Elsevier

Hydrometallurgy

Volume 90, Issues 2–4, February 2008, Pages 92-102
Hydrometallurgy

Novel atmospheric scorodite synthesis by oxidation of ferrous sulfate solution. Part I

https://doi.org/10.1016/j.hydromet.2007.09.012Get rights and content

Abstract

A new atmospheric scorodite synthesis process was investigated. A large size and good crystalline scorodite was precipitated at 95 °C even in a short time of 1 to 7 h when ferrous ions were oxidized by oxygen gas in the presence of As(V) ion with concentrations as high as 50 g/L As in sulfuric acid solution. The key point of the process is the ferrous state of iron in solution and the oxidation of ferrous ions during the scorodite precipitation. This scorodite has a particle size of 15 μm with approximately 10% moisture content (wet base) even under atmospheric conditions. The arsenic content was about 30% by mass. The results of the leach test are very desirable. The process can be applied to a primary smelter which produces copper, zinc, lead and other secondary materials.

Introduction

Chalcophile elements (e.g., As, Sb, Bi, Se, Te) are typically present in non-ferrous metals that are smelted (e.g., Cu, Zn, Pb) and are eliminated out of smelters as a slag to a minor extent. A considerable portion of undesired elements that remain in the smelting process are recovered and condensed from intermediate refinery products. Among these recovered undesired materials, arsenic-bearing compounds are retrieved in high volume and find a very limited market due to the severe arsenic toxicity to humans and the environment. Considerable amounts of arsenic by-products, therefore, are stored and impounded in a concentrated form that prevents subsequent leaching because the demand is much smaller than the supply. Fixation of arsenic is therefore a key technology to a sustainable copper industry.

Chemical stabilization methodologies for controlling arsenic have long been studied and implemented, as recently reviewed by Harris (2003). Based on a survey of industrial practices, Harris summarized the methods currently applied for bulk removal and disposal of arsenic as follows:

  • 1.

    Neutralization with lime: Not effective at completely removing arsenic, easily releasing arsenic with decreased pH.

  • 2.

    Neutralization with lime plus ferric iron: Removes arsenic down to < 0.1 mg/L, high molar Fe/As, high volume residues.

  • 3.

    Pressure oxidation to form scorodite or Type I/Type II mineral: Eliminates As < 0.1 mg/L but requires autoclave.

  • 4.

    Arsenic sulfide: Formed residue is unstable at pH  4.

  • 5.

    Arsenic trioxide: Not environmentally stable, requires a market for later use.

  • 6.

    Atmospheric scorodite: Pilot study only.

Scorodite or crystalline ferric arsenate dihydrate is a very stable mineral, suitable for arsenic stabilization and storage. This species of ferric arsenate (FeAsO4) compounds has very low solubility and high stability under acidic to neutral pH conditions. Scorodite crystals are more stable to dissolution when their crystal size is larger (Dove and Rimstidt, 1985). Conversely, amorphous ferric arsenate compounds are very unstable, and dissolved arsenic levels have been reported to reach 20 mg/L (Krause and Ettel, 1989).

Researchers have proposed hydrothermal precipitation of iron-arsenate compounds at temperatures above 100 °C (Dutrizac et al., 1987, Dutrizac and Jambor, 1988). Such reactions are more likely to produce well-crystallized precipitates with a lower molar Fe/As ratio, which allows for a higher As content. Typically the precipitates synthesized in this way are in the scorodite form (FeAsO4·2H2O), although Type I or Type II crystalline arsenic compounds have been reported to occur depending on the hydrothermal temperature conditions (Monhemius and Swash, 1996, Monhemius and Swash, 1999, Swash and Monhemius, 1994).

The formation of arsenic minerals by autoclaving demands a large capital investment since it requires sealed vessels that are resistant to high pressure, heat, acid, and abrasion as well as incidental facilities such as mechanical seals, heat exchangers, flash tanks, and so forth. Moreover, this process is unfavorable with respect to high running costs due to vapor heat loss. Furthermore, the autoclave pressure oxidation process is still incapable of properly controlling arsenic leaching. As pointed out by Harris (2003), this is partly because the resulting fine scorodite particles are not satisfactorily filtered out and thus require a polishing step for the resulting waste liquors.

The hydrothermal synthetic reaction process, therefore, requires the following key improvements: (1) arsenic dissolution levels should be lowered, and (2) construction and variable expenses should be decreased—i.e. the reaction temperature lowered, reaction volume reduced (reaction concentration increased), and the reaction time shortened.

For these reasons, scorodite synthesis at temperatures below 100 °C under atmospheric conditions has been a matter of great concern throughout industry. Demopoulos and colleagues at McGill University have been vigorously studying this field (Demopoulos, 1996, Demopoulos et al., 1994, Demopoulos et al., 2003, Droppert et al., 1996, Filippou and Demopoulos, 1997, Debekaussen et al., 2001). They proposed a synthetic scorodite approach which can be summarized as: (1) oxidation of arsenic in effluents to As(V) (AsO43−), (2) addition of Fe3+, H2SO4, and MgO to adjust the pH, (3) addition of crystal seeds, and (4) 3–4 incremental increases of pH. Their ambient pressure precipitation process involves the following two key techniques:

  • Addition of scorodite seed to maintain a low supersaturation level of ferric arsenate.

