Physical chemistry of copper smelting slags and copper losses at the Paipote smelter Part 1 Thermodynamic modelling

Transcription

1 Physical chemistry of copper smelting slags and copper losses at the Paipote smelter Part 1 Thermodynamic modelling N. Cardona* 1, P. Coursol 2, P. J. Mackey 3 and R. Parra 1 Thermodynamic modelling was used in conjunction with the results of a sampling campaign to investigate the liquidus temperature and the copper losses in slag from the Teniente converter (TC) and from the slag cleaning electrical furnace (EF) at the Paipote smelter in Chile. The present paper (Part 1) describes the smelter flow sheet and discusses the results of thermodynamic calculations. The impact of process parameters such as the Fe/SiO 2 ratio, the minor oxide levels in slag (CaO, Al 2 O 3, MgO and ZnO), the sulphur dioxide partial pressure p(so 2 ), temperature and matte composition were examined with respect to the slag liquidus and copper level in the slags. The operating window of the Paipote TC to produce good slag quality correspond to a vessel temperature between 1200 and 1250uC, and an Fe/SiO 2 ratio in slag between 1?5 and 1?8 when the iron content of the matte is in the range of 3 6 wt-%fe (or 76?5 73?2 wt-%cu). Under these conditions, the TC slag (average total copper in slag,6 wt-%cu) contains less than 1 wt-%cu as soluble copper with approximately 90% of total copper in the slag is entrained matte; it is noted that 0 10 wt-% solid magnetite is in suspension in the slag. As regards the EF, over the nominal temperature range from 1200 and 1270uC, the EF slag is fully molten over a wide range of Fe/SiO 2 ratios for slag containing the present level of minor oxides. An operating temperature of about 1225uC and a Fe/SiO 2 ratio in the range 1?4 1?8 are considered good conditions for optimum EF performance. The EF slag typically averages 0?82 wt-%cu and it was estimated that under present conditions, about 50% of this copper was as soluble copper with the balance as entrained matte. La modélisation thermodynamique a été utilisée en conjonction avec les résultats d une campagne d échantillonnage pour examiner la température de liquidus et les pertes en cuivre dans le laitier du convertisseur de Teniente (TC) et du four électrique (EF) de nettoyage du laitier de la fonderie de Paipote au Chili. Le présent article (1 iere Partie) décrit le schéma du procédé de la fonderie et discute les résultats des calculs thermodynamiques. On a examiné l impact des paramètres du procédé, comme le rapport Fe/SiO2 dans le laitier, le niveau de CaO, Al2O3, MgO et ZnO dans le laitier, la pression partielle du dioxyde de soufre p(so2), la température et la composition de la matte, sur le líquidus du laitier et sur le niveau de cuivre soluble dans le laitier. La fenêtre d opération du TC de Paipote permettant de produire une bonne qualité de laitier correspond à une température entre 1200 et 1250C et à un rapport Fe/SiO2 dans le laitier entre 1?5 1?8 lorsque la teneur en fer de la matte est dans la gamme de 3 à 6% en poids (ou 76?5 à 1 Department of Metallurgical Engineering, Universidad de Concepción, CP , Chile 2 Coursol Consultants, 485 Av. Humphrey Apt. 3, Sept-Iles, Que. G4R-2G4, Canada 3 P.J. Mackey Technology Inc., 295 Kirkland Blvd, Kirkland, Que. H9J-1P7, Canada *Corresponding author, nubiacardonavalencia@gmail.com 318 ß 2011 Canadian Institute of Mining, Metallurgy and Petroleum Published by Maney on behalf of the Institute Received 27 November 2010; accepted 1 June 2011 DOI / X Canadian Metallurgical Quarterly 2011 VOL 50 NO 4

2 73?2%Cu). Sous ces conditions, le laitier du TC contenant un cuivre moyen total d environ 6% contient moins que 1% en poids de Cu sous forme de cuivre soluble. Ce rapport permet de déduire qu approximativement 90% du cuivre total dans le laitier est sous forme de matte entraînée. On note également qu entre 0 et 10% en poids de magnétite solide est en suspension dans le laitier, dans les conditions normales d opération. En ce qui concerne le EF, le laitier est complètement fondu sur une large gamme de rapports de Fe/SiO2 pour une température comprise entre 1200 à 1270uC en considérant les niveaux actuels en CaO, Al2O3, MgO et ZnO dans le laitier. Une température de fonctionnement d environ 1225C et un rapport Fe/SiO2 dans une gamme de 1?4 à1?8 sont considérés comme des bonnes conditions pour le rendement optimal du EF. Le laitier du EF contient typiquement une moyenne de 0?82% Cu et l on a estimé que sous les conditions présentes, environ 50% de ce cuivre se trouvait sous la forme de cuivre soluble, le reste étant sous la forme de matte entraînée. Keywords: Paipote smelter, copper smelting, slag cleaning, copper losses, thermodynamics Introduction The Paipote smelter also known as the Hernan Videla Lira smelter is part of ENAMI (Empresa Nacional de Minería) in Chile. Located 8 km