ALKALINE HYDROTHERMAL ZEOLITES SYNTHESIZED FROM FLY ASH SIO 2 AND AL 2 O 3

Transcription

1 ALKALINE HYDROTHERMAL ZEOLITES SYNTHESIZED FROM FLY ASH SIO 2 AND AL 2 O 3 V.S. Somerset 1, L.F. Petrik 1, R.A. White 1, M.J. Klink 1, D. Key 2 and E. Iwuoha 2 1 Inorganic Porous Media Group. 2 Department of Chemistry, University of the Western Cape, Private Bag X17, Bellville, Tel: Fax: vsomerset@uwc.ac.za ABSTRACT A co-disposal reaction was used wherein fly ash (FA) was reacted with acid mine drainage (AMD), to collect filtrates for zeolite synthesis. The Si and Al contents of the fly ash (FA) filtrates were used as precursor species for the alkaline hydrothermal conversion of the fly ash filtrates into zeolites. These filtrates were then analysed by XRF spectrometry for quantitative determination of SiO 2 and Al 2 O 3. The [SiO 2 ]/[Al 2 O 3 ] ratio obtained in the filtrates range from 1.4 to 2.5. The [SiO 2 ]/[Al 2 O 3 ] ratio was used to predict whether the fly ash filtrates could successfully be converted into faujasite zeolitic material by the synthetic method of Rayalu et al. (1). If the [SiO 2 ]/[Al 2 O 3 ] ratio is higher than 1.5 in the co-disposal filtrates, it favours the formation of faujasite. The zeolite synthesis included an alkaline fusion of the co-disposal filtrates, followed by aging for eight hours and hydrothermal conversion by crystallisation at 100 ºC. Different variables were investigated during the synthesis of zeolite to ascertain their influence on the end product. These variables include the amount of water in the starting material, composition of fly ash related starting material and the FA:NaOH ratio used for fusing the starting material (2,3). INTRODUCTION The ever-increasing demand for electricity has resulted in the burning of large quantities of coal to produce only a relatively small portion of the electricity needed. With most of the coal-fired power stations in South Africa operating on the coal combustion principle, large quantities of fly ash (FA) are produced annually. Despite the many environmental issues associated with coal combustion, it will remain a major source of electrical power generation for many years to come. It has therefore become necessary to look at methods that can be used to produce or manufacture value-added products from fly ash. Another persistent environmental problem also encountered in South Africa is acid mine drainage (AMD), as it is an unavoidable by-product of the mining and mineral industry. AMD forms when pyrite and other sulphide minerals associated with coal seams are exposed to water and oxygen. AMD are waters with typical high concentrations of dissolved heavy metals and sulphate, and can have a ph as low as 2 (4-6). When AMD is discharged untreated, it can pollute receiving streams and aquifers and the resulting overall effect on streams and waterways can be very dramatic. Some of the harmful effects of AMD on the environment include the disappearance of all aquatic life, river bottoms become coated with layers of rust-like particles, and the ph of the water and streams decreases. It is therefore essential that methods are in place to treat mine drainage in order to limit its negative impart on the environment (4). South African FA contains relatively high concentrations of SiO 2, Al 2 O 3 and CaO, with CaO considered as a liming agent to neutralise AMD (7). The possibility was explored to employ a co-disposal reaction, firstly to neutralise AMD by co-disposing it with FA, and secondly to collect the filtrates of the reaction for zeolite synthesis. Proceedings of the 2004 Water Institute of Southern Africa (WISA) Biennial Conference 2 6 May 2004 ISBN: Cape Town, South Africa Produced by: Document Transformation Technologies Organised by Event Dynamics

