PROCESS FOR PREPARING A GRANULAR CERAMIC MIXTURE

Abstract

A process for preparing a granular ceramic mixture includes the steps of: (a) contacting fluid bed combustion fly ash with an acidic aqueous solution to obtain acidic fluid bed combustion fly ash slurry; (b) removing excess acid from the slurry obtained in step (a) to obtain solid acid treated fluid bed combustion fly ash; and (c) contacting together: (i) the solid acid treated fluid bed combustion fly ash obtained in step (b); (ii) clay; (iii) optionally, feldspar; and (iv) optionally, other ingredients.

Claims

1. A process for preparing a granular ceramic mixture, wherein the process comprises the steps of: (a) contacting fluid bed combustion fly ash with an acidic aqueous solution to obtain an acidic fluid bed combustion fly ash slurry; (b) removing excess acid from the slurry obtained in step (a) to obtain solid acid treated fluid bed combustion fly ash; (c) contacting together: (i) the solid acid treated fluid bed combustion fly ash obtained in step (b); (ii) clay; (iii) optionally, feldspar; and (iv) optionally, other ingredients. to form the granular ceramic mixture.

2. A process according to claim 1, wherein the acidic aqueous solution is selected from an aqueous solution of acetic acid (ethanoic acid), aqueous solution of ascorbic acid ((2)-2-[(1S)-1,2-dihydroxyethyl]-3,4-dihydroxy-2H-furan-5-one), an aqueous solution of hydrochloric acid, an aqueous solution of nitric acid, an aqueous solution of oxalic acid (ethandioic acid), and any combination thereof.

3. A process according to claim 1, wherein the acidic aqueous solution is an aqueous solution of acetic acid (ethanoic acid).

4. A process according to claim 1, wherein the acidic aqueous solution has a molarity of from 0.2M to 3.0M.

5. A process according to claim 1, wherein step (b) comprises the steps of rinsing the slurry, and removing a supernatant from a solid content to obtain the solid acid treated fluid bed combustion fly ash.

6. A process according to claim 1, wherein the granular ceramic mixture comprises: (a) from 10 wt % to 60 wt % fluid bed combustion fly ash; (b) from 15 wt % to 55 wt % clay; (c) from 0 wt % to 35 wt % feldspar; and (d) optionally, other ingredients to 100 wt %.

7. A process according to claim 1, wherein the granular ceramic mixture comprises: (a) from 20 wt % to 50 wt % fluid bed combustion fly ash; (b) from 15 wt % to 35 wt % clay; (c) from 0 wt % to 25 wt % feldspar; and (d) optionally, other ingredients to 100 wt %.

8. A process according to claim 1, wherein the fluid bed combustion fly ash is circulating fluid bed combustion fly ash.

9. A process according to claim 1, wherein the fluid bed combustion fly ash comprises greater than 5.0 wt % oxide of sulphur.

10. A process according to claim 1, wherein the fluid bed combustion fly ash comprises greater than 10 wt % oxide of sulphur.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0040] Process for Preparing a Granular Ceramic Mixture

[0041] The process for preparing a granular ceramic mixture comprises the steps of: [0042] (a) contacting fluid bed combustion fly ash with an acidic aqueous solution to obtain acidic fluid bed combustion fly ash slurry; [0043] (b) removing excess acid from the slurry obtained in step (a) to obtain solid acid treated fluid bed combustion fly ash; [0044] (c) contacting together: [0045] (i) the solid acid treated fluid bed combustion fly ash obtained in step (b); [0046] (ii) clay; [0047] (iii) optionally, feldspar; and [0048] (iv) optionally, other ingredients.

[0049] Treating the FBC fly ash with aqueous acid before contacting the treated fly ash with other ceramic materials such as clays has multiple advantages. For example, this minimises the amount of acid that is needed since in this application it is only the fly ash that actually needs acid treatment. The inventive process may require the removal of the soluble products of the chemical reaction between the acid and the fly ash.

[0050] In addition, contacting the fly ash only with acid makes filtering of the solid suspension easier since fly ash does not swell (and hence gel) in aqueous mixes as many clays do. The gelling and thickening associated with washing clays as well as fly ashes would make separation of the supernatant liquid slow and complex unless very dilute solutions are used.

