CHARACTERIZATION AND RECOVERY OF VALUABLES FROM WASTE COPPER SMELTING SLAG
Sarfo Prince1, Jamie Young1, Guojun Ma1, 2, Courtney Young1
1 Department of Metallurgical and Materials Engineering, Montana Tech of the University of Montana, Butte, MT, USA, 59701
2 The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan, China, 430081
Keywords: slag; recovery; characterization; carbothermal; reduction
Abstract
Silicate slags produced from smelting copper concentrates contains valuables such as Cu and Fe as well as heavy metals such as Pb and As which are considered hazardous. In this paper, various slags were characterized with several techniques: SEM-MLA, XRD, TG-DTA and ICP-MS. A recovery process was developed to separate the valuables from the silicates thereby producing value-added products and simultaneously reducing environmental concerns. Results show that the major phases in air-cooled slag are fayalite and magnetite whereas the water-cooled slag is amorphous. Thermodynamic calculations and carbothermal reduction experiments indicate that most of Cu and Fe can be recovered from both types using minor amounts of lime and alumina and treating at 1350°C (1623K) or higher for 30 min. The secondary slag can be recycled to the glass and/or ceramic industries.
1. Introduction
Over the years, a large amount of slag has been produced because of the increased demand for copper. About 80% of the world’s copper is produced from sulfide ores by concentrating, smelting and refining [1,2]. During copper smelting, two liquid phases are formed, namely a copper-rich matte and a slag [3]. It is estimated that, for a ton of copper produced, 2.2 to 3 tons of slag is generated [2-5]. These molten copper slags are dumped near the smelter site and is either allowed to cool slowly under air forming a dense, hard crystalline product or granulated with water resulting in a glassy, amorphous material.
Copper smelted slag typically contains about 1% copper (Cu) and 40% iron (Fe) with the balance being significant amounts of silica (SiO2) and minor amounts of other elements (e.g., zinc, Zn; molybdenum, Mo; lead, Pb; and arsenic, As). Some of these minor elements can be deleterious to the environment. Notwithstanding this negative aspect, the copper slag does contain valuables which can be recovered by recycling the slag, hence mitigating its negative impact by producing economic benefits and returning the affected site to its near-natural setting after the slag is removed.
Metal recovery from the copper slag in this study was performed at a high temperature with graphite as a reducing agent and some fluxes to adjust the slag properties. Based on their thermodynamic stability, the carbothermal reduction is able to produce a Cu and Fe rich alloy as a pig iron as well as a secondary slag herein referred to as glass for simplicity purposes.
2. Thermodynamic calculations
In this study, theoretical thermodynamic calculations of chemical reactions, binary alloy phase diagrams of Cu-Fe-C-Si-Mo-As systems and CaO-SiO2-Al2O3 ternary phase diagrams were considered thoroughly using a typical slag composition.
Advances in Molten Slags, Fluxes, and Salts: Proceedings of The 10th International Conference on Molten Slags, Fluxes and Salts (MOLTEN16) Edited by: Ramana G. Reddy, Pinakin Chaubal, P. Chris Pistorius, and Uday Pal TMS (The Minerals, Metals & Materials Society), 2016
2.1. Selection of slag forming system
Based on the pyrometallurgical principles, the overall reduction reaction of waste copper slag by carbon can be illustrated simply by the following equation:
Copper slag + Carbon + Fluxes = Pig iron + Glass + Gas
In order to improve the productivity of metal, fluxes such as CaO, Al2O3, and Na2O, are usually added in the system. In the reduction process, the oxides (and sulfides) of Fe and Cu would be reduced to form pig iron. Other oxides of Mo, P and As would also be reduced and dissolved into the pig iron along with small amounts of silica. Carbon is typically saturated in the molten iron- based alloy, while zinc and lead oxides in waste slag would be vaporized.
Fig. 1 shows that the phase diagrams of Fe-Cu, Fe-Mo, Fe-Si, Fe-P, and Fe-As, as well as Cu content variation with temperature in Fe-Cu-C system [6-10]. It indicates that ~10% copper can be dissolved into the liquid phase at 1573K (Fig. 1a), while the solubility of copper increases with decreasing of carbon content and temperature increase (Fig. 1b). Silicon and molten iron at high temperature are miscible (Fig. 1d) but silicon can decrease the solubility of carbon in molten iron [3]. Phosphorous exists in the pig iron in the forms of Fe2P and Fe3P (Fig. 1e).
