Extractive Metallurgy of Copper 4th ed. - W. Davenport_ et. al. (2002) WW Part 14 pptx

CHAPTER 22 Melting and Casting About 95% of the copper currently produced in the United States has existed as cathode copper at some time during its processing (Edelstein, 2000) The cathodes are produced by electrorefining pyrometallurgical anodes (from ore and scrap) and by electrowinning copper leached from 'oxide' and chalcocite ores To make it useful, this copper must be melted, alloyed as needed, cast and fabricated Much of the fabrication process for copper and its alloys is beyond the scope of this book; see Joseph (1999) for more information However, melting and casting are often the last steps in a copper smelter or refinery A discussion of these processes is, therefore, in order 22.1 Product Grades and Quality The choice of melting and casting technology is defined by: (a) the quality of the input copper (b) the required chemistry of the desired product (c) the type of final product, e.g wire or tube Table 22.1 lists the copper cathode impurity limits specified by various national standards (Joseph, 1999; ASTM B115-00) Customers usually require purer coppcr than in these specifications Fortunately, recent adoption of stainless steel cathodes for electrorefining and electrowinning has improved cathode purity to match these customer requirements The tightest impurity limits in copper cathode are for selenium, tellurium and bismuth All three of these elements are nearly insoluble in solid copper They form distinct grain boundary phases upon casting and solidification 361 368 Extractive Metallurgv of Copper Table 22.1 Upper impurity limits for copper cathodes as specified in the United States, Great Britain and Chile (IWCC) Impurity limits specified for the Southwire Continuous Rod (SCR) systems are also shown (ASTM = American Society for Testing and Materials; BS = British Standards; ppm =parts per million.) ASTM B-I15 Grade Grade 99.95 Se b p m ) Te (PPm) Bi (ppm) Bi+Se+Te (ppm) 10 Sb (PPm) Pb b p m ) As (ppm) Fe b p m ) Ni k p m ) Element % Cu+Ag Sn (ppm) s bpm) Ag ( p P c o @pm) Mn ( p P d Zn (ppm) Total (ppm) 10 15 40 15 25 20 15 25 10 25 70 5 10 65 IWCC High SCR (Southwire) Classification System Grade Class I Class Class 2 0.5 0.5 10 4 5 10 5 0.5 0.5 0.1 10 10 20 10 20 12 30 30 0.5 3 10 30 5 20 10 15 BSEN 1978 15 25 65 2 15 25 0.1 15 65 Selenium and tellurium form CuzSe and Cu2Te, while bismuth exists as pure Bi (Zaheer, 1995) These phases are brittle and cause rod cracking and poor drawability The Unified Numbering System currently recognizes about 35 grades of wrought 'coppers' (99.3% Cu or better) and six grades of cast coppers (Joseph, 1999) Several of these coppers are alloyed with small amounts of phosphorus to combine with oxygen when they are being welded Unalloyed coppers can be divided into two general classes The first is tough pitch copper, which purposefully contains -250 ppm dissolved oxygen (Table 22.2; ASTM B49-98; Feyaerts et al., 1996) Dissolving oxygen in molten copper accomplishes two goals The first is 369 Melting and Casting Table 22.2 Upper impurity limit specificationsfor tough pitch copper in the United States and Great Britain (CDA = Copper Development Association; ASTM = American Society for Testing and Materials; BS = British Standards;ppm = parts per million.) CDA CDA ASTM BS Cu-ETP-1 CU-ETP-2 B2 16-97 1038 Element (Grade 1) (Grade 2) %Cu (min.) Ag (PPm) As (PPm) Sb (PPW Bi @pm) Fe bpm) Pb (PPm) Ni @pm) QJpm) Se bpm) s @pm) Te bpm) Sn @pm) 99.95 25 05 04 02 10 05 99.90 99.88 99.85 120 030 0200 0050 005 030 0030 050 040 500 550 250 60 02 15 02 060 Total (ppm) 65 300 0100 0100 0500 1000 0300 0100 removal of inadvertently absorbed hydrogen during melting by the reaction: (22.1) This reduces the amount of porosity created by HlO(g) formation during casting and welding The second is reaction of the oxygen with metallic impurities, precipitating them as oxides at grain boundaries during solidification These oxide precipitates have a smaller adverse effect on drawability than compounds which would form if oxygen were not present Most copper i s cast and fabricated s tough pitch impurities are shown in Table 22.2 Specified limits for its The second class of pure coppers are the oxygen free (oxygen free copper [OFC] or oxygen free high conductivity copper [OFHC]) grades The amount of 370 Extractive Metallurgy o Copper f oxygen in these grades is so low that no visible amount of CuzO is present in the solid copper microstructure The maximum permissible oxygen level in OFC is 10 ppm In the best grades it is only ppm (ASTM B49-98; Nogami et al., 1993) Because no Cu20 is generated in the grain boundaries, the electrical conductivity of OFC