Preparation and Characterization of Nano structured Materials from Fly Ash: A Waste from Thermal Power Stations, by High Energy Ball Milling
© to the authors 2007
Received: 27 April 2007
Accepted: 15 June 2007
Published: 11 July 2007
The Class F fly ash has been subjected to high energy ball milling and has been converted into nanostructured material. The nano structured fly ash has been characterized for its particle size by using particle size analyzer, specific surface area with the help of BET surface area apparatus, structure by X-ray diffraction studies and FTIR, SEM and TEM have been used to study particle aggregation and shape of the particles. On ball milling, the particle size got reduced from 60 μm to 148 nm by 405 times and the surface area increased from 0.249 m2/gm to 25.53 m2/gm i.e. by more than 100%. Measurement of surface free energy as well as work of adhesion found that it increased with increased duration of ball milling. The crystallite was reduced from 36.22 nm to 23.01 nm for quartz and from 33.72 nm to 16.38 nm for mullite during ball milling to 60 h. % crystallinity reduced from 35% to 16% during 60 h of ball milling because of destruction of quartz and hematite crystals and the nano structured fly ash is found to be more amorphous. Surface of the nano structured fly ash has become more active as is evident from the FTIR studies. Morphological studies revealed that the surface of the nano structured fly ash is more uneven and rough and shape is irregular, as compared to fresh fly ash which are mostly spherical in shape.
Nanoscience and nanotechnology has become the buzzword in recent years since its inception in 1990’s. It literally means any technology performed in the nanoscale down to molecular level. Nanotechnology encompasses the production and application of physical, chemical and biological systems at scales ranging from individual atoms or molecules to submicron level as well as integration of the resulting nano structure to larger systems . Nanomaterial is defined as the materials with the microstructure having at least one dimension in nanometer range. It has appeal of miniaturization; also it imparts enhanced electronic, magnetic, optical and chemical properties to a level that cannot be achieved by conventional materials. The key characteristics of nanomaterials are its small size, narrow size distribution, low levels of agglomeration and high dispersability .
A variety of ways have been reported to synthesize nano level materials such as plasma arcing, chemical vapor deposition, electro deposition, sol–gel synthesis, high intensity ball milling etc . Among these methods high energy milling has advantages of being simple, relatively inexpensive to produce, applicable to any class of materials and can be easily scaled up to large quantities . In this mechanical treatment, powder particles are subjected to a severe plastic deformation due to the repetitive compressive loads arising from the impacts between the balls and the powder. The high concentration of defects and the continuous interfaces renewal, associated with the milling-induced enhanced atomic mobility, promote different phenomena depending on the materials being milled [5–7]. This produces novel crystalline and amorphous materials with crystallite sizes at the nanometer scale.
Coal-burning power plants that consume pulverized solid fuels produce large amounts of fly ash. These are the finely divided mineral residue resulting from the combustion of ground or powdered coal in electric power generating plant. The fly ash consists of inorganic, incombustible matter present in the coal that has been fused during combustion into a glassy, amorphous structure. This material is solidified while suspended in the exhaust gases and is collected by particulate emission control devices, such as electrostatic precipitators or filter fabric bag houses. Fly ash, often called pulverized fuel ash, is the largest produced industrial waste in the world, mainly due to the global reliance on the coal-fired power plants . Since the particles solidify while suspended in the exhaust gases, fly ash particles are mostly spherical in shape and range in size from 0.5 μm to 100 μm. They consist mostly of mullite(3Al2O3 · 2SiO2), quartz (SiO2), aluminium oxide (Al2O3), hematite (Fe2O3), lime(CaO) and gypsum(CaSO4 · 2H2O). As a result it possesses various physical, chemical and mineralogical properties, depending on the mineralogical composition of the used coal and on the combustion technology .
About 75% of India’s energy supply is coal based and shall be so for the next few decades. There are about 82 utility thermal power stations to produce approximately 110 million tonnes of fly ash per annum in the Country . Nearly 38% of the fly ash waste is utilized in the Country at present , in various fields including landfills, cement making and concrete product making such as bricks, blocks and tiles, in road making, in filling of the mines. Attempts have been made earlier to utilize this fly ash waste in the polymer industry in making polymeric composites where fly ash is being used as inorganic particulate filler without much breakthrough. The utilization of fly ash is determined by their properties such as fineness, specific surface area, particle shape, hardness, freeze-thaw resistance, etc. Many investigations have been carried out towards the effective utilization of fly ash and with understanding of potential environmental and health impacts associated with its disposal by land filling.
In this paper an attempt has been made to modify the fly ash by transforming the micro sized fly ash into nanostructured fly ash using high energy ball mill. The smooth, glassy and inert surface of the fly ash can be altered to a rough and more reactive by this technique. The nano structured fly ash thus obtained may be characterized using sophisticated analytical techniques. Thus, nano level mineral filler can be used as reinforcing filler in making polymer composites, in particular rubber based composites.
