Study of extracting alumina from high-alumina PC fly ash by a hydro-chemical process

•A high alumina extraction efficiency of 96.03% has been achieved.•Effects of temperature and reaction time on alumina extraction has been studied.•A two-stage theory was proposed as the reaction mechanism for this process.•The kinetics of the decomposition of Na8Al6Si6O24(OH)2(H2O)2 has been studied.•Engineering suggestions on reactors design and prevention of scaling have been offered.

Alumina extraction from high-alumina pulverized coal (PC) fly ash by a mild hydro-chemical process characterized by a high alkaline sodium aluminate solution was studied. A high alumina extraction efficiency of 96.03% has been achieved. The effects of temperature and reaction time on alumina extraction ratio, the phase transformation and the morphology change of the extraction residues have been investigated. The alumina extracting process involves two steps: transformation from mullite and corundum to an intermediate product aluminasilicate, Na8Al6Si6O24(OH)2(H2O)2, and then from the aluminasilicate to calcium sodium hydrate silicate (NaCaHSiO4). The reaction temperature has been found to be critical for the alumina extraction and especially for the decomposition of the intermediate product Na8Al6Si6O24(OH)2(H2O)2 at high temperatures (> 250 °C). With the aid of a specially designed rapid real-time sampling device, the kinetics of the decomposition of Na8Al6Si6O24(OH)2(H2O)2 at high temperatures has also been studied and fitted by the Avrami–Erofeev equation. Furthermore, some engineering suggestions about the reactors design and prevention of pipe scaling have been proposed based on the kinetic results.

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