Investigation of Copper Losses to Synthetic Slag at Different
Oxygen Partial Pressures in the Presence of Colemanite
AYDIN RUSEN ,1,4BORA DERIN,2AHMET GEVECI,3 and YAVUZ ALI TOPKAYA3
1.—Department of Metallurgical and Materials Engineering, Karamanoglu Mehmetbey Univer-sity, 70100 Karaman, Turkey. 2.—Department of Metallurgical and Materials Engineering, Ist-anbul Technical University, 34469 IstIst-anbul, Turkey. 3.—Department of Metallurgical and Materials Engineering, Middle East Technical University, 06800 Ankara, Turkey. 4.—e-mail: aydinrusen@hotmail.com
Copper losses to slag are crucial for copper matte smelting and converting stages. One factor affecting the copper losses to slag during these processes is partial pressure of oxygen. In this study, theoretical and experimental investigations of oxygen partial pressure effect on copper losses to fayalite type slag in the presence of colemanite were investigated. Theoretical con-siderations include liquidus temperatures and phase diagrams of the fayalite type slag calculated by the FactSage software program. In the experiments, a synthetic matte–slag (SM–SS) was produced by melting certain amounts of reagent grade Fe2O3-SiO2and metallic Fe as starting materials. Experiments were carried out with SM–SS pair by the addition of calcined colemanite (from 0% to 6%) under various partial pressures of oxygen (107, 109, 1011atm) at 1250°C for 2 h. From the experimental results, it was found that the amount of copper in slag decreased slowly when colemanite was increased under all oxidizing atmospheres. The lowest copper content in synthetic slag was ob-tained as 0.38% after 6% colemanite addition.
INTRODUCTION
In the world, about 80% of copper is produced from copper mines (i.e. primary copper) and the remaining amount is obtained from industrial wastes (scraps) called secondary copper. Secondary resources have become a very important source for copper, like other metals, due to depletion of high-grade ores and increasing demand of this metal. More than two tons of slag containing 0.7–2% Cu are formed in the production of one ton of copper matte in the smelting stage. About 25 million tons of slag discarded annually by copper manufacturers in the world is thought to be another main source for recovering copper. Also, these huge quantities of discarded slag containing a considerable amount of valuable metals cause very important economic and environmental problems for all copper plants. How-ever, due to processing difficulties, recovery of the valuable metals from smelting slag is not easy. It should be evaluated according to appropriate pro-cesses like flotation, magnetic concentration,
leaching, etc.1–6 These methods have been investi-gated many times; however, desired economical results have not been obtained. Apart from these recovery methods, another option is to minimize the copper losses to slag resulting from entrained Cu droplets during the smelting of the concentrate. This can be achieved by decreasing slag viscosity, decreasing total slag amount, or increasing settling time. Because of the negative effects on the produc-tion rate of the plant and extra cost, decreasing slag viscosity is more convenient than increasing set-tling time for the same slag amount. However, it should be noted that too low of a viscosity of slag may lead to higher refractory wear; therefore, viscosity of slag may be decreased to an optimum level.7,8
It is known from the literature9–13 that copper could be lost to slag as both chemical and mechan-ical losses. In addition, it is noted by several researchers9–13 that the considerable amounts of copper losses to slag arise from mechanically entrained particles such as metallic copper or matte Ó2016 The Minerals, Metals & Materials Society
droplets that do not settle to the matte phase due to the high-viscosity slag layer or limited settling duration. Taking into consideration this situation, viscosity as a property of slag phase can be lowered by adding some fluxing agents like CaF2, CaO, and B2O3 in several industries.
14–19
Recent studies20,21 also showed that the mechanical type of copper loss can be prevented by adding colemanite as a fluxing agent. However, previous studies were carried out under inert gas (N2or Ar) atmosphere. By taking a more realistic approach that considers the atmo-sphere in copper smelting plants, the present study has been performed on copper losses to slag with colemanite addition under different partial pres-sures of oxygen. The results of this study are also supported by the FactSage22 computer modeling program to calculate and simulate the related liquidus temperatures and phase diagrams of the fayalite-type slag.
EXPERIMENTAL PART Materials
For the production of synthetic slag, defined amounts of hematite, silica, and metallic iron powder were used as starting reagents. Certain amounts of Cu, S, and Fe powders as starting reagents were used for the synthesis of matte. They were melted in a SiC crucible above 1300°C in an induction furnace under an argon atmosphere in the Department of Metallurgical and Materials Engi-neering at Middle East Technical University (METU). The resulting synthetic slag and synthetic
matte (labeled as ‘‘SS’’ and ‘‘SM’’) were cast and then ground to powder form (below 150 lm) by a disk mill.
