US3434825A - Process for purifying copper base alloys - Google Patents

Description

United States Patent Oflice 3 ,434,825 Patented Mar. 25, 1969 US. CI. 7576 8 Claims ABSTRACT OF THE DISCLOSURE A process for removing both metallic impurities and gaseous impurities from copper base alloys characterized by bubbling a gaseous monomeric, halogen-containing, lower aliphatic hydrocarbon containing at least one fluorine atom through a molten copper base alloy.

This application is a continuation-in-part of co-pending application Ser. No. 312,898, filed Oct. 1, 1963, by J. E. Dore, now US. Patent 3,282,680.

The present invention relates to a new and improved process for purifying copper or copper base alloys, hereinafter referred to as copper base alloys.

It is highly desirable to develop effective means for removing both metallic impurities and gaseous impurities from copper base alloys. For example, a number of the major difficulties encountered in the fabrication of copper base alloy strip, rod or tubing can be directly traced to defects in the starting process ingot produced by evolution of gases from the melt during solidification. Gas filled voids created in this manner prevail through the rolling and thermal treatment operation and eventually produce such gross defects as center plane separation or blistering in the finished copper or brass strip. In a similar manner, the formation of gases during solidification can cause undesirable defects and voids in copper base alloy castings.

In order to minimize gaseous unsoundness in copper base alloys, it is necessary to reduce the hydrogen content of the melt close to or below the equilibrium solid solubility level at the melting point or solidus temperature and either substantially eliminate oxygen or the gas formers such as carbon and cuprous sulfide from the melt. Unfortunately, presently used degassing practices cannot accomplish this elfectively. These practices generally involve an oxidation reduction type treatment of the melt. The melt is first oxidized by either percolating dry airthrough it or by the addition of a glassy flux containing an oxidizing agent, such as manganese dioxide or cuprous oxide. This serves to reduce the hydrogen content of the melt to the desired level, by the reaction:

Subsequently, the melt is deoxidized using materials such as metallic phosphorus and lithium or calcium boride to reduce the oxygen content to a satisfactory level. The oxidation-reduction degassing method is very time consuming and can only be used with reasonable success on virgin metal charges. In scrap charges, oxidation must be carried out until all residual reducing elements, e.g., tin and phosphorus, are burned out before the melt can be effectively oxidized. This results in the loss of expensive alloying additions, such as tin, from the melt. Also, this method is not always elfective as evidenced by fairly frequent epidemics of blistering and center plane separation in finished strip. A further problem is that the procedure cannot be used in the production of low oxygen copper of high electrical conductivity since the addition of either oxidizing (e.g., manganese dioxide) or reducing agents (e.g., phosphorus) will substantially reduce the electrical conductivity of this alloy. Further, the presence of cuprous oxide renders the alloy susceptible to hydrogen embrittlement.

Furthermore, the existing method of producing oxygen free copper is both complex and costly. In the existing method, the starting material is select grade high purity cathode copper. All subsequent melting and casting processes must be conducted under a gas cover usually consisting of carbon monoxide and nitrogen. The sole function of this gas cover is to prevent oxygen and hydrogen pickup during processing, since this mixed gas cover cannot effectively reduce either the oxygen or hydrogen level in a contaminated system. Thus, it can be seen that the raw material requirements are very rigid and the melting and casting processes must be exacting.

It is also desirable to develop an effective method for removing undesirable metallic impurities from copper base alloys. At the present time, great care must be exercised in the choice of the starting raw materials used in copper and copper alloy remelt operations. Selected grades of virgin copper, alloying elements and scrap must be used to avoid contaminating the melt with unwanted impurity elements that will adversely affect physical and/ or mechanical proper-ties of the final cast or wrought products. Depending on. the particular type of copper or copper alloy being prepared, all of the following elements can cause problems: lead; zinc; iron; chromium; aluminum; silicon; tin; phosphorus; antimony; and bismuth.

Several examples are:

(1) Lead in relatively small concentrations in certain alloys produces hot shortness and drastically reduces both strength and ductility.

