Co-H2O Eh-pH diagram: dissolution paths for Co3O4: 21-Mat-B2 Pyrometallurgy, May 2015, Q6

Problem 6 — Co-H2O Eh-pH diagram: dissolution paths for Co3O4 (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

[Figure not reproduced: Co-H2O Eh-pH diagram, as printed on the exam. See the official exam paper or the cited reference text.]

Fig. 5 — the printed Co-H2O Eh-pH (Pourbaix) diagram: CoO2 (top, most oxidized), Co2+ (upper-left, low pH), Co3O4(s) (centre), Co(OH)2(s) (lower-right, high pH), and Co metal (bottom, most reduced). The two dashed lines are the standard water-stability reference lines (upper = O2/H2O, lower = H2/H+), not Co-species boundaries.

Reading the diagram. Vertically (at fixed pH), the field order from bottom to top is Co (metal, oxidation state 0) → Co2+/Co(OH)2(s) (state +2) → Co3O4(s) (mixed-valence, average state +8/3 ≈ +2.67) → CoO2 (state +4) — i.e. Eh increases with the average Co oxidation state, as it must. Co3O4 sits at a HIGHER average oxidation state than the aqueous Co2+ ion, so dissolving Co3O4 to the only aqueous species shown (Co2+) is inherently at least partly a REDUCTION (2 of every 3 Co atoms must go from +3 to +2). The diagram shows three distinct routes across the Co3O4 boundary, matching the question's explicit mention of "combined thermochemical/hydrochemical treatment."

Fig. 6 — schematic (not to scale) field layout with the three dissolution paths sketched: (1) direct reductive acid leach straight into the Co2+ field; (2)/(3) a thermochemical reduction step (roast) down into the Co(OH)2/Co(metal) fields, followed by a hydrochemical acid-leach step back to Co2+ in solution.

Path 1 — direct reductive acid leach (single hydrochemical step). Crossing straight from Co3O4 into the Co2+ field at low-to-moderate pH, using an added chemical reductant together with acid:

$$\text{Co}_3\text{O}_4(\text{s})+\text{SO}_2(\text{g})+2\text{H}_2\text{SO}_4(\text{aq})\rightarrow 3\text{CoSO}_4(\text{aq})+2\text{H}_2\text{O}.$$

Classification: REDUCTIVE (SO2 is oxidized to sulfate, supplying the 2 electrons that convert the 2 Co3+ per formula unit down to Co2+; H2SO4 supplies the acid to dissolve all three Co as the sulfate salt).

Fig. 7 — Path 1: single-stage reductive acid leach (SO2 + H2SO4) directly dissolving Co3O4 to CoSO4(aq).

Path 2 — combined thermochemical (partial reduction) + hydrochemical (non-oxidative leach). A reducing roast first converts Co3O4 to the +2 oxide CoO (thermochemical step, no aqueous chemistry involved), then a plain acid leach dissolves CoO without any further change in oxidation state:

$$\text{Thermochemical: } \text{Co}_3\text{O}_4(\text{s})+\text{H}_2(\text{g})\rightarrow 3\text{CoO}(\text{s})+\text{H}_2\text{O}(\text{g})$$

$$\text{Hydrochemical: } \text{CoO}(\text{s})+\text{H}_2\text{SO}_4(\text{aq})\rightarrow \text{CoSO}_4(\text{aq})+\text{H}_2\text{O}(\text{l})$$

Classification: the roast step is REDUCTIVE (H2 supplies the electrons, thermally rather than electrochemically); the leach step is NON-OXIDATIVE (Co stays at +2 throughout — simple acid dissolution of an already-Co2+ solid).

Fig. 8 — Path 2: reducing roast (H2) to CoO, then a non-oxidative acid leach to CoSO4(aq).

