Linear sweep voltammetry as a technique to characterize mining wastes

Francisco Carranza, Pablo González, Rafael Romero & Nieves Iglesias* Manuel J. García

MINING WASTE

GOAL

The aim of this study is the development of a methodology for the mining waste

characterization and the prediction of their potential long-term degradation, based on

electrochemical tests of short duration.

Linear sweep voltammetry

The characteristics of the linear sweep voltammogram recorded depend on: • The rate of the electron transfer reaction(s) • The chemical reactivity of the electroactive species • The voltage scan rate

Sample

1 2 3 4 5 6 7 8 9 10

Marl Stockwork Carbonated

sulfide Gossan sulfide

Massive sulfide

Arseno-pyrite

Low-grade sulfide

Copper pyrite

Slate Porphyry

Ag (mg/kg) 29,2 26,6 41,4 127,2 18,2 6,8 25,0 36,7 51,1 5,5 As (mg/kg) < 50 1358,7 1807,6 2109,4 1712,6 1436,5 3300,8 1676,3 679,6 < 50 B (mg/kg) 0,9 0,2 0,2 0,1 0,1 0,0 1,3 0,2 0,0 0,2 Bi (mg/kg) < 20 < 20 89,9 823,4 72,3 < 20 57,1 301,5 436,0 < 20 Ca (%) 10,17 < 0,01 < 0,01 1,49 0,13 0,01 0,12 0,49 < 0,01 7,44 Cd (mg/kg) < 20 < 20 20,0 < 20 < 20 < 20 33,6 80,4 < 20 < 20 Co (mg/kg) 11,4 48,4 127,3 20,3 62,4 15,5 133,1 116,9 25,5 19,5 Cr (mg/kg) 43,0 79,1 50,2 57,6 81,4 67,1 45,0 27,4 84,1 73,7 Cu (%) 0,004 0,071 0,565 0,260 1,608 0,013 0,584 1,066 0,697 0,003 Fe (%) 3,4 28,2 39,9 20,4 16,5 4,0 14,5 15,1 7,6 5,3

Hg (mg/kg) < 5,0 < 5,0 < 5,0 < 5,0 < 5,0 8,9 < 5,0 < 5,0 < 5,0 < 5,0 Li (mg/kg) 22,8 22,9 108,8 < 10 < 10 < 10 45,1 < 10 < 10 < 10

Mn (mg/kg) 875,3 994,9 1182,8 208,8 264,0 516,8 458,3 288,1 21,9 724,8

Mo (mg/kg) < 10 < 10 < 10 59,3 < 10 < 10 < 10 32,1 < 10 < 10

Na (mg/kg) 1112,9 < 100 < 100 < 100 < 100 < 100 < 100 < 100 < 100 75,7 Ni (mg/kg) 36,7 38,7 < 20 < 20 < 20 38,0 < 20 35,6 46,6 < 20 Pb (%) 0,003 0,033 0,426 4,065 0,174 0,000 0,182 0,501 0,132 0,000 Sb (mg/kg) 60,9 66,1 689,3 1170,0 141,2 11,4 563,9 716,5 75,4 143,7 Se (mg/kg) < 50 < 50 < 50 < 50 361,3 < 50 < 50 < 50 57,5 < 50 Sn (mg/kg) 30,9 310,8 485,7 155,1 455,1 39,2 458,8 393,2 72,4 48,4 Tl (mg/kg) < 50 < 50 < 50 < 50 < 50 < 50 < 50 < 50 80,8 < 50 V (mg/kg) < 50 53,5 65,4 < 50 < 50 < 50 < 50 < 50 < 50 < 50 Zn (%) 0,008 0,071 1,299 0,012 0,009 0,010 0,620 0,930 0,018 0,008

C R W

A

V

Linear sweep voltammetry

CV-27 Bioanalytical Systems

Linear sweep voltammetry

C R W

A

V

Three electrodes system: W: working electrode (CPE, PE) R: Reference electrode calomel, +0.2444V vs. SHE C: counter electrode Pt 1mm

Experimental conditions: • Potential applied: from -0.1 V to +2.5 V • Distance between working and reference electrodes: 1-2 mm • Luggin capillary • Sweep rate: 5 mV/s. • Electrolyte: solution of pH 1.5 (sulfuric acid in ultrapure water). • Electrolyte volume: 25 mL • Atmosphere: Nitrogen is bubbled for 1 minute before each test.

