Elsevier

Hydrometallurgy

Volume 87, Issues 3–4, July 2007, Pages 163-177
Hydrometallurgy

The incongruent dissolution of scorodite — Solubility, kinetics and mechanism

https://doi.org/10.1016/j.hydromet.2007.03.003Get rights and content

Abstract

This work reports the results of a laboratory investigation on the long-term stability of crystalline scorodite conducted at fixed pH (5–9) and temperature (22 °C, 50 °C and 75 °C). The scorodite used in this work was prepared via a hydrothermal synthesis procedure. The dissolution of scorodite at 22 °C was found to be extremely slow. At neutral pH, the arsenic concentration stabilized after 24 weeks at 5.9 mg/L. Analysis of the solubility data as a function of temperature yielded a scorodite solubility model equation. The solubility product of scorodite was recalculated as 10 25.4 using the solubility data generated by the study and the geochemical code PHREEQC for solution modelling. As scorodite dissolved, the iron re-precipitated as 2-line ferrihydrite. The growth and re-crystallization of ferrihydrite was apparently retarded by arsenate adsorption. The dissolution rate of scorodite was modeled with a decreasing exponential equation. The initial rate approached first order dependency on OH concentration while the apparent activation energy suggested that scorodite dissolution is chemically controlled.

Introduction

Arsenic is a major contaminant of non-ferrous ores. It enters in metallurgical operations where it is generally discarded from aqueous effluents or smelter gases as a waste. Arsenic-containing wastes can create environmental problems; therefore, strict limits have been set to regulate their disposal. Consequently, the mining/metallurgical industry has been challenged, to find arsenic-carriers having low solubility and high long-term stability (Riveros et al., 2001). Scorodite (FeAsO4·2H2O) is a crystalline ferric arsenate found in nature. Its production has been proposed as a potentially good carrier for the fixation of arsenic in the case of arsenic-rich and iron-deficient effluents (Demopoulos et al., 1994). Among scorodite's advantages are its high arsenic content, its stoichiometric (Fe/As molar ratio = 1) iron demand and its excellent dewatering characteristics. Scorodite can be either produced by hydrothermal precipitation at temperatures higher than 150 °C (Swash and Monhemius, 1994) or by controlled neutralization at 95 °C under atmospheric pressure (Filippou and Demopoulos, 1997, Demopoulos et al., 2003, Singhania et al., 2005, Singhania et al., 2006).

Because of its high toxicity for plants and animals, the scorodite solubility and long-term stability must be thoroughly evaluated. Scorodite passes the EPA TCLP test limit of 5 mg/L (Riveros et al., 2001) and has a low solubility (typically < 0.5 mg/L) between pH 2.8 and 5.3 (Krause and Ettel, 1989). However, it tends to dissolve incongruently in contact with water (see Eq. (1)). Hence concerns have been expressed that over the long-term, scorodite would convert to ferric oxyhydroxide with concomitant release of arsenic to solution (Welham et al., 2000).FeAsO4··2H2O (s)  FeOOH (s) + H2AsO4 + H+

Predictions claiming that scorodite is not stable are mainly based on thermodynamic calculations and are as good as the thermodynamic data used and the assumptions made. Up to about four orders of magnitude difference exists between the published scorodite solubility products (Demopoulos, 2005) depending if their calculation involved solubility data (Dove and Rimstidt, 1985, Krause and Ettel, 1988, Robins, 1990, Rochette et al., 1998, Swash et al., 2000) collected from “amorphous”, poorly-crystalline or crystalline materials (Langmuir et al., 2006). Moreover the solubility itself, especially when measured through short-term equilibration experiments (varying typically from 1 (Swash et al., 2000) to 2 (Krause and Ettel, 1988) to 3 (Rochette et al., 1998) to 8 (Dove and Rimstidt, 1985) weeks), is not a sufficient indicator of long-term stability. The physical and chemical changes that the scorodite solids undergo during the aging period must also be determined (Harris and Krause, 1993). The critical factor in considering scorodite as a safe disposal option would seem to be the rate of arsenic release (Welham et al., 2000). The kinetics of scorodite dissolution (both in terms of rate and mechanism) have not been characterized previously1 but it was thought to be relatively slow in comparison to amorphous ferric arsenate precipitates (Demopoulos et al., 1994).

The objective of this work was to assess the potential of arsenic fixation in the form of crystalline scorodite by equally considering its incongruent dissolution equilibria and kinetics over the environmentally relevant pH range 5 to 9. To this end highly crystalline and pure scorodite material was prepared and tested. New solubility data were generated by employing extended equilibration times (up to 66 weeks) to ensure the attainment of equilibrium. A new solubility product for scorodite was calculated. The phase changes occurring during scorodite dissolution were assessed and the kinetics of scorodite dissolution were quantified. Finally, the temperature effect on equilibrium arsenic concentration was determined.

Section snippets

Experimental

The solubility and long term (up to 66 weeks) stability of crystalline scorodite was studied in the incongruent dissolution region at different pH values (5, 6, 7, 8, and 9) and temperatures (22 °C, 50 °C, and 75 °C). Elevated temperatures were included to (i) accelerate the equilibration process, (ii) facilitate the production of crystalline phases that can be more easily characterized and (iii) allow for a tool be developed suitable for predicting dissolution rate and solubility as a function

Characterization of the starting material

The material produced had the characteristic light green colour of scorodite. A chemical analysis indicated that the product was composed of 33.0 wt.% As and 24.5 wt.% Fe corresponding to a Fe/As molar ratio of 1. The XRD pattern of the solid and its comparison with the standard 26-0778 from JCPDS (PCDF WIN) confirmed that the material was highly crystalline scorodite (see Fig. 1). Its crystal lattice parameters were determined by Le Berre et al. (in press) to be respectively: a = 10.36 Å, b = 

Conclusions

The dissolution of scorodite at 22 °C over the environmentally important pH range 5 to 9 was found to be extremely slow. After more than 60 weeks, equilibrium was not reached at pH ≥ 8. At neutral pH, the arsenic concentration stabilized at 5.8 mg/L. From the pH 5 and 7 experiments, the solubility product of scorodite was calculated as 10 25.4 by taking into consideration Fe(III)–AsO4 complexes. This value is close to the solubility product recently calculated by Langmuir et al. (1999), which is

Acknowledgment

The support of this work by the Natural Sciences and Engineering Research Council (NSERC) via a strategic project grant is gratefully acknowledged, as is the sponsorship of Hatch, Areva Resources Inc., and Barrick Gold Corporation. NSERC is also acknowledged for the financial help provided to M.C. Bluteau in the form of a post-graduate scholarship.

References (30)

  • G.P. Demopoulos et al.

    The atmospheric scorodite process

  • P.M. Dove et al.

    The solubility and stability of scorodite, FeAsO4·2H2O

    American Mineralogist

    (1985)
  • D. Filippou et al.

    Arsenic immobilization by controlled scorodite precipitation

    JOM

    (1997)
  • G.B. Harris et al.

    The disposal of arsenic from metallurgical processes: its status regarding ferric arsenate

  • M.C. Harvey et al.

    Scorodite dissolution kinetics: implications for arsenic release

    Environmental Science & Technology

    (2006)
  • Cited by (133)

    View all citing articles on Scopus
    View full text