Method of inhibiting metal leaching from incineration bottom ash

Abstract

The invention provides a method of inhibiting metal leaching from incineration bottom ash (IBA), the method comprising providing a carbonated IBA, and subjecting the carbonated IBA to one or more solidification reactions comprising adding a binder to the carbonated IBA. The carbonated IBA may be provided by a process comprising mixing an aqueous solution comprising a carbonation agent with the IBA for a period of time to obtain a carbonated IBA mixture and filtering the carbonated IBA mixture to obtain the carbonated IBA and a filtrate. The invention also provides a method of regenerating a carbonation agent, the method comprising providing the filtrate and subjecting the filtrate to carbon dioxide to obtain the regenerated carbonation agent.

Description

[0001] METHOD OF INHIBITING METAL LEACHING FROM INCINERATION BOTTOM ASH

[0002] FIELD OF INVENTION

[0003] The present invention provides methods of inhibiting metal leaching from incineration bottom ash, more particularly, methods of inhibiting metal leaching from incineration bottom ash, the methods comprising carbonation and solidification reactions.

[0004] BACKGROUND

[0005] The listing or discussion of a prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.

[0006] Incineration bottom ash (IBA) is a residual waste material generated from the incineration of municipal and industrial solid waste. While waste incineration offers a viable means of waste reduction and energy recovery, the management of IBA presents a complex and multifaceted challenge, spanning environmental concerns, disposal challenges, and opportunities for resource utilization. As societies worldwide grapple with increasing waste generation and the imperative to adopt more sustainable waste management practices, understanding and addressing the intricacies of IBA is paramount.

[0007] In addition to environmental concerns, the management of IBA entails economic and logistical considerations. Disposing of IBA in landfills can incur substantial costs, and its sheer volume further strains limited disposal capacities (The only Singapore landfill Semakau island is expected to be fully filled by 2035 at the current disposal rate of waste like IBA). However, within this challenge lies an opportunity — the potential recovery and reuse of valuable materials contained within IBA. These materials may include metals, aggregates, and minerals that can find applications in various industries, contributing to a circular economy and reducing the environmental footprint of waste disposal.

[0008] The management of IBA extends beyond conventional waste disposal. It intersects with environmental regulations, sustainability goals, and resource recovery strategies. Notably, the leaching of heavy metals from IBA into the environment is a pressing concern, especially given the stringent standards set by governments, such as the Singaporean authorities. Thus, there is a need for alternative and / or improved methods of inhibiting metal leaching from IBA for solving at least the above-mentioned problems.

[0009] SUMMARY

[0010] Aspects and embodiments of the current invention will now be described by reference to the following numbered clauses.

[0011] 1. A method of inhibiting metal leaching from incineration bottom ash (IBA), the method comprising:

[0012] (i) providing a carbonated IBA; and

[0013] (ii) subjecting the carbonated IBA to one or more solidification reactions comprising adding a binder to the carbonated IBA.

[0014] 2. The method according to Clause 1 , wherein the carbonated IBA is provided by contacting an IBA with an aqueous solution comprising a carbonation agent.

[0015] 3. The method according to Clause 2, wherein the carbonated IBA is provided by a process comprising:

[0016] (ia) mixing the aqueous solution comprising the carbonation agent with the IBA for a period of time to obtain a carbonated IBA mixture; and

[0017] (ib) filtering the carbonated IBA mixture to obtain the carbonated IBA and a filtrate.

[0018] 4. The method according to Clause 3, wherein the period of time in step (ia) is from 15 min to 48 hours, such as from 20 min to 5 hours, optionally wherein the period of time is from 30 min to 2 hours.

[0019] 5. The method according to any one of Clause 3 or Clause 4, wherein step (ia) is conducted at a temperature of from 10°C to 40°C (such as 20°C) and at a pressure of about 105Pa (1 bar).

[0020] 6. The method according to any one of Clauses 2 to 5, wherein the carbonation agent is one or both of a bicarbonate salt and a carbonate salt, optionally wherein: when the carbonation agent is a bicarbonate salt, the bicarbonate salt comprises a cation selected from a group consisting of a Group I metal ion (e.g., Na+and K+), a Group II metal ion (Mg2+and Ca2+), a quaternary ammonium ion, a quaternary phosphonium ion, an imidazolium ion and a phosphazenium ion; and when the carbonation agent is a carbonate salt, the carbonate salt comprises a cation selected from a group consisting of a Group I metal ion (e.g., Na+and K+), a quaternary ammonium ion, a quaternary phosphonium ion, an imidazolium ion and a phosphazenium ion.

[0021] 7. The method according to Clause 6, wherein the carbonation agent is sodium bicarbonate.

[0022] 8. The method according to any one of Clauses 2 to 7, wherein the carbonation agent is provided in an amount of from 5% w / w to 200% w / w, such as from 10% w / w to 150% w / w, such as from 25% w / w to 100% w / w, relative to the weight of the IBA, optionally wherein the carbonation agent is provided in an amount of about 50% w / w relative to the IBA.

[0023] 9. The method according to any one of Clauses 3 to 8, wherein the method further comprises a step:

[0024] (iA) contacting the carbonated IBA with an aqueous solution comprising a complexing agent before conducting step (ib).

[0025] 10. The method according to Clause 9, wherein one or both of the following applies:

[0026] (a) the contacting of the carbonated IBA with the complexing agent in step (iA) is from 15 min to 24 hours, such as from 20 min to 2.5 hours, optionally wherein the period of time is from 30 min to 1 hour; and

[0027] (b) step (iA) is conducted at a temperature of from 10°C to 40°C (such as 20°C) and at a pressure of about 105Pa (1 bar).

[0028] 11 . The method according to any one of Clauses 9 to 10, wherein the complexing agent forms a complex with one or more metals present in the carbonated IBA mixture.

[0029] 12. The method according to any one of Clauses 9 to 11 , wherein the complexing agent is one or more selected from a group consisting of sodium dimethyldithiocarbamate, sodium sulfide and more particularly, sodium diethyldithiocarbamate.

[0030] 13. The method according to Clause 12, wherein the complexing agent is sodium diethyldithiocarbamate. 14. The method according to any one of Clauses 9 to 13, wherein the complexing agent is provided in an amount of from 0.05% w / w to 5% w / w relative to the weight of the IBA, optionally wherein the complexing agent is provided in an amount of about 0.1% w / w relative to the weight of the IBA.

[0031] 15. The method according to any one of Clauses 3 to 14, wherein following an initial reaction cycle, when the pH of the filtrate in step (ib) is from 7 to 11 , such as from 7 to <11 , such as from 7 to 10, then the filtrate is reused as the aqueous solution comprising the carbonation agent in step (ia).

