WO2017222396A1

WO2017222396A1 - Magnesium containing compositions

Info

Publication number: WO2017222396A1
Application number: PCT/NZ2017/050086
Authority: WO
Inventors: Allan Scott; Christopher John-Paul OZE
Original assignee: The University Of Canterbury
Priority date: 2016-06-24
Filing date: 2017-06-23
Publication date: 2017-12-28

Abstract

Disclosed are magnesium containing compositions and their use in the production of concrete. In particular, disclosed are methods of producing Mg(OH)2 comprising digesting olivine with water.

Description

Magnesium containing compositions

TECHNICAL FIELD

The present invention generally relates to magnesium containing compositions, and their use in the production of concrete.

BACKGROUND ART

While traditional calcium-based cements are well known, the development of a magnesium binder that can compete with Portland cement is fairly recent. Wei et al. (2006) developed one of the first successful magnesium-silica pastes by combining MgO (produced at 800°C) with a microsilica (and some unspecified additives) to achieve compressive strengths comparable to those of Portland cement.

Most commercially available reactive MgO is obtained from the calcination of magnesite (MgC03) at around 700°C. MgO is also commonly obtained from seawater. Mg(OH)2, derived from the seawater, is calcined at temperatures of approximately 400°C, compared to 700°C to 1000°C for MgCOs.

Serpentine has been suggested as an alternative source of both MgO and silica. Weathering of serpentine produces both MgO and silica via a slow process occurring naturally over many years or even millennia. Hrsak et a/. (2005) state that serpentine heated to 660°C will break down to yield a mixture of free oxides of MgO and S1O2. MgO derived from serpentine is not currently commercially available.

The known art methods for producing a magnesium-based cement system can be summarized as

1. MgCOs + Heat (> 700°C)→ MgO + CO2

2. MgO + Silica + Water→ Magnesium Cement (Magnesium silicate hydrate)

3. Mg²⁺ + CO2 → Magnesium Cement (MgC03).

Accordingly, it is an object of the present invention to go some way to avoiding the above disadvantages; and/or to at least provide the public with a useful choice.

Other objects of the invention may become apparent from the following description which is given by way of example only. Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date.

SUMMARY OF THE INVENTION

The present inventors have developed an alternative process for producing Mg(OH)2. This material may be used to produce a magnesium-based concrete. In some embodiments the process results in reduced manufacturing-related carbon dioxide emissions and lower energy consumption when compared to the production of conventional calcium-based cement.

In a first aspect, the invention provides a method of producing Mg(OH)2 comprising digesting olivine particles with water at a temperature of 200°C or less, wherein the olivine particles have a mean particle size of less than ΙΟΟμηη, and wherein at least 10% of the olivine particles have a mean particle size of less than ΙΟμηη.

Preferably, the temperature is 90°C or less. More preferably the temperature is 50°C or less.

Preferably, the olivine particles have a mean particle size of less than 20μηι.

Preferably, at least 30% of the olivine particles have a mean particle size of less than ΙΟμηη. In a second aspect, the invention provides a method of producing Mg(OH)2 comprising digesting olivine particles with water at a temperature of 200°C or less, wherein the olivine particles have a cumulative specific surface area of at least 18.2 m²/kg.

Preferably, the olivine particles have a cumulative specific surface area of at least

150m²/kg.

Preferably, the water has a pH from 5 to 9. In some embodiments the water has a pH from 6 to 8. In some embodiments the water has a pH from 5 to 7. In some embodiments the water has a pH of 5. In some embodiments the water has a pH of 6. In some embodiments the water has a pH of 7.

In some embodiments a method of the invention further produces hydrogen gas. In some embodiments a method of the invention further produces magnetite. In some embodiments a method of the invention further produces silica dioxide.

In some embodiments a method of the invention further comprises precipitating the Mg(OH)2. In some embodiments the Mg(OH)2 is precipitated using an alkali solution.

In some embodiments the method of the invention further comprises recovering the precipitated Mg(OH)2.

In some embodiments the method of the invention further comprises dehydrating the precipitated Mg(OH)2 to produce MgO.

In some embodiments the method of the invention further comprises combining the MgO obtained by a method of the invention with silica to produce a magnesium silica binder. In some embodiments the silica is amorphous. In some embodiments the silica has a similar fineness to Portland cement. In some embodiments the silica particles have a diameter of less than ΙΟΟμηη, preferably less than 10 μηη. In some embodiments the silica particles have a surface area of 300-400m²/kg.

In some embodiments the method of the invention further comprises combining the MgO obtained by a method of the invention with quartz to produce a magnesium silica binder. In some embodiments the quartz particles have a diameter of less than Ιμηη.

In some embodiments the method of the invention further comprises using the magnesium silica binder obtained by a method of the invention to produce concrete.

In some embodiments the Mg(OH)2 forms a layer on the olivine particles. In some embodiments the olivine particles further comprise a layer of serpentine beneath the Mg(OH)2 layer. In some embodiments Mg(OH)2 forms a layer on serpentine particles.

In a third aspect, the invention provides Mg(OH)2 produced by a method of the invention. In a fourth aspect, the invention provides a magnesium silica binder produced by a method of the invention. In a fifth aspect, the invention relates to the use of Mg(OH)2 produced according to the invention to produce concrete.

In a sixth aspect, the invention relates to the use of a magnesium silica binder according to the invention to produce concrete.

