COMPOSITIONS, METHODS OF MANUFACTURE AND USE IN CARBON DIOXIDE (CO2) SEQUESTRATION

TECHNICAL FIELD

Described herein is a composition, methods of manufacture of compositions and, use of compositions for carbon dioxide (CO2) sequestration.

BACKGROUND

Carbon dioxide (CO2) emissions are a key driver of climate change and ways to minimise or avoid such emissions are of heightened interest. One method of reducing CO2 emissions is to sequester or capture the CO2 before it enters the atmosphere.

Soda lime is a compound that has been used to sequester CO2 from industrial systems as well as to maintain breathable air in enclosed spaces.

Soda lime is effective at removing CO2 at low and high concentrations.

Soda lime is not practical for large scale carbon sequestration as a means of reducing global CO2 levels. Existing sequestrants such as soda lime are capable of sequestering CO2; however, the method of soda lime production from calcium carbonates releases CO2. Hence, there is no benefit for using soda lime as a net zero to negative CO2 sequestrant. The reason for the above loss of CO2 during manufacture is that the source of material for soda lime is typically a carbonate mineral which evolves CO2 during production. There is also a considerable energy requirement to convert the carbonate into soda lime which may also contribute to the release of CO2 via combustion of fossil fuels.

Some prior art has considered carbon sequestration.

Singh US2013/0280152 describes methods and apparatuses for removing carbon dioxide from a gas stream using an alkaline absorbing solution and Singh teaches mixing water and one or more alkaline components selected from the group consisting of sodium hydroxide, calcium hydroxide, magnesium hydroxide, and potassium hydroxide. Singh does not however describe a combination of a magnesium hydroxide material with water and a metal hydroxide nor the catalysing effect of metal hydroxide in this combination on carbon sequestration. Further, magnesium hydroxide is merely mentioned as one of several alkaline reagents in Singh and no examples are provided in Singh using magnesium hydroxide at all.

Dreisinger WO2022/113025 describes a method in which successive steps of hydrometallurgical value extraction may be carried out using the products of carbon capture and an electrolytic reagent-generating process. The electrolytic process provides an acid leachate and an alkali hydroxide, with the alkali hydroxide then available for use either directly as a precipitant in the hydrometallurgical steps, or available for conversion by carbon capture to an alkali metal carbonate that can in turn be used as the precipitant in the selective hydrometallurgical steps. Dreisinger does not describe after an acid wash or sodium hydroxide addition and at no point is a magnesium salt solution produced. Base washing is completed after multiple washes using sodium carbonate and base washing is done in a final step to produce magnesium hydroxide. In addition, the process of Dreisinger produces carbon dioxide during manufacture leading to producing more carbon dioxide - the opposite of what is desired from a sequestration agent since the net effect may be either balanced carbon production and sequestration or a net loss of carbon.

Ideally, a CO2 sequestrant should be sourced from a non-carbonate material and/or have a manufacturing process that does not release CO2, resulting in a net sequestration of CO2.

Further aspects and advantages of the composition, methods of manufacture and methods of CO2 sequestration will become apparent from the ensuing description that is given by way of example only.

SUMMARY

A composition, methods of manufacture of compositions and use of the compositions for carbon dioxide sequestration are described herein. The composition may comprise a mixture of magnesium hydroxide, water and metal hydroxide to create a composition that may be highly effective at sequestering carbon dioxide. Further, the composition may be manufactured in a variety of ways that do not release CO2 during manufacture.

In a first aspect, there is provided a composition comprising: magnesium hydroxide material; water; and metal hydroxide, the metal hydroxide not being magnesium hydroxide.

In a second aspect, there is provided a composition configured to be exposed to and react with CC to form stable magnesium carbonates and hydrated magnesium carbonates, and hence, sequester CO2, the composition comprising: a CO2 sequestering concentration of magnesium hydroxide material; water; and a catalysing concentration of metal hydroxide, wherein said metal hydroxide is a metal alkali and is not magnesium hydroxide.

In a third aspect, there is provided a method of manufacturing a composition substantially as described above comprising: selecting said magnesium hydroxide material; selecting said metal hydroxide; and mixing said water with the magnesium hydroxide containing material and the metal hydroxide to form the composition.

In a fourth aspect, there is provided a method of manufacturing the composition substantially as described above, comprising: selecting a magnesium containing silicate source; subjecting the selected magnesium containing silicate source to an acid wash, to produce an acid digested solution; subjecting the acid digested solution to a base wash by adding a base solution to the acid digested solution to produce a magnesium salt solution and, during this step, removing silica, iron or other metals and minerals from the magnesium salt solution; subjecting the magnesium salt solution to electrolysis to recover magnesium hydroxide material; mixing the magnesium hydroxide material that is recovered with a solution containing said water and said metal hydroxide to produce the composition.

In a fifth aspect there is provided a method of manufacturing the composition substantially as described above comprising: selecting a magnesium containing silicate source; subjecting the selected magnesium containing silicate source to an acid wash, to produce an acid digested solution; subjecting the acid digested solution to a base wash by adding a base solution to the acid digested solution to produce a magnesium salt solution and, during this step, removing silica, iron or other metals and minerals from the magnesium salt solution; completing a further base wash of the magnesium salt solution to produce magnesium hydroxide material in solution and filtering the magnesium hydroxide material in solution to recover magnesium hydroxide material and a separate metal salt solution; mixing the recovered magnesium hydroxide material with a solution containing said water and said metal hydroxide to produce the composition.

In a sixth aspect there is provided a method of manufacturing the composition substantially as described above comprising: selecting a magnesium containing silicate source; subjecting the selected magnesium containing silicate source to a base wash by adding a base solution to the magnesium containing silicate source to produce a magnesium hydroxide material solution; mixing the produced magnesium hydroxide material solution with a solution containing said water and said metal hydroxide to produce the composition.

