Study of alumina-magnesia binary phase diagram reveals that around 40-50 wt% alumina dissolves in spinel (MgAl 2 O 4 ) at 1600°C. Solid solubility of alumina in spinel decreases rapidly with decreasing temperature, which causes exsolution of alumina from spinel phase. Previous work of one of the authors revealed that the exsolution of alumina makes some interlocking structures in between alumina and spinel phases. In the present investigation, refractory grade calcined alumina and spinel powder were used to make different batch compositions. Green pellets, formed at a pressure of 1550 kg cm -2 were fired at different temperatures of 1500°, 1550° and 1600°C for 2 h soaking time. Bulk density, percent apparent porosity, firing shrinkage etc were measured at each temperature. Sintering results were analysed to understand the mechanism of spinel-alumina interactions. SEM study of present samples does not reveal the distinct precipitation of needle shaped α-alumina from spinel, but has some effect on densification process of spinel-alumina composites. Microstructural differences between present and previous work suggest an ample scope of further work in spinel-alumina composites.

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Precipitation of magnesium aluminate hydrate with faster addition of ammonia at desired pH causes agglomeration. Agglomerated powder, without any further treatment, on calcination forms intermediate compounds at low temperatures (≤ 900°C). The intermediate compounds on further heat treatment (≥ 1000°C) decompose into MgO, MgAl 2 O 4 and α-Al 2 O 3 . Effect of agglomeration and absorption of foreign ions such as Cl -, SO 2 4-, and NH + 4 in complex compounds probably cause loss of Al 3+ and Mg 2+ ions during heat treatment, and stoichiometry changes. Powders prepared by continuous method with better control of process parameters than batch process yields better spinellization.

Graphite has extraordinary properties, viz. high corrosion resistance against liquid metals and slag s, good spalling resistance and impact on reduction of porosity and finer pore size distribution in oxides -carbon composite refractories. However, it has poor oxidation resistance which restricts its use i n pyrometallurgical operation and gives poor campaign life. Different antioxidants such as Al, Si, Al/M g, B 4 C, SiC, etc have been used in commercial production of carbon bearing composite refractories for improvement of their oxidation resistance. Coating of graphite with oxides such as Al 2 O 3 , SiO 2 , TiO 2 , ZrO 2 , etc has been the recent trend of R & D work to improve the oxidation resistance of graphite in composite refractories. In the present investigation, graphite powders have been coated with MagnesioAluminate Hydrate (M AH). The coated powders were characterized by TGA/DTGA, DTA and calcinations at 1000°C to study their oxidation loss. Liquid resin was also specially treated with M AH. Both MAH treated graphite and resin were used to make M gO-C composites. Present work reveals remarkable improvement of oxidation resistance of both graphite and M gO-C composites when graphite powders were coated with M AH. IntroductionGraphite has excellent refractory properties that include its non-wetting behaviour and high chemical inertness against liquid metal and slag, extreme low value of coefficient of linear expansion and high mechanical strength at elevated temperatures. Combined refractory properties of oxide and graphite make oxide-graphite composite refractories more corrosion resistant and spalling resistant in pyrometallurgical applications. Among oxide-graphite refractories, MgO-C refractories are the most suitable in steel melting processes, particularly in converter lining.The wear mechanism of MgO-C refractories is caused due to the following reasons : a) Oxidation of carbon that is also classified as oxidation of O 2 and CO 2 in ambient gas (gas phase oxidation) and oxidation by iron oxides in slag (liquid phase oxidation). b) Dissolution of MgO aggregates by slag. It is dependent on the crystal size and purity of the aggregates, nature of impurities and their distribution as mineral phases in the aggregates. c) Oxidation-reduction reaction between the brick components (i.e. MgO and carbon at high temperatures near hot face of the bricks), which causes structural degradation. d) Physical properties of composite bricks, such as brick porosity during use (i.e. coked porosity), its pore size distribution, hot strength, etc. e) Mechanical stress during use.

The sintering and microstructural evaluation of Indian magnesite was carried out in presence of zirconia. Zirconia in monoclinic form was added in the range 3-6 wt% with respect to raw magnesite and the sintering temperature selected were 1500-1600°C for 2 h. The main impurities present in the magnesite were Fe 2 O 3 , CaO, SiO 2 . The natural crystalline magnesite could be sintered with bulk density of 3⋅38 g/cc (A.P. 1⋅54%) at 1550°C/2 h. But the higher bulk density (3⋅50 g/cc) and minimum apparent porosity (A.P. 0⋅25%) was attained at 1550°C/2 h with the 3 wt% zirconia additive. On firing magnesite with zirconia as additive, a crystalline phase, magnesio-zirconate, was identified at the triple point regions of the direct bonded periclase grains. The morphology of the periclase grains were changed from subrounded/rounded to angular shaped in presence of zirconia as additive.