In a seventh aspect, the invention provides a method of making concrete, comprising mixing the magnesium silica binder of the invention with aggregate and water.

In an eighth aspect, the invention provides concrete, when made by a method of the invention.

In a ninth aspect, the invention provides a concrete premix comprising the binder of the invention and aggregate, and optionally comprising additional chemical and/or mineral admixtures typically used in the production of concrete. For example, in some embodiments the additional chemical and/or mineral admixtures are selected from plasticisers, set retardants, set accelerants, viscosity modifiers, water reducing agents, and air entraining agents.

In a tenth aspect, the invention provides a method of producing hydrogen gas comprising digesting olivine particles with water at a temperature of 200°C or less, wherein the olivine particles have a mean particle size of less than ΙΟΟμηη, and wherein at least 10% of the olivine particles have a mean particle size of less than ΙΟμηη.

In an eleventh aspect, the invention provides a method of producing hydrogen gas comprising digesting olivine particles with water at a temperature of 200°C or less, wherein the olivine particles have a cumulative specific surface area of at least 18.2 m²/kg.

In a twelfth aspect, the invention provides a method of producing magnetite comprising digesting olivine particles with water at a temperature of 200°C or less, wherein the olivine particles have a mean particle size of less than ΙΟΟμηη, and wherein at least 10% of the olivine particles have a mean particle size of less than ΙΟμηη.

In a thirteenth aspect, the invention provides a method of producing magnetite comprising digesting olivine particles with water at a temperature of 200°C or less, wherein the olivine particles have a cumulative specific surface area of at least 18.2 m²/kg. In a fourteenth aspect, the invention provides a method of producing silica dioxide comprising digesting olivine particles with water at a temperature of 200°C or less, wherein the olivine particles have a mean particle size of less than ΙΟΟμηη, and wherein at least 10% of the olivine particles have a mean particle size of less than ΙΟμηη.

In a fifteenth aspect, the invention provides a method of producing silica dioxide comprising digesting olivine particles with water at a temperature of 200°C or less, wherein the olivine particles have a cumulative specific surface area of at least 18.2 m²/kg.

In a sixteenth aspect, the invention provides a method of producing serpentine comprising digesting olivine particles with water at a temperature of 200°C or less, wherein the olivine particles have a mean particle size of less than ΙΟΟμηη, and wherein at least 10% of the olivine particles have a mean particle size of less than ΙΟμηη.

In a seventeenth aspect, the invention provides a method of producing serpentine comprising digesting olivine particles with water at a temperature of 200°C or less, wherein the olivine particles have a cumulative specific surface area of at least 18.2 m2/kg.

Other preferred embodiments of the tenth to seventeenth aspects of the invention incorporate features of the various preferred embodiments of the first and second aspects of the invention described herein.

This invention 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, and where specific integers are mentioned herein which have known equivalents in the art to which this invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.

In addition, where features or aspects of the invention are described in terms of Markush groups, those persons skilled in the art will appreciate that the invention is also thereby described in terms of any individual member or subgroup of members of the Markush group.

As used herein "(s)" following a noun means the plural and/or singular forms of the noun. As used herein the term "and/or" means "and" or "or" or both. The term "comprising" as used in this specification means "consisting at least in part of". When interpreting each statement in this specification that includes the term "comprising", features other than that or those prefaced by the term may also be present. Related terms such as "comprise" and "comprises" are to be interpreted in the same manner.

It is intended that reference to a range of numbers disclosed herein (for example, 1 to 10) also incorporates reference to all rational numbers within that range (for example, 1, 1.1, 2, 3, 3.9, 4, 5, 6, 6.5, 7, 8, 9 and 10) and also any range of rational numbers within that range (for example, 2 to 8, 1.5 to 5.5 and 3.1 to 4.7) and, therefore, all sub-ranges of all ranges expressly disclosed herein are hereby expressly disclosed. These are only examples of what is specifically intended and all possible combinations of numerical values between the lowest value and the highest value enumerated are to be considered to be expressly stated in this application in a similar manner.

Although the present invention is broadly as defined above, those persons skilled in the art will appreciate that the invention is not limited thereto and that the invention also includes embodiments of which the following description gives examples.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the Figures in which :

Figure 1 shows the particle size distribution for olivine particles ground using a plate mill, a ring mill for 20 seconds, a ring mill for one minute or a horizontal rod mill, wherein the particle size is expressed as a percentage mass passing;

Figure 2 shows an embodiment of the overall process of the invention, including the digestion of olivine, the recovery of the digestion products, and their use to make a magnesium cement;

Figure 3 shows the pH of the reaction mixture after olivine was digested for 3 hours at 20°C and 50°C;

Figure 4 shows the change in pH of the reaction mixture over time when olivine was digested for 168 hours at 20°C and 50°C; Figure 5 shows the change in magnesium concentration in the reaction mixture over time when olivine was digested at 20°C and 50°C;

Figure 6 shows the change in silica concentration in the reaction mixture over time when olivine was digested at 50°C;

Figure 7 shows the relationship between dissolved silica in the reaction mixture and pH at 20°C over time;

Figure 8 shows hydrogen concentration over time when olivine is digested at a) 20°C and b) 50°C;

Figure 9 shows particle size distribution for olivine pa rticles ground using a plate mill, a ring mill for 20 seconds, a ring mill for one minute or a horizonta l rod mill, expressed as specific surface area (m²/kg) .

