Speciation in Size and Density Fractionated Fly Ash II. Characterization of a Low-Calcium, High-Iron Fly Ash
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II.
SPECIATION IN SIZE AND DENSITY FRACTIONATED FLY ASH CHARACTERIZATION OF A LOW-CALCIUM, HIGH-IRON FLY ASH
R.T. HEMMINGS*, E.E. BERRY*, B.J. CORNELIUS* and B.E. SCHEETZ** *Ontario Research Foundation, Sheridan Park, Mississauga, Ontario, Canada, L5K 1B3. **Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16802 Received 25 February, 1987; Communicated by G.J. McCarthy ABSTRACT Morphological, chemical and mineralogical speciation of a low-Ca, high-Fe fly ash from a bituminous coal has been investigated by examination of size, density and magnetic fractions. Fractionation by size revealed little information as to speciation among particle types. However, separation of the ash into eight density fractions and into magnetic and non-magnetic components showed major differences in particle properties. It was found that glasscontaining particles can be divided into three general types: Type 1, being low-Fe content, low-density hollow spheres comprising aluminosilicate/mullite glass ceramics; Type 2, of intermediate density, being ferroaluminosilicate/ mullite glass ceramics; and Type 3, high density composite particles of spinel/hematite crystals embedded in an iron-substituted glass. It is proposed that Type 1 and Type 2 particles are derived from thermal decomposition of clay minerals with a range of Fe contents. Type 3 particles are considered to arise from thermal decomposition of pyrite in the presence of small quantities of aluminosilicate minerals. Two general types of glass were distinguished: Glass I(f), being largely a low-iron aluminosilicate; and Glass II(f), being a ferroaluminosilicate of high Fe-content. XRD and vibrational spectroscopic evidence suggest that, in both glass types, Fe is substituted for Al in an aluminosilicate-type structure. INTRODUCTION In early studies of the chemical and mineralogical properties of fly ashes [1-3], it was recognized that Fe was present in various component phases. Crystalline iron-containing phases were identified by x-ray diffraction (XRD) as magnetite (Fe 3 0 4 ), hematite (Fe 2 0 3 ) and glass [1]. Watt and Thorne [3] examined density fractions of a number of ashes from bituminous coals and found both Fe and Ca to be more concentrated in the heavier fractions (SG 2.5-2.6). These authors concluded that most of the Fe found analytically in the fly ashes occurred in separate, Fe-rich particles and not in glassy siliceous particles. In related work, Lauf [4] identified Fe-rich spheres in reflected light microscopy by their high reflectivity and optical isotropy. The particles were seen to comprise a "light" phase and an internal pattern comprising a dark phase. The light phase was attributed to a spinel or ferrite [5]; the dark phase was optically anisotropic and contained Al and Si. Lauf [4] further suggested that the dark phase was mullite or other clay decomposition products, and that the high-Fe particles may be pseudomorphs of pyrite framboids [6] found in the feed coal. Studies of ashes from North American bituminous coals [7
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