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

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