[0032] 16. The method according to any one of the preceding clauses, wherein step (ii) comprises:

[0033] (a) subjecting the carbonated IBA to a first solidification reaction with a first binder and a first solvent to provide first solidified particles;

[0034] (b) washing the first solidified particles with a solvent (e.g. water) to provide washed first solidified particles; and

[0035] (c) subjecting the washed first solidified particles to a second solidification reaction with a second binder and a second solvent to provide second solidified particles.

[0036] 17. The method according to Clause 16, wherein one or more of the following applies:

[0037] (a) the first solidification reaction comprises mixing the first binder, the first solvent and the carbonated IBA to obtain a mixture and curing the mixture for from 12 hours to 36 hours, such as 24 hours to provide the first solidified particles;

[0038] (b) the washing step of the first solidified particles with the solvent is at a liquid-to-solid ratio (L / S) of from 2 to 3, such as 2.5 and is from 2 to 6 hours, such as about 4 hours to provide the washed first solidified particles; and

[0039] (c) the second solidification reaction comprises mixing the second binder, the second solvent and the washed first solidified particles to obtain a mixture and curing the mixture for at least 5 days, such as at least 7 days to provide the second solidified particles.

[0040] 18. The method according to any one of the preceding clauses, wherein the first binder and the second binder are independently selected from one or more selected from a group consisting of water glass and, more particularly, Ordinary Portland Cement (OPC), fast drying cement, and white cement. 19. The method according to Clause 18, wherein the first binder is fast drying cement and the second binder is white cement or wherein the first binder and the second binder are white cement.

[0041] 20. The method according to any one of the preceding claims, wherein the first and second binders are provided in an amount of from 10% w / w to 75% w / w, such as 15% w / w to 50% w / w, such as 20% w / w to 35% w / w, relative to the weight of the IBA, optionally wherein the first and second binder are provided in an amount of about 15% w / w relative to the weight of the IBA, or optionally wherein the first binder is provided in an amount of about 15% w / w relative to the weight of the IBA and the second binder is provided in an amount of about 35% w / w relative to the weight of the IBA, or optionally wherein the first binder is provided in an amount of about 20% w / w relative to the weight of the IBA and the second binder is provided in an amount of about 50% w / w relative to the weight of the IBA.

[0042] 21 . A method of regenerating a carbonation agent, the method comprising:

[0043] (A) providing a filtrate obtained from step (ib) in any one of Clauses 3 to 20;

[0044] (B) subjecting the filtrate to carbon dioxide to obtain the regenerated carbonation agent.

[0045] 22. The method according to Clause 21 , wherein the pH of the filtrate is more than about pH 11 , optionally wherein the filtrate is substantially free of one or both of carbonate or bicarbonate ions.

[0046] 23. The method according to Clause 21 or Clause 22, wherein step (B) comprises either: (Bi) subjecting the filtrate to carbon dioxide at a pressure of at least 2 bar for a period of time; or

[0047] (Bii) subjecting the filtrate to a countercurrent flow with a flue gas comprising carbon dioxide in an absorption tower.

[0048] 24. The method according to Clause 23, wherein one or more of the following applies:

[0049] (a) the pressure in step (Bi) is about 2 bar;

[0050] (b) the period of time in step (Bi) is from 15 min to 24 hours, such as from 20 min to 2.5 hours, such as from 30 min to 1 hour;

[0051] (c) step (Bi) is conducted at a temperature of from 10°C to 40°C (such as 20°C);

[0052] (d) the concentration of carbon dioxide in the flue gas in step (Bii) is from 3% (v / v) to 100% (v / v), such as 10% (v / v);

[0053] (e) step (Bii) is conducted at a temperature of from 10°C to 70°C (such as 50°C); (f) the flow rate of filtrate in step (Bii) is from 10 mL / min to 100 mL / min from a top end of the absorption tower;

[0054] (g) the flow rate of flue gas in step (Bii) is from 2 L / min to 100 L / min from a bottom end of the absorption tower.

[0055] 25. The method according to any one of Clauses 21 to 24, wherein the method further comprises step (Ai) subjecting the filtrate to a treatment step for removing one or more organic species and / or halogen before conducting step (B).

[0056] 26. The method according to Clause 25, wherein step (Ai) comprises subjecting the filtrate to one or both of activated carbon or hydrotalcite, optionally wherein step (Ai) comprises subjecting the filtrate to activated carbon.

[0057] BRIEF DESCRIPTION OF DRAWINGS

[0058] FIG. 1 is a flowchart of an integrated method for inhibiting heavy metal leaching from IBA via carbonation and solidification with cement according to an embodiment of the present invention.

[0059] FIG. 2 depicts the sequestrated CO2in carbonated IBA based on TGA analysis according to Example 2 of the present disclosure: weight loss rate vs reaction time. Reaction conditions: 5 g of IBA, 2.18 g of sodium bicarbonate in 25 mL of water, room temperature.

[0060] DESCRIPTION

[0061] The present inventors have surprisingly found that integrating carbonation and solidification techniques with cement effectively inhibits metal leaching from Incinerated Bottom Ash (IBA) which may be used for further applications, such as construction. The aqueous medium involved in the carbonation reaction can be reused (e.g., up to 5 reaction cycles with IBA) and can be regenerated with ease by subjecting the used aqueous medium to carbon dioxide. The carbonation reaction also results in the permanent sequestration of CO2 within the carbonated IBA (e.g., approximately 10% by weight), contributing to carbon capture efforts and reducing greenhouse gas emissions. The present method is easily scalable and does not require any specialized equipment or harsh reaction conditions.

[0062] Thus, in a first aspect of the invention, there is provided a method of inhibiting metal leaching from incineration bottom ash (IBA), the method comprising: (i) providing a carbonated I BA; and

[0063] (ii) subjecting the carbonated IBA to one or more solidification reactions comprising adding a binder to the carbonated IBA.

[0064] In embodiments herein, the word “comprising” may be interpreted as requiring the features mentioned, but not limiting the presence of other features. Alternatively, the word “comprising” may also relate to the situation where only the components / features listed are intended to be present (e.g. the word “comprising” may be replaced by the phrases “consists of” or “consists essentially of”). It is explicitly contemplated that both the broader and narrower interpretations can be applied to all aspects and embodiments of the present invention. In other words, the word “comprising” and synonyms thereof may be replaced by the phrase “consisting of” or the phrase “consists essentially of’ or synonyms thereof and vice versa.

[0065] The phrase, “consists essentially of” and its pseudonyms may be interpreted herein to refer to a material where minor impurities may be present. For example, the material may be greater than or equal to 90% pure, such as greater than 95% pure, such as greater than 97% pure, such as greater than 99% pure, such as greater than 99.9% pure, such as greater than 99.99% pure, such as greater than 99.999% pure, such as 100% pure.

[0066] As used herein, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a compound” includes mixtures of two or more such compounds, reference to “a composition” includes mixtures of two or more such compositions, and the like.

