Year 2008, Volume 28

Abstract

Five sample mixtures of Ugandan kaolin, ball clay, feldspar and flint were formulated and porcelain samples fabricated by extrusion through a vacuum pugmill. Samples were evaluated for formability. The degree of densification of sintered specimens fired at 1200 to 1350 °C was evaluated by measuring the firing shrinkage, bulk density, water absorption and bending strength. Crystalline phases and mullite morphology were studied using XRD and SEM respectively. Dielectric strength measurements were carried out on disc specimens. Dielectric and bending strength properties deteriorated at high temperatures because of pore formation and decreasing amount of undissolved quartz in the crystalline phase. An optimum composition of 30% kaolin, 15% ball clay, 30% feldspar and 25% quartz yielded an unglazed body with highest bending strength of 72MPa and dielectric strength of 19MVm^-1 after firing at 1250 °C. The body consisted of small, closely packed mullite needles and undissolved quartz crystals embedded in a glass matrix.

Abstract

Objective of this study is to investigate and optimize an industrial batch ball mill by using the population-balance modeling. Experimental work involved taking samples after different grinding times from an industrial batch ball mill. The parameters of specific rate of breakage and breakage function were calculated from the population-balance modeling equation using the back-calculation method. As the time of grinding increases, the specific rate of breakage increases to a peak value at 6 h grinding, and then decreases. One of the main reasons for the above is that the particle size decreases continuously in a batch grinding process; however the ball size remains constant. Therefore, allowing an excessively long grinding time without changing the media size is an inefficient method of production of fines. The specific rate of breakage values indicated maximum selection at the end of 6 h. Any further grinding after this time is considered inefficient. In order to obtain finer particles, the samples were taken from the industrial ball at after 6 h milling and fed to a laboratory stirred media mill obtaining the desired product within 15 min of grinding.

Abstract

This article relates to a presentation given at the 11th International Ceramics Congress of CIMTEC 2006, Acireale, Sicily, Italy, June 4-9, 2006, and discusses early work carried out on a 3-year European framework VI Collective Research Project entitled “Studies aimed at assisting legislation and encouraging continual improvement in the field of Respirable Crystalline Silica”. This project was ultimately completed in September 2007. The article provides an overview of the project before discussing the development of novel equipment for sampling airborne factory dusts. The envisaged role for the equipment in achieving the project objectives and so assisting SMEs is emphasized.

Abstract

This paper reports an investigation carried out on the deskulling problem of tundishes at the Mobarakeh Steel Plant in Iran. Bad deskulling of tundishes is a major problem in continuous casting of steel, and can occur due to the chemical reactions between the refractory lining and the slag carry over from ladle to tundish. Post-mortem analysis of the slag/commercial magnesia-olivine mix refractory interface by XRF, XRD, OM, TEM, SEM and EDS techniques showed that the volume of slag carry over from ladle to tundish has a direct effect on refractory wear in the tundish. The percentage of tundishes where the skull was locked into the safety lining was dependent on the slag basicity and amount of Al₂O₃ in the slag inside the tundish. It has been shown that a direct corrosion mechanism occurs in basic slag and also in acidic slag containing low alumina, but an indirect corrosion mechanism occurs in acidic slag containing high alumina.

Abstract

Spinel formation was studied by phase analysis and dilatomeric experiment using commercially available caustic magnesia and technical alumina in the temperature range of 1000 °C to 1400 °C. AlF₃ as mineralizer was added up to 3 wt%. AlF₃ was found to reduce the spinel formation temperature significantly, but also reduces the reactivity of the calcined products, measured by specific surface area.

Abstract

SiC/(W,Ti)C functionally graded ceramic composites were produced by hot pressing for use as sand-blasting nozzles, and their microstructure and wear behaviours were examined. The wear resistance of the graded nozzles and of a stress-free nozzle with the same composition, was assessed by sand blasting surface treatments. Results showed that the surface hardness (nozzle entry zone) of the functionally graded nozzle is greatly improved compared to the homologous stress-free nozzle. The functionally graded nozzle shows higher wear resistance than the homologous stress-free nozzle, and the ceramic nozzle graded both at the entry and at the exit area exhibited higher wear resistance over the one graded only at the entry area.

Abstract

A rapid manufacturing method for fabrication of 3D ceramic parts will be presented. The structural information is printed by ink jet in powder layers of 80 μm thickness. Different granulated powders can be used, such as zirconia and alumina if they show a good flow ability. After printing the structures a heating process takes place. After these the parts can picked out from the powder bed. The manufactured parts can be impregnated with epoxy. Another option is the infiltration with ceramic slurries and glass with sintering to higher density. The sintering process has been studied and the shrinkage and material properties evaluated. The interrelationship between the raw material qualities, infiltration  media and the sinter parameters as well as the material-specific properties such as density and stability will be presented. The manufacturing method is used for fabrication of moulds and cores for casting processes. Otherwise the process can be used for fast fabrication of models and prototypes. The possibilities to use these methods for implant manufacturing will be shown. A cost analysis has been performed comparing direct manufacturing of small batches of components to mould injection processes.

