Method of precipitating magnesium carbonate

Abstract

In some embodiments, the present invention relates to a method of precipitating magnesium carbonate, the method comprising the step of contacting a reactive magnesium containing solid material with an aqueous solution comprising magnesium bicarbonate to thereby precipitate magnesium carbonate.   The method may relate to a method of precipitating magnesium carbonate, the method comprising the steps of: (i) Contacting a first reactive magnesium containing solid material with carbon dioxide in an aqueous solution, to thereby provide an aqueous solution comprising magnesium bicarbonate; and (ii) Contacting the aqueous solution comprising magnesium bicarbonate with a second reactive magnesium containing solid material to thereby precipitate magnesium carbonate. In other embodiments, the present invention relates to a method of producing magnesium oxide, an apparatus for precipitating magnesium carbonate, and magnesium carbonate when produced by the above methods.

Description

METHOD OF PRECIPITATING MAGNESIUM CARBONATE

[0001] This application claims priority from Australian provisional patent application number 2024903896, filed on 26 November 2024, and from Australian provisional patent application number 2024900507, filed on 28 February 2024. The entire contents of these provisional patent applications are incorporated herein by cross-reference.TECHNICAL FIELD

[0002] The present invention relates, inter alia, to a method of precipitating magnesium carbonate, magnesium carbonate when precipitated by the method, and to an apparatus for precipitating magnesium carbonate. The present invention also relates to the production of magnesium oxide by the method, and to an apparatus for producing magnesium oxide. BACKGROUND ART

[0003] It will be clearly understood that, if a prior art publication is referred to herein, this reference does not constitute an admission that the publication forms part of the common general knowledge in the art in Australia or in any other country.

[0004] The increasing effect of human activity on the carbon dioxide levels of the planet is well known. Whilst some countries are making progress towards a more renewable energy supply and reafforestation, others are continuing with a dependence on fossil fuels for energy and continuing deforestation. Even if the global move to renewable energy is accelerated, to the inventor’s knowledge there are no economically viable processes for eliminating carbonaceous fuels from large scale industrial processes such as cement, steel and fertiliser manufacture, or for air transportation. Nor to the inventor’s knowledge are there any credible methods for large scale capturing and storing of carbon dioxide from the air, to redress some of the historical damage to the atmosphere that has arisen with the increasing human living standards that are sought by all.

[0005] Various methods have been proposed for the capturing and storing of carbon dioxide from industrial processes. These are primarily focussed on storage of carbon dioxide in rock formations from which oil or gas has been previously extracted. This is termed carbon capture and underground storage (CCUS). Whilst CCUS provides the potential for significant carbon dioxide storage, the carbon dioxide must first be concentrated, pressurised, and then pumped through an extended pipeline network to the injection point at some considerable cost. The suitable locations for such storage are limited, and the longevity of the carbon dioxide retention in such storage areas remains an ongoing concern. Early attempts at CCUS have often not delivered on their design expectations.

[0006] Carbon dioxide has also been captured using calcined lime or magnesia. However, as this rock has been calcined, it ultimately emits more carbon dioxide than the quantity of carbon dioxide that can be scrubbed with the resulting lime or magnesia.

[0007] A different approach is to react the carbon dioxide in the atmosphere with rocks to form carbonates. For example, WO2007 / 069902 discloses a method for manufacturing pure magnesium carbonate from an olivine containing species of rock, in which the process is conducted in at least two separate steps: (i) a first step in which a solid magnesium silicate dissolves in carbonic acid, to form magnesium ions, bicarbonate ions, and silica; and (ii) a second step in which the CO2 pressure is reduced and the magnesium ions react with bicarbonate ions to form magnesium carbonate. The CO2 which is released by the precipitation must be recovered and repressurised for further dissolution.

[0008] A similar approach is detailed in WO2019 / 213705 which relates to an integrated process for carbon dioxide capture, sequestration and utilisation. This process comprises first contacting a carbon dioxide containing gas stream with an aqueous slurry at a first pressure to dissolve magnesium from a magnesium silicate mineral to provide a slurry comprising a magnesium ion enriched carbonated aqueous liquid and a magnesium depleted solid residue. Secondly, magnesium carbonate is precipitated from magnesium ions dissolved in the previous step by multiple successive stage-wise reductions in pressure.SUMMARY OF INVENTION

[0009] With the foregoing in view, the present invention in one embodiment is directed towards a method that allows storage of carbon dioxide into a solid material. In a further embodiment, the present invention is directed towards a method of precipitating magnesium carbonate. In a further embodiment, the present invention is directed to a method which may at least partially overcome at least one of the abovementioned disadvantages or provide the consumer with a useful or commercial choice.

[0010] In a first aspect, the present invention provides a method of precipitating magnesium carbonate, the method comprising the step of contacting a reactive magnesium containing solid material with an aqueous solution comprising magnesium bicarbonate to thereby precipitate magnesium carbonate.

[0011] In a second aspect, the present invention provides a method of precipitating magnesium carbonate, the method comprising the steps of:(i) Contacting a first reactive magnesium containing solid material with carbon dioxide in an aqueous solution, to thereby provide an aqueous solution comprising magnesium bicarbonate; and(ii) Contacting the aqueous solution comprising magnesium bicarbonate with a second reactive magnesium containing solid material to thereby precipitate magnesium carbonate.

[0012] With reference to the second aspect of the invention, in the step of contacting a first reactive magnesium containing solid material with carbon dioxide in an aqueous solution, the carbon dioxide dissolves in the aqueous solution thereby producing carbonic acid, which in turn lowers the pH. The lower pH assists in the dissolution of magnesium from the first reactive magnesium containing solid material, and this produces a solution comprising magnesium bicarbonate. The step of contacting the aqueous solution comprising magnesium bicarbonate with a second reactive magnesium containing solid material results in the reactive magnesium containing solid material reducing the amount of magnesium bicarbonate / carbonic acid in the aqueous solution, which in turn increases the pH. As the pH increases this in turn results in the precipitation of magnesium carbonate. Both of these steps are reversible reactions, and exemplary equations that describe the chemical reactions occurring are as follows:Step (i): MgO + H2O + CO2  Mg2+ + 2HCO3- H2O + CO2  H2CO3Step (ii): MgO + CO2  MgCO3MgO + Mg2+ + 2HCO3-  2MgCO3 + H2O

[0013] The protons in carbonic acid have pKa’s of about 6.4 and 10.3, so for carbonic acid at a pH at or approaching 10, CO32- is favoured in solution, which results in precipitation of magnesium carbonate. In contrast, at a pH at or approaching 6, H2CO3 is favoured in solution.

[0014] The method described in the first and second aspects includes a number of advantages over the processes described above for WO2007 / 069902 and WO2019 / 213705. In WO2007 / 069902 and WO2019 / 213705 temperature and / or pressure is used to reduce the amount of carbon dioxide present in the aqueous solution to progress the precipitation of magnesium carbonate. As the reaction is reversible, this has the potential to decrease the concentration of magnesium bicarbonate in solution prior to precipitation (which in turn would decrease yields of precipitated magnesium carbonate). Furthermore, in effect only 50% of the total carbon present in the pregnant solution can precipitate in any cycle. Also, use of temperature or pressure to remove carbon dioxide is very energy intensive and would increase operational costs when performing these reactions at scale. In both WO2007 / 069902 and WO2019 / 213705 the magnesium bicarbonate solutions are separated to remove solids before precipitation of magnesium carbonate.

[0015] In contrast in step (ii) of the second aspect and in the first aspect of the present invention the use of reactive magnesium containing solid material (or second reactive magnesium containing solid material) depletes the concentration of magnesium bicarbonate (and possibly also carbonic acid) in solution, may result in dissolution of magnesium from the reactive magnesium containing solid material and generates additional magnesium carbonate. In turn, this increases yields of magnesium carbonate at the time of precipitation. Furthermore, this process may be performed at atmospheric temperature and pressure, which would decrease operational costs when performing these reactions at scale. Therefore, in the second aspect of the present application, the reactions and change in pH for the two steps may be controlled through the addition, or not, of carbon dioxide.

[0016] The reactive magnesium containing solid material (or the first or second reactive magnesium containing solid material) may be a material that has a natural pH of greater than 8, and from which at least 10% of the magnesium can be dissolved at a pH of around 6. In one embodiment, the reactive magnesium containing solid material (or the first or second reactive magnesium containing solid material) is or comprises magnesia (MgO). The reactive magnesium containing solid material (or the first or second reactive magnesium containing solid material) may have a p80 of less than 500 μm, or less than 450 μm, or less than 400 μm, or less than 350 μm, or less than 300 μm, or less than less than 250 μm, or less than 200 μm, or less than 150 μm, or less than 100 μm or less than 50 μm. The reactive magnesium containing solid material may have a p80 of from about 1 μm to about 300 μm, or from about 10 μm to about 300 μm, or from about 5 μm to about 250 μm, or from about 10 μm to about 200 μm, or from about 50 μm to about 150 μm, or from about 10 μm to about 100 μm, or from about 15 μm to about 150 μm, or from about 20 μm to about 100 μm, or from about 25 μm to about 75 μm, or from about 35 μm to about 65 μm, or about 50 μm. The first and second reactive magnesium containing solid material may be the same or from the same source, or they may be from a different source.

[0017] In one embodiment, the reactive magnesium containing solid material (or the first or second reactive magnesium containing solid material) is an activated magnesium containing solid material (or a first or second activated magnesium containing solid material). In one embodiment, the method of the first and / or second aspects of the present invention may comprise a step of activating a magnesium containing solid material to provide an activated magnesium containing solid material. The step of activating may comprise grinding, crushing or comminuting the magnesium containing solid material, and / or may comprise heating (or calcining) the magnesium containing solid material. The step of activating may produce a form of solid material in which the magnesium is sufficiently reactive to dissolve under ambient or near ambient temperature or pressure. The step of activating may produce a form of solid material in which the magnesium is sufficiently reactive to dissolve under ambient or near ambient pH conditions, such as a pH greater than 6. The step of activating may produce a form of solid material in which magnesium is sufficiently reactive to reduce the magnesium and bicarbonate concentrations in solution to enable the recycle of this solution for a subsequent stage of dissolution of rock at a lower pH. The reactive magnesium containing solid material may be a reactive magnesium containing particulate material. In one embodiment, the reactive magnesium containing solid material (or the first or second reactive magnesium containing solid material) is particulate.

[0018] The magnesium containing solid material (or the first or second reactive magnesium containing solid material, or the activated magnesium containing solid material) may comprise, consist essentially of, or consist of an ultramafic material (or ultramafic rock). The ultramafic material may comprise, consist essentially of, or consist of olivine, pyroxene (such as ferromagnesian pyroxene), serpentinite and / or asbestos; especially olivine or serpentinite. The ultramafic rock may be serpentinized or non-serpentinized. An exemplary non-serpentinized material is olivine. Exemplary serpentinized materials may include serpentinite and asbestos. The ultramafic rock may comprise one or more of (or all of) calcium, magnesium, and silica. The magnesium containing solid material may comprise magnesium and silica (or silicate). The magnesium containing solid material may be or comprise a magnesium silicate. The magnesium containing solid material may comprise at least 20%, at least 25%, at least 30%, at least 35%, or at least 40% (by weight) magnesium oxide; especially at least 35%. It may be advantageous to use serpentinite as this is a common rock type, which may allow the method to be scaled up and for the method to be performed closer to the carbon dioxide source. In one embodiment, the magnesium containing solid material (or the first or second reactive magnesium containing solid material or the activated magnesium containing solid material) comprises, consists of, or consists essentially of serpentinite. In one embodiment, the magnesium containing solid material (or the first or second reactive magnesium containing solid material, or the activated magnesium containing solid material) comprises at least 40% serpentinite, or at least 60% serpentinite, or at least 70% serpentinite. The serpentinite may consist of or comprise chrysotile, antigorite and / or lizardite.

[0019] The magnesium containing solid material may be from any suitable source. In one embodiment, the magnesium containing solid material may be or be derived from fly ash, slag from an iron-nickel smelter, or material that has been subject to or is to be subject to recovery of other values such as nickel, asbestos or diamonds.

[0020] The step of grinding, crushing or comminuting the magnesium containing solid material may comprise grinding, crushing or comminuting the magnesium containing solid material to a p80 of less than 500 μm, or less than 450 μm, or less than 400 μm, or less than 350 μm, or less than 300 μm, or less than less than 250 μm, or less than 200 μm, or less than 150 μm, or less than 100 μm or less than 50 μm. The step of grinding, crushing or comminuting the magnesium containing solid material, may comprise grinding, crushing or comminuting the magnesium containing solid material to a p80 of from about 1 μm to about 300 μm, or from about 10 μm to about 300 μm, or from about 5 μm to about 250 μm, or from about 10 μm to about 200 μm, or from about 10 μm to about 100 μm, or from about 15 μm to about 150 μm, or from about 20 μm to about 100 μm, or from about 25 μm to about 75 μm, or from about 35 μm to about 65 μm, or about 50 μm. The step of grinding, crushing or comminuting the magnesium containing solid material may be suitable to activate a material that is or comprises olivine.

