US3767383A

US3767383A - Refining copper pyrometallurgically by two-stage subatmospheric treatment

Info

Publication number: US3767383A
Application number: US00198984A
Authority: US
Inventors: M Bell; J Pargeter
Original assignee: International Nickel Co Inc
Current assignee: Huntington Alloys Corp
Priority date: 1971-11-15
Filing date: 1971-11-15
Publication date: 1973-10-23
Anticipated expiration: 1990-10-23
Also published as: FR2160439A1; SE396770B; NO131550B; DE2255977B2; PH9937A; DE2255977A1; JPS4861322A; NO131550C; ES408574A1; AU4869872A; BE791287A; JPS524250B2; ZM17772A1; GB1351089A; ZA727948B; CA973720A; FI60239C; DE2255977C3; FR2160439B1; FI60239B

Abstract

Copper precipitated from solution is melted in air to slag iron and to form a copper bath containing sulfur and oxygen, with the oxygen content being at least in excess of the sulfur content without producing a separate cuprous oxide phase, and at least one other volatile impurity selected from the group consisting of arsenic, bismuth, lead, selenium, tellurium, tin and zinc. The copper bath is subjected to a subatmospheric pressure treatment while passing a purge gas, that can contain free oxygen, through the bath to rapidly remove sulfur from the bath. The flow of the inert gas through the bath is terminated and the bath is then subjected to even lower subatmospheric pressures to remove other volatile impurities.

Description

United States Patent [191 Bell et al.

[ 1 Oct. 23, 1973 [73] Assignee: The International Nickel Company,

Inc., New York, N.Y.

22 Filed: Nov. 15, 1971 21 Appl. No.: 198,984

[52] U.S. Cl 75/76, 75/72, 75/74, 75/75 [51] Int. Cl LS/QQ [58] Field of Search ..75/7276, 109, 2

[56] References Cited UNITED STATES PATENTS 2,732,294 1/1956 Hewitt et al. 75/76 3,212,883 10/1965 Cech et a1 75/72 3,258,330 6/1966 Ito et a1. 75/76 3,282,675 11/1966 Parker 75/2 3,288,599 11/1966 Keyes 75/109 '9 3,298,070 11/1966 Yurko et a1... 75/76 3,424,575 1/1969 Long 75/109 3,470,936 10/1969 Jandras 75/76 3,490,897 3/1969 Dore et al.. 75/76 3,490,899 1/1970 Krivsky 75/109 3,123,466 3/1964 Crampton.. 75/76 103,434 5/1870 DuMotay... 75/76 1,348,457 8/1920 Strasser 75/76 3,622,304 1 1/1971 Khalafalla et a1 75/72 3,630,722 12/1971 Chew 75/76 3,634,065 1/1972 Worner... 75/75 X 3,664,828 5/1972 Worner... 75/73 3,669,646 6/1972 Cullom 75/72 X 3,674,463 7/1972 Yannopoulos 75/74 X FORElGN PATENTS OR APPLlCATlONS 510,861 8/1939 Great Britain 75/76 698,758 10/1953 Great Britain 75/109 1,052,907 12/1966 Great Britain..... 75/72 1,071,127 6/1967 Great Britain..... 75/76 1,160,984 8/1969 Great Britain 75/109 Primary Examiner-L. Dewayne Rutledge Assistant Examiner-W. R. Satterfield Attorney-Maurice Pine] [57] ABSTRACT Copper precipitated from solution is melted in air to slag iron and to form a copper bath containing sulfur and oxygen, with the oxygen content being at least in excess of the sulfur content without producing a separate cuprous oxide phase, and at least one other volatile impurity selected from the group consisting of arsenic, bismuth, lead, selenium, tellurium, tin and zinc. The copper bath is subjected to a subatmospheric pressure treatment while passing a purge gas, that can contain free oxygen, through the bath to rapidly remove sulfur from the bath. The flow of the inert gas through the bath is terminated and the bath is'then subjected to even lower subatmospheric pressures to remove other volatile impurities.

17 Claims, No Drawings REFINING COPPER PYROMETALLURGICALLY BY TWO-STAGE SUBATMOSPHERIC TREATMENT The present invention pertains to refining copper precipitated from solution, and more particularly to refining metallic copper precipitates by pyrometallurgical techniques.

