Field of the invention

The present invention relates to sintered articles comprising coal combustion fly ash. The sintered articles are uncoated and homogenous. The sintered articles typically have an appearance and texture like sandstone, typically having a rough surface. The sintered articles have low water absorption, are strong, and are suitable for use as floor and wall files, especially floor tiles having a sandstone-like appearance and texture.

Background of the invention

There are major environmental and economic benefits in using coal combustion fly ash, which is amongst the most abundant waste material on earth, as a raw material in as many products as possible.

Sintered articles, such as sintered tiles, are one type of product that can use coal combustion fly ash as a raw material to replace conventional sintered materials such as clay. There are many types of sintered tiles, for example wall tiles and porcelainized floor tiles, which can have a range of properties and aesthetic appearances.

Natural stone tiles are also very popular, and many different types of stone and surface finishes are available. Granite, marble, limestone and sandstones are common varieties and there are many different grades. Sandstone tiles, when used internally, are usually fine-cut and smoothed so as not to be unpleasantly rough to the touch but are often not polished. This leaves enough texture and surface roughness so as to emphasise the natural stone origins of the tile and give anti-slip properties but smooth enough to not feel rough to the touch and looking fine enough for internal domestic use. The textures of sandstone tiles arise from the grains of sand originally forming the stone as well as any surface smoothing. This gives a uniformity and consistency to the surface roughness. The sand grains that originally formed a sandstone will have been of consistent and similar size due to how they were laid down. One downside of many natural stone products is that they can be quite porous and are often unsuitable for use in areas where liquids could be spilt.

The difficulties and costs associated with natural stone tile production, such as waste and inefficiencies from the cutting, means that there are many artificial stone sintered products on the market. However, many of these do not have fully convincing aesthetic properties or are complex to make. A common issue is how to get the texture as well as the appearance of the sintered product to convincingly match that a natural stone article, such as a sandstone tile. For example, any chips or damage would expose the internal matrix whose appearance would not match that of any surface coating or layer.

The present invention provides for a sintered article that typically has a sandstone-like appearance and texture. The article comprises significant amounts of coal combustion fly ash and provides for an environmentally friendly use of coal combustion fly ash. In addition to having a sandstone-like appearance and texture, the articles of the present invention have very low water absorption properties, which enables their use in a much broader range of applications compared to natural stone articles.

There is benefit in controlling and defining certain physical properties, including the composition, of a sintered article, such as a tile, that comprises coal combustion fly ash, such that it can simulate the appearance and texture of fine sandstone tiles whilst actually having superior physical properties. This maximises consumer acceptance and hence use of this waste coal combustion fly ash material.

Summary of the invention

The present invention provides an uncoated homogeneous sintered article, wherein the article comprises: (a) at least 30wt% coal combustion fly ash; and (b) at least 30wt% clay, wherein the article has: (i) an Ra value of greater than 5 0pm; and (ii) a water absorption of less than 3.0%.

Detailed description of the invention

Sintered Article. The sintered article is an uncoated homogeneous sintered article, typically having a sandstone-like appearance and texture, wherein the article comprises:

(a) at least 30wt% coal combustion fly ash;

(b) at least 30wt% clay; and

(c) optionally other ingredients, such as ingredients selected from feldspar and/or glass; wherein the article has: (i) a Ra surface roughness of greater than 5.0mih, or greater than 6.0pm, or greater than 7.0mih, or greater than 8.0mih, or greater than IOmih;

(ii) a water absorption of less than 3.0wt%, or less than 2.0wt%, or less than 1.5wt%, or less than 1.0wt%, or less than 0.5wt%; and

(iii) optionally, a Rz surface roughness of less than 70 pm, or less than 60pm, or less than 50pm, or less than 40pm.

Typically, the article is an artificial sandstone article.

Preferably, the article comprises at least 40wt%, or at least 50wt%, or even at least 60wt%, coal combustion fly ash.

Preferably, the article comprises at least 35wt% clay, or at least 40wt%, or 45wt%, or 50wt% clay.

Preferably, the article comprises at least 5.0wt%, or least 6.0wt%, or at least 7.0wt%, or at least 8.0wt% ingredients selected from feldspar and/or glass.