southeast of the city of Copiapó, the plant s mandate upon commissioning in 1952 was to treat concentrates from small and medium size mining companies of the Atacama region. The plant continues to handle such feed, covering a wide range of copper, gold and silver concentrates, copper precipitates from hydrometallurgical operations and direct smelting high grade ores. With such a wide range of feed materials, the smelter has a well developed procedure for planning and controlling the feed blend so as to provide as uniform a charge as possible to the Teniente converter (TC) smelting furnace. In spite of this, it is noted that even small unexpected changes in the feed material can affect the slag quality, copper losses and the deportment of minor elements. It is now recognised that copper losses in smelting slags are made up of chemically soluble copper and as mechanically entrained copper (as matte or a metallic phase). 1 The chemically soluble copper is in the form of cationic copper (Cu z ) 2,3 and thus dependent on the partial pressure of oxygen p(o 2 ), 4,5 the matte composition 1,2,5 and the slag composition. 6,7 The mechanical loss component 8 10 mainly depends on a number of physical factors including the density of matte and slag, bath fluid dynamics and slag viscosity. In the present study, thermodynamic modelling was performed using the FactSage 6?1software to evaluate the chemistry and copper losses in TC and electric furnace (EF) slags at the Paipote smelter. The main parameters evaluated were: the Fe/SiO 2 ratio in slag, the matte composition, the partial pressure of SO 2 p(so 2 )andthe minor oxide levels in slag (CaO, MgO, Al 2 O 3 and ZnO). The liquidus temperature and the proportion of solid phases present were calculated under smelting conditions. The copper solubility and the slag sulphide capacity were obtained from the equilibrium calculations. Description of smelter flow sheet The Paipote smelter in Chile has a nominal capacity of t/year of dry concentrate producing an average of t/year of copper anodes. Figure 1 presents a simplified flow diagram of the plant. The TC operates as the main smelting unit with slag treated in an EF for copper recovery. Operating with oxygen enriched air, the TC typically treats 1000 t/day of copper concentrate plus solid reverts in a range of t/day, with an average of 80 t/day of silica flux. Matte at,74?5 wt-%cu and 4 wt-%fe is transferred to Peirce Smith converters, with the resulting blister refined in the anode furnace before anode casting. The anodes are normally shipped to Codelco s las Ventanas plant for electrorefining. After skimming, Peirce Smith converter slag is cast and later crushed and returned to the TC and/or the EF as internal reverts; the same procedure is followed for anode furnace slag. Upon tapping, the TC slag is transferred by ladle to the EF. The key operating parameters of the TC and EF units are given in Table 1. The Paipote EF can treat about 800 t/day of liquid TC slag and can handle up to 150 t/day of solid reverts. The added coarse sized coke serves to reduce magnetite (as solid and liquid) in the slag. The produced matte is tapped and returned by ladle to the TC, while EF slag is transported to the slag dump. Typical matte and slag compositions for both the TC and EF furnaces are given in Table 2. At the EF, the metallurgical coke premixed with reverts is continuously introduced as solid charge through drop holes in the furnace roof. It has been found that adding coke this way improves the reduction efficiency by minimising the tendency for the coke to burn at the slag surface. It has also been found that optimum conditions in the EF include maintaining a good slag bath and minimum matte levels (by proper scheduling), and regulating correct coke/revert additions. Overreduction of slag needs to be controlled as this can lead to the formation of excessive amounts of metallic copper at the furnace bottom 15 ; this is undesirable as the copper layer tends to retain metallic impurities such as As, Sb and Bi, 6,16 thus affecting plant anode quality when such copper is recycled back to the smelter. As would be expected, the performance of the EF for slag cleaning depends on both the EF operation itself and also the TC slag chemistry. The present paper is intended to provide both an understanding and optimisation of the slag chemistry and copper losses in slag at Paipote. Canadian Metallurgical Quarterly 2011 VOL 50 NO 4 319

