WO2021214289A1

WO2021214289A1 - Process for making a refractory article

Info

Publication number: WO2021214289A1
Application number: PCT/EP2021/060660
Authority: WO
Inventors: Erik Severin; John Vincent Adap MISA; Erwin N. FERNANDEZ; Sandor Koszo; Peter Ma
Original assignee: Vecor Ip Holdings Limited; Vecor Ceramic Tiles Italia S.R.L.
Priority date: 2020-04-23
Filing date: 2021-04-23
Publication date: 2021-10-28
Also published as: GB202005989D0

Abstract

The present invention relates to a process for making a refractory article, the process comprises the steps of: (a) preparing a particulate refractory mixture by: (I) obtaining coal combustion fly ash having a bimodal particle size distribution such that: (i) the smaller peak is in the range of from greater than 32μm to 75μm; and (ii) the larger peak is in the range of from greater than 90μm to 250μm; and (II) contacting the coal combustion fly ash with binder and/or water, wherein the particulate refractory mixture comprises: (i) from 90wt% to 99wt% coal combustion fly ash (I); and (ii) from 1wt% to 10wt% binder and/or water (II); (b) optionally, contacting the particulate refractory mixture to water; (c) pressing the particulate refractory mixture to form a green article; (d) optionally, subjecting the green article to an initial drying step; (e) subjecting the green article to a firing step in a kiln to form a hot article; and (f) cooling the hot article to form a refractory article.

Description

Process for making a refractory article

Field of the invention

The present invention relates to a process for making a refractory article. The present invention uses coal combustion fly ash particles having a specific bimodal particle size distribution, which improves the strength of the resultant refractory article whilst maintaining a low thermal conductivity profile.

Background of the invention

Coal combustion fly ash is one of the most abundant waste materials on earth. The burning of coal to generate power has resulted in the production of huge amounts of ash products. The large majority of ash produced by coal combustion power stations is in the form of coal combustion fly ash, which is the fine ash that is carried out in the exhaust gases. Much smaller amounts of ash are in the form of large cinders. This ash is called bottom ash. This coal combustion fly ash is an environmental problem. Traditionally, the coal combustion fly ash was disposed of in landfill sites and thousands of square miles of land are now taken up by these landfill sites.

More recently, attempts have been made to use this coal combustion fly ash. For example, the cement industry uses some of the finer coal combustion fly ash material. However, a large amount of coal combustion fly ash, and especially the coarser coal combustion fly ash material, is still disposed of in landfill sites.

There are environmental benefits in using coal combustion fly ash, as a raw material, in as many products as possible. Using coal combustion fly ash as a raw material in other products reduces the amount going to landfill sites, and reduces the amounts of other raw materials, such as clay, that need to be used in these other products. This has environmental benefits. Increasing the range of products that can incorporate coal combustion fly ash and increasing the proportions of coal combustion fly ash that can be incorporated into such products is highly desirable.

Using coal combustion fly ash in refractory articles is not straight forward. It is difficult to incorporate very high levels of coal combustion fly ash into a refractory article and obtain a refractory article having good strength, low thermal conductivity and good thermal resistance.

A refractory article is thermally resistant, being capable of withstanding high temperatures without deformation. Refractory articles are typically used to line the interiors of furnaces, ovens and boilers, amongst other applications. A good refractory article needs to be thermally resistant and it is very desirable for the refractory article to also have a low thermal conductivity, be resistant to thermal shock, be chemically inert and physically robust. The majority of refractory articles are used in iron and steel production. Such refractory articles need to combine good thermal properties with good strength.

There are many types of refractory articles, with the majority being based on the oxides of aluminium, magnesium and silicon. Refractory articles typically have significant internal porosity to reduce their thermal conductivity. Usually, the higher the porosity, the lower the thermal conductivity.

However, increasing the porosity of the article typically reduces its strength. Highly porous refractory articles usually have poor (low) strengths (e.g. as shown by low cold crushing strength values and/or a low Modulus of Rupture (MOR) values. Typically, refractory articles are by necessity a balance of these conflicting requirements of good thermal properties and good strength.

The inventors have found that coal combustion fly ash can be incorporated into refractory articles at very high levels. The present invention provides a method of making a refractory article. The process of the present invention uses coal combustion fly ash particles having a specific bimodal particle size distribution, which further improves the strength of the resultant refractory article whilst maintaining high thermal resistance and a low thermal conductivity profile.

