METHOD FOR EXTRACTING SILICA

TECHNICAL FIELD THIS INVENTION relates to methods for extracting silica from organic material and more particularly plant material, such as rice hulls, rice straw and sugarcane bagasse. Lignin may also be produced by the method.

BACKGROUND

Plant material, such as rice hulls and straw, can be used, amongst other things, as a source of silica. In this regard, the prior art provides a number of methods to extract silica from plant material, such that it can then be applied to a number of downstream uses. Seemingly the most commonly used method includes the use of potassium hydroxide or sodium hydroxide to solubilise and thereby extract silica from the ashed plant material, before the addition of an acid (e.g., HC1 or H ₂ SO ₄ ) to subsequently precipitate the silica. Other methods demonstrate the use of C0 ₂ gas as a way to precipitate the silica from solution after alkaline extraction. Alternative methods of silica extraction may include a pyrolysis step, where the plant material, for example, may be first treated with an acid or an ionic liquid and subsequently pyrolysed to produce a silica extract. The above methods, however, represent relatively complex chemistry, are not environmentally friendly (e.g., issues of effluent/waste treatment) and/or would be capitally intensive when performed at an industrial scale.

Accordingly, improved methods for the extraction of silica from organic material are required.

SUMMARY

The present invention is initially predicated in part on a surprising discovery that plant-based silica may be solubilised in aqueous ammonia, and that an extraction process based on aqueous ammonia affords a liquid fertilizer coproduct, avoiding the production of waste salt solutions. However, silica solubility in aqueous ammonia is not high enough for the process to be economically viable. Notwithstanding this, the liquid fertilizer coproduct (and the minimization of waste salt solutions) can be achieved by the unique coupling of alkali silica extraction with a nearly reversible ion-exchange of ammonia and sodium or other cations. In this coupling of extraction and ion-exchange the starting materials and products are the same as those of an ammonia extraction of silica. Furthermore, using alkali for the extraction of silica from organic material may result in the production of other useful by-products, such as lignin and a residual biomass suitable for use as an animal feed ingredient and/or as a feedstock for the production of a partially hydro lysed lignocellulosic material therefrom.

In a first aspect, the invention provides method of extracting silica from an organic material, including the steps of:

(i) separating a liquids fraction and a solids fraction from the organic material treated with an alkali;

(ii) adding a first acid to the liquids fraction so as to facilitate at least partial precipitation of silica therefrom;

(iii) separating precipitated silica from the liquids fraction;

(iv) optionally adding a second acid to the liquids fraction so as to facilitate at least partial precipitation of lignin therefrom;

(v) contacting the liquids fraction of step (iv) with an ion-exchanger so as to facilitate at least partial substitution of a first cation of the liquids fraction with a second cation of the ion-exchanger, such that the liquids fraction is then suitable as a liquid fertilizer;

thereby extracting silica from the organic material.

In one embodiment, the method further includes the step of separating precipitated lignin from the liquids fraction of step (iv) or step (v).

In a second aspect, the invention provides a method of extracting lignin from an organic material, including the steps of:

(i) separating a liquids fraction and a solids fraction from the organic material treated with an alkali;

(ii) adding a first acid to the liquids fraction so as to facilitate at least partial precipitation of silica therefrom;

(iii) separating precipitated silica from the liquids fraction;

(iv) adding a second acid to the liquids fraction so as to facilitate at least partial precipitation of lignin therefrom;

(v) separating precipitated lignin from the liquids fraction

thereby extracting lignin from the organic material. Suitably, the method of the second aspect further includes the step of contacting the liquids fraction of step (v) with an ion-exchanger so as to facilitate at least partial substitution of a first cation of the liquids fraction with a second cation of the ion-exchanger, such that the liquids fraction is then suitable as a liquid fertilizer. In particular embodiments, the liquid fertilizer comprises ammonium sulphate and/or ammonium phosphate.

In particular embodiments, the first cation is Na ⁺ and/or K ⁺ .

In certain embodiments, the second cation is NH ⁴⁺ .

Preferably, the method of the above aspects further includes the step of contacting the ion-exchanger with a regenerating agent comprising the first cation so as to facilitate at least partial substitution of the second cation of the ion exchanger by the first cation of the regenerating agent. In one embodiment, the regenerating agent is or comprises sodium hydroxide.

Referring to the first and second aspects, the method suitably further includes the step of contacting the ion exchanger with a further agent comprising the second cation so as to facilitate at least partial substitution of the first cation of the ion- exchanger by the second cation of the further agent. Preferably, substitution of the second cation of the further agent with the first cation of the ion-exchanger results in production of the alkali for treating the organic material. In one particularly preferred embodiment, the further agent is or comprises ammonium hydroxide.

Accordingly, in view of the above, the method of the above aspects is preferably cyclical.

With respect to the method of the first and second aspects, the first and/or second acid are suitably selected from the group consisting of sulphuric acid, hydrochloric acid, phosphoric acid, hydrofluoric acid, hydrobromic acid, nitric acid, acid metal salts and any combination thereof. Preferably, the first and/or second acid is sulphuric acid and/or phosphoric acid.

Suitably for the aforementioned aspects, the alkali is selected from the group consisting of sodium hydroxide, potassium hydroxide, alkali metal salts and any combination thereof. Preferably, the alkali is sodium hydroxide and/or potassium hydroxide.

In certain embodiments, the first acid is added in an amount effective to lower the pH of the liquids fraction of step (i) to a pH of about 8 to about 10.

In particular embodiments, the second acid is added in an amount effective to lower the pH of the liquids fraction of step (iii) to a pH of about 2.5 or less.

Suitably, the method of the first and second aspects further includes the step of treating the organic material with the alkali.

In one embodiment, the alkali is present in an amount of about 500% to about 5000% by weight of the organic material.

With respect to the method of the above aspects, the organic material is suitably treated at a temperature from about 70°C to about 200°C. In one embodiment, the organic material is treated at a temperature of about 160°C. In an alternative embodiment, the organic material is treated at a temperature of about 90°C.

Suitably, the organic material is treated for a period of time from about 30 minutes to about 120 minutes. Preferably, the organic material is treated for a period of time of about 30 minutes.

In one embodiment, treatment of the organic material with the alkali is carried out at a solids to liquids ratio of about 2% to about 20%.

Preferably, the organic material is or comprises plant material.

In regards to the first and second aspects, the method suitably further includes the step of pretreating the organic material with a further acid, a further alkali and/or a polyol.

In one embodiment, the precipitated silica and/or precipitated lignin are at least partly separated from the liquids fraction by filtration and/or centrifugation.

In particular embodiments, the solids fraction is suitable for use as an animal feed ingredient.

In certain embodiments, the solids fraction: (i) is suitable for use in producing a partially hydro lysed lignocellulosic material therefrom; and/or (ii) is or comprises a partially hydro lysed lignocellulosic material. Preferably, the partially hydro lysed lignocellulosic material is suitable for the production of a fermentable sugar therefrom.

In a third aspect, the invention provides a silica precipitate produced by the method of the first or second aspect.

In a fourth aspect, the invention provides a lignin precipitate produced by the method of the first or second aspect.

Throughout this specification, unless otherwise indicated, "comprise" , "comprises" and "comprising" are used inclusively rather than exclusively, so that a stated integer or group of integers may include one or more other no n- stated integers or groups of integers. Conversely, the terms "consist", "consists" and "consisting" are used exclusively, such that a stated integer or group of integers are required or mandatory, and no other integers may be present. The phrase "consisting essentially of' indicates that a stated integer or group of integers are required or mandatory, but that other elements that do not interfere with or contribute to the activity or action of the stated integer or group of integers are optional.