[0051] Step (a) Obtaining the Acidic Fluid Bed Combustion Fly Ash Slurry

[0052] Fluid bed combustion fly ash is contacted with an acidic aqueous solution to obtain acidic fluid bed combustion fly ash slurry. Preferably, the pH of step (a) is in the range of from 2.0 to less than 7.0, preferably from 2.0 to 6.0, or from 2.0 to 5.0, or even from 2.3 to 4.0.

[0053] Step (b) Obtaining the Solid Acid Treated Fluid Bed Combustion Fly Ash

[0054] Excess acid is removed from the slurry obtained in step (a) to obtain solid acid treated fluid bed combustion fly ash.

[0055] Typically, step (b) comprises the steps of rinsing the slurry, and removing the supernatant from the solid content. This typical rinsing step can be repeated a number of times, for example two or more times, or even three or more times, or even four or more times.

[0056] Step (c) Forming the Granular Ceramic Mixture

[0057] During step (c), the following ingredients are contacted together: [0058] (i) the solid acid treated fluid bed combustion fly ash obtained in step (b); [0059] (ii) clay; [0060] (iii) optionally, feldspar; and [0061] (iv) optionally, other ingredients

[0062] The Granular Ceramic Mixture

[0063] The granular ceramic mixture comprises solid acid treated fluid bed combustion fly ash, clay, feldspar and optionally other ingredients.

[0064] Preferably, the granular ceramic mixture comprises: [0065] (a) from 10 wt % to 60 wt %, or from 20 wt % to 50 wt % solid acid treated fluid bed combustion fly ash; [0066] (b) from 15 wt % to 55 wt % clay; [0067] (c) from 0 wt % to 35 wt %, or from 5 wt % to 25 wt %, feldspar; and [0068] (d) optionally, other ingredients to 100 wt %.

[0069] Preferably, the granular ceramic mixture comprises: [0070] (a) from 20 wt % to 50 wt % solid acid treated fluid bed combustion fly ash; [0071] (b) from 15 wt % to 35 wt % clay; [0072] (c) from 0 wt % to 35 wt %, or from 5 wt % to 25 wt %, feldspar; and [0073] (d) optionally, other ingredients to 100 wt %.

[0074] Fluid Bed Combustion Fly Ash

[0075] Suitable fluid bed combustion fly ash can be atmospheric fluid bed combustion fly ash, pressurized fluid bed combustion fly ash, or a combination thereof.

[0076] Suitable fluid bed combustion fly ash can be circulating fluid bed combustion fly ash, bubbling fluid bed combustion fly ash, or a combination thereof.

[0077] A preferred fluid bed combustion fly ash is circulating fluid bed combustion fly ash.

[0078] Typically, the fluid bed combustion fly ash comprises greater than 4.0 wt % oxide of sulfur, or greater than 5.0 wt %, or greater than 6.0 wt %, or greater than 6.5 wt %, or greater than 7.0 wt %, or greater than 10 wt % oxide of sulfur.

[0079] Typically, the fluid bed combustion fly ash is derived from coal, typically fluid bed combustion coal fly ash.

[0080] Solid Acid Treated Fluid Bed Combustion Fly Ash

[0081] Typically, the solid acid treated the fluid bed combustion fly ash comprises greater than 4.0 wt % oxide of sulfur, or greater than 5.0 wt %, or greater than 6.0 wt %, or greater than 6.5 wt %, or greater than 7.0 wt %, or greater than 10 wt % oxide of sulfur.

[0082] Oxide of Sulfur

[0083] Analysis of the elemental composition of fly ash is most commonly done by X-ray fluorescence (XRF) techniques. This measures the levels of the heavier elements, such as iron, aluminium, silicon, sulfate and calcium. The convention is that these are then reported as the equivalent stoichiometric level of oxide. Sulfur is reported as SO.sub.3.

[0084] SO.sub.3 is often referred to as “sulfate” in the ceramic literature even though the term “sulfate” technically refers to the SO.sub.4.sup.2− ion. Sometimes sulfur is reported as elemental sulfur but how the sulfur is reported makes no difference to the actual levels present. The present invention therefore uses the term “oxide of sulfur” to be more general. “Oxide of sulfur”, SO.sub.3 and “sulfate” are interchangeable terms when used herein.

[0085] The level of oxide of sulfur present in the fly ash can be determined using the following XRF method.