Physical, chemical, and mineralogical character-izations of the SM–SS couple were previously described in detail.20 According to this previous study, when the FeO-Fe2O3-SiO2 system is consid-ered, a synthetic slag containing 37.6% SiO2,60.0% FeO, and 2.4% Fe2O3 without any copper was obtained near silica saturation and synthetic matte was obtained with the composition of 51.6% Cu, 24.4% Fe, and 24.0% S.
Colemanite (2CaOÆ3B2O3Æ5H2O) supplied by Eti Mine Works (Turkey) was used in all experiments after calcination at 400°C for 24 h in a muffle furnace. The calcined colemanite (labeled as CC) with 51.7% B2O3, 27.7% CaO, 8.6% CaCO3, 7.9% SiO2, and 4.1% other oxides (Al2O3, MgO and SrO) was kept in a desiccator to prevent moisture pick-up.
Experimental Setup
Figure1shows a schematic diagram of the exper-imental setup mainly consisting of a high-temper-ature vertical tube furnace, a gas supplying system with silica gel columns, gas cleaning furnaces, and gas washing columns.
The required oxygen partial pressure of the system was supplied by introducing a carbon diox-ide–carbon monoxide (CO2–CO) gas mixture into the furnace according to the following reaction (Eq.1):23
Fig. 1. Experimental setup. Presence of Colemanite
COþ1=2O
2¼ CO2;DG
Total
¼ 282400 þ 86:81 T jouleð Þ ð1Þ The flow rates of CO2–CO gases were controlled by two capillary flowmeters calibrated for measure-ment of small flows. This system includes two leveling bottles filled with CuSO4solution to adjust the height of the liquid (dibutyl phthalate) in manometers. Hence, the CO2–CO flow rates were determined by adjusting the height of liquids in manometers. PO2 values corresponding to (PCO2/ PCO) ratios for 1250°C were given in TableI.
Prior to the main experiments, the capillary flow meters were calibrated by the soap bubble method, which is commonly used to measure the volume flow rate of gases. Consequently, the calibrated CO2–CO gases were mixed in a glass bead mixer and sent into the furnace to assure predefined oxygen partial pressure of the system.
In each experiment, the oxygen partial pressure of the system was also measured by means of an oxygen probe (supplied from Australian Oxytrol System Co.). It was suitable for the measurement of oxygen partial pressures down to 1020atmosphere at a temperature range from 700°C to 1700°C, which covered the experimental conditions given in this study (PO2: 107–1011 atm, Temp.: 1250°C). Since the oxygen probe output was DC millivolt, a potentiometer was connected to its output. This millivolt signal was used to calculate the oxygen partial pressure in the furnace by means of Nernst Equation24 This equation with atmospheric air as reference is given as:
PO2¼ 0:209 exp 46:421 E=T½ ð Þ ð2Þ
where T is the experiment temperature (Kelvin) and E is the sensor electromotive force (mV) measured from the potentiometer in the experiments. All calculations of PO2depending on the CO2/CO ratio
were in good agreement with oxygen probe
measurements.
Methods and Models
Before each experiment, the reaction tube was flushed with purified argon. The experiments were carried out under a predefined partial pressure of oxygen that was generated by a mixture of CO2–CO gases with the CO2/CO ratio in the range of 0.47–47.
A predefined amount of calcined colemanite was uniformly mixed with 30 g of slag and 30 g of matte. The mixture was melted in a silica crucible. Ini-tially, it was positioned in the furnace, and then the furnace was heated up to 1250°C with 4°C/min heating rate. After reaching the experimental tem-perature, the furnace was kept at that temperature for 2 h and then cooled slowly in the same gas atmosphere. Finally, the cooled samples were sep-arated into slag and matte parts and ground to 150 lm to analyze chemically for Cu, Fe, SiO2, B2O3, and S.
Chemical analyses of all samples were performed by inductively coupled plasma mass spectroscopy (ICP-MS) at METU Central Laboratory. The wet chemical analyses were conducted at Eti Copper Inc. Analysis Laboratory. The magnetite amount of each sample was determined by a saturated mag-netite analyzer (SATMAGAN S135) with a maxi-mum error of 0.2% of the measured values. Scanning electron microscopy (SEM) analyses were also performed by using a JEOL JSM-6400 model with energy-dispersive x-ray spectroscopy (EDS).