Cement copper, copper precipitated on metallic iron from acidic process solutions, is separated from the solution and can be treated by magnetic separation and- /or screening techniques to provide upgraded cement copper. Cement copper, whether upgraded or not, has been considered an intermediate product, either being recycled to smelting operations or being added in small amounts to copper electrolytes in electrorefining operations. The use of cementcopper as an intermediate product places a burden on the smelting or electrorefining capacity with no corresponding benefits being realized thereby.

Metallic copper precipitates can also be obtained by hydrogen reduction of copper-containing process solutions at elevated pressures of 350 pounds per inch at 2- 90C. These. powders are generally finely divided and contain up to about 0.1% sulfur and 0.01% iron.

It has been suggested that cement copper be treated by screening and magnetic separation techniques to provide cement copper of at least anode quality. The very definition of this product implies that further electrorefining operations are required. After electrorefining, the material would have to be further processed to produce copper in useable form. Thus, after the cement copper is concentrated by magnetic separation and screening techniques, it must be melted to form anodes which are electrorefined to cathodes that are eventually melted to produce commercial forms of copper.

It has now been discovered that cement copper can be pyrorefined by a special two stage subatmospheric pressure treatment after which the copper can be deoxidized and cast.

It is an object of the present invention to provide a process for pyrorefining metallic copper precipitates.

Another object of the present invention is to provide a process for pyrorefining metallic copper precipitates to such an extent that the refined copper can be used directly in commercial applications.

Other objects and advantages will becomeapparent from the following description.

According to the present invention, metallic copper that contains up to about 5% iron, up to about oxygen, up to about 1% sulfur and at least one volatile impurity selected from the group consisting of arsenic, bismuth, lead, selenium, tellurium, tin, and zinc, after any preliminary beneficiation treatment, is melted in a free-oxygen-containing atmosphere to slag iron and to form a copper bath containing sulfur and oxygen, with the oxygen content being at least in excess of the sulfur content without producing a separate cuprous oxide phase, and at least one other volatile impurity selected from the group consisting of arsenic, bismuth, lead, selenium, tellurium, tin, and zinc. After removing the slag from the copper bath, a purge gas, which can contain free oxygen, is passed through the copper bath while the bath is maintained at subatmospheric pressures below about 0.001 atmosphere to rapidly lower the sulfur content to less than about 0.001%. At this point the single phase copper bath contains at least about 0.1%

oxygen. The flow of the purge gas is terminated after the sulfur content of the copper bath is lowered to less than about 0.001%, and the bath is subjected to subatmospheric pressures of less than about 0.0002 atmosphere for further refining by volatilizing at least one impurity selected from the group consisting of arsenic, bismuth, lead, selenium, tellurium, tin and zinc from the bath. The copper bath is then deoxidized. Before casting, the copper bath can be optionally treated with phosphorus to produce oxygen-free copper.

As noted hereinbefore, cement copper is produced by the precipitation of copper from acidic process solutions on metallic iron. If the iron used as a precipitant is coarse or massive, the precipitated copper eventually coats the entire surface of the iron and the cementation reaction comes to a complete halt. As the particle size of the metallic iron used in precipitating copper decreases, the iron content of the cement copper also decreases. When coarse metallic iron particles are employed for precipitating copper, the cement copper can be treated by magnetic separation and/or by screening techniques to upgrade the copper so that it will contain less than about 3% iron and more than about copper. Thus, as an advantageous embodiment of the present invention, cement copper is initially treated by magnetic separation techniques and/or by screening techniques to provide upgraded cement copper.

Any metallic copper precipitate that contains up to about 5% iron, up to about 1% sulfur, up to about 10% oxygen, up to about 0. 1% arsenic, up to about 0.1% bismuth, up to about 0.1% lead, up to about 0.01% selenium, up to about 0.01% tellurium, up to about 0.1% tin, and up to about 0.1% zinc can be treatedby the process in accordance with the present invention. As noted hereinbefore, cement copper can advantageously be upgraded by magnetic separation and screening techniques. Such upgraded cement copper can contain up to about 3% iron, up to about 1% sulfur, up to about 10% oxygen, up to about 0.1% arsenic, up to about 0.1% bismuth, up to about 0.1% lead, up to about 0.01% selenium, up to about 0.01% tellurium, up

to about 0.1% tin, and up to about 0.1% zinc. Copper powder produced by hydrogen precipitation from process solutions and containing up to 0.5% sulfur, 0.01% iron and arsenic, bismuth, lead, selenium, tellurium, tin and zinc within the hereinbefore specified ranges-can also be treated by the process in accordance with the present invention. It is to be noted that, except where otherwise expressly stated, solid and liquid compositions given herein are on a weight basis and gaseous compositions are on a volumetric basis.

