In the process of melting copper and copper alloy, it is necessary to cover and protect the copper and alloy liquid to reduce the oxidation and volatilization of alloy elements and the inhalation of copper liquid. The improper use of covering agent not only causes the loss of metal smelting, but also is the main cause of the metallurgical defects such as Blowhole, inclusion and crack in copper products. The metal loss of copper processing in China is 960,000 tons per year through oxidation, slagging and pickling, and 480,000 tons of charcoal through melting and casting. Charcoal, Rice Bran, salt and glass are the common covering agents for copper smelting, which contain a certain amount of adsorbed water. Charcoal is the most widely used mulching agent, the standard required that the total moisture of charcoal less than 7% . Because charcoal is an active material, in the actual transport, storage process, often adsorb a lot of oxygen, carbon dioxide and water, water content even more than 20% . By covering HSi80-3 with charcoal, the hydrogen content of copper solution was increased by 0.5ー1.5 Ml/100gcu, the hydrogen content of Tin Brass was increased by 0.3ー0.5 Ml/100gcu, the carbon content of white copper alloy was even discarded. Rice Bran and bran make the copper absorb hydrogen, oxygen and phosphorus. The salt covering agent can react with furnace lining and oxide, resulting in the loss of valuable metals, but it can not effectively prevent the alloy from oxidation, volatilization, high viscosity, difficulty in slag cleaning and poor permeability. As an excellent conductive and heat-conducting material, the primary task of pure copper smelting is to prevent oxidation, and covering protection is the most effective means to prevent oxidation. The covering agent used in modern copper processing can not protect the liquid copper effectively, and even causes some pollution to the liquid copper. The covering protection of copper melt is related to the mass transfer process of oxygen to copper liquid, the oxygen-slag-copper mass transfer behavior related to the loss of copper smelting, and the covering protection of copper melt related to the consumption of charcoal, in recent years, few related studies have been reported. In view of the current situation of high metal loss and high charcoal consumption in copper smelting, this project adopts the methods of Scanning electron microscope, x-ray diffraction (XRD) , etc. , the structure, composition and morphology of oxygen-free copper smelting slag without covering, solid alumina film, borax + cryolite, borax + carbon black were studied, it provides theoretical support for reducing metal loss and charcoal consumption in copper smelting.

1. Experiment 

1.1 four groups of oxygen-free copper were prepared by high temperature resistance furnace. The oxygen-free copper was studied under the conditions of no covering, solid oxide film, Borax + Cryolite, borax + carbon black, the structure, composition and morphology of smelting slag and the law of copper loss in slag. 1.2 The corundum pot was heated to constant weight in a high temperature crucible resistance furnace with high purity nitrogen to protect the copper liquid. The melting temperature was 1180 ~ 1220 °C, four groups of oxygen-free copper were prepared. 1.3 experiments of pure copper smelting with different covering agents were carried out to study the protective effects of oxygen-free copper and copper-aluminum alloys under the conditions of no covering, solid oxide film protection, Borax + cryolite and borax-carbon black protection melting with the apparatus shown in Fig. 2, the test device consists of gas source, coil gas heating device, gas pressure regulating device, high temperature electric furnace and other parts. The electric furnace is a silicon carbon rod high temperature electric furnace, the highest working temperature can reach 1673K, the test temperature is 1180 ~ 1240 °c, the highest temperature is 1280 °C.

