Design of a Leaching Test Framework for Coal Fly Ash Accounting for Environmental Conditions

Abstract

Fly ash from coal combustion contains trace elements which, on disposal or utilisation, may leach out, and therefore be a potential environmental hazard. Environmental conditions have a great impact on the mobility of fly ash constituents as well as the physical and chemical properties of the fly ash. Existing standard leaching methods have been shown to be inadequate by not representing possible disposal or utilisation scenarios. These tests are often criticised on the grounds that the results estimated are not reliable as they are not able to be extrapolated to the application scenario. In order to simulate leaching behaviour of fly ash in different environmental conditions and to reduce deviation between measurements in the fields and the laboratories, it is vital to study sensitivity of the fly ash constituents of interest to major factors controlling leachability. pH, liquid-to-solid ratio, leaching time, leachant type and redox potential are parameters affecting stability of elements in the fly ash. Sensitivity of trace elements to pH and liquid to solid ratio (as two major overriding factors) has been examined. Elements have been classified on the basis of their leaching behaviour under different conditions. Results from this study have been used to identify leaching mechanisms. Also the fly ash has been examined under different standard batch leaching tests in order to evaluate and to compare these tests. A Leaching Test Framework has been devised for assessing the stability of trace elements from fly ashes in different environments. This Framework assists in designing more realistic batch leaching tests appropriate to field conditions and can support the development of regulations and protocols for the management and disposal of coal combustion by-products or other solid wastes of environmental concern.

Leaching behavior of selected trace elements in coal fly ash samples from Yenikoy coal-fired power plants

“…According to previous studies, some elements such as Ca, Cr and Mg are reported to the surface of the fly ash particles, while Al, Si, K, Pb and many other trace elements are distributed throughout the particle [40,41]. Most of the cationic elements including Mg, Cu, Mn and Ni, show lower leaching rates due to their chemical behavior in solution below pH 10.…”

“…The solubility of these elements tends to increase at pH levels above and below 7. Solubility of Cr increases with increasing pH for pH levels over 7 and decreases in strong basic conditions (pH > 11) [38][39][40].…”

Leaching behaviour of elements from coal combustion fly ash: An overview

“…In the light of the pronounced mobility reported throughout the literature, special consideration should be given to these elements in disposal FIGURE CAPTIONS Figure 1. General trends in the leaching behaviour of selected elements vs pH after Chandler et al (1997), de Groot (1989, Dubikova et al (2006), Jankowski et al (2004), Jones (1995), Moreno et al (2005), van der Sloot (1990) and Zandi and Russell (2007). …”

“…A detailed discussion on the worldwide standard leaching tests is out of the scope of this study and can be found elsewhere (Chandler et al, 1997;Kim, 2002;van der Sloot, 1997;Zandi and Russell, 2007).…”

“…Given that a number of elements contained in fly ash are markedly pH sensitive, unified curves presenting the pH-dependent solubility of various elements are plotted where possible (Figure 1), after the compilation of data from various sources (Chandler et al, 1997;de Groot, 1989;Dubikova et al, 2006;Jankowski et al, 2004;Jones, 1995;Moreno et al, 2005;van der Sloot, 1990;Zandi and Russell, 2007). In interpreting the curves is important to bear in mind that a number of factors (e.g.…”