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LECTURE NOTES. MG – 4111 H Y DRO-ELE C TROMETAL L URG Y Semester I, 2010/2011. DR. M. Zaki Mubarok Department of Metallurgical Engineering, Faculty of Mining and Petroleum Engineering (FTTM)-ITB. Course Outline. Introduction to Hydrometallurgy
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LECTURE NOTES MG – 4111 HYDRO-ELECTROMETALLURGYSemester I, 2010/2011 DR. M. Zaki Mubarok Department of Metallurgical Engineering, Faculty of Mining and Petroleum Engineering (FTTM)-ITB
Course Outline • Introduction to Hydrometallurgy • Thermodynamic and Kinetic Aspects in Hydrometallurgy • Leaching and Solid-Liquid Separation • Solution Purification and Metals Recovery Methods from Pregnant Leach Solution
Course Outline V. Leaching and Recovery of Metals and Oxides Ores (Au, Ag, Zn, Al, Cu, Ni) • Leaching and Recovery of Sulphide Ores (Zn, Ni, Cu) • Introduction to Electrometallurgy • Metals Production by Electrolysis in Aqueous Solution • Fused Salt Electrolysis
Literatures • Havlik,T.,”Hydrometallurgy:Principles and Applications,”CRC publisher, 2008. • Habashi,F. ”A Textbook of Hydrometallurgy”, Metallurgie Extractive, Quebec,1993 • Norman L. Weiss, “SME Mineral Processing Handbook“, Volume II, SME, 1985 • Unit Processes in Extractive Metallurgy: Hydrometallurgy, A Modular Tutorial Course of Montana College of Mineral Science and Technology • Biswas, A.K. And Davenport, W.G., “Extractive Metallurgy of Copper”, Pergamon, Oxford, fourth edition, 2002
Literatures • Unit Processes in Extractive Metallurgy: Electrometallurgy, A modular tutorial course of Montana College of Mineral Science and Technology • Yannopoulus, J.C,”The Extractive Metallurgy of Gold”, Von Nostrand Reinhold, New York, 1991
Course Structure and Mark Distribution • Course Structure • Lecture • Tutorial • Assignment and Lab Work • Mark Distribution • 45% Midterm Exam • 45% Final Exam • 5% Assignment • 5% Lab Work • Attendance: 70% minimum
CHAPTER I INTRODUCTION TO HYDROMETALLURGY Hydrometallurgy Extraction, recovery and purification of metals, through processes in aqueous solutions. Metals are also recovered in the other forms such as oxides, hydroxides. Electrometallurgy Recovery and purification of metals through electrolytic processes by using electrical energy.
Hydrometallurgy Scope • Traditionally, hydrometallurgy is emphasized for metals extraction from ores. • Hydrometallurgical processing may be used for the following purposes: • Production of pure solutions from which high purity metals can be produced by electrolysis, e.g., copper, zinc, nickel, gold, and silver. • Production of pure compounds which can be subsequently used for producing the pure metals by other methods. For example, pure alumina to produce smelter grade aluminium. • However, hydrometallurgy principles can be applied to a variety of areas such as metals recycling from scrap, slag, sludge, anode slime, waste processing, etc.
Unit Processes in Hydrometallurgy • In general, hydrometallurgyinvolves 2 (two) mainsteps: • Leaching Selectivedissolution of valuable metals fromore. • Recovery Selectiveprecipitation of thedesired metals from a pregnant-leachsolution.
General outline of hydrometallurgical processes Ore/concentrate Leaching agent Oxidant leaching Solid residu to waste Solid-liquid separation Pregnant Solution Precipitant or electric current Solution purification Precipitation Pure compound Metals
Commonly, solution purification is conducted prior to metals recovery from the solution. • Solution purification is aimed at obtaining a concentrated solution from which valuable metals can be precipitated in the next processes effectively • Solution purification methods which are commonly used are as follows: • Adsorption by activated carbon • Adsorption by ion exchange resins • Solvent extraction (using organic solvents) • Precipitation with metals (cementation)
Solution purification • Solution purifications by adsorption with activated carbon, ion exchange resins (IX) and solvent extraction (SX) have the same unit operations, namely: • Loading, and • Elution • In the elution step, the adsorbers are usually regenerated for another process cycle.
Hydrometallurgy development • Hydrometallurgy is developed after pyrometallurgy. Metals smelting has been practiced since thousands years ago. • Hydrometallurgy was developed after the people discovered acid and base solutions. However, modern hydrometallurgy development is commonly associated with the invention of Bayer Process for bauxite leaching and cyanidation for gold extraction at the end of 19th century (1887). • One of important highlights of hydrometallurgy development is uranium extraction (Manhattan Project) aimed at nuclear weapon production in second world war (1940‘s).
