitation step. The use of a “self-produced” pH adjusting agent in one or more of the precipitation steps of the seawater treatment process is advantageous for a variety of reasons. Perhaps most importantly, it reduces, or eliminates entirely, the need to use externally produced pH adjusting agents (recognising that NaOH is expensive to purchase), thereby lowering overall energy consumption and cost. When the pH adjusting agent is destined solely for use in the precipitation steps of the seawater treatment process (as opposed to being sold commercially), a lower purity threshold can be tolerated, thereby further lowering energy consumption and cost. Suitably, the pH adjusting agent (e.g., NaOH) prepared from the brine is used to adjust the pH in step (b), step (e) and the third precipitation step (where present).

[0046] In particular embodiments, the process comprises a third precipitation step, as described hereinbefore; brine produced by the third precipitation step is used to prepare NaOH by an electrochemical (e.g., BMED) process; and the NaOH produced by the electrochemical process is used to adjust the pH in step (b) and / or step (e) and / or the third precipitation step (e.g., in all 3 precipitation steps). Suitably, step (b) comprises adjusting the pH of the seawater provided in step (a) to pH 9.5–10.25 and / or step (e) comprises adjusting the pH of the brine to pH 12.0– 13.25 to precipitate Mg2+ions and / or in the third precipitation step, the pH of the brine is adjusted to pH 11.1–12.25. More suitably, step (b) comprises adjusting the pH of the seawater provided in step (a) to pH 9.6–9.9 and / or step (e) comprises adjusting the pH of the brine to pH 12.4–12.8 to precipitate Mg2+ions and / or in the third precipitation step, the pH of the brine is adjusted to pH 11.2–12.0. In any such embodiment, the seawater resulting from step (c) is suitably subjected to a pre-treatment step before reverse osmosis step (d), e.g., a filtration step, such as an ultrafiltration process and / or a multimedia filtration process.

[0047] In particular embodiments, step (b) comprises adjusting the pH of the seawater provided in step (a) to pH 9.6–9.9; in step (c), the precipitated DIC and any other precipitated solids are separated from the seawater by a filtration process; the seawater resulting from step (c) is subjected to a ultrafiltration process and / or a multimedia filtration process pre-treatment step conducted before reverse osmosis step (d); the seawater subjected to reverse osmosis in step (d) has a pH of 9.1–9.4; step (e) comprises adjusting the pH of the brine to pH 12.4–12.8 to precipitate Mg2+ions; in step (f), the precipitated Mg2+(e.g., Mg(OH)2) and any other precipitatedsolids are separated from the brine by a filtration process; the brine resulting from step (f) is concentrated (e.g., by a LSRRO process); the process comprises a third precipitation step conducted at pH 11.4–11.8; brine produced by the third precipitation step is used to prepare NaOH by an electrochemical (e.g., BMED) process; and the NaOH produced by the electrochemical process is used to adjust the pH in step (b), step (e) and the third precipitation step.

[0048] In an embodiment, the process is as defined in Figure 1B.

