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    COPPER LOSSES TO SLAGS OBTAINED FROM THE EL TENIENTE PROCESS 177

    IntroductionCurrent El Teniente pyrometallurgical process for copperconcentrate was commissioned at Caletones Smelter duringthe period 19881991, following an intensive R&Dprogram that led to several improvements to the originalprocess developed during the seventies. Several industrialapplications of the process, in Chile and abroad, haveshown their capability to treat copper concentrates in a widerange of chemical and mineralogical compositions. Mainoperational parameters that determine the performance of the process are oxygen enriched air flow rate, degree of oxygen enrichment, moisture content of the solid materialsprocessed, molten material levels inside the vessel,frequency of molten materials tapping, bath temperatureand copper losses in slags.

    The copper losses in the slags from El Tenientepyrometallurgical process predicted by calculation fromthermodynamic data has been compared with thosedetermed by microscopic examination of the slag samplesfrom El Teniente pyrometallurgical processes and someslag industrial data.

    El Teniente pyrometallurgical processSimplified flowsheet of the current pyrometallurgicalprocess at Caletones Smelter with production capacity of 383 000 tons of copper annually, is presented in Figure 1.Basically, the current process involves four main stages totransform copper sulphide concentrate into blister copper,as follows:

    Wet concentrate drying down to 0.2 wt.% moisture Smelting bone dried concentrate and partial convertingto white metal

    White metal converting to blister copper Slag cleaning directed to recover copper from the slag

    produced during the smelting and partial converting

    steps.In the particular application of the process at CaletonesSmelter, wet concentrate with about 8 wt.% moisture isreceived from Coln and Andina Concentrator Plants andthen conveyed to storage bins. Then concentrate is split intotwo parallel lines to be conveyed to two direct-firedFluidized Bed Dryers. After being dried, concentrate with0.2 wt.% moisture is separated from the gaseous products inbag-filters, the gases being ducted to dedicated stacks andexhausted to the atmosphere, while the bone driedconcentrate is pneumatically conveyed to storage binslocated adjacent to the Teniente Converters Nos. 1 and 2.

    The El Teniente converter process involves the smeltingand partial converting of bone dried copper concentrate in athermally autogeneous operation. This operation modetakes into account an improvement to the El Tenienteconverter original process. The bone dried concentrate isdirectly injecting to the molten bath, with a mixture of oxygen-enriched air up to 34 vol.% O2 through speciallydesigned tuyeres. Siliceous flux and reverts are fed into thereactor through a garr-gun feeder. The products of smeltingand oxidizing of copper concentrate with flux are twoseparate molten phases, the high grade matte or whitemetal with the degree of oxidation being controlled so as toyield white metal with about 75 wt.% Cu and a slag thatcontains 710 wt.% Cu and 1216 % Fe3O4. These moltenphases are intermittently tapped in a countercurrent flow

    pattern, white metal at 1220C and slag at 1240C, throughespecially designed water cooled tapholes located at eachside of the endplates. White metal is poured into ladles and

    IMRIS, I., SANCHEZ, M., and ACHURRA, G. Copper losses to slags obtained from the El Teniente process.VII International Conference on Molten SlagsFluxes and Salts , The South African Institute of Mining and Metallurgy, 2004.

    Copper losses to slags obtained from the El Teniente process

    I. IMRIS*, M. SANCHEZ, and G. ACHURRA*Department of Power Engineering, Faculty of Mechanical Engineering, Technical University of Kosice, Slovak Republic

    Department of Metallurgical Engineering, University of Concepcion, ChileMetallurgical Engineering and Strategic Planning, Smelter Business Unit, Chile

    The Smelting and Converting El Teniente Process for copper concentrates as well as SlagCleaning Furnace Technologies to treat slags coming out from the El Teniente Furnaces arewidely applied today in Chile and around the world.

    Several industrial applications of this process have shown their capability to treat copperconcentrates in a wide range of chemical and mineralogical compositions. Main operationalparameters determining performance of the process are: oxygen enriched air flow rate, degree of oxygen enrichment, moisture content of the solid materials processed, molten material levelsinside the vessel, frequency of molten materials tapping, bath temperature and copper losses in

    slags.In this work, the copper losses in the slags from El Teniente pyrometallurgical process arepredicted by calculation from thermodynamic data and compared with those determined bymicroscopic examination and slag industrial data.

    Results obtained show that the main part of copper losses in the slag from El Teniente Furnaceare mechanically entrapped or floated unsettled droplets of the coexisting matte phase. by thecontrary the very low copper losses in the slag from slag cleaning furnace are limited by thephysico-chemical form.