DETAILED DESCRIPTION OF THE INVENTION

Olivine occurs naturally in ultramafic basalt rock. When exposed to subsurface water and/or to the elements, the olivine is subject to alteration, a nd in particular hydrolysis. In natural environments the hydrolysis of olivine typically occurs over a range of temperatures and pressures and is commonly thought to take thousands of yea rs. The present inventors have surprisingly determined that if olivine is ground very finely the reaction can be rapid even at low temperatures.

The reactions occurring during the low temperature hydrolysis of olivine are summarized in Table 1.

Table 1

1 1 2 2Mg₂Si0₄ + 3H₂0→ Mg₃Si20₅(OH)₄ + Mg²⁺ + 2(OH^")

4 Mg²⁺ + 2(OH^")→ Mg(OH)2 (or a wide va riety of

3 2

5 (oxy) hyd roxides)

6 3 7 3Fe₂Si0₄ + 2H₂0→ 2Fe₃0₄ + 3Si0₂ + 2H₂

(Mgo.88Feo. i₂)2Si0₄ + 1.34H₂0 = 0.5Mg₃Si₂O5(OH)₄ + 0.08Fe3O₄ +

A

0.26Mg(OH)₂ + 0.08H₂ The invention provides a method of producing Mg(OH)2 comprising digesting olivine particles with water at a temperature of 200°C or less, wherein the olivine particles have a mean particle size of less than ΙΟΟμηη, and wherein at least 10% of the olivine particles have a mean particle size of less than 10μη-ι.

The invention also provides a method of producing Mg(OH)2 comprising digesting olivine particles with water at a temperature of 200°C or less, wherein the olivine particles have a cumulative specific surface area of at least 18.2 m²/kg.

Other products of the process may include hydrogen gas, and a solid residue. The solid residue may comprise one or more of serpentine, magnetite (Fe30₄), unreacted olivine and precipitated Mg(OH)₂.

The olivine suitable for use in the method of the invention includes ultramafic or mafic (e.g ., basalt) rock containing a suitable concentration of olivine. In those embodiments wherein the method is directed to producing Mg(OH)2, MgO, a magnesium silica binder or concrete the olivine is preferably predominantly magnesium rich (i.e., forsteritic in character). In those embodiments wherein the method is directed to producing hydrogen gas, magnetite, silica dioxide or serpentine the person skilled in the art will appreciate that other forms of olivine may be preferred. In some embodiments the ultramafic or mafic rock contains at least 10% olivine by mass. For example, in some embodiments the ultramafic or mafic rock contains at least 10%, at least 15%, at least 20%, at least 25%, at least 30%, at least 40%, at least 45%, at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, or at least 80% olivine by mass. In some embodiments the ultramafic or mafic rock contains 30%-80% olivine by mass. In some embodiments the ultramafic or mafic rock contains at least 40% olivine by mass. A person skilled in the art can analyse a sample of an ultramafic or mafic rock to determine the percentage by mass of olivine using, for example, X-ray diffraction.

Olivine is mined in many locations around the world. In some embodiments the mined olivine may be subject to a preliminary processing step to reduce the size of the mined material. Accordingly, in some embodiments the olivine useful in the invention has an initial particle size of 20-50mm. Any suitable apparatus can be used to reduce the initial particle size of the olivine to provide the mean particle sizes or cumulative specific surface area useful in the invention.

Examples of suitable apparatus include, but are not limited to, a plate mill, a ring mill, a horizontal rod mill, a jet mill, a ball mill and a vertical rolling mill. The person skilled in the art will appreciate how to operate these apparatus to obtain particles that are suitable for use in the invention.

Particle size can be determined by mass or by volume. The term "mean particle size" as used herein is calculated as Zxirrii, where m, is the fraction by mass at a given size Xi . In some embodiments a sample is subject to multiple analyses and the average result determined. Examples of suitable means for analysing particle size include, but are not limited to, laser particle size analysis, scanning electron microscope imaging and other optical techniques.

As used herein, the term "specific surface area" of particles means the area of the exterior surface of the particles. Specific surface area may be calculated by assuming that the particles are more or less spherical, and calculating the surface area based on particle size. Specific surface area may be calculated as surface area per mass, for example as surface area per kg of material. The term "cumulative surface area" as used herein takes into account the particle size distribution as well as individual surface area . Cumulative surface area can be calculated by adding together the specific surface area of the individual size fractions.

In some embodiments the olivine particles have a mean particle size of less than δθμηη. For example, the olivine particles may have a mean particle size of less than 75μηΊ, less than 70μη-ι, less than 65μηΊ, less than δθμηη, less than 55μηΊ, less than 50μηΊ, less than 45μηΊ, less than 40μη-ι, less than 35μηΊ, less than 30μηΊ, less than 25μηΊ, less than 20μηΊ, less than Ιδμηη, less than ΙΟμηη, less than 5μηΊ, or less than Ι μηη. Preferably the olivine particles have a mean particle size of less than 40μηι. Even more preferably, the olivine particles have a mean particle size of less than 20μηι.

