A Shaft Furnace Line for Producing Oxygen-Free Copperfor Electron Devices Applications
by Satoshi Teshigawara * , Akiyuki Ohta * 2 , Kouichi Higashi * 2 ,Hiromichi Konishi * 3 , Chizuna Kamata * 4 and Kensaku Oda *In recent years the level of environmental awareness has been rising world-wide,with increased emphasis being placed on measures to stop global warming andenergy-saving initiatives, but these have been largely ineffective. In this work we present a shaft
furnace as a replacement for induction furnaces in the melting of oxygen-free copper, and it has
been possible, using this shaft furnace line, to produce oxygen-free copper that exceeds both
the JIS standard for use in electron devices applications and the ASTM Grade 1 standard. We
also succeeded in achieving an energy saving of approximately 10 % in terms of primary crude
oil equivalence units.

ABSTRACT1.

INTRODUCTION

From the viewpoint of preventing global warming inrecent years, there have been increasing demands
for energy-saving measures to reduce the emission ofgreenhouse gases such as CO2, and the accumulationof small improvements by various companies has beeneffective. The same is true in molten copper casting,where large amounts of gas and electricity continue to be
consumed in melting, so that the need for energy saving
becomes more urgent every year. And it is obvious thatgas is superior to electricity in terms of lower use of crude
oil overall, including electric power generation for melting.
Analyzing by product category, the use of gas-fired shaft
furnaces is predominant for tough pitch copper, andenergy efficiency is high. In the manufacture of oxygenfree copper, on the other hand, the properties requiredof the material mean that the admixture of oxygen andhydrogen must be avoided. It is thus necessary to melt
the base metal using electrical induction heating, so thathuge amounts of energy continue to be used. In an effort
to promote energy savings in the production of oxygenfree copper there are manufacturers that produce roughdrawn oxygen-free copper wire by means of the shaft
furnaces normally used in making tough pitch copperand phosphorus deoxidized copper, but this has not been
extended to the oxygen-free copper ingots (slabs andbillets) supplied for electron devices applications.
In this work, we have, in the course of replacing our line

* Production Engineering Dept., Metal Company* 2 Plant & Facilities Div.
* 3 Quality Assurance Dept., Metal Company* 4 Metal Research Center, R&D Div.

for the production of oxygen-free copper, installed a shaft
furnace in place of the conventional induction furnace asa means of saving energy, where oxygen-free copper ofthe highest quality is required. We therefore report here
our success in producing oxygen-free copper satisfying
both JIS standards for electron devices applications 1) and
ASTM Grade 1 2).
In terms of the special properties required forsuperconductivity, the oxygen-free copper manufactured
through this development has a residual resistivity ratio(RRR) in excess of 500. And because of the outstanding
energy saving results in terms of crude oil equivalence
units, this project has been recognized as a project
supported by the New Energy and Industrial Technology
Development Organization (NEDO).

2.

GENERAL DESCRIPTION OF THE
SHAFT FURNACEIn the 50 years since they were developed, shaft furnaces
have achieved the status of a mature technology. They
have been used for the manufacture of tough pitch
copper having oxygen contents of from 300 to 500 ppm(i.e., g/ton) and phosphorus deoxidized copper, andFurukawa Electric at present has seven in operation at
our Chiba, Mie, Osaka and other plants. A shaft furnaceis one that melts copper using a burner fueled by gas(natural gas, butane or propane) that is characterized bylarge-scale operation to improve productivity, and by highenergy efficiency.
As can be seen from Figures 1 and 2, gas burners aredisposed radially around the lower part of a cylindrical
furnace. The base metal is melted, and a path for theexhaust gas produced by combustion is provided in

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A Shaft Furnace Line for Producing Oxygen-Free Copper for Electron Devices Applications

4.

DEVELOPMENT STATUS

4.1���� �����

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Behavior of Melt Constituents due to Shaft
Furnace
In the production of oxygen-free copper its characteristics
require that the amounts of oxygen and hydrogen in themelt be reduced as much as possible. It can, however,be expected that if a shaft furnace is used, melting
will proceed until the concentration of gases in the
combustion atmosphere reaches equilibrium with those in
the melt. The equilibrium relationship between the partialpressure of steam in the furnace atmosphere and oxygenand hydrogen in the melt may be expressed as:→ 2H+OH2O ←K = [ H ]2 . [ O ] / PH2O

Figure 2

Appearance of shaft furnace.

the upper part of the furnace so that it pre-heats thebase metal that is stacked there. For this reason thetemperature of the exhaust gas when exhausted isapproximately 473~673 K, making for a furnace of very
high energy efficiency.

3.

where:
K is the equilibrium constant,
O is the oxygen concentration in the melt,
H is the hydrogen concentration in the melt, andPH2O is the partial pressure of steam.

Structure of shaft furnace.

