AU2017246245A1 - Mineral recovery and method for treatment of water having carbonate alkalinity - Google Patents

Abstract

A method for mineral recovery comprising: conditioning water having carbonate alkalinity with a source of hydroxide; adding a source of magnesium ions to the conditioned water, thereby forming a magnesium carbonate compound; and recovering the magnesium carbonate compound, wherein the source of hydroxide is added to the water in an amount sufficient to achieve a predetermined carbonate equilibria in the conditioned water.

Description

MINERAL RECOVERY AND METHOD FOR TREATMENT OF WATER HAVING CARBONATE ALKALINITY

FIELD OF INVENTION

The present invention relates to methods for mineral recovery and, also, methods for the treatment of water having alkalinity, specifically carbonate/bicarbonate alkalinity, and optionally salinity (i.e. sodium salts of chloride), such as, but not limited to, coal seam brine. The present invention also provides methods for the amelioration or elimination (i.e. suppression) of carbon dioxide emission during the treatment of water having carbonate/bicarbonate alkalinity. The present invention further relates to methods for the recovery of magnesium carbonate compounds from water having carbonate/bicarbonate alkalinity, such as, but not limited to, hydromagnesite, dypingite and nesquehonite. Products of such methods also form another aspect of the invention. Systems suitable for use in the methods are further considered.

BACKGROUND ART

Carbonate Alkalinity

Carbonate alkalinity in water has traditionally been dealt with through treatments using acid. In the case of coal seam gas, carbonate alkalinity which includes both carbonate and bicarbonate is high enough to make the addition of acid a costly exercise. The evolution of carbon dioxide gas during this process is an undesirable consequence of existing treatment schemes, since it is a greenhouse gas and also because it causes significant processing issues.

In common water treatment schemes, conventional thinking has been to adjust alkalinity with acid and not caustic. Addition of magnesium generates magnesium carbonate (MgCO3) with no addition of caustic. Generally, precipitants generate another waste stream that must be dealt with and this is not desirable. Furthermore, it is generally not possible to produce commercial products from such waste streams. The stream is chemically ‘dirty’, and full of unwanted impurities, which cannot be removed when required.

The coal seam gas industry has spent 100’s of millions of dollars and several years investigating how to process brines into commercial, saleable products, primarily looking at separating sodium salts of chloride and bicarbonate and carbonate. This approach was taken up by the industry and subsequently abandoned due to massive capital investment requirements and expensive ongoing operating costs.

Alkalinity and pH in the above systems have a relationship. Generally, above a pH of 10.2, constituents are hydroxide ion and carbonate ion; between a pH of 10.2 and 8.3, constituents are carbonate and bicarbonate; between a pH of 8.3 and 4.3, constituents are bicarbonate and carbon dioxide; and below 4.3 free acidity and carbon dioxide are present. Other species present in the water and operating conditions may distort this. Graphically:

Toft TcoT C02 Acid

Magnesium Carbonate

Magnesium carbonate, MgCO3, is a white solid that occurs in nature as a mineral. Several hydrated and basic forms of magnesium carbonate also exist as minerals. In addition, MgCO3 has a variety of applications. The most common magnesium carbonate forms are the anhydrous salt called magnesite (MgCO3) and the di, tri, and pentahydrates known as barringtonite (MgCO3-2H2O), nesquehonite (MgCO3-3H2O), and lansfordite (MgCO3-5H2O), respectively. Some basic forms such as artinite (MgCO3-Mg(OH)2-3H2O), hydromagnestite (4MgCO3-Mg(OH)2-4H2O), and dypingite (4MgCO3- Mg(OH)2-5H2O) also occur as minerals.

Magnesium carbonate can be prepared in laboratory by reaction between any soluble magnesium salt and sodium bicarbonate:

MgCI2(aq) + 2NaHCO3(aq) —> MgCO3(s) + 2NaCI(aq) + H2O(I) + CO2(g)

When the solution of magnesium chloride (or sulfate) is treated with aqueous sodium carbonate, a precipitate of basic magnesium carbonate is formed: 5MgCI2(aq) + 5Na2CO3(aq) + 5H2O(I) Mg(OH)2-3MgCO3-3H2O(s) + Mg(HCO3)2(aq) + 10NaCI(aq)

High purity industrial routes include a path through magnesium bicarbonate: combining magnesium hydroxide and carbon dioxide. A slurry of magnesium hydroxide is treated with 3.5 to 5 atm of carbon dioxide below 50°C, giving the soluble bicarbonate, then vacuum drying the filtrate, which returns half of the carbon dioxide as well as water:

Mg(OH)2 + 2 C02 Mg(HCO3)2 Mg(HCC>3)2 —► MgCO3 + C02 + Η20

Water Treatment

Brackish water is often produced as a by-product of coal seam gas extraction/exploitation in Queensland, Australia. This water is of no practical use as the salinity is too high for irrigation and not fit for livestock watering purposes.

At the volumes produced, this water poses an environmental threat to agricultural and native land and water courses due to the salinity of the water. Thus, the water is confined to being collected and treated. Additionally, coal seam gas operators are increasingly confined to a zero liquid discharge (ZLD) requirement, and so treating the formation water for beneficial use is compulsory. It is noted that ZLD means that, eventually, solids must be produced and disposed of. It means that the only streams ‘discharged’ are essentially ‘dry’ solids. Dry may also mean, for example, an 85 wt% slurry going to landfill.

The treatment of this water generates a significant reject or waste stream that has no useful end point. It is noted that ‘reject’ is generally a technical term defining a part of processed brine. ‘RO reject’ is also used to describe this stream. Moreover, government regulations dictate that the salt or waste stream must be treated to create useable products wherever feasible. In the case of no commercial solution, the brine must be further processed until the dissolved salts are made solid. These ‘solids’ generally have to be disposed of in a regulated waste facility.

Water produced from each well is collected in a water gathering system. The water gathering system connects all of the individual wells to a central conduit, which delivers all of the collected water to a water treatment facility.

The treatment of coal seam gas produced water is primarily focussed on recovering useable water which can be distributed to end users, such as agriculture industries and nearby communities. This reduction in volume is also desirable to the CSG operators since it is cheaper than storing the sheer volume produced in engineered dams.

Before the water is treated, it usually undergoes pre-treatment, which prepares the water for the treatment processes. Typical pre-treatment processes which may be employed include filtration, dissolved air flotation, ion exchange, chemical addition, and electrodialysis.

Reverse osmosis (RO) is the primary treatment process used to treat and purify CSG water. Subsequent processing may be employed, including distillation and solar evaporation. In the case of RO, the formation water is divided into a permeate stream and a reject stream. The reject stream, also known as RO reject, is generally more concentrated in dissolved solids than the formation water feedstock. Reverse osmosis plants are run at high permeate recoveries, since the object is to reduce volume of the feed water so that the problem becomes more manageable. This generates a significant volume of useable, high quality water, which is easy to offload for the operator. It also generates a lesser, but problematic volume of unusable brine.