In some embodiments at least 10% of the olivine particles have a mean particle size of less than ΙΟμηη. For example, at least 15%, at least 20%, at least 25%, at least 30%, at least 35%, at least 40%, at least 45%, at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95% or at least 100% of the olivine particles have mean particle size of less than ΙΟμηη. Preferably at least 20% of the olivine particles have mean particle size of less than ΙΟμηη. Even more preferably, at least 30% of the olivine particles have mean particle size of less than ΙΟμηη. In some embodiments the olivine particles have a cumulative specific surface area of at least 18.2m²/kg. in some embodiments the olivine particles have a cumulative specific surface area of at least 150m²/kg, at least 250m²/kg, at least 500m²/kg, at least 750m²/kg, at least 1000m²/kg, at least 1250m²/kg, at least 1500m²/kg or at least 1800m²/kg.

Preferably the olivine particles have a cumulative specific surface area of at least 45m²/kg. Even more preferably, the olivine particles have a cumulative specific surface area of at least 90m²/kg.

Water is used to digest the olivine particles. In some embodiments the water has a pH of from 5 to 9. In some embodiments the water has a pH of from 6 to 8.5. In some embodiments the water has a pH of 7. Those persons skilled in the art can adjust the pH of the water using an appropriate acid or base. In some embodiments, the pH of the water is adjusted by dissolution of carbon dioxide.

In some embodiments the method comprises mixing the olivine particles with water. The temperature of the reaction mixture of olivine particles and water will typically be less than 200°C. For example, the temperature may be less than 180°C, less than 160°C, less than 140°C, less than 120°C, less than 100°C, less than 90°C, less than 60°C, less than 40°C or less than 20°C. In some embodiments the temperature may be 50°C. Generally, the olivine is mixed with cold water and the reaction mixture heated to the required

temperature. Alternatively, the olivine can be mixed with water that is already heated. In some embodiments the ratio of water to olivine particles is from 1000: 1 to 1 : 10. In some embodiments the ratio of water to olivine particles is from 100 : 1 to 1 : 10. In some embodiments the ratio of water to olivine particles is from 10 : 1 to 1 : 10. In some embodiments the ratio of water to olivine particles is 5 : 1, 2: 1, 1 : 2, or 1 : 5. In some embodiments the ratio of water to olivine particles is 1 : 1. In some particular embodiments the method comprises digesting olivine particles with water at a temperature of 150°C or less, wherein the olivine particles have a mean particle size of less than δθμηη, and wherein at least 30% of the olivine particles have a mean particle size of less than 10μη-ι.

In some particular embodiments the method comprises digesting olivine particles with water at a temperature of 90°C or less, wherein the olivine particles have a mean particle size of less than 40μη-ι, and wherein at least 50% of the olivine particles have a mean particle size of less than 10μη-ι.

In some particular embodiments the method comprises digesting olivine particles with water at a temperature of 90°C or less, wherein the olivine particles have a mean particle size of less than 40μη-ι, and wherein at least 30% of the olivine particles have a mean particle size of less than ΙΟμηη.

In some particular embodiments the method comprises digesting olivine particles with water at a temperature of 90°C or less, wherein the olivine particles have a mean particle size of less than 30μηΊ, and wherein at least 30% of the olivine particles have a mean particle size of less than ΙΟμηη.

In some particular embodiments the method comprises digesting olivine particles with water at a temperature of 150°C or less, wherein the olivine particles have a cumulative specific surface area of at least 66 m²/kg.

In some particular embodiments the method comprises digesting olivine particles with water at a temperature of 90°C or less, wherein the olivine particles have a cumulative specific surface area of at least 103m²/kg.

In some particular embodiments the method comprises digesting olivine particles with water at a temperature of 90°C or less, wherein the olivine particles have a cumulative specific surface area of at least 78m²/kg.

In some particular embodiments the method comprises digesting olivine particles with water at a temperature of 90°C or less, wherein the olivine particles have a cumulative specific surface area of at least 87m²/kg.

In the particular embodiments above, the pH of the water is preferably from 6 to 8.5. In the particular embodiments above, the ratio of water to olivine particles is preferably 2 : 1. In some embodiments the process is performed in one or more reaction vessels. For example, in some embodiments the reaction vessel is a stainless steel or fibreglass tank. In some embodiments the size of the reaction vessel may be 20 to 100m³.

In some embodiments the process is a batch process. However, the invention is not limited thereto and other embodiments, in which the process is a continuous process, are also contemplated.

The reaction mixture of olivine and water may be agitated. In some embodiments agitation is the result of stirring, a bubbling system, and/or the use of a pump or filter. In some embodiments air, carbon dioxide, or a mixture thereof are bubbled through the reaction mixture of olivine and water. In some embodiments currents can be created during the digestion process by feeding a slurry of olivine and water into the bottom and/or top of the reaction vessel.