[0021] The step of heating (or calcining) the magnesium containing solid material may comprise heating (or calcining) to a temperature of at least 500 °C, at least 550 °C, at least 600 °C, or at least 650 °C. The step of heating (or calcining) the magnesium containing solid material may comprise heating to a temperature of from about 500 °C to about 1200 °C, or from about 550 °C to about 1150 °C, or from about 550 °C to about 800 °C, or from about 600 °C to about 750 °C, or about 650 °C to about 750 °C. The step of heating may be performed for a duration of up to 4 hours, or up to 3 hours, or up to 2 hours or up to 1 hour. The step of heating may be performed for from 10 minutes to 4 hours, or from 10 minutes to 3 hours, or from 10 minutes to 2 hours or from 10 minutes to 1 hour. The step of heating may be performed in the presence of an ambient concentration of oxygen, or in the presence of a lower concentration of oxygen than the ambient concentration. The magnesium containing solid material may be subject to grinding, crushing or comminuting as discussed in the preceding paragraph prior to heating. In one embodiment, the magnesium containing solid material is subject to grinding, crushing or comminuting to a p80 of less than 400 μm (especially less than 300 μm) prior to heating.The step of heating may be performed in a rotary kiln. The step of heating may be suitable to activate a material comprising or consisting of serpentinite. The conditions for heating (or calcining) may be selected for a temperature and duration to convert serpentinites to a more soluble form and to minimise conversion to olivine. The step of heating (or calcining) may convert magnesium in the material to a more soluble form, such as a solid material which comprises MgO.

[0022] In one embodiment, the method comprises the step of activating a magnesium containing solid material to provide the activated magnesium containing solid material; wherein the step of activating comprises: comminuting the magnesium containing solid material to a p80 of less than 300 μm; and / or calcining the magnesium containing solid material to a temperature of from about 650 °C to about 750 °C; especially comminuting the magnesium containing solid material to a p80 of less than 300 μm, and calcining the magnesium containing solid material to a temperature of from about 650 °C to about 750 °C.

[0023] The aqueous solution comprising magnesium bicarbonate in the first aspect, or in step (ii) in the second aspect may comprise any suitable concentration of magnesium bicarbonate. In one embodiment, the concentration of magnesium in the aqueous solution at the start of the precipitation step (or after step (i)) is at least 1 gL-1, especially at least 3 gL-1, or at least 5 gL-1, or at least 7 gL-1, or at least 10 gL-1, or at least 15 gL-1. In one embodiment, the aqueous solution at the start of the precipitation step (or after step (i)) is a substantially saturated magnesium bicarbonate solution. In one embodiment, the concentration of magnesium in the aqueous solution at the end of the precipitation step is less than 5 gL-1, less than 3 gL-1, less than 1 gL-1, or less than 0.5 gL-1.

[0024] The step of contacting a reactive magnesium containing solid material with an aqueous solution comprising magnesium bicarbonate to thereby precipitate magnesium carbonate may be referred to herein as “the precipitation step”, or “the step of precipitating magnesium carbonate”. The precipitation step may be performed at any suitable temperature and pressure. The precipitation step may be performed at ambient pressure. The precipitation step may be performed at about 1 bar. The precipitation step may be performed without applying external heating or cooling. The precipitation step may be performed at ambient temperature. The temperature of the reactor or vessel may increase due to the exothermic nature of the dissolution of magnesium into the aqueous solution, and this increased temperature may further accelerate precipitation of magnesium carbonate.

[0025] In one embodiment, the precipitation step is performed without adding carbon dioxide. In another embodiment the precipitation step is performed in the presence of carbon dioxide. However, when the precipitation step is performed in the presence of carbon dioxide the most reactive magnesium component of the reactive magnesium containing material would not be entirely used for the precipitation of magnesium carbonate. The inventor believes that contacting an aqueous solution comprising magnesium bicarbonate with reactive magnesium containing material in the absence of carbon dioxide would assist to maximise precipitation of magnesium carbonate in the dissolution and precipitation method as described in embodiments of the present invention (for example as outlined with reference to Figure 1). However, if the precipitation step is performed using a plurality of reactors, one of the reactors may operate in the presence of carbon dioxide. When carbon dioxide is added to a precipitation reactor, there may be both precipitation of magnesium carbonate and dissolution of magnesium in that reactor. Such a reactor may operate at a pH of about 6-9, especially 7-8. In one embodiment, when the precipitation step is performed in a plurality of reactors, if at least one reactor operates in the presence of carbon dioxide, at least one reactor operates without addition of carbon dioxide. In one embodiment, step (ii) may have a reduced CO2 activity in the solution, compared to step (i). The reduced CO2 activity may be achieved by vacuum distillation of the CO2, heating the solution, or contacting the solution with a material that is reactive to CO2 (such as a reactive magnesium containing solid material).

[0026] The precipitation step may be performed (or controlled) at a pH of at least 6.5, or at least 7. In one embodiment, the precipitation step may be performed (or controlled) at a pH of from 7.0 to 10.0, especially from 7.5 to 9 or from 7.5 to 9.5. In one embodiment, the precipitation step may be performed at a terminal pH of above 7.0, or from 7.0 to 10.0, or about 7.5, or from 7.5 to 9, or from 7.5 to 9.5, or about 8.0. When the step of precipitation is complete, the pH of the aqueous solution (the terminal pH) may be greater than 7.5, especially greater than 8.0. The inventor believes that at a pH of greater than 8.0 the proportion of carbonates in the aqueous solution is less than 1 % in the form of CO2. The precipitation step may be performed with addition of a pH adjusting agent. In one embodiment, the sole pH adjusting agent is the reactive (or activated) magnesium containing solid material.

[0027] The precipitation step may be performed at any suitable concentration of solids or pulp density. In one embodiment, the average pulp density in the precipitation step can be varied with the level of carbon dioxide being carried by the aqueous solution comprising magnesium bicarbonate. In one embodiment, the pulp density is selected to precipitate most of the magnesium carbonate from the aqueous solution as it passes through the precipitation step. In this embodiment, the pulp density may be greater than 5% solids, or greater than 10% solids, or greater than 15% solids, or greater than 20% solids, or greater than 25% solids.

[0028] The precipitation step may be performed with addition of a basic material. The addition of basic material may raise the pH. The basic material may comprise a magnesium containing material. The basic material may comprise magnesium oxide. The basic material may comprise, for example, magnesium oxide, slag (such as blast furnace slag), fly ash, brucite or caustic soda. In one embodiment, use of magnesium oxide may efficiently raise the pH to assist in the precipitation step. In one embodiment, the precipitation step may further comprise a step of removing carbon dioxide from the aqueous solution.

[0029] The precipitation step may comprise forming a slurry from the aqueous solution comprising magnesium bicarbonate and the reactive magnesium containing solid material.

[0030] The precipitation step may comprise adding a seed material to the aqueous solution. Use of a seed material may encourage precipitation of magnesium carbonate discrete from other solid surfaces, thus enhancing later beneficiation / separation of magnesium carbonate.

[0031] The precipitation step may be performed in any suitable vessel or reactor. In one embodiment, the precipitation step may be performed in a reactor (such as an agitated tank reactor). In another embodiment, the precipitation step may be performed in flow through reactor such as a dam (such as an enclosed dam) or a very large tank (such as a thickener). In one embodiment, the precipitation step may be performed in a precipitation reactor. The precipitation reactor may be an agitated tank, or a very large container such as an enclosed dam through which solution is circulated. The use of dams may enable a much longer residence time at acceptable cost. Furthermore, the carbon dioxide activity during this step is low, so enclosure of such reactors is simplified.

[0032] The precipitation step may be performed over any suitable length of time. The residence times during the precipitation step may be selected depending on the reactivity of the magnesium containing solid material, with a less reactive material requiring a longer residence time and a more reactive material requiring a shorter residence time. In one embodiment, the residence time is at least 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, or 24 hours, or at least 1, 2, 3, 4, 5, 6, or 7 days, or at least 1, 2, 3, 4, or 5 weeks, or at least 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, or 12 months, or at least 1, 2, 3, 4, 5, 6, 7, or 8 years. In one embodiment, the residence time is from 10 minutes to 24 hours, especially from 10 minutes to 10 hours, or from 10 minutes to 5 hours.

[0033] The precipitated magnesium carbonate may be in any suitable form. In one embodiment, the magnesium carbonate is or comprises magnesite, basic magnesium carbonate or nesquehonite (MgCO3·3H2O). Nesquehonite may form needle-like crystals which may be discrete from, for example, a magnesium depleted solid material (such as a magnesium depleted silicate). Without wishing to be bound by theory, it is believed that the crystalline shape of nesquehonite greatly enhances the subsequent separation (especially physical separation) of the phases. For example, nesquehonite may have discrete, lighter and finer crystals which may have high natural buoyancy, in comparison to the magnesium depleted solid material (such as magnesium depleted silicate) which may have a more spherical shape. If the intent is to store carbon dioxide, the magnesium carbonate may be stored in perpetuity. If the intent is for the magnesium carbonate to be used for subsequent application, the magnesium carbonate may be used directly or further processed.

[0034] The precipitation step may produce precipitated magnesium carbonate, a magnesium depleted aqueous solution and a magnesium depleted solid material.

[0035] The aqueous solution comprising magnesium bicarbonate in the first aspect of the present invention may be produced by step (i) of the second aspect, that is by contacting a first reactive magnesium containing solid material with carbon dioxide in an aqueous solution, to thereby provide an aqueous solution comprising magnesium bicarbonate. This step (which may be described herein “as the dissolution step” or “the step of dissolution”) may generate a high tenor of pregnant magnesium bicarbonate solution suitable for precipitation, whilst also dissolving a high proportion of the soluble magnesium content in the reactive (or activated) material, and hence maximising the CO2 storage per tonne of material.

[0036] The first reactive magnesium containing solid material may be as described above. The first reactive magnesium containing solid material may comprise, consist essentially of, or consist of an ultramafic material (or ultramafic rock). The ultramafic material may comprise, consist essentially of, or consist of olivine, pyroxene (such as ferromagnesian pyroxene), serpentinine and / or asbestos. The ultramafic rock may be serpentinized or non-serpentinized. An exemplary non-serpentinized material is olivine. Exemplary serpentinized materials may include serpentinite and asbestos. In one embodiment, the first reactive magnesium containing solid material comprises, consists of, or consists essentially of serpentinite. The ultramafic rock may comprise one or more of (or all of) calcium, magnesium, and silica. The first reactive magnesium containing solid material may comprise magnesium and silica (or silicate). The first reactive magnesium containing solid material may be or comprise a magnesium silicate. The first reactive magnesium containing solid material may comprise at least 20%, at least 25%, at least 30%, at least 35%, or at least 40% (by weight) magnesium oxide; especially at least 35%. In one embodiment, the reactive magnesium containing solid material comprises, consists of, or consists essentially of serpentinite. The first reactive magnesium containing solid material may have a p80 of less than 500 μm, or less than 450 μm, or less than 400 μm, or less than 350 μm, or less than 300 μm, or less than less than 250 μm, or less than 200 μm, or less than 150 μm, or less than 100 μm or less than 50 μm. The first reactive magnesium containing solid material may have a p80 of from about 1 μm to about 300 μm, or from about 10 μm to about 300 μm, or from about 5 μm to about 250 μm, or from about 10 μm to about 200 μm, or from about 10 μm to about 100 μm, or from about 15 μm to about 150 μm, or from about 20 μm to about 100 μm, or from about 25 μm to about 75 μm, or from about 35 μm to about 65 μm, or about 50 μm.

[0037] Step (i) of the second aspect (or the dissolution step) may comprise forming a slurry from the aqueous solution and the first reactive magnesium containing solid material. The step may be performed at any suitable concentration of solids or pulp density. In one embodiment, the pulp density may be greater than 5% solids, or greater than 10% solids, or greater than 15% solids, or greater than 20% solids, or greater than 25% solids. The step may be performed to generate a substantially or near saturated magnesium bicarbonate aqueous solution (such as a at least 75%, at least 80%, at least 85%, at least 90% or at least 95% saturated magnesium bicarbonate aqueous solution). The step may be performed to dissolve an increased proportion of the magnesium from the magnesium containing material.

[0038] Step (i) of the second aspect (or the dissolution step) may be performed in any suitable vessel or reactor (such as a dissolution reactor). In one embodiment, the step is performed in a pressurised reactor. In one embodiment, the atmosphere in the reactor may be (or step (i) may be performed in an atmosphere of) greater than 70% carbon dioxide, or greater than 80% carbon dioxide or greater than 90% carbon dioxide or greater than 95% carbon dioxide, or substantially pure carbon dioxide. The pressure in the reactor may be (or the pressure in step (i) may be) from about 1 to about 100 bar, or from about 1 to about 50 bar, or from about 1 to about 25 bar, or from about 1 to about 20 bar, or from about 1 to about 10 bar. Use of pressure and an atmosphere that is substantially pure carbon dioxide more rapidly increases the acidity of the aqueous solution and may allow the step to be performed over a shorter period of time. In one embodiment, step (i) is performed in an atmosphere of greater than 70% carbon dioxide, at a pressure of from about 1 to about 100 bar.

