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 in-house 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].

Galvanizing, 47%

Brass, 18%

Die Casting, 17%

Sheet, 7%

Chemicals, 7%

Miscellaneous, 4%

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.

5.0

5.5

6.0

6.5

7.0

7.5

8.0

Jan-

99

Jan-

00

Jan-

01

Jan-

02

Jan-

03

Jan-

04

Jan-

05

Jan-

06

Jan-

07

Wes

tern

Mar

ket Z

inc

Supp

ly a

nd D

eman

d (M

t/yr)

DemandSupply

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.

$0

$500

$1,000

$1,500

$2,000

$2,500

$3,000

$3,500

$4,000

Janu

ary-

05

Mar

ch-0

5

May

-05

July

-05

Sept

embe

r-05

Nov

embe

r-05

Janu

ary-

06

Mar

ch-0

6

May

-06

July

-06

Sept

embe

r-06

LME

Cas

h Pr

ice

(US$

/Ton

ne)

ZincHDG

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.

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.

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.

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24

Aluminum (wt%)

Iron

(wt%

)

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

L + δ

L

L + ξ

L + η

contain minimal amounts of oxide particles (<1%) and the percentage of zinc entrained in the removed dross volume is proportional to the operator skimming practices. Hence, when these drosses are discarded and sold to recycling or metal trading companies, a phenomenal amount of “good” zinc is also being lost with the waste material. A typical batch galvanizing plant may generate up to 18 tonnes of ash (i.e. top dross) each month. If this dross contains 70% metallic zinc, then it equates to over US$40,000 per month in lost zinc. A nominal buyback rate may be recouped from a zinc recycling company, but, in general, a significant financial loss is being incurred by the discarded zinc.

Metallic Zinc Recovery In the past, minimal research efforts have focused on reducing zinc waste and recovering metallic zinc from galvanizing residues due to the low cost of primary zinc. In the late 1990’s Barakat [Refs. 12, 13] determined that it was possible to develop the thermal processing parameters necessary to allow separation of metallic zinc from dross particles in general galvanizing top drosses. Accordingly, back in the early 1900’s, zinc was readily recovered from drosses by remelting the skimmings in a large cast iron pot. Davies noted [Ref. 17], “When it is thoroughly hot, the [zinc] will begin to trickle out. It is requisite to work it about well with a small iron scraper, to coax the [zinc] out.” Along these lines, many general galvanizing plants attempt to mechanically “work” the ash (top dross) while it is still floating on the molten zinc bath. This task can be difficult and often dangerous with mixed results pertaining to the amount of metallic zinc retained in the pot. In 1946, Daesen [Ref. 18] stated the following: “Attempts to save some of the ‘good’ zinc by chopping or shaking the dross to allow the zinc to run off are not successful, and are based on a misconception of the particle size of the dross crystals and also on the deceiving appearance of so-called ‘dry’ and ‘wet’ dross. Chemical analysis proves that there is very little difference in the iron content of these two samples. The particles of the zinc-iron compound are so small they run off with the ‘good’ zinc in almost the same proportion in which they existed in the dross.” In response to these issues, a thermo-mechanical system was developed to provide a safe, automated means for efficiently extracting viable metallic zinc from the waste drosses of a galvanizing facility. The patented Metaullics Zinc Recovery System (or MZR System) (Figure 5) equips a galvanizing plant with the capability to reheat small batches of dross (up to 1 tonne) and recover up to 90% of the interred metallic zinc contained within the waste material [Refs. 19 & 20]. The MZR System creates the dross separation by an optimized thermo-mechanical process which rotates a specially designed drum (containing a given quantity of dross) at a specified rotational velocity within a temperature controlled chamber. After a specified processing time, an operator can manually decant the liquid zinc (Figure 6) from the floating dross, which has now stratified within the heated drum.

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]

Recovered

Dross Metallic ResidualAnalysis Zinc Ash

Zn: 87.37 99.66 74.86Fe: 0.61 0.316 0.637Al: 1.33 0.001 1.99Si: 0.21 <0.001 0.929

Ca: 0.01 0.001 1.08Pb: 0.002 0.008 0.024Mg: 0.003 <0.001 0.059

Inadvertent disposal of large volumes of metallic zinc in conjunction with dross waste from a galvanizing bath is a significant problem that many coating companies may not be aware they have. However, by utilizing a safe, effective thermal processing system, such as the Metaullics MZR System, galvanizing operations can reclaim their own clean zinc, minimizing waste and substantially impacting their bottom line.

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: 1. Smale, D., “Zinc in galvanizing: a world view, with special reference to China”,

Intergalva2006 Conference, Naples, Italy, June 2006 2. Zinc Recycling: The General Picture, 1996, International Zinc Association 3. Deller, G., “Zinc price forecasting – explaining the limitations”, Intergalva2006

Conference, Naples, Italy, June 2006 4. Schauman, M., “Outlook for zinc market: will supply meet growing demand?”, 11th

International Galvanizing and Coil Coating Conference, Metal Bulletin, September 2006 5. Platts Metals Daily LME Close Edition, September 21, 2006, McGraw-Hill Co. 6. Market Analysis, Metal Bulletin Monthly, September 2006, issue 429, pp. 15 7. MBR Coated Steels Monthly, August 2006, issue 91, Metal Bulletin 8. London Metals Exchange, www.lme.com 9. Lopez, F. A., Medina, F., Medina, J., Palacios, M. A., “Process for recovery of non-

ferrous metals from steel mill dusts involving pelletising and carbothermic reduction”, 1991, Ironmaking and Steelmaking, vol. 18, no. 4, pp. 292 – 295

10. Everitt, M., “Profitable recycling of galvanized scrap”, 11th International Galvanizing and Coil Coating Conference, Metal Bulletin, September 2006

11. Shastry, C. R., Galka, J. J., “Analysis of zinc melt for aluminum, iron and dross intermetallics”, Galvanizer’s Association Annual Meeting, 1998, Indianapolis, Indiana

12. Barakat, M. A., “Recovery of zinc from zinc ash and flue dust by pyrometallurgical processing”, Recycling of Metals and Engineered Materials, 2000, TMS, pp. 211 – 223

13. Barakat, M. A., “The pyrometallurgical processing of galvanizing zinc ash and flue dust”, Journal of Metals, August 2003, TMS, pp. 26 – 29

14. Unpublished research, Metaullics Systems (div. of Pyrotek Inc.), Solon, Ohio 2004-2006 15. Marder, A. R., “The metallurgy of zinc-coated steel”, Progress in Materials Science,

2000, vol. 45 16. DuBois, M., “Top drosses: the undocumented gold mine”, Galvanizer’s Association

Annual Meeting, 2003, Monterrey, Mexico 17. Davies, J., Galvanized Iron: Its Manufacture and Uses, 1914, Spon and Chamberlain, pp.

99 - 101 18. Daesen, J. R., Galvanizing Handbook, 1946, Reinhold Publishing, pp. 133 – 136 19. Fryatt, J., Vanska, J., “In-house recovery of metallic zinc and other metals from

continuous process drosses and residues”, 11th International Galvanizing and Coil Coating Conference, Metal Bulletin, September 2006

20. Deem, N., “In-house recovery of metallic zinc from galvanizing bath skimmings”, AGA TechForum Conference, American Galvanizers Association, October 5-7, 2005

21. Shastry, C. R., Galka, J. J., “A comparative evaluation of jumbo ingots produced from primary zinc and from zinc recovered from dross”, Galvanizer’s Association Annual Meeting, 2000, Toronto, Ontario