3 1 Simplified flow diagram for Paipote smelter (TC: Teniente converter; EF: electric furnace; PSC: Peirce Smith converter; AF: anode furnace) Methodology In mapping the process chemistry, the number of degrees of freedom were fixed for the TC and EF according to the Gibbs phase rule 4 using industrial parameters (such as gas, slag, matte compositions, etc) to define the reference values (refer to Table 2). In the Table 1 Key operational date for TC and EF (Paipote smelter ) calculations, one parameter was varied at a time in order to evaluate its impact on the slag liquidus, the copper solubility in slag and magnetite levels. It is noted that minor elements, such as Pb, As, Sb, Bi, Cr and Ni, are at very low levels in Paipote matte and slag, and because of this were neglected in the present calculations. Furnace Key operating parameters Range Teniente converter Dry concentrate feed/t/day (avg. 1000) Solid reverts charge/t/day (avg. 80) Oxygen enrichment/% (avg. 38) Specific blowing rate/nm 3 per tonne of concentrate Slag production/t/day Slag temperature/uc (avg. 1220) Fe/SiO 2 in slag/wt-%/wt-% (avg ) Fe 3 O 4 in slag (Satmagan)/wt-% (avg ) Matte production/t/day Matte temperature/uc Matte grade/wt-%cu (avg ) Electric furnace Solids reverts charge/t/day Coke charge/kg/tonne of slagzreverts Energy consumption/kwh/tonne of solidszslag Södeberg electrodes 3 of m diameter Slag production/t/day Slag temperature/uc (avg. 1220) Fe/SiO 2 in slag/wt-%/wt-% (avg. 1. 6) Fe 3 O 4 in slag (Satmagan)/wt-% (avg. 6. 9) Matte production/t/day Matte temperature/uc Matte grade/wt-%cu (avg ) 320 Canadian Metallurgical Quarterly 2011 VOL 50 NO 4

4 Table 2 Slag and matte compositions for the TC and for the EF Paipote s averages for Furnace Oxide or element Teniente converter Electric furnace Slag composition/wt-% FeO SiO Al 2 O ZnO CaO MgO PbO Cu S Others 0. 1* 0. 25{ Fe/SiO Matte composition/wt-% Cu Fe S Zn Pb As Sb Gas p(so 2 )/atm *Totals: As and Sb. {Totals: As, Sb and Cr 2 O 3. The liquidus calculations were performed using the FactSage TM databases 13 and the modified quasichemical model 11,12 for matte, copper and slag liquid solutions It considers the copper solubility in slag as Cu z and the related parameters are based mainly on data from sulphur free experiments (some information included data obtained under matte slag equilibrium conditions 17 ). The solubility of sulphur in the present multicomponent slag was calculated using the Reddy Blander model as modified by Pelton et. al. 20 In order to establish the slag liquidus lines delimitating the operating windows for each furnace, all possible solid phases in the particular system were considered: spinel ([Fe 2z,Zn 2z,Mg 2z ]{Fe 3z,Al 3z } 2 O 4 ), olivine ([Mg 2z,Fe 2z,Zn 2z,Ca 2z ] 2 SiO 4 ), pyroxene ([Fe 2z,Mg 2z, Ca 2z ]SiO 3 ) and tridymite (SiO 2 ); a full description of the chemistry of the solid solutions has been provided by Jung et al. 21,22 The thermodynamic properties of tridymite (pure SiO 2 ) were taken from the FactSage database. The following parameters for calculations on the TC were fixed according to the current operational conditions and slag composition: [Fe] matte, [Cu] matte, [Fe/SiO 2 ] slag, [Al 2 O 3 ] slag, [MgO] slag, [CaO] slag, [ZnO] slag, p(so 2 ), the total pressure (1 atm) and the temperature. As regards the EF, the chemical activities of Cu 2 S, FeS and Fe were fixed according to the matte composition (%Cu, %Fe and %S). It can be determined from plant data that the Paipote EF matte is deficient in sulphur (that is, contains less sulphur than that required to fully account for the metals as stoichiometric sulphides). In the present study, the sulphur deficiency was approximated to be 0?25 wt-%, based on the average Cu, Fe and S levels in matte as given in Table 2, and with an estimated oxygen level of 0?35 wt-% oxygen. 17,23 Equation (1) was then used to calculate the molar sulphur deficiency (this was then converted to a wt-% term). Sulphur deficiency (mol)~x s { X Cu 2 {X FezX o (1) The sulphur deficiency value was assumed to be constant over the range of matte grades considered. The other degrees of freedom were fixed according to the Paipote EF slag composition ([Fe/SiO 2 ] slag, [Al 2 O 3 ] slag, [MgO] slag, [CaO] slag and [ZnO] slag ), the operating temperature and total gas pressure (1 atm). Results and discussion Impact of Fe/SiO 2 ratio on TC slag liquidus During FeS oxidation in the TC, the resulting local equilibrium with the matte and slag could be represented by reaction (2) 3(FeS) matte z5o 2 ~(Fe 3 O 4 ) slag z3so 2 (2) Magnetite in slag is controlled with silica flux according to reaction (3) (Fe 3 O 4 ) slag z2sio 2 ~(Fe 2 SiO 4 ) slag z 3 = 2 O 2 (3) Maintaining the correct Fe/SiO 2 ratio in slag is thus quite important for the production of a good quality slag. Running with more silica than required is undesirable as in addition to requiring more smelting heat, it can also lead to slag having a higher viscosity. Figure 2 illustrates the effect of the Fe/SiO 2 ratio of the TC slag liquidus for conditions when the minor oxide components, the iron level in matte and the p(so 2 ) are fixed (Table 2). It is noted that the resulting partial pressure of oxygen p(o 2 ) under such conditions was calculated as atm. Bath smelting processes such as the TC or the Noranda reactor process usually operate at spinel saturation (magnetite saturation). 24,25 The ranges of normal operating conditions (Fe/SiO 2 ratio and temperature) are shown by the dotted square in Fig. 2; the average condition for Paipote are indicated by the star point. It is seen that the slag is at about 20uC below the calculated slag liquidus. The spinel saturation line can be Canadian Metallurgical Quarterly 2011 VOL 50 NO 4 321

