Process for refining high-impurity copper to anode copper

A process for refining high-impurity blister copper to anode quality copper is disclosed. In an oxidation step of a blister copper refining stage, soda ash fluxing removes antimony and arsenic while also removing sulfur and iron. In a deoxidation step of the blister copper refining stage, sulfur hexafluoride is injected at a controlled oxygen concentration to remove bismuth while reducing the oxygen content. Slag is continuously or periodically skimmed from the surface of the molten blister copper to prevent re-entry of impurities. The process may be carried out in batch operation or, in a preferred embodiment, in continuous flow-through operation.

BACKGROUND OF THE INVENTION 
The present invention relates to refining "blister" copper to anode quality 
copper and, more specifically, to refining blister copper to remove 
arsenic (As), antimony (Sb), and bismuth (Bi) impurities. 
All existing flash- and bath-smelting processes that refine copper sulfide 
concentrates into blister copper consist of two stages: smelting and 
converting. In the smelting stage, copper-iron sulfide concentrates, which 
also contain "minor elements," i.e., gold (Au), silver (Ag), arsenic, 
antimony, and bismuth, are introduced with silica flux into a smelting 
furnace, into which air or oxygen-enriched air at 1150.degree. 
C.-1200.degree. C. is also injected. The air oxidizes some of the sulfur 
in the concentrates to sulfur dioxide, which, in turn, exits the furnace 
as part of a smelting "off-gas" stream. Concurrently, some of the iron 
reacts with the silica and oxygen to form slag. The remaining sulfur and 
iron combine with copper to form a copper rich matte (Cu.sub.2 S-FeS). As 
a result, three main products are generated by the smelting stage of a 
two-stage smelting and converting process--matte, slag, and off-gas. The 
minor elements are distributed among these products in the approximate 
proportions listed in Table 1. 
TABLE 1 
______________________________________ 
Approximate Distribution (%) of Minor Components 
Among Smelting Stage Products 
Matte Slag Off-Gas 
______________________________________ 
Au, Ag 99 1 -- 
As 35 55 10 
Sb 30 55 15 
Bi 10 10 80 
______________________________________ 
The matte produced in the smelting stage of a two-stage smelting and 
converting process is transferred to a converting furnace, such as a 
Pierce-Smith converter ("PS converter"), which is maintained at about 
1150.degree.-1250.degree. C. The converting is then carried out by further 
oxidizing the sulfur in the matte to sulfur dioxide (SO.sub.2), which 
exits the converter as off-gas. Concurrently, the iron in the matte 
consolidates in a slag which is periodically skimmed from the surface of 
the melt, leaving "blister copper," which is about 98.5%-99.5% Cu. 
Accordingly, three products are primarily produced in the converting 
stage--blister copper, slag, and off-gas. The approximate distribution of 
minor elements among the three converting stage products is provided in 
Table 2. 
TABLE 2 
______________________________________ 
Approximate Distribution (%) of Minor Components 
Among Converting Stage Products 
Blister Copper 
Slag Off-Gas 
______________________________________ 
Au, Ag 90 10 -- 
As 15 10 75 
Sb 20 20 60 
Bi 5 -- 95 
______________________________________ 
One disadvantage of conventional two-stage smelting and converting 
processes is that the matte produced in the smelting furnace must be 
physically transferred to the converter furnace. During this transfer, 
fugitive emissions of SO.sub.2 are generated in large quantities. In 
addition, slag must be periodically skimmed from the converter, and the 
process is not energy efficient. Due to these drawbacks, there has been a 
need to develop environmentally benign single-stage smelting and 
converting techniques that are both cost and energy efficient. 
At least one of the existing copper smelting and converting techniques, the 
Noranda continuous smelting and converting process, has the capability of 
producing blister copper in a single furnace. Themelis, N. J., "The 
Noranda Process for the Continuous Smelting and Converting of Copper 
Concentrates," Journal of Metals, April 1972, pp. 25-32. This single-stage 
copper refining technique offers environmental and energy advantages over 
the conventional two-stage copper smelting and converting processes. Other 
single-stage copper refining techniques, such as the Outokumpu flash 
smelting process, can produce blister copper from chalcocite concentrates 
(which have low iron content) and have the future potential to produce 
blister copper from chalcopyrite concentrates. Themelis, N. J., "Rate 
Phenomena in the Outokumpu Flash Reaction Shaft," in Physical Chemistry of 
Reactive Metallurgy, (Kudryk and Rao, ed., TMS, Warrendale, Pa.), 1985, 
pp. 289-309. A significant drawback of all single-stage copper refining 
processes is that they each produce blister coppers which contain high 
levels of impurities, specifically arsenic, antimony, and bismuth. 
Blister copper currently produced in conventional two-stage copper smelting 
and converting processes typically contains about 0.02-0.1 wt. % sulfur. 
