Seydişehir alüminasının sinterlenebilirliğinin arttırılması ve tane büyüme kinetiğinin incelenmesi
Seydişehir alüminasının sinterlenebilirliğinin arttırılması ve tane büyüme kinetiğinin incelenmesi
Dosyalar
Tarih
1996
Yazarlar
Avcı, Şenol
Süreli Yayın başlığı
Süreli Yayın ISSN
Cilt Başlığı
Yayınevi
Fen Bilimleri Enstitüsü
Özet
Son yıllarda, dünyada, seramik sanayinde görülen gelişmeye paralel olarak ülkemizde de saf alüminaya dayalı teknik seramiklerin yapımında artış görülmüştür. Talepteki bu artış ulusal kaynakların değerlendirilmesi çalışmalarına zemin hazırlamıştır. Alüminyum endüstrisinin bütün kademelerini içeren Seydişehir Alüminyum Tesislerinde üretilen alumina metalurjik amaçlı bir malzeme olup çalışmalarda "normal alümina" olarak adlandırılmıştır. Diğer numune grubu ise kalsinasyon esnasında bacalarda biriken ve elektrofiltre altı alümina tozu olarak adlandırılan alüminadır. Numuneler normal alümina için "N Grubu", baca tozu için "B Grubu", şeklinde kodlanmıştır. Her iki numune grubu da oksit katkıların ilavesinden sonra atritör cihazında öğütülmüş, elde edilen N ve B grubu tozlar etüvde 12 saat kurutulmuştur. Granülasyon çalışmalarından sonra numuneler preslenmiştir. Sinterleme çalışmaları hem zamanın, hem de sıcaklığın fonksiyonunu ölçmek amacıyla her grup numune için 1450, 1500, 1550, 1600, 1650, 1700°C lerde ve 1, 2, 3,4, 5 saat için yapılmıştır. Karakterizasyon çalışmalarında suda kaynatma yöntemi ile numunelerin yoğunlukları tesbit edilmiştir. Numunelerin kırık yüzeylerinden SEM görüntüleri alınmıştır. Parlatılan numuneler üzerinde Vickers sertlik ölçümü yapıldıktan sonra HF ile dağlama işlemi gerçekleştirilmiş ve optik mikroskop görüntüleri alınarak lineer kesişme yöntemi ile tane boyutları tespit edilmiştir. Sertlik ve yoğunluk değerlerinin sinterleme süresi ve sıcaklığı ile arttığı tespit edilmiştir. Tane boyutu sinterleme süresi ve sıcaklıkla artmaktadır. Yapılan aktivasyon enerjisi hesaplamaları sonucunda N grubu numuneler için aktivasyon enerjisi 465.17 kJ/mol; B Grubu numuneler için 393.92 kJ/mol olarak bulunmuştur.
