Year 2007, Volume 27

Abstract

The influence of the correlation between the drying constant and the diffusion coefficient of the liquid on the behavior of two raw materials for brick manufacturing during their drying process was examined. The test was performed in a laboratory dryer with air recirculation. It has been observed that the behavior of some raw materials during “fast” drying process regimes does not depend only on external drying conditions, but also depend on internal controlling mechanisms, in this case moisture diffusion in the slab.

Abstract

The present research is concerned with the production and characterization of ceramic materials obtained using demolition debris alone or in mixtures with bottom ash from an incinerator of municipal solid waste, incinerated sewage sludge and steelmaking slag. Blending of the different components was performed by attrition milling. Milled and dried powders were uniaxially pressed into specimens which were sintered and characterized by shrinkage on firing, density, water absorption, hardness, strength, fracture toughness measurements as well as by XRD and SEM investigations. The fired samples were also tested in acid environment (HCl) in order to evaluate their leaching behaviour. All materials produced have low water absorption, fine microstructures, good mechanical properties and the release of most of the hazardous metals contained in the starting powders is low. On the contrary, shrinkage on firing is high for most of the compositions studied.

Abstract

Carbon-containing refractory bricks are used extensively in the steel industry worldwide. Since the first generation became commercially available in the 1970’s, their processing, microstructures and properties have been improved dramatically, and the service lives of industrial furnaces have thus been extended substantially. In addition to the work on carbon-containing refractory bricks, much effort has been, and is being, made worldwide towards the development of carbon-containing refractory castables. In this paper, the latest R & D towards new generation carbon-containing refractory bricks as well as carbon-containing refractory castables have been highlighted. In the first part, current techniques used to improve mechanical properties of carbon-containing refractory bricks are summarised. A new concept using a catalytic-growth technique to create in-situ oxide nanofibres and/or carbon nanotubes in carbon-containing refractory bricks is introduced. The second part addresses some important technical issues of low carbon carbon-containing refractory bricks. Besides the concern about thermal shock resistance, other new problems arising from the use of nanosized carbon, such as the accelerated MgO-C reaction and carbon oxidation, are discussed. In the final part of the paper, technical difficulties hindering the development of carbon-containing castables as well as measures to overcome them are discussed. A novel molten salt synthesis technique developed recently at Sheffield to prepare high quality carbide coatings on graphite is introduced.

Abstract

Refractory bodies based on magnesia with different amounts of zircon and lime were sintered through two firing stages. The bodies were characterized for their mineralogy composition by XRD and microstructure by SEM. Periclase, calcium zirconate, zirconia, and merwinite were detected. The densification and mechanical properties were correlated with their minerology and microstructure. An improvement of refractoriness under-load was recorded with increasing zircon content. The fracture mode in the thermally loaded bodies was both intergranular and transgranular. The parepared bodies showed a good corrosion resistance to attack by Portland cement clinker.

Abstract

Relations between technological factors such as grain size distribution of mixes, forming pressure and firing temperature and properties of magnesia spinel refractories have been investigated. The model investigation was based on magnesia clinker with low Fe₂O₃ content and different content of fused spinel. Grain composition of mixtures was calculated according to Dinger and Funk model. Apparent density, open porosity, compression strength, permeability and pore size distribution of products were determined. The results clearly showed that higher amount of fine grains fraction in mixes of magnesia spinel products led to the increase of open porosity and crushing strength but simultaneously led to the decrease of bulk density and permeability. The increase of forming pressure for mixes with constant grain size distribution led to the increase of crushing strength and bulk density with simultaneous open porosity and permeability decrease. Higher forming pressure of samples led to the increase of the share of the pores below 10 mm. Generally, the higher was the amount of pores below 10 mm, the lower was the permeability of the material.

Abstract

Alumina-zirconia fibres (Al₂O₃-10 wt.% ZrO₂) were prepared by the sol-gel method with organic additives viz. hydroxy ethyl cellulose (HEC) and polyvinyl alcohol (PVA). As the organic content is increased, the phase transition to α-Al₂O₃ is retarded in the presence of hydroxy ethyl cellulose and enhanced in the presence of polyvinyl alcohol. The phases present in the sintered fibres are a-Al₂O₃, t-ZrO₂ and m-ZrO₂. Infra red spectra of the sintered fibres show bands corresponding to α-Al₂O₃. The surface morphology of fibres prepared with higher amount of organics shows equiaxed grains with relatively dense microstructure for fibre prepared with hydroxy ethyl cellulose, whereas the structure is porous with polyvinyl alcohol. Tensile strength decreases with the increase in organic content.

