Elsevier

Hydrometallurgy

Volume 90, Issues 2–4, February 2008, Pages 85-91
Hydrometallurgy

Novel atmospheric scorodite synthesis by oxidation of ferrous sulfate solution. Part II. Effect of temperature and air

https://doi.org/10.1016/j.hydromet.2007.09.011Get rights and content

Abstract

We have previously reported a novel method for producing stable scorodite (FeAsO4·2H2O) under atmospheric conditions in which ferrous sulfate was oxidized by oxygen in the presence of high concentrations of arsenic(V). This work examines the effects of reaction temperature (95, 70, 50 °C) and the oxidizing agent (air, pure oxygen gas) to optimize scorodite formation in a practical process. Crystalline scorodite of low solubility could be prepared at 70 °C within only 7 h using either oxygen or air sparging and small particles were even formed at 50 °C using air oxidation.

Introduction

Arsenic is a common impurity in non-ferrous metallurgical processes (e.g. copper smelting), and the removal of arsenic still remains a major theme for research. As discussed in Part I (Fujita et al., in press) various fixation processes of arsenic have been proposed from the arsenic containing dust generated during smelting processes. The formation of environmentally stable scorodite typically requires a high-temperature treatment; but various atmospheric procedures for scorodite synthesis have been reported — although many of them are not suitable for industrial applications. Conventional synthetic processes have disadvantages in that the crystalline scorodite particles formed are agglomerates of smaller nano-sized particles. Such physical properties make it difficult to wash the particles and also contribute to the long-term uncertainty for the storage of waste scorodite.

To overcome these obstacles, we have investigated a new process for producing large-sized scorodite particles with improved filtration and washability properties under conditions of normal atmospheric pressure and at temperatures below 100 °C (Fujita et al., in press). When oxygen gas was blown into solutions of high arsenic(V) concentration that also contained ferrous ion, the ferrous ion was oxidized to ferric ion and formed large-sized scorodite grains in a short period of time. This follow-up study reports on the use of air for the oxidation process and on the effect of lower reaction temperatures on the properties of the scorodite products formed.

Section snippets

Experimental

The materials, apparatus, and procedures for producing and examining the scorodite are the same as those reported in our previous study, with the exception that oxidation was partly performed with ambient air and the reaction temperatures were 50, 75, or 95 °C. The experimental conditions are summarized in Table 1.

Effects of reaction temperature

The effects of changes in the temperature on pH, oxidation–reduction potentials (ORP's) and solution arsenic and iron concentrations were shown in Fig. 1 for O2 gas oxidation of ferrous ion. With increasing the reaction temperature, the final ORP rose and the pH decreased after precipitation of scorodite. The final concentration of arsenic at 70 °C was same as that at 95 °C, yielding 98% arsenic precipitation. When the reaction temperature was 50 °C, however, the post-reaction arsenic level was

Effects of reaction temperature

The above experimental results showed scorodite precipitation under atmospheric pressure at low reaction temperatures of 50 to 70 °C. As discussed in our previous paper, these results are similar to the formation of iron arsenate at 25 °C as reported by Nishimura (1996). A striking difference, however, is that a 7-hour reaction period was sufficient with our approach. The difference was distinctly ascribable to the reaction in which the scorodite formation proceeded as the ferrous ion was

Conclusions

The effects of reaction temperature and oxygen partial pressure on the properties of the scorodite were investigated under atmospheric conditions using mixtures of ferrous ion and concentrated arsenic(V). Our results indicate that controlled synthesis at 70 °C achieved sufficiently stable scorodite particles and that air oxidation produced scorodite even at 50 °C. This study primarily showed that controlling the degree of supersaturation in the vicinity of the precipitation boundary is crucial

Acknowledgements

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

References (4)

  • Fujita, T., Taguchi, R., Abumiya, M., Matsumoto, M., Shibata, E., Nakamura, T., in press. Novel atmospheric scorodite...
  • Mori, K., Kawabata, M., Horiishi, N., Toda, K., 1987. Method of the preparation of spherical fine particles of...
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