Temperature and seeding effects on the precipitation of scorodite from sulfate solutions under atmospheric-pressure cond
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NTRODUCTION
ARSENIC is a major contaminant in the nonferrous metallurgical industry. It is usually deported and fixed as an amorphous Fe(III)–As(V) co-precipitate with high ironto-arsenic molar ratio—usually higher than four.[1,2] This practice, however, produces voluminous sludges with notwell-understood stability.[3] Disposal of arsenic as crystalline scorodite (FeAsO4•2H2O) seems to offer several advantages[4] over the current practice of arsenic disposal. Despite some concerns about the long-term stability of crystalline ferric arsenate in acidic-to-neutral pH environments,[5] well-crystallized scorodite can pass as an acceptable residue as it may decompose very slowly releasing only small amounts of arsenic back into the aqueous environment. Scorodite can be easily produced by hydrothermal precipitation at temperatures above 150 °C.[6] Nonetheless, the hydrothermal precipitation of scorodite requires the use of autoclaves and it is economically attractive only if it can be combined with the processing of a valuable concentrate such as gold-carrying pyrite, copper concentrate, etc. In the last 10 years, Demopoulos and co-workers[1,4,7–13] from McGill University have worked extensively on the fixation of arsenic from chloride and sulfate solutions under
atmospheric-pressure conditions at temperatures about 95 °C. It was thus proved that it is possible to precipitate wellcrystallized scorodite at temperatures below the solution boiling point. The ambient-pressure precipitation technique developed at McGill University is based on controlled crystallization, that is, on careful pH control that favors crystal growth against excessive homogeneous nucleation. The technique has the advantage of being less capital intensive than its hydrothermal counterpart, and can be operated independently of another metal-recovery process. As part of that on-going research on ambient-pressure scorodite precipitation, the authors have lately examined the effects of temperature and seeding on the effectiveness of arsenic removal and fixation. As outlined in Section II, there is a significant effect on the kinetics of scorodite precipitation by varying the temperature from 80 °C up to the solution boiling point. The authors have also noted that other mineral phases, namely, hematite and gypsum, can serve as seed where scorodite can grow. The study is complemented by extensive characterization data, from scanning-electron microphotographs and X-ray diffractograms to results from toxicity-characterization leachability tests.
II. EXPERIMENTAL SHALABH SINGHANIA, formerly Master’s Student, Department of Mining, Metals and Materials Engineering, McGill University, is with Telarix, Inc., Vienna, VA. QIANKUN WANG, formerly Postdoctoral Fellow, Department of Mining, Metals and Materials Engineering, McGill University, is with Inco Ltd., Mississauga, ON, Canada. DIMITRIOS FILIPPOU, formerly Doctorate Student and Postdoctoral Fellow, Department of Mining, Metals and Materials Engineering, McGill University, is with Rio Tinto Iron & Titanium Inc.,
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