Abstract

A solid oxide fuel cell (SOFC) electrochemically converts chemical energy of a fuel into electricity at temperatures from about 650 to 1000 °C. SOFCs offer certain advantages over lower temperature fuel cells, notably ability to use CO as a fuel rather than being poisoned by it, and high grade exhaust heat for combined heat and power, or combined cycle gas turbine applications. This paper reviews the operating principle, materials for different cell and stack components, cell designs, and applications of SOFCs. Among different designs of solid oxide fuel cells (SOFCs), the electrical resistance of tubular SOFCs is high, and areal power density (W/cm²) and volumetric power density (W/cm³) low. Planar SOFCs, in contrast, are capable of achieving very high power densities.

Abstract

The nature as well as composition of the Egyptian clay deposits exposed all over the country are summarized. The influence of composition on their ceramic properties in terms of plasticity as well as drying and firing behaviour is discussed. According to their clay mineral composition, they are classified into kaolinite-, smectite- and illite- rich clays. The kaolinite rich clays are mainly exposed in Aswan as well as Gulf of Suez provinces and generally characterized by a slow rate of slaking in water, low plasticity, high Al₂O₃ as well as a high and long vitrification range (1100-1500 °C). Hence, they are used in the production of shaped and unshaped refractories as well as vitreous china and white Portland cement. However, they are not suitable for the production of white wares due to their high colouring iron and titanium oxides content. On the other hand, smectiterich clays cover most of the Eastern as well as Western Deserts and have low Al₂O₃ content with variable amounts of quartz, calcite, gypsum and iron minerals as well as alkali water-soluble salts. They show high plasticity, drying and firing shrinkage as well as a low and short vitrification range (850-1000 °C). Therefore, they are applied for the production of ordinary Portland cement and building bricks. Also, they are recommended for the manufacture of light-weight clay aggregate due to their bloating on firing up to 1250 °C. Meanwhile, illite-rich clays are encountered in El-Bahariya Oasis, Western Desert and contain predominant iron-rich illite clay minerals, i.e. glauconite in addition to variable amounts of kaolinite, smectite, quartz and goethite minerals. They have moderate plasticity, vitrification range (950-1150 °C) as well as drying and firing shrinkage. Hence, they are recommended for manufacturing building bricks and other heavy-clay products, after blending their varieties.

Abstract

Slag was progressively substituted upto 15 (w/w) % to feldspar in a normal porcelain composition and compacts prepared by the usual ceramic processing were sintered at 1100-1300 °C. The effect of thermal cycling on flexural strength of the vitrified samples was studied and the behaviour was correlated with phase and microstructural features. It was found that slag incorporation increased residual strength in thermally cycled samples compared to normal porcelain. The microstructural features of thermally cycled slag-containing porcelain showed limited quartz grain cracking compared to normal porcelain composition, which results in better strength. Formation of anorthite, which has low thermal expansion, in slag-bodies, is also significant in the development of better thermal stability.

Abstract

Slag penetration into a magnesia refractory castable was investigated by the crucible test method. A synthetic calcium aluminate slag system has been used to corrode a commercial magnesia mix refractory for 1, 2, 3, 4, 5 and 6 h at 1450 °C and 1600 °C. It has been shown that the penetration rate was controlled by a diffusion mechanism at 1450 °C, capillaries being the main channels of initial slag penetration into the refractory. In the penetration process, calcium silicate was formed on the surfaces of MgO grains, and around them by reaction between the grain boundary and mayenite, as a main phase of slag with a low melting point. Dissolution of the refractory components in the slag supported the penetration process at 1600 °C. In this case, dissolution of the refractory components in the slag not only makes new open channels, but also changes the local slag composition, resulting in a decrease of viscosity and an increase of surface tension of the slag.

Abstract

One of the major problems facing Blast Furnaces is the occurrence of cracks in taphole mud, as the underlying causes are not easily identifiable. The absence of this knowledge makes it difficult the use of conventional techniques for predictability and mitigation. This paper will address the application of Probabilistic Neural Network using the Matlab software as a means to detect and control such cracks. The most relevant BF operational variables were picked through the statistic tool “Principal Component Analysis - PCA.” Based upon the selection of these variables a probabilistic neural network was built. A set of BF operational data, consisting of 30 controlling variables, was divided into 2 groups, one of which for network training, and the other one to validate the neural network. The neural network got 98% of the cases right. The results show the effectiveness of this tool for crack prediction in relation to clay intrinsic properties and as a result of the fluctuation in operational variables.