AU2009200846A1 - A Method for Improving the Colour of Fly Ash - Google Patents

Description

A METHOD FOR IMPROVING THE COLOUR OF FLY ASH Field of the Invention. The present invention relates to treatment of fly ash and particularly to treatments that improve the colour of fly ash. 5 Background Art. Fly ash is a fine powder collected as the combustion residue of coal in the exhaust gases from the combustion chambers of the pulverized coal fired boilers at power plant stations. Individual particles are mostly solid, irregular or spherical with a proportion of hollow spherical particles called cenoshperes. The size, chemical 10 composition and the colour of fly ash varies depending on the coal type used in coal power stations. Australia's fly ashes are light to mid-grey in colour with sizes range from less than 1 pm to above 200 pm and predominantly silicon and aluminium oxide with a range of other metal oxides being present in smaller quantities. For decades, fly ash has been used in construction applications 15 predominantly in ready mixed and other concrete but also in smaller amounts in cement bricks, road stabilisation, road bases, and structural fills. In Australial, these applications consume less than 20% of fly ash and the remainder is disposed of to landfill adjacent to the source. Recently research has been initiated to use fly ash for other potential applications, for example, as fillers in polymer to produce a particulate 20 reinforced polymer composite. Compared to other particulate fillers, for example calcium carbonate (CaCO 3 ), fly ash has advantages when added to polymer. Fly ash is cheaper and less dense for the same particle size of CaCO 3 . This means that for the same volume fraction of filler in a composite, less fly ash is needed to replace CaCO 3 .However as a 25 natural product, fly ash has many disadvantages when directly added to a polymer to make a composite. One of them is that fly ash contains contaminants, such as carbon black which gives fly ash and therefore the final product a darker colour. It would therefore be advantageous to provide a method for improving the colour of fly ash such that it could then be incorporated as a particulate filler with 30 minimal colour change to composite materials, particularly light coloured composite materials. It will be clearly understood that, if a prior art publication is referred to herein, this reference does not constitute an admission that the publication forms part 2 of the common general knowledge in the art in Australia or in any other country. Summary of the Invention. The present invention is directed to a method for improving the colour of fly ash, which may at least partially overcome at least one of the abovementioned 5 disadvantages or provide the consumer with a useful or commercial choice. With the foregoing in view, the present invention in one form, resides broadly in a method for improving the colour of fly ash including the steps of heating the fly ash to a temperature, Te, and holding the fly ash at or above Tc for a time, te then cooling the fly ash, wherein T, and te are dependent upon the chemical 10 composition of the fly ash. Preferably, Te is a critical temperature above which oxidation of colouring components or unburned carbon contained in the fly ash occurs. Preferably, te is a critical time for which the fly ash is held at or above the critical temperature to achieve oxidation of colouring components or unburned 15 carbon, in order to affect the colour of the fly ash. With regard to the heating step, the fly ash will preferably be heated in a furnace or similar piece of process equipment. The rate of heating of the fly ash from ambient temperature in terms of temperature increase per unit time until the critical temperature is reached, it is 20 preferably approximately 5*-20*C per minute. The critical temperature is typically at least 500*C, and according to the most preferred embodiment, will normally be at least 1000*C. The fly ash will typically be cooled, preferably using controlled cooling after the heating and holding stages. Typically, the fly ash will be cooled back to 25 ambient temperature. There may be some loss of weight of the fly ash during the method of the present invention. However the loss of weight will normally be less than 3% as this is usually the maximum proportion of unburnt carbon in the fly ash. The critical temperature, Tc, and the critical time, te, may vary for 30 different particulate sizes of the fly ash. In particular, the critical temperature, Te, and the critical time, te, may be determined in a direct relationship according to the particulate size fraction of the fly ash. Normally, the critical time, te, will increase for larger particulate sizes.