In a seventh aspect, there is provided a method of sequestration of CO2 by: selecting the composition substantially as described above, exposing the composition to CO2; and wherein the composition reacts with the CO2 to form stable magnesium carbonates and hydrated magnesium carbonates, and hence, sequesters the CO2.

The inventors have identified a composition that acts in a synergistic manner to sequester CO2. Magnesium hydroxide materials do sequester CO2. The inventors found that by mixing metal hydroxide and water with magnesium hydroxide, the rate of CO2 sequestration by the magnesium hydroxide may rapidly accelerate or be catalysed, to the point of being a rapid and useful method of reducing CO2 emissions. Sequestration may be by direct air capture or from a point source.

A further advantage identified is that the composition may be made without the method itself producing CO2, and hence negating any benefits of subsequent sequestration.

BRIEF DESCRIPTION OF THE DRAWINGS

Further aspects of the composition, methods of manufacture and methods of CO2 sequestration will become apparent from the following description that is given by way of example only and with reference to the accompanying drawings in which:

Figure 1 illustrates a flow diagram of a method of producing magnesium hydroxide along with subsequent manufacture of the composition;

Figure 2 illustrates a flow diagram of an alternative method of producing magnesium hydroxide material along with subsequent manufacture of the composition;

Figure 3 illustrates a flow diagram of a further alternative method of producing magnesium hydroxide material along with subsequent manufacture of a composition;

Figure 4 illustrates a flow diagram where solid composition is used to sequester CO2 where air is passed over the composition material and where the composition is in a pelletised form and reacts with carbon dioxide gas in the air to produce a hydrated magnesium carbonate end product;

Figure 5 illustrates an apparatus for direct air capture according to the flow diagram of Figure 4;

Figure 6 illustrates a flow diagram of an alternative sequestration method using a slurry of composition in a wet scrubber configuration where the slurry is distributed as a fine mist in a column of hot gas;

Figure 7 illustrates a further flow diagram of a method of sequestration via bubbling of CO2 through a slurry containing the composition; Figure 8 illustrates a graph showing the results of a CO2 sequestration trial;

Figure 9 illustrates XRD graph results showing the results of a CO2 sequestration trial;

Figure 10 illustrates a trial testing the catalysing effect of metal hydroxide on CO2 sequestration by magnesium hydroxide.

DETAILED DESCRIPTION

As noted above, a composition, methods of manufacture of the compositions and use of the compositions for carbon dioxide sequestration are described. The composition may comprise a mixture of magnesium hydroxide material, water and metal hydroxide to create a composition that may be highly effective at sequestering carbon dioxide.

For the purposes of this specification, the term 'about' or 'approximately' and grammatical variations thereof mean a quantity, level, degree, value, number, frequency, percentage, dimension, size, amount, weight or length that varies by as much as 30, 25, 20, 15, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1% to a reference quantity, level, degree, value, number, frequency, percentage, dimension, size, amount, weight or length.

The term 'substantially' or grammatical variations thereof refers to at least about 50%, for example 75%, 85%, 95% or 98%.

The term 'comprise ¹ and grammatical variations thereof shall have an inclusive meaning - i.e. that it will be taken to mean an inclusion of not only the listed components it directly references, but also other non-specified components or elements.

The term 'alkali metals' and grammatical variations thereof as used herein refers to both alkali and alkaline metals.

The term 'hydrate' and grammatical variations thereof as used herein refers to a compound, composition or substance that has variable amounts of water present.

A Composition

In a first aspect, there is provided a composition comprising: magnesium hydroxide material; water; and metal hydroxide, the metal hydroxide not being magnesium hydroxide. In a second aspect, there is provided a composition configured to be exposed to and react with CC to form stable magnesium carbonates and hydrated magnesium carbonates, and hence, sequester CO2, the composition comprising: a CO2 sequestering concentration of magnesium hydroxide material; water; and a catalysing concentration of metal hydroxide, wherein said metal hydroxide is a metal alkali and is not magnesium hydroxide.

The inventors have identified that the above composition acts in a synergistic manner to sequester CO2. Magnesium hydroxide materials do sequester CO2 but only slowly and not at a commercially useful reaction rate. The inventors found that by mixing metal hydroxide and water with magnesium hydroxide material, the rate of CO2 sequestration by the magnesium hydroxide may rapidly accelerate, to the point of being a rapid and useful method of reducing CO2 emissions. Sequestration may be by direct air capture or from a point source.

Magnesium Hydroxide Material

The magnesium hydroxide material may be present as Mg(0H>2. The magnesium hydroxide material may be present as a hydrate of magnesium hydroxide.

The magnesium hydroxide material may be obtained from magnesium silicate bearing minerals.

The magnesium hydroxide material may be obtained from: olivine, serpentine group minerals, pyroxenes, amphiboles, and combinations thereof.

The magnesium hydroxide material may be obtained from minerals such as olivine. Olivine is a magnesium-rich silicate present in widely distributed mafic and ultramafic rock deposits. For these reasons, olivine may be an ideal source material to manufacture magnesium hydroxide used to form the composition.

The magnesium silicate bearing minerals may be sourced as: sands, sediment, rock, and combinations thereof. These material sources are common natural forms for magnesium silicate bearing materials.

The composition may comprise approximately 1-99%, or 5-95%, or 20-90%, or 60-90%, or approximately 70-80% by weight magnesium hydroxide material. These ranges may provide an improvement in operating efficiencies which may be based on the relative proportion of magnesium hydroxide material, water and metal hydroxide.

Water Water may be used to allow for reaction of magnesium hydroxide material and metal hydroxides present, with CO2 for carbon sequestration purposes. For example, the metal hydroxide may act as a catalyst to accelerate, or shift, or accelerate and shift, the reaction equilibrium of magnesium hydroxide material with carbon dioxide to favour carbonation of the magnesium hydroxide Mg(0H)2 material. Water may be used to help this reaction by allowing intimate mixing of the compounds used.

