N. B. Gray

Imaging the solidification of molten metal by eddy currents: I

This paper presents an eddy-current-based technique for imaging the solidification of molten metal inside a metal pipe. This technique is motivated by the fact that the ac (alternating current) impedance seen from a solenoid placed outside the pipe is dependent on the distribution of the solidification inside the pipe provided that the ac frequency is properly chosen and the overall thickness of the pipe and solidification is small. This paper first establishes a simplified mathematical model based on the fundamental electromagnetic theory, which reveals the exact relationship between the solidification inside the pipe and the scattered field outside the pipe. Based on this model, an iterative algorithm for reconstructing the solidification distribution is then developed. (Some figures in this article appear in colour in the electronic version; see www.iop.org)

Thermal separation of arsenic and antimony oxides

Experiments were carried out to remove arsenic from antimony trioxide by two techniques: first, by selectively volatilizing the more volatile arsenic trioxide from a mixed oxide sample; and, second, by selectively condensing the less volatile antimony tetroxide at high temperatures, leaving arsenic trioxide to condense out at a lower temperature. Thermodynamic analysis of the As-Sb-O system indicated that if arsenic and antimony oxides behave like pure solid phases, then there is no limitation to producing pure antimony oxide by these proposed techniques. The selective volatilization experiments were carried out at 379 °C to 587 °C, using both nitrogen and air as carrier gases and an industrial antimony trioxide fume containing 13.8 wt pct As. The results showed it is difficult to achieve an arsenic content below 5.0 wt pct using either air or nitrogen, and formation of solid solutions between arsenic and antimony trioxides appears to be the main barrier to the removal of arsenic. Selective condensation experiments were carried out in which a mixed oxide vapor was progressively cooled through a series of condensers over a controlled temperature profile. Injection of oxygen into the vapor improved separation, and antimony tetroxide containing as low as 0.23 wt pct As was obtained. The recovery of antimony tetroxide in the experiments seems to have been limited by kinetic factors, and the results sug-gest that high conversions to antimony tetroxide are likely to be achieved only in antimony-rich vapors.

Study of refractory wear in the tuyere region of a peirce-smith nickel convertor

Abstract Profiles of the composition variation with distance from the hot face were mapped out by SEM analysis of chrome-magnesite tuyere refractory bricks from a Peirce-Smith nickel converter. The presence of magnesium sulphate was confirmed by XRD, SEM and wet analysis. The work showed that periclase was highly susceptible to attack by sulphur dioxide and formed magnesium sulphate in the cooler regions of the refractory brick where the reaction was favourable. Thermal cycling caused the periclase to alternate between the sulphate and the oxide leading to increased pore formation and decreased brick strength. This combined with tuyere punching contributed to the enhanced loss of refractory at the tuyere line. The accretion layer had a high nickel ferrite content which agreed with the high oxygen potential expected at the tuyere region. An interface region between the accretion layer and the refractory showed that counterdiffusion of iron and magnesium assisted in removing periclase from the refractory. Nickel and nickel compounds were limited to the first two mm of the hot face and no infiltration was found. The chrome-spinel is more resistant to attack than the periclase. The pore structure in the refractory brick was identified as the key variable in resisting sulphate formation. An ability to close or fill the pores will reduce the possibility of sulphur dioxide diffusion into the cooler regions for magnesium sulphate formation.

Heat-transfer and pressure-drop considerations in the design of Sirosmelt lances

The Sirosmelt submerged combustion smelting process utilizes top injection lances to deliver fuel and air into a metallurgical melt. Helical vanes are used within the annular lance to impart swirl to the flowing air and enhance heat transfer from the lance wall to the air. To improve the understanding of transport phenomena within the lance, a detailed study of the fluid-flow and heat-transfer characteristics of decaying swirling flow in a heated annulus has been performed. Swirl strength and type were varied, the outer wall of the test section was heated uniformly, and Reynolds numbers ranged from 85,000 to 175,000. The swirl decay characteristics, heat-transfer coefficient, pressure losses, and heat transfer per unit pumping power were determined, and heat-transfer mechanisms were identified. It was found that the entrance pressure losses associated with the helical vane swirlers contributed up to 80 pct of the total pressure loss, indicating that improved swirler entrance design could significantly reduce operation costs. Recommendations are made for optimizing the shape of the swirler by using variable pitch/variable span swirlers and optimizing swirler position by aligning the entrance of the swirlers with the local flow angle. These changes will significantly improve the heat transfer per unit pumping power within a Sirosmelt lance.

Experimental study of splash generation in a flash smelting furnace

A survey of previous studies of splash formation in metallurgical vessels revealed that little information is available to characterize and describe the processes involved in splash formation. An experimental study of splash formation by top submerged gas injection was carried out in the settler region of the nickel flash smelting furnace at the Kalgoorlie Nickel Smelter (KNS) both to obtain some visualization of the splash mechanisms that occur on a plant scale and to measure the amount of splash being formed. Video images taken of the splashing showed that large sheets of melt were formed by the escaping gas and subsequently thinned into ligaments which then broke up into large splash drops. The video could only resolve a minimum size of 2 cm. The large splash drops visible on video have an initial velocity between 1 and 2 m/s, are unstable, and fall back into the bath after traveling a short distance. The analysis identified two major splash forming mechanisms. First, the gas injected resulted in the bulk movement of the melt to form a cavity and large sheets of melts being thrown around the point of injection. The area affected by this splash mechanism can be predicted successfully by using an energy balance between the removal of the melt in the cavity and the energy of the gas being injected. Second, the slag free surface within the cavity is highly unstable, and through the Kelvin-Helmholtz instability mechanism, small splash droplets are generated which are carried into the furnace’s top space. A model proposed for the formation of the smaller splash droplets predicted that the splash collected decreases exponentially with increasing height above the slag free surface from the point of splashing, and this is in agreement with the experimental results obtained.