Türk boksitlerinin kullanılması ile refrakter kalsiyum alüminatların üretimi

Abstract

Bu çalışmada refrakter kalsiyum alüminatları üretmek amacı İle GUllUk'ten farklı bölgelere ait boksit numune leri alınarak kimyasal analizleri yapılmıştır. Boksit ve kalker taşı 100/nm'in altına öğütülerek homojen olarak karıştırılmıştır. üretimi amaçlanan refrakter yapısını verecek şekilde harmanlanan boksit ve kalker taşı 9DD C ile 1400 C aralığındaki değişik sıcaklıklarda ve değişik sürelerde pişirme işlemine tabi tutulmuştur. Pişirme iş lemi ile sinterlenen numuneler elektrikli havanda 1 Gü /nm ' un altına öğütülerek dökülebilir formdaki kalsiyum alümi- nat refrakteri elde etmek için öğütülmüş numune içine 1350 Cin üzerinde kalsine edilmiş değişik tane boyutla rındaki (1,2 ve 3 mm) şamot partikülleri ilave edilerek homojen olarak karıştırılmışlardır. öğütülmüş numune ve şamot partiküllerinden oluşan malzemeye değişik oranlarda su katılarak (.% 30,40 ve 50) en iyi mukavemet değirini ve ren su oranı belirlenmiştir. Bunun yanında değişik sıcak lık ve değişik sürelerde pişirilen harmanlanmış numunenin nihayi faz yapısını belirlemek amacı ile X-ışınları anali zi yapılmıştır. 1250 C'ta elde edilen fazlar; CA,CA",. C12A2,<*-A12D, ve CF2, 1300 ve 1350 C'ta elde edilen fazlar ise CA,CA2, C12A7 ve CF'dir. Ayrıca boksit ve harmanlan mış numunelere ait DTA eğrileri çıkarılmıştır. Üretilen dökülebilir refrakterin, kullanım sıcaklıklarında maruz kaldığı eğme ve basma kuvvetlerine karşı gösterdiği diren ci belirlemek amacı ile 900 ile 1400°C sıcaklık aralığında pişirilen numuneler, eğme ve basma deneylerine tabi tutul muştur, üretilen dökülebilir formdaki kalsiyum alüminat refrakterin 110°C'taki kuruma kısalması ve 900 ile 1400 C aralığındaki sıcaklıklarda meydana gelen pişme kısalması değerleri belirlenerek, hem bu değerler ve hemde eğme ve basma mukavemeti değerleri, değişik oranlarda perlit ve diatomit içeren izole dökülebilir refrakterlerle karşılaş- tırılmıştır.

