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Neutralization and Dissolution of High-Calcium Fly Ash¹

ABSTRACT

The burning of coal for the production of energy is anticipated to increase tremendously during this decade. This will result in large quantities of fly ash that must be disposed of or utilized. One possible use of Ca-rich fly ash is as a supplement to agricultural lime.

The neutralization reactions of three high-Ca fly ashes with HCl, and the concomitant dissolution of Ca, Mg, and Al are reported. The end products of the dissolution of the ash were measured under controlled and replicated conditions after 2 weeks of incubation. The acid neutralization was measured after 2 weeks and 6 months. The ratio of acid to fly ash varied from 0 to 15 meq H₃O⁺/g ash. Plots of pH and the percent dissolution of Ca, Mg, and Al are reported for each system.

The neutralization reaction for each fly ash showed three distinct buffer zones: a high-pH region (pH 12.0-10.5) accompanied by dissolution of Ca, a second region (pH 9.2-8.5) with the concomitant dissolution of Mg, and a third region (below pH 4.2), in which Al was released from the fly-ash matrix. The neutralization curve for an artificially prepared fly-ash material followed the same trend.

The neutralization capacity (NC) in meq H₃O⁺/g ash at a particular pH value was predicted from the percent Ca and Mg contents over the pH range 8.0-5.0 by the equation: NC = 4.08 + 1.20 (Mg) + 0.37 (Ca) – 0.75 (pH) (n = 63, r² = 0.99, p < 0.0001) for the 2-week period, and NC = 3.64 + 1.44 (Mg) + 0.32 (Ca) – 0.57 (pH) n = 48, r² = 0.98, p < 0.0001), for the 6-month period.

Electron micrographs of untreated and acid-treated fly ash exhibited deterioration of the particles and re-crystallization of dissolved salts onto particle surfaces.

Key Words: acid weatherings • soil acidity • neutralization capacity

¹ Contribution of the Department of Agronomy, Arkansas Agric. Exp. Stn. Published with the approval of Director of the Arkansas Agric. Exp. Stn., Fayetteville. Supported in part by grants from the Arkansas Power and Light Co., which are gratefully acknowledged.

² Graduate Assistants and Professor, respectively, Department of Agronomy, University of Arkansas, Fayetteville, AR 72701.

Received for publication April 20, 1981.

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