Performance Analysis of a Smelting Reactor for Copper Production

Oct 9, 2008 - The enriched air derives from three different sources: industrial oxygen with 95% purity, blowing air from air compressors, and annular ...
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Ind. Eng. Chem. Res. 2009, 48, 1120–1125

Performance Analysis of a Smelting Reactor for Copper Production Process Pimporn Chamveha, Kattiyapon Chaichana, Anon Chuachuensuk, Suthida Authayanun, and Amornchai Arpornwichanop* Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn UniVersity, Bangkok 10330, Thailand

A primary smelting reactor (PSR) is an important unit in the pyrometallurgical process for the production of copper. This article presents a performance analysis of a PSR with respect to the effects of various operating parameters. The model of the PSR was developed using a metallurgical process simulator, METSIM. Reactor parameters, i.e., heat loss and phase distributions of matte and slag, were estimated from actual plant data. The model predictions were validated with plant data, and good agreement was observed. The developed PSR model was used to analyze the influence of the feed rates of copper concentrate, oxygen, silica flux, and revert on the PSR performance in terms of the percentage of copper in white metal and PSR slag, the percentage of magnetite and silica in PSR slag, and reactor temperature. 1. Introduction Copper is an important commodity that is widely used in electrical/electronic products, building construction, industrial machinery, and general consumer goods. In the production of copper, most producers generate full plate copper cathodes as a final product from the complex manufacturing process consisting of smelter and refinery units. However, some producers ship their output as wire bars to hot rolling mills, and a few specialized producers provide copper powders and bronze and brass alloys.1 In general, a pyrometallurgical process is applied to extract copper from copper-iron sulfide (Cu-Fe-S) ores consisting of 0.5-2% Cu.2,3 In this process, a primary smelting reactor (PSR) is the major unit used to smelt “copper concentrate”, which is obtained from the isolation of Cu-Fe-S minerals in ores via physical means, i.e., a froth floatation process. The aim of the smelting reactor is to oxidize S and Fe in the copper concentrate to produce a high-Cu molten sulfide (“white metal”) and a molten oxide slag.1 Rosales et al.4 showed that the metallurgical efficiency of smelting reactors depends strongly on the utilization of oxygen within the reactor during the smelting and converting reactions of Cu-Fe-S minerals in the copper concentrate that leads to the production of white metal. Luraschi and Canas5 performed a thermodynamic analysis of a conversion process in a smelting reactor and compared the model predictions with operating data. Considering the copper production process, it has been accepted that major copper loss is found in a discarded slag.6 Therefore, the avoidance of copper loss in the discarded slag is an important issue for enhancing the efficiency of the copper smelting process, resulting in higher yields. To achieve this objective, an understanding of the effects of operating parameters on the performance of smelting reactors is needed in order to operate the reactor efficiently and to minimize copper losses in the copper smelting process.7 Therefore, the aim of this study was a performance analysis of a PSR with respect to various operating parameters. Modeling of the PSR was performed using a metallurgical process simulator, METSIM. Reactor model parameters used for simulations of the PSR were estimated from actual smelting plant data. The model developed was used to investigate the influence of the feed rates of copper concentrate, oxygen, and * To whom correspondence should be addressed. Tel.: +66-22186878. Fax: +66-2-2186877. E-mail: [email protected].

silica flux on the performance of the PSR in terms of the percentages of copper in the white metal and the slag. 2. Description and Modeling of a Primary Copper Smelting Reactor A schematic diagram of a primary smelting reactor is shown in Figure 1. Dry copper concentrate is fed into the reactor with revert and silica flux. Oxygen is injected into the reactor in order to promote oxidation reactions.8 Minerals in the concentrate are decomposed and oxidized (eqs 1-7)1 at temperatures above 1250 °C, resulting in segregated liquid layers: matte and slag. The matte, a mixture of copper and iron sulfide (Cu2S and FeS) containing 45-75% Cu, is further processed to obtain the final copper product, whereas the slag, composed of iron oxide (FeO) and silica, is treated for copper recovery. The off-gas comprising mainly sulfur dioxide (SO2) is sent to a sulfuric acid production plant. Decomposition of Cu-Fe-S mineral 4CuFeS2 w 2Cu2S(matte) + 4FeS(matte) + S2

