Sürekli karıştırmalı tank reaktöründe zeolit a sentezi

Abstract

Deterjanların içerdikleri fosfat, karbon, azot, potasyum; güneş ışığı ile birlikte yüzey sularındaki biyolojik verimlilik için gerekli elemanlardandırlar. Ancak ortamda fosfatın gereğinden fazla bulunması 'ötrofikasyon' olayma sebep olur. Deterjanda bu kötü etkiye sahip olan sodyum tripolifosfatlar (STPP) yerine aynı etkiye sahip, suda çözünmeyen, iyi dağılabilen, iyon değiştirici zeolitlerin kullanılması düşünülmüş ve ilk olarak Almanya'da zeolit A içeren deterjanlar üretilmiştir. Zeolit A temel olarak sodyum aluminat ve sodyum silikat çözeltilerinin hidrotermal reaksiyonu sonucu elde edilir. Zeolit A genellikle ya kesikli sistemlerde ya da otoklav adı verilen kapalı sistemlerde üretilmiştir. Bu çalışmada ise zeolit A kristallerinin çok miktarda ve daha ucuza elde edilebilmesi için sürekli sistemde üretimi denenmiştir. Başlangıç maddeleri olarak sodyum aluminat ve sodyum silikat çözeltileri kullanılmış, sisteme devamlı hammadde beslemesi yapılmıştır. Sistemden devamlı ürün çıkışı yapılarak hacim sabit tutulmuştur. Sürekli sistemde elde edilen zeolit A kristallerinin yapısı incelenmiş ve iyon değiştirme kapasiteleri de bulunmuştur

