Chemical and mineralogical characterization of chromite ore processing residue from two recent Indian disposal sites
1. Descriptions of methods
X-ray fluorescence spectroscopy (XRF)
Powdered samples were mixed with a flux material and melted into glass beads. To determine the loss on ignition, 1000 mg of sample material was heated to 1030 °C for 10 minutes. After mixing the residue with 5.0 g of lithium metaborate and 25 mg of lithium bromide, it was fused at 1200 °C for 20 minutes. The calibrations were validated regularly by analysis of reference materials, and 130 certified reference materials were used for the correction procedures. Due to the importance of accurate determination of the total Cr content of the waste, two different total digestion methods were evaluated.
Digestions
For the pressure bomb digestion, 0.05 g of hand-ground samples was weighed into polytetra-fluoroethylene (PTFE) liner and mixed with HF, HClO4, and HNO3 (2:1:1 vol., concentrated, Suprapur, Merck). The acid mixture was heated slowly during five hours to 160 °C and then left at this temperature for 50 hours. After cooling, the samples were evaporated twice at 180 °C on a heating plate and then made up to a final volume of 50 mL with deionized water.
Because the COPR was not totally dissolved, a second pressure bomb digestion with finer material was implemented. Therefore the samples were ground with cyclohexane for 5 minutes with a McCrone micronizing mill using a corundum grinding set. Under these conditions the samples were completely dissolved. The chromite-containing reference materials SARM 8 (Mintex, South Africa), CHR-Bkg (GIT-IWG, Geostandards, France), and CHR-Pt+ (GIT-IWG, Geostandards, France) were run simultaneously for quality control.
Due to the complexity of the pressure bomb digestion, a modified total microwave digestion application was evaluated. For this purpose, 0.05 g of hand-ground material was weighed into PTFE liner and mixed with concentrated HNO3 (Suprapur, Merck), concentrated H3PO4 (Pro Analysis, Merck), and concentrated HBF4 (pure, AppliChem) (5:3:2 vol.). The mixture was heated to 180 °C within 7 minutes in a microwave (Ethos, MLS). Seven minutes later, the temperature reached 260 °C and was held for 2.5 hours. After cooling, the samples were filled up to a final volume of 50 mL with deionized water. Aluminum, calcium (COPR samples), Cr, Fe, Mg, and Mn were analyzed by flame atomic absorption spectrometry (AAS, iCE 3500, Thermo) with a nitrous oxide–acetylene flame at 309.3 nm (Al), 422.7 nm (Ca), 357.9 nm (Cr), 248.3 nm (Fe), 285.0 nm (Mg), and 279.5 nm (Mn). The use of this type of flame was essential
Chemical and mineralogical characterization of chromite ore processing residue from two
re-cent Indian disposal sites 37
due to the formation of hardly atomizing phosphate compounds. To overcome ionization inter-ferences, 0.2% (m/v) Cs [CsCl Al(NO3)3 buffer solution according to Schuhknecht and Schinkel, Merck] was used as an ionization buffer, except for Al, where 0.2% m/v K (KCl, Merck) was used. Sodium and K were measured with an air–acetylene flame and an ionization buffer as described above at 589.0 (Na) and 766.5 nm (K). Silicon and Ca (reference materials) were analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES, Ultima 2, Horiba Scientific) due to their low concentrations.
For the aqua regia digestion, 0.2 g of ground sample was weighed into a PTFE liner. Then 2 mL of concentrated HNO3 and 6 mL of concentrated HCl were added. After microwave di-gestion, the samples were filtered through a quantitative filter (5 to 10 μm, VWR) and made up to a final volume of 50 mL with deionized water. Chromium was measured as described before.
Chromate extraction
For this purpose, 50 mL of the extraction solutions were added to 2.5 g of non-milled samples.
In the case of step 1 and 2 reactions took place at room temperature, for step 3 at 85 °C and for step 4 at room temperature in a sonication bath (Haver USC 200, Haver & Boecker).
X-ray powder diffraction (XRD)
The samples were carefully pre-ground by hand in an agate mortar. Afterwards the samples were finely ground in a McCrone micronizing mill using a corundum grinding set. Isopropanol was used as a cooling agent during grinding; the grinding time was 5 minutes for each sample.
The preparation for the measurements was done by frontloading in a steel sample holder.
Phase identification was done by reference to patterns in the powder diffraction file database, which was released in 2001 (Table S1).
Scanning electron microscopy (SEM)
Samples of dried COPR were vacuum impregnated with a low viscosity epoxy resin for BSE imaging, qualitative and quantitative EDX analysis, and element mapping. Afterwards, samples were polished with diamond paste up to 1 µm. Sections were coated with a thin layer (approx.
5 nm) of carbon to improve electrical conductivity. The contrast in BSE images from flat pol-ished samples is determined by variations in average atomic number. Phases of a higher av-erage atomic number are brighter than phases with a lower atomic number. Spectra for quan-titative analyses were collected in the spot mode at intervals of 60 s using count rates of 10,000 counts s–1. Spectra were then processed by a standardless-based ZAF correction. Ini-tially, separate maps for each element were simultaneously recorded over a scan area of 512 × 400 points in the X and Y directions. A special software package (Genesis,
Chemical and mineralogical characterization of chromite ore processing residue from two
re-cent Indian disposal sites 38
Edax/Ametek) was applied to deduce phase maps from quantitative EDX maps for visualiza-tion of spatial distribuvisualiza-tion and relavisualiza-tion of individual phases.
Fig. S1 Phase maps of COPR obtained from (a) Rania and (b) Chhiwali with the spatial distribution of the main mineral phases (white: magnesiochromite; red: CAC (Cr-rich); green: brownmillerite (Cr-poor); blue: periclase; pink: brucite; yellow: CAC phases (Cr-poor, Fe-rich).
Fig. S2 Energy-dispersive X-ray spectrometer analyses of grimaldiite crystals in the Chhiwali COPR.
b) a)
Chemical and mineralogical characterization of chromite ore processing residue from two
re-cent Indian disposal sites 39
Table S1 Minerals identified in COPR using X-ray powder diffraction, backscattered electron images, and secondary electron images.
Mineral Symbol Chemical formula PDF Rania Chhiwali
Aragonitea A CaCO3 41-1475 __ x
Brownmillerite B Ca2(Fe,Al)2O5 30-0226 x x
Brucite Br Mg(OH)2 07-0239 x x
Calcite C CaCO3 05-0586 x x
CAC-14b CAC Ca4Al2O6(CrO4)·14H2O 52-0654 x x Ettringitea E Ca6Al2(SO4)3(OH)12·26H2O 41-1451 x __
Magnesiochro-mite
Cr MgCr2O4 10-0351 x x
Grimaldiite G CrO(OH) 85-1372 x x
Katoite K (CaO)3Al2O3(H2O)6 24-0217 x x
Larnite L Ca2SiO4 33-0302 x x
Periclase P MgO 87-0652 x x
Portlandite Pt Ca(OH)2 04-0733 x x
Quartz Q SiO2 46-1045 x x
Sjogrenite S (Mg6Fe2(OH)16)(CO3)(H2O)4 24-1091 x __
Voltaite V K2Fe5Fe3Al(SO4)12(H2O)18 71-0718 x x
a Not identified by XRPD but by BSE and SEI
b CAC: Calcium aluminum chromium oxide hydrate
Chemical and mineralogical characterization of chromite ore processing residue from two
In-dian disposal sites 40
Table S2 Comparison of total element contents (g kg–1) of two Indian COPR samples (Rania and Chhiwali) and three certified reference materials determined by microwave di-gestion, pressure bomb didi-gestion, and X-ray fluorescence (XRF).
Rania Chhiwali SARM 8 CHR-Bkg CHR-Pt+
Leaching of hexavalent chromium from young chromite ore processing residue 41