Bigadiç klinoptilolit rezervinin NH+4 değişimi ve CO2 adsorpsiyonu yardımıyla karakterizasyonu

thumbnail.default.placeholder
Tarih
1993
Yazarlar
Sirkecioğlu, Ahmet
Süreli Yayın başlığı
Süreli Yayın ISSN
Cilt Başlığı
Yayınevi
Fen Bilimleri Enstitüsü
Institute of Science and Technology
Özet
Endüstride kullanımı son 30 yıl içinde hızla yaygınlaşan zeolitler, alkali ve toprak alkali metallerin sulu aluminosilikat kristalleridir. Düzgün dağılmış, tekdüze gözenekleri, gözeneklerdeki değişebilen katyonları ve yüzlerce metrekarelik iç hacimleriyle zeolitler, iyon değiştirici, adsorban ve katalizör olarak çeşitli kullanım alanları bulmuşlardır. Günümüzde, 40'a yakın doğal, çoğunun doğal analoğu bulunmayan 200 civarında da sentetik zeolitin varlığı bilinmektedir. Ticari olarak yaygın kullanım alanı bulmuş olanların çoğu sentetik türlerdir. Yeryüzünde ekonomik olarak değerlendirilebilecek potansiyelde birçok zeolit oluşumu bulunmasına karşın, doğal zeolitlerin fiziksel ve kimyasal özelliklerinin rezervden rezerve, ve hatta aynı rezervde yatay ve dikey kesitlerde farklılık göstermesinden dolayı endüstriyel kullanımları sınırlıdır. Doğal zeolitlerin endüstriyel kullanımları ancak ayrıntılı olarak karakterize edilmeleri ve bir performans testinin geliştirilmesi ile sağlanabilir. Bu çalışmada Bigadiç bölgesinde iki borat katmanı arasında, üstte ince, altta ise kaba taneli türlerden oluştuğu gözlenen zengin klinoptilolit rezervinin iyon değişimi ve CO2 adsorpsiyonu ile karakterizasyonu gerçekleştirilmiş, kapasiteleri artıracak ön işlemler araştırılmış ve rezervden alınacak herhangi bir örneğin hızla karakterizasyonunda kullanılabilecek bir performans testi geliştirilmiştir. Karakterizasyonda kimyasal analizler, XRD ve İR analizlerinden de yararlanılmıştır. İnce ve kaba taneli tüf içeren bölgelerden alınmış temsili örneklerin iyon değişim kapasitelerini artırmak amacıyla asit, baz ve tuz çözeltileri ile bir dizi önişlem test edilmiş, etkinliği saptananlar, değişen oranlarda klinoptilolit içeren adresli örneklere de uygulanmıştır. CO2 adsorpsiyonu, asit ve baz ile işlem görmüş ve Na, K, Ca ve H formları hazırlanmış temsili örnekler ile, yalnızca Na ve H formları hazırlanmış farklı oranlarda klinoptilolit içeren örnekler üzerinde gerçekleştirilmiştir. Adsorpsiyon deneylerinde elde edilen izotermlere Dubinin-Astakhov (D-A) ve Langmuir izoterm model denklemleri uyarlanmış ve bu denklemlerin karakteristik parametreleri hesaplanmıştır. Daha sonra, farklı oranlarda klinoptilolit içeren örneklerin iyon değişim ve adsorpsiyon kapasiteleri ile klinoptilolit yüzdeleri ve kapasitelerin kendi aralarındaki ilişkiler araştırılmıştır. Deneylerde, ince ve kaba temsili örneklerin 353 K'deki iyon değişim kapasitelerinin sırasıyla 1.83 ve 1.71 meq/g olduğu saptanmış, NaCl çözeltisinin tekrarlı ve NaCl+NIfyCl çözeltilerinin ardışık olarak tekrarlı kullanılması ile bu kapasitelerin, sırasıyla, 2. 1 1 ve 2.26 meq/g değerlerine yükseldiği gözlenmiştir. Farklı oranlarda klinoptilolit içeren örneklerin iyon değişim kapasitelerinin artırılması için uygulanan önişlemlerden ince ve kaba taneli örneklerin farklı şekilde etkilendiği, NaCl çözeltilerinin tekrarlı kullanıldığı yöntemin en etkin yöntem olduğu saptanmıştır. Çalışma boyunca en yüksek kapasite (2.36 meq/g) bu yolla elde edilmiştir. Adsorpsiyon deneylerinde, Na+K açısından zengin kaba taneli örneğin, Ca+Mg açısından zengin ince taneli örnekten daha fazla CO2 adsorpladığı, adsorpsiyon kapasitesinin K>Na>H>Ca katyon sırasıyla azaldığı saptanmıştır. Adsorpsiyon izotermlerini en iyi D-A modelinin açıkladığı gözlenmiştir. Kapasiteler ile klinoptilolit içeriği arasındaki ilişkilerin araştırılmasında, Na formundaki zeolitlerin adsorpsiyon kapasiteleri ile klinoptilolit yüzdesi arasındaki ilişkinin en güçlü ilişki olduğu, Na formunda bir örneğin 100 kPa denge başmandaki adsorpsiyon kapasitesinin saptanmasıyla klinoptilolit içeriğinin de güvenilir olarak öngörülebileceği belirlenmiştir. 
