10
Cross-cutting Issues for the Materials Processing Industries

Introduction

The materials processing industries have extremely diverse needs for separation technologies. However, the panel identified seven issues that are common to two or more of the materials processing industries described in this report. These are: removal of impurities from feedstock; metal purification; separation of scrap; prevention of gas inclusions; separation of components from dilute gaseous streams; separation of components from dilute aqueous streams; and water remoral. In addition, the panel identified one technology area with the potential to meet some of these needs and three enabling technologies.

Common Needs

The removal of impurities from feedstock is an important issue for the aluminum and steel industries. The removal of impurities from alumina feedstock minimizes the need for metal purification at later processing stages. Improving the quality of iron-bearing materials for blast furnace feed and DRI feed is important to the steel industry.

Metal purification is important for the aluminum, steel, and metal casting industries. The purity of molten metal is important for the aluminum industry because almost all other metals are more noble than aluminum, and once an impurity is introduced into the melt, it is almost impossible to remove it. Removal



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--> 10 Cross-cutting Issues for the Materials Processing Industries Introduction The materials processing industries have extremely diverse needs for separation technologies. However, the panel identified seven issues that are common to two or more of the materials processing industries described in this report. These are: removal of impurities from feedstock; metal purification; separation of scrap; prevention of gas inclusions; separation of components from dilute gaseous streams; separation of components from dilute aqueous streams; and water remoral. In addition, the panel identified one technology area with the potential to meet some of these needs and three enabling technologies. Common Needs The removal of impurities from feedstock is an important issue for the aluminum and steel industries. The removal of impurities from alumina feedstock minimizes the need for metal purification at later processing stages. Improving the quality of iron-bearing materials for blast furnace feed and DRI feed is important to the steel industry. Metal purification is important for the aluminum, steel, and metal casting industries. The purity of molten metal is important for the aluminum industry because almost all other metals are more noble than aluminum, and once an impurity is introduced into the melt, it is almost impossible to remove it. Removal

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--> of nitrogen and phosphorus from steel during steelmaking is an important separation issue for the steel industry. The metal casting industry would benefit from improved methods of removing impurities from molten metal. Sorting scrap materials is important for the aluminum, steel, metal casting, and glass industries. For the aluminum industry, energy savings can be obtained by melting scrap instead of raw materials. Improved scrap sorting can also lead to improved metal purity. The steel industry needs better methods for removing copper and other impurities from shredded automobile scrap. The metal casting industry would benefit from efficient and cost-effective methods of separating gate and riser scrap, as well as nonfoundry scrap. Recycling by the glass industry would be improved by better methods of sorting nonglass contaminants from postconsumer glass, different types of glass from each other, and different colored cullet. The glass industry would also benefit from technologies for separating glass particles from grinding media and technologies for separating lead flit and lead funnel glass from nonlead panel glass for recycling. The prevention of gas inclusions is important for the steel and metal casting industries. The steel industry would be helped by a better understanding of the mechanisms of separation of inclusions from liquid steel. The metal casting industry would benefit from improved methods for preventing or minimizing gas inclusions in gates and risers. Separation of components from dilute gaseous streams are important for all of the materials processing industries. For the aluminum industry, increasingly stringent regulations on emissions from several processes used to produce aluminum have made this a necessity. In the steel industry, the recovery of VOCs and particulates from gases in coke production is an important separation issue. Better methods for capturing gases and fine particles at high temperatures would benefit the glass industry. The forest products industry needs separation technologies for better separation of the contaminants from the hot gases produced in black liquor gasification and of VOCs from wet air streams. Separation of components from dilute aqueous streams are an issue for all of the materials processing industries. The aluminum industry needs better separation methods, particularly for the Bayer process. In the steel industry, the recovery and reuse of quench water from coke production is an important consideration. The glass industry would benefit from methods of separating solvents from wastewater streams. The forest products industry needs improved technologies for the separation of inorganic contaminants from bleach effluent streams, VOCs from aqueous streams, and dissolved organics and inorganics from paper machine process water. Water removal is an important issue for the aluminum, metal casting, and forest products industries. Improvements are needed in water removal from the Bayer process used by the aluminum industry. In the metal casting industry, reducing the drying time of investment casting moulds is an important consideration. In addition, the reclamation of core room sand would be improved by better drying of sand and dewatering of sludge. The forest products industry would benefit from technologies

