Arsenic Condensation and Reaction Mechanisms in Flash ...

Materials and Experimental Setup

The raw materials were pure metallic arsenic lump (Alfa Aesar, 99.999 pct pure) and silver shots (Boliden Harjavalta, 99.999 pct pure). The experimental atmosphere was achieved by mixing SO2 (99.98 vol pct pure, Aga-Linde), air (<0.001 g/m3 H2O), and N2 gases (99.999 vol pct pure, Woikoski, Finland). The gas flow rates were regulated using digital mass flow controllers DFC26 (Aalborg). Silver was selected as solvent for arsenic due to its stability against oxidation in the gas mixtures used.

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The novel experimental apparatus used for the condensation study was a modified transportation setup where arsenic was vaporized freely from a molten silver-arsenic alloy at a fixed temperature to a premixed carrier gas containing O2(g), SO2(g), and N2(g). Thus, the initial situation in the gas at 800 °C was similar to that in the FSF where arsenic compounds are dissociated to elemental form.[19] The industrial smelter off-gas contains dust which was not included in the present study. The selected setup design allows condensation of arsenic compounds in a controlled temperature gradient,[20] sometimes called thermochromatography.[21] The vertical design avoided settling of the homogeneously nucleated material on the fused silica riser tube, and the deposit was formed by electrophoretic forces. The lead time of the gas in the riser from the alloy surface to the outlet was estimated to be 10.5 seconds.

The setup shown in Figure 1 was designed specifically for these experiments. It comprised a vertical resistance tube furnace LHA12/300 (Lenton, UK) equipped with metal heating elements. On top of the furnace, brass flanges with a cooling water circulation were placed to hold an alumina work tube (Kyocera AL23, Germany; o.d. = 40 mm) closed in one end, and a fused silica dust-collecting tube, &#;riser&#; (Finnish Special Glass, Finland; i.d. = 12.5 mm). A rubber plug was used to seal the silica riser on top, having a hole for inserting either a calibrated Pt100 resistance thermometer (tolerance class B 1/10 DIN) or the gas outlet tube #2. Temperature in the furnace was controlled by a Eurotherm PID controller, and the data were logged in with a LabVIEW software. The resistance of Pt100 probe was measured using a Keithley DMM digital multimeter (USA) connected to a PC for data logging (with a frequency of 0.2 1/s) during the measurements. The design allowed As additions in the furnace during the experiment without cooling.

Schematic of the experimental furnace, the controls, and data-logging device. Note the two off-gas lines (#1 and #2) for leading the carrier gas stream from the riser tube during heat up and cooling

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The furnace off-gas was cleaned with hydrogen peroxide solution to oxidize the arsenic to higher valence and to reduce its toxicity, and then SO2 was absorbed by a NaOH solution with BTS as pH indicator (bromothymol blue, C27H28Br2O5S, Sigma-Aldrich) before venting to the fume hood.

Two alumina crucibles were located in the bottom of the work tube in the constant temperature zone of the furnace. The larger crucible was used to insert and lift up the As source and the smaller contained the Ag-As alloy. The bottom part of the silica riser tube was placed next to the smaller crucible for better mixing the carrier gas and arsenic vapors before leading them into the vertical riser tube.

Experimental Procedure

Before the experiments, the volatile material samples were loaded in the furnace with an iron wire. In each experiment, the initial volatile material was composed of 1 g arsenic and 10 g silver. Then, nitrogen was lead through the gas inlet with a flow rate of 150 mL/min. We kept the valve #1 open and valve #2 closed when the furnace temperature increased to 800 °C with the rate of 7 °C/min. After reaching 800 °C, the valve #2 was opened and valve #1 turned off, directing the gas flow through the silica riser to start the sample collection process. At the same time, the atmosphere was changed to a mixture of SO2, air, and N2. Dust-collecting time was 1 h in each experiment. After that, off-gas valve #2 was closed and valve #1 was turned on for nitrogen flow with a rate of 150 mL/min during cool down the furnace. Three experimental atmospheres tested in this work are shown in Table I.

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Temperature of the silica riser tube at different heights varied and its temperature profile were measured before the experiments at 150 mL/min nitrogen flow, as shown in Figure 2. Thus, the specific atmosphere and temperature conditions of the collected dust samples were precisely controlled.

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Silica riser tube sections used for collecting the deposits (a) and the temperature profile at a constant furnace temperature of 800 °C (b), with a total gas flow rate of 150 mL N2/min

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After the furnace reached room temperature, the silica riser was taken out from the furnace and dust in the inside wall was collected, as shown in Figure 2. The dust samples were separated from different heights of the silica tube, corresponding to the different temperature regions.

Analytical Methods

The samples were prepared with dry metallographic methods and first analyzed with a Mira 3 scanning electron microscope (SEM, Tescan, Czech Republic) equipped with an UltraDry silicon drift energy-dispersive X-ray spectrometer (EDS, Thermo Fisher Scientific, Waltham, MA) coupled to a NSS micro-analysis software (Thermo Fisher Scientific). The standards (Astimex, Canada) utilized in the EDS analyses were quartz (O Kα and Si Kα), pentlandite (S Kα), metallic silver (Ag Lα) and arsenic (As Kα).

The particle sizes were measured from the SEM pictures, the mineralogical phases and their compositions were first analyzed by EDS. To further confirm the mineralogical phases and chemical speciation, samples were investigated by X-ray diffraction (XRD), electron probe micro-analysis (EPMA), and X-ray photoelectron spectroscopy (XPS).

Due to limited amounts of the collected samples, a zero-background silicon disk was used in the XRD analysis. The samples were first ground and homogenized in an agate mortar; then an appropriate amount of ethanol was added into the mortar to form a slurry. The slurry was poured on the zero-background silicon disk and the ethanol was evaporated naturally in 10-15 mins, after which the sample was forwarded to XRD analysis. The X&#;Pert PRO MPO Alpha1 powder XRD (PANalytical, Netherlands) analysis used Cu Ka radiation and a scan rate of 2°/min from 10° to 90° (acceleration voltage 40 kV, current 40 mA).

The EPMA used for confirming the EDS analyses was a Cameca SX100 (France) equipped with five wavelength dispersive spectrometers. The samples were repolished using water-based suspension. The accelerating voltage used was 20 kV, beam current 10 nA, and beam diameter 5 and 10 µm. The analyzed characteristic X-ray lines and external standards (Astimex) used were S Kα (sphalerite), O Kα (obsidian), Ag Lα (Ag), and As Kα (cobaltite). The average detection limits were 300 ppmw, 940 ppmw, ppmw, and 120 ppmw for Ag, As, O, and S, respectively.

The XPS spectra were recorded using a Kratos Axis Ultra spectrometer at 45 W power (instead of typical 200 W[22]) and Al anode (.7 eV) using exposure times of 1, 5, and 10 minutes. The measurements were performed with a 20 keV pass energy and 0.1 eV energy step. Survey spectra and the high-resolution (HR) spectra of As 3d, O 1s, S 2p, Ag 3d, and C 1s regions for selected 13 samples from the experimental series were measured. The instrument has been calibrated (a two-point calibration) using Au 4f and Ag 3d peaks. For each measurement/sample, a one-point energy correction was made where the binding energy (B.E.) of C 1s was set to a reference value (285.0 eV). The peak fitting was performed with the CasaXPS software.[23]

The speciation of arsenic was estimated from the As 3d peaks, combined with information collected from the S 2p peaks. It could be observed that the XPS data obtained from the deposits generated at the lowest oxygen partial pressure (p(O2) = 0.5 vol pct) were not stable from one electron radiation exposure time to another. Moreover, due to the very low concentration of silver in the deposits, its HR spectra exhibited a low S:N ratio.

The fitting included base line approximation by Shirley extrapolation. The HR peak shapes were fitted with gmix values of 0.6 for metal and 0.3 for non-metal peaks.[24] The speciation of arsenic included metallic component (As° with B.E of 41.4 eV), trivalent oxide component (As(III-O) with B.E. of 44.9 eV), pentavalent oxide component (As(V-O) with B.E. of 45.8 eV), and sulfidic components (As(III-S), orpiment (with B.E. of 43.5 eV) and As(II-S), and realgar (with B.E. of 43.4 eV).[22,25,26] The HR spectrum of sulfur was fitted using the peak of sulfur in orpiment and realgar with B.E. of 161.05 eV,[26] elemental sulfur with B.E of 164.1 eV, and +6-valent sulfur (sulfate) with B.E. of 168.0 eV.[26,27,28] Some fitting results are shown numerically in Table S-4 in electronic supplementary file.

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