If the metallic copper precipitate contains more than about 2% oxygen, the cement copper is treated with a reductant during meltdown to lower the oxygen content to avoid the formation of an immiscible cuprous oxide phase that is highly corrosive to furnace refractories and to minimize copper losses to the ironcontaining slag. The copper precipitate, whether upgraded or not, is advantageously mixed with a carbonaceous reductant, the amount of the carbonaceous reductant being correlated with the iron and sulfur contents of the copper precipitate so that the final oxygen content of the copper bath after melting is at least in excess of the sulfur content and less than that which would produce a separate immiscible cuprous oxide phase. The carbonaceous reductant can be either liquid or solid. The amount of carbonaceous reductant added to the metallic copper precipitate is dependent upon the iron and oxygen contents of the copper precipitate. The additions of the carbonaceous reductant are controlled so that the carbonaceous reductant, the sulfur and the iron are effective in producing molten copper having an oxygen content below about 1.5%. In most instances, the amount of the carbonaceous reductant added to the copper precipitate is between about 0.5% and 5%, based on the weight of the cement copper to insure that the oxygen content of the molten copper is below about 1.5%. The oxygen content can, of course, be controlled by melting in an atmosphere reducing to cuprous oxide.

Copper precipitates generally have a substantial distribution of particles finer than minus 325 mesh which upon heating can cause dusting with concomitant metal losses. Finely divided copper precipitates can also present materials handling problems and can render the melting operation less efficient when induction furnaces are used. Therefore, it is advantageous to form agglomerates of the mixture of the copper precipitates and the carbonaceous reductant by conventional techniques. The mixture of the copper precipitates and the carbonaceous reductant is formed into pellets by known balling techniques or into briquettes by conventional pressing operations. Regardless of the manner in which the mixture of cement copper and carbonaceous reductant is agglomerated, the agglomerates should have a particle size with a minimum dimension of at least about 0.1 millimeter, and advantageously at least about 20 millimeters, in order to minimize the problems of dusting, materials handling, and less efficient melting.

The copper precipitate, whether agglomerated or not, is melted to provide a copper bath and a floating dross that contains iron oxide. In order to permit rapid removal of the iron oxide dross and to facilitate subsequent impurity removal, the mixture of cement copper and carbonaceous reductant is heated to a temperature of at least about 1,200C., and advantageously to a temperature between about 1,250C. and 1,400C. At temperatures within the foregoing range, the molten copper is sufficiently liquid to permit iron oxide formed in the copper to rapidly rise to the surface of the bath where the iron oxide is removed as a solid dross. Efficient and substantially complete removal of the iron oxide dross from the bath is an advantageous embodiment of the present invention because removal of the iron oxide dross facilitates subsequent subatmospheric pressure desulfurization and refining treatments. Higher temperatures are also effective in increasing the kinetics of the subsequent desulfurizing and other refining reactions.

After the iron content of the copper bath has been lowered to less than about 0.01% and the iron oxide dross has been removed from the bath, the bath is subjected to subatmospheric pressures below about 0.01 atmosphere of mercury, and advantageously between about 0.001 atmosphere of mercury and 0.0002 atmosphere of mercury, while a purge gas, which can contain free oxygen, is passed through the bath to lower the sulfur content of the bath to less than about 0.001%, and advantageously to less than about 0.0005%. Desulfurization is quite rapid at subatmospheric pressures but the rate of desulfurization is surprisingly increased by the passage of a purge gas through the melt while the melt is maintained under subatmospheric pressures. It is believed, although the invention is not limited thereto, that the bubbles of the inert gas passing through the melt overcome the high energies required to nucleate sulfur dioxide bubbles, establish a large difference in chemical potential between the sulfur dioxide in the bubble and the sulfur dioxide dissolved in the bath and vigorously agitate the bath to minimize concentration gradients therein. The purge gas is at least one member selected from the group consisting of nitrogen, argon, air, carbon dioxide and oxygen. In most instances, gas flow rates between about 1 and 20 standard cubic feet per hour per square foot of bath surface area are employed in order to realize all the advantages derived from purging and stirring with gas while minimizing apparatus requirements.

