US3529956A - Refining copper - Google Patents

Description

United States Patent 3,529,956 REFINING COPPER William Harold Foard, Miami, and Robert D. Lear, Tucson, Ariz., assignors to The Anaconda Company, New York, N.Y., a corporation of Montana No Drawing. Continuation-impart of application Ser. No. 468,959, July 1, 1965. This application June 3, 1969, Ser. No. 830,141

Int. Cl. C22b 15/14 US. Cl. 75-76 4 Claims ABSTRACT OF THE DISCLOSURE Molten copper containing excess oxygen is deoxidized in a refining furnace in which a hydrocarbon fuel is incompletely burned above the surface of the metal to form a reducing furnace atmosphere containing hydrogen, water vapor, carbon monoxide and carbon dioxide. Poling fluid comprising a hydrocarbon fuel is introduced into the molten copper below its surface, resulting in formation of carbon monoxide, hydrogen and water vapor by decomposition and/ or reaction of the poling fuel, accompanied by deoxidation of the copper. At the conclusion of the deoxidation treatment tough pitch copper is withdrawn and cast into suitable shapes. The rate of admission and total amount of poling fuel introduced is correlated with the amount of copper oxide in the molten metal and the amount of oxygen and fuel admitted above the charge to establish and maintain in the furnace atmosphere low ratios of H O/I-I and CO /CO neither of which should exceed 0.5 at the conclusion of the deoxidation treatment and which preferably are kept below 2.75 and 1.50, respectively, throughout the deoxidation treatment.

CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of our copending application Ser. No. 468,959, filed July 1, 1965, now abandoned.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to a fire refining process in which copper in a molten state is subjected to a sequential oxidation and deoxidation operation to remove impurities contained therein. More particularly, this invention relates to the use of a conventional gaseous or nongaseous fuel in the deoxidation step of the refining process to reduce the excess oxygen contained in the copper.

Description of the prior art In the fire refining of molten copper, unstable impurities are first removed by blowing air or oxygen through the molten metal. The blowing operation is customarily continued until the copper contains 0.6% to 0.9% oxygen at which time the impurities will have been substantially completely oxidized to products which will have been vaporized or have risen to the surface of the bath from 'Where they are removed by skimming. The excess oxygen in the form of copper oxide thus introduced into the molten copper must be removed before the copper can be cast into anodes for further refining in an electrolytic process, or into other shapes for commercial use. Conventionally, the unwanted oxygen in the copper is removed in a poling operation in which wooden poles are immersed in the bath of molten copper. The immersed poles decompose rapidly in the hot metal bath generating a large quantity of inert and reducing gases which cause violent agitation and simultaneous deoxidation of the molten copper. The poles used for the deoxidation may be hardwood or softwood and the wood consumption is about to pounds per ton of copper.

Many attempts have been made to replace comparatively expensive wooden poles with a more convenient and less expensive reducing agent for the poling operation. Natural gas, fuel oil, and other forms of hydrocarbon have been suggested as possible poling fuels. None of them, however, have been used with consistent success in commercial scale operations. In more recent attempts, natural gas and other low molecular Weight parafiinic hydrocarbons are reformed by partial oxidation to gaseous mixtures containing high percentages of hydrocarbon and carbon monoxide before they are used for the deoxidation process. The utilization of reformed gas represents an improvement over the prior gaseous poling process. Its use, however, fails to produce consistent results and the extent to which the molten copper can be deoxidized by reformed gas is also found to be limited.

SUMMARY OF THE INVENTION We have now found that natural gas and other gaseous as well as nongaseous hydrocarbons can be used advantageously without the necessity of using an external reforming process for the deoxidation of molten copper to yield consistently high-grade tough pitch copper products. Their use is made possible by the discovery that a reducing atmosphere over the molten copper, in which the partial pressures of reducing gases and nonreducing gases are such as to be in final equilibrium with the grade of copper desired, must be established in the furnace. More specifically, we have found that the composition of the furnace atmosphere immediately above the molten copper, which contains both potential oxidants (water vapor and carbon dioxide) and reductants (hydrogen and carbon monoxide) must be controlled during the deoxidation process so thatthe molar ratios of Hgo/Hz and 001 /00 therein are brought to predetermined values.

