Field of the invention

The present invention relates to the incorporation of fluid bed combustion fly ash, and especially circulating fluid bed combustion fly ash into a granular ceramic mixture.

Background of the invention

Fluid bed combustion (FBC) power plants versus pulverized coal combustion (PCC) power plants

Large quantities of fly ash are produced as a result of coal-fired electricity generation. This will continue into the foreseeable future. There is interest in how to utilise this fly ash waste material. Much fly ash is currently used in concrete as a pozzolan or cementitious material. Other uses include brick making and as a soil stabilisation material. However, much fly ash continues to go to landfill. This has obvious environmental, as well as economic, costs. There is therefore ongoing value and interest in developing products and processes that can use fly ash as a raw material. This minimises the amount of fly ash going to landfill and reduces the amounts of other virgin raw materials used.

The question of how to re-utilise the fly ash from power generation has become harder due to the introduction of fluidised bed combustion (FBC) technologies for thermal power generation and incineration. Fluidised bed combustion (FBC) plant designs are quite different to the pulverised coal combustion (PCC) plant designs that have been standard for power plants for many decades. The fly ash produced by FBC plants is different to PCC fly ash, and FBC fly ash is much harder than PCC fly ash to re-use in other applications such as ceramic production.

Fluidised bed combustors burn the coal in a heated fluidised bed of ash and/or sand at lower temperatures than PCC designs. Fluidised bed combustion (FBC) designs include “bubbling” fluidised beds as well as “circulating” fluidised bed designs. A bubbling fluid bed is also referred to as a “boiling fluid bed”. Circulating fluidised beds, known as CFBs, are most common. The various FBC designs can be further divided based on the pressures at which they run, either atmospheric or pressurised.

A circulating fluid bed (CFB) furnace operates by continuously recycling most of the hot ash (including any fine unburnt fuel) from the exhaust stream coming from the combustion zone back into the base of the fluidised bed combustion zone. A proportion of the finest fly ash is continuously removed from the exhaust and fresh fuel and additives are continuously added to the combustion zone. This system has many advantages including very high levels of carbon bum (due to the repeated passes of burning particles through the combustion zone) plus the fuel does not have to be pulverised before addition to the combustion zone. The extended time at elevated temperatures and the high levels of particle:particle interactions that ash particles experience in a CFB design gives opportunity for mineral phases that are not normally seen in PCC ashes to form.

Fluidised bed combustion (FBC) technology is becoming increasingly popular since plants using such technologies are less polluting. FBC plants emit much lower levels of nitrous oxides than conventional PCC plants, the removal of sulfur oxides is easier, and FBC plants can burn a wider range of fuels such as low-grade coal, and even fuels such as tyres and oil. Often these lower-grade fuels have a high sulfur content.

The solid/solid contact with hot particles in FBC plant designs gives very high heat exchange coefficients. This means that FBC plants can produce power efficiently at much lower temperatures: typically, between 800 - 900 °C, compared to 1400 - 1700 °C in a PCC plant. Being able to operate efficiently at lower combustion temperatures has large advantages. In particular, the formation of nitrous oxides is lower in FBC plants, and NO _x pollution is therefore reduced.

The removal of sulfur oxides is also simpler in FBC plants as compared to PCC plants. Typically, PCC plants burn higher quality, lower sulfate coals such as anthracite. Typically, PCC plants have wet scrubbers which treat the exhaust gases to chemically remove the sulfur oxides via a process called flue gas desulfurisation (FGD). This is a costly and intensive process. In contrast to PCC plants, FBC plants typically reduce their sulfur oxide emissions by burning a mixture of fuel and limestone/chalk/dolomite. The limestone material (calcium carbonate) forms calcium oxide within the fluidised bed. This reacts with sulfur oxides from the combustion of sulfur compounds in the fuel to form calcium sulfate in-situ. This is possible as the temperatures are low enough in the fluidised bed for calcium sulfate minerals such as anhydrite to be stable and readily formed.

Such reactions would not be possible in a PCC plant due to the high temperatures used. Hence their need for a separate FGD system.

The addition of limestone material to the coal to allow sulfur oxides to react in-situ is a much simpler process than having to scrub the flue gases.

The differences between fluid bed combustion (FBC) fly ash and pulverised coal combustion (PCC) fly ash

The addition of significant amounts of limestone material to the boiler in a fluid bed combustion (FBC) plant means that the fluid bed combustion (FBC) fly ash typically comprises high levels of calcium species and sulfur species.

The levels of calcium species in FBC fly ash, usually reported as the equivalent calcium oxide level, are often higher than even high calcium oxide (Type C) fly ashes from PCC plants.

