4th International Conference on Molten Slags and Fluxes, 1992, Sendai, ISIJ

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1 Synopsis: CRYOLITE-METAL OXIDE BINARY PHASE DIAGRAMS AND PURIFICATION OF LEAD BULLION AND BLISTER COPPER Gill Won Suh and Young Hyun Paik Dept. of Met. Eng. College of Eng. Korea Univ., Korea Partial binary phase diagrams of ryolite-metal oxide were introdued and briefly disussed. Most of them were found to form a simple euteti in the ryolite rih orner. Their euteti points were loated between 1146 and 1281 K and between 1. 1 and 32.2 mole% of metal oxide. Removal of antimony, arseni, tin and bismuth in the system of molten ryolite-lead bullion was investigated in the presene of dissolved oxygen. Contents of the first three elements dereased rapidly from 1 OOO ppm to 40, 30 and 35 ppm, respetively at dissolved oxygen ontent of 0.2 wt% and 973 K. Removal of impurities, suh as antimony, arseni, tin, lead and bismuth from blister opper by ryolite flux was also attempted at K. Contents of the first three elements also dereased rapidly from 1200, 1130 and 700 ppm to 60, 14 and 10 ppm at dissolved oxygen ontent of 0.5 wt%. The ryolite flux, however, had only a moderate effet on the removal of lead and virtually no effet on the removal of bismuth in both ases. A brief thermodynami analysis was arried out to estimate the solubilities of impurity oxides and experimental results were disussed, aordingly. Keywords: ryolite, flux, lead refining, opper refining 1 Introdution Industrial metal refining proesses very often adopt the oxidation tehnique for the removal of impurity elements from the melt. The formation of insoluble oxides may serve as a refining step in removing the metalli omponent of the oxide. In this proess the effetive removal of the oxide formed must ertainly be our major onern. It is well known that ryolite(na3aifg) has a remarkable dissolving power for metalli oxides. It also has thermal stability, high fluidity and low speifi gravity at moderately high temperatures[1]. The utilization of ryolite in metal fire-refining, suh as opper and lead refining may provide new perspetive in pyrometallurgy. Knowledge of ryolite-metal oxide phase diagrams gives information on the solubility limits of impurity oxides in the melt and on the seletion of the ontainer materials as well. In the early 50's Hayakawa and Kido[2) have reported on the binary phase diagrams of Na3AIFG-CaO, -CdO, -MgO, -Ti02, -ZnO and -Zr02. Foster[3,4) has reported on that of Na3AIFG-Al203, and Sterten et al.[5) studied the systems of Na3AIFG-Al203, -NiO, -Cr203, -Cu20, -Ti02, -Zr02 and -FeO by means of thermal and hemial analyses. More reently, the present authors[6, 7,8,9) have reported on the systems of Na3AIFG-O, - NiO, -Cu20, and -Co304 by the thermal analysis and X-ray tehnique. Those data reported for phase diagrams are rather disordant and improved data are learly needed. The interest in using speial fluxes in the fire refining of rude opper has been growing in the last few years due to the inreasing need to apply impure, omplex raw materials in the pyrometallurgial prodution of primary opper. It has been known for several deades that the alkali slag treatment is an effetive method for removing arseni, antimony and bismuth from -620-