[0067] As used herein, the term “incineration bottom ash (IBA)” includes residual waste material generated from the incineration of municipal and / or industrial solid waste. The term “carbonated IBA” refers to IBA that has been subjected to a carbonation reaction, which is well known to a skilled person in the art and refers to one where one or more metal oxide or metal hydroxide present in the IBA (e.g., CaO) is converted to a carbonate or bicarbonate (e.g., CaCOs). Advantageously, the carbonation of IBA reduces heavy metal leaching by binding the heavy metal in the IBA within the carbonate form / phase. Further advantageously, the carbonation reaction results in the permanent sequestration of CO2 within the carbonated IBA (approximately 10% by weight), contributing to carbon capture efforts and reducing greenhouse gas emissions In certain embodiments, the carbonated IBA may be provided by contacting an IBA with an aqueous solution comprising a carbonation agent. In certain particular embodiments, the carbonated IBA may be provided by a process comprising (ia) mixing the aqueous solution comprising the carbonation agent with the IBA for a period of time to obtain a carbonated IBA mixture and (ib) filtering the carbonated IBA mixture to obtain the carbonated IBA and a filtrate.

[0068] Step (ia) may be conducted for any suitable period of time. As demonstrated in the Examples, carbonation of IBA may take place rapidly (e.g., 10 min). As such, in certain embodiments, wherein the period of time in step (ia) may be from 15 min to 48 hours, such as from 20 min to 5 hours. In certain exemplary embodiments, the period of time may be from 30 min to 2 hours. As will be appreciated, the carbonated IBA may be contacted with other reagents (e.g., a complexing reagent) during the period of time before conducting step (ib).

[0069] Step (ia) may be conducted at any suitable temperature and pressure. In certain embodiments, step (ia) may be conducted at a temperature of from 10°C to 40°C (such as 20°C) and at a pressure of about 105Pa (1 bar). Advantageously, the present invention does not require high temperature and pressure.

[0070] Any suitable carbonation agent that is capable of reacting with the IBA to obtain the carbonated IBA may be used provided that it does not interfere with the present method. Without wishing to be bound by theory, any soluble bicarbonate or carbonate may be used. In certain embodiments, the carbonation agent may be one or both of a bicarbonate salt and a carbonate salt. In certain particular embodiments, when the carbonation agent is a bicarbonate salt, the bicarbonate salt may comprise a cation selected from a group consisting of a Group I metal ion (e.g., Na+and K+), a Group II metal ion (Mg2+and Ca2+), a quaternary ammonium ion, a quaternary phosphonium ion, an imidazolium ion and a phosphazenium ion. In other certain particular embodiments, when the carbonation agent is a carbonate salt, the carbonate salt may comprise a cation selected from a group consisting of a Group I metal ion (e.g., Na+and K+), a quaternary ammonium ion, a quaternary phosphonium ion, an imidazolium ion and a phosphazenium ion. In certain exemplary embodiments, the carbonation agent may be sodium bicarbonate.

[0071] Any suitable amount of the carbonation agent may be used. The carbonation agent may be provided in an amount of from 5% w / w to 200% w / w, such as from 10% w / w to 150% w / w, such as 25% w / w to 100% w / w, relative to the weight of the IBA. In certain exemplary embodiments, the carbonation agent may be provided in an amount of about 50% w / w relative to the IBA. As will be appreciated, the amount of carbonation agent provided will determine the number of reaction cycles that the aqueous solution comprising the carbonation agent may be reused. For example, when the carbonation agent is provided in an amount of 50% w / w carbonation agent relative to the weight of the IBA, the aqueous solution comprising the carbonation agent may be reused for about 5 reaction cycles. In yet another example, when the carbonation agent is provided in an amount of 10% w / w carbonation agent relative to the weight of the IBA, the aqueous solution comprising the carbonation agent may be used for 1 reaction cycle before it has to be regenerated again.

[0072] In certain embodiments, the method may further comprise a step (iA) contacting the carbonated IBA with an aqueous solution comprising a complexing agent before conducting step (ib). When used herein, the term “complexing agent” refers to one that forms a complex with one or more metals present in the carbonated IBA mixture. In certain embodiments, the complexing agent may be one or more selected from a group consisting of sodium diethyldithiocarbamate, sodium dimethyldithiocarbamate and sodium sulfide. In certain exemplary embodiments, the complexing agent may be sodium diethyldithiocarbamate. Advantageously, the complexing agent may complex and precipitate one or more metals in the IBA, further inhibiting leaching of certain metals from the IBA.

[0073] Step (iA) may be conducted for any suitable period of time. In certain embodiments, the contacting of the carbonated IBA with the complexing agent in step (iA) may be from 15 min to 24 hours, such as from 20 min to 2.5 hours. In certain exemplary embodiments, the period of time may be from 30 min to 1 hour.

[0074] Step (iA) may be conducted at any suitable temperature and pressure. In certain embodiments, step (iA) may be conducted at a temperature of from 10°C to 40°C (such as 20°C) and at a pressure of about 105Pa (1 bar). Advantageously, step (ia) and step (iA) may be performed sequentially under the same reaction conditions before conducting step (ib).

[0075] Any suitable amount of the complexing agent may be used. In certain embodiments, the complexing agent may be provided in an amount of from 0.05% w / w to 5% w / w relative to the weight of the IBA. In certain exemplary embodiments, the complexing agent may be provided in an amount of about 0.1% w / w relative to the weight of the IBA.

[0076] The term “about” as used herein can allow for a degree of variability in a value or range, for example, within 10%, within 5%, within 1%, within 0.5%, within 0.1%, within 0.05%, within 0.01 %, within 0.005%, or within 0.001% of a stated value or of a stated limit of a range, and includes the exact stated value or range.

[0077] As discussed in the Examples, only a portion of the carbonation agent (e.g., NaHCO3) may be used in the carbonation reaction. As such, the conjugate product (e.g., Na2CO3) may reach an equilibrium with the unreacted carbonation agent (e.g., NaHCO3), thereby forming a buffer solution with controlled pH, which is crucial for amphoteric metals (e.g., Zn) that are sensitive to pH changes and exist as soluble species at high pH levels.

[0078] As mentioned above, the aqueous medium involved in the carbonation reaction can be reused (e.g., up to 5 reaction cycles with IBA). As such, in certain embodiments, following an initial reaction cycle, when the pH of the filtrate in step (ib) is from 7 to 11 , such as from 7 to <11 , such as from 7 to 10, then the filtrate may be reused as the aqueous solution comprising the carbonation agent in step (ia).

[0079] Solidification reaction is well known to a skilled person in the art and refers to one that immobilizes heavy metals and other harmful substances in the IBA within a stable matrix, preventing their release into the environment. The solidification reaction may involve binding the IBA with cement or other binders to create a solid, less leachable material.