Arsenic has similar behavior with that of phosphorus (Fig. 1f) and would be completely reduced and dissolved in the pig iron [11].
A) B)
C) D)
E) F)
Fig. 1 Phase diagrams of alloys [6-10] (a) Cu-Fe; (b) Effects of T and C content on Cu content; (c) Fe-Mo; (d) Fe-Si; (e) Fe-P; and (f) Fe-As.
Considering the overall slag forming system and reaction temperature, CaO, SiO2 and Al2O3
are the major components in the waste slag so only the iron oxides would be reduced. After modifying the chemical composition of the slag, their approximate positions of various slags present in the CaO-SiO2-Al2O3 phase diagram are shown in Fig. 2 [12]. The chemical composition of the original copper slag sample is plotted in Fig. 2 along with that of a typical Blast Furnace (BF) ironmaking slag.
It can be seen from Fig. 2 that there are two areas with low melting points less than 1573K, i.e., Slag I after reduction and Slag II after reduction (marked areas). The advantages for this targeted slag composition are mainly due to its lower melting point (the lowest melting point is
~1443K) and possibly higher silicon in pig iron which is essential for its fluidity. However, in the area for Slag I after reduction, since the final slag phase has a lower basicity (B) (%CaO/%SiO2 = 0.4~0.5 at ~25% CaO, ~60% SiO2 and ~15% Al2O3), the desulphurization ratio of the slag would be reduced and could also seriously corrode the refractory lining of the smelting furnace. Therefore, it is also suggested that Slag II after reduction be examined as a second targeted final slag area. This has a basicity of ~1.0-1.2 and a composition of ~40% CaO,
~40% SiO2 and ~20% Al2O3 [12]. This might be good for impurities removal from the pig iron.
Moreover, because the alumina content in Slag II is higher than that of Slag I, a higher strength slag could result assuming that property is important to have.
Moreover, Maweja et al. suggested that a good viscosity of less than 5 poise of the slag would be easy to separate the slag and molten metal [13]. According to the viscosity diagram of CaO- SiO2-Al2O3 at 1673K (Fig. 3), it is possible to obtain the Slag II composition after reduction.
Fig. 2 Phase diagram of CaO-SiO2-Al2O3 [12].
Original copper slag
Slag I after reduction
Slag II after reduction
Blast Furnace slag
Fig. 3 Viscosity diagram of CaO-SiO2-Al2O3 at 1673K (Poise) [13].
2.2. Chemical reactions in the reduction system
The chemical composition of the copper slag is very complex and it mainly exists as oxides or sulfides in the slag [5,14]. Many chemical reactions would occur in the reduction systems, such as the direct and indirect reduction of oxides or silicates (see Table 1) as well as the reduction of oxides by molten iron. Table 1 shows that most of the major oxides (except silica) in the slag can be directly reduced by carbon, only iron oxides can be reduced indirectly with carbon monoxide. Moreover, the on-set temperatures of these chemical reactions would decrease when the fluxes are added in the reduction system. The reduction temperature can be set as >1623K after considering the possibility of reduction reactions, melting point and viscosity of the final slag.
3. Materials and methods 3.1. Materials
In this study, copper slags from the inactive Anaconda Copper Company smelter in Anaconda, Montana and the active Freeport McMoRan smelter in Miami were used. Samples obtained from both sites had size distributions ranging from .015 to .323 inches.
3.2. Methods
3.2.1. Chemical characterization
Elemental compositions of the slags were measured by the Montana Bureau of Mining and Geology (MBMG) using inductively coupled plasma mass spectrometry (ICP-MS) after lithium tetraborate fusion and acid digestion with 5% hydrochloric acid. Results are shown in Table 2 as oxide or element in weight percent.
3.2.2. Mineralogical characterization
The slags were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermogravimetric and differential thermal analysis (TG-DTA)
3.2.3. XRD
X-ray diffraction (XRD) was carried out with a Rigaku Ultima IV X-ray Diffractometer using Cu-Kα radiation at 40kV and 40mA. Results were analyzed using Rigaku PDXL software (Fig.