Summary of the invention

The present invention provides a process for making a refractory article, the process comprises the steps of: (a) preparing a particulate refractory mixture by: (I) obtaining coal combustion fly ash having a bimodal particle size distribution such that: (i) the smaller peak is in the range of from greater than 32pm to 75pm; and (ii) the larger peak is in the range of from greater than 90pm to 250pm; and (II) contacting the coal combustion fly ash with binder and/or water, wherein the particulate refractory mixture comprises: (i) from 90wt% to 99wt% coal combustion fly ash (I); and (ii) from lwt% to 10wt% binder and/or water (II); (b) optionally, contacting the particulate refractory mixture to water; (c) pressing the particulate refractory mixture to form a green article; (d) optionally, subjecting the green article to an initial drying step; (e) subjecting the green article to a firing step in a kiln to form a hot article; and (f) cooling the hot article to form a refractory article.

Brief description of the drawings

Figure 1: Figure 1 shows a suitable bimodal particle size distribution of the coal combustion fly ash.

Figure 2: Figure 2 shows a suitable bimodal particle size distribution of the coal combustion fly ash.

Detailed description of the invention

Process of making a refractory article. The process comprises the steps of: (a) preparing a particulate refractory mixture by: (I) obtaining coal combustion fly ash having a bimodal particle size distribution such that: (i) the smaller peak is in the range of from greater than 32pm to 75pm; and (ii) the larger peak is in the range of from greater than 90pm to 250pm; and (II) contacting the coal combustion fly ash with binder and/or water, wherein the particulate refractory mixture comprises: (i) from 90wt% to 99wt% coal combustion fly ash (I); and (ii) from lwt% to 10wt% binder and/or water (II); (b) optionally, contacting the particulate refractory mixture to water; (c) pressing the particulate refractory mixture to form a green article; (d) optionally, subjecting the green article to an initial drying step; (e) subjecting the green article to a firing step in a kiln to form a hot article; and (f) cooling the hot article to form a refractory article.

Step (a) preparing the particulate refractory mixture. During step (a) a particulate refractory mixture is prepared by: (I) obtaining coal combustion fly ash having a bimodal particle size distribution such that: (i) the smaller peak is in the range of from greater than 32mih to 75mih; and (ii) the larger peak is in the range of from greater than 90mih to 250mih and (II) contacting the coal combustion fly ash with binder and/or water.

Typically, the particulate refractory mixture obtained in step (a) comprises at least 90wt% coal combustion fly ash.

Step (a)(1) obtaining the coal combustion fly ash. During step (a)(1) the coal combustion fly ash is prepared. The coal combustion fly ash has a bimodal particle size distribution such that: (i) the smaller peak is in the range of from greater than 32pm to 75pm; and (ii) the larger peak is in the range of from greater than 90pm to 250pm. The coal combustion fly ash can be prepared by any suitable means, for example by size classification, for example by sieving.

Preferably, step (a)(1) comprises the step of blending fine coal combustion fly ash with coarse coal combustion fly ash. The fine coal combustion fly ash and the coarse coal combustion fly ash are described in more detail below.

Preferably, the coal combustion fly ash obtained in step (a)(1): (a) comprises from 5wt% to 40wt%, or from 5wt% to 35wt%, or from 10wt% to 35wt%, fine coal combustion fly ash; and (b) comprises from 60wt% to 95wt%, or from 65wt% to 95wt%, or from 65wt% to 90wt%, coarse coal combustion fly ash.

Preferably, during step (a)(1) the coal combustion fly ash is subjected to an air classification step to separate coarse coal combustion fly ash from the remainder of the coal combustion fly ash. The remainder of the coal combustion fly ash is then preferably subjected to a milling step to obtain fine coal combustion fly ash. Preferably, the coarse coal combustion fly ash is contacted with the fine coal combustion fly ash to obtain coal combustion fly ash.

Preferably, the coal combustion fly ash has a bimodal particle size distribution such that the ratio of (i) the particle size of the larger peak to (ii) the particle size of the smaller peak is in the range of from 2:1 to 4:1, or from 2.5:1 to 3.5:1.

In one preferred embodiment, the coal combustion fly ash has a bimodal particle size distribution such that: (i) the smaller peak is in the range of from greater than 32pm to 53 pm; and (ii) the larger peak is in the range of from greater than 90pm to 150pm.