Accordingly, the process of this invention for deoxidizing molten copper containing copper oxide is carried out as follows: The copper to be deoxidized is maintained in the molten state in a furnace in which a hydrocarbon fuel is incompletely burned with air above the surface of the molten metal to form a reducing atmosphere comprising hydrogen, Water vapor, carbon monoxide and carbon dioxide in direct contact with the molten copper. A poling fluid comprising a hydrocarbon poling fuel is introduced into the molten copper below the surface thereof, causing ebullition of the molten metal and resulting in formation of carbon monoxide and hydrogen by decomposition of the poling fuel and reaction of it with at least a portion of the copper oxide in the molten copper, with accompanying deoxidation of the copper. When at the conclusion of the deoxidation treatment the molten metal has been brought to the tough pitch stage (corresponding to a residual oxygen content of about 0.02 to 0.04 percent) it is withdrawn from the furnace and cast into a desired shape.

It is characteristic of the invention that the rate of admission and total amount of poling fuel introduced is correlated with the amount of copper oxide present in the molten metal and the amount of oxygen and fuel ad mitted above the charge to establish in the furnace atmosphere in contact with the molten copper, at least near the end of the deoxidation treatment when the copper has been reduced to the tough pitch stage, molar ratios of H O/H and CO /CO less than 0.5. Preferably the furnace atmosphere in contact with the molten copper sub stantially throughout the time required for the deoxidation treatment is established and maintained with a molar ratio of H O/H less than 2.75 and a molar ratio of CO /CO less than 1.50 and these ratios are brought to less than 0.5 by the time the deoxidation treatment comes 3 to an end with reduction of the copper to the tough pitch stage.

It is desirable that the molar ratios of Hzo/Hz and CO /CO both be substantially below 0.5, say below 0.4 or even below 0.3, at the end of the deoxidation treatment, to insure that the residual oxygen content of the tough pitch copper is desirably low.

DESCRIPTION OF THE PREFERRED EMBODIMENT The hydrocarbon fuel which is incompletely burned with air above the molten copper generally is at least in part fuel introduced through conventional burners above the molten metal surface and burned for the purpose of of liberating heat to maintain the copper in the molten condition. It may also be (in whole or in part) excess of poling fuel which is introduced beneath the molten metal and burns when it rises to above the molten metal into contact with atmospheric oxygen. However supplied, it comprises both hydrogen and carbon and it is incompletely burned so that the combustion products contain both carbon monoxide and hydrogen as well as carbon dioxide and water vapor. Oxidation-reduction reactions occur between these constituents of the furnace atmosphere and the copper and copper oxide of the molten metal. Hence the amount of copper oxide present in the molten metal plus the amount of oxygen admitted for burning fuel above the molten metal must be correlated with the amount of poling fuel (and amount of fuel admitted directly above the molten metal) to establish the desired H O/H and CO /CO ratios.

It is to be noted that it is the ratios of water vapor to hydrogen and of carbon dioxide to carbon monoxide that is critical, and not the absolute concentrations of any of these substances or the relative proportion of hydrogen and carbon. Hence the composition of the poling fuel in terms of the relative proportion of hydrogen to carbon it contains is not critical. As a practical matter, all commercial fuels contain some amounts of both hydrogen and carbon, and most such fuels contain substantial amounts of both.

In the process of this invention, conventional fuels both mainly carbonaceous and hydrocarbons, in gas, liquid, and solid forms, can be used as the poling fuel and as the fuel burned above the molten metal. Among them, they include, for example, natural gas, diesel oil, and pulverized coal and coke. The solid and liquid poling fuels are advantageously introduced into the molten copper by a carrier gas which may be an inert gas, steam, or an active (carbonaceous or hydrocarbon) gas such as natural gas. The carrier gas and the liquid or solid fuel are thoroughly preblended before charging into the molten copper. When the fuel used is mainly carbonaceous, such as coal or coke, the carrier gas (e.g., steam or natural gas) supplies the hydrogen and water vapor. When an active gas which is capable of forming hydrogen, carbon and carbon monoxide, in situ, is used as the carrier gas, additional poling fuel may not be required.