If the copper bath does not contain sufficient oxygen to eliminate sulfur as sulfur dioxide and to provide an oxygen content between about 0. 1% and 1.5% after desulfurization, some additional oxygen must be added to the bath. This can be accomplished by lancing the bath with air or oxygen or by adding oxidized copper powder to the bath.

Desulfurization is advantageously conducted at sub atmospheric pressures of less than about 0.01 atmosphere and advantageously between about 0.001 atmosphere and 0.0002 atmosphere. Within this range desulfurization proceeds at commercially attractive rates without placing undue demands on the vacuum equipment. At higher subatmospheric pressures the rate of desulfurization at sulfur contents below about 0.001% is not commercially attractive while the use of lower pressures would entail the use of high capacity, complex and expensive equipment to maintain such low pressures, particularly when purging with an inert gas.

When the sulfur content has been lowered to predetermined levels, e.g., to less than about 0.001% or even to less than about 0.0005 the gas purge is terminated and final refining to lower the level of at least one impurity selected from the group consisting of arsenic, bismuth, lead, selenium, tellurium, tin, and zinc is be gun. The oxygen content of the bath is adjusted to between about 0.1% and 1.5%, and the copper bath is subjected to a subatmospheric pressure of less than about 0.0002 atmosphere, advantageously less than about 0.000] atmosphere, to further refine the bath by volatilizing at least one impurity selected from the group consisting of arsenic, bismuth, lead, selenium,

' tellurium, tin, and zinc. The copper melt, during this stage, is mechanically or inductively maintained in a state of turbulence to facilitate volatilization. of any of the foregoing impurities. Although vigorous agitation could be supplied by passage of an inert gas through the melt, it has been found that the lower subatmospheric pressures obtainable in the absence of an inert gas are more effective in eliminating the foregoing impurities than are the overall effects of the inert gas passing through the melt. This refining operation is effective in lowering the arsenic, bismuth, lead, tin, and zinc contents to less than about 0.001% while the selenium and tellurium contents are at least halved.

The copper bath, after refining, contains between about 0.1% and 1.5% oxygen and is advantageously deoxidized before casting. At least partial deoxidation can be effected by passing a reducing gas, such as hydrogen, carbon monoxide, natural gas or propane through the copper bath. Passage of a reducing gas through the copper bath works reasonably well; but it has been surprisingly found that as the oxygen content approaches about 0.05% a solid reductant, such as solid carbon or coke, is kinetically more effective in lowering the oxygen content. Therefore, a solid reductant is added to the copper bath either during the entire deoxidation treatment or during the latter stages thereof, i.e., when the oxygen content of the bath falls to about 0.05% or less, to lower the oxygen content to about 0.01%, advantageously about 0.005%. If oxygenfree copper is desired, the copper melt can optionally be completely deoxidized by phosphorus additions thereto. The deoxidation treatment with solid carbon is advantageously conducted at subatmospheric pressures less than about 0.01 atmosphere, e.g., between about 0.005 atmosphere and 0.0005 atmosphere. During deoxidation, the bath is maintained in a state of turbulence by purging with a non-oxidizing gas, i.e., an inert or reducing gas. Inert gases, such as nitrogen or argon, are advantageously employed as purge gases to minimize the problems associated with dissolved hydrogen at low oxygen contents. When deoxidizing at subatmospheric pressures, purge gas flow rates between about 1 and standard cubic feet per hour per square foot of bath surface area are employed to provide the re-.

quired turbulence without placing undue burdens on the vacuum equipment. A wide range of purge gas flow rates can be employed when deoxidizing at ambient pressures as long as the copper bath is adequately agitated. After deoxidation, the copper melt is cast into commercial shapes.

The copper bath is desulfurized, refined and deoxidized at temperatures of at least about l,200C. and advantageously at temperatures between about 1,250and 1,400C. to insure rapid and substantially complete desulfurization, refining, and deoxidation. The various operations can be conducted in any type of furnace but it has been found advantageous to use induction furnaces to take advantage of the stirring effect of these furnaces. Induction furnaces have the additional advantages of completely eliminating the possibility of contaminating the copper by combustion products of the fuel. An even further advantage of induction furnaces is that they can be readily equipped with appropriate vacuum equipment.

The following examplesillustrate the results that can be obtained by practice of the present invention.