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--> to optimize the removal of suspended and dissolved contaminants in the recycle fiber stream, better paper drying methods, and lumber drying strategies that minimize the release of organics or generate concentrated streams of organics that can be treated easily. Separation Technologies Methods of Separating and Sorting Solids Based on Physical Properties The panel identified one separation technology with the potential for meeting a number of the cross-cutting needs of the materials processing industries. The aluminum, steel, metal casting, and glass industries each identified separation needs that could be classified as materials handling and sorting or separation methods based on the physical properties of components. In many cases, the analytical technology has already been developed, and scrap separation processes are in place. In order to make these processes more economical, a higher speed sorting technology, such as the current air jet and conveyer belt technology, must be further developed. In other cases, the analytical technology has not been developed yet. Separation based on physical properties is an area of research and development where similar technologies could benefit these four industries. This family of separation methods is based on the classical physical properties of solids, such as magnetic and electrical properties, densities, and melting points. A number of these techniques are already being used, and refinements of existing techniques are commonplace. These processes are often part of the process flow of an industrial operation. Separation Processes Based on Melting Points Separation processes based on melting points are one example of separation methods based on physical properties. Sweat furnaces have been used in the scrap processing industry for many years for separating metals according to their melting points. The sweat furnace operates at a temperature at which one metal is selectively melted from a component, leaving the metal with the higher melting point, usually a ferrous metal, as a recoverable solid. Incipient melting of one phase in an alloy mixture has also been used as a means of separation. Separation Processes Based on Gravity As the name implies, the force of gravity comes into play in all gravity separations, but only one relies entirely on gravity. In this case, particles of different

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--> densities are separated in a medium of intermediate density. Depending on the densities of the particles, the medium can range from water to solutions denser than water to suspensions of fine, heavy particles in water. In practice, solutions of salts, such as calcium chloride and various bromides, have been used to a limited extent because of recovery problems and the consequent costs. Water is used as the medium if one of the particles floats; many useful separations are actually done in this way. True ''heavy liquids,'' such as thallium malonate/formate solutions, are used for analytical separations, but they are much too expensive and toxic to be used industrially. For separations of particles heavier than water, suspensions of fine solids can be used: magnetite and ferro-silicon are commonly used because their suspensions can be reconstituted by magnetic separation. In use, these suspensions must be agitated to prevent or limit settling of the medium. For separations of coarse particles, such as metals from the nonferrous components of shredded automobile scrap, large, relatively quiescent vessels can be used, but finer separations may require a second force, often a centrifugal force, to hasten and sharpen the density difference. Coal cleaning is probably the largest and most highly developed use for this "dense media" technology. Air separators take advantage of the fact that very light particles, even though they are heavier than air, can be swept aside by a current of air flowing normally to the suspended stream. Ancient farmers took advantage of this phenomenon to winnow their grain; highly developed machines are now available that can effect a wide variety of air-induced separations, including heavy from light metals (e.g., lead from aluminum in scrap processing). Obviously, materials to be air-separated must be dry. Throughout history, water has been the most common separation medium, even though the particles being separated may be heavier, sometimes much heavier, than water. When an assemblage of particles is allowed to settle freely in water, they tend not to separate cleanly according to density because mass and shape come into play. When the settling assemblage reaches a certain density, a condition of "hindering settling" is reached, and separation according to mass and shape then becomes possible. If the bulk material is then shaken or dilated rapidly, it is possible to segregate particles and separate the coarse and fine particles of the denser constituent in a single product. This principle was known in antiquity; today the machines that do it are called "jigs." Another phenomenon observed and used in ancient times was the behavior of particles being carried in traction by a flowing stream. Early peoples developed smooth stone tables on which fine particles of metal-bearing ores could be separated in a thin film of flowing water, and, in due course, they learned to flit the tables and direct some of the water across the flowing stream. Eventually, impediments to the flow called "riffles" were introduced to protect very fine heavy particles from being washed away. Ninety years ago the modern shaking table was introduced; an