(1)

4Cu5FeS4 w 10Cu2S(matte) + 4FeS(matte) + S2

(2)

Pyritic sulfur oxidation S2+2O2 w 2SO2

(3)

FeS/FeS2 oxidation 2FeS + 2O2 w 2FeO(slag) + 2SO2

(4)

2FeS2+5O2 w 2FeO + 4SO2

(5)

Cu2S/CuS oxidation 3 Cu2S + O2 w Cu2O(slag) + SO2 2

(6)

2CuS + O2 w Cu2S + SO2

(7)

As FeO is highly reactive liquid, particularly in the presence of oxygen, it can further react with oxygen (eq 8). This leads to the formation of magnetite (Fe3O4), which is maintained in the solid phase at the operating temperature of the smelting reactor (about 1250 °C), thus complicating reactor operation. Because of agitation in the reactor, the FeS in the matte phase reacts with Fe3O4 to generate SO2 (eq 9). Upon the generation of SO2 gas, bubbles are produced that allow the white metal particles (Cu2S) to be dragged to the slag by floatation, resulting

10.1021/ie800618a CCC: $40.75  2009 American Chemical Society Published on Web 10/09/2008

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Figure 1. Primary smelting reactor. Table 1. Comparison of Simulation Results and Plant Data key variables

plant data

simulation

Cu percentage in white metal white metal flow rate (tph) Cu percentage in slag slag flow rate (tph) T (°C) Fe/SiO2 ratio Fe3O4 percentage in PSR slag

72.43 530 6.02 880 1199.5 1.473 18.5

72.40 530.92 6.29 884.24 1197.15 1.60 18.18

Table 2. Plant Data for Model Validation garr injection dry blowing oxygen gun air air data concentrate flux revert air 3 3 3 set (tpd) (tpd) (tpd) (Nm /h) (Nm /h) (Nm /h) (Nm3/h)

Figure 2. Flow diagram of a primary smelting reactor.

in a higher loss of copper in the final slag. To reduce the chemical activity of FeO, silica (SiO2) as the flux is added to the reactor (eq 10) in order to form ferrous orthosilicate, 2FeO · SiO2 (fayalite), as slag. Magnetite formation 6FeO + O2 w 2Fe3O4

(8)

3Fe3O4 + FeS w 10FeO + SO2

(9)

FeO oxidation 2FeO + SiO2 w 2FeO·SiO2(slag)

(10)

It is noted that the heat generated by exothermic oxidations inside the reactor is used for the endothermic decomposition of the copper concentrate. This process is known as an autogenous smelting process because no external heat is supplied. The heat generation from the smelting process depends on the standard enthalpies of reaction of the minerals contained in the copper concentrate. In general, the total oxygen flow is a key parameter for improving energy balance and controlling matte quality.4 In this study, the modeling of a copper smelting reactor was performed using METSIM, simulation software for metallurgical reaction processing. The FRL furnace module in METSIM was used to represent the reactor (Figure 2). Feed streams of the PSR comprise dry copper concentrate, revert, silica flux, and enriched air. Using data from actual plant operation, mineral compositions of the copper concentrate obtained from a mixture of three different copper concentrates (Antamina, Escondida, and Collahuasi) were determined to be mainly chalcopyrite (CuFeS2), pyrrotite (FeS), and pyrite (FeS2). Revert is a byproduct of the copper smelting process and is recycled to the PSR because of its high copper content (40% Cu), whereas the silica flux used for the formation of slag has a major mineral content of silica (95% SiO2) and alumina (5% Al2O3). The enriched air derives from three different sources: industrial

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7500 7865 7867 7886 7889 7860 7883 7169 7099 7035