[0039] In one embodiment, step (i) may be performed using a plurality of reactors, and the final reactor may operate at a pressure (or carbon dioxide pressure) of from 1-20 bar, especially 3-15 bar, more especially 5-10 bar. Furthermore, the preceding reactors may operate at a lower pressure (or carbon dioxide pressure) than the final reactor, especially from 1-15 bar, or from 1-10 bar, or from 1-5 bar, or about 1 bar, or atmospheric pressure.

[0040] In an alternative embodiment, the pressure in the reactor or vessel may be atmospheric pressure, and the atmosphere in the reactor or vessel may be slowly circulated to maintain carbon dioxide levels at close to saturation in the aqueous solution. In this and in other embodiments, aqueous solution comprising magnesium bicarbonate may be continuously removed from the reactor for use in the precipitation step. This embodiment allows for longer reaction durations, such as months or even years. Especially if the reactor or vessel is at atmospheric pressure, then the reactor or vessel may be a dam or tank.

[0041] Step (i) of the second aspect (or the dissolution step) may be performed at a pH of less than 8.0, especially less than 7.0. In one embodiment, step (i) of the second aspect (or the dissolution step) may be performed at a terminal pH of less than 8.0, especially less than 7.0, or less than 6.0. In one embodiment, step (i) of the second aspect (or the dissolution step) may be performed at a pH (or a terminal pH) of from about 4.0 to about 8.0, or from about 4.5 to about 7.5, or from about 5.0 to about 7.0, or from about 5.0 to about 6.0, or about 5.0. In one embodiment, the pH of the solution at the end of step (i) of the second aspect (or the dissolution step) is less than about 5.5. In one embodiment, the aqueous solution at the end of step (i) of the second aspect (or the dissolution step) is substantially saturated with magnesium bicarbonate. The pH may change as step (i) proceeds, as without wishing to be bound by theory the inventor believes that as the magnesium in the solid material dissolves into the aqueous solution the pH increases due to increased buffering by the presence of dissolved bicarbonate and carbonate ions. In one embodiment, magnesium carbonate may precipitate during step (i) or the dissolution step, but the magnesium carbonate may redissolve (especially when this step includes more than one reactor, as magnesium carbonate may precipitate in one reactor and redissolve in a subsequent reactor).

[0042] Step (i) of the second aspect (or the dissolution step) may be performed over any suitable length of time. The residence times during the dissolution may be selected depending on the reactivity of the first reactive magnesium containing solid material, with a less reactive material requiring a longer residence time and a more reactive material requiring a shorter residence time. In one embodiment, the residence time is at least 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, or 24 hours, or at least 1, 2, 3, 4, 5, 6, or 7 days, or at least 1, 2, 3, 4, or 5 weeks, or at least 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, or 12 months, or at least 1, 2, 3, 4, 5, 6, 7, or 8 years. In one embodiment, the residence time is from 10 minutes to 24 hours, especially from 10 minutes to 10 hours, or from 10 minutes to 5 hours.

[0043] Step (i) of the second aspect (or the dissolution step) may be performed at any suitable temperature. The step may be performed without applying external heating or cooling. The step may be performed at ambient temperature. The temperature during the step may increase due to the exothermic nature of the dissolution of magnesium into the aqueous solution.

[0044] Step (i) of the second aspect (or the dissolution step) may comprise contacting the aqueous solution and carbon dioxide with more than one reactive magnesium containing solid material. The more than one reactive magnesium containing solid material may comprise an activated magnesium containing solid material.

[0045] The carbon dioxide in step (i) of the second aspect (or the dissolution step) may be from any suitable source. In one embodiment, the carbon dioxide may be produced from an industrial process (an industrial gas stream). In one embodiment, the method of the present invention may be integrated with an industrial process which produces carbon dioxide (such as conversion of biofuel into carbon dioxide and hydrogen gas), or an industrial scrubbing process to remove carbon dioxide from a gas stream. In one embodiment, the method of the present invention may be integrated with a process for the direct air capture of carbon dioxide. In one embodiment, the present invention can be used to remove carbon dioxide from carbon dioxide gas streams, preferably by scrubbing. In one embodiment, the present invention is suitable for scrubbing carbon dioxide from industrial gas streams and / or from impure gas streams, such as those generated during burning of fossil fuels, or reduction of metals, or as part of the gas streams in steam methane reforming.

[0046] In one embodiment, the carbon dioxide is from the formation of hydrogen generated by steam methane reforming. In one embodiment, the source of carbon dioxide is biomass (including biofuel or biodiesel) which has been converted to carbon dioxide and another valuable product such as ethanol or hydrogen (which may provide a carbon negative product), or a byproduct from the conversion of carbonaceous fuels to hydrogen gas or a lesser carbon rich fuel. In one embodiment, the source of carbon dioxide is an off gas from a combustion or reduction process such as power generation, metal (such as steel) production or cement production. In one embodiment, the carbon dioxide is produced from an industrial process; optionally wherein the industrial process is selected from the group consisting of: burning of fossil fuels, burning of biofuels, reduction of metals, cement production and steam methane reforming.

[0047] For scrubbing, the gas may be contacted with a solution or slurry at a basic pH. The solution or slurry may be generated by the dissolution of magnesium from the magnesium containing material. The solution or slurry may be maintained at a pH of greater than 7 or greater than pH 8. In one embodiment, carbon dioxide is passed into a scrubbing solution (converting the gas to the liquid phase in the scrubber) enabling a gas almost barren in carbon dioxide to exit the scrubber. In one embodiment, the magnesium depleted aqueous solution in step (ii) is suitablefor use as the scrubbing solution (and may be combined with a first reactive magnesium containing solid material, especially to form a slurry), and may be at a pH greater than 7 or greater than 8 for this use. In one embodiment, the concentration of bicarbonate in the scrubbing solution may be maintained by a bleed of the scrubbing solution into step (i) (forming the aqueous solution in step (i)). This may allow for additional dissolution of reactive magnesium containing solid material at a lower pH.

[0048] The scrubbing of gases may be achieved whilst also achieving a high uptake of carbon dioxide per tonne of reactive magnesium containing solid material. In one embodiment, the purification of the carbon dioxide from the gas stream can be optimised to provide only the stream of 100% carbon dioxide necessary for dissolving the remaining magnesium from the reactive magnesium containing solid material.

[0049] In one embodiment, the carbon dioxide is scrubbed from gas streams containing about 10% to about 35% carbon dioxide. Preferably, the pH of the scrubbing solution is maintained above about 7. In this embodiment, the less reactive magnesium containing solid material is introduced to step (ii) of the method of the second aspect and the partially carbon dioxide laden solution is introduced to step (i) of the second aspect.

[0050] In one embodiment, the precipitated magnesium carbonate in the first aspect, or in step (ii) of the second aspect is in the form of a slurry. In one embodiment, after step (ii) of the second aspect, or after the precipitation step of the first aspect, the method comprises a further step of separating the precipitated magnesium carbonate from the magnesium depleted solid material. In one embodiment, the step of separating may also comprise separating the precipitated magnesium carbonate from the magnesium depleted aqueous solution and the magnesium depleted solid material. The step of separating may be or include a physical beneficiation step.

[0051] The step of separating may comprise solid-liquid separation to separate the magnesium depleted aqueous solution from the solid material (magnesium carbonate and magnesium depleted solid material). The step of separating may comprise removal of carbon dioxide from the mixture of magnesium depleted aqueous solution, magnesium carbonate and magnesium depleted solid material, prior to separating the magnesium carbonate from the magnesium depleted solid material (the removal of the carbon dioxide may be achieved through the solid-liquid separation discussed earlier in this paragraph).

[0052] The step of separating (or physical beneficiation) may: (i) remove most of the magnesium carbonate from the magnesium depleted solid material; or (ii) remove most of the magnesium depleted solid material from the magnesium carbonate. As used in this paragraph “most” means at least 50%, at least 60%, at least 70%, at least 80%, or at least 90%. If solid magnesium depleted solid material following the separation step is contacted with carbon dioxide in an aqueous solution (in a dissolution step), and magnesium carbonate is present in the solid material at that time, then that magnesium carbonate will fully or partially redissolve. However, as the magnesium carbonate dissolves it will exhibit a buffering effect and form magnesium bicarbonate (which does not exist as a solid), and in turn this increases the pH which slows the dissolution of magnesium from the reactive magnesium containing solid material (especially as the magnesium containing solid material is less reactive).

[0053] The step of separating (or physical beneficiation) may provide solid magnesium carbonate, and magnesium depleted solid material. In one embodiment, the step of separating the magnesium carbonate from the magnesium depleted solid material may comprise at least one selected from the group consisting of: attritioning to physically separate the magnesium carbonate from the magnesium depleted solid material; flotation separation; an apparent density-based separation process; and gravity separation. Exemplary apparent density-based separation processes may include, for example, a hydrocyclone and / or a hydraulic classifier (such as an elutriator or a reflux classifier).

[0054] Separated magnesium carbonate may be subsequently stored or utilised (for example in water treatment, in construction materials including wall board, fire retardants, fillers for plastics or fertilisers). The separated magnesium carbonate may be subject to further refining.

[0055] In one embodiment, the precipitated magnesium carbonate solid from step (ii) (or the precipitation step) may be physically associated (or adsorbed) with the magnesium depleted solid material. In one embodiment, the magnesium carbonate is substantially a discrete solid in the aqueous solution / slurry.

[0056] In one embodiment, the magnesium carbonate and magnesium depleted solid material is ground or crushed, for example through attritioning, to physically separate the magnesium carbonate from the magnesium depleted solid material.

[0057] Various separation or physical beneficiation steps may be performed separately or in combination, including flotation and gravity separation. Gravity separation may achieve separation based on the differential size and density of the magnesium carbonate and the magnesium depleted solid material. For example, gravity separation may be used either before flotation (for preparing the feed), or after flotation (to further purify the flotation product). The step of separation may comprise at least one, at least two or at least three separate separation (or physical beneficiation) steps.

[0058] Flotation separation is generally more selective then gravity separation, and there are many collectors and depressants that are known to differentially float magnesium carbonates from serpentinites and similar silicates, for example see Wonyen (A review of flotation separation of Mg carbonates (Dolomite and Magnesite), (2018) Minerals, 8(8), 354) and Yao (Research on the floatability of magnesite and its gangue minerals with sodium oleate and lauryl amine as collectors, Advanced Minerals Research, 798-799, 328-332). For example, a collector that absorbs selectively on the carbonate fraction may enable bubble attachment, and further promote a buoyancy differential. Flotation separation may achieve separation based on differential surface chemical compositions enabling different bubble attachment to the magnesium carbonate and the magnesium depleted solid material.

[0059] During flotation, the pH may be greater than 7.0, especially greater than 7.5. A pH of greater than 7.5 may avoid redissolution of magnesium carbonate. The flotation may comprise flotation of magnesium carbonate, or flotation of magnesium depleted solid material (which may be reverse flotation). The flotation may include multiple stages of scavenging and cleaning of the products formed in the initial flotation step.

[0060] The separated magnesium carbonate may comprise a small quantity of entrained magnesium depleted solid material. In one embodiment, the separated magnesium carbonate may comprise less than 10%, less than 6%, less than 4%, less than 2% or less than 1% magnesium depleted solid material. The separated magnesium depleted solid material may comprise a small quantity of entrained magnesium carbonate. In one embodiment, the separated magnesium depleted solid material may comprise less than 10%, less than 6%, less than 4%, less than 2% or less than 1% magnesium carbonate.

[0061] The solid material during step (i) may be ground, crushed or comminuted. The solid material during step (ii) may be ground, crushed or comminuted. The magnesium depleted solid material may be ground, crushed or comminuted. The solid material (during step (i) and / or during step (ii)) may be ground, crushed or comminuted to a p80 of less than 500 μm, or less than 450 μm, or less than 400 μm, or less than 350 μm, or less than 300 μm, or less than less than 250 μm, or less than 200 μm, or less than 150 μm, or less than 100 μm or less than 50 μm. The solid material (during step (i) and / or during step (ii)) may be ground, crushed or comminuted to a p80 of from about 1 μm to about 300 μm, or from about 10 μm to about 300 μm, or from about 5 μm to about 250 μm, or from about 10 μm to about 200 μm, or from about 10 μm to about 100 μm, or from about 15 μm to about 150 μm, or from about 20 μm to about 100 μm, or from about 25 μm to about 75 μm, or from about 35 μm to about 65 μm, or about 50 μm. The solid material after step (ii) (which may be magnesium carbonate and magnesium depleted solid material) may be ground, crushed or comminuted (for example to a p80 described above), especially prior to separation (especially gravity separation). After grinding, crushing or comminuting the magnesium carbonate may be finer and lighter, and therefore better recoverable by gravity separation. Magnesium carbonate may be the least dense and softest material and therefore the most easily recoverable solid material from gravity separation. Separation can be achieved based on the density of the solid material. Exemplary density-based separation processes may comprise dense media separation and / or hydraulic classification.

