Year 2005, Volume 25

Abstract

One of the major environmental issues within the ceramic industry is the fluorine emission in the flue gases exhausted during the firing process. European and National regulations limit these emissions, and there is a trend towards the harmonization and restrictions of these limits among the EU countries.The present study reports the investigation of the fluorine emissions process, from the raw materials (chemical composition and fluorine content, crystal nature, effect of selected body additives) to the thermal process control (heating rate, peak temperature, oxygen and water vapour pressure and overall flow of gases in kiln atmosphere).A laboratory scale apparatus was developed to study the effect of several parameters on the emissions during firing.The experiments results show that some primary measures can be adopted in industrial operation to reduce fluorine emissions of clay based ceramic materials by means of adjustments in body composition and the control of the atmosphere and firing cycle.

Abstract

The objective of this work is to evaluate the feasibility of using alum sludge, a by-product from potable water plants, as raw material for ceramic production.Thus, 10, 20 and 30 wt% of alum sludge were added to a clay used for the production of building materials. In order to increase the densification of the ceramic bodies, a silicate-based fluxing agent was also added.The clay and alum sludge were characterised by chemical and mineralogical composition and particle size distribution. Porosity, linear shrinkage, mechanical strength and leaching tests according to Brazilian standards were determined on the fired bodies which showed properties comparable to similar commercial products.The results indicated that the ceramic products are inert to leaching according to Brazilian environmental regulations, and present an interesting potential for recycling alum sludge as raw material for ceramic products.

Abstract

The addition of 9, 14, 15 wt. % Afyon Volcanic Tuff (AVT) into a standard wall tile body has been studied. Dry pressed samples were produced by adding volcanic tuff into the standard wall tile batch composed of Istanbul Etiler clay (IEc), Istanbul clay (Ic), kaolin(a), pegmatite, calcite, kaolin(b) and clay and fired at 1090°C for 40 min.The possible use of AVT has been investigated through a number of tests including viscosity, compressive strength, water absorption and firing shrinkage.Tests were performed on the fired specimens and compared with specifications.As a result, it was found that experienced AVT addition into the standard body can be used for the production of wall tiles. Alkaline properties and viscosity of recipes and water absorption and compressive strength of specimens are slightly affected by AVT addition.

Abstract

The introduction of metallic fibres in a ceramic refractory matrix can limit its intrinsic brittle behaviour. Microstructural investigations, micromechanical and macromechanical tests have been performed for a better understanding of the thermomechanical behaviour of such composites. Results of pullout tests are presented and discussed. For non-inclined fibres, results allow to quantify the effect of a first heating on the microstructure and on the pullout performances. During the second heating the development of a fretting pressure at fibre/matrix interfaces has a beneficial effect on the pullout performances.When introducing a fibre inclination angle, supplementary pullout micromechanisms are observed and described.They can have a beneficial effect on the pullout performances. Results of macroscopic tensile tests are shown and discussed too. Analytical models have been developed to predict and to quantify the effect of variations in material parameters and/or in testing conditions. Links are established between the results of the micromechanical and macromechanical approaches both from an experimental and from a modelling point of view.

Abstract

The thermo mechanical behavior of magnesia refractory with CaO/SiO₂<1 containing 0-12 wt% CaZrO₃ clinker was investigated after sintering at 1700°C for 6 hr. Results show that CaZrO₃ additions improve sintering behavior of magnesia with formation of monticellite. Samples containing CaZrO₃ have a higher strength at room temperature while their hot strength substantially decreases.Thermal shock is improved by the addition of CaZrO₃ due to the decrease of the thermal expansion coefficient.

Abstract

Mullite formation by reaction at high temperature of kaolinite with two types of alumina (aluminium hydroxide gel and reactive alumina) was investigated in the presence of three types of additives containing monovalent, divalent and trivalent cations i.e, MgO, Cr₂O₃ and TiO₂. Role of these additives on product development was studied by bulk density, apparent porosity, true density, compressive strength measurements and XRD analysis of the sintered compacts. It was observed aluminium hydroxide gel containing samples to densify better and to contain more mullite after firing. Among the additives, MgO was found to influence the densification process to the maximum extent.

Abstract

The role of magnesia and iron oxide additives on the synthesis and properties of aluminum titanate formed from reactive alumina and rutile powders was investigated. For this purpose the precursor powder mixtures containing MgO and Fe₂O₃ additives were compacted and sintered at 1300°C, 1400°C and 1500°C. Different properties of the sintered samples e.g. firing shrinkage, bulk density, apparent porosity, specific gravity, phase composition and microstructure were studied. It was observed that the simultaneous use of MgO and Fe₂O₃ additives facilitates the formation of aluminium titanate and its densification.

