PHASE EQUILIBRIA IN THE SYSTEM MgO-Mn2O3-MnO-CaSiO3 IN AIR

The Mg0-Mn203-Mn0 system in air exhibits two series of solid solutions, (Mg,Mn)Mn204 and (Mg,Mn)O, the boundaries of which have been located. At 1600 +5' C a tie-triangle exists in which these two solid solution phases and a liquid phase coexist with the gas phase. The Mn3+/Mn2+ ratio of the crystalline phases is lowered by the addition of CaSi03 and this has the effect of stabilising the (Mg,Mn)O phase at lower temperatures. I INTRODUCTION Magnesia (MgO) refractories have increased in importance in recent years mainly because of their high refractoriness, resistance to iron oxide attack, alkalies and high lime fluxes. Natural magnesite (MgC03) is still a major source of raw material for magnesia-based refractories despite the increasing production of sea-water and brine well magnesias of high purity. One of the largest deposits of refractory grade magnesite is reported to be in Brazil /1,2/ where different grades of dead-burned magnesias of low impurity content are produced by enriching the run-of-mine ore followed by a single or double firing process. One of the impurities present in these natural magnesites is MnO (up to % 0.8 wt.%) and in this paper we describe the results of a study of phase equilibria in the Mg0-Mn203-Mn0 system and the effect of small additions of CaSi03 (CS) on that system. The phases formed depend ontheoxidation state of the transition metal which is a function of temperature and oxygen partial pressure (Po2). The work was carried out under constant Po2 equal to 0.021 MPa by opening the system to air. I1 LITERATURE In the Yi-0 system the oxides m02, ~ n ~ o j , MngO4 and MnO exist as stable phases and can be converted from one form to another by carefulchoice of oxygen partial pressure and temperature /1-5/. The region Mn20-3-MnO is of principal interest to this study and brief mention of results reported in that region will be made. MnpOj is reported to covert in air to Mn304 at 877'~ /3/ and 871°c /1/ but the conversion back to Mn203 was found to be much slower than the reduction reaction. The melting (1) Magnesita S.A., 30.000 Belo Horizonte MG, Brazil. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986168