3 The critical time, t, will typically be in excess of 20 minutes and usually will be in excess of 60 minutes. Normally, the critical time, te does not include the time taken to heat the fly ash to the critical temperature, Te. When during the treatment phase, the fly ash will usually be held in an 5 oxygenated environment. Treatment of the fly ash may be improved by mixing or aeration. For example, aeration, agitation or fluidisation may improve the whiteness of the fly ash at a fixed critical time, te or may reduce the critical time, to for a given amount of colour change. The fly ash may be subjected to further treatment steps. For example, 10 iron present in the fly ash normally contributes to the "redness" of the fly ash. Iron present in a silicate form in the fly ash is typically protected from oxidation by the silicate. In order to treat iron in this form present in the fly ash, a further treatment step may be used prior to the heating step in order to disrupt the silicates to allow (further) oxidation of the iron, thus reducing the "redness" of the fly ash. 15 As stated above, the chemical composition of the fly ash to be treated, and particularly the "coloured" components of the fly ash to be treated such as iron, chromium and magnesium, is important. The method of the present invention will typically be varied according to the composition of the fly ash to be treated. In another form, the present invention resides in a plastic/fly ash 20 composite material including a polymeric material and fly ash treated using a heat treatment process to heat the fly ash to a temperature, Te, and holding the fly ash at or above T, for a time, te then cooling the fly ash, wherein T, and t, are dependent upon that the chemical composition of the fly ash. In still a further form, the present invention resides in a composite 25 material including a polymeric base material and heat treated fly ash. Brief Description of the Drawings. Various embodiments of the invention will be described with reference to the following drawings, in which: Figure 1 is a graph illustrating the loss of weight of samples of fly ash 30 at different temperatures established using thermogravimetric analysis. Figure 2 is a graph illustrating the loss of weight of samples of fly ash at different temperatures established using the furnace method. Figure 3 is a graph illustrating the normalized spectral power 4 distribution curves of fly ashes as a function of wavelength. Figure 4 illustrates specimens of polypropylene with added fly ash from different samples compared to pure polypropylene. Figure 5 illustrates representative fracture surfaces of a T63-1000-filled 5 polypropylene composite at low magnification. Figure 6 illustrates representative fracture surfaces of a T63-1000-filled polypropylene composite at high magnification. Detailed Description of the Preferred Embodiment. According to a particularly preferred aspect, a method for improving 10 the colour of fly ash is provided. The method for improving the colour of fly ash of the preferred embodiment includes the steps of heating the fly ash to a temperature, T,, and holding the fly ash at or above To for a time, te then cooling the fly ash, wherein Te and t, are dependent upon the chemical composition of the fly ash. 15 In the following example which were carried out in the initial testing phase, the critical temperature, Te was 10004C and the critical time, te was 60 minutes. As mentioned before the colour depends on the composition of fly ashes. Therefore, the fly ashes were analysed using ICP-EAS and the results are tabulated in Table 1. The fly ash samples are designated with a "T" or "S" indicating 20 their origin from Tarong or Swanbank power stations respectively. It can be seen that the fly ashes were relatively clean with a Loss on Ignition (LOI) maximum of 1.08 wt %. Furthermore, it can be seen that in general, sieving fly ashes into three different particle sizes gives the same order in chemical composition. Only a few exceptions can be noticed here such as iron and manganese 25 content in Tarong fly ash and sodium in Swanbank fly ashes. There is no significant difference in the weight losses when fly ashes loss was measured using the thermogravimetric analysis (TGA) method (illustrated in Figure 1) and the furnace method (illustrated in Figure 2). However the TGA analysis tends to give the same or lower order of weight loss compared to the furnace method, 30 except for T63, where it was the other way round. Table 1 is a chemical composition of the fly ashes [wt %] in oxide form used in the first testing phase.

