METE 215 MATERIALS PROCESSING. Prepared by: Prof.Dr. İshak Karakaya

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1 Experiment 6: METE 215 MATERIALS PROCESSING LEACHING AND ELECTROWINNING Prepared by: Prof.Dr. İshak Karakaya THEORY Leaching is the process of extracting a soluble constituent from a solid by means of a solvent. It is the process of dissolving certain constituents from an ore, concentrate, or from metallurgical products such as calcines, mattes, etc. Leaching processes have been used for many years as a parallel treatment method to pyrometallurgy in treating some metals such as zinc. Leaching of zinc concentrates followed by electrowinning is rapidly increasing in popularity for its cleaner environmental operations, energy saving possibilities and ability to treat lower grade and complex ores. The relations between chemical change and electrical energy have theoretical as well as practical importance. Chemical reactions can be used to produce electrical energy (voltaic or galvanic cells). Electrical energy can be used to bring about chemical transformations (electrolytic cells). Electrowinning is important for the very reactive light metals (Al, Mg) which are produced by electrolysis of fused salts. For other metals such as Cu and Zn, electrowinning from aqueous solutions represent an alternative to pyrometallurgical processes. Electrowinning (electrolytic deposition) is used for extraction, particularly from acid leach solutions where the form of the leach solution makes a satisfactory aqueous solution to be used as an electrolyte. The calcine is leached in dilute sulphuric acid solution according to: ZnO(s) + H 2 SO 4 (aq) = ZnSO 4 (aq) + H 2 O(l) (1) Where ZnSO 4 (aq) exists as Zn ++ and SO 4 = ions in aqueous solution. When current is passed to cause decomposition of this solution, zinc is precipitated by electrolysis. The main cathodic reaction is: Zn e - = Zn e o (cathode) = V (2) The following reaction occurs at the anode:

2 H 2 O(l) = ½ O 2 (g) + 2 H e - e o (anode) = V (3) The solution becomes more acidic during electrolysis and can be recycled as a leaching agent. This solution is called spent electrolyte and it could be used to leach new concentrate or calcine (ZnO). The metallic zinc will be deposited at the cathode and can be stripped from there after sufficient metal buildup has occurred. The deposited metal is relatively pure and needs only to be melted and cast into slabs or ingots for commercial use or sale. The cathodes can be either a pure metal starting sheet of the same metal (zinc in this case) to be deposited or they can be a different metal (aluminum in this case) from which deposited layer will be stripped at regular intervals. The anodes are merely insoluble electrical conductors, such as sheet lead or a lead-1% silver alloy. The cell will have one more anode than cathodes. A typical industrial cell will have 46 anodes and 45 cathodes. Cell temperature is important and is generally held at 35 to 45 C. Heat generated during electrolysis is dissipated by circulating cool water through lead coils immersed in the cell. High temperature can increase deposition but also intensifies the effect of impurities in the cell. The decomposition of zinc sulfate solution takes place according to reaction: ZnSO 4 (aq) + H 2 O(l) = Zn(s) + H 2 SO 4 (aq) + ½ O 2 (g) (4) Since ZnSO 4 (aq) is Zn ++ and SO 4 =, similarly H 2 SO 4 (aq) is 2H + and SO 4 = ; reaction (4) can also be written as: Zn ++ + SO 4 = + H 2 O(l) = Zn(s) + 2H + + SO 4 = + ½ O 2 (g) (5) This is the summation of cathodic and anodic reactions given in equations (2) and (3) respectively. Then, the theoretical standard decomposition voltage of zinc sulfate is: E o = e o (anode) + e o (cathode) = volts. (6) Theoretically 1.99 volts of potential should be applied between the electrodes for the electrolysis of a zinc sulfate solution under standard conditions. Under electrolysis conditions, the theoretical voltage required for the electrolysis of a zinc sulfate solution is 2.35 volts, but in actual practice voltages higher than this are used. In practical electrolysis, there will be additional potentials. These are the voltage drop due to ohmic resistance in the electrolyte, voltage drop due to ohmic resistance in the electrode leads and electrodes and voltage drop due to overvoltage, η, of the cell. The overvoltage is caused either by the reactants not being supplied to the electrodes as fast as they are removed or by the reaction products not being removed as fast as they are supplied. Thus, the total applied voltage may be expressed as:

3 V = - E + ( R e + R l ) I + η (7) Where; V: The total applied voltage R l : Resistance of leads E: Decomposition Voltage I: Current R e : Resistance of electrolyte η: Overvoltage The decomposition voltage of zinc sulfate is above that of hydrogen, and normally it would be expected that hydrogen evolve instead of zinc being deposited. However, the hydrogen overvoltage, which is the voltage actually required to evolve hydrogen in excess of theoretical decomposition voltage, with respect to zinc in an acid solution is high enough to let the zinc plate out of a zinc sulfate solution without the evolution of a great amount of hydrogen at the cathode. Most plants operate with current density of 220 to 450 amperes per square meter of cathode area, with the choice of the current density for each particular plant adjusted against the voltage used per cell, the acid strength of the electrolyte, the volume and temperature of the cooling water, and the volume and metal content of the solution, to give the maximum deposition of metal in each case. OBJECTIVE A series of tests on leaching and electrowinning of metal values from aqueous solutions will be performed to analyze various principles involved. APPARATUS 1. D.C power supply 2. Ampermeters ml. beakers 4. aluminum cathodes 5. lead anodes 6. Zinc sulfate 7. Sulfuric acid 8. Balance and weights

