Basic Oxygen

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<p>2.4Recycling Technologies2.4.1Metal Recycling Technologies</p> <p>Electronic scrap consists of highly multi-component wastes containing besides steel, plastics mostly all main non-ferrous metals such as aluminium, copper, zinc, tin and lead, ferrous metals and precious metals like gold, palladium and silver. Next to the extraction of harmful components as lead, cadmium, hexavalent chromium and mercury are the high trade prices for gold, copper and silver the determining factor for recovery of the precious metals of electrical and electronic systems. Nonetheless metal recycling technologies are demanding high requirements to achieve an efficient output, a proper disposal of problematic substances and a reliable evaluation of the precious metal contents has to be provided, too.</p> <p>As exemplified in table 24 precious metals are used in components such as pin connectors, contact points, silver coated wire, terminals, capacitators, plugs and relays (PCB components).</p> <p>Table 24: Metal compounds in EES componentsMetal compoundMain use in EES components</p> <p>Copper</p> <p>Conductors (wire harness, coils, electric motors, PCB)</p> <p>IronHousings and structural parts, magnetic cores (in coils, solenoids, motors, transformers)</p> <p>Aluminium</p> <p>Housings of components, heat sinks</p> <p>LeadBattery (lead acid), piezoelectrical components (sensors + actuators), solder (PCB, junction boxes and all components soldered to wires), pyrotechnical initiators, carbon brushes of electric motors</p> <p>TinSolder (PCB, junction boxes and all components soldered to wires)</p> <p>NickelBattery (NiMH), PCB</p> <p>ZincBattery, metal coatings</p> <p>MagnesiumAluminium alloys, castings</p> <p>MercuryDischarge lamps (also used in LCD screens)</p> <p>SilverPCB components, batteries, heating wires of window heating</p> <p>GoldPCB components and electrical contacts, relays, connectors</p> <p>PalladiumPCB components</p> <p>PlatinPCB components, electrodes of sensors / actuators</p> <p>Metallic alloys (CuZn, CuSn)Wire harness</p> <p>LithiumBattery (Li-ions)</p> <p>TungstenDecandescent lamps</p> <p>IndiumLCD panels</p> <p>Zr (ZrO2)Ceramics (e.g. lambda control sensor, piezoelectrics)</p> <p>Titanium (TiO2)Ceramics (e.g. lambda control sensor, piezoelectrics)</p> <p>Other (bismuth, antimony, tantalum)PCB</p> <p>In the following chapter the pyrometallurgical processes, which are applied for the recovery of the major amounts of metals from end-of-life vehicles copper, aluminium, iron and steel and the hydrometallurgical processes, which are applied for the recovery and refinement of secondary non-ferrous metals and precious metals, are described.</p> <p> Processes Smelting</p> <p>Smelting consists of separation and purification of specific metals by means of heating and treating the scrap fractions. The non-ferrous metals alloys can be treated by means of pyrometallurgical processes by taking advantage of their different smelting point meaning that the material is processed in high-temperature reactions to separate metals from impurities. The heavy fraction of scrap consisting of lead, copper and zinc can be extracted by selective smelting processes. In general, the extraction of metals from electroscrap is a complex process, integrated with several process lines to refine various primary and secondary metals.</p> <p>The recovery of metals in a selective smelting process is done on special sweating furnaces (rotary, reverberatory, or muffle furnaces) where the metal fraction is slowly heated. The oxidation of the finely divided scrap is minimised by a multi zone temperature control, which allows the controlled melting of the different metals present [Rosseau 1991].</p> <p>The TS-process (Trennschmelz-Verfahren) described by Rosseau (1991) for metal recovery is based on the selective smelting of a particular metal by partial immersion of the previously dried scrap in a bath consisting of the same metal. Consequently only one metal can be recovered in each step, starting with the lowest melting point, followed subsequently by the rest of the components. The most frequent metals recovered by this process are lead-rich (90% Pb), zinc-rich (92% Zn) and copper-rich (45 50% Cu) products by the performance of two subsequent TS-treatments.</p> <p>A variety of furnaces can be used for melting metal scrap. The choice of furnace depends upon the quality and composition of the metal scrap, the desired production rate, and the mode of operation desired. Other factors influencing furnace selection are capital costs, refractory lifetime, and metal losses.