The reaction mechanism and operating window for the decomposition of hydrated magnesium sulfate under reducing conditions

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School of Chemical Technology | Doctoral thesis (monograph) | Defence date: 2015-10-16
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Aalto University publication series DOCTORAL DISSERTATIONS, 120/2015
Magnesium compounds commonly occur in ores that are treated hydrometallurgically for the recovery of valuable metals. The metals of interest can be recovered by pH control, using a neutralizing agent. Magnesium sulfate remains in the waste leach solution when sulfuric acid is used as the leaching medium. The magnesium sulfate can be crystallized from the solution and decomposed into magnesium oxide and sulfur dioxide. The magnesium oxide can be utilized as a neutralizing agent, thus avoiding the need for additional materials such as lime, in the process, whereas the sulfur dioxide can be used for the production of sulfuric acid, which is then used for leaching of the ore.  The decomposition of magnesium sulfate is a strongly endothermic reaction, but the decomposition temperature can be lowered by using a reducing agent. The effect of different reductants on the decomposition mechanism of magnesium sulfate and on the operating window for decomposition of magnesium sulfate was tested experimentally. Sulfur was used as the main reductant in the experimental work, as it does not produce any carbon-containing off gases; carbon and carbon monoxide were used for comparison. The main equipment for the experimental work was a fluidized bed reactor, as the heat and mass transfer rates are high in this type of reactor. The intermediate samples and products were analyzed using XRD analysis, total sulfur analysis and extensive SEM/EDS analysis of both powder samples as well as polished sections.  Several factors affecting the reaction kinetics were tested and evaluated, including temperature, the type of magnesium sulfate raw material, amount of reductant, particle size and the presence of minor amounts of manganese sulfate, iron sulfate, and calcium sulfate. It was found that sulfur as the reductant not only strongly increases the reaction rate, but that it also forms a protective magnesium oxide layer around the magnesium sulfate particles, which prevents fluidization problems. A clear reaction front was observed for the majority of the particles with SEM/EDS analysis. Kinetic model fitting showed that the rate-controlling step with magnesium sulfate monohydrate and sulfur as the reductant was chemical reaction control at the unreacted interface, and the control shifted to ash layer diffusion control when dried magnesium sulfate heptahydrate was used instead. Gas film diffusion was found to be the rate-controlling step with carbon and carbon monoxide as the reductant, due to carbon deposition on the surface of the particles.
Supervising professor
Taskinen, Pekka, Prof., Aalto University, Department of Materials Science and Engineering, Finland
Thesis advisor
Metsärinta, Maija-Leena, Dr.
magnesium sulfate, decomposition, fluidized bed, reductant, kinetics
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