Improvement of a Cr(VI) Extraction Method for Chromite Ore Processing Residue-contaminated Materials
Oral Presentation
Prepared by R. Wolf1, C. Mills1, C. Bern1, J. Morrison1, M. Goldhaber1, A. Foster2
1 - U. S. Geological Survey, Denver Federal Center, MS 964D, Denver, CO, 80225, United States
2 - U. S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA, 94025, United States
Contact Information: [email protected]; 303-236-2470
ABSTRACT
Chromite ore processing residue (COPR) produced by the high lime process contains high concentrations of total Cr (typically near 5 weight %). A substantial fraction (25 to 30%) of total Cr is often in the toxic Cr(VI) oxidation state. COPR mineralogy is well-documented and is dominated by calcite [CaCO3], periclase [MgO], brucite [Mg(OH)2], hydrated calcium aluminate minerals, brownmillerite (Ca4Al2Fe2O10), unreacted chromite [(Mg,Al,Fe)Cr2O4], and amorphous material. Cr(III) is present mainly in chromite while Cr(VI), which forms an oxyanion, is found substituting for other anions in the hydrated calcium aluminate minerals hydrocalumite, ettringite, and hydrogarnet. The mineralogy of COPR typically results in alkaline pH of COPR-contaminated soils. Cr(VI) is redox stable and quite mobile at alkaline pH so that leaching of Cr(VI) poses a serious environmental issue. Accurate determination of total Cr(VI) concentration and mineralogical residence in COPR-contaminated soils is essential to address this issue, yet current methods fail to quantitatively extract Cr(VI) from COPR-contaminated materials. For example, Mahlerbe et al. (2011) showed that the certified Cr(VI) concentration of NIST SRM 2701 (551±35 mg kg-1) determined by EPA Method 3060A is <20% of the total Cr(VI) concentration of SRM 2701 determined by X-ray absorption near edge structure spectroscopy (XANES). We found that intensive grinding NIST 2701 prior to extraction resulted in significant improvement in Cr(VI) extraction efficiency. Grinding was performed by micronization with both water and methanol as lubricants. Up to 1.6 times as much Cr(VI) was extracted from ground versus unground samples of SRM 2701. A similar improvement in extraction efficiency was observed for additional COPR-contaminated soil from New Jersey. We are analyzing pre- and post-extraction materials by SEM-EDX, XRD, and XANES as well as extraction solution chemistry to understand whether grinding enhances the dissolution of a specific mineral residence of Cr(VI).
Mahlerbe et al. (2011) Environmental Science and Technology 45, 10492-10500.
Oral Presentation
Prepared by R. Wolf1, C. Mills1, C. Bern1, J. Morrison1, M. Goldhaber1, A. Foster2
1 - U. S. Geological Survey, Denver Federal Center, MS 964D, Denver, CO, 80225, United States
2 - U. S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA, 94025, United States
Contact Information: [email protected]; 303-236-2470
ABSTRACT
Chromite ore processing residue (COPR) produced by the high lime process contains high concentrations of total Cr (typically near 5 weight %). A substantial fraction (25 to 30%) of total Cr is often in the toxic Cr(VI) oxidation state. COPR mineralogy is well-documented and is dominated by calcite [CaCO3], periclase [MgO], brucite [Mg(OH)2], hydrated calcium aluminate minerals, brownmillerite (Ca4Al2Fe2O10), unreacted chromite [(Mg,Al,Fe)Cr2O4], and amorphous material. Cr(III) is present mainly in chromite while Cr(VI), which forms an oxyanion, is found substituting for other anions in the hydrated calcium aluminate minerals hydrocalumite, ettringite, and hydrogarnet. The mineralogy of COPR typically results in alkaline pH of COPR-contaminated soils. Cr(VI) is redox stable and quite mobile at alkaline pH so that leaching of Cr(VI) poses a serious environmental issue. Accurate determination of total Cr(VI) concentration and mineralogical residence in COPR-contaminated soils is essential to address this issue, yet current methods fail to quantitatively extract Cr(VI) from COPR-contaminated materials. For example, Mahlerbe et al. (2011) showed that the certified Cr(VI) concentration of NIST SRM 2701 (551±35 mg kg-1) determined by EPA Method 3060A is <20% of the total Cr(VI) concentration of SRM 2701 determined by X-ray absorption near edge structure spectroscopy (XANES). We found that intensive grinding NIST 2701 prior to extraction resulted in significant improvement in Cr(VI) extraction efficiency. Grinding was performed by micronization with both water and methanol as lubricants. Up to 1.6 times as much Cr(VI) was extracted from ground versus unground samples of SRM 2701. A similar improvement in extraction efficiency was observed for additional COPR-contaminated soil from New Jersey. We are analyzing pre- and post-extraction materials by SEM-EDX, XRD, and XANES as well as extraction solution chemistry to understand whether grinding enhances the dissolution of a specific mineral residence of Cr(VI).
Mahlerbe et al. (2011) Environmental Science and Technology 45, 10492-10500.