(2) Small amounts of iron, aluminum, silicon and phosphorus reduce the electrical conductivity of copper.

(3) Iron and silicon undesirably alter the recrystallization and annealing characteristics of copper zinc alloys.

These are only a few of many problems caused by undesirable metallic impurities in copper and copper alloys.

As evidenced from above, the regulation of metallic impurities in solution in copper alloy melts is extremely important in the cast shop. Unfortunately, no effective methods of controlling or removing impurities once they enter the melt are presently available. The conventional oxidation-reduction procedure used in fire refining is of limited value for the following reasons:

(1) Excessively long treatment times are required, thus, severely limiting productivity. The economic implications here are obvious.

(2) Either unwanted elements such as phosphorus or lithium must be added to deoxidize the melt after the oxidation step, or the archaic poling technique or carbonaceous melt covers must be utilized.

(3) Lead cannot be removed.

As a consequence, impurity control is limited to the selective choice of raw materials as outlined above.

Accordingly, it is the principal object of the present invention to provide an improved process for purifying copper base alloys.

It is a further object of the present invention to provide a simple, convenient and effective method for degassing copper base alloys.

It is a still further object of the present invention to provide a method as above which overcome the difliculties heretofore encountered, which does not result in the loss of expensive alloying additions from the melt, which is effective as evidenced by the lack of blistering in the finished strip and which is otherwise convenient and effective.

It is an additional object of the present invention to provide an inexpensive and simple method for preparing low oxygen copper of high electrical conductivity.

It is a still further object of the present invention to provide a method as aforesaid capable of conveniently removing undesirable metallic impurities.

Further objects and advantages of the present invention will appear hereinafter.

In accordance with the process of the present invention it has now been found that the foregoing objects and ad vantages may be readily accomplished and an improved process for purifying copper base alloys may be provided by providing molten copper base alloy; providing a monomeric, halogen-containing, lower aliphatic hydrocarbon containing at least one fluorine atom; adding said hydrocarbon to said molten alloy, thereby removing impurities from said alloy.

In accordance with the simple and convenient process described above, it has now been found that the aforementioned disadvantages of the art are readily overcome.

The process of the present invention attains numerous advantages over the prior art. Some of the more important of these include the fact that only one operation is required to remove both metallic impurities and gaseous impurities from the molten alloy. In addition, the reaction proceeds rapidly; therefore, the total time required to treat the melt is relatively short. Furthermore, melts prepared from both virgin metal and scrap metal charges can be effectively treated in accordance with the process of the present invention. Alloys used in applications where electrical conductivity properties of the alloy are important can be adequately and effectively degassed in the molten state in accordance with the process of the present invention without contamination of the melt. Heretofore, this has not been possible since degassing practices require the use of reducing agents, such as phosphorus and lithium, which remain as an alloying ingredient and thus adversely affecting the electrical conductivity properties, or utilize the ineffective poling or carbonaceous melt cover. In fact, in accordance with the present invention, electrical conductivities in excess of 100% IACS may be obtained in low oxygen coppers.

Furthermore, in accordance with the present invention it is possible to obtain a material with high ductility, high impact strength, good weldability, freedom from hydrogen embrittlement, and low volatility under high vacuum. In addition, almost any copper of moderate purity may be used.

An additional and significant advantage of the present invention is the provision of a new and unique method for removing undesirable metallic impurities from copper base alloy melts in the cast shop. For example, the present invention enables the following significant advantages:

(1) The ability to produce high quality cast and wrought products exclusively from low grade scrap. This will result in a significant reduction in production costs.

(2) The ability to beneficiate plant generated and/or purchased scrap by removing trace metallic impurities and also hydrogen and oxygen.

(3) The ability to produce a material with improved mechanical properties by removing those impurities which reduce strength and ductility.

(4) The ability to increase electrical conductivity by improving the purity level of the treated metal.

(5) The ability to reduce plant generated scrap by maintaining a positive control over those elements known to have adverse effects on the cast or wrought product.