Although they were known since 1756, zeolites have been used in large quantities for hydrocarbon conversion, size/shape selective heterogeneous catalysis, gas separation and purification, as well as for ion exchange, only after the first synthetic preparation of these aluminosilicate materials by Barrer and Milton in 1940's. Most of the applications for zeolites are based on their chemical and physical properties. With their uniformly distributed micropores having uniform pore sizes, zeolites act as molecular sieves allowing certain hydrocarbon molecules to enter the crystals while rejecting others depending on their molecular size. The exchanging cations in the cavities, allow to perform ion exchange reactions, and give the ability to develop acidity that makes zeolites interesting materials for catalytic reactions. Chemically zeolites are alkali and alkaline earth metal hydroaluminosilicates. They are represented by the following empirical formula; M2/nO.Al203.ySi02.wH20 where y is equal to or greater than two, M is an alkali or alkaline earth cation such as Na, K, Ca, Mg or Ba, n is the cation balance and w is the number of the water molecules contained in the voids of zeolites. Structurally, zeolites are complex, three dimensional crystalline inorganic polymers that consist of four connected frameworks of AIO4 and SİO4 tetrahedra linked to each other by sharing the neighboring oxygen ions. Each tetrahedron, in the framework, having Al atoms at the center instead of Si, bears a net negative charge which is balanced by alkali and alkaline earth metal cations. The framework structure contains channels or interconnected voids that are occupied by the charge balancing cations and water molecules. The cations are mobile and can undergo ion exchange to varying degree in aqueous solutions with counter ions. The water molecules can be removed reversibly by applying heat, leaving a compact crystalline structure permeated by micropores and voids. Up to date, with the help of x-ray diffraction (XRD) analysis, almost 40 distinct species of natural zeolite occurrences were reported in basaltic and sedimentary rocks. Only nine of the known zeolite occurrences commonly make up the major part of zeolitic rocks. These are analcime, chabazite, erionite, ferrierite, heulandite, laumontite, mordenite, phillipsite and clinoptilolite. Beside natural zeolites, there are more than 200 synthetic zeolites that are synthesized and structurally identified. Some of them are counterparts to the natural zeolites, whereas others have no natural analog. In the preparation of zeolites, early investigators attempted to simulate the natural processes. The first synthetic zeolites were prepared by Union Carbide scientists based on the use of highly reactive aluminosilicate gels. As was mentioned before, zeolites have obtained widespread applications in industry, agriculture and environmental protection. Synthetic zeolites of ZSM-5, A, X, Y, L, Omega, Zeolon (Mordenite) and natural minerals such as mordenite, chabazite, erionite, phillipsite and clinoptilolite are the zeolites which are commercially utilized, primarily as adsorbents, catalysts and ion exchangers. Although several zeolite minerals such as clinoptilolite, mordenite and phillipsite are available in mineable deposits having relatively high purity, extensive application has not yet been achieved. This problem arises from the fact that zeolite ores contain a variety of minerals such as unreactive volcanic glass, quartz, feldspar, calcite and clay minerals. Moreover, zeolitic rocks vary in mineralogical composition and particle size from deposit to deposit. In addition, the zeolites themselves may vary chemically from deposit to deposit and even in the same deposit. Thus, before a particular use, a zeolite reserve must be thoroughly characterized with respect to its chemical composition, crystal structure, and other physical and chemical properties that will be useful in the proposed applications. Several techniques can be used for the characterization of the zeolites. These include XRD, IR, adsorption, ion exchange and Si and Al NMR. The estimation of the zeolite content of the rock is generally made by measuring physical properties of the rock that are directly related to the amount of the zeolite present. The best technique proposed for quantifying the zeolite content is XRD, since the intensities of x-ray reflections are directly proportional to the amount of the zeolite contained in a sample. Some other techniques proposed to estimate the zeolite content of the zeolitic