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Weathering Processes in Relation to Leachate Properties of Alkaline Fly Ash¹

ABSTRACT

Results are presented for a column leaching experiment designed to evaluate the relationship among leachate properties, weathering processes, and solid phase characteristics of an alkaline fly ash. Fly ash was weathered with 0.005 mol/L H₂SO₄ in a series of five leaching columns. Acidification of the ash leachates occurred in a stepwise fashion involving three buffer stages. Chemical dissolution and mobilization of elements was greatest in the first column and decreased to minimal dissolution and maximum accumulation of weathered products in ash of the fifth column. Results indicated that unreacted fly ash consists of spherical micron-sized particles composed of mullite enclosed in a two-phase glassy matrix. The external glass was enriched in Ca, Mg, Fe, and Al. The interior glass matrix was composed primarily of Si and a major portion of the total Na and K. The ash also contained a highly reactive inorganic phase composed largely of submicron-sized CaO fragments on the surfaces of ash particles. Initial dissolution characteristics were dictated by the reactions of the CaO fragments and surface-associated soluble salts. Leachate characteristics were largely controlled by dissolution of the external glassy matrix after the rapid dissolution of surface salts. Solid state migration and removal of Na from the interior glassy matrix was active under acidic leaching conditions.

Key Words: coal ash • solid waste • dissolution • energy waste • fossil fuel • mineralogy

¹ Contribution from the Dep. of Soil Science, The Univ. of Alberta, Edmonton, Alberta, Canada T6G 2E3.

² Graduate student and associate professor of soil science, respectively, Dep. of Soil Science, The Univ. of Alberta.

Received for publication December 7, 1983.

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