It will also be appreciated that the indefinite articles "a" and "an" are not to be read as singular indefinite articles or as otherwise excluding more than one or more than a single subject to which the indefinite article refers. For example, "a" protein includes one protein, one or more proteins or a plurality of proteins.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example only, an embodiment of the invention is described more fully hereinafter with reference to the accompanying drawings, in which:- Figure 1 demonstrates a silica precipitate formed from aqueous ammonia extraction of rice husks.

Figure 2 demonstrates a lignin precipitate formed at a pH of -2.0 from sodium hydroxide extraction of rice husks.

Figure 3 demonstrates the effect of alkaline extraction on the structure of rice straw.

Figure 4 demonstrates a schematic of an embodiment of the invention.

Figure 5 demonstrates: (i) a sample of silica extracted from rice husk without a pretreatment step (left panel) and; (ii) a sample of silica extracted from rice husk previously pretreated with sulphuric acid and then glycerol.

Figure 6 shows the reversible ion-exchange of ammonia cations bound to a strong cation exchange resin (Mitsubishi UBK04H converted to the ammonia form) with sodium hydroxide and sodium sulphate solutions.

DETAILED DESCRIPTION

The present invention arises, in part, from the identification of novel methods of extracting silica from organic material, which may also produce a number of useful by-products, including lignin, a liquid fertilizer and a residual biomass that may be suitable as an animal feed ingredient and/or as a feedstock for the production of a partially hydro lysed lignocellulosic material. Accordingly, the methods described herein typically have fewer effluent or waste treatment issues, lower capital and operational costs and/or are more efficient than those previously described in the art.

In one aspect, the invention provides a method of extracting silica from an organic material, including the steps of:

(i) separating a liquids fraction and a solids fraction from the organic material treated with an alkali;

(ii) adding a first acid to the liquids fraction so as to facilitate at least partial precipitation of silica therefrom;

(iii) separating precipitated silica from the liquids fraction;

(iv) optionally adding a second acid to the liquids fraction so as to facilitate at least partial precipitation of lignin therefrom;

(v) contacting the liquids fraction of step (iv) with an ion-exchanger so as to facilitate at least partial substitution of a first cation of the liquids fraction with a second cation of the ion-exchanger, such that the liquids fraction is then suitable as a liquid fertilizer;

thereby extracting silica from the organic material.

In one embodiment, the method of this aspect further includes the step of separating precipitated lignin from the liquids fraction of step (iv).

In a related aspect, the invention provides a method of extracting lignin from an organic material, including the steps of:

(i) separating a liquids fraction and a solids fraction from the organic material treated with an alkali;

(ii) adding a first acid to the liquids fraction so as to facilitate at least partial precipitation of silica therefrom;

(iii) separating precipitated silica from the liquids fraction;

(iv) adding a second acid to the liquids fraction so as to facilitate at least partial precipitation of lignin therefrom;

(v) separating precipitated lignin from the liquids fraction

thereby extracting lignin from the organic material.

The statements which follow apply equally to the two aforementioned aspects of the invention.

It would be well understood by the skilled artisan that silica, also known as silicon dioxide (i.e., Si0 ₂ ), is a chemical compound that is generally ubiquitous in nature. To this end, it is well established that silica may be found in a variety of organic materials, such as sugar cane, rice and microalgae (e.g., diatoms).

The term "organic material" as used herein refers to silica-containing material obtained or obtainable from living organisms or component parts thereof, such as plants, animals and/or algae inclusive of microalgae and diatoms. Preferably, the organic material is or comprises plant material.

The term "plant material", as used herein, generally refers to material of plant origin. Plant material (e.g., plant biomass) can be derived from a single material or a combination of materials and/or can be non-modified and/or modified. Additionally, plant material can be transgenic (i.e., genetically modified). Plant material can be derived from living or previously living plant material. Plant material typically includes, for example, the fibres, pulp, stems, leaves, hulls, canes, husks, and/or cobs of plants or fibres, leaves, branches, bark, and/or wood of trees and/or bushes.

Examples of plant materials include, but are not limited to, agricultural biomass, e.g., farming and/or forestry material and/or residues, branches, bushes, canes, forests, grains, grasses, short rotation woody crops, herbaceous crops, and/or leaves; oil palm fibre waste such as empty fruit bunch and palm trunk; energy crops, e.g., corn, millet, and/or soybeans; energy crop residues; paper mill residues; sawmill residues; municipal paper waste; orchard prunings; Willow coppice and Mallee coppice; wood waste; wood chip, logging waste; forest thinning; short-rotation woody crops; bagasse, such as sugar cane bagasse and/or sorghum bagasse, duckweed; wheat straw; oat straw; barley straw; rye straw; flax straw; soy hulls; rice hulls; rice straw; tobacco; corn gluten feed; oat hulls; corn kernel; fibre from kernels; corn stover; corn stalks; corn cobs; corn husks; canola; miscanthus; energy cane; prairie grass; gamagrass; foxtail; sugar beet pulp; citrus fruit pulp; seed hulls; lawn clippings; cotton, seaweed; trees; shrubs; wheat; wheat straw; products and/or by-products from wet or dry milling of grains; yard waste; plant and/or tree waste products; herbaceous material and/or crops; forests; fruits; flowers; needles; logs; roots; saplings; shrubs; switch grasses; vegetables; fruit peels; vines; wheat midlings; oat hulls; hard and soft woods; or any combination thereof.

Of known plant materials, however, the rice plant is somewhat unique owing to the typically high concentrations of silica that it contains. By way of example, rice straw and hulls have been found to have silica concentrations of 130g/kg and 230g/kg respectively compared with concentrations of around 10 to about 50g/kg for wheat, oat and barley straw (Van Soest, 2006). In particular embodiments, the organic material has a silica concentration of about 50 g/kg to about 300 g/kg (e.g., about 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 250, 260, 270, 280, 290, 300 g/kg and any range therein).

As such, in a preferred embodiment, the organic material, preferably with a high silica concentration (i.e., preferably greater than about 50g/kg, more preferably greater than about 75g/kg and even more preferably greater than about lOOg/kg) is selected from the group consisting of rice hulls, rice straw, sugar cane bagasse, oil palm empty fruit bunch and any combination thereof. Rice hulls are the natural sheaths that form on rice grains during their growth. They are typically removed during the refining of rice and are a waste or a low-value by-product of the rice milling industry. Rice straw generally includes the stem, leaf sheathes, leaf blades and the remains of the panicle after harvesting.

For the present invention, the organic material may have been processed by a processor selected from the group consisting of a rice mill, a sugar cane factory, palm oil mill or any combination thereof.

In particular embodiments, the organic material may be ashed, pyrolysed or combusted prior to inclusion in the methods described herein. In alternative preferred embodiments, the organic material has not been ashed, pyrolysed or combusted prior to inclusion in the methods described herein. To this end, it will be apparent that such methods advantageously not only provide for silica extraction, but also lignin extraction and a valuable biomass-based by-product of, for example, a partially hydrolysed lignocellulosic material.

Suitably, the methods described herein further include the step of treating the organic material with an alkali, such as an alkaline solution or an aqueous alkaline solution.

As used herein, "treating" or "treatment" may refer to, for example, contacting, soaking, steam impregnating, spraying, suspending, immersing, saturating, dipping, wetting, rinsing, washing, submerging, percolating and/or any variation and/or combination thereof.

As would be readily understood by the skilled artisan, "alkali", as used herein, refers to various water-soluble compounds with a pH of greater than 7 that can be reacted with an acid to form a salt. By way of example, an alkali can include, but is not limited to, sodium hydroxide, potassium hydroxide, ammonium hydroxide, magnesium hydroxide and alkali metal salts such as, but not limited to, sodium carbonate and potassium carbonate.

Preferably, the alkali is selected from the group consisting of sodium hydroxide, potassium hydroxide, alkali metal salts and any combination thereof.

