Minerals come in a variety of shapes and sizes, as well as varying composition. While some minerals form precious or semiprecious stones, others are important for their abrasive properties. They smoothen or rub off surface matter by friction. Some commonly used abrasive minerals include diamond, silicon dioxide, garnet and aluminum oxide. Abrasives are used in a wide range of applications including mechanical planing of components for computers or DVDs, on sandpaper, grinding wheels, cutting wheels, cosmetic surgery for skin rejuvenation, removal of blemishes or birthmarks, giving anti-skid properties to a floor, in toothpaste for tooth whitening, milling, cleaners for kitchens and bathrooms, or hand cleaners.
When coal is burnt it produces sulfur dioxide (SO2), particles of soot, carbon dioxide and nitrogen oxides, among many other substances. These emissions have come down over the last three decades or so, with the use of Clean Coal Technology. Scrubbers are devices used since 1980 to desulfurate waste gases, by what is called flue-gas desulfurization, and this has helped reduce sulfur dioxide emission in the USA by about 40% since then.
Both wet and dry scrubbers are in use, using dry or slurried limestone to react with a stream of sulfur dioxide flowing past it to form calcium sulfate. This reduces the emission of SO2 by about 90%. The end product is gypsum, CaSO4, thus recycling the emitted SO2 to a useful substance.
Wet-flue systems are preferred for their cost-effectiveness, reliable operation and high efficiency of scrubbing. The SO2 is finally accumulated at the bottom in solid or slurry form, or using an electrostatic precipitator or bag-house. In Figure 1, the size distribution of diffracted particles can been seen as measured by the Microtrac Sync instrument.
The gas is removed using the following chemical reaction:
The sludge collected in the scrubber is rich in gypsite, but also has several impurities such as CaSO4 • 1/2 H2O and CaCO3. The gypsite can be oxidized to gypsum under natural conditions, or it may undergo forced oxidation as in Figure 2, leading to conversion of almost all the gypsite to synthetic gypsum. The use of the scrubber therefore leads to the industrial production of gypsum in addition to the traditional mining source.
This is important because gypsum is the main component of wallboard. Purification is necessary since the presence of impurities prolongs the time required for the wallboard to harden. The purified gypsum produced by this method is widely used because it is less expensive to produce than mined gypsum.
By-Products Used in Portland Cement
Synthetic gypsum can also be added to cement clinker or to the final cement product, as well as bottom ash and fly ash removed from the bag houses or from electric precipitators. Cement manufacturing processes thus absorb more than 420 000 tons of scrubber products generated from power plants each year, using two byproducts of coal burning in different ways.
The addition of fly ash reduces the permeability and boosts the durability of cement, while gypsum is added in the finish mills during Portland cement manufacture. Cement kiln dust is another byproduct of cement processing that is added to synthetic gypsum to yield a powder that is used in the finish mills.
Recycling two or more of these byproducts of power generation to produce useful goods reduces the final emissions and also decreases the amount of waste sent to landfills. In this way the environment is protected against undue stress.
Plastic Filler Substitution with Coal-Burning By-Products
Electric power plants also help with plastic production. Fly ash is used as filler in plastics manufacture, to enhance the performance characteristics while bringing down the cost. According to its properties, fly ash is a mineral filler, just like alumina, mica, talc and kaolin, but the particles of fly ash have a size distribution that is skewed to above 5 microns. These coarse particles must be removed or treated before fly ash is used as a filler. Figure 3 shows a comparison between the results of Sync diffraction of fly ash particles for limestone, gypsum and fly ash.
Sync Image Analysis
Gypsum was bought from a local store selling building supplies and used for image analysis, to compare the results from this technology with those obtained by laser diffraction. The latter uses light scattering by particles to obtain volume distributions of particle sizes. Dynamic image analysis (DIA) uses photographs of each particle to perform a number of calculations as to the shape and size.
Both these methods yield separate data sets as to particle size based on different theories, employing different calculation methods. This often leads to some discordance between the results of the two measurement methods. On the other hand, their simultaneous use on the same sample ensures that particle size and shape are more fully characterized using a single instrument, the Microtrac Sync. Figure 4 shows the measurements obtained by Sync diffraction on a gypsum sample bought from a commercial supplier.
The use of laser diffraction to get a volume-based particle size distribution is complemented by the additional shape information obtained by image analysis. This combination helps to describe the shape and size of the particles more completely.
Table 1 shows that the greatest agreement between the two measurements is the FWidth, which may be considered equivalent to sieve size. However, the FWidth obtained by imaging analysis technology may not be suitable for defining specifications since it may differ from the historical settings, which were obtained using diffraction.
The Sync offers the advantage of measuring diffraction size and images using the same sample. Figure 5 shows how the parameters Da, Sphericity (column 3), FLength, L/W ratio (column 4) and FWidth, Sphericity (column 5) are constant for this material, indicating that the particles have similar shapes and the degree of segregation by size is low. They are also not likely to be spherical and therefore do not segregate by shape. The constant FWidth suggests a sieve cut.
The same data is represented as a graph in Figure 6. Diffraction technology does not give the same information, so the addition of image analysis actually rounds out the sample characteristics.
Sample Preparation and Measurement
The particles of the sample dissolve slightly in water. In most cases, the sample comes in dry cake form because of processing, and further drying can be done to reduce it to powder form. This may result in the formation of agglomerates which must then be dispersed by ultrasonic application, adding a little more time to the process.
The carrier fluid in the recirculator is 99% Isopropyl Alcohol, IPA, with a refractive index of 1.38, while the particle has a refractive index of 1.53, with irregular shape.
The powder is mixed properly and placed in 60 ml of IPA in a 100 ml beaker. The sample is examined for agglomerates under a microscope. If present, 10 seconds of ultrasonic pulsation is used to break them up in the IPA-powder slurry, if a cake was used, or 60 seconds if the cake was further dried. Microscopic examination is recommended to confirm the complete break up of agglomerated particles.
After complete dispersion has thus been achieved, the beaker sample is stirred with an overhead stirrer. Meanwhile, some parts of the dispersed solution are removed with a pipette and put into the recirculator which contains IPA. This is repeated until an acceptable loading level is reached.
The use of the Microtrac Particle Size Analyzers has been adopted in more than 20 large and ultramodern electric power plants using coal with Clean Coal Technology. More methods are under development to decrease the environmental toll caused by power generation due to plant emissions, and Microtrac instrumentation to provide laser measurements of particle size will be useful in these technologies as well.
The data obtained from these experiments shows that combining laser diffraction with imaging analysis technology yields a lot of information on particle size and shape over a wide range of sizes. The simultaneous approach allows the complete characterization of the tested sample so that the issue of whether the measurement of a sample was representative of the whole batch does not arise when a separate or new sample has to be measured.
- Huang,X., Hwang, J.Y., and Gillis, J.M., Processed Low NOx Fly Ash as a Filler in Plastics. Journal of Minerals and Materials Characterization & Engineering, Vol2, No 1, p11-31, 2003
- Kawatra, S.K., Eisele, T.C., and Shoop, K., Separation of Flue Gas Scrubber Sludge into Marketable Products, Michigan Technological University, Dept of Metallurgical and Materials Eng., For the US DOE, Pittsburgh Energy tech Center, Pittsburgh, PA August 1997.
- Portland Cement Association, Sustainable Manufacturing Fact Sheet. Power Plant Byproducts. July 2005, PCA Website.
This information has been sourced, reviewed and adapted from materials provided by Microtrac, Inc.
For more information on this source, please visit Microtrac, Inc.