  • Strict control of ferric arsenate solution pH by means of magnesium or calcium hydroxides.

Practical application of this process in industry settings, however, still needs further elaboration. The addition of seed crystals indicates that surplus residues are fed back to the preceding step, which is an unfavorable aspect for production efficiency. Moreover, the addition of magnesium or calcium compounds requires neutralizing agents; and the use of calcium salts results in gypsum being mixed into the sediment. This poses a problem for producing low-volume residues since the addition of gypsum results in a decreased arsenic content. A patent application filed in Japan (Miyagawa et al., 2005) has demonstrated a low-efficiency arsenic processing method that employs the addition of crystal seed and requires more than 12 h of reaction time to convert amorphous Fe–As precipitates into crystalline form.

In this situation, we embarked on a study to develop a scorodite synthesis process that satisfies the following points: (1) atmospheric process (2) easy re-pulp and filtration (3) stable for leaching tests (4) low operation cost. Elimination of iron in a hydrometallurgy process is typically performed by precipitation as jarosite, goethite, or hematite. In an actual industrial environment, this process is performed following oxidation of ferrous to ferric ions. Therefore, we investigated the possibility of inducing arsenic co-precipitation in the atmospheric iron elimination process by controlling the iron oxidation process. Extensive studies have achieved atmospheric scorodite formation that is free from the use of seed crystals and neutralizing agents such as calcium salts. The scorodite precipitate particles we have produced have a diameter of 10 to 20 μm — larger than the often reported range of sub-microns to 5 μm. The precipitated grains were uniformly crystallized, not an agglomeration of smaller particles. The major focus of this work is the superiority of the increased crystalline scorodite particle size for arsenic fixation and disposal.

Section snippets

Materials

Aqueous arsenic solution, derived from the purification and concentration of refinery arsenic compounds, was used for our experiments. The physical–chemical properties and the chemical composition of this solution are shown below (Table 1).

Analytical-reagent grade iron(II) sulfate hepta-hydrate was used as the iron source. Oxidation was performed by directly introducing oxygen gas (99.9% purity) into the reactor.

Experimental apparatus

The apparatus used in our experiments is illustrated in Fig. 1. The reaction

Effect of reaction time

Fig. 3 illustrates changes with time of the pH, solution arsenic and iron concentrations for O2 gas oxidation reaction. The pH decreased from an initial value of 1.02 to 0.68 in 5 min, then further to below pH 0 in 60 min. No significant pH changes were observed thereafter. The ORP gradually increased from an initial value of 160 mV to above 450 mV in 7 h.

Precipitation started at 5 min, and > 80% of the arsenic content precipitated within 60 min. Precipitation continued further with 97%

Iron precipitation associated with oxidation of ferrous ion

Our experimental results showed that scorodite was readily formed and precipitated by co-precipitation of ferric and arsenic ions in an O2 gas oxidation reaction of ferrous ion present in a mixture containing high-concentration arsenic(V). The precipitation reaction can be expressed as:4H3AsO4 + 4FeSO4 + O2(g) + 6H2O  4FeAsO4  2H2O + 4H2SO4.

Various scorodite precipitation reactions have been previously reported (Dutrizac et al., 1987, Dutrizac and Jambor, 1988, Monhemius and Swash, 1996, Monhemius and

Conclusions

An atmospheric scorodite synthesis reaction has been investigated in which precipitation is produced by oxidising ferrous ion with oxygen gas in the presence of up to 50 g/L As(V). The results indicate that well-crystallized scorodite precipitates as large as 15 μm are formed in a short time of 1 to 7 h. with less than 10% in moisture content. They were readily washed and feature excellent packing properties. Moreover, the precipitated samples yielded a very low arsenic concentration of

Acknowledgements

Dr. Kazuteru Tozawa and Dr. Kazuo Koike are thanked for their helpful discussion and advice.

References (24)

  • J.E. Dutrizac et al.

    The synthesis of crystalline scorodite FeAsO4 × 2H2O

    Hydrometallurgy

    (1988)
  • E. Krause et al.

    Solubilities and stabilities of ferric arsenate compounds

    Hydrometallurgy

    (1989)
  • J. Babcan

    Synthesis of jarosite KFe3(SO4)2(OH)6

    Geoloski Zbornik

    (1971)
  • R. Debekaussen et al.

    Ambient pressure hydrometallurgical conversion of arsenic trioxide to crystalline scorodite

    CIM Bulletin

    (2001)
  • G.P. Demopoulos

    Effluent treatment by crystallization

  • G.P. Demopoulos et al.

    Options for the immobilization of arsenic as crystalline scorodite

  • 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.J. Droppert et al.

    Ambient pressure production of crystalline scorodite from arsenic-rich metallurgical effluent solutions

  • J.E. Dutrizac et al.

    The behaviour of arsenic during jarosite precipitation: reactions at 150°C and the mechanism of arsenic precipitation

    Canadian Metallurgical Quarterly

    (1987)
  • D. Filippou et al.

    Arsenic immobilization by controlled scorodite precipitation

    Journal of Metals

    (1997)
  • B. Harris

    The removal of arsenic from process solutions: theory and industrial practice

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