Molten metal holdup

A mixture of air, molten metal, and molten slag is sucked with a suction pipe. The pipe is 400 mm long and has inner and outer diameters of 4 and 6 mm, respectively. The suction volume of the mixture is approximately 8 ml in every experimental run. After a sufficiently long settling time, the three phases are separated as shown in the upper right part of Fig. 6.2. The length of each phase is denoted by A, B,or C. The gas holdup a and the molten metal holdup e are defined by... 

The molten metal holdup, s, including the elevated molten metal and accumulated molten metal droplets can be measured in a real situation by using the expression ... 

Figure 6.4 shows the radial distributions of total molten metal holdup s in the molten slag layer at three representative axial positions. It should be noted that the axial distance z is not measured from the initial, horizontal interface between the two layers but from the actual horizontal interface after a steady state has been established. The actual horizontal interface is lower than the initial interface due to the existence of elevated molten metal and accumulated molten metal droplets. This actual interface is here simply called a horizontal interface between the two layers. 

The holdup of accumulated molten metal droplets in the molten slag layer located outside the bubble dispersion region, and designated by /I as above, increases linearly toward the horizontal interface between the molten metal and slag layers, and then suddenly increases after p exceeds 30% (see Fig. 6.5). On the other hand, in the absence of accumulated molten metal droplets, is at most 10% even just (say 1 cm) above the horizontal interface between the two layers. The contour lines of the total molten metal holdup, s, calculated from the data plotted in Fig. 6.4, are shown in Fig. 6.6. 

Horizontal Distribution of Elevated Molten Metal Holdup... 

Two types of distribution are assumed for the accumulated molten metal holdup, /I, as illustrated in Figs. 6.7 and 6.8. In Fig. 6.7, contour lines of are parallel to the horizontal interface between the molten slag and metal layers. In this case, P decreases as the radial distance decreases in the elevated region, and the distribution function expressed by (6.4) seems reasonable. 

Figure 6.9 shows the elevated molten metal holdup s calculated using the data on s of Fig. 6.4 and (6.6). Also, Fig. 6.10 shows the distribution of e calculated by using the same data on s and (6.7). In each figure, s follows a Gaussian error curve similar to the s distribution. 

The contour lines of elevated molten metal holdup s calculated from (6.6) are given in Fig. 6.11 and those calculated from (6.7) are shown in Fig. 6.12. Both figures indicate that the elevated region is parabolic in shape. When bubbles escape from the vicinity of the top of the elevated region, molten metal is engulfed in the wakes of the bubbles and lifted up into the upper slag layer. The filament-like molten metal consequently breaks up into many droplets. 

The horizontal distribution of the total molten metal holdup, e, in the elevated region can be approximated by a Gaussian distribution. 

The shape of successively rising bubbles in molten metal baths is closely related to the horizontal distribution of gas holdup, as shown in Fig. 1.3. Thus the shape can be predicted from the results of gas holdup measurements. 

For primary insulation or cable jackets, high production rates are achieved by extmding a tube of resin with a larger internal diameter than the base wke and a thicker wall than the final insulation. The tube is then drawn down to the desked size. An operating temperature of 315—400°C is preferred, depending on holdup time. The surface roughness caused by melt fracture determines the upper limit of production rates under specific extmsion conditions (76). Corrosion-resistant metals should be used for all parts of the extmsion equipment that come in contact with the molten polymer (77). 

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