Extractive Metallurgy of Copper part 14 docx

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.. Induction furnaces are usua

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.)

IWCC SCR (Southwire) Classifica-

1978 Element Grade 1 Grade 2 Grade Class I Class 2 Class 3

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

Melting and Casting 369

Table 22.2 Upper impurity limit specifications for 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.)

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 of Copper

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 5 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)

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

372 Extractive Metallurg), of Copper

Table 22.4 Operating details of Asarco cathode melting shaft furnaces, 2001

Melting plant Nexans Phelps Dodge Norddeutsche Palabora

Montreal El Paso, U.S Germany South Africa

& rod mill

1.9 giga- joules

50G 000

250 000

cathodcs

Hazelett caster

& rod mill

75

elevator with automatic trip

2400

1.8 giga- joules

cathodes

Southwire caster & rod mill

1100

26 (furnace only)

500 000

300 000

cathodes and recycled scrap Southwire caster & rod mill

50 x IO6 kJ/h

a 2 1 5 t Cu/h 2.34 giga- joules

3zkO.5 years 3h0.5 vears

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 high- velocity 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 quality scrap is also occasionally melted

High-

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 high- quality 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 0 2 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 graphite- ceramic 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 of Copper

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 ROW

Cross section rim mould

Steel Band

Fig 22.3 Southwire casting machine for continuously casting copper bar (Adams and Sinha, 1990) The inset shows the cross-section of the rim mold

Melting and Casting 3 77

The Hazelett twin-band caster is shown in Fig 15.3 in its role as an anode- casting machine Molten copper is fed from a pour pot into the space between two sloped moving steel bands The bands are held apart by moving alloyed copper dam blocks on each side, creating a mold cavity ranging between 5-15

cm in width and 5-10 cm in thickness Both separations are adjustable, allowing variable product size Solidification times are similar to those of the Southwire

and Properzi machines (Strand et al., 1994)

The three types of moving-band casting devices have several features in common All require lubrication of the bands and mold wheel or dam blocks, using silicone oil or acetylene soot (Adams and Sinha, 1990) Leftover soot is removed from the bands after each revolution, then reapplied This ensures an even lubricant thickness and a constant heat transfer rate

Fig 22.4 System for controlling molten copper level in Southwire continuous casting machine (Adams and Sinha, 1990) Reprinted courtesy TMS

378 Extractive Metallurgy of Copper

T a b l e 22.5 Operating details of Hazelett and Southwire continuous casting machines, 2001,

Casting plant Nexans Phelps Dodge Norddeutsche Palabora

Canada Refinerv Affinerie Minim

7 x 1 3

48

electromagnetic pool level measurement

1 I25

-950

250 Electro-nite cell

in launder;

Tempolab in holding furnace;

Leco on rod manual

Twin band details

caster length, m 3.7

band material low carbon steel

dam block material Si bronze

dam block life 100 000 tonnes

Hazelett twin band

7 x 13.2

63

electromagn- etic pool level measurement

1 I30

1015

250 Leco on rod

compressed air injection into molten

c u

3.7 titanium steel

1300 tonnes

c u Union Carbide Lb-300x oil

Cu with 1.7- 2% Ni & 0.5-

0.9% Si -300 hours

Southwire wheel & band 5.8 x 11.7

45

X-ray

1 1 10-1 125

900 160-250 Leco

protective gas, larger or smaller quan- tity 3.05 1.33 Cu-Cr-Zr

100 000 cold rolled steel

72 hours Lubro 30 FM

Southwire wheel &band 2.15 x 15 21.5

infrared scan- ner

1100-1 130

890-930 180-250 Leco on rod

holding fur- nace CO and launder burner

co

2.44 1.8 Cu-Cr-Zr

45 000 steel low split C 1000-1800 t

Cu per band Thermia B

(Shell)

Melting and Casting 379

The casters all use similar input metal temperatures, 11 10-1 130°C, Table 22.5 All require smooth, low-turbulence metal feed into the mold cavity, to reduce defects in the solidified cast bar Lastly, all require steady metal levels in the pour pot and mold

Control of mold metal level is done automatically, Fig 22.4 Metal level in the mold cavity is measured electromagnetically (Hazelett) or with a television camera (Southwire) It is controlled with a stainless-steel metering pin in the pour pot

Metal level in the pour pot is determined using a conductivity probe or load cell

It is controlled by changing the tilt of the holding furnace which feeds it (Nogami et ul., 1993; Shook and Shelton, 1999)

The temperature of the solidified copper departing the machine is controlled to 940- 101 5°C by varying casting machine cooling-water flow rate

