Entrainment of a two-layer liquid through a taphole

Entrainment through a taphole of a two-layer system has been investigated. The results showed that for liquids with low viscosities, the minimum dimensionless clearance from the taphole before entrainment occurs can be predicted theoretically and was related to the Froude number as Fr^0.4. At low Froude numbers less than 1, fluids with low viscosities resulted in the entrainment showing a positive deviation from the critical Froude number. This allowed tapping to be carried out without entrainment occurring, even when the liquid-liquid interface was within the region of the taphole diameter. For liquids with appreciable viscosities, the entrainment showed a negative deviation from the critical Froude number. This occurs for the tapping of slag and results in entrainment at a higher liquid-liquid interface height than is predicted theoretically. Entrainment was more likely to happen for slag tapping than for matte tapping. A ledge below the taphole was found to reduce entrainment, enabling the liquid-liquid interface to be closer to the taphole during tapping. This resulted in improved slag removal efficiencies and reduced matte or metal entrainment, coupled with less slag left in the furnace after tapping. The length of the ledge from the taphole into the slag was found to be the controlling parameter for entrainment, while the ledge thickness had no effect on entrainment.