Council for Mineral Technology

Oxidative Precipitation of Mn from Cobalt Leach Solutions using Dilute

SO2/air Mixtures

Ndinanwi Mulaudzi

SAIMM Hydrometallurgy conference 24 – 26 February 2009

Contents

Introduction Objectives Background Experimental Results Conclusions

Introduction

Dilute SO2/air (or O2) gas mixtures have been used to oxidatively precipitate Mn from Zn(II), Co(II) and Ni(II) solutions.

SO2/air (or O2) gas mixture is a cheaper oxidant compared to other strong oxidising agents such as O3 and H2O2.

Objectives

Evaluate effects of:– SO2/air ratio

– pH – temperature

Minimum cobalt losses targeted (< 1%).

Thermodynamic considerations

SO3*-/SO32- and SO5*-/SO5

2- redox couples

(Das et.al, 1999).

S(IV) speciation

SO32-

HSO3-

S2O52-

SO2

[S(I

V)]

x 1

0-2, M

pH 1 2 3 4 5 6 7

9

7

5

3

1

Distribution of sulphur(IV) oxide species (Brandt et.al, 1994)

Reaction stoichiometry

MnSO4 + SO2 + O2 + 2H2O → MnO2 + 2H2SO4 (1)

2MnSO4 + SO2 + O2 + 3H2O → Mn2O3 + 3H2SO4 (2)

SO2 + 0.5O2 + H2O → H2SO4 (3)

H2SO4 + Ca(OH)2 → CaSO4 + 2H2O (4)

SO2 flowrate:- calculated from reaction 1 for a stoichiometric runtime of 5 hours; actual runtime was 6 hours- was kept constant at 11 ml/l for all the tests

Experimental matrix

Parameter RangepH 2 - 4Temperature, oC 30 - 60% SO2 in air, % (v/v) 0 - 6

SO2 flowrate, mL/min 11

Air flowrate, mL/min 170 - 1439% SO2 in O2, % (v/v) 3.5 - 23O2 flowrate, mL/min 36 - 301All initial solutions contained: 2 g/L Mn(II) and 6.5 g/L Co(II)

Moles of O2 used in SO2/air and SO2/O2 mixtures the same

Experimental set-up

pH controller

lime pump

Air

SO2

redox meter

condensor

Effect of SO2/air ratio on kinetics

0

10

20

30

40

50

60

0 1 2 3 4 5 6 7Time, hr

% M

n(I

I) p

rec

ipit

ate

d

0% 0.75% 1.5% 3% 4.5% 6%

pH 3; 30oC

Mn precipitation and Redox

0

10

20

30

40

50

0 1 2 3 4 5 6 7% SO2, (v/v)

% M

etal

pre

cip

itate

d

0.85

0.90

0.95

1.00

1.05

1.10

Red

ox,

V (S

HE

)

Mn Co redox

pH 3; 30oC; 6 hours

SO2/air versus SO2/O2

0

10

20

30

40

50

60

70

80

3 6 9 12 15 18 21 24

% SO2, (v/v)

% M

n p

reci

pita

ted

Air O2

pH 3; 30oC; 6 hours

Effect of pH on kinetics

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7Time, hr

% M

n(I

I) p

rec

ipit

ate

d

2 2.5 3 3.5 4

3% SO2; 30oC

Effect of pH on Mn and Co

40

50

60

70

80

90

100

2 2.5 3 3.5 4pH

% M

n(I

I) p

rec

ipit

ate

d

0

2

4

6

8

10

12

% C

o(I

I) p

rec

ipit

ate

d

Mn Co

3% SO2; 30oC; 6 hours

Redox: O2 vs SO2/air

0.75

0.85

0.95

1.05

1.15

1.25

1.35

2 2.5 3 3.5 4pH

Red

ox,

V(S

HE

)

O2 3% SO2 MnO2/Mn(II) Co3O4/Co(II)

3% SO2; 30oC; 6 hours

Effect of temperature on kinetics

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7Time, hr

% M

n(II

) pre

cip

itate

d

30oC 40oC 50oC 60oC

pH 3; 3% SO2

Effect of temperature on Mn and Co

40

50

60

70

80

90

100

30 35 40 45 50 55 60

Temperature, oC

% M

n(I

I) p

rec

ipit

ate

d

0

5

10

15

20

25

% C

o(I

I) p

rec

ipit

ate

d

Mn Co

pH 3; 3% SO2; 6 hours

Gypsum phase

Mn phase

Conclusions

For specific experimental set-up:Efficiency increased with increased SO2/air

ratio up to 3% SO2 in airO2 instead of air: Mn precipitation improvedSO2/air ratio affected redox potentialSolution pH criticalTemperature: increased rate and extent of

Mn pptRedox potential not only parameter

determining Mn removal rate and extent of removal

Co losses to be improved

Thank you

www.mintek.co.za