20 JOM • August 1999 Aluminum Research Summary INTRODUCTION In the recycling of post-consumer materials, there are two options for preventing the loss of valuable materials: product reuse, in which the product is applicable several times, and materials reuse, in which the materials are returned to the materials cycle for the production of new materials and products. After the consumer phase of most products, it is often difficult to separate the different metals present, particularly nonferrous metals and stainless steels, for materials recycling; a mixing of metal scrap will occur. When this mixture is directly melted, a metal with high amounts of impurities is produced, which is suitable for application only as a casting alloy. Rewinning nonferrous metals from waste is essential for many reasons. First, metals have an important scrap value. Second, remelting aluminum requires only five percent of the energy necessary for the production of primary aluminum, resulting in less consumption of energy and raw materials. Third, rewinning decreases the amount of waste. Finally, both compost production and incineration require that a limited amount of metals are present, and sales of incinerator ashes are only possible when the metal levels are below set limits. Upgrading Nonferrous Metal Scrap for Recycling Purposes Gerrit H. Nijhof and Peter C. Rem Reusing materials in the post-consumer phase of a product is very important; how- ever, it is often difficult to separate the met- als, resulting in a scrap that is high in impu- rities and limited in application. To prevent the mixing of scrap, separation at the source is required. Recently, several techniques for separating nonferrous metals have become available, including eddy-current separation to separate nonferrous metals from a mix- ture of waste, fluid-bed separation to sepa- rate light and heavy metals, and image analy- sis to separate cast and wrought alloys. SEPARATION TECHNOLOGIES Table 1. The Effectiveness of Separation Electric Conductivity Density Metal (σ29 (ρ29 σ/ρ Al 0.35 2.7 13.0 Cu 0.59 8.9 6.7 Ag 0.63 10.5 6.0 Zn 0.17 7.1 2.4 Bronze 0.14 8.5 1.7 Sn 0.09 7.3 1.2 Pb 0.05 11.3 0.4 Figure A. The trajectories of (a) 10 mm and (b) 18 mm aluminum in a magnetic drum. b a Much theoretical work on separation technology is being performed at the Delft University of Technology, Faculty of Applied Earth Sciences, Netherlands. 4 These various techniques are tested on a laboratory scale, and industrial validation is performed in the household handling plants of VAM and VAGRON. 5–7 Eddy-Current Separation In eddy-current separators, an alternating magnetic field induces a force on conductors (e.g., the nonferrous metals in waste streams). The effectiveness of particle separation in an eddy-current separator is proportional to the electric conductivity of particles, σ, and dis- proportional to its density, ρ. This combination makes the technique suitable for the separation of aluminum and copper, which have the highest values for the quotient σ/ρ from mixed waste streams (Table 1). The theoretical basis for eddy-current separation has been known for more than 100 years. A vast amount of progress was made in the 1980s due to the industrial applications of permanent magnets Nd-Fe-B and the development of rotating magnetic drums. Theo- retical modeling made it possible to predict particle trajectories and to design a magnetic drum with an optimal number and form of the magnets. 4 An example is shown in Figure A; the large circles are the calculated values, and the experimental points are the small circles. This new type of magnet led to an improvement in the separation efficiency of the eddy-current separa- tors. Also, the operating conditions, the speed of the conveyor belt, the thickness of the belt (distance of Table I. The Increase in Separation Efficiency with the New Magnetic Drum Al + Cu + Particle Size Alloys (%) Alloys (%) >60 mm 19.3 32.9 16–60 mm 4.4 3.3 4–16 mm 3.3 57.0 Table II. Production Results of the Eddy Current Percentage Result Kilograms of Input Total Weight 546.7 — Separated by the Eddy Current 11.80 2.16 Amount Nonferrous Metals 10.65 1.95 Aluminum 3.67 0.67 particles to the magnetic field), and the rotation direction of the magnetic drum are of large importance. Various experiments are described in Reference 4. An industrial application is shown in Figure B. A research program at Delft University and Bakker Magnetics has led to a new generation of nonferrous separators. Theoretical calculations based on the fun- damental physical principles have led to an optimal magnetic system, including the number of magnets and the shape of the individual magnets. Dry-Density Separation Dry-density separation is based on the differential movement of particles in a dry fluidized bed consisting of fine particles, such as sand, zirconium silicate, or hematite, typically 100–400 μm in size. The feed parti- cles range in size from 10–50 mm, but smaller particles ●●●●●●●●●●●●●●● ●●●●●●● ●● ●● ●●●●● ● ●●●●●●● ● ● ●●● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●●●●●●●●●●●●●●●●●●● ● ●●●●●●●● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●