Metals droplet assembling mechanism during carbon reduction of stainless steel pickling sludge

Stainless steel sludge contained large amounts of heavy metals like Cr. These sludge are now mainly disposed by landfills, occupying a large amount of land and polluting the environment. The objective of the paper is to explore the synergistic reduction of Cr and Fe, and the phase transition during the process. Find out the forming condition of Fe–Cr–C alloy. Study the growth and migration trend of low melting point Fe–Cr–C droplets. Thus we could find a way to recover heavy metals from the sludge with less energy and make the tailings harmless to the environment. Through X-Ray Diffraction and Energy Dispersive Spectrometer analysis, the results showed that chromium mainly presented in the FeCr2O4 phase before it was reduced, and the reduction product was Fe–Cr–C metal carbide. By observing samples reduced at different temperatures with metallographic microscope, it could be found that the melted metal carbides flowed under the surface tension to form larger droplets at temperatures around 1250 °C. On the basis, the method of semi-melting sludge reduction at a temperature of 1275 °C which was lower than the existing method of 1350 °C, was first proposed. Under this condition, the average diameter of the obtained Fe–Cr–C particles was 2.7 mm, which was easy to separate from the tailings. The yield of Fe and Cr reached 86.7% and 71.3% respectively, which means 0.22 ton of metal particles could be recovered per ton of sludge, and could be used in steel making. The obtained non-magnetic tailings with a Cr content of less than 0.04% was harmless, and the remained magnetic powder could be used as a nucleating agent for the reduction of sludge.