In addition, the levels of sulfur species in FBC fly ash, usually reported as the equivalent sulfur oxide level, are higher than the sulfur oxide level of PCC fly ash.

The ASTM-C618 standard is commonly used to define suitable fly ash quality for use as a pozzolan or cementitious product. The upper SO ₃ limit for materials to meet ASTM-C618 is 5wt%. FBC fly ash typically comprises a much higher level of oxide of sulfur.

Physically, the fly ash from FBC designs is quite different to conventional PCC fly ash. The FBC fly ash has not been subjected to the very high temperatures encountered in the exhaust systems of conventional PCC plants. The fly ash produced in PCC plants has been suspended in the very hot effluent combustion gases. The temperatures experienced are high enough to melt particles. This means that the large majority of PCC fly ash particles are spherical and formed of glassy amorphous phases.

In contrast, fly ash from FBC plants, and especially CFB combustion plants, will not have been melted due to the lower temperatures of the FBC. As a result, the FBC fly ash particles have an irregular shape and do not contain glassy phases. Another difference is that the time for which the fly ash has been subjected to high temperatures is typically much longer in FBC plants, especially those plants where high levels of fly ash are recirculated, such as in a CFB combustion plant. This means, for example, that, whilst the iron in PCC fly ash is often present as magnetite and hematite, in FBC fly ash, and CFB combustion fly ash, it is mostly present as ferrite. This has major implications, for example, the ease of iron removal.

PCC and FBC fly ash are different chemically (e.g. typically having different levels of calcium species and sulfate species), different physically (e.g. typically having different morphologies, e.g. regular/glassy (PCC) compared to irregular/non-glassy (FBC)), and different mineralogically. FBC fly ash also has a smaller diameter than PCC fly ash (due to a self-grinding action) and has a lower residual carbon level compared to PCC fly ash.

The problem of incorporating fluid bed combustion (FBC) fly ash, and especially circulating fluid bed (CFB) combustion fly ash, in a granular ceramic mixture

Most of the FBC fly ash is currently sent to landfill or used as a very low value soil stabilisation agent. It is less effective than PCC fly ash as a pozzolan. In addition, the high sulfate levels can cause problems. There is a growing need to find alternative uses for FBC fly ash. A potential high-value use is in ceramic articles such as ceramic floor tiles and porcelain floor tiles.

Fly ash can be used as a partial replacement for clays in ceramic articles. Fly ash can be combined with clay and other materials such as feldspar to form granular ceramic mixtures. The granular ceramic mixtures can then be formed into ceramic articles, such as ceramic tiles and especially porcelain tiles. Such ceramic articles can be made with significant levels of fly ash and this is known in the art. The replacement of clay by fly ash is beneficial as supplies of suitable clay are becoming limited. Maximising the practical level of clay that can be replaced by fly ash is beneficial.

However, the fly ashes used in the art are mostly PCC fly ash. FBC, and particularly CFB combustion, technology is a relatively recent development, and was not in use when earlier ceramic art was developed. Hence issues relating specifically to the use of FBC fly ash in ceramics applications were not recognised, or even relevant, to most of this body of fly ash work. For example, the art describes fly ash as consisting of spheres of amorphous, glassy phases, which is a description of PCC fly ash and not FBC fly ash. It is clear that the art is referring to PCC fly ash.

The inventors have discovered that simply replacing PCC fly ash with FBC fly ash, and especially CFB combustion fly ash in ceramic compositions that also contain clays, feldspars and optionally other ingredients, can cause defects. Ceramic articles made using FBC fly ash have been observed to crack during the firing cycle. This is particularly problematic when making high quality, large ceramic items such as ceramic floor tiles, and especially porcelain floor tiles, where such defects are particularly unacceptable. The incorporation of fly ash into porcelain floor tiles, which need to have low water absorption and high flexural strength, is particularly challenging.

The inventors have also seen that this problem is exacerbated when the FBC fly ash is used at higher levels in ceramics applications.

Without wishing to be bound by theory, it is hypothesised that the high energy environment of a typical ceramics production process, and the high level of intense surface/surface contacts between clay and FBC fly ash particles results in the formation of higher levels of specific mineral phases which change the firing behaviour of the material. This in turn, leads to cracking of the ceramic articles.

The inventors have discovered that the problems associated with the incorporation of FBC fly ash, and especially CFB combustion fly ash, into granular ceramic mixtures can be overcome if silica glass is incorporated into the granular ceramic mixture. This allows FBC fly ash, and especially CFB combustion fly ash, to be incorporated into granular ceramic mixture, that can then be formed into ceramic articles, such as ceramic floor tiles and porcelain tiles, without the problem of cracking.