Recent years the utilization of alumina ceramics has increased in our country. The increasing area of the use of alumina ceramics has given a rise to the need for alumina powder. As a result of this recent increase, it has become important to enhance the properties of alumina which is being produced as metallurgical grade at Seydişehir plant, so that it can be used in technical ceramic applications. Alumina has got different polymorphic types, such as; 1) a (hexagonal) 2) 8 (tetragonal) 3) E (hexagonal) 4) T (cubic) 5) K (hexagonal) 6) e (monoclinic) 7) X (hexagonal) There is only one crystal modification types of pure alumina, CI-AI2O3, which exagonal structure. has a hexagonal structure Alumina is extremely brittle since the dislocations are immobile, grain- boundary sliding does not occur. So, material exhibits anisotropic behaviour for both thermal expansion and elasticity. Hence, even the densest materials when subject to tensile stresses fail at the grain boundaries. Alumina represents about 25% of the earth's crust. The commonest sources of alumina are hydrargillite and gibbsite (Al(OH)3) Although aluminium has many kinds of ore, bauxite is the main one in the extraction of aluminium, which is the most common metal in the world. By 1977, world production of bauxite was over 920.000 tones. Over two-thirds of this was coming from Guyana and the remainder from Surinam, America and China. Bauxite is a group of minerals rich in aluminum. Bauxite, Al20(OH)4, often written as A1203.2H20; Diaspora, AI2O3.H2O, or more correctly AlOOH. This forms represent differ stages of hydration and frequently occur mixed together. Processes that are used in the extraction of aluminum differ according to the type of raw material. Bayer process, which is the more common one in the world and in our country, consist of five stages; raw material preparation, digestion, clarification, precipitation, and calcination. Equation summarizes the reactions...^.-^.T ^TT TT _ 4 atmospheres....... Aİ20(HO)4+NaOH +H20------------------------> sodium aluminate in solution + waste 160-170°C The separated sodium aluminate is concentrated to saturation. After seeding with a charge of fine gibbsite (AI2O3.3H2O), a heavy precipitate of Al(OH)3 is produced by hydrolysis for subsequent calcining. Final calcination is achieved at temperatures around 1200°C to convert gibbsite into ceramic grades of alumina. Impurities removed at the digestion stage are titanium, silicon and iron. These impurities are called red mud. Approximately 35-40% of the bauxite ore processed goes into waste as red mud and later diluted with water to be pumped into a waste storage. In Turkey, exploration activities started in 1960, to establish the aluminium industry. In 1962, Bauxite suitable to be treated industrially by the Bayer process was found in the region of Seydişehir-Mortaş, Doğankuzu, by MTA At Seydişehir plant, 200.000 ton alumina and 60.000 ton aluminium are produced per year. The bonding of fine powders into more or less dense solid bodies is called sintering. This is one of the earliest methods of fabricating metals and ceramics. It is used to, make solid bodies from ceramic powder. The size of particles normally employed for sintering ranges from 0.5 micron to about 200 microns. A mass of particles which are small in size, but large in specific surface area, adsorbs a large volume of gases and other impurities. These impurities has great practical importance in the process of sintering. XI Preparation a powder for sintering normally involves pressing or consolidating it. Sintering is effected when powder particles are brought together at a temperature sufficient to bond them together. The sintering of a material usually causes many changes in its properties such as; strength, thermal conductivity, density, transparency and translucency. It is fact that Al20.i can be sintered to theoretical density. The necessary condition to achieve theoretical density in solid state sintering consist in eliminating or suppressing the occurrence of discontinuous grain growth. Then, grain boundaries will remain attached to the pores, and the normal grain growth will be sufficiently slow that the pores can follow the movement of the grain boundaries and do not become trapped inside the grains. In the production of metallurgical grade alumina by Bayer process, during calcination some small particle size alumina powders were caught in electrofilters. The purpose of this study is to find out density, hardness and to compute the activation energies of the powders. The first type of the powder is as-received powder calcinated at 1200°C. The other one is metallurgical grade (electrofilter residue) alumina powder with certain impurities from the Seydişehir Plant in Turkey. The powders were grained for 2 h and were pressed by using hydraulic press. Samples were dried in the drying oven and sintered at 1450, 1500, 1550, 