Seydişehir Alüminasından Kalıplamayla Seramik Malzemelerin Üretilmesi

Abstract

Ülkemizde, gelişen seramik sektöründe alümina kullanımı günden güne artmaktadır. Özellikle elektrik seramikleri, tekstil seramikleri, refrakter uygulamaları ve geleneksel seramiklerde kullanımının ciddi boyutlarda olduğu bilinmekle beraber son ürün olarak alümina seramikleri kullanan bir çok sanayi kuruluşunun olduğu bilinmektedir. Bu sektörler için ihtiyaç duyulan alüminanın büyük bir miktarı ne yazık ki ithal edilmektedir. Bu çalışma, Seydişehir alüminasının seramik değerli alüminaya dönüştürülerek özellikle elektrik seramikleri ve enjeksiyonla kalıplamaya yönelik olarak bu sektörlerde kullanılabilir nitelikteki alümina üretimi amacıyla gerçekleştirilmiştir. Bu çerçevede, XRD, SEM, yaş kimyasal analiz, görünür yoğunluk ve tane boyutu çalışmalarıyla Seydişehir alüminası karakterize edildikten sonra ıslah çalışmalarına başlanmıştır. Alüminanın elektrik seramiklerinde kullanımını engelleyen Na20 içeriği yıkama çalışmalarıyla kabul edilebilir seviye olan %10 mertebelerine düşürülmüş, 1200 °C'de yapılan kalsinasyon işlemiyle yüksek orandaki geçiş faz alüminalannın kararlı faz a-AfeCVya dönüşümü sağlanmış, vibrasyonlu öğütme işlemiyle de tane boyutu 10 um'nun altına düşürülmüştür. Islah edilen Seydişehir alüminası bu işlemden sonra enjeksiyon kalıplama için hazırlanmıştır. Bunun için çeşitli bağlayıcı sistemleriyle çalışılmış ve ana bağlayıcı olarak polipropilen uygun görülmüştür. Enjeksiyon kalıplama ve çok düşük hızlarda bağlayıcı giderme çalışmalarından sonra sinterleme işlemlerine geçilmiş ve sinter sıcaklığını düşürücü katkılarla 1400 °C'de teorik yoğunluğun %90'ına, 1600 °C'de ise teorik yoğunluğun %95'ine ulaşılmıştır. Yapılan tüm işlemler sonrası metalurjik değerli Seydişehir alüminası seramik değerli alüminaya dönüştürülerek ülkemiz sanayisinin hizmetine sunulmuştur.

The alumina chemicals industry has become a global enterprise in which most of the world's major aluminum firms participate. Current world production capacity for alumina is almost 40 million tonnes annually. Approximately 92% of this for smelting to aluminum metal. The remaining 8% is converted to alumina chemicals products. Today, a wide variety of alumina chemicals products with a broad range of properties is available on the world market from numerous suppliers. These products are used in many commercial applications involving thousands of end products used daily by people all over the world. Products containing alumina chemicals in one form or another include electrical insulators, abrasives, industrial ceramics, electronic substrates, refractories, fine chine, glass, cement, coating, airborne ceramics, biomedical ceramics, and many others. Figure 1 traces aluminum oxide from its origin at the bauxite mine trough various processing stages to a wide variety of end uses. Alumina is produced worldwide in tonnage quantities for the aluminum and ceramics industries using the Bayer process. The principal operations in the Bayer process are the physical beneficiation of the bauxite, digestion (in the presence of caustic soda NaOH at an elevated temperature and pressure), clarification, precipitation, and calcination, followed by crushing, milling, and sizing. During the digestion, most of the hydrated alumina goes into solution as sodium aluminate, and insoluble compounds of iron, silicon, and titanium are removed by settling and filtration. Al(OH)3 + NaOH -> Na+ + A!(OH)'4 ( 1 )BAUXITE MIKE CRUSH ER ryer ' caiximrg ETUI ABRASIVE CRABE wjxrn electric arc furnace csode »row n.