The reject stream, or RO reject, can be further reduced in volume by brine concentrators which are able to generate a distillate stream of water and a concentrated liquor stream of high temperature and high salinity, referred to as CSG brine.

Coal seam gas brine, which may generally be described as a brine having >40,000 mg/l TDS, is further treated by evaporation, thermal or mechanical separation, desalination processes or the like. The initial aim is to reduce the volume of the coal seam brine so that it is manageable. The ultimate aim is to reduce the brine to crystalline solids and then dispose of those solids according to enviro-legal obligations. This is also the subject of significant cost issues.

Currently operating brine concentrators use mechanical vapour recompression to operate, which means that they effectively operate at (near) atmospheric pressures.

Attempts have been made to commercially recover minerals from coal seam brine or formation water, but these have met with little, if any, success. Evaporation of the RO reject stream (i.e. CSG brine), which may have been exposed to the environment in intermediate storage ponds/dams, ultimately results in a waste mixed salt that is fated for disposal in a regulated waste facility. A large cost is usually associated with the disposal of the waste salt since it requires the construction of an engineered salt storage facility and transportation to the waste salt facility (i.e. if it is not collocated within the evaporation facility).

The method of the invention generally relies heavily on an understanding of the carbonate system in the formation water as it is processed, as well as crystallisation phenomena and aqueous chemistry. As such, these issues will be discussed briefly below.

The carbonate system involves equilibrium of carbon dioxide speciation in water. The species present and the proportioning between species are dependent on the pH of the solution, as discussed above.

Carbonate equilibria may be represented by the following equations: CO, ,i 4 Liquid Water 4- Pressure 4 CO, CO, 4 H,0 4 fH,CO,T_.,

i, i I i, iJi,Li.£iJ 2[HCO3-](ai?) 4 [ft2CO3]^5 4 [CO32-]^} [HCO5-]Ce<s 4 [HX 4 [CQ/-]^, [H-χ 4 [OH-]iag) 4 fi20a)

Ultimately, carbon dioxide equilibria in aqueous environments is dictated by the partial pressure of CO2 gas, and the presence of hydroxide in the liquid. CSG formation water (in coal seam/subterranean aquifers) is composed of predominantly sodium bicarbonate and sodium chloride in solution. As soon as the formation water leaves in the aquifer, carbon dioxide starts evolving from solution. The reason for this is that the partial pressure of carbon dioxide outside the aquifer is less than the pressure in the aquifer.

Some carbon dioxide also comes out of solution as the formation water is progressively treated. An observable effect of this is that the pH of solution generally increases as the formation water is progressively treated. The pH at equilibrium in the aquifer may be as low as 7.1. However, by the time it is rejected by RO, the pH may be around 9. If the brine is then evaporated, more CO2 will evolve. Concentrated brine may have a pH closer to 10.

Taking the example of hydromagnesite, there are several routes for production. One example is the bicarbonate route: SMg£S2 fa(5) 4 10NaHCOs^3 MgsCC0s)4(0H>2.4B20 ω 4 6COS 410Na£3

The bicarbonate route involves a reaction of sodium bicarbonate and magnesium chloride at elevated temperature. In this process, 60% of the moles of bicarbonate evolve as CO2 gas. This may create operational problems (from associated foaming), and the reaction time is excessive, since CO2 takes a long time to completely evolve from solution. The reaction forms magnesium bicarbonate, Mg(HCO3)2, as an intermediate species, which is unstable and has to lose a carbon dioxide molecule so that it can participate in the formation of hydromagnesite chains. Endpoint control may not be possible or practical and, because of this, further concentration of the effluent is difficult. This is because carbon dioxide foaming remains a real issue. Additionally, a blunt end point leaves either an excess of remaining bicarbonate/carbonate alkalinity, or an excess of magnesium ions in solution which impacts on the subsequent recovery of a useable sodium chloride product from the effluent.

For reference only, Figure 1 illustrates a chart that examines the solution chemistry of the bicarbonate route. For the bicarbonate route, there is a lack of convergence of species at the end point of the reaction (0.5 moles MgCI2). There is still carbon in solution, but there is also magnesium remaining in solution. Carbon dioxide gas is also generated throughout the reaction.

Crystallisation is a purification technique. The main factor determining the recovery of crystal from a liquor within specification is the composition of the starting liquor. In terms of NaCI crystallisation, the ratios of sodium to other cations and chloride to other anions are of critical importance. Of some concern during the development of the present invention were the ratios of CI:Br, Cl:l and Na:Mg.

The first crystals produced by a liquor are the purest. As NaCI crystallises, the liquor becomes more concentrated with impurities, such as bromine and magnesium. This means that the critical ratios are adversely changing as the brine is processed. Since the ratios in the starting solution determine the possible salt recoverable before a specification is exceeded, altering the ratios allows significantly more salt to be recovered.

The subject matter claimed herein is not limited to embodiments that solve any disadvantages or that operate only in environments such as those described above. Rather, this background is only provided to illustrate exemplary technology areas where some embodiments described herein may be practiced.

Various aspects and embodiments of the invention will now be described.

SUMMARY OF INVENTION

As mentioned above, the present invention relates generally to methods for mineral recovery and, also, methods for the treatment of water having alkalinity, specifically carbonate/bicarbonate alkalinity, and optionally salinity (i.e. sodium salts of chloride), such as, but not limited to, coal seam brine. The present invention also provides methods for the amelioration or elimination (i.e. suppression) of carbon dioxide emission during the treatment of water having carbonate/bicarbonate alkalinity. The present invention further relates to methods for the recovery of magnesium carbonates from water having carbonate/bicarbonate alkalinity, such as, but not limited to, hydromagnesite, dypingite and nesquehonite. Products of such methods also form another aspect of the invention, as do systems for use in the methods.

According to one aspect of the invention there is provided a method for mineral recovery comprising: conditioning water having carbonate alkalinity with a source of hydroxide; adding a source of magnesium ions to the conditioned water, thereby forming a magnesium carbonate compound; and recovering the magnesium carbonate compound, wherein the source of hydroxide is added to the water in an amount sufficient to achieve a predetermined carbonate equilibria in the conditioned water.

According to a related aspect of the invention there is provided a method for the treatment of water having carbonate alkalinity comprising: conditioning the water with a source of hydroxide; adding a source of magnesium ions to the conditioned water, thereby forming a magnesium carbonate compound; wherein the source of hydroxide is added to the water in an amount sufficient to achieve a predetermined carbonate equilibria in the conditioned water.

