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Synthesis of zeolite (ZSM-5 and Faujasite) and geopolymer from South African coal fly ash
Author(s)
Ndlovu, Nkululeko Zenzele Neville
Date Issued
2016
Type
Thesis
Publisher
Cape Peninsula University of Technology
Abstract
Population growth in South Africa has led to a direct increase in electricity demand. Due to the
abundance of coal in the country, most of the energy requirement is met through coal combustion.
Although there is a vast coal resource, the natural high grade coal is mainly exported, while the
low grade coal is exploited for electricity generation. The combustion of low grade coal during
electricity production results in huge quantities of coal fly ash (CFA) that require careful
disposal, due to its toxicity. Poor management of this waste constitutes serious human and
environmental problems, such as respiratory diseases, contamination of soil, surface water and
groundwater. This is in part due to the fact that only a small percentage of fly ash is utilised
efficiently in the construction industry. Several studies have recently been conducted into the use
of CFA as a starting material for the synthesis of zeolites and geopolymers, due to its high
silicon and aluminium content. However, the synthesis of zeolites from CFA has been subject to
criticism, because the synthesis of zeolites from the bulk CFA results in zeolite products that are
mixed with non-reacted fly ash and toxic elements. On the other hand, pure phase zeolites can only
be synthesised from CFA extracts, which results in a small yield of the zeolite products and a huge
amount of solid waste. Therefore, this does not facilitate either the use of fly ash-based zeolites
as catalysts in advanced chemical processes or scaling up of the synthesis process. This study seeks to make optimal use of CFA by developing a method for optimal extraction of Si and Al for the synthesis of ZSM-5 and faujasite zeolites, and use the resulting solid waste for the synthesis of geopolymers such that the resultant waste is minimised or completely eliminated. Two distinct processes are employed in this study to synthesise ZSM-5 or faujasite zeolite from CFA extracts, while the solid residue is transformed into a geopolymer. In the first process, an alkaline leaching method is employed for extraction of Si from CFA using 8 M NaOH at 150 0C for 24 h. It was found that the Si extract contained a certain amount of Al, enough for the synthesis of a high silica zeolite such as ZSM-5. However, the Si extract had to be treated with oxalic acid in order to remove the excess Na in the extract, since this could prevent the formation of ZSM-5. The obtained Si extract was then used as a feedstock for the synthesis of Zeolite ZSM-5 with NaOH and tetraethyl ammonium hydroxide (TEAOH) as mineralising and structure directing agents respectively. The obtained gel underwent hydrothermal synthesis at 160 °C for 72 h, while the solid residue obtained after Si extraction was used in the synthesis of geopolymer at 70 °C for 5 days. The obtained ZSM-5 and geopolymer products were characterised using ICP, XRD, SEM, FTIR and NMR techniques.
abundance of coal in the country, most of the energy requirement is met through coal combustion.
Although there is a vast coal resource, the natural high grade coal is mainly exported, while the
low grade coal is exploited for electricity generation. The combustion of low grade coal during
electricity production results in huge quantities of coal fly ash (CFA) that require careful
disposal, due to its toxicity. Poor management of this waste constitutes serious human and
environmental problems, such as respiratory diseases, contamination of soil, surface water and
groundwater. This is in part due to the fact that only a small percentage of fly ash is utilised
efficiently in the construction industry. Several studies have recently been conducted into the use
of CFA as a starting material for the synthesis of zeolites and geopolymers, due to its high
silicon and aluminium content. However, the synthesis of zeolites from CFA has been subject to
criticism, because the synthesis of zeolites from the bulk CFA results in zeolite products that are
mixed with non-reacted fly ash and toxic elements. On the other hand, pure phase zeolites can only
be synthesised from CFA extracts, which results in a small yield of the zeolite products and a huge
amount of solid waste. Therefore, this does not facilitate either the use of fly ash-based zeolites
as catalysts in advanced chemical processes or scaling up of the synthesis process. This study seeks to make optimal use of CFA by developing a method for optimal extraction of Si and Al for the synthesis of ZSM-5 and faujasite zeolites, and use the resulting solid waste for the synthesis of geopolymers such that the resultant waste is minimised or completely eliminated. Two distinct processes are employed in this study to synthesise ZSM-5 or faujasite zeolite from CFA extracts, while the solid residue is transformed into a geopolymer. In the first process, an alkaline leaching method is employed for extraction of Si from CFA using 8 M NaOH at 150 0C for 24 h. It was found that the Si extract contained a certain amount of Al, enough for the synthesis of a high silica zeolite such as ZSM-5. However, the Si extract had to be treated with oxalic acid in order to remove the excess Na in the extract, since this could prevent the formation of ZSM-5. The obtained Si extract was then used as a feedstock for the synthesis of Zeolite ZSM-5 with NaOH and tetraethyl ammonium hydroxide (TEAOH) as mineralising and structure directing agents respectively. The obtained gel underwent hydrothermal synthesis at 160 °C for 72 h, while the solid residue obtained after Si extraction was used in the synthesis of geopolymer at 70 °C for 5 days. The obtained ZSM-5 and geopolymer products were characterised using ICP, XRD, SEM, FTIR and NMR techniques.
Additional information
Thesis (MTech (Chemical Engineering))--Cape Peninsula University of Technology, 2016.
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