Issue No. 3, December

Abstract

A method has been developed to produce mullite from Trinidad porcellanite. Involving high temperature calcination followed by aqueous leaching of kalsilite with caustic soda, the process does not generate solid waste and is projected to yield some 0.65 kg of mullite mixed with silica and hematite per 1 kg of porcellanite. Depending on the concentration of the caustic soda used (in the range 200-500 g/l), the proportion of mullite, silica and hematite range between 49-53%, 42-44% and 5-7% respectively. The mullite agglomerated crystals are of mixed acicular and equiaxed morphology.

Abstract

In this study the use of the kaolinite, which contains very low levels of pyrophyllite, as a ceramic tiles material was studied. Chemical, mineralogical analysis and thermal behaviours of the clay were determined. After analysing the chemical, mineralogical content and thermal behaviour of the clay, the shaping properties were determined according to Atterberg and Pfefferkorn plasticity tests. The samples that had been shaped with vacuum extrusion were fired at between 600 °C to 1200 °C with 100 °C internals. Mineralogical, physical-mechanical properties and microstructure of fired samples were determined. As a result, it was seen that the grain size of the clay and its plasticity values are suitable for ceramic tile and formation of secondary mullite as well as lamellar shaped pore structure at low temperatures (1100 °C) has positive effect on mechanical properties. It was determined that ceramic tile can be produced at 1000 °C for Group AIIb and at 1100 °C for Group AIIa according to Turkish Standard (TS EN ISO 14411). The colour and aesthetic appearance of the fired products correspond the market demands with smooth surface.

Abstract

Aim of this study was to investigate the relations between chemical composition and firing behaviour of porcelain stoneware formulations. The effects of magnesia and/or calcia incorporation in varying amounts into a standard stoneware formulations through combined use of calcite and magnesite on firing behaviour was studied by means of a non-contact optical dilatometer. It was shown that suitable combination of magnesite/calcite noticeably lowered peak firing temperature without narrowing the firing range. Peak firing temperatures of these bodies were also at commercially acceptable levels. Further formulations prepared with calcite, kaolin and a local clay high in iron oxide content were also shown to give acceptable chromatic co-ordinates. It was also possible to develop bodies with high whiteness values without employing zircon.

Abstract

Generally, the deformations of ceramic materials during and after firing may have a complex origin. If the products are made up of a single material, such deformations are mainly due to pyroplastic phenomena. In the case of glazed materials, two further factors must be considered: the state of tension established between glaze and body, and their differences in behaviour during sintering. A novel optical technique is described to measure the state of tension of ceramic glazed bodies and applied to the study of delayed crazing and pyroplastic deformation of the body. This article outlines how can ceramists avail themselves of new optical equipment in order to design high quality products and avoid ordinary deflects. Combining dilatometric tests and bending analysis a full study of residual stresses on glazed ceramic can be performed therefore this is how planarity problems can be solved. Furthermore the Optical Fleximeter MISURA® is able to get information about viscosity changing during firing process in porcelain stoneware tiles: study of pyroplastic behaviour.

Abstract

Turkey is the fifth biggest ceramic tile exporter of the world. Considering the expectation that the Turkish tile sector continues to expand, further studies both on the improvement and the development of the tile formulations are necessary in order to make the producers advantaged in the world market while maintaining the capital and running costs at reasonably competitive levels as well. In addition to such studies, waste management in tile manufacturing has also become an important issue. Accordingly, the industrial use of wastes in the traditional ceramic industry has been intensively investigated in recent years. The present paper describes a research project carried out on the possible re-use of filter-pressed mud (sludge) from waste water treatment plant of a local tile company in commercial wall, floor and porcelain tile formulations. In order to achieve this aim, firstly the complete characterisation of the mud was carried out. Then, the particular focus was given to the effect of the waste material in increasing amounts (up to 10 wt. %) on the thermal behaviour and physical and technological properties of the re-formulated tile bodies. During the studies, a range of characterisation techniques, namely XRF, XRD, SEM in combination with EDS, DTA, TGA and non-contact dilatometry, were employed. The results showed that it was possible to use the waste material in tile production without deteriorating the relevant technological properties of the dried and fired bodies.

Abstract

Refractory castables containing alumina-magnesia/self-forming spinel (MgAl₂O₄) are used in impact pads of steel ladles in steelmaking processes. In order to understand the wear mechanisms of refractory materials, several recipes were tested from a corrosion, slag resistance and thermal shock point of view. The results show that the corrosion is extremely limited at the interface slag/refractory for all cases. Nevertheless, for higher cement alumina content castables, the formation of micro cracks is observed in refractory castables into which slag deposit can penetrate. The slag reacts with alumina to form a new phase such as hibonite (CA6) and calcium dialuminate (CA2). The expansive reactions involving CA2 and CA6 lead to macro-cracks. Thus, the penetration of slag and steel are increased causing hot mechanical properties to degrade. For lower cement alumina castables, the formation of micro cracks is avoided by controlling volume expansion. Thus, the slag deposit reacts with alumina grains and the matrix at the slag/refractory interface to produce a monomineral layer of hibonite. In this way, the monomineral layer acts as a barrier and limits the penetration of slag and steel into the refractory lining. Thus, to increase the life time of refractory castables containing alumina-magnesia/self-forming spinel, it is advisable to control volume expansion in order to avoid the formation of cracks and to limit the penetration of secondary metallurgy steel ladle slag.

Abstract

The aim of the present work is to prepare and assess developed LCC and ULCC refractory alumino-silicate castables containing 60-99% alumina as compared with traditional RCC and MCC types within the CaO-Al₂O₃-SiO₂ system. The materials used were graded calcined kaolin (grog) and bauxite as well as tabular alumina grains as aggregates. Also, mixtures of 50 or 70% Al₂O₃ calcium-aluminate cement (CAC), ultra-fine calcined alumina (CA) and water-reducing agents (WRA) were used as binding matrices. It is shown that the addition of 0.20% of a combined admixture as well as increasing CA of both the CAC-Secar- 51 and 71 binding batches generally lead to a gradual decrease of water of consistency (WOC) as well as initial and final setting times of their pastes. These batches were applied as binding matrices for the traditional and developed castables with minimum water contents. The developed ULCC and LCC types show generally limited linear change and reduction the cold crushing strength after preheating up to 1000 °C as compared with the traditional MCC and RCC types. After preheating up to 1600 °C, the developed types also give higher bulk density with accepted cold crushing strength. Highly refractory developed LCC and ULCC corundum castables with 97-99 % total alumina as well as minimum contents of CAC₇₁ (3-6%), WOC (6-7%) and glassy phase were prepared and recommended for application at ≥ 1600 °C. Similarly LCC and ULCC sillimanite (60-65% Al2O3) and bauxite (85-88% Al2O3) castables with relatively higher WOC (7-11%) and lower refractory grade were also prepared and recommended for application up to only 1500 °C.

Abstract

Four sets of precursors of aluminium and magnesium compounds have been taken to prepare magnesium aluminate (MgAl₂O₄) spinel powders. Among them the most cost effective route has been selected to synthesize spinel at significantly lower temperature than needed for solid state synthesis. The evolution of spinel has been evaluated and confirmed by XRD, IR, SEM (EDS), DTG, DSC studies. The pore size distributions in fired pellets were characterized to examine their suitability as humidity sensors. Finally the capacitance values at different humidity levels have been measured to ensure their commercial suitability.