The amount of hydroxide added is preferably sufficient to at least ameliorate carbon dioxide evolution in the conditioned water. More preferably, though not necessarily, the amount of hydroxide added is sufficient to eliminate carbon dioxide evolution in the conditioned water.

In a preferred embodiment, the conditioned water has a carbonate:hydroxide ratio of at least 2:1, preferably approximately 2:1.

The source of hydroxide is preferably NaOH, though other sources of hydroxide may be equally applicable, such as Brucite (Mg(OH)2). The source of hydroxide may be an aqueous source of hydroxide, or may be some other form. The source of magnesium ions is preferably MgCI2, though Mg(NO3)2 or MgSO4 may also be used. The latter options may introduce economic and processing disadvantages compared with MgCI2. The source of magnesium ion may be an aqueous source of magnesium ion, or may be some other form.

The temperature of the conditioned water may be selected depending on the desired magnesium carbonate compound. For example, the temperature of the conditioned water may be up to 50°C, from 50 to 55°C, or higher than 55°C. Generally, where hydromagnesite is the desired magnesium carbonate compound, the temperature of the conditioned water is above about 55°C. In preferred embodiments, the temperature is maintained throughout the method or treatment. This advantageously maintains the phase of the magnesium carbonate compound (e.g. hydromagnesite).

As will be appreciated from the above discussion, in one particular embodiment the water having carbonate alkalinity is coal seam gas brine. For example, the coal seam gas brine may have a dissolved solids content of up to up to saturation point, for example up to about 30 wt%. It will generally be preferred that the coal seam gas brine be pre-treated prior to conditioning with the source of hydroxide, such as with filtration of organics, scrubbing, concentration and/or reverse osmosis. In most cases, the coal seam brine is preferably pre-treated to remove unwanted components, such as silica and/or fluoride.

In certain embodiments, the methods of the invention further comprise removing contaminating halides following formation of the magnesium carbonate compound. The removed contaminated halides may comprise bromide and iodide, the removal preferably comprising acidification of water containing the halides, followed by oxidation.

Additional value may be realised through the recovery of other compounds using the methods of the invention. For example, in certain embodiments the methods further comprise recovering remanent salt, such as NaCI, following removal of the magnesium carbonate compound.

In certain embodiments, the magnesium carbonate compound is hydromagnesite and/or dypingite. Recovery of the magnesium carbonate compound may comprise dewatering, washing and drying.

It should further be appreciated that the methods described above may be batch or semi-batch processes or may be continuous. It is envisaged that the methods of the invention will preferably be continuous, particularly in the cases where the water feed for processing is voluminous, such as in the case of waste water treatment (e.g. coal seam gas brine treatment). Batch or semi-batch processing may be advantageous if there is a need to account for variations in the feed stock.

The method may comprise maintaining pH and/or Mg ion in the conditioned water by adjusting addition of the source of hydroxide, if needed, to achieve the predetermined carbonate equilibria in the conditioned water. For example, the pH may be monitored to ensure it remains at or above 8.3 to suppress any CO2 generation. Mg ion content may be monitored as a rise in Mg ion may be accompanied by CO2 generation from the system.

According to another aspect of the invention there is provided use of a source of hydroxide in the suppression of carbon dioxide formation during the treatment of water having carbonate alkalinity with a source of magnesium ions.

According to another aspect of the invention there is provided a method for the production of a magnesium carbonate compound comprising: providing a feed of water having carbonate alkalinity; conditioning the feed with a source of hydroxide to a carbonate:hydroxide ratio of at least 2:1; and adding a source of magnesium ions to the conditioned feed to form said magnesium carbonate compound.

The above preferred features and examples are equally applicable to the above further aspects of the inventions and are explicitly incorporated into those aspects of the invention by reference.

The invention also provides magnesium carbonate compounds when recovered or produced by a method as described above.

According to a further aspect of the invention there is provided a system for mineral recovery or treatment of water having carbonate alkalinity, the system comprising: a feed of water having carbonate alkalinity; a reactor having an inlet feeding the reactor with a source of magnesium ions; an inlet to the feed of water and/or the reactor, feeding the feed of water and/or reactor with a source of hydroxide; a separator in fluid communication with the reactor and adapted to separate a magnesium carbonate compound from treated water, wherein the source of hydroxide is used to condition the water having carbonate alkalinity to achieve a predetermined carbonate equilibria in the conditioned water prior to feeding the reactor with the source of magnesium ions.

Throughout this specification, unless the context requires otherwise, the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated step or element or integer or group of steps or elements or integers, but not the exclusion of any other step or element or integer or group of steps, elements or integers. Thus, in the context of this specification, the term “comprising” is used in an inclusive sense and thus should be understood as meaning “including principally, but not necessarily solely”.

The present invention consists of features and a combination of parts hereinafter fully described and illustrated in the accompanying drawings, it being understood that various changes in the details may be made without departing from the scope of the invention or sacrificing any of the advantages of the present invention.

BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS

To further clarify various aspects of some embodiments of the present invention, a more particular description of the invention will be rendered by references to specific embodiments thereof, which are illustrated in the appended drawings. It should be appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting on its scope. The invention will be described and explained with additional specificity and detail through the accompanying drawings in which: FIG. 1 illustrates a chart that examines the solution chemistry of a known bicarbonate route for production of hydromagnesite. FIG. 2 illustrates a simplified flowchart of a method in accordance with an embodiment of the invention. FIG. 3 illustrates a graph of magnesium addition (Mg in ppm : carbonate species in mM : pH based on an unconditioned brine). FIG. 4 illustrates a graph of magnesium addition (Mg in ppm : carbonate species in mM : pH based on a conditioned brine (2 CO3: 0.5 OH)). FIG. 5 illustrates a graph of magnesium addition (Mg in ppm : carbonate species in mM : pH based on a conditioned brine (2 CO3: 1 OH)). FIG. 6 illustrates a graph of magnesium addition (Mg in ppm : carbonate species in mM : pH based on a conditioned brine (OH excess)). FIG. 7 illustrates a flowchart of a method for the production of hydromagnesite according to one embodiment of the present invention. FIG. 8 illustrates a chart that examines the solution chemistry of the production of hydromagnesite according to a method of the invention. FIG. 9 illustrates a thermographic analysis of hydromagnesite product. FIG. 10 illustrates the hydromagnesite commercial standard.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, this specification will describe the present invention according to the preferred embodiments. It is to be understood that limiting the description to the preferred embodiments of the invention is merely to facilitate discussion of the present invention and it is envisioned without departing from the scope of the appended claims. Specifically, pre-treatment and post-treatment stages discussed in the following detailed description should be understood to be described as optional stages that are not limiting on the scope of the central, essential stages of the method of the invention.