[0049] The following numbered statements 1 to 50 are not claims, but instead describe particular aspects and embodiments of the invention: 1. A seawater treatment process, the process comprising the following steps: (a) providing seawater; (b) adjusting the pH of the seawater to pH 9.0–10.5 (e.g., pH 9.5–10.5) to precipitate dissolved inorganic carbon (DIC); (c) separating the precipitated DIC from the seawater; (d) subjecting the seawater resulting from (c) to a reverse osmosis process to produce freshwater and brine; (e) adjusting the pH of the brine to pH 10.0–13.25 (e.g., pH 11.5–13.25) to precipitate Mg2+ions; and (f) separating the precipitated Mg2+ions from the brine. 2. The process as described in statement 1, wherein step (b) comprises adjusting the pH of the seawater provided in step (a) to pH 9.5–10.25 to precipitate dissolved inorganic carbon (DIC). 3. The process as described in statement 1, wherein step (b) comprises adjusting the pH of the seawater provided in step (a) to pH 9.5–10.0 to precipitate dissolved inorganic carbon (DIC). 4. The process as described in statement 1, wherein step (b) comprises adjusting the pH of the seawater provided in step (a) to pH 9.6–9.9 to precipitate dissolved inorganic carbon (DIC).5. The process as described in any one of the preceding statements, wherein DIC is precipitated in step (b) as CaCO3. 6. The process as described in any one of the preceding statements, wherein step (c) comprises separating the precipitated DIC from the seawater by a solid-liquid separation process. 7 The process as described in statement 6, wherein the solid-liquid separation process is a filtration process. 8. The process as described in any one of statements 1 to 4, wherein DIC is precipitated in step (b) as CaCO3; and step (c) comprises separating the precipitated DIC from the seawater by a filtration process. 9. The process as described in any one of the preceding statements, wherein the seawater resulting from (c) has a pH of 9.0–10.0. 10. The process as described in statement 9, wherein the seawater resulting from (c) has a pH of 9.0–9.5. 11. The process as described in statement 9, wherein the seawater resulting from (c) has a pH of 9.1–9.4. 12. The process as described in any one of the preceding statements, wherein step (d) comprises subjecting the seawater resulting from (c) to a single pass reverse osmosis process. 13. The process as described in any one of the preceding statements, wherein the brine resulting from step (d) has a pH of 9.0–10.0. 14. The process as described in statement 13, wherein the brine resulting from step (d) has a pH of 9.3–9.7. 15. The process as described in any one of the preceding statements, wherein the freshwater resulting from step (d) has a boron concentration of <0.5 mg / L. 16. The process as described in statement 15, wherein freshwater resulting from step (d) has a boron concentration of <0.4 mg / L.17. The process as described in statement 15, wherein freshwater resulting from step (d) has a boron concentration of <0.3 mg / L. 18. The process as described in any one of statements 1 to 8, 10 to 12, 14, and 16 to 17, wherein the seawater resulting from (c) has a pH of 9.0–10.0; the brine resulting from step (d) has a pH of 9.0–10.0; and the freshwater resulting from step (d) has a boron concentration of <0.5 mg / L. 19. The process as described in any one of the preceding statements, wherein prior to step (e), the brine resulting from step (d) is concentrated. 20. The process as described in statement 19, wherein the brine is concentrated thermally, electrochemically and / or using a membrane. 21. The process as described in any one of the preceding statements, wherein step (e) comprises adjusting the pH of the brine to pH 12.0–13.25 to precipitate Mg2+ions. 22. The process as described in statement 21, wherein step (e) comprises adjusting the pH of the brine to pH 12.0–13.0 to precipitate Mg2+ions. 23. The process as described in statement 21, wherein step (e) comprises adjusting the pH of the brine to pH 12.4–12.8 to precipitate Mg2+ions. 24. The process as described in any one of the preceding statements, wherein step (f) comprises separating the precipitated Mg2+ions from the brine by a solid-liquid separation process. 25. The process as described in statement 24, wherein the solid-liquid separation process is a filtration process. 26. The process as described in any one of the preceding statements, wherein Mg2+ions are precipitated in step (e) as Mg(OH)2. 27. The process as described in any one of statements 1 to 20 and 22 to 26, wherein step (e) comprises adjusting the pH of the brine to pH 12.0–13.25 to precipitate Mg2+ions; and Mg2+ions are precipitated in step (e) as Mg(OH)2.28. The process as described in statement 26 or 27, wherein the Mg(OH)2is processed into MgO (e.g., by calcining the Mg(OH)2at 350-700˚C). 29. The process as described in statement 28, wherein the MgO is suitable for use as an additive in the manufacture of cement. 30. The process as described in any one of the preceding statements, wherein at least a portion of the brine resulting from step (f) is used for sequestering CO2. 31. The process as described in any one of the preceding statements, wherein at least a portion of the brine resulting from step (f) is concentrated (e.g., thermally, electrochemically and / or using a membrane). 32. The process as described in any one of the preceding statements, wherein the pH of at least a portion of the brine resulting from step (f) is adjusted to pH 11.0–14.0 (e.g., pH 11.0– 12.5) to precipitate one or more divalent metal ions (i.e., a third precipitation step). 33. The process as described in statement 32, wherein the pH of at least a portion of the brine resulting from step (f) is adjusted to pH 11.25–12.0 to precipitate one or more divalent metal ions. 34. The process as described in statement 32 or 33, wherein the precipitated one or more divalent metal ions are separated from the brine by a solid-liquid separation process. 35. The process as described in statement 34, wherein the solid-liquid separation process is a filtration process. 36. The process as described in any one of the preceding statements, wherein brine produced by the seawater treatment process (e.g., brine resulting from step (f)) is used to prepare a pH adjusting agent. 37. The process as described in statement 36, wherein the pH adjusting agent is produced by an electrochemical process. 38. The process as described in statement 37, wherein the electrochemical process is a bipolar membrane electrodialysis process or a chlor-alkali electrolysis process.39. The process as described in statement 36, 37 or 38, wherein the pH adjusting agent is NaOH. 40. The process as described in any one of statements 36 to 39, wherein the pH adjusting agent is used to adjust the pH in step (b) and / or (e) and / or the third precipitation step (when present). 41. The process as described in any one of statements 1 to 31, 33 to 35, and 37 to 38, wherein the pH of at least a portion of the brine resulting from step (f) is adjusted to pH 11.0– 12.5 to precipitate one or more divalent metal ions (i.e., a third precipitation step); brine produced by the seawater treatment process (e.g., brine produced by the third precipitation step) is used to prepare a pH adjusting agent; the pH adjusting agent is NaOH; and the pH adjusting agent is used to adjust the pH in step (b) and / or (e) and / or the third precipitation step. 42. The process as described in any one of the preceding statements, wherein the seawater is subjected to at least one pretreatment step prior to being subjected to reverse osmosis in step (d). 43. The process as described in statement 42, wherein the at least one pretreatment step reduces membrane fouling during reverse osmosis in step (d). 44. The process as described in statement 42 or 43, wherein the at least one pretreatment step occurs before step (b) and / or after step (c). 45. The process as described in statement 42, 43 or 44, wherein the at least one pretreatment step is selected from a filtration process, a chlorination process, a coagulation process, a flocculation process, a sedimentation process, an adsorption process, a flotation process and an oxidant scavenger process. 46. The process as described in any one of statements 42 to 45, wherein the at least one pretreatment step is a filtration process. 47. The process as described in statement 46, wherein the filtration process is an ultrafiltration process and / or a multimedia filtration process.48. The process as described in any one of statements 1 to 41, 46 and 47, wherein the seawater is subjected to at least one pretreatment step prior to being subjected to reverse osmosis in step (d); the at least one pretreatment step occurs before step (b); and the at least one pretreatment step is selected from a filtration process, a chlorination process, a coagulation process, a flocculation process, a sedimentation process, an adsorption process, a flotation process and an oxidant scavenger process. 49. The process as described in any one of statements 1 to 41, 46 and 47, wherein the seawater is subjected to at least one pretreatment step prior to being subjected to reverse osmosis in step (d); the at least one pretreatment step occurs after step (c); and the at least one pretreatment step is selected from a filtration process, a chlorination process, a coagulation process, a flocculation process, a sedimentation process, an adsorption process, a flotation process and an oxidant scavenger process. 50. The process as described in any one of statements 1 to 41, 46 and 47, wherein the seawater is subjected to two pretreatment step prior to being subjected to reverse osmosis in step (d); the first pretreatment step occurs before step (b); the second pretreatment step occurs after step (c); and the first and second pretreatment steps are independently selected from a filtration process, a chlorination process, a coagulation process, a flocculation process, a sedimentation process, an adsorption process, a flotation process and an oxidant scavenger process. EXAMPLES

