L . Dezincing Galvanized Scrap producing regions in Europe and the Pacific

Rim.

Frederick J. Dudek Edward J. Daniels

Argonne National Labor 9700 S. Cass Aven

Argonne, Illinois 60439 (630) 252-7797

Charles Braun

The use of galvanized steel scrap is most problematic for integrated (basic oxygen

cess) steel mills, which historically have d on home and prompt scrap sources such amping plants to supply clean black scrap.

e availability of home scrap has decreased due to increased efficiency of continuous O 8 X E asting and consequently has resulted in -

Metal Recovery Technologies, Inc. 415 East 151st Street

East Chicago, Indiana 463 12 (219) 397-6261

ABSTRACT

A caustic leach dezincing process is being developed for upgrading galvanized stamping plant scrap into clean scrap with recovery of the zinc. With further development the technology could also process galvanized scrap from obsolete automobiles. This paper will review: 1. the status of recent pilot plant operations in East Chicago, Indiana and plans for a commercial demonstration facility with a dezincing capacity of up to 250,000 tonnes/year, 2. the economics of caustic dezincing, and 3. benefits of decreased cost of environmental compliance, raw material savings, and improved operations with use of dezinced scrap.

INTRODUCTION

Zinc contained in galvanized ferrous scrap recycled to steelmaking furnaces can incur costs for environmental compliance and cause operational problems [l-41. The presence of zinc in furnace fumes reduces the on-site recyclability of the recovered dusts, and increases regulatory compliance costs to control emissions of zinc to the shop floor environment, air and wastewater. Low concentrations of zinc, in combination with other residuals, have also been shown to have modest negative effects in the metallurgy of ductile iron [SI.

Concerns about zinc content have come to the forefront as the consumption of galvanized sheet steel for automobiles has increased in North America, doubling in the ten years since 1985. Similar consumption trends are seen for the other major car-

great& use of purchased scrap. Further, as the use of galvanized steel rises and galvanized- steel-based automobiles are retired, the combined prompt and obsolete ferrous scrap supply is becoming richer in zinc. The reduction in black scrap availability will lead to the increased use of other low residual iron units, such as direct reduced iron (DRI), hot briquetted iron (HBI), iron carbide, and pig iron, whose cost in use has historically been higher and which consume more energy and material resources than use of ferrous scrap. Consequently, the U.S. iron and steelmaking industry has identified the commercialization of dezincing technology as a research and development priority [6].

Since 1987, Argonne National Laboratory has been working with Metal Recovery Industries, U.S., Inc., or predecessors, to develop and commercialize a new technology for converting galvanized steel scrap from auto-stamping plants into clean scrap for steelmaking with recovery of the zinc. Processes for removal of tramp copper are also being investigated to upgrade obsolete ferrous scrap that contains both zinc and copper.

DEZINCING PROCESS DESCRIPTION

The degalvanking technology involves two process steps: (1) galvanic-corrosion- promoted leachmg in hot caustic and, (2) electrowinning to recover zinc and regenerate sodium hydroxide for recycle. The process schematic is shown in Figure 1. The process chemistry is:

(1) Leaching:

Zn, + 2H,O ------- > Zn(0H) , + H, Feo

>3 M NaOH Zn(OH), + 2NaOH ------------ > NaJn(OH),

DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes a n y legal liability or responsibility for the accuracy, completeness, or use- fulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any spe- cific commercial product, process, or service by trade name, trademark, manufac- turer, or otherwise does not necessarily constitute or imply its endorsement, m m - mcndktion. or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

DISCLAIMER

Portions of this document may be illegible electronic image products. images are produced from the best available original document.

(2) Electrowinning:

The overall reaction results in decomposition of one mole of water for every mole of zinc leached and then electrowon.