In this study, the theoretical calculations of the ternary phase and P–X isopleth diagram were performed by using FactSage 7.0 software devel-oped by CRCT-ThermFact and GTT-Technologies.22 In the calculations, FactPS and FToxide were chosen as the most appropriate databases.
RESULTS AND DISCUSSIONS Theoretical Considerations
Industrial smelting slags (frequently based on fayalite-type slag) contain mainly iron oxide (as FeO and Fe3O4) and SiO2. They also have low amounts of CaO, Al2O3, ZnO, PbO, MgO. When minor oxides are ignored, the working smelting slags can be identified by the components of Fe-O-SiO2. Since it is not possible for metallic iron to exist in the slag under highly oxidizing atmosphere, iron can be found as Fe+2 and/or Fe+3. For this reason, iron-silicate slags without any metal or liquid matte can be represented by the ternary system FeO-Fe2O3 -SiO2, given in Fig.2. The solid lines calculated in the figure by using FactPS represent liquidus isotherms at the temperatures between 1200°C and 1400°C, whereas the dot lines show oxygen partial pressures (logPO2) at 1250°C. The liquid slag phase (called the ‘‘fayalite-type slag’’) is bor-dered by c-Fe, SiO2, Fe3O4 (magnetite), and FeO (wustite). In addition, it can be observed that the composition of the condensed phases is strongly dependent on the partial pressure of oxygen of the gas phase. Furthermore, this figure verifies that the Fe2O3/FeO ratio increases with increasing partial pressure of oxygen from too low levels (for equilib-rium with solid iron, PO2< 1011atm at 1250°C) to relatively high values (for equilibrium with solid magnetite, PO2> 107atm at 1250°C).25
Table I. PO2 values corresponding to (PCO2/PCO)
ratio
(PCO2/PCO) or (VCO2/VCO) ratio PO2values (atm)
0.47 1.1011
4.70 1.109
The industrial flash smelting furnaces generally operate with slags near the silica saturation (at 35– 38% SiO2 in the slag), which corresponds to the vicinity of the liquidus line AD in Fig.2, while the converting process is performed at 22–28% SiO2 in the slag under more oxidizing atmosphere. Oxygen partial pressure values for smelting and converting processes are ranging between 1010.5to 108.5and 108.5 to 106.5 atm, respectively.26 The ternary phase diagram FeO-Fe2O3-SiO2with oxygen isobars shows that the oxygen partial pressure varies from 107 to 1011 atm on the line of silica saturation. Therefore, as a first stage, the effect of calcined colemanite (Ca2B6O11) addition labeled as ‘‘CC’’ and oxygen partial pressure on the iron silicate slags was investigated theoretically by calculation with Fact-Sage 7.0 software. Oxygen partial pressure versus Fe/(Fe + SiO2) stability diagram for Fe-O-CaO-B2O3 -SiO2 system at 1250°C can be seen in Fig.3. Four P–X isopleths (PO2versus Fe/(Fe + SiO2)) of the
five-component system Fe-O-CaO-B2O3-SiO2 at 1250°C are superimposed at different CC additions (0, 2, 4, and 6 wt.%). It is apparent that CC addition expands the liquid slag region toward both lower and higher Fe/(Fe + SiO2) ratios. This is due to the combined effect of CaO and B2O3, which shift the liquid region to silica saturation or to wustite saturation area, respectively.27 It is also obvious that the silica-magnetite double saturation point gradually moves to higher oxygen partial pressures with calcined colemanite addition and that the liquid slag region expands with decreasing oxygen partial pressure at a fixed temperature.
Another figure calculated by FactSage (Fig.4a and b) shows the difference in liquid slag region area between ternary phase diagrams of 6 wt.% CC added (in Fig.4a) and pure (in Fig. 4b) FeO-Fe2O3 -SiO2slag system at 1250°C. Oxygen isobars for both slag systems were calculated and inserted into the phase diagrams. It can be readily seen that the liquid slag region expands noticeably toward silica, magnetite, and wustite saturation zones with 6 wt.% CC addition so that the liquid slag region can be stable even at higher PO2levels.