[0050] Examples of the invention will now be described, for the purpose of further illustration only, with reference to the accompanying figures: Fig. 1. A) Process layout for high quality permeate and green cement production. B) Process layout showing optimal processing parameters. Abbreviations: DIC=Dissolved inorganic carbon, UF / MF=Ultra / microfiltration, RO=Reverse osmosis, LSRRO=Low salt rejection RO, BMED=Bipolar membrane electrodialysis. Fig.2. Aqion initial system conditions. Fig.3. Aspen HYSYS simulation setup of heating and cooling units. Fig.4. Single-pass setup based on acidification to remove CO2 [Ophek et al.2015]. Fig.5. RO setup inside WAVE simulation.Fig. 6. LSRRO with recycling permeates to previous stage where the first stage has 100% rejection, and the latter is lower (~35%) [Wang et al.2020]. Fig.7. Different NaOH production technologies setups and reactions [Kumar et al.2021]. Fig.8. BMED with AEM, BPM and CEM triplet stack setup [Culcasi et al.2022]. Fig.9. X-ray diffraction of original CEM (a) and base stream side of used CEM (b) [Wang et al. 2011]. Fig.10. Precipitant formation at different seawater pH levels. Fig.11. Aqion precipitation results. Fig.12. Aqion charge balance check. Fig.13. Aspen HYSYS energy analysis and specification of thermal heating and cooling. Fig.14. WAVE pH values for the feed and concentrate of the RO process. Fig.15. WAVE simulation results of Boron concentration in RO permeate with respect to the feed pH level. Fig.16. Silicic acid concentration with varying pH levels. Fig.17. The amount of NaOH consumption per mineral precipitated. Fig.18. CaCO3 precipitation from processed brine with varying pH levels. Fig.19. LSRRO Simulink / MATLAB model. Methods

[0051] A new proposed seawater treatment process flowsheet is presented in this work. Having regarding to Figure 4A / B, the process allows for a reduction in energy consumption and synergistically combines desalination with the production of industrially useful materials, such as Mg(OH)2, which may be used in the green cement manufacturing. The work is simulation based, with varying software, tools, and data sources utilized to measure the outcomes. Pre-treatment

[0052] The first precipitation step can be done prior to or after the general RO pre-treatment processes, which generally include a combination of dissolved air flotation (if high algae loading), dual media filtration, granular activated carbon and / or microfiltration / ultrafiltration. Different types of fouling are detailed in Table 1 with their causes and pre-treatment options.Table 1: Fouling types, causes and mitigation technologies Fouling Cause Appropriate Pre-treatment Bacteria, microorganisms, Biological fouling Chlorination viruses, protozoan sand, clay (turbidity, Particle fouling Filtration suspended solids) Organic and inorganic Coagulation + Filtration Colloidal fouling complexes, colloidal Optional: Flocculation / particles, micro-algae sedimentation Natural Organic Coagulation + Filtration + Organic fouling Matter (NOM) : humic and Activated carbon adsorption / fulvic acids, biopolymers Coagulation+ Ultrafiltration Calcium, Magnesium, Barium or Antiscalant dosing Mineral fouling Strontium sulfates and Acidification carbonates Oxidant scavenger dosing: Oxidant fouling Chlorine, Ozone, KMnO4 Sodium (meta)bilsulfite Granulated Activated Carbon [Lenntech, 2023] 1stPrecipitation Stage

[0053] The first seawater precipitation stage is installed to remove DIC and increase pH levels before the RO step. Decarbonizing the system as a first step is beneficial (e.g., reducing DIC), to lower the pre-treatment load and aids in fouling reduction resulting in savings both in capital and operating costs [Ophek et al.2015]. This limits any scaling issues caused by CaCO3, which are harmful to the RO performance by limiting the freshwater production through formation of a fouling layer on the membrane surface. Precipitation of DIC will create deposition sites for organic fouling to form on it and to be filtered out with CaCO3, thus contributing to less fouling. Moreover, the increased pH will make rejecting boron in a single-pass possible as it will be demonstrated later. Furthermore, removing these contaminants early on, will ensure a higher purity of Mg(OH)2 when produced for calcination in a later separate precipitation step, thus reducing the energy needed for calcination due to lower overall volume, while maintaining a higher purity.

[0054] The mechanism behind the precipitation is demonstrated by the following reactions:CO2(g) ⇌ CO2(aq)(1)CO2(aq) + H2O ⇌ H2CO3(2)H2CO3 ⇌ H++ HCO3- (3) HCO3- ⇌ H++ CO32-(4) Ca2++ CO32-⇌ CaCO3(aq) (5) CaCO3(aq) ⇌ CaCO3(g) (6)

[0055] Most of the seawater contained DIC under normal seawater pH levels (e.g. ~8) are in the form of Bicarbonate (HCO3-). A previous study demonstrated the effects of NaOH addition to increase the pH level and Bicarbonate formation eq(7-9) [La Plante et al.2021], leading to overall precipitation of Calcite (CaCO3) following eq(4-6). NaOH ⇌ Na++ OH- (7)CO2(aq) + OH- ⇌ HCO3- (8)NaOH + HCO3- ⇌ CO32-+ Na++ H2O (9) Table 2: Typically concentration of seawater Element Cl- Na+SO42-Mg2+Ca2+K+Br-Concentration 18980 10556 2649 1262 400 380 50 (mg / L) Element DIC- Sr2+B F- Si4+Total Concentration 24 13 4.6 1 1 34321 (mg / L) [Cotruvo 2005; Lenntech 2023]