Even more preferably, the alkali is sodium hydroxide. For the method of the above aspects, the alkali solution is suitably present in an amount from about 500% to about 5000% by weight of the organic material or any range therein, such as, but not limited to, about 500% to about 4000%, about 1000% to about 3000%, about 1500% to about 2500%, of the organic material. In particular embodiments of the present invention, the alkali is present in an amount of about 500%, 600%, 700%, 800%, 900%, 1000%, 1100%, 1200%, 1300%, 1400%, 1500%, 1600%, 1700%, 1800%, 1900%, 2000%, 2100%, 2200%, 2300%, 2400%, 2500%, 2600%, 2700%, 2800%, 2900%, 3000%, 3100%, 3200%, 3300%, 3400%, 3500%, 3600%, 3700%, 3800%, 3900%, 4000%, 4100%, 4200%, 4300%, 4400%, 4500%, 4600%, 4700%, 4800%, 4900%, 5000% or any range therein, by weight of the organic material. In particularly preferred embodiments of the present invention, the alkali is present in an amount of about 1000% to about 3000% by weight of the organic material.

It would be appreciated that the alkali treatment may solubilise a portion of the silica present in the organic material treated thereby. For the present invention, treatment with the alkali preferably solubilises a portion of the silica present in the organic material. Accordingly, in some embodiments of the present invention, the method results in the removal of about 100% or less (e.g., about 100%, 95% 90%, 85%, 80%, 75%, 70%, 65%, 60%, 55%, 50%, 45%, 40%, 35%, 30%, 25%, 20%, etc.), or any range therein, of the silica in the organic material compared to the amount of silica present in the organic material prior to the treatment with the alkali.

With respect to step (i), the solids fraction may be separated from the liquid fraction by any means known to those skilled in the art. Methods of separating the solids fraction from the liquid fraction may include, but are not limited to, vacuum filtration, membrane filtration, sieve filtration, partial or coarse separation, or any combination thereof. Accordingly, the separating step can produce a liquid fraction that includes, for example, a filtrate or hydrolysate, and a solids fraction, that includes, for example, the treated organic material, residual alkali and/or by-products from the treatment process. In some embodiments of the present invention, water and/or a wash solution is added to the solids fraction before, during and/or after separation. A wash solution may be or comprise water, an acidic solution, an alkaline solution and/or an organic solvent, but without limitation thereto.

With regard to step (ii), the method suitably results in the precipitation of about 20% or more (e.g., about 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100%) or any range therein of the silica in the liquids fraction with the addition of the first acid.

Regarding step (iii), the method results in the separation or recovery of about 20% or more (e.g., about 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100%) or any range therein of the precipitated silica from the liquids fraction.

In one embodiment, the precipitated silica is at least partly separated from the liquids fraction by filtration and/or centrifugation. This may include any means of filtration (e.g., vacuum filtration, membrane filtration) and/or centrifiguation (e.g., microcentrifugation, high-speed centrifugation, ultracentrifugation) as are known in the art.

In particular embodiments, the method further comprises washing the precipitated silica after separation from the liquids fraction, to at least partly, remove any liquids fraction residue therefrom. In this regard, washing may be carried out with a wash solution and/or water. The precipitated silica may be washed with water and/or a wash solution one or more times, such as 2, 3, 4, or more times. After one or more water and/or wash solution washes, the precipitated silica can be separated from the water and/or wash solution via methods such as, but not limited to, filtration and centrifugation as are known in the art.

In view of steps (ii) and (iv), the skilled person would readily understand that the term "acid", as used herein, refers to various water-soluble compounds with a pH of less than 7 that can be reacted with an alkali to form a salt. Examples of acids can be monoprotic or polyprotic and can comprise one, two, three, or more acid functional groups. Examples of acids include, but are not limited to, mineral acids, Lewis acids, acidic metal salts, organic acids, solid acids, inorganic acids, or any combination thereof. Specific acids include, but are not limited to hydrochloric acid, sulfuric acid, phosphoric acid, hydrofluoric acid, hydrobromic acid, hydroiodic acid, nitric acid, formic acid, acetic acid, methanesulfonic acid, toluenesulfonic acid, boron trifluoride diethyletherate, scandium (III) trifluoromethanesulfonate, titanium (IV) isopropoxide, tin (IV) chloride, zinc (II) bromide, iron (II) chloride, iron (III) chloride, zinc (II) chloride, copper (I) chloride, copper (I) bromide, copper (II) chloride, copper (II) bromide, aluminum chloride, chromium (II) chloride, chromium (III) chloride, vanadium (III) chloride, molybdenum (III) chloride, palladium (II) chloride, platinum (II) chloride, platinum (IV) chloride, ruthenium (III) chloride, rhodium (III) chloride, zeolites, activated zeolites, or any combination thereof.

Preferably, the first and/or second acid are selected from the group consisting of sulphuric acid, hydrochloric acid, phosphoric acid, hydrofluoric acid, hydrobromic acid, nitric acid, acid metal salts and any combination thereof.

Even more preferably, the first and/or second acid is or comprises sulphuric acid and/or phosphoric acid.

In certain embodiments, the first acid is added in an amount effective to lower the pH of the liquids fraction of step (i) to a pH of about 8 to about 10, or any range therein, such as, but not limited to, about 8.25 to about 9.75 and about 8.5 to about 9.5. In particular embodiments of the present invention, the first acid is suitably added in an amount effective to lower the pH of the liquids fraction to a pH of about 8.0, 8.1, 8.2, 8.3, 8.4, 8.5, 8.6, 8.7, 8.8, 8.9, 9.0, 9.1, 9.2, 9.3, 9.4, 9.5, 9.6, 9.7, 9.8, 9.9, 10.0 or any range therein. Preferably the first acid is added in an amount effective to lower the pH of the liquids fraction of step (i) to a pH of about 9.

In particular embodiments, the second acid is added in an amount effective to lower the pH of the liquids fraction of step (iii) to a pH of about 2.5 or less, or any range therein, such as, but not limited to, about -1.0 to about 2.5 and about 0.0 to about 2.0. In particular embodiments of the present invention, the second acid is suitably added in an amount effective to lower the pH of the liquids fraction to a pH of about 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5 or any range therein. Preferably the second acid is added in an amount effective to lower the pH of the liquids fraction of step (iii) to a pH of about 2.

As would be readily understood by the skilled artisan, the alkali may also solubilise a portion of the lignin present in the organic material, particularly plant material. To this end, it would be appreciated that the term "lignin", refers to a complex polymer that is generally the principle noncarbohydrate constituent of plant material. The lignin in plant material is typically bound to cellulose fibres and functions to harden and/or strengthen plant cell walls. The term encompasses all available forms of lignin, either naturally occurring or formed from conventional processes (e.g., non-naturally occurring), such as pulping processes. Lignin may be removed from the organic material by hydrolysis of the chemical bonds that hold the various components (e.g., lignin, cellulose, hemicellulose, xylan, etc.) of the organic material together, such as by treatment with the alkali. Accordingly, in some embodiments of the present invention, the method results in the removal of about 80% or less (e.g., about 80%, 75%, 70%, 65%, 60%, 55%, 50%, 45%, 40%, 35%, 30%, 25%, 20%, etc.) or any range therein of the lignin in the organic material compared to the amount of lignin present in the organic material prior to the treatment with the method. In some embodiments, the method results in the precipitation of about 20% or more (e.g., about 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100%) or any range therein of the lignin in the liquids fraction with the addition of the second acid in step (iv). In some embodiments, the method results in the recovery of about 20% or more (e.g., about 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100%) or any range therein of the precipitated lignin in the liquids fraction.

In one embodiment, the precipitated lignin is at least partly separated or removed from the liquids fraction by filtration and/or centrifugation. This may include any means of filtration (e.g., vacuum filtration, membrane filtration) and/or centrifiguation (e.g., microcentrifugation, high-speed centrifugation, ultracentrifugation) as are known in the art.