Referring to Figure 2, generally the invention relates to a method for the treatment of a feed of water having carbonate alkalinity 100. The carbonate alkalinity may primarily contain carbonate, with some additional bicarbonate to provide the carbonate alkalinity. The feed water may also have hydroxide alkalinity. It may have other constituents, such as but not limited to sodium, chloride, iodide, bromide and so on.

Depending on the desired products of the method, the temperature of the water having carbonate alkalinity 100 may vary. For example, if the water 100 is at an elevated temperature of greater than 55°C basic Magnesium Carbonate Light (Hydromagnesite) may be formed as Mg5(CO3)4(OH)2.4H2O. Under specific temperature conditions of from 52-55°C, Magnesium Carbonate Heavy (Dypingite) may be formed as Mg5(CO3)4(OH)2.5H2O is formed. The temperature of the water 100 may be as a result of prior processing, or may be achieved through heating or cooling of the water 100.

The water having carbonate alkalinity 100 is conditioned 110 with hydroxide from a source of hydroxide 120. The amount of hydroxide added may be determined based on the desired carbonate equilibria to be achieved in the system and the nature of the water having carbonate alkalinity 100. As previously mentioned, the amount of hydroxide added will be such that the carbonate equilibria is predetermined to provide for a system generally having constituents of hydroxide, carbonate and bicarbonate, while supressing carbon dioxide production. In terms of pH, this would generally include pH’s of above about 8.3, though lower pH’s may be acceptable based on acceptability of an amount of carbon dioxide formation. Put another way, the method is generally operated to avoid carbonate depletion and magnesium formation in the conditioned water 110 on the addition of a source of magnesium 130, as discussed in more detail below. The amount of hydroxide added advantageously provides a carbonate:hydroxide ratio of about 2:1.

The source of hydroxide 120 is generally a sodium hydroxide solution. For example, the hydroxide feed could be a solution of sodium hydroxide, with hydroxide beads hoppered in. This may be pumped and mixed in with the water feed (e.g. brine solution). Dosage could be provided as a batch dosage or in-line dosage to achieve the desired ratio. A source of magnesium ion 130, such as a magnesium chloride solution, is added to the conditioned brined to precipitate a magnesium carbonate compound for recovery 140. As mentioned above, the magnesium carbonate compound may be hydromagnesite or dypingite, though other magnesium carbonate compounds are also considered within the scope of the present invention. The source of magnesium ion 130 could be from hydrated chlorides (e.g. hexahydrate), dehydrates and anhydrous forms. For example, magnesium chloride as a solid may be dissolved in the carbonate depleted water (e.g. brine), which may then be recirculated into the process to reduce water requirements. This can be added, for example, to the pre-conditioned brine.

An additional pretreatment step may comprise using formed hydromagnesite, or some other absorbent, in the removal of silica and fluoride to improve brine purity. The hydromagnesite product can be recycled by dissolving with acid and silica and fluoride recovered if economically viable. It may also be processed further into other commercial products.

Referring to Figures 3-6, graphs are provided supporting the viability of the method of the invention generally. In these graphs pH is on the right axis, all carbonate species are in mM and Mg is in ppm.

Figure 3 illustrates an addition curve of magnesium on addition to an unconditioned brine at around 70-80°C. The brine contained primarily carbonate (56200mg/L as CaC03) with a little bit of bicarbonate (9700mg/L as CaC03).

As illustrated, there is immediate production of bicarbonate and linear reduction of carbonate on the addition of magnesium ion. No noticeable carbon dioxide gas was observed until fraction 0.5, where Mg is also observed. The production of carbon dioxide gas affects settling of the solids from 0.5 onwards. At this point, the carbonate is virtually depleted and the bicarbonate to carbonic acid equilibrium starts to take effect. From fraction 0.5 onwards carbon dioxide was evolved.

As mentioned above, from 0.5 onwards the amount of magnesium starts to increase. With an excess of magnesium and an excess of bicarbonate, a resultant brine that contains inorganic carbon and magnesium, along with the sodium and chloride, is produced. This is still recoverable with the addition of hydroxide, although loss occurs with carbon dioxide production.

This graph clearly shows that not all of the carbonate can be removed without hydroxide addition. Although some product can be recovered, an undesirable brine is left. Recovery of magnesium compounds, for example hydromagnesite, is achievable up to 0.5.

In this case the reaction up to fraction 0.5 would be: 5Mg2+ + 6CO32" + 2H2O -> Mg5(CO3)4(OH)2.4H2O + 2HCO3'

As illustrated, 2 molecules of water generate 2 hydroxide ions (to hydromagnesite) and 2 hydrogen ions (bicarbonate). To convert two moles of bicarbonate back to carbonate, 2 moles of hydroxide will be required.

Figure 4 illustrates an addition curve of magnesium to a conditioned brine at a 2 : 0.5 ratio of caustic. Compared to the above example (i.e. no addition of caustic), the point at which carbonate is depleted and magnesium starts to appear is shifted from 0.5 to 0.8. As such, more carbonate is recovered but there is still an amount of inorganic carbon left in the water. Also, from 0.8 carbon dioxide was evolved.

As will be noted from Figure 4, once hydroxide is depleted bicarbonate is formed. Reaction kinetics and mechanics are slow once bicarbonate is involved. With evolving carbon dioxide (which is slow) all of the inorganic carbon may be removed, but the rate will be much slower. The rate is limited by the conversion of carbonic acid to carbon dioxide gas.

Bicarbonate: 5Mg2+ + 6HCO3" + 2H2O -> Mg5(CO3)4(OH)2.4H2O + 2H2CO3

Carbonic Acid to Carbon Dioxide Gas:

H2CC>3 <—> CO2(g) + H2O

Referring to Figure 5, an addition curve of magnesium to conditioned brine is illustrated (carbonate:hydroxide ratio of approximately 2:1). This graph shows a very good correlation to modelling. Virtually no bicarbonate speciation is present.

As illustrated, the end point is between 0.8 to 0.9. The hydroxide is taken up at fraction 0.2. Very little bicarbonate is present, which is considered due to the presence of brucite pH in the order of 9. There is a little rise from the end point, which is considered more a reflection of the method (high range method) than an actual carbonate result. An increase in the magnesium is seen beyond the end point.

Referring to Figure 6, an addition curve of magnesium to conditioned brine with an excess of hydroxide is illustrated. This example has a clear excess of hydroxide (i.e. approximately 10% excess), as part of the conditioned brine. An endpoint is evident with magnesium again appearing with the exhaustion of carbonate. All of the lines almost extrapolate to fraction 1.0 at the end point. With excess brucite the pH curve is masked.

The graphs show a correlation of Mg appearing after depletion of carbonate. As such, it is anticipated that the methods of the invention may be monitored with just a pH meter and magnesium detection (ICP-MS or ion selective electrode). It is considered that it may generally be possible to control the process by adding hydroxide to maintain a pH of around 8.8 to 9.0 (excess OH) and a small amount of magnesium in solution.