To further reduce the sulfur content, the blister copper is subjected to 
"blister copper refining" to produce molten "anode quality" copper. Anode 
quality copper has a very low sulfur content (0.001-0.003 wt. % sulfur), 
low oxygen content (0.05-0.3 wt. % oxygen as Cu.sub.2 O), and only trace 
amounts of precious and other minor elements. Blister copper refining, 
which is the subject of this invention, is conventionally carried out in 
two steps. In the first step, batches of molten blister copper are 
introduced into a cylindrical "anode furnace" and an oxygen-containing gas 
is injected until the oxygen content reaches a level of about 0.8 wt. %, 
which concurrently decreases the sulfur concentration. The oxidized molten 
copper thus produced is then deoxidized to an oxygen level of about 0.1% 
by injecting a reducing gas, such as natural gas, or a reducing liquid, 
such as oil, or by "poling" with wood. 
In a final purification stage, the molten anode quality copper is cooled, 
cast into anodes, and electrorefined to produce a very high purity copper 
product. Impurity levels in the copper product are reduced to less than 40 
ppm (including sulfur), and the resulting copper purity is at least 99.99% 
Cu. In addition, gold, silver, and other valuable metals are recovered 
during this stage as by-products. However, if the impurity concentration 
in the anode copper starting material is too high, an excessive amount of 
floating "slime" forms in the electrolytic solution (or "cells"). The 
slime deposits on the surface of the cathodes, affects the copper quality, 
and decreases the energy efficiency of the electrolysis and the purity of 
the cathode copper produced. 
Despite the energy and environmental advantages of single-stage smelting 
and converting techniques, the inability of these techniques to 
effectively remove arsenic, antimony, and bismuth presents a problem 
because the impurities pass through conventional blister copper refining 
stages and accumulate in the anode copper at levels which, as indicated in 
Table 3, are unacceptably high for the subsequent stage of 
electrorefining. As the grade and quality of copper ores decrease with 
time, even blister copper from conventional two-stage smelting processes 
may contain high levels of these undesirable elements. 
TABLE 3 
______________________________________ 
Impurity Levels in Anode Copper Produced by 
Different Converting and Smelting Processes (ppm) 
Single- 
Impurity Two-stage stage Max. Allowed 
______________________________________ 
Arsenic (As) 
600-900 3000 1500 
Antimony (Sb) 
100-300 900 300 
Bismuth (Bi) 
30-100 300 150 
______________________________________ 
To compensate for these increased impurity levels, researchers have 
recently attempted to modify existing blister copper refining techniques 
to remove some of the impurities. Some of these efforts are described in 
Themelis, N. J., "Injection Refining of Directly-Smelted Copper," 
International Symposium on Injection in Process Metallurgy, TMS Minerals, 
Metals and Materials Society (1991), pp 229-251. One effort described by 
Themelis entails fluxing the blister copper with soda ash (principally 
sodium carbonate--Na.sub.2 CO.sub.3) to remove As and Sb. Another effort 
described by Themelis is SF.sub.6 injection to remove Bi. The author 
examines the potential application of these techniques in a modified anode 
furnace operation that would combine the conventional 
oxidation/deoxidation treatment of blister copper with impurity removal by 
injection refining. The author explains that soda ash fluxing is effective 
for removing As and Sb, but not Bi, and that SF.sub.6 injection has been 
proposed for Bi removal, but has not been used commercially due to the 
lack of thermodynamic and kinetic data to support a commercial design. 
Data is presented in the reference relating to oxidation and fluxing with 
sodium carbonate (i.e., soda ash), specifically for Sb removal, and a 
conclusion is drawn that the rate of removal is dependent on the rate and 
depth of soda ash injection. Similar data and conclusions are presented 
with regard to Bi removal by SF.sub.6 injection, and the author notes that 
the oxygen concentration should be maintained above 0.4% (4000 ppm) during 
SF.sub.6 injection. The reference does not address the overall amount of 
SF.sub.6 required. 
Stapurewicz, T. T., and Themelis, N. J., "Removal of Antimony from Copper 
by Injection of Soda Ash," Metallurgical Transactions, Vol. 21B (1990), 
pp. 967, indicates that a principal obstacle to single-stage smelting of 
copper is that impurities such as As, Sb, and Bi are concentrated in the 
copper phase, necessitating a complementary process for removing these 
impurities. Consequently, the reference provides a study of the 
thermodynamics of Sb removal from blister copper by soda ash fluxing. The 
authors indicate that this purification step may be performed during the 
blister copper refining stage, but that it has never been commercialized 
due to low utilization efficiencies. Among other things, the reference 
studies the effects of oxygen concentration by developing thermodynamic 
relationships, including a distribution coefficient, to relate the 
concentration of Sb in the copper to its concentration in the slag formed 
by the injection of soda ash and oxygen. In addition, the disclosed data 
(e.g., FIG. 1 of the reference) show that Sb removal is more efficient at 
higher oxygen concentrations. The reference further indicates that a 
proper choice of process conditions allows the Sb concentration to be 
lowered in 25 minutes from 1000 to 100 ppm. The reference also states that 
Bi cannot be removed by oxidation and fluxing. Although the reference 
mentions As as an impurity, it limits the study to Sb removal. The 
reference makes no mention of SF.sub.6. 