1600, 1650, 1700°C for 1,2,3,4 and 5 h. Various test that given bellow, were performed on sintered samples. - Hardness - Bulk density - SEM and optic metallography - Grain size analysis Hardness of samples was obtained by Vickers method. To Obtain the densities, samples were boiled in the pure water for 3 h., then weighed by using digital balance. Densities were obtained as follows; XII ¦d = -^ wy-w. For microstructural observations, the samples were mounted in bakelites, then abraded by SiC abrasive papers, and finally policed with 1 u,m alumina powder. Optic micrographs of the samples were taken to obtain the particle-size. Linear intercept method was used to measure the grain-size. Six horizontal lines were drawn on the micrographs, then the lines were scanned to make a count of the number of intersections they made with the boundaries in the structure. The average grain size G was obtained as; G=1.56L where L is the average grain-boundary intercept length of a series of random lines on the micrographs. It is obvious that the average grain size increases with increasing sintering time. The kinetic grain growth equation can be readily plotted in the form; n(logG) = logD + logt + 0.434(-E/RT) From the slope of the log(grain size) versus log(time) line, which is 1/n, the grain growth kinetic exponent is readily determined. It was found that average Grain Growth Kinetic exponents were equal to 3.11 for as-received powder, and 3.25 for electroflilter residue powder. The apparent activation energy for grain growth can be calculated as; Gn=Dtexp(-E/RT) log(Gn/t) is plotted versus the reciprocal of the temperature (1/T). From the slope of the line, the apparent activation energy can be calculated. xill The experimental results can be summarised as follows; As sintering temperatures and sintering time increase, bulk density increases because of weight loss, decreasing porosity in structure and shrinkage. After 1600°C bulk density decreases, because of the grain growth. As sintering temperatures and sintering time increase, hardness increases. After 1600°C hardness decreases, because of the grain growth. As sintering temperatures and sintering time increase, the grain size increases because of rising diffusion rate in the grain size. It is found that activation energy is equal to 465.17 kJ/mol for as-received powder, and 393.92 kJ/mol for electrofilter residue.
Recent years the utilization of alumina ceramics has increased in our country. The increasing area of the use of alumina ceramics has given a rise to the need for alumina powder. As a result of this recent increase, it has become important to enhance the properties of alumina which is being produced as metallurgical grade at Seydişehir plant, so that it can be used in technical ceramic applications. Alumina has got different polymorphic types, such as; 1) a (hexagonal) 2) 8 (tetragonal) 3) E (hexagonal) 4) T (cubic) 5) K (hexagonal) 6) e (monoclinic) 7) X (hexagonal) There is only one crystal modification types of pure alumina, CI-AI2O3, which exagonal structure. has a hexagonal structure Alumina is extremely brittle since the dislocations are immobile, grain- boundary sliding does not occur. So, material exhibits anisotropic behaviour for both thermal expansion and elasticity. Hence, even the densest materials when subject to tensile stresses fail at the grain boundaries. Alumina represents about 25% of the earth's crust. The commonest sources of alumina are hydrargillite and gibbsite (Al(OH)3) Although aluminium has many kinds of ore, bauxite is the main one in the extraction of aluminium, which is the most common metal in the world. By 1977, world production of bauxite was over 920.000 tones. Over two-thirds of this was coming from Guyana and the remainder from Surinam, America and China. Bauxite is a group of minerals rich in aluminum. Bauxite, Al20(OH)4, often written as A1203.2H20; Diaspora, AI2O3.H2O, or more correctly AlOOH. This forms represent differ stages of hydration and frequently occur mixed together. Processes that are used in the extraction of aluminum differ according to the type of raw material. Bayer process, which is the more common one in the world and in our country, consist of five stages; raw material preparation, digestion, clarification, precipitation, and calcination. Equation summarizes the reactions...^.