Using the method of argon (with specified oxygen content) blowing, the possibility of copper deoxidation has been considered on the basis of specific thermodynamic data. Despite the oxygen exceeding its equilibrium content in argon, a significant copper deoxidation during blowing has been experimentally observed. The explanation of this phenomenon can be the reaction of oxygen with the crucible material. In the present study, graphite and Al2O3 crucibles were used. The stirring effect of gaseous bubbles promotes passing the substrates to and removing the products from the reaction zone.Koristeći postupak propuhivanja argonom (s određenim sadržajem kisika) razmatrana je mogućnost dezoksidacije bakra na temelju posebnih termodinamičkih podataka. Neovisno, što kisik prelazi ravnotežnu koncentraciju u argonu, praktički je utvrđena znakovita dezoksidacija bakra propuhivanjem argonom. Objašnjenje ove pojave može biti reakcijom kisika s materijalom lonca. U ovom radu rabljeni su lonci od grafita i Al2O3. Utjecaj miješanja plinskih mjehura podupire prolaz substrata i također odstranjivanje produkata iz zone reakcije

G. Siwiec

J. Willner

English

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G. SIWIEC, J. WILLNERTHE MECHANISM OF LIQUIDCOPPER DEOXIDATION DURING ARGON BLOWINGReceived – Prispjelo: 2008-06-22Accepted – Prihva}eno: 2008-09-20Original Scientific Pages – Izvorni znanstveni radINTRODUCTIONDissolution of gases in liquid metals is a negative as-pect of metallurgical processes. For example, oxygen orhydrogen, when dissolved in liquid metal, causes cre-ation of bubbles in solidifying ingots and, as a conse-quence, leads to formation of defects during the processof rolling. A series of deoxidation techniques are em-ployed, one of which is the method of metal blowingwith inert gas, most frequently argon 1, 2.A relationship of gas solubilities in liquid metals (cA)and their partial pressures above metal (pA) is describedby Sievert’s law:c k pA s A (1)Gases or diatomic pairs, such as H2, O2, N2 or S2, typ-ically dissociate when dissolved in metal. In the case ofoxygen, for example, the following reaction occurs:12 2O O % (2)The equilibrium constant in this reaction is the valueof kS from Sievert’s equation. The direction of the reac-tion course is determined here by the change of its freeenthalpy G. G values in processes of oxygen dissolu-tion in various metals, depending on temperature, arelisted in Table 1. Table 1 also contains the values ofequilibrium constant kS for respective temperatures,equilibrium pressures corresponding to these tempera-tures and respective values of oxygen content in liquidmetal.Table 1 The changes of free enthalpy of the oxygendissolution processes in chosen liquid metals,equilibrium constants and equilibrium pres-sures for given oxygen concentration in me-tals 3, 4.MetalG /J/molEquilibr.constantT /KEquilibr.Pressure /PaOxygenconcent./% wgtAg-17054+27,7T0,32 1173 9,93×103 0,1Co-97912+10,4T156 1873 4,15×10-4 0,01Cu-86361+7,17T6,21 1473 2,63×10-1 0,01Fe-116760+2,88T2690 1873 1,42×10-6 0,01Ni-96744+16,39T70 1873 2,03×10-3 0,01Pb-106043+31,4T8×109 673 1,6×10-17 0,1Sn-192070+50T8×108 673 1,5×10-17 0,01METALURGIJA 48 (2009) 2, 75-77 75Using the method of argon (with specified oxygen content) blowing, the possibility of copper deoxidation hasbeen considered on the basis of specific thermodynamic data. Despite the oxygen exceeding its equilibriumcontent in argon, a significant copper deoxidation during blowing has been experimentally observed. The ex-planation of this phenomenon can be the reaction of oxygen with the crucible material. In the present study,graphite and Al2O3 crucibles were used. The stirring effect of gaseous bubbles promotes passing the substratesto and removing the products from the reaction zone.Key words: metallurgy, liquid copper, deoxidation, argonMehanizam dezoksidacije teku}eg bakra propuhivanjem argonom. Koriste}i postupak propuhivanjaargonom (s odre|enim sadr`ajem kisika) razmatrana je mogu}nost dezoksidacije bakra na temelju posebnihtermodinami~kih podataka. Neovisno, {to kisik prelazi ravnote`nu koncentraciju u argonu, prakti~ki jeutvr|ena znakovita dezoksidacija bakra propuhivanjem argonom. Obja{njenje ove pojave mo`e biti reakcijomkisika s materijalom lonca. U ovom radu rabljeni su lonci od grafita i Al2O3. Utjecaj mije{anja plinskih mjehurapodupire prolaz substrata i tako|er odstranjivanje produkata iz zone reakcije.Klju~ne rije~i: metalurgija, teku}i bakar, dezoksidacija, argonISSN 0543-5846METABK 48(2) 75-77 (2009)UDC – UDK 