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Study of Arsenic release of atmosphericScorodite in reductive environments
Csar Verdugo1, Gustavo Lagos1
Levente Becze2,Mario Gmez2& George Demopoulos2
1Departamento de Ingeniera de Minera, Pontificia Universidad Catlica de Chile ([email protected])2Departament of Mining and Material Engineering, McGill University ([email protected])
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Outline1. IntroductionProblems and challenges of arsenic in metallurgic process
2. Objectives
What are the goals of this research?
3. MethodologySynthesis
Stability test
Characterization
4. Results
Characterization of initial solidsStability test
Characterization final
5. Discussion and Conclusions
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Introduction
Arsenic (As) is major contaminant present in wastes from metallurgical and mining
industries of non-ferrous.
Due to its high toxicity, environmental regulations are becoming increasingly more
stringent regardings its environmental disposal.
Many efforts have been made to confine As for disposing in relatively stable phases
and therefore, immobilize this toxic element.
Mainly two
methods
Co-precipitation of As(V) with Fe(III) by lime
neutralization in the form of poorly-
crystalline Fe(III)-AsO4solids (molar ratio > 3)
Crystalline scorodite:1.- Autoclave : 150 C or higher
2.- Atmospheric: 95 C, involving
supersaturation control, avoiding nucleation.
Introduction- Methodology - Results - Conclusions
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Previous work on scorodite has focused on the determination of its stability in oxidizing
conditions. In fact, can dissolve either congruently or incongruently, depending on pH.
Congruent dissolution: FeAsO4 2H2O + H+=> H2AsO4
-+ Fe(OH)2++ H2O (Dove and Rimstidt, 1985)
Incongruent dissolution: FeAsO4 2H2O + H2O => H2AsO4-+ Fe(OH)3(s)+ H
+(Zhu and Merkel, 2001)
FeAsO4 2H2O + H2O =>HAsO42-+ Fe(OH)3(s)+ 2H
+ (Zhu and Merkel, 2001)
However little is known under reducing conditions.
Why would be important to study this condition?
The dissolved oxygen concentration decreases as the depth increases in tailings ponds, contributing to
various reducing media, generating the potential release of As to the environment.
The reductive dissolution of scorodite could be catalyzed by Fe (III)-reducing bacteria, such as
Shewanella algaor Desulfuromonas palmitatisamongst others, which release dissolved arsenate as a
result of dissimilatory reduction of Fe(III) to Fe(II) (Cummings, et al., 1999; Papassiopi et al., 2003).
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Introduction- Methodology - Results - Conclusions
IntroductionFirst motivation
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Gypsum!
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Introduction- Methodology - Results - Conclusions
IntroductionSecond motivation
Lime is widely used for neutralization of sulphate-containing
acidic effluents (Langmuir et al., 1999; Jia & Demopoulos, 2008).
Due to this, the porewater of mining and
metallurgical tailings ponds are gypsum-saturated
(Bluteau et al.2009).
This opens the possible interaction of scorodite
and gypsum or its constituents Ca2+or SO42-
Some facts:
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Objectives
This research considers 2 goals under concentrated aqueous solutions of As (40 g/L):
1. To determine the stability of atmospheric scorodite from oxic to anoxic conditions
at pH 7 until its equilibrium is reached.
2. To evaluate the behavior of scorodite particles under Redox conditions and pH 7 in
the presence of gypsum-saturated environments.
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Introduction- Methodology - Results - Conclusions
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Methodology
Introduction
Methodology
Results -Conclusions 7
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Methodology
Introduction
Methodology
Results -Conclusions 8
Methodology
1.- Synthesis and characterization of
atmospheric scorodite.
2.- Stability test and full characterization of
scorodite particles.
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Methodology
Introduction
Methodology
Results -Conclusions 9
XRDRaman
ATR-IR
SEM
Chemical
analysis
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Methodology
Introduction
Methodology
Results -Conclusions 10
Methodology
1.- Synthesis and characterization of
atmospheric scorodite.