Referring to Figure 7, a flowchart of a method of an embodiment of the invention is illustrated. Also illustrated in Figure 8 is a chart that examines the solution chemistry of the production of hydromagnesite according to a method of the invention.

The brine, which is received from a coal seam gas process 0000, has a high temperature. The brine concentrator supplies brine typically around 115°C (105120°C). The method of this embodiment of the invention for hydromagnesite production utilises this high temperature as hydromagnesite is less soluble at evated temperatures.

The major components in the brine received from the process 0000 are chlorides, carbonates, and some bicarbonates of sodium. During the process 0000 a significant amount of caustic may be added to convert some bicarbonate to carbonate to reduce the evolution of carbon dioxide gas from solution in the brine concentrator.

Samples received from the process 0000 may have high organics content. This can be managed/mitigated through changes in the process 0000 operating scheme (e.g. filtration prior to brine concentration, as well as a reduction in, or prevention of intermediate storage time between reverse osmosis and brine concentration stages).

It is expected that the process may supply brine at a dissolved solids concentration of up to about 30-40 wt% (but at high temperature) if this was desirable.

The brine concentrator is operated under mechanical vapour recompression, so it is essentially operating at atmospheric pressure and atmospheric boiling point. The desired temperature range will depend on the crystal structure or form of nesquehonite (low temperature <50°C), dypingite (mid temperature 50-55°C) or Hydromagnesite (high temperature >55°C). All of these forms can be calcined into magnesia.

Brine Preparation A brine preparation unit 1000 is made up of sub-units, including filtration, brine scrubbing and brine conditioning, each of which will be discuss in more detail below.

Filtration

The initial step in the brine preparation unit includes polishing to light duty filtration of the incoming feedstock to remove particles. The workload of this initial stage may be reduced by installing upstream filtration on the process 0000 and better management of the overall process 0000.

Brine Scrubbing or Hydromagnesite Stripping A brine scrubbing unit may be utilised to remove silica and fluoride in the brine, thereby improving brine purity. The consequences of not scrubbing may be that more silica and fluoride enters the hydromagnesite product.

The mechanism for silica removal is adsorption to the surface of hydromagnesite. The mechanism for fluoride removal is reaction with magnesium ion to form magnesium fluoride. It is envisaged that this process will operate continuously by dosing a small quantity of magnesium chloride into the incoming brine stream. Hydromagnesite waste may be removed at the same rate of magnesium addition. It is envisaged that this would only be a low grade product. There may also be a solid/liquid separation aspect to this unit.

Scrubbing may be conducted in a continuously stirred system, or a fluidised bed, or some other arrangement. An opportunity exists to step this process out by first removing silica (via hydromagnesite scrubbing) and then removing fluoride (by Mg2+ addition). This would allow the recovery of both the silica and the fluoride as saleable products and eliminate a waste stream.

It may be possible to strip both silica and fluoride by adding sufficient magnesium in a pre-reactor to generate hydromagnesite which may then scrub and remove silica, fluoride and possibly barium and strontium. Testing has identified that there is an affinity for F and Si in the hydromagnesite matrix.

It is noted that the mechanism for silica removal is adsorption onto the surface of the hydromagnesite. The fluoride removal mechanism involves a reaction with magnesium ion. It is considered that the key to effective fluoride removal is a high degree of dispersion of Mg ion into solution. Hydromagnesite kinetics are rapid, as are brucite kinetics, though MgF2 are faster. Therefore, a higher concentration MgF2 waste stream can be produced (less carbonate in the waste stream) with a high degree of dispersion of the Mg ion (i.e. rapid mixing). It has been observed that a significant reduction in fluoride (e.g. from 150ppm to <20ppm), and only a minor reduction in carbonate (e.g. a 5% reduction), was achieved by adding Mg ion and mixing quickly. If the kinetics for MgF2 were the same as for hydromagnesite, then the proportion in reduction would be expected to be equivalent.

Brine Conditioning

Brine conditioning involves adding hydroxide to the scrubbed brine until the ratio of carbonate to hydroxide in the stream is approximately 2:1. This allows (near) complete removal of carbonate from the stream. The 2:1 ratio can be explained by noting that the ratio of hydroxide to carbonate in the hydromagnesite mineral is 2:1. If this is not done, bicarbonate will remain in solution and the process effluent will have significant levels of residual magnesium (present as [Mg(HCO3)2](aq)). Excess caustic will result in the presence of brucite in the hydromagnesite produced. Small quantities of brucite will not impact filtration and product specification, however it may be present in the filter cake.

Carbon dioxide will also form towards the endpoint of the reaction and may cause processing problems such as foaming and retarded settling ability. If the intention is to deliver a viable zero liquid discharge solution, then near-complete removal of carbonate is required. It is noted that for near-complete NaCI recovery to occur, contaminating halides (i.e. bromide and iodide) must also be removed. Failure to remove these contaminating halides will impact on the amount of NaCI that can be recovered commercially. Bromine and Iodine can be removed by oxidation, but this can only be done at low pH ranges. Oxidation at pH values higher than 3 will cause the formation of soluble bromates and iodates, which cannot be separated from the stream. At pH values lower than 3, bromide is oxidised to bromine liquid, and iodide is oxidised to iodine solid. These can then be easily separated from the NaCI stream. To lower the pH, an acid (ideally HCI) is required. If any carbonate is left in solution, then excess acid will be required to lower the pH so that contaminating halides can be removed.

It is noted that brine conditioning, as described above, does not have to be done at the front end of the process in the case of multiple stage reactions. It may be preferable to have multiple stages if multiple grade products are desirable. For example, scrubbed brine may be reacted with MgCI2 down to a pH of about 8.3 (at which point CO2 starts to come out of solution), and then fed into a separation device such as a thickener. The brine overflowing the thickener would then primarily be bicarbonate. This brine could then be conditioned, as described above, and fed into a second reactor where the remaining carbon in solution is removed by precipitation. It is thought that the disadvantage of multi-stage reactions would be that multiple solids processing trains would be required. Unconditioned brine coming from the brine concentrator would have a typical pH of about 9.5-10.3 (typical bicarbonate/carbonate pH), noting that pH changes with temperature. Conditioned brine may typically have an elevated pH of about 11.7. However, this depends on the concentration of inorganic carbon in solution. Again, it is noted that pH of conditioned brine is affected by both temperature and the concentration of inorganic carbon in the brine. The more inorganic carbon, the more hydroxide is required, and so lower pH. The proportion of carbonate and hydroxide in the conditioned brine is considered more important than the pH of solution.

