Chromium steel from chromite ore processing residue——A valuable construction material from a waste

As species we humans generate excessive amounts of waste and hence for sustainability we should explore innovative ways to recover them.The primary objective of this study is to demonstrate an efficient and optimum way to recover chromium and iron from chromite ore processing residues(COPR)for the production of chrome steel and stainless steel.In Hudson County,New Jersey,there are more than two million tons of leftover COPR.Part of COPR was used as fill materials for construction sites,which spread the problem to a larger area.With high solubility along with their toxicity leached chromate from COPR is threatening the environment as well as human health.In this research,COPR was thermally treated to recover iron with chromium by applying techniques used in steel manufacturing.An extensive experimental program was performed using a Thermo-Gravimetric Analyzer(TGA)and bench scale tests to thermally treat the processed chromium contaminated soils with carbon and sand at varying temperatures and under reducing environment.The optimum chemical composition of COPR and additives to be used in the melts were evaluated based upon the thermodynamic properties of the mixture to ensure good phase separation,least amounts of iron and chromium oxides in the slag and minimum variability of final product(steel or iron with chromium).The impact of other oxides on the steel making process was evaluated to minimize the adverse impact on the process.The research demonstrated the feasibility of recovering a valuable construction material(chrome steel)from a waste(COPR).

Extraction of Cr(Ⅵ) from chromite ore processing residue via hydrothermal-assisted phase transformation

The effective extracting Cr(Ⅵ) from chromite ore processing residue(COPR) is the key to achieve COPR detoxification and recovery.We developed an effective method to extract Cr(Ⅵ) from COPR via controlling the phase transformation of Cr(Ⅵ)-containing minerals.Characteristic analysis showed that Cr(Ⅵ) was mainly incorporated in the hydrocalumite(NaCa4Al2O6(SO4/CrO4)1.5-15H2O) in COPR,which was a layered-double hydroxide(LDH) with multilayer structure.In the hydrothermal treatment experiments,the Na2CO3 solution showed significant extraction effect of Cr(Ⅵ) and detoxification effect of COPR.After treatment,95% of Cr(Ⅵ) was removed and the Cr(Ⅵ) concentration in the leachate was decreased to 1.6 mg/L by the toxicity characteristic leaching procedure(TCLP),within the regulatory limit disposal standard(HJ/T 301-2007,3 mg/L).Further study revealed that,during the treatment,hydrocalumite transformed into calcite(CaCO3) under the effect of mineralizer,therefore,the layered structure collapsed and the incorporated Cr(Ⅵ) was released to the supernatant.Meanwhile,the Cr(Ⅵ)desorbed from calcite with the calcite particles grew into large size with smooth surface.Stir-flow experiment revealed that the amount of chromium released from CORP to the environment was significantly reduced after treatment,and it is safer for landfill disposal.This work will provide an instructive guidance for the detoxification and recovery of COPR.