WO2025007185A1

WO2025007185A1 - Methods of preparing a zeolite

Info

Publication number: WO2025007185A1
Application number: PCT/AU2024/050719
Authority: WO
Inventors: John VOGRIN
Original assignee: Zeotech Ltd
Current assignee: Zeotech Ltd
Priority date: 2023-07-03
Filing date: 2024-07-02
Publication date: 2025-01-09
Anticipated expiration: 2026-01-03

Abstract

Disclosed herein is a method to produce zeolite A comprising forming a slurry of a blend of coal combustion products or a leached residue thereof with metakaolin and aqueous alkaline solution; and maintaining the slurry at a temperature below the boiling point of the aqueous alkaline solution at atmospheric pressure for a time sufficient to dissolve aluminate and silica species into solution and precipitating zeolite A from the dissolved aluminate and silica species.

Description

METHODS OF PREPARING A ZEOLITE

Field

[0001] The invention relates to a method for producing zeolite, and a zeolite product produced by such method.

Background

[0002] Zeolites are typically synthesised from pure reagents: a mixture of sodium hydroxide (NaOH), sodium aluminate (NaAlCh), sodium silicate (NaiSiOs) and water. A hydrothermal process is generally employed, whereby all reactants are added in some order into a stirred reaction vessel, potentially aged, heated and allowed to react for a specified period. During this ageing and heating phase, the raw materials are converted to a zeolite material of formula aM2/«O;A12O3;nSiO2;Z?H2O with varied nature based on the base and ratio of the mixture. The product is then recovered by some means of solid-liquid separation. The hydrothermal route to production is the most widely used method of synthesis in the industry and has been employed since the 1960s.

[0003] Another synthesis route, which has been investigated to great extent but not known to be widely implemented for commercial zeolite production, is alkali fusion. In this method, the solid feed, typically an impure mineral phase, is mixed with solid or highly concentrated sodium hydroxide (>12 M) to form a paste, which is then transferred into a furnace at elevated temperature (up to ~600 °C) to facilitate the breakdown of phases into solid sodium silicate and sodium aluminate. The sintered mixture is then crushed and combined with water and/or additional sodium hydroxide in a stirred reaction vessel at a specified temperature and period, and the zeolite product recovered by some means of solid-liquid separation. The alkali fusion route is known to be the preferred route when processing caustic-inert phases in the feed material, such as quartz (SiCh).

[0004] One raw material that has been used to produce zeolites is coal fly ash (CFA). CFA refers to the by-product material that is captured by filters, electrostatic precipitators, and other air control devices at coal power plants. Coal combustion products (CCPs) refer to the entire byproduct process residue leaving a coal power plant inclusive of CFA, bottom ash, and desulfurization gypsum. The chemical and mineralogical composition of CFA/CCP is known to vary significantly based on the coal being used, however are generally concentrated in amorphous alumina and silica components, mullite (AleSiiOr,), quartz and other predominantly Fe or Ca based compounds.

[0005] The most researched method of producing zeolite from CFA is through alkali fusion, which often results in the formation of an impure zeolite X (NaiO- AhCh ^.SSiCh yFhO) product, due to the high Si:Al ratio of the raw material. The alkali-fusion method is often used with CFAs, as CFAs are concentrated in quartz and mullite, which are known for their chemical inertness and unreceptive dissolution in sodium hydroxide, even at elevated temperatures. In the alternative, if the hydrothermal route is employed for producing zeolite from CFAs, it is well reported that the resultant zeolite will be in the form of zeolite Pl (Na2O-AhO3-2-5SiO2-5H2O), or a mixture of zeolite Pl and analcime (NaAlSi2O6-H2O), alongside the unreacted gangue minerals. Of note, zeolites X and Linde Type A (Na2O A12O3-2SiO2-2.25H2O) are not typically formed through conventional hydrothermal synthesis of CFAs.

[0006] It is important to reiterate that most zeolite synthesis efforts on CFA have used the alkali- fusion method, as this provides a higher purity product than the hydrothermal method and is thus more accepted by the market. During the hydrothermal method, a higher percentage of gangue mineral impurities present in CFA persist through the reaction and end up invariably as part of the product, diluting the concentration of the zeolitic portion. However, though the resultant zeolite product is of higher purity, the high-energy requirements of alkali-fusion and the nature of the method to operate in batches, have limited its commercial viability.

[0007] It is desirable to provide improved or alternate methods for forming zeolites, and in particular zeolites having particular zeolitic structures. It is thus an object of the invention to address at least one shortcoming of the prior art and/or provide a useful alternative.

