The advantages of recycling metallic zinc from the ... - Pyrotek

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Galvanized steel that has been either scrapped or recycled is typically just remelted through a specialized system (quite prevalent in Electric Arc Furnaces) and the mill dust and volatized galvanized coating ash are collected and stored. The zinc content of this mill dust has been measured to be greater than 20wt% Zn [Ref. 9]. Although a significant amount of research has been devoted toward converting this zinc-rich ash into a functional product, the results have been mixed and a consistent, economically viable process has not been widely utilized. On the other hand, it has been proven that metallic zinc may be recovered from unpainted galvanized steel using a dedicated electrochemical process [Ref. 10] and the practicality of these procedures is now being realized. Thus, only a minimal quantity of metallic zinc is currently recycled from galvanized steel substrates. Conversely, the largest source of zinc waste for galvanizing plants is zinc drosses and residues from the molten galvanizing bath. Dross forms as a result of either oxidation of the zinc at the bath surface or by intermetallic reactions with the iron in the bath that has been introduced by the steel substrates being coated. In the two typical categories of galvanizing, “general (or batch) galvanizing” and “continuous galvanizing (CGL)”, the zinc baths possess variable amounts of aluminum (0-0.05wt%Al: General & 0.12-0.3wt%Al: Continuous) resulting in different dross compounds being formed. Looking at the zinc-rich corner of the Zn-Fe-Al phase diagram [Figure 4], the Fe solubility and subsequent dross compounds can be observed for each of the galvanizing regimes. Iron (wt%) 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 L + ξ L L + δ 0 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.180.2 0.22 0.24 Aluminum (wt%) L + η 465°C FIGURE 4: Zinc-Rich Corner of Zn-Fe-Al Ternary Phase Diagram at 465°C [Ref. 11] By analysis [Refs. 12, 13, 14] the floating “top” drosses (also known as “ash” or “skimmings”) for general galvanizing consist primarily of ZnO with varying amounts of Zn5(OH)8Cl2 while the dross on the bottom of the kettle is FeZn13 (ξ) [Ref. 15]. Analogously, the top dross in continuous galvanizing is Fe2Al5 (η) and the bottom dross is FeZn10 (δ) [Refs. 11, 15]. However, during typical cleaning procedures to remove the accumulation of these drosses, a tremendous amount of “clean” metallic zinc is also extracted from the galvanizing pots. General galvanizing dross can contain between 40% to 80% metallic zinc [Refs. 12, 13, 14] while CGL dross may contain up to 95% usable zinc [Ref. 14]. In 2003, DuBois [Ref. 16] noted that CGL drosses
contain minimal amounts of oxide particles (
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The advantages of recycling metallic zinc from the ... - Pyrotek

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