The production of nickel-zinc alloys by powder injection
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I.
INTRODUCTION
A L L O Y S of nickel and zinc are used for the hot dip galvanizing of reactive steels (containing around 0.1 pct Si), where the nickel helps to reduce the thickness of the zinc coating and improve its appearanceJU Zinc alloys containing between 0.15 and 2.5 wt pct nickel are produced commercially. The optimum galvanizing bath composition for reactive steels is 0.15 wt pct Ni. The production of nickelzinc alloys is hindered by the low solubility of nickel in zinc at the temperatures where zinc is typically available in zinc casting shops (450 ~ to 550 ~ The addition of nickel in the form of pellets is not conducive to rapid dissolution, as these settle to the bottom of the zinc melt, where they form a clinker. While the nickel solubility is fixed by the temperature of the zinc bath, there is scope to significantly increase the rate of nickel dissolution by reducing the particle size of the nickel additive. Submerged injection of fine nickel powder through a lance into the melt has the potential to produce intimate contact between the nickel and the zinc melt to achieve rapid alloying, while also mixing the alloy to achieve a uniform composition. In this investigation, the use of powder injection for the production of nickel-zinc alloys was tested at plant scale. The plant investigations followed laboratory studies which established that the dissolution of nickel in zinc is a masstransfer-controlled process. The diffusion coefficient of nickel in molten zinc was measured as a function of temperature. A mathematical model for the dissolution time of particles suspended in an agitated bath was formulated. The combination of the mathematical model with the diffusion data allowed the dissolution time of nickel powder in a gasstirred zinc melt to be predicted after specifying melt tern-
D.E. LANGBERG, Senior Research Fellow, and M. NILMANI, Senior Lecturer, are with the G.K. Williams Cooperative Research Centre for Extractive Metallurgy, Department of Chemical Engineering, University of Melbourne, Parkville 3052, Australia. Manuscript submitted January 5, 1995.
780~VOLUME 27B. OCTOBER 1996
perature and composition, the initial particle size, and the specific mixing power.
lI.
MODEL
The thermodynamic and phase relationships of the nickel-zinc system have been reviewed by Nash and Pan.E21 The melting point of the pure nickel (1455 ~ is much higher than the temperature of a typical zinc alloying bath (450 ~ to 550 ~ Under these circumstances, GuthrieI31 has observed that the rate of dissolution is often limited by mass transfer through the liquid boundary layer surrounding the additive. A mathematical model to predict the dissolution time of high melting point additives was developed by Apetian et al. [4j and applied to the manganese-aluminum system. However, this model relied on mass transfer coefficients calculated from their experimental data and was therefore not a truly predictive model. Langberg et aLE53 incorporated a correlation for the mass transfer coefficient of a particle suspended in an
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