The selection of any combination of poling fuels and carrier gases depends on operational variables and particularly on their availability at the plant site and their relative costs. In general, we find diesel oil and steam mixtures to be effective. Natural gas used alone or in combination with steam or further enriched with oil, and pulverized coal or coke and steam mixtures, are also found to be eminently suitable.

In carrying out the process of this invention, the poling is accomplished by blowing the thoroughly blended mixture of poling fuel and carrier gas into the bath of molten copper by tuyeres or pipes opening below the surface thereof until the copper is reduced to a suitable oxygen content. Sufiicient pressure is used to overcome the hydrostatic pressure of the copper. The amount of poling fuel is controlled so that the furnace atmosphere above the bath is maintained in a reducing condition having the required partial pressures of reductants and oxidants. In general, we find that the optimum deoxidation conditions cannot readily be attained by the poling mixtures alone. Such is best attained by controlled burning of fuel above the molten metal to establish reducing furnace atmosphere. A convenient control method is to control the amount of combustion air admitted at the furnace burner to bring about the required reducing conditions. Once the required atmospheric condition is established, the ebullition of molten copper enables the equilibrium between charge and atmosphere to be more quickly attained and hence substantially reduces the poling time.

The required furnace atmosphere for the successful deoxidation is governed principally by the complex equilibrium relationships which exist between the reducing atmosphere and the refining furnace charge. Predicting the precise effect of different furnace atmospheric conditions upon these relationships is not always possible, and the task is further complicated by other existing equilibrium relationships, such as those between oxidants and reductants in the furnace atmosphere between copper and copper oxide in the metal, and between impurities and the principal reactants contained in both the liquid and gaseous phases. Fortunately optimum atmospheric condition for reducing the copper to a tough pitch stage is readily found by controlling the admission of the poling fuel and combustion air to produce a reducing furnace atmosphere having the specified molar (or partial pressure) ratios of H O/H and CO /CO.

Under normal operation conditions in which the melt temperature is about 2000 F. to 2250 F., the molar ratios of H O/H and CO /CO in the reducing atmosphere must be less than about 0.5 at the final stage of the deoxidation in order to produce copper castings having sufliciently low oxygen content to meet the specification for tough pitch copper. Preferably, these ratios are maintained at about 0.40 for a flat set and 0.30 for a crown set copper. The suitable reducing atmosphere generally contains at least 20% by volume of CO and H and preferably above about 30%. The critical molar ratios are easily determined from gas composition measurements expressed as volume percent. The reducing atmosphere is maintained at a pressure between about 0.7 atmosphere to slightly above about one atmosphere.

Except in the final stage of the process, in which the molar ratios of oxidants and reductants must be as specified, above, the deoxidation can be carried out in a reducing atmosphere having higher ratios of H O/H and CO /CO than those stated. For best results, the ratios for H O/H should not exceed about 2.75 and for CO /CO should not exceed about 1.50 at any time throughout the deoxidation treatment, although a higher ratio of H O/H than CO /C0 can be tolerated especially near the beginning of the deoxidation treatment. Accordingly, the furnace burner and the rate of poling fuel injected into the molten bath should be adjusted to provide maximum attainable hydrogen concentration and minimum attainable carbon dioxide concentration in the reducing gas atmosphere above the molten metal. It is understood when higher molar ratios of H O/H and CO /CO are used during the early and intermediate stages of the deoxidation process, they must be brought down to the desired values near the end of such process. To allow the reduction to proceed at a faster rate, ratios lower than those specified may also be used. Maintaining such low ratios in the furnace, however, entails very tight control of the furnace burner and the rate and quantity of poling fuel addition.

The technique for adjusting a furnace burner to provide a reducing atmosphere is well-developed and hence need not be described. The adjustment, however, must be coordinated with the rate and amount of poling fluid addition in order to maintain the desired atmospheric conditions. With respect to the poling fluid, which may be a homogeneous gaseous hydrocarbon or a heterogeneous mixture of gas and solid or liquid, there is no definite parametric limitation on its feeding rate because of the wide selection of poling fuels and carrier gases that can be used. The optimum rate, therefore, must be separately and generally determined for each individual operation. Preferably, the rate should not be so high as to dominate the reducing atmosphere immediately above, for then the relatively easy control of the furnace atmosphere conditions that results from adjusting the furnace burner is not possible. On the other hand, if the rate is so low that the major reduction of the copper oxide must occur through reaction of the reducing atmosphere, the deoxidation may take too long to be commercially economical.