The feedstock to the process is prompt manufacturing scrap, preferably from auto stamping plants. Prompt scrap is clean and auto scrap now contains the largest percentage of galvanized material - on average 50%. This material is sheared and then shredded in a h a m m e d to fist size chunks. The abrasion in the hammermill increases the number of zinc/iron electrochemical corrosion sites and promotes more rapid and effective dezincing. The loose, shredded galvanized scrap is fed to a rotary dezincing reactor that is partially immersed in a wann ( 7 0 - 9 0 O C ) water solution of 2040% sodium hydroxide. As the scrap steel is tumbled €?om the feed to discharge end of the rotary reactor, coating metals (zinc, lead, aluminum) on the surface are leached into the solution. The degalvanized scrap exiting the reactor is rinsed in water and prepared for shipment to a steel mill or foundry. The ferrous product from dezincing plants is a shredded zinc-free low-carbon steel. Zinc, lead, aluminum, and other coating constituents (except nickel) on loose scrap are reduced by a minimum of 98%. The zinc concentration is reduced to below 0.1 wt%.

The pregnant leach solution is fed to electrowinning cells, where finely divided zinc is collected on magnesium cathodes. The power utilization is about 4 kW-h/kg Zinc recovered. The effluent slurry is filtered to recover the zinc which is then washed, dried and packed for sale. The caustic solution is regenerated during electrowinning and is recycled to the leaching section. The process consumes only small quantities of sodium hydroxide to compensate for drag-out losses of the leach solution and the amount consumed by reactions with aluminum in the coatings.

PROCESS ECONOMICS

Table 1 shows the cost profile for a degalvanizing facility with a capacity of 250,000 tonnedyear of steel scrap feed. The steel scrap contains, on average, 1.7 wt% zinc. Because of the volatility in ferrous scrap prices, the cost of scrap steel feedstock and value of product dezinced steel are not included

in these estimates. The value of the steel is carried through the process. An estimated tolling fee of about $15 per tonne of steel is required to recover the net operating and capital costs for the dezincing process. The zinc powder is credited at 70% of its London Metal Exchange (LME) price. Development efforts are directed at increasing the value of the zinc product.

The value of the recovered zinc is critically important to the economics of dezincing. At a price of $1,25O/tonne for SHG slab zinc, the estimated 17 kg of zinc recoverable per tonne of galvaruzed scrap dezinced is worth about $22. Figure 2 shows the sensitivity of the dezincing premium to the market price for zinc.

STATUS/FUTURE RESEARCH

Development of the dezincing process has been carried in phases from bench-scale studies in the laboratory to semi-works operation. Studies of the fundamental electrochemical behavior of zinc and steel in hot sodium hydroxide helped define the operating conditions of the process [7]. Two pilot plants were built for caustic dezincing process development. The first was located in Hamilton, Ontario and dezinced batches of sheet, loose, and baled galvanized scrap. Lots as large as ten tonnes were dezinced with anodic assistance in Hamilton culminating with accumulation of 580 tonnes of baled scrap for an AIS1 BOF melt campaign [8]. A total of 900 tonnes of galvanized scrap of various forms and coating types were treated in these batch processing studies [9]. This led to construction of a pilot plant in East Chicago, Indiana for continuous degalvanizing of loose clips or shredded scrap with a 5.7 cubic meter reactor capable of dezincing 1-4 tonnehour. This plant operated in 1993 for dezincing a total of 1,500 tonnes of loose and shredded stamping plant scrap. The scrap was conveyed through the caustic bath on a pan belt conveyor with corrosive dissolution of the zinc on the steel scrap substrate. Small-scale melt tests with the dezinced steel product at foundries showed melt chemistries equivalent to that obtained with black scrap. In 1996 the plant was dismantled and refurbished with the present rotary reactor. The new reactor has an active volume of 19 cubic meters and a dezincing design capacity on shredded scrap of 12 tonnehour. Operating experience with the first 1,500 tonnes of scrap dezinced in this plant

indicates the reactor is exceeding its design performance.

A commercial demonstration plant capable of dezincing 250,000 tonne/year with recovery of 4,300 tonne/year zinc is planned for start-up in 1998. Operation of the plant will provide data to determine the following: optimum design of the rotary dezincing reactor; zinc removal effectiveness as a function of throughput; power utilization and current efficiency of electrowinning as a function of current density, temperature, and composition of the electrolyte; mass balances; reliability of operations; process improvements; and controls needed. Market evaluation and quality specification development will continue for the degalvanized ferrous scrap and zinc metal products.