[0080] In certain embodiments, step (ii) may comprise (a) subjecting the carbonated IBA to a first solidification reaction with a first binder and a first solvent to provide first solidified particles, (b) washing the first solidified particles with a solvent (e.g. water) to provide washed first solidified particles and (c) subjecting the washed first solidified particles to a second solidification reaction with a second binder and a second solvent to provide second solidified particles.

[0081] The first solidification reaction may be conducted and cured for any suitable period of time. In certain embodiments, the first solidification reaction may comprise mixing the first binder, the first solvent and the carbonated IBA to obtain a mixture and curing the mixture for from 12 hours to 36 hours, such as 24 hours to provide the first solidified particles.

[0082] The washing step may be conducted at any suitable liquid-to-solid ration (L / S) for any suitable period of time. In certain embodiments, the washing step of the first solidified particles with the solvent may be at a liquid-to-solid ratio (L / S) of from 2 to 3, such as 2.5 and may be from 2 to 6 hours, such as about 4 hours to provide the washed first solidified particles. The second solidification reaction may be conducted and cured for any suitable period of time. In certain embodiments, the second solidification reaction may comprise mixing the second binder, the second solvent and the washed first solidified particles to obtain a mixture and curing the mixture for at least 5 days, such as at least 7 days to provide the second solidified particles.

[0083] Any suitable binder may be used. In certain embodiments, the first binder and the second binder may be independently selected from one or more selected from a group consisting of water glass and more particularly, Ordinary Portland Cement (OPC), fast drying cement, and white cement.

[0084] For the avoidance of doubt, the first binder and the second binder may be the same or may be different. For example, the first binder may be fast drying cement and the second binder may be fast drying cement as well i.e., both the first and second binders may be fast drying cement. In another example, both the first and second binders may be a mixture of fast drying cement and white cement. In yet another example, the first binder may be fast drying cement and the second binder may be a mixture of fast drying cement and white cement. In certain exemplary embodiments, the first binder may be fast drying cement and the second binder may be white cement. In other exemplary embodiments, both the first and second binders may be white cement.

[0085] Any suitable amount of the binder may be used. The amount of the first binder and the second binder may be the same or may be different. In certain embodiments, the first and second binder may be provided in an amount of from 10% w / w to 50% w / w, such as 15% w / w to 50% w / w, such as 20% w / w to 35% w / w, relative to the weight of the IBA. In certain exemplary embodiments, the first and second binder may be provided in an amount of about 15% w / w relative to the weight of the IBA. In another exemplary embodiment, the first binder may be provided in an amount of about 15% w / w relative to the weight of the IBA and the second binder may be provided in an amount of about 35% w / w relative to the weight of the IBA. In yet another exemplary embodiment, the first binder may be provided in an amount of about 20% w / w relative to the weight of the IBA and the second binder may be provided in an amount of about 50% w / w relative to the weight of the IBA.

[0086] In a second aspect of the invention, there is provided a method of regenerating a carbonation agent, the method comprising: (A) providing a filtrate obtained from step (ib) as disclosed hereinbefore;

[0087] (B) subjecting the filtrate to carbon dioxide to obtain the regenerated carbonation agent.

[0088] The carbonation agent can be regenerated at any suitable stage. In certain embodiments, the pH of the filtrate may be more than about pH 1 1 . As demonstrated in the Examples, amphoteric metals such as aluminium may appear in the filtrate after consecutive reuse of the aqueous medium (such as the fourth and fifth reaction cycle) due to the rising pH after each reaction cycle. In certain particular embodiments, the filtrate may be substantially free of one or both of carbonate or bicarbonate ions. The word "substantially" does not exclude "completely" e.g. a mixture which is "substantially free" from carbonate or bicarbonate ions may be completely free from the carbonate or bicarbonate ions. Moe particularly, the term “substantially free” may be one 5 that has less than >95 wt%, such as >96 wt%, such as >97 wt%, such as >98 wt%, such as >99 wt%, such as >99.5 wt%, such as >99.9 wt%, such as >99.99 wt% free of carbonate or bicarbonate ions. As will be appreciated, no carbonate or bicarbonate ions may be present.

[0089] The filtrate can be regenerated via any suitable means known to those skilled in the art. In certain embodiments, step (B) may comprise either (Bi) subjecting the filtrate to carbon dioxide at a pressure of at least 2 bar for a period of time or (Bii) subjecting the filtrate to a countercurrent flow with a flue gas comprising carbon dioxide in an absorption tower. The present invention is not particularly limited by the exact mode of regeneration.

[0090] In certain particular embodiments, one or more of the following may apply:

[0091] (a) the pressure in step (Bi) is about 2 bar;

[0092] (b) the period of time in step (Bi) is from 15 min to 24 hours, such as from 20 min to 2.5 hours, such as from 30 min to 1 hour;

[0093] (c) step (Bi) is conducted at a temperature of from 10°C to 40°C (such as 20°C);

[0094] (d) the concentration of carbon dioxide in the flue gas in step (Bii) is from 3% (v / v) to 100% (v / v), such as 10% (v / v); and

[0095] (e) step (Bii) is conducted at a temperature of from 10°C to 70°C (such as 50°C).

[0096] As will be appreciated, the flow rate of the filtrate and the flue gas during the countercurrent flow / exchange are dependent on the size (or handling capacity) of the absorption tower and can be readily arrived at by the person skilled in the art. As such, in certain embodiments, when the absorption tower size is of the dimensions 200 mm x 1200 mm (Depth x Height): (f) the flow rate of filtrate in step (Bii) is from 10 mL / min to 100 mL / min from a top end of the absorption tower; and

[0097] (g) the flow rate of flue gas in step (Bii) is from 2 L / min to 100 L / min from a bottom end of the absorption tower.

[0098] In certain embodiments, the method may further comprise step (Ai) subjecting the filtrate to a treatment step for removing one or more organic species and / or halogen before conducting step (B). In certain particular embodiments, step (Ai) may comprise subjecting the filtrate to one or both of activated carbon or hydrotalcite. In certain exemplary embodiments, step (Ai) may comprise subjecting the filtrate to activated carbon for removing one or more organic species.

[0099] Further aspects and embodiments of the invention will now be discussed by reference to the following non-limiting examples.

[0100] EXAMPLES

[0101] Example 1 : Materials and Characterisation

[0102] IBA samples were sourced from a local municipal solid waste (MSW) incineration plant and subjected to drying at 100eC until achieving a constant weight prior to utilization. For an in- depth examination of the chemical and mineralogical composition of the IBA, the IBA samples were meticulously sieved through mesh sizes of 0.6, 1.12, 2.0, and 2.8 mm. Subsequently, small portions of the samples of varying sizes were selected and ground into a powder with particle sizes below 200 pm, facilitating X-ray fluorescence (XRF) and X-ray diffraction (XRD) analyses. IBA particles exceeding 2.8 mm were excluded from analysis and were not studied further.