4 is presented and discussed later).
Table 1 Major chemical reactions in the carbothermal reduction system [15].
No. Chemical reaction ∆Go(J/mol) On-set T(K)
1 Cu2O(l) + C(s) = 2Cu(l) + CO(g) 9410-122.31T 76.9
2 Fe3O4(s) + C(s)= FeO(s)+ CO(g) 207510-217.62T 953.5
3 1/2MoO2(l) + C(s)= Mo(l) + CO(g) 161120-173T 931.3 4 ZnO(l) + C(s)= Zn(l) + CO(g) 231902-187.55T 1236.5 5 PbO(l) + C(s)= Pb(l) + CO(g) 73520-158.79T 463.0 6 FeO(l) +C(s) = Fe(l) + CO(g) 141740-140T 1012.4
7 SiO2(s) + 2C(s) = Si(s) + 2CO(g) 675889-363.71T 1858.3
8 2FeO.SiO2(s) + C(s) = Fe(s)+ SiO2(s) + CO(g) 354140-341.59T 1036.7 9 Cu2O(l) + CO(g) =2Cu(l) + CO2(g) -161380+52.24T 3089.2
10 Fe3O4(s) + CO(g) = FeO(s) + CO2(g) 35380-40.16T 881.0
11 1/2MoO2(l) + CO(g) = Mo(l) + CO2(g) 9670+1.55T - 12 ZnO(l) + CO(g) = Zn(l) + CO2(g) 61112-13T 4700 13 PbO(l) + CO(g) = Pb(l) + CO2(g) -97270+15.76T 6172.0 14 FeO(l) + CO(g) = Fe(l) + CO2(g) -24910+31.6T 788.3 15 1/2SiO2(s) + CO(g) = Si(l) + CO2(g) 194021-15.07T 12874.7 16 1/2(2FeO.SiO2(l)) + CO(g) =
Fe(l) + SiO2(s) + CO2(g) -20890+32.34T 645.9
17 2C(s) + O2(g) = 2CO(g) -221840-178.01T -
18 C(s) + CO2(g) = 2CO(g) 170790-174.55T -
19 1/2(2FeO.SiO2(l)) + C(s) + 1/2CaO(s) =
Fe(l) + 1/2CaO.SiO2(l) + CO(g) 43345-72.36T 599.0 20 1/2(2FeO.SiO2(l))+ CO(g)+ CaO(s) =
Fe(l) +1/2CaO.SiO2(l) + CO2(g) -84100+29.83T 2819.3 21 1/4Fe3O4(s)+3/8SiO2(s)+3/4CaO(s)+C(s) =
3/4Fe(l) + 3/8(2CaO.SiO2(l)) + CO(g) 118000-168.015T 702.3 22 1/4Fe3O4(s)+3/8SiO2(s)+3/4CaO(s)+CO(g) =
3/4Fe(l) + 3/8(2CaO.SiO2(l)) + CO2(g) -52790+6.535T 8078.0
Table 2. Oxide and elemental composition of Anaconda and Freeport slags (wt. %).
Oxides/
elements SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O Cu As Mo Zn Pb Anaconda % 34.61 5.48 43.93 6.65 0.33 1.39 1.36 0.50 0.28 - 1.77 0.16
Freeport % 28.07 3.01 59.64 2.76 0.53 1.43 1.21 0.68 - 0.53 0.33 -
3.2.4. SEM-EDS-MLA
The scanning electron microscope was a LEO 1430 VP with a tungsten filament, coupled with an EDAX Genesis XM 4 Energy Dispersive Spectrometer (EDS) and Mineral Liberation Analysis (MLA) software to convert the results into mineralogical percentages. Slag samples were mounted as grains in EpoFix resin and hardener using 1-inch diameter molds. Resulting pucks were cut with diamond saw and remounted in fresh resin/hardener with cross-sections as the exposed face in order to minimize orientation effects due to density differences of locked particles. This face was then polished using alumina paste, coated with carbon, and analyzed by SEM-EDS-MLA (Fig.’s 5 and 6 are presented and discussed later).