Taskinen, P., "Distribution Equilibria of As, Bi, Cu, Pb and Sb between 
Molten Copper and Soda at 1200.degree. C." Scandinavian Journal of 
Metallurgy, vol. 11 (1982), pp. 150-154, describes the distribution 
equilibria of, inter alia, As, Bi, and Sb between copper and soda ash. The 
reference reports that equilibrium favors the removal of As and Sb over 
that of Bi. The reference also notes that equilibrium removal is favored 
by maintaining oxygen concentrations above 4,000 ppm. 
Eddy, C. T., "Arsenic Elimination in the Reverbatory Refining of Native 
Copper," Transactions of the Metallurgical Society of the American 
Institute of Mining and Metallurgical Engineers, vol. 96 (1931), pp. 
104-118, discusses the removal of As by soda ash fluxing at approximately 
10,000 ppm oxygen, and generally emphasizes the importance of maintaining 
high oxygen concentration. 
Peacey, J. G., Kubanek, G. R., and Tarassoff, P., "Arsenic and Antimony 
Removal from Blister Copper by Blowing and Fluxing," Noranda Research 
Center, TMS Paper No. A80-54, Proceedings of the Las Vegas annual meeting, 
Nevada, 1980, discusses the equilibria resulting from fluxing low-sulfur 
copper with soda ash and limestone. The reference stresses that skimming 
the soda ash slag creates a favorable equilibrium, and shows that higher 
oxygen concentrations (e.g., 4-6,000 ppm) favor faster As and Sb removal. 
Riveros, G. A., Salas, R. I., Zuniga, J. A., and Jimenez, O. H., "Arsenic 
Removal in Anode Refining by Flux Injection," Mining in America, Institute 
of Mining & Metallurgy, Chatman & Hall, London, 1994, discloses the 
relationship between soda ash fluxing rates, impurity removal time and 
removal efficiency observed in the operation and optimization of the soda 
ash fluxing process at the Chuquicamata smelting facility. 
Zhao, B. and Themelis, N. J., "Removal of As, Sb and Bi from Molten Copper 
by SF.sub.6 injection," Proceedings of International Symposium on 
Co-products and Minor Elements in Nonferrous Smelting, TMS Las Vegas 
Annual Meeting, February 1995, pp. 39-52, discloses a process for removing 
impurities, particularly As, Sb, and Bi, from blister copper by SF.sub.6 
injection. The authors identify SF.sub.6 as one of the most promising 
reagents for removing these impurities and suggest that the process can be 
practiced in existing anode furnaces. The reference states that the rate 
of Bi removal by SF.sub.6 injection is significantly lower than that of Sb 
and As, and shows the relationship between impurity removal and oxygen 
concentration. The disclosed data suggests that oxygen concentration has 
little effect on Sb and As removal, but that Bi removal occurs 
significantly faster at lower oxygen concentrations. Based on the 
foregoing, Zhao and Themelis concludes that Bi should be removed at low 
oxygen concentrations. 
Like Zhao and Themelis, U.S. Pat. No. 4,010,030, filed Sep. 8, 1975 and 
issued Mar. 1, 1977 to French, discloses a process for removing As, Sb, 
and Bi by SF.sub.6 injection. The patent further indicates that high 
injection rates can be used to agitate the melt and that SF.sub.6 in the 
head-space continues to react with Bi in the melt to form BiF.sub.3. The 
patent also teaches that re-formation of CuS can be prevented by 
simultaneously injecting an oxygen-containing gas. 
Archer, G., "Thermodynamic and Kinetic Considerations in the Removal of 
Blister Copper by Sulfur-hexafluoride Injection," Ph.D. Thesis, Columbia 
University, New York, 1987, discloses a study of SF.sub.6 injection to 
remove Bi, responding in part to the need for a continuous impurity 
removal process which could complement the Noranda and Outokumpu smelting 
processes. Soda ash fluxing is also discussed, but the thesis does not 
suggest combining it with SF.sub.6 injection. 
Known techniques to remove impurities from blister copper suffer from 
several disadvantages. In the case of soda ash fluxing, the technique is 
ineffective at removing bismuth, which causes subsequent fouling of the 
cathodes in the electrorefining stage. In the case of sole SF.sub.6 
injection, inordinately large amounts of SF.sub.6 are required to 
sufficiently remove the contained impurities. Accordingly, existing copper 
refining technology does not provide, and there exists a need for, an 
environmentally acceptable and cost effective technique capable of 
substantially removing not only As and Sb, but also Bi from future as well 
as current grades of high impurity blister copper. 
SUMMARY OF THE INVENTION 
The above-discussed shortcomings of the prior art are overcome, and the 
aforementioned need is substantially satisfied by the present invention, 
which in one aspect is a process for refining high-impurity blister copper 
to anode quality copper by: (1) soda ash fluxing in the oxidation step of 
a blister copper refining stage to remove antimony and arsenic while also 
removing sulfur and iron; and (2) sulfur hexafluoride injection in the 
deoxidation step of a blister copper refining stage at a controlled oxygen 
concentration to remove bismuth while reducing the oxygen content. This 
process overcomes the disadvantages of existing soda ash processes that 
use fluxing, which does not effectively remove Bi, and substantially 
eliminates the environmental and cost disadvantages associated with 
proposed methods that only use SF.sub.6 injection. 