[0103] The chemical composition of the IBA samples underwent analysis through XRF, utilizing the Bruker S8 TIGER Series. Concurrently, XRD analysis, employing the Bruker D8 advance A25 / XRK900 diffractometer, was conducted to ascertain the mineral phases of the diverse samples. Cu-Ka radiation (A = 1 .54060 A) was employed in continuous scan mode, with a step size of 0.02° in the 29 range spanning from 15° to 90°. Thermal gravimetric analysis (TGA) was performed using the TA Instruments SDT Q600 unit. In this process, 10 to 20 mg of IBA samples underwent ramping at 10 °C min-1from 30 °C to 900 °C under a 100 cm3min-1flow of N2.

[0104] Results and Discussion:

[0105] The physicochemical properties of IBA are influenced by both the waste profile of the population and the entire waste management process, from collection and sorting to incineration. Understanding the composition of IBA from a specific area is crucial for assessing waste generation patterns, the effectiveness of waste management practices, and determining appropriate treatment or recovery procedures. Despite variations in its source, IBA from different locations generally share common physical traits and chemical compounds. It is important to note that freshly incinerated MSW IBA is typically washed with water to reduce its temperature before collection, and the present Example discusses the physical properties and chemical composition of water quenched IBA.

[0106] IBA comprises a heterogeneous mix of particles with varying shapes, roughness, and sizes, characterized by a porous microstructure. Its particle size distribution ranges from micrometers to centimeters, which can evolve gradually due to natural weathering. The majority of particles in IBA range from several hundred micrometers to several millimeters in size. For the purpose of screening and validation of the process, IBA with particle size of 1.12 to 2 mm was used.

[0107] IBA typically consists of several material classes: minerals (50-85%), glass and ceramics (10- 30%), ferrous metals (5-15%), non-ferrous metals (1 -5%), and organic carbon materials. Among these fractions, the most abundant are Ca, Si, Fe, and Al in their oxides form, present in various mineralogical phases, collectively contributing to over 80% of the mass fraction. Table 1 presents the compositions of metal oxides and mineralogical phases that have been identified on one of IBA samples analyzed by XRF and XRD.

[0108] Table 1 Typical composition of metal oxides and mineralogical phases found in IBA.

[0109] The non-ferrous metal fraction of IBA includes metallic aluminum (Al), primarily sourced from disposed beverage containers, household furnishings, and appliances. However, significant concern arises from toxic heavy metals within this fraction. The prevalent heavy metal elements in IBA are zinc (Zn), copper (Cu), lead (Pb), chromium (Cr), cobalt (Co), titanium (Ti), nickel (Ni), manganese (Mn), tin (Sn), and strontium (Sr) (Huber F., Blasenbauer D., Aschenbrenner P., Fellner J., Waste Manag. 2019, 95, 593-603). These heavy metals mainly originate from electronic waste (e.g., wires, batteries, printed circuit boards), automotive waste, metallic coatings, paints, and additives in structural metal alloys (Patra S., Whaung S. T., Kwan W. L., Energy Procedia 2017, 143, 454-459). Understanding the leaching behaviour of heavy metal elements from IBA is crucial to assess their potential impact on the environment. Leaching tests, conducted under equilibrium conditions, measure the concentration of heavy metal species extracted from the leachate. Contrary to intuition, there is no direct correlation between the heavy metal composition in IBA and the amount of heavy metal leaching (Hyks J., Astrup T., Chemosphere 2009, 76, 1178-1184). Instead, factors such as availability, solubility, liquid-to-solid (L / S) ratio, and particle size control the release of constituent heavy metals and other elements from IBA. Availability refers to the maximum quantity of an element that can be released into the leachate under aggressive conditions, with some elements bound to stable compounds exhibiting lower availability. Solubility, influenced by pH and additives like complexing agents, affects leaching at low L / S ratios. Smaller particles in IBA generally exhibit higher leaching due to their increased surface area, although they also possess higher adsorption capacity, leading to a complex relationship between particle size and heavy metal leaching potential (Lu Y., Tian A., Zhang J., Tang Y., Shi P., Tang Q., Huang Y., Adv. Civ. Eng. 2020, 8886134). Many countries utilize standardized leaching tests to evaluate heavy metal leaching from IBA in accordance with national environmental regulations (Astrup T., Muntoni A., Polettini A., Pomi R., Van Gerven T. & Van Zomeren A., in Environmental Materials and Waste (eds Prasad M. N. V. and Shih K.), 607-645 (Academic Press, 2016)). In Europe, procedures established by the European Standardization Technical Committee (CEN / TC) or the British Standards Institution are followed, while the United States employs the Toxicity Characteristic Leaching Procedure (TCLP) set by the US Environmental Protection Agency (EPA). Singapore utilizes a one-stage batch leaching test, following the UNE-EN-12457-2 standard, to assess heavy metal release from IBA. Table 2 displays the leaching results of an IBA sample tested by the accredited lab MLS.

[0110] Table 2 Leaching results of IBA (1.2 - 2 mm).*

[0111]

[0112] Environmental Agency (NEA) for the application of waste material for construction purposes.

[0113] The leaching results indicate that while the IBA is acceptable for landfill disposal, it falls short of suitability for additional applications, such as construction. Several heavy metals, including Sb, Cr, Cu, along with other anions such as chloride, bromide, and organic residues, surpass the regulatory values set by NEA. To address this, carbonation and solidification with cement to curb heavy metal leaching while enhancing the strength of the treated IBA were implemented. The schematic diagram of the whole process is illustrated in FIG. 1 .

[0114] Example 2:

[0115] Example 2.1 : Carbonation Kinetic Study

[0116] A series of reactions were conducted by mixing 2.18 g of sodium bicarbonate in 25 mL of water with 5 g of IBA at room temperature for 10 minutes, 30 minutes, 1 hour, and 2 hours. After each reaction, the mixture was filtered, washed with water, and then dried overnight at 70 degrees Celsius. The dried samples were subjected to Thermogravimetric Analysis (TGA) to determine the rate of CO2 sequestration by analysing the weight loss within the temperature range of 600 to 800 degrees.

[0117] Example 2.2: Carbonation of IBA

[0118] Initially, a solution comprising 100 g of sodium bicarbonate in 900 mL of deionized water was prepared in a 2 L beaker and reacted with 200 g of IBA (particle size between 1.12 and 2 mm). The suspension was stirred at room temperature for 30 minutes. Following this, 0.2 g of sodium diethyldithiocarbamate in 5 mL of water was added, and the reaction mixture was stirred for another 30 minutes. Subsequent to filtration, the reaction mixture underwent washing with 50 to 100 mL of water. A 25 mL aliquot of filtrate was then sampled and diluted to 500 mL for metals and other parameter analysis. The pH of the collected filtrate was measured at 8.69 using a pH meter. This filtrate was directly utilized for the next cycle of carbonation until the pH exceeded 1 1. In this case, pH of the filtrate from the 5thcycle of carbonation was 12.30.

