Sintering Studies on Magnesia-Rich Chromium-Free Spinel-Bonded Basic Refractories

摘要

Basic bricks with Cr_2O_3 from chrome ore, as the spinel forming oxide, are used in the non-ferrous industry because of their corrosion resistance against fayalite-type slags, rich in FeO. Our objective in this study was to replace Cr~(3+) with Me~(4-) ions, which along with Fe~(3+) could maintain the spinel formation capability with MgO and perform similarly against fayalite slags in non-ferrous furnaces. Our preliminary research studies showed that Cr-free spinels in the MgO-Al_2O_3-FeOx-Me~(4+)O_2 systems could perform against fayalite slags similar to the complex (Mg~(2+), Fe~(2-))O-(Cr~(3-), Fe~(3+), A1~(3+))_2O_3 spinel, the main corrosion resistant component in the magnesia-chrome bricks. The incorporation of iron oxide in the MgO-Al_2O_3-Me~4 O_2 systems would contribute to reactive sintering and also in decreasing the solubility of both the ferrous and ferric ions present in the fayalite slag. Phase analysis on stoichiometric mixes showed that the use of tetravalent cation oxides like tin dioxide (SnO_2) and titanium dioxide (TiO_2) can induce high solubility of spinel in magnesia. In order to maintain charge balance, two trivalent cations were replaced by a tetravalent and a bivalent cation causing the additional bivalent cation to occupy the octahedral position thereby creating an inversion in position of the bivalent ions similar to the behaviour exhibited by Fe~(3-) occupying tetrahedral site in complex spinel phase of magnesia-chrome ceramics. Most of the magnesia-chrome refractories have -60 wt. % MgO and hence our experimental mixes contained that amount and called "magnesia-rich" compositions, to be distinguished from the stoichiometric MgAl_2O_4 spinel. Our findings showed that the incorporation of nano TiO_2 powders reduces the temperature of spinel formation as the diffusion path is shortened and thus activates both synthesis and sintering. Compositions containing 60 wt. % magnesia with alumina, nano TiO_2 and Fe_2O_3 fired below 1500°C for 3 hours resulted in complete spinel formation and open porosity less than 5%.