Seydişehir bayer çözeltisinden zeolit A üretimi

Abstract

DZET Gelişen teknoloji çevre kirliliğini de beraberinde getirmektedir. Detarjanlı sular, kirliliğe yal açan önem li bir evsel atık grubudur. Sert ve asidik suda sabundan daha iyi temizleme gücüne sahip olan deterjanlar yüzey aktif maddeleri ve polifosf atları ihtiva ederler. Fosfat, karbon, azot ve potasyum elementleri gibi güneş ışığı ile birlikte yüzey sularındaki biyolojik verimliliğin zaruri elemanıdır. Buna karşın bu maddelerin ortamda gereğinden fazla bulunması yosun ve diğer su bitkilerinde aşırı büyü meye sebep olarak doğanın bozulmasına sebep olur. Bu ola ya `ütrif ikasyon` adı verilir. Deterjan katkı maddesi alarak palifasf atlarla aynı etkiye sahip suda çözünmeyen, iyi dağılabilen, iyon değiş tirici zeolitler denenmiştir. ilk olarak Almanya'da zeolit A içeren deterjanlar kullanılmaya başlanmıştır. Zeolit A nın doğada eşdeğeri yoktur, ve genellikle sodyum aluminat ve sodyum silikat çözeltilerinin hidrotermal reaksiyonu sonucu elde edilir. Zeolit üretimi için birçok başlangıç materyali denenmiş ve en ekonomik olanları araştırılmıştır. Seydişehir Alüminyum Tesisleri Bayer Prosesi ile 'alu mina üretmektedir. Bayer Prosesi esnasında elde edilen zayıf ve kuvvetli Sayer çözeltileri zeolit A için sodyum aluminat kaynağı alarak kullanılabilmektedir. Bu çalışma da sodyum aluminat, zayıf ve kuvvetli Bayer çözeltileri kullanılarak açık sistemde ve otoklavlarda zeolit A sen tezi gerçekleştirilmiş, eldeedilen zeolit A kristallerinin yapısı incelenmiştir.