5 (FeS) matte z3(fe 3 O 4 ) slag ~(FeO) slag zso 2 (4) 2 Impact of Fe/SiO 2 ratio on calculated TC slag liquidus: dotted square represents current operational window; star symbol represents average operational point (p(so 2 )50?25 atm, [Fe] matte 54wt-%, [Cu] matte 575?0 wt-%, ([Al 2 O 3 ] slag 54?0 wt-%, [ZnO] slag 52?1 wt-%, [MgO] slag 50?8 wt-%, [CaO] slag 50?8 wt-%) seen to also be quite sensitive to the Fe/SiO 2 ratio, with the liquidus increasing by y14uc per increment of 0?1in the Fe/SiO 2 ratio. Impact of minor oxides components (CaO, Al 2 O 3, MgO and ZnO) on TC slag liquidus Minor oxides such as CaO, Al 2 O 3, MgO and ZnO enter in the slag via the concentrate feed, flux, from coal ash and from refractory dissolution. Such oxides can then affect the slag liquidus temperature depending on the slag composition and level of oxidation Figure 3 illustrates the effect of the Fe/SiO 2 ratio and minor oxide components on the TC slag liquidus when the iron level in matte and the p(so 2 ) are held constant. According to the equilibrium calculations, CaO, Al 2 O 3, MgO and ZnO all increase the slag liquidus under spinel saturation. The oxides CaO and the Al 2 O 3 increase the liquidus by approximately 12 and 10uC per wt-% of oxide respectively; the oxides MgO and the ZnO similarly affect the liquidus but to a lesser degree, unless the level of MgO is above about 4%. The above behaviour of these minor oxides (Al 2 O 3, MgO and CaO) is in general agreement with that reported by Kongoli and Yazawa 27 and Henao et al. 28 Impact of p(so 2 ) and iron level in matte on on TC slag liquidus The slag liquidus increases with p(so 2 ) and with a lower content of Fe in matte (or higher matte grade); these two parameters are influenced by the oxygen potential; 24,25 thus, a high oxygen potential can raise the liquidus at spinel saturation. 27 The effect of p(so 2 ) and FeS activity on the spinel liquidus can be explained by the equilibrium between the matte and the slag containing magnetite (as Fe 2z and Fe 3z cations) in reaction (4) ðfe 3 O 4 Þ slag ~ 1 1=3 pso 2 ðfeoþ K 4 a(fes) slag (5) When the Fe/SiO 2 ratio and the slag composition are fixed, the chemical activity of FeO is therefore also fixed. In this case, it can be seen that the magnetite activity in slag in equation (5) is related to the p(so 2 ) and to the %Fe in matte which in effect fixes the a(fes). It can be seen that a lower p(so 2 ) level and a higher %Fe in matte (lower matte grade) will reduce the Fe 3 O 4 activity, and hence lower the slag liquidus at magnetite saturation. As seen in Fig. 4a, the slag liquidus at the typical Fe/ SiO 2 ratio of 1?7 increases by 20uC as the p(so 2 ) is increased from 0?1 to 0?4. However, for the present conditions at Paipote, the p(so 2 ) is maintained at steady between 0?2 and 0?3 atm (since variations in percentage of oxygen enrichment are small), leading to small variations in liquidus temperature of 4uC; hence, at Paipote, this factor being essentially constant does not contribute to changes in the slag melting temperature. Figure 4b shows the impact of the %Fe in matte on the slag liquidus. At Paipote, the iron content of the matte can vary between 3 and 7 wt-%, leading to significant liquidus variations (y15uc). The TC operates best at a steady blowing rate and matte grade and under such conditions, maintaining a correspondingly constant slag temperature and a uniform Fe/SiO 2 ratio in slag are then the key in maintaining good slag quality. Solids fraction in TC slag As shown above, the Paipote TC operates at an average temperature just below the slag liquidus; this implies that some solid magnetite would be in suspension in the liquid slag. Such a condition advantageously allows for a protective solid layer to build up on the refractory face; on the other hand, an excessive amount of such solid would rapidly increase the slag viscosity, 31 hindering good matte slag separation thereby increasing the amount of entrained matte in the slag. Figure 5 shows the effect of the Fe/SiO 2 ratio and of the iron level in matte respectively on the percentage of solid spinel formed in the slag (essentially as magnetite) within the normal temperature range of the TC operation. As shown in Fig. 5a, solid spinel in slag is estimated to occur at 1180uC when the Fe/SiO 2 ratio is above 1?3. At the average operating temperature of 1220uC, solid precipitation begins at an Fe/SiO 2 ratio of about 1?5, while the slag would be fully liquid with the Fe/SiO 2 ratio up to 1?8 at 1250uC; however, under the latter conditions, there would be no protective refractory layer formed. From the results shown on Fig. 5b and from industrial experience, it is recognised that relatively small variations in the %Fe level in matte can significantly disturb the process chemistry of the TC. At the target matte grade range at the Paipote smelter (74 75 wt-%cu or 4?5 3?5 wt-%fe), with the Fe/SiO 2 ratio of 1?7 and a temperature of 1220uC, it was estimated that the slag would contain y6 wt-% solids (essentially as magnetite); this is illustrated by the star 322 Canadian Metallurgical Quarterly 2011 VOL 50 NO 4

