2009:056 MASTER'S THESIS Optimisation of Influential Factors in Electrowinning of Tellurium by Means of PLS Modelling Seyed Mohammad Khosh Khoo Sany Luleå University of Technology Master Thesis, Continuation Courses Minerals and Metallurgical Engineering Department of Chemical Engineering and Geosciences Division of Process Metallurgy 2009:056 - ISSN: 1653-0187 - ISRN: LTU-PB-EX--09/056--SE Optimisation of Influential Factors in Electrowinning of Tellurium by Means of PLS Modelling Master Thesis By: Seyed Mohammad Khoshkhoo Sany [email protected] Supervisors: Prof. Åke Sandström (Luleå University of Technology) Dr. Nils Johan Bolin (Boliden Mineral AB) Abstract Electrowinning of tellurium is a relatively simple process that can be carried out effectively by employing the existing technology. In order to obtain tellurium with the least amount of impurities, the effects of three main factors were studied, namely current density, free concentration of caustic soda and initial concentration of tellurium. Five elements were chosen as the main impurities: Ag, Bi, Cd, Ni and Pb. A series of 17 experiments arranged in a CCF (central composite faced-centred) design has been carried out and the results were fitted using PLS (partial least squares) method. The effect of free caustic concentration was found to be the most important of the three parameters studied. Current density was also important, yielding the least amount of impurities at high current densities. Tellurium concentration had the smallest effect of the three parameters studied. From the model, current densities of 330-350 A/m2, tellurium concentrations of 90-120 g/l and free sodium hydroxide concentration of around 120 g/l are suggested as optimal conditions for obtaining the purest tellurium. Table of Contents Abstract..................................................................................................................... 1 Acknowledgment ...................................................................................................... 3 Aim of the project ..................................................................................................... 4 Chapter 1 – Tellurium ............................................................................................... 5 1.1 Introduction ..................................................................................................... 5 1.2 Geochemistry .................................................................................................. 5 1.3 Resources ........................................................................................................ 6 1.4 Production ....................................................................................................... 6 1.5 Purification...................................................................................................... 9 1.5.1 Vacuum Distillation.................................................................................. 9 1.5.2 Zone Refining........................................................................................... 9 1.6 Applications .................................................................................................. 10 1.7 Economy ....................................................................................................... 10 Chapter 2- Electrowinning........................................................................................12 2.1 Introduction to Electrowinning ...................................................................... 12 2.1.1 Introduction to electrometallurgy ............................................................ 12 2.1.2 Basic Facts.............................................................................................. 12 2.1.3 Faraday Law ........................................................................................... 14 2.1.4 Current Efficiency................................................................................... 14 2.1.5 Electrode Potential and Nernst’s Equation............................................... 14 2.1.6 Cell Voltage and energy consumption..................................................... 15 2.1.7 Metal Deposition..................................................................................... 15 2.2 Electrochemistry of Tellurium ....................................................................... 16 2.2.1 Cathodic Reduction of Tellurite .............................................................. 16 2.3 Electrowinning of Tellurium.......................................................................... 18 Chapter 3 - Experimental Part ..................................................................................21 3.1 Design of Experiments................................................................................... 21 3.1.1 Introduction ............................................................................................ 21 3.1.2 Definition of Factors ............................................................................... 21 3.1.3 Definition of Responses .......................................................................... 22 3.1.3 Design of Model ..................................................................................... 23 3.2 Experiments................................................................................................... 25 3.2.1 Investigating the Effect of Periodic Short-Circuited Current.................... 25 3.2.2 Running the Designed Experiments......................................................... 26 3.3 Analysing ...................................................................................................... 27 Chapter 4- Results and Discussion............................................................................29 4.1 Results........................................................................................................... 29 4.2 Fitting the Model and Evaluation of Fit.......................................................... 31 4.3 Using the Model (Optimisation)..................................................................... 35 4.4 Conclusion..................................................................................................... 40 4.5 Future Work .................................................................................................. 41 References................................................................................................................43 2 Acknowledgment I am grateful to the financial support given by Boliden Mineral AB which enabled me to carry out this project. Also, I wish to thank Prof. Åke Sandström for his guidance throughout my master studies at Luleå University of Technology and for giving me the opportunity to perform this thesis. Dr. Nils-Johan Bolin is greatly acknowledged for his help with the theoretical and experimental parts of the project. I am very thankful to Paul Kruger who always had time to answer my questions. In addition, I appreciate all the assistance I received from Rolf Danielsson, Amang Saleh, Mikael Widman, Mikael Eriksson and Carina Andersson during the laboratory work. Lars Eric Carlsson is sincerely thanked for analysing my samples and teaching me how to play badminton (you have been very patient). Boliden’s musical band, Anders Nyström, Conny Jacobsson, Gösta Pettersson and his lovely wife, Per Ivar Marklund, Mehran Mousavipour, Ulrika Holmgren and Daniel Holmgren, is appreciated for inviting me to their group and for their open mindedness attitude towards my music. There is no enough space to name all other nice friends that I have found in Boliden. They made a nice and joyful stay for me during the time I was working on the thesis. I am grateful to all of you. If it wasn’t for my friend Sepehr, I would have never been able to come so far in my education. He was the one who gave me his invaluable consults helping me finding the right path. A considerable part of my success is because of him. Thank you Sepehr. Above all, I can’t find suitable words to express thanks to my family for all they have done for me. I am indebted to my dad for his never-ending care and support, and to my mum for her unconditional love. My lovely sisters have been emotionally supporting me during all these times. I love you all! 3 Aim of the project Tellurium is a rare metal with a very promising market. Demand for highly pure tellurium to be used in strategic applications is increasing day by day. Boliden AB possesses considerable resources of tellurium both in its gold bearing minerals and in the copper anode slimes. Recovery and purification of this tellurium indeed is a promising business in a near future. This thesis work focuses on a single stage of every tellurium processing line; electrowinning. Electrowinning is an important stage to produce commercial grade tellurium of higher value. Interaction between important factors and their influence on the purity of electrowon tellurium have been investigated in this project. In order to obtain tellurium with a minimum content of impurities, optimum values are suggested by modelling the process, using the partial least square (PLS) method. 4 Chapter 1 – Tellurium 1.1 Introduction Tellurium with atomic number of 52 is one of the rarest elements in the earth crust with an abundancy of only 10 ppb. It was first discovered by a Hungarian mineralogist Franz-Joseph Müller von Reichenstein in 1782. The route of the word comes from Tellus which means earth in Latin.