New technologies are also being developed to address two needs: (1) improving process economics by increasing the recovery and purity of the zinc produced from the zinc slurry generated during electrowinning, and (2) incorporating tramp copper removal capabilities to extend the applicability of the dezincing technology to obsolete scrap. To increase recovery and purity of zinc, studies are required on zinc melting techniques, and aluminate and impurity removal from dezincing solutions. Development of a novel fluidized bed electrochemical cell is ongoing at the University of California Berkeley which produces, with low energy consumption, a low porosity and low oxygen content zinc pellet; this form of zinc should have greater value than the zinc powder produced conventionally from alkaline solution. Work is ongoing to extend the dezincing process to obsolete scrap, but shredded obsolete auto scrap contains about 0.25% of tramp metallic copper as well as zinc coated material. The copper is more troublesome than the zinc because it causes metallurgical problems such as hot shortness. Excluding diluting the copper content with other low residual scrap or iron units, copper removal, as well as zinc removal, would have to be a part of a successful treatment of obsolete auto scrap.

COMPETING PROCESSES The need for cost effectively

degalvanizing scrap steel was the subject of a Center for Metals Production report [2] prepared by 0. F. Angeles and E. F. Petras of United States Steel in 1986. This seminal report identified the problems in using zinc

coated scrap and estimated the cost of degalvanizing processes. They found no process economic but suggested that further work in the area would be desirable.

Of the several chemical, electrochemical, and pyro-me tallurgical ways for recycling galvanized scrap that have been the subject of R&D subsequent to the CMP report, two technical approaches to dezincing survive in pilot-scale development. One is the electrochemical corrosive attack of zinc on steel in hot aqueous caustic; this is the method employed in the MRTUANL process. This approach is also being developed by Hoogovens Groep in Europe [ lo]. The second approach uses vacuum evaporation of the zinc from the steel substrate; this is most recently being done by Ogihara and also Toyokin in Japan [ll,lZ]. The Hoogovens process is being piloted successfully in France in a 50,000 net ton per year (Nuyear) plant using essentially the same flowsheet as the MRT/ANL process but they do not use a rotary reactor. The W A N L process is being piloted in a 75,000 tonne/year plant in East Chicago, Indiana. Oghara is operating a 1,000 tordmonth plant in a batch mode on bales and Toyokin has a vacuum-aided recycling system in pilot stage with a capacity of 5,000 Numonth of shredded scrap.

TECHNICAL AND ECONOMIC BENEFITS

Given a market penetration of five million tonnes, a scrap dezincing process could save the U.S. steel industry an estimated $140 million per year compared to the historical cost of using HBI; and the U.S. could realize a $100 million decrease in foreign exchange deficit through a reduction in the need to import 85,000 tonnes of zinc and doubling the zinc recycling rate. The process also has major environmental benefits. The removal of zinc from steel scrap increases the recyclability of steelmaking fume and eliminates zinc from wastewater streams. Dust disposal and wastewater treatment costs are estimated at $1- 3 per tonne of hot metal for BOP operations. Operational problems that would be mitigated include shortened furnace lining life, porosity in billets, and zinc on walls of continuous casting molds [4]. Koros and Bauer [lo] have reviewed the consequences of use of galvanized scrap in iron foundries and have concluded they are predominantly costs of environmental compliance.

ACKXOWLEDGMENT 9. Work supported by the U.S .

Department of Energy, Assistant Secretary for Energy Efficiency and Renewable Energy, under Contract W-3 1 - 109-Eng-38 with cost sharing by Metal Recovery Industries, US, Inc .