In one preferred embodiment, the coal combustion fly ash has a bimodal particle size distribution such that: (i) the smaller peak is in the range of from greater than 53pm to 75pm; and (ii) the larger peak is in the range of from greater than 125pm to 250pm. Step (a)(1) may comprise the step of blending fine coal combustion fly ash with coarse coal combustion fly ash, wherein the fine coal combustion fly ash has a weight average particle size in the range of from in the range of from greater than 32pm to 75pm, and wherein the coarse coal combustion fly ash has a weight average particle size in the range of from in the range of from greater than 90pm to 250pm.

The fine coal combustion fly ash may have a particle size in the range of from in the range of from greater than 32pm to 53 pm, and the coarse coal combustion fly ash may have a weight average particle size in the range of from in the range of from greater than 90pm to 150pm.

The fine coal combustion fly ash may have a particle size in the range of from in the range of from greater than 53pm to 75pm, and the coarse coal combustion fly ash may have a weight average particle size in the range of from in the range of from greater than 125 pm to 250pm.

The particulate refractory mixture comprises:

(i) from 90wt% to 99wt% coal combustion fly ash (I); and

(ii) from lwt% to 10wt% binder and/or water (II).

Typically, the particulate refractory mixture obtained in step (a) comprises from 95wt% to 99wt% coal combustion fly ash (I); and from lwt% to 5wt% binder and/or water (II).

Step (a)(II) contacting the coal combustion fly ash with binder and/or water.

During step (a)(II) the coal combustion fly ash is contacted with binder and/or water. The binder can be dissolved in water and contacted with the coal combustion fly ash during a humidification step. Alternatively, the binder can be added as a powder. Typically, a binder powder is added to the coal combustion ash prior to a humidification step and uses the agitation of the humidification step to disperse and blend the powder binder.

Step (b) optional humidification step. During the optional step (b) the particulate refractory mixture may be contacted with water. The humidification step is typically done in a high-speed mixer where water is dispersed uniformly throughout the powder blend by the action of tools rotating at high speed. Suitable equipment for the humidification step would be the Schugi Flexomixer^(RTM) series from Hosokawa Bepex^(RTM) operating at speeds greater than 1000 rpm. Typical water levels for humidification are typically up to 10% and binder levels of up to 5% can be used. In lab-scale use, mixing for 30 seconds in a kitchen mixer such as the Kenwood FP120^(RTM) is suitable.

Step (c) pressing step. During step (c) the particulate refractory mixture is pressed to form a green article. The mixture is typically spread uniformly in a mold or belt with constraining device. Pressure is then applied for several seconds, typically less than 15 seconds, to compress the powder. The compression pressure can be greater than 30 MPa or greater than 50 MPa or greater than 70 MPa. Higher compression pressures such as 150 MPa or higher are not required or beneficial but normally do not have a negative effect.

Step (d) optional initial drying step. During the optional step (), the green article may be subjected to an initial drying step. This drying step can be done by placing the green article in an oven at 100 °C for one hour. High drying temperatures are not commonly used as this can cause defects from steam generation.

Step (e) firing step. During step (e) the green article is subjected to a firing step in a kiln to form a hot article. Preferably, the green article is sintered during the firing step (e). Typically, the temperature is increased to at least 1450°C, or at least 1475°C, or at least 1500°C, or at least 1525°C, or at least 1550°C, or at least 1575°C, during the firing step (e).

The firing step can be done in an electric kiln with programmable temperature control. A heating ramp rate of 2.5°C/min can be used to increase the temperature in the kiln to the maximum target temperature, e.g. 1550°C. Typically, the temperature is held at the maximum temperature for an extended period of time, for example 30 min.

Step (f) cooling step. During step (f) the hot article is cooled to form a refractory article. Typically, the hot article is allowed to cool slowly, for example by letting the kiln cool down naturally, so as to avoid the generation of thermal stresses.

Particulate refractory mixture. The particulate refractory mixture comprises: (a) coal combustion fly ash; and (b) binder and/or water. Typically, the particulate refractory mixture comprises at least 90wt% coal combustion fly ash. The particulate refractory mixture can comprise from lwt% to 10wt% ingredients selected from binder and/or water. Coal combustion fly ash. The coal combustion fly ash has a bimodal particle size distribution such that: (i) the smaller peak is in the range of from greater than 32pm to 75pm; and (ii) the larger peak is in the range of from greater than 90pm to 250pm.