EXAMPLE I Cement copper, which was derived from the precipitation of copper from sulfate leach solutions on shredded, detinned cans and which contained 0.7% sulfur, 6% oxygen, 1.2% iron, 0.029% arsenic, 0.038% lead, 0.0023% selenium, 0.0007% tellurium and minor amounts of calcium oxide, silica and alumina, was melted in air in an induction furnace, which furnace was equipped with a vacuum unit, to slag the iron, calcium oxide, silica and alumina and to form a copper bath which contains 0.01% iron, 0.30% sulfur, and 1.49% oxygen. The surface of the bath was cleared of the slag, and the pressure within the vacuum unit was then lowered to 0.0003 atmosphere while nitrogen at a rate of 15 standard cubic feet per hour per square foot of bath surface area is passed through the bath to lower the sulfur'content to 0.0005%. Purging of the bath with nitrogen is then terminated, and the pressure within the furnace is lowered to 0.0001 atmosphere to further refine the bath. During this stage of the refining operation, the levels of arsenic, lead, selenium, and tellurium are lowered to 0.01%, less than 0.002%, 0.001%, and 0.0003%, respectively. Carbon is then added to the bath, and the bath is again purged with nitrogen at a flow rate of 12 standard cubic feet per square foot of melt surface per hour to lower the oxygen content of the bath to 0.02%. The bath is then cast.

EXAMPLE I] This example confirms thatmolten copper is more readily desulfurized by a simultaneous inert gas purge and vacuum treatment than by a vacuum treatment alone, even with pressures of one-tenth or less of that employed with an inert gas purge. Two copper baths, one containing 1.2% oxygen and 120 parts per million (ppm) sulfur and the other containing 0.95% oxygen and ppm sulfur, were established by heating cement copper to 2,300F. The bath initially containing ppm sulfur was simultaneously subjected to subatmospheric pressures and a nitrogen purge at a rate of about 15 standard cubic feet per square foot of surface melt per hour. An equilibrium pressure between about 200 microns and 250 microns was rapidly established over the bath, and the sulfur content was lowered to less than 1 ppm in about 1 hour. The sulfur content and the pressure over the bath for various times are shown in Table 1A. The copper bath containing 100 ppm sulfur was subjected to such subatmospheric pressures between about 15 and 32 microns and the sulfur content of the bath was measured at various intervals with the results being shown in Table 1B. Without the nitrogen purge nearly two hours were required to lower the sulfur content to 1 ppm even with pressures less than onetenth of the pressures used with the nitrogen purge.

TABLE 1A Time, Pressure, S content, mins. mm Hg ppm 0 300 120 12 0.24 9 22 0.25 4 33 0.24 1 46 0.20 1 63 0.21 1

TABLE 18 Time, Pressure, S content, mins. mm Hg ppm 0 300 100 20 0.032 20 50 0.025 I 4 80 0.018 3 110 0.016 1 0.015 1 By comparing the results in Tables 1A and 113, it is evident that the simultaneous use of a purge and subatmospheric pressures greatly improves the rate of subatmospheric desulfurization.

EXAMPLE III Although an inert gas purge greatly improves subatmospheric pressure desulfurization, this example confirms that the inert gas purge interferes with other refining reactions by precluding the attainment of the necessary low pressures and that two-stage subatmospheric pressure treatments are advantageous in providing an overall refining operation. Two copper baths, one containing 19 ppm selenium and 13 ppm tellurium and the other containing 29 ppm selenium and 11.5 ppm tellurium, were established by heating cement copper to 2,300F. Both baths were oxidizing in that oxygen was present in amounts of more than 1%. The bath containing 19 ppm selenium was simultaneously purged with nitrogen at a rate of 15 standard cubic feet per square foot of melt surface area per hour and subjected to subatmospheric pressure between 200 and 250 microns. Samples were taken at various intervals and analyzed for selenium and tellurium contents with the results being shown in Table 2A. The bath initially containing 29 ppm selenium was merely subjected to subatmospheric pressures between 15 and 32 microns. Again, samples were periodically taken and analyzed for selenium and tellurium, and the results are reported in Table 2B.