Without wishing to be bound by theory, it is hypothesised that the inclusion of silica glass into the granular ceramic mixture results in the formation of different mineral compositions and phases that do not crack as readily. For example, the addition of silica glass could result in the formation of higher levels of wollastonite, which is a calcium aluminium silicate. This is known to have low levels of thermal expansion and could lead to a lower cracking behaviour compared to when the silica glass is not present.

Alternatively, without wishing to be bound by theory, it is possible that the presence of the lower melting point silica glass in the granular ceramic mixture changes the pyroplastic properties of the ceramic article during firing and makes it more deformable and better able to handle strain without cracking.

Discussion of the art

WO96/02477 relates the incorporation of glass into a ceramic article. The ceramic articles of WO96/02477 may also comprise fly ash. WO96/02477 teaches that the vibrancy and depth of colour of the resultant ceramic article was not significantly affected by the inclusion of the fly ash (c.f. page 6, line 31 to page 7, line 5). WO96/02477 does not disclose or suggest the inclusion of fluid bed combustion (FBC) fly ash. The fly ash disclosed in WO96/02477 as being useful is Type F or Type C fly ash, with type F fly ash being the most preferred (c.f. page 8, line 36 to page 9, line 8).

Fluid bed combustion fly ash is different to Type C and Type F fly ash. Most notably, fluid bed combustion fly ash typically comprises higher levels of oxide of sulfur. Without wishing to be bound by theory, the high level of sulfur oxide leads to the formation different mineral phases when FBC fly ash is incorporated into ceramic articles compared to when different fly ash with lower levels of sulfur oxide (such as PCC fly ash) is incorporated into ceramic articles. These different mineral phases formed in the presence of higher levels of sulfur oxide are more prone to defects and cracking, which is why the incorporation of FBC fly ash into ceramic articles is particularly problematic in this regard. WO96/02477 does not disclose, teach or suggest that the incorporation of fluid bed combustion fly ash into a ceramic article causes any particular problem. WO96/02477 does not disclose, teach or suggest that the incorporation of fluid bed combustion fly ash into a ceramic article leads to problems of defects and cracking. Furthermore, WO96/02477 does not disclose, teach or suggest that these problems of defects and cracking can be overcome by the incorporation of silica glass into the ceramic article together with the fluid bed combustion fly ash.

US5935885 relates to the incorporation of fly ash into ceramic articles. US5935885 teaches to sufficiently oxidize the organic material and metal contaminants in the fly ash to ensure the uniform quality of the ceramic article (c.f. column 2, lines 11-21).

Suitable fly ash disclosed in US5935885 are utility boiler ash, municipal solid waste incinerator ash and auto shredder residue ash (c.f. column 3, lines 1-4). US 5935885 also discloses that other additives may also be present in the ceramic article, soda-lime-silica glass cullet is disclosed along with many other ingredients as a suitable additive (c.f. column 3, lines 38-42). US5935885 teaches that the additional additives like soda-lime-silica glass cullet, are to be chosen to render a composition that can be melted with relative ease and can be crystalized to produce a glass ceramic (c.f. column 3, lines 35-38).

However, US5935885 does not disclose that fly ash can be combined with clay, and US5935885 does not disclose, teach or suggest that the incorporation of fluid bed combustion fly ash into a ceramic article causes any particular problem. US5935885 does not disclose, teach or suggest that the incorporation of fluid bed combustion fly ash into a ceramic article leads to problems of defects and cracking. Furthermore, US5935885 does not disclose, teach or suggest that these problems of defects and cracking can be overcome by the incorporation of silica glass into the ceramic article together with the fluid bed combustion fly ash.

Summary of the invention

The present invention provides a process for preparing a granular ceramic mixture, wherein the process comprises the step of contacting fluid bed combustion fly ash with silica glass, clay, optionally feldspar, and optionally other ingredients, to form the granular ceramic mixture. Detailed description of the invention

Process for preparing a granular ceramic mixture

The process for preparing a granular ceramic mixture comprises the step of contacting fluid bed combustion fly ash with:

(a) silica glass;

(b) clay;

(c) optionally, feldspar; and

(d) optionally, additional ingredients, to form the granular ceramic mixture.

Granular ceramic mixture

Preferably, the granular ceramic mixture comprises:

(a) from 10wt% to 60wt%, or from 20wt% to 50wt%, fluid bed combustion fly ash;

(b) from 10wt% to 55wt%, or from 15wt% to 55wt% clay;

(c) from 0wt% to 35wt%, or from 5wt% to 25wt% feldspar;

(d) from 5wt% to 50wt%, or from 10wt% to 24wt%, silica glass; and

(e) optionally, other ingredients to 100wt%.