5 SiO 2 3 Fe 2 0 3 TiO 2 CaO MgO I K20 Na 2 O [MnO Cr 2 0 3 LOI S59 71.9 22.3 1.84 1.62 0.43 0.53 0.24 0.20 0.02 0.05 0.57 S60 73.7 20.7 1.28 1.61 0.48 0.47 0.22 0.20 0.01 0.05 0.91 S64 73.6 18.65 1.90 1.36 1.02 0.46 0.27 1.00 0.01 0.06 0.98 T59 63.0 28.4 5.14 1.65 0.42 0.25 0.34 0.08 0.11 0.09 0.25 T60 66.2 25.5 4.37 1.70 0.19 0.23 0.33 0.08 0.08 0.06 1.08 T63 66.3 28.0 1.10 1.78 0.41 0.15 0.21 0.36 0.01 0.05 0.56 The colours of as received fly ashes are different depending on the source of the fly ashes; however it was difficult to differentiate the colour of fly ash from the same source with different code (particle size). 5 It is difficult to visibly justify which is the most white of the fly ashes. Therefore, colour analysis using colour measurement, known as colour by number, was necessary. Ideally, the measurement covers all factors influencing the colour such as light source, surroundings, condition of visual system, shape and orientation of motif, and also angular size. However it is almost impossible to have a physical 10 measurement for it, but by fixing some of the parameters for example the light source, angular size and geometry for standard white, the colour measurement can give a usable value. In this analysis, Chromaticities and luminous reflectance were calculated using CIE (Commission Internationale de I'Eclairage) Standard Illuminant 15 D65 (day light under blue sky), the CIE 10' Standard Observer, and geometry of 0 (normal incidence). The fly ash particles were filled into a sample holder, a cylindrical recess with a diameter of 10 mm and 8 mm deep. The surface (top) was smoothened to be visually level. The sample is illuminated at normal incidence (from above) by a quartz halide tungsten filament lamp energised with a current supply stable to 20 ±0.01%. Measurements were made at 450 to the surface using a Topcon SR-3 telespectroradiometer from 380 nm to 780 nm with interval of Inm. In determining the colour of materials, a relative value or normalized value is often used since the absolute reproduction was impractical. The normalized radiation curve as function of all wavelengths for the fly ashes is depicted in Figure 3 25 below. Colourless (white, grey or black) can be seen as flatness of the spectral power distribution curve, and it will be black when relative intensity is 0 and white when it is 1. If there is deviation from perfect flatness, then a colour is present.

