Electrochemistry of Enargite:Reactivity in Alkaline Solutions
R. Nick GowMontana Tech / University of Montana
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?