6 3 Calculated TC slag liquidus (p(so 2 )50?25 atm, [Fe] matte 54 wt-% and [Cu] matte 575?0 wt-%): LzTr, liquid saturated with tridymite; LzOl, liquid saturated with olivine. (a) impact of Al 2 O 3 ([ZnO] slag 52?1 wt-%, [MgO] slag 50?8 wt-%, [CaO] slag 50?8 wt-%); (b) impact of CaO ([Al 2 O 3 ] slag 54?0 wt-%, [MgO] slag 50?8 wt-%, [ZnO] slag 52?1 wt-%); (c) impact of MgO ([Al 2 O 3 ] slag 54?0 wt-%, [CaO] slag 50?8 wt-%, [ZnO] slag 52?1 wt-%); (d) impact of ZnO ([Al 2 O 3 ] slag 54?0 wt-%, [MgO] slag 50?8 wt-%, [CaO] slag 50?8 wt-%) point in Fig. 5. The calculated total magnetite content of the slag including the above solid component plus the soluble magnetite was estimated at y20 wt-%. This level of total magnetite compares well with reported plant data for Paipote TC slag of between 18 and 23 wt-%fe 3 O 4. 15,32 It is noted that the above plant 4 Calculated TC slag líquidus (p(so 2 )50?25 atm. ([Al 2 O 3 ] slag 54?0wt., [ZnO] slag 52?1wt.%, [MgO] slag 50?8wt.%, [CaO] slag 50?8wt.%). (a) Impact of p(so 2 ) at Fe/SiO 2 51?7. (b) Impact of the iron level in matte ( [Fe] matte ) Canadian Metallurgical Quarterly 2011 VOL 50 NO 4 323

7 5 Calculated solid fraction in TC slag (p(so 2 ) 50?25 atm, [Al 2 O 3 ] slag 54?0wt.%, [ZnO] slag 52?1wt.%, [MgO] slag 50?8wt.%, [CaO] slag 50?8wt.%, Fe/SiO 2 51?7), the star symbol represents the average operational point. (a) Impact of the Fe/SiO 2 ratio at [Fe] matte 54?0wt.%, and [Cu] matte 574?8wt. (b) Impact of the [Fe] matte at T51220uC magnetite data were obtained at Paipote using the Satmagan instrument. 33 Copper solubility in TC liquid slag Figure 6 shows the effect of temperature, Fe/SiO 2 ratio and %Fe in matte on the soluble copper in slag when the minor oxide levels are fixed at the average values. According to Fig. 6a, the copper level in the slag (considered as soluble Cu 2 O) is between 0?53 and 0?63 wt-%cu at the current operating conditions. Both the Fe/SiO 2 ratio and temperature are seen to have only a small influence on the copper solubility in slag at a constant matte grade. Based on the present calculations, the proportion of copper as soluble copper in slag was estimated to be less than 10 wt-% of the total measured copper in Paipote TC slag, with the balance present as entrained matte. As shown in Fig. 6b, the level of soluble copper in slag increases rapidly when there is less than about 3?5 wt-%fe in matte (,75?5 wt-%cu). Thus, in changing from 3?5 wt-%fe in matte to 2 wt-%fe, the soluble copper increases from 0?7 wt-% to nearly 1 wt-%cu. It is noted that for the present average Fe level in TC matte (4 wt-%) and with Fe/SiO 2 at,1?7, the soluble sulphur in slag is approximately 0?2 wt-% (mainly as FeS). This level is consistent with reported data for similar operations. 5,24 The above level corresponds to,13% of the total reported sulphur content (Table 2). It is noted that most of the reported sulphur in slag is associated with entrained matte. At such low levels of soluble sulphur in the slag, the oxide dissolution of copper is expected to be the predominant over the copper sulphide dissolution. Impact of Fe/SiO 2 ratio and minor oxides (MgO, Al 2 O 3, CaO and ZnO) on EF slag liquidus At the Paipote EF, there is no provision to add any flux for example, to adjust the Fe/SiO 2 ratio if required; 6 Copper solubility in TC slag (P(SO 2 )50?25 atm, [Al 2 O 3 ] slag 54?0wt.%, [ZnO] slag 52?1wt.%, [MgO] slag 50?8wt.%, [CaO] slag 5 0?8wt.%), the star symbol represents the average operational point. (a) Effect of Fe/SiO 2 ratio at [Fe] matte 54?0wt.%, [Cu] matte 574?8wt.%. (b) Impact of the %Fe in matte at T51250uC 324 Canadian Metallurgical Quarterly 2011 VOL 50 NO 4