[0056] The precipitation data was generated according to a simulation carried out by hydrochemistry software Aqion (Pro version 8.1.3) shown in Figure 2 using inputs shown in Table 2.Filter Press

[0057] The formed sludge contains seawater containing CO2captured as a solid in the form of CaCO3. This sludge can be settled and / or filtered and disposed of via effluent discharge into the ocean or can be used as a cement additive. A conventional filter press can decrease the water content to 55% with a specific energy consumption (SEC) of 370 kWh / t of dried product [Lee et al.2007]. Filter presses are installed after each precipitation step to dewater the precipitate for further processing. If the formed CaCO3is not used as a cement additive, then no filter press is required and all CaCO3can for example be removed from the softened seawater stream solely using sedimentation / filtration. Calcination

[0058] The precipitate formed in the 1stprecipitation stage is typically not calcinated as it could lead to the release of the captured CO2 demonstrated in eq(10) which is counterproductive compared to Mg(OH)2calcination eq(11).

[0059] Mg(OH)2produced may be calcinated to produce reactive Magnesia MgO, the active ingredient in cement binders.

[0060] The energy calculations will be carried out using Aspen HYSYS for drying, heating, and cooling in a setup of heater and cooler modules seen in Figure 3. Their thermal heat requirement is converted into an electrical power consumption by using the a conversion factor discussed in eq(13).

[0061] The temperature needed for calcinating Mg(OH)2could be as low as 350-700oC compared to 1450oC for CaCO3(Limestone) / PC (Portland Cement), which was determined experimentally by earlier studies [Wu et al.2018; Don et al.2018; Shand 1980] . On top of the lower temperature range, Mg(OH)2releases water vapor instead of carbon when CaCO3is calcinated as seen in eq(10-11).

[0062] A previous study analysed the decomposition of brucite seen in eq(11) during the calcination process [Shand 1980]. The study presented the heat (Q) required for the reaction based on heat of formation (Hf) as follows:Q= △HfMgO + △HfH₂O - △Hfbrucite = 1304 kJ / kg Mg(OH)2(12)

[0063] For drying, a heat pump with a Coefficient of Performance (CP) equal to 2 is used [Jiang et al.2022]. For electrical power calculation, it could be described by the ratio of thermal power to CP: Electrical Power = Thermal Power / CP (13)

[0064] For heating up to 500oC and calcination, a heating setup based on ohmic resistance is adapted with CP equal to 0.9 [Tokheim et al.2019], and then it could be cooled down to 25oC by a heat exchanger with water. Single-Pass Reverse Osmosis

[0065] A single pass RO process is operated preferably at pH 9 to 9.5 to ensure high boron and silica rejection. The pH from the 1stprecipitation stage can be left unchanged if already in the required pH range or can be lowered using HCl in case the pH in the first precipitation stage exceeds 9.5.

[0066] The prior work shown in Figure 4 [Ophek et al.2015; Chaudhury et al.2021], suggested acidification at 4.3 pH to remove CO2and thus limit the scaling formation of CaCO3(whereas in this work CO2and CaCO3were addressed through pH increase as seen at the first precipitation step), then using a strong base to turn the neutral boron (B) present as B(OH)3into B(OH)4- ion at 9.53 pH that could be easily removed and rejected by a single pass SWRO membrane as seen at Figure 4A / B, due to membrane ability to reject charged ions [Ophek et al.2015]. Moreover, the molecular weight of B(OH)3is 61.83 g / mol compared to 78.84 g / mol for B(OH)4-, which makes the later a larger molecule that could be easier to filter out. This approach saves on pre-treatment, post-treatment, and brine disposal costs lowering both Opx and CapX. Moreover, Calcium and DIC removal saves on the costs of anti-scalant chemicals to be added to protect from membrane scaling and a higher pH RO operation ensures high separation performance as seen in Table 3. In another study, it was estimated that a single-pass setup could achieve a 25% reduction in the overall costs compared to a two-pass configuration, and an overall reduction of 20% is assumed from the standard SEC which is equal to 4 kWh / m3[Kim et al.2018].

[0067] To test different pH operating conditions on Boron rejection a simulation using Water Application Value Engine (WAVE ver.1.82.824), is carried out. Table 3: Previous work operational costs (USD cents per m3of freshwater) Two-Pass Single-Pass (Normal Process) (Optimised Process) Electrical power 17.36 15.62 Acid + Base 0 2.51 + 1.21 Anti-Scalant 0.94 0 Degasification 0 0.33 Pre-Treatment 12 10.08 Brine Disposal 5.5 3.83 Boron removal Post-Treatment 4.7 0 Total 40.5 33.58 Scaling limiting solid CaCO3 Mg(OH)2 [Ophek et al.2015]

[0068] The simulation adapted a standard FilmTecTMmembrane under the brand name of SW30HRLE-440, which is considered and marketed to be an excellent choice for boron rejection and is a conventional membrane module used for seawater desalination [DuPont 2023]. The number of stages is set to 1 with 11 pressurised vessels (PV) in parallel per stage. Each PV contains 8 membrane elements, totalling 88 modules at the RO setup with a standard feed of 100 m3 / h per RO system. The overall recovery is set to reach 50% with a flow factor of 0.9 and a pressure drop of 0.31 bar and standard seawater type from the software library. The operating pressure is around 60 bar. The pH change is carried out by adding NaOH while assuming a complete prior DIC removal to eliminate CaCO3scaling, which is the case in this setup. The WAVE simulation is detailed in Figure 5. 2ndPrecipitation Stage