669.2/:8:669.3:546.293=111G. Siwiec, J. Willner, Faculty of Materials Sciences and Metallurgy,Silesian University of Technology, Katowice, PolandGas contained above the metal dissolves in it accord-ingly to Sievert’s law. The rate of this process in mild,non-stirred bath depends on the diffusion rate of the gasfrom the metal surface into the bath. The diffusion pro-cess lasts as long as the equilibrium of oxygen in themetal layer and inside the bath has been reached. If thegas pressure above the metal is less than it is defined bythe equilibrium condition determined by the solubilitylaw, the gas will difuse from the metal to the atmosphereabove the metal surface. In Table 1, it is shown that oxy-gen can be very easily removed from silver 5.The thermodynamic data on oxygen solubility incopper (Table 1) show that oxygen equilibrium pressureabove copper containing 0,01% of dissolved oxygenequals 2,63×10-1 Pa. It means that if vacuum better than2,63×10-1 Pa is applied at 1473 K, the concentration ofoxygen remaining in liquid copper after deoxidation canbe less than 0,01%. It also defines the condition of thepurity of inert gas which can be a recipient of dissolvedgas. To be the carrier gas for 0,01% oxygen dissolved inliquid copper, inert gas should theoretically contain lessthan 2,63×10-4 % of oxygen. It is determined by the con-dition of oxygen diffusion into a bubble of argon. As-suming the process of carrying of dissolved oxygen intoargon is stationary, it can be described by a simplifieddiffusion equation:ddmtA k c cp d m g ( ) (3)where:ddmt– amount of dissolved gas passing into an inertgas bubble within a time unit,Ap – liquid-gas (bubble) interface,kd – diffusion coefficient,cm – oxygen concentration in metal,cg – oxygen concentration in a gas bubble.If Sievert’s law is applied, the value of gas concen-tration in a bubble in equation (3) should be substitutedby k ps O2 . When equilibrium pressure in a gas bubbleis reached, cm will reach the value of k ps O2 and diffu-sion ends. Inert gas should not contain an amount of ox-ygen greater than equilibrium pressure, because it wouldmean diffusion of this gas into the metal. Thedeoxidation kinetics is complicated by the fact that theprocess of liquid metal deoxidation using the method ofblowing with argon is not stationary as well as the pres-ence of surface active elements.On the basis of the thermodynamic data, the possibil-ity of liquid copper deoxidation through blowing it withargon containing a given amount of oxygen was consid-ered as well as experiments with the use of graphite cru-cibles and Al2O3 crucibles were conducted.STUDYIn the study, electrolytic copper containing 0,018 –0,022 % argon was used. For blowing, argon containing0,03 % oxygen (on the average) was applied, which the-oretically was too much contamination for reducing thecontent of oxygen dissolved in copper to the value of0,01 %. The equilibrium pressure of oxygen above cop-per containing 0,018 % of this gas equals 9,49×10-1 Pa,which corresponds to the 8,4×10-4 % of oxygen contentin argon. Despite this, a significant decrease in the con-tent of oxygen in copper was observed when blowingcopper with argon at 1500 K, both in graphite and Al2O3crucibles.In the case of graphite crucibles, the deoxidationmechanisms may consist in binding of oxygen dissolvedin liquid copper to the crucible carbon in the followingreaction:OCu + Cst = COg (4)This reaction is facilitated by small partial pressureof carbon monoxide. The free enthalpy value of thementioned G1 reaction can be determined when thefree enthalpies for the reactions of oxygen dissolution incopper and combustion of solid graphite giving the car-bon monoxide product are known. If the enthalpychange G2 corresponds to the reaction OCu = 0,5O2gand G3 corresponds to the reaction Cst + 0,5O2g = COg,the free enthalpy for the reaction (4) is as follows:G1 = G2 + G3 (5)If G2 = +86361 – 7,17T 6 and G3 = -111340 –87,36T 7, the equation (5) allows to obtain the standardfree enthalpy of the binding reaction of oxygen dis-solved in copper to the carbon G1, expressed in J/mol:G1 = -25000-94,53T (6)Summing up, copper deoxidation with the use ofgraphite is determined by carbon monoxide pressure pCOin the reaction zone. The equilibrium constant for the re-action (4) is described as follows:kpa COO(7)where:aO – oxygen activity in copper.The constant determined in the (6) equals k =7×15 at1473 K, which means that liquid copper deoxidation oc-curs. Blowing copper with inert gas only enhances thisreaction through stirring and restoring