4006 3996 4007 3997 3999 4003 3997 4000 4004 4013

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oxygen with 95% purity, blowing air from air compressors, and annular air that protects tuyeres from aggressive oxidation reactions. To perform a simulation of the FRL furnace module, the reactor model parameters of the heat loss and phase distribution within the reactor are required. A least-squares parameter estimation problem was formulated and solved with the objective of minimizing the difference between the model predictions and real plant operating data. The optimization results showed that the reactor heat loss is 5.1 Mcal/h, the white metal product derives from 87% matte, and the PSR slag consists of 13% matte and 100% slag. Table 1 reports a comparison of plant data and the model prediction based on the estimate reactor parameters; good agreement of the results is observed. To apply the FRL furnace model of METSIM to the simulation of a primary smelting reactor unit with confidence, the reliability of the model was tested by comparing simulation results with various available plant data sets. With the operating conditions given in Table 2, the model results and actual plant data in terms of the percentage of copper in the white metal, the percentage of copper in the PSR slag, the percentage of Fe3O4 in the PSR slag, and the reactor temperature were compared as shown in Figures 3-6, respectively. It can be seen that the copper content in the white metal, the magnetite content in the PSR slag, and the reactor temperature as calculated by METSIM are in good agreement with the plant data. However, it is observed that the copper content in the PSR slag is slightly different from the actual plant data. This can be explained by the way in which slag sampling was performed. Different operators take samples at different times and positions in the

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Figure 3. Comparison of the Cu percentage in white metal from model predictions and plant data.

Figure 6. Comparison of the reactor temperature from model predictions and plant data. Table 3. Major Mineral Composition (%) of Copper Concentrates copper concentrate component chalcopyrite chalcocite covellite bornite pyrrotite pyrite

CuFeS2 Cu2S CuS Cu5FeS4 FeS FeS2

25% Cu

30% Cu

35% Cu

60 0.5 1 5 3 12

43 6 1 15 8.5 13

25 13 0 25 14 14

Table 4. Operating Ranges of Parameters for Simulation Studies

Figure 4. Comparison of the Cu percentage in slag from model predictions and plant data.

Figure 5. Comparison of the Fe3O4 percentage in slag from model predictions and plant data.

PSR. Because of the maldistribution of the white metal in the slag within the reactor, the PSR slag product removed from a slag tap hole consists mainly of the slag phase. Therefore, the Cu percentage in the PSR slag obtained from plant data is lower than the predicted result. To reduce the error from different operators sampling the slag, a work instruction should be prepared. The schedule of tapping can affect these data values because of the different phase levels in the smelting reactor. It is noted that, by considering the content of Fe3O4 in the PSR slag, the method of slag sampling has a slight effect on the deviation between the model and plant data as Fe3O4 is dissolved only in the slag phase. 3. Simulation Results and Discussion A sensitivity analysis of the smelting reactor performance in terms of the percentages of copper (%Cu) in the white metal

parameter

nominal value

range

dry concentrate feed rate oxygen flow rate flux feed rate revert feed rate

70 tph 9836 Nm3/h 6 tph 0 tph

60-80 tph 6000-10 000 Nm3/h 2-10 tph 0-20 tph

and slag, the percentage of magnetite (%Fe3O4) in the slag, the percentage of silica (%SiO2) in the slag, and the reactor temperature as functions of key operating parameters was performed. Because the composition of the copper concentrates from the many sources around the world varies, the effect of the copper concentrate composition on the reactor performance was also investigated. Table 3 lists the major components of the copper concentrates used in this study. It is noted that, to illustrate the effect of chemical compositions, the main mineral components in the copper concentrate such as chalcopyrite, chalcocite, and covellite were calculated to obtain copper concentrates with 25%, 30%, and 35% Cu. Table 4 lists the ranges of the adjusted parameters that can normally be changed to continue stable operation without production interruption. 3.1. Effect of Concentrate Feed Rate. Figure 7 shows the effects of the variation of the Cu content in the copper concentrate from 25% to 35% and the variation of the feed rate of the copper concentrate from 60 to 80 ton/h (tph) on the performance of the smelting reactor. It was found that an increase in the concentrate feed rate increases the Cu percentage in the white metal only slightly compared to an increase in the Cu percentage in the copper concentrate. It can be seen from Figure 7a that an increase of 5 tph in the concentrate feed rate can increase the Cu percentage in the white metal by 0.1% whereas a 5% increase in the copper content in the concentrate can increase the Cu content in the white metal by 1%. This is mainly due to the higher degree of mineral decomposition. A greater quantity of copper in the concentrate can increase the copper content in the white metal and, consequently, has a greater effect on the composition of the white metal than the feed rate. Increasing the concentrate feed rate also increases the Cu content in the slag slightly because the amount of white metal

