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Kinetics of the zinc slag fuming process Richards, Gregory George


A study involving in-plant measurements, laboratory analysis and mathematical modelling was conducted to elucidate the kinetics of the zinc slag fuming process. The traditional assumption has been that the process operates at thermodynamic equilibrium. The results of industrial measurements at five different companies has demonstrated that this approach is not correct. Chemical assays of the slag show carbon levels in the range of 0.1 - 1.0% and char particles have been extracted from slag samples. Tuyere back-pressure measurements revealed that the predominant mode of gas injection behavior is bubbling. This evidence indicates that a portion of the coal injected into the furnace is entrained in the slag. A model of the direct coal particle-slag reaction was developed and incorporated into an overall model of the slag bath. This model included the behavior of the water-jacketed wall, a treatment of coal combustion in the tuyere gas stream, and a model of the entrained coal residence time. Fitting of the data to eleven industrial fuming cycles showed that the fraction of coal entering the bath was consistently about 35%. About 50% of the coal is combusted in the tuyere gas stream and 10% passes through the bath unconsumed. Calculated oxygen utilization ranged from 70-95%, dependent on slag depth. The slag fuming process is therefore kinetically controlled. There are essentially two critical parameters: the fraction of coal entrained in the slag, and the rate of ferrous iron oxidation. The rate of ferric reduction balances ferric inputs to the bath by displacing previously reduced zinc from the entrained coal-slag reaction bubbles. Process efficiency can be increased therefore by increasing entrainment of coal in the bath, perhaps by the use of high pressure injection, and by reducing ferrous iron oxidation. The latter objective may be achieved by more complete combustion of tuyere coal or pre-combustion. A significant control advantage might be gained by separating these two functions to different sets of tuyeres. In continuous fuming operations the model would suggest that improved efficiencies could be obtained by using a more coarsely ground coal, higher fixed carbon coals, and operating at intermediate temperatures.

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