[0069] At the second stage the precipitation targets Mg(OH)2production. A major benefit of precipitating Mg at a 2ndstage after RO instead of the 1ststage, is the faster kinetics when the Mg concentration is higher in the concentrated brine compared to that of raw seawater, which is evident by the nature of reaction and nucleation presented in previous studies [Carson et al. 1994; Alamdari et al.2008]. At higher concentrations nucleation can be accelerated by several orders of magnitude. That means doubling the concentration by reducing the saline water volume by half (e.g., 50% RO recovery in previous step) can lead to an 8 (23) times reduction induration.Membrane or thermal concentration

[0070] To minimize OpX and CapX of the expensive electrolysis / electrodialysis process, an energy-efficient LSRRO (or osmotically assisted RO (OARO)) process is implemented to further recover a low salinity permeate that can be further desalinated to increase freshwater production and to reduce the volume of the concentrated Mg- and Ca-free brine stream to be electrolysed which contributes to a great reduction in SEC [Oner et al.2021]. The stream should be sufficiently free of Mg and Ca for scaling not to be an issue. Stage 1 in Figure 6 represents the single-pass RO system where ‘Brine 1’ is used for Mg(OH)2 precipitation and is then further dewatered using nanofiltration (NF) ‘2ndstage’ . The permeate from the NF stage is recirculated to the feed of the single-pass RO system to enhance freshwater production. The energy consumption of the process is modelled at a specific energy consumption SEC equal to 4.2 kWh / m3of freshwater [Wang et al.2020; Wang et al.2021], this setup could be seen in Figure 19.

[0071] The LSRRO model is based on a previous study discussing the mechanism and performance of LSRRO as a brine concentration method [Wang et al.2021]. ^^^^ = ^^^^ (A1)^^^^ + ^^^^^^ = ^^^^^^ (A2)where Q0 and C0 are the flow rate and concentration of the feed, respectively, QB and CB are the flow rate and concentration of the final brine, respectively, QP2 and CP2 are the flow rate and concentration of the recycled permeate, respectively, QB1 and CB1 are the flow rate and concentration of the first stage brine, respectively.

[0072] The overall and the second stage water balance yields the following equations: ^^ = ^^ + ^^ (A3)^^^ = ^^ + ^^^ (A4)where QP is the final permeate / freshwater produced.

[0073] A salt balance based on salt rejection could be demonstrated as follows:where r2 is the salt rejection from the second stage.

[0074] Based on RO working principal (i.e., the osmotic pressure difference is equal to the applied pressure ∆^) and van’t Hoff’s law, the following formula is acquired: ∆^ ^^^= (A6) ^^^^ where k is the overpressurization factor (e.g., taken as 1.1), ^ is dimensionless van’t Hoff index (e.g., taken as 2 for NaCl), R is the gas constant, T is the absolute temperature.

[0075] The SEC could be measured as the power needed to drive the process by the amount of pressure applied per unit of freshwater produced, while considering pressure recovery from the final brine:

[0076] The analysis is based on total recovery of 50%, a pressure of 80 bar (up to theoretical 120 bar), and a membrane rejection of 100% at the first stage (RO membrane) and 80% at the second stage (NF membrane). NaOH Production

[0077] The concentrated brine stream is electrolysed in a BMED or chlor-alkali electrolysis process with the reactions indicated under each technology in Figure 7 [Kumar et al.2021]. NaOH does not have to be at high concentrations and purity as it is solely mixed with seawater and not sold as a commercial product. This is an important factor as it significantly lowers the energy consumption and no high-purity and concentrated NaOH needs to be produced as it is solely used in this process [Thiel et al.2017].

[0078] The selection for this process is based on BMED setup due to its lower energy consumption (because of the lower needed potential of 1.23 V to drive the reaction as seen in Figure 7).

[0079] The SEC is going to be scaled from a previous experimental study at 1 kWh / kg NaOH at a concentration of 0.5M [Culcasi et al. 2022], with a triplet stack made of Anion Exchange Membrane (AEM), Bipolar Membrane (BPM), and Cation Exchange Membrane (CEM), seen in Figure 8.

[0080] The BMED unit used (under the brand name FT-ED-100, acquired from Fumatech BWT Gmbh, Germany) equipped with commercial ion-exchange membranes; "fumasep® FAB as AEM, fumasep® FKB as CEM, and fumasep® FBM as BPM" [Culcasi et al.2022]. BMED pre-treatment

[0081] The BMED process is very sensitive to divalent ions (e.g., Mg2+and Ca2+), which could cause a scaling issue on the BPM and CEM, thus lowering performance. This is because of the high pH environment inside the unit (i.e., inside the base compartment when divalent ions pass through the CEM), which could lead to the precipitation of these ions in the form of solid minerals (e.g., Mg(OH)2, CaCO3 and Ca(OH)2) [Du et al.2021]. This could be seen from x-ray results in Figure 9 where calcite, brucite and portlandite (Calcium Hydroxide) were found to be the main scaling minerals on top of the membranes [Wang et al.2021].

[0082] To avoid the scaling issue a possible avenue could be by using a mono-selective membranes to prevent divalent ions migrations to the high pH side. But as of now, the selectivity of those membranes is yet to reach satisfactory levels, and pre-treatment with precipitation is still needed and the only effective way to prevent scaling inside the BMED [Chaudhury et al.2021]. Alternatively, a nanofiltration pre-treatment system could be used to reject divalent ions from the BMED process. However, the proposed precipitation step further sequesters CO2, and produces cement additives which could be of value in a simple process.