[0086] Suitable XRF equipment is the Epsilon 4 XRF analyser from Malvern Panalytical using sample disks prepared using an Aegon 2 automatic fusion equipment for sample disk preparation from Claisse. The ash sample is automatically dissolved in molten lithium borate flux and formed into a disk. This is then placed in the Epsilon 4 for analysis. Equipment should be operated as per manufacturer's instructions. When measuring for SO.sub.3, the Epsilon 4 should be set to a voltage of 4.5 kV, a current of 3000 μa, use helium as the medium, not use a filter, and have a measurement time of 450 s.

[0087] Clay

[0088] A suitable clay is a standard clay such as Ukrainian clay or illitic clay. A preferred clay is a combination of standard clay and high plasticity clay. The weight ratio of standard clay to high plasticity clay may in the range of from 2:1 to 5:1. A suitable clay is a high plasticity clay such as bentonite clay. Typically, a high plasticity clay has an Attterburg Plasticity Index of greater than 25.0. Typically, a standard clay has an Atterburg Plasticity Index of 25.0 or less. The amount of high plasticity clay can be selected to provide sufficient robustness and flowability for granular ceramic mixtures.

[0089] Feldspar

[0090] Suitable feldspars include sodium and/or potassium feldspars.

[0091] Optional Other Ingredients

[0092] Other optional ingredients include chemical additives and binders.

[0093] Acidic Aqueous Solution

[0094] The acidic aqueous solution can be an organic acidic aqueous solution, an inorganic acidic aqueous solution, or a combination thereof. The acidic aqueous solution is preferably a weak acid.

[0095] Suitable acidic aqueous solutions are selected from acetic acid (ethanoic acid), ascorbic acid ((2R)-2-[(1S)-1,2-dihydroxyethyl]-3,4-dihydroxy-2H-furan-5-one), hydrochloric acid, nitric acid, oxalic acid (ethandioic acid), sulfuric acid, and any combination thereof.

[0096] Preferably, the acidic aqueous solution is an aqueous solution of acetic acid (ethanoic acid). A suitable source of acetic acid (ethanoic acid) is vinegar.

[0097] Typically, during step (a), the acidic aqueous solution has a molarity of from 0.2M to 3.0M, or from 0.4M to 2.0M, or even 0.5M to 1.5M.

[0098] Preferably, the pH of the acidic aqueous solution during step (a) is in the range of from 2.0 to less than 7.0, preferably from 2.0 to 6.0, or from 2.0 to 5.0, or even from 2.3 to 4.0.

[0099] Preferably, the acid is not sulfuric acid, and the acidic aqueous solution is not an aqueous solution of sulphuric acid.

EXAMPLES

Inventive Example

[0100] Acid treated FBC fly ash was prepared by taking FBC fly ash and washing it in 10% (by volume) acetic acid (ethanoic acid) aqueous solution at the ratio of 50 g FBC fly ash to 500 ml of acetic acid (ethanoic acid) aqueous solution. The mixture was vigorously stirred at ambient for 60 mins.

[0101] After mixing, the mixture was allowed to settle, and the supernatant liquid poured off. The mix was then rinsed with fresh water by swirling, allowing the solid to settle and decanting the supernatant liquid. The wet ash was then dried at 120° C. for 1 hour to form dried acid treated FBC fly ash.

[0102] The dried acid treated FBC fly ash was then mixed with clay and feldspar and made into a ceramic article according to the process below.

[0103] 100 g of acid treated fluid bed combustion fly ash was mixed with 50 g of illitic clay and 50 g sodium feldspar to form the granular ceramic mixture. The mixture was milled, sieved and wetted. 140 g of the above granular ceramic mixture was then uniaxially pressed in a rectangular mild steel mold (155×40 mm) to a pressure of 40 MPa which was held for 1.5 min (90 sec). The formed body was released from the mold and placed into a 110° C. oven to dry.

[0104] The dried body was fired in an electric kiln at a ramp rate of 2.5° C./min to 1160° C. The temperature was held at the top temperature for 30 min. The fired body was then allowed to cool down naturally (hence slowly) to room temperature.

[0105] No cracking was observed in the fired body.

Comparative Examples

[0106] The same procedure was followed as above, except that the fluid bed ash, whilst being identical in every other way to the acid treated fluid bed combustion fly ash apart from the acid treatment step, was not subjected to the acid washing step and was instead directly mixed with the other ingredients.

[0107] Cracking was observed in this fired body.

[0108] A comparative example of an identical composition to the FBC fly ash comparative example, made in exactly the same manner but made from PCC fly ash, did not show any cracking.