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Green Seed-Assisted Fly Ash Zeolitization At Room Temperature

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It was successfully synthesized zeolite X from fly ash produced by combustion of lignite coal in “Maritsa Iztok 2” TPP using a two-stage process – fusion with sodium hydroxide followed by hydrothermal treatment at room temperature. By decreasing the amount of NaOH, crystallization of zeolite NaX begins later. In order to optimize synthesis process seed-assisted procedure was introduced. Preliminary synthesized zeolite X from pure chemicals was used as seed. Addition of seed results not only in directing the synthesis to the desired zeolite structure (obtaining a monophasic zeolite X product) and in reducing synthesis time, but also in reducing the amount of sodium hydroxide upon melting. In this way, the process of zeolitization is economically more cost-effective and more eco-efficient.

INTRODUCTION

Coal-fired thermal power plants are a major source of fly ash (FA). Depending on the type of source and composition of the combustion coal, components of solid by-products may vary significantly, but all types of FA include large amounts of silica (amorphous and crystalline) and alumina, both of which are part of many coal bedrocks. World production of FA is over 750 million tons per year. The areas where the production waste from the TPPs is deposited are huge terrains. FA represents an environmental hazard for soils, waters and air due to their dusting during the drying and release of the harmful substances they contain. Discharges of the ash create environmental hazards due to increased acid content and infiltration of heavy metals and radioactive elements in the soil. Toxic components in FA depend on the specific structure of the deposit, but they could include a variety of the following elements found in negligible quantities: arsenic, beryllium, boron, cadmium, chromium, cobalt, lead, mercury, selenium, thallium, vanadium. Many approaches to the use of FA have been developed, given their composition and degree of crystallinity.

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Part of deposited ash is used in the construction of roads and buildings, in the form of gypsum, gypsum board, cement and concrete, in the ceramic industry and other building mixes. Another solution for utilizing of FA can be represented by the synthesis of zeolites. The production of zeolite from FA results in a good with higher added value compared to the use of ash as a cement additive. In recent years, many studies have also been done on the synthesis of zeolites from other alternative sources of SiO2 and Al2O3 due to the fact that the production of synthetic zeolites from pure sources of SiO2 and Al2O3 is quite expensive. For alternative sources of Si and Al, industrial waste or geothermal materials rich in Si and Al may be used, as well as FA, which is largely composed of Si and Al units. So naturally one of the approaches to using FA is their zeolitization. Zeolites synthesized from fly ash have many applications, including ion exchangers, molecular sieves and adsorbents, similar to the zeolites obtained by conventional methods.

Generally, zeolites represent a group of natural or synthetic crystalline microporous alumosilicates having a pore size of 0.3 to 2.0 nm. Zeolite pores form a system of channels and cavities with well-defined shape and dimensions. These microporous materials are used as adsorbents, for ion exchange processes, in the catalysis. Thus, the synthesis of coal ash zeolites and their use as adsorbents, catalysts, and ion exchangers is considered to be one of the most effective applications of fly ash from an environmental point of view.

FA is fine particles that are trapped by the thermal power plant’s filters. They are collected by electrostatic or mechanical precipitation. The ash mainly contains amorphous and crystalline SiO2 as well as hematite, mullite and magnetite. It has been found that different zeolite structures can be synthesized from the same ash but under different crystallization conditions, with the zeolites A, NaX and NaY being of utmost importance. One of them, namely zeolite NaX, which is an analogous to the natural faujasite, has a great ecological application. This zeolite is characterized by a highly developed specific surface area and numerous pores ranging in size from 5.0 Å to 7.5 Å, allowing for the adsorption of 3.2 Å carbon dioxide molecules.

Primarily, zeolite NaX is used as a catalyst in biodiesel production and CO2 adsorption. The purpose of the present work is the synthesis of this type of zeolite structure from fly ash obtained after the combustion of lignite coal at Maritsa – Iztok 2 TPP by using of two stages process – fusion with sodium hydroxide followed by hydrothermal synthesis at room temperature. The first step, fusion of the alkaline–ash mixture leads to the formation of soluble sodium aluminate and sodium silicate, enhancing the zeolite formation and facilitating the complete utilization of the waste as a raw material. The hydrothermal activation is the second stage which usually is held at a relatively high temperature (90-120 °C). The goal of the present study is to perform this stage at room temperature. The synthesis time at room temperature is between two weeks and one month, whereas the crystallization at 90 °C takes several hours (4-8 h). In order to reduce crystallization time, seed-induced synthesis was applied. Preliminary synthesized zeolite X from pure chemicals was used as seed.