[0119] Results and Discussion:

[0120] IBA naturally undergoes slow carbonation via weathering, but accelerated carbonation using CO2expedites this process, achieving in days to months what would take months to years naturally. This artificial method speeds up the conversion of ash minerals into stable metal carbonate compounds, reducing heavy metal leaching potential by chemically binding them within carbonate phases. This process also converts lime and portlandite into calcite (CaCO3), lowering the IBA pH and promoting heavy metal immobilization (Van Gerven T., Van Keer E., Arickx S., Jaspers M., Wauters G., Vandecasteele C., Waste Manag. 2005, 25, 291-300). Using carbonate salt for carbonation can significantly accelerate the reaction rate, completing the process within minutes to an hour. Both sodium carbonate (Atanes E., Cuesta-Garcia B., Nieto-Marquez A., Fernandez-Martinez F., J. Environ. Manag. 2019, 240, 359-367) and ammonium carbonate (Ye S., Ou Yong R., Kouk Q. Y., Goh W., Tham K. H., Bu J., Wang C. Waste Manag. Bull. 2024, 2, 153-160) can serve as effective carbonation agents for IBA in an aqueous medium.

[0121] Sodium bicarbonate was utilised as the initial carbonation reagent, which was in excess. The resulting carbonated IBA samples were isolated, dried, and analyzed using TGA to determine the carbonate content, given that CaCO3decomposes within the temperature range of 600 to 800 degrees (refer to FIG. 2). The rapid carbonation with sodium bicarbonate was evident, reaching its full capacity within 10 minutes. To ensure completion of the reaction and facilitate handling and processing, the subsequent screening for carbonation was extended to 30 minutes.

[0122] Example 3: Recycling of Carbonation Medium

[0123] Once the pH of the filtrate exceeds 1 1 , indicating the conversion of most sodium bicarbonate into sodium carbonate and / or sodium hydroxide, regeneration of sodium bicarbonate becomes necessary. This is achieved by treating the solution with CO2 while carefully controlling the pH during the carbonation reaction. In the 5th cycle of reaction, the filtrate appeared red, largely due to dissolved organic species from the IBA. To remove these organics, the filtrate was initially treated with 30 g of activated carbon, resulting in a pale-yellow solution. Subsequently, this solution was transferred to a Parr reactor and exposed to a CO2atmosphere of 2 bars at room temperature for 30 minutes. The pH was lowered to around 7, indicating complete formation of bicarbonate solution. This regenerated sodium bicarbonate solution was then utilized for the 6th cycle of carbonation.

[0124] Alternatively, the filtrate after carbonation was introduced at a flow rate of 40 mL / min from the top of an absorption tower (D x H: 20 cm x 120 cm) packed with structured material. Simulated flue gas (10% CO2in air, v / v) was introduced from the bottom at a flow rate of 10 L / min. The column was maintained at 50 °C to simulate the real conditions under which flue gas exits a power plant as a hot gas mixture. Although lower temperatures are generally preferred for CO2absorption and the conversion of carbonate to bicarbonate. The regenerated solution collected from the bottom exhibited a pH of 9.21 , indicating that more than 90% of the carbonate was successfully converted to bicarbonate.

[0125] Results and Discussion:

[0126] During carbonation, only a portion of sodium bicarbonate reacts with CaO (or Ca(OH)2), converting into sodium carbonate. This sodium carbonate reached equilibrium with the unreacted sodium bicarbonate, forming a buffer solution with controlled pH (see eq 1 and eq 2). Maintaining pH control is crucial, especially for amphoteric metals like Zn, which are sensitive to pH changes and exist as soluble species at high pH levels. In our experiments, an initial input of 100 g of NaHCOs in 900 mL of water was sufficient to carbonate 5 batches of 200 g of IBA, with the final reaction cycle resulting in a filtrate with pH > 11 .

[0127] The pH exceeding 1 1 indicates the conversion of most sodium bicarbonate into sodium carbonate or sodium hydroxide, necessitating the regeneration of sodium carbonate using CO2. Before regeneration, the filtrate from the 5 cycles of reaction underwent activated carbon treatment to remove dissolved organic residues from raw IBA, resulting in a faded coloration of the solution. The activated carbon-treated solution was then exposed to a CO2atmosphere of 2 bar at room temperature for 30 minutes with stirring, leading to a pH of approximately 7 (refer to eq 3 and eq 4). In addition to treating the filtrate with pure CO2, regeneration can also be achieved using flue gas (10% CO2v / v) through counter-current contact in an absorption tower, where the filtrate is introduced from the top and the flue gas from the bottom. This regenerated sodium bicarbonate was recycled for the subsequent cycle of carbonation.

[0128] 2 NaOH + CO2- Na2CO3+ H2O Eq. 3

[0129] To ensure the long-term recyclability of the carbonation medium and minimize environmental impacts while promoting sustainability, it is essential to monitor the accumulation of heavy metals and other species leaching after each cycle. The filtrate from each cycle underwent analysis for heavy metals, other elements, and species by the accredited lab MLS. The results are detailed in Table 3. With the exception of Al, chloride, bromide, and COD, all species reached their peak concentration and showed no further increase in subsequent carbonation cycles, likely due to their limited solubility in water in various forms. Aluminium appeared in the fourth and fifth cycles of filtrates due to the rising pH after each cycle, reflecting its amphoteric properties. However, aluminium can be easily removed during the regeneration of sodium bicarbonate as pH is adjusted back to around 7. Dissolved organic species (TOC and COD) increased with each cycle but were effectively removed by activated carbon treatment. Accumulation of chloride and bromide poses a potential issue, which could be addressed through electrolysis or treatment with hydrotalcite, as its easily exchanged carbonates make it suitable for wastewater treatment applications.

[0130] In conclusion, the carbonation medium can be effectively recycled with minimal associated waste production, primarily comprising activated carbon or hydrotalcite.

[0131] Table 3 Species in the carbonation medium after each cycle (dilution factor: 20, concentration: in mg / L).

[0132]

[0133] Example 4: Solidification with Cement

[0134] The solidification reaction comprises the following. A. Pretreatment with Cement: The wet carbonated I BA from carbonation was treated with a first portion of 30 g of cement, thoroughly mixed, and a controlled amount of deionized water (DI H2O) was added until small particles formed. The resulting IBA particles were then cured for 24 hours.

