Selenium Recovery in Precious Metal Technology Fabian Kling Willig Degree Project in Engineering Chemistry, 30 hp Report passed: February 2014 Supervisors: Tomas Hedlund, Umeå University Mikhail Maliarik, Outotec (Sweden) AB Abstract Selenium is mostly extracted from copper anode slimes because the selenium-rich ores are too rare to be mined with profit. When copper anode slimes are processed in a precious metal plant the first step is to remove copper by pressure leach in an autoclave. The anode slime is then dried and fed into a Kaldo furnace. The Kaldo is heated and reduction and smelting begins. During reduction, slag builders are added to form a slag with impurities and fluxes with coke breeze are added to reduce precious metal. In the oxidation step, selenium dioxide, sulphur dioxide and some tellurium dioxide are removed along with the process gas. The dioxides and the process gas are captured in a circulating venturi solution in the gas cleaning system. The dioxides lowers the pH of the venturi solution. Sodium hydroxide is added to the solution to keep pH above 4 in order to prevent selenium from precipitating in the circulation tank. After a Kaldo cycle, the venturi solution is transferred to a precipitation tank where sulphur dioxide is added in order to precipitate selenium. The precipitated selenium is finally collected in a filter press and sold as crude (99.5%) selenium. During commissioning of a precious metal plant and during processing of 2 batches anode slime, data such as pH and electrochemical potential was collected from the venturi solution and later shown in a Pourbaix diagram. The purpose of this degree project was to get a better understanding of selenium's redox chemistry and investigate if the potential could explain why selenium precipitates despite pH regulation. Results show that the redox environment in the venturi solution is suitable for selenium precipitation and that pH regulation is not enough. A material balance of selenium was also performed in order to follow selenium recovery. Data such as selenium concentration was taken from the circulating venturi solution in the circulation tank and from the precipitation tank. Selenium amount data was collected from anode slime, converter slag, bag filter dust, smelt slag, venturi slime and slime from the precipitation tank. It was not possible to make a thorough material balance as the results obtained for the determination of selenium concentration and amount of selenium were contradictory. I Acknowledgments I would like to thank all who have been involved in this project. In particular I would like to mention my supervisor, Mikhail Maliarik at Outotec Sweden (AB) for ideas, discussions and help during report writing. Thanks also to my supervisor at Umeå University, Tomas Hedlund for input and help during the start-up of this project and report writing. Thank you Kjell-Åke Johansson at Boliden and Bert Ögren at Outotec Sweden (AB) for showing me around the precious metal plant. Thank you Rickard Lindh at Outotec Sweden (AB) for introducing me to Outotec. Thanks also to the Chinese personnel for all the analyse data and help during sampling during this project. Finally I would thank my wife for supporting me. II Contents 1. Introduction and background ............................................................................................................... 1 1.1 Aim of thesis ................................................................................................................................. 1 1.2 Literature search ............................................................................................................................ 1 1.3 Criticism of the sources ................................................................................................................. 2 1.4 General information of selenium ................................................................................................... 2 1.4.1 History of selenium ................................................................................................................ 2 1.4.2 Occurrence. ............................................................................................................................. 2 1.4.3 Meteorites ............................................................................................................................... 3 1.4.4 Volcanic matter ...................................................................................................................... 3 1.4.5 Soils ........................................................................................................................................ 3 1.4.6 Plants ...................................................................................................................................... 3 1.4.7 Humans ................................................................................................................................... 4 1.4.8 Physical properties of Selenium ............................................................................................. 4 1.4.9 Uses and applications ............................................................................................................. 5 1.4.10 Chemical properties of Selenium ......................................................................................... 5 1.4.10.1 Oxides and oxyacids ...................................................................................................... 5 1.4.10.2 Hydrides ........................................................................................................................ 6 1.4.10.3 Halides ........................................................................................................................... 6 1.5 Production ..................................................................................................................................... 7 1.6 Recovery of selenium .................................................................................................................... 9 1.6.1 Leaching minerals .................................................................................................................. 9 1.6.2 Recovery of selenium from copper refinery slimes ................................................................ 9 1.6.2.1 Roasting of copper refinery slimes .................................................................................. 9 1.6.2.1.1. Roasting to form selenites ....................................................................................... 9 1.6.2.1.2 Roasting to produce SeO2 ....................................................................................... 10 1.6.2.1.3 Roasting to form selenates ...................................................................................... 10 1.6.2.1.4 Outotec selenium roasting process ......................................................................... 12 1.6.3 Outotec selenium recovery process during Kaldo processing of copper anode slimes ........ 12 1.6.3.1 Charging ........................................................................................................................ 12 1.6.3.2 Smelting / Reduction ..................................................................................................... 12 1.6.3.3 Converting / Refining .................................................................................................... 13 1.6.3.4 Outotec gas cleaning process and equipment ................................................................ 13 III 1.6.3.4.1 Venturi system ........................................................................................................ 14 1.6.3.4.2 Wet-ESP separator.................................................................................................. 15 1.6.3.4.3 Absorption tower .................................................................................................... 15 1.6.3.4.4 Treatment of the Venturi solution .......................................................................... 15 1.6.3.4.5 Selenium recovery from Venturi solution .............................................................. 16 1.6.3.4.5.1 Precipitation of selenium, step 1 ...................................................................... 16 1.6.3.4.5.2 Precipitation of selenium, step 2 ...................................................................... 16 1.7 Selenium precipitation reaction in the venturi solution ............................................................... 16 1.8 Other selenium recovery techniques ........................................................................................... 17 1.8.1 Leaching of copper anode slimes ......................................................................................... 17 1.8.1.1 Leaching with sodium hydroxide .................................................................................. 17 1.8.1.2 Sulphuric acid leaching in the presence of oxygen. ...................................................... 18 1.8.2 Recovery from aqueous solutions ........................................................................................ 18 1.8.2.1 Recovery from the -2 valence state ............................................................................... 18 1.8.2.2 Recovery from the +4 valence state .............................................................................. 19 1.8.2.3 Recovery from the +6 valence state .............................................................................