The composition may comprise at least approximately 1-99%, or 5-99%, or 5-95%, or 5-60%, or 5-35%, or approximately 20% by weight water. In one example, greater than approximately 5% by weight of the composition may be water. Moisture may be required to facilitate the uptake of CO2 by the magnesium hydroxide material. Moisture content may be varied to change the workability and reactivity needed for end-user applications and products.

The amount of water present in the composition may depend on the final form of the composition.

Metal Hydroxide

The metal hydroxide may be a metal alkali.

The metal hydroxide may not be magnesium hydroxide.

The metal hydroxide may be selected from: sodium hydroxide (NaOH), lithium hydroxide (LiOH), potassium hydroxide (KOH), calcium hydroxide (Ca(OH)2), and combinations thereof. In one embodiment, the metal hydroxide is sodium hydroxide (NaOH) alone. Sodium hydroxide may be advantageous given how widely available an inexpensive this metal hydroxide is, although, as noted, other metal hydroxides such as those listed may also be used.

As noted above, the presence of metal hydroxide appears to catalyse the reaction between the magnesium hydroxide material and CO2, causing far higher CO2 sequestration than for magnesium hydroxide material alone. The listed metal hydroxides are abundant and relatively inexpensive hence why they may be advantageous. Other metal hydroxides may be used and this list should not be seen as limiting.

The composition may comprise approximately 1-20%, or 1-10%, or 2-10%, or 3-10%, or 4-10%, or 5-10%, or approximately 5-8% by weight metal hydroxide. These ranges may provide an improvement in operating efficiencies which may be based on the relative proportion of magnesium hydroxide material, metal hydroxide and water. Process efficiencies are capable of being modified to suit the requirements of chemical reactivity needed for end use applications (i.e. CO2 reactivity and sequestration). For amounts of metal hydroxide below 1% by weight, the catalysing effect does not appear to take effect or at least not to any significant extent. Between 1 and 10% there is a steady improvement on reactivity of magnesium hydroxide at sequestering CO2. Beyond 10% the catalysing effect plateaus and sequestering decreases as the amount of magnesium hydroxide becomes diluted to not sequester CO2 at a useful rate. Maximising magnesium hydroxide content is optimal for providing the key reagent for sequestration and up to 20% by weight, the metal hydroxide still appears to catalyse the reaction although between 10% and 20% the extent of catalysation plateaus. To minimise process cost and materials, it may be useful to minimise metal hydroxide content hence an amount of 1-10% may be optimal.

Pellet or Granule Form

The composition may be configured as a pellet or granule. Pellets or granules may be useful for ease of handling.

The pellet or granule may approximately comprise (% by weight): magnesium hydroxide material from 1-99%, or 5-95%, or from 20-90%, or from 60-90%, or 70- 80%, or approximately 75%; water from 1-99%, or from 5-95%, or from 5-60%, or from 5-35%, or approximately 20%; and metal hydroxide 0.1-99%, or 1-95%, or 0.1-20%, or 1-20%, or 1-10%, or 2-10%, or 3-10%, or 4- 10%, or 5-10%, or approximately 5-8%.

Relative percentages within the ranges described above may be used as described in other parts of this specification.

The pellets or granules produced may be approximately 1 pm to 10 mm, or 0.5 to 2 mm, or approximately 1 mm in diameter. Note that the term 'diameter' is used for brevity. The pellets or granules do not need to be round or spherical as alluded to by referring to diameter and the pellets or granules may take a variety of shapes all of a size within the ranges described.

Slurry Form

The composition may be configured as a slurry.

Slurry configuration may be useful to reduce or prevent dust and inhalation handling issues.

The solid content of the slurry may be variable. In one example, the solid content may be from 1-50%, or 1-40%, or 1-30%, or 1-20%, or 5-15%, or approximately 10% by weight. Liquid contents at these rates may be useful to allow the slurry to be mobilised for transfer.

The slurry may approximately comprise (% by weight): magnesium hydroxide from 1-99%; water from 5-99%; and metal hydroxide from 0.1-99%, or 1-95%, or 0.1-20%, or 1-20%, or 1-10%, or 2-10%, or 3-10%, or 4-10%, or 5-10%, or approximately 5-8%. Relative percentages within the ranges described above may be used as described in other parts of this specification.

Silica Containing Materials

The composition may further comprise silica containing materials. In one example, the silica containing materials if used, may be added as a powder. The silica containing materials may comprise less than approximately 50%, or 40%, or 30%, or 20%, or 10% by weight of the composition.

The silica containing materials may be clays, quartz or other siliceous minerals.

The silica containing materials may be kaolin, smectite, bentonite or vermiculite.

The silica containing materials are understood by the inventors to increase the rate at which the composition described above may sequester CO2. Without being bound by theory, the mechanism for why silica containing materials increase sequestration rate may be because these materials give the composition a higher void space, higher pore reaction area, and higher surface area for contact and reaction between the magnesium hydroxide material and CO2.

Inert Materials

The composition may further comprise inert materials. The inert materials may comprise: carbonates, silicates, oxides, and sulphates and combinations thereof. The inert materials may be selected from: clay minerals, quartz, zeolites, calcite, magnetite, magnesite, and combinations thereof.

The inert materials used may be selected based on the ability of the inert material to increase the overall porosity of the composition and improve the carbon sequestration rates of the composition. The inert materials used may also be selected based on the ability of these materials to meet certain physical characteristics for the end product.

The composition may comprise 5%, or 10%, or 15%, or 20% by weight inert materials. The composition may approximately comprise 5-20%, or 10-20%, or 5-15%, or 15-20% by weight inert materials. The amount of inert materials present may be varied to allow for changes in porosity to meet the requirements of the end application for the composition. The inventors have found that 5-20% by weight of inert materials may be suitable for many CO2 sequestration applications.

Metal Salts

The composition may further comprise metal salts. Metal salts may be present in the composition as residue from magnesium hydroxide material manufacturing. By way of example, various metal salts may be used to produce the magnesium hydroxide material and at least some of these metal salts may remain in the magnesium hydroxide material used to form the composition.