2 Several studies on zeolite synthesis (8-10) have shown that zeolites can be obtained by hydrothermal treatment of fly ash. Zeolites have important industrial applications such as in catalysis, sorbents for removal of ions and molecules from wastewaters, radioactive wastes and gases, and as replacements for phosphates in detergents (8,11). In this study a co-disposal reaction between FA and AMD was explored to determine whether the two harmful effluents could be co-disposed to form filtrates at a near neutral ph. The sources of fly ash for this study were from Eskom s Arnot and Matla coal-fired power stations. AMD were taken from Navigation colliery and the Brugspruit AMD treatment plant. XRF analysis of FA and the co-disposal filtrates was done to evaluate the appropriateness of the FA compounds for alkaline hydrothermal conversion into zeolites. If the [SiO 2 ]/[Al 2 O 3 ] ratio is higher than 1.5 in the FA or co-disposal filtrates, it favours the formation of faujasite. EXPERIMENTAL Fly Ash (FA) Filtrate Sample Preparation The FA filtrates were collected by using a co-disposal reaction wherein fly ash is reacted with acid mine drainage in a specific FA:AMD ratio (e.g. 1:3.5, 1:4, 1:5). The readings for ph and EC of the co-disposal reaction mixture were taken at regular time intervals until a near neutral ph of 7 was obtained. Solids and liquids were separated using filter paper. The co-disposal filtrates were dried in the oven at a temperature of 70 ºC and then transferred into a plastic container. The co-disposal filtrate samples were milled and ground with an agar mortar and pestle, to ensure that a powder of even particle size was obtained (2). XRF Analysis of Sample Material The chemical composition, calculated as major oxides, of the FA and co-disposal filtrate samples was obtained by XRF spectrometry. A Phillips 1404 XRF Wavelength Dispersive Spectrometer employing an array of 6 analysing crystals and an Rh X-ray tube target was used. A vacuum was used as the medium of analyses to avoid interaction of X-rays with air particles (2). Zeolite Synthesis In the zeolite synthesis method the FA filtrates was fused with sodium hydroxide (NaOH) in a 1:1.2 ratio at 600 ºC for about 1 2 hours. The fused product was then mixed thoroughly with distilled water and the slurry was subjected to aging for 8 hours. After aging the slurry was subjected to crystallisation at 100 ºC for 24 hours. The solid product was recovered by filtration and washed thoroughly with deionised water until the filtrate had a ph of 10 to 11. The product was then dried at a temperature of 70 ºC (1). The synthesized zeolitic material was then prepared for mineralogical, chemical and physical characterisation. X-Ray Diffraction (XRD) Analysis The zeolite sample mineralogy was evaluated by conducting XRD spectrometry (Philips Analytical graphite monochromator and Cu Kα radiation samples were scanned for 2θ ranging from 7 to 70 º). The data files presented by X Pert Graphics & Identify data collection software were used to identify the minerals present in the samples (2). Scanning Electron Microscopy (SEM) Analysis A Hitachi X-650 Scanning Electron Microanalyzer was used to take the micrographs of the samples. Samples were mounted on aluminium stubs using conductive glue and were then coated with a thin layer of carbon (2). Icp-Ms Analysis The presence of trace elements in the post-synthesis filtrate and acid digested zeolite samples was determined by ICP-MS using a Perkin Elmer Elan ICP-MS unit (2).

3 Fourier Transformed Infrared (FTIR) Spectrometry Approximately 15 mg of the zeolite sample, plus 1 g of KBr was weighed out, milled and ground in an agate mortar and pestle for 5 minutes, until a fine smooth powder of even particle size was obtained. A quarter (~ 0.25 g) of the zeolite and KBr mixture was then pressed with a steel die at 10 ton.cm -2 into a pellet (or wafer) IR spectra of the zeolite samples were recorded on a Perkin Elmer, Paragon 1000 PC, FT-IR Spectrometer, in the range between cm -1 (2). Cation Exchange Capacity (CEC) Approximately 5.0 g of the zeolite sample was weighed out and placed in a 100 ml polyethylene bottle. A 25 ml ammonium acetate solution was added and the mixture shaken for 1 hour. The supernatant was filtered directly into a 100 ml volumetric flask through filter paper, and care was taken not to pour any sample into the filter funnel. The extract was then made up to 100 ml with deionised water and the concentration of exchangeable cations (Ca 2+, Mg 2+, Na +, and K + ) determined by ICP-MS analysis (2). N 2 -Bet Surface Area Determination The specific surface area of the zeolite samples was determined by using a Micromeritics, ASAP 2010, Micropore Analyser. Nitrogen adsorption isotherms were obtained at liquid nitrogen temperature. Prior to the determination of the adsorption isotherm, the sample was outgassed (2). RESULTS AND DISCUSSION Neutralisation of AMD with FA In Figure 1 the data for the Navigation AMD neutralisation experiment using Arnot FA is shown. A major ph change occurs within the first 20 minutes of the experiment, raising the ph of the reaction mixture to between 6 and 7, and then there are some minor changes during the remainder of the experiment. The electrical conductivity (EC) also decreases sharply during the first 20 minutes of the reaction and reaches a minimum as the reaction continues. The results of the experiment provide good evidence that FA is a suitable neutralising agent in the treatment of AMD (2). ph & EC measurements for Arnot FA reacted with Navigation AMD with ratio FA:AMD = 1: ph ph EC EC (ms/cm) Reaction time (minutes) Figure 1. ph and EC measurements for Arnot FA reacted with Navigation AMD with ratio FA:AMD = 1:2. The neutralisation of AMD is commonly achieved by the addition of chemicals such as CaO, Ca(OH) 2, CaCO 3, NaOH and Na 2 CO 3. The use of FA for AMD neutralisation, involves a process whereby the ph of the AMD is raised from 2 3 up to a neutral ph of 7, or even above ph 10. FA ash contains significant quantities of total alkalinity in the form of CaO, MgO, K 2 O and Na 2 O, thereby increasing the neutralisation potential of FA. Calcium oxide (CaO) is formed upon FA by the oxidation of calcium during coal combustion in the burner of a coal-fired power station. FA may therefore be a substitute for limestone or lime treatment in the neutralisation of AMD. Furthermore, FA has a very high surface area and small particle size resulting in the FA having a relatively high

