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Precipitation of hematite and recovery of hydrochloric acid from aqueous iron(II, III) chloride solutions by hydrothermal processing

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McGill University
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Keywords
  • Engineering - Metallurgy

Abstract

Regeneration of HCl from process solutions is an important unit operation in chloride hydrometallurgy. Currently, it is accomplished by the highly capital and energy intensive technology of pyrohydrolysis. The research described in this thesis, has focused on an alternative HCl regeneration system from iron(II, III) chloride solutions that has been coined "Hydrolytic Distillation". This system relates to the earlier developed (in the 1970s) PORI Process. Basically this system involves two steps: (1) the oxidation of ferrous chloride solution by oxygen sparging under reflux conditions (atmospheric pressure) at 150ºC; and (2) the hydrolytic decomposition under atmospheric pressure of ferric chloride solution by continuous controlled water addition at 180ºC. As a result of the latter step iron is recovered as hematite and chloride units as superazeotropic (8-9 M) hydrochloric acid in the vapor phase condensate. The study on oxidation determined that 2/3 of FeCl2 is converted to FeCl3 and 1/3 to Fe2O3 at a rate that is controlled by oxygen mass transfer. The hematite product from the oxidation step proved to be crystalline but nanostructured consisting of 1-2 μm porous aggregated particles possessing 12-28 m2/g specific surface area and to be essentially pure α-Fe2O3 (70% Fe, <0.1% Cl-). With reference to the hydrolytic precipitation/distillation step it was determined that at 180ºC, the temperature at which superazeotropic HCl (8-9 M) was consistently produced, the composition of the liquid phase is "FeCl32H2O". Controlled continuous addition of H2O (or feed solution) was found to be critical in maintaining pseudo-steady state and driving the reaction to ~95% conversion efficiency. The hematite consisted of coarse spherical compact aggregated particles (~40 μm) that exhibited excellent filtration and washing properties. Its %Fe content was 68.5% and it contained ~4% H2O and ~3% Cl- that could be re

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