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Figure 7. Effect of copper concentrate feed rate on the performance of a PSR at different copper percentages in the concentrate feed.

distributed to the slag phase is slightly increased. In addition, although the slag-forming reaction is still the same, there is an increase in the quantity of slag formed. The combined effects result in a slight increase in the percentage of copper in the slag phase. In comparison, there is higher copper loss in the slag with a higher copper content in the concentrate than with an increased concentrate feed rate. As can be seen from Figure 7b, a 5% increase in the copper content of the concentrate results in a 1.5% increase in the Cu content in the slag because of the higher amount of the white metal and the higher copper content of the white metal. It is shown from the trends in Figure 7c that the amount of Fe3O4 increases slightly when the concentrate feed rate and copper concentration in the dry concentrate are increased. The copper concentrate with 25% Cu has the lowest magnetite content. It is possible that, with the lower Cu percentage in the white metal, the FeO for the slag-forming reaction is also at its lowest and, consequently, the magnetite content is reduced. On the other hand, the amount of SiO2 in the slag shows a decreasing trend when the flow rate of the concentrate is increased, as shown in Figure 7d. This is because of the increased amount of FeO in the slag and the addition of slagforming reactions. It should also be noted that the concentrate feed with 25% Cu has the highest percentage of SiO2 in the slag because of the assumption that the gangue that is used to balance the mineral compositions of the copper concentrate at different percentages of copper is represented by silica so that the concentrate with less copper has higher silica content. The effects of increasing the concentrate feed rate and the Cu percentage in the concentrate are to increase the reactor temperature, as shown in Figure 7e. This is a consequence of the higher net heat generated from the decomposition and combustion of the copper concentrate. It is noted that the proportion of increasing the Cu content in the concentrate from 25% to 30% has an effect on the PSR temperature in a different range than the case of increasing from 30% to 35% Cu. This

Figure 8. Effect of oxygen flow rate on the performance of a PSR at different copper percentages in the concentrate feed.

could be due to the differences in mineralogical composition as the copper content in the concentrate increases. As a consequence of the higher reactor temperature resulting from the higher concentrate feed rate, an increasing trend of the yield in the PSR is also observed (Figure 7f). It is noted that the yield of the PSR is defined as the total content of copper in the white metal divided by the total input amount of copper, which is derived from the copper concentrate and the revert. 3.2. Effect of Oxygen Flow Rate. The oxygen supply to the reactor is one of the key parameters in a PSR because most important reactions in the PSR involve oxygen. It is noted that the heat generated from these combustion reactions is required for an autogenous operation. For this reason, the optimum oxygen supply to the furnace should be calculated. Oxygen comes from two main sources: industrial oxygen and blowing air. This study considered the variation of industrial oxygen as the adjusted parameter. Figure 8a demonstrates that there is a slight change in the percentage of copper in the white metal with variations in the oxygen flow rate because the quantity of copper that reacts with oxygen is still the same and the use of oxygen is in the same proportion. The unreacted oxygen goes to the off-gas stream. The utilization of oxygen in the reactor is generally called the oxygen efficiency. In normal operation, for every copper concentrate blend, the oxygen coefficient, which indicates the amount of oxygen feed for combustion and slag-forming reactions per ton of concentrate feed, is determined. The coefficient of oxygen in this simulation case increased as a consequence of the increase in the industrial oxygen flow rate. Normally, operation of a PSR with a higher coefficient of oxygen than required for the concentrate blend results in a higher Cu content in white metal and a higher temperature. The dependence of the Cu percentage in the slag on the oxygen flow rate shows a flat trend, as can be seen in Figure 8b. It is observed that, when a concentrate with higher copper is used, the copper loss in the slag is higher because the quantity

