Fluid Extraction of Metals from Coal Fly Ash: Geochemical Simulation of Natural leaching

• Main Author: Kim, Ann Gallagher
• Format: Thesis
• Language: English
• Published: 2002
• Subjects: DML
• Online Access: http://d-scholarship.pitt.edu/7825/; http://d-scholarship.pitt.edu/7825/1/dissertationagk.pdf

The objective of this study was to develop data that are broadly applicable to the release of trace metals from fly ash, and to quantify the rate of release as a function of the composition of the ash. Thirty-two samples of Class F fly ash from pulverized coal combustion boilers were leached with seven leaching solutions simulating natural fluids. The leachate was analyzed for 21 cations that were major, minor, or trace constituents of the ash. The rate at which metal ions are released from fly ash is a complex function of the alkalinity of the ash, the distribution of elements in various chemical compounds or minerals, and characteristics of the leachant solution, particularly its pH. In this experiment, the release of cations is defined as a solubility function with respect to the volume of leachant solution. During the first leaching interval, the ashes alkalinity is neutralized, and the release of metal ions, except for Ca, is relatively low. At some point, the release of metal ions increases by one or more orders of magnitude, and remains at that level, until the readily soluble ions are released. Then the elemental release decreases, again by one or more orders of magnitude. The solubility of an element is defined by the three volumetric functions and the median volumes for those functions. The N LF (neutralization leaching function) is describes the release of cations until the sample is neutralized (dMN/dVN, meq/L). The RLF (rapid leaching function) rate is the average slope of cumulative curve between inflection points (dML/dVL, meq/L). The TLF (terminal leaching function) is the average slope of cumulative curve after 2nd inflection point (dMT/dVT, meq/L). In a natural setting, if the infiltration rate is known (L/d), the time dependent release of the elements can be estimated. The results of this study show that most cations in fly ash are only slightly soluble, that elements, other than arsenic, tend to be most soluble in acid solutions, and that non-silicates tend to be more soluble than silicates.