Enhanced Resource Recovery by Beneficiation and Direct Acid Leaching of Fly Ash
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ENHANCED RESOURCE RECOVERY BY BENEFICIATION AND DIRECT ACID LEACHING OF FLY ASH E.E. BERRY*, R.T. HEMMINGS* and D.M. GOLDEN** *Ontario Research Foundation, Sheridan Park, Mississauga, Ontario, Canada, L5K 1B3 ** Electric Power Research Institute, P.O. Box 10412, Palo Alto, CA 94303. Received 30 January,
1987; Communicated by G.J.
McCarthy
ABSTRACT An investigation was conducted into the use of beneficiated ash fractions as feed materials for direct acid leaching to recover Al and other metals. A low-Ca (1.4Z CaO) ash was selected from which four fractions (classifier fines, classifier rejects, magnetic concentrate, and non-magnetics) were produced by mineral beneficiation methods. The raw ash and the four ash fractions were leached with 6 M HCI under reflux. Mass balance data were obtained to determine extraction efficiency for Si, Al, Fe, Ca, Mg, Na, K and Ti. It was found that size separation of the ash had a marked influence in enhancing extraction efficiency from the fine fraction for all of the elements studied except Fe. Extraction of Fe from the magnetic fraction was less efficient than from the non-magnetic fraction. Examination of the solid residue after leaching showed that, although most of the acid-soluble components were removed from the glassy phases of the ash, a significant part of the ash comprised non-reactive glass of high-Si content. A relationship between extractable Al in ash and leachable alkali and alkaline earth metals was identified and discussed in terms of glass modification theory. INTRODUCTION Since 1979, EPRI has sponsored research to examine several processes for the recovery of metal values from coal combustion residues [1-4]. An early conclusion from these studies was the identification of direct acid leaching (DAL) with HCI as a promising minimum-treatment method for the recovery of aluminum and other metals from fly ash. The process requires refluxing fly ash in HC1, whereby Al, Fe and other metal values are dissolved and a largely aluminosilicate mass remains (DAL-residue). For economical application of the DAL-process, it is necessary that: (a) process conditions are optimized, including pre-leach process operations; and (b) suitable markets are found for DAL-residues or the products made from them [4]. The work described in this paper comprises part of a study conducted by ORF for EPRI directed to the following primary objectives: -Demonstration of the use of pre-treatment (beneficiation) of a selected raw fly ash to upgrade it as a feed for the DAL-process; -Investigation of the extractability of aluminum, iron and other components from beneficiated fractions of the selected ash; -Confirmation of the scale-up of the DAL-process from the laboratory to pilot-plant level; -Demonstration of the production of filler grade products from the DALresidue, evaluation of their performance as fillers in polymer composites and evaluation of the potential marketability of DAL-residues in the fillers industry. This paper summarizes the findings of work related to the first two of these o
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