1 Traditional processing technology of copper rod

Traditional copper rod used to make electric wire and cable is cathode copper with copper content equal to or more than 99.95% being cast into tough pitch copper (ETP copper) wire bar and then receiving hot rolling.

1.1 Traditional Manufacturing technique of tough pitch copper wire bar

Manufacturing process: Melting in reverberatory furnace — oxidation and drossing — poling (reduction) — casting of wire bar.

① Melting

Melting in reverberatory furnace.

In addition to cathode copper, a few amount of scrap wire can be added to furnace charge.

Furnace capacity: 10 t ~ 300 t

Fuel: Coal gas, natural gas or fuel oil

Smelting and casting cycle: about 17h for a 50t furnace

② oxidation and drossing

After melting of furnace charges, compressed air will be blown into melted copper for oxidation. After air blowing oxidation, sulfur and such impurities as arsenic and antimony in melted copper will become volatile oxidizing gas and be carried away together with furnace gas. Oxides of other impurities and certain amount of cuprous oxide (CuO) will become slags, float on the surface of melted copper and then be removed outside the furnace. During the whole process of oxidation, slags will be removed successively until there is no sticky slag formed on the surface of melted copper and only a layer of CuO is left. Then, oxidation can be considered to be finished.

After oxidation, copper sample is taken to observe if there are minute bubbles generated due to sulfur dioxide separated. Copper sample is fragile, the section of such samples is in brick red, and the crystal is coarse and loose without luster. By this moment, melted copper has a copper oxide (Cu₂O) content of 6% ~ 10%.

During oxidation, the quantity of Cu₂O in melted copper is related to temperature. The higher the temperature, the more the Cu₂O. Thus, the temperature for oxidation should not be excessively high and generally is controlled between 1,070°C and 1,090°C.

③ Poling (reduction)

After completely removal of slags in the pool of melted copper, dry charcoal will be used to cover the surface of melted copper. Then, freshly green wooden pole will be inserted into the melted copper for reduction.

The pole will receive destructive distillation under the high temperature of the melted copper and release such gases as H₂O, CO₂, H₂ and other carbohydrates. Such gases will generate an intensive agitation of the melted copper. In addition, these reducing gases will gradually reduce copper oxide (Cu₂O) to metallic copper. Poling is also useful to remove oxygen in the melted copper and then make electrolytic tough pitch (ETP) copper into high-conductivity oxygen-free (HCOF) copper.

During the reduction process, samples should be taken continuously. When the reduction level is sufficiently high, copper samples will be difficult to be broken. If they are broken, the section should be in rose red, with radial-shape fine crystal which has silky luster. At this time, copper will reach its tenacious point, so the copper is called tough pitch copper.

The tough pitch copper has an oxygen content of about 300 ~ 500 ppm.

In case of poling, properly low temperature should be kept, so as to shorten reduction duration. If poling is done excessively, excessive reducing gases (H₂ and CO) will be dissolved in the melted copper and be separated out in large quantities during solidification of the melted copper. This will not only reduce the electric conductivity of copper wire bar cast and embrittle the wire bar, but also generate a large number of air pores, even make the surface of the wire bar be full of such pore and difficult to be used.

④ Casting of wire bar

During casting of wire bar of tough pitch copper, oxygen-enriched bubble layer, 10 ~ 12 mm thick, will be formed on the surface of copper bar. This fragile and loose layer, in case of wire bar rolling into a copper rod, will be made thin through rolling and be wrapped around the copper rod. A small part of this layer will be sandwiched into the copper rod. Copper wires drawn with this type of rod have a surface not smooth and easy to chip and be broken. This rod is difficult to draw fine wire. Obviously, this is the disadvantage of traditional technique used to manufacture copper rod.

The thickness of the oxygen-enriched bubble layer is related to casting temperature. The higher the casting temperature, the thicker the bubble layer. In terms of actual casting, the temperature of melted copper at the casting head is better to be controlled between 1,100°C and 1,120°C.

In addition, there exists relatively large Cu₂O eutectic particles (about 100μm) on the α Cu/ Cu₂O eutectic surface. Such particles are very unfavorable for drawing fine wires and are difficult to be removed even with planing of copper bar and descaling of copper rod.

