Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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Thermal Transformation of Mixed E‑Waste Materials into Clean SiMn/FeMn Alloys Samane Maroufi,* Rasoul Khayyam Nekouei, Kamrul Hassan, and Veena Sahajwalla
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Centre for Sustainable Materials Research and Technology (SMaRT), School of Materials Science and Engineering, University of New South Wales, Sydney 2052, Australia ABSTRACT: In this paper, the use of three fractions of e-waste (i.e., glass, alkaline batteries, and developer kit) in production of clean alloys of silicomanganese (SiMn) and ferromanganese (FeMn) is verified. The need for producing high quality steels with lower impurities has put strain on manganese alloy-making industries to improve the quality of the product by tighter specifications on harmful elements, particularly sulfur and phosphorus. Commercial production of manganese alloys from ordinary ores inevitably introduces many impurities in the final metal, although in small amounts. In recent years, e-waste has turned into a global concern despite the attempts being made worldwide to tackle this critical issue. Using a blend of glass taken from monitors (GCM), the cathode part of alkaline batteries (CAB), and a developer kit (DK) (as rich sources of silicon, Mn/C, and Fe, respectively), a carbothermal process was employed at 1550 °C under argon purge at atmospheric pressure to produce clean alloys. SEM-EDS studies revealed that the produced alloys (i.e., SiMn (30 wt % Si and 70 wt % Mn) and FeMn (7% Si, 15% Mn, and 78% Fe)) were highly pure. X-ray diffraction spectra of the SiMn and FeMn alloys showed strong peaks corresponding to Mn5Si3 and Si/ Mn rich Fe phases, respectively. Reaction of FeMn alloy formation was complete in a shorter time compared to SiMn. The use of such a global waste stream in production of highly pure SiMn and FeMn can be a promising pathway to tackle the problem of depleting natural resources while simultaneously reducing the burden of e-waste. KEYWORDS: E-waste, Recycling, SiMn, FeMn, Thermal processing
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INTRODUCTION The silicon and manganese required for steel making are mainly supplied by ferromanganese (FeMn) and silicomanganese (SiMn) with a rising trend toward silicomanganese.With increasing annual demand for steel worldwide, steel production is expected to reach ∼2.2 billion tonnes of crude steel in 2050 from ∼1.4 billion tonnes in 2010, indicating a strong demand for manganese alloys whose production has increased over the past few years. More than 90% of the manganese used in the world today is produced as FeMn or SiMn. Total manganese alloy produced in 2012 (17.5 Mt) included 5.9 Mt of high carbon FeMn and 11.5 Mt of SiMn. The rest of the manganese is produced as Mn-metal and manganese dioxide.1−3 The primary use of manganese is in the steel industry where manganese is added mainly in the form of ferroalloys. Silicomanganese normally has a silicon content from 17% to 20%, corresponding respectively with carbon contents from approximately 2% down to 1.5%.4 FeMn and SiMn are used in steel production for deoxidation and immobilization of sulfur and as a source of manganese for steel alloying.1 Silicomanganese is a more efficient deoxidiser than silicon or manganese.1,5 This alloy introduces many fewer impurities, such as phosphorus and carbon, compared to FeMn, which stimulates steel manufacturers to use SiMn in steel production.1 The need for producing higher quality steels © XXXX American Chemical Society
with lower impurities has put strain on manganese alloy producers to improve the quality of the product by tighter specifications on harmful elements, particularly sulfur and phosphorus.1 Commercial production of manganese alloys from ordinary raw materials inevitably introduces many impurities in the final metal, although in small amounts. Massive consumption of natural resources as charge materials in such operations are critical issues. As high quality raw resource materials are being depleted worldwide, beneficiation and utilization of secondary resource materials become more important. On the other side, as a result of continuous technological advancement and global digital connectivity, and owing to high consumer demand and the early obsolescence of electronic devices, e-waste has turned into a critical concern. While most metal and plastic components of e-waste are considered of value for recycling, the glass component is of low purity and is technically challenging and economically unattractive for recyclers. Computer monitors, televisions, and other devices with glass screens are a major component of e-waste destined for landfills. In Australia, for example, around 88% of 4 million Received: June 18, 2018 Revised: August 3, 2018