[0008] Reference to any prior art in the specification is not an acknowledgment or suggestion that this prior art forms part of the common general knowledge in any jurisdiction or that this prior art could reasonably be expected to be understood, regarded as relevant, and/or combined with other pieces of prior art by a skilled person in the art. Summary of Invention

[0009] In one aspect of the invention there is provided a method to produce zeolite Linde Type A comprising: forming a slurry of a blend of coal combustion products or a leached residue thereof with metakaolin and aqueous alkaline solution; and maintaining the slurry at a temperature below the boiling point of the aqueous alkaline solution at atmospheric pressure for a time sufficient to dissolve aluminate and silica species into solution and precipitating zeolite A from the dissolved aluminate and silica species.

[0010] In an embodiment, the method is carried out for a time of up to 24 hours. Preferably up to 18 hours, more preferably up to 9 hours. Most preferably up to 6 hours.

[0011] In an embodiment, the slurry is agitated.

[0012] In another aspect of the invention there is provided a method to produce zeolite A comprising: leaching a blend of coal combustion products or leached residue thereof with metakaolin using an aqueous alkaline solution to provide a leach solution comprising dissolved aluminate and dissolved silica; and precipitating zeolite A from the leach solution; wherein the method is carried out at a temperature that is less than boiling point of the aqueous alkaline solution at atmospheric pressure.

[0013] In an embodiment, the step of precipitating zeolite is a step of crystallising zeolite.

[0014] In an embodiment of the first and second aspects, the method is carried out at atmospheric pressure.

[0015] In an embodiment of the second aspect, the leaching step and/or the precipitating step is carried out at atmospheric pressure.

[0016] In an embodiment of the first and second aspects, the method is carried out at a temperature of at least 70 °C, and preferably at least 75 °C. [0017] In an embodiment of the second aspect, the leaching step and/or the precipitating step is carried out at a temperature of at least 70 °C, and preferably at least 75 °C.

[0018] In an embodiment of the first and second aspects, the method is carried out at a temperature of less than 100 °C, and preferably at a temperature of up to 95 °C.

[0019] In an embodiment of the second aspect, the leaching step and/or the precipitating step is carried out at a temperature of less than 100 °C, and preferably at a temperature of up to 95 °C.

[0020] In an embodiment of the first and second aspects, the blend of coal combustion products or leached residue thereof with metakaolin has a ratio of coal combustion products or leached residue thereof to metakaolin of 10:90 to 90: 10. According to the mineral properties of the coal combustion products, the blend ratio is adjusted to maintain LTA product purity.

[0021] In an embodiment of the first and second aspects the coal combustion products comprise, consist of, or consist essentially of coal fly ash, bottom ash, desulfurization gypsum, and combinations thereof.

[0022] In an embodiment of the first and second aspects, the aqueous alkaline solution is a NaOH solution, preferably a 2 to 6 M NaOH solution.

[0023] In an embodiment of the second aspect, the leaching step provides a solid residue, and the zeolite A is crystallised in the presence of the solid residue.

[0024] In an embodiment of the second aspect, the precipitation step is carried out for a time of up to 24 hours. Preferably up to 18 hours, more preferably up to 9 hours. Most preferably up to 6 hours.

[0025] In an embodiment of the second aspect, the leaching step comprises forming a slurry of the mixture of coal combustion products and metakaolin with the aqueous alkaline solution to dissolve aluminate and silica in the mixture of coal combustion products and metakaolin and provide the dissolved aluminate and dissolved silica.

[0026] In one form of the above embodiment, the slurry is agitated. [0027] In an embodiment of the first and second aspects, the method further comprises a solidliquid separation step to provide a solid stream comprising zeolite A and an aqueous liquid stream comprising NaOH.

[0028] In one form of the above embodiment, the solid stream comprises zeolite A in an amount of at least 50 wt% and up to 95 wt%. Preferably, the solid stream comprises zeolite A in an amount of at least 75 wt% in the zeolitic portion.

[0029] In one form of the above embodiment, the aqueous liquid stream is recycled.

[0030] In an embodiment of the first and second aspects, the method is carried out as a batch or continuous reaction process.

[0031] In an embodiment of the first and second aspects, the solid concentration of the blend in the aqueous alkaline solution is from 50 g/L to 400 g/L.

[0032] In a further aspect of the invention there is provided a zeolite A produced according to the method of the first or second aspects of the invention, or embodiments or forms thereof.

