Sodur/Konya doğal magnezit atık tozlarının sinterlenme ve karakterizasyonu

Abstract

Magnezit, refrakter sanayimde bazik refrakter hammaddesi olarak büyük önem taşıyan bir cevherdir. Türkiye magnezit cevherleri açısından önemli rezerve sahip bir ülkedir. Bu çalışma, Çitosan'a bağlı Konya Krom Magnezit Tuğla Sanayii A.Ş. ait işletmede birikmiş %46.16 MgO içeren Sodur/Konya magnezit atık tozlarının değişik alanlarda kullanılabilirliğinin araştırılması amacı ile yapılmıştır. Uygulanan seri deneylerde, 1 mm altı tane boyutunda doğal magnezit tozlan önce (450-950)° C'lar arasında (30-180) dakikalık değişen sürelerde kalsinasyon işlemine tabi tutularak ideal kalsinasyon koşullan saptanmıştır. Sinterleme işlemi için atık tozlar 1000° C'da 5 saat kalsine edildikten sonra, kalsine tozlar manyetik aymadan geçirilerek, demir bileşiklerinden kısmen arındırılmıştır. 74 /zm altı tane boyutuna öğütülen konsantre ve doğal kalsine magnezit tozlan, 14 kg/mm ve 23 kg/mm şekillendirme basınçlarında tablet biçiminde şekillendirilmiş ve (1100-1450)° C'lar arasmda, farklı sürelerde oksidan ortamda sinterlenmiştir. Sinterlemenin tüm aşamalan taramalı elektron mikroskobu (SEM) ve X- Işınlan difraktometresi ile izlenmiş, numunelere mekanik testler de uygulanmıştır. Deney sonuçlan, kalsinasyon ve şekillendirme sırasmda oluşan aglomerasyonun sinterlenme ve dolayısı ile densifikasyon kinetiğine etki ettiğini göstermiştir. MgO tabletlerinin sinterlenmesinde sıcaklık artışına bağlı olarak farklı densifikasyon mekanizmalan yer almaktadır. Sinterlenmede (1100-1300)° C arasmda yeniden kristalleşme ve 1400° Cm üzerinde ise sıvı faz smterlenmesinin önem kazandığı ve densifikasyonu arttıncı yönde etkilediği saptanmıştır. Magnezit cevherinde bulunan empürite oksitler sinterlenme kinetiğini olumlu etkilemekle beraber, düşük sıcaklıklarda ergiyerek oluşturduklan viskoz film, tane yüzeylerini kaplayarak periklas tanelerinin büyümesini engellediği saptanmıştır. Magnezitin kalsinasyonu ile mikronaltı boyutlarda ince MgO tanecikleri oluşmaktadır. MgO tanelerinin yüksek sıcaklıklarda sıvı faz sinterlenmesi ile birlikte şekillendirilmesi sonucu yüksek densiteye sahip ürün elde edilmekte, bu özellik yüksek teknoloji seramiklerin üretimi için avantaj teşkil etmektedir.

Sintering, the agglomeration of a loosely packed or pressed powder by heat treatment, is conventionally divided into three stages. In stage I, neck growth between particles is rapid and there are marked changes in the shapes of the pores. The neck growth stage is predominant until about 5 percent linear shrinkage occurs, and is then succeeded by stage II where grain growth usually commences, characterized by pores being interconnected and nearly circular in cross section. EUmination of isolated pores is difficult at this point and approximately 70% to 92% of theoretical densities are achieved. The grain growth stage is important in detennining the properties of the sintering compact. In the final stage, the pores become isolated, are approximately spherical in shape, and further densification is severely hampered both by the absence of grain boundaries and the slow kinetics of gas removal, trapped in pores. Final stage sintering is a slow process wherein spherical pores shrink by a diffusion mechanism. Any atmosphere trapped in the pores will inhibit densification. Sintering is the result of atomic motion stimulated by high temperatures. Several potential mass transport paths can be active during sintering. In crystalline bodies, six transport mechanisms can contribute to neck growth. These are volume and surface diffusion from the surface of the particles to the neck, evaporation and condensation, grain boundary and volume diffusion from the grain boundary between the two particles to the neck, and dislocation climb. The first three mechanisms do not lead to densification of the compact and will only give appreciable contribution to neck growth during stage I. The fourth and fifth mechanisms will contribute to both neck growth and densification in all three stages as long as the grain boundary remains pinned to the pore. Several variables influence the rate of sintering. These include the initial density, material, powder size, sintering atmosphere, temperature, time and heating rate. Some key processing factors are: VI - High sensitivity to the inverse particle size, smaller particle sizes giving more rapid sintering, - In all cases, temperature appears in an exponential term meaning that small temperature changes can have a large effect, - Time has a relatively small effect in comparison to temperature and powder size. Agglomerates and aggregates are formed during several stages of powder processing. The problems of agglomeration and aggregation are most pronounced in powder with a submicron particle size. The voids between aggregates and agglomerates are much larger than those between the constituent particles, and the larger voids obviously require a much