Preferably, the article has a Ra surface roughness of greater than 10pm.

Preferably, the article has a Rz surface roughness of less than 60pm.

Preferably, the surface of the sintered article has a roughness such that: (i) the Ra value is greater than 5.0pm, or greater than 6.0pm, or greater than 7.0pm, or greater than 8.0pm, or greater than 10.0pm, or greater than 12pm, or even greater than 14.0pm; and (ii) optionally, the Rz value is less than 70pm, or less than 60pm, or less than 50pm, or less than 40pm. This is especially preferred when the sintered article is a floor tile.

There are multiple well-established parameters for measuring the roughness of a surface, that is the variability of a surface profile. Different parameters are used for different applications. The most used parameters for tiles and bricks are (i) Ra, the arithmetic mean deviation of the profile and (ii) Rz, the maximum height of the profile. Rz measures the maximum distance between a peak and a trough along the surface profile.

A smooth (but not polished) natural stone tile has a degree of uniform surface roughness whilst not being too rough to the touch. Natural stone tiles can have various different surface finishes ranging from rough-cut slabs for exterior use to highly polished finishes for some internal decorative applications. Highly polished stone tiles, such as polished sandstone tiles, will have attractive visual aesthetics but may not feel authentic and can feel slippery when walked on. Rough-cut natural stone tiles may not be aesthetically consistent with other internal surface finishes and will often feel too rough for many internal surfaces where they may be touched. Smooth, but not polished, natural stone tiles are therefore often used internally. A smooth natural stone, such as a sandstone, is characterised by a uniform roughness partly coming from the grains forming the stone.

Ra, which is a commonly used measure for surface roughness (based on profilometry) is an average. Hence similar Ra values can be obtained for a surface with (i) many small peaks/troughs or (ii) a surface with a fewer but larger peaks/troughs. This difference is better shown by the Rz values. The tactile perception is better for surfaces with multiple smaller peaks/troughs as compared to surfaces with fewer, but larger peaks/troughs. Such surfaces can feel rougher or scratchy to the touch. High Rz values mean a “scratchier” surface. Acceptable surfaces can be defined by specifying Ra and Rz values.

The inventors have found that the tactile perception of articles such as smooth stone tiles is better when they have a Ra value which is large enough such that the surface feels like a uniform high quality natural stone but the Rz value (which measures the largest peak/trough) is within an upper limit such that the surface does not feel unpleasantly rough or scratchy. Controlling the surface to have the desired Ra and Rz values keeps the surface uniformly rough enough to be convincing as a natural stone but to not feel unpleasantly rough or scratchy.

One issue for articles, such as tiles, is how to increase Ra value to a desired level without the Rz value becoming too high. As stated before, the Ra value of a surface could be increased by the inclusion of a few larger lumps or peaks on the surface. However, this would not be acceptable as it would feel “scratchy” to the touch. The Rz value would be too high. Increasing or decreasing the mean particle size of the fly ash used to make the article can be used to increase or decrease the Ra of a sample. Changing the size of the fly ash is preferred as fly ash is typically less affected by the firing temperature, and hence will deform less, compared to materials such as feldspars and clays.

Changing the amount of coarse coal combustion fly ash in a sintered article changes the surface roughness, especially the Ra value. This effect is not linear, and a minimum amount of coarse coal combustion fly ash is needed to provide sufficient roughness. Changing the size of the coarse coal combustion fly ash fraction, as well as the proportion used, changes the Ra and the Rz values in a different manner compared to just changing the amount of the coarse fraction. The maximum distance between the peaks and troughs of a profile (i.e. the Rz value) will be partially driven by the size of the coarse coal combustion fly ash particles protruding above the surface. The other components of the article, such as the smaller clay and feldspar particles, will typically sinter and melt around the larger coal combustion fly ash particles leaving them protruding above the surface. In addition, the physical form of coal combustion fly ash has some advantages for the tactile perception of surface texture such that it will not feel too rough. This is because coal combustion fly ash is predominantly made up of spherical particles. These are obviously smoother and less angular than, for example, equivalent sized crushed sintered frit particles. Hence the use of coarse coal combustion fly ash to control surface roughness so as to simulate stone has many advantages.