[0033] As used herein, except where the context requires otherwise, the term "comprise" and variations of the term, such as "comprising", "comprises" and "comprised", are not intended to exclude further additives, components, integers, or steps.

[0034] Further aspects of the present invention and further embodiments of the aspects described in the preceding paragraphs will become apparent from the following description, given by way of example and with reference to the accompanying drawings.

Brief Description of Drawings

[0035] Figure 1: Process flow diagram illustrating an embodiment of a process of the invention.

[0036] Figure 2: Process flow diagram illustrating another embodiment of a process of the invention. [0037] Figure 3: X-ray diffractograms of a) calcined kaolin showing full transformation to metakaolin, b) raw kaolin. Letters annotated above peaks indicate corresponding phases K: kaolinite, T : anatase.

[0038] Figure 4: X-ray diffractograms of the CCP. Letters annotated above peaks indicate corresponding phases M: mullite, Q: quartz, N: andradite.

[0039] Figure 5: X-ray diffractograms of zeolite products produced from CCPs as follows a) in a 75% blend with metakaolin, b) with an initial source of aluminate equal to 0.25M, c) untreated CCP. The letters annotated above peaks indicate corresponding phases A: zeolite A, S: sodalite, P: zeolite Pl, Q: quartz.

[0040] Figure 6: Scanning electron micrographs of a) the CCPs, b) zeolite A product after synthesis from CCP-metakaolin.

Description of Embodiments

[0041] The inventors have devised a process to produce zeolite A from a blended feed of coal combustion products (or leached residue thereof), such as coal fly ash, bottom ash, desulfurization gypsum, and the like, with metakaolin.

[0042] The process broadly includes forming a slurry of the blended feed with aqueous sodium hydroxide at a temperature below the boiling point thereof for a time sufficient to dissolve aluminate and silica species into solution and precipitate zeolite A from the dissolved aluminate and silica species. Advantageously, the dissolution and precipitation steps can be carried out concomitantly in a ‘one pot’ synthesis step, e.g., without an intervening solid / liquid separation step.

[0043] In contrast with prior art processes, surprisingly, the inventors have found that presence of the metakaolin in the blended feed provides a slow source of dissolved aluminium, which forms an amorphous zeolite precursor, to promote the zeolite reaction. The metakaolin thereby acts as a source for the templating agent to form the final LTA product, and provides a large window of time to structure-direct away from other higher-silica zeolite phases. If the source of aluminate is not slow dissolving, such as in the presence of an initially supersaturated source of soluble aluminate, undesirable zeolite phases such as sodalite (Nae(AlSiO4)6) can form as a result.

[0044] Figure 1 is a process flow diagram of a first embodiment of the invention in which coal combustion products (CCP) 100 and metakaolin 102 are provided as a blended feed to a stirred reaction vessel 104. The feed may be blended upstream of, on entry to, or in stirred reaction vessel 104. The feed may be a blend of the CCP and metakaolin in the range of about 10:90 to 90: 10 by weight.

[0045] Aqueous sodium hydroxide solution is added to stirred reaction vessel 104. The sodium hydroxide may be fresh sodium hydroxide 106 (labelled as make-up NaOH in Figure 1), a recycled NaOH stream 108 which is recovered from one or more downstream processes, or a combination thereof. In any event, the blended feed and the aqueous sodium hydroxide are mixed to form a slurry. In this example, the solid loading of the slurry is from about 50 to about 400 g /L.

[0046] The sodium hydroxide acts as a leachant causing aluminate and silica to be dissolved out from the coal combustion products and metakaolin. Advantageously, the presence of the slow- dissolving metakaolin aids in control of the Al concentration in solution. In the context of the embodiment of Figure 1, the leaching is carried out with 2-6 M NaOH solution under atmospheric pressure and at a temperature that is less than the boiling point of the solution, in this case at a temperature in the range of 75-95 °C. The slurry is retained in stirred reaction vessel 104 for sufficient time to crystallise or otherwise precipitate zeolite A. Typically the slurry is retained in stirred reaction vessel 104 for up to 6 hours. However, longer retention times e.g., of up to 24 hours can be used. The inventors have found that the blended feed in combination with the aforementioned reaction conditions is conducive to forming zeolite A rather than zeolites of other structures.

