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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).

4


IntroductionFirst motivation


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Gypsum!

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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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Methodology

Introduction

Methodology

Results -Conclusions 7


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Methodology

Introduction

Methodology


Methodology

1.- Synthesis and characterization of

atmospheric scorodite.

2.- Stability test and full characterization of

scorodite particles.


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Methodology

Introduction

Methodology


XRDRaman

ATR-IR

SEM

Chemical

analysis


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Methodology

Introduction

Methodology


Methodology

1.- Synthesis and characterization of

atmospheric scorodite.

2.- Stability test and full characterization of

scorodite particles.


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Methodology

Introduction

Methodology


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

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

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

2.- Stability test and full characterization of scorodite particles.


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Introduction

Methodology

Results-Conclusions

16

Results


Figure 7: 0,5 CaO - Gypsum saturated.Figure 6: 1M NaOH - Gypsum saturated.


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Introduction Methodology Results- Conclusions17

Results


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


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])

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