Suitably, the method further includes the step of contacting the liquids fraction of step (v) with an ion-exchanger so as to facilitate at least partial substitution of a first cation of the liquids fraction with a second cation of the ion-exchanger, such that the liquids fraction is suitable as a liquid fertilizer.

As generally used herein, "ion-exchanger" refers to a substrate or structure, such as an ion-exchange membrane or ion-exchange resin, that participates in an ion- exchange reaction. Ion-exchangers typically include a high molecular compound or polymer having an acidic group (cation exchange group) such as a sulfonic acid or phosphonic acid group or a basic group (anion exchange group) such as an amino group or imidazoyl group.

It will be understood, that an ion-exchange reaction is generally an adsorption phenomenon where the mechanism of adsorption is electrostatic. In this regard, electrostatic forces act to hold ions to charged functional groups on the surface of the ion-exchanger. The adsorbed ions normally replace ions that are on the resin surface on a 1: 1 charge basis. In some cases there may also be more than electrostatic forces holding the ion to the functional group of the ion-exchanger. For example, for weakly acid functional groups, such as carboxylic groups, hydrogen bonding is partially responsible for attracting H ⁺ ions. Another example is the strong bonding between Ca and P0 ₄ when P0 ₄ is used for a functional group. A detailed discussion of ion exchange can be found in ION EXCHANGE by F. G. Helfferich (McGraw-Hill, New York, 1962), which is incorporated by reference herein.

The ion-exchanger of the present invention may be any of the various types known in the art, including natural (e.g., proteins, soils, lignin, coal, metal oxides, alumino silicates (zeolites) (NaOA1203.4Si02)) and synthetic (e.g., zeolite gels, polymeric resins (macroreticular, large pores)) ion-exchangers, albeit without limitation thereto. Preferably, the ion-exchanger is a low cross-linked strong acid ion- exchanger.

In one particular embodiment, the ion-exchanger is or comprises an ion- exchange resin. Ion-exchange resins typically include an insoluble matrix (or support structure) in the form of small beads fabricated from an organic polymer substrate. The material generally has a structure of pores on the surface that, upon chemical activation, can comprise exchange sites that trap and release ions.

Polymeric resins are typically made into 3-D networks by cross-linking hydrocarbon chains. The resulting resin is typically insoluble, inert and relatively rigid. Ionic functional groups can then be attached to this framework. Further, ion- exchange resins are generally manufactured by polymerizing neutral organic molecules such as sytrene (to form polystrene) and then cross-linked with divinyl benzene (DVB). Functional groups can then be added according to the intended use thereof. For example, the resin can be sulfonated by adding sulfuric acid.

Resins are normally classified based on the type of functional group they contain and their % of crosslinking. Illustrative examples of suitable ion-exchange resins include anion exchange resins, cation exchange resins, and mixed-mode chromatography resins, also sometimes called herein as mixed-mode ion exchange resins. Cationic exchange resins may be described as strongly acidic (i.e., functional groups are derived from strong acids e.g., R-S03H (sulfonic)) or weakly acidic (i.e., functional groups are derived from weak acids, e.g., R-COOH (carboxylic)). Similarly, anionic exchange resins may be described as strongly basic (e.g., functional groups derived from quaternary ammonia compounds, R-N-OH) or weakly basic (e.g., functional groups derived from primary and secondary amines, R-NH30H or R- R'-NH20H). The exchangeable ion form is generally one or more of Na ⁺ , H ⁺ , OH ^" , or CI ^" ions, depending on the type of ion-exchange resin

Polymer matrices of ion-exchange resins may include polystyrene, polystyrene and styrene copolymers, polyacrylate, aromatic substituted vinyl copolymers, polymethacrylate, phenol-formaldehyde, polyalkylamine, combinations thereof, and the like. In a preferred embodiment, the polymer matrix is or comprises sulfonated polystyrene.

Ion-exchange resins are generally designed, when placed in a particular solution, to reach an equilibrium state between one or more ions in solution (e.g., the first cation in the liquids fraction) and one or more ions on the resin (e.g., the second cation). From this equilibrium state, selectivity coefficients (i.e., equilibrium constants) can be defined based on the ratios of ions in solution versus ions on the resin. Effectively, these selectivity coefficients are a measurement of a resin' s preference for an ion. Normally, the greater the selectivity coefficient, the greater the preference for the ion.

The skilled artisan will appreciate that ion-exchangers, such as ion-exchange resins or membranes, are widely used for the purpose of purifying substances. For example, ion-exchangers may be used, for example, in water treatment, wastewater treatment, food production, drug separation and refining, hydrometallurgy, analyses, and catalysis applications.

In particular embodiments, the first cation is Na ⁺ and/or K ⁺ .

In certain embodiments, the second cation is NH ₄ ⁺ .

Preferably, the method of the above aspects further includes the step contacting the ion-exchanger with a regenerating agent comprising the first cation so as to facilitate at least partial substitution of the second cation of the ion exchanger by the first cation of the regenerating agent. To this end, it will be appreciated that after contacting the ion exchanger with the liquids fraction there may be incomplete substitution of the second cation on the ion exchanger thereby leaving some residual second cation thereon. By contacting the ion exchanger with the regenerating agents preferably removes, at least partly, this residual second cation therefrom to be replaced by the first cation.

The expression "regenerating" or "regeneration" as used herein refers to a process in which the ion-exchange capacity of a used ion-exchanger, such as an ion- exchange resin or membrane, is returned to a level whereby it is rendered suitable for use in subsequent ion-exchange processes. A chemical agent capable of regeneration is referred to as a "regenerating agent" herein. Non-limiting examples of the regenerating agent, include an sodium hydroxide solution usable for a Na ⁺ form ion- exchanger.

The ion-exchanger used in the method of the present invention preferably comprises anionic groups which provide suitable sites for the adsorption of the first cation from the liquids fraction. These anionic groups have associated cations (e.g., the second cation) which exchange, for example, with the first cation during the ion- exchange reaction. The regeneration process of the present invention involves the displacement, substitution or exchange of the second cation which remains adsorbed to the ion-exchanger after contacting with the liquids fraction by contacting the ion- exchanger with a regenerating agent preferably having a high concentration of the first cation. It is not necessary for all ion exchange sites to be regenerated for an ion- exchanger to be considered "regenerated" for the purposes of the present invention. It is sufficient that the regeneration process has occurred to an extent that the ion- exchanger is useful in subsequent ion exchange processes. Preferably, more than 80% of the ion exchange sites previously taken up by the second cation or other compounds or cations are regenerated, more preferably greater than 90% and most preferably greater than 98%.

In addition to exhaustion, ion-exchangers may suffer a deterioration in performance due to the adsorption thereto of organics, contaminating ions or other impurities. As noted above, the performance of an exhausted ion-exchanger can usually be recovered through reversible regeneration treatment thereof with an acid, an alkali or the like. In addition, such treatment may also be sufficient to remove such impurities. It will be apparent, however, that depending upon the selectivity coefficient of the contaminating ion/s, a more concentrated regenerating agent may be required in order to remove said contaminating ion/s than that used to regenerate the ion-exchanger. In one embodiment, the regenerating agent is or comprises ammonium hydroxide and/or sodium hydroxide.

Referring to the aforementioned aspects, the method suitably further includes the step of contacting the ion exchanger with a further agent comprising the second cation so as to facilitate at least partial substitution of the first cation of the ion- exchanger by the second cation of the further agent. In this regard, the present step may be considered to be a priming step for the ion-exchanger prior to contacting the liquids fraction therewith. In one particularly preferred embodiment, substitution of the second cation of the further agent with the first cation of the ion-exchanger results in production of the alkali for treating the organic material. In one particularly preferred embodiment, the further agent is or comprises ammonium hydroxide.