Importantly, alkalinity (rather than the pH) is adjusted so that there is approximately twice as much carbonate alkalinity as hydroxide alkalinity (on a molar basis), so that nearly all inorganic carbon can be removed prior to NaCI crystallisation. From a processing perspective, one may want to run with an excess carbonate compared to alkalinity to leave a little bit of carbonate/bicarbonate in the stream to be removed by hydrochloric acid. It is considered that any inorganic carbon left in solution will impact the NaCI crystallisation downstream. If the downstream objective is to recover high purity NaCI as a product, then bromide and iodide will preferably need to be removed from solution and the pH brought down to ~2.5 to achieve this. It is considered that any inorganic carbon left in solution will act as a buffer and cause more acid to be required to lower the pH. It is considered that it may be preferable to run excess carbonate and remove this with hydrochloric acid to avoid magnesium passing through to the NaCI stream.

Hydromaqnestie Reaction

The method of this embodiment of the invention includes a hydromagnesite reaction 2000. It is considered that several types of reactor may be suitable (CSTR, Batch Reactor, Pipe Reactor, Fluidised Bed, etc.). Current thinking is that a pipe reactor may be preferred due to the fast reaction kinetics (i.e. a quick precipitation reaction).

Assuming appropriate conditioning of the brine, the pH endpoint will be about 8.3. If the brine has been under conditioned (not enough hydroxide) then the pH endpoint may be much lower and both magnesium and inorganic carbon will remain in solution. Excess caustic will result in a higher pH at endpoint. It is noted that brucite is virtually insoluble at pH 9, so even if excessive caustic is added, the endpoint pH couldn’t be higher than 9. Brucite may contaminate the product. It is considered that minor quantities of brucite may not be of concern, while large quantities may.

Carbon dioxide may also form in under-conditioned brine and impact solid-liquid separation downstream. Carbon dioxide may form according to the bicarbonate route of hydromagnesite formation. If the brine has been over conditioned (too much hydroxide) then the endpoint may be higher, and brucite impurity (Mg(OH)2) will be present in the hydromagnesite. If the conditioned brine is not taken to endpoint, then brucite impurity may be present in the hydromagnesite.

The reaction temperature is at least 55°C, otherwise nesquehonite or dypingite may form. The optimal temperature of the reactor effluent is likely to be above 55°C. Hydromagnesite is least soluble, in pure water, at about 92°C. In concentrated brine, hydromagnesite is less soluble as the temperature increases. It is considered that one of the drivers is to have as little residual magnesium ions in the brine as possible, so that magnesium does not contaminate the sodium chloride process downstream. The reaction may be represented by the following equation: ys.eSds SMgCT (a£5) + 4Na2COsUi?) + ZNaOH --* Μ§δ(£Ο3)4(0%,4Η20 ω + 1 ONaCl

Unlike the above mentioned bicarbonate route, this route emits no carbon dioxide, so operational issues in that regard are avoided and the reaction can be controlled virtually to completion. The ratio of hydroxide to carbonate in the brine is 1:2, as discussed above, which is the same ratio of these components in the hydromagnesite mineral. At the completion of this reaction, neither carbonate nor magnesium is left in solution. The only significant species left in solution is sodium chloride. This is ideal for downstream crystallisation.

As illustrated in Figure 9, thermographic analysis of hydromagnesite product displays two characteristic peaks at about 240°C and 400°C. The first peak represents a loss of water of hydration and the second step a loss of carbon dioxide. Reference is also made to the hydromagnesite commercial standard illustrated in Figure 10.

Solid-Liquid Separation A solid-liquid separation unit 3000 is made up of a number of sub-units, including a hydromagnesite thickener (or other solid/liquid separation device) and a spent brine tank.

Hydromagnesite Thickener (or other solid/liquid separation device)

The inventors have identified that hydromagnesite has excellent settling properties and is self-coagulating. Carbon dioxide production (usually below a pH of 8.3) will affect the separation of hydromagnesite from the brine and is avoided, as discussed above. Hydromagnesite has been produced according to the method of the invention with a wide range of particle sizes, and it has been identified that the finer material produced settles reasonably well, although noticeably slower relative to the larger particle material that has been produced. Separation may be achieved, for example, using filter press and centrifuge.

It is considered that flocculent is not likely to be required. Furthermore, it is considered that an overly large thickener will not be required.

Spent Brine Tank A spent brine tank handles the supernatant overflowing the thickener (or other solid/liquid separation device). According to this embodiment of the invention, the spent brine tank feeds a magnesium chloride preparation circuit, and also a sodium chloride processing train. In the absence of a sodium chloride processing train, the spent brine tank may overflow to a brine dam.

Hydromaqnesite Processing A hydromagnesite processing unit 4000 is made up of a number of sub-units, including a dewatering unit, a washing unit, a drying unit and a packaging and load-out unit. Each will be discussed in turn below.

Dewatering

It is considered that available vendor equipment will be suitable for dewatering purposes. Ideally, it is considered that a vacuum belt filter with integrated washing may be preferred.

Hydromagnesite Washing

Hydromagnesite washing may possibly be integrated with the dewatering unit. In the case of feed with high organic content, one of the washing stages may require a small dose of oxidant (e.g. peroxide) to ensure optimal product whiteness (i.e. to remove any organic colouration from the product).

Hydromagnesite Drying

Again, it is considered that available vendor equipment may be suitable for drying of the hydromagnesite product.

Hydromagnesite Packaging and Load-out

Again, it is considered that available vendor equipment may be suitable for packaging and load-out of the hydromagnesite product. In that regard, the needs of the customer need to be considered. Ideally, the hydromagnesite could be loaded into container-bags (i.e. 35+ m3 bags installed in shipping containers). Smaller packaging may be required depending on customer requirements (e.g. FIBC sized).

Liquor Preparation A liquor preparation unit 5000 is made up of a number of sub-units, including a filtration unit, a pH reduction unit, a bromine and iodine removal, recovery, and load-out unit and a pH increase unit. Generally, preconcentration will be conducted with an evaporator, with addition of caustic.

Filtration

The filtration unit removes any solids, if they are at all present, to avoid them entering the salt processing train. As such, this represents a polishing duty only. pH Reduction

In the pH reduction unit, the pH is reduced to 2.5 so that contaminating halides can be separated after they are oxidised. This may be achieved using hydrochloric acid as other acids may introduce contaminants to the sodium chloride. If the pH is not lowered prior to oxidation, halates may form and remain dissolved in solution. An advantage of near complete removal of inorganic carbon is that there is only a small amount remaining that will consume acid.

Bromine and Iodine Removal, Recovery, and Load-out

Bromine and iodine may be oxidised and separated in a currently available package. It is anticipated that chlorine will be an appropriate oxidant for use in this stage. pH Increase

If necessary the pH may be increased for crystallisation. This may be driven by material selection.