-sn> "«.oraBim mist abrasives CESQCAL GRADE uman ^ ftETRACTORY CRASt Munra METAL CRAPE RABXITE rod MILL BAYER PROCES! REFIIIStT AUMJJU _ TKiRnnun" flo» flasb calcirer CALCIREP SCALE _L PELLLIJUR »FBMKACOTICALS- » flame kexarqarts- -raxEBS t Cannes - -»ACTIVATES ALBKIRA. PRIMARY ALimiira ma murua * shaft icnjr ismnuc) TAMLAR ALOHltU ¦LOW MS» AlOMHA- -Biee purity aumma-. FILLERS < COATIRSS- - D1UIU6EI1TS -noKDAHTS, wis. orcakic lakes - paper nuu»cnniE -SÎBADE TEEATMTn - CEMENT UATERPROOFIVC -emmlkik fluid -LEATHER lARJJIPG -hater minai -CLASS l CERAMICS -TOOTHPASTE - DEODORARTS -ARTACIBS -COSMETICS -CARPET 1ACKIK -roAM s solid elastics -PLASTIC «IU IKSBLATIOK ¦E electric CRUDE KBITE, Pin, I MOT FUSE» ALUM1RA " jxmua "ı suns' il 1 PAIRTS. PAW ADBCSTVKS, KUBBER ¦ULLPAPEK. UAXE5 DTI, PAIRI. PAPER, BUBSER 'CLAOS PROCESS SOLFISt RECOVERY ADSORSERT Ft» CASES. LIQUIDS DESICCA8TE PETROLEDH REFHIKC CATALYST SUPPORT. ABTO EXHAUST VBITEUARX. PIRI CURA PORCELAIR UnDLATORS. SPARK FLOSS KIUI PQRWITORE miiuic BOlttS. SAPPHIRES TOOLS tülısbırs cohpoutos substrates ı microchips, cataltsts trahslvcewt aurora prosbcts class. ceramics retractories keldik electrode coat1rc EPDXY. SILICOIIE, POLYESTER CATALYST «9PP0RT BEDS ABRASIVES HHITE FUSED ABRASIVE BRAIR flR FDSB> ABRASIVE GRAll IH POSE» ABRASIVE CM» Figure 1. Speciality Alumina Production Flowchart! l,p 7]. After cooling, the filtered sodium aluminate solution is seeded with very fine gibbsite Al(OH)3, and at the lower temperature the aluminum hydroxide reforms as the stable phase. The agitation time and temperature are carefully controlled to obtain a consistent gibbsite precipitate. The gibbsite is continuously classified, washed to reduce sodium content, and then calcined at 1 100-1200 C. Al(OH> must be calcined at 1150-1200 °C for obtain a-Al203. Corundum, a-Al203, is the only thermodynamically stable form of aluminum oxide. Its excellent XIthermal, mechanical and dielectric properties make it one of the most important ceramic raw materials. If the calcination temperature lower than 1150 °C thermal decomposition of trihydroxides and oxide hydroxides is not accomplished (except diaspore) and metastable transition phases of alumina (gamma, delta, theta, kappa, chi, eta, rho) occurs. These transition phases have poor sinterability and poor grindability. They are also induce laminations, shrinkage, and distortions in the final ceramic product. Bayer-processed alumina consists of solid agglomerates rather than individual primary particles. Breaking down the agglomerates by grinding results in many advantages, including denser compacts, lower firing shrinkage, lower firing temperatures, lower tendency for lamination and warpage, and, most important of all, better-fired micro structures. Several different types of grinding equipment have become available for preparing ceramic materials in the finer sizes for forming operations. These include, vibratory grinding, Attrition, conventional ball milling, and fluid energy milling. Alumina is a dominant ceramic material not only because of its superior mechanical, thermal, and chemical properties, but also because of outstanding electrical properties that make it excellent low-tension and high-tension insulator. The electrical properties make it adaptable for use in applications ranging from electronic substrates to spark plug insulators. Resistivity is affected by phases present, secondary-phase distribution, alkali content, and temperature. Dielectric strength is a function of temperature, specimen thickness, porosity, and silicate glass- phase concentration and distribution. Owing to Bayer process (digestion in the presence of caustic soda NaOH) alumina has Na2Ü contamination. Presence of Na2 has significant effect on the conductivity or dielectric losses for alumina dielectrics. Most commercial electrical insulator compositions in the high-alumina class have been based on the use of Bayer process alumina of reduced alkalinity (below 0.2% Na20). The selection of alumina dielectric forming operation is very dependent on the size, shape, and dimensional tolerances of the product. Forming types of alumina ceramics are pressing, plastic-forming, and casting. XIIIn plastic forming, feed material in a continuous, sectioned, or granular form that exhibits plastic behaviour when compressed is consolidated and deformed into a particular shape. Injection molding is a major polymer fabrication process used especially for the high-productivity forming of the part with a thin wall and a complex shape. For alumina injection molding, special abrasion-resistant steels must be used in high wear areas. The organic binder system must be formulated to facilitate both flow in forming and burnout during firing. In Seydişehir, the purpose of the process is production of the metallurgical grade alumina. Owing