TABLE 2A Time, Pressure, Se, Te, Mins. mm Hg ppm ppm 12 0.24 21 23 22 0.25 23 15 33 0.24 18 24 46 0.20 19 23 63 0.21 23 24 TABLE 2B Time, Pressure, Se, Te, Mins. mm Hg ppm ppm 20 0.032 29 l 1.1 50 0.025 28 7.6 80 0.018 21 7.3 l 0.016 17 5.8 140 0.015 12 8.7

The results shown in Tables 2A and 2B confirm that lower subatmospheric pressures are more effective in refining copper with respect to selenium and tellurium, particularly tellurium.

EXAMPLE IV This example confirms that molten copper is more effectively deoxidized by carbon at subatmospheric pressures with a simultaneous inert gas purge than by hdyrogen bubbled through molten copper. A copper bath containing 0.26% oxygen was established, and a hydrogen-nitrogen gaseous mixture containing 75% hydrogen was bubbled through the bath at a rate of 6.9 standard cubic feet per hour which was equivalent to 100 standard cubic feet per square foot of melt surface area per hour, which flow rate insured vigorous agitation of the bath. The oxygen content of the bath was determined at various intervals wtih the results being reported in Table 3A. Another copper bath containing 0.27% oxygen was established, and granular graphite in an amount equivalent to 1% of the bath was floated on the surface of the bath. The bath was subjected to subatmospheric pressures of between 400 microns and 500 microns while the bath was purged with nitrogen at a rate of 0.8 standard cubic feet per hour which was equivalent to 1 1 standard cubic feet per square foot of melt surface area per hour. The baths oxygen content was determined at various intervals, and the results are reported in Table 3B.

TABLE 3A Time, Oxygen content, mins. Weight 0 0.26 13 0.14 22 0.066 30 0.045 40 0.021 50 0.017 60 0.0105

TABLE 38 Time, Oxygen content, mins. Weight The results reported in Tables 3A and 3B confirm that deoxidation of copper with carbon at subatmospheric pressures, even though kinetically slower liquid-solid reactions are involved, proceeds at materially faster rates than deoxidation with hydrogen, which is thought of as being kinetically more reactive.

A separate deoxidation test at atmospheric pressure using granular graphite and an inert gas purge was conducted for comparative purposes. After 50 minutes the oxygen content of the bath was 0.01% which was four times the oxygen content of the bath deoxidized for 50 minutes with carbon and an inert gas purge at subatmospheric pressure. In 65 minutes the oxygen content was lowered to 0.0032%, confirming that effective deoxidation with granular graphite and an inert gas purge can also be realized without resorting to subatmospheric pressures.

It will be observed that the present invention provides a pyrometallurgical process for refining metallic copper precipitated from aqueous solutions. Although the present invention has been described in conjunction with the treatment of cement copper, those skilled in the art will appreciate that the process can be employed to refine melts of electrorefined or electrowon copper that are contaminated with sulfur during melting operations or contain objectionably high quantities of arsenic, bismuth, lead, selenium, tellurium, tin, and zinc.

Although the present invention has been described in conjunction with advantageous embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. For example, copper melts containing oxygen contents that are high in comparison to the sulfur content can, during the latter stage of desulfurization, be simultaneously desulfurized and partially deoxidized while purging with a non-oxidizing gas is continued and employing the second stage operation to eliminate other impurities. Such modifications and variations are considered to be within the purview and scope of the.

invention and appended claims.

We claim:

1. A process for refining metallic copper that contains up to about 5% iron, up to about 10% oxygen, up to about 1% sulfur and at least one volatile impurity selected from the group consisting of arsenic, bismuth, lead, selenium, tellurium, tin, and zinc which comprises: melting the metallic copper precipitate in a free-oxygen-containing atmosphere to slag the iron and to form a copper bath containing sulfur, oxygen, and at least one other volatile impurity selected from the group consisting of arsenic, bismuth, lead, selenium, tellurium, tin and zinc, the oxygen content being at least in excess of the sulfur content without forming a separate cuprous oxide phase; removing the slag from the copper bath; then passing a purge gas, which can contain free oxygen, through the copper bath while maintaining the bath at a subatmospheric pressure below about 0.01 atmosphere to rapidly lower the sulfur content of the bath to less than about 0.001%; then terminating the flow of the purge gas and lowering the pressure above the copper bath to less than about 0.0002 atmosphere to volatilize the volatile impurity; deoxidizing the copper bath; and then casting the deoxidized copper bath.

2. The process as described in claim 1 wherein the metallic copper precipitate is cement copper which is upgraded by screening prior to being melted.

