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

Light Metals 2007 Edited by
TMS (The Minerals, Metals & Materials Society), 2007
The advantages of recycling metallic zinc from the processing wastes of
industrial molten zinc applications
Mark A. Bright, Nathan J. Deem, John Fryatt
Metaullics Systems
a division of Pyrotek Inc.
31935 Aurora Road
Solon, Ohio
email: marbri@pyrotek-inc.com
Keywords: Recycling, Waste Management, Zinc, Dross, Galvanizing
Abstract
With the rising cost of zinc reaching historic levels, more emphasis is being placed on
technologies to increase the efficient utilization of zinc. One area targeted for increased
efficiency is in-house recycling of metallic zinc industrial wastes. Metallic zinc is utilized in a
variety of applications including die casting and brass manufacturing, but the largest consumer of
zinc is the hot-dip galvanizing of steel. Through the processing of zinc-coated steel,
considerable quantities of usable zinc are discarded with the skimmings from the zinc bath. It
has been demonstrated that metallic zinc can be extracted from these drosses by utilizing a
relatively simple pyro-mechanical process. By obtaining a better understanding of the
metallurgical constituents which compose the zinc waste products and acknowledging the
relative ease by which metallic zinc separation may be achieved, the practical justification of inhouse
zinc dross recycling may be utilized to further support obvious economic and
environmental advantages.
Introduction
Zinc has been widely used over the past century for numerous products such as precision die
castings and zinc sheet products, brass components and platings, and industrial chemicals
utilizing zinc oxide and other zinc derivatives. Moreover, due to its electrochemical features, the
predominant application for zinc is as a galvanic protective coating for steel and, as outlined in
Figure 1, nearly 50% of the annual zinc tonnage is consumed by galvanizing processes [Ref. 1].

Sheet, 7%
Die Casting,
17%
Miscellaneous,
4%
Chemicals, 7%
Brass, 18%
Galvanizing, 47%
FIGURE 1: End-Use Zinc Consumption Distribution [Ref. 1]
Over the past fifty years the production of metallic zinc has increased by over three times [Ref.
2] with consumption levels now exceeding 7 million tonnes annually [Ref. 3]. However, due to
this increasing demand as well as numerous other economic issues, the production supply of
primary zinc has been unable to keep pace with required zinc tonnage. As shown in Figure 2, the
demand for primary zinc has steadily grown over the past five years, but the zinc supply has been
both erratic and diminishing. In addition it is widely accepted that this deficit in zinc production
will not subside until mid-2007 or beyond [Refs. 1, 3-6]. At present the scarcity of zinc is
making many zinc consumers invest in secured zinc supply just to avoid a potential interruption
in raw material.
Western Market Zinc Supply and Demand (Mt/yr)
8.0
7.5
7.0
6.5
6.0
5.5
5.0
Jan-99
Jan-00
Jan-01
Jan-02
Jan-03
Jan-04
Jan-05
Jan-06
Jan-07
Demand
Supply
FIGURE 2: Zinc Supply and Demand Trends with Projections for 2007 [Ref. 3]

Consequently, the negative impact that has developed as a result of the depleted zinc production
has been a sharp rise in primary zinc prices. Over a twelve month period from May 2005 to May
2006 (Figure 3), zinc prices rose at an unprecedented pace from US$0.565/lb. to US$1.625/lb.
In addition, these LME cash prices did not reflect surcharges, delivery fees and other “extras”
imposed by the zinc producers.
LME Cash Price (US$/Tonne)
$4,000
$3,500
$3,000
$2,500
$2,000
$1,500
$1,000
$500
$0
January-05
March-05
May-05
July-05
September-05
November-05
FIGURE 3: LME Cash Price for Zinc and Hot-Dipped Galvanized Steel (HDG) [Ref. 7 & 8]
Meanwhile, as the costs of zinc were rising rapidly, galvanized steel producers tried to maintain
analogous price increases for the finished hot-dip galvanized sheet steel (Figure 3), however, the
cost differential still impacted the bottom line for many producers. “Zinc is now reported to
account for 25% to 30% of the cost of HDG steel, as compared to around 5% before the price of
zinc headed skywards [Ref. 5].” Hence, steel companies must now start to consider alternative
technologies for minimizing their zinc consumption such as tighter coating weight control and
recycling of metallic zinc waste materials.
January-06
March-06
Zinc Recycling
Zinc, like most other metals, is easily recycled and approximately 30% of the overall zinc
consumption is currently from secondary sources [Ref. 2]. Many zinc-based products can
reintroduce scrap components back into the production process with little variation. Brass scrap
and zinc die castings can be directly remelted and recast as new parts much in the same manner
as aluminum and iron castings. However, zinc-coated steel scrap and galvanizing process wastes
are not as readily returned to the manufacturing process flow. In 1996, it was reported [Ref. 2]
that the utilization of secondary zinc ingot by galvanizing facilities equated to less than 40% of
the tonnage of the zinc scrap, drosses, residues and wastes produced by the coating process.
May-06
July-06
September-06
Zinc
HDG

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 (

FIGURE 5: Metaullics MZR Zinc Recovery System [Ref. 19]
FIGURE 6: Pouring Recovered Zinc from a MZR System [Ref. 20]

As one example of the zinc chemistry effects in the MZR, Table 1 provides an outline of the ICP
(Inductively Coupled Plasma) analysis of a representative processing run. Approximately
550kgs. (1210 lbs.) of top dross (skimmings) from a general galvanizing pot were heated for 3
hours with over 400kgs. (880 lbs.) metallic zinc extracted, resulting in a metal recovery in excess
of 70%. Furthermore, the zinc purity of the recovered metal was >99.6wt% Zn and was
analogous to the chemistry of the production galvanizing bath. Previous research [Ref. 21] has
indicated that controlled usage of secondary recycled zinc ingot in galvanizing operations can
provide a functional enhancement to primary zinc supply. Moreover, the zinc-depleted ash
(“residual ash”) from the MZR processing was still viable for sale at a reasonable rate due to the
induced purity of the retained ZnO and residual elemental Zn content.
TABLE 1: Identification of Zinc Chemistry for Input and
Output Products from a MZR System [Ref. 20]
Dross
Recovered
Metallic Residual
Analysis Zinc Ash
Zn: 87.37 99.66 74.86
Fe: 0.61 0.316 0.637
Al: 1.33 0.001 1.99
Si: 0.21

Conclusions
• Rising primary zinc prices have reached historic levels and are greatly impacting the raw
material costs of zinc consuming industries such as galvanizing, die casting and brass
manufacturing.
• The quantity of metallic zinc contained within the waste materials of these zinc processing
facilities is extremely high, especially for producers of galvanized steel products.
• The metallic zinc content of the waste drosses which are discarded from a molten zinc pot in a
galvanizing operation may exceed 90%.
• Unlike die casting and brass manufacturing, currently only a minimal volume of recycled zinc
is directly returned to an industrial galvanizing pot.
• Implementation of defined in-house procedures by galvanizing operations could help extract
higher fractions of valuable zinc from their process wastes.
• Utilization of a Metaullics MZR Zinc Recovery System may enhance retention of zinc
inventories by providing an on-site secondary processing technique for recovery of usable
metallic zinc.

References:
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Intergalva2006 Conference, Naples, Italy, June 2006
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metals from steel mill dusts involving pelletising and carbothermic reduction”,
1991, Ironmaking and Steelmaking, vol. 18, no. 4, pp. 292 – 295
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