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Figure 9. Effect of silica flux feed rate on the performance of a PSR at different copper percentages in the concentrate feed.

of white metal is increased and, consequently, the amount of white metal distributed to the slag phase increases. It should be considered from the simulation results shown in Figure 8a-c that increasing oxygen flow rate has only a minimal effect on the quality of white metal and slag (percentages of Cu and Fe3O4) because of the fixed extent of the reactions. It is assumed that, when all of the copper and iron react with the oxygen at the specified extent of reaction, all of the excess oxygen added will not react any further but instead will go to the off-gas system. However, the trend in the silica content shown in Figure 8d is different because of the effect of the copper content in the concentrate. It can be seen that the concentrate with 35% Cu has the lowest silica content in the slag, as the gangue used to balance the concentrate composition is assumed to be silica. It is noted that the range of oxygen flow rates used, 7000-10 000 Nm3/h, is high enough to make the FeO react completely with the oxygen, resulting in a constant percentage of Fe3O4 in slag. The effect of the oxygen flow rate on the reactor temperature is shown in Figure 8e: the temperature decreases when the flow of oxygen is increased. This indicates that an oxygen flow rate of 7000 Nm3/h is sufficient to react with the copper concentrate. Increasing the oxygen further does not result in further reactions but rather causes heat loss from the PSR. Because the Cu percentage in the white metal does not change with oxygen flow rate, the PSR yield is constant (Figure 8f). 3.3. Effect of Silica Flux Feed Rate. It can be seen from the trends in Figure 9a that flux addition has no effect on the copper in the white metal. With an increase in the feed rate of flux, the silica in the flux reacts with iron oxide to form slag and has no consequence on the white metal, which is mainly composed of copper sulfide and iron sulfide. It can be seen from the trends in Figure 9b that the Cu percentage in the slag decreases with increasing flux feed rate because the amount of slag increases with the addition of silica whereas the quantity

Figure 10. Effect of revert flow rate on the performance of a PSR at different copper percentages in the concentrate feed.

of white metal that distributes to the slag phase remains the same. This results in a decrease in the proportion of copper in the slag. Figure 9c,e shows that flux feed rates below 4 tph result in high magnetite levels in the slag and high temperatures, respectively. The high magnetite-forming reaction generates more heat, thus increasing the PSR temperature. This reaction occurs when the amount of silica added is not enough to form fayalite slag and the remaining FeO is overoxidized to form Fe3O4. When the flux feed rate is increased to more than 4 tph, the quantity of silica is sufficient to react with FeO to produce fayalite slag, and the magnetite content is gradually reduced. In Figure 9d, the trend shows the opposite way. As more silica is added in excess, the silica goes to the slag phase. Considering the yield of PSR, it can be seen from the trends in Figure 9f that the flux feed rate does not has an influence on the PSR yield, as it has no significant effect on the Cu percentage in the white metal, as shown in Figure 9a. In general, silica flux has a greater effect on the slag quality and quantity. 3.4. Effect of Revert Feed Rate. The variation of the flow rate of the revert, which consists mainly of Cu and Fe, was determined by the design capacity to be 0-20 tph. The purpose of revert addition to the PSR is to recover the copper in the smelting process and to control the reactor temperature. The composition and quantity of the revert added have many effects on the process, as shown in Figure 10a-e. It can be seen from the trends in Figure 10a that the Cu percentage in the white metal increases with revert addition, as the copper sulfide in revert, when melted, goes directly to the matte phase. The effect of revert addition on the Cu percentage in the slag is shown in Figure 10b, which indicates that the low-copper-content concentrate (25% Cu) is more sensitive to the variation of the revert feed rate. Figure 10c shows that the Fe3O4 in the slag increases with increasing revert feed rate. The magnetite present in the revert is melted and dissolved in the slag phase. In Figure 10d, the reduction of silica in the slag

