Fly ash is classified as a natural pozzolan and finds significant use as a cement additive. It has been found to increase the plasticity of the cement mixture while lowering the amount of mix water needed. In addition to improving leveling and finishing properties, fly ash improves pumpability as well as decreases the heat of hydration of the concrete mix which is particularly important in massive concrete structures such as dams where the concrete needs to cool as quickly as possible.
Experimental
Approximately 600 pound samples of Class F fly ash were collected from several coal fired PC boilers in the eastern United States. The samples were collected over a one week period during normal operation to ensure that the fly ash was representative. The composite samples were thoroughly mixed at the Center for Applied Energy Research (CAER) using a V-blender to prevent size segregation. A representative sample of each composite was obtained by riffling for size analysis while approximately 400 pounds of each composite sample was shipped to General Electric for classification tests. After completion of classification tests, all samples were returned to CAER where they were again, thoroughly mixed to obtain a representative sample of each by riffling.
Results and Discussion
The lowest grade obtained was 2.85 %C with 5% secondary air. The corresponding yield was 60.9%. At higher levels of secondary air, the carbon content of the fines increased to 2.9% carbon while the yield improved to 77.8%. For the GR sample, a product was produced within specification with 3% secondary air (2.94% C) with a yield of 50%. Higher amounts of secondary produced higher yields, but also increased the carbon content of the fines to above 3%.
Classifier efficiency was determined for 100, 200, 325 and 500 mesh separations with the SP sample. Efficiencies were calculated using the formula:
Efficiency = c(f-t)/f(c-t)
where c is the weight % finer than the desired size in the fines, f is the weight % finer than the desired size in the feed and t is the weight % finer than the desired size in the coarse product.
Conclusions
The results obtained in this study show that the centrifugal classifier is capable of producing a product within carbon specification (<3.0%) for both samples at low amounts of secondary air. More favorable yield was obtained with the SP sample under these conditions. At higher amounts of secondary air, the results obtained with the SP sample increased slightly but were still within spec, while those obtained with the GR sample were above spec. The lower carbon content of the fines produced with the SP sample is attributed to both the lower carbon content of the feed and higher classification efficiency for the fine fractions.