rocks are O2-CO2 adsorption and cation exchange. For certain samples these techniques are highly reliable. However, the presence of other adsorbent or ion exchanging minerals in the sample sometimes limits the applicability of these methods. The measurement of the cation exchange capacity of a sample for a given ion is quite a difficult problem because no standard method has been established yet. Many ion exchange procedures presently employed are strongly affected by the composition of the zeolite itself. Zeolite bearing mineral assemblages in Neogene basins are widespread in Western Anatolia. The most important occurrence, with a large reserve, estimated to approach 2 billion tons and a high clinoptilolite content, is found in Bigadiç Neogene basin, which extends for 300 square kilometers in the region. The sequence contains vui upper borate formation, upper tuffs, lower borate formation, lower tuffs, limestones and basal volcanics from top to bottom. The upper tuffs which are rich in their zeolite content, form a layer of 250 meters in thickness. These tuffs are rhyolitic in composition and are observed to be mainly in the form of coarse grained glassy ash tuffs at the bottom and fine grained glassy dust tuffs at the upper section. There are also localizations containing finer grains and altered zones of coarser particles in the coarse and fine grained zones, respectively. The zeolites in the tuffs are classified as Ca-rich clinoptilolites, based on their thermal stability and canonic composition. Quartz, sanidine, plagioclase and biotite are the phenocrystals in the tuffs. Authigenic mineral is clinoptilolite associated with clays, silica minerals, feldspars and carbonates. The size of phenocrystals are observed to be larger in the coarse grained zone, which also contains a large amount of coarse pumice fragments, and a higher amount of carbonate minerals. The samples from this zone have higher alkali cation content and thermal stability than those obtained from fine grained zone which are rich in alkaline earth metal cations. Most of the samples from the coarse and fine grained zones of the upper tuffs were observed to have clinoptilolite contents of above 80 %. The purpose of this study is to characterize the Bigadiç clinoptilolite deposit via the investigation of the variation of ammonium exchange and C02 adsorption capacities across the reserve and their dependence on zeolite content in both fine and coarse grained zones. Since the ion exchange capacities of the natural zeolite samples depend significantly on the way they are pretreated and on the procedures of ion exchange, several procedures and pretreatments with acid (HC1), base (NaOH and KOH) and salt (NaCl) solutions were conducted on the representative samples. Then selected procedures were applied on the samples with varying clinoptilolite contents in order to investigate the effects of pretreatmets on the ion exchange capacities and to correlate these capacities with zeolite content. It is known that the adsorption properties of zeolites can be changed by the modification of pore size via ion exchange. Because of this, near homoionic forms (Na, K, Ca and H) of representative coarse and fine grained samples prepared by the exhaustive ion exchange with the related salt solutions and, acid and base treated samples are used in C02 adsorption capacity measurements. To investigate the dependence of adsorption capacities on clinoptilolite content, Na and H forms of the samples with varying clinoptilolite contents are used. C02 adsorption capacities are measured by using a volumetric glass adsorption system designed and constructed specifically for this study. Prior to measuring adsorption capacities, the samples are activated to remove water from intercrystalline voids and channels of the clinoptilolites. All samples are activated by heating at 400 °C and 10-5 mbar for 6 hours. The sorption isotherms are then measured up to 100 kPa. In order to check the reversibility of the adsorption, desorption measurements are also carried out. IX Experimental data are then evaluated by using a BASIC program and isotherms are analyzed in terms of Langmuir and Dubinin-Astakhov isotherm models by using a statistical software package called