8 7 Impact of Fe/SiO 2 ratio on calculated EF slag liquidus: dotted square represents current operational window; star symbol represents average operational point ([Fe] matte 5 6?5 wt-%, [Cu] matte 572?0 wt-%, [S] matte 521?5 wt-%, [Al 2 O 3 ] slag 5 4?0 wt-%, [ZnO] slag 52?0 wt-%, [MgO] slag 50?9 wt-%, [CaO] slag 5 0?9 wt-%) hence, the Fe/SiO 2 ratios in the TC and EF slags are very similar. Figure 7 shows the effect of the Fe/SiO 2 ratio on the EF slag liquidus when the minor oxide levels in slag and the iron level in matte are fixed. Figure 8 shows the impact of MgO, Al 2 O 3, CaO and ZnO on the EF liquidus temperature over the full range of Fe/SiO 2 ratios. The main solid phase which could precipitate out under present EF operating conditions is seen to be the olivine. The liquidus under this condition is only mildly influenced by the Fe/SiO 2 ratio. For a Fe/SiO 2 ratio less than 1?1, the slag readily becomes saturated with tridymite, and the slag liquidus temperature increases significantly. According to the present work, the Paipote EF slag is typically at some 50 to 100uC above the liquidus temperature, which is considered sufficient to avoid/control any solid phase precipitation that could occur; this condition is also effective for matte settling. The oxide MgO has a significant effect in raising the olivine liquidus (,25uC/1 wt-%) as shown in Fig. 8a; higher MgO levels promote a higher concentration of Mg 2 SiO 4 in the olivine solid solution, and with the Mg 2 SiO 4 melting point being much higher than that of Fe 2 SiO 4, the olivine liquidus then raises sharply. However, it is seen that the slag can contain up to 3 wt-%mgo without increasing the slag liquidus beyond the normal temperature range. This result is consistent with data reported by Zhao et al. 29 As seen in Fig. 8b, there is a lowering of the slag temperature corresponding to 7uC for every additional Al 2 O 3 in the 0 5 wt-% range at the Fe/SiO 2 ratio of 1?6. This is also in agreement with Zhao et al. 30 who showed that under reducing conditions (experiments carried out at iron saturation), additions of up to 6 wt-%al 2 O 3 expanded the olivine primary phase field at lower Fe/SiO 2 ratios and decreased the liquidus temperatures. At higher alumina levels, the slag becomes saturated with spinel, and the slag liquidus increases rapidly. The slag liquidus temperature at olivine saturation is also lowered by CaO additions (,5uC/1 wt-% at a Fe/ SiO 2 ratio of 1?6), but the slag becomes saturated with spinel at Fe/SiO 2 ratios greater than 1?6. The behaviour of CaO, Al 2 O 3 and MgO in EF slag is in agreement with work reported by Kongoli and Yazawa 27 in slag under reducing conditions: p(o 2 ) atm. Finally, it can be seen from Fig. 8d that ZnO in slag increases the slag liquidus by only,1uc/1 wt-% (at Fe/ SiO 2 ratio of 1?6); hence, the impact is quite small. Impact of iron level in matte on EF slag liquidus Liquidus calculations were performed on EF slag over a range of Fe in matte from 1?75 to 16 wt-%fe (78 60 wt-%cu) with the results presented in Fig. 9. The impact of %Fe in matte for levels higher than about 3 wt-% is negligible; this is in contrast with the situation for TC slag, where it has a stronger influence on the spinel liquidus. This is not surprising since changes in the %Fe in matte involve a change in the oxygen potential in the system, mostly affecting the Fe 2z /Fe 3z ratio in slag. This ratio does not influence the olivine composition (as olivine contains iron only as Fe 2z ). Magnetite solubility in EF liquid slag As noted above, the added coke in the EF provides suitable reducing conditions in the furnace to reduce magnetite and copper in slag. Under certain conditions, e.g., slag close to the electrode surfaces or at the coke/ slag interface, iron oxide reduction can proceed to the point where metallic iron can be formed and then may dissolve in matte. The overall reduction reaction for this is shown by reaction (6) a(fe 3 O 4 ) slag z2c~3(fe) matte z2co 2 (6) As noted above, the EF slag is considered to be fully molten at the present temperature range; this implies that all the iron (as both Fe 3z and Fe 2z ) is fully dissolved in the slag. Optical microscopic observations on solidified slag samples taken at the plant confirm the absence of solids in the EF slag. 32 The equilibrium between the iron oxides and metallic iron can be represented by reaction (7) 3(Fe 3 O 4 ) slag z(fe) matte ~4(FeO) slag (7) aðfe 3 O 4 Þ slag ~ 1 K 6 h i 4 a(feo)slag a(fe) matte (8) The magnetite activity (equation (8)) is then related to the chemical activity of FeO (afeo) and the %Fe in matte (which fixes the a(fe)). The a(feo) depends essentially on the Fe/SiO 2 ratio. Figure 10 shows the effect of the Fe/SiO 2 ratio and of %Fe in matte on the soluble magnetite level in the liquid TC slag at 1220uC. It is seen that the amount of soluble magnetite increases with the Fe/SiO 2 ratio. At iron levels lower than about Canadian Metallurgical Quarterly 2011 VOL 50 NO 4 325