[0062] In another embodiment, the separated magnesium carbonate can be refined by dissolution to form a pregnant solution, and subsequent vacuum crystallisation. The high dissolution rates of nesquehonite, and the low level of silicates in the physically beneficiated feed may allow such a process to produce a pure MgCO3 for the chemical market. This is similar to the dissolution of calcined magnesite described by Canterford (Magnesia from magnesite by calcination / carbon dioxide leaching, AusIMM Proceedings 1981), used in combination with the vacuum crystallisation to form pure MgCO3, as described in WO2019 / 213705.

[0063] In another embodiment, magnesium carbonate and the magnesium depleted solid material (or the solid material following precipitation) is heated to above 60°C, or above 70°C, or above 80°C, or above 90°C or above 95°C or above 100°C. In this embodiment, the heat converts nesquehonite into a much less soluble form of magnesium carbonate. After heating the resulting solids may be cooled. The solids after heating may be subject to dissolution (for example in step (i) of the second aspect of the present invention). In the dissolution step preferential dissolution of the remaining magnesium in the magnesium depleted solid material occurs, and the insoluble magnesium carbonate remains mostly in solid form. This embodiment may be especially useful if separation of the solid mixture of magnesium carbonate and the magnesium depleted solid material is less essential. In one embodiment, the step of separating comprises heating the magnesium carbonate and the magnesium depleted solid material (or the solid material following precipitation) to above 60°C, or above 70°C, or above 80°C, or above 90°C or above 95°C or above 100°C; and contacting the magnesium carbonate and the magnesium depleted solid material with an aqueous solution.

[0064] In one embodiment of the first or second aspects, the method further comprises a step of calcining the precipitated magnesium carbonate to provide magnesium oxide and carbon dioxide. This step may be described herein as “the calcination step” or “the step of calcination”.

[0065] The calcination step may take place at temperatures below 1000 °C. This may ensure the formation of an active form of magnesium oxide. The calcining conditions can be adjusted according to the desired product specifications and the desired characteristics of the magnesium oxide. For example, the calcination may take place at at least 500 °C, at least 550 °C, at least 600 °C, or at least 650 °C. The calcination step may comprise heating to a temperature of from about 500 °C to about 1000 °C, or from about 550 °C to about 1000 °C, or from about 550 °C to about 800 °C, or from about 600 °C to about 750 °C, or about 650 °C to about 750 °C, or about 600 °C to about 700 °C, or about 650 °C. The calcination step may be performed for a duration of up to 4 hours, or up to 3 hours, or up to 2 hours or up to 1 hour. The calcination step may be performed for from 10 minutes to 4 hours, or from 10 minutes to 3 hours, or from 10 minutes to 2 hours or from 10 minutes to 1 hour. The calcination step may be performed in the presence of an ambient concentration of oxygen, or in the presence of a lower concentration of oxygen than the ambient concentration. In one embodiment, the calcination may take place in a kiln or fluid bed or similar vessel with the calcination temperature and residence times selected to meet the product qualities sought in the magnesium oxide product. In one embodiment, the magnesium oxide may be slaked with water to form magnesium hydroxide prior to use in the scrubbing reactor. The calcination step may produce magnesium oxide with a disordered crystal structure, which may be more reactive to acidic dissolution (including dissolution by aqueous carbon dioxide). Typical reaction times for carbonic acid leaching of this activated form of rock, as recorded in the literature, are in the order of 1-3 hours. The activated rock, mixed with water and CO2 gas, causes part of the magnesium content of the rock to dissolve, to form a magnesium containing solution, and under some conditions to exceed the solubility limit and precipitate a magnesium carbonate.

[0066] In one embodiment, the step of calcination generates carbon dioxide for recycle and further reaction of the reactive magnesium containing solid material in accordance with the present invention. For example, in one embodiment, the carbon dioxide generated from the calcination step may be used in the dissolution and / or precipitation steps of the first or second aspects. In one embodiment, the carbon dioxide stream from calcination may require upgrading to close to 100% prior to reuse in the dissolution and / or precipitation steps. Such methods of upgrading carbon dioxide streams are conventional technology. Advantageously, with this recycle of carbon dioxide to be used in the precipitation and / or dissolution steps, the magnesium oxide is generated without net production or consumption of carbon dioxide. In effect, the generated magnesium oxide is carbon neutral. In one embodiment, the calcination step may be performed using a calcination reactor fired with fossil fuel, and in this case the resulting carbon dioxide may be captured and re-incorporated into the overall system to ultimately be converted to MgCO3. This may retain the carbon neutrality of a MgO product.

[0067] In one embodiment, the magnesium oxide is suitable for scrubbing carbon dioxide. This step may be described herein as “scrubbing”, “the scrubbing step” or “the step of scrubbing”.

[0068] The magnesium oxide may be used to scrub carbon dioxide from industrial gas streams and / or from impure gas streams, such as those generated during burning of fossil fuels, or reduction of metals, or as part of the gas streams in steam methane reforming. In one embodiment, the magnesium oxide may be used to scrub carbon dioxide from off gasses that are generated by a various sources (including industrial processes). These sources may range from large production facilities like steel and cement manufacture, down to small, localised facilities such as boutique alcohol producers, and combustion ovens and furnaces used for heating or local power generation. Advantageously, by the use of scrubbing in accordance with the present invention, these industries can become carbon neutral. Without wishing to be bound by theory, the inventor believes that the magnesium oxide produced through the calcination step enables much more rapid and efficient scrubbing of carbon dioxide containing gases then could be achieved using a reactive (or activated) magnesium containing material such as calcined serpentinite.

[0069] In one embodiment, the generated magnesium oxide is used for scrubbing gases containing intermediate concentrations of carbon dioxide, such as those generated by combustion or fermentation reactions. The scrubbing may form a carbon dioxide depleted gas suitable for venting to atmosphere. For this type of application, the calcination step may take place at temperatures below 1000 °C, to ensure the formation of an active form of magnesium oxide and the magnesium oxide may be slaked with water to form magnesium hydroxide prior to use in the scrubbing reactor. The magnesium oxide may be slaked prior to being utilised in scrubbing.

[0070] In one embodiment, the scrubbing step may comprise addition of magnesium carbonate. The scrubbing may be accelerated by the addition of magnesium carbonate. Without being bound by theory, is believed that the magnesium carbonate provides additional surface area to react with carbon dioxide to form a solution comprising magnesium bicarbonate which can subsequently react with magnesium oxide to form magnesium carbonate. Magnesium carbonate may also be added to step (i) or step (ii) of the second aspect or to the first aspect. This may compensate for losses in carbon dioxide that may occur, such as through imperfect separation of magnesium carbonate and magnesium depleted solid material.

[0071] In one embodiment, the scrubbing step may be performed over any suitable residence time for the solids. In one embodiment, the scrubbing step may be performed over 1-3 hours, or greater than 1 hour, or greater than 3 hours, or less than 3 hours, or less than 1 hour. In one embodiment, the scrubbing step is performed at ambient temperature and pressure. The solids content of the scrubbing slurry containing magnesium oxide may be adjusted to enable both efficient scrubbing and efficient conversion of magnesium oxide to magnesium carbonate. The slurry may contain more than 1% solids, or more than 10% solids, or more than 20% solids, or more than 30% solids.

[0072] In one embodiment, the scrubbing step may provide magnesium bicarbonate or magnesium carbonate. The magnesium bicarbonate or magnesium carbonate may be in an aqueous solution. The magnesium bicarbonate, magnesium carbonate and / or aqueous solution may be used in the precipitation step. In one embodiment the magnesium oxide may be used directly in the precipitation step. Use of the magnesium oxide in the precipitation step may generate a solution that is further depleted in bicarbonate, for subsequent dissolution of magnesium from the magnesium depleted solid material.

[0073] In one embodiment, the produced magnesium oxide provides a more rapid and / or efficient scrubbing of carbon dioxide than could be achieved using magnesium containing solid material alone.

[0074] In one embodiment, the produced magnesium oxide may be used as a fertiliser or soil enhancer. For example, in one embodiment, magnesium oxide reacts with carbon dioxide present in the soil before this carbon dioxide ultimately escapes back to the atmosphere.

[0075] In one embodiment, magnesium oxide may be used to produce refractories.

[0076] In one embodiment, the present invention enables carbon negative or carbon neutral energy. For example, the carbon dioxide used in any one of the embodiments disclosed herein may be provided from the fermentation or combustion of biomass. For example, as algae or crops grow, they absorb carbon dioxide from the atmosphere. This stored carbon in biomass ultimately returns to the atmosphere as the plants die and eventually decompose. For example, biomass can be used to generate thermal energy, or liquid fuels such as ethanol or biodiesel, or hydrogen. In one embodiment, when biomass processing is combined with the present invention, the permanent storage of the absorbed carbon may be achieved.

[0077] In one embodiment, the methods disclosed herein enable the capture of carbon dioxide from the many distributed point sources of intermediate carbon dioxide concentrations, to permanently store this carbon dioxide locally, as a stable and potentially valuable solid, thus delivering a net neutral or net negative carbon dioxide emissions to / from the atmosphere.

[0078] In an embodiment of the second aspect, step (i) is to thereby provide an aqueous solution comprising magnesium bicarbonate and residual solid material; wherein step (ii) provides a magnesium depleted aqueous solution and a magnesium depleted solid material; and wherein the method further comprises: (iii) separating the magnesium carbonate from the magnesium depleted solid material.

[0079] In a third aspect, the present invention provides a method of precipitating magnesium carbonate, the method comprising the steps of:(i) Contacting a first reactive magnesium containing solid material with carbon dioxide in an aqueous solution, to thereby provide an aqueous solution comprising magnesium bicarbonate and residual solid material; and(ii) Contacting the aqueous solution comprising magnesium bicarbonate with a second reactive magnesium containing solid material to thereby precipitate magnesium carbonate and provide a magnesium depleted aqueous solution and a magnesium depleted solid material; and(iii) Separating the magnesium carbonate from the magnesium depleted solid material.

[0080] In an embodiment of the third aspect, the magnesium depleted solid material in step (iii) is used as the first reactive magnesium containing solid material in step (i). In one embodiment, step (i) receives the magnesium depleted solid material from step (iii) as the first reactive magnesium containing material.

[0081] The magnesium depleted aqueous solution in step (ii) may be used as the aqueous solution in step (i). In one embodiment, step (i) receives the magnesium depleted aqueous solution from step (ii) as the aqueous solution. In one embodiment, the magnesium depleted aqueous solution from step (ii) is low in magnesium bicarbonate due to the higher pH in step (ii). Due to the separation of magnesium carbonate in step (iii), there may be a reduction in the buffering capacity of the magnesium depleted solid material from step (iii) (which may be used as the first reactive magnesium containing material in step (i)). Thus, upon addition of carbon dioxide in step (i), the pH in the dissolution reactor may be lowered, which may enable faster and more complete dissolution of magnesium from the magnesium depleted solid material to form a magnesium bicarbonate solution.

[0082] In one embodiment, step (ii) may be performed with at least a first precipitation step and a second precipitation step. The first precipitation step may be performed in the absence of carbon dioxide, and the second precipitation step may be performed in the presence of carbon dioxide. The second precipitation step may be performed under pressure. The magnesium depleted aqueous solution in the first precipitation step may be used as the aqueous solution in step (i). Aqueous solution comprising magnesium bicarbonate from step (i) may be added to the first and second precipitation steps (or all precipitation steps).

[0083] In one embodiment of the third aspect, the method further comprises a step of calcining the magnesium carbonate from step (iii) to produce magnesium oxide and carbon dioxide. In one embodiment, the calcination step is as defined for the first or second aspects. In another aspect, the method further comprises a step of scrubbing carbon dioxide with the magnesium oxide. In an embodiment of the third aspect, the residual solid material from step (i) is separated from the aqueous solution comprising magnesium bicarbonate. The residual solid material may be discarded. In another embodiment, the residual solid material may be stored in a tailings storage facility or used in another application (such as building materials). In another embodiment, the residual solid material may comprise silica, which may be further refined or used.

