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| Plešingerová B., Kriššaková S., Števulová N., Trpčevská J. |
| CHARACTERISATION OF MA-SPINEL OBTAINED BY SOLID-STATE SYNTHESIS FROM POWDERY MG AND AL OXIDES AND HYDROXIDES |
Abstract
Magnesium aluminate spinel is widely used as a refractory raw material. In common spinel materials are produced by solid-state reaction between MgO and Al2O3 at high temperature (above 1500-1800°C). The manufacture of spinel is a much energy and time-demanding process.
This study is a first step to the investigation of the solid-state synthesis of the spinel to be prepared by mechanochemical treatment and activation reaction of mixtures of raw materials (oxide and hydroxide) by intensive grounding. An investigation was done to evaluate different methods, which may be used to the determination of conversion to spinel and its properties.
Spinel samples were prepared from four binary powder mixtures: Al2O3 + MgO; Al2O3 + Mg(OH)2; Al(OH)3 + MgO a Mg(OH)2+ Al(OH)3. The prisms 60x10x ? 5 mm were uniaxialy pressed (40MPa) from the homogenous mixtures then have been fired at a temperature of 1500°C for 4 hours. Parameters like the degree of conversion to spinel, phase composition, morphology and densification, were tested on the obtained reaction products. Conversion to spinel was determined by the titration analytical method (after leaching of the powder samples in hydrochloric acid was determined the concentration Mg2+ in the solution) and X-ray diffraction analysis.
The results of conversion to spinel determined by titration are 0,9-0,98. The time of leaching, 15 minutes, is sufficient to the dissolution of the free, MgO not entered into reaction in the sample of the spinel. These small differences in the composition of products are not detected by the X-ray analysis. The figures from the electron microscopy show that the growth of the particles is significant during the synthesis. It corresponds to the decreased specific surface and increased volume mass - density of the block materials after synthesis. The mean value of the thermal expansion coefficient over the interval 600-1200°C (? (600-1200°C) ~ 8,5. 10-6 K-1) of the porous spinel blocks is similar, a little lower than the value mentioned in the literature.
The determination of the conversion by titration method (Mg2+ after leaching of the spinel product in acid) is very accurate. This method is applicable at the study of the kinetics of the solid-state reaction. However, all the other tested parameters of the spinel material i.e. specific surface, morphology, dilatation, strength and density are important for the characterisation of the spinel material.
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| Keywords: MA-spinel|solid-state synthesis|conversion to spinel|mechanochemical reaction| |
| No 4 (1999), p. 257-266 |
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