Common practice for copper cast in the Hazelett, Properzi and Southwire ma- chines is direct feeding of the solidified bar into a rolling machine to give con- tinuous production of copper rod Southwire Continuous Rod and Hazelett Contirod are prominent (Buch et al., 1992; Hugens and DeBord, 1995; Zaheer, 1995) Both systems produce up to 60 tonnes of 8-14 mm rod per hour, Table 22.5

22.3.3 Oxygen free copper casting

The low oxygen and hydrogen content of oxygen free copper minimizes porosity

when this metal is cast As a result, the rolling step which is used to turn tough

pitch copper bar into rod is not necessary This has led to the development of processes for direct casting of OFC copper rod These include both horizontal and vertical casting machines (Joseph, 1999)

Horizontal rod-casting machines use a graphite crucible and a submerged casting die They generally operate as multi-strand machines Their capacities are limited to about 0.6 tonnes per hour They cannot produce very small diameter rod

Upward vertical casting machines use a vacuum to draw metal into water- cooled graphite-lined dies partially submerged in the molten copper As it freezes, the rod is mechanically drawn upward and coiled (Eklin, 1999; Rautomead, 2000) It is about the same size as rolled rod

22.3.4 Strip casting

The development of strip casting for copper and copper alloys parallels

380 Extractive Metallurgy of Copper

developments in the steel industry, in that continuous processes are favored The newer the technology, the less rolling is required One approach taken by small- volume producers is to roll strip from the bar produced by a Hazelett caster (Roller et al., 1999) This can be combined with continuous tube rolling/welding

to make optimum use of the casting machine for a mix of products

However, direct strip casting which avoids rolling is the goal Current horizontal casters can produce 'thick strip' (15-20 mm), which requires some rolling (Roller and Reichelt, 1994) Development efforts are being made to develop 'thin-strip' (5-12 mm) casting to avoid rolling completely

22.4 Summary

The last step in copper extraction is melting and casting of electrorefined and electrowon cathodes The main products of this melting and casting are:

(a) continuous rectangular bar for rolling to rod and drawing to wire

(b) round billets ('logs') for extrusion and drawing to tube

( c ) flat strip for rolling to sheet and forming into welded tube

The copper in these products is almost always 'tough pitch' copper, Le cathode copper into which -250 ppm oxygen has been dissolved during meltinghasting This dissolved oxygen:

(a) ensures a low level of hydrogen in the copper and thereby avoids steam porosity during casting and welding

(b) ties up impurities as innocuous grain boundary oxide precipitates in the cast copper

The remainder of unalloyed copper production is in the form of oxygen free high conductivity copper with 5 to 10 ppm dissolved oxygen This copper is

expensive to produce so it is only used for the most demanding high conductivity applications It accounts for less than 2% of copper production

These pure copper products account for about 70% of copper use remainder is used in the form of copper alloy, mainly brass and bronze

(a) as rectangular bar in continuous wheel-and-band and twin-band casters (b) as round billets ('logs') in horizontal and vertical direct chill casters

Melting and Casting 381

The bar casters are especially efficient because their hot bar can be fed directly into continuous rod-rolling machines

The quality of cathode copper is tested severely by its performance during casting, rolling and drawing to fine wire Copper for this use must have high electrical conductivity, good drawability and good annealability These properties are all favored by maximum cathode purity

Suggested Reading

Adams, R and Sinha, U (1990) Improving the quality of continuous copper rod Journal of

Metals, 42(5), 3 1 34

Hugens, J.R and DeBord, M (1995) Asarco shall melting and casting technologies '95 In

Copper 95-Cobre 95 Proceedings of the Third International Conference, Vol IV Pyrometallurgy of Copper, ed Chen, W.J., Diaz, C., Luraschi, A and Mackey, P.J., The

Metallurgical Society of CIM, Montreal, Canada, 133 146

Joseph, G (1999) Copper: Its Trade, Manufacture, Use and Environmental Status, ed

Kundig, K.J.A., ASM International, Materials Park, OH, 141 154; 193 217

Schwarze, M (1994) Furnace systems for continuous copper rod production Wire Industry,

American Society for Testing and Materials (1998) Standard specification for copper rod drawing stock for electrical purposes (B49-98) In Annual Book of Standards, Section 2, Nonferrous Metal Products, ASTM, Philadelphia, PA

American Society for Testing and Materials (2000) Standard specification for electrolytic

cathode copper (B115-00) In Annual Book of Standards, Section 2, Nonferrous Metal Products, ASTM, Philadelphia, PA

Back, E., Paschen, P., Wallner, J and Wobking, H (1993) Decrease of hydrogen and oxygen contents in phosphorus-free high conductivity copper prior to continuous casting

BIIMs 138,22 26

Bebber, H and Phillips, G (1998) Induction furnace technology for horizontal casting

Metallurgia 65,349 35 1