2 What Are These Things Again? Eh–pH diagram: aka Pourbaix diagram, potential-pH diagram, electro-chemical phase diagramInvented in 1930’s by Marcel Pourbaix (Belgian)Used in lots of places: extractive metallurgy, corrosion (their original purpose), environmental engineering, geochemistryClosely tied to aqueous thermodynamics
3 The Basicsx-axis is pH; usually 0–14, but sometimes as low as –3, and sometimes up to 16pH = –log [H+]; change of 1.0 pH unit changes [H+] by factor of 10y-axis is electrode potential relative to SHE (range varies); positive is oxidiz-ing condition, negative is reducingAssumes constant temperature, aH2O = 1Diagram is divided into predominance regions, where one phase prevailsRequires definition of predominance in terms of chemical potentialFor solids, activity = 1; for gases, set a partial pressure; for solutions, set an activity
4 The BasicsTwo lines shown here are present on nearly all Eh-pH diagramsLine (a) is for 2 H+ + 2 e– = H2 (g)Usually presumes pH2 = 1 atmSince ΔG° = 0, applying Nernst equation,E = 0 – pHResult: E = 0 at pH = 0 (SHE), slope of straight line = –When conditions are below line, reduction reaction generates H2 (g); when conditions are above line, H2 (g) oxidizes to H+Line (b) is for 4 H+ + O2 + 4 e– = 2 H2O; E = 1.23 – pHAbove line, oxidizing conditions generate O2; below line, reduction reaction generates H2OMost hydrometallurgical processes operate between the lines
5 Add A Metal Eh-pH diagram shows Cu–H2O system Dotted lines represent water stability region; solid lines represent equi-libria between copper speciesTwo aqueous species, Cu2+ and CuO22-Oxidation state of Cu as Cu0 is 0Oxidation state of Cu as Cu2O is +1Oxidation state of Cu in Cu2+, CuO, and CuO22- is +2Lower oxidation states are stable at bottom, higher oxidation states at topActivity of solid compounds = 1 when predominant; varies for aqueous species (1 in this case, could be as low as 10–6)Predominance activity determined by purpose, value of metal
6 More on Metal – H2O Diagrams Type of stable ion depends on pHFor CuO + 2 H+ = Cu2+ + H2O, low pH drives reaction to rightSimple ions like Cu2+ are stable at low pHFor CuO + H2O = 2 H+ + CuO22–, high pH drives reaction to rightOxyions like CuO22– are stable at high pHSolid oxides, hydroxides most stable in center of diagram
7 More on Metal – H2O Diagrams Three kinds of lines separate copper species in this diagramFirst is vertical: CuO + 2 H+ = Cu2+ + H2O; CuO + H2O = CuO22– + 2 H+Reactions involve exchange of H+, but no electrons (no oxidation/reduc-tion); independent of ESecond type of line is horizontal: Cu e– = CuReaction involves oxidation/reduction, but no H+; independent of pHThird type of line is diagonal: Cu2O + 2 H+ + 2 e– = 2 Cu + H2OReaction involves both oxidation/reduction and H+ exchange, so line is a function of E and pH(No curved lines in most diagrams.)
8 Why Does This Matter? (Part I) Diagram at bottom left is Cu–H2O systemPresence of stability region between lines for Cu and ions shows that Cu can be produced hydrometallurgicallyDiagram at bottom right is Au–H2O systemNo stability region for gold ions between lines; can’t dissolve Au in aqueous solutions (for now)
9 The Effect of Ion Activity Diagram shows Co–H2O systemTiny 0, –2, –4, –6 represent base-10 log of ion activity (Co2+, HCoO2–)As required activity of ions decreases, predominance area for ions grows (sideways and vertically)Easier to “produce” ions if desired concentration isn’t as highEasier to reduce ions to metal is con-centration of ions is higher
10 The Effect of Temperature Partial Eh-pH diagrams below show Cu–H2O system at 25° (left) and 100°C(Use log aCu(2+) = 0 lines for low–temperature diagram)Notice slight change in slope of diagonal linesCu2+ region shrinks (unusual), Cu2O region is smaller, CuO and Cu regions ↑Water stability region also movesCan use changes in temperature to our advantage
11 Eh-pH Diagrams for Anions Diagram shows S–H2O system at 25°CH2S is dissolved in solution, not gasCan do this for other anions as wellMatters because pure oxide minerals are uncommon, and anions are used for leaching, precipitation; need the right one!
12 Why This Matters (Part II) Diagrams below show Au–H2O and Au–CN–H2O diagrams at 25°CDiagram at left shows why we can’t dissolve gold; diagram at right shows how we can(This is why cyanide is used)Notice vertical line at bottom for H+ + CN– = HCN (g); impacts other linesAlso notice curvature of lines; reflects changing activity coefficients
13 Add An Anion And Another Metal (Hope you’re taking notes!)Diagram shows Cu–Fe–S–H2O sys-tem at 25°CRequires setting activity for aqueous Cu, Fe, and S speciesCuFeS2 is chalcopyrite, main copper mineralCu5FeS4 is borniteFeS2 is pyrite; FeS is pyrrhotiteNotice separate predominance regions for several species; impact of changing predominant S species
14 Why This Matters (Part III) Chalcopyrite contains copper ($3/lb) and iron ($0.08/lb). How to separate?Could smelt, oxidize iron to slag; re-quires energy, flux, slag disposalWhy not leach?Where on this diagram can I put Cu into solution and leave Fe behind?
15 Limitations of Eh-pH Diagrams • Doesn’t include impact of kinetics• Presumes only one predominant species (sometimes activities of ions are nearly equal)• Depends on accurate thermodynamic data (not always available for complex compounds)
16 For More Information… University of Montana Geology Department University of Idaho Geology Department