Sodium Chloride Processing A sodium chloride processing unit 6000 is made up of a number of sub-units, including a vacuum crystallisation unit, a NaCI washing unit, a NaCI drying unit and a NaCI packaging and load-out unit.

Vacuum Crystallisation

Vacuum crystallisation may be achieved user a currently available package. It is expected that 2 to 3 fractions may be obtained at a maximum. It also expected that a very high overall recovery of a salt (i.e. >95% recovery) a very high quality product may be achieved.

The hydromagnesite plant effluent is expected to be ideally suited for sodium chloride crystallisation, potentially far more suited and desirable than seawater. NaCI makes up in excess of 99% of the weight in the dissolved portion of the starting liquor. The inventors have also altered the CI:Br ratio, and the Na:K ratio to advantage. If bromide is oxidised prior to crystallisation, then the ratio in the liquor again improves. The only significant impurity is potassium, and potassium is not of great concern for a chlor-alkali plant, since it behaves like sodium does and it only contaminates the caustic stream.

NaCI Washing

All conventional processes provide for NaCI washing. As such, it is expected that currently available units will be suitable for use in the NaCI washing according to this embodiment of the invention.

NaCI Drying

It is considered that NaCI will be required and may be achieved using currently available units.

NaCI Packaging and Load-out

Again, it is expected that currently available units will be suitable for NaCI packaging and load-out.

Reagent Systems A reagent systems unit 7000 is provided having a number of sub-units, including magnesium chloride storage and handling, magnesium chloride reagent preparation, sodium hydroxide storage and handling and other small volume reagents (e.g. HCI, oxidants, etc.).

Magnesium Chloride Storage and Handling

It is expected that this unit will include a special vendor package.

Magnesium Chloride Reagent Preparation

Magnesium chloride is delivered to the reagent preparation system. The MgCI2 is dissolved in spent brine via a process recycle stream. There is an opportunity for capturing the MgCI2 heat of solution and using it for other purposes. In that regard, a cooling circuit may be desirable. A very high thermal duty is apparent due to the (violently) exothermic nature of dissolving anhydrous magnesium chloride (+/- 1.8 MW of energy per brine concentrator treated).If the heat of solution is not removed by a cooling coil, then a significant circulating load is required to prevent boiling in this unit. On the other hand, if the circulating load is too small, then NaCI will crystallise in the dissolving unit. This is not ideal, since it will contain high magnesium impurities, and will cause operational issues.

Sodium Hydroxide Storage and Handling

The sodium hydroxide storage and handling unit includes transfer facilities, storage facilities and dosing facilities.

Other Small Volume Reagents

Other reagents are generally required, albeit at minor volumes. For example, HCI is required to reduce pH of spent brine and oxidant is required to remove iodine and bromine.

Magnesia Calcination A magnesium calcination unit 8000 may be provided. Some hydromagnesite may be converted to magnesia in this unit by calcination. It is considered that several types of magnesia can be produced.

In the process of embodiments of the invention carbonate ions are exchanged with chloride ions, thereby altering the CI:Br ratio to a more favourable one for downstream crystallisation. In the QGC Brine, the initial CI:Br ratio is about 300 by mass. By the time the carbonate has been removed with MgCI2, the CI:Br ratio is 550:1. It is considered that the implications of this are significant.

Bromide and iodide may be removed by oxidation. Since bromide is undesirable in a chlor-alkali plant the production of highly pure salt is possible once the ratios have been altered, and at near total salt recovery.

The magnesium concentration may also be controlled by controlling the endpoint of the reaction. If the reaction is performed as intended then only minor levels of magnesium will be present in the effluent from the hydromagnesite plant.

It is considered that potassium will be the largest impurity by mass percentage. This is because the Na:K ratio in the QGC brine is low and because potassium ions substitute well with sodium ions in the crystal lattice. Results indicate that potassium will represent about 80 to 90% of the total impurities in the crystal formed from treated brine. If the crystal is used as feed to a chlor-alkali plant potassium is not of great concern since it will end up in the caustic stream as KOH. Some mitigation for potassium may also be provided through the use of some NaOH as a reagent. That is, addition of sodium to the brine may favourably modify the Na:K ratio.

EXAMPLES

The following examples are provided for exemplification only and should not be construed as limiting on the invention in any way.

Examples 1-4

Experiments were conducted to identify the effect of the addition of hydroxide to a brine having carbonate alkalinity. Specifically, experiments were conducted to consider the effect of magnesium addition to an unconditioned brine, a conditioned brine with a carbonate:hydroxide ratio of 2:0.5, a conditioned brine with a carbonate:hydroxide ratio of 2:1, and a conditioned brine with an excess (about 10%) of hydroxide.

Results are graphically represented in Figures 3-6 and discussed in more detail above.

Methods for experimentation

The brine was titrated using the T and P alkalinity method whereby HCI acid is added and a titration curve is plotted volume vs pH. The alkalinity OH, CO3 and HCO3 is calculated from two titration points 8.3 and 4.2.

The calculation is performed as per the table below:

Alkalinity Titration (2) ......... .......................

Result of Hydroxide CaGxwUs Bioaffeonafo tiuaGon alkalinity alkalinity alkalinity P 0 o o' τ'

P<1/2T G 2P T~2P

P-1/2T G 2F G

P > 1/2 I 2P-T 2(T~P) G *Key; ateimity, TMofal AaHGly

The results from the brine titration were used to calculate the amount of hydroxide by the following equation:

Required [OH ] = ([CO3] + [HCO3]) / 2 + [HCO3 ]

Hydroxide was added to convert bicarbonate to carbonate and provide a 2:1 ratio of carbonate to hydroxide.

The brine was conditioned with hydroxide according to the appropriated experiment: • No hydroxide; • 0.5 part hydroxide; • 1 part hydroxide to 2 parts carbonate; and • 10% overdosed 1 part hydroxide to 2 parts carbonate. 50ml_ samples of conditioned brine were aliquoted out and fractional magnesium was added from a standard solution of 2M Mg.

Fraction 1.0 means stoichiometry (5 parts Mg to 4 parts carbonate). 0.1 is 10% of the required magnesium and so forth. Each sample generated a solid which was filtered by a 0.45 uM filter. The filtrate was measured for speciated titration, pH and magnesium. Speciated titration was done by the T and P alkalinity method.

The pH was measured with an electrode. Magnesium was measured with an electrode.

Unless the context requires otherwise or specifically stated to the contrary, integers, steps or elements of the invention recited herein as singular integers, steps or elements clearly encompass both singular and plural forms of the recited integers, steps or elements.

It will be appreciated that the foregoing description has been given by way of illustrative example of the invention and that all such modifications and variations thereto as would be apparent to persons of skill in the art are deemed to fall within the broad scope and ambit of the invention as herein set forth.