Experiment one no cover protection: High pure nitrogen is fed into the furnace, oxygen-free copper preparation reference 1.2, oxygen-free copper liquid is poured into a constant weight crucible (the same below) . Open the switch shown in Fig. 2, blow oxygen to the surface of the copper liquid by 9, and carry out the test of no covering protection oxidation and slagging. Analysis of slag removal after 0.5 h test. Experiment 2 solid oxide film protection: Blowing the furnace with high purity nitrogen, pouring oxygen-free copper into the crucible, adding 0.9% of pure aluminum to the copper liquid, stirring the copper rod on the surface of the copper liquid and making the aluminum completely dissolved, oxygen is blown on the surface of copper liquid to form the protective film of Alumina on the surface. After 1.5 h of test, the slag is taken for analysis. Experiment Three borax-cryolite, borax-carbon black cover protection: Using high-purity nitrogen to drive the oxidation atmosphere in the furnace, the oxygen-free copper liquid into the crucible. The mixed salt is heated to 1100 °C, and the salt liquid is poured into the surface of the copper liquid, then the copper liquid and the salt liquid are clarified and layered, and the surface of the salt liquid is swept with oxygen medium. At the end of the experiment, the covering agent was replaced by borax-carbon Black and the BORAX + cryolite experiment was repeated. The slag sample is taken after 1.5 h of test.

2. Results and discussion 2.1 figure 3 of solid slag morphology and phase analysis is a picture of solid slag on the surface of pure copper cast sample melted by different covering agents, it can be seen from Fig. 3(a) and (b) that under the conditions of no covering protection and solid alumina covering protection, the oxidation of liquid copper is serious, and there is thick black oxide slag on the surface of solidified sample. Fig. 3(C) is a photo of solid slag and cast sample solidified under borax + cryolite cover. There is a yellow solid slag protective film on the surface of pure copper cast sample. The rose red copper cast sample can be seen when the surface slag layer is removed, the results show that Borax + cryolite has better protective effect on pure copper smelting. When pure copper is smelted by borax + cryolite, the smelted copper will be smoky and have strong pungent smell. The smoke and pungent smell are related to the high temperature decomposition of cryolite and the hydrolysis of aluminum fluoride, the reaction process can be described as follows: NA3ALF6→3NAF + ALF3(1) ALF3 + H2o → Al2O3 + HF (2) cryolite can be decomposed into sodium fluoride and aluminum fluoride at copper smelting temperature, and aluminum fluoride can be vaporized without melting at 1290 °C, sublimated aluminum fluoride in cold air re-condensed into tiny grains and floating in the air, the formation of smoke. Aluminum Fluoride Smoke Reacts With Water Vapor in the air to produce pungent hydrogen fluoride gas. 3(D) under the condition of borax + carbon black covering, the solidified photos of solid slag and cast sample have a semi-transparent protective film between red and brown color on the surface of pure copper cast sample, however, the thickness of the protective film is not uniform. In the upper right corner of the thin protective film, there is a black oxide slag layer on the surface of the sample. As can be seen from Fig. 3(D) , the amount of slag formed by covering pure copper with Borax and carbon black is small, it has certain protective effect on pure copper smelting. XRD analysis results of slag obtained by melting pure copper at 1180 ~ 1240 °C under different covering conditions. The slag is mainly composed of Cuo, Cu2O, chalcocite and so on. The slag phase is mainly chalcocite. The slag is mainly composed of Cuo, Cu2O and Cuprammonite, and the slag phase is mainly Cu2O and Cuprammonite. The slag is mainly composed of CUF2, Cu2O, Cu (OH)2, Na3AlF6 and Cuo. Pure copper is smelted under the condition of borax + carbon black covering. The slag is mainly composed of copper, Cu2O, cuprammonite and so on. 3.2 microstructure observation of solid slag Fig. 5 is a SEM analysis picture of smelting slag under the protection of pure copper without covering, solid oxide film, borax + cryolite, borax + carbon black. Fig. 5(a) is a SEM analysis picture of oxygen-free copper smelting slag without covering protection. According to Fig. 4(a) , the slag is an oxidation product of copper at high temperature. Fig. 5(b) is a SEM analysis photo of a smelting slag covered with solid oxide film under protective conditions. From Fig. 5(B) , it can be seen that the slag is densely covered with pores of 10 ~ 300μm. As can be seen from Fig. 3(a) and Fig. 4(b) , the slag is also an oxidation product of copper at high temperature. In Fig. 5(B) , the reason for the formation of air holes is that the aluminum has a good deoxidation effect on the liquid copper, and the oxide film formed after Aluminum Oxidation has a good protection effect on the liquid copper. With the increase of melting temperature and melting time, the dense oxide film on the surface of copper liquid occurs thickening and cracking. Secondary Oxidation and film formation of aluminum in liquid copper occur, and the process of thickening, cracking and film formation is repeated until the consumption of aluminum on the surface of liquid copper reaches a low level and the surface of liquid copper can not form dense oxide film again, the high temperature copper liquid reacts with the Water Vapor in the furnace gas and absorbs hydrogen. The hydrogen in the surface layer of the copper liquid rapidly diffuses into the copper liquid, the resulting Cuprous oxide reacts with hydrogen in the Liquid Copper to Form Water Vapor, which escapes from the liquid copper and accumulates and grows under the oxide residue, forming numerous dispersed bubbles. The process of Deoxidation, hydrogen absorption and secondary oxidation of high temperature copper liquid can be expressed as follows: Al + o 2→ Al 2o 3(3) Cu2O + Al → AL 2O 3 + Cu (4) Cu + H2o → Cu2O + [ h ](5) Cu2O + [ h ]→ Cu + [ h 2o ] Cu + [ h 2o ] Cu + H2o (6) figure 5(C) is under borax + cryolite cover, pure copper smelting solid slag SEM analysis photos, from the picture can be seen: smelting slag from each other separate black phase and surface cracks gray phase composition. Fig. 5(d) is a SEM analysis photo of solid slag covered with borax and carbon black. As can be seen from Fig. 5(D) , a large number of gray and white second phases are uniformly distributed in the black solid slag matrix.