In view of the foregoing, it will be appreciated that the present methods of extracting silica and/or lignin preferably include the reversible and repeatable exchange of a first cation, such as Na ⁺ and/or K ⁺ , and a second cation, such as NH ₄ ⁺ , between the liquids fraction, the regenerating agent, the further agent and the ion- exchanger. Additionally, the skilled person will understand that ion exchange adsorption and regeneration are usually repeatable and/or cyclical, but not necessarily reversible. In this regard, the ion-exchange of Na ⁺ and NH4 ⁺ on low cross-linked strong acid ion-exchange resins can be uniquely very close to reversible.

It would be readily apparent that the liquids fraction may include a number of components, such as a nitrogen source (e.g., ammonia), a phosphorus source (e.g., phosphate) and/or a sulphur source (e.g., sulphate), that represent vital or important plant nutrients and as such form part of a liquid fertilizer. As generally used herein, the term "liquid fertilizer ^' " refers to an aqueous solution or suspension that may be added to soil, land or other plant growth medium in order to increase the fertility thereof. Of the basic nutrients that plants need for healthy development, most crops and soils generally require significant amounts of nitrogen. Accordingly, the liquid fertilizer preferably includes a source of nitrogen, such as urea, ammonium hydroxide, ammonium nitrate, ammonium sulphate, ammonium pyrophosphate, ammonium thiosulfate, ammonium chloride or combinations thereof.

In particularly preferred embodiments, the liquid fertilizer comprises ammonium sulphate and/or ammonium phosphate.

Suitably, the method of the aforementioned aspects further includes the step of treating the organic material with the alkali. In particular embodiments, one or a plurality of alkalis are used to treat the organic material. For example, 1, 2, 3, 4, 5, or more alkalis may be added in this step.

For the method of the above aspects, the alkali is suitably added in an amount effective to raise the pH of the liquids fraction of step (i) to a pH of about 8 to about 14 or any range therein, such as, but not limited to, about 8.5 to about 13.5 and about 9 to about 13 so as to treat the organic material. In particular embodiments of the present invention, the alkali is suitably added in an amount effective to raise the pH of the liquids fraction of step (i) to a pH of about 8.0, 8.1, 8.2, 8.3, 8.4, 8.5, 8.6, 8.7, 8.8, 8.9, 9.0, 9.1, 9.2, 9.3, 9.4, 9.5, 9.6, 9.7, 9.8, 9.9, 10.0, 10.1, 10.2, 10.3, 10.4, 10.5, 10.6, 10.7, 10.8, 10.9, 11.0, 11.1, 11.2, 11.3, 11.4, 11.5, 11.6, 11.7, 11.8, 11.9, 12.0, 12.1, 12.2, 12.3, 12.4, 12.5, 12.6, 12.7, 12.8, 12.9, 13.0, 13.1, 13.2, 13.3, 13.4, 13.5, 13.6, 13.7, 13.8, 13.9, 14.0 or any range therein.

Suitably, the organic material is treated at a temperature from about 70 to about 200°C, or any range therein, such as, but not limited to, about 130°C to about 190°C or about 140°C to about 180°C. In particular embodiments, treatment of the organic material is carried out at a temperature of about 70°C, 71°C, 72°C, 73°C, 74°C, 75°C, 76°C, 77°C, 78°C, 79°C, 80°C, 81°C, 82°C, 83°C, 84°C, 85°C, 86 °C, 87 °C, 88 °C, 89 °C, 90 °C, 91 °C, 92 °C, 93 °C, 94, 95 °C, 96 °C, 97 °C, 98 °C, 99 °C, 100 °C, 101 °C, 102 °C, 103 °C, 104 °C, 105 °C, 106 °C, 107 °C, 108 °C, 109 °C, 110 °C, 111 °C, 112 °C, 113 °C, 114 °C, 115 °C, 116 °C, 117 °C, 118 °C, 119 °C, 120°C, 121°C, 122°C, 123°C, 124°C, 125°C, 126°C, 127°C, 128°C, 129°C, 130°C, 131°C, 132°C, 133°C, 134°C, 135°C, 136°C, 137°C, 138°C, 139°C, 140°C, 141°C, 142°C, 143°C, 144°C, 145°C, 146°C, 147°C, 148°C, 149°C, 150°C, 151°C, 152°C, 153°C, 154°C, 155°C, 156°C, 157°C, 158°C, 159°C, 160°C, 161°C, 162°C, 163°C, 164°C, 165°C, 166°C, 167°C, 168°C, 169°C, 170°C, 171°C, 172°C, 173°C, 174°C, 175°C, 176°C, 177°C, 178°C, 179°C, 180°C, 181°C, 182°C, 183°C, 184°C, 185°C, 186°C, 187°C, 188°C, 189°C, 190°C, 191°C, 192°C, 193°C, 194°C, 195°C, 196°C, 197°C, 198°C, 199°C, 200 °C or any range therein.

In one preferred embodiment, the organic material is treated at a temperature of about 160°C to about 200°C. This temperature range is preferred if both silica and lignin are to be extracted from the organic material. More preferably, the organic material is treated at a temperature of about 160°C.

In an alternative embodiment, the organic material is treated at a temperature of about 70°C to about 120°C and more preferably between about 70°C to about 100°C. This treatment temperature range is preferred if only silica (with little to no lignin extraction or contamination) is required to be extracted from the organic material, as the present inventors have found that little to no lignin will be extracted if the present method is performed at such a temperature. More preferably, the organic material is treated at a temperature of about 90°C. It will be appreciated that treatment temperatures less than 100°C, typically allow for the present methods to be performed, at least in part, within an atmospheric pressure reactor or vessel having a generally lower capital cost rather than in a pressurised reactor or vessel that would be required for higher treatment temperatures.

Treatment of the organic material with the alkali is suitably performed or carried out for a period of time from about 30 to about 120 minutes or any range therein, such as, but not limited to, about 40 minutes to about 80 minutes, or about 50 minutes to about 70 minutes. In certain embodiments, treatment of the organic material with the alkali is carried out for a period of time of about 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90 minutes, or any range therein. In particularly preferred embodiments, treatment of the organic material with the alkali is carried out for a period of time of about 60 minutes.

In one embodiment, the step of treating the organic material with the alkali is carried out at a solids to liquids ratio of about 2% to about 20%, or any range therein, such as, but not limited to, about 3% to about 12%, or about 5% to about 10%. In certain embodiments, the step of treating the organic material with the alkali is carried out at a solids to liquids ratio of about 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10%, 10.5%, 11%, 11.5%, 12%, 12.5%, 13%, 13.5%, 14%, 14.5%, 15%, 16.5%, 17%, 17.5%, 18%, 18.5%, 19%, 19.5%, 20% or any range therein. In particularly preferred embodiments, the step of treating the organic material with the alkali is carried out at a solids to liquids ratio of about 2.5% to about 10%.

In one embodiment, the method of the above aspects further includes the step of pretreating the organic material with a further acid, a further alkali and/or a polyol. The further acid and the further alkali may be any as are known in the art, such as those hereinbefore described. The term "polyoF as used herein refers to an alcohol containing multiple hydroxyl groups. Examples of polyols of the present invention include, but are not limited to, 1,2-propanediol, 1,3-propanediol, glycerol, 2,3- butanediol, 1,3-butanediol, 2-methyl- 1,3-propanediol, 1,2-pentanediol, 1,3- pentanediol, 1,4-pentanediol, 1,5-pentanedial, 2,2-dimethyl- 1,3-propanediol, 2- methyl-l,4-butanediol, 2-methyl- 1,3-butanediol, 1,1,1-trimethylolethane, 3-methyl- 1,5-pentanediol, 1,1,1-trimethylolpropane, 1,7-heptanediol, 2-ethyl-l,6-hexanediol, 1,9-nonanediol, 1,11-undecanediol, diethylene glycol, triethylene glycol, oligoethylene glycol, 2,2'-thiodiglycol, diglycols or polyglycols prepared from 1,2- propylene oxide, propylene glycol, ethylene glycol, sorbitol, dibutylene glycol, tributylene glycol, tetrabutylene glycol, dihexylene ether glycol, trihexylene ether glycol, tetrahexylene ether glycol, 1,4-cyclohexanediol, 1,3-cyclohexanediol, or any combination thereof. Preferably, the polyol is selected from the group consisting of glycerol, ethylene glycol and any combinations thereof. Even more preferably, the polyol is glycerol.

