Carbothermic Reduction of the Waste Chromium Oxide Rods from Thermal Spray Processes

Abstract

The waste chromium-oxide rods from thermal spray processes contained the valuable components of high chromium. The carbothermic reduction with graphite powders can be used to recover the chromium from the chromium-oxide rod. The waste rods were prepared into small pieces with a half-dimensions of 6.3 mm diameter and 5 mm length, and heated under the reducing atmosphere at 1150 – 1350 °C. Besides, the effect of different types and sizes of reducing agent on the reduction was observed. During the heating and cooling period, argon gas was fed into the atmosphere. Chromium carbide formation was promoted when the temperature and time were increased. Cr-O and Cr-O-Al-Si were observed in the microstructures before and after the reduction. The outermost part of the waste rod can be reduced to chromium because of the interphase zone. Consequently, the carbothermic reduction is one possible recycling method for utilizing ceramic rod wastes from the thermal spray process.

The high temperature degradation of ferritic stainless steel in solid carbon atmospheres

“…Using ferritic stainless steel at high temperatures is one possible way to reduce material costs for general stainless-steel components under low aggressive environments in power plants. However, intergranular corrosion and deterioration of passive film may present during the combustion process due to carbon diffusion [9][10][11]. Solid carbon from soot particles can diffuse into austenite grain at temperatures between 830℃ to 1400℃ and form chromium carbide along grain boundaries following Equation (1-3) [5,12,13].…”

“…Besides, the chromium oxide film on steel surfaces can decay and transform to chromium carbides as expressed in Equation (4). The chromium carbides of Cr3C2, Cr7C3, and Cr23C6 can be formed when the sensitization temperature above 475℃ [9,[13][14][15]. They can be predicted using the standard Gibbs free energy free change of chromium carbide formation calculated by thermodynamic data from Kubaschewski [16].…”