[0084] In one embodiment, the second reactive magnesium containing solid material is activated magnesium containing solid material as described above. An advantage of this arrangement is that the dissolution of magnesium into an aqueous solution is exothermic. In one embodiment the first reactive magnesium containing solid material in step (i) is the magnesium depleted solid material. The magnesium dissolution in step (i) may be slow, as much of the most reactive magnesium has already been dissolved from the magnesium depleted solid material in step (ii). Because the dissolution may be slow, the associated temperature increase from the exothermic dissolution in step (i) is small. So, as the temperature of step (i) may increase only marginally, more carbon dioxide will remain in solution aiding further dissolution in step (i). In contrast, by contacting the activated magnesium containing solid material with an activated solution in step (ii), heat would be generated due to the dissolution of magnesium in that step, which is advantageous for the formation and precipitation of magnesium carbonate.

[0085] In one embodiment the third aspect of the present invention provides a method of precipitating magnesium carbonate, the method comprising the steps of:(i) Contacting a first reactive magnesium containing solid material with carbon dioxide in an aqueous solution, to thereby provide an aqueous solution comprising magnesium bicarbonate and residual solid material; and(ii) Contacting the aqueous solution comprising magnesium bicarbonate with an activated magnesium containing solid material to thereby precipitate magnesium carbonate and provide a magnesium depleted aqueous solution and a magnesium depleted solid material; and(iii) Separating the magnesium carbonate from the magnesium depleted solid material;wherein step (i) receives the magnesium depleted solid material from step (iii) as the first reactive magnesium containing material; wherein step (i) receives the magnesium depleted aqueous solution from step (ii) as the aqueous solution; and wherein the residual solid material from step (i) is separated from the aqueous solution comprising magnesium bicarbonate.

[0086] In one embodiment, the magnesium depleted solid material in step (iii) is subjected to grinding, crushing or comminuting before use as the reactive magnesium containing solid material in step (i). The magnesium depleted solid material may be subjected to grinding, crushing or comminuting to a p80 of less than 500 μm, or less than 450 μm, or less than 400 μm, or less than 350 μm, or less than 300 μm, or less than less than 250 μm, or less than 200 μm, or less than 150 μm, or less than 100 μm or less than 50 μm. The step of grinding, crushing or comminuting the magnesium depleted solid material may comprise grinding, crushing or comminuting to a p80 of from about 1 μm to about 300 μm, or from about 10 μm to about 300 μm, or from about 5 μm to about 250 μm, or from about 10 μm to about 200 μm, or from about 10 μm to about 100 μm, or from about 15 μm to about 150 μm, or from about 20 μm to about 100 μm, or from about 25 μm to about 75 μm, or from about 35 μm to about 65 μm, or about 50 μm.

[0087] In one embodiment the third aspect of the present invention provides a method of precipitating magnesium carbonate, the method comprising the steps of:(i) Contacting a first reactive magnesium containing solid material with carbon dioxide in an aqueous solution, to thereby provide an aqueous solution comprising magnesium bicarbonate and residual solid material; and(ii) Contacting the aqueous solution comprising magnesium bicarbonate with an activated magnesium containing solid material to thereby precipitate magnesium carbonate and provide a magnesium depleted aqueous solution and a magnesium depleted solid material; and(iii) Separating the magnesium carbonate from the magnesium depleted solid material;wherein step (i) receives the magnesium depleted solid material from step (iii) as the first reactive magnesium containing material; wherein step (i) receives the magnesium depleted aqueous solution from step (ii) as the aqueous solution; wherein step (ii) is performed with at least a first precipitation step and a second precipitation step, wherein the first precipitation step is performed in the absence of carbon dioxide, and the second precipitation step is performed in the presence of carbon dioxide.

[0088] The magnesium carbonate after step (iii) may be in a fraction that comprises impurities (especially magnesium depleted solid material). The magnesium carbonate after step (iii) may be in a fraction which is physically discrete from the residual material. The fraction comprising magnesium carbonate may comprise at least 40%, at least 50%, at least 60%, at least 70%, at least 80%, at least 90%, or at least 95% (w / w) magnesium carbonate.

[0089] Features of the third aspect of the present invention may be as described for the first and second aspects of the present invention. In one embodiment, the second reactive magnesium containing solid material is an activated magnesium containing solid material. The activated magnesium containing solid material may be as described above.

[0090] In one embodiment, the total balance of magnesium bicarbonate formation and magnesium carbonate precipitation between the dissolution and precipitation reactors is maintained by increasing the number of passes of solution between the reactors. This may enable use of a lower pH for precipitation than some prior processes.

[0091] In one embodiment, step (i) (or the dissolution reactor(s)) can be batch or may be arranged to enable counter current flows of solids and liquids as described herein. In another embodiment, step (ii) (or the precipitation reactor(s)) can be batch or may be arranged to enable counter current flows of solids and liquids as described herein.

[0092] In one embodiment, the step of contacting a first reactive magnesium containing solid material with carbon dioxide in an aqueous solution is performed in a plurality of stages or reactors / vessels, such as in two, three, four or five stages or reactors / vessels, especially three stages or reactors / vessels. In some embodiments, the solid and liquid material in the plurality of stages or reactors / vessels may pass through the reactors / vessels in a counter current arrangement or system. In one embodiment, a first reactor / vessel receives magnesium depleted aqueous solution from step (ii). This aqueous solution is passed from the first reactor / vessel to the second and so on until it reaches the final reactor / vessel. The final vessel / reactor in this step receives magnesium depleted solid material from step (iii). This solid material is passed from the final reactor / vessel to the penultimate reactor / vessel and so on until it reaches the first reactor / vessel. In one embodiment, the step of contacting a first reactive magnesium containing solid material with carbon dioxide in an aqueous solution comprises a plurality of reactors / vessels, and a first reactor / vessel receives magnesium depleted aqueous solution from step (ii) and a final reactor / vessel receives magnesium depleted solid material from step (iii); and wherein the magnesium depleted aqueous solution is passed from the first through to the final reactor / vessel, and wherein the magnesium depleted solid material is passed from the final through to the first reactor / vessel. In one embodiment, the step of contacting a first reactive magnesium containing solid material with carbon dioxide in an aqueous solution comprises a plurality of reactors / vessels, and a first reactor / vessel receives magnesium depleted aqueous solution from step (ii) and a second reactor / vessel receives magnesium depleted solid material from step (iii); and wherein the magnesium depleted aqueous solution and the magnesium depleted solid material is passed through the plurality of reactors / vessels. The reactors / vessels may be in series. After the magnesium depleted solid material has passed through the first reactor / vessel it may be discarded. In this arrangement, the most magnesium depleted solution is contacting the most magnesium depleted solid material, thus extending the extraction of magnesium from the material.

[0093] In one embodiment, the step of contacting an aqueous solution comprising magnesium bicarbonate with a second reactive magnesium containing solid material (or an activated magnesium containing material) is performed in a plurality of stages or reactors / vessels, such as in two, three, four or five stages or reactors / vessels, especially three stages or reactors / vessels. In some embodiments, the solid and liquid material in the plurality of stages or reactors / vessels may pass through the reactors / vessels in a counter current arrangement or system. In one embodiment, a first reactor / vessel receives activated magnesium containing solid material, as described above. This solid material is passed from the first reactor / vessel to the second and so on until it reaches the final reactor / vessel. The final vessel / reactor in this step receives the aqueous solution comprising magnesium bicarbonate from step (i). This aqueous solution is passed from the final reactor / vessel to the penultimate reactor / vessel and so on until it reaches the first reactor / vessel. In one embodiment, the step of contacting an aqueous solution comprising magnesium bicarbonate with a second reactive magnesium containing solid material (or an activated magnesium containing material) comprises a plurality of reactors / vessels, and a first reactor / vessel receives activated magnesium containing solid material and a final reactor / vessel receives the aqueous solution comprising magnesium bicarbonate from step (i); and wherein the second reactive magnesium containing solid material (or activated magnesium containing solid material) is passed from the first through to the final reactor / vessel, and wherein the aqueous solution comprising magnesium bicarbonate is passed from the final through to the first reactor / vessel. In one embodiment, the step of contacting an aqueous solution comprising magnesium bicarbonate with a second reactive magnesium containing solid material (or an activated magnesium containing material) comprises a plurality of reactors / vessels, and a first reactor / vessel receives a second reactive magnesium containing solid material (or an activated magnesium containing solid material) and a second reactor / vessel receives the aqueous solution comprising magnesium bicarbonate from step (i); and wherein the second reactive magnesium containing solid material (or the activated magnesium containing material) and the aqueous solution comprising magnesium bicarbonate is passed through the plurality of reactors / vessels. The reactors / vessels may be in series. After the magnesium containing solid material has passed through the final reactor / vessel it may be separated in step (iii). In this arrangement, the most reactive magnesium containing solid material is contacting the most magnesium depleted aqueous solution, thus extending the extraction of magnesium from the material.

[0094] Therefore, in one embodiment the third aspect of the present invention provides a method of precipitating magnesium carbonate, the method comprising the steps of:(i) Contacting a first reactive magnesium containing solid material with carbon dioxide in an aqueous solution; wherein the step comprises a plurality of reactors in series to thereby provide an aqueous solution comprising magnesium bicarbonate and a residual solid material; wherein the plurality of reactors comprises at least a first and a final reactor; and(ii) Contacting the aqueous solution comprising magnesium bicarbonate with an activated magnesium containing solid material; wherein the step comprises a plurality of reactors in series to thereby precipitate magnesium carbonate and provide a magnesium depleted aqueous solution and a magnesium depleted solid material; wherein the plurality of reactors comprises at least a first and a final reactor; and(iii) Separating the magnesium carbonate from the magnesium depleted solid material;wherein in step (i), the first reactor receives the magnesium depleted aqueous solution from step (ii) and the final reactor receives magnesium depleted solid material from step (iii); and wherein the magnesium depleted aqueous solution is passed from the first reactor through to the final reactor to thereby provide the aqueous solution comprising magnesium bicarbonate, and wherein the magnesium depleted solid material is passed from the final reactor through to the first reactor to thereby provide the residual solid material;wherein in step (ii), the first reactor receives the activated magnesium containing solid material and the final reactor receives the aqueous solution comprising magnesium bicarbonate from step (i); and wherein the activated magnesium containing solid material is passed from the first reactor through to the final reactor to thereby provide the magnesium depleted solid material, and wherein the aqueous solution comprising magnesium bicarbonate is passed from the final reactor through to the first reactor to thereby provide the magnesium depleted aqueous solution.

[0095] Therefore, in another embodiment the third aspect of the present invention provides a method of precipitating magnesium carbonate, the method comprising the steps of:(i) Contacting a first reactive magnesium containing solid material with carbon dioxide in an aqueous solution; wherein the step comprises a plurality of reactors in series to thereby provide an aqueous solution comprising magnesium bicarbonate and a residual solid material; wherein the plurality of reactors comprises at least a first and a second reactor; and(ii) Contacting the aqueous solution comprising magnesium bicarbonate with an activated magnesium containing solid material; wherein the step comprises a plurality of reactors in series to thereby precipitate magnesium carbonate and provide a magnesium depleted aqueous solution and a magnesium depleted solid material; wherein the plurality of reactors comprises at least a first and a second reactor; and(iii) Separating the magnesium carbonate from the magnesium depleted solid material;wherein in step (i), the first reactor receives the magnesium depleted aqueous solution from step (ii) and the second reactor receives magnesium depleted solid material from step (iii); and wherein the magnesium depleted aqueous solution and the magnesium depleted solid material is passed through the plurality of reactors to thereby provide the aqueous solution comprising magnesium bicarbonate and the residual solid material;wherein in step (ii), the first reactor receives the activated magnesium containing solid material and the second reactor receives the aqueous solution comprising magnesium bicarbonate from step (i); and wherein the activated magnesium containing solid material and the aqueous solution comprising magnesium bicarbonate is passed through the plurality of reactors to thereby provide the magnesium depleted solid material and the magnesium depleted aqueous solution.

[0096] In one embodiment, in step (i) the first reactor is at a pH of less than 7, or less than 6.5, or less than 6.0 or less than 5.5. The first reactor may operate at far from magnesium bicarbonate saturation.

[0097] In one embodiment, the residual solid material from step (i) is separated from the aqueous solution comprising magnesium bicarbonate.

[0098] In the above counter current arrangement, in step (i), the highest acidity and most magnesium depleted solution may first contact the more magnesium depleted magnesium containing solid material, thus extending the extraction of magnesium from the rock. This solution may then proceed to contact the magnesium depleted solid material (which would comprise more readily soluble solids), thus increasing the magnesium bicarbonate concentration in solution. Furthermore, in step (ii), the least concentrated magnesium bicarbonate solution may contact the activated magnesium containing solid material, encouraging a maximum proportion of magnesium precipitation from solution. In effect, the counter-current flows may allow more effective use of the magnesium containing solid material, and reduce the duration for the reactions, by altering the acidity in each stage.