EXPERIMENTAL

Sample preparation Ash residues, containing 52.66 mass % of SiO2 and 23.37 mass % of Al2O3, were subjected to a dual stage fusion-hydrothermal transformation. Initially, the coal ash was homogenized well with sodium hydroxide, and then the resulting mixture was poured into a nickel pot. Melting is carried out in a furnace at a temperature of 550 °C for a period of 1 hour. To the well-sharpened sample is added the distilled water and the seed, and then the resulting mixture is poured into a polypropylene vessel. The crystallisation of zeolite NaX is carried out at room temperature with crystallization time varying from 15 days to two months depending on the NaOH / fly ash ratio and the amount of crystal seed added to the resulting mixture.

The separated sample is filtered and allowed to dry at room temperature. In order to reduce the crystallization time, an increase the yield, 1, 2 and 5 wt % crystalline seed of zeolite NaX were used. The weight % is calculated regarding silica and alumina in the fly ash. The obtained solid substance was carefully washed with distilled water and then dried at 105 °C for an hour. Seed was synthesized by using of pure chemicals from initial gel with molar composition NaAlO2: 4SiO2: 16NaOH:325H2O at 90 °C for 8 h. Samples characterization The products morphology was observed by scanning electron microscopy (SEM) on a JEOL JSM6390 microscope, coupled with an energydispersive X-ray (EDX) analyser (Oxford Instruments). The phase identification was performed by X-ray diffraction (XRD) technique on a Brucker D2 Phaser diffractometer with CuKα-radiation and a Ni filter.

RESULTS AND DISCUSSIONS

The hydrothermal synthesis of zeolite NaX from fly ash is carried out at room temperature. In these experiments, the duration of the synthesis varies between 15 and 60 days. Data on the crystallization time of zeolite X obtained from fly ash without seed and with 1, 2 and 5% seed, respectively at different NaOH/FA ratio. The NaOH / FA ratios used are: 1.2/1; 1.0/1; 0.8/1 and 0.6/1. At higher amounts of NaOH, the crystallization of zeolite NaX begins much earlier. From the X-ray powder patterns it is seen that the best results are obtained when the amount of used sodium hydroxide is higher – respectively when the ratio NaOH/FA is 1.2/1 and 1.0/1.

The result is expected, because with more sodium hydroxide, a larger amount of the silica and alumina phases present in the fly ash pass into soluble silicates and, respectively, aluminates. This makes these phases more mobile and more inclined to form zeolite nuclei around which crystal growth takes place to build zeolite structure. By using of lower amount of sodium hydroxide experiments for synthesis of zeolite NaX failed. From the X-ray powder patterns of the resulting products synthesized at the ratio of NaOH/FA = 1/1, it was observed that crystalline phase began to be produced around the 29th day after crystallization at room temperature and a high crystalline product of this zeolite is only obtained by the 45th day of the synthesis. When the NaOH/FA ratio used in the studies is 1.2/1, the crystallization of zeolite NaX begins about 25 days after the beginning of the synthesis.

In order to reduce the crystallization time and to direct the synthesis to the desired zeolite structure, seed-assisted synthesis of zeolite NaX was applied with using of 1, 2 or 5 weight percent crystalline zeolite NaX. The weight % is calculated regarding total amount of silica and alumina in the fly ash. At ratio NaOH/FA = 1.2/1, the crystallization of zeolite NaX begins earlier when 1, 2 and 5% of seed are used. For comparison, in studies of zeolite NaX synthesis carried out in the absence of seed and with the same NaOH/FA ratio, crystallization begins later. By using 1, 2 or 5% by weight of crystalline seed crystals, zeolite NaX crystallization begins 21 days after the beginning of the synthesis at room temperature. The crystallization time of zeolite NaX synthesized from fly ash in the presence of 1, 2 or 5% seed at room temperature depending from used ratios of NaOH/FA NaOH/FA No seeds 1% crystal seed 2% crystal seed 5% crystal seed 1.2/1 25 days 21 days 21 days 21 days 1 29 days 29 days 29 days 21 days 0.8/1 – 37 days 37 days 24 days 0.6/1 – – 43 days 37 days.