JIS STANDARDS FOR OXYGEN-FREE
COPPER FOR ELECTRON DEVICESAPPLICATIONS AND ASTM GRADE 1

In comparison with standards governing ordinary oxygenfree copper (C1020 specified in JIS), those governing
oxygen-free copper for electron devices applications(C1011 specified in JIS) specify superior impurity levels,
oxygen and hydrogen gas levels, electrical conductivity,and hydrogen embrittlement characteristics, for use
originally in vacuum tubes and at present in magnetronsand electron tubes. ASTM Grade 1 specifies evenhigher performance in terms of impurities and hydrogen
embrittlement characteristics than does C1011.

It is this, together with the balance between CO,
CO2, H2O and H 2 is the combustion atmosphere, that
determines the concentrations of oxygen and hydrogenin the melt discharged from the shaft furnace. Also theadjustment of CO, CO2, H2O and H2 in the combustion
atmosphere can be controlled by adjusting the excess
combustion air ratio using CO as an index. Figure 3
shows the results of measurements of oxygen andhydrogen at the discharge port of the shaft furnace 3)~5).
It can be seen that the oxygen-hydrogen equilibrium
conditions in molten copper at 1453 K and 1493 K are
in good agreement. Thus when a shaft furnace is used
for the melting of oxygen-free copper, it is essential thatoxygen and hydrogen be adequately removed, and it
is for this reason that in the past it has been induction
furnaces that were used for melting oxygen-free copper.
4.2

Oxygen and Hydrogen Concentrations Requiredin Shaft Furnace Melts of Oxygen-Free Copper
In producing oxygen-free copper to the ASTM Grade 1
standard, Furukawa Electric has a target of 2 ppm or less���������� �����

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Figure 3

Measured concentrations of oxygen and hydrogen incopper melt at discharge port of shaft furnace.

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A Shaft Furnace Line for Producing Oxygen-Free Copper for Electron Devices Applications

4.3 Technologies Introduced with the Shaft Furnace
With this introduction of the shaft furnace, attention
has been given to developing a system that allows forfully automated operation. We therefore establishedautomated charge control for the base metal andautomated combustion control for the burners.
Specifically in the case of burner control, a system thatkeeps constant track of combustion conditions in eachof the burners automatically controls excess air ratios inreal time, and a system that automatically controls theamount of combustion in response to the amount of melt
discharged from the shaft furnace. By these means fully
automated operation is being carried out.

of oxygen and 0.2 ppm or less of hydrogen. This requires
oxygen- and hydrogen-removal treatment to be applied
to the shaft furnace melt. Thus with a view to the ease of
implementation of various methods of melt treatment, wehave constructed a facility with target concentrations forthe shaft furnace melt set at 100 ppm for oxygen and 1
ppm for hydrogen.
Stable production of oxygen-free copper also requires
the stable discharge of the melt from the shaft furnace,
but as has been mentioned above, the furnace melt isstrongly affected by the combustion atmosphere fromthe burners so that if any changes should occur in the
combustion atmosphere, fluctuations in the oxygen andhydrogen concentrations of the shaft furnace discharge
melt will also occur. This is not a desirable situation in the
manufacture of oxygen-free copper for electron devices
applications or ASTM Grade 1 applications, and gives thegreatest importance to controlling burner combustion to
obtain a stable melt.
However in the interior of a shaft furnace, unlike thatof a conventional furnace, the base metal for the melt
appears in front of the burner in a condition that changes
from moment to moment making it difficult to maintain aconstant pressure loss in front of the burner. As a result
minute changes are made to the excess air ratio of theburner. This makes it necessary to constantly monitor andcontrol this combustion air ratio, but with the problems
with the response and control range of existing systems,
it is difficult to obtain a stable air ratio conducive tomaintaining the melt level required for the production ofoxygen-free copper for electron devices applications.
To solve these problems we have constructed a novel
system that provides real-time monitoring of all 15
burners of the shaft furnace and individual control of theexcess air ratios of each, and have been successful inobtaining the target melt level in a stable manner.
We have also carried out melt treatment in subsequent
process steps using previously developed technologies
for the removal of oxygen and hydrogen, succeeding in
the production of oxygen-free copper for electron devices
applications having an oxygen content of 2 ppm or lessand a hydrogen content of 0.2 ppm or less.

Table 1

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EVALUATION OF PROPERTIES OFOXYGEN-FREE COPPER

5.1 Analysis of Impurity Concentrations
Table 1 shows the results of analyses of the constituentsof the oxygen-free copper for electron devices
applications that was successfully produced in thiswork. Results were satisfactory with respect to all of
the impurities, significantly exceeding the levels set
forth in ASTM Grade 1. With the start-up of the new line
there were concerns relating to the admixture of iron inparticular, but by changing the structural materials used
for the furnace and by coating them, it was possible toreduce admixture to a negligibly low level. Sulfur was
also at an extremely low value, and it is thought that
this was due to the full benefit of the pre-heating effectof the shaft furnace, a particular phenomenon having
a desirable effect on the production of oxygen-freecopper for electron devices applications. This meansa further improvement with respect to iron and sulfur
in comparison with the conventional line. It was also
possible to achieve similar levels for other impurities.
5.2 Analysis of Oxygen and Hydrogen
As Table 1 shows it was possible to achieve the target
values of 2 ppm or less for oxygen and 0.2 ppm or lessfor hydrogen. The results for oxygen were substantiallythe same as with the conventional line, but for hydrogen,the improvements in the melt treatment effect of the new