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    MOLTEN SLAGS FLUXES AND SALTS178

    transferred to the Peirce-Smith Converters, where finalblowing to blister copper takes place. Normally threePeirce-Smith Converters are operating while another unitremains on stand-by or under maintenance. Slag producedis transferred by ladles or directly by launder, to the ElTeniente slag cleaning furnaces for decoppering.

    The El Teniente slag cleaning furnace process is based onan intensive reduction of magnetite and copper content inmolten slag produced at the El Teniente converter, byinjection of a solid, liquid or gaseous reductant directly into the molten slag through specially designed tuyeresfollowed by sedimentation stage. The reduction of magnetite decreases the slag viscosity and enhances thesettling of copper enriched phase. The molten productsobtained after the settling stage are, a discard slag with acopper content less than 1 wt.% Cu, and a high gradecopper matte with 6070 wt.% Cu. The decoppered slag ispoured into ladles, and then transported to dump by pot-carriers. The high-grade copper matte is tapped into ladlesand then recycled to the Peirce-Smith converters.

    Sulphur dioxide and other gases produced inside the ElTeniente reactors, continuously exhausted through the off-gas mouths, are cooled, dedusted and used for sulphuricacid production.

    Thermodynamic considerationsThe thermodynamic study of the copper losses in the slagsfrom El Teniente Process may be discussed on the basis of

    the Cu-Fe-S-O-SiO2 system, in which the metals, sulphurand silica represent the major components of the condensedphases and oxygen is the major component of the gaseousphase. From the available thermodynamic data and phaserelations the predominance area diagram was calculated forthe system Cu-Fe-S-O-SiO2 at 1250C equally, as wasdescribed in the authors previous paper1 and the resultswere plotted in Figure 2.

    The equilibrium between matte, slag and gas phase in ElTeniente converter and El Teniente slag cleaning furnacecan be represented in Figure 2 by the dotted and the dashedlines. The dotted line corresponds to the condition of coexistence of white metal containing about 75 wt.% of copper, slag nearly saturated with magnetite and silica withhigh copper oxide activity 10-210-3 and gas phase withpartial pressure of oxygen about 10-2 Pa and partial pressureof sulphur dioxide in the order of between 104105 Pa in ElTeniente converter. This means that intensive smelting andpartial converting of bone dried copper concentrates in ElTeniente converter produces highly oxidized slag with highmagnetite and copper oxide contents. The dashed lines inFigure 2 corresponds to the condition of co-existence of matte with about 60 wt.% of copper, slag nearly saturatedwith silica and iron with low copper oxide activity 10-310-4and gas phase with low partial pressure of oxygen about

    10-5

    Pa and low partial pressure of sulfur dioxide in theorder between 1010-1 Pa in El Teniente slag cleaningfurnace. This means that the decoppering process, based on

    Figure 1. Simplified flow-sheet of the current El Teniente pyrometallurgical process at Caletones smelter

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    COPPER LOSSES TO SLAGS OBTAINED FROM THE EL TENIENTE PROCESS 179

    the slag reduction by injection of a solid, liquid or gaseousreductant directly into the molten slag with followingsettling stage, produce slag with low magnetite and copperoxide contents.

    Copper losses in slagsThe pyrometallurgical extraction of copper from coppersulphide concentrate consists of two essential steps that arechemical reactions followed by phase separation. Thecopper content in sulphide copper concentrates is limitingby impurities. The main impurities are iron sulphideminerals. Part of iron sulphide minerals is selectivelyoxidized and fluxed as fayalite (2FeO.SiO2) and second partis oxidized and fluxed as magnetite (Fe3O4) which increasesthe viscosity of the slag and decreasing the matte-slagseparation. The corelation between the silica, magnetite andcopper content in a sequence of 100 samples taken at theend of the first period of matte converting is illustrated inFigure 3. From the above it may be concluded, that themagnetite content in the slag will have great influence onthe total copper content in discharged slags (%CuT) fromthe El Teniente slag cleaning furnace.

    Despite the contradictory opinions regarding the forms of copper losses in the slag that have appeared recently317 , thetwo forms considered here are mechanical and physico-chemical losses.

