A Co-S-H2O Eh-pH diagram involving CoS as the only cobalt sulfide species is provided below. An expanded diagram at around pH 2 is shown below. The detail is not so obvious in the first diagram.

  

1. In principle, can a mixture of ferric and ferrous sulfates be used to leach CoS at pH 1 (E^o = 0.68 V)? 

 

2. Indicate all possible pairs of Co and S species that can be formed by leaching CoS. Sulfur products may be soluble or insoluble. 

 

3. Over what pH range can CoS be leached to dissolve cobalt under these conditions (as per the diagram)? 

 

4. Estimate the concentration of ferric ion in contact with Fe(OH)₃ at pH 5. The K_sp for Fe(OH)₃ is 1.6 x 10^-39. 

 

5. Do you think it is feasible to use ferric sulfate to leach CoS at pH 5? Explain. 

 

6. Equations for several Eh lines are provided in the table below in the form Eh = slope x pH + constant. Consider the leaching of CoS by oxygen gas in air (air contains 21% oxygen) at 1 atm pressure, 25^oC and pH = 2.5. Calculate the Eh for the O₂/H₂O half reaction and for the appropriate cobalt-containing half reaction. Report to three decimal places. Include the units. Calculate ∆E for the reaction (and report the units). 

 

7. Is the reaction favourable? Explain briefly. 

Half reaction	Slope	Constant
Co+2/Co		-0.3116
Co(OH)3/Co+2	-0.1775	1.8163
Co+2, S/CoS		0.1899
Co+2, SO42-/CoS	-0.05917	0.3121
Co(OH)2, SO42-/CoS	-0.07396	0.4110
HSO4-/S	-0.06903	0.3333
SO42-/S	-0.07889	0.3528

Transcribed Image Text:

Eh (V) 1.8 Co-S-H2O Eh-pH diagram, 0.1 m Co and 1 m S solute activities, 25C, 1 atm 1.3 02 HO Co+2 0.8 HSO4 SO 0.3 -0.2 -0.7 +H H Co -1.2 0 5 COS Co(OH)3 Co(OH)2 HS HS pH 10 Eh (V) 1.8 Co-S-H2O Eh-pH diagram, 0.1 m Co and 1 m S solute activities, 25C, 1 atm 1.3 02 HO Co+2 0.8 HSO4 SO 0.3 -0.2 -0.7 +H H Co -1.2 0 5 COS Co(OH)3 Co(OH)2 HS HS pH 10