</p> <p>The processing of several metals by smelting processes as listed in table 25 will be described in the next paragraphs, on the basis of the different furnace descriptions :</p> <p>Table 25: Overview of pyrometallurgical processes for the recovery of metals</p> <p>Pyrometallurgical processMetals</p> <p>SmeltingCu, Al, Zn, Pb, Mg</p> <p>Reverbatory furnaceCu, Al, Zn</p> <p>Electric furnacesteel, Fe</p> <p>Multichamber furnaceAl, Zn, non-ferrous metals</p> <p>Blast furnacesteel, Cu</p> <p>Crucible furnaceZn, Mg</p> <p>Gas fired furnacePb, Zn, Cu, Cd, Al</p> <p>Rotary furnaceCu, Al, Zn</p> <p>Induction furnaceAl, Zn</p> <p>Blast oxidation furnacesteel</p> <p>Electric arc furnacesteel</p> <p>Reverberatory Furnace</p> <p>Reverberatory furnaces heat the required metals to melting temperatures with direct fired wall mounted burners. The primary mode of heat transfer is through radiation from the refractory brick walls to the metal scrap, but convective heat transfer also provides additional heating from the burner to the metal. Reverberatory furnaces are available with capacities ranging up to 150 tons of molten metals. The advantages provided by reverberatory melters are the high volume processing rate, and low operating and maintenance costs. The disadvantages include the high metal oxidation rates, low efficiencies, and large floor space requirements.</p> <p>Figure 56: Reverberatory Furnace []</p> <p>Electric Reverberatory Furnace</p> <p>Electric reverberatory furnaces are used primarily as holding furnaces. These furnaces are refractory lined vessels using resistance heating elements mounted in the furnace roof above the hearth. These furnaces are used for smaller melting applications where limitations on emissions, product quality, and yield are of high priority.</p> <p>Advantages over gas-fired reverberatory furnaces include low emissions, low metal oxidation, and reduced furnace cleaning. Disadvantages include high fuel costs, low production rates, higher capital costs, and frequent replacement of heating elements.</p> <p>Multichamber Furnace</p> <p>This type of furnace consists of two rooms. The scrap is fed into one of the chambers. In the other room, the metal is heated using a flame, identical to the reverberatory furnace. Through a system with natural or forced convection, the warm metal is transported to the room with the scrap. By circulating the liquid metal through it, the scrap is melted. This type of furnace is mostly used for melting moderately polluted types of scrap.</p> <p>Blast Furnace</p> <p>A blast furnace is a smelting furnace consisting of a vertical cylinder atop a crucible, into which lead-bearing charge materials are introduced at the top of the furnace and combustion air is introduced at the bottom of the cylinder. It operates in temperature greater than 980C in the combustion zone that metal compounds are chemically reduced to elemental metals (e.g. lead and elemental lead).</p> <p>Figure 57: Blast Furnace []</p> <p>Crucible Furnace</p> <p>The crucible furnace consists of a crucible of refractory material in which the metal is melted by direct heating with a flame, or by an induction spiral. In order to tap the furnace, it is turned over manually or by using a hydraulic device. This type of furnace is used in the recycling industry to remelt thin-walled, clean types of scrap. Advantages provided by the electric crucible furnace is the near elimination of emissions and low metal oxidation losses. Disadvantages include increased fuel costs and size limitation.</p> <p>Figure 58: Diagram of a crucible furnace with an inward radiating metal fibre burner []</p> <p>Gas Fired Crucible Furnaces</p> <p>Crucible furnaces are small capacity, indirect melters/holders typically used for small melting applications or exclusively as a holding furnace. The metal scrap is placed our poured into a ceramic crucible which is contained in a circular furnace which is fired by a gas burner. The energy is applied indirectly to the metal by heating the crucible. The advantages of crucible furnaces are their ability to change alloys quickly, low oxidation losses, and their low maintenance costs. Disadvantages include low efficiency, (as low as 12%), high emissions, and size limitations [Metal Advisor 2004].</p> <p>Energy efficiency can be improved by 50% by adding a ceramic matrix recuperator to the exhaust system to recover waste heat for preheating the combustion air.</p> <p>Rotary Furnace</p> <p>Rotary furnaces are used almost exclusively for reclaiming low grade scrap. The Furnace operates by rotating the charge through the furnace which comes in direct contact with a gas burner or with a refractory wall which was directly heated by the burner. Typical rotary furnaces have holding capacities of 2 to 5 tons and are usually charged with salt which acts as a flux to improve metal recovery and reduce oxidation.</p> <p>The advantage provided by rotary furnaces is their ability to process dross and low-grade scrap which is difficult to process in other types of furnaces. The disadvantages are low efficiency, higher maintenance requirements, and considerable salt cake production which must be disposed of as hazardous waste.