9 8 Calculated EF slag liquidus. ([Fe] mte 56?5wt.%, [Cu] mte 572?0wt.%, [S] mte 521?5wt.%). LzTr: Liquid saturated with Tridymite, LzSp: Liquid saturated with Spinel. (a) Impact of MgO ([Al 2 O 3 ] slag 54?0, [ZnO] slag 52?0wt.%, [CaO] slag 50?8wt.%). (b) Impact of [Al 2 O 3 ] slag ([ZnO] slag 52?0wt.%, [MgO] slag 50?9wt.%, [CaO] slag 50?9wt.%). (c) Impact of CaO ([Al 2 O 3 ] slag 54?0, [MgO] slag 5 0?9wt.%, [ZnO] slag 52?0wt.%) (d) Impact of ZnO wt.% ([Al 2 O 3 ] slag 54?0, [MgO] slag 50?9wt.%, [CaO] slag 50?9wt.%) 3?3 wt-%fe in matte (,76 wt-%cu), the magnetite solubility increases rapidly and solid magnetite can be formed. Sulphur and copper solubilities in EF liquid slag The reduction of copper oxide in slag in the EF occurs by reaction with the coke (as a direct or indirect mechanism), or by other mechanisms such as coreduction of copper oxide and magnetite with iron oxide or liquid iron The overall reactions for the reduction with coke, or co-reduction by metallic iron can be represented as follows 2(Cu 2 O) slag zc~4(cu) matte zco 2 (9) (Cu 2 O) slag z(fe) matte ~2(Cu) matte z(feo) slag (10) As noted above, the EF slag contains some dissolved sulphur (mainly as FeS). This can react with dissolved copper oxide in slag according to reaction (11) (Cu 2 O) slag z(fes) slag ~(Cu 2 S) matte z(feo) slag (11) The presence of soluble sulphur in slag can also increase the total level of soluble copper in slag which is in equilibrium with matte below 65 wt-%cu as discussed by Nagamori, 4 Yazawa 37 andalsobycoursolet al. 26 But in the present case, even though the level of sulphur in EF slag is small, knowledge of the slag sulphide capacity is nevertheless considered important in understanding the mechanisms of copper losses. This aspect is explored as follows. The effect of the Fe/SiO 2 ratio on the copper solubility in EF slag in the temperature range of uC and for the slag in equilibrium with sulphur deficient matte containing 72 wt-%cu (6?5 wt-%fe) is shown in Fig. 11. It is seen that the Fe/SiO 2 has only a small impact on copper solubility. This is due to the rather constant oxygen potential (calculated as 10 29?6 atm) over the range of Fe/SiO 2 ratios examined. The temperature has a greater influence on copper solubility than the Fe/SiO 2 ratio, confirming that a lower slag temperature favours a lower copper solubility in slag. 24 Figure 12 shows the effect of the iron level in EF matte on both the soluble copper and soluble sulphur levels. As expected, the iron level in matte has an important influence on the levels of both copper and sulphur dissolved in slag. It can be seen that copper 326 Canadian Metallurgical Quarterly 2011 VOL 50 NO 4

10 9 ImpactofironlevelinmatteoncalculatedEFslagliquidus (sulphur deficiency in matte of 0?25 wt-%): star symbol represents average operational point ([Al 2 O 3 ] slag 5 4?0 wt-%, [ZnO] slag 52?0 wt-%, [MgO] slag 50?9 wt-%, [CaO] slag 5 0?9 wt-%) solubility decreases, while the sulphur solubility rises linearly, for iron levels in matte between 0?5 and 16 wt-% (78 60 wt-%cu). The effect on copper solubility is more marked below about 4 wt-%fe in matte. Figure 13 shows that a lower Fe/SiO 2 ratio results in lower sulphur levels in slag, over the same range of Fe (wt-%) in matte as shown in Fig. 12. When the iron level in matte approaches 0 wt-%, the activity of FeS and its solubility in slag decreases to nearly zero as discussed by Shimpo et al. 38 The slope of the solubility lines in Fig. 13 changes with different Fe/SiO 2 ratios, thus suggesting a lower FeS activity coefficient at the higher values of the Fe/SiO 2 ratio. Iron sulphide is considered to dissolve in the iron silicate melt by a common ion mechanism (Fe 2z / SiO 4{ 4,S 22 ), but when more SiO 2 is introduced (i.e. lower Fe/SiO 2 ratios), the slag becomes more glass-like, hence having a lower solubility for S 22 anions as shown. The FactSage model used in the present work to calculate the sulphide capacity of the slag 20 is considered more reliable when the activity of an oxide is much greater than the activity of the sulphide for a given element (for example, a(feo) and a(fes)). This situation is considered valid for the FeS FeO sulphide oxide couple but not for the Cu 2 S Cu 2 O couple. From this perspective and from the authors experience, the model appears to predict well the S 22 solubility in the slag present as FeS, but does not seem to reproduce well the interaction between the soluble sulphur and the soluble copper. The model appears to underestimate the interaction between the soluble sulphur and the soluble copper, leading to calculated copper losses lower than levels reported of industrial slag, 5,39 or those reported in laboratory experiments. 6,40 Given that a high interaction is in fact expected between soluble sulphur and copper, an optimum sulphur level in slag would then exist for a 10 Calculated soluble magnetite levels in EF slag at 1220uC; star symbol is the average operational point. (sulphur deficiency in matte of 0?25 wt-%, [Al 2 O 3 ]slag5 4?0 wt-%, [ZnO]slag52?0 wt-%, [CaO]slag50?8 wt-%, [MgO]slag50?8 wt-%) pyrometallurgical slag cleaning process at which the total copper soluble in slag would be minimised. Conclusions Based on the present thermodynamic modelling of the TC and EF processes, the key parameters for the production of a good quality slag with low copper losses were examined. It was shown that matte entrainment in the TC slag is the most important component of the observed copper level (average Cu in this slag is,6 wt-%). The calculated level of dissolved copper in this 11 Impact of Fe/SiO 2 ratio on calculated copper solubility in EF slag: star symbol is average operational point ([Fe] matte 56?5 wt-%, [Cu] matte 572?0 wt-%, [S] matte 521?5 wt-%, [Al 2 O 3 ] slag 54?0 wt-%, [ZnO] slag 52?0 wt-%, [MgO] slag 5 0?8 wt-%, [CaO] slag 50?8 wt-%) Canadian Metallurgical Quarterly 2011 VOL 50 NO 4 327