[0083] Different studies indicated different possible concentration targets for both Mg2+and Ca2+to avoid any scaling issues. For magnesium, a range as low as 4.5-10 mg / L should be reached [Tran et al.2013; Ibanez et al.2013]. While for calcium it is higher at a range of 17-39 mg / L [Reig et al.2016; Lin Teng Shee et al.2008]. In general Mg(OH)2 seems to be the more critical mineral in terms of scaling, as 5mg / L of Mg caused more severe scaling than 334 mg / L of Ca [Tran et al.2013; Lin Teng Shee et al.2008].Results and Discussion 1stPrecipitation Stage

[0084] The process starts with sodium hydroxide (NaOH) at a concentration of 0.5-1M and a flowrate of 13.8 t / h being mixed with 3,100,000 m3 / d (129,167 m3 / h the capacity of Ras Al-Khair plant in Saudi Arabia assuming 50% recovery) of raw seawater at typical concentrations presented in Table 2 (or more general Ca- and Mg-containing brine) to increase the seawater pH from 8.1 to ~9.5-10 for dissolved inorganic carbon (DIC) removal via CaCO3precipitation. As shown in Figure 10, at this pH mostly CaCO3precipitates with a minimal Mg(OH)2precipitation. Mg(OH)2can then be optimally recovered in the pH range of ~12.5-13. Higher pH precipitation of Mg is possible, however, DIC removal is affected due to higher Ca precipitation as Ca(OH)2instead of CaCO3.

[0085] The consumption of OH- in the precipitation reactions will lead to a drop in the pH level below 9.5 after the mineralization of CaCO3 as seen in Figure 11.

[0086] The precipitation data is measured with high accuracy and low absolute charge balance error of 0.04%, which is adjusted and accounted for in pH level as seen in Figure 12.The results show a 96% reduction rate for DIC.

[0087] Although DIC removal could be achieved by acidification instead of increasing pH levels

[0029] , the approach of increasing alkalinity is chosen here due to the higher RO boron rejection and the possibility of green cement production further downstream. Filter Press

[0088] The scaling of filter press output is measured to be 55.42 t / h of sludge made of 24.94 t / h CaCO3 and the rest is water. The power needed is measured at 9.23 MW. Calcination

[0089] A calcination step is implemented when it is desired to process the Mg(OH)2 into MgO, e.g., for use in manufacturing green cement.

[0090] Removing the CaCO3 prior to Mg(OH)2 precipitation step lowers the calcination load as a 381.8 t / h of higher purity brucite Mg(OH)2 sludge is only dewatered and fed through the calcination step. This results in a 6% reduction in calcination load from wasting heat on CaCO3 calcination and hence, in a direct energy saving of an energy-intensive process. Also, that means the produced cement is made of a purer MgO without CaCO3 reaching a maximum purity of 99.8% compared to 93.8% if the CaCO3 isn’t removed in a separate precipitation stage.

[0091] If an equal amount of CaCO3is calcinated, it would have produced 168 t / h of CO2, which is avoided by solely taking the Mg(OH)2calcination route.

[0092] The calcination results in 263.8 t / h of MgO produced, which is blended with a mixture mainly consisting of silica, alumina, and ferrite. MgO constitutes 70% of the whole cement mixture

[0050] , with a production rate of 376 t / h.

[0093] According to a previous experimental study MgO and CO2are mixed at a 1:0.8 molar ratio to maximize the strength of MgO-based cement. One day of CO2curing is recommended, where the CO2uptake is measured at 34% (129.27 t / h) of the total cement weight

[0025] , to form a cured concrete at 506.13 t / h. According to the same study, longer curing duration could lead to more than +50% uptake of CO2, which makes it more valuable as CO2sequestration method, but that might reduce the tensile strain capacity of the produced concrete.

[0094] The heat duty data for thermal energy analysis can be seen in Figure 13.

[0095] The electrical power consumption is measured at 166.9 MW for water drying and 93 MW for heating Mg(OH)2, this is expected knowing that heat of enthalpy needed for evaporating water is very high due to phase change. While calcination required 153.65 MW and the cooler needed to discharge 88.1 MW. Slight Acidification

[0096] When seawater is filtered in an RO process, the pH of the brine increases slightly due to hydroxide ions rejection [Kezia et al.2014], which may lead to membrane scaling / fouling in the last RO membrane modules. This should be prevented where possible. Hence, the pH of the treated seawater can be slightly lowered below the pH in the 1stprecipitation stage by, e.g., self- produced HCl (which is available if a BMED process is used) to >9 to ensure high boron and silica rejection in a single pass RO process. However, since the pH level is not high to pose a concern after DIC removal at the first precipitation stage, it could be directly sent to the RO process. According to WAVE results, the pH level of concentrate will increase from 9.36 to 9.55, as seen in Figure 14 and the permeate’s pH will be at 8.36 which is within drinking water health standards [Dietrich et al.2015]. The flow rate entering the RO process is equal to 129,802 m3 / h with a total dissolved solids (TDS) at 34,281 ppm. Single-Pass Reverse Osmosis

[0097] As seen in Figure 15, the boron concentration in the RO permeate at normal operating condition (e.g., 8.1 pH) is 0.9 mg / L, which is higher than the 0.5 B mg / L standard for drinkingwater in most countries [Ophek et al.2015]. There is a need for an increased pH operation to avoid the necessity of a 2ndRO pass to target regulatory acceptable boron levels. At a pH of 9 the process barely reaches 0.49 mg / L, which is close to the regulatory limits and may exceed it due to process fluctuations and how sensitive the results could be to external factors (e.g., inlet water salinity, and membrane stability). Therefore, operating at a range closer to 9.5 pH (e.g., 9.36 pH) ensures a constant removal of boron. However, operating at higher pH level (e.g., beyond 10 pH) may trigger some Mg(OH)2precipitation and scaling as seen in Figure 10.

[0098] The ideal process will drop the boron concentration in permeate from 0.9 mg / L to a range of 0.49 to 0.25 mg / L, which means a 46% to 72% reduction in boron concentration meeting drinking water standards without a need for a second RO pass. Working closer to the limit of 10 pH is possible since at that limit only less than 0.5% of Mg will precipitate, which may be needed depending on the inlet concentration and membrane ability to remove boron. That will lead to boron equal to 0.15 mg / L, which represents 83.3% reduction in its original concentration in the permeate. That means boron rejection is raised from 90.2% up to 98.4%.