Thus, one aspect of the invention is a process for refining molten blister 
copper into anode copper comprising two steps. Specifically, the first 
step comprises simultaneously injecting into the molten blister copper a 
first oxygen-containing gas in an amount to provide at least 5 Nm.sup.3 of 
oxygen and at least 10 kg of soda ash for each ton of molten blister 
copper being refined, the injecting taking place while the molten blister 
copper is maintained at a temperature between 1150.degree. C. and 
1300.degree. C. Concurrently, slag is skimmed from the surface of the 
molten blister copper at a rate sufficient to remove impurities from the 
molten blister copper. The partially refined molten copper thus produced 
is then subjected to the second step of deoxidation, which comprises 
simultaneously injecting into the partially refined molten copper a 
hydrocarbon, a second oxygen-containing gas in an amount sufficient to 
maintain an oxygen concentration of between 500 to 2000 ppm in the 
partially refined molten copper, and at least 0.05 kg of sulfur 
hexafluoride for each ton of molten blister copper being refined. The 
injecting takes place while the partially refined molten copper is 
maintained at a temperature between 1150.degree. C. and 1300.degree. C. 
The process may be carried out in batch operation or, in a preferred 
embodiment, in a continuous flow-through basis. Excess oxygen may be 
provided, e.g., 10 Nm.sup.3 or more of oxygen for each ton of molten 
blister copper being refined, and the oxygen may be supplied by injecting 
air or, in a preferred embodiment, bulk oxygen. When bulk oxygen is used, 
a shield gas should be provided to protect the refractory walls. 
Preferably, such shield gas is injected as an annular gas stream 
surrounding the gas stream containing the bulk oxygen. 
In another aspect, the injection rates of the process are based on the 
amounts of impurities contained in the molten blister copper being 
refined. In this aspect, the first step comprises injecting into the 
molten blister copper a first oxygen-containing gas in an amount to 
provide at least 0.7 Nm.sup.3 of oxygen for each kg of sulfur contained in 
the molten blister copper being refined. Soda ash is concurrently injected 
in an amount to provide at least 0.7 kg of soda ash for each kg of arsenic 
and at least 0.4 kg of soda ash for each kg of antimony contained in the 
molten blister copper being refined. The injecting takes place while the 
molten blister copper is maintained at a temperature between 1150.degree. 
C. and 1300.degree. C., and slag is concurrently skimmed from the surface 
of the molten blister copper at a rate sufficient to remove impurities. 
The partially refined molten copper thus produced is then subjected to the 
second step, which comprises injecting into the partially refined molten 
copper a hydrocarbon, a second oxygen-containing gas in an amount 
sufficient to maintain an oxygen concentration of between 500 to 2000 ppm 
in the partially refined molten copper, and at least 0.35 kg of sulfur 
hexafluoride for each kg of bismuth contained in the molten blister copper 
being refined. The injecting takes place while the partially refined 
molten copper is maintained at a temperature between 1150.degree. C. and 
1300.degree. C. These steps may be carried out in batches or, in a 
preferred embodiment, on a continuous flow-through basis. Excess oxygen 
may be provided, e.g., 0.7 Nm.sup.3 or more of oxygen for each kg of 
sulfur contained in the molten blister copper being refined, and the 
oxygen may be provided by injecting air or, in a preferred embodiment, 
bulk oxygen. When bulk oxygen is used, a shield gas is preferably provided 
to protect the refractory walls. 
In another aspect, the invention is a refractory furnace vessel for 
refining blister copper into anode copper. At least one gaseous discharge 
port is provided in the vessel above the molten copper to permit gases to 
escape, and at least one skimming port is provided in a position to permit 
slag on the surface of the molten copper to flow out of the vessel. 
Beneath the surface of the molten copper, at least one tuyere is provided 
for introducing a first oxygen-containing gas and soda ash into the molten 
copper, and at least one tuyere is provided for introducing a hydrocarbon, 
a second oxygen-containing gas, and sulfur hexafluoride into the molten 
copper. At least one copper outlet port is provided in a position to 
permit the refined molten copper to flow out of the vessel. In a preferred 
embodiment of this furnace, at least one tuyere is located in a bottom 
wall of the vessel. In another embodiment, at least one tuyere for 
introducing the soda ash into the molten copper is also used for 
introducing the sulfur hexafluoride. In another preferred embodiment, the 
furnace is cylindrical and may be rotated. 
In still another aspect, the invention is a refractory furnace vessel for 
refining blister copper into anode copper on a continuous flow-through 
basis. The vessel of this aspect comprises two compartments separated by a 
dam, the first and second compartments being in communication with one 
another through a passage adjacent a lower portion of the dam near the 
bottom of the vessel. The first and second compartments each contain at 
least one gaseous discharge port located above the respective surfaces of 
molten copper to permit gases to escape. Similarly, the first and second 
compartments each contain at least one skimming port positioned to permit 
slag on the respective molten copper surfaces to flow out of the vessel. 