In some embodiments olivine is digested with water until at least 20%, at least 50% or at least 80% of the olivine has reacted. One method for following progression of the digestion comprises monitoring the pH of the reaction mixture of olivine and water. In some embodiments olivine is digested with water until the pH of the reaction mixture has increased to 9.5 and then decreased by at least 0.5 pH. Another method for determining whether the digestion has sufficiently progressed comprises monitoring the rate at which hydrogen gas is produced. In some embodiments olivine is digested with water until the rate of hydrogen production increases less than 10%/day (or equivalent for different time steps). Another method for determining whether the digestion has sufficiently progressed comprises measuring the concentration of magnesium ions in the reaction mixture. In some embodiments the digestion continues for about one to about three days. Another method for determining whether the digestion has sufficiently progressed comprises monitoring the change in density as the reaction proceeds. Olivine has a density of approximately 3.3 g/cm³ and serpentine (i.e. the hydrolysis product of olivine) has a density of approximately 2.5-2.6 g/cm³. The person skilled in the art will appreciate that this method and/or others may be applied to both a batch process and a continuous process. Digesting olivine with water produces Mg(OH)2 in solution in the reaction mixture. The Mg(OH)2 can be precipitated by any conventional means. For example, the Mg(OH)2 can be precipitated using an aqueous alkali solution with a pH greater than approximately 10.4. Suitable alkali solutions include but are not limited to aqueous solutions of alkali metal hydroxides, for example 0.1M solutions of NaOH or KOH. In some embodiments the supernatant containing Mg(OH)2 is continuously removed from the initial reaction vessel into a second reaction vessel and reacted with an aqueous alkali solution. The precipitated Mg(OH)2 may then be recovered from the second reaction vessel. In some embodiments olivine and/or serpentine particles are removed from the supernatant. Suitable means include, but are not limited to, a filter or a settling tank.

In some embodiments the recovered Mg(OH)2 is dehydrated to MgO. For example, in some embodiments the recovered Mg(OH)2 is heated, preferably to 300°C to 400°C. In some embodiments the recovered Mg(OH)2 may be subject to mechanical processing, such as centrifugation, before being heated. In some embodiments, after the precipitated Mg(OH)2 has been removed from the aqueous alkali solution, the aqueous alkali solution can be processed to obtain alkali hydroxides and water. In some embodiments, where a second reaction vessel is being used to precipitate the Mg(OH)2, the alkali hydroxides obtained from the aqueous alkali solution may be used to maintain the pH in the second reaction vessel. In some embodiments the water obtained from the aqueous alkali solution is used to digest further olivine with the method of the invention.

Alternatively, in some embodiments the Mg(OH)2 can be precipitated in the initial reaction vessel.

If the Mg(OH)2 precipitates in the initial reaction vessel, the Mg(OH)2 may not readily be separated from the residual solid. Accordingly, if separated Mg(OH)2 is required, the Mg(OH)2 concentration in the reaction mixture should be monitored so that the supernatant can be removed before precipitation occurs. In some embodiments the pH of the primary reaction vessel is maintained below approximately 10.5. For example, the pH can be kept below 10.5 by adding water to the reaction vessel. The solid residue produced by the method may be separated from the liquid component of the reaction mixture by, for example, filtering the reaction mixture. In some embodiments, the solid residue is separated from the liquid component by vacuum filtration. Other methods of separating the solid residue of the reaction mixture from the liquid component may also be used, such as a phase separator, a hydrocyclone, extractor, or centrifuge. In some embodiments the solid residue may be separated from the liquid component by gravity settlement.

In some embodiments the hydrogen gas produced by the method is collected. In some embodiments the method of the invention is carried out in a sealed reaction vessel. In some embodiments the method of the invention is carried out under reduced pressure. Advantageously, these embodiments simplify recovery of the hydrogen gas. The person skilled in the art can readily envisage alternative methods of recovery. The collected hydrogen gas may be used as an energy source either in its own right, or to supplement the energy requirements of the process.

Serpentine and magnetite reaction products may also be recovered from the solid residue. In some embodiments the use of larger olivine particles in the method of the invention results in particles comprising a core of unreacted olivine surrounded by a layer of serpentine. In some embodiments these particles are recovered, reground and used as feedstock for the method of the invention.

In some embodiments, if the Mg(OH)2 precipitates in the initial reaction vessel, the serpentine or olivine/serpentine particles may also have a layer of Mg(OH)2. In some embodiments, if the Mg(OH)2 precipitates in the initial reaction vessel, the magnetite reaction products may have an outer layer of Mg(OH)2. In some embodiments these particles with a layer of Mg(OH)2 may be heated to dehydrate the Mg(OH)2, leaving a MgO layer on the surface of the particle.

In some embodiments the MgO obtained from the method of the invention can be combined with a finely divided amorphous silica to produce a construction material. In some embodiments the construction material is a magnesium silica binder. In some embodiments the ratio of silica to MgO is from 70: 30 to 30 : 70, preferably from 60:40 to 40 : 60. The silica can be from a variety of sources including but not limited to fly ash, ground granulated blast furnace slag, naturally occurring amorphous silica (pumice for example) and silica fume. Preferably the silica has a particle size similar to or smaller than the MgO.

In some embodiments the MgO produced by the method of the invention can be used to prepare concrete. For example, in some embodiments concrete is prepared by mixing the MgO produced by the invention with aggregate, silica, sand and water. In some

embodiments the ratio of water to MgO is from 0.3 to 33 by mass. In some embodiments the ratio of water to MgO is from 1 to 3.3 by mass.

In some embodiments the magnesium silica binder of the invention can be used to prepare concrete. The person skilled in the art is familiar with methods for making concrete from binders, and the magnesium silica binder of the invention can be used in a manner comparable to traditional Portland cement. The person skilled in the art will be able to select the appropriate concrete preparation method depending on the particular application and the desired result. For example, in some embodiments concrete is prepared by mixing the magnesium silica binder of the invention with aggregate, sand and water. In some embodiments, the ratio of water to magnesium silica binder is 0.1 to 10. In some embodiments the ratio of water to magnesium silica binder is 0.3 to 1.

Alternatively, in some embodiments a concrete premix can be prepared by mixing the MgO produced by a method of the invention with aggregate, silica and sand.