In one particular embodiment, the organic material is pretreated with a further acid and/or a further alkali and then a polyol, such as described in WO2016/004482, which is incorporated by reference herein.

Suitably, the pretreatment step is carried out at a temperature from about 20°C to about 200°C (e.g., 20 °C, 25 °C, 30 °C, 35 °C, 40 °C, 45 °C, 50 °C, 55 °C, 60 °C, 65 °C, 70 °C, 75 °C, 80 °C, 85 °C, 90 °C, 95 °C, 100 °C, 105 °C, 110 °C, 115 °C, 120 °C, 125 °C, 130 °C, 135 °C, 140 °C, 145 °C, 150 °C, 155 °C, 160 °C, 165 °C, 170 °C, 175 °C, 180 °C, 185 °C, 190 °C, 195 °C, 200 °C or any range therein).

Suitably, the pretreatment step is carried out for a period of time from about 5 minutes to about 120 minutes (e.g., 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120 minutes or any range therein).

It would be apparent, that because of their high silica content, some organic materials, such as rice hulls and rice straw offer little nutritive value as components of an animal feed. Nonetheless, after treatment with the method of the present aspect the solids fraction of the organic material will typically have a reduced silica content and additionally typically be of improved digestibility thereby potentially increasing the nutritive value thereof. Accordingly, in particular embodiments, the solids fraction is suitable for use as an animal feed ingredient.

In certain embodiments, the solids fraction is suitable for use in producing a partially hydrolysed lignocellulosic material therefrom. Alternatively, if the organic material has been previously subjected to a pretreatment step, such as that previously described herein, the solids fraction suitably is or comprises a partially hydrolysed lignocellulosic material.

It would be appreciated that lignocellulosic material, such as that contained within the treated organic material, can be used, amongst other things, to produce biofuels (e.g., bioethanol) and biochemicals. For efficient biofuel production from lignocellulosic materials, the cellulose and/or hemicellulose components of lignocellulosic material need to be converted to monosaccharides (i.e., monosugars) that are capable of being fermented into ethanol, butanol or other fermentation products as are known in the art. Generally, the production of fermentable sugars from lignocellulosic material involves an initial chemical and/or physical pretreatment or hydrolysis step to disrupt the natural structure of the lignocellulosic material. In this regard, the treated organic material of the solids fraction, including any lignocellulosic material therein, may be suitable for and/or may have undergone during the pretreatment step, at least partial hydrolysis thereof by any such means or method known in the art.

Accordingly, in one particular embodiment, the partially hydro lysed lignocellulosic material produced or derived from the solids fraction is suitable for the production of a fermentable sugar, such as glucose, xylose, arabinose, galactose, mannose, rhamnose, sucrose and fructose, therefrom. This may be achieved by any means or method known in the art, such as by enzymatically hydrolysing the partially hydrolysed lignocellulosic material to produce the fermentable sugar. It will be appreciated that enzymatically hydrolysing the partially hydrolysed lignocellulosic material may be performed, at least in part, by contacting the partially hydrolysed lignocellulosic material with one or more enzymes, such as cellulases, ligninases, hemicellulases, xylanases, lipases, pectinases, amylases and proteinases, albeit without any limitation thereto.

The terms "lignocellulosic" or "lignocellulose", as used herein, refer to material comprising lignin and/or cellulose. Lignocellulosic material can also comprise hemicellulose, xylan, proteins, lipids, carbohydrates, such as starches and/or sugars, or any combination thereof.

By "hydrolysis" is meant the cleavage or breakage of the chemical bonds that hold the lignocellulosic material together. For instance, hydrolysis can include, but is not limited to, the breaking or cleaving of glycosidic bonds that link saccharides (i.e., sugars) together, and is also known as saccharification. Lignocellulosic material, in some embodiments, can comprise cellulose and/or hemicellulose. Cellulose is a glucan, which is a polysaccharide. Polysaccharides are polymeric compounds that are made up of repeating units of saccharides (e.g., monosaccharides or disaccharaides) that are linked together by glycosidic bonds. The repeating units of saccharides can be the same (i.e., homogenous) to result in a homopolysaccharide or can be different (i.e., heterogeneous) to result in a heteropoly saccharide. Cellulose can undergo hydrolysis to form cellodextrins (i.e., shorter polysaccharide units compared to the polysaccharide units before the hydrolysis reaction) and/or glucose (i.e. a monosaccharide). HemiceUulose is a heteropolysaccharide and can include polysaccharides, including, but not limited to, xylan, glucuronoxylan, arabinoxylan, glucomannan and xyloglucan. Hemicellulose can undergo hydrolysis to form shorter polysaccharide units, and/or monosaccharides, including, but not limited to, xylose, mannose, glucose, galactose, rhamnose, arabinose, or any combination thereof. "Partial hydrolysis" or "partially hydrolyses" and any grammatical variants thereof, as used herein, refer to the hydrolysis reaction cleaving or breaking less than 100% of the chemical bonds that hold the lignocellulosic material together.

In a related aspect, the invention provides a silica precipitate produced by the method hereinbefore described.

In a further aspect, the invention provides a lignin precipitate produced by the method hereinbefore described.

All computer programs, algorithms, patent and scientific literature referred to herein is incorporated herein by reference.

In order that the invention may be readily understood and put into practical effect, particular preferred embodiments will now be described by way of the following non-limiting examples.

EXAMPLE 1

The objective of Example 1 was to evaluate methods of silica extraction from rice husks and straw. Additionally, Example 1 assessed the application of a pretreatment process to rice husks, straw and mixtures with and without prior extraction of silica.

Materials and Methods

Rice husks and straw

The husks were used as supplied. The straw was subjected to comminution in a garden mulcher (in the absence of a suitable laboratory mill several alternatives were tested - while not optimal, the garden mulcher provided the best outcome). Due to limitations of the equipment and the bulk density of the material, the resulting amount of useable rice straw was lower than expected. The scale of each experiment using straw was adjusted accordingly to match the limited supply of processed straw. All other chemicals were reagent or analytical grade chemicals. Alkaline extraction of silica

Samples of rice husks and straw were extracted in aqueous 3 % mass NaOH and 10 % mass NH ₄ OH at a dry solid to liquid ratio of 1: 12 for husks and at a dry solid to liquid ratio of 1:40 for straw. The suspensions of straw and husks in alkaline solutions were heated to boiling point for 1 hour with stirring, left to stand overnight and then filtered (Whatman 1 filter paper). The solid residues were washed in water until the filtrate pH was 7.0 and dried at 60°C overnight. The liquors were subjected to a fractional precipitation process by stepwise addition of concentrated H ₂ SO ₄ .

In all liquors a precipitate formed between 10.0 and 8.0 pH. These precipitates were collected by centrifugation, washed in water and again centrifuged to produce a gel- like pellet (see Figure 1). Upon further addition of acid (to a pH of ca. 2.0) a precipitate formed which was likely to be mostly dissolved lignin with some silica (see Figure 2).

Pretreatment of rice husks, straw and mixtures with and without prior silica extraction

Subsamples of the dried & extracted husks and straw were combined in equal mass ratios to form mixtures, and similarly husks and straw that were not subjected to extraction were combined to form mixtures. By this means 9 materials were obtained, viz. husks only, straw only, and husk and straw mixtures that were extracted in NaOH or NH ₄ OH, or not extracted. To each of these 9 materials, a calculated mass of 10% H ₂ SO ₄ was administered via a spray to give a final acid loading of 1% acid to dry biomass (w/w). The acid impregnated husk was then air dried at room temperature.