(6) The ability to improve overall quality by maintaining a consistent purity level in the finished product.

In accordance with the process of the present invention, the halogen-containing hydrocarbon is added to the molten copper base alloy. The particular materials which can be used are the monomeric, halogen-containing, lower aliphatic hydrocarbons, both cyclic and open chain, containing at least one fluorine atom, particularly the saturated hydrocarbons, for example: dichlorodifluoromethane; trichlorofiuoromethane; trifluorochloromethane; trifiuorochloroethane; pentafiuorochloroethane; tetrafluorodichloroethane; octafluorobutane; trifiuorobromomethane; difluorochloromethane; trifluoro methane; difiuoroethane; chlorotrifluoroethylene, etc.

Due to convenience of introduction into the melt, it is preferred to use those materials which are gases at room temperature, although solids may also be used.

The hydrocarbons are preferably saturated and should contain at least one fluorine atom, with the balance of the molecule substituted with, for example: chlorine; bromine; and hydrogen. It is preferred to use fully halogenated hydrocarbons since species containing hydrogen may have adverse effects on the treated metal.

In accordance with the present invention, the hydrocarbon is added to the molten copper base alloy. Naturally, the temperature of the melt will vary depending on the particular alloy.

Several alternate methods of treating the melt may, of course, be used. As stated above, it is preferred to use a hydrocarbon which is a gas at room temperature. The hydrocarbon may be used in its pure form or diluted with an inert gas, with the inert gas-hydrocarbon mixture containing as little as 0.1% by volume of hydrocarbon. The mixture with small amounts of hydrocarbon is particularly useful as a. melt cover in furnaces and transfer launders, etc.

The copper base alloy charge, for example, copper scrap, tough pitch copper, etc., may be melted in an induction furnace in either an inert or chemically active (graphite or charcoal cover) environment.

The hydrocarbon or hydrocarbon-inert gas mixture is then preferably bubbled through the melt through suitable fluxing wands. As the halogenated hydrocarbon passes into the melt, it thermally decomposes. The fluorine preferentially reacts with impurity elements in solution in the melt, such as aluminum, iron, chromium, etc., to form insoluble fluorides which separate from the melt by gravity.

In addition, the decomposed hydrocarbon will react with available cuprous oxide in liquid solution in the alloy to form insoluble reaction products separable from the melt by gravity. In accordance with the present invention, by reducing the oxygen content of the melt to a sufliciently low level, the formation of reaction gases (C0, C0 S0 and H 0) can be prevented even when substantial quantities of gas formers such as carbon, cuprous sulfide and hydrogen are present. It should be noted that hydrogen by itself is a cause of gas porosity in the casting. Degassing agents of this type will also minimize the occurrence of hydrogen porosity. By using an excess of the agent and allowing the vapors to percolate through the melt, hydrogen will diffuse to, disorb in the vapor bubbles, and be swept from the melt. In other words, for hydrogen removal it is essential that the reagent be in the gaseous state in the melt, and preferably also that one of the reaction products be in the gaseous state in the melt.

The duration of the treatment of the present invention is dictated by the impurity levels of the initial charge, the method of introduction and the composition of the gas. The rate of introducing the hydrocarbon is not critical. Similarly, the method used to introduce the material can be widely varied.

After the desired level of purity has been obtained, the treated metal is transferred to a casting station for casting or is alloyed in the furnace before casting. The treated metal should be transferred to the casting station in a closed launder. Recontamination during both transfer and ingot casting should be prevented, for example, by the use of a protective cover of a mixture of dichlo rodifluorornethane and an inert gas, such as nitrogen.

Naturally, the processof the present invention is readily amenable to batch or continuous operation, for example, continuous treatment, such as that obtained by using a separate degassing chamber in flow sequence from the furnace to the casting site where there is maintained a protective atmosphere of excess hydrocarbon or decomposed hydrocarbon and reaction products.