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DOI: 10.1021/acssuschemeng.8b02884 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
A waste developer kit (DK) was supplied in the form of a fine powder. After the moisture was removed in an oven overnight, the sample was subjected to XRF and XRD analyses. Powders of GCM and CAB were subsequently mixed in equal amounts, and then about 3 g of the resulting mixture was placed in a graphite crucible (1 cm diameter and 3 cm height) which was inserted gently into a tubular furnace (100 cm length × 5 cm diameter) that was already at 1550 °C under argon purge (1 L min−1). The reaction was carried out for different times. The gas emitted from the sample during the process of reduction was monitored using an IR gas analyzer connected to the furnace.The sample was then removed from the hot zone of the furnace to the cold zone (i.e., furnace mouth) and left to cool for 10 min under argon purge at different time intervals. Each sample was characterized using XRD, SEM-EDS, and EPMA. Using the same process, GCM, CAB, and DK were mixed with ratio of 0.25:0.25:0.5 and heated isothermally in the tubular furnace. The resulting alloys were further characterized using the techniques mentioned above. To determine the volatile portion, thermal degradation behavior, and carbon content of CAB, thermogravimetric analysis (TGA, PerkinElmer STA 8000 and Pyris V.11 software) using alumina crucibles with a 5 °C/min heating rate was carried out, followed by differential thermogravimetric (DTG) plotting and interpretation. The thermal degradation behavior of the CAB powder was studied by TGA; the sample was heated from room temperature to 900 °C at heating rate of 5 °C/min under oxygen purge of 20 mL min−1. XRD analysis was carried out using PANalytical X0Pert Pro multipurpose X-ray diffractometer (45KW, 40 mA), with step scans from 0° to 100°, a step interval of 0.02°, and a 58 s count time per step. The XRD patterns were analyzed qualitatively using X′PertHighScore Plus software. Morphological investigation as well as qualitative analysis of the samples was carried out using scanning electron microscopy (SEM), with a Hitachi S3400 instrument equipped with energy dispersive spectroscopy (EDS) (Bruker). Prior to a microscopic session, the samples, which were in the form of droplets, were cut, ground, and polished with 3 μm alumina powder. The samples were then mounted in conductive resin, and the accelerating voltage of the probe was selected to be 20 kV in order to be compatible with EDS analysis.
computers and 3 million TVs purchased annually ends up in landfills.6 Glass contains more than 70% silica that can be considered a valuable source of silicon in the production of SiMn alloy. Global battery demand has also entered a period of extremely rapid growth in recent years, and the implications for Australia are potentially highly significant. Driven by their versatility, low maintenance, reduced cost, and their requirements in the electronic industry, the growth in global consumption of batteries is expected to continue. Approximately 400 million hand-held batteries weighing 5 kg or less were sold in Australia in 2012−13.4 For the same time period, 14703 tonnes of batteries were disposed, of which only 403 tonnes were collected for recycling.4 Recovery rates for sealed lead acid, nickel cadmium, lithium primary, and nickel metal hydride batteries were all in the range of 4.4−5.5%, while the recovery rate for alkaline and zinc carbon batteries was estimated at 1.6%.4 A significant proportion of end-of-life batteries are landfilled, stockpiled, incinerated,7 or illegally exported to developing nations, where informal processing exposes poor communities to serious risks of contamination.8,9 Batteries such as alkaline batteries are rich in manganese that can be an alternative and precious source of manganese for production of FeMn and SiMn alloys. Developer kits (DKs) from printers are a valuable source of iron with almost no impurities that can be potentially considered as an alternative source of iron in FeMn production. With depleting natural resources and rapidly increasing demand for steel worldwide, beneficiation and utilization10,11 of low grade materials become more important for steelmaking industries. Currently, the direct use of low grade raw materials (i.e., waste) are considered as a more appropriate approach compared to the conventional natural resources in terms of environmental protection and cost effectiveness. In addition, several attempts are being made worldwide to tackle the problem associated with e-waste. Along with the increased interests in environmental issues and natural resource usage worldwide, this research aims to use e-waste, i.e., glass, alkaline batteries, and developer kits, as a valuable source of silicon, manganese, and iron to produce SiMn and FeMn alloys.