The vessel also contains beneath the surface of the molten copper in the 
first compartment at least one tuyere for introducing a first 
oxygen-containing gas and soda ash into the molten copper in the first 
compartment; and contains beneath the surface of the molten copper in the 
second compartment at least one tuyere for introducing a hydrocarbon, a 
second oxygen-containing gas, and sulfur hexafluoride into the molten 
copper in the second compartment. The second compartment further contains 
at least one copper outlet port positioned to permit refined molten copper 
to flow out of the vessel. In a preferred embodiment, the tuyeres in the 
first and second compartments are located in the bottom of the vessel. In 
a further preferred embodiment, the furnace is cylindrical and may be 
rotated.

DETAILED DESCRIPTION 
In conventional blister copper refining, iron and sulfur (about 0.1% Fe and 
0.1% sulfur as traces of copper matte) are removed by raising the blister 
copper oxygen concentration to about 6000 ppm in a conventional anode 
furnace or refining ladle. When the blister copper is produced in a single 
stage smelting furnace, the blister copper contains high levels of 
arsenic, antimony and bismuth. In several aspects, the present invention 
provides for the removal of these impurities from the blister copper. 
One aspect of the present invention, described with reference to FIG. 1, is 
a batch process in which arsenic and antimony (As and Sb) are removed in a 
first "oxidation" step 101 and bismuth removed in a second "deoxidation" 
step 102, each carried out at copper smelting temperatures of 1150.degree. 
C. to 1250.degree. C. on single batches of blister copper in a modified 
anode furnace or refining ladle. In the first step, the anode furnace or 
refining ladle is charged with molten blister copper 103 that contains up 
to 1-2% of sulfur (S) and up to 1% iron (Fe), principally in the form of 
entrained matte (FeS and CuS) and copper sulfide (Cu.sub.2 S). Oxygen in 
the form of air, bulk oxygen (i.e., greater than 98 percent oxygen), or 
other gaseous mixture, is introduced into the melt by injection 107 
through tuyeres located at the bottom of the furnace, upon which the 
oxygen reacts with sulfur and FeS to form SO.sub.2 and FeO. If bulk oxygen 
is injected, the refractory walls around the tuyere should be protected 
from excessive localized reaction temperatures by a co-injection 107 of a 
shield gas such as nitrogen, hydrocarbon, steam, or a combination of 
appropriate gases. Preferably, such shield gas is injected as an annular 
gas stream surrounding the bulk oxygen-containing gas stream. The oxygen 
is injected until the sulfur content in the melt is lowered to about 30 
ppm, corresponding to a chemical equilibrium oxygen concentration in the 
melt of about 6000 ppm. 
In accordance with this aspect of the invention, arsenic and antimony are 
removed during this oxidation-removal of sulfur and iron by an injection 
107 of soda ash into the melt. The soda ash particles react with As and Sb 
to form Na.sub.2 O.As.sub.2 O.sub.5 and Na.sub.2 O.Sb.sub.2 O.sub.5, which 
accumulate in a sodium carbonate slag on the surface of the melt. Slag 
removal 114 by skimming the surface of the melt is performed to prevent 
reentry of arsenic into the copper phase, which would otherwise occur due 
to the reversible nature of the reaction between soda ash and arsenic. 
Sufficient soda ash is added to lower the arsenic concentration to below 
700 ppm, corresponding an antimony concentration of less than 100 ppm. A 
removal 115 of exhaust gases is accomplished by allowing the gases to 
escape through an exhaust port located above the surface of the melt. 
Upon the removal 114 of much of the arsenic and antimony in the 
oxidation-step 101, a deoxidation step 102 is initiated by an introduction 
121 of a chemically reducing carbonaceous material, such as natural gas, 
into the melt. Sufficient carbonaceous material is introduced to cause an 
oxygen concentration reduction from 6000 ppm to 1000 ppm. The products of 
this reaction are primarily carbon dioxide and steam, which exit the 
furnace as part of an overall exhaust gas removal 124. In accordance with 
the invention, sulfur hexafluoride (SF.sub.6) is also introduced into the 
melt by injection 121. Due to the decreased concentration of arsenic and 
antimony, a majority of the SF.sub.6 reacts with bismuth to form 
BiF.sub.3. Although remaining arsenic and antimony also react with 
SF.sub.6 to form AsF.sub.3 and SbF.sub.3, the impact of these reactions on 
considerations relating to SF.sub.6 injection are greatly reduced. To 
limit the re-formation of copper sulfide (CuS), sufficient oxygen is 
introduced by an injection 121 of bulk oxygen, air, or other oxygen 
mixtures to maintain an oxygen concentration of about 1000 ppm in the 
melt. If the oxygen is injected as bulk oxygen, a shield gas should also 
be injected to protect the refractory walls. Preferably, such shield gas 
is injected as an annular gas stream surrounding the gas stream containing 
the bulk oxygen. 
In another aspect, the invention is a process and apparatus for carrying 
out the aforementioned steps on batches of molten blister copper in a 
vessel comprising a modified anode furnace. As shown in FIG. 2a, which 
depicts such a vessel 200 in the first step of the above-described 
process, a port 201 is located above the surface of the melt for 
introducing blister copper into the vessel. At least one tuyere 202 is 
located in a bottom wall of the vessel, through which a first 
oxygen-containing gas is injected, along with soda ash and possibly a 
shield gas. Slag is continuously or periodically skimmed from the surface 
of the melt through a skimming port 203 located in a side wall of the 
vessel. A gaseous discharge port 201 is provided above the surface of the 
melt to permit gases to escape. 