Fly ash samples collected from Kolaghat Thermal Power Station, West Bengal, India having a specific gravity of 2.33 gm/cc and total evaporable moisture content of 1.54% is used. The particle size of fly ash falls in the range of 60–100 μm. Loss on ignition, which was measured by burning the sample in muffle furnace at 800 °C for 3 h, was 3%. Fresh fly ash has been washed in distilled water and removed the carbon that creamed up during washing. It is then dried at 100 °C for 48 h to remove water. Dried fly ash has been sieved using ASTM meshes ranging in size from 72 to 350. Fly ash fractions after passing through 200 mesh has been taken for ball milling since it gave 45% by weight of the total fly ash taken for sieving, the other size ranges providing less quantity.
High Energy Pulverization of Fly Ash
The reduction in particle size of fly ash from micron level to the nano level was carried out using a high-energy planetary ball mill (Pulverisette, Fritsch, Germany). The total duration of milling was 60 hours. The following milling conditions were maintained: loading of the ball mill with 10:1 ratio of balls to fly ash and milling chamber and balls were of tungsten carbide, the ball diameter was 10 mm. Toluene was used as the medium with an anionic surface active agent to avoid agglomerations; rotation speed of the planet carrier was 300 rev min−1.
Particle Size, Surface Area and Surface Energy Measurements
Particle size of ball milled fly ash at different time of milling was determined using dynamic light scattering technique in a Brookhaven particle size analyzer. Specific surface area of the ground fly ash was found out by using BET method. The samples were degassed at 350 °C before testing. The surface energy of the samples was calculated by measuring contact angle. The powder contact angles were found out using Dynamic Contact angle Tester (DCAT) from Dataphysics, UK.
X-Ray Diffraction Studies
The X-ray diffraction measurements were carried out with the help of a Goniometer model PW1710 using CuKα radiation (Kα = 1.54056 A) at an accelerating voltage of 40 kV and a current of 20 mA. The samples were scanned in the range from 10 to 90 degrees 2-theta.
Infrared Spectroscopy Studies
A Fourier Transform Infrared Spectroscopy (Perkin Elmer FTIR) was employed for examining the functional groups on the fresh as well as ball milled fly ash. The powder samples were ground with spectroscopic grade KBr and made into pellets according to the specified sample preparation procedure.
The size and dimensions of fresh as well as ball milled fly ash were examined by means of electronic microscopy. Scanning Electron Microscope (JEOL JSM 850) and Transmission Electron Microscope (Philips CM 12) were used for the particle surface as well as surface texture analysis.
Results and Discussion
Composition of the fresh fly ash (Class F)
% Elemental composition
% Oxide composition
As per ASTM C 618  fly ash has been classified into two categories, Class F and Class C. The fly ash that contains more than 70% oxides of silicon, aluminium and iron of the total composition with Fe2O3 content higher than CaO is termed as Class F type. According to the calculations carried out based on EDX analysis the overall composition of fly ash obtained for this study consists of major proportion of SiO2, Al2O3 and Fe2O3, which seems up to 97.42%. More over the percentage of calcium oxide, which is 0.99%, is less than that of iron which is 5.03%. This observation reveals that the procured fly ash is Class F type.
Variation of oxide composition with milling hours
Particle Size, Surface Area and Surface Energy Measurements of Fly Ash
Surface energy and work of adhesion of fresh as well as ball milled fly ash
Fly ash sample
Surface energy (mJ/m2)
Work of adhesion in water (mJ/m2)
Work of adhesion in formamide (mJ/m2)
Fresh fly ash
Ball milled for 40 hours
Ball milled for 60 hours
X-Ray Diffraction Studies
where D is the particle diameter, λ is the X-Ray wavelength, B is the FWHM of the diffraction peak, θ is the diffraction angle and K is the Scherrer’s constant of the order of unity for usual crystals.
Although fly ash exhibits lower degree of crystallinity, but it shows a number of crystalline peaks in the diffractogram. Mullite (Alumino silicate) and quartz (Silica) peaks are significant. Mullite shows strong peaks at 16.402°, 25.999°, 26.22° and 40.821° 2θ values (d spacing of 5.3998, 3.4243, 3.3959 and 2.2087 Å). The quartz exhibits strong peaks at of 20.763° and 26.579° 2θ values (d spacing of 4.2745 and 3.3508 Å). Iron oxide phase shows a peak at 34.856° 2θ value (d spacing of 1.82 Å). An amorphous hump is observed in the diffraction pattern between approximately 14° 2θ to 35° 2θ may be due to the presence of amorphous glassy materials .
Infrared Spectroscopy Studies
The size reduction of fly ash from micrometer level to nano levels has been achieved by high energy ball milling. The average particle size has been reduced from 60 μm to 148 nm, a reduction of nearly 405 times in magnitude, by this process. The surface area shows a tremendous increase by around 102 times in magnitude. The total surface free energy has increased by 300% after ball milling for 60 h. The characteristic –OH stretching vibration peak intensity increases by ball milling. The fly ash becomes more amorphous and the crystallite size reduces drastically. The shape and surface texture of the fly ash has been changed by ball milling which is evident from TEM and SEM studies. The nanostructured fly ash may be effectively used as reinforcing filler in polymer matrices.
The authors would like to thank Ms. Sasmitha Mohapatra, Department of Chemistry, Indian Institute of Technology, Kharagpur for particle size analyses and IRMRA, Thane, India for surface area measurements.
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