4 reactive surface area (2,12). XRF Analysis of FA and Co-Disposal Filtrate Samples XRF analysis of the co-disposal filtrates and other FA related starting material was done for quantitative determination of SiO 2 and Al 2 O 3. From this data the [SiO 2 ]/[Al 2 O 3 ] ratio was determined. The XRF results for the FA and co-disposal filtrate samples are given in Table 1. Table 1. XRF analysis of starting materials. Oxide Samples (Wt%) CDPptE FAPptE LFAPpt ALFAPpt SiO TiO Al 2 O Cr 2 O Fe 2 O 3 T MnO NiO MgO CaO Na 2 O K 2 O P 2 O H 2 O Total [SiO 2 ]/[Al 2 O 3 ] Sample CDPptE represents the co-disposal filtrate of the reaction between Arnot FA and Navigation AMD. Sample FAPptE represents the Arnot FA, with LFAPpt representing the water-leached FA, while sample ALFAPpt is the HCl acid leached FA material. From the results in Table 1 it can be seen that the [SiO 2 ]/[Al 2 O 3 ] ratio range from 1.95 to Thus samples CDPptE, FAPptE, LFAPpt and ALFAPpt can be converted to faujasite with the synthesis steps outlined earlier in this study (2,3). Zeolite Synthesis Results The effects of 3 different factors on the hydrothermal synthesis steps were investigated. These factors included the amount of water in the starting material, composition of fly ash related starting material and the FA:NaOH ratio used for fusing the starting material. In the following paragraphs the results obtained from the XRD analysis of the different starting materials are discussed. Effect of Different Amounts of Water With the addition of different amounts of water to 1 g of the co-disposal material, different zeolite products were synthesized. With the addition of 2, 3 or 6 ml of deionised water to 1g of sample material CDPptE, the following results were obtained. With the addition of 2 ml of water to sample CDPptE no zeolite phase was obtained. Adding 3 ml of water produced the phases faujasite, sodalite and zeolite A. With the addition of 6 ml of water only faujasite or a combination of faujasite and sodalite were present as zeolite phases. The results therefore indicate that the amount of water added effect the end zeolite phase obtained and samples with a [SiO 2 ]/[Al 2 O 3 ] ratio higher than 1.5, were successful in delivering faujasite as zeolitic material (2). Effect of Different Fly Ash Related Starting Material The use of different fly ash related starting materials were also investigated to assess the influence of fly ash quality on the zeolite phases produced. The starting material included fresh fly ash, water leached fly ash and HCl leached fly ash. Sample material ALFAPpt produced faujasite and sodalite as zeolite phases. The zeolite phase faujasite was present in sample FAPptE, while the zeolite phases zeolite A and sodalite were present in sample LFAPpt. Thus the FA produced faujasite as