6 From Figure 3, it can be seen that the as-received fly ashes at room temperature are almost colourless with normalized intensity is below 0.5 except for T63-RT is around 0.5. It means that T63-RT is the whitest among them, followed by S60, S59, S64, T59 and then T60-RT as the blackest fly ash.| 5 However, when they were heated at 1000"C, the colours were present, that is, the curves are not flat. The highest intensity is for T63-1000 and the lowest is for T59-1000. Here it can be said that the T63-1000 is the whitest among others, while T59-1000 is the blackest fly ash after heating at 1000'C for one hour. It is however, difficult to say what colours are present using this curve. 10 There are several possible colour coordinate systems that are designed to better fit the perception of colours, the most well known is the CIE L*a*b*-system which is the most complete colour model to describe all the colour visible for human eyes. The colour value in CIE L*a*b* coordinate is tabulated in table 2 below. Firstly, the intensity is transferred to CIE XYZ system which is the 15 basis of another colour space system. In CIE XYZ colour space, the tristimulus X, Y, and Z value which roughly correspond to three primary colours, red, green and blue respectively, is calculated using CIE colour matching functions with following formula. 780 X= I(A)x(A)dA. 380 20 (1) 780 Y= fI(A)y(A)dA. 380 (2) 780 Z= fI(A)z()ca . 380 (3) 25 Wherein: I(A) : Normalized intensity per small constant width wavelength interval (2) throughout the spectrum in spectral power distribution curve (in this case is the interval was 1 nm) x(A), y(A), z(A): the heights of the CIE colour-matching function at the central 30 wavelength of each interval. After obtaining an X, Y, and Z value, these values are transferred to L*, 7 a*, and b* using formula below, and the result is tabulated in Table 2 below. L*=116(Y/Y,'" -16. (4) 5 a*= 500(X / X)" (Y / Y")' (5) b* = 200[(Y/Y)'" - (z/Z)1". (6) 10 where X,, Yn, Z, are the Value of X, Y, and Z for chosen reference white. In Table 2, L* represent lightness (whiteness) ranging from 0 (black) to 100 (white). a* means reddish when positive and greenish when negative, and b* means yellowish when positive and bluish when negative. When the value of both a* 15 and b* are zero (0), it means no colour is present and the material has black, grey, or white colour depending on the L* value.| Table 2. The L*, a* and b* value of fly ashes in the testing phase. L* a* b* Raw Fly Ash S59-RT 71.5 0.2 3.6 S60-RT 71.7 0.2 3.6 S64-RT 70.8 0.2 2.7 T59-RT 69.8 0.2 4 T60-RT 69.0 0.2 4.7 T63-RT 77.2 0.2 2.1 Post Treatment S59-1000 87.1 0.2 6.5 S60-1000 88.8 0.2 7.5 S64-1000 87.8 1.2 8 T59-1000 76.1 1.4 9.4 T60-1000 79.6 2.1 13.1 T63-1000 89.8 1.9 5.7 From the above results, it can be seen that Swanbank fly ashes prior to 20 heating had the same order of whiteness (-70), while the whiteness of Tarong fly ashes depend on its chemical composition, especially iron and manganese content. After heating, the whiteness of Swanbank fly ashes increased relatively in the same 8 order, and resulting in the same order of whiteness (-88). On the other hand, in the Tarong fly ashes after heating, the whiteness varied. Table 2 shows that whitest fly ash is T63 at 1000 C (L* value 89.8), while the most black (grey) is T60 at room temperature (L* value 69). 5 From Table 2, the increasing whiteness ranged from 9.03 % (T59) to 24.01 (S64), redness from 0% (S59 and S60) to 950% (T60) and yellowness from 81% for S59 fly ash to 196% for S64 fly ash. It is an interesting phenomenon that there was no increasing of redness for the S59 and S60 fly ashes (see Table 2). This result indicates that either there was 10 no oxidation of iron or there was oxidation of iron but the increasing in redness was compensated by oxidation of chromium (Cr) and manganese (Mn) (see Table 1). However, since the concentration of Cr and Mn in the fly ashes was low compared to that of iron, the first is more likely. This phenomenon is likely to occur because the iron is present in the fly ash as an inclusion in other materials such as silica which 15 protects iron from oxidation.1 It is an interesting phenomenon that there was no increasing of redness for the S59 and S60 fly ashes (see table 2). It indicates that redness which produced by the oxidation of iron was elaborated by greenness which produced by oxidation of chromium (Cr) and oxidation of manganese (Mn) (see Table 1). 20 From Table 2 can be seen that heating the T59 fly ash at 1000C could not increase its whiteness to the level of whiteness of T63 at room temperature. This result clarifies that the chemical composition plays an important role that can result in significant differences in colour although fly ashes come from the same source. Furthermore, from chemical composition analysis found that the iron 25 content plays an important role in colouring especially after heating. The iron oxide content of Tarong fly ashes were 5.14 wt % (T59), 4.37 wt% (T60) and 1.1 wt % (T63) with whiteness of 76.1, 79.6 and 89.8 respectively. In other words, the smaller iron contents in a fly ash, the whiter the fly ash. For Swanbank fly ashes, aside from the iron content, particle size also played important role. Based on iron content, the 30 whiteness of Swanbank fly ashes should be S60 > S59 >S64. However since the particle size of S64 is much smaller compared to S59 (see Table 3),: the proportion of iron to oxidize was bigger, that made S64 whiter than S59. However 64 was redder (1.2) and more yellow (8) compared to S59 which had redness of 0.2 and yellowness 9 of 6.5 due to iron oxidation. Since Tarong fly ashes contain less iron when the particle size was smaller, it indicates that there is possibility to increase the whiteness by reducing the particle size to the commercial particulate fillers (<5 Lm) thereby reducing the iron content. 5 Table 3. Particle size data of fly ashes (pm) Mean Median Mode (peak) Raw Fly Ash S59-RT 44.12 32.92 37.96 S60-RT 26.02 20.33 26.14 S64-RT 10.61 6.84 7.08 T59-RT 85.89 67.37 105.9 T60-RT 21.17 13.44 12.4 T63-RT 8.96 7.44 11.29 Post Treatment S59-1000 46.09 34.44 37.96 S60-1000 28.32 22.14 23.81 S64-1000 12.62 9.79 11.29 T59-1000 80.64 66.30 105.9 T60-1000 24.69 15.76 16.40 T63-1000 9.56 7.90 11.29 It was also found that oxidation oFT63 was still continuing when it was heated for 1 hour. This occurs even though the fly ash is not under heated conditions, because the oxidation of magnetite (Fe 3 0 4 ) to at-hematite (a-Fe 2 0 3 ) can occur below 600*C 10 and since a-Fe 2 0 3 was already present in the fly ash as result of oxidation previously. When this situation occurs, based on the color measurement, T63 may not be the best fly ash for obtaining whiter product when stored for long periods. Rather, S60-1000 can be considered as replacement since there is no increasing in redness for S60. In conclusion, the whitest fiv ash was T63-1000 with the whiteness of 89.8, 15 followed by S60-1000 (88.8), S64-1000 (87.8), S59-1000 (87.1), T60-1000 (79.6), T63-RT (77.2), T59-1000 (76.1), S60-RT (71.7), S59-RT (71.5), S64-RT (70.8), T59 RT (69.8), and T60-RT (69.0). As illustrated in Figure 4, the visible colour of composites using injection moulding with polypropylene (PPI) polymer differs widely. Figures 5 and 6 20 respectively presents representative fracture surfaces of a T63-1000-filled 10 polypropylene composite at low and high magnification. In the present specification and claims (if any), the word "comprising" and its derivatives including "comprises" and "comprise" include each of the stated integers but does not exclude the inclusion of one or more further integers. 5 Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all 10 referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more combinations. In compliance with the statute, the invention has been described in language more or less specific to structural or methodical features. It is to be understood that the invention is not limited to specific features shown or described 15 since the means herein described comprises preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims (if any) appropriately interpreted by those skilled in the art.