is higher than that of tough pitch copper As a result, OFC is primarily used for demanding electrical applications, such as bus tube and wave guides (Joseph, 1999) Specific numbers are unavailable, but the fraction of copper sold as OFC is not large Koshiba et al (2000) and the Copper Development Association (2001) estimate that OFC accounts for less than two percent of total copper use Table 22.3 U.S copper processing in 1999, kilotonnes (Copper Development Association, 2001) Processing Facility Wire rod mills Brass mills Foundaries Powder plants Other Copper processed in 1999, kilotonnes 2259.6 1878.2 167.3 18.1 82.8 22.2 Melting Technology 22.2.1 Furnace types Table 22.3 shows the 1999 distribution of copper in the U S by type of processing plant (Copper Development Association, 200 1) Over half of copper production is drawn into copper wire, a fraction which remained largely unchanged in the 1990's Also, about half of the 'brass mill product' shown in Table 22.3 is unalloyed copper It is mostly fabricated into pipe and tube As a result, most current melting and casting technology produces (i) copper rod for drawing into wire or (ii) billets for extrusion to pipe and tube The vast majority of this copper is tough pitch Most tough pitch copper is produced from cathode in Asarco type shaft furnaces, Fig 22.1, Table 22.4 Ninety-five Asarco furnaces were operating in 1995, processing about half the world's copper (Hugens and DeBord, 1995) Melting and Casting 371 PB! U JI Sillcon Cnrblde Refractory Prsmlx TunnelBurners per Row Caatable retracttry Fig 22.1 Asarco shaft fkmace for melting cathodes Descending cathodes are melted by ascending combustion gases Table 22.4 gives industrial operating data 372 Extractive Metallurg), o Copper f Table 22.4 Operating details of Asarco cathode melting shaft furnaces, 2001 Melting plant Inputs Molten copper destination Melting rate, tonnes of copper per hour Feed system Furnace details, m height, taphole to charge floor inside diameter at charge floor inside diameter at taphole Burner details number of burners rows of burners fuel Nexans Canada Montreal cathodes and 'runaround' scrap rod Phelps Dodge Refinery El Paso, U.S cathodcs Norddeutsche Affnerie Germany cathodes Palabora Mining South Africa cathodes and recycled scrap Hazelett caster & rod mill Hazelett caster & rod mill Southwire caster & rod mill Southwire caster & rod mill 48 75 45 35 capacity, 21 operating skip hoist elevator with automatic trip forklift truck & skip hoist forklift truck 13 12.2 10 7.9 1.7 1.75 1.8 1.6 1.3 1.37 1.3 1.3 23 32 22 23 3 natural gas natural gas natural gas propane 2400 1100 50 x IO6 kJ/h a t Cu/h 1.8 gigajoules 26 (furnace only) 2.34 gigajoules 500 000 300 000 3zkO.5 years 3h0.5 vears combustion rate, Nm3/hour Nm3 of natural gas burnt per tonne of copper melted Refractory life, tonnes of copper above burners below burners 1.9 gigajoules 50G 000 250 000 Melting and Casting 313 The furnace operates counter currently, with rising hot hydrocarbon combustion gas heating and melting descending copper cathodes Natural gas is the usual fuel, Table 22.4 The process is continuous An important feature of the furnace is its burner The burner uses a highvelocity premix flame in a burner tile, accomplishing the premix within the burner itself rather than in an external manifold This design reduces accretions, shortens downtime for cleaning and allows individual control of each burner Automatic burner control using CO analysis of the offgas is a common feature of these furnaces (Schwarze, 1994) The flame is intended to generate a moderately reducing atmosphere, resulting in molten metal with about 50 ppm oxygen and 0.3-0.4 ppm hydrogen Other impurity concentrations are largely unaffected The most common feed to Asarco shaft furnaces is copper cathodes Highquality scrap is also occasionally melted Lower-quality scrap is less suitable for Asarco shaft furnaces, which have no refining ability As a result, some produccrs use reverberatory furnaces as an adjunct to their Asarco units (Schwarze, 1994; McCullough et al., 1996) Metal charged to these furnaces can be fire refined This allows the furnaces to be used for melting lower grade copper and scrap Another melting option is the induction furnace, either the channel or coreless type (Schwarze, 1994) Induction furnaces are usually used to melt oxygen free copper, since the absence of a combustion atmosphere prevents oxygen and hydrogen from inadvertently being absorbed into the molten copper Feed to induction furnaces which produce oxygen free copper is limited to highquality cathode and scrap Melting capacities are generally less than two tonnes per hour (Vaidyanath, 1992; Nogami et al., 1993) Molten