3.2.5. TG-DTA
Non-isothermal oxidation of the slag samples was carried out using TA Instruments SDT Q600 simultaneous thermal analysis (STA) with thermogravimetric (TG) and differential thermal analysis (DTA) capabilities. Argon gas and corundum crucibles were used at atmospheric pressure from room temperature to 1300°C under dynamic conditions at heating rates of 20°C/min and gas flow rates of 10 ml/min (Fig.’s 7 and 8 are presented and discussed later).
3.2.6. Carbothermal reduction
Based on thermodynamic calculations as well as chemical and mineralogical characterization, carbothermal reduction experiments were performed using a Sentro Tech furnace with Eurotherm 2404 controller. For the Anaconda slag experiments, 30g of ground slag was mixed with 3.627g of graphite, 2.829g of calcium oxide, and 1.353g of alumina. For the Freeport slag experiments, 30g of ground slag was mixed with 3.627g of graphite, 8.925g of calcium oxide, and 3.966g of alumina. These amounts were determined from thermodynamic and mass-balance considerations of the reactions listed in Table 1. Mixtures were placed in alumina crucibles and heated to 1400°C at 5°C/min. Samples were left at the targeted temperature for 30 minutes.
Cooling to room temperature at approximately 2°C/min was performed in the furnace (Fig.’s 9 and 10 are presented and discussed later). The heating and cooling rates were chosen for safety reasons (i.e., prevents boiling and shattering, respectively) which prevented damage to the furnace and extended the life of the WC heating elements. The reaction time was considered the minimum time needed for the tests to work. Many of these variables will be examined via statistical experimental design in a subsequent study [15].
4. Results and discussions 4.1. Chemical composition
As can be seen from the ICP-MS results in Table 2, the slag samples from Anaconda and Freeport are made up of significant amount of SiO2, Fe2O3 and Cu. However, the Zn-content in Anaconda slag is more than that of the Freeport slag, while the Freeport slag contains more Mo.
4.1.1 XRD analysis
XRD powder diffraction patterns for the slag samples are shown in Fig. 4. Anaconda slag is absent of any definitive pattern thereby suggesting it is amorphous. By comparison, Freeport slag exhibits a strong pattern not only indicating it is crystalline but an analysis of the 2θ values between 25 and 40 indicates two major phases are present: magnetite and fayalite. The amorphous feature of the Anaconda slag further suggests it must have been water quenched such that fast cooling rates did not allow crystalline phases to develop. On the other hand, Freeport slag exhibits crystalline features because it solidified slowly as would occur with air-cooling.
10 20 30 40 50 60 70 80 90 Anaconda slag
Intensity
2 Theta (Degree)
Freeport slag Fe2SiO4 Fe3O4
Fig. 4 XRD result of the copper slags.
4.1.2. Microscopy analysis
SEM-EDS-MLA was needed to determine the chemical compositions of all phases in each slag particularly the amorphous features in Anaconda slag and the minor phases not detected in Freeport slag. Several particulate samples of each slag were randomly selected and presumably representative, each measuring at least 3-mm in shortest dimension, and mounted in the epoxy pucks. After polishing the pucks with a series of grit papers and ultimately 0.05-m alumina powder, all exposed surfaces were examined. Example back-scatter-electron (BSE) images and EDS elemental determinations from spot analyses are shown in Fig.’s 5 and 6. Typically, bright inclusions are Cu, Fe and Pb sulfides, gray areas are Fe oxides and silicates, and dark gray masses are Al and Ca silicates, often incorporated with Fe. Mineral Liberation Analyzer (MLA) was then used to determine the chemical compositions from the EDS signals.
Fig. 5 SEM/EDS results of Anaconda slag. Fig. 6 SEM/EDS results of Freeport slag.
2 Theta (Degree)
Fe2SiO4
Fe3O4
Freeport Slag
Anaconda Slag
Results presented in Table 3 are overall averages expressed as minerals in weight percent.
Interestingly, the major phases are Fe1.2Ca0.5Al0.3SiO4 and Fe2SiO4 (fayalite) with Anaconda slag measuring 71.8 and 27.47% and Freeport slag having reverse percentages at 22.90 and 60.56%, respectively. Freeport slag also has more magnetite (11.41%) than the Anaconda slag (0.05%).