As shown in FIG. 2b, which depicts the vessel 200 in the second step of the 
above-described process, at least one tuyere 202 provides a means for 
introducing a second oxygen-containing gas, a hydrocarbon, and sulfur 
hexafluoride into the melt. The gaseous discharge port 201 again allows 
exhaust gases to escape, while a copper outlet port 204 is positioned to 
allow refined copper to flow out of the vessel. 
In a preferred aspect, the aforementioned oxidation removal of As and Sb, 
and deoxidation removal of Bi, are carried out on a continuous or 
semi-continuous flow-through basis in a single furnace divided by a 
refractory partition wall into a first "oxidation" compartment and a 
second "deoxidation" compartment. Similar dual-compartment furnaces have 
been used industrially in the QSL process for producing lead from lead 
concentrates (see L. Deininger et al., "The QSL plants in Germany and 
Korea" in: EPD Congress 1994, TMS, G. Warren, ed. , (Warren, Pa. 1994), 
pp. 477-501), but have not heretofore been used in blister copper 
refining. As shown by the apparatus diagram of FIG. 3, and referring also 
to the process flow diagram of FIG. 1, a dual-compartment furnace 300 for 
refining blister copper on a continuous flow-through basis may be supplied 
by modifying a known cylindrical anode furnace to include a refractory 
partition 303 dividing the furnace into two compartments. An opening 304 
near the bottom of the partition is incorporated to allow copper to 
continuously flow from the first compartment to the second compartment. 
In accordance with the preferred embodiment, oxidation-removal of sulfur is 
effectuated continuously in the first compartment 301 by an injection 107 
of bulk oxygen, air, or another oxygen-containing gas mixture into the 
melt. A removal 114 of much of the arsenic and antimony is accomplished by 
a co-injection 107 of soda ash and by continuously or periodically 
skimming 114 slag from the surface of the melt contained in the first 
compartment. The resulting partially refined copper, from which sulfur, 
iron, arsenic and antimony have been largely removed, continuously flows 
into the second compartment 302, where an oxygen concentration reduction 
is accomplished by a continuous or semi-continuous introduction 121 of 
carbonaceous material. Concurrently, bismuth is removed by a continuous or 
periodic injection 121 of sulfur hexafluoride (SF.sub.6) into the melt. 
Again, although SF.sub.6 reacts with remaining arsenic and antimony in the 
melt, the levels of these impurities are sufficiently low that 
environmental and cost concerns raised by SF.sub.6 injection are 
alleviated. Sufficient injection 121 of an oxygen-containing gas 
suppresses the re-formation of copper sulfide caused by a reaction of 
copper with SF.sub.6. 
Referring now to FIG. 3, there is shown an anode furnace according to a 
preferred embodiment of the present invention in which impurities are 
removed from blister copper on a continuous flow-through basis. The 
furnace 300 depicted in FIG. 3 includes a first "oxidation" compartment 
301 and a second "deoxidation" compartment 302, each separated by a 
refractory partition 303. In this example, the blister copper refining 
operation is downstream of a Noranda single-stage smelting and converting 
process capable of producing about 100,000 metric tons of copper per year 
and refining 15 t/h of blister copper. Without limiting the geometrical 
configuration, the blister copper refining furnace is a modified 
cylindrical anode furnace with approximate internal dimensions of 3-meters 
in diameter by 9-meters in length, as depicted in FIG. 3. The refractory 
partition 303 dividing the two compartments has a bottom opening to allow 
the flow of copper from the first compartment into the second compartment. 
Such a furnace could be provided by installing a partitioning refractory 
wall into a known cylindrical anode furnace, along with appropriate 
tuyeres and orifices. 
Molten blister copper may be charged continuously or intermittently into 
the first compartment 301 by turning the furnace through an appropriate 
angle so that the opening 305 at the left end of the top of the first 
compartment 301 moves downward from a gas hood (not shown) into a position 
that allows contents of a copper ladle to be emptied into the first 
compartment 301. This operation is similar to that used to charge existing 
conventional anode furnaces. The compositions are those typical of blister 
copper produced by single stage refining operations, namely 1-2% sulfur 
and about 0.5% iron, principally in the form of entrained matte, and the 
"single-stage" impurity levels provided in Table 1. 
In the exemplary apparatus shown in FIG. 3, the oxidation compartment 301 
is provided with two oxygen injection tuyeres (not shown) at the bottom of 
the compartment 301, each of which injects approximately 150 Nm.sup.3 /h 
of oxygen (O.sub.2) and sufficient shielding gas (N.sub.2) to thermally 
protect the refractory walls. (The metric unit, Nm.sup.3 /h, is a "normal 
cubic meter per hour," and corresponds to the equivalent amount of gas 
contained in one cubic meter at normal temperature (0.degree. C.) and 
pressure (1 atm.)). The injected gas forms a gas-liquid plume above the 
orifice and also results in a relatively large recirculating velocity 
(approximately 0.5-1.0 m/s) of liquid metal in the rest of the melt. At an 
estimated 80% utilization, this amount of oxygen sufficiently oxidizes the 
sulfur and iron in the blister copper, and provides a first-compartment 
copper effluent stream that contains about 30 ppm sulfur and 6000 ppm 
oxygen. These concentrations correspond to an equilibrium state between 
the sulfur and oxygen, which is achieved because the agitation caused by 
the injected oxygen-containing gas makes the compartment a nearly 
perfectly mixed reactor. 