Experiments with Synthetic Slag–Matte (SS– SM)
The previous theoretical considerations have shown that the fayalite slag region widens with the addition of colemanite. This means that the slag will be molten at a lower temperature and that if the furnace continues to operate at the same tempera-ture, it is expected to increase the fluidity of the slag. Hence, the copper or matte particles in the slag will settle with a higher rate and their losses will be lower. To verify this fact, experiments with the synthetic slag and matte (SS–SM) were conducted under different oxygen partial pressures with cal-cined colemanite addition. For this purpose, 107, 109, and 1011atm oxygen partial pressures were applied to the system for 2 h at 1250°C with the addition of varying calcined colemanite (0, 2, 4, and 6 wt.%) to the matte-slag mixture. The results of all 12 experiments are given in TableIIand plotted in Fig.5.
Balancing values of the chemical analyses for the resultant slags analyzed by x-ray fluorescence (XRF) included all other oxides from colemanite and crucible, such as Al2O3 (2.1–3.1%), CaO (0– 3.3%), K2O (<0.3%), and MgO (<0.1%). The mag-netite level measured by SATMAGAN in these synthetic slags was between 1.9 wt.% and 3.8 wt.%. Figure5 shows the effect of oxygen partial pres-sure and CC additions at 1250°C for 2 h. The results of these experiments are evaluated separately in the following in terms of the effects of oxygen partial pressure and CC additions.
As mentioned, magnetite (Fe3O4) formation increases with the extent of oxidation in the flash smelting furnace. Several studies26,28demonstrated Fig. 2. FeO-Fe2O3-SiO2calculated ternary phase diagram with
liq-uidus isotherms at different temperatures and logPO2 isobars at 1250°C.
Fig. 3. Oxygen partial pressure versus Fe/(Fe + SiO2) stability dia-gram for Fe-O-CaO-B2O3-SiO2system at 1250°C. CC/(Fe + SiO2) ratios are selected as 0/100, 2/98, 4/96, and 6/94.
that copper content is strongly dependent on the magnetite level in slag. Increasing magnetite in slag leads to precipitation of Fe3O4 crystals, which causes an increase in slag viscosity and conse-quently in mechanical-type copper losses (entrain-ment of molten copper/matte droplets) due to slag increases. In this study, the results obtained from SATMAGAN show that magnetite levels of the slags remain nearly constant (1.9–3.8% Fe3O4) as the PO2 value decreases. This may arise from settlement of the magnetite to the bottom of crucible due to its higher density than that of matte. In every exper-iment in these series, there was a thin magnetite
layer between the crucible and the matte phase. Unfortunately, this magnetite layer could not be separated from the crucible.
According to earlier studies,29–32 most copper is dissolved in cuprous form (Cu2O) in intermediate oxygen potentials and the concentration of dissolved copper in the silica saturated iron silicate slag is directly affected by the oxygen partial pressure. In other words, the copper solubility in slag increases with increasing oxygen potential. Similarly, it could be seen from the results of the present study (Fig.5) that in the experiments without CC addition, the amount of copper lost in slag decreases with the Fig. 4. The calculated ternary diagrams of (a) 6 wt.% CC added and (b) pure FeO-Fe2O3-SiO2slag system at 1250°C with logPO2isobars.
Table II. Chemical analysis results of experiments with SS and SM with various additions of CC and under different partial pressure of oxygen atmosphere (as wt.%; at 1250°C for 2 h)
Exp. Code Po2(at.) CC add. (%)
Slag analyses Matte analyses
Cu SiO2 *Fetotal S B2O3 Cu Fe S
B–1 107 0 1.80 34.9 42.1 1.7 – 45.5 27.1 20.0 B–2 107 2 0.80 34.1 42.5 1.8 1.80 45.4 26.4 18.9 B–3 107 4 0.56 35.3 41.8 1.8 3.29 45.2 26.3 20.4 B–4 107 6 0.47 35.9 43.5 1.6 4.61 46.0 26.6 18.9 B–5 109 0 1.72 33.4 42.0 2.1 – 47.1 25.1 21.9 B–6 109 2 0.81 32.8 41.9 1.6 1.77 45.2 27.4 19.9 B–7 109 4 0.63 33.5 40.8 1.8 3.38 45.6 26.6 19.1 B–8 109 6 0.43 34.8 38.8 1.5 4.54 46.9 26.8 20.4 B–9 1011 0 1.28 34.3 43.0 1.2 – 45.1 28.7 18.2 B–10 1011 2 0.68 33.9 42.7 1.1 1.77 46.5 27.4 18.5 B–11 1011 4 0.60 33.5 41.4 1.1 3.22 46.2 26.8 19.8 B–12 1011 6 0.38 33.6 41.0 1.0 4.48 45.9 26.3 19.0
decreasing partial pressure of oxygen. Some researchers31 also derived several empirical rela-tions from their experimental results to estimate the solubility of copper in fayalite slag. They stated that there is a linear relationship between copper solubility and PO2 at a constant copper activity. This makes it possible to calculate physicochemical losses to slag as Cu2O.