[0135] B. Washing with Water: To remove soluble salts such as chloride, bromide, and sulfate, the sample was washed with 500 mL of water in a 1 L round-bottom flask (RBF) on a rotavapor set at a rotation speed of 10 rpm for 4 hours. Subsequently, the mixture was filtered using a Corning filter (PES 0.22 micron, 1 L), and the collected filtrate was sent to MLS for analysis to ensure compliance with watercourse discharge standards.

[0136] C. Final Treatment with Cement: The wet IBA sample from step B underwent treatment with a second portion of 30 g of cement using the same procedure as in step A. Subsequently, the resulting IBA particles were cured for a minimum of 5 days.

[0137] Results and Discussion:

[0138] The use of cement as a binder in IBA solidification is a vital strategy to create a more durable and less leachable material, reducing environmental impact. The selection of binders significantly influences heavy metal immobilization effectiveness, as different binders possess unique chemical properties and reactivities that impact the final product. For example, Anastasiadou et al. demonstrated the effectiveness of Ordinary Portland Cement (OPC) in reducing toxic heavy metal leaching from medical waste IBA, enabling its reuse in construction or environmentally friendly disposal (Anastasiadou K., Christopoulos K., Mousios E., Gidarakos E., J. Hazard. Mater. 2012, 207-208:165-170). Mixing IBA with wet OPC at varying ratios and curing it at room temperature in a humid environment for up to 28 days enhanced both compressive strength and heavy metal stabilization efficiency. The resulting material met regulatory limits for safe landfill disposal.

[0139] In the present Example, different types of cement (fast-dry and white cement) were utilized as binders and a two-step treatment process was applied to the carbonated IBA. The first portion of cement was applied to the IBA and allowed to age for one day before undergoing water washing. This washing step, conducted at a liquid-to-solid ratio (L / S) of 2.5 and room temperature for 4 hours under stirring, aimed to remove soluble salts such as chloride, bromide, and sulfate. These salts may have adverse effects when the final treated IBA is used in construction materials. The washing water was then filtered and analyzed for species according to NEA standards for discharge into watercourses (refer to Table 4). All parameters were met except for COD, likely caused by dissolved organic species, which was easily remedied by treatment with activated carbon (refer to the last column).

[0140] Table 4 Species in the washed water from 5 IBA samples with first portion of cement from each cycle.

[0141]

[0142] MLS-SOP-WQ-036 Rev 0 (using Lovibond Tintometer)

[0143] The water-washed IBA samples were subsequently treated with second portions of cement and cured at room temperature for a minimum of 5 days. The resulting treated IBA samples underwent analysis according to the UNE-EN-12457-2 standard to evaluate heavy metal release. Table 5 presents the leaching results of the IBA samples tested by the accredited lab MLS.

[0144] Table 5 Leaching results from IBA samples with different treatment methods.

[0145]

[0146] C: carbonation; S: treatment with sodium diethyldithiocarbamate; FDC: treatment with fast dry cement; WC: treatment with white cement. *becoming worse from left to right with more treatments (no “*” means improvement or no change) Carbonation significantly enhances the inhibition of metal leaching, with exceptions noted for Sb, Se, and sulfate. Sodium diethyldithiocarbamate is recognized as an effective reagent for the complexation and precipitation of copper ions, as evidenced by its absence in the recycled carbonation medium. However, its efficacy in solid IBA is less clear. Carbonation followed by treatment with sodium diethyldithiocarbamate may further inhibit leaching of certain metals, although some parameters may worsen. Additional treatment with cement can improve heavy metal leaching management, though in some instances, heavy metals associated with the cement can elevate leaching values, such as Cr and ammonia (N) from fast-dry cement. This was corroborated by analysing benchmark samples made with acid-washed sand and fastdry cement.

[0147] Ensuring consistency in metal leaching behavior of treated IBA samples is crucial. Table 6 presents leaching results from IBA samples treated using the same method sequence: carbonation, sodium diethyldithiocarbamate, fast-dry cement, and fast-dry cement with recycling of the carbonation medium. Generally, the final IBA samples exhibit similar or improved leaching behavior compared to the initial cycle. Particularly noteworthy is that IBA samples from carbonation using regenerated sodium bicarbonate solution demonstrate consistent performance, confirming the recyclability of the carbonation medium using sodium bicarbonate.

[0148] Table 6 Leaching results from IBA samples after each cycle.*

[0149]

[0150] All IBA samples were treated in sequence: C>S>FDC>FDC. ** A benchmark sample was prepared using acid washed sand (200 g) and two portions of FDC (30 g + 30 g). Since commercial cements can vary in composition and may contain heavy metals, solidification with these cements can introduce such metals, which may be detected during leaching tests, as shown in Table 6 (last column). To evaluate the effect of different solidification materials on the leaching behavior of the final product, we tested various combinations of cements in the first and second treatment steps of carbonated I BA. Except for the changes noted, all operating parameters remained the same. The results are presented in Table 7 (first treatment with 30 g white cement followed by a second treatment with 70 g white cement) and Table 8 (new batch of white cement).

[0151] It is worth noting that even different batches of white cement can vary in composition. For instance, the new batch resulted in higher leaching values of Ba, F, Pb, and even organic species. Selecting an appropriate cement material is therefore crucial to ensure all parameters comply with the limits set by Singapore NEA (Table 8, last column).

[0152] Table 7 Leaching results of carbonated IBA with the treatment of two portions of white cement.

[0153]

[0154] Table 8. Leaching results of carbonated IBA with new batch of white cement and OPC.

[0155]

[0156] Carbonation: 200g of IBA, 1000 mL of 5% NaHCOs, IBA:S = 500g:1 g, RT, new batch of white cement (WC). Sample A: Carbonated IBA treated with 40 g of WC, washed and dried; Sample B: the wet sample A was further treated with 100 g of WC; Sample C: the wet Sample A was further treated with 100 g of OPC.

Claims

CLAIMS1. A method of inhibiting metal leaching from incineration bottom ash (IBA), the method comprising:(i) providing a carbonated IBA; and(ii) subjecting the carbonated IBA to one or more solidification reactions comprising adding a binder to the carbonated IBA.

2. The method according to Claim 1 , wherein the carbonated IBA is provided by contacting an IBA with an aqueous solution comprising a carbonation agent.

3. The method according to Claim 2, wherein the carbonated IBA is provided by a process comprising:(ia) mixing the aqueous solution comprising the carbonation agent with the IBA for a period of time to obtain a carbonated IBA mixture; and(ib) filtering the carbonated IBA mixture to obtain the carbonated IBA and a filtrate.

4. The method according to Claim 3, wherein the period of time in step (ia) is from 15 min to 48 hours, such as from 20 min to 5 hours, optionally wherein the period of time is from 30 min to 2 hours.

5. The method according to any one of Claim 3 or Claim 4, wherein step (ia) is conducted at a temperature of from 10°C to 40°C (such as 20°C) and at a pressure of about 105Pa (1 bar).