In some embodiments the concrete premix may further comprise a set retarding additive. As used herein, the term "set retarding additive" refers to an additive that retards the setting of concrete. Examples of suitable set retarding additives include, but are not limited to, ammonium, alkali metals, alkaline earth metals, metal salts of sulfoalkylated lignins, organic acids (e.g., hydroxycarboxy acids), copolymers that comprise acrylic acid or maleic acid, and combinations thereof. In some embodiments the set retarding additive is included in the concrete premix in an amount sufficient to provide the desired set retardation. The person skilled in the art will be able to select the appropriate amount of the set retarding additive to include for a chosen application. For example, in some embodiments the set retarding additive may be present in the concrete premix an amount in the range of 0.1% to 5% (by weight).

Optionally, other additional additives may be added to the concrete premix as considered appropriate by the person skilled in the art. Examples of such additives include, but are not limited to, viscosity modifiers, water reducing agents, air entraining agents, strength- retrogression additives, set accelerators, weighting agents, lightweight additives, gas- generating additives, mechanical property enhancing additives, lost-circulation materials, filtration-control additives, dispersants, fluid loss control additives, defoaming agents, foaming agents, oil-swellable particles, water-swellable particles, thixotropic additives, and combinations thereof. Specific examples of these, and other, additives include limestone, salts, fibers, hydratable clays, microspheres, rice husk ash, elastomers, elastomeric particles, resins, latex, combinations thereof, and the like. In some embodiments ground limestone is used to bulk up the concrete premix and control particle distribution. The person skilled in the art can determine the type and amount of additive useful for a particular application and desired result.

Figure 2 shows an embodiment of the overall reaction process. In particular, it shows the digestion of olivine with water in a primary reaction vessel to produce serpentine, Mg(OH)2, magnetite and hydrogen gas. In this embodiment, the supernatant is mixed with an alkali solution in a secondary reaction vessel to precipitate the Mg(OH)2. The precipitated

Mg(OH)2 is then heated to produce MgO and combined with silica for use in producing a magnesium based cement. Figure 2 also shows that serpentine may be recovered, and that some of the solid particles may have a layer of Mg(OH)2. The particles with a layer of Mg(OH)2 may be heated to produce a magnesium based cement.

Figure 2 further shows that the hydrogen gas produced by the digestion can be collected. The hydrogen gas may be used to provide heat for the dehydration of Mg(OH)2.

The following non-limiting examples are provided to illustrate the present invention and in no way limit the scope thereof.

EXAMPLES

Materials and methods Olivine was sourced from Dunn Mountain, New Zealand. All other chemicals and reagents were standard laboratory supplies.

Sample preparation :

The weathering products were removed from the exterior of the olivine samples prior to grinding. The olivine samples were initially prepared in a jaw crusher resulting in material with a maximum particle size of less than 2 mm. Four samples were then ground using either a plate mill, 20 seconds in a ring mill, 1 minute in a ring mill or 8 hours in a horizontal rod mill with a load capacity of 6L.

Due to the limited capacity of the ring mill, approximately 50 grams, the olivine had to be processed in batches. The individual batches were blended to create a homogenous material for analysis. For each batch, the ring mill was charged with approximately 50g of olivine. Two sizes of 106 μηη and 42 μηη were obtained by operating the machine for 20 seconds and 1 minute respectively.

The plate mill was set with a distance between the plates equal to approximately 0.11mm. The olivine was added to the hopper at a rate of approximately 1 kg per minute. The average particle size was approximately 152 m.

The rod mill was charged with approximately 10 kg of olivine. The mill containing the steel rods rotated about the horizontal axis at a speed of approximately 30 revolutions per minute. The olivine was left in the rod mill for 8 hours resulting in an average fineness of approximately 41 m.

The average particle size for the different samples is set out in Table 2.

Table 2: Average particle size

Grinding Type Mean Size ( m) Median Size ( m)

Plate mill (01) 152.5 138.2

20 sec ring mill (02) 103.4 59.7

1 min ring mill (03) 44.0 19.1

8 hr rod mill (04) 41.8 19.6

Mean calculated = Zxirrii, where m, is the fraction by mass at a given size x The particle size distribution, expressed as a percentage mass passing, is shown in Figure 1 and appears consistent with average particle sizes presented in Table 2.

Digestion of olivine to produce MqQ

Triplicate olivine samples were prepared for each level of grinding, temperature and duration. 3g of olivine and 6 ml of de-ionized water were added to 12 ml glass bottles and sealed with a septum lid to allow for extraction and analysis of the gas. The hydration of olivine was evaluated after 3 hours, 1 day, 3 days and 7 days at temperatures of 20°C and 50°C. The gas present in the head space above the olivine and water sample was drawn off with a syringe and analysed with a gas chromatograph to determine the hydrogen, oxygen, nitrogen and carbon dioxide concentrations. A summary of the specimen test conditions is provided in Table 3.

Table 3: Sample description

Approximately 4 ml of reaction mixture was filtered for ICPMS analysis.

Within 3 hours of mixing the ground olivine with water, the pH increased from

approximately 7 to at least 9.2, as shown in Figure 3. The rapid increase in pH indicates the formation of hydroxides as given in Equation 1 in Table 1. It is also evident from Figure 3 that the pH increased with the fineness of the olivine.