For the effect of CC addition, it is seen from Fig.5 that the amount of copper in slag decreases signif-icantly from 1.80% to 0.47% Cu for a constant PO2 = 107 atm. However, as seen from TableII, the decrease is slightly different for both PO2= 109 and 1011 atm. Considering that a typical Pierce Smith converter operated at an oxygen potential of nearly PO2= 106 atm, CC can be used in the converter stage for a low copper content in slag at oxygen partial pressure atm (PO2= 107atm).
In the previous study,33the temperature effect on copper losses to slag with CC addition as flux were investigated at controlled partial pressure of oxygen (PO2= 109 atm), which refers to the atmosphere condition in the flash smelting furnace. Apart from the effect of other oxides (CaO, Al2O3, ZnO, etc.) present in the flash furnace slag (FFS), it is expected that the results obtained using an SS– SM sample resemble those obtained using flash furnace slag–flash furnace matte (FFS–FFM) under the same conditions.33 Comparing the results, the amount of copper in the resultant synthetic slags was slightly higher than that in the slag series where the experiments were done with FFS–FFM. With the addition of 6% CC for 2 h at 1250°C under PO2= 109 atm, 0.34% Cu was obtained as the lowest copper content in the slag for experiments with flash furnace slags, while the minimum copper content for experiments with synthetic slags was 0.43% Cu under the same conditions. This differ-ence can be considered due to the fluidity-increasing effect of other oxides present in FFS slag.
Color mapping techniques (by SEM–EDS) are being commonly used by researchers to separate available phases or to determine distribution of the constituents in a sample. The color mapping char-acterization applied to the B-3 series of experiments is given in Fig. 6 to observe the distribution of constituents (especially Cu) in the slag. In color mapping, Cu is represented by red, S by dark blue, Fe by light blue, Si by yellow, and O by green. The dark regions in each picture indicate that the element does not exist or is present in trace amounts. It could be deduced from Fig.6 that the copper was present in the slag in sulfide (or metallic) form due to the mechanically entrained particles. Based on the pictures belonging to Fe, O, and Si, it can be concluded that both SiO2and iron oxide (as Fe3O4 or FeO) phases exist in the repre-sentative slag.
Fig. 5. Effect of partial pressure of oxygen and addition of calcined colemanite on copper losses to a synthetic fayalite slag (at 1250°C for 2 h).
Fig. 6. Color mapping of representative sample (B-3). Presence of Colemanite
CONCLUSION
In this study, the effects of oxygen partial pres-sure on copper losses to slag with colemanite addition were theoretically and experimentally studied for the fayalite-type slags under smelting and converting atmospheric conditions. From the FactSage program calculations, it was demon-strated that calcined colemanite addition expands the liquid slag region toward both lower and higher Fe/(Fe + SiO2) ratios. In addition, when calcined colemanite is added to the system, the oxygen partial pressure can be adjusted in a larger range at the fixed temperature.
The addition of 2%, 4%, and 6% CC to synthetic matte–slag (SM–SS) was investigated experimentally at 1250°C for 2 h under different PO2atmospheres of 107, 109, and 1011atm, respectively. The effects of oxygen partial pressure and CC addition on liquidus temperatures and phase diagrams of fayalite-type slag have been discussed, and the experimental results have showed that the copper contents in the slags without CC addition exhibited an increase along with increasing oxygen partial pressure, which was reported in the previous studies.26,30 However, this effect decreased with the calcined colemanite addition. In any case, it can be concluded from the experimental results that the copper amount in the slag decreases with the increasing additions of CC under all oxidizing atmospheres.
ACKNOWLEDGEMENTS
This study was supported by the National Boron Research Institute with Project BOREN-2007-C0141 and METU. The support given by Eti Copper Inc. by performing the wet chemical analysis is gratefully acknowledged.
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