Abstract: in view of the quality defects such as slag inclusion and hollow in shaft furnace continuous casting and rolling production of bright copper rod with recycled copper as raw material, the measures and methods such as equipment optimization and quality control process are adopted to improve the quality of bright copper rod. The results show that the defects are reduced and the product quality of bright copper rod is improved.

    Key words: recycled copper; bright copper rod; quality control

    China is a country lacking in copper resources, and the supply of primary copper is tight. It relies on a large number of imported scrap copper to meet the consumption demand of the domestic market every year. In recent years, the state has vigorously supported the development of recycled copper industry to ease the tension. Recycled copper has been widely used in the fields of copper, copper alloy strip, bar and wire rod, etc.

    After extensive market research, it is found that although there are few enterprises using recycled copper to produce bright copper rod, and this kind of bright copper rod is easy to break in the subsequent drawing of fine wire. The main reason for this phenomenon is the quality defects of copper rod such as slag inclusion and hollow.

    In view of the above problems, the author's enterprise has also taken a series of measures to improve the above defects. For example, * mixing and using different batches of recycled copper raw materials; standardizing the slag stripping operation at the copper outlet, specifying the slag stripping times in unit time; adding filter bricks in the tundish, etc., the improvement effect on slag inclusion and hollow is not obvious through these measures. In order to improve the quality of bright copper rod produced by recycled copper raw materials, the control methods and measures such as equipment optimization and quality control process are adopted in this paper.

    1. Quality defects

    1.1 slag inclusion

    Under normal conditions, all slag particles float up to the surface of copper water and are removed by slag raking process. However, there are still slag inclusion particles wrapped in copper water and enter into the ingot during the copper water flow. The main reason is that the slag removal effect of the previous process is not ideal, and the slag inclusion enters the ingot from the tundish. When the copper water enters the tundish from the runner, it directly impacts the copper water surface of the tundish, resulting in a large amount of slag inclusion mixed into the copper water and failed to float up in time. At the same time, affected by the temperature, the copper water in the tundish has poor fluidity and is brought into the ingot. On the other hand, the slag formed in the tundish or at the edge of the nozzle will also enter the ingot and cause slag inclusion (Fig. 1).

    Fig. 1 slag inclusion in slab

    Fig．1Castingbilletslag

    1.2 pores

    Hydrogen and oxygen are mainly dissolved in copper by shaft furnace continuous casting and rolling. Due to the existing technical conditions, it is difficult to avoid oxidation and oxygen absorption, and the processes of oxidation and deoxidation, hydrogen absorption and dehydrogenation exist at the same time, so there will always be a large amount of hydrogen and oxygen in copper water. When the copper water is cooled from the liquid state to the solidification temperature, the solubility of hydrogen in the copper decreases sharply, and the supersaturated hydrogen will precipitate in the boundary layer of the crystallization front, and a part of the bubbles generated by the reaction will not be eliminated in time and remain in the billet, forming pores (Fig. 2).

    2. Quality control measures

    According to the analysis, the core problems of the company are slag removal and exhaust. The direct and effective way of slag removal is to increase the fluidity of copper water, improve the scum capacity of copper water, and with effective slag raking and filtering measures, the slag inclusion can be completely controlled. For exhaust gas, on the one hand, increasing the fluidity of copper water, on the other hand, appropriately prolonging the exposure time of copper water and giving sufficient gas precipitation time can also reduce the probability of porosity. Therefore, the follow-up improvement measures mainly focus on how to improve the slag removal capacity and how to make the gas discharge copper water as much as possible.

    Fig. 2 air hole of slab

    Fig．2Castingbilletporosity

    2.1 reduce the inclination angle of shaft furnace bottom and increase the size of copper outlet

    The bottom of shaft furnace is shown in Fig. 3. According to the actual production, due to the increase of the inclination angle at the bottom, the fluidity of copper water can be increased at the initial stage of operation. However, with the increase of operation time, due to the larger corner from furnace bottom to copper discharge, some oxide slag of copper water began to gather at the corner, and gradually formed slag dam. This slag dam will lead to very harmful phenomenon of copper water, and copper block will melt and fall to the corner, and the burner is easy to accumulate copper and block. ***Finally, the copper melting speed will be affected, and the flame will become smaller, the copper water temperature will be reduced, and the fluidity will be poor, which is not conducive to scum.