Weighed amounts of the 9 materials were placed into 2 L stainless steel pressure reactors (bombs) with weighed amounts of glycerol/water mixtures or water only to give a liquids to solids ratio of 6: 1, and where glycerol was included, a glycerol to water ratio of 32:1 (taking into account the moisture content of materials). The bombs were then placed in the air bath reactor and heated from ambient (approx. 18°C) to 160°C over ca. 1 hr. The reactor was then maintained at 160°C+5°C for 1 hr before the bombs were removed and cooled in a water bath. Once cooled, samples were removed and separated into solid and liquid components using a sieve (mesh size ΙΟΟμιη). The liquid portion being collected for analysis, with sodium azide (1%) added as a preservative. The solid components were then washed to remove the solvent (glycerol and water) and solutes (primarily dissolved hemicellulose sugars) and collected for analysis. Due to the limited amount of straw processed in the garden mulcher, three of the planned runs of solely straw had to be excluded. One extra run of husk was included in place of straw, as seen in Table 1 below.

Table 1. Experimental design for pretreatment trials.

Silica analysis

Silica was analysed by two methods, viz., inductively coupled plasma optical emission spectrometry (ICP-OES), and wavelength dispersive X-ray fluorescence (WD-XRF). For ICP-OES, samples were dried at 65°C to constant mass, ground to particle size of <2mm, ashed at 600°C for 4h, fused with NaOH at 500°C, neutralised with aqueous HNO ₃ and subjected to ICP-OES analysis. For WD-XRF by fused glass method, samples were dried at 105°C, 12-22 lithium borate flux (4 g) was added, and the mixtures were heated to 750°C overnight to slowly oxidise the organic component. The oven dried mixtures were then heated to 1050°C in Pt/Au crucibles for 20 minutes to completely dissolve the sample then poured into a 32mm Pt/Au mould heated to a similar temperature. The melt was cooled rapidly over a compressed air stream and the resulting glass disks were analysed by WD-XRF using a silicates calibration program (note: there was not enough extracted straw material to analyse by this method). For WD-XRF by pressed powder method, samples were dried (85°C), finely ground for 5 minutes in a chromium plated steel rotary mill, and pressed in a 32mm die at 12 tonnes pressure and the resulting pellets were analysed by WD-XRF using a biomass calibration program.

Biomass analysis

Samples of starting materials, digested materials and liquors were analysed by standard methods of biomass compositional analyses developed and published by the National Renewable Energy Laboratory

(http://www.nrel.gov/biomass/analytical_procedures.html).

Results and discussion

The compositions of starting materials are shown in Table 2. The results of husk and straw materials are consistent with literature values. In general terms the NaOH and NH ₄ OH extractions removed some ash components, some lignin and likely some hemicellulose. The NaOH extraction solubilised substantially more of these biomass components, and in the case of extraction of straw effected a complete loss of plant architecture (see Figure 3). Husks were softened to some extent by both treatments, but the husk structure was still intact.

Table 2. Composition of starting materials.

The mixtures of husks and straw were obtained by combining equal masses (as is) of husks and straw of common treatments (i.e. not extracted, NaOH extracted and NH ₄ OH extracted). The compositions of these mixtures were calculated from starting materials (i.e. moisture weighted averages) and are shown in Table 3 below.

Table 3. Calculated compositions of mixed materials.

(with the exception of moisture, all results are % mass dry basis) Silica extraction

The results of silica analyses on husks, straw and extracted materials are shown in Tables 4 to 6. The analysis of silicon/silica (as Si or S1O ₂ ) in biomass materials is typically complex and prone to errors. To address this issue silicon/silica was analysed by several methods. Contrary to the claim of Van Soest (2006), silica is soluble in aqueous NH ₄ OH, albeit the results confirm that silica is more soluble in aqueous NaOH. It should be noted that lignin and hemicellulose are also very soluble in hot aqueous NaOH, and the extraction of these components in the process targeting silica production will reduce yields of these components in the pretreatment process and may impact on the purity of the silica precipitate obtained by pH adjustment. Consequently, on this basis alone NH ₄ OH extraction may be the preferred approach.

In these experiments the extraction efficiencies of the alkaline solutions were quantified by analysis of the solid materials before and after extraction. The extractions have not been optimised for silica yield or purity. The low extraction yield from husks relative to straw is likely due to differences in plant architecture, and rather than due to low solubility of silica in aqueous ammonia. Preparation of the plant material prior to extraction (e.g. hammer milling) will likely result in higher extraction efficiencies. Extraction efficiencies in the order of 70% would seem to be a reasonable target for an optimised process.

The precipitation of silica was qualitatively assessed only, i.e. acid was added until a precipitate was observed and a sample of the precipitate was obtained by centrifugation. Analysis of a precipitate showed Si0 ₂ contents of 85%. Other elements (e.g. Na, Fe and S as oxides) amount to less than 1% of the mass. The remaining impurities are organic (presumably lignin derived).

Table 4. ICP-OES analysis of extracted materials.

Table 5. WD-XRF (fused glass) analysis of extracted materials.

Table 6. WD-XRF (pressed powder) analysis of extracted materials.

Pretreatment digestion trials

Subsamples of the husks, straw and extracted materials were subjected to the pretreatment process as described above. The detailed analyses of the digested materials are shown in Tables 7 and 8 below. Generally, the materials behaved as expected. Up to 77% of the hemicellulose was dissolved by digestion, while cellulose and lignin was not dissolved. Again, this outcome is not optimised; it should be possible to dissolve substantially more of the hemicellulose under optimal conditions. In our experience this material is no different to other agricultural residues (e.g. sugarcane bagasse, wheat straw and corn stover).

The only unusual results were the observations that plant architecture of the rice husks was preserved during the digestion and that some glycerol remained in the biomass after washing. Husks were softened by digestion but were still essentially intact. The preservation of the husk plant architecture may also provide an explanation for the difficulty in removing glycerol by washing. While glycerol was easily washed from the outer surfaces of the husks, the pore structures may have held onto some glycerol. It is probable some milling of the husks prior to acid impregnation (either before or after silica extraction) may result in an improved outcome from pretreatment digestion.

At pilot scale the pretreatment process is typically run at a liquid to solid ratio of ca. 3 and a glycerol to biomass (dry basis) ratio of 1.0 or less. Free water (water not bound to the biomass) is reduced to limit detrimental hydrolytic reactions that lead to product decomposition and formation of compounds known to inhibit fermentation processes (an undesirable outcome for a sugar syrup feedstock to a fermentation process). The results of experiments described herein are only indicative of the pretreatment process. Under optimised conditions at pilot scale, we would expect to obtain results that are typical of other agricultural residues that we have processed at pilot.

Table 7 - Analysis of pretreated rice husks and straw materials (solids fraction) with and without prior silica extraction.

Table 8 - Analysis of pretreatment liquids fraction with and without prior silica extraction

Conclusion

Our silica extraction results indicate that a simple process based on aqueous ammonia extraction and precipitation with sulphuric acid, and resulting in little or no effluent/waste treatment issues may be feasible. This process of silica extraction produces a silica product, aqueous ammonium sulphate fertilizer and a husk residue, which may further be used as an improved animal feed ingredient or a suitable feedstock to a pretreatment process.

EXAMPLE 2

In Example 1 it is noted the most commonly cited methods for extraction of silica from biomass use aqueous alkaline solutions. While literature indicated that aqueous ammonia solution is not suitable for silica extraction, the observations in Example 1 indicated that it is possible to extract silica from rice-based biomass with aqueous ammonia and that the silica product was >85% amorphous silica. The use of ammonia for this purpose has some distinct advantages over other alkaline solvents, viz. precipitation of silica with sulphuric acid produces a liquid fertilizer co-product and literature indicates the residual extracted material is an animal feed of superior quality when compared to the starting material. A preliminary assessment of the economics of this method with animal feed as a coproduct indicated the economic viability of the process is strongly linked to the solubility of silica in aqueous ammonia.

