dross

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dross

the scum formed, usually by oxidation, on the surfaces of molten metals

Dross

 

an intermediate product obtained from the liquation separation of heavy nonferrous metals with low melting points: lead, bismuth, and tin. As the metals are cooled during the refining process, impurities of low solubility, such as copper and iron, crystallize. Denser and possessed of a higher melting point than the metal being refined, the impurities (in the elemental state or in compounds) float to the surface of the bath as solids to form dross. Additives of sulfur or carbon are sometimes introduced into the metal being refined in order to remove a higher percentage of impurities. The deeper the cooled metal, the more complete the crystallization and purification; however, there is also an increase in dross yield and in the amount of refined metal found in the dross.

Dross is removed from the surface of the liquid bath with a skimmer. The liquid metal entrained mechanically with the dross drains into the bath. Dross is highly complex with respect to its chemical and phase composition; the refined metal constitutes its base. The dross obtained in the refining of lead is reprocessed by smelting in a reverberatory or electric furnace with the addition of a sulfidizing agent and fluxes. The smelted lead is returned to the primary production cycle, and the matte and speiss produced are sent to copper mills. The dross obtained in the refining of bismuth and tin is recycled.

REFERENCE

Loskutov, F. M. Metallurgiia svintsa. [Moscow] 1965.

M. P. SMIRNOV

dross

[drȯs]
(metallurgy)
An impurity, usually an oxide, formed on the surface of a molten metal.