Freeze-lining Formation and Microstructure in a Direct-to-blister Flash Smelting Slag

Abstract

Copper smelting in one processing step directly from concentrates to blister copper has been realized on an industrial scale in a few smelters, using concentrates with a high Cu/Fe-ratio. The fluxing of any direct-to-blister slag is demanding task as it must be fluid and maintain suitable properties in the oxidising conditions of copper making, and the reducing conditions of slag cleaning. The smelting slags in the direct-to-blister furnaces contain much more chemically dissolved copper than typical matte making slags. In this investigation, an industrial direct-to-blister slag was used in a freeze lining growth kinetics study. The freeze lining was formed on a water cooled metal finger at typical smelting temperatures using different dipping times from 5 to 120 min. The growth kinetics of the lining was very fast in the initial stage of the slag contact with the cooled metal surface. The quenched samples showed characteristic solidification zones from the cold end towards the hot side of the freeze lining and the molten slag shown already in other freeze linings and different slag types. The slag chemistry modifies the solidification pattern very much and thus the crystalline phases in the lining included also phases created by the high copper oxide concentration as well as the specific gangue assay of the smelters feed mixture. The thermal stability of the freeze lining in high-in-copper DB slags is discussed as well as the mechanism of delafossite precipitation.

Freeze lining formation on water cooled refractory wall

“…4. The microcrystalline zone and its structure were similar to those obtained earlier with the same DtoB slag and described in the earlier publication (Taskinen et al , 2013). It contained randomly distributed, relatively large magnetite crystals in a very fine crystalline matrix next to the probe surface in the cold end.…”

“…The microstructures of the freeze linings formed on the refractory sleeve did not have the familiar and characteristic glassy layer in the cold end, as reported on the previous studies (Taskinen et al , 2011; Taskinen et al , 2012; Fallah-Mehrjardi et al , 2013a). Figure 2 shows a panorama composed of several SEM BSE images from the cold end (upper left) to the hot end (lower right) of the formed freeze lining.…”

“…In copper matte smelting slags, the crystalline phases are mostly magnetite but depending on the gangue mineralogy and age, also fayalite (‘olivine’) and other silicates may be present (Taskinen et al , 2011; Fallah-Mehrjardi et al , 2012). In the silica fluxed copper converting or direct to blister (DtoB) slags, the prevailing high oxygen pressure increases the chemically dissolved copper concentration and causes growth of crystalline delafossite (CuFeO 2 ) at lower temperatures of the freeze lining, thus binding also a fraction of ferric iron oxide from the slag (Taskinen et al , 2012). Towards the hot end, the porosity increases, and thus, mechanical strength of the freeze lining decreases.…”

Copper-Bearing Magnetite and Delafossite in Copper Smelter Slags

“…This phase is not entirely formed within the furnace but results from the heating of SiO 2 flux that is preserved as undigested pockets observed in some samples (Figure 2b). A high oxygen partial pressure or a high copper oxide concentration in the slag are the conditions considered for the precipitation of delafossite [21]. Whereas such conditions control the formation of delafossite at high temperature in the furnace, in a geological environment, delafossite only forms at low temperature in the oxidation zone of primary Cu ores (http://www.webmineral.com/, accessed on 11 September 2023).…”

“…In contrast to their restricted occurrence in the natural environment, Cu-bearing magnetite and delafossite are common co-existing phases in slags produced during copper smelting in flash furnaces (e.g., [21,22]).…”