Important milestones in the development of hydro-electrometallurgy • Cementation & Aqua Regia Use - 8th Century • • Cyanidation - 1887 • • Bayer Process - 1887 • • Hall-HeroultProcess - 1886, 1888 • • CopperElectrowinning - 1912 • • ZincElectrolyticProcess - 1916 • • Manhattan Project (IX/SX) - 1940’s • • Biooxidation of Sulphide Concentrates - 1960’s • • PressureLeaching • – Sherrit Gordon Nickel Process - 1954 • – Pressure Acid Leaching of Ni Laterites - 1955 • • Large Scale Copper SX/EW - 1960’s
Important milestones in the development of hydro-electrometallurgy • Carbon in Pulp (CIP)/Carbon in Leach (CIL) • for Gold Recovery - 1980’s • • Pressure Oxidation of Zinc Sulphides - 1981 • • Two-Stage Zinc Pressure Leach - 1993 • • Atmospheric Leaching of Zinc Sulphides • – Albion (1993), Outokumpu (1999) • RecentDevelopments: • • Skorpion Project (Anglo American) – 2003 (Zn from ZnS) • • Hydrozinc (TeckCominco) - 2004 • • Inco’s Goro and Voisey Bay Projects - 2007 • • Leaching of Chalcopyrite (CuFeS2) Ores • Hydrocopper (Outokumpu) Cu fromsulfidicores • Atmospheric leaching of nickel laterite ore: 2008?
Spontaneous Reaction, Equilibrium State • As has been learned in basic engineering courses, chemical reaction will spontaneously occur when the Gibbs free (G) < 0. G = Go + RT ln K • G = 0 process is in equilibrium state • Go = standard Gibbs free energy • R = ideal gas constant = 8,314 J/K.mol • T = absolute temperature of the system (K) • K = equilibrium constant • Standard Gibbs free energy is determined at: • Gaseous components partial pressure = 1 atm • Temperature = 25 oC (298 K) • Ions activity = 1
Equilibrium Constant • For reaction: aA + bB cC + dD = activity coefficient of component A
Nernst Equation • Hydro-electrometallurgical processes often involve electrochemical reactions. • For electrochemical reaction G = -nFE, Go = -nFEo, therefore Nernst Equation In which, E = potential for reduction-oxidation reaction Eo =standard potential for reduction-oxidation reaction n = number of electron involved in the electrochemical reaction, F = Faraday constant = 96485 Coulomb/mole of electron • Spontaneous process E > 0 G < 0
Chemical reactions usually perform in leaching processes • Dissolutionbyacid • Example: ZnO(s) + 2H+ → Zn2+(aq) + H2O(l) • Dissolutionbybase • Example: Al2O3(s) + 2OH- → 2AlO2-(aq) + H2O(l) • Dissolutionbycomplexionformation • Example: CuO(s) + 2NH4+(aq) + 2NH3(aq) → Cu(NH3)42+(aq) + H2O(l)
Chemicalreactionsusuallyperform in leachingprocesses • Dissolution by oxidation • Ex: CuS(s)+ 2Fe3+ → Cu2+(aq) + 2Fe2+ + So(s) Other oxidators: O2, ClO-, ClO3-, MnO4-, HNO3, H2O2, Cl2 • Dissolution by reduction mechanism • Ex: MnO2(s) + SO2(aq) → Mn2+(aq) + SO42-(aq)
Correlation of free energy (G) and heat (enthalphy = H) G = H - TS Go = Ho - TSo ∆Ho = Standard enthalpy (kJ/mol) ∆Go = Standard entropy (kJ/mol) ∆Go (reaction) = ∆Go (products) - ∆Go (reactants) ∆Ho (reaction) = ∆Ho (products) - ∆Ho (reactants) ∆So (reaction) = ∆So (products) - ∆So (reactants) Cp = heat capacity at constant pressure (J/molK) Where possible, processes are designed to be autothermal → maintain constant temperature by the heat given by the reaction
Calc. example 1 • Find K for each reaction using a) Standard free energy data b) Standard electrode potential data
Calc. Example 2 a) What is the electrode potential of the Ni2+/Ni reaction in sulphate solution at 25°C at a Ni2+ concentration of 0.005 M (assumption: activity of Ni2+ is equal to its molar concentration) b) At what pH is H2 at 10 atm at equilibrium with this solution and pure nickel? Ni2+ + 2 e = Ni E° = -0.26 V 2H+ + 2 e = H2 E° = 0.00 V
Pourbaix Diagram • Pourbaix Diagram = Potensial (Eh) – pH Diagram. • The diagram represents thermodynamic equilibrium of metal, ions, hydroxides (or, oxides) in aqueous solution at certain temperature (isothermal). • The boundary of stability regions of metal, ion, hydroxides (or oxides) are equilibrium lines. • Does not reflect reaction kinetics.
Pourbaix Diagram • Three possible types of equilibrium lines: • Horizontal • Vertical • Slope • Variations in ion activities are plotted as contours/dashed lines • Horizontal Line: for equilibrium reactions that are independent of pH.
Horizontal Line Fe3+ + e = Fe2+ Eo = 0.77 V • Example: R = 8.314 J/Kmol, T = 298 K, F = 96500 C/mol e-, n = 1 mol e- If all ion concentrations are assumed to be equal to their molar concentrations 10-6 M.