As mentioned, the tuyeres used for injecting bulk oxygen are of the 
gas-shielded type, wherein the shield gas (such as nitrogen, hydrocarbon, 
or steam) is injected through an annulus surrounding the oxygen stream. 
Such tuyeres are known and have been used successfully in steel making 
converters and in lead smelting. 
While the sulfur is being removed by oxygen injection, arsenic and antimony 
are removed by introducing soda ash into the injected gas stream by means 
of a conventional solids feeder (not shown) at an average rate in this 
example of 150 kg/h, i.e., about 10 kg per ton of copper refined. The soda 
ash addition is discontinued briefly when new batches of blister copper 
are added to the oxidation compartment. The entrained sodium carbonate 
particles, which have a lower melting point (851.degree. C.) than copper 
(1083.degree. C.), melt instantaneously and react with As and Sb either as 
they rise through the melt or after they reach the slag-metal interface. 
The size of these particles are preferably less than 20 mesh in size. 
The rising droplets of sodium carbonate (Na.sub.2 CO.sub.3) react with the 
impurities in the first compartment copper melt according to the following 
reactions: 
EQU 2As!.sub.Cu +5O!.sub.Cu +Na.sub.2 CO.sub.3 .revreaction.Na.sub.2 
O.multidot.As.sub.2 O.sub.5 +CO.sub.2, 
and 
EQU 2Sb!.sub.Cu +5O!.sub.Cu +Na.sub.2 CO.sub.3 .revreaction.Na.sub.2 
O.multidot.Sb.sub.2 O.sub.5 +CO.sub.2. 
The resulting sodium carbonate slag accumulates on the surface of the melt 
and is skimmed periodically or continuously through a skimming port, which 
is shown in FIG. 3 in the left wall of the oxidizing compartment. Skimming 
is facilitated by the presence of plumes which push the slag towards the 
decreased surface level near the skimming port. It is advantageous to 
continuously or periodically remove this high impurity slag from the 
furnace because a failure to do so decreases the effectiveness of the 
impurity removal. The skimmed sodium carbonate may be treated by simple 
water leaching and crystallization to recover reusable sodium carbonate 
and arsenic and bismuth salts. 
As shown in FIG. 3, an opening 304 located at the bottom of the refractory 
partition 303 permits copper to continuously flow from the first 
compartment 301 oxidizing zone into the second compartment 302 deoxidizing 
zone. The size of the opening 304 is designed so that back-flow caused by 
turbulent circulation in the second compartment 302 is much less than the 
bulk flow of copper into the second compartment. Preventing back-flow and 
incident mixing provides a beneficially substantial difference in impurity 
concentrations between the two compartments. 
Under the foregoing operating conditions, the arsenic distribution 
coefficient, L.sub.As, is about 1600, where L.sub.As is defined as the 
ratio of arsenic content in the slag to arsenic content in the 
metal-phase. The corresponding distribution coefficient for antimony, 
L.sub.Sb, is about 40. Under these conditions, arsenic and antimony 
decrease from about 3400 ppm to about 200 ppm and from about 980 ppm to 
about 700 ppm, respectively. Bismuth removal is insignificant and the 
bismuth content in the molten metal is about 300 ppm. The flow rate of 
exhaust gas from the first compartment is about 1000 Nm.sup.3 /h, which 
includes an estimated 100% infiltration of atmospheric air through the gap 
between the furnace entrance and gas collection hood. This gas may be 
treated (e.g., scrubbed or recycled) with other smelter gases. 
In the second compartment, the oxidized blister copper is chemically 
reduced by introducing carbonaceous material into the melt, which reacts 
with the oxygen and thereby reduces the oxygen concentration from about 
6000 ppm to about 1000 ppm. The carbonaceous material may be natural gas 
(principally methane (CH.sub.4)) or another hydrocarbon, such as propane, 
oil, or even fine coal particles. The apparatus depicted in FIG. 3 
illustrates an example in which a mixture of natural gas and oxygen is 
introduced by means of two gas injection tuyeres (not shown) located along 
the bottom of the second compartment. The oxygen, which may itself be 
supplied as bulk oxygen, air, or other oxygen-containing mixtures, serves 
to partially combust, preheat and reform the hydrocarbon. Sufficient 
oxygen is supplied to maintain a 0.1-0.3 oxygen/hydrocarbon injection 
lambda factor, which is defined as the ratio of oxygen injected to the 
amount required for stoichiometric combustion of the hydrocarbon. For 
example, when natural gas is injected, the stoichiometric combustion 
requirement is approximately two volumes of oxygen for each volume of 
natural gas, as estimated by the stoichiometric equation for methane: 
EQU CH.sub.4 +2O.sub.2 .revreaction.CO.sub.2 +2H.sub.2 O. 