The past decade has been an interesting and exciting era far both the development and application of refractory castable technology. There has been more research and development on castable technology in the past 1Q years than in the previous half century. Castable producers and consumers have gained insight and understanding in both basic and advanced castable systems as a result of extensive study and the charac terization of physical behavior and thermomechanical properties. Castables have evolved from secondary materials of choice for general applications to primary materials of design in many critical high-temperature processes. Refractory castable tecnology progressed because of new developments and/or improvements in several key areas including the folloujing: raw materials (neui and improved); advanced castable materials (low cement, ultralotij cement, and no cement); additons(stainless steel and organic fibers); and installation methods. Most of these areas of development had their origins or roots in laborotory work during the 1970's but major commercial applications (and commercial acceptance) did not occur until the 1980 s. The last 10 years have seen the introduction of a variety of new or improved refractory ram materials. The refractories technologist of today has a much uiider choice of materials than ever before. This is exemplified by the fact that castables are no longer slmpleaggregate-cement blends, as they were in the past. Some of today1 s castables represent the most Vll complex refractory formulations, reguiring high- quality precision-sized aggregates, modifying fillers, binder, and additives. Rau-material suppliers have responded by making higher quality materials is avaible and have implemented statistical process control programs to assure consistency. Aggregate choices have expanded uiith the use of new synthetic and naturel materials from around the world, especially the People's Republic of Chin» and South America. In the United states, the consumption of Chinese bauxites and calcines has increased steadily since Î980. Domestic calcined aggregates have also improved in quality, making possible dramatic improve ments in castable quality. Calcium aluminate cements also were the subject of considerable development, resulting in mare calcium aluminate cements avaible today, in wider ranges of purity, than ever before. Several cement suppliers have worked to develop specific cements for use with law-cement castable systems to optimize setting, flow, and strength characteristics, even though these suppliers recognized that the application of low-cement tecnology meant lower consumption of their products per unit of castable. Aggressive development and marketing by many chemical companies of new additive packages for def locculatian, set control, flow, etc., was yet another response to the castable manufacturers needs, supporting continued growth in the field use of advanced technology castable materials. Refractory calcium aluminate cements are a successfull class of materials. They are widely used and their use and range of applicability are still increasing. Nonetheles, a better understanding of the processes which determine the fired microstructure and strength is essential if these materials are to be used to their full potential. Current production and use is still very much dominated by'trial and error1 thinking it has been shown that the entire history of hydration, curing and dehydration affects the fired product. It seems reasanable, therefore, to say that the initial hydrate microstructure, any subsequent conversion of the hydrates, together with the dehydration and the firing of the refractory cement, are all important in determining its final microstructure and Vlll physical properties. Unfortunately It Is clear from the literature that there Is no simple uay of relating the mlneraloglcal changes which occur on heating to the changes In strength. Neither Is It possible to determine the best combination of anhydrous phases in the starting cement to produce a strong concrete at a given temperature with our current knowledge. Various workers have shown that differences in mineralogy may have a considerable effect on the strength of a fired cement, but the differences caused by different initial curing temperatures may be still greater with a greater understanding of these relation ships it may be possible to produce stronger and more refractory cements in the future. No-cement castables have found use in numerous molten iron and steel contact applications, where the elimination of lime in the refractory matrix is a distinct advantage. No-cement castables use a variety of banding mechanisms. Band systems used include clay bonding, gel bonding, q-alumina bonding, and phosphate bonding, among others. No-cement castables normally do not posses the superior physical and mechanical properties of low-or ultralow-cement castables, but do possess better corrosion resistance in cnntact with metals and slags. In the late 197Q's, initial trials of low-cement castables technology were in stell mill applications Since then, the application list has grown to caver essentially every area of refractory applications. To day low-cement castables are widely used in critical abrasion areas in petrochemical processing, for aluminum contact, for ferrouB metal contact, and for abrasion and alkali attack situations in mineral processing and incinerators, to name a few areas of application. No- cement systems continue to be used mainly in the steel and foundry industries for blast furnace trough and runners, injection lances, ladles, etc. Briebach has classified refractory hydraulic cements into four groups: their chemical compositions are different* The first group, Portland cements, contains ordinary Portland cement-the common' cement' used in the production of concrete and mortar for the construction industry. IX Dther farms of Portland cement (such as sulphate- resisting cement) differ only slightly in chemical composition, by a modification of the relative propor tions of the four major phoses; C3S, B-C2S, C3A and Portland cement is by for the cheapest of the four types of cement under consideration, but, because of undesirable phase changes that occur during heating (including the formation of free lime), it is severely limited as a refractory material. Ciment fondu was first developed in France around 1914 as on alternative construction material for use inhere acid waters, especially those containing sulphate, might lead to the deterioration of concrete made from Pnrtland cement. Because this material has a higher alumina content than Portland cement, it is often called.High-Alumina cement'. The alumina contents of the more refractory aluminous cements are, however, much higher than that of ciment foundn; the title "High- Alumina cement", applied to the latter material is, therefore a misnomer, and it is best used as a general description for all aluminous cements. The name"ciment fondu" is less confusing, and indicates its made of manufacture-by melting the raw materials together. Although ciment fondu was developed for its chemical resistance to sulphates, its rapid hardening and refractory properties were soon discovered. Ciment fondu does not set more rapidly than Portland cement, yet it gains appreciable strength very rapidly (so much so that its compressive strength after Zk hours of hydration is comparable with that of so-called rapid- hardening Portland cement after Ik days of hydrotion). This property of ciment fondu was explaited in the U.K. and elsewhere, especially during the 1960's, to produce factory-made, pre-cast beams for buildings. The failure of at least three beams is the U.K. in 1972-73 led to a virtual banon the use of ciment fondu as a construction material in the U.K. The imposition of this ban, and the possible dargers associated with the buildings that still contain ciment fondu beams, remsin matters of controversy. The gradual conversion of metastable to stable cement hydrates in high-alumina cements led to these failures. ('Conversion may be discussed in more detail later, in this study, we will discuss it briefly). Opinions are divided about the extent to which this conversion process limits the use of ciment fondu as a building material. Neville believes that ciment fondu should not be used a structural material, because "its satis factory behaviour cannot be maintained in the lf>ng term", whereas George believes that, in manufactured at a IqU mater/cement ratio (less than 0.4) and using a minimum cement content to make the concrete (400 kg/m3) ciment fondu is a durable structural material. As a refractory material, ciment fondu has been more succesful. It does not undergo the undesirable phase changes that restrict the use of Portland cement Instead, ciment fondu is only limited by the temperature at which a eutectic liquid forms. The presence of impurities, and a relatively low alumina content, limit the gyrometric cone equivalent (PCE) of ciment fondu to 1270 C. In association uiith a carefully chosen aggreagate, however, ciment fondu may be used in a refractory concrete at temperatures up to 1400 C. The purer refractory cements of groups 3 and 4 were developed in order to allow concretes to be submitted to higher temperatures. These refractory calcium alüminates are the main concern of many reviews. Their higher purity and higher alumina contents increase the temperature at which a eutectic may form, as we will discuss later it is, however, worth noting here that these refractory cements posses the same rapid hardening properties as ciment f ondıı. Although they are substantially more expensive than even ciment fondu, and consequently of no interest to the construction industry, they are valuable to a user who requires rapid placement of a furnace lining. The purpose of this study is the production of calcium alüminate refractories by using Turkish bauxites. 5 different kinds of bauxites from Güllük were used in the experiments. Bauxite and lime stone were ground to minus 100 /nm and mixed homogeneously. Suitable mixtures have been heated from 900°C to 1400 C for different periods of time. As a result of heating process, a sintered material was obtained by solid-state diffusion mechanism. Then, sintered materials have been ground to minus 100 /nm. The calcined fire brick with different particle size (1,2 and 3mm) have been added into powder refractory material to get a castable form of calcium aluminate refractories. The result material has been mixed in the mixer for 15 minutes. After that, different amounts of water (30,40 and 50 %) have been added into mixed material. Of these, the 50% water has given the best machanical strength. XI X-ray analysis have been done to determine the final phases uihic mere CA,CA2, o<-AlpD_, C. 2A7 and CF" (at 1250 C for 160 minutes heating time) and CA,CA2, C12A3 and CF2 (at 1 300°C for 120 minutes heating time) aha CA,CA", C. "A" and CF" (at 1 350 C for 120 minutes h...C. \ c / c. eating time). The effect of the particle size of fire brick on the mechanical properties of the refractories has been determined by mechanical test methods. DTA curves have been established. The compressive and bending strengths of the castable refractories have been determined and the results have been compared with the isolated castable refractories. The shrinkages of both castable and isolated refractories due to drying and firing processes have been determined.