Redox Equilibria of Chromium in Calcium Silicate Base Melts

  • PDF / 536,969 Bytes
  • 11 Pages / 593.972 x 792 pts Page_size
  • 104 Downloads / 236 Views

DOWNLOAD

REPORT


INTRODUCTION

CHROMIUM can be present in slag melts, as it is in solid phases and aqueous solutions, in three valences, namely, in divalent (Cr2+), trivalent (Cr3+), and hexavalent (Cr6+) states.[1–8] Several studies have been carried out on the redox reactions between Cr2+ and Cr3+. It has been established that, in the low-oxygenpressure range of the slag/metal equilibrium, chromium is mainly divalent. But the data deviate and several influences are not clear, so that more measurements would be useful. On the other hand, little is known about the reactions involving hexavalent chromium. Hexavalent chromium forms under oxidizing conditions.[1–3] Phases containing hexavalent chromium are found in electric furnace dust from stainless steel making. There is much interest in this matter, because the hexavalent chromium makes toxic substances that may leak into groundwater if the dust is stockpiled, dumped, or used as landfill. We have determined the oxidation state of chromium in calcium silicate base melts with selected CaO/ SiO2 ratios as a function of oxygen pressure at 1600 C and 1430 C. The investigation was confined to the range of low chromium content levels in the slags from approximately 0.1 to 2 mass pct, which is the level in the final slags of stainless steel making. The results are reported.

A.-M. MIRZAYOUSEF-JADID, Scientist, and KLAUS SCHWERDTFEGER, University Professor Emeritus, are with the Institut fu¨r Metallurgie, Technische Universita¨t Clausthal, 38678 Clausthal-Zellerfeld, Germany. Contact e-mail: [email protected] Manuscript submitted July 29, 2008. Article published online May 27, 2009. METALLURGICAL AND MATERIALS TRANSACTIONS B

II.

GENERAL EXPERIMENTAL PROCEDURE

Oxide samples of 1.5 to 6 g were melted in platinumrhodium crucibles and were equilibrated with the gas phases of a controlled oxygen pressure. Two techniques were applied to determine the oxidation state of the chromium: thermogravimetry and the quenching of samples with subsequent chemical analysis. A Tammann furnace was used. A gas-tight alumina tube was inserted into the carbon tube of the furnace; this was connected at its upper end either to the thermal balance or to a water-cooled brass head for quenching. The temperature was controlled electronically within ±2 C via two Pt/ Rh-18 thermocouples. The gas mixtures were composed with capillary flow meters. The starting gases were high-purity argon, oxygen, carbon dioxide, and carbon monoxide. Further purification (from moisture with phosphorus pentoxide and from oxygen with a heated copper column and an oxisorb cartridge) was carried out in the usual manner. The oxide samples were prepared from reagent-grade CaCO3, SiO2, Al2O3, and Cr2O3. The materials were dried at 100 C before weighing and then mixed by grinding in an agate mortar. The mixtures for the quenching method were heated to 900 C under air prior to the equilibration experiments, to decompose the carbonate. In the thermogravimetric method, the mixture was suspended in the furnace without prior calcination under ai