Analysis Of Chemical Valence Of Chromium In Cr₂O₃-containing Refractories Used In Different High-temperature Furnaces

There is a heavy concern that hexavalent chromium compound is generated inCr2O3-containing refractories after use in all different high-temperature furnaces.However，all of the former studies are still lack of systemtic and quantificationalanalysis. Up to now, there are many reports at home and abroad on Cr6+generated inmagnesia-chrome refractories used in cement kilns while little study on that used inother furnaces.In this work, Cr2O3-containing refractories used in different furnaces, such ascement kiln, copper converter, RH furnace, LF refining furnace and gasifier werestudied. First of all, all used bricks were cut into different zones according to thedistance from the hot face. X-ray Photoelectron Spectroscopy (XPS) and InductivelyCoupled Plasma (ICP) were used to determine the chemical valence of chromium inCr2O3-containing refractories used under different working environments. In addition,the phase compositions of the compounds containing chromium were investigated byXRD.XRD patterns of used magnesia-chrome refractory for cement kiln show thatgeneration of the Cr6+compound of CaCrO4was confirmed for all the samples. It isnoticeable that Cr2p3/2spectrum is deconvoluted into two components at576.8eVand581eV. The peak at581eV may be attributed to the presence of Na2CrO4(Cr6+)and indicates the valence of chromium has been changed from Cr3+to Cr6+. The peakat576.8eV is assigned to the presence of Cr2O3. ICP results indicate that the contentof water-soluble hexavalent chromium in this sample after soaking for every24h areas follows:2.1mg·L-1,0.18mg·L-1and0.02mg·L-1. Therefore, XRD, XPS and ICPresults indicate that Cr6+compound formed in the used Cr2O3-containing bricks forcement kiln. CaCrO4is considered to be a general Cr6+compound generated by thereaction between Cr2O3and the uncombined CaO in cement. The temperature rangeof CaCrO4formation is600~1000°C. It is therefore thought that the CaCrO4isgenerated during cooling. Moreover, it is found that the penetration of K+from the hot face to the rear part plays a significant role in the formation of hexavalentchromium under oxidizing conditions.The XRD patterns do not show the presence of the Cr6+compound in thepostmortem magnesia-chrome brick for RH furnace. According to the Cr2p3/2spectrum of sample representing the zone with a distance of0-30mm from the hotface, the peak at576.8eV is attributed to the Cr3+compound. So both XRD and XPSresults indicate that the valence of chromium has not been changed in the zone witha distance of0-30mm from the hot face. ICP results show that no water-solublehexavalent chromium is detected in the same sample after soakng for7days in a row.This is attributed to the cooling rate which is too fast for the formation of hexavalentchromium compound and the protective reducing atmosphere.XRD patterns of used magnesia-chrome refractory for copper converter indicatethat MgCr2O4, Cr2O3and CaCr2O4were generated in all the samples. According tothe Cr2p3/2spectrum of sample representing the zone with a distance of0-10mmfrom the hot face, the peak at576.9eV is attributed to the Cr2O3. Similar topostmortem magnesia-chrome brick for RH furnace, ICP results show that nowater-soluble hexavalent chromium is detected in the used magnesia-chromerefractory for copper converter after soakng for7days in a row. The formation ofCr6+under the operation condition of copper converter is less possibly due to thereducing atmosphere teeming with SO2.X-ray diffraction peak of Cr2O3、ZrO2and CrF3were indentified in usedhigh-chrome refractory for gasifier. However, Cr2p3/2spectrum of slag notchrefractory sample with a distance of0-30mm from hot face is deconvoluted into twocomponents at576.1eV and578.8eV. The maximum intensity peak at578.8eV isassigned to the presence of CaCrO4. The peak at576.1eV may be attributed to thepresence of Cr2O3. ICP results suggest that the content of water-soluble hexavalentchromium in this sample after soaking for every24h are as follows:13.2mg·L-1,0.90mg·L-1and0.25mg·L-1, which indicates the valence of chromium has partiallybeen changed from Cr3+to Cr6+. Despite of reducing atmosphere and acidic slag, ahigh oxygen partial pressure in gasifier leads to Cr2O3oxidizing to Cr6+.XRD patterns of used chromium corundum purging plug for refining ladle show that generation of Al1.98Cr0.02O3was confirmed for all the samples, that is to say,Cr2O3is solid dissolved into Al2O3in high temperature. Cr2p3/2spectrum ofchromium corundum purging plug for refining ladle is deconvoluted into twocomponents at576.5eV and578.3eV. The maximum intensity peak at578.3eV isassigned to the presence of CrO3. The peak at576.5eV may be attributed to thepresence of Cr2O3. ICP results suggest that the content of water-soluble hexavalentchromium in this sample after soaking for every24h are as follows:0.42mg·L-1,0.025mg·L-1and not detected, which indicates the valence of chromium has partiallybeen changed from Cr3+to Cr6+. In addition to the tremendous repeated changes intemperature, being exposed to slag which has a high calcium level is the main reasonwhy valence of chromium has partially been changed from Cr3+to Cr6+.In conclusion, hexavalent chromium does not exist in all Cr2O3-containingrefractories after use.