the reaction zonelocated on the crucible sides.In the case of graphite crucibles, it was observed thatafter blowing copper with argon, the oxygen content incopper was approximately 0,004 %. The oxygen contentin copper was measured with the Leco apparatus.During the process of copper blowing in Al2O3 cruci-bles, it was observed that after blowing the oxygen con-tent in metal was 0,005 %. While the mechanism of liq-uid copper deoxidation through blowing it with argon is76 METALURGIJA 48 (2009) 2, 75-77G. SIWIEC et al.: THE MECHANISM OF LIQUID COPPER DEOXIDATION DURING ARGON BLOWINGobvious in a graphite crucible, the oxygen decrease incopper while blowing it in an Al2O3 crucible is morecomplicated. Oxygen is a surface active element notonly at the liquid copper-gas interface 8, but also at theinterface of liquid metal-refractory material 9. Themaximum surface oxygen excess at the liquid cop-per-gas interface equals 21×1014 mol/cm2, and it equals34×1014 mol/cm2 at the liquid copper-solid aluminiumoxide interface. A high oxygen activity at the interfaceof liquid copper-solid aluminium oxide indicates that achemical reaction occurs. On this surface, a signifi-cantly greater amount of excessive oxygen is seen and,regarding the structure of copper surface layer 6, itshould be assumed that there are advantageous condi-tions for the formation of compounds such asCuOAl2O3 or Cu2OAl2O3. This hypothesis was con-firmed by X-ray radiography analysis of the surface ofthe crucible in which copper was molten, as the presenceof CuOAl2O3 spinels was observed.DISCUSSIONThe study of copper deoxidation through copperblowing with argon containing oxygen above the equi-librium value for oxygen dissolution in liquid coppershowed that it also caused metal deoxidation despite theexcessive amount of oxygen in argon. However, themechanism of this process is different from vacuumdeoxidation or deoxidation with gas containing oxygenof the content far less than the equilibrium value for agiven temperature. Nevertheless, the results obtainedduring deoxidation in the graphite crucibles (oxygen de-crease from 0,02 % to 0,004 %) and Al2O3 crucibles (ox-ygen decrease to 0,005 %) proved a good efficiency ofthis process in both types of crucibles.The process is based on the reaction of oxygen dis-solved in copper with the crucible material. The reactionis facilitated by the stirring effect of the gas introducedinto the crucible. Intensive stirring enhances passingsubstrates to and removing products from the reactionzone. When metal blowing with argon containing oxy-gen above the equilibrium concentration is applied, theadsorption of excessive oxygen at the metal-gas (bub-ble) interface should be taken into account. In this pro-cess, a specific layer of oxygen and metal ions is formed9. Contact of the bubble with the crucible surfacecauses the bubble’s surface oxygen binding and leads tothe decrease in oxygen content in the bubble. As a con-sequence, oxygen adsorption from the metal occurs,which means the decrease in oxygen content in the liq-uid metal volume.SUMMARYCopper deoxidation through blowing copper with ar-gon containing more oxygen than its equilibrium valueis caused by the reaction of oxygen with the crucible ma-terial. For graphite crucibles, the reaction product is car-bon monoxide, and for Al2O3 crucibles, the products arespinels, such as CuOAl2O3 or Cu2OAl2O3. A similarmechanism is observed during the reactions with refrac-tory lining and covering slag materials which causedeoxidation during blowing other metals (mainly alltypes of steel) with argon in ladles.REFERENCES1 Saternus, M.; Botor, J.; Siwiec, G.: Acta Metallurgica Slo-vaka, 3 (2004), 6502 Saternus, M.; Obcarski, M.; Botor, J.: Arch. Metall. Mater.,2 (2006), 8913 W. A. Fischer, W. Ackerman, Arch. Eisenhut., 43 (1966),374 C. Gatellier, M. Olette, Comptes Rendus Acad. ,Sci., 266(1968), 11335 B. Sikora, S. Tochowicz, Rudy Metale, 22 (1977), 1296 A. D. Kulkarni, Metal. Trans., 4 (1973), 17137 O. Kubaschevski, E. Evans, La thermochimie en metallur-gie, Cauthier-Villars, Paris, 19648 G. Siwiec, J. Willner, Rudy i Metale Nie¿elazne, 6 (2007),3529 J. W. Najdicz, W. N. Jerjemjenko, W. W., Fjesjenko, M. I.Wasilin, £. F. Kiri}jenko, ¯urn. Fiz. Chimii, 3 (1961), 694Note: The responsible translator for English language is G. Siwiec.METALURGIJA 48 (2009) 2, 75-77 77G. SIWIEC et al.: THE MECHANISM OF LIQUID COPPER DEOXIDATION DURING ARGON BLOWING

Mehanizam dezoksidacije tekućeg bakra propuhivanjem argonom

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