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On the precipitation and stability of scorodite produced from sulphate media at 950C

Authors
Publisher
McGill University
Publication Date
Keywords
  • Engineering
  • Metallurgy.
Disciplines
  • Ecology
  • Geography

Abstract

Arsenic is a major contaminant in the non-ferrous extractive metallurgical industry and its disposal poses a serious environmental threat. Precipitation of arsenic from aqueous acidic solutions in the form of crystalline scorodite (FeAsO4.2H2O), which is a naturally-occurring mineral, is proposed as an arsenic fixation method. The production of scorodite is usually done in autoclave at high temperature and pressure. However, in a breakthrough at McGill University, a step-wise neutralization procedure has been developed where scorodite can be formed at ambient pressure (95°C). This work investigates the relation between precipitation conditions in sulphate media with the crystallinity, yield, and above all, solubility of scorodite produced. This work has investigated the effect of (1) type of seed, (2) type of base, (3) cationic and anionic additives, (4) Fe/As ratio, and (5) presence of Fe(II) in the precipitation media. It was found that crystalline scorodite can be produced via the use of various types of seed such as hematite and gypsum in addition to scorodite. The stability of the product was not found to be affected measurably upon change of base from MgO to CaO and in the presence of various cations (Cu, Zn, Ni, Co, and Mn) and anions (sulphate, nitrate and phosphate). Most solubilities measured were between 1 and 3 mg/L at pH 5. Solubilities as low as 0.5 mg/L were found when Fe(III):As(V) was 3:1 but the yield was low. Following these results, Fe(II) was added to the system and solubilities as low as 0.2 ppm were observed with yields of over 80% in a single precipitation step.

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