[0047] The slurry containing zeolite A is then passed to a filtration step 110 for solid liquid separation whereby the liquid stream containing NaOH is recovered as recycle stream 108 for reuse in the synthesis of further zeolite A. A source of make-up aluminate 112 may be added to this stream to maintain a recycle-able ratio of Al: Si. The skilled person will appreciate that make-up aluminate 112 may also be added directly to stirred reaction vessel 104 via an independent stream or with the make-up NaOH or via an independent stirred vessel. The solid stream 114 contains zeolite A in a concentration typically greater than 50 wt%.

[0048] Figure 2 is a process flow diagram illustrating a second embodiment of the invention for forming zeolites, and in particular zeolite A. In this embodiment, the CCP, in this case fly ash 200, is subjected to an initial silicon leaching step, and the leached residue of this fly ash is then blended with metakaolin which is then treated to form zeolite A.

[0049] In more detail, coal combustion products 200 are provided to first stirred reaction vessel 202 where it is mixed with aqueous NaOH at a concentration of around 4 M to form a slurry. The slurry has a solid loading of about 25-400 g/L similar to the embodiment of Figure 1. The NaOH solution may be provided fresh as make-up NaOH stream 204 or as a recycle stream 206 from a downstream part of the process. The NaOH solution acts as a leachant to primarily leach silicon from the fly ash. The leaching step is carried out at a temperature below the boiling point of the leachant, and at a temperature in the range of 75-95 °C. Leachate 208 containing primarily dissolved silica and solid leached fly ash residue 210 are then separated via a solidliquid separation process, which in this case is filtration step 212.

[0050] Leachate 208 can then be fed to reactor 214 where it is treated with an aluminate source 216 for the purpose of crystallising zeolites. The reaction conditions used in the step can be varied to target zeolites of different structures. The resulting zeolites are then separated from the leachate in filtration step 218 to provide a product zeolite stream 220 and NaOH recycle stream 206. High purity zeolites 220 can be produced and recovered using this method.

[0051] Returning to solid leached fly ash residue 210, this is blended with metakaolin for the purpose of producing zeolite A in a process like that described in relation to the embodiment of Figure 1. Briefly, solid leached fly ash residue is blended with metakaolin in a ratio of from about 10:90 to about 90: 10 and treated with aqueous 2-6 M NaOH to a solid loading of 50-400 g/L. Leaching and precipitation 222 are carried out under atmospheric pressure at a temperature from 75-95 °C. Solid-liquid separation in the form of filtration step 224 is then used to recover a solids stream 226 which contains zeolite A in a concentration typically greater than 50 wt% with the liquid stream 228 containing NaOH being recycled to an earlier step in the process.

[0052] The invention is described below in relation to a preferred embodiment thereof. [0053] This example broadly reports results from a process for forming zeolite A according to one embodiment of the invention that comprises the following steps (i) calcining a kaolin containing material to form metakaolin, (ii) blending metakaolin with coal combustion products to produce a blended feed, (iii) mixing the blended feed with an aqueous caustic solution to form a slurry, (iv) leaching the feed materials and crystallising zeolite A from the slurry, and (v) separating the zeolite A from at least the liquid fraction of the slurry.

[0054] The skilled person will appreciate that this embodiment is intended to be illustrative of the process and is intended to be non-limiting. In other embodiments, certain process steps may be omitted, for example, metakaolin may be sourced directly which would obviate the need for the initial calcining step. Nevertheless, in some embodiments of the invention, it is advantageous to include the calcining step since this may result in improved process control and/or a superior metakaolin product.

[0055] Elemental assays of the CCP and kaolin used in this example were determined by X-ray fluorescence (XRF) and are provided in Table 1.

Table 1. Chemical composition of the materials used determined by XRF (wt. %)

Sample AI2O3 CaO Fe₂O₃ K₂O MgO Na₂O P₂O₅ SiO₂ TiO₂ LOI

CCP 21.95 4.81 6.72 1.54 1.35 0.25 1.26 54.76 1.28 5.11

Kaolin 37.4 0.01 0.36 0.25 0.06 0.1 0.15 45.5 1.58 13.55

Kaolin activation

[0056] The raw kaolin was dry screened at 18 mesh before being thermally activated (calcined) in a muffle furnace at a temperature of 850 °C for a period of 6 hours to fully transform the kaolinite proportion into metakaolin. The temperature and period of calcination are not known to have a substantial influence on the effectiveness of the process, as long as the kaolin has been fully transformed into metakaolin. The resultant metakaolin was dry screened again at 60 mesh. The metakaolin was confirmed to be fully calcined by the distinct lack of kaolinite peaks in the X-ray diffraction (XRD) pattern, with minor amounts of titania present as anatase (TiCh) persisting through calcination. The XRD patterns of the metakaolin and initial raw kaolin are provided in Figure 3. Figure 3 shows X-ray diffractograms of a) calcined kaolin showing full transformation to metakaolin, and b) raw kaolin. The letters annotated above peaks indicate corresponding phases K: kaolinite, T : anatase.