Preferred amounts of coarse coal combustion fly ash in the mixture used to make the inventive sintered article are between 30wt% and 70wt%. The coarse coal combustion fly ash can be obtained by classifying or sieving coal combustion fly ash to have a size distribution such that preferably at least 20wt% of the total mixture consists of coal combustion fly ash having a particle of greater than 125pm to less than 250pm. For example, the coarse coal combustion fly ash can be classified to have a particle size of between 125 pm and 250pm and blended with clay to form a mixture with 35wt% coal combustion fly ash. Alternatively, the coarse coal combustion fly ash can be classified to a wider particle size distribution, for example between 75pm and 250pm, and blended with clay. Preferably, the particle size distribution of the size classified coarse coal combustion fly ash is such that greater than 20wt% of the resulting mixture comprises coarse coal combustion fly ash having a particle size of greater than 125 pm. This may allow for process simplifications. The upper size limit of the coarse combustion coal ash can be reduced so as to reduce Rz value of the resultant article.

It may be preferred for the particulate mixture used to make the sintered article to comprise fine coal combustion fly ash. The particulate mixture may comprise at least 20wt%, or at least 30wt% fine coal combustion fly ash. The particulate mixture may comprise from 20wt% to 40wt% fine coal combustion fly ash. The inclusion of fine coal combustion ash can contribute to higher sintered article strengths and water resistance and also enable the use of lower firing temperatures. It also increases the overall amount of coal combustion fly ash that can be used in the particulate mixture which has environmental benefits. The mixture can also comprise fluxing materials such as feldspars and other materials such as glass. When feldspars, such as sodium or potassium feldspars, are added they are typically added at levels of from 4wt% to 40wt%, preferably from 5wt% to 35wt%, or from 6wt% to 25wt% of the mixture. Other materials such as glass can optionally be added at levels of up to 30wt%, such as from lwt% to 30wt% of the mixture. Preferably, the article has a water absorption of less than 1.0%. Preferably, the sintered article, has a water absorption of 3.0% or less, or even 2.0% or less, or 1.0% or less, or even 0.5% or less. This is especially preferred when the sintered article is a floor tile.

The water absorption can be altered by changing the level and type of other additives in the mixture in addition to the coal combustion fly ash and/or by changing the firing conditions. Materials such as feldspar and/or glass can be added to the mixture. These will flux the clay causing it to melt at lower temperatures than the large coal combustion fly ash particles and can seal and fill the pores in the sintered article. This reduces water absorption. Alternatively, the firing temperature can be increased to increase the degree of sintering and melting of the various components such that the pores are filled with molten material. Both approaches, which can be combined, can be used to control water absorption.

Typically, the coal combustion fly ash comprises less than 10wt%, or less than 8wt%, or less than 6wt%, or less than 4wt% iron oxide. This can help keep the colour of the sintered article from being too dark or the product too speckled.

Preferably, the sintered article has a modulus of rupture of at least 35MPa, or at least 40MPa, or even at least 50MPa. These MOR values are beneficial for tiles being used as floor tiles where robustness is required to avoid cracking and breakage. The MOR values can be controlled by the other materials in the mixture apart from the fly ash and the firing temperatures. Inclusion of high alumina ingredients typically provides more strength to the sintered articles. This can include high alumina clay, or alumina rich aluminium silicates from ash that has been highly beneficiated. The clay and other optional ingredients can be milled prior to blending with the fly ash.

Typically, the coal combustion fly ash is distributed in a homogeneous manner throughout the article from the article’s surface to the article’s core.

Typically, the colour of the article’s surface is the same as the colour of the article’s interior.

Coal combustion fly ash. Preferably, the coal combustion fly ash has a particle size of greater than 53 pm.

Clay. A suitable clay is a standard clay such as Ukrainian 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’s plasticity index of greater than 25.0. Typically, a standard clay has an Atterburg’s plasticity index of 25.0 or less.

Process of making the sintered article. The sintered article is made by classifying or sieving coal combustion fly ash to produce coarse coal combustion fly ash. The coarse coal combustion fly ash is then typically blended with clay and optionally other materials. The clay, and other optional materials such as glass or feldspar, can be milled prior to the addition of the coarse coal combustion fly ash so to reduce their particle size. This can improve the physical properties of the final sintered article. The mixture may also be humidified and then pressed to form a green sintered article. The green sintered article is then typically fired to sinter the sintered materials together to form the sintered article.