Path 3 — combined thermochemical (full reduction to metal) + hydrochemical (oxidative leach). A stronger reducing roast (excess H2) drives Co3O4 all the way to metallic cobalt, which is then dissolved oxidatively by acid (the metal itself is oxidized, evolving H2 gas):

$$\text{Thermochemical: } \text{Co}_3\text{O}_4(\text{s})+4\text{H}_2(\text{g})\rightarrow 3\text{Co}(\text{s})+4\text{H}_2\text{O}(\text{g})$$

$$\text{Hydrochemical: } \text{Co}(\text{s})+\text{H}_2\text{SO}_4(\text{aq})\rightarrow \text{CoSO}_4(\text{aq})+\text{H}_2(\text{g})$$

Classification: the roast step is REDUCTIVE (thermal, as in Path 2 but carried further); the leach step is OXIDATIVE (Co0 loses 2 electrons to H+, which is reduced to H2 — the metal itself is the species being oxidized).

Fig. 9 — Path 3: full reducing roast (excess H2) to metallic Co, then an oxidative acid leach (H2 evolved) to CoSO4(aq).

Path	Route	Classification
1	Co3O4 $\xrightarrow{\text{SO}_2+\text{H}_2\text{SO}_4}$ Co2+(aq) — single-step	Reductive (hydrochemical)
2a	Co3O4 $\xrightarrow{\text{H}_2,\ \Delta}$ CoO(s)	Reductive (thermochemical)
2b	CoO(s) $\xrightarrow{\text{H}_2\text{SO}_4}$ Co2+(aq)	Non-oxidative (hydrochemical)
3a	Co3O4 $\xrightarrow{4\text{H}_2,\ \Delta}$ Co(s)	Reductive (thermochemical)
3b	Co(s) $\xrightarrow{\text{H}_2\text{SO}_4}$ Co2+(aq) + H2	Oxidative (hydrochemical)

Check. The schematic in Fig. 6 reproduces the printed diagram's field ORDER and general shape from the labelled regions and the two standard water-stability dashed reference lines (whose printed values at pH 0 and pH 14 match the textbook O2/H2O and H+/H2 lines almost exactly, confirming they are NOT Co-specific boundaries).

Concept & Theory

Topic. Eh-pH (Pourbaix) diagram interpretation for selecting a leaching route, and the thermochemical/hydrochemical distinction in extractive metallurgy.

Key relations. Average Co oxidation state fixes vertical (Eh) field order: Co(0) < Co2+/Co(OH)2(+2) < Co3O4(+2.67) < CoO2(+4). Water stability window: $E_{\text{O}_2/\text{H}_2\text{O}}=1.229-0.0592\,\text{pH}$ (upper dashed line), $E_{\text{H}^+/\text{H}_2}=-0.0592\,\text{pH}$ (lower dashed line).

Why this works. A Pourbaix diagram's fields are ordered vertically by oxidation state precisely because Eh is the "oxidizing power" axis; any solid whose average oxidation state lies between two aqueous/solid species of interest can therefore always be reached from either side by a suitable choice of Eh (redox conditions) and pH (acidity) — which is exactly why Co3O4, sitting between Co2+ and CoO2, admits BOTH a direct reductive-leach route and staged thermochemical-then-hydrochemical routes through the lower fields.

Common pitfall. Assuming any acid leach of Co3O4 must be purely non-oxidative because it "just adds acid" — in fact, since Co3O4 has a HIGHER average oxidation state than Co2+, converting it to Co2+(aq) always requires net reduction of two of every three Co atoms, whether that reduction happens electrochemically in solution (Path 1) or thermally beforehand (Paths 2/3).

Source. Free, Hydrometallurgy: Fundamentals and Applications, Ch. 3 (Eh-pH diagrams); Habashi, A Textbook of Hydrometallurgy, Ch. 2 (reductive leaching) and Ch. 4 (roasting pretreatment).

Question 6 of 6: Co-H2O Eh-pH diagram: dissolution paths for Co3O4

Problem 6 — Co-H2O Eh-pH diagram: dissolution paths for Co3O4 (20 marks)