Recovery of Metals from Aluminum Dross and Saltcake
J.Y. Hwang*, X. Huang, and Z. Xu Institute of Materials Processing, Michigan Technological University Houghton, MI 49931, USA *Corresponding author’s e-mail address: firstname.lastname@example.org
ABSTRACT Variousaluminum-smelting by-products from three production sources were received and characterized. The waste materials were tested for compound identification and environmental acceptance. A coarse metallic aluminum recovery test using an Eddy Current separator (ECS) was performed using two different Circuit configurations. White dross performed equally well with either Circuit, while black drossprocessing shows significant difference on the separation results. It was found that ECS technology was effective for particle sizes down to 6-10 mesh. Keywords: Eddy current, recycling, aluminum slag
INTRODUCTION Aluminum is a critical material in the U.S. construction, packaging, and transportation industries. The aluminum industry produces approximately one million tons of waste by-products fromdomestic aluminum smelting annually. The most significant by-products are called salt cake and dross, and are generated in the smelting process. Over the last two decades, aluminum recycling has grown rapidly in terms of both size and importance to the U.S. economy. Between 1950 and 1974, recycled aluminum constituted only about 5% of the total domestic aluminum market . Since then, both thefraction of recycled materials and the total domestic aluminum market have grown substantially. In January 1997, for example, total aluminum shipments to domestic markets were 1,591 million lbs., an increase of 12.5% over January 1996 levels. Of this total, 639 million lbs., or about 40%, was recovered from new and old metallic scrap. In most applications, recycled aluminum materials perform as wellas primary material, and provide significant savings in both production costs and energy usage. At present, most aluminum-bearing scrap is recycled through a smelting process. Although the details of the smelting process differ between various installations, most involve melting the scrap in the presence of chloride-based slag, generally using either a reverberatory or rotary furnace. This slagis typically a eutectic or near-eutectic mixture of sodium and potassium chlorides containing low levels of fluorides (cryolite) or other additives. It serves two primary functions. First, since the material is molten and fairly
J. Y. Hwang, X. Huang, and Z. Xu
fluid at typical aluminum smelting temperatures, the slag coats the metallic aluminum being melted andminimizes oxidation losses during processing. Second, the presence of the fluorides and other additives assists in breaking down prior surface oxide layers on the aluminum charge and promotes improved separation between the aluminum and the residual nonmetallics in the charge. The aluminum-bearing scrap for recycle may be either reclaimed metallic aluminum products (e.g. castings or used beveragecontainers) or metal-bearing aluminum oxide drosses skimmed from primary aluminum melting furnaces. Drosses obtained from primary melting operations (so-called “white drosses“) consist primarily of aluminum oxide (with some oxides of other alloying elements such as magnesium and silicon) and may contain from 15 to 70% recoverable metallic aluminum. Drosses from secondary smelting operations(so-called “black drosses“) typically contain a mixture of aluminum/alloy oxides and slag, and frequently show recoverable aluminum contents ranging from 12 to 18%. Commercial smelting of both white and black drosses is often done in a rotary salt furnace. The nonmetallic byproduct residue, which results from such dross smelting operations is frequently termed “salt cake“ and contains 3 to 5% residual...