Dezincing Scrap Steel

Steel is one of the most recycled resources in the U.S.; half of the steel produced is derived from scrap. Since 1980, automobile and appliance manufacturers have increased their use of galvanized steel almost five-fold, with a resulting increase in the amount of galvanized steel scrap returned to steel producers.

The steel galvanizing process involves the application of a zinc-coating, which provides corrosion resistance. When galvanized scrap is melted in a steelmaking furnace, the zinc that it contains volatizes. The costs of treating the resulting zinc-laden dust and sludge by-products are significant. The economic removal and recovery of the zinc coating from scrap prior to melting would provide a number of cost and environmental advantages. Currently, however, most iron and steel-making operators put zinc-coated steel into their furnaces and absorb the costs of the waste zinc.

In 1987, Argonne and its industrial partner, Metal Recovery Industries, U.S., Inc., sought to develop a new technology for converting galvanized scrap to clean scrap for steel making. Three pilot plants were built. The first, located in Hamilton, Ontario, dezinced batches of sheet, loose, and baled galvanized scrap. A second pilot plant was constructed at MRTI (Metal Recovery Technologies, Inc.) in East Chicago, Indiana, for dezincing prompt galvanized scrap from auto stamping plants. This plant processed loose chips and shredded scrap in a 200-cubic-foot reactor using a belt conveyor. The third pilot plant used a rotary-drum conveyor, which is the basis for a commercial-scale plant.

Research led to the identification of the most cost-effective dezincing approach: stripping in a hot caustic solution, with the subsequent removal of the zinc from the solution. As shown in the diagram, shredded, zinc-bearing scrap is charged into a rotary electrolytic reactor that contains a hot-water solution (70° to 90°C [158° to 194°F]) of about 20% to 32% sodium hydroxide. The zinc dissolves in the solution, and the clean steel scrap is removed, rinsed and recycled. The zinc-containing solution is then pumped into a second cell, where the zinc is removed from the solution electrolytically.

The process consumes no chemicals other than drag-out losses, and it produces little waste. Concentrations of zinc, lead, aluminum and other coating constituents (except nickel) on the scrap are reduced by at least 98%; zinc is reduced to below 0.1%.

Depending on the form and zinc content of the steel scrap, the cost of processing is about $15 to $40 per ton of scrap for plant capacities of 50,000 to 500,000 tons/year, respectively. These costs include capital recovery and reflect the credits for the co-product zinc.

The dezincing technology has been patented and successfully transferred to industry.

Schematic of the dezincing process developed at Argonne.

May 2008