[0099] In one embodiment, the aqueous solution comprising magnesium bicarbonate is only generated by dissolution of reactive magnesium containing solid material (especially magnesium depleted solid material after separation). In one embodiment, the conditions may be controlled across the method such that the aqueous solution comprising magnesium bicarbonate contributes most or all of the carbonate required to balance the total carbon dioxide uptake per tonne of reactive magnesium containing solid material. For example, if the total carbon dioxide uptake arose from dissolution of 70% of the magnesium, the balance may be located around 35% of the magnesium dissolved in step (ii), and 35% dissolved in step (i), depending on the efficiency of the separation in step (iii).

[00100] The solution flow rate in steps (i) and (ii) may be varied to balance the pH, and hence desired reaction rates, in these steps. If step (ii) is slower than desired, the flow of aqueous solution relative to the flow of solids can be increased. This reduces the ‘bite’ (i.e. precipitation) from each pass of the solution but increases the acidity and hence the rate of magnesium leaching in step (ii). In one embodiment, in the final reactor in step (ii) the carbon dioxide vapour pressure is low (such as less than about 0.1 atmosphere, or less than about 0.01 atmosphere). The pH in the final reactor in step (ii) may be controlled to between 7.5 and 9.5.

[00101] In one embodiment, the ‘bite’ of magnesium removed in each pass of the solution step (ii) is not as critical as prior art situations where vaporisation of carbon dioxide is used for precipitation, as the energy requirements per pass of solution in the current invention may not be material. In contrast, in the present invention the precipitation of MgCO3 is induced by contacting a reactive magnesium containing solid material with a solution comprising magnesium bicarbonate. The only penalty of a reduced ‘bite’ per pass may be greater transfer of liquors between the dissolution and precipitation reactors. When the precipitation of magnesium carbonate occurs in a plurality of stages (or when a plurality of reactors / vessels are used for precipitation), application of the most highly reactive magnesium containing solid material in the final stages of precipitation enables the pH of the solution exiting precipitation to be above 7.5 and preferably around 8.5.

[00102] In one embodiment, the method of the present invention may achieve a balance such that the quantity of carbonate precipitated in step (ii), matches the magnesium level dissolved in step (i). The pH differential between steps (i) and (ii) may be maximised, allowing the maximum amount of carbon dioxide to be stored for a given tonnage of reactive magnesium containing solid material. The efficiency of the method may be influenced by the reactivity of the reactive magnesium containing solid material, the efficiency of the separation step in removing magnesium carbonate, and the residence time and reaction conditions in each of steps (i) and (ii). It may be possible to increase precipitation of magnesium carbonate by adding additional base in step (ii), or reduce precipitation required by introducing disproportionation to independently consume some of the carbon dioxide present in the aqueous solution in step (ii).

[00103] In a fourth aspect, the present invention provides magnesium carbonate when precipitated by the method of the first, second or third aspects. Features of the fourth aspect of the present invention may be as described for any of the first, second or third aspects.

[00104] In a fifth aspect, the present invention provides an apparatus for precipitating magnesium carbonate; the apparatus comprising:(i) A dissolution reactor for contacting a first reactive magnesium containing solid material with carbon dioxide in an aqueous solution, to thereby provide an aqueous solution comprising magnesium bicarbonate;(ii) A precipitation reactor for contacting the aqueous solution comprising magnesium bicarbonate with a second reactive magnesium containing solid material to thereby precipitate magnesium carbonate; and(iii) A separator for separating the magnesium carbonate from the magnesium depleted solid material.

[00105] In one embodiment, the dissolution reactor comprises a plurality of reactors, especially in series. In another embodiment, the precipitation reactor comprises a plurality of reactors, especially in series.

[00106] In one embodiment of the fifth aspect, the apparatus further comprises a calcination reactor for calcining the magnesium carbonate to provide magnesium oxide and carbon dioxide. In another embodiment, the apparatus further comprises a scrubbing reactor for scrubbing carbon dioxide with the magnesium oxide.

[00107] Features of the fifth aspect of the present invention may be as described for any of the first, second or third aspects.

[00108] In a sixth aspect, the present invention provides a method of producing magnesium oxide, the method comprising the steps of:(i) Contacting a first reactive magnesium containing solid material with carbon dioxide in an aqueous solution, to thereby provide an aqueous solution comprising magnesium bicarbonate; (ii) Contacting the aqueous solution comprising magnesium bicarbonate with a second reactive magnesium containing solid material to thereby precipitate magnesium carbonate; and(iii) Separating the magnesium carbonate from the magnesium depleted solid material; (iv) Calcining the magnesium carbonate of step (iii) to provide magnesium oxide; and(v) Scrubbing carbon dioxide with the magnesium oxide.

[00109] The features of the sixth aspect may be as defined for the first, second, and / or third aspects.

[00110] Steps (iv) and (v) of the method of the sixth aspect may be performed in different locations.

[00111] In one embodiment, step (iv) provides magnesium oxide and carbon dioxide. The carbon dioxide may be used in step (i) or step (ii), especially step (i).

[00112] In one embodiment, step (v) may provide a carbon dioxide depleted gas and an aqueous solution comprising magnesium carbonate solid and / or magnesium bicarbonate.

[00113] In a seventh aspect, the present invention provides magnesium oxide when generated (or produced) by the method of any one of the first, second, third, and / or sixth aspects.

[00114] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as would be commonly understood by those of ordinary skill in the art to which this invention belongs.

[00115] In describing and claiming the present invention, the following terminology will be used in accordance with the definitions set out below. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments of the present invention only and is not intended to be limiting.

[00116] Unless the context clearly requires otherwise, throughout the description and the claims, the terms “comprise”, “’comprising”, and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”. For example, an apparatus, composition, mixture or method that comprises a list of elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such apparatus, composition, mixture or method.

[00117] The transitional phrase “consisting of” excludes any element, step, or ingredient not specified. If in the claim, such would close the claim to the inclusion of materials other than those recited except for impurities ordinarily associated therewith. When the phrase “consisting of” appears in a clause of the body of a claim, rather than immediately following the preamble, it limits only the element set forth in that clause; other elements are not excluded from the claim as a whole.

[00118] The transitional phrase “consisting essentially of” is used to define a composition, process or method that includes materials, steps, features, components, or elements, in addition to those literally disclosed, provided that these additional materials, steps, features, components, or elements do not materially affect the basic and novel characteristic(s) of the claimed invention. The term “consisting essentially of” occupies a middle ground between “comprising” and “consisting of”.

[00119] Where the applicant has defined an invention or a portion thereof with an open-ended term such as “comprising”, it should be readily understood that (unless otherwise stated) the description should be interpreted to also describe such an invention using the terms ‘’consisting essentially of” or “consisting of.” In other words, with respect to the terms “comprising”, “consisting of”, and “consisting essentially of”, where one of these three terms is used herein, the presently disclosed and claimed subject matter may include the use of either of the other two terms. Thus, in some embodiments not otherwise explicitly recited, any instance of “comprising” may be replaced by “consisting of” or, alternatively, by “consisting essentially of”. Also, the indefinite articles “a” and “an” preceding an element or component of the invention are intended to be non-restrictive regarding the number of instances (i.e., occurrences) of the element or component. Therefore “a” or “an” should be read to include one or at least one, and the singular word form of the element or component also includes the plural unless the number is obviously meant to be singular.

[00120] As used herein, with reference to numbers in a range of numerals, the terms “about,” “approximately” and “substantially” are understood to refer to the range of -10% to +10% of the referenced number, preferably -5% to +5% of the referenced number, more preferably -1 % to + 1 % of the referenced number, most preferably -0 .1 % to +0 .1 % of the referenced number. Moreover, with reference to numerical ranges, these terms should be construed as providing support for a claim directed to any number or subset of numbers in that range. For example, a disclosure of from 1 to 10 should be construed as supporting a range of from 1 to 8, from 3 to 7, from 1 to 9, from 3.6 to 4.6, from 3.5 to 9.9, from 8 to 10, and so forth.

[00121] As used herein, wt.% refers to the weight of a particular component relative to total weight of the referenced composition.

[00122] The term “and / or” used in the context of “X and / or Y” should be interpreted as “X,” or “Y,” or “X and Y.” Similarly, “at least one of X or Y” should be interpreted as “X,” or “Y,” or “both X and Y.”

[00123] The terms “preferred” and “preferably” refer to embodiments of the invention that may afford certain benefits, under certain circumstances. However, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful, and is not intended to exclude other embodiments from the scope of the invention.

[00124] Where otherwise indicated, all numbers expressing quantities of ingredients or reaction conditions used herein are to be understood as modified in all instances by the term “about”. The examples are not intended to limit the scope of the invention. In what follows, or where otherwise indicated, “%” will mean “weight %” (wt%), “ratio” will mean “weight ratio” and “parts” will mean “weight parts”.

[00125] As used herein, “reactive magnesium containing solid material” refers to a solid material comprising a magnesium species in which the magnesium is capable of dissolving (or is soluble) in an aqueous solution at an acidic pH. When assessing whether or not a solid material is a “reactive magnesium containing solid material”, the presence or absence of magnesium carbonate or magnesium bicarbonate in the solid material should be disregarded. Consequently, in one embodiment, the magnesium species is not magnesium carbonate or magnesium bicarbonate. An “activated magnesium containing solid material” is a reactive magnesium containing solid material that has been activated, for example by a method as discussed above. A “reactive magnesium containing solid material” or an “activated magnesium containing solid material” is not magnesium carbonate or magnesium bicarbonate.

[00126] As used herein, the term “separated”, as in “separating the magnesium carbonate from the magnesium depleted solid material” does not necessarily mean that all of the magnesium carbonate is separated from the magnesium depleted solid material. Some magnesium carbonate may be present in the separated magnesium depleted solid material, and some magnesium depleted solid material may be present in the separated magnesium carbonate.

[00127] As used herein, “deplete” as in “magnesium depleted aqueous solution” and “magnesium depleted solid material” means that the amount or concentration of magnesium in the solution or in the solid material has reduced or decreased as a result of the step. It does not mean that all magnesium is removed from the solution or the solid material.

[00128] Reference throughout this specification to ‘one embodiment’ or ‘an embodiment’ means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases ‘in one embodiment’ or ‘in an embodiment’ in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more combinations.

[00129] Any of the features described herein can be combined in any combination with any one or more of the other features described herein within the scope of the invention. For example, any of the features described herein in relation to step (i) of the second aspect (or the dissolution step) may be combined with any other features of step (i) (or the dissolution step). Furthermore, any of the features described herein in relation to step (ii) of the second aspect (or the precipitation step) may be combined with any other features of step (ii) (or the precipitation step). Furthermore, any of the features described herein in relation to step (iii) (or the separation step) may be combined with any other features of step (iii) (or the separation step). Moreover, any of the features described herein in relation to step (i) of the second aspect (or the dissolution step), step (ii) of the second aspect (or the precipitation step) and step (iii) (or the separation step) may be combined with any other features of these steps. BRIEF DESCRIPTION OF DRAWINGS

[00130] Preferred features, embodiments and variations of the present invention may be discerned from the following Detailed Description of the invention which provides sufficient information for those skilled in the art to perform the invention. The Detailed Description of the invention will now be described by way of example with reference to the accompanying Figures, in which:

[00131] Figure 1 shows a first embodiment of the present invention;

[00132] Figure 2 shows a counter-current arrangement of the precipitation and dissolution reactors in a second embodiment of the present invention;

[00133] Figure 3 shows an embodiment of the present invention for the step of contacting a first reactive magnesium containing solid material with carbon dioxide in an aqueous solution;

[00134] Figure 4 shows an embodiment of the present invention for the step of containing the aqueous solution comprising magnesium bicarbonate with a reactive magnesium containing solid material;

[00135] Figure 5 shows an embodiment of the present invention;

[00136] Figure 6 shows an embodiment of the present invention which includes a step of calcining the magnesium carbonate to generate magnesium oxide;

[00137] Figure 7 shows an embodiment in which the magnesium oxide generated in accordance with the present invention is used in a scrubbing step to scrub carbon dioxide;

[00138] Figure 8 shows a further embodiment of the present invention;

[00139] Figure 9 shows an electron micrograph of magnesium carbonate produced in accordance with the present invention;

[00140] Figure 10 shows an electron micrograph of magnesium carbonate produced in accordance with the present invention and is an expanded view of Figure 9. In this embodiment, the magnesium carbonate has a needle-like nesquehonite structure;

[00141] Figure 11 shows a graph of CO2 uptake by an activated calcined sample after 1, 2, 3 and 4 hours’ respectively;

[00142] Figure 12 shows an electron micrograph of precipitated nesquehonite produced in accordance with the invention; and

[00143] Figure 13 shows an electron micrograph of precipitated nesquehonite produced in accordance with the invention.

[00144] The following Detailed Description are not to be regarded as limiting the scope of the preceding Summary of the Invention in any way.DETAILED DESCRIPTION OF INVENTION

[00145] The following examples illustrate the compositions, methods, and results according to the disclosed subject matter. These examples are not intended to be inclusive of all aspects of the subject matter disclosed herein, but rather to illustrate representative methods and results.