longer sintering time. Furthermore, densification of the individual agglomerates or aggregates leads to their shrinkage from each other and voids between them become even larger. Additive phases that improve diffusion rates during sintering are used in many ceramic materials. These phases can be used to stabilize desirable crystal structures or more typically, to form a liquid phase to increase the rate of sintering. Several studies have been reported on sintering of MgO powder compacts and effect of dopants and atmosphere. In these studies chemically pure magnesia (MgO) samples produced from Mg(OH)2 were used. Large initial shrinkage in MgO compacts were observed by previous investigators and it was suggested that the shrinkage was due to the effect of water absorbed. Later on large shrinkage during the early stage assumed an instantaneous rearrangement and it was proposed that the diffusion mechanism controlling densification can not be determined from shrinkage versus time type plot. Experimental data were insufficient to analyze the stages of sintering. Several sintering models were proposed including the grain boundary diffusion, viscous or plastic flow. Structural rearrangement during the initial stage of sintering is another factor that causes difficulties in the analysis of shrinkage. The existence of different processes, such as grain growth and rearrangement during sintering, leads to misinterpretation of shrinkage data. The large number of factors which influence the properties of the product MgO any research in this area must involve a high experimental effort in precursor, processing and product characterization if understanding of the calcining process and the-products resulting is to lead to improvements of either the process used in. Such comprehensive studies do not appear to have been vn carried out to date, but could be expected to be extremely fruitful in terms of improved processing methods to yield sintered magnesia with particular properties. The objective of the present investigation has been two-fold; to study the nature of calcination and influence of the sintering behaviour of impure Sodur/Konya natural waste magnesite powder and to provide experimental verification for the possibility of utilization of approximately 15.000 ton of waste magnesite powder. This also would be an interesting and significant basic research area for further understanding of the kinetics of sintering. Experimental Procedures In this investigation, Sodur/Konya natural waste magnesite powder having chemical composition of 46.16 %MgO, 0.32 %Fe203, 1.15 %CaO, 1.56 Si02 were studied. Magnesite powder was supplied by "Konya Krom Magnezit Tuğla Sanayii" belonging to Çitosan Company of Turkey. As-received magnesite powder having powder size < 1 mm were ground and sieved through a -200 mesh screen. The materials were dried at 110°C and stored in desiccator for experimental studies. X-ray analysis of the Sodur/Konya magnesite shows the presence of MgC03 and quartz peaks. Calcination The magnesite powder was calcined at 50° C intervals from 450° C to 950° Ç for 30, 60, 90, 120 and 150 minutes. D.T.A. and T.G. test results of Sodur/Konya natural magnesite revealed that a complete decomposition occurs at 630° C for 1 hour. Consequently, in the lower temperature range, i.e. 650-750° C, decomposition product with very fine particles in the submicron regime were observed. Decomposition at higher temperatures results in agglomeration of the fine particles. X-ray studies showed that calcination at 950° C results in the nucleation of some periclase crystals. Sintering MgO powder which is used for sintering experiments was prepared by calcining Sodur/Konya magnesite in air at 1000° C for 5 hours. Immediately following calcination, the MgO powder was ground in a porcelain mortar to break up the calcined aggregates and then stored in a vacuum desiccator until it was required for the preparation of MgO compacts. Iron was partially eliminated from the calcined powder using a magnetic separator. The chemical composition of this concentrated calcined magnesia powder was 92.50 %MgO, 0.40 %Fe203, 3.70 %Si02 and 3.4 %CaO. X-ray analysis of the calcined viii powder revealed the presence of MgO and periclase peaks. Compacts of MgO were prepared by compacting of 2 gr. dry calcined powder (< 0.0074 mm) without binder into a brass mould, 16.5 mm in diameter using the cross-head speed of 0.1 mm/min under pressures of 14 kg/mm and 23 kg/mm. Apparent porosities of the pressed MgO compacts were 28% and 35%, respectively. Experimental sintering studies of calcined magnesite compacts were conducted at 1100, 1200, 1300, 1350, 1400 and 1450° C in air for different soaking times up to 6 hour. After each sintering run, the specimens were cooled in furnace conditions. According to X-ray diffraction studies, the intensity of the periclase peaks were increased with increasing sintering temperatures up to 1350° C, indicating