    ① Shaft furnace; ② copper outlet; ③ upflow trough

    Figure 3 shaft furnace bottom

    Fig．3Bottomofverticalfurnace

    2.2 adjust the flame length and atmosphere of shaft furnace burner

    The burner distribution of shaft furnace is shown in Fig. 4. According to the calculation, the preheating section is from the feeding mouth of shaft furnace to the starting point of copper melting, and the temperature is 300 ℃ ~ 1083 ℃; from the copper melting point to the copper outlet, the short flame combustion is used, and the copper outlet temperature is 1090 ℃ ~ 1100 ℃; when the long flame combustion is used, the copper outlet temperature is 1105 ℃ ~ 1115 ℃. Therefore, when using a long flame, the fluidity of copper water will increase. The oxides of low-temperature impurities and some high-temperature impurities can emit into the flue gas with the impinging flame and be discharged from the furnace body.

    When the burner is in a micro reducing atmosphere, the flame is long, the temperature is low, and the jet airflow is fierce; when the burner is kept in the micro reduction atmosphere and the number of burners is increased, the higher the copper melting starting point is from the bottom, the longer the heating time of copper water reaches the copper outlet, the higher the temperature of copper water at the copper outlet and the better fluidity.

    ① Exhaust burner

    Remarks: ① increase the flame of row burner; ② increase the angle of row burner by 5 ° upward

    Figure 4 burner distribution of shaft furnace

    Fig．4Distributionofburnerofverticalfurnace

    2.3 increasing the vertical drop between the shaft furnace and the caster and the angle of the runner, the vertical drop between the shaft furnace and the caster was not implemented according to the standard due to the influence of the site conditions in the early stage of equipment installation, which was 0.5m lower than the design. There are two problems: one is the poor fluidity of copper water, which is not as expected; the other is that the position of the observation port of the upper flow tank coincides with the burner, which is not convenient to observe the copper water situation. On the other hand, the angle between the copper outlet and the corner of the launder is only 100 ° which leads to slag retention at the corner, which increases the risk of slag inclusion in copper rod.

    When the shaft furnace is raised as a whole, the vertical distance between the pouring nozzle and the casting wheel is increased, and the flow time in the air is long, which is conducive to the gas precipitation (Fig. 5). However, adjusting the angle between the copper outlet and the chute can reduce the occurrence of slag deposition and reduce the risk of slag inclusion in copper rod (Fig. 6).

    ① Shaft furnace; ② holding furnace; ③ casting machine

    Remarks: the shaft furnace is raised by 0.5m as a whole

    Fig. 5 melting, heat preservation and casting system

    Fig．5Melting，heatpreservation，castingsystems

    2.4 add burner and flame in tundish

    After the first several treatments, the slag inclusion into the tundish is limited, but it is inevitable that some fine copper oxide residue and enriched slag at the edge of tundish will be produced. Therefore, ensuring the temperature of copper water in tundish can increase the fluidity of copper water and make the slag inclusion float up in time and be removed by slag blocking brick. On the other hand, it can also reduce the generation of enriched slag at the edge of tundish (Fig. 7).

    ① Shaft furnace

    Remarks: the angle between copper outlet and chute is increased by 20 degrees

    Fig. 6 structure of shaft furnace and up flow trough

    Fig．6Verticalfurnaceanduppergroovestructure

    ① Downflow trough; ② tundish; ③ casting wheel

    Remarks: add a burner to tundish and increase flame

    Figure 7 tundish structure

    Fig．7Tundishstructure

    3. Implementation effect

    Through the above optimization, according to the production records in the past two months, the slag inclusion and hollow situation is obviously improved. Before the improvement, the number of slag inclusion and wire breaking was 20.78 times / 10000 t, and the wire breaking was serious; * after optimization, the number of slag inclusion and wire breaking was 16.88 times / 10000 t, slightly improved; after the equipment optimization, the number of slag inclusion and wire breaking was 3.78 times / 10000 t, with obvious effect. The number of broken wires with slag decreased from 20.78 times / 10000 T before improvement by 3.78 times / 10000 t, and the product quality of bright copper rod was significantly improved.

    4. Conclusion

    It has been proved by practice that the control methods and measures for equipment optimization and quality control process improvement are effective. However, how to further improve the quality of bright copper rod by using recycled copper raw materials needs to be further explored, such as adding rare earth or other trace elements into copper water, increasing the length and depth of launder, and adding automatic slag removing device, etc.

    Source: CNKI Author: Du Minsheng, Li Shiwei, fan Jinjin

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