Preferably, the coal combustion fly ash is a blend of fine coal combustion fly ash and coarse coal combustion fly ash. The coal combustion fly ash may comprise from 5wt% to 40wt%, or from 5wt% to 35wt%, or from 10wt% to 35wt%, fine coal combustion fly ash.

The coal combustion fly ash may comprise from 60wt% to 95wt%, or from 65wt% to 95wt%, or from 65wt% to 90wt% coarse coal combustion fly ash.

In one preferred embodiment, the coal combustion fly ash has a bimodal particle size distribution such that: (i) the smaller peak is in the range of from greater than 53pm to 75pm; and (ii) the larger peak is in the range of from greater than 125pm to 250pm.

The coal combustion fly ash may comprise a blend of fine coal combustion fly ash with coarse coal combustion fly ash. The fine coal combustion fly ash and coarse coal combustion fly ash are described in more detail below.

The coal combustion fly ash may comprise a blend of fine coal combustion fly ash and coarse coal combustion flay, wherein the fine coal combustion fly ash has a weight average particle size in the range of from in the range of from greater than 32pm to 75pm, and wherein the coarse coal combustion fly ash has a weight average particle size in the range of from in the range of from greater than 90pm to 250pm.

The coal combustion fly ash may comprise a blend of fine coal combustion fly ash and coarse coal combustion flay, wherein the fine coal combustion fly ash may have a particle size in the range of from in the range of from greater than 32pm to 53 pm, and the coarse coal combustion fly ash may have a weight average particle size in the range of from in the range of from greater than 90pm to 150pm.

The coal combustion fly ash may comprise a blend of fine coal combustion fly ash and coarse coal combustion flay, wherein the fine coal combustion fly ash may have a particle size in the range of from in the range of from greater than 53 pm to 75 pm, and the coarse coal combustion fly ash may have a weight average particle size in the range of from in the range of from greater than 125 pm to 250pm.

Preferably, the coal combustion fly ash comprises less than 10wt%, or less than 8wt%, or less than 5wt%, carbon.

Fine coal combustion fly ash. Fine coal combustion fly ash typically has a weight average particle size in the range of from in the range of from greater than 32pm to 75pm.

Typically, at least 90wt%, or at least 95wt%, or at least 99wt%, or even substantially all, of the fine coal combustion fly ash has a particle size in the range of from in the range of from 32pm to 75pm.

One preferred fine coal combustion fly ash typically has a weight average particle size in the range of from in the range of from greater than 32pm to 53 pm. Typically, at least 90wt%, or at least 95wt%, or at least 99wt%, or even substantially all, of the fine coal combustion fly ash has a particle size in the range of from in the range of from 32pm to 53pm.

One preferred fine coal combustion fly ash typically has a weight average particle size in the range of from in the range of from greater than 53pm to 75pm. Typically, at least 90wt%, or at least 95wt%, or at least 99wt%, or even substantially all, of the fine coal combustion fly ash has a particle size in the range of from in the range of from 53 pm to 75pm.

Coarse combustion fly ash. Coarse coal combustion fly ash typically has a weight average particle size in the range of from in the range of from greater than 90pm to 250pm, or to 249pm, or to 240pm , or to 230pm, or to 220pm, or to 210pm, or to 200pm. Typically, at least 90wt%, or at least 95wt%, or at least 99wt%, or substantially, of the coarse coal combustion fly ash has a particle size in the range of from in the range of from greater than 90pm to 250pm, or to 249pm, or to 240pm , or to 230pm, or to 220pm, or to 210pm, or to 200pm.

One preferred coarse coal combustion fly ash typically has a weight average particle size in the range of from in the range of from greater than 90pm to 150pm. Typically, at least 90wt%, or at least 95wt%, or at least 99wt%, or substantially, of the coarse coal combustion fly ash has a particle size in the range of from in the range of from 90pm to 150pm. One preferred coarse coal combustion fly ash typically has a weight average particle size in the range of from in the range of from greater than 125pm to 250pm, or to 249pm, or to 240pm , or to 230pm, or to 220pm, or to 210pm, or to 200pm. Typically, at least 90wt%, or at least 95wt%, or at least 99wt%, or substantially, of the coarse coal combustion fly ash has a particle size in the range of from in the range of from greater than 125pm to 250pm, or to 249pm, or to 240pm , or to 230pm, or to 220pm, or to 210pm, or to 200pm.