5 zeolite phase, the water-leached FA produced zeolite A and sodalite, and the HCl leached FA produced sodalite or sodalite/faujasite mixtures as zeolite phases. The results therefore indicate that the samples with a [SiO 2 ]/[Al 2 O 3 ] ratio higher than 1.5, were successful in delivering faujasite as zeolitic material (2). Effect of Different FA:Naoh Ratios During Fusion The third variable investigated was the use of different FA:NaOH ratios during fusion of the starting material. In the synthesis steps outlined by Rayalu et al. (1), a FA:NaOH of 1:1.2 was used. This ratio was changed to 1:1.1 and 1:1.5 to investigate the effect it had on the zeolite phase produced. In using the co-disposal material CDPptE, with different FA:NaOH ratios, the following results were obtained. The ratio of 1:1.1 produced faujasite and sodalite as products, while the ratio of 1:1.5 produced only faujasite as zeolite product, compared to faujasite and sodalite as products for a ratio of 1:1.2. A change in the FA:NaOH ratios of the water-leached FA material, sample LFAPpt, had the following effect on the zeolite product formed. For both the ratios of 1:1.1 and 1:1.5 sodalite was produced as zeolitic material, compared to sodalite and zeolite A as products for a ratio of 1:1.2. For the acid (HCl) leached FA material, sample ALFAPpt, a FA:NaOH ratio of 1:1.1 produced no zeolite phase, with the ratios of 1:1.5 and 1:1.2 producing the same zeolite phases of faujasite and sodalite (2,3). Results of Morphological Observations Scanning Electron Microscopy (SEM) was used to determine the size distribution and morphology of the zeolite crystals. Figure 1(a) shows a SEM photomicrograph of the raw Arnot FA and Figure 1(b) shows the crystals of the faujasite phase obtained when the fly ash (FA), HCl acid leached FA and co-disposal precipitates are subjected to alkaline hydrothermal conversion into zeolites. The photomicrographs clearly show the transformation of FA into a zeolite phase, as the morphology of single crystals look well defined (2,3). Figure 1(a) Arnot fly ash. Figure 1(b) Fly ash faujasite. Surface Area Results of Zeolitic Material The BET surface area and the pore volume of the synthesized zeolitic materials were determined for a range of starting material used. The values of the surface areas for the zeolitic material prepared from the co-disposal precipitates range from m²/g. These samples had the phases faujasite and sodalite as zeolitic material. The pore volume for all the co-disposal precipitates was approximately the same and has an average value of 0.25 cm 3 /g. A very high surface area of 515 m²/g was obtained for the FA sample used as starting material in the zeolite synthesis. This sample contains faujasite as zeolitic material and also has the highest

6 pore volume of 0.45 cm 3 /g. The results of the surface area for the water and HCl acid leached fly ash used in the zeolite synthesis range from m²/g. These samples also had faujasite/sodalite and sodalite/zeolite A combinations as zeolite phases. The values for the pore volume range from cm 3 /g (2,3). Cation Exchange Capacity (CEC) The CEC was determined for zeolitic material synthesized from the co-disposal precipitates, FA, and HCl acid leached samples. The results are shown in Table 2. Table 2. CEC results of zeolitic material. Sample name Exchangeable cations in meq./100g Ca 2+ Mg 2+ Na + K + Σ CEC ALFAPpt FAPptE CDPptE In Table 2 sample ALFAPpt represents the HCl acid leached FA, sample FAPptE is the FA itself and sample CDPptE represents the co-disposal precipitate material. The first two samples contained a mixture of faujasite and sodalite as zeolite phases, with only faujasite present in the FA sample. The amount of exchangeable Ca 2+ cations is far less for sample CDPptE, compared to the other two samples, and the highest for sample ALFAPpt. Sample CDPptE has the lowest amount of exchangeable Mg 2+ cations, with sample FAPptE having the most, although the amount of Mg 2+ cations that are exchanged for each of the 3 samples is not very high. For sample FAPptE the amount of exchangeable K + is the lowest, while the highest value is obtained for the sample CDPptE. The amount of exchangeable Na + cations is higher for all the samples, when compared to the amount of exchangeable Ca 2+, Mg 2+ and K +. It also indicates that more Na + sites are available for cation exchange in the zeolite material, compared to the other cations. For samples ALFAPpt and CDPptE the amount of exchangeable Na + cations is approximately the same, while the highest amount of exchangeable Na + cations is recorded for sample FAPptE. These results therefore indicate, as with other zeolite materials, that mainly Na + is available for cation exchange with other more toxic ion and metal species (2,3). Ftir Spectrometry Results In the FTIR spectrum of the zeolite obtained from the co-disposal material, OH bands were observed as a single strong broad band occurring at approximately 3480 to 3500 cm -1. These strong broad bands were attributed to the presence of hydroxyls in the faujasite supercage and the hydroxyls in the sodalite cage. The faujasite supercage consists of sodalite cages as its building blocks and therefore its presence will be shown in the FTIR spectrum (2,13,14). The FTIR spectrum also showed strong medium bands at 1646 and 1649 cm -1 respectively. These bands were attributed to the H 2 O deformation mode normally seen at 1650 cm -1. The presence of the H 2 O mode shows that complete dehydration has not been achieved for the zeolite samples (2,15).