-^.T ^TT TT _ 4 atmospheres....... Aİ20(HO)4+NaOH +H20------------------------> sodium aluminate in solution + waste 160-170°C The separated sodium aluminate is concentrated to saturation. After seeding with a charge of fine gibbsite (AI2O3.3H2O), a heavy precipitate of Al(OH)3 is produced by hydrolysis for subsequent calcining. Final calcination is achieved at temperatures around 1200°C to convert gibbsite into ceramic grades of alumina. Impurities removed at the digestion stage are titanium, silicon and iron. These impurities are called red mud. Approximately 35-40% of the bauxite ore processed goes into waste as red mud and later diluted with water to be pumped into a waste storage. In Turkey, exploration activities started in 1960, to establish the aluminium industry. In 1962, Bauxite suitable to be treated industrially by the Bayer process was found in the region of Seydişehir-Mortaş, Doğankuzu, by MTA At Seydişehir plant, 200.000 ton alumina and 60.000 ton aluminium are produced per year. The bonding of fine powders into more or less dense solid bodies is called sintering. This is one of the earliest methods of fabricating metals and ceramics. It is used to, make solid bodies from ceramic powder. The size of particles normally employed for sintering ranges from 0.5 micron to about 200 microns. A mass of particles which are small in size, but large in specific surface area, adsorbs a large volume of gases and other impurities. These impurities has great practical importance in the process of sintering. XI Preparation a powder for sintering normally involves pressing or consolidating it. Sintering is effected when powder particles are brought together at a temperature sufficient to bond them together. The sintering of a material usually causes many changes in its properties such as; strength, thermal conductivity, density, transparency and translucency. It is fact that Al20.i can be sintered to theoretical density. The necessary condition to achieve theoretical density in solid state sintering consist in eliminating or suppressing the occurrence of discontinuous grain growth. Then, grain boundaries will remain attached to the pores, and the normal grain growth will be sufficiently slow that the pores can follow the movement of the grain boundaries and do not become trapped inside the grains. In the production of metallurgical grade alumina by Bayer process, during calcination some small particle size alumina powders were caught in electrofilters. The purpose of this study is to find out density, hardness and to compute the activation energies of the powders. The first type of the powder is as-received powder calcinated at 1200°C. The other one is metallurgical grade (electrofilter residue) alumina powder with certain impurities from the Seydişehir Plant in Turkey. The powders were grained for 2 h and were pressed by using hydraulic press. Samples were dried in the drying oven and sintered at 1450, 1500, 1550, 1600, 1650, 1700°C for 1,2,3,4 and 5 h. Various test that given bellow, were performed on sintered samples. - Hardness - Bulk density - SEM and optic metallography - Grain size analysis Hardness of samples was obtained by Vickers method. To Obtain the densities, samples were boiled in the pure water for 3 h., then weighed by using digital balance. Densities were obtained as follows; XII ¦d = -^ wy-w. For microstructural observations, the samples were mounted in bakelites, then abraded by SiC abrasive papers, and finally policed with 1 u,m alumina powder. Optic micrographs of the samples were taken to obtain the particle-size. Linear intercept method was used to measure the grain-size. Six horizontal lines were drawn on the micrographs, then the lines were scanned to make a count of the number of intersections they made with the boundaries in the structure. The average grain size G was obtained as; G=1.56L where L is the average grain-boundary intercept length of a series of random lines on the micrographs. It is obvious that the average grain size increases with increasing sintering time. The kinetic grain growth equation can be readily plotted in the form; n(logG) = logD + logt + 0.434(-E/RT) From the slope of the log(grain size) versus log(time) line, which is 1/n, the grain growth kinetic exponent is readily determined. It was found that average Grain Growth Kinetic exponents were equal to 3.11 for as-received powder, and 3.25 for electroflilter residue powder. The apparent activation energy for grain growth can be calculated as; Gn=Dtexp(-E/RT) log(Gn/t) is plotted versus the reciprocal of the temperature (1/T). From the slope of the line, the apparent activation energy can be calculated. xill The experimental results can be summarised as follows; As sintering temperatures and sintering time increase, bulk density increases because of weight loss, decreasing porosity in structure and shrinkage. After 1600°C bulk density decreases, because of the grain growth. As sintering temperatures and sintering time increase, hardness increases. After 1600°C hardness decreases, because of the grain growth. As sintering temperatures and sintering time increase, the grain size increases because of rising diffusion rate in the grain size. It is found that activation energy is equal to 465.17 kJ/mol for as-received powder, and 393.92 kJ/mol for electrofilter residue.
Açıklama
Tez (Yüksek Lisans) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 1996
Anahtar kelimeler
Alümina,
Sinterleme,
Alumina,
Sintering