Results from pure CCP and CCP-metakaolin blends

[0057] The XRD pattern of the CCPs used in these trials is provided in Figure 4. Figure 4 contains X-ray diffractograms of the CCP used in this example. The letters annotated above peaks indicate corresponding phases M: mullite, Q: quartz, N: andradite. The material is predominantly amorphous alumina and silica as indicated by the broad hump at 15-30 29, with a large make-up of crystalline quartz and mullite and lesser amounts of andradite (CasFciSisOii).

[0058] The CCP with an elevated concentration of CFA was contacted with hot, concentrated NaOH in a stirred reactor vessel, either on its own or blended with metakaolin, in a prior known mass ratio. The resultant slurry was allowed to react for a specified time to crystallise zeolites and then pressure filtered through a 2pm filter paper. The filter cake was dried overnight at 105 °C. The solids were subsequently collected and analysed for phase and elemental composition. Several experiments were conducted following the experimental matrix given in Table 2. Optimal conditions were determined by maximising the zeolite A mass proportion in the final product compared to the amount of feed material processed.

Table 2. Zeolite synthesis conditions from coal combustion products. Optimal conditions are donated by bracketed () values.

NaOH Solid/liquid ratio CCP/ metakaolin Temperature Time

(M) (g/L) ratio (°C) (hours)

2-6 (4) 50-400 (300) 10-90 (75) 75-95 (85) 1-24 (5)

[0059] Three experimental results at optimised synthesis conditions are discussed herein. The conventional hydrothermal method led to the dissolution of amorphous alumina and silica present in the CCP which was expected. In the trial with pure CCPs, zeolite Pl formed early in the reaction and persisted without any further phase transformation. Significant portions of unreacted quartz and mullite also remained in the final product. Tests with CCP-metakaolin blends resulted in the preferential synthesis of zeolite A over other zeolite phases as determined by XRD peak intensity in the final product. After leaving the reaction for a prolonged period, the zeolite A can slowly transform into zeolite P, and hence the reaction residence time must be controlled at or around the time where leached Si is at a maximum.

[0060] To isolate the effect of the slow-dissolving metakaolin, the same experiment was completed with an addition of NaAlCh equal to 0.25 M in the starting solution. This caused significant precipitation of sodalite, indicating that the same result cannot be replicated with a non-metakaolin aluminium source present in the reaction. Without wishing to be bound by theory, the inventors are of the view that this is likely due to the high initial supersaturation of aluminium in solution which promotes the formation of sodalite, which itself then acts as a template to create more sodalite, as observed in processes such as the Bayer Process. These observations are shown in Figure 5. Figure 5 reports X-ray diffractograms of CCPs as follows a) in a 75% blend with metakaolin, b) with an initial source of sodium aluminate equal to 0.25M, c) untreated CCP. The letters annotated above peaks indicate corresponding phases A: zeolite A, S: sodalite, P: zeolite Pl, Q: quartz.

[0061] Advantageously, if the blend ratio is sufficiently high in CFA such that an accumulation of Si in solution occurs, the subsequent filtrate is of high quality and concentration to precipitate other zeolites of near 100% purity as shown in Figure 2.

[0062] Scanning electron microscopy (SEM) imaging performed of the raw CCP and final zeolite produced from synthesis conditions disclosed in this patent are shown in Figure 6. Figure 6 reports SEM images of: a) the CCPs, and b) zeolite A product after synthesis from CCP-metakaolin. The distinct cubic particle morphology of zeolite A is evident.

Resultant zeolite composition

[0063] The XRD patterns presented in Figure 5 indicates that the resultant solid is a product with a majority composition of zeolite A. The chemical composition of the produced zeolite A (referred to as Zeolite-CCP below) was determined by XRF and is provided in Table 3 along with a comparison to commercial zeolite A. XRF reconciliation on Na data, assuming that LT A is the only Na-bearing phase, indicates that zeolite-CCP products were synthesised with greater than 55% purity zeolite A.