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RESULTS AND DISCUSSION Characterization of Starting Materials. The elemental composition of the GCM was identified by XRF analysis. As shown in Table 1, the GCM consisted primarily of SiO2 (70.3%) and some oxide minerals such as Na2O (13.9%), Al2O3, CaO, and MgO. Table 1. Chemical Composition of GCM
MATERIALS AND EXPERIMENTAL PROCEDURE
Spent computer monitors and alkaline type batteries were collected from the Reverse E-waste Company, Sydney, Australia. A residual (waste) developer kit from a discarded printer was supplied by TESAMM. The glass fraction of the computer monitor (GCM) was dismantled and pulverized using a ring mill. The elemental composition of the GCM was identified by XRF analysis. For XRF analysis, 1 g of powdered pre-dried sample was mixed with 10 g of type 1222 flux thoroughly in a glass vial and transferred to a Pt/Au crucible. The crucible was placed in a high temperature muffle furnace (Fusilux) at a temperature of 1050 °C and allowed to melt and mix for 15 min. The molten flux and sample were poured into 40 mm Pt/ Au casting dishes and left to cool before labeling. Prepared glass beads were measured on an Axios Advanced WDXRF spectrometer (end window Rh tube, 4 kW). An appropriate analytical program was selected, which was calibrated using certified reference materials and used optimal instrumental operating conditions. The cathode part of an alkaline battery (CAB) was manually separated from the supporting metal and plastic case. The powder was dried in an oven overnight and then analyzed using XRD and thermogravimetric analysis (TGA).
oxides
Na2O
SiO2
Al2O3
CaO
MgO
MnO
Fe2O3
weight percent
13.9
70.3
1.8
7.8
4.6
0.01
0.1
The cathode part of the battery was analyzed using XRD. The X-ray diffraction pattern of the CAB powder sample, Figure 1b, shows several peaks corresponding to graphite and smaller peaks belonging to MnO2. The thermal degradation behavior of the CAB powder presented in Figure 1a shows three degradation steps starting around 20 °C. The three variations of the TGA thermogram of the CAB sample are in the temperature ranges of 20−325 °C, 325−535 °C, and 510−750 °C. TGA thermograms of the CAB sample shown in Figure 1a were employed to determine the actual content of each component in the sample. The first 5% weight loss at 20−310 °C is attributed to dehydration of the CAB powder. The second 5% weight loss in the range of 310−510 °C corresponds to the water molecule removed from MnO2 crysallization12 and also partial oxidation B
DOI: 10.1021/acssuschemeng.8b02884 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
Figure 1. (a) Mass loss and Gaussian fit of derivative thermogravimetric (DTG) curves of waste CAB at heating rates of 5 °C/min under purge. (b) XRD spectrum of the CAB sample. (c) XRD spectrum of the DK sample.
Figure 2. Generated gas concentration in terms of CO and CO2 in the production of (a) SiMn and (b) FeMn.
Produced CO2 then quickly reacts with carbon according to the Boudouard reaction (reaction 4). IR-gas analysis clearly shows that at early stages of reaction, the concentration of CO2 increased and quickly dropped, indicating that CO2 was consumed by carbon. MnO and SiO2 are relatively stable, and their reduction takes longer time. Further reduction of MnO to Mn and SiO2 to Si produces CO gas. Gibbs free energy of the reactions are presented in Figure 3. Reduction of MnO to metallic manganese proceeds through carbon or carbon-saturated alloy9 in accordance with reaction 5.