Fig. 5 SEM photos of pure copper oxide slag under different covering conditions. Fig. 6(a) SEM photos of pure copper oxide slag and EDS photos. Fig. 6(b) EDS selected spots distribution of pure copper oxide slag. Fig. 6(B) Energy Spectrum of Particle A, fig. 6(C) is energy spectrum of b region of slag sample Matrix, Fig. 6(D) is energy spectrum of c region of slag sample matrix. Fig. 6 the chemical composition of slag phase of particle a, Matrix B and Matrix C are shown in Table 2. From Table 2, it can be seen that particle a of oxide slag on pure copper surface is mainly composed of Al2O3, Sio2, Cuo and solid carbon, among them: Al2O3, SIO2 are mainly caused by the erosion and spalling of Refractory particles, CuO is formed by the oxidation of copper, solid Carbon is the residue of carbon black and charcoal in the preparation of oxygen-free copper. The Matrix B and C are CuO and Cu2o, which are the chemical mixture of CU2O and CU2O formed by copper oxidation and slag formation during copper smelting.

Fig. 7 is the SEM image of surface oxide slag and EDS formed under the condition of pure copper solid oxide film protection (copper and aluminum oxygen, the same below) . Fig. 7(a) is the selected distribution of EDS samples of surface oxide slag of pure copper solid oxide film protection melting, fig. 7(b ~ D) energy spectrum of Particle A, b region of slag-like Matrix and c region of slag-like Matrix. In Fig. 7, the microstructure and chemical composition of slag phase of particle a, Matrix B, Matrix C, Particle D and Particle e are shown in Table 3. From Table 3, it can be seen that the oxide slag particles a and e on the surface of copper-aluminum alloy by oxygen smelting are mainly composed of Al2O3, Sio2, Cuo, FEO and solid carbon, fEO is mainly caused by the erosion and spalling of the Refractory particles of the crucible. Cuo is formed by the oxidation of copper to form slag. The solid carbon is carbon black and charcoal residue during the preparation of oxygen free copper. The Matrix B and C are Cuo, CU2O chemical mixture formed by oxidation and slag formation of copper during copper smelting. Copper is dissolved in red copper ore. The Particle D is Cuo, a mechanical mixture of metal Cu and solid carbon. Under the condition of pure copper protected by solid oxide film, oxygen smelting still causes obvious loss of metal oxidation.