At ratio NaOH/FA = 1.2/1 the synthesis of zeolite X is the most sensitive to the presence of even small amounts of seed. Upon addition of 1 % seed, the crystallization time decreases to 21 days, whereas without seed, crystallization begins after 25th day. Using 5 wt % seed leads to a product with a higher degree of crystallinity.

When the NaOH/FA ratio is 1/1 and the crystallization takes place in the presence of 1 or 2 % seed, zeolite NaX begins to crystallize before the 29th day. By using a higher amount of seed (5 %), crystallization of zeolite NaX begins earlier than when 1 or 2 % crystal seed are used. In the X-ray powder patterns of the synthesized products, it is observed that zeolite NaX begins to crystallize before the 21st day of the synthesis. From these results, it can be concluded that the use of a larger amount of crystal seed favors a significant decrease in crystallization time.

In order to investigate the morphology and size of the crystal particles of zeolite NaX the scanning electron microscopy (SEM) was applied. The SEM image of zeolite NaX synthesized from fly ash and sodium hydroxide in a ratio of 1/1 and in the presence of 5% crystalline seed is presented. From the micrographs, it can be seen that aggregates are formed. The size of the zeolite NaX aggregates varies between 1 and 10 μm, and some aggregates have size in the submicron range (500-1000 nm). SEM micrograph of zeolite NaX synthesized from fly ash at room temperature in the presence of 5% of seeds, ratio of NaOH/FA = 1/1 and crystallization time of 39 days. Bar length 10 μm; magnification: 5 000.

When the duration of crystallization time is longer – up to 70 days a product with higher crystallinity is formed. X-ray powder pattern of a product synthesized over a period of 70 days at NaOH/FA = 1/1 ratio in the presence of 2% of seeds is presented. It is observed that the product obtained has a higher crystallinity compared with those synthesized for shorter period. X-ray powder patterns of zeolite NaX synthesized from fly ash at room temperature in the presence of 2% of seeds and ratio of NaOH/FA=1/1.

When using less sodium hydroxide at NaOH/FA = 0.8/1) and without seed, zeolite NaX synthesis fail despite the long crystallisation period of 50 days. By adding of 1 or 2% crystalline seed to the reaction mixture, zeolite NaX begins to crystallize 37 days after the beginning of the synthesis. With the use of a larger amount of crystalline seed (5%), zeolite NaX began to crystallize significantly earlier – the 24th day. Despite the use of a larger amount of seed and the crystallization duration of nearly 50 days, completely crystallized zeolite NaX phase does not observed. The X-ray powder patterns of products which are synthesized in the presence of 5% seed are presented.

At a NaOH/FA = 0.6/1 ratio, the studies for the synthesis of zeolite NaX without seed or in the presence of 1% seed were unsuccessful. In these studies, crystallization of zeolite NaX was not observed despite the long synthesis period of 50 days. The X-ray powder patterns of the resulting products synthesized in the presence of 5 % crystalline seed. When synthesis occurs in the presence of 2% of seed, only 43 days after the onset of synthesis, zeolite NaX begins to crystallize. From the X-ray powder patterns of the products synthesized in the presence of a higher amount of crystalline seed (5%) it was observed that the crystallisa-tion of zeolite NaX started relatively earlier – at 37 days.

From the results obtained, it can be concluded that a larger amount of seed contributes to the earlier formation of a zeolite phase. X-ray powder patterns of zeolite NaX synthesized from fly ash at room temperature in the presence of 5% of seeds and ratio of NaOH/FA=0.8/1. X-ray powder patterns of zeolite NaX synthesized from fly ash at room temperature in the presence of 5% of seeds and ratio of NaOH/FA=0.6/1.

It is evident that the lowest NaOH/FA ratio, in which zeolite NaX is successfully synthesized at room temperature and without the presence of seeds, is 1/1. Further reduction of the sodium base in this ratio does not result in a crystalline phase of zeolite NaX. It has been found that by the addition of crystalline seed to the reaction mixture it is possible to use less NaOH in the zeolite NaX synthesis at room temperature.

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