1. Council for Mineral Technology Oxidative Precipitation of Mn from Cobalt Leach Solutions using Dilute SO 2 /air Mixtures Ndinanwi Mulaudzi SAIMM Hydrometallurgy conference 24 – 26 February 2009

2. Contents  Introduction  Objectives  Background  Experimental  Results  Conclusions

3. Introduction  Dilute SO 2 /air (or O 2 ) gas mixtures have been used to oxidatively precipitate Mn from Zn(II), Co(II) and Ni(II) solutions.  SO 2 /air (or O 2 ) gas mixture is a cheaper oxidant compared to other strong oxidising agents such as O 3 and H 2 O 2.

4. Objectives  Evaluate effects of: –SO 2 /air ratio –pH –temperature  Minimum cobalt losses targeted (< 1%).

5. Thermodynamic considerations

6. SO 3 * - /SO 3 2- and SO 5 * - /SO 5 2- redox couples (Das et.al, 1999).

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

8. Reaction stoichiometry  MnSO 4 + SO 2 + O 2 + 2H 2 O → MnO 2 + 2H 2 SO 4 (1)  2MnSO 4 + SO 2 + O 2 + 3H 2 O → Mn 2 O 3 + 3H 2 SO 4 (2)  SO 2 + 0.5O 2 + H 2 O → H 2 SO 4 (3)  H 2 SO 4 + Ca(OH) 2 → CaSO 4 + 2H 2 O (4)  SO 2 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

9. Experimental matrix All initial solutions contained: 2 g/L Mn(II) and 6.5 g/L Co(II) Moles of O 2 used in SO 2 /air and SO 2 /O 2 mixtures the same

10. Experimental set-up

11. Effect of SO 2 /air ratio on kinetics pH 3; 30 o C

12. Mn precipitation and Redox pH 3; 30 o C; 6 hours

13. SO 2 /air versus SO 2 /O 2 pH 3; 30 o C; 6 hours

14. Effect of pH on kinetics 3% SO 2 ; 30 o C

15. Effect of pH on Mn and Co 3% SO 2 ; 30 o C; 6 hours

16. Redox: O 2 vs SO 2 /air 3% SO 2 ; 30 o C; 6 hours

17. Effect of temperature on kinetics pH 3; 3% SO 2

18. Effect of temperature on Mn and Co pH 3; 3% SO 2 ; 6 hours

19. Gypsum phase

20. Mn phase

21. Conclusions For specific experimental set-up:  Efficiency increased with increased SO 2 /air ratio up to 3% SO 2 in air  O 2 instead of air: Mn precipitation improved  SO 2 /air ratio affected redox potential  Solution pH critical  Temperature: increased rate and extent of Mn ppt  Redox potential not only parameter determining Mn removal rate and extent of removal  Co losses to be improved

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