The same procedures as those used in conventional poling operations may be used to determine the extent the copper is deoxidized. In the conventional procedure, test buttons are taken regularly and examined to determine the extent of the reduction of the charge. When the test button contains the desired amount of oxygen, it will solidify upon cooling with the desired surface characteristic, such as a fiat wrinkled surface for fiat set copper or a slightly convex surface for crown set copper. When reduction has reached the end point, casting of the finished charge into copper anodes suitable for further electrolytic refining, or into other shapes such as wirebars, billets, or cakes can be carried out in the conventional manner.

The following examples will illustrate the practical applications of the process of this invention:

EXAMPLE I A ZOO-ton charge of copper contaminated with sulfur was prepared for poling by the standard practice of blowing with air until the charge was substantially free of sulfur and other oxidizable impurities and contained 0.6% to 0.9% oxygen. The vessel used was a 13 x 27' rotating refining furnace of conventional design. Hydrocarbon fuel supplied through the burners of the furnace was burned with a stochiometric deficiency of combustion air to produce a furnace atmosphere 2%5% combustibles. Four pipe lances were inserted into the molten charge to a depth of about one foot and a gaseous poling mixture was introduced into the charge at a pressure of from 20 to 30 p.s.i.g. The poling mixture comprised natural gas (analyzing at about 95% saturated aliphatic hydrocarbons) as a carrier gas into which diesel fuel had been atomized by means of a conventional carburetor device. Neither the natural gas nor the diesel fuel was preheated (a substantial commercial advantage). Fuel was delivered at a rate equal to 2 gallons of fuel per ton of copper per hour; natural gas at a rate of 8500 standard cubic feet per hour.

During lancing, the furnace atmosphere was analyzed and contained an average of 26% H 13% CO and 12% C (Percentages are by volume and the mol ratio of H O/H was about 0.5 and of co /co was about 1.0.) Precautions were taken to prevent excessive air seepage into the furnace. As reduction of the copper progressed, gas composition was altered to produce a more highly reducing atmosphere containing 30% H 14% CO and 6% CO so that as the deoxidation neared completion, the mol ratio of H O/H was about 0.35 and of CO /CO was about 0.43. After a total lancing time of 4 hours, the charge had been reduced to a crown set containing about 0.02% oxygen by weight.

EXAMPLE II A 200-ton charge was prepared as in Example I. Similar preheating procedures were adopted. Four lances were inserted into the charge and poling was commenced using steam as a carrier and diesel fuel as the poling fuel. Delivery was at the rate of 85 gallons of fuel per hour and 600 pounds of steam per hour. After 4 /2 hours, the charge had been reduced to a flat set analyzing at about 0.05% oxygen by weight.

It might be noted in passing that, in the case of both Examples I and II, the introduction of a gaseous combustible mixture into the charge caused vigorous ebullition of the charge with consequent continual splashing of copper into the furnace atmosphere.

During the procedure described in Example II, the volumetric ratio of H 0 to H was at no time permitted to exceed 2.75 nor was the ratio of CO to CO permitted to exceed 1.5. During the final stages of fire refining, the reducing character of the furnace atmosphere was altered so that the volumetric ratio of H 0 to H was less than 0.4 and the volumetric ratio of CO to CO was near or below 0.4.