of carbon. It has been proved that the weight loss associated with the removal of oxygen from the manganese oxide lattice, due to the phase transition of MnO2 to Mn2O3 and the transition of Mn2O3 to Mn3O4 up to 900 °C, is not significant.13,14 Therefore, the weight loss in the temperature range of 510−750 °C belongs to the further oxidation of carbon particles. From TGA results, it can be concluded that the carbon content of the CAB powder was 9−12 wt %. The chemical composition of DK was examined using XRF, and it was confirmed that developer kit is in the form of metallic iron. The X-ray diffraction spectrum of DK is shown in Figure 1c. LECO analysis was also performed and confirmed that DK contains around 4 wt % carbon, 0.09 wt % N, and less than 0.009 wt % sulfur. Reactions of Formation of Si−Mn and Fe−Mn Alloys. Figure 2 shows the volume percentage of CO and CO2 from off-gas measurements over time. The high concentration of CO and CO2 in off-gas during the process of SiMn formation is attributed to a higher content of oxides (SiO2 and MnO2) in the initial mixture. The MnO2 is relatively unstable and easily reduced in solid state in the presence of CO(g) generated from the Boudouard reaction (reaction 4): 2MnO2 + CO(g) = Mn2O3 + CO2 (g)
(1)
3Mn2O3 + CO(g) = 2Mn3O4 + CO2 (g)
(2)
Mn3O4 + CO(g) = 3MnO + CO2 (g)
(3)
C(s) + CO2 (g) = 2CO(g)
(4)
As shown in reactions 1−4, reduction of MnO2 to MnO via CO results in producing CO2 at early stages of the process.
Figure 3. Gibbs free energy diagram of the main reactions involved in producing SiMn alloy. C
DOI: 10.1021/acssuschemeng.8b02884 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
Figure 4. XRD spectra of produced (a) SiMn and (b) FeMn alloys.
Figure 5. (a) Silicon and manganese content (wt %) in SiMn alloy. (b) SEM image, elemental mapping, and line scanning of the resulting SiMn alloy.
MnO(l) + C(s) = Mn(l) + CO(g)
(5)
SiO2 (l) + 2C(s) = Si(l) + 2CO
(6)
SiO2 (l) + 2Mn(l) = Si(l) + 2MnO(l)
This reaction is considered to be much faster than the carbothermal reactions 5 and 6. The solubility of carbon in the alloy decreases with increasing Si content. Graphite is the stable phase coexisting with liquid Si−Mn alloy until the Si-content reaches a certain value, approximately 17−18% Si, depending on the temperature. At higher Si-content, SiC is the stable carbon-containing phase. Formation of SiC can be presented by the following reaction:
When the MnO phase is consumed, MnO content in the molten slag decreases below the saturation level, and therefore, MnO activity in slag becomes less than one. In simple MnO− SiO2 slag, the thermodynamic activity of MnO strongly depends on the silica content in the slag. In more complex multicomponent slags such as the SiMn slag, a similar dependency of MnO activity on the concentration of silica in the slag is to be expected.1 The Gibbs free energy versus reaction temperature is plotted in Figure 3. In the case of FeMn alloy, carbon starts diffusing into iron (CC) which reduces the melting point of iron. Carbonsaturated iron melts at 1150 °C. Partial reduction of MnO and SiO2 can also occur via dissolved carbon in iron (reactions 7 and 8). MnO(l) + C = Mn(l) + CO(g)
(7)
SiO2 (l) + 2C = Si(l) + 2CO(g)
(8)
(9)
SiO2 + 3C = SiC + 2CO
(10)
Silica can be reduced by SiC by the reaction:1 SiO2 (l) + 2SiC = 3Si(l) + 2CO(g)
(11)
With addition of DK as a source of iron, MnO is first reduced by reaction 5 (or 7), forming Mn−Fe−C alloy with a high manganese concentration. Silica is then reduced via reaction 6 (or 8). In this case the solubility of carbon in the alloy decreases with increasing Si content. Graphite is the stable phase coexisting with liquid Mn−Fe−Si alloy until the Si-content reaches a certain value, depending on the temperature and the Fe-content of the alloy. Characterization of the Resulting SiMn and FeMn. The resulting SiMn and FeMn were subjected to XRD analysis to identify phases that are formed during the process of the alloy solidification. The X-ray diffraction spectrum of the resulting SiMn (Figure 4a) shows strong peaks corresponding
In the production of SiMn alloy, high temperature is necessary to obtain metal with a sufficiently high content of Si and discard slag with a low amount of MnO. In the SiMn production, MnO is first reduced by reaction 5 (or 7), and silica is then reduced via reaction 6 (or 8). In a system with simultaneous reduction of MnO and SiO2, a slag/metal exchange reaction of manganese and silicon takes place. D
DOI: 10.1021/acssuschemeng.8b02884 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
Figure 6. (a) Silicon, manganese, and iron content (wt %) in FeMn alloy. (b) SEM image, elemental mapping, and line scanning of the resulting FeMn alloy.