Claims (40)

1. A method for mineral recovery comprising: conditioning water having carbonate alkalinity with a source of hydroxide; adding a source of magnesium ions to the conditioned water, thereby forming a magnesium carbonate compound; and recovering the magnesium carbonate compound, wherein the source of hydroxide is added to the water in an amount sufficient to achieve a predetermined carbonate equilibria in the conditioned water.

2. A method for the treatment of water having carbonate alkalinity comprising: conditioning the water with a source of hydroxide; adding a source of magnesium ions to the conditioned water, thereby forming a magnesium carbonate compound; wherein the source of hydroxide is added to the water in an amount sufficient to achieve a predetermined carbonate equilibria in the conditioned water.

3. A method according to claim 1 or 2, wherein the amount of hydroxide added is sufficient to at least ameliorate carbon dioxide evolution in said conditioned water.

4. A method according to claim 3, wherein the amount of hydroxide added is sufficient to eliminate carbon dioxide evolution in said conditioned water.

5. A method according to claim 1 or 2, wherein the conditioned water has a carbonate:hydroxide ratio of at least 2:1, preferably approximately 2:1.

6. A method according to any one of the preceding claims, wherein said source of hydroxide is an aqueous hydroxide, such as NaOH, or wherein said source of hydroxide is brucite Mg(OH)2.

7. A method according to any one of the preceding claims, wherein said source of magnesium ions is an aqueous magnesium, preferably MgCI2.

8. A method according to any one of the preceding claims, wherein the temperature of the conditioned water is up to 50°C, from 50 to 55°C, or higher than 55°C, preferably the temperature being maintained throughout the method.

9. A method according to any one of the preceding claims, wherein said water having carbonate alkalinity is coal seam gas brine.

10. A method according to claim 9, wherein said coal seam gas brine has a dissolved solids content of up to saturation point, for example up to about 30 wt%.

11. A method according to claim 9 or 10, wherein said coal seam gas brine has been pre-treated prior to conditioning with said source of hydroxide, such as with filtration of organics, scrubbing, concentration and/or reverse osmosis.

12. A method according to claim 12, wherein said coal seam brine has been pre-treated to remove unwanted components, such as silica and/or fluoride.

13. A method according to any one of claims 9 to 12, further comprising removing contaminating halides following formation of said magnesium carbonate compound.

14. A method according to claim 13, wherein said removed contaminated halides comprise bromide and iodide and said removal preferably comprises acidification of water containing said halides and oxidation.

15. A method according to any one of claims 9 to 14, further comprising recovering remanent salt, such as NaCI, following removal of said magnesium carbonate compound.

16. A method according to any one of the preceding claims, wherein said magnesium carbonate compound is hydromagnesite and/or dypingite.

17. A method according to any one of the preceding claims, wherein recovery of said magnesium carbonate compound comprises dewatering, washing and drying.

18. A method according to any one of the preceding claims, wherein said process is a batch process, semi-batch process, or is continuous.

19. A method according to any one of the preceding claims, comprising maintaining pH and/or Mg ion in the conditioned water by adjusting addition of the source of hydroxide, if needed, to achieve said predetermined carbonate equilibria in the conditioned water.

20. Use of a source of hydroxide in the suppression of carbon dioxide formation during the treatment of water having carbonate alkalinity with a source of magnesium ions, wherein said treatment of said water with said source of magnesium ions is after addition of said source of hydroxide.

21. Use according to claim 19, wherein the amount of hydroxide added is sufficient to at least ameliorate carbon dioxide evolution in said conditioned water.

22. Use according to claim 21, wherein the amount of hydroxide added is sufficient to eliminate carbon dioxide evolution in said conditioned water.

23. Use according to claim 20, wherein the water, after addition of said source of hydroxide, has a carbonate:hydroxide ratio of at least 2:1, preferably approximately 2:1.

24. Use according to any one of claims 20 to 23, wherein said source of hydroxide is an aqueous hydroxide, such as NaOH, or wherein the source of hydroxide is brucite Mg(OH)2.

25. Use according to any one of claims 20 to 24, wherein said source of magnesium ions is an aqueous magnesium, preferably MgCI2.

26. Use according to any one of claims 20 to 25, wherein the temperature of the water is up to 50°C, from 50 to 55°C, or higher than 55°C, preferably the temperature being maintained throughout the treatment.

27. Use according to any one of claims 20 to 26, wherein said water having carbonate alkalinity is coal seam gas brine.

28. Use according to claim 27, wherein said coal seam gas brine has a dissolved solids content of up to saturation point, for example up to about 40 wt%.

29. Use according to claim 27 or 28, wherein said coal seam gas brine has been pre-treated prior to conditioning with said source of hydroxide, such as with filtration of organics, scrubbing, concentration and/or reverse osmosis.

30. Use according to claim 29, wherein said coal seam brine has been pretreated to remove unwanted components, such as silica and/or fluoride.

31. Use according to any one of claims 20 to 30, further comprising removing contaminating halides following formation of said magnesium carbonate compound.

32. Use according to claim 31, wherein said removed contaminated halides comprise bromide and iodide and said removal preferably comprises acidification of water containing said halides and oxidation.

33. Use according to any one of claims 20 to 32, further comprising recovering remanent salt, such as NaCI, following removal of said magnesium carbonate compound.

34. Use according to any one of claims 20 to 33, wherein said magnesium carbonate compound is hydromagnesite and/or dypingite.

35. Use according to any one of claims 20 to 34, wherein recovery of said magnesium carbonate compound comprises dewatering, washing and drying.

36. Use according to any one of claims 20 to 35, wherein said treatment is a batch process or is continuous.

37. Use according to any one of claims 20 to 36, comprising maintaining pH and/or Mg ion in the conditioned water by adjusting addition of the source of hydroxide, if needed, to achieve said predetermined carbonate equilibria in the conditioned water.

38. A method for the production of a magnesium carbonate compound comprising: providing a feed of water having carbonate alkalinity; conditioning the feed with a source of hydroxide to a carbonate:hydroxide ratio of at least 2:1; and adding a source of magnesium ions to the conditioned feed to form said magnesium carbonate compound.

39. Magnesium carbonate compounds when recovered or produced by a method according to any one of claims 1,3-19 and 38.

40. A system for mineral recovery or treatment of water having carbonate alkalinity, said system comprising: a feed of water having carbonate alkalinity; a reactor having an inlet feeding the reactor with a source of magnesium ions; an inlet to the feed of water and/or the reactor, feeding said feed of water and/or reactor with a source of hydroxide; a separator in fluid communication with said reactor and adapted to separate a magnesium carbonate compound from treated water, wherein said source of hydroxide is used to condition said water having carbonate alkalinity to achieve a predetermined carbonate equilibria in the conditioned water prior to feeding said reactor with said source of magnesium ions.