4 PROCEDURE 1. Dissolve proper amount of ZnSO 4 in a 150 ml beaker (as instructed) by using distilled water to prepare 100 ml electrolyte with desired concentration. Find your group number and corresponding Zn concentration from the table below. 2. Add proper amount of H 2 SO 4 given for your group in the table below. ZnSO 4 (g/lt) H 2 SO 4 (ml/lt) Cathode Area, cm² Group (2x2) Group (2x2) Group (2x2) Group (2x2) Group (2x2) Group (2x2) Group (2x2) Group (2x2) Group (2x2) 3. Weigh the cathode and record the weight. 4. Place the electrodes in the electrolyte and position them so that the anode and cathode are parallel to each other (see Fig. 1). Place spacers on the bus bars to keep the electrode distance uniform and constant (about 2 cm). (Do not let the hooks supporting the electrodes touch the electrolyte or they will dissolve.) 5. Connect anode bus bar to the (+) source and cathode bus bar to the (-) source and adjust the voltage to 3.3 volts. During the course of the experiment make adjustments to maintain this voltage if necessary. 6. Measure current passing through the cell every 5 minutes (every minute at the beginning) and record them. 7. After one hour remove the electrodes, rinse in water, dry them, then weigh the cathode and record its weight change. 8. Weight the deposited zinc metal, if possible, as an alternative. QUESTIONS 1. Calculate the current density in amperes per square meter of cathode surface. 2. Draw power consumption and current density vs. time graphs. 3. Calculate the percent current and energy efficiencies and energy consumptions for zinc deposition. 4. Discuss and draw conclusions from the data obtained based on your observations. 5. Discuss the importance of e.m.f. series in electrowinning.

5 SUGGESTED READINGS 1. J. O M. Bockris and A.K.N. Reddy, Modern Electrochemistry, Vol. 1 and 2, Plenum/Rosetta Ed., C.L. Mantell, Electrochemical Engineering, McGraw-Hill, New York, H. H. Kellog, Energy Use in Zinc Extraction, Lead-Zinc-Tin 80, TMS-AIME World Symposium on Metallurgy and Environmental Control, Editors: J. M. Cigan, T. S. Mackey and T. J. O Keefe, (1979). ELECTROWINNING CELL D.C. Voltage source + - bus Solution level Aluminum cathode 150 ml Fig. 1: Schematic representation of experimental electrowinning cell

6 APPENDIX Faraday s Laws: Faraday showed that the amount of chemical substance liberated at an electrode is directly proportional to the amount of current passed through the cell. Also, he found that coulombs is required to deposit or dissolve 1 gm-equivalent substance, where 1 gm equivalent = atomic weight valency The theoretical weight of substance dissolution or deposition after a certain electrolysis time can be expressed as; Weight of substance deposited in gm (theor.) = MW I t n F Where: MW: Atomic or Molecular Weight n: Valency I: Current (ampere) F: Faraday s constant t: Time (sec) Due to deposition of unwanted products, electronic conductivity in the electrolyte, chemical and electrical short-circuiting in actual cases substance deposition is less than theoretical value. The ratio between the weight of actually deposited and theoretically deposited substance is called current efficiency; Decomposition Potentials: C. E. = Wt Actual deposited Wt Theoretical deposited If an electrochemical reaction is carried out reversibly i.e at infinitely low current density, the cell voltage is denoted E and is called electromotive force (e.m.f) of the cell. We have; G = -n E F Where G is the change in Gibbs energy, n is the number of electrons transferred (valency), and F is Faraday s number (96500 coulombs). If in a cell reaction all components are present in their standard states the Gibbs energy change is denoted as G and the corresponding e.m.f. as E. If for a reaction AX = A + X the components are present with activities a AX, a X and a A, then we have: G = G + RT ln [ a A a X a AX ]

7 and E = E - RT/nF ln [ a A a X a AX ] ENERGY CONSUMPTION: The energy consumption per unit mass of metal produced is an important factor in assessing and comparing the economics of electrolytic processes. It is generally expressed in unit of kw.h/kg of metal deposited, and is given by; Energy consumption = (V* I* t)/(w*3600*1000) Where V is the applied potential in volts, I is the current passing through the circuit in amperes, t is the time of deposition in seconds, and W is the actual weight of deposition in kilograms. ENERGY EFFICIENCY: It is also an important factor in the electrolytic processes. Because, it combines two factors: energy consumption and current efficiency. It is given by formula of; Energy Efficiency = (V t /V a ) * C.E. Where V t is the theoretical cell voltage in volts (V t of zinc electrowinning cell is 2.35 volts), V a is the applied voltage in volts, and C.E. is the current efficiency of process. OHMIC RESISTANCE: Resistance of electrolyte affects the rate of electrolytic processes. Therefore, it is also an important parameter in the electrowinning. It may be calculated from equation; Resistance of electrolyte= R = (L/A) Where L is the distance between electrodes, A is the electrode area, and is the resistivity of electrolyte.

Name AP CHEM / / Collected Essays Chapter 17

Name AP CHEM / / Collected Essays Chapter 17 1980 - #2 M(s) + Cu 2+ (aq) M 2+ (aq) + Cu(s) For the reaction above, E = 0.740 volt at 25 C. (a) Determine the standard electrode potential for the reaction