The preliminary work in Example 1 did not provide an optimised process for the extraction of silica (i.e. the 60% extraction yield of 85% pure silica was not optimised for yield or purity). It was speculated that while extraction yields were higher with aqueous sodium hydroxide than with aqueous ammonia as extraction solvent, the low extraction yield from husks relative to straw was likely due to differences in plant architecture, and not due to low solubility of silica in aqueous ammonia. As such, mechanical preparation of the plant material prior to extraction (e.g. hammer milling) should result in higher extraction efficiencies and extraction efficiencies in the order of 70% seemed to be a reasonable target for an optimised process.

Example 2 has been conducted in an air-bath reactor with pressure vessels that allows the method of Example 1 to be performed at higher extraction temperatures. The work focussed on ground rice husks as feedstock. Hammer milling, extraction at higher temperatures, and increasing the ammonia concentration did not increase the silica extraction yields. Yield is, in fact, limited by the solubility of silica in aqueous ammonia. By comparison, extractions with aqueous sodium hydroxide at 160 °C have yields approaching 100%.

To overcome this, we have developed a process wherein an ion-exchange process (e.g., Novasep's I-SEP process) is coupled to silica extraction to effect a high yielding NaOH extraction but still obtain ammonium sulphate or phosphate as a co- product (liquid fertilizer). A schematic of this process is shown in Figure 4. The use of the ion-exchanger avoids the production of sodium salt waste and allows for production of a liquid fertilizer product (e.g., ammonium sulphate and/or ammonium phosphate).

The embodiment of the process outlined in Figure 4 employs a single ion- exchange unit operation that cycles between Na ⁺ and NH ₄ ⁺ ionic forms. The affinity for NH4+ (1.44) of a low cross-linked strong acid exchanger (e.g., DOWEX™ Ion Exchange Resins by Dow) is only a bit more than for Na+ (e.g., 1.2), whilst K+ is, for example, 1.72 (see Table 9) . The selectivity coefficient for exchanging Na+ with resin bound NH4+ is 0.833 and in the other direction it's 1.2. By our calculations this means that you can exchange 98.8% of the resin with Na+ and 99.2% of the resin with NH4+ before exhaustion, it's nearly reversible and only needs a small excess of sodium to make the process reversible (e.g., only about 1% more), which can be washed off the bed or could go to the phosphate/sulphate product. Accordingly, use of a counterflow regenerated fixed-bed ion exchange column may be suitable to substantially or completely avoid leakage.

Contaminating or impurity cations as a result of the method in Figure 4 will likely build up on the ion-exchanger as they all have selectivity coefficients greater than K+ (e.g., for Ca2+ it's 3.14) - so the NH4+/Na+ cycles will become shorter over successive runs. As a result, the ion-exchanger may require a rejuvenation step periodically, such as washing or contacting the ion-exchanger with concentrated NaOH or NH ₄ OH to thereby remove any contaminating cations.

It has also been observed that pretreatment of rice husks, such as by treatment with an acid, alkali and/or a polyol (e.g., glycerol; such as that described in WO2016/004482, which is incorporated by reference herein) prior to silica extraction, which removes extractives (including salts other than silica) and hemicellulose, has the benefit of producing a higher purity silica product (see Figure 5).

Throughout the specification, the aim has been to describe the preferred embodiments of the invention without limiting the invention to any one embodiment or specific collection of features. Various changes and modifications may be made to the embodiments described and illustrated without departing from the present invention.

The disclosure of each patent and scientific document, computer program and algorithm referred to in this specification is incorporated by reference in its entirety. Table 9. Selectivity coefficients of various cations (compared with the hydrogen ion) on DOWEX™ sulfonated polystyrene cation exchange resins of different crosslinkage

EXAMPLE 3

The objective of Example 3 was to determine the relative solubilities of silica (in the absence of the plant matrix) in aqueous ammonia, sodium hydroxide and potassium hydroxide. Rice husk (300 g) was milled and then heated in a muffle furnace (600 °C for 4 hrs) to produce rice husk ash. The ash was then subjected to a two-step extraction. This involved an acid extraction to remove impurities followed by a water wash and then extraction with alkali at 80 °C, 100 °C and 170 °C at a liquid:solid ratio of 10:1 for lhr.. Sodium hydroxide, ammonia and potassium hydroxide solutions at varying concentrations were tested. The acid washed ash completely dissolved in both sodium hydroxide and potassium hydroxide, while an insoluble residue remained after ammonia extraction. The extracts were filtered and acidified, and the mass of the dried silica precipitates were determined. Yields for silica extraction with sodium hydroxide ranged from 72 % to 94 %. Yields from potassium hydroxide extraction were similar to those of sodium hydroxide extraction. Yields of ammonia extraction ranged between 35 and 45 %.

The results of testing in Example 3 indicated that 0.5 N sodium hydroxide at 80 °C was an effective extraction solvent of silica, while 15 % ammonia extractions over all tested temperatures were less effective and probably not commercially viable.

Table 10. Silica yields from ash extraction with aqueous NaOH, KOH and NH ₄ OH solutions at 80 °C, 100 °C and 170 °C.

EXAMPLE 4

The objective of Example 4 was to demonstrate the ion-exchange of sodium and ammonium cations on a strong acid ion-exchange resin is effectively reversible. A strong cation exchange resin with low cross-linking (Mitsubishi UBK04H) was utilised for this experiment. As the resin was provided in hydrogen form, it was necessary to first convert to sodium form. The resin (ca. 100 ml) was added to a solution of 0.5 M NaOH (500 ml), and allowed to sit with occasional stirring for approx. 20mins. The resin (30 ml) was transferred to a column which was connected by silicon tubing (ID 1.5, OD 4mm) to a pump on the inlet side and a fraction collector on the outlet side of the column. The column containing the sodium form resin was washed with water to remove excess NaOH (until the pH of the eluate was neutral).

An ammonia- salicylate colourimetric assay (Abs 687 nm) was used to measure ammonia in eluates.

A solution of 1M NH ₄ OH solution was prepared, applied to the ion-exchange column (pumped at ca. 2 ml/min), and the eluate was collected and analysed for ammonia. The exchange of column bound sodium with ammonium in solution is straight forward as ammonia has a higher affinity for the resin than sodium. The calculated volume of ammonia solution to completely exchange the sodium cations bound to the column was 39 ml. Ammonia appeared in the elute (break through) after 43 ml of the solution was applied to the column. After ammonia break through, the column was washed with water until the pH was near neutral.

The column (now in the ammonia form) was eluted with 0.5 M NaOH (60 ml), and the ammonia concentration in the eluate was measured. A second column (prepared in an identical manner) was eluted with 0.157 M sodium sulphate (60 ml). The elution profiles (the ammonia concentration in the eluate and % complete exchange) are shown for both exchange experiments in Figure 6.

The exchange process described here is very nearly reversible, and has utility as described in the body of this document (i.e. it can be used to cycle from ammonia to sodium bound forms, and facilitate the described extraction).

It is understood that sodium sulphate solutions obtained by acidification (with sulphuric acid) of sodium hydroxide extracts of silica rich biomass will contain other cationic species (e.g. potassium, calcium and iron) that unlike silica remain in solution after acid addition. Furthermore, these cations all have higher affinity for strong cation exchange resins than both sodium and ammonium. These cations will build up on the resin column over several cycles of use, and consequently the sodium- ammonium exchange efficiency will reduce over successive cycles of use. This problem can be easily overcome by periodic washing of the column with a more concentrated sodium hydroxide solution.