Densification and properties of magnesia-rich magnesium-aluminate spinel derived from natural and synthetic raw materials

Magnesia rich magnesium aluminate spinel (MgO: Al2O3=2:1) was developed by reaction sintering of Indian natural magnesite of Salem region as well as from synthetic caustic magnesia with calcined alumina. Dilatometric study of the green compacts was carried out to evaluate the spinelisation and sintering behaviour of both the samples. Green samples were heat treated between 1400 to 1600 degrees C and characterised in terms of densification behaviour, high temperature flexural strength, microstructure and phase development. Spinel and periclase are the major phases in both the samples, where as forsterite is found only in the sample developed from Indian magnesite due to presence of silica as impurities. (C) 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved

Effect of ZrO2 on the densification behavior and properties of Indian magnesite

Natural magnesite is the primary source for magnesia-based refractory materials. India has vast deposits of magnesite in Salem and Almora regions. However, due to the presence of large amount of impurities which forms low-melting compounds at elevated temperature, its high-temperature application is restricted. Raw magnesite was evaluated in terms of chemical analysis, differential thermal analysis, thermogravimetric analysis, and phase assemblage. Zirconia (1-5 wt %) was added to Indian natural magnesite of Salem region to minimize the low-melting phase formation at high temperatures. Samples were sintered in the temperature range of 1550-1700 degrees C. Sintered samples were characterized in terms of densification, mechanical and thermo-mechanical properties, phase assemblage, and microstructure. It was found that the addition of zirconia reduced the formation of detrimental phases like monticellite and thereby improved the high-temperature mechanical properties

Problematic aspects of the consequences of infringement in criminal procedure

When the criminal procedure ends, pretrail investigators, prosecutors and judges usually don’t excite about the consequences of the proceedings, which they made in the finished case. Usually officers associate such consequences only with their own disciplinary responsibility. But disciplinary responsibility of the concrete officer is not the only consequences of illegal proceedings. Systemic analysis of the procedural and other legal acts lets to the Author to distinguish four main groups of the consequences of illegal proceedings: 1) criminal or disciplinary responsibility of the officer; 2) data, collected in the pretrail investigation, will not be admitted as evidence; 3) consequences which eliminate infringement from the further criminal procedure; 4) duty of the state to compensate person’s harm, which he experienced by illegal proceedings. In the article Author notes that investigators and other officers usually are not informed about the court’s decisions, which are related with the concrete procedural infringement. Such situation doesn’t let to improve officers qualification and also let’s to reiterate the same procedural infringements. Another problem of the analyzed theme is related with the legal regulation of the compensation to the person for the harm, which he experienced by illegal proceedings. According to the Lithuanian laws, person acquires the right to require such compensation only if he experienced the harm by the illegal temporary detention, arrest, other procedural coercive measures, or by the illegal conviction. Such situation is contrary to the international treaties and general principles of the law. That’s why Author suggests to consider possibility to change such present legal regulation, refusing an exhaustive list of procedural infringements and providing person’s right to require compensation in all cases if he experienced harm by illegal proceedings

Rationalizing the role of magnesia and titania on sintering of alpha-alumina

The rationalization of selection of sintering additives for alpha-alumina was investigated using two oxides (MgO and TiO2) to discern their individual roles. Using both dynamic heating study in a thermo-mechanical analyzer and static heat treatment, the precise role of each oxide was established. Grain growth trajectory of different doped samples sintered at 1700 degrees C revealed that MgO neither significantly affected densification nor facilitated grain growth upto 1700 degrees C. MgO reacted with alumina to form spinel prior to the densification process. Thus it could not generate further extrinsic defects in corundum lattice during sintering, which usually facilitate densification. In contrast, TiO2 significantly enhanced the densification and promoted grain growth in alpha-alumina. At 1700 degrees C, the average grain size of titania doped samples were 7.7x larger than undoped ones and 10x larger than magnesia dopes samples. The sintered grains developed higher aspect ratio when TiO2 was used which may be ascribed to preferred growth of the 012 and 024 planes of corundum. The nearly perfect junction of grain boundaries meeting at similar to 120 degrees indicates absence of liquid phase and that the entire sintering process most probably took place in solid state for both MgO and TiO2 doped samples. (C) 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved

Magnesium Aluminate Spinel: Structure, Properties, Synthesis and Applications

Magnesium aluminate (MA) is a spinel group of material and rarely available in nature. It exhibits several advantageous thermal, thermo-mechanical, optical, chemical properties which renders it suitable for wide gamut applications starting from refractory to sensor to IR transmitting window. It has face centered cubic structure and has wide solubility to its end members at elevated temperatures. As a result of this solubility, non-stoichiometric compositions exist in the phase field of spinel in the phase diagram. Due to its unavailability in nature, MA spinels are synthetically prepared through different routes and using different starting materials. Among these, solid state reaction sintering or conventional mixed oxide (CMO) method is the most techno-economical viable process. However, the challenge lies with the preparation of dense MA spinel from its oxide precursors in a single stage process is the expansion due to spinellization. Several attempts have been made to overcome this deterring factor through improving the reactivity of the precursors, by controlling the processing parameters, or by using the mineralizers. In this paper crystal structure, stoichiometry in spinel composition, mechanism of spinel formation, different synthesis method, properties and applications are reviewed

