Mapping of Settling Processes at Boliden Rönnskär and Harjavalta Smelters
Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
In copper extraction, the Cu-Fe-S minerals from which about 80% of world’s copper originates from are insoluble in aqueous solutions hence much of the copper extraction is through the pyrometallurgical route. In this pyrometallurgical route, the Cu-Fe-S and Cu-S mineral particles are separated from gangue minerals by froth flotation to obtain a copper concentrate (20-30% Cu) which is then melted at about 1200oC and oxidised (smelting) to produce a sulphide-rich melt called matte (45-75% Cu), and an oxide-rich melt called slag (1-2% Cu). The separation of these two melts (matte and slag) in the smelting process is enhanced by addition of silica (SiO2) flux which makes them immiscible. The molten matte from the smelting process is further oxidised in the converting process where blister copper (99% Cu) and slag (4-8% Cu) are produced.Boliden Mineral AB is a metals company with two of its five smelters being copper smelters namely Rönnskär and Harjavalta, which use the copper extraction processes (smelting and converting) described above. Rönnskär smelter is located on the coast of Skellefteå in Sweden, while Harjavalta smelter is located in Finland with operations in Harjavalta and Pori. The separation of different molten phases (matte, white metal, metal, speiss and slag) in copper and other base metals production is critical in that it affects the efficiency of the smelting and converting processes as the direct and total yield of the processes get affected. Since in both smelting and converting processes the phase separation is mostly mechanical through settling (gravity separation), these processes are collectively being referred to as settling processes in this study. It is therefore, important to have an in-depth understanding of the degree of separation of the molten phases in these settling processes, and the factors that could influence the settling. The known factors that could influence settling are time, temperature, density, viscosity, composition and vessel geometry.Therefore, in this dissertation, mapping (graphic symbolic representation of the degree of separation) of different settling processes at the Boliden Rönnskär and Harjavalta smelters is done to enable assessment of the quality of settling in each process studied, and compare the different settling processes. The result of this study could then be used to improve processes efficiency as the yields would be improved. Also, process operating parameters that affect separation such as settling time would be optimised.The Scanning Electron Microscopy (SEM) micrographs of the selected slag samples from Boliden Rönnskär and Harjavalta smelters settling processes gave an understanding of the characteristics of the different slags. Based on this understanding, Light Optical Microscopy (LOM) slag characterization was used to come up with a mapping of the different settling processes at the two smelters which included the electric smelting furnace, flash furnace, electronic scrap kaldo (E-Kaldo) furnace, lead kaldo furnace, precious metals (PMs) kaldo furnace, and Peirce-Smith (PS) converter. Comparing LOM micrographs of smelting processes to converting processes proved that the degree of separation was better in smelting processes. Slag composition was observed to be a key factor in settling processes as with it other factors such as viscosity, temperature and settling time become interlinked. As slag composition would be controlled to improve settling, it would also indirectly help manage impurities in the processes. It is recommended that the degree of separation be quantified so that a proper assessment of process optimization is made.
Place, publisher, year, edition, pages
2015. , 111 p.
Technology, Smelting, Converting, Settling processes, Degree of separation, Slag characterization, Mapping
IdentifiersURN: urn:nbn:se:ltu:diva-53198Local ID: a3ec70e5-a991-4aa9-99b6-913b5371d9dbOAI: oai:DiVA.org:ltu-53198DiVA: diva2:1026572
Subject / course
Student thesis, at least 30 credits
Chemical Engineering, master's level
Validerat; 20150706 (global_studentproject_submitter)2016-10-042016-10-04Bibliographically approved