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Since the oxidation of the liquid iron droplet involves reaction between the oxygen in the air and the liquid metal the following reaction steps should occur:

(i) transfer of oxygen to the gas/metal interface from bulk gas,

(ii) chemical reaction at the interface, and

(iii) transfer of the dissolved oxygen to the interior of the droplet

When the reaction commences, the deoxidised iron and oxygen are in direct contact at high temperature. The chemical reaction rate is probably so high under these conditions that the overall rate is controlled by one of the transport processes (i) or (iii). Of these two transport processes, (i) is most likely in the

initial stages as (iii), the diffusion of ions through the oxide film, may be important in the later stages of reaction.

To test this hypothesis, a mathematical model was developed to calculate the rate of mass transfer by forced convection from the gaseous phase to accelerating droplets. Using this model, the rate of oxidation of liquid iron was calculated assuming that the equilibrium value of partial pressure of oxygen at the inter­ face is negligible compared with that of the bulk gas. The model is developed in Appendix III. A number of differential equations were generated and these were solved by computer programmes. The results of the mahematical model are drawn in Figs. 157 to 164* The figures clearly show that droplet size is an important factor in oxygen pick-up i.e. the smaller the mass of the droplet the greater the oxygen pick-up at any particular oxidation time, indicating that the oxygen concentration in a droplet is directly related to the surface area to volume ratio.

In the experiments carried out on the oxidation of iron drop­ lets it was found that the temperature of the droplet did not have a pronounced effect. The temperature of the droplets were kept, as far as possible at 1600°C. Small variations in tempera­

ture, mainly within the band i 20°C, occurred. Vig and L u ^ ^ performed their oxidation studies over a greater range of tempera*- tures and noted that the oxidation rate increased with increasing temperatures, however, this increase was also only small.

The results from the mass transfer model indicate that the effect of temperature on the oxidation rate is not great.

Increasing the film temperature from 800°C to 1600°C only increases the oxygen level from 0.04*/$ to 0.052$, after 0 .3 seconds or 1 metre fall in air, according to the model. Although the value of film temperature is incorporated in the formula there are a number of temperature dependent factors such as diffusion coefficient, density and viscosity of air which tend to outweigh the temperature effect. 5.2.2 Kinetics of the Oxidation of Molten Iron Droplets

The mathematical model generates an almost linear oxidation rate for iron. For the six values generated in the experimental arrangement the best fit for the results is a straight line.

The oxidation of free falling iron droplets as carried out by Vig and L u ^ ^ for the 1580°C and 1638°C temperatures are reported in Table 127. Adopting the same basis of normalising (see Section

4.2.1) the oxygen pick-up for a 0.7 g mass equivalent droplet was determined from Vig and Lu!s results. Their figures were the mean of as many as ten values obtained by analysing quenched specimens reacted under the same conditions. They did not indicate the range of values obtained for each result quoted. Their normalised values are plotted in Fig.165. It can be seen that there is good agreement between their normalised values and the ones determined in this experiment.

The residence time in the air was only up to O.35 seconds. It has been noted^) that when liquid iron is exposed to an

oxidising atmosphere the absorption process occurs in two distinct stages. The first stage is one of rapid absorption where the volume absorbed is dependent on oxygen pressure and gas/metal

interfacial area. This stage involves rapid exothermic chemical reaction between oxygen gas and the melt. This gives way to a second stage observed to consist of two competing processes, that is, growth of the oxide layer and simultaneous dissolution of oxygen in the melt at the oxide-metal interface. Choh and Inouye' confirmed that oxygen absorption from the gas phase is represented by a model where most of the gaseous oxygen dissolves into the liquid iron through the oxide free interface although some oxygen

forms iron oxide at the interface and this in turn dissolves into the liquid iron.

For this work, it is evident that the results obtained from the oxidation of iron in air relate to the first stage of rapid absorption and are characterised by a linear rate. As the droplet falls through the air, the deoxidised metal surface at 1600°C will absorb oxygen from the air. Chemical reaction rate under these conditions would be expected to be very high and transfer of

oxygen to the gas-metal interface from the bulk gas or transfer of the dissolved oxygen to the interior of the droplet could be rate controlling.

The mathematical model predicts the rate of mass transfer by forced convection from the gaseous phase to accelerating iron droplets. Figure 166 shows the experimental results and those

predicted by the model at 1600°C for droplets of 0.7 g mass for oxidation in air. The calculated rate line is of the same form and slope as the experimental line but lies below it. The experimental line is a factor of approximately 1 .6 greater than the line predicted by the model. Such a line is drawn in and it can be seen that the lines are almost coincident, just starting to diverge at the 0.2b level. The divergence can be explained on the basis that, as the droplets accelerate, the turbulence in the gas surrounding the droplet increases and consequently mass transfer to the droplet increases.

The increased rate of oxidation of the experimental values compared to those predicted by the model can be explained in terms of circulation within the droplet. Internal circulation can

promote oxidation in two ways. Firstly, it has been s h o w n ^ ^ that internal circulation in liquid drops raises the effective diffusivity to a value equal to 2.25 times the molecular

diffusivity. It is difficult to estimate mass transfer coefficients for the interior of a droplet because there is no way of knowing precisely the degree of circulation set up within the drop either during levitation or in free fall. Secondly, it is known that oxygen is surface active in molten iron and lowers the surface tension strongly(-^>^9). It is therefore quite feasible that in these studies, eddies in the metal may bring about sufficiently large local changes in surface tension for interfacial turbulence to be set up and promote mass transfer in this way.

The morphology of the oxidised iron droplets is shown in Plates 10 and 11. At low magnification the surface shows a

rippled effect. This effect is shown in more detail in Plate 11 where it can be seen that most of the surface presents a rounded,

smooth effect. There are slight hollows in this topography where a second surface feature presenting a rough, more uneven type of

oxide can be seen.

5.3 The Role of Alloying Elements on the Oxidation Rate of