Refractories of alumina-silica system

The inorganic phases belonging to the binary alumina-silica system exhibit a plethora of excellent high temperature properties that make them useful for refractory applications. Synthesized from abundant aluminosilicate minerals, these refractories are ubiquitous in high temperature industrial applications. The refractories of alumina-silica system can be engineered to generate a range of high temperature properties by varying the Al2O3/SiO2 ratio, presence of other oxides and texture. This versatility is unique to this system and is reflected in widely varied fields of applications of these refractories. Composed predominantly of mullite and corundum phases, the noncrystalline phases also play important role in determining the property and end use of these refractories. This review covers different theoretical and practical aspects of refractories of aluminosilicate system spanning 30-100% alumina. Important role of microstructure in aluminosilicate refractory has been discussed in light of phase diagram, raw materials and thermo-chemical reactions. The applications of these refractories in different areas have been discussed in detail with structure-property correlation

Effect of Yb2O3 and TiO2 on reaction sintering and properties of magnesium aluminate spinel

Magnesium aluminate spinel with an initial MgO: Al2O3 molar ratio of 2:1 was prepared from its constituent oxides through a solid-state sintering process at temperatures ranging from 1550 to 1700 degrees C in a normal air atmosphere. The effect of varying amount (0.25-1.0 wt%) of TiO2 and Yb2O3 on densification, phase assemblage, mechanical, thermo-mechanical properties and microstructure of magnesia-rich spinel were investigated under static heating condition. The addition of TiO2 and Yb2O3 favours the densification of magnesia-rich spinel, which is discernible up to 1650 degrees C. This beneficial effect may be attributed to the development of the secondary phase and formation of solid solution due to the dissolution of the additive ions in the spinel structure. A marginal increase in the average grain size of the samples along with a narrower grain size distribution occurred with the incorporation of both the additives. Both the additives improved the mechanical properties of the magnesia-rich spinel; however, better room temperature flexural strength was achieved with Yb2O3 as compared to TiO2 addition. For the samples sintered at 1550 degrees C, 1.0 wt% Yb2O3 addition resulted in 30% increase in flexural strength; however, same amount of TiO2 addition increased the strength by 20%. In case of thermal shock resistance, 1.0 wt% TiO2 and 0.25 wt% Yb2O3 addition demonstrated promising result among all the samples

Reverse Flotation of Natural Magnesite and Process Optimization Using Response Surface Methodology

Indian natural magnesite containing silica and lime as main impurities was beneficiated using reverse froth flotation technique. Pine oil was used as frother, Flotigam EDA as collector and sodium hexametaphosphate as depressant of carbonate group. Operating parameters were optimized using response surface methodology and a quadratic model equation was formulated for the experiment. The effect of different process parameters was studied using Box-Behnken design. Statistical analysis suggested that the model was significant. Moreover, optimum process conditions were predicted after analyzing the experimental data. The beneficiated sample for which the highest silica in froth was achieved was characterized in terms of X-ray diffraction analysis. Further, quantification of the crystalline phases showed that amount of quartz was reduced from 2.3% in raw sample to 1.8% in processed sample

Kinetic analysis of magnesium aluminate spinel formation: Effect of MgO:Al2O3 ratio and titania dopant

The mechanistic pathway of MgO-Al2O3 reaction in solid state to form MgAl2O4 spinel was investigated to correlate the kinetic parameters with ratio of reactants (MgO:Al2O3) and with the presence of a doping agent, TiO2. The time-temperature-expansion data of oxide compacts was analyzed using several model free analyses and model based (linear and non-linear) kinetic algorithms. These indicated that spinel formation process can be best described by single step with n-dimensional Avrami equation for every MgO:Al2O3 ratio, irrespective of titania dopant. The activation energy (Ea) of the process was proportional to % spinel formed in each system and validated with quantitative XRD analysis. The higher value of Avrami coefficient (n) in 90 wt% Al2O3 compositions has been explained with geometric considerations of powder packing. Incorporations of 1% TiO2 in the MgO: Al2O3 oxide compact did not markedly affect the reaction model, frequency factor and Activation energy

Kinetic modelling of solid state magnesium aluminate spinel formation and its validation

The kinetic pathway for magnesia-excess magnesium aluminate spinel (MgAl2O4) formation is determined from the dilation behaviour of MgO-Al2O3 (34:66 w/w) oxide compacts during reaction sintering under dynamic heating regime at different constant heating rates (beta). It is calculated from expansion values at different beta, analysed through model-free and model-based kinetic methods to arrive at the most suitable and unambiguous kinetic pathway, where spinelisation is best described (r(2)=0.986) with Avrami model with exponent n=0.65 and activation energy (E-a) 409 kJ mol(-1). To validate the kinetic model, an isothermal heat treatment of the oxide compact was done and % spinelisation was determined by Rietveld refinement of the XRD data. The results matched the predicted values very closely and validate our kinetic model. This method can be extended to several industrially important thermally activated solid state reactions which require thermal treatment in order to minimise the energy requirement by optimising the heating protocol. (C) 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved