Hydrothermal reaction chemistry and characterization of ferric arsenate phases precipitated from Fe2(SO4)3–As2O5–H2SO4 solutions

https://doi.org/10.1016/j.hydromet.2011.01.007Get rights and content

Abstract

The hydrothermal reaction chemistry and characterization of high temperature (150–225 °C) ferric arsenate phases produced from sulfate media was studied. In particular, the effect of Fe (III)/As (V) molar ratio, temperature, and time on the phases formed was examined. Three major arsenate-bearing phases were produced in our studies: (a) sulfate-containing scorodite (Fe(AsO4)1  0.67x(SO4)x·2H2O where x  0.20) at an Fe(III)/As(V) molar ratio of 0.7–1.87, 150–175 °C and 2–24 h reaction time; (b) ferric arsenate sub-hydrate (FAsH; Fe(AsO4)0.998(SO4)0.01·0.72H2O) at Fe(III)/As(V) molar ratio of 0.69–0.93, 200–225 °C and 10–24 h reaction time; (c) basic ferric arsenate sulfate (BFAS; Fe(AsO4)1  x(SO4)x(OH)x·(1  x)H2O, where 0.3 < x < 0.7) at Fe(III)/As(V) molar ratio of 1.67–4.01, 175–225 °C and 4–24 h reaction time. Scorodite was found to be metastable at an Fe(III)/As(V) molar ratio of one, 1 h reaction time and 200 °C. Extension of retention time to 10 h under the same conditions gave predominantly FAsH with minor presence of scorodite, while additional retention time to 24 h resulted in the formation of only FAsH. XRD, Raman and ATR-IR, proved that our FAsH is the same phase with that labeled previously as “Type 1” and the “Phase 4”; similarly our BFAS was identical to the so-called “Type 2” and “Phase 3”. The Fe L-edge XANES analysis showed that scorodite, FAsH and BFAS exhibited an octahedral type of crystal field with a paramagnetic high spin anti-ferromagnetic character as in hematite. Short term leachability tests determined FAsH to be slightly more soluble than scorodite and BFAS after 24 h, while after several TCLP-like contacts there was < 0.1 mg/L arsenic release from all phases. Long term (> 8 months) stability testing of FAsH and BFAS found FAsH to yield somewhat higher arsenic release than BFAS. The latter's arsenic release potential was evaluated to be equivalent or slightly better than that of scorodite.

Graphical abstract

Research highlights

► Three arsenate phases were formed: scorodite, FAsH and BFAS. ► FAsH was found to be identical with Type 1 and Phase 4. ► BFAS was found to be identical with Type 2 and Phase 3. ► Kinetics were found to affect the formation of these phase. ► BFAS had arsenic retention properties slightly better than scorodite.

Introduction

Autoclave processing of copper and/or gold-bearing mineral feedstocks is associated with the in-situ precipitation of iron (III) arsenates (Berezowsky et al., 1999, Dymov et al., 2004). These precipitates report with the leach residues into tailings ponds. Therefore, characterization and evaluation of the arsenic release (leachability) of these iron (III) arsenates is of great environmental interest.

In contrast to the poorly crystalline Fe(III)–AsO4 waste solids produced by co-precipitation during normal neutralization of hydrometallurgical process effluents (Langmuir et al., 1999, Moldovan et al., 2003, Jia and Demopoulos, 2008, Chen et al., 2009), controlled precipitation (Filippou and Demopoulos, 1997, Singhania et al., 2006) or autoclave processing leads to the precipitation of crystalline phases. Swash and Monhemius (1994) were the first to report on the precipitation and characterization of Fe(III)–AsO4 compounds from sulfate solutions – the type of solutions encountered in industrial processes – under autoclave processing conditions. According to their work four distinct crystalline phases were found to form: scorodite, FeAsO4·2H2O; basic iron sulfate, FeOHSO4 (BFS); “Type1”, Fe2(HAsO4)3·zH2O with z < 4; and “Type 2”, Fe4(AsO4)3(OH)x(SO4)y with x + 2y = 3. The formation of these phases was correlated to temperature (150° to 225 °C) and Fe(III) to As(V) molar ratio (1/1–9/1) and fixed retention time of 24 h. Thus according to these authors, Type 1 formed in the whole temperature range 150 °C to 225 °C, when Fe(III)/As(V) molar ratio < 1.5. Scorodite was reported to form at 150 °C and 175 °C when Fe(III)/As(V) ratio > 1.5. Finally Type 2, in mixture with basic iron sulfate, was reported to form at 200 °C  T  225 °C and Fe(III)/As(V) ratio > 1.5. Of the two new phases, only “Type 2” (< 0.34 mg/L As) was found to meet the TCLP leachability criterion exhibiting similar behavior with scorodite (< 0.8 mg/L As).

More recently, Dutrizac and Jambor (2007) reported on an extensive experimental program involving the precipitation of Fe(III)–AsO4–SO4 phases in the temperature range of 175–225 °C. In their program, the effects of time (1–24 h), initial acidity (0 to 0.71 M H2SO4) and variable Fe(III), As(V) concentrations were considered. Characterization of the precipitated phases, which were all crystalline in nature, identified two new phases in addition to scorodite (FeAsO4·2H2O) and basic ferric sulfate (BFS: FeOHSO4). The two new phases, labeled as “Phase 3” and “Phase 4”, were determined to have the following stoichiometries respectively: Fe(AsO4)x(SO4)y(OH)v(H2O)w where x + y = 1 and v + w = 1; and “Phase 4”, FeAsO4·3/4H2O. Phase 3 was proposed to be a monoclinic polytype of basic ferric sulfate produced via solid-solution uptake of AsO4. Phase 3 precipitated at 175–210 °C, but mixtures of Phase 3 and BFS were found to form at higher temperatures from solutions with Fe/As ~ 4. At Fe/As molar ratio ~ 1 and 205 °C, Phase 4 was found to form instead. Finally in the same work, it was found that scorodite (containing a small amount of sulfate) formed in the 150–175 °C range from solutions with initial Fe/As molar ratio ~ 3 (as calculated by the present authors). Short term (40 h) leachability tests (terminal pH in the range 3.5 to 4.5) that were conducted on the two new phases yielded < 0.1 mg/L As for Phase 3 and 1–3 mg/L As for Phase 4. This observation led the authors to suggest that Phase 3 might be an acceptable carrier for the disposal of arsenic.

From the above brief review, it becomes evident that the hydrothermal Fe(III)–As(V)–SO4 system is very complex resulting in the formation of different phases, the true identity and environmental stability of which is a matter of industrial importance. In this study the hydrothermal precipitation of iron (III) arsenate–sulfate phases is revisited (a preliminary brief communication was made during the Hydrometallurgy 2008 Conference by Gomez et al., 2008) with the objectives of identifying the true nature of the precipitated iron (III) arsenate phases via comprehensive characterization and correlating their formation to prevailing solution chemistry in terms of reaction stoichiometries and temperature. Furthermore, the obtained results are compared to those of Swash and Monhemius (1994) as well as Dutrizac and Jambor (2007) with the view of clarifying the apparent differences between the two previous studies and contributing to the fuller understanding of the overall chemistry of this system.

Section snippets

Precipitation procedure

For the preparation of the starting solutions, analytical-reagent grade As2O5·xH2O and Fe2(SO4)3·xH2O were dissolved in water in the desired molar proportions (CFe = 0.30 M and CAs = 0.075–0.40 M) to give different starting Fe(III) to As(V) molar ratios. The resulting solutions (with natural pH ~ 1) were placed in a two-liter Parr autoclave equipped with a glass liner. The solutions were then heated (typical heat-up period ~ 45 min) to the desired temperature (150–225 °C) and held there for different times

Results and discussion

The precipitation of iron (III) arsenate–sulfate phases was investigated over the temperature range 150–225 °C. Typically 0.3 M Fe(SO4)1.5 solutions containing arsenic (V) at various molar ratios (0.7  Fe(III)/As(V)  4) were treated from 1 to 24 h. There were three iron (III) arsenate phases found to form: (1) sulfate-substituted scorodite [Fe(AsO4)1  0.67x(SO4)x·2H2O] (where 0.00  x  0.22) (2) ferric arsenate sub-hydrate (FAsH) [FeAsO4·0.75H2O], and (3) basic ferric arsenate sulfate (BFAS) [Fe(AsO4)1  x

Characterisation studies

In addition to chemical composition analysis (ICP-AES), the solid products were further analyzed by several analytical techniques (TGA, XRD, SEM, TEM, ATR-IR, Micro-Raman, and XANES). The detailed ATR-IR and Raman vibrational analysis has been already published (Gomez et al., 2010) and the crystallographic electron and X-ray synchrotron based work is currently under way. Emphasis is given in the characterization of the least known phases FAsH and BFAS and their comparison to the phases labeled

Short term arsenic release

Fig. 14 shows the sequential TCLP-like leachability response of scorodite, FAsH and BFAS (details on the TCLP method used in this study maybe found in Bluteau and Demopoulos, 2007). As it can be seen after the first 24 h contact, scorodite and BFAS were well below 1 mg/L As but FAsH yielded about one order higher i.e. ~ 5 mg/L. However, with subsequent contacting all samples yielded values < 0.1 mg/L. The initial higher values measured for FAsH are attributed to possible trace contamination (via

Conclusions

In this work, the hydrothermal reaction chemistry, detailed characterization and experimental arsenic release behavior of ferric arsenate phases produced in the temperature range 150–225 °C from sulfate media was reported. Here are the major conclusions from this research:

  • (1)

    The domain of formation of these phases depends upon four main experimental variables: temperature, Fe(III)/As(V) molar ratio, acidity and time.

  • (2)

    There were four arsenate-bearing phases produced according to the following

Acknowledgements

The support of this project by NSERC via a strategic project grant is acknowledged as well as the sponsorship of the following companies: Areva Resources Inc., Barrick Gold, Cameco, Hatch and Teck Cominco. The authors are also grateful to staff at the SGM beam line and the industrial science group (Tom Regier, Robert Blyth and Jeff Warner) at the Canadian Light Source located at the University of Saskatchewan for their technical support. Canadian Light Source is supported by NSERC, NRC, CIHR,

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