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Chromium ore processing residue (COPR) is a hazardous waste generated during the production of chromate. Currently, approximately 10% of Cr 2 O 3 cannot be extracted after chromite sodium roasting and remains in COPR, wasting valuable Cr resources. In this study, Mg was selectively removed by using (NH 4) 2 SO 4 roasting in combination with H 2 SO 4 leaching. The results showed that the selective removal of 79.55% Mg from COPR could be achieved under the optimum (NH 4) 2 SO 4 roasting conditions (80 mmol (NH 4) 2 SO 4 , 800 °C, 2 h). During the subsequent sodium roasting and acid leaching stages, the Cr extraction rate was 84.63% for the COPR direct roasting and 95.39% for the Mg removal residue roasting. The increased Cr extraction efficiency is attributed to the transformation of Mg-rich spinel and diopside (the Mg & Cr coexisting phases) in COPR converted into easily extractable (Fe,Cr) 2 O 3 and Cr 2 O 3 after the Mg treatment. This study investigated that the phase transformation of the Cr host phases is crucial for the sufficient extraction of Cr and provides inspiration for the development of efficient and practical Cr extraction techniques. Moreover, the method can be extended to the effective extraction of Cr from other Cr-containing wastes. [Display omitted] • The Cr host phases in COPR are Mg-containing spinel and Mg-containing diopside. • Selective pre-removal of Mg can effectively enhance the extraction of Cr from COPR. • The phase transformation of Cr-containing wastes is critical for Cr extraction. [ABSTRACT FROM AUTHOR]