[00146] Embodiments of the invention will now be described with reference to Figures 1 to 13. In the figures, like reference numerals refer to like features.

[00147] An embodiment of the present invention is shown in Figure 1. A reactive magnesium containing solid material (12) (a second reactive magnesium containing material) is contacted with an aqueous solution comprising magnesium bicarbonate (42) at (20) (which may be a precipitation reactor (or a slurry precipitation reactor)). The step of contacting may be performed by forming a slurry of the aqueous solution comprising magnesium bicarbonate (42) and the reactive magnesium containing solid material (12). As this step proceeds, the pH of the aqueous solution increases as the reactive magnesium containing solid material (12) dissolves, and as the pH increases the bicarbonate solution is converted to carbonate. The magnesium carbonate (24) then precipitates, leaving lower concentrations of magnesium and bicarbonate in the solution phase (the magnesium depleted aqueous solution (23)). A magnesium depleted solid material (22) is also produced.

[00148] The dissolution of the reactive magnesium containing solid material (12) in the precipitation step (20) is exothermic, and this raises the temperature during (20), thus further accelerating the precipitation of magnesium carbonate (24).

[00149] At (30), the magnesium depleted solid material (22) and the magnesium carbonate (24) may be separated from the magnesium depleted aqueous solution (23) or may be directly subjected to physical beneficiation to separate the solid fractions into magnesium depleted solid material (22) and magnesium carbonate (24). The physical beneficiation (30) can be achieved through such attritioning as may be required to physical separate the grains of magnesium carbonate from the residual rock, then using flotation and / or gravity separation to separate the two fractions. The physical beneficiation may remove most the magnesium carbonate (24) from the magnesium depleted solid material (22), and hence avoids its redissolution at step (40). The magnesium carbonate (24) may then be processed or used further as desired. The magnesium carbonate fraction (24) may contain predominantly magnesium carbonate with minor impurities of magnesium depleted solid material (22).

[00150] The magnesium depleted aqueous solution (23) and the magnesium depleted solid material (22) pass to (40) where dissolution occurs. The dissolution step (40) may be performed in a dissolution reactor. At (40) a first reactive magnesium containing solid material in the form of magnesium depleted solid material (22) is contacted with carbon dioxide (35) in an aqueous solution in the form of a magnesium depleted aqueous solution (23). Step (40) provides an aqueous solution comprising magnesium bicarbonate (42) (which is passed to step (20)) and a residual solid material (45). The residual solid material (45) may be separated and may be discarded or subsequently used.

[00151] Consequently, the method illustrated in Figure 1 addresses the separation of the magnesium carbonate from the reactive magnesium containing solid material, without the need for a carbon dioxide pressure swing reactor and reconcentration of the carbon dioxide generated by magnesium carbonate disproportionation. Hence, the energy required for the precipitation of magnesium carbonate may be much lower than the equivalent task achieved with vacuum distillation. The scale up of equipment to manage the large material flows may be greatly simplified.

[00152] In an embodiment of Figure 1, the reactive magnesium containing solid material (12) may be produced by activating a magnesium containing solid material. For example, the magnesium containing solid material may be an ultramafic material, and may include serpentinite or olivine. The step of activating may comprise grinding (especially fine grinding) and / or calcining. The step of activating may provide an activated magnesium containing solid material, which is used as the reactive containing solid material at (12).

[00153] In one embodiment, the magnesium depleted solid material (22) may be activated after separation (30), preferably by calcining and / or grinding (or crushing or comminuting). This activation may take place prior to or during step (40), including in a multi-stage grinding (or crushing or comminuting) and dissolution process.

[00154] Step (40) may comprise a single batch reactor or a counter current arrangement of reactors to maximise the concentration of magnesium bicarbonate in the aqueous solution and / or the dissolution of magnesium from the depleted magnesium containing solid material (22). In one embodiment, step (40) comprises one or more agitated pressure reactors or one or more large containers such as sealed dams or the equivalent, through which solution can be circulated. In one embodiment, the overflow of a substantially or near saturated aqueous solution comprising magnesium bicarbonate (42) can be recovered.

[00155] In climates with a large seasonal variation in temperature, the equilibrium concentration of magnesium carbonate in solution will increase in colder conditions. When using a dam style reactor in (40), removal of an aqueous solution comprising magnesium bicarbonate can take place in the cold weather.

[00156] For less reactive magnesium containing solid material, dissolution of magnesium may be more effectively achieved by arranging a multi-stage counter-current flow of solution and / or solids in one or both of (20) and / or (40).

[00157] Without wishing to be bound by theory, the inventor believes that the very reactive component of the reactive magnesium containing solid material can be utilised in step (20) (which may be a precipitation reactor), to precipitate twice the quantity of MgCO3, when the aqueous solution is already saturated with magnesium bicarbonate, rather than when utilising CO2 alone. This is illustrated in the equations below, CO2 alone: MgO (fast reacting) + CO2  MgCO3Step (20): MgO (fast reacting) + Mg2+ + 2HCO3-  2MgCO3 + H2O As such, recycling of the aqueous solution comprising magnesium bicarbonate may mean that the most reactive component of the reactive magnesium containing solid material is better utilised for magnesium carbonate precipitation, rather than half of it being consumed in forming the bicarbonate ions as has occurred in prior art processes which only use carbon dioxide to leach the magnesium into the aqueous solution. The use of this most reactive proportion of the reactive magnesium containing solid material is important, as the CO2 activity providing the means to dissolve the reactive magnesium containing solid material, is significantly lower than in normal dissolution. In some embodiments, this additional magnesium dissolution is combined with the simplicity and reduced costs of a precipitation reactor which can operate without the energy intensive removal and reconcentration of carbon dioxide gas.

[00158] Figure 2 illustrates a method of precipitating magnesium carbonate in a counter current arrangement. The reference numbers in this figure correspond to the same features as in Figure 1. In this method step (20) comprises contacting the aqueous solution comprising magnesium bicarbonate (42) with an activated magnesium containing solid material (12); wherein the step comprises a plurality of reactors in series (A, B and C) to thereby precipitate magnesium carbonate (24) and provide a magnesium depleted aqueous solution (23) and a magnesium depleted solid material (22). Step (40) comprises contacting a first reactive magnesium containing solid material (which is the magnesium depleted solid material (22)) with carbon dioxide (35) in an aqueous solution (the magnesium depleted aqueous solution (23)); wherein the step comprises a plurality of reactors in series (A, B and C) to thereby provide an aqueous solution comprising magnesium bicarbonate (42) and a residual solid material (45). Step (30) comprises separating the magnesium carbonate (24) from the magnesium depleted solid material (22).

[00159] In this arrangement in step (40), the highest acidity and most magnesium depleted solution (23) first contacts the more magnesium depleted solid material at reactor A, thus extending the extraction of magnesium from the rock. The aqueous solution then passes to reactors B and then C where there is contact with more readily soluble solids in residual solid material (45), thus increasing the magnesium bicarbonate concentration in solution. In step (20), in reactor C the aqueous solution with the lowest concentration of magnesium bicarbonate contacts the freshest solid material from (12), encouraging a maximum proportion of magnesium precipitation from solution. In effect, the counter-current flows allows more effective use of the solid material, and reduces the duration for the reactions, by altering the acidity in each stage.

[00160] Preferably, the balance between (20) and (40) when combined with a counter-current arrangement and expeditious use of carbon dioxide (35),pressure and temperature enables a favourable pH swing to be achieved between (20) and (40) (which may be the precipitation and dissolution reactors). In one embodiment, the pH swing ensures a high “bite” (i.e., precipitation) of magnesium carbonate from each cycle of solution and the most effective use of the reactive magnesium containing solid material (12) which declines in reactivity as further material is reacted.

[00161] Figure 3 illustrates another embodiment of step (40), of contacting a first reactive magnesium containing solid material in the form of magnesium depleted solid material (22) with carbon dioxide (35) in an aqueous solution in the form of a magnesium depleted aqueous solution (23). This arrangement may improve (or maximise) the magnesium concentration at the time of precipitation of magnesium carbonate, and / or may maximise the pH or carbon dioxide pressure swing. In this embodiment, the stages of dissolution (40) may be to saturate or substantially saturate the leaching solution. The embodiment of Figure 3 may be used in the method shown in Figures 1 or 2.

[00162] In Figure 3 the magnesium depleted aqueous solution (23) is added to reactor F along with residual solid material from reactor D and carbon dioxide (35). Reactor F may be a long term reactor (with a residence time of up to months or years), in which the magnesium bicarbonate concentration in the aqueous solution is relatively low (it may be far from saturation). The residual solid material from reactor F (45) may be discarded, or used elsewhere. The aqueous solution from reactor F is assigned to reactor E, which also receives magnesium depleted solid material (22) from separation (30) and carbon dioxide (35). Reactor E may be a medium term reactor (with a residence time of from days to weeks). Reactor E may operate at saturation of magnesium bicarbonate and may contain solid magnesium carbonate. The slurry from reactor E then is assigned to reactor D. Reactor D is a short term (with a residence time in minutes) pressure reactor operating to dissolve the magnesium carbonate to form a higher concentration of magnesium bicarbonate. The aqueous solution comprising magnesium bicarbonate (42) is then assigned to step (20), and the residual solid material from reactor D is assigned to reactor F as discussed above. Reactor D may operate with carbon dioxide over-pressure (such as about 5-10 bar) whereas reactors E and F may operate at 1 bar CO2 pressure.

[00163] Figure 4 illustrates another embodiment of step (20), of contacting the aqueous solution comprising magnesium bicarbonate (42) with a reactive magnesium containing solid material (12) to thereby precipitate magnesium carbonate (24) and provide a magnesium depleted aqueous solution (23) and a magnesium depleted solid material (22). This embodiment may be considered to illustrate a different step (20) to that shown in Figure 2. For reference, in Figure 4 reactors A and C of step (40) in Figure 2 have been illustrated.In this embodiment the concentration of magnesium bicarbonate in the magnesium depleted aqueous solution (23) assigned to the last stage of dissolution (reactor A of step (40)) is very low, maximising the potential pH swing.

[00164] In the embodiment of Figure 4, the aqueous solution comprising magnesium bicarbonate (42) from reactor C in step (40) is assigned to reactor G (which may be a precipitation reactor) in step (20), along with solids from reactor H (which also may be a precipitation reactor). Reactor H is assigned reactive magnesium containing solid material (12), which may be activated magnesium containing solid material. The magnesium depleted aqueous solution (23) from reactor H is assigned to reactor A of step (40). Reactor H may operate at higher than the pulp density achieved by simple counter-current flow. This configuration may raise the pH in this reactor to a high level, precipitating most of the magnesium bicarbonate in the solution from reactor G. Furthermore, because of the very low magnesium carbonate concentration in the magnesium depleted aqueous solution, reactor A of step (40) may operate at a lower dissolution pH when carbon dioxide is added to reactor A.

[00165] Figure 5 illustrates a further embodiment of the invention. This embodiment is similar to the embodiment illustrated in Figure 1, except that the precipitation step comprises two reactors, Reactor K and Reactor J. Reactor K receives reactive magnesium containing solid material (12), as well as solution from Reactor J. The solids from Reactor K following the precipitation step are passed to Reactor J, which also receives carbon dioxide (35) and an aqueous solution comprising magnesium bicarbonate (42) from Dissolution (40). At the end of the precipitation step, magnesium depleted solid material (22) and magnesium carbonate (24) from Reactor J are separated (30). In this embodiment, some dissolution and most precipitation would occur in Reactor J (which may operate at a more intermediate pH (e.g. pH 7-8)), whereas Reactor K is only intended to precipitate magnesium carbonate so it may operate at a more basic pH. Similarly, the dissolution reactor (40) may be run at a more acidic pH as this reactor is only intended for dissolution of magnesium.

[00166] Figure 6 illustrates one embodiment of the present invention in which the magnesium carbonate (24) is calcined to generate magnesium oxide (52) and carbon dioxide (54). This embodiment is similar to the embodiment shown in Figure 1 with the addition of a calcination step. In the calcination step, magnesium carbonate (24) is calcined to produce magnesium oxide (52) and carbon dioxide (54). In this embodiment, the carbon dioxide (54) generated from calcining is further recycled to precipitate magnesium carbonate (24) in the precipitation reactor (20).

[00167] Figure 7 illustrates another embodiment of the present invention which includes a scrubbing step (60) after calcining. In this embodiment, magnesium oxide (52) produced in the step of calcination (50) is used to scrub carbon dioxide (62) to produce carbon dioxide depleted gas (66) and magnesium carbonate (64). In this embodiment, the precipitation and dissolution steps are as defined in Figure 2 with a counter-current between the precipitation (20) and dissolution reactors (40).

