The injection of solids using a reactive carrier gas
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INTRODUCTION
EFFECTIVEinjection of powders into melts requires the efficient disengagement of the particles from the cartier gas in order to become exposed to the melt and sufficient residence time for them to react or dissolve before surfacing or sedimenting. The disengagement of coarse particles injected at low solids loadings, which occurs by penetration through a bubble-liquid interface, has been the subject of several studies3~,2,3] It is generally concluded that large, dense, wettable particles are relatively easy to inject, but poor injection efficiencies are encountered when injecting fine, low density, nonwettable particles. When fine particles are injected at high solids loadings, coupling between the phases results in gas-solid jet formation at the lance tip. [4,5,6] Disengagement between the particles and the gas when the jet disintegrates is not well understood. It is likely that many particles remain attached to the bubbles due to the low relative velocity between the solid and gas phases in the coupled flow. F] The residence time of the particles is determined by their velocity relative to the melt and by the bulk flow velocity of the melt. If the particles have low settling velocities, their motion will be controlled by the bulk circulation generated by the bubble plume. If they remain in the vicinity of the lance tip upon injection, they will be rapidly carded by the plume to the upper surface of the melt with a low utilization efficiency. In many systems of practical interest such as lime injection with oxygen as the cartier gas into steel ]s~ and flux injection with air through tuyeres into a PierceSmith converter, tgj the carrier gases strongly react with the melt. In some important systems (Fe-O, Cu-O), the presence of a reactive gas can reduce the surface tension of the gas-melt interface[~~ and, hence, lower the interfacial resistance to particle penetration of the bubble surface. Furthermore, the elimination of the bubble plume by the dissolution of the carrier gas will significantly reduce the bulk flow velocity of the melttt2,'3] and D.E. LANGBERG, Research Fellow, and M. NILMANI, Senior Lecturer, are with G.K. Williams Cooperative Research Centre for Extractive Metallurgy, Department of Chemical Engineering, University of Melbourne, Parkville, Victoria 3052, Australia. Manuscript submitted September 20, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS B
potentially increase the residence time of the injected particles. This article addresses experimentally the differences in the hydrodynamic behavior that occur when a soluble rather than an insoluble carrier gas is used for solids injection. A cold-model analog system was used in which polyethylene particles were injected into a cylindrical vessel containing water, through a top-submerged vertical lance. The cartier gases used were air (insoluble) and ammonia (soluble). The degree of dispersion of the injected particles and the penetration distance of the particles into the liquid were measured as the gas composition, gas flow rate, s
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