Method of measuring Ra and Rz surface roughness. Suitable equipment for this test is the Surtronic ^(RTM) range of Roughness Testers from Taylor Hobson ^(RTM) , including the Duo and S-series. This equipment can measure roughness parameters according to ASTM 4287 including Rz (maximum height of the profile) and Ra (the arithmetic mean deviation of the profile). The equipment uses a very fine stylus and the probe is moved across the surface by simply drawing the probe along the length of the surface being tested. Typically, five or so different measurements are taken across the surface so as to get a broader average and ensure all surfaces are measured. The equipment can automatically calculate the various surface roughness measurements including Ra and Rz when correctly following the manufacturer’ s instructions.

Method of measuring water absorption. A sample of the sandstone article to be measured is heated to a constant weight at 100°C to ensure the sample is dry. The weight of the sample is then measured (original sample weight). The dry sandstone article is then immersed in boiling water (taking care that the sample does not contact any heating surfaces). The sample is then allowed to cool in the water for 12 hours making sure that no surface is exposed to air during this time. The sample is then taken from the water and touch dried with a dampened cloth to remove surface droplets. The sample is then weighed (absorbed weight). The amount of absorbed water in the sample is the difference of these two weights (absorbed sample weight - original sample weight = weight of absorbed water). The wt% water absorption is calculated by dividing the weight of absorbed water by the original sample weight and multiplying the result by 100.

Method of measuring the modulus of rupture (MOR). The MOR of a sample can be measured by testing a sample according to ASTM C1505-15. The sandstone article being tested 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 article has a width b (in mm) and a thickness h (in mm). A third rod is placed across the middle of the test article and parallel to the others. An increasing load is applied to the middle rod until the test article ruptures or breaks at the breaking load P (in N). This can be used to calculate B, the breaking strength, using the equation B = (P x L)/b. The modulus of rupture (MOR) is then given by the equation MOR = 3B/2h ² .

Method of measuring iron oxide. The level of iron oxide can be measured by X-ray fluorescence. The technique works by the excitation of the sample using high energy gamma or X-rays. This causes an ionisation of the atoms present which then emit characteristic frequency EM radiation which is dependent on the type of atom. Analysis of the intensity of different frequencies allows an elemental analysis to be made. Suitable equipment would be the Varta ^(RTM) range of XRF analyzers supplied by 01ympus ^(RTM) . The equipment detects elemental iron, and the result is most usually converted to the corresponding level of FeiC .

Examples

A mixture is prepared as follows:

170 grams of clay and 30 grams of sodium feldspar are milled in a ball mill (MITR, model YXQM-8L) having a container of 152 mm and depth of 172 mm. The container has approx 1250g of alumina grinding balls added to it. The alumina balls (density 3.95 g/ml) have the following size distribution: 5 mm (50%wt), 10 mm (32%wt), 20 mm (18%wt). The mixture is then milled at 180 rpm for 60 min.

Next, coal combustion fly ash is sieved through a 250pm sieve and a 90pm sieve so as to collect the fraction > 90pm and < 250pm.

Following this, lOOg of the milled mixture is mixed with lOOg of the 90pm - 250pm coal combustion fly ash. Then 16g of water are sprayed on to 200g of this mix as a binder. The wetted powder mix is then shaken through the 500pm mesh and granulated prior to pressing.

140 g of the mix is then uniaxially pressed in a rectangular mild steel mold (155x40mm) to a pressure of 40 MPa which is held for 1.5 min (90 sec). The formed body is released from the mold and placed into a 110°C oven to dry (for 2hrs).

The dried body is fired in an electric kiln at a ramp rate of 2.5°C/min to 1400°C. The temperature is held at the top temperature for 30 min. The fired body is then allowed to cool down naturally (hence slowly) to room temperature.

The fired sintered article has the appearance and colour of sandstone. The average Ra is 7 microns, the Rz is less than 50 microns and the water absorption is 1.0wt%.