1.2 Traditional rolling technology of copper rod

Before rolling, copper wire bar, the copper wire bar will be heated to about 900°C in a reheating furnace and then fed into a close-line rolling mill for three hot rollings (rough milling, intermediate rolling and finish rolling) with 14 holes of different patterns. Finally, Ф7.2mm black copper rods are produced.

Then, such copper rods will receive acid pickling and cold draw to become copper wires.

Due to defects of wire bar of tough pitch copper, it is inevitable that wrinkles, rolled edges and oxide skins will appear on the copper rod manufactured with close-line rolling mill.

Traditional manufacturing technique of high-conductivity oxygen-free copper bars

With a “deaerator” (a special closed container lined with refractory materials and full of high-quality charcoals) for deoxidation, melted copper having oxygen content of 300 ~ 500 ppm and copper content of over 99.95% can also be used to produce high-conductivity oxygen-free copper. The specific procedures are as follows:

Via a launder covered by charcoals, the melted copper at the temperature of 1,150°C continuously flows into the “deaerator” for deoxidation. After the liquid copper is injected onto the surface of charcoals in the “deaerator”, residual oxygen will be removed:

C + Cu₂O        2 Cu + CO
CO  + Cu₂O        2 Cu + CO2

After the melted copper flows out from the “deaerator”, the oxygen in it can be completely removed. Oxygen-free liquid copper then enters a closed holding furnace via a launder. Both the launder and the furnace are filled with protective gases: CO 27%, CO₂ 0.5% and the rest is N₂. Protective gas is not allowed to contain H₂, H₂O and other carbohydrates.

Modern continuous manufacturing process of ETP copper rod

Copper rod manufactured with traditional technique has an oxygen content of 300 ~ 500 ppm. The oxygen is distributed outside 90% section of cast wire bar, while the rest 10% section (the upper surface of the cast wire bar, oxygen-enriched layer, 10 ~ 12mm thick, ) also absorbs oxygen of corresponding proportion. Thus, only through surface planing, copper bars manufactured with traditional technique can be used to manufacture copper rods having an oxygen content of 300 ~ 500 ppm.

A feature of modern continuous melting, casting and rolling technique of ETP copper rods is that through continuously inclined or vertical (up-casting) casting, cast wire bar will not have oxygen-enriched layer or bubble layer. Copper rods manufactured with this continuous manufacturing technique should still be ETP copper rods (with oxygen content of 250 ~ 500 ppm). Electrical property and physical and mechanical properties of this kind of copper rod are almost the same with those of oxygen-free copper (with oxygen content being less than 20 ppm), so it is wrong to call it low-oxygen copper rod.

In fact, no matter with new technique or traditional technique, “low-oxygen copper rod” (having an oxygen content of 100 ~ 250 ppm) is not manufactured internationally. The reason is as follows: Even with Grade A cathode copper having a copper content of 99.95% as original material, the cathode copper when it is reduced to have an oxygen content of 100 ~ 250ppm, will also absorb a great quantity of H₂ if without protection by protective gas, and then copper wire bar cast will have a lower electric conductivity.

2.1 Melting of cathode copper in shaft furnace

In the manufacturing system of continuously cast and rolled copper rod, the countries in the world have shaft furnaces adopted to melt copper, so as to realize continuous melting (redox in reverberatory furnace is canceled). If sulfur-free (desulfurated) natural gas or liquefied petroleum gas is directly sprayed in the shaft furnace, thermal efficiency can be increased to 65%, with less pollution caused by external impurities and less time to start up the furnace (about 30 minutes for black furnace and 5 minutes for heated furnace). Besides, there is no melted copper in the furnace, so only 1 minute is needed to blowout the furnace.

Cathode copper or waste copper material (with a copper content exceeding 99.9%) is lifted by charging equipment and poured into charge door. After heating and melting, the copper materials glide down along the slideway in the shaft furnace and then will be heated by risen combustion gas ejected by several rows of burner nozzles close to the bottom of the furnace. Fuel-air ratio will be automatically controlled to maintain reducing atmosphere in the furnace.

Since the furnace is only used to melt rather than refining, the fuel is required to be free of sulfur or to have a low sulfur content.

2.2 Transfer system of melted copper from melting furnace to casting machine

The transfer system runs as follows: Flowing out from shaft furnace - closed launder - slag separating tank- to fore-hearth launder - fore-hearth - launder (oxygen control)- slag separating tank (oxygen control)- place for pouring- casting machine.