Binder. Any suitable binder can be used. A suitable binder is an organic polymer, such as polyvinyl alcohol. A preferred binder is a polysaccharide. A most preferred binder is dextrin.

Refractory article. The refractory articles made by the inventive process have a porous internal structure resulting from the larger fly ash particles being sintered together to give a porous interconnected lattice structure in which the individual fly ash particles, especially the larger fly ash particles, still retain substantial and discernible aspects of their original shape and size. A majority of the particles have discernible dimensions of greater than 150pm. The temperatures used do not melt the larger particles together to form a single vitrified mass as such a mass would have a high thermal conductivity due to the low porosity. However, the inclusion of a limited proportion of fine particles of fly ash (or other ceramic material) allows for the sintered bridges connecting particles to be larger, thus increasing ceramic article robustness. The fine particles are more susceptible to temperature than the coarse particles and help sinter the coarser particles together. The refractory articles are characterised by good thermal resistance in that they can withstand high temperatures without deformation and low thermal conductivity. The refractory article typically has an apparent porosity of between 30% and 50%. This range ensures a balance of article robustness and low thermal conductivity. The refractory articles show increased robustness compared to many commercial refractory tiles thus permitting a wide range of use and the use of waste fly ash lowers the production cost.

Method of measuring particle size. The particle size distribution is typically measured by laser diffraction. A suitable standard for size analysis by laser diffraction is given in ISO 13320:2009. Suitable size analysers are the Mastersizer^(RTM) 2000 and 3000 instruments by Malvern Instrum ents^(RTM). It is preferred to disperse the samples by compressed air (usually with a Scirocco 2000^(RTM) unit) where the material is tested as a powder stream, rather than the wet method where the test material is dispersed in a fluid first. However, it is possible to disperse and test these mixtures in non-aqueous liquids. The measurement is typically done as per the manufacturer’s instruction manual and test procedures.

The results are typically expressed in accordance with ISO 9276-2.

Method of measuring carbon content. The amount of carbon is typically measured by the Loss on Ignition (LOI) test as per ASTM D7348. In this test, 1 g of sample is first dried at 110 °C to dry the sample. The sample is then cooled to room temperature and weighed. The dried sample is then heated in a step wise manner over a two-hour period to reach 950 °C and allowed to cool to room temperature and the weight loss measured.

The Modulus of Rupture Test. The MOR of a sample can be measured by testing a sample according to ASTM C1505-15. The article being tested, such as a tile, is placed resting on two parallel, cylindrical support rods such that the edges of the tile are parallel to the axis of the rods. The span between the rods is a defined distance L and the edges of the tile need to overhang the support rods. The test tiles have a width b (mm) and a thickness h (mm). A third rod is placed across the middle of the tile and parallel to the others. An increasing load is applied to the middle rod until the test tile ruptures or breaks at the breaking load P (N). This can be used to calculate B, the breaking strength, using the equation B = (P x L)/b. The Modulus of Rupture R is then given by the equation R = 3B/2h².

Examples

Inventive Example 1

Coal combustion fly ash is sieved to produce two fractions (i) between 90pm and 250pm and (ii) between 32pm and 75pm. Sample (i) has a peak of 130pm and sample (ii) has a peak of 45pm.

A sample of fly ash having the desired bimodal size distribution is prepared as follows.

170 grams of fraction (i) are mixed with 30g of fraction (ii).

The fly ash mixture from above (200g) is then mixed with 6g of fine powdered dextrin with agitation, followed by lOg of water which are sprayed onto the mix with agitation to form the humidified mixture binder.

140g of the mix are then uniaxially pressed in a rectangular mild steel mold (155x40mm) to a pressure of 40MPa which is held for 1.5min (90sec) to form a green body. The green body is released from the mold and placed into a 110°C oven for one hour to dry.

The dried green body is then fired in an electric kiln at a ramp rate of 2.5°C/min to 1550°C. The temperature is held at the top temperature for 30min. The fired body is then allowed to cool down naturally (hence slowly) to room temperature.

On testing, the fired refractory body has a MOR of greater than 20MPa.

Comparative Example 2

The above test is repeated but now without the addition of the finer fly ash. Only the coarser fly ash is used. 200g of fly ash having a size range of 90pm - 250pm is mixed with 6g of fine powdered dextrin and lOg of water in the same manner as before. The humidified mixture is formed into a green article and fired in exactly the same manner as the previous sample.