a complete recrystallisation. Sintering at 1450° C for 3 hour, showed the presence of (MgQ 64FeQ 36)Si03 compound with 30% loss in the intensity of periclase peaks. Corresponding microstructural changes of specimens during sintering were examined under Scanning Electron Microscope (SEM). Kinetics and microstructural data were analyzed to determine pertinent sintering mechanisms during the initial and intermediate stages of sintering. Microstructural Observations The microstructure can be described as interconnected dense regions (i.e., domains or agglomerates) separated by interconnected pores. Therefore, initially two types of porosity were present; primary porosity within the agglomerates and secondary porosity between the aggregates. Recrystallisation is initiated within domains followed by grain growth within the agglomerate. The domain appeared fully dense at 1200° C for the compacts compressed using pressures 23 kg/mm whereas a compression pressure of 14 kg/mm was sufficient to fully density domains at 1300° C. Compacts prepared under 23 kg/mm pressure result in large agglomerates compared to the agglomerates of the compacts prepared under 14 kg/mm and it was found that this feature play an important role on the recrystallization kinetics of the compacts. Due to presence of impurities, sintering over 1350° C cause liquid-phase sintering to great extent and enhanced densification. However as a result of aggregates some stable macro porosity was observed at high temperature regimes. IX It was thought that formation of silicate viscous film around the grains prevents the growth of periclase crystals. It might be concluded that grain growth and rearrangement processes are the two phenomena that control the densification of an agglomerated powder compact. Results of this study show that the process of sintering can not be described by a single progress parameter such as shrinkage. Microstructural observations show that the rearrangement due to liquid-phase sintering extends through the entire stages of sintering. Results of Mechanical Tests Mechanical behaviour of sintered compacts were tested using Instron Universal Testing machine in compression with 0.1 mm/min. crosshead speed. Compression strength for the samples compressed under 23 kg/mm increases gradually- with the sintering temperature up to 1100°C and then turn to a steep increase. For the samples compressed under 14 kg/mm compression strength shows steep increases at 1300° C due to onset of recrystallization. Based on the microstructural observations, in addition to high initial densification, it was concluded that pellets compressed at higher pressure cause the complete recrystallization at lower temperatures. Compacts made from the original magnesite powder have higher strength values due to liquid phase sintering. Hydration Resistance of Sintered Compacts After sintering, compacts were tested for hydration resistance. For this purpose, samples were placed in a boiling water for 2 hr, the hydration was determined by weighing the compacts before and after the treatment. It was observed that hydration resistance increases with the increases of the compression pressure and the sintering temperature of the compacts. Due to liquid-phase sintering at sintering temperatures over 1400° C, amount of initial porosity becomes less important and hydration resistance below 4% can be achieved. Conclusions The sintering of MgO compacts derived from Sodur/Konya natural magnesite was studied systematically. The important results are summarized as follows; - Calcination of magnesite occurred in two distinct steps: (1) Loss of gases at temperatures of 300° C to 500° C, and (2) recrystallization or sintering at temperatures above 900° C. At lower calcination temperatures, the loss of gases leaves a very porous structure with a large internal surface area. - Decomposition temperature for Sodur/Konya natural magnesite was found as 630° C. - X-ray diffraction studies of calcined powder at 1000° C for 5 hour, revealed the presence of periclase peaks. Intensity of the periclase peaks were increased during sintering between the sintering temperature of 1100° C to 1350° C. However, intensity of the periclase peaks were decreased due to formation of (Mgg 64Fe0 36)Si03 compound for the sintering temperatures of 1400° C and 1450° C - At temperatures of 1200° C recrystallization becomes significant and the particles grow in size. Compacts were almost completely dense after sintering at 1200° C for 3 hour. - Depending on the sintering temperature, different densification mechanisms take place. Recrystallization and grain growth play important roles at the sintering temperatures between the 1100-1300° C, and liquid-phase sintering contribute to densification in addition to grain growths above 1300° C. - It is suggested that proper calcination of Sodur/Konya magnesite and hot pressing (or HIP) might facilitate the preparation of dense ceramics.