The metal salts present in the composition may be selected from: lithium chloride (LiCI), sodium chloride (NaCI), potassium chloride (KCI), magnesium chloride (MgC ), sodium sulphate (NazSC ), magnesium sulphate (MgSC ), calcium sulphate (CaSC ), lithium sulphate (IJ2SO4), calcium chloride (CaC ), and combinations thereof.

Metal salts, if present, may comprise less than approximately 20%, or 19%, or 18%, or 17%, or 16%, or 15%, or 14%, or 13%, or 12%, or 11%, or 10%, or 9%, or 8%, or 7%, or 6%, or 5%, or 4%, or 3%, or 2%, or 1% by weight of the composition. In the inventor's experience, metal salt concentrations above 20% by weight may impair the effectiveness/ability of the composition to react with and sequester CO2.

In the inventors' experience, metal salts, if present at low concentrations, have minimal if any impact on the composition activity in regards to CO2 reaction and sequestration.

Magnesium Oxide

The composition may further comprise magnesium oxide (MgO). The magnesium oxide may also be present as a residue from magnesium hydroxide material manufacturing. In the inventors experience, MgO if present has minimal if any impact on the composition activity in regards to CO2 reaction and sequestration.

CO2 During Manufacture

The composition may have low to zero carbon emissions during production as it is made from silicate minerals that have no inherent CO2, such as carbonate groups seen in the materials used to form other sequestration materials like soda lime.

The composition may also react with carbon dioxide to capture and sequester carbon dioxide which is discussed further below.

Comparison to Soda Lime

The composition may have similar properties to soda lime and may be used interchangeably with soda lime. 'Similar properties' in the context of this specification refers to the ability of the composition to sequester or reduce CO2 preferentially and quickly, even at low concentrations. Soda lime, however, has the drawback that carbon dioxide is released from the soda lime manufacturing process. This means that later sequestration only results in a net neutral CO2 effect (i.e. the amount of CO2 sequestered is similar to the amount produced to manufacture soda lime). By contrast, CO2 is not released during production of the composition described herein and, hence, there may be a net benefit from the composition to reduce CO2 emissions to the atmosphere.

Method of Manufacture - Magnesium Hydroxide - Any Source

In a third aspect, there is provided a method of manufacturing a composition substantially as described above comprising: selecting said magnesium hydroxide material; selecting said metal hydroxide; and mixing said water with the magnesium hydroxide containing material and the metal hydroxide to form the composition.

Magnesium Hydroxide Material

The magnesium hydroxide material may be produced via a processing method prior to completing the above method or may be supplied prior to commencement of the above method.

As noted above, the magnesium hydroxide material may be present as Mg(OH)2, or as a hydrate of magnesium hydroxide.

Other aspects about the magnesium hydroxide, water and the metal hydroxide may be as described above and are not repeated here for brevity.

Initial Grinding

Optionally, and if applicable, the magnesium hydroxide material may be ground prior to the above method to produce an average particle size less than 1 mm. Particle sizes less than 1 mm may be useful as the surface area is higher and hence, the reaction rate for CO2 sequestration may be higher. Further, manufacturing techniques to produce the magnesium hydroxide material may progress faster using higher surface area mixtures.

Water Removal

Some water removal may occur from the composition once produced.

Water removal may be via drying. Drying may be via methods including: filtration, conventional drying, and vacuum evaporation drying. Drying may be to reduce the water content. Water removal noted above may result in greater than approximately 5%, or 10%, or 20%, or 30%, or 40%, or 50%, or 60%, or 70%, or 80%, or 90% by weight of water being removed from the composition described above.

As noted elsewhere, sufficient water removal may occur to produce a composition with a water content of at least approximately 1-99%, or 5-99%, or 5-95%, or 5-60%, or 5-35%, or approximately 20% by weight water.

Water removal to the extent described may be useful in order to decrease energy requirements for drying the materials where water is beneficial for reactivity, transport, and minimising dust/inhalation issues.

Pellet/Granule or Slurry

The composition produced above may be formed into a pellet or granule, or alternatively, into a slurry. The pellet, granule or slurry may have the characteristics noted earlier in this specification.

Other Materials

The composition produced may have further materials added comprising the silica containing materials, inert materials, and metal salts, described above.

Method of Manufacture - Acid/Base Wash and Magnesium Salt Electrolysis

In a fourth aspect, there is provided a method of manufacturing the composition substantially as described above, comprising: selecting a magnesium containing silicate source; subjecting the selected magnesium containing silicate source to an acid wash, to produce an acid digested solution; subjecting the acid digested solution to a base wash by adding a base solution to the acid digested solution to produce a magnesium salt solution and, during this step, removing silica, iron or other metals and minerals from the magnesium salt solution; subjecting the magnesium salt solution to electrolysis to recover magnesium hydroxide material; mixing the magnesium hydroxide material that is recovered with a solution containing said water and said metal hydroxide to produce the composition. Magnesium Containing Silicate Source

As noted elsewhere in this specification, the magnesium containing silicate may be obtained from magnesium silicate bearing minerals comprising: olivine, serpentine group minerals, pyroxenes, and amphiboles. The magnesium silicate bearing minerals may be sourced as: sands, sediment, rock, and combinations thereof.

Crushing and Grinding

The magnesium containing silicate may be crushed prior to processing above. Crushing may be to sand size, or less than approximately 1 mm average diameter.

A portion of iron (if present) in the magnesium containing silicate may be removed from the raw magnesium containing silicate. Removal may be through magnetic separation or acid washing. The magnetic iron may be recovered as a by-product.

Remaining magnesium containing silicate may be further ground. Grinding may be to a particle size of less than 100 pm. The ground magnesium containing silicate may be a finely ground magnesium containing silicate prior to processing in the above method.

Iron removal (if needed) may also be completed using the finely ground magnesium containing silicate described.