Each launder should be as closed as possible, so as to avoid severe oxygen absorption and temperature decrease and prevent impurities from entering the launder.

After start-up of the system, if it is found that the oxygen content in the melted copper is excessively high, dry charcoals can be put onto melted copper at the two slag separating tanks and the place for pouring , and burner can be adjusted to obtain more reducing atmosphere (generally 15 ~ 20 heating burners are mounted throughout the transfer system). In case of normal conditions, most burners are closed.

Quality control of melted copper during melting and transfer

① Control of oxygen content in melted copper

Main influence factors of oxygen content in melted copper:

a Charging and proportioning (such scrap returns as head of wire bar and fertilizer generally will not influence oxygen content);

b Flowing status of copper materials in shaft furnace (blockage of furnace charges will lead to increase of oxygen content);

c Atmosphere adjustment in burner (proper adjustment of fuel-air ratio in burner);

d Situation of melted copper in transfer system (ETP copper rod is required to have an oxygen content of 250 ~ 500 ppm, so oxygen content of melted copper flowing out from shaft furnace should be 100 ~ 200 ppm. Thus, such melted copper will gradually absorb oxygen during transferring process and the final oxygen content will not exceed 500 ppm.

② Control of sulfur content in melted copper

Fuel is desulfurated gas, so it will not influence sulfur content.

Sulfur content of melted copper is the same with that of cathode copper plate. In fact, sulfur in cathode copper plate has already been removed during melting, so the melted copper has a very low sulfur content and thus its quality will not be impaired. (Attention shall be paid to eliminating raised copper particles on the cathode copper plate.)

③ Control of hydrogen content in melted copper

On the basis of given oxygen content and certain melting temperature, the minimum hydrogen content (air content) can be realized through adjusting burner and controlling the transfer system. Under normal conditions, because the oxygen content of wire bars exceeds 500 ppm, some parts of the bars are inevitable to be cut off and sent back to the furnace. By this time, flame atmosphere of burner should be adjusted and dry charcoals should be put onto the two slag separating tanks. Generally, after cutting 1 ~ 2t wire bars, the compositions of copper wire bars will be proper.

2.4 Casting system of melted copper (omitted)

2.5 Rolling of copper rod (omitted)

2.6 Characteristics of continuously cast and rolled ETP copper rod

① Continuously cast and rolled ETP copper rod has an oxygen content similar to that of traditional ETP copper rod, 250 ~ 500 ppm. However, oxygen in oxygen-enriched layer of traditional wire bars occupied half of the total oxygen (this is the main cause for poor quality of traditional copper rods), while continuously cast and rolled ETP copper rod does not contain oxygen-enriched layer, which is the outstanding advantage of the new technique.

② With continuous manufacturing technique of ETP copper rod, cathode copper is melted quickly under neutral atmosphere or slightly aerated atmosphere. Since poling is not required, so large quantity of gas will not enter the melted copper. The casting temperature can also be decreased to 1,110°C~ 1,130°C, which makes the oxygen absorbed by the melted copper be lower. With such characteristics of cast wire bars as quick solidification and continuous casting, the wire bars cast will not have large quantity of air pores, and furthermore, these air pores will not meet together. Thus, loose texture of wire bars can be avoided.

There might be a few micro bubbles and other minor defects on the two upper corner edges of the wire bar. Continuous casting and angle planing can be used to remove these defects.

③ Both in traditional ETP copper rods and continuously cast ETP copper rods, there exist large Cu₂O particles, but these particles in the latter is much less and smaller than those in the former. The largest particle in the latter has a diameter of 4 ~ 5 μm and those with a diameter exceeding 1.7 μm only account for 1%. The largest particle in the former has a diameter of 8 μm and those with a diameter exceeding 1.7 μm account for 8%.

Oxygen in large particles accounts for 70% of the total oxygen in copper rod,
so high content of large Cu₂O particles is the greatest threat to wire drawing.

④ The whole manufacturing process of  continuously cast and rolled RTP copper is favorable for strict quality control and on-line control.

⑤ The electric conductivity of ETP copper rod with a copper content of 99.95% ~ 99.96% can reach up to about 101.3% IACS, but  the same electric conductivity of the oxygen-free copper rod (HCOP copper rod) requires a higher grade of cathode copper (with a copper content being 99.97% ~ 99.98%).