Microstat. The same software is used to correlate the experimental adsorption capacities and characteristic adsorption model parameters with the clinoptilolite contents of the samples. Although the zeolitic tuffs are classified as Ca-rich clinoptilolites, the chemical analysis conducted on single crystals show that the coarse grained samples are rich in Na+K, whereas fine grained samples are rich in Ca+Mg. This observation is important because most of the properties of clinoptilolite samples are affected by the cationic composition. For example, fine grained samples have relatively low thermal stability than coarse grained samples. Chemical analyses of the original and near homoionic forms of the representative samples show that despite the severe experimental conditions it is not possible to achieve complete ion exchange in all cases as it is reported in several studies. The ammonium ion exchange capacities of the representative samples which are 2.11 and 2.26 meq/g for fine and coarse grained samples respectively, are comparable with those reported in the literature. Ion exchange capacities depend on the way that the samples are treated before and during ion exchange experiments. Capacities obtained from the experiments conducted at high temperatures are quite higher than those obtained with the experiments conducted at room temperature. Acid and base pretreatments prior to the ion exchange have no significant effect on the ion exchange capacities of the samples. Only when NaCl treatment follows acid and base treatments increased ion exchange capacities for some samples can be obtained. Preparation of near homoionic Na forms of the representative samples is observed to be the most effective treatment which increased the capacity by more than 40 %. Clinoptilolite rich tuffs from two different zones in Bigadiç basin also have differences in their ion exchange behavior and react differently to pretreatments. The capacities obtained at room temperature vary from 0.81 to 1.92 meq/g. It was observed that the ion exchange capacities of fine grained samples in general, are greater than those of coarse grained samples. Prior to the ion exchange with NH4CI solutions, preparation of near homoionic Na forms of the samples is seen to increase the ion exchange capacities of eight samples out of fifteen samples by more than 9 %. Two samples have ion exchange capacities higher than 2 meq/g and the highest exchange capacity of 2.36 meq/g observed during the study, was obtained by this procedure. NaCl treatment improves the capacities of most of the samples, but acid treatment followed by a salt treatment is significantly beneficial in increasing the capacities, by 8-30 % for a group of coarse grained samples which are containing acid soluble minerals such as carbonates. If not followed by conversion to Na form after acid treatment, only dilute acid treatment can improve the capacities to an extent, for samples from the same zone. NaOH treatment also increases the ion exchange capacities of the samples but only one of the samples has a capacity significantly greater than those obtained by acid + salt treated samples. C02 adsorption experiments conducted on the fine and coarse grained samples show that tiie samples from the two zones have significant differences also in their adsorption capacities. The adsorption capacities are a strong function of the cationic composition of the samples. C02 adsorption capacity of the representative coarse grained sample is higher than that of the fine grained sample since as a cation probe molecule C02 is adsorbed with higher capacity on the Na+K rich coarse grained samples rather than on Ca+Mg rich fine grained sample. The C02 adsorption capacities of the coarse and fine grained samples are found to decrease in the order K>Na>H>Ca. In the case of fine grained samples it was observed that the cationic forms of K, Na and H have capacities higher than the untreated sample. C02 adsorption experiments conducted on acid treated samples show that the adsorption capacity decreases with increasing acid concentration and treatment time depending on the decationization of the samples as a result of H3Û+ exchange and dealumination. The base treatment also decreases the C02 adsorption capacity