6. The method according to any one of Claims 2 to 5, wherein the carbonation agent is one or both of a bicarbonate salt and a carbonate salt, optionally wherein: when the carbonation agent is a bicarbonate salt, the bicarbonate salt comprises a cation selected from a group consisting of a Group I metal ion (e.g., Na+and K+), a Group II metal ion (Mg2+and Ca2+), a quaternary ammonium ion, a quaternary phosphonium ion, an imidazolium ion and a phosphazenium ion; and when the carbonation agent is a carbonate salt, the carbonate salt comprises a cation selected from a group consisting of a Group I metal ion (e.g., Na+and K+), a quaternary ammonium ion, a quaternary phosphonium ion, an imidazolium ion and a phosphazenium ion.

7. The method according to Claim 6, wherein the carbonation agent is sodium bicarbonate.

8. The method according to any one of Claims 2 to 7, wherein the carbonation agent is provided in an amount of from 5% w / w to 200% w / w, such as from 10% w / w to 150% w / w, such as from 25% w / w to 100% w / w, relative to the weight of the IBA, optionally wherein the carbonation agent is provided in an amount of about 50% w / w relative to the IBA.

9. The method according to any one of Claims 3 to 8, wherein the method further comprises a step:(iA) contacting the carbonated IBA with an aqueous solution comprising a complexing agent before conducting step (ib).

10. The method according to Claim 9, wherein one or both of the following applies:(a) the contacting of the carbonated IBA with the complexing agent in step (iA) is from 15 min to 24 hours, such as from 20 min to 2.5 hours, optionally wherein the period of time is from 30 min to 1 hour; and(b) step (iA) is conducted at a temperature of from 10°C to 40°C (such as 20°C) and at a pressure of about 105Pa (1 bar).

11. The method according to any one of Claims 9 to 10, wherein the complexing agent forms a complex with one or more metals present in the carbonated IBA mixture.

12. The method according to any one of Claims 9 to 11 , wherein the complexing agent is one or more selected from a group consisting of sodium dimethyldithiocarbamate, sodium sulfide and more particularly, sodium diethyldithiocarbamate.

13. The method according to Claim 12, wherein the complexing agent is sodium diethyldithiocarbamate.

14. The method according to any one of Claims 9 to 13, wherein the complexing agent is provided in an amount of from 0.05% w / w to 5% w / w relative to the weight of the IBA, optionally wherein the complexing agent is provided in an amount of about 0.1% w / w relative to the weight of the IBA.

15. The method according to any one of Claims 3 to 14, wherein following an initial reaction cycle, when the pH of the filtrate in step (ib) is from 7 to 11 , such as from 7 to <11 , such as from 7 to 10, then the filtrate is reused as the aqueous solution comprising the carbonation agent in step (ia).

16. The method according to any one of the preceding claims, wherein step (ii) comprises:(a) subjecting the carbonated IBA to a first solidification reaction with a first binder and a first solvent to provide first solidified particles;(b) washing the first solidified particles with a solvent (e.g. water) to provide washed first solidified particles; and(c) subjecting the washed first solidified particles to a second solidification reaction with a second binder and a second solvent to provide second solidified particles.

17. The method according to Claim 16, wherein one or more of the following applies:(a) the first solidification reaction comprises mixing the first binder, the first solvent and the carbonated IBA to obtain a mixture and curing the mixture for from 12 hours to 36 hours, such as 24 hours to provide the first solidified particles;(b) the washing step of the first solidified particles with the solvent is at a liquid-to-solid ratio (L / S) of from 2 to 3, such as 2.5 and is from 2 to 6 hours, such as about 4 hours to provide the washed first solidified particles; and(c) the second solidification reaction comprises mixing the second binder, the second solvent and the washed first solidified particles to obtain a mixture and curing the mixture for at least 5 days, such as at least 7 days to provide the second solidified particles.

18. The method according to any one of the preceding claims, wherein the first binder and the second binder are independently selected from one or more selected from a group consisting of water glass and, more particularly, Ordinary Portland Cement (OPC), fast drying cement, and white cement.

19. The method according to Claim 18, wherein the first binder is fast drying cement and the second binder is white cement or wherein the first binder and the second binder are white cement.

20. The method according to any one of the preceding claims, wherein the first and second binder are provided in an amount of from 10% w / w to 75% w / w, such as 15% w / w to 50% w / w, such as 20% w / w to 35% w / w, relative to the weight of the IBA, optionally wherein the first andsecond binders are provided in an amount of about 15% w / w relative to the weight of the IBA, or optionally wherein the first binder is provided in an amount of about 15% w / w relative to the weight of the IBA and the second binder is provided in an amount of about 35% w / w relative to the weight of the IBA, or optionally wherein the first binder is provided in an amount of about 20% w / w relative to the weight of the IBA and the second binder is provided in an amount of about 50% w / w relative to the weight of the IBA.21 . A method of regenerating a carbonation agent, the method comprising:(A) providing a filtrate obtained from step (ib) in any one of Claims 3 to 20;(B) subjecting the filtrate to carbon dioxide to obtain the regenerated carbonation agent.

22. The method according to Claim 21 , wherein the pH of the filtrate is more than about pH 11 , optionally wherein the filtrate is substantially free of one or both of carbonate or bicarbonate ions.

23. The method according to Claim 21 or Claim 22, wherein step (B) comprises either:(Bi) subjecting the filtrate to carbon dioxide at a pressure of at least 2 bar for a period of time; or(Bii) subjecting the filtrate to a countercurrent flow with a flue gas comprising carbon dioxide in an absorption tower.

24. The method according to Claim 23, wherein one or more of the following applies:(a) the pressure in step (Bi) is about 2 bar;(b) the period of time in step (Bi) is from 15 min to 24 hours, such as from 20 min to 2.5 hours, such as from 30 min to 1 hour;(c) step (Bi) is conducted at a temperature of from 10°C to 40°C (such as 20°C);(d) the concentration of carbon dioxide in the flue gas in step (Bii) is from 3% (v / v) to 100% (v / v), such as 10% (v / v);(e) step (Bii) is conducted at a temperature of from 10°C to 70°C (such as 50°C);(f) the flow rate of filtrate in step (Bii) is from 10 mL / min to 100 mL / min from a top end of the absorption tower;(g) the flow rate of flue gas in step (Bii) is from 2 L / min to 100 L / min from a bottom end of the absorption tower.

25. The method according to any one of Claims 21 to 24, wherein the method further comprises step (Ai) subjecting the filtrate to a treatment step for removing one or more organic species and / or halogen before conducting step (B).

26. The method according to Claim 25, wherein step (Ai) comprises subjecting the filtrate to one or both of activated carbon or hydrotalcite, optionally wherein step (Ai) comprises subjecting the filtrate to activated carbon.

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