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--> asymmetrical lengthwise forward and backward shaking action was added to adjustable tilt and water controls. At this stage, gravity or density differences were still operative but were not sufficient to effect a practical separation. Flowing film separations can be made using devices that are simpler than shaking tables and have higher capacities per unit capital investment and operating cost. One of these devices, the "spiral," is essentially an open half-cylinder in a spiral shape. The device has no moving parts; particles and water flow down the spiral and are influenced by gravity flow, wash water introduced along the inside edge of the spiral, and centrifugal force, which spreads the flow out and up the curved surface. Ports located along the spiral "cut" and divert heavier materials as they are separated. Perhaps the ultimate development of flowing film separators is the "pinched sluice," a device even simpler and cheaper than the spiral. The sluice is a simple inclined tray, wider at the bottom than at the top. Particles and water introduced at the top flow down the surface; the film spreads out and slows as it nears the bottom where cutters intercept the separated products. The sluice is usually "pinched" along its length so that the change in width is not uniform from top to bottom. This design feature accentuates the separation, but the efficiency of a single sluice is usually low, so that a succession of sluices must be used. This is not a great impediment because sluices are usually made of fiberglass or fiber-reinforced plastic and are inexpensive. An important variant of the sluice is the cone separator, which is essentially a sluice integrated through 360 degrees. Cones have very high capacities and are often used ahead of spirals or sluices to make rough, highrecovery separations. Separation Processes Based on Magnetic and Electric Forces Solids can be classified as ferromagnetic, paramagnetic, or diamagnetic. Relatively few are ferromagnetic, but those that are respond to magnetic force so strongly that magnetic separation is the most common and efficient method for separating them. The ferromagnetic class includes iron and most steels, some other metals, and a few minerals, such as magnetite. The separation environment can be wet or dry, and the equipment is designed to lift the ferromagnetics out of mixtures. Permanent magnets can be used, but most industrial processes employ electromagnets. Paramagnetic materials exhibit some susceptibility to magnetic force, but it is much weaker than for ferromagnetics. This difference dictates that much stronger magnets be used and that equipment be designed to minimize the "air gap" over which the field must operate. Capacities would also be lower because the weaker forces require longer separation times. Nevertheless, paramagnetic separations are very widely used, and major advances have been made in both magnet strength and equipment design. Cryogenic magnets are being used more widely, and equipment is designed for

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--> optimally configured field gradients. A vivid example is provided by the machines used to separate paramagnetic iron minerals from kaolin slurries intended for use in making very high quality paper. The magnetic force is so weak that time must be allowed for the iron-bearing particles to overcome the drag forces of water. High gradients are achieved by using stainless-steel wool as a secondary magnet in a field generated by a powerful cryogenic magnet. Magnetic separation does not depend on the electrical conductivity of the material being treated; in fact, many conductors are diamagnetic. Because they are conductors, some materials experience a force when passed through a variable magnetic field due to the generation of eddy currents, which are induced in conductive particles as a result of time dependent variations of a magnetic field. Eddy currents, in turn, interact with the magnetic field to generate repulsive forces, the magnitude of which are related to the conductivity, shape, mass, and size of the particles and the intensity of the magnetic field. Eddy current separators are based on either rotating disc permanent magnets or linear motor electromagnets for the generation of time dependent magnetic fields. The particles are passed through the magnetic field and physically separated according to the degree of thrust exerted on individual particles by the magnetic field. This method is now used for certain industrial separations, the separation of various nonferrous metals from the product of shredding old white goods and automobiles, for example, and the separation of aluminum cans from mixed packaging materials. Electrical separations take advantage of charges, either natural or induced, on solid particles. In a simple form, particles passed between two oppositely charged plates are attracted or repelled according to their own charges and can thereby be separated. This technique is called electrostatic separation because the charge relationships are not changed. In electrodynamic separation, however, charged particles brought into contact with a grounded drum lose their charges at different rates and are repelled more or less strongly, which is the basis for the separation. The natural differences between discharging rates can sometimes be accentuated by treating the particles with chemical reagents or by heating, but the material being separated must be dry. Dust is also a problem, but the principal limitation of the method is that the particles must usually be fed to the drum in a layer only one particle deep. Tribo-electrification is based on the fact that two dissimilar materials contacted or rubbed together will become charged. If one material is an insulator, it will retain its charge; if both are insulators, the retained charges will be in the order of the dielectric constants of the two. Separators based on this phenomenon are now being developed. Separation Processes Based on Optical Properties Optical properties of materials, such as color, induced fluorescence, or reflectivity can be used as a means of identification for separation. For example, certain

