New method for selective Cr recovery from stainless steel slag by NaOCl assisted alkaline leaching and consecutive BaCrO4 precipitation

• A hypochlorite assisted alkaline leaching approach was developed to leach Cr selectively from stainless steel slag.
• An effective extraction of Cr occurs at low temperature (<105 °C) and low mass ratio alkali to stainless steel slag (0.13).
• At optimum conditions a Cr leaching of 68% is obtained.
• Cr could be recovered from the leachate for 99.9% by precipitation as BaCrO4.
• Carbonation of the residue after leaching yields a strong and environmentally safe material.

A new hydrometallurgical method was investigated for selective leaching of chromium from stainless steel slag (SS slag) consisting of temperature controlled extraction with NaOH in the presence of NaOCl, followed by water leaching. After parameter optimization of the NaOCl–NaOH extraction step, a selective Cr leaching of 68% was reached, while dissolution of matrix materials was low (Al 0.3%, Ca 2.0%, Si 0.4%). The optimum conditions for the investigated system are: 105 °C, 6 h, SS slag particle size <63 μm, mass ratio of NaOH to SS slag 0.13, and 3.3 mmol NaOCl to 1 g SS slag. The described oxidative alkaline leaching process by hypochlorite enables selective recovery of Cr at a significantly lower temperature and required amount of alkaline agent than molten salt or alkaline roasting processes. BaCrO4 was precipitated to purify and concentrate Cr from the leachate in which also minor amounts of Mn and V were present. This method allowed for a 99.9% Cr recovery rate. The obtained leaching residue shows no alterations of the SS slag’s mineralogy with respect to untreated material. Therefore, a known carbonation treatment of the slag can be applied to prepare novel construction materials with a lowered Cr content.

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