Bigadiç Kolemanit Konsantrelerinin Yapısal Seramik Üretiminde Alüminaya Katkısının İncelenmesi

Abstract

Bu çalışmada öncelikle dünya rezervlerinin büyük bir kısmı ülkemiz sınırları dahilinde olan bor cevherlerinin ana minerallerinden olan kolemanit cevheri, Eti Maden Bigadiç işletmelerinden konsantre olarak temin edilmiştir. Bu ürünün dekrepitasyon ve eleme işlemleri ile B2O3 tenörü %33,08’den %58,40’a çıkarılmıştır. Elde edilen tozun karakteristik özellikleri belirlenerek, yapısal seramiklerden Al2O3’ün bünyesine katılabilir hale getirilmiştir. Bunun için toz karakterizasyonu işlemlerinden partikül boyutu ölçümü ve dağılımı, piknometre yoğunluğu, Arnold görünür yoğunluğu, doldurma yoğunluğu ve dekrepitasyon öncesi ve sonrası için kimyasal analiz deneyleri yapılarak sonuçlar değerlendirilmiştir. Daha sonra elde ettiğimiz zenginleştirilmiş kolemanit tozu %1-3-5 oranlarında ticari Alcoa™ alüminasına katılmıştır. PVA ile peletlenen karışımlardan kuru presleme ile 175 MPa’da ham numuneler elde edilmiştir. Bu ham numunelerin yoğunlukları ölçülmüştür. Preslenen numuneler 1350C, 1450C ve 1550C’de aynı sürede sinterlenmiştir. Sinterlenmiş numunelerin karakterizasyonunda SEM taramalı elektron mikroskobu ile mikroyapı incelenmiştir. Bunun yanında su emme değerlerine, arşimed yoğunluklarına, XRD faz analizlerine ve mikro sertlik değerlerine bakılmıştır. Yapılan bu toz karakterizasyon ve sinterlenmiş numunelerin karakterizasyonu işlemleriyle elde edilen sonuçlar irdelenmiştir.

Alumina is the most widely used oxide ceramic material. Its applications are widespread, and include spark plugs, tap washers, pump seals, electronic substrates, grinding media, abrasion resistant tiles, cutting tools, bioceramics, (hip-joints), body armour, laboratory ware and wear parts for the textile and paper industries. Very large tonnages are also used in the manufacture of monolithic and brick refractories. It is also used mixed with other materials such as flake graphite where even more severe applications are envisaged, such as pouring spouts and sliding gate valves. Sintering of alumina ceramics at relatively low temperatures to obtain dense and fine grained microstructures with sufficient mechanical properties is one of tecnological investigations. Alumina ceramics are extensively used for structural applications due to alumina’s high melting point, chemical stability, corrosion resistance and mechanical properties such as hardness and wear resistance at elevated temperatures. Restrictions to the applications of alumina arise due to the problems related to low thermal shock resistance and low fracture toughness. In present day society, boron, an important element of both chemical and biological interest, has a variety of uses. Modern uses of boron-bearing minerals and boron derivatives include heat resistant glass (e.g., pyrex), fiberglass, ceramics, washing products (e.g., detergents and soaps), special alloys, fertilizers, fire retardants, wood treatment agents, insecticides and microbiocides. Colemanite (Ca2B6O11.5H2O), a major source of boron mineral, is semi-soluble calcium-borate hydrate that is found in massive beds with other calcium containing minerals such as calcite and gypsium, and a variety of clays. Colemanite is the most important calcium containing commercial borate mineral with 5 mol crystal water. Direct use of colemanite is problematic for some industrial applications, e.g., in glasses and ceramics. Moreover, transporting raw colemanite and removing impurities and crystal water is expensive and energy inefficient, as colemanite must then undergo heat treatment before use, or the production process must be carried out under certain conditions. Ideally, the removal of impurities and crystal water at the same time will increase the use of the use of colemanite as amorphous B2O3. Therefore, decrepitation or calcination is receiving increased attention. When raw colemanite is heated up to its decomposition temperature, only colemanite explodes, breaking up into a fine powder. This unique characteristic could be utilized in the upgrading of colemanite. Densification is the outcome of a combination of numerous parameters during processing, which include the powder characteristics, process variables, and sintering kinetics. The use of fine starting powder without agglomeration in the green preforms may improve densification. Regarding the latter, specific additives such as low melting oxides can be used to enhance the sinterability of powder. Additives in small amounts are deliberately and commonly used in ceramic systems to influence densification processes either by reducing sintering temperature or suppressing/promoting grain growth or enhancing mechanical and physical properties. Such additives can promote liquid phase formation at lower sintering temperatures and may considerably increase the rate of sintering. During sintering process, a viscous liquid may promote an additional diffusion mechanism of dissolution, particle rearrangement and capillary forces and finally may improve the densification. A variety of oxides such as B2O3, MgO, TiO2, SiO2, MnO2, Cr2O3, Fe2O3 are some of the typical additives that have been commonly used in alumina ceramics. The fundamental explanation of anisotropic grain growth in alumina is not totally understood but includes several proposals: inhomogeneous distribution of the dopant and the presence of a liquid phase on the grain boundary. In this study, initially, colemanite mineral which is found in our country as large scale deposits along worldwide, provided as concentrate from Eti Maden Bigadiç enterprices. The tenor of B2O3 increased from %33,08 to %58,40 by decrepitation at 4500C – 2 hours and dry screening processes after cooling down to the room temperature. The characteristic of the obtained colemanite powder determined by experiment studies and powder is processed for incorporating with Al2O3 in order to produce structural ceramics. The experiments for powder characterization are laser particle size distribution, pycnometer density, arnold density, tapped density and chemical analysis before&after decrepitation for the colemanite powder. Afterwards, enriched colemanite powder added to commercial Alcoa™ alumina powder with the ratios of %1-3-5. These powder mixtures pelletized by PVA for dry pressing and pressed at 175 MPa to obtain green samples. Green density measurements made for the obtained samples to compare with theorical densities. Pressed samples have been sintered at 13500C, 14500C and 15500C for 2 hours and freely cooled down to room temperature. SEM scanning electron microscope has been used for investigating the microstructure of the sintered samples. Besides, water absorption rates, sintered densities, relative densities, XRD phase analysis and micro hardness values measured in order for investigation of the effects of colemanite added alumina structural ceramics. Eventually, the examination of powder characterization and sintered samples characterization have been made and commented for the obtained results. After sintering at 13500C, %3 colemanite added alumina exhibits an increase in micro hardness values, %1 colemanite added alumina exhibits an increase in sintered density values, accordingly undoped alumina. For the same sintering temperature and soaking time, when the addition of colemanite increases, grain growth is also increases according to the scanning electron microscope images. It has been investigated that, while the colemanite additions inside alumina increase, water absorption rates are also increase at the samples which are sintered at 14500C and 15500C. XRD measuring results shows corundum and aluminum borate structures obtained during sintering of the samples. The results in this thesis have revealed that, the place of use where the porosity and hardness is not important like undoped alumina, 1350C sintered and %3 added colemanite to alumina products can be substituted for industrial usage. 200C decrease in sintering temperature brings many advantages such as; saving up the energy consumption, highly increasing the working life of the sintering furnaces, thermal isolation elements and sintering casettes, reducing amortisation costs of the sintering furnaces, also brings in an alternative for the investing lower temperature furnaces which are cheaper, substantially reduces maintenance, investment and production costs.