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REFERENCES

Koros, P. J. and R. P. Muhlhan, “Zinc and Steel Scrap - Continued Recyclability and Zinc Recovery,” Proceedings of the Second International Zinc Conference, AZA, Scottsdale, Arizona, Feb. 21-24, 1993, Section 7, pp. 1-15. Angeles, 0. F. Jr. and E. F. Petras, ”Dezincing of Galvanized Sheet Steel Scrap, A Scoping Study,” Center for Metal Production Report 86-1F, October 1986. Koros, P. J., “Recycling of Galvanized Steel Scrap - Issues-and Solutions,” 75th Steelmakinp Conference Proceedings, ISS-AIME, 75, , Toronto, Ontario, Apr. 5-8, 1992, pp. 679-685. C. L. Nassaralla, “Update on the Michigan Tech Ferrous Metallurgy Grant Program Project,” Iron & Steelmaker,

Koros, P. J. and M. E. Bauer, “Consequences of Galvanized Scrap Use in Iron Foundries and Technologies for Degalvanizing Scrap,” lOlst Conaess of the A F S Proceedings, Seattle, WA, April 20-23, 1997, AFS Paper No. 97-162. American Iron and Steel Institute & Steel Manufacturers Association, Steel: Industry Technolog Roadma?, August 1997.

24( 1997), pp. 49-50.

Dudek, F. J., E. J. Daniels, Z. Nagy, S. b o m b and R. M. Yonko, “Electrolytic Separation and Recovery in Caustic of Steel and Zinc from Galvanized Steel Scrap,” Separation Science and Technoloa, 25(13-15), (1990), pp. 2 109-213 1. Dudek, F. J., E. J. Daniels, W. A. Morgan, A W. Kellner and J. Harrison, “A Recycling Process for Dezincing Steel Scrap,” 75th Steelmaking Conference Proceedings, I S S - A M , Toronto, Ontario. Am. 5-8, 1992, DD. 743-748.

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Dudek, F. J., E. J. Daniels, W. A. Morgan, “Recycling Galvanized Steel: Operating Experience and Benefits,” Proceedings of the World Zinc ‘93 Svmposium, AusIMM and the CIM, Hobart, Tasmania, Oct. 10-13, 1993, pp. 5 15-522. Mooij, J. N. and J. €I. 0. J. Wijeberg, “Dezincing of Galvanized Steel Scrap in Hot Caustic Soda,” Proceedings of the 2nd International Conference on the Recvcling: of Metals, ASM International, 1994, pp. 199-206. Saotome, Y., et al., “Vacuum Aided Degalvanizing as a High Quality Materials Recycling Technology of Ecofactory,” Galvatech ‘95 Conference Proceedings,

Okada, Y., et al., “Development of a Method for Removal of Zinc from Automobile Body Scraps,” Galvatech ‘95 Conference ProceedinPs, pp. 549-554.

pp. 569-576.

n

Purn p ce4

8; Pump Ek&owirnn

Figure 1. Schematic of rotary reactor caustic dezincing and zinc recovery circuit.

OPERATING COSTS $/m t Variable Dezincing 6.44 Variable zinc recovery 4.07 Fixed 7.54

TOTAL DEZINC PROCESS COSTS 18.05 CREDIT, ZINC RECOVERY (15.15)

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38.1 lb/mt scrap dezinced 70% of LME @ $1,25O/mt = $0.40/lb zinc

RETURN OF CAPITAL, 20% 12.00 NET COST OF DEZINCING 14.90

Table 1. Process economics for a 250,000 muyear caustic dezincing plant recovering 1.7% zinc.

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18

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.....

.".I .......... .................... .................... .................... ..................

.I...... ....... 2 .................... A.... ........... .................... .................... .................. 250,00(i mtla, 1 9 % zin4

Capita6 $15 rni4ion 1 ... ..."""...+................+.............".....+.."............... i .................... .................. Zinc c9dit: 70%j of LME t

900 1000 1100 1200 1300 1400 1500 1600 1700

LME Zinc, $/metric ton

Figure 2. Sensitivity of the cost of caustic dezincing to the market price of zinc. Historical pricing source: IZA Pocket Guide to World Zinc. Zinc pricing in constant 1994 dollars.

The submitted manuscript has been authored , by a contractor of the U. S. Government under contract No. W-31-104ENG-38. Accordingly, the U. S. Government retains a , nonexclusive. royalty-free license to publish or reproduce the published form of t h i s contributlon, or allow others to do SO, for i U. S. Government purposes.