Table 3 microstructure and chemical composition of the second phase of copper-aluminum alloy melting slag Fig. 8 is the SEM picture of surface oxidation slag and EDS formed by melting pure copper under borax + cryolite cover, fig. 8(A, b) is the EDS distribution of oxide slag samples on the surface of protective melting with borax and cryolite. Fig. 8(C, D) is the energy spectrum of Particle A, Matrix B and Matrix C. In Fig. 8, the chemical composition of slag phase of particle a, Matrix B and Matrix C are shown in table 4.

It can be seen from Table 4 that the oxide slag particle A on the surface of smelted pure copper covered by borax cryolite is mainly composed of AlF3, NaF, Na3AlF6, Na3BO7, CuO and solid carbon, among which CuO is formed by copper oxide slag-making, and solid carbon is the carbon black and charcoal residue in the process of anaerobic copper preparation. Matrix B is a fluorine-rich and borax depleted region with a slightly higher CuO content. Matrix C is rich in borax and fluorine-depleted, with slightly lower CuO content. As can be seen from Figure 8 and Table 4, when borax cryolite covers and protects pure copper smelting, the covering slag generated will form separated fluorine-salt rich area and boron-salt rich area, and the slagging loss of copper in the fluorine-salt rich area is slightly higher than that in the boron-salt rich area.

Figure 9 shows SEM and EDS images of surface oxidation slag formed by smelting pure copper under the condition of borax carbon black covering protection. Figure 9(a) shows EDS distribution of oxidation residue sample on the surface of borax carbon black covering protection smelting. Figure 9(b) shows energy spectrum of slag sample matrix A, Figure 9(b) shows energy spectrum of slag sample matrix A, and Figure 9(C, d) shows energy spectrum of particle B and C. The histochemical composition of slag phase of slag-like matrix A, particle B and particle C in FIG. 9 is shown in Table 5.

As can be seen from Table 5, the matrix A of oxide slag on the surface of pure copper smelting covered by borax carbon black is mainly composed of Na3BO7, CuO, Al2O3, SiO2 and solid carbon, among which CuO is formed by copper oxide slagging, Al2O3 and SiO2 are mainly caused by erosion and spalling of crucible refractory particles. Solid carbon is the carbon black and charcoal residue in the preparation process of oxygen-free copper. Particle B is a mixture of borax, Al2O3 and CuO. Matrix C is composed of a group of particles, and its main components are borax, Al2O3, CuO and solid carbon. As can be seen from Figure 9 and Table 5, when the borax carbon black covers and protects the smelting of pure copper, the substrate and particle group of the generated covering slag contain high copper, resulting in a large metal loss. 3. Conclusion Solid oxide film, borax cryolite and borax carbon black can protect liquid copper to a certain extent in pure copper smelting process. The phase composition and metal loss of slag produced by different mulching agent composition schemes are also different. 1. The slag phase of unprotected smelting is mainly chemical mixture of CuO and Cu2O, and the metal loss is large during smelting. 2. The solid oxide film protection slag is mainly a chemical mixture of CuO and Cu2O, and copper is solidly dissolved in copper ore. Solid oxide film protection smelting can still cause significant metal oxidation loss. 3. The slag of borax cryolite smelting furnace is composed of CuF2, Cu2O, Cu(OH)2, Na3AlF6 and other phases, forming separated fluoride-rich zone and boron-rich zone. The slagging loss of copper in fluoride-rich zone is slightly higher than that in boron-rich zone. 4. The main phases of the slag are borax, Al2O3, CuO and solid carbon. The substrate and particle group of the smelting slag contain high copper, and the metal loss is large. 

Source: copper alloy casting

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