    Mechanical copper losses in slagsThe mechanical copper losses in slags (%Cumech) can be

    considered as particles of mechanically entrapped orfloating unsettled droplets of the coexisting matte phase,their sizes varying from several millimeters to a fewmicrons. Poggi and co-workers4 suggested that matte-phasedroplets are floated into the slag by SO2 bubbles thatoriginate in the melt according to the reaction:

    [1]By evolving SO2 gas bubbles the droplets of matte phase

    are floated to the slag surface. After coagulation of veryfine droplets on the slag surface the large particles sinkthrough the surface and settle down to the bottom. Thesettling rate of the matte particles mechanically entrappedor floated in the slag may be theoretically calculated byStokes law:

    [2]

    wherev is the rate of settling (cm.s-1), 1 is the density of matte (g.cm-3), 2 is the density of slag (g.cm-3), 2 is theviscosity of the slag (P), andr D is the diameter of theparticle (cm).

    Assuming that the variation of the densities of slag andmatte on composition is negligibly low and the matteparticle diameter is constant, the main factor to influencethe rate of matte settling will be the slag viscosity. Thesignificant effect for viscosity of fayalite slag has magnetitecontent. According to Figure 2 the homogeneous liquidmatte-slag region is limited by the lines corresponding tosolid iron and solid magnetite separation. Between theselines in the fayalite slags the ferrous and ferric contentdepends on partial pressure of oxygen in gaseous phase.The higher oxygen potential in the gas phase the higher isthe magnetite content in the slag as is documented inFigure 4.

    Higgins and Jones18 showed that the viscosity of the slag

    is rapidly increased when magnetite saturation is reachedand the solid magnetite crystals are separated inhomogeneous fayalite melt. To verify this claim themagnetite content in the slag after decoppering with pyrite,under constant settling conditions, were analysed andsummarized in Figure 5. From Figure 5 it is evident that thecopper content in the slag is altered rapidly when magnetic

    vg r D

    = ( )2

    91 2

    2

    2

    3 103 4 2Fe O FeS FeO SOs matte slag g( ) ( )+ [ ] = ( ) +

    Figure 2. System Cu-Fe-S-O-SiO 2 at 1250C

    Figure 3. The correlation between the silica, magnetite andcopper content in a sequence of 100 samples taken at the end of

    the first period of matte converting

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    MOLTEN SLAGS FLUXES AND SALTS180

    saturation is reached. Microscopic analysis showed that thehigher copper contents in slags at higher magnetite contentsafter washing with pyrite concentrate are due to the fact thatthe mechanically entrained copper and matte particles didnot settle down19.

    Physico-chemical copper losses in slagsThe physico-chemical losses in slags (%Cusol) are causedby solubility of copper in the sulphide and oxide forms sothe following common equation is valid:

    [3]

    A function of the physico-chemical copper losses insilica-saturated slags of temperature and matte grade wascalculated based on thermodynamic investigations of theCu-Fe-S-O-SiO2 system1,6,22 and can be described by thefollowing equations:

    [4]

    [5]

    [6]where A and B are constants depending on temperature,T istemperature (K), and N Cu2S is the mole fraction of Cu 2S inmatte coexisting with slag.

    The first term in Equation [4] represent copper losses inslags arising fromCu 2O solubility, which at constanttemperature and silica saturation is only a function of mattegrade andFeO activity in slag. The dissolved cuprous oxidein molten slag may react with ferric oxide producingdelaphosite23 by following chemical reaction:

    [7]

    According to Rosenqvist24

    cuprous oxide may react with

    ferrous oxide by chemical reaction:[8]

    This means that the separated copper would be found insolid slag together with magnetite crystal as copper prills17.

    The second term represents copper losses in slags owingto Cu2S dissolution, which, under the conditions given,depends only on matte grade.

    DiscussionThe total physico-chemical copper losses in slags calculatedfromEquations [4] [5] [6] at 1250C are plotted inFigure 6 as a function of the matte grade, together withresults obtained by the other investigators517 and industrialdata from El Teniente processes. The measured andcalculated data from different investigators summarized inFigure 6 are significantly scattered, mainly at low mattegrade where sulphidic copper is the predominant dissolvedform, which depends on matte grade, slag composition andpartial pressures of sulphur and oxygen in gas atmosphere.If matte grade rises to near 80 wt.% Cu and the oxidiccopper is the predominantly dissolved form because the

    partial pressure of oxygen is high and slag is almostsaturated by magnetite, all the solubility curves for totalsolubility of copper are similar and rise sharply. This is in agood agreement with thermodynamic calculations,illustrated in Figure 2.