In accordance with the present invention, numerous metallic impurities may 'be readily removed from copper, for example, zinc, iron, chromium, aluminum, silicon, boron, lead, tin, phosphorus, and bismuth. 'In the case of alloys applicability of the process is dependent on the major alloying elements present and the specific impurities one desires to remove. For example, numerous impurities can be removed from cupronickcl alloy melts since nickel, the major alloying element, is not alfected by this treatment. Similarly, numerous impurities may be removed from silver bearing copper since silver is not affected by the treatment. Also, Zinc, iron, chromium, aluminum, silicon, boron and lead can be removed from copper-tin alloy melts since the removal efficiency of tin is substantially lower than the elements listed. On the other hand, the processis not directly applicable to copper-zinc, copper-aluminum, copper-chromium, and copper-silicon alloy melts, since the major alloying elements in these melts are preferentially removed by the treatment.

In the case of alloys that cannot be treated directly because of compositional considerations, alternate treatment routes may be employed. For example, with copper-zinc or copper-aluminum alloys, the following treatment scheme may 'be used.

(1) Melt down the copper portion of the charge in the furnace and treat same to remove undesirable impurities.

(2) Add the Zinc or aluminum to obtain the desired alloy composition and cast.

The present invention and the improvements resulting therefrom will be more readily apparent from a consideration of the following illustrative examples.

Example I A 12 pound charge of electrolytic, tough pitch copper was melted down in a silicon carbide crucible under an argon cover and brought to a temperature of 2200 F. The starting oxygen content of the melt was 196 parts per million of oxygen. The melt was then treated with gaseous dichlorodifluoromcthane by bubbling through the melt for 30 minutes at a flow rate of 0.80 s.c.f.h. (standard cubic feet per hour). Immediately after treatment the oxygen content was two parts per million of oxygen.

Example II A 15 pound charge of oxygen free copper was melted down in a silicon carbide crucible in an argon environment and brought to a temperature of about 2200 F. The charge was saturated with hydrogen to a level of 3.4 parts per million. The melt was treated with dichlorodifiuoromethane in a manner after Example I by bubbling the gaseous material through the melt for about minutes at a flow rate of 1.2 s.c.f.h. Immediately after treatment the hydrogen level was reduced to 0.2 part per million.

Example 111 A 12 pound charge oxygen free copper was melted down in an argon environment as in Example II and doped with silicon to a level of 0.115%. The melt was treated with gaseous dichlorodifiuoromethane in a manner after Example II for one hour at a flow rate one 1) s.c.f.h. Immediately after treatment the silicon level was reduced to 0.005%.

Example IV A 12 pound charge of electrolytic, tough pitch copper was melted down as in Example I. The starting oxygen content of the melt was 274 parts per million of oxygen and the starting hydrogen level was about 1 p.p.m. The starting conductivity was 101% LACS. The melt was treated with dichlorodifluoromethane in a manner after Example I for one hour at a flow rate of one (1) s.c.f.h. Immediately after treatment the following results were obtained: oxygen level 4.5 parts per million; hydrogen level 0.1 part per million; electrical conductivity, 102.0% IACS.

Example V In a manner after Example III 12 pound copper melts containing various levels of metallic impurities were treated with gaseous dichlorodifluoromethane by bubbling through the melts for 60 minutes at flow rates of one (1) s.c.f.h. In all cases the level of metallic impurities was sharply reduced. The following table shows the metallic impurity, the percent starting level and the percent final level after treatment and the percent removal efficiency. The table also shows hydrogen removal.

A 12 pound charge of tough pitch copper containing 1.1% zinc was melted in a silicon carbide crucible under an argon atmosphere and the temperature adjusted to between 2150 F. and 2200 F. Dichlorodifluoromethane was then bubbled through the melt with a fluxing wand for two (2) hours at a flow rate of 1 s.c.f.h. A final zinc level of 22 p.p.m. (0.0022%) was achieved.

Example VII A 15 pound charge of oxygen free copper containing 0.10% aluminum was melted in a silicon carbide crucible under an argon atmosphere and the temperature adjusted to between 2150 F. and 2200 F. Dichlorodifluoromethane was then bubbled through the melt with a fluxing wand for one (1) hour at a flow rate of 1.0 s.c.f.h. A final aluminum level of 14 p.p.m. (0.0014%) was achieved.