Abstract

Up to now most of the efforts for developing HTS technology have been directed to devices for grid applications. However, HTS synchronous machines as motors and generators become more and more interesting within the world-wide development programs. Replacing the copper winding of the rotor by an HTS one and introducing an iron-less air-core stator winding the very compact HTS machine has less than half the weight and volume, higher efficiency and an excellent operational behavior when compared to the conventional devices. These features make HTS rotating machines very attractive for e.g. ship drives and industrial applications for the processing industry and power generation in power plants and wind parks. World-wide, HTS machines have already demonstrated their advantages and technical feasibility. The prototypes are ranging from the 5 MW low-speed high-torque propulsion motor to the highspeed 3600 rpm 4 MVA HTS generator. Feasibility studies clearly show the financial benefits when introducing high-efficient HTS wire based rotors into a large scale power generator systems. At present, most of the industry driven activities take place in the United States, Germany, Japan and Korea and are mainly directed towards applications aboard ships. Further potential applications as well as the technical and economic benefits will be discussed.

Abstract

Four clay samples from Mayouom deposit were studied to evaluate their potential uses in the ceramic industry. Characterization of crude clays involved chemical and mineralogical composition, plasticity and particle size distribution. Fired properties of clays were measured in terms of densification parameters viz linear firing shrinkage, bulk density, open porosity and water absorption. The samples denoted as MY38, MY41g and MY45g were found to be poor in sand and highly plastic, whereas MY45s is sandy and has low plasticity. The alkali and alkaline earth oxides compositions are low (< 0.9 %) while the colouring oxides (TiO₂+Fe₂O₃) are 0.4- 6.1%. In all the samples, kaolinite is the dominant mineral associated with quartz, illite, goethite, apatite, rutile or anatase. Test-discs from sand-poor samples show important linear firing shrinkage vitrifying above 1300°C, whereas the shrinkage in MY45s was small, its ceramics also vitrified at high temperatures. These properties show that these clays can be successfully used as refractories. In addition, due to its white firing colour, MY45s can be very suitable for use in the production of porcelain and vitreous china.

Abstract

The possibility to obtain a cheap ceramic material based on 50 wt % basaltic tuffs and 50 wt % industrial clays was studied. The tuffs were characterized by optical microscopy, X-ray diffraction and differential thermal analysis. It was shown that this material is appropriate for the ceramic industry: it is easy for milling and is characterized by low liquidus temperature. The sintering behaviour of the ceramic was evaluated by hot-stage microscopy and dilatometry. The structure of samples, heat-treated at different temperatures, were observed by scanning electron microscopy while the densification degree was evaluated by dry flow and gas pycnometery. Samples with 13% total porosity and 4% water absorption were obtained at 1150 °C for 30 min soaking.

Abstract

Population balances are the most useful tool in solving many engineering problems including comminution process modeling. Studying batch grinding in terms of material balances inevitably demands for determining two basic functions: breakage rate function, also known as selection function and breakage distribution function. They depend on many factors such as initial fraction size or distribution of sizes, material characteristics, comminution machine characteristics etc. This paper investigates the influence of process parameters (grinding ball material and diameter, mill speed, relative ball filling ratio) on dry batch grinding behavior of single size fraction (- 3.36+2.25 mm) dolomite ground by using two different planetary ball mills. Effects are expressed in terms of breakage rate (selection) function and breakage distribution function.

Abstract

In this study, conventional ball mill and hybrid (roll crusher + ball mill) grinding techniques were compared in the preparation of porcelainized stoneware tile body mix in terms of energy consumption. In the conventional ball mill grinding technique, raw materials for porcelainized stoneware body mix were ground in the fast mill alone. In the case of hybrid grinding technique, the same material was first crushed using roll crusher and then subsequently ground in the fast mill. Specific energy consumptions were determined for both techniques. Comparing the results it was found that hybrid grinding appears to offer considerable energy saving over conventional ball milling alone. In this example energy saving was about 40%. Some of the important technological properties of products obtained from conventional and hybrid grinding techniques were found to be similar indicating that hybrid grinding offers energy saving without altering technological properties of the products.

Abstract

Feldspar porcelains are ceramic materials that allow full or partial dental ceramic restorations similar to natural teeth, combining aesthetics (color, shape and texture) to mechanical strength. The ceramic restorations are made of a ceramic support (alumina, zirconia, spinel or glass-ceramics) on which layers of porcelains with pigments are applied and fired to provide color, translucency and fluorescence. In the present work, the ceramic support was a densely sintered alumina plate for an easier colorimetric evaluation using CIELab method. A previous manufacture of finely milled frits for each different batch of porcelain was essential, in order to obtain a nice layering by firing at relatively low temperatures (900 to 980 °C). Among four different formulations (pigments and frits) for porcelain layers using Brazilian raw materials, the so named B1 and B2 were those which presented shades and textures near to dentine-3 reference.