[00168] Figure 8 illustrates another embodiment of the invention. In step A (20) an activated (or calcined) magnesium containing solid material (12) (such as calcined serpentinite) is contacted with an aqueous solution containing magnesium bicarbonate (42) to precipitate magnesium carbonate and provide a magnesium depleted aqueous solution (23). Some (or all) of the magnesium depleted aqueous solution (23) is passed to step D (40), and the remaining matter passes to step B (21). In step B an aqueous solution containing magnesium bicarbonate (42) is added along with carbon dioxide to provide bulk precipitation of magnesium carbonate (24), and also provide magnesium depleted solid material (22) and magnesium depleted aqueous solution (23). Step B (21) may be performed under pressure, and combines precipitation of magnesium carbonate with dissolution of magnesium in the same reactor. 22, 23 and 24 all pass to step C (30), where the magnesium carbonate (24) is separated from 22 and 23. Step C (30) may be a beneficiation step. The magnesium carbonate (24) may be in the form of a hydrate salt, especially MgCO3.3H2O. In step D (40) the magnesium depleted solid material (22) and magnesium depleted aqueous solution (23) from step C, together with carbon dioxide and magnesium depleted aqueous solution (23) from step A, are used in a dissolution step. Step D (40) may be used to dissolve any residual magnesium carbonate from step C (30), and this may avoid a buffering effect when magnesium carbonate exists as a solid. The remaining solids after step D may be disposed of as residual solid material (45).

[00169] Magnesium oxide (which may be in a slaked form), may be used to scrub carbon dioxide, for example from a lower grade carbon dioxide source, to product magnesium carbonate. The magnesium carbonate may be stored or sold directly. Alternatively, or in addition, the magnesium carbonate may be: (a) added to step A (20) of Figure 8; and / or (b) added to step B (21) of Figure 8. When the magnesium carbonate is added to step A or B, it may assist to reduce the concentration of bicarbonate passing to step D.

[00170] Figure 9 shows a micrograph of magnesium carbonate produced in accordance with the present invention. In this embodiment, the magnesium carbonate and magnesium depleted solid material are present in discrete phases with large blobs of magnesium depleted solid material and smaller needle-like structures of magnesium carbonate.

[00171] The compositions of both phases were confirmed by quantitative spectrographic analysis using electron microscopy. As illustrated in Figure 10, the magnesium carbonate in this embodiment precipitates as needle-like nesquehonite crystals.

[00172] In another example, a serpentinite rock from Canada was ground to a p80 of 75µm and calcined at 650 °C for 15 minutes, to generate an activated calcined sample. The sample underwent a 4 hour CO2 uptake kinetic test. The sample was mixed with fresh water at a solid:liquid ratio of 1:17.5. The kinetic test was undertaken at 25 °C with continuous agitation under a pressure of 14.5 bar CO2. Subsamples were taken every 60 minutes for the duration of the test. Figure 11 illustrates the CO2 uptake by the activated calcined sample after 1, 2, 3 and 4 hours’ respectively. The CO2 uptake primarily occurs in the first hour (280 kgCO2 / t) and then marginally increases for the remainder of the experiment to reach 310 kgCO2 / t. The inventor believes that this small increase in CO2 uptake after the first hour is partially due to depletion of activated serpentinite, and partially due to the pH buffering effect of MgCO3.3H2O. The CO2 was stored mainly as solid nesquehonite, with minor quantities of soluble magnesium bicarbonate in solution.

[00173] In a further example, for a different sample of serpentinite ground to a p80 of 120 micron, and calcined above 650 °C for around 2 hours, the total CO2 uptake in a single stage of carbonation at 1 atmosphere CO2 was 150kg CO2 / t, present as soluble magnesium bicarbonate, with a magnesium concentration of 4.4 gpl Mg and a pH of around 7.2. The 4.4 gpl Mg solution from this dissolution test was decanted into a closed vessel to which a similar mass of the fresh calcined serpentinite was added. The Mg concentration in solution fell within 1 hour to 2.0 gpl Mg, as MgCO3.3H2O precipitated, with the pH rising to 8.6.

[00174] This experiment demonstrates that in a counter-current arrangement, the activated solids are sufficiently reactive to lower the CO2 activity in solution, precipitating over 50% of the saturated bicarbonate at 1 atmosphere of CO2. This precipitation occurs even in the presence of MgCO3 buffering. The precipitated nesquehonite (Figures 12 and 13) was in very fine needle like crystals (around 1 micron diameter and 10-20 micron long, amongst the larger more cubic residue of partially Mg-depleted serpentinite. The discrete crystal structure and the size and density differences all aid physical separation of the magnesium carbonate.

[00175] In various embodiments, the experimental data illustrates at least three essential elements, that enable the many possible embodiments of the invention:High levels of MgCO3 can be generated by converting the activated solid to nesquehonite.The activated solid can precipitate nesquehonite from a magnesium bicarbonate solution, generating a solution which can absorb more CO2, for further dissolution of Mg from the partially magnesium depleted solid, particularly when MgCO3 has been removed.Nesquehonite is precipitated in discrete crystals, enabling its physical separation from the magnesium depleted rock.

[00176] Embodiments of the present invention may provide one or more of the following advantages:(i) Storage of large quantities of carbon dioxide in a stable form, permanently, without the longevity concerns about future carbon dioxide leakage (as in CCUS) or inadvertent combustion (as in forestry); (ii) A pathway to carbon neutral hydrogen which may be cost competitive with conventional fuels, and may be independent of local constraints around efficiency of renewable energy generation or storage to support hydrogen production by electrolysis; (iii) The production of carbon negative fuels and chemical feedstocks such as biodiesel, ethanol and hydrogen from biomass, and in so doing may act as a net sink for the carbon dioxide currently in the atmosphere; (iv) Large scale production of a magnesium carbonate precursor for such products as fire retardant building products, fire extinguishers, fertilisers and fillers for plastics; (v) Production of a fine silica residue which can be utilised as an additive in building products; (vi) A common rock such as serpentinite may be used as the reactive magnesium containing solid material, which allows flexibility in where the method may be performed, for example close to the carbon source, close to the market for the fuel or close to the market for magnesium carbonate;(vii) The direct scrubbing of carbon dioxide from a wide variety of industrial waste gas streams; and / or(viii) Reduction in operating costs since little or no physical removal or reconcentration of carbon dioxide may be needed to precipitate magnesium carbonate.

[00177] Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms in particular features of any one of the various described examples may be provided in any combination in any of the other described examples. Various modifications and alterations to this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention. It should be understood that this invention is not intended to be unduly limited by the illustrative embodiments and examples set forth herein and that such examples and embodiments are presented by way of example only with the scope of the invention intended to be limited only by the claims set forth herein as follows.

Claims

1. A method of precipitating magnesium carbonate, the method comprising the steps of:(i) Contacting a first reactive magnesium containing solid material with carbon dioxide in an aqueous solution, to thereby provide an aqueous solution comprising magnesium bicarbonate; and(ii) Contacting the aqueous solution comprising magnesium bicarbonate with a second reactive magnesium containing solid material to thereby reduce the concentration of bicarbonate in the aqueous solution and precipitate magnesium carbonate;wherein the second reactive magnesium containing solid material comprises an ultramafic material. 2. The method of claim 1, wherein step (ii) is to thereby reduce the concentration of magnesium and bicarbonate in the aqueous solution and precipitate magnesium carbonate. 3. The method of claim 1 or claim 2, wherein the second reactive magnesium containing solid material is activated magnesium containing solid material. 4. The method according to any one of claims 1 to 3, wherein step (i) is to thereby provide an aqueous solution comprising magnesium bicarbonate and residual solid material; wherein step (ii) provides a magnesium depleted aqueous solution and a magnesium depleted solid material; and wherein the method further comprises: (iii) Separating the magnesium carbonate from the magnesium depleted solid material;optionally wherein step (i) receives the magnesium depleted solid material from step (iii) as the first reactive magnesium containing solid material; and wherein step (i) receives the magnesium depleted aqueous solution from step (ii) as the aqueous solution. 5. A method of precipitating magnesium carbonate, the method comprising the steps of:(i) Contacting a first reactive magnesium containing solid material with carbon dioxide in an aqueous solution; wherein the step comprises a plurality of reactors in series to thereby provide an aqueous solution comprising magnesium bicarbonate and a residual solid material; wherein the plurality of reactors comprises at least a first and a second reactor; and(ii) Contacting the aqueous solution comprising magnesium bicarbonate with an activated magnesium containing solid material; wherein the step comprises a plurality of reactors in series to thereby precipitate magnesium carbonate and provide a magnesium depleted aqueous solution and a magnesium depleted solid material; wherein the plurality of reactors comprises at least a first and a second reactor; and wherein the concentration of bicarbonate is reduced in the magnesium depleted aqueous solution; and(iii) Separating the magnesium carbonate from the magnesium depleted solid material;wherein in step (i), the first reactor receives the magnesium depleted aqueous solution from step (ii) and the second reactor receives magnesium depleted solid material from step (iii); and wherein the magnesium depleted aqueous solution and the magnesium depleted solid material is passed through the plurality of reactors to thereby provide the aqueous solution comprising magnesium bicarbonate and the residual solid material;wherein in step (ii), the first reactor receives the activated magnesium containing solid material and the second reactor receives the aqueous solution comprising magnesium bicarbonate from step (i); and wherein the activated magnesium containing solid material and the aqueous solution comprising magnesium bicarbonate is passed through the plurality of reactors to thereby provide the magnesium depleted solid material and the magnesium depleted aqueous solution; andwherein the activated magnesium containing solid material comprises an ultramafic material.  6. The method of claim 4 or claim 5, wherein the step of separating the magnesium carbonate from the magnesium depleted solid material comprises at least one selected from the group consisting of: attritioning to physically separate the magnesium carbonate from the magnesium depleted solid material; flotation separation; an apparent density-based separation process; and gravity separation. 7. The method of any one of claims 4 to 6, wherein the step of separating comprises heating the magnesium carbonate and the magnesium depleted solid material to above 70°C, and contacting the magnesium carbonate and the magnesium depleted solid material with an aqueous solution. 8. The method of any one of claims 4 to 7, wherein the separated magnesium carbonate is refined by dissolution to form a pregnant solution and subsequent vacuum crystallization. 9. The method of any one of claims 4 to 8, wherein the residual solid material comprises silica, and wherein the method comprises refining the silica. 10. The method of claim 3 or claim 5, wherein the ultramafic material comprises serpentinite. 11. The method of any one of claims 3, 5 and 10, wherein the method further comprises the step of activating a magnesium containing solid material to provide the activated magnesium containing solid material; wherein the step of activating comprises:Comminuting the magnesium containing solid material to a p80 of less than 300 μm; and / orCalcining the magnesium containing solid material to a temperature of from about 650 °C to about 750 °C;optionally wherein the step of activating comprises comminuting the magnesium containing solid material to a p80 of less than 300 μm, and calcining the magnesium containing solid material to a temperature of from about 650 °C to about 750 °C. 12. The method of any one of claims 1 to 11, wherein step (i) is performed in an atmosphere of greater than 70% carbon dioxide, at a pressure of from about 1 to about 100 bar. 13. The method of any one of claims 1 to 12, wherein the carbon dioxide is produced from an industrial process; optionally wherein the industrial process is selected from the group consisting of: burning of fossil fuels, burning of biofuels, reduction of metals, cement production and steam methane reforming. 14. The method of any one of claims 1 to 13, wherein step (i) is performed at a terminal pH of less than 6.0. 15. The method of any one of claims 1 to 14, wherein the step of contacting with an aqueous solution comprising magnesium bicarbonate to thereby precipitate magnesium carbonate is performed:(i) at a terminal pH of greater than 8.5; (ii) with a concentration of magnesium in the aqueous solution at the start of the step of at least 5 gL-1; and / or(iii) with addition of a basic material; optionally wherein the basic material comprises magnesium oxide, slag, fly ash, brucite or caustic soda. 16. The method of any one of claims 1 to 15, further comprising a step of calcining the magnesium carbonate to provide magnesium oxide. 17. The method of claim 16, further comprising a step of scrubbing carbon dioxide with magnesium oxide.  18. The method of claim 17, wherein the carbon dioxide is from industrial sources.  19. An apparatus for precipitating magnesium carbonate; the apparatus comprising:(i) A dissolution reactor for contacting a first reactive magnesium containing solid material with carbon dioxide in an aqueous solution, to thereby provide an aqueous solution comprising magnesium bicarbonate;(ii) A precipitation reactor for contacting the aqueous solution comprising magnesium bicarbonate with a second reactive magnesium containing solid material to thereby reduce the concentration of bicarbonate in the aqueous solution and precipitate magnesium carbonate; wherein the second reactive magnesium containing solid material comprises an ultramafic material; and(iii) A separator for separating the magnesium carbonate from the magnesium depleted solid material. 20. The apparatus of claim 19, which is suitable for generating magnesium oxide, further comprising:(iv) a calcination reactor for calcining the magnesium carbonate of step (iii) to generate magnesium oxide, optionally wherein the apparatus further comprises(v) a scrubbing reactor for scrubbing carbon dioxide with the magnesium oxide of step (iv).

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