Linear sweep voltammetry

Linear voltammetry of CPE 2 H2O → 4 H+ + O2 + 4 e-

Linear voltammetry of CPE

Residues sorted in order of increasing corrosion threshold

(I = 1 mA)

Material Potential (V) Copper pyrite 0.06 Massive sulfide 0.23 Carbonated sulfide 0.32 Gossan/sulfide 0.41 Arsenopyrite 0.46 Low-grade sulfide 0.48 Slate 0.50 Porphyry 0.56 Marl 0.57 Blank 0.91 Stockwork 0.98

Material Intensity (mA) Copper pyrite 70.1 Carbonated sulfide 19.6 Massive sulfide 15.8 Low-grade sulfide 14.7 Gossan/sulfide 10.9 Slate 8.6 Marl 5.8 Arsenopyrite 4.3 Porphyry 3.2 Stockwork 1.2 Blank 1.4

Residues sorted according intensity of degradation

(V =1 V)

Washing

PE Air washing Anoxic washing

Metal analysis ICP

Evolution of Cu, Fe and Zn concentrations in air-washing

Time (min) Sample ppm 30 195 560 1315 1875 2910 4308

[Cu] 582 610 595 645 465 650 635 Massive sulfide

[Fe ] 150 254 332 402 382 417 467

[Zn] 0.6 0.6 0.6 0.7 0.7 0,8 0.9 [Cu] 6.0 9.1 8.4 23 29 40 53 Low-grade

sulfide [Fe ] 219 755 1150 1175 1205 1235 1235

[Zn] 32 50 73 100 125 140 160 [Cu] 26 55 77 100 102 111 121

Copper pyrite

[Fe ] 128 156 187 238 225 258 295

[Zn] 30 47 70 138 150 175 203 [Cu] 1.6 3.0 3.8 21 29 46 59

Carbonated sulfide

[Fe ] 155 618 573 608 543 1040 1080

[Zn] 18 55 50 78 83 200 240

Evolution of Cu, Fe and Zn concentrations in

N2-washing

t (s)

t (s)

t (s)

Linear voltammetry of copper pyrite PE

Linear voltammetry of massive sulfide PE

Linear voltammetry of low-grade sulfide PE

Linear voltammetry of carbonated sulfide PE

Electrochemical parameters

WASTE

Threshold Potencial

(V)

Potencial required to 0.2 mA

(V) I/V

Q

(0- 1.3 V) (C)

Massive sulfide 0.67 0.59 6.1 654

Low-grade sulfide 0.71 0.75 3.6 146

Copper pyrite 0.71 0.72 7.0 551

Carbonated sulfide 0.70 0.81 3.2 156

CONCLUSIONS • A low intensity signal corresponds to relatively insulating materials that are not

susceptible to weathering and therefore do not generate acid mine drainage. • A threshold potential higher than 1.3 V (electrolysis of water potential)

corresponds to non-electroactive materials at ambient conditions that will not generate acid mine drainage.

• The materials which threshold potential below 1.3 V are electroactive materials

that are susceptible to environmental weathering and generation of acid mine drainage. Among them, the most active are the copper pyrite and massive sulfide.

• The use of Press Electrodes prevents the inhomogeneity introduced by the

presence of carbon in Carbon Paste Electrodes, but the electric contact between particles may be inefficient due to the degradation products around the particles. In order to avoid this effect, a washing before voltammetry test is needed.

Linear sweep voltammetry as a technique to characterize mining wastes

Francisco Carranza, Pablo González, Rafael Romero & Nieves Iglesias* Manuel J. García

Thank you!