Concentration of impurities in oxygen-free copper.
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A Shaft Furnace Line for Producing Oxygen-Free Copper for Electron Devices Applications

line gave even better results, with a lower concentration
than in the conventional line. Analysis for oxygen was
carried out by the JIS quantitative method using inert gas
atmosphere melting and infrared absorption 6).
5.3 Ingot Density
Ingot density was improved to 8.932 g/cm3, from the
8.930 g/cm 3 obtained using the conventional line.
Provided there is no change in the solidified state,ordinary density indicates the possibility that defects
such as bubbles are present, but the improvement of theratio in this case is also backed up by the fact that, asshown in Sec. 5.2, the gas content has been reduced.
This inevitably produced the result that gas exhaustcharacteristics are also improved.
5.4 Results of Hydrogen Embrittlement Tests
Hydrogen embrittlement is the most importantcharacteristics in oxygen-free copper for electron devices
applications. Figure 4 shows the results of heating ina hydrogen gas stream. The tests were conducted on
ingots this time. JIS provides for 30 min at 1123 K, but inanticipation of more rigorous conditions, we used 30 min
at 1273 K. In checking for the presence of voids in the
microstructure of the ingots we found that in all cases novoids occurred and no hydrogen embrittlement occurred.
In repetitive bending tests as well, 15 times or more wasobtained --satisfactory results that were equivalent tothose obtained in the past.
5.5 Annealing Characteristics
Figure 5 shows the annealing properties of oxygenfree copper manufactured under a high reduction rate
(95 %) and a low reduction rate (9.6 %) using the shaft
furnace line and the conventional line. In all cases noabnormalities were found in comparison with previous
oxygen-free copper. As shown in Figure 6, no changes
were observed in the growth of crystal grain in either
temperature range.
5.6 Residual Resistivity Ratio
Fo r s p e c i a l i z e d a p p l i c a t i o n s , s u c h a s i n s u p e rconductivity, the ratio between the electrical resistance atthe temperature of helium liquefaction (4 K) and normal
temperature, known as the residual resistivity ratio(RRR), is an index to how good electrical conductivity is
at low temperatures and is thus an extremely importantcharacteristic. Indexes in excess of the 350 required
for superconductivity applications were obtained,with a maximum of 503. When impurities are present
in the copper, the conductivity decreases with theirconcentration, so this result shows that the impurity
concentration is very low.

6.

EFFECTIVENESS OF ENERGY-SAVING
MEASURES

Figure 7 shows the effectiveness of energy-saving
measures. In the conventional line melting isaccomplished primarily by inductive heating using
electricity. In the new shaft furnace line, on the other
hand, the heat is provided by gas burners. Since thismakes it difficult to compare primary units directly, we
have used a measure of crude oil equivalence. Basing
the comparison on normal output, with conventional
prime crude oil equivalence unit of 0.102 kl/ton, the figure
was 0.090 kl/ton for the shaft furnace line, a reduction inenergy of approximately 10 %. At this plant we have beenworking towards a target reduction of 1 % per annum,
and if the savings obtained by the shaft furnace line arecompared in terms of primary units for the plant as awhole, there has been a saving of 1.3 % in energy used interms of primary crude oil equivalence units.

7.

CONCLUSION

As part of our efforts toward energy saving we haveintroduced a shaft furnace into the line for producing
oxygen-free copper. We have succeeded in ourdevelopment objective of producing oxygen-freecopper satisfying the JIS standard for electron devices
applications and ASTM Grade 1. The oxygen-free copper
produced here was satisfactory in terms of the levelsof oxygen, hydrogen and impurities, and a residual
resistivity ratio of 350 or more was achieved, confirming
its usability in specialized applications involving
superconductivity. And in terms of other mechanical��� ���� �� ���� �

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Results of hydrogen embrittlement tests.

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Figure 7

Comparison of energy efficiency of conventionaland shaft furnace lines in terms of primary crude
oil equivalence units.

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characteristics, it was possible to obtain satisfactory
oxygen-free copper, with properties in no way different
from those of earlier oxygen-free copper. In terms ofenergy savings as well we succeeded in a reduction of 10% on a shaft furnace plant basis in terms of primary crude
oil equivalence units.
In closing we would like to extend our thanks to all
those who cooperated with us in this development work.

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A Shaft Furnace Line for Producing Oxygen-Free Copper for Electron Devices Applications

REFERENCES
1) JIS H 3510 C1011
2) ASTM B-170
3) S. Otsuka and Z. Kozuka: Metall. Trans. B, 12B (1981), 501.
4) E. Kato and H. Orimo: Journal of the Japan Institute of Metals, 33(1969), 1165. (in Japanese)5) O. Kubaschewski and C.B. Alock: Metallurgical Thermochemistry,
5th ed., Pergamon, (1979).
6) JIS H 1067

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