Determination of solid solution limits based on the thermal behaviour of aluninium substituted iron hydroxides and oxides

Abstract

The formation of substitutional solid solutions of the isostructural oxyhydroxides o~-FeOOH-a-A1OOH (goethite-diaspore and "/-FeOOH-y-A1OOH (lepidocrocite-boehmite) was investigated by X-ray powder diffraction technique and by thermal analysis.The unit cell parameters of both orthorhombic structures of a-Fel.xAlxOOH (Pbnm) and ~-Fel.xAlxOOH (Cmcm) decrease clearly with the increase of alumininm content up to x ~0.10.Thermal analysis reveals the sensitivity of DTA and DTG effects to the composition of solid solutions. The temperatures of topotactic formation of rhombohedral ~-(Fe, A1)203 and of the defect-spinel y-(Fe, A1)203 during the dehydroxylation of ot-(Fe, A1)OOH and y-(Fe, A1)OOH respectively increase as the aluminium concentration rises up to 10 tool%. The dehydration curves of samples with greater aluminium content exhibits the same maximum Am/AT as for the 10 mol% Al-preparation, indicating indirectly the end-members of the solid solution ranges.

Thermochemistry of jarosite-alunite and natrojarosite-natroalunite solid solutions

“…This is in agreement with earlier results (Brophy et al, 1962), showing that the dehydroxylation of alunites requires higher temperatures than the dehydroxylation of jarosites. Note that the same general trend is observed for Fe and Al oxyhydroxides (Wolska et al, 1992;Kloprogge et al, 2002;Mitov et al, 2002): the higher the Al content, the higher the decomposition temperatures. According to Brophy et al (1962), these observations may be indicative of stronger bonds formed between Al and hydroxyl groups.…”

Kinetics and energetics of phosphate sorption in a multi-component Al(III)–Fe(III) hydr(oxide) sorbent system

“…Characterization studies of multi-component sorbents [5,[14][15][16][17] indicate that the physico-chemical properties of these solids differ significantly from those of their single-component constituents. It is these differences in physico-chemical properties that are believed to be the primary reason for differences in * sorption behavior between multi-and single-component solids.…”

Low-temperature thermal conversion of Al-substituted goethite in gibbsitic bauxite for maximum alumina extraction

“…20,21 A general agreement exists on the fact that the temperature of thermal conversion increases with the Al substitution in Al-goethite. 22 With the Al substitution for Fe of up to 14 mol%, the conversion temperature range of Al-goethite shied to 247-320 C. 23 In addition, the difficulty of converting Al-goethite increases with the crystallinity of Al-goethite, and the conversion temperature can increase from 260 C to 320 C. [24][25][26] However, the mechanism of thermal conversion of Al-goethite has two points. Wolska used X-ray diffraction (XRD) and infrared spectroscopy (IR) to observe that intermediate structures referred to as "proto-hematite" and/or "hydrohematite" were formed by the dehydration of goethite in the temperature range of 180-250 C and completely transformed into hematite at 800-1050 C. 27 Other researchers insist that the process is a direct conversion from goethite to hematite and does not form any intermediate.…”

“… 22 With the Al substitution for Fe of up to 14 mol%, the conversion temperature range of Al-goethite shifted to 247–320 °C. 23 In addition, the difficulty of converting Al-goethite increases with the crystallinity of Al-goethite, and the conversion temperature can increase from 260 °C to 320 °C. 24–26 However, the mechanism of thermal conversion of Al-goethite has two points.…”