to this kind of process Al(OH)3 is calcined at 1000 °C for dehydration of aluminum trihydroxide. This is because, Seydişehir alumina powders have metastable transition phases. Bayer-processed Seydişehir alumina powders have coarse particle sizes. Another property of these aluminas is high Na20 contamination. All of these properties make it non-ceramic grade. The aim of this study is to find out process conditions for production of injection molded electrical ceramics from Seydişehir alumina powders. The first step of the process is characterization of Seydişehir aluminas. This step involves: particle size analysis, SEM analysis, chemical analysis, and XRD analysis. Particle size analysis and SEM analysis show that Bayer-processed Seydişehir alumina powders have broad range of particle size distribution from submicron to higher than 125 um. Chemical analysis shows that these aluminas have high soda contamination about 0.3 percent. Also XRD analysis shows that Bayer-processed Seydişehir alumina powders have both transition and alpha phases. The second step of the process is improvement of these aluminas. For improvement, these operations are used: calcination (at different temperatures), XRD analysis, washing (in different solutions), chemical analysis, grinding (with different techniques), particle size analysis, and SEM analysis. With calcination, the XRD results are show that all of the transition phases are converted to corundum at 1200 °C. By the help of washing, Na20 content of Bayer- processed Seydişehir alumina powders is reduced. Chemical analysis shows that Na20 content of washed in cold water aluminas is 0.11 percent. The last operationgrinding is done by the help of attritory and vibratory grinding in dry and wet conditions before and after calcination. Particle size and SEM analysis show that grinding after calcination is very effective. With 16 hours grinding after calcination is enough for obtain smaller particles than 15 urn. After all these operations Bayer-processed Seydişehir alumina powders are became ceramic grade alumina, and this alumina can be used for ceramic forming. Owing to purpose of this thesis only injection molding type of forming is achieved. Before forming, the binder system is prepared by various thermoplastics, waxes and lubricants. Optimum binder composition is consisted of polypropylene and paraffin waxes, and binder-alumina composition is mixed with the screw of the injection molding machine. After the mixing operations plastic forming is done at different binder-alumina ratio. The forming step is done in the screw type injection- molding machine. The molding conditions show that suitable temperature was between 160 °C and 200 °C. Before final sintering, binder phase must be removed from the molded part. Binder removal is a key step for successful ceramic injection molding. In this process binder removal is achieved by two different types of thermal degradation. One of is evaporation and the other is draining through a porous bed. The results of the binder removal show that porous bed method is more effective than the evaporation of binder. The last step of the process is sintering. Injection molded alumina parts are sintered at different temperatures of 1400 °C to 1600 °C. To achieve 80% theoretical density of final product, molded parts must be sintered at 1600 °C for 2 hours. 80% theoretical density is inadequate for electrical alumina ceramics applications. Some additives are used (for example MgO, SİO2, TİO2, Mn304, Cr203) to enhanced higher density values. The specimens are prepared with pressing. By using Mn3Û4 and TİO2 additives 95% theoretical density is achieved. Finally, by the help of improvements of Bayer-processed Seydişehir alumina powders injection molded parts are produced. XIVIn our country, ceramic sector have grown up. Day by day consumption of ceramic grade aluminas are increased. Especially, it is used for electrical ceramics, textile ceramics, ordinary ceramics and refractories but huge quantities of aluminas have been importing. With this thesis, Seydişehir alumina powders are converted to ceramic grade alumina. Thus, these powders can be used for above mentioned ceramic sectors