Accordingly, an injection gas that contains one volume of O.sub.2 per 
volume of natural gas has an oxygen/hydrocarbon injection lambda factor of 
about 0.5. 
The hydrocarbon injection rate is determined by: a) the amount of 
hydrocarbon necessary to decrease the inlet flow oxygen concentration from 
6000 ppm to the required exit concentration of 1000 ppm; b) thermodynamic 
constraints on the chemical reduction reaction in the bath; and c) the 
injection rate of oxygen into the bath. In view of these considerations, 
it is desirable to maintain less than 0.9 as "bath lambda factor," which 
is defined as the ratio: 
##EQU1## 
For the assumed blister copper processing rate of 15 t/h, and 
oxygen/hydrocarbon injection lambda factor of 0.1, the calculated total 
injection rate is 75 Nm.sup.3 /h of CH.sub.4 and 15 Nm.sup.3 /h of 
O.sub.2, and the corresponding bath lambda factor is 0.9. This gas flow, 
along with the required flow of nitrogen or other shielding gas, is 
injected through tuyeres. The injection creates a gas-liquid plume, where 
most of the reducing reaction takes place. The gas injection also creates 
a recirculating flow of somewhat lower velocity than in the oxidizing 
compartment, but sufficient to mimic a perfectly mixed reactor. Therefore, 
at least in the second half of the second compartment, the melt has 
essentially the same composition as the exiting molten copper stream. 
Even at a bath lambda factor of 0.9, the gas exiting the melt continues to 
have a reducing potential, so that some reducing reaction continues 
between the hydrocarbon in the gas atmosphere above the melt surface and 
the oxygen in the melt. To further utilize the remaining unreacted gas, it 
is advantageous to inject air or oxygen from or near the roof of the 
furnace to scavenge the remaining hydrocarbon and thereby release 
sufficient heat of combustion to satisfy the temperature requirements of 
the process. Also, auxiliary burners are appropriate for the first and 
second compartments to preheat the furnace and to maintain the furnace 
temperature during stand-by periods. 
At an oxygen/hydrocarbon injection lambda factor of 0.1 and a bath lambda 
factor of 0.9, the percent utilization efficiency (% UE--the fraction of 
injected hydrocarbon reacting with oxygen in the melt) of the input 
natural gas for deoxidation is 60%. This is nearly double the % UE 
reported for conventional batch anode furnaces, in which nearly 300 tons 
of copper are treated over a 2-3 hour period. A much higher efficiency is 
provided by the continuous flow furnace of the present invention, at least 
in part because: 
a) the gas flow rate per injector is much lower; 
b) the volume of melt associated with each injector is much less; 
c) the injectors are more effective due to their deeper location in the 
bath; and 
d) the injection of some oxygen along with the gas assists in pre-reforming 
and pre-heating the reducing gas. 
Removal of bismuth is effected in the second compartment by adding sulfur 
hexafluoride (SF.sub.6) to the gas injection stream. Experimental results 
have shown that the bismuth rate of reaction is slower than that of 
arsenic and antimony. Thus, although SF.sub.6 is injected primarily to 
remove bismuth, much of the remaining antimony and arsenic is removed as 
well. The principal chemical reactions involved are as follows: 
EQU 2Bi!.sub.Cu +SF.sub.6 +2O!.sub.Cu =2BiF.sub.3 +SO.sub.2, 
EQU 2Sb!.sub.Cu +SF.sub.6 +2O!.sub.Cu =2SbF.sub.3 +SO.sub.2, and 
EQU 2As!.sub.Cu +SF.sub.6 +2O!.sub.Cu =2AsF.sub.3 +SO.sub.2. 
By using the proposed furnace shown in FIG. 3 and assuming 50% loading of 
the furnace (i.e., 50% of the furnace being occupied by the copper melt), 
the average residence time of copper melt in the reducing zone is about 10 
hours, which provides ample time for the metal-phase reactions. 
In a 15 t/h blister copper refining operation, adding 1.8 Nm.sup.3 /h of 
sulfur hexafluoride constitutes about 2.0% of the total injection flow. 
Under such operating conditions, test results indicate that the copper 
exiting the reducing compartment contains about 30 ppm sulfur, 1000 ppm 
oxygen, and less than 100 ppm each of As, Sb and Bi. If a higher level of 
As is desired by the electrorefinery, then part of the arsenic salt 
recovered from the slag skimmed from the surface of the melt in the first 
compartment can be dissolved in the molten copper from which bismuth has 
been removed. The overall utilization of SF.sub.6, i.e., SF.sub.6 used for 
As, Sb and Bi removal, is about 60-70%. Experiments have shown that the 
utilization efficiency is increased at lower gas injection rates by 
lowering the SF.sub.6 partial pressure in the injection gas. 
The purified copper exits the second compartment of the furnace either 
periodically or continuously, and is suitable for casting into copper 
anodes to be used in the subsequent electrorefining process. When refining 
15 t/h of blister copper, the exhaust gas flow rate from the second 
compartment is estimated to be about 500 NM.sup.3 /h, which includes air 
infiltration through the furnace hood. Wet scrubbing, as used in the 
aluminum industry for treating fluoride-containing gases, is 
advantageously used to treat this gaseous effluent.