2 blister opper[ 10, 11]. Several equilibruim studies on opper fire-refining with soda slags[ 12, 13] have been arried out partiularly during the last ten years. An analogous proess well known as the Harris proess adopts austi flux to remove arseni, antimony, tellurium and tin from lead bullion under the oxidizing atmosphere. Those treatments, however, have some disadvantages, suh as metal loss to the slag, flux onsumption, treatment time and metalli oxide inlusions in the metal phase. The aim of this study was to examine ryolite as a fire-refining flux. Removal of antimony, arseni, tin and bismuth from lead bullion and blister opper was attempted with ryolite flux. A brief thermodynami analysis was also arried out to estimate the solubilities of impurity oxides both in liquid opper and lead. 2 Cryolite - Metal Oxide Binary System Hayakawa et al.[2] was 1273 K and The experimental apparatus used in this work is shown in Fig. 1 The working tube was loated in a vertial tube furnae heated by a SiC heating element. Water ooled brass heads were used to seal the working tube. Temperature of the ruible was ontrolled by an automati temperature ontroller to better than ±3 K. Atmosphere over the melt was dry argon whih was purified before use by passing through silia gel and pyrogalli aid traps. Its flow rate was ontrolled to 3 ml/se(stp). Syntheti ryolite, with a purity of 99.9% Na3AIFs was used through the experiment. Eletri opper and lead were used for the metal phase. Sb, As, Sn and Bi were hemial pure and taken as impurity elements. As seen in Fig. 1 a small stainless steel ontainer was plaed inside of the arbon ruible to avoid possible ontamination of the melt by redued impurity elements. A 300g of eletri opper(lead) was melted in a stainless steel ruible at 1473K (973K) under 3 Experimental Euteti points for the ryolite-metal oxide(mx0y) binary systems are shown in Table 1. As seen in Table 1 euteti points are somewhat disordant. Reently Sterten et al.[5] reported the melting point of ryolite(t ml to be 1285 K, whih agrees well with the present authors' results. However, that measured by Berul et al. [ 14] and 1273 K, respetively. The low melting point of ryolite obvserved may be mainly due to impurities inluded in ryolite. The lowering of the euteti temperature as well as the euteti omposition is also attributed to those impurities. Metal oxides of group II in the periodi table generally form a euteti system with a solid solution region in the ryolite rih orner. On the other hand, metalli oxide of group VI (FeO, Co304 and NiO) form a simple euteti and the euteti temperature inreases in the order of FeO, Co304 and NiO, whereas the euteti omposition dereases in the same order. In the ase of lanthanides, only few systems(ce02,ndz03 and Sm203) were reported and the phase diagrams lak to give useful informations on phase relations. The majority of works to date are not well defined and improved phase diagrams are needed. It is also neessary to extend experimental work to other metal oxide systems for the utilization of ryolite flux in the fire-refining proess. Table 1 Na3AIFs-MxOy binary systems Group Binary System T(K) Tt(K) NE(mole %) Soure li N33AIFo MgO HayalawaU -C.O HayakawaZ> -ZnO HayakawaU -CdO Hayakawaz> N Na3AIF6-Ti I Hayak:awa2> -TiOz BelyaevS> -TiO:z Madhavens> -TiOz Rolin 5 > -TiOz 1284±1 1268± ±0.3 StertenS> -ZrOz HayakawaZ> -ZrO:z 1284±1 1268± ±0.4 Slertn5l VI N33Alf,.fO Choi9> -FO 1284± ±2 14.2±0.5 SterleoS> -Co Choi 9 > -NiO Lee -NiO 1284± ±1 2.80±0.2 StertenS> Others Na,AIFo-Al,O Fostr3J Al, Suh -AJ,Q, 1284± ±1 19.1±0.5 StrtnS> -O Suh&> -Cuz O Suh O -CuzO 1284± ±1 1.1±0.I S1ertens> -Crz ± ±1.S 0.95±0.l Stertens> la nth. Na3AJf&-Ce (1155) (5.4) Berull3> -Nd,o, 1263 BruJll> -Smz (1173) (2.0) Berutl :D T: Melting point of ryolite in Kelvin, T: Euteti temperature in Kelvin and Nr: Euteti omposition in mole perent,{ ): the point of lowest solubility.

