Electrochemistry of Enargite:Reactivity in Alkaline Solutions

R. Nick GowMontana Tech / University of Montana

(E) robert.gow@flsmidth.com

Introduction to Enargite

Project Objectives

Thermodynamic Database

Raman Spectroscopy

Cyclic Voltammetry

Updated EH - pH Diagram

Conclusions

Further Work

Cu-As Sulfosalts CuAsS Lautite Cu3AsS4 Luzonite (tetragonal) Cu3AsS4 Enargite (orthorhombic) Cu6As4S9 Sinnerite Cu12As4S13 Tennantite

Usually some Sb substitution(will see in the sample MLA scans)

Environmental/Processing Issues

Smelter limitations 0.5% As with futureconsideration to be lowered to 0.3%

One of the more refractory copper sulfides

Higher reagent consumption for gold processes Copper competes for cyanide Arsenic competes for oxygen

Potential Processing Methods Reductive Pre Roast Selective Flotation Sulfuric Acid Bake

Bioreduction of Sulfur Bioleach of Copper

Albion -Ultrafine Grind

Galvanox Pyrite Catalyzed Leach Acidic Pressure Leach Nitrogen Species Catalyzed

Hypochlorite Sunshine -Alkaline Sulfide

PROJECT OBJECTIVES

Task 1 Compile a thermodynamic database for Cu-As-S system Comparison of Cu-As-S Mass Balanced and Line Plot Diagrams

(Oxidation to Sulfate vs Oxidation to Sulfide)

Task 2 Modification and Validation of Thermodynamic Models using acombination of Raman Spectroscopy and Cyclic Voltammetry

Reactivity in Alkaline Solutions (Selective As Leach) Reactivity in Acidic Solutions (Co-dissolution of As and Cu) Addition of HS- as a lixiviant

Task 3 Determine Viability of Arsenic Adsorption using aNanographene Material

Novel nano-graphene material as an arsenic adsorbent

Task 1 Compile a thermodynamic database for Cu-As-S system Comparison of Cu-As-S Mass Balanced and Line Plot Diagrams

(Oxidation to Sulfate vs Oxidation to Sulfide)

Task 2 Modification and Validation of Thermodynamic Models using acombination of Raman Spectroscopy and Cyclic Voltammetry

Reactivity in Alkaline Solutions

Reactivity in Acidic Solutions (Co-dissolution of As and Cu) Addition of HS- as a lixiviant

Task 3 Determine Viability of Arsenic Adsorption using a Nanographene Material Novel nano-graphene material as an arsenic adsorbent

THERMODYNAMIC DATABASE

Over 100 Cu-As-S Species compiled from severalsources Minteq (primary database) USGS Several other species taken from individual authors

Species G (kcal) Species G (kcal)

Cu3AsS4 Enargite -49.808 Cu(AsO2)2 -169.2

Cu12As4S13 Tennantite -180.908 Cu3As - Domeykite -3.184

Cu6As4S9 Sinnerite -110.256 H3AsSO2 -103.412

StabCal Thermodynamic Stability Program (Huang, 2013)

Mass balanced EH pH diagrams (@ 25C) at the followingmolecular ratios (Cu:As:S)

0.66 - 0.44 - 1 (Sinnerite) 0.75 - 0.25 - 1 (Enargite) 0.92 - 0.77 - 1 (Tennanite) 1 - 1 - 1 (Lautite)

Sulfur vs Sulfate Oxidation States

Previous Cu-As-S Stability Diagram

Activity of soluble species 0.1, 25C

Cu + AsH3 vs

Padilla et al, 2008.

Cu3As

CuO + HAsO42- vsCu2AsO4OH

Cu-As-S Stability Diagrams:Oxidation to Sulfate

Cu-As-S Stability Diagrams:Oxidation to Sulfur only

RAMAN SPECTROSCOPYMineral phase determination in conjunction with RRUFF database

(Downs, 2006)

Raman Spectroscopyn Renishaw Raman 100 InVian 100 mW He-Ne laser (632.8 nm)

n Pt counter electroden Ag/AgCl Sat. KCl ref. electroden Mineral working electrode

MLA - Butte MLA - Peru

Characteristic Raman of Enargite

Covellite (CuS) pH 9, 300 mV, 60 minChalcocite (Cu2 S) pH 12, -600 mV, 10 min

Enargite Surface Progression - pH 9, -1000 mV vs SHE, 60 min

Enargite Surface Progression - pH 9, 0 mV vs SHE, 60 min

Possible Olivenite

Enargite Surface Progression - pH 9, 500 mV vs SHE

Arsenate Hydroxyl (Frost et al, 2002)

CYCLIC VOLTAMMETRYMultifile Raman program to frequently scan the surface as the potentialcycled

Voltammetry ProfileScan direction Negative or PositiveScan speed 5 mV/sUpper Potential 800 mV vs Ag/AgCl saturated KClLower Potential -1200 mVStart Potential -200 mVNumber of cycles 3Not stirred

Raman Spectra 5 sec per scan100% Laser power

pH 8 CV Scan

pH 8 CV Scan

C1C2

C3C4

Cycle12

3

Cu3AsS4 Cu12As4S13 Cu2S + As Cu0

C1 C2 C3

CuS CuxS Cu2S

C4

pH 8 CV ScanCycle

12

3A1

A3A2

(A5)

(A4)

Cu0 Cu2S CuS

Cu(OH)2

Cu2AsO4OH

A1 A2

A3 As HAsO43-(A4)

Cu3AsS4 Cu2AsO4OH

(A5)

MODIFICATIONS TO THE EH PHDIAGRAMS

Oxidation to sulfur only database.

Based on Raman results - removed CuO, Cu2O, and Cu3As.

Region of elemental sulfur formation included

The dotted line indicates the regions of water stability, copper-only transitions are indicated bythe dashed line, and area above the bolded line indicates stability of elemental sulfur.

Updated EH-pH Diagram for the Cu-As-S system overlaid with CV Inflection Points

Nonstoichiometric transition between CuS and Cu2S

Potential operating region: ~ -300 mV, pH >12

CONCLUSIONS & FINDINGS Potential identification of olivenite

Arsenic depletion layer (CuxS/S) causes sulfurformation above -200 mV

Cu3AsS4 AsO43- + CuxS/Cu(OH)2 + 3 S0

Non-stoichiometric metastable copper sulfides may explainsome of the CV transitions

Operating region corresponds to the expected regionfor alkaline sulfide

Acknowledgements

Co-authors H. Huang and C. Young Montana Tech G. Hope Griffith University

QUESTIONS?