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Fundamental Aspects of Nickel Electrowinning from Chloride Electrolytes

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Fundamental Aspects of Nickel Electrowinning from Chloride Electrolytes FUNDAMENTAL ASPECTS OF NICKEL ELECTROWINNING FROM CHLORIDE ELECTROLYTES by JINXING JI B.Eng., Shanghai University of Technology, 1982 M.Eng., Shanghai University of Technology, 1985 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES The Department of Metals and Materials Engineering We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA February 1994 ©JinxingJi, 1994 _____________________________ In presenting this thesis in partial fulfillment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. Ifurther agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. (Signature): Department of Metals and Materials Engineering The University of British Columbia Vancouver, B.C., Canada Date: February 14, 1994 Abstract ii Abstract Nickel electrowinning from chloride electrolytes is an innovative and efficient process developed and commercialized mainly by Falconbridge Ltd. Several fundamental aspects related to this process have been addressed in this thesis, including the thermodynamic study of nickel electrolytes, the measurement and modelling ofthe cathode surface pH during nickel electrowinning and the kinetic study of nickel reduction and hydrogen evolution. The major apparatus and equipment used include a surface pH measuring device, an EG&G rotating disc electrode, a SOLARTRON 1286 Electrochemical Interface and a RADIOMETER titrator system. All of the experiments were carried out via computer control. The thermodynamic study includes the activity coefficient of the hydrogen ion and the spe ciation of nickel electrolytes to obtain a better understanding of the properties of nickel electrolytes. The activity coefficient of the hydrogen ion (y11+) was measured using a combination glass pH electrode. It was found that was greater than 1 in concentrated NiCl2 solutions and increased significantly with increasing NiCl2 concentration. The addition of NaC1 increases ‘y11÷ , whereas the addition of Na2SO4 decreases it. Theoretically, several useful equations were derived based on Meissner’ sand Stokes-Robinson’s theories to calculate the single-ion activity coefficients including These equations are the two-parameter (q and h) functions, capable of predicting with rea sonable accuracy single-ion activity coefficients in any concentrated pure electrolytes and in mixed electrolytes of the type 1:1 + 1:1, 2:1 + 1:1 and 2:1 + 1:1 + 1:1. The accuracy of the calculations may be further improved when the Meissner parameter q is adjusted properly and the effect of ionic strength on the hydration parameter h is taken into account. A series of speciation diagrams for nickel species was plotted with and the effect of the ionic strength on the equilibrium constants being taken into account. It was discovered that the predominant nickel species in the acidic region are Ni2 and NiCl in concentrated pure NiC12 solutions and Ni2, NiCl and NiSO4 in concentrated sulfate-containing NiCl2 solutions. The traditionally accepted electroactive species NiOH is negligible until the NiCl2 concentration is lowered to the order of 106 M. When the pH increases, the formation of insoluble Ni(OH)$) should be expected if the NiCl2 concentration is higher than 106 M. The pH where Ni(OH)S) starts to form decreases with increasing NiCl2 concentration and temperature. A limited number of electrowinning tests were carried out under conditions similar to those employed in the industrial process in order to obtain information concerning the current efficiency of nickel deposition. It was found that higher nickel concentration, higher pH and the addition of NaCl, H3B0 and NH4C1 improved the currentefficiency ofnickel deposition. However, the addition Abstract iii of sulfate decreased the current efficiency of nickel. In 0.937 M NiC12 at 60°C, the pH may go as low as 1.5 for a current efficiency above 96 %. Nickel deposition was also found to be a steady-state process since the amount of acid added to the electrolyte at a constant pH increased linearly with time. To acquire data on the cathode pH behaviour during nickel deposition, the cathode surface pH was measured using a flat-bottom combination glass pH electrode and a fine mesh gold gauze as cathode. Nickel was deposited on the front side ofthe gold gauze and the pH electrode was positioned in the back and in direct contact with the nickel-plated gold gauze. The cathode surface pH was always found to be higher than the pH in the bulk electrolyte, and if the current density was suf ficiently large, it would eventually reach a level causing precipitation of insoluble Ni(OH)S) on the cathode surface. Lower bulk pH, higher nickel concentration, higher temperature and the addition of H3B0 and