Example VIII A 14 pound charge of oxygen free copper containing 0.10% aluminum and 0.097% iron wasmelted in a silicon carbide crucible under an argon atmosphere and the temperature adjusted to between 2150 F. and 2200 F. Dichlorodifluoromethane was then bubbled through the melt with a fluxing wand for one (1) hour at a flow rate of one (1) s.c.f.h. Final aluminum and iron levels obtained were as follows: aluminum, 10 p.p.m. (0.001%), iron, 40 p.p.m. (0.004%

Example IX A 13 pound charge of oxygen free copper was melted in a silicon carbide crucible under an argon cover and then doped with zinc to a level of 0.13 Monochloropentafluoroethane was then bubbled through the melt with a fluxing wand for one (1) hour at a flow rate of one (1) s.c.f.h. Melt samples taken directly after the treatment were analyzed for zinc. These samples showed that a final level of 0.001% zinc resulted from this treatment.

Example X An 11 pound charge of commercial purity, electrolytic tough pitch copper containing 274 p.p.m. oxygen Was melted in a high purity alumina crucible under an argon cover. The melt was treated by bubbling dichlorodifluoromethane through the molten metal for one (1) hour at a flow rate of one (1) s.c.f.h. The melt was then allowed to solidify in the crucible under argon. When the charge reached room temperature, it was taken from the crucible and sectioned; a bar 3%" x 1" x W being taken from the center of the solidified charge. A bar of similar dimensions was then cut from the same electrolytic tough pitch copper bar which was used for the charge but which had not been treated. Both bars were then cold rolled to a thickness of 0.5" at which point the untreated bar began to crack, demonstrating that an increase in ductility had resulted from this treatment. Chemical analysis of the treated bar showed an oxygen level of 5 p.p.rn. and a hydrogen level of 0.1 ppm. The change in impurity level was measured by the change in electrical resistivity, the electrical resistivity of the untreated bar being 101% IACS while the electrical resistivity of the treated specimen had increased to 102% IACS indicating that there had been an overall reduction in the impurity level of the treated charge.

Example XI Example I was repeated using the following hydrocarbons: octafluorobutane, and chlorotrifiuoroethylene.

Similar reductions in oxygen levels were obtained.

This invention may be embodied in other forms or carried out in other ways without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered as in all respects illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and all changes which come within the meaning and range of equivalency are intended to be embraced therein.

What is claimed is:

1. A process for purifying copper base alloys which comprises: providing molten copper base alloy; providing gaseous monomeric, halogen-containing, lower aliphatic hydrocarbon containing at least one fluorine atom; bubbling said hydrocarbon through said alloy, thereby thermally decomposing said hydrocarbon; forming insoluble reaction products in the melt separable from the melt by gravity; and removing impurities from said alloy.

2. A process according to claim 1 wherein said hydrocarbon is fully halogenated.

3. A process according to claim 1 wherein said hydrocarbon is admixed with an inert gas, said hydrocarboninert gas mixture containing at least 0.1% of said hydrocarbon.

4. A process according to claim 2 wherein said hydrocarbon is dichlorodifluoromethane.

5. A process according to claim 2 wherein said hydrocarbon is monochloropentafluoroethane.

6. A process according to claim 2 wherein said hydrocarbon is octafluorobutane.

7. A process according to claim 2 wherein said hydrocarbon is chlorotrifluoroethylene.

8. A process according to claim 1 wherein said hydrocarbon is saturated.

References Cited UNITED STATES PATENTS 1,998,467 4/1935 Stroup -93 XR 2,965,477 12/1960 Kondic et a1. 75-53 X 3,282,680 11/1966 Dore 7576 FOREIGN PATENTS 603,213 7/1945 Great Britain.

L. DEWAYNE RUTLEDGE, Primary Examiner.

J. E. LEGRU, Assistant Examiner.