Pressure Vessel Measurements and Modelling of Metal Solubility in Aqueous Processes Vladimiros G. Papangelakis Dept. of Chemical Engineering & Applied Chemistry, University of Toronto, Canada Reagents Discharge Vent Feed

1  Acid concentration measurements  pH measurements  Chemical modeling  Examples  Conclusions Outline

2 AUTOCLAVE CaCO 3 Feed PRESSURE LEACHING NEUTRALIZATION DISPOSAL PURIFICATION T = 260 o CH 2 SO 4 Steam Aqueous Processing - Hydrometallurgy

3 High temperature on-line acid sensor l Many industrial chemical processes are acid driven: n Produce adequate yields – Equilibria n Reduce process times - Kinetics n Often added in excess l Excess acid must be partially or completely neutralized within the process: n Cost of base addition (CaO – CaCO 3 ) n Waste management problem (CaSO 4. 2H 2 O) l Other factors: n Excess acid detrimental to equipment n Puts more impurities into solution l Monitoring solution acidity is crucial to process control

4  Hydrogen ion is more mobile than other ions  Moves by jumping on water molecules Grotthuss Conduction

5 Limiting equivalent conductivities of different ions in water up to 250°C

6 Electrodeless Conductivity

7

8 Leach Temperature250°C Solids Loading27%wt. Acid/Ore Ratio0.2 Divalent Metal Sulphates0.01 to 0.17M Trivalent Metal Sulphates0.009 to 0.04M Absolute Average Difference = 4.6%, S.D. = 3.0%

9 Leach Temperature250°C Solids Loading40%wt. Acid/Ore Ratio (pre-acidified to 0.2)0.3 Divalent Metal Sulphates0.22 to 0.27M Trivalent Metal Sulphates0.05 to 0.15M Absolute Average Difference = 3.6%, S.D. = 0.9%

10 Leach Temperature250°C Solids Loading40%wt. Acid/Ore Ratio (pre-acidified to 0.2)1/8 stoich. NaOH Divalent Metal Sulphates0.20 to 0.23M Trivalent Metal Sulphates0.06 to 0.002M Absolute Average Difference = 1.5%, S.D. = 0.7% 3Fe 2 (SO 4 ) 3 + 2NaOH + 10H 2 O = 2NaFe 3 (SO 4 ) 2 (OH) 6 (s) + 5H 2 SO 4

11 High temperature pH measurements

12 The yttria-stabilized zirconia (YSZ) pH sensor l The YSZ pH sensor consists of an oxygen ion conducting ZrO 2 (9 wt% Y 2 O 3 ) ceramic tube l The sensor can be represented as: H 2 O, H + | ZrO 2 (Y 2 O 3 ) | HgO | Hg l The reactions that occur at the membrane interfaces can be represented as: External: O o + 2H + = V o.. + H 2 O Internal: V o.. + HgO + 2e - = O o + Hg where: O o – oxygen ion in a normal anion site in the lattice V o.. – oxygen ion vacancy in the lattice

13 Potentials in the flow-through electrochemical cell l Irreversible thermodynamic contributions:  STR – streaming potential (left - RE, right - YSZ)  TD – thermal diffusion potential  D – diffusion potential  TE – thermoelectric potential

14 Calculation of the diffusion potential (Henderson equation) l (A) – acidic solution l (B) – reference electrode solution l (i) – ith ionic species – ionic conductivity, taken from OLI Systems software l z – valence l m – molality, taken from OLI Systems software

15 Leach solutions L 1 and L 2

16 Diluted, acid-adjusted leach solutions D 1 to D 5

17 Prediction of [H 2 SO 4 ] 25C to attain pH T =1 based on [Mg 2+ ] 25C and [Ni 2+ ] 25C

18 Solid - Aqueous Equilibria Solid Leaching Precipitation Aqueous solution REDOX Reactions

19 Simulating Concentrated Electrolyte Solutions at High Temperatures: Challenges Extrapolation of already uncertain thermodynamic data!  Inadequate theory to account for the physics of ionic interactions and structures  Inconsistent-incomplete thermodynamic databases  Experimental data is hard to obtain due to corrosion, and lack of in situ sensors  Weak mathematical framework, when it based on the “infinite dilution” – “ideal solution” hypothetical standard state

20 Water Properties

21 Trends at High Temperature Reactions that are ignored for kinetic reasons at 25ºC can proceed rapidly at high T Transition metals (e.g., Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Pt, Pd, Rh, Au) present a complicated case: 1. High ionic charge 2. Tendency to form coordination complexes 3. Tendency to exist in more than one oxidation state

22 Mixed Solvent Electrolyte (MSE) Model  Features  Electrolytes in organic or water or mixed organic + water solvents from infinite dilution to pure electrolytes  Unit scale: mole fraction x  Reference state: Symmetrical reference state  The activity coefficient expression LR : Long Range electrostatic interactions between ions, Pitzer- Debye-Hückel expression is used MR: Middle Range interactions involving charged ions, Ion-Ion, Ion-Molecule SR: Short Range interactions between all species, Ion-Ion, Ion- Molecule, Molecule-Molecule, UNIQUAC equation is used C A M C A CA M2M2 M1M1

23 MSE Middle Range Interaction Term c ij = CMD0+CMD1  T+CMD2/T b ij = BMD0+BMD1  T+BMD2/T B ij : Middle range parameters, ionic strength dependent B ii = B jj =0, B ij = B ji =0

24 Software  OLI Systems  An extensive databank of over 3,000 species Advanced thermodynamic framework to calculate thermodynamic properties like free energy, entropy, enthalpy, heat capacity, pH, ionic strength, density, conductivity, osmotic pressure etc. Built-in data regression capabilities to obtain thermodynamic model parameters based on experimental data Wide applicability for the aqueous phase: -50<T<300  C, 0<P<1500 Bar, 0 < I < 30 molal

25 Sulphuric Acid Species at 25 ° C in the Whole Acid Concentration Range SO 4 2- HSO 4 - H 2 SO 4(aq) 25 o C Clegg S.L., Brimblecombe P., Journal of Chemical Engineering Data, 40, Walrafen G.E., Yang W.H., Chu Y.C., Hokmabadi M.S., Journal of Solution Chemistry, 29(10), T.F. Young, L.F. Maranville, H.M. Smith, The Structure of Electrolyte Solutions, 1959, p. 35.

26 MSE model prediction of sulphuric acid species vs. temperature in H 2 SO 4 -NaCl-H 2 O system Dickson A.G., Wesolowski D.J., Palmer D.A., Mesmer R.E., Dissociation Constant of Bisulfate Ion in Aqueous Sodium Chloride Solutions to 250°C. J. of Phys. Chemistry, 94,

27 MSE model prediction for sulphuric acid species vs. temperature at different acid concentrations

28 Anhydrite Solubility in PAL Solutions vs. NiSO 4 concentration from 150 to 200°C

29 Anhydrite Solubility in PAL Solutions vs. H 2 SO 4 concentration from 150 to 250°C

30 Prediction of anhydrite solubility in different electrolyte solutions: Scaling potential

31 Conclusions  New sensors have been developed for stoichiometric acid and pH measurements as in multicomponent systems at high temperatures  Application of thermodynamics is becoming an essential tool in industrial process design and development  OLI Systems offer the best available software for chemical modelling of both low and high temperature industrial processes  Proprietary databanks have been developed at the UofT and are growing for applications in hydrometallurgy

32 Acknowledgments Anglo American plc Barrick Gold Corporation Norilsk Nickel Sherritt International Corporation Vale Inco Ltd.