Two grinding steps may be completed. The number of grinding steps completed may vary depending on the size and type of source material used. Two grinding steps may allow for easier removal of iron followed by a finer grinding to increase reactivity and dissolution rate of the magnesium containing silicate in the above method.

Note that the terms 'crushing' and 'grinding' are used herein, however, this should not be seen as limiting as the process relates to reducing particle size and increasing surface area which may be achieved by various methods not limited to use of a crusher or grinder.

Acid Wash

Acid washing may be completed for example using an acid selected from hydrochloric acid (HCI) or sulphuric acid (H2SO4). These acids may be useful because they are easy to obtain and low cost. Other acids may be used and reference to HCI or H2SO4 should not be seen as limiting.

Acid washing may occur at an elevated temperature. The elevated temperature may be from approximately 40-95°C, or 50-90°C, or 60-85°C, or approximately 80°C. As the temperature increases, the acid effectiveness at washing also increases up to an optimum that, in the inventor's experience is at around 80°C, although other temperatures may be used. Acid washing may occur over a period of time. Acid washing may take place over approximately 1-24, or 1-12, or 1-6, or 1-5, or 1-4, or 1-3, or approximately 2 hours. Acid washing times may vary due to variable mineralogy of the magnesium containing silicate source material.

In the inventor's experience, the pH at the beginning of acid washing may be below pH 2 or even negative pH. Over time, and as the magnesium containing silicate is digested by the acid conditions, the pH may tend to rise. In one example, if olivine is acid washed in 2 M HCI at 80°C for 2 hours then, after 2 hours, the pH becomes positive (to approximately 0.5 from less than zero).

There may be a further ripening step after the above acid wash where the pH is increased to pH 1 or greater, and left for another 1-24 hours. Ripening in this case may allow the silica to develop further prior to separation.

Base Wash

The base solution may be selected from: lithium hydroxide (LiOH), sodium hydroxide (NaOH), potassium hydroxide (KOH), magnesium hydroxide (Mg(OH)2), calcium hydroxide (Ca(OH)2). Other metal hydroxides may also be used in the inventor's experience. Sodium hydroxide (NaOH) was typically used by the inventors as this is easily available and lowest cost.

Base washing may occur at an elevated temperature. An 'elevated temperature' may be a temperature above ambient conditions of approximately 15-25°C. The elevated temperature may be from approximately 40-95°C, or 40-80°C, or approximately 50-70°C. Base washing temperatures may vary due to variable mineralogy of the magnesium containing silicate source material. At elevated temperatures, many of the magnesium-bearing silicate minerals will sufficiently react or react to an optimum extent.

Base washing may occur over a period of time. Base washing may take place over approximately 10 min- 12 hours, or 10 min-3 hours, or 10-120 min, or 10-90 min, or, or less than 90 min. Base washing times may vary due to variable mineralogy of the magnesium containing silicate source material.

Base washing may occur in two steps. A first step may involve base washing for a first period of time (less than approximately 60 min) at which point the pH is increased to approximately 2 or greater. A second step may involve addition of further base solution and base washing for another time period at which point the pH may be increased to approximately 4 or greater to precipitate and remove silica and any precipitated iron. Two (or more) steps of base washing may be useful to allow for separation of side products from the reaction which may be collected as separate product streams. The time period for this second step may be less than approximately 24 hours, or less than 60, or 50, or 40, or 30, or 20, or 10 min. Electrolysis

As may be appreciated, electrolysis apparatus operating characteristics may vary depending on the device operating details. In one example, the inventors found that electrolysis apparatus used provided good separation at approximately 40-90°C, an electrical current density of at least approximately 0.1 kA/m ² and with an anode being a mixed metal oxide titanium and the cathode being stainless steel or nickel.

Other materials could be used for the anode and cathode and reference to these materials are provided by way of illustration only. Similarly, other temperatures and electrical current densities could be used as well.

Water Removal

At least some water removal may occur from the recovered magnesium hydroxide material in the above method. Water removal may occur prior to said composition manufacture. Water removal may be via drying. Drying may be via methods including: air drying, filtration, conventional drying, vacuum evaporation drying, and combinations thereof.

Drying may be used to reduce the water content in this aspect as well. Water removal noted above may result in greater than approximately 5%, or 10%, or 20%, or 30%, or 40%, or 50%, or 60%, or 70%, or 80%, or 90% by weight of water being removed from said composition described above. As noted elsewhere, sufficient water removal may occur to produce said composition with a water content of at least approximately 1-99%, or 5-99%, or 5-95%, or 5-60%, or 5-35%, or approximately 20% by weight water.

Method of Manufacture - Mineral Precipitation Using Metal Hydroxide Addition, Metal Salt Electrolysis Option

In a fifth aspect there is provided a method of manufacturing the composition substantially as described above comprising: selecting a magnesium containing silicate source; subjecting the selected magnesium containing silicate source to an acid wash, to produce an acid digested solution; subjecting the acid digested solution to a base wash by adding a base solution to the acid digested solution to produce a magnesium salt solution and, during this step, removing silica, iron or other metals and minerals from the magnesium salt solution; completing a further base wash of the magnesium salt solution to produce magnesium hydroxide material in solution and filtering the magnesium hydroxide material in solution to recover magnesium hydroxide material and a separate metal salt solution; mixing the recovered magnesium hydroxide material with a solution containing said water and said metal hydroxide to produce the composition.

Magnesium Containing Silicate Source

As noted elsewhere in this specification, the magnesium containing silicate may be obtained from magnesium silicate bearing minerals comprising: olivine, serpentine group minerals, pyroxenes, and amphiboles and combinations thereof. The magnesium containing silicate may be sourced as: sands, sediment, rock, and combinations thereof.

Acid Wash

Acid washing noted above may be completed in a similar manner to that described above.

First Base Wash

The first base wash noted above may be completed in a similar manner to that described above. In one example, the first base wash may be completed in one step or in two steps. For example, a first base wash may comprise two base washes being to first remove the silica and second, to remove the iron. Silica removal may occur at a pH of approximately 2 or greater. Iron removal may occur at a pH of approximately 4 or greater.