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--> metals in a mixed stream can be identified by their reflectivity and the identification used to trigger pulsed air jets that physically separate the elements of the stream. Glass can be identified by type using optical sorting based on color, coupling the identification signal to a physical method of particle separation. Enabling Technologies The panel identified three enabling technologies that would help the materials processing industries meet their separation needs: lower cost oxygen; particle characterization; and on-line diagnostics and sensors. Lower Cost Oxygen Several industries, including the aluminum, steel, and glass industries, have stated that inexpensive, high-purity oxygen would be beneficial in combustion processes. As noted in Chapter 5, oxy-fuel burners are already being used in some sectors of the aluminum industry, chiefly in recycling furnaces. The attractions of oxy-fuel include low NOx emissions compared with conventional fuel-air burners and energy savings from not having to heat the nitrogen component of air. Currently, the relatively high cost of oxygen remains a significant barrier to wider use of this technology. Particle Characterization Particles are ubiquitous in industrial processes and can be found as raw materials, products, by-preducts, and contaminants. The materials processing industries would benefit from improved characterizations of particles in terms of size, shape, and composition. Because of the wide range of particles, conditions, and parameters to be monitored, particle characterization encompasses a wide range of technologies. The properties of the particle that must be characterized depend on the process and the nature of the particle. In some cases, such as precipitation reactions, the size, shape, and density of the crystals are important. Size determination, however, is exceptionally difficult when the shape of the particle varies substantially from spherical. In other cases, characterizing the particle's chemical composition may be necessary for determining its source. For settling operations, particle density becomes important. For other separation processes, surface area, electrostatic charge, surface energy, and chemical characteristics may be important. In sorting operations, for example, in the recycling of cullet for glass manufacture, particle color is critical. Finally, an enormous number of separation processes are based on

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--> crystallization or precipitation processes, many of which operate with little or no control on particle size because it is impossible to measure size or transitions in size on line. Particle characterization has many uses for in process applications. Characterization methods must, therefore, be rugged and have response times that are appropriately rapid for the system. In some cases, such as the monitoring of particulates in stack gases, the characterization device must either withstand a harsh environment or function via remote sensing. On-line Diagnostics and Sensors On-line detection of the composition of material streams in separation processes would benefit all of the materials processing industries. For example, knowing the composition of molten glass in a tank would allow for real-time adjustments in composition and would increase product yield. On-line detection of inorganics, such as dissolved metals and transition metals in aqueous systems, would benefit the forest products industry. In fact, the on-line detection of the amount, size, and shape of particles is a significant challenge for all of the IOF industries (Klimpel, 1997). Some examples are the composition of papermaking furnishes and the control of particle emissions from manufacturing and combustion. On-line detection of the composition of process streams and the makeup of individual objects in these streams is essential to the sorting and reuse of materials, such as glass and aluminum (Kenny, 1997). Sensitive analytical procedures that can detect new species in process emissions have stimulated efforts to eliminate them. For example, recent detections of chlorinated wastes in the aqueous streams leaving pulp and paper processes have stimulated research and development on nonchlorine-based bleaching.

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--> III CROSS-CUTTING SEPARATION ISSUES

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