3 the argon atmosphere. The impurity elements were added in metalli form and the bath was sampled for hemial analysis. A 50g of ryolite in the form of ompated tablet was added into the ruible. Molten materials in the ruible were well mixed for 2 to 3 min by a quartz rod through the sampling hole and held for 15 min to allow the slag-metal phase separation. Oxygen ontents in the melt was ontrolled by suessive additions of Cu 20 and O for opper and lead melts respetively. The slag and metal samples were suked from the ruible using a quartz tube through the sampling hole. The metal phase was analyzed spetrosopially and by the l.c.p. method, and the slag phase by the standard wet method tt--fl-::fl r+-::---'11_1rmiih...t--14 llmlt Results and Disussion In view of the fat that oxygen plays a major role in the liquid metal refining proess, equilibria between dissolved oxygen and other elements i.e. impurity elements in the melt being refined are of prime importane. Reation equilibrium between the dissolved oxygen and the resulting oxide may be expressed by the reation, xm+yq=mxoy ( 1) Fig. 1 Experimental apparatus 1. Sampling tube 2. Ar gas outlet 3. Brass water jaket 4. 0-ring 5. Mullite tube 6. Carbon ruible 7. Thermoouple well 8,9. Refratory support 10. SiC furnae 11. Ar gas inlet 12. Thermoouple well 13. Flux 14. Blister opper 15. Stainless steel ruible where M and Q represent the impurity metal and oxygen dissolved in the bulk metal, respetively. Assuming that the metalli oxide, MxOy is at its standard state and that the dissolved elements obey the Henry's law the solubility produt of MxOy, Ksr may be expressed by the equation, Ksr =[M]x[Q]Y =exp( LIG 0 1/RT) (2) where LI G 0 1 stands for the standard free energy hange of eq( 1). The value of LI G 0 1 may be alulated from thermodynami quantities at the given temperature i.e. the standard free energy of formation of MxOy, LI G 0 1 and Henrian ativity oeffiients of dissolved elements, r 0 The standard free energy of formation[ 15] and the ativity oeffiients[ 16] are presented in Table 2 and 3, respetively. Table 2 Standard free energy of formation Reation Temp.(K) 4G (al) 2Sb(I) + 3/20z(g) = SbzOJ(s) 92S-169S S4TlogT XIO 'T' 0.3XIO'T I T 2Bi(I) + 3/20z(g) = BizOJ(s) TlogT 3.25XJ0 3T' 0.3XIO'T I T TlogT XIO lt' 0.3XIO'T I T Sn(I) + Oz(g) = SnOz(s) 505-IS9S mogT. 0.70XIO.T' 2.3SX IO'T I T (I) + l/20z(g) = O(s) TlogT Xl0-'T' - 0. IOX!OST I + IS.OST SO 12.94TlogT XIO-lT' - 0. IOX IO'T I T Cu(I) + l/20z(g) = CuO(s) TlogT XIO-lT' - O. IOX!OST I ST l/20z(g) = Q(l WI!' in ) S300 + S.47T l/20z(g) = Q(l wt!' in Cu) T Table 4 shows solubility produts estimated for oxides of impurity elements in lead and opper melts at 973 K and K, respetively. Those for arseni ouldn't be estimated beause thermodynami data were not available. Equilibrium plots of dissolved elements in lead and opper melts with respet to the oxygen ontent were made from solubility produt values and are shown respetively in Figs. 2 and 3. One an easily see that in both ases antimony and tin are readily oxidized from lead and opper melts to the ryolite flux, while bismuth is never oxidized in lead and opper melts before lead and opper are oxidized. In general, stability areas of oxides derease as oxygen ontent in the melt dereases. It is also interesting to point out that antimony oxide, Sb203 beomes the more stable phase than opper oxide, CuO in the opper melt ontaining oxygen over 2.63 wt%

4 Table 3 Henrian ativity oeffiients in liquid lead and opper Element Bi Sb Sn Solvent me1al Cu Cu Temp.(K) r Atomi wt Table 4 Solubility produts of oxides in melts Reations Solvent metal Temp.(K) log Ksr 2SJ! + 3Q = SbzO>(s) Cu !!i + 3Q = BizO,(s) Cu ,W + 2Q = SnOz(s) Cu ,W + Q = SnO(s) The thermodynami analysis suggests that antimony and tin an easily be oxidized and dissolved into the ryolite flux. The oxidation, however, is not an effetive proess to remove bismuth from lead bullion as well as blister opper. This observation is in good agreement with the industrial pratie known as the Harris proess where other methods must be adopted for the bismuth removal[17]. Figs. 4 and 5 show experimental results for removal of impurity elements in lead and opper melts to the ryolite flux, respetively. The initial onentrations of impurities in the lead melt was 0. 