Preferably, the granular ceramic mixture comprises:

(a) from 20wt% to 60wt% fluid bed combustion fly ash;

(b) from 10wt% to 35wt%, or from 15wt% to 35wt% clay;

(c) from 0wt% to 35wt%, or from 5wt% to 25wt%, feldspar;

(d) from 10wt% to 40wt%, or from 10wt% to 24wt% silica glass; and

(e) optionally, other ingredients to 100wt%. Fluid bed combustion fly ash

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

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

A preferred fluid bed combustion fly ash is circulating fluid bed combustion fly ash.

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

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

The fluid bed combustion fly ash can be obtained from the appropriate fluid bed combustion plant.

Oxide of sulfur

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 level of oxide. Iron levels are reported as Fe ₂ 0 ₃ even if the iron is not actually in that form. Calcium is reported as CaO and sulfur is reported as SO3. Hence, a material such as CaSCri is typically reported as CaO and SO3.

SO3 is often referred to as “sulfate” in the ceramic literature even though the term “sulfate” technically refers to the SO4 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”, SO3 and “sulfate” are interchangeable terms when used herein. The level of oxide of sulfur present in the fly ash can be determined using the following XRF method.

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 SO3, the Epsilon 4 should be set to a voltage of 4.5 kV, a current of 3000 pa, use helium as the medium, not use a filter, and have a measurement time of 450 s.

Silica glass

The term silica glass refers to a glass composed predominantly of amorphous SiCk. It can also be referred to as “silicate glass” or “sodadime glass” or just “glass”. The source of silica glass can be from a variety of waste sources. The source of the silica glass added to the ceramic granular composition is preferably crushed recycled domestic and industrial glass, such as from bottles.

Clay

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.

Feldspar

Suitable feldspars include sodium and/or potassium feldspars. Optional other ingredients

Other optional ingredients include chemical additives and binders.

Process of preparing a ceramic article

The process of preparing a ceramic article comprises the steps:

(a) preparing a granular ceramic mixture by a process according to the above process of the present invention;

(b) pressing the ceramic granular mixture obtained in step (a) to form a green article;

(c) optionally, subjecting the green article to a drying step;

(d) subjecting the green article to a sintering step in a kiln to form a hot sintered article; and

(e) cooling the hot sintered article to form a ceramic article.

Step (a) preparing a granular ceramic mixture Step (a) is described in more detail above.

Step (b) pressing the granular ceramic mixture

The granular ceramic mixture obtained in step (a) is pressed to form a green article.

Step (c) optional drying step

Optionally, the green article may be subjected to a drying step. The temperature during this optional drying step (c) is typically limited to temperatures below 250°C, or below 200°C, or even below 150°C, so this optional step (c) is not a sintering step. This optional drying step (c) can be used to remove water from the green article in a controlled manner.

Step (d) sintering step The green article is subjected to a sintering step in a kiln to form a hot sintered article.

Step (e) cooling step

The hot sintered article is cooled to form a ceramic article.

Green article

The green article is formed by pressing the granular ceramic mixture to form the desired shape. The green article typically has sufficient strength to withstand handling operations before the sintering step.

Hot sintered article

The green article is heated, typically under controlled conditions, to sinter the particles of the green article together to form the hot sintered article.

Ceramic article

A preferred ceramic article is a ceramic floor tile. A highly preferred ceramic floor tile is a ceramic porcelain floor tile.

Examples

Three mixtures were prepared: a 50 parts PCC fly ash, 25 parts illitic clay, 25 parts soda feldspar (comparative) b 50 parts PCC fly ash, 16.7 parts illitic clay, 16.7 parts soda feldspar, 16.7 parts crushed silica glass (comparative). c 50 parts FBC fly ash, 25 parts illitic clay, 25 parts soda feldspar (comparative) d 50 parts FBC fly ash, 16.7 parts illitic clay, 16.7 parts soda feldspar, 16.7 parts crushed silica glass (inventive).

200g of mix (a), 200g of mix (b) 200g of mix (c) and 200g of mix (d) were placed in separate containers, milled, wetted and sieved.

140g of each mix was then uniaxially pressed in a rectangular mild steel mold (155x40mm) to a pressure of 40MPa which was held for 1.5min (90sec). Each formed body was released from the mold and placed into a 110°C oven to dry.

The dried bodies were 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 30min. The fired bodies were then allowed to cool down naturally (hence slowly) to room temperature.

No cracking was observed in the fired body made from mixes (a) and (b).

Cracking was observed in the fired body made from mix (c).

No cracking was observed in the fired body made from mix (d).

IB