[0099] The overall boron rejection with reference to the feed according to the experimental results from the previous study of high pH single-pass RO was 93.5% [Ophek et al.2015], while it is estimated to be ~96.6% using WAVE simulation at 9.36 pH. For the overall process, the RO freshwater recovery is set to 50%, which is a conservative target compared to the experimental study that reached 56% due to pH pre-treatment [Ophek et al.2015]. The brine and permeate flow rates of 64,901 m3 / h with brine TDS at 68,554 ppm and permeate TDS less than 300 ppm, meeting the guidelines of World Health Organization (WHO) limit of 500-600 ppm [WHO 2022]. The power consumption is measured at 207.7 MW, while the power savings in the single-pass RO desalination step is estimated to reach 51.9 MW in comparison to a conventional two-pass RO process.

[0100] When it comes to the possibility of silica scaling issues, the higher pH operation helps in reducing the effect and even reduce the load on any pre-treatment needed. This is seen in Figure 16, where silicic acid H4SiO4 concentration is reduced with higher pH levels by converting it into anions (e.g., H3SiO4-) going from 3.31 mg / L at 8.1 pH to 2.11 mg / L at 9.36 pH and reaching a low of 0.933 mg / L at 10 pH. That means the process could achieves up to 71.8% reduction in silicic acid concentration, thus limiting the possibility of scaling. 2ndPrecipitation Stage

[0101] As shown in Figure 10, the pH should be increased to ~12.5-13 to ensure maximum Mg(OH)2 precipitation. According to Aqion hydrochemistry results, going any higher than 13 will reduce the efficiency of the precipitation step, which would lead to a waste in the used NaOH asseen in Figure 17. This is due to Ca(OH)2formation, which also competes with DIC removal in the form of CaCO3seen in Figure 10 and eq(14-15).

[0102] As seen in Figure 17, the ideal range for Mg(OH)2production per NaOH consumed is between 12-13 pH where the lowest point represents the biggest return on caustic soda consumed per brucite produced, while the highest point nets a higher recovery. Noting earlier precipitation starts from pH 10 with substantial brucite production after 10.5 pH. For CaCO3it is preferable to operate in a range of pH 9-10 during the first precipitation step. Operating at the higher end of the range will ensure a greater recovery as seen in Figure 10, but with higher NaOH consumption.

[0103] The NaOH flow rate is 522.6 t / h at a concentration of 0.5 M, this accounts for 2 moles of Na+ per 1 mole of Mg2+, which is stoichiometrically ideal as seen from simulation results in Figure 17, and previous experimental studies [Dong et al. 2018; Ruan et al. 2021]. The total mixed flow is 91,029 m3 / h of water and NaOH. This leads to 97.2% Mg recovery and 67.4% DIC removal (98.7% total DIC removal) in the form of 381.8 t / h of Mg(OH)2 and 0.71 t / h CaCO3. The pH reached a high of 12.65 before settling down to 10.08 after OH- consumption in the mineralization process seen in eq(14).

[0104] The precipitant is recovered via coarse filtration / sedimentation, a filter press to dry the material, and then calcinated. Like the first filter press setup, the flow rate is made of water and minerals where the total flow is 849.97 t / h, where water accounts for 55% of the total. The higher purity MgO product can be used for cement production and further CO2 sequestration can be achieved here by injecting CO2 during the concrete curing process. Effluent discharge and CO2 sequestration

[0105] 80% of the high pH and scalant free brine can be discharged. Due to its high pH, CO2 can be directly injected and sequestered in the stream to lower the pH of the stream to adequate discharge levels (pH~8) which accounts for 6.08 t / h of CO2. This is demonstrated by eq(1-4) [Bang et al.2017].

[0106] The remaining 20% of the high pH and scalant free brine can be used for NaOH production, which is required within the process for all precipitation processes. NaCl is used here as the source of caustic soda production as demonstrated by the following electrolysis equation: NaCl + H2O ⇌ NaOH + HCl(16)

[0107] Prior to the BMED process, however, CO2injection into the high pH brine may lead to CaCO3precipitation as this product is generally formed at a pH>10. This may be used to further recover the remaining Ca from the brine and to lower the scaling potential of the BMED process.

[0108] Initial results from hydrochemistry simulations suggest that the ideal pH level for that would be pH 9-9.25 as seen in Figure 18. At pH 10, the carbon content is depleted and limiting the precipitation process, however, increasing CO2concentrations will lead to more acidification (increased H+release), which will favour eq(4) to shift to the left (Le Chatelier's principle) towards bicarbonate formation instead of CaCO3. Thus, a strong base will be needed again (but to a lesser degree) to maintain the ideal pH for a high Ca recovery. LSRRO (brine concentration)

[0109] The feed to LSRRO is measured at 17,566 m3 / h with a TDS of 50,900 ppm and 50% recovery, which is well within the working limits of LSRRO used in brine concentration studies. The power consumption is 52.7 MW. 3rdPrecipitation Stage

[0110] At the third precipitation stage a total of 9.47 t / h of CO2 can injected to precipitate Ca2+as CaCO3 as demonstrated earlier in eqs(1-6), while 21 t / h of NaOH at 0.5M is needed to counteract the CO2 acidification and to increase the pH level to 11.63. This pH level is chosen to also ensure that any remaining Mg2+ions precipitate as Mg(OH)2 as shown in eq(14).

[0111] This results in a 98.2% and 99.4% removal of Ca2+and Mg2+, respectively. Hence, the brine concentration of scaling precursor ions leaving the 3rdprecipitation stage is below the safe / free scaling limits of BMED, namely 14.8 mg / L and 0.53 mg / L of Ca2+and Mg2+, respectively.