1 wt%. The oxygen onentrations studied were in the range of 0 to 1 wt%. As an be seen antimony, arseni and tin exept bismuth were easily oxidized from the lead melt and dissolved into the ryolite flux. Their onentrations were lowered below the resolution of the analytial methods used at about 0.3 wt% oxygen. Fig.4 indiates that onentrations of antimony, arseni and tin were dereased to below 40, 35 and 30 ppm at 0.2 wt% oxygen, respetively. This result is exellent ompared to the industrial praties. Similar experimental results ould also be observed in the opper-oxygen-ryolite system(see Fig. 5). Antimony, arseni and tin ontents in the opper melt were readily dereased from 0.12, Fig. 2 Equilibrium plot of dissolved elements in melt at 973 K; M stands for Bi,, Sn and Sb Fig. 3 Equilibrium plot of dissolved elements in Cu melt at 1473 K; M stands for Bi,, Cu, Sb and Sn 0.5 LO log <2 wt%) log (2 wt%) ZnO en... 0 O Cu Cu fl! + Q = O(s) Cu l& + Q = CuO(s) Cu

5 t>--t>--t>--l>--t>---t> !'.l e Cl.:: ' ' : Bi..t..---.A. : Sb 0--0 : Sn... : As Fig. 4 Effet of oxygen onentration on the elimination of impurity elements from melt at 973 K and 0.07 wt% to 60, 14 and 10 ppm at 0.5 wt% oxygen, respetively. Lead and bismuth, in partiular, were not signifiantly affeted in oxygen-bearing opper melts. Those results are in good agreement with thermodynami preditions As for the effet of time on the removal of the impurity elements in opper melt(see Fig. 6). it only took about fifteen minutes for equilibrium to be established between the ryolite flux and the metal phase at 0.5 wt% oxygen and 1473K. This is very advantageous ompared to other fluxes(e.g.na2c03) whih require substantial amount of time for the treatment. Cryolite thus is proved to be a pratial flux with great appliable potential. 5 Conlusion An attempt was made to remove impurity elements from lead bullion and blister opper by using the ryolite flux. Antimony, arseni and tin ould easily be removed from oxygen-bearing lead and opper melts at 973 ' :::r- - u... : Bi : ' 0--0 : Sb As e 6--n: A : Sn...,, ' ' E D 0- H Fig. 5 Effet of oxygen onentration on the elimination of impurity elements from Cu melt at 1473 K :;; u E.,...,,... ' Ti me (mini -: Bi 0--0: Sb,,o. D_D Fig. 6 Effet of time on the elimination of impurity elements from Cu melt at K, and oxygen ontent, 0.5 wt% K and 1473 K, respetively. Those elements dereased rapidly from ppm to 1060 ppm depending upon melts and oxygen ontent in the melts. The oxygen ontent was the most important fator in those systems. The optimum oxygen ontents were 0.2 and 0.5 wt% for the lead and opper melts, respetively. The ryolite flux, however, had only a moderate effet on the removal of lead from the opper melt and virtually no effet on the removal of bismuth in both lead and opper melts. Those results are in good agreement with thermodynami predition. The equilibrium in the opper-oxygen-ryolite system ould be attained in 15 min at 0.5 wt% oxygen and 1473K. This is an advantageous fator ompared to other fluxes oxygen added in (wt%) l : 6--f:!,. : As.a...-.A. : Sn Oxygen added in Cu (wt\)

6 Aknowledgements The authors wish to thank the Korea Siene and Engineering Foundation for their finanial support. Referenes ) G. Mamantov and R. Marassi: Molten Salt Chemistry, D. Reidel Pub. Co.,Dordreht, 1986, 447 2) Y. Hayakawa and H. Kido: Si. Rept., Saitama Univ., 1 (1952) 41 3) P.A. Foster, Jr.: J. Am. Ceram. So., 43 (1960) 66 4) P.A. Foster, Jr.: J. Eletrohem. So., 106 (1959) 971 5) A. Sterten and O.S. Trondheim : Aluminum, 64 (1988) ) G.W. Suh et al: J. Korean Inst. of Met. and Mater., 28 (1990) 129 7) 1.K. Lee et al : ibid., 28 (1990) 83 8) Y.H. Paik: Bull. of the Korean Inst. of Met. and Mater., 4, No.2 (1991) 108 9) J.H. Choi: J. Korean Inst. of Met. and Mater., 29 (1991) ) W.J. Hillenbrand et al: Trans. AIME, 106 (1933) ) T.P. Philip: J. Met., 21 (1969) 38 12) T. Nakamura et al: Can. Met. Quart., 23, No. 4 (1984) ) P. Taskinen: San. J. of Met., 11 (1982) ) S.I. Berul and N.K. Voskresenskaya: Z. Neorgan. Khim., 8 (1963) ) R.C. Weast: Handbook of Chemistry and Physis, CRC Press In., Boa Raton, 1974, D-45 16) Y.K. Rao: Stoihiometry and Thermodynamis of Metallurgial Proesses, Cambridge Univ. Press, London, 1985, ) R. Harris: Metal I. Trans(B)., 1 5B ( 1984) 251