An investigation into the removal of aluminosilicates scaling species by activated alumina

Master Thesis


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University of Cape Town

Gas condensates from the coal conversion plants contains trace amounts of inorganic species such as Si, Ca and Al ions, which cause scaling in downstream processes. Silica has been identified as the main constituent of the scale materials in geothermal plants. In order to prevent scaling, silica ions need to be removed or reduced. Alumina has been shown to successfully remove both silica and calcium from waste water streams. However, it also causes an increase in the aluminum concentration through dissolution. The mechanism of the silica and calcium uptake by alumina is not fully understood. In this study, the mechanism of silica uptake by alumina was investigated through an extensive literature review and experimental work on the alumina and silica chemistry when in solution. The chemistry of the alumina in suspension can be used to explain its reactions with other species (both inorganic and organic) in solutions. Activated alumina chemistry in suspension under alkaline conditions was investigated. The results showed that small amounts of alumina particles can undergo transformation into its hydrated phases and consequently aluminate (Al(OH)4 -) species are leached out from the pellets and dissolve in solution with subsequent precipitation when in solution. The inorganic species uptake can be attributed to the species interacting with the Al in solution and the hydrated phases of alumina. The results on the inorganic species uptake by alumina showed that a break through point is never attained. This indicates that the inorganic species removal by alumina cannot be attributed exclusively to an adsorption process. Hence, the mechanism of species removal was suggested to be a combination of adsorption and surface precipitation/reaction. Since alumina is costly, its application in wastewater treatment is dependent on its ability to be regenerated. As a result, the second objective of this study was to investigate the regeneration of the alumina by unloading the silica from the loaded alumina using various reagents and subsequently testing the effectiveness of the alumina with a second loading. The reagents used to unload the loaded alumina were sulphuric acid, sodium hydroxide and sodium gluconate at varying concentrations. The three reagents showed an increase in Si unloading with an increase in reagent concentrations. Sulphuric acid showed an unloading capacity of up to about 50% and 70% for batch and continuous unloading respectively. On the other hand, sodium hydroxide showed Si unloading of up to about 50% and 40% for the batch and continuous unloading of loaded alumina respectively under the investigated concentrations. The unloading of Si from saturated alumina using sodium gluconate was only conducted batch-wise as it only achieved a 6% unloading for the concentrations investigated. However, even though the scaling species were eluted from the alumina bed this did not improve/restore the loading capacity of alumina but rather kept the performance of the alumina at the same level that it was before the regeneration process. Also during the unloading of silica from alumina, excessive alumina dissolution was observed when using 0.25M and 0.65M NaOH.

Includes abstract.

Includes bibliographical references (leaves 69-74).