Elimination of copper from the molten steel by NH3 blowing under reduced pressure

Abstract

Based on the finding that blowing of NH3 gas onto molten pure copper at atmospheric pressure causes an evaporation of an enormously high rate, attempts have been made to eliminate copper dissolved in molten steel by NH3 gas blowing under reduced pressure. Typical behaviours of molten steel when attacked by NH3 gas are boiling, splashing and, in a violent case, slopping from the crucible. These phenomena are accompanied by an immense increase in the surface area from which volatile species are able to evaporate and may be useful for vacuum metallurgical processes. In this investigation, the decuprization experiments have been conducted to measure the rate at which the copper level in the molten steel is reduced using a vacuum induction furnace and blowing NH3 gas through a vertical water‐cooled copper nozzle. The data were represented in terms of the amount of copper transferred from the melt to the gaseous phase per unit surface area and time. According to these experiments, complete decuprization was attainable confirming selective evaporation of copper from molten steel under reduced pressure. So far as the mechanism of copper elimination by NH3 gas, besides the enhanced vacuum evaporation of metal by the cooling effect of endothermic decomposition of NH3, the formation of volatile copper compound was suggested from the fact that the copper could be removed down to an extra‐low level of several ppm, even under a residual gas pressure of the order of 10 Torr.