Surprisingly, the non-water quenched Freeport slag contained more hydrated minerals (biotite, 0.02%; muscovite, 0.03%) than the Anaconda slag (muscovite, 0.01%). Conichalcite [CaCu(AsO4)(OH)] was found in both slags but in negligible amounts.
Table 3. Chemical/mineral compositions of Anaconda and Freeport slags (wt %).
Mineral Formula Anaconda Freeport
(FeCa)SiO Fe1.2Ca0.5Al0.3SiO4 71.80 22.90
Al2O3 Al2O3 0.00 0.03
Albite NaAlSi3O8 0.00 0.01
Biotite K(Mg,Fe)3(AlSi3O10)(OH)2 0.00 0.02
Bornite Cu5FeS4 0.20 0.29
Calcite CaCO3 0.00 0.00
Chalcocite Cu2S 0.06 0.68
Conichalcite CaCu(AsO4)(OH) 0.00 0.00
Fayalite Fe2SiO4 27.47 60.56
FeO Fe3O4 0.05 11.41
Galena PbS 0.00 0.00
Ilmenite FeTiO3 0.00 0.01
Iron Fe 0.02 0.20
Muscovite KAl2(AlSi3O10)(OH)2 0.01 0.03
Plagioclase (Na,Ca)(Al,Si)4O8 0.01 0.00
Pyrite FeS2 0.00 0.00
Pyroxene CaMgSi2O6 0.00 0.00
Pyrrhotite FeS 0.00 0.24
Quartz SiO2 0.18 0.05
Quartz_FeAlCa Fe1.4Ca0.5AlSi5O7 0.18 3.56
Total 100.00 100.00
4.1.3. TG-DTA
The thermogravimetric and differential thermal analysis (TG-DTA) results are shown in Fig. 7 and 8 and indicate that the waste slags gain weight (~7%) due to oxidation of fayalite to hematite and silica (~750°C) as well as transformation of magnetite to hematite (~400°C) as indicated below with Eq. 1 and Eq. 2 [16]. The weight loss of Anaconda slag below 100°C is attributed to the moisture in the slag. It is also noted that a weight loss near 400°C in the Freeport slag possibly resulting from the dehydration of biotite and muscovite. Moreover, the plateaus of the curves at 950-1050°C concur with the oxidation of sulfides as illustrated in Eq. (3).
2FeO·SiO2 + 0.5O2 → αFe2O3 + SiO2 (1) Fe3O4 → γFe2O3 → αFe2O3 (2) MxSy + yO2 → xM + ySO2 (3)
Figure 7. TG-DTA of Freeport slag. Figure 8. TG-DTA of Anaconda slag.
4.1.4. Carbothermal reduction
Duplicate carbothermal reduction experiments were conducted for each slag. All results confirmed that the final products was pig iron and glass which were separated by breaking with a hammer. After weighing the products, 89.1 and 99.6% of metal was reduced for Anaconda slag (see Fig. 9) and 99.9 and 93.1% of metal was reduced for Freeport slag (see Fig. 10). Resulting products can be recycled to the steel, glass and ceramic industries.
Figure 9: (A) product formed from Anaconda slag after the reduction test, (B) metal recovered from the Anaconda slag.
Figure 10: (A) product formed from the Freeport slag after the reduction test, and (B) metal recovered from the Freeport slag.
5. Conclusions
According to the thermodynamic calculations and the analysis of phase diagrams of alloys and oxides, it is possible to reduce waste copper slag with graphite into pig iron and glass. Most of
A B
A B
1
2
3
1
2
3
Cu and Fe can be recovered from both types using minor amounts of lime and alumina and treating at 1400°C or higher for 30 min. The residual slags are similar with the ironmaking slag and therefore can be recycled to the glass and/or ceramic industry.
6. Ackowledgements
The authors gratefully acknowledge the support from the China Scholarship Council (CSC) for Dr. Guojun Ma and his visit to Montana Tech, USA. Thanks are also extended to Mr. Gary Wyss of the Center for Advanced Mineral & Metallurgical Processing (CAMP) for analytical work and to Dr. Rod James for his contributions to the research and manuscript.
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