Formation of hexavalent chromium by reaction between slag and magnesite-chrome refractory
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INTRODUCTION
MAGNESITE-CHROME refractories have been used in steel, copper, and glass industries. The service life of magnesite-chrome refractory varies with the process and severity of conditions to which it is exposed. Under normal industrial temperatures and oxidizing conditions (in air), Cr 3+, which is present in the refractory, can oxidize to Cr 6+ . Spent magnesite-chrome refractory is classified as a solid hazardous waste when it has over 5 ppm Cr 6+, because Cr 6+ is soluble in water and is known to be carcinogenic. Iron and steel industries are increasingly conscious of this problem due to more stringent environmental regulations.[1] Therefore, bigger efforts have been made to recycle and reduce solid wastes.[2,3] The factors that influence the formation of Cr 6+ in the magnesite-chrome refractory have been previously studied for the design of environmentally friendly, magnesitechrome refractory. They are chromite phase size, temperature, and basicity (CaO/SiO2).[4] This study will deal with the formation of Cr 6+ caused by the chemical reaction between several slags and chromite particles to simulate slag/refractory interactions in an industrial environment. Slag is formed during the steelmaking process, or added for a specific purpose, such as to thermally insulate a liquid metal bath, entrap inclusions and impurities, and protect a liquid metal bath against oxidation. The main components of slag are CaO, Al2O3, SiO2, MgO, and FeO. The final composition of slag is affected by the composition of metal and controlled to minimize refractory attack. The slag/refractory interaction can be seen in Figure 1, which is shown in the backscattered electron image of the interface between magnesite-chrome refractory and slags. If slag compositions are not carefully designed and controlled, slags can drastically dissolve the magnesia and Y. LEE, Graduate Research Assistant, and C.L. NASSARALLA, Assistant Professor, are with the Department of Metallurgical and Materials Engineering, Michigan Technological University, Houghton, MI 49931-1295. Manuscript submitted July 30, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS B
interact with chromite at the interface between slag and magnesite-chrome refractory to form Cr 6+. Cr 6+ is usually formed at the slag/chromite interface during cooling of a magnesite-chrome lined furnace (below 1228 7C[4]). It is well known that industrial furnaces experience heating and cooling cycles; for example, during the ladle refining process of liquid steel, the ladle experiences high temperatures (1600 7C). After teeming of the liquid steel into tundish, the ladle furnace is cooled to 1100 7C and then reheated for another cycle. In addition, some of the slag that is present to protect the liquid steel remains adhered to the wall of the ladle; this slag is usually rich in CaO. Unfortunately, lower temperatures and high CaO are ideal conditions to form Cr 6+. The information generated from this study has the potential to minimize the final amount of Cr 6+ bearing solid wastes by carefully
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