The pH increase did not continue over the period of the investigation but slowly decreased with further reaction times as shown in Figure 4. The system was closed to the atmosphere but some residual CO2 would have been present in the head space of each sample after sealing. Without wishing to be bound by theory, it is thought that this residual CO2 from the air dissolved into the reaction mixture to reduce pH.

The magnesium and silica concentrations in the reaction mixture were at a maximum 3 hours after mixing and gradually decreased over the exposure period. The 20°C samples consistently had higher concentrations than 50°C samples for the comparable particle size. The change in magnesium concentration over the exposure period is provided in Figure 5 while the pronounced decrease in silica concentration for 20°C is show in Figure 6.

The relationship between dissolved silica and pH is provided in Figure 7.

The overall production of magnesium can be inferred from the accumulation of hydrogen in the head space of the container which is shown as an increase in H2 concentration in Figure 8.

The increase in temperature from 20°C to 50°C resulted in a noticeable increase in the measured hydrogen concentration and by extension the amount of MgO which would have been produced.

Analysis of Particle Size Distribution

A summary of the average particle size data for the four levels of grinding is shown above in Table 2. It can be seen from Table 2 that there is very little difference in either the mean or median particle size for the 1 minute ring mill or 8 hour rod mill while there is a clear difference between the plate mill, 20 second ring mill and the remaining two.

An examination of the hydrogen evolved from the different levels of grinding at 50°C, Figure 8b, shows a slightly different relationship. After 24 hours exposure the plate mill, 20 second ring mill, and 1 minute ring mill samples showed a similar level of reactivity as indicated by the hydrogen concentration despite very different particle size distributions. The results from the 8 hour rod mill samples are significantly different from the other samples with a hydrogen concentration approximately double that of the next highest.

Examination of the particle size distribution in Figure 9, expressed as specific surface area (m²/kg), revealed some noticeable difference between the various grinding levels.

While the average mean particle size is very similar between the 1 minute ring mill and 8 hour rod mill there is a greater quantity of very fine material for the 8 hr rod mill samples. At 2.5 μΓη the cumulative surface area of the 8 hour rod mill samples is almost double that of the 1 minute ring mill. The difference in surface area decreases as the particle size diameter increases but even at 5 m the difference is approximately 50% and at 10 μηη 23%.

Without wishing to be bound by theory, it is thought this lower fine fraction is important for accelerating the rate of digestion.

Effect of digestion conditions

Three 4 gram samples of olivine were prepared as described above. Each sample had an average particle size of 41 μηη. Each sample was added to 80 ml of either soda water (CO2 injected) or deionized (DI) water. The soda water was a commercially available, off the shelf product, initially pressurized to approximately 2 bar. The pH of the soda water was approximately 5. Sample Paerated contained deionized water with air continuously bubbled through at a rate of approximately 1 l/min.

The reaction vessels were un-sealed and the concentration of dissolved CO2 would be expected to decrease with time for sample P5.

The reaction rates for olivine in all the solutions, based on changes in the measured pH, was fairly rapid, achieving quasi steady state pH values within approximately 1 to 10 minutes. After two hours at 20°C the solution was extracted, filtered at 0.45 μηη and stabilized with 0.03 ml of HNO3 (0. 1 M) and the magnesium concentration was measured.

Table 4: Reaction conditions and results

Solution Final pH Additional Mg Concentration

Sample starting

er 2 hrs conditions (mg/L)

condition aft

pH 5 soda

P5 6 162

water

P6 pH 6 DI water 10 21

2 hr

Paerated pH 6 DI water 8.7 48

aerated

Note: the pH values given are approximate values based on limited testing.

Table 4 shows the concentration of magnesium ions in solution increased for the reaction in soda water and, to a lesser extent, the aerated reaction, compared with the reaction in deionized water. The highest level of dissolved magnesium was measured for the soda water solution.

References:

Hrsak, D. Malina, J. Hadzipasic, A. 2005, The Decomposition of Serpentine by Thermal Treatment, Materiali in Technologije, Vol. 39, pp. 225-227.

Wei, J. Chen, Y., Li, Y. 2006, The Reaction Mechanism between MgO and Microsilica at Room Temperature, Journal of Wuhan University of Technology, Mater. Sci. Ed., Vol. 21, No. 2, pp. 88-91.

Constantz, Laurence Clodic, Cecily Ryan, Miguel Fernandez, Kasra Farsad, Sidney Omelon, Phil Tuet, Paulo Monteiro, Jr. Gordon E. Brown, Katharine Geramita, 2010, Production of carbonate-containing compositions from material comprising metal silicates, WO

2010/006242.

Blencoe James G., Palmer Donald A., Anovitz Lawrence M., Beard James S., 2004,

Carbonation of metal silicates for long-term C02 sequestration, US 2004/0213705.

Nikolaos Vlasopoulos, Christopher Robert Cheeseman, 2009, Binder composition, WO 2009/156740.

Nikolaos Vlasopoulos, Christopher Robert Cheeseman, 2011, Binder composition, US

20110290155.

Nikolaos Vlasopoulos, 2011, Process for producing cement binder compositions containing magnesium, US 20130213273.

Amutha Rani Devaraj, Hai Xiang Lee, Diego Alfonso Martinez Velandia, Nikolaos

Vlasopoulos, 2014, Binder composition, US 20140290535. It is not the intention to limit the scope of the invention to the abovementioned examples only. As would be appreciated by a skilled person in the art, many variations are possible without departing from the scope of the invention.