11 12 Calculated soluble Cu (as Cu 2 O) and S (as FeS) levels in EF liquid slag at 1220uC: sulphur deficiency in matte of 0?25 wt%; star symbol is average operational point ([Al 2 O 3 ] slag 54?0 wt-%, [ZnO] slag 52?0 wt-%, [MgO] slag 5 0?8 wt-%,[cao] slag 50?8 wt-%, Fe/SiO 2 51?6) slag under current operating conditions was found to be between 0?50 and 0?63 wt-%, representing less than 10% of the total copper content. It was estimated that there is about 6 wt-% of solid magnetite in suspension in TC slag (at a Fe/SiO 2 ratio of 1?7); conveniently, some of this refractory-like material precipitates on the refractory face of the vessel, thus forming a protective layer. At Fe/SiO 2 ratios above 1?8, or with mattes containing less than 3 wt-%fe, higher amounts of solid magnetite would be present in the slag, thus also contributing to higher levels of entrained matte. The present study confirmed the optimal conditions for TC operations as follows: %Fe in matte of between 3 and 6 wt-%, the temperature range between 1200 and 1250uC and the Fe/SiO 2 ratio in slag between 1?5 and 1?8. As regards the EF, it was found that Paipote EF slag would be fully molten (no solids present) under current operating conditions (Fe/SiO 2 ratio in slag of 1?6 and temperature range of uC). The slag superheat was found to be between 50 and 100uC, thus allowing for efficient settling of entrained matte. On the other hand, at Fe/SiO 2 ratios above 1?8, there is an increase in the soluble magnetite content, along with higher sulphide slag capacity in slag, while at lower Fe/SiO 2 ratios, the slag viscosity increases, thus hindering good settling of entrained matte. Based on the present study, the level of soluble copper in Paipote EF discard slag is expected to be in the range 0?36 0?53 wt-%cu. This is considered to represent the minimum achievable level of copper in EF discard slag. Plant data show an average of 0?82 wt-%cu in slag; hence, the soluble copper represents y50% of the total copper losses (range: 43 65%), with the balance being as entrained matte. 13 Impact of Fe/SiO 2 ratio and of %Fe in matte on EF liquid slag at 1220uC on calculated sulphur solubility in EF slag: sulphur deficiency in matte of 0?25 wt-%; star symbol is average operational point ([Al 2 O 3 ] slag 54?0 wt-%, [ZnO] slag 52?0 wt-%,[mgo] slag 50?8 wt-%,[cao] slag 50?8 wt-%) As a practical application of this work, the graphs presented in the present paper can be used by smelter personnel to evaluate the effect of changes in feed materials and/or changes in furnace operating conditions at the plant on slag characteristics. Acknowledgements The first author (NC) wishes to thank Comision Nacional de Investigación Científica y Tecnológica (CONICYT) of Chile for its financial support during this study. The authors also thank the personnel of the Hernan Videla Lira Smelter; their collaborative attitude has led to great synergies between fundamentals and applied aspects of this project. References 1. P. J. Mackey: The physical chemistry of copper smelting slags a review, Can. Metall. Q., 1982, 2, (3), L. R. Sridhar, J. M. Toguri and S. Simenov: Copper losses and thermodynamic considerations in copper smelting, Metall. Trans. B, 1997, 28B, E. M. Ripley and J. G. Brophy: Solubility of copper in a sulphurfree mafic melt, Geochim. Cosmochim. Acta, 1995, 59, (23), M. Nagamori: Metal loss to slag: Part I. Sulfidic and oxidic dissolution of copper in fayalite slag from low grade matte, Metall. Trans., 1974, 5, I. Imris, S. Rebolledo, M. Sanchez, G. Castro, G. Achura and F. Hernandez: The copper losses in the slags from the El Teniente Process, Can. Metall. Q., 2000, 39, (3), M. Nagamori, P. J. Mackey and P. Tarassoff: Copper solubility in FeO Fe 2 O 3 SiO 2 Al 2 O 3 slag and distribution equilibria of Pb, Bi, Sb and as between slag and metallic copper, Metall. Trans. B, 1975, 6B, H. G. Kim and H. Y. Sohn: Effects of CaO, Al 2 O 3 and MgO additions on the copper solubility, ferric/ferrous ratio, and minorelement behavior of iron-silicate slags, Metall. Trans. B, 1998, 29B, Canadian Metallurgical Quarterly 2011 VOL 50 NO 4

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