[0112] After mineral precipitation, 19.8 t / h of CaCO3 and 2.16 t / h of Mg(OH)2 are removed from the brine stream via settling and filtration. It’s worth noting that at this high pH operation and CO2 injection, some SrCO3 precipitants are formed at a flow rate of 0.46 t / h. The pH at the 3rdprecipitation stage outlet is 11.09.

[0113] The outlet brine is then fed to a filter press with 49.81 t / h of sludge to be dewatered. Similar to the previous filter press stages, it is assumed that the minerals account for 45% of the sludge with the remainder being water. The power consumption is estimated at 8.29 MW.

[0114] Since the largest share of the 3rdstage precipitated minerals is CaCO3 with less Mg(OH)2 , it may not be necessary to introduce this dewatered sludge to any calcination mix for MgO production.BMED

[0115] The concentrated brine stream at 9,740 t / h and 92,038 ppm (Mainly NaCl) is electrolysed in the BMED unit, which results in the equimolar (eq.16) production of 557.4 t / h and 508.1 t / h of NaOH and HCl respectively, at a concentration of 0.5 M. The power needed to drive the unit is 557.39MW, which accounts for 52% of the total process power requirement of 1,374.56 MW. It is estimated that more than 30% of power can be saved by using BMED instead of the Chlor Alkali electrolyzer process.

[0116] Dewatering (i.e., filter press and drying) and calcination accounts for the highest portion of power consumption at 41.66%, while the BMED is close second by 40.55%, and the least desalination (RO and LSRRO) at 17.79%.

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Claims

CLAIMS 1. A seawater treatment process, the process comprising the following steps: (a) providing seawater; (b) adjusting the pH of the seawater to pH 9.0–10.5 to precipitate dissolved inorganic carbon (DIC); (c) separating the precipitated DIC from the seawater; (d) subjecting the seawater resulting from (c) to a reverse osmosis process to produce freshwater and brine; (e) adjusting the pH of the brine to pH 10.0–13.25 to precipitate Mg2+ions; and (f) separating the precipitated Mg2+ions from the brine.

2. The process as claimed in claim 1, wherein step (b) comprises adjusting the pH of the seawater provided in step (a) to pH 9.0–10.5 to precipitate dissolved inorganic carbon (DIC).

3. The process as claimed in claim 1 or 2, wherein DIC is precipitated in step (b) as CaCO3. 4 The process as claimed in claim 1, 2 or 3, wherein step (c) comprises separating the precipitated DIC from the seawater by a solid-liquid separation process, and wherein the solid- liquid separation process is a filtration process.

5. The process as claimed in any one of the preceding claims, wherein the seawater resulting from (c) has a pH of 9.0–10.

0.

6. The process as claimed in any one of the preceding claims, wherein step (d) comprises subjecting the seawater resulting from (c) to a single pass reverse osmosis process.

7. The process as claimed in any one of the preceding claims, wherein the brine resulting from step (d) has a pH of 9.3–9.

7.

8. The process as claimed in any one of the preceding claims, wherein the freshwater resulting from step (d) has a boron concentration of <0.5 mg / L.

9. The process as claimed in any one of the preceding claims, wherein prior to step (e), the brine resulting from step (d) is concentrated (e.g., thermally, electrochemically and / or using a membrane).

10. The process as claimed in any one of the preceding claims, wherein step (e) comprises adjusting the pH of the brine to pH 11.5–13.25 to precipitate Mg2+ions.

11. The process as claimed in any one of the preceding claims, wherein step (f) comprises separating the precipitated Mg2+ions from the brine by a solid-liquid separation process, and wherein the solid-liquid separation process is a filtration process.

12. The process as claimed in any one of the preceding claims, wherein Mg2+ions are precipitated in step (e) as Mg(OH)2.

13. The process as claimed in claim 12, wherein the Mg(OH)2is processed into MgO.

14. The process as claimed in any one of the preceding claims, wherein at least a portion of the brine resulting from step (f) is concentrated (e.g., thermally, electrochemically and / or using a membrane).

15. The process as claimed in any one of the preceding claims, wherein the pH of at least a portion of the brine resulting from step (f) is adjusted to pH 11.0–14.0 to precipitate one or more divalent metal ions (i.e., a third precipitation step).

16. The process as claimed in claim 15, wherein the precipitated one or more divalent metal ions are separated from the brine by a solid-liquid separation process, and wherein the solid- liquid separation process is a filtration process.

17. The process as claimed in any one of the preceding claims, wherein brine produced by the seawater treatment process is used to prepare a pH adjusting agent.

18. The process as claimed in claim 17, wherein the pH adjusting agent is produced by a bipolar membrane electrodialysis process or a chlor-alkali electrolysis process.

19. The process as claimed in claim 17 or 18, wherein the pH adjusting agent is NaOH.

20. The process as claimed in claim 17, 18 or 19, wherein the pH adjusting agent is used to adjust the pH in step (b) and / or step (e) and / or the third precipitation step.

21. The process as claimed in claim 17, 18 or 19, wherein the pH adjusting agent is used to adjust the pH in step (b) and step (e), and optionally the third precipitation step.

22. The process as claimed in any one of the preceding claims, wherein the seawater is subjected to at least one pretreatment step prior to being subjected to reverse osmosis in step (d).

23. The process as claimed in claim 22, wherein the at least one pretreatment step reduces membrane fouling during reverse osmosis in step (d).

24. The process as claimed in claim 22 or 23, wherein the at least one pretreatment step occurs before step (b) and / or after step (c).

25. The process as claimed in claim 22, 23 or 24, wherein the at least one pretreatment step is a filtration process, and wherein the filtration process is an ultrafiltration process and / or a multimedia filtration process.

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