Claims

We claim:

1. A method of producing Mg(OH)2 comprising digesting olivine particles with water at a temperature of 200°C or less, wherein the olivine particles have a mean particle size of less than ΙΟΟμηη, and wherein at least 10% of the olivine particles have a mean particle size of less than ΙΟμηη.

2. A method according to claim 1, wherein the olivine particles have a mean particle size of less than 20μηι.

3. A method according to claim 1 or claim 2, wherein at least 30% of the olivine particles have a mean particle size of less than ΙΟμηη.

4. A method of producing Mg(OH)2 comprising digesting olivine particles with water at a temperature of 200°C or less, wherein the olivine particles have a cumulative specific surface area of at least 18.2 m²/kg.

5. A method according to claim 4, wherein the olivine particles have a cumulative specific surface area of at least 150m²/kg.

6. A method according to any one of claims 1 to 5, wherein the temperature is 90°C or less.

7. A method according to claim 6, wherein the temperature is 50°C or less.

8. A method according to any one of claims 1 to 7, wherein the water has a pH from 5 to 7.

9. A method according to any one of claims 1 to 8, wherein the water has a pH of 5.

10. A method according to any one of claims 1 to 8, wherein the water has a pH of 6.

11. A method according to any one of claims 1 to 8, wherein the water has a pH of 7.

12. A method according to any one of claims 1 to 7, wherein the water has a pH from 5 to 9.

13. A method according to any one of claims 1 to 7 and 12, wherein the water has a pH from 6 to 8.

14. A method according to any one of claims 1 to 10, wherein the water comprises dissolved carbon dioxide.

15. A method according to any one of claims 1 to 14, wherein the digesting further comprises agitating the olivine particles and water.

16. A method according to claim 15, wherein the agitating is by bubbling.

17. A method according to claim 16, wherein the agitating is by bubbling air, carbon dioxide or a mixture thereof.

18. A method according to any one of claims 1 to 17, wherein the method further produces hydrogen gas.

19. A method according to any one of claims 1 to 18, wherein the method further produces magnetite.

20. A method according to any one of claims 1 to 19, wherein the method further produces silica dioxide.

21. A method according to any one of claims 1 to 20, wherein the method further comprises precipitating the Mg(OH)2.

22. A method according to claim 21, wherein the Mg(OH)2 is precipitated using an alkali solution.

23. A method according to claim 21 or claim 22, wherein the method further comprises recovering the precipitated Mg(OH)2.

24. A method according to claim 23, wherein the method further comprises dehydrating the precipitated Mg(OH)2 to produce MgO.

25. A method according to claim 24, wherein the method further comprises combining the MgO with silica to produce a magnesium silica binder.

26. A method according to claim 25, further comprising using the magnesium silica binder to produce concrete.

27. A method according to any one of claims 1 to 20, wherein Mg(OH)2 forms a layer on the olivine particles.

28. A method according to claim 24, wherein the olivine particles further comprise a layer of serpentine beneath the Mg(OH)2 layer.

29. Mg(OH)2 produced by the method according to any one of claims 1 to 23, 27 or 28.

30. A magnesium silica binder produced by the method of claim 25.

31. Use of Mg(OH)2 according to claim 29 to produce concrete.

32. Use of a magnesium silica binder according to claim 30 to produce concrete.

33. A method of making concrete, comprising mixing the magnesium silica binder of claim 30 with aggregate and water.

34. Concrete, when made by a method of claim 26 or claim 33.

35. A concrete premix comprising the binder of claim 30 and aggregate.

36. A method of producing hydrogen gas comprising digesting olivine particles with water at a temperature of 200°C or less, wherein the olivine particles have a mean particle size of less than ΙΟΟμηη, and wherein at least 10% of the olivine particles have a mean particle size of less than ΙΟμηη.

37. A method of producing hydrogen gas comprising digesting olivine particles with water at a temperature of 200°C or less, wherein the olivine particles have a cumulative specific surface area of at least 18.2 m²/kg.

38. A method of producing magnetite comprising digesting olivine particles with water at a temperature of 200°C or less, wherein the olivine particles have a mean particle size of less than ΙΟΟμηη, and wherein at least 10% of the olivine particles have a mean particle size of less than ΙΟμηη.

39. A method of producing magnetite comprising digesting olivine particles with water at a temperature of 200°C or less, wherein the olivine particles have a cumulative specific surface area of at least 18.2 m²/kg.

40. A method of producing silica dioxide comprising digesting olivine particles with water at a temperature of 200°C or less, wherein the olivine particles have a mean particle size of less than ΙΟΟμηη, and wherein at least 10% of the olivine particles have a mean particle size of less than ΙΟμηη.

41. A method of producing silica dioxide comprising digesting olivine particles with water at a temperature of 200°C or less, wherein the olivine particles have a cumulative specific surface area of at least 18.2 m²/kg.

42. A method of producing serpentine comprising digesting olivine particles with water at a temperature of 200°C or less, wherein the olivine particles have a mean particle size of less than ΙΟΟμηι, and wherein at least 10% of the olivine particles have a mean particle size of less than ΙΟμηη.

43. A method of producing serpentine comprising digesting olivine particles with water at a temperature of 200°C or less, wherein the olivine particles have a cumulative specific surface area of at least 18.2 m²/kg.