When tested, this fired refractory body only has a MOR of 1 lMPa. Comparative Example 3

Fly ash is sieved as above to produce a coarse fraction (i) between 90pm and 250pm with a peak of 130pm. Fine fly ash (ii) of less than 20pm with a peak of 12pm is obtained by classification.

A sample of fly ash having a bimodal size distribution but where the fine fly ash is too fine and less than 32pm is prepared as follows.

170g of fraction (i) is mixed with 30g of fraction (ii).

The mixture is then pressed into a mold to form a green article and fired in exactly the same manner as Inventive Sample 1.

When tested, this sample has a MOR of 15 MPa.

Claims

1. A process for making a refractory article, the process comprises the steps of:

(a) preparing a particulate refractory mixture by:

(I) obtaining coal combustion fly ash having a bimodal particle size distribution such that:

(i) the smaller peak is in the range of from greater than 32pm to 75pm; and

(ii) the larger peak is in the range of from greater than 90pm to 250pm; and

(II) contacting the coal combustion fly ash with binder and/or water, wherein the particulate refractory mixture comprises:

(i) from 90wt% to 99wt% coal combustion fly ash (I); and

(ii) from lwt% to 10wt% binder and/or water (II);

(b) optionally, contacting the particulate refractory mixture to water;

(c) pressing the particulate refractory mixture to form a green article;

(d) optionally, subjecting the green article to an initial drying step;

(e) subjecting the green article to a firing step in a kiln to form a hot article; and

(f) cooling the hot article to form a refractory article.

2. A process according to claim 1 wherein the coal combustion fly ash has a bimodal particle size distribution such that the ratio of the particle size of the larger peak to the particle size of the smaller peak is in the range of from 2: 1 to 4: 1.

IB

3. A process according to claim 1, wherein the coal combustion fly ash has a bimodal particle size distribution such that:

(i) the smaller peak is in the range of from greater than 32pm to 53 pm; and

(ii) the larger peak is in the range of from greater than 90pm to 150pm.

4. A process according to claim 1, wherein the coal combustion fly ash has a bimodal particle size distribution such that:

(i) the smaller peak is in the range of from greater than 53pm to 75pm; and

(ii) the larger peak is in the range of from greater than 125pm to 250pm.

5. A process according to any preceding claim, wherein step (a)(1) comprises the step of blending fine coal combustion fly ash with coarse coal combustion fly ash, wherein the fine coal combustion fly ash has a weight average particle size in the range of from in the range of from greater than 32pm to 75pm, and wherein the coarse coal combustion fly ash has a weight average particle size in the range of from in the range of from greater than 90pm to 250pm.

6. A process according to claim 5, wherein the fine coal combustion fly ash has a particle size in the range of from in the range of from greater than 32pm to 53 pm, and wherein the coarse coal combustion fly ash has a weight average particle size in the range of from in the range of from greater than 90pm to 150pm.

7. A process according to claim 5, wherein the fine coal combustion fly ash has a particle size in the range of from in the range of from greater than 53 pm to 75 pm, and wherein the coarse coal combustion fly ash has a weight average particle size in the range of from in the range of from greater than 125pm to 250pm.

8. A process according to any of claims 5-7, wherein the coal combustion fly ash obtained in step (a)(1): (a) comprises from 5wt% to 40wt% fine coal combustion fly ash; and

(b) comprises from 60wt% to 95wt% coarse coal combustion fly ash.

9. A process according to any of claims 5-7, wherein during step (a)(1) the coal combustion fly ash is subjected to an air classification step to separate coarse coal combustion fly ash from the remainder of the coal combustion fly ash, wherein the remainder of the coal combustion fly ash is subjected to a milling step to obtain fine coal combustion fly ash, and wherein the coarse coal combustion fly ash is contacted with the fine coal combustion fly ash to obtain coal combustion fly ash having a bimodal particle size distribution.

10. A process according to any preceding claim, wherein the particulate refractory mixture obtained in step (a) comprises from 95wt% to 99wt% coal combustion fly ash (I) and from lwt% to 5wt% binder and/or water (II).

11. A process according to any preceding claim, wherein the binder is an organic polymer.

12. A process according to claim 11, wherein the binder is dextrin.

13. A process according to any preceding claim, wherein during the firing step (e), the green article is sintered.

14. A process according to any preceding claim, wherein during the firing step (e), the temperature is increased to at least 1550°C.

15. A process according to any preceding claim, wherein the coal combustion fly ash comprises less than 10wt% carbon.