Table 3. Chemical composition of the materials used determined by XRF (wt. %) Sample A1₂O CaO Fe₂O K₂O MgO Na₂O P₂O₅ SiO₂ TiO₂ LOI

Zeolite-CCP 26.0 2.74 3.78 0.48 0.79 11.25 0.55 39.6 1.25 11.81

Zeolite-Commercial 27.2 0.07 0.02 0.12 0.01 20.4 0.01 31.8 0.01 20.1

[0064] Advantageously, analysis of the zeolite-CCP product shows that it contains a substantially lower content of heavy metals which can be considered environmentally toxic (Pb, Ni, Zn, etc.) than the starting feed material due to the increased uptake of sodium and dilution with metakaolin components. The heavy metals may also be advantageously incorporated into the zeolitic structure, negating long-term environmental leaching. This unlocks use of this products for water and fertiliser industries. Heavy metal concentrations were determined by inductively coupled plasma mass spectroscopy (ICP-MS) following a 4-acid dissolution method. The concentrations of the heavy metals determined in the products and their feed source are presented in Table 4.

Table 4. Heavy metal concentration of the materials used determined by ICP-MS (ppm)

Sample As Cd Co Cr Hg Ni Pb Rb Sn Zn

CCP 3.5 <0.5 15 31 0.07 14 32 34 5.6 140

Zeolite-CCP 1.5 <0.5 12 59 0.02 28 21 15 3.2 45

[0065] It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.

Claims

1. A method to produce zeolite A comprising: forming a slurry of a blend of coal combustion products or a leached residue thereof with metakaolin and aqueous alkaline solution; and maintaining the slurry at a temperature below the boiling point of the aqueous alkaline solution at atmospheric pressure for a time sufficient to dissolve aluminate and silica species into solution and precipitating zeolite A from the dissolved aluminate and silica species.

2. A method to produce zeolite A comprising: leaching a blend of coal combustion products or leached residue thereof with metakaolin using an aqueous alkaline solution to provide a leach solution comprising dissolved aluminate and dissolved silica; and precipitating zeolite A from the leach solution; wherein the method is carried out at a temperature that is less than boiling point of the aqueous alkaline solution at atmospheric pressure.

3. The method of claim 1 or 2, wherein the method is carried out at atmospheric pressure.

4. The method of any one of the preceding claims, wherein the method is carried out at a temperature of at least 70 °C, and preferably at least 75 °C.

5. The method any one of the preceding claims, wherein the method is carried out at a temperature of less than 100 °C, and preferably at a temperature of up to 95 °C.

6. The method of any one of the preceding claims, wherein the blend of coal combustion products or leached residue thereof with metakaolin has a ratio of coal combustion products or leached residue thereof to metakaolin of 10:90 to 90: 10.

7. The method of any one of the preceding claims, wherein the coal combustion products comprise, consist of, or consist essentially of coal fly ash, bottom ash, desulfurization gypsum, and combinations thereof.

8. The method of any one of the preceding claims, wherein the aqueous alkaline solution is a NaOH solution, preferably a 2 to 6 M NaOH solution.

9. The method of claim 2, wherein the leaching step provides a solid residue, and the zeolite A is crystallised in the presence of the solid residue.

10. The method of claim 2, wherein the precipitation step is carried out for a time of up to 24 hours, preferably up to 18 hours, more preferably up to 9 hours, most preferably up to 6 hours.

11. The method of claim 1, wherein the method is carried out for a time of up to 24 hours, preferably up to 18 hours, more preferably up to 9 hours, most preferably up to 6 hours.

12. The method of claim 2, wherein the leaching step comprises forming a slurry of the mixture of coal combustion products and metakaolin with the aqueous alkaline solution to dissolve aluminate and silica in the mixture of coal combustion products and metakaolin and provide the dissolved aluminate and dissolved silica.

13. The method of claim 1 or 12, wherein the slurry is agitated.

14. The method of any one of the preceding claims further comprising a solid-liquid separation step to provide a solid stream comprising zeolite A and an aqueous liquid stream comprising NaOH.

15. The method of claim 14, wherein the solid stream comprises zeolite A in an amount of at least 50 wt% and preferably up to 95 wt%, more preferably, the solid stream comprises zeolite A in an amount of at least 75 wt% in the zeolitic portion.

16. The method of claims 14 or 15, wherein the aqueous liquid stream is recycled.

17. The method of any one of the preceding claims, wherein the method is carried out as a batch or continuous reaction process.

18. The method of any one of the preceding claims, wherein a solid concentration of the blend in the aqueous alkaline solution is from 50 g/L to 400 g/L.

19. Zeolite A produced according to the method of any one of the preceding claims.