Vertical Line • Reactions do not involve electron → n = 0, no potensial , the equilibrium depends only on pH. • Example: Fe2O3 + 6H+ = 2Fe3+ + 3H2O K = [Fe3+]2/[H-]6 For certain Fe3+ concentration we can determine the equilibrium pH for the above reaction.
Slope Line • For reactions that depend both on potensial (Eh) dan pH. • Example: If all ion concentrations are assumed to be equal to their molar concentrations 10-6 M.
Water stability region (dotted lines) • Upper boundary line • Lower boundary line At pO2 = 1 atm At pH2 = 1 atm
Application of Eh – pH diagram in hydrometallurgy • Predicting potential leaching behaviour for certain mineral system • Predicting the possibility of metals ion precipitation at the purification of pregnant-leach solution
Application of Eh – pH diagram in hydrometallurgy Fe(OH)3 or Fe2O3 can be precipitated from Fe3+ at lower pH than the precipitation of Zn2+ to Zn(OH)2 or ZnO. Fe2+ have to be oxidized to Fe3+ to gain lower pH value for Fe(OH)3 precipitation.
Pourbaix Diagram can be constructed at various temperature for more than two systems Eh-pH diagram of Zn-S-H2O system at 25oC
Diagram Pourbaix in Presence of Complex Ion • Example: Au-H2O system with the presence of cyanide (CN-) ion (case of gold cyanidation leaching) • Equilibrium of Au3+/Au Standard reduction potential for Reaction 1:
(2) • Equilibrium reaction of O2/H2O Eo = 1.23 V. Therefore, Au3+ions are not stable in water and readily reduced to Au by oxidation of H2O to O2 (the opposite of Reaction 2). In the other word, gold can not be oxidized (dissolved) in water only with the presence O2.
Potensial – pH diagram of Au–H2O system without the presence of complexing agent
With the presence of CN-,Au3+ forms STABLE COMPLEX of “aurocyanide“ (Au(CN)2-) and the potential-pH diagram for Au changes significantly as follow: Eh-pH Diagram of Au-CN-H2O system at 25 oC for [Au] = 10-4 M and [CN-] = 10-3 M
By the presence of cyanide ions, Au+ + e = Au E = 1.69 – 0.0591 log [Au+] Au+ + 2CN- = Au(CN)2- (K = 2 x 1038) Au(CN)2-+ e = Au + 2CN- ...........................(3) In comparison to the first reaction that has Eoof 1.69 V, Reaction (3) has much lower Eo at -0.57 V. Dissolution of Au is limited by the following equilibrium of Reaction (3). • During cyanidation leaching, dissolved oxygen is required to oxidize Au prior to the formation of stable complex of Au(CN)2-.
Interactions in Electrolyte Solution • Two types of interactions in electrolyte: • Ion-ion interaction, and • ion-solvent interaction • Knowledge of interaction in electrolyte solution is important because theinteractions affect solvation effects, diffusion, conductivity, ionicstrengthand activitycoefficients of ions in solution. • Interactions in electrolytesolutioninfluence the transport properties of ions in solution.
Ionic Strength and Activity Coefficient • Ionic strength (I), expresses the ionic concentration that includes the effects of ionic charge. • Ionic strength (I) is defined as follow: • It is found that activity coefficient, electrical conductivity and the rates of ionic reactions are all the functions of ionic strength. in which ci = concentration of ion i in molar (mol/L) and zi = the charge of ion i.
Ionic Strength for unit concentration in molal • Remember, molality = moles of solute in 1 kg solvent. Molality can be converted to molality by the following correlation: • in which Mi = the molar mass of each solute in kg/mol (not in g/mol), ci = molarity of solute i, and is the density of the solution in kg/m3 (=g/L) • In dilute solutions, ci 0.001mio (in which o = density of pure solvent).
Ionic Strength for unit concentration in molal • Therefore for dilute solution, • If the solvent is water at 25oC (density 1000 kg/m3), then: Similar form with ionic strength in molarity
- Molar activity coefficient can be converted to molal activity coefficient by the following correlation:) for salt, or for single ion. in which = total moles of ion formed during complete dissociation, m = ionic molality and Ms = molecular weight of solvent (kg/mol).
Activity and Activity Coefficient, DEBYE-HUCKEL LAW • Debye Huckel Law correlates the activity coefficient (fi , i)with ionic strength (I). • Forms of Debye-Huckel equations depend on concentration of solution and the unit concentration used. • For dilute solution at 25 oC and I given in molar (M), • The above equations are known as LIMITING DEBYE HUCKEL LAW. for single ion, and for salt.
The limitation of LIMITING Debye-Huckel Equation • The D-H Limiting Law is called a ”limiting” law because it becomes increasingly accurate as the limit of infinite dilution is approached. • Up to concentrations of about 0.01m THE LIMITING D-H LAW gives reasonable values, but at higher concentrations the calculated activity coefficient become inaccurate (high %error compared to the values determined experimentally).