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from the addition of revert is the effect of the slag-forming reaction where the amount of FeS increases from revert melting and consumes silica to form slag. Figure 10e illustrates that the PSR temperature decreases when the feed rate of revert is increased. This is consistent with more heat being required to melt the revert. It is noted that the adjustment of the revert feed rate is used to control the reactor temperature when it exceeds 1250 °C. The effect of revert addition on the PSR yield is shown in Figure 10f. When the revert feed rate is greater than 10 tph, the increase in yield is lower than for revert flow rates in the range of 0-10 tph. The addition of more revert to the reactor can improve the yield because the copper in the revert reacts and increases the amount of copper sulfide in the white metal. Copper in the slag also increases because of the amount of copper oxide. However, the amount of copper oxide in the revert is less than the amount of copper sulfide, so there is still an improvement in the yield. It is noted that the revert composition has a significant impact on the yield, so the analysis of the revert composition should have a certain level of accuracy. In this study, the revert composition was obtained through reconciliation with the laboratory data for the analysis of Cu, Fe, S, Fe3O4, and SiO2. 4. Conclusion In this study, a performance analysis of a primary smelting reactor (PSR) was carried out to investigate the effects of reactor operating parameters. An FRL furnace module of the METSIM9 simulator was used to model the reactor. The validation of the model coupled with estimation of reactor parameters was performed with actual industrial process data. From simulations, it was found that the feed rate of copper concentrate has a significant influence on the reactor performance. The effect of varying the oxygen flow rate, if the amount of oxygen is not sufficient to oxidize the minerals, is to decrease the quality of the white metal. The silica flux feed rate is a key variable for controlling slag quality because it is used to generate fayalite slag. Finally, the revert feed rate affects PSR performance because revert components consist of both white metal and slag materials.

Acknowledgment Support from the Thailand Research Fund (TRF), the Office of Small and Medium Enterprises Promotion (OSMEP), and the Graduate School of Chulalongkorn University is gratefully acknowledged. The authors thank Dr. Romeo U. Pagador for useful discussions and suggestions and Thai Copper Industries Public Co. Ltd. for providing the technical process data used in this study. Literature Cited (1) Davenport, W. G.; King, M.; Schlesinger, W. G.; Biswas, A. K. ExtractiVe Metallurgy of Copper, 4th ed.; Elsevier: New York, 2000. (2) Mackey, P. J.; Campos, R. Modern Continuous Smelting and Converting by Bath Smelting Technology. Can. Metall. Q. 2001, 40, 355. (3) Moskalyk, R. R.; Alfantazi, A. M. Review of copper pyrometallurgical practice: Today and tomorrow. Miner. Eng. 2003, 16, 893. (4) Rosales, M.; Ruz, P.; Fuentes, R.; Moyano, A. Oxygen Efficiency Calculation in Teniente Converters. In Proceedings of Copper 2003-Cobre 2003: The Hermann Schwarze Symposium on Copper Pyrometallurgy; Diaz, C., Landolt, C., Utigard, T., Eds.; Canadian Institute of Mining, Metallurgy and Petroleum: Montreal, Quebec, Canada, 2003; Vol. VI-Pyrometallurgy of Copper, pp 241-249. (5) Luraschi, A.; Can˜as, J. D. Thermodynamic Fundamentals of Teniente ConVerter Smelting; CADE IDEPE: Santiago, Chile, 1997. (6) Sridhar, R.; Toguri, J. M.; Simeonov, S. Copper Losses and Thermodynamic Considerations in Copper Smelting. Metall. Trans. B. 1997, 28B, 191. (7) Harjunkoski, I.; Borchers, H. W.; Fahl, M. Simultaneous Scheduling and Optimization of a Copper Plant. Comput.-Aided Chem. Eng. 2006, 21, 1197. (8) Alvarado, R. ; Lertora, B.; Hernandez, F.; Moya, C. Recent development in the Teniente converter. In Proceedings of Copper 95COBRE 95 International Conference; Chen, W. J., Diaz, C., Luraschi, A., Mackey, P. J. Eds.; Canadian Institute of Mining, Metallurgy and Petroleum: Montreal, Quebec, Canada, 1995; Vol. IV-Pyrometallurgy of Copper, pp 83-101. (9) METSIM Simulations Modules (http://www.metsim.com).

ReceiVed for reView April 17, 2008 ReVised manuscript receiVed August 27, 2008 Accepted August 28, 2008 IE800618A