Alternatively, the first base wash may occur in one step as one base wash to remove silica and iron together. In this example, the pH may be approximately 4 or greater.

Further Base Wash

A further base wash may be used to precipitate and recover magnesium hydroxide material Mg(OH)2. This further base wash may in one example be completed using NaOH or other metal hydroxide.

Recovery may be by filtration, the retentate in the filter comprising the magnesium hydroxide material and the permeate from the further base wash being primarily a sodium salt solution (the metal salt solution).

In this further base wash, the pH may be increased to approximately 10 or greater. The further base wash may be maintained for a period of time of at least approximately 10 minutes, or 1 hour or up to approximately 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22 or 24 hours (or possibly more) to ensure precipitation and development of the magnesium hydroxide material.

The further base wash may be completed at a temperature from 20-90‘C, or at approximately 20, or 25, or 30, or 35, or 40, or 45, or 50, or 55, or 60, or 65, or 70, or 75, or 80 C.

Electrolysis

Electrolysis may be completed in the above method to recover reagents. Electrolysis is not needed to produce magnesium hydroxide via this method.

The separate metal salt solution may undergo electrolysis to produce an acid solution, a base solution, and optionally, evolved gases. The acid solution produced may be recycled for acid washing in the above method; and the base solution produced may be recycled for base washing in the above method.

For example, if hydrochloric acid is used for acid washing and sodium hydroxide is used for base washing, the metal salt may be sodium salt in solution and in the electrolyser, the sodium salt solution may be consumed and chlorine Cl 2 (gas) recovered from the anode and hydrogen H2 (gas) and sodium hydroxide (NaOH) recovered from the cathode. The H2 and CI2 gas may be combined to form hydrochloric acid HCI that may be recycled for use in acid wash described above.

If sulphuric acid (H2SO4) (optionally including NaHSC ) is used for the acid washing process, then hydrogen and NaOH may be generated at the cathode and oxygen may be generated at the anode during the electrolysis of Na2$O4 (produced from reacting the MgSO4 with NaOH). In addition a sulphuric acid (H2SO4) solution may also be generated at the anode. This may also be recycled for use for further acid wash steps.

Other acid/base wash solutions may also be used in the above method and electrolysis used to separate resulting metal salt in solution to acid and base solutions.

The electrolyser may operate using similar operating characteristics to the electrolyser described above.

Washing

The recovered magnesium-hydroxide may be washed. Washing may be completed to remove contaminants and increase the purity of the magnesium hydroxide material. The magnesium hydroxide material may typically be washed with water using a counter current method to minimise water usage (i.e., recycled water used for first rinse and progressively cleaner water for final rinses). Other Steps

Other steps described above around the base and acid solutions, slurry or pellet/granule formation and use of other materials may also be used in this method but are not repeated here for brevity. By way of example, inert materials may be added as described further below including: carbonates, silicates, oxides, and sulphates and combinations thereof. The inert materials may be selected from: clay minerals, quartz, zeolites, calcite, magnetite, magnesite and combinations thereof.

Method of Manufacture - Base Wash

In a sixth aspect there is provided a method of manufacturing the composition substantially as described above comprising: selecting a magnesium containing silicate source; subjecting the selected magnesium containing silicate source to a base wash by adding a base solution to the magnesium containing silicate source to produce a magnesium hydroxide material solution; mixing the produced magnesium hydroxide material solution with a solution containing said water and said metal hydroxide to produce the composition.

The base solution may be a metal hydroxide. In one example the base solution may be sodium hydroxide.

Magnesium hydroxide material may form in solution during base washing as noted above. In one example, the resulting magnesium hydroxide material may be at least partly separated from the liquid in the solution. Separation may be via decanting.

Silica, or iron, or both silica and iron, may also be removed from the magnesium hydroxide material. Removal of silica and iron may occur prior to mixing the magnesium hydroxide material with metal hydroxide and water.

Sequestration

In a seventh aspect, there is provided a method of sequestration of CO2 by: selecting the composition substantially as described above, exposing the composition to CO2; and wherein the composition reacts with the CO2 to form stable magnesium carbonates and hydrated magnesium carbonates, and hence, sequesters the CO2. Reaction

Magnesium hydroxide may naturally react with carbon dioxide CO2 gas under both atmospheric (and near atmospheric) conditions and at elevated CO2 levels, such as industrial point flue gas sources for example. This reaction forms stable magnesium carbonates and hydrated magnesium carbonates. This reaction, however, may not be a commercially optimal way to sequester significant volumes of CO2. As noted elsewhere, mixing magnesium hydroxide with metal hydroxide and water accelerates the reaction process substantially, resulting in a commercially useful CO2 sequestrant.

Direct Air Capture or Point Source Capture

The inventors have found that the composition is sensitive and reactive with CO2 so that it may be used for direct air capture or capture about a point source.

Exposure of the composition in this environments directly captures CO2 from the air hence the term 'direct air capture'. This method of sequestration may be a useful passive means to sequester general CO2 in the environment.

Point source capture may refer to exposing the composition at or about a source of CO2. Point sources may for example be from industrial sources, from residential sources or form commercial sources.

Composition Form

The composition may be used in a partially dried form such as in pellets or granules or as a slurry with compositions substantially as described above.

Reaction Rate

The rate of the reaction may be aided by an increase in CO2 concentration and temperature. For example, the more CO2 there is in the atmosphere or gas (e.g. exhaust gas) that the composition is exposed to, the faster the reaction rate and CO2 sequestration. This means that the composition may be well suited to sequestration of high CO2 producing sources.

The sequestration reaction rate may proceed more quickly when water is present. Water may be present as moisture. The moisture may be present at a moderate range of humidity. For example, at a relative humidity (RH) from 30-90%, or from 45-75%, or approximately 60%. Dry RH (low RH < 40%) or very wet RH, > 75% tends to be less effective in the inventor's experience for CO2 sequestration rate although sequestration still occurs.