    It is clear from Figures 2 and 6 that if the copper contentin the matte is around 75 wt.% and slag is nearly saturatedwith magnetite and silica corresponding to smelting andpartial converting conditions in the El Teniente converter,the mechanically entrapped matte particles form thepredominant copper losses in the slag. From the flotationtests of the finely ground samples of slags from the ElTeniente converter slag21 , was suggested that 87.7090.70% of the copper in the slag is presented in the form of mechanical losses, whereas the remaining 9.3012.30%represent the physico-chemical copper losses in the slag.The physico-chemical copper losses are aproximately 70%in the form of dissolvedCu 2S and 30% in the form of dissolved asCu 2O . The microscopic examination of polished samples of the slags from the El Teniente

    Cu O FeO Cu Fe O2 3 43 2+ = +

    1 2 1 22 2 3 2 / / Cu O Fe O CuFeO+ =

    log / . B T = +22500 14 707

    log / . A T = +5000 2 101

    %Cu A N

    N B N sol

    Cu S

    Cu S Cu S =

    + 2

    2

    21

    % % %Cu Cu Cusol Cu Cu S O= +2 2

    Figure 4. Relationship between the magnetic content in slag andoxygen pressure at 1250C

    Figure 5. Copper content in slag and matte versus magneticcontent in slag at 1250C

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    COPPER LOSSES TO SLAGS OBTAINED FROM THE EL TENIENTE PROCESS 181

    converter showed (Figure 7) that the primary octahedral

    and secondary dendritic crystals of magnetite and needle-shaped crystals of fayalite are present in the slag and thecopper losses in the slag are mainly as mechanicallyentrained matte particles their sizes varying from severalmillimeters to a few microns. Some metallic copper prillsassociated with magnetite crystals were also found.Dissolved cuprous sulphide precipitated during the slagsolidification in the form of beta chalcocite (Cu 2S ) formingfinely dispersed particles mainly in the spinel slag matrix.

    During the decoppering of slag by reduction in the ElTeniente slag cleaning furnace, reduction of magnetite tookplace, which improved the condition of matte-phaseseparation, as is documented in Figure 8. Because thesettling time was not sufficient the copper content in theslag after reduction and matte settling is slightly higher aspredicted from theoretical calculation by Equations [4], [5]and [6] in Figure 6. If the reduction conditions in the ElTeniente slag cleaning furnace is high, the liquid systembecomes saturated by iron and silica, and the settling time issufficient for matte phase separation the copper content0.79 wt.% in discarded slag is mainly in the form of physico-chemical losses which represents the minimumcopper content in the slag after reduction process21. This isin a good agreement with the microscopic examination of polished samples21 of decoppered slag from the El Tenienteslag cleaning furnace (Figure 9). The copper content in theslag was mainly in the form of very fine particles of beta

    chalcocite (Cu 2S ) precipitated during the slag solidificationin a glass spinel matrix with large needle-shape crystals of fayalite. Some metallic iron particles were also found. Only

    a few droplets with very small particle diameters of mechanically entrapped unsettled matte particles wasdetected. From these results it could be concluded, thatcopper losses in the slag saturated by iron and silica fromthe El Teniente slag cleaning furnace are present mainly asphysico-chemical copper losses which could be predictedfrom Equations [4], [5] and [6] and hence represent theminimum copper content in a discard slag after reductionprocess.

    Figure 6. Solubility of copper in silica saturated slags versuscopper content in matte

    Figure 7. Microstructure of El Teniente converter slag.Magnification x 300

    Figure 8. Vertical and longitudinal dynamic composition of copper in the El Teniente slag cleaning furnace

    Figure 9. Microstructure of El Teniente slag cleaning furnaceslag Magn x 750

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    MOLTEN SLAGS FLUXES AND SALTS182

    ConclusionsThe mechanical copper losses in slags are difficult topredict but the effect of slag viscosity is significant.Therefore, there is a reason to believe that good slagcomposition with low viscosity and well-mixed reactor withgood coagulation of fine matte particles will result in thelowest mechanical copper losses in a discard slag. The ElTeniente slag cleaning furnace is such as reactor.

    During the decoppering of molten slag, produced at theEl Teniente converter, by reduction process in the ElTeniente slag cleaning furnace the reduction conditions areso high that the reduction of magnetite took place and theliquid system became saturated by iron and silica. Thereduction of magnetite decreasing the slag viscosity andimproving the condition of matte phase separation whichmeans that a substantial part of copper in discarded slagexists in the form of physico-chemical losses which couldbe predicted from Equations [4], [5] and [6] and, hence,represent the minimum copper content in a discard slagafter reduction process.

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