to Mn5Si3 and also carbon. In the XRD spectrum of FeMn, Fe and C phases were detected (Figure 4b). Figure 5a illustrates silicon and manganese content (wt %) in the resulting SiMn alloy versus reaction time in the range of 30−250 min. The results presented in Figure 5a for different times is the average of EDS analysis data corresponding to different points. Silicon concentration shows a gradual increase from 13 to 30 wt % over the reaction time from 30 to 250 min, while the Mn content decreased from 87 to 70 wt %. As can be seen from Figure 5a, reduction of SiO2 and MnO was complete in 120 min reaction time. Given that the concentration of Si reached 17 wt % in a few minutes, according to reactions 8 and 9, silica was also reduced by SiC. However, X-ray diffraction pattern of the final product did not show any peaks corresponding to SiC. It implies that all of the SiC phase was consumed in the reduction of SiO2 to Si. Figure 5b represents elemental mapping and EDS line scanning of the resulting SiMn alloy after 180 min of heat treatment. EDS analysis confirms a relatively high level of purity of the resulting SiMn alloy. Iron, silicon, and manganese contents (wt %) in the resulting FeMn alloy versus reaction time from 30 to 180 min are shown in Figure 6a. Silicon and manganese concentrations gradually increased to 7 and 15 wt % in 90 min, while the content of iron decreased to 78 wt % and remained constant after 90 min reaction time, implying that the reaction was complete in a shorter time compared to SiMn. Figure 6b represents elemental mapping and EDS line scanning of the resulting FeMn alloy after 120 min of heat treatment. As can be seen clearly from elemental mapping, the produced Fe−Mn was highly pure and two phases are present in the synthesized alloy. The presence of these two phases, i.e., Fe3Si and Mn2Fe3Si3, was also confirmed by XRD. In the high magnification SEM image of the resulting FeMn alloy illustrated in Figure 7, several areas of different color can be observed. Elemental line scanning indicates that the two large, light gray circular areas contain high concentrations of iron. The content of Si and Mn in the remaining area with dark gray is higher. According to the Fe−Si−Mn phase diagram, for the Fe (78 wt %)−Mn(15 wt %)−Si(7 wt %) ternary system, Fe with FCC and BCC structure is expected to be the dominant phase, while Mn and Si are dissolved in the iron. Therefore, the circular area shown in the SEM image (Figure
Figure 7. High magnification SEM image and elemental line scan of the FeMn alloy.
7) is representative of the Fe phase and the dark gray area is iron with different amounts of dissolved Si and Mn. The produced clean alloys can be further used in steel production for the purpose of deoxidation and immobilization of sulfur and as a source of manganese for steel alloying.