EXAMPLE III The furnace of Example I, having been modified by installing a tuyere system, was charged with 200 tons of sulfur-contaminated blister copper and was prepared for poling as in Example I. The furnace atmosphere was adjusted to contain 25% combustibles. Natural gas, at ambient temperature, was blown through the tuyeres after rotating the furnace to submerge the tuyere openings below the surface of the molten copper. Flow rate of the gas was 50,000 s.c.f.h. After 2% hours the charge was reduced to a crown set analyzing at about 0.02% oxygen by weight. The volumetric ratio of CO to CO was at no time permitted to exceed 1.0; the ratio of H 0 to H was not permitted to exceed 2.75, and these ratios at the end of the poling period were both below 0.3. The sum of CO+H in the furnace atmosphere exceeded 25% by volume at all times. Average gas analysis: 25% H 13% CO, 13% CO with final analysis: 32% H 14% CO, 3% CO EXAMPLE IV The furnace of Example III was charged and readied for poling as in Example I. The furnace atmosphere was adjusted to contain 25% combustibles. Steam as the carrier and diesel oil as poling fuel were blown through the tuyeres into the molten copper. The flow rate of the steam Was 1850 lbs./hr.; the flow rate of the diesel fuel was 210 g.p.h. After 2% hours the charge was reduced to a crown set analyzing at about 0.02% oxygen by weight. The ratios H O/H and CO /CO were not permitted to exceed the values 2.75 and 1.0, respectively, nor was the H +CO content allowed to fall below 25%. Average analysis: 27% H 14% CO, 12% CO Final analysis: 30% H 13% CO, 5% CO In the final stages of fire refining, the volumetric ratios of H 0 and H and CO to CO were not permitted to exceed 0.3.

We claim:

1. A process for deoxidizing molten copper containing copper oxide to produce tough pitch copper which comprises:

(a) maintaining the copper molten in a furnace (1) in which a hydrocarbon fuel is incompletely burned with air above the surface of the molten copper to form a reducing atmosphere comprising hydrogen, water vapor, carbon monoxide and carbon dioxide in direct contact with the molten copper,

(2) and in which a poling fluid comprising a hydrocarbon poling fuel is introduced into the molten copper below the surface thereof causing ebullition of said molten copper and resulting in formation of carbon monoxide and hydrogen with accompanying deoxidation of the copper,

(b) and withdrawing molten tough pitch copper from the furnace at the conclusion of the deoxidation treatment,

(0) characterized in that the rate of admission and total amount of poling fuel introduced is correlated with the amount of copper oxide present in the molten copper and the amount of oxygen and fuel admitted above the charge to establish in the atmosphere in contact with the molten copper, at least near and at the end of the deoxidation treatment when the copper has been reduced to the tough pitch stage, molar ratios of H2O/H2 and CO /CO less than 0.5.

2. A process for deoxidizing molten copper containing copper oxide to produce tough pitch copper which comprises:

(a) maintaining the copper molten in a furnace (1) in which a hydrocarbon fuel is incompletely burned with air above the surface of the molten copper to form a reducing atmosphere comprising hydrogen, water vapor, carbon monoxide and carbon dioxide in direct contact with the molten copper,

(2) and in which a poling fluid comprising a hydrocarbon poling fuel is introduced into the molten copper below the surface thereof causing ebullition of said molten copper and resulting in formation of carbon monoxide and hydrogen with accompanying deoxidation of the copper,

(b) and withdrawing molten tough pitch copper from the furnace at the conclusion of the deoxidation treatment,

(0) characterized in that the rate of admission and total amount of poling fuel introduced is correlated with the amount of copper oxide present in the molten copper and the amount of oxygen and fuel admitted above the charge to establish and maintain in the atmosphere in contact with the molten copper throughout the deoxidation treatment a molar ratio of H O/H less than 2.75 and a molar ratio of 8 CO /CO less than 1.50, and to bring both such ratios to less than 1.5 at the end of the deoxidation treatment when the copper has been reduced to the tough pitch stage.

3. The process according to claim 2 wherein the molar ratios of H O/H and CO /CO are both less than 0.4 at the end of the deoxidation treatment.

4. The process according to claim 2 wherein the molten copper is maintained during deoxidation at a temperature between 2000" F. and 2250 F., and the molar ratios of H O/H and CO /CO are both less than 0.3 at the end of the deoxidation treatment.

References Cited UNITED STATES PATENTS 57,969 9/1866 Reese 26625 X 433,086 7/1880 Keys 7576 1,958,754 5/1934 Holley 7576 X 2,989,397 6/1961 Kuzell 7576 3,258,330 6/1966 Ito 7575 L. DEWAYNE RUTLEDGE, Primary Examiner J. E. LEGRU, Assistant Examiner US. Cl. X.R. 75-75, 93