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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.

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The initial growth rate offreeze linings on water-cooled elements submerged in molten iron silicate slag is fast. The freeze lining microstructure forming on water cooled steel surface in a high-silica, slag cleaning furnace slag o f a direct-to-blister copper smelter is mostly glassy or amorphous. It contains 5-30 pm magnetite crystals, very small and larger copper droplets as well as small magnetite and silicate nuclei embedded in the glassy silica-rich matrix. Chemically the form ed freeze linings are more silica-rich than the slag from which they were generated. Magnetite (spinel) is the primary phase o f the solidifying SCF slag but it does not form a continuous network through the freeze lining. Its strength is given by the intergranular silica-rich phase which initially is glassy or microcrystalline. Due to only partial slag reduction in the SCF process, large magnetite crystals are present in the freeze lining and seem to interact physically with copper droplets.