In the inventor's experience to date, approximately 1.3 tonnes of magnesium hydroxide in the described composition may be required to sequester 1 tonne of CC at 100% efficiency. The actual operating efficiency will be below 100%, hence, these figures are presented by way of example only and should not be seen as limiting.

Capture Apparatus

The composition described, may be incorporated into a method or apparatus or both method and apparatus, to increase mass transfer and mixing. The methods/apparatus may for example comprise: a fluidized bed, wet scrubber, dry scrubber, ambient exposure (spread on a field for example).

In point source applications, a slurry of the composition may be used in a wet scrubber configuration as one example where the slurry may be distributed as a fine mist in a column of hot gas about the point source. Alternatively, a dry scrubbed arrangement may use solid composition such as a pellet or granule. This may be for example, in a fluidized bed type scrubber.

For direct air capture applications, solid forms of said composition may be used where air may be passed over the magnesium material.

Advantages

As noted above, the inventors have identified a composition acts in a synergistic manner to sequester CO2. Magnesium hydroxide materials do sequester CO2. The inventors found that by mixing metal hydroxide and water with magnesium hydroxide, the rate of CO2 sequestration by the magnesium hydroxide is catalysed and may rapidly accelerate, to the point of being a rapid and useful method of reducing CO2 emissions.

A further advantage identified is that the composition may be made without the method itself producing CO2 and hence negating any benefits of subsequent sequestration.

The examples described above may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, and any or all combinations of any two or more said parts, elements or features.

Further, where specific integers are mentioned herein which have known equivalents in the art to which the examples relate, such known equivalents are deemed to be incorporated herein as if individually set forth.

WORKING EXAMPLES

The above described composition, methods of manufacture and methods of CO2 sequestration are now described by reference to specific examples. EXAMPLE 1

An example composition in the form of a pellet or granule produced by the inventors is demonstrated in Table 1 below: Table 1 - Example Pellet/Granule Composition

The pellets or granules produced may be approximately 1pm to 10mm in diameter.

EXAMPLE 2 An example composition in the form of a slurry produced by the inventors is demonstrated in Table 2 below:

Table 2 - Example Slurry Composition

EXAMPLE 3

In this example a basic method of producing a composition is described by the steps of: - Selecting a magnesium hydroxide material. This material may be produced via a processing method described in further examples below or supplied in purified form prior to commencement;

Optionally, grind the magnesium hydroxide material to an average particle size less than 1mm; Selecting a metal hydroxide e.g. NaOH; and - Mixing water with the magnesium hydroxide material and the metal hydroxide together to form the composition;

Optionally adding silica containing materials, inert materials, and metal salts, described above; Optionally, remove water via drying;

Optionally, forming the composition produced into a pellet or granule or a slurry.

EXAMPLE 4

In this example a method of producing magnesium hydroxide is described along with subsequent manufacture of the above final composition with reference to Figure 1. Steps completed are as follows: Selecting olivine as the magnesium containing silicate source as olivine sand or processed olivine-rich rock. In this example, the olivine is sourced with an average particle size of <lmm; Magnetic iron is removed from the olivine sand (approximately 13% of total mass of the olivine sand);

Olivine is then ground in a puck mill to produce average particle size of 30 pm;

An acid wash is completed at a rate of 50 g of ground olivine added to 500 ml of 2 M HCI acid. The acid wash is completed at 60°C for approximately 3 hours;

A base wash is then completed by adding sodium hydroxide to bring pH up to approximately 4.0 to precipitate silica and approximately 8.0 for iron removal;

The solution produced is filtered and retentate (filter cake) washed;

Additional base solution (sodium hydroxide) is added to the permeate to bring the pH up to 6 to precipitate any remaining iron;

The solution is filtered and retentate (filter cake) washed;

The permeate produced is primarily ~1 M magnesium chloride (MgCh) solution with some sodium chloride (NaCI) present;

The MgC solution is placed in an electrolyser and electrolysis completed. Chlorine (Cl 2) gas is recovered from the anode and hydrogen (H2) gas is recovered from the cathode. Optionally, the gases evolved may be combined to form HCI acid for acid washing of further magnesium containing silicate source;

The magnesium hydroxide Mg(OH>2 material recovered from the cathode contains approximately 50% moisture;

The composition is then produced by adding 2 g of sodium hydroxide NaOH powder to 10 g of recovered magnesium hydroxide Mg(OH>2 to create high moisture content pellets.

Note that hydrochloric acid may be replaced with sulphuric acid or other acids in the above example.

EXAMPLE 5

In this example a method of producing magnesium hydroxide is described along with subsequent manufacture of the above final composition with reference to Figure 2. Steps completed are as follows:

Selecting olivine as the magnesium containing silicate source as olivine sand or processed olivine-rich rock. In this example, the olivine is sourced with an average particle size of <lmm; Magnetic iron is removed from the olivine sand (approximately 13% of total mass of the olivine sand);

Olivine is then ground in a puck mill to produce average particle size of 30 pm;

An acid wash is completed at a rate of 50 g of ground olivine added to 500 ml of 2 M HCI acid. The acid wash is completed at 60°C for approximately 3 hours; A first base wash is then completed by adding sodium hydroxide to bring pH up to approximately 4.0 to precipitate silica and approximately 8.0 to remove iron;

The solution produced is filtered and retentate (filter cake) washed;

Additional base solution (sodium hydroxide) is added to the permeate to bring the pH up to 6 to precipitate any remaining iron;

The solution is filtered and retentate (filter cake) washed;

A further base wash is the completed by adding base solution NaOH to the permeate to bring the solution pH up to 11 to precipitate magnesium hydroxide. The magnesium hydroxide produced has a moisture content of approximately 50%;

The solution is then filtered and retentate (filter cake) may be washed or left with residual NaOH. The permeate produced is primarily approximately 2 M NaCI solution;

Optionally, the permeate solution may be placed in an electrolyser and electrolysis completed. Chlorine (C ) gas is recovered from the anode and hydrogen (H2) gas is recovered from the cathode. Optionally, the gases evolved may be combined to form HCI acid for acid washing of further magnesium containing silicate source;

The magnesium hydroxide Mg(OH>2 material recovered from the cathode contains approximately 50% moisture;

The composition is then produced by drying (approximately 50% reduction in mass) of recovered Mg(OH>2 material to create 25% moisture content pellets. NaOH is also present due to additional base solution used in the magnesium-precipitation stage;

Additional NaOH may be added if necessary to the recovered magnesium hydroxide material Mg(OH) ₂ ;

Silica or clay may also be added.