2.- Stability test and full characterization of
scorodite particles.
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Methodology
Introduction
Methodology
Results -Conclusions 11
No gypsum
Saturated- gypsum1M NaOH
0,5M CaO
1. Chemical analysis
2. XRD3. SEM
4. ATR-IR
5. RAMAN
Redox conditions:DI water 400 mV 250mV -100mV
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Introduction
Methodology
Results- Conclusions 12
Results
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Introduction
Methodology
Results- Conclusions 13
Results
1.- Synthesis and characterization of atmospheric scorodite.
Atm. scorodite
Intensity(relative
)
Figure 1: XRD patterns.
Ramanintensity(
relative)
Figure 2: Raman spectroscopy.
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Introduction
Methodology
Results- Conclusions 14
Results
1.- Synthesis and characterization of atmospheric scorodite.
Figure 3: ATR-IR spectrum. Figure 4: SEM images.
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Figure 5: 1M NaOH No Gypsum.
Introduction
Methodology
Results- Conclusions 15
Results
2.- Stability test and full characterization of scorodite particles.
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Introduction
Methodology
Results-Conclusions
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Results
2.- Stability test and full characterization of scorodite particles.
Figure 7: 0,5 CaO - Gypsum saturated.Figure 6: 1M NaOH - Gypsum saturated.
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Introduction Methodology Results- Conclusions17
Results
2.- Stability test and full characterization of scorodite particles.
Wavenumber (cm-1)
Figure 8: XRD patterns after stability test. Figure 9: Raman spectroscopy after stability test.
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Introduction Methodology Results- Conclusions18
Results
2.- Stability test and full characterization of scorodite particles.
Wavenumber (cm-1)
Figure 10: ATR-IR spectra after stability test. Figure 11: SEM images after stability test.
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Introduction Methodology Results - Conclusions19
Conclusions
It was found that the use of NaOH as base during stability test, caused problems
resulting in high arsenic release, due to apparent strong local release of alkalinity.
The release of arsenic from scorodite particles was dramatically mitigated, due to
the presence of gypsum.
With the exception of the 100 mV severe anoxic environment, the release of
arsenic from atmospheric scorodite in the presence of calcium/gypsum did not
exceed 5 mg/L, even under mild reducing potential of about 250 mV.
This implies that from a disposal point of view scorodites arsenic release rate is
manageable when it is codeposited with gypsum near neutral pH and not less than250 mV reducing environment.
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Introduction Methodology Results - Conclusions20
Conclusions
In fact, some publications support this phenomenon:
i. Harris and Monette, 1987 and Krause and Ettel 1989 found that Ca(OH)2yielded lower residual As concentration than NaOH.
ii. Similar results were found by Emett and Khoe 1994, where the presence
of calcium ions decreased significantly the dissolved As in neutral toalkaline pH range conditions.
iii. Latter Jia and Demopoulos 2005, observed soluble calcium sulphate
enhanced the uptake of arsenate by ferrihydrite.
iv. Bluteau and colleagues 2009, postulated that the co-adsortion of Ca2+with
arsenate on ferrihydrite was one of the As retention enhancement
The real interaction of scorodite with Ca2+ and/or SO42- is still unknown
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Acknowledgment
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This research was carried out at McGills
Hydrometallurgy Laboratory supported
via NSERC CRD grant.
Additional support was provided by:
Beca apoyo de realizacin de
tesis doctoral CONICYT N
24091003. Vicerrectora de Investigacin
(VRI), Pontificia Universidad
Catlica de Chile.
Direccin de Postgrado de la
Escuela de Ingeniera UC (DIPEII),
from Pontificia UniversidadCatlica de Chile.
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Study of Arsenic release of atmosphericScorodite in reductive environments
Csar Verdugo1, Gustavo Lagos1
Levente Becze2,Mario Gmez2& George Demopoulos2
1Departamento de Ingeniera de Minera, Pontificia Universidad Catlica de Chile ([email protected])2Departament of Mining and Material Engineering, McGill University ([email protected])