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CONCLUSIONS SiMn and FeMn alloys were produced via carbothermal processing of three problematic components of e-waste: the glass fraction of computer monitors (GCM), the cathode part of alkaline batteries (CAB), and developer kit (DK). Carbothermal processing was carried out at 1550 °C under argon purge at atmospheric pressure to produce clean alloys. The produced alloys (i.e., SiMn (30 wt % Si and 70 wt % Mn) and FeMn (7% Si, 15% Mn, and 78% Fe)) were highly pure. In X-ray diffraction patterns of resulting SiMn and FeMn, strong peaks corresponding to Mn5Si3 and Si/Mn rich Fe phases were detected, respectively. The formation of FeMn alloy was complete in a shorter reaction time compared to the formation of SiMn alloy. The proposed sustainable process could simultaneously demonstrate a sustainable low cost means of producing pure alloys and the benefits of transforming waste to value. Such sustainable, cost-effective approaches to transforming waste into secondary resources and products can help address the challenge of global resource depletion. E
DOI: 10.1021/acssuschemeng.8b02884 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
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AUTHOR INFORMATION
Corresponding Author
*E-mail: s.maroufi@unsw.edu.au. ORCID
Samane Maroufi: 0000-0001-5553-8519 Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Olsen, S.; Tangstad, M.; Lindstad, T. Chapters 2, 7, and 8. In Production of Manganese Ferroalloys; SINTEF and Tapir Academic Press: Trondheim, Norway, 2007. (2) Kononov, R. Carbothermal Solid State Reduction of Manganese Oxide and Ores in Different Gas Atmosphere. Ph.D. Thesis, The University of New South Wales, Sydney, Australia, 2008. (3) Peacy, J. G; Davenport, W. G. The Iron Blast Furnace; Hopkins, D. W., Ed.; Pergamon Press: Sydney, 1979. (4) Ringdalen, E.; Gaal, S.; Tangstad, M.; Ostrovski, O. Ore Melting and reduction in silicomanganese production. Metall. Mater. Trans. B 2010, 41, 1220−1229. (5) Maroufi, S.; Ciezki, G.; Jahanshahi, S.; Sun, S.; Ostrovski, O. Dissolution Rate and Diffusivity of Silica in SiMn Slag. Metall. Mater. Trans. B 2015, 46 (1), 101−108. (6) See http://www.abs.gov.au/ausstats/
[email protected]/Products/4602.0. 55. 005~2013~Main+Features~Electronic+and+Electrical+Waste?Open Document. (7) De Michelis, I.; Ferella, F.; Karakaya, E.; Beolchini, F.; Vegliò, F. Recovery of zinc and manganese from alkaline and zinc-carbon spent batteries. J. Power Sources 2007, 172 (2), 975−983. (8) Electronics TakeBack Coalition. Recycle It Right: Guide to Recycling Your Electronics; http://www.electronicstakeback.com/howto-recycle-electronics/. (9) Hilty, L. M. Electronic Waste-an emerging risk? Environmental Impact Assessment Review 2005, 25 (5), 431−435. (10) Sahajwalla, V.; Zaharia, M.; Mansuri, I.; Rajarao, R.; Dhunna, R.; Nur Yunos, F.; Khanna, R.; Saha-Chaudhury, N.; O’Kane, P.; Fontana, A.; Jin, Z.; Skidmore, C.; Vielhauer, P.; O’Connell, D.; Knights, D. The Power of SteelmakingHarnessing High-Temperature Reactions to Transform Waste into Raw Material Resources, Proceedings of the Iron and Steel Technology Conference, Pittsburgh, Pennsylvania, May 6−9, 2013; Hickey, K. D., Ed.; Association for Iron and Steel Technology: 2013; Vol. 1, pp 68−83. (11) Zaharia, M.; Sahajwalla, V.; Saha-Chaudhury, N.; O’Kane, P.; Fontana, A.; Skidmore, C.; Knights, D. Recycling of rubber tyres in electric arc furnace steelmaking: Carbon/slag reactions of coke/ rubber blends’. High Temp. Mater. Processes 2012, 31, 593−602. (12) Zolfaghari, A.; Naderi, H. R.; Mortaheb, H. R. Carbon black/ manganese dioxide composites synthesized by sonochemistry method for electrochemical supercapacitors. J. Electroanal. Chem. 2013, 697, 60−67. (13) Reza Naderi, H.; Reza Ganjali, M.; Norouzi, P. The study of supercapacitive stability of MnO2/MWCNT nanocomposite electrodes by fast Fourier transformation continues cyclic voltammetry. Int. J. Electrochem. Sci. 2016, 4267−4282. (14) Reddy, R. N.; Reddy, R. G. J. Power Sources 2004, 132, 315.
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DOI: 10.1021/acssuschemeng.8b02884 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
