Fuming of stannous oxide from silicate melts
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I.
INTRODUCTION
THE metallurgy of tin is based
on the two-stage reduction of cassiterite using solid carbonaceous reductant. Though the principle of the process is very simple, complications arise due to the amphoteric nature of stannous oxide. When silica is used as a flux, tin is lost to the slag as silicate. On the other hand, when limestone is used as a flux, tin enters the slag as calcium stannate. In either case, extensive cleaning of the slag becomes essential to avoid loss of tin. Conventional recovery of tin from slag by fuming tl] consists of heating tin slags in the presence of sulfur under reducing conditions. The main volatile product is stannous sulfide, which is subsequently oxidized and cooled to give stannic oxide. The stannic oxide, in turn, is returned to the smelter. Effective disposal of SO2 to avoid environmental pollution and the prevention of FeSnS matte formation are important requirements for the operation of the sulfide fuming process. Although SnO is less volatile than SnS, fuming of SnO from slags is attractive, since both of the abovementioned problems pertaining to sulfide fuming can be avoided. Recovery of tin from slags by oxide fuming has been a subject of several previous investigations. Decroly and Ghodsi t21 found that more than 90 pct of tin could be recovered by heating tin slags with 23.3 pct Sn under a pressure of 0.1 mm Hg at 1423 K. Decroly e t al. [31 also demonstrated the oxide fuming of tin from cassiterite concentrates in the presence of iron under reduced pressure. While the gaseous reduction of tin slags is known to be a slow process, t4,5] the importance of SnO vaporization from tin slag during injection of reducing gas was demonstrated earlier, t61 A significant amount of oxide fuming from tin slags can also be achieved by electrical heating.t7] During oxide fuming, tin is extracted primarily as SnO (g). However, SnO (g) can polymerize t81 to form SnzOz (g), 5n303 (g), and S n 4 0 4 (g). Unlike vaporization from slags, metal vaporization has been studied extensively in alloy systems. If the rate of vaporization of a metal is controlled by the diffusion of the metal vapor in the gas boundary layer, the rate can
T. DEBROY, Professor, A. PATANKAR, Graduate Student, and G. SIMKOVICH, Professor, are with the Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802. Manuscript submitted July 1, 1988. METALLURGICAL TRANSACTIONS B
be enhanced by allowing the vaporization to occur in a reactive environment, tgJ If the metal vapor is consumed by a reaction in the boundary layer, the diffusion distance is reduced, and consequently, the vaporization rate is enhanced. Since tin is present in the vapor phase as SnO (g) and, to a lesser extent, as its polymers and since these oxide vapors can be reduced by a gaseous reductant, occurrence of enhanced vaporization is possible, at least in principle. It is to be noted that the enhanced vaporization phenomenon has not been demonstrated in any slag system. However, since fuming
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