Waste reduction and recycling of hexavalent chromium bearing waste oxide

The purpose of the present work is to study the formation of Cr 6+ and the reduction of Cr6+ to Cr3+ or Cr in several oxide/metal systems under various oxygen pressures and temperatures. The findings from this study can be used to minimize the formation of Cr 6+ during the production or use of chromium oxide bearing refractories.; In the magnesite-chrome refractory (MgO-Cr₂O₃-CaO-SiO ₂-Al₂O₃-Fe₂O₃ system), the formation of Cr6+ is mainly due to the reaction between CaO and Cr₂O₃. This work has found that: (i) a decrease in temperature of the magnesite-chrome refractory increases the formation of Cr6+, because Cr6+ is stable at low temperatures; (ii) an increase in the ratio of CaO/SiO₂ increases the content of Cr6+; (iii) Cr6+ forms at the interface between the calcium silicate and chromite; and (iv) an increase in chromite particle size decreases the content of Cr6+, because large chromite particles have a smaller surface area per unit volume for the formation of Cr6+ to take place.; The Cr6+ also forms by the interaction of slag systems with chromite (Cr₂O₃) in magnesite-chrome refractory. This investigation has found that: (i) calcium aluminate slag reacts with chromite to yield higher Cr6+ than calcium silicate slag, because calcium aluminate slags have a higher amount of uncombined CaO that can react with chromite to form Cr6+; (ii) the addition of SiO₂ to calcium aluminate slags decreases the level of Cr 6+; (iii) the role of CaF₂ on the formation of Cr 6+ is similar to CaO in slag; and (iv) an increase in cooling rate decreases the content of Cr6+ because a fast cooling rate limits the kinetics of the formation of Cr6+.; The standard free energy for CaCrO₄ was also determined and assessed in this work because CaCrO₄ is the only stable Cr 6+ containing compound formed at low temperatures under atmospheric conditions. The temperature of formation of CaCrO₄ estimated from the free energy of CaCrO₄ measured in this work is in good agreement with the value reported in Kaiser's CaO-Cr₂O₃ phase diagram [14].; It was also found that the oxidation states of chromium depend on oxygen partial pressures at a given temperature. The major oxidation state of chromium is trivalent at 1560°C and oxygen partial pressures from 10−6 atm. to 10−8 atm. Furthermore, Cr6+ is reduced to Cr by silicon dissolved in iron alloy. When a chromium ion reacts with silicon at the liquid metal/slag interface, the overall rate of reduction of the chromium ion to Cr is controlled by mass transfer of the chromium ion through the slag boundary layer. (Abstract shortened by UMI.)...