7 Composition of Post-Synthesis Zeolite Filtrates An ICP-MS study of the zeolite filtrates collected after zeolite synthesis was done, in order to gain a better understanding of the trace and heavy metal species contained in the filtrates, and also to determine which element species stays trapped in the zeolite sample and which are released. The results have indicated that the Al, Si and Na concentrations of the samples are relatively high. This can be expected for the Al and Si concentrations originating from the FA, as it provided the Al and Si building blocks for the zeolitic material. The concentration of Na is high due to the fusion step with NaOH during synthesis. The concentrations of elements such as Hg, Pb, Cu, Ni, Cd and Se are relatively low, while higher concentrations for B and As were observed. However, these concentrations should not prove difficult to reduce with standard water treatment technologies (2,3). In order to determine the fate of the heavy metals that may have been encapsulated in the zeolitic materials during synthesis, a study of the heavy metal content of the synthesized zeolites, by acid digestion of the solid zeolite materials was also performed. The ICP-MS results for the metal content of the acid digested zeolites compared to the post-synthesis filtrates, shows that the concentrations of metal species Al, Fe, Ni, Cu, Zn, Ba and Hg are lower in the post-synthesis filtrates. On the other hand the concentrations of metal species Na, As, Cr and Pb are higher in the post-synthesis filtrates than in the zeolitic material, while mixed levels of concentrations were observed for metal species Se, Cd, Mn, K and Ca (2,3). CONCLUSIONS The results of this study has shown that Arnot and Matla FA can be co-disposed with Navigation and Brugspruit AMD for the collection of co-disposal filtrates at near neutral ph. These co-disposal filtrates provide a rich source of SiO 2 and Al 2 O 3 as feedstock for zeolite synthesis. The starting material with a [SiO 2 ]/[Al 2 O 3 ] ratio higher than 1.5 can be successfully converted into faujasite zeolitic material, while for some sample material the zeolite phases sodalite and zeolite A can also be obtained. What makes the co-disposal process somewhat unique is the fact that FA (ph of in solution) can first be used to treat AMD (ph of 2 3), and then the filtrate residues can also be used for the synthesis of zeolite minerals. Pollution control of South African mining and coal powered utility waste streams has been shown to be possible by the application of a co-disposal reaction between the two waste products, i.e. acid mine drainage and fly ash. The co-disposal of coal mine derived AMD utilising South African fly ash is thus a suitable method for the low cost treatment of AMD, delivering wastewater at a near neutral ph (suitable for further treatment) and high quality zeolite adsorbents by post process synthesis of solid process filtrates. ACKNOWLEDGEMENTS The authors wish to express their gratitude to the Water Research Commission (WRC), Coaltech 2020 Consortium, and the National Research Foundation (NRF) for funding and financial support to perform this study. We would also like to express our gratitude to the CSIR and ESKOM, for their assistance in the collection of samples. Special thanks to the Chemistry Department of the University of the Western Cape for the assistance provided during this study. REFERENCES 1. S. Rayalu, S.U. Meshram, M.Z. Hasan, Journal of Hazardous Materials, B77 p.123 (2000). 2. V.S. Somerset, Masters Dissertation, University of the Western Cape (2003). 3. L.F. Petrik, R.A. White, M.J. Klink, C. Burgers, V.S. Somerset, Water Research Commission (WRC) Project. Report No K5/1242 (2003). 4. B. Gazea, K. Adam, A. Kontopoulos, Minerals Engineering, 9 (1) p.23 (1996). 5. D. Feng, C. Aldrich, H. Tan, Minerals Engineering, 13 (6) p.623 (2000).

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