but the change in capacity with increasing treatment time follows a different path. It was observed that the C02 adsorption capacity shows a maximum with increasing time that was attributed to the dissolution and recrystallization of the clinoptilolite structure in basic medium. C02 adsorption isotherms of the Na and H forms of the samples with varying clinoptilolite contents show that Na forms of the samples, in general, have higher adsorption capacities than the of H forms, with a few exceptions. Langmuir and Dubinin-Astakhov (D-A) isotherm model analyses of the experimental data show that only D-A adsorption isotherm model can represent the full sorption data satisfactorily, although the saturation capacities calculated from Langmuir adsorption isotherm model are in close agreement with those obtained experimentally at 100 kPa. Characteristic adsorption energies of Na clinoptilolites, calculated from D-A equation, are higher than those of H clinoptilolites. The energies fall into the ranges of 9-15 and 11-17 kJ/mol for H and Na clinoptilolite samples, respectively. The correlations of ion exchange capacities obtained at room and high temperature ion exchange with clinoptilolite contents of the samples are not very strong. Although the ion exchange capacities obtained at high temperature are higher than those obtained at room temperature, the relationship between capacities and clinoptilolite content is weak and there is a considerable amount of scatter in the data around the linear regression line. The relationship becomes stronger if the exchange is carried out after acid treatment and preparation of near homoionic Na forms of the samples. The scattering around the linear regression lines diminished with increasing correlation coefficients. The relationship between the capacities of near homoionic Na samples and clinoptilolite content was observed to have the highest coefficient of correlation (r= 0.79). The dependence of the exchange capacities on clinoptilolite content is stronger when the samples are evaluated separately according to the zones that the samples XI originated. The relationships become even more stronger when the samples taken from the altered zones are included in fine and coarse grained sample sets where the correlation coefficients of the relationships for the coarse grained sample sets are higher than those of fine grained sample sets. The relationship between clinoptilolite content and ion exchange capacities obtained after acid treatment indicate that the coarse grained samples benefited more than the fine grained samples from this treatment. The CO2 adsorption capacities are highly dependent on the clinoptilolite content. It was observed that the coefficient of correlations are higher in case of CO2 adsorption than those of ion exchange capacities. As it was mentioned before Na clinoptilolites adsorbed more CO2 than H clinoptilolites. The relationship between the adsorption capacities and clinoptilolite content is stronger for Na clinoptilolites than that of H clinoptilolites. The relationships become stronger when the samples are grouped according to the zones that they are taken from. It was observed that although the D-A isotherm model could represent the isotherms more accurately, there is no strong linear relationship between the characteristic model parameters and clinoptilolite content. In this study, relationships between the capacities are also investigated. It was observed that there is a strong relationship between the ion exchange capacities and CO2 adsorption capacities of Na homoionic samples when all of the samples are taken into consideration. The coefficients of correlations get stronger when the samples are grouped as fine and coarse grained samples and the samples taken from altered zones are included. As a conclusion, it can be said that, the clinoptilolite content of a sample from the Bigadiç reserve can be predicted by measuring the CO2 adsorption capacity at 100 kPa with more accuracy than obtaining the NH4+ exchange capacity. This method is much easier and faster than the XRD method.
Açıklama
Tez (Doktora)-- İTÜ Fen Bil. Enst., 1993.
Anahtar kelimeler
Doğal zeolit, Zeolitler, Çanakkale-Bigadiç, Natural zeolite, Zeolites, Çanakkale-Bigadiç
Alıntı