</p> <p>Figure 59: Rotary Furnace []</p> <p>Induction Furnace</p> <p>There are two general types of induction furnaces: channel and coreless. Channel furnaces are used almost exclusively as holding furnaces. Channel furnaces operate at 60 Hz where the electromagnetic field heats the metal between two coils and induces a flowing pattern of the molten metal which serves to maintain uniform temperatures without mechanical stirring. Coreless furnaces heat the metal via an external primary coil. Coreless furnaces are slightly less efficient than channel furnaces, but their melt capacity per unit floor area is much higher. Coreless furnaces are used mainly for melting of finely shredded scrap where they are most cost competitive with gas-fired furnaces. Advantages of induction furnaces include high melting efficiency (5070%), low emissions, low metal oxidation losses, and high allow uniformity due to increased mixing. Disadvantages are primarily their high capital and operating costs.</p> <p>Figure 60: Coreless Induction Furnace []</p> <p>Figure 61: Channel Induction Furnace []</p> <p>Basic Oxygen Furnace (BOF)</p> <p>The BOF is a pear-shaped furnace, lined with refractory bricks, that refines molten iron from the blast furnace and scrap into steel. Up to 30% of the charge into the BOF can be scrap, with hot metal accounting for the rest. Scrap is dumped into the furnace vessel, followed by the hot metal from the blast furnace. A lance is lowered from above, through which blows a high-pressure stream of oxygen to cause chemical reactions that separate impurities as fumes or slag. Once refined, the liquid steel and slag are poured into separate containers. The main advantages include its rapid operation, lower cost and ease of control.</p> <p>Figure 62: Diagram of the Basic Oxygen Furnace []</p> <p>Electric Arc Furnace</p> <p>An Electric Arc Furnace is a steel melting furnace in which heat is generated by an arc between graphite electrodes and the metal. The basic material is metal scrap in place of molten metal, and both carbon and alloy steels are produced. Furnaces with capacities up to 200 tonnes are now in use. The Electric Arc Furnace (EAF) offers an alternative method of bulk steel manufacture, utilising scrap as a metal source, e.g. car scrap.</p> <p>The EAF has evolved into a highly efficient melting apparatus and modern designs are focused on maximising the melting capacity of the EAF. Melting is accomplished by supplying electrical or chemical energy to the furnace interior. The melting point is reached at around 1520 C and the steelmaking efficiency is about 55-65 % [Jones, 2004].</p> <p>The first step in the production is to select the grade of steel to be made. Many operations add some lime and carbon in the scrap and supplement this with injection. After the scrap is loaded in the furnace, the roof is lowered and then the electrodes are lowered to strike an arc on the scrap, this commences the melting portion of the cycle.</p> <p>Once the final scrap charge is melted, flat bath conditions are reached. The analysis of the bath chemistry will allow the melter to determine the amount of oxygen to be blown during refining and arrange the alloy additions to be made. Refining operations in the electric arc furnace have traditionally involved the removal of phosphorus, sulphur, aluminium, silicon, manganese and carbon from the steel with the addition of oxygen throughout the cycle, as a result some of the melting and refining operations occur simultaneously. In recent times, dissolved gases, especially hydrogen and nitrogen, has been recognised as a concern. Other operation include the oxidation of impurities by de-slagging [Jones, 2004].</p> <p>Once the desired steel composition and temperature are achieved in the furnace, the tap-hole is opened, the furnace is tilted, and the steel pours into a ladle for transfer to the next batch operation (usually a ladle furnace or ladle station).</p> <p>Figure 63: Electric Arc Furnace []</p> <p>The following section characterises the individual smelting procedures by metal type present in automotive EES:</p> <p>Copper Smelting</p> <p>Low-grade copper scrap is melted in either blast or rotary furnace resulting in slag and impure copper. The smelting point of Cu is approximately 1080C. In the blast furnace, the copper is charged to a converter, where the purity is increased to about 80 to 90%, and then to a reverberatory furnace, where purity levels of 99% are achieved [EPA 1995]. In these fire-refining furnaces, flux is added to the copper and air is blown upward through the mixture to oxidise impurities. Then by reduced atmosphere, cuprous oxide (CuO) is converted into copper. Fire-refined copper is cast into anodes, which are used during electrolysis. The anodes are submerged in a sulphuric acid solution containing copper sulphate. As copper is dissolved from the anodes, it deposits on the cathode, with a purity up to 99.99%, where it is extracted and recast.</p> <p>The...</p>


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