Minimization of hexavalent chromium in magnesite-chrome refractory

The fundamentals on the formation of Cr6+ have been studied from two points of view: thermodynamic and kinetic. Thermodynamically, the CaO-Cr2O3 phase diagram can be used as a guide to understand the formation of Cr6+ as a function of temperature and composition. The Cr6+ content, in the CaO-Cr2O3 phase diagram, increases with exposure to temperatures below 1022 degrees C and with an increase in CaO (from 0 to 42 pet CaO). In kinetics, the chromite phase size also plays a major role in the formation of Cr6+. A decrease in chromite phase size increases the Cr6+ content in the refractory. The use of fused grains also decreases the formation of Cr6+. In the case of magnesite-chrome refractories, temperature, basicity (CaO/SiO2), and chromite phase size play important roles in Cr6+ formation. The formation of Cr6+ can be minimized by carefully controlling the amount of calcium oxide in the refractory and by avoiding the use of a fine chromite phase during brickmaking.