copper from the above described melting furnaces flows into a holding furnace before being directed to continuous casting This ensures a steady supply of molten copper to the casting machines Holding furnaces vary considerably in size and type, but they are usually induction-heated to minimize hydrogen pickup from combustion gases The copper may also be covered with charcoal to minimize oxygen pickup Automation of the holding furnace to produce a steady flow of constant temperature metal has become an important part of casting operations (Shook and Shelton, 1999) Ceramic filters have also begun to appear in copper casting plants, to remove 374 Extractive Metallurgy ofcopper inclusions caused by erosion of the furnace refractories or precipitation of solid impurities from the molten copper (Strand et al., 1994; Zaheer, 1995) Introduction of multi-chamber induction furnaces is also a recent development (Bebber and Phillips, 1998) The 'storage' chambers in these furnaces eliminate the need for multiple holding furnaces 22.2.2 Hydrogen and oxygen measurementkontrol As previously mentioned, control of hydrogen and oxygen in molten copper is critical Oxygen is monitored one of two ways The first is Leco infrared absorbance, which measures the amount of C generated when the oxygen in a heated sample of copper reacts with admixed carbon black This method requires external sample preparation, so does not offer an immediate turnaround The second approach is an oxygen sensor, which is applied directly to the molten copper The electrode potential of the dissolved oxygen in the copper is measured against a reference electrode in the sensor This relative potential is converted to an equivalent oxygen content in the metal at the measurement temperature Dion et al (1995) have shown that the two methods yield similar results The amount of oxygen in the molten copper is controlled by adjusting burner flames and by injecting compressed air into the copper, Table 22.5 Hydrogen is more difficult to monitor and control Analysis of solid samples is usual practice (Strand et al., 1994), but efforts have been made to adapt aluminum industry technology to on-line measurement of hydrogen in molten copper (Hugens, 1994) Hydrogen pickup is minimized by melting the copper with oxidizing flames However, the molten copper always contains a small amount of hydrogen from entrapped electrolyte in the cathode feed (Chia and Patel, 1992; Back et al., 1993) 22.3 Casting Machines Casting machines can be divided into three main types: (a) billet ('log') casting, for extrusion and drawing to tube, Fig 22.2 (b) bar casting, for rolling to rod and drawing towire, Figs 22.3, 22.4, Table 22.5 (c) strip casting, for rolling to sheet and forming of welded tube 22.3 I Billet casting Billet casting is usually performed in vertical direct-chill casters, such as that Melting and Casting 315 shown in Fig 22.2 (Nussbaum, 1973) Graphite-lined copper or graphiteceramic molds are used Diameters up to 30 centimeters are cast (Hugens and DeBord, 1995) Oscillation of the water-cooled molds (60-360 m i d ) improves surface quality and prevents sticking in the mold Over the past decade, horizontal casters have begun to replace vertical billet casters, due to their lower cost (Owen, 1990) A recent innovation is horizontal continuous casting of hollow billets (Rantanen, 1995; Taylor, 1992) These billets are rollcd directly to tube, eliminating the need for extrusion and piercing They give a low-cost, high quality product Fig 22.2 Continuous direct-chill casting machine for casting copper billet (Nussbaum, 1973) Reprinted with permission of TMS 376 Extractive Melallurgy o Copper f 22.3.2 Bar and rod casting Copper bar is mostly cast in continuous wheel-and-band and twin-band casting machines, Table 22.5 and Figs 22.3 and 15.3 Figure 22.3 shows a Southwire wheel-and-band caster Its key features are: (a) a rotating copper-zirconium alloy rimmed wheel with a mold shape machined into its circumferencc (b) a cold-rolled steel band which moves in the same direction and at the same speed as the wheel circumference Molten copper is poured from a 'pour pot' into the mold just as the steel band joins the wheel to form the fourth side of the mold The wheel and band move together through water sprays as the copper solidifies After 180-250" of rotation, the band moves off to an idler wheel and the solidified copper bar is drawn away (under minimum tension) to a rolling mill Pouring to bar separation takes about 0.25 minutes (Adams and Sinha, 1990) The cast bar is removed at about 0.25 d s The Properzi casting machine is similar Extractor Pinch R W O Cross section rim mould for 35 cm2 bar Cast Copper Presser Wheel Steel Band