Investigation of Origin of Attached Cu-Ag Droplets to Solid Particles During High-Temperature Slag/Copper/Spinel Interac
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INTRODUCTION
THE overall metal recovery in primary and secondary copper production is limited by copper droplet losses.[1] Limiting these metal losses requires a fundamental understanding of their characteristics and origin. Copper losses in slags are in literature generally subdivided into chemical and mechanical losses.[2–4] Chemical copper losses represent the formation and dissolution of copper sulfide and/or copper oxide and are inherent to pyrometallurgical processes. The chemical losses are linked to the system’s thermodynamics, i.e., the oxygen partial pressure,[2,4–6] the temperature, and the composition of the slag and matte.[2,4–6] Mechanically entrained copper refers to entrapped or floating unsettled droplets. In primary copper production, these include both matte and metal droplets, whereas mainly metallic droplets are entrapped in
INGE BELLEMANS and KIM VERBEKEN are with the Department of Materials, Textiles and Chemical Engineering, Ghent University, Technologiepark 903, Zwijnaarde, 9052 Ghent, Belgium. Contact e-mail: [email protected] EVELIEN DE WILDE is with Umicore R&D, Kasteelstraat 7, 2250 Olen, Belgium. BART BLANPAIN and NELE MOELANS are with the Department of Materials Engineering, KU Leuven, Kasteelpark Arenberg 44, Bus 2450, Heverlee, 3001 Leuven, Belgium. Manuscript submitted December 28, 2016.
METALLURGICAL AND MATERIALS TRANSACTIONS B
secondary copper production. For secondary production, which is based on for example e-scrap, some noble metals are also collected in the metallic phase and even though these noble metals have limited solubility, their loss has a potentially significant economic impact. One of the causes of the mechanical entrainment of the droplets is the precipitation of copper or matte droplets within the slag due to a decrease in the solubility of copper in the slag. This decrease finds its origin in the inhomogeneity of the process, e.g., there are zones with a different local oxygen potential or a lower temperature.[7] A second cause of the mechanical entrainment is the dispersion of metal into the slag by gas-producing reactions. Minto and Davenport[8] suggested that SO2 bubbles nucleated at the bottom of the furnace rise to the matte/slag interface and elevate a surface film of matte into the slag.[7,8] This surface film can break down because of capillary instability, dispersing fine droplets into the slag. Operational actions performed in pyrometallurgical processes, such as tapping or charging, are another possible cause for mechanically entrained droplets. During tapping, the denser liquid can rise when flowing around obstacles in the vessel and hence mechanical entrainment can take place.[3] Moreover, gas injections, turbulence, or pouring of one phase into the other can cause the physical dispersion of the denser layer into the slag.[3,9] These causes were already examined and discussed extensively in literature, but another possible source for
the mechanical entrainment of copper droplets is often not taken into account: the attachment of droplets to solids
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