NH4C1 effectively depress the rise of the cathode surface pH. Additions of NaCl and Na2SO4 also depress the rise of the cathode surface pH but to a much smaller degree. Also, agitation of the electrolyte decreases the cathode surface pH. In order to predict the cathode surface pH, mathematical modelling in the case of 0.937 M NiCl2 and 2 M NiCl2 was carried out. The model was in reasonably good agreement with the experimental data. Nickel deposition and hydrogen evolution were studied using a rotating disc electrode. The hydrogen evolution was found to be affected strongly by the RPM. The rate of nickel deposition was first order with respect to the activity of nickel ion and zero order with respect to the activities of chloride and hydrogen ions. The rate of hydrogen evolution was found to be first order with respect to the activity of hydrogen ion and to be zero order with respect to the activities of nickel and chloride ions. These findings indicate that nickel deposition and hydrogen evolution proceed independently. The Tafel slopes obtained from the partial polarization curves were 94 mV/decade for nickel deposition and 112 mV/decade for hydrogen evolution. Hydrogen evolution was also studied using a rotating nickel-coated Pt disc electrode in 2.5 M NaCl solution in the absence of nickel ions. The rate of hydrogen evolution was first order with respect to the activity of hydrogen ion and zero order with respect to the activity of chloride ion. According to the relationship between the limiting current density and the square root of rotational speed, hydrogen evolution was mass transfer controlled under the limiting conditions and the buffering actions of H3B0 and NH4C1 were negligible. The magnitude of the limiting current density at a given pH or a given acidity in the presence of sulfate can be well explained considering the activity coefficient of the hydrogen ion. Further studies of nickel electrowinning should be directed towards hydrogen evolution on the nickel substrate in nickel-containing electrolytes, focusing on the hydrogen bubble’s nucleation, Abstract iv growth, coalescence and detachment. The use of addition agents affecting hydrogen evolution by way of adsorption, change in interfaciai tension or destruction of atomic hydrogen is worth investigating. The identity of intermediate species during nickel reduction is not clear. The identification of these species would be quite rewarding in clarifying the mechanism of nickel reduction. The nucleation of nickel and crystal growth in the initial stages of deposition on various substrates including titanium, stainless steel, copper and nickel are other important aspects of nickel electrowinning which should be investigated. Table of Contents v Table of Contents Abstract Table of Contents v List of Tables ix List of Figures xii Acknowledgements xix Nomenclature xx Introduction 1 Chapter 1 Literature Review on Nickel Electrodeposition 7 1.1 Nickel matte chlorine leaching process 7 1.2 Plant practice of nickel electrowinning 10 1.3 Nickel electrodeposition in chloride and chloride-sulfate electrolytes 15 1.4 Kinetics and mechanism of nickel electrodeposition 20 1.5 Hydrogen evolution during nickel electrodeposition and on a nickel substrate in acidic media 26 Chapter 2 Thermodynamics of Nickel Chloride Solutions 34 2.1 Activity coefficients in multicomponent nickel chloride solutions 34 2.1.1 Measurement of activity coefficient of hydrogen ion 35 2.1.2 Calculation of mean activity coefficients and water activity 47 2.1.3 Calculation of single-ion activity coefficients 55 2.1.3.1 Single-ion activity coefficients in aqueous solutions ofpure electrolytes 56 2.1.3.2 Single-ion activity coefficients in aqueous solution of mixed NiCI2-HC1- NaCZ 61 2.2 The pH for the formation of insoluble nickel hydroxide 67 2.3 Distribution of nickel species in aqueous solutions as a function of pH 72 Table of Contents vi Chapter 3 Electrodeposition of Nickel in Various Electrolytes 84 3.1 Experimental apparatus and set-up for nickel electrodeposition 84 3.2 Electrodeposition of nickel at 25°C 85 3.3 Electrodeposition of nickel at 60°C 87 3.4 Electrodeposition of nickel in 2 M NiC12 + 6 M HC1 95 3.5 Measurement of current efficiency of nickel from the acid volume 98 Chapter 4 Surface pH Measurement during Nickel Electrodeposition 103 4.1 Experimental apparatus and set-up for surface pH measurement 104 4.2 Characterization of gold gauze 106 4.2.1 Electrochemical properties of gold in chloride solution 106 4.2.2 Investigation of new 500-mesh gold gauze 107 4.2.3 Investigation of nickel-coated 500-mesh gold gauze 111 4.3 Effect of nickel concentration on the surface pH in pure NiCl2 solutions at 25°C 116 4.4 Effect of sulfate on the surface pH in NiCl2-NaSO4 solutions at 25°C 118 4.5 Effect of sodium chloride on the surface pH in NiCl2-NaCl solution at 25°C .
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