Zeolites are hydrous aluminosilicates of alkali and alkaline earth metals, a large group of inorganic minerals, which have gained considerable importance as industrial materials, mainly because of their uses as catalysts and sorbents in petroleum refining industry and numerous other applications as desiccants or ion exchangers. They were first recognized by the Swedish mineralogist Cronstedt in 1756, with his discovery of stilbite who named them "zeolites" meaning "boiling stone" in Greek. Many zeolite structures exist. There are natural zeolites, synthetic analogues of natural zeolites, and synthetic zeolites with no natural counterparts. Although zeolites were discovered in 1756, their large scale production and commercial applications did not begin until 1950's. In 1953 sodalite structure was reported. However, it was the synthesis of zeolite A that really initiated the large-scale use of zeolites as adsorbents and catalysts. Zeolites are classified according to their morphological characteristics, crystal structure, chemical composition, effective diameter and natural occurrence. The framework structure consists of corner-linked tetrahedra in which small atoms (collectively denoted T atoms) lie at the centers of tetrahedra and oxygen atoms lie at the corners. The T sites of all natural zeolites are dominated by Al and Si atoms but chemically related atoms such as Ga,Ge, and P can also be incorparated into synthetic zeolite structures. The (Al,Si)04 tetrahedra are called "Primary Building Units" and the relatively simple configurations resulting from their linkage are called "Secondary Building Units", which further link to form various framework structures. Secondary x Building Units are made up four and six membered rings in addition to complex and multiple units. The basic formula for all crystalline zeolites where "M" represent a metal and "n" its valance may be represented as follows: M2/"0.A1203. xSi02.yH20 In general, a particular crystalline zeolite will have values for x and y that fall in a definite range. The large ions in the cavities in natural zeolites are mono or diavent alkali and/or alkaline earth metal cations neutralizing the negative charge of the framework. The weakly held water molecules, termed "zeolitic water", can be removed by heating and/or under vacuum, without distorting the ctystalline framework, thereby providing a large internal surface area, typically on the order of several hundred square meters for most zeolites. The most important properties of zeolite crystals can be classified as the adsorption and ion-exchange properties, both of which are exploited for catalytic applications. In addition to their ability to separate gas molecules on the basis to then- size, the unusual charge distributions within the pores of dehydrated zeolite crystals allow many species with permanent dipole moments to be adsorbed with a selectivity unlike that of other sorbents. The exchangable cations of zeolites are only loosely bonded to the tetrahedral framework and can be exchanged easily, using solutions of other cations. The ion-exchange properties of zeolites have been exploited for their usage as water softeners for many years. Processes for the manufacture of commercial zeolite products may be classified into two groups: a) Preparation from aluminosilicate gels or hydrogels. xi b) Preparation from clay minerals. The first process for preparing zeolites was based on the results of original laboratory synthesis using amorphous hydrogels. The gel is defined as a hydrous metal aluminosilicate which is prepared from aluminate and silicate solutions. The gel is crystallized in a closed hydrothermal system at temperatures varying generally from room temperature to about 175 °C. In some cases higher temperatures up to 300 °C are used. The time required for crystallization varies from a few hours to several days. The gel preparation and crystallization is represented shematically using Na20-Aİ203-Si02-H20 system as an example: NaOH(aq) + NaAl (OH)4 (aq) + Na2Si03 (aq) i T1=25 °C I Naa (A102)b (Si02)c. NaOH. H20 1 gel i T2=25 to 175 °C Nax I (A102)b (Si02)y |. mH20 + Solution (zeolite crystals) The aluminosilicate framework of zeolite A can be described in terms of two types of polyhedra; one is a simple cubic arangement of eight tetrahedra, the D4R, the other is the truncated octahedron of 24 tetrahedra or P-cage. The aluminosilicate framework of zeolite A is generated by placing the cubic D4R units (AUSLjOiö) in the centers of the edges of a cube of edge 12.3 A0. The unit cell of zeolite A contains 24 tetrahedra, 12 A104 and 12 Si02. When fully hydrated, there are 27 water molecules. Zeolite A is useful as an adsorbent primarily since it has almost a 50% volid fraction. However the largest use of zeolite A, in terms of tons per year, is as a phosphate substitute detergent builder. When zeolite A is used as a detergent builder, the sodium ion exchanges for Ca+2 and Mg+2 ions responsible for the hardness in waters. Although there are a number of cations that may be present in zeolite A, it is synthesized in its sodium form since the reactants in this case are readily available and water soluble. The sodium in the sodium form of zeolite A may then be easily exchanged for other cations. XI i In making the sodium form of zeolite, typical sources of silica that can be used are silica gel, silicic acid or sodium silicate. Alumina may be obtained from activated alumina or sodium aluminate. A solution of the reactants in the proper proportions is generally mixed in a closed container. Reactants are mixed with agitation. The system is stirred until a homogenous gel is obtained. Satisfactory results have been obtained at temperatures as low as about 21°C and as high as about 150 °C. Increasing reaction temperature increases rate of reaction. Products are filtered, washed with distilled water and dried at a temperature between 25 °C and 150 °C. The individual crystals of synthetic zeolite A usually appear to be cubic. Most of the crystals are observed to be in the range of 0. 1 micron to 10 microns, but smaller and larger crystals can occur covering the size range of 0.001 micron to 100 microns. In the synthesis of zeolite A, it has been found that the composition of the reacting mixture is critical. The crystallizing temperature and the length of reaction are important variables in determining the yield of the crystalline material. Extreme conditions may also result in the production of materials other than zeolite A. The present invention relates to a continuous method of preparating zeolites, particularly zeolite A by effecting precipitation and crystallization of solutions of sodium silicate and sodium aluminate at a temperature of 40 °C to 90 °C. The resulting slurry is moved to a crystallization reactor or reactors, maintained at a temperature of 75 °C to 100 °C. The slurry is aged in these reactors until the desired crystallinity is obtained. The crystalline product is recovered, washed and dried. To promote the exchange capacities of Ca+2 and the selectivity of the zeolite, the latter should be as pure as possible and consequently be well crytallized, any impurity being either inactive or less selective. The Na+ exchange selectivity by Ca+2 is also improved by the use of zeolite grains constituted by agglomeration of crytals so that the diameter of the pores is just slightly larger than the diameter of the Ca+2 ion surrounded by the sphere of the coordinated water molecules. xm In this study we performed zeolite A crystallization in a CSTR and then we ion-exchanged the zeolite A with 0.5 M CaCl2 to calculate ion-exchange capacities. The same gel formulation was used through the synthesis experiments. Initially 2 cc/min NaAlC>2 and 1 cc/min sodium silicate were fed into the reactor which is the same volume with 600 ml. During the synthesis experiments, three different synthesis temperatures (90,100,110) were used. Second we reduced the feed flow by keeping other parameters fixed, and the same experiments were performed in autoclaves. After crystallization reactions were completed, synthesized crystals were ion-exchanged with 0.5 M CaCl2 to bring them to their Ca+2 form.