Abstract

The resistance to deep abrasion of porcelain stoneware tiles (standard ISO 10545-6) corresponds to a model of tribological behaviour for threebody abrasion, based on mechanical characteristics (hardness and fracture toughness) and wear conditions (applied load, contact area and size of abrasive particles).The mechanical properties are affected by the total porosity (2-8%) of porcelain stoneware; thus there is a certain dependence of the material removed by abrasion on the pore volume, that however does not explain the entire variance of the measurements of wear resistance.The phase composition plays a fondamental role and a simple predictive model has been developed, indicating that the larger is the content of mullite, zircon and quartz, the higher is the resistance to deep abrasion. In particular, mullite and zircon ensure the main contribution to the tribological properties, owing to both their hardness and a toughening mechanism originated by differences of thermal expansion and elastic moduli between glassy and crystalline phases, inducing a compression at grain boundaries. Other crystalline phases, such as quartz and corundum, have a less relevant effect on the wear resistance, for the unfavourable differences of thermal expansion with the vitreous matrix.To improve the resistance to deep abrasion of tiles, it is possible to act on the body formulation, in order to increase significantly the amount of "hard" phases, mainly mullite and zircon, without altering the amount of residual porosity, that influences heavily both hardness and fracture toughness of porcelain stoneware.

Abstract

An attempt has been made to develop a “Singular Blended Clay” (SBC) having plastic properties and dry strength required for the production of vitreous sanitary porcelain and tableware. A pre-determined mixture of a crude china clay, plastic ball clay and a plastic fire clay from Indian origin were crushed, blended together and impurities removed by wet processing including gravity settling, screening, deironing and filter pressing. The physico-chemical properties of feed raw materials have also been studied with emphasis on singular blended clay and its influence on the thermo-mechanical properties and microstructure of vitreous sanitary porcelain compositions. Progressive substitution of singular blended clay up to 60 wt % leading to almost complete replacement of traditional crude clays resulted in an increase of flexural strength (~ 32%) and a decrease in thermal expansion (~ 22.7%) due to decrease in residual quartz content in the compositions. The addition of SBC had although little effect on the mullite content of the fired bodies but decreased the vitrification temperature by 50°C and also enhanced the vitrification range.

Abstract

A municipal sewage sludge (MSS) from a traditional wastewater treatment plant and a paper mill sludge (PS) were incinerated and transformed into powders which were blended in different proportions to give powders with different composition. Blendings were performed by attrition milling in order to reduce the milling time with respect to the traditional wet milling process. Powders were then pressed into specimens which were submitted to thermodilatometric tests to evaluate their shrinkage and their softening temperatures on heating up to 1350°C. Samples were then air sintered by means of a muffle furnace and characterized by density, strength, hardness, fracture toughness measurements and XRD and SEM investigations.The mechanical properties of the sintered specimens are satisfactory for all compositions and this may be due to the formation of small grains that, in some cases, are surrounded by a vitreous phase. One binary composition (75% MSS and PS 25%) showed a strength over 150 MPa.A mixture containing 33 % wt MSS, 33 % wt PS and 33 % wt steelworks slag (SS) was also prepared, processed and characterized like the other blends. All sintered samples were finally aged in an acidic (HCl) water solution to evaluate the elution of the components. It was observed that it is possible to limit the release of many of the most hazardous metals contained in the starting powders by an appropriate sintering schedule.

Abstract

A new technology aimed at protecting Torpedo Car refractory lining through the Formation of a Protective Layer - FPL - was developed.The technique consists of doping the process slag, thereby leading to an increased resistance of Torpedo Car refractory lining wear. Preliminary laboratory simulations were run, with a view to streamlining slag doping elements concentration, followed by trials in a rotary oven to simulate Torpedo Car refractory wettability by means of doped slag. As a result, the FPL technology was implemented on an industrial scale, thus helping protect CSN Torpedo Car refractory lining.

Abstract

High performance composite fibre or platelet reinforced ceramics were widely expected to have been implemented into a relatively wide range of industrial applications but did not achieve the expected potential whilst for general high temperature refractory applications the vast majority of products have very anisotropic microstructures although they may actually behave as bulk isotropic. Finite element modelling of these complicated 3 dimensional microstructures is relatively difficult due to the very large model size and obtaining accurate material property data.This paper demonstrates the use of advanced parametric finite element analysis modelling techniques of composite ceramics and structures for industrial applications. The accurate modelling of these composite materials and related industrial refractory structures in a reasonable time scale would drastically reduce the materials technology development cycle for existing and new high temperature refractory applications compared to the historically empirical materials and industrial application development cycle.An industrial case study of this approach is demonstrated.