Claims (17)

1. A method for improving the colour of fly ash including the steps of heating the fly ash to a temperature, Te, and holding the fly ash at or above T. for a time, te then cooling the fly ash, wherein Tc and tc are dependent upon the chemical 5 composition of the fly ash.

2. A method for improving the colour of fly ash as claimed in claim I wherein, Te is a critical temperature above which oxidation of colouring components or unburned carbon contained in the fly ash occurs.

3. A method for improving the colour of fly ash as claimed in claim 1 or claim 2 10 wherein, te is a critical time for which the fly ash is held at or above the critical temperature to achieve oxidation of colouring components or unburned carbon, in order to affect the colour of the fly ash.

4. A method for improving the colour of fly ash as claimed in any one of the preceding claims wherein the rate of heating of the fly ash from ambient 15 temperature in terms of temperature increase per unit time until the critical temperature is reached is between approximately 5* to 20*C per minute.

5. A method for improving the colour of fly ash as claimed in any one of the preceding claims wherein the temperature Tc is at least 500*C.

6. A method for improving the colour of fly ash as claimed in any one of the 20 preceding claims wherein the temperature Te is at least 1000*C.

7. A method for improving the colour of fly ash as claimed in any one of the preceding claims wherein the fly ash is cooled using controlled cooling after the heating and holding stages.

8. A method for improving the colour of fly ash as claimed in any one of the 25 preceding claims wherein the critical temperature, Tc, and the critical time, te, is determined in a direct relationship to the particulate size fraction of the fly ash.

9. A method for improving the colour of fly ash as claimed in any one of the preceding claims wherein the critical time, t, is in excess of 20 minutes.

10. A method for improving the colour of fly ash as claimed in any one of the 30 preceding claims wherein the critical time, t; is in excess of 60 minutes.

11. A method for improving the colour of fly ash as claimed in any one of the preceding claims wherein the critical time, t, does not include the time taken to heat the fly ash to the critical temperature, T. 12

12. A method for improving the colour of fly ash as claimed in any one of the preceding claims wherein, the fly ash is held in an oxygenated environment during the treatment phase.

13. A method for improving the colour of fly ash as claimed in any one of the 5 preceding claims wherein the fly ash is aerated, agitated or fluidised during treatment.

14. A method for improving the colour of fly ash as claimed in any one of the preceding claims wherein the fly ash is subjected to at least one further treatment step to treat iron in the fly ash, the further treatment step prior to the heating step 10 in order to disrupt silicates to allow oxidation of the iron, thus reducing the "redness" of the fly ash.

15. A plastic/fly ash composite material including a polymeric material and fly ash treated using a heat treatment process to heat the fly ash to a temperature, Tc, and holding the fly ash at or above Tc for a time, to then cooling the fly ash, wherein 15 Tc and te are dependent upon that the chemical composition of the fly ash.

16. A plastic/fly ash composite material including a polymeric material and fly ash treated according to a method claimed in any one of claimed I to 14.

17. A composite material including a polymeric base material and heat treated fly ash. 20