THE SYNTHESIS QF ZEOLIT A FROM THE BAYER SOLUTIONS OF SEYDİŞEHİR ALUMINUM PLANT SUMMARY ZealitHS are hydrous aluminasilicates 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 mere first recognized by the Swedish mineralogist Cranstedt in 1756, with his discovery of stilbite who named them `zeolites` meaning `bailing stane` in Greek. Many zeolite structures exist. There are natural zeolites, synthetic anallogues of natural zeolites, and synthetic zeolites with na natural counterparts. Although zeolites were discovered in 1755, 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 occurence. The framework structure consists of corner-linked tetrahedra in which small atomsCcallectively denoted T atoms) lie at the centers of the tetrahedra and oxygen atoms lie at the corners. The T sites of all natural zeolites are dominated by Al and Si atoms but chemically VIrelated atama such as Ga,Ge, and P can also be incorporated into synthetic zeolite structures. The (Al,Si)Q, tetra- hedra are called `primary Building Units` aod the relatively simple configurations resulting from their linkage are called `Secondary Building Units`, which further link to form various frometuork structures. Secondary Building Units are made up of four and six- membered rings in addition to complex and multiple units. The basic formula for all crystalline zeolites where `M` represents a metal and `n` its valence may be represented as follows: M2/n0*âL2a3* xSiD2*VH2Q In general, a particular crystalline zeolite will have values far x and y that fall in a definite range. The large ions in the aavities in natural zeolites are mono or divalent alkali and/or alkaline earth metal cations neutralizing the negative charge of the fromework. The weakly held water molecules, termed `zealitic water`, can be removed by heating and or under vacuum, without distorting the crystalline fromework, there by providing a large internal surface area, typically on the order of several hundred square meters for most zeolites. The most important properties of the zeolite crystals can be classified as the adsorption and ion- exchange-praperties, bath af which are exploited for catalytic applications. In addition to their ability to separate gas molecules on the basis of their 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. vxiThe exchanable cations of zeolites are only loosely banded to the tetrahedral framework and can be exchanged easily, using strong solutions of other cations. The ion-exchange properties of zeolites have been exploited for their usage as water softeners for many years. Processes far the manufacture of commercial zeolite products may be classified into two groups: a)- Preparation from aluminosilicate gels or hydrogels. b)- Preparation from clay minerals. The first process far preparing zeolites was based an the results ar original labaratory synthesis using amorphous hydragels. The gel is defined as a hydrous metal aluminosilicate which is prepared from aluminate and silicate solutions. The gels are crystallized in a closed hydrathermal system at temperatures varying generally from roam temperature to about 175 C. In some cases higher temperatures up to 300 C are used. The time required for crystallizations varies from a few hours to several days. The gel preparation and crystallization is represented schematically using Na-O-Al^Q-.-SiOp-H-Q system as an example: NaOH(aq)+NaAlCOH)it(aq) + Na2Si03(aq) I T1= 25°C Naa (A10`)h(Si0`)n.Ma0H.H`0l al 1 a 2b 2c 2 ' gel 1 T2= 25 to175DC Na A1D`) (SiD0) I.mH`D + Solution x ' 2 x 2 y ' 2 (zeolite crystals^ The aluminosilicate framework of zeolite A can be described in terms of two types of polyhedra; one is a simple cubic arrangement of eight tetrahedra, the D4R, vixxthe other is the truncated octahedron of 2k tetrahedra or p-cage. The aluminaailinate framework of zeolite A is generated by placing the cubic D^R units (Al,Si,G1fi) in the centers of the edges of a cube of edge 12.3 A. The unit cell of zeolite A contains 2k tetrahedra, 12 AID, and 12 SiCL. When fully hyrated, there are 27 water molecules. Zeolite A is useful as an adsorbent primarily since it has almost a 50 % void fraction. However the largest use of zeolite A, in terms of tans per year, is as a phospate substitue detergent builder. When zeolite A is used`as a detergent builder, the sodium ion exchanges for Ca and Mg ~ ions responsible for the h_ardness in waters. Although there are a number of cations that may be present in zeolite A, it is synthesized in its sodium farm since the reactants in this case are readily, availahle and water soluble. The sadium in the sadium farm of zeolite A may then be easily exchanged for other cations. In making the sodium farm of the 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 homogeneous gel is obtained. Satisfactory results have been obtained at temperatures as low as about 21DC and as o high as about 150 C. Increasing reaction temperature increases the rate of reaction. Products are filtered, washed with distilled water and dried at a temperature between 25DG 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, ixThe 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. In the Seydişehir Alumina Plant, aluminum can be extracted by treating bauxite uiith sodium hydroxide solution in the digester at higher temperatures, and from the liquor, supersaturated after cooling, aluminum hydroxide can be partially precipitated by seeding with gibbsite. Alumina is obtained by calcining the aluminum hydroxide which is electrolized to obtained the aluminum metal. During the Bayer process, the weak Bayer solution is obtained after the extraction step, and distilled and circulated into the process as the strong Bayer solution. The purpose of this study is the synthesis of zeolite A from both Bayer solutions, using them as the source of alumina. At the beginning sodium aluminate solution was used as the source of alumina. Experiments were carried out in a glass flask mounted on a shaker apparatus. Some experiments were repeated in modified Morey type autoclaves. Later the two Bayer alumina solutions were used as the alumina sources far zeolite A synthesis, with the addition of sodium silicate to the reactant mixtures in appropriate amounts. To investigate the effect of temperature on crystallization of zeolite A, a series of autoclave tests were conducted using. Bayer sol utions. The zeolite A crystals which were synthesised during batch and autoclave tests were characterized various chemical analyses, partical size analyses, X-ray diffraction and SEM analyses. All these laboratory experiments indicated that zeolite A was successfully synthesized from Seydişehir Bayer solutions. The crystals produced during the experiments appear to be cubic in form and the size of the crystals were observed to be in a range of 1 micron to 1D microns.The zeolite A crystals synthesized From bath Bayer solutions were seen to have a very pronounced bevelled edge morphology, which is desirable far detergent applications. During the experiments carried out at 90,1DQir11D C with weak Bayer solution in the autoclaves it has been observed that an increase in temperature results in a decrease in the crystallization period along with a slight deformation in the crystal morphology. Synthesis of zeolite A crystals as detergent builders, with high crystallinity, appropriate size range, and morphology, especially from the weak Bayer solution which is an economically and environmentally important process, is also shown to be technically feasible in this study. Further assessment of the ion-exchange performance of the synthesized zeolite A in detergent formulations may be required far prases development. XI