Note that hydrochloric acid may be replaced with sulphuric acid or other acids in the above example.

EXAMPLE 6

Referring to Figure 3, an example of a method of producing composition is described below by direct NaOH addition (note: this differs from base washing described in other examples where washing causes a pH increase for the purpose of product separation):

Magnesium hydroxide material is obtained from any source (i.e. commercially available).

If applicable, the magnesium hydroxide material can be ground to produce an average particle size less than 1mm.

A solution containing a mixture of NaOH (or other metal hydroxide) with water can be added to the magnesium hydroxide material. The metal hydroxide concentration of the solution is adjusted to produce a final material with the composition as described in Table 1 (pelletized) or Table 2 (slurry). Alternatively, solid NaOH is combined with magnesium hydroxide and water added to achieved the composition outlined in Table 1 and Table 2.

Clays, silica or other material may be added to the mixture.

The mixture maybe processed into pellets. The pellets will typically be approximately 1 mm in diameter (but may be 1 urn to 10mm).

Alternatively, the mixture can be maintained in slurry.

EXAMPLE 7

Referring to Figures 4-8, methods of CO2 sequestration are described using the composition.

Direct Air Capture

Typically, the composition may be used where air is passed over the composition. Figure 4 illustrates this reaction where the composition is in a pelletised form and naturally reacts with carbon dioxide gas (atmospheric or point source) to produce a hydrated magnesium carbonate end product.

The above reaction may occur using an apparatus as shown in Figure 5. In an experiment completed by the inventors using the apparatus of Figure 5 as a closed system, a first measurement was a control where no composition was added to the flow through the cell shown. Steady state atmospheric CO2 readings of approximately 400 ppm were recorded. 5 g of pelletised composition was placed in the flow through cell and readings then taken over time of the CO2 levels. The CO2 readings decreased from a starting value of approximately 400 ppm to less than 200 ppm over 24 hours illustrating the CO2 absorbing properties of the composition.

Point source carbon capture

As an alternative, a slurry of the composition may be used in a wet scrubber configuration where the slurry is distributed as a fine mist in a column of hot gas as illustrated in Figure 6.

In a further alternative method, a dry scrubber arrangement may use a pelletised composition (for instance in a fluidised bed type scrubber). To demonstrate the carbon absorption reaction for this alternative method, an experiment was completed by the inventors as follows:

~ 5 grams of composition was placed in a beaker containing deionised water;

Concentrated CO2 (~99%) was passed through the cell for a period of approximately 1 hour.

Following the addition of CO2, the thermogravimetric analysis results and acid test verified the presences of a highly carbonated material demonstrating carbon absorption. Aqueous carbon capture

A further method for sequestration may be via bubbling of CO2 through a slurry containing the composition as illustrated in Figure 7.

To further illustrate this method of sequestration, an experiment was completed as follows:

~ 21 grams of the composition was placed in a beaker containing deionised water;

Concentrated CO2 (~99%) was bubbled through the beaker for a period of ~ 15 minutes.

The thermogravimetric results after exposure of the composition to CO2 bubbling shown in Figure 8, illustrates the material was a hydrated magnesium carbonate similar to hydromagnesite.

The X-ray diffraction (XRD) results after exposure of the composition to CO2 bubbling shown in Figure 9, illustrates the material was a hydrated magnesium carbonate similar to hydromagnesite.

EXAMPLE 8

A trial was completed to determine the catalysing effects of metal hydroxide on the rate of sequestration.

A sequestration demonstration set up was prepared like that noted above in Example 7. Trials were completed using sodium hydroxide as the metal hydroxide and sequestration of carbon dioxide measured over a 10 minute time period.

Figure 10 shows a graph of the trial results. Samples were taken for a magnesium hydroxide only slurry (labelled 0% NaOH), a 10% sodium hydroxide slurry, a water sponge alone and a no DAC sample. The water (labelled 'H2O sponge') was to control assess the ability of water to uptake CO2 and the 'no DAC' label was with no sorbent/water in the absorption loop. As expected, the controls of 'no DAC' and the H2O sponge had no effect at all on CO2 sequestration. The 0% slurry had a mild sequestration effect with CO2 levels decreasing from 500ppm/min to 450ppm/min after 10 minutes. The 10% slurry by contrast showed a rapid and immediate decrease in CO2 moving from 500ppm/min at the trial start to approximately 350ppm/min after 10 minutes. With 10% NaOH addition a 30% reduction in CO2 occurred while with no NaOH, the reduction was only 10%.

At values less than 1% metal hydroxide (not shown in Fig 10), CO2 capture was similar to the 0% NaOH plot shown. At values greater than 1% NaOH, CO2 capture pulled away from the 0% line. At 10% NaOH the inventors achieved a maximum sequestration rate. Increasing the values above 10% NaOH did not appreciably change CO2 uptake rates. 100% NaOH as expected had minimal effect on CO2 sequestration. Amounts above 20% NaOH were disregarded as having that much sodium is not ideal for Mg carbonate formation due to size and charge considerations of the sodium. Higher sodium levels would also be unfavourable for ends uses due to potential residues. Aspects of the composition, methods of manufacture and methods of CO2 sequestration have been described by way of example only and it should be appreciated that modifications and additions may be made thereto without departing from the scope of the claims herein.

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