3. The process as described in claim 1 wherein the metallic copper precipitate is cement copper which is upgraded by magnetic separation prior to being melted.

4. The process as described in claim 1 wherein the metallic copper precipitate is cement copper which is upgraded by magnetic separation and screening and then mixed with a carbonaceous reductant prior to melting.

5. The process as described in claim 1 wherein the metallic copper precipitate is mixed with a carbonaceous reductant prior to melting.

6. The process as described in claim wherein the mixture of the metallic copper precipitate and the carbonaceous reductant is agglomerated before melting.

7. The process as described in claim 4 wherein the metallic copper precipitate is mixed with between about 0.5% and 5% of a carbonaceous reductant and the mixture is then agglomerated.

8. The process as described in claim 7 wherein the carbonaceous reductant is fuel oil.

9. The process as described in claim 1 wherein the copper bath is maintained at a temperature between 10 about l,250 and 1,400C. during desulfurization and impurity volatilization.

10. The process as described in claim 1 wherein the purge gas is at least one member selected from the group consisting of nitrogen, argon, air and oxygen.

11. The process as described in claim 1 wherein the purge gas is at least one member selected from the group consisting of nitrogen, argon, air and oxygen and is passed through the copper bath at a rate between about 1 and 20 standard cubic feet per hour per square foot of bath surface area.

12. The process as described in claim 1 wherein the copper bath after desulfurization contains between about 0.1% and 1.5% oxygen.

13. The process as described in claim 1 wherein additional oxygen is added to the bath after desulfurization to adjust the oxygen content to between about 0.1% and 1.5%.

14. The process as described in claim 1 wherein the copper bath is deoxidized by the addition of granular carbon and by purging with a non-oxidizing gas.

15. The process as described in claim 14 where deoxidation is conducted at subatmospheric pressures of less than about 0.01 atmosphere.

16. The process as described in claim 15 wherein the purge gas is passed through the copper bath at a rate of between about 1 and 20 standard cubic feet per hour per square foot of bath surface area.

17. The process as described in claim 1 wherein the copper bath is desulfurized at subatmospheric pressures of less than about 0.001 atmosphere.

Claims

2. The process as described in claim 1 wherein the metallic copper precipitate is cement copper which is upgraded by screening prior to being melted.
3. The process as described in claim 1 wherein the metallic copper precipitate is cement copper which is upgraded by magnetic separation prior to being melted.
4. The process as described in claim 1 wherein the metallic copper precipitate is cement copper which is upgraded by magnetic separation and screening and then mixed with a carbonaceous reductant prior to melting.
5. The process as described in claim 1 wherein the metallic copper precipitate is mixed with a carbonaceous reductant prior to melting.
6. The process as described in claim 5 wherein the mixture of the metallic copper precipitate and the carbonaceous reductant is agglomerated before melting.
7. The process as described in claim 4 wherein the metallic copper precipitate is mixed with between about 0.5% and 5% of a carbonaceous reductant and the mixture is then agglomerated.
8. The process as described in claim 7 wherein the carbonaceous reductant is fuel oil.
9. The process as described in claim 1 wherein the copper bath is maintained at a temperature between about 1,250* and 1,400*C. during desulfurization and impurity volatilization.
10. The process as described in claim 1 wherein the purge gas is at least one member selected from the group consisting of nitrogen, argon, air and oxygen.
11. The process as described in claim 1 wherein the purge gas is at least one member selected from the group consisting of nitrogen, argon, air and oxygen and is passed through the copper bath at a rate between about 1 and 20 standard cubic feet per hour per square foot of bath surface area.
12. The process as described in claim 1 wherein the copper bath after desulfurization contains between about 0.1% and 1.5% oxygen.
13. The process as described in claim 1 wherein additional oxygen is added to the bath after desulfurization to adjust the oxygen content to between about 0.1% and 1.5%.
14. The process as described in claim 1 wherein the copper bath is deoxidized by the addition of granular carbon and by purging with a non-oxidizing gas.
15. The process as described in claim 14 where deoxidation is conducted at subatmospheric pressures of less than about 0.01 atmosphere.
16. The process as described in claim 15 wherein the purge gas is passed through the copper bath at a rate of between about 1 and 20 standard cubic feet per hour per square foot of bath surface area.
17. The process as described in claim 1 wherein the copper bath is desulfurized at subatmospheric pressures of less than about 0.001 atmosphere.