A method for generating uniform size-segregated pyrite particle fractions
© Wolfe et al; licensee BioMed Central Ltd. 2007
Received: 02 March 2007
Accepted: 10 October 2007
Published: 10 October 2007
Standardized sample preparation techniques allow comparison of pyrite dissolution experiments under diverse conditions. Our objective was to assess dry and wet sieving preparation methodologies, and to develop a reproducible technique that yields uniformly size-distributed material within a limited size range of interest.
Here, we describe a wet sieving preparation method that successfully concentrates pyrite particles within a 44–75 μm diameter range. In addition, this technique does not require a post-processing cleanup step to remove adhering particles, as those particles are removed during the procedure. We show that sample preparation methods not only affect the pyrite size distribution, but also apparent dissolution rates.
The presented methodology is non-destructive to the sample, uses readily available chemical equipment within the laboratory, and could be applied to minerals other than pyrite.
Pyrite, FeS2, is one of the most abundant sulfide minerals at the Earth's surface and represents an important reservoir for iron and sulfur within the Earth's crust. It exists in a variety of forms and is prevalent in numerous environments including hydrothermal ore zones, modern lake and ocean sediments, and sedimentary rocks [1–6]. Regardless of its source, the weathering of pyrite via oxidative dissolution can result in the acidification and degradation of water resources [[6–13] and references therein]. The rates and mechanisms governing this process are only partially understood despite numerous experimental studies of pyrite oxidation [14–22].
Pyrite preparation methodologies used in previous studies. In each method listed, the pyrite was hydrothermal in origin.
Target Size Fraction (μm)
125 – 250 μm
The pyrite was crushed, soaked overnight in hot hydrofluoric acid, washed in distilled water, dried in air and sieved. Sieved pyrite was ultrasonically cleaned in ethanol, rinsed with 1 M nitric acid for one minute, triply rinsed with distilled water, and then with ethanol. The pyrite was dried with air and stored briefly in beakers.
 McKibben, M.A., and Barnes, H.L. (1986)
40 – 80 μm
Powders were prepared by grinding in an agate mortar. The oxidation products were eliminated by rinsing with 10-2 MHNO3.
 Bonnisell – Gissinger, P. et al. (1998)
Samples were crushed using an agate mortar and pestle. The crushed pyrite was soaked overnight in hot hydrofluoric acid, washed in deoxygenated deionized water, dried in air, and sieved.
 Kamei, G., and Ohmoto, H. (2000)
105 – 150 μm
Samples were dry ground in two steps: 1) a glass-cleaned ring pulverizer was used to reduce grain size and 2) an agate mortar was used to crush the particles to the desired particle size range. The ground pyrite was dry sieved. Samples were kept in a glass desiccator under vacuum after preparation to avoid surface oxidation.
 Cruz, R., et al. (2001)
Pyrite was ground using an agate mortar, sieved with ethanol, and then washed in an ultrasonic bath. Procedure was repeated until the ethanol was clear and free of fine particles after the ultrasonic bath treatment.
 Descostes, M., et al. (2004)
150 – 500 μm
Crushed minerals were sieved, ultrasonically treated and washed repeatedly to remove fine particles, and then treated with 10% HCl for 2 hours to remove any preexisting oxide layer. The crushed mineral particles were rinsed with ethanol and allowed to dry.
 McGuire, M.M., et al. (2001)
Material was classified into various size fractions by wet-dry screening. Prior to leaching experiments, samples of the ground material were soaked in 3 M hydrochloric acid solution for 36 h, filtered, rinsed with double-distilled water, dried with acetone, and kept under vacuum in a desiccator.
 Caldeira, C.L. et al. (2003)
250 – 420 μm
The pyrite was crushed, sieved, and rinsed with ethyl alcohol three to five times until the supernatant was clear. The samples were then sonicated in ethyl alcohol (repeated at least three times until the supernatant was clear). The grains were dried at 70°C for 12 h.
 Jerz, J.K., and Rimstidt, D. (2004)
37 – 74 μm
Pyrite was ground in air for different periods. After grinding, samples were sieved under dry conditions and the size fraction between 200 and 400 mesh collected.
 Sasaki, K. (1994)
The objective of this work was to develop an effective, reproducible procedure for isolating pyrite grains in the 44–75 μm range for dissolution studies. This work was conducted as part of a comparative investigation of dissolution rates for pyrite from different petrogenetic environments. Previous pyrite dissolution experiments (Table 1) involved hydrothermal pyrite particles >75 μm in diameter, while our experiments called for a smaller size fraction, 44–75 μm, to better simulate dissolution of finely disseminated pyrite in some sedimentary environments. We compared dry and wet sieving preparation methodologies with the goal of developing a reproducible technique that yields clean material within our size range of interest. The methods were evaluated through a combination of SEM analysis and batch dissolution experiments.
Crushing and Sieving Procedures
Samples used in this study.
Wards Natural Science
Rock Currier, personal communication
Lower Kittanning coal, OH
sedimentary, within coal
sedimentary, within coal
Calvert Bluff Formation, Texas
sedimentary, within coal
pyrite with minor quartz
Powder Preparation and Characterization
Powder preparation techniques using both a mixer mill and a mortar and pestle were evaluated. Most samples of the crushed pyrite (~10 g) were milled into a powder using a tungsten carbide mixer mill for approximately 3 minutes. An aliquot of one sample (HY-001) was also ground in an agate mortar and pestle as a comparison to the mixer mill. Samples were placed in a desiccator under vacuum immediately after being powdered. X-ray diffraction analysis (Philips XRD PW3710; Almelo, Netherlands) indicates that all samples are pyrite, although SED-003 contains minor (≤ 10%) quartz. Additional aliquots (~0.1 g) of pyrite powder were completely dissolved in 10 mL concentrated nitric acid and further diluted to 5% nitric acid for iron and sulfur analysis by ICP-AES. The results (Table 2) indicate that the samples consist of stoichiometric FeS2.
Prior to sieving, the sample was dried in an oven for approximately 30 minutes at 105°C to drive off adsorbed moisture. Given the results of previous pyrite oxidation experiments, the appearance of oxidation products on pyrite surfaces is most likely minor, given the short period of time in which the samples were in the oven [19, 25]. In addition, the samples were treated prior to dissolution experiments to ensure the removal of any possible oxidation products (see section on dissolution experiment procedures). Approximately 5–6 grams of material were transferred to a polypropylene sieve set equipped with nylon mesh (41 μm, 63 μm and 75 μm mesh sizes were used) and shaken for 10 minutes in a sieve shaker. Following the dry sieving procedure, a coating of pyrite grains much finer than the smallest sieve size remained on mineral surfaces. To address this issue, two surface cleaning procedures were evaluated: (1) About 10 grams of sieved pyrite were added to ~200 ml tetrabromoethane (density = 2.89 g/cm3) within a 250 ml separatory funnel. After 20 minutes, the settled pyrite was collected from the bottom of the column and cleaned with acetone. (2) Two to three grams of sieved pyrite grains were placed into a 50 ml polypropylene test tube. Approximately 35 ml of 70% ethanol were added to each tube and the sample was ultrasonicated for 1 minute. Suspended material within the solution was decanted and discarded. Following both procedures, the remaining samples were oven-dried 30–60 minutes at 105°C, then transferred to a desiccator and placed under vacuum. Sub-samples were collected for SEM analysis to determine the size range of the particles collected.
To begin the procedure, powdered material was transferred into 50-ml polypropylene test tubes. Ethanol (35 ml of 70%) was then added to each tube and the sample was ultrasonicated for 1 minute. The material was then poured onto the top of the sieve stack to begin the wet sieving process. Alternating aliquots of ultrapure water and 50% ethanol (ending with ethanol) were added until the entire sample had been sieved. Ethanol was used to prevent pyrite oxidation during sieving. Finally, the remaining samples were oven-dried 30–60 minutes at 105°C, then transferred to a desiccator and placed under vacuum. Sub-samples were collected for SEM analysis to determine the size range of the particles collected.
Samples SED-001 and SED-002, both of which were extracted from a coal matrix, appeared to contain a significant fraction of organic carbon (not revealed during XRD analyses), based on the formation of an opaque black solution after addition of 5 mL concentrated nitric acid to approximately 1 gram of a powdered pyrite sample. This phenomenon was also observed by Lord . Based on the method of Huerta-Diaz and Morse , these samples were treated with concentrated H2SO4 for approximately 10 minutes and rinsed with ultrapure water, followed by ethanol. This appeared to eliminate the organic carbon.
The dissolution behavior of pyrite material that had undergone dry sieving was compared to the dissolution of those that had been wet sieved. Prior to experimentation, all pyrite samples were treated to remove any surface iron oxides or iron sulfates that could have been produced when the samples were exposed to the atmosphere. This procedure, a modified version of a method used by Paschka and Dzombak , involved boiling 7–8 g of pyrite in 50 ml concentrated HCl for approximately 10 minutes. The sample was rinsed with boiling concentrated HCl at least twice, then rinsed with 25 ml deionized water, followed by a boiling acetone rinse using a vacuum filter. The acetone rinse was repeated at least 3 times. The sample was dried in the oven at 105°C for about 10 minutes and stored in a desiccator. Specific surface area measurements were conducted prior to the cleaning procedure.
Dissolution experiments were carried out in a batch reactor under tightly controlled conditions: pH = 3 ± 0.05, a constant temperature of 25 ± 0.01°C, fixed dissolved oxygen (8–11 ppm), and electrolyte solution of 0.01 M NaCl initial ionic strength . A precise 5.355 ± 0.005 g aliquot of cleaned pyrite was added to 1.5 L of deionized water in a stirred, jacketed glass vessel with a lid having sealed ports for insertion of reagents and withdrawal of samples from the reactor. During the experiments, the reactor was covered with aluminum foil to exclude light. pH was maintained through the addition of HCl or NaOH via acid/base pumps and a pH-stat. Pyrite dissolution was monitored by measuring total dissolved iron. Five milliliters of sample were collected periodically over an 8 hour time period, and then filtered through a 0.45 μm disposable filter into a 20 ml polyethylene scintillation vial containing 5 ml 10% HNO3 for sulfur and iron analysis. Iron and sulfur concentrations were measured using ICP-AES, with replicate measurements of Fe by flame and graphite furnace AA.
Specific surface area of each sample was measured by the nitrogen adsorption multipoint BET method with a Quantosorb instrument (Quanta Chrome, Boynton Beach, Florida). The accuracy of the instrument was verified by measurements on alumina and black carbon standards of known surface area. Particle surfaces were examined pre- and post- cleaning using a Philips XL-30 FEG field emission scanning electron microscope (Almelo, Netherlands). Sulfur and iron concentrations were measured on a SpectroFlame EOP ICP-AES (Kleve, Germany) using EPA Method SW 846 . Accuracy of measurements are within ± 5% of true values. Replicate analyses of total dissolved iron were measured using a GBC 908AA atomic absorption spectrometer (GBC Scientific Equipment, Hampshire, IL). Instrument calibration was carried out using a suite of different concentrations of iron standard solution (Fisher Scientific) in 5% nitric acid matrix. All the aqueous samples were preserved in 5% nitric acid matrix before ICP-AES and AA measurements.
Surface area and dissolution rates for pyrite samples after preparing material using the dry and wet sieving technique.
Dry Sieving Preparation 63 – 75 μm
Wet Sieving Preparation 45 – 75 μm
Initial Dissolution Rate μg of Fe/min
Surface Area m 2 /g
Initial Dissolution Rate μg of Fe/min
Surface Area m 2 /g
Results of the dissolution experiments are reported in Table 3. Initial dissolution rates (Table 3) were calculated using iron concentrations measured one hour into the experiment. Dissolution rates calculated for dry sieved pyrite samples were highest for sedimentary samples and lowest for hydrothermal samples, with SED-002>SED-003>SED-001>HY-001. Dissolution rates obtained for hydrothermal samples yielded the lowest rates, regardless of whether they were wet or dry sieved. The highest dissolution rates were obtained from sedimentary samples that had been prepared using the dry sieve technique.
Dry sieving vs. wet sieving of crushed pyrite
Early efforts using the dry sieving technique to achieve a narrow range of fine particles yielded poor results; dry sieving resulted in a wide range of sizes from very fine particles adhering to the surface of individual grains to smaller particles (<44 μm) scattered throughout the larger matrix. Initially we attributed the aggregation of smaller sized particles to electrostatic charges being induced across the nylon mesh material, thus prohibiting movement through the sieve mesh, while the samples were sieved using the sieve shaker. However, changing from the polypropylene sieves to brass sieves did not improve the yield. The addition of tetrabromoethane to the sieving procedure helped marginally when the tetrabromoethane followed dry sieving, as the tetrabromoethane tended to clear the particle surfaces of finer particles. However, the finer particles were not removed from the sample, and using tetrabromoethane prior to dry sieving had little to no effect on the distribution of particles in the final sample.
Several workers ultrasonicated their respective samples in aqueous suspensions after dry sieving to remove fine particles from the pyrite surface [14, 17, 23]. However, this is not an effective particle separation method for fine grained (<100 μm) samples. Particles within our range of interest (45–75 μm) tend to remain suspended in the solution after ultrasonication. Decantation removes these particles along with other fine particles, ultimately biasing the method to retain larger sized particles within the collected size range.
The wet sieving technique was significantly more successful in producing a uniform distribution of particles in the size range of interest and is an effective method to produce a uniform, fine, and restricted pyrite particle size range for experiments. In addition, this technique does not require a post-processing cleanup step to remove adhering particles, as the particles are removed during the wet sieving procedure.
Application to pyrite dissolution experiments
Results from the dissolution experiments indicate that pyrite preparation methods can affect the rate of dissolution significantly (Fig. 4). Pyrite powder prepared by dry sieving exhibited the highest surface area measurements and dissolution rates of all samples. Examination by SEM indicates that these results are likely caused by the presence of particles finer than 63 μm that adhere to larger particles even after separation with tetrabromoethane, and regardless of the type of sieve used.
There is clearly a need to standardize sample preparation techniques to allow accurate comparisons of pyrite dissolution experiments under diverse conditions. We have developed a wet sieving procedure using vacuum filtration techniques to obtain fine (<100 μm) particle size fractions of pyrite for use in geochemical experiments. Compared to traditional methodologies, this procedure is far more successful at acquiring a narrow range (45–75 μm) of pyrite particles, as reflected in batch dissolution experiments and SEM analysis. This method uses readily available materials and equipment, and has potential application to other minerals as well.
We gratefully acknowledge the staff and faculty associated with the Materials Micro-Characterization Laboratory of the Department of Materials Science and Engineering, University of Pittsburgh, for assistance with the x-ray diffraction and scanning electron microscopy conducted during this study. We thank W. Winters, Office of Surface Mining, and R. Currier for providing pyrite samples, and B.K. Games for technical support with the ICP-AES analyses. This manuscript benefited greatly from the detailed comments and suggestions of three anonymous reviewers. This work was supported by NSF EAR-0229019, 0214212, and 0517313 (R. C. Capo/B. W. Stewart), NSF EAR-0